Simultaneous identification of viral proteins and nucleic acids in cells infected with Aleutian mink disease parvovirus

Replication of Aleutian mink disease parvovirus in vivo is influenced by residues in the VP2 protein

Aleutian mink disease parvovirus (ADV) is the etiological agent of Aleutian disease of mink. Several ADV isolates have been identified which vary in the severity of the disease they elicit. The isolate ADV-Utah replicates to high levels in mink, causing severe Aleutian disease that results in death within 6 to 8 weeks, but does not replicate in Crandell feline kidney (CrFK) cells. In contrast, ADV-G replicates in CrFK cells but does not replicate in mink. The ability of the virus to replicate in vivo is determined by virally encoded determinants contained within a defined region of the VP2 gene (M. E. Bloom, J. M. Fox, B. D. Berry, K. L. Oie, and J. B. Wolfinbarger. Virology 251:288-296, 1998). Within this region, ADV-G and ADV-Utah differ at only five amino acid residues. To determine which of these five amino acid residues comprise the in vivo replication determinant, site-directed mutagenesis was performed to individually convert the amino acid residues of ADV-G to those of ADV-Utah. A virus in which the ADV-G VP2 residue at 534, histidine (H), was converted to an aspartic acid (D) of ADV-Utah replicated in CrFK cells as efficiently as ADV-G. H534D also replicated in mink, causing transient viremia at 30 days postinfection and a strong antibody response. Animals infected with this virus developed diffuse hepatocellular microvesicular steatosis, an abnormal accumulation of intracellular fat, but did not develop classical Aleutian disease. Thus, the substitution of an aspartic acid at residue 534 for a histidine allowed replication of ADV-G in mink, but the ability to replicate was not sufficient to cause classical Aleutian disease.

Identification of a cell surface protein from Crandell feline kidney cells that specifically binds Aleutian mink disease parvovirus

Aleutian mink disease parvovirus (ADV) is the etiological agent of Aleutian disease of mink. The acute disease caused by ADV consists of permissive infection of alveolar type II cells that results in interstitial pneumonitis. The permissive infection is experimentally modeled in vitro by infecting Crandell feline kidney (CrFK) cells with a tissue culture-adapted isolate of ADV, ADV-G. ADV-G VP2 empty virions expressed in a recombinant baculovirus system were analyzed for the ability to bind to the surface of CrFK cells. Radiolabeled VP2 virions bound CrFK cells specifically, while they did not bind either Mus dunni or Spodoptera frugiperda cells, cells which are resistant to ADV infection. The binding to CrFK cells was competitively inhibited by VP2 virions but not by virions of cowpea chlorotic mottle virus (CCMV), another unenveloped virus similar in size to ADV. Furthermore, preincubation of CrFK cells with the VP2 virions blocked infection by ADV-G. The VP2 virions were used in a virus overlay protein binding assay to identify a single protein of approximately 67 kDa, named ABP (for ADV binding protein), that demonstrates specific binding of VP2 virions. Exogenously added VP2 virions were able to competitively inhibit the binding of labeled VP2 virions to ABP, while CCMV virions had no effect. Polyclonal antibodies raised against ABP reacted with ABP on the outer surface of CrFK cells and blocked infection of CrFK cells by ADV-G. In addition, VP2 virion attachment to CrFK cells was blocked when the VP2 virions were preincubated with partially purified ABP. Taken together, these results indicate that ABP is a cellular receptor for ADV.

A comparison between permissive and restricted infections with Aleutian mink disease parvovirus (ADV): characterization of the viral protein composition at nuclear sites of virus replication

We used three-color fluorescent labeling and confocal microscopy to compare the permissive and the antibody-mediated, restricted replication of Aleutian mink disease parvovirus (ADV). In both permissive (CRFK cells) and restricted (K562 cells) situations, both ADV non-structural proteins (NS1 and NS2) concentrated at focal sites in the nucleus, which also contained viral DNA. Bromodeoxyuridine labeling demonstrated that these sites also supported active ADV single-strand DNA synthesis, indicating that they were replication compartments. ADV capsid proteins were located in intranuclear shells surrounding the replication compartments. At later time points, NS2 was readily detected in the cytoplasm of permissively infected CRFK cells, whereas the cytoplasmic presence of NS2 was much less pronounced in the K562 cells. These results showed that both permissive and restricted ADV replication are associated with a tight nuclear subcompartmentalization of viral products. Furthermore, differences between the permissive and restricted virus-cell interactions were noted, suggesting that there may be a morphological basis for examining the outcome of ADV infection.

Subcellular localization of Aleutian mink disease parvovirus proteins and DNA during permissive infection of Crandell feline kidney cells

Confocal microscopy allowed us to localize viral nonstructural (NS) and capsid (VP) proteins and DNA simultaneously in cells permissively infected with Aleutian mink disease parvovirus (ADV). Early after infection, NS proteins colocalized with viral DNA to form intranuclear inclusions, whereas VP proteins formed hollow intranuclear shells around the inclusions. Later, nuclei had irregular outlines and were virtually free of ADV products. In these cells, inclusions of viral DNA with or without associated NS protein were embedded in cytoplasmic VP protein. These findings implied that ADV replication within an infected cell is regulated spatially as well as temporally.

