Using routinely processed, paraffin-embedded tissue specimens, osteoclast-like giant cells in giant cell tumour of bone (GCT), chondroblastoma, osteoblastoma and osteoblastic osteosarcoma were examined histochemically for osteoclast-specific enzymes tartrate-resistant acid phosphatase (TRAP) and carbonic anhydrase isoenzyme II (CA-II). Osteoclast-like giant cells and some mononuclear cells possessed TRAP activity. These were further classified with respect to CA-II immunoreactivity, i.e. cells with CA-II were seen in GCT and chondroblastoma, while those in osteoblastoma and osteoblastic osteosarcoma were negative for CA-II. All the cellular components in malignant fibrous histiocytoma and various extraosseous inflammatory lesions including malignant giant cells and macrophage polykaryons were negative for both TRAP and CA-II. These results indicate that osteoclast-like giant cells in GCT, chondroblastoma, osteoblastoma and osteoblastic osteosarcoma are all osteoclasts and generated by fusion of mononuclear cells with the same histochemical characteristics as osteoclast-like giant cells. The difference in CA-II immunoreactivity suggests the functional or maturational difference between osteoclast-like giant cells in GCT and chondroblastoma and those in osteoblastoma and osteosarcoma.

The immunoreactivity for class II antigens of the major histocompatibility complex and interleukin-1 (IL-1) in Warthin's tumor (WT) cells was studied. In addition to macrophages, dendritic cells, and capillary endothelia, the luminal tumor cells and some keratinocytes in the metaplastic squamous foci exhibited immunoreactivity for both class II antigens and IL-1. The distribution of the class II antigens in the luminal tumor cells was limited to their basolateral membrane. These data, together with previous findings, strongly suggest that the luminal tumor cells of WT introduce the luminal antigen to the underlying lymphoid tissue and, thus, act as an antigen-presenting cell.