Peptides recognized by class I restricted T cells also bind to MHC class II molecules

Sarcopenia and Cognitive Decline in Older Adults: Targeting the Muscle-Brain Axis

Declines in physical performance and cognition are commonly observed in older adults. The geroscience paradigm posits that a set of processes and pathways shared among age-associated conditions may also serve as a molecular explanation for the complex pathophysiology of physical frailty, sarcopenia, and cognitive decline. Mitochondrial dysfunction, inflammation, metabolic alterations, declines in cellular stemness, and altered intracellular signaling have been observed in muscle aging. Neurological factors have also been included among the determinants of sarcopenia. Neuromuscular junctions (NMJs) are synapses bridging nervous and skeletal muscle systems with a relevant role in age-related musculoskeletal derangement. Patterns of circulating metabolic and neurotrophic factors have been associated with physical frailty and sarcopenia. These factors are mostly related to disarrangements in protein-to-energy conversion as well as reduced calorie and protein intake to sustain muscle mass. A link between sarcopenia and cognitive decline in older adults has also been described with a possible role for muscle-derived mediators (i.e., myokines) in mediating muscle-brain crosstalk. Herein, we discuss the main molecular mechanisms and factors involved in the muscle-brain axis and their possible implication in cognitive decline in older adults. An overview of current behavioral strategies that allegedly act on the muscle-brain axis is also provided.

Mass spectrometry and peptide-based vaccine development

The development of new vaccines against pathogens is an important part of infectious disease control. In the last decade, a variety of proteins giving rise to naturally processed pathogen-derived antigenic peptides, representing B-cell and T-cell epitopes, have been characterized. Numerous candidate vaccines consisting of synthetic peptides are being designed and evaluated, with encouraging results. In this context, the application of mass spectrometry based on the isolation and identification of pathogen-derived peptides from the human leukocyte antigen (HLA) molecules is a major focus of peptide-based vaccine development. Dramatic improvements have been made in mass spectrometer performance for peptide sequencing in terms of increased sensitivity, the ability to rapidly obtain data-directed tandem mass spectra, and the accuracy of mass measurement. This review focuses on the efforts to identify T-cell epitopes for viral and microbial pathogens for directed vaccine development.

Identification of a mutated receptor-like protein tyrosine phosphatase kappa as a novel, class II HLA-restricted melanoma antigen

Recent studies increasingly point to a pivotal role of CD4(+) T cells in human anti-tumor immune response. Here we show that lymphocytes purified from a tumor-infiltrated lymph node of a melanoma patient that had remained disease free for 10 years after surgical resection of a lymph node metastasis comprised oligoclonal class II HLA-restricted CD4(+) T cells recognizing the autologous tumor cells in vitro. In fact, the CD4(+) T cell clones isolated from these lymphocytes displayed a tumor-specific, cytotoxic activity in addition to a Th1-like cytokine profile. By a genetic approach, a peptide derived from a mutated receptor-like protein tyrosine phosphatase kappa was identified as a novel HLA-DR10-restricted epitope for all the melanoma-specific CD4(+) T cell clones. The immunogenic peptide was shown to contain the mutated residue that was crucial for T cell recognition and activation. Moreover, a systemic immunity against the mutated peptide was detectable in the patient's peripheral blood T lymphocytes obtained during the disease-free period of follow-up. These findings further support the relevance of CD4(+) T cells directed against mutated epitopes in tumor immunity and provide the rationale for a possible usage of mutated, tumor-specific Ags for immunotherapy of human cancer.

Specific and general HLA-DR binding motifs: comparison of algorithms

Using panels of peptides well characterized for their ability to bind to HLA DR1, DRB1*1101, or DRB1*0401 molecules, algorithms were deduced to predict binding to these molecules. These algorithms consist of blocks of 8 amino acids containing an amino acid anchor (Tyr, Phe, Trp, Leu, Ile, or Val) at position i and different amino acid combinations at positions i+2 to i+7 depending on the class II molecule. The sensitivity (% of correctly predicted binder peptides) and specificity (% of correctly predicted non-binder peptides) of these algorithms, were tested against different independent panels of peptides and compared to other algorithms reported in the literature. Similarly, using a panel of 232 peptides able to bind to one or more HLA molecules as well as 43 non-binder peptides, we deduced a general motif for the prediction of binding to HLA-DR molecules. The sensitivity and specificity of this general motif was dependent on the threshold score used for the predictions. For a score of 0.1, the sensitivity and specificity were 84.7% and 69.8%, respectively. This motif was validated against several panels of binder and non-binder peptides reported in the literature, as well as against 35, 15-mer peptides from hepatitis C virus core protein, that were synthesized and tested in a binding assay against a panel of 19 HLA-DR molecules. The sensitivities and specificities against these panels of peptides were similar to those attained against the panels used to deduce the algorithm. These results show that comparison of binder and non-binder peptides, as well as correcting for the relative abundance of amino acids in proteins, is a useful approach to deduce performing algorithms to predict binding to HLA molecules.

