Tumor-specific CD4(+) T cells eradicate myeloma cells genetically deficient in MHC class II display

CD4(+) T cells have been shown to reject tumor cells with no detectable expression of major histocompatibility complex class II (MHC II). However, under certain circumstances, induction of ectopic MHC II expression on tumor cells has been reported. To confirm that CD4(+) T cell-mediated anti-tumor immunity can be successful in the complete absence of antigen display on the tumor cells themselves, we eliminated MHC II on tumor cells using CRISPR/Cas9. Our results demonstrate that ablation of the relevant MHC II (I-E(d)) in multiple myeloma cells (MOPC315) does not hinder rejection by tumor-specific CD4(+) T cells. These findings provide conclusive evidence that CD4(+) T cells specific for tumor antigens can eliminate malignant cells in the absence of endogenous MHC class II expression on the tumor cells. This occurs through antigen uptake and indirect presentation on tumor-infiltrating macrophages.