Binding of a major T cell epitope of mycobacteria to a specific pocket within HLA-DRw17(DR3) molecules

The in vivo expressed Mycobacterium tuberculosis (IVE-TB) antigen Rv2034 induces CD4⁺ T-cells that protect against pulmonary infection in HLA-DR transgenic mice and guinea pigs

Tuberculosis (TB) remains a life-threatening infectious disease of global proportions with serious negative health and economic consequences. The lack of sufficient protection induced by Mycobacterium bovis BCG, the current vaccine for TB, as well as the impact of HIV co-infection and the emergence of drug resistant Mycobacterium tuberculosis (Mtb) strains all urge for improved vaccines against TB. A minimal requirement for Mtb vaccine antigens is their in vivo expression during Mtb infection and ability to trigger significant immune responses. Recently we identified a new class of Mtb antigens, designated IVE-TB (in vivo expressed) antigens. These included Rv2034, a protein that was expressed during pulmonary infection and strongly recognized by human T-cells. Here, the in vivo immunogenicity and protective efficacy of Rv2034 was further analyzed using HLA-DR transgenic mice that lack endogenous murine MHC class II molecules. The Rv2034 protein indeed was highly immunogenic in HLA-DR3 transgenic mice and induced HLA-DR3 restricted IFN-γ(+)/TNF(+) and IFN-γ(+) CD4(+) T-cells, specific for an epitope encoded in peptide 31-50. CD4(+) T-cell responses were optimally induced when using TLR9- and TLR3-ligand-adjuvants or CAF09. Rv2034-specific antibodies were observed following immunization with either TLR2-, TLR3-, TLR4-, TLR5-, TLR7- or TLR9-ligands or CAF09. Importantly, immunization with Rv2034 or the hybrid-protein Ag85B-ESAT6-Rv2034 adjuvanted with CpG or CAF09, induced over one log reduction, relative to unvaccinated controls, in the number of bacilli in the lungs of Mtb challenged HLA-DR3 transgenic mice and guinea pigs. These data demonstrate the potential of Rv2034 as a novel, IVE-TB antigen for future TB vaccination.

Identification of T cell epitopes of Mycobacterium tuberculosis with biolistic DNA vaccination

Tuberculosis (TB) has been listed as one of the most prevalent and serious infectious diseases worldwide. The etiological pathogen of TB is Mycobacterium tuberculosis (Mtb), a facultative intracellular bacterium. Mycobacterium bovis bacillus Calmette-Guérin (BCG) is the only approved vaccine against TB to date. BCG has been widely used, but the efficacy is questionable, especially in adult pulmonary TB. Therefore, more effective, safe and reliable TB vaccines have been urgently needed. T cell-mediated cellular immune response is a key immune response for effective protective immunity against TB. DNA vaccines using Mtb antigens have been studied as promising future TB vaccines. Most TB DNA vaccine studies so far reported used intramuscular or intradermal injection with needles, as these methods tend to induce a type 1 helper T lymphocyte (Th1)-type immune response that is critical for the protective immunity. We have been using DNA vaccines with gene gun bombardment for T cell epitope identification of various Mtb antigens. We show here our strategy to identify precise Mtb T cell epitopes using DNA vaccines with gene gun bombardment.

Vaccines against tuberculosis: where are we and where do we need to go?

In this review we discuss recent progress in the development, testing, and clinical evaluation of new vaccines against tuberculosis (TB). Over the last 20 years, tremendous progress has been made in TB vaccine research and development: from a pipeline virtually empty of new TB candidate vaccines in the early 1990s, to an era in which a dozen novel TB vaccine candidates have been and are being evaluated in human clinical trials. In addition, innovative approaches are being pursued to further improve existing vaccines, as well as discover new ones. Thus, there is good reason for optimism in the field of TB vaccines that it will be possible to develop better vaccines than BCG, which is still the only vaccine available against TB.

Analysis of the binding of gluten T-cell epitopes to various human leukocyte antigen class II molecules

Celiac disease is a prevalent disorder of the small intestine that is caused by an inflammatory reaction to dietary gluten in genetically susceptible individuals. More than 90% of patients express the HLA-DQ2 molecule, whereas DQ8 is carried by most of the remaining patients. DQ2- and DQ8-mediated presentation of gluten peptides to CD4+ T cells is a key event in the pathogenesis of the disease. The association of celiac disease with these human leukocyte antigen (HLA) molecules is explained by a preferential binding of gluten peptides to these HLA molecules, although the actual data on this in the literature are scarce. The objective of this study was to test this hypothesis. A panel of peptides representing DQ2-restricted gluten T-cell epitopes was tested for binding to various HLA class II molecules using various experimental approaches. The results demonstrate that the gluten T-cell epitopes mainly bind to the DQ2 molecule.

Crystallographic structure of the human leukocyte antigen DRA, DRB3*0101: models of a directional alloimmune response and autoimmunity

We describe structural studies of the human leukocyte antigen DR52a, HLA-DRA/DRB3*0101, in complex with an N-terminal human platelet integrin alphaII(B)betaIII glycoprotein peptide which contains a Leu/Pro dimorphism. The 33:Leu dimorphism is the epitope for the T cell directed response in neonatal alloimmune thrombocytopenia and post-transfusion purpura in individuals with the alphaII(B)betaIII 33:Pro allele, and defines the unidirectional alloimmune response. This condition is always associated with DR52a. The crystallographic structure has been refined to 2.25 A. There are two alphabeta heterodimers to the asymmetric unit in space group P4(1)2(1)2. The molecule is characterized by two prominent hydrophobic pockets at either end of the peptide binding cleft and a deep, narrower and highly charged P4 opening underneath the beta 1 chain. Further, the peptide in the second molecule displays a sharp upward turn after pocket P9. The structure reveals the role of pockets and the distinctive basic P4 pocket, shared by DR52a and DR3, in selecting their respective binding peptide repertoire. We observe an interesting switch in a residue from the canonically assigned pocket 6 seen in prior class II structures to pocket 4. This occludes the P6 pocket helping to explain the distinctive "1-4-9" peptide binding motif. A beta57 Asp-->Val substitution abrogates the salt-bridge to alpha76 Arg and along with a hydrophobic beta37 is important in shaping the P9 pocket. DRB3*0101 and DRB1*0301 belong to an ancestral haplotype and are associated with many autoimmune diseases linked to antigen presentation, but whereas DR3 is susceptible to type 1 diabetes DR52a is not. This dichotomy is explored for clues to the disease.

