Ultraviolet radiation from sunshine seems to thwart multiple sclerosis, but perhaps not the way most researchers had assumed, a new study in mice suggests.
If validated in further research, the finding could add a twist to a hypothesis that has gained credence in recent decades. The report appears online March 22 in the Proceedings of the National Academy of Sciences.
Scientists have hypothesized that MS is rare in the tropics because people synthesize ample vitamin D from exposure to the UV radiation in equatorial sunlight. What’s more, MS is more common in the high latitudes of northern parts of Europe and North America than in regions farther south. That pattern has led to the assumption that higher levels of vitamin D might prevent people from developing MS, what became known as the latitude hypothesis.
But a direct cause-and-effect relationship between vitamin D deficiency and MS has never been established. In past experiments, giving vitamin D supplements to mice with an MS-like disease required giving the animals harmful amounts of the nutrient, notes Hector DeLuca, a biochemist at the University of Wisconsin–Madison.
“It just didn’t add up,” he says. “We decided to go back and see if maybe UV light by itself was doing something.”
In MS, the fatty myelin sheaths that insulate nerves in the central nervous system are damaged by attacks by the immune system. In a series of experiments in mice, DeLuca and his team induced a condition comparable to human MS by injecting the animals with proteins that instigate similar myelin damage.
The researchers exposed some mice to UV radiation before and after giving the animals the damaging injection. Another group of mice got the injection but not the UV exposure.
The mice exposed to UV rays suppressed the effects of MS-like disease better than the control mice, the researchers found, even though the amount of radiation wasn’t enough to greatly increase the animals’ blood concentrations of vitamin D.
In another test, the researchers gave injected mice varying doses of vitamin D supplements, but no UV radiation. The supplements failed to control the disease onset, severity or progression.
“We concluded that UV light is doing something beyond [making] vitamin D,” DeLuca says.
There’s no question that the latitude hypothesis has merit, says George Ebers, a neurologist at the University of Oxford in England. “MS risk is geographically related.” But that risk is more complicated than exposure to UV radiation during an MS attack, as this mouse model used. For example, previous research has shown that children in northern latitudes who are born in May, after their mothers had spent a winter with little sunshine, are more likely to develop MS than are kids born in November, he says.
Ebers notes that mice in this study were exposed or not exposed to UV over a matter of weeks and were in the throes of an MS-like disease during the study. “That’s completely separate ... from the question of whether your risk is boosted or diminished by where your mother lived,” he says.
Apart from the timing issue, MS risk might well be influenced by a biological mechanism apart from vitamin D blood levels, but many questions remain, Ebers says. Those include how UV radiation might inhibit MS and, more specifically, what is the effect of UV rays in suppressing the immune system. “It’s quite possible that UV exposure will have a number of other mechanisms and be involved in hormonal circuits,” he says.DeLuca and his colleagues speculate that UV radiation is playing a mysterious role in MS that is independent of vitamin D production. “We're doing experiments trying to find out what it is," he say