Study results of individuals with various skin diseases, including psoriasis and atopic eczema, found that narrowband UV light increased regulatory T cells and thus inhibited the immune system, particularly when 25-hydroxyvitamin D levels were low.
The study’s participants (n=22) were recruited from December 2010 to March 2011 in Scotland, a time when there are fewer hours of daylight in the northern portion of the country and when 25-hydroxyvitamin D [25(OH)D] levels for individuals are at their lowest each year. None of the participants had taken vitamin D supplements or fish oil with vitamin-D before the study.
After receiving full-body phototherapy treatment three times a week with narrowband UV-B, each participant’s blood was measured for 25(OH)D levels at study entry, at 2 weeks, and at 4 weeks. Also measured were the numbers of regulatory T (Treg) cells and proliferative and cytokine responses to stimulation by anti-CD3 and anti-CD28 beads, which stimulate Treg cells.
The participants received UV-B doses equivalent to approximately 25% of the sunlight they would be exposed to during summer months; this is a standard phototherapy dose to treat erythema. The levels of 25(OH)D increased from a mean standard deviation of 34±17 nmol/L at baseline to 78±19 nmol/L after 4 weeks.
Circulating Treg cells increased from a mean of 0.5% to a mean of 1.6%, a significant correlation with the increased levels of 25(OH)D.
The UV treatments caused a reduction in the proliferative and IL-10 responses to anti-CD3 and anti-CD28.
“It is likely that vitamin D status is a key determinant of regulatory T-cell numbers,” the investigators said.