Lung dendritic cells and the inflammatory response

CME review article This feature is supported by an unrestricted education grant from AstraZeneca LP

Lung dendritic cells and the inflammatory response Mitchell H. Grayson, MD

Objective: To discuss the role of conventional and plasmacytoid dendritic cells in inducing and modulating immune responses in the lung. Data Sources: The primary literature and selected review articles studying the role of dendritic cells in both rodent and human lungs as identified via a PubMed/MEDLINE search using the keywords dendritic cell, antigen-presenting cell, viral airway disease, asthma, allergy, and atopy. Study Selection: The author’s knowledge of the field was used to identify studies that were relevant to the stated objective. Results: Dendritic cells are well positioned in the respiratory tract and other mucosal surfaces to respond to any foreign protein. These cells are crucial to the initiation of the adaptive immune response through induction of antigen specific T-cell responses. These cells also play an important role in the regulation of developing and ongoing immune responses, an area that is currently under intense investigation. This review discusses the various subsets of human and rodent dendritic cells and the pathways involved in antigen processing and subsequent immune regulation by dendritic cells in the lung using both viral and nonviral allergenic protein exposure as examples. Conclusions: Conventional and plasmacytoid dendritic cells are uniquely situated in the immune cascade to not only initiate but also modulate immune responses. Therapeutic interventions in allergic and asthmatic diseases will likely be developed to take advantage of this exclusive position of the dendritic cell. Ann Allergy Asthma Immunol. 2006;96:643–652. Off-label disclosure: Dr Grayson has indicated that this article does not include the discussion of unapproved/investigative use of a commercial product/device. Financial disclosure: Dr Grayson has indicated that in the last 12 months he has served on the speaker’s bureau for Merck and has received research support from Genentech/Novartis. Instructions for CME credit 1. Read the CME review article in this issue carefully and complete the activity by answering the self-assessment examination questions on the form on page 2. To receive CME credit, complete the entire form and submit it to the ACAAI office within 1 year after receipt of this issue of the Annals.

INTRODUCTION The generation of a specific immune response begins with activation of professional antigen-presenting cells. Chief among these is the dendritic cell, a cell derived from the pluripotent hematopoietic stem cell.1 Dendritic cells are found in most peripheral tissues and are able to dynamically monitor mucosal surfaces. After activation, they move to secondary lymphoid tissues (eg, lymph nodes), where the dendritic cell presents antigen to T cells and initiates the adaptive (ie, T and B cell) immune response. In addition to this primary function of initiating the adaptive immune re-

Division of Allergy and Immunology, Department of Internal Medicine, Washington University School of Medicine, Saint Louis, Missouri. This review was supported by National Institutes of Health grant AI1800. Received for publication March 21, 2005. Accepted for publication in revised form April 5, 2006.

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sponse, dendritic cells are also capable of modulating and even suppressing developing and ongoing responses. Thus, the dendritic cell is critical to the normally functioning immune system. This review discusses the role of conventional and plasmacytoid dendritic cells in inducing and modulating immune responses in the lung. The primary literature and selected review articles on the role of dendritic cells in both rodent and human lungs were identified via a PubMed/MEDLINE search dendritic cell, antigen-presenting cell, viral airway disease, asthma, allergy, and atopy. The author’s knowledge of the field was used to identify studies that were relevant to the stated objective. DENDRITIC CELL SUBTYPES All dendritic cell subtypes are thought to develop in the bone marrow and travel to peripheral tissues in an immature state.2

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Two broad subtypes of dendritic cells exist: conventional and plasmacytoid. The conventional subset can be further divided into myeloid and lymphoid dendritic cells based on surface marker expression.3 The conventional dendritic cells are the major antigen-presenting cells in the body, whereas plasmacytoid dendritic cells are major producers of interferon-␣ (IFN-␣) (type I IFN) and play an important role in modulating immune responses rather than initiating them. Although some early evidence in mice suggested that conventional myeloid dendritic cells preferentially skewed T-cell responses toward TH2 and lymphoid dendritic cells toward TH1, subsequent investigations have shown that the T-cell–skewing event may be more plastic with either dendritic cell subtype being capable of inducing TH1 or TH2 responses.4 – 6 Plasmacytoid dendritic cells are capable of migrating to draining lymph nodes but lack significant antigen-presenting ability; therefore, in the draining lymph nodes they influence conventional dendritic cell skewing of T-cell responses via their IFN-␣ production. Although much of our understanding of dendritic cells is based on work in the rodent system, the conclusions are assumed to be equally applicable to human dendritic cells. The human DC1 subset is analogous to the conventional dendritic cells found in mice, whereas the human DC2 subset is analogous to the plasmacytoid dendritic cells found in mice. Although peripheral mouse conventional dendritic cells can be subdivided further into myeloid and lymphoid cells, no such differentiation has been described in the human. A

