PGE₂ signals via cyclic adenosine monophosphate (cAMP) and PKA to trigger IDO. (A) The adenylate cyclase activator forskolin mimics PGE₂ activation of EP2/4 and induces IDO activity in the presence of TNFα. Monocyte-derived DCs (MoDCs) were matured during 48 hours in the presence of increasing doses of forskolin with or without TNFα. IDO activity was assessed spectrophotometrically after 4 hours at the end of the culture. (B) The adenylate cyclase inhibitor SQ22536 and the PKA inhibitor H-89 block IDO expression. Monocyte-derived DCs were cultured in the presence of PGE₂ or forskolin, in addition to TNFα for 48 hours. The effect of inhibiting adenylate cyclase during this culture using SQ22536, as well as the effect of the PKA inhibitor H-89, were assessed on IDO mRNA levels.