Effect of Fibroblast Signaling on Macrophage Polarization

Systemic and organ-specific fibrotic disorders are a leading cause of death worldwide. Crosstalk between fibroblasts and macrophages has been suggested as a key event leading to either resolution or aberrant remodeling and fibrosis. This study sought to identify the impacts of the timing and effects of exposure to quiescent (basal) and transforming growth factor-β-stimulated (activated) fibroblast-secreted products on macrophage polarization and function. Naïve (M0 macrophages), lipopolysaccharide/interferon-γ-stimulated (M1 macrophages), and IL-4-stimulated (M2 macrophages) macrophages were exposed to basal or activated fibroblast conditioned media (FBCM) for 24 hours before, after, or during macrophage polarization. Macrophage function and polarization were quantified by phagocytosis, nitric oxide, and arginase activity assays and by cytokine array. FBCM from activated fibroblasts led to a pronounced up-regulation of arginase-1 compared with that from quiescent fibroblasts in M0 macrophages. Moreover, treatment with FBCM from activated fibroblasts resulted in significant increases in arginase-1 immunoexpression as well as urea production in M2 macrophages when applied antecedent, concurrent, or subsequent to M2 macrophage polarizing cytokines. Activated FBCM enhanced several proinflammatory cytokines, such as IL-1β and IL-6, in all macrophage subsets while only increasing tumor necrosis factor-α in M1 macrophages. This study elucidates multiple proinflammatory and profibrotic effects of fibroblasts on M1 and M2 macrophages, providing insights into the complex orchestration of macrophage-fibroblast crosstalk in fibrosis and the critical role of fibroblasts in the inflammatory response to injury.