THE SIGNIFICANCE OF THE 3'-NONTRANSLATED REGION AND E2 AMINO-ACID MUTATIONS IN THE VIRULENCE OF SEMLIKI-FOREST-VIRUS IN MICE

We have recently shown that the 3'-nontranslated region (3'-NTR) of th e avirulent Semliki Forest virus A7(74) [SFVA(74)] contains a unique s equence of 101 nucleotides and five repetitive nucleotide units wherea s the 3'-NTR of the neurovirulent SFV4 has only two repeats. A chimeri c virus was constructed by replacing the entire 3'-NTR of the SFV4 clo ne with the A7(74) 3'-NTR. The hybrid replicated efficiently in the ce ntral nervous system (CNS) of adult Balb/c mice and, similarly to SFV4 , led to high mortality after intraperitoneal inoculation. In contrast , another chimeric virus, CME2, containing the E2 gene of the avirulen t SFVA7(74) virus in the SFV4 clone was recently shown to be avirulent for mice. Several derivatives with single-site or a constellation of amino acid mutations were constructed. Two single-site E2 mutants, Val 37Ile and Asn212Ser, displayed an attenuated phenotype in mice with mo rtality reduced from 90 to 48 and 43%, respectively, None of the multi ple site mutants were significantly attenuated. Adult female mice show ed a greater resistance to SFV infection than male mice. The SFV hybri d viruses CM3NTR and CME2, reached the CNS similarly to the parental v iruses, but the single-site 52 mutants were only sporadically found in the CNS. We conclude that in mice the 3'-NTR does not play a signific ant role in the pathogenesis of Semliki Forest virus and that specific 52 amino acid mutations reduce the virulence, especially in female mi ce. The results additionally suggest that individual amino acid mutati ons in the E2 glycoprotein affect the efficiency of migration into the CNS. (C) 1998 Academic Press.