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Is HIV infection a TNF receptor signalling-driven disease?
Opinion
Is HIV infection a TNF receptor signalling-driven disease? Georges Herbein and Kashif Aziz Khan Department of Virology, EA3186, IFR133, Franche-Comte´ University, Hoˆpital Saint-Jacques, F-25030 Besanc¸on cedex, France
Recent studies indicate that TNF (tumor necrosis factor) receptor signalling is a key player in HIV infection. HIV proteins have been shown to target TNF receptor signalling, leading both to apoptosis of uninfected bystander T cells and to sustained viral replication in infected T cells and macrophages. This article proposes a model that highlights the role of HIV proteins in the modulation of TNF receptor signalling and could explain both immune suppression and the formation of viral reservoirs during HIV infection. Introduction The progressive depletion of both CD4+ and CD8+ T cells and the formation of viral reservoirs are two of the hallmarks of human immunodeficiency virus (HIV) infection [1]. Aside from a Th1-Th2 switch, AIDS pathogenesis could be explained by an immune dysregulation involving proinflammatory cytokines, especially tumor necrosis factor (TNF) a (TNF). TNF is secreted by activated macrophages and lymphocytes and induces diverse responses, including inflammation and apoptosis [2]. TNF binds to the cell transmembrane receptors TNFR1 and TNFR2, mainly to modulate immunity (Box 1). Microbial pathogens have evolved several mechanisms by which to modulate host responses mounted by TNF [3]. Even though two decades have now elapsed since the identification of HIV as the cause of acquired immunodeficiency syndrome (AIDS), knowledge about how HIV affects TNF receptor (TNFR) signalling is only now starting to emerge, and sometimes in contradictory ways. On the one hand, TNF is directly or indirectly involved in the modulation of T cell apoptosis via members of the TNFR superfamily, such as TNFR1, TNFR2, and Fas, whereas on the other hand, TNF stimulates HIV-1 replication in infected cells [4–6]. In addition, HIV-1 proteins target the TNFR signalling pathway, modulating gene expression, especially HIV long terminal repeat (LTR) stimulation and T cell apoptosis, leading, in turn, both to immune suppression and to the formation of viral reservoirs in HIV infection. Therefore, targeting the TNFR signalling pathway might confer a crucial advantage to HIV via increased viral replication in the context of immune suppression. In the early stages of infection, HIV proteins mimic TNFR signalling and favour sustained viral growth In the early stages of HIV infection, and in the context of low levels of plasma TNF [5,7,8], virally encoded proteins, Corresponding author: Herbein, G. ([email protected]).
particularly Nef, Vpr, and Tat, mimic TNFR signalling, resulting in sustained viral growth within infected cells, especially macrophages (Figure 1). Nef is a 27 kDa myristoylated protein that is expressed early during the virus life cycle and modulates several signalling pathways. The Nef protein mimics TNFR signalling, especially in monocytic cells and primary macrophages, resulting in the positive regulation of the HIV LTR. Recombinant Nef protein stimulates NF-kB activation in promonocytic U937 cells and monocyte-derived macrophages, leading to sustained LTR activation [9,10]. Moreover, recombinant Nef protein activates AP-1, and induces the rapid phosphorylation of mitogen-activated protein kinase (MAPK)-family members (i.e. ERK1/2, p38, and JNK), interferon regulatory factor 3 (IRF-3), and both the a and b subunits of the IkB kinase (IKK) complex, required for the activation of the NF-kB pathway in monocyte-derived macrophages [9,11]. Thus, the features observed in promonocytic cells and primary macrophages after exposure to exogenous Nef are very similar to those observed after TNF treatment (see Table 1). Both exogenous Nef and TNF activate NF-kB, AP-1, and MAPK, suggesting that they might modulate the cellular machinery in a similar way, and therefore might have the same effect on HIV-1 replication in mononuclear phagocytes. Like the Nef protein, Vpr – a 96 amino acid virionassociated protein – plays the role of an early viral protein [12] and mimics the biological effects of TNF. Although it is not essential for viral replication in T cells, Vpr is crucial for HIV replication in nondividing cells such as macrophages and acts by transactivating the viral promoter and the HIV-1 LTR, resulting in increased viral replication. An interaction between Vpr and the transcription factors Sp1 and TFIIB is required for Vpr-mediated transcriptional enhancement of the HIV-1 LTR in lymphoid and myeloid cells [12]. The ability of Vpr to activate HIV transcription is mediated by the p300/CBP transcriptional coactivator, which, in turn, enhances the ability of Vpr to activate NF-kB and the HIV enhancer [13]. Synthetic Vpr protein activates NF-kB, AP-1, and JNK in promonocytic U937 cells and in primary macrophages, resulting in enhanced HIV replication [14]. A structural and functional interaction between Vpr and Tat synergistically enhances the transcriptional activity of the HIV-1 LTR [15]. In fact, Tat by itself displays biological activities that mimic those mediated by TNF. Like TNF, Tat activates NF-kB, AP-1, and MAPK, as well as c-Jun N-terminal kinase/stressactivated protein kinase (JNK/SAPK), p38, and ERK1/2; however, unlike TNF, signalling does not modify the
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Opinion Box 1. TNFR signalling TNFa binds to two distinct cell transmembrane receptors, 55 kDa TNFR1 (also called TNF R55, TNF Rb, p55, or CD120a) and 75 kDa TNFR2 (also called TNF R75, TNF Ra, p75, and CD120b). They belong to the large TNFR superfamily that is defined by the cysteine-rich architecture of the ligand-binding region. Although TNFR1 is more ubiquitous, both TNFR1 and TNFR2 are present on virtually all cell types, except, most notably, for red blood cells. The trimeric TNF ligand is able to bind to TNFR1 and/or TNFR2 and thereby initiates the signal transduction process. Although TNF exhibits higheraffinity binding to TNFR2 than to TNFR1, most of the biological responses of TNF are thought to be mediated through TNFR1. The two receptors share 28% identity in their extracellular ligandbinding domain. In its intracytoplasmic portion, TNFR1, but not TNFR2, has a death domain (DD) of 80 amino acids that allows the transduction of proapoptotic signals. In addition, TNFRs mediate the activation of NF-kB, AP-1, c-Jun N-terminal kinase, and p38 or p42/ 44 mitogen-activated protein kinase (MAPK), leading to enhanced gene expression.
