HIV-1 infects the brain in the early stage of systemic viral infection, but HIV-associated cognitive/motor complex develops later in the course of disease, suggesting that host responses mediated by brain cells may inhibit the early productive infection in the brain and the development of neurologic disease. Macrophages play a critical role in the pathogenesis of HIV infection, both as targets for viral replication and as a source of multifunctional cytokines. Our previous studies revealed that monocyte-derived ET-1 is stimulated during HIV infection and suggested that its potent vasoactive properties could cause alterations in the cerebral perfusion pattern associated with AIDS dementia complex (JI 1993). However, our studies with monocyte derived macrophages (MDM) showed that HIV-1 infection neither induces secretion of ET-1 nor potentiates its production by the known inducer, LPS, in vitro. In addition, expression of the genes for ET-1 and nitric oxide synthase (NOS), an enzyme responsible for production of the potent vasodilator, NO, are not detected by RT-PCR in mRNA prepared from HIV-infected MDM. Thus, the in vitro system looking only at MDM most likely fails to reflect the more complex cellular interactions occurring in vivo. Since HIV-1 producing cells in the brain, such as blood-derived macrophages and brain microglia, are surrounded by astrocytes, we studied the effect of astrocytes on HIV-1 expression in MDM. Co-culture of HIV-infected MDM with primary human astrocytes resulted in diminished virus replication. This effect was mediated in part by an unidentified secreted factor, since astrocyte conditioned medium could suppress HIV replication MDM and paraformaldehyde fixed astrocytes unable to secrete cytokines failed to inhibit HIV. In contrast, cell-to-cell contact with astrocytes augmented HIV-1 replication in MDM, which correlated with enhanced production of M-CSF (AIDS 1999). Additional studies revealed the expression of inducible NOS (iNOS)and production of NO by astrocytes, but not MDM, when astrocytes were cocultured with HIV-infected MDM. This coincided with decreased HIV replication. Supernatants from cocultures of HIV-infected MDM and astrocytes stimulated iNOS expression in astrocytes, but cytokines known to induce iNOS expression were not detected, suggesting a novel factor may mediate this effect. A competitive NOS inhibitor partially reversed the HIV-1 suppressive effect of astrocytes (BLOOD 1999), consistent with a dual mechanism for astrocyte-mediated HIV inhibition. Furthermore, studies utilizing NO donor compounds with variable half lives and flux of production reveal that NO inhibits HIV-1 replication directly via a mechanism most likely involving NFkB. This suggests that astrocytes play a pivotal role in determining the course of neurologic HIV disease via production of HIV modulating cytokines and expression of iNOS/NO. It also leads us to speculate that neurologic damage observed in HIV disease may ensue from prolonged, high level production of NO by astrocytes, which may reflect a host attempt to inhibit virus replication.
