The biology of maedi-visna virus--an overview

Differential gene expression and immune cell infiltration in maedi-visna virus-infected lung tissues

BACKGROUND

Maedi-visna virus (MVV) is a lentivirus that infects monocyte/macrophage lineage cells in sheep, goats, and wild ruminants and causes pneumonia, mastitis, arthritis, and encephalitis. The immune response to MVV infection is complex, and a complete understanding of its infection and pathogenesis is lacking. This study investigated the in vivo transcriptomic patterns of lung tissues in sheep exposed to MVV using the RNA sequencing technology.

RESULT

The results indicated that 2,739 genes were significantly differentially expressed, with 1,643 downregulated genes and 1,096 upregulated genes. Many variables that could be unique to MVV infections were discovered. Gene Ontology analysis revealed that a significant proportion of genes was enriched in terms directly related to the immune system and biological responses to viral infections. Kyoto Encyclopedia of Genes and Genomes analysis revealed that the most enriched pathways were related to virus-host cell interactions and inflammatory responses. Numerous immune-related genes, including those encoding several cytokines and interferon regulatory factors, were identified in the protein-protein interaction network of differentially expressed genes (DEGs). The expression of DEGs was evaluated using real-time polymerase chain reaction and western blot analysis. CXCL13, CXCL6, CXCL11, CCR1, CXCL8, CXCL9, CXCL10, TNFSF8, TNFRSF8, IL7R, IFN-γ, CCL2, and MMP9 were upregulated. Immunohistochemical analysis was performed to identify the types of immune cells that infiltrated MVV-infected tissues. B cells, CD4+ and CD8+ T cells, and macrophages were the most prevalent immune cells correlated with MVV infection in the lungs.

CONCLUSION

Overall, the findings of this study provide a comprehensive understanding of the in vivo host response to MVV infection and offer new perspectives on the gene regulatory networks that underlie pathogenesis in natural hosts.

In silico discovery of epitopes of gag and env proteins for the development of a multi-epitope vaccine candidate against Maedi Visna Virus using reverse vaccinology approach

Maedi Visna Virus (MVV) causes a chronic viral disease in sheep. Since there is no specific therapeutic drug that targets MVV, development of a vaccine against the MVV is inevitable. This study aimed to analyze the gag and env proteins as vaccine candidate proteins and to identify epitopes in these proteins. In addition, it was aimed to construct a multi-epitope vaccine candidate. According to the obtained results, the gag protein was detected to be more conserved and had a higher antigenicity value. Also, the number of alpha helix in the secondary structure was higher and transmembrane helices were not detected. Although many B cell and MHC-I/II epitopes were predicted, only 19 of them were detected to have the properties of antigenic, non-allergenic, non-toxic, soluble, and non-hemolytic. Of these epitopes, five were remarkable due to having the highest antigenicity value. However, the final multi-epitope vaccine was constructed with 19 epitopes. A strong affinity was shown between the final multi-epitope vaccine and TLR-2/4. In conclusion, the gag protein was a better antigen. However, both proteins had epitopes with high antigenicity value. Also, the final multi-epitope vaccine construct had a potential to be used as a peptide vaccine due to its immuno-informatics results.

Small ruminant lentivirus-induced arthritis: clinicopathologic findings in sheep infected by a highly replicative SRLV B2 genotype

We describe the clinicopathologic features of an arthritis outbreak in sheep induced by small ruminant lentivirus (SRLV), linked to the presence of a new SRLV isolate phylogenetically assigned to caprine arthritis encephalitis virus-like subgroup B2. Thirteen SRLV seropositive Rasa Aragonesa adult ewes were selected from 5 SRLV highly infected flocks (mean seroprevalence, 90.7%) for presenting uni- or bilateral chronic arthritis in the carpal joint. A complete study was performed, including symptomatology, histopathology, immunocytochemistry, immunohistochemistry, in situ hybridization, and microbiology. The carpus was the joint almost exclusively affected, with 10 sheep (76%) showing a moderate increase in carpal joint size (diameter range, 18-20 cm; normal range, 15-16 cm) without signs of locomotion problems and with 3 ewes (23%) showing severe inflammation with marked increase in diameter (21-24 cm), pain at palpation, and abnormal standing position. Grossly, chronic proliferative arthritis was observed in affected joints characterized by an increased thickness of the synovial capsule and synovial membrane proliferation. Microscopically, synovial membrane inflammation and proliferation and hyperplasia of synoviocytes were observed. More positive cases of SLRV infection were detected by immunocytochemistry of articular fluid than of bronchoalveolar lavage fluid. Immunohistochemistry and in situ hybridization also detected positive cells in the subsynovial connective tissue, lung, mediastinal lymph node, mammary gland, and mammary lymph node. All animals were negative for the presence of Mycoplasma or other bacteria in the articular space. The present outbreak likely represents an adaptation of a caprine virus to sheep. Our results underline the importance of the arthritis induced by SRLV in sheep, a clinical form that might be underestimated.

