Neuroinvasion by ovine lentivirus in infected sheep mediated by inflammatory cells associated with experimental allergic encephalomyelitis.

Maedi Visna Virus (MVV) is a prototypic lentivirus that causes infection only in cells of macrophage lineage, unlike the primate lentiviruses which infect both CD4+ T lymphocytes and macrophages. In primates, the earliest viral invasion is associated with the ability of the virus to infect and activate T cells which convey virus to the brain. Infected monocytes in blood rarely cause CNS infection in absence of activation of CD4+ T cells. In the face of lack of infection or activation of T cells by MVV in sheep, the question arises, how does MVV gain access to the brain to cause the classical lesions of visna? In previous studies on experimental induction of visna, sheep were inoculated with virus directly in the brain. In this study, we asked whether neuroinvasion by MVV would occur if sheep were inoculated with virus in a non-neural site. Nine sheep were inoculated intratracheally and all developed systemic infection when examined 3 weeks later. At this time, five were injected intramuscularly with brain white matter homogenized in Freund's complete adjuvant to induce EAE. None of the four animals inoculated with virus alone developed CNS infection despite typical lentiviral infection in lungs, lymphoid tissues and blood-borne mononuclear cells. In contrast, all five of the sheep injected with brain homogenate developed infection in the brain. Virus was produced by macrophages associated with the EAE lesions. This study illustrated that both activated T cells specific for antigen in the CNS and infected macrophages are essential for lentivirus neuropathogenesis.