What this research adds
Working in mice, researchers have found that CRB1 contributes to regulate the integrity of the lower gastrointestinal tract. Dampening the expression of CRB1 enables bacteria to travel from the gut through the body and into the eye, leading to lesions in the retina. Giving antibiotics to mice lacking CRB1 prevented damages to their eyes.

Conclusions
The findings suggest that inherited eye diseases are caused in part by gut bacteria that travel to the retina.

More than 5 million people worldwide suffer from inherited retinal diseases such as retinitis pigmentosa — a condition that makes cells in the retina break down over time, causing blindness. Now, researchers have found that some of these diseases may be caused in part by gut bacteria that travel to the eye.

The findings, published in Cell, also indicate that antimicrobial drugs may ease the symptoms associated with retinal damage. “We found an unexpected link between the gut and the eye, which might be the cause of blindness in some patients,” says study co-author Richard Lee at University College London in the United Kingdom.

Retinitis pigmentosa and other inherited eye diseases are caused by mutations in a gene called CRB1. This gene is known to be expressed in the retina and brain, and it is crucial to building the protective barrier around the eye. But whether CRB1 is expressed in other tissues outside of the retina and brain is unknown.

Lee, Shanzhen Peng at Sun Yat-sen University in Guangzhou, China, and their colleagues set out to answer this question by studying mice that lack the gene CRB1.

Gut barrier integrity

The researchers found that CRB1 is not only crucial for maintaining the integrity of the eye’s barriers, but it also contributes to regulate the integrity of the lower gastrointestinal tract. In mouse lacking CRB1, bacteria were able to travel from the gut through the body and into the eye, leading to lesions in the retina, the researchers found.

In the retina of mice lacking CRB1, the team observed an enrichment of five bacterial species, including Anaerostipes hadrus, Bifidobacterium pseudocatenulatum and Oscillibacter valericigenes.

Giving antibiotics to mice lacking CRB1 prevented damages to their eyes. However, the antimicrobial treatment did not rebuild the damaged cell barriers in the eye.

Transforming treatment

Further experiments showed that restoring the expression of CRB1 in the gut also ameliorated the retinal damage associated with the gene.

Although the findings reveal that CRB1-associated retinal degeneration is partly dependent on bacterial translocation from the gut to the retina, more work is needed to understand whether this applies in humans, the researchers say.

“Our findings could have huge implications for transforming treatment for CRB1-associated eye diseases,” Lee says. “We hope to continue this research in clinical studies to confirm if this mechanism is indeed the cause of blindness in people, and whether treatments targeting bacteria could prevent blindness.”

The findings, he adds, may also have implications for a broader range of eye diseases beyond those associated with CRB1.

What is already known on this topic
Uveitis is a form of eye inflammation that causes eye redness, pain and blurred vision, and it can even lead to blindness. In many cases, the specific cause of uveitis isn’t clear, and the disorder is considered an autoimmune disease. Recent studies have showed that an altered gut microbiota may modulate immune responses in people with uveitis.

What this research adds
Researchers have found decreased levels of secondary bile acids, which result from gut bacteria’s metabolism, in the stool and blood of mice with autoimmune uveitis. Restoring the levels of microbiota-derived bile acid deoxycholic acid (DCA) reduces the severity of uveitis, likely by inhibiting the production of pro-inflammatory molecules in immune cells called dendritic cells. DCA appears to inhibit the production of inflammatory molecules by dendritic cells through the activation of the bile acid receptor TGR5. Compounds that activate TGR5 also inhibit dendritic cells.

Conclusions
The findings indicate that bile acids metabolism plays an important role in immune responses. The work also suggests that microbiota-derived bile acids can be a therapeutic target in autoimmune uveitis.

Uveitis is a form of eye inflammation that causes eye redness, pain and blurred vision, and it can even lead to blindness. In many cases, the specific cause of uveitis isn’t clear, and the disorder is considered an autoimmune disease. Now, researchers have found that the levels of microbiota-derived bile acids can influence the severity of uveitis — at least in mice. 

The findings, published in Cell Reports, indicate that bile acids metabolism plays an important role in immune responses. The work also suggests that microbiota-derived bile acids can be a therapeutic target in autoimmune uveitis.

Uveitis is usually treated with anti-inflammatory and immunosuppressive drugs. “But despite this therapy, many eyes still go blind,” the researchers say. “Therefore, there is an ongoing search to elucidate the mechanisms involved in the pathogenesis of uveitis, which might lead to the discovery of potential therapeutic targets,” they add.

Recent studies have showed that an altered gut microbiota may modulate immune responses in people with uveitis. For example, transferring feces from people with uveitis to mice worsens eye inflammation in the animals. Researchers have also observed an altered microbiota composition in individuals with uveitis and other autoimmune or inflammatory diseases.

To further investigate the link between uveitis and gut microbes, a team led by Hong Li at Chongqing Branch of National Clinical Research Center for Ocular Diseases tested bile acids and microbiota composition in a mouse model of autoimmune uveitis.

Bile acid balance

Produced in the liver and metabolized by gut bacteria, bile acids are involved in the control of the immune system and have been implicated in conditions such as inflammatory bowel disease and type 1 diabetes. 

First, Li and colleagues analyzed the gut microbiota of mice with autoimmune uveitis and healthy mice. Ruminococcaceae, Lachnospiraceae, and Eggerthellaceae were abundant in control mice, whereas Prevotellaceae were predominant in mice with uveitis, the researchers found.

