Expression of cysteine proteinases cathepsins B and K and of cysteine proteinase inhibitor cystatin C in giant cell tumor of tendon sheath

The expression of cysteine proteinases cathepsins B and EC and of the endog enous inhibitor of cysteine proteinases, cystatin C, was investigated in ti ssue specimens of patients with giant cell tumor of tendon sheath (GCTTS). Expression of both enzymes was examined by immunohistochemistry in tissue s pecimens of 14 patients with GCTTS. Applying double-labeling techniques, th e coexpression of cathepsin B and its major endogenous inhibitor cystatin C was additionally studied. Cells expressing the respective proteins were fu rther characterized with the macrophage markers HAM56 and anti-CD68 (clone PG-M1). Cathepsin B could be detected in numerous HAM56-positive mononuclea r cells (MC), but only in very few giant cells (GC). In contrast, cathepsin K was predominantly identified in GC that were also strongly immunoreactiv e for cystatin C and CD68, Coexpression of cathepsin B and cystatin C occur red only in a few MC. The strong expression of both cathepsin B and K sugge sts that in GCTTS, bone erosion might be mediated not only by pressure of t he proliferative tissue, but also by matrix-degrading cysteine proteinases. Because previous studies showed that osteoclasts express high levels of CD 68, cathepsin K, and cystatin C but not of cathepsin B, our study contribut es to the view that GC of GCTTS and osteoclasts are closely associated.