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Distinct roles for DC-SIGN+-dendritic cells and Langerhans cells in HIV-1 transmission
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Distinct roles for DC-SIGN+-dendritic cells and Langerhans cells in HIV-1 transmission Lot de Witte, Alexey Nabatov and Teunis B.H. Geijtenbeek Department of Molecular Cell Biology and Immunology, VU University Medical Center, Amsterdam, The Netherlands
Dendritic cells (DCs) are thought to mediate HIV-1 transmission but it is becoming evident that different DC subsets at the sites of infection have distinct roles. In the genital tissues, two different DC subsets are present: the Langerhans cells (LCs) and the DC-SIGN+-DCs. Although DC-SIGN+-DCs mediate HIV-1 transmission, recent data demonstrate that LCs prevent HIV-1 transmission by clearing invading HIV-1 particles. However, this protective function of LCs is dependent on the function of the C-type lectin Langerin: blocking Langerin function by high virus concentrations enables HIV-1 transmission by LCs. Here, we will discuss the molecular mechanisms involved in HIV-1 transmission and viral clearance. A better understanding of these processes is crucial to understand and develop strategies to combat transmission. Dendritic cells and sexual HIV-1 transmission A better understanding of the mechanisms governing HIV-1 transmission (see Glossary) is crucial for the development of strategies to prevent new HIV-1 infections. Sexual transmission is the major route of acquiring HIV-1 worldwide [1,2]. However, the sequential events that occur from the moment HIV-1 encounters the genital epithelial barrier until the virus has caused a systemic infection remain obscure. HIV-1 is thought to target dendritic
Glossary Dendritic cells Birbeck granules: Organelles, consisting of pentalamellar and zippered membranes that can be visualized using electron microscopy. Langerin has been shown to induce their formation and the organelles have only been found in LCs. The function of Birbeck granules is not known yet. CD1a: Family member of the CD1 cell surface receptors that function as a microbial recognition system for the activation of T cell responses. CD1a has been shown to be involved in the presentation of mycobacterial lipids to CD1a-restricted T cells. CD34+-derived LCs: A model to study LCs. CD34+ stem cells are isolated from bone marrow and cord blood and cultured specific cytokines. After 6 days these cells acquire a DC morphology, express Langerin and contain Birbeck granules. C-type lectins: A family of cell surface receptors that belong to the pattern-recognition receptor family. These receptors recognize
Corresponding author: Geijtenbeek, T.B.H. ([email protected]). Available online 4 December 2007. www.sciencedirect.com
carbohydrate structures and are thought to play an important role in the immune system. DC-SIGN: A C-type lectin that is expressed by a subset of immature DCs and specific macrophages. This receptor is involved in the capture of various pathogens and mediates internalization and antigen presentation. DC-SIGN also mediates cellular communication between different immune cells. Moreover, DC-SIGN modulates the immune response by activating intracellular signaling pathways. Dendritic cells (DCs): These cells are professional antigen presenting cells, since these cells are essential to the activation naı¨ve T cells. Different DC subsets have been described, such as the myeloid and plasmacytoid DCs, and the Langerhans cells. All these subsets are thought to have special capacities and specialties. Langerin: A C-type lectin that is specifically expressed by immature LCs and is present in Birbeck granules. Moreover, Langerin induces the formation of Birbeck granules. Monocyte-derived DCs (moDCs): A model to study DCs. Human monocytes are cultured in the presence of IL-4 and GM-CSF. After 6 days these cells acquire a typical DC morphology and express different DC markers such as CD80, CD86, DC-SIGN and are able to stimulate naı¨ve T cells HIV-1 APOBEC3G: A host factor with anti-viral activity that can restrict HIV-1 infection. APOPEC3G is a cytidine deaminase that is incorporated into virions during viral production and subsequently triggers massive G-to-U hypermutation in the nascent retroviral genome. In addition, cellular APOPEC3G can function as a post-entry restriction factor for HIV during reverse transcription in resting CD4+ T cells. HIV-1 infection in cis: A receptor (e.g. DC-SIGN) captures HIV-1 and facilitates infection of the same cell via the primary HIV-1 receptor CD4 and the chemokine receptors. HIV-1 infection in trans: A receptor (e.g. DC-SIGN) captures HIV-1 and mediates infection of another cell by the same virion (also referred to as first phase transmission) HIV-1 transmission: The process in which the virus is transferred from an infected person to a non-infected person. Sexual transmission is the main route of acquiring HIV-1. The mechanisms how the virus finds its way from the site of sexual contact in mucosal tissues to infect the CD4+-T cells, which are the main target cells for HIV-1 and reside in lymphoid tissues, have not been elucidated yet. Recent studies indicate that DCs are involved in the transmission of HIV-1. Microbicides: Apart from the use of condoms, there is no method to prevent the transmission of HIV-1. Therefore, there is a need to develop microbicides. A microbicide is a topical agent, such as a cream or gel, that can be applied on the genital tissues to prevent the transmission of HIV-1. SIV and SHIV: SIV is the simian immunodeficiency virus that infects macaques and SHIV is an abbreviation used for chimeric replicating viruses containing different combinations of genes from both HIV and SIV. X4- and R5-tropic HIV-1: To infect a cell HIV-1 needs the expression of the entry receptor CD4 as well as the expression of a co-receptor. X4- and R5-tropic HIV-1 are strains that use the co-receptor CXCR4 and CCR5, respectively.
1471-4914/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.molmed.2007.11.001
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Figure 1. DCs mediate HIV-1 clearance and antigen presentation but also viral transmission to T cells. DCs are antigen-presenting cells. They capture antigens in the periphery and migrate to the lymphoid tissues to initiate a specific T-cell response. (a) LCs (pink) and DC-SIGN+-DCs (green) capture HIV-1 in the genital mucosal tissues, process the antigens and migrate to the lymphoid tissues where they present HIV-1 antigens on MHC I and MHC II to T cells, resulting in a T-cell response. Moreover, LCs clear the virus efficiently and thereby are thought to function as a protective barrier. However, it is also thought that HIV-1 subverts DC function at the moment of sexual transmission. (b) DC-SIGN+-DCs capture the virus in the submucosal tissues and LCs, exposed to high concentrations of HIV-1, loose their protective function and become infected. DC-SIGN+-DCs and LCs migrate to the lymphoid tissues and mediate transmission of the virus to T cells, resulting in a systemic infection.
cells (DCs) in the mucosal genital tissues to subvert their function and to reach their main target cells: the CD4+ T cells [3]. DCs have a pivotal role in the adaptive defense against invading pathogens. Immature DCs capture foreign antigens in the periphery. Next, the DCs acquire a mature phenotype and migrate to the lymphoid tissues. Here, the mature DCs present the processed antigens on MHC to naı¨ve T cells, resulting in an adaptive immune response against the pathogen. HIV-1 is thought to hijack DC function: DCs capture HIV-1, migrate to the lymphoid tissues but, instead of – or at the same time as – inducing an efficient immune reaction against the virus, DCs mediate HIV-1 transmission to T cells in the lymphoid tissues (Figure 1). However, recent data demonstrate that, in contrast to the current dogma, not all DC subsets mediate HIV-1 transmission but that the epithelial DC subset, the Langerhans cells (LCs), form a protective barrier that prevents HIV-1 transmission [4]. Here, we will summarize the recent developments concerning the function of mucosal DC subsets in HIV-1 infection. We will discuss how this knowledge has changed our view of the mechanisms that might be involved at the moment of sexual HIV-1 transmission and what the implications are for the development of better strategies to prevent HIV-1 transmission. www.sciencedirect.com
Different DC subsets in genital mucosal tissues To understand the role of DCs in the sexual transmission of HIV-1, it is important to identify the different DC subsets at the site of HIV-1 infection. Genital mucosa contain at least two important DC subsets: the LCs and a DC subset that is characterized by the expression of the C-type lectin receptor DC-SIGN (DC-SIGN+-DCs) (Figure 2). Both subsets have a specific anatomical localization and express unique receptors. LCs reside in the stratified mucosal epithelia of the vagina, ectocervix and foreskin but not in the columnar epithelium of the rectum and the endocervix [4–6]. Moreover, LCs are present abundantly in the epidermis of foreskin, glans penis and skin [5]. LCs are characterized by the expression of CD1a and Langerin and contain Birbeck granules, which are LC-specific cell organelles. Although LCs contribute to less than 1% of the total epithelial cells, their long dendrites form a continuous network throughout the epithelial layer and extend into the lumen of the mucosa to maximize their surveillance function (Figure 2). Owing to these anatomical and functional characteristics, LCs are the first DC subset that encounters HIV-1 within intact genital epithelial tissues. The low transmission rate of HIV-1 suggests that the epithelial barrier might be an important barrier to HIV-1. Indeed, HIV-1 transmission is enhanced under conditions of trauma and ulcerative anogenital
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Figure 2. Langerhans cells reside in the epithelia, whereas DC-SIGN+-DCs reside in the subepithelia. Different DC subsets are present at the mucosal epithelium where HIV-1 enters the body. To illustrate these subsets, cryosections of human foreskin were fixed and stained with primary antibodies against CD1a, Langerin and DC-SIGN. Sections were counterstained with isotype specific Alexa-488 (green) or Alexa-594 (red) anti-mouse antibodies and nuclei were stained blue with Hoechst. The slides were analyzed by immunofluorescence microscopy and the mucosal epithelium is depicted. LCs were visualized using antibodies against (a,b) CD1a (red) and (b) Langerin (green) (adapted, with permission, from Ref. [4]). The DC-SIGN+-DCs were stained using antibodies against (c) DC-SIGN (green). (d) A cartoon is displayed to show the specific localization of the LCs and DC-SIGN+-DCs in the genital mucosal epithelia.
