Collagen morphology influences macrophage shape and marker expression in vitro

Fibrosis is a process in which an accumulation of extracellular matrix (ECM) leads to an impaired function of the affected organ. Pulmonary fibrosis is the end-stage of several lung diseases, characterized by scarring of the lungs. Although macrophages are known to be important players in ECM homeostasis, their ability to respond to fibrosis-related morphological and mechanical changes of the ECM is relatively unexplored. In this study we aimed to elucidate the effect of ECM stiffness and morphology on macrophage polarization, by using a collagen type I-based in vitro system. Collagen morphology, but not stiffness, affected the relative expression of CD206 (the mannose receptor) and Ym1 (a murine marker of pro-healing M2 macrophages). Higher expression of Ym1 was found when macrophages were cultured on fibrous collagen. Globular collagen led to higher expression of CD206, a marker known to be upregulated on alveolar macrophages in idiopathic pulmonary fibrosis. Moreover, macrophages exhibited distinct differences in shape with actin-rich protrusions on fibrous collagen and more filopodia on globular collagen. In addition to these cytoskeletal changes, transmigration was higher when macrophages were cultured on fibrous collagen. Together these findings indicate that macrophages are sensitive to collagen morphology, responding with subtle changes in marker expression, shape and behavior rather than a complete polarization switch. This study emphasizes the complex interaction between macrophages and their surroundings, and the need for further exploration of both mechanical and morphological aspects.