Ocorrência de anticorpos contra o vírus Maedi-Visna em ovinos Santa Inês, no Estado de Sergipe, Brasil

As Lentiviroses de Pequenos Ruminantes (LVPR) são enfermidades infecciosas de etiologia viral causadas por retrovírus, caracterizadas por uma doença de degeneração progressiva e lenta. O objetivo deste estudo foi determinar a ocorrência de anticorpos contra o lentivírus no rebanho ovino Santa Inês, de Sergipe. Para analisar o número mínimo de amostras utilizou-se o cálculo de Astudillo, com prevalência esperada (p) de 10%, margem de erro (α) de 20% e grau de confiança de 95% (g = 1,96), obtendo-se o número mínimo de amostras (n) de 861. Foram colhidas 941 amostras de soro ovino oriundas de 54 propriedades e 19 municípios distribuídos nas três mesorregiões do Estado, sendo o número de amostras proporcional ao efetivo ovino do município. O soro foi acondicionado a -20°C até a realização do teste de Imunodifusão em Gel de Ágar (IDGA). Os resultados considerados positivos ou suspeitos na IDGA foram reavaliados através da técnica de Western blotting. Dos animais amostrados, 194 (20,62%) eram machos e 747 (79,38%) eram fêmeas. Quanto à estratificação por idade, 300 ovinos eram jovens (31,88%), entre seis meses e um ano, 308 (32,73%) eram jovens adultos, entre 1 e 3 anos, e 333 (35,39%) tinham idade estimada superior a 3 anos. Os resultados obtidos demonstraram que anticorpos contra o vírus Maedi-Visna (MVV) ocorrem no rebanho ovino do estado de Sergipe em baixa frequência (0,11%). Assim, relata-se a ocorrência da infecção por Maedi-Visna em ovinos no estado de Sergipe e ressalta-se a participação do trânsito de animais na epidemiologia da doença.

Identification of the ovine mannose receptor and its possible role in Visna/Maedi virus infection

This study aims to characterize the mannose receptor (MR) gene in sheep and its role in ovine visna/maedi virus (VMV) infection. The deduced amino acid sequence of ovine MR was compatible with a transmembrane protein having a cysteine-rich ricin-type amino-terminal region, a fibronectin type II repeat, eight tandem C-type lectin carbohydrate-recognition domains (CRD), a transmembrane region, and a cytoplasmic carboxy-terminal tail. The ovine and bovine MR sequences were closer to each other compared to human or swine MR. Concanavalin A (ConA) inhibited VMV productive infection, which was restored by mannan totally in ovine skin fibroblasts (OSF) and partially in blood monocyte-derived macrophages (BMDM), suggesting the involvement of mannosylated residues of the VMV ENV protein in the process. ConA impaired also syncytium formation in OSF transfected with an ENV-encoding pN3-plasmid. MR transcripts were found in two common SRLV targets, BMDM and synovial membrane (GSM) cells, but not in OSF. Viral infection of BMDM and especially GSM cells was inhibited by mannan, strongly suggesting that in these cells the MR is an important route of infection involving VMV Env mannosylated residues. Thus, at least three patterns of viral entry into SRLV-target cells can be proposed, involving mainly MR in GSM cells (target in SRLV-induced arthritis), MR in addition to an alternative route in BMDM (target in SRLV infections), and an alternative route excluding MR in OSF (target in cell culture). Different routes of SRLV infection may thus coexist related to the involvement of MR differential expression.

Development and comparison of strain specific gag and pol real-time PCR assays for the detection of Visna/maedi virus

The aim of this study was the development of gag and pol dual labelled probe real-time PCR and RT PCR assays to quantify the proviral load and the transcripts of the British Visna/maedi virus EV1 strain. Primers and probes were chosen based on the consensus sequences of gag and pol clones representative of EV1 genetic variants. Both PCRs had a detection limit of 3 copies of target gene, with a linearity over 6 orders of magnitude. The performances of the two PCRs in vivo were evaluated and compared on a panel of DNAs extracted from blood of sheep infected experimentally with EV1. The pol assay detected in most cases lower numbers of viral molecules than gag assay, yielding some false negative results. The gag real-time RT PCR had a detection limit of 100 RNA molecules with a linearity over 5 orders of magnitude. This did not result in a lower performance of the RT PCR compared to the PCR in cells permissive for virus replication, which contain higher numbers of viral transcripts than proviral genomes. The real-time assays developed in this study, particularly the gag assay, provide a sensitive tool which can be used to quantify the viral load in experimental infections.

