SIRPα-antibody fusion proteins stimulate phagocytosis and promote elimination of acute myeloid leukemia cells

// Laia Pascual Ponce 1, 5 , Nadja C. Fenn 1 , Nadine Moritz 1 , Christina Krupka 2, 3 , Jan-Hendrik Kozik 1 , Kirsten Lauber 4 , Marion Subklewe 2, 3 , Karl-Peter Hopfner 1, 5 1 Gene Center Munich, Department of Biochemistry, Ludwig-Maximilians-Universität München, Munich, Germany 2 Department of Internal Medicine III, Klinikum der Universität München, Ludwig-Maximilians-Universität München, Munich, Germany 3 Gene Center and Clinical Co-operation Group Immunotherapy at the Helmholtz Zentrum München, Munich, Germany 4 Department of Radiation Oncology, Klinikum der Universität München, Ludwig-Maximilians-Universität München, Munich, Germany 5 Graduate School of Quantitative Biosciences Munich, Ludwig-Maximilians-Universität München, Munich, Germany Correspondence to: Karl-Peter Hopfner, email: hopfner@genzentrum.lmu.de Keywords: therapeutic antibody, immunotherapy, CD47, SIRPα, acute myeloid leukemia Received: August 02, 2016      Accepted: December 12, 2016      Published: January 04, 2017 ABSTRACT CD47, expressed on a variety of tumor cells, confers immune resistance by delivering an inhibitory “don’t eat me” signal to phagocytic cells via its myeloid-specific receptor SIRPα. Recent studies have shown that blocking the CD47-SIRPα axis with CD47-directed antibodies or antibody-derivatives enhances phagocytosis and increases antitumor immune effects. However, CD47 expression on healthy cells creates an antigen sink and potential sites of toxicity, limiting the efficacy of CD47-directed therapies. In this study, we first characterized CD47 expression in Acute Myeloid Leukemia (AML) patients ( n = 213) and found that CD47 is highly expressed on both AML bulk and stem cells irrespective of the disease state. Furthermore, to inhibit the CD47-SIRPα signaling pathway at the tumor site, we developed a so-called local inhibitory checkpoint m onoclonal antibody (licMAB) by grafting the endogenous SIRPα domain to the N-terminus of the light chain of an antibody targeting CD33, a surface antigen expressed in AML. LicMABs selectively bind CD33-expressing cells even in the presence of a large CD33-negative CD47-positive antigen sink, stimulate phagocytosis of AML cells and eliminate AML cell lines and primary, patient-derived AML cells. Our findings qualify licMABs as a promising therapeutic approach to confine the benefit of disrupting the CD47-SIRPα axis to tumor antigen-expressing cells.