Loss of CD20 and bound CD20 antibody from opsonized B cells occurs more rapidly because of trogocytosis mediated by Fc receptor-expressing effector cells than direct internalization by the B cells

We previously reported that 1 h after infusion of CD20 mAb rituximab in patients with chronic lymphocytic leukemia (CLL), > 80% of CD20 was removed from circulating B cells, and we replicated this finding, based on in vitro models. This reaction occurs via an endocytic process called shaving/trogocytosis, mediated by Fc gamma R on acceptor cells including monocytes/macrophages, which remove and internalize rituximab-CD20 immune complexes from B cells. Beers et al. reported that CD20 mAb-induced antigenic modulation occurs as a result of internalization of B cell-bound mAb-CD20 complexes by the B cells themselves, with internalization of similar to 40% observed after 2 h at 37 degrees C. These findings raise fundamental questions regarding the relative importance of shaving versus internalization in promoting CD20 loss and have substantial implications for the design of mAb-based cancer therapies. Therefore, we performed direct comparisons, based on flow cytometry, to determine the relative rates and extent of shaving versus internalization. B cells, from cell lines, from patients with CLL, and from normal donors, were opsonized with CD20 mAbs rituximab or ofatumumab and incubated for varying times and then reacted with acceptor THP-1 monocytes to promote shaving. We find that shaving induces considerably greater loss of CD20 and bound mAb from opsonized B cells in much shorter time periods (75-90% i

Complement Activation on B Lymphocytes Opsonized with Rituximab or Ofatumumab Produces Substantial Changes in Membrane Structure Preceding Cell Lysis.

Binding of the CD20 mAb rituximab (RTX) to B lymphocytes in normal human serum (NHS) activates complement (C) and promotes C3b deposition on or in close proximity to cell-bound RTX. Based on spinning disk confocal microscopy analyses, we report the first real-time visualization of C3b deposition and C-mediated killing of RTX-opsonized B cells. C activation by RTX-opsonized Daudi B cells induces rapid membrane blebbing and generation of long, thin structures protruding from cell surfaces, which we call streamers. Ofatumumab, a unique mAb that targets a distinct binding site (the small loop epitope) of the CD20 Ag, induces more rapid killing and streaming on Daudi cells than RTX. In contrast to RTX, ofatumumab promotes streamer formation and killing of ARH77 cells and primary B cells from patients with chronic lymphocytic leukemia. Generation of streamers requires C activation; no streaming occurs in media, NHS-EDTA, or in sera depleted of C5 or C9. Streamers can be visualized in bright field by phase imaging, and fluorescence-staining patterns indicate they contain membrane lipids and polymerized actin. Streaming also occurs if cells are reacted in medium with bee venom melittin, which penetrates cells and forms membrane pores in a manner similar to the membrane-attack complex of C. Structures similar to streamers are demonstrable when Ab-opsonized sheep erythrocytes (non-nucleated cells) are reacted with NHS. Taken together, our findings indicate that the membrane-attack complex is a key mediator of streaming. Streamer formation may, thus, represent a membrane structural change that can occur shortly before complement-induced cell death