JUNCTIONAL DIVERSITY IN THE ABSENCE OF N-REGIONS - NEONATAL T-CELL RECEPTOR BETA-CHAIN JUNCTIONAL SEQUENCES ARE MORE HETEROGENEOUS THAN NEONATAL T-CELL RECEPTOR-GAMMA-DELTA OR IGH JUNCTIONS

Early in ontogeny, Ig, TCR-alphabeta, and TCR-gammadelta lack N region s. in addition, Ig and TCR-gammadelta junctions preferentially occur a t regions of short sequence homology, thus limiting junctional diversi ty for these neonatal lymphocyte populations. Here, we analyze the ext ent of heterogeneity in TCR-beta chain junctions made early in ontogen y. DNA and cDNA from fetal/neonatal thymocytes were amplified by polym erase chain reaction, and the V-D and D-J junctions from these randoml y generated sequences were analyzed. The D-J junctions were very heter ogeneous, and displayed little evidence of homology-directed recombina tion. The Vbeta8-D and Vbeta5-D junctions that we analyzed each had a particular junctional sequence that was created at the site of a two-n ucleotide homology, but in each case that sequence only comprised 10 t o 17% of the total sequences. This junctional heterogeneity of N regio n lacking TCR-beta chains can be partially explained by a relative pau city of homologies at the appropriate locations near the coding ends, particularly at the D-J junction, but other factors such as the sequen ce surrounding the homology may also contribute. Thus, TCR-beta chains have extensive junctional heterogeneity early in ontogeny before N re gions begin to be added. Since TCR-alphabeta CDR3 plays a major role i n binding antigenic peptides, this junctional heterogeneity is likely to be advantageous for establishing a diverse TCR repertoire. We sugge st that the sequences of the coding ends of the TCR-alphabeta have bee n selected through evolution to avoid the restricted junctional divers ity resulting from homology-directed recombination.