Immunogenic (tum−) variants obtained by mutagenesis of mouse mastocytoma P815

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Abstract
Mutagen treatment of mouse mastocytoma P815 produces highly inununogenic “tum−” variants. Most of these variants express potent transplantation antigens which are not present on the original P815 tumor cells. These tum− antigens, which appear to be specific for each variant, elicit a strong cytolytic T lymphocyte (CTL) response, but do not seem to induce a specific antibody response. As a first step in the isolation of the gene of a tum− antigen, we attempted DNA-mediated gene transfer. As a DNA recipient cell we used P1.HTR, a highly transfectable P815 cell line, whose selection has been previously described. For the detection of antigen-expressing cells in transfected populations we developed a procedure that relies on the ability of these cells to stimulate the proliferation of the relevant CTL. Using DNA from tum− variant P91 mixed with a plasmid carrying an antibiotic resistance gene, we obtained several independent transfectants expressing a tum− antigen, at a frequency of approximately 1 in 13 000 antibiotic-resistant transfectants. These transfectants express only one of the two tum− antigens that were identified on P91, suggesting that these tum− antigens correspond to different genes. We expect that the detection procedure described here will be-suitable for the identification of transfectants for any gene that determines the expression of an antigen recognized by CTL.