Ocular surface inflammation associated with Sjögren’s syndrome is characterized by a loss of secretory function and alteration in numbers of mucin secreting goblet cells. Such changes are a prominent feature of ocular surface inflammatory diseases and are attributed to inflammation; however, the exact effect of the inflammatory cytokines on conjunctival goblet cell function remains largely unknown. In this study, we developed a primary culture of mouse goblet cells from conjunctival tissue and evaluated the effects on their function by inflammatory cytokines detected in the conjunctiva of mouse model of Sjögren’s syndrome (Thrombospondin-1 deficient mice). We found that apoptosis of goblet cells was primarily induced by TNF-αand IFN-γ. These two cytokines also inhibited mucin secretion by goblet cells in response to cholinergic stimulation, whereas IL-6 enhanced such secretion. No changes in secretory response were detected in the presence of IL-13 or IL-17. Goblet cells proliferated to varying degrees in response to all the tested cytokines with the greatest response to IL-13 followed by IL-6. Our results therefore reveal that inflammatory cytokines expressed in the conjunctiva during an ocular surface disease directly disrupt conjunctival goblet cell functions, compromising the protective function of tears, thereby contributing to ocular surface damage.

Goblet cells are secretory epithelial cells of mucosal tissues that confer protection from environmental agents or pathogens via expression and secretion of soluble mucins. Loss of these cells is associated with several chronic inflammatory disorders of the mucosa. Although demonstrated to transfer antigens from the luminal surface to stromal cells in the intestinal mucosa, it is not known if goblet cells contribute to the regulation of an immune response. In this study we report that similar to intestinal and respiratory mucosal epithelia, mouse ocular surface epithelia predominantly express the TGF-ß2 isoform. Specifically, we demonstrate the ability of goblet cells to express TGF-ß2 and increase it in response to Toll-Like Receptor 4 mediated stimulus in cultures. Goblet cells not only express TGF-ß2, but are also able to activate it in a thrombospondin-1 (TSP-1) dependent manner via their cell surface receptor CD36. Furthermore, goblet cell derived soluble factors that possibly include TGF-ß2, alter dendritic cell (DC) phenotype to a tolerogenic type by downregulating DC expression of MHC class II and co-stimulatory molecules CD80, CD86 and CD40. Thus our study demonstrates goblet cells as a cellular source of active TGF-ß2 in ocular mucosa and implicates their immunomodulatory function in maintaining mucosal immune homeostasis.

Lacrimal gland inflammation during autoimmune Sjögren’s syndrome (SS) leads to ocular surface inflammation – Keratoconjunctivitis sicca (KCS). This condition afflicts both the cornea and conjunctiva that form the ocular surface. Thrombospondin-1 (TSP-1) deficiency in mice results in lacrimal gland and corneal inflammation that resembles the human disease. In this study we report conjunctival pathology in this mouse model of SS. We found that TSP-1 null mice develop inflammation in the conjunctiva and associated loss of goblet cell function similar to that seen in patients with SS. Increased expression of Th1 (IFN-γ, TNF-α) and Th17 (IL-6, IL-17A) inflammatory cytokines and related transcription factors (Tbet and RORγt) were detected in TSP-1 null conjunctiva as well as their draining lymph nodes (LNs). The conjunctival inflammation was also accompanied by an increase in local lymphatic vessels. Interestingly, migration of antigen-bearing dendritic cells (DCs) from the ocular surface to the LNs was dependent on the TSP-1 available in the tissue. These results not only reveal potential immunopathogenic mechanisms underlying KCS in SS but also highlight the therapeutic potential of TSP-1.

Thrombospondins are a family of large multi-domain glycoproteins described as matricelluar proteins based on their ability to interact with a broad range of receptors, matrix molecules, growth factors or proteases, and to modulate array of cellular functions including intracellular signaling, proliferation and migration. Two members of the thrombospondin family, thrombospondin 1 (TSP-1) and thrombospondin 2 (TSP-2) are studied extensively to determine their structure and function. While expressed at low levels in normal adult tissues, their increased expression is seen predominantly in response to cellular perturbations. Despite structural similarities, a notable functional difference between TSP-1 and TSP-2 includes the ability of former to activate of latent TGF-β and its competitive inhibition by the latter. Both these thrombospondins are reported to play important roles in TGF-β rich ocular environment with most reports related to TSP-1. They are expressed by many ocular cell types and detectable in the aqueous and vitreous humor. TSP-1 and TSP-2 influence many cellular interactions in the eye such as angiogenesis, cell migration, wound healing, TGF-β activation and regulation of inflammatory immune responses. Together, these processes are known to contribute to the immune privilege status of the eye. Emerging roles of TSP-1 and TSP-2 in ocular functions and pathology are reviewed here.