THE BALANCE BETWEEN DELETION AND ACTIVATION OF CD4(+)8(+) THYMOCYTES IS CONTROLLED BY T-CELL RECEPTOR-ANTIGEN INTERACTIONS AND IS AFFECTED BY CYCLOSPORINE-A

The sensitivity of immature thymocytes to antigen-induced deletion has been shown to correlate with their differentiation status. By using an in vitro approach we have investigated whether parameters of antigenic stimulation may also affect the response of thymocytes. Two T cell receptor (TcR)-transgenic (Tg) mouse models have been compared, both of which recognize the allo-antigen H-2K(b) but are functionally CD8''-dependent'' (KB5.C20-Tg) and ''-independent'' (BM3.3-Tg). Presentation of the antigen H-2K(b) on the surface of fibroblasts, to thymocytes in vitro, resulted in the apoptosis of CD4(+)8(+) thymocytes. In contrast to in vivo deletion, in vitro deletion was much greater for KB5.C20-Tg than for BM3.3-Tg. In the absence of engagement of CD8 (using an altered H-2K(b)-alpha 3 domain or CD8-specific antibodies), the H-2K(b)-induced deletion of CD4(+)8(+) thymocytes was decreased for KB5.C20-Tg, but no change in the pattern of deletion for BM3.3-Tg occurred. CD4(+)8(+) thymocytes which remained viable after in vitro exposure to antigen, were shown to have been activated. Cyclosporin A (CsA), which has been reported to inhibit activation-induced cell death, did not affect antigen-induced deletion of CD4(+)8(+) thymocytes from KB5.C20-Tg. More strikingly, deletion of CD4(+)8(+) thymocytes from BM3.3-Tg increased, whilst activation was partially inhibited by CsA. These results provide direct evidence that presentation of antigen to thymocytes can result in deletion or activation, depending on not only the differentiation status of the cell, but also parameters of TcR-antigen interaction. Additionally, the effects of CsA suggest that activation can prevent the induction of deletion.