How herpes simplex virus in the cornea spreads and what halts its progress

Little is known about how herpes simplex virus type 1 (HSV-1) enters the cornea, or how it spreads. Lead researcher, WIMR’s Professor Tony Cunningham AO said, “To examine the earliest stages of infection and spread of HSV-1 in the cornea, we developed a new model in the lab. We used a highly sensitive method for detecting HSV DNA, RNAscope (the ability to look at single RNA molecules) and fluorescently labelled virus.” This project was the first HSV-1 study to use RNAscope and to demonstrate how the virus spreads in cornea. It also identified the mechanisms that stop the virus from spreading further.

The cornea comprises three main layers – the epithelium, stroma and endothelium. The anterior limiting membrane (ALM) or Bowman’s layer sits between the epithelium and stroma.

“To examine the initial infection and spread of HSV-1 in detail, we adapted a model developed at WIMR and previously used to map viral infection in human genital mucosa,” said Professor Cunningham.

“We discovered that for HSV-1 to enter the cornea, there must be some existing trauma to the uppermost layer of the epithelium, allowing the virus to enter.

“Interestingly, while the HSV-1 virus spreads quickly, we observed that it remained in the epithelium layer and did not penetrate the Bowman’s layer. So, while the virus can spread and cause significant damage to sight, it is contained within the epithelium. This means that the design of new effective treatments and vaccines for herpes keratitis must take into account how quickly the virus spreads.

“We identified the presence of a protein called SPRR1A in the uppermost epithelium layer. When the cornea was infected, we also found SPRR1A in the Bowman’s layer.

“We know that SPRR1A contributes to the protective barrier function of human skin. Its presence in the cornea suggests that this barrier function contributes to limiting both superficial and deep HSV-1 spread.”

The research team is now using this model to examine donated human corneas from patients undergoing corneal transplant for blindness caused by herpes simplex keratitis. The next phase will aim to discover why some people with herpes simplex keratitis develop scarring and blindness, while others do not.

Joint lead investigator, Associate Professor Nicole Carnt is Research Director at the School of Optometry and Vision Science, UNSW and is also a researcher with WIMR’s Centre for Vision Research. Associate Professor Carnt concludes, “This study reveals the cornea’s impressive defenses against HSV1-related scarring and vision loss. Beyond aiding vaccine development, it identifies gene targets to enhance these protective mechanisms in keratitis patients.”