Lentiviruses such as Maedi-Visna Virus (MVV) in sheep and Human
Immunodeficiency Virus (HIV) often cause a variety of neurological symptoms in
later stages of infection. In sheep, MVV infection results in paralysis and ataxia,
with post-mortem investigations revealing astrocytosis, demyelination and axonal
damage. At present it is unclear how such neurodegeneration is mediated as MVV,
like HIV, does not directly infect neurons. There is much debate as to the
mechanisms involved in lentivirus related brain damage, centred around three main
possibilities. Firstly, there may be a direct detrimental action of a viral fragment on
neuronal cell types. Secondly, MVV infected cells may be releasing diffusible
mediators such as cytokines which could cause neural damage or act on astrocytes
and microglia causing them to produce factors to induce neurodegeneration. Thirdly,
the prospect of an immune response occuring within the brain, involving infected
macrophages or microglia, is highly probable.In order to investigate the possible mechanisms at work in lentivirus induced
neurodegeneration, several sets of experiments were carried out. Firstly, a putative
model of MVV encephalopathy was established in severe combined immunodeficient
(,scid) mice. Scid mice lack T and B cells and are thus unable to mount any immune
response. Blood derived macrophages were obtained from healthy sheep and
infected with MVV in vitro. These were intracerebrally injected into scid mice, as
were uninfected macrophages, in order to assess the occurrence of any neural cell
damage. Cell preparations of peripheral blood mononuclear cells (PBMCs),
uninfected, supplemented with virus, or obtained from productively MVV infected
sheep, were also administered to scid mice in the same manner, to evaluate the role
of immune cells in causing any neurodegeneration. The results show that the
administration of MVV infected macrophages produces greater gliosis and astrocytosis in the brains of these mice as compared to those injected with control
uninfected macrophages or vehicle. However, the injections of PBMCs did not cause
any detectable differences in astrocytes and microglia between the groups. No
neuronal loss or demyelination was evident with any of the injections. These results
suggest that neural cell disruption in MVV encephalopathy may be due to the
presence of MVV infected macrophages in the Central Nervous System (CNS) of
infected sheep, acting indirectly through the release of diffusible mediators.To investigate further the underlying causes of these changes, the possible
detrimental contribution of cytokines in this model was evaluated through the use of
the reverse transcriptase-polymerase chain reaction (RT-PCR) of MVV infected and
uninfected blood derived macrophages in vitro. The results show an increase in the
levels of the pro-inflammatory cytokine interleukin-1 beta (TL-113) mRNA in MVV
infected macrophages. Interpretation of this model therefore suggests that MVV
exerts an indirect disruptive effect on neural cells via the release of pro-inflammatory
cytokines from infected macrophages, causing microgliosis and astrocytosis which in
turn lead to major disturbances of brain function.The possibility of a neurotoxic action of a viral product was also investigated.
Peptides derived from the basic region of the transactivating protein Tat from both
MVV and HIV have previously been shown to be lethal to neurons in vivo and in
vitro, through a mechanism believed to involve Nitric Oxide (NO)-mediated
glutamate neurotoxicity and cytokines. The actions of the MVV tat peptide when
injected intracerebrally in rats are diminished one week post-operatively by the Nmethyl-D-aspartate (NMDA) receptor antagonist (+)-5-methyl-10,ll-dihydro-5Hdibenzo[a,d]cyclohepten-5,10-imine maleate (MK801) and the Nitric Oxide Synthase
(NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME). Here, its short term
effects were examined and its neurotoxic mechanisms probed through the use of in
vivo stereotaxic injections and pharmacologic manipulations. The results show that
the MVV and HIV tat peptides were very rapidly neurotoxic, causing neuronal cell
death within 0.5 hours, and displayed a distinctive and unusual lesion profile.
Changes in astrocytes and microglia were also observed. The acute effects of the
MVV tat peptide were blocked by MK801, an NMDA receptor antagonist. However, the administration of L-NAME, a NOS inhibitor, alpha melanocyte stimulating
hormone (aMSH), a TNFa inhibitor, or 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo-
(F)-quinoxaline (NBQX), a non-NMDA glutamate receptor antagonist, did not
reduce the volume of the lesion. This suggests that the MVV tat peptide acts at the
NMDA receptor to cause cell death either directly or via an excitotoxic mechanism
