MORPHOLOGY OF CYTOMEGALOVIRUS (SALIVARY GLAND VIRUS)

Human Cytomegalovirus gH/gL/gO Promotes the Fusion Step of Entry into All Cell Types, whereas gH/gL/UL128-131 Broadens Virus Tropism through a Distinct Mechanism

Interaction between gH/gL and the fusion protein gB is likely a conserved feature of the entry mechanism for all herpesviruses. Human cytomegalovirus (HCMV) gH/gL can be bound by gO or by the set of proteins UL128, UL130, and UL131, forming gH/gL/gO and gH/gL/UL128-131. The mechanisms by which these complexes facilitate entry are poorly understood. Mutants lacking UL128-131 replicate well on fibroblasts but fail to enter epithelial/endothelial cells, and this has led to the general assumption that gH/gL/UL128-131 promotes gB-mediated fusion on epithelial/endothelial cells whereas gH/gL/gO provides this function on fibroblasts. This was challenged by observations that gO-null mutants were defective on all of these cell types, suggesting that entry into epithelial/endothelial cells requires both of the gH/gL complexes, but the severe replication defect of the gO mutants precluded detailed analysis. We previously reported that the ratio of gH/gL/gO and gH/gL/UL128-131 in the virion envelope varied dramatically among HCMV strains. Here, we show that strains not only differ in the ratio, but also vary in the total amount of gH/gL in the virion. Cell-type-specific particle-to-PFU ratios of HCMV strains that contained different amounts of gH/gL/gO and gH/gL/UL128-131 were determined. Infection of both fibroblasts and epithelial cells was generally correlated with the abundance of gH/gL/gO, but not with that of gH/gL/UL128-131. The low infectivity of virions rich in gH/gL/UL128-131 but low in gH/gL/gO could be overcome by treatment with the chemical fusogen polyethylene glycol (PEG), strongly arguing that gH/gL/gO provides the conserved herpesvirus gH/gL entry function of promoting gB-mediated fusion for entry into all cell types, whereas gH/gL/UL128-131 acts through a distinct mechanism to allow infection of select cell types.

IMPORTANCE

The functions of HCMV gH/gL complexes in entry are unclear. Unlike the well-studied Epstein-Barr virus (EBV), where gH/gL and gH/gL/gp42 complexes both seem capable of promoting gB fusion during entry into different cell types, our studies here suggest that for HCMV, gH/gL/gO promotes gB fusion on all cell types, whereas gH/gL/UL128-131 broadens virus tropism through a distinct, as yet unknown mechanism. To our knowledge, this is the first suggestion of a herpesvirus gH/gL that does not act by promoting gB fusion, which might make HCMV a useful model to study the fundamental mechanisms by which herpesvirus gH/gL regulates gB fusion. Moreover, gH/gL/UL128-131 is a candidate vaccine target. Our findings help to explain the cell-type-dependent virus neutralization exhibited by anti-gH/gL/UL128-131 antibodies and underscore the importance of gH/gL/gO as another important part of vaccine or therapeutic strategies.

Immune-based therapies for cytomegalovirus infection

Immune fitness is critical in the pathogenesis and outcome of cytomegalovirus (CMV) infection. CMV disease is seen almost exclusively among individuals with an immature or defective immune system, such as patients with AIDS, transplant recipients and the developing fetus. These observations have generated interest in immune-based strategies for the management of CMV disease. Among the immune-based therapies that have been investigated in experimental and clinical settings are: passive immunotherapy with immunoglobulin; CMV vaccination; adoptive CMV-specific T-cell immunotherapy; and immune reconstitution strategies (HAART in AIDS patients, and a reduction in pharmacologic immunosuppression among transplant recipients). However, except for immune reconstitution strategies, there is no widely accepted immune-based strategy that is proven to be highly effective for CMV disease management. The benefits of immunoglobulins remain debated in an era when antiviral therapy is widely available. CMV vaccination and adoptive immunotherapy, on the other hand, remain experimental, but have had encouraging preliminary results.

Early diagnosis of cytomegalovirus infection in renal transplant and dialysis patients by DNA-DNA hybridisation assay

One hundred forty-eight urine specimens were collected from 47 renal transplant and dialysis patients and screened for the detection of cytomegalovirus (CMV). Diagnosis of CMV infection was suggested in 17 out of 47 patients (36.2%) by more than one of the five methods used. DNA hybridisation assay (DNA HA) using 32P-labelled probe detected CMV DNA in 15 (31.9%) of 47 patients, whereas virus isolation on conventional tube cell cultures (CTC), immunofluorescence incorporating monoclonal antibodies on centrifugation vial cultures (IF), complement fixation test (CFT), and electron microscopy (EM) yielded positive results in only nine (19.2%), 12 (25.2%), 11 (23.4%), and one (2.1%) of 47 patients, respectively. The significance of these results obtained by DNA HA lies not only in the apparent increase in number of patients diagnosed, but also in both early and rapid detection of CMV DNA. More importantly, the DNA HA is highly specific in that it correlates accurately with clinical and laboratory data characteristic of CMV disease. In respect of clinically manifest CMV disease, the specificity of DNA HA, CTC, IF, CFT, and EM was 87.5, 43.7, 56.3, 43.7, and 6.3%, respectively. These advantages of DNA HA make it the test of choice for early diagnosis of CMV infections in immunosuppressed patients.

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.

