MMP12 identified as a macrophage-intrinsic driver of fibrosis: its inhibition reduces myocardial fibrosis and improves cardiac conduction

The study identifies a fibrogenic macrophage (FibMac) population in a cardiac sarcoidosis model, characterized by Cd63, Spp1, Gpnmb, and Fabp5 expression, comprising more than 40% of all cells in cre/+ mice, with monocytes also significantly increased to around 25%. A fibrogenic macrophage population expressing Cd9, Cd63, Spp1, Gpnmb, and Fabp5 had been described in experimental and human lung and liver fibrosis, but this population and its origin had not been defined in cardiac sarcoidosis. Based on single-cell RNA sequencing of 6 samples (+/+ n = 3; cre/+ n = 3), with Louvain clustering and the Wilcoxon rank-sum test (false discovery rate-adjusted P < .05), and immunofluorescence validation of GPNMB and SPP1 expression in MAC2-positive macrophages.

The study proposes a TGF-β- and mTORC1-driven monocyte-to-FibMac differentiation trajectory culminating in a late fibrogenic module enriched for Spp1, Gpnmb, Mmp12, Cela1, Ctsk, and collagen catabolic enzymes. Previous work had implicated mTORC1 signaling in promoting granuloma formation and fibrosis in sarcoidosis, but how this signaling state is translated into a stable pathological cell identity had remained unresolved. Based on ligand-target analysis identifying monocyte-derived TGF-β as the ligand with the highest predicted regulatory influence on FibMac gene expression, strong nuclear pSMAD2/3 staining in SPP1+ macrophages, trajectory inference and pseudotime analysis, and gene expression module analysis.

MMP12 is identified as a macrophage-intrinsic inducer and dominant effector: recombinant MMP12 was sufficient to induce fibrogenic markers (SPP1, FABP5) and to promote F-actin, γ-catenin, and E-cadherin-associated epithelioid features and macrophage clustering, whereas genetic loss or pharmacologic inhibition of MMP12 attenuated these responses. Previous studies found MMP12 expression in pulmonary sarcoidosis and implicated MMP12 in carbon nanotube and lung fibrosis models, but this study places MMP12 as an upstream driver conferring structural competence to FibMacs rather than a secondary consequence of differentiation. Based on recombinant MMP12 stimulation of wild-type bone marrow-derived macrophages, TGF-β stimulation of Mmp12−/− versus control bone marrow-derived macrophages, pharmacologic inhibition in Tsc2-deficient bone marrow-derived macrophages, and experiments in human peripheral blood mononuclear cell-derived macrophages.

In vivo, two selective MMP12 inhibitors (linvemastat administered orally via gavage for 3 weeks; aderamastat administered twice daily for 5 weeks) reduced interstitial, perivascular, and epicardial fibrosis, decreased αSMA+ and FAPα+ myofibroblasts, reduced MAC2+ and CD68+MMP12+ macrophages and macrophage clustering, and, with aderamastat, substantially decreased QRS duration approaching ventricular conduction time values like wild-type mice. Current strategies for cardiac fibrosis largely target fibroblast-intrinsic pathways or broadly suppress upstream inflammatory mediators such as TGF-β, approaches that often carry substantial toxicity; this study proposes targeting immune-cell effectors that organize and sustain fibrotic niches. Based on in vivo administration of two inhibitors in mice, quantitative analysis of collagen deposits and myofibroblast markers, morphometric quantification of macrophage clustering, and surface ECG analysis (6 subcutaneously placed electrodes in a standard limb lead configuration).

The results apply to the myeloid-specific Tsc2 deletion mouse model that recapitulates key features of human cardiac sarcoidosis, as well as to the human cardiac sarcoidosis tissue and human peripheral blood mononuclear cell-derived macrophages used for validation. They open directions for future work: cell type-specific perturbation studies to resolve whether granuloma organization and fibrotic remodeling are mechanistically coupled or represent parallel MMP12-dependent programs; testing MMP12 as a rate-limiting node in the context of chronic, nonresolving granulomatous inflammation; and exploring whether MMP12 inhibition can translate into a therapeutic strategy for cardiac sarcoidosis and other fibrotic diseases.

The extent to which granuloma organization and fibrotic remodeling are mechanistically coupled versus representing parallel MMP12-dependent programs remains unresolved and will require future cell type-specific perturbation studies. The use of middle-aged and aged mice to model late-stage disease introduces age heterogeneity across cohorts; although no significant differences were observed, age-related variability in immune and fibrotic responses cannot be fully excluded. Because the model is driven by chronic mTORC1 activation, the extent to which MMP12-dependent effects are contingent on sustained mTORC1 signaling remains to be fully defined. In addition, although this is the full text, figures and supplementary tables are not included in the loaded content, so specific effect sizes, sample sizes, and statistical details can only be summarized from the narrative text rather than from the figures themselves.