Specificity of antileukemia sera prepared by immunization with leukemia cells admixed with normal antigen-blocking sera.

Two types of antisera were tested, namely, conventional antileukemia serum (ALKS), prepared by immunization of rabbits with mouse leukemia cells, and antileukemia normal antigen-blocked serum (ALK-NABS), prepared by immuniza tion of rabbits with a mixture of mouse leukemia cells plus normal antigen-blocking serum (NABS). The latter was raised in rabbits by immunization with normal mouse lymphoid cells. Two types of mouse leukemias were used: Gardner leukemia transplanted in C3HeB/FeJ mice and L1210-MTX murine leukemia cells transplanted in DBA/2J mice. Compared with ALKS reagents, ALK-NABS reagents had cytolytic potencies against leukemia cells that were depressed 3or 4-fold. Long-continued immunization was necessary to elicit increased specificity of ALK-NABS for immune cytolysis of leukemia cells relative to normal splenic lymphocytes. If only small amounts of NABS were used to prepare ALKNABS, no increase in specificity was obtained. Small amounts of allogeneic NABS prepared against H-2 and 0 incompati bilities were also ineffective. Absorption of ALKS or ALK-NABS with normal mouse lymphoid cells increased their specificity against leukemia cells. However, this increase was greater for ALK-NABS, even after compensation for its lower initial cytolytic potency. Thus, ALK-NABS reagents appear to contain a higher proportion of antibodies that react with leukemia cells. In therapy experiments, ALK-NABS were less toxic, required less normal mouse tissue for absorption, and gave slightly better results than ALKS.