Mycobacterium avium subsp paratuberculosis Inhibits Gamma Interferon-Induced Signaling in Bovine Monocytes: Insights into the Cellular Mechanisms of Johne's Disease

Mycobacterium avium subsp. paratuberculosis is the causative agent of Johne's disease in cattle and may have implications for human health. Establishment of chronic infection by M. avium subsp. paratuberculosis depends on its subversion of host immune responses. This includes blocking the ability of infected macrophages to be activated by gamma interferon (IFN-gamma) for clearance of this intracellular pathogen. To define the mechanism by which M. avium subsp. paratuberculosis subverts this critical host cell function, patterns of signal transduction to IFN-gamma stimulation of uninfected and M. avium subsp. paratuberculosis-infected bovine monocytes were determined through bovine-specific peptide arrays for kinome analysis. Pathway analysis of the kinome data indicated activation of the JAK-STAT pathway, a hallmark of IFN-gamma signaling, in uninfected monocytes. In contrast, IFN-gamma stimulation of M. avium subsp. paratuberculosis-infected monocytes failed to induce patterns of peptide phosphorylation consistent with JAK-STAT activation. The inability of IFN-gamma to induce differential phosphorylation of peptides corresponding to early JAK-STAT intermediates in infected monocytes indicates that M. avium subsp. paratuberculosis blocks responsiveness at, or near, the IFN-gamma receptor. Consistent with this hypothesis, increased expression of negative regulators of the IFN-gamma receptors SOCS1 and SOCS3 as well as decreased expression of IFN-gamma receptor chains 1 and 2 is observed in M. avium subsp. paratuberculosis-infected monocytes. These patterns of expression are functionally consistent with the kinome data and offer a mechanistic explanation for this critical M. avium subsp. paratuberculosis behavior. Understanding this mechanism may contribute to the rational design of more effective vaccines and/or therapeutics for Johne's disease.