Circulating cytotoxic T lymphocyte precursors in maedi-visna virus-infected sheep.

Maedi-visna virus (MVV)-specific cytotoxic T lymphocytes (CTL) were detected, after in vitro culture with MVV antigen and recombinant human interleukin-2, in the efferent lymph and peripheral blood of sheep chronically infected with MVV. Cytotoxicity was mediated by CD8+ lymphocytes and was specific for particular strains of MVV. These precursor CTL were detected in the blood between day 23 and day 100 after infection via the skin. In one out of seven persistently infected sheep MVV-specific cytotoxicity was seen in uncultured peripheral blood cells. Again the effector population consisted of CD8+ lymphocytes. The only other viral infections in which CTL have been detected in peripheral blood mononuclear cells prior to secondary stimulation are those caused by the simian and human immunodeficiency viruses.

The biology of maedi-visna virus--an overview.

This review aims to summarize the current understanding of the biology of maedi-visna virus (MVV), the prototype virus of the family lentivirinae. The paper provides a short overview of the historical background to the discovery of MVV. Detailed descriptions of the structure and organization of the MVV genome and of the virion encoded polypeptides are given and the MVV life cycle in vitro and in vivo are compared and contrasted and the tropism of the virus discussed. The clinical consequences of infection are considered and the mode of transmission, immune response to the virus and possible mechanisms of pathogenesis are discussed.

Pathogenesis of lymphoid interstitial pneumonia in natural and experimental ovine lentivirus infection.

Ovine lentivirus (OvLV), as a member of the lentivirinae subfamily of Retroviridae, shares morphological, genomic, and cytopathic features with human immunodeficiency virus (HIV). Although OvLV infection does not induce profound immune deficiency in sheep, it has many similarities with HIV infection, such as the capacity to infect macrophages, undergo antigenic variation in vivo, and induce slow progressive diseases involving the pulmonary, lymphoid, and central nervous systems. Studies of the pathogenesis of disease in sheep naturally or experimentally infected by OvLV are providing clues to the pathogenesis of HIV infection, including the significance of viral load, the emergence of cytopathic variants, the mechanisms and significance of viral antigenic variation, and viral neutralization, and mechanisms of lymphoproliferation and tissue destruction induced by the virus. Preliminary evidence suggests that infection by other microbial agents, including Mycoplasma species, may play a cofactor role in the pathogenesis of lentivirus-associated lymphoid interstitial pneumonia in sheep, but further studies are required to address this issue.

Ovine lentivirus is macrophagetropic and does not replicate productively in T lymphocytes.

The lentiviruses of sheep, goats, and horses cause chronic multiorgan disease in which macrophages are highly permissive for viral replication. Monocytes, which mature into macrophages, are thought to be latently infected with lentivirus, but the extent to which other leukocytes are infected is unknown. Dendritic cells have not been studied separately from monocytes and T-cell subsets have not been examined in previous attempts to identify infected cells in peripheral blood mononuclear cells (PBMC). We found no evidence of T-cell tropism using an animal-passaged, pathogenic ovine lentivirus. Phytohemagglutinin-stimulated infectious PBMC produced 20-fold less virus than differentiated macrophages, and cocultivation of infectious PBMC with fresh, uninfected phytohemagglutinin blasts did not facilitate virus replication. Furthermore, central lymph cells, the best in vivo source of purified lymphocytes, lacked virus and did not yield virus upon in vitro cultivation. In contrast, cultivated blood-derived macrophages were highly permissive for viral replication. To identify the latently infected PBMC, PBMC from infected sheep were selectively depleted of monocytes and B cells by passage over nylon wool and then of nonadherent cells bearing CD4, CD8, T19, gamma delta T-cell receptor, CD45RA, or major histocompatibility complex class II antigens by panning. Removal of adherent monocytes and B cells or of adherent cells and the three major T-cell subsets (CD4+, CD8+, T19+) did not decrease the infectivity of PBMC. The richest sources of infected cells in fresh PBMC were CD45RA+ and major histocompatibility complex class II+ nonadherent cells, which are three characteristics of dendritic cells. Thus, the dendritic cell, and not the monocyte or the CD4+ cell, is probably the predominant infected cell type in blood.

Nucleotide sequence of EV1, a British isolate of maedi-visna virus.

We have isolated a maedi-visna-like virus from the peripheral blood mononuclear cells of a British sheep displaying symptoms of arthritis and pneumonia. After brief passage in fibroblasts this virus (designated EV1) was used to infect choroid plexus cells. cDNA clones of the virus were prepared from these cells and sequenced. Gaps between non-overlapping clones were filled using gene amplification by the polymerase chain reaction. The genome structure is similar to that described for visna virus strain 1514, and differs from that described for visna virus strain SA-OMVV in not having a W reading frame. Overall the genome differs by about 20% between each of these strains, but there is fivefold variation in the amount of divergence of derived amino acid sequences of different open reading frames. Two sequenced EV1 clones each contain only one copy of the 43 bp repeat, with paired AP-1 sites, which is a feature of other ruminant lentiviral long terminal repeats (LTRs). However, analysis of viral DNA in infected cells by gene amplification shows that LTRs with two repeats do occur, albeit at a relatively low frequency.

In situ amplification of visna virus DNA in tissue sections reveals a reservoir of latently infected cells.

