Histopathologic and ultrastructural studies of disseminated cytomegalovirus infection in strain 2 guinea pigs

Inbred strain 2 guinea pigs developed severe disseminated disease during acute experimental guinea pig cytomegalovirus (GPCMV) infection. A high mortality rate (100%) resulted, with most animals dying between 10 and 14 days after high dose (7.5 X 10(5) TCID50) virus inoculation. Infectious virus was recovered from many tissues, including spleen, lungs, liver, pancreas, heart, adrenals, kidneys, and salivary glands. The rate of GPCMV isolation from these tissues ranged from 50 to 100%. Gross lesions were observed in the spleen, liver, and lungs. On histologic examination, lesions were also seen in many other organs, including heart, pancreas, kidneys, adrenals, brain, intestines, and salivary glands. Intranuclear viral inclusions were present in many cell types of various organs. Under electron microscopic examination, cells with viral inclusions were easily found in the spleen, and liver, but less readily in the lungs, kidneys, salivary glands, and other organs. Most of the intranuclear inclusions consisted of electron-dense fibrils (10 nm diameter), viral nucleocapsids (100 nm), and tubular structures (60 nm diameter). Dense bodies and enveloped dense virions containing single or multiple capsids were present in the cytoplasm of many infected cells. The morphologic developments of GPCMV in these visceral tissues of strain 2 guinea pigs resembled those seen in GPCMV-infected cultured guinea pig cells but differed from those observed in the infected salivary gland duct cells. Strain 2 guinea pigs are a useful animal model for studying disseminated infection in CMV-associated human diseases.     

Comparison of guinea pig cytomegalovirus and guinea pig herpes-like virus: growth characteristics and antigentic relationship.

The growth characteristics of guinea pig cytomegalovirus (GPCMV) and guinea pig herpes-like virus (GPHLV) in cell cultures were compared. Guinea pig fibroblast cells were highly susceptible to infection with both viruses, whereas guinea pig kidney cells were sensitive only to GPHLV. No cytopathic effect was observed in the latter cell system after infection with GPCMV,nor was there an increase in virus titer, although the cirus persisted in the kidney cells for 2 to 3 weeks postinfection. Electron microscope studies showed nonvirion tubular structures in GPCMV -infected fibroblast cells, but not in GPHLV- infected cells. Large packages of enveloped nuclear virus particles were commonly seen in GPHLV -infected cells, especially kidney epithelial cells, but none were found in the GPCMV -infected fibroblasts. Complete enveloped extracellular virus particles were present in both virus-cell systems. Both viruses showed narrow host spectra and replicated well only in guinea pig cells although GPHLV multiplied to some degree in rabbit cells. No antigenic relationship could be demonstrated between the two viruses using antisera specific for each virus that was produced in rabbits and guinea pigs. Rabbits produced high neutralizing antibody titers to GPHLV, whereas guinea pigs were the animals of choice for GPCMV antiserum production.     

Ultrastructural development and persistence of guinea pig cytomegalovirus in duct cells of guinea pig submaxillary gland

Summary Salivary glands from Hartley guinea pigs were experimentally infected with guinea pig cytomegalovirus (GPCMV) and examined by light and electron microscopy at different time intervals. Characteristic intranuclear and intracytoplasmic viral inclusions were observed in duct cells of infected animals. Viral inclusion counts and infectivity titers in the salivary gland reached maximum levels by 3 to 4 weeks after infection; infectivity persisted, though at reduced levels, for at least 30 weeks. Electron microscopic examination of viral inclusions revealed several developmental events including nucleocapsid assembly, envelopment of nucleocapsids at the inner nuclear membrane and their enclosure by a thin vacuolar membrane. While contained within cytoplasmic vacuoles, enveloped virions acquired surface spikes. Cytoplasmic vacuoles containing virions subsequently coalesced and discharged mature virions at the cell surface into the lumen of the salivary gland duct. The data indicate that the ultrastructural development of GPCMV in the guinea pig salivary gland shows many similarities to that of human cytomegalovirus in humans. The salivary gland may provide a primary locus for virus shedding and horizontal transmission of cytomegalovirus.With 7 Figures     

Ultrastructural development of guinea pig cytomegalovirus in cultured guinea pig embryo cells.

