The emergence and evolution of the mammalian germinal center: analysis of the splenic microarchitecture of the teleost threespine stickleback (Gasterosteus aculeatus)

Abstract

Lymphoid cell-based adaptive immunity first arose in jawed fish ~500 mya and fish lymphoid cells share many conserved features with their mammalian counterparts. Both fish and mammalian B cells respond to T-dependent antigen by proliferating, differentiating into plasma cells, and generating high-affinity, antigen-specific antibody. In mammals, this response occurs in the germinal center (GC) and is highly dependent on GC architecture and cellular dynamics, which direct B cell SHM and clonal selection. Though most cellular components are conserved, little is known with regard to the immune architecture of the fish humoral adaptive response. Therefore, we sought to investigate the organization of B cell activation within the lymphoid tissues of the fish, threespine stickleback (Gasterosteus aculeatus). Stickleback splenic B cells are organized around melanomacrophages (MM), highly pigmented melanomacrophages which are proposed homologues to mammalian follicular dendritic cells (FDCs). We find that MM aggregates, known as melanomacrophage centers (MMCs), increase in size in response to immunization, but not control injection with adjuvant alone, suggesting that the MMC response is antigen driven. Analysis of lymphoid tissue architecture indicates B and T lymphocytes inhabit distinct regions of the stickleback spleen. For instance, IgM mRNA-expressing cells aggregate in discrete regions surrounding MMC clusters, comparable to the organization of mammalian B cells around FDCs. These findings support the hypothesis that MMCs are evolutionary precursors to GCs. Furthermore, it suggests that the microarchitecture necessary to sustain somatic hypermutation and clonal selection evolved early in vertebrates.