The foreign body reaction (FBR) is a chronic host response that can negatively impact the performance of an implanted material or device (e.g., drug delivery devices, biosensors), through the persistent release of inflammatory mediators and by impeding integration between the implanted material and the surrounding tissue1. This innate immune response is initiated by the implantation procedure and is characterized by the long-term presence of innate immune cells and fibrous capsule formation around the implant1. Within the context of material host responses, macrophage-material interactions have a significant impact on the progression of the host response and development of a FBR1. Macrophages are a diverse innate immune cell population, recruited to the implant site either from tissue-resident macrophage populations or from the blood as monocyte-derived macrophages. They begin to accumulate at the implant site shortly after implantation, and within days become the predominant cell population in the implant microenvironment. Material-adherent macrophages, along with foreign body giant cells (FBGC) formed through macrophage fusion, can persist at the material surface for the lifetime of the implant2,3. Consequently, macrophages are considered to be key players in the foreign body response due to their roles orchestrating the characteristic steps of the FBR: acute inflammatory response, tissue remodeling, and formation of fibrotic tissue1.
Toll-like receptors (TLRs) are a family of pattern recognition receptors that are expressed by many immune cells, including macrophages, and have been shown to play a significant role in inflammation and wound healing. In addition to pathogen-derived ligands, TLRs are able to bind endogenous molecules, known as damage-associated molecular patterns (DAMPs), which are released during cell necrosis and activate inflammatory signaling pathways resulting in the production of proinflammatory cytokines4. We and others have proposed that damage incurred during soft tissue biomaterial implantation procedures release DAMPs, which then adsorb to biomaterial surfaces in addition to blood proteins and modulate subsequent cell-material interactions5,6. When macrophages interact with the adsorbed protein layer on an implant, their surface TLRs may recognize adsorbed DAMPs and activate proinflammatory signaling cascades, leading to NF-κB and AP-1 transcription factor activation and production of proinflammatory cytokines. We have previously shown that murine macrophages have significantly increased NF-κB/AP-1 activity and tumor necrosis factor α (TNF-α, proinflammatory cytokine) secretion in response to DAMP-containing adsorbed protein layers on a variety of polymeric surfaces compared to surfaces with adsorbed serum or plasma only (i.e., no DAMPs present), and that this response is largely mediated by TLR2, while TLR4 plays a lesser role5.
The NF-κB/AP-1 reporter macrophage cell line (Table of Materials) used in this protocol is a convenient method to measure relative NF-κB and AP-1 activity in macrophages5,7,8. In combination with TLR pathway inhibitors, this cell line is a useful tool for investigating TLR activation and its role in inflammation in response to a variety of stimuli5,7,8. The reporter cells are a modified mouse macrophage-like cell line that can stably produce secreted embryonic alkaline phosphatase (SEAP) upon NF-κB and AP-1 transcription factor activation9. The colorimetric enzymatic alkaline phosphatase assay (Table of Materials) can then be used to quantify relative amounts of SEAP expression as an indirect measure of NF-κB/AP-1 activity. As NF-κB and AP-1 are downstream of many cell signaling pathways, neutralizing antibodies and inhibitors targeting specific TLRs (e.g., TLR2) or TLR adaptor molecules (e.g., MyD88) can be used to verify the role of a specific pathway. The methodology described in this article provides a simple and rapid approach for assessing the contribution of TLR signaling in murine macrophage responses to a variety of polymeric surfaces with adsorbed protein layers containing both blood proteins and DAMPs as an in vitro model of implanted biomaterials.