Professor Scott Byrne, based at the University of Sydney in Australia, is one of a team of researchers around the world looking into how sunlight may affect human health in many ways which are not yet fully understood. Here he answers some questions about how UV rays could affect the body’s immune system
How do you think sunlight affects the immune system via the skin?
A: This is a really good question. When you consider that sunlight doesn’t actually penetrate very deep into the skin, it is remarkable that it can have distant, whole body effects. We and others have shown that when sunlight hits the skin, it causes the production and release of lots of different immune modulating molecules that can act locally at the site of irradiation. Some of these molecules, and the skin cells they affect, find their way to other, more distant sites of the body. This is how we think sunlight affects the immune system via the skin.
Are UVB rays the key ones or UVA as well?
A: We’ve shown that both UVA and UVB can suppress the immune system, but via different mechanisms. This means both UVA and UVB make major contributions to skin cancer development and that protection from both is needed to combat the skin cancer problem. However, it also means that both UVA and UVB are likely to be involved in the systemic effects of UV radiation.
What about the link to MS? Is this very tenuous?
A: We’ve known for some time now that the association between sunlight exposure and MS is quite strong. In our animal models of MS we can demonstrate a clear link between UV and suppression of the disease. These studies have allowed us to discover a key role for a particular immune cell which we call a “UV-regulatory B cell” (see here: https://www.ncbi.nlm.nih.gov/pubmed/27289166). We’re now working with Prof Prue Hart at the Telethon Kids in Australia on the PhoCIS study to determine whether the same types of UV-activated immune cells are also involved in protecting humans from developing MS.
MS is more common in northern latitudes. Is this a red flag that UVB plays a part? is this to do with Vitamin D deficiency or just UV?
A: And more common in southern latitudes! (think Tasmania vs Queensland in Australia, Nth vs Sth Chile in South America and the southern vs northern islands of New Zealand)! MS is what we call a “latitude dependent autoimmune disease” – that is, the further you move away from the equator, the more prevalent MS is. This suggests, but does not prove, that sunlight is involved in MS pathogenesis. This is where our animal models are so valuable and why the PhoCIS study led by Prof Hart is so important.
I’m often asked whether the latitude gradient is due to Low Vit D or is it sunlight that’s more important? To be honest we don’t know, it’s probably both, but much more research is required to really know the answer to this one. What I can say, is that genetic studies have clearly shown that Vit D genes are a risk factor for MS, and many people with MS present with low Vit D levels. So Vit D is likely to be involved. However, Vit D supplementation trials have been mostly disappointing which has led some to argue that what we may need is better Vitamin D analogues that are more effective and/or more sun – but this is a difficult message to sell, particularly in countries like Australia and New Zealand which have serious and realproblems with UV-induced skin cancers. Our approach has been to understand the mechanisms involved in UV-induced immune suppression so that we can harness the power of the sun, safely, without increasing the risk of skin cancer.
Would you suggest that people wear lower SPF or even go ‘skin naked’ in the UK to get the right amount of sunlight?
A: I’m not really qualified to comment on this. What I would say is that we really don’t know how much, or what type of solar UV is required to protect us from autoimmune diseases like MS. Due to the lack of investment from funding bodies, we don’t even know how much, or the type of UV that is required to cause skin cancer! It is routinely advised to avoid getting a sunburn, but doses much lower than this can cause significant DNA damage and suppress the immune response.
In a recently published consensus statement, it was concluded that “People living in Australia and New Zealand are now advised to apply sunscreen every day when the UV index is predicted to reach 3 or above”.
Are there some areas of the skin which are ‘thinner’ and better UV absorption areas than others? ie elbows?
A: The skin varies in thickness throughout the body, but the thickness of the skin does not dictate UV absorption, this is a physical property of the light itself. Longer wavelengths like UVA penetrate deeper than shorter wavelengths like UVB. However, with thinner skin, not only UVA, but more UVB might be able to penetrate deeper which could be causing unknown effects on the underlying blood vessels and connective tissue. We have no idea whether this would be “better” or “worse”
To prevent skin ageing on the face, would the ideal solution be to cover up on the face with high SPF while walking round in shorts and short-sleeved shorts for HOW long in the summer sun in the UK?
A: I’m not familiar enough with UK sun to answer this question. What I can say about photoaging is that this is caused mainly from UVA radiation so protection using a broad spectrum, high SPF sunscreen is always advised.