Mechanisms of rabies virus neutralization

The number of immunoglobulins necessary to neutralize rabies virus (CVS strain) was estimated using IgG and IgM monoclonal antibodies (MAb) specific to the three antigenic sites of the glycoprotein. It was estimated that below 130 IgG or 30 IgM bound per virions, infectivity was totally preserved. Neutralization occurred for an average of 1 or 2 IgG for 3 spikes and 1 IgM for 9 to 10 spikes on the virus surface. Saturation was obtained for 1 to 3 IgG per spike, depending on the antibody, and 1 IgM for 4 to 5 spikes. This result was confirmed by electron microscopy. Neutralization-resisting mutants which continued to fix the selecting MAb in ELISA were also investigated. In two cases, the lack of neutralization was due to the fact that the maximum number of immunoglobulins bound per virion was below the neutralizing dose. In one case, however, the mutant was able to fix the same number of IgG as the parental strain and was not neutralized, even at saturation. The capacity of the antibodies to reduce the attachment of the virus onto BSR cells was also examined. Every intermediate between no inhibition of the attachment and inhibition by a factor of 20 was found; even in this last case, inhibition of attachment was insufficient to explain the extent of neutralization. No correlation was found between the antigenic site recognized by the antibody and the level of inhibition. IgM inhibited attachment more than IgG and one IgG2b antibody did not inhibit attachment at all. The fusion of virions saturated with this antibody with artificial liposomes was totally inhibited, either specifically or because virus-antibody complexes did not attached to the liposomes.