CD75s Represents a Therapeutic Target for Antibody-Based Immunotherapy of B Cell Lymphoma and Multiple Myeloma

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.