comparison of murine and human dendritic cell subset phenotypes is given in Table 1. Most of the information presented in this review has come from rodent studies, and although surface marker expression may be somewhat different between the 2 species, the broader concepts of dendritic cell function are similar. LUNG DENDRITIC CELLS The rodent lung is well endowed with dendritic cells, and it has been estimated that there are between 400 and 800 dendritic cells per cubic millimeter of epithelial surface in the rat airway.7 The turnover of these cells depends on antigen exposure, but, in general, dendritic cells rapidly turn over, with an estimated 85% changing every 36 to 48 hours. In the same set of experiments, it was estimated that the half-life of lung dendritic cells ranged from 1.5 to 2 days in the airway to 3 to 4 days in the periphery. This is compared with a half-life of 9 to 15 or more days in skin.2,8 Another difference between lung and skin is that in the skin the Langerhans cells (a specialized skin dendritic cell) are capable of maintaining their numbers by cell division, whereas no self-renewing population of lung dendritic cells have been identified.9 In fact, homeostatic maintenance (recruitment and/or retention) of lung dendritic cell numbers appears to depend primarily on the chemokine CCL5 (RANTES) and other agonists for the chemokine receptors CCR1 and CCR5.7 Most, if not all, conventional dendritic cells in the lung exist in the immature state, which is defined by low-level

Table 1. Lung Dendritic Cell Subsets in Mice and Humans* Conventional dendritic cells Mouse

Surface markers CD11c CD11b CD4 CD8 Gr-1 mPDCA-1 B220 440c 120G8 BDCA-1 (CD1c) BDCA-2 BDCA-3 BDCA-4 CD123 (IL-3R␣ chain) Key cytokines produced Cytokines needed to generate cells from CD34⫹ precursor

Human

Mouse

Human

Plasmacytoid

DC2

Myeloid

Lymphoid

DC1

⫹ ⫹ ⫹/⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺

⫹ Low/⫺ ⫺ ⫹/⫺ ⫺ ⫺ ⫺ ⫺ ⫺

⫹ ⫹† ⫹† ⫺

IL-12 and IL-23 GM-CSF and IL-4 or TNF

Plasmacytoid dendritic cells

⫹ ⫺ ⫹ ⫺ ⫺

Low-intermediate ⫺ ⫺ ⫺ ⫹ ⫹‡ ⫹ ⫹‡ ⫹‡

⫺ ⫺ ⫹ ⫺

⫺ ⫹ ⫺ ⫹ ⫹

IFN-␣ and TNF Flt-3 ligand (IL-3 and CD40 ligand if generating from blood)

Abbreviations: GM-CSF, granulocyte-macrophage colony-stimulating factor; IL, interleukin; TNF, tumor necrosis factor. * ⫹ indicates high expression; ⫹/⫺, mixed expression; ⫺, no expression. † Subset of human DC1 cells do not express this marker. ‡ Found on a subset of murine plasmacytoid dendric cells.

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expression of major histocompatibility complex (MHC) class I and II molecules and a lack of costimulatory molecules such as CD80/CD86 (B7.1/B7.2).7 Most lung dendritic cells display no specific subset markers or are of the myeloid-type conventional dendritic cell.10 In addition, immature murine dendritic cells express specific chemokine receptors (CCR1, CCR5, and CCR6), Fc␥R, Fc␧R, and various pattern receptors (such as the mannose receptor and toll-like receptors [TLRs]).7,11,12 Via pinocytosis and these receptors, as shown in Figure 1, the dendritic cell samples its environment by binding various sugars (mannose), complement (via CR3), lipopolysaccharide (via TLR-4), nucleic acids (via TLR-9), and immunoglobulins.3 Thus, the immature dendritic cell can ingest virtually any foreign protein or nucleic acid that appears in the tissue. As shown in Figure 2, dendritic cells are well positioned at mucosal surfaces to sample proteins and invading organisms that might be present. In the gut, dendritic cells are located

