Recently, with Daratumumab and Elotuzumab two monoclonal antibodies targeting malignant plasma cells have been approved. Despite convincing clinical activity, still not all patients benefit and novel well-defined surface receptors with an expression profile restricted to mature B cells may represent promising target structures. A monoclonal antibody designated EBU-141 was generated in our laboratory and targets CD75s. The antibody is of mouse IgM isotype and reacts with mature B- cells as well as plasma cells. Of note, EBU-141 specifically binds to myeloma/plasma cell leukemia cells and B cell lymphomas, including Burkitt's lymphoma and CLL, while no binding to precursor B-cell leukemias was found. Non-B-lineage hematolymphatic tumors showed no significant staining with EBU-141. The aim of the present study was to investigate effector mechanisms mediated by EBU-141 and to develop therapeutically useful EBU-141-based chimeric antibody variants. The potential of these different formats for immunotherapy of B cell lymphomas and multiple myeloma was evaluated.
The variable regions of EBU-141 were isolated and a chimeric IgG1k antibody called chEBU-141 was generated. chEBU-141 was produced by transient transfection and purified from cell culture supernatants by affinity chromatography. To investigate the antibodies' direct inhibitory effects and Fc-mediated modes of action, cell proliferation assays and chromium release experiments with myeloma and lymphoma cell lines were performed. The parentalhybridoma antibody EBU-141 significantly induced programmed cell death by cross-linking the target antigen on the cell surface of lymphoma cells. It also triggered strong complement-dependent cytotoxicity (CDC) against lymphoma cells using human serum as source of complement. Interestingly, EBU-141 was not capable in triggering CDC against myeloma cell lines and patient-derived malignant plasma cells despite significant expression of CD75s. When compared with the IgM antibody EBU-141, the bivalent IgG1 chEBU-141 lacked CDC activity. In contrast, chEBU-141 was capable to trigger potent antibody-dependent cell-mediated cytotoxicity (ADCC) against lymphoma and myeloma cells using human mononuclear cells or isolated NK cells as effector cells.
Here, CD75s could be identified as a promising target for immunotherapy of mature B cell malignancies. The selection of the chimeric IgG1 variant chEBU-141 allows especially effective triggering of ADCC. The potential of CD75s antibody therapy may be further improved by applying Fc-engineering technologies that enhance CDC activity, an effector function potently triggered by the murine IgM antibody.
Klausz: Affimed: Research Funding. Kellner: Affimed: Research Funding. Peipp: Affimed: Research Funding.
Asterisk with author names denotes non-ASH members.