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Within Peripheral Blood Mononuclear Cells, Antibody-Dependent Cellular Cytotoxicity of Rituximab-Opsonized Daudi cells Is Promoted by NK Cells and Inhibited by Monocytes due to Shaving¹

Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA 22908

Treatment of chronic lymphocytic leukemia patients with anti-CD20 mAb rituximab (RTX) leads to substantial CD20 loss on circulating malignant B cells soon after completion of the RTX infusion. This CD20 loss, which we term shaving, can compromise the therapeutic efficacy of RTX, and in vitro models reveal that shaving is mediated by effector cells which express FcRI. THP-1 monocytes and PBMC promote shaving, but PBMC also kill antibody-opsonized cells by antibody-dependent cellular cytotoxicity (ADCC), a reaction generally considered to be due to NK cells. We hypothesized that within PBMC, monocytes and NK cells would have substantially different and competing activities with respect ADCC or shaving, thereby either enhancing or inhibiting the therapeutic action of RTX. We measured ADCC and RTX removal from RTX-opsonized Daudi cells promoted by PBMC, or mediated by NK cells and monocytes. NK cells take up RTX and CD20 from RTX-opsonized B cells, and mediate ADCC. PBMC depleted of NK cells show little ADCC activity, whereas PBMC depleted of monocytes have greater ADCC than the PBMC. Pre-treatment of RTX-opsonized B cells with THP-1 cells or monocytes suppresses NK cell-mediated ADCC, and blockade of FcRI on monocytes or THP-1 cells abrogates their ability to suppress ADCC. Our results indicate NK cells are the principal cells in PBMC that kill RTX-opsonized B cells, and that monocytes can suppress ADCC by promoting shaving. These results suggest that RTX-based immunotherapy of cancer may be enhanced based on paradigms which include infusion of compatible NK cells and inhibition of monocyte shaving activity.

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 grant to the University of Virginia Cancer Center from the James and Rebecca Craig Foundation and by the University of Virginia Cancer Center Support Grant.

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; ADCC, Ab-dependent cellular cytotoxicity; CLL, chronic lymphocytic leukemia; Al, Alexa; E/T: effector cell to target cell ratio; MESF, molecules of equivalent soluble fluorochrome.

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