Abstract 73: Role of macrophages in the antitumor activity of an anti-transferrin receptor 1 antibody ch128.1 in a xenograft model of multiple myeloma
Authors: Leoh Lai Sum , Kim Yoon Kyung , Candelaria Pierre V. , Martínez-Maza Otoniel , Daniels-Wells Tracy R. , Penichet Manuel L. 
Published: July 2017

Abstract
The transferrin receptor 1 (TfR1), also known as CD71, is a membrane glycoprotein involved in cellular iron uptake and regulation of cell growth. The high level of TfR1 expression on malignant cells and its key role in cancer cell pathology make this receptor an attractive target for antibody cancer therapy. We previously developed a mouse/human chimeric IgG3 specific for human TfR1 (ch128.1). This antibody exhibits direct cytotoxicity against certain human malignant B cells in vitro through TfR1 degradation and iron deprivation. Importantly, ch128.1 shows remarkable anti-tumor activity in xenograft models of disseminated multiple myeloma (MM) in immunosuppressed mice (SCID-Beige). Interestingly, this anti-tumor protection was observed even against MM cells (KMS-11 cells) that show no sensitivity to this antibody in vitro, suggesting the in vivo contributions of antibody effector functions. This possibility was supported by the lack of anti-tumor protection observed using a ch128.1 Fc mutant with impaired binding to FcγRs and to the complement component C1q. To examine host effector functions involved in ch128.1-mediated protection in our mouse model bearing KMS-11 tumors, depletion studies of complement and macrophages were performed. Complement depletion using cobra venom factor (CVF) did not affect protection, suggesting that complement-mediated cytotoxicity (CDC) is not a relevant mechanism of action. Notably, we now report that macrophage depletion using clodronate liposomes (clodrolip) significantly reduced protection, suggesting that these effector cells play a relevant role in the anti-tumor activity. Consistent with this result, we also report that ch128.1 is capable of eliciting antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cell-mediated phagocytosis (ADCP) against KMS-11 cells in the presence of the murine macrophage cell line J774.2 or murine bone marrow-derived macrophages. To examine the role of iron deprivation in ch128.1-mediated tumor death in vivo, animals treated with ch128.1 were systemically supplemented with iron in a preliminary study. However, no difference in survival was observed, suggesting that iron deprivation is not a contributor to the effects of ch128.1 in our in vivo model or the iron dose tested was not optimal. Our results suggest that macrophages play a key role in ch128.1-mediated anti-tumor protection in our model and that ch128.1 can be an effective therapy of incurable human B-cell malignancies such as MM.
Citation Format: Lai Sum Leoh, Yoon Kyung Kim, Pierre V. Candelaria, Otoniel Martínez-Maza, Tracy R. Daniels-Wells, Manuel L. Penichet. Role of macrophages in the antitumor activity of an anti-transferrin receptor 1 antibody ch128.1 in a xenograft model of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 73. doi:10.1158/1538-7445.AM2017-73