Mechanism of cytotoxicity induced by chimeric mouse human monoclonal antibody IDEC-C2B8 in CD20-expressing lymphoma cell lines

With a new flow cytometric cytotoxicity assay, we examined the mechanism of action of chimeric mouse human anti-CD20 monoclonal antibody IDEC-C2B8. ID EC-C2B8 alone induced direct cytotoxicity in four of eight examined CD20-ex pressing lymphoma cell lines (RAJI, DAUDI, JOK-1, and WT100) at a concentra tion above 100 ng/ml. Moreover, after 4 h incubation in human serum, only a moderate complement-dependent cellular cytotoxicity (CDCC) was observed, w hereas cytotoxicity increased markedly after 3 days of culture, indicating that combined direct cytotoxicity and CDCC were responsible. IDEC-C2B8 indu ced an effective antibody-dependent cellular cytotoxicity (ADCC) in seven o f eight tested lymphoma cell lines when peripheral blood mononuclear cells were used as effector cells. ADCC was moderately enhanced by cytokine inter leukin-2, whereas interleukin-12 interferon-alpha, and GM-CSF had no influe nce. Interestingly, we could demonstrate a correlation between CD32 express ion on lymphoma cell lines and IDEC-C2B8-induced direct cytotoxicity, indic ating that crosslinking of CD20 with CD32 may be involved in the mechanism of cytotoxicity. We propose that direct cytotoxicity, CDCC, and ADCC result in the marked elimination of CD20-expressing tumor cells observed after tr eatment with IDEC-C2B8. (C) 2000 Academic Press.