Germinal center-like melanomacrophage centers (MMCs): species-specific variation in both organization and responses to immunization 4508

Abstract Description Ectotherm melanomacrophage centers (MMCs), aggregates of pigmented phagocytes, are germinal center (GC)-like lymphoid microstructures. Recent work in trout showed that MMC-associated lymphocytes form discrete B and T cells zones and are the sites of B cell proliferation and clonal expansion. However, fish are a highly diverged taxa, therefore, to broaden our understanding of teleost lymphoid tissue architecture and function, we assessed splenic MMC lymphoid microstructures in threespine stickleback. Stickleback melanomacrophages form exceptionally large and dense MMC aggregates. Spatial analysis revealed that splenic IgM+ and CD4+ cells density is highest at the MMC border. A zone of IgM+ cells surrounds the MMC however, unlike trout, no discrete T cell zones were detected. Instead, individual CD4+ and TCRB+ cells were found throughout the spleen. While CXCR5+ lymphocytes were present throughout, this species lacks the gene for the CXCL13 ligand, which may explain the disorganization of the stickleback spleen. MMC-associated B/T cell populations increased following immunization with T-dependent antigen, however lymphocyte density was unchanged following immunization with T-independent antigen. While our work supports a GC-like model for MMCs, it also raises the question: does disorganization lead to inefficiency in adaptive immunity? The heterogeneity in lymphoid microstructure architecture we observed highlights the importance of immunologic study of diverse taxa. Funding Sources NIH NIAID RO1AI146168 Topic Categories Veterinary and Comparative Immunology (VET)