Latent varicella zoster virus (VZV) resides in ganglionic neurons in 90% of humans and can reactivate to produce herpes zoster (shingles) in the elderly. Zoster is associated with increased dementia risk, notably Alzheimer’s disease (AD). However, studying how VZV contributes to dementia is challenging due to its exclusive infectivity to humans.

Part 1: Simian varicella virus (SVV) produces a similar if not identical disease from chickenpox to latency to zoster in primates (NHP). We used the SVV infection in NHP to examine how infection may recapitulate AD pathology, induction of amyloidogenic peptides and amyloid deposition, thereby accelerating disease progression. Rhesus macaques (RM) were infected with SVV, leading to chickenpox, latency, and zoster under immune suppression. Blood, tissues, and arteries were collected at various stages for analysis. Blood was examined for the presence of SVV DNA by PCR from pre-inoculation to zoster. Serum was analyzed by ELISAs and Thio-T assays to confirm an amyloid-promoting environment. Sections of tissues and arteries in the periphery and central nervous system were examined by immunofluorescence for SVV antigen and amyloid. DNA extracted from adjacent sections were analyzed by PCR for virus DNA. During peak viremia, elevated amylin levels and an amyloidogenic environment were seen in serum compared to pre-inoculation and viremia resolution. SVV antigen co-localized with amyloid in the periphery (pancreas, GI tract, temporal and carotid artery, olfactory bulb) and amyloid was found in the central nervous system (cerebral arteries). These findings suggest that varicella infection may hasten AD progression by increasing the amyloid burden. Notably, the presence of amyloid in cerebral arteries of infected monkeys supports the link between varicella infection and cerebrovascular diseases, including cerebral amyloid angiopathy, observed in AD brains.

Part 2: VZV Reactivation not only heightens the risk of stroke and other neurological complications but also increases susceptibility to co-infections with various viral and bacterial pathogens at sites distant from the original infection. The mechanism by which VZV results in complications remote from the initial foci remains unclear. Small extracellular vesicles (sEVs) are membranous signaling structures that can deliver proteins and nucleic acids to modify the function of distal cells and tissues during normal physiological conditions. Although viruses have been documented to exploit the sEV machinery to propagate infection, the role of non-infectious sEVs released from VZV-infected neurons in viral spread and disease has not been studied. Using multi-omic approaches, we characterized the content of sEVs released from VZV-infected human sensory neurons (VZVsEVs). Out of the 70 viral proteins one protein (immediate-early 62) was detected, as well as numerous immunosuppressive and vascular disease-associated host proteins and miRNAs that were absent in sEVs from uninfected neurons. Notably, VZV sEVs are non-infectious yet transcriptionally altered primary human cells, suppressing the antiviral type 1 interferon response and promoting neuroinvasion of a secondary pathogen in vivo. These results challenge our understanding of VZV infection, proposing that the virus may contribute to distant pathologies through non-infectious sEVs beyond the primary infection site. Furthermore, this study provides a previously undescribed immune-evasion mechanism induced by VZV that highlights the significance of non-infectious sEVs in early VZV pathogenesis.

Dr Ravi Mahalingam is a Professor of Neurology at the University of Colorado School of
Medicine. He is also a visiting professor at the Sri Ramachandra Medical College & Research Institute, Chennai, India. He did his Masters in Chemistry at the Indian Institute of Technology in Mumbai, India in 1976 and completed his PhD in Biochemistry and Molecular Biology in 1980 at Southern Illinois University, Carbondale, Illinois, USA. For the past 3 decades, Dr Mahalingam has been working on the pathogenesis, latency, and reactivation of the human varicella zoster virus. He established the unique animal model using simian varicella virus infection in non-human primates. In collaboration with Prof Wayne Gray, Dr Mahalingam sequenced the complete primate virus genome in 2001. His laboratory currently focusses on how herpesvirus infections increase Alzheimer’s Disease risk in the elderly.