Role of LIGHT in the pathogenesis of joint destruction in rheumatoid arthritis

Abstract

Aim: To characterise the role of substitutes for receptor-activator nuclear factor kappa-B ligand (RANKL) in rheumatoid arthritis (RA) joint destruction.

Methods: Synovial fluid (SF) macrophages isolated from the knee joint of RA patients were incubated with 25 ng/mL macrophage-colony stimulating factor (M-CSF) and 50 ng/mL LIGHT (lymphotoxin-like, exhibits inducible expression and competes with herpes simplex virus glycoprotein D for herpes virus entry mediator, a receptor expressed by T lymphocytes) in the presence and absence of 25 ng/mL RANKL and 100 ng/mL osteoprotegerin (OPG) on glass coverslips and dentine slices. Osteoclastogenesis was assessed by the formation of multinucleated cells (MNCs) expressing tartrate-resistant acid phosphatase (TRAP) on coverslips and the extent of lacunar resorption pit formation on dentine slices. The concentration of LIGHT in RA and osteoarthritis (OA) synovial fluid was measured by an enzyme-linked immunosorbent assay (ELISA) and the expression of LIGHT in RA and OA synovium was determined by immunohistochemistry using an indirect immunoperoxidase technique.

Results: In cultures of RA SF macrophages treated with LIGHT and M-CSF, there was significant formation of TRAP + MNCs on coverslips and extensive lacunar resorption pit formation on dentine slices. SF-macrophage-osteoclast differentiation was not inhibited by the addition of OPG, a decoy receptor for RANKL. Resorption pits were smaller and less confluent than in RANKL-treated cultures but the overall percentage area of the dentine slice resorbed was comparable in LIGHT- and RANKL-treated cultures. LIGHT significantly stimulated RANKL-induced lacunar resorption compared with RA SF macrophages treated with either RANKL or LIGHT alone. LIGHT was strongly expressed by synovial lining cells, subintimal macrophages and endothelial cells in RA synovium and the concentration of LIGHT was much higher in RA compared with OA SF.

Conclusion: LIGHT is highly expressed in RA synovium and SF, stimulates RANKL-independent/dependent osteoclastogenesis from SF macrophages and may contribute to marginal erosion formation.

Keywords: LIGHT; Osteoclast; Receptor-activator nuclear factor kappa-B ligand; Resorption; Rheumatoid arthritis.

Figure 1

LIGHT induces receptor-activator nuclear factor kappa-B ligand-independent osteoclastogenesis from rheumatoid arthritis synovial fluid macrophages. A: Osteoclast differentiation in 14-d cultures of RA SF macrophages incubated with M-CSF and LIGHT showing (left) TRAP+ multinucleated osteoclasts and (right) F actin-ring formation; B: Dentine slices stained with Toluidine blue showing lacunar resorption in 14-d RA SF macrophage cultures treated with M-CSF and sRANKL, LIGHT or LIGHT ± OPG; C: Percentage surface area lacunar resorption on dentine slices in LIGHT- (± OPG) treated SF macrophage cultures relative to sRANKL - treated controls; data is expressed as mean ± SEM of three independent experiments where each condition was carried out in triplicate. RANKL: Receptor-activator nuclear factor kappa-B ligand; RA: Rheumatoid arthritis; SF: Synovial fluid; M-CSF: Macrophage-colony stimulating factor; OPG: Osteoprotegerin.

Figure 2

LIGHT augments receptor-activator nuclear factor kappa-B ligand-induced osteoclastogenesis from rheumatoid arthritis synovial fluid macrophages. A: Dentine slice stained with Toluidine blue showing lacunar resorption pits in 14-d RA SF macrophage cultures incubated with M-CSF and sRANKL ± LIGHT; B: Percentage surface area lacunar resorption on dentine slices in 14-d RA SF macrophage cultures incubated with M-CSF and sRANKL ± LIGHT. Data is expressed as mean ± SEM of three independent experiments where each condition was carried out in triplicate; ^aP < 0.05. RANKL: Receptor-activator nuclear factor kappa-B ligand; RA: Rheumatoid arthritis; SF: Synovial fluid; M-CSF: Macrophage-colony stimulating factor.

Figure 3

LIGHT levels are significantly higher in rheumatoid arthritis than osteoarthritis synovial fluid. LIGHT concentration is significantly increased in SF of RA patients compared with OA joints. Data is expressed as mean ± SEM of three independent experiments where each condition was carried out in triplicate; ^aP < 0.05. RA: Rheumatoid arthritis; SF: Synovial fluid; OA: Osteoarthritis.

Figure 4

Expression of LIGHT in rheumatoid arthritis and osteoarthritis synovium. Immunohistochemical staining of (A) RA and (B) OA synovium, showing expression of LIGHT on synovial lining cells and subintimal macrophages in RA synovium with no staining for LIGHT in OA synovium. Magnification × 200. RA: Rheumatoid arthritis; OA: Osteoarthritis.