A new study shows that the complement system, part of the innate immune system, plays a protective role to slow retinal degeneration in a mouse model of retinitis pigmentosa, an inherited eye disease.
This surprising discovery contradicts previous studies of other eye diseases suggesting that the complement system worsens retinal degeneration.
The research was performed by scientists at the National Eye Institute (NEI), part of the National Institutes of Health, and appears in the Journal of Experimental Medicine.
Retinitis pigmentosa is an incurable and unpreventable blinding eye disease that affects 1 in 4,000 people.
“Much research is devoted to studying therapies that attempt to alter the immune system’s role in inherited diseases such as retinitis pigmentosa because such treatments would have broad applicability, regardless of a patient’s causative mutation,” said the study’s principal investigator Wai T Wong, M.D., Ph.D., chief the Neuron-Glia Interactions in Retinal Disease Section at NEI.
In previous studies, activation of the complement system, which mediates some aspects of inflammation, worsens damage in age-related macular degeneration (AMD), a leading cause of blindness in people age 65 years and older.
“The current study involving retinitis pigmentosa underscores the notion that the complement system may in fact exacerbate or curb retinal degeneration depending on the context.
Appreciating this complexity is important for guiding the development of therapies that target the complement immune system to treat degenerative diseases of the retina,” Dr. Wong said.
Sean Silverman, Ph.D., an NEI postdoctoral researcher in Dr. Wong’s lab and the lead author on the study, and colleagues monitored the genetic expression of the complement system in a transgenic mouse model of retinitis pigmentosa.
They found that upregulation of complement expression and activation coincided with the onset of photoreceptor degeneration.
What’s more, this upregulation occurs in the exact location of the degeneration.
“Having found complement at the scene of the crime, we then wanted to know whether it was helping or hurting the degenerative process,” Dr. Wong said.
Using the retinitis pigmentosa mouse model, the researchers examined the role of C3 and CR3, the central component of complement and its receptor, by comparing mice with genetically ablated C3 or CR3 to mice with normal expression.
They found that the absence of C3 or CR3 made degeneration worse.
Rod photoreceptors, the light-sensing cells that die off first in retinitis pigmentosa, were precipitously lost along with a surge in the expression of neurotoxic inflammatory cytokines.
They pieced together that C3 gets secreted by microglia, trash-collecting cells that in a healthy retina clear away dead cells by phagocytosis to keep the tissue working properly.
Once secreted, C3 lands on dead photoreceptors labeling them for destruction and removal.
The receptor, CR3, recognizes the C3 markers and conveys the information to microglia.
“Breakdown of this C3-CR3 interaction results in a decreased ability of microglia to phagocytose dead photoreceptors, which then accumulate in the retina, stimulating greater inflammation and degeneration,” Dr. Wong said.
“Degeneration accelerates pretty quickly.”
When placed alongside each other in a dish, microglia from C3- or CR3-ablated retinas turned out to be toxic to photoreceptors.
Taken together, the results show that in the context of retinitis pigmentosa, complement activation is actually helpful for clearing away dead cells and maintaining a state of homeostasis, a physiological balance, in the retina.
However, in the context of AMD, harmful effects observed from complement activation have spurred clinical trials testing complement inhibitors.
“Our findings suggest that this approach may be appropriate for some disease scenarios, but may induce complex responses in other disease scenarios by inhibiting helpful and homeostatic functions of inflammation,” Dr. Wong said.
What is retinitis pigmentosa?
Retinitis pigmentosa (RP) is the name given to a group of inherited eye diseases that affect the retina (the light-sensitive part of the eye).
RP causes the breakdown of photoreceptor cells (cells in the retina that detect light).
Photoreceptor cells capture and process light helping us to see.
As these cells breakdown and die, patients experience progressive vision loss.
The most common feature of all forms of RP is a gradual breakdown of rods (retinal cells that detect dim light) and cones (retinal cells that detect light and color).
Most forms of RP first cause the breakdown of rod cells.
These forms of RP, sometimes called rod-cone dystrophy, usually begin with night blindness.
Night blindness is somewhat like the experience normally sighted individuals encounter when entering a dark movie theatre on a bright, sunny day.
However, patients with RP cannot adjust well to dark and dimly lit environments.
What are the symptoms of retinitis pigmentosa?
As the disease progresses and more rod cells breakdown, patients lose their peripheral vision (tunnel vision).
Individuals with RP often experience a ring of vision loss in their periphery, but retain clear central vision.
Others report the sensation of tunnel vision, as though they see the world through a straw.
Many patients with retinitis pigmentosa retain a small degree of central vision throughout their life.
