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J Virol. 1970 July; 6(1): 42-48
Copyright © 1970 American Society for Microbiology. All Rights Reserved.

Cells Persistently Infected with Newcastle Disease Virus

II. Ribonucleic Acid and Protein Synthesis in Cells Infected with Mutants Isolated from Persistently Infected L Cells

Department of Microbiology, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15213

ABSTRACT

A comparison of the replication patterns in L cells and in chick embryo (CE) cell cultures was carried out with the Herts strain of Newcastle disease virus (NDV_o) and with a mutant (NDV_pi) isolated from persistently infected L cells. A significant amount of virus progeny, 11 plaque-forming units (PFU)/cell, was synthesized in L cells infected with NDV_o, but the infectivity remained cell-associated and disappeared without being detectable in the medium. In contrast, in L cells infected with NDV_pi, progeny virus (30 PFU/cell) was released efficiently upon maturation. It is suggested that the term "covert" rather than "abortive" be used to describe the infection of L cells with NDV_o. In both L and CE cells, the latent period of NDV_pi was 2 to 4 hr longer than for NDV_o. The delay in synthesis of viral ribonucleic acid (RNA) in the case of NDV_pi coincided with the delay in the inhibition of host RNA and protein synthesis. Although both NDV_o and NDV_pi produced more progeny and more severe cell damage in CE cells than in L cells, the shut-off of host functions was significantly less efficient in CE cells than in L cells. Paradoxically, no detectable interferon was produced in CE cells by either of the viruses, whereas in L cells most of the interferon appeared in the medium after more than 90% of host protein synthesis was inhibited. These results suggest that the absence of induction of interferon synthesis in CE cells infected with NDV is not related to the general shut-off of host cell synthetic mechanisms but rather to the failure of some more specific event to occur. In spite of the fact that NDV_pi RNA synthesis commenced 2 to 4 hr later than that of NDV_o, interferon was first detected in the medium 8 hr after infection with both viruses. This finding suggests that there is no relation between viral RNA synthesis and the induction of interferon synthesis.