Air exposure induced squamous metaplasia of human limbal epithelium.

PURPOSE: Squamous metaplasia is a pathologic process that frequently occurs in nonkeratinized stratified ocular surface epithelia. The mechanism for this occurrence is largely unknown except for vitamin A deficiency.
METHODS: Human limbal explants were cultured under airlift with or without p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 or in a submerged manner for different durations up to 2 weeks. Epithelial cell proliferation, differentiation, limbal stem cell maintenance, and expansion were studied using certain markers such as Ki67, p63, K10 and K12 keratins, filaggrin, Pax6, ABCG-2, and Musashi-1. Expression of phospho-p38 MAPK and its downstream transcription factors, C/EBPalpha and C/EBPbeta, were studied by immunohistochemistry. Epithelial cells harvested from explants after 2 weeks of culturing under different conditions were seeded onto 3T3 feeder layers and cultured for 12 days. The differentiation of clonal epithelial cells was investigated by double staining to K12 and K10 keratins.
RESULTS: The squamous metaplasia model was successfully created by culturing human limbal explants at an air-liquid interface (airlift) for 2 weeks. Increased stratification and hyperproliferation only happened in the limbal, but not the corneal, epithelium in airlift, but not submerged, cultures. Epithelial proliferation was associated with a transient increase of limbal epithelial stem cells. Abnormal epidermal differentiation-evidenced by positive expression of K10 keratin in suprabasal cells and filaggrin in superficial cells-ensued. Clones generated from epithelial cells harvested from airlift culture only expressed K12 keratin without K10. As early as 2 days in airlift cultures, p38 expression emerged in limbal basal epithelial cells and gradually extended to the cytoplasm and nuclei. Furthermore, addition of the p38 inhibitor SB203580 abolished abnormal epidermal differentiation without affecting limbal epithelial proliferation. Expression of C/EBPalpha and C/EBPbeta, downstream of the p38 MAPK signaling pathway, was strongly induced by airlift culture and partially was inhibited by SB203580.
CONCLUSIONS: Dryness resulting from exposure activates p38 MAPK signaling coupled with abnormal epidermal differentiation without intrinsic alteration of stem cells in the limbus. On the ocular surface, p38 inhibitors may have the potential to revert the pathologic process of squamous metaplasia induced by dryness.