TLR-2-mediated induction of vascular endothelial growth factor (VEGF) in cartilage in septic joint disease

Bacterial arthritis is a progressive joint disease which includes rapid destruction of articular cartilage even after clearance of the causal factor. The resulting post-infectious arthropathy is mainly characterized by self-perpetuating joint destruction and extensive angiogenesis in the emerging pannus-like synovial membrane, but the underlying molecular mechanisms of the bacteria-initiated process remain incompletely understood. This study was conducted to elucidate the expression and regulation of angiogenic and cartilage-destructive vascular endothelial growth factor (VEGF) in septic arthritis. For that purpose, aspirates of synovial fluid from patients with pyogenic arthritis were examined for VEGF levels by ELISA. In vitro studies with primary and immortalized chondrocytes were performed to determine whether Gram-positive and Gram-negative bacteria induce VEGF expression, by using real-time RT-PCR, ELISA, and immunohistochemistry. Activation of the transcription factor AP-1 was assessed by EMSA experiments. The necessity of the Toll-like receptor-2 (TLR-2), ERK-1/-2, and AP-1 pathway for infectious VEGF induction in chondrocytes was examined by using specific blocking reagents. ELISA experiments revealed that aspirates of synovial fluid from patients with pyogenic arthritis contain elevated levels of VEGF. The in vitro results confirmed the transcriptional induction of VEGF in chondrocytes after bacterial challenge by real-time RT-PCR, ELISA, and immunohistochemistry. This activation was mediated by a TLR-2-, ERK-1/-2-, and AP-1-dependent pathway. The findings demonstrate the expression of Toll-like receptors on mesenchymal articular chondrocytes and reveal TLR-2-mediated VEGF induction in human chondrocytes after Gram-positive bacterial sensing. Since VEGF is a potent angiogenic and tissue remodelling factor, evidence that Toll-like receptors contribute to destructive arthropathy after microbial joint infection is provided. VEGF may be a therapeutic target in the future for the prevention of post-infectious cartilage degradation in articular joints.