Life after the thymus: CD31+ and CD31 human naive CD4+ T-cell subsets

Abstract

Early in life, thymic export establishes the size and the diversity of the human naive T-cell pool. Yet, on puberty thymic activity drastically decreases. Because the over- all size of the naive T-cell pool decreases only marginally during ageing, peripheral postthymic expansion of naive T cells has been postulated to account partly for the maintenance of T-cell immunity in adults. So far, the analysis of these pro- cesses had been hampered by the inabil- ity to distinguish recent thymic emigrants from proliferated, peripheral, naive T cells. However, recently, CD31 has been intro- duced as a marker to distinguish 2 subsets of naive CD4+ T cells with distinct T-cell receptor excision circle (TREC) content in the peripheral blood of healthy humans. Here, we review studies that have character- ized TREChi CD31+thymicnaive CD4+ T cells and have accordingly used the assess- ment of this distinct subset of naive CD4+ T cells as a correlate of thymic activity. We will discuss further potential clinical applications and how more research on CD31+thymicnaive and CD31-centralnaive CD4+ T cells may foster our knowledge of the impact of thymic involution on im- mune competence. © 2009 by The American Society of Hematology.