PROJECT SUMMARY The inability of tumor infiltrating lymphocytes (TILs) to target and kill tumor cells is a major hurdle in treating many malignancies. New treatment strategies that block immune inhibitory mechanisms, such as antibodies that block the interaction of programmed death-1 (PD-1) with its ligand, B7 homolog 1 (B7-H1, also known as PD-L1), have shown promising efficacy in the clinic. Termed checkpoint inhibitor therapy, these drugs have been approved for many indications, including melanoma, Hodgkin lymphoma and lung cancer, and are increasingly being used in combination or in conjunction with other cancer therapies, such as chemotherapy and radiation. Although checkpoint inhibitor treatments have resulted in durable clinical responses in a large proportion of cases, many patients present with tumor types that do not respond to treatment. For instance, ~26% of NSCLC cases, which are negative for B7-H1/PD-L1 and positive for TILs, have been shown to be resistant to anti-PD-1/B7-H1(PD-L1) (anti-PD) therapy. This type of NSCLC, denoted as Type III, is suspected to harbor a mechanism of immune inhibition distinct from other NSCLC types, which has been found to be driven, at least in part, by sialic acid binding immunoglobulin-like lectin 15 (siglec-15). Siglec-15 expression is mutually exclusive from B7-H1/PD-L1 expression in NSCLC cohorts and has been shown to inhibit T cell proliferation and effector function. Blocking of siglec-15 using anti-siglec-15 (S15) monoclonal antibody (mAb) is therapeutic in mouse models and human cell culture systems and results in amplified T cell responses. Based on these findings, a phase I/II, dose escalation, safety and tolerability clinical trial for S15 mAb treatment in patients with advanced or metastatic solid tumors is on-going. Although preliminary studies have generated promising results with regard to the potential efficacy of S15 mAb in the clinic, the mechanism of S15-mediated immune suppression remains unknown. Furthermore, to enhance and improve treatment response rates, more work must be done to identify pertinent biomarkers for S15 mAb therapy and modes that modulate S15 expression. Finally, developing combination strategies that alter the tumor microenvironment (TME), such that conversion of the tumor Type is achieved, is imperative for successful targeting and killing of tumor by immune cells and in attaining increased patient response rates to available checkpoint inhibitor therapies. A newly generated immune PDX (iPDX) mouse model, which uses patient-derived tumor tissue to recapitulate and manipulate immune cell responses in the TME, will be utilized to investigate these topics specifically in the NSCLC setting. A proposed investigator-initiated phase II clinical trial in patients with S15+ advanced NSCLC who have progressed on PD-1 axis inhibitor therapy will evaluate S15 mAb efficacy and support biomarker validation studies. Strategies to combine S15 mAb with other agents, such as anti-FGL1 and anti-4-1BB/CD137, to improve therapeutic effect will also be explored. Taken together, the studies proposed here will improve our understanding of the NSCLC TME and enhance therapeutic approaches.