Cytotoxicity of anti-CD64-ricin A chain immunotoxin against human acute myeloid leukemia cells in vitro and in SCID mice

Blast cells from patients with acute myeloid leukemia (AML) commonly expres s CD64, the high-affinity receptor for immunoglobulin G (Fc gamma RI). An i mmunotoxin (MDX-44) was constructed by coupling humanized anti-CD64 monoclo nal antibody (mAb) H22 via a bivalent linker to deglycosylated ricin A-chai n (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 [H-3] thymidine incorporation. In the presence of interferon-gamma (IFN- gamma), MDX-44 significantly inhibited the proliferation of CD64(+) HL-60, NB4, and U937 cells in 72-h cultures in a dose-dependent manner. The mechan ism of action appeared to be the induction of apoptosis, as measured by pro pidium iodide staining and flow cytometry analysis. However, CD64(-) KG-1a and Daudi cells were not affected by MDX44/IFN-gamma. Incubating HL-60 cell s with MDX-44/IFN-gamma 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 in hibited by MDX-44/IFN-gamma, and the inhibition was correlated with CD64 ex pression on these cells (r = 0.65). In a human AML model in NOD/SCID mice, MDX-44/IFN-gamma inhibited 95-98% of peritoneal exudate AML cell proliferat ion and 85-90% of solid leukemia masses. The effect of MDX-44 on AML cells was dependent on activation of cells by IFN-gamma. MDX-44/IFN-gamma may hav e value in the therapy of AML cells expressing cell-surface CD64.