Immune pathways that regulate neutrophil activation and replenishment prevent persistent pneumococcal colonization

Abstract

Here, we examine immune signaling pathways affecting the duration of primary nasopharyngeal colonization by Streptococcus pneumoniae (Spn), the first step in its pathogenesis. Spn colonization which lasts days to weeks in WT mice was persistent (> 6 months) in the absence of IL-17RA-signaling. RNA-seq analysis confirmed the role of IL-17RA signaling in neutrophil-associated pathways. Rather than driving neutrophil specific chemokine expression, IL-17RA-signaling was required to replenish neutrophils in nasal tissue that were otherwise depleted during infection. Enhanced neutrophil trafficking correlated with IL-17RA-dependent expression of endothelial cell adhesion molecules that promote neutrophil trafficking from the circulation into nasal tissue. Persistent colonization was also observed in mice lacking IL-1R-signaling. Recognition of IL-1-family cytokines, however, was not necessary for the expression of IL-17A or neutrophil recruitment. Instead, IL-1R-signaling was associated with the activation of neutrophils in nasal tissue that displayed increased levels of the surface marker CD11b, an important receptor for the complement-opsonized phagocytosis of Spn. Our findings provide insight into the requirement for sustained neutrophil presence and activity to prevent persistent mucosal infection by a leading opportunistic mucosal pathogen. Author SummaryColonization of the upper airway by Streptococcus pneumoniae (Spn) is common and often asymptomatic, particularly in early life, yet prolonged colonization increases the risk of transmission and invasive disease. Despite its importance, the immune mechanisms that prevent long-term persistence of Spn in the upper airway are not fully understood. In this study, we investigated how two immune signaling pathways, the IL-17RA and IL-1R pathways, contribute to preventing persistent Spn colonization. Using a mouse model, we found that the IL-17RA pathway supports continued replenishment of neutrophils into nasal tissue by promoting expression of molecules that allow these cells to exit the bloodstream and enter the nasal tissue. In contrast, the IL-1R pathway activates neutrophils in a way that makes them more effective at clearing Spn. Loss of either pathway resulted in persistent Spn colonization, with an additive effect seen when both pathways were removed. Collectively, our results highlight how two distinct immune pathways cooperate to control pneumococcal colonization by sustaining neutrophil recruitment and function.