Antibody-dependent enhancement of hepatitis C virus infection

Hepatitis C virus (HCV) often causes a persistent infection associated with hypergammaglobulinemia, high levels of antiviral antibody and circulating immune complexes, and immune complex disease. We previously reported that only a limited neutralizing activity to vesicular stomatitis virus or HCV pseudotype is generated in animals immunized with recombinant HCV envelope proteins and chronically infected HCV patient sera. Interestingly, when some of these neutralizing sera were diluted into a range of concentrations below those that reduced virus plaque number, an increase in pseudotype plaque formation was observed. Purified HCV E2-specific human monoclonal antibodies were used to further verify the specificity of this enhancement, and one- to twofold increases were apparent on permissive Huh-7 cells. The enhancement of HCV pseudotype titer could be inhibited by the addition of a Fc-specific anti-human immunoglobulin G Fab fragment to the virus-antibody mixture prior to infection. Treatment of cells with antibody to Fc receptor I (FcRI) or FcRII, but not FcRIII, also led to an inhibition of pseudotype titer enhancement in an additive manner. Human lymphoblastoid cell line (Raji), a poor host for HCV pseudotype infection, exhibited a four- to sixfold enhancement of pseudotype-mediated cell death upon incubation with antibody at nonneutralizing concentrations. A similar enhancement of cell culture-grown HCV infectivity by a human monoclonal antibody was also observed. Taken together, antibodies to viral epitopes enhancing HCV infection need to be taken into consideration for pathogenesis and in the development of an effective vaccine.