just below the epithelial surface, and they extend projections up between epithelial cells into the lumen. This allows the dendritic cell access to the protein loads in the gut. Whether a similar arrangement occurs in the lungs is not known but assumed to be the case. Further, in the respiratory tract dendritic cells have been shown to be capable of migrating out of the tissue into the air space and back again into the parenchyma. This sentinel activity allows the dendritic cell to constantly inform the immune system of current and pending insults. Once the dendritic cell has taken up a foreign antigen, the maturation program begins. This changes the dendritic cell from an antigen-uptake cell into an antigen-presenting cell. Rather quickly the chemokine receptors involved in homing of dendritic cells to peripheral tissues (primarily CCR1, CCR5, and CCR6) are down-regulated.11 This allows the maturing dendritic cell to escape from the tissue and enter the lymph flow. Associated with this shift is up-regulation of

Figure 1. Dendritic cell processing pathways. A, Dendritic cells ingest foreign proteins primarily through attachment of pattern recognition receptors, such as immunoglobulin receptors (FcR), carbohydrate receptors (mannose receptor), complement (CR3), and toll-like receptors (TLR). Once bound and ingested by pinocytosis, the proteins begin to breakdown in the endolysosome. B, The usual pathway for these peptides is to merge with a major histocompatibility complex (MHC) class II– containing vesicle, which allows the peptides to bind to the MHC class II molecules and be exported to the surface, where they are presented to T cells. C, Intracellular proteins, such as those from infecting viruses, are broken down by the proteosome and then transported into the endoplasmic reticulum (ER), where they bind and stabilize MHC class I molecules. The peptide-containing MHC class I molecules are then transported to the plasma membrane for presentation. D, Cross-presentation of extracellular proteins occurs when vesicles with recycled MHC class I molecules fuse with the endolysosome, allowing for the ingested peptides to bind to the MHC class I molecules. This binding stabilizes the MHC class I molecules and allows for their transport back to the membrane. Thus, an extracellular protein is presented in terms of MHC class I. E, Another route for cross-presentation is if the aspartic protease cathepsin D causes release of the contents in the late endosome. This allows peptide fragments from the extracellular environment to be released into the cytosol, where the proteosome can digest them. F, Subsequent transport to the ER and loading on MHC class I molecule occurs, and an extracellular peptide is expressed in terms of MHC class I. G, A newly discovered pathway, termed cross-dressing, involves the fusion of membrane fragments from injured or dying cells with the dendritic cell plasma membrane. If these membrane fragments contain peptide-loaded MHC class II molecules, the dendritic cell then presents these molecules to passing T cells and can induce a response against an antigen that the dendritic cell never actually processed. Whether this pathway exists for MHC class I molecules is not known, although it is intriguing to speculate that it may be an important mechanism in the generation of an antiviral immune response.

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Figure 2. Dendritic cells in lung inflammation. Immature conventional dendritic cells traffic to the lung parenchyma via the use of specific chemokine receptors. Once in the lung they may extend processes out into the airways, where they can sample the external environment. In addition, dendritic cells are capable of exiting into the airway to ingest foreign protein before returning to the parenchyma. As shown in the left panel, ingestion of a nonviral antigen in the presence of a “danger” signal (for example, endotoxin or lipopolysaccharide [LPS]) causes the immature dendritic cell (iDC) to migrate to the draining lymph node, where it undergoes a phenotypic change and becomes a mature and differentiated dendritic cell (mDC)–presenting antigen to rare naı¨ve antigen-specific T cells. These CD4 T cells proliferate and induce B cells to produce IgE, among other immunoglobulins, whereas the mDC dies by apoptosis. Effector TH2 cells then migrate into the lung parenchyma, where they release their cytokines, inducing an allergic or asthmatic response. Plasmacytoid dendritic cells (pDC), in part through their production of interferon-␣ (IFN-␣), are able to negatively regulate this response. In the right panel, a similar role for dendritic cells occurs in viral airway disease. However, the primary T-cell response consists of CD8 T cells producing IFN-␥, as well as neutralizing antiviral IgG. Plasmacytoid dendritic cells produce IFN-␣ in the lymph node, which helps skew the T-cell response toward a primary IFN-producing antiviral response. Direct viral infection of pDC leads to the generation of IL-10 regulatory T cells that can inhibit the activated CD8 T cells and help down-regulate the immune response.

