Effect of nicotine on IL-18-initiated immune response in human monocytes

Nicotine is thought to inhibit the production of proinflammatory cytokines from macrophages through an anti-inflammatory pathway that is dependent on nicotinic acetylcholine receptor α7 subunit (α7-nAChR). IL-18, an important proinflammatory cytokine, is reported to induce the expression of adhesion molecules on monocytes, thus enhancing cell-to-cell interactions with T-cells and contributing to IL-18-initiated cytokine production. Accordingly, inhibition of IL-18 suppresses systemic inflammatory responses. In the present study, we found that nicotine inhibited the IL-18-enhanced expression of ICAM-1, B7.2, and CD40 on monocytes, and the production of IL-12, IFN-γ, and TNF-α by PBMC. A nonselective and a selective α7-nAChR antagonist, mecamylamine, and α-bungarotoxin abolished the effects of nicotine, suggesting that this depends on α7-nAChR stimulation. It is reported that nicotine induces prostaglandinE2 (PGE₂) production in PBMC through the up-regulation of cyclooxygenase (COX)-2 expression. PGE₂ is known to activate the EP2/EP4-receptor, leading to an increase in cyclic adenosine monophosphate (cAMP) levels and protein kinase A (PKA) activity. Consistent with this, we found that COX-2 and PKA inhibitors prevented the effects of nicotine on adhesion molecule expression and cytokine production, indicating that the mechanism of action of nicotine may be via endogenous PGE₂ production.