CARD14 alterations in Tunisian patients with psoriasis and further characterization in European cohorts

BACKGROUND

Rare highly penetrant gain-of-function mutations in caspase recruitment domain family, member 14 (CARD14) can lead to psoriasis, a chronic inflammatory disease of the skin and other organs.

OBJECTIVES

To investigate the contribution of rare CARD14 variants to psoriasis in the Tunisian population and to expand knowledge of CARD14 variants in the European population.

METHODS

CARD14 coding exons were resequenced in patients with psoriasis and controls from Tunisia and Europe, including 16 European cases with generalized pustular psoriasis (GPP). Novel variants were evaluated for their effect on nuclear factor (NF)-κB signalling.

RESULTS

Rare variants in CARD14 were significantly enriched in Tunisian cases compared with controls. Three were collectively found in 5% of Tunisian cases, and all affected the N-terminal region of the protein harbouring its caspase recruitment domain or coiled-coil domain. These variants were c.349G>A (p.Gly117Ser), c.205C>T (p.Arg69Trp) and c.589G>A (p.Glu197Lys). c.589G>A (p.Glu197Lys) led to upregulation of NF-κB activity in a similar manner to that of previously described psoriasis-associated mutations. p.Arg69Trp led to sevenfold downregulation of NF-κB activity. One Tunisian case harboured a c.1356+5G>A splice alteration that is predicted to lead to loss of exon 9, which encodes part of the coiled-coil domain. No cases of GPP harboured an interleukin-36RN mutation, but one of 16 cases of GPP with a family history of psoriasis vulgaris harboured a c.1805C>T (p.Ser602Leu) mutation in CARD14.

CONCLUSIONS

These observations provide further insights into the genetic basis of psoriasis in the Tunisian population and provide functional information on novel CARD14 variants seen in cases from Tunisia and other populations.

Gene signature critical to cancer phenotype as a paradigm for anticancer drug discovery

Malignant cell transformation commonly results in the deregulation of thousands of cellular genes, an observation that suggests a complex biological process and an inherently challenging scenario for the development of effective cancer interventions. To better define the genes/pathways essential to regulating the malignant phenotype, we recently described a novel strategy based on the cooperative nature of carcinogenesis that focuses on genes synergistically deregulated in response to cooperating oncogenic mutations. These so-called 'cooperation response genes' (CRGs) are highly enriched for genes critical for the cancer phenotype, thereby suggesting their causal role in the malignant state. Here, we show that CRGs have an essential role in drug-mediated anticancer activity and that anticancer agents can be identified through their ability to antagonize the CRG expression profile. These findings provide proof-of-concept for the use of the CRG signature as a novel means of drug discovery with relevance to underlying anticancer drug mechanisms.

Bortezomib and gemcitabine in relapsed or refractory Hodgkin's lymphoma

BACKGROUND

Given the significant activity and tolerability of gemcitabine in patients with relapsed Hodgkin's lymphoma (HL), the critical role that nuclear factor kappa B (NF-kappaB) appears to play in the pathogenesis of this tumor, the ability of bortezomib to inhibit NF-kappaB activity, and laboratory studies suggesting synergistic antitumor effects of gemcitabine and bortezomib, we hypothesized that this combination would be efficacious in patients with relapsed or refractory HL.

PATIENTS AND METHODS

A total of 18 patients participated. Patients received 3-week cycles of bortezomib 1 mg/m(2) on days 1, 4, 8, and 11 plus gemcitabine 800 mg/m(2) on days 1 and 8.

RESULTS

The overall response rate for all patients was 22% (95% confidence interval 3% to 42%). Three patients developed grade III transaminase elevation: one was removed from the study and two had doses of gemcitabine held. Almost all patients exhibited inhibition of proteasome activity with treatment.

CONCLUSIONS

The combination of gemcitabine and bortezomib is a less active and more toxic regimen in relapsed HL than other currently available treatments. It poses a risk of severe liver toxicity and should be pursued with caution in other types of cancer.

