Macrophages are key players in the foreign body response (FBR) to implanted biomaterials, in which an avascular fibrous capsule walls off the implant. However, the cellular mechanisms that contribute to the fibrous capsule have not yet been elucidated. As a result, synthetic-based biomaterials have been limited to those that the body tolerates and which function despite a FBR and does not lead to biomaterial-host integration. In this highly exploratory project, we put forth a novel hypothesis that is based on our team?s recent published findings in tissue fibrosis. We demonstrated that inhibition of cellular FLICE-like inhibitory protein (cFLIP), which is a major regulator of macrophage cell fate, can prevent tissue fibrosis. In this project, we test the hypothesis that that macrophage persistence in the FBR is mediated by intracellular cFLIP and inhibiting cFLIP resensitizes macrophages to apoptotic death signals to prevent or resolve fibrous encapsulation. We developed two specific aims to test the hypothesis and to translate this idea to a biomaterial strategy that targets cFLIP in macrophages to prevent fibrous encapsulation. In specific Aim #1, we will determine the kinetics of macrophage persistence in the FBR to distinct implants. This aim will use the hCD68-rtTA transgenic mouse that is coupled with a Tet-on Cre system and fluorescent tdTomato expression. Using this mouse model, a series of lineage tracing experiments will be performed that combine multiparameter flow cytometry to identify myeloid subsets, including recruited and tissue-resident macrophages, distinguish their temporal patterns in the FBR and determine changes in their expression profiles for fibrosis-relevant genes. In specific Aim #2, we will temporally inhibit c-FLIP in macrophages to promote their programmed cell death and attenuate formation and maintenance of the fibrous capsule in the FBR. The first part of Aim #2 will determine the temporal effects of cFLIP inhibition in macrophages using a similar transgenic mouse model as in Aim 1, but which is coupled with a tet-On Cre system that deletes cFLIP. This mouse model will enable the temporal effects of cFLIP deletion to be determined on both the formation of fibrous capsule and on its dissolution. The second part of Aim #2 will focus on designing a phototriggerable biomaterial to inhibit cFLIP temporally and locally in macrophages. This will be achieved through photo-labile microparticles that are embedded within a biomaterial, which when triggered by light lead to the slow release of YM155, a small molecule inhibitor of cFLIP. By tightly controlling when YM155 is released, the temporal effects of local cFLIP inhibition by a biomaterial-based strategy will be determined. At the conclusion of this project, we will have a) determined the temporal patterns of macrophage accumulation and when they begin to persist in the FBR, b) elucidated the role of cFLIP in mediating pro-survival programming in macrophages and its effect on fibrous encapsulation, c) identified the optimal timing for depleting cFLIP, and d) developed novel strategies that can be applied for preventing or resolving the FBR.