Differential eicosanoid synthesis by murine fetal thymic non-lymphoid cells

Cyclooxygenases and prostaglandins: shaping up the immune response

The relevance of cyclooxygenases (COX)-1 and -2 and their products to inflammation, thrombosis and gastroprotection are well known. Their importance in the immune response was first recognized more than 25 years ago, but has only gained widespread attention recently. In this review, we attempt to integrate information on prostanoids and both the innate and acquired immune responses, including effects on leukocytes, antigen presenting cells, dendritic cells, T and B lymphocytes. Prostanoids may be relevant to immunotolerance, autoimmune disorders, transplantation, immunologic defense against tumors, acquired immunodeficiencies and viral infections. Insight into the role of prostanoids in immune function may afford novel therapeutic opportunities.

Prostaglandin E2 inhibits T cell activation-induced apoptosis and Fas-mediated cellular cytotoxicity by blockade of Fas-ligand induction

Prostanoids exhibit both pro-apoptotic and anti-apoptotic functions depending on the maturation stage and tissue localization of target cells. Prostaglandin (PG) E2 has been shown to protect T lymphocytes from TCR-mediated activation-induced cell death, but the mechanism by which PGE2 inhibits apoptosis of T cells has not been established. We show that this protection involves the down-regulation of Fas-ligand (Fas-L) mRNA levels in T cells. Modulation of cell surface Fas-L expression by physiological concentrations of PGE2 was shown to be both anti-apoptotic as well as capable of inhibiting Fas-L-mediated cytotoxicity of Fas-transfected P815 target cells. Thus, this study provides direct evidence of the likely biological means by which PGE2 down-regulates T cell apoptosis.

Distinct roles of prostaglandin H synthases 1 and 2 in T-cell development

Prostaglandin G and H synthases, or cyclooxygenases (COXs), catalyze the formation of prostaglandins (PGs). Whereas COX-1 is diffusely expressed in lymphoid cells in embryonic day 15.5 thymus, COX-2 expression is sparse, apparently limited to stromal cells. By contrast, COX-2 is predominant in a subset of medullary stromal cells in three- to five-week-old mice. The isozymes also differ in their contributions to lymphocyte development. Thus, experiments with selective COX-1 inhibitors in thymic lobes from normal and recombinase-activating gene-1 knockout mice support a role for this isoform in the transition from CD4(-)CD8(-) double-negative (DN) to CD4(+)CD8(+) double-positive (DP). Concordant data were obtained in COX-1 knockouts. Pharmacological inhibition and genetic deletion of COX-2, by contrast, support its role during early thymocyte proliferation and differentiation and, later, during maturation of the CD4 helper T-cell lineage. PGE2, but not other PGs, can rescue the effects of inhibition of either isoform, although it acts through distinct EP receptor subtypes. COX-dependent PG generation may represent a mechanism of thymic stromal support for T-cell development.

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-1, while COX-2 predominated in the medulla. IFN-γ up-regulated expression and activity of COX-2 in medullary cells, in which COX-2 was expressed constitutively. In contrast, IFN-γ 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.

Effects of PGE2 upon differentiation and programmed cell death of suspension cultured CD4-CD8- thymocytes

Recently, several works have focused on the modulation of the immune response by arachidonic acid metabolites. Some of these metabolites, such as prostaglandins, have been shown to influence thymocyte "education" in vitro. However, the effect of one of them, prostaglandin E2 (PGE2), in the education of CD4-CD8- double negative immature thymocytes (DN cells) remained unclear. Using a flow cytometry analysis of DN cells cultured for 24 h in the presence of PGE2, we observed, compared with DN thymocytes cultured without PGE2, an increase in the CD4+CD8-CD3- immature thymocytes and in the CD4+CD8- and CD8+CD4- mature single positive thymocytes and a decrease in the DN and CD4highCD8high double positive thymocytes. Other differentiation thymocyte surface markers, such as CD3, CD5, TCR alpha beta, TCR delta gamma and HSAg, revealed an increasing number of thymocytes bearing these first four markers and a lower expression of the HSAg. Furthermore, we observed an accumulation of CD4lowCD8low thymocytes and an increasing proportion of hypodiploid nuclei. These two findings have been shown to be markers of the programmed cell death process. These findings suggest that PGE2 probably acts on thymocyte differentiation through at least two distinct pathways. On the one hand, PGE2 seems to promote differentiation of DN cells into CD4+CD8-CD3- immature cells and drive CD4+CD8+CD3+ thymocyte to a CD4+CD8- and CD8+CD4- mature phenotype. On the other hand, PGE2 is probably implicated directly or indirectly in the increase or the acceleration of the programmed cell death process of immature CD4+CD8+CD3+ thymocytes, which is linked to the positive and/or negative selection.

Immunocytochemical demonstration of prostaglandin synthase (cyclooxygenase) in thymic macrophages of normal and cyclosporin-treated rats

As revealed with ED1 and ED2 monoclonal antibodies, macrophages are scattered throughout the thymic tissue. However, in contrast to the cortex and medulla, in the cortico-medullary zone macrophages are large and show strong reactivity with rabbit polyclonal antisera to cyclooxygenase. Only few smaller cortical macrophages also show weaker presence of prostaglandin synthase. After cyclosporin treatment cortical macrophages become strikingly similar to the macrophages of the cortico-medullary zone of the normal thymus. Cortical macrophages become enlarged and develop the strong expression of prostaglandin synthase. Our results show that a specific type of macrophages (with distinct histochemical characteristics, enzyme profile and ultrastructural organization, which is strategically positioned within the thymic tissue--as we demonstrated earlier) possesses the enzyme capacity required for prostaglandin synthesis. After cyclosporin treatment, which interferes with the maturation of thymocytes, cortical macrophages thoroughly change and develop the strong prostaglandin synthase expression, similar to that of normal cortico-medullary zone macrophages.