The ultrastructural development of guinea pig cytomegalovirus (GPCMV) in guinea pig embryo cells was studied using electron microscopy. Tubular structures were found in nuclei of virus infected cells, followed by the appearance of intranuclear inclusions containing virus nucleocapsids. While some nucleocapsids were enveloped at the inner nuclear membrane, others were released into the cytoplasm where they were associated with, or within, dense matrix which was subsequently enveloped by cytoplasmic membranes to form enveloped dense virions. Dense bodies without virus capsids were formed in the cytoplasm and enveloped in a similar manner. An involvement of the nuclear pores in the release of unenveloped virus capsids from the nucleus to the cytoplasm was postulated. Evidence that the enveloped dense virions and dense bodies shared common envelope antigen(s) was obtained by immunoelectron microscopy. The similarities and differences in the ultrastructural development of GPCMV and other cytomegaloviruses are discussed.     

Quantitative-competitive PCR monitoring of viral load following experimental guinea pig cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common cause of congenital viral infection in the developed world, and can lead to significant morbidity. Animal models of HCMV infection are required for study of pathogenesis, because of the strict species-specificity of cytomegalovirus (CMV). Among the small animal CMV models, the guinea pig CMV (GPCMV) has unique advantages, in particular its propensity to cross the placenta, causing disease in utero. In order to develop quantitative endpoints for vaccine and antiviral therapeutic studies in the GPCMV model, a quantitative-competitive PCR (qcPCR) assay was developed, based on the GPCMV homolog of the HCMV UL83 gene, GP83. Optimal amplification of GPCMV DNA was observed using primers spanning a 248 base pair (bp) region of this gene. A 91 bp deletion of this cloned fragment was generated for use as an internal standard (IS) for PCR amplification. Standard curves based upon the fluorescent intensity of full-length external target to IS were compared with signal intensity of DNA extracted from blood and organs of experimentally infected guinea pigs in order to quantify viral load. Viral load in newborn guinea pigs infected transplacentally was determined and compared with that of pups infected with GPCMV as neonates. Viral loads were highest in pups infected as neonates. The most consistent isolation and highest quantities of viral DNA were observed in liver and spleen, although viral genome could be readily identified in brain, lung, and salivary gland. Viral load determination should be useful for monitoring outcomes following vaccine studies, as well as responses to experimental antiviral agents.     

Evidence of diffusion artefacts in diaminobenzidine immunocytochemistry revealed during immune electron microscope studies of the early interactions between influenza virus and cells

Anti-haemagglutinin-labelled antibodies have been used to search for influenza entry into cells by fusion of viral and plasma membranes. The plasma membranes of infected cells were stained by immunoperoxidase but not by immunoferritin reagents. It is suggested that the staining obtained with the peroxidase conjugate was due to diffusion of the diaminobenzidine reaction product away from the enzymic site. Immunoferritin labelling provided no evidence for entry of influenza by fusion of viral and plasma membranes under conditions of physiological pH.     

Comparison of guinea pig cytomegalovirus and guinea pig herpes-like virus: pathogenesis and persistence in experimentally infected animals.

The pathogenesis of guinea pig cytomegalovirus (GPCMV) and guinea pig herpes-like virus (GPHLV) in guinea pigs was compared. Animals were inoculated with the two viruses by different routes and sacrificed after varying periods of time. GPCMV was consistently isolated from salivary gland 2 weeks postinoculation and thereafter following intraperitoneal or subcutaneous incoulaton. Virus was less frequently found in other tissues including blood, spleen, and kidney. Intranuclear inclusions were seen in tissue sections of salivary gland after inoculation with GPCMV- infected tissue suspension, but were only rarely found after inoculation with tissue culture virus. In GPHLV-infected guinea pigs, consistent latent infection of leukocytes and other tissues was detected by cocultivation techniques. Intranuclear inclusions were not found in the spleen, salivary gland, or other infected tissues after GPHLV infection with either tissue culture virus or infected tissue suspension. Guinea pigs inoculated with GPCMV produced high titers of specific neutralizing antibody to the homologous virus; those inoculated with GPHLV developed long-term viremia accompanied by minimal neutralizing antibody levels to the virus.     

