Synergistic cytolytic activity by combined populations of peritoneal T lymphocytes and macrophages during the L5178Y cell tumor-dormant state in DBA/2 mice

It was previously reported that the establishment of the L5178Y cell tumor-dormant state in DBA/2 mice is mediated principally by a peritoneal cytolytic T-cell response that reaches peak levels 4 days after L5178Y cell challenge, lyses more than 99% but less than 100% of peritoneal L5178Y cells, and gradually wanes to background levels by 40–70 days postchallenge (DPC). At this time the majority of mice are clinically normal, and contain a relatively small number of L5178Y cells in the peritoneal cavity. During the tumor-dormant state, mice that harbor more than 10⁴ L5178Y cells contain peritoneal macrophage-mediated cytolytic activity. We report here that tumor-dormant mice that contain fewer than 10⁴ peritoneal L5178Y cells also produce cytolytic activity in vitro, but that it is synergistic, in that the cytolytic activity of adherent (AD) peritoneal cells (PEC) and nonadherent (NAD) PEC cultured together is greater than the additive lysis produced by these cell populations when cultured separately. This synergistic cytolytic activity is: (1) effector cell density dependent, (2) dependent on the tumor-dormant status of the NAD and AD PEC donor mice, (3) protracted in its kinetics during a 48-hr in vitro assay, and (4) dependent on an interaction between NAD T cells and AD phagocytic macrophages. The consistent detection of this in vitro-assayed cytolytic activity in PEC of tumor-dormant mice which harbor small endogenous tumor burdens suggests that it reflects an in vivo cytotoxic effector mechanism involved in the long-term maintenance of the tumor-dormant state.