PrPSC in Icelandic sheep naturally infected with scrapie.
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Animals↗
Biomedical subjects
G Georgsson
Publications and source records attributed to G Georgsson.
Nucleotide sequence and biological properties of a pathogenic proviral molecular clone of neurovirulent visna virus.
Intracerebral serial passage of visna virus KV1514 through three Icelandic sheep was used to select for strains with increased neurovirulence. A strain (KV1772) with increased neuropathogenicity was obtained. We isolated several proviral molecular clones from a plaque-purified biological clone of KV1772 that induced typical visna virus pathology in young sheep. One of the clones (kv72) was infectious, while others contained mutations or were permuted and required gene recombination with other proviral clones to generate infectious virus after transfection. Stable plasmids containing functional, full-length, visna virus KV1772 genomes were constructed from the proviral molecular clones. The in vitro cytopathic effects of virus derived from these clones varied depending upon the tissue origin of the infected cells. A goat cell line became persistently infected with molecularly cloned KV1772 virus; these cells resisted the cell-killing effects and continuously shed high levels of infectious virus. We determined the complete nucleotide sequence of a KV1772 provirus; it contains open reading frames for all structural and accessory genes previously identified in the visna virus genome and is highly homologous to other published visna virus sequences. Progeny of molecularly cloned KV1772 virus rapidly induced both a pronounced neuropathology and an unexpected, strong, neutralizing antibody response in experimentally infected young Icelandic sheep. The availability of stable plasmids of replication-competent and pathogenic proviral molecular clones of visna virus should now enable the study of the genetic determinants of neurovirulence and their interaction with the host immune system in visna virus pathogenesis.
Amino Acid Sequence↗
Quantitative assessment of the astrocytic response in natural scrapie of sheep.
Scrapie of sheep and goats belongs to the spongiform encephalopathies, a term derived from the characteristic vacuolar degeneration in the central nervous system. Astrocytosis has been described, but a systematic quantitative study has not been made. Such a study is important in resolving the still controversial issues as to whether the astrocytic response is caused directly by the infectious agent or represents a secondary reaction to tissue damage. In this study the numbers of astrocytes in 12 sheep with scrapie and 12 healthy sheep of a similar age were assessed and compared. Sections from eight planes of the brain were immunostained with anti-glial fibrillary acidic protein (GFAP) for astrocytes and their numbers counted in 40 areas of 1 mm2. The main findings were as follows. (1) A significant increase of astrocytes was detected in sheep with scrapie. (2) Astrocytosis was not usually related to the severity of the characteristic vacuolar lesions, indicating that it was at least partly due to a direct effect of the infectious agent but was not a secondary response to the neuronal damage. (3) The astrocytic response varied considerably between individual affected sheep; this may have been due to differences in agent strains, host response, or both.
Animals↗
On the role of monocytes/macrophages in the pathogenesis of central nervous system lesions in hereditary cystatin C amyloid angiopathy.
The pathogenesis of the deposition of a variant cystatin C as amyloid in hereditary cystatin C amyloid angiopathy (HCCAA) is not known. To address this question the synthesis and secretion of cystatin C in cultured monocytes from 9 carriers of the mutated cystatin C gene (5 symptomatic and 4 asymptomatic) was examined. The quantity of cystatin C in cells and supernatants was determined by the ELISA method, Western blots were done and selected samples immunostained for cystatin C. Monocytes from individuals carrying the gene defect synthesized cystatin C that was apparently not truncated, a form found in the cerebral amyloid deposits in HCCAA, but showed a distinctly lower rate of cystatin C synthesis than monocytes from healthy controls. The main difference was that the quantity of cystatin C was significantly lower in the supernatants in monocyte cultures from carriers of the gene defect than from healthy controls, possibly due to a partial block in its secretion. This abnormal processing of the cystatin C could explain the low cerebrospinal fluid levels of cystatin C in HCCAA and might be a part of the pathogenetic pathway of amyloid deposition. Furthermore it could, through a lower extracellular concentration of this inhibitor of cysteine proteinases, contribute to destruction of the amyloidotic blood vessels, leading to the most serious clinical manifestation in HCCAA, intracerebral hemorrhage.
