Abstract 538: Mesenchymal stem cell subset prevents cycling of KG1a leukemic cells by cell-cell interaction

The interactions that occur between leukemic cells and the microenvironment are currently poorly understood. The definition of novel pathways that deliver survival and drug resistance signaling from the milieu to leukemic cells may lead to the discovery of new drugs that are able to target microenvironmental cues perpetuating the leukemic clones. Since the bone marrow (BM) environment is comprised of a complex assortment of different cell types, matrices and soluble and membrane-bound factors, it is crucial to start delineating specific cell-cell interactions and their effects on leukemic cell survival. Mesenchymal stem cells (MSC) are a fundamental component of the BM niche as they support hematopoiesis and are able to differentiate into neighboring cellular types. Therefore, in these studies we investigated the effect of Stro-1+ MSC on the growth and survival of KG1a, a primitive, differentiation-resistant subtype of acute myeloid leukemia. MSC and KG1a cells were co-cultured for 3 days under contact or non-contact conditions, at incremental ratios of 1:1, 1:5, 1:10 and 1:100 respectively, while maintaining a constant initial total number of cells for each dilution (n=3). In transwell non-contact cultures, KG1a cells proliferated at the same rate as KG1a alone with a 3 to 5 fold increase in cell numbers, regardless of the ratio. By contrast, when KG1a were cultured in direct contact with MSC, KG1a growth inhibition was directly proportional to the MSC:KG1a ratio. Specifically, MSC cultured with KG1a cells at ratios of 1:1, 1:5, 1:10, and 1:100 inhibited the growth of KG1a cells by 84%, 76%, 74%, and 45%, respectively while remaining 99-100% viable. Thus, we next performed cell cycle analysis using flow cytometry and found that while the cell cycle distribution of KG1a cultures were: 23% in G0/G1; 19% in G2/M; and 58% in S phase, KG1a co-cultured with MSC had undergone cell cycle arrest in S-phase (98%). Although our results contrast with prior studies showing that MSC inhibited KG1a in G1phase through the production of soluble factors, it is possible that the differences seen are due to the population of MSC used. Within the MSC, Stro-1+ cells constitute a primitive population of cells in intimate relation with the osteoblastic niche, the role of which is to retain stem cells in a quiescent state. Therefore, studies are currently underway to identify molecules responsible cell cycle arrest upon KG1a/MSC interaction.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 538.

A new human somatic stem cell from placental cord blood with intrinsic pluripotent differentiation potential

Here a new, intrinsically pluripotent, CD45-negative population from human cord blood, termed unrestricted somatic stem cells (USSCs) is described. This rare population grows adherently and can be expanded to 10¹⁵ cells without losing pluripotency. In vitro USSCs showed homogeneous differentiation into osteoblasts, chondroblasts, adipocytes, and hematopoietic and neural cells including astrocytes and neurons that express neurofilament, sodium channel protein, and various neurotransmitter phenotypes. Stereotactic implantation of USSCs into intact adult rat brain revealed that human Tau-positive cells persisted for up to 3 mo and showed migratory activity and a typical neuron-like morphology. In vivo differentiation of USSCs along mesodermal and endodermal pathways was demonstrated in animal models. Bony reconstitution was observed after transplantation of USSC-loaded calcium phosphate cylinders in nude rat femurs. Chondrogenesis occurred after transplanting cell-loaded gelfoam sponges into nude mice. Transplantation of USSCs in a noninjury model, the preimmune fetal sheep, resulted in up to 5% human hematopoietic engraftment. More than 20% albumin-producing human parenchymal hepatic cells with absence of cell fusion and substantial numbers of human cardiomyocytes in both atria and ventricles of the sheep heart were detected many months after USSC transplantation. No tumor formation was observed in any of these animals.

