Measuring CDR3 length variability in individuals during ontogeny

The portion of the antibody sequence that contributes most to the confornation of the antigen-combining site is the third complementarity-determining region (CDR3). The CDR3 in antibody heavy chains are somatically produced by the rearrangement of three different immunoglobulin (Ig) gene segments, V_H, D and J_H, and are therefore highly variable in sequence and length. We have devised a technique for rapid analysis of CDR3 length variability from a large number of cDNA samples. This technique allows comparative evaluation of IgH heterogeneity in individuals without recourse to DNA cloning and sequencing. We also present data on Ig sequences from different stages of development in the amphibian, Xenopus laevis. Most of the larval and post-metamorphic Ig samples contained CDR3 of 3-10 codons, two codons shorter than those of the adult CDR3, which were 5-12 codons. Antibodies made by immunized tadpoles have been shown to be much less diverse than those produced by adults to the same antigen. The shorter tadpole CDR3 perhaps generates a more limited spectrum of antigen-combing site structures and, in this way, makes for a more restricted antibody repertoire.