Phenotypic and Functional Characterization of CD4 T Cells Expressing Killer Ig-Like Receptors

Killer Ig-like receptors (KIR) are commonly found on human NK cells, gammadelta T cells, and CD8 T cells. Although KIR(+) CD4 T cells are found in certain patients, their prevalence in healthy donors is controversial. We now provide definitive proof that such cells are present in most individuals, and report on their frequency, surface phenotype, cytokine profile, and Ag specificity. The number of KIR(+) CD4 T cells detected in peripheral blood increased with age. In contrast with regular KIR(-) CD4 T cells, the majority of KIR(+) CD4 T cells lacked surface expression of CD27, CD28, CCR4, and CCR7, but did express CD57 and 2B4. In addition, KIR were detected on approximately one-tenth of CD28(-) and CD57(+) memory CD4 T cells. In line with the absence of the Th2 marker CCR4, the KIR(+) CD4 cells produced mainly IFN-gamma and little IL-4, IL-10, or IL-17 upon TCR triggering. Furthermore, the KIR(+) population contained cells that responded to recall Ags in an HLA class II-restricted fashion. Together, our data indicate that KIR-expressing CD4 T cells are predominantly HLA class II-restricted effector memory Th1 cells, and that a significant, previously unrecognized fraction of effector memory Th1 cells expresses KIR.

The HLA molecules DQA1*0501/B1*0201 and DQA1*0301/B1*0302 share an extensive overlap in peptide binding specificity

Assays to measure the binding capacity of peptides for HLA-DQA1*0501/B*0201 (DQ2.3) and DQA1*0301/B*0302 (DQ3.2) were developed using solubilized MHC molecules purified from EBV-transformed cell lines. These quantitative assays, based on the principle of the inhibition of binding of a high-affinity radiolabeled ligand, were validated by examining the binding capacity of known DQ-restricted epitopes or ligands. The availability of these assays allowed an investigation of patterns of cross-reactivity between different DQ molecules and with various common DR molecules. DQ2.3 and DQ3.2 were found to have significantly overlapping peptide binding repertoires. Specifically, of 13 peptides that bound either DQ2.3 or DQ3.2, nine (69.2%) bound both. The molecular basis of this high degree of cross-reactivity was further investigated with panels of single substitution analogs of the thyroid peroxidase 632-645Y epitope. It was found that DQ2.3 and DQ3.2 bind the same ligands by using similar anchor residues but different registers. These data suggest that in analogy to what was previously described for HLA-DR molecules, HLA-DQ supertypes characterized by largely overlapping binding repertoires can be defined. In light of the known linkage of both HLA-DQ2.3 and -DQ3.2 with insulin-dependent diabetes mellitus and celiac disease, these results might have important implications for understanding HLA class II autoimmune disease associations.

A 320-kilobase artificial chromosome encoding the human HLA DR3-DQ2 MHC haplotype confers HLA restriction in transgenic mice

MHC class II haplotypes control the specificity of Th immune responses and susceptibility to many autoimmune diseases. Understanding the role of HLA class II haplotypes in immunity is hampered by the lack of animal models expressing these genes as authentic cis-haplotypes. In this study we describe transgenic expression of the autoimmune prone HLA DR3-DQ2 haplotype from a yeast artificial chromosome (YAC) containing an intact similar320-kb region from HLA DRA to DQB2. In YAC-transgenic mice HLA DR and DQ gene products were expressed on B cells, macrophages, and dendritic cells, but not on T cells indicating cell-specific regulation. Positive selection of the CD4 compartment by human class II molecules was 67% efficient in YAC-homozygous mice lacking endogenous class II molecules (Abeta(null/null)) and expressing only murine CD4. A broad range of TCR Vbeta families was used in the peripheral T cell repertoire, which was also purged of Vbeta5-, Vbeta11-, and Vbeta12-bearing T cells by endogenous mouse mammary tumor virus-encoded superantigens. Expression of the HLA DR3-DQ2 haplotype on the Abeta(null/null) background was associated with normal CD8-dependent clearance of virus from influenza-infected mice and development of CD4-dependent protection from otherwise lethal infection with Salmonella typhimurium. HLA DR- and DQ-restricted T cell responses were also elicited following immunization with known T cell determinants presented by these molecules. These findings demonstrate the potential for human MHC class II haplotypes to function efficiently in transgenic mice and should provide valuable tools for developing humanized models of MHC-associated autoimmune diseases.

HLA-DR restricted peptide candidates for bee venom immunotherapy

T cell epitopes containing peptides have been recently proposed as an alternative to conventional immunotherapy of allergic diseases because they are expected to be better tolerated than allergen extracts. A principal limitation to their clinical use is that they present an important diversity, which primarily results from the polymorphism of HLA class II molecules. In Caucasian populations, however, seven alleles of the most expressed molecules (namely DRB1*0101, DRB1*0301, DRB1*0401, DRB1*0701, DRB1*1101, DRB1*1301, and DRB1*1501) predominate. Peptides from allergens that would efficiently bind to them should be potential candidates for specific immunotherapy. In this paper, we have determined the peptides present in the major bee venom allergen by investigating the capacity of synthetic peptides that encompass its whole sequence to bind to each allele. Several efficient binders have been identified and are either allele-specific or common to several HLA-DR molecules. Interestingly enough, the 81-97 sequence is universal in the sense that it binds to all studied molecules. This sequence is surrounded by several active regions, which make the 76-106 sequence particularly rich of binding determinants and a good candidate for specific immunotherapy. Statistical analyses of the binding data also provide an overview of the preponderant HLA-DR alleles specificity.

Binding of human thyrotropin receptor peptides to a Graves' disease-predisposing human leukocyte antigen class II molecule

There are many reports that Graves' disease (GD) is associated with certain human leukocyte antigen (HLA) molecules, in particular DR3. Here we examined the characteristics of binding of human TSH receptor (TSHR) peptides to this disease-associated HLA class II molecule. DR3 molecules bind TSHR immunodominant peptide epitopes with intermediate affinity. On the contrary, DR3 binds nonimmunogenic peptides either with poor affinity or not at all, with one exceptional peptide that has extremely high affinity. These results suggest that susceptibility to GD associated with inheritance of a specific HLA class II gene is due to the influence of the HLA molecule-TSHR peptide complex on the T cell repertoire.

