Discussion
In this study, we demonstrated that CD14, TLR4 and TLR-9 are involved in ocular surface inflammatory flare-ups in patients with VKC. Specifically, tear sCD14 and conjunctival epithelium CD14 expression were decreased in patients with VKC suggesting that CD14 plays a potential protective role on the development of inflammatory reaction. In fact, it has been clearly demonstrated that CD14 modulates immune response to microbial products via TLRs, and recent evidence extended its role in the development of the allergic reaction.15, 26 Although the role of CD14 in the pathogenesis of allergic diseases has not yet been completely understood, it has been hypothesized that CD14 is involved in atopy susceptibility.26 Several studies demonstrated that CD14 binding with LPS stimulates cytokine release through TLRs’ signalling, and deviates the adaptive immune reaction toward a Th1 response and consequent Th2 suppression.12, 13, 27, 28As a consequence, alterations in the CD14 pathway may influence Th1/Th2 balance and may induce a switch toward atopy.29 In a paediatric population, an association between circulating sCD14 levels and low IL-4 response has been reported.30, 31Moreover, sCD14 has been shown to interact with B cells by inhibiting IL-6 and IgE production.32 It has been also demonstrated that during acute asthma attacks in children, plasma sCD14 levels were inversely correlated with asthma severity, suggesting that the expression of circulating sCD14 could have a protective effect in the development of asthma attacks.33, 34
Accordingly, VKC is characterized by infiltration of Th2 cells, eosinophils and mast cells and by the release of Th2 cytokines including IL-4, IL-5, IL10, TNF-alpha and IL-13.35 In vitro studies showed that IL-4 and IL-13 inhibit sCD14 release in monocyte, and macrophage cultures.36, 37 Based on this evidence, we may hypothesize that the decrease of ocular surface CD14 expression observed in VKC patients may be an epiphenomenon of the inflammatory reaction during active VKC. This hypothesis may be supported by the significant correlation between the severity of conjunctival hyperaemia and the decrease of tear sCD14. However, after seven days of topical corticosteroid treatment and resolution of the ocular inflammatory reaction, we did not find significant changes of expression of tear sCD14 and conjunctival CD14. This result may support the alternative hypothesis that the lower expression of ocular surface CD14 is associated with the higher risk to develop recrudescence of VKC inflammation. On the other hand, ocular surface CD14 expression may require longer time to normalize after topical corticosteroid treatments, as suggested by the observation that VKC patients in the quiescent phase showed CD14 expression levels similar to healthy subjects.38
Our results also showed that conjunctival epithelium expression of TLR-4 and TLR-9 decreased during active inflammation of VKC, suggesting that TLRs may contribute to the inhibition of development of the allergic reaction. This finding is in line with previous studies, which showed that TLR-4 and TLR-9 were reduced in epithelial and inflammatory cells in allergic rhinitis.39, 40 The protective role of TLR-9 against allergic inflammation is supported by several studies.40, 41 In addition, it has been reported that TLR-9 expression was decreased in conjunctival biopsies of VKC patients in the active phase.9, 10 Conversely, the role of TLR-4 in VKC should be further clarified. Our results showed that conjunctival epithelium TLR-4 expression was decreased while it has been reported that immune cells infiltrating the conjunctiva of VKC patients showed increased expression of TLR-4.10 These findings suggest a possible different role played by resident cells (conjunctival epithelium) and the infiltrating immune cells in the development of allergic reaction in VKC. The active role of conjunctival epithelium in the development of allergic reaction has been demonstrated.24 In fact, it is known that environmental factors, including microbial agents or hormonal influence may induce changes of conjunctival epithelium expression of TLRs by which may predispose to atopic conditions. 42-44 On the other hand, the hypothesis of a protective role of CD14 and TLR-4 in the development of ocular allergic reaction is supported by the results of an experimental study showing that that LPS interaction with TLR-4 was able to decrease Th2 response through TLR-4 signalling.18, 44 In addition, Iovieno et al. showed that administration of topical Lactobacillus was effective in reducing inflammatory reaction in VKC patients and that the clinical improvement was associated with decrease of TLR-4 conjunctival expression. 45 Accordingly, our results demonstrated that topical steroid treatment in VKC induced significant inhibition of TLR-4 conjunctival expression.
This study showed that ocular surface decreased expression of CD14 and its ligands TLR-4 and 9 may foster the development of allergic reaction in VKC, suggesting tevaluate the potential therapeutic role of agents targeting these receptors.