TCR gene rearrangements and expression of the pre-T cell receptor complex during human T-cell differentiation

Recent studies have identified several populations of progenitor cells in t he human thymus. The hematopoietic precursor activity of these populations has been determined. The most primitive human thymocytes express high level s of CD34 and lack CD1a. These cells acquire CD1a and differentiate into CD 4(+)CD8(+) through CD3(-)CD4(+)CD8(-) and CD3(-)CD4(+) CD8 alpha(+)beta(-) intermediate populations. The status of gene rearrangements in the various TCR loci, in particular of TCR delta and TCR gamma, has not been analyzed i n detail. In the present study we have determined the status of TCR gene re arrangements of early human postnatal thymocyte subpopulations by Southern blot analysis. Our results indicate that TCR delta rearrangements initiate in CD34(+)CD1a(-) cells preceding those in the TCR gamma and TCR beta loci that commence in CD34(+)CD1a(+) cells. Furthermore, we have examined at whi ch cellular stage TCR beta selection occurs in humans. We analyzed expressi on of cytoplasmic TCR beta and cell-surface CD3 on thymocytes that lack a m ature TCR alpha beta. In addition, we overexpressed a constitutive-active m utant of p56(lckF505) by retrovirus-mediated gene transfer in sequential st ages of T-cell development and analyzed the effect in a fetal thymic organ culture system, Evidence is presented that TCR beta selection in humans is initiated at the transition of the CD3(-)CD4(+)CD8(-) into the CD4(+)CD8 al pha(+)beta(-) stage. (C) 1999 by The American Society of Hematology.