Passive transfer of antiviral antibodies restricts replication of Aleutian mink disease parvovirus in vivo

Strong selection signatures for Aleutian disease tolerance acting on novel candidate genes linked to immune and cellular responses in American mink (Neogale vison)

Aleutian disease (AD) is a multi-systemic infectious disease in American mink (Neogale vison) caused by Aleutian mink disease virus (AMDV). This study aimed to identify candidate regions and genes underlying selection for response against AMDV using whole-genome sequence (WGS) data. Three case-control selection signatures studies were conducted between animals (N = 85) producing high versus low antibody levels against AMDV, grouped by counter immunoelectrophoresis (CIEP) test and two enzyme-linked immunosorbent assays (ELISA). Within each study, selection signals were detected using fixation index (FST) and nucleotide diversity (θπ ratios), and validated by cross-population extended haplotype homozygosity (XP-EHH) test. Within- and between-studies overlapping results were then evaluated. Within-studies overlapping results indicated novel candidate genes related to immune and cellular responses (e.g., TAP2, RAB32), respiratory system function (e.g., SPEF2, R3HCC1L), and reproduction system function (e.g., HSF2, CFAP206) in other species. Between-studies overlapping results identified three large segments under strong selection pressure, including two on chromosome 1 (chr1:88,770-98,281 kb and chr1:114,133-120,473) and one on chromosome 6 (chr6:37,953-44,279 kb). Within regions with strong signals, we found novel candidate genes involved in immune and cellular responses (e.g., homologous MHC class II genes, ITPR3, VPS52) in other species. Our study brings new insights into candidate regions and genes controlling AD response.

Epidemiology, pathogenesis, and diagnosis of Aleutian disease caused by Aleutian mink disease virus: A literature review with a perspective of genomic breeding for disease control in American mink (Neogale vison)

Aleutian disease (AD) is a multi-systemic infectious disease in American mink (Neogale vison) caused by the Aleutian mink disease virus (AMDV). Commonly referred to as mink plasmacytosis, AD is an economically significant disease in mink-breeding countries. Aleutian disease mainly induces weight loss, lower fertility, and dropped pelt quality in adults and can result in acute interstitial pneumonia with high mortality rates in kits. In this review, we employed the scientific literature on AD over the last 70 years to discuss the historical and contemporary status of AD outbreaks and seroprevalence in mink farming countries. We also explained different forms of AD and the differences between the pathogenicity of the virus in kits and adults. The application of the available AD serological tests in AD control strategies was argued. We explained how selection programs could help AD control and proposed different approaches to selecting animals for building AD-tolerant herds. The advantages of genomic selection for AD tolerance over traditional breeding strategies were discussed in detail. We also explained how genomic selection could help AD control by selecting tolerant animals for the next generation based on genome-wide single nucleotide polymorphisms (SNP) data and the challenges of implementing genomic selection for AD tolerance in the mink industry. This review collected the information required for designing successful breeding programs for AD tolerance. Examples of the application of information are presented, and data gaps are highlighted. We showed that AD tolerance is necessary to be among the traits that animals are selected for in the mink industry.

Serum Analytes of American Mink (Neovison Vison) Challenged with Aleutian Mink Disease Virus

Black American mink (Neovison vison), which had been selected for tolerance to Aleutian mink disease virus (AMDV) for more than 20 years (TG100) or were from herds that have been free of AMDV (TG0), along with their progeny and crosses with 50% and 75% tolerance ancestry, were inoculated with a local isolate of AMDV. Blood samples were collected from 493 mink between 120 and 1211 days post-inoculation, and concentrations of 14 serum analytes were measured. Distributions of all analytes significantly deviated from normality, and data were analyzed after Box-Cox power transformation. Significant differences were observed among tolerant groups in the concentrations of globulin (GLO), total protein (TP), alkaline phosphatase, urea nitrogen, and calcium. Concentrations of GLO and TP linearly and significantly decreased with an increasing percentage of tolerance ancestry. Eleven analytes had the smallest values in the tolerant groups (TG100 or TG75), and eight analytes had the greatest values in the non-selected groups (TG0 or TG50). Antibody titer had the greatest correlation coefficients with GLO (0.62), TP (0.53), and creatinine (0.36). It was concluded that selection for tolerance decreased the concentrations of most serum analytes, and TP and GLO were the most accurate biomarkers of tolerance to AMDV infection. Males had significantly greater values than females for phosphorus and total bilirubin concentrations, but females had significantly greater amylase, cholesterol, and BUN concentrations than males.

AMDV Vaccine: Challenges and Perspectives

Aleutian mink disease virus (AMDV) is known to cause the most significant disease in the mink industry. It is globally widespread and manifested as a deadly plasmacytosis and hyperglobulinemia. So far, measures to control the viral spread have been limited to manual serological testing for AMDV-positive mink. Further, due to the persistent nature of this virus, attempts to eradicate Aleutian disease (AD) have largely failed. Therefore, effective strategies to control the viral spread are of crucial importance for wildlife protection. One potentially key tool in the fight against this disease is by the immunization of mink against AMDV. Throughout many years, several researchers have tried to develop AMDV vaccines and demonstrated varying degrees of protection in mink by those vaccines. Despite these attempts, there are currently no vaccines available against AMDV, allowing the continuation of the spread of Aleutian disease. Herein, we summarize previous AMDV immunization attempts in mink as well as other preventative measures with the purpose to shed light on future studies designing such a potentially crucial preventative tool against Aleutian disease.

Amdoparvoviruses in small mammals: expanding our understanding of parvovirus diversity, distribution, and pathology

Many new viruses have been discovered recently, thanks in part to the advent of next-generation sequencing technologies. Among the Parvoviridae, three novel members of the genus Amdoparvovirus have been described in the last 4 years, expanding this genus that had contained a single species since its discovery, Aleutian mink disease virus. The increasing number of molecular and epidemiological studies on these viruses around the world also highlights the growing interest in this genus. Some aspects of amdoparvoviruses have been well characterized, however, many other aspects still need to be elucidated and the most recent reviews on this topic are outdated. We provide here an up-to-date overview of what is known and what still needs to be investigated about these scientifically and clinically relevant animal viruses.

Implementation and validation of a sensitive PCR detection method in the eradication campaign against Aleutian mink disease virus

Aleutian mink disease virus (AMDV) is a severe progressive disease causing multiple different clinical syndromes in mink. In Denmark, the disease is notifiable and under official control. The control programme, based on serological screening, has confined successfully AMDV to the northern part of Denmark. However, re-infections and new introductions of virus into farms require a confirmatory virological test to verify the positive test results of single animals and ultimately to investigate disease transmission. A one step PCR amplifying a 374-base fragment of the NS1 gene of AMDV was compared to the counter-current immune electrophoresis (CIE) routinely used in the serological screening programme. Mink organs (n=299) obtained from 55 recently infected farms and 8 non-infected farms from 2008 to 2010 were tested by PCR, and the results were found to have a high correlation with the serological status of the mink. The relative diagnostic sensitivity of the PCR was 94.7%, and the relative diagnostic specificity was 97.9% when read in parallel with the CIE. PCR positive samples were sequenced and phylogenetic analysis revealed high similarity within the analysed AMDV strains and to AMDV strains described previously.

