Birch pollen rupture and the release of aerosols of respirable allergens

Backgound Birch pollen allergens have been implicated as asthma triggers; however, pollen grains are too large to reach the lower airways where asthmatic reactions occur. Respirable-sized particles containing birch pollen allergens have been detected in air filters, especially after rainfall but the source of these particles has remained speculative.

Objective To determine the processes by which birch pollen allergens become airborne particles of respirable size with the potential to contribute to airways inflammation.

Methods Branches with attached male catkins were harvested and placed in a controlled emission chamber. Filtered dry air was passed through the chamber until the anthers opened, then they were humidified for 5 h and air-dried again. Flowers were disturbed by wind generated from a small electric fan. Released particles were counted, measured and collected for immuno-labelling and high-resolution microscopy.

Results Birch pollen remains on the dehisced anther and can rupture in high humidity and moisture. Fresh pollen takes as long as 3 h to rupture in water. Drying winds released an aerosol of particles from catkins. These were fragments of pollen cytoplasm that ranged in size from 30 nm to 4 μm and contained Bet v 1 allergens.

Conclusion When highly allergenic birch trees are flowering and exposed to moisture followed by drying winds they can produce particulate aerosols containing pollen allergens. These particles are small enough to deposit in the peripheral airways and have the potential to induce an inflammatory response.

Localisation of allergens in ryegrass pollen and in airborne micronic particles

In Melbourne, Australia, grass pollen allergens, especially from ryegrass, are a major cause of allergic hayfever and asthma. This review outlines recent developments in our understanding of how grass pollen allergens find their way into the atmosphere and how they are transported in particulate form. Much of this work has relied on antibody technology in immunological and immunocytochemical investigations. The localisation of allergens in situ has proved difficult due to their water-soluble character. Recently, allergens have been localised in developing ryegrass pollen by dryfixation, rapid-freeze and freeze-substitution techniques. This involved anthers being substituted in a mixture of aldehydes, organic solvents, and 2,2-dimethoxypropane. Incubation in dimethylsulfoxide prior to embedding in LR Gold resin provided good infiltration with freeze-substituted material. Immunogold-labelled sections show that the major allergens, Lol p 1 and Lol p 5, are synthesised in the pollen cytoplasm from the early bicellular stage, soon after the first starch granules are formed. From the early tricellular stage, Lol p 5 moves into the starch granules where it remains until maturity. Lol p 1 is localised in the cytoplasm of mature pollen grains. The incidence of airborne grass pollen, as measured in pollen traps, correlates with hayfever symptoms. Forecasting models which rely on rainfall and temperature data have been produced for the grass pollen (daily and seasonal) counts in Melbourne. Research over the past six years has shed light on the causes of grass-pollen-induced asthma. Micronic particles in the atmosphere may be starch granules originating from pollen grains osmotically ruptured by rainwater. Ultrastructural and immunological characterisation of micronic particles collected from outdoor air filters confirm the presence of airborne starch granules. These are loaded with grass pollen allergens, occur in the atmosphere especially after rainfall, and correlate significantly with instances of allergic asthma. Diesel particles might also play a role in the transmission of grass pollen allergens and thus become an extra asthma trigger. A variation in the mode of release of micronic particles occurs in other species, such as birch, where such particles are derived from burst birch pollen tubes. These particles are positive for Bet v 1 and are starch granules which are released into the atmosphere after light rain as a result of pollen germination on, e.g., leaves. After subsequent rupture of pollen tubes their contents are released when conditions become drier.