House dust mite allergens (HDM) cause bronchoconstriction in asthma patients and induce an inflammatory response in the lungs due to the release of cytokines, chemokines and additional mediators. The mechanism how HDM components achieve this is largely unknown. The objective of this study was to assess whether HDM components of Dermatophagoides pteronissinus with protease activity (Der p 1) and unknown enzymatic activity (Der p 2, Der p 5) induce biological responses in a human airway-derived epithelial cell line (A549), and if so, to elucidate the underlying mechanism(s) of action. A549 cells were incubated with HDM extract, Der p 1, recombinant Der p 2 and recombinant Der p 5. Cell desquamation was assessed by microscopy. The proinflammatory cytokines, IL-6 and IL-8, were measured by ELISA. Intracellular Ca2+ levels were assessed in A549 cells and in mouse fibroblasts expressing the human protease activated receptor (PAR)1, PAR2 or PAR4. HDM extract, Der p 1 and Der p 5 dose-dependently increased the production of IL-6 and IL-8. Added simultaneously, Der p 1 and Der p 5 further increased the production of IL-6 and IL-8. The action of Der p 1 was blocked by cysteine-protease inhibitors, while that of Der p 5 couldn't be blocked by either serine- or cysteine protease inhibitors. Der p 5 only induced cell shrinking, whereas HDM extract and Der p1 also induced cell desquamation. Der p 2 had no effect on A549 cells. Der p 1's protease activity causes desquamation and induced the release of IL6 and IL-8 by a mechanism independent of Ca2+ mobilisation and PAR activation. Der p 5 exerts a protease-independent activation of A549 that involves Ca2+ mobilisation and also leads to the production of these cytokines. Together, our data indicate that allergens present in HDM extracts can trigger protease-dependent and protease-independent signalling pathways in A549 cells.