Development of transgenic sheep that express the visna virus envelope gene

The ovine lentiviruses cause encephalitis, pneumonia, and arthritis in sheep worldwide. Visna virus is a prototype of this family and the pathogenesis and molecular biology of the virus has been well characterized. The envelope proteins of visna virus are responsible for binding of virus to host cells and for causing cell fusion. The surface glycoprotein also elicits cellular and humoral immune responses to the virus, the former being thought to be responsible for eliminating infected cells as well as causing inflammatory lesions. In this study, transgenic sheep were constructed that expressed the envelope genes of visna virus under the control of the visna LTR to investigate the role of the env gene in the pathogenesis of lentiviral disease in its natural host. Three transgenic lambs were identified that contain the env transgene and express the envelope glycoproteins. These transgenic animals have remained healthy and expression of the viral gene has had no obvious deleterious effect. Expression of the visna envelope protein was demonstrated by cell fusion mediated by the envelope gene as well as by immunoprecipitation of the envelope proteins with monoclonal antibodies and immunofluorescence analyses of Env protein in cells. The target cell for visna virus replication in infected animals is the monocyte/macrophage. In natural infection, the level of viral gene expression in these cells increases with cell maturation. In the transgenic sheep, monocytes did not express the envelope glycoproteins until they differentiated into macrophages in vitro. Expression of the env mRNA in macrophages was quantitated by an RNase protection assay. In addition to expression in macrophages, the transgene was expressed by fibroblasts isolated from skin of the transgenic sheep. Expression of both the Env and Rev proteins was detected by immunoprecipitation and immunofluorescence. Two of the three lambs responded immunologically to the expression of the transgene by producing binding antibodies to the envelope glycoproteins. Thus, these transgenic sheep provide a model to study whether a lentivirus glycoprotein will prevent infection or modulate disease in its natural host after virus challenge.