Loss of fibronectin fiber tension is inherent to ECM remodeling in human myocarditis and post-inflammatory fibrosis.

Background: Viral myocarditis (MC) is associated with extracellular matrix (ECM) remodeling and involves excessive deposition of collagen and other ECM proteins by cardiac fibroblasts, potentially leading to scarring and ECM stiffening, which may subsequently contribute to impaired cardiac functions. Clarifying whether changes in ECM fiber tension are detectable during either the acute or scarring phase could provide a novel, mechanistically relevant mechanobiological signature. Objectives: Our goal was to ask whether ECM mechano-markers can be identified in the myocardium, beyond excessive collagen fiber deposition, that are associated with the acute infection or the pathological scarring of the human heart, and how this might be associated with the infiltration of macrophages. Methods: Left-ventricular (LV) endomyocardial biopsies were obtained from patients (N = 39) with acute myocarditis MC (N = 21) including COVID-19 (N = 4) patients suspected with myocarditis MC and/or impaired LV function, dilated cardiomyopathy (N = 6), inflammatory dilated cardiomyopathy (N = 12) to specify diagnosis and treatment options. Endomyocardial biopsies were analyzed for viral genomes, immune cells infiltration and ECM remodeling. Fibronectin fiber tension was assessed using the fibronectin-binding tension-sensor (FnBPA5), while collagen fiber deposits were visualized using second harmonic generation (SHG) microscopy. Results: While fibronectin fibers were tensed in healthy hearts, histological staining with our novel tension probe FnBPA5 showed that fibronectin fibers had lost their tension in distinct loci in all patient groups. In the acute inflammatory phase (MC), loci with untensed fibronectin fibers were found in close proximity to infiltrated macrophages. In already dilated hearts, biopsies which presented low densities of infiltrated macrophages, thick collagen I/III fiber bundles as visualized by SHG were found in proximity to untensed fibronectin fibers and myofibroblasts, which together are indicative of fibrotic ECM niches where the inflammation has subsided. Comparison of clinical diagnostic and experimental histological data showed clear correlations between altered ECM niche properties and cardiac function deterioration. Conclusions: Our data suggest that relaxed fibronectin fibers are a recurrent feature of myocarditis and associate with measures of cardiac dysfunction. These findings suggest that fibronectin fiber tension might be a mechanobiological signature that warrants validation in larger, longitudinal cohorts and evaluation of in-vivo measurability.