Immune cells that disrupt the ability of eyelid glands to keep the eyes moisturized could be indirectly to blame for dry eye, a condition that affects millions of people worldwide.
This was the conclusion that researchers came to after studying eyelid gland dysfunction in mice and analyzing human tear samples.
They found that neutrophils, which are a type of immune cell that normally protects the eye during inflammation, can also disrupt the eyelid's "meibomian glands."
These glands release oil when the eyelid opens. The oil moisturizes the eyes and prevents evaporation of tears.
Blockage of the glands causes meibomian gland dysfunction (MGD), a condition that results in increased eyelid bacteria, inflammation, and dry eye.
Senior study author Daniel R. Saban, an associate professor of ophthalmology at Duke University School of Medicine in Durham, NC, explains that the neutrophils do not "directly" block the gland cells.
"They're being recruited around the gland and changing the actual glandular cells, which is causing them to malfunction," he adds.
Prof. Saban and his colleagues report their findings in a paper now published in the journal Science Translational Medicine.
Dry eye, MGD, and neutrophils
MGD is the most common cause of dry eye, a chronic disease that is "considered a major health problem internationally."
Dry eye is the most common reason that people seek medical care for their eyes. Estimates suggest that up to a third of the population can be affected. In the United States, the annual cost of treating dry eye is around $3.84 billion.
The disease is uncomfortable and painful and diminishes quality of life. Having dry eye makes it more difficult to read, work effectively, use a computer, watch television, and drive. The disease can also restrict use of contact lenses and cosmetics.
MGD is sometimes visible because it forms small "off-white beads" on the eyelid. But this does not always occur, leaving many cases undiagnosed.
The study authors note that although conditions "such as allergic eye disease" that cause the eyelids to be chronically inflamed are linked to MGD, "it is not clear whether inflammatory processes contribute" to the obstruction of the glands.
Neutrophils are immune cells that are present in many types of tissue. They are summoned to help deal with bacteria and other pathogens.
Scientists are not sure why neutrophils enter tears as we sleep. Perhaps they are taking advantage of the fact that the eyes are shut to collect waste. Normally, the cells depart when the eyes open. But in people with MGD, they stay.
Treatment of underlying cause
In their study, Prof. Saban and team examined neutrophil levels in mice that had "eye inflammation in the form of allergic eye disease."
They found that a type of neutrophil called polymorphonuclear neutrophils "promoted" obstruction of meibomian glands.
The researchers then examined tear samples from 64 people with and without MGD. They found that tear samples from those with MGD had higher levels of polymorphonuclear neutrophils.
In addition, they found that samples with the highest levels of the neutrophils came from those with most severe forms of MGD and who also had other inflammation-related conditions, such as rosacea, allergies, and autoimmune disorders.
The study authors point out that not all MGD is caused by inflammation and autoimmunity, and that conversely, not all eye inflammations cause MGD.
However, the results should be relevant for all those who have MGD and dry eye and should lead to better treatments that tackle the underlying cause rather than just relieve the symptoms.
Prof. Saban also notes that it could be possible that a drug already approved for another condition might be "repurposed" for treating MGD.
"In addition to providing new treatment strategies, the presence of neutrophils in the eye could provide a biomarker to detect the disease or measure its severity."
Prof. Daniel R. Saban