CCR7, which allows the dendritic cell to migrate toward a CCL19/CCL21 gradient. CCL19 and CCL21 are the chemokines that are found in the high endothelial venules of the draining lymph node. Thus, the dendritic cell attains the ability to hone to the lymph node.13 Interestingly, mice deficient in Runx3, a transcription factor that represses expression of CCR7, develop asthma-like disease in the absence of any exogenous stimuli, presumably due to enhanced CCR7 expression on dendritic cells, which results in a higher frequency of dendritic cell migration to the draining lymph nodes.14 As the dendritic cell enters the lymph node, several other phenotypic changes occur. In particular, there is an increase in expression of costimulatory and antigen presentation molecules—for example, increased CD80/CD86 (B7.1/B7.2), CD1, intercellular adhesion molecule 1, lymphocyte function associated antigen 3, and MHC class I and II expression.1,3 Along with these antigen presentation molecules, the dendritic cell undergoes significant shape change. Although immature dendritic cells (and those in transit in the draining lymph) are basically rounded cells, mature dendritic cells have many long extensions (giving rise to the name dendritic cell). All these changes make the dendritic cell a better antigen-presenting cell, and the dendrites provide a greater surface area with which to contact T cells.

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Although these phenotypic changes are occurring on the dendritic cell, extracellular proteins that were ingested or taken up by pattern recognition receptors are processed as shown in Figure 1. This processing involves breaking down the proteins into peptide fragments in the acidic endolysosome. Peptide fragments of 13 amino acids or larger are then capable of binding in the grove on MHC class II molecules, and these complexes are then transported to the dendritic cell plasma membrane for presentation. Intracellular proteins, on the other hand, are continuously being diced into fragments of 8 to 10 amino acids by the proteosome. These peptides are transported into the endoplasmic reticulum (ER) by the transporter associated with the antigen-processing protein. In the ER, the peptides bind and stabilize MHC class I, leading to expression of the peptide-loaded MHC class I molecule on the dendritic cell plasma membrane. Alternative pathways exist to load extracellular proteins onto MHC class I molecules, and these are discussed in terms of viral infections in the section that follows. In general, once the dendritic cell has matured and entered the lymph node, it presents antigen to any passing T cell. Rapidly moving rare antigen-specific T cells percolate past the slowly moving dendritic cells in the node.15 During this intricate dance, the antigen-specific receptor on the T cell (the TCR) scans the MHC molecules on dendritic cell extensions.

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When a dendritic cell is presenting the appropriate antigen in the correct MHC context, the TCR will attach. On this initial engagement of the TCR, a more stable interaction develops with a well-characterized immunologic synapse, forming between the T cell and dendritic cell.16,17 Interference with the formation or maintenance of the synapse will lead to release of the T cell, which will then become anergic. The strong interaction initiated by synapse formation leads to stimulation of T cells for up to 10 hours, during which time the cells remain tightly bound to the dendritic cell.18 After this period, the T cells disengage from the dendritic cell and begin to proliferate, giving rise to the clonal expansion that characterizes the immune response. Once the antigen-specific response is generated, the dendritic cell is thought to undergo apoptosis, thus down-regulating the immune response.

REGULATION BY DENDRITIC CELLS As shown in Figure 3, dendritic cells not only initiate the immune response but also are important modulators of it. Conventional dendritic cells are able to skew developing CD4 or CD8 T-cell responses by selectively producing various cytokines during antigen presentation. Production of interleukin 12 (IL-12), for example, will skew developing CD4 T cells toward a TH1 phenotype, whereas IL-4 skews toward TH2. Interestingly, it was shown recently that the transcription factor t-bet, which had been known to be selectively expressed in CD4 TH1 cells, needs to be expressed in the dendritic cell to generate fully functional TH1 responses.19 Although several TH2-promoting transcription factors have been identified (GATA-3 and NF-ATc1), as of yet their expression in dendritic cells and their role in TH2 skewing

