The ''early'' CD4(+)CD8(+) stage of thymocyte differentiation hallmarks the end of a strong positive correlation between extracellular matrix receptor expression and protein tyrosine phosphorylation

We used irradiation-induced thymic regression/reconstitution to study phosp hotyrosine (PTyr) levels and expression of extracellular matrix receptors i n thymocyte subsets by flow cytometry. High PTyr levels (PTyr(hi)) characte rized cells from the CD4(-)CD8(-)(DN)CD25(in/hi) to the "early" CD4(+)CD8()(DP)CD25(-) stage. Correlation indexes (R) between the percentages of thes e PTyr(hi) cells and cells with up-regulated expression of alpha4 integrin (alpha4(hi)) were strongly positive (R= 0.91, P= 0.002, for DN; R= 0.98, P= 0.0001 for DP). At the "early" DP stage, R between PTyr(hi) cells and cell s with up-regulated expression of alpha5 integrin and L-selectin (alpha5(hi ) and L-sel(hi)) also rendered strongly positive (R>0.95, p<0.0003). "Late" expanding DP cells exhibited intermediate PTyr levels (PTyr(hi)), associat ed with a down-regulation of the adhesion receptors assessed. Triple-labeli ng suggested that in most early CD3(-/lo) cells, <alpha>4(hi) and alpha5(hi ), but not L-sel(hi) expression preceded a PTyr(hi) content. CD3(in/hi)-enr iched CD8(+) cells were also PTyr(hi), but conversely to the immature ones exhibited a tendency for a negative R between PTyr(hi) and alpha4(hi) (R= - 0.93, P= 0.067, n= 4) or alpha5(hi) cells (R= -0.77, P= 0.23, n= 4). CD4(+) cells were either PTyr(hi) or PTyr(hi), exhibiting a tendency for a positi ve R (R= 0.59, P= 0.124, n= 8) between PTyr(hi) and L-sel(hi) cells only. I n conclusion, our results associate an up-regulation of alpha4 and alpha5 c hains expression with PTyr(hi) levels and, as elsewhere published, with inc reased adhesion to fibronectin up to the "early" DP stage, but not afterwar ds.