activation of MEK [16–18]. Altogether, Nef, Vpr, and Tat mimic TNFR signalling, especially in mononuclear phagocytes, resulting in enhanced HIV replication. Both macrophages and peripheral blood lymphocytes (PBLs) have been reported to produce less TNF and C-C chemokines after treatment with extracellular Vpr [19,20]. Thus, this
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hijacking of TNFR signalling by Nef, Vpr, and Tat might occur in the context of low levels of proinflammatory cytokine and C-C chemokine production. Low levels of TNF and C-C chemokines could also result from low levels of immune activation, as previously reported in HIV asymptomatic patients at early stages of the disease or in long-term nonprogressors [5,8,21]. HIV proteins favour inhibition of T cell apoptosis through interference with TNFR signalling HIV proteins favour persistence of the virus not only through activation of LTR activity, but also through inhibition of host cell apoptosis (Figure 1). In fact, the inhibition of apoptosis in HIV-1-infected T cells enhances virus production and facilitates persistent infection [22]. Both viral and cellular factors are involved in the controlled and sustained production of virions in infected CD4+ T lymphocytes in the vicinity of macrophages [23], and they could expand the viral reservoir and fuel the progression of the disease. At more advanced stages of the disease, when TNF expression (plasma and membrane-bound) becomes more pronounced, Nef prevents TNFR-mediated death in HIV-infected T cells via interaction with the apoptosis
Figure 1. Targeting of TNF signalling pathways by HIV proteins. The early-expressed proteins Nef, Tat, and Vpr target PI3K, ASK1, PAK2, c-Flip, p53, members of the Bcl-2 family, and caspases for inhibition of apoptosis while favouring viral replication through activation of NF-kB and AP-1. The late-expressed viral proteins gp120 and Vpu increase apoptosis by enhancing TNFR signalling, activating proapoptotic pathways (e.g. p53), and inhibiting antiapoptotic pathways (Bcl-2) and NF-kB. Green arrows indicate activation; red arrows indicate inhibition.
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Table 1. HIV proteins: from TNF mimicry to enhancement of TNF activity Viral protein Cellular target Action TNF signal mimicry and sustained viral growth (early stage of infection) p56Lck Activation of NF-kB, AP-1, and members of the MAPK family Tat (ERK1/2, p38, and JNK) NF-kB Activation of NF-kB and stimulation of the LTR Nef AP-1 Activation of AP-1 and stimulation of the LTR Members of the MAPK family Activation of ERK1/2, p38, and JNK NF-kB, AP-1, and JNK Activation of NF-kB, AP-1, and JNK and stimulation of the LTR Vpr p300/CBP transcriptional coactivator Activation of NF-kB and the HIV enhancer Inhibition of T cell apoptosis (early-mid stage of infection) ASK-1 Blockade of TNF-mediated apoptosis Nef PI3K Phosphorylation of Bad, a proapoptotic member of the Bcl-2 family Inhibition of mitochondrial-related apoptosis PAK2 Inactivation of Bad Akt Formation of a Nef-Akt-PI3K complex and inhibition of apoptosis
p53 Bcl-2 c-FLIP Immune suppression (late stage of infection) CXCR4 gp120 Bcl-XL Tat
Vpu
TrCp Bcl-xL, A1/Bfl-1, and TRAF1
Blockade of p53-mediated apoptosis Protection from apoptosis Decreased levels of caspase 10 to escape TRAIL cytotoxicity T cell apoptosis Decreased intracellular levels of the apoptosis-protective protein Bcl-XL, leading to apoptosis Reduction of NF-kB activity, contributing to the virus-induced cytopathic effects Caspase 3 activation, resulting in induction of apoptosis
signal-regulating kinase-1 (ASK-1). Nef inhibits ASK-1, caspase 3, and caspase 8 activation, resulting in the blockade of apoptosis in HIV-infected cells [24]. In addition, Nef coprecipitates phosphatidylinositol-3-kinase (PI3K) in 293T cells, especially its p85 subunit, via sequences present within the N and C termini of Nef [25]. This interaction leads to the activation of PI3K, which, in turn, phosphorylates Bad, the proapoptotic member of the Bcl2 family. Phosphorylation of Bad results in its inactivation and subsequently inhibits mitochondrial-related apoptosis. Nef-mediated inactivation of Bad is also dependent on the activation of p21-activated kinase 2 (PAK2) in T cells. In fact, Nef interacts with PAK2 via its PXXP motif [25,26]. In recent years, the PI3K-Akt signalling pathways have emerged as important and general mechanisms of antiapoptosis [27]. We observed that Nef binds directly to Akt in primary PBLs, suggesting the potential formation of a Nef-Akt-PI3K complex in HIV-infected T cells (G.H. and T. Fulop, unpublished data). Another antiapoptotic effect of Nef in T cells involves the interaction with the tumor suppressor protein p53 via its N-terminal extremity (amino acids 1–57), both in the cytoplasm and in the nucleus of the cell. This interaction results in a decrease of p53 half-life and in a decrease of p53-mediated transcriptional activity, and it ultimately blocks p53-mediated apoptosis [28]. Therefore, Nef, via interaction with ASK1, PAK2, PI3K, and p53, can protect infected T cells against apoptosis mediated by the TNFRs (Table 1). In addition to that of Nef, an antiapoptotic role for Tat has been reported. Endogenous Tat stably expressed in Jurkat clones protects cells from activation-induced apoptosis induced by TNF [18]. Tat can also protect lymphoid and myeloid cells from TRAIL (TNF-related apoptosisinducing ligand)-mediated apoptosis by upregulating Bcl-2 [29]. Furthermore, upregulation of c-FLIP and decreased levels of caspase 10 in lymphoid T cells have
Refs [16,17] [9,10] [9] [9,11] [14] [13] [24] [25] [25] G.H. and Fulop, unpublished [28] [29] [30] [5,43–45] [47] [48] [50]
been reported as potential molecular mechanisms by which to escape TRAIL cytotoxicity [30]. Thus, in the context of increasing expression of TNF throughout the disease [5], HIV proteins such as Nef, Vpr, and Tat favour a sustained viral replication, especially in mononuclear phagocytes, while at the same time preventing TNF-mediated death of infected T cells, thereby expanding the formation of cellular reservoirs of virions. HIV proteins favour immune suppression through the enhancement of TNFR signalling in the late stages of disease In late stages of the disease, high levels of TNF are produced, resulting in dramatic TNF-mediated death of uninfected T cells and subsequent major immune suppression [5,31]. HIV proteins, such as gp120 and Vpu, might further participate in immune failure by enhancing the cytopathic effects mediated through TNFR signalling. In vitro culture models demonstrate that uninfected CD4+ T cells undergo apoptosis upon contact with HIVinfected cells, such as mononuclear phagocytes. Macrophages play a major role in this process, suggesting that apoptosis-inducing ligands expressed by macrophages can mediate apoptosis of susceptible CD4+ and CD8+ T cells [23,32]. TNF is expressed on the surface of activated macrophages, and most of the CD4+ T lymphocyte death is mediated via TNF-mediated apoptosis and via Fas-Fas ligand and TRAIL-DR5 (death receptor) interactions [4,5,33,34]. The engagement of TNFR2 by TNF results in degradation of TRAF2 (TNFR-associated factor). Because TRAF2 recruits cIAP (cellular inhibitor of apoptosis proteins) to TNFR1, its degradation can facilitate apoptosis. TRAF2 degradation also attenuates TNFR1-mediated NF-kB activation, further promoting apoptosis [35]. Cell viability can be further impaired by the actions of HIV-1 proteins, especially gp120 and Vpu, 63
