The MHC class II molecule H2-M is targeted to an endosomal compartment by a tyrosine-based targeting motif

Human leukocyte Antigen-DM polymorphisms in autoimmune diseases

Classical MHC class II (MHCII) proteins present peptides for CD4⁺ T-cell surveillance and are by far the most prominent risk factor for a number of autoimmune disorders. To date, many studies have shown that this link between particular MHCII alleles and disease depends on the MHCII's particular ability to bind and present certain peptides in specific physiological contexts. However, less attention has been paid to the non-classical MHCII molecule human leucocyte antigen-DM, which catalyses peptide exchange on classical MHCII proteins acting as a peptide editor. DM function impacts the presentation of both antigenic peptides in the periphery and key self-peptides during T-cell development in the thymus. In this way, DM activity directly influences the response to pathogens, as well as mechanisms of self-tolerance acquisition. While decreased DM editing of particular MHCII proteins has been proposed to be related to autoimmune disorders, no experimental evidence for different DM catalytic properties had been reported until recently. Biochemical and structural investigations, together with new animal models of loss of DM activity, have provided an attractive foundation for identifying different catalytic efficiencies for DM allotypes. Here, we revisit the current knowledge of DM function and discuss how DM function may impart autoimmunity at the organism level.

Comprehensive analysis of MHC class II genes in teleost fish genomes reveals dispensability of the peptide-loading DM system in a large part of vertebrates

Background

Classical major histocompatibility complex (MHC) class II molecules play an essential role in presenting peptide antigens to CD4⁺ T lymphocytes in the acquired immune system. The non-classical class II DM molecule, HLA-DM in the case of humans, possesses critical function in assisting the classical MHC class II molecules for proper peptide loading and is highly conserved in tetrapod species. Although the absence of DM-like genes in teleost fish has been speculated based on the results of homology searches, it has not been definitively clear whether the DM system is truly specific for tetrapods or not. To obtain a clear answer, we comprehensively searched class II genes in representative teleost fish genomes and analyzed those genes regarding the critical functional features required for the DM system.

Results

We discovered a novel ancient class II group (DE) in teleost fish and classified teleost fish class II genes into three major groups (DA, DB and DE). Based on several criteria, we investigated the classical/non-classical nature of various class II genes and showed that only one of three groups (DA) exhibits classical-type characteristics. Analyses of predicted class II molecules revealed that the critical tryptophan residue required for a classical class II molecule in the DM system could be found only in some non-classical but not in classical-type class II molecules of teleost fish.

Conclusions

Teleost fish, a major group of vertebrates, do not possess the DM system for the classical class II peptide-loading and this sophisticated system has specially evolved in the tetrapod lineage.

HLA-DO and Its Role in MHC Class II Antigen Presentation

Helper T cells are stimulated to fight infections or diseases upon recognition of peptides from antigens that are processed and presented by the proteins of Major Histocompatibility Complex (MHC) Class II molecules. Degradation of a full protein into small peptide fragments is a lengthy process consisting of many steps and chaperones. Malfunctions during any step of antigen processing could lead to the development of self-reactive T cells or defective immune response to pathogens. Although much has been accomplished regarding how antigens are processed and presented to T cells, many questions still remain unanswered, preventing the design of therapeutics for direct intervention with antigen processing. Here, we review published work on the discovery and function of a MHC class II molecular chaperone, HLA-DO, in human, and its mouse analog H2-O, herein called DO. While DO was originally discovered decades ago, elucidating its function has proven challenging. DO was discovered in association with another chaperone HLA-DM (DM) but unlike DM, its distribution is more tissue specific, and its function more subtle.

Pathways of antigen processing

T cell recognition of antigen-presenting cells depends on their expression of a spectrum of peptides bound to major histocompatibility complex class I (MHC-I) and class II (MHC-II) molecules. Conversion of antigens from pathogens or transformed cells into MHC-I- and MHC-II-bound peptides is critical for mounting protective T cell responses, and similar processing of self proteins is necessary to establish and maintain tolerance. Cells use a variety of mechanisms to acquire protein antigens, from translation in the cytosol to variations on the theme of endocytosis, and to degrade them once acquired. In this review, we highlight the aspects of MHC-I and MHC-II biosynthesis and assembly that have evolved to intersect these pathways and sample the peptides that are produced.

Forced expression of HLA-DM at the surface of dendritic cells increases loading of synthetic peptides on MHC class II molecules and modulates T cell responses

Adoptive transfer of autologous dendritic cells (DCs) loaded with tumor-associated CD4 and CD8 T cell epitopes represents a promising avenue for the immunotherapy of cancer. In an effort to increase the loading of therapeutic synthetic peptides on MHC II molecules, we used a mutant of HLA-DM (DMY) devoid of its lysosomal sorting motif and that accumulates at the cell surface. Transfection of DMY into HLA-DR(+) cells resulted in increased loading of the exogenously supplied HA(307-318) peptide, as well as increased stimulation of HA-specific T cells. Also, on transduction in mouse and human DCs, DMY increased loading of HEL(48-61) and of the tumor Ag-derived gp100(174-190) peptides, respectively. Interestingly, expression of DMY at the surface of APCs favored Th1 differentiation over Th2. Finally, we found that DMY(-) and DMY(+) mouse APCs differentially stimulated T cell hybridomas sensitive to the fine conformation of peptide-MHC II complexes. Taken together, our results suggest that the overexpression of HLA-DMY at the plasma membrane of DCs may improve quantitatively, but also qualitatively, the presentation of CD4 T cell epitopes in cellular vaccine therapies for cancer.

H2-O, a MHC class II-like protein, sets a threshold for B-cell entry into germinal centers

Upon antigen (Ag) encounter, B cells require T-cell help to enter the germinal center (GC). They obtain this help by presenting Ag-derived peptides on MHC class II (MHCII) for recognition by the T-cell receptor (TCR) of CD4(+) T cells. Peptides are loaded onto MHCII in endosomal compartments in a process catalyzed by the MHCII-like protein H2-M (HLA-DM in humans). This process is modulated by another MHCII-like protein, H2-O (HLA-DO in humans). H2-O is a biochemical inhibitor of peptide loading onto MHCII; however, on the cellular level, it has been shown to have varying effects on Ag presentation. Thus, the function of H2-O in the adaptive immune response remains unclear. Here, we examine the effect of H2-O expression on the ability of Ag-specific B cells to enter the GC. We show that when Ag specific WT and H2-O(-/-) B cells are placed in direct competition, H2-O(-/-) B cells preferentially populate the GC. This advantage is confined to Ag-specific B cells and is due to their superior ability to obtain Ag-specific T-cell help when T-cell help is limiting. Overall, our work shows that H2-O expression reduces the ability of B cells to gain T-cell help and participate in the GC reaction.