Adolescent↗
Pathogenesis of central nervous system lesions in visna: cell-mediated immunity and lymphocyte subsets in blood, brain and cerebrospinal fluid.
There are several indications that central nervous system (CNS) lesions in visna are immune-mediated and that cell-mediated immunity (CMI) may be of importance in the initiation of the lesions. To study the role of CMI in the pathogenesis of CNS lesions, five sheep were infected by intracerebral inoculation with visna virus and observed for 1 year. The following parameters were monitored at regular intervals: (1) neutralizing and ELISA antibodies; (2) visna virus-specific stimulation of lymphocytes from peripheral blood; (3) lymphocyte subpopulations in peripheral blood, cerebrospinal fluid (CSF) and brain at sacrifice. The CNS lesions were graded and compared with other parameters. The time course and titers of antibodies did not correlate with the severity of CNS lesions whereas the CMI did, indicating that CMI may play an important role in lesion development. The correlation of the number of CD8-positive cells in the CSF with the severity of lesions and the reversed ratio of CD4/CD8-positive cells in the diffusely infiltrated neuroparenchyma indicates that the CD8-positive T lymphocyte may be an important effector cell in the induction of CNS lesions.
Animals↗
Human and ovine lentiviral infections compared.
Maedi-visna virus (MVV) of sheep was the first lentivirus to be isolated. The genomic organization of MVV is very similar to that of human immunodeficiency virus (HIV) with several genes regulating the expression of the viral genome. Viral replication is severely restricted in the host and some cells apparently contain the genetic information in a DNA provirus form with little or no expression of viral antigens. This seems to be a major factor in causing the "slowness" of lentiviral infections and the persistence of the virus in the host since the immune system may not recognize the provirus-containing cells. The target cells for HIV and MVV are similar although T4 lymphocytes are not specifically destroyed in maedi-visna. There are also certain similarities in the pathological changes in both diseases, both in the central nervous system, the lungs and the lymphatic system. Although the severe final immunodeficiency state characteristic of AIDS has not been observed in maedi-visna, the basic biological features of the MVV and its interaction with host cells are so similar to HIV infection, that we consider ovine maedi-visna useful animal model for the human lentivirus infections.
Acquired Immunodeficiency Syndrome↗
[Visna and AIDS--various comparative aspects].
Visna, a lingering meningo-encephalitis in sheep, was one of the diseases on which B. Sigurdsson based his theory of a special group of disorders called slow infections. The cause of the disease, a retrovirus, was isolated in 1957. Visna is now classified in a subgroup of retroviridae, lentivirinae, together with virus types in animals and the human immune deficiency virus HIV. Notwithstanding that Visna has been eradicated among Icelandic sheep for 25 years research into this virus continues since it bears many similarities to HIV and also MS.
Antigenic Variation↗
Expression of viral antigens in the central nervous system of visna-infected sheep: an immunohistochemical study on experimental visna induced by virus strains of increased neurovirulence.
Icelandic sheep were infected by intracerebral inoculation with visna virus strains of increased neurovirulence. The character and severity of pathological lesions were studied in brains from four sheep that developed clinical signs 5 to 12 weeks after infection. Viral antigens were identified by immunostaining using mouse monoclonal antibodies against two core proteins and the Avidin-Biotin method of detection. The pathological lesions were in general more severe than observed following infection with the parent strain K1514. Primary demyelination, a late manifestation of infection with K1514, was detected. Thus, in addition to causing more severe pathological lesions, these neurovirulent strains apparently have an increased potential to induce primary demyelination. Viral antigens were detected in lymphocytes, plasma cells, macrophages, endothelial cells, pericytes, fibroblasts and choroidal epithelial cells. Neurons and glial cells were antigen negative. The spectrum of infected cells in the brain was similar to that observed in infections with human immunodeficiency virus. These results do not support the view that the demyelination is caused by immunological damage to infected oligodendrocytes. A perturbation of the function of oligodendrocytes through a non-productive infection could be the underlying pathogenetic mechanism and/or a non-specific demyelination due to the intense inflammatory reaction.