T-Cell Expansions With Conserved T-Cell Receptor β Chain Motifs in the Peripheral Blood of HLA-DRB1*0401 Positive Patients With Necrotizing Vasculitis

T lymphocytes are implicated in the pathogenesis of systemic vasculitis such as Wegener’s granulomatosis (WG) and polyarteritis nodosa (PAN). In the present study, we have characterized in detail the T-cell receptor (TCR) of peripheral blood T cells from eight vasculitis patients of known HLA class II genotypes. We used flow cytometry to outline the exact TCR V gene expression, complementarity determining region 3 (CDR3) fragment analysis to estimate the degree of clonality and cDNA sequencing to define the exact TCR  or β chain sequences. The TCR CDR3 region interacts with antigenic peptides presented by HLA molecules, and it is normally immensely diverse. It was therefore of particular interest to identify a common dominating TCR BV8-F/L-G-G-A/Q-G-J2S3 β chain sequence in the CD4+T cells of four unrelated vasculitis patients. Furthermore, this BV8-associated CDR3 motif was linked to the HLA-DRB1*0401 allele, as well as to active disease and/or an established BV8+ CD4+ T-cell expansion. In contrast, age- and HLA-matched patients with rheumatoid arthritis did not harbor the described BV8 motif. These results strongly suggest that BV8+ CD4+ T cells with the described CDR3 motif recognize a specific antigen presented by DR4 molecules, indicating the existence of a common vasculitis-associated antigen.

T-Cell Expansions With Conserved T-Cell Receptor β Chain Motifs in the Peripheral Blood of HLA-DRB1*0401 Positive Patients With Necrotizing Vasculitis

T lymphocytes are implicated in the pathogenesis of systemic vasculitis such as Wegener’s granulomatosis (WG) and polyarteritis nodosa (PAN). In the present study, we have characterized in detail the T-cell receptor (TCR) of peripheral blood T cells from eight vasculitis patients of known HLA class II genotypes. We used flow cytometry to outline the exact TCR V gene expression, complementarity determining region 3 (CDR3) fragment analysis to estimate the degree of clonality and cDNA sequencing to define the exact TCR  or β chain sequences. The TCR CDR3 region interacts with antigenic peptides presented by HLA molecules, and it is normally immensely diverse. It was therefore of particular interest to identify a common dominating TCR BV8-F/L-G-G-A/Q-G-J2S3 β chain sequence in the CD4+T cells of four unrelated vasculitis patients. Furthermore, this BV8-associated CDR3 motif was linked to the HLA-DRB1*0401 allele, as well as to active disease and/or an established BV8+ CD4+ T-cell expansion. In contrast, age- and HLA-matched patients with rheumatoid arthritis did not harbor the described BV8 motif. These results strongly suggest that BV8+ CD4+ T cells with the described CDR3 motif recognize a specific antigen presented by DR4 molecules, indicating the existence of a common vasculitis-associated antigen.

An epitope delivery system for use with recombinant mycobacteria

We have developed a novel epitope delivery system based on the insertion of peptides within a permissive loop of a bacterial superoxide dismutase molecule. This system allowed high-level expression of heterologous peptides in two mycobacterial vaccine strains, Mycobacterium bovis bacille Calmette-Guérin (BCG) and Mycobacterium vaccae. The broader application of the system was analyzed by preparation of constructs containing peptide epitopes from a range of infectious agents and allergens. We report detailed characterization of the immunogenicity of one such construct, in which an epitope from the Der p1 house dust mite allergen was expressed in M. vaccae. The construct was able to stimulate T-cell hybridomas specific for Der p1, and it induced peptide-specific gamma interferon responses when used to immunize naive mice. This novel expression system demonstrates new possibilities for the use of mycobacteria as vaccine delivery vehicles.

HLA-DR/DQ transgenic, class II deficient mice as a novel model to select for HSP T cell epitopes with immunotherapeutic or preventative vaccine potential

Protective immunity against mycobacteria is dependent on antigen/MHC class II specific, CD4+ Th1 cells. HLA-DR3-restricted Th1 cells respond to a subset of mycobacterial antigens, including the immunodominant hsp65, and recognize a single epitope in hsp65, notably p1-20. Altered peptide ligands (APL) of p1-20 can inhibit p1-20/hsp65-induced proliferation of DR3-restricted T cells in an allele specific manner in vitro. In order to develop a preclinical model in which p1-20 APL can be tested in vivo in the context of HLA, we have used murine class II deficient, HLA transgenic (Ab0) mice, in which all CD4+ T cells are restricted by the tg HLA molecule. BCG-immunized DR3.Ab0 and DQ8.Ab0 mice both responded well to hsp65. Furthermore, DR3.Ab0 mice recognized precisely the same p1-20 epitope as DR3-restricted human T cells, whereas DQ8.Ab0 mice responded to a different set of hsp65 peptides. This shows that (i) the same immunodominant protein and peptide epitope are recognized by T cells from DR3.Ab0 mice and DR3+ humans and (ii) indicates the major role of HLA-polymorphism in controlling the human T cell response to mycobacterial antigens. Thus, HLA-transgenic, Ab0 mice provide a novel, preclinical model system to analyze APL and vaccines in the context of HLA polymorphism.

Human monoclonal antibody with T-cell-like specificity recognizes MHC class I self-peptide presented by HLA-DR1 on activated cells

Alloreactive T cells recognize peptides presented in the binding groove of major histocompatibility complex molecules (MHCs), whereas B cells mainly recognize the MHCs independent of bound peptides. Here, we demonstrate that the human B-cell repertoire comprises B cells which can be stimulated during pregnancy to produce antibodies reacting with MHCs in a way similar to T cells. The human monoclonal antibody UL-5A1 recognizes DR1(DRA/DRB1*0101) molecules on lymphoblastoid cell lines only if they co-express HLA-A2 or if they have been loaded with HLA-A2-derived peptides. The effect of the HLA-A2 peptide 105-117 on UL-5A1 reactivity was specific, time and dose-dependent. Reactivity increased when naturally processed peptides were removed from DR1 molecules before the HLA-A2 peptide 105-117 was loaded. UL-5A1 reacted specifically with cells that had been activated. The results imply a role of activation of cells in peptide processing and/or loading.

Liposome-mediated peptide loading of MHC-DR molecules in vivo

Amino acid residues 3-15 of mycobacterial HSP60 define a dominant T-cell epitope for HLA-DR3+ve humans and Mamu-DR3+ve rhesus monkeys. Our results show that Mamu-DR3 molecules on PBMC can be efficiently loaded in vivo with the above-mentioned peptides when they are intravenously injected encapsulated in liposomes, but not in the free form. Mamu-DR3 loading is abolished by encapsulation of a nonstimulatory peptide. These results have implications for the delivery of therapeutic peptides in vivo.

