title: Complement-induced cell death by rituximab depends on CD20 expression level and acts complementary to antibody-dependent cellular cytotoxicity.
abstract: OBJECTIVE
  The use of the CD20-specific antibody rituximab has greatly improved the response to treatment of CD20+ follicular lymphoma. Despite the success of rituximab, resistance has been reported and prognostic markers to predict individual response are lacking. The level of CD20 expression on tumors has been related to response, but results of several studies are contradictory and no clear relationship could be established. Complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) are thought to be important effector mechanisms, but the exact mechanism of rituximab-mediated cell kill is still unknown. Importantly, no data have been reported on the combined contribution of CDC and ADCC.
  
  
  METHODS
  We have developed a system of clonally related CEM-CD20 cells by retroviral transfer of the human CD20 cDNA (n = 90). This set of cells, with the CD20 molecule as the only variable, was used to study the importance of CD20 expression level on rituximab-mediated CDC, ADCC, and the combination.
  
  
  RESULTS
  We show a sigmoidal correlation of CD20 expression level and rituximab-mediated killing via CDC but not ADCC. On both high and low CD20-expressing cells, all CD20 molecules were translocated into lipid rafts after rituximab binding. Furthermore, CDC and ADCC act simultaneously and CDC-resistant cells are sensitive to ADCC and vice versa.
  
  
  CONCLUSIONS
  These findings suggest that CDC depends on CD20 expression level and that both CDC and ADCC act complementary. These data give new insights into novel strategies to improve the efficacy of CD20-specific antibodies for the treatment of CD20+ tumors.
authors.affiliation: Department of Haematology, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Tom
authors.initials: T
authors.lastname: van Meerten
authors.forename: Rozemarijn S
authors.initials: RS
authors.lastname: van Rijn
authors.forename: Samantha
authors.initials: S
authors.lastname: Hol
authors.forename: Anton
authors.initials: A
authors.lastname: Hagenbeek
authors.forename: Saskia B
authors.initials: SB
authors.lastname: Ebeling
mesh_terms: D000911:Antibodies, Monoclonal / Q000494:pharmacology*; D058846:Antibodies, Monoclonal, Murine-Derived; D000920:Antibody-Dependent Cell Cytotoxicity / Q000187:drug effects* / Q000276:immunology; D000936:Antigen-Antibody Complex / Q000276:immunology; D018951:Antigens, CD20 / Q000096:biosynthesis* / Q000276:immunology; D016923:Cell Death / Q000187:drug effects; D045744:Cell Line, Tumor; D003167:Complement Activation / Q000187:drug effects / Q000276:immunology; D003601:Cytotoxicity Tests, Immunologic; D003602:Cytotoxicity, Immunologic / Q000187:drug effects* / Q000276:immunology; D006801:Humans; D018448:Models, Immunological; D000069283:Rituximab
chemical_list: D000911:Antibodies, Monoclonal; D058846:Antibodies, Monoclonal, Murine-Derived; D000936:Antigen-Antibody Complex; D018951:Antigens, CD20; D000069283:Rituximab