Derivation, culture, and characterization of thymic epithelial cell lines

The major histocompatibility complex (MHC)-restricted presentation of self-peptides, generated from tissue-specific antigens, by thymic epithelial cells (TECs) is essential for development of central tolerance and for generation of the regulatory T-cell repertoire in the thymus. However, the mechanisms by which self-peptides are generated in and presented by TECs have not been well defined. To elucidate the processes involved in MHC class II-restricted presentation of self-peptides by TECs, cortical and medullary TEC lines may be established from C57BL/6 mouse thymi. Localization of a variety of molecules, including the MHC class II molecules critically involved in the presentation of antigen by TECs, may be investigated by both immunofluores-cence microscopy and Western blotting analyses. Our own studies using these approaches have demonstrated that such molecules are localized in the H2-DM+ lysosomal compartments isolated from both cortical and medullary TECs.

Right place, right time, right peptide: DO keeps DM focused

Peptide loading of major histocompatibility class II molecules is catalyzed in late endosomal and lysosomal compartments of cells by the catalytic action of human leukocyte antigen (HLA)-DM (H-2M in mice). In B cells, dendritic cells and thymic epithelial cells, the peptide loading of class II molecules is modified by the expression of the non-classical class II molecule, HLA-DO (H-2O in mice). Collectively, studies to date support that DO/H-2O expression inhibits the presentation of antigens acquired by cells via fluid phase endocytosis. However, in B cells, the expression of H-2O promotes the presentation of antigens internalized by the B-cell receptor. In this review, we summarize the literature pertaining to DO assembly, transport, and function, with an emphasis on the function of DO/H-2O.

A defective viral superantigen-presenting phenotype in HLA-DR transfectants is corrected by CIITA

Activation of T lymphocytes by mouse mammary tumor virus superantigen (vSAg) requires binding to MHC class II molecules. The subcellular location where functional interactions occur between MHC class II molecules and vSAgs is still a matter of debate. To gain further insight into this issue, we have used human epithelial HeLa cells expressing HLA-DR1. Surprisingly, the human cells were unable to present transfected vSAg7 or vSAg9 to a series of murine T cell hybridomas. The defect is not related to a lack of vSAg processing, because these cells can indirectly activate T cells after coculture in the presence of B lymphocytes. However, after IFN-gamma treatment, the HeLa DR1(+) cells became apt at directly presenting the vSAg. Furthermore, transfection of CIITA was sufficient to restore presentation. Reconstitution experiments demonstrated the necessity of coexpressing HLA-DM and invariant chain (Ii) for efficient vSAg presentation. Interestingly, inclusion of a dileucine motif in the DRbeta cytoplasmic tail bypassed the need for HLA-DM expression and allowed the efficient presentation of vSAg7 in the presence of Ii. A similar trafficking signal was included in vSAg7 by replacing its cytoplasmic tail with the one of Ii. However, sorting of this chimeric Ii/vSAg molecule to the endocytic pathway completely abolished both its indirect and direct presentation. Together, our results suggest that functional vSAgs-DR complexes form after the very late stages of class II maturation, most probably at the cell surface.

Coevolution of TCR-MHC interactions: conserved MHC tertiary structure is not sufficient for interactions with the TCR

The specificity for self-MHC that is necessary for T cell function is a consequence of intrathymic selection during which T cell antigen receptors (TCRs) expressed by immature thymocytes are tested for their affinity for self-peptide:self-MHC. The germ-line-encoded segments of the TCR, however, are believed to have an innate specificity for structural features of MHC molecules. We directly tested this hypothesis by generating a transgenic mouse system in which the protein HLA-DM is expressed at the surface of thymic cortical epithelial cells in the absence of classical MHC molecules. The specialized intracellular function of HLA-DM has removed this MHC class II-like protein from the evolutionary forces that have been hypothesized to shape TCR-MHC interactions. Our study shows that a structural mimic of MHC class II is not sufficient to appropriately interact with the TCRs expressed by developing thymocytes. This result emphasizes the unique complementarity of TCR-MHC interactions that are maintained by the evolutionary pressures dictated by positive selection.

A point mutation in the groove of HLA-DO allows egress from the endoplasmic reticulum independent of HLA-DM

B lymphocytes express the nonclassical class II molecule HLA-DO, which modulates the peptide loading activity of HLA-DM in the endocytic pathway. Binding to HLA-DM is required for HLA-DO to egress from the endoplasmic reticulum (ER). To gain insights into the mode of action of DO and on the role of DM in ER release, we sought to identify DM-binding residues on DO. Our results show that DOalpha encompasses the binding site for HLA-DM. More specifically, mutation of residue DOalpha41 on an exposed lateral loop of the alpha1 domain affects the binding to DM, ER egress, and activity of DO. Using a series of chimeric DR/DO molecules, we confirmed the role of the alpha chain and established that a second DM-binding region is located C-terminal to the DOalpha80 residue, most probably in the alpha2 domain. Interestingly, after mutation of a buried proline (alpha11) on the floor of the putative peptide-binding groove, HLA-DO remained functional but became independent of HLA-DM for ER egress and intracellular trafficking. Collectively, these results suggest that the binding of HLA-DM to DOalpha allows the complex to egress from the ER by stabilizing intramolecular contacts between the N-terminal antiparallel beta-strands of the DOalphabeta heterodimer.

Tails of wonder: endocytic-sorting motifs key for exogenous antigen presentation

Antigen-presenting molecules, including MHC I, II and CD1, have central roles in the induction of T cell-mediated immunity against pathogens and tumors and also in the maintenance of tolerance towards self-antigens. The presentation of exogenously derived peptide and lipid antigens to specific T cells by professional antigen-presenting cells (pAPCs) is an essential part of both processes. Exogenous antigen loading takes place mostly within specialized endocytic and phagocytic compartments of pAPCs and targeting of antigen-presenting molecules to these intracellular compartments is mediated by highly conserved cytoplasmic sorting motifs. Recent data have revealed that the cytoplasmic tails of antigen-presenting molecules, by controlling the access of these molecules to exogenously derived antigens, have a crucially important and largely underappreciated role in the generation of tolerance and T-cell mediated immunity.

