Antigen Presentation

Antigen Presentation

N gene data. Indeed, there is a clear need for further phylogenetic studies within the dimarhabdovirus supergroup, particularly with respect to the demarcation of genera, which currently is influenced more by genome structure than host/vector relationships. There is some evidence that some of these viruses contain additional genes that are not present in lyssaviruses and vesiculoviruses. Although the functions of these additional proteins are not understood, revealing the evolution of genome complexity may be an important factor in resolving the taxonomy of this supergroup.

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Fu ZF (2005) Genetic comparison of the rhabdoviruses from animals and plants. In: Fu ZF (ed.) Current Topics in Microbiology and Immunology, Vol. 292: The World of Rhabdoviruses, p. 1. Berlin: Springer. Hogenhout SA, Redinbaugh MG, and Ammar ED (2003) Plant and animal rhabdovirus host range: A bug’s view. Trends in Microbiology 11: 264–271. Karabatsos N (1985) International Catalogue of Arboviruses Including Certain Other Viruses of Vertebrates. San Antonio, TX: American Society of Tropical Medicine and Hygiene. Kuzmin IV, Hughes GJ, and Rupprecht CE (2006) Phylogenetic relationships of seven previously unclassified viruses within the family Rhabdoviridae using partial nucleoprotein gene sequences. Journal of General Virology 87: 2323–2331.

See also: Chandipura Virus; Rabies Virus.

Relevant Websites Further Reading Bourhy H, Cowley JA, Larrous F, Holmes EC, and Walker PJ (2005) Phylogenetic relationships among rhabdoviruses inferred using the L polymerase gene. Journal of General Virology 86: 2849–2858. Calisher CH, Karabatsos N, Zeller H, et al. (1989) Antigenic relationships among rhabdoviruses from vertebrates and hematophagous arthropods. Intervirology 30: 241–257.

http://www.pasteur.fr – CRORA Report. This report involves all the data collected by Pasteur Institute and ORSTOM, since 1962, more than 6000 isolated strains of 188 arboviruses or mixed arboviruses.For each virus identified by the CRORA, all the observed hosts or vectors are given, with the number of collected strains in each country, viral properties of collected strains, and bibliographical references concerning them. http://www.ncbi.nlm.nih.gov – ICTV database on Rhabdoviridae, International Committee on Taxonomy of Viruses, NCBI.

Antigen Presentation E I Zuniga, D B McGavern, and M B A Oldstone, The Scripps Research Institute, La Jolla, CA, USA ã 2008 Elsevier Ltd. All rights reserved.

Introduction The immune system is responsible for the tremendous task of fighting a wide range of pathogens to which we are constantly exposed. This system can be broadly subdivided in innate and adaptive components. The innate immune system exists in both vertebrate and invertebrate organisms and represents the first barrier against microbial invasion. This arm of the immune system rapidly eliminates the vast majority of microorganisms that we daily encounter and is responsible for limiting early pathogen replication. The adaptive response is a more sophisticated feature of vertebrate animals involving a broad repertoire of genetically rearranged receptors that specifically recognize microbial antigens (antigen is a generic term for any substance that can be recognized by the adaptive immune system). The hallmark of the adaptive response is the generation of a potent and long-lasting defense specifically directed against the invading pathogen. B and T lymphocytes represent the effector players of adaptive immunity and carry on their surface antigenspecific receptors, B-cell receptors (BCRs) and T-cell

