Keeping an Eye on COVID-19

Studies show ocular surface a potential portal of entry for COVID-19 virus. Roibeard Ó hÉineacháin reports

The question of whether the eye can be the route of entry for SARS-CoV-2, the virus that causes COVID19, remains controversial. However, recent research shows ocular surfaces, particularly the conjunctiva, are susceptible to SARS-CoV-2 infection.

A study presented at the 2021 ARVO virtual conference showed the angiotensin-converting enzyme 2 (ACE2) receptor and the transmembrane protease (TMPRSS2) that enable cells to become infected with SARS-CoV-2 virus were present in conjunctival and corneal tissues in post-mortem human eyes and surgical specimens.

“Our key findings were that both ACE2-receptor and TMPPRS2 were prominently expressed in conjunctiva, limbus, and cornea, especially on the surface cells. Together with other studies, our findings highlight the importance of safety guidelines, including eye protection and ocular contact precautions, in preventing the spread of COVID-19,” said Elia J. Duh MD, Wilmer Eye Institute, Johns Hopkins School of Medicine, Baltimore, Maryland, USA.

Previous evidence suggesting the eye could serve as a portal of entry for the SARS-CoV-2 virus in the human body includes the conjunctival abnormalities reported in some patients with COVID19 and the detection of SARS-CoV-2 in ocular surface specimens. In addition, epidemiological studies suggest goggles and face shields can help reduce virus transmission in high-risk settings.

To determine the presence of ACE2 and TMPRSS2 on the ocular surface, Dr Duh and his associates analysed post-mortem human eyes as well as surgical specimens of conjunctival and corneal tissue and human corneal epithelial lysates using immunohistochemistry and immunoblot assays.

Western blotting showed ACE2 and TMPRSS2 were both expressed in corneal epithelial lysates. Immunohistochemical staining for ACE2 receptors was mild to moderate in the corneal epithelial cells, but prominent staining was especially prominent in the superficial epithelial limbus cells. There was also positive staining for ACE2 receptors in the conjunctival cells, particularly in the superficial epithelial cells. Goblet cells had largely negative staining.

In addition, there was strong staining for TMPRSS2 in both the corneal epithelium and endothelium. There was intense staining for TMPRSS2 in the multilayered epithelium throughout the entire conjunctival epithelial layer and in the limbus. OCULAR SURFACE CELLS INFECTED IN LAB The intercellular environment of some ocular surface cells appears to be conducive to viral replication under laboratory conditions, reported Timothy Blenkinsop PhD, Department of Ophthalmology, Icahn School of Medicine at Mount Sinai, New York, USA, in a separate presentation.

He noted that in a series of post-mortem eyes of COVID-19 patients, immunohistochemical staining showed the presence of SARS-CoV-2 in the corneal and limbal tissues. However, this still did not show whether the cells were infected by aerosols or a result of systemic infection reaching the eye.

To explore that question, his team exposed cultured cornea limbus and sclera cells to the virus. They found evidence of viral replication and RNA sequencing showed signs of SARSCoV-2 present in all three cell types—most pronounced in the limbal cells. In addition, they also found the infected limbal cells exhibited the same gene expression as those associated with SARSCoV-2 infection—such as upregulated gene expression of TNF-α signalling via NF-кB—and an interferon-gamma response.

They found similar results using a whole eye stem cell model of eye development that includes many cell types of the eye, such as the cornea, the limbus, the lens, the retina, the RPE, and the CNS. Their research showed that ocular surface cells accounted for 90% of those infected with an area of limbus carrying the bulk of the infection. Gene expression changes found by way of single-cell RNA sequencing demonstrated increased expression of TNF-α compared to less infected cells from the stem cell whole eye model and uninfected colonies. Infected cells also exhibited lower expression of interferon-gamma pathway genes compared to uninfected colonies.

“Together, these data suggest the human eye can be directly infected by SARS-CoV-2 and implicates limbal cells as an alternative portal for viral entry,” Dr Blenkinsop said.