Intravenous inoculation of a bat-associated rabies virus causes lethal encephalopathy in mice through invasion of the brain via neurosecretory hypothalamic fibers

The majority of rabies virus (RV) infections are caused by bites or scratches from rabid carnivores or bats. Usually, RV utilizes the retrograde transport within the neuronal network to spread from the infection site to the central nervous system (CNS) where it replicates in neuronal somata and infects other neurons via trans-synaptic spread. We speculate that in addition to the neuronal transport of the virus, hematogenous spread from the site of infection directly to the brain after accidental spill over into the vascular system might represent an alternative way for RV to invade the CNS. So far, it is unknown whether hematogenous spread has any relevance in RV pathogenesis. To determine whether certain RV variants might have the capacity to invade the CNS from the periphery via hematogenous spread, we infected mice either intramuscularly (i.m.) or intravenously (i.v.) with the dog-associated RV DOG4 or the silver-haired bat-associated RV SB. In addition to monitoring the progression of clinical signs of rabies we used immunohistochemistry and quantitative reverse transcription polymerase chain reaction (qRT-PCR) to follow the spread of the virus from the infection site to the brain. In contrast to i.m. infection where both variants caused a lethal encephalopathy, only i.v. infection with SB resulted in the development of a lethal infection. While qRT-PCR did not reveal major differences in virus loads in spinal cord or brain at different times after i.m. or i.v. infection of SB, immunohistochemical analysis showed that only i.v. administered SB directly infected the forebrain. The earliest affected regions were those hypothalamic nuclei, which are connected by neurosecretory fibers to the circumventricular organs neurohypophysis and median eminence. Our data suggest that hematogenous spread of SB can lead to a fatal encephalopathy through direct retrograde invasion of the CNS at the neurovascular interface of the hypothalamus-hypophysis system. This alternative mode of virus spread has implications for the post exposure prophylaxis of rabies, particularly with silver-haired bat-associated RV.

Rabies virus (RV) infects mammalian neurons and cycles in regionally distinct animal populations such as the red fox in Europe, domestic canines in Asia, or raccoons, skunks and bats in Northern America. Although human rabies can be prevented by pre- and post-exposure prophylaxis, more than 50,000 people die annually from the severe encephalopathy caused by RV. Recently, two cases of RV transmission by organ transplantation were reported. In our study, using intravenous inoculation of mice, we evaluated the pathogenetic relevance of virions that reach the bloodstream. Mice inoculated intravenously with a canine-derived RV survived the infection in contrast to intramuscularly injected mice, while mice infected with a silver-haired bat-related RV succumbed to the disease regardless of the route of inoculation. We found that the silver-haired bat-related RV was able to transit from the blood to the brain by invading neurosecretory fibers of the hypothalamus, which form neurohemal synapses lacking a blood-brain-barrier. This newly described route of brain invasion might reflect how RV reached the central nervous system from transplanted organs, since it takes longer to establish the neural connections between host and grafted tissue necessary for classical RV migration than the time until the infection became symptomatic in the two reported cases.

Respiratory syncytial virus infects and abortively replicates in the lungs in spite of preexisting immunity

Respiratory syncytial virus (RSV) is a major cause of bronchiolitis and viral pneumonia in young children and a serious health risk in immunocompromised individuals and the elderly. Immunity to RSV is not completely understood. In this work, we established a method for monitoring RSV infection by real-time PCR and applied this method for analysis of RSV replication in vivo in the cotton rat model in naïve animals and in animals rendered immune to RSV by prior RSV infection. We found that even though no virus could be isolated from the lungs of RSV-challenged immune animals, RSV infection in fact took place and an accumulation of viral RNA transcripts was observed. This type of replication, therefore, can be termed "abortive," as RSV is capable of entering the cells in the lungs of immune animals, yet the production of progeny viruses is impaired. Similar patterns of RSV gene expression gradient were observed between naïve and reinfected animals, indicating that the skewing of mRNA gradient of viral gene expression, a mechanism documented during latent infection by other viruses, is not likely to be responsible for abortive replication of RSV during reinfection. We found that passive administration of antibodies to RSV prevents productive infection normally accompanied by viral release in the lung, but it does not prevent abortive replication of the virus. To the best of our knowledge, this is the first evidence of abortive replication of RSV in vivo.

The transcription profile of Aleutian mink disease virus in CRFK cells is generated by alternative processing of pre-mRNAs produced from a single promoter

A reevaluation of the transcription profile of Aleutian mink disease parvovirus (AMDV)-infected CRFK cells at either 32 degrees C or 37 degrees C has determined that strain AMDV-G encodes six species of mRNAs produced by alternative splicing and alternative polyadenylation of a pre-mRNA generated by a single promoter at the left end of the genome. Three different splicing patterns are used, and each type is found polyadenylated at either the 3' end of the genome (the distal site) or at a site in the center of the genome (the proximal site). All spliced species accumulate similarly over the course of infection, with the R2 RNA predominant throughout. The R2 RNA, which contains and can express the NS2 coding region, encodes the viral capsid proteins VP1 and VP2.

Pathogenesis of aleutian mink disease parvovirus and similarities to b19 infection

Aleutian mink disease parvovirus (ADV) is an unusual member of the autonomous parvoviruses in both its replication and pathogenesis. Infection of newborn mink kits results in an acute disease typified by virus replication in type II pneumocytes in the lung. This replication is permissive and cytopathic, characterized by the production of high levels of viral replicative intermediates and infectious progeny. However, infection of adult Aleutian mink leads to a chronic form of the disease termed Aleutian disease (AD). In this case, virus replication occurs predominantly in lymph node macrophages and is restricted, with viral DNA replication, RNA transcription, protein expression and production of infectious progeny occurring at low levels. B19 is the only autonomous parvovirus known to infect humans. The primary site of virus replication in both children and adults is in erythrocyte precursors in the blood and bone marrow, although viral genomes have been detected in various other tissues. B19 infection often causes a self-limiting disease although persistent infection of B19 can occur in both immuno-compromised and -competent people. Perhaps the most striking similarity between infection with ADV or with B19 is the important role the humoral immune response to infection has in pathogenesis. It can be both protective and pathogenic. Due to of the central role of antibody in the disease caused by either virus, understanding the specific roles of antibody production in protection, antibody-mediated enhancement of infection, the establishment of persistent infection and immune-mediated pathology will provide insight into the pathogenesis of these infections. A second similarity between the two viruses is the ability to establish persistent infection. Persistence of ADV is associated with restricted replication. Although many cellular factors may contribute to restricted virus replication, the interactions between the major non-structural protein, NS1, and the cells are likely to be critical. Parallels exist between the expression and post-translational modification of ADV and B19 NS1 proteins that may contribute to restriction of virus replication. Thus, a study of the regulation of NS1 expression and its interactions with cell signalling pathways may lead to increased understanding of the restricted replication of these two viruses, and perhaps of persistent infection.

