CATHEPSIN-B IN OSTEOARTHRITIS - CYTOCHEMICAL AND HISTOCHEMICAL ANALYSIS OF HUMAN FEMORAL-HEAD CARTILAGE

Objective-To localise the cysteine endopeptidase cathepsin B in chondr ocytes and cartilage from normal and osteoarthritic (OA) human femoral heads in order to provide qualitative information on its cellular exp ression and distribution at possible sites of action. Methods-OA artic ular cartilage was obtained at surgery for total hip replacement; cont rol cartilage was obtained at postmortem. Chondrocytes were isolated b y sequential enzymatic digestion and cathepsin B analysed by immunocyt ochemistry and activity staining with a fluorogenic substrate. Lysosom es were visualised by fluorescence microscopy after staining of living cells with acridine orange. Using a histochemical reaction, enzyme ac tivity was measured in cryosections of full thickness cartilage. Resul ts-Chondrocytes from normal cartilage contained very few lysosomes and only a minor cell population was cathepsin B positive. A high proport ion of chondrocytes from active OA cartilage contained a large number of lysosomes and an excess of cathepsin B in intracellular organelles; the enzyme was stored in an active form. In this respect, OA chondroc ytes closely resembled normal cells that had been phenotypically modul ated by serial subcultures. No cathepsin B activity could be detected by histochemistry in either chondrocytes or matrix of normal cartilage . While apparently intact and severely degraded OA cartilage was also cathepsin B negative, tissue at sites of active destruction and, parti cularly, at repair sites was highly positive. Conclusion-The presence and the particular distribution of active cathepsin B in OA cartilage at 'more involved' sites suggest a pathological role for this enzyme i n sustaining and perpetuating cartilage degradation. While other stimu li may also be responsible for cathepsin B expression in OA chondrocyt es, the similarity with artificially modulated cells indicates fibrobl astic metaplasia as a plausible mechanism.