In vivo growth of tumor cell spheroids after in vitro hyperthermia.

The effect of hyperthermia on the host-tumor interrelation ship was investigated using an in vitro-in vivo multiceli spheroid tumor model. The experiments were designed to attempt to discriminate between the relative importance of direct effects of heat on the tumor cells and of indirect delayed effects possibly related to modifications of immune responses to heated tumors or of damage to tumor vasculature. EMT6/Ro mammary tumor cells were first grown in vitro as spheroids to model the tridimensional structure and microenvironments known to affect responsiveness to hyperthermia. The spheroids were heat treated in vitro and immediately implanted into the peritoneal cavities of normal or sensitized syngeneic and allogeneic mice in which their growth, clonogenicity, and infiltration by host cells were determined by retrieving spheroids in vivo, and the lethal action on the host was also characterized. In all cases, except for the initial cell kill, the growth kinetics and clonogenicity of the spheroids were found to be little affected by hyperthermia, suggesting similar host responsiveness to ward normal and heat-modified spheroids. No marked qualita tive or quantitative differences were found in the different types of host cells which infiltrated the spheroids. The effect of heat treatment of the spheroids on survival kinetics of syngeneic mice was investigated. Treatments, which reduced by at least 30% the initial number of clonogenic cells per spheroid, were required to prolong the life of the animal, but lighter treatments increased spheroid lethality to the mice. The serial events leading to animal death after implantation of spheroids suggest that cell shedding from the spheroid surface, which can be modified by hyperthermia, and their subsequent seeding to the peritoneal lining represent the major factors that determine the toxicity of the implanted spheroids. Compared with results obtained previously by others when EMT6 tumors were heated in situ, the present data suggest that vascular and not immunological events play a role of central importance in the effect of hyperthermia on this tumor.