Efficacy of in vitro sensitized cells generated by in vivo priming with OK-432 for adoptive immunotherapy of the poorly immunogenic B 16-BL6 melanoma

Abstract We investigated the efficacy of the streptococcal preparation OK-432 as an adjuvant for in vivo priming in induction of sensitized cells for adoptive immunotherapy of the poorly immunogenic BI6-BL6 (BL6) melanoma. C57BL/6 (B6) mice were immunized subcutaneously (s.c.) with 3 × 10 6 viable BL6 tumor cells admixed with various doses of OK-432 ranging from 1 to 100 μg in the foot-pad. Draining popliteal lymph nodes (LNs) were harvested 7 days after immunization and LN cells were further sensitized with irradiated tumor cells in the presence of 60–300 IU/ml of IL-2 for 11 days. These in vitro sensitized (IVS) cells (2 × 10 6 ) were transferred intravenously (i.v.) to B6 mice bearing 4-day pulmonary metastases established by i.v. injection of 2–4 × 10 5 viable BL6 cells. The mice were also received intraperitoneally (i.p.) 4 × 10 4 IU/day of IL-2 for 4 days after adoptive transfer. Transfer of IVS cells from mice immunized by s.c. injection of tumor cells admixed with 10 μg of OK-432 significantly reduced the numbers of BL6 pulmonary metastases compared with that of control IVS cells without the administration of OK-432 ( P = 0.003). These effective IVS cells also significantly prolonged the survival of treated animals ( P =0.003). Functional IVS cells required in vitro stimulation with tumor cells. However, addition of OK-432 in the vaccine resulted in no enhancement of in vitro cytotoxicity and no characteristic change of phenotype of IVS cells. These results suggest that in vivo priming of OK-432 facilitates the sensitization of tumor-reactive T-cells. The procedure of in vivo priming with OK-432 may be beneficial in the adoptive immunotherapy of melanoma.