Role Of Monocytes In Immunopathology

Dental & Craniofacial Research

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Abstract

Research in the Immunopathology Section focuses on the biological mediators and signal transduction pathways involved in the modulation of human monocyte functions that may contribute to the immunopathology associated with various inflammatory lesions. Connective tissue destruction is associated with many diseases in which the monocytemacrophage is a prominent cell. Examples of these chronic inflammatory lesions include periodontal disease and rheumatoid arthritis where destruction of connective tissue leads to loss of structural integrity and atherosclerosis where degradation of connective tissue in vulnerable plaques leads to rupture and subsequent ischemic events. Additionally, the interaction of monocytesmacrophages with cancer cells has the potential for regulating or contributing to metastasis through the induction of connective tissue degrading enzymes. Since matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs) are believed to play a major role in the destruction and remodeling of connective tissue, a major emphasis has been placed on how these enzymes and inhibitors are regulated in the human monocyte and how the monocyte may regulate the function of other cell types. Determining the inflammatory agents and signal transduction pathways that lead to the production of MMPs by monocytes is important to potential therapeutic intervention. [unreadable] [unreadable] Phosphorylation and dephosphorylation of signaling proteins is controlled by precise interactions with upstream kinases and can also be strongly regulated by phosphatases. We have examined the potential role of MAPK phosphatase 1 (MKP1), one of the dual-specific protein phosphatases, in the regulation of monocyte signaling and the production of MMPs. To carry out this study THP-1 cells (a human acute monocytic leukemia cell line) were compared with human primary monocytes. THP-1 cells were found to be deficient in MKP1 mRNA which resulted in constitutive activation of the transcription factors, c-Myc, and NF-kB, culminating in the proliferation of these cells and increased production of MMPs. Transfection of THP-1 cells with MKP1 suppressed constitutive activation of c-Myc and NF-kB resulting in decreased proliferation and MMP production. Unlike THP-1 cells, primary monocytes expressed mRNA for MKP1 and did not have constitutively activated c-Myc or NF-kB. Additionally, primary monocytes required stimulation to produce MMPs. Furthermore, in parallel experiments with primary monocytes and inhibitors of MEK12 and phosphatase, we demonstrate an intrinsic balance between phosphatase activity and phosphorylation of ERK12. These findings indicate that MKP1 may function as a putative tumor suppressor.[unreadable] [unreadable] Urokinase plasminogen activator (uPA) and plasmin, in addition to their role in fibrinolytic activity and activation of pro-matrix metalloproteinases (MMPs), have been shown to transduce intracellular signals through specific receptors. We hypothesized that components of the urokinase system may induce signals that regulate the production of MMPs by monocytes. We speculated that binding of uPA to the uPA receptor (uPAR) andor plasmin to specific receptors would induce signaling leading to MMP production. In a submitted manuscript we show that catalytically active high molecular weight uPA (HMW-uPA), which binds to the uPAR, and low molecular weight uPA (LMW-uPA), which does not bind uPAR, both significantly enhanced MMP-1 by activated primary human monocytes. In contrast, the amino terminal fragment of uPA (ATF-uPA) which lacks the catalytic site but binds to uPAR failed to induce MMP-1 production, indicating that uPA-stimulated MMP-1 synthesis was plasmin-dependent and independent of uPAR binding. Plasmin generated endogenously by the action of uPA or exogenously added plasmin increased MMP-1 by signaling through annexin A2. This was demonstrated by inhibition of uPA or plasmin-induced MMP-1 production with antibodies against annexin A2 and S100A10, a dimeric protein associated with annexin A2, as well as by inactive plasmin. Interaction of plasmin with annexin A2 resulted in the stimulation of ERK12 and p38 MAPKs, cyclooxygenase-2 and PGE2 leading to increased MMP-1 production. These findings demonstrate that the urokinase plasminogen activation system, in addition to activating pro-MMPs, directly regulates the production of MMP-1 by activated monocytes and thus plays a major role in the turnover of connective tissue at an inflammation site. Furthermore, the inhibition of plasmin induction of MMP-1 by inactive plasmin as a result of its binding to annexin A2 suggests that inactive-plasmin may useful in suppressing inflammatory responses. [unreadable] [unreadable] Polyreactive antibodies bind to a variety of structurally unrelated antigens, however their function has remained an enigma and because of their low binding affinity their biological relevance has been questioned. We have collaborated with investigators in the Experimental Medicine Section of the Oral Infection and Immunity Branch who are examining the functional role of polyreactive antibodies. Through the use of a panel of monoclonal polyreactive antibodies it was shown that these antibodies: bind to both gram-negative and gram-positive bacteria and acting through the classical complement pathway inhibit bacterial growth by lysis; enhance phagocytosis; and neutralize endotoxin as shown by the ability of these antibodies to block LPS induction of MMP-1 in monocytes. From these findings it is concluded that polyreactive antibodies are a major contributor to the broad anti-bacterial activity of the natural antibody repertoire.