Parvovirus variation for disease: a difference with RNA viruses?

The Parvoviridae, a family of viruses with single-stranded DNA genomes widely spread from invertebrates to mammal and human hosts, display a remarkable evolutionary capacity uncommon in DNA genomes. Parvovirus populations show high genetic heterogeneity and large population sizes resembling the quasispecies found in RNA viruses. These viruses multiply in proliferating cells, causing acute, persistent or latent infections relying in the immunocompetence and developmental stage of the hosts. Some parvovirus populations in natural settings, such as carnivore autonomous parvoviruses or primate adeno associated virus, show a high degree of genetic heterogeneity. However, other parvoviruses such as the pathogenic B19 human erythrovirus or the porcine parvovirus, show little genetic variation, indicating different virus-host relationships. The Parvoviridae evolutionary potential in mammal infections has been modeled in the experimental system formed by the immunodeficient scid mouse infected by the minute virus of mice (MVM) under distinct immune and adaptive pressures. The sequence of viral genomes (close to 10(5) nucleotides) in emerging MVM pathogenic populations present in the organs of 26 mice showed consensus sequences not representing the complex distribution of viral clones and a high genetic heterogeneity (average mutation frequency 8.3 x 10(-4) substitutions/nt accumulated over 2-3 months). Specific amino acid changes, selected at a rate up to 1% in the capsid and in the NS2 nonstructural protein, endowed these viruses with new tropism and increased fitness. Further molecular analysis supported the notion that, in addition to immune pressures, the affinity of molecular interactions with cellular targets, as the Crml nuclear export receptor or the primary capsid receptor, as well as the adaptation to tissues enriched in proliferating cells, are major selective factors in the rapid parvovirus evolutionary dynamics.

Enhanced cytoplasmic sequestration of the nuclear export receptor CRM1 by NS2 mutations developed in the host regulates parvovirus fitness

To investigate whether a DNA virus can evade passive immunotherapy with a polyclonal antiserum, we analyzed the protection of a neutralizing capsid antiserum against a lethal infection of the immunosuppressive strain of the parvovirus minute virus of mice (MVMi) in 42 immunodeficient mice over a period of 200 days. A few mice were effectively protected, but most developed a delayed lethal leukopenic syndrome during the treatment or weeks afterwards. Unexpectedly, viruses isolated from treated but also from control leukopenic mice showed no amino acid changes throughout the entire capsid coding region, although the viral populations were genetically heterogeneous, mainly in the second exon of the coding sequence of the NS2 nonstructural protein. The NS2 point amino acid changes (T88A, K96E, L103P, and L153 M) that were consistently selected in several mice clustered within the nuclear exportin CRM1 binding domain, in a reading frame that did not alter the overlapping NS1 coding region. These mutations endowed emerging viruses with an increased fitness that was demonstrable by their relative resistance to the neutralizing capsid antiserum in a postentry plaque-forming assay, the rapid overgrowth of a competing wild-type (wt) population in culture, and a larger yield of infectious particles. Mutant NS2 proteins interacted with a higher affinity and sequestered CRM1 in the perinuclear region of the cytoplasm more efficiently than the wt. Correspondingly this phenomenon, as well as the following timely ordered release of the NS1 nonstructural protein and the empty capsid from the nucleus to the cytoplasm, occurred markedly earlier in the infection cycle of the mutant viruses. We hypothesize that the enhanced cytoplasmic sequestration of CRM1 by the NS2 mutations selected in mice may trigger pleiotropic effects leading to an accelerated MVMi life cycle and thus to increased fitness. These results strengthen our earlier report on the rapid evolutionary capacity of this mammalian-specific DNA virus in vivo and indicate that the NS2-CRM1 interaction is an important determinant of parvovirus virulence that can be modulated in nature, hampering the effectiveness of passive antibody therapies in the long term.

High mutant frequency in populations of a DNA virus allows evasion from antibody therapy in an immunodeficient host

The degree of genetic heterogeneity of DNA virus populations in nature and its consequences for disease control are virtually unknown. The parvovirus minute virus of mice (MVMi) was used here to investigate (i) the frequency of antibody-escape mutants in populations of a DNA virus and (ii) the ability of a DNA virus to evade in the long-term a passive monoclonal antibody (MAb) therapy in an immunodeficient natural host. Independent clonal populations of MVMi harbored a high proportion of mutants resistant to neutralizing MAb (mutant frequency = [2.8 +/- 0.5] x 10(-5)) that rapidly evolved under antibody pressure in culture to become mixtures dominated by genotypically diverse escape mutants. Immunodeficient mice naturally infected with clonal populations of MVMi and subsequently treated by intravenous injections of MAb were initially protected from the characteristic viral induced lethal leukopenia. However, some treated animals developed a delayed severe leukopenic syndrome associated with the emergence of genetically heterogeneous populations of MAb-resistant mutants in the MVMi main target organs. The 11 plaque-purified viruses analyzed from an antibody-resistant population obtained from one animal corresponded to four different mutant genotypes, although their consensus sequence remained wild type. All cloned escape mutants harbored single radical amino acid changes within a stretch of seven residues in a surface-exposed loop at the threefold axes of the capsid. This antigenic site, which can tolerate radical changes preserving MVMi pathogenic potential, may thereby allow the virus to evade the immune control. These findings indicate a high genetic heterogeneity and rapid adaptation of populations of a mammal DNA virus in vivo and provide a genetic basis for the failure of passive immunotherapy in the natural host.

Identification of aleutian mink disease parvovirus capsid sequences mediating antibody-dependent enhancement of infection, virus neutralization, and immune complex formation

Aleutian mink disease parvovirus (ADV) causes a persistent infection associated with circulating immune complexes, immune complex disease, hypergammaglobulinemia, and high levels of antiviral antibody. Although antibody can neutralize ADV infectivity in Crandell feline kidney cells in vitro, virus is not cleared in vivo, and capsid-based vaccines have proven uniformly ineffective. Antiviral antibody also enables ADV to infect macrophages, the target cells for persistent infection, by Fc-receptor-mediated antibody-dependent enhancement (ADE). The antibodies involved in these unique aspects of ADV pathogenesis may have specific targets on the ADV capsid. Prominent differences exist between the structure of ADV and other, more-typical parvoviruses, which can be accounted for by short peptide sequences in the flexible loop regions of the capsid proteins. In order to determine whether these short sequences are targets for antibodies involved in ADV pathogenesis, we studied heterologous antibodies against several peptides present in the major capsid protein, VP2. Of these antibodies, a polyclonal rabbit antibody to peptide VP2:428-446 was the most interesting. The anti-VP2:428-446 antibody aggregated virus particles into immune complexes, mediated ADE, and neutralized virus infectivity in vitro. Thus, antibody against this short peptide can be implicated in key facets of ADV pathogenesis. Structural modeling suggested that surface-exposed residues of VP2:428-446 are readily accessible for antibody binding. The observation that antibodies against a single target peptide in the ADV capsid can mediate both neutralization and ADE may explain the failure of capsid-based vaccines.

