Neutrophil subsets enhance the efficacy of host-directed therapy in pneumococcal pneumonia

Host-directed therapy, using nasal administration of the Toll-like receptor 5 agonist flagellin in combination with antibiotics, has proven effective against pneumococcal pneumonia. In this study, we investigated the immune mechanisms underlying the therapy-induced protective effects. Transcriptomic analysis of lung tissue during infection revealed that flagellin not only enhanced pathways associated with myeloid cell infiltration into the airways and antimicrobial functions, but also promoted the early and transient mobilization of neutrophils and inflammatory monocytes. Neutrophils were identified as crucial for the protective effects of flagellin. The adjunct activity of flagellin correlated with the increased recruitment of neutrophils into airways, their localization at the periphery of bronchi, alveoli, and lung vessels, along with alterations in phagocytic activity. Clustering analysis identified seven neutrophil subsets; notably, flagellin adjunct treatment expanded clusters involved in recruitment and antibacterial activity, and primed augmented functionality. In conclusion, this study highlights specific neutrophil subsets as a promising target for host-directed therapy in infection.