The role of mast cell proteases in allergic disease and apoptosis

Mast cells (MCs) are key effector cells in allergic reactions, through the release of a wide variety of granule-stored and de novo synthesized inflammatory mediators. The MC secretory granules contain exceedingly high levels of serglycin proteoglycan and the heparin-binding proteases chymase, tryptase and carboxypeptidase A. In this thesis the contribution of mouse mast cell protease (mMCP)-4, which is thought to be the functional homolog to the human chymase, was studied in the context of allergic airway inflammation. Using two models of allergic airway inflammation, wild-type (WT) and mMCP-4 deficient (mMCP-4-/-) mice were treated with ovalbumin (OVA) or with house dust mite (HDM) extract. We found that the OVA challenged mMCP-4-/- mice displayed increased airway hyperreactivity and lung eosinophilia and in the HDM model they displayed increased serum IgE levels. Moreover, the level of IL-33, a pro-inflammatory cytokine, was enhanced in the lung tissue in mMCP-4-/- mice compared to WT mice after HDM-treatment. The active proteases stored in MC granules have the ability to cleave a number of components upon degranulation. We could demonstrate that proteolytic degradation of IL-13 by MCs is mediated by a serine protease, dependent on serglycin proteoglycan for its storage. Permeabilization of lysosomal membranes often leads to apoptosis and the released proteases take part in this process, activating pro-apoptotic compounds. We have found that serglycin-/- MCs are more resistant to apoptosis induced by secretory granule damage. We showed that serglycin-/- MCs exhibited reduced caspase-3 activity and protease activity in the cytosol compared to WT cells. Taken together, the studies in this thesis suggest that MCs chymase plays a protective role in the development of allergic airway inflammation and this could possibly be explained by chymases ability to degrade the pro-inflammatory cytokine, IL-33. In addition, we also suggest that serglycin proteoglycan and serglycin-dependent MC proteases participate in IL-13 degradation as well as in MC apoptosis induced by secretory granule damage.