ABSTRACT – Project 1 The mononuclear phagocyte system is critical in the host's response to pathogens and inflammation, but it is also critical for removing old or malignant cells. In non-Hodgkin lymphoma (NHL), monocytic cells are central components of the innate immune system and CD14+ monocytes in the peripheral blood as well as CD68+ tumor-associated macrophages (TAMs) in tissue influence the prognosis of patients. We have previously shown that CD14+HLADRlow monocytes are increased in the peripheral blood of NHL patients, are induced by IL-10, and profoundly suppresses T-cell function. However, in lymphoma tissue biopsies, we found that many CD68+ TAMs downregulate CD14 expression. While CD14+ TAMs typically expressed common macrophage markers, we found that the CD68+CD14- fraction expressed very few of these markers. To determine whether CD68+CD14- TAMs retained macrophage function, we measured the expression of signal-regulatory protein α (SIRPα), a receptor that inhibits phagocytic function. Based on SIRPα expression, we identified 2 distinct populations of TAMs in sites involved by lymphoma – those that were CD14+SIRPαhigh and those that were CD14-SIRPαlow/- – but the phagocytic ability and role of these two populations is unknown. SIRPα regulates macrophage-mediated removal of apoptotic cells that upregulate `eat-me' signals such as calreticulin. The induction of phagocytosis by `eat-me' signals on tumor cells is countered by `don't-eat-me' signals such as CD47, which binds macrophage SIRPα to inhibit phagocytosis. CD47 has been shown to be highly expressed on lymphoma cells and is a mechanism by which malignant B-cells protect themselves from phagocytosis by activated macrophages. However, CD47/SIRPα interaction not only regulates phagocytosis but also has a role in modulating T-cell function by enhancing antigen presentation, effectively making the CD47/SIRPα axis an immune checkpoint for the innate immune system. We hypothesize that CD14+SIRPαhigh TAMs are highly functional, able to phagocytose malignant cells, present tumor antigens and activate the immune system but are inhibited by CD47. In contrast, CD14-SIRPαlow/- TAMs are immature, fail to phagocytose malignant cells and suppress immune function. To improve the outcome of lymphoma patients, the phagocytosis and T-cell activation by both CD14+SIRPαhigh and CD14-SIRPαlow/- TAMs needs to be augmented. We therefore propose to determine the phagocytic function and immune activation of both CD14+SIRPαhigh and CD14-SIRPαlow/- TAMs, and determine whether blocking CD47/SIRPα signaling clinically using SIRPα-Fc can enhance the tumor-directed phagocytic function of both populations of TAMs. We anticipate that results from this project will lead not only to a comprehensive understanding of the subtypes of TAMs in lymphoma, but an innovative therapy that harnesses the power of both the innate and adaptive immune systems.