Opinion which potentiate TNF proapoptotic activity (Figure 1). HIV-1 gp120 has been reported to trigger the production of TNF and C-C chemokines, especially in mononuclear phagocytes [36–38]. Interactions between HIV-1 gp120 and CD4 stimulates signal transduction pathways, such as activation of protein kinase C (PKC); the generation of PKC-dependent phosphorylation of CD4; activation of the ERK and MAPK pathways, which, in turn, stimulates transcription factors, such as NF-kB, AP-1, Elk-1; and the induction of proinflammatory cytokine and C-C chemokine gene expression [36,39–41]. In addition, HIV-1 gp120, via interaction with CD4, activates uninfected CD4+ T cells, resulting in CD4+ T cell priming, and renders them susceptible to TNF-mediated apoptosis [42]. Furthermore, the binding of a cell-associated gp120 molecule to CXCR4 induces mitochondrial transmembrane depolarization; cytochrome C release from the mitochondria to the cytosol; and, ultimately, activation of downstream caspases, resulting in T cell apoptosis [43,44]. CD8+ T cell apoptosis in HIV infection is observed at the onset of AIDS in the context of high TNF expression [5,45,46], and it is triggered via interaction of HIV-1 gp120 with CXCR4 and via contacts between membrane-bound TNF on macrophages and TNFR2 on CD8+ T cells [5,45]. TNFR2 stimulation of T cells results in decreased intracellular levels of the apoptosis-protective protein Bcl-XL, a member of the Bcl-2 family [47]. Therefore, TNFR2 stimulation on CD8+ T cells by membrane-bound TNF expressed on the surface of macrophages might decrease the intracellular levels of antiapoptotic proteins, resulting in CD8+ T cell death. Vpu is a 16 kDa viral membrane protein that regulates the release of virions from infected cells and induces the disruption of an envelope gp120-CD4 interaction in the endoplasmic reticulum. Vpu functions as a competitive inhibitor of TNFR-complex proteins, such as TRAF1 [48,49]. In Vpu-expressing cells, the levels of TRAF1 in response to TNF stimulation are reduced and are no longer sufficient to inhibit the cytokine-induced activation of caspase 8. Activated caspase 8 induces the release of cytochrome C from the mitochondria. The release of cytochrome C is usually inhibited by members of the Bcl-2 protein family. In Vpu-expressing cells, the levels of these proteins are limited, resulting in increased release of cytochrome C and, subsequently, in caspase 3 activation [50]. Thus, expression of Vpu might augment the proapoptotic role of TNF during HIV infection, especially in late stages of the disease, when high TNF levels are detected [5,8]. Thus, in HIV-infected CD4+ T cells, Vpu blocks the TNFR1 signalling pathway and the activation of NF-kB and enhances TNFR-mediated apoptosis. In addition, TNFR signalling is involved in downregulation of the antiviral T cell response during persistent viral infection, and it acts by determining the fate of antigenspecific T cells. The inhibitory receptor programmed death-1 (PD-1), a negative regulator of activated T cells, is markedly upregulated on the surface of exhausted, HIV-specific CD8+ and CD4+ T cells [51]. PD-1 expression is upregulated by TNF and correlates with impaired, HIV-specific T cell functions as well as with predictors of disease progression: it is correlated positively with plasma viral load and inversely with CD4 T cell count [52,53]. 64
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Model of HIV pathogenesis based on modulation of TNFR signalling by HIV proteins: from TNF mimicry to enhancement of TNF activity Because of the various effects of HIV proteins on TNF signalling and apoptosis (Table 1), we would like to present a new, to our knowledge, model that highlights the role of TNFR signalling in the modulation of both T cell apoptosis and viral persistence and could thereby further enhance our understanding of the pathogenesis of HIV-mediated disease (Figure 2). Early in the disease, when the levels of proinflammatory proteins and C-C chemokines are low and chronic immune activation is not yet predominant [8], viral proteins are crucial for establishing a productive infection. Viral proteins expressed early in the viral cycle, such as Nef, Tat, and virion-associated Vpr, activate the TNFR pathway to partially mimic TNF biological effects, suggesting that these viral proteins can fuel the progression of the disease even in the absence of proinflammatory cytokines. These viral proteins play a role in the formation of viral reservoirs by activating transcription from the LTR and interfering with apoptotic machinery. In the meantime, Vpr protein blocks the production of proinflammatory cytokines and chemokines [19], and soluble Nef and Tat proteins favour the recruitment of both T cells and monocytes and macrophages [54–56], further indicating that viral proteins play a crucial role in taking control of HIV-1 replication. At a later stage of the disease, at the onset of AIDS, proinflammatory cytokines such as TNF and C-C chemokines are produced abundantly due to chronic immune stimulation [5,57,58], and viral proteins, rather than mimicking TNFR signalling, will, in fact, enhance TNFmediated T cell apoptosis. At that point, a high level of TNF has been reached; HIV-1 proteins expressed late in the viral cycle, such as gp120 and Vpu, are detected; and TNF-mediated T cell apoptosis is maximal, resulting in accelerated immune suppression. The T cell proapoptotic effect results from both increased cell-surface expression of TNF and TNFR molecules, especially TNFR1, in the context of a high level of proinflammatory cytokines triggered by gp120 [36] and from impaired antiapoptotic effects mediated via the TNFR-TRAF1 pathway triggered by Vpu [50]. In addition, high levels of proinflammatory cytokines and C-C chemokines are produced by infected cells, leading to enhanced T cell and monocyte and macrophage chemotaxis [58] and favouring T cell apoptosis in the context of chronic immune activation. This will result in accelerated T cell death and in increased release of mature, infectious virions from the infected cells, thus favouring immune failure. Although our model suggests a dual role for HIV proteins throughout the progression of the disease, from TNF mimicry to the enhancement of TNF activity, additional features have to be taken into account. First, although Nef, Vpr, and Tat play important roles in early viral persistence, each of these proteins might play a dual role and contribute to late immune suppression, owing to differential properties of its N and C termini (Box 2). Second, expression of TNF and TNFRs on
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Figure 2. A model of HIV pathogenesis based on modulation of TNFR signalling by HIV proteins. (a,b) During early HIV infection, the level of proinflammatory cytokines and chemokines is low. Soluble Nef and Tat favour T cell, monocyte, and macrophage chemotaxis, and T cells are infected upon contact with macrophages. Early viral proteins Nef, Tat, and Vpr stimulate HIV-1 transcription and block apoptosis in T cells, favouring viral persistence and the formation of viral reservoirs. In addition, Vpr blocks the production of proinflammatory cytokines and chemokines. (c,d) At a later stage of the disease, gp120 triggers the abundant production of proinflammatory cytokines and chemokines and increases the expression of TNF and TNFR1, resulting in CD4+ T cell apoptosis. In the meantime, Vpu accelerates the process by impairing antiapoptotic machinery. TNF upregulates PD-1 on HIV-specific CD8+ T cells, resulting in CD8+ T cell apoptosis as well. Thus, accelerated T cell death and increased release of virions from the infected cells favour immune failure in the late stages of disease. Mf, macrophage; T4, CD4+ T cell; T8, CD8+ T cell.
