Nitric oxide synthesis enhances human immunodeficiency virus replication in primary human macrophages

Macrophages are suspected to play a major role in human immunodeficiency vi rus (HIV) infection patho genesis, not only by their contribution to virus dissemination and persistence in the host but also through the dysregulatio n of immune functions. The production of NO, a highly reactive free radical , is thought to act as an important component of the host immune response i n several viral infections. The aim of this study was to evaluate the effec ts of HIV type 1 (HIV-1) Ba-L replication on inducible nitric oxide synthas e (iNOS) mRNA expression in primary cultures of human monocyte-derived macr ophages (MDM) and then examine the effects of NO production on the level of HIV-1 replication. Significant induction of the iNOS gene was observed in cultured MDM concomitantly with the peak of virus replication. However, thi s induction was not accompanied by a measurable production of NO, suggestin g a weak synthesis of NO. Surprisingly, exposure to low concentrations of a NO-generating compound (sodium nitroprusside) and L-arginine, the natural substrate of MOS, results in a significant increase in HIV replication. Acc ordingly; reduction of L-arginine bioavailability after addition of arginas e to the medium significantly reduced HIV replication. The specific involve ment of NO was further demonstrated by a dose-dependent inhibition of viral replication that was observed in infected macrophages exposed to N-G-monom ethyl L-arginine and N-G-nitro-L-arginine methyl eater (L-NAME), two inhibi tors of the iNOS. Moreover, an excess of L-arginine reversed the addition o f L-NAME, confirming that an arginine-dependent mechanism is involved. Fina lly, inhibitory effects of hemoglobin which can trap free NO in culture sup ernatants and in biological fluids in vivo confirmed that endogenously prod uced NO could interfere with HIV replication in human macrophages.