co-infections [2,7,8], which damage the epithelial LC-rich layer and provide access for HIV-1 to the subepithelial DCSIGN+-DCs (Figure 2). Moreover, the rectal epithelium consists of columnar epithelial cells that lack LCs and contain DC-SIGN+-DCs [6]. Thus, HIV-1 encounters different DC subsets depending on the site of infection and other influencing factors (e.g. the integrity of the epithelial layer). A better understanding of the precise HIV-1–DC interactions during infection will require detailed investigation into the function of the different DC subsets in HIV-1 transmission. The paucity and difficulty of isolating primary DCSIGN+-DCs has led to the use of human monocyte-derived DCs (moDCs) as a model for DCs at the site of HIV-1 transmission because these moDCs also express DC-SIGN (Box 1). Here, we will first discuss the current knowledge regarding the molecular interactions of HIV-1 with moDCs and then we will discuss and speculate on the recent results obtained with primary human DC-SIGN+-DCs and LCs. MoDCs capture HIV-1 efficiently MoDCs are used as a model to study the interaction of HIV1 with the subepithelial DC-SIGN+-DCs. MoDCs capture HIV-1 efficiently; however, the receptors that are important for this binding are under debate. There is consensus that the C-type lectin DC-SIGN expressed on cell lines binds HIV-1 efficiently [9,10] (Box 1). In addition, various research groups have demonstrated that DC-SIGN is important for the binding of HIV-1 to moDCs [9,11,12]. www.sciencedirect.com
However, other reports have stated that moDCs bind HIV1 independently of DC-SIGN [10,13] and suggested other receptors might be involved, such as the glycospingholipid galactosyl ceramide [14], the mannose receptor [15,16] and HIV-1 receptor CD4 [16]. A recent study demonstrates that binding of HIV-1 to moDCs is dependent on both DC-SIGN and the heparan sulphate proteoglycan Syndecan-3 [17]. Differences in binding assays, DC culture and activation protocol and virus production might have contributed to these conflicting results. Therefore, it is now essential to define the role of the different HIV-1 receptors on primary DC-SIGN+-DCs rather than on cultured moDCs. The interaction of HIV-1 with the different receptors on DC-SIGN+-DCs is thought to dictate the subsequent fate of the virus within the DCs, resulting in either degradation and antigen presentation or viral capture, infection and transmission. MoDCs mediate viral transmission of HIV-1 MoDCs mediate transmission of HIV-1 to T cells efficiently and it is now evident that different pathways are involved [18] (Figure 3). DCs can transmit HIV-1 without becoming infected. This was shown by experiments in which DCs capture single round replication-defective HIV-1 efficiently and transfer the same virus particles to T cells [9,19]. This mechanism is referred to as first-phase transmission or trans-infection because it occurs fast and independently of de novo virus synthesis [20]. After capture, the virus is internalized into endosomes or multivesicular bodies,
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Box 1. DC-SIGN versus Langerin: the similarities and the differences Similarities DC-SIGN and Langerin belong to the family of C-type lectins: receptors that recognize carbohydrates in a calcium-dependent manner. DC-SIGN and Langerin are both type II transmembrane proteins with an extracellular region consisting of a neck involved in multimerization and a C-terminal C-type carbohydrate-recognition domain. Both DC-SIGN and Langerin recognize carbohydrate structures with high mannose specificities and, therefore, both bind HIV-1 gp120. Differences Langerin has a unique expression pattern because it is only expressed by LCs, which reside abundantly in epidermis and mucosal stratified epithelia. DC-SIGN is expressed by DCs in dermis, submucosa and rectal epithelia but also on DCs in other tissues, such as the lymphoid tissues. Moreover, DC-SIGN can be expressed by macrophages during inflammation. The carbohydrate-recognition domain of the DC-SIGN gene is highly conserved, whereas Langerin polymorphisms are present that
protected from degradation and transferred subsequently to the infectious synapse formed by DCs and T cells during HIV transmission [11,21]. Moreover, transmission from multivesicular bodies might also occur through exosomes [22]. Recently, it was suggested that cell-surface-bound HIV-1 primarily is transferred from moDCs to CD4+ T cells [23], although the question of whether cell surfacebound HIV-1 is protected remains to be investigated. By
result in different carbohydrate-binding capacities. Future research needs to further elucidate whether Langerin polymorphisms result in different HIV-1 transmission rates. In contrast to DC-SIGN, the murine homologue of Langerin shows large similarities to human Langerin, including the formation of Birbeck granules and expression on LCs. This enables future research on Langerin function in small animal models. The carbohydrate-recognition spectrum of both C-type lectins overlaps partly (both recognize high mannose and fucose structures) but also differs (Langerin recognizes a broader variety of carbohydrate structures). The intracellular domain of DC-SIGN and Langerin is distinct and might reflect differences in function. Langerin induces the formation of the Birbeck granules in Langerin+ cells and mediates HIV-1 degradation, whereas DC-SIGN mediates HIV-1 transmission. Future research will demonstrate whether the extracellular domains of Langerin and DC-SIGN, which are similar in the respect of binding mannose- and fucose-containing structures, might also contribute to the differences in function.
targeting moDCs, HIV-1 has not only found an efficient way to reach and infect T cells but, being captured inside DCs, it is also protected from extracellular antiviral factors. Therefore, it is essential for future studies to focus on virus survival and not just virus transmission. Notably, DC infection by HIV-1 might provide long-term survival to HIV-1 within DCs [19,20,24] and, as such, might contribute to viral transmission. This mechanism
Figure 3. LCs and DC-SIGN+-DCs interact differently with HIV-1. Different interactions of HIV-1 with the different DC subsets have been described. (a) Langerin is a receptor for HIV-1 on LCs and internalizes HIV-1 through the Birbeck granules (i). Langerin interaction with HIV-1 results in HIV-1 degradation, viral clearance and inhibition of HIV-1 transmission. However, the specific organelles in which HIV-1 is degraded need to be elucidated. (ii) A block of Langerin function through drugs, co-infections or mutation confers the ability on LCs to transmit HIV-1 efficiently to T cells (right) through infection of the LCs. (b) DC-SIGN is a receptor for HIV-1 on DCs and internalizes HIV-1. (i) HIV-1 is targeted to the lysosomes, resulting in viral degradation and MHC II presentation. (ii) HIV-1 is targeted to the multivesicular body, resulting in HIV-1 transmission to T cells. (iii) LSP-1 targets HIV-1 from the endosome to the proteasome, resulting in MHC I presentation. (iv) DC-SIGN+-DCs can be infected with HIV-1. DC-SIGN enhances DC infection in cis. Infection of DC-SIGN+-DCs can result in HIV-1 transmission. www.sciencedirect.com
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is referred to as second-phase transmission because of its importance in transmission several days after HIV-1 capture [20]. MoDCs express the HIV-1 (co-)receptors (CD4, CCR5 and CXCR4), enabling infection of the cells. Moreover, infection can be enhanced by DC-SIGN in cis; DC-SIGN captures HIV-1 efficiently and facilitates the interaction of HIV-1 with CD4 and the co-receptors on the same cell, resulting in infection [19,25]. The reported low levels of CD4, CXCR4 and CCR5 on moDCs can be responsible for their weaker susceptibility to HIV-1 infection in vitro compared with that of CD4+ T cells [4]. However, cellular restriction factors, such as APOBEC3G (see Glossary), and HIV-1 degradation in moDCs might also restrict infection of DCs [26,27]. Thus, DC-SIGN+-DCs mediate HIV-1 transmission through different pathways (Figure 3). By studying human moDCs, the molecular mechanisms that direct these different pathways, including the specific receptors and intracellular signaling molecules, are being unraveled rapidly. DC-SIGN is involved in routing of HIV-1 to the infectious synapse [11]. Moreover, a recent study demonstrated that DC-SIGN triggering by HIV-1 leads to increased rho-GTPase activity, which is required for the formation of virus–T-cell synapses [28]. These results imply that DC-SIGN might have an important role in HIV-1 transmission and pathogenesis and require further investigations of HIV-1–DC interactions in vivo to finally combat the virus. MoDCs mediate antigen presentation of HIV-1 Adaptive immune responses to HIV-1 can subdue HIV-1 infection but are not sufficient to clear infection. The presence of HIV-1-specific CD4+ and CD8+ T cells in vivo suggest that MHC I and II presentation by DCs is essential for HIV-1 pathogenesis [29]. Indeed, vaccination with autologous moDCs pulsed with inactivated HIV-1 improved virological control in chronically infected HIV-1 patients [30]. Interestingly, mechanisms governing antigen presentation by moDCs appear to be closely related or even part of the viral transmission process. HIV-1 capture by moDCs is essential to both transmission and antigen presentation. Although a fraction of the virus escapes degradation and is transmitted to T cells, most of the internalized virions are degraded, leading to MHC II presentation [20,26,31]. So far, little is known about the molecules involved in the different routing of HIV1. A recent study demonstrated that the intracellular leukocyte specific protein (LSP)-1 binds to the intracellular domain of DC-SIGN and mediates targeting of the virus to the proteasome for degradation and probably also mediates MHC I presentation [32]. Indeed, after exposure to HIV-1, moDCs can present HIV-1 on MHC I [26,33]. Besides antigen presentation, DC maturation is essential to initiate an efficient immune response. Mature moDCs are highly efficient in not only activating but also differentiating T cells in specific antiviral effector cells. High concentrations of HIV-1 initiate DC maturation, including upregulation of co-stimulatory molecules [34], which could enhance antigen presentation but also viral www.sciencedirect.com
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transmission. However, HIV-1 can also modulate DC function, presumably to suppress adaptive immune responses. HIV-1 modulates Toll-like receptor (TLR) signaling through DC-SIGN, which results in increased production of the anti-inflammatory cytokine interleukin (IL)-10 [35], whereas HIV-1 also suppresses the T-cell stimulatory function of moDCs [36]. Thus, moDCs mediate viral transmission and antigen presentation. However, the ‘signals’ that direct these DCSIGN+-DCs down the lane of transmission or antigen presentation need to be further elucidated. Primary subepithelial DC-SIGN+-DCs versus moDCs Although moDCs have been investigated extensively, little is known about the role of primary DC-SIGN+-DCs at the site of sexual transmission. Primary subepithelial DCs in the cervix, foreskin and glans penis express DC-SIGN and are therefore thought to mediate HIV-1 transmission [4,5], however, few studies have studied these primary DCSIGN+-DCs. DC-SIGN+-DCs are present throughout the whole rectal epithelium [6] and, after isolation, these cells mediate transmission to T cells through the receptor DCSIGN [12]. Moreover, dermal DC-SIGN+-DCs, which, based on their expression of surface markers, such as DC-SIGN, resemble the subepithelial DC-SIGN+-DCs at the site of sexual transmission, transmit HIV-1 efficiently ex vivo [37]. Thus, DC-SIGN+-primary rectal and dermal DCs mediate HIV-1 transmission. However, more research is essential to investigate the other primary DC-SIGN+DCs and DC-SIGN+-DCs present in the subepithelia and to determine the mechanisms that are involved in HIV-1 transmission in vivo. This is underscored by a recent study demonstrating that primary LCs might actually protect against HIV-1 infection [4]. LCs capture HIV-1 efficiently through the C-type lectin Langerin Different mucosal DC subsets express different receptors that are involved in HIV-1 binding. LCs express the HIV1 entry receptor CD4 and co-receptor CCR5 and some reports have also shown CXCR4 expression [5,38–40]. In contrast to moDCs and subepithelial DCs, LCs are negative for DC-SIGN but are unique in the expression of the C-type lectin Langerin [41] (Box 1). Epidermal LCs capture HIV-1-gp120 efficiently through the receptor Langerin [4]. Langerin has a broad specificity for glycans that overlaps partly with DC-SIGN, including high mannose structures that are present on the envelope of HIV-1 (www.functionalglycomics.org). However, the intracellular domain and trafficking of both lectins is highly distinct. Based on the similarities in carbohydrate-binding specificities, it was thought, until recently, that Langerin and DC-SIGN have a similar function in HIV-1 transmission [16]. However, recent data demonstrate that Langerin has a different function than DC-SIGN and protects against HIV-1 infection [4]. Therefore, although LCs interact with HIV-1 in a similar way to subepithelial DCs, the function of both DC subsets in HIV-1 transmission is different owing to the expression of the different C-type lectins, Langerin and DC-SIGN, respectively.