Characterization of mouse parvovirus infection by in situ hybridization

Infection of young adult BALB/cByJ mice with mouse parvovirus-1, a newly recognized, lymphocytotropic, nonpathogenic parvovirus, was examined by in situ hybridization. Virus appeared to enter through the small intestine and was disseminated to the liver and lymphoid tissues. Strand-specific probes detected virion DNA in a consistently larger number of cells than replicative forms of viral DNA and/or viral mRNA. The number of signal-positive cells in the intestinal mucosa, lymph nodes, spleen, and thymus increased through day 10 after oral inoculation but decreased after seroconversion. Positive cells were still detected, however, in peripheral lymphoid tissues of mice examined at 9 weeks postinoculation. The results underscore the need to assess potential effects of persistent mouse parvovirus-1 infection on immune function in mice.

Maintenance of high virus load even after seroconversion in newborn cats acutely infected with feline immunodeficiency virus

The viral loads in adult and newborn cats have been compared following injection with feline CD4+ FeL-039 line cells acutely infected with feline immunodeficiency virus (FIV). The level of virus genome in peripheral blood mononuclear cells (PBMC) increased progressively despite seroconversion in the newborn cats, whereas the virus genome was apparently cleared after seroconversion in the adult cats. Immunohistochemical staining of thymus of the FIV-infected newborn cats showed clusters of viral antigen-positive cells. These results indicate that FIV infection of the newborn cat results in higher virus loads than infection of the adult cat. We discuss these findings in relation to FIV as a model system for studies of the infection of neonates with an immunosuppressive retrovirus.

Characterization of chimeric full-length molecular clones of Aleutian mink disease parvovirus (ADV): identification of a determinant governing replication of ADV in cell culture

The ADV-G strain of Aleutian mink disease parvovirus (ADV) is nonpathogenic for mink but replicates permissively in cell culture, whereas the ADV-Utah 1 strain is highly pathogenic for mink but replicates poorly in cell culture. In order to relate these phenotypic differences to primary genomic features, we constructed a series of chimeric plasmids between a full-length replication-competent molecular clone of ADV-G and subgenomic clones of ADV-Utah 1 representing map units (MU) 15 to 88. After transfection of the plasmids into cell culture and serial passage of cell lysates, we determined that substitution of several segments of the ADV-Utah 1 genome (MU 15 to 54 and 65 to 73) within an infectious ADV-G plasmid did not impair the ability of these constructs to yield infectious virus in vitro. Like ADV-G, the viruses derived from these replication-competent clones caused neither detectable viremia 10 days after inoculation nor any evidence of Aleutian disease in adult mink. On the other hand, other chimeric plasmids were incapable of yielding infectious virus and were therefore replication defective in vitro. The MU 54 to 65 EcoRI-EcoRV fragment of ADV-Utah 1 was the minimal segment capable of rendering ADV-G replication defective. Substitution of the ADV-G EcoRI-EcoRV fragment into a replication-defective clone restored replication competence, indicating that this 0.53-kb portion of the genome, wholly located within shared coding sequences for the capsid proteins VP1 and VP2, contained a determinant that governs replication in cell culture. When cultures of cells were studied 5 days after transfection with replication-defective clones, rescue of dimeric replicative form DNA and single-stranded progeny DNA could not be demonstrated. This defect could not be complemented by cotransfection with a replication-competent construction.

Comparison of promoter activity in Aleutian mink disease parvovirus, minute virus of mice, and canine parvovirus: possible role of weak promoters in the pathogenesis of Aleutian mink disease parvovirus infection

