Unopposed Matrix Metalloproteinase-9 Expression in Human Tuberculous Granuloma and the Role of TNF-α-Dependent Monocyte Networks

Tuberculosis is characterized by granuloma formation and caseous necrosis, but the factors causing tissue destruction are poorly understood. Matrix metalloproteinase (MMP)-9 (92-kDa gelatinase) secretion from monocytes is stimulated by Mycobacterium tuberculosis (M. tb) and associated with local tissue injury in tuberculosis patients. We demonstrate strong immunohistochemical MMP-9 staining in monocytic cells at the center of granuloma and adjacent to caseous necrosis in M. tb-infected patient lymph nodes. Minimal tissue inhibitor of MMPs-1 staining indicated that MMP-9 activity is unopposed. Because granulomas characteristically contain few mycobacteria, we investigated whether monocyte-monocyte cytokine networks amplify MMP-9 secretion. Conditioned medium from M. tb-infected primary human monocytes or THP-1 cells (CoMTB) stimulated MMP-9 gene expression and a >10-fold increase in MMP-9 secretion by monocytes at 3-4 days (p < 0.009, vs controls). Although CoMTB stimulated dose-dependent MMP-9 secretion, MMP-1 (52-kDa collagenase) was not induced. Anti-TNF-alpha Ab but not IL-1R antagonist pretreatment decreased CoMTB-induced MMP-9 secretion by 50% (p = 0.0001). Anti-TNF-alpha Ab also inhibited MMP-9 secretion from monocytic cells by 50%, 24 h after direct M. tb infection (p = 0.0002). Conversely, TNF-alpha directly stimulated dose-dependent MMP-9 secretion. Pertussis toxin inhibited CoMTB-induced MMP-9 secretion and enhanced the inhibitory effect of anti-TNF-alpha Ab (p = 0.05). Although chemokines bind to G protein-linked receptors, CXCL8, CXCL10, CCL2, and CCL5 did not stimulate monocyte MMP-9 secretion. However, the response to cholera toxin confirmed that G protein signaling pathways were intact. In summary, MMP-9 within tuberculous granuloma is associated with tissue destruction, and TNF-alpha, critical for antimycobacterial granuloma formation, is a key autocrine and paracrine regulator of MMP-9 secretion.