Replication of Aleutian mink disease parvovirus in mink lymph node histocultures

Aleutian mink disease parvovirus (ADV), causes an immune disorder with a persistent infection of lymphoid organs in adult mink. We studied replication of ADV in gel-supported histocultures prepared from adult mink mesenteric lymph node (MLN). Evidence of virus replication in the histocultures was first observed by indirect immunofluorescence 72 h after incubation with virus. Cells resembling lymphocytes and macrophages contained both ADV capsid (VP2) and nonstructural (NS1 and NS2) proteins, and were present in a distribution suggestive of infected cells within germinal centres. ADV replicative form and encapsidated virion DNA were also detected in infected histocultures at time-points after 72 h. In addition, we were able to passage ADV-Utah to a new round of histocultures. These results suggested that the infected cells were actual target cells for ADV replication and that productive ADV-Utah replication, complete with the generation of virus, was occurring in the histocultures. The mink MLN histocultures provide a system to study the replication and molecular pathogenesis of ADV in target tissues.

Construction of pathogenic molecular clones of Aleutian mink disease parvovirus that replicate both in vivo and in vitro

The ADV-G isolate of Aleutian mink disease parvovirus (ADV) replicates permissively in Crandell feline kidney (CRFK) cells but is nonpathogenic for mink, whereas the highly pathogenic ADV-Utah isolate is nonviable in CRFK cells. To assign control of host range in CRFK cells and pathogenicity to specific regions of the ADV genome, we constructed a full-length molecular clone chimeric between ADV-G and ADV-Utah. If either the map unit (MU) 54-65 (clone G/U-5) or MU 65-88 (clone G/U-7) sections were ADV-Utah, viability in CRFK cells was abolished, thus indicating that in vitro host range was controlled by two independent determinants: A in the MU 54-65 segment and B in the MU 65-88 segment. Determinant B could be divided into two subregions, B1 (MU 65-69) and B2 (MU 73-88), neither of which alone could inhibit replication in CRFK cells, an observation suggesting that expression of the B determinant required interaction between noncontiguous sequences. Adult mink of Aleutian genotype inoculated with G/U-8 or G/U-10 developed viremia, antiviral antibody, and histopathological changes characteristic of progressive Aleutian disease. The capsid sequences of G/U-8 and G/U-10 differed from ADV-G at five and four amino acid residues, respectively. Our results suggested that the host range and pathogenicity of ADV are regulated by sequences in the capsid protein gene.

Two parvoviruses that cause different diseases in mink have different transcription patterns: transcription analysis of mink enteritis virus and Aleutian mink disease parvovirus in the same cell line

The two parvoviruses of mink cause very different diseases. Mink enteritis virus (MEV) is associated with rapid, high-level viral replication and acute disease. In contrast, infection with Aleutian mink disease parvovirus (ADV) is associated with persistent, low-level viral replication and chronic severe immune dysregulation. In the present report, we have compared viral transcription in synchronized CRFK cells infected with either MEV or ADV using a nonradioactive RNase protection assay. The overall level of viral transcription was 20-fold higher in MEV- than in ADV-infected cells. Furthermore, MEV mRNA encoding structural proteins (MEV mRNA R3) was dominant throughout the infectious cycle, comprising approximately 80% of the total viral transcription products. In marked contrast, in ADV-infected cells, transcripts encoding nonstructural proteins (ADV mRNA R1 and R2) comprised more than 84% of the total transcripts at all times after infection, whereas ADV mRNA R3 comprised less than 16%. Thus, the ADV mRNA coding for structural proteins (ADV mRNA R3) was present at a level at least 100-fold lower than the corresponding MEV mRNA R3. These findings paralleled previous biochemical studies analyzing in vitro activities of the ADV and MEV promoters (J. Christensen, T. Storgaard, B. Viuff, B. Aasted, and S. Alexandersen, J. Virol. 67:1877-1886, 1993). The overall low levels of ADV mRNA and the paucity of the mRNA coding for ADV structural proteins may reflect an adaptation of the virus for low-level restricted infection.

Effect of maternally acquired Aujeszky's disease (pseudorabies) virus-specific antibody in pigs on establishment of latency and seroconversion to differential glycoproteins after low dose challenge

This study investigated whether (1) passively immune pigs could become latently infected after challenge with low doses of wild type pseudorabies virus (PRV) and (2) if seroconversion to PRV could be consistently detected using two commercially available differential diagnostic ELISAs. Three litters of piglets with passively acquired PRV serum neutralizing (SN) antibody (geometric mean titers 47.03 to 95.10) were challenged at 6 to 12 days of age with 236 to 500 TCID50 of Shope strain virus; pigs were vaccinated at 11 weeks of age with a commercially available genetically engineered vaccine (TK- gE- gG- Iowa S62 strain PRV). Vaccination was intended to reduce the risk of reactivation of latent infection resulting in spread of virulent PRV infection to previously uninfected pigs during the experiment. Vaccination at this age also approximated common field practices in infected herds. After 15 weeks, all challenged pigs were seropositive on the PRV glycoprotein (g or gp) E differential ELISA but were seronegative on the gG differential ELISA. All three challenge groups had pigs that were latently infected as evidenced by the detection of PRV DNA by polymerase chain reaction (PCR) assay of their trigeminal ganglia (TG). There was a significant inverse relationship observed for age at challenge and the proportion of PCR positive pigs in the group 15 weeks postchallenge (p = 0.0004). This trend was independent of the passively acquired PRV SN antibody titers at challenge. In this study, passively acquired antibody did not provide protection against establishment of latent infection in piglets after exposure to low doses of virulent PRV. These latent infections were detected serologically by only one of two available differential diagnostic ELISA.

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.

