Clinical Considerations in Proinflammatory Cytokine Profiling of Tears from Patients with Dry Eye by Means of Antibody Microarrays
Investigative Ophthalmology and Visual Science
We read with interest the recently published article by Boehm et al. 1 This is an important research area and certainly the findings presented are highly interesting. The authors found that cytokines are elevated in tears from aqueous deficient dry eye (DRYaq) and dry eye with combined aqueous deficiency and lipid deficiency (DRYaqlip), but not in lipid deficient dry eye (DRYlip). This seems to be a reasonable finding since, traditionally, the aqueous deficient dry eye subtype is associated
... e is associated with autoimmunity and especially the systemic autoimmune conditions such as Sjögren's syndrome. However, we feel that the readers should consider a few points when interpreting these data. After a thorough reading of the data presented, we feel that the controls may have been improperly selected or insufficiently characterized, leading to puzzling findings as well as the apparent discrepancy with a previous study. 2 First, the authors 1 classified subjects as controls based on Schirmer test results of Ͼ10 mm and tear break-up times (TBUT) of Ͼ10 seconds. It is not clear from the paper if the meibomian gland dysfunction (MGD) score of Ͻ18 had to be fulfilled in this group. Without knowledge of the mean and SD of the MGD score, this group is insufficiently characterized. Second, the positions of "*" and " †" in Table 1 appear to have been swapped. The text in the Materials and Methods section states the MGD score criterion is mandatory for the DRYlip group rather than the DRYaq group, and vice versa in Table 1 . Third, the very strong and significant correlations between tear cytokines (IL-6, IL-8, TNFalpha, and IFN gamma) with TBUT in the control subjects (r values 0.80, 0.74, 0.67, and 0.78, respectively), as reported in Supplementary Table 1B, 1 are puzzling. These correlations are of a higher magnitude than that of other correlations reported elsewhere in this study (e.g., between Schirmer test readings and tear cytokine in dry eye). It is difficult to explain the direct and not inverse association of cytokines with TBUT. Fourth, even if the MGD score of Ͻ18 is obligatory in controls, it still encompasses a broad range of clinical MGD severity. MGD is an important consideration because the lipid alterations may have effects other than tear instability. We analyzed seven classes of meibum lipids from people with dry eye and MGD, and found that certain members of lipids are actually elevated in disease. 3 For example, cholesteryl esters 18:0, 20:0, 21:0, and the ceramide d18:1/19:0 were significantly increased in moderate compared with mild dry eye. It may be that some altered lipids play an important role in promoting inflammation; free fatty acids 4 and ceramides 5 have been known to play such roles, including generation of free radicals. Such phenomena may add to the effect of inflammatory tear proteins and, given sufficient time, they may actually elevate tear cytokines such as IFN gamma and IL-8, which were previously observed in MGD-induced dry eye. 2 Because MGD can potentially affect inflammation in a direct way other than just destabilize tears, it should be a primary consideration in the dry eye subtype. Given that MGD may have effects on tear proteins, 6 it is helpful to report the MGD scores (as a continuous variable) 7 in the control group and, additionally, in each of the dry eye subtypes, rather than using a criterion of Ն18 in the DRYlip group and optionally including it in the DRYaqlip group. To investigate the effect of aqueous deficiency on tear cytokines, the authors can compare groups with similar extents of MGD but different Schirmer test values.