Adsorption of human milk proteins to tear lipids for enhancing tear stability in dry eye

Purpose: Dry eye disease is the most common eye condition presenting with symptoms of drying, burning, and gritty sensation in the eye. It is characterised by inability of tears to form a stable film on the ocular surface. Symptoms of dry eye can be relieved by enhancing tear stability. Our previous research explored the use of human breast milk (HBM) proteins for treating dry eye and found these to be more effective surfactants than natural tear proteins in influencing film stability. Since proteins enhance tear stability by adsorbing to lipids in the tear film, this study aimed at studying adsorption of HBM proteins to tear lipids for assessing their application in treating dry eye disease.

Methods: HBM samples were fresh frozen, thawed, and centrifuged to obtain the aqueous layer. Adsorption of HBM proteins to model tear lipids (nonpolar: cholesterol oleate (CO), cholesterol (Chl), oleic acid (OA); and polar: dioleoyl phosphatidylcholine (DOPC)) was studied using a Langmuir trough. Lipids were spread on an artificial tear (AT) solution containing salt composition of tears at the physiological pH (7.4) and temperature (35°C) of tears. The lipid film was compressed and expanded to record pre-adsorption surface pressure-area (Π-A) cycles. The film was then compressed to a pressure of 10 mN/m and the area of the film was held constant. HBM proteins were injected into the AT solution and changes in surface pressure with time (Π-T) were recorded. Π-A cycles of post-adsorption lipid film were also recorded.

Results: The Π-T profiles showed that HBM proteins adsorbed to the nonpolar lipid films and increased the surface pressure by 6, 13, and 10 mN/m for CO, Chl, and OA, respectively. HBM proteins also adsorbed to the polar lipid film and showed a small increase in the surface pressure by 3 mN/m for DOPC. Comparison of pre- and post-adsorption Π-A cycles showed that the protein-adsorbed lipid film had 20 mN/m higher surface pressure than the lipid-only film for CO but other lipids had similar surface pressures.

Conclusions: HBM proteins being effective surfactants adsorb to the model tear lipids increasing their surface pressure and film stability. These proteins adsorb more to the nonpolar lipids than the polar lipids. Since tear lipids are predominantly nonpolar in nature, adsorption of HBM proteins can increase the stability of the lipid layer of the tear film and thereby relieve the symptoms of dry eye disease.