Figure 3. Immunoregulation by dendritic cells. Broadly dendritic cells can influence immune responses in 3 ways: activation, abrogation, and inhibition. Activation includes secretion of TH1-, TH2-, or TH17-promoting cytokines (interleukin 12 [IL-12], IL-4, and IL-23, respectively) during antigen presentation. This leads to expansion of clonally restricted T cells with the appropriate phenotype. TH1 skewing depends on expression of the transcription factor t-bet in dendritic cells and T cells; it is unclear if similar transcription factor requirements exist for other T-cell skewing. TH17 cells induce secretion of IL-6, which has been linked to mucous production and may be important in the development of asthma. Dendritic cell–produced IL-6 and glucocorticoid-induced tumor necrosis factor–receptor related ligand allow effector T cells to escape regulation by regulatory T cells. Abrogation involves preventing an effector T-cell response from occurring at the point of antigen presentation. Release of antigen-specific T cells before they have received a complete activation stimulus leads to anergy of the clone. Dendritic cells can produce indoleamine-2,3-deoxygenase (IDO), which catabolizes tryptophan and starves developing T cells. IL-12p40 induces dendritic cells to express Fas ligand (FasL), which drives Fas-expressing T cells into apoptosis. The final way in which dendritic cells can modulate an immune response is through inhibition of ongoing reactions. Production of CCL17 (TARC) by dendritic cells recruits regulatory T cells, and these cells can induce the dendritic cell to increase expression of IDO. In addition, these cells induce release of transforming growth factor ␤ (TGF␤) by dendritic cells, which inhibits effector T-cell activation. Plasmacytoid dendritic cells, when infected with a virus, induce IL-10 –producing regulatory T cells in a type I IFN– dependent fashion. Thus, dendritic cells are capable of extensively modulating the immune environment during an adaptive response.

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have not been fully characterized.20,21 A relatively new CD4 T-cell subset, the IL-17–producing TH17 cell, develops when IL-23 is released from the conventional dendritic cell during antigen presentation.22 The function of these cells is not known, but they are thought to increase inflammatory responses in general. However, a mouse with transgenic overexpression of IL-17 in the lung developed airway remodeling and leukocyte infiltration in the absence of an exogenous stimulus, and in vitro IL-17 has been shown to induce IL-6 – dependent mucous production from epithelial cells.23,24 In humans, IL-17 has been found to be increased in the airways of asthmatic patients.25 Therefore, dendritic cells may be important in the development of asthma and airway remodeling through their induction of TH17 cells. Conventional dendritic cells are also able to regulate T-cell responses through several mechanisms. Release of a T cell before full stimulation during antigen presentation will lead to an anergic response by the T cell. Dendritic cells also express the enzyme indoleamine-2,3-deoxygenase (IDO), which catabolizes tryptophan, a critical amino acid for T-cell proliferation.26 Therefore, by increasing expression of IDO, the dendritic cell can starve developing T cells and abrogate an otherwise productive immune response. Dendritic cells can also induce developing T cells to undergo apoptosis via IL-12p40 – dependent expression of Fas ligand by the dendritic cell.27 Finally, conventional dendritic cells are a major source of CCL17, a chemokine that recruits naturally occurring regulatory T cells (Tregs).28 Production of this chemokine, therefore, in the draining lymph node could lead to an influx of Tregs that dampen or shut down the developing or ongoing immune response. Indeed, this may be an important mechanism to protect against development of autoimmune disease. Tregs can also use the dendritic cell to accomplish their regulatory function. In particular, Tregs can induce IDO expression in dendritic cells through Treg binding to costimulatory molecules on the dendritic cell.29 Tregs can also induce dendritic cells to produce transforming growth factor ␤, which by binding to a receptor on effector T cells leads to inhibition of their activation.30 Finally, dendritic cells can produce IL-6 or glucocorticoid-induced tumor necrosis factor–receptor related ligand, cytokines that make responding effector cells unresponsive to control by Tregs.31,32 Plasmacytoid dendritic cells, although primarily not presenting antigen, have the ability to skew T-cell responses through production of IFN-␣. Indeed, IL-10 –producing Treg development depends on type I IFN signaling, and at least one report has shown that virally infected plasmacytoid dendritic cells are capable of generating IL-10 –producing Tregs.33–35 Similar to the conventional dendritic cell, plasmacytoid dendritic cells also produce IDO and can reduce microenvironment tryptophan levels, leading to starvation of T cells.36,37 The functional relevance of plasmacytoid dendritic cell regulation of T-cell responses has been shown in a mouse model of asthma, where depletion of these cells led to a worsening asthmatic phenotype and exogenous addition of these cells reduced the response.38 A similar phenomenon

