Live Cell Imaging Captures Immune Cell-Mediated Killing of Precursor-B Acute Lymphoblastic Leukemia Cells Targeted with Anti-CD19 (Medi-551) Antibodies: Support for Macrophage and NK Cell In Vivo Activity

Abstract 1522

Precursor-B acute lymphoblastic leukemia (pre-B ALL) is the most common malignancy in children and can be cured in a majority of patients. However, cure remains elusive in approximately 20% of patients for reasons that are not well understood. Importantly, survivors commonly develop morbidities that result from dose-intensified treatment with cytotoxic drugs. Here, we investigate the tumoricidal effects of a novel humanized anti-CD19 monoclonal antibody (Medi-551). The a-fucosylated form of this antibody has increased affinity to human FcgammaRIII (CD16) receptor, present on the surface of NK cells and macrophages, mediating antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP). Medi-551/CD19 complexes internalize slowly and thus remain accessible for effector cells for prolonged periods. We evaluated in vitro ADCC and ADCP activities of primary human NK cells and macrophages (effector cells) against four pre-B ALL cell lines (697, Nalm 6, MHH-Call 3, RS 4;11), as well as freshly isolated patient blasts. We report results of live cell fluorescent imaging studies, characterizing the formation of immunological synapses between Medi 551-bound target leukemia cells and effector cells, as well as the kinetics of both NK-mediated killing and macrophage phagocytosis. The number of the CD19 receptors present on the cell surface is shown to be a factor in effector-mediated killing of Medi-551 targeted leukemia cells. Further, genetic polymorphisms in FcgammaRIII (158 F/V, V/V or F/F) affected in vitro ADCC and ADCP activities with FcgammaRIII 158 V homo- or heterozygotes showing the strongest activity. We also evaluated the efficacy of Medi-551 in a human pre-B ALL murine xenograft model. SCID mice were engrafted with 697 pre-B ALL cells and received either vehicle alone or Medi-551 (3 mg/kg; twice weekly for a total of 5 doses); treatment was started at day 5 after engraftment. Medi-551 treatment markedly lowered disease burden in blood, liver and bone marrow. The lack of cure is consistent with impaired roles for NK cells in this model, since murine NK cells lack FcgammaRIV. Experiments are in progress to improve the model through adoptive transfer of human NK cells. Taken together, the in vitro and in vivo data show that Medi-551 has strong activity against pre-B ALL and support a move forward to early phase trials in this disease.