Mycobacterium avium-induced SOCS contributes to resistance to IFN-gamma-mediated mycobactericidal activity in human macrophages.

Mycobacterium avium is an opportunistic pathogen that commonly infects individuals colonized with HIV-1, although it is less frequent in the post-HAART era. These microorganisms invade macrophages after interacting with TLR2 and/or CD14 co-receptors, but signaling pathways promoting survival in macrophages are not well defined. Although IFN-gamma plays an important role in protective immunity against bacterial infections, IFN-gamma responses are compromised in AIDS patients and evidence suggests that exogenous IFN-gamma is inadequate to clear the mycobacteria. To determine the mechanism by which M. avium survives intracellularly, even in the presence of IFN-gamma, we studied the effect of mycobacteria infection in macrophages during early IFN-gamma signaling events. M. avium infected cells exhibited a reduced response to IFN-gamma, with suppressed phosphorylation of STAT-1 compared with uninfected cells. Interaction of M. avium with macrophage receptors increased gene expression of the suppressors of cytokine signaling (SOCS) to diminish IFN responsiveness. Specifically, we observed an increase in mRNA for both SOCS-3 and SOCS-1, which correlates with elevated levels of SOCS protein and positive immunostaining in M. avium/HIV-1 co-infected tissues. We also linked the p38 MAPK signaling pathway to mycobacterial-induced SOCS gene transcription. The induction of SOCS may be part of the strategy that allows the invader to render the macrophages unresponsive to IFN-gamma, which otherwise promotes clearance of the infection. Our data provide new insights into the manipulation of the host response by this opportunistic pathogen and the potential for modulating SOCS to influence the outcome of M. avium infection in immunocompromised hosts.