Visna/maedi virus serology in sheep: survey, risk factors and implementation of a successful control programme in Aragón (Spain)

A serological survey of Visna/maedi virus (VMV) infection involving 274,048 sheep from 554 flocks was undertaken during 2002-2007 in Aragón, North-East Spain. One hundred and two of these flocks enrolled in a VMV control programme to reduce seroprevalence by selecting replacement lambs from seronegative dams and gradual culling of seropositive sheep. Twenty-five flocks were also visited to collect flock management and housing data. All study flocks had seropositive animals and 52.8% of animals tested were seropositive. Among flocks that joined the control programme 66 adopted the proposed measures and reduced seroprevalence significantly by between 26.1% and 76.9% whereas the remaining 36 flocks did not apply the measures and seroprevalence significantly increased. Seroprevalence increased with flock size and the number of days the sheep were housed, and decreased with increasing weaning age and shed open area, suggesting a reduced risk of VMV infection in sheep associated with better ventilation. At the end of the period, 24 flocks were certified as VMV-controlled with a seroprevalence <5%, and seven as VMV-free with 0% seroprevalence. These are the first officially recognised VMV-free flocks in Spain and represent a nucleus of VMV-free replacement animals for other flocks. Moreover, they are evidence of the possibility of eliminating VMV infection without resorting to whole-flock segregation or culling of seropositive sheep.

Detection of ovine lentivirus in the cumulus cells, but not in the oocytes or follicular fluid, of naturally infected sheep

The aim of this study was to examine the Maedi-Visna virus (MVV) infection status of oocytes, cumulus cells, and follicular fluid taken from 140 ewes from breeding flocks. MVV proviral-DNA and MVV RNA were detected using nested-PCR and RT-PCR MVV gene amplification, respectively in the gag gene. Nested-PCR analysis for MVV proviral-DNA was positive in peripheral blood mononuclear cells in 37.1% (52/140) of ewes and in 44.6% (125/280) of ovarian cortex samples. The examination of samples taken from ovarian follicles demonstrated that 8/280 batches of cumulus cells contained MVV proviral-DNA, whereas none of the 280 batches of oocytes taken from the same ovaries and whose cumulus cells has been removed, was found to be PCR positive. This was confirmed by RT-PCR analysis showing no MVV-viral RNA detection in all batches of oocytes without cumulus cells (0/280) and follicular fluid samples taken from the last 88 ovaries (0/88). The purity of the oocyte fraction and the efficacy of cumulus cell removal from oocytes was proved by absence of granulosa cell-specific mRNA in all batches of oocytes lacking the cumulus cells, using RT-PCR. This is the first demonstration that ewe cumulus cells harbor MVV genome and despite being in contact with these infected-cumulus cells, the oocytes and follicular fluid remain free from infection. In addition, the enzymatic and mechanical procedures we used to remove infected-cumulus cells surrounding the oocytes, are effective to generate MVV free-oocytes from MVV-infected ewes.

Preventing the spread of maedi-visna in sheep through a voluntary control programme in Finland

The sheep disease maedi-visna (MV) was introduced into Finland in 1981 and had spread to eight flocks in the southwestern part of the country when first detected in a survey in 1994. Six more seropositive flocks were subsequently traced, bringing the total to 14. MV has a notifiable disease status in Finland that provides for official restrictive measures to which all infected herds are subject. These measures are withdrawn once the seropositive animals and their progeny are culled and the flock has showed negative signs in the test done twice, or after total culling. A voluntary control programme was initiated in January 1995 to extend official control efforts. The programme furnishes a guideline for culling, restrictions on contacts, and a timetable for testing the flock to attain MV-free status. Seven flocks of the 14 were slaughtered either immediately or after a period under restrictive measures. One flock finished sheep production after four years under restrictive measures. Selective culling and repeated testing was attempted with the other six flocks, three of which attained MV-free status. One flock finished sheep production after two years in the control programme, the other two dropped out of the programme when the restrictive measures were withdrawn. It was concluded that the control programme was salient in eradicating MV from Finland and that serological monitoring of the situation must be continuous.

