| Abstract
| - The immune surveillance hypothesis suggests that cancer evolves as a multistage process. Further, it predicts that cells intermediate on the pathway to cancer are susceptible to host protective mechanisms, and only those variants that are able to escape the protective mechanisms are able to grow as tumors. We have isolated, as lineages, fibroblast lines that express phenotypes predicted by the surveillance hypothesis. The lineages were derived by treating nontransformed cells (N-cells) with chemical carcinogens and by isolating transformed variants in vitro. From the transformants that are tumorigenic in immune-depressed ATXFL mice but rejected by normal mice (I-cells), variants were selected in vivo that had escaped the rejection mechanism(s) and had grown as tumors in normal mice (C-cells). Thus lineages were established comprised of sequentially derived cell lines with the following phenotypes: nontransformed, transformed but susceptible to host protective mechanisms, transformed and resistant to host protective mechanisms (i.e., N→I→C). With the use of in vivo cross-protection experiments, two independently derived I-cell lines were shown to express non-cross-reactive antigens that are not expressed by the parental nontransformed N-cells (i.e., transformation-associated antigens). The transformation-associated antigens are expressed at an equivalent level on the cells that are susceptible to rejection (i.e., I-lines) and those that have escaped rejection (i.e., C-lines). In addition, although the transformation-associated antigens expressed by I-cells induce an effective immune response capable of rejecting both the I-line and C-line, the expression of these antigens on C-cells does not induce an effective immune response. The role of host defense mechanisms in the rejection of these chemically transformed I-cells and the possible mechanisms by which C-cells escape rejection are discussed.
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