New models to investigate mechanisms of disease genesis from primitive BCR-ABL(+) hematopoietic cells

Three years ago we described a novel autocrine IL-3/G-CSF mechanism active in the leukemic CD34(+) cells from chronic myeloid leukemia (CML) patients in chronic phase (PNAS 96: 12804-12809, [1999]). We also showed that exposure of the most primitive CD34(+) cells from normal human bone marrow to excess IL-3 stimulates not only the division of these cells but also their differentiation. In contrast, both IL-3 and G-CSF cause an expansion of the more mature types of normal CD34(+) progenitors. These findings suggested that the autocrine IL-3/G-CSF mechanism active in CML stem cells can compromise their self-renewal in spite of increasing their proliferative activity, which, in turn, might explain the paradoxically slow rate of expansion of this compartment over time in patients with latent disease. To investigate this hypothesis, we have begun to characterize the numbers and types of cells generated from chronic phase CML patients' cells transplanted into adult immunodeficient mice or fetal sheep, and also from transplants of primitive murine and human hematopoietic cells transduced with a retroviral BCR-ABL vector. Our findings to date using these models reinforce the importance of the autocrine IL-3/G-CSF mechanism in the development of CML. BCR-ABL appears to directly activate IL-3 and G-CSF production in primitive hematopoietic cells and this is important to their transplantable leukemogenic activity. However, the development in vivo of an overt leukemia from primitive BCR-ABL(+) hematopoietic cells can be very delayed. These models thus offer new opportunities for analyzing the molecular events that underlie the pathogenesis of human CML and the future testing of new therapeutic approaches.

Ex vivo expanded cord blood cells provide rapid engraftment in fetal sheep but lack long-term engrafting potential

OBJECTIVE

Cord blood (CB) products are becoming routinely used in unrelated allogeneic transplantation for smaller pediatric patients. Because of the low numbers of cells in CB compared to bone marrow or peripheral blood progenitor cells, their use is more limited in larger adults. Therefore, we developed ex vivo expansion conditions for CB and currently are transplanting ex vivo expanded CB products to patients receiving high-dose chemotherapy. As there is concern that ex vivo expansion may exhaust long-term engrafting cells, the current clinical protocols consist of both an expanded fraction and an unexpanded fraction. To determine the effect of expansion culture on long-term engrafting cells, we evaluated the short- and long-term engrafting potential of ex vivo expanded CB using a fetal sheep xenogeneic transplant model.

MATERIAL AND METHODS

CD 34(+) cells were selected from CB products and cultured in a two-step procedure in the presence of stem cell factor, megakaryocyte growth and differentiation factor, and granulocyte colony-stimulating factor for 14 days. Starting cells (CD34(+) cells), and cultured cells (day 7 and day 14 cells) were transplanted in 60-day-old fetal sheep and evaluated at various time points post transplant for the presence of human cells. Long-term engrafting cells were assessed by serial passage into secondary and tertiary recipients.

RESULTS

Day 14 expanded CB cells provided more rapid engraftment than either the day 7 expanded cells or the day 0 cells; however, this engraftment was transient, and no human cells were detectable at 16 months post transplant in the animals that received the day 14 expanded cells. Day 0 cells had engrafted animals at 2 months post transplant and both the day 0 and day 7 cells persisted to 16 months or longer. In the secondary animals, the day 0 and day 7 cells engrafted equivalently at 3 months post transplant; however, no secondary engraftment resulted from the day 14 cells. The levels of engraftment in secondary animals receiving day 7 cells decreased with time to barely detectable levels at 12 months post transplant.

CONCLUSIONS

Ex vivo expansion of CB CD34(+) cells under the conditions described results in the generation of increased mature cells and progenitors that are capable of more rapid engraftment in fetal sheep compared to unexpanded CB CD34(+) cells. The expanded cells engrafted primary sheep but lacked secondary and tertiary engrafting potential. These studies demonstrate that although ex vivo expanded cells may be able to provide rapid short-term engraftment, the long-term potential of expanded grafts may be compromised. Therefore, clinical protocols may require transplantation of two fractions of cells, an expanded CB graft to provide rapid short-term engraftment and an unmanipulated fraction of CB graft to provide stem cells for long-term engraftment.

