Immunogenic capacity of tum--variants isolated from a rat rhabdomyosarcoma

An increasing number of reports highlight the fact that tumour cells are able to give rise in vitro to immunogenic variants, which are defined in vivo as being non tumorigenic, tum-. We have observed the emergence of immunogenic variants, derived from a primary nickel-induced rat rhabdomyosarcoma established in culture (RMS 9-4/0), resistant to treatment with the chloronitrosourea, chlorozotocin (CZT) (R-lines). They were separated from the whole population of cells by a cloning procedure. Furthermore, we demonstrate that the cloning procedure by itself allows the isolation of tum- variant designated as C-lines. In both cases, the tum- phenotype was observed after s.c. injection of cells into syngeneic rats with a broad range of R9 or C8 cells (10(4) to 10(7). This characteristic was inherited in a stable manner. Athymic mice developed tumours of rat rhabdomyosarcoma origin when grafted with 10(5) cells. Immunization of rats with one R variant (R9) tum- protected the rats grafted with the parental RMS 9-4/0 cells against metastatic invasion of the lungs, but not against local tumour growth, and rats grafted with a CZT-resistant tum+ cell variant S4T (in vivo-derived) against its hepatic and pulmonary metastases, while the local tumour progressed as usual. Immunization of rats with one C variant (C8) tum- cells did not protect them against either metastases or local growth of the implanted tumours. Both R and C lines cells became progressively resistant to NK- and macrophage-induced cytotoxicity. Splenic lymphocyte transfer from immune rats into nude mice, i.e., the Winn test, showed a complete degree of protection against C8 or R9 tumour growth. We conclude that two different antigenicities were revealed, one common to R9 and C8 cells in relation with their selection procedure by repeated cloning. Another antigenicity appeared in the R9 line, selected by CZT-resistance. The anti R9 cell immunization against CZT-resistant tum+ S4T could argue in favour of CZT action in the acquisition of R9 cell antigenicity. More likely, an amplification of antigens rather than induction of a new antigen could explain the protection of anti R9 immunized rats against parental tumour metastases.