B-Cell Responses in the Lung during Acute Pneumocystis Infection

Abstract

Abstract Pneumocystis jiroveci is an opportunistic fungal organism that causes Pneumocystis pneumonia (PCP) in humans with HIV-AIDS or other immunocompromised individuals. Pneumocystis can replicate in the alveoli where it disrupts gas exchange and promotes inflammation in the lung. Although Pneumocystis association is with T cell immunodeficiency, recent evidence shows that B cells play a key role as APCs in the lung to prime fungal T cell responses. We hypothesized that B cells are recruited and undergo tissue specific CSR during PCP infection and that these cells are critical to the priming of PCP specific T cells. To test this hypothesis we inoculated 4 to 6wk old male and female C57Bl/6 mice with Pneumocystis murina by oral pharyngeal aspiration. Flow cytometry analysis revealed that IgG+ and IgA+ B cells were at higher abundance in infected lung tissue at days 14 and 24. Single cell BCR-sequencing revealed an increase in class switched B cell clonotypes (both IgG and IgA) at day 14 compared to spleen. Our lab has previously shown that germ encoded IgM has specificity to PC antigens. To understand the role of germline encoded IgM and class-switched antibody responses, 4 to 6 wk old Aicda, Ighm −/− or C57Bl/6 mice were inoculated with P. murina for 14 days. We found at day 14 that fungal burden loads were slightly higher in the Aicda,Ighm −/− mice than C57Bl/6 mice, but H&E staining showed that Aicda,Ighm −/− mice possessed high levels of immunopathology. Bulk RNA sequencing revealed a lack of Type II immune responses, which have been shown to control Pneumocystis lung burden. Our data suggests that both germline encoded IgM and class switched antibody responses are critical in controlling pneumocystis infection.