The Myeloid Cell Surface Receptor IREM-1: A Novel Monoclonal Antibody Target for Acute Myeloid Leukemia (AML)

Introduction: Immune Receptor Expressed by Myeloid Cells (IREM-1), a member of the CD300L gene family, is an inhibitory receptor expressed by myeloid cells.  We reported the characterization of IREM-1 expression in AML blasts and demonstrated the therapeutic potential of IREM-1 specific mouse monoclonal antibodies (mAb) against AML blasts through complement-dependent cytotoxicity assays.  Here we present our latest expression and in vivo studies to further evaluate the potential of IREM-1 for antibody-mediated AML therapy.  Methods: High affinity mAbs to IREM-1 were generated with Kd values in low nM range.  For in vivo models, the IREM-1 chimeric (huIgG1) monoclonal antibody D12 was generated.  In these models, growth of established tumors in an HL-60 xenograft model in nude mice was monitored upon dosing with anti-IREM1 mAb.  Immunohistochemistry (IHC, Discovery, Ventana Medical Systems) or flow cytometry (FC, FACSCalibur, BD Bioscience) were performed using MAbs (Alexa-488 conjugated for FC).  Results: We compared the expression of IREM-1 and CD33 in normal human peripheral blood (n=10) by flow cytometry.  Both molecules are expressed in monocytes, dendritic cells and granulocytes, while no expression in lymphocytes was observed.  The expression pattern of CD33 was similar to the IREM-1 pattern.  IHC staining also demonstrated the expression of IREM-1 in AML and normal bone marrow.  Flow cytometry indicated IREM-1 was expressed in 37/52 (71.2%) of AML cases with a range of positive blasts from 20-99% (mean 72.1%).  The IREM-1 chimeric monoclonal antibody D12 showed antibody-dependent cell-mediated cytotoxicity activity against IREM-1 transfected human embryonic kidney 293 cells with EC50 at 65 ng/ml.  Antibody-mediated IREM-1 internalization was observed in both transfected cells as well as AML cell lines.  In an HL-60 AML xenograft model, anti-tumor activity of D12 antibody delayed tumor growth by 40%.  Conclusion:  Our results further confirm IREM-1 as a cell surface antigen expressed in majority of AML cases and IREM-1 has a similar expression pattern as CD33 among normal blood specimen.  Our high affinity chimeric monoclonal antibody against IREM-1 showed anti-tumor activity both in vitro and in vivo. IREM-1 internalization upon antibody engagement also identifies IREM-1 as a suitable target antigen for toxin-conjugated antibody therapy. Taken together, this study supports the rationale for further development of anti-IREM-1 D12 as an immunotherapeutic agent in AML.