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A4723 - Lung-Localized B Cells Mediate Heterotypic Immunity Against Pneumococcal Pneumonia
Author Block: K. Barker1, N. Stauffer2, G. A. Wasserman3, E. Lipsitz1, I. Martin1, M. R. Jones4, L. J. Quinton5, J. P. Mizgerd4; 1The Pulmonary Center, Boston University, Boston, MA, United States, 2Pulmonary Center, Boston University, Boston, MA, United States, 3Microbiology/ Pulmonary Center, Boston University School of Medicine, Boston, MA, United States, 4Boston Univ School of Med, Boston, MA, United States, 5Pulm Ctr, Boston Univ Sch of Med, Boston, MA, United States.
RATIONALE Pneumococcal pneumonia remains a substantial public health burden, partially due to the reliance of current vaccines on serotype-specific antibodies. Recent work demonstrated the importance of lung resident memory Th17 cells in clearing pneumococcal colonization and developing a serotype-independent (heterotypic) protective response (NM Smith et al., Mucosal Immunology 2017). How B cells contribute to lung immunity is less understood, especially for bacterial infections. We aimed to investigate the hypothesis that memory B cells and airspace antibodies contribute to naturally acquired heterotypic protection against pneumococcal lung infection. METHODS We used a model of naturally acquired immunity in which mice were repeatedly infected in the lung with low virulence pneumococcal serotypes, yielding protection against later challenge with a more virulent serotype (Sp3). The role of B cells in this model was investigated using μMT mice, which lack mature B cells. Immunophenotyping was done using flow cytometry and an intravital CD45 stain to exclude intravascular leukocytes. Antibodies in bronchoalveolar lavage fluid (BALF) were measured using ELISAs with Sp3 as the capture agent. RESULTS Wild type mice displayed better defense than μMT mice after prior heterotypic exposures, indicating that B cells are required for maximum protection. In a pilot RNAseq experiment, B cell signatures were prominent in the pneumonic lung transcriptomes of previously infected mice compared to those of control mice. These data suggested that B cells may contribute locally to heterotypic lung protection. In experiments to quantify and immunophenotype lung B cells, lungs of mice that had recovered from prior pneumococcal pneumonias contained increased numbers of extravascular B cells, some of which were double positive for the memory markers PD-L2 and CD80. These markers have been ascribed to B cells poised to secrete antibody upon rechallenge. Thus, lung memory B cells may provide local pulmonary defense against subsequent infections by secreting heterotypic antibodies. To investigate the role of heterotypic antibodies in healthy lungs that received prior pneumococcal infections, we measured antibody levels in BALF. Mice previously exposed to serotype-mismatched pneumococci developed heterotypic IgA responses in the lung airspaces, while control mice did not. CONCLUSIONS Heterotypic immunity to pneumococcus after prior respiratory infections requires B cells for maximal efficacy and includes heterotypic lung IgA and a novel population of lung-localized memory B cells, which now demand further investigation. We are pursuing the premise that memory B cells and antibodies within the lung contribute to naturally acquired serotype-independent protection against pneumococcal pneumonia.