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 CD3(high)CD4(+)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