In the present study, use of a commercially available EIA kit did not detect IL-1β in the tear fluid of any subject. Because the EIA kit had a restricted range of the standard curve (3.2–500 pg/mL), tear fluid IL-1β levels were measured with a more sensitive assay based on cytokine bead technology. Very low levels of IL-1β were detected in the tear film of a few subjects with the cytokine bead assay, but there was no difference between the subjects with moderate dry eye and healthy controls. Additionally, the tear film IL-1β level in two subjects in each group was below that of the lowest standard, suggesting that these subjects had extremely low or undetectable IL-1β levels in their tear film. These low levels did not agree with levels measured in human tear fluid in other published studies. For example, one study
28 showed that the IL-1β level in the tear film of subjects with dry eye was 80 to 180 pg/mL, whereas that of healthy controls was 30 pg/mL. This earlier study,
28 performed on patients with Sjögren syndrome or ocular rosacea-associated Meibomian gland disease, made use of a commercially available EIA kit similar to the one used in the present study. It should be noted that in this study,
28 IL-1β was detected in immunostained CIC specimens in 2 of 6 healthy controls and 15 of 16 subjects with Sjögren syndrome, suggesting that IL-1β protein expression in the healthy population is low and variable but that it is highly expressed in patients with Sjögren syndrome. These results demonstrate some concurrence with our data with respect to IL-1β expression in CIC samples from healthy controls, and they suggest that the definite presence of an immune-mediated disease process (such as Sjögren syndrome) may be essential to routinely detect significant conjunctival epithelial inflammatory cytokine expression. The lack of agreement between previous studies and the present investigation with respect to the presence of tear fluid IL-1β in patients with moderate dry eye cannot be definitively explained. One shortcoming of our study is the small number of participants; sampling from a greater number of healthy controls and subjects with dry eye would illuminate the variability regarding IL-1 levels in tear film. With this caveat, it can be speculated that the results of the present study could be attributed to the moderate nature of the disease in the study subjects. The subjects with moderate dry eye in the present study demonstrated very low scores on corneal and conjunctival staining with fluorescein and lissamine green, suggesting minimal ocular surface damage. These scores, especially the low lissamine green staining scores, clearly demonstrate the moderate nature of the disease. Mean corneal and conjunctival staining scores were slightly higher in the dry eye group compared with those in healthy controls, though this difference did not reach statistical significance. It is notable from our present and previous
2 data that small differences in ocular surface health (as determined by vital dye staining) can lead to large differences in ocular irritation symptoms. Even though this small amount of ocular surface epithelial damage was adequate to cause significant dry eye symptoms in our study, it might not have been enough to upregulate IL-1β and the other inflammatory cytokines to a detectable level. The low levels of IL-1β detected in the tear fluid, coupled with the absence of IL-1β in the CIC samples, suggest that IL-1β and perhaps other inflammatory cytokines may be just below the detection level of the assays performed. Therefore, a very low grade inflammatory reaction or an inflammatory pathway mediated by proinflammatory cytokines such as TNF-α cannot be ruled out in these subjects with moderate dry eye. It can also be argued, however, that the low levels of IL-1β in the tear fluid and the absence of IL-1β in the CIC samples suggest that IL-1β may not play a role in causing the symptoms observed in patients with moderate dry eye. In keeping with this argument, for example, the upregulation of hBD-2 observed in subjects with moderate dry eye in an earlier study
28 might have been mediated by other cytokines (such as TNF-α). Clarification of this speculative interpretation awaits an investigation with a larger cohort of subjects expanded to include patients with severe dry eye disease in whom a role for IL-1 appears likely.