ALK2/3 are components of the immune synapse crucial for T cell activation and death

Antigen recognition by TCR triggers T cell activation and activation-induced cell death (AICD). To investigate additional mechanisms beyond the classical model of T cell activation and death, we conducted a survival-based kinome-wide CRISPR-Cas9 knockout screens and identified the BMP receptors ALK2 and ALK3 as components required for AICD. ALK2 and ALK3 were interdependently required for induction of a subset of effector genes and AICD in activated T cells, and the functions of ALK2/3 in these processes are independent of their BMP ligands. Upon T cell activation, ALK2/3 were recruited to the immunological synapse and phosphorylated by PKC-θ at the conserved T203, resulting in their enhanced kinase activities. The activated ALK2/3, in the absence of BMP, phosphorylated SMAD1/5 at S57, which is reciprocally antagonistic to BMP-induced phosphorylation of SMAD1/5 at S463/465. The S57-phosphorylated SMAD1/5 associated with c-Fos to induce effector genes upon T cell activation. Disruption of Alk2 in T cells attenuated T cell–mediated immunity to Listeria, whereas blocking BMPs enhanced host defense to Listeria in wild-type but not Alk2-deficient mice. Our findings suggest that the BMP-independent ALK2/3-SMAD1/5 axis plays essential roles in T cell activation and AICD, which is reciprocally antagonistic with BMP-triggered inhibition of T cell-mediated immunity.