CpG-oligodeoxynucleotide-induced TLR9 activation regulates macrophage TREM-1 expression and shedding

CpG-oligonucleotide (ODN)-induced TLR9 activation exerts anti-inflammatory effects. TREM-1 is a DAP12-associated receptor, which is up-regulated in response to LPS-mediated TLR4 activation, and plays an essential role in innate immune response by augmenting the production of pro-inflammatory chemokines and cytokines. TREM-1 up-regulation resulted in a grave outcome in animal models, and in patients with sepsis and rheumatoid arthritis, while its soluble form (sTREM-1) exerted anti-inflammatory effects. We hypothesized that CpG-ODN regulates membrane TREM-1 expression and sTREM-1 shedding. The effect of CpG-ODN-induced TLR9 activation on TREM-1 expression and shedding was studied in mouse peritoneal macrophages and the mouse macrophage cell line RAW 264.7. While TREM-1 expression was not altered by CpG-ODN alone, stimulation with both LPS and CpG-ODN significantly abrogated TREM-1 LPS-induced up-regulation. Moreover, CpG-ODN-induced TLR9 activation either alone or in combination with LPS resulted in a significant increase of supernatant sTREM-1. The release of sTREM-1 was correlated positively with MMP-9 activity and was inhibited by chloroquine. These results suggest (i) a novel CpG-ODN-induced TLR9 pathway for the regulation of macrophage TREM-1 expression and MMP-9-mediated TREM-1 shedding; and (ii) a novel mechanism for an anti-inflammatory effect of CpG-ODN through abrogation of LPS-induced membrane TREM-1 up-regulation and increased MMP-9-mediated TREM-1 shedding.