As CD4+CD25+ regulatory T cells (nTreg) have been reported to regulate immunity in a number of auto-immune and infectious diseases [14]–[18] and can exert their regulatory role through secretion of IL-10 and/or TGF-β we investigated, using intracellular staining for Foxp3 as well as transgenic Foxp3-GFP reporter mice [24], whether nTreg activation is correlated with the virulence of PyL infection. Here, CD4 is linked to infectious disease.