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.