Glucocorticoid-induced tumour necrosis factor receptor family related protein (GITR) mediates inflammatory activation of macrophages that can destabilize atherosclerotic plaques.

Glucocorticoid-induced tumour necrosis factor receptor family related protein (GITR) is the 18th member of the tumour necrosis factor receptor superfamily (TNFRSF18) and is known to interact with its cognate ligand GITRL (TNFSF18). We investigated the potential role of GITR in the pro-inflammatory activation of macrophages. Immunohistochemistry and in situ hybridization analyses of human atherosclerotic plaques demonstrated that GITR and its ligand are expressed mainly in lipid-rich macrophages. We then investigated the role of GITR in human and mouse monocyte/macrophage functions. Stimulation of GITR caused nuclear factor (NF)-kappaB-dependent activation of matrix metalloproteinase-9 (MMP-9) and pro-inflammatory cytokine expression in both the human and mouse monocytic/macrophage cell lines, THP-1 and RAW264.7, respectively. These cellular responses were also observed when the THP-1 cells were treated with phorbol-12 myristate-13 acetate (PMA), which is known to induce macrophage differentiation. To demonstrate that these responses are not restricted to cultured cell lines, we tested primary macrophages. Both peritoneal and bone marrow-derived macrophages responded to GITR stimulation with induction of MMP-9 and tumour necrosis factor-alpha (TNF-alpha). Furthermore, the GITR staining pattern overlapped with those of MMP-9 and TNF-alpha in atherosclerotic plaques. These data indicate that GITR-mediated macrophage activation may promote atherogenesis via the induction of pro-atherogenic cytokines/chemokines, and destabilize the atherosclerotic plaques via the induction of the matrix-degrading enzyme, MMP-9.

Involvement of TL1A and DR3 in induction of pro-inflammatory cytokines and matrix metalloproteinase-9 in atherogenesis.

TL1A (VEGI/TNFSF15) is the ligand for DR3 (TNFRSF12) and is a newly identified member of the tumor necrosis factor superfamily (TNFSF). Previously, DR3 has been shown to have a role in atherogenesis through stimulation of matrix degrading enzymes including matrix metalloproteinase (MMP)-9. Immunohistochemical staining of human carotid atherosclerotic plaques revealed a high-level expression of TL1A in regions rich in macrophage/foam cells. To investigate the role of TL1A and DR3 in the functioning of macrophage/foam cells in relation to atherogenesis, we have analyzed cellular events mediated by TL1A and DR3 in a human macrophage-like cell line, THP-1. Treatment of THP-1 cells with immobilized anti-DR3 monoclonal antibody in combination with IFN-gamma caused induction of pro-atherogenic cytokines/chemokines such as TNF-alpha, monocyte chemoattractant protein (MCP)-1, and interleukin (IL)-8. Treatment of THP-1 cells with recombinant TL1A in combination with IFN-gamma also caused induction of MMP-9 and IL-8. Furthermore, the expression of DR3 in peripheral blood monocytes was induced after atherogenic stimulation. These data suggest that TL1A and DR3 is involved in atherosclerosis via the induction of pro-inflammatory cytokines/chemokines and decreasing plaque stability by inducing extracellular matrix degrading enzymes.