Synergistic proinflammatory effects of the antiviral cytokine interferon-α and toll-like receptor 4 ligands in the atherosclerotic plaque

Background - Interferon (IFN)-alpha is a pluripotent inflammatory cytokine typically induced by viral infections. In rupture- prone atherosclerotic plaques, plasmacytoid dendritic cells produce IFN-alpha. In the present study we explored the contribution of IFN-alpha to inflammation and tissue injury in the plaque microenvironment. Methods and Results - In 53% of carotid plaques (n = 30), CD123(+) plasmacytoid dendritic cells clustered together with CD11c(+) myeloid dendritic cells, a distinct dendritic cell subset specialized in sensing danger signals from bacteria and tissue breakdown. Tissue concentrations of IFN-alpha and tumor necrosis factor (TNF)- alpha transcripts were tightly correlated (r = 0.76, P < 0.001), suggesting a regulatory role of IFN-alpha in TNF-alpha production. Plaque tissue stimulation with CpG ODN, a Toll- like receptor (TLR) 9 ligand, increased IFN-alpha production (57.8 +/- 23.7 versus 25.9 +/- 8.6 pg/ mL; P < 0.001), whereas the TLR4 ligand lipopolysaccharide induced TNF-alpha secretion (225.1 +/- 3.0 versus 0.7 +/- 0.2 pg/mL; P < 0.001). Treating plaque tissue with IFN-alpha markedly enhanced lipopolysaccharide-triggered TNF-alpha secretion (559.0 +/- 25.9 versus 225.1 +/- 3.0 pg/mL; P < 0.001). IFN-alpha pretreatment also amplified the effects of lipopolysaccharide on interleukin-12, interleukin-23, and matrix metalloproteinase-9, suggesting that the antiviral cytokine sensitized myeloid dendritic cells and macrophages toward TLR4 ligands. Mechanistic studies demonstrated that IFN-alpha modulated the myeloid dendritic cell response pattern by upregulating TLR4 expression (P< 0.001) involving both the STAT (signal transducer and activator of transcription) and the PI(3) K pathway. Conclusions - In the atherosclerotic plaque, IFN-alpha functions as an inflammatory amplifier. It sensitizes antigen- presenting cells toward pathogen-derived TLR4 ligands by upregulating TLR4 expression and intensifies TNF-alpha, interleukin-12, and matrix metalloproteinase-9 production, all implicated in plaque destabilization. Thus, IFN-alpha- inducing pathogens, even when colonizing distant tissue sites, threaten the stability of inflamed atherosclerotic plaque.