Unique molecular and cellular features of acute myelogenous leukemia stem cells

It is well known in the field of acute myelogenous leukemia (AML) that many different translocations and genetic aberrancies are found with the various forms of the disease. Indeed, specific translocations are often associated with disease subtypes that manifest themselves through the accumulation of immature myeloid cells at varying stages of differentiation. Moreover, the differentiation state of myeloid blast populations has been utilized as a means of categorizing different AML subtypes (French, American, British, or FAB classification system). Thus, the notion that AML is a family of related but distinct diseases is a common view. Interestingly, however, studies in recent years that have formalized the concept of a leukemic stem cell (LSC) have also begun to define shared developmental, cellular and molecular features amongst the malignant stem cells that give rise to different AML subtypes. Moreover, some of these conserved features appear to be unique to the leukemia stem/progenitor cell population, and are not found in normal hematopoietic stem cells (HSCs). This article will summarize data emerging from the study of LSCs and suggest how distinct molecular and cellular characteristics of the LSC population may provide new opportunities for AML therapy.

Nuclear factor-kappaB is constitutively activated in primitive human acute myelogenous leukemia cells

Human acute myelogenous leukemia (AML) is thought to arise from a rare population of malignant stem cells. Cells of this nature, herein referred to as leukemic stem cells (LSCs), have been documented for nearly all AML subtypes and appear to fulfill the criteria for stem cells in that they are self-renewing and give rise to the cells found in many leukemic populations. Because these cells are likely to be critical for the genesis and perpetuation of leukemic disease, the present studies sought to characterize unique molecular properties of the LSC population, with particular emphasis on the transcription factor, nuclear factor-kappaB (NF-kappaB). Previous experiments have shown that unstimulated human CD34(+) progenitor cells do not express NF-kappaB. In contrast, primary AML CD34(+) cells display readily detectable NF-kappaB activity as assessed by electrophoretic mobility shift assay and gene expression studies. Furthermore, detailed analyses of enriched AML stem cells (CD34(+)/CD38(-)/CD123(+)) indicate that NF-kappaB is also active in the LSC population. Given the expression of NF-kappaB in leukemic, but not normal primitive cells, the hypothesis that inhibition of NF-kappaB might induce leukemia-specific apoptosis was tested by treating primary cells with the proteasome inhibitor MG-132, a well-known inhibitor of NF-kappaB. Leukemic CD34(+)/CD38(-) cells displayed a rapid induction of cell death in response to MG-132, whereas normal CD34(+)/CD38(-) cells showed little if any effect. Taken together, these data indicate that primitive AML cells aberrantly express NF-kappaB and that the presence of this factor may provide unique opportunities to preferentially ablate LSCs.

The use of adenoviral vectors for genetic manipulation and analysis of primitive hematopoietic cells

Gene transfer into stem cells has long been studied as a means by which primitive hematopoietic cells could be characterized and manipulated. While a variety of strategies have been attempted, it still remains relatively difficult to perform direct stem cell analysis. In this review, we examine recent studies using adenovirus-based vectors as a means to achieve high-level gene transfer into primitive hematopoietic cell types.

Expression of tumor-suppressor genes interferon regulatory factor 1 and death-associated protein kinase in primitive acute myelogenous leukemia cells

Previous studies indicate that human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells. Cells of this nature can initiate and maintain leukemic cell growth in both long-term cultures and nonobese diabetic/severe combined immune-deficient mice. To characterize the biology of primitive AML cells, gene expression screens were performed with 7 primary AML and 3 normal specimens. For each sample, stem cell populations (CD34(+)/CD38(-)) were isolated and used to synthesize radiolabeled complementary DNA (cDNA). AML vs normal probes were then hybridized to cDNA arrays containing genes related to cancer and apoptosis. Of approximately 1400 genes analyzed, 2 tumor-suppressor genes were identified that were overexpressed in all 7 of the AML CD34(+)/CD38(-) cell populations: death-associated protein kinase and interferon regulatory factor 1. Expression of each gene was confirmed by reverse-transcription polymerase chain reaction and immunoblot analysis. It is proposed that tumor-suppressor proteins play a role in the biology of primitive AML cells. (Blood. 2001;97:2177-2179)

Clinical infection control in gene therapy: a multidisciplinary conference

Gene therapy is being studied for the treatment of a variety of acquired and inherited disorders. Retroviruses, adenoviruses, poxviruses, adeno-associated viruses, herpesviruses, and others are being engineered to transfer genes into humans. Treatment protocols using recombinant viruses are being introduced into clinical settings. Infection control professionals will be involved in reviewing the safety of these agents in their clinics and hospitals. To date, only a limited number of articles have been written on infection control in gene therapy, and no widely available recommendations exist from federal or private organizations to guide infection control professionals. The goals of the conference were to provide a forum where gene therapy experts could share their perspectives and experience with infection control in gene therapy and to provide an opportunity for newcomers to the field to learn about issues specific to infection control in gene therapy. Recommendations for infection control in gene therapy were proposed.

