Date Published: April 8, 2019
Publisher: Public Library of Science
Author(s): Andrey L. Matveev, Irina V. Kozlova, Oleg V. Stronin, Yana A. Khlusevich, Elena K. Doroshchenko, Ivan K. Baykov, Oksana V. Lisak, Ljudmila A. Emelyanova, Olga V. Suntsova, Vera A. Matveeva, Julia S. Savinova, Nina V. Tikunova, Nicholas J. Mantis.
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.
Tick-borne encephalitis virus (TBEV), a positive-sense single-stranded RNA virus from the Flaviviridae family, is a causative agent of one of the most severe human neuroinfections [1–3]. TBEV is mostly transmitted via bites by Ixodes ticks that inhabit the forested areas of Eurasia from Western Europe to the Far East and from the Scandinavian peninsula to the Mediterranean . Three main subtypes of TBEV are currently recognized: the Far-Eastern (TBEV-FE), European (TBEV-Eu), and Siberian (TBEV-Sib) subtypes [5–7]. TBEV-FE is considered to cause the most severe disease, while TBEV-Eu causes mostly mild infections [6,8–10]. Tick-borne encephalitis (TBE) is documented in many European countries, Russia, China, Mongolia, and Kazakhstan, and, in the past several years, the highest incidence of the disease was recorded in Russia, Slovenia, and the Baltic states . Although most human cases of TBE are asymptomatic, TBEV can cause severe TBE, which is often lethal [3,6,12]. Several vaccines for the prevention of TBE in adults and children are currently available in Europe, Russia, and China. These vaccines are safe, highly immunogenic, and efficient; however, vaccination coverage is low in many regions, which leads to a substantially higher frequency of TBE cases in these regions. Moreover, TBEV vaccine breakthrough cases have been recorded in several countries [13–15].
In this study, the therapeutic efficacy of the chimeric antibody chFVN145 was investigated in mice infected with various subtypes of TBEV. In previous experiments, ch14D5a purified from culture medium after transient expression in CHO-K1 cells showed protective efficacy against lethal infection of BALB/c mice with the TBEV strain Absettarov (TBEV-Eu) when ch14D5a was administered one day after infection . A similar experiment was designed to test the ability of chFVN145 produced by the stable clone CHO-S/FRT/chFVN145 to protect mice challenged with the same TBEV strain. Commercially available anti-TBE-Ig, which is routinely applied for post-exposure prophylaxis and treatment of TBE in Russia [3,16], was used as a positive control. Since antibody preparations used in this study were diluted in 0.9% NaCl, mice treated with 0.9% NaCl were used as one of the controls. Clear dose-dependent efficacy of chFVN145 was observed, and the dose of 100 μg/mouse provided substantially better protection than the dose of 10 μg/mouse. The protective efficacy of chFVN145 was significantly higher than that of the anti-TBE-Ig that is probably due to the substantially lower amount of specific anti-TBEV immunoglobulins in the total protein in this preparation. The anti-TBE-Ig was not effective at a dose of 10 μg/mouse, resulting in the exclusion of this dose from further experiments.