Characterization of RFM mouse T lymphocyte anti-oncofetal antigen immunity in apparent tumor-free, long-term survivors of sublethal X-irradiation by limiting dilution T lymphocyte cloning.

Fractionated sublethal x-irradiation induces thymic lymphomas in up to 60% of RFM mice by 6 mo after irradiation, but no thymomas arise after 6 mo. All radiation-induced tumors expressed oncofetal Ag (OFA) and thymic OFA expression significantly preceded detectable thymomas. To determine whether lymphoma-free, radiation survivor mice had anti-OFA T cell immunity, we analyzed their clonable 5T lymphoma-reactive T lymphocytes and determined the resistance of such mice to challenge with RFM lymphoma cells. RFM mice that were irradiated 6 to 6.5 mo earlier, but which had no apparent tumors, had no more resistance to OFA+ 5T lymphoma cell challenge than nonirradiated age-matched mice. These mice actually developed maximal tumor incidence significantly faster. Analyses of 5T lymphoma-reactive T cell clones established from RFM mice 11 mo after irradiation, however, suggest that activation of anti-OFA immunity occurred subsequent to irradiation. Previously irradiated RFM mice yielded 257% more tumor-reactive T cell clones/mouse than non-irradiated controls. All clones from both sets of mice specifically proliferated to purified OFA. Each irradiated mouse yielded some clones that proliferated to fivefold lower doses of OFA than did any T cell clones from nonirradiated mice. Only these low-dose OFA-reactive clones responded equally to irradiated 4T and 5T cells by proliferation, IFN-gamma secretion, and target lymphoma cell killing. 4T cells express less OFA/cell than 5T cells. Some noncytotoxic CD8 T cells that inhibited cytotoxic T cell function were cloned only from irradiated RFM mice.