Identification of a subpopulation of cells with cancer stem cell properties in head and neck squamous cell carcinoma

Like many epithelial tumors, head and neck squamous cell carcinoma (HNSCC) contains a heterogeneous population of cancer cells. We developed an immunodeficient mouse model to test the tumorigenic potential of different populations of cancer cells derived from primary, unmanipulated human HNSCC samples. We show that a minority population of CD44(+) cancer cells, which typically comprise <10% of the cells in a HNSCC tumor, but not the CD44(-) cancer cells, gave rise to new tumors in vivo. Immunohistochemistry revealed that the CD44(+) cancer cells have a primitive cellular morphology and costain with the basal cell marker Cytokeratin 5/14, whereas the CD44(-) cancer cells resemble differentiated squamous epithelium and express the differentiation marker Involucrin. The tumors that arose from purified CD44(+) cells reproduced the original tumor heterogeneity and could be serially passaged, thus demonstrating the two defining properties of stem cells: ability to self-renew and to differentiate. Furthermore, the tumorigenic CD44(+) cells differentially express the BMI1 gene, at both the RNA and protein levels. By immunohistochemical analysis, the CD44(+) cells in the tumor express high levels of nuclear BMI1, and are arrayed in characteristic tumor microdomains. BMI1 has been demonstrated to play a role in self-renewal in other stem cell types and to be involved in tumorigenesis. Taken together, these data demonstrate that cells within the CD44(+) population of human HNSCC possess the unique properties of cancer stem cells in functional assays for cancer stem cell self-renewal and differentiation and form unique histological microdomains that may aid in cancer diagnosis.

Gene transfer by lentiviral vectors is limited by nuclear translocation and rescued by HIV-1 pol sequences

Gene-transfer vectors based on lentiviruses are distinguished by their ability to transduce non-dividing cells. The HIV-1 proteins Matrix, Vpr and Integrase have been implicated in the nuclear import of the viral genome in non-dividing cells. Here we show that a sequence within pol is also required in cis. It contains structural elements previously associated with the progress of reverse transcription in target cells. We restored these elements in cis within late-generation lentiviral vectors. The new vector transduced to a much higher efficiency several types of human primary cells, when both growing and growth-arrested, including haematopoietic stem cells assayed by long-term repopulation of NOD/SCID mice. On in vivo administration into SCID mice, the vector induced higher plasma levels of human clotting factor IX (F.IX) than non-modified vector. Our results indicate that nuclear translocation of the genome is a rate-limiting step in lentiviral infection of both dividing and non-dividing cells, and that it depends on protein and nucleic acid sequence determinants. Full rescue of this step in lentivirus-based vectors improves performance for gene-therapy applications.

Retroviral marking of acute myelogenous leukemia progenitors that initiate long-term culture and growth in immunodeficient mice

Rare primitive progenitors among the malignant cells from most patients with AML include AML long-term culture-initiating cells (AML LTC-IC) and NOD/SCID mouse leukemia-initiating cells (NOD/SL-IC). To evaluate the feasibility of genetic modification of these progenitors for gene marking and/ or gene therapy strategies, cells from patients with newly-diagnosed AML were cocultured with retroviral producer cells and then placed in colony (AML-CFC) assays, LTC, and injected intravenously into NOD/SCID mice. Southern blotting demonstrated transfer of the neo(r) gene to 30% to 80% of leukemic blasts when cells were cultured for 48 hours in the presence of IL-3 and steel factor (SF) prior to 48-hour coculture with viral producers. Three of six retrovirally-infected AML samples showed both engraftment in NOD/SCID mice and the presence of the neo(r) transgene in mouse tissues 8-15 weeks after injection of transduced cells. Thirteen weeks after injection of one of these samples, >80% of cells from mouse bone marrow were the progeny of two retrovirally-transduced AML progenitors. Four of the remaining five samples showed markedly reduced ability to engraft in mice after retroviral infection. Subsequent experiments demonstrated that the loss of engraftment potential took place within 24 hours of culture initiation in the absence of retroviral producers and regardless of the cytokines present. Interestingly, the majority of AML-CFC or AML LTC-IC survived the 24-hour culture period. A retroviral vector containing the murine cell surface marker heat stable antigen (HSA), which allows purification of transduced cells on immunomagnetic columns, was used to obtain an enriched population of gene-modified AML cells following an infection protocol that eliminated the 48 hours of prestimulation in IL-3 and SF and reduced coculture with viral producers to 10-36 hours. These modifications failed to improve engraftment of the infected cells. In addition, in these experiments more than 10 hours of cocultivation with viral producer cells was necessary to achieve gene transfer and expression in AML LTC-IC. These data demonstrate that although retroviral-mediated gene transfer can be achieved to AML progenitors, including NOD/SL-IC, improved culture conditions will be required before substantial numbers of such transduced primitive progenitors can be obtained. In addition, the difference in the ability of AML LTC-IC and NOD/SL-IC to survive ex vivo suggests that these assays may detect different populations of cells or that changes are induced in vitro in primitive cells which can only be detected in the mouse assay.