The interleukin-3 receptor alpha chain is a unique marker for human acute myelogenous leukemia stem cells

Recent studies suggest that the population of malignant cells found in human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells (LSCs). LSCs have been documented for nearly all AML subtypes and have been phenotypically described as CD34+/CD38- or CD34+/HLA-DR-. Given the potentially critical role of these primitive cells in perpetuating leukemic disease, we sought to further investigate their molecular and cellular characteristics. Flow cytometric studies using primary AML tissue showed that the interleukin-3 receptor alpha chain (IL-3Ralpha or CD123) was strongly expressed in CD34+/CD38- cells (98 +/- 2% positive) from 16 of 18 primary specimens. Conversely, normal bone marrow derived CD34+/CD38- cells showed virtually no detectable expression of the CD123 antigen. To assess the functional role of IL-3Ralpha positive cells, purified CD34+/CD123+ leukemia cells were transplanted into immune deficient NOD/SCID mice. These experiments showed that CD123+ cells were competent to establish and maintain leukemic populations in vivo. To begin to elucidate a biological role for CD123 in leukemia, primary AML samples were analyzed with respect to signal transduction activity in the MAPK, Akt, and Stat5 pathways. Phosphorylation was not detected in response to IL-3 stimulation, thereby suggesting CD123 is not active in conventional IL-3-mediated signaling. Collectively, these data indicate that CD123 represents a unique marker for primitive leukemic stem cells. Given the strong expression of this receptor on LSCs, we propose that targeting of CD123 may be a promising strategy for the preferential ablation of AML cells.

Genetic manipulation of primitive leukemic and normal hematopoietic cells using a novel method of adenovirus-mediated gene transfer

Gene transfer into early hematopoietic cells has been problematic due to the quiescent nature of primitive cells and the lack of gene transfer vehicles with high efficiency for hematopoietic cell types. Previously, we have shown that adenoviral vectors can be used for the transduction of normal human progenitors with gene transfer efficiencies of approximately 30%. However, this approach is limited by relatively slow uptake kinetics (24-48 h) and a strong dependence on the presence of exogenous cytokines. Thus, we have modified this approach by combining adenoviral vectors with polycations to generate a virus-polycation complex, or VPC. Vehicles of this nature, when composed of conventional adenoviral vectors and polyamidoamine dendrimers, are a highly efficient means of transducing both normal and acute myelogenous leukemia (AML) cells. Moreover, the kinetics of gene transfer are markedly increased using the VPC strategy, with approximately 70% of transduction complete within 2 h. In this study, using viruses that encode green fluorescence protein (GFP), or the T cell costimulatory molecule B7.1 (CD80), we show that VPC-mediated gene transfer is an effective means of transducing normal and AML cells, including those with a highly primitive phenotype. Our data suggest that transient genetic manipulation of primitive hematopoietic cells can readily be achieved and should therefore permit a variety of research and clinical endeavors.

Efficient and durable gene marking of hematopoietic progenitor cells in nonhuman primates after nonablative conditioning

