Anti-thymocyte globulin's activity against acute myeloid leukemia stem cells

Rabbit anti-thymocyte globulin (ATG (Thymoglobulin)) kills T cells in vitro and probably also in vivo as it prevents graft-vs-host disease (GvHD) in patients. Recently we demonstrated that ATG at a clinically relevant concentration (10-50 mg/L) kills in vitro not only T cells but also leukemic blasts. In the present study, we investigated whether ATG kills not only leukemic blasts but also leukemic stem cells (LSCs). We used a flow cytometric assay of complement-mediated cytotoxicity (CDC). ATG-induced death of acute myeloid leukemia (AML) cells from patients newly diagnosed with AML was measured among blasts as well as LSCs. At 10 mg/L ATG, blasts but not LSCs were killed. At 50 mg/L ATG, both blasts and LSCs were killed. We also measured ATG-mediated killing of healthy individuals' hematopoietic stem cells (HSCs). Median 2% HSCs from blood and 15% HSCs from filgrastim-mobilized grafts were killed with 50 mg/L ATG, compared to 30% LSCs from the blood of AML patients (p = 0.001 and 0.022, respectively). In conclusion, LSCs are sensitive to ATG, however, only at a relatively high ATG concentration. At that concentration, LSCs are killed to a higher degree than HSCs.