CD4+ T-cell-induced inflammatory killing controls immune evasive tumors

Most clinically applied cancer immunotherapies rely on the ability of CD8+ cytolytic T-cells to directly recognise and kill tumor cells. These strategies are limited by the emergence of MHC-deficient tumor cells and the development of an immunosuppressive tumor microenvironment. The ability of CD4+ effector cells to contribute to anti-tumor immunity independently of CD8+ T-cells is increasingly recognised, but strategies to unleash their full potential remain to be identified. Here, we describe a mechanism through which a small number of CD4+ T-cells is sufficient to eradicate MHC-deficient tumors that escape direct CD8+ T-cell targeting. The CD4+ effector T-cells preferentially cluster at tumor invasive margins where they interact with MHC-II+CD11c+ antigen-presenting cells. We show that Th1-directed CD4+ T-cells and innate immune stimulation reprogram the tumor-associated myeloid cell network towards IFN-activated antigen-presenting and iNOS-expressing tumoricidal effector phenotypes. Together, CD4+ T-cells and tumoricidal myeloid cells orchestrate a remote inflammatory cell death process that indirectly eradicates IFN-unresponsive and MHC-deficient tumors. These results warrant to clinically exploit the ability of CD4+ T-cells and innate immune stimulators as a strategy to complement the direct cytolytic activity of CD8+ T- and NK-cells and advance cancer immunotherapies. Overall design: Myeloid immune cells were isolated by FACS from HCmel12 melanomas in C57BL/6 mice treated with TRP-1 TCRtg CD4+ T-cells or left untreated and analyzed using scRNAseq.