Cytokines and epigenetic regulation of matrix metalloproteinases in tuberculosis

Abstract

Tuberculosis (TB) remains a leading cause of death globally. Some of the morbidity and mortality associated with TB arises from excessive or inappropriate immune activation in response to infection. In pulmonary TB tissue destruction, cavitation and fibrosis drive disease transmission and chronic lung dysfunction. The matrix metalloproteinases (MMPs) are host enzymes controlling extracellular matrix turnover, immune cell recruitment and activation, and they play a key role in the pathology of TB. This project explores the regulatory mechanisms controlling MMP expression in TB using a tissue culture model of respiratory epithelial cell and macrophage responses to Mycobacterium tuberculosis (Mtb). The major human collagenase MMP-1 and its activator MMP-3 are expressed in response to Mtb stimulation. The influence of exogenously added cytokines IL-4, IL-10, IL-13, TGFβ and IFNγ on MMP expression is investigated to examine how immunopathology is driven by acquired immune responses. Epigenetic mechanisms regulating MMP expression in response to Mtb are examined using inhibitors of histone deacetylases (HDACs) and histone acetyltransferases, HDAC siRNA and chromatin immunoprecipitation assays. Mtb-induced collagenase expression by epithelial cells and macrophages is selectively inhibited by the Th2 cytokines IL-4 and IL-13, but unaffected by IL-10. TGFβ enhances epithelial cell MMP secretion but does not affect macrophage MMP expression. IFNγ has divergent effects, driving epithelial cell MMP secretion but inhibiting MMP-1 and -3 expression in macrophages. The intracellular pathways mediating these effects are explored. Mtb-driven MMP expression is sensitive to HDAC and HAT inhibition, and altered HDAC expression is observed in Mtb-stimulated cells. Chromatin immunoprecipitation reveals changes in histone acetylation at the MMP-1 promoter in Mtb-stimulated epithelial cells. In pulmonary TB, production of matrix-degrading MMPs by epithelial cells and macrophages is modulated by Th2 cytokines and IFNγ. Key MMPs implicated in pulmonary immunopathology are subject to epigenetic regulatory mechanisms, which may represent potential targets for selective MMP inhibition.