A cytosolic pathway for MHC class II–restricted antigen processing that is proteasome and TAP dependent

By convention, presentation of major histocompatibility complex (MHC) class I-restricted epitopes involves processing by cytosolic proteasomes, whereas MHC class II-restricted epitopes are generated by endosomal proteases. Here, we show that two MHC class II-restricted epitopes within influenza virus were generated by a proteasome- and TAP-dependent pathway that was accessed by exogenous virus in dendritic cells (DCs) but not cell types with less permeable endosomes. Both epitopes were presented by recycling MHC class II molecules. Challenging mice with influenza or vaccinia viruses demonstrated that a substantial portion of the MHC class II-restricted response was directed against proteasome-dependent epitopes. By complementing endosomal activities, this pathway broadens the array of MHC class II-restricted epitopes available for CD4(+) T cell activation.

Dissecting MHC class II export, B cell maturation, and DM stability defects in invariant chain mutant mice

Invariant (Ii) chain loss causes defective class II export, B cell maturation, and reduced DM stability. In this study, we compare Ii chain and class II mutant mouse phenotypes to dissect these disturbances. The present results demonstrate that ER retention of alphabeta complexes, and not beta-chain aggregates, disrupts B cell development. In contrast, we fail to detect class II aggregates in Ii chain mutant thymi. Ii chain loss in NOD mice leads to defective class II export and formation of alphabeta aggregates, but in this background, downstream signals are misregulated and mature B cells develop normally. Finally, Ii chain mutant strains all display reduced levels of DM, but mice expressing either p31 or p41 alone, and class II single chain mutants, are indistinguishable from wild type. We conclude that Ii chain contributions as a DM chaperone are independent of its role during class II export. This Ii chain/DM partnership favors class II peptide loading via conventional pathway(s).

Mechanisms governing B cell developmental defects in invariant chain-deficient mice

Invariant chain (Ii)-deficient mice exhibit profound B cell defects that have remained poorly understood, because they could not be simply explained by impaired Ag presentation. We found that Ii deficiency induced cell autonomous defects of two distinct B cell lineages. The life span of mature follicular (FO) B cells was reduced, accounting for their markedly decreased frequency, whereas, in contrast, marginal zone (MZ) B cells accumulated. Other Ii-expressing lineages such as B1 B cells and dendritic cells were unaffected. Surprisingly, the life span of FO B cells was fully corrected in Ii/I-Abeta doubly deficient mice, revealing that Ii-free I-Abeta chains alter FO B cell survival. In contrast, the accumulation of MZ B cells was controlled by a separate mechanism independent of I-Abeta. Interestingly, in Ii-deficient mice lacking FO B cells, the MZ B cells invaded the FO zone, suggesting that intact follicules contribute to the retention of B cells in the MZ. These findings reveal unexpected consequences of Ii deficiency on the development and organization of B cell follicles.

Tab2 is a novel conserved RNA binding protein required for translation of the chloroplast psaB mRNA

The chloroplast psaB mRNA encodes one of the reaction centre polypeptides of photosystem I. Protein pulse-labelling profiles indicate that the mutant strain of Chlamydomonas reinhardtii, F14, affected at the nuclear locus TAB2, is deficient in the translation of psaB mRNA and thus deficient in photosystem I activity. Genetic studies reveal that the target site for Tab2 is situated within the psaB 5'UTR. We have used genomic complementation to isolate the nuclear Tab2 gene. The deduced amino acid sequence of Tab2 (358 residues) displays 31-46% sequence identity with several orthologues found only in eukaryotic and prokaryotic organisms performing oxygenic photosynthesis. Directed mutagenesis indicates the importance of a highly conserved C-terminal tripeptide in Tab2 for normal psaB translation. The Tab2 protein is localized in the chloroplast stroma where it is associated with a high molecular mass protein complex containing the psaB mRNA. Gel mobility shift assays reveal a direct and specific interaction between Tab2 and the psaB 5'UTR. We propose that Tab2 plays a key role in the initial steps of PsaB translation and photosystem I assembly.

Class II transactivator-induced expression of HLA-DO(beta) in HeLa cells

HLA-DO is an intracellular nonclassical MHC class II molecule expressed in the endocytic pathway of B lymphocytes. It shapes the repertoire of peptides bound to classical class II molecules such as HLA-DR by regulating the activity of HLA-DM. Using a peptide corresponding to the cytoplasmic tail of HLA-DO(beta), we have developed a mouse monoclonal antibody, HKC5. Immunofluorescence microscopy revealed that HKC5 recognizes HLA-DO molecules present in the endoplasmic reticulum as well as those in vesicular compartments of the endocytic pathway. In addition, the antibody detects the isolated beta chain on Western blots. Using mutants of the DO(beta) cytoplasmic tail fused to a reporter molecule and expressed in epithelial cells, we showed by flow cytometry that the antibody epitope includes one or both of the leucine residues forming the lysosomal sorting signal. Finally, we have used HKC5 to evaluate the presence of the HLA-DO(beta) chain in HeLa cells expressing the class II transactivator protein CIITA. Our flow cytometry and confocal microscopy analyses showed a marked expression of DO(beta) suggesting that HLA-DO could accumulate under the influence of CIITA in non-B cells.

Functional analysis of tryptophans alpha 62 and beta 120 on HLA-DM

In the endocytic pathway of antigen-presenting cells, HLA-DM catalyzes the exchange between class II-associated invariant chain peptide (CLIP) and antigenic peptides onto major histocompatibility complex class II molecules. At low pH of lysosomal compartments, both HLA-DM and HLA-DR undergo conformational changes, and it was recently postulated that two partially exposed tryptophans on HLA-DM might be involved in the interaction between the two molecules. To define contact regions on HLA-DM, we have conducted site-directed mutagenesis on those two hydrophobic residues. The HLA-DM alphaW62A,betaW120A (DM(W62A/W120A)) double mutant was expressed in HLA-DR(+) HeLa cells expressing invariant chain, and the activity of this DM molecule was assessed. Flow cytometry analysis of cell surface DR-CLIP complexes revealed that DM(W62A/W120A) removes CLIP as efficiently as its wild-type counterpart. DM(W62A/W120A) was found in the endocytic pathway by immunofluorescence, and DM-DR complexes were immunoprecipitated from these cells at pH 5. Finally, mutations alphaW62A and betaW120A on HLA-DM did not affect the association with HLA-DO. The complex egresses the endoplasmic reticulum and accumulates in endocytic vesicles. Moreover, DO and DM(W62A/)W120A were co-immunoprecipitated at pH 7. We conclude that the alpha62 and beta120 tryptophan residues are not required for the activity of DM, nor are they directly implicated in the interaction with DR or DO.

