In vivo efficacy of B43 (anti-CD19)-pokeweed antiviral protein immunotoxin against BCL-1 murine B-cell leukemia.

We show that a highly aggressive subclone of murine BCL-1 B-lineage leukemia expresses a single 2.4-kb transcript hybridizing to the human CD19 cDNA probe and reacts strongly with the anti-human CD19 monoclonal antibodies (MoAb) B43, B4, Leu-12, and J3-119. In contrast to their strong reactivity with anti-human CD19 MoAb, BCL-1 cells show no reactivity with MoAb directed against human CD22, CD72, HLA-DR, IgD, or IgM. Western blot analysis of BCL-1 whole cell lysates with the anti-human CD19 MoAb J3-119 showed a single 69-Kd protein band, which was not detected by the negative control MoAb G19.4 (anti-CD3). In contrast to BCL-1 cells, normal BALB/c splenocytes or mouse splenocyte/myeloma hybridoma cell lines did not (1) express any transcripts that hybridized to the human CD19 cDNA probe, (2) react with B43/anti-CD19 MoAb, or (3) express the 69-Kd protein that reacts with the anti-human CD19 MoAb J3-119. Murine BCL-1 B-cell leukemia thus provides a unique model of disseminated B-lineage leukemia to evaluate the antileukemic efficacy of anti-CD19 immunotoxins. This model was subsequently used to evaluate the in vivo homing ability, pharmacokinetics, and antileukemic efficacy of B43 MoAb conjugated to the plant hemitoxin pokeweed antiviral protein (PAP). B43-PAP immunotoxin (1) showed strong and antigen-specific reactivity with BCL-1 cells, (2) promptly penetrated the spleens of leukemic mice, (3) rapidly reduced the BCL-1 leukemia burden of leukemic mice and, most importantly, (4) improved survival. Finally, B43-PAP immunotoxin was more effective against BCL-1 leukemia than 700 cGy (LD100/30) total body irradiation (TBI) followed by syngeneic bone marrow transplantation (BMT).