Cytotoxicity of Anti-CD64-Ricin A Chain Immunotoxin Against Human Acute Myeloid Leukemia Cells In Vitro and in SCID Mice

Abstract

Blast cells from patients with acute myeloid leukemia (AML) commonly express CD64, the high-affinity receptor for immunoglobulin G (FcγRI). An immunotoxin (MDX-44) was constructed by coupling humanized anti-CD64 monoclonal antibody (mAb) H22 via a bivalent linker to deglycosylated ricin A-chain (RA). Human leukemia cell lines were incubated with MDX-44 or H22/free RA. The effect of MDX-44 on the proliferation of leukemia cells was assessed by [³H]thymidine incorporation. In the presence of interferon-γ (IFN-γ), MDX-44 significantly inhibited the proliferation of CD64⁺ HL-60, NB4, and U937 cells in 72-h cultures in a dose-dependent manner. The mechanism of action appeared to be the induction of apoptosis, as measured by propidium iodide staining and flow cytometry analysis. However, CD64^- KG-1a and Daudi cells were not affected by MDX-44/IFN-γ. Incubating HL-60 cells with MDX-44/IFN-γ resulted in a 99% decrease in colony-forming units, whereas colony-forming cells in normal bone marrow were not significantly suppressed by such treatment. Cells from 60% of AML patients (6/10) were inhibited by MDX-44/IFN-γ, and the inhibition was correlated with CD64 expression on these cells (r = 0.65). In a human AML model in NOD/SCID mice, MDX-44/IFN-γ inhibited 95-98% of peritoneal exudate AML cell proliferation and 85-90% of solid leukemia masses. The effect of MDX-44 on AML cells was dependent on activation of cells by IFN-γ. MDX-44/IFN-γ may have value in the therapy of AML cells expressing cell-surface CD64.