Chemoimmunotherapy of a Friend Leukemia With Cells Secondarily Sensitized In Vitro: Effect of Culture Duration on Therapeutic Efficacy23

Abstract

Although in vivo-primed cells are known to be rendered more effective in adoptive tumor therapy by secondary sensitization in vitro, they were tested only at the time of maximum in vitro cytolytic reactivity. Since the requirements for in vitro cytotoxicity and tumor therapy differ, the present study was designed to evaluate and compare the influence of culture duration on these two effector functions. Spleen cells from inbred C57BL/6 mice primed in vivo with the Friend virus-induced leukemia FBL-3 were secondarily sensitized in vitro by culture with tumor and tested for cytotoxicity in vitro in a 4-hour ⁵¹Cr release assay and for therapeutic efficacy in vivo against established tumor. In vivo-primed cells tested directly without prior culture were effective in therapy but were not cytotoxic in vitro. Culture of primed cells for 3 days rendered them cytotoxic as measured in vitro. This cytotoxicity increased through day 5, then it plateaued. Culture duration modified the in vivo efficacy of primed cells differently. Cells cultured for 3 days with secondary sensitization by tumor became more effective in tumor therapy than noncultured primed cells. Increasing the duration of in vitro sensitization from 3 to 5 days did not enhance in vivo therapeutic efficacy, despite a concurrent increase in in vitro cytotoxicity. However, further lengthening of the culture duration to 7 days rendered cells more effective in vivo. These discrepancies presumably reflect different effector cell requirements and/or regulation operative for tumor lysis during a short-term in vitro assay and for tumor eradication following adoptive transfer of immune cells in vivo.