Airway neutrophils actively enhance the therapeutic efficacy of flagellin in treating pneumococcal pneumonia

Host-directed therapy, using nasal administration of the Toll-like receptor 5 agonist flagellin in combination with antibiotics, has demonstrated effectiveness against pneumococcal pneumonia. In this study, we investigated the immune mechanisms underlying the 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. Lastly, single-cell analysis highlighted that the treatment stimulated the recruitment of specific neutrophil subsets capable of enhancing antibacterial effectiveness. In conclusion, this study highlights neutrophils as an appealing target for host-directed therapy in infection. Overall design: Eight weeks-old female mice were infected with Streptococcus pneumoniae and treated 12 hours post-infection with amoxicillin alone or in combination with flagellin. Bronchoalveolar lavages were sampled 4 hours post-treatment and processed for RNA isolation and sequencing.