HUMAN CYTOMEGALOVIRUS. MORPHOLOGY BY NEGATIVE STAINING

MCMV Centrifugal Enhancement: A New Spin on an Old Topic

Human cytomegalovirus (HCMV) is a ubiquitous pathogen infecting a majority of people worldwide, with diseases ranging from mild to life-threatening. Its clinical relevance in immunocompromised people and congenital infections have made treatment and vaccine development a top priority. Because of cytomegaloviruses' species specificity, murine cytomegalovirus (MCMV) models have historically informed and advanced translational CMV therapies. Using the phenomenon of centrifugal enhancement, we explored differences between MCMVs derived in vitro and in vivo. We found centrifugal enhancement on tissue culture-derived virus (TCV) was ~3× greater compared with salivary gland derived virus (SGV). Using novel "flow virometry", we found that TCV contained a distinct submicron particle composition compared to SGV. Using an inhibitor of exosome production, we show these submicron particles are not extracellular vesicles that contribute to centrifugal enhancement. We examined how these differences in submicron particles potentially contribute to differing centrifugal enhancement phenotypes, as well as broader in vivo vs. in vitro MCMV differences.

Current and emerging antivirals for the treatment of cytomegalovirus (CMV) retinitis: an update on recent patents

Cytomegalovirus (CMV) retinitis is the most common ocular opportunistic complication and a serious cause of vision loss in immunocompromised patients. Even though, a rise in human immunodeficiency virus (HIV) infected individuals seems to be a major factor responsible for the prevalence of CMV retinitis, the introduction of highly active antiretroviral therapy (HAART) significantly reduced the incidence and severity of CMV retinitis. Thorough evaluation of the patient's immune status and an exact classification of the retinal lesions may provide better understanding of the disease etiology, which would be necessary for optimizing the treatment conditions. Current drugs such as ganciclovir, valganciclovir, cidofovir and foscarnet have been highly active against CMV, but prolonged therapy with these approved drugs is associated with dose-limiting toxicities thus limiting their utility. Moreover development of drug-resistant mutants has been observed particularly in patients with acquired immunodeficiency syndrome (AIDS). Continuous efforts by researchers in the industry and academia have led to the development of newer candidates with enhanced antiviral efficacy and apparently minimal side effects. These novel compounds can suppress viral replication and prevent reactivation in the target population. Though some of the novel therapeutics possess potent viral inhibitory activity, these compounds are still in stages of clinical development and yet to be approved. This review provides an overview of disease etiology, existing anti-CMV drugs, advances in emerging therapeutics in clinical development and related recent patents for the treatment of CMV retinitis.

Microvesicles and viral infection

Cells secrete various membrane-enclosed microvesicles from their cell surface (shedding microvesicles) and from internal, endosome-derived membranes (exosomes). Intriguingly, these vesicles have many characteristics in common with enveloped viruses, including biophysical properties, biogenesis, and uptake by cells. Recent discoveries describing the microvesicle-mediated intercellular transfer of functional cellular proteins, RNAs, and mRNAs have revealed additional similarities between viruses and cellular microvesicles. Apparent differences include the complexity of viral entry, temporally regulated viral expression, and self-replication proceeding to infection of new cells. Interestingly, many virally infected cells secrete microvesicles that differ in content from their virion counterparts but may contain various viral proteins and RNAs. For the most part, these particles have not been analyzed for their content or functions during viral infection. However, early studies of microvesicles (L-particles) secreted from herpes simplex virus-infected cells provided the first evidence of microvesicle-mediated intercellular communication. In the case of Epstein-Barr virus, recent evidence suggests that this tumorigenic herpesvirus also utilizes exosomes as a mechanism of cell-to-cell communication through the transfer of signaling competent proteins and functional microRNAs to uninfected cells. This review focuses on aspects of the biology of microvesicles with an emphasis on their potential contributions to viral infection and pathogenesis.

