A certain hormone works in tandem with vitamin D to control how skin cells produce a natural microbe-fighting agent, and can compensate for a lack of vitamin D, scientists have found.
The new results help explain something that has confused researchers for a long time: although it is known that vitamin D plays a role in the immune defense, there are very few clinical trials that show that taking supplemental vitamin D helps prevent infection.
The work was led by Richard Gallo, a professor of medicine and pediatrics and chief of the Division of Dermatology at the University of California, San Diego. It was published this week in the journal Science Translational Medicine.
The skin makes natural antimicrobial compounds (protein fragments called peptides) to kill unwanted bacteria, fungi, and viruses. Eczema patients produce these compounds at lower levels than normal; psoriasis patients, at higher levels. Vitamin D initiates production of cathelicidin, a broad-spectrum antimicrobial.
Gallo and colleagues showed that human skin cells produce parathyroid hormone (PTH) when treated with a bacterial compound known to trigger the immune system. The same cells, stimulated with vitamin D, manufactured copies of the receptor for PTH. And the skin cells produce far more cathelicidin when they are treated with parathyroid hormone and vitamin D than with either compound alone.
The results suggest a model in which, in humans, vitamin D can stimulate cathelicidin production by itself—but PTH is doing so by a parallel pathway, which vitamin D can amplify.
The scientists also showed that PTH helps reduce the severity and extent of Streptococcus skin infections in mice—but it does so much more strongly in normal mice, compared to mice genetically engineered to be unable to convert vitamin D to its active form. (Apparently it is very difficult to make a mouse deficient in vitamin D.)
What this means for eczema patients is not clear yet. The research gets us further toward understanding how vitamin D and other factors participate in the skin’s immune response. If I were a doctor, it would make me hesitant to recommend that patients with normal vitamin D levels should take supplements.