Background:
The prevalence of allergy to aeroallergens is rising and attributed to lifestyle- and environmental changes such as nitrogen pollution, although causality is often difficult to infer. Despite its known ecological impact, the impact on pollen allergenicity of environmental nitrogen pollution remains poorly circumscribed. We hypothesize that soil nitrogen pollution may directly affect pollen production and/or properties, resulting in pollen collected from different environmental nitrogen deposition conditions exhibiting different allergenicity in sensitized patients.
Methods:
Fresh Betula pollen samples were collected throughout Europe (e.g. Belgium, Sweden, Ireland, Poland, Ukraine, Spain, France). In 2022, 66 samples were collected in a paired design in Belgium. Groups of birch trees close to each other but with different environmental nitrogen conditions (natural vs agricultural land) were sampled. In 2023, samples from Belgium (n = 35) and Europe (n = 63) were collected over a gradient of environmental nitrogen based on nitrogen deposition map data (Eutrophication caused by atmospheric nitrogen deposition in Europe, European Environment Agency, EEA). Nitrogen deposition values were extracted from modeled air concentrations and depositions from the European Monitoring and Evaluation Program (EMEP). Fresh pollen, leaf nitrogen and carbon content were measured (CHNS elemental analyzer, EA1108 by Carlo Erba). Soluble proteins were extracted and normalized for protein concentration (Bradford). A selection of good quality pollen (10/66 for 2022 and 20/98 for 2023) extracts representing the nitrogen gradient, was used to determine their allergenicity in pollen-allergic adult patients (n = 60 in total) using ex vivo basophil activation testing (BAT) and specific IgE (sIgE) determination (ImmunoCAP, Phadia). For BAT, area under the curve (AUC), half maximal effective concentration (EC50), and maximal reactivity were compared.
Results:
No difference in BAT reactivity was observed in a subset of 20 patients. Also no difference in sIgE titer was seen in the natural vs. agricultural group (mean 12.51 vs. 12.47 kUA/mL, paired t-test, p = 0.9075). For pollen obtained in 2023 in Europe (10/63) and Belgium (10/35) over a nitrogen deposition gradient, a significantly higher estimated reactivity was observed in samples with lower predicted nitrogen deposition (BAT AUC estimated effect for total N-deposition -402.631 (EU, p = 0.041) and -539.178 (BE, p = 0.014)), linear mixed model analysis). We are awaiting results that correlate sIgE with nitrogen deposition values. We did observe a significant positive estimated effect of initial pollen protein concentration on BAT reactivity (AUC 25.053 (EU, p < 0.001) and 8.213 (BE, p < 0.001)), despite our correction for this in our experimental setup. Initial protein concentrations did not correlate with nitrogen deposition estimates or measurements (total N-deposition, n = 41, ρ = -0.3078, p = 0.0503).
Conclusion:
Betula pollen collected during two consecutive sampling rounds in Belgium and across Europe from sites with varying levels of environmental nitrogen deposition did not consistently impact pollen allergenicity. Further analysis will assess how environmental nitrogen enrichment affects pollen production capacity and its protein characteristics.