Survival and characteristics of murine leukaemic and normal stem cells after hyperthermia: a murine model for human bone marrow purging

Heat shock proteins and Bcl-2 expression and function in relation to the differential hyperthermic sensitivity between leukemic and normal hematopoietic cells

A major problem in autologous stem cell transplantation is the occurrence of relapse by residual neoplastic cells from the graft. The selective toxicity of hyperthermia toward malignant hematopoietic progenitors compared with normal bone marrow cells has been utilized in purging protocols. The underlying mechanism for this selective toxicity has remained unclear. By using normal and leukemic cell line models, we searched for molecular mechanisms underlying this selective toxicity. We found that the differential heat sensitivity could not be explained by differences in the expression or inducibility of Hsp and also not by the overall chaperone capacity of the cells. Despite an apparent similarity in initial heat-induced damage, the leukemic cells underwent heat-induced apoptosis more readily than normal hematopoietic cells. The differences in apoptosis initiation were found at or upstream of cytochrome c release from the mitochondria. Sensitivity to staurosporine-induced apoptosis was similar in all cell lines tested, indicating that the apoptotic pathways were equally functional. The higher sensitivity to heat-induced apoptosis correlated with the level of Bcl-2 protein expression. Moreover, stable overexpression of Bcl-2 protected the most heat sensitive leukemic cells against heat-induced apoptosis. Our data indicate that leukemic cells have a specifically lower threshold for heat damage to initiate and execute apoptosis, which is due to an imbalance in the expression of the Bcl-2 family proteins in favor of the proapoptotic family members.

Differences in heat sensitivity between normal and acute myeloid leukemic stem cells: feasibility of hyperthermic purging of leukemic cells from autologous stem cell grafts

OBJECTIVES

In autologous stem cell transplantation contamination of the graft with malignant cells is frequently noticed and necessitates the use of in vivo or in vitro purging modalities. The hematopoietic recovery after transplantation depends on the number of stem and progenitor cells in the transplant. Therefore, in the present study the effects of hyperthermic treatment on the human normal and acute myeloid leukemic (AML) stem cell compartment were investigated.

METHODS

Normal bone marrow and AML blasts were heat treated up to 120 minutes at 43 degrees C. The surviving fractions of the different stem cell subsets were determined using in vitro methylcellulose and cobblestone area-forming cell (CAFC) clonogenic assays, as well as the in vivo NOD/SCID repopulating assay. The leukemic nature of the colonies from AML cells was confirmed by RT-PCR analysis. In order to increase the therapeutic index of the hyperthermic purging modality, the heat treatment was preceded by a 3-hour incubation at 37 degrees C with the ether lipid ET-18-OCH(3) (25 microg/mL).

RESULTS

It could be demonstrated that normal progenitor cells are far more resistant to hyperthermia than leukemic progenitor cells (56%+/-7% vs 9.9%+/-2.6% survival after 60 minutes at 43 degrees C, respectively). Furthermore, normal hematopoietic stem cells appear to be extremely resistant to the heat treatment (94%+/-9% survival after 60 minutes at 43 degrees C). In contrast, in the leukemic stem cell compartment no significant differences in heat sensitivity between the stem cells and progenitor subsets could be observed (12.3%+/-2.9% vs 9.9%+/-2.6% survival after 60 minutes at 43 degrees C, respectively). The combined treatment resulted in a survival for normal progenitor and stem cells of 32%+/-6% and 85%+/-15% after 60 minutes at 43 degrees C, respectively. Under these conditions the number of leukemic stem cells was reduced to 1%+/-0.3%. After 120 minutes at 43 degrees C, no AML-colonies could be detected anymore.

CONCLUSIONS

Our data demonstrate that leukemic stem cells have an increased hyperthermic sensitivity compared to their normal counterparts and that this difference can be further increased in combination with ET-18-OCH(3). These striking differences in heat sensitivity warrant the use of hyperthermia as a clinically applicable purging modality in autologous stem cell transplantation.

Purging of acute myeloid leukaemia cells from stem cell grafts by hyperthermia: enhancement of the therapeutic index by the tetrapeptide AcSDKP and the alkyl-lysophospholipid ET-18-OCH(3)

Hyperthermia has been shown to be a potential purging modality in autologous stem cell transplantation settings owing to its selective toxicity towards leukaemic cells. We describe two approaches to further increase the therapeutic index of the hyperthermic purging modality by using normal murine bone marrow cells and a murine model for acute myeloid leukaemia. First, the tetrapeptide AcSDKP was used to protect the normal haematopoietic progenitor cells against hyperthermic damage. Pretreatment for 8 h at 37 degrees C with 1 x 10(-9) mol/l AcSDKP resulted in a decrease in hyperthermic sensitivity of only normal haematopoietic progenitor cells. This combined treatment protocol revealed a therapeutic index (ratio of surviving fractions of normal vs. leukaemic cells) of > 500, which was considered to be sufficient for purging. This was confirmed in vivo by the survival of lethally irradiated recipients transplanted with purged simulated remission bone marrow (1 x 10(6) normal bone marrow cells and 5 x 10(4) leukaemic cells). A further increase of the therapeutic index cells was achieved by the alkyl-lysophospholipid ET-18-OCH(3). An incubation for 4 h at 37 degrees C with 25 microg/ml in the presence of 5% fetal calf serum preferentially enhanced the cytotoxic effect towards the leukaemic stem cell. The combination of AcSDKP and ET-18-OCH(3) with hyperthermia resulted in a therapeutic index of > 5000. This enabled a reduction of the hyperthermic treatment and will further minimize the toxicity to normal haematopoietic stem cell subsets, while a therapeutic index far above the required value is achieved. This tripartite purging treatment therefore offers a safe and fast purging protocol for the elimination of residual leukaemic cells in autografts.

