Abstract. Chimeric antigen receptor (CAR) T cell therapy has transformed clinical care against blood malignancies and is seeing encouraging progress against solid tumors. While scientific advancement has been rapid, our mechanistic understanding of intrinsic features of CAR-engineered T cells is still evolving. CAR products typically consist of CD4+ and CD8+ T cell subsets at variable ratios, yet a clear understanding of how each subset contributes together and independently to therapeutic response is lacking. CD8+ CAR T cells are well-characterized for their perforin-dependent killing effects; however, the role of CD4+ CAR T cells as ‘helpers’ versus ‘killers’ has been variable across models and warrants more in-depth investigation. A recent study by Boulch and colleagues published in Nature Cancer demonstrates that CD4+ CAR T cells, alone, can exert potent antitumor activity through a mechanism involving IFNγ. CD4+ CAR T cell production of IFNγ creates a cytokine field that can act a