Viral Infections of the Lung

The lungs are among the most vulnerable to microbial assault of all organs in the body. From a contemporary vantage, lower respiratory tract infections are the greatest cause of infection-related mortality in the United States, and rank seventh among all causes of deaths in the United States.2,3 From a global and historic perspective, the scope and scale of lower respiratory tract infection is greater than any other infectious syndrome, and viral pneumonias have proven to be some of the most lethal and dramatic of human diseases. The 1918–1919 influenza pandemic, perhaps the most devastating infectious disease pandemic in recorded history, resulted in an estimated 40 million deaths worldwide, including 700,000 deaths in the U.S.4 The global outbreak of severe acute respiratory syndrome (SARS) during 2003, although considerably smaller in scale, resulted in 8098 cases and 774 deaths5 and is a dramatic contemporary example of the ability of viral pneumonias to rapidly disseminate and cause severe disease in human populations.

Cytomegalovirus replication in cultured human retinal pigment epithelial cells

Retinal pigment epithelium (RPE) was isolated from the globe of a donor positive for human immunodeficiency virus (HIV) who had cytomegalovirus (CMV) retinitis secondary to acquired immunodeficiency syndrome (AIDS). In culture, the cells exhibited normal epithelioid morphology by phase contrast microscopy. After two weeks the cells developed cytomegaly and dense intranuclear and cytoplasmic inclusions and, eventually, died. Transmission electron microscopy (EM) demonstrated intranuclear nonenveloped virus particles 80-120 nm in diameter consistent with a herpes type infection. Immunofluorescence staining demonstrated the presence of CMV antigens. Conditioned medium from the infected cells caused infection in RPE cells isolated from normal donors. Hybridization assay demonstrated the presence of CMV DNA and indicated that the time course of the infection was similar, but not identical to infection in MRC-5 and HEL cells. We conclude that cultured human RPE is a permissive host for CMV.

Virus-like particles in human vestibular ganglion cells

We found intracytoplasmic aggregates of virus-like particles in human vestibular ganglion cells. These particles were always observed in the peripheral area of the cytoplasm. Morphological characteristics of the cytoplasm are similar to those of other ganglion cells. The inclusion bodies are round and measure about 1.7 micron in maximum diameter. They consist of a porous convoluted dense material and virus-like particles. The overall diameter of 118 randomly selected particles varies from 36 nm to 73 nm, and the mean value is 53 nm. Most of the particles are spherical while a few possess a hexagonal or semicircular profile. The particles exhibit a double external membrane or vesiculated external layer. Most of the particles are empty. There are, however, some particles which show vesicular structures in their content. Although our present data are insufficient to determine these particles as viral, their hexagonal shape and size are similar to true virus. With these data in mind, we suggest that these particles might be a dormant form of virus and may possibly produce infectious disease in the inner ear.

Isolation and characterization of a noninfectious virion-like particle released from cells infected with human strains of cytomegalovirus

Three types of virus particles have been recovered from the culture medium of human foreskin fibroblasts infected with human strains of cytomegalovirus (HCMV). Two of these, virions and dense bodies, are routinely observed and have been described by others. The third, produced in lesser amounts, has not been previously characterized. This particle, separable from virions by rate-velocity sedimentation, is morphologically distinguished from them only by core structure. Radiolabeling and biological assays have established that these particles, like dense bodies, lack DNA and are not infectious. Based on these properties, we have designated this virion-like structure as a noninfectious enveloped particle (NIEP). Comparisons of the protein constituents of these three particles has shown that dense bodies have the simplest composition. Approximately 95% of their protein mass is represented by a 69,000 Da (69K) matrix-like protein. While dense bodies appear to have a normal complement of virion glycoproteins, they completely lack other predominant virion species. The protein compositions of virions and NIEPs are more complex than that of dense bodies, and are distinguished from one another by the presence in NIEPs of a 35,000 Da (35K) protein absent from the two other particles. Biosynthetic radiolabeling and cell fractionation experiments have demonstrated that this 35K protein is produced only in infected cells, is phosphorylated and partitions with the nuclear fraction. These and other results suggest that this protein is the HCMV counterpart of the previously described B-capsid proteins VP22a of herpes simplex and 37K of CMV (strain Colburn). NIEPs are produced by all HCMV strains examined and have not been observed in preparations of herpes simplex virus- or Old World monkey CMV-infected cells. Although this particle is generally present in much lower amounts than virions, strain AD169 overproduces NIEPs by approximately 10-fold. We have also found that the additional NIEP protein of AD169 has an apparently larger size (i.e., 36K) than the corresponding protein of other strains. The correlation between AD169 NIEP overproduction and its altered protein suggests that the two may be causally related.

