Differential Effects of CpG DNA on IFN-β Induction and STAT1 Activation in Murine Macrophages versus Dendritic Cells: Alternatively Activated STAT1 Negatively Regulates TLR Signaling in Macrophages

Abstract

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-β 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-β in mouse bone marrow-derived macrophages (BMM) and a CpG-B oligonucleotide did not induce IFn-β 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 STAT1S727A 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-β mRNA and consequently triggered STAT1 phosphorylation on S727, but not Y701, in human monocyte-derived macrophages. The differential activation of Ifn-β 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.

Regulation of innate immunity by suppressor of cytokine signaling (SOCS) proteins

“…Indeed, it turned out that IFN-b signaling, which uses Tyk2, is able to mediate a feed-forward (Thomas et al, 2006) amplification route in TLR signaling and Tyk2 knockout mice have decreased LPS sensitivity (Karaghiosoff et al, 2003). Following this line, we now show that even some direct TLR target genes are sensitive to amplification by IFN-b and this might explain some of the contrasting results (Schroder et al, 2007). Recent data furthermore suggest that TLR4 stimulation might activate JAK2, thereby becoming sensitive to SOCS1 (Kimura et al, 2005).…”

CpG-containing immunostimulatory DNA sequences elicit TNF-α–dependent toxicity in rodents but not in humans

“…This helps explain the apparent discrepancy with our findings that pDC depletion did not reduce ISS-induced pathology or BALF levels of cytokines other than IFN-α and that the reduction of IFN-α was not complete (Figure 3). Thus, in pDC-depleted mice, the remaining levels of IFN-α, possibly along with IFN-β and IFN-Ω, were sufficient to support the subsequent induction of key inflammatory cytokines from non-pDC cells (33) in the lung. In humans and nonhuman primates, while the IFNAR signaling pathway is intact, induction of TNF-α does not occur because of lack of expression of TLR9 on non-pDC cells, principally macrophages and DCs (25).…”

The Tpl2 Mutation Sluggish Impairs Type I IFN Production and Increases Susceptibility to Group B Streptococcal Disease

“…TLR3 and TLR4 primarily elicit IFN-␤ production through a MyD88-independent pathway (24,25), while TLR7 and TLR9 signal through MyD88 to induce production of type I IFNs primarily in dendritic cells (3,23,26). Following IFN-␥ priming, macrophages are able to induce IFN-␤ production in response to CpG-B (27,28).…”