Targeting cFMS signaling to restore immune function and eradicate HIV reservoirs

While combination anti-retroviral therapy (cART) has improved the length and quality of life of individuals living with HIV-1 infection, the prevalence of HIV-associated neurocognitive disorders (HAND) has increased and remains a significant clinical concern. The neuropathogenesis of HAND is not completely understood, however, latent HIV infection in the central nervous system (CNS) and chronic neuroinflammation are believed to play a prominent role. CNS-associated macrophages and resident microglia are significant contributors to CNS inflammation and constitute the chief reservoir of HIV-1 infection in the CNS. Previous studies from our lab suggest monocyte/macrophage invasion of the CNS in HIV may be driven by altered monocyte/macrophage homeostasis. We have reported expansion of a monocyte subset (CD14+CD16 +CD163+) in peripheral blood of HIV+ patients that is phenotypically similar to macrophages/microglia that accumulate in the CNS as seen in post-mortem tissue. The factors driving the expansion of this monocyte subset are unknown, however, signaling through cFMS, a type III receptor tyrosine kinase (RTK), may play a role. Macrophage-colony stimulating factor (M-CSF), a ligand of cFMS, has been shown to be elevated in the cerebral spinal fluid (CSF) of individuals with the most severe form of HAND, HIV-associated dementia (HAD). M-CSF promotes a Macrophage-2-like phenotype and increases CD16 and CD163 expression in cultured monocytes. M-CSF has also been shown to increase the susceptibility of macrophages to HIV infection and enhance virus production. These findings, in addition to the known function of M-CSF in promoting macrophage survival, supports a role for M-CSF in the development and maintenance of macrophage viral reservoirs in tissues where these cells accumulate, including the CNS. Interestingly, a second ligand for cFMS, IL-34, was recently identified and reported to share some functions with M-CSF, suggesting that both ligands may contribute to HIV-associated CNS injury and AIDS pathogenesis. Through immunohistochemical studies using a relevant animal model of HIV infection, SIV infected rhesus macaques, we reported the presence of M-CSF and IL-34 in the brains of seronegative and SIV+ animals, for the first time, and identified spatial differences in the expression of these ligands. Important to our interest in viral persistence in the CNS, we observed the predominance of M-CSF expression in brain to be by cells that comprise perivascular cuffs and nodular lesions, which contain monocytes/ macrophages that have migrated into the CNS. IL-34 appeared to be a tissue-specific ligand expressed by resident microglia. Like M-CSF, we found that IL-34 also increased the frequency of CD16 +CD163+ monocytes in vitro. We further investigated the potential of cFMS inhibition as a means to abrogate macrophage-2-like immune polarization using the small molecule tyrosine kinase inhibitor (TKI), GW2580. The addition of GW2580 abolished cFMS ligand-mediated increases in CD16+CD163+ monocyte frequency in human peripheral blood mononuclear cells (PBMC) as well as virus production in HIV infected primary human microglia. Furthermore, we found cFMS-mediated upregulation of CD16 and CD163 to be relevant to an additional disease process, high-grade astrocytomas, suggesting that M-CSF and IL-34 may be mediators of other neuroinflammatory diseases, as well. We hope these findings will provide insight into the role of altered monocyte/macrophage homeostasis in HIV disease and identify a novel strategy for targeting long-lived cellular reservoirs of HIV infection through restored immune homeostasis.