Monovalent single‐chain Fv fragments and bivalent miniantibodies bound to vesicular stomatitis virus protect against lethal infection

AbstractSeveral antibody‐dependent mechanisms have been postulated to mediate neutralization of different animal viruses, including blocking of docking to receptors, induction of conformational changes in the virus coat, and Fc‐dependent opsonization. We have studied the molecular requirements for antibody‐mediated neutralization of vesicular stomatitis virus (VSV) in vitro and protection against lethal disease in vivo with a single‐chain Fv fragment (scFv) and the corresponding bivalent miniantibody (scFv‐dHLX) generated from a VSV‐neutralizing monoclonal antibody. Both monovalent scFv and bivalent scFv‐dHLX miniantibodies were able to neutralize VSV in vitro and to protect interferon‐αβ receptor‐deficient (IFN‐αβR−/−) mice against lethal disease after intravenous injection of 50 plaque‐forming units (pfu) VSV pre‐incubated with the scFv reagents. Similarly, severe‐combined immunodeficient (SCID) mice infected with immune complexes of 108 pfu VSV and bivalent scFv‐dHLX were protected against lethal disease; however, mice infected with immune complexes of 108 pfu VSV and monovalent scFv were not. Although repeated scFv‐dHLX treatment reduced virus quantities in the blood, neither SCID nor IFN‐αβR−/− mice were protected against lethal disease after passive immunization and subsequent VSV infection. This was due to the short half‐life of 17 min of scFv‐dHLX in the circulation. These data demonstrate that neutralization of VSV and protection against lethal disease do not require Fc‐mediated mechanisms and that cross‐linking is not crucial for protection against physiologically relevant virus doses in vivo.