Leishmania: fine mapping of the Leishmanolysin molecule's conserved core domains involved in binding and internalization

The Leishmanolysin molecule's role in the uptake of Leishmania parasites by the human U937 pro-myelocytic cell line was studied, using synthetic peptides representing the complete Leishmania (Viannia) guyanensis Leishmanolysin protein amino acid sequence. The particular peptides present in two protein's core domains efficiently impaired the internalization of promastigotes from four different Leishmania species and modified the kinetics of the binding of heterologous recombinant Leishmanolysin protein. The functional domains which exhibited this property represent a highly conserved portion of the sequence among different Leishmania species. The peptides' inhibitory activity correlated with their ability to bind molecules present on the surface of the human cell line. One of the two functional core domains identified involves the previously described adhesive sequence (SRYD) and the putative zinc-binding motif (HExxH). The second functional core domain includes a third histidine residue coordinated with zinc which determines the molecule's structural features. These findings indicate that the molecular interactions between Leishmanolysin's conserved domains and the macrophage surface molecules efficiently contribute to the parasite's internalization. Induction of neutralizing immune responses, which impair the early parasite-host interaction described here, may be an important alternative in designing synthetic subunit human leishmaniasis vaccines.

Immunoglobulin E-independent major histocompatibility complex-restricted T cell peptide epitope-induced late asthmatic reactions

Intradermal administration of short overlapping peptides derived from chain 1 of the cat allergen Fel d 1 (FC1P) that did not cross-link IgE, elicited isolated late asthmatic reactions with no visible early or late cutaneous response in 9/40 cat-allergic asthmatics. Four of the nine were human histocompatibility leukocyte antigen DR13-positive, as compared with only 1/31 nonreactors. The other five reactors expressed either DR1 or DR4. To confirm major histocompatibility complex restriction, fibroblast cell lines transfected with HLA-DR molecules were used to present FC1Ps to cat allergen-specific T cell lines derived from subjects before peptide injection. FC1P3 (peptide 28-44 of Fel d 1 chain 1) was recognized in the context of DR13 alleles (DRB1*1301, 1302) and induced specific T cell proliferation and IL-5 production. T cells from a DR1(+) responder proliferated and produced IL-5 in the presence of FC1P3 and DR1 (DRB1*0101) fibroblast cell lines, whereas T cells from a DR4(+) subject recognized FC1P2 (peptide 22-37) when presented by DRB1*0405. We conclude that short, allergen-derived peptides can directly initiate a major histocompatibility complex-restricted, T cell-dependent late asthmatic reaction, without the requirement for an early IgE/mast cell-dependent response, in sensitized asthmatic subjects.

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 relative importance of individual DR binding motif positions as defined by peptide anchor analysis of influenza hemagglutinin peptide 306-318 and human myelin basic protein peptide 152-165 binding to several DR molecules: definition of a common extended DR binding motif

Definition of peptide binding motifs for DR molecules has proven difficult as the peptides that bind to a DR molecule have shown extensive variability at putative motif positions. Recent studies suggest that specific peptide anchor residues (motif positions) and specific DR residues can differ in importance for peptide binding to a DR molecule. To assess further the relevance of individual peptide anchor residues, the binding of serial alanine-substituted analogs of influenza virus hemagglutinin (HA) 306-318 and human myelin basic protein (MBP) 152-165 to a panel of transfected wild-type DR molecules was examined. This analysis included DR molecules from a wide range of allelic families and, unlike most earlier studies, multiple members of single DR allelic families. The data show that different peptide residues serve as critical anchors for binding to different DR molecules. For example, MBP binding to DR(alpha, beta 1*0303) required peptide residues F154 (i), R159 (i + 5) and R162 (i + 8). In contrast, MBP binding to DR(alpha, beta 1*0102) required peptide residues I153 (i) and L156 (i + 3). More importantly, the combination of critical anchor residues in HA and MBP differed for binding to a single DR molecule [e.g. V309 (i) for HA and I153 (i) and L156 (i + 3) for MBP binding to DR(alpha, beta 1*0102)]. Although the location of the binding pocket in each DR molecule compared to the DR (alpha, beta 1 *0101) crystal is expected to be similar and suggests a common extended DR binding motif, the present results suggest that the relative importance of individual peptide anchor residues and of the corresponding DR binding pockets will differ for each DR/peptide complex.

