The biochemical mechanisms through which eosinophils contribute to
asthma pathogenesis are unclear. Here we show
eosinophil peroxidase (EPO), an abundant granule
protein released by activated eosinophils, contributes to characteristic
asthma-related phenotypes through oxidative posttranslational modification (PTM) of
proteins in asthmatic airways through a process called carbamylation. Using a combination of studies we now show EPO uses plasma levels of the pseudohalide
thiocyanate (SCN-) as substrate to catalyze protein carbamylation, as monitored by PTM of
protein lysine residues into Nϵ-carbamyllysine (
homocitrulline), and contributes to the pathophysiological sequelae of eosinophil activation. Studies using EPO-deficient mice confirm EPO serves as a major enzymatic source for protein carbamylation during eosinophilic inflammatory models, including aeroallergen challenge. Clinical studies similarly revealed significant enrichment in carbamylation of airway
proteins recovered from atopic asthmatics versus healthy controls in response to segmental
allergen challenge.
Protein-bound
homocitrulline is shown to be co-localized with EPO within human asthmatic airways. Moreover, pathophysiologically relevant levels of carbamylated
protein either incubated with cultured human airway epithelial cells in vitro, or provided as an aerosolized exposure in non-sensitized mice, induced multiple
asthma-associated phenotypes including induction of
mucin, Th2
cytokines, IFNγ, TGFβ, and epithelial cell apoptosis. Studies with scavenger receptor-A1 null mice reveal reduced
IL-13 generation following exposure to aerosolized carbamylated
protein, but no changes in other
asthma-related phenotypes. In summary, EPO-mediated protein carbamylation is promoted during
allergen-induced
asthma exacerbation, and can both modulate immune responses and trigger a cascade of many of the inflammatory signals present in
asthma.