Toxicity of xanthine oxidoreductase to malignant B lymphocytes.

Reactive oxygen species (ROS) generated by xanthine oxidoreductase (XOR) were toxic to lymphoma-derived Raji cells (positive for 8A monoclonal antibody, mAb). The sensitivity of these malignant cells to the hypoxanthine/XOR system was higher than that observed in peripheral human lymphocytes. Cell death mostly had the feature of apoptosis and post-apoptotic necrosis and depended on the activity of XOR. Catalase, but not superoxide dismutase, protected cells from the toxicity of XOR, thus indicating that cell damage depended on the production of hydrogen peroxide. XOR was covalently linked either to the 8A mAb, which recognises antigens expressed on the B cell lineage, or to IgG anti-mouse polyclonal antibodies. Both direct (8A-XOR) and indirect (8A mAb plus anti-mouse IgG-XOR) immunotoxins induced apoptotic death to target cells in a dose-dependent manner. These conjugates showed no aspecific cytotoxicity in conditions very similar to the ex vivo treatment of cell suspension for bone marrow transplantation. Moreover, the prevalence of apoptotic death over necrosis may reduce the in vivo inflammatory response and its local and systemic consequences, thus becoming relevant in the construction of immunotoxins with therapeutic potential.