EVOLUTION OF SPECIFIC ANTIGEN RECOGNITION - SIZE-REDUCTION AND RESTRICTED LENGTH DISTRIBUTION OF THE CDRH3 REGIONS IN THE RAINBOW-TROUT

The immunoglobulin heavy chain repertoire in fish was investigated by cloning a total of 88 rearranged VDJ junctions from the head kidney B cell mRNA of a salmonid, the rainbow trout (Oncorhynchus mykiss). Trou t D-H segments are short and cannot be classified into independent D-H families. Several of the ten identified putative D-H segments had str etches of nucleotide sequence identity with mouse (DQ52, DFL 16.2 and Dsp 2.1), human (DM1) and chicken (D(H)4) D-H. There was a clear prefe rence for one or two of the three putative D-H reading frames and a st op codon is often present in the less used reading frame. Four of the six J(H) segments are preferentially used, and analysis of the V-H-D-H and D-H-J(H) junctions suggest the presence of N-nucleotides. The abs olute size and size heterogeneity of the rainbow trout CDRH3 are small er than those of the Xenopus, mouse and human CDRH3. About 75% of the 84 in-frame trout CDRH3 have 8, 9 or 10 residues and none of them have more than 11 residues. This homogeneization of the CDRH3 loop size ma y partly explain the restricted antibody diversity in lower vertebrate s.