Differential expression and regulation of cyclooxygenase isozymes in thymic stromal cells

Prostaglandins (PGs) are lipid-derived mediators of rapid and localized cellular responses. Given the role of PG in supporting thymic T cell development, we investigated the expression of the PG synthases, also known as cyclooxygenases (COX)-1 and -2, in the biosynthesis of PGs in thymic stromal cell lines. The predominant isozyme expressed in cortical thymic epithelial cells was COX-I, while COX-2 predominated in the medulla, IFN-gamma, up-regulated expression and activity of COX-2 in medullary cells, in which COX-2 was expressed constitutively. In contrast, IFN-gamma down-regulated COX-1 activity, but not expression, in cortical cells. Stromal cells support T cell development in the thymus, although the mediators of this effect are unknown. Selective inhibition of COX-2, but not COX-1, blocked the adhesion of CD4(+)CD8(+) and CD4(+)CD8(-) thymocytes to medullary cell lines. No effect of the inhibitors was observed on the interactions of thymocytes with cortical epithelial lines, These data further support the differential regulation of COX-1 and COX-2 expression and function in thymic stromal cells. PGs produced by COX-2 in the medullary thymic stroma may regulate the development of thymocytes by modulating their interaction with stromal cells.