Proper wound healing still remains a clinical challenge. Fibrin is a major component of the provisional extracellular matrix that forms initially after injury, enabling cell infiltration and anchoring to the wound site. Fibrinogen, the precursor of fibrin and a blood plasma protein, is also important in inflammation, wound healing, angiogenesis, and other biological functions. Of the many cells present during the wound healing process, macrophages play a key role in modulating the immune response. The plasticity of macrophages permits them to adopt behaviors along a spectrum of phenotypes depending on signals in their microenvironment. While much is known about how chemokines, cytokines, and other soluble mediators influence macrophages, less is understood about the roles of the ECM and the physical properties of the surrounding tissue. In this study, we examined the interactions between fibrin(ogen) and macrophages to gain a better fundamental understanding of how physical and biochemical properties of the provisional matrix together influence immune cell behavior. We separately looked at the roles fibrinogen and fibrin on macrophage inflammatory or anti-inflammatory functions and explored how mechanical properties of fibrin(ogen) such as substrate stiffness and tethering affect macrophage activation and behavior.

First, we investigated the differential regulation of macrophage activation by fibrinogen and fibrin. The presentation of fibrin(ogen), either presented in the soluble form or incorporated within an insoluble matrix, plays an important role in regulating macrophage phenotype. In particular, fibrin exerts a protective effect on macrophage inflammatory activation. Additionally, we altered mechanical properties of fibrin by controlling matrix stiffness and explored methods of adsorbing or tethering fibrinogen. Substrate rigidity alters the expression of integrins, proteins receptors that facilitate macrophage-ECM interactions. Adsorption of fibrinogen also inhibits inflammatory activation. Finally, we utilized a light-based crosslinking technique to stiffen fibrin matrices. Increased substrate rigidity enhances inflammatory macrophage behavior. Moreover, macrophages are more motile, both in terms of velocity and distance traveled, on stiffer fibrin matrices. Together, these findings contribute to our understanding of how the extracellular matrix regulates macrophages during wound healing and disease and offer new insight for designing biomaterials that can precisely control the host response and promote proper tissue regeneration.