Temperature-sensitive mutants of herpesviruses

A modified screening system for loss-of-function and dominant negative alleles of essential MCMV genes

Inactivation of gene products by dominant negative mutants is a valuable tool to assign functions to yet uncharacterized proteins, to map protein-protein interactions or to dissect physiological pathways. Detailed functional and structural knowledge about the target protein would allow the construction of inhibitory mutants by targeted mutagenesis. Yet, such data are limited for the majority of viral proteins, so that the target gene needs to be subjected to random mutagenesis to identify suitable mutants. However, for cytomegaloviruses this requires a two-step screening approach, which is time-consuming and labor-intensive. Here, we report the establishment of a high-throughput suitable screening system for the identification of inhibitory alleles of essential genes of the murine cytomegalovirus (MCMV). In this screen, the site-specific recombination of a specifically modified MCMV genome was transferred from the bacterial background to permissive host cells, thereby combining the genetic engineering and the rescue test in one step. Using a reference set of characterized pM53 mutants it was shown that the novel system is applicable to identify non-complementing as well as inhibitory mutants in a high-throughput suitable setup. The new cis-complementation assay was also applied to a basic genetic characterization of pM99, which was identified as essential for MCMV growth. We believe that the here described novel genetic screening approach can be adapted for the genetic characterization of essential genes of any large DNA viruses.

A beta-herpesvirus with fluorescent capsids to study transport in living cells

Fluorescent tagging of viral particles by genetic means enables the study of virus dynamics in living cells. However, the study of beta-herpesvirus entry and morphogenesis by this method is currently limited. This is due to the lack of replication competent, capsid-tagged fluorescent viruses. Here, we report on viable recombinant MCMVs carrying ectopic insertions of the small capsid protein (SCP) fused to fluorescent proteins (FPs). The FPs were inserted into an internal position which allowed the production of viable, fluorescently labeled cytomegaloviruses, which replicated with wild type kinetics in cell culture. Fluorescent particles were readily detectable by several methods. Moreover, in a spread assay, labeled capsids accumulated around the nucleus of the newly infected cells without any detectable viral gene expression suggesting normal entry and particle trafficking. These recombinants were used to record particle dynamics by live-cell microscopy during MCMV egress with high spatial as well as temporal resolution. From the resulting tracks we obtained not only mean track velocities but also their mean square displacements and diffusion coefficients. With this key information, we were able to describe particle behavior at high detail and discriminate between particle tracks exhibiting directed movement and tracks in which particles exhibited free or anomalous diffusion.

Herpesvirus genetics has come of age

The genetic analysis of the large and complex herpesviruses has been a constant challenge to herpesvirologists. Elegant methods have been developed to produce mutants in infected cells that rely on the cellular recombination machinery. Bacterial artificial chromosomes (BACs), single copy F-factor-based plasmid vectors of intermediate insert capacity, have now enabled the cloning of complete herpesvirus genomes. Infectious virus genomes can be shuttled between Escherichia coli and eukaryotic cells. Herpesvirus BAC DNA engineering in E. coli by homologous recombination requires neither restriction sites nor cloning steps and allows the introduction of a wide variety of DNA modifications. Such E. coli-based technology has provided a safe, fast and effective approach to the systematic mining of the information stored in herpesvirus genomes as a result of their intimate co-evolution with their specific hosts for millions of years. Use of this technique could lead to new developments in clinical virology and basic virology research, and increase the usage of viral genomes as investigative tools and vectors.

