Ly6G⁺ granulocytes-derived IL-17 limits protective host responses and promotes tuberculosis pathogenesis

The protective correlates of Mycobacterium tuberculosis (Mtb) infection-elicited host immune responses are incompletely understood. Here, we report pro-pathogenic crosstalk involving Ly6G⁺ granulocytes (Ly6G⁺Gra), IL-17, and COX2. We show that in the lungs of Mtb-infected wild-type mice, either BCG-vaccinated or not, most intracellular bacilli are Ly6G⁺Gra-resident 4 weeks post-infection onwards. In the genetically susceptible ifng^-/- mice, excessive Ly6G⁺Gra infiltration correlates with severe bacteremia. Neutralizing IL-17 (anti-IL17mAb) and COX2 inhibition by celecoxib reverse Ly6G⁺Gra infiltration, associated pathology, and death in ifng^-/- mice. Surprisingly, Ly6G⁺Gra also serves as the major source of IL-17 in the lungs of Mtb-infected WT or ifng^-/- mice. The IL-17-COX2-Ly6G⁺Gra interplay also operates in WT mice. Inhibiting RORγt, the key transcription factor for IL-17 production or COX2, reduces the bacterial burden in Ly6G⁺Gra, leading to reduced bacterial burden and pathology in the lungs of WT mice. In the Mtb-infected WT mice, COX2 inhibition abrogates IL-17 levels in the lung homogenates and significantly enhances BCG's protective efficacy, mainly by targeting the Ly6G⁺Gra-resident Mtb pool, a phenotype also observed when IL-17 is blocked by RORγt inhibitor. Furthermore, in pulmonary TB patients, high neutrophil count and IL-17 correlated with adverse treatment outcomes. Together, our results suggest that IL-17 and PGE2 are the negative correlates of protection, and we propose targeting the pro-pathogenic IL-17-COX2-Ly6G⁺Gra axis for TB prevention and therapy.