We are conducting studies to examine the role of human viruses in CNS disease, focusing on the role of human herpesvirus 6. HHV-6 is associated with a variety of neurologic diseases including multiple sclerosis, encephalitis, epilepsy, and brain cancer. The virus is ubiquitous and has two variants HHV-6A and HHV-6B. Exposure to HHV-6B occurs at a young age and is the etiologic agent of Roseola. Given the ubiquitous nature of the virus and the early exposure time, it is difficult to prove causation of neurologic disease by the virus. The generation of an animal model of HHV-6 infection would allow studies on the potential of this virus to cause neurologic disease. We initially inoculated marmosets with HHV-6A, HHV-6B, or vehicle control intravenously and followed disease development clinically, radiologically, and immunologically. Marmosets were exposed to virus every 30 days for a total of 4 exposures. None of the virus inoculated animals showed signs of a primary infection; however shortly after the second exposure HHV-6A inoculated marmosets developed signs of neurologic disease. The neurologic disease was seen in 3 of 4 exposed marmosets and was characterized primarily by sensory deficits. We saw no changes in MRI in the brains of affected marmosets during the initial disease development. We were able to observe transient areas of hyperintensity in the corpus callosum on T2 weighted images 25 weeks post-inoculation in one marmoset. Marmosets exposed to HHV-6A generated a rapid antibody response to HHV-6 generating both IgM and IgG antibodies against the virus. This was not seen in the HHV-6B inoculated marmosets only 2 of the 4 animals generated HHV-6 specific IgG antibodies and no IgM antibodies were detected. We detected very little HHV-6 DNA, using nested PCR, in the PBMC, plasma, and saliva of virus exposed marmosets. Currently we are developing methods to look at cellular immune responses to HHV-6 in cells from the virus-exposed marmosets. The route of HHV-6 exposure may also influence the disease development seen as a result of viral infection. Human exposure to HHV-6 occurs most likely through mucous membranes, and we have recently shown that the nasal mucous is a reservoir for the virus. To examine a more physiologic route of exposure we inoculated another group of marmosets with HHV-6A intranasally. In this group of marmosets we did not observe signs of neurologic disease or detect HHV-6 specific antibody responses. We do however consistently find HHV-6 DNA in the PBMC, plasma, and saliva of exposed marmosets. Experiments in this cohort of marmosets are ongoing and data is still being collected and analyzed. We intend to continue optimizing and defining the HHV-6 infection model in the marmosets. With this model we have generated a system in which to study independently the biology of the 2 HHV-6 variants. This is an ongoing problem in studying HHV-6 due to the high homology between the variants and early exposure time to HHV-6B. Reagents to measure immune responses to the individual variants are not available and the marmoset infection models provide a system to develop these reagents. The marmoset infection model also provides a system in which to test anti-herpesvirus drugs to improve current anti-viral therapies. We are also beginning experiments to examine whether exposure to HHV-6 can alter the disease pathogenesis of EAE, to look at whether the virus may be a cofactor in the development of CNS autoimmunity. We have successfully established two models of EAE a model in the marmoset. In these models, either recombinant human myelin oligodendrocyte glycoprotein (MOG) or human white matter homogenate is used as the auto-antigen to drive disease. Using these methods we are able to follow clinical and MRI parameters that will allow us to test new disease modifying therapies. Using the 7T MRI, we now have a MRI protocoloptimized to follow lesion development and quantitate lesion loads in affected marmosets. We are currently working to expand the MRI protocol in collaboration with Afonso Silva in NINDS to obtain images of the spinal cord in the marmosets, to better track disease development. We are investigating the radiological correlates of the tissue changes that accompany the formation of inflammatory, demyelinating lesions in the brain the histopathological hall-marks of multiple sclerosis (MS). We are studying a primate model of MS experimental autoimmune encephalomyelitis (EAE) in the common marmoset which develops white matter lesions that bear remarkable immunological and pathological similarity to those seen in MS. On a 7 tesla MRI scanner, we acquired T1 maps and T2-weighted (T2) images every 2 weeks after immunization and weekly after the first lesion appeared. Comparing scans obtained before and after intravenous injection of contrast dye, we estimated changes over time in blood-brain barrier (BBB) permeability within lesion areas and compared the results to postmortem tissue examination. On average, we found that BBB permeability increased 3.5 fold (p<0.0001) over a 4-week period prior to the appearance of a lesion as a focal area of perivenular signal abnormality on T2. BBB permeability gradually decreased after lesion appearance, with changes in the distribution of inflammatory cells (predominantly macrophages and microglia) and demyelination within and around both the lesion itself and the central vessel around which it forms. We also identified small perivascular foci of microglia and T cells without blood-derived macrophages or associated demyelination. These foci had no associated T2 MRI changes, though MRI-estimated BBB permeability within the foci, but not outside of them, became elevated in the weeks before the animal died (p<0.0001). Perivascular inflammatory cuffing and microglial activation have been described in MS tissue samples as well, and our findings in marmoset EAE suggest that these processes precede the development of the focal inflammatory, demyelinated lesions that cause neurological symptoms. Our findings identify the cell types that set the stage for tissue damage in these disorders and identify a temporal window of opportunity for its prevention.
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