The antigen and virus of Aleutian disease in mink

Analyses of leucocytes in blood and lymphoid tissues from mink infected with Aleutian mink disease parvovirus (AMDV)

Mink were infected with Aleutian Mink Disease Parvovirus (AMDV) and sacrificed at monthly intervals after infection. During this time humoral immune responses and leucocyte numbers in blood, mesenteric lymph node, spleen and thymus were monitored. Serum hypergammaglobulinaemia was observed together with elevated antibody responses to AMDV NS1 and VP1/2 proteins. In blood, a highly significant increase in CD8+ lymphocytes was observed. However, (presumed)CD4+ cells defined as CD3+CD8- cells, and B lymphocytes remained relatively constant throughout the study. The (presumed)CD4+/CD8+ ratio decreased significantly from greater than 2 to less than 0.5 and MHC-II+ blood leucocytes increased significantly during infection, a large proportion of these being CD8+. Similar changes were observed in the mesenteric lymph node and spleen. Immunohistology of lymph nodes showed a massive expansion of the paracortical area due to increased numbers of CD8+ cells. The staining intensity of B lymphocytes in lymph nodes with a CD79a reactive monoclonal antibody was decreased in the late infection, indicating a possible greater number of plasma cells. Thymic involution was observed during the AMDV infection, although relative increases in CD3high (presumed)CD4+ and CD3highCD8+ single positive cells were observed. These increases were countered by a corresponding reduction in the CD3low(presumed)CD4+CD8+ double positive cell population. Immunohistology of the thymus in normal mink showed that most of the matured CD3+ T cells were present in the inner medulla, while only few CD3+ cells could be found in the outer cortex. In severely infected mink the thymic structural organisation vanished, and CD3+ cells were found throughout the organ.

Mink infected with Aleutian disease virus have an elevated level of CD8-positive T-lymphocytes

Lymphocytes, monocytes, granulocytes, B-lymphocytes and CD8-positive T-lymphocytes of non-infected mink and mink infected with Aleutian disease virus (ADV) were measured by flow cytometry. The gammaglobulin levels of the sera were also measured. Besides development of hypergammaglobulinaemia in the infected mink, the most pronounced finding was that the number of CD8-positive lymphocytes doubled on average during development of Aleutian disease, while the number of B-lymphocytes did not change dramatically. The enhanced CD8 frequency was still apparent 6 months after initial ADV infection of the mink. The present experiments contribute to a better understanding of the immune deficiency stage seen in mink infected with ADV.

Analysis of the quantity of antiviral antibodies from mink infected with different Aleutian disease virus strains

Mink persistently infected with Aleutian disease virus (ADV) develop hypergammaglobulinaemia and immune complex disease. Radiolabelled antibodies from mink infected with ADV-G, DK, Pullman , and Utah I strains of ADV were reacted against all four ADV strains in radioimmunoassay (RIA). The amount of anti-ADV antibody in two equally hypergammaglobulinaemic serum pools varied from 13% (anti- Pullman ) to 57% (anti-Utah I). Serum pools from two other sources (anti-DK and anti-ADV-G), although less hypergammaglobulinaemic , had 5% and 13%, respectively, indicating that 43-95% of the Ig in the sera of mink with AD was not specific antibody to ADV structural antigens. The possibility of a general polyclonal activation of the humoral immune system is being discussed. Comparison of plateau RIA binding levels for the four serum pools against the four viral antigens suggested three patterns of reactivity: DK and Utah I reacted similarly, but Pullman and ADV-G reacted serologically different.

Mink with Aleutian disease have high-affinity antiviral antibodies

Mink persistently infected with Aleutian disease virus (ADV) develop plasmacytosis (hypergammaglobulinaemia) and immune complex disease. Mink of different colour phases were infected with different strains of ADV and bled at different times after infection. The average antibody affinities (Kav) were measured in the sera and found to fall in the range of 2 X 10(9) - 2 X 10(10) M-1, thus indicating good-quality antibodies. In sera of non-Aleutian genotype mink a decline in Kav during development of plasmacytosis was observed. Moreover, the antibody heterogeneity (alpha values) tended to decrease during the disease progress. In contrast, the Kav values in sera of infected Aleutian genotype mink remained relatively high after hypergammaglobulinaemia developed, and the antibody heterogeneity for certain of the mink sera indicated restricted heterogeneity (high alpha values). In agreement with the clonal selection theory, low virus burden (for instance, during infection with a low-virulence ADV strain) generated relatively higher affinity antibodies than a high virus burden for instance, the highly virulent Utah I strain of ADV). Furthermore, antibodies present in low concentration were of higher affinity than antibodies present in high concentrations. The relatively high affinity antibodies found in this study indicate that if the immune complex disease seen in AD is caused by virus-anti-virus antibodies, good-quality antibodies are likely to be responsible for the pathological findings.

