In humans, dysregulation in the equilibrium between natural T reg cells and effector T cells has been associated with impaired immune responses to chronic infections, such as Helicobacter pylori, hepatitis C virus, and HIV (18???20), and depletion of CD4+CD25+ cells from PBMCs has been shown to increase antigen-specific proliferative and IFN-?? responses against several microbial antigens (21???23