Tear Protein Alteration in Dogs with Keratoconjunctivitis Sicca

Keratoconjunctivitis sicca (KCS) in dogs is an immune-mediated disorder characterized by aqueous tear deficiency, ocular surface inflammation, and risk of vision loss. Although tear quantity is routinely evaluated using the Schirmer tear test (STT), the accompanying qualitative alterations in tear protein composition remain poorly understood. In this exploratory study, we identified and characterized qualitatively differentially expressed tear proteins in samples collected from seven Beagle dogs with KCS and five healthy Beagles. Samples were collected using filter paper, extracted in phosphate-buffered saline, concentrated by trichloroacetic acid precipitation, and then separated via two-dimensional electrophoresis. Differential protein spots were identified by MALDI-TOF-MS-based peptide mass fingerprinting. Total protein concentrations were determined by measuring UV absorbance at 280 nm and were found to be significantly higher in dogs with KCS (30.7 ± 13.5 mg/mL) than in healthy dogs (11.5 ± 1.8 mg/mL, p < 0.05). Five proteins were identified as differentially expressed: serum albumin, lactotransferrin isoform 1, immunoglobulin gamma heavy chain C, major allergen Can f 1, and lysozyme C. High-molecular-weight proteins were upregulated in KCS, whereas low-molecular-weight proteins (<10 kDa, proline-rich protein-like components) were markedly reduced or absent. These compositional shifts suggest that KCS alters both the quantity and qualitative integrity of the tear proteosome, reflecting impaired tear film homeostasis and diminished ocular surface defense. The results support the potential utility of the tear proteome as a source of diagnostic and therapeutic biomarkers in canine KCS.