Aleutian disease virus, a parvovirus, is proteolytically degraded during in vivo infection in mink.

The polypeptides of the highly virulent mink-passaged Utah I and the nonvirulent cell culture-adapted ADV-G strain of Aleutian disease virus (ADV) were compared. When CRFK cells infected with either Utah I or ADV-G were analyzed by immunoprecipitation, both viruses induced proteins with molecular weights characteristic of the ADV-G 85,000 ( 85k )- and 75k-dalton structural proteins (p85 and p75) as well as the 71k -dalton nonvirion protein p71 . However, when Utah I, Pullman ADV, and DK ADV (a Danish isolate of ADV) were purified from infected mink, only polypeptides with molecular weights between 27k and 30k could be identified. In addition, trypsin treatment of ADV-G degraded p85 and p75 to smaller antigenic proteins with molecular weights of 24k and 27k, similar to those found for the virulent in vivo viruses. The effect of proteolytic treatment of ADV was then studied in detail. Purification of Utah I ADV from mink organs in the presence of protease inhibitor did not prevent the appearance of the low-molecular-weight proteins and ADV-G proteins were not degraded upon purification from a homogenate of normal mink organs, suggesting that artifactual proteolysis was not occurring. When a serum pool from terminally diseased mink was analyzed by radioimmunoassay for antibody reactivity against trypsinized and nontrypsinized ADV-G, five times higher reactivity was found for the trypsinized ADV-G than for the nontrypsinized ADV-G, an effect which could not be elicited by chymotrypsin or V8 protease treatment, implying that in vivo-produced ADV was being modulated in vivo by trypsin or a trypsin-like enzyme. Trypsinization was shown not to cause a change in ADV virion density, but to decrease the in vitro infectivity of ADV-G for CRFK cells. These studies suggested that during infection of mink ADV proteins are degraded to highly antigenic smaller polypeptides.     

Characterization of the Aleutian disease virus genome and its intracellular forms

Aleutian disease virus (ADV) of mink is a nondefective parvovirus with a single-stranded DNA genome. We characterized the viral DNA forms found in infected cells prepared by a modified Hirt extraction procedure. Double-stranded DNA molecules corresponding in size to 4.8-kilobase-pair duplex monomers and 9.6-kilobase-pair duplex dimers were identified in agarose gels by blot hybridization to 32P-labeled ADV DNA. A rapidly reannealing ADV duplex monomer was isolated on a preparative scale and physically mapped with a series of restriction endonucleases. The map derived was similar to one derived from double-stranded ADV DNA produced by self-primed synthesis on virion DNA, but differed from restriction endonuclease maps reported for other parvovirus DNAs. The purified duplex monomer could be labeled with [32P]dCTP by nick translation and used as a probe in blot hybridization to detect ADV sequences in DNA from small numbers of infected cells. Additional studies indicated that double-stranded ADV DNA could first be detected at 24 h after infection.     

Monoclonal antibodies against Aleutian disease virus distinguish virus strains and differentiate sites of virus replication from sites of viral antigen sequestration.

Monoclonal antibodies (mAbs) were used to study antigenic differences among strains of Aleutian disease virus (ADV) and to characterize viral proteins in vitro and in vivo. A number of ADV field strains could be discriminated, and highly virulent Utah I ADV was clearly delineated from the tissue culture-adapted avirulent ADV-G strain. This specificity could be demonstrated by indirect immunofluorescence against infected cultures of Crandell feline kidney cells or against tissues of Utah I ADV-infected mink. Viral antigens were demonstrated in both the nuclei and the cytoplasm of infected tissue culture cells. However, in mink mesenteric lymph node, spleen, and liver, viral antigen was observed only in the cytoplasm. Absence of nuclear fluorescence suggested that the detected antigen represented phagocytized viral antigens rather than replicating virus. This conclusion was supported by the finding that mAbs reactive only against low-molecular-weight polypeptides derived from intact viral proteins gave the same pattern of in vivo fluorescence as mAbs with broad reactivity for large or small (or both) viral polypeptides. The distribution of infected cells was the same as that described for macrophages in these tissues and suggested that cells of the reticuloendothelial system had sequestered viral antigens.     

Aleutian mink disease parvovirus infection of mink macrophages and human macrophage cell line U937: demonstration of antibody-dependent enhancement of infection.

