Figure 1. Complement Dependent Cytotoxicity induced by Monoclonal Antibodies and Serum.

Complement dependent cytotoxicity (CDC) induced by monoclonal antibodies (mAb) was evaluated in vitro in CLL cells from 21 patients with untreated CLL using counting bead calibrated flow cytometry after propidium iodide staining. The results were normalized to measurements on CLL cells treated with 10% normal human serum (10%NHS) as a source of complement (C). These studies showed that CDC could result in cellular disintegration (lysis) or loss of cell membrane integrity without lysis (intact dead cells) and our results showed that these events occurred with similar frequency. Addition of rituximab and 10%NHS (RTX + C) promoted a low level of CDC compared to CLL cells treated with 10%NHS alone. However, ofatumumab and 10%NHS (OFA + C) did appreciably increase CDC (median 23%, p<0.0001) but with a wide range of responses (range 0% – 71%). Alemtuzumab and 10%NHS (ALM + C) induced considerably more CDC (median 86%, p<0.0001) with a narrower range of responses (49% - 96%). Addition of OFA to ALM +C caused significant additional increases in CDC (median 91%, p=0.01) with further narrowing of the range of response (83 – 96%) but this combination achieved > 95% cytotoxicity in only 3 patients. The median %CDC and %cell lysis for each mAb treatment is shown in the box below the dot plot and * indicate significant differences (p≤0.01).