Cloning and expression of a single-chain antibody fragment specific for foot-and-mouth disease virus.

The gene for a single-chain antibody (VHK) to a conformational epitope on the type A12 foot-and-mouth disease virus (FMDV) particle was assembled and expressed in Escherichia coli. The VHK, purified from periplasmic extracts immunoprecipitated virus as efficiently as its parental monoclonal antibody (MAb) and exhibited the same binding specificity when tested against panel of natural and genetically engineered virus particles. The VHK neutralized type A12 virus in the presence of goat anti-mouse IgG; however, in the absence of the second antibody, only weak neutralizing activity was detected. Preliminary analysis of the mechanism of viral neutralization indicated that both the MAb and the VHK neutralize by the same mechanism. Small amounts of the VHK allowed infection of cells via Fc receptor-mediated adsorption in the presence of the second antibody. These data represent the first report of a single-chain neutralizing antibody for a picornavirus and provide insights into the mechanisms of viral neutralization and virus uptake.