We have shown that both regressor and progressor clones can be isolated from a UV regressor tumor, RD-1024. Although the daughter clones are characterized by differences in tumorigenic potential in normal transplant hosts, they nevertheless seem to express the same major tumor rejection antigens, because immunization with either the regressor parent tumor, RD-1024, or with regressor Cl 8 protects against subsequent challenge with progressor C1 4 or Cl 9. Consistent with the in vivo-generated data is the evidence that draining lymph node cells with functional specificity for regressor Cl 8 are capable of cross-reactive cytotoxicity in an in vitro chromium release assay.

We have demonstrated an indirect interaction occurring in vivo between regressor and progressor cells, in that Cl 8 cells have the ability to influence the outcome of simultaneous or sequential challenge with Cl 4 or Cl 9 cells. Because 500 rad of gamma irradiation has been shown to compromise the ability of mice to respond to a primary challenge with tumor, an immunological mechanism is implicated in the ultimate rejection of progressor tumor in a doubly challenged host.

The importance of these results lies in the knowledge that these interacting subpopulations have been isolated directly from a tumor growing in vivo and that no selection pressure has been exerted on the cells greater than the short in vitro culture period necessary for the isolation and expansion of individual clones. The apparent immunoregulatory potential in a tumor-bearing animal is thus seen to be modified in accordance with the phenotypic heterogeneity of the cells within that tumor.

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