The Role of Alternaria and its Major Allergen, Alt a 1, in the Pathogenesis of Allergic Airway Disorders

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Virginia Tech

Chronic exposure to the ubiqutious airborne fungus, Alternaria alternata, has long been implicated in the development and exacerbation of human allergy and asthma. Alt a 1 was identified previously by several groups as the major allergen secreted by A. alternata, due to its IgE-specific reactivity with sera from atopic patients. Despite the well-documented clinical importance of Alternaria and its major allergen, little knowledge exists regarding their role and interaction with the innate immune system. Here for the first time we characterize the innate immune response to A.alternata and verify the significance of Alt a 1 in contributing to this response in human airway cells and murine models. Our studies establish a baseline response for both a chronic and single-challenge murine inhalation model with Alternaria spores. Both models demonstrate live conidia induce a robust response, arguably more pathologically relevant compared to studies employing Alternaria extracts. We also elucidate the overall importance of Alt a 1 by utilizing recombinant Alt a 1 protein, A. alternata (Δalt a 1) deletion mutants, and an A.alternata (Alt a 1+) overexpression mutant. Both Alt a 1 protein and A. alternata conidia stimulated production of pro-inflammatory cytokines/chemokines in mice after a single intranasal challenge. Infiltration of effector cells (macrophages, neutrophils, eosinophils, and lymphocytes) into the lungs along with other hallmarks of airway inflammation was observed. In addition, Alt a 1 protein and conidia evoked secretion of pro-inflammatory cytokines in treated human airway epithelial cells while the Alt a 1+ overxpression mutant induced a significantly higher response. In contrast, spores of Δalt a 1 caused an attenuated response in both human cells and murine lungs suggesting that this single protein may play a major role in inducing the innate immune response in airway epithelium at the organismal level. Finally, we identified key biochemical properties of the Alt a 1 protein including a single histidine required for esterase activity and a unique RXLR-like motif which controls Alt a 1's ability to bind external lipids and enter human airway cells. Overall, these results improve our understanding of how Alternaria induces innate immunity and identifies possible therapeutic targets within allergenic proteins.

Alternaria, innate immunity, asthma, cellular entry, allergy, Alt a 1, lipid binding, IRAK, RXLR-motifs