Important Role for CC Chemokine Ligand 2-Dependent Lung Mononuclear Phagocyte Recruitment to Inhibit Sepsis in Mice Infected with Streptococcus pneumoniae

The monocyte chemoattractant CCL2 is of major importance in inflammatory monocyte recruitment to the lungs in response to bacterial infection. Streptococcus pneumoniae is the most prevalent pathogen in community-acquired pneumonia causing significant morbidity and mortality worldwide. In the current study, we examined the role of CCL2 in lung-protective immunity against two strains of S. pneumoniae exhibiting different virulence profiles. Both wild-type mice and CCL2 knockout (KO) mice became septic within 24 h of infection with serotype 3 S. pneumoniae and died of infection by day 4 after challenge. In contrast, wild-type mice challenged with serotype 19 S. pneumoniae did not become septic or succumb to pneumococcal pneumonia, whereas CCL2 KO mice showed an early bacterial outgrowth in their lungs and sepsis starting by day 2 after infection, finally resulting in approximately 50% decreased survival compared with wild-type mice. This phenotype was not due to impaired lung neutrophil recruitment in the KO mice, but was characterized by a significantly reduced recruitment of lung exudate macrophages and conventional lung dendritic cells, suggesting that these two phagocyte subsets critically regulate protection against septic disease progression in mice. In conclusion, we show here a differential role for CCL2-dependent lung exudate macrophage and conventional dendritic cell recruitment that critically contributes to lung protective immunity against S. pneumoniae.