Growth of Newcastle disease virus (NDV) in L cells, where the virus undergoes limited replication, has been compared to that in fully permissive BHK-21 host cells. The synthesis of viral proteins and the production of infectious progeny were found to occur normally at early times of infection in L cells. However, the subsequent amplification of viral protein synthesis did not take place and instead of infectious virions, non-infectious haemagglutinin was the predominant product. This shift of replication pattern from complete to incomplete virus production seemed to be temporally related to the appearance and accumulation of interferon (IFN) in the system. The addition of specific antiserum against mouse IFN to the infected L cell cultures was able to circumvent such restriction of virus growth. In the presence of the antiserum, synthesis of viral proteins was found to progress normally, with the production of a high amount of infectious progeny comparable to that of the permissive system. These results suggest that the limited replication of NDV in L cells may be due to interference by the endogenously produced IFN during the course of infection.
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