Objective. To show the eventual presence and extent of production of matrix metalloproteinase 13 (MMP‐13, or collagenase 3) in rheumatoid synovial tissue samples and extracts, and to assess the inhibition characteristics of recombinant MMP‐13.

Methods. Immunohistochemical avidin‐biotin‐peroxidase complex staining/morphometry was used to analyze MMP‐13‐positive cells in situ. Neutral salt extraction of synovial tissue, electrophoresis of the extract in different buffer systems, and Western blotting were also used. The inhibitory properties of doxycycline, clodronate, pamidronate, and D‐penicillamine for recombinant enzyme were determined with a soluble type II collagen assay.

Results. MMP‐13 was detected in fibroblast‐ and macrophage‐like mononuclear cells in the synovial lining and stroma and in vascular endothelial cells. The overall expression of MMP‐13 in these cells in the synovial stroma was high in rheumatoid arthritis (86 ± 12%) compared with osteoarthritis (17 ± 5%) patient samples (P = 0.0027). In a high‐pH native electrophoresis gel, immunoreactivity to anti‐MMP‐1 and anti‐MMP‐13 were clearly separated, with anti‐MMP‐13‐immunoreactive material migrating faster than anti‐MMP‐1‐immuno‐reactive material. Finally, in contrast to MMP‐1 and MMP‐8, MMP‐13 was found to be relatively resistant to the inhibitory effects of doxycycline and clodronate in vitro.

Conclusion. Due to its localization in synovial tissue, its substrate profile, increased expression, and relative resistance to known MMP inhibitors, MMP‐13 is suggested to play a major role in the pathogenesis of tissue destruction in rheumatoid arthritis.