CD31, a Valuable Marker to Identify Early and Late Stages of T Cell Differentiation in the Human Thymus.

Although CD31 expression on human thymocytes has been reported, a detailed analysis of CD31 expression at various stages of T cell development in the human thymus is missing. In this study, we provide a global picture of the evolution of CD31 expression from the CD34+ hematopoietic precursor to the CD45RA+ mature CD4+ and CD8+ single-positive (SP) T cells. Using nine-color flow cytometry, we show that CD31 is highly expressed on CD34+ progenitors and stays high until the early double-positive stage (CD3-CD4+CD8α+β-). After β-selection, CD31 expression levels become low to undetectable. CD31 expression then increases and peaks on CD3highCD4+CD8+ double-positive thymocytes. However, following positive selection, CD31 expression differs dramatically between CD4+ and CD8+ lineages: homogeneously high on CD8 SP but lower or negative on CD4 SP cells, including a subset of CD45RA+CD31- mature CD4+ thymocytes. CD31 expression on TCRγδ thymocytes is very similar to that of CD4 SP cells. Remarkably, there is a substantial subset of semimature (CD45RA-) CD4 SP thymocytes that lack CD31 expression. Moreover, FOXP3+ and ICOS+ cells are overrepresented in this CD31- subpopulation. Despite this CD31-CD45RA- subpopulation, most egress-capable mature CD45RA+ CD4 SP thymocytes express CD31. The variations in CD31 expression appear to coincide with three major selection processes occurring during thymopoiesis: β-selection, positive selection, and negative selection. Considering the ability of CD31 to modulate the TCR's activation threshold via the recruitment of tyrosine phosphatases, our results suggest a significant role for CD31 during T cell development.