A daily dose of sunlight may help the immune system guard against invading pathogens and sun-induced skin damage, according to a new study. The findings reveal how immune cells specialize to protect the skin and suggest that staying out of the sun could cause harm if carried too far.
Immune cells called T cells battle infections and guard against cancer. But first they need to be tipped off about the threat. The informants are a group of cells called dendritic cells, which chew up infected and damaged cells and present the regurgitated pieces to T cells. If the T cells judge the pieces to be foreign or in need of removal, they reproduce, forming an army of clones that hunt down infected and rogue cells in the body.
The body's a big place, however, and scientists have long wondered whether these roving T cells receive other help finding their targets. Studies on the gut suggest that dendritic cells there release a chemical that induces T cells to produce a receptor that helps them home in on the intestine. Immunologist Eugene Butcher of Stanford University and colleagues wondered whether something similar happened in the skin.
What they found surprised them. Skin cells harness sunlight to make an inert form of vitamin D. Biologists had long thought that to become active--and thus usable by the body--this vitamin D precursor had to be processed by the kidney and liver. But Butcher's team showed that dendritic cells in the skin could accomplish the same task. Once they did, vitamin D made its way to nearby T cells, prompting the cells to make receptors specific to skin chemokines. With a better "nose" for these chemicals, the altered T cells make a beeline for the outer layer of the skin, where they began destroying defective and infected cells, the team reports in the February issue of Nature Immunology.
Because sunlight gets the whole process going, Butcher says the findings "suggest that getting some sun is good" for building the skin's defenses. It doesn't take much sun to get the vitamin D the body needs, however, and too much sun still poses a dangerous risk of skin cancer, the authors note.
"It's work of extremely high standards," says vitamin D expert Chantal Mathieu of the Catholic University of Leuven in Belgium. Hints of the story had been known from previous studies, but no one had made sense of it until now, she says.