EZH2 and intracellular Ca²⁺ signals interdependently coordinate alloreactive and CAR-T-cell responses

In graft-versus-host disease (GVHD), Ca²⁺ signals in alloreactive T cells are carefully controlled to determine whether cells survive or thrive, although how this is accomplished during GVHD remains poorly defined. We demonstrate that EZH2, a chromatin-modifying enzyme, promotes alloreactive T-cell survival in GVHD by acting as a Ca²⁺ signaling brake to limit excessive intracellular Ca²⁺ responses. Ezh2 loss led to the upregulation of gene programs that promote effector differentiation in activated T cells, coincident with enhanced intracellular Ca²⁺ responses that ultimately caused massive cell death. Conditional deletion of Stim1 (required for cytosolic Ca²⁺ entry) led to "synthetic rescue" of Ezh2-null T cells by protecting them from cell death without interfering with effector differentiation, resulting in severe GVHD. Interestingly, Stim1 expression was unaffected by EZH2, whereas the expression of the endoplasmic reticulum Ca²⁺ release channel inositol 1,4,5-trisphosphate receptor 2 (Itpr2) was suppressed by EZH2. Notably, EZH2 and Ca²⁺ signals served mutually opposing roles in controlling the expression of genes in chimeric antigen receptor (CAR) T cells. Inhibiting Ca²⁺ signaling restored EZH2 function in CAR-T cells, significantly improving their antitumor activity. Our findings reveal the interdependent roles of EZH2 and Ca²⁺ signals in coordinating antigen-activated T-cell responses that mediate alloimmunity and tumor immunity.