2 REGIONS IN THE CD80 CYTOPLASMIC TAIL REGULATE CD80 REDISTRIBUTION AND T-CELL COSTIMULATION

CD28 is a major T cell costimulatory molecule, delivering signals dist inct from those of the CD3/TCR complex, which regulate cytokine and cy tokine receptor expression, cell proliferation, and cell viability, CD 28 needs to be cross-linked to initiate signals, yet both of its ligan ds, CD80 and CD86, are expressed as monomers, Previously, we determine d the cytoplasmic tail of CD80 is required for CD28-mediated costimula tion and subcellular relocalization of CD80 in lymphocytes, In this st udy, we report that Reh B cell transfectants expressing CD80 with muta tions in the cytoplasmic tail region either at 275-278 (RRNE-->AAAA, C D80/4A) or serine 284 (S-->A, CD80/SA) can bind ligand similar to tran sfectants expressing wild-type CD80, Set are unable to costimulate T c ell proliferation. These mutant CD80 molecules are expressed on the su rface of the Reh cells in small clusters or foci indistinguishable fro m those of wild-type CD80 molecules. However, mutant CD80 molecules un like wild-type CD80 cannot be readily induced by ligand into caps. Thu s, small clusters of CD80 found on APC are insufficient to initiate CD 28-mediated signals, and the formation of CD80 caps appears to be a cr itical factor regulating the initiation of T cell costimulation, A 30- kDa phosphoprotein that associates with the cytoplasmic tail of CD80 i n activated cells may play a role in CD80 redistribution and thus CD28 -mediated costimulation, These results indicate two distinct regions o f the CD80 cytoplasmic tail regulate its costimulatory function, and b oth regions are required for CD80 function.