CD3‐induced apoptosis of CD4+CD8+ thymocytes in the absence of clonotypic T cell antigen receptor

Clonal selection of T cells mediated through the T cell antigen receptor (TCR) mostly occurs at the CD4+CD8+ double positive thymocyte stage. Immature CD4+CD8+ thymocytes expressing self-reactive TCR are induced to die upon clonotypic engagement of TCR by self antigens. CD3 engagement by antibody of the surface TCR-CD3 complex is known to induce apoptosis of CD4+CD8+ thymocytes, a process that is generally thought to represent antigen-induced negative selection in the thymus. The present study shows that the CD3-induced apoptosis of CD4+CD8+ thymocytes can occur even in TCR alpha- mutant mice which do not express the TCR alpha beta/CD3 antigen receptor. Anti-CD3 antibody induces death of CD4+CD8+ thymocytes in TCR alpha- mice either in cell cultures or upon administration in vivo. Interestingly, most surface CD3 chains expressed on CD4+CD8+ thymocytes from TCR alpha- mice are not associated with clonotypic TCR chains, including TCR beta. Thus, apoptosis of CD4+CD8+ thymocytes appear to be induced through the CD3 complex even in the absence of clonotypic antigen receptor chains. These results shed light on previously unknown functions of the clonotype-independent CD3 complex expressed on CD4+CD8+ thymocytes, and suggest its function as an apoptotic receptor inducing elimination of developing thymocytes.