Recent advances in herpesvirus genetics using bacterial artificial chromosomes

Although the members of the Herpesvirus family are responsible for a wide variety of human diseases, advances in the understanding of viral molecular mechanisms of pathogenesis have been hampered by the large size of herpesvirus genomes, rendering the viruses difficult to experimentally manipulate. Better techniques have been needed to facilitate mutagenesis of herpesvirus genomes, allowing for the assessment of the role of specific viral gene products in replication, immunity, and pathogenesis. Homologous recombination with plasmids containing genes of interest flanked by selectable markers has been a successful method for generating viral mutants, as has the generation of recombinant virus from transfection of cosmid clones. Although these efforts to generate recombinant viruses have met with modest success, the protocols have been cumbersome. More recently, a novel technique for the manipulation of herpesvirus genomes has been developed. This technology utilizes bacterial F plasmids, and allows for the stable cloning of herpesvirus genomes as bacterial artificial chromosomes (BACs) in Escherichia coli. Once cloned, such BACs are stable, and DNA purified from E. coli is infectious, fully capable of reproducing replication-competent virus. Manipulation of herpesvirus genomes is now feasible using the powerful techniques of bacterial genetics, and should facilitate a better understanding of the molecular pathogenesis of herpesvirus infections.

Forward with BACs: new tools for herpesvirus genomics

The large, complex genomes of herpesviruses document the high degree of adaptation of these viruses to their hosts. Not surprisingly, the methods developed over the past 30 years to analyse herpesvirus genomes have paralleled those used to investigate the genetics of eukaryotic cells. The recent use of bacterial artificial chromosome (BAC) technology in herpesvirus genetics has made their genomes accessible to the tools of bacterial genetics. This has opened up new avenues for reverse and forward genetics of this virus family in basic research, and also for vector and vaccine development.

Establishment of latency in vitro with herpes simplex virus temperature-sensitive mutants at nonpermissive temperature

This report describes a latency model using human embryo lung cells that were infected with herpes simplex virus type 1 (HSV-1) temperature-sensitive (ts) mutants and cultivated at nonpermissive temperature (40.5 degrees C). ts mutants tsG8 (parental strain HSV-1 KOS) and tsG5 (parental strain HSV-1 13) could be maintained in a latent state at 40.5 degrees C for at least 40 days without exhibiting virus infectivity. During this time, viable virus could be reactivated by reducing the incubation temperature to the permissive level (34 degrees C). Virus replication could be detected 2 to 6 days after temperature reduction and the virus reactivated from the latent state seemed to retain the same ts phenotype as the input virus for at least 14 days.

Characterization of a herpes simplex virus regulatory protein: aggregation and phosphorylation of a temperature-sensitive variant of ICP 4

The viral polypeptide ICP 4 (or Vmw 175) is synthesized during the immediate early phase of infection by herpes simplex virus (HSV) and is required during the viral reproductive cycle for efficient transcription of delayed early viral genes. Replication of mutant strains of HSV-1 such as tsLB 2 that encode a temperature-sensitive variant of ICP 4 does not proceed beyond the immediate early phase in cells that are infected and maintained at the nonpermissive temperature (NPT). Under these conditions, the immediate early viral polypeptides accumulate to levels that are 10 to 100 fold greater than normal. We have investigated the use of tsLB 2-infected cells maintained at the NPT as a source for substantial amounts of ICP 4 for further characterization. Extraction of ICP 4 from tsLB 2-infected cells requires 0.5 M NaCl and yields aggregates that contain ICP 4, ICP 6, ICP 27, and lesser amounts of other proteins. These large aggregates cannot be disrupted under nondenaturing conditions and thus are not a suitable source for native ICP 4. We have used this overproduced ICP 4 as an antigen to generate ICP 4-specific antibody and for characterization of the primary structure of ICP 4. Analysis of acid-hydrolysed 32P-labeled ICP 4 revealed that the major phosphorylated residues in ICP 4 are phosphoserine and phosphothreonine.

