Sunlight, in addition allowing us to make vitamin D, energizes T cells that play a central role in human immunity, Georgetown University Medical Center researchers have found. The findings suggest how the skin, the body’s largest organ, stays alert to the many microbes that can nest there.
The study’s senior investigator, Gerard Ahern, PhD, associate professor in the Georgetown’s Department of Pharmacology and Physiology, said:
“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. Some of the roles attributed to vitamin D on immunity may be due to this new mechanism."
“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 added that although 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.
T Cell Pathways
The researchers further decoded how blue light makes T cells move more by tracing the molecular pathway activated by the light.
“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.
What drove the motility response in T cells was synthesis of hydrogen peroxide, which then activated a signaling pathway that increases T cell movement. Hydrogen peroxide is a compound that white blood cells release when they sense an infection in order to kill bacteria and to “call” 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, but he suggests that if blue light T cell activation has only beneficial responses, it might make sense to offer patients blue light therapy to boost their immunity.
Thieu X. Phan, Barbara Jaruga, Sandeep C. Pingle, Bidhan C. Bandyopadhyay & Gerard P. Ahern Intrinsic Photosensitivity Enhances Motility of T Lymphocytes Scientific Reports 6, Article number: 39479 (2016) doi:10.1038/srep39479
Image: A. Walker, L. Sharp & J. Pryde, Wellcome Images. Human T cells showing nuclei