2621 Background: Complement-dependent cytotoxicity (CDC) is a key mechanism of Rituximab action in killing NHL cells both in vitro and in vivo. Here we present the studies of a mouse/human chimeric Mab (ETI-210) specific for human complement C3 cleavage products, C3b and iC3b, in enhancing Rituximab ability to kill NHL cells in vitro and ex vivo.Methods: In the CDC assay, Raji cells, DB cells, primary NHL and CLL cells were treated with Rituximab and the anti-C3b/iC3b in the presence of normal human serum at 37 degrees for 1 hour or longer. Cell death, determined by propidium iodide staining, was measured by a flow cytometric method. Results: The anti-C3b/iC3b Mab increased the cell killing of NHL cell lines tested in the one-hour treatment of Rituximab plus anti-C3b/iC3b Mab in the presence of human complement. The difference in cell killing between Rituximab plus anti-C3b/iC3b and Rituximab alone was 40%–70%. The enhancing effect of the anti-C3b/iC3b was also seen in long-term treatment (1 - 2 days). In the absence of human serum, the cell killing by Rituximab plus the anti-C3b/iC3b Mab was much lower than in the presence of serum. The addition of excess purified iC3b to the CDC assay under the experimental conditions only caused a minimal inhibition of cell killing. The anti-C3b/iC3b Mab was also tested ex vivo against primary cells from 11 NHL and 10 CLL patients. In 3 NHL samples, the anti-C3b/iC3b Mab enhanced Rituximab-mediated killing; in the remaining samples enhancement was observed but was not significant since Rituximab alone already caused above 90% killing. The antibody also increased Rituximab-mediated killing in 6 out of 10 CLL samples. Conclusions: The anti-C3b/iC3b Mab is able to substantially enhance Rituximab-mediated killing of lymphoma cells via complement-mediated lysis. This Mab has therapeutic applications in NHL and CLL treatment. No significant financial relationships to disclose.