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LCs can become infected by HIV-1 and mediate viral transmission Only low numbers of primary LCs from human genital tissues can be isolated. Therefore, most research on the molecular interaction of HIV-1 with LCs has been performed using skin-derived LCs. HIV-1 can infect LCs within epidermal sheets and after the cells migrate out of the tissues, they can mediate HIV-1 transmission [37,42]. In contrast to dermal DC-SIGN+-DCs, LCs are infected primarily by CCR5-using (R5-tropic) and not by CXCR4-using (X4-tropic) HIV-1 [37,38,43] and infectivity is associated with the CCR5 genotype [42]. These studies support a role for selective R5-transmission at the level of the LCs [44]. Several in vivo and ex vivo data support a role for LC infection in HIV-1 transmission [39,45,46]. Vaginal simian immunodeficiency virus (SIV) infection of Rhesus macaques resulted in infected LCs beneath the vaginal epithelium within the first day of infection [46]. Infection of biopsies of human cervical and foreskin tissue show that LCs can be infected [39]. However, detection of HIV-1infected LCs in the genital mucosal tissues in situ and in vivo is complicated because infection is a rare event and the existence of HIV-1-specific proteins cannot distinguish between viral uptake and virus replication in LCs. In contrast to moDCs, LCs transmit HIV-1 primarily through de novo synthesis of the virus. Transmission of single-round, replication defective HIV-1 was not detected using primary immature LCs, indicating that LCs do not capture and transfer the same virus particles and that HIV-1 trafficking in LCs is different than in moDCs [4]. Owing to the paucity of primary LCs, CD34+ stem cells have been used to generate LCs. These cells mediate firstphase transmission after maturation with tumor necrosis factor (TNF)-a and lipopolysaccharide (LPS) [47]. Although a useful model, the mechanisms governing transmission by CD34+ stem cell-derived LCs might be different from primary LCs, such as the response to LPS, because primary LCs do not express TLR4 [48,49]. Future investigations have to clarify whether activated LCs mediate first-phase transmission, similarly to primary LCs. LCs scavenge HIV-1 and thereby protect against HIV-1 transmission Compared with other viruses, the transmission rate of HIV-1 is highly inefficient and variable from one person to another. Therefore, it is likely that HIV-1 transmission is dependent on certain conditions and mediated by a onehit event. This makes it complicated to define the exact scenario happening at the moment of HIV-1 exposure. To obtain results in the laboratory using in vitro or primate studies, researchers use optimized systems, which do not mimic transmission inefficiency and complexity. LCs can be infected in vitro and ex vivo with HIV-1 and in vivo in macaques with SIV or simian/human immunodeficiency virus (SHIV) (see Glossary). However, high concentrations of HIV-1 and/or specific infection procedures have been used to infect LCs, such as spinoculation, a nonphysiological method that forces HIV-1 to attach to the cellsurface of target cells through centrifugation of target cells with the inoculated HIV-1, which increases infection www.sciencedirect.com
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[37,42,45]. This suggests that LC infection is not very efficient. Strikingly, using low concentrations of virus, we have shown recently that LCs inhibit the infection of T cells in a co-culture system [4], whereas, under similar conditions, moDCs enhanced HIV-1 infection of T cells. The ability of LCs to inhibit infection was dependent on the Ctype lectin Langerin. Langerin captures HIV-1 for rapid degradation and thereby clears the virus before T cells can become infected. In addition, and maybe more importantly in vivo, this ability of Langerin protects LCs from becoming infected and thereby prevents LC-mediated HIV-1 transmission. Conditions in which Langerin is blocked by blocking antibodies or saturated by high concentrations of virus abrogate the protection and enable efficient HIV-1 infection of LCs and subsequent transmission to T cells. Data suggest that the LC-specific intracellular pathway of the internalized Langerin is essential to the protective function of LCs [4]. Thus, Langerin protects against HIV-1 transmission and the C-type lectins Langerin and DCSIGN have opposite roles in HIV-1 transmission (Figure 3). LCs, Birbeck granules and antigen presentation Langerin recycles between the plasma membrane and early endosomes and is associated with the formation of Birbeck granules, which are subdomains of the endosomalrecycling compartment (the signature organelles identifying LCs) [41,50]. LCs capture HIV-1 efficiently through Langerin and internalize the virus through the Birbeck granules [4]. Capture by Langerin results in rapid degradation of the virus. Notably, similar to DC-SIGN, LSP-1 binds to the cytoplasmic tail of Langerin [32], suggesting that this might promote HIV degradation by the proteasome. However, owing to the specific internalization pathway of Langerin in LCs, it is possible that LSP-1 is necessary for targeting HIV-1 to Birbeck granules. So far, no role of Langerin in MHC I and II presentation has been reported but it will be interesting to determine whether virus degradation by Birbeck granules reflects efficient antigen processing by LCs. Antigen-processing components that mediate MHC I and II presentation have not yet been identified in these unique organelles [41]. However, Birbeck granules might route captured viruses efficiently to lysosomes for processing. Interestingly, CD1a is highly expressed in Birbeck granules. Birbeck granules and Langerin have been implicated in the CD1a-dependent antigen presentation of lipoproteins of Mycobacterium leprosy [51]. Thus, the role of LCs and Langerin in antigen presentation of HIV-1 antigens is largely unknown. Risk factors for acquiring HIV-1: a role for LCs and DC-SIGN+-DCs? Many known and unknown biological factors influence the transmission rate for HIV-1, such as bacterial or viral co-infection, epithelial trauma during intercourse, menstrual cycle and use of oral contraceptives [2,7]. Interestingly, these are all conditions that might alter LC and DC phenotype and, subsequently, their interaction with HIV-1. This is further supported by the actual functions of DC subsets: the ability to sense and react to changes within the environment that reflect infection or trauma.
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Trauma and co-infection can create ulcers and rupture the epithelial LC-rich layer. Moreover, the density of LCs might decrease on co-infection [52]. LCs protect against HIV-1 [4] and a breach in the LC barrier might result in an increased risk of HIV-1 infection. The function of Langerin is essential to the protective function of LCs; however, this function can be saturated with a high viral inoculum [4]. Moreover, when Langerin function is blocked or impaired, LCs mediate efficient HIV-1 transmission. Langerin expression is downregulated on activation of LCs [41]. In these situations of impaired Langerin function, the LCs might change from protective virus-clearing cells into the key players for HIV-1 transmission. Next to a breach in the Langerin barrier, these conditions enable interaction of HIV-1 with the DC-SIGN+-DCs. Interestingly, the number of DC-SIGN+ cells in the cervix increases on HSV-2 infection, a risk factor for acquiring HIV-1 [53]. DCs are efficient in mediating HIV-1 transmission also at low concentration of HIV-1 virus. Moreover, DC infection and transmission are dependent on the maturation state of the cells. This could be attributed to the differential expression of co-receptors, C-type lectin receptors and restriction factors. Thus, different conditions might alter DC function and therefore the susceptibility to acquire HIV-1. HIV-1 transmission in vivo and microbicide development The HIV-1 pandemic is expanding rapidly [2]. Because a curative treatment or a preventive vaccine against HIV-1 has not been discovered yet, there is great importance in developing a microbicide, a compound that could be applied topically, to prevent HIV-1 transmission [8]. To rationally design a microbicide, it is essential to know exactly what is happening at the moment of HIV-1 transmission and to define the key cellular and molecular player(s) to target to prevent transmission. In vitro, human moDCs and primary DC-SIGN+-DCs mediate transmission of HIV-1. Moreover, human LCs can be infected in human ex vivo models using high virus concentration. These experiments suggest a role for DC subsets in HIV-1 transmission; however, the in vivo role of the different DC subsets in humans has not been defined yet. The best available in vivo model to date is infection of macaques with SIV or SHIV. A role for DCs has been shown, although another report demonstrated that CD4+ T cells are the first cell type that is infected by HIV-1 [54]. Drawbacks of these models so far are that the probability of infection has to be increased to obtain reproducible infections, by high virus concentrations or hormone treatments before infection, which reduces the usefulness of modeling the inefficient transmission of HIV-1 in humans. LCs capture HIV-1 through Langerin, clear the virus and thereby protect against HIV-1 [4]. Only when the cells are exposed to high concentrations of virus or when the receptor Langerin is blocked, LCs transmit HIV-1 efficiently. Thus, to prevent HIV-1 transmission through LCs, Langerin function should be protected or even enhanced, whereas CCR5 and CD4 are molecules to target to prevent transmission. By contrast, moDCs and primary rectal DC-SIGN+-DCs mediate HIV-1 transmission and DC-SIGN is thought to have an www.sciencedirect.com
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Box 2. Outstanding questions Pathogenesis What is the role of different DC subsets in HIV-1 transmission in vivo: protection against or transmission of HIV-1? Cellular When are LCs protective against infection and when do LCs become infected and promote HIV-1 transmission? Do primary DC-SIGN+-DCs behave similar to cultured moDCs? Do LCs mediate antigen presentation of HIV-1? Molecular (targets for microbicides) What are the receptors on the DC subsets that are involved in HIV1 transmission or degradation? What are the intracellular signaling molecules that are involved in HIV-1 transmission or degradation?
important role. Carbohydrate structures that block DC-SIGN have been proposed as possible microbicides [9,55] but, considering the similarities of DC-SIGN to Langerin, it is important that these compounds do not interfere with Langerin function. Moreover, because little is known about the actual function of primary DC-SIGN+-DCs in HIV1 transmission, it is possible that primary subepithelial genital DC-SIGN+-DCs also capture, degrade and clear the virus, similarly to LCs. LCs, and maybe also primary DC-SIGN+-DCs, might function as a viral filter instead of the key target. Concluding remarks DCs are thought to be essential to induce a specific immune response during viral infections. Moreover, in vitro data indicate that different DC subsets either protect against or mediate HIV-1 transmission. More research is needed to define the roles of the different target cells at the site of HIV-1 transmission (Box 2). The unraveled mechanisms in cultured cells that we have discussed here should be tested carefully on primary cells. Specific research focus should be aimed towards understanding the protective function of cells against HIV-1. A better model that resembles the in vivo situation would be beneficial to confirm the mechanisms that are unraveled by the human in vitro and ex vivo as well as macaque in vivo HIV-1 transmission experiments. Acknowledgements Financial support was provided by the Dutch Scientific Research program (L.dW.: NWO. 917–46–367) and the Dutch AIDS Foundation (A.N.: 20005033).
