Studies on the different conditions for rabies virus neutralization by monoclonal antibodies #1-46-12 and #7-1-9

Virus-neutralizing activity of two monoclonal antibodies (mAbs), #7-1-9 and #1-46-12, against rabies virus glycoprotein (G) was compared. Although these mAbs affected the virion's ability to bind to host cells similarly, a big difference was found in the titres of virus neutralization (1 :7132 and 1:32 for mAbs #1-46-12 and #7-1-9, respectively, at a concentration of 10 mug protein/ml). Although no big difference in virion-binding affinity between the two mAbs was found, the number of antibodies required for virus neutralization was very low, less than or equal to 20 molecules for mAb # 1-46-12 and greater than or equal to 250 molecules for mAb #7-1-9. In the latter case, the mAbs cover a major part of the virion surface and cause steric hindrance of viral receptor-binding activity. The infectivity of an epitope-preserved escape mutant virus (R-61) was not affected by the binding of high numbers of mAb # 1-46-12 to the virion, which implies that mAb binding does not mask the receptor-binding site of the viral spikes. Based on these results, it is hypothesized that mAb # 1-46-12 affected virus infectivity by a mechanism different from covering the virion spikes. Possible virus-neutralizing mechanisms by low numbers of mAb # 1-46-12 in comparison to that of mAb #7-1-9 are discussed.