Project Summary/Abstract: Macrophages are one of the major cellular elements in the tumor microenvironment (TME). Tumor associated macrophages (TAMs) markedly outnumber dendritic cells (DCs) and represent a prominent population of antigen presenting cells (APCs) in the TME. Assessment of their number and distribution in human solid tumors provides elusive and paradoxical information regarding their roles in human tumor immunity. Based on studies in murine systems, macrophages are historically classified into M1 and M2 subsets. However, the phenotype, characteristics, and functions of macrophages are determined by the environmental milieu, in which macrophages reside. Like the PD-L1/PD-1 pathway, we aim at identification of previously unknown immune resistance target(s), which may be potentially targetable with specific monoclonal antibody (mAb). Hence, we have attempted to explore the relationship between transmembrane proteins in APCs (and tumor cells as well) and ICB efficacy in patients with cancer and in tumor bearing animal models. We discovered that (a) membrane-spanning 4-domains subfamily A member 7 (MS4A7) was predominantly expressed in TAMs; (b) MS4A7 contributed to poor anti-tumor immunity and correlated with immune resistance to ICB in both in humans and mice; and (c) MS4A7 interacted with and prevented TREM2 (Triggering receptor expressed on myeloid cells 2) cleavage, resulting in a reduced expression of key functional genes (such as MHC and CD86) in macrophages, an impaired macrophage- mediated phagocytosis, and a reduced T cell activation. Based on these surprising and novel findings, our central hypothesis is that MS4A7 endows TAM phenotype and shapes ICB efficacy; and blocking MS4A7 and its pathway could reprogram TAMs. Therefore, MS4A7 is a myeloid cell associated, previously unknown immune resistance membrane protein, and a potential novel ICB target. In this application, we propose two specific aims with 7 subaims to test this central hypothesis mechanistically, functionally, and translationally by engaging bioinformatics, genomic, proteomic, immunological tools, and novel MS4A7-neutralizing antibodies. Aim 1 is to test our hypothesis that MS4A7 contributes to TAM-mediated immune suppression. Aim 2 is to test our hypothesis that the MS4A7-TREM2 axis controls TAMs and tumor immunity. The proposal will define the role of MS4A7 in tumor immunity and the underlying molecular mechanisms in both human and mouse cancers, set the stage to explore other likely cancer immune-relevant membrane- spanning family members, and provide a proof of principle that targeting specific membrane-spanning protein may augment tumor immunity and sensitize ICB efficacy.