The biological properties of an Indian isolate (GP78) of Japanese encephalitis virus (JEV) were characterized in tissue-cultured cells and mice and these were compared with the JaOArS982 strain from Japan. The GP78 strain had a markedly extended lag phase during its growth in porcine stable kidney (PS) cells. There were no obvious defects in the penetration of GP78 into PS cells. However, viral RNA and protein synthesis were significantly delayed in GP78-infected PS cells. Fusion-from-within assays carried out in C6/36 cells indicated that GP78 was less fusogenic than the JaOArS982 strain of JEV. Moreover, maximum fusion in GP78-infected cells occurred at pH 5.5, whereas JaOArS982-infected cells showed maximum fusion at pH 6.0. These results suggested that there may be a lesion in the virus-cell fusion process. The GP78 strain also showed delayed growth in brains of 1-week-old BALB/c mice. Although JEV GP78 was as virulent as the JaOArS982 strain in these mice, the appearance of clinical symptoms of JEV infection was delayed by a day in mice infected with the GP78 strain and these animals showed an increased average survival time. Comparison of the nucleotide sequences of the GP78 and the JaOArS982 strains of JEV identified a number of amino acid substitutions in structural proteins. Of these, a Thr --> Met substitution at residue 76 of the envelope protein is predicted to be causally associated with the altered biology of the GP78 strain during growth.


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