CD1d endosomal trafficking is independently regulated by an intrinsic CD1d-encoded tyrosine motif and by the invariant chain

Endosomal trafficking is an essential component of the CD1 pathway of lipid antigen presentation to T cells. We demonstrate that CD1d access to endosomal compartments is under dual regulation by an intrinsic tyrosine-based motif, which governs intense recycling between the plasma membrane and the endosome, and by the invariant chain, with which CD1d associates in the endoplasmic reticulum. Both pathways independently enhance antigen presentation to V(alpha)14(+) NKT cells, the main subset of CD1d-restricted T cells. These results reveal the complexity of CD1d trafficking and suggest that the invariant chain was a component of ancestral antigen presentation pathways prior to the evolution of MHC and CD1.

Reorganization of multivesicular bodies regulates MHC class II antigen presentation by dendritic cells

Immature dendritic cells (DCs) sample their environment for antigens and after stimulation present peptide associated with major histocompatibility complex class II (MHC II) to naive T cells. We have studied the intracellular trafficking of MHC II in cultured DCs. In immature cells, the majority of MHC II was stored intracellularly at the internal vesicles of multivesicular bodies (MVBs). In contrast, DM, an accessory molecule required for peptide loading, was located predominantly at the limiting membrane of MVBs. After stimulation, the internal vesicles carrying MHC II were transferred to the limiting membrane of the MVB, bringing MHC II and DM to the same membrane domain. Concomitantly, the MVBs transformed into long tubular organelles that extended into the periphery of the cells. Vesicles that were formed at the tips of these tubules nonselectively incorporated MHC II and DM and presumably mediated transport to the plasma membrane. We propose that in maturing DCs, the reorganization of MVBs is fundamental for the timing of MHC II antigen loading and transport to the plasma membrane.

Amino acid substitutions in the putative MHC class II "dimer of dimers" interface inhibit CD4+ T cell activation

Activation of T lymphocytes is dependent on multiple ligand-receptor interactions. The possibility that TCR dimerization contributes to T cell triggering was raised by the crystallographic analysis of MHC class II molecules. The MHC class II molecules associated as double dimers, and in such a way that two TCR (and two CD4 molecules) could bind simultaneously. Several subsequent studies have lent support to this concept, although the role of TCR cross-linking in T cell activation remains unclear. Using DRA cDNAs modified to encode two different C-terminal tags, no evidence of constitutive double dimer formation was obtained following immunoprecipitation and Western blotting from cells transiently transfected with wild-type DRB and tagged DRA constructs, together with invariant chain and HLA-DM. To determine whether MHC class II molecules contribute actively to TCR-dependent dimerization and consequent T cell activation, panels of HLA-DR1beta and H2-E(k) cDNAs were generated with mutations in the sequences encoding the interface regions of the MHC class II double dimer. Stable DAP.3 transfectants expressing these cDNAs were generated and characterized biochemically and functionally. Substitutions in either interface region I or III did not affect T cell activation, whereas combinations of amino acid substitutions in both regions led to substantial inhibition of proliferation or IL-2 secretion by human and murine T cells. Because the amino acid-substituted molecules were serologically indistinguishable from wild type, bound antigenic peptide with equal efficiency, and induced Ag-dependent CD25 expression indicating TCR recognition, the reduced ability of the mutants to induce full T cell activation is most likely the result of impaired double dimer formation. These data suggest that MHC class II molecules, due to their structural properties, actively contribute to TCR cross-linking.

CLIP-derived self peptides bound to MHC class II molecules of medullary thymic epithelial cells differ from those of cortical thymic epithelial cells in their diversity, length, and C-terminal processing

Medullary thymic epithelial cells (mTEC) are able to present soluble antigens to CD4+ helper T cell lines, whereas cortical thymic epithelial cells (cTEC) are not (Mizuochi, T., et al., J. Exp. Med. 1992. 175: 1601-1605). In addition, class II heterodimers from mTEC migrated with apparently less relative molecular mass in SDS-PAGE than those from cTEC (Kasai, M., et al., Eur. J. Immunol. 1998. 28:1867-1876). To investigate the cause of the distinct migration profiles of class II heterodimers in both TEC types, class II heterodimer-associated peptides were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Self peptides from cTEC were shown to vary moderately in length and to be highly diverse, including low amounts of CLIP (class II-associated invariant chain peptide) variants. On the other hand, self peptides from two mTEC consisted predominantly of two CLIP variants with exceptional C-terminal extensions. C-terminally overhanging residues of CLIP in mTEC may be responsible for the distinct migration of class II heterodimers in SDS-PAGE. Differences in migration of class II heterodimers on SDS gels was also observed in H2-DM+ vesicles isolated from both TEC. The possible contribution of self peptides bound to class II heterodimers in TEC to positive or negative selection of T cells in the thymus is discussed.

Functional characterization of a lysosomal sorting motif in the cytoplasmic tail of HLA-DObeta

HLA-DO is an intracellular non-classical class II major histocompatibility complex molecule expressed in the endocytic pathway of B lymphocytes, which regulates the loading of antigenic peptides onto classical class II molecules such as HLA-DR. The activity of HLA-DO is mediated through its interaction with the peptide editor HLA-DM. Here, our results demonstrate that although HLA-DO is absolutely dependent on its association with DM to egress the endoplasmic reticulum, the cytoplasmic portion of its beta chain encodes a functional lysosomal sorting signal. By confocal microscopy and flow cytometry analysis, we show that reporter transmembrane molecules fused to the cytoplasmic tail of HLA-DObeta accumulated in Lamp-1(+) vesicles of transfected HeLa cells. Mutagenesis of a leucine-leucine motif abrogated lysosomal accumulation and resulted in cell surface redistribution of reporter molecules. Finally, we show that mutation of the di-leucine sequence in DObeta did not alter its lysosomal sorting when associated with DM molecules. Taken together, these results demonstrate that lysosomal expression of the DO-DM complex is mediated primarily by the tyrosine-based motif of HLA-DM and suggest that the DObeta-encoded motif is involved in the fine-tuning of the intracellular sorting.

