An examination of tumor antigen loss in spontaneous metastases

Metastases arising from a subcutaneous injection of the DBA/2 tumor, MDAY-D2, as well as four drug-resistant variants (either wheat germ agglutinin-resistant, ouabain-resistant, or both, i.e., WGAR/OuaR) of MDAY-D2, were examined for the presence of a tumor-associated antigen (TAA). Of 15 mice examined, tumor antigen-loss variants were detected in only 1 animal. These antigen-loss metastases arose in a mouse injected with the WGAR variant called MDW4. The tumor at the site of inoculation retained the TAA, whereas all four of the metastases removed from liver, spleen and other tissues were antigen-loss variants. The antigen-loss variants were not killed by cytotoxic T cells (CTL) directed against the TAA of the parental tumor, did not competitively inhibit CTL lysis of MDW4 targets in a 'cold target' inhibition test, and were not able to elicit a CTL response. In vivo immunization-protection (challenge) experiments also showed that the metastases did not express the TAA of MDAY-D2. Unlike the WGAR phenotypes, which were lost in all spontaneous metastases recovered from MDW4-injected mice, loss of the TAA appeared to be an uncommon event. Antigen-loss tumor cell variants are discussed in terms of their relevance to metastasis, and in regard to their use in the study of T cell-mediated cytotoxicity of tumor cell populations.