The overall goal of this proposal is to characterize senescent cells (SnCs) and their communication networks during fibrosis induced by the foreign body response (FBR), specifically with respect to biomaterials, across the lifespan. SnCs are rare but impactful cells that contribute to multiple biological processes; to date, their identity and function in vivo, including interactions with the immune system, remain elusive. We recently discovered that SnCs contribute to fibrosis induced by the FBR in young animals. In preliminary studies, we found senescent pericytes and fibroblasts in the FBR that each had a unique senescence-associated secretory phenotype (SASP) and that communicated with different macrophage subpopulations. We also found differences in SnC induction and phenotype in older animals, and sex differences in the immune response to implants in both young and aged mice. In older animals, for example, SnC communication shifted toward increased communication with T cells. We developed an approach that defines senescence signatures (SenSigs) from transgenic p16-Cre reporter mice that can be used in combination with transfer learning methods to predict which cells are senescent in single-cell datasets. Using these transgenic tools and computational approaches, the proposed research will address long -standing questions of SnC phenotype, heterogeneity, function, and immunological interactions in the context of fibrosis in the foreign body response. We anticipate that this will enable development of more sophisticated and targeted senolytic therapies for the FBR and for fibrosis in general. Our Aims are: Specific Aim 1. Define senescent cell types and their senescence signature in the FBR across lifespan in male and female p16-reporter mice. Specific Aim 2. Define SnCs and their communication patterns in the FBR. Specific Aim 3. Determine the functional role of SnCs and SnC-subtypes on immune response and tissue structure.