IMMUNOBIOLOGY Brief report Defective circulating CD25 regulatory T cells in patients with chronic immune

is characterized by the presence of anti-platelet autoantibodies as a result of loss of tolerance. CD4CD25 regulatory T cells (Tregs) are important for mainte-nance of peripheral tolerance. Decreased levels of peripheral Tregs in patients with ITP have been reported. To test whether inefficient production or reduced immuno-suppressive activity of Tregs contributes to loss of tolerance in patients with chronic ITP, we investigated the fre-quency and function of their circulating CD4CD25hi Tregs. We found a com-parable frequency of circulating CD4CD25hiFoxp3 Tregs in patients and controls (n 16, P>.05). However, sorted CD4CD25hi cells from patients with chronic ITP (n 13) had a 2-fold reduction of in vitro immunosuppressive activity compared with controls (n 10, P <.05). The impaired suppression was specific to Tregs as shown by cross-mixing experiments with T cells from con-trols. These data suggest that functional defects in Tregs contribute to breakdown of self-tolerance in patients with chronic ITP. (Blood. 2008;112:1325-1328