Macrophage-Dependent Trafficking and Remodeling of Extracellular Matrix Barriers

Abstract

Macrophages are evolutionarily-conserved immune cells distributed throughout all tissues in the body, which rapidly mobilize to defend against a range of insults. In executing events ranging from wound healing and host defense functions to regulating the tumor microenvironment, macrophages traverse and remodel extracellular matrix (ECM) barriers, i.e. the basement membrane and interstitial matrix. To date, the molecular mechanisms operative during macrophage migration and remodeling of ECM barriers have relied on non-physiologic in vitro constructs whose relevance to the in vivo environment remains unclear. As such, we have adopted an ex vivo native tissue model as well as a 3-dimensionsal type I collagen hydrogel model that retain structural crosslinks integral to the barrier characteristics of the in vivo ECM. Using primary mouse and human macrophages in conjunction with high-resolution confocal microscopy, we characterize a program wherein macrophages degrade the basement membrane and infiltrate the interstitial matrix. We find that of the dozens of proteases that macrophages express in response to immune stimuli, only the membrane-anchored metalloprotease, MT1-MMP, is absolutely required for basement membrane degradation. Furthermore, we discover a unique hybrid ability of macrophages to either degrade the basement membrane in an MT1-MMP-dependent fashion or alternatively, mobilize actomyosin-mediated mechanical forces to non-proteolytically traverse preformed portals that exist in the basement membrane. Though macrophages can transmigrate the basement membrane via either mechanism, the transcriptional program of tissue-invasive macrophages is alternatively regulated during proteinase-dependent versus independent invasion. Following basement membrane transmigration, macrophages then confront a high-density interstitial matrix that is dominated by type I collagen. Under these conditions, macrophages must again mobilize MT1-MMP to create passageways through the interstitial matrix that permit the transit of the rigid macrophage nucleus. Strikingly, in the absence of MT1-MMP activity, the macrophage is incapable of creating matrix tunnels that support efficient invasion. Instead, the macrophage traverses the matrix while the rigid nucleus remains trapped and distorted above the surface of the collagen matrix. These studies, together with preliminary data from mouse models of cancer, establish new paradigms for MT1-MMP-dependent macrophage trafficking and remodeling of physiologically-relevant ECM barriers.