Superantigens (SAgs) bind to class II Major Histocompatibility Complex (MHC) proteins and stimulate T cells. They recognize T cells on the basis of their interaction with the variable region of the beta chain of the T cell receptor (TCR) with little or no regard to the clonotypic and combinatorial elements of the TCR. Because there are a limited number of TCR variable elements encoded in the germ line, a given SAg can recognize a large number of the T cells found in an organism. T cell responses to typical peptide antigens are considerably more restricted. The ability to induce the proliferation of T cells has led to the use of SAgs by murine and human pathogens, which either infect or exploit some useful byproduct of the proliferating lymphocytes. The SAgs made by some bacteria (e.g., the Staphylococcal enterotoxins) cause shock-like symptoms in humans, and are well characterized at the biochemical and immunological levels. The mouse mammary tumor virus (MMTV) has also been shown to produce a SAg, and although the MMTV SAg (vSAG) has been characterized in detailed immunological level, it is relatively uncharacterized biochemically. Our previous data has indicated that the prototypical MMTV SAg (vSAG) is a Type II transmembrane glycoprotein, that it undergoes proteolytic processing prior to its expression on the cell surface, and that it interacts physically with class II MHC proteins. Proteolytic processing appears to be required for normal vSAG function. The goal of the proposed research is the characterization of the interactions between vSAGs and class II MHC proteins at the molecular level. Qualitative and quantitative measurements will be made of the binding of vSAGs to purified class II proteins, and to class II proteins expressed on antigen presenting cells (APCs). The vSAG proteins will be obtained from APCs, or will be produced in a soluble form using a yeast protein expression system. In addition, studies will be undertaken to investigate the role and requirement of proteolytic processing for vSAG function. Mutagenesis studies will also be performed on the vSAG7 protein in order to identify amino acid residues that are important for the binding of vSAG7 to class II MHC proteins. Because it is possible that yet undiscovered SAgs may be used by human pathogens, the studies proposed here will provide a biochemical paradigm for human viral or retroviral Sags. A detailed biochemical characterization of these proteins is important for our ability to use them as immunological tools, as potential agents for immune intervention, and for our understanding of basic problems in immunology and virology.