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was reported recently in mice where blockade of the complement receptor C5aR was associated with an increased myeloid-to-plasmacytoid dendritic cell ratio, as well as increased TH2 cell development.39 Indeed, it is appealing to speculate that the atopic diathesis relates to an imbalance in conventional and plasmacytoid dendritic cells. VIRUS AND LUNG DENDRITIC CELLS Although some viruses are capable of directly infecting dendritic cells (herpes simplex virus, vaccinia virus, and measles virus, for example) and being presented in MHC class I molecules, most do not directly infect the dendritic cell.40 – 43 To take up virus without being directly infected, the dendritic cell may express receptors that bind the virus and internalize it. Examples of viruses taken up in this manner are human immunodeficiency virus, cytomegalovirus, Ebola, dengue, and hepatitis C, all of which are capable of binding to the C-type lectin, dendritic cell–specific intercellular adhesion molecule grabbing nonintegrin.44 Dendritic cell–specific intercellular adhesion molecule grabbing nonintegrin normally functions as an adhesion receptor for rolling and tethering on endothelium and firmly adhering to T cells during antigen presentation. Since many viruses do not have known receptors and cannot directly infect dendritic cells, the cell must ingest virally infected cells (and virus) from the extracellular environment to obtain viral proteins to process. These types of antigens are primarily presented by MHC class II; however, antiviral responses are most often characterized by CD8 responses. Since CD8 T cells recognize antigen presented in MHC class I, the use of MHC class II by conventional dendritic cells would fail to induce a productive CD8 response. Studies of mechanisms of tolerance and recent examinations of antiviral responses have shown that extracellular antigens can be expressed in terms of MHC class I on dendritic cells.45,46 This act of displaying peptides from the extracellular environment in context of MHC class I has been termed cross-presentation (Fig 1).47 One way exogenous antigen can be presented in MHC class I molecules is if apoptotic or necrotic material is ingested by dendritic cells and processed via the endosomal pathway. At the late endosome, the aspartic protease cathepsin D can induce release of the endosomal protein contents into the cytosol. Once the partially processed proteins have been released into the cytosol, they can be degraded by the proteosome into appropriate peptides. These peptides are exported to the ER by the transporter associated with the antigen-processing protein. Once in the ER, the peptides are able to be loaded onto MHC class I molecules for transport to the cell surface. Another reported mechanism for cross-presentation involves recycling MHC class I molecules encountering antigenic peptides in endosomal compartments. In this case, the peptides stabilize the MHC class I, which is then capable of returning to the cell surface displaying the antigenic peptide. It appears that cross-presentation is a regular occurrence in dendritic cells and primarily initiates tolerance— especially if

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it occurs in the absence of dendritic cell activation.48,49 However, in viral infections the inflammatory response rapidly leads to activation of dendritic cells, and the cross-presentation pathway leads to a productive immune response. Another nontraditional mechanism for expressing peptide-loaded MHC molecules on dendritic cells was reported in a study of tumor cell vaccines. In this model, tumor cells expressing MHC class II loaded with tumor peptides were found to induce a productive antitumor response; however, this response was dependent on the host’s dendritic cells.50 On further examination, it was discovered that fragments of tumor cell membranes with peptide-loaded MHC molecules were integrated into the dendritic cell plasma membrane in a manner that appeared to require an unidentified surface receptor on the dendritic cell. Whether this novel mechanism, entitled cross-dressing, also functions for MHC class I molecules is not known, although it is plausible to speculate that it occurs and may be an important route for inducing CD8 antiviral immunity. In the case of influenza, it has been shown that pulmonary dendritic cells rapidly become activated and migrate to the draining lymph node. Indeed, within 6 hours of exposure to the virus, the draining lymph nodes contain numerous activated and mature pulmonary dendritic cells. Interestingly, the dendritic cell response was limited to the first inflammatory antigen encountered in the lung.51 If an additional dendritic cell maturation stimulus (CpG, for example) was given 54 hours after the viral inoculation, there was no increase in dendritic cell migration to lymph nodes. This finding suggests that in a viral response (at least a pulmonary viral response) there is only an initial wave of dendritic cells that go to the node to present antigen; however, since most productive viral infections last for approximately 7 to 10 days, it remains unclear whether dendritic cells are capable of mounting a second wave of maturation and presentation after the initial lag period. Although this control of dendritic cell migration may be a means to prevent autoimmune responses to the many self-antigens that would be released during the productive viral infection, it also suggests that the host is at an increased risk of a superinfection, since lung dendritic cells would not be able to respond to a secondary insult, such as a bacterial infection occurring during the viral illness. As a result of activation, maturation, and migration of the pulmonary dendritic cell, a brisk induction of the adaptive immune response occurs. Within several days antigen-specific T cells (both CD8 and to a lesser extent CD4) arrive in enormous numbers at the lung (Fig 2). Rapidly, the virus is cleared and the inflammatory response ratcheted down. The role the dendritic cell plays in this process remains greatly underexplored. Mature dendritic cells can be found in the lung during the resolution phase of the infection, and it is intriguing to speculate that these mature dendritic cells provide a brake for the response—perhaps by activating Tregs or inducing tolerance in CD4 and CD8 T cells through some other means. Thus, the dendritic cell is a prime candidate for

