The virulence spectrum of isolates from the 49th HK passage of OCT-541 (JBE) virus at 37°C was examined using an infectious center technique in singly infected HK cells. Thirty per cent of that portion of the virus population that could be examined in this manner were found to be less virulent for the weanling mouse than was the parent virus. Ten per cent of the isolates were of very low virulence and one isolate designated T-11 virus was sufficiently different to be studied in depth.The T-11 virus infectivity titer for cell cultures exceeds the intracerebral LD50 for weanling mice 1 million-fold. Intracerebral inoculation of 250 TCID50 of virus produces a non-lethal immunizing infection in weanling mice. Peripheral inoculation of this host does not result in infection. T-11 virus retains complete neurovirulence for the suckling mouse when given intracerebrally. It will produce a non-lethal immunizing infection in 30% of weanling hamsters inoculated by peripheral routes.Comparison of the attenuated virus with the virulent OCT-541 virus by several methods did not reveal the mechanism or defect by which this virus is different. Thus, T-11 virus has a half-life at 38°C and pH 7.1 similar or superior to that of OCT-541 virus. The two strains are equally sensitive to interferon and T-11 virus can not be demonstrated to induce more interferon than the parent virus. Single cycle growth curves in cell culture are identical for the two viruses. T-11 virus is inhibited in multicycle growth curves and is more sensitive to the saline-soluble fraction of unpurified commerical agars. It has several other plaque markers common to other tissue culture-passaged viruses.
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