Biological characterization of a herpes simplex virus intertypic recombinant which is completely and specifically non-neurovirulent

In this study, a pre-existing "library" of Herpes Simplex Virus (HSV) intertypic recombinants was found to be not useful for mapping HSV genes controlling viral neurovirulence in mice. Most of these agents were significantly less virulent than either parental type following intracranial inoculation, and in the general case this lessened virulence could be attributed to inefficient replication in any cell type at 38.5 degrees (the normal temperature of the mouse). One agent tested (recombinant RE6) was completely non-neurovirulent following intracranial inoculation of as much as 3.2 X 10(7) PFU. Since about 10 PFU of either 17 Syn+ or HG52 (the "parental" strains of this recombinant) were lethal for mice, RE6 is at least 10 million-fold less neurovirulent than the wild-type strains from which it was produced. The function of the defective gene(s) in RE6 is not yet known, but it is not required for the expression of viral thymidine kinase, efficient replication in cultured cells at 38.5 degrees, or replication in non-neural mouse tissue in vivo. Therefore, the defect in RE6 is in an HSV gene function(s) which is absolutely required for neurovirulence but not for general viral replication. Several possibilities for the molecular nature of the defect in RE6 are discussed.

Functions of the major nonstructural DNA binding protein of a herpesvirus (pseudorabies)

Eight mutants of pseudorabies virus belonging to complementation group 3 and situated between 0.14 and 0.18 units on the physical map of the genome were analyzed. All the mutants tested in this respect (seven) recombined with one another, indicating that the mutations were located in different regions of the gene. All mutants were DNA-; the first round, as well as subsequent rounds, of DNA replication was completely blocked at the nonpermissive temperature in the mutant-infected cells. After shift-up from the permissive to the nonpermissive temperature, viral DNA synthesis continued for a short period of time only and viral DNA which had accumulated at the permissive temperature became degraded. Parental viral DNA, however, retained its integrity at the nonpermissive temperature and viral DNA synthesis started immediately after shift-down of the mutant-infected cells from the nonpermissive to the permissive temperature (even in the absence of protein synthesis). All mutants belonging to complementation group 3 tested (5 out of 8) produced a thermolabile nonstructural DNA binding protein (136K). In some of the mutant virus-infected cells this protein failed to migrate to the nucleus. We conclude that the pseudorabies virus mutants in complementation group 3 code for a defective 136K protein and that this protein is not only essential to the process of viral DNA synthesis but also plays a role in the stabilization of progeny DNA (but not of nonreplicating parental DNA) within the infected cells.

Genetic studies of cell fusion induced by herpes simplex virus type 1

Eight cell fusion-causing syn mutants were isolated from the KOS strain of herpes simplex virus type 1. Unlike the wild-type virus, the mutants produced plaques containing multinucleated cells, or syncytia. Fusion kinetics curves were established with a Coulter Counter assay for the mutants and wild-type virus in single infections of human embryonic lung (HEL) cells, for the mutants and wild-type virus in mixed infections (dominance test), and for pairs of mutants in mixed infections (complementation test). In single infections, fusion began 4 to 6 h after infection and proceeded with an exponential decrease in the number of small single cells. At some later time that was characteristic of the mutant, there was a significant reduction in the rate of fusion for all but possibly one of the mutants. Although the wild-type virus did not produce syncytial plaques, it did induce a small amount of fusion that stopped abruptly about 2 h after it started. These data are consistent with the hypothesis that both mutants and wild type induce an active fusion inducer and that the activity of this inducer is subsequently inhibited. The extent of fusion is apparently determined by the length of the interval during which the fusion inducer is active. That fusion is actively inhibited in wild-type infections is indicated by the observation that syn mutant-infected cells fused more readily with uninfected cells than with wild-type infected cells. Fusion was decreased in mixed infections with the mutants and wild-type virus, but the mutants displayed a codominant fusion phenotype. Fusion was not decreased in mixed infection with pairs of mutants, indicating that the mutants, with one possible exception, are members of the same complementation group. A linkage map was established for six of the mutants by analysis of recombination frequencies.