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therapeutic manipulation to alter the antiviral immune response. The plasmacytoid dendritic cell is thought to play a major role in the antiviral response primarily through production of the antiviral cytokine IFN-␣. In a mouse model of viral respiratory infection, few plasmacytoid dendritic cells are noted within the lung parenchyma at baseline; however, rapidly on initiation of a productive infection, plasmacytoid dendritic cells are recruited into the lung. Shortly thereafter, these cells appear in the draining lymph node, allowing them to modulate the developing immune response. Through the production of IFN-␣, the plasmacytoid dendritic cells are capable of skewing the reaction toward a predominant TH1 and TC1 antiviral response. In addition, these cells may be important in regulating the immune response as described herein. ALLERGEN AND LUNG DENDRITIC CELLS In many ways the development of an airway allergic response is similar to that of the antiviral response. Dendritic cells pick up the antigen from the airways and then begin to mature as they migrate back to the draining lymph node. Interestingly, the presence of antigen alone does not appear to cause dendritic cells to mature and in mouse models appears to induce tolerance through either a direct effect on T cells or the generation of Tregs.52,53 Therefore, a dendritic cell maturation stimulus needs to be present along with the antigen. Given the popularity of the hygiene hypothesis, the maturation stimulus often used in these models is lipopolysaccharide.54 Once exposed to lipopolysaccharide along with antigen, dendritic cells migrate to the lymph nodes and generate a specific T-cell response (Fig 2). Plasmacytoid dendritic cells have been shown to play an important role in protection against development of allergic airway disease.55 Whether this protection is mediated by direct inhibition of T-cell proliferation, interference with myeloid dendritic cell antigen presentation, or induction of Tregs is unknown. Further, there is significant heterogeneity in the ratio of plasmacytoid to myeloid dendritic cells found among mouse strains.56 Whether this heterogeneity applies to other species is not known. Finally, there appears to be a role for dendritic cells in the ongoing allergic inflammatory response. With the onset of inflammation there is a rapid increase in dendritic cells in the lung. In contrast to the situation with a viral infection, dendritic cells in the chronic inflammatory phase are capable of migrating back to the draining lymph node. It is presumed that they lead to increased T-cell proliferative responses. The importance of dendritic cells in both the initiation and maintenance phases of the allergic airway inflammatory response has been shown using mice where these cells can be selectively depleted.57 Further, repeated allergen challenge has been shown to deplete airway myeloid dendritic cell numbers.58

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CONCLUSION This review has focused on the role of the lung conventional and plasmacytoid dendritic cell in generating and modulating a respiratory immune response. Both with viral and allergenic protein exposure, the dendritic cell is at the helm of initiating the adaptive immune response and, as such, is a prime candidate for therapeutic attention. Modulating dendritic cell function early in a paramyxoviral infection, for example, may reduce the risk of subsequent asthma. Alternatively, and perhaps even more intriguing, is the possibility of altering dendritic cell regulatory activities, which could allow for the “turning off” of an allergic response even after it had begun. The dendritic cell family is clearly important and likely will be the target of many future therapeutic endeavors. It is important for the clinician to be aware of these specialized cells and the critical roles they play in respiratory immunology. ACKNOWLEDGMENTS I thank Dorothy Cheung, MD, for critical reading of the manuscript and Michelle M. Rohlfing for excellent technical assistance. REFERENCES 1. Shortman K, Liu YJ. Mouse and human dendritic cell subtypes. Nat Rev Immunol. 2002;2:151–161. 2. Holt PG, Haining S, Nelson DJ, Sedgwick JD. Origin and steady-state turnover of class II MHC-bearing dendritic cells in the epithelium of the conducting airways. J Immunol. 1994;153: 256 –261. 3. Kelsall BL, Biron CA, Sharma O, Kaye PM. Dendritic cells at the host-pathogen interface. Nat Immunol. 2002;3:699 –702. 4. Lambrecht BN, De Veerman M, Coyle AJ, Gutierrez-Ramos JC, Thielemans K, Pauwels RA. Myeloid dendritic cells induce Th2 responses to inhaled antigen, leading to eosinophilic airway inflammation. J Clin Invest. 2000;106:551–559. 5. Dodge IL, Carr MW, Cernadas M, Brenner MB. IL-6 production by pulmonary dendritic cells impedes Th1 immune responses. J Immunol. 2003;170:4457– 4464. 6. De Smedt T, Butz E, Smith J, et al. CD8alpha(-) and CD8alpha(⫹) subclasses of dendritic cells undergo phenotypic and functional maturation in vitro and in vivo. J Leukoc Biol. 2001;69:951–958. 7. Stumbles PA, Strickland DH, Pimm CL, et al. Regulation of dendritic cell recruitment into resting and inflamed airway epithelium: use of alternative chemokine receptors as a function of inducing stimulus. J Immunol. 2001;167:228 –234. 8. Ghaznawie M, Papadimitriou JM, Heenan PJ. The steady-state turnover of murine epidermal Langerhans cells. Br J Dermatol. 1999;141:57– 61. 9. Merad M, Manz MG, Karsunky H, et al. Langerhans cells renew in the skin throughout life under steady-state conditions. Nat Immunol. 2002;3:1135–1141. 10. Vermaelen KY, Carro-Muino I, Lambrecht BN, Pauwels RA. Specific migratory dendritic cells rapidly transport antigen from the airways to the thoracic lymph nodes. J Exp Med. 2001;193: 51– 60. 11. Luster AD. The role of chemokines in linking innate and adaptive immunity. Curr Opin Immunol. 2002;14:129 –135.