Primitive human hematopoietic cells displaying differential efflux of the rhodamine 123 dye have distinct biological activities

Human bone marrow (BM) CD34+ cells were stained with the vital dye, rhodamine 123 (Rh123), and analyzed for their biological properties based on the level of dye retention. Heterogeneous rhodamine staining is seen within the CD34+ population, and the staining patterns differ dramatically between fetal BM (FBM), adult BM (ABM) and mobilized peripheral blood (MPB). Kinetic analysis of the efflux of Rh123 from ABM CD34+ cells showed that efflux of Rh123 was most rapid from the most primitive Thy-1+ subset. The efflux of Rh123 could be inhibited by verapamil, suggesting that rhodamine efflux from primitive hematopoietic cells is primarily due to the P-glycoprotein (P-gp) pump or another intracellular transport system affected by verapamil. When four CD34+ subpopulations were plated onto SyS1 BM stromal cell cocultures after 1 to 2 weeks, only wells plated with CD34+ Thy-1+Rh123lo (low-level Rh123 retention) or CD34+Thy-1+Rh123mid (mid-level Rh123 retention) cells maintained greater than 50% of cells in an uncommitted CD34+33- stage. CD34+Lin- (lineage-negative) cells were fractionated based on Rh123 dye staining into Rh123hi (high-level Rh123 retention), Rh123mid, and Rh123lo and deposited as single cells into long-term SyS1 BM stromal cell cultures. The Rh123mid fraction had immense early proliferative activity in vitro, but lost the ability to form cobblestone areas after 5 to 6 weeks in culture. In contrast, the Rh123lo fraction proliferated more slowly but sustained long-term in vitro hematopoiesis as evidenced by continued cobblestone area-forming cells (CAFC) activity for at least 6 weeks. The Rh123hi fraction showed a plating efficiency similar to that of the Rh123lo or Rh1123mid fractions but did not extensively proliferative in vitro and did not show evidence of CAFC activity. We predicted from these in vitro results that the Rh123lo subsets possesses long-term engrafting potential. Indeed, on transplantation into the SCID-hu bone assay, all long-term engrafting potential and multilineage differentiation potential resided within the Rh123lo-mid but not Rh123hi subset. Furthermore, human marrow subpopulations derived from chimeric sheep after in utero transplantation with CD34+Thy-1+Lin- cells were reisolated based on Rh123 staining. Again, CD34+Lin- subsets showing Rh123lo-mid had long-term growth in culture, whereas Rh123hiCD34+Lin- cells did not. These results show that, after injection of CD34+Thy-1+Lin- cells into an in utero microenvironment, primitive CD34+ cells maintain a Rh123 phenotype that correlates with their in vitro CAFC activity. Thus, Rh123 staining is an effective way to define functional subsets of primitive hematopoietic cell populations.

Identification of CD34+ subsets after glycoprotease selection: engraftment of CD34+Thy-1+Lin- stem cells in fetal sheep

Epitopes on the CD34 molecule detected by some CD34 antibodies can be cleaved by a unique glycoprotease from Pasteurella haemolytica, which cleaves only glycoproteins rich in O-linked glycans. A method to isolate CD34+ cells from adult bone marrow was developed subsequently, in which CD34+ cells were isolated in high purity and yield following immunomagnetic bead selection and detachment with the glycoprotease. Using a variety of other cell-surface markers shown here to be insensitive to glycoprotease, committed progenitors of T lymphoid, B lymphoid, monomyeloid, megakaryoblastic, or erythroid lineages could be identified. Significantly, candidate hematopoietic stem cells (HSC) that are contained within a CD34+Lin- (CD2-, CD14-, CD15-, CD16-, CD19-) (or CD34+CD38-) subset expressing the Thy-1 antigen (CDw90), c-kit receptor (CD117), and CDw109 but lacking expression of CD71 and HLA-DR antigens also were detected. Functionally distinct subsets of glycoprotease-selected CD34+ cells were identified and subfractionated using flow cytometry and fluorescence-activated cell sorting (FACS). These subsets included candidate HSCs expressing the CD34+Thy-1+Lin- phenotype, which were sorted from a CD34+ fraction of a mobilized peripheral blood (MPB) sample. In a fetal sheep model, when CD34+Thy-1+Lin- cells were injected intraperitoneally, they were capable of homing to the marrow, where they generated long-term multilineage hematopoiesis and maintained human CD34+ cells, indicating that candidate HSC subsets of CD34+ cells selected with this highly specific enzyme were capable of engraftment in vivo. The ability to identify and purify virtually any phenotypically defined subset of glycoprotease-selected CD34+ stem/progenitor cells should facilitate the study of hematopoiesis in vitro and in animal models in vivo as well as the development of novel genetic techniques for the correction of specific blood cell disorders in humans.

