Conditional ablation of SFB-induced IL-17A<sup>+</sup>CD4<sup>+</sup> T cells, precursors of tumour-associated T<sub>H</sub>1-like cells, abolishes anti-PD-1-mediated tumour control and markedly impairs tumour-specific CD8<sup>+</sup> T cell recruitment and effector function within the TME. Here, IL17A is linked to neoplasm.