Decreased chymase activity is associated with increased levels of proteaseinhibitors in mast cells of psoriatic lesions

Mast cells contain large amounts of the powerful serine proteinases, trypta se and chymase, of which only chymase can be inactivated by serum protease inhibitors. In this study, 20 patients with psoriasis and a control group o f 13 with atopic dermatitis were biopsied for lesional and non-lesional ski n specimens. The presence of chymase inhibitor alpha(1)-proteinase inhibito r (alpha(1)-PI), alpha(1)-antichymotrypsin (alpha(1)-AC), alpha(2)-macroglo bulin (alpha(2)-MG) and C1-esterase inhibitor (C1-Inh) immunoreactivity in mast cells was verified using the sequential double-staining method. Trypta se- and chymase-positive mast cells were stained enzyme-histochemically. Tr yptase-positive mast cells were increased in number in the upper dermis of the psoriatic lesion compared with lesion-free psoriatic skin (308 +/- 109 vs. 100 +/- 29 cells/mm(2), respectively, mean +/- SD, p < 0.0005, t-test) while the percentage of mast cells showing chymase activity Has decreased ( 76.8 +/- 22.1% vs. 28.6 +/- 14.4%, p < 0.0005). These findings are consiste nt with our previous ones. In contrast to the decreased percentage of chyma se-positive mast cells, a novel finding was that the percentages of alpha(1 )-AC(+) (86.9 +/- 7.2% vs. 59.5 +/- 12.6%, p < 0.0005), alpha(1)-PI+ (72.2 +/- 14.9% vs. 33.4 +/- 18.6%, p < 0.0005) and alpha(2)-MG(+) (16.8 +/- 7.0% vs. 6.2 +/- 3.5%, p < 0.002) mast cells were significantly higher in the p soriatic lesion with the exception of the percentage of C1-Inh(+) mast cell s (13.7 +/- 10.0% vs. 11.0 +/- 6.1%, p < 0.7). The localization of these in hibitors in mast cells is not a characteristic feature of psoriasis, since mast cells in atopic dermatitis skin also showed immunoreactivity though in slightly lower percentages. Previously, we have shown that MCTC (tryptase( +), chymase(+)) mast cells increase in number in the psoriatic lesion but c hymase becomes inactive. The results of this study show again the decreased chymase activity, which could he due to increased levels of its inhibitors (alpha(1)-AC, alpha(1)-PI and alpha(2)-MG) in the same mast cells, Thus, a ctive tryptase could promote inflammation but chymase seems not to be an im portant mediator in the pathomechanism of psoriasis.