To more clearly determine the role of Tregs in the pathogenesis of experimental African trypanosomiasis, we adoptively transferred highly enriched (>97%, Fig. 6A) naïve syngeneic CD4+CD25+ (Tregs) and CD4+CD25− (non Tregs) cells (4 million cells/mouse) into naïve C57BL/6 mice and after 24 hr infected them with T. congolense. This evidence concerns the gene CD4 and human African trypanosomiasis.