Cell culture studies with the IMC-Hz-1 nonoccluded virus

Studies were conducted on an adventitious agent (Hz-1V) isolated from the IMC-Hz-1 cell line. It appeared identical to the virus first obtained by Granados et al (R. R. Granados, T. Nguyen, and B. Cato, 1978, Intervirology, 10, 309-317) from a persistent infection of this cell line. Restriction endonuclease digestion of Hz-1V DNA indicated the agent was different from the S nuclear polyhedrosis virus (HzSNPV) of the host species, Heliothis zea, from which the IMC-Hz-1 cell line was derived by Hink and Ignoffo (W. F. Hink and C. M. Ignoffo, 1970, Exp. Cell Res.60, 307-309). Hz-1V caused extensive cytopathic effects (CPE) in cultures of TN-368 cells and was moderately infectious to several other insect cell lines. The lytic nature of the infection permitted development of a plaque assay employing TN-368 cells. Following acute infections of TN-368 cells, persistently infected cultures were readily established by the surviving cells. Attempts to induce persistent infections of clones 10 and 13 of TN-368 were, however, unsuccessful. Infectious Hz-1V was shed continuously into the medium throughout subculturings of persistently infected cells. The infectious titer dropped sharply during initial subculturings, but subsequently reached a basal level at about the 25th passage. These cultures were resistant to superinfection by homologous Hz-1V, but remained as susceptible as normal TN-368 cells to heterologous Autographa californica M NPV (AcMNPV).