A DR17-restricted T cell epitope from a secreted Mycobacterium tuberculosis antigen only binds to DR17 molecules at neutral pH

The assembly of peptide-major histocompatability class II complexes in vitro is accelerated at low pH, comparable to that found in the intracellular compartments of metabolically active antigen-presenting cells (APC). Mycobacteria such as Mycobacterium tuberculosis reside in phagosomes with only mildly acidic pH. Therefore, we investigated the pH dependency of peptide-HLA-DR binding for several T cell epitopes of mycobacterial proteins, focussing particularly on well-defined, immunodominant HLA-DR17(3)-restricted T cell epitopes: peptide (p) 3-13 from the cytoplasmic 65-kDa heat shock protein of M. tuberculosis/M. leprae, and peptide 56-65 from the secreted 30/31-kDa protein from M. tuberculosis/M. leprae. p3-13 bound to purified, cell-free DR17 under both acidic and neutral conditions. Four other, unrelated DR17-binding peptides showed the same pH-dependent binding characteristics as p3-13. p56-65, however, only bound to purified DR17 at pH 7 but not at all at pH 4.5. These DR17 peptide binding data were confirmed in cell-bound DR17, in T cell stimulation assays in which fixed APC were peptide-pulsed at acidic or neutral pH before addition of peptide-specific DR17-restricted T cells. As far as we are aware, p56-65 is the only human T cell epitope binding to HLA exclusively at neutral pH. The binding characteristics of p56-65 may reflect dominant processing in alternative, less acidic vacuolar compartments specifically related to the generation of epitopes from (secreted) mycobacterial proteins. The observation that p56-65 is an immunodominant epitope for anti-mycobacterial T cells suggests the relevance of such novel processing compartments in T cell-mediated immunity.

Epitope specificity of anti-heat shock protein 65/60 serum antibodies in atherosclerosis

Levels of specific antibodies (Ab) against mycobacterial and human heat shock protein (hsp) 65/60 are increased in the sera of patients with atherosclerotic lesions and have been demonstrated to be capable of mediating endothelial cytotoxicity. To clarify the antigen epitopes recognized by these serum Abs, Ab binding to hsp65 deletion mutants (Dms), as well as to overlapping 15-mer and 8-mer hsp65 peptides, was assessed. Western blotting of hsp65 Dms indicated the presence of at least one epitope between amino acid (aa) residues 171 and 276, recognized by both high-titer sera and affinity-purified anti-hsp65/60 Ab. Fluorescence immunoassays using 53 15-mer peptides and Pin ELISA using 526 7-mer peptides demonstrated three distinct, conserved sequences with high affinity to high-titer sera and purified anti-hsp65/60 Ab. Two N-terminal sequences, aa 97-109 and aa 179-187, and one C-terminal sequence, aa 504-512, were identified. These three epitopes recognized by anti-hsp65/60 Ab may serve as autoantigens in certain circumstances in vivo. This phenomenon could contribute to the initiation of atherosclerosis by an autoimmune reaction.

Flexibility in T-cell receptor ligand repertoires depends on MHC and T-cell receptor clonotype

T-cell receptors (TCR) recognize peptides complexed to self-major histocompatibility complex (MHC) molecules. Recognition of peptide/MHC ligands by the TCR is highly peptide specific. However, certain TCRs can also recognize sequence-related and -unrelated ('mimicry') epitopes presented by homologous MHC molecules. Using two human, human leucocyte antigen-DR1 (HLA-DR1)-restricted T-cell clones specific for HA p307-319, we identified several diverse combinations of peptide-MHC complexes that are functionally equivalent in their ability to trigger T-cell stimulation. These findings demonstrate that a single TCR can productively interact with different peptides complexed to self- as well as non-self-MHC molecules. This extended reactivity is human leucocyte antigen (HLA) allele and TCR clonotype dependent, as the peptide repertoire recognized depends on the presenting HLA-DR molecule and varies among different TCRs that both recognize the HA p307-319/DR1 complex. Importantly, certain peptide analogues can completely change the HLA-restriction pattern of the TCR: T-cell recognition of the wild-type peptide that was absent in the context of a non-self HLA-DR molecule, was restored by complementing substitutions in altered peptide ligands, that could not be presented by the original restriction element. This mechanism may play an important role in allorecognition.

A sensitive fluorometric assay for quantitatively measuring specific peptide binding to HLA class I and class II molecules

A sensitive, highly reproducible assay was developed for measuring binding of peptides to various HLA class I and II alleles. The assay is based on competition for binding to HLA between a peptide of interest and a fluorescent labelled standard peptide. This mixture is incubated with HLA to obtain equilibrium binding, and subsequently separated on an HPLC size-exclusion column in (i) a protein fraction containing HLA and bound peptide and (ii) a free peptide fraction. Each assay uses only 100 fmol labelled peptide and approximately 10 pmol of HLA. The analytical system contains an autosampler that samples from 96-well microtiter plates. Injections and data recording/evaluation is fully automated. Typical analysis time is 10-12 min per sample. The fluorescence in the HLA-bound peptide and free peptide containing fractions is measured on-line. The ratios of fluorescence signal in protein and peptide fractions at various concentrations of the peptide of interest are determined. IC50 values are calculated from the binding curve as obtained by curve fitting of the data. Here we show results for peptide binding to HLA-DR1 and -DR17 molecules purified from detergent solubilized cell lysates. and for recombinant HLA-A*0201 and HLA-A*0301 expressed in E. coli. The assay reported is sensitive and reproducible. It is non-radioactive and is non-labor intensive due to the high degree of automation.

Peptide specificity of alloreactive CD4 positive T lymphocytes directed against a major histocompatibility complex class I disparity

The mouse strains C57BL/6 (B6, H2b) and Kbm1 mutant bm1 have a defined difference of three amino acids at position 152, 155, and 156 in the MHC class I K molecule. This causes a change in the side and the bottom of the antigen presenting groove of the K molecule resulting in strong allogeneic responses in vitro and in vivo. Here we report on the peptide specificity of CD4+ T cells of B6 origin directed against the Kbm1 mutant and speculate on the peptide specificity of CD8+ bm1-specific T lymphocytes of B6 origin. Bm1-specific CD4+ T helper cells recognized a peptide derived from the Kbm1 molecule encompassing the three mutations, presented by MHC class II molecules on syngeneic cells. The ability of this peptide to bind to MHC class II resulted from amino acid mutations at positions 155 and 156. Furthermore, the recognition of the natural peptide derived from the Kbm1 molecule presented by MHC class II I-Ab molecules on cells of bml origin could be blocked by addition of an MHC class II I-Ab binding competitor peptide. Thus, due to the mutations in an MHC class I molecule, indirect presentation via MHC class II molecules and MHC class II-restricted recognition of a peptide derived from such a MHC class I molecule is demonstrable.

