Surprise Benefits of Sunlight in Immunity

    Sunlight allows us to make vitamin D, credited with healthier living, but a surprise research finding could reveal another powerful benefit of getting some sun.  There is something new under the sun—speedier T cells. They are found in the skin, which harbors a larger share of cells in humans, about twice the number found in blood. T cells, it turns out, are intrinsically photosensitive, and when they manage to catch some rays, they react by moving more quickly, potentially enhancing their ability to do their work, which is to travel to infection sites and mount orchestrate an immune response.

    The new findings about light-responsive T cells comes from Georgetown University, where researchers indicate that the sun-enhanced immune response is independent of the production of vitamin D, another sun-catalyzed process that has been linked to immunity, among other things.

    “We found a completely separate role of sunlight on immunity,” said Gerard Ahern, Ph.D., an associate professor in Georgetown University’s Department of Pharmacology and Physiology. “Some of the roles attributed to vitamin D on immunity may be due to this new mechanism.”

    Dr. Ahern is the lead author of a study (“Intrinsic Photosensitivity Enhances Motility of T Lymphocytes”) that appeared December 20 in the journal Scientific Reports. The study emphasizes that whereas vitamin D production requires UV light, which can promote skin cancer and melanoma, enhanced T-cell motility makes do with blue light, which is safer.

    A T cell, or T lymphocyte, is a type of lymphocyte (a subtype of white blood cell) that plays a central role in cell-mediated immunity.

    “We know that blue light can reach the dermis, the second layer of the skin,” Dr. Ahern noted. “This study shows that sunlight directly activates key immune cells by increasing their movement.”

    This finding, Dr. Ahern continued, applies to both helper and killer T cells in the skin. Once these cells are activated by blue light, which may be provided by the sun or special lamps, they can move throughout the body.

    The current study illuminated the mechanism by which light affects immune function. 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.

    “Blue light irradiation at low doses (<300 mJ cm⁻²) triggers synthesis of hydrogen peroxide (H₂O₂) in T cells revealed by the genetically encoded reporter HyPerRed,” wrote the authors of the Science Reports article. “In turn, H₂O₂ activates a Src kinase/phospholipase C-γ1 (PLC-γ1) signaling pathway and Ca²⁺ mobilization. Pharmacologic inhibition or genetic disruption of Lck kinase, PLC-γ1 or the T cell receptor complex inhibits light-evoked Ca²⁺ transients.”

    “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,” Dr. Ahern explained. “This all fits together.”

    Dr. Ahern added that there is much work to do to understand the impact of these findings, but he suggested 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.