Human anergic/suppressive CD4+CD25+ T cells:: a highly differentiated and apoptosis-prone population

Anergic/suppressive CD4(+)CD25(+) T cells exist in animal models but their presence has not yet been demonstrated in humans. We have identified and characterized a human CD4(+)CD25(+) T cell subset, which constitutes 7-10 % of CD4(+) T cells in peripheral blood and tonsil. These cells are a CD45RO(+)CD45RB(low) highly differentiated primed T cell population that is anergic to stimulation. Depletion of this small subset from CD4(+) T cells significantly enhances proliferation by threefold in the remaining CD4(+)CD25(-) T cells, while the addition of isolated CD4(+)CD25(+) T cells to CD4(+)CD25(+) T cells significantly inhibits proliferative activity. Blocking experiments suggest that suppression is not mediated via IL-4, IL-10 or TGF-beta and is cell-contact dependent. Isolated CD4(+)CD25(+) T cells are susceptible to apoptosis that is associated with low Bcl-2 expression, but this death can be prevented by IL-2 or fibroblast-secreted IFN-beta. However, the anergic/suppressive state of these cells is maintained after cytokine rescue. These human regulatory cells are therefore a naturally occurring, highly suppressive, apoptosis-prone population which are at a late stage of differentiation. Further studies into their role in normal and pathological situations in humans are clearly essential.