Immunomodulatory agents blocking the immune checkpoint PD-1/PD-L1 (PD) pathway have shown remarkable clinical benefits and constitute a new standard of care for cancer treatment. The success of these agents is due to the prominent immunosuppressive function of the PD-1 receptor alongside the selective expression of its ligand, PD-L1, in the tumor microenvironment (TME), leading to a favorable efficacy-to-toxicity ratio. However, despite the vast advances provided by anti-PD therapy, only a subset of patients develops a long-term response, illustrating the need for identifying new immune modulators in the TME, especially in PD-L1 negative tumors. To achieve this, we built the first genome-scale T-cell activity screen of human membrane proteins and identified Siglec-15 as a critical immune suppressor with broad upregulation on various cancer types and a new target for cancer immunotherapy. Siglec-15 has unique molecular features compared with many other known checkpoint inhibitory ligands. It shows dominant expression on cancer cells, besides tumor-associated macrophages, and exhibits PD-L1 mutually exclusive expression pattern in human cancers. Siglec-15 blockade by a monoclonal antibody shows therapeutic efficacy in mouse tumor models and human cancer cell culture systems and results in amplified T cell responses. Siglec-15 may represent a novel class of immune checkpoint ligands with tumor- associated expression and divergent mechanisms of action to PD-L1, with important implications for anti-PD unresponsive patients. Based on these results, we have developed an anti-human Siglec-15 antibody (NC318) with the related phase I/II clinical trials currently ongoing for PD-1 refractory metastatic solid cancers. Although the preliminary clinical trial results are quite promising, the clear elucidation of the immune suppressive mechanism of Siglec-15 in the tumor-microenvironment will advance the clinical translation of this novel program. Other than modulating T cell function, our new findings also suggest that Siglec-15 represents a potential novel axis of control for myeloid cells, a key immune cell subset critical in shaping the tumor microenvironment which have a relative lack of successful clinical targets. Beyond this, important questions remain to optimize Siglec-15- related therapy, such as its receptor(s), induction mechanisms, and potential additive effects or synergies with existing treatments. In this project, through two complementary aims, we will leverage our expertise in cancer immunology and immunotherapy to: 1) determine the potential immune function of Siglec-15 beyond T cells, particularly on myeloid cells, and to characterize functional Siglec-15 receptors on both myeloid cells and T cells; 2) examine the role of oncogenic KRAS mutants as induction mechanism of Siglec-15 expression on cancer cells and identify better anti-Siglec-15 combinatorial therapies using two oncogenic KRAS mutant-associated syngeneic orthotopic mouse models of pancreatic and ovarian cancer. Collectively, our proposed studies will facilitate our understanding of a novel PD-L1 mutually exclusive immune checkpoint and enhance Siglec-15- based therapeutic approaches for anti-PD-1/PD-L1 insensitive cancers.

Project Narrative This proposal seeks to elucidate the immunosuppressive mechanism of Siglec-15, a novel PD-L1 mutually exclusive T cell immune checkpoint, on the regulation of myeloid cell responses and the identification of its functional receptors on both myeloid cells and T cells. We will further characterize its potential cancer-associated induction mechanism and strategies to improve Siglec-15-targeted therapeutic responses in anti-PD-1- insensitive cancers. The successful completion of our proposed studies will provide important insights to improve the efficacy of our established clinical program and suggest new approaches for next-generation cancer immunotherapies.