Image 1: Art by Madeleine Popofsky.
When we think about immunizations, we typically imagine shots administered to the upper arm or the thigh. But in some cases, the eye might be even better—especially when it comes to fighting herpes.
Research has shown that herpes can spread to the brain, triggering inflammation and potentially leading to encephalitis, which is characterized by the swelling of the brain. Severe, uncontrolled swelling can result in seizures and even death. Herpes is the leading cause of sporadic fatal encephalitis (SPE), a subset of infectious encephalitis stemming from viral infection. Since certain regions of the brain are too sensitive for traditional herpes simplex virus (HSV) treatments, researchers have shifted their focus to surrounding structures that might have a direct immunological connection to the brain. Their first stop was the eye.
A recent research collaboration led by associate research scientists Eric Song GSAS ’20, MD ’22 and Xiangyun Yin at the Yale School of Medicine described a biological pathway linking the eye and the brain. Using this link, the researchers pioneered an intravitreal immunization technique, which involves injecting a vaccine directly into the back of the eye, to protect mice from herpes.
The Eye-Brain Connection
The eye and the brain are unique structures within the body. Studies dating back to the mid-twentieth century have demonstrated that both structures are immune-privileged, meaning that they can tolerate antigens without triggering the inflammatory response typical of other parts of the body. This immune privilege helps to safeguard the delicate structures of the brain and eye from potential damage caused by inflammation, particularly in the case of the eye, where inflammation could otherwise impair vision.
“All tissues in the rest of the body have lymphatic vessels in the actual tissue, but in the brain and eye, there are no lymphatic vessels in the tissue itself,” said Song, the corresponding author of the study. The lymphatic system plays a vital role in the body’s defense against disease, with lymphatic vessels connecting to crucial structures known as lymph nodes.
When vaccines are administered, they typically contain antigens that mimic pathogens, triggering an immune response in the body. These antigens need to reach the lymph nodes, where T-cells and B-cells—specialized immune cells that recognize and eliminate diseased cells or invading pathogens—are located. Lymphatic vessels serve as the conduit for transporting these antigens from the site of injection to the lymph nodes.
Once antigens reach the lymph nodes, they are presented to immune cells, initiating the production of antibodies and memory cells that provide immunity against the targeted pathogen. The efficient delivery of antigens to the lymph nodes is crucial for the efficacy of vaccines.
Lymphatic vessels are also crucial in the body’s drainage system, which helps remove waste and toxins from tissues. Because the brain and the eye lack these vessels within their tissues, they are somewhat isolated from the typical immune surveillance and response mechanisms of the rest of the body. This isolation creates a protective barrier that limits the immune response within the brain and eye, hence their immune privilege. Because of these shared immune properties, the researchers were curious whether the eye-brain connection played a role in immunity. If it did, then they could potentially tackle herpes in the brain, through the eye.
The researchers focused on the area around the optic nerve, which is the critical link between the eye and the brain. They employed sophisticated methods, including spatial transcriptomics to analyze gene expression in tissues and an imaging technique to visualize and map the lymphatic vessels that surround the optic nerve. If there were lymphatic vessels, that would mean an immunological connection between the eye and the brain exists, and therefore a potential route for immune cells to get to the brain. This would support the feasibility of using the eye as a route for vaccination to reach the brain.
Using the imaging technique, the researchers observed that two initial lymphatic markers, LYVE1 and VEGFR3, stained the membrane surrounding the nerve rather than the nerve itself. This led to the hypothesis that the optic nerve sheath, a system of membranes covering the nerve, was crucial to a potential pathway for an immune response. The researchers ultimately concluded that the optic nerve sheath contains a lymphatic vessel network that connects the eye and the brain to a set of lymph nodes.
The researchers found a connection between the intravitreal chamber, located at the back of the eye, and lymph nodes connected to the central nervous system, unique from the rest of the eye. When they injected fluorescent dyes into the intravitreal chamber at the back of the eye, they found that the dyes localized to the deep cervical lymph nodes (dCLN) and the superficial cervical lymph nodes (sCLN) in the neck, indicating the presence of a distinct drainage system. They also found that trace amounts of the dye could be detected in the cerebrospinal fluid of the mice, following intravitreal chamber injection.
This meant a vaccine delivered to the back of the eye could potentially trigger an immune response mediated by CLNs that would extend to the brain through their shared lymphatic network. Thus, the vaccine would bolster immune defenses in both the eye and the brain.
Immunity Against Herpes
With these initial findings, the researchers knew there was an eye-brain connection in immunity and that a drainage system through the eye would make delivering sporadic fatal encephalitis (SPE) treatment to the brain possible. So they proceeded to their main experimental goal: testing HSV immunizations to decrease the risk of fatal encephalitis. They compared four administration routes to deliver the HSV: intraperitoneal, intracranial, anterior chamber, and intravitreal chamber. The first route involves delivery via the abdominal cavity, the second directly into the brain, the third through the anterior chamber at the front of the eye, and the fourth through the back of the eye. “The most difficult part of the research was working with the mice. [Human] eyes are already incredibly small and delicate compared to the rest of the body. Now scale that down to the size of a mouse; patience and practice were key to making sure [the team] could continue this study,” Song said.
Vaccination through the intraperitoneal route resulted in all mice succumbing to the virus, while vaccination via the intracranial route provided robust protection. This suggests that conventional systemic immunization is inadequate in protecting against brain HSV infection. Now, what about the eye? None of the mice injected via the AC route survived. However, a majority of those injected via the intravitreal chamber route did. This outcome was attributed to the dCLN, which proved vital in responding to herpes in the brain.
Injecting at the front of the eye via the AC was ineffective because there was no distinct drainage pathway to the dCLN. Injecting at the back of the eye allowed the SPE treatment to drain directly to the dCLN and fight the infection.
Next Steps
After a series of additional confirmation experiments, the researchers affirmed the specific relevance of the eye-brain immunological connection in protecting against pathogens affecting the brain, such as HSV. Moreover, there are indications that this pathway could be explored for the treatment of various other diseases of the central nervous system, including ocular diseases, bacterial infections, and even tumors.
There’s even a chance that we haven’t yet uncovered all the immunological connections to the brain. Discovering more connections could open up even more treatment possibilities. “It has been shown that there is a connection between the nose and the brain. There might be more connections [to the brain] left to be discovered,” Yin said.
If further developed, this targeted treatment approach could deliver relief to hundreds of thousands of patients, and physicians would have access to a larger range of treatment options based on each patient’s condition. Having more options would not only increase the likelihood of patients finding relief but also speed up the process.