A 2ND 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 C L domain of the new isotype shares only 29% residues with a recently c loned trout IgL isotype, termed IgL1, which has some similarities to C kappa and C lambda isotype domains of several vertebrate species, Usi ng 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 an remarkably simila r in CDR3, i. e., at the junction between VL and JL segments. VL eleme nts are rearranged to navel JL elements which differ from those descri bed for VL1-CL1 rearrangements, Two cDNA clones contained JL-CL2 segme nts but no VL segments. The JL segments were preceded by typical rearr angements signal sequences [RSS, nonamer-23 base pair (bp) spacer-hept amer]. Further upstream of RSS were located two to three near identica l 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 ge nes. These sequences are believed to act as binding sites for the prot ein KLP, which could be a transcriptional factor involved in the synth esis of germline Jk transcripts, Their phylogenic conservation in vert ebrates suggests that they have an important role in B-cell differenti ation. Remarkably, an RNA species of about 0.7 kilobase is the predomi nant IgL mRNA in trout spleen and coincides in size with JLCL2 transcr ipts. Genomic DNA blot analysis indicates that the trout L2 locus has a cluster-like organization similar to the trout LI locus and the IgL locus of several teleost fish. A phylogenic analysis of VL3 and CL2 co rroborates their low similarity to other vertebrate IgL chains and sug gests an ancient diversification of the IgL locus.