Signal transducer and activator of transcription-3/suppressor of cytokine signaling-3 (STAT3/SOCS3) axis in myeloid cells regulates neuroinflammation.

Suppressor of cytokine signaling (SOCS) proteins are feedback inhibitors of the JAK/STAT pathway. SOCS3 has a crucial role in inhibiting STAT3 activation, cytokine signaling, and inflammatory gene expression in macrophages/microglia. To determine the role of SOCS3 in myeloid cells in neuroinflammation, mice with conditional SOCS3 deletion in myeloid cells (LysMCre-SOCS3(fl/fl)) were tested for experimental autoimmune encephalomyelitis (EAE). The myeloid-specific SOCS3-deficient mice are vulnerable to myelin oligodendrocyte glycoprotein (MOG)-induced EAE, with a severe, nonresolving atypical form of disease. In vivo, enhanced infiltration of inflammatory cells and demyelination is prominent in the cerebellum of myeloid-specific SOCS3-deficient mice, as is enhanced STAT3 signaling and expression of inflammatory cytokines/chemokines and an immune response dominated by Th1 and Th17 cells. In vitro, SOCS3-deficient macrophages exhibit heightened STAT3 activation and are polarized toward the classical M1 phenotype. SOCS3-deficient M1 macrophages provide the microenvironment to polarize Th1 and Th17 cells and induce neuronal death. Furthermore, adoptive transfer of M2 macrophages into myeloid SOCS3-deficient mice leads to delayed onset and reduced severity of atypical EAE by decreasing STAT3 activation, Th1/Th17 cells, and proinflammatory mediators in the cerebellum. These findings indicate that myeloid cell SOCS3 provides protection from EAE through deactivation of neuroinflammatory responses.

Suppressor of cytokine signaling 3 inhibits antiviral IFN-beta signaling to enhance HIV-1 replication in macrophages.

HIV-1 replication within macrophages of the CNS often results in cognitive and motor impairment, which is known as HIV-associated dementia (HAD) in its most severe form. IFN-beta suppresses viral replication within these cells during early CNS infection, but the effect is transient. HIV-1 eventually overcomes this protective innate immune response to resume replication through an unknown mechanism, initiating the progression toward HAD. In this article, we show that Suppressor of Cytokine Signaling (SOCS)3, a molecular inhibitor of IFN signaling, may allow HIV-1 to evade innate immunity within the CNS. We found that SOCS3 is elevated in an in vivo SIV/macaque model of HAD and that the pattern of expression correlates with recurrence of viral replication and onset of CNS disease. In vitro, the HIV-1 regulatory protein transactivator of transcription induces SOCS3 in human and murine macrophages in a NF-kappaB-dependent manner. SOCS3 expression attenuates the response of macrophages to IFN-beta at proximal levels of pathway activation and downstream antiviral gene expression and consequently overcomes the inhibitory effect of IFN-beta on HIV-1 replication. These studies indicate that SOCS3 expression, induced by stimuli present in the HIV-1-infected brain, such as transactivator of transcription, inhibits antiviral IFN-beta signaling to enhance HIV-1 replication in macrophages. This consequence of SOCS3 expression in vitro, supported by a correlation with increased viral load and onset of CNS disease in vivo, suggests that SOCS3 may allow HIV-1 to evade the protective innate immune response within the CNS, allowing the recurrence of viral replication and, ultimately, promoting progression toward HAD.