The major histocompatibility complex (MHC) class II molecules are important cell recognition molecules for the immune system. They are characterized by a very high degree of polymorphism within a species. Because the T cells of the immune system depend upon the MHC polymorphisms for the discrimination of self and nonself, it is important to understand how the observed polymorphisms relate to the function of the MHC molecules. In previous studies this laboratory has shown that one of the four polymorphic regions in the class II A-beta polypeptide specifies the epitopes recognized by antibodies. A model of the three-dimensional structure of the class II molecule has been proposed based on the recently determined x-ray crystal structure of the HLA A2 class I molecule. In this model the region that has been determined to specify the antibody epitopes is a series of residues between two conserved alpha-helices. The structure of this region has not been predicted, but its position in the molecule suggests that it would be accessible to solvent and thus to antibody molecules and potentially also to the T cell receptor. Of the other three polymorphic regions in the beta 1 domain, two are likely to be inaccessible, while the other is likely to be accessible. In the studies now proposed, the role of each of the polymorphic regions in determining the epitopes recognized by T cells will be analyzed. The naive T cell responses to a panel of cell lines expressing limited and defined alterations in the class II I-A molecule will be quantitated by limiting dilution analysis of responder cell frequencies. The set of cell lines expressing variant I-A molecules has been produced by site-directed mutagenesis and DNA-mediated gene transfer. These studies should indicate which of the polymorphic regions of the MHC class II molecule determine epitopes recognized by T cells. The detailed information on the functional protions of the class II molecule that will be obtained in these studies can then be correlated with the details of the structure of the molecule as they become available.
