SummaryTo investigate the functional roles of individual HLA-DR residues in T cell recognition, transfectants expressing wild-type or mutant DR(o~, B1"0401) molecules with single amino acid substitutions at 14 polymorphic positions of the DI~1"0401 chain or 19 positions of the DRo~ 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 DRB 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 DRB positions 70, 71, 78, and 86 on the oe helix eliminated recognition by each of the dones, and mutations at positions 74 and 67 eliminated recognition by four and two clones, respectively. Most of the DRol mutations had minimal or no effect on most of the clones, although one clone was very sensitive to changes in the DRo~ 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 B86 mutation drastically decreased peptide binding. Single amino acid substitutions at polymorphic positions in the central part of the DRB c~ helix disrupted T cell recognition much more frequently than substitutions in the floor, suggesting that DRB residues on the ol helix make relatively greater contributions than those in the floor to the ability of the DR(oe,~l*0401) molecule to present HA307-19. The data indicate that DRq8 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.T he MHC molecules on the cell surface have evolved the remarkable capacity to bind and present to T lymphocytes an extremely large number of structurally diverse peptides. Recognition of the appropriate peptide-class II complexes by the antigen-specific TCR leads to CD4 + T cell proliferation and a cascade of cellular immune responses. HLA class II molecules are highly polymorphic, and this structural polymorphism determines the distinct peptide-binding and antigen presentation characteristics of each molecule. However, the functional roles of individual residues in HLA class II molecules in peptide binding and T cell recognition have not been dearly defined. Elucidation of the ways in which class II residues interact with peptides and TCR may have important implications for understanding the function of the This study was presented in part as an abstract at the 19th annual meeting of the American Society for Histocompatibility and Immunogenetics, Phoenix, Arizona, 2-7 October, 1993. immune system, as well as for the treatment ofimmunologically mediated diseases.Considerable progress has been made in the understanding of the structure of HLA class II molecules and the nature of class II-peptide interactions in recent years. Based ...

The human class II-associated chondroitin sulfate proteoglycan (CSPG) was analyzed biochemically and immunologically to determine a possible relationship with the human invariant chain (gamma 1) and its related components. The CSPG was purified by a three-step procedure involving associative ion-exchange chromatography, immunoprecipitation, and dissociative ion-exchange chromatography. Treatment of the CSPG with chondroitinase revealed core proteins of Mr approximately 46,000, 38,000, and 28,000, with the 38,000 species most highly represented. Tryptic peptide analysis revealed identity of the peptides of the 38,000 Mr core protein and gamma 1, and of the 28,000 Mr species and p25. The CSPG and its core proteins were shown to react directly with the mouse anti-human invariant chain monoclonal antibody VIC-Y1 and a rabbit antiserum produced against a synthetic peptide corresponding to the C-terminal end of invariant chain. These results demonstrate that the invariant chain is the core protein of the class II-associated CSPG. In addition, virtually all the CSPG was shown to be present on the cell surface.

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HLA class II MHC molecule alpha- and beta-chains are normally synthesized in the presence of a third molecule, the invariant chain (Ii). Although Ii is not required for surface expression of HLA class II molecules, the influence of Ii on post-translational processing and maturation HLA class II molecules has not been thoroughly studied. In the present study, BALB/c 3T3 cells were transfected with HLA-DR alpha- and beta-chains with or without co-transfection with human Ii. Although Ii had no effect on the surface expression of DR, Ii did have a profound effect on the post-translational processing of both the alpha- and beta-chains. In the absence of Ii, the major species of alpha- and beta-chains were of lower m.w. than when expressed in the presence of Ii. The differences in m.w. were shown to be caused by differences in glycosylation with the majority of alpha- and beta-chains remaining unprocessed and endo H sensitive in the absence of Ii. The small proportion of alpha-chains that were processed in the absence of Ii showed an altered m.w. and altered sensitivity to treatment with endo H relative to alpha-chains processed in the presence of Ii. Pulse/chase studies demonstrated that although the majority of the alpha- and beta-chains remained unprocessed in the absence of Ii, the small amount that was processed was done so at a rate similar to that observed for alpha- and beta-chains processed in the presence of Ii. These studies demonstrate that Ii influences the post-translational processing of human class II molecules by affecting the proportion of alpha- and beta-chains that are processed and by determining the degree of processing of oligosaccharides on mature alpha-chains.

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