Discovery of a new class of immunoglobulin heavy chain from fugu

In teleosts, the genomic organization of the immunoglobulin (Ig) heavy (H)-chain locus was thought to follow a typical translocon-type multigene structure; however, recent studies have indicated a variation in the structure and this might be teleost specific. Isotypes of the Ig H-chain, namely IgM, IgD, IgZ and IgT, have been identified. In this study, we report the discovery of a new class of IgH from fugu. This isotype was first identified from the genomic sequence of the fugu IgH locus. This novel IgH gene is composed of two constant (C) domains, a hinge region, and two exons encoding membrane regions. Surprisingly, the new IgH gene is present between the variable (V)H and Cmu regions of the locus. The C domains of the new isotype do not show any significant similarity to mammalian or fish IgH genes. The cloned cDNA from the new isotype has typical Ig H-chain characteristics and is expressed as both secretory and membrane form. Transcript analyses suggest that the new IgH from fugu might only use the joining (J)H segments present in front of the new CH domains and that the usage of DH and JH segments is specific to the isotype expressed. The expression pattern of the gene has been confirmed by in situ hybridization and PCR studies.