Nonclassical MHC class II molecules

Major histocompatibility complex (MHC) class II molecules are cell surface proteins that present peptides to CD4(+) T cells. In addition to these wellcharacterized molecules, two other class II-like proteins are produced from the class II region of the MHC, HLA-DM (DM) and HLA-DO (DO) (called H2-M, or H2-DM and H2-O in the mouse). The function of DM is well established; it promotes peptide loading of class II molecules in the endosomal/lysosomal system by catalyzing the release of CLIP peptides (derived from the class II-associated invariant chain) in exchange for more stably binding peptides. While DM is present in all class II- expressing antigen presenting cells, DO is expressed mainly in B cells. In this cell type the majority of DM molecules are not present as free heterodimers but are instead associated with DO in tight heterotetrameric complexes. The association with DM is essential for the intracellular transport of DO, and the two molecules remain associated in the endosomal system. DO can clearly modify the peptide exchange activity of DM both in vitro and in vivo, but the physiological relevance of this interaction is still only partly understood.

H2-DMalpha(-/-) mice show the importance of major histocompatibility complex-bound peptide in cardiac allograft rejection

The role played by antigenic peptides bound to major histocompatibility complex (MHC) molecules is evaluated with H2-DMalpha(-/)- mice. These mice have predominantly class II-associated invariant chain peptide (CLIP)-, not antigenic peptide-bound, MHC class II. H2-DMalpha(-/)- donor heart grafts survived three times longer than wild-type grafts and slightly longer than I-A(beta)(b)-(/)- grafts. Proliferative T cell response was absent, and cytolytic response was reduced against the H2-DMalpha(-/)- grafts in vivo. Residual cytolytic T cell and antibody responses against intact MHC class I lead to eventual rejection. Removal of both H2-DMalpha and beta2-microglobulin (beta2m) in cardiac grafts lead to greater (8-10 times) graft survival, whereas removal of beta2m alone did not have any effect. These results demonstrate the significance of peptide rather than just allogeneic MHC, in eliciting graft rejection.

Phagosomes acquire nascent and recycling class II MHC molecules but primarily use nascent molecules in phagocytic antigen processing

Phagosomes contain class II MHC (MHC-II) and form peptide:MHC-II complexes, but the source of phagosomal MHC-II molecules is uncertain. Phagosomes may acquire nascent MHC-II or preexisting, recycling MHC-II that may be internalized from the plasma membrane. Brefeldin A (BFA) was used to deplete nascent MHC-II in murine macrophages to determine the relative contributions of nascent and recycling MHC-II molecules to phagocytic Ag processing. In addition, biotinylation of cell-surface proteins was used to assess the transport of MHC-II from the cell surface to phagosomes. BFA inhibited macrophage processing of latex bead-conjugated Ag for presentation to T cells, suggesting that nascent MHC-II molecules are important in phagocytic Ag processing. Furthermore, detection of specific peptide:MHC-II complexes in isolated phagosomes confirmed that BFA decreased formation of peptide:MHC-II complexes within phagosomes. Both flow organellometry and Western blot analysis of purified phagosomes showed that about two-thirds of phagosomal MHC-II was nascent (depleted by 3 h prior treatment with BFA) and primarily derived from intracellular sites. About one-third of phagosomal MHC-II was preexisting and primarily derived from the plasma membrane. BFA had little effect on phagosomal H2-DM or the degradation of bead-associated Ag. Thus, inhibition of phagocytic Ag processing by BFA correlated with depletion of nascent MHC-II in phagosomes and occurred despite the persistent delivery of plasma membrane-derived recycling MHC-II molecules and other Ag-processing components to phagosomes. These observations suggest that phagosomal Ag processing depends primarily on nascent MHC-II molecules delivered from intracellular sites, e.g., endocytic compartments.

Invariant chain controls H2-M proteolysis in mouse splenocytes and dendritic cells

The association of invariant (Ii) chain with major histocompatibility complex (MHC) class II dimers is required for proper antigen presentation to T cells by antigen-presenting cells. Mice lacking Ii chain have severe abnormalities in class II transport, T cell selection, and B cell maturation. We demonstrate here that H2-M, which is required for efficient class II antigenic peptide loading, is unexpectedly downregulated in splenocytes and mature dendritic cells (DCs) from Ii(-/-) mice. Downregulation reflects an increased rate of degradation in Ii(-/-) cells. Degradation apparently occurs within lysosomes, as it is prevented by cysteine protease inhibitors such as E64, but not by the proteasome inhibitor lactacystin. Thus, Ii chain may act as a lysosomal protease inhibitor in B cells and DCs, with its deletion contributing indirectly to the loss of H2-M.

Functional HLA-DM on the surface of B cells and immature dendritic cells

HLA-DM (DM) plays a critical role in antigen presentation through major histocompatibility complex (MHC) class II molecules. DM functions as a molecular chaperone by keeping class II molecules competent for antigenic peptide loading and serves as an editor by favoring presentation of high-stability peptides. Until now, DM has been thought to exert these activities only in late endosomal/lysosomal compartments of antigen-presenting cells. Here we show that a subset of DM resides at the cell surface of B cells and immature dendritic cells. Surface DM engages in complexes with putatively empty class II molecules and controls presentation of those antigens that rely on loading on the cell surface or in early endosomal recycling compartments. For example, epitopes derived from myelin basic protein that are implicated in the autoimmune disease multiple sclerosis are down-modulated by DM, but are presented in the absence of DM. Thus, this novel concept of functional DM on the surface may be relevant to both protective immune responses and autoimmunity.

Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed antigens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and associated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cells are thus potentially important for antigen presentation. Here we review some cell biological aspects of polarized sorting of MHC class II and the associated invariant chain and the signals that are involved in the sorting process to the basolateral domain. The molecules involved in sorting and loading of peptide may modulate antigen presentation, and in particular we discuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.

Intracellular transport and peptide loading of MHC class II molecules: regulation by chaperones and motors

MHC class II molecules are important in the onset and modulation of cellular immune responses. Studies on the intracellular transport of these molecules has provided insight into the way pathogens are processed and presented at the cell surface and may result in future immunological intervention strategies. Recent reviews have extensively described structural properties and early events in the biosynthesis of MHC class II (1-3). In this review, the focus will be on the function of the dedicated chaperone proteins Ii, DM and DO in the class II assembly, transport and peptide loading as well on proteins involved in transport steps late in the intracellular transport of MHC class II.

