Comprehensive lipid analysis of human meibum and tears

Abnormalities in the tear film lipid layer, which plays a critical role in preventing water evaporation and protecting the corneal surface, lead to dry eye disease. The lipids in this layer include both meibum lipids (from the meibomian glands) and phospholipids of other origins. Meibum lipids include cholesteryl esters, wax monoesters, wax diesters (WdiEs), (O-acyl)-ω-hydroxy fatty acids (OAHFAs), and cholesteryl OAHFAs. Nonetheless, the exact composition of these lipid classes remains largely unclear. Here, we analyze the composition of cholesteryl esters, wax monoesters, WdiEs, OAHFAs, cholesteryl OAHFAs, phosphatidylcholines, and sphingomyelins in human meibum and tears using multiple reaction monitoring mode liquid chromatography-tandem mass spectrometry, which is highly sensitive, selective, and quantitative. This revealed that the WdiEs in meibum and tears fall within the type 1ω and 2ω classes. Among the lipids examined, the type 1ω WdiEs in particular comprised diverse species. The lipid composition of most of the lipid classes, except for the phosphatidylcholines, was similar in meibum and tears. The findings of this comprehensive lipid analysis contribute to elucidating the overall composition of human meibum and tear lipids.