A novel chimeric Ig heavy chain from a teleost fish shares similarities to IgD

IgD is considered to be a recently evolved Ig, being previously found only in primates and rodents. Here we describe, from a teleost fish (the channel catfish, Ictalurus punctatus), a novel complex chimeric Ig heavy chain, homologous, in part, to the heavy chain (δ) of IgD. In addition to alternative secretory or membrane-associated C termini, this chimeric molecule contains a rearranged variable domain, the first constant domain of μ, and seven constant domains encoded by a δ gene homolog. Identification of the catfish gene as δ is based on the following properties: sequence relatedness to mammalian δ; a location within the IgH locus that is immediately downstream of the μ gene; separate terminal exons for the secretory and membrane forms; coexpression with the complete μ chain in some but not all B cells. These results (i) suggest that IgD is an ancient immunoglobulin that was present in vertebrates ancestral to both the mammals and the ray-finned fishes, and (ii) raise the possibility that this Ig isotype may have served an as yet unidentified important function early in the evolution of the immune system.