Role of IL-10 in the crossregulation of prostaglandins and cytokines in monocytes

In the present study we have focused mainly on the role of IL (interleukin)-10 in the crossregulation of prostaglandins and cytokines in human monocytes. We first determined the effects of tumor necrosis factor-α (TNF-α) and IL-10 on monocyte prostaglandin E₂ (PGE₂) production. Unstimulated monocytes constitutively produced a small but significant amount of PGE₂ in the culture supernatants. Both TNF-α and lipopolysaccharide (LPS) caused a remarkable increase in monocyte PGE₂ production. On the other hand, IL-10 alone was without effect on constitutive PGE₂ production but drastically inhibited LPS-induced PGE₂ production in monocytes. Moreover, this inhibitory effect of IL-10 was not simply attributable to its inhibition of TNF-α production in LPS-stimulated monocytes. Next, we determined the effect of PGE₂ on TNF-α mRNA expression in monocytes. Treatment of monocytes with or without PGE₂ showed no detectable TNF-α mRNA. Activation of monocytes by LPS resulted in a remarkable accumulation of TNF-α mRNA and PGE₂ efficiently inhibited this expression. Finally, we determined the effect of PGE₂ on IL-10 mRNA expression in monocytes. Similar to TNF-α mRNA, unstimulated monocytes showed no detectable IL-10 mRNA. Interestingly, PGE₂ alone drastically induced IL-10 mRNA. Besides, activation of monocytes by LPS resulted in a remarkable accumulation of IL-10 mRNA, and PGE₂ further enhanced this expression. These results indicate that TNF-α and PGE₂ are key molecules for the induction of IL-10 in monocytes, and that IL-10, in turn, plays a crucial role in terminating the inflammatory cascade via downregulation of production of proinflammatory molecules including TNF-α and PGE₂.