References 1 Shattock, R.J. and Moore, J.P. (2003) Inhibiting sexual transmission of HIV-1 infection. Nat. Rev. Microbiol. 1, 25–34 2 Anon. (2005) UNAIDS: global AIDS epidemic just beginning. AIDS Policy Law 20, 1 3 Steinman, R.M. et al. (2003) The interaction of immunodeficiency viruses with dendritic cells. Curr. Top. Microbiol. Immunol. 276, 1–30 4 de Witte, L. et al. (2007) Langerin is a natural barrier to HIV-1 transmission by Langerhans cells. Nat. Med. 13, 367–371 5 McCoombe, S.G. and Short, R.V. (2006) Potential HIV-1 target cells in the human penis. AIDS 20, 1491–1495 6 Jameson, B. et al. (2002) Expression of DC-SIGN by dendritic cells of intestinal and genital mucosae in humans and rhesus macaques. J. Virol. 76, 1866–1875
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7 Galvin, S.R. and Cohen, M.S. (2004) The role of sexually transmitted diseases in HIV transmission. Nat. Rev. Microbiol. 2, 33–42 8 Lederman, M.M. et al. (2006) Microbicides and other topical strategies to prevent vaginal transmission of HIV. Nat. Rev. Immunol. 6, 371–382 9 Geijtenbeek, T.B.H. et al. (2000) DC-SIGN, a dendritic cell-specific HIV-1-binding protein that enhances trans-infection of T cells. Cell 100, 587–597 10 Trumpfheller, C. et al. (2003) Cell type-dependent retention and transmission of HIV-1 by DC-SIGN. Int. Immunol. 15, 289–298 11 Arrighi, J.F. et al. (2004) DC-SIGN-mediated infectious synapse formation enhances X4 HIV-1 transmission from dendritic cells to T cells. J. Exp. Med. 200, 1279–1288 12 Gurney, K.B. et al. (2005) Binding and transfer of human immunodeficiency virus by DC-SIGN+ cells in human rectal mucosa. J. Virol. 79, 5762–5773 13 Boggiano, C. et al. (2007) Dendritic cell-mediated trans-enhancement of human immunodeficiency virus type 1 infectivity is independent of DC-SIGN. J. Virol. 81, 2519–2523 14 Magerus-Chatinet, A. et al. (2007) Galactosyl ceramide expressed on dendritic cells can mediate HIV-1 transfer from monocyte derived dendritic cells to autologous T cells. Virology 362, 67–74 15 Nguyen, D.G. and Hildreth, J.E. (2003) Involvement of macrophage mannose receptor in the binding and transmission of HIV by macrophages. Eur. J. Immunol. 33, 483–493 16 Turville, S.G. et al. (2002) Diversity of receptors binding HIV on dendritic cell subsets. Nat. Immunol. 3, 975–983 17 de Witte, L. et al. Syndecan-3 is a dendritic cell-specific attachment receptor for HIV-1. Proc. Natl. Acad. Sci. U. S. A. (in press) 18 Wu, L. and Kewal Ramani, V.N. (2006) Dendritic-cell interactions with HIV: infection and viral dissemination. Nat. Rev. Immunol. 6, 859–868 19 Burleigh, L. et al. (2006) Infection of dendritic cells (DCs), not DCSIGN-mediated internalization of human immunodeficiency virus, is required for long-term transfer of virus to T cells. J. Virol. 80, 2949– 2957 20 Turville, S.G. et al. (2004) Immunodeficiency virus uptake, turnover, and 2-phase transfer in human dendritic cells. Blood 103, 2170–2179 21 McDonald, D. et al. (2003) Recruitment of HIV and its receptors to dendritic cell-T cell junctions. Science 300, 1295–1297 22 Wiley, R.D. and Gummuluru, S. (2006) Immature dendritic cellderived exosomes can mediate HIV-1 trans infection. Proc. Natl. Acad. Sci. U. S. A. 103, 738–743 23 Cavrois, M. et al. (2007) In vitro derived dendritic cells trans-infect CD4 T cells primarily with surface-bound HIV-1 virions. PLoS. Pathog. 3, e4 24 Kwon, D.S. et al. (2002) DC-SIGN-mediated internalization of HIV is required for trans-enhancement of T cell infection. Immunity 16, 135–144 25 Lee, B. et al. (2001) cis expression of DC-SIGN allows for more efficient entry of human and simian immunodeficiency viruses via CD4 and a coreceptor. J. Virol. 75, 12028–12038 26 Moris, A. et al. (2003) DC-SIGN promotes exogenous MHC-I restricted HIV-1 antigen presentation. Blood 130, 2648–2654 27 Pion, M. et al. (2006) APOBEC3G/3F mediates intrinsic resistance of monocyte-derived dendritic cells to HIV-1 infection. J. Exp. Med. 203, 2887–2893 28 Hodges, A. et al. (2007) Activation of the lectin DC-SIGN induces an immature dendritic cell phenotype triggering Rho-GTPase activity required for HIV-1 replication. Nat. Immunol. 8, 569–577 29 Betts, M.R. et al. (2001) Analysis of total human immunodeficiency virus (HIV)-specific CD4(+) and CD8(+) T-cell responses: relationship to viral load in untreated HIV infection. J. Virol. 75, 11983–11991 30 Lu, W. et al. (2004) Therapeutic dendritic-cell vaccine for chronic HIV-1 infection. Nat. Med. 10, 1359–1365 31 Moris, A. et al. (2006) Dendritic cells and HIV-specific CD4+ T cells: HIV antigen presentation, T-cell activation, and viral transfer. Blood 108, 1643–1651 32 Smith, A.L. et al. (2007) Leukocyte-specific protein 1 interacts with DCSIGN and mediates transport of HIV to the proteasome in dendritic cells. J. Exp. Med. 204, 421–430
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33 Buseyne, F. et al. (2001) MHC-I-restricted presentation of HIV-1 virion antigens without viral replication. Nat. Med. 7, 344–349 34 Harman, A.N. et al. (2006) HIV induces maturation of monocytederived dendritic cells and Langerhans cells. J. Immunol. 177, 7103–7113 35 Gringhuis, S.I. et al. (2007) C-type lectin DC-SIGN modulates Toll-like receptor signaling via Raf-1 kinase-dependent acetylation of transcription factor NF-kB. Immunity 26, 605–616 36 Granelli-Piperno, A. et al. (2004) HIV-1-infected monocyte-derived dendritic cells do not undergo maturation but can elicit IL-10 production and T cell regulation. Proc. Natl. Acad. Sci. U. S. A. 101, 7669–7674 37 Reece, J.C. et al. (1998) HIV-1 selection by epidermal dendritic cells during transmission across human skin. J. Exp. Med. 187, 1623–1631 38 Zaitseva, M. et al. (1997) Expression and function of CCR5 and CXCR4 on human Langerhans cells and macrophages: implications for HIV primary infection. Nat. Med. 3, 1369–1375 39 Patterson, B.K. et al. (2002) Susceptibility to human immunodeficiency virus-1 infection of human foreskin and cervical tissue grown in explant culture. Am. J. Pathol. 161, 867–873 40 Prakash, M. et al. (2004) Chemokine receptor expression on mucosal dendritic cells from the endocervix of healthy women. J. Infect. Dis. 190, 246–250 41 Valladeau, J. et al. (2000) Langerin, a novel C-type lectin specific to Langerhans cells, is an endocytic receptor that induces the formation of Birbeck granules. Immunity 12, 71–81 42 Kawamura, T. et al. (2003) R5 HIV productively infects Langerhans cells, and infection levels are regulated by compound CCR5 polymorphisms. Proc. Natl. Acad. Sci. U. S. A. 100, 8401–8406 43 Kawamura, T. et al. (2004) PSC-RANTES blocks R5 human immunodeficiency virus infection of Langerhans cells isolated from individuals with a variety of CCR5 diplotypes. J. Virol. 78, 7602–7609 44 Zhu, T. et al. (1993) Genotypic and phenotypic characterization of HIV1 patients with primary infection. Science 261, 1179–1181 45 Hladik, F. et al. (2007) Initial events in establishing vaginal entry and infection by human immunodeficiency virus type-1. Immunity 26, 257– 270 46 Miller, C.J. and Hu, J. (1999) T cell-tropic simian immunodeficiency virus (SIV) and simian-human immunodeficiency viruses are readily transmitted by vaginal inoculation of rhesus macaques, and Langerhans’ cells of the female genital tract are infected with SIV. J. Infect. Dis. 179, S413–S417 47 Fahrbach, K.M. et al. (2007) Activated CD34-derived Langerhans cells mediate transinfection with human immunodeficiency virus. J. Virol. 81, 6858–6868 48 van der Aar, A.M. et al. (2007) Loss of TLR2, TLR4, and TLR5 on Langerhans cells abolishes bacterial recognition. J. Immunol. 178, 1986–1990 49 Flacher, V. et al. (2006) Human Langerhans cells express a specific TLR profile and differentially respond to viruses and Gram-positive bacteria. J. Immunol. 177, 7959–7967 50 McDermott, R. et al. (2004) Reproduction of Langerin/CD207 traffic and Birbeck granule formation in a human cell line model. J. Invest. Dermatol. 123, 72–77 51 Hunger, R.E. et al. (2004) Langerhans cells utilize CD1a and langerin to efficiently present nonpeptide antigens to T cells. J. Clin. Invest. 113, 701–708 52 Jimenez-Flores, R. et al. (2006) High-risk human papilloma virus infection decreases the frequency of dendritic Langerhans’ cells in the human female genital tract. Immunology 117, 220–228 53 Rebbapragada, A. et al. (2007) Negative mucosal synergy between Herpes simplex type 2 and HIV in the female genital tract. AIDS 21, 589–598 54 Zhang, Z. et al. (1999) Sexual transmission and propagation of SIV and HIV in resting and activated CD4+ T cells. Science 286, 1353–1357 55 Veazey, R.S. et al. (2005) Protection of macaques from vaginal SHIV challenge by vaginally delivered inhibitors of virus-cell fusion. Nature 438, 99–102
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Distinct roles for DC-SIGN+-dendritic cells and Langerhans cells in HIV-1 transmission Lot de Witte, Alexey Nabatov and Teunis B.H. Geijtenbeek Department of Molecular Cell Biology and Immunology, VU University Medical Center, Amsterdam, The Netherlands
Dendritic cells (DCs) are thought to mediate HIV-1 transmission but it is becoming evident that different DC subsets at the sites of infection have distinct roles. In the genital tissues, two different DC subsets are present: the Langerhans cells (LCs) and the DC-SIGN+-DCs. Although DC-SIGN+-DCs mediate HIV-1 transmission, recent data demonstrate that LCs prevent HIV-1 transmission by clearing invading HIV-1 particles. However, this protective function of LCs is dependent on the function of the C-type lectin Langerin: blocking Langerin function by high virus concentrations enables HIV-1 transmission by LCs. Here, we will discuss the molecular mechanisms involved in HIV-1 transmission and viral clearance. A better understanding of these processes is crucial to understand and develop strategies to combat transmission. Dendritic cells and sexual HIV-1 transmission A better understanding of the mechanisms governing HIV-1 transmission (see Glossary) is crucial for the development of strategies to prevent new HIV-1 infections. Sexual transmission is the major route of acquiring HIV-1 worldwide [1,2]. However, the sequential events that occur from the moment HIV-1 encounters the genital epithelial barrier until the virus has caused a systemic infection remain obscure. HIV-1 is thought to target dendritic
