Targeting TREM2 to boost anti-cancer therapy

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PROJECT SUMMARY Cancer is a leading cause of death and disease. The recent success of immune checkpoint therapy (ICT) has revolutionized tumor therapy, indicating that manipulation of the immune system is an effective strategy to treat cancer. MAbs inhibiting CTLA-4 and PD-1 have been extensively shown to unleash T cell effector functions to control tumors in both mice and some cancer patients. However, ICT is incompletely effective for certain tumors, which escape using multiple mechanisms, one of which is the generation of a tumor microenvironment rich in immunosuppressive myeloid cells. TREM2 is an immune receptor expressed by tissue macrophages that binds phospholipids and lipoproteins and transmits intracellular signals through the ITAM pathway. Recently, TREM2+ macrophages have been reported in many human tumors. In our preliminary data, we demonstrate that TREM2-deficiency or mouse TREM2 blockade with the mAb 178 curbs subcutaneous tumor growth of the 3- methylcholanthrene (MCA) cell line and leads to complete tumor regression when associated with suboptimal PD-1 immunotherapy. Furthermore, high-resolution analysis of the tumor cell infiltrate in the MCA model reveals complex remodeling of the myeloid cell landscape in Trem2–/– and anti-TREM2 treated mice. The overall goal of this application is to advance our understanding of the therapeutic impact of TREM2 blockade in mouse models and human cancer. In Aim 1 we show that TREM2 targeting enhances ICT mediated by anti-PD1; we propose to determine whether TREM2 deficiency or blockade impact other tumor therapies, such as anti-CTLA4 and chemotherapy, which elicit different types of immune responses. The impact of TREM2 will be assessed using injected MCA cell lines and the spontaneous MMTV-PyMT model of breast cancer. In Aim 2 we will define the mechanisms through which anti-TREM2 impacts the tumor microenvironment. Given that a) immunosuppressive macrophages depend on lipid metabolism and accumulate lipid droplets; b) TREM2 promotes foam cell formation by binding lipoproteins; and c) anti-TREM2 mAb blocks lipid binding to TREM2, we will test the hypothesis that TREM2 blockade converts tumor macrophages from immunosuppressive to immunostimulatory by blocking lipid droplet accumulation and foam cell formation. We will also test an alternative mechanism based on the observation that TREM2 is cleaved from the cell surface by ADAM metalloproteases, generating soluble TREM2 (sTREM2), which promotes survival of macrophages in various disease models. We will test the hypothesis that lack of sTREM2 in a transgenic mouse with uncleavable TREM2 prevents survival of immunosuppressive tumor macrophages. In Aim 3, we show unpublished data indicating that anti-human TREM2 mAb 21E10 delays tumor growth of an injected MCA cell line in mice expressing human TREM2 in place of mouse TREM2. Therefore, we will determine whether TREM2 blockade with a specific mAb can be extended to a preclinical model expressing the human TREM2 receptor. Overall, this proposal will advance our knowledge of a novel therapeutic approach based on TREM2 that broadens our armamentarium for targeting immunosuppressive myeloid cells in tumors.