The role of H2-O and HLA-DO in major histocompatibility complex class II-restricted antigen processing and presentation

The function of major histocompatibility complex (MHC) class II molecules is to sample exogenous antigens for presentation to CD4+ T helper cells. After synthesis in the endoplasmic reticulum, class II molecules are directed into the endosomal system by association with the invariant chain (Ii), which is sequentially cleaved, generating class II dimers loaded with Ii-derived peptides (CLIP). These class II-peptide complexes are physiological substrates for H2-M/HLA-DM, a resident of the endosomal/lysosomal system which facilitates the removal of CLIP from newly synthesised class II alpha beta dimers. Exchange of CLIP for antigenic class II-binding peptides is also promoted by the action of H2-M/HLA-DM, resulting in stable peptide-class II complexes that are transported to the cell surface for presentation to CD4+ T cells. Recent evidence suggests that this H2-M/HLA-DM-mediated 'peptide editing' is influenced by another MHC class II-encoded molecule, H2-O/HLA-DO. This non-polymorphic alpha beta heterodimer is associated with H2-M/HLA-DM during intracellular transport and within the endosomal system of B cells. H2-O/HLA-DO alters the peptide exchange function of H2-M/HLA-DM in a pH-dependent manner, so that H2-M/HLA-DM activity is limited to more acidic conditions, corresponding to lysosomal compartments. Indeed, H2-O/HLA-DO may serve to limit the presentation of antigens after fluid phase uptake by B cells, while augmenting presentation of antigens internalised via membrane Ig receptors. Such a mechanism may maintain the fidelity of the B-cell-CD4+ T-cell interaction, counteracting self reactivity arising from less stringent lymphocyte activation. Here, data evaluating the role of H2-O/HLA-DO shall be reviewed and its putative function discussed.

The impact of the non-classical MHC proteins HLA-DM and HLA-DO on loading of MHC class II molecules

Peptide binding to classical major histocompatibility complex (MHC) class II molecules is known to be determined by the properties of the class II peptide binding groove but recently it turned out to be co-controlled by the activity of the non-classical MHC molecules HLA-DM and HLA-DO: HLA-DM functions as a mediator of peptide exchange. In addition, HLA-DM is a chaperone for MHC class II molecules in endosomal and lysosomal loading compartments because it stabilizes the empty MHC class II peptide binding groove and keeps it receptive for peptide loading until appropriate peptide ligands are captured. Since HLA-DM favors the generation of high-stability peptide-MHC class II complexes by releasing low-stability peptide ligands, DM activity affects the peptide repertoire presented on the cell surface of antigen-presenting cells. HLA-DO is expressed mainly in B cells and binds tightly to HLA-DM thereby modulating its activity. Together, HLA-DM and HLA-DO are critical factors in shaping the MHC class II-associated self or foreign peptide repertoire of antigen presenting cells and, hence, govern initiation or prevention of an immune response.

Quality control of MHC class II associated peptides by HLA-DM/H2-M

For many years the crucial components involved in MHC class II mediated antigen presentation have been thought to be known: polymorphic MHC class II molecules, the monomorphic invariant chain (li) and a set of conventional proteases that cleave antigenic proteins thereby generating ligands able to associate with MHC class II molecules. However, in 1994 it was found that without an additional molecule, HLA-DM (DM), efficient presentation of protein antigens cannot be achieved. Biochemical studies showed that DM acts as a molecular chaperone protecting empty MHC class II molecules from functional inactivation. In addition, it serves as a peptide editor: DM catalyzes not only the release of the invariant chain remnant CLIP, but of all sorts of low-stability peptides, and simultaneously favors binding of high-stability peptides. Through this quality control of peptide loading, DM enables APCs to optimize MHC restriction and to display their antigenic peptide cargo on the surface for prolonged periods of time to be scrutinized by T cells.

Accurate intracellular localization of HLA-DM requires correct spacing of a cytoplasmic YTPL targeting motif relative to the transmembrane domain

HLA-DM is an MHC class II-related heterodimer that is targeted to lysosomal compartments by a tyrosine-based signal YTPL, present in the cytoplasmic tail of the beta chain. Similar signals in other proteins control transport to different intracellular locations and can be recognized at several sorting sites within the cell including the trans-Golgi network, the plasma membrane and the early or sorting endosome. Therefore, in addition to recognizing the basic tyrosine motif, the sorting machinery must be sensitive to additional features associated with these elements. Here we show that efficient trafficking of HLA-DM to lysosomal compartments is dependent upon the proximity of its tyrosine motif to the transmembrane domain. Constructs in which the spacing is altered are rapidly internalized but are expressed at the cell surface. We conclude that the spacing of the HLA-DMB-encoded tyrosine motif relative to the transmembrane domain is an important feature controlling DM sorting in endosomes.

Targeting human papillomavirus type 16 E7 to the endosomal/lysosomal compartment enhances the antitumor immunity of DNA vaccines against murine human papillomavirus type 16 E7-expressing tumors

DNA vaccination is an attractive approach for tumor immunotherapy because of its stability and simplicity of delivery. Advances demonstrate that helper T cell responses play a critical role in initiating immune responses. The aim of the current study is to test whether targeting HPV-16 E7 to the endosomal/lysosomal compartment can enhance the potency of DNA vaccines. We linked the lysosome-associated membrane protein 1 (LAMP-1) to HPV-E7 to construct a chimeric DNA, Sig/E7/LAMP-1 DNA. For in vivo tumor prevention experiments, mice were vaccinated with E7 DNA or Sig/E7/LAMP-1 DNA via gene gun, followed by tumor challenge. For in vivo tumor regression experiments, mice were first challenged with tumor cells and then vaccinated with E7-DNA or Sig/E7/LAMP-1 DNA. Intracellular cytokine staining with flow cytometry analysis, cytotoxic T lymphocyte (CTL) assays, enzyme-linked immunoabsorbent assay (ELISA), and enzyme-linked immunospot (ELISPOT) assays were used for in vitro E7-specific immunological studies. In both tumor prevention and tumor regression assays, Sig/E7/LAMP-1 DNA generated greater antitumor immunity than did wild-type E7 DNA. In addition, mice vaccinated with Sig/E7/LAMP-1 DNA had greater numbers of E7-specific CD4+ helper T cells, higher E7-specific CTL activity, and greater numbers of CD8+ T cell precursors than did mice vaccinated with Sig/E7 or wild-type E7 DNA. Sig/E7 generated a stronger E7-specific antibody response than did Sig/E7/LAMP-1 or wild-type E7 DNA. Our results indicate that linkage of the antigen gene to an endosomal/lysosomal targeting signal may greatly enhance the potency of DNA vaccines.

