Abstract 3629: Overcoming macrophage immunosuppression in small cell lung cancer with high-affinity SIRPa variants
Authors: Weiskopf Kipp , Schnorr Peter J. , Jahchan Nadine , Ring Aaron M. , Maute Roy L. , Volkmer Anne K. , Volkmer Jens-Peter , Garcia Kenan C. , Sage Julien , Weissman Irving L. 
Published: May 2015

Abstract
CD47 allows cancer cells to evade the immune system by signaling through SIRPa, an inhibitory receptor on macrophages. Therapies that block CD47 convert tumor-promoting macrophages to a tumoricidal state within the tumor microenvironment. We recently developed next-generation CD47 antagonists by engineering the extracellular domain of SIRPa. As single-domain polypeptides, these “high-affinity SIRPa variants” have an affinity for human CD47 (KD) as low as 11.1 pM, approximately 50,000-fold improved over wild-type SIRPa. By themselves, the high-affinity SIRPa variants are inert and therefore non-toxic in mouse and primate studies. However, when combined with tumor-specific antibodies, the high-affinity SIRPa variants act as immunotherapeutic adjuvants to antibody therapies by maximizing the ability of macrophages to destroy cancer cells.
In our current study, we hypothesized these novel CD47-blocking agents could be applied to the treatment of small cell lung cancer (SCLC), a cancer with poor prognosis for which no clinically-approved antibodies or immunotherapies exist. We examined a panel of human SCLC samples and found all samples tested expressed high levels of CD47 on their surface. Using purified macrophages in vitro, we found that CD47-blocking therapies were able to induce macrophage phagocytosis of SCLC cell lines and primary patient samples. As a proof-of-concept, treatment of mice bearing primary SCLC tumors with CD47-blocking antibodies was able to inhibit tumor growth and significantly prolong survival.
To identify novel SCLC antigens that can be targeted in combination with the high-affinity SIRPa variants, we screened SCLC samples by high-throughput flow cytometry using LEGENDScreen comprehensive antibody arrays. We identified several new and established therapeutic targets on the surface of SCLC cells, including CD99, CD56, CD166, CD326, and CD164. We identified antibodies to these antigens that could elicit macrophage phagocytosis in vitro, validating these antigens as targets for immune-based therapies. The ability of these antibodies to induce phagocytosis was dramatically enhanced when combined with the high-affinity SIRPa variants. Future studies will test these immunotherapeutic combinations in vivo against SCLC samples to develop novel therapeutic combinations for patients. We propose this strategy as a universal method to identify new tumor antigens and overcome macrophage immunosuppression within the tumor microenvironment.
Citation Format: Kipp Weiskopf, Peter J. Schnorr, Nadine Jahchan, Aaron M. Ring, Roy L. Maute, Anne K. Volkmer, Jens-Peter Volkmer, Kenan C. Garcia, Julien Sage, Irving L. Weissman. Overcoming macrophage immunosuppression in small cell lung cancer with high-affinity SIRPa variants. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3629. doi:10.1158/1538-7445.AM2014-3629