Rapid detection and quantitation of human cytomegalovirus in urine through DNA hybridization

We detected cytomegalovirus DNA in clinical urine specimens after immobilization on nitrocellulose filters and hybridization with a radioactively labeled, cloned fragment of cytomegalovirus DNA. We accomplished the specific detection and quantitation of viral DNA within 24 hours with 39 urine specimens from nine patients with cytomegalovirus viruria, mostly at a tissue-culture infective titer of 10(3) per milliliter or higher. None of 57 urine specimens from 21 patients that were culture-negative for cytomegalovirus gave false-positive results. Analysis of specimens from patients with cytomegalovirus viruria showed a correlation of the infective titer with the intensity of DNA hybridization (r = 0.77). Hybridization of sequential urine specimens from a patient undergoing treatment with interferon for cytomegalovirus retinitis revealed quantitative variations in hybridizable viral DNA over a period that correlated with clinical findings. This assay can be useful in the selection of patients for antiviral therapy and for the assessment of its efficacy.

Ovine pulmonary cytomegalovirus

Cytomegalic cells with intranuclear inclusions were identified in lung tissue from one of two weanling Western lambs with pneumonia and Mycoplasma ovipneumoniae infection. On the basis of light and electron microscopy, cytomegalovirus infection was diagnosed. Arrays of viral particles whose morphology was consistent with that of viruses of the herpes group were in the intranuclear inclusions of cytomegalic cells.

Syncytium formation caused by human cytomegalovirus in human embryonic lung fibroblasts

Large syncytia were regularly produced after prolonged infection in vitro of human embryonic fibroblasts by AD169 and other strains of human CMV. The syncytia showed typical CMV intranuclear inclusion bodies and herpesvirus particles in the nuclei and cytoplasm. It is proposed that syncytium formation follows abortive infection of the fibroblasts with defective virus.

Cytomegalovirus in the submandibular and sublingual glands of the southern grasshopper mouse (Onychomys torridus torridus)

The infection observed within the submandibular and sublingual glands of the grasshopper mouse, Onychomys torridus torridus, is a cytoinegalovirus.

Herpes simplex virus and human cytomegalovirus replication in WI-38 cells. I. Sequence of viral replication

A comparison, under standardized conditions, of herpes simplex virus (HSV) and human cytomegalovirus (CMV) revealed differences in viral morphology, in the timing of their infectious cycles, and in several morphological events during those cycles. Structural distinctions between the two viruses included the coating of unenveloped cytoplasmic CMV capsids, but not those of HSV, and a variation in the structure of their cores. Since the two cycles were carried out in the same host cell strain under conditions of one-step growth (input multiplicity = 10 PFU/cell), it was possible to construct time scales locating the major events of each cycle. Comparison of the two showed that HSV replicated and released progeny within 8 h postinfection, whereas CMV required 4 days. These results correlated well with those of concurrent plaque assays. Within the longer CMV cycle, most of the major events appeared retarded to a similar degree, and no obvious limiting step in particle production could be identified. Distinctions between the two cycles included the following: condensation of the chromatin in HSV- but not CMV-infected cells; the greater tendency of HSV to produce membrane alterations; and the appearance of cytoplasmic dense bodies in CMV- but not HSV-infected cells. Identification of these differences even under identical conditions of culture and infection strongly implies that they result from intrinsic differences in the nature of the viruses, and are not caused by variations in experimental conditions.

Cytomegalovirus replication in cells pretreated with 5-iodo-2'-deoxyuridine

The replication of human cytomegalovirus (CMV) in cells pretreated with 5-iodo-2'-deoxyuridine (IUdR) was studied. Pretreatment of cells with IUdR enhanced several parameters of virus replication. Virus grown in drug-treated cells exhibited a shorter eclipse period and the cells produced more infectious virus sooner than did untreated cells. There was an approximate fivefold increase in virus yield per cell in the drug-treated samples when compared to control cultures. The time required for plaque development was shortened by 6 days in drug-treated cultures. Pretreatment of cells with IUdR also increased plaquing efficiency of the virus by approximately 10-fold. The enhancement of virus replication by IUdR was further demonstrated by varying the multiplicity of infection. In a 7-day period there was a 100-fold increase in sensitivity of the cultures for virus detection when the cells had been previously exposed to IUdR. The data presented indicate the possibility that IUdR interferes with the production of a cellular product inhibitory for CMV replication.

Virus-like particles in the spiral ganglion of the guinea pig cochlea

Double-walled spherical particles 100 millimicrons in diameter were accidentally discovered in the cytoplasm of spiral ganglion neurons of apparently healthy guinea pigs with normal startle responses. These particles in some ways resemble viruses of the herpes group and may represent a latent form of neuronal infection.

Enhancement of infectivity of murine cytomegalovirus in vitro by centrifugal inoculation

Centrifugation of murine cytomegalovirus inocula from a variety of sources onto secondary mouse embryo cell monolayers at 1,900 x g for 30 min regularly revealed 10- to 100-fold more infectious virus than could be found in the same materials using standard inoculation methods. Virus demonstrable only by centrifugation was present throughout the entire growth cycle in a constant proportion to virus measured without centrifugation. Extracellular growth curves of both populations revealed an 18- to 21-hr latent period, followed by a long-linear increase over the next 12 hr; final yield was 30 plaque-forming units (PFU) per cell. Centrifugation of cells prior to inoculation or after standard adsorption and removal of inoculum failed to result in any significant change in measured virus titer. However, even after 4-hr adsorption, the supernatant inoculum could be transferred and centrifuged onto a fresh monolayer resulting in the same increment of measurable virus. Neutralizing antibody and interferon were equally efficacious against 100 PFU of virus as defined by either method. Thus, this newly identified population of cytomegalo-virus represents the vast majority of potentially infectious units and appears to differ solely in ease of adsorption onto cell monolayers.

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.

Fine structure of a virus in cultured lymphoblasts from Burkitt lymphoma

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HUMAN CYTOMEGALOVIRUS INFECTION. ELECTRON MICROSCOPIC AND HISTOCHEMICAL CHANGES IN CULTURES OF HUMAN FIBROBLASTS

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