Polarized Transport of MHC Class II Molecules in Madin-Darby Canine Kidney Cells Is Directed by a Leucine-Based Signal in the Cytoplasmic Tail of the β-Chain

MHC class II molecules are found on the basolateral plasma membrane domain of polarized epithelial cells, where they can present Ag to intraepithelial lymphocytes in the vascular space. We have analyzed the sorting information required for efficient intracellular localization and polarized distribution of MHC class II molecules in stably transfected Madin-Darby canine kidney cells. These cells were able to present influenza virus particles to HLA-DR1-restricted T cell clones. Wild-type MHC class II molecules were located on the basolateral plasma membrane domain, in basolateral early endosomes, and in late multivesicular endosomes, the latter also containing the MHC class II-associated invariant chain and an HLA-DM fusion protein. A phenylalanine-leucine residue within the cytoplasmic tail of the β-chain was required for basolateral distribution, efficient internalization, and localization of the MHC class II molecules to basolateral early endosomes. However, distribution to apically located, late multivesicular endosomes did not depend on signals in the class II cytoplasmic tails as both wild-type class II molecules and mutant molecules lacking the phenylalanine-leucine motif were found in these compartments. Our results demonstrate that sorting information in the tails of class II dimers is an absolute requirement for their basolateral surface distribution and intracellular localization.

Differential expression of alternative H2-M isoforms in B cells, dendritic cells and macrophages by proinflammatory cytokines

Major histocompatibility (MHC) class II heterodimers bind peptides generated by degradation of endocytosed antigens and display them on the surface of antigen presenting cells (APCs) for recognition by CD4+ T cells. Efficient loading of MHC class II molecules with peptides is catalyzed by the MHC class II-like molecule H2-M. The coordinate regulation of MHC class II and H2-M expression is a prerequisite for efficient MHC class II/peptide assembly in APCs determining both the generation of the T cell repertoire in the thymus and cellular immune responses in the periphery. Here we show that expression of H2-M and MHC class II genes is coordinately and cell type-specific regulated in splenic B cells, splenic dendritic cells (DCs) and peritoneal macrophages (Mphi) in response to proinflammatory and immunoregulatory cytokines, including GM-CSF, IFN-gamma, TGF-beta2, IL-4, IL-10 and viral IL-10. In addition, ratio-RT-PCR expression analysis of the duplicated H2-Mbeta-chain loci demonstrates for the first time that Mbl and Mb2 genes are differentially expressed in individual APC types. Mb2 is preferentially expressed in IL-4, GM-CSF, IL-10, vIL-10 and IFN-gamma stimulated splenic B cells, whereas splenic DCs express both Mb genes at almost equal levels. In contrast, peritoneal Mphi express predominantly Mb2 but stimulation with IFN-gamma induces a switch towards Mb1 expression. These data suggest a common mechanism that regulates coordinate expression of H2-M and MHC class II genes in professional APCs. Differential expression of Mb1 and Mb2, and by consequence alternative H2-M isoforms (Malphabeta1 or Malphabeta2), may influence the nature of the peptide repertoire presented by different APC types.

Conserved structural features between HLA-DO beta and -DR beta

HLA-DO is a non-classical MHC class II molecule presumed to play a specialized role in the antigen processing pathway. We have modeled the HLA-DO beta-chain and found its overall structure compatible with the one of DR beta. Functional studies further highlighted the similarity between these beta-chains of the class II family of proteins. Indeed, a mixed heterodimer composed of the DR alpha and a chimeric DO beta-chains presented bacterial superantigens to T cells and was shown to interact with CD4. The implications of such structural conservation for the in vivo functions of HLA-DO are discussed.

Crystal structure of mouse H2-M

H2-M (HLA-DM in humans) resides in an acidic endosomal compartment, where it facilitates the loading of antigenic peptides into the peptide-binding groove of class II MHC. The crystal structure of a soluble form of H2-M has been solved to 3.1 A resolution, revealing a heterodimer with structural similarities to the MHC family of proteins. In contrast to its antigen-presenting cousins, the membrane distal alpha helices of H2-M pack closely together, occluding most of the binding groove except for a single large pocket near the center. The structure of H2-M has several unique features that may play a role in its function as a molecular chaperone and peptide exchange factor.

Protein transport from the secretory to the endocytic pathway in mammalian cells

The trans-Golgi network (TGN) is the last station of the secretory pathway where soluble and membrane proteins are sorted for subsequent transport to endocytic compartments. This pathway is primarily followed by two distinct but related mannose 6-phosphate receptors which exhibit complementary functions in soluble lysosomal enzyme targeting. These transmembrane proteins and their bound ligands are packaged in transport intermediates coated with clathrin and the AP-1 assembly complex. Their segregation is determined by the interaction of tyrosine- and di-leucine-based sorting determinants present in their cytoplasmic domains with AP-1. Other membrane proteins such as the lysosomal membrane glycoproteins or envelope glycoproteins of herpes viruses, which contain similar sorting signals, may also follow the same pathway. In this review, we will summarize our current understanding of the molecular mechanisms leading to membrane protein sorting in the TGN and the formation of AP-1-coated transport intermediates.

A role for HLA-DO as a co-chaperone of HLA-DM in peptide loading of MHC class II molecules

In B cells, the non-classical human leukocyte antigens HLA-DO (DO) and HLA-DM (DM) are residents of lysosome-like organelles where they form tight complexes. DM catalyzes the removal of invariant chain-derived CLIP peptides from classical major histocompatibility complex (MHC) class II molecules, chaperones them until peptides are available for loading, and functions as a peptide editor. Here we show that DO preferentially promotes loading of MHC class II molecules that are dependent on the chaperone activity of DM, and influences editing in a positive way for some peptides and negatively for others. In acidic compartments, DO is engaged in DR-DM-DO complexes whose physiological relevance is indicated by the observation that at lysosomal pH DM-DO stabilizes empty class II molecules more efficiently than DM alone. Moreover, expression of DO in a melanoma cell line favors loading of high-stability peptides. Thus, DO appears to act as a co-chaperone of DM, thereby controlling the quality of antigenic peptides to be presented on the cell surface.

