Effects of a constitutively active form of calcineurin on T cell activation and thymic selection

Calcineurin is a calcium/calmodulin-dependent phosphatase whose activity is required for the induction of T cell lymphokine production and proliferati on. Although its specific role in T cell development is less well defined, studies with the immunosuppressive drugs cyclosporin A and FK-506 suggest t hat it is involved in both positive and negative selection of immature thym ocytes. To more completely characterize a role for calcineurin in T cell de velopment in vivo, we have generated transgenic mice that express an activa ted form of this enzyme in thymocytes and peripheral T cells. We find that the transgene causes a block in early thymic development, resulting in a re duction in the steady-state number of CD4 and CD8 double positives, but not on the number of mature T cells. We also find that thymocytes and mature T cells expressing this transgene are more sensitive to signals through thei r TCR. In thymocytes this sensitivity difference is manifested as an increa se in positive selection, although negative selection seems to remain unaff ected. Therefore, these studies confirm and extend past reports that sugges ted a role for calcineurin in thymic development and selection.