CD4 and infection: The modest and sustained systemic upregulation of TGF-β1 in HIV-1 and pathogenic SIV infection contrasts sharply with the pattern of TGF-β1 in non-pathogenic SIVagm infection of African green monkeys, where a robust CD4+ Treg cell response is activated one day after infection, with an associated pronounced increase in systemic TGF-β1, but circulating TGF-β1 concentrations during subacute and early chronic infection are in fact reduced compared those present prior to infection (24).