Interferon production and sensitivity of Semliki Forest virus variants

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Abstract
Semliki Forest virus was passaged 10 times in series in calf kindly cells, starting with virus passaged in the mouse brain (MB virus). A variant was obtained (termed CK virus). The two viruses were antigenically indistinguishable. When grown on L cells, CK virus formed smaller plaques than MB virus. In parallel growth curve studies in calf kidney cells, a small inoculum of CK virus grew more rapidly and to a higher final titre than a comparable inoculum of MB virus, and usually stimulated production of less interferon. Pre-treatment of cells with calf interferon reduced the growth of both viruses, but especially that of MB virus. The results are considered in relation to the phenomenon of virus adaptation. It is suggested that such differences between the two variants in their relative sensitivity to and production of interferon could have played a part in the emergence of CK virus as the predominant type during serial passages in calf cells.I wish to thank Mr H. Moores and Mr R. Sims for skilful technical assistance.