Elevation of cathepsin L levels in the synovial lining of rabbits with antigen-induced arthritis.

The activity and distribution of the collagenolytic lysosomal enzyme, cathepsin L, has been assessed in the synovial lining of the rabbit during the initiation and development of chronic (antigen-induced) arthritis. Biochemical assay of homogenates of synovial lining revealed a marked increase in the activity of lysosomal enzymes, including cathepsin L, between 1 and 5 days following initiation of arthritis. Elevated levels of these enzymes were still present at 4 weeks duration of arthritis. Using a monospecific antibody to cathepsin L. enzyme was immunolocated only in synovial lining cells in normal joints. In arthritic joints the enzyme was found in synovial lining cells, synovial fibroblasts and in infiltrating macrophages. Measurement, by scanning and integrating microdensitometry, of the peroxidase-reaction product in synovial lining cells revealed an increased content of cathepsin L in these cells in the diseased tissues.