Box 2. Differential role of the N and C termini of viral proteins in HIV pathogenesis Although our model suggests that the HIV-1 proteins can be distinguished from one another based on their involvement either in early viral persistence (Nef, Tat, Vpr) or in late immune suppression (gp120, Vpu), each of the proteins involved in early viral persistence might also play a role in late immune suppression. The first coding exon of Tat protein is crucial for the stimulation of HIV-1 LTR, and, subsequently, for the formation of cellular reservoirs of virions; by contrast, the C terminus of the Tat protein has been reported to be proapoptotic [60]. Similarly, the N terminus of the Vpr protein has been reported to be involved in NF-kB activation and LTR stimulation, whereas the C terminus of Vpr is proapoptotic [14,61]. In addition, the N terminus and especially the myristoylation signal of the Nef protein have been reported to be involved in NF-kB activation [62]. By contrast, the C terminus of the Nef protein triggers apoptosis [63]. Thus, all three ‘‘early’’activating viral proteins, Tat, Nef, and Vpr, could play a dual role in HIV pathogenesis and could both favour the formation of cellular reservoirs of virions via their N termini and subsequently participate with gp120 and Vpu via their C termini to a proapoptotic stage, resulting ultimately in immune suppression.
lymphocytes and monocytes from HIV-1-infected patients shows significant variations in relation to disease stage and numbers of CD4+ lymphocytes in peripheral blood. Circulating levels of soluble TNF components among HIV-1-infected patients show a gradual increase with disease stage, with the highest levels present in the AIDS group [5]. Patients in earlier stages of the disease, with only a moderate decrease in numbers of CD4+ lymphocytes, are characterized by increased expression of membrane-bound TNFa (mTNF) and TNFRs (mTNFRs) on lymphocytes and monocytes as compared with healthy controls, whereas AIDS patients with markedly decreased numbers of CD4+ lymphocytes showed a significantly decreased expression of mTNFR2 on both lymphocytes and monocytes, even compared with healthy individuals [59]. Thus, a complex interplay between several HIV proteins and both soluble and membrane-bound forms of TNF and TNFRs could explain the two main characteristics of HIV infection: viral persistence and immune failure. 65
Opinion Conclusion The formation of viral reservoirs and the loss of T lymphocytes are central factors in the progression of HIV. TNF favours HIV replication and can induce apoptosis, and, interestingly, several HIV-1 proteins share similar functions. HIV-1 proteins mimic, interfere with, or enhance TNFR signalling at different stages of the disease and therefore represent potential targets that could ultimately lead to new treatments controlling both viral replication and immune activation in HIV-infected subjects. Acknowledgements We thank Aurelie Vacheret for secretarial assistance. Support was provided by grants from Franche-Comte´ University (to G.H.), from Societe Francaise D’Exportation des Ressources Educatives, and from the Higher Education Commission, Pakistan (to K.A.K.).
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41 Del Corno, M. et al. (2001) HIV-1 gp120 and chemokine activation of Pyk2 and mitogen-activated protein kinases in primary macrophages mediated by calcium-dependent, pertussis toxin-insensitive chemokine receptor signaling. Blood 98, 2909–2916 42 Westendorp, M.O. et al. (1995) Sensitization of T cells to CD95mediated apoptosis by HIV-1 Tat and gp120. Nature 375, 497–500 43 Roggero, R. et al. (2001) Binding of human immunodeficiency virus type 1 gp120 to CXCR4 induces mitochondrial transmembrane depolarization and cytochrome c-mediated apoptosis independently of Fas signaling. J. Virol. 75, 7637–7650 44 Ritsou, E. et al. (2007) CD4/CXCR4-mediated cell death in AIDS. Cell Death Differ. 14, 634–636 45 Herbein, G. et al. (1998) Apoptosis of CD8+ T cells is mediated by macrophages through interaction of HIV gp120 with chemokine receptor CXCR4. Nature 395, 189–194 46 Chun, T.W. et al. (2004) Relationship between the frequency of HIV-specific CD8+ T cells and the level of CD38+CD8+ T cells in untreated HIV-infected individuals. Proc. Natl. Acad. Sci. U. S. A. 101, 2464–2469 47 Lin, R.H. et al. (1997) TNF receptor-2-triggered apoptosis is associated with the down-regulation of Bcl-xL on activated T cells and can be prevented by CD28 costimulation. J. Immunol. 158, 598–603 48 Bour, S. et al. (2001) The human immunodeficiency virus type 1 Vpu protein inhibits NF-kB activation by interfering with b TrCP-mediated degradation of IkB. J. Biol. Chem. 276, 15920–15928 49 Besnard-Guerin, C. et al. (2004) HIV-1 Vpu sequesters b-transducin repeat-containing protein (bTrCP) in the cytoplasm and provokes the accumulation of b-catenin and other SCFbTrCP substrates. J. Biol. Chem. 279, 788–795 50 Akari, H. et al. (2001) The human immunodeficiency virus type 1 accessory protein Vpu induces apoptosis by suppressing the nuclear factor kB-dependent expression of antiapoptotic factors. J. Exp. Med. 194, 1299–1311 51 Riley, J.L. and June, C.H. (2007) The road to recovery: translating PD-1 biology into clinical benefit. Trends Immunol. 28, 48–50
