Early events in the experimental interstitial lung disease induced in sheep by the Visna-maedi virus

Interstitial lung disease associated with Equine Infectious Anemia Virus infection in horses

EIA (Equine Infectious Anemia) is a blood-borne disease primarily transmitted by haematophagous insects or needle punctures. Other routes of transmission have been poorly explored. We evaluated the potential of EIAV (Equine Infectious Anemia Virus) to induce pulmonary lesions in naturally infected equids. Lungs from 77 EIAV seropositive horses have been collected in Romania and France. Three types of lesions have been scored on paraffin-embedded lungs: lymphocyte infiltration, bronchiolar inflammation, and thickness of the alveolar septa. Expression of the p26 EIAV capsid (CA) protein has been evaluated by immunostaining. Compared to EIAV-negative horses, 52% of the EIAV-positive horses displayed a mild inflammation around the bronchioles, 22% had a moderate inflammation with inflammatory cells inside the wall and epithelial bronchiolar hyperplasia and 6.5% had a moderate to severe inflammation, with destruction of the bronchiolar epithelium and accumulation of smooth muscle cells within the pulmonary parenchyma. Changes in the thickness of the alveolar septa were also present. Expression of EIAV capsid has been evidenced in macrophages, endothelial as well as in alveolar and bronchiolar epithelial cells, as determined by their morphology and localization. To summarize, we found lesions of interstitial lung disease similar to that observed during other lentiviral infections such as FIV in cats, SRLV in sheep and goats or HIV in children. The presence of EIAV capsid in lung epithelial cells suggests that EIAV might be responsible for the broncho-interstitial damages observed.

Role of alveolar macrophages in respiratory transmission of visna/maedi virus

A major route of transmission of Visna/maedi virus (VMV), an ovine lentivirus, is thought to be via the respiratory tract, by inhalation of either cell-free or cell-associated virus. In previous studies, we have shown that infection via the lower respiratory tract is much more efficient than via upper respiratory tissues (T. N. McNeilly, P. Tennant, L. Lujan, M. Perez, and G. D. Harkiss, J. Gen. Virol. 88:670-679, 2007). Alveolar macrophages (AMs) are prime candidates for the initial uptake of virus in the lower lung, given their in vivo tropism for VMV, abundant numbers, location within the airways, and role in VMV-induced inflammation. Furthermore, AMs are the most likely cell type involved in the transmission of cell-associated virus. In this study, we use an experimental in vivo infection model that allowed the infection of specific segments of the ovine lung. We demonstrate that resident AMs are capable of VMV uptake in vivo and that this infection is associated with a specific up-regulation of AM granulocyte-macrophage colony-stimulating factor mRNA expression (P < 0.05) and an increase in bronchoalveolar lymphocyte numbers (P < 0.05), but not a generalized inflammatory response 7 days postinfection. We also demonstrate that both autologous and heterologous VMV-infected AMs are capable of transmitting virus after lower, but not upper, respiratory tract instillation and that this transfer of virus appears not to involve the direct migration of virus-infected AMs from the airspace. These results suggest that virus is transferred from AMs into the body via an intermediate route. The results also suggest that the inhalation of infected AMs represents an additional mechanism of virus transmission.

Presence of a unique parainfluenza virus 3 strain identified by RT-PCR in visna-maedi virus infected sheep

The presence in farm sheep of a paramyxovirus closely related to parainfluenza virus type 3 (Pi3) from humans or cattle was confirmed using RT-PCR on RNA samples from lung cells from slaughtered animals. Sequencing and restriction enzyme patterns of the amplified fragment of the F gene confirms the distinctness of the isolate, and suitable PCR primers allow specific detection of the ovine virus. A study of the incidence of ovine Pi3 in samples from sheep with or without distinctive histopathological signs of maedi shows that it is uncommon in aged sheep with overt lentiviral disease, but it occurs at moderate frequency in lambs and may, in the presence of visna-maedi virus, contribute to early lesion formation.

Visna-maedi virus-induced expression of interleukin-8 gene in sheep alveolar cells following experimental in vitro and in vivo infection

Visna-maedi virus is a lentivirus which causes inflammatory disorders in sheep, including a chronic interstitial lung disease resembling that observed in human immunodeficiency virus type 1 (HIV 1) infection. In view of our previous demonstration of the production of neutrophil chemotactic activity by alveolar macrophages, and given the lymphocytic and neutrophilic nature of the alveolar cell infiltrate in both naturally and experimentally infected animals, we hypothesized that interleukin-8 (IL8) could be a candidate for at least part of the chemotactic activity we described. In this study, we investigated IL8 mRNA expression following visna-maedi virus infection. Northern analysis of total RNA using an ovine IL8-specific probe demonstrated that the IL8 gene is upregulated in alveolar macrophages as a consequence of in vitro infection and in alveolar cells from experimentally infected animals. Using a semi-quantitative RT-PCR method, we showed that various levels of IL8 mRNA are expressed by alveolar cells from infected animals and that they correlate with the intensity of the lesions. In conclusion, visna-maedi virus is able to induce IL8 mRNA expression in sheep alveolar cells. Results from in vivo infected animals suggest that IL8 could play a role in the early build-up of visna-maedi virus-induced lesions.

Increased expression of tissue factor mRNA and procoagulant activity in ovine lentivirus-infected alveolar macrophages

To link ovine lentivirus infection to lung tissue damage, we studied the procoagulant response in alveolar macrophages from experimentally infected lambs and in in vitro infected alveolar macrophages. We cloned ovine tissue factor cDNA and analysed its in vitro expression by Northern blotting. Visna-maedi virus induced tissue factor mRNA. In order to correlate this mRNA induction with its cellular function, we analysed macrophage procoagulant activity after in vitro and in vivo infection. The procoagulant activity was increased by interaction with the virus in both cases. Thus, visna-maedi virus-induced expression of tissue factor mRNA was associated with enhanced macrophage procoagulant activity. These findings indicate an active role of alveolar macrophages in the pathogenesis of these inflammatory lung lesions.