Glossary Dendritic cells Birbeck granules: Organelles, consisting of pentalamellar and zippered membranes that can be visualized using electron microscopy. Langerin has been shown to induce their formation and the organelles have only been found in LCs. The function of Birbeck granules is not known yet. CD1a: Family member of the CD1 cell surface receptors that function as a microbial recognition system for the activation of T cell responses. CD1a has been shown to be involved in the presentation of mycobacterial lipids to CD1a-restricted T cells. CD34+-derived LCs: A model to study LCs. CD34+ stem cells are isolated from bone marrow and cord blood and cultured specific cytokines. After 6 days these cells acquire a DC morphology, express Langerin and contain Birbeck granules. C-type lectins: A family of cell surface receptors that belong to the pattern-recognition receptor family. These receptors recognize
Corresponding author: Geijtenbeek, T.B.H. ([email protected]). Available online 4 December 2007. www.sciencedirect.com
carbohydrate structures and are thought to play an important role in the immune system. DC-SIGN: A C-type lectin that is expressed by a subset of immature DCs and specific macrophages. This receptor is involved in the capture of various pathogens and mediates internalization and antigen presentation. DC-SIGN also mediates cellular communication between different immune cells. Moreover, DC-SIGN modulates the immune response by activating intracellular signaling pathways. Dendritic cells (DCs): These cells are professional antigen presenting cells, since these cells are essential to the activation naı¨ve T cells. Different DC subsets have been described, such as the myeloid and plasmacytoid DCs, and the Langerhans cells. All these subsets are thought to have special capacities and specialties. Langerin: A C-type lectin that is specifically expressed by immature LCs and is present in Birbeck granules. Moreover, Langerin induces the formation of Birbeck granules. Monocyte-derived DCs (moDCs): A model to study DCs. Human monocytes are cultured in the presence of IL-4 and GM-CSF. After 6 days these cells acquire a typical DC morphology and express different DC markers such as CD80, CD86, DC-SIGN and are able to stimulate naı¨ve T cells HIV-1 APOBEC3G: A host factor with anti-viral activity that can restrict HIV-1 infection. APOPEC3G is a cytidine deaminase that is incorporated into virions during viral production and subsequently triggers massive G-to-U hypermutation in the nascent retroviral genome. In addition, cellular APOPEC3G can function as a post-entry restriction factor for HIV during reverse transcription in resting CD4+ T cells. HIV-1 infection in cis: A receptor (e.g. DC-SIGN) captures HIV-1 and facilitates infection of the same cell via the primary HIV-1 receptor CD4 and the chemokine receptors. HIV-1 infection in trans: A receptor (e.g. DC-SIGN) captures HIV-1 and mediates infection of another cell by the same virion (also referred to as first phase transmission) HIV-1 transmission: The process in which the virus is transferred from an infected person to a non-infected person. Sexual transmission is the main route of acquiring HIV-1. The mechanisms how the virus finds its way from the site of sexual contact in mucosal tissues to infect the CD4+-T cells, which are the main target cells for HIV-1 and reside in lymphoid tissues, have not been elucidated yet. Recent studies indicate that DCs are involved in the transmission of HIV-1. Microbicides: Apart from the use of condoms, there is no method to prevent the transmission of HIV-1. Therefore, there is a need to develop microbicides. A microbicide is a topical agent, such as a cream or gel, that can be applied on the genital tissues to prevent the transmission of HIV-1. SIV and SHIV: SIV is the simian immunodeficiency virus that infects macaques and SHIV is an abbreviation used for chimeric replicating viruses containing different combinations of genes from both HIV and SIV. X4- and R5-tropic HIV-1: To infect a cell HIV-1 needs the expression of the entry receptor CD4 as well as the expression of a co-receptor. X4- and R5-tropic HIV-1 are strains that use the co-receptor CXCR4 and CCR5, respectively.
1471-4914/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.molmed.2007.11.001
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Figure 1. DCs mediate HIV-1 clearance and antigen presentation but also viral transmission to T cells. DCs are antigen-presenting cells. They capture antigens in the periphery and migrate to the lymphoid tissues to initiate a specific T-cell response. (a) LCs (pink) and DC-SIGN+-DCs (green) capture HIV-1 in the genital mucosal tissues, process the antigens and migrate to the lymphoid tissues where they present HIV-1 antigens on MHC I and MHC II to T cells, resulting in a T-cell response. Moreover, LCs clear the virus efficiently and thereby are thought to function as a protective barrier. However, it is also thought that HIV-1 subverts DC function at the moment of sexual transmission. (b) DC-SIGN+-DCs capture the virus in the submucosal tissues and LCs, exposed to high concentrations of HIV-1, loose their protective function and become infected. DC-SIGN+-DCs and LCs migrate to the lymphoid tissues and mediate transmission of the virus to T cells, resulting in a systemic infection.
cells (DCs) in the mucosal genital tissues to subvert their function and to reach their main target cells: the CD4+ T cells [3]. DCs have a pivotal role in the adaptive defense against invading pathogens. Immature DCs capture foreign antigens in the periphery. Next, the DCs acquire a mature phenotype and migrate to the lymphoid tissues. Here, the mature DCs present the processed antigens on MHC to naı¨ve T cells, resulting in an adaptive immune response against the pathogen. HIV-1 is thought to hijack DC function: DCs capture HIV-1, migrate to the lymphoid tissues but, instead of – or at the same time as – inducing an efficient immune reaction against the virus, DCs mediate HIV-1 transmission to T cells in the lymphoid tissues (Figure 1). However, recent data demonstrate that, in contrast to the current dogma, not all DC subsets mediate HIV-1 transmission but that the epithelial DC subset, the Langerhans cells (LCs), form a protective barrier that prevents HIV-1 transmission [4]. Here, we will summarize the recent developments concerning the function of mucosal DC subsets in HIV-1 infection. We will discuss how this knowledge has changed our view of the mechanisms that might be involved at the moment of sexual HIV-1 transmission and what the implications are for the development of better strategies to prevent HIV-1 transmission. www.sciencedirect.com
Different DC subsets in genital mucosal tissues To understand the role of DCs in the sexual transmission of HIV-1, it is important to identify the different DC subsets at the site of HIV-1 infection. Genital mucosa contain at least two important DC subsets: the LCs and a DC subset that is characterized by the expression of the C-type lectin receptor DC-SIGN (DC-SIGN+-DCs) (Figure 2). Both subsets have a specific anatomical localization and express unique receptors. LCs reside in the stratified mucosal epithelia of the vagina, ectocervix and foreskin but not in the columnar epithelium of the rectum and the endocervix [4–6]. Moreover, LCs are present abundantly in the epidermis of foreskin, glans penis and skin [5]. LCs are characterized by the expression of CD1a and Langerin and contain Birbeck granules, which are LC-specific cell organelles. Although LCs contribute to less than 1% of the total epithelial cells, their long dendrites form a continuous network throughout the epithelial layer and extend into the lumen of the mucosa to maximize their surveillance function (Figure 2). Owing to these anatomical and functional characteristics, LCs are the first DC subset that encounters HIV-1 within intact genital epithelial tissues. The low transmission rate of HIV-1 suggests that the epithelial barrier might be an important barrier to HIV-1. Indeed, HIV-1 transmission is enhanced under conditions of trauma and ulcerative anogenital
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Figure 2. Langerhans cells reside in the epithelia, whereas DC-SIGN+-DCs reside in the subepithelia. Different DC subsets are present at the mucosal epithelium where HIV-1 enters the body. To illustrate these subsets, cryosections of human foreskin were fixed and stained with primary antibodies against CD1a, Langerin and DC-SIGN. Sections were counterstained with isotype specific Alexa-488 (green) or Alexa-594 (red) anti-mouse antibodies and nuclei were stained blue with Hoechst. The slides were analyzed by immunofluorescence microscopy and the mucosal epithelium is depicted. LCs were visualized using antibodies against (a,b) CD1a (red) and (b) Langerin (green) (adapted, with permission, from Ref. [4]). The DC-SIGN+-DCs were stained using antibodies against (c) DC-SIGN (green). (d) A cartoon is displayed to show the specific localization of the LCs and DC-SIGN+-DCs in the genital mucosal epithelia.
