Urokinase, a constitutive component of the inflamed synovial fluid, induces arthritis

Abstract

Urokinase plasminogen activator (uPA) is an important regulator of fibrinolysis in synovial fluid. An increase of uPA activity and expression of its receptor have been reported in joints of patients with rheumatoid arthritis (RA). The aim of the present study was to assess the arthritogenic capacity of uPA and the mechanisms by which this effect is mediated. uPA was injected into the knee joints of healthy mice, and morphological signs of arthritis were assessed 4 days after the injection. The prerequisite of different leukocyte populations for the development of uPA-triggered arthritis was assessed by selective cell depletion. The inflammatory capacity of uPA was assessed in vitro. Finally, levels of uPA were measured in 67 paired blood and synovial fluid samples from RA patients. The synovial fluid from RA patients displayed higher levels of uPA compared with blood samples. Morphological signs of arthritis were found in 72% of uPA-injected joints compared with in only 18% of joints injected with PBS (P < 0.05). Synovitis was characterised by infiltration of CD4-Mac-1+ mononuclear cells, by the formation of pannus and by occasional cartilage destruction. The absence of monocytes and lymphocytes diminished the frequency of synovitis (P < 0.01), indicating an arthritogenic role of both these leukocyte populations. Synthetic uPA inhibitor downregulated the incidence of uPA-triggered arthritis by 50%. uPA induced arthritis, stimulating the release of proinflammatory cytokines IL-6, IL-1beta and tumour necrosis factor alpha. Accumulation of uPA locally in the joint cavity is a typical finding in erosive RA. uPA exerts potent arthritogenic properties and thus may be viewed as one of the essential mediators of joint inflammation.

Figure 1

Accumulation of urokinase plasminogen activator (uPA) in the synovial fluid of patients with rheumatoid arthritis (RA). The level of uPA is presented as the mean ± SEM (ng/ml). Significance regarding differences between the groups is indicated.

Figure 2

Histopathological appearance of arthritis after intra-articular injection of low molecular weight urokinase plasminogen activator (LMW-uPA). (a) Arthritic knee joint 4 days after the LMW-uPA injection (60 pmol/joint). Infiltration of mononuclear cells in the synovial tissue is apparent. Original magnification × 10. (b) Knee joint injected with PBS revealed no signs of inflammation. Original magnification × 10. JC, joint cavity; C, cartilage; ST, synovial tissue; P, pannus. Arrows indicate inflammatory cell infiltration in the synovium and pannus formation.

Figure 3

Immunohistochemical staining of an arthritic knee joint injected with low molecular weight urokinase plasminogen activator. (a) Abundance of mononuclear cells expressing Mac-1 (red–brown colour) in the inflamed synovial tissue. Original magnification × 20. (b) Arthritic knee joint stained with irrelevant goat IgG. JC, joint cavity; C, cartilage; ST, synovial tissue.

Figure 4

Essential role of monocytes and lymphocytes for the development of urokinase plasminogen activator (uPA)-induced arthritis. Frequency of histological signs of arthritis 4 days after a single injection of low molecular weight uPA (60 pmol/knee). Significance regarding the difference of incidence of arthritis between the groups is indicated.

Figure 5

In vitro cytokine production following stimulation with low molecular weight urokinase plasminogen activator for 48 hours. (a) IL-6 (n = 10), (b) tumour necrosis factor alpha (TNF-α) (n = 6), and (c) IL-1β (n = 4). Levels of cytokines are presented as the mean ± SEM (pg/ml). Significant difference in the groups is indicated; n.s., not significant.