Host responses within solid tumors: Non‐thymus‐derived specific cytotoxic cells within a murine mammary adenocarcinoma

The nature of host‐derived, specifically cytotoxic cells infiltrating solid murine mammary adenocarcinomas (line T1699) was investigated. Cell suspensions obtained from enzymatically dispersed tumors were separated by sedimentation velocity. The host cell fraction was heterogeneous and contained T‐lymphocytes, non‐phagocytic cells bearing Fc receptors, eosinophils and monocytes. Host cells equivalent in size to small lymphocytes and bearing Fc receptors were found to be the predominant cell type responsible for colony inhibition. These colony‐inhibiting cells were insensitive to lysis with either anti‐θ or anti‐IgG serum and complement; and they were specific both in their induction and in their mediation of cytotoxicity. Host cells made up the predominant population in spontaneously regressive tumors but only a minor component in progressive tumors, and they were totally absent from progressive tumors in X‐irradiated animals.