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52 Day, C.L. et al. (2006) PD-1 expression on HIV-specific T cells is associated with T-cell exhaustion and disease progression. Nature 443, 350–354 53 Trautmann, L. et al. (2006) Upregulation of PD-1 expression on HIVspecific CD8+ T cells leads to reversible immune dysfunction. Nat. Med. 12, 1198–1202 54 Swingler, S. et al. (1999) HIV-1 Nef mediates lymphocyte chemotaxis and activation by infected macrophages. Nat. Med. 5, 997–1003 55 Park, I.W. et al. (2001) HIV-1 Tat promotes monocyte chemoattractant protein-1 secretion followed by transmigration of monocytes. Blood 97, 352–358 56 Lehmann, M.H. et al. (2006) Extracellular HIV-1 Nef increases migration of monocytes. Exp. Cell Res. 312, 3659–3668 57 Hornung, F. et al. (2000) TNF-a-induced secretion of C-C chemokines modulates C-C chemokine receptor 5 expression on peripheral blood lymphocytes. J. Immunol. 164, 6180–6187 58 Fantuzzi, L. et al. (2003) Monocyte/macrophage-derived CC chemokines and their modulation by HIV-1 and cytokines: a complex network of interactions influencing viral replication and AIDS pathogenesis. J. Leukoc. Biol. 74, 719–725 59 Hestdal, K. et al. (1997) Dysregulation of membrane-bound tumor necrosis factor-a and tumor necrosis factor receptors on mononuclear cells in human immunodeficiency virus type 1 infection: low percentage of p75-tumor necrosis factor receptor positive cells in patients with advanced disease and high viral load. Blood 90, 2670–2679 60 Campbell, G.R. et al. (2005) The C terminus of HIV-1 Tat modulates the extent of CD178-mediated apoptosis of T cells. J. Biol. Chem. 280, 38376–38382 61 Roumier, T. et al. (2002) The C-terminal moiety of HIV-1 Vpr induces cell death via a caspase-independent mitochondrial pathway. Cell Death Differ. 9, 1212–1219 62 Wang, J.K. et al. (2000) The Nef protein of HIV-1 associates with rafts and primes T cells for activation. Proc. Natl. Acad. Sci. U. S. A. 97, 394– 399 63 Roshal, M. et al. (2001) Apoptosis in AIDS. Apoptosis 6, 103–116
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Is HIV infection a TNF receptor signalling-driven disease? Georges Herbein and Kashif Aziz Khan Department of Virology, EA3186, IFR133, Franche-Comte´ University, Hoˆpital Saint-Jacques, F-25030 Besanc¸on cedex, France
Recent studies indicate that TNF (tumor necrosis factor) receptor signalling is a key player in HIV infection. HIV proteins have been shown to target TNF receptor signalling, leading both to apoptosis of uninfected bystander T cells and to sustained viral replication in infected T cells and macrophages. This article proposes a model that highlights the role of HIV proteins in the modulation of TNF receptor signalling and could explain both immune suppression and the formation of viral reservoirs during HIV infection. Introduction The progressive depletion of both CD4+ and CD8+ T cells and the formation of viral reservoirs are two of the hallmarks of human immunodeficiency virus (HIV) infection [1]. Aside from a Th1-Th2 switch, AIDS pathogenesis could be explained by an immune dysregulation involving proinflammatory cytokines, especially tumor necrosis factor (TNF) a (TNF). TNF is secreted by activated macrophages and lymphocytes and induces diverse responses, including inflammation and apoptosis [2]. TNF binds to the cell transmembrane receptors TNFR1 and TNFR2, mainly to modulate immunity (Box 1). Microbial pathogens have evolved several mechanisms by which to modulate host responses mounted by TNF [3]. Even though two decades have now elapsed since the identification of HIV as the cause of acquired immunodeficiency syndrome (AIDS), knowledge about how HIV affects TNF receptor (TNFR) signalling is only now starting to emerge, and sometimes in contradictory ways. On the one hand, TNF is directly or indirectly involved in the modulation of T cell apoptosis via members of the TNFR superfamily, such as TNFR1, TNFR2, and Fas, whereas on the other hand, TNF stimulates HIV-1 replication in infected cells [4–6]. In addition, HIV-1 proteins target the TNFR signalling pathway, modulating gene expression, especially HIV long terminal repeat (LTR) stimulation and T cell apoptosis, leading, in turn, both to immune suppression and to the formation of viral reservoirs in HIV infection. Therefore, targeting the TNFR signalling pathway might confer a crucial advantage to HIV via increased viral replication in the context of immune suppression. In the early stages of infection, HIV proteins mimic TNFR signalling and favour sustained viral growth In the early stages of HIV infection, and in the context of low levels of plasma TNF [5,7,8], virally encoded proteins, Corresponding author: Herbein, G. ([email protected]).
