HIV-1 infects the brain in the early stage of systemic viral infection. However, HIV-1-associated cognitive/ motor complex develops later in the course of disease, suggesting that host responses mediated by brain cells may contribute to inhibition of 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 sources of multifunctional cytokines. We previously reported that monocyte-derived ET-1 is stimulated during HIV infection and suggested that its potent vasoactive properties could potentially mediate alterations in the cerebral perfusion pattern associated with AIDS dementia complex. However, our studies with monocyte derived macrophages (MDM) revealed that in vitro infection with macrophage tropic HIV-1 isolates neither induces secretion of ET-1 nor potentiates its production by the known inducer, LPS. In addition, mRNA prepared from HIV-infected and uninfected MDM and examined using RT-PCR techniques revealed that genes for ET-1 and nitric oxide synthase (NOS), an enzyme responsible for production of the potent vasodilator, NO, are not expressed. It is probable that the in vitro system looking only at MDM 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 determined 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 likely mediated by a secreted factor, since astrocyte conditioned medium also suppressed HIV replication MDM and paraformaldehyde fixed astrocytes unable to secrete cytokines failed to inhibit HIV. In contrast, cell-to-cell contact with astrocytoma cells can enhance replication of HIV-1 in MDM, which correlates with enhanced production of M-CSF (submitted, AIDS). Additional studies revealed the expression of inducible NOS (iNOS) with concomitant production of NO in astrocytes, but not MDM, when astrocytes were cocultured with HIV-infected MDM. This coincided with decreased HIV replication. Although supernatants from cocultures of HIV-infected MDM and astrocytes stimulated iNOS expression in astrocytes, cytokines known to induce iNOS expression were not detected. A competitive NOS inhibitor partially reversed the HIV-1 suppressive effect of astrocytes (submitted, BLOOD). 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, dysregulated production of NO.
