Immunologic Mechanisms In Experimental Autoimmune Diseases Of The Nervous System

National Institute Of Neurological Disorders And Stroke

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Abstract

We have examined the role of human herpesviruses in CNS disease. In particular, we have focused on the ubiquitous virus HHV-6 (and HHV6B) that is associated with a variety of neurologic diseases including multiple sclerosis, encephalitis, epilepsy, and brain cancer. The generation of an animal model of HHV-6 infection would allow studies on the potential of this virus to cause neurologic disease. Human exposure to HHV-6 occurs most likely through mucous membranes, and we have shown that nasal mucous is a reservoir for the virus. We have successfully infected marmosets intranasally with HHV-6A, to examine a more physiologic route of exposure with the virus that was comparatively more immunogenic and neuropathogenic. As HHV6 is a human virus it cannot infect rodents since they lack the appropriate receptor for virus binding but marmosets are susceptible to HHV6 infection. In contrast to what we observed with the intravenous inoculations, marmosets inoculated intranasally with the HHV-6A strain did not exhibit clinical symptoms, did not mount robust anti-HHV-6 antibody responses, and had a marked increase of detectable virus in the periphery (plasma, PBMC, saliva). These data suggested an inverse correlation between antibody production and circulation of viral DNA in the periphery, and moreover, that the clinical symptoms observed in the intravenously inoculated marmosets may have been tied to the robust antibody responses. With this model, we have generated a system in which to independently study the biology of the two HHV-6 species. This is an ongoing problem in the field, due to the high homology between HHV-6A and HHV-6B, the early exposure time to HHV-6B and the unknown time of exposure to HHV-6A. This also enables us to compare the development of neurologic disease with experimental autoimmune encephalomyelitis (EAE), a well-known established model for MS. EAE is one model of autoimmune inflammatory-mediated CNS demyelination, created by immunizing animals with CNS (usually white matter) peptides or proteins. Although EAE is typically studied in rodents, marmoset EAE presents with greater radiologic and pathologic similarities to MS, reflecting the genetic, immunological, and CNS anatomical proximity of marmosets to humans. EAE in the common marmoset develops cortical and white matter lesions with remarkable immunological and pathological similarity to those seen in MS. We have successfully established EAE in the marmoset, using human white matter homogenate as the antigen(s) to drive inflammatory CNS demyelination, with progressive or relapsing phenotypes. Using these methods of induction, we are able to implement clinical and MRI parameters that will enable us to test new disease modifying therapies. We have demonstrated that intranasal inoculations with HHV-6A and HHV-6B accelerate the MS-like neuroinflammatory disease, EAE. Although those animals inoculated intranasally with HHV-6 (virus/EAE marmosets) were asymptomatic, they exhibited significantly accelerated clinical EAE compared with control animals. In addition, expansion of a proinflammatory CD8+ T cell subsets correlated with post-EAE survival in virus/EAE marmosets, suggesting that a peripheral (viral?) antigen-driven expansion may have occurred post-EAE induction. HHV-6 viral antigen in virus/EAE marmosets was markedly elevated and concentrated in brain lesions, similar to previously reported localizations of HHV-6 in MS brain lesions. Collectively, we demonstrate that asymptomatic intranasal viral acquisition accelerates subsequent neuroinflammation in a nonhuman primate model of MS. We have also characterized the distribution of the 18-kDa translocator protein (TSPO) in marmoset EAE. TSPO is weakly expressed in the healthy CNS but is strongly upregulated in CNS lesions, where it is abundantly expressed in activated microglia and macrophages and consequently of interest as a potential biomarker of neuroinflammation. Numerous studies have shown that TSPO is highly expressed in the inflammatory lesions of multiple sclerosis (MS) patients and animal with experimental models of MS, such as rodents with EAE. We have shown that TSPO was present in the CNS tissues of control non-EAE and EAE-affected marmosets. In non-EAE control tissue, we find TSPO expression in the meninges, ependyma, and approximately half of all Iba1+ microglia, but not neurons or astrocytes. Moreover, we have characterized temporal changes in TSPO expression in acute and chronic lesions. In acute marmoset EAE, we find the constitutive expression of TSPO in the ependyma and meninges persists but is accompanied in a massive increase in TSPO expression by over 90% of all microglia and macrophages in inflammatory lesions. While we find microglia and macrophages to be the principal contributors of TSPO density in these acute lesions, we find their contribution declines in chronic lesions and is associated with a concomitant increase in astrocytic TSPO expression. In chronic lesions, the percentage of microglia and macrophages expressing TSPO declines with time. TSPO expression was also observed in neurons in regions of the cortical gray matter. Finally, we characterize the immunophenotypes of TSPO-expressing microglia/macrophages in healthy and diseased primate brain. In keeping with studies in MS brain tissue, we find TSPO expression in both M1- and M2-biased microglia/macrophages.