Damage Associated Molecular Patterns (DAMPs) Mediate the Foreign Body Response to Poly(ethylene glycol) Diacrylate Hydrogels via Toll like Receptors.

Poly(ethylene glycol) hydrogels prepared from PEG diacrylate (PEGDA) monomers are widely investigated for biological applications including drug delivery and tissue engineering. Despite their high-water contents, PEGDA hydrogels when implanted into the body induce a foreign body response (FBR). The overall goal of this study was to investigate the role of surface adsorbed proteins in the FBR to PEGDA hydrogels and determine whether they act as damage associated molecular patterns (DAMPs) to initiate inflammation. Toll-like receptors (TLRs) 2 and 4 are one of the primary receptors that recognize DAMPs. In vitro and in vivo studies were performed using wildtype (Wt), TLR2^-/-, TLR4^-/-, and TLR2^-/-TLR4^-/- double knockout (DKO) mice. In vitro, Wt neutrophils were activated in response to the PEGDA hydrogels as measured by myeloperoxidase, and this response was partially mediated by TLR4 but not TLR2. Wt macrophages predisposed to an inflammatory state responded to the PEGDA hydrogel itself and to a greater extent to surface-adsorbed plasma by producing the pro-inflammatory cytokines interleukin 6 and tumor necrosis factor α. TLR4 and to a lesser extent TLR2 mediated this response. To link DAMPs to the FBR in vivo, PEGDA hydrogels were implanted subcutaneously in mice. The thickness of the inflammatory cell layer was mediated by both TLR2 and TLR4 as knocking out both TLRs led to significantly fewer inflammatory cells. The fibrous capsule was reduced by 50% in both single KO mice as well as the DKO mice. Taken together, this study determined that DAMPs formed from surface-adsorbed plasma activate TLR4. In the more complex in vivo environment, both TLR2 and TLR4 are major contributors to the inflammatory response and partial contributors to the fibrous encapsulation. Overall, these findings provide a critical link between DAMPs, TLRs, immune cells, and the FBR to PEGDA hydrogels.