Mycobacterium avium Infection of Mouse Macrophages Inhibits IFN-γ Janus Kinase-STAT Signaling and Gene Induction by Down-Regulation of the IFN-γ Receptor

Abstract

Macrophage activation is required to control the growth of intracellular pathogens. Recent data indicate that macrophages become functionally deactivated during mycobacterial infection. We studied macrophage deactivation by examining the expression of a panel of IFN-γ-inducible genes and activation of Janus Kinase (JAK)-STAT pathway in Mycobacterium avium-infected macrophages. Reduced expression of IFN-γ-inducible genes—MHC class II gene Eβ; MHC class II transactivator; IFN regulatory factor-1; and Mg21, a gene coding for a GTP-binding protein—was observed in M. avium-infected macrophages. Decreased tyrosine phosphorylation and DNA binding activity of STAT1 in M. avium-infected macrophages stimulated with IFN-γ was observed. Tyrosine phosphorylation of JAK1, JAK2, and IFN-γRα was also reduced in infected cells. Northern and Western blot analyses showed that a down-regulation of IFN-γR α- and β-chain mRNA and protein occurred in M. avium-infected macrophages. The down-regulation of IFN-γR and inhibition of STAT1 activation were time dependent and required 4 h of infection for down-regulation of the IFN-γR and 8 h for STAT1 inhibition. These findings suggest that M. avium infection inhibits induction of IFN-γ-inducible genes in mouse macrophages by down-regulating IFN-γR, resulting in reduced phosphorylation of IFN-γRα, JAK1, JAK2, and STAT1.