CD4(+)CD25(+) regulatory T cells (Tregs) are critical for the peripheral immune 
tolerance. Understanding the signals for the generation of Tregs is important 
for the clinical immunotherapy, but only limited progress has been made on 
obtaining enough peripheral Tregs. The aim of this study was to evaluate the 
role of trichosanthin (Tk) extracted from Chinese medicinal herb Trichosanthes 
kirilowi on the function of Tregs in vitro and in vivo. We reported here that Tk 
is needed for the expansion of freshly isolated CD4(+)CD25(+)Tregs (nTregs) into 
Tk-expanded CD4(+)CD25(+)Tregs (Tk-Tregs) through up-regulating CD25 and Foxp3 
expression. The dose-response analyses indicated that 100 ng/ml Tk was the most 
appropriate dose. The result of real-time PCR showed that Tk-Tregs expressed 
1.5-fold higher levels of Foxp3 than those observed in nTregs. Tk-Tregs markedly 
suppressed activation of effector T cells at a suppressor/responder ratio of 
1:1, 1:2, 1:4, 1:8 or 1:16, and their effect was dose dependent. Moreover, 
Tk-Tregs secreted more immunosuppressive cytokines interleukin (IL)-10 and 
transforming growth factor (TGF)-beta1 after stimulating with antigen and 
antigen-presenting cells (APC). Transwell experiments showed that not only 
cell-to-cell contact but also soluble cytokines were involved in suppressive 
mechanism of Tk-Tregs. And Tk-Tregs were more efficient in suppressing CD25(-)T 
cell response to specific antigen than to irrelative antigen. Most importantly, 
it was revealed for the first time that Tk-Tregs could prolong the survival 
duration of mice with acute graft-versus-host disease (aGVHD). In conclusion, 
the study suggests a possible therapeutic potential of Tk-Tregs for clinical 
treatment on aGVHD.