Prior studies have demonstrated that cholera toxin (CT) and other cAMP-inducing factors inhibit interleukin (IL)-12 production from monocytes and dendritic cells (DCs). We show that CT inhibits Th1 responses in vivo in mice infected with Toxoplasma gondii. This correlated with low serum IL-12 levels and a selective reduction in the numbers of CD8alpha(+) conventional DCs (cDCs) in lymphoid organs. CT inhibited the function of interferon (IFN) regulatory factor (IRF) 8, a transcription factor known to positively regulate IL-12p35 and p40 gene expression, and the differentiation of CD8alpha(+) and plasmacytoid DCs (pDCs). Fluorescence recovery after photobleaching analysis showed that exposure to CT, forskolin, or dibutyryl (db) cAMP blocked LPS and IFN-gamma-induced IRF8 binding to chromatin. Moreover, CT and dbcAMP inhibited the binding of IRF8 to the IFN-stimulated response element (ISRE)-like element in the mouse IL-12p40 promoter, likely by blocking the formation of ISRE-binding IRF1-IRF8 heterocomplexes. Furthermore, CT inhibited the differentiation of pDCs from fms-like tyrosine kinase 3 ligand-treated bone marrow cells in vitro. Therefore, because IRF8 is essential for IL-12 production and the differentiation of CD8alpha(+) cDCs and pDCs, these data suggest that CT and other Gs-protein agonists can affect IL-12 production and DC differentiation via a common mechanism involving IRF8.