Antithymocyte globulin induces complement-dependent cell lysis and caspase-dependent apoptosis in myeloma cells

Objective Allogeneic stem cell transplantation is a potentially curative therapy for patients with multiple myeloma. Polyclonal antithymocyte globulins (ATG) or monoclonal anti-CD52 (Alemtuzumab) are included in conditioning regimens to enhance engraftment and reduce risk of severe graft-vs-host disease. Because both agents have been reported to induce depletion of B cells, we sought to investigate their cytotoxic activity on myeloma cells. Materials and Methods Complement-mediated and complement-independent activity of ATG-Fresenius and Alemtuzumab was investigated on four myeloma cell lines (RPMI-8226, U266, KMS-12-BM, and EJM) and bone marrow samples from six myeloma patients. Cytotoxicity was determined by staining with annexin V-fluorescein isothiocyanate and 7-amino-actinomycin D followed by flow cytometry. Results ATG at a concentration of 500 μg mL−1 induced up to 100% and 85% complement-dependent killing of myeloma cell lines and primary myeloma samples respectively. In the absence of complement ATG still could induce up to 50% and 80% apoptosis in myeloma cell lines and primary myeloma samples, respectively. Preincubation of myeloma cells with a general caspase inhibitor abrogated ATG-induced complement-independent cell death. Alemtuzumab-mediated myeloma cytotoxicity was only observed in KMS-12-BM cells, and in none of the patient samples. Conclusion ATG induces marked cytotoxic activity both in myeloma cell lines and in primary myeloma samples. Further elucidation of antibodies and antigens involved may pave the way for antibody-based myeloma therapy.