pBiFRT-145, and the plasmid pOG44 (Life Technologies, Carlsbad, CA) were co-transfected into suspension CHO-S/FRT cells using PEIPro (Polyplus-transfection SA, Illkirch, France) for site-directed genomic integration. Two days after transfection, the culture medium was replaced with the selective medium CD OptiCHO (Life Technologies) containing 50 μg/mL hygromycin B, and the stable clone CHO-S/FRT/FVN145 was selected according to the manufacturer’s instructions.

Tick-borne encephalitis virus (TBEV) is the most important tick-transmitted pathogen. It belongs to the Flaviviridae family and causes severe human neuroinfections. In this study, protective efficacy of the chimeric antibody chFVN145 was examined in mice infected with strains belonging to the Far-Eastern, European, and Siberian subtypes of TBEV, and the antibody showed clear therapeutic efficacy when it was administered once one, two, or three days after infection. The efficacy was independent of the TBEV strain used to infect the mice; however, the survival rate of the mice was dependent on the dose of TBEV and of the antibody. No enhancement of TBEV infection was observed when the mice were treated with non-protective doses of chFVN145. Using a panel of recombinant fragments of the TBEV glycoprotein E, the neutralizing epitope for chFVN145 was localized in domain III of the TBEV glycoprotein E, in a region between amino acid residues 301 and 359. In addition, three potential sites responsible for binding with chFVN145 were determined using peptide phage display libraries, and 3D modeling demonstrated that the sites do not contact the fusion loop and, hence, their binding with chFVN145 does not result in increased attachment of TBEV to target cells.