The dynamic equilibrium between dimeric and monomeric cell surface B7-1 regulates T cell activation and CD28 expression (88.1)

Abstract

Abstract Numerous studies have identified differences between B7-1:CD28/CTLA-4 and B7-2:CD28/CTLA-4 complexes that form at the immunological synapse. Our earlier studies have established that B7-1 is expressed as a mixture of predominant non-covalent dimers and monomers on the cell surface. In order to test the hypothesis that altering the nature of B7-1:CD28 complexes will have functional consequences for T cell activation, we generated a disulfide-linked covalent dimer of B7-1 (B7-1:Cys), as well as mutants of B7-1 (L58D and I68D) that exist as a monomer on the cell surface. We used modulation of cell surface CD28 to assess the outcome of binding of B7-1 mutants to CD28 as one measure of the dynamics of B7-1:CD28 interaction since earlier studies have shown that CD28 is transiently downregulated upon binding to WT B7-1. As expected, L58D and I68D mutants resulted in reduced T cell activation and reduced CD28 downregulation when compared to WT B7-1, potentially due to low avidity interaction between monomeric B7-1 and monovalent CD28. Surprisingly, B7-1:Cys also resulted in less T cell activation than WT B7-1. Moreover, B7-1:Cys also resulted in reduced downregulation of CD28. These data suggest a novel model in which optimal B7-1 function for CD28 downmodulation and T cell activation requires CD28 engagement by non-covalent B7-1 dimers, followed by dissociation of B7-1 to a monomeric state, potentially facilitating disengagement from CD28.