Macrophages play a critical role in the pathogenesis of HIV infection, both as targets for virus replication and as sources of multifunctional cytokines. Our early studies showed that normal human monocytes stimulated with the HIV-1 envelope protein, gp120, produce the HIV-modulatory cytokines, TNF-a, IL-1b, IL-6 and GM-CSF (JI 1991) and the potent vasoactive peptide, ET-1 (JI 1993). Later studies with monocyte derived macrophages (MDM) revealed that HIV-1 infection fails to induce these latter cytokines in vitro, but consistently induces M-CSF production (JI 1995) which facilitates monocyte differentiation and increases the susceptibility of MDM to HIV infection by enhancing expression of the HIV receptors, CD4 and CCR5. Viral specificity of M-CSF production in MDM was then determined using a panel of single-stranded RNA viruses, including HIV-1, HIV-2, measles and respiratory syncytial viruses (MV and RSV). Only monocytropic strains of HIV-1 (JI 2000) and HIV-2 (in prep, 2002) caused enhanced production of M-CSF. Chemokines MIP-1a/b and MCP-1 were induced by HIV-1, but production was extremely diminished with HIV-2, suggesting that chemokines and M-CSF are needed to establish a viral reservoir, but chemokine production may correlate more closely with pathogenicity. Other studies using anti-retroviral agents (AZT and Ritonavir) confirmed that virus replication and M-CSF production are inextricably linked in HIV-infected MDM, such that inhibition of one leads to comcommitant inhibition of the other (ARHR 2002). Since HIV arose from cross-species transmission of SIV and progressed from a benign to highly pathogenic disease with initial infections targeting MDM, we investigated the ability of SIV to infect human MDM. We found that 12 of 16 SIV isolates belonging to 5 different primate lentivirus families were capable of infecting MDM and 11 of these were also able to replicate in human PBMC. Isolates that replicated in MDM also induced production of M-CSF, but varied with regard to pattern of chemokines induced (in prep, 2002). Ongoing studies will provide insight into determinants of these lentiviruses that are pathogenic for the human population with the hope of identifying novel targets for therapeutic intervention. Since astrocytes surround HIV-producing cells in the brain and would have the potential to influence virus replication, we studied their effect on HIV expression in human MDM. Co-culture of HIV-infected MDM with primary human astrocytes caused reduced HIV replication, mediated in part by an unidentified astrocyte secreted factor (AIDS 1999), in addition to expression of inducible nitric oxide synthase (iNOS) and production of NO by astrocytes (Blood 1999). Our data indicate 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. This 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.
