SUBCELLULAR-LOCALIZATION OF CD80 RECEPTORS IS DEPENDENT ON AN INTACT CYTOPLASMIC TAIL AND IS REQUIRED FOR CD28-DEPENDENT T-CELL COSTIMULATION

CD28 provides a major costimulatory signal to T cells when it is cross -linked with mAb, immobilized recombinant ligand (CD80Ig or CD86Ig), o r ligand-bearing cells but not when it is bound by specific Fab fragme nts or monomeric ligand. We wanted to determine how monomeric CD80 cou ld cross-link CD28 since CD80 is expressed as a monomer on the surface of APC, We found that CD80 may interact with the actin-based cytoskel eton, To test whether the interaction of CD80 with the cytochalasin B- sensitive cytoskeleton was necessary for T cell costimulation through CD28, we constructed a tailless form of CD80 and generated stable tran sfectants of Chinese hamster ovary epithelial cells and Reh B cells ex pressing either the tailless or wild-type CD80 molecules, Unlike contr ol cells expressing wild-type CD80, the tailless CD80 transfectants ex pressing equivalent levels of surface CD80 were not able to provide a costimulatory signal for anti-CD3-induced T cell proliferation, up-reg ulation of CD25 (IL-2R alpha) expression, or the induction of IL-2 sec retion, Thus, the cytoplasmic tail of CD80 apparently is required to s ignal T cells, Confocal microscopic studies revealed that wild-type CD 80 and tailless CD80 have different patterns of subcellular distributi on in both epithelial and lymphoid cells, Furthermore, T cell contact induces more patching and capping of CD80 in wild-type CD80-expressing cells than in tailless CD80-expressing cells, This suggests that the cytoplasmic region of CD80 functions to localize CD80 in complexes req uired for effective T cell costimulation.