Suppression of Toll-like receptor-mediated innate immune responses at the ocular surface by the membrane-associated mucins MUC1 and MUC16

Abstract

Membrane-associated mucins (MAMs) expressed on the ocular surface epithelium form a dense glycocalyx, which is hypothesized to protect the cornea and conjunctiva from external insult. In this study, the hypothesis that the MAMs MUC1 and MUC16, expressed on the apical surface of the corneal epithelium, suppress Toll-like receptor (TLR)-mediated innate immune responses was tested. Using an in vitro model of corneal epithelial cells that are cultured to express MAMs, we show that reduced expression of either MUC1 or MUC16 correlates with increased message and secreted protein levels of the proinflammatory cytokines IL-6, IL-8, and TNF-α following exposure of cells to the TLR2 and TLR5 agonists, heat killed Listeria monocytogenes and flagellin, respectively. Since mice express MUC1 (but not MUC16) in the corneal epithelium, a Muc1-/- mouse model was used to extend in vitro findings. Indeed, IL-6 and TNF-α message levels were increased in the corneal epithelium of Muc1-/- mice, in comparison to wild type mice, following exposure of enucleated eyes to the TLR2 and TLR5 agonists. Our results suggest that the MAMs MUC1 and MUC16 contribute to the maintenance of immune homeostasis at the ocular surface by limiting TLR-mediated innate immune responses.

Pseudomonas aeruginosa stimulates tyrosine phosphorylation of and TLR5 association with the MUC1 cytoplasmic tail through EGFR activation

“…These observations support prior reports showing EGFR tyrosine phosphorylates MUC1-CT in breast cancer cells (21), and that tyrosine-phosphorylated MUC1-CT interacts with TLR5 in lung cancer cells (11). Recently, suppression of TLR5-dependent proinflammatory responses by MUC1 also has been documented in telomerase-transformed, immortalized human corneal epithelial cells (30), MNK7 human gastric cancer cells (31), and murine dendritic cells (32). To our knowledge, however, this is the first report demonstrating both EGFR-driven MUC1-CT phosphorylation and MUC1-CT/TLR5 association in nontransformed, normal human airway epithelial cells.…”

Mucosal immune tolerance at the ocular surface in health and disease

“…). Tear film clearance and regular blinking continuously remove antigens and microbes away from the OS, whereas the glycocalyx and the tight junctions in the apical cell layers of the conjunctival and corneal epithelia serve as a formidable physical barrier with immunomodulatory properties . In addition, tears favour the protective, tonic activation of stress response‐associated transcription factors NF‐ κ B and activator protein 1 (AP‐1) in these cells, which influence mucosal immune responses .…”

“…Tear film clearance and regular blinking continuously remove antigens and microbes away from the OS, 3 whereas the glycocalyx and the tight junctions in the apical cell layers of the conjunctival and corneal epithelia serve as a formidable physical barrier with immunomodulatory properties. 4,21,22 In addition, tears favour the protective, tonic activation of stress response-associated transcription factors NF-jB and activator protein 1 (AP-1) in these cells, 23 which influence mucosal immune responses. 24,25 The epithelial layer itself is not a mere barricade, but an active component of the immune system: corneal and conjunctival epithelial cells secrete microbicidal and immunomodulatory peptides and cytokines and can respond to pathogen-and danger-associated molecular patterns through their functional Toll-and NOD-like receptor signalling system.…”

Glycosylation pathways of human corneal and conjunctival epithelial cell mucins

“…Mucin O-glycosylation is also an important regulator of cell surface functions and immunity [9–11]. On the ocular surface, MUC1 is a docking protein for cell surface receptors and contributes to cell signaling and the innate immune responses [12–15]. In conditions of dryness of the eye and salivary glands (dry eye disease and Sjögren’s syndrome) mucin expression, secretion and their glycan structures are affected, associated with inflammation [16–18].…”