SPRED2 Negatively Regulates CD8⁺ T Cell-Mediated Antitumor Immunity in Breast Cancer

Activation of the RAS/RAF/ERK pathway is crucial for adaptive immunity. Here, we provide evidence that Sprouty-related EVH1 domain containing 2 (SPRED2), an endogenous inhibitor of this pathway, negatively regulates CD8⁺ T cell-mediated antitumor immunity in breast cancer. In EO771 and 4T1 mouse models, Spred2^-/- mice exhibit reduced tumor growth, with lower endpoint lung metastatic burden in the 4T1 model, accompanied by increased T-cell infiltration and activation. Spred2^-/- T cells show enhanced cytokine expression, proliferation, survival, in vitro cytotoxicity, and memory-like phenotypes, particularly among CD8⁺ T cells. Adoptive transfer of Spred2^-/- CD8⁺ T cells into tumor-bearing wild-type hosts similarly reduces endpoint lung metastatic burden. MEK inhibition with U0126 attenuates increased interferon-gamma and granzyme B expression in Spred2^-/- CD8⁺ T cells. Transcriptomic analyses reveal an inverse correlation between SPRED2 expression and antitumor CD8⁺ T cell states in both humans and mice. In tumor-infiltrating T cells from patients with breast cancer, SPRED2^low T cells show enriched effector and cytotoxicity programs and are associated with "immune-hot" tumors, whereas survival associations vary with CD8⁺ T cell context. Collectively, these findings suggest that SPRED2 functions as a cell-intrinsic negative regulator of CD8⁺ T-cell activation and highlight its potential as a therapeutic target to enhance T cell-based cancer immunotherapy.