ONTOGENY OF TCR V-BETA-1 EXPRESSION REVEALED NOVEL INVARIANT ALTERNATIVE TRANSCRIPTS

Analysis of TCR-beta gene recombination and expression was performed b y quantitative PCR amplification technique throughout chicken embryoge nesis and development. Our data demonstrated that TCR V beta 1 promote rs were turned on by day 10 of embryogenesis, 2 days before detection of TCR-beta gene recombination. The V to D recombination step was firs t detected by day 11 of embryogenesis whereas DJ and V(D)J rearranged genes were detected 1 day later, on day 12 of embryogenesis. Thus, tra nscription of unrearranged TCR-beta genes in chickens precedes the exp ression of V(D)J recombinase activity as in mammals. In contrast, alth ough TCR-beta rearrangement starts with the D to J recombination step in mammals, it can start either by the VD or the DJ step in chickens. Furthermore, reverse transcriptase-PCR amplification of TCR-beta trans cripts revealed the presence of two kinds of alternative transcripts. These novel alternatively spliced products appeared in thymocytes from embryonic thymus during colonization periods and were absent in trans formed T cell lines. Splicing sites are located in the middle of V bet a 1 segments and lead to Delta V beta 1-C beta and Delta V beta 1-D be ta-J beta-C beta transcripts. Delta V beta 1-C beta transcripts might lead to synthesis of invariant truncated TCR beta-chains containing th e aminoterminal portion of the V beta 1 region followed by the C beta region. Because this type of splicing can be generated by using all kn own V beta 1 members, these invariant forms could play a role in thymo cyte development.