Identification of antigen-specific, lung resident memory B cells after influenza infection

B cells display phenotypic and functional heterogeneity in multiple anatomical locations following vaccination or infection. Influenza-specific memory B cells (Flu+B MEM cells) are found in both lymphoid tissues and lung. It is unclear whether these cells represent circulating or resident memory B cell (BRM) populations. We hypothesized that a portion of the Flu+B MEM cell population in the lung would be non-circulating, BRMs. To determine whether Flu+B MEM cells in the lung are circulating or resident, we parabiotically joined previously-infected, congenically-mismatched mice for 2 weeks and marked those cells currently in circulation by infusing anti-B220 and identified those that have trafficked between the partners by their expression of the CD45 congenic marker. The lungs of these mice had large populations of hemagglutinin and nucleoprotein -specific B MEM cells that did not attain equilibrium within 2 weeks of parabiosis, suggesting that they are non-circulating. The Flu+ BRMs in the lungs consisted of 56% IgM+ and 43.9% isotype-switched BRMs. They were established as early as 15 days after infection and maintained for at least 60 days. The formation of Flu+ BRMs required the germinal center (GC), as blocking CD40L with MR1 antibody, during the primary infection abrogated BRM. However, MR1-treatment of mice with established BRM did not affect BRMs in the lung, even though Flu+ GC B cells could be detected in the LN for up to 90 days. These data suggest that GC-dependent lung-BRMs are established early after infection and maintained independently of GCs. These findings are important in the development of vaccines that elicit BRMs and they will provide mechanistic information into the function of Ag+B MEM cells residing in the mucosa.