Discovery of a unique Ig heavy-chain isotype (IgT) in rainbow trout: Implications for a distinctive B cell developmental pathway in teleost fish.

During the analysis of Ig superfamily members within the available rainbow trout (Oncorhynchus mykiss) EST gene index, we identified a unique Ig heavy-chain (IgH) isotype. cDNAs encoding this isotype are composed of a typical IgH leader sequence and a VDJ rearranged segment followed by four Ig superfamily C-1 domains represented as either membrane-bound or secretory versions. Because teleost fish were previously thought to encode and express only two IgH isotypes (IgM and IgD) for their humoral immune repertoire, we isolated all three cDNA isotypes from a single homozygous trout (OSU-142) to confirm that all three are indeed independent isotypes.