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J. Biol. Chem., Vol. 282, Issue 50, 36724-36735, December 14, 2007

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Resistance to Neutralization by Antibodies Targeting the Coiled Coil of Fusion-active Envelope Is a Common Feature of Retroviruses^*

From the Biomedical Research Centre, Ninewells Hospital and Medical School, the University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom

The human T-cell leukemia virus transmembrane glycoprotein (TM) is a typical class 1 membrane fusion protein and a subunit of the viral envelope glycoprotein complex. Following activation, the TM undergoes conformational transitions from a native nonfusogenic state to a fusion-active pre-hairpin intermediate that subsequently resolves to a compact trimer-of-hairpins or six-helix bundle. Disruption of these structural transitions inhibits membrane fusion and viral entry and validates TM as an anti-viral and vaccine target. To investigate the immunological properties of fusion-active TM, we have generated a panel of monoclonal antibodies that recognize the coiled-coil domain of the pre-hairpin intermediate. Antibody reactivity is highly sensitive to the conformation of the coiled coil as binding is dramatically reduced or lost on denatured antigen. Moreover, a unique group of antibodies are 100-1000-fold more reactive with the coiled coil than the trimer-of-hairpins form of TM. The antibodies recognize virally expressed envelope, and significantly, some selectively bind to envelope only under conditions that promote membrane fusion. Most importantly, many of the antibodies potently block six-helix bundle formation in vitro. Nevertheless, viral envelope was remarkably resistant to neutralization by antibodies directed to the coiled coil. The data imply that the coiled coil of viral envelope is poorly exposed to antibody during membrane fusion. We suggest that resistance to neutralization by antibodies directed to fusion-associated structures is a common property of retroviral TM and perhaps of other viral class I fusion proteins. These observations have significant implications for vaccine design.

Received for publication, August 16, 2007 , and in revised form, October 4, 2007.

^* This work was supported by a grant from the Leukemia Research Fund (to D. W. B.) and in part by a grant from the Association for International Cancer Research (to D. W. B.). 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 To whom correspondence should be addressed. Tel.: 44-1382-660111 (Ext. 33513); Fax: 44-1382-669993; E-mail: brighty{at}cancer.org.uk.

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