Aleutian mink disease parvovirus (ADV) infects macrophages in adult mink. The virulent ADV-Utah I strain, but not the cell culture-adapted ADV-G strain, infects mink peritoneal macrophage cultures and the human macrophage cell line U937 in vitro. However, preincubation of ADV-G with ADV-infected mink serum enhanced its infectivity for U937 cells. the enhancing activity was present in the protein A-binding immunoglobulin G fraction in the serum, but F(ab')2 fragments failed to enhance the infection. On the other hand, the same sera inhibited ADV-G infection of Crandell feline kidney (CRFK) cells. Although U937 cells were not fully permissive for antibody-enhanced ADV-G infection, ADV mRNA expression, genome amplification, and protein expression were identical to those found previously for ADV-Utah I infection of U937 cells. Preincubation of ADV-Utah I with soluble protein A partly inhibited the infection of U937 cells but did not affect infection of CRFK cells. In mink peritoneal macrophages, preincubation with the infected mink serum did not make ADV-G infectious. However, the infectivity for mink macrophages of antibody-free ADV-Utah I prepared from the lungs of infected newborn mink kits was enhanced by ADV-infected mink serum. Moreover, protein A partly blocked ADV-Utah I infection of mink macrophage cultures. These results suggested that ADV-Utah I enters mink macrophages and U937 cells via an Fc receptor-mediated mechanism. This mechanism, antibody-dependent enhancement, may also contribute to ADV infection in vivo. Furthermore, since ADV infection in mink is characterized by overproduction of anti-ADV immunoglobulins, antibody-dependent enhancement may play a critical role in the establishment of persistent infection with ADV in vivo.     

Studies on the sequential development of acute interstitial pneumonia caused by Aleutian disease virus in mink kits.

We studied different parameters during the development of acute interstitial pneumonia in mink kits caused by neonatal infection with Aleutian disease virus (ADV). When histological lesions, presence of intranuclear inclusion bodies, and intranuclearly localized ADV antigen were correlated with levels of single-stranded virion and duplex replicative forms of ADV DNA in the different tissues, it was concluded that the lung, probably alveolar type II cells, is the major primary target for viral replication and cytopathology. The presence of the duplex dimeric replicative-form DNA, a strong marker of parvovirus replication, was also observed in low amount in the mesenteric lymph node, suggesting replication of ADV in this organ, although no viral cytopathology could be demonstrated. Moreover, a few intranuclear inclusion bodies were demonstrated in kidney and liver from affected kits, but intranuclearly localized ADV antigen could not be demonstrated in liver sections, and neither could duplex dimer replicative-form DNA, suggesting that these organs are nevertheless not a major site of ADV replication. When the data were compared with results previously reported for ADV-infected adult mink and ADV-infected permissive cell cultures, the data suggested that the pattern of ADV replication in alveolar type II cells is similar to that seen in infected cell cultures but that the replication in the other kit organs resembles the restricted pattern seen in adult mink.     

Aleutian mink disease parvovirus infection of mink peritoneal macrophages and human macrophage cell lines.

Aleutian mink disease parvovirus (ADV) mRNAs are found in macrophages in lymph nodes and peritoneal exudate cells from ADV-infected mink. Therefore, we developed an in vitro infection system for ADV by using primary cultures of mink macrophages or macrophage cell lines. In peritoneal macrophage cultures from adult mink, virulent ADV-Utah I strain showed nuclear expression of viral antigens with fluorescein isothiocyanate-labeled ADV-infected mink serum, but delineation of specific viral proteins could not be confirmed by immunoblot analysis. Amplification of ADV DNA and production of replicative-form DNA were observed in mink macrophages by Southern blot analysis; however, virus could not be serially propagated. The human macrophage cell line U937 exhibited clear nuclear expression of viral antigens after infection with ADV-Utah I but not with tissue culture-adapted ADV-G. In U937 cells, ADV-Utah I produced mRNA, replicative-form DNA, virion DNA, and structural and nonstructural proteins; however, virus could not be serially passaged nor could [3H]thymidine-labeled virions be observed by density gradient analysis. These findings indicated that ADV-Utah I infection in U937 cells was not fully permissive and that there is another restricted step between gene amplification and/or viral protein expression and production of infectious virions. Treatment with the macrophage activator phorbol 12-myristate 13-acetate after adsorption of virus reduced the frequency of ADV-positive U937 cells but clearly increased that of human macrophage line THP-1 cells. These results suggested that ADV replication may depend on conditions influenced by the differentiation state of macrophages. U937 cells may be useful as an in vitro model system for the analysis of the immune disorder caused by ADV infection of macrophages.     

