CD31, a Valuable Marker to Identify Early and Late Stages of T Cell Differentiation in the Human Thymus [IMMUNE SYSTEM DEVELOPMENT]

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^highCD4⁺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 TCR 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.

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