The relationship between capsid protein (VP2) sequence and pathogenicity of Aleutian mink disease parvovirus (ADV): a possible role for raccoons in the transmission of ADV infections

Aleutian mink disease parvovirus (ADV) DNA was identified by PCR in samples from mink and raccoons on commercial ranches during an outbreak of Aleutian disease (AD). Comparison of DNA sequences of the hypervariable portion of VP2, the major capsid protein of ADV, indicated that both mink and raccoons were infected by a new isolate of ADV, designated ADV-TR. Because the capsid proteins of other parvoviruses play a prominent role in the determination of viral pathogenicity and host range, we decided to examine the relationship between the capsid protein sequences and pathogenicity of ADV. Comparison of the ADV-TR hypervariable region sequence with sequences of other isolates of ADV revealed that ADV-TR was 94 to 100% related to the nonpathogenic type 1 ADV-G at both the DNA and amino acid levels but less than 90% related to other pathogenic ADVs like the type 2 ADV-Utah, the type 3 ADV-ZK8, or ADV-Pullman. This finding indicated that a virus with a type 1 hypervariable region could be pathogenic. To perform a more comprehensive analysis, the complete VP2 sequence of ADV-TR was obtained and compared with that of the 647-amino-acid VP2 of ADV-G and the corresponding VP2 sequences of the pathogenic ADV-Utah, ADV-Pullman, and ADV-ZK8. Although the hypervariable region amino acid sequence of ADV-TR was identical to that of ADV-G, there were 12 amino acid differences between ADV-G and ADV-TR. Each of these differences was at a position where other pathogenic isolates also differed from ADV-G. Thus, although ADV-TR had the hypervariable sequence of the nonpathogenic type 1 ADV-G, the remainder of the VP2 sequence resembled sequences of other pathogenic ADVs. Under experimental conditions, ADV-TR and ADV-Utah were highly pathogenic and induced typical AD in trios of both Aleutian and non-Aleutian mink, whereas ADV-Pullman was pathogenic only for Aleutian mink and ADV-G was noninfectious. Trios of raccoons experimentally inoculated with ADV-TR and ADV-Utah all became infected with ADV, but only a single ADV-Pullman-inoculated raccoon showed evidence of infection. Furthermore, none of the ADV isolates induced pathological findings of AD in raccoons. Finally, when a preparation of ADV-TR prepared from infected raccoon lymph nodes was inoculated into mink and raccoons, typical AD was induced in Aleutian and non-Aleutian mink, but raccoons failed to show serological or pathological evidence of infection. These results indicated that raccoons can become infected with ADV and may have a role in the transmission of virus to mink but that raccoon-to-raccoon transmission of ADV is unlikely.

Suppression of measles virus expression by noncytolytic antibody in an immortalized macrophage cell line

Immune regulation of measles virus (MV) expression was studied in a persistently infected mouse macrophage cell line. Synthesis of both membrane-associated and internal MV antigens was suppressed when infected macrophages were treated with polyclonal rabbit anti-MV antibody that was specific for MV proteins. Persistently infected macrophages were treated for 3, 5, or 7 days with increasing doses of anti-MV antibody. All MV proteins were down-regulated 2 days after treatment was terminated. One week after treatment was terminated, down-regulation was still evident but to a lesser degree. MV protein synthesis was suppressed whether or not complement components were inactivated by heating all serum supplements and antibodies. However, when complement was active, cell lysis accounted for some of the reduced MV protein synthesis. When lytic destruction of infected cells by antibody and complement was prevented by inactivation of complement, antibody alone reduced the cellular synthesis of viral proteins by noncytolytic mechanisms. The absence of cell death in the absence of complement was confirmed by the lack of 51Cr release from labeled cells, the lack of reduction in cell number, and the lack of a decrease in total protein synthesis when radiolabeled infected cells were treated with antibody. It is noteworthy that low doses of antibody were optimal for suppression in the longer-term experiments and did not cause lysis, even in the presence of active complement. Since infected macrophages disseminate virus in measles infection, noncytolytic regulation of these cells by antibody may supplement viral clearance by cytolytic T cells and other immune mechanisms.

Sequence comparison of the non-structural genes of four different types of Aleutian mink disease parvovirus indicates an unusual degree of variability

The present work shows that at least four different sequence types of Aleutian mink disease parvovirus (ADV) are present in ADV isolates from mink. We here report the nucleotide sequences of these four types of ADV from nucleotide 123 to 2208 (map unit 3 to 46). This part of the genome encodes three non-structural (NS) proteins of ADV. Comparison of the deduced amino acid sequences of these NS proteins showed that the ADV proteins are much less conserved than the NS proteins from other members of the autonomous group of parvoviruses. In general, we found that the middle region of the ADV NS-1 protein was relatively well conserved among the types, while both the amino- and carboxy-terminal ends of the protein had higher amino acid variability. Interestingly, the putative NS-3 protein from type 3 ADV is truncated in the carboxy-terminal end. The molecular evolutionary relationship among the four types of ADV was examined. This analysis, taken together with the unusually high degree of variability of the ADV types, indicates that the ADV infection in mink is likely to be an old infection compared to the other parvovirus infections or, alternatively, that ADV accumulates sequence changes much faster than other parvoviruses.

Role of alveolar type II cells and of surfactant-associated protein C mRNA levels in the pathogenesis of respiratory distress in mink kits infected with Aleutian mink disease parvovirus

Neonatal mink kits infected with Aleutian mink disease parvovirus (ADV) develop an acute interstitial pneumonia with clinical symptoms and pathological lesions that resemble those seen in preterm human infants with respiratory distress syndrome and in human adults with adult respiratory distress syndrome. We have previously suggested that ADV replicates in the alveolar type II epithelial cells of the lung. By using double in situ hybridization, with the simultaneous use of a probe to detect ADV replication and a probe to demonstrate alveolar type II cells, we now confirm this hypothesis. Furthermore, Northern (RNA) blot hybridization showed that the infection caused a significant decrease of surfactant-associated protein C mRNA produced by the alveolar type II cells. We therefore suggest that the severe clinical symptoms and pathological changes characterized by hyaline membrane formation observed in ADV-infected mink kits are caused by a dysfunction of alveolar surfactant similar to that observed in respiratory distress syndrome in preterm infants. However, in the infected mink kits the dysfunction is due to the replication of ADV in the lungs, whereas the dysfunction of surfactant in preterm infants is due to lung immaturity.

