Differential effects of CpG DNA on IFN-beta induction and STAT1 activation in murine macrophages versus dendritic cells: alternatively activated STAT1 negatively regulates TLR signaling in macrophages

Classical STAT1 activation in response to TLR agonists occurs by phosphorylation of the Y701 and S727 residues through autocrine type I IFN signaling and p38 MAPK signaling, respectively. In this study, we report that the TLR9 agonist CpG DNA induced Ifn-beta mRNA, as well as downstream type I IFN-dependent genes, in a MyD88-dependent manner in mouse myeloid dendritic cells. This pathway was required for maximal TNF and IL-6 secretion, as well as expression of cell surface costimulatory molecules. By contrast, neither A- nor B-type CpG-containing oligonucleotides induced Ifn-beta in mouse bone marrow-derived macrophages (BMM) and a CpG-B oligonucleotide did not induce IFn-beta in the macrophage-like cell line, J774. In BMM, STAT1 was alternatively activated (phosphorylated on S727, but not Y701), and was retained in the cytoplasm in response to CpG DNA. CpG DNA responses were altered in BMM from STAT1(S727A) mice; Il-12p40 and Cox-2 mRNAs were more highly induced, whereas Tlr4 and Tlr9 mRNAs were more repressed. The data suggest a novel inhibitory function for cytoplasmic STAT1 in response to TLR agonists that activate p38 MAPK but do not elicit type I IFN production. Indeed, the TLR7 agonist, R837, failed to induce Ifn-beta mRNA and consequently triggered STAT1 phosphorylation on S727, but not Y701, in human monocyte-derived macrophages. The differential activation of Ifn-beta and STAT1 by CpG DNA in mouse macrophages vs dendritic cells provides a likely mechanism for their divergent roles in priming the adaptive immune response.

G-protein-coupled receptor expression, function, and signaling in macrophages

G-protein-coupled receptors (GPCRs) are widely targeted in drug discovery. As macrophages are key cellular mediators of acute and chronic inflammation, we review here the role of GPCRs in regulating macrophage function, with a focus on contribution to disease pathology and potential therapeutic applications. Within this analysis, we highlight novel GPCRs with a macrophage-restricted expression profile, which provide avenues for further exploration. We also review an emerging literature, which documents novel roles for GPCR signaling components in GPCR-independent signaling in macrophages. In particular, we examine the crosstalk between GPCR and TLR signaling pathways and highlight GPCR signaling molecules which are likely to have uncharacterized functions in this cell lineage.