Mast cell tryptase and chymase in developing and mature psoriatic lesions

The number and distribution of mast cells in nonlesional and lesional skin samples from 13 psoriatic patients were analyzed enzyme- and immunohistochemically. Mast cell tryptase was stained with the sensitive substrate Z-Gly-Pro-Arg-4-methoxy-2-naphthylamide, and chymase with Suc-Val-Pro-Phe-MNA and monoclonal B7 anti-chymase antibody. In addition, healthy-looking skin from 27 psoriatic patients was tape-stripped resulting in induction of the Köbner response in 9 patients. Sequential biopsies were taken before and after (7, 14 and 21 days) tape-stripping, and both tryptase and chymase were stained enzyme-histochemically. In nonlesional psoriatic skin, 70 ± 24% (mean ± SD) of the mast cells contained chymase enzyme activity, and 78 ± 18% chymase immunoreactivity. About 10% of the chymase-immunoreactive cells lacked chymase activity. In lesional psoriatic skin, tryptase-positive cells were increased in number throughout the dermis but especially beneath the epidermis. Chymase immunoreactivity paralleled the tryptase activity, whereas chymase activity was strongly diminished both in terms of mast cell numbers and in staining intensity in the papillary dermis. The apparent inactivation of chymase may be due to the action of the chymase inhibitors, ₁-antitrypsin and ₁-antichymotrypsin, localized immunohistochemically in mast cells of lesional and nonlesional psoriatic skin. In the developing psoriatic lesion, mast cells displaying chymase activity were already 27–38% decreased in number in the upper dermis on day 7 after tape-stripping, along with the first clinical signs of psoriasis. Earliest alterations in tryptase-positive cells were observed on day 14 as increased mast cell contacts with the epidermis combined with only a slight increase in mast cell numbers in the upper dermis. During the development of a psoriatic lesion, TC mast cells (tryptase⁺, chymase⁺) increase in number in the upper dermis, but chymase becomes inactive at an early stage. The abundant presence of active trypase but inactive chymase in the upper dermis may have a potential role in psoriasis since both of these enzymes can process several biologically active peptides and proteins.