The impact of DR3 microvariation on peptide binding: the combinations of specific DR beta residues critical to binding differ for different peptides

HLA-DR molecules are a group of highly polymorphic glycoprotein heterodimers that present peptide antigens to T lymphocytes for immune surveillance. To assess the significance of limited polymorphism on the functional differentiation of DR molecules, the binding of several immunogenic peptides to the DR3 microvariants [DR(alpha, beta 1*0302) and DR(alpha, beta 1*0301)] and to mutants of these DR3 molecules was examined. This analysis has shown that each residue (DR beta 26, DR beta 28, DR beta 47, and DR beta 86), which differentiates these two DR3 molecules, contributes to their functional distinction and that the relative contribution of each residue varies for different peptide/DR3 complexes. For example, DR beta 28 and DR beta 86 controlled the mycobacterium tuberculosis 65-kD heat shock protein peptides 3-13 and 4-15 (HSP) binding specificity to DR (alpha, beta 1*0301). [HSP does not bind to DR(alpha, beta 1*0302)], whereas DR beta 26, DR beta 28, and DR beta 86 controlled the influenza hemagglutinin peptide 306-318 (HA) binding specificity to DR(alpha, beta 1*0302). [HA does not bind to DR(alpha, beta 1*0301).] In comparison, DR beta 86 alone controlled the binding level difference of sperm whale myoglobin peptide 132-151 (SWM) and of myelin basic protein peptide 152-170 (MBP) [both bind to DR(alpha, beta 1*0301) at levels five times greater than to DR(alpha, beta 1*0302)] to the DR3 molecules. Although not critical, additional DR beta residues influenced the binding level of individual peptides of each of the DR3 molecules and, again, the combinations of these residues differed for different peptide/DR3 complexes. These data showed that individual DR residues vary in their relative contribution to the interaction between a specific DR molecule and different peptides and that limited polymorphism can create substantial differences in the peptide binding profiles among DR molecules.

The invariant chain derived fragment CLIP is an efficient in vitro inhibitor of peptide binding to MHC class II molecules

The invariant chain derived peptide CLIP inhibits association of peptides to the class II peptide binding site. Two DR3 specific peptides, the microbacterial heat shock protein 65 derived peptide hsp3-13 and the naturally occurring invariant chain derived peptide Ii131-149 were employed to study binding inhibition by CLIP (Ii82-102) in a series of combinations. Incubation of detergent solubilized DR polypeptides from Ii-free cells with 500 microM of synthetic CLIP almost completely prevents binding of 50 microM subsequently added DR3-specific peptides. When CLIP and the peptides were added simultaneously to DR3 molecules, binding of hsp3-13 was abolished, whereas binding of Ii131-149 was only partially blocked. This indicates apparent affinity differences of the peptides. The addition of CLIP to preformed DR-peptide complexes substantially reduced binding of hsp3-13,while there was little effect on the DR associated Ii131-149. The profound inhibitory ability of CLIP, which in vivo would diminish binding of antigenic peptides, suggests an intracellular mechanism that abrogates the persistence of the CLIP-DR complex. The HLA-DM molecules have been suggested as candidates for this function. The strong in vitro binding of the naturally occurring peptide Ii131-149 to DR3 may suggest that only limited amounts of this peptide are available in vivo for competition of exogenous peptide binding to class II molecules.

Epitopes for human CD4+ cells on diphtheria toxin: structural features of sequence segments forming epitopes recognized by most subjects

The sequence regions of diphtheria toxin (DTX) recognized by CD4+ T cells of seven healthy humans of different major histocompatibility complex haplotypes were identified. Overlapping synthetic peptides, screening the DTX sequence, were used to test in proliferation assays unselected blood CD4+ cells, or DTX-specific CD4+ lines propagated by stimulation with DTX of blood mononuclear cells. Blood CD4+ cells and DTX-specific CD4+ lines gave consistent results. Although each subject had an individual pattern of peptide recognition, six peptide sequences (residues 271-290, 321-340, 331-350, 351-370, 411-430 and 431-450) were recognized by all subjects. In the native DTX molecule, these sequence regions are flanked by sequence loops exposed on the DTX surface. They overlap uncharged segments of the DTX sequence. These structural properties may be general requirements for immunodominance in CD4+ cell sensitization in humans.

Clonal dominance and selection for similar complementarity determining region 3 motifs among T lymphocytes responding to the HLA-DR3-associated Mycobacterium leprae heat shock protein 65-kd peptide 3-13

In order to establish whether specific MHC class II-peptide complexes are capable of selecting TCR V regions, we investigated in detail the TCR beta chain used in the recognition of HLA-DR3 restricted hsp65 peptide 3-13 in a tuberculoid leprosy patient. Using RT-PCR, a clear dominance of the TCRBV5 gene family was observed in a hsp65 peptide 3-13-specific T-cell line; however, not in fresh, unstimulated PBMCs, PHA-stimulated PBMCs, or a T-cell line specific for tetanus toxoid. DNA sequence analysis of the TCR V regions, comprising TCRBV5 genes, derived from the hsp65 peptide 3-13-specific T-cell line revealed the exclusive usage of the TCRBV55S1 gene segment and a predominance of one V-D-J gene rearrangement, which is indicative of clonal expansion of these T lymphocytes. Additional highly similar V-D-J gene rearrangements were detected at a low level in this hsp65 peptide 3-13 specific T-cell line. These conserved junctional regions (CDR3 regions) could not be detected within the TCRBV5 gene family of fresh PBMCs, PHA-stimulated PBMCs, hsp65, and tetanus-toxoid-specific T-cell lines from this patient. The observations in this tuberculoid leprosy patient reveal that an HLA class-II-restricted T-cell response results in selection of TCRBV regions which are highly similar in amino acid composition to the CDR3 region within the expanding TCRBV regions.

HLA-DM induces CLIP dissociation from MHC class II alpha beta dimers and facilitates peptide loading

Human leukocyte antigen DM (HLA-DM) molecules are structurally related to classical MHC class II molecules and reside in the lysosome-like compartment where class II-restricted antigen processing is thought to occur. Mutant cell lines lacking HLA-DM are defective in antigen processing and accumulate class II molecules associated with a nested set of invariant chain-derived peptides (class II-associated invariant chain peptides, CLIP). Here we show that HLA-DM catalyzes the dissociation of CLIP from MHC class II-CLIP complexes in vitro and facilitates the binding of antigenic peptides. The reaction has an acidic pH optimum, consistent with its occurrence in a lysosome-like compartment in vivo. Antibody blocking experiments suggest that a transient interaction between HLA-DM and the MHC class II-CLIP complex is required.

