Comparison of the expression of TGFβ2 activating molecules in conjunctival inflammation

Abstract

Purpose: Increased expression of TGFβ2 is reported in the conjunctiva of dry eye patients despite the decline in TGFβ2-expressing goblet cells suggesting a lack of anti-inflammatory activity of TGFβ2 in the context of conjunctival inflammation. While integrins expressed in inflamed tissue are unable to activate this isoform of TGFβ, Thrombospondin-1 (TSP1) is known to activate it efficiently via ligation of its receptor CD36. Our aim was to compare expression of molecules associated with TGFβ2 activation during murine conjunctival inflammation and assess their correlation with inflammatory conjunctival epithelial apoptosis.

Methods: Human conjunctival tissue from cadaveric donors, primary human conjunctival epihelial, stromal cells and murine conjunctiva were immunostained for CD36, TSP1 or latent TGFβ2. Inflamed conjunctival tissues were obtained from scopolamine-injected C57BL/6 (WT) mice induced to develop Experimental Dry Eye (EDE) with 5 days of desiccating conditions and TSP1 deficient (TSP1-/-) mice, which spontaneously develop Sjögren’s syndrome associated conjunctival inflammation with age. Immunostaining intensities were compared with ImageJ analysis. Apoptosis was assessed by detecting activated caspase-3/7 using CellEvent detection kit (Life Technologies).

Results: Both CD36 and TSP1 were detectable in human conjunctival tissue as well as primary conjunctival epithelial and stromal cells just as in normal WT mouse conjunctiva that lacked caspase-3/7 positive cells. However, epithelial cells positively stained from caspase-3/7 were detected in conjunctiva derived from both EDE and TSP1-/- mice indicative of apoptosis in line with local inflammation. Increased immunostaining of latent TGFβ2 was detected in TSP1-/- as compared with WT mice, supporting lack of its activation in inflamed murine conjunctiva. Interestingly, compared to WT conjunctiva increased TSP1 and reduced CD36 immunostaining was detected in EDE mice. Conversely increased CD36 immunostaining was seen in TSP1-/- conjunctiva in comparison to WT mice.

Conclusions: The absence or reduced expression of one of the molecules, CD36 or TSP1, involved in TGFβ2 activation supports pro-inflammatory conditions in the conjunctiva that lead to apoptotic cell death of conjunctival epithelial cells. Therapeutic strategies directed towards restoring activation of latent TGFβ2 may help treat chronic conjunctival inflammation.