The Hallé subacute sclerosing panencephalitis (SSPE) measles virus isolate and its plaque-purified progeny were investigated to determine whether any unusual properties could be associated with its ability to cause persistent infection. Three types of plaque-purified progeny were isolated. One population appeared to be similar in biological and biochemical properties to laboratory-adapted measles virus and had the ability to induce syncytia (syn). A second population (syn) plaqued more efficiently at 39 °C than at 33 °C, did not cause normal cell fusion at either temperature, and produced particles that interfered with the replication of other measles virus isolates and . This syn virus was further plaquepurified to eliminate the interfering particles, producing the syn P2 virus. This virus also plaqued more efficiently at 39 °C than at 33 °C, but caused cell fusion only at 39 °C. Both syn viruses and the parental virus were significantly less virulent than the syn virus and caused a more prolonged infection. Biochemical analysis showed that the syn P2 population produced particles that banded at two different densities in potassium tartrate gradients; both densities were less than those of the standard laboratory measles virus and the syn virus. Although the syn P2 virus did not cause cell fusion at 33 °C, [S]methionine labelling demonstrated that the haemolysin/cell fusion protein was present in syn P2 virions. The production of interfering particles, the inability to cause cell fusion at 33 °C, and the cold-sensitive nature of the syn population appear to play a role in the ability of the Hallé SSPE virus to establish persistent infection.


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