Background: Proteomics and mass spectrometry are useful tools for peptide screening in body fluids. In thyroid-associated orbitopathy (TAO), evidence for lacrimal gland involvement with altered composition of tears has been reported. Our objective was to detect and evaluate potential changes in the proteomic patterns of tear fluid in TAO.
Methods: Tear fluid was collected from 45 patients with TAO and 15 healthy controls. Tear proteins were analyzed using surface-enhanced laser desorption/ionization time-of-flight mass spectrometry, and peptides were identified using matrix-assisted laser desorption/ionization time-of-flight technology.
Results: Peptides with molecular weights 3808 Dalton (Da, p=0.004), 3734 Da (p=0.034), and 3837 Da (p=0.042), respectively, were downregulated in patients with TAO versus controls. They were identified as proline-rich protein 4 (PRP4) or as its variant nasopharyngeal carcinoma-associated PRP4. The peptide 3837 Da correlated positively with the basal secretory test (r=0.506, p<0.001) and negatively with the clinical activity score (r = -0.334, p<0.05) and age (r=-0.431, p<0.001). Also, a 12,003-Da peptide was downregulated (p=0.019) in patients and identified as ß2-microglobulin. This peptide decreased in tear fluid with increased clinical severity of TAO (p=0.027). In comparison, a 5815-Da peptide was upregulated (p=0.045) and identified as lysozyme C. When differentiating between treated and untreated patients with TAO, an 11,770-Da peptide (p=0.0072) that was also upregulated was identified as cystatin S.
Conclusions: Altered regulation of proinflammatory and protective proteins in tears of patients with TAO was demonstrated, reflecting an autoimmune- and/or inflammatory-induced dysfunction of the lacrimal gland.