Lung resident memory B cells protect against bacterial pneumonia

Lung resident memory B cells (BRM cells) are elicited after influenza infections of mice, but connections to other pathogens and hosts have yet to be determined, as has their functional significance. We postulate that BRM cells are core components of lung immunity. To test this, we examined whether lung BRM cells are elicited by the respiratory pathogen pneumococcus, are present in humans, and are important in pneumonia defense. Lungs of mice recovered from pneumococcal infections did not contain organized lymphoid structures, but did have non-circulating memory B cells that expressed distinctive surface markers (including CD69, PD-L2, CD80, and CD73) and were poised to secrete antibodies upon stimulation. Human lungs also contained B cells with a resident memory phenotype. In mice recovered from pneumococcal pneumonia, depletion of PD-L2+ B cells, including lung BRM cells, drastically compromised bacterial clearance. These data define lung BRM cells as a common feature of pathogen-experienced lungs and provide direct evidence of a role for these cells in pulmonary anti-bacterial immunity. Overall design: Whole blood mRNA samples from 10 naïve C57BL/6 mice vs. 10 C57BL/6 mice experienced with serial intranasal exposures to Streptococcus pneumoniae