The Plight of the Metabolite: Oxidative Stress and Tear Film Destabilisation Evident in Ocular Allergy Sufferers across Seasons in Victoria, Australia

Abstract

Ocular allergy (OA) is characterised by ocular surface itchiness, redness, and inflammation in response to allergen exposure. The primary aim of this study was to assess differences in the human tear metabolome and lipidome between OA and healthy controls (HCs) across peak allergy (spring-summer) and off-peak (autumn-winter) seasons in Victoria, Australia. A total of 19 participants (14 OA, 5 HCs) aged 18-45 were recruited and grouped by allergy questionnaire score. Metabolites and lipids from tear samples were analysed using mass spectrometry. Data were analysed using TraceFinder and Metaboanalyst. Metabolomics analysis showed 12 differentially expressed (DE) metabolites between those with OA and the HCs during the peak allergy season, and 24 DE metabolites were found in the off-peak season. The expression of niacinamide was upregulated in OA sufferers vs. HCs across both seasons (p ≤ 0.05). A total of 6 DE lipids were DE between those with OA and the HCs during the peak season, and 24 were DE in the off-peak season. Dysregulated metabolites affected oxidative stress, inflammation, and homeostasis across seasons, suggesting a link between OA-associated itch and ocular surface damage via eye rubbing. Tear lipidome changes were minimal between but suggested tear film destabilisation and thinning. Such metabolipodome findings may pave new and exciting ways for effective diagnostics and therapeutics for OA sufferers in the future.

Keywords: human tears; immunometabolism; inflammation; lipidomics; mass spectrometry; metabolomics; ocular allergy; oxidative stress; tear film.

Figure 1

Overview of the tear film structure and a detailed view of the lipid and metabolite components of the lipid layer and aqueous layers, including fatty acids, WEs, OAHFAs, CEs, vitamin a, niacinamide, gluconolactone, and lysozymes. WEs, phospholipids, and CEs typically are closer to the surface of the tear film exposed to air, while fatty acids (represented by oleic acid) and triglycerides are distributed throughout the lipid layer. Lysozyme–fatty acid complexes cross the border between the aqueous and lipid layers, and the aqueous layer contains a variety of metabolites, including vitamin A, niacinamide, gluconolactone, and many more.

Figure 2

ROC curve analysis of niacinamide expression during peak allergy season between ocular allergy sufferers (PA) and healthy controls (PHCs). Niacinamide had an area under the curve (AUC) value of 0.86 and a p-value of 0.043, indicating a moderate ability to predict OA status.

Figure 3

MetaboAnalyst enrichment analysis conducted using RaMP consisting of HMDB, KEGG, Reactome, and WikiPathways simultaneously. Only upregulated metabolites underwent enrichment analysis due to the low numbers of downregulated metabolites (<5 during both peak allergy season and off-peak season). (A) Upregulated metabolites in PA vs. PHCs; (B) upregulated metabolites in OPA vs. OPHCs.