PREDOMINANT ROLE OF NEUTROPHILS IN THE INACTIVATION OF ALPHA-2-MACROGLOBULIN IN ARTHRITIC JOINTS

We studied the state of alpha-2-macroglobulin (alpha-2M), an important inhibitor of cartilage-degrading proteinases, in relation to activation of neutrophils in 82 patients with several types of arthritis, including 52 with rheumatoid arthritis and 11 with osteoarthritis. Levels of total inactive alpha-2M (i-alpha-2M), which comprises alpha-2M complexed to proteinases and alpha-2M inactivated by oxidation or hydrolysis, were measured with a monoclonal antibody specific for i-alpha-2M. In addition, levels of alpha-2M complexed to proteinases were quantitated with specific assays. Neutrophil activation was assessed by measuring elastase-alpha-1-antitrypsin complexes and lactoferrin. In 83% of the 82 patients tested, the synovial fluid (SF) to plasma ratio of i-alpha-2M exceeded 1, indicating an intraarticular generation. Levels of i-alpha-2M significantly correlated with neutrophil numbers (P < 0.0005) and with levels of elastase-alpha-1-antitrypsin complexes and of lactoferrin (P < 0.00001 for both). Moreover, part of i-alpha-2M consisted of alpha-2M complexed to elastase-like and chymotrypsin-like proteinases, presumably, neutrophil elastase and cathepsin G, respectively. However, the amount of i-alpha-2M was approximately 10-fold larger than the amount complexed to these proteinases. In vitro inactivation of alpha-2M by activated neutrophils was only partly inhibitable by eglin C, a specific inhibitor of both elastase and cathepsin G. Release of reactive oxygen species was presumably responsible for the additional inactivation of alpha-2M, because eglin C completely abolished the inactivation of alpha-2M by cell-free supernatant of activated neutrophils. Thus, our results suggest a predominant role of neutrophils in the inactivation of alpha-2M in the SF of patients with inflammatory joint diseases. However, this inactivation could be explained only in part by the release of neutrophilic proteinases. We propose that the inactivation of alpha-2M in SF was due to the concerted action of both reactive oxygen species and lysosomal proteinases.