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.

Comparative pathogenicity of four strains of Aleutian disease virus for pastel and sapphire mink

Information was sought on the comparative pathogenicity of four North American strains (isolates) of Aleutian disease virus for royal pastel (a non-Aleutian genotype) and sapphire (an Aleutian genotype) mink. The four strains (Utah-1, Ontario [Canada], Montana, and Pullman [Washington]), all of mink origin, were inoculated intraperitoneally and intranasally in serial 10-fold dilutions. As indicated by the appearance of specific antibody (counterimmunoelectrophoresis test), all strains readily infected both color phases of mink, and all strains were equally pathogenic for sapphire mink. Not all strains, however, regularly caused Aleutian disease in pastel mink. Infection of pastel mink with the Utah-1 strain invariably led to fatal disease. Infection with the Ontario strain caused fatal disease nearly as often. The Pullman strain, by contrast, almost never caused disease in infected pastel mink. The pathogenicity of the Montana strain for this color phase was between these extremes. These findings emphasize the need to distinguish between infection and disease when mink are exposed to Aleutian disease virus. The distinction has important implications for understanding the natural history of Aleutian disease virus infection in ranch mink.

Mitogen- and viral antigen-induced transformation of lymphocytes from normal mink and from mink with progressive or nonprogressive Aleutian disease

Peripheral blood lymphocytes (PBL) from mink with progressive Aleutian disease (AD) were shown to be significantly less responsive to phytohemagglutinin, concanavalin A, and pokeweed mitogen than were PBL from normal mink and from mink with a nonprogressive form of AD. Response to the virus of AD was significantly greater in PBL cultures from mink with nonprogressive AD than in those from normal mink or mink with progressive AD. After experimental infection with AD virus, mink PBL were responsive to viral antigen only transiently. These findings suggest that lymphocyte responsiveness as indicated by transformation induced by mitogens or viral antigen may be an important aspect of host response to infection with the parvovirus of AD.

Characterization of Aleutian disease virus as a parvovirus

We characterized a strain of Aleutian disease virus adapted to growth in Crandall feline kidney cells at 31.8 degrees C. When purified from infected cells, Aleutian disease virus had a density in CsCl of 1.42 to 1.44 g/ml and was 24 to 26 nm in diameter. [3H]thymidine could be incorporated into the viral genome, and the viral DNA was then studied. In alkaline sucrose gradients, Aleutian disease virus DNA was a single species that cosedimented at 15.5S with single-stranded DNA from adeno-associated virus. When the DNA was analyzed on neutral sucrose gradients, a single species was again observed, which sedimented at 21S and was clearly distinct from 16S duplex adeno-associated virus DNA. A similar result was obtained even after incubation under annealing conditions, implying that the bulk of Aleutian disease virus virions contained a single non-complementary strand with a molecular weight of about 1.4 X 10(6). In addition, two major virus-associated polypeptides with molecular weights of 89,100 and 77,600 were demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of virus purified from infected cultures labeled with [35S]methionine. These data suggest that Aleutian disease virus is a nondefective parvovirus.

Properties of Aleutian disease virus assayed with feline kidney cells

Properties of Aleutian disease virus (ADV) were studied using feline kidney cells, line CRFK, to assay virus by the induction of nuclear antigen. ADV nuclear antigen was detected by immunofluorescent staining. Titers of virus obtained from mink spleens at 10-8 days after infection were usually between 10(3) and 10(5) infectious units per gram of spleen. ADV was purified by fluorocarbon extraction, differential centrifugation, biogel A-15 chromatography and CsCl equilibrium centrifugation. The molecular weight of the virus was estimated to be 3-5 X 10(5) daltons. The density of antigen-inducing virus in equilibrium CsCl gradients was 1.32--1.34 g/cm3. On velocity sucrose gradients, antigen-inducing virus had a sedimentation coefficient of approximately 110S. The virus was not neutralized by sera from chronically infected mink and ferrets and by sera from experimentally infected mink. ADV was resistant to ionic and nonionic detergents and lipid solvents. The titer of partially purified virus was reduced as much as 700-fold by proteolytic enzymes but not by DNase or RNase. The virus was inactivated slowly at 56 degrees C; the initial half-life was 90 minutes. It is concluded that the properties of ADV can be determined by assay in CRFK cells, thus facilitating virological study of the disease.