The cellular context of AID expressing cells in fish lymphoid tissues.

It has long been held that the cold-blooded vertebrates lack mammalian-like germinal centers, though they do have affinity maturation and the immunoglobulin mutator activation-induced cytidine deaminase or AID. Using AID as a marker of sites of somatic hypermutation, we have identified discrete cell clusters of up to several thousand cells, in the spleen and kidney of channel catfish (Ictalurus punctatus), which may be primordial germinal centers. In situ hybridization revealed that AID expressing cells are interspersed or surrounded by a population of pigmented CSF1-R expressing cells called melano-macrophages.