co-infections [2,7,8], which damage the epithelial LC-rich layer and provide access for HIV-1 to the subepithelial DCSIGN+-DCs (Figure 2). Moreover, the rectal epithelium consists of columnar epithelial cells that lack LCs and contain DC-SIGN+-DCs [6]. Thus, HIV-1 encounters different DC subsets depending on the site of infection and other influencing factors (e.g. the integrity of the epithelial layer). A better understanding of the precise HIV-1–DC interactions during infection will require detailed investigation into the function of the different DC subsets in HIV-1 transmission. The paucity and difficulty of isolating primary DCSIGN+-DCs has led to the use of human monocyte-derived DCs (moDCs) as a model for DCs at the site of HIV-1 transmission because these moDCs also express DC-SIGN (Box 1). Here, we will first discuss the current knowledge regarding the molecular interactions of HIV-1 with moDCs and then we will discuss and speculate on the recent results obtained with primary human DC-SIGN+-DCs and LCs. MoDCs capture HIV-1 efficiently MoDCs are used as a model to study the interaction of HIV1 with the subepithelial DC-SIGN+-DCs. MoDCs capture HIV-1 efficiently; however, the receptors that are important for this binding are under debate. There is consensus that the C-type lectin DC-SIGN expressed on cell lines binds HIV-1 efficiently [9,10] (Box 1). In addition, various research groups have demonstrated that DC-SIGN is important for the binding of HIV-1 to moDCs [9,11,12]. www.sciencedirect.com
However, other reports have stated that moDCs bind HIV1 independently of DC-SIGN [10,13] and suggested other receptors might be involved, such as the glycospingholipid galactosyl ceramide [14], the mannose receptor [15,16] and HIV-1 receptor CD4 [16]. A recent study demonstrates that binding of HIV-1 to moDCs is dependent on both DC-SIGN and the heparan sulphate proteoglycan Syndecan-3 [17]. Differences in binding assays, DC culture and activation protocol and virus production might have contributed to these conflicting results. Therefore, it is now essential to define the role of the different HIV-1 receptors on primary DC-SIGN+-DCs rather than on cultured moDCs. The interaction of HIV-1 with the different receptors on DC-SIGN+-DCs is thought to dictate the subsequent fate of the virus within the DCs, resulting in either degradation and antigen presentation or viral capture, infection and transmission. MoDCs mediate viral transmission of HIV-1 MoDCs mediate transmission of HIV-1 to T cells efficiently and it is now evident that different pathways are involved [18] (Figure 3). DCs can transmit HIV-1 without becoming infected. This was shown by experiments in which DCs capture single round replication-defective HIV-1 efficiently and transfer the same virus particles to T cells [9,19]. This mechanism is referred to as first-phase transmission or trans-infection because it occurs fast and independently of de novo virus synthesis [20]. After capture, the virus is internalized into endosomes or multivesicular bodies,
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Box 1. DC-SIGN versus Langerin: the similarities and the differences Similarities DC-SIGN and Langerin belong to the family of C-type lectins: receptors that recognize carbohydrates in a calcium-dependent manner. DC-SIGN and Langerin are both type II transmembrane proteins with an extracellular region consisting of a neck involved in multimerization and a C-terminal C-type carbohydrate-recognition domain. Both DC-SIGN and Langerin recognize carbohydrate structures with high mannose specificities and, therefore, both bind HIV-1 gp120. Differences Langerin has a unique expression pattern because it is only expressed by LCs, which reside abundantly in epidermis and mucosal stratified epithelia. DC-SIGN is expressed by DCs in dermis, submucosa and rectal epithelia but also on DCs in other tissues, such as the lymphoid tissues. Moreover, DC-SIGN can be expressed by macrophages during inflammation. The carbohydrate-recognition domain of the DC-SIGN gene is highly conserved, whereas Langerin polymorphisms are present that
protected from degradation and transferred subsequently to the infectious synapse formed by DCs and T cells during HIV transmission [11,21]. Moreover, transmission from multivesicular bodies might also occur through exosomes [22]. Recently, it was suggested that cell-surface-bound HIV-1 primarily is transferred from moDCs to CD4+ T cells [23], although the question of whether cell surfacebound HIV-1 is protected remains to be investigated. By
result in different carbohydrate-binding capacities. Future research needs to further elucidate whether Langerin polymorphisms result in different HIV-1 transmission rates. In contrast to DC-SIGN, the murine homologue of Langerin shows large similarities to human Langerin, including the formation of Birbeck granules and expression on LCs. This enables future research on Langerin function in small animal models. The carbohydrate-recognition spectrum of both C-type lectins overlaps partly (both recognize high mannose and fucose structures) but also differs (Langerin recognizes a broader variety of carbohydrate structures). The intracellular domain of DC-SIGN and Langerin is distinct and might reflect differences in function. Langerin induces the formation of the Birbeck granules in Langerin+ cells and mediates HIV-1 degradation, whereas DC-SIGN mediates HIV-1 transmission. Future research will demonstrate whether the extracellular domains of Langerin and DC-SIGN, which are similar in the respect of binding mannose- and fucose-containing structures, might also contribute to the differences in function.
targeting moDCs, HIV-1 has not only found an efficient way to reach and infect T cells but, being captured inside DCs, it is also protected from extracellular antiviral factors. Therefore, it is essential for future studies to focus on virus survival and not just virus transmission. Notably, DC infection by HIV-1 might provide long-term survival to HIV-1 within DCs [19,20,24] and, as such, might contribute to viral transmission. This mechanism
Figure 3. LCs and DC-SIGN+-DCs interact differently with HIV-1. Different interactions of HIV-1 with the different DC subsets have been described. (a) Langerin is a receptor for HIV-1 on LCs and internalizes HIV-1 through the Birbeck granules (i). Langerin interaction with HIV-1 results in HIV-1 degradation, viral clearance and inhibition of HIV-1 transmission. However, the specific organelles in which HIV-1 is degraded need to be elucidated. (ii) A block of Langerin function through drugs, co-infections or mutation confers the ability on LCs to transmit HIV-1 efficiently to T cells (right) through infection of the LCs. (b) DC-SIGN is a receptor for HIV-1 on DCs and internalizes HIV-1. (i) HIV-1 is targeted to the lysosomes, resulting in viral degradation and MHC II presentation. (ii) HIV-1 is targeted to the multivesicular body, resulting in HIV-1 transmission to T cells. (iii) LSP-1 targets HIV-1 from the endosome to the proteasome, resulting in MHC I presentation. (iv) DC-SIGN+-DCs can be infected with HIV-1. DC-SIGN enhances DC infection in cis. Infection of DC-SIGN+-DCs can result in HIV-1 transmission. www.sciencedirect.com
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is referred to as second-phase transmission because of its importance in transmission several days after HIV-1 capture [20]. MoDCs express the HIV-1 (co-)receptors (CD4, CCR5 and CXCR4), enabling infection of the cells. Moreover, infection can be enhanced by DC-SIGN in cis; DC-SIGN captures HIV-1 efficiently and facilitates the interaction of HIV-1 with CD4 and the co-receptors on the same cell, resulting in infection [19,25]. The reported low levels of CD4, CXCR4 and CCR5 on moDCs can be responsible for their weaker susceptibility to HIV-1 infection in vitro compared with that of CD4+ T cells [4]. However, cellular restriction factors, such as APOBEC3G (see Glossary), and HIV-1 degradation in moDCs might also restrict infection of DCs [26,27]. Thus, DC-SIGN+-DCs mediate HIV-1 transmission through different pathways (Figure 3). By studying human moDCs, the molecular mechanisms that direct these different pathways, including the specific receptors and intracellular signaling molecules, are being unraveled rapidly. DC-SIGN is involved in routing of HIV-1 to the infectious synapse [11]. Moreover, a recent study demonstrated that DC-SIGN triggering by HIV-1 leads to increased rho-GTPase activity, which is required for the formation of virus–T-cell synapses [28]. These results imply that DC-SIGN might have an important role in HIV-1 transmission and pathogenesis and require further investigations of HIV-1–DC interactions in vivo to finally combat the virus. MoDCs mediate antigen presentation of HIV-1 Adaptive immune responses to HIV-1 can subdue HIV-1 infection but are not sufficient to clear infection. The presence of HIV-1-specific CD4+ and CD8+ T cells in vivo suggest that MHC I and II presentation by DCs is essential for HIV-1 pathogenesis [29]. Indeed, vaccination with autologous moDCs pulsed with inactivated HIV-1 improved virological control in chronically infected HIV-1 patients [30]. Interestingly, mechanisms governing antigen presentation by moDCs appear to be closely related or even part of the viral transmission process. HIV-1 capture by moDCs is essential to both transmission and antigen presentation. Although a fraction of the virus escapes degradation and is transmitted to T cells, most of the internalized virions are degraded, leading to MHC II presentation [20,26,31]. So far, little is known about the molecules involved in the different routing of HIV1. A recent study demonstrated that the intracellular leukocyte specific protein (LSP)-1 binds to the intracellular domain of DC-SIGN and mediates targeting of the virus to the proteasome for degradation and probably also mediates MHC I presentation [32]. Indeed, after exposure to HIV-1, moDCs can present HIV-1 on MHC I [26,33]. Besides antigen presentation, DC maturation is essential to initiate an efficient immune response. Mature moDCs are highly efficient in not only activating but also differentiating T cells in specific antiviral effector cells. High concentrations of HIV-1 initiate DC maturation, including upregulation of co-stimulatory molecules [34], which could enhance antigen presentation but also viral www.sciencedirect.com
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transmission. However, HIV-1 can also modulate DC function, presumably to suppress adaptive immune responses. HIV-1 modulates Toll-like receptor (TLR) signaling through DC-SIGN, which results in increased production of the anti-inflammatory cytokine interleukin (IL)-10 [35], whereas HIV-1 also suppresses the T-cell stimulatory function of moDCs [36]. Thus, moDCs mediate viral transmission and antigen presentation. However, the ‘signals’ that direct these DCSIGN+-DCs down the lane of transmission or antigen presentation need to be further elucidated. Primary subepithelial DC-SIGN+-DCs versus moDCs Although moDCs have been investigated extensively, little is known about the role of primary DC-SIGN+-DCs at the site of sexual transmission. Primary subepithelial DCs in the cervix, foreskin and glans penis express DC-SIGN and are therefore thought to mediate HIV-1 transmission [4,5], however, few studies have studied these primary DCSIGN+-DCs. DC-SIGN+-DCs are present throughout the whole rectal epithelium [6] and, after isolation, these cells mediate transmission to T cells through the receptor DCSIGN [12]. Moreover, dermal DC-SIGN+-DCs, which, based on their expression of surface markers, such as DC-SIGN, resemble the subepithelial DC-SIGN+-DCs at the site of sexual transmission, transmit HIV-1 efficiently ex vivo [37]. Thus, DC-SIGN+-primary rectal and dermal DCs mediate HIV-1 transmission. However, more research is essential to investigate the other primary DC-SIGN+DCs and DC-SIGN+-DCs present in the subepithelia and to determine the mechanisms that are involved in HIV-1 transmission in vivo. This is underscored by a recent study demonstrating that primary LCs might actually protect against HIV-1 infection [4]. LCs capture HIV-1 efficiently through the C-type lectin Langerin Different mucosal DC subsets express different receptors that are involved in HIV-1 binding. LCs express the HIV1 entry receptor CD4 and co-receptor CCR5 and some reports have also shown CXCR4 expression [5,38–40]. In contrast to moDCs and subepithelial DCs, LCs are negative for DC-SIGN but are unique in the expression of the C-type lectin Langerin [41] (Box 1). Epidermal LCs capture HIV-1-gp120 efficiently through the receptor Langerin [4]. Langerin has a broad specificity for glycans that overlaps partly with DC-SIGN, including high mannose structures that are present on the envelope of HIV-1 (www.functionalglycomics.org). However, the intracellular domain and trafficking of both lectins is highly distinct. Based on the similarities in carbohydrate-binding specificities, it was thought, until recently, that Langerin and DC-SIGN have a similar function in HIV-1 transmission [16]. However, recent data demonstrate that Langerin has a different function than DC-SIGN and protects against HIV-1 infection [4]. Therefore, although LCs interact with HIV-1 in a similar way to subepithelial DCs, the function of both DC subsets in HIV-1 transmission is different owing to the expression of the different C-type lectins, Langerin and DC-SIGN, respectively.
