Eye Microbiome Trains Immune Cells to Fend Off Pathogens In Mice

Bugs in your eyes may be a good thing. Resident microbes living on the eye are essential for immune responses that protect the eye from infection, new research shows. The study, which appeared in the journal Immunity on June 27, demonstrates the existence of a resident ocular microbiome that trains the developing immune system to fend off pathogens. The research was conducted at NEI.

“This is the first evidence that a bacterium lives on the ocular surface long-term,” said Dr. Rachel Caspi, senior investigator in NEI’s Laboratory of Immunology. “This work addresses a longstanding question about whether there is a resident ocular microbiome.” 

For years, the ocular surface was thought to be sterile because of the presence of an enzyme called lysozyme that destroys bacteria, antimicrobial peptides and other factors that rid the eye of microbes that may land from the air (or from our fingers) onto the surface of the eye.

Dr. Anthony St. Leger, research fellow in Caspi’s laboratory, was able to culture bacteria from the mouse conjunctiva, the membrane that lines the eyelids. He found several species of Staphylococci, which are commonly found on the skin, and Corynebacterium mastitidis (C. mast). But it wasn’t clear whether those microbes had just arrived on the eye and were en route to being destroyed or whether they lived on the eye for extended periods of time. 

The researchers found that C. mast, when cultured with immune cells from the conjunctiva, induced the production of interleukin (IL)-17, a signaling protein critical for host defense. Upon further investigation, they found that IL-17 was produced by gamma delta T cells, a type of immune cell found in mucosal tissues. IL-17 attracted other immune cells called neutrophils—the most abundant type of white blood cell—to the conjunctiva and induced the release of anti-microbial proteins into the tears. The researchers are currently investigating the unique features that can make C. mast resistant to the immune response that it itself provokes and allow it to persist in the eye.

The researchers are also investigating whether other bacteria play a role in regulating eye immunity.

“We’ve established the proof of concept of a central ocular microbiome,” St. Leger said. “It’s well known that there are good bacteria in the gut that modulate the immune response. Now we show that this relationship exists in the eye. That’s important for how we think about treating ocular disease.”