Cells from a secondary tumor developing at the site of a regressed Moloney sarcoma virus-induced tumor could be passaged in adult STU mice by intramuscular and intraperitoneal inoculation. The tumors induced by these cells, as well as by a cell line derived from it, grew progressively and led to death of the animals between 3 and 7 wk after tumor transplantation. No evidence for production of virus from these cells was obtained or for the presence of viral antigens (p30, gp69/71). From both cell variants, sarcoma virus genome could be rescued by infection with helper virus, resulting in the establishment of a cell line producing focus- and XC plaque-forming virus. The rescued producer cells very frequently also produced tumors which finally grew progressively. The nonproducer cells were not immunogenic, as was demonstrated in cross transplantation tests and in studies for cell-mediated cytotoxicity (CMC) and complement-dependent antibody-mediated cytotoxicity (AMC). The producer cells, however, were demonstrated to possess a strong immunogenicity. The nonproducer cells, though nonimmunogenic, revealed a weak immunosensitivity when used for challenge in the transplantation protection assay or as target cell for the demonstration of AMC and CMC, if the immune response was induced by cells producing the sarcoma-helper virus complex, but not by cells producing only helper virus. The nonproducer cells, as well as their rescued producer derivative, showed a stronger reactivity with cytotoxic antibodies than with cytotoxic cells, whereas the helper virus-producing cell line was comparably suitable as target cell for AMC and CMC. The recurrence of a regressed Moloney sarcoma is assumed to be the result of the occurrence of transformed nonproducer cells escaping immune destruction, and not as a consequence of a depleted immune resistance in the host.

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