Synthetic peptides coupled to the lipotripeptide P3CSS induce in vivo B and Thelper cell responses to HIV-1 reverse transcriptase

To evaluate the ability of the lipotripeptide P3CSS to increase peptide-specific immune responses in vivo, we immunized mice from different inbred strains (BALB/c, C3H/HeJ, C57BL/6) with the 22-mer lipopeptide conjugates P3CSS-[RT-(522-543)] and P3CSS-[RT-(528-549)] of HIV-1 reverse transcriptase (RT) which included an immunodominant Th epitope [i.e. RT-(528-543)] characterized previously. Analysis of T and B cell responses to these lipopeptide conjugates indicated that specific Th responses could be readily induced in vivo. The peptide segments could also efficiently prime mice for secondary recognition of native RT. The use of shorter peptides permitted a delineation of the minimal T cell recognition site of this RT C-terminal region [i.e. RT-(528-540)]. Close to this T cell epitope we identified a B cell determinant containing the motif EQVD [RT-(546-549)] which was recognized in three different strains of mice (H-2b, H-2d and H-2k). A comparison with X-ray analysis of the C-terminal region of HIV-1 reverse transcriptase indicated exposed positions of these Th and B cell epitopes. Both the presence of T and B cell sites and its limited polymorphism make the region RT-(528-549) a promising candidate for vaccine design. The use of the P3CSS adjuvant/carrier principle as a nontoxic adjuvant may be of major importance in the development of vaccines applicable to humans.

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.

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.

Human immunodeficiency virus-1 reverse transcriptase immunodominant CD4+ T cell epitopes: a peptide-based multiparametric assessment in the mouse

We previously identified an immunodominant CD4+ T cell determinant in the carboxy-terminal region of HIV-1 reverse transcriptase (RT528-543). The present study aimed at enumerating all the potential sites of HIV-1 RT recognized by Th cells in the BALB/c (H-2d) mouse model. To achieve this we used a panel of 62 overlapping 15-mer synthetic peptides covering the whole RT sequence to assay the following parameters: (i) immunogenicity in naive BALB/c mice injected either with peptides pools or individual peptides; (ii) antigenicity, as detected by their ability to restimulate in vitro T cells from BALB/c mice primed with native RT; (iii) MHC class II (Ad)-binding capacity as measured by the inhibition of the antigen-specific, Ad-restricted presentation of unfolded apamin (4-Acm) by fixed antigen-presenting cells to Ad/4-Acm-specific, interleukin-2-producing T hybridoma cells; and (iv) the presence of typical or degenerate consensus Ad-binding motifs. The results in this study permitted identification of three novel immunodominant RT mouse CD4+ T cell sites (RT276-290, RT375-389 and RT411-425) located in regions of limited polymorphism among RT from several HIV isolates. Some of these RT segments were found to be in the vicinity of B cell or H-2Kk- or HLA-A2-restricted cytotoxic T lymphocyte epitopes. Finally, the approach used in this study was found to be very efficient for enumerating most T cell recognition sites in a complex protein, a result that would have not been achieved by a single parameter-based analysis.

Allogeneic recognition of class I molecules: anti-H-2Ld repertoire of H-2b mice includes T cells recognizing mutant class II H-2b (Abm12) molecules

Two major histocompatibility complex (MHC) class I-reactive T cell clones derived from H-2b mice, generated against the allogeneic Ld molecule, were found to recognize the H-2b class II mutant Abm12 molecule as well. In addition, these clones also recognize the class II A(s) molecule, and display a class II-dependent reactivity to staphylococcal enterotoxin B. Neither the class I nor the class II alloreactivities of the clones were found to be dependent on other MHC molecules. Both clones express CD4+CD8- phenotypes. The CD4 molecule appears to be involved in their class II reactivity, while little or no role for CD4 could be detected in the class I reactivity. This is the first report of a class I/class II cross-reactivity being mediated by CD4+ T cells. The structural basis for this cross-reactivity is discussed.

