Heat-shock protein 72 in human osteosarcoma: T-lymphocyte reactivity and cytotoxicity. Heat-shock proteins (hsp) have been shown to be involved in tumor immunity. The expression of hsp72, the inducible form of the 70-kDa family, is increased in human osteosarcoma and correlates with a good response to neoadjuvant chemotherapy. It is selectively expressed on the surface of osteosarcoma cell lines where it acts as a target molecule for natural killer cells. Because hsps are strongly immunogenic, this study investigates the role of hsps as antigens in human osteosarcoma. Osteosarcoma cells and infiltrating T lymphocytes were isolated from osteosarcoma specimens from 3 patients with high-grade osteosarcoma. Two of the tumors immunohistochemically expressed hsp72, whereas one did not. T-cell lines isolated from both hsp72-positive osteosarcomas had a significantly proliferative response upon stimulation with recombinant human hsp72, whereas the T lymphocytes from the hsp72-negative osteosarcoma did not recognize hsp72. The lines had a significantly proliferative response upon stimulation with autologous osteosarcoma cells and exerted cytotoxicity. Cytotoxicity and proliferation could be increased by heat treatment of the target cell in the hsp72 responsive lines. These results demonstrate that hsp72 is involved in the interaction of T lymphocytes and osteosarcoma cells in a specific group of osteosarcomas expressing hsp72. Because of the cytotoxic potential of these T lymphocytes, induction of hsp72 in osteosarcomas might lead to an increased immune response and rejection of the osteosarcoma.