An Fc-engineered CD19 Antibody Eradicates MRD in Patient-Derived MLL-rearranged Acute Lymphoblastic Leukemia Xenografts

Antibody therapy constitutes a major advance in the treatment of B-cell precursor acute lymphoblastic leukemia (BCP-ALL). To evaluate the efficacy and the mechanisms of action of CD19 monoclonal antibody therapy in pediatric BCP-ALL, we tested an Fc-engineered CD19 antibody carrying the S239D/I332E mutation for improved effector cell recruitment (CD19-DE). Patient-derived xenografts (PDX) of pediatric mixed-lineage leukemia gene (MLL)-rearranged ALL were established in NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ (NSG) mice. Antibody CD19-DE was efficient in prolonging the survival of NSG mice in a minimal residual disease (MRD) model. The majority of surviving mice remained polymerase chain reaction (PCR)-MRD negative after treatment. When antibody therapy was initiated in overt leukemia, antibody CD19-DE was still efficient in prolonging survival of xenografted mice in comparison with nontreated control animals, but the effects were less pronounced than in the MRD setting. Importantly, the combination of antibody CD19-DE and cytoreduction by chemotherapy (dexamethasone, vincristine, PEG-asparaginase) resulted in significantly improved survival rates in xenograftedmice. Antibody CD19-DE treatment was also efficient in a randomized phase 2-like PDX trial using 13 MLL-rearranged BCP-ALL samples. Macrophage depletion by liposomal clodronate resulted in a reversal of the beneficial effects of CD19-DE, suggesting an important role for macrophages as effector cells. In support of this finding, CD19-DE was found to enhance phagocytosis of patient-derived ALL blasts by human macrophages in vitro. Thus, Fc-engineered CD19 antibodies may represent a promising treatment option for infants and children with MLL-rearranged BCP-ALL who have a poor outcome when treated with chemotherapy only.