Acute interstitial pneumonia in mink kits inoculated with defined isolates of Aleutian mink disease parvovirus

The present study addressed the causal role of Aleutian mink disease parvovirus (ADV) in acute interstitial pneumonia in mink kits. All the examined isolates of ADV caused interstitial pneumonia in newborn kits, although the severity of disease and the mortality varied. These findings indicate that ADV is the direct causal agent of this disease in mink kits and that cofactors, which could have been present in the original ADV-K isolate, do not play a role. Acute interstitial pneumonia characterized by hypertrophy and hyperplasia of alveolar type II cells, intranuclear viral inclusions, interstitial edema, and hyaline membrane formation was experimentally reproduced in mink kits infected as newborns with five different isolates of ADV. Four hundred forty-nine newborn mink kits were included in the study, of which 247 were necropsied. The lesions caused by the different isolates were indistinguishable by histopathologic examination, but the incidence (50-100%) and severity (mortality of 30-100%, n = 218) of disease among the mink kits varied. Also, the content of ADV antigens in the lungs of infected kits varied among the groups. According to these features, the examined isolates could be placed in groups of high and low virulence. ADV-K, ADV-Utah I, and ADV-DK were in a highly virulent group producing a mortality of 90-100% (n = 110) in mink inoculated as newborns. ADV-GL and ADV-Pullman belonged to a group of low virulence, with an incidence of clinical disease of 50-70% and a mortality of approximately 30-50% (n = 118) in kits inoculated as newborns. The mortality in the control group receiving a mock inoculum was around 12% (n = 34). The period from infection to development of fatal disease varied from approximately 12 days for the highly virulent isolates up to around 20 days for the isolates of low virulence. The 107 mink kits that survived inoculation with ADV as newborns developed lesions typical of classical Aleutian disease irrespective of the ADV isolate used. The lesions consisted of chronic immune complex-mediated glomerulonephritis and infiltrations with mononuclear cells, including plasma cells in lung, liver, spleen, kidney, mesenteric lymph node, and intestine. Surviving kits also had hypertrophy of the bronchus-associated lymphoid tissue and focal subpleural, intraalveolar accumulations of large cells with foamy cytoplasm, so-called lipid pneumonia.

Pathogenesis of Aleutian mink disease parvovirus infection: effects of suppression of antibody response on viral mRNA levels and on development of acute disease

We suppressed the B-cell development and antibody response in mink by using treatment with polyclonal anti-immunoglobulin M (anti-IgM) to study the effects of antiviral antibodies on development of Aleutian mink disease parvovirus (ADV)-induced disease in more detail. Newborn mink kits were injected intraperitoneally with 1 mg of either anti-IgM or a control preparation three times a week for 30 to 34 days. At 21 days after birth, groups of mink kits were infected with the highly virulent United isolate of ADV. At selected time points, i.e., postinfection days 9, 13, 29, and 200, randomly chosen mink kits were sacrificed, and blood and tissues were collected for analyses. The efficacy of immunosuppressive treatment was monitored by electrophoretic techniques and flow cytometry. Effects of treatment on viral replication, on viral mRNA levels, and on development of acute or chronic disease were determined by histopathological, immunoelectrophoretic, and molecular hybridization techniques. Several interesting findings emerged from these studies. First, antiviral antibodies decreased ADV mRNA levels more than DNA replication. Second, suppression of B-cell development and antibody response in mink kits infected at 21 days of age resulted in production of viral inclusion bodies in alveolar type II cells. Some of these kits showed mild clinical signs of respiratory disease, and one kit died of respiratory distress; however, clinical signs were seen only after release of immunosuppression, suggesting that the production of antiviral antibodies, in combination with the massive amounts of free viral antigen present, somehow is involved in the induction of respiratory distress. It is suggested that the antiviral antibody response observed in mink older than approximately 14 days primarily, by a yet unknown mechanism, decreases ADV mRNA levels which, if severe enough, results in restricted levels of DNA replication and virion production. Furthermore, such a restricted ADV infection at low levels paves the way for a persistent infection leading to immunologically mediated disease. The potential mechanisms of antibody-mediated restriction of viral mRNA levels and mechanisms of disease induction are discussed.

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.

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)

cis-acting sequences in the Aleutian mink disease parvovirus late promoter important for transcription: comparison to the canine parvovirus and minute virus of mice

We are currently investigating the regulation of transcription of the Aleutian mink disease parvovirus (ADV). ADV causes a chronic immune complex-mediated condition known as classical Aleutian disease, characterized by slow viral replication. This slow replication is an intrinsic property of ADV and distinguishes it from the more prototypic parvoviruses such as minute virus of mice (MVM) and canine parvovirus (CPV). We have previously suggested a role for the weak ADV promoters in the slow replication and thereby the absence of acute cytopathology and instead establishment of persistent ADV infection with progressive immune complex-mediated chronic lesions. In this study, we have mapped the cis-acting sequences around the ADV P36 promoter responsible for both constitutive transcription and transactivation mediated by the nonstructural protein 1. The mapping was performed by using endpoint deletions of the ADV P36 promoter and by making chimeras between the ADV P36 and MVM P38 promoters. We found the weak constitutive activity of the ADV P36 promoter to be caused by suboptimal promoter proximal sequences, while the low level of transactivation was caused mainly by an upstream region including sequences with homology to the transactivation responsive element (tar) of the H-1 parvovirus (M.-L. Gu, F.-X. Chen, and S. L. Rhode, Virology 187:10-17, 1992). We also found the corresponding regions in the MVM and CPV P38 promoters to be important for transactivation of these promoters by making 5' deletions of the promoter region. In addition, it was found that MVM tar-like and upstream sequences could transfer high nonstructural protein 1 responsiveness to the ADV promoter even though the distance between the tar-like element and the TATA box was significantly changed. On the basis of comparative data for ADV, MVM, CPV, and H-1, a new clustered motif (TTGGTT) is proposed to be the responsive cis-acting element for transactivation. Homology comparison of the specific transcriptional elements of the ADV P36, MVM P38, and CPV P38 promoters suggests that few, but crucial, changes in the ADV P36 promoter and upstream region are responsible for the weak constitutive activity and low level of transactivation of the ADV P36 promoter.

Identification of alternatively spliced mRNAs encoding potential new regulatory proteins in cattle infected with bovine leukemia virus

The polymerase chain reaction was used to detect and characterize low-abundance bovine leukemia virus (BLV) mRNAs. In infected cattle we could detect spliced mRNA with a splice pattern consistent with a Tax/Rex mRNA, as well as at least four alternatively spliced RNAs. Two of the alternatively spliced mRNAs encoded hitherto unrecognized BLV proteins, designated RIII and GIV. The Tax/Rex and alternatively spliced mRNAs could be detected at their highest levels in BLV-infected cell cultures; the next highest levels were found in samples from calves experimentally infected at 6 weeks postinoculation. Alternatively spliced mRNAs were also expressed, albeit at lower levels, in naturally infected animals; they were detected by a nested polymerase chain reaction. Interestingly, the GIV mRNA was specifically detected in naturally infected cows with persistent lymphocytosis and in two of five calves at 6 months after experimental infection with BLV. Furthermore, the calf with the strongest signal for GIV had the highest lymphocyte counts. These data may suggest a correlation between expression of the GIV product and development of persistent lymphocytosis. Some of the donor and acceptor sites in the alternatively spliced mRNAs were highly unusual. The biological mechanisms and significance of such a choice of unexpected splice sites are currently unknown.

