Plasmin(ogen) Mediates Macrophage Migration in a Fibrin(ogen) Dependent Mechanism
Authors: Shaw Maureen A. , Gao Zhen , Mullins Eric S. 
Published: October 2019

Abstract
Mounting evidence ties both fibrin(ogen) and plasmin(ogen) to inflammatory diseases. Indeed, both fibrin(ogen) and plasmin(ogen) have been linked to critical macrophage functions in multiple disease processes. Migration of macrophages to sites of sterile inflammation is, at least partially, dependent on plasmin(ogen). Mice lacking plasminogen, when challenged with sterile thioglycollate-induced peritonitis, have both diminished overall leukocyte migration and decreased macrophage migration. Additionally, macrophage migration defects have been identified in both mice lacking plasminogen and plasminogen receptors. Plasmin has many targets that may play a role in supporting macrophage migration. In addition to proteolysis of fibrin(ogen), plasmin activates matrix metalloprotease (MMP) 2 and MMP9 and cleaves collagen and laminin. Indeed, mice that lack MMP9 have a migration defect similar to mice that lack plasminogen, suggesting that MMP9 is a biologically relevant proteolytic target in this context. To further examine the targets of plasmin that regulate macrophage migration, we challenged animals that have individual and combined genetic deficiencies in fibrinogen and plasminogen with thioglycollate-induced peritonitis. We have found that mice that lack fibrinogen alone have a significantly increased migration of macrophages to the peritoneal cavity. Mice with lack of plasminogen alone demonstrated the expected diminution in macrophage migration to the peritoneal cavity. However, mice that were deficient in both plasminogen and fibrinogen demonstrated macrophage migration that was indistinguishable from wildtype. These data suggest that fibrin(ogen) impedes macrophage migration to the peritoneal cavity. To further confirm this mechanism, we examined macrophage migration in a transwell assay in vitro, in response to macrophage chemoattractant protein-1 (MCP-1). Here, a macrophage cell line (RAW 264.7) migration was examined in the absence and presence of fibrin matrices. Macrophages, in the absence of plasminogen, did demonstrate a modest, but statistically significant, increase in migration across the transwell membrane in the absence of fibrinogen. When a fibrin matrix was generated on the transwell membrane, macrophages were essentially unable to cross in the absence of plasminogen. These data further support the concept that macrophages require plasmin(ogen) to cross fibrin matrices. To further explore the plasmin(ogen)-fibrin(ogen) interaction in macrophage migration, we assessed the migration of macrophages to fibrin degradation products (FDPs). First, we examined macrophage transwell migration in response to MCP-1 in the presence of FDPs to assess if FDPs impede macrophage migration. Instead of impeding macrophage migration, FDPs significantly increased macrophage migration across the transwell membrane. Indeed FDPs initiated macrophage migration even in the absence of MCP-1. To confirm that this was FDP induced migration, and not direct plasmin signaling on macrophages, we examined macrophage migration to FDPs in the presence of an irreversible plasmin inhibitor. We again found that plasmin degradation of fibrin was needed for migration, however, further plasmin activity was not required. Taken together, these data suggest that macrophages require plasmin(ogen) to navigate fibrin matrices and that the by-product of this degradation (FDPs) is a signal for additional macrophage migration to sites of fibrin deposition.

Disclosures
Mullins: Baxalta (now part of Shire): Honoraria; US WorldMeds: Membership on an entity's Board of Directors or advisory committees.