receptors (TCRs), respectively. There are two major classes of T lymphocytes: CD8 cytotoxic and CD4 helper T cells. Upon antigen encounter, lymphocytes undergo clonal expansion and differentiation of their unique functional features. B cells differentiate into plasma cells and secrete antibodies that specifically bind the corresponding antigen. CD8 T cells directly kill infected cells or release cytokines that interfere with viral replication, while CD4 Tcells activate other cells such as B cells and macrophages. Unlike B cells, which can directly bind native free antigen, T cells only recognize antigen-derived peptides displayed on cell surfaces in the context of major histocompatibility complex (MHC) class I (MHC-I, CD8 T cells) or class II (MHC-II, CD4 T cells) molecules. Different pathogens preferentially replicate in distinct cellular compartments. While viruses and intracellular bacteria replicate in the cytosol, microbes such as mycobacterium and protozoan parasites are intravesicular and colonize the endosomal and/or lysosomal compartments. In addition, extracellular bacteria release antigens, such as toxins, that are engulfed by antigen-presenting cells (APCs) to also reach the endosomal pathway. Antigenic

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peptides derived from these sources are exhibited on cell surfaces by MHC molecules. This process, which represents the major focus of this article, is named ‘antigen presentation’ and is a fundamental pillar of antimicrobial host defense.

Antigen-Presenting Cells For initiation of an immune response, naive T cells need to be activated or ‘primed’. For that, they require both the recognition of the specific MHC–peptide complex (signal 1) and simultaneous co-stimulation (signal 2). Although all nucleated cells express MHC-I and can potentially display MHC-I –microbial peptide complexes after infection, only a specialized group of leukocytes, named APCs, expresses both MHC-I and MHC-II as well as co-stimulatory molecules. The best-characterized co-stimulatory molecules are B7-1 and B7-2, which bind to the CD28 molecule on the T-cell surface. In addition, T cells express CD40 ligand, which interacts with CD40 on APC further enhancing costimulation and enabling T-cell response. Finally, there is another group of adhesion molecules such as lymphocyte function-associated antigen-1 (LFA-1) on APCs which

binds to ICAM-1 on T cells that seal the APC–T-cell interface. During APC–T-cell interactions, all these molecules cluster together forming a highly organized supramolecular adhesion complex (SMAC), enabling the intimate contact between the two cells that is referred to as the immunological synapse (Figure 1). APCs are composed of macrophages, B cells, and dendritic cells (DCs). They differ in location, antigen uptake, and expression of antigen-presenting and co-stimulatory molecules. Macrophages are localized in connective tissues, body cavities, and lymphoid tissues. Within the secondary lymphoid tissues, macrophages are mainly distributed in the marginal sinus and medullary cords. They specialize in phagocytosis and engulf particulate antigens through scavenger germline receptors such as the mannose receptor. On the other hand, B cells form follicular structures within secondary lymphoid organs and recirculate through the blood stream and lymph seeking their specific antigen. B cells recognize antigens specifically through a rearranged BCR. DCs are the most professional and robust of the APCs. They are widely distributed through the body at an ‘immature’ stage of development, acting as sentinels in peripheral tissues. They continuously sample the antigenic environment by both phagocytosis and macropinocytosis,

Figure 1 Interactions between virus-specific T cells and APCs. Three-color confocal microscopy was used to demonstrate immunological synapse formation between lymphocytic choriomeningitis virus-specific T cells (blue) and MHC-IIþ APCs (red) in the central nervous system. Immunological synapses were indicated by the polarization of the adhesion molecule LFA-1 (green) between the CTL and APC. Asterisks denote the engaged APC, and arrows denote the contact point between the two cells. LFA-1 is expressed on both CTLs and APCs, but note that all of the CTL-associated LFA-1 is focused toward a contact point at the CTL–APC interface. Reproduced from Lauterbach H, Zu´n˜iga EI, Truong P, Oldstone MBA, and McGavern DB (2006) Adoptive immunotherapy induces CNS dentritic cell recruitment and antigen presentation during clearance of a persistent viral infection. Journal of Experimental Medicine 203 (8): 1963–1975, with permission from Rockefeller University Press.