Antibodies to varicella-zoster virus modulate antigen distribution but fail to induce viral persistence in vitro

Varicella-zoster virus (VZV) persists in human sensory ganglia. One of the hypotheses to explain the induction or the maintenance of VZV latency is that it could be promoted by the immune response itself. It is known that in the case of viruses which bud off the infected cell membrane, virus-specific antibodies can induce antigenic modulation, i.e., spatial redistribution of viral antigens and modulation of their synthesis. To determine whether antigenic modulation occurs during VZV infection in vitro and could possibly be involved in viral persistence, we have grown infected cells in the presence of anti-VZV antibodies either transiently or permanently. The distribution of immune complexes and viral proteins was then analyzed. In transient immunomodulation experiments, the distribution of one or more viral antigens was modified not only in the cytoplasmic membranes but also in the cytoplasm and nucleoplasm of infected cells. When infected cells were kept permanently in the presence of antibodies, the same pattern of redistribution of immune complexes was observed and the localization of internal viral glycoproteins was significantly modified. However, antibodies did not prevent the lytic effect of infection; they altered neither the infectious virus yield nor the Western immunoblot pattern of viral proteins, suggesting that immunomodulation is not the primary effector of viral persistence.

Persistence of viral RNA in mouse brains after recovery from acute alphavirus encephalitis

Little is known about the relationship between recovery from acute viral encephalitis and the clearance of viral genetic material from the central nervous system. In a mouse model of Sindbis virus encephalitis, we have previously shown that clearance of infectious virus is mediated by antibody-induced restriction of viral gene expression rather than by cytotoxic destruction of virally infected cells. To explore whether Sindbis virus genomes persist in mouse brain after the clearance of infectious virus, we used reverse transcriptase-polymerase chain reaction amplification methods to detect Sindbis virus RNA in brain samples from immunocompetent BALB/c and antibody-treated immunodeficient scid/CB17 mice. RNA sequences from both the nonstructural region (NSP1 gene) and structural regions (E2 gene) of Sindbis virus were detected in the brains of all BALB/c and antibody-treated scid mice examined at 1, 2, and 3 months after infection. Additional BALB/c mouse brains were also positive at 8, 12, and 17 months after infection. To determine whether persistent RNA was capable of resuming unrestricted replication in the absence of the continuous presence of antiviral antibodies, viral titers were measured in the brains of scid mice at 1, 2, 3, and 6 months after antibody treatment. Viral reactivation was seen in scid mice treated with hyperimmune serum or a low dose of monoclonal antibody to the E2 envelope glycoprotein, but not in mice treated with a high dose of monoclonal antibody to E2. Replication of infectious virus isolated from scid mouse brain could be restricted by repeat treatment with immune serum, indicating that viral reactivation is not due to antibody-escape mutations. These results demonstrate that Sindbis virus can persist long term in a nonproductive form in mouse brain and suggest that the humoral immune response plays an important role in preventing viral reactivation.

Antibody-dependent transcriptional regulation of measles virus in persistently infected neural cells

Application of neutralizing anti-hemagglutinin antibodies to mouse neuroblastoma cells (NS20Y/MS) persistently infected with measles virus (MV) leads to a significant reduction of viral structural proteins within 6 days. While the transcriptional gradient for MV-specific mRNAs remained unaffected upon antibody treatment, the total amount of MV-specific transcripts dropped by 80% after 24 h. The expression of genomic RNA was affected similarly, with slightly slower time kinetics. Both transcription and expression of the viral structural proteins could be completely reactivated when viral antibodies were removed from the tissue culture. The same findings could be obtained in rat glioma cells persistently infected with subacute sclerosing panencephalitis virus (C6/SSPE) but not in cells of nonneural origin. The data indicate that antibody-induced antigenic modulation affects the early stages of viral transcription within a few hours after the addition of antibodies and leads to an almost complete repression of viral gene expression in cells of neural origin.

Follicular dendritic cell-B cell interactions in virus disease. Common localization but different cell damage caused by antibody immobilized virus?

Follicular dendritic cells (FDC) are involved in the trapping and retention of antigen-antibody complexes in lymphoid follicles. This FDC immobilized antigen is thought to be involved in the generation of memory B-lymphocytes. Follicular trapping of both Aleutian disease virus and HIV particles has been demonstrated. However as far as known their affects on FDC and follicular B-cells are completely different. It is hypothesized that the trapping of (antibody-complexed) virus particles by the FDC-network may have an important role in several virus diseases.

Characterization of early pathogenic effects after experimental infection of calves with bovine immunodeficiency-like virus

The early pathogenic effects of bovine immunodeficiency-like virus (BIV) were studied in calves experimentally inoculated with BIV. All animals inoculated with BIV R29-infected cells seroconverted by 6 weeks postinoculation, and BIV was recoverable from each animal at 2 weeks postinoculation. However, levels of BIV replication in vivo appeared to be low. In situ hybridization studies indicated that during peak periods of viral replication in vivo, less than 0.03% of peripheral blood mononuclear cells were expressing detectable levels of viral RNA. Moreover, the levels of viral RNA in these cells in vivo were less than 1/10 the levels observed in persistently infected cells in vitro. BIV-inoculated calves had significantly higher numbers of circulating lymphocytes, and follicular hyperplasia was observed in lymph nodes, hemal nodes, and spleen. The histopathological changes observed in BIV-infected calves were similar to changes found early after infection with the immunosuppressive lentiviruses, including human immunodeficiency virus type 1.

Persistent rat parvovirus infection in individually housed rats

The duration of infection with rat virus (RV), an autonomous rodent parvovirus, was examined at multiple intervals over 6 months in rats inoculated by the oronasal route at 2 days of age or 4 weeks of age and individually housed after weaning to prevent cross-infection. Infectious virus was recovered by explant culture from 32 of 80 rats inoculated as pups and was detected as late as 6 months after inoculation. Rats inoculated as juveniles developed acute infection, but virus was not detected beyond 7 weeks after inoculation. Tissues from rats in both age groups were surveyed for RV DNA by Southern blotting using a double-stranded DNA probe made from a 1700 bp cloned fragment of RV spanning map units 0.19-0.52. Band patterns representative of acute infection (juvenile rats) were consistent with the replicating form of RV DNA, whereas patterns representative of persistent infection (rats inoculated as pups) were suggestive of defective or non-productive viral replication.

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

A method combining in situ hybridization and immunohistochemistry was used to characterize cells infected with Aleutian mink disease parvovirus (ADV). Single-stranded RNA hybridization probes specific for obligate replicative intermediates and antisera specific for virion or non-structural proteins were employed. Crandell feline kidney cells in which the ADV-G strain of ADV was permissively replicating contained virion and non-structural proteins, large amounts of single stranded virion DNA, duplex replicative form (RF) DNA, and mRNA. Late in the infectious cycle, however, cells containing non-structural proteins but little nucleic acid were observed, probably representing cells in the end stage of viral cytopathology. Sections of lung prepared from mink kits infected with the ADV-Utah 1 strain were then examined. Alveolar type II cells permissively replicating ADV contained viral nucleic acids and proteins in patterns nearly identical to CRFK cells, suggesting that permissive ADV replication was similar in vitro and in vivo. Another population of ADV containing cells that had cytoplasmic virion antigen, but undetectable levels of non-structural protein was found in vivo. Furthermore, although virion DNA was present in the cytoplasm of these cells, RF DNA or mRNA could not be detected. These cells may have been alveolar macrophages sequestering viral particles.