Detection of guinea pig cytomegalovirus nucleic acids in cultured cells with biotin-labelled hybridization probes

Biotin labelled hybridization probes prepared from recombinant plasmids containing segments of the guinea pig cytomegalovirus (GPCMV) genome were used to detect GPCMV nucleic acids in guinea pig cells by in situ hybridization. The time course of GPCMV infection was assessed in two cultured cell types, guinea pig embryo (GPE) cells and 104C1 cells, a transformed and cloned guinea pig cell line. Detection of GPCMV nucleic acids was accomplished in both cell types with individual GPCMV DNA fragments and with mixtures of GPCMV DNA fragments. When compared to other established methods of GPCMV detection, the method of in situ hybridization enabled the detection of a higher percentage of positive cells early during the course of the infection. In addition, differences in the replication cycle of GPCMV in the two cultured cell lines could be demonstrated. These findings will facilitate future studies of GPCMV tissue tropism in vivo.     

Identification of essential and non-essential genes of the guinea pig cytomegalovirus (GPCMV) genome via transposome mutagenesis of an infectious BAC clone

We report application of a transposition methodology that allows the easy characterization and mutation of genes encoded on an infectious bacterial artificial chromosome (BAC) clone. We characterized mutants generated by transposome (Tn) mutagenesis of a BAC clone of guinea pig cytomegalovirus (GPCMV). A pool of Tn mutant GPCMV BACs were screened initially by restriction profile analysis to verify they were full-length, and subsequently GPCMV BAC DNA from individual mutants was transfected onto guinea pig lung fibroblast cells in order to generate virus. Tn GPCMV BAC mutants were classed as either essential or non-essential gene insertions, depending upon their ability to regenerate viable, replication-competent virus. Representative mutants were more fully characterized. Analysis by sequencing the Tn insertion site on the mutated BACs, and by regeneration of virus using transfection of guinea pig fibroblasts (GPL), demonstrated that a recombinant with a Tn insertion in the UL35 homolog gene (GP35) was a non-essential gene for viral replication in tissue culture. A mutant with an insertion in the UL46 homolog (GP46) was nonviable, a phenotype which could be rescued by homologous recombination of BAC DNA with wild-type UL46 sequences, suggesting an essential role of this putative capsid gene in virus replication.     

Immunofluorescence of simian adenovirus 7 (SA7)--specific antigens in infected LLC-MK 2 cells and SA7 transformed cells

Antigens synthesized in LLC-MK₂ cells infected with SA7 were described using indirect immunofluorescence. When cells were reacted with sera from tumor-bearing hamsters, “T” antigens were observed as early as 10 hr post-infection (PI). In certain cells a second type of exclusively intranuclear fluorescence was observed at 16 hr PI. Specific antiviral sera reacted with viral structural antigens approximately 15–16 hr PI. Sera from hamsters immunized with density gradient-purified virus reacted with both “T” antigens and viral antigens.     

Ultrastructural localization of viral antigens in the CNS of mice persistently infected with lymphocytic choriomeningitis virus (LCMV).

Lymphocytic choriomeningitis virus (LCMV) produces a persistent infection of the nervous system in susceptible mice. To map the localization of viral antigens in the central nervous system (CNS), the authors studied, by means of ultrastructural immune peroxidase techniques, 4-6-month-old mice persistently infected with LCMV following an intracerebral inoculation at birth. The greatest number of infected neurons was observed in the cortex, particularly of the limbic system, and certain nuclei of the hypothalamus. In the cerebellum, Purkinje cells selectively expressed viral antigens. Moderate numbers of infected neurons were found in the anterior horns of the spinal cord, basal ganglia, and thalamus. The immunoperoxidase technique using monoclonal antibodies showed that persistently infected neurons primarily expressed the nucleocapsid protein antigens of LCMV. Glycopeptide antigens were minimally expressed. Electron-microscopic examination of selected individual infected neurons showed viral antigens exclusively associated with ribosomes. No staining was seen on cell surfaces. Glutaraldehyde-fixed CNS tissue studied by electron microscopy did not reveal morphologic abnormalities or mature viral particles. This study demonstrates that LCMV persistently infects specific neuronal populations. Infected neurons express viral antigens in association with host ribosomes and show no significant morphologic alterations.     

Serodiagnosis of tsutsugamushi fever (scrub typhus) by the indirect immunoperoxidase technique.

