The cells of giant cell tumor of tendon sheath resemble osteoclasts.

To determine whether the mononuclear cells (MC) and multinucleated giant cells (GC) of giant cell tumor of tendon sheath (GCTTS) exhibit evidence of monocyte/macrophage lineage, we studied their antigenic features (seven cases, paraffin sections; two cases, frozen sections) and enzymatic features in situ (four cases, plastic sections). Both MC and GC expressed a monocyte/macrophage phenotype: HLA-A,B,C+, HLA-DR+, T200+ (leukocyte common antigen), Leu-M3+ and Leu-3+. MC and GC also expressed similar enzymatic phenotypes which resembled that of osteoclasts. Both were rich in acid phosphatase and contained smaller, variable amounts of ATPase, beta-glucuronidase, alpha-naphthyl acetate esterase, and 5'-nucleotidase. Both lacked alkaline phosphatase. Reactive osteoclasts in plastic and paraffin sections were also T200+, a finding strongly supporting their bone marrow derivation and leukocytic differentiation. In plastic sections, osteoclasts were additionally reactive with macrophage antigen EBM11. In aggregate, these data suggest that GCTTS is a true histiocytic lesion of monocyte/macrophage lineage composed of phenotypically similar MC and GC that most closely resemble osteoclasts. We found no evidence that GCTTS cells resemble osteoblasts, fibroblasts, or synovial sarcoma cells. Furthermore, expression of the Ki-67 nuclear antigen by 1-2% of MC but not by GC suggests that the proliferating cells in GCTTS are restricted to its MC component.