Release of erythropoietin from macrophages by treatment with silica

An erythropoietic stimulating factor (ESF) can be shown to be released from preincubated macrophage-containing cell suspensions from mice by the macrophage-specific, cytotoxic agent, silica. A concentrated silica-treated spleen cell supernatant containing ESF is shown to cause a dose-dependent increase in 59Fe incorporation into red blood cells using the in vivo polycythemic mouse bioassay. The ESF from the same supernatant can also be neutralized by anti-erythropoietin. A second concentrated supernatant fractionated using wheat germ lectin-Sepharose 6MB and compared to either unfractionated or fractionated step III erythropoietin (Ep), tested in vitro using the erythroid colony-forming technique and 12-day fetal liver as target cells, indicates parallelism of all linear dose-response lines. This, together with the in vivo data, strongly suggests that the ESF released from macrophages treated with silica is, in fact, Ep. Substituting Ca2+ ions for fetal calf serum in the preincubation procedure results in the same activity being released compared to the presence of 1% or 20% fetal calf serum.

Cooperative erythropoietic assay of several steroid metabolites in polycythemic mice

A blinded cooperative assay of several androstane and pregnane steroid metabolites has been carried out in order to determine whether 5beta-H derivatives are as active as testosterone in stimulating in vivo erythropoiesis. The steroids tested were: testosterone, 5alpha-dihydrotestosterone, 5beta-dihydrotestosterone, 5beta-pregnane-3,20-dione, 3alpha-dihydroxy-5beta-pregnane-11,20-dione and 3beta-hydroxy-5beta-pregnan-20-one. The incorporation of radioactive iron into newly formed red cells in exhypoxic polycythemic mice was used to compare the effects of the steroids. Testosterone and 5alpha-dihydrotestosterone both produced significant increases in 59Fe incorporation. 5beta-dihydrotestosterone, 5beta-pregnane-3,20-dione, 3alpha-hydroxy-5beta-pregnane-11,20-dione and 3beta-hydroxy-5beta-pregnan-20-one were all devoid of significant erythropoietic activity in polycythemic mice in almost all instances. Thus, under the conditions chosen, this study failed to demonstrate that 5beta-steroids increase radioactive iron incorporation in red cells of exhypoxic polycythemic mice.

Inappropriate erythropoietin secretion in polycythemia vera

A patient with classical polycythemia vera (PV) was found to have an inappropriately elevated serum erythropoietin (Ep) level. Investigations did not reveal any lesion or blood abnormality known to be associated with excessive Ep production and erythrocytosis. Sudden withdrawal of blood to reduce the Hb and Hct from 18.5 gm% and 56% to 13.6 gm% and 41.5%, respectively, resulted in an increment of serum Ep to abnormal level. With iron treatment there was a brisk return of Hb and Hct to prebleeding levels which was associated with reduction in the serum Ep. The inverse relationship between the Ep and Hb or Hct is inconsistent with the presence of excessive Ep-producing lesion. These results suggested that the threshold for Ep secretion from normal Ep-secreting tissue to Hb and Hct levels is set at an abnormal level. This patient's marrow cells when cultured in vitro in the absence of Ep, unlike other PV patients' (except one) marrow cells, did not grow erythroid colonies. In the presence of Ep, however, the colonies comparable to those formed from normal marrow cultures were obtained. These results suggested that his marrow erythropoietic cells were neither Ep independent nor Ep-hyperresponsive, as has been suggested by some investigators for erythropoiesis in PV. This patient presents phenomena that hitherto have not been reported.