Specificity of antigens on UV radiation-induced antigenic tumor cell variants measured in vitro and in vivo

In vitro exposure of nonimmunogenic murine tumor cells to UV radiation (UVR) generates highly antigenic variants that are immunologically rejected by normal, syngeneic mice. The purpose of this study was to determine whether these antigenic variants cross-react immunologically with the parental tumor and whether the UVR-associated antigen unique to UVR-induced tumors is also present on the variants. Antigenic (regressor) variants and nonimmunogenic (progressor) clones derived from UV-irradiated cultures of the C3H K1735 melanoma and SF19 spontaneous fibrosarcoma cell lines were used to address these questions. In an in vivo immunization and challenge assay, the antigenic variants did not induce cross-protection among themselves, but each induced immunity against the immunizing variant, the parent tumor cells, and nonimmunogenic clones derived from UV-irradiated parent cultures. Therefore, the variants can be used to induce in mice a protective immunity that prevents the growth of the parent tumor and nonimmunogenic clones, but not other antigenic variants. In contrast, immunization with cells of the parental tumor or the nonimmunogenic clones induced no protective immunity against challenge with any of the cell lines. Utilizing the K1735 melanoma-derived cell lines in vitro, T-helper (Th) cells isolated from tumor-immunized mice were tested for cross-reactivity by their ability to collaborate with trinitrophenyl-primed B-cells in the presence of trinitrophenyl-conjugated tumor cells. Also, the cross-reactivity of cytotoxic T-lymphocytes from tumor-immunized mice was assessed by a 4-h 51Cr-release assay. Antigenic variants induced cytotoxic T-lymphocytes and Th activity that was higher than that induced by the parent tumor and nonimmunogenic clones from the UVR-exposed parent tumor and cross-reacted with the parental tumor cells and nonimmunogenic clones, but not with other antigenic variants. Furthermore, upon transplantation, the UVR-induced antigenic variants grew in UV-irradiated and immunosuppressed mice, but not in untreated mice indicating that the variants expressed the determinant recognized by suppressor T-cells present in UV-irradiated mice. These results demonstrate that highly antigenic cells generated by the in vitro exposure of two different murine tumors to UV radiation express a determinant shared with the parental tumor cells and nonimmunogenic clones, a unique variant-specific determinant and the suppressor cell-defined determinant present on UVR-induced tumors. Based on these results, two models are proposed to explain the make-up of the antigenic determinants present on the UVR-induced antigenic variants.