Varicella zoster virus (VZV) infection may activate dormant herpes simplex virus (HSV-1), leading to neuroinflammation and accumulation of Alzheimer's disease (AD)–related proteins in the brain, new research suggests.
"Our results suggest one pathway to Alzheimer's disease, caused by a VZV infection which creates inflammatory triggers that awaken HSV in the brain," lead author Dana Cairns, PhD, research associate, Department of Biomedical Engineering at Tufts University, Boston, Massachusetts, said in a news release.
The findings were published online August 2 in Journal of Alzheimer's Disease.
Previous research has suggested a correlation between HSV-1 and AD and involvement of VZV. However, the sequence of events that the viruses create to set the disease in motion has been unclear.
"We think we now have evidence of those events," co-senior author David Kaplan, PhD, chair of the Department of Biomedical Engineering at Tufts, said in the release.
Working with co-senior author Ruth Itzhaki, PhD, University of Oxford, United Kingdom, the researchers infected human-induced neural stem cells (hiNSCs) and 3D brain tissue models with HSV-1 and/or VZV. Itzhaki was one of the first to hypothesize a connection between herpes virus and AD.
The investigators found that HSV-1 infection of hiNSCs induces amyloid-beta and P-tau accumulation: the main components of AD plaques and neurofibrillary tangles, respectively.
On the other hand, VZV infection of cultured hiNSCs did not lead to amyloid-beta and P-tau accumulation but instead resulted in gliosis and increased levels of proinflammatory cytokines.
"Strikingly," VZV infection of cells quiescently infected with HSV-1 caused reactivation of HSV-1, leading to AD-like changes, including amyloid-beta and P-tau accumulation, the investigators report.
This suggests that VZV is unlikely to be a direct cause of AD but rather acts indirectly via reactivation of HSV-1, they add.
Similar findings emerged in similar experiments using 3D human brain tissue models.
"It's a one-two punch of two viruses that are very common and usually harmless, but the lab studies suggest that if a new exposure to VZV wakes up dormant HSV-1, they could cause trouble," Cairns said.
The researchers note that vaccination against VZV has been shown previously to reduce risk for dementia. It is possible, they add, that the vaccine is helping to stop the cycle of viral reactivation, inflammation, and neuronal damage.
"A First Step"
Commenting for Medscape Medical News, Heather M. Snyder, PhD, vice president of Medical & Scientific Relations at the Alzheimer's Association, said that the study "is using artificial systems with the goal of more clearly and more deeply understanding" the assessed associations.
She added that although it is a first step, it may provide valuable direction for follow-up research.
"This is preliminary work that first needs replication, validation, and further development to understand if any association that is uncovered between viruses and Alzheimer's/dementia has a mechanistic link," said Snyder.
She noted that several past studies have sought to help the research field to better understand the links between different viruses and Alzheimer's and other forms of dementia.
"There have been some challenges in evaluating these associations in our current model systems or in individuals for a number of reasons," said Snyder.
However, "the COVID-19 pandemic has created an opportunity to examine and investigate the relationships between different viruses and Alzheimer's and other dementias by following individuals in more common and well-established ways," she added.
She reported that her organization is "leading and working with a large global network of studies and investigators to address some of these questions" from during and after the COVID pandemic.
"The lessons we learn and share may inform our understanding of how other viruses are, or are not, connected to Alzheimer's and other dementia," Snyder said.
More information on the Alzheimer's Association International Cohort Study of Chronic Neurological Sequelae of SARS-CoV-2 is available online.
The study was funded by the National Institutes of Health. Cairns, Kaplan, Itzhaki, and Snyder have reported no relevant financial relationships.
J Alz Dis. Published online August 2, 2022. Abstract
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Cite this: Mechanistic Link Between Herpes Virus, Alzheimer's Revealed? - Medscape - Aug 16, 2022.