Accuracy of a structural homology model for a class II histocompatibility protein, HLA-DR1: comparison to the crystal structure

Structural homology modeling is used to test the accuracy by which a Class I major histocompatibility complex (MHC) could be used to model a Class II MHC. The crystal structure of HLA-aw68 served as a reference molecule to model HLA-DR1. The resulting model was compared to the recently released crystal structure by Brown et al. (Nature, Vol. 364, p. 33-39 (1993)). The overall tertiary structure motif (two alpha-helices and a beta-sheet forming a peptide binding cleft) was maintained. However, significant deviations in the secondary structure elements were found between the model and the DR1 crystal structure. These deviations were consistent with the differences between Class I and Class II crystal structures. In regions where the model and DR1 crystals structures are most similar, side chain orientations are also similar. Specific peptide-MHC interactions are discussed and compared with the crystal structure results.

Differences in peptide binding of DR11 and DR13 microvariants demonstrate the power of minor variation in generating DR functional diversity

The influence of subtle HLA diversification on antigen binding was explored using murine L-cell transfectants expressing alleles in the DR11/DR13 family and a panel of peptides. The levels of binding among this family of DR microvariants were as diverse as the levels of binding among distantly related DR molecules. Even a single amino acid difference between allelic products had a profound effect on peptide binding. Specific amino acid substitutions, generated using site-directed mutagenesis to alter polymorphic residues at DR beta 32, 37, 57, 58, 67, 71, 86, demonstrated that a specific change within the context of a single DR molecule differed in its effect on the binding of specific peptides. In addition, a specific amino acid substitution had a differential effect on the binding level of a peptide to different DR molecules. Each polymorphic amino acid appeared to play a role in the binding of some peptide. Studies using the amino-terminal portion of the invariant chain CLIP peptide suggested that this peptide may offer varying degrees of competition in the binding of the cellular peptide pool in cells expressing different DR molecules. Finally, the results obtained with two strain-specific peptides from an immunodominant region of a malarial parasite show differential binding to two DR13 molecules, suggesting that immune pressure may promote parasite diversity. A dynamic interaction may exist between pathogens and the immune system shaping the HLA profile in a population. Thus even subtle diversification of the HLA molecules, possibly pathogen driven, can have a substantial effect on peptide binding and immune recognition.

Pocket 4 of the HLA-DR(alpha,beta 1*0401) molecule is a major determinant of T cells recognition of peptide

To investigate the functional roles of individual HLA-DR residues in T cell recognition, transfectants expressing wild-type or mutant DR(alpha,beta 1*0401) molecules with single amino acid substitutions at 14 polymorphic positions of the DR beta 1*0401 chain or 19 positions of the DR alpha chain were used as antigen-presenting cells for five T cell clones specific for the influenza hemagglutinin peptide, HA307-19. Of the six polymorphic positions in the DR beta floor that were examined, mutations at only two positions eliminated T cell recognition: positions 13 (four clones) and 28 (one clone). In contrast, individual mutations at DR beta positions 70, 71, 78, and 86 on the alpha helix eliminated recognition by each of the clones, and mutations at positions 74 and 67 eliminated recognition by four and two clones, respectively. Most of the DR alpha mutations had minimal or no effect on most of the clones, although one clone was very sensitive to changes in the DR alpha chain, with loss of recognition in response to 10 mutants. Mutants that abrogated recognition by all of the clones were assessed for peptide binding, and only the beta 86 mutation drastically decreased peptide binding. Single amino acid substitutions at polymorphic positions in the central part of the DR beta alpha helix disrupted T cell recognition much more frequently than substitutions in the floor, suggesting that DR beta residues on the alpha helix make relatively greater contributions than those in the floor to the ability of the DR(alpha,beta 1*0401) molecule to present HA307-19. The data indicate that DR beta residues 13, 70, 71, 74, and 78, which are located in pocket 4 of the peptide binding site in the crystal structure of the DR1 molecule, exert a major and disproportionate influence on the outcome of T cell recognition, compared with other polymorphic residues.

Microvariation creates significant functional differences in the DR3 molecules

Two DR3 molecules differ by four amino acids whose side chains point into the DR antigen-binding groove. To begin to assess the role of microvariation on DR3 function, DRB1*0302 residues were replaced with DRB1*0301 residues at beta-chain positions 26, 47, 86, and 47 plus 86. Murine fibroblast cell lines expressing DR(alpha, beta 1*0301), DR(alpha, beta 1*0302), and the four mutant 0302 molecules were examined for alloproliferative DR(alpha, beta 1*0302)-specific TLC stimulation and peptide binding. Changing position 26 had the most profound effect on T-cell recognition (seven of nine TLCs did not respond). Two TLCs did not respond to the mutant 0302V86 molecule and four TLCs that did respond to this mutant lost responsiveness when positions 47 and 86 were mutated together. These data suggest that each of these variant residues, including position 47, influence T-cell recognition. Surprisingly, none of the mutations had an effect on the absolute binding of HA 307-319 (DR[alpha, beta 1*0302] specific) and HSP 3-13 (DR[alpha, beta 1*0301] specific); however, the mutant 0302 molecules changed at position 86 (glycine to valine) consistently bound HA 307-319 at significantly higher levels than DR(alpha, beta 1*0302). These data for position 86 are in contrast to other DR molecules and indicate that peptide contact residues for a specific DR molecule cannot be predicted based on binding results obtained with other DR molecules. These data suggest that each of these variant groove residues, although not accessible to the TCR, contribute to the significant functional differences between the DR3 microvariants through subtle influences on the DR3-peptide complex.

