Diversity and repertoire of IgW and IgM VH families in the newborn nurse shark

IgM and IgW VH cDNA clones isolated from newborn nurse shark primary and secondary lymphoid tissues had highly diverse and unique CDR3 with N-region addition and VDJ gene rearrangement, implicating functional TdT and RAG gene activity. Despite the clear presence of N-region additions, newborn CDR3 were significantly shorter than those of adults. The IgM clones are all included in a conventional VH family that can be classified into five discrete groups, none of which is orthologous to IgM VH genes in other elasmobranchs. In addition, a novel divergent VH family was orthologous to a published monotypic VH horn shark family. IgW VH genes have diverged sufficiently to form three families. IgM and IgW VH serine codons using the potential somatic hypermutation hotspot sequence occur mainly in VH framework 1 (FR1) and CDR1. Phylogenetic analysis of cartilaginous fish and lungfish IgM and IgW demonstrated they form two major ancient gene groups; furthermore, these VH genes generally diversify (duplicate and diverge) within a species.As in ratfish, sandbar and horn sharks, most nurse shark IgM VH genes are from one family with multiple, heterogeneous loci. Their IgW VH genes have diversified, forming at least three families. The neonatal shark Ig VH CDR3 repertoire, diversified via N-region addition, is shorter than the adult VDJ junction, suggesting one means of postnatal repertoire diversification is expression of longer CDR3 junctions.The major components of adaptive immunity, including Ig, T cell receptors, and MHC class I and class II, have been identified in the ancient jawed vertebrate taxon cartilaginous fish (Chondrichthyes), which includes sharks, skates, rays (elasmobranchs), and ratfish (holocephali) (Figure 1) [1,2]. Additionally, genes required for lymphopoiesis and Ig gene rearrangement/diversification, such as PU.1, Ikaros, RAG1, RAG2 and TdT, as well as primary and secondary lymphoid tissues, are also present within this vertebrate class [3-8]. Cartilaginou