CD8⁺ T cell-derived CD40L mediates noncanonical cytotoxicity in CD40-expressing cancer cells

T cells and their effector functions, in particular the canonical cytotoxicity of CD8⁺ T cells involving perforin, granzymes, Fas ligand (FasL), and tumor necrosis factor related apoptosis inducing ligand (TRAIL), are crucial for tumor immunity. Here, we reveal a previously unidentified mechanism by which CD40L-expressing CD8⁺ T cells induce cytotoxicity in cancer cells. In murine models, up to 50% of tumor-specific CD8⁺ T cells expressed CD40L, and conditional CD40L ablation in CD8⁺ T cells alone led to tumor formation. Mechanistically, CD40L⁺CD8⁺ T cells can induce cell death in CD40-expressing cancer cells by triggering caspase-8 activation. We demonstrate that a gene signature for resistance to CD40 signaling-induced cell death strongly correlates with worse survival in different human cancer cohorts. Our results introduce CD40L as a rather counterintuitive, noncanonical cytotoxic factor that complements the capabilities of CD8⁺ T cells to combat cancers and has the potential to enhance the efficacy of immunotherapies.