Investigation of the pathogenesis of transplacental transmission of Aleutian mink disease parvovirus in experimentally infected mink.

The transplacental transmission of Aleutian mink disease parvovirus (ADV) was studied in experimental infection of 1-year-old female non-Aleutian mink. The ADV-seronegative female mink were inoculated with ADV prior to mating or after the expected implantation of the embryos during pregnancy. A group of uninfected females served as a control group. Animals from each group were killed prior to or shortly after parturition. The in situ hybridization technique with radiolabeled strand-specific RNA probes was used to determine target cells of virus infection and virus replication. In both infected groups, ADV crossed the endotheliochorial placental barrier, although animals infected before mating already had high antibody titers against ADV at the time of implantation. The percentage of dead and resorbed fetuses was much higher in dams infected before mating. In the placentae of these mink, virus DNA and viral mRNA were detected in cells in the mesenchymal stroma of the placental labyrinth and hematoma but only occasionally in the cytotrophoblast of the placental hematoma. Placentae of animals infected during pregnancy showed in addition very high levels of virus and also viral replication in a large number of cytotrophoblast cells in the placental hematoma, which exhibited distinct inclusion bodies. In both groups, neither virus nor virus replication could be detected in maternal endothelial cells or fetal syncytiotrophoblast of the placental labyrinth. Fetuses were positive for virus and viral replication at high levels in a wide range of tissues. Possible routes of transplacental transmission of ADV and the role of trophoblast cells as targets for viral replication are discussed.     

Reduced severity of lesions in mink infected transplacentally with Aleutian disease virus.

Inoculation of mink late in the second trimester of pregnancy with Aleutian disease virus (ADV) produces a persistent infection in the offspring. When these mink were analyzed at 83 days of age and compared with adolescent mink infected for a similar length of time, the transplacentally infected mink show: 1) a marked reduction in plasmacytosis, immunoglobulin level and specific ADV antibody; 2) increased amounts of infectious ADV and numbers of cells containing viral antigen; 3) a marked reduction in immune complex glomerulonephritis and absence of immune complex arteritis; 4) free ADV antigen in the glomeruli; and 5) a striking accumulation of eosinophils in the tissues. The findings suggest that the degree of ADV expression is partially immunologically controlled.     

Identification of a nonvirion protein of Aleutian disease virus: mink with Aleutian disease have antibody to both virion and nonvirion proteins.

We studied Aleutian disease virus polypeptides in Crandall feline kidney (CRFK) cells. When CRFK cells labeled with [35S]methionine at 60 h postinfection were studied by immunoprecipitation with sera from infected mink, the major Aleutian disease virus virion polypeptides (p85 and p75) were consistently identified, as was a 71,000-dalton nonvirion protein (p71). The peptide maps of p85 and p75 were similar, but the map of p71 was different. p85, p75, and p71 were all precipitated by sera from Aleutian disease virus-infected mink, including those with signs of progressive disease, but heterologous sera raised against purified Aleutian disease virus did not precipitate the nonvirion p71. These results indicated that the nonvirion p71 was unrelated to p85 and p75 and further suggested that mink infected with Aleutian disease virus develop antibody to nonvirion, as well as structural, viral proteins.     

Demonstration of Aleutian disease virus-specific lymphocyte response in mink with progressive Aleutian disease: comparison of sapphire and pastel mink infected with different virus strains

Lymphocyte blastogenesis was used to study the antiviral lymphocyte response of sapphire (Aleutian) and pastel (nonAleutian) mink inoculated with Pullman or Utah 1 Aleutian disease virus (ADV). Both mink genotypes developed a virus-specific response when inoculated with Utah 1 ADV. In contrast, after inoculation of Pullman ADV, sapphire mink had a positive virus-specific response, whereas pastel mink did not. Response occurred late after infection (8 wk) and correlated with the development of progressive Aleutian disease (AD). The response to keyhole limpet hemocyanin (KLH) and concanavalin A (Con A) was also determined. Most mink of either genotype, inoculated with either virus strain, maintained an anti-KLH response during disease. Most mink also responded to Con A, although some exhibited suppressed Con A response late in the disease course. These results indicated that mink develop an anti-ADV lymphocyte response during progressive AD and are not immunosuppressed with regard to other antigens or mitogens.     

