# Tear Protein Alteration in Dogs with Keratoconjunctivitis Sicca

## TL;DR

This study finds that dry eye disease in dogs changes the proteins in their tears, which could help diagnose and treat the condition.

## Contribution

The study identifies specific tear protein alterations in dogs with KCS using proteomic analysis.

## Key findings

- Dogs with KCS have higher levels of high-molecular-weight proteins like serum albumin and immune-related proteins.
- Low-molecular-weight protective proteins are reduced in dogs with KCS.
- Tear proteome changes suggest impaired tear film homeostasis and diminished ocular defense.

## Abstract

Keratoconjunctivitis sicca (KCS) is a common cause of dry eye in dogs and leads to chronic ocular surface inflammation. While reduced tear production is a well-known feature of KCS, changes in tear protein composition are less well understood. Beagles are not considered a breed highly predisposed to KCS and typically develop the disease later in life; therefore, this study focused on Beagles to minimize breed-related effects. We compared tear proteins from healthy Beagles and Beagles with naturally occurring KCS using proteomic analysis. Dogs with KCS showed increased levels of high-molecular-weight proteins, such as serum albumin and immune-related proteins, while small protective proteins were reduced. These findings suggest that KCS not only reduces tear volume but also alters the protective protein components of tears, potentially compromising ocular defense. This pilot study highlights the value of tear proteomics in advancing our understanding of KCS pathology and in identifying potential biomarkers for diagnosis and therapeutic monitoring.

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.