Optimization of mobilization, harvest, and transduction of hematopoietic stem cells is critical to successful stem cell gene therapy. We evaluated the utility of a novel protocol involving Flt3-ligand (Flt3-L) and granulocyte colony-stimulating factor (G-CSF) mobilization of peripheral blood stem cells and retrovirus transduction using hematopoietic growth factors to introduce a reporter gene, murine CD24 (mCD24), into hematopoietic stem cells in nonhuman primates. Rhesus macaques were treated with Flt3-L (200 microgram/kg) and G-CSF (20 microgram/kg) for 7 days and autologous CD34(+) peripheral blood stem cells harvested by leukapheresis. CD34(+) cells were transduced with an MFGS-based retrovirus vector encoding mCD24 using 4 daily transductions with centrifugations in the presence of Flt3-L (100 ng/mL), human stem cell factor (50 ng/mL), and PIXY321 (50 ng/mL) in serum-free medium. An important and novel feature of this study is that enhanced in vivo engraftment of transduced stem cells was achieved by conditioning the animals with a low-morbidity regimen of sublethal irradiation (320 to 400 cGy) on the day of transplantation. Engraftment was monitored sequentially in the bone marrow and blood using both multiparameter flow cytometry and semi-quantitative DNA polymerase chain reaction (PCR). Our data show successful and persistent engraftment of transduced primitive progenitors capable of giving rise to marked cells of multiple hematopoietic lineages, including granulocytes, monocytes, and B and T lymphocytes. At 4 to 6 weeks posttransplantation, 47% +/- 32% (n = 4) of granulocytes expressed mCD24 antigen at the cell surface. Peak in vivo levels of genetically modified peripheral blood lymphocytes approached 35% +/- 22% (n = 4) as assessed both by flow cytometry and PCR 6 to 10 weeks posttransplantation. In addition, naïve (CD45RA(+) and CD62L(+)) CD4(+) and CD8(+) cells were the predominant phenotype of the marked CD3(+) T cells detected at early time points. A high level of marking persisted at between 10% and 15% of peripheral blood leukocytes for 4 months and at lower levels past 6 months in some animals. A cytotoxic T-lymphocyte response against mCD24 was detected in only 1 animal. This degree of persistent long-lived, high-level gene marking of multiple hematopoietic lineages, including naïve T cells, using a nonablative marrow conditioning regimen represents an important step toward the ultimate goal of high-level permanent transduced gene expression in stem cells.

Stable transduction of quiescent CD34(+)CD38(-) human hematopoietic cells by HIV-1-based lentiviral vectors

We compared the efficiency of transduction by an HIV-1-based lentiviral vector to that by a Moloney murine leukemia virus (MLV) retroviral vector, using stringent in vitro assays of primitive, quiescent human hematopoietic progenitor cells. Each construct contained the enhanced green fluorescent protein (GFP) as a reporter gene. The lentiviral vector, but not the MLV vector, expressed GFP in nondivided CD34(+) cells (45.5% GFP+) and in CD34(+)CD38(-) cells in G0 (12.4% GFP+), 48 hr after transduction. However, GFP could also be detected short-term in CD34(+) cells transduced with a lentiviral vector that contained a mutated integrase gene. The level of stable transduction from integrated vector was determined after extended long-term bone marrow culture. Both MLV vectors and lentiviral vectors efficiently transduced cytokine-stimulated CD34(+) cells. The MLV vector did not transduce more primitive, quiescent CD34(+)CD38(-) cells (n = 8). In contrast, stable transduction of CD34(+)CD38(-) cells by the lentiviral vector was seen for over 15 weeks of extended long-term culture (9.2 +/- 5.2%, n = 7). GFP expression in clones from single CD34(+)CD38(-) cells confirmed efficient, stable lentiviral transduction in 29% of early and late-proliferating cells. In the absence of growth factors during transduction, only the lentiviral vector was able to transduce CD34(+) and CD34(+)CD38(-) cells (13.5 +/- 2.5%, n = 11 and 12.2 +/- 9.7%, n = 4, respectively). The lentiviral vector is clearly superior to the MLV vector for transduction of quiescent, primitive human hematopoietic progenitor cells and may provide therapeutically useful levels of gene transfer into human hematopoietic stem cells.

Recent progress in identifying genes regulating hematopoietic stem cell function and fate

Significant advances in the use of genetic and molecular biology strategies have recently begun to identify genes that have a major impact on the determination, commitment and developmental potential of hematopoietic stem cells. Using a variety of experimental strategies, genes such as SCL, GATA-2, HoxB4, Flk-2, c-mpl, dlk, and others have been implicated as important regulators of stem cell growth. In addition, genetic mapping has identified several loci that correlate strongly with stem cell numbers and proliferation.

Adenovirus vectors for gene transduction into mobilized blood CD34+ cells

Mobilized blood CD34+ cells from cancer patients were ex vivo infected by a recombinant adenovirus vector carrying an alkaline phosphatase gene, whose expression is evaluable by flow cytometry. A mean of 40% CD34+ cells were infected by the vector, with high levels of expression of the transgene. Among attempts to improve infection efficiency by manipulating culture conditions, only reinfection by the same vector achieved a 10% increase of transgene expression. Transduced CD34+ cells were induced to differentiate along the myeloid and the dendritic lineage, and in either case AP+ cells were detectable among the differentiated cell population. We conclude that adenovirus vectors may be useful tools for gene transduction into mobilized blood CD34+ cells, particularly for those applications in which high transgene expression for limited periods of time is required.

