Background: Dermatophagoides pteronyssinus is a trigger of atopic dermatitis. Many D pteronyssinus allergens are proteases that can elicit airway inflammation by stimulating the release of cytokines and chemokines by bronchial epithelial cells. Objective: We sought to investigate whether D pteronyssinus allergens can exert a similar activity on skin keratinocytes. Methods: Primary cultures of keratinocytes from healthy subjects or patients with atopic dermatitis and normal human bronchial epithelial cells were compared for cytokine production in response to D pteronyssinus extract. Results: Keratinocytes, but not bronchial epithelial cells, displayed a modest dose-dependent release of IL-1⍺ and IL-1 receptor antagonist but no induction of their mRNA after exposure to D pteronyssinus . However, D pteronyssinus also degraded these cytokines. On the other hand, D pteronyssinus extract induced bronchial epithelial cells, but not keratinocytes, to increased expression of IL-8/CXCL8 and GM-CSF mRNA and protein. These effects were efficiently abrogated by a mixture of cysteine and serine protease inhibitors. Both IL-8 and GM-CSF were fully resistant to D pteronyssinus proteolytic attack. No induction of monocyte chemoattractant protein 1/CCL2, RANTES/CCL5, or IFN-γ-induced protein of 10 kd/CXCL10 was detected in either cell type. Only bronchial epithelial cells expressed protease-activated receptor (PAR) 4 mRNA, whereas PAR-1, PAR-2, and PAR-3 mRNA was found in both cell types. D pteronyssinus did not affect PAR mRNA signals. Conclusions: Although D pteronyssinus can cause proteolysis-dependent release of cytokines from keratinocytes, it appears incapable of activating de novo expression of cytokines and chemokines, arguing against a direct proinflammatory activity of house dust mite on the skin. (J Allergy Clin Immunol 2002;109:532-8.)