The human immunodeficiency virus type 1 regulatory protein Tat inhibits interferon-induced iNos activity in a murine macrophage cell line

1 Department of Biochemistry, University of Southampton, Southampton SO16 7PX, UK 2 University Clinical Biochemistry Department, University of Southampton Medical School, Southampton General Hospital, Southampton SO16 6YD, UK Human immunodeficiency virus type 1 (HIV-1) infection is frequently associated with concurrent infection by opportunistic pathogens, against which production of nitric oxide by host macrophages provides a first line of defence. We have investigated whether regulatory HIV-1 proteins, such as Tat, can modulate the activity of the inducible nitric oxide synthase ( iNos ) gene when expressed in stable transfectant lines of RAW264.7 cells. A bioassay for Tat, based on transactivation of an HIV-1 LTR-CAT reporter gene, allowed selection of Tat-expressing cells. Parental and Tat-expressing macrophages accumulated identical levels of nitrite following lipopolysaccharide (LPS) stimulation. Interferon (IFN- ) stimulation however, resulted in reduced levels of nitrite accumulation as a direct consequence of Tat expression. Conditioned media from Tat-expressing cells reduced the level of nitrite accumulation in parental cells following IFN- stimulation but not stimulation with LPS. These results implicate HIV-1 Tat as a modulator of the IFN- -specific signal transduction pathways leading to iNos expression. Received 31 January 1996; accepted 17 April 1996.