Ultrastructural characterization of an early, nonstructural polypeptide of herpes simplex virus type 1

An immunoperoxidase procedure was employed to study the expression of a large-molecular-weight, virus-induced polypeptide (VP175; molecular weight, 175,000) at the light and electron microscopic levels in Vero cells infected with herpes simplex virus type 1 or with tsB2, a DNA-negative, temperature-sensitive mutant of herpes simplex virus type 1. In cells infected with herpes simplex virus type 1 and in cells infected with tsB2 at the permissive temperature (34 degrees C), VP175 was found within the nucleus. The protein was detected as early as 2 h postinfection and, by 3 h postinfection, was generally distributed in a marginated pattern contiguous with, and extending from, the inner lamella of the nuclear membrane. At 6 h postinfection, protein accumulations were dispersed throughout the nucleus, and, by 9 h postinfection, these accumulations tended to be localized in a marginated pattern near the nuclear membrane. It was also noted that, at 9 h postinfection, under permissive conditions, VP175 was not found in association with nucleocapsids or enveloped particles. In contrast, in cells infected with tsB2 at the nonpermissive temperature (39 degrees C) and harvested at 6 or 9 h postinfection, accumulations of VP175 were identified not only within the nucleus, but also within the cytoplasm in the form of annular or globular aggregates. These aggregates consisted of a granular matrix and were not bound by membranes.

Biogenesis of poxviruses: mirror-image deletions in vaccinia virus DNA

Restriction endonuclease analysis of viral DNA extracted from wild-type and temperature-sensitive mutants of vaccinia IHD-W (Dales et al., 1978) revealed sequence alterations in approximately 20% of all ts clones examined. The rearrangements were due to deletions up to 250 nucleotide pairs long. Using Eco RI, Sal I, Bam I, Hpa I and Ava I, the deletions were always observed in the same fragments, while analysis with Hind III demonstrated deletions of identical size in the two terminal fragments. Since vaccinia virus contains inverted terminal repeats of more than 10 kb, these clones possess identical deletions of opposite orientation at both ends of the genome. Analysis of several revertants of the ts mutants demonstrated that the deletions probably arise as events independent from those producing ts lesions and are generated spontaneously at high frequency. This implies that a single event during replication caused the elimination of nonessential information, and suggests that circular intermediates must exist transiently during viral replication.

The role of the macrophages in Marek's disease: in vitro and in vivo studies

Macrophages form S- and K-strain Leghorn chickens, susceptible and restant to Marek's disease (MD) respectively, were studied to determine the macrophage contribution to the dynamics of MD infection, tumorigenesis and genetic resistance to this disease. In vitro studies demonstrated that macrophages from bothstrains were similar in their responses toward JM strain of Marek's disease virus (MDV) and JM-1 tumor cells. Macrophages were observed to phagocytize JM virus, but the interiorized virus was not seen to replicate within the macrophage or induce antigenic changes of the cell membrane. Clearance of JM-1 tumor cells was by both cytolytic and phagocytic mechanism. In vivo selective suppression of macrophage functions by antimacrophage serum or trypan blue inoculations resulted in significantly elevated viral titers and increased tumorigenesis, as compared to infected, non-suppressed or non-infected control groups. Results from this study indicate that genetic susceptibility or resistance to MD, as exhibited by S- and K-strain chickens, respectively, is not controlled at the macrophage level. The role of the macrophage in MD infection appears to be specifically surveillance.

Viral vaccines under development: a third generation

In summary then, my purpose has been two-fold: on the one hand, I have tried to highlight the kinds of basic science advances in both cellular and virologic research that can (and should) be focussed both on vaccines under development and, retrospectively, on those whose origins were strictly empiric. On the other hand, I have attempted a partial survey of some of the prominent members of a potential new generation of vaccines to point out areas where these advances can and should contribute either to progress or to a sense of caution about the further reliance on pure empiricism. It is clear that we are not finished with new viral vaccines. It is equally clear that narrowing the persistent gap between basic science and its application to public health needs will require much energy and attention as vaccine development progresses.