Cross-talk between intracellular pathogen infection and IFNγ signaling drives enhanced pro-inflammatory responses
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Many intracellular pathogens have evolved to evade immune responses and establish a secure niche inside host cells. One such stealth pathogen is the obligate intracellular bacterium Coxiella burnetii , the causative agent of Q-fever. Coxiella translocates an array of bacterial proteins (‘effectors’) into the host cell through a type IVB secretion system (T4BSS) that mediates suppression of pathogen sensing and innate immunity. Yet, at a systemic level, immunocompetent hosts often restrict pathogens through Th1-mediated and cell-autonomous immunity through the expression of immune-inducible genes. However, the expression and regulation of chemokines, particularly, the CXC-ligands (CXCL9,-10,-11) that are considered biomarkers of Q-fever, is poorly understood. We observed minimal to no CXCL10 transcript levels during Coxiella infection. However, Coxiella -infected cells robustly augmented IFNγ-activated expression of CXCL10 in both phagocytic and non-phagocytic cells, and this process was dependent on viability and T4BSS in epithelial cells. This phenomenon extends to other highly pro-inflammatory cytokines and other pathogens including Salmonella, Mycobacteria (H37Ra) and Toxoplasma . Synergistic increase in CXCL10 expression in Coxiella -infected, IFNγ-activated cells requires ISRE and NF-κB transcriptional elements in the promoter, and the transcription factors STAT1, STAT3 and IRF9. Inhibition of STAT3 by small molecule inhibitors potently decreased the excess promoter activity of CXCL10 . In addition, treatment of Coxiella -infected cells with IFNγ is associated with decreased expression of SOCS1 , a negative regulator of the IFNγ signaling axis and relatively higher detection of extracellular bacteria. Altogether, these data demonstrate that intracellular pathogens including those conventionally considered to be “immunologically silent”, robustly synergize with IFNγ signaling, with STAT3 activation emerging to be a nodal point for promoting both persistent infection as well as synergism in the expression of immune genes.
Author summary
Acute host immune response is often associated with production of soluble messenger molecules called cytokines/chemokines which direct the migration, recruitment and activation of leukocytes and serve as biomarkers in infectious and inflammatory diseases. The regulation of expression of these molecules and their influence on the infection process is not well-understood. In particular, interferon-gamma (IFNγ), a potent pro-inflammatory cytokine produced by activated T and NK cells, activates signaling pathways involved in host defense and inflammation in macrophages and other cell types. We observed that infection with many intracellular bacterial/parasitic pathogens that employ sophisticated immune evasion strategies, synergize with IFNγ signaling and significantly amplify the levels of pro-inflammatory mediators implicating the origin of adverse immune pathologies. We investigated the mechanistic basis of this seemingly counter-intuitive phenomenon, underlying host and bacterial factors involved in distinct cell types, and identified the small molecule-targetable-transcription factor STAT3 as a host determinant in promoting excess cytokine synthesis.
