Anti-PD-1 antibody-activated Th17 cells subvert re-invigoration of antitumor cytotoxic T-lymphocytes via myeloid cell-derived COX-2/PGE₂

Anti-PD-1 antibody-mediated activation of type 17 T-cells undermines checkpoint inhibitor therapy in the LSL-Kras^G12D murine lung cancer model. Herein, we establish that the Th17 subset is the primary driver of resistance to therapy demonstrate that the ontogeny of dysplasia-associated Th17 cells is driven by microbiota-conditioned macrophages; and identify the IL-17-COX-2-PGE₂ axis as the mediator of CD8⁺ cytotoxic T-lymphocyte de-sensitization to checkpoint inhibitor therapy. Specifically, anti-PD-1 treatment of LSL-Kras^G12D mice, in which CD4⁺ T-cells were deficient for RORc, resulted in a 60% increase in CTL cytotoxicity and a 2.5-fold reduction in tumor burden confirming the critical role of Th17 cells in resistance to therapy. Lung-specific depletion of microbiota reduced Th17 cell prevalence and tumor burden by 5- and 2.5-fold, respectively; establishing a link between microbiota and Th17 cell-driven tumorigenesis. Importantly, lung macrophages from microbiota sufficient, but not from microbiota-deficient, mice polarized naïve CD4⁺ T-cells to a Th17 phenotype, highlighting their role in bridging microbiota and Th17 immunity. Further, treatment with anti-PD-1 enhanced COX-2 and PGE₂ levels, whereas neutralization of IL-17 diminished this effect. In contrast, inhibition of COX-2 rescued CTL activity and restored tumor suppression in anti-PD-1-treated mice, revealing the molecular basis of IL-17-mediated resistance to checkpoint blockade. Clinical implications of these findings are discussed.