Refractory VKC shows low levels of tear SL
Interestingly, tear sphingolipids showed differential levels based on the refractory nature of the VKC disease. Specific S1Ps and C1P (4 of them) were found to be characteristically lowered in R-VKC compared to control as well as NR-VKC. While Cer(d18:1/17:0) was significantly increased (by 30%) in NR- VKC than control, this response was not observed in R-VKC. Looking at the cumulative levels of tear sphingolipids a relatively lower level of the total sphingolipids, S1P, C1P and ceramide were seen in R-VKC compared to NR-VKC, while NR-VKC showed an increase compared to the control (Supplementary Figure 1 ). Of the refractory cases classified based on intervention, those who were on topical immunomodulators > 8 weeks and still active, showed maximal lowering of tear ceramide and raised S1P/ceramide ratio thus correlating with the more severe/refractory grade of VKC (Supplementary Figure 1). Ceramide is an important component of the lipid raft in cell membranes and plays a pivotal role in signal transduction via FceRI receptors in mast cells. Among the multiple roles of ceramide, apoptosis is considered as the key function which it induces by both intrinsic and extrinsic pathways. The effect depends on the chain length of the fatty acids in the ceramide as short (C16; C18) or long (C24)(26,27). Apoptosis is necessary to resolve inflammation and delayed apoptosis has been observed in autoimmune diseases(28). A higher or raised ceramide probably induces apoptosis and the inflammation eventually subsides in the non-refractory group whereas reduced levels of ceramide in the refractory group with a raised S1P/ceramide ratio inhibits apoptosis and the inflammation persists contributing to the refractory status. Epithelial barrier permeability is influenced by ceramide and its deficiency reportedly contributes to the pathogenesis of atopic dermatitis(29). Similarly, a lower level of ceramide in the conjunctival epithelium probably alters the permeability making them more susceptible to allergens and reduced apoptosis further contributes to the refractory behavior in a select group of VKC patients. The reduced apoptosis in VKC and persistence of inflammation was further validated by the lower expression levels of Bcl2 and Bax and a 2-fold increase in the gene expression of IL6 expression in the conjunctival cells of VKC cases noted in our study(30). Moreover, there was an increase in the epithelial mesenchymal transition (EMT) marker namely vimentin, indicating pro-survival and EMT changes in VKC. EMT and sphingolipid metabolism has been explored in cancer and each inducing the changes in other is reported(31).
Since altered sphingolipid metabolism has been reported in allergic asthma, atopic dermatitis and many other autoimmune diseases, they have also been considered as therapeutic targets(23). Topical ceramide cream has been found to improve the transepidermal barrier permeability in patients with atopic dermatitis(32). Topical liposomal C6-ceramide was found to inhibit corneal inflammation in murine models(33). SphK inhibitors like SK1-I and DMS have been found to reduce airway inflammation in mice(16,34). S1P receptor modulators like FTY 720 are being used in allergic rhinitis, asthma, allergic skin diseases such as contact hypersensitivity or atopic dermatitis(35). Owing to its lesser side effects than immunomodulators, topical FTY720 has been attempted to reduce rejection in corneal allograft as it was found to decrease infiltration of CD4 T cells in corneal grafts in animal experiments(36). A similar approach based can be explored in VKC.