COMPLETE TCR-DELTA REARRANGEMENTS AND PARTIAL (D-J) RECOMBINATION OF THE TCR-BETA LOCUS IN CD34(-BLOOD - EVIDENCE FOR T-CELL LINEAGE COMMITMENT()7(+) PRECURSORS FROM HUMAN CORD)

In the neonatal human thymus, early immature precursors co-express CD3 4 and CD7 cell surface Ags, and we have recently shown that its most p rimitive CD34(+)7(+)1(-) fraction includes TCR-beta-rearranging cells, Bone marrow and cord blood also contain a CD34(+)7(+) population. Alt hough this population is heterogeneous in terms of both phenotype and differentiation capacities, it may include T cell-committed thymus col onizing precursors (prothymocytes), Recently, it has been shown in the mouse that initiation of TCR-beta rearrangements is not restricted to early thymocytes, In the present study, we examined the TCR-beta and TCR-delta rearrangement and transcription status of cord blood CD34(+) 7(-) and CD34(+)7(+) subpopulations. RNA and DNA were isolated from th ese cell subsets purified by two consecutive rounds of fluorescence-ac tivated cell sorting from CD3-depleted cord blood cells. Using D-J bet a or V-J beta primer sets and genomic DNA PCR amplification, we showed that CD34(+)7(+), but not CD34(+)7(-), cells contained D-J beta rearr angements without concomitant V-D beta recombination. These partial TC R-beta rearrangements within CD34(+)7(+) progenitors were also confirm ed at the transcriptional level, Furthermore, whereas none of the CD34 cord blood cell fraction expressed TCR-a transcripts, mature V delta 1-C delta mRNA could be amplified from the CD34(+)7(+) subset, The TdT gene was also transcriptionally active in CD34(+)7(+) cells, thus con firming their lymphoid progenitor content, These data indicate that co rd blood CD34(+)7(+) cells, like CD34(+)7(+)1(-) neonatal thymocytes, can initiate TCR-beta gene recombination, reinforcing the idea that T cell commitment may occur before prothymocyte migration to the thymus.