ENHANCEMENT OF TUMOR-ANTIGEN EXPRESSION AND INHIBITION OF PULMONARY METASTASIS OF RAT FIBROSARCOMA CELLS BY LOCAL RADIOTHERAPY

Pulmonary metastasis formation after local radiotherapy against a rat fibrosarcoma was investigated. KMT-17 fibrosarcoma cells were transpla nted into the hind leg in syngeneic WKA rats and two different doses ( 30Gy, 60Gy) of irradiation from a Co-60 source were applied 5 days aft er transplantation. Pulmonary metastasis was inhibited by 30Gy irradia tion rather than 60Gy irradiation, which was enough to almost complete ly cure the local tumors. This inhibitory effect of 30Gy irradiation w as induced by the continued presence of irradiated tumors. As for pulm onary metastasis, the different effects of irradiation doses were not recognized when the tumor was removed surgically 1 day after irradiati on, but when it was removed 4 days after 30Gy irradiation significantl y inhibited metastasis. Expression of tumor-associated antigen (TAA), termed CE7 antigen, on the cell surface was enhanced effectively and c ontinuously by 30Gy irradiation rather than by 60Gy. With this increas e in CE7-expressing cells, the enhancement of anti-tumor immunity of s pleen cells was observed in an in vitro I-125-IudR release assay and a n in vivo tumor-neutralizing assay (Winn assay). The above results sug gest that an appropriate dose of irradiation such as 30Gy, to a local tumor can efficiently enhance the TAA expression and that TAA-expressi ng cells may stimulate anti-tumor immunity, resulting in inhibition of pulmonary metastasis. This phenomenon may offer the possibility of re sistance to micrometastasis through the induction of antitumor effecto r cells.