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LCs can become infected by HIV-1 and mediate viral transmission Only low numbers of primary LCs from human genital tissues can be isolated. Therefore, most research on the molecular interaction of HIV-1 with LCs has been performed using skin-derived LCs. HIV-1 can infect LCs within epidermal sheets and after the cells migrate out of the tissues, they can mediate HIV-1 transmission [37,42]. In contrast to dermal DC-SIGN+-DCs, LCs are infected primarily by CCR5-using (R5-tropic) and not by CXCR4-using (X4-tropic) HIV-1 [37,38,43] and infectivity is associated with the CCR5 genotype [42]. These studies support a role for selective R5-transmission at the level of the LCs [44]. Several in vivo and ex vivo data support a role for LC infection in HIV-1 transmission [39,45,46]. Vaginal simian immunodeficiency virus (SIV) infection of Rhesus macaques resulted in infected LCs beneath the vaginal epithelium within the first day of infection [46]. Infection of biopsies of human cervical and foreskin tissue show that LCs can be infected [39]. However, detection of HIV-1infected LCs in the genital mucosal tissues in situ and in vivo is complicated because infection is a rare event and the existence of HIV-1-specific proteins cannot distinguish between viral uptake and virus replication in LCs. In contrast to moDCs, LCs transmit HIV-1 primarily through de novo synthesis of the virus. Transmission of single-round, replication defective HIV-1 was not detected using primary immature LCs, indicating that LCs do not capture and transfer the same virus particles and that HIV-1 trafficking in LCs is different than in moDCs [4]. Owing to the paucity of primary LCs, CD34+ stem cells have been used to generate LCs. These cells mediate firstphase transmission after maturation with tumor necrosis factor (TNF)-a and lipopolysaccharide (LPS) [47]. Although a useful model, the mechanisms governing transmission by CD34+ stem cell-derived LCs might be different from primary LCs, such as the response to LPS, because primary LCs do not express TLR4 [48,49]. Future investigations have to clarify whether activated LCs mediate first-phase transmission, similarly to primary LCs. LCs scavenge HIV-1 and thereby protect against HIV-1 transmission Compared with other viruses, the transmission rate of HIV-1 is highly inefficient and variable from one person to another. Therefore, it is likely that HIV-1 transmission is dependent on certain conditions and mediated by a onehit event. This makes it complicated to define the exact scenario happening at the moment of HIV-1 exposure. To obtain results in the laboratory using in vitro or primate studies, researchers use optimized systems, which do not mimic transmission inefficiency and complexity. LCs can be infected in vitro and ex vivo with HIV-1 and in vivo in macaques with SIV or simian/human immunodeficiency virus (SHIV) (see Glossary). However, high concentrations of HIV-1 and/or specific infection procedures have been used to infect LCs, such as spinoculation, a nonphysiological method that forces HIV-1 to attach to the cellsurface of target cells through centrifugation of target cells with the inoculated HIV-1, which increases infection www.sciencedirect.com
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[37,42,45]. This suggests that LC infection is not very efficient. Strikingly, using low concentrations of virus, we have shown recently that LCs inhibit the infection of T cells in a co-culture system [4], whereas, under similar conditions, moDCs enhanced HIV-1 infection of T cells. The ability of LCs to inhibit infection was dependent on the Ctype lectin Langerin. Langerin captures HIV-1 for rapid degradation and thereby clears the virus before T cells can become infected. In addition, and maybe more importantly in vivo, this ability of Langerin protects LCs from becoming infected and thereby prevents LC-mediated HIV-1 transmission. Conditions in which Langerin is blocked by blocking antibodies or saturated by high concentrations of virus abrogate the protection and enable efficient HIV-1 infection of LCs and subsequent transmission to T cells. Data suggest that the LC-specific intracellular pathway of the internalized Langerin is essential to the protective function of LCs [4]. Thus, Langerin protects against HIV-1 transmission and the C-type lectins Langerin and DCSIGN have opposite roles in HIV-1 transmission (Figure 3). LCs, Birbeck granules and antigen presentation Langerin recycles between the plasma membrane and early endosomes and is associated with the formation of Birbeck granules, which are subdomains of the endosomalrecycling compartment (the signature organelles identifying LCs) [41,50]. LCs capture HIV-1 efficiently through Langerin and internalize the virus through the Birbeck granules [4]. Capture by Langerin results in rapid degradation of the virus. Notably, similar to DC-SIGN, LSP-1 binds to the cytoplasmic tail of Langerin [32], suggesting that this might promote HIV degradation by the proteasome. However, owing to the specific internalization pathway of Langerin in LCs, it is possible that LSP-1 is necessary for targeting HIV-1 to Birbeck granules. So far, no role of Langerin in MHC I and II presentation has been reported but it will be interesting to determine whether virus degradation by Birbeck granules reflects efficient antigen processing by LCs. Antigen-processing components that mediate MHC I and II presentation have not yet been identified in these unique organelles [41]. However, Birbeck granules might route captured viruses efficiently to lysosomes for processing. Interestingly, CD1a is highly expressed in Birbeck granules. Birbeck granules and Langerin have been implicated in the CD1a-dependent antigen presentation of lipoproteins of Mycobacterium leprosy [51]. Thus, the role of LCs and Langerin in antigen presentation of HIV-1 antigens is largely unknown. Risk factors for acquiring HIV-1: a role for LCs and DC-SIGN+-DCs? Many known and unknown biological factors influence the transmission rate for HIV-1, such as bacterial or viral co-infection, epithelial trauma during intercourse, menstrual cycle and use of oral contraceptives [2,7]. Interestingly, these are all conditions that might alter LC and DC phenotype and, subsequently, their interaction with HIV-1. This is further supported by the actual functions of DC subsets: the ability to sense and react to changes within the environment that reflect infection or trauma.
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Trauma and co-infection can create ulcers and rupture the epithelial LC-rich layer. Moreover, the density of LCs might decrease on co-infection [52]. LCs protect against HIV-1 [4] and a breach in the LC barrier might result in an increased risk of HIV-1 infection. The function of Langerin is essential to the protective function of LCs; however, this function can be saturated with a high viral inoculum [4]. Moreover, when Langerin function is blocked or impaired, LCs mediate efficient HIV-1 transmission. Langerin expression is downregulated on activation of LCs [41]. In these situations of impaired Langerin function, the LCs might change from protective virus-clearing cells into the key players for HIV-1 transmission. Next to a breach in the Langerin barrier, these conditions enable interaction of HIV-1 with the DC-SIGN+-DCs. Interestingly, the number of DC-SIGN+ cells in the cervix increases on HSV-2 infection, a risk factor for acquiring HIV-1 [53]. DCs are efficient in mediating HIV-1 transmission also at low concentration of HIV-1 virus. Moreover, DC infection and transmission are dependent on the maturation state of the cells. This could be attributed to the differential expression of co-receptors, C-type lectin receptors and restriction factors. Thus, different conditions might alter DC function and therefore the susceptibility to acquire HIV-1. HIV-1 transmission in vivo and microbicide development The HIV-1 pandemic is expanding rapidly [2]. Because a curative treatment or a preventive vaccine against HIV-1 has not been discovered yet, there is great importance in developing a microbicide, a compound that could be applied topically, to prevent HIV-1 transmission [8]. To rationally design a microbicide, it is essential to know exactly what is happening at the moment of HIV-1 transmission and to define the key cellular and molecular player(s) to target to prevent transmission. In vitro, human moDCs and primary DC-SIGN+-DCs mediate transmission of HIV-1. Moreover, human LCs can be infected in human ex vivo models using high virus concentration. These experiments suggest a role for DC subsets in HIV-1 transmission; however, the in vivo role of the different DC subsets in humans has not been defined yet. The best available in vivo model to date is infection of macaques with SIV or SHIV. A role for DCs has been shown, although another report demonstrated that CD4+ T cells are the first cell type that is infected by HIV-1 [54]. Drawbacks of these models so far are that the probability of infection has to be increased to obtain reproducible infections, by high virus concentrations or hormone treatments before infection, which reduces the usefulness of modeling the inefficient transmission of HIV-1 in humans. LCs capture HIV-1 through Langerin, clear the virus and thereby protect against HIV-1 [4]. Only when the cells are exposed to high concentrations of virus or when the receptor Langerin is blocked, LCs transmit HIV-1 efficiently. Thus, to prevent HIV-1 transmission through LCs, Langerin function should be protected or even enhanced, whereas CCR5 and CD4 are molecules to target to prevent transmission. By contrast, moDCs and primary rectal DC-SIGN+-DCs mediate HIV-1 transmission and DC-SIGN is thought to have an www.sciencedirect.com
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Box 2. Outstanding questions Pathogenesis What is the role of different DC subsets in HIV-1 transmission in vivo: protection against or transmission of HIV-1? Cellular When are LCs protective against infection and when do LCs become infected and promote HIV-1 transmission? Do primary DC-SIGN+-DCs behave similar to cultured moDCs? Do LCs mediate antigen presentation of HIV-1? Molecular (targets for microbicides) What are the receptors on the DC subsets that are involved in HIV1 transmission or degradation? What are the intracellular signaling molecules that are involved in HIV-1 transmission or degradation?
important role. Carbohydrate structures that block DC-SIGN have been proposed as possible microbicides [9,55] but, considering the similarities of DC-SIGN to Langerin, it is important that these compounds do not interfere with Langerin function. Moreover, because little is known about the actual function of primary DC-SIGN+-DCs in HIV1 transmission, it is possible that primary subepithelial genital DC-SIGN+-DCs also capture, degrade and clear the virus, similarly to LCs. LCs, and maybe also primary DC-SIGN+-DCs, might function as a viral filter instead of the key target. Concluding remarks DCs are thought to be essential to induce a specific immune response during viral infections. Moreover, in vitro data indicate that different DC subsets either protect against or mediate HIV-1 transmission. More research is needed to define the roles of the different target cells at the site of HIV-1 transmission (Box 2). The unraveled mechanisms in cultured cells that we have discussed here should be tested carefully on primary cells. Specific research focus should be aimed towards understanding the protective function of cells against HIV-1. A better model that resembles the in vivo situation would be beneficial to confirm the mechanisms that are unraveled by the human in vitro and ex vivo as well as macaque in vivo HIV-1 transmission experiments. Acknowledgements Financial support was provided by the Dutch Scientific Research program (L.dW.: NWO. 917–46–367) and the Dutch AIDS Foundation (A.N.: 20005033).
