The multicellular tumor spheroid model: I. Characterization of the allograft response in sensitized mice

Intraperitoneal implants of multicellular spheroids of EMT6 mammary sarcoma cells into specifically sensitized allogeneic mice have been used as a model to study in situ immunity to solid tumor allografts. Morphological examination of the spheroids recovered from sensitized mice revealed massive infiltration by host cells. There was an early but apparently nonspecific infiltration by large numbers of polymorphonuclear leukocytes followed by the infiltration of large numbers of macrophages and smaller numbers of lymphocytes. The later infiltration correlated with tumor cell killing as determined by a cloning assay for surviving EMT6 cells, and with the presence of cells with cytotoxic activity in in vitro assays. The kinetics of tumor cell killing indicated that tumor cell survival decreased within a few hours of spheroid implantation and less than 1% of the tumor cells survived after 48 h. Purification of the infiltrating cells by nylon wool fractionation and by treatment with anti-T-cell serum and complement indicated that the primary cytotoxic cell is a T lymphocyte.