Diurnal tear cycle: evidence for a nocturnal inflammatory constitutive tear fluid

To investigate the tear film in the closed eye, microliter tear samples were collected without overt reflex stimulation throughout the diurnal cycle, with closed eye samples recovered immediately upon eye opening. Samples were subjected to agarose, polyacrylamide, and two-dimensional electrophoresis, coupled with immunofixation, immunoblot, and lectin blot assays. Major protein constituents were densitometrically and immunologically quantified. Results revealed a distinct progression in composition from reflex to open to closed eye tear samples. Total protein increased from 6.0 to 9.0 to 18.0 mg/ml, secretory IgA increased from less than 0.23 to 0.85 to 8.40 mg/ml, and serum albumin increased from 0.02 to 0.06 to 1.10 mg/ml. In contrast, concentrations of the major reflex tear components (lysozyme, lactoferrin, and tear specific prealbumin) remained essentially static. Immunoblot assay for complement C3 and C3c revealed that eye closure was associated with C3 activation. Results indicate that: (1) the reflex and closed eye tear layers represent opposite extremes in composition and likely origins, with open eye tear film suggesting an intermediate origin; (2) reflex tears are derived from a neurologically inducible lacrimal or accessory gland secretion composed almost exclusively of lysozyme, lactoferrin, tear specific prealbumin, and a minor mixed alpha to beta globulin fraction; (3) upon eye closure, reflex secretion ceases or greatly diminishes, with ongoing slower flow maintained by a constitutive secretion composed almost exclusively of secretory IgA; (4) the closed eye environment induces a subclinical inflammation, accounting in part for the marked rise in albumin concentration. This increase, coupled with that of secretory IgA, may play a critical role in protecting the closed eye environment from pathogens. However, this may render the closed eye environment particularly vulnerable to inflammatory and immune-mediated pathological processes, such as those seen with extended wear soft contact lenses.