Tissue resident macrophages are derived from embryonic yolk sac progenitors, develop prior to birth, are long- lived and radioresistant, and have critical functions in maintaining tissue homeostasis and in the pathogenesis of many human diseases that are of broad interest to NIH. This proposal is to build cutting-edge humanized mouse resources that support the efficient development of human tissue resident macrophages in disease relevant models and is in response to RFA-22-013: “Resource-Related Research Projects for Development of Animal Models and Related Materials”. The rationale for our approach is based on the paucity of translational animal models that are readily available to the research community to study human tissue resident macrophages in vivo and evaluate their role in human diseases. While several humanized platforms expressing human cytokines have been reported to improve development of human macrophage populations, many of these models are not available to basic research programs. Moreover, these models are often focused on engraftment of single macrophage populations and thus lack components of the adaptive immune system and other innate immune cells. An additional limitation for humanized models is that the mouse resident macrophage populations occupy the “niches” available for tissue specific macrophages and resist replacement with human HSC-derived macrophages. We recently knocked out the fms-intronic regulatory element (FIRE) enhancer in the CSF1 receptor gene in immunodeficient NSG mice (NSG-Fire) that impairs the development of mouse tissue resident macrophages, and we are crossing NSG-(Fire) mice with cytokine transgenic NSG mice to promote the development of human myeloid cells. Our overall hypothesis is that genetic modification of NSG mice to express species-specific, myeloid promoting cytokines and to reduce mouse tissue resident macrophage populations will enhance the development of functional human tissue resident macrophages and provide a robust small animal model for the study of inflammatory immune responses mediated by human macrophage populations. We will test this hypothesis in three Specific Aims. Aim 1) Generation of novel NSG- (Fire) mouse stocks that are deficient in mouse tissue resident macrophages and express human cytokines for development of human myeloid cells. Aim 2) Engraftment of novel NSG-(Fire) mouse stocks with CD34+ UCB- HSC and evaluation of human tissue resident macrophage development. Aim 3) Testing NSG-(Fire) strains as a model for tissue resident macrophages acting as a reservoir for HIV during latency and for microglia in Alzheimer’s Disease-relevant neurodegeneration. These new mouse models will allow for the in vivo investigation of human tissue resident macrophages in multiple diseases and accelerate the development of human specific therapies.

Tissue resident macrophages have critical functions in maintaining tissue homeostasis and in the pathogenesis of many human diseases, including Alzheimer’s Disease and multiple sclerosis and in HIV infection. This project will fill a critical need by providing validated humanized platforms for investigating human tissue resident macrophages in vivo during development of disease.