Discussion DISCUSSION

Discussion

DISCUSSION

The results of this study confirmed the presence of a non-purulent encephalitis in natural infections of the three viral diseases considered. In all cases, the lesions were characterized by mononuclear perivascular cuffs, hallmark of the inflammatory response in the brain (Summers et al., 1995). However, the three diseases showed differences in the other histopathological changes studied, such as neuronal necrosis. These differences could suggest that the diseases examined have a different pathogenesis of the nervous lesions. Louping-ill is a neuronotropic virus, and it is supposed to replicate only in neurons (Kruger and Reid, 1994) while Maedi Visna virus replicates mainly in macrophages/microglia lineage cells (Gendelman et al., 1986; Ebrahimi et al. 2000).

Louping-ill virus replicates inside the cell and the virus replication is supposed to be the main cause of neuronal damage and thus of the death of the animal (Doherty et al., 1972). In our study, histopathological changes in the neurons were more frequent in the brain stem, which agrees with previous reports, in either experimental or natural cases (Doherty and Reid, 1971; Krueger and Reid, 1994). However, the inflammatory lesions were present throughout the brain, with no clear preponderance in any area, which suggests that the presence and the severity of the inflammation is not dependent on the neuronal damage, and that possibly immunopathological mechanisms other than or in addition to the direct effect of the virus are involved in the pathogenesis of the encephalitis. Previous experimental infections of this disease have shown that the extent of perivascular cuffing is inversely proportional to the number of neurons

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containing viral antigen, suggesting that the presence of sensitised cells is not essential for the maintenance of the inflammatory response in the brain (Doherty and Vantsis, 1973: Reid et al., 1980). Moreover, infection of immunocompromised lambs with Louping-ill virus (Reid et al., 1980) or mice with tick borne encephalitis virus (Ruzek et al., 2009) have shown that the cellular immunoresponse develops in the brain, and more precisely that the response carried out by CD8+ cells has a detrimental effect in the host, decreasing its survival time. However, in the total absence of an immune response, tick-borne encephalitis virus on its own is also capable to determine encephalitis and the death of the host (Ruzek et al., 2009), therefore a dual pathogenesis is likely to exist, thus either a direct damage on the neurons by the virus or the immunopathogenic response could be involved in the development of Louping-ill encephalitis.

In the two Borna cases studied, the presence of diffuse gliosis and the neuronal damage (denoted by neuronal necrosis and neuronophagia) had a similar distribution to the inflammatory changes. This suggests a close relation between the inflammatory reaction and the neuronal damage or the presence of the virus in the neuroparenchyma. In previous studies on BDV it has been suggested that the inflammatory reaction follows the neuronal changes, and that the inflammation is possibly triggered by the expression of viral antigen in the neurons (reviewed by Gosztony and Ludwig, 1995).

The main finding in the Visna cases studied was a diffuse infiltration of the neuroparenchyma by lymphocytes, macrophages and glial cells, together with thick mononuclear perivascular cuffs. The development of the lesions in the brain is triggered by the presence of the virus in the neuroparenchyma (Georgsson et al., 1989). The perivascular cuffs are the earliest inflammatory changes induced by the virus, as shown in

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experimental lentivirus encephalitides (Ryan et al., 2005). This occurs because the perivascular space constitutes the first compartment of the brain reached by lentiviruses (Peluso et al., 1985), which enter into CNS through the blood-brain barrier (Chebloune et al., 1998). The presence of a diffuse infiltrate in the neuropile in the cases studied, also observed in previous natural and experimental infections with MVV (Sigurdsson et

al.,1962; Georgsson et al., 1990), could suggest a spread of the inflammation

beyond the perivascular space. Previous studies have demonstrated that in other lentiviral encephalitis the virus can spread through the neuroparenchyma (Mankowski et al., 1997), thus sensitized lymphocytes could migrate into the brain, following the presence of the virus. Several studies have shown that the amount of viral antigen related to the inflammatory lesions is very scarce, suggesting that an immunopathological mechanisms could be involved in the pathogenesis of the lesions, more than direct effect of the virus (Zink et al., 1991).

In this work a different distribution of the lesions was noted in the three diseases studied. Although all these viruses affect the brain, they reach it through different routes: blood in Louping-ill and Visna and axons in Borna (Summers et al., 1995). Different routes to entry into the brain could determine a different distribution of the lesions, even in infections in the same host species with the same virus, as shown in experimental infections of Louping-ill (Sheahan et al., 2002). In Borna, the fact that olfactory and limbic structures are usually involved is congruent with the hypothesis that the agent enters the CNS through the nasal neuroepithelium (Gosztonyi et

al., 2008). The areas affected in the Visna and in the Louping-ill cases were

also different, despite both being blood-borne viruses, and this could be explained by the different cellular tropism shown by these agents. A

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regional distribution of the lesions could be due to differences in local factors, such as neuroregulatory neuroenvoirement (Phillips et al., 1999), blood brain barrier permeability to cytokines (Pan et al., 1997) or leukocyte trafficking (Phillips and Lampson, 1999) among the different CNS regions. These differences could allow a more successful presentation of the antigens in certain areas, leading to more severe inflammatory changes. Previous studies have shown that in Borna and Louping-ill the age of the animals could determine variation in the severity and in the distribution of the lesions (Doherty, 1969, review by Gosztonyi and Ludwig, 1995). When discussing the different regional distribution of the lesions in the Louping-ill and in the Borna cases, the elective vulnerability of certain neuronal populations to neurodegenerative disorders, such as a BDV-induced apoptosis of specific neurons, should be considered (Weissenbock et al., 2000). According to Gosztonyi et al. (2008), BDV only enters neurons that possess receptors of the glutamate receptor family; in fact its distribution is intimately related to certain constituents of the glutamatergic system. The widespread distribution of BDV infection within the CNS supports this postulation. This elementary affinity of BDV to the glutamate system could also explain its widespread distribution among vertebrates (Gosztonyi et al., 2008). In Louping-ill, neuronal necrosis and neuronophagia are most prominent in the motor neurons (Doherty et al., 1971a), which is congruent with our findings regarding the distribution of the neuronal damage in this disease, which shows a clear distribution in areas rich in motor neurons. However, most probably, a combination of several factors is responsible for the differences observed in the regional distribution of lesions in these diseases.