CD20 levels determine the in vitro susceptibility to rituximab and complement of B-cell chronic lymphocytic leukemia: further regulation by CD55 and CD59

Complement-dependent cytotoxicity is thought to be an important mechanism o f action of the anti-CD20 monoclonal antibody rituximab. This study investi gates the sensitivity of freshly isolated cells obtained from 33 patients w ith B-cell chronic lymphocytic leukemia (B-CLL), 5 patients with prolymphoc ytic leukemia (PILL), and 6 patients with mantle cell lymphoma (MCL) to be lysed by rituximab and complement in vitro. The results showed that in B-CL L and PLL, the levels of CD20, measured by standard immunofluorescence or u sing calibrated beads, correlated linearly with the lytic response (coeffic ient greater than or equal to 0.9; P <.0001). Furthermore, the correlation remained highly significant when the 6 patients with MCL were included in t he analysis (coefficient 0.91; P <.0001), which suggests that CD20 levels p rimarily determine lysis regardless of diagnostic group. The role of the co mplement inhibitors CD46, CD55, and CD59 was also investigated. All B-CLL;a nd PLL cells expressed these molecules, but at different levels. CD46 was r elatively weak on all samples (mean fluorescence intensity less than 100), whereas CD55 and CD59 showed variability of expression (mean fluorescence i ntensity 20-1200 and 20-250, respectively). Although CD55 and CD59 levels d id not permit prediction of complement susceptibility, the functional block of these inhibitors demonstrated that they play an important role in regul ating complement-dependent cytotoxicity. Thus, lysis of poorly responding B -CLL samples was increased 5- to 6-fold after blocking both CD55 and CD59, whereas that of high responders was essentially complete in the presence of a single blocking antibody. These data demonstrate that CD20, CD55, and CD 59 are important factors determining the in vitro response to rituximab and complement and indicate potential strategies to improve the clinical respo nse to this biologic therapy. (Blood. 2001;98:3383-3389) (C) 2001 by The Am erican Society of Hematology.