Comparative heat sensitivity of murine and human hemopoietic progenitors and clonogenic leukemia cells

The aim of this study was to compare the thermal sensitivity of normal murine and human hemopoietic progenitors to that of leukemic murine and human clonogenic cells in order to assess the clinical relevance of experimental data. Colony forming units-granulocyte-macrophage (CFU-GM) from normal human bone marrow and from bone marrow of patients with acute lymphocytic leukemia (ALL), acute myelogenous leukemia (AML), and Hodgkin's disease in complete remission proved to be less sensitive to 42.5 degrees C in vitro hyperthermia (D0: 93.9 min) than murine bone marrow CFU-GM (D0: 49.6 min). Leukemic colony forming cells (CFU-L) from HL-60 suspension culture--when compared to human CFU-GM--showed significantly increased thermal sensitivity (D0: 22.8 min). While the thermal sensitivity of CFU-L from a murine leukemia cell line (WEHI 3-B) was not statistically significant when compared to that of CFU-L from HL-60 (D0 values 17.0 versus 22.8 min), the vertical difference between the parallel regression lines suggested an approximately three-fold greater survival for human CFU-L. Although carefully controlled hyperthermia is an easy purging technique, the relevance of murine data to human clinical practice must be considered critically.

Total body hyperthermia in combination with etoposide and melphalan in a child with acute myelomonocytic leukemia

In vitro and clinical studies have shown antineoplastic effects of hyperthermia alone and in combination with other treatment modalities. Synergistic cytotoxic effects of chemotherapy and hyperthermia have been demonstrated on leukemic cell clones in vitro. It seems that hyperthermia is effective in overcoming chemotherapy resistance. Several groups treated solid tumors by using total body hyperthermia (TBHT). However, only a few studies have been reported investigating the clinical effects of TBHT in myeloproliferative disorders. We report the case of a 7-year-old boy with myelomonocytic leukemia treated with TBHT (2 hours, 42 degrees C) combined with etoposide (600 mg/m2), melphalan (30 mg/m2) and hyperglycemia (200-300 mg/dl). Within 24 hours after TBHT, the leukemic cells decreased after TBHT from 53,000/microliters to zero. Skin leukemic infiltrates, resistant to conventional treatment, also responded well. Although our patient relapsed 34 days after TBHT, these results indicate that TBHT in combination with cytotoxic treatment may be a useful treatment modality in refractory leukemia.

In vitro purging of clonogenic leukemic cells from human bone marrow by heat: simulation experiments for autologous bone marrow transplantation

In order to apply a simple purging method by heat to autologous bone marrow transplantation (ABMT), we have revaluated the ability to purge clonogenic leukemic cells from the simulated marrow mixture of normal marrow cells and leukemic cell lines (HL-60, Molt-3 and HEL) in vitro by heat, using two different clonogenic assays for normal granulocyte-macrophage progenitors (CFU-GM) and leukemic cell lines. It appeared that in vitro hyperthermia (42 degrees C for 120 min) is able to selectively remove clonogenic leukemic cells from simulated tumor cell-normal marrow mixtures even when leukemic cell concentrations are increased up to 3 x 10(6) cells/ml in vitro, and results in a 4-6 log destruction of clonogenic leukemic cells/ml according to a limiting dilution assay, while leaving half of normal CFU-GM surviving. The hyperthermic purging of clonogenic leukemic cells was not affected in the presence of normal marrow cells in vitro. This high level of clonogenic leukemic cell depletion by heat correlated with that of immunologic and pharmacologic studies. These results suggest that in vitro hyperthermia could be applied effectively and safely for the elimination of residual clonogenic leukemic cells in autologous marrow grafts before ABMT.

A simple elimination of clonogenic tumor cells from human bone marrow in vitro by heat: its application to autologous bone marrow transplantation for B-cell lymphoma

The application of hyperthermia to the treatment of neoplastic disease has focused on solid tumors. Since the hyperthermic sensitivity of human B-cell lymphoma cells is not known, we have examined the effect of hyperthermia on the growth of B-cell lymphoma cell lines (Raji and Daudi) in vitro to evaluate the ability to purge tumor cells from normal bone marrow by heat, utilizing a limiting-dilution assay to measure log depletion of tumor cells in a 20-fold excess of normal bone marrow. When exposed at 42 degrees C and 43 degrees C for 120 min, both clonogenic Raji and Daudi cells were dramatically decreased (a 4- to 6-log reduction) with exposure time, while leaving over half of the normal granulocyte-macrophage progenitor cells surviving at 42 degrees C and 10% at 43 degrees C. This high level of lymphoma-cell depletion by heat correlated with that obtained in immunologic and pharmacologic studies. These results suggest that in vitro hyperthermia might be applied effectively for the elimination of residual lymphoma cells in autologous marrow grafts before autologous bone marrow transplantation in B-cell non-Hodgkin's lymphoma.