• Yinxing Liu, Mengjie Zheng, Zengzu Lai, Dongsheng Xiong, Dongmei Fan, Yuanfu Xu, Hui Peng, Xiaofeng Shao, Yuansheng Xu, Ming Yang, Jinhong Wang, Hanzhi Liu, Yong Xie, Chunzheng Yang, and Zhenping Zhu.
• State Key Laboratory of Experimental Hematology, Institute of Hematology, Chinese Academy of Medical Sciences, 288 Nanjing Road, Tianjin 300020, China.
• Cancer Lett. 2004 Mar 18; 205 (2): 143-53.

AbstractMonoclonal antibodies (mAb) directed against CD20, either unmodified or in radiolabeled forms, have been successfully used in clinic as effective therapeutic agents in the management of non-Hodgkin's B-cell lymphoma. Despite all clinical success the exact mechanisms of action of various anti-CD20 antibodies remains mostly unclear. Several mechanisms have been proposed to be responsible for the therapeutic activity of anti-CD20 antibodies, including antibody-dependent cell-mediated cytotoxicity, complement-mediated cytotoxicity, and direct inhibition of tumor growth via induction of apoptosis. We previously produced an anti-CD20 mAb, HI47, and showed that the antibody effectively blocked human B-cell proliferation in vitro and inhibited xenografted B-cell lymphoma in nude mice. In this study, we engineered the chimeric versions of both the Fab and F(ab)'2 fragments of HI47 and produced the fragments in E. coli. Both fragments competed efficiently with HI47 for binding to CD20+ B cells, and inhibited proliferation of B-lymphoma cells in a dose-dependent manner. Mechanistic studies revealed that both antibody fragments induced significant degree of B-cell apoptosis that is independent of any cross-linking agents. Further, both the F(ab)'2 and Fab fragments when administered in vivo significantly inhibited the growth of human B-cell lymphoma xenografts in nude mice. The bivalent F(ab)'2 fragment showed consistently better efficacy compared to its monovalent Fab counterpart in inducing apoptosis and inhibiting B-cell lymphoma growth both in vitro and in vivo. Taken together, these observations suggest that HI47 and its fragments most likely exert their antitumor activity through induction of cell apoptosis, and cross-linking/dimerization of CD20 molecules on B- cell surface is an important, but not essential, process for therapeutic efficacy of HI47 and its fragments.

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