which is the engulfment of large volume of surrounding liquid. Within the secondary lymphoid organs, some DCs strategically localize within T-cell areas where they can optimally encounter circulating naive T lymphocytes that actively scan the DC network. APCs are able to detect components of invading pathogens which trigger their activation/maturation. Specifically, pathogen-associated molecular patterns (PAMPs), as these components are termed, range from lipoproteins to proteins to nucleic acids carried by potential invaders. These PAMPs are recognized by evolutionary conserved ‘pattern recognition receptors’ (PRRs) on APCs. Among PRRs, the Toll-like receptors (TLRs) have emerged as critical players in determining APC imprinting on the ensuing immune response. TLR triggering has pleiotropic effects on APCs, promoting survival, chemokine secretion, expression of chemokine receptors, migration, cytoskeletal and shape changes, and/or endocytic remodeling. After interacting with these pathogen signatures, the microbial antigens are processed and presented as peptides associated with MHC molecules and activated APCs upregulate both antigen-presenting and co-stimulatory molecules initiating a ‘maturation’ process. As part of this process, APCs in peripheral tissues change their chemokine receptors and initiate migration to secondary lymphoid organs where the adaptive immune response is initiated. The strategic migration and location of DCs into T-cell areas of the secondary lymphoid organs coupled to their superior antigen-presenting capacity make them the most powerful APCs. Indeed, DCs are about 1000 more efficient than B cells or macrophages in stimulating naive T cell. This has been shown by several experiments in which elimination of DCs prevented the initiation of antigen-specific T-cell responses. Interestingly, DCs are a heterogeneous cell population composed of different subtypes which present unique and overlapping functions. As many as six different subsets of DCs occupy the lymph nodes. Three major defined populations of DCs have been recognized in mouse spleen and humans: CD8þ conventional DCs (cDCs), CD11bþ cDCs, and plasmacytoid DCs. These subpopulations differ not only in surface phenotype but also in functional potential and localization. In this regard, cDCs are potent activators of naive T lymphocytes, as CD8þ DCs are believed to be specialized in cross-presentation of exogenous antigens. A recent study suggests that CD8þ and CD11bþ cDCs differ from each other in their intrinsic antigen-processing capacity being specialized in MHC-I and MHC-II antigen presentation, respectively. In contrast, plasmacytoid DCs are poorer activators of T cells, even after stimulation in vitro. They likely play a more protagonist role during innate immunity by secreting specific cytokines and chemokines, such as type 1 interferons (IFNs an important antiviral mediator), and activation of a broad range of effector cells, such as natural killer (NK)

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cells. Thus, the heterogeneity inherent to DC populations significantly influences the varieties of immune responses to different pathogens, which are subsequently amplified by cross talk between the various subsets.

Major Pathways of Antigen Presentation Although in all healthy individuals MHC molecules play the same crucial role of antigen presentation, these molecules are highly polymorphic. There are hundreds of different alleles encoding the MHC molecules in the whole population and each individual exhibits only few of them. The major allelic variants of MHC are found in key amino acids forming the peptide-binding cleft. Thus, although a given MHC-I molecule can bind several different peptides; particular amino acids are preferred in certain positions of the peptide resulting in differential peptide sets for particular MHC variants. Importantly, T-cell specificity involves corecognition of a particular antigenic peptide together with a particular MHC variant, a feature known as T-cell MHC restriction. Like other polypeptide chains of proteins destined to arrive at the cell surface, MHC molecules are translocated to the lumen of the endoplasmic reticulum (ER) during synthesis. In this compartment, the subunits of MHC molecules are assembled together and the peptidebinding groove or cleft is formed. However, MHC molecules are unstable in the absence of bound peptide. In the following sections, we will consider how MHC molecules are folded and generated peptides are bound to MHC-I or MHC-II molecules. After binding, MHC–peptide complexes travel to the cell surface where they are recognized by antigen-specific T cells. Although not discussed in this article, it should be noted that other MHC-like molecules (i.e., CD1) also display peptide and lipid antigens contributing to antigen presentation, especially during mycobacteria infections. MHC-I Antigen Presentation MHC-I molecules are expressed in most if not all nucleated cells. MHC-I molecules are heterodimers of a highly polymorphic a-chain (43 kDa) that binds noncovalently to b2-microglobulin (12 kDa), which is nonpolymorphic. The a-chain contains three domains. The a3 domain crosses the plasma membrane while the a1 and a2 domains constitute the antigen-binding site. The peptides that bind the MHC-I molecule are usually 8–10 amino acids long and contain key amino acids at two or three positions that anchor the peptide to the MHC pocket and are called anchor residues. As mentioned above, the peptide-binding site of MHC molecules is formed in the ER. However, all proteins,

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including viral-derived antigens, are synthesized in the cytosol. Numerous studies in the last years outlined the molecular events connecting the antigen generation in the cytosol with the peptide binding to the MHC-I molecule in the ER. A highly conserved multicatalytic proteasome complex is in part responsible for cytosolic protein degradation into small peptides. The proteasome contains 28 subunits forming a cylindrical structure composed of four rings, each of seven units. Under normal conditions, the proteasome complex exists in a constitutive form. During viral infections, IFNs released by cells of the innate immune system induce the synthesis of three different proteasome subunits, which replace their constitutive counterparts to form the immunoproteasome. This inflammatory form of the proteasome favors the production of peptides with a higher chance of MHC binding. Moreover, IFNs can also enhance the rate of proteasome peptide degradation increasing the availability of peptides and reducing their excessive cleavage. It is important to highlight that other cytosolic proteases also contribute to MHC-I peptide generation and further cleavage can occur within the ER before MHC binding. The source of peptides for MHC-I complexes still holds its secrets. Proteasome substrates may encompass de novo synthesized, mature stable, and/or defective proteins. It is believed that defective ribosomal products (DRiPs), which are proteins targeted for degradation due to premature termination or misfolding, constitute an important source of MHC-I peptides. Peptides available in the cytosol are transported into the lumen of the ER by ATP-dependent transportersassociated antigen-processing 1 and 2 (TAP-1 and TAP-2) proteins. TAP proteins are localized in the ER membrane forming a channel through which peptides can pass. Within the ER, the newly synthesized MHC-I a-chain binds to a chaperone molecule called calnexin, which retains the incomplete MHC molecule in the ER. After binding to the b2-microglobulin, calnexin is displaced and the emerging MHC molecule binds to a loading complex composed by the chaperone protein calreticulin, TAP, the thiol oxidoreductase Erp57, and tapasin, which bridges MHC-I molecule and TAP. After peptide binding, the fully folded MHC-I molecule and its bound peptide are released from the complex and transported to the cell membrane. Importantly, under steady-state conditions, the MHC-I molecules in ER are in excess with respect to peptides allowing the rapid appearance of microbial peptides onto the cell surface during infection. However, since MHC-I molecules are unstable without bound peptide, they also present self antigens under normal conditions. Because of the absence of microbial signatures, antigen presentation of self peptides by inactivated/immature APCs leads to T-cell tolerance rather than activation. This is one of the important ways anti-self or autoimmune responses are controlled.

For several years, intracellular peptides were believed to be the only source of MHC-I molecules. However, it is now clear that exogenous proteins also have access to the cytosolic compartment and bind MHC-I in the ER. This mechanism is known as cross-presentation and is believed to be particularly important for enabling MHC-I presentation by cells that are not directly infected by the virus but instead are engulfing viral particles by phagocytosis or micropinocytosis. The molecular mechanism by which MHC-I molecules access exogenous peptides is of considerable interest. Different nonexclusive possibilities have been proposed, including sampling of phagosomegenerated peptides by MHC-I molecules, transference of ER molecules (including MHC-I and its loading complex) into phagosomes, re-entry of plasma membrane MHC-I into recycling endosomes with the subsequent peptide exchange, and finally the acquisition of peptides from other cells through GAP junctions. MHC-II Antigen Presentation The MHC-II molecule is composed by two noncovalently bound transmembrane glycoprotein chains, a (34 kDa) and b (29 kDa). Each chain has two domains and altogether form a four-domain heterodimer similar to the MHC-I molecule. a1 and b1 domains form the peptidebinding cleft resulting in a groove which is open at the ends, which is different from the MHC-I groove in which the extremes of the peptide are buried at the ends. Peptides that bind to MHC-II are larger than those that bind to MHC-I molecules, being 13–17 amino acids long or even much longer. Since MHC-I is a surface protein, its biosynthesis is initiated in the ER. To prevent newly synthesized MHC-II molecules from binding cytosolic peptides that are abundant in the ER, its peptide-binding cleft is covered by a protein known as MHC-II-associated invariant chain (Ii). Through a targeting sequence in its cytoplasmic domain, the Ii also directs MHC-II molecules to acidified late endosomal compartments, where Ii is cleaved leaving only the Ii pseudopeptide (CLIP) covering the peptidebinding groove. MHC-II molecules bound to CLIP cannot bind other peptides, indicating that CLIP must be dissociated or displaced by the antigenic peptide. Proteins that enter the cell through endocytosis or are derived from pathogens that replicate in vesicles are degraded by endosome proteases. These proteases become activated as the endosome pH progressively decreases. The final set of peptides available in the endosomal compartment is a result of antigen processing by several acid proteases that exist in endosomes and lysosomes. For instance, the cathepsin S is a very predominant acid protease and mice deficient in this enzyme have a compromised antigen-processing capacity. Vesicles carrying


peptides fuse with the vesicles carrying MHC-II molecules, achieving CLIP dissociation and the incorporation of antigenic peptides to MHC-II molecules. An MHC-IIlike molecule that is predominant in the endosome facilitates this process. This molecule contributes to ‘peptide editing’, removing weakly bound peptides and assuring that the emerging MHC-II–peptide complexes are stable enough to be scanned by CD4 T cells. MHC-II molecules seem to be in excess and are rapidly degraded unless microbial peptides become available to fill the groove. This excess is important to permit MHC-II availability upon infection. However, during infection, APCs are exposed to both self and microbial peptides. How APCs discriminate between self and nonself represents a fundamental question in immune biology. Recent evidence suggests that the efficiency of presenting antigens from phagocytosed cargo is dependent on the presence of a TLR ligand within the cargo. Thus, TLR signaling would mark a particular phagosome for an inducible mode of maturation dictating the fate of the cargo-derived peptides and favoring their presentation by MHC-II molecules in a phagosome autonomous fashion. Because they travel through the endocytic pathway, which can be considered a topological continuation of the extracellular space, MHC-II molecules were believed to be specialized in the presentation of exogenous antigens. However, the analysis of MHC-II peptidome revealed many peptides of cytosolic or even nuclear origin. Autophagy or ‘self-eating’ explains MHC-II access to cytosolic peptides. This highly conserved pathway could be accomplished by several mechanisms including microphagy (when lysosomal invagination sequesters cytosolic componets), macrophagy (when a double membrane structure that encloses and isolates cytoplasmic components and eventually fuses with lysosome), and chaperone-mediated autophagy (when cytosolic proteases generate peptides that are transported into lysosomes).

Viral Subversion of Antigen Presentation Considering the crucial role of antigen presentation for host defense, it is not surprising that many viruses have evolved maneuvers to evade or divert this process. Particularly, the essential role played by APCs in host defense to pathogens makes them an ideal target for viruses to suppress the immune response, thereby maximizing their chances of survival, replication, and transmission. Indeed, many viruses that cause major health problems are able to interfere with the ability of APCs to prime an efficient and effective antiviral immune response. In fact, many viruses have developed different mechanisms to subvert each stage of APC biology. Furthermore, with the greater understanding of antigen presentation pathways comes the discovery of

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novel viral immune-evasion strategies. In this section, we illustrate selective viral strategies to subvert antigen presentation by describing particular cases. An interesting example of virus blockade of antigen presentation from very initial steps is the ability of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV), to dramatically block DC development from early hematopoietic progenitors. Fms-like tyrosine kinase 3 ligand (Flt3L) is known to induce the expansion of undifferentiated progenitors into DCs within the spleen and bone marrow (approximate 20-fold increase), both in mice and humans. In contrast, LCMV-clone (CL)-13 that suppresses the immune response and causes a persistent infection in mice is associated with DC early progenitors that become refractory to the stimulatory effects of Flt3L. TLRs function in APCs as an early sensor against pathogens; therefore, impairment in TLR signaling confers another selective advantage to certain infectious agents. As an example, vaccinia virus (VV) blocks TLR signaling and the subsequent maturation of APCs. Specifically, two proteins of VV suppress intracellular signaling of interleukin-1 (a potent pro-inflammatory host factor) and TLR-4. Migration of DCs is a crucial step in initiating the adaptive immune response. Examples of viruses that have developed mechanisms to prevent migration of infected DCs to lymphoid organs are herpes simplex virus (HSV) 1 and human cytomegalovirus (HCMV). In both cases, there is an inhibition of complete DC maturation and subsequent expression of chemokine homing receptors. In addition, HCMV inhibits DC migration one step further by preventing APCs from arriving at a site of infection by producing homologs of chemokines that interfere with host pro-inflammatory chemokine gradients. Another effective immune-evasion strategy used by viruses to disrupt APCs is the prevention of or interference with antigen-specific T-cell activation. The ability to disrupt MHC–peptide binding has evolved in many different virus species including adenovirus and human immunodeficiency virus (HIV). Herpesviruses have also evolved to block host cell antigen presentation. Some mechanisms utilized by herpesviruses to disrupt the antigen-presentation pathway include blocking peptide transport to the ER through interference with TAP proteins (HSV ICP47, HCMV US6), transport of particular MHC-I heavy chains from the ER to the cytosol where they are destroyed (HCMV US11, HCMV US2), retention of specific MHC-I heavy chains in the ER (HCMV US3, murine CMV–MCMV-m152), and disruption of T-cell recognition of MHC-I on the cell surface (MCMV m04). That viruses have independently evolved numerous mechanisms to disrupt MHC–peptide presentation indicates the effectiveness and importance of this strategy to the survival of viruses with different infectious life cycles.

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Maturation of APCs results in upregulation of costimulatory molecules and expression of cytokines that enable them to stimulate naive T cells. Viruses that can impair T-cell stimulation by preventing the upregulation of co-stimulatory molecules include Ebola virus, Lassa fever virus (LFV), HSV-1, and HIV. Additionally, a number of viruses (hepatitis C virus (HCV), HIV, measles virus (MV), and dengue virus (DV)) are also able to inhibit interleukin (IL)-12 production, which is often required for effective T-cell response. HCV does this through the action of its core and nonstructural protein 3 (NS3), which induces production of IL-10. DV, on the other hand, is able to inhibit IL-12 production through an IL-10 independent mechanism. In addition, compelling evidence showed that in vivo persistent infection of mice with LCMV, as well as persistent HCV infection in humans, induces IL-10 production by APCs resulting in the blunting of the CD8 T-cell response and chronic viral infection. Remarkably, antibodies blocking IL-10/IL-10R interactions correct T-cell exhaustion by restoring T-cell function, which results in purging of virus from mice persistently infected with LCMV. Finally, a novel immunosuppressive molecule, programmed cell death-1 (PD-1), is upregulated in nonfunctional CD8 T cells during chronic infections (LCMV, HIV, HCV). Interaction of PD-1 with PD-ligands on APCs (or parenchymal cells) inhibits lymphocyte activation. As for IL-10 blockade, antibodies interfering with PD-1/PDL interactions also promote viral clearance from a persistently infected host. The fact that not all viruses are able to block APC maturation does not necessarily represent a failure of the pathogen or a success for the host. A good example of this is observed following MV infection that exploits the ability of DCs to mature and migrate to lymphoid organs in response to infection. MV benefits greatly by having infected DCs home to lymphoid compartments where the infected cells are able to actively suppress T-cell proliferation (mediated through T-cell contact with surface viral glycoproteins) and also facilitate virus spread to more lymphoid cells. Therefore, the full understanding of the virus–host relationship requires not only studying the active mechanisms that viruses use to disable the immune system, but by also asking how a virus benefits by not altering a particular immune function.

Concluding Remarks Co-evolution of certain hosts and pathogens for millions of years has resulted in a fine-tuned equilibrium that enables survival of both. Antigen presentation is one of the critical elements in this balance. While antigen presentation is an essential process for long-term effective

host defense, targeting APCs represents a common maneuver of many viruses to avoid host surveillance and establish a chronic or persistent infection. A major challenge in biomedical research is to thwart microbial APC subversion to promote eradication of the pathogen. A better understanding of the mechanisms used by APC to display microbial antigens as well as the virus strategies to subvert APC functions during immune responses will provide new tools for designing novel vaccination approaches and immunotherapeutic treatments for human infectious diseases.

Acknowledgments This is publication no. 18909 from Molecular and Neuroscience Integrative Department, The Scripps Research Institute (TSRI). This work was supported by NIH grants AI 45927, AI 05540, and AI 09484. E. I. Zuniga is a Pew Latin American Fellow. See also: Cytokines and Chemokines; Immune Response to viruses: Antibody-Mediated Immunity; Immune Response to viruses: Cell-Mediated Immunity; Immunopathology; Persistent and Latent Viral Infection; Vaccine Strategies.

Further Reading Bevan MJ (2006) Cross-priming. Nature Immunology 7: 363–365. Blander JM and Medzhitov R (2006) On regulation of phagosome maturation and antigen presentation. Nature Immunology 7: 1029–1035. Dudziak D, Kamphorst AO, Heidkamp GF, et al. (2007) Differential antigen processing by dendritic cell subsets in vivo. Science 315: 107–111. Itano AA and Jenkins MK (2003) Antigen presentation to naive CD4 T cells in the lymph node. Nature Immunology 4: 733–739. Janeway CA, Travers P, Walport M, and Shlomchik MJ (2005) Immunobiology: The Immune System in Health and Disease. New York: Garland Science Publishing. Lauterbach H, Zu´n˜iga EI, Truong P, Oldstone MB, and McGavern DB (2006) Adoptive immunotherapy induces CNS dendritic cell recruitment and antigen presentation during clearance of a persistent viral infection. Journal of Experimental Medicine 203(8): 1963–1975. Menendez-Benito V and Neefjes J (2007) Autophagy in MHC class II presentation: Sampling from within. Immunity 26: 1–3. Norbury CC and Tewalt EF (2006) Upstream toward the ‘DRiP’-ing source of the MHC class I pathway. Immunity 24: 503–506. Oldstone MB (2007) A suspenseful game of ‘hide and seek’ between virus and host. Nature Immunology 8: 325–327. Reis e Sousa C (2006) Dendritic cells in a mature age. Nature Reviews Immunology 6: 476–483. Shen L and Rock KL (2006) Priming of T cells by exogenous antigen cross-presented on MHC class I molecules. Current Opinion in Immunology 18: 85–91. Shortman K and Liu YJ (2002) Mouse and human dendritic cell subtypes. Nature Reviews Immunology 2: 151–161. Strawbridge AB and Blum JS (2007) Autophagy in MHC class II antigen processing. Current Opinion in Immunology 19: 87–92. Yewdell JW and Nicchitta CV (2006) The DRiP hypothesis decennial: Support, controversy, refinement and extension. Trends in Immunology 27: 368–373.