Macrophages express membrane bound form of APRIL that can generate immunomodulatory signals.

Members of the tumour necrosis factor superfamily play an essential role in inducing various biological responses including proliferation, differentiation, survival and cell death. A proliferation-inducing ligand (APRIL), first identified as a stimulant of tumour proliferation, is now known as a regulator of B-cell-mediated immune responses through the modulation of B-cell survival and activation. However, the role of APRIL in macrophage function has not been explored. High level expression of APRIL was detected on the surface of cells of the monocytic lineage including the human macrophage-like cell line, THP-1. To identify the role of APRIL in macrophage functions, THP-1 cells were stimulated with either its counterpart (TACI : Fc fusion protein) or a monoclonal antibody that is specific to APRIL. Stimulation of APRIL resulted in the expression of pro-inflammatory mediators such as interleukin-8 and matrix metalloproteinase-9 through the activation of mitogen-activated protein kinase and nuclear factor-κB. In contrast, stimulation of APRIL had an inhibitory effect on processes that require cytoskeletal movement such as phagocytosis of opsonized zymosan and chemotaxis through an inhibition of phosphatidylinositol 3-kinase activity. These observations demonstrate that macrophages express a membrane-bound form of APRIL which, upon stimulation, modulates the activities of macrophages through stimulation or inhibition of processes associated with inflammation.

Reverse signaling through BAFF differentially regulates the expression of inflammatory mediators and cytoskeletal movements in THP-1 cells.

Most members of the tumor-necrosis factor superfamily have been reported to mediate reverse signaling in T cells, macrophages, and/or dendritic cells. BAFF has been reported to have important functions in B-cell survival through forward signaling, but the presence of reverse signaling has not been explored. To investigate the possibility of BAFF-mediated reverse signaling, the expression patterns and functions of BAFF were analyzed in monocytic cell lines including the human macrophage-like cell line, THP-1. The expression of BAFF and its receptors was detected in monocytic cell lines, either before or after activation. The stimulation of BAFF induced the expression of matrix metalloproteinase (MMP)-9, interleukin -8, and transforming growth factor-beta-induced gene product (beta ig-h3) and the upregulation of intercellular adhesion molecule-1 in THP-1 cells. The activation of mitogen-activated protein kinase extracellular signal-regulated kinase1/2 and nuclear factor-kappaB was required for these responses. In addition to these stimulatory effects, BAFF-mediated signaling inhibited processes involving cytoskeletal movement such as phagocytosis and transmigration through blocking the activation of phosphatidylinositol 3-kinase/AKT and Rac-1. Furthermore, murine primary macrophage culture such as peritoneal macrophages expressed BAFF and stimulation of it induced the expression of MMP-9. These observations show that the reverse signaling initiated from BAFF induces the expression of inflammatory mediators while suppressing the cytoskeletal movements associated with phagocytosis and transmigration.

Decursin inhibits induction of inflammatory mediators by blocking nuclear factor-kappaB activation in macrophages.

In the course of screening inhibitors of matrix metalloproteinase (MMP)-9 induction in macrophages, we isolated decursin, a coumarin compound, from the roots of Angelicae gigas. As a marker for the screening and isolation, we tested expression of MMP-9 in RAW264.7 cells and THP-1 cells after treatment with bacterial lipopolysaccharide (LPS), the TLR-4 ligand. Decursin suppressed MMP-9 expression in cells stimulated by LPS in a dose-dependent manner at concentrations below 60 microM with no sign of cytotoxicity. The suppressive effect of decursin was observed not only in cells stimulated with ligands for TLR4, TLR2, TLR3, and TLR9 but also in cells stimulated with interleukin (IL)-1beta, and tumor necrosis factor (TNF)-alpha, indicating that the molecular target of decursin is common signaling molecules induced by these stimulants. In addition to the suppression of MMP-9 expression, decursin blocked nitric oxide production and cytokine (IL-8, MCP-1, IL-1beta, and TNF-alpha) secretion induced by LPS. To find out the molecular mechanism responsible for the suppressive effect of decursin, we analyzed signaling molecules involved in the TLR-mediated activation of MMP-9 and cytokines. Decursin blocked phosphorylation of IkappaB and nuclear translocation of NF-kappaB in THP-1 cells activated with LPS. Furthermore, expression of a luciferase reporter gene under the promoter containing NF-kappaB binding sites was blocked by decursin. These data indicate that decursin is a novel inhibitor of NF-kappaB activation in signaling induced by TLR ligands and cytokines.

Cyclophilin A may contribute to the inflammatory processes in rheumatoid arthritis through induction of matrix degrading enzymes and inflammatory cytokines from macrophages.

Cyclophilin A (CypA) levels increase in the sera and synovial fluids of rheumatoid arthritis (RA) patients, but the cell types expressing CypA and the function of CypA in the pathogenesis of RA are not known yet. Immunohistochemistry analyses revealed high level CypA staining in the macrophages in the lining layers of human RA and osteoarthritis synovium. Low level CypA staining was also detected in endothelial cells, lymphocytes, and smooth muscle cells in RA synovium. Further investigation of the CypA function using monocyte/macrophage cell lines revealed that CypA induced expression of cytokine/chemokines such as TNF-alpha, IL-8, MCP-1, and IL-1beta and matrix metalloproteinase (MMP)-9 through a pathway that is dependent on NFkappaB activation. Furthermore, MMP-9 staining pattern overlapped with that of CypA in both RA and OA synovium. Our data suggest that CypA may stimulate macrophages to degrade joint cartilage via MMP-9 expression and promote inflammation via pro-inflammatory cytokine secretion.