Maedi and visna are, respectively, the pulmonary and neurological manifestations of slowly progressive infections of sheep caused by retroviruses of the lentivirus subfamily. Lentivirus infections are also persistent infections in which host defenses are generally not successful in eliminating the infectious agent because of restricted viral gene expression in many infected cells. In this report, we describe a method for amplifying and detecting viral DNA in tissue sections which has made it possible to verify experimentally the postulated existence of this reservoir of latently infected cells, as well as to estimate the actual number of cells which harbor viral genomes in infected tissues. In the discussion, we present a simple mathematical model that relates this number to the rate at which inflammatory lesions develop. This model can account for both the slow progression of natural infections and for the rapid accumulation of inflammatory foci in the high dosage experimental system analysed in our studies.

[Development of lymphocyte subsets in milk from ewes excreting Maedi virus]. / Evolution des sous-populations lymphocytaires dans le lait de brebis au moment de l'excrétion du virus Maedi.

Milk is the most important route of lentivirus spread in sheep and goat. Blood and milk cell populations were characterised with specific monoclonal antibody at lambing time in 4 primiparous seropositive ewes. Two of the ewes (3/4 half udders) produced milk with infected cells. The cell number/ml was always higher in milk and blood from virus-producing animals. In ewes which spread infected macrophages CD8 lymphocyte number was increased in blood and milk. Serological tests are able to detect infected animals but virus production is reflected immediately by an increase of CD8 lymphocytes in milk.

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.

Survey for antibodies against maedi-visna in sheep in Poland.

Eighteen flocks of sheep were serologically tested for antibody to maedi-visna virus. A total of 4284 serum samples were tested by agar gel diffusion test and 1015 (24%) were found positive. Prevalence of infection increased with the age of sheep. Within age groups, the lowest prevalence was in the group at 1 year or less (7%) and gradually increased to the highest prevalence at 5 years of older (52%). A difference in prevalence of infection among breeds and sexes was seen but not proven because of inadequate numbers. All flocks tested were infected with a range of serological prevalence from 1.2% to 45.9%.

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.

Ovine progressive pneumonia (maedi-visna) in sheep.

Ovine progressive pneumonia (OPP) is a multi-systemic disease of sheep caused by a nononcogenic exogenous retrovirus belonging to the Lentiviridae subfamily. Characteristics of the disease are chronic lymphocytic pneumonitis, encephalitis, arthritis, mastitis and vasculitis associated with progressive wasting, dyspnea, lameness, indurated udder and, rarely, paralysis. Any one or all of the characteristics may be manifest. Transmission of the virus is predominantly through the colostrum to newborn lambs, however, transmission can occur by contact and in utero. Treatment of the disease is only symptomatic and prevention of infection is only by avoiding the virus.

Spontaneous interferon production by pulmonary leukocytes is associated with lentivirus-induced lymphoid interstitial pneumonia.

Ovine lentiviruses share genome sequence, structural features, and replicative mechanisms with HIV, the etiologic agent of AIDS. A lamb model of lentivirus-induced lymphoid interstitial pneumonia, comparable to lymphoid interstitial pneumonia associated with pediatric AIDS, was used to investigate production of leukocyte-soluble mediators. Lentivirus-infected lambs and adult sheep with severe lymphoid interstitial pneumonia had significantly elevated levels of spontaneous interferon (IFN) production from pulmonary leukocytes compared with ovine lentiviruses-infected animals with mild or no lesions of lymphoid interstitial pneumonia or non-infected controls. However, peripheral blood mononuclear cells from lentivirus-infected lambs did not spontaneously release significant amounts of IFN. IFN production by pulmonary lymph node lymphocytes was enhanced in the presence of lentivirus-infected alveolar macrophages. Animals with lentivirus-induced disease and spontaneous IFN production had enhanced virus replication within tissues. The ovine lentiviruses-induced IFN had a m.w. of between 25,000 and 35,000 and was resistant to freeze/thawing procedures. The IFN activity was sensitive to trypsin and stable to low pH and heat. IFN with similar physical and biochemical properties was produced when ovine lentiviruses was added to control leukocyte cultures. IL-2 and PGE2 production and responses to mitogen by pulmonary lymph node lymphocytes of lentivirus-diseased lambs were not statistically different from control animals. Increased local production of IFN in lentivirus-infected host tissues may serve to accelerate the entry of leukocytes into virus-induced lesions promoting cell-mediated tissue damage and also provide increased numbers of cells for virus replication.

Lesions and retroviruses associated with naturally occurring ovine pulmonary carcinoma (sheep pulmonary adenomatosis).

Five sheep with ovine pulmonary carcinoma were markedly dyspneic and had sporadic coughing; two had copious watery nasal exudate. In four, lesions consisted of multifocal nodules of neoplastic cuboidal epithelial cells in acinar or papillary patterns. Electron microscopically, cells had microvilli, tight junctions, and cytoplasmic lamellar bodies typical of alveolar type II cells. One sheep had a single lung tumor of nonciliated bronchiolar epithelial cells. Vacuolated alveolar macrophages surrounded adenomatous foci. One sheep had a metastatic lesion in the caudal mediastinal lymph node. All sheep had histologic lesions of lymphoid interstitial pneumonia (LIP, ovine progressive pneumonia) consisting of peribronchiolar and interstitial lymphoid hyperplasia, and fibromuscular proliferation; all had serum precipitating antibodies to ovine lentivirus. Lung fluids or tumor homogenates contained a 26-kd peptide that crossreacted with a primate-derived type D retrovirus as detected by immunoblotting or interspecies competition radioimmunoassay. Ovine lentivirus was isolated from concentrated lung fluids or tumor tissues of four sheep tested and from tumor cell DNA of one animal transfected into ovine muscle cells. These studies document the presence of type D-related retrovirus antigen in ovine pulmonary carcinoma (OPC) in the United States and indicate that lentivirus-induced LIP is a lesion frequently associated with this disease.