Cancer stem cells represent the most important target cells for anti-tumor therapy as they are uniquely capable to maintain tumor growth and to induce relapse. For sustained anti-tumor activity, anti-cancer therapy has to address and eliminate cancer stem cells. Unfortunately in acute lymphoblastic leukemia (ALL), cancer stem cells / leukemia stem cells (LSCs) can not be reliably defined by cell surface markers complicating studies on cancer stem cells. To overcome this obstacle, we used the functional definition of CSCs as surrogate and studied leukemia-initiating cells (LICs) by their name-defining capability to initiate leukemia in immune-compromised mice.
TRAIL (TNF-related apoptosis-inducing ligand) is a member of the TNF family which induces apoptosis in a wide variety of tumor cells while sparing normal cells. TRAIL represents an important candidate anti-cancer agent which is currently tested in phase I and II clinical trials, yet its ability to target cancer stem cells is currently unknown.
Here we tested the effect of TRAIL on LSCs and LICs using tumor cells from children with precursor B-cell acute lymphoblastic leukemia (pre-B ALL). Primary ALL cells were engrafted and passaged in NSG mice. Cells were freshly isolated from NSG mice and stimulated with TRAIL in vitro for 2 days and afterwards re-engrafted into mice in dilution curves upon limiting dilution transplantation assay comparing non-treated with TRAIL-treated cells and using about 25 mice per stimulation. After 12–16 weeks, mice were evaluated for leukemic engraftment by staining for leukemia cells in organs like bone marrow, spleen, liver and blood using FACscan analysis and immune histochemistry.
In all 3 pre-B ALL samples tested, in vitro treatment with TRAIL prior to transplantation of cells into mice significantly reduced their engraftment capability. TRAIL disabled leukemic engraftment by > 95 % in all 3 samples. Accordingly, in vitro TRAIL treatment significantly increased the time to engraftment and completely disabled engraftment below a defined threshold.
In a second, additive approach, TRAIL was used in a preclinical mouse model. Pre-B ALL samples were engrafted in NSG mice and leukemia-bearing mice were treated with TRAIL systemically at 7.5 mg / kg daily i.p. for 10 days. In this preclinical in vivo model, TRAIL treatment completely cured a proportion of animals harbouring patient-derived pre-B ALL xenografts.
Taken together, TRAIL significantly disabled the leukemia-initiating function of LICs from patient-derived pre-B ALL xenografts in vitro and that TRAIL eliminated leukemia together with its LSCs in vivo. Although the methods used do not allow the study of signalling mechanisms, TRAIL might most probably have induced apoptosis in LICs and LSCs. Our data show that it is feasible, although technically demanding, to test the apoptosis sensitivity of LICs and LSCs.
We conclude from these data that TRAIL constitutes an attractive future drug for treatment of ALL.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.