Inhibitory role of the host apoptogenic gene pkr in

Inhibitory role of the host apoptogenic gene PKR in the establishment of persistent infection by encephalomyocarditis virus in U937 cells
Michael C. Yeung, Danny L. Chang, Randell E. Camantigue, and Allan S. Lau*
The Moses Grossman Infectious Diseases Laboratory, San Francisco General Hospital, and Department of Pediatrics, University of California, San Francisco, CA 94110 Communicated by I.Robert Lehman, Stanford University School of Medicine, Stanford, CA, July 21, 1999 (received for review January 20, 1999)

Persistent infections by viruses such as HIV-1 and hepatitis B virus can pose long-term health hazards. Because establishment of persistent infections involves close interactions and adjustments in both host and virus, it would be informative to establish a paradigm with whicha normally cytolytic viral infection can be easily converted to persistent infection, so that the different stages in developing persistent infection can be examined. Such a model system is described in this paper. Highly cytolytic encephalomyocarditis virus (EMCV) infection was shifted to persistent infection as a result of repressed expression of the double-stranded RNAdependent protein kinase(PKR) in the promonocytic U937 cells. Because of the apoptogenic potential of PKR, a deficiency of PKR resulted in a delay in virus-induced apoptosis in EMCV-infected U937 cells, allowing the eventual establishment of persistent EMCV infection in these cells (U9K-AV2). That this was a bona fide persistent infection was demonstrated by the ability of infected cells to propagate as long-termvirus-shedding cultures; electron microscopy studies showing presence of intracellular EMCV virions and chromatin condensation; detection of virus-induced chromosomal DNA fragmentation and sustained expression of apoptogenic p53 and IL-1 converting enzyme; and demonstration of active EMCV transcription by reverse transcription–PCR. In addition, a host-virus coevolution was observed in U9K-AV2 cultures overtime: U9K-AV2 cells exhibited slower growth rates, resistance to viral super-infection, and cessation of IFN- synthesis, whereas the infectivity of EMCV was drastically attenuated. Finally, data are presented on the suitability of this model to study establishment of persistent infection by other viruses such as Sendai virus and reovirus.

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uring acute viral infections, the effectivenesswith which the host’s immune response can clear viruses dictates that the invading virus must infect new hosts with matching efficiency. Some viruses, such as poliovirus and rotavirus, can survive without a host for some time. Alternatively, viruses may find it advantageous to establish persistent infection with periodic transmission to new hosts. For the host, persistent viral infections can bepotentially devastating. Classic examples include clinical progression to AIDS and hepatocarcinoma as a result of chronic infection by HIV-1 and hepatitis B virus, respectively. Thus, it is important to study at the cellular level the molecular mechanisms of how viruses evade the host’s antiviral defenses and are allowed to establish persistent infections in susceptible host cells. Apoptosis, orprogrammed cell death, is a self-destruct process whereby unwanted individual cells in metazoan animals undergo a genetically determined program of new protein synthesis and morphological changes, culminating in chromosomal DNA fragmentation and eventual cell death (1). A cell can have more than one apoptotic pathway in its arsenal. Depending on the death signal and the cell type, a cell turns on anappropriate apoptotic pathway with associated death genes, which can either be unique or shared by different apoptotic programs (2). One wellcharacterized apoptotic pathway important for viral clearance is induced by tumor necrosis factor (TNF- ). Apparently, there
PNAS October 12, 1999 vol. 96 no. 21

is a cross-talk between the TNF- and the IFN signaling pathways: a factor with a pivotal...