Mechanism of antibody-mediated viral clearance in immunotherapy of respiratory syncytial virus infection of cotton rats

Antibody-mediated clearance of respiratory syncytial virus from cotton rat pulmonary tissues occurs in the absence of complement and in the absence of the Fc portion of the immunoglobulin G molecule, suggesting that complement-independent, cell-independent neutralization is the major mechanism of clearance.

Analysis of experimental mink enteritis virus infection in mink: in situ hybridization, serology, and histopathology

Strand-specific hybridization probes were used in in situ hybridization studies to localize cells containing mink enteritis virus (MEV) virion DNA or MEV replicative-form DNA and mRNA. Following the experimental MEV infection of 3-month-old unvaccinated mink, a significant increase in serum antibodies to MEV was detected at postinfection day (PID) 6, 2 days after the onset of fecal shedding of virus. Prior to the appearance of virus in feces, viral DNA could be detected in the mesenteric lymph node and intestine. The largest percentage of cells positive for virion DNA was 10% and was detected in the intestine on PID 6. However, replication of the virus apparently peaked at PID 4. The number of MEV replicative-form DNA and mRNA molecules was found to be approximately 250,000 copies per infected lymph node cell or crypt epithelial cell. The localization, levels, and time course of viral replication have important implications for the pathogenesis of MEV-induced disease. The data presented on MEV are correlated with earlier results on the other mink parvovirus, Aleutian mink disease parvovirus, and a possible explanation for the remarkable differences in pathogenesis of disease caused by these two parvoviruses is discussed.

Aleutian disease parvovirus infection of mink and ferrets elicits an antibody response to a second nonstructural viral protein

A second nonstructural protein of the Aleutian disease parvovirus was predicted from nucleotide sequence analysis and a detailed transcription map. Western immunoblotting analysis showed that infected mink and ferrets show an antibody response to this predicted protein.

Violet mink develop an acute disease after experimental infection with Aleutian disease virus (ADV) isolate ADV SL3

Six-Aleutian (aa)-genotype violet mink were infected intraperitoneally with the Aleutian Disease Virus (ADV) bone marrow derived isolate ADV SL3. All animals developed virus-specific antibodies and hypergammaglobulinaemia. Mortality during the fourteen week duration of the infection was 50%. The virus induced (histo)pathological lesions typical for Aleutian Disease. By immunohistochemical examination using a virus capsid-specific monoclonal antibody viral antigen was detected in lymph nodes, spleen, kidneys and once in hepatic Kupffer cells. By Southern blot and in situ hybridization studies with strand-specific RNA probes able to distinguish viral replicative forms from merely sequestered genomic DNA, ADV replication was detected in mesenteric lymph nodes and spleen. In one mink DNA replicative forms were also found in bone marrow cells or mononuclear cells of the peripheral blood, respectively. Only single-stranded viral DNA was detected in liver, kidney, gut and lung of infected animals. From Southern blot hybridization results a different, possibly organ-specific permissiveness of ADV in vivo is suggested.

Antibody-induced restriction of viral gene expression in measles encephalitis in rats

After infection with the neurotropic CAM/RBH measles virus (MV) strain, newborn Lewis rats succumb to an acute necrotizing encephalopathy. Passive transfer of neutralizing monoclonal antibodies directed against MV hemagglutinin prevented this disease process. Instead, either an antibody-induced acute or subacute measles encephalitis developed after a prolonged incubation period with a restricted expression of MV structural proteins. The molecular biological analysis of MV gene expression in brain tissue of rats treated with MV-neutralizing antibodies revealed a transcriptional restriction of viral mRNAs, particularly for the envelope proteins, leading to a steep expression gradient. Based on in situ hybridization, it was concluded that the efficiency of transcription of viral genes at the single-cell level is reduced compared with that of controls. Passive immunization with monoclonal antibodies directed against other MV structural proteins proved to be ineffective. Similar results were obtained in MV-infected weanling Brown Norway rats. These rats developed a clinically silent encephalitis in the presence of high titers of neutralizing antibodies. In such animals, a pronounced attenuation of the viral gene transcription was observed. These findings indicated that neutralizing antibodies directed against a restricted set of specific antigenic sites on the viral hemagglutinin protein expressed on cell membranes exert a modulating effect on the viral gene expression at the level of transcription. This phenomenon contributes to the switch from the acute cytopathic effect to a persistent infection in the central nervous system.

Analysis of parvovirus infections using strand-specific hybridization probes

The autonomous parvoviruses cause a broad spectrum of acute and chronic infections of animals and man. The discrimination of sites of viral replication from sites of viral sequestration is an important goal in elucidating the pathogenesis of these diseases. It is possible to employ strand-specific RNA hybridization probes in such analyses because a 'plus' sense probe will react with single stranded virion DNA and duplex replicative form DNA, but a 'minus' sense probe will react preferentially with obligate replicative intermediates (duplex replicative form DNA and mRNA). Strand-specific RNA hybridization probes were developed for the Aleutian mink disease parvovirus (ADV) and were used to study acute and chronic infections of mink. Such probes were capable of differentiating replicative intermediates (duplex replicative form DNA and mRNA) from single-stranded virion DNA in Southern blot analysis and in strand-specific in situ hybridization. ADV infection of seronegative newborn mink kits causes an acute, cytopathic infection of type II alveolar cells. Replication in these cells is highly permissive and is characterized by high levels of replicative intermediates and virion DNA. A fatal respiratory distress syndrome and hyaline membrane formation result from impaired surfactant production by the infected type II cells. On the other hand, ADV infection of adult mink is associated with a persistent infection and a disorder of the immune regulation. The target cells for viral replication in adult mink are confined to the lymphoid system and the bone marrow. Replication in these cells, which are probably lymphocytes, is restricted, and characterized by greatly reduced levels of replicative intermediates and virion DNA. It, therefore, seems that disease in the infected adult mink results from a restricted infection by ADV. Large amounts of virion DNA can also be demonstrated in locations where replication cannot be detected and apparently represents sequestration of virion particles by elements of the reticuloendothelial system. Thus, replication and sequestration can, in fact, be distinguished by the strand-specific in situ hybridization. These studies indicate that strand-specific in situ hybridization is a potentially valuable method for studying the pathogenesis of parvovirus infections.