Importance of antitumor immunity for complete cure of highly drug-sensitive leukemia in mice.

Seven transplantable leukemia lines were established from spontaneous leukemias and screened for 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-1-(chloroethyl)-1-nitrosourea hydrochloride (ACNU) sensitivity in DDD mice. Three of them were classified as highly sensitive, two as sensitive and two as resistant to ACNU. A highly sensitive line, DL812, was extensively characterized from a therapeutic point of view. DL812 cells were so invasive as to produce enlargement of spleens and lymph nodes but not local tumors when injected s.c., markedly sensitive to in vitro ACNU exposure and moderately immunogenic. The invasion process of DL812 cells differed with the status of host immunity. Advanced DL812 leukemias were macroscopically completely cured with normalization of spleen and lymph node sizes 3-7 days after an i.p. injection of 0.5 mg ACNU, but more ACNU-resistant leukemias with splenomegaly and enlarged lymph nodes recurred thereafter. Recurring DL812 cells were approximately four times more resistant to in vitro ACNU exposure but maintained similar immunogenicity as compared to the original ones. Permanent cures of advanced leukemias were achieved by ACNU treatment plus subsequent adoptive transfer of immune splenocytes in 15% of diseased mice. The results suggest the importance of host antitumor immunity for permanent cures of highly drug-sensitive leukemias, overcoming drug resistance and relapse.