Identification of a pharyngeal mucosal lymphoid organ in zebrafish and other teleosts: tonsils in fish?

The constant exposure of the fish branchial cavity to aquatic pathogens causes local mucosal immune responses to be extremely important for their survival. Here, we used a marker for T lymphocytes/natural killer cells (ZAP70) and advanced imaging techniques to investigate the lymphoid architecture of the zebrafish branchial cavity. We identified a sub-pharyngeal lymphoid organ, which we tentatively named “Nemausean Lymphoid Organ” (NELO). NELO is enriched in T/NK cells, plasma/B-cells and antigen-presenting cells embedded in a network of reticulated epithelial cells. Presence of activated T-cells and lymphocyte proliferation but not V(D)J recombination or hematopoiesis, suggests NEMO is a secondary lymphoid organ. In response to infection, NELO displays structural changes including the formation of T/NK cells clusters. NELO and gill lymphoid tissues form a cohesive unit within a large mucosal lymphoid network. Collectively, we reveal an unreported mucosal lymphoid organ reminiscent of mammalian tonsils that evolved in multiple teleost fish families.