Genetic and lifestyle factors, along with repeated head trauma, have long been associated with an increased risk of Alzheimer’s disease (AD).¹ In comparison, the hypothesis that viruses like herpes simplex virus-1 (HSV-1), could contribute to the pathogenesis of AD has received relatively little attention until recently.²

In a new study published in Science Signaling, researchers from Tufts University and the University of Oxford have used human brain organoids to demonstrate that head injuries can reactivate dormant HSV-1 in the brain in carriers of a common genetic variant, triggering the onset of pathological features of AD.³  The findings support mounting evidence that viruses may increase susceptibility to or contribute to the progression of AD, highlighting their potential role in neurodegenerative processes.

HSV-1 is highly prevalent in the population, infecting around 80 percent of people by the age of 60. The virus can lurk in the body in a dormant state for years and is thought to travel to the brain in older people as their immune system gradually declines. Biophysicist Ruth Itzhaki of the University of Oxford was an early proponent of the link between HSV-1 and AD.⁴ Yet for decades, her research was met with hostility and largely dismissed.

Dana Cairns, a stem cell biologist in David Kaplan’s laboratory at Tufts University and coauthor of the study, teamed up with Itzhaki to investigate the connection between HSV-1 and AD from a new angle. The team hypothesized that inflammation from head injuries may trigger the reactivation of latent viruses, which could contribute to the onset of AD. To test their theory, they used brain organoids: three-dimensional cultures that resemble miniature brains.

“They really lend themselves to a lot of different techniques for analysis and because they have a scaffold, they’re quite robust in the sense that you can do all sorts of manipulations and different kinds of damage to them,” remarked Cairns.

Cairns and her colleagues created the organoids using induced neural stem cells from humans that carry one copy of the E4 variant of the apolipoprotein E gene (APOE4), the most common known risk factor for AD.⁵ After infecting the organoids with HSV-1 they induced latency by treating with antivirals to prevent replication.

After letting the organoids mature, they encased them in small, fake skulls, crafted with the help of mechanical engineers. “We add fluid to mimic cerebral spinal fluid, and then there’s a cap that goes on, so it really fits snugly in the way that a skull does around your brain,” said Cairns.

Next, the team needed to knock the tiny brains around to simulate real head injuries. Using a small piston, Cairns and her colleagues delivered precise impacts to the skulls, causing the brains to jostle around. They followed this up with immunostaining to search the injured brains for morphological changes associated with AD and neurodegeneration, including β-amyloid plaque accumulation.⁶

The results were more dramatic than Cairns expected. While the mock-infected controls showed no significant morphological changes beyond the expected injury-associated inflammation, the organoids with latent HSV-1 exposed to repeated trauma displayed prominent AD-associated phenotypes, such as β-amyloid plaques. Quantitative PCR revealed elevated levels of HSV-1 transcripts, confirming the reactivation of the latent virus.

“When we looked at these brain organoids that had been latently infected and then injured, they had these massive plaques, which was super cool,” said Cairns.

The success of the experiments inspired the team, many of whom were new to research. “I was working with a team of female undergraduates, and they were just so enthusiastic,” Cairns reflected. “Those initial pilot experiments, fortunately, worked really well, and it got them very excited to stay in the lab a lot longer!”

“[The authors] insulted the brain and saw effects, and they had all the appropriate controls. I was quite impressed with the quality of the work that came out of it,” said biochemist and AD expert Ralph Martins of Macquarie University, who was not involved in the study. “The findings are quite convincing in this model.”

Cairns and her team found that HSV-1 reactivation was associated with an increase in the pro-inflammatory cytokine interleukin-1β (IL-1β) in injured organoids, leading them to wonder if blocking IL-1β production could prevent the formation of AD phenotypes. By scratching the surface of 2D cultures with a needle and treating them with an antibody that blocks IL-1β, they significantly reduced β-amyloid production.

Cairns says these results could eventually contribute to the development of prophylactic measures for professional athletes who experience repeated concussions and head trauma from a young age­­­­—something they plan to test in future experiments. “[For a concussion] the standard of care is currently just rest and maybe occasional painkillers,” she commented. “To us, it seems reasonable to consider antiviral therapy as a prophylactic measure to stave off neurodegeneration in the future.”

However, Martins noted that further research is needed before the results can be translated for AD prevention. “You have to remember that [the model] is a brain in a dish. It’s not a human brain…But it’s a good step in the direction of seeing whether this is going to make a difference,” he remarked.