Role of brain microglial cells in neuropathogenesis of Maedi-visna virus infection

Abstract

A prominent feature of lentivirus infections is involvement of the central nervous system (CNS).The ovine Maedi-Visna Virus (MVV) is the prototypic lentivirus, which like all other known lentiviruses, results in infection of the CNS with protracted neuropathology.The complications in deciphering the mechanisms of lentivirus-induced neuropathology arise because of discordance between extent of pathological changes and scarcity of virus antigen-positive cells in the CNS.These and other observations have lent support to the indirect bystander damage hypothesis, whereby released soluble factors of host and/or viral origin may result in extensive tissue damage.Brain macrophages/microglia are frequently viral antigen-positive in lentivirus-infected neural tissues.Primary ovine glial cultures were set up to study the cellular events that may contribute to MW-initiated neuropathology.The ovine glial cells showed both morphological as well as phenotypic similarities to glial cells from other species. The role of microglial cells in visna neuropathology was studied further by use of highly enriched (purity > 98%) primary cultures.Ovine microglial cells showed extensive phenotypic similarities with cells of the monocyte/macrophage lineage.However, they also exhibited distinct morphological characteristics.Further, ovine microglial cultures could be productively infected with MTVV in vitro.This was demonstrated by a combination of immunocytochemical and molecular assays.Assessment of ovine cytokine production in MW-infected microglial cells in vitro demonstrated significant increases in the message RNA levels of pro-inflammatory cytokines tumour necrosis factor-a and interleukin-6.Intrathecal infection experiments carried out in immunosuppressed animals had previously demonstrated an association between early visna lesions and the virus dose. Severe combined immunodeficient (SCID) mice were used to assess whether MVV-infected microglial cells could contribute to neuropathological changes commonly associated with lentivirus infections and independent of specific cellular immune responses. Intrastriatal injection of MVVinfected ovine microglial cells into SCID mice resulted in significantly increased acute gliosis in vivo.The neurotoxicity of MVV-infected microglial cells in visna neuropathology was also demonstrated by in vitro cultures of murine neuronal cells.The supernatants from ovine microglial cells infected with MVV in vitro contained factors, which resulted in both morphological changes and loss of murine trigeminal cultures. Similar observations were made with a synthetic peptide homologue encoded by the MVV regulatory protein Tat.The findings in this thesis demonstrate that infection of ovine microglial cells with MVV can result in release of soluble factors, which may contribute to pathological changes associated with lentiviral infection of the CNS.