Infectious bronchitis virus was propagated in chicken kidney cell culture, showing cytopathogenic effect (CPE). It also multiplied in chick-embryo cell culture, but failed to exhibit CPE. Neither virus propagation nor CPE was observed in cell cultures of first generation from the kidney of cattle, swine, goat, dog, rabbit, guinea pig, or hamster, or from the mouse embryo, or in passage cells of BHK-21 (hamster kidney origin), PK-15 or PS (swine kidney origin), or Vero (green-monkey kidney origin). There were no differences in the appearance of CPE on culture cells, resistance to heating at 37 or 56°C, or antigenicity after virus inactivation between a strain obtained from passage in chicken kidney cells and the initial strain. The former was distinguished from the latter, since it was well propagated in chicken kidney cells, delayed in multiplying in embryonated hen's eggs, and weakly pathogenic for chickens and chick embryos.
The Nerima strain is infectious bronchitis virus attenuated by passage in chicken kidney cells. It was proved to be high in safety, since it produced no reaction to 3-day-old chicks showing an antibody titer of 0.5≥or 30-to 70-day-old chicks with no antibody. The amount of virus required for immunization was 104.0 TCID50≤per capita. Neutralizing antibody began to increase 12 days after administration and reached a maximum 18 days. It was maintainedat a high level for 172 days or more. Contact infection by cohabitation was established up to 10 days after administration. Attenuated virus was recovered from the trachea, lung, and an emulsion of liver, spleen, and kidney, although it was small in amount. Attenuated virus was examined for recurrence of pathogenicity. The virus was successfully subjected to five passages in chickens. The resultant virus showed no enhancement in pathogenicity for chicks. Therefore, this virus is assumed to be available as strain for the preparation of live virus vaccine.