Molecular cloning of the Aleutian disease virus genome: expression of Aleutian disease virus antigens by a recombinant plasmid

Three nonoverlapping segments representing approximately 80% of the 4.8-kilobase pair Aleutian disease virus (ADV-G) duplex genome were molecularly cloned into either bacteriophage M13mp9 (M13bm2 = 0.07 to 0.15 map unit; M13bm1 = 0.15 to 0.54 map unit) or plasmid pUC8 (pBM1 = 0.54 to 0.88 map units). In addition the 0.54- to 0.88-map unit segment of a Danish isolate of ADV (DK ADV) was also cloned into pUC8 (pBM2). The recombinant plasmids pBM1 and pBM2 induced expression of several polypeptides in Escherichia coli JM103 that were specifically recognized by sera from mink infected with ADV. The same three proteins with approximate molecular weights of 55,000, 34,000, and 27,000 were detected both by immune blotting and by immunoprecipitation of [35S]methionine-labeled JM103 (pBM1). None of these proteins were recognized in JM103 or JM103 (pUC8), nor were they detected by sera from normal mink. Purified pBM1 and pBM2 DNA appeared identical in size by gel analysis and contour length measurement, and electron microscopic heteroduplex mapping revealed no visible areas of heterology. However, restriction endonuclease mapping showed that pBM2 was different from pBM1, indicating that this segment of the ADV genome was similar but not identical for two strains of ADV (ADV-G and DK ADV). Furthermore, when cloned DNA from ADV-G was labeled with [32P]dCTP by nick translation, DNA relatedness to several field strains of ADV (Utah I, Pullman, and DK), but not to mink enteritis virus or cellular DNA, was shown by Southern blot hybridization.     

Interferon response in normal and Aleutian disease virus-infected mink.

Studies were done to determine whether differences in interferon production are responsible for the resistance of pastel mink to Aleutian disease. The abilities of normal pastel and sapphire mink to produce interferon when inoculated with either Newcastle disease virus or a synthetic polyribonucleotide, poly (I):poly (C), were identical, even to the production of a novel, acid-labile interferon. The resistance of pastel mink to Aleutian disease did not correlate with interferon production, because neither sapphire nor pastel mink produced detectable amounts of interferon when infected with either the Pullman strain of Aleutian disease virus (ADV) or the highly virulent Utah I strain. Sapphire mink infected with the Pullman strain responded normally to poly (I):poly (C) early in the course of the disease, but interferon production was impaired late, when the mink were hypergammaglobulinemic and had renal, vascular, and hepatic lesions. These data suggest that ADV Pullman neither stimulates nor interferes with interferon production in infected mink and may represent a mechanism whereby ADV can more readily establish 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.     

Molecular comparisons of in vivo- and in vitro-derived strains of Aleutian disease of mink parvovirus.

DNA from one cell culture-adapted and two pathogenic strains of Aleutian disease of mink parvovirus (ADV) was molecularly cloned into the vectors pUC18 and pUC19. The DNA from the two pathogenic strains (ADV-Utah I and ADV-Pullman) was obtained from virus purified directly from the organs of infected mink, whereas the DNA from the nonpathogenic ADV-G was derived from cell culture material. The cloned segment from all three viruses represented a 3.55-kilobase-pair BamHI (15 map units) to HindIII (88 map units) fragment. Detailed physical mapping studies indicated that all three viruses shared 29 of 46 restriction endonuclease recognition sites but that 6 sites unique to the pathogenic strains and 5 sites unique to ADV-G were clustered in the portion of the genome expected to code for structural proteins. Clones from all three viruses directed the synthesis of two ADV-specific polypeptides with molecular weights of approximately 57 and 34 kilodaltons. Both species reacted with sera from infected mink as well as with a monoclonal antibody specific for ADV structural proteins. Because production of these ADV antigens was detected in both pUC18 and pUC19 and was not influenced by isopropyl-beta-D-thiogalactopyranoside (IPTG) induction, their expression was not regulated by the lac promoter of the pUC vector, but presumably by promoterlike sequences found within the ADV DNA. The proteins specified by the clones of ADV-G were 2 to 3 kilodaltons smaller than those of the two pathogenic strains, although the DNA segments were identical in size. This difference in protein molecular weights may correlate with pathogenicity, because capsid proteins of pathogenic and nonpathogenic strains of ADV exhibit a similar difference.     