T cell receptor and peptide-contacting residues in the HLA-DR17(3) beta 1 chain

Previously, we have proposed that the beta 1 residues 9-13, 26, 28 and 86 in HLA-DR17, the most common subtype of DR3, might be critical for the binding of an immunodominant, mycobacterial epitope (peptide 3-13 of the 65-kDa heat shock protein). In order to examine directly (i) which DR17 residues are involved in peptide binding, (ii) whether the same or other DR17 residues are involved in the binding of different peptides, and (iii) whether subtle differences in the mode of peptide binding can influence T cell stimulation, we have now systematically mutated 15 highly polymorphic DR17 beta 1 residues, located in the proposed peptide binding groove of DR17, and examined the effect thereof on binding and presentation of two peptides, hsp65 p3-13 and p56-65 of the 30/31-kDa secreted mycobacterial protein. Mutations in residues 28 (D-->H) and 86 (V-->G) completely eliminated binding of p3-13 and significantly reduced binding of p56-65. A mutation in residue 26 (Y-->F) decreased binding of p3-13 but did not affect binding of p56-65. Substitutions of amino acid residues 28, 67, 71 and 86 in the DR17 beta 1 chain abrogated peptide-specific stimulation of both the p3-13- and the p56-65-specific T cell clones, while specific stimulation by only one peptide was eliminated by substitution at positions 26 and 74 (p3-13) and by substitution of residues 11 and 37 (p56-65). The observation that substitution of several other peptide-contacting DR17 beta 1 chain residues does not significantly affect peptide binding but does affect T cell stimulation, suggests that these substitutions alter the conformation of the bound peptide.

Development of high potency universal DR-restricted helper epitopes by modification of high affinity DR-blocking peptides

Pan DR-binding peptides engineered by introducing anchor residues for different DR motifs within a polyalanine backbone bound 10 of 10 DR molecules tested, with affinities, in most cases, in the nanomolar range. Because of the small methyl group exposed for T cell recognition, these peptides were poor immunogens but effective blockers of DR-restricted antigen presentation. Introduction of bulky and charged residues at positions accessible for T cell recognition yielded extremely powerful Pan DR epitope peptides (PADRE). These peptides elicited powerful responses in vitro from human peripheral blood mononuclear cells (PBMC). Because these cells also cross-react on certain mouse class II alleles, we could also demonstrate that PADRE peptides are active in vivo. In one example of their capacity to elicit T help, they were approximately 1000 times more powerful than natural T cell epitopes. We propose that PADRE peptides may be useful in the development of subunit vaccines.

Definition of a human suppressor T-cell epitope

The quality of the response produced by regulatory or helper T (Th) cells presently receives much attention because of its possible implications for vaccine development and immunomodulation. Apart from cytokines and so-called costimulatory signals, antigens and the presenting major histocompatibility complex (MHC) molecules may play a role in determining the type of T-cell response generated toward antigens. To examine the role of antigen and/or HLA in control of T-cell subset activation, we have studied a special case, namely CD4+ suppressor T (Ts) cells in leprosy. Mycobacterium leprae-induced Ts cell clones have been previously isolated from peripheral blood and skin lesions of lepromatous leprosy patients and were shown to specifically down-regulate mycobacterium-specific Th cell responses. Despite considerable effort, the antigens recognized by these Ts cells have thus far not been identified. Here we report that all HLA-DR2-restricted CD4+ Ts cell clones derived from a lepromatous leprosy patient recognize an epitope that maps between the amino acid residues 439 and 448 of the mycobacterial hsp65. The peptide was presented to these Ts cells by HLA-DRB1*1503, a recently discovered HLA-DR2 variant. Non-suppressor T-cell clones derived from the same patient recognized antigens other than the hsp65 and were also stimulated by other HLA-DR2 variants. In independent cloning experiments peptide 435-449 and recombinant hsp65 induced exclusively Ts cells in this lepromatous leprosy patient. The Ts clones recognizing this particular epitope were derived from at least seven different progenitors, as they expressed different T-cell receptor alpha and beta chains. Thus, our data indicate that a specific peptide-HLA class II combination may exclusively activate Ts cells.

Identification of an HLA-DQ2 peptide binding motif and HLA-DPw3-bound self-peptide by pool sequencing

Molecules of the major histocompatibility complex (MHC) present antigenic peptides to T cells. Sequencing peptide pools eluted from MHC class I molecules has established allele-specific peptide binding motifs. We applied pool sequencing to analyze human MHC class II-bound peptides and found that HLA-DQ2-eluted peptides predominantly contained lysine, isoleucine, and phenylalanine at relative position i, i + 3 and i + 8, respectively. These residues putatively represent anchor residues for MHC binding. Analysis of a heterogeneous HLA-DPw3/DPw4-eluted peptide pool yielded a sequence matching an epitope from the endogenous enzyme glyceraldehyde-3-phosphate dehydrogenase. This self-peptide and a partially identical, known allo-epitope bound specifically to DPw3 and DR13 molecules, suggesting the sharing of a binding motif. In particular, the presence of an arginine at relative position 4 appeared important for binding to these HLA class II specificities. Thus, pool sequencing is applicable for the analysis of MHC class II-eluted peptides.

A mutant human histocompatibility leukocyte antigen DR molecule associated with invariant chain peptides

From a human histocompatibility leukocyte antigen (HLA)-DR/DQ hemizygous, B lymphoblastoid progenitor, we isolated a cell line, 10.24.6, with a DR alpha missense mutation (96P-->96S), which results in an N-linked carbohydrate addition at position 94 in the DR alpha 2 domain. Several features of 10.24.6 cells suggest that the mutation disrupts normal intracellular formation of peptide/DR complexes. The mutant HLA-DR dimers, though expressed at the cell surface, lack the conformation of the mature, peptide-loaded class II molecules of the progenitor cell, as assessed by their loss of binding of certain antibodies and by the lack of stability in detergent (sodium dodecyl sulfate) solution. In addition, presentation of endocytosed antigen to HLA-DR-restricted T cells is defective in the mutant, but can be restored by transfection of a wild type DRA gene. Assays with synthetic peptides indicate that the 10.24.6 phenotype is not due to an intrinsic inability of the mutant DR molecules to bind peptides. Therefore, to directly evaluate peptide occupancy of the mutant molecules, we analyzed acid-eluted, HLA-DR-associated peptides. The predominant species from the 10.24.6 mutant is a nested set of invariant chain (Ii)-derived peptides that are undetectable in the DR eluate from progenitor cells. The region of DR alpha altered in the mutant molecules is thus implicated in normal formation of peptide/DR complexes. Further, the same set of Ii peptides associated with the DR molecules is present in the eluate from an antigen presentation mutant with a defect in an major histocompatibility complex (MHC)-linked gene. These results suggest that DR molecules in 10.24.6 and in certain presentation mutants are affected at the same or related steps in class II molecule biosynthesis, raising the possibility that class II molecules interact with an MHC-encoded accessory molecule during antigen presentation.