Growth characteristics of acute myelogenous leukemia progenitors that initiate malignant hematopoiesis in nonobese diabetic/severe combined immunodeficient mice

The use of immunodeficient mice, particularly of the nonobese diabetic/severe combined immunodeficient (NOD/SCID) strain, has allowed detection of very primitive malignant progenitors from patients with acute myelogenous leukemia (AML). To define the sensitivity and reproducibility with which the engraftment of different AML cells can be detected, 61 different samples from patients with newly diagnosed AML representing a variety of cytogenetic and French-American-British (FAB) subtypes were injected into NOD/SCID mice. Eight weeks after intravenous injection of 10(7) AML cells, the average percent of human cells in mouse bone marrow was 13.3%, with 70% of samples showing easily detectable engraftment of CD45(+) cells. AML samples with cytogenetic changes associated with a poor clinical prognosis tended to engraft to higher levels than those with changes associated with a good prognosis. Cells with FAB subtypes M3 and, to a lesser extent, M2, engrafted more poorly (P =.002 and.06, respectively) than those from other subtypes. Intraperitoneal injection of human interleukin-3 and Steel factor thrice weekly for 4 weeks did not enhance the levels of AML cell engraftment. However, AML samples that showed cytokine-independent colony growth in methylcellulose assay or expressed growth-factor mRNA in malignant blasts achieved significantly higher levels of engraftment than those which were cytokine dependent in culture or failed to express cytokine message (P <.03 and P <.02, respectively). In 6 patient samples, the frequency of NOD/SCID leukemia-initiating cells (NOD/SL-IC) varied from 0.7 to 45 per 10(7) cells, which was 200- to 800-fold lower than the frequency of AML long-term culture-initiating cells (AML LTC-IC) in the same samples. Each NOD/SL-IC will produce more than 10(6) leukemic blasts as well as many AML-CFC and AML LTC-IC as detected 8 weeks postinjection into mice. Serial transplant experiments showed the ability of NOD/SL-IC to maintain their own numbers over at least 3 to 4 weeks in vivo. The ability of these progenitors to self-renew combined with their potential to differentiate to produce large numbers of more mature progenitors and leukemic blasts suggests that the NOD/SL-IC assay identifies leukemic 'stem cells' that may maintain the malignant clone in human patients. The further use of this assay should facilitate studies of AML stem cell biology and the evolution of novel therapeutic strategies.

Cytokine responsiveness of primitive progenitors in acute myelogenous leukemia

Long-term cultures (LTC) and immunodeficient (NOD/SCID) mice have been used to quantitate and characterize primitive malignant progenitors from patients with acute myelogenous leukemia (AML). In 5-week-old LTC of cells from newly diagnosed patients with AML cytogenetically abnormal as well as normal progenitors could be easily detected and their numbers increased by cytokine supplements to the cultures. Sixty percent of AML samples will engraft in NOD/SCID mouse marrow. The frequency and level of engraftment of human cells detected appears to vary among the different subtypes of AML but is not generally affected by treatment of the mice with human cytokines. Both the LTC and NOD/SCID mouse assay show promise as tools to allow characterization of differences between leukemic stem cells which maintain malignant hematopoiesis in individual patients and, more importantly, between these cells and their normal stem cell counterparts.

