Researching herpes treatments with 'skin-on-chip' technology

A small square device made of plexiglass, which could fit in the palm of your hand, has shown promise as a model of human disease that could speed research for treatments for herpes and other infections.

Inside the square are sandwiched several layers of human skin cells, seeded and grown into a thin swatch of skin about the size of a dime. A grid of living blood vessels lies beneath the tissues, keeping them alive.

Through one of the many holes on the top of the plexiglass, you can insert a tiny metal probe to punch a perfect circle through the layers of living skin. Next, you drop into the wound a dose of herpes simplex virus.

The skin gets infected with herpes.

At Fred Hutchinson Cancer Center in Seattle, immunologist and virologist Dr. Jia Zhu leads a group of researchers studying herpes simplex viruses, which are responsible for periodic outbreaks of cold sores in the mouth (HSV-1) or for recurrent ulcers in genitals (HSV-2).

With their device, which she calls a “skin-on-chip” platform, Zhu and her team have built and tested a prototype tool that could help researchers speed the development of human herpes treatments.

Under a grant from the National Center for Advancing Translational Sciences, part of the National Institutes of Health, they are preparing to test it to screen for promising antiviral compounds.

“Ultimately, we want to use it to study mechanisms dictating disease outcomes,” said Zhu. “Because in herpes, you can have a wide spectrum of symptoms. Some people may be asymptomatic, while others have very severe disease and constantly have reactivations.”

In the journal Nature Communications, the researchers describe how they built their model; infected its laboratory-grown patches of human skin; how the infected skin provoked a response from human immune cells; and how the herpes infection was suppressed by the commonly prescribed antiviral drug acyclovir.

The device has already yielded an important clue about human herpes infection. Zhu’s team noticed that cells in the bottom layer of developing skin cells — known as basal keratinocytes — were by far the most susceptible to invading HSV-1 particles during the experiments. They also discovered that infected cells in the skin-on-chip device cranked out a chemical alarm (a cytokine protein called IL-8) that attracts a type of immune cell called neutrophils.

A better model than mice for human disease?

First responders to infections, neutrophils were known to flood herpes-infected skin and help to suppress the virus, but the role of IL-8 was a surprise. That could be because most laboratory research on herpes is conducted on mice — which don’t produce human IL-8.

When the researchers flushed neutrophils through into the blood vessel grid of their device, the immune cells responded to HSV infection by swarming up to infected keratinocytes, literally devouring them. It was an important validation that this skin-on-chip model mimicked the immune defense behavior of human skin as observed in patients. 

Finding models of human disease to test at scale in a laboratory is a longstanding challenge in biology. Mouse studies are valuable but imperfect models for human diseases. Testing living human tissue from biopsies is difficult, because such tissue does not live long.

When keratinocytes are just grown in a lab dish, they produce a uniform layer of skin cells. Zhu calls this tissue 2D, or two-dimensional. She describes her skin-on-chip platform as 3D, because the immature skin cells differentiate and mature into multiple layers.

The multi-layered tissue in the chip is just as thick as the tender skin targeted by herpes in patients. It appears to mimic not just the structure of human skin, but its biology as well.

“Whenever we infected 2D tissue cultures, we did not detect IL-8. So again, this was unexpected,” she said. “With this system, you can really test which cytokines are at work.”

Five years in the making

The newly published study is the first report about the device, which the Fred Hutch team has been developing and testing for five years. Lead author Dr. Sijie Sun worked on the project as a postdoctoral fellow mentored by Zhu but returned home to China after the outbreak of COVID-19.

While scientists have been constructing methods of growing human skin for laboratory research since the 1980s, Zhu’s approach is different. One example is the 3D structure, which closely resembles the multi-layered architecture of human skin. The device is also designed to be placed above the lens of a powerful microscope, so the real-time activities of the living cells can be observed.

Perhaps its most important innovation is the grid of blood vessels, through which oxygen and CO₂ are transported in and out, and through which human immune cells and micro-doses of drugs can be circulated. The miniscule cavities for the grid of living pipes were first stamped into a gel-like block of collagen protein. It stiffens and then is filled with human endothelial cells, which proceed to self-assemble. They line the cavities and become a network of living, functioning blood vessels.

That network was put to the test when various concentrations of the herpes antiviral drug acyclovir were delivered through the vessels.

“You can test a drug and do quantitative analysis, like which concentration works best,” Zhu said.

Another path to precision therapy

Using microscopes and proteins designed to light up in infected cells, her team saw how the tissue responded well to the drug when it was given early after infection. But the medication works less effectively if it was given one day later — just as it performs in patients treated with suppressive therapy.

Zhu said her goal is to expand the types of immune cells that can be tested through the skin-on-chip device — including infection-fighting T cells that lodge themselves in tissue to monitor and respond to herpes virus reactivation. For this model to work, the seeded skin- and blood-vessel cells grown in the device need to come from the same person providing the T cells — otherwise those immune cells might attack the laboratory-grown skin as if it were foreign tissue, like a mismatch between donor and recipient in an organ transplant.

Her team envisions a time when potential herpes therapies could be tested through the device by customizing it for each patient — seeding it with keratinocytes and immune cells from an individual who perhaps suffers more than others from recurrent infections.

Correct doses and the right combination of therapies could be determined using that person’s own skin-on-chip. It is another path to precision therapy, tailored for each patient.

“We all have different cells,” Zhu said. “We can get them and grow them. We can make different chips and we can compare them. It’s personalized medical research.”

This research was supported by grants from the National Institutes of Health.