HLA and disease

Many human diseases are associated with HLA class I, class II and class III antigens. It appears that the class III antigen disease associations can be explained by a direct defect operating at the level of either the class III gene or its gene product. The mechanism underlying class I and class II antigen disease associations is at present unknown. In this review we have considered thirty diseases which have been ranked according to their relative risk as defined by the frequency of a given HLA antigen in patient and control populations. The chronic inflammatory disorder, ankylosing spondylitis and its association with HLA B27 has been used as a model to study the HLA linked diseases. We have suggested that the disease may be caused by the Gram-negative microorganism Klebsiella which has antigenic similarity to HLA B27. It is proposed that some antibodies made against Klebsiella bind to HLA B27, thereby acting as autoantibodies leading to the pathological sequelae of chronic inflammatory arthritis. This is the crosstolerance hypothesis or molecular mimicry model and it has been compared to the receptor model. It is further suggested that the crosstolerance hypothesis can be utilised as a general theory to explain the association of other diseases with the class I and class II antigens, and offer a possible explanation for the polymorphism of HLA.

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

CD4+ T cells recognize antigenic peptides bound to the polymorphic peptide-binding site of major histocompatibility complex (MHC) class II molecules. The polymorphism of this site is thought to dictate which peptides can be bound and thus presented to the T cell receptor. The mycobacterial 65-kDa heat-shock protein (hsp65) peptide 3-13 is an important T cell epitope: it is immunodominant in the mycobacterium-specific T cell response of HLA-DR3+ individuals but, interestingly cannot be recognized in the context of any other HLA-DR molecules. We, therefore, have tested whether the hsp65 epitope p3-13 is selected for T cell recognition in the context of only HLA-DR3 molecules by an unique binding specificity for HLA-DR3. Using biotinylated peptides and EBV-transformed BLCL comprising all known HLA class II specificities, we find that p3-13 binds to HLA-DRw17(DR3) but not to any other HLA-DR molecule. Conversely, a control peptide p307-319 influenza hemagglutinin binds to all known HLA-DR molecules but only weakly to HLA-DRw17 and HLA-DR9. Peptide binding could be inhibited by excess unbiotinylated competitor analogue as well as by anti-DR monoclonal antibodies but not by anti-class I-, anti-DP- or anti-DQ monoclonal antibodies. The amino acid sequence of DRw17 molecules differs uniquely at five positions from the other DR beta 1 sequences. Three of these five residues (positions 26, 71 and 74) are potential peptide contacting residues. These residues map closely together in the hypothetical three-dimensional model of the DR molecule and, thus, most probably form a positively charged pocket, critical for the binding of p3-13. Interestingly, p3-13 does not bind to a DR3 variant, the DRw18 molecule. The DRw18 beta 1 chain differs from DRw17 at two major positions, close to or within the DRw17-specific pocket. These substitutions drastically change the structure and charge of the pocket and thus presumably abrogate its ability to bind p3-13.

Synthetic peptides as vaccines

The economics of vaccines has been a major limitation in the commercial research and development of new approaches. This coupled with the natural scientific desire to simplify and define the composition of effective vaccines argues that the future of vaccines lies in novel approaches that will discover effective and less expensive components. Peptides, whether they are chemically synthesized or produced in bacteria, are an attractive possibility. To substitute linear peptides for complex mixtures of proteins would be a major technical advance and would stimulate tremendous commercial interest. However, at the present time I view this approach still unlikely to be of major practical importance. I conclude this because of the complexity of immunological responses to microorganisms. Even though, in some instances, a cytotoxic T-cell response or even the majority of the antibody response to a pathogen can be defined by a short linear peptide, most people believe that multiple effector functions of the immune system should be stimulated in optimal vaccines. For a small cocktail of peptides to reproduce the diversity of responses elicited by a virus, parasite, or bacterium is unlikely. However, I fully realize that remarkable progress has occurred towards understanding the structural requirements necessary to stimulate cellular and humoral immune responses, and peptides have been integral in the development of this field. Also, the success of several research groups in developing effective antiviral vaccines using short linear peptides argues that I might be painting too dark of a picture. As someone who has used this strategy to explore peptide-MHC and peptide-antibody interactions, I am a strong scientific supporter of the approach. In this forum I am purposely cautious in my optimism. As the details of the complex molecular and cellular interactions that control the immune system are elucidated, both the number of strategies and the possible applications of modulating the immune response will increase as well. In addition to protective immunity to pathogens, cancer therapy could be revolutionized if tumor-specific cytotoxic T-cells could be generated routinely. Novel therapeutic approaches to allergy, autoimmunity, and transplantation can be envisioned if the T-lymphocytes responsible for these syndromes could be modulated without total immune suppression. Consequently, I am confident that the experiments described in this chapter will be central to developing exciting new therapeutic and prophylactic compounds, but I am not sure that they will resemble naturally occurring peptides. The one aspect I am confident of is that the capacity of the immune response to protect the organism will continually surprise us.

