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), or ligand-bearing cells but not when it is bound by specific Fab fragments or monomeric ligand. We wanted to determine how monomeric CD80 could 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 cytoskeleton. 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 transfectants of Chinese hamster ovary epithelial cells and Reh B cells expressing either the tailless or wild-type CD80 molecules. Unlike control cells expressing wild-type CD80, the tailless CD80 transfectants expressing equivalent levels of surface CD80 were not able to provide a costimulatory signal for anti-CD3-induced T cell proliferation, up-regulation of CD25 (IL-2Ralpha) expression, or the induction of IL-2 secretion. Thus, the cytoplasmic tail of CD80 apparently is required to signal T cells. Confocal microscopic studies revealed that wild-type CD80 and tailless CD80 have different patterns of subcellular distribution 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 required for effective T cell costimulation.