F. Vasseur et al., Scheduled kinetics of cell proliferation and phenotypic changes during immature thymocyte generation, EUR J IMMUN, 31(10), 2001, pp. 3038-3047
Precursor CD4(-)CD8(-) (DN) thymocytes rearrange their TCR-beta genes, and
only those which succeed in beta -selection subsequently expand and differe
ntiate into immature CD4(+)CD8(+) (DP) thymocytes. The cell subsets corresp
onding to the successive steps of this transition can be defined in terms o
f CD44 and CD25 expression. We partially synchronized the differentiation p
rocess by eliminating cycling cells with the anti-mitotic agent demecolcine
. Using in vivo pulse labeling with bromodeoxyuridine, we determined the or
der of entry into DNA synthesis of the different DN and transitory (CD4(-/l
o)CD8(+)) cell subsets. Two independent proliferation phases were identifie
d. The first cells to enter the cell cycle were CD44(-)CD25(lo), and CD4/CD
8/TCR-/BrdU four-color staining showed that they all expressed a low densit
y of the TCR-beta chain, an element of the pre-TCR (the TCR-alpha locus is
still in germ-line configuration at this stage). Cycling of CD44(+)CD25(+)
cells was detected later, and no starting point was observed at the CD44(-)
CD25(hi) stage. CD8 expression was immediately detectable in cycling cells,
but they took 24 In to reach the DID stage. The study of TCR-C alpha -defi
cient mice showed that P gene rearrangement occurred once proliferation had
ceased at the DP stage, and that it had no influence on the DN-DP transiti
on. These data show that precursor thymocytes undergo two independent waves
of expansion, and that the second wave is restricted to cells capable of p
re-TCR expression.