Previous work has suggested that, in the presence of bile acids, Ruminococcaceae and Lachnospiraceae convert primary bile acids into secondary bile acids. To investigate whether the composition of bile acids could contribute to the development of autoimmune uveitis, the researchers looked at the levels of bile acids in the mice’s stool and blood. Those with autoimmune uveitis had lower levels of secondary bile acids compared to healthy mice, the team found. 

Immune interaction

Restoring the levels of microbiota-derived bile acids reduced the severity of uveitis, the researchers found. The team discovered that feeding mice with a diet rich in the secondary bile acid deoxycholic acid (DCA) improved symptoms of uveitis. DCA acts by regulating the function of immune cells called dendritic cells and reducing the production of specific inflammatory immune molecules. 

Further experiments indicated that DCA inhibits the production of inflammatory molecules through the activation of the bile acid receptor TGR5. Both DCA and TGR5 blocked the activation of human-derived dendritic cells, and so did compounds that activate TGR5, the researchers found.

“We show that gut microbiota-derived secondary bile acids might be key regulators in the pathogenesis of autoimmune uveitis,” the authors say. “Gut microbiota and secondary bile acid composition as well as TGR5 signaling may provide potential therapeutic targets for the treatment of autoimmune and inflammatory diseases, including uveitis.”

What is already known on this topic
Dry eye disease affects millions of people worldwide and is associated with ocular surface inflammation. Long-term inflammation may lead to permanent damage of the corneal epithelium. Animal models have linked two subsets of CD4+ T lymphocytes—T-helper type 17 cells (Th17) and regulatory T cells (Treg)—to chronic dry eye disease.

What this research adds
Using a daily sterile saline wash, researchers collected tears from human subjects upon first awakening and characterized the bacterial microbiome of closed-eye tears based on 16S rRNA genes. They applied statistical models and machine learning tools to differentiate the microbial communities in patients with dry eye disease and healthy subjects.

Conclusion

The analysis revealed that closed-eye tears in patients with moderate or severe dry eye disease house a distinctly different and more diverse microbiome than in healthy subjects. Through immunological interactions with Th17 cells and Treg cells, the resident ocular microbiota could potentially trigger and perpetuate inflammation in dry eye disease.

Dry eye disease affects millions of people worldwide and is associated with ocular surface inflammation. Long-term inflammation may lead to permanent damage of the corneal epithelium.  Two subsets of CD4+ T lymphocytes—T-helper type 17 cells (Th17) and regulatory T cells (Treg)—with opposing immunological roles are known to interact with resident microbiota and have been linked to chronic dry eye disease in animal studies.

Sterile saline eye washes are a proposed non-pharmaceutical treatment for patients with dry eye disease. As an adjunct to a randomized clinical trial to evaluate this treatment, a team of researchers led by Kent A. Willis, M.D. at the University of Tennessee Health Sciences Center in Memphis, collected daily closed-eye tears –those tears generated upon waking –from the study subjects. Upon enrolling in the study, subjects were evaluated for dry eye disease, stratified by level of severity, and randomly assigned to receive the saline eye wash treatment or no intervention. The researchers sequenced the bacterial microbiome of the closed eye tears using the 16S rRNA gene, and statistical analyses revealed that patients with moderate or severe dry eye disease had significantly more diverse and distinctly different microbiomes from those of normal subjects. Furthermore, the daily saline wash intervention did not significantly alter the ocular microbial communities in the treatment groups.

This study is published in Scientific Reports, a Nature Research journal. 

T-helper type 17 cells (Th17) and regulatory T cells (TREG)

Th17 cells are pro-inflammatory and produce cytokines including IL-17 to mount an immune response to pathogens. Treg cells are anti-inflammatory under normal conditions and play a role in immunologic tolerance. The ratio of these two subsets of lymphocytes modulates immune function, and an imbalance is associated with onset of several inflammatory diseases as well as autoimmunity. Previous studies have shown that resident gut microbes and their metabolites can influence this ratio. Therapies targeted at restoring optimal Th17/Treg cell balance may offer viable mechanisms to treat dry eye disease.

Bacterial Diversity

Typically, more diverse bacterial communities are considered more stable and less prone to disruption, but this research suggests that within the human eye increased microbial diversity is a marker of dry eye disease and remains largely unaffected by daily saline washes. The ecological measures of alpha diversity such as richness—the count of different taxonomic units, and evenness—the degree to which the taxonomic units are evenly distributed, revealed significant differences between subjects with dry eye disease and normal subjects. Measures of beta diversity reflected the same distinctions. The most significant distinctions in relative abundance were observed in genera including OPB56, Methylobacteriaceae, Bacteroidetes, Pseudomonas, and Meiothermus. Moreover, the daily eye rinses with sterile saline did not yield significant changes in diversity from the baseline microbiome in subjects with and without dry eye disease. The authors suggest that the mechanism to clear microbes from the eye surface is more complex than moisture level and tear clearance.

The research team utilized machine learning tools to predict with 94% accuracy which closed eye tear samples at baseline were from patients with dry eye disease. This approach has diagnostic potential.

This study demonstrated that closed-eye tears in patients with moderate or severe dry eye disease host a distinctly different and more diverse microbiome than in healthy subjects. Interactions between resident microbes and host Th17 cells and Treg cells may play a role in pathogenesis of dry eye disease. While further research is needed to determine causality, these findings present new clinical insights into the closed eye microbiome and a potential target for diagnosis and treatment of dry eye disease.