NEUTRALIZING F(AB')(2) FRAGMENTS OF PROTECTIVE MONOCLONAL-ANTIBODIES TO YELLOW-FEVER VIRUS (YF) ENVELOPE PROTEIN FAIL TO PROTECT MICE AGAINST LETHAL YF ENCEPHALITIS

Monoclonal antibodies (MAbs) prepared against the yellow fever virus ( YF) vaccine strain 17D (17D YF) envelope E protein were used to invest igate Fc piece involvement in antibody-mediated protection against YF encephalitis in mice. 17D YF passaged either in Vero cells or in mouse brain (P-YF) to increase neurovirulence was used. To avoid uncertaint y concerning antibody clearance and blood-brain barrier penetration, a nd to directly compare protective activity with neutralization in vitr o, pre-formed antibody-virus complexes were injected intracerebrally o r assayed for plaque formation in parallel. F(ab')(2) fragments of an IgG2a MAb that strongly neutralized both YF strains retained molar equ ivalent neutralizing activity in vitro, but did not protect. However, further incubation of such F(ab')(2)-virus antibody complexes with rab bit IgG, but not F(ab')(2) anti-mouse IgG resulted in protection. To u nambiguously test for Fc piece involvement in this model we derived an IgG2a isotype switch variant from a protective IgG1 MAb-secreting hyb ridoma and prepared F(ab')(2) fragments of the derivative. Intact and fragmented antibodies exhibited weak neutralizing activity. The varian t antibody failed to protect against P-YF, but against considerably le ss neurovirulent 17D YF its protective capacity was 10-fold higher tha n that of its IgG1 parent. F(ab')(2) fragments of the variant did not protect. Together, these results provide strong evidence of an in vivo protective function for the anti-virion antibody Fc piece and indicat e that in vitro neutralizing activity as a predictor of antibody prote ctive capacity is dependent on Fc piece integrity and isotype.