Infection of dendritic cells by the Maedi-Visna lentivirus

The early stages of lentivirus infection of dendritic cells have been studied in an in vivo model. Maedi-visna virus (MVV) is a natural pathogen of sheep with a tropism for macrophages, but the infection of dendritic cells has not been proven, largely because of the difficulties of definitively distinguishing the two cell types. Afferent lymphatic dendritic cells from sheep have been phenotypically characterized and separated from macrophages. Dendritic cells purified from experimentally infected sheep have been demonstrated not only to carry infectious MVV but also to be hosts of the virus themselves. The results of the in vivo infection experiments are supported by infections of purified afferent lymph dendritic cells in vitro, in which late reverse transcriptase products are demonstrated by PCR. The significance of the infection of afferent lymph dendritic cells is discussed in relation to the initial spread of lentivirus infection and the requirement for CD4 T cells.

Serological survey and epidemiological investigation of maedi-visna in sheep in Finland

A survey for antibodies to maedi-visna virus (MV) in the Finnish sheep surveillance flocks was conducted in 1994. Examination of a total of 12931 serum samples from animals over 1 year of age from 545 flocks (81% of all flocks) revealed eight seropositive flocks and the subsequent epidemiological investigation yielded one additional seropositive flock, indicating a low prevalence of 1.6%. The infection was very probably imported from Sweden in 1981, but it was not detected until the survey was conducted 13 years later. The entire primary infection flock was slaughtered in 1995. 77% of the sheep were seropositive but the animals were clinically healthy and only one (5%) of the contact flocks of the primary infection flock had contracted the infection. This secondary infection flock, 77% of which was seropositive, was slaughtered in 1994; however, animals in this flock had respiratory problems and the lungs of three sheep showed typical MV lesions. Seven (24%) of its contact flocks had contracted the infection and these each had one or two seropositive animals except for one flock which had seven (18%) seropositive animals. The results show that the initial spread of MV can be insidious and wide before infection is revealed in surveys or any clinical cases are encountered.

Maedi-Visna and ovine progressive pneumonia

Maedi-Visna and ovine progressive pneumonia are disease of sheep that are caused by ovine lentivirus and characterized by chronic inflammation of the lungs, mammary glands, joints, and central nervous system. Although tremendous progress in research has led to a better understanding of the pathogenesis of these diseases, many questions still remain. Much of the mystery is the result of the complexity of the ovine lentivirus genome and the intricate interactions of the virus with the host during replication. Discoveries in molecular virology are shedding light on these interactions and novel approaches to prevent and control lentivirus infections are being explored. There is hope that some of these approaches will eventually be used to eradicate these diseases.

Respiratory infections of sheep

Sheep respiratory infections appear as differing clinical syndromes. Mild, acute infections are usually due to parainfluenza 3 (PI3) virus. A mild but chronic respiratory problem in lambs under 1 year old is thought to be caused by Mycoplasma ovipneumoniae probably in association with Pasteurella and PI3. Acute bacterial pneumonia usually results from infection with Pasteurella of biotype A. Infection with PI3 can initiate invasion by Pasteurella. Bordetella parapertussis infection has also been implicated. Serotypes of biotype T P. haemolytica cause an acute septicaemia. Stressful management practices may be a predisposing factor. Chronic proliferative pneumonia results from infection by retroviruses of pulmonary adenomatosis or maedi-visna. Both infections have incubation periods extending into years. The former produces fatal tumorous masses in the lungs. Diagnostic tests are being actively sought. Maedi-visna can present as several clinical problems, frequently as an insidious but fatal proliferative pneumonia.

Genome analysis of North American small ruminant lentiviruses by polymerase chain reaction and restriction enzyme analysis

The polymerase chain reaction (PCR) was used to amplify portions of the gag and env structural genes of 8 ovine and 1 caprine lentivirus isolates of North American origin. Three sets of primers were used to amplify p16, p25, and N'-gp40 gene fragments, and 1 set, annealing highly conserved portions of long terminal repeat (LTR) among small ruminant lentiviruses, was used as a positive control. Variable PCR amplification efficiency was observed. Different stringency conditions of hybridization with specific DNA probes were used to maximize detection of the PCR product. The p25 primers detected all strains, the gp40 primers detected 1 ovine and the caprine strain, and the p16 primers detected only 1 ovine isolate. All strains were detected by LTR primers. Restriction endonuclease analysis of 5 amplified p25 and 2 N'-gp40 gene fragments revealed extensive heterogeneity among these North American small ruminant lentiviruses.