A modified spleen colony assay was employed to evaluate the effect of various tumor-related factors upon the in vivo effectiveness of anti-fetal immunity. Reduction of fetal liver (hemopoietic) colony formation by in vitro incubation of fetal liver cells with lymph node cells sensitized to syngeneic fetal liver or plasma cell tumor was blocked by (1) solubilized fetal antigen, (2) serum from mice recently immunized with syngeneic fetal liver and (3) serum from patients with metastatic colon cancer. In the latter, the degree of blocking correlated with plasma CEA levels. Both humoral and cell-mediated mechanisms of anti-fetal immunity were blocked in plasma cell tumor-bearing mice, suggesting in vivo suppression of the immune response via circulating tumor-associated fetal antigen.
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