Costimulatory molecules play important roles in immune responses. In the present study we investigated the effects of PGE2 on the expression of ICAM-1, B7.1, and B7.2 on monocytes in IL-18-stimulated PBMC using FACS analysis. Addition of PGE2 to PBMC inhibited ICAM-1 and B7.2 expression elicited by IL-18 in a concentration-dependent manner. We examined the involvement of four subtypes of PGE2 receptors, EP1, EP2, EP3, and EP4, in the modulatory effect of PGE2 on ICAM-1 and B7.2 expression elicited by IL-18, using subtype-specific agonists. ONO-AE1–259-01 (EP2R agonist) inhibited IL-18-elicited ICAM-1 and B7.2 expression in a concentration-dependent manner with a potency slightly less than that of PGE2, while ONO-AE1-329 (EP4R agonist) was much less potent than PGE2. The EP2/EP4R agonist 11-deoxy-PGE1 mimicked the effect of PGE2 with the same potency. ONO-D1-004 (EP1R agonist) and ONO-AE-248 (EP3R agonist) showed no effect on IL-18-elicited ICAM-1 or B7.2 expression. These results indicated that EP2 and EP4Rs were involved in the action of PGE2. Dibutyryl cAMP and forskolin down-regulated ICAM-1 and B7.2 expression in IL-18-stimulated monocytes. As EP2 and EP4Rs are coupled to adenylate cyclase, we suggest that PGE2 down-regulates IL-18-induced ICAM-1 and B7.2 expression in monocytes via EP2 and EP4Rs by cAMP-dependent signaling pathways. The fact that anti-B7.2 as well as anti-ICAM-1 Ab inhibited IL-18-induced cytokine production implies that PGE2 may modulate the immune response through regulation of the expression of particular adhesion molecules on monocytes via EP2 and EP4Rs.

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Lipopolysaccharide (LPS) is recognized as a key molecule in the pathogenesis of Gram negative sepsis and septic shock. In the present study, we demonstrate that LPS (1-1000 pg/ml) concentration dependently up-regulated the expression of intercellular adhesion molecule (ICAM)-1, B7.1, and B7.2 on human monocytes using fluorescence-activated cell sorting analysis, and that tumor necrosis factor (TNF)-␣ production induced by LPS in peripheral blood mononuclear cells (PBMCs) was inhibited by the addition of antibodies against these adhesion molecules, suggesting the dependence of TNF-␣ production on cell-cell interaction through these adhesion molecules. Moreover, we found that histamine (10 Ϫ7 -10 Ϫ4 M) concentration dependently inhibited the expression of ICAM-1 and B7.1, but not B7.2 on monocytes induced by LPS. Histamine also inhibited the responses of TNF-␣ production induced by LPS. The modulatory effects of histamine on ICAM-1 and B7.1 expression and TNF-␣ production were all concentration dependently antagonized by famotidine but not by d-chlorpheniramine and thioperamide, and were mimicked by selective H2-receptor agonists but not by H1-, H3-, and H4-receptor agonists, indicating the involvement of H2-receptors in the histamine action. Dibutyryl cAMP down-regulated ICAM-1 and B7.1 expression on monocytes stimulated by LPS, suggesting the mediation by the cyclic adenosine monophosphate-protein kinase A pathway of H2-receptor activation. These results as a whole indicated that histamine via H2-receptor inhibited the LPS-induced TNF-␣ production through the regulation of ICAM-1 and B7.1 expression, leading to the reduction of innate immune response stimulated by LPS.

These findings as a whole indicated that endogenously produced histamine in Kupffer cells/macrophages plays a very important role in preventing excessive innate immune response in endotoxin-induced fulminant hepatitis through the stimulation of H2-receptors.

In the previous study, we demonstrated that interleukin (IL)-18 up-regulated intercellular adhesion molecule-1 (ICAM-1) expression on monocytes in human peripheral blood mononuclear cells (PBMC) and that heterotypic interaction between monocytes/T or NK cells through ICAM-1/LFA-1 intensified the production of IL-12, interferon-␥ (IFN-␥), and tumor necrosis factor-␣ (TNF-␣) in PBMC. In the present study, we demonstrate that histamine inhibited the ICAM-1 expression in monocytes induced by IL-18 using flow cytometry and that the responses of IL-12, IFN-␥, and TNF-␣ induced by IL-18 were concentration dependently inhibited by coexisting histamine, whereas IL-18-inhibited IL-10 production was reversed by the same concentrations of histamine. The modulatory effects of histamine on ICAM-1 expression and cytokine production were