Temperature-sensitive alteration in fusion activity of subacute sclerosing panencephalitis virus during serial passages in vitro and expression of hemagglutinin on the infected cells.

The Biken strain of subacute sclerosing panencephalitis (SSPE) virus, a maturation-defective variant of measles virus, was serially passed in human embryonic lung (HEL) cells at 37 C. The strain formed syncytial giant cells (GC) at both 37 C and 39 C, but the surface of infected cells did not show hemadsorption at early passages of the strain. However, GC-forming activity of the strain diminished at 39 C after 25 passages or more and hemadsorption on the infected cells became positive at both 37 C and 39 C after 40 passages or more of the strain. Hardly any infectious cell-free virus was detected in the culture fluid even after hemadsorption became positive. Possible mechanisms for the defect of SSPE virus were discussed.