Comparison of class I- and II-restricted T cell recognition of the identical peptide

There is structural and functional evidence that both class I- and II-restricted T cells recognize short processed peptides bound to MHC molecules. Although the structural conformation of bound peptides remains unknown, no evidence of distinct structural motifs of class I- or class II-restricted peptides has been described. Conversely, two algorithms proposed to predict T cell epitopes, and based on primary amino acid sequence or tertiary structure, are both compatible with many observed class I- and class II-restricted peptides. We previously identified eight class I-restricted peptides which were also recognized by class II-restricted T cells. Based on functional and direct binding studies, additional examples of peptides with both class I and II restrictions have been identified. In this study, we have directly compared the fine specificity of T cell recognition of a single epitope in a single mouse strain in the context of both class I- and class II-restricted responses. Based on a panel of analogue peptides with amino acid substitutions and peptides of various lengths, we observed several striking similarities in the recognition patterns of both class I- and class II-restricted T cells. In addition, some characteristics of recognition were different in the two systems indicating that the recognition processes were similar but not identical.

Dissociation of beta 2-microglobulin leads to the accumulation of a substantial pool of inactive class I MHC heavy chains on the cell surface

A large pool of free class I heavy chains is detected in situ on the plasma membrane of living cells. These chains are present on cells of different MHC genotypes and appear to exist under physiological conditions in vivo. These molecules arise from the dissociation of previously assembled class I heterodimers at the cell surface. The ratio of intact to dissociated heterodimers is strongly affected by the occupancy of the peptide-binding site of the class I molecule. Upon dissociation of the heterodimer, the class I molecule is functionally inactive. These findings may help to explain why class I molecules on the cell surface are unreceptive to binding peptides yet readily associate with peptides in the presence of exogenous beta 2-microglobulin. These results have implications for understanding the distinct functions of class I versus class II molecules and how the immunological identity of cells is preserved.

The distinctive specificity of antigen-specific suppressor T cells

Although suppressor T cells have been cloned in only a few instances, the existence of a functional cadre of T cells that acts to downregulate the immune response is well documented. In this review Eli Sercarz and Urszula Krzych describe studies on suppressor T-cell (TS-cell) specificity that provide some support for the conclusion that the TS cell is a distinctive cell type with an expressed repertoire that is different from that expressed by helper T (TH) cells. They go on to explore the interaction between cells recognizing TS-cell-inducing determinants (SDs) and TH-cell-inducing determinants (HDs), and their relationship to immunogenicity and Ir gene effects.

The cell biology of antigen processing

T lymphocytes recognize antigen only after a series of intracellular events known as antigen processing. The result of antigen processing is the production of short segments of the primary peptide sequence bound to a polypeptide-binding groove on major histocompatibility complex (MHC) molecules. Antigen originates from one of two sites: intracellular or extracellular. There are two corresponding pathways for antigen processing and two corresponding classes of MHC molecule. Analysis of each pathway has demonstrated that their separation is not purely anatomical, but is maintained by molecular interactions with other molecules. Antigen processing has been shown to regulate the overall immune response, but the mechanisms involved remain obscure.

Delineation of antigen contact residues on an MHC class II molecule

This report describes a detailed mutational analysis of a major histocompatibility complex class II molecule--the alpha chain of the Ak complex. Each residue from 50-79 was replaced by an alanine, and the effects on recognition of Ak by panels of antibodies and T cells determined. The results provide the strongest existing experimental evidence that the antigen binding site on a class II molecule can be modelled on the crystal structure of a class I molecule. The data have also permitted the delineation of residues that actually contact antigenic peptides.

Detection of peptide-MHC class II complexes on the surface of intact cells

The interaction of peptides with major histocompatibility complex proteins on the surface of cells is required for their recognition by T lymphocytes. Many studies characterizing the formation of peptide-MHC class II complexes have used either assays for T cell responses or for peptide binding to purified class II molecules. Recently, specific peptide-class II interactions have been demonstrated convincingly on the surface of intact cells. The effects of varying peptide and class II structure have been examined in order to identify structural requirements for binding to cell surface class II molecules and to examine the conformation adopted by immunogenic peptides when bound.