Repeated respiratory bacterial exposures elicit lung resident memory B cells in the absence of organized tertiary lymphoid tissue

Abstract Recent work has shown that resident memory B cells (BRM) are generated in the lung after flu infection. However, it is unclear whether the existence of lung BRM is unique to influenza, which often elicits iBALT, or alternatively if these cells represent a common feature of the lung immune landscape. We aimed to determine whether bacterial lung exposures experienced by nearly all human children might also elicit lung BRM. To model this scenario, we gave young mice repeated respiratory exposures to low virulence pneumococci. After at least 4 weeks, we then analyzed lung B cells using flow cytometry and an intravital CD45 stain to exclude intravascular leukocytes. Pneumococcus-exposed mice had many more extravascular IgD- lung B cells than controls, and these B cells remained in the lung at least 12 weeks after the last exposure. Although unorganized aggregates of B and T cells were observed in previously exposed lungs, no iBALT structures were generated. Most of the IgD-B cells expressed the memory markers PD-L2, CD80, and CD73; a majority of the B cells expressed two or more of these markers, indicating they are poised to become antibody secreting upon rechallenge. Notably, the hallmark lung TRM phenotype (CD11abrightCD69+CD44brightCD62Llow) was also observed on lung B cells; these markers may represent a common resident memory lymphocyte phenotype. Although all lung B cells were CD20+, they were not affected by αCD20 antibody treatment, which requires circulation of bound cells for depletion to occur. BRM appear to be a common feature of the adaptive immune cell repertoire of experienced lungs. These cells do not require iBALT for their maintenance and represent a previously unrecognized pool of B cells resistant to depletion with αCD20 therapy.