particularly Nef, Vpr, and Tat, mimic TNFR signalling, resulting in sustained viral growth within infected cells, especially macrophages (Figure 1). Nef is a 27 kDa myristoylated protein that is expressed early during the virus life cycle and modulates several signalling pathways. The Nef protein mimics TNFR signalling, especially in monocytic cells and primary macrophages, resulting in the positive regulation of the HIV LTR. Recombinant Nef protein stimulates NF-kB activation in promonocytic U937 cells and monocyte-derived macrophages, leading to sustained LTR activation [9,10]. Moreover, recombinant Nef protein activates AP-1, and induces the rapid phosphorylation of mitogen-activated protein kinase (MAPK)-family members (i.e. ERK1/2, p38, and JNK), interferon regulatory factor 3 (IRF-3), and both the a and b subunits of the IkB kinase (IKK) complex, required for the activation of the NF-kB pathway in monocyte-derived macrophages [9,11]. Thus, the features observed in promonocytic cells and primary macrophages after exposure to exogenous Nef are very similar to those observed after TNF treatment (see Table 1). Both exogenous Nef and TNF activate NF-kB, AP-1, and MAPK, suggesting that they might modulate the cellular machinery in a similar way, and therefore might have the same effect on HIV-1 replication in mononuclear phagocytes. Like the Nef protein, Vpr – a 96 amino acid virionassociated protein – plays the role of an early viral protein [12] and mimics the biological effects of TNF. Although it is not essential for viral replication in T cells, Vpr is crucial for HIV replication in nondividing cells such as macrophages and acts by transactivating the viral promoter and the HIV-1 LTR, resulting in increased viral replication. An interaction between Vpr and the transcription factors Sp1 and TFIIB is required for Vpr-mediated transcriptional enhancement of the HIV-1 LTR in lymphoid and myeloid cells [12]. The ability of Vpr to activate HIV transcription is mediated by the p300/CBP transcriptional coactivator, which, in turn, enhances the ability of Vpr to activate NF-kB and the HIV enhancer [13]. Synthetic Vpr protein activates NF-kB, AP-1, and JNK in promonocytic U937 cells and in primary macrophages, resulting in enhanced HIV replication [14]. A structural and functional interaction between Vpr and Tat synergistically enhances the transcriptional activity of the HIV-1 LTR [15]. In fact, Tat by itself displays biological activities that mimic those mediated by TNF. Like TNF, Tat activates NF-kB, AP-1, and MAPK, as well as c-Jun N-terminal kinase/stressactivated protein kinase (JNK/SAPK), p38, and ERK1/2; however, unlike TNF, signalling does not modify the
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Opinion Box 1. TNFR signalling TNFa binds to two distinct cell transmembrane receptors, 55 kDa TNFR1 (also called TNF R55, TNF Rb, p55, or CD120a) and 75 kDa TNFR2 (also called TNF R75, TNF Ra, p75, and CD120b). They belong to the large TNFR superfamily that is defined by the cysteine-rich architecture of the ligand-binding region. Although TNFR1 is more ubiquitous, both TNFR1 and TNFR2 are present on virtually all cell types, except, most notably, for red blood cells. The trimeric TNF ligand is able to bind to TNFR1 and/or TNFR2 and thereby initiates the signal transduction process. Although TNF exhibits higheraffinity binding to TNFR2 than to TNFR1, most of the biological responses of TNF are thought to be mediated through TNFR1. The two receptors share 28% identity in their extracellular ligandbinding domain. In its intracytoplasmic portion, TNFR1, but not TNFR2, has a death domain (DD) of 80 amino acids that allows the transduction of proapoptotic signals. In addition, TNFRs mediate the activation of NF-kB, AP-1, c-Jun N-terminal kinase, and p38 or p42/ 44 mitogen-activated protein kinase (MAPK), leading to enhanced gene expression.
activation of MEK [16–18]. Altogether, Nef, Vpr, and Tat mimic TNFR signalling, especially in mononuclear phagocytes, resulting in enhanced HIV replication. Both macrophages and peripheral blood lymphocytes (PBLs) have been reported to produce less TNF and C-C chemokines after treatment with extracellular Vpr [19,20]. Thus, this
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hijacking of TNFR signalling by Nef, Vpr, and Tat might occur in the context of low levels of proinflammatory cytokine and C-C chemokine production. Low levels of TNF and C-C chemokines could also result from low levels of immune activation, as previously reported in HIV asymptomatic patients at early stages of the disease or in long-term nonprogressors [5,8,21]. HIV proteins favour inhibition of T cell apoptosis through interference with TNFR signalling HIV proteins favour persistence of the virus not only through activation of LTR activity, but also through inhibition of host cell apoptosis (Figure 1). In fact, the inhibition of apoptosis in HIV-1-infected T cells enhances virus production and facilitates persistent infection [22]. Both viral and cellular factors are involved in the controlled and sustained production of virions in infected CD4+ T lymphocytes in the vicinity of macrophages [23], and they could expand the viral reservoir and fuel the progression of the disease. At more advanced stages of the disease, when TNF expression (plasma and membrane-bound) becomes more pronounced, Nef prevents TNFR-mediated death in HIV-infected T cells via interaction with the apoptosis
Figure 1. Targeting of TNF signalling pathways by HIV proteins. The early-expressed proteins Nef, Tat, and Vpr target PI3K, ASK1, PAK2, c-Flip, p53, members of the Bcl-2 family, and caspases for inhibition of apoptosis while favouring viral replication through activation of NF-kB and AP-1. The late-expressed viral proteins gp120 and Vpu increase apoptosis by enhancing TNFR signalling, activating proapoptotic pathways (e.g. p53), and inhibiting antiapoptotic pathways (Bcl-2) and NF-kB. Green arrows indicate activation; red arrows indicate inhibition.
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Table 1. HIV proteins: from TNF mimicry to enhancement of TNF activity Viral protein Cellular target Action TNF signal mimicry and sustained viral growth (early stage of infection) p56Lck Activation of NF-kB, AP-1, and members of the MAPK family Tat (ERK1/2, p38, and JNK) NF-kB Activation of NF-kB and stimulation of the LTR Nef AP-1 Activation of AP-1 and stimulation of the LTR Members of the MAPK family Activation of ERK1/2, p38, and JNK NF-kB, AP-1, and JNK Activation of NF-kB, AP-1, and JNK and stimulation of the LTR Vpr p300/CBP transcriptional coactivator Activation of NF-kB and the HIV enhancer Inhibition of T cell apoptosis (early-mid stage of infection) ASK-1 Blockade of TNF-mediated apoptosis Nef PI3K Phosphorylation of Bad, a proapoptotic member of the Bcl-2 family Inhibition of mitochondrial-related apoptosis PAK2 Inactivation of Bad Akt Formation of a Nef-Akt-PI3K complex and inhibition of apoptosis
p53 Bcl-2 c-FLIP Immune suppression (late stage of infection) CXCR4 gp120 Bcl-XL Tat
Vpu
TrCp Bcl-xL, A1/Bfl-1, and TRAF1
Blockade of p53-mediated apoptosis Protection from apoptosis Decreased levels of caspase 10 to escape TRAIL cytotoxicity T cell apoptosis Decreased intracellular levels of the apoptosis-protective protein Bcl-XL, leading to apoptosis Reduction of NF-kB activity, contributing to the virus-induced cytopathic effects Caspase 3 activation, resulting in induction of apoptosis
