SINGLE AMINO-ACID CODON CHANGES DETECTED IN LOUPING ILL VIRUS ANTIBODY-RESISTANT MUTANTS WITH REDUCED NEUROVIRULENCE

Seven mutant viruses were derived from a Scottish strain of louping il l virus using a virus envelope-specific neutralizing monoclonal antibo dy. None of the mutants was neutralized and immunofluorescence microsc opy confirmed that they did not bind to this antibody. Four mutants sh owed reduced mouse neurovirulence compared with parent virus and two m utants failed to induce protective immune responses in mice challenged with virulent tick-borne encephalitis virus. The mutants with the low est virulence showed poor or undetectable haemagglutinating activity. The nucleotide sequence of the envelope glycoprotein gene of each of t he seven mutants was determined and the deduced amino acid sequence wa s compared with parent virus. For each mutant, only a single amino aci d codon change was detected and all the amino acid substitutions occur red within amino acid positions 308 to 311. A change from the amino ac id aspartate to asparagine at amino acid position 308, which represent ed a potential glycosylation site, was the most effective substitution in reducing mouse neurovirulence. The results demonstrate the importa nce of critical sites within the envelope glycoprotein as determinants of virus virulence.