UV suppression of mast cell-mediated wealing in human skin.

Prior exposure of normal skin to subthreshold doses of UVR from a "UVA" phototherapy cabinet resulted in a small but significant reduction of the wealing response to intradermally injected codeine phosphate, a mast cell degranulating agent. The flare component of the reaction was not affected, nor were the responses to intradermally injected histamine. Both UVA and UVB from banks of filtered fluorescent tubes appeared to produce a similar but less pronounced effect. Dose-response studies revealed that the inhibition of mast cell-mediated wealing required a certain cumulative threshold dose of UVR (about 35 J/cm2 UVA + 20 mJ/cm2 UVB). The observed reduction in wealing could not be entirely explained by differences in the content of tissue histamine. It is concluded that repeated exposure to a broad range of UV wavelengths can depress the response of mast cells in vivo to degranulating agents by an unknown mechanism. This finding may have relevance to some of the observed therapeutic effects of phototherapy in certain inflammatory dermatoses and especially cutaneous mast cell disorders.