Hyaluronan for Regenerative Medicine Applications: A Macrophage-Centered Approach

Biomaterials with the innate ability to inhibit inflammation and promote tissue integration and angiogenesis show positive outcomes in regenerative medicine. Of the cells that contribute toward the outcome of the biomaterials, macrophages are one of the most influential. Macrophages are highly plastic host immune cells, which persist for the lifetime of the biomaterial and have the robust ability to influence inflammation, tissue integration, and angiogenesis dependent on their polarization. Developing biomaterials that elicit desired macrophage polarization is a key component to their success in the host environment. Hyaluronan (HA) has been extensively explored in regenerative medicine due to its natural derivation and favorable mechanical properties. Biologically, HA exists in a variety of molecular weights in the soluble form and may become crosslinked during inflammatory events. We explored the influence of soluble high and low molecular weight HAs as well as crosslinked HA on macrophage polarization in cytokine naïve and activated (classical or alternative) macrophages. Soluble low molecular weight HA (LMWHA) was found to stimulate classical activation of the macrophage, promoting inflammation, tissue destructive, and anti-angiogenic functions of the macrophage. On the other hand, high molecular weight HA (HMWHA) promoted tissue integrative, anti-inflammatory, and pro-angiogenic functions of macrophages, suggesting it polarized and reprogramed macrophages to the alternatively activated form. We exploited the favorable interactions of HMWHA with macrophages to bioengineer hydrogel scaffolds suitable for regenerative medicine purposes. These hydrogels exhibited macrophage-polarizing properties dependent on the molecular weight between crosslinks. HA hydrogels with high molecular weight between crosslinks retained the alternatively activating effects of soluble HMWHA, whereas those with low molecular weight between crosslinks elicited characteristics of both classically and alternatively activated macrophages. Our study suggested that HMWHA retains properties favorable for regenerative medicine when it is loosely crosslinked. Prior to this study, macrophage polarization and reprogramming from soluble and crosslinked HA had never determined. Beyond regenerative medicine, the understanding of HA-macrophage interactions has implications in rheumatology, sterile inflammation, and cancer. Our study supports further understanding of biomaterial macrophage interactions to derive therapeutic benefit.