Animals↗
Maedi-visna in sheep: host-virus interactions and utilization as a model.
Controlled animal experiments with the ovine maedi-visna virus, the prototype lentivirus, have been carried out for almost 40 years. This non-oncogenic virus leads to a life-long, persistent infection with slow development of lesions in the lungs and in the central nervous system. The virus is present in many cells in a DNA provirus state and its replication and expression is highly restricted in vivo. The basic biological features of maedi-visna virus are quite similar to those of HIV and this ovine lentiviral disease may be useful as a model for infection with human lentiviruses.
Acquired Immunodeficiency Syndrome↗
Virus spread and initial pathological changes in the nervous system in genital herpes simplex virus type 2 infection in mice. A correlative immunohistochemical, light and electron microscopic study.
Mice were infected by the vaginal route with the MS strain of herpes simplex virus type 2 (HSV-2). Serial vaginal cultures were used to confirm infection and to select mice for this study. Two mice were killed by perfusion on days 2-6 post infection (p.i.) and lumbar and sacral cord with cauda were fixed and embedded for electron microscopy. Semithin Epon-sections were stained for viral antigen using a rabbit anti-HSV-2 antiserum and the Avidin-Biotin (ABC) method. Thin sections from antigen-positive blocks were examined by electron microscopy, and the number and types of infected cells detected by these two methods were compared. A good correlation was found between detection of infected cells by these methods. Infected cells included neurons of dorsal root ganglia and spinal cord, satellite cells of dorsal root ganglia, non-myelinating Schwann cells, astrocytes, oligodendrocytes and arachnoidal cells. Infected cells were first detected in the cauda on day 3 p.i. and in the spinal cord on day 5 p.i. The temporal and spatial distribution of infected cells was consistent with neural spread to and within the CNS. The pathological lesions showed a good correlation with the distribution and number of infected cells and are probably due to a direct virus effect. The similar sensitivity of the Epon-ABC method to electron microscopy in detecting infected cells indicates that this method may have useful applications in both experimental and diagnostic work.
Animals↗
Immunohistochemical staining of cells in the brain of a patient with acquired immune deficiency syndrome (AIDS) with a monoclonal antibody to visna virus.
Several monoclonal antibodies to different epitopes of the two major core proteins of visna virus, p25 and p15, were tested with the Avidin-Biotin immunostaining method on formaldehyde-fixed and paraffin-embedded sections of brains from patients with acquired immune deficiency syndrome (AIDS) who had shown neurological symptoms at death. In one of five AIDS cases a few cells, mainly inflammatory cells, showed a positive staining with a monoclonal antibody to the p25 core protein of visna.
Acquired Immunodeficiency Syndrome↗
[Icelandic research. Slow virus infections and AIDS].
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Acquired Immunodeficiency Syndrome↗
Detection of visna virus antigens and RNA in glial cells in foci of demyelination.
Visna is a slow virus infection of sheep in which the characteristic pathological change is demyelination in foci of inflammation. The latter is thought to be the result of an immunopathological process directed against cellular and antigenic targets that have been difficult to define because of restricted viral gene expression. A new simultaneous detection assay is used to demonstrate viral RNA in cells identified unambiguously as oligodendrocytes and astrocytes. These cells were found in inflammatory foci. With a new strain of virus that causes a rapid form of visna in Icelandic sheep, viral antigens were demonstrated in cells in the inflammatory lesions. These findings are consistent with the postulated immunopathological mechanism of demyelination: cells that maintain intact myelin sheaths in the central nervous system are destroyed by the inflammatory response to viral antigens expressed in these cells.
Animals↗
Precipitating antibodies in experimental visna and natural progressive pneumonia of sheep.
