Study Finds Surprise Benefit of Sunshine

Spring/Summer 2017

Sunlight allows us to make vitamin D, but a new research finding could reveal another powerful benefit of getting some sun. Georgetown University Medical Center researchers have found that sunlight, through a mechanism separate from vitamin D production, energizes T cells that play a central role in human immunity.

“We all know sunlight provides vitamin D, which is suggested to have an impact on immunity, among other things. But what we found is a completely separate role of sunlight on immunity,” says the study’s senior investigator, Gerard Ahern, PhD, associate professor in pharmacology and physiology. “Some of the roles attributed to vitamin D with regard to immunity may actually be due to this new mechanism.”

They specifically found that blue light, found in sunlight, makes T cells move faster—marking the first report of a human cell responding to sunlight by speeding its pace.

“T cells, whether they are helper or killer, need to move to do their work, which is to get to the site of an infection and orchestrate a response,” Ahern says. “This study shows that sunlight directly activates key immune cells by increasing their movement.”

Ahern also added that while production of vitamin D required UV light, which can promote skin cancer and melanoma, blue light from the sun, as well as from special lamps, is safer.

And while the human and T cells they studied in the laboratory were not specifically skin T cells they were isolated from mouse cell culture and from human blood —the skin has a large share of T cells in humans, he says, approximately twice the number circulating in the blood.

“We know that blue light can reach the dermis, the second layer of the skin, and that those T cells can move throughout the body,” he says. The researchers further decoded how blue light makes T cells move more by tracing the molecular pathway activated by the light.

What drove the motility response in T cells was synthesis of hydrogen peroxide, which then activated a signaling pathway that increases Tcell movement. Hydrogen peroxide is a compound that white blood cells release when they sense an infection, in order to kill bacteria and summon T cells and other immune cells to mount an immune response.

“We found that sunlight makes hydrogen peroxide in T cells, which makes the cells move. And we know that an immune response also uses hydrogen peroxide to make T cells move to the damage,” Ahern says. “This all fits together.”

Ahern says there is much work to do to understand the impact of these findings.