Sequential studies of serum blocking activity in rats bearing chemically induced primary bowel tumors.

Colon and small-bowel tumors were induced in WF rats by treatment with one of three chemical carcinogens. Tumor occurrence and growth were monitored by multiple double-contrast examinations. Roentgenologic diagnoses and histologic examination of tumors were verified at tumor resection or autopsy. Serial serum samples from each of 11 rats were tested in microcytotoxicity assays for their ability to block the cytotoxicity of lymph node cells from rats with isografts of colon carcinoma NG-W1 against NG-W1 target cells. Sera from all tumor-bearing rats demonstrated specific blocking activity. With two exceptions, serum blocking activity preceded roentgenologic evidence of tumor. Sera of the 2 exceptional rats lacked blocking 2 and 8 weeks before tumor detection, respectively. The sera of only one animal specifically inhibited lymphocyte cytotoxicity despite consistently negative double-contrast examinations. At autopsy this rat was found to have an adenocarcinoma of the distal rectum, impossible to visualize by the roentgenologic technique used. In serum samples obtained from animals after successful tumor excision and with no radiologic evidence of recurrence, blocking activity could no longer be demonstrated. Rats that had received carcinogen but developed no tumor and had no abnormalities on double-contrast examination demonstrated no serum blocking activity.