Other forms of RP, sometimes called cone-rod dystrophy, first affect central vision. Patients first experience a loss of central vision that cannot be corrected with glasses or contact lenses. With the loss of cone cells also comes disturbances in color perception.
As the disease progresses, rod cells degenerate causing night blindness and peripheral vision.
Symptoms of RP are most often recognized in children, adolescents and young adults, with progression of the disease continuing throughout the individual’s life. The pattern and degree of visual loss are variable.
What causes retinitis pigmentosa?
Retinitis pigmentosa is an inherited disorder, and therefore not caused by injury, infection or any other external or environmental factors.
People suffering from RP are born with the disorder already programmed into their cells.
Doctors can see the first signs of retinitis pigmentosa in affected children as early as age 10. Research suggests that several different types of gene mutations (changes in genes) can send faulty messages to the retinal cells which leads to their progressive degeneration. In most cases, the disorder is linked to a recessive gene, a gene that must be inherited from both parents in order to cause the disease.
But dominant genes and genes on the X chromosome also have been linked to retinitis pigmentosa.
In these cases, only one parent has passed the disease gene. In some cases, a new mutation causes the disease to occur in a person who does not have a family history of the disease. The disorder also can show up as part of other syndromes, such as Bassen-Kornzweig disease or Kearns-Sayre syndrome.
How is retinitis pigmentosa treated?
There is no known cure for retinitis pigmentosa.
However, there are few treatment options such as light avoidance and/or the use of low-vision aids to slow down the progression of RP.
Some practitioners also consider vitamin A as a possible treatment option to slow down the progression of RP.
Research suggests taking high doses of vitamin A (15,000 IU/day) may slow progression a little in some people, but the results are not strong.
Taking too much vitamin A can be toxic and the effects of vitamin A on the disease is relatively weak. M
ore research must be conducted before this is a widely accepted form of therapy.
Research is also being conducted in areas such as gene therapy research, transplant research, and retinal prosthesis.
Since RP is usually the result of a defective gene, gene therapy has become a widely explored area for future research.
The goal of such research would be to discover ways healthy genes can be inserted into the retina.
Attempts at transplanting healthy retinal cells into sick retinas are being made experimentally and have not yet been considered as clinically safe and successful.
Retinal prosthesis is also an important area of exploration because the prosthesis, a man-made device intended to replace a damaged body part, can be designed to take over the function of the lost photoreceptors by electrically stimulating the remaining healthy cells of the retina.
Through electrical stimulation, the activated ganglion cells can provide a visual signal to the brain.
The visual scene captured by a camera is transmitted via electromagnetic radiation to a small decoder chip located on the retinal surface.
Data and power are then sent to a set of electrodes connected to the decoder.
Electrical current passing from individual electrodes stimulate cells in the appropriate areas of the retina corresponding to the features in the visual scene.
What do we know about heredity and retinitis pigmentosa?
Since RP is an inherited disorder, it can potentially affect another member of the family.
With retinal cells being among the most specialized cells in the human body, they depend on a number of unique genes to create vision.
A disease-causing mutation in any one of these genes can lead to vision loss.
Researchers have discovered over 100 genes that can contain mutations leading to retinitis pigmentosa.
Approximately 50 percent of RP cases are isolated and have no previous family history.
The cause of these cases cannot be explained. Other cases of RP, where family history has been determined, fall into three main categories: autosomal recessive, autosomal dominant, and X-linked recessive.
Autosomal recessive RP occurs when both parents are unaffected carriers of the same defective gene.
The chances of a child being affected is one in four.
This means the affected child must inherit the defective gene from each parent.
The chances of a parent having an unaffected child who would be a carrier of the defective gene is one in two. The chance of parents having a child completely free of the RP gene is one in four.
In autosomal dominant RP, the disease is present in males or females only when a single copy of the gene is defective.
Typically, one of the parents is affected by the disease.
The chance is one in two of any given offspring being affected by the disease, if the affected parent has one normal and one defective gene.
X-linked recessive RP may occur in offspring in two ways.
The fathers can be affected or mothers can be carriers of the defective gene.
If the father is affected, all sons will be unaffected and all daughters will be carriers. I
f the mother is the carrier, 1 in 2 sons will be affected and 1 in 2 daughters will be carriers.
In families with the X-linked type, only males are affected, while females carry the genetic trait but do not experience serious vision loss.
More information: Silverman SM, Ma W, Wang Z, Zhao L, Wong WT. “C3- and CR3-dependent microglial clearance protects photoreceptors in retinitis pigmentosa.” Published online June 17, 2019 in the J Exp Med.
Journal information: Journal of Experimental Medicine
Provided by National Eye Institute