Role of CD8-positive cells in radioimmunotherapy utilizing ^sup 177^Lu-mAbs in an immunocompetent rat colon carcinoma model

Background CD8-positive cells might play a crucial role in the therapeutic response to radiation, which has however not been investigated in radioimmunotherapy (RIT). The aim of this study was to evaluate whether cytotoxic T cells affect the response of established tumors and, above all, if they delay or prevent the development of distant metastases after RIT, using an immunocompetent syngeneic rat colon carcinoma model. Methods The cytotoxic T cells were depleted in 15 rats by anti-CD8 before the injection of the radioimmunoconjugate (400 MBq/kg body weight ^sup 177^Lu-BR96, which binds to the tumor-associated antigen Lewis Y). Fifteen other rats were treated with RIT only. Both groups were followed for 99 days. Blood samples were collected at least once weekly, and tumors were monitored twice weekly. Results Twenty-nine of the 30 animals exhibited local complete response. The non-responder was treated with anti-CD8 and RIT but succumbed later due to metastases. Five animals in the group given anti-CD8+RIT were sacrificed due to metastatic disease, and 4 additional animals were found to have metastases at autopsy. In the group given RIT, 4 animals developed metastatic disease, but no metastases were found in the remaining 11 animals at autopsy. Thus, at the end of the study, 6 animals in the anti-CD8+RIT group were free from metastases, while 11 were free from metastases in the group receiving RIT. CD3^sup +^CD4^sup -^CD8^sup +^ lymphocytes were consistently depleted by the anti-CD8 treatment. The myelosuppression was otherwise similar in the two groups. The initial depletion of CD8-positive cells in our syngeneic rat colon carcinoma model resulted in a higher frequency of animals developing metastases. Conclusions Depletion of CD8-positive cells during RIT in an immunocompetent rat tumor model might influence the number of animals developing metastases, indicating that the immune system may be important in the long-term outcome of RIT.