POSITIVE AND NEGATIVE REGULATION OF HUMAN T-CELL ACTIVATION MEDIATED BY THE CTLA-4 CD28 LIGAND CD80/

CD28 and CTLA-4 are related receptors that differentially regulate T c ell activation. Despite the fact that they bind the same ligands, CD28 is a classical costimulator enhancing proliferation whereas CTLA-4 ap pears to perform negative regulatory functions. In this study, we have utilized the natural ligand for CD28 and CTLA-4 (CD80) to determine u nder what circumstances positive and negative effects are operative. W e show here that the stimulation of purified human T cells with phorbo l ester and ionomycin is inhibited in the presence of Chinese hamster ovary (CHO) cells expressing CD80. This inhibition is reversed by bloc king with both anti-CD80 or Fab fragments of anti-CTLA-4 but also requ ires CD28 engagement. Furthermore, we show that the inhibitory functio n of CD80 requires elevated intracellular calcium since inhibition was observed only in the presence of ionomycin, In the absence of intrace llular calcium elevation, CTLA-4 was not expressed at the cell surface , and CD80 acted positively as a costimulator of T cells, via CD28. Th ese results demonstrate that the natural ligand CD80 can either costim ulate or inhibit T cell responses depending on the conditions of T cel l stimulation.