SUMMARY:An epicutaneous application of 2,4-dinitrofluorobenzene (DNFB) to a mouse ear caused a transient skin swelling, and the repetition of the challenge enlarged the contact dermatitis. The repeated challenge with DNFB also induced eosinophil infiltration on the application site. Administration of a chymase inhibitor significantly inhibited the ear swelling as well as eosinophil accumulation. An intradermal injection of human chymase to the mouse ear also elicited transient skin swelling and eosinophil infiltration, both of which were augmented in proportion to the number of injections. Human serum albumin and heat-inactivated chymase failed to induce such skin reactions, suggesting the participation of proteolytic activity of the enzyme. In addition, chymase stimulated eosinophil migration in vitro in a concentration-dependent manner. Taken together, these observations suggest that mast cell chymase may contribute to development of the DNFB-induced dermatitis, probably by promoting eosinophil infiltration. It is therefore possible that chymase plays a role in pathogenesis of chronic dermatitis such as atopic dermatitis. (Lab Invest 2002, 82:789 -794).A topic dermatitis (AD) is a chronic eczematous skin disorder characterized by dry and itchy skin. The patients with AD often have a family history of other allergic diseases including asthma and hay fever. Although AD is known to be a manifestation of immediate hypersensitivity mediated by IgE, delayed-type hypersensitivity is also involved in the skin reaction of the patients (Tanaka et al, 1994;Varela et al, 1999). Interestingly, in mice, repeated application of contact sensitizing agents, such as 2,4-dinitrofluorobenzene (DNFB) and trinitrochlorobenzene, develops an immediate-type reaction, in contrast to a single application that causes a typical delayed-type reaction (Kitagaki et al, 1995(Kitagaki et al, , 1997. Repeated elicitation with such agents not only increases the serum level of IgE but also induces Th2-type cytokine expression (Nagai et al, 1997b). In addition, mast cell accumulation occurs in the dermis because of the repeated challenge (Kitagaki et al, 1995), as is often observed in AD patients. Therefore, this animal model is thought to be valuable for studying human AD.Chymase is a chymotrypsin-like serine protease and is primarily stored in secretory granules of mast cells (Schwartz and Austen, 1980). Chymase cleaves a variety of physiological substances, including metalloproteases (Fang et al, 1997), procollagen (Kofford et al, 1997), precursor of interleukin 1†(IL-1â€) (Mizutani et al, 1991b), and membrane-associated stem cell factor (Longley et al, 1997), while its precise role is not clear. Human mast cells are classified into two types, MC TC and MC T , based on their composition of serine protease (Irani et al, 1986). MC TC contains both chymase and tryptase, while MC T expresses tryptase but not chymase. Because MC TC predominates in the skin (Irani et al, 1989), chymase has been implicated in the pathogenesis of skin disorders complic...