PROJECT SUMMARY/ABSTRACT The goal of this proposal is to develop the applicant into an independent investigator with a research focus on peritoneal immunity controlled by resident peritoneal macrophages. As of 2020, the PI has received almost three years of postdoctoral training, published his first postdoctoral project that laid foundation of the current proposal, and presented his work in several prestigious international conferences. Other than experimental techniques, most importantly, the PI has established skills to initiate, develop, and complete new projects on his own. Large peritoneal macrophages (LPMs) (also known as resident peritoneal macrophages) comprise ~50% of all mouse peritoneal cells in the steady state and float freely in peritoneal fluid, but the specific roles of LPMs against tumor and infection are not well understood. LPMs express specialized sets of genes that are distinct from macrophages in other organs, blood monocytes, and monocyte-derived macrophages, including many clotting factors. We recently demonstrated that LPM-specific expression of Factor V facilitates the formation of leukocyte- rich peritoneal clots that trap bacteria and particulates in models of infectious and irritant peritonitis. The production of coagulation factors by local macrophages may further be linked to literature from other groups emphasizing that patients with peritoneal metastasis of ovarian tumors express high levels of coagulation products in ascites. Thus, for reasons ranging from host defense to combatting cancer in the peritoneal cavity, it seems important to better understand how LPMs generate a link between peritoneal clotting and immunity. Thus, the main aim of my proposal is to define the cellular and molecular interactions that result in clotting of LPMs in the context of peritonitis and peritoneal carcinomatosis. As described in my recently published studies of peritonitis, integrin-mediated adhesion and LPM-mediated clotting cooperatively cause LPMs to rapidly become irretrievable in peritoneal exudate, a phenomenon historically known as the macrophage disappearance reaction. We show that this “disappearance” is a crucial player in host defense for capture of phagocytic cargos, in turn preventing dissemination of such cargo. In other, unpublished preliminary data, we find that LPMs, in contrast to other resident or recruited macrophages in the peritoneum, are critical for preventing the spread of ovarian tumor cells on mesothelium and expanding in the peritoneal cavity. Thus, we hypothesize that LPMs, likely through their promotion of clotting and likely requiring their adhesion to mesothelial surface, are critical for immune protection against microbial and tumor cell insults in the peritoneal cavity. This application is designed to test this hypothesis and examine underlying mechanisms that facilitate macrophage function in the peritoneum through adhesion and coagulation. I will use specific genetic mouse models to test whethe...