The biologic importance of conserved major histocompatibility complex class II motifs in primates

Phylogenetic comparisons of polymorphic second-exon sequences of MHC class II DRB genes showed that equivalents of the HLA-DRB1*03 alleles are present in various nonhuman primate species such as chimpanzees, gorillas, and rhesus macaques. These alleles must root from ancestral structure(s) that were once present in a progenitor species that lived about 35 million years ago. Due to accumulation of genetic variation, however, sequences that cluster into a lineage are generally unique to a species. To investigate the biologic importance of such conservation and variation, the peptide-binding capacity of various Mhc-DRB1*03 lineage members was studied. Primate Mhc-DRB1*03 lineage members successfully binding the p3-13 peptide of the 65-kD heat-shock protein of Mycobacterium tuberculosis/leprae share a motif that maps to the floor of the peptide-binding site. Apart from that, some rhesus macaque MHC class-II-positive cells were able to present the p3-13 peptide to HLA-DR17-restricted T cells whereas cells obtained from great ape species failed to do so. Therefore, these studies open ways to understand which MHC polymorphisms have been maintained in evolution and which MHC residues are essential for peptide binding and T-cell recognition.

Evolutionary conservation of major histocompatibility complex-DR/peptide/T cell interactions in primates

Many major histocompatibility complex (MHC) polymorphisms originate from ancient structures that predate speciation. As a consequence, members of the Mhc-DRB1*03 allelic lineage are not only present in humans but in chimpanzees and rhesus macaques as well. This emphasizes that Mhc-DRB1*03 members must have been present in a common ancestor of these primate species that lived about 30 million years ago. Due to the accumulation of genetic variation, however, alleles of the Mhc-DRB1*03 lineage exhibit species-unique sequences. To investigate the biological importance of such conservation and variation, we have studied both the binding and antigen presentation capacity of various trans-species Mhc-DRB1*03 lineage members. Here we show that p3-13 of the 65-kD heat-shock protein (hsp65) of Mycobacterium leprae and M. tuberculosis binds not only to HLA-DR17(3) but also to some chimpanzee and rhesus macaque class II-positive cells. Comparison of the corresponding human, chimpanzee, and rhesus macaque Mhc-DRB1*03 lineage members revealed the presence of uniquely shared amino acid residues, at positions 9-13 and 26-31, of the antigen-binding site that are critical for p3-13 binding. In addition it is shown that several nonhuman primate antigen-presenting cells that bind p3-13 can activate HLA-DR17-restricted T cells. Certain amino acid replacements, however, in Mhc-DRB1*03 lineage members did not influence peptide binding or T cell recognition. Therefore, these studies demonstrate that some polymorphic amino acid residues (motifs) within the antigen-binding site of MHC class II molecules that are crucial for peptide binding and recognition by the T cell receptor have been conserved for over 30 million years.

Functional analysis of the antigen binding region of an MHC class II molecule

The MHC class II molecules bind antigenic peptides and present them to T cells. Their ability to carry out these functions depends, in a critical way, on the detailed structure of the membrane-distal alpha 1 and beta 1 domains of these molecules. Using the I-Ak molecule and a series of hen egg lysozyme (HEL) peptide-specific, I-Ak-restricted T cell hybridomas as a model, we have examined the effect of altering essentially all of the polymorphic residues of the murine class II molecule on its ability to present Ag. Our results support the following conclusions: (1) both the location and the structural alteration introduced in a specific amino acid interchange are important in determining the effect the interchange will have on Ag presentation; and (2) changes in amino acids in the floor of the putative Ag binding cleft of the class II molecule can exert a major influence on the presentation of peptides to T cells. By carrying out direct binding experiments between the HEL(46-61) peptide and two mutant I-A molecules that fail to present HEL(46-61) to appropriate T cells, we were able to assess, in a quantitative fashion, the role played by peptide binding in the failure to present Ag. Our results suggest that, in the two cases studied, the failure to bind the HEL(46-61) peptide was not primarily responsible for the failure of the mutant class II molecule to present that peptide. Specifically, an A beta chain mutant that possesses d allelic residues at positions 65-67 in the second PMR of the Ak beta chain actually binds HEL(46-61) at wild type (I-Ak) levels. In contrast, an A alpha chain chimera in which b allelic residues are inserted in the third PMR of the Ak alpha chain, binds HEL(46-61) about three- to four-fold less well than wild type. While this decrease in binding affinity may be partially responsible for the inability of the latter chimeric molecule to present HEL(46-61), it can not be the total explanation because increasing the peptide concn even by an order of magnitude does not restore Ag presentation by APC expressing this chimeric molecule. These results are discussed in terms of the currently accepted model of the class II molecule.

Factors governing the binding and recognition of foreign and self-peptides by MHC class II

Considerable progress has been made in understanding the basis of T cell recognition and T cell activation. This knowledge has recently been used to modulate T cell activation in animal models of experimental autoimmune disease by two means--selective MHC blockade and peptide-induced tolerance. The use of peptides to interfere with the binding of autoantigenic peptides to MHC requires knowledge of both the class II allele which presents the immunodominant peptide to autoimmune T cells and the identification of peptide analogs that bind with high affinity to that allele. The alternative strategy of peptide-induced tolerance will require identification of the autoantigen and its immunodominant peptides. While the latter approach holds great promise for immunointervention, its wide application will require full knowledge of the mechanisms by which tolerance to self is maintained and how it can be broken.

HLA-DR molecules from an antigen-processing mutant cell line are associated with invariant chain peptides

The invariant chain, which associates with the major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum, serves two functions important in antigen processing. First, it prevents class II molecules from binding peptides in the early stages of intracellular transport. Second, it contains a cytoplasmic signal that targets the class II-invariant chain complex to an acidic endosomal compartment. Proteolytic cleavage and subsequent dissociation of the invariant chain then occurs, allowing peptides derived from endocytosed proteins to bind to released class II molecules before their expression at the cell surface. Certain human cell lines that are mutant in one or more MHC-linked genes are defective in class II-restricted antigen processing. Here we show that in transfectants of one of these cell lines, T2, this deficiency results in the association of a large proportion of class II molecules with a nested set of invariant-chain-derived peptides (class II-associated invariant chain peptides, or CLIP). HLA-DR3 molecules isolated from T2 transfectants can be efficiently loaded with antigenic peptides by exposure to a low pH in vitro, perhaps reflecting the in vivo conditions in which peptides associate with class II molecules. Addition of synthetic CLIP inhibits the loading process, indicating that CLIP may define the region of the invariant chain responsible for obstructing the class II binding site.

Heat-shock proteins and stress tolerance in microorganisms

Heat-shock proteins help microorganisms cope with the toxic effects of a wide variety of stresses. Some help the organism grow under moderately stressful conditions, others help it to survive more extreme conditions. Surprisingly, the relative importance of individual proteins differs between organisms.