- Exposure to sunlight is protective against autoimmune diseases, including multiple sclerosis.
- While vitamin D has often been implicated in this effect, there are other effects of UV light on the immune system.
- An MS Australia-funded study has identified new lipid molecules induced by UV that can suppress the immune response.
We have known for many years that exposure to UV radiation from the sun can reduce the immune response in the skin, and this can increase the risk of developing skin cancer.
However, UV exposure can also weaken the immune response more widely throughout the body. This can be beneficial in protecting against certain autoimmune diseases like multiple sclerosis (MS).
When UV light hits the skin, immune cells in the skin travel to the nearby lymph nodes. In these “hubs” of immune activity, they “talk” with many other cells to reduce the immune response.
UV suppresses immunity by trapping immune cells
In earlier work, Professor Scott Byrne and his team at the University of Sydney identified a profound effect of UV exposure: that it traps immune cells inside lymph nodes near the UV-treated skin.
Trapping the immune cells means they cannot travel to other parts of the body to cause inflammation and damage, such as to the brain and spinal cord in MS.
Interestingly, the MS medications Gilenya, Mayzent and Zeposia (fingolimod, siponimod and ozanimod) work in the same way by trapping immune cells in the lymph nodes.
The team previously showed that a fat (lipid) molecule called sphingosine-1-phosphate, was responsible for this trapping effect. This provided the first evidence that UV can suppress the immune system by altering lipids in the lymph nodes.
What was the aim of this study?
In this new study, the researchers asked whether there may be other lipids induced by UV that can suppress the immune system.
What did the researchers do?
Using a laboratory model, the team conducted experiments investigating lipids in lymph nodes near the skin, before and after UV exposure.
Six unique, previously unidentified lipids were increased in response to UV.
Using cutting-edge microscopic imaging technology, researchers showed that levels of these lipids increased within the lymph nodes in areas where particular immune cells, called T cells, are grouped together.
To understand the effect of these UV-induced lipids, researchers extracted them from the lymph nodes and tested them in the lab. They found that these lipids prevented T cells from multiplying.
UV suppresses immunity by preventing immune cell expansion
T cells play a major role in causing inflammation and damage to the brain and spinal cord in MS.
They induce a strong immune response when they recognise a “danger signal”. The T cells then multiply rapidly to form an immune “army” against the danger.
This research has shown that UV induces lipids that suppress this rapid T cell expansion, so that the immune response is dramatically weakened.
In the case of MS, this could result in reducing the autoimmune attack on the brain and spinal cord.
What does this mean for people with MS?
This discovery could lead to a better understanding of how exposure to sunlight is beneficial in the context of MS.
But as Professor Byrne explains, “UV from sunlight can cause cancer. For people with MS who often struggle with heat intolerance, it can also make them uncomfortably hot.”
“Finding the molecules responsible for the UV effect on the immune system opens the door for development of new therapies that could mimic the immune suppressive effects of sunlight without these side effects.”
Moving from the laboratory models used in this study, Professor Byrne said, “Our next steps are to investigate whether similar UV-induced changes to these molecules occur in humans.”
This research was supported by an MS Australia Incubator grant and was published in the prestigious journal, Frontiers in Immunology.
Incubator grants are a type of funding that helps to develop creative and innovative ideas that may be “outside the square”. These grants aim to support the discovery of new and better ways to prevent, treat, and cure MS.
IMAGE Credit: shared with permission from PhD thesis: Lipids in ultraviolet-radiation induced immunomodulation, University of Sydney (2020), Benita Tse.”