The indirect immunoperoxidase technique was assessed for the serodiagnosis of tsutsugamushi fever (scrub typhus). The antigens were peritoneal smears prepared from mice infected intraperitoneally with the Karp, Kato, and Gilliam strains of Rickettsia tsutsugamushi. Treatment of the mice with cyclophosphamide apparently increased the number of the rickettsiae, and it minimized the exudate that interfered with the specific staining. The rickettsiae were seen as clusters in the juxtanuclear region of the mesothelial cells and also as free particles outside of the cells. By the indirect immunoperoxidase technique, the sera from all of the patients (49 samples from 30 patients) were positive for the R. tsutsugamushi antibody. The antibody titers (immunoglobulin G [IgG] and IgM) determined by the indirect immunoperoxidase technique correlated with those determined by the indirect immunofluorescence technique. Thus, the indirect immunoperoxidase technique was useful for quantifying both IgG and IgM antibodies to the rickettsia.     

Sperm autoantigens and fertilization. III. Ultrastructural localization of guinea pig autoantigens

Guinea pig sperm autoantigens have been localized by direct and indirect immunoferritin techniques in (1) plasma membrane over the entire sperm head, (2) acrosomal contents, (3) fibrous sheath and outer dense fibers of the tail filament, and (4) the inner acrosomal membrane of 50% of acrosome reacted spermatozoa. These cellular structures are known to be involved in guinea pig sperm rouleaux formation, acrosome reaction, interaction of acrosome-reacted sperm with zona pellucida and with the vitellus of guinea pig ova. Since IgG and Fab of autoantiserum to guinea pig spermatozoa have been shown to interfere with these cellular reactions, this study provides further evidence, albeit indirect, that sperm autoantigens are involved in these cellular events.     

Cellular localization of cytomegalovirus nucleic acids in guinea pig salivary glands by in situ hybridization

In situ hybridization with guinea pig cytomegalovirus (CMV) DNA probes was used to detect guinea pig CMV nucleic acid sequences in salivary glands during the course of infection. Optimum conditions for detection of guinea pig CMV gene sequences at the cellular level were determined. The technique of nucleic acid hybridization was compared to antigen detection and histologic examination for cellular localization of guinea pig CMV during acute and chronic infection. Tissue sections fixed for 1 h in 4% paraformaldehyde demonstrated darker specific staining and contained significantly larger numbers of cells positive for guinea pig CMV nucleic acids and antigens than sections fixed longer and in other fixatives. The method of in situ hybridization detected more guinea pig CMV infected cells than did routine histological evaluation. Histologically normal salivary gland duct cells as well as cells bearing typical inclusions were found to contain guinea pig CMV nucleic acids. Guinea pig CMV nucleic acids were also detected although less frequently in cells outside of the ducts. These results suggest that in situ hybridization allows for the detection of histologically inapparent guinea pig CMV infections at the cellular level.     

Molecular cloning of the guinea pig cytomegalovirus (GPCMV) genome as an infectious bacterial artificial chromosome (BAC) in Escherichia coli

Since cytomegalovirus (CMV) infection is highly species-specific, it is necessary to study animal cytomegaloviruses to assess viral factors which contribute to pathogenesis. The generation of recombinant viruses carrying reporter genes would provide useful tools for studying the genetics of CMV pathogenicity in vivo. We evaluated whether the guinea pig cytomegalovirus (GPCMV) was amenable to such manipulation. Metabolic selection using the guanosylphosphoribosityl transferase (gpt) gene facilitated recovery of a recombinant virus, vAM403, containing a gpt/green fluorescent protein (eGFP) cassette introduced into the HindIII "N" region of the viral genome. This virus had replication kinetics identical to wild-type virus. We next attempted to clone the GPCMV genome as a bacterial artificial chromosome (BAC). A BAC plasmid containing a gpt/eGFP cassette and the chloramphenicol resistance marker was introduced into HindIII "N" to generate another GPCMV recombinant, vAMBGPCMV. Circular viral DNA isolated from vAMBGPCMV-infected cells was used to transform Escherichia coli. Restriction profiles revealed that the GPCMV genome had been cloned as a BAC plasmid, and transfection of BAC plasmid DNA confirmed that the BAC clone was infectious. A novel strategy based on a unique PmeI site was devised to quickly modify the BAC GPCMV plasmid. Recombinants retained the capability to replicate and express reporter genes in guinea pigs, suggesting that these viruses will be useful for in vivo pathogenesis studies.     

