Abortive infection with human cytomegalovirus induces an alteration of growth pattern: morphological changes with cytocidal effect in rabbit kidney epithelial cells. Brief report

Human cytomegalovirus gene expression is silenced by Daxx-mediated intrinsic immune defense in model latent infections established in vitro

In addition to productive lytic infections, herpesviruses such as human cytomegalovirus (HCMV) establish a reservoir of latently infected cells that permit lifelong colonization of the host. When latency is established, the viral immediate-early (IE) genes that initiate the lytic replication cycle are not expressed. HCMV IE gene expression at the start of a lytic infection is facilitated by the viral pp71 protein, which is delivered to cells by infectious viral particles. pp71 neutralizes the Daxx-mediated cellular intrinsic immune defense that silences IE gene expression by generating a repressive chromatin structure on the viral major IE promoter (MIEP). In naturally latently infected cells and in cells latently infected in vitro, the MIEP also adopts a similar silenced chromatin structure. Here we analyze the role of Daxx in quiescent HCMV infections in vitro that mimic some, but not all, of the characteristics of natural latency. We show that in these "latent-like" infections, the Daxx-mediated defense that represses viral gene expression is not disabled because pp71 and Daxx localize to different cellular compartments. We demonstrate that Daxx is required to establish quiescent HCMV infections in vitro because in cells that would normally foster the establishment of these latent-like infections, the loss of Daxx causes the lytic replication cycle to be initiated. Importantly, the lytic cycle is inefficiently completed, which results in an abortive infection. Our work demonstrates that, in certain cell types, HCMV must silence its own gene expression to establish quiescence and prevent abortive infection and that the virus usurps a Daxx-mediated cellular intrinsic immune defense mechanism to do so. This identifies Daxx as one of the likely multiple viral and cellular determinants in the pathway of HCMV quiescence in vitro, and perhaps in natural latent infections as well.

Differential detection of viral DNA and RNA in situ in cells infected with human cytomegalovirus

We have analyzed the ability to use in situ cytohybridization to distinguish between human cytomegalovirus (HCMV) DNA and RNA in human cells infected in vitro. Two different viral-specific probes were used, one for an abundantly expressed late gene, and one which includes at least two genes coding for immediate early (IE) proteins. In productively infected cells, hybridization of the late gene probe extended over both the nucleus and cytoplasm and was RNase sensitive, whereas hybridization of the IE probe was restricted to the nucleus and was DNase-sensitive. In nonproductively infected cells hybridization of the IE probe was localized to the cytoplasm and was RNase-sensitive. The specific nuclease sensitivities indicate that a cytoplasmic hybridization pattern correlates with detection of viral RNA sequences, whereas a nuclear pattern represents detection of viral DNA. These results demonstrate that in situ cytohybridization can potentially be used to determine the extent of HCMV infection in a particular tissue or cell type by distinguishing between transcription and replication of specific viral genes.

Rabbit kidney cells abortively infected with human cytomegalovirus are arrested in mitotic phase

In rabbit kidney epithelial cells (RK13) abortively infected with human cytomegalovirus (HCMV), DNA synthesis at 1 or 2 days post-infection was enhanced 4 to 5 fold, compared to mock-infected cells. DNA analysis by isopycnic centrifugation revealed that the DNA newly synthesized in the virus infected RK13 cells was of cellular origin. HCMV infection also caused a marked increase in the mitotic activity of RK13 cells. When semi-confluent RK13 cells were infected more than 20 per cent of cells demonstrated mitosis at 72 hours post-infection although the rate of cell growth was considerably reduced compared to that of uninfected cells. The most frequent chromosomal change observed was fragmentation although other aberrations, gap, break, deletion etc. occurred also. Two immediate-early viral polypeptides with apparent molecular weights 72,000 (72K) and 76,000 (76K) daltons were produced in both RK13 cells and human embryonic lung cells (HEL) by 3 hours post-infection. Synthesis of the 76K polypeptide was greater than that of the 72K polypeptide in non-permissive RK13 cells whereas the reverse occurred in permissive HEL cells. Furthermore, of three early polypeptides which were expressed in productively infected HEL cells two, 88K and 80K, were not detected in abortively infected RK13 cells. These results suggest that the arrest in mitosis of the abortively infected RK13 cells and the subsequent chromosomal changes are associated with the altered expression of immediate-early or early virus functions in these cells.