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A clonal line of highly oncogenic spontaneously transformed mouse cells (104C) was transformed in tissue culture by simian virus 40 (SV40) and subsequently recloned (106CSC). This 106CSC cell line expressed T antigen and transplantation antigen but was about 100 times less tumorigenic than the 104C parent. When 10(5) 106CSC cells were injected into immunocompetent syngeneic mice, tumors were produced. From such tumors, cell lines were established in culture, all of which were consistently negative for T antigen. We found previously by solution DNA hybridization methods that the tumor cells were depleted in the early region of SV40 DNA which codes for the T antigen. We postulated that this loss occurs through a DNA rearrangement of unknown mechanism in one or a few 106CSC cells and that the tumors are then produced from such a cell or cells, whereas all the T-antigen-positive 106CSC cells are rejected by immunologic means. In this investigation we showed by the DNA transfer method using appropriately selected SV40 DNA probes that indeed the tumor cell clone (130CSCT) we selected to investigate came from one 106CSC cell in which the T-antigen-coding SV40 DNA sequences (but not all the early SV40 DNA sequences) were lost by an excision and recombination mechanism. We also showed that the 130CSCT cells, which are highly tumorigenic, could again be transformed by SV40 and that the resulting T-antigen-positive cloned derivative cells became much less tumorigenic (approximately 10(5)-fold), apparently again because of immunologic recognition and rejection. Indeed, when 10(7) T-antigen-positive cloned cells were injected, all the T-antigen-positive cells were rejected and the tumor was produced again from one or more T-antigen-negative cells. Thus, a one-step in vivo transplantation experiment allowed a selection (for tumorigenicity and against the SV40 T antigen) of a mutant mammalian cell with a DNA deletion at a definable site.

Immunologic selection of simian virus 40 (SV40) T-antigen-negative tumor cells which arise by excision of early SV40 DNA