Rubella virus-induced superinfection exclusion studied in cells with persisting replicons

For the first time, homologous superinfection exclusion was documented for rubella virus (RUB) by using Vero cells harbouring persisting RUB replicons. Infection with wild-type RUB was reduced by tenfold, whereas Sindbis virus infection was unaffected. Replication following infection with packaged replicons and transfection with replicon transcripts was also restricted in these cells, indicating that restriction occurred after penetration and entry. Translation of such 'supertransfecting' replicon transcripts was not impaired, but no accumulation of supertransfecting replicon RNA could be detected. We tested the hypothesis favoured in the related alphaviruses that superinfection exclusion is mediated by cleavage of the incoming non-structural precursor by the pre-existing non-structural (NS) protease, resulting in an inhibition of minus-strand RNA synthesis. However, cleavage of a precursor translated from a supertransfecting replicon transcript with an NS protease catalytic-site mutation was not detected and the event in the replication cycle at which superinfection exclusion is executed remains to be elucidated.