signal-regulating kinase-1 (ASK-1). Nef inhibits ASK-1, caspase 3, and caspase 8 activation, resulting in the blockade of apoptosis in HIV-infected cells [24]. In addition, Nef coprecipitates phosphatidylinositol-3-kinase (PI3K) in 293T cells, especially its p85 subunit, via sequences present within the N and C termini of Nef [25]. This interaction leads to the activation of PI3K, which, in turn, phosphorylates Bad, the proapoptotic member of the Bcl2 family. Phosphorylation of Bad results in its inactivation and subsequently inhibits mitochondrial-related apoptosis. Nef-mediated inactivation of Bad is also dependent on the activation of p21-activated kinase 2 (PAK2) in T cells. In fact, Nef interacts with PAK2 via its PXXP motif [25,26]. In recent years, the PI3K-Akt signalling pathways have emerged as important and general mechanisms of antiapoptosis [27]. We observed that Nef binds directly to Akt in primary PBLs, suggesting the potential formation of a Nef-Akt-PI3K complex in HIV-infected T cells (G.H. and T. Fulop, unpublished data). Another antiapoptotic effect of Nef in T cells involves the interaction with the tumor suppressor protein p53 via its N-terminal extremity (amino acids 1–57), both in the cytoplasm and in the nucleus of the cell. This interaction results in a decrease of p53 half-life and in a decrease of p53-mediated transcriptional activity, and it ultimately blocks p53-mediated apoptosis [28]. Therefore, Nef, via interaction with ASK1, PAK2, PI3K, and p53, can protect infected T cells against apoptosis mediated by the TNFRs (Table 1). In addition to that of Nef, an antiapoptotic role for Tat has been reported. Endogenous Tat stably expressed in Jurkat clones protects cells from activation-induced apoptosis induced by TNF [18]. Tat can also protect lymphoid and myeloid cells from TRAIL (TNF-related apoptosisinducing ligand)-mediated apoptosis by upregulating Bcl-2 [29]. Furthermore, upregulation of c-FLIP and decreased levels of caspase 10 in lymphoid T cells have
Refs [16,17] [9,10] [9] [9,11] [14] [13] [24] [25] [25] G.H. and Fulop, unpublished [28] [29] [30] [5,43–45] [47] [48] [50]
been reported as potential molecular mechanisms by which to escape TRAIL cytotoxicity [30]. Thus, in the context of increasing expression of TNF throughout the disease [5], HIV proteins such as Nef, Vpr, and Tat favour a sustained viral replication, especially in mononuclear phagocytes, while at the same time preventing TNF-mediated death of infected T cells, thereby expanding the formation of cellular reservoirs of virions. HIV proteins favour immune suppression through the enhancement of TNFR signalling in the late stages of disease In late stages of the disease, high levels of TNF are produced, resulting in dramatic TNF-mediated death of uninfected T cells and subsequent major immune suppression [5,31]. HIV proteins, such as gp120 and Vpu, might further participate in immune failure by enhancing the cytopathic effects mediated through TNFR signalling. In vitro culture models demonstrate that uninfected CD4+ T cells undergo apoptosis upon contact with HIVinfected cells, such as mononuclear phagocytes. Macrophages play a major role in this process, suggesting that apoptosis-inducing ligands expressed by macrophages can mediate apoptosis of susceptible CD4+ and CD8+ T cells [23,32]. TNF is expressed on the surface of activated macrophages, and most of the CD4+ T lymphocyte death is mediated via TNF-mediated apoptosis and via Fas-Fas ligand and TRAIL-DR5 (death receptor) interactions [4,5,33,34]. The engagement of TNFR2 by TNF results in degradation of TRAF2 (TNFR-associated factor). Because TRAF2 recruits cIAP (cellular inhibitor of apoptosis proteins) to TNFR1, its degradation can facilitate apoptosis. TRAF2 degradation also attenuates TNFR1-mediated NF-kB activation, further promoting apoptosis [35]. Cell viability can be further impaired by the actions of HIV-1 proteins, especially gp120 and Vpu, 63
Opinion which potentiate TNF proapoptotic activity (Figure 1). HIV-1 gp120 has been reported to trigger the production of TNF and C-C chemokines, especially in mononuclear phagocytes [36–38]. Interactions between HIV-1 gp120 and CD4 stimulates signal transduction pathways, such as activation of protein kinase C (PKC); the generation of PKC-dependent phosphorylation of CD4; activation of the ERK and MAPK pathways, which, in turn, stimulates transcription factors, such as NF-kB, AP-1, Elk-1; and the induction of proinflammatory cytokine and C-C chemokine gene expression [36,39–41]. In addition, HIV-1 gp120, via interaction with CD4, activates uninfected CD4+ T cells, resulting in CD4+ T cell priming, and renders them susceptible to TNF-mediated apoptosis [42]. Furthermore, the binding of a cell-associated gp120 molecule to CXCR4 induces mitochondrial transmembrane depolarization; cytochrome C release from the mitochondria to the cytosol; and, ultimately, activation of downstream caspases, resulting in T cell apoptosis [43,44]. CD8+ T cell apoptosis in HIV infection is observed at the onset of AIDS in the context of high TNF expression [5,45,46], and it is triggered via interaction of HIV-1 gp120 with CXCR4 and via contacts between membrane-bound TNF on macrophages and TNFR2 on CD8+ T cells [5,45]. TNFR2 stimulation of T cells results in decreased intracellular levels of the apoptosis-protective protein Bcl-XL, a member of the Bcl-2 family [47]. Therefore, TNFR2 stimulation on CD8+ T cells by membrane-bound TNF expressed on the surface of macrophages might decrease the intracellular levels of antiapoptotic proteins, resulting in CD8+ T cell death. Vpu is a 16 kDa viral membrane protein that regulates the release of virions from infected cells and induces the disruption of an envelope gp120-CD4 interaction in the endoplasmic reticulum. Vpu functions as a competitive inhibitor of TNFR-complex proteins, such as TRAF1 [48,49]. In Vpu-expressing cells, the levels of TRAF1 in response to TNF stimulation are reduced and are no longer sufficient to inhibit the cytokine-induced activation of caspase 8. Activated caspase 8 induces the release of cytochrome C from the mitochondria. The release of cytochrome C is usually inhibited by members of the Bcl-2 protein family. In Vpu-expressing cells, the levels of these proteins are limited, resulting in increased release of cytochrome C and, subsequently, in caspase 3 activation [50]. Thus, expression of Vpu might augment the proapoptotic role of TNF during HIV infection, especially in late stages of the disease, when high TNF levels are detected [5,8]. Thus, in HIV-infected CD4+ T cells, Vpu blocks the TNFR1 signalling pathway and the activation of NF-kB and enhances TNFR-mediated apoptosis. In addition, TNFR signalling is involved in downregulation of the antiviral T cell response during persistent viral infection, and it acts by determining the fate of antigenspecific T cells. The inhibitory receptor programmed death-1 (PD-1), a negative regulator of activated T cells, is markedly upregulated on the surface of exhausted, HIV-specific CD8+ and CD4+ T cells [51]. PD-1 expression is upregulated by TNF and correlates with impaired, HIV-specific T cell functions as well as with predictors of disease progression: it is correlated positively with plasma viral load and inversely with CD4 T cell count [52,53]. 64
