Apoptosis of CD8 T cells is an essential mechanism that maintains immune system homeostasis, prevents autoimmunity, and reduces immunopathology. CD8 T cell death also occurs early during the response to both inflammation and costimulation blockade (CoB). In this article, we studied the effects of a combined deficiency of Fas (extrinsic pathway) and Bim (intrinsic pathway) on early T cell attrition in response to lymphocytic choriomeningitis virus infection and during CoB during transplantation. Loss of Fas and Bim function in Bcl2l11 2 / 2 Fas lpr/lpr mice inhibited apoptosis of T cells and prevented the early T cell attrition resulting from lymphocytic choriomeningitis virus infection. Bcl2l11 2 / 2 Fas lpr/lpr mice were also resistant to prolonged allograft survival induced by CoB targeting the CD40-CD154 pathway. These results demonstrate that both extrinsic and intrinsic apoptosis pathways function concurrently to regulate T cell homeostasis during the early stages of immune responses and allograft survival during CoB.