Administration of 8-methoxypsoralen and ultraviolet A irradiation (PUVA) induces turnover of mast cells in the skin of C57BL/6 mice.

Administration of 8-methoxypsoralen followed by ultraviolet A irradiation (PUVA treatment) has been used as a therapy for urticaria pigmentosa. The effect of PUVA treatment on cutaneous mast cells in mice was investigated by using giant granules of mast cells from C57BL/6-bgJ/bgJ (Chediak-Higashi syndrome) mice as a marker. C57BL/6-(+)/+ mice were lethally irradiated and rescued by bone marrow transplantation from C57BL/6-bgJ/bgJ mice. In the radiation chimeras, mast cells in the skin were of +/+ type and mast-cell precursors migrating in the bloodstream were bgJ/bgJ. When PUVA treatment was applied to the skin of the radiation chimeras, the total number of mast cells continued to decrease until the third week after the treatment and then recovered to pre-treatment levels. The initial reduction was attributed to the decrease of +/(+)-type mast cells, and the subsequent recovery to be as a result of the increase of bgJ/bgJ-type mast cells. This observation may explain the fact that the therapeutic effect of PUVA treatment is transient. Symptoms of urticaria pigmentosa become manifest after the cessation of PUVA treatment probably because new mast cells differentiate from bone marrow-derived precursors.