Transduction efficiency of cell lines and hematopoietic stem cells correlates with retrovirus receptor mRNA levels

The low transduction efficiency of hematopoietic stem cells (HSC) using amphotropic retroviruses continues to plague gene therapy protocols. This low transduction efficiency may be related to a low level of amphotropic retrovirus binding to target cell receptors. We have assayed murine and human cell lines as well as primary bone marrow HSC populations for mRNA encoding retrovirus receptors. Total cellular RNA was amplified by reverse transcriptase-polymerase chain reaction and the level of ecotropic and amphotropic receptor mRNA was compared to the level of beta 2-microglobulin mRNA in the same cell populations. Cell lines that are easily transduced by ecotropic and amphotropic retroviruses have high levels of receptor mRNA. In studies using murine HSC-enriched populations obtained from bone marrow, we observed a high correlation between transduction efficiency and the level of ecotropic and amphotropic receptor mRNA. We predict from these findings that purification of monkey and human HSC populations with high levels of amphotropic receptor mRNA will enable us to obtain improved efficiency of gene transfer.

Relative to adult marrow, fetal liver repopulates nearly five times more effectively long-term than short-term

Multilineage precursor cells from 14-day B6 (C57B1/6J) mouse fetal liver and adult bone marrow that repopulate both the lymphoid and myeloid systems were compared by competitive repopulation. Cells were assayed in normally functioning populations, and enrichment, tissue culture, and induced marking were avoided since these manipulations might affect cell function. Fetal or adult donor cells were mixed with marked adult competitor cells and transplanted into irradiated recipients whose blood was tested at short (25-33-day) or long (105-245-day) time periods after transplantation. Proportions of lymphocytes, granulocytes, and platelets descended from donor precursors were measured by GPI (glucosephosphate isomerase) isozyme genetic markers in congenic mice, and represent the repopulating abilities of these precursors relative to the standard competitor. For short-term repopulation 25-33 days after transplantation, fetal and adult donor cells were similar; in three studies, fetal liver contributed 0.8, 1.1, and 1.4 times as much as adult marrow per 10(5) cells transplanted. However, when long-term (105-245-day) repopulation was tested in the same recipients, fetal liver contributed 3.5, 5.0, and 7.1 times as much as adult marrow. Ratios of long-term/short-term repopulating abilities in fetal liver relative to standard adult marrow competitors were 2.5, 8.9, and 4.7, while in marrow controls, these ratios remained approximately one (1.14 and 0.80). Thus, 14-day fetal liver contains several times more long-term repopulating cells relative to short-term repopulating cells than does adult marrow. Ratios of long-term/short-term fetal cells were unchanged by precursor enrichment. The AA4.1+, Ly-6A/E+, lineage low fraction had a ratio of 4.4, although it repopulated 276 times better than unenriched fetal cells whose ratio was 4.7. There are two hypotheses that explain these data most simply: 1) There may be only a single multilineage precursor, but after transplantation cells seed in different microenvironments that support either long-term or short-term function. 2) Conversely, the difference may be at the stem cell level rather than the microenvironmental level, so that there are tow types of stem cells with multilineage differentiating ability, but only one functions over the long-term. The current report defines new conditions required by each hypothesis. If functional life spans are defined by seeding sites, as in hypothesis 1, fetal cells seed much higher proportions of long-term sites than adult cells. If different types of stem cells function short-term and long-term, as in hypothesis 2, they are not distinguished by markers allowing a 276-fold enrichment to 1367 times the repopulating ability of fresh marrow.

The level of mRNA encoding the amphotropic retrovirus receptor in mouse and human hematopoietic stem cells is low and correlates with the efficiency of retrovirus transduction

The low level of amphotropic retrovirus-mediated gene transfer into human hematopoietic stem cells (HSC) has been a major impediment to gene therapy for hematopoietic diseases. In the present study, we have examined amphotropic retrovirus receptor (amphoR) and ecotropic retrovirus receptor mRNA expression in highly purified populations of mouse and human HSC. Murine HSC with low to undetectable levels of amphoR mRNA and relatively high levels of ecotropic retrovirus receptor mRNA were studied. When these HSC were analyzed simultaneously for ecotropic and amphotropic retrovirus transduction, ecotropