Carrier-mediated uptake and presentation of a major histocompatibility complex class I-restricted peptide

Antigenic peptides derived from endogenous or viral proteins can associate with class I or class II major histocompatibility complex (MHC) molecules, while exogenous antigens are endocytosed, processed intracellularly and presented on MHC class II molecules. Here we describe a method that allows the presentation of an MHC class I-restricted antigenic peptide on MHC class I molecules, although it was taken up from the outside. The HLA-A2-restricted influenza virus matrix protein-derived peptide (flu, 57–68) was used either in soluble form or coupled via an S-S bridge to transferrin (Tf-flu). Target cells were incubated with flu or Tf-flu and the effective antigen presentation was detected in a cytotoxicity assay using flu peptide-specific, HLA-A2-restricted CD8⁺ cytotoxic T lymphocytes. Sensitization of target cells with Tf-flu required 5 to 10 times higher molar concentrations of peptide compared to sensitization with soluble free peptide. The Tf-flu construct was taken up by the cells via the Tf receptor (CD71) as the binding of Tf-flu was blocked by an excess of Tf. In contrast to the flu peptide, cytotoxicity elicited by Tf-flu was blocked by brefeldin A but not by chloroquine nor inhibitors of intracellular reducing steps, like 1-buthionine-(s, r)-sulfoximine or n-ethylmaleimide. Presentation of the flu peptide derived from Tf-flu construct is not hindered in the mutant T2 cell line, which lacks genes coding for transporter proteins for antigenic peptides (TAP1/TAP2) and proteasomes subunits, suggesting that the processing pathway described in this report may involve TAP-independent steps.     

Peptide transporter-independent,stress protein-mediated endosomal processing of endogenous protein antigens for major histocompatibility complex class I presentation

The peptide transporter-defective cell line RMA-S expressing the wild-type simian virus 40 large T antigen (wtT-Ag) from a transfected gene did not present two well-defined, H-2 class I (D^b)-restricted epitopes of T-Ag to cytotoxic T lymphocytes (CTL). Hence, “endogenous” processing and presentation of the wtT-Ag depended on a functional peptide transporter heterodimer. In contrast, both T-Ag epitopes were efficiently presented to CTL by transfected RMA-S cells expressing a truncated, cytoplasmic T-Ag variant (cT-Ag) or a karyophilic, amino-terminal 272-amino acid T-Ag fragment. Transporter-independent “endogenous” processing of mutant T-Ag molecules correlated with their association with the constitutively expressed heat shock protein 73 (hsp 73). Class I-restricted presentation of both epitopes processed from these hsp73-associated protein antigens was sensitive to NH₄Cl and chloroquine. These data indicate that selected intracellular proteins access an alternative, hsp73-mediated pathway for class I-restricted presentation that operates independent of peptide transporters in an endosomal compartment.     

Major histocompatibility complex class I binding predictions as a tool in epitope discovery

Over the last decade, in silico models of the major histocompatibility complex (MHC) class I pathway have developed significantly. Before, peptide binding could only be reliably modelled for a few major human or mouse histocompatibility molecules; now, high‐accuracy predictions are available for any human leucocyte antigen (HLA) ‐A or ‐B molecule with known protein sequence. Furthermore, peptide binding to MHC molecules from several non‐human primates, mouse strains and other mammals can now be predicted. In this review, a number of different prediction methods are briefly explained, highlighting the most useful and historically important. Selected case stories, where these ‘reverse immunology’ systems have been used in actual epitope discovery, are briefly reviewed. We conclude that this new generation of epitope discovery systems has become a highly efficient tool for epitope discovery, and recommend that the less accurate prediction systems of the past be abandoned, as these are obsolete.     

Major histocompatibility complex class I presentation of ovalbumin peptide 257–264 from exogenous sources: protein context influences the degree of TAP-independent presentation

Peritoneal macrophages from C57BL/6 mice process antigens from bacteria or coated on polystyrene beads for presentation by major histocompatibility complex (MHC) class I molecules. To investigate this antigen processing pathway, peritoneal macrophages from homozygous TAP1^−/− mice, which lack the transporter associated with antigen processing (TAP) and are defective in presenting endogenous antigens on MHC class I, were used. TAP1^−/− or C57BL/6 macrophages were co-incubated with either bacteria or polystyrene beads containing the 257–264 epitope from ovalbumin [OVA(257–264)], which binds the mouse class I molecule K^b. The source of the OVA(257–264) epitope was either the Crl-OVA(257–264) (Crl-OVA) fusion protein, the maltose binding protein (MBP)-Crl-OVA fusion protein, native OVA or bacterial recombinant OVA (rOVA); Crl-OVA, MBP-Crl-OVA and rOVA were each expressed in bacteria, and Crl-OVA and MBP-Crl-OVA purified from bacterial lysates and native egg OVA were coated onto polystyrene beads. The data reveal that peritoneal macrophages from C57BL/6 and TAP1^−/− mice can process bacteria expressing Crl-OVA, MBP-Crl-OVA and rOVA as well as beads coated with native OVA, purified Crl-OVA, and purified MBP-Crl-OVA and present OVA(257–264) for recognition by OVA(257–264)/K^b-specific T hybridoma cells, albeit with different relative processing efficiencies. The processing efficiency of TAP1^−/− macrophages co-incubated with bacteria or beads containing Crl-OVA or MBP-Crl-OVA was reduced approximately three to five times compared to C57BL/6 macrophages, but OVA(257–264) was presented 100 times less efficiently when the source of OVA(257–264) was full-length OVA. Chloroquine inhibition studies showed a differential requirement for acidic compartments in C57BL/6 versus TAP1^−/− macrophages, which also depended upon the source of the OVA (257–264) epitope (Crl-OVA versus full-length OVA). These data suggest that TAP1^−/− and C57BL/6 macrophages may process Crl-OVA and full-length OVA in different cellular compartments and that the protein context of the OVA(257–264) epitope influences the extent of TAP-independent processing for MHC class I presentation.     

Differential contribution of TAP and tapasin to HLA class I antigen expression

Expression of class I human leucocyte antigens (HLA) on the surface of malignant cells is critical for their recognition and destruction by cytotoxic T lymphocytes. Surface expression requires assembly and folding of HLA class I molecules in the endoplasmic reticulum with the assistance of proteins such as Transporter associated with Antigen Processing (TAP) and tapasin. Interferon-gamma induces both TAP and tapasin so dissection of which protein contributes more to HLA class I expression has not been possible previously. In this study, we take advantage of a human melanoma cell line in which TAP can be induced, but tapasin cannot. Interferon-gamma increases TAP protein levels dramatically but HLA class I expression at the cell surface does not increase substantially, indicating that a large increase in peptide supply is not sufficient to increase HLA class I expression. On the other hand, transfection of either allelic form of tapasin (R240 or T240) enhances HLA-B*5001 and HLA-B*5701 antigen expression considerably with only a modest increase in TAP. Together, these data indicate that in the presence of minimal TAP activity, tapasin can promote substantial HLA class I expression at the cell surface.     

Major histocompatibility class II molecules prevent destructive processing of exogenous peptides at the cell surface of macrophages for presentation to CD4 T cells

We studied factors affecting major histocompatibility complex class II (MHC-II)-restricted presentation of exogenous peptides at the surface of macrophages. We have previously shown that peptide presentation is modulated by surface-associated proteolytic enzymes, and in this report the role of the binding of MHC-II molecules in preventing proteolysis of exogenous synthetic peptides was addressed. Two peptides containing CD4 T-cell epitopes were incubated with fixed macrophages expressing binding and non-binding MHC-II, and supernatants were analysed by high-performance liquid chromatography and mass spectrometry to monitor peptide degradation. The proportion of full-length peptides that were degraded and the number of peptide fragments increased when non-binding macrophages were used, leading to reduction in peptide presentation. When MHC-II molecules expressed on the surface of fixed macrophages were blocked with monoclonal antibody and incubated with peptides and the supernatants were transferred to fixed macrophages, a significant reduction in peptide presentation was observed. Peptide presentation was up-regulated at pH 5.5 compared to neutral pH, and the latter was found to be the pH optimum of the proteolytic activity of the surface enzymes involved in the degradation of exogenous peptides and proteins. The data suggest that MHC-II alleles that bind peptides protect them from degradation at the antigen-presenting cell surface for presentation to CD4 T cells and we argue that this mechanism could be particularly pronounced at sites of inflammation.     

Hepatitis B virus small surface antigen particles are processed in a novel endosomal pathway for major histocompatibility complex class I-restricted epitope presentation

We investigated the major histocompatibility complex (MHC) class I-restricted presentation of an epitope of the hepatitis B virus small surface (S) antigen particle to cloned murine cytotoxic T lymphocytes (CTL). Efficient L^d-restricted presentation of the S_28–39 epitope to CTL is observed in cells of different tissue origin pulsed in vitro, either with the antigenic S_28–39 12-mer S-peptide, or with particulate S-antigen. The kinetics of epitope presentation differ in S-peptide-pulsed and in S-particle-pulsed cells: while a 15-min pulse with the antigenic peptide sensitizes targets for class I-restricted CTL lysis, presentation of S-particles requires 30–60 min to sensitize cells for CTL lysis. Uptake of antigenic material and active metabolism of the presenting cell are required for processing of S-particles, but not for sensitizing targets with S-peptides. Intracellular processing and presentation of S-particles is blocked in cells treated with chloroquine, NH₄Cl, primaquine, or leupeptin, but not by treatment with cycloheximide or brefeldin A. This processing pathway operates efficiently in peptide-transporter-deficient, L^d-transfected T2 cells, revealing a novel endosomal/lysosomal processing pathway for class I-restricted presentation of peptides derived from exogenous S-particles.     

Major histocompatibility complex class I-restricted alloreactive CD4+ T cells

Although it is well established that CD4+ T cells generally recognize major histocompatibility complex (MHC) class II molecules, MHC class I-reactive CD4+ T cells have occasionally been reported. Here we describe the isolation and characterization of six MHC class I-reactive CD4+ T-cell lines, obtained by co-culture of CD4+ peripheral blood T cells with the MHC class II-negative, transporter associated with antigen processing (TAP)-negative cell line, T2, transfected with human leucocyte antigen (HLA)-B27. Responses were inhibited by the MHC class I-specific monoclonal antibody (mAb), W6/32, demonstrating the direct recognition of MHC class I molecules. In four cases, the restriction element was positively identified as HLA-A2, as responses by these clones were completely inhibited by MA2.1, an HLA-A2-specific mAb. Interestingly, three of the CD4+ T-cell lines only responded to cells expressing HLA-B27, irrespective of their restricting allele, implicating HLA-B27 as a possible source of peptides presented by the stimulatory MHC class I alleles. In addition, these CD4+ MHC class I alloreactive T-cell lines could recognize TAP-deficient cells and therefore may have particular clinical relevance to situations where the expression of TAP molecules is decreased, such as viral infection and transformation of cells.     

Cross-priming utilizes antigen to the direct presentation pathway

CD8+ T cells play a crucial role in protective immunity to viruses and tumours. Antiviral CD8+ T cells are initially activated by professional antigen presenting cells (pAPCs) that are directly infected by viruses (direct-priming) or following uptake of exogenous antigen transferred from virus-infected or tumour cells (cross-priming). In order to efficiently target each of these antigen-processing pathways during vaccine design, it is necessary to delineate the properties of the natural substrates for either of these antigen-processing pathways. In this study, we utilized a novel T-cell receptor (TCR) transgenic mouse to examine the requirement for both antigen synthesis and synthesis of other cellular factors during direct or cross-priming. We found that direct presentation required ongoing synthesis of antigen, but that cross-priming favoured long-lived antigens and did not require ongoing antigen production. Even after prolonged blockade of protein synthesis in the donor cell, cross-priming was unaffected. In contrast, direct-presentation was almost undetectable in the absence of antigen neosynthesis and required ongoing protein synthesis. This suggests that the direct- and cross-priming pathways may utilize differing pools of antigen, an observation that has far-reaching implications for the rational design of vaccines aimed at the generation of protective CD8+ T cells.     

Lipopeptides: a novel antigen repertoire presented by major histocompatibility complex class I molecules

Post‐translationally modified peptides, such as those containing either phosphorylated or O‐glycosylated serine/threonine residues, may be presented to cytotoxic T lymphocytes (CTLs) by MHC class I molecules. Most of these modified peptides are captured in the MHC class I groove in a similar manner to that for unmodified peptides. N‐Myristoylated 5‐mer lipopeptides have recently been identified as a novel chemical class of MHC class I‐presented antigens. The rhesus classical MHC class I allele, Mamu‐B*098, was found to be capable of binding N‐myristoylated lipopeptides and presenting them to CTLs. A high‐resolution X‐ray crystallographic analysis of the Mamu‐B*098:lipopeptide complex revealed that the myristic group as well as conserved C‐terminal serine residue of the lipopeptide ligand functioned as anchors, whereas the short stretch of three amino acid residues located in the middle of the lipopeptides was only exposed externally with the potential to interact directly with specific T‐cell receptors. Therefore, the modes of lipopeptide–ligand interactions with MHC class I and with T‐cell receptors are novel and fundamentally distinct from that for MHC class I‐presented peptides. Another lipopeptide‐presenting MHC class I allele has now been identified, leading us to the prediction that MHC class I molecules may be separated on a functional basis into two groups: one presenting long peptides and the other presenting short lipopeptides. Since the N‐myristoylation of viral proteins is often linked to pathogenesis, CTLs capable of sensing N‐myristoylation may serve to control pathogenic viruses, raising the possibility for the development of a new type of lipopeptide vaccine.     

Presentation of Listeria monocytogenes antigens by major histocompatibility complex class I molecules to CD8 cytotoxic T lymphocytes independent of listeriolysin secretion and virulence

Virulence and intracellular persistence of Listeria monocytogenes markedly depend on secretion of listeriolysin (Hly), which promotes invasion of the pathogen from the endosome into the cytosol. Recent studies have provided compelling evidence that Hly also facilitates recognition of listerial antigens, in association with major histocompatibility complex (MHC) class I molecules, by CD8 T lymphocytes. Data presented here confirm that the Hly-deficient strains, the prfA^? mutant L. monocytogenes SLCC53 and the transposon mutants L. monocytogenes M3 and M20 are avirulent for mice, and unable to replicate inside bone marrow-derived macrophages (BMMΦ). Furthermore, BMMΦ infected with M3, M20 or SLCC53 were as efficiently lysed as BMMΦ infected with the Hly-positive wild-type strain EGD by MHC class I-dependent CD8 cytotoxic T lymphocytes. Using the highly sensitive polymerase chain reaction method, hly mRNA was detectable in BMMΦ infected with L. monocytogenes EGD or SLCC53, but totally absent in M3-infected BMMΦ. In the case of M20, an excision of the transposon occurred, but the excision was not precise and the hly gene was approximately 400 base pairs shorter. These findings argue against a unique role for Hly in MHC class I presentation of listerial antigens, although Hly appears central to virulence and intracellular replication. Thus, virulence of L. monocytogenes is dissociable from MHC class I presentation of listerial antigens.     

Major histocompatibility complex class I-restricted presentation of influenza virus nucleoprotein peptide by B lymphoma cells harboring an antibody gene antigenized with the virus peptide

We analyzed the capacity of B cells to process and present a peptide from the variable region of an endogenous immunoglobulin heavy (H) chain to a major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocyte (CTL) clone. The H-chain gene was engineered to express 14-amino acid peptide from the sequence of the influenza virus nucleoprotein (NP) antigen in the third complementarity-determining region (CDR3). This NP peptide is presented in association with the D^b allele in H?2^b mice. We demonstrate that B lymphoma cells (H-2^b) harboring the antigenized H-chain gene process and present the NP peptide in association with the D^b molecule and are lysed by a CTL clone specific for that peptide in an MHC-restricted way. In contrast, the soluble antigenized antibody failed to mediate lysis of H?2^b target cells. The endogenously processed immunoglobulin CDR3 peptide could be eluted from surface D^b molecules in transfected cells. This study formally demonstrates that peptides from the hypervariable loops of endogenous immunoglobulin are processed through the endogenous degradative pathway and are presented to CD8⁺ T cells in the context of MHC class I molecules. The implication of these findings for processing and presentation of endogenous immunoglobulin peptides in B cells and network regulation by idiopeptides is discussed.     

Epitope selection in major histocompatibility complex class I-mediated pathway is affected by the intracellular localization of an antigen

We analyzed the mode of antigen presentation of an endogenous antigen localized in the cytoplasm or in the mitochondria. Pseudomonas aeruginosa PAO leucine-, isoleucine-, valine-binding protein (LIVAT-BP) encoded by the braC gene was used as a model antigen. Using mouse BALB/3T3 cells, we established two LIVAT-BP transfectants by transfection of a plasmid harboring the intact braC or braC gene fused with the mitochondrial transport signal derived from the yeast COXIV gene. One of the resulting transfectants, BC-15, expressed LIVAT-BP in the cytoplasm, while YZ-710 cells expressed LIVAT-BP in the mitochondria. The splenic effector cells derived from BALB/c mice primed with BC-15 cells exhibited cytotoxic T lymphocyte (CTL) activity against BC-15 cells, but not against YZ-710 cells, whereas splenic effector cells primed with YZ-710 cells exhibited CTL activity against YZ-710 cells, but not against BC-15 cells. Neither group of splenic effector cells showed CTL activity against parental BALB/3T3 cells. These CTL belonged to the CD8⁺ αβ T cell subset. Furthermore, we observed that the CTL activity against BC-15 cells or YZ-710 cells was blocked with anti-H2-K^d mAb, but not with anti-H2-D^d or H2-L^d mAb. The CTL against BC-15 or YZ-710 cells could kill parental BALB/3T3 cells in the presence of peptides produced by alkali lysis of the LIVAT-BP. suggesting that these CTL indeed recognized the peptide(s) derived from LIVAT-BP. We determined that the epitope for the CTL against BC-15 cells was QYGEGIATEV, corresponding to residues 162–171, and that the epitope recognized by the CTL against YZ-710 cells was GYKLIFRTI, corresponding to residues 123–131 of LIVAT-BP, respectively. Thus, we show here that epitope selection for MHC class I expression is affected by the intracellular localization of the antigenic protein.     

Trypanosoma cruzi-infected macrophages are defective in major histocompatibility complex class II antigen presentation

Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas' disease, interferes with the host immune response to establish a persistent infection. In this report, we demonstrate that macrophages infected with T. cruzi are unable to effectively present antigens to CD4 T cells. The interference is due to defective antigen-presenting cell (APC) function, as antigen-independent stimulation of the T cell in the presence of infected macrophages is not affected. The defect is distal to antigen processing and is not due to decreased major histocompatibility complex (MHC) class II expression, decreased viability, defective peptide loading in the infected macrophages, nor absence of CD28 co-stimulation. There was a role for gp39:CD40 co-stimulation during antigen presentation to the T cells we studied, but the expression of CD40 on T. cruzi-infected macrophages was not decreased. Antigen-specific adhesion between macrophages and T cells was reduced by infection. Equivalent levels of the adhesion molecules lymphocyte function-associated antigen-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 or very late antigen-4 are found on infected and uninfected APC, suggesting that reduced expression of these adhesion molecules was not responsible for the defect in antigen-specific adhesion. The defective T cell:macrophage adhesion may be due to the reduced expression of other adhesion molecules or other changes in the cell induced by infection. Interfering with MHC class II antigen presentation in infected macrophages may help T. cruzi to blunt the immune response by the host.     

Mechanisms of major histocompatibility complex class II-restricted processing and presentation of the V antigen of Yersinia pestis

We mapped mouse CD4 T-cell epitopes located in three structurally distinct regions of the V antigen of Yersinia pestis. T-cell hybridomas specific for epitopes from each region were generated to study the mechanisms of processing and presentation of V antigen by bone-marrow-derived macrophages. All three epitopes required uptake and/or processing from V antigen as well as presentation to T cells by newly synthesized major histocompatibility complex (MHC) class II molecules over a time period of 3-4 hr. Sensitivity to inhibitors showed a dependence on low pH and cysteine, serine and metalloproteinase, but not aspartic proteinase, activity. The data indicate that immunodominant epitopes from all three structural regions of V antigen were presented preferentially by the classical MHC class II-restricted presentation pathway. The requirement for processing by the co-ordinated activity of several enzyme families is consistent with the buried location of the epitopes in each region of V antigen. Understanding the structure-function relationship of multiple immunodominant epitopes of candidate subunit vaccines is necessary to inform choice of adjuvants for vaccine delivery. In the case of V antigen, adjuvants designed to target it to lysosomes are likely to induce optimal responses to multiple protective T-cell epitopes.     

Linkage disequilibrium between HLA class II (DR, DQ, DP) and antigen processing (LMP, TAP, DM) genes of the major histocompatibility complex

TAP, LMP and DM genes map within the major histocompatibility complex (MHC) class II region between the DQB1 and DPB1 loci, and are involved in the processing of peptides bound to HLA class I or class II molecules. In order to determine the various linkage disequilibria existing between these genes and HLA class II genes, we have analyzed TAP1, TAP2, LMP2, DMA, DMB, DRB1, DQA1, DQB1 and DPB1 polymorphisms in 162 unrelated healthy Caucasian individuals. Many positive or negative associations were observed between alleles at these loci, such as between DR/DQ and TAP2, DM or LMP, between DP and DMB, and between TAP2 and DM, TAP2 and LMP. Conversely, no linkage disequilibrium was detected between some closely related genes (DR/DQ and TAP1, TAP1 and TAP2, LMP2 and DM), in agreement with the existence of recombination hot spots in this region. Other weak linkage disequilibria are likely to exist in this region. These data allow to define some conserved MHC class II haplotypes including HLA class II and TAP, LMP and DM alleles. Furthermore, the knowledge of such linkage disequilibria is of outstanding importance in order to avoid misinterpretation of the data when studying MHC class II associations with autoimmune diseases.     

TAP-dependent major histocompatibility complex class I presentation of soluble proteins using listeriolysin

Immunization of mice with mixtures of listeriolysin, a pore-forming hemolysin secreted by the pathogenic bacterium Listeria monocytogenes, together with soluble ovalbumin, nucleoprotein of influenza virus, or β-galactosidase of Escherichia coli, resulted in strong cytotoxic CD8 T cell responses to each of the respective passenger proteins in vivo. Also, the concomitant addition of either protein with listeriolysin to target cells elicited efficient sensitization of these cells which could be attributed to the pore-forming activity of listeriolysin. This response was dependent upon a functional TAP transporter and was inhibitable by brefeldin A, indicating the transfer of the soluble proteins into the cytosol and the classical major histocompatibility (MHC) class I presentation pathway. The treatment of target cells with listeriolysin under our experimental conditions did not affect cell viability and the pores generated by listeriolysin treatment were repaired within 60 min. Introduction of soluble proteins into the MHC class I presentation pathway by listeriolysin provides a powerful system to study the cytotoxic response towards intracellular pathogens and would allow for rapid screening of potential antigens in vaccine formulations.     

Sequence of the canine major histocompatibility complex region containing non-classical class I genes

Abstract:  We have sequenced a segment of 150,102 nucleotides of canine major histocompatibility complex (MHC) DNA, corresponding to the junction of the class I and class III regions. The distal portion contained five class III genes including two tumor necrosis factor genes and the proximal portion contained five genes or pseudogenes belonging to the class I region. The order of the class III region genes was conserved as in the porcine and human MHC regions. The order of the class Ib loci from the proximal side outwards was DLA-53, DLA-12a, DLA-64, stress-induced phosphoprotein-1, followed by DLA-12. Only DLA-64 and DLA-12 display an overall predicted protein sequence compatible with the expression of membrane-anchored glycoproteins. The other class 1b loci do not appear to be functional by sequence analysis. In all, these 10 genes spanned 24% of the total sequence. The remaining 76% comprised of a number of non-coding and repetitive DNA elements including long interspersed nuclear element (LINE) fragments, short interspersed nuclear elements (SINE), and microsatellites.     

Evasion and subversion of antigen presentation by Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the most successful of human pathogens and has acquired the ability to establish latent or progressive infection and persist even in the presence of a fully functioning immune system. The ability of M. tuberculosis to avoid immune-mediated clearance is likely to reflect a highly evolved and coordinated program of immune evasion strategies, including some that interfere with antigen presentation to prevent or alter the quality of T-cell responses. Here, we review an extensive array of published studies supporting the view that antigen presentation pathways are targeted at many points by pathogenic mycobacteria. These studies show the multiple potential mechanisms by which M. tuberculosis may actively inhibit, subvert or otherwise modulate antigen presentation by major histocompatibility complex class I, class II and CD1 molecules. Unraveling the mechanisms by which M. tuberculosis evades or modulates antigen presentation is of critical importance for the development of more effective new vaccines based on live attenuated mycobacterial strains.