Detection and characterization of primitive malignant and normal progenitors in patients with acute myelogenous leukemia using long-term coculture with supportive feeder layers and cytokines

Analysis of the mitogenic activity of interleukin-3 (IL-3), Steel factor (SF), and flt-3 ligand (FL) on acute myelogenous leukemia (AML) blasts using the short-term endpoints of proliferation in 3H-thymidine (3H-Tdr) incorporation assays or methylcellulose cultures (colony assays) showed that greater than 90% of samples contained cells that were responsive to one or more of these cytokines. With this information, culture conditions that were known to support normal long-term culture-initiating cells (LTC-IC) were tested, with or without supplements of one or more of these three growth factors, for their ability to support primitive progenitors from 10 cell samples from patients with AML. In all cases cytogenetically abnormal colony forming cells (CFC) were detected after 5 weeks when AML peripheral blood or marrow cells were cocultured on preestablished, normal human marrow feeders (HMF) and/or SI/SI mouse fibroblast feeders and the number of CFC detected in these 5-week-old LTC maintained a linear relationship to the number of input AML cells. Limiting dilution analysis, performed on 6 of the 10 samples, showed the frequency of AML cells initiating LTC (AML LTC-IC) to be 5- to 300-fold lower than the frequency of AML-CFC in the same cell sample, whereas the average number of CFC produced per LTC-IC varied from 1 to 13. Surprisingly, in each case the concentration of cytogenetically normal LTC-IC detected in AML patient blood was at least 10-fold higher than that previously observed in the blood of normal individuals. "Mixed" mouse fibroblast feeders engineered to produce human G-CSF, IL-3, and SF did not enhance detection of AML LTC-IC but did increase the output of cytogenetically normal CFC from LTC of 3 of 4 patient samples. Supplementation of AML LTC with IL-3 and exogenously provided SF and/or FL increased the output of AML-CFC from 5-week-old LTC by greater than or equal to twofold with 5 of 9 patient samples, whereas in one case exogenous addition of FL reduced the output of malignant CFC from LTC. These studies show that conditions that support normal LTC-IC also allow a functionally analogous but rare AML progenitor cell type to be detected. In addition, differences in the responses of normal and leukemic cells to various cytokines active on normal LTC-IC were revealed. Further analysis of these differences may enhance our understanding of leukemogenesis and lead to observations that could be exploited therapeutically.

Lack of expression of Thy-1 (CD90) on acute myeloid leukemia cells with long-term proliferative ability in vitro and in vivo

Acute myeloid leukaemia (AML) is thought to be maintained by a small population of leukemic progenitor cells. To define the phenotype of such cells with long-term proliferative capacity in vitro and in vivo, we have used the production of leukemic clonogenic cells (CFU) after 2 to 8 weeks in suspension culture as a measure of these cells in vitro and compared their phenotype with that of cells capable of engrafting nonobese diabetic severe combined immune deficient (NOD/SCID) mice. Leukemic blast peripheral blood cells were evaluated for expression of CD34 and Thy-1 (CD90) antigens. The majority of AML blast cells at diagnosis lacked expression of Thy-1. Most primary CFU-blast and the CFU detected at up to 8 weeks from suspension cultures were CD34+/Thy-1-. AML cells that were capable of engrafting NOD/SCID mice were also found to have the CD34+/Thy-1- phenotype. However, significant engraftment was achieved using both CD34+/Thy-1- and CD34- subfractions from one AML M5 patient. These results suggest that while heterogeneity exists between individual patients, the leukemic progenitor cells that are capable of maintaining the disease in vitro and in vivo differ from normal hematopoietic progenitor cells in their lack of expression of Thy-1.