Immunity to Murine Sarcoma Virus Induced Tumors. III. Analysis of the Cell Populations Involved in Protection from Lethal Tumour Progression of Sublethally Irradiated, MSV Inoculated, Mice

A comparison was made between the cells responsible for demonstrable activity against MSV antigens, using both in vivo and in vitro assays. Similar cells (in terms of size and sensitivity to anti-theta serum) were detected in both assays. However, while lymphoid cells from animals at all stages post-MSV infection were active in protecting irradiated mice from the lethal effect of induction of MSV sarcomata, cells from animals at early stages post-MSV infection (when the tumour was in a progressive phase of growth) were not active in the in vitro assay. By manipulation of the in vivo assay conditions a situation was observed in which cells from “progressor animals” were able to suppress both the in vitro and in vivo activity of regressor lymphoid cells. The potential physiological role of this cell type is disussed.