The immunoperoxidase technique for rapid human cytomegalovirus identification

Sixteen cytomegalovirus (CMV) isolates from both ill and healthy patients were identified by the immunoperoxidase technique (IP). CMV detection was accomplished by direct examination of the primary isolate using either direct (DIP) or indirect immunoperoxidase (IIP) techniques. In thirteen of the isolates, confirmation of identification was achieved by indirect immunofluorescence (IFA) and by demonstration of herpes particles by electron microscopy (EM). Further, in four cases of non-CMV alterations in the tissue culture which might be confused with actual infection, the IP test was negative as were the confirmatory tests. The IIP is preferred over the DIP test since the latter shows a certain amount of background stain of uninfected cells. Tissue culture cells showing focal CMV cytopathic effect contained both nuclear and cytoplasmic inclusions stained by IP technique. Nonspecific staining was associated with cytoplasmic inclusion bodies. The IP technique can detect individual cell CMV infection at 24 hours when EM reveals only unenveloped viral particles. It is sensitive, specific, and allows direct identification of infected cells in the primary isolate in as little as 90 minutes. Furthermore, it can be performed in standard isolation tissue culture tubes, whereas IFA requires the transfer of the infected cells onto slides or the routine use of Leighton tubes.

Human cytomegalovirus. II. Lack of relatedness to DNA of herpes simples I and II, Epstein-Barr virus, and nonhuman strains of cytomegalovirus

The purified DNA of human cytomegalovirus, radiolabeled in vitro, was examined for homology to Epstein-Barr virus, herpes simplex type I and II, and simian and murine cytomegalovirus DNA by DNA-DNA reassociation kinetics analyses with the S1 enzyme differential digestion technique. Cross-matching of the DNAs showed no relatedness or less than 5% detectable homology.

Toxoplasma gondii and cytomegalovirus: mixed infection by a parasite and a virus

Human fibroblasts infected in vitro with cytomegalovirus are relatively resistant to infection by Toxoplasma gondii during the first 4 days of virus infection. After 5 days, however, the cytomegalovirus-infected cells become susceptible to the parasites. The toxoplasmas replicate in paracentral rosettes surrounded by host cell mitochondria. This growth configuration differs from that seen in human fibroblasts infected in vitro with toxoplasma only but resembles the pattern seen in doubly infected cells found in human necropsy tissue.

Correlation between infectivity and physical virus particles in human cytomegalovirus

Benyesh-Melnick, Matilda (Baylor University College of Medicine, Houston, Tex.), Fern Probstmeyer, Robert McCombs, Jean P. Brunschwig, and Vladimir Vonka. Correlation between infectivity and physical virus particles in human cyto-megalovirus. J. Bacteriol. 92:1555-1561. 1966.-Infectivity titers [measured as plaque-forming units (PFU)] and particle counts by the sedimentation pseudo-replication technique were determined for crude, unpurified, intracellular preparations of two different strains of human cytomegalovirus. Unlike the high particle-infectivity ratio of 10(6) to 10(8) previously reported for these viruses, the number of total particles per PFU ranged from 160 to 490 with strain AD-169 and from 176 to 1,050 for strain C-87. Interpretation of particle-PFU ratios of intracellular cytomegalovirus in terms of particle morphology is not conclusive at this time. The number of enveloped forms found varied between 0 and 34% of the total particles counted. However, the true proportion is probably greater, because envelopes were found to be destroyed by the enzyme treatment used in preparing the specimens for examination in the electron microscope. The number of full particles found ranged between 4 and 31% of the total particles counted. The particle per PFU ratio of extracellular virus was found to be three- to fivefold lower than that of intracellular virus.