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,
* Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA 22908; and
Genmab,
Department of Hematology, and
Immunotherapy Laboratory, Department of Immunology, University Medical Center, Utrecht, The Netherlands
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.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This work was supported by a research grant from Genmab.
2 Address correspondence and reprint requests to Dr. Ronald P. Taylor, Department of Biochemistry and Molecular Genetics, P.O. Box 800733, University of Virginia, Charlottesville, VA 22908. E-mail address: rpt{at}virginia.edu
3 Abbreviations used in this paper: RTX, rituximab; Al, Alexa; C, complement; CDC, C-dependent cytotoxicity; CLL, chronic lymphocytic leukemia; FM, fluorescence microscopy; GVB, gelatin veronal buffer; MAC, membrane attack complex of C; NHS, normal human serum; OFA, Ofatumumab; SDCM, spinning disk confocal microscopy; 7-AAD, 7-aminoactinomycin D.
4 The online version of this article contains supplemental material.
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  A. W. Pawluczkowycz, F. J. Beurskens, P. V. Beum, M. A. Lindorfer, J. G. J. van de Winkel, P. W. H. I. Parren, and R. P. Taylor Binding of Submaximal C1q Promotes Complement-Dependent Cytotoxicity (CDC) of B Cells Opsonized with Anti-CD20 mAbs Ofatumumab (OFA) or Rituximab (RTX): Considerably Higher Levels of CDC Are Induced by OFA than by RTX J. Immunol., July 1, 2009; 183(1): 749 - 758. [Abstract] [Full Text] [PDF]
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