References 1 Shattock, R.J. and Moore, J.P. (2003) Inhibiting sexual transmission of HIV-1 infection. Nat. Rev. Microbiol. 1, 25–34 2 Anon. (2005) UNAIDS: global AIDS epidemic just beginning. AIDS Policy Law 20, 1 3 Steinman, R.M. et al. (2003) The interaction of immunodeficiency viruses with dendritic cells. Curr. Top. Microbiol. Immunol. 276, 1–30 4 de Witte, L. et al. (2007) Langerin is a natural barrier to HIV-1 transmission by Langerhans cells. Nat. Med. 13, 367–371 5 McCoombe, S.G. and Short, R.V. (2006) Potential HIV-1 target cells in the human penis. AIDS 20, 1491–1495 6 Jameson, B. et al. (2002) Expression of DC-SIGN by dendritic cells of intestinal and genital mucosae in humans and rhesus macaques. J. Virol. 76, 1866–1875
Review
TRENDS in Molecular Medicine
7 Galvin, S.R. and Cohen, M.S. (2004) The role of sexually transmitted diseases in HIV transmission. Nat. Rev. Microbiol. 2, 33–42 8 Lederman, M.M. et al. (2006) Microbicides and other topical strategies to prevent vaginal transmission of HIV. Nat. Rev. Immunol. 6, 371–382 9 Geijtenbeek, T.B.H. et al. (2000) DC-SIGN, a dendritic cell-specific HIV-1-binding protein that enhances trans-infection of T cells. Cell 100, 587–597 10 Trumpfheller, C. et al. (2003) Cell type-dependent retention and transmission of HIV-1 by DC-SIGN. Int. Immunol. 15, 289–298 11 Arrighi, J.F. et al. (2004) DC-SIGN-mediated infectious synapse formation enhances X4 HIV-1 transmission from dendritic cells to T cells. J. Exp. Med. 200, 1279–1288 12 Gurney, K.B. et al. (2005) Binding and transfer of human immunodeficiency virus by DC-SIGN+ cells in human rectal mucosa. J. Virol. 79, 5762–5773 13 Boggiano, C. et al. (2007) Dendritic cell-mediated trans-enhancement of human immunodeficiency virus type 1 infectivity is independent of DC-SIGN. J. Virol. 81, 2519–2523 14 Magerus-Chatinet, A. et al. (2007) Galactosyl ceramide expressed on dendritic cells can mediate HIV-1 transfer from monocyte derived dendritic cells to autologous T cells. Virology 362, 67–74 15 Nguyen, D.G. and Hildreth, J.E. (2003) Involvement of macrophage mannose receptor in the binding and transmission of HIV by macrophages. Eur. J. Immunol. 33, 483–493 16 Turville, S.G. et al. (2002) Diversity of receptors binding HIV on dendritic cell subsets. Nat. Immunol. 3, 975–983 17 de Witte, L. et al. Syndecan-3 is a dendritic cell-specific attachment receptor for HIV-1. Proc. Natl. Acad. Sci. U. S. A. (in press) 18 Wu, L. and Kewal Ramani, V.N. (2006) Dendritic-cell interactions with HIV: infection and viral dissemination. Nat. Rev. Immunol. 6, 859–868 19 Burleigh, L. et al. (2006) Infection of dendritic cells (DCs), not DCSIGN-mediated internalization of human immunodeficiency virus, is required for long-term transfer of virus to T cells. J. Virol. 80, 2949– 2957 20 Turville, S.G. et al. (2004) Immunodeficiency virus uptake, turnover, and 2-phase transfer in human dendritic cells. Blood 103, 2170–2179 21 McDonald, D. et al. (2003) Recruitment of HIV and its receptors to dendritic cell-T cell junctions. Science 300, 1295–1297 22 Wiley, R.D. and Gummuluru, S. (2006) Immature dendritic cellderived exosomes can mediate HIV-1 trans infection. Proc. Natl. Acad. Sci. U. S. A. 103, 738–743 23 Cavrois, M. et al. (2007) In vitro derived dendritic cells trans-infect CD4 T cells primarily with surface-bound HIV-1 virions. PLoS. Pathog. 3, e4 24 Kwon, D.S. et al. (2002) DC-SIGN-mediated internalization of HIV is required for trans-enhancement of T cell infection. Immunity 16, 135–144 25 Lee, B. et al. (2001) cis expression of DC-SIGN allows for more efficient entry of human and simian immunodeficiency viruses via CD4 and a coreceptor. J. Virol. 75, 12028–12038 26 Moris, A. et al. (2003) DC-SIGN promotes exogenous MHC-I restricted HIV-1 antigen presentation. Blood 130, 2648–2654 27 Pion, M. et al. (2006) APOBEC3G/3F mediates intrinsic resistance of monocyte-derived dendritic cells to HIV-1 infection. J. Exp. Med. 203, 2887–2893 28 Hodges, A. et al. (2007) Activation of the lectin DC-SIGN induces an immature dendritic cell phenotype triggering Rho-GTPase activity required for HIV-1 replication. Nat. Immunol. 8, 569–577 29 Betts, M.R. et al. (2001) Analysis of total human immunodeficiency virus (HIV)-specific CD4(+) and CD8(+) T-cell responses: relationship to viral load in untreated HIV infection. J. Virol. 75, 11983–11991 30 Lu, W. et al. (2004) Therapeutic dendritic-cell vaccine for chronic HIV-1 infection. Nat. Med. 10, 1359–1365 31 Moris, A. et al. (2006) Dendritic cells and HIV-specific CD4+ T cells: HIV antigen presentation, T-cell activation, and viral transfer. Blood 108, 1643–1651 32 Smith, A.L. et al. (2007) Leukocyte-specific protein 1 interacts with DCSIGN and mediates transport of HIV to the proteasome in dendritic cells. J. Exp. Med. 204, 421–430
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33 Buseyne, F. et al. (2001) MHC-I-restricted presentation of HIV-1 virion antigens without viral replication. Nat. Med. 7, 344–349 34 Harman, A.N. et al. (2006) HIV induces maturation of monocytederived dendritic cells and Langerhans cells. J. Immunol. 177, 7103–7113 35 Gringhuis, S.I. et al. (2007) C-type lectin DC-SIGN modulates Toll-like receptor signaling via Raf-1 kinase-dependent acetylation of transcription factor NF-kB. Immunity 26, 605–616 36 Granelli-Piperno, A. et al. (2004) HIV-1-infected monocyte-derived dendritic cells do not undergo maturation but can elicit IL-10 production and T cell regulation. Proc. Natl. Acad. Sci. U. S. A. 101, 7669–7674 37 Reece, J.C. et al. (1998) HIV-1 selection by epidermal dendritic cells during transmission across human skin. J. Exp. Med. 187, 1623–1631 38 Zaitseva, M. et al. (1997) Expression and function of CCR5 and CXCR4 on human Langerhans cells and macrophages: implications for HIV primary infection. Nat. Med. 3, 1369–1375 39 Patterson, B.K. et al. (2002) Susceptibility to human immunodeficiency virus-1 infection of human foreskin and cervical tissue grown in explant culture. Am. J. Pathol. 161, 867–873 40 Prakash, M. et al. (2004) Chemokine receptor expression on mucosal dendritic cells from the endocervix of healthy women. J. Infect. Dis. 190, 246–250 41 Valladeau, J. et al. (2000) Langerin, a novel C-type lectin specific to Langerhans cells, is an endocytic receptor that induces the formation of Birbeck granules. Immunity 12, 71–81 42 Kawamura, T. et al. (2003) R5 HIV productively infects Langerhans cells, and infection levels are regulated by compound CCR5 polymorphisms. Proc. Natl. Acad. Sci. U. S. A. 100, 8401–8406 43 Kawamura, T. et al. (2004) PSC-RANTES blocks R5 human immunodeficiency virus infection of Langerhans cells isolated from individuals with a variety of CCR5 diplotypes. J. Virol. 78, 7602–7609 44 Zhu, T. et al. (1993) Genotypic and phenotypic characterization of HIV1 patients with primary infection. Science 261, 1179–1181 45 Hladik, F. et al. (2007) Initial events in establishing vaginal entry and infection by human immunodeficiency virus type-1. Immunity 26, 257– 270 46 Miller, C.J. and Hu, J. (1999) T cell-tropic simian immunodeficiency virus (SIV) and simian-human immunodeficiency viruses are readily transmitted by vaginal inoculation of rhesus macaques, and Langerhans’ cells of the female genital tract are infected with SIV. J. Infect. Dis. 179, S413–S417 47 Fahrbach, K.M. et al. (2007) Activated CD34-derived Langerhans cells mediate transinfection with human immunodeficiency virus. J. Virol. 81, 6858–6868 48 van der Aar, A.M. et al. (2007) Loss of TLR2, TLR4, and TLR5 on Langerhans cells abolishes bacterial recognition. J. Immunol. 178, 1986–1990 49 Flacher, V. et al. (2006) Human Langerhans cells express a specific TLR profile and differentially respond to viruses and Gram-positive bacteria. J. Immunol. 177, 7959–7967 50 McDermott, R. et al. (2004) Reproduction of Langerin/CD207 traffic and Birbeck granule formation in a human cell line model. J. Invest. Dermatol. 123, 72–77 51 Hunger, R.E. et al. (2004) Langerhans cells utilize CD1a and langerin to efficiently present nonpeptide antigens to T cells. J. Clin. Invest. 113, 701–708 52 Jimenez-Flores, R. et al. (2006) High-risk human papilloma virus infection decreases the frequency of dendritic Langerhans’ cells in the human female genital tract. Immunology 117, 220–228 53 Rebbapragada, A. et al. (2007) Negative mucosal synergy between Herpes simplex type 2 and HIV in the female genital tract. AIDS 21, 589–598 54 Zhang, Z. et al. (1999) Sexual transmission and propagation of SIV and HIV in resting and activated CD4+ T cells. Science 286, 1353–1357 55 Veazey, R.S. et al. (2005) Protection of macaques from vaginal SHIV challenge by vaginally delivered inhibitors of virus-cell fusion. Nature 438, 99–102