S-phase-dependent cell cycle disturbances caused by Aleutian mink disease parvovirus.

We examined replication of the autonomous parvovirus Aleutian mink disease parvovirus (ADV) in relation to cell cycle progression of permissive Crandell feline kidney (CRFK) cells. Flow cytometric analysis showed that ADV caused a composite, binary pattern of cell cycle arrest. ADV-induced cell cycle arrest occurred exclusively in cells containing de novo-synthesized viral nonstructural (NS) proteins. Production of ADV NS proteins, indicative of ADV replication, was triggered during S-phase traverse. The NS+ cells that were generated during later parts of S phase did not undergo cytokinesis and formed a distinct population, termed population A. Formation of population A was not prevented by VM-26, indicating that these cells were arrested in late S or G2 phase. Cells in population A continued to support high-level ADV DNA replication and production of infectious virus after the normal S phase had ceased. A second, postmitotic, NS+ population (termed population B) arose in G0/G1, downstream of population A. Population B cells were unable to traverse S phase but did exhibit low-level DNA synthesis. Since the nature of this DNA synthesis was not examined, we cannot at present differentiate between G1 and early S arrest in population B. Cells that became NS+ during S phase entered population A, whereas population B cells apparently remained NS- during S phase and expressed high NS levels postmitosis in G0/G1. This suggested that population B resulted from leakage of cells with subthreshold levels of ADV products through the late S/G2 block and, consequently, that the binary pattern of ADV-induced cell cycle arrest may be governed merely by viral replication levels within a single S phase. Flow cytometric analysis of propidium iodide fluorescence and bromodeoxyuridine uptake showed that population A cells sustained significantly higher levels of DNA replication than population B cells during the ADV-induced cell cycle arrest. Therefore, the type of ADV-induced cell cycle arrest was not trivial and could have implications for subsequent viral replication in the target cell.     

In situ molecular hybridization for detection of Aleutian mink disease parvovirus DNA by using strand-specific probes: identification of target cells for viral replication in cell cultures and in mink kits with virus-induced interstitial pneumonia.

Strand-specific hybridization probes were utilized in in situ molecular hybridization specifically to localize replicative form DNA of Aleutian mink disease parvovirus (ADV). Throughout in vitro infection, duplex replicative form DNA of ADV was located in the cell nuclei. Single-stranded virion DNA and capsid proteins were present in the nuclei early in infection, but were later translocated to the cytoplasm. In neonatal mink, ADV causes acute interstitial pneumonia, and replicative forms of viral DNA were found predominantly in alveolar type II cells of the lung. Viral DNA was also found in other organs, but strand-specific probes made it possible to show that most of this DNA represented virus sequestration. In addition, glomerular immune complexes containing intact virions were detected, suggesting that ADV virions may have a role in the genesis of ADV-induced glomerulonephritis.     

Cytokine profiles in adult mink infected with Aleutian mink disease parvovirus

The aim of this study was to examine the levels of gamma interferon (IFN-gamma)-, interleukin 4 (IL-4)-, and IL-8-producing cells in peripheral blood mononuclear cells from mink infected with the Aleutian mink disease parvovirus (ADV). As expected, ADV-infected mink developed high plasma gamma globulin values (hypergammaglobulinemia) and enhanced quantities of CD8-positive (CD8(+)) cells in the blood during the infection. We quantified the percentages of IFN-gamma- and IL-4-positive lymphocytes and IL-8-positive monocytes up to week 38 after virus challenge. The results clearly indicated marked increases in the percentages of IFN-gamma- and IL-4-producing lymphocytes during ADV infection. The total number of IL-8-producing monocytes in the blood of ADV-infected mink stayed fairly constant during the infection. In order to characterize the phenotype of the cytokine-producing cells, we performed double-labeling fluorescence-activated cell sorter (FACS) experiments with CD8 surface labeling in one channel and cytokine intracellular staining in the other. We found that most IFN-gamma and IL-4 in ADV-infected mink was produced by CD8(+) cells, while in the uninfected mink, these cytokines were primarily produced by a cell type that was not CD8 (possibly CD4-positive cells). We also observed that IL-8 was almost exclusively produced by monocytes. All of the above findings led us to conclude that both Th1- and Th2-driven immune functions are found in mink plasmacytosis.