HIV type 1 Tat protein inhibits interleukin 12 production by human peripheral blood mononuclear cells

HIV-1 Tat protein, which trans-activates HIV-1 expression, exerts many effects on host immune function. Meanwhile, PBMCs and pulmonary macrophages from HIV-1-infected patients produce only a small amount of IL-12, which plays an essential role in the development of helper T type 1 (Th1) cells, and in the generation of cytotoxic T lymphocytes. We examined the possibility that Tat suppresses IL-12 production by PBMCs from healthy donors. Tat significantly inhibited IL-12 production by human PBMCs stimulated with Staphylococcus aureus Cowan 1 strain (SAC) at concentrations between 5 and 40 ng/ml. Immunoabsorption by using polyclonal antibody to Tat abolished the suppression of the IL-12 production by Tat. Tat at the same concentrations did not affect IL-10, IL-6, or TNF-alpha production. Other HIV-1 proteins (Nef and gp120) did not influence IL-12 production. Tat also suppressed the expression of mRNA encoding the p40 chain of IL-12, whereas it did not affect the expression of mRNA encoding IL-10 and beta-actin. IL-12 production by monocytes, separated from PBMCs by the adhesion method, was also inhibited by Tat. These results suggest that Tat protein is one of the main causes of decreased IL-12 production by PBMCs (mostly by monocytes) from HIV-1-infected individuals.