Serological responses of Icelandic sheep experimentally infected with visna virus (vv) were contrasted with responses in American Targhee sheep naturally infected with progressive pneumonia virus (PPV). Precipitating antibodies assayed by immunodiffusion were compared with the neutralising and complementing fixing antibody response. In experimental infections with vv, complement fixing and neutralising antibodies appeared early after infection and rose to high levels in all sheep, while precipitating antibodies were detected only at minimal titre. In natural infections with PPV, immune responses were less consistent and precipitating antibodies were detected more frequently than complement fixing or neutralising antibodies against PPV. These results may suggest important biological differences between the lytic fibroblast-tropic virus strains used for experimental infection of Icelandic sheep and the nonlytic macrophage-tropic strains of PPV circulating in nature. Lytic strains evoke a brisk response against the viral glycoprotein with high titre neutralising antibody while nonlytic strains induce a less consistent response to the glycoprotein.
Animals↗
Antigenic drift in visna: virus variation during long-term infection of Icelandic sheep.
A group of 20 Icelandic sheep were infected intracerebrally with visna virus strain 1514, and 209 virus isolates were obtained from the blood, cerebrospinal fluid, and central nervous system (CNS) over a period of 7 years, during which eight animals developed clinical signs of visna necessitating sacrifice. (i) Using type-specific antisera, it was found that 12 (16%) of 76 isolates tested escaped neutralization. These 12 variant viruses were distributed randomly among animals and over time, and did not replace the infecting strain even though all sheep developed homotypic antibody within 3 months of infection. The one exception was sheep no. 1557 (an animal without clinical visna), where the last six isolates were variants. (ii) A total of 35 blood and CNS isolates from seven of these sheep (including five with clinical visna) were tested against serial samples of their own sera. Autologous antisera neutralized all isolates tested with the exception of isolates from sheep 1557. None of the isolates obtained at sacrifice from the five sheep with clinical visna escaped neutralization with autologous antisera. These data suggest that although variant viruses are encountered at considerable frequency during long-term infection of Icelandic sheep, the variants usually do not replace the infecting strain. Antigenic drift does not appear to be essential for virus persistence or for the development of clinically evident CNS lesions.
Animals↗
Primary demyelination in visna. An ultrastructural study of Icelandic sheep with clinical signs following experimental infection.
Two Icelandic sheep with clinical signs of visna appearing 6-7 years after intracerebral infection with visna virus were killed, fixed by perfusion and the central nervous system lesions examined by light and electron microscopy. Both sheep showed similar pathological changes. In the brain there was a severe periventricular inflammatory process with small foci of liquefaction necrosis and scattered small granulomas. In some areas of inflammation there was evidence of primary demyelination but it was not prominent. In the spinal cord there were focal plaques of primary demyelination. At the ultrastructural level the spinal cord lesions showed unambiguous primary demyelination with many naked axons; various stages of remyelination with peripheral type of myelin were also common. These observations indicate that the CNS lesions of visna, as seen in Icelandic sheep, fall into two categories: (a) an inflammatory process which often begins within weeks of infection and which occurs in the majority of infected animals in the absence of clinical paresis; and (b) focal demyelinating lesions of the spinal cord which are seen in sheep with clinical paresis but are uncommon in animals prior to onset of clinical signs. Both types of lesions may coexist.
Animals↗
Cerebrospinal fluid immunoglobulins in sheep with visna, a slow virus infection of the central nervous system.
Icelandic sheep were injected intracerebrally with visna virus, which produces a persistent infection of the CNS accompanied by encephalomyelitis and focal demyelinating lesions. Studies were conducted on two groups of sheep, with short-term infections (25 sheep sampled 1-3 months after infection) and long-term infections (14 sheep sampled 5-6 years after infection). Quantitative determination of CSF immunoglobulin levels 5 years after infection indicated that IgM concentration was usually elevated, IgG2 was occasionally elevated and IgG1 was rarely elevated. CSF oligoclonal bands were seen in about half the sheep examined 5 years after infection. There was a correlation between high titers of CSF antiviral antibody and both elevated CSF IgM concentration and CSF oligoclonal bands. Serum/CSF IgG1 ratios indicated that the blood-brain barrier was apparently intact in long-term visna infection, consistent with intrathecal synthesis of IgM and of antiviral antibody. The alterations in CSF immunoglobulins in visna resemble those found in other persistent CNS virus infections and in multiple sclerosis.
Animals↗
The effect of post-infection immunization on the severity of experimental visna.
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Animals↗