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Requests for reprints should be addressed to: Mitchell H. Grayson, MD [email protected] Campus Box 8122 660 South Euclid Ave Saint Louis, MO 63130 E-mail: [email protected]

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Objectives: After reading this article, participants should be able to demonstrate an increased understanding of their knowledge of allergy/asthma/ immunology clinical treatment and how this new information can be applied to their own practices. Participants: This program is designed for physicians who are involved in providing patient care and who wish to advance their current knowledge in the field of allergy/asthma/immunology. Credits: ACAAI designates each Annals CME Review Article for a maximum of 2 category 1 credits toward the AMA Physician’s Recognition Award. Each physician should claim only those credits that he/she actually spent in the activity. The American College of Allergy, Asthma and Immunology is accredited by the Accreditation Council for Continuing Medical Education to sponsor continuing medical education for physicians.

CME Examination 1–5, Grayson MH. 2006;96:643– 652. CME Test Questions 1. Which dendritic cell subset represents the major antigen-presenting cells in the body? a. DC2 b. plasmacytoid c. DC1 d. traditional e. Kupfer dendritic cells 2. Dendritic cells are capable of regulating a developing T-cell response in all of the following ways EXCEPT: a. secretion of T-cell–skewing cytokines b. expression of enzyme(s) that lead to starvation of developing T cells c. induction of Fas ligand, leading to apoptosis of the developing T cell d. expression of glucocorticoid-induced tumor necrosis factor–receptor related ligand, which induces apoptosis in developing T cells e. secretion of CCL17, which attracts regulatory T cells to the tissue 3. Extracellular proteins normally are presented by dendritic cells in the grove of which molecule? a. major histocompatibility complex (MHC) class I b. MHC class II c. MHC class III d. B7.1 e. dendritic cell–specific intercellular adhesion molecule grabbing nonintegrin 4. When the T-cell–dendritic cell interaction is interrupted during antigen presentation, what happens to the T cell? a. it undergoes apoptosis b. it resets and can develop into an effector cell with encounter of the same antigen

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c. it becomes anergic d. it develops into an interleukin 10 (IL-10)–producing regulatory T cell e. it proliferates and becomes a fully functional effector cell 5. When conventional dendritic cells ingest a foreign protein and begin to mature, which of the following occurs that allows them to migrate to the draining lymph node? a. immunoglobulin receptor expression increases and chemokine receptors are shed b. reduction in certain chemokine receptors (CCR1, CCR5, and CCR6), with expression of other chemokine receptors (CCR7) c. development of “foamy” vacuoles and ruffled extensions d. release of IL-6 after binding of glucocorticoid-induced tumor necrosis factor–receptor related ligand e. shedding of MHC class II molecules 6. In the lung, dendritic cells are excellent sentinel cells because: a. they can traffic through the airspaces in the lung looking for foreign proteins b. they express numerous pattern recognition receptors that allow them to bind to foreign proteins c. they are present in a fairly high density within the lung parenchyma and may have long processes that they stick between epithelial cells, similar to what is found in the gastrointestinal tract d. all of the above e. none of the above Answers found on page 678.

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