Influence of climate change on the allergenicity of pollen: the involvement of pollen transglutaminase.

Climatic trends resulting from increases in temperature and outdoor air pollution influence the allergenicity of pollen, which in turn is responsible for the dramatic raise in respiratory allergic diseases attributed to climate change (hay fever, bronchial asthma, seasonal allergic conjunctivitis). Although the link between climate change and pollen allergenicity is proven, the underlying mechanism is little understood. Increased pollen production and higher airway sensitization have been blamed; however the effect of climate changes on the allergenicity of pollen grains has never been investigated. In mammals, type 2 TG (TG2) is implicated in autoimmune inflammatory conditions and enhances the activity of pro-inflammatory secretory phospholipase A2 (sPLA2). Moreover TG2 leads to protein modifications in conditions of cell stress in mammals and plants. Recently, a calcium-dependent transglutaminase activity has been identified in the pollen cell wall, raising the possibility that pollen TG may have a role in pollen antigen modification and in the transamidation of proteins present in the human mucosa interacting with surface pollen TG. In the present study we have investigated the role played by TG in the allergenic potential of aero allergenic pollen (Corylus avellana) and pollen of enthomophylus plants (Malus domestica) exposed to climate changes. Simulations of environmental conditions, to which biological aerosols are likely to be exposed prior to penetrating the airway mucosa and accessing the immune system, were conducted by incubating pollen grains in the environmental Phytotron chamber for 15h. The treatments included simulation of different atmospheric temperatures relative humidity (rH), simulation of acid rain at pH 5.6 and copper pollution (3.10 g/l). To correlate the effect of climate stress factors with pollen surface TG activity when pollen grains contact the airways mucosa in humans, the level of TG activity was measured in whole pollen particles in wet conditions at 37C, by means of a cross-linking plate assay. Pollen surface TG activity clearly increased in Corylus a pollen exposed to elevated temperature and decreased humidity (20C, 50% rH) compared to simulated natural conditions (10C, 70% rH) (+38%) while Malus d pollen TG was not significantly affected by temperature increases. Acid rain simulation led to a significant increase in pollen surface TG activity in Corylus a pollen in natural conditions (10C 70% hydration) (+76%;) with a worsening in warmer conditions (20C 50% rH; 30C 100% rH). Copper pollution also led to significant changes in pollen surface TG activity (+65%). The TG activity could be reversed by a variety of known irreversible and competitive inhibitors of TGs. Western blotting of pollen TG did suggest an induced expression of TG immunoreactive protein after exposure to stressors, while detection of in situ TG activity and morphological examinations of pollen indicated pollen damage and release of TG activity. Since an interesting feature of plant TGs lie in their ability to recognise not only plants substrates but also animal substrates, the cross- talk of pollen TG with inflammatory molecules from animal cells like sPLA2, is presently being investigated.