Localization of the Angiogenesis Inhibitor Thrombospondin in Human Synovial Tissues

Abstract

Recently, we have shown that a macrophage subpopulation isolated from the synovial tissue of patients with rheumatoid arthritis was potently angiogenic and that a secreted inhibitor of angiogenesis, which is controlled by a tumor suppressor gene in hamster cells, was similar to thrombospondin. In order to investigate the potential role of thrombospondin in human arthritic disorders, we employed immunohistochemistry to examine frozen synovial tissue sections from normal controls (n = 3), patients with rheumatoid arthritis (n = 14) and with osteoarthritis (n = 5). The synovial tissues were stained with monoclonal antibody (mAb) A2.5, which reacts with the heparin-binding domain of thrombospondin, mAb A6.1 which reacts with the epidermal growth factor repeat motif of thrombospondin, and with mAb A4.1 which reacts with the properdin-repeat domain of thrombospondin. In rheumatoid synovial tissues the anti-thrombospondin mAbs reacted with vascular endothelial cells, and to a lesser extent with vascular smooth muscle. Pericytes were stained, particularly with mAb 6.1. Reactivity was also found with isolated macrophages and with the macrophage-derived synovial lining layer in over half the tissues. In osteoarthritis synovial tissues, mAb A2.5 stained fewer macrophages than in rheumatoid arthritis synovial tissues. Slightly fewer blood vessels reacted with mAb A2.5 in normal compared to diseased synovia. The mAbs reacted with capillaries, venules and arterioles in all synovial tissues. We conclude that mAbs to thrombospondin react primarily with blood vessels and macrophages in synovial tissues. Perhaps thrombospondin may function as an adhesive glycoprotein mediating cellular interactions, or it may serve to counteract the effects of the angiogenic factors produced by cells within diseased synovial tissues.

Identification of known and novel genes in activated monocytes from patients with rheumatoid arthritis

“…Differential hybridization and RT-PCR documented a significant activation of the TSP-1 gene (Table 3), a multifunctional extracellular matrix glycoprotein (59) with angiogenesisinhibiting properties (60). TSP-1 can be produced by many cell types involved in wound repair, such as keratinocytes, fibroblasts, and endothelial cells (61), and interestingly, it is also overexpressed in situ in M of the RA synovial lining cell layer (60). In RA, therefore, TSP-1 could act as an adhesion glycoprotein to mediate cell interaction, or it could be involved in counteraction/…”

The N-terminal Module of Thrombospondin-1 Interacts with the Link Domain of TSG-6 and Enhances Its Covalent Association with the Heavy Chains of Inter-α-trypsin Inhibitor

“…One inflammatory site where these two proteins are clearly co-expressed is in synovial fluid. TSP1 is induced in rheumatoid synovium (74) and accumulates on the surface of fibroblast-like synoviocytes, where it engages T cells through their CD47 receptor (8). Similarly, TSG-6 is elevated in arthritic synovial fluid (75) and joint tissues (76) and is induced by interleukin-17 on fibroblast-like synoviocytes of rheumatoid arthritis patients (77).…”

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“…Thrombospondin is an inhibitor of angiogenesis and localises primarily to blood vessels, including endothelial cells, and to macrophages in the RA synovium [69,90] Another angiogenesis regulator is angiotensin II, and angiotensin converting enzyme localises to endothelial cells and to macrophages in RA synovia [91]. …”