A second immunoglobulin light chain isotype in the rainbow trout.

A novel immunoglobulin (Ig) light chain isotype, termed IgL2, has been isolated from trout lymphoid tissues both by reverse transcription - polymerase chain reaction (PCR) and screening of cDNA libraries. The CL domain of the new isotype shares only 29% residues with a recently cloned trout IgL isotype, termed IgL1, which has some similarities to Ckappa and Clambda isotype domains of several vertebrate species. Using anchored PCR, a VL element rearranged to CL2 was isolated. It is a member of a new VL family (VL2) of which four members were sequenced. These differ in the sequence of CDR1 and CDR2 but are remarkably similar in CDR3, i. e., at the junction between VL and JL segments. VL elements are rearranged to novel JL elements which differ from those described for VL1-CL1 rearrangements. Two cDNA clones contained JL-CL2 segments but no VL segments. The JL segments were preceded by typical rearrangements signal sequences [RSS, nonamer-23 base pair (bp) spacer-heptamer]. Further upstream of RSS were located two to three near identical 53 bp repeats, each of which included a 16 bp sequence similar to KI and KII sequences located at similar places in human and mouse Jk1 genes. These sequences are believed to act as binding sites for the protein KLP, which could be a transcriptional factor involved in the synthesis of germline Jk transcripts. Their phylogenic conservation in vertebrates suggests that they have an important role in B-cell differentiation. Remarkably, an RNA species of about 0.7 kilobase is the predominant IgL mRNA in trout spleen and coincides in size with JLCL2 transcripts. Genomic DNA blot analysis indicates that the trout L2 locus has a cluster-like organization similar to the trout L1 locus and the IgL locus of several teleost fish. A phylogenic analysis of VL2 and CL2 corroborates their low similarity to other vertebrate IgL chains and suggests an ancient diversification of the IgL locus.