Aleutian mink disease parvovirus (ADV) infection causes both acute and chronic disease in mink, and we have previously shown that it is the level of viral gene expression that determines the disease pattern. To study the gene regulation of ADV, we have cloned the P3 ADV and P36 ADV promoters in front of a reporter gene, the chloramphenicol acetyltransferase (CAT) gene, and analyzed these constructs by transient transfection in a feline kidney cell line and mouse NIH 3T3 cells. The genes for ADV structural proteins (VP1 and VP2) and the nonstructural proteins (NS-1, NS-2, and NS-3) were cloned into a eukaryotic expression vector, and their functions in regulation of the P3 ADV and P36 ADV promoters were examined in cotransfection experiments. The ADV NS-1 protein was able to transactivate the P36 ADV promoter and, to a lesser degree, the P3 ADV promoter. Constitutive activities of the P3 ADV and P36 ADV promoters were weaker than those of the corresponding promoters from the prototypic parvovirus minute virus of mice (MVM) and canine parvovirus (CPV). Also, the level of transactivation of the P36 ADV promoter was much lower than those of the corresponding P38 MVM and P38 CPV promoters transactivated with MVM NS-1. Moreover, the ADV NS-1 gene product could transactivate the P38 MVM promoter to higher levels than it could transactivate the P36 ADV promoter, while the P36 ADV promoter could be transactivated by MVM NS-1 and ADV NS-1 to similar levels. Taken together, these data indicated that cis-acting sequences in the P36 ADV promoter play a major role in determining the low level of transactivation observed. The P3 ADV and P4 MVM promoters could be transactivated to some degree by their respective NS-1 gene products. However, in contrast to the situation for the late promoters, switching NS-1 proteins between the two viruses was not possible. This finding may indicate a different mechanism of transactivation of the early promoters (P3 ADV and P4 MVM) compared with the late (P36 ADV and P38 MVM) promoters. In summary, the constitutive levels of expression from the ADV promoters are weaker than the levels from the corresponding promoters of MVM and CPV. Moreover, the level of NS-1-mediated transactivation of the late ADV promoter is impaired compared with the level of transactivation of the late promoters of MVM and CPV.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of acute rat parvovirus infection by in situ hybridization

In situ hybridization and virus titration were used to characterize early stages of rat virus (RV) infection of rat pups after oronasal inoculation. Results suggest that virus enters through the lung and that early viremia leads rapidly to pantropic infection. Cells derived from all three germ layers were infected with RV, but those of endodermal and mesodermal origin were the predominant targets. Infection of vascular endothelium was widespread and was associated with hemorrhage and infarction in the brain. Convalescence from acute infection was accompanied by mononuclear cell infiltrates at sites containing RV DNA. Viral DNA was also detected in endothelium, fibroblasts and smooth muscle myofibers four weeks after inoculation. Further examination of these cells as potential sites of persistent infection is warranted.

Computer assisted signal co-localization for simultaneous detection of antigen by immunohistochemistry and DNA by non-isotopic in situ hybridization

A method has been developed to co-localize signals for antigen and DNA using a desktop microcomputer system (computer assisted signal co-localization). Antigens were detected by standard immunohistochemical methods and DNA was detected by non-isotopic in situ hybridization (NISH). Using this method, NISH signals can be precisely located in cells with well-preserved morphology captured by computer. The removal of the first immunohistochemical reaction products and reagents eliminates possible interference with hybridization and non-specific binding to the probe; therefore the sensitivity of the original NISH method remains. The captured NISH signals can be converted to any other colour which contrasts with the immunostaining. We have used detection of Epstein-Barr virus (EBV) and keratins as a model system. This method is straightforward, and with necessary modifications, will be applicable to any type of combined immunohistochemistry and in situ hybridization technique for simultaneous detection of antigen and nucleic acids or two types of nucleic acids in the same cells.

Production of monoclonal antibodies reactive with a denatured form of the Friend murine leukemia virus gp70 envelope protein: use in a focal infectivity assay, immunohistochemical studies, electron microscopy and western blotting

Four monoclonal antibodies were selected for their ability to recognize the envelope protein of Friend murine leukemia virus (F-MuLV) in methanol-fixed tissue culture cells. Each of these monoclonal antibodies was found to react only with F-MuLV. By using recombinant retroviruses, it was determined that each of the monoclonal antibodies recognized the C-terminal one-third of the F-MuLV gp70 envelope protein. The monoclonal antibodies were effective in radioimmunoprecipitation of F-MuLV proteins, and one of the antibodies, 720, was also effective in Western blotting. The ability of antibody 720 to react with F-MuLV in methanol-fixed cells facilitated the use of a sensitive immunoperoxidase method with a focal virus infectivity assay. In immunohistochemical studies using light microscopy, antibody 720 could specifically label F-MuLV-infected cells in acetone-fixed tissue sections from F-MuLV-infected animals. Finally, in immuno-gold labelling studies using electron microscopy, antibody 720 could be used to distinguish F-MuLV from amphotropic MuLV.

Murine retrovirus-induced spongiform encephalopathy: productive infection of microglia and cerebellar neurons in accelerated CNS disease

We have examined the pathological lesions and sites of infection in mice inoculated with a highly neurovirulent recombinant wild mouse ecotropic retrovirus (FrCasE). The spongiform lesions appeared initially as swollen postsynaptic neuronal processes, progressing to swelling in neuronal cell bodies, all in the absence of detectable gliosis. Infection of neurons in regions of vacuolation was not detected. However, high level infection of cerebellar granule neurons was observed in the absence of cytopathology, wherein viral protein was found associated with both axons and dendrites. Infection of ramified and amoeboid microglial cells was associated with cytopathology in the brain stem, and endothelial cell-pericyte infection was found throughout the CNS. No evidence of defective retroviral expression was observed. These results are consistent with an indirect mechanism of retrovirus-induced neuropathology.