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82 Binding and Neutralization of Primary Isolates by IgG of Infected Patients
R. Burrer, S. Haessig, V. Roques, A.-M. Aubertin, and C. Moog*
Inst. de Virologie, Strasbourg, France
Background: The mechanism(s) of HIV-1 primary isolate (PI) antibody-mediated neutralization are still poorly understood and a subject of controversy. Some studies have suggested that the viral particle is neutralized when the number of viral spikes available for interaction with the target cell falls below a threshold number. In this model, most of the antibodies directed against epitopes exposed on the free virus would contribute to neutralization, irrespective of epitope specificity, and the number of antibodies binding the particle is critical for neutralization. On the other hand, antibodies directed to cryptic epitopes exposed only after attachment of the virus to its receptors have been shown to display a powerful neutralizing activity against PI. The description of at least 1 broadly neutralizing monoclonal antibody directed to an epitope of gp41 (2F5) supports the importance of less accessible regions of the viral envelope. The aim of our study is to investigate the involvement of epitopes exposed on the free PI in neutralization.
Methods: The neutralizing activity of antibodies collected from infected patients was determined on 4 HIV-1 PI in a PBMC-based assay. Viruses purified from free gp120 were used in the binding assays of virions to seric antibodies. To prevent any potential bias inherent to the use of polyclonal samples, we have also designed a new protocol in which virion-IgG immune complexes were separated from nonreacting IgG before the capture step.
Results: No correlation could be found between neutralization and binding of IgG to PI. Some neutralizing samples were unable to capture detectable amount of virus, whereas most of the highly binding IgG fractions were devoid of any neutralizing activity. In addition, the residual infectivity of the purified immune complexes has been evaluated and compared with virus incubated with control IgG. Antibodies that bound the viral particle before its attachment to the cell were able to neutralize virus infectivity.
Conclusions: These results suggest that the neutralization of HIV-1 PI by polyclonal sera is not primarily determined by the amount of antibodies able to bind the viral particle and that some of the epitopes targeted by neutralizing antibodies are exposed on the free virion.
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