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Model of HIV pathogenesis based on modulation of TNFR signalling by HIV proteins: from TNF mimicry to enhancement of TNF activity Because of the various effects of HIV proteins on TNF signalling and apoptosis (Table 1), we would like to present a new, to our knowledge, model that highlights the role of TNFR signalling in the modulation of both T cell apoptosis and viral persistence and could thereby further enhance our understanding of the pathogenesis of HIV-mediated disease (Figure 2). Early in the disease, when the levels of proinflammatory proteins and C-C chemokines are low and chronic immune activation is not yet predominant [8], viral proteins are crucial for establishing a productive infection. Viral proteins expressed early in the viral cycle, such as Nef, Tat, and virion-associated Vpr, activate the TNFR pathway to partially mimic TNF biological effects, suggesting that these viral proteins can fuel the progression of the disease even in the absence of proinflammatory cytokines. These viral proteins play a role in the formation of viral reservoirs by activating transcription from the LTR and interfering with apoptotic machinery. In the meantime, Vpr protein blocks the production of proinflammatory cytokines and chemokines [19], and soluble Nef and Tat proteins favour the recruitment of both T cells and monocytes and macrophages [54–56], further indicating that viral proteins play a crucial role in taking control of HIV-1 replication. At a later stage of the disease, at the onset of AIDS, proinflammatory cytokines such as TNF and C-C chemokines are produced abundantly due to chronic immune stimulation [5,57,58], and viral proteins, rather than mimicking TNFR signalling, will, in fact, enhance TNFmediated T cell apoptosis. At that point, a high level of TNF has been reached; HIV-1 proteins expressed late in the viral cycle, such as gp120 and Vpu, are detected; and TNF-mediated T cell apoptosis is maximal, resulting in accelerated immune suppression. The T cell proapoptotic effect results from both increased cell-surface expression of TNF and TNFR molecules, especially TNFR1, in the context of a high level of proinflammatory cytokines triggered by gp120 [36] and from impaired antiapoptotic effects mediated via the TNFR-TRAF1 pathway triggered by Vpu [50]. In addition, high levels of proinflammatory cytokines and C-C chemokines are produced by infected cells, leading to enhanced T cell and monocyte and macrophage chemotaxis [58] and favouring T cell apoptosis in the context of chronic immune activation. This will result in accelerated T cell death and in increased release of mature, infectious virions from the infected cells, thus favouring immune failure. Although our model suggests a dual role for HIV proteins throughout the progression of the disease, from TNF mimicry to the enhancement of TNF activity, additional features have to be taken into account. First, although Nef, Vpr, and Tat play important roles in early viral persistence, each of these proteins might play a dual role and contribute to late immune suppression, owing to differential properties of its N and C termini (Box 2). Second, expression of TNF and TNFRs on
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Figure 2. A model of HIV pathogenesis based on modulation of TNFR signalling by HIV proteins. (a,b) During early HIV infection, the level of proinflammatory cytokines and chemokines is low. Soluble Nef and Tat favour T cell, monocyte, and macrophage chemotaxis, and T cells are infected upon contact with macrophages. Early viral proteins Nef, Tat, and Vpr stimulate HIV-1 transcription and block apoptosis in T cells, favouring viral persistence and the formation of viral reservoirs. In addition, Vpr blocks the production of proinflammatory cytokines and chemokines. (c,d) At a later stage of the disease, gp120 triggers the abundant production of proinflammatory cytokines and chemokines and increases the expression of TNF and TNFR1, resulting in CD4+ T cell apoptosis. In the meantime, Vpu accelerates the process by impairing antiapoptotic machinery. TNF upregulates PD-1 on HIV-specific CD8+ T cells, resulting in CD8+ T cell apoptosis as well. Thus, accelerated T cell death and increased release of virions from the infected cells favour immune failure in the late stages of disease. Mf, macrophage; T4, CD4+ T cell; T8, CD8+ T cell.
Box 2. Differential role of the N and C termini of viral proteins in HIV pathogenesis Although our model suggests that the HIV-1 proteins can be distinguished from one another based on their involvement either in early viral persistence (Nef, Tat, Vpr) or in late immune suppression (gp120, Vpu), each of the proteins involved in early viral persistence might also play a role in late immune suppression. The first coding exon of Tat protein is crucial for the stimulation of HIV-1 LTR, and, subsequently, for the formation of cellular reservoirs of virions; by contrast, the C terminus of the Tat protein has been reported to be proapoptotic [60]. Similarly, the N terminus of the Vpr protein has been reported to be involved in NF-kB activation and LTR stimulation, whereas the C terminus of Vpr is proapoptotic [14,61]. In addition, the N terminus and especially the myristoylation signal of the Nef protein have been reported to be involved in NF-kB activation [62]. By contrast, the C terminus of the Nef protein triggers apoptosis [63]. Thus, all three ‘‘early’’activating viral proteins, Tat, Nef, and Vpr, could play a dual role in HIV pathogenesis and could both favour the formation of cellular reservoirs of virions via their N termini and subsequently participate with gp120 and Vpu via their C termini to a proapoptotic stage, resulting ultimately in immune suppression.
lymphocytes and monocytes from HIV-1-infected patients shows significant variations in relation to disease stage and numbers of CD4+ lymphocytes in peripheral blood. Circulating levels of soluble TNF components among HIV-1-infected patients show a gradual increase with disease stage, with the highest levels present in the AIDS group [5]. Patients in earlier stages of the disease, with only a moderate decrease in numbers of CD4+ lymphocytes, are characterized by increased expression of membrane-bound TNFa (mTNF) and TNFRs (mTNFRs) on lymphocytes and monocytes as compared with healthy controls, whereas AIDS patients with markedly decreased numbers of CD4+ lymphocytes showed a significantly decreased expression of mTNFR2 on both lymphocytes and monocytes, even compared with healthy individuals [59]. Thus, a complex interplay between several HIV proteins and both soluble and membrane-bound forms of TNF and TNFRs could explain the two main characteristics of HIV infection: viral persistence and immune failure. 65
Opinion Conclusion The formation of viral reservoirs and the loss of T lymphocytes are central factors in the progression of HIV. TNF favours HIV replication and can induce apoptosis, and, interestingly, several HIV-1 proteins share similar functions. HIV-1 proteins mimic, interfere with, or enhance TNFR signalling at different stages of the disease and therefore represent potential targets that could ultimately lead to new treatments controlling both viral replication and immune activation in HIV-infected subjects. Acknowledgements We thank Aurelie Vacheret for secretarial assistance. Support was provided by grants from Franche-Comte´ University (to G.H.), from Societe Francaise D’Exportation des Ressources Educatives, and from the Higher Education Commission, Pakistan (to K.A.K.).
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