[Changes in the reproduction of tick-borne encephalitis virus in cell cultures]

The currently used tick-borne encephalitis virus vaccines are based on the inactivation of tick-borne encephalitis virus (TBEV) of Far Eastern or West European genetic types from the primary cultures of chick embryo fibroblasts. Since the WHO recommends that vaccines should be designed using continuous cell cultures rather than chick embryos as a substrate, this investigation has compared the infection of continuous monolayer SPEV, Vero E6, and vaccine line Vero (B) cell cultures with TBEV strains of the Siberian and Far Eastern genetic types dominating in the endemic regions of Russia. After cell infection with Far Eastern (Sofyin and 205 strains) or Siberian (Aina, 2530, 2689, and 2703 strains) TBEV genetic types, the viable TBEV titers reached 2.8 Ig CPD50 for Vero (B) cells, 5.5 Ig CPD50 for Vero E6 cells, and up to 9 Ig CPD50 for SPEV cells. The quantitative scores of TBEV E antigen in enzyme immunoassay (EIA) and genome equivalents by reverse-transcription polymerase chain reaction (PCR), followed by real-time PCR, permitted one to estimate as high as 108 virions in 1 ml of culture fluid, which corresponded to those of the microscopic observations of CPD for SPEV cells and substantially exceeded the values for Vero E6 cells, and for Vero (B) cells in particular. The data of TBEV strain titration, EIA, and realtime reverse-transcription PCR suggest that the Russian vaccine Vero (B) cell line defined as meeting the WHO requirements, as well as Vero E6 cells may be used to design tick-borne encephalitis vaccine.