Cloning and expression of the guinea pig cytomegalovirus glycoprotein B (gB) in a recombinant baculovirus: utility for vaccine studies for the prevention of experimental infection

The guinea pig cytomegalovirus (GPCMV) is unique among the cytomegaloviruses of small mammals, insofar as during pregnancy it crosses the placenta, causing infection of the fetus. Although the guinea pig model is well suited to vaccine studies, the lack of cloned, recombinant forms of immunogenic GPCMV proteins, such as envelope glycoproteins, has hindered experimental evaluations of subunit immunization for prevention of fetal disease. Since the glycoprotein B (gB) is a major target of neutralizing antibody responses, the GPCMV gB was cloned and expressed in a recombinant baculovirus. A recombinant was generated which expressed gB, truncated at codon 692, upstream of the putative transmembrane domain. Processing and expression of the recombinant protein, designated Bac-gB, was assessed, and the protein was characterized immunologically. Anti-gB antibodies were immunoreactive with Bac-gB by enzyme linked immunosorbent assay (ELISA) and immunoblot assay. Immunoprecipitation with polyclonal anti-GPCMV antisera identified protein species of 120, 80 and 30 kDa by reducing SDS-PAGE, suggesting that authentic cleavage and processing of Bac-gB occurred in insect cells. Sera from guinea pigs immunized with lectin-column purified native glycoproteins had high ELISA titers to Bac-gB. Recombinant GPCMV gB expressed in insect cells should prove useful in defining correlates of protective immunity in the GPCMV congenital infection model.     

The Langerhans cell

In all mammalian species so far examined, Langerhans cells or their precursors are the only epidermal cells expressing Ia antigens or their equivalents. In man, xeno-antisera raised in rabbits against purified B-lymphocyte cell membrane antigens were utilized to stain the Langerhans cells by either fluorescent or immunoferritin methods. As high proportion of the indeterminate cells in the epidermis also expressed HLA-DR antigens, and a relationship to Langerhans cells is suggested. Confirmation of these results was obtained in mouse. Alloantisera raised against I-A and I-EC subregion products again stained only Langerhans cells. Fluorescent, immunoperoxidase, and immunoferritin methods were used and confirmation of the specificity of the reaction was achieved at the electron microscope level. Langerhans cells were shown by ATPase staining to be absent from the epithelium of the central cornea, but present in the limbus. Population of the entire corneal epithelium surface was induced by application of irritants or contact sensitizing agents such as DNCB. Grafting of corneas either deficient or populated with Langerhans cells, to skin beds, may answer the question of the influence of such cells on allograft rejection.     

Detection of swine pox and buffalo pox viruses in cell culture using a protein A-horseradish peroxidase conjugate

Buffalo pox virus antigen was detected in Vero cells and swine pox virus antigen in the cytoplasm and nucleus of PK-15 and IB-RS-2 cells as early as 6 hr post infection (p.i.) by indirect immunoperoxidase technique using a Protein A-horseradish peroxidase (HRP) conjugate. The viral antigens localized in the cytoplasm of infected cells were the most prominent after 24 hr p.i.     

Immunoelectron microscopic study of the detection of the glycoproteins of influenza and Sendai viruses in infected cells by the immunoperoxidase method

The envelope glycoproteins of influenza virus (HA and NA) and paramyxovirus (HN and F) were visualized on the surface of infected cells by immunoelectron microscopy using the indirect immunoperoxidase technique. In X7 influenza virus-infected fibroblasts, the hemagglutinin (HA) and the neuraminidase (NA) were observed on the cell membrane respectively 2 and 3--4 h after infection. The antigens were initially seen as discrete patches and later evenly distributed along the plasma membrane prior to budding. Antibody induction of HA and NA was observed as cytoplasmic inclusions, with peroxidase-positive activity, attributed to endocytosis. The redistribution of HA and NA supports the hypothesis of lateral mobility of the viral glycoproteins in cellular membranes as visualized by the immunoperoxidase method. The glycoproteins of Sendai virus, in infected Madin--Darby bovine kidney cells, were found to be evenly distributed along the plasma membrane and endoplasmic reticulum, the latter by the indirect microperoxidase method. The immunoperoxidase methods may be useful for investigating the polarized distribution of envelope glycoproteins.