Tumor-specific CD4 T cells cooperate with myeloid cells to remodel the pancreatic tumor microenvironment and enable effective immunotherapy.

We interrogate antigen-specific CD4 T cells during immunotherapy in pancreatic ductal adenocarcinoma. Vaccination with MHC-II-restricted tumor epitopes impart superior protection compared to an immunodominant MHC-I epitope, prompting development of MHC-II affinity-enhanced tetramers to track tumor-specific CD4 T cells. As tumors progress, tumor-specific CD4 T cells decline, and remaining cells acquire features of regulation. Agonistic anti-CD40 increases Th1 cell clonal expansion and transiently decreases Tregs. Anti-PD-L1 promotes Tfh clonal expansion in draining lymph nodes and tumor while preventing Treg rebound after anti-CD40. Treatment with anti-CD40 promotes intratumoral Stat1 + macrophages, tertiary lymphoid structures (TLS), and immune triads. MHC-II on myeloid cells but not B cells is required for immunotherapy-induced TLS formation and antitumor effects. IL-15 complex enhances immunotherapy-induced Th1 effectors without promoting Tregs. Human immunotherapy transcriptomics shows conserved Th1 programming and Treg destabilization as a feature of response. Thus, tumor-specific CD4 T cells are central mediators of effective immunotherapy in solid tumors.