DR/CLIP (Class II-Associated Invariant Chain Peptides) and DR/Peptide Complexes Colocalize in Prelysosomes in Human B Lymphoblastoid Cells

In APCs, MHC class II molecules (MHC class II) bind antigenic peptides after HLA-DM mediated removal of CLIP. To characterize intracellular sites of peptide loading in human B lymphoblastoid cell lines, we conducted immunoelectron microscopy studies with Abs recognizing MHC class II associated with CLIP or bound peptide, respectively, together with Abs to HLA-DM and endocytic markers. The distribution of these molecules indicates that peptide binding occurs in compartments with characteristics of normal late endosomes, and in compartments that show characteristics of late endosomes, but are not detectably accessed by endocytosed BSA-gold. The latter compartments may represent or give rise to recycling vesicles that deliver peptide-loaded class II molecules to the cell surface. In addition, we have compared cells in which HLA-DM and HLA-DR interaction is defective with cells in which this interaction is intact, and find that DM/DR interaction is not required for the proper localization of either molecule to peptide-loading compartments.

Novel Glycosylation of HLA-DRα Disrupts Antigen Presentation Without Altering Endosomal Localization

The HLA-DR hemizygous B lymphoblastoid cell line, 10.24.6, has a DRA mutation (Pro96→Ser) that creates a novel glycosylation site at Asn94. The mutant DR molecules are primarily associated with nested fragments of invariant chain (class II-associated invariant chain peptides), and their interaction with HLA-DM is impaired. Here we further analyzed the defect in 10.24.6 cells. Expressing Ser96 mutant DRA cDNA in DRA-null cells recapitulated the 10.24.6 phenotype, indicating that the mutation causes the Ag presentation defect. A mutation to Ala96α, which does not introduce an extra glycan, generated a normal phenotype; the critical role of the glycan was further supported by experiments in which N-glycosylation was blocked by tunicamycin. We also evaluated whether the 10.24.6 mutation affected DR3 maturation or trafficking. Metabolic labeling and subcellular fractionation showed that assembly, endosomal transport, and invariant chain proteolysis of mutant DR3 molecules were similar to wild-type. A slight delay in export from the endoplasmic reticulum to the Golgi apparatus in 10.24.6 cells probably did not contribute significantly to the Ag presentation defect, because the abundance of DM and mutant DR in peptide-loading compartments was normal at steady state. Our results indicate that proper localization of these molecules does not depend on their interaction.

Endocytic clathrin-coated pit formation is independent of receptor internalization signal levels

The mechanisms responsible for coated pit formation in cells remain unknown, but indirect evidence has argued both for and against a critical role of receptor cytoplasmic domains in the process. If the endocytic motifs of receptors are responsible for recruiting AP2 to the plasma membrane, thereby driving coated pit formation, then the level of constitutively internalized receptors at the membrane would be expected to govern the steady-state level of coated pits in cells. Here we directly test this hypothesis for broad classes of receptors containing three distinct constitutive internalization signals. Chimeric proteins consisting of an integral membrane reporter protein (Tac) coupled to cytoplasmic domains bearing tyrosine-, di-leucine-, or acidic cluster/casein kinase II-based internalization signals were overexpressed to levels that saturated the internalization pathway. Quantitative confocal immunofluorescence microscopy indicated that the number of plasma membrane clathrin-coated pits and the concentration of their structural components were invariant when comparing cells expressing saturating levels of the chimeric receptors to nonexpressing cells or to cells expressing only the Tac reporter lacking cytoplasmic internalization signals. Biochemical analysis showed that the distribution of coat proteins between assembled coated pits and soluble pools was also not altered by receptor overexpression. Finally, the cellular localizations of AP2 and AP1 were similarly unaffected. These results provide a clear indication that receptor endocytic signals do not determine coated pit levels by directly recruiting AP2 molecules. Rather, the findings support a model in which coated pit formation proceeds through recruitment and activation of AP2, likely through a limited number of regulated docking sites that act independently of endocytic signals.

Multiple signals regulate the intracellular trafficking of HLA-DM in B-lymphoblastoid cells

Peptide loading by major histocompatibility complex (MHC) class II molecules occurs in the endocytic pathway and is critically dependent upon the function of the class II-related molecule human leucocyte antigen-DM (HLA-DM). We have previously shown that a tyrosine-based lysosomal targeting signal present in the cytoplasmic tail of DMB has the capacity to target HLA-DM to peptide-loading compartments in HeLa cells. Here we investigate the importance of this signal in directing HLA-DM to processing compartments in professional antigen-presenting cells. We reconstituted a DMB-negative B-lymphoblastoid cell line with native or targeting-deficient DMB and show that in the absence of its tyrosine signal, DMB-Y230A is as efficient as the wild-type molecule in inducing MHC class II SDS stable dimer formation; restoring expression of the conformation-dependent DR3 epitope 16:23; the removal of CLIP; and accessing lysosomal peptide-loading compartments. By transient transfection in HeLa cells we show that Ii is able to compensate for loss of DMB-encoded targeting information. These data imply that in cells expressing physiological levels of class II, Ii and DM, there is sufficient association with Ii to direct the majority of DM into the endocytic pathway. Thus MHC class II and HLA-DM may follow similar intracellular trafficking pathways on route to antigen-processing compartments.

CD1--a new paradigm for antigen presentation and T cell activation

Despite identification of the CD1 family of molecules in the late 1970s, the function of CD1 was undetermined for more than a decade. Recent evidence has established that CD1 molecules comprise a novel lineage of antigen-presenting molecules, distinct from major histocompatibility complex (MHC) class I and class II molecules. Unlike the MHC molecules, which bind short peptides in their antigen-binding groove for presentation to either CD4+ or CD8+ T cells bearing alpha beta T cell receptors, the CD1 molecules appear to accommodate lipid and glycolipid antigens in their hydrophobic cavity for presentation to a wide variety of T cells, including double-negative alpha beta and gamma delta T cells and CD8+ alpha beta T cells. By using a unique cytoplasmic signal, some CD1 molecules traffic to endosomal compartments for sampling mycobacteria-derived lipid antigens, and subsequently lipid antigen-loaded CD1 molecules are expressed on the cell surface to activate specific T cells. These CD1-restricted T cells kill mycobacteria-infected cells and secrete interferon-gamma, indicating a potential role of CD1-mediated T cell responses in clearing mycobacterial infection. The identification of an MHC-independent antigen presentation pathway for nonpeptide antigens provides new insights into immunoregulation and host defense.