Reverted virulence of attenuated tick-borne encephalitis virus mutant is not accompanied with the changes in deduced viral envelope protein amino acid sequence.

Serial passages of tick-borne encephalitis (TBE) virus strain 4387 isolated from the liver and lungs of the bank vole through the salivary glands of Ixodes ricinus ticks led to a reduction of its virulence for laboratory mice infected via peripheral route. When attenuated mutants were passaged through mouse brains, virulent phenotypes have appeared in the 3rd mouse passage. After 5 consecutive passages the virus was more pathogenic for mice after peripheral inoculation than the parental 4387 strain. The nucleotide sequence of the envelope proteins of the strain 4387 was studied after passaging through ticks salivary glands and subsequently through mice. The sequences coding for the envelope protein E of the virus from the first, third and fifth mouse passages were compared with those of parental virus and mutant attenuated in ticks. The attenuated mutant differing from the parental strain 4387 by the amino acid substitution from glutamic acid to lysine at position 84, and from isoleucine to threonine at amino acid position 319 revealed strongly reduced pathogenicity for adult laboratory mice after peripheral inoculation. The attenuated mutant regained its virulence after 3 - 5 mouse brain passages, but the two amino acid substitutions were still conserved.