B7-CD28 INTERACTION IS A LATE ACTING COSTIMULATORY SIGNAL FOR HUMAN T-CELL RESPONSES

The interaction of CD28 with one of the B7 molecules (CD80 and CD86) o n professional antigen-presenting cells (APC) is generally considered as the most important co-stimulatory signal for T cell activation, APC in a resting condition express either no or only low levels of B7 mol ecules, These are up-regulated as a result of interactions with activa ted T cells, thus suggesting that B7-CD28 interaction is not required at initiation of T cell activation, To study this issue, we blocked B7 -CD28 interaction at various time points after in vitro stimulation of peripheral blood T cells with allogeneic monocytes, Epstein-Barr viru s-transformed a cells or soluble antigens, We observed that T cell pro liferation and IL-2 production were inhibited by B7-blocking agents (C TLA-4-lg or anti-B7 mAb) almost to the same degree when added either a t initiation of culture or 24 h later, B7-blocking agents still result ed in significant inhibition of allogeneic T cell activation when adde d after 48 h, Furthermore, when CTLA-4-lg was added at the start of an allogeneic T cell stimulation, addition of anti-CD28 mAb after 24 h o f culture nearly fully restored T cell proliferation to control revels . Finally, we demonstrate that delayed addition of B7-blocking agents together with cyclosporin A 1 day after the onset of culture of T cell s with allogeneic a cells is highly efficient to induce anergy as eval uated by lack of proliferation, cytotoxic T lymphocyte reactivity and IFN-gamma or IL-5 production upon alloantigen rechallenge, Taken toget her, our data can explain why B7 expression on APC is not required at the time of initial APC-T cell contact, and suggest that the effect of the CD28 signal indeed consists in prolonging IL-2 production and amp lifying T cell responses, rather than in providing a critical co-stimu latory signal at the time of initial TCR triggering.