Eomesodermin⁺ CD4⁺ T cells are critical for curative immunotherapy outcomes

Interleukin 10 (IL-10)-producing CD4⁺ type-1 regulatory T cells (Tr1) promote immune tolerance during chronic infection, autoimmunity, and transplantation. However, specific Eomesodermin (Eomes)-dependent stages of Tr1 differentiation and function remain unclear. Using preclinical models of bone marrow transplantation (BMT), we demonstrated a Tr1 differentiation trajectory in vivo from Eomes⁺IL-10^- to Eomes⁺IL-10⁺ subsets with the acquisition of cytokine, cytolytic, and exhaustion features. The Eomes⁺CD4⁺ fraction represented the dominant cytotoxic subset after BMT, mediating graft-versus-leukemia effects while limiting inflammation. In CD19-targeted chimeric antigen receptor (CAR) T cell immunotherapy, Eomes drove the same CD4⁺ Tr1 phenotype that controlled cytolysis, while mitigating immune toxicity and promoting persistence. In individuals with high-grade B cell lymphomas that had long-term disease control after receiving commercial CD19-targeted CAR T cells, Eomes⁺ Tr1 cells represented a stable population comprising 40%-80% of the CD4⁺ CAR T cell population. Hence, Eomes controls both regulatory and cytotoxic programs in CD4⁺ T cells, essential for curative immunotherapy outcomes.