Destruction of articular cartilage by alpha2 macroglobulin elastase complexes:: role in rheumatoid arthritis

Objective-Neutrophil elastase accounts for the ability of some fresh rheumatoid synovial fluids to degrade cartilage matrix in vitro. The aim of this study was to determine if enzyme activity could result from depletion of synovial fluid inhibitors or protection of the enzyme from inhibition. Methods-The ability of synovial fluids to inhibit porcine pancreatic elastase was investigated together with chemical pretreatments capable of inactivating alpha, protease inhibitor (alpha(1)PI) or preventing formation of alpha, macroglobulin (alpha(2)M) elastase complexes. Subsequently, complexes of human neutrophil elastase with alpha(2)M were prepared and applied to frozen sections of cartilage. Proteoglycan loss was quantified by alcian blue staining and scanning and integrating microdensitometry. Parallel studies were carried out using a low molecular weight chromogenic elastase substrate. The effects of alpha(1)PI and SF on these systems were investigated. Finally, synovial fluids were subjected to gel filtration and the fractions assayed for elastase activity. High molecular weight fractions were pooled, concentrated, and tested for their ability to degrade cartilage sections. Results-All synovial fluids reduced the activity of porcine pancreatic elastase, the inhibition mainly being attributable to alpha(1)PI, whereas remaining activity resulted from complexes of elastase with alpha(2)M. Complexes of human neutrophil elastase with alpha(2)M were shown to cause proteoglycan degradation in frozen sections of human articular cartilage. Alpha(1)PI prevented alpha(2)M elastase complexes from degrading cartilage but not the chromogenic substrate. The data suggested that alpha(1)PI does not inhibit elastase bound to alpha(2)M but sterically hinders the complex. However, only one of five synovial fluids was able to completely block the actions of alpha(2)M elastase complexes against cartilage. Gel filtration of rheumatoid synovial fluids showed elastase and cartilage degrading activity to be associated with fractions that contained alpha(2)M, and not with fractions expected to contain free enzyme. Conclusions-The data suggest that synovial fluid alpha(2)M elastase complexes can degrade cartilage matrix in rheumatoid arthritis.