Identification and characterization of antigen retaining cells in the putative germinal centres of goldfish (Carassius auratus)

Abstract

Follicular dendritic cells (FDCs) present within the germinal centres (GCs) capture the antigen in the form of an immune complex on their cell surface and facilitates the selection of high-affinity antibody-producing B-cells. Recruitment of B-cell to GC and the survival of B-cells within GC is regulated by FDCs. Production of cytokines by FDCs facilitates the clearance of apoptotic cells within GC, thereby preventing the production of auto-reactive B-cells. Fish were initially thought to lack antibody affinity maturation (AAM) partially because they lack a histologically distinct germinal centre. Later studies in channel catfish showed the presence of fully functional AID (activation-induced cytidine deaminase – mutator enzyme required for AAM) co-localized among aggregates of immune cells referred to as melanomacrophage clusters (MMCs). Studies have shown that experimentally injected antigen gets trapped in and around the MMCs. Based on the similarities between MMs and GCs we hypothesized that MMCs in fish could be analogues to mammalian GCs. The goal of this work was to identify FDC-like cell type in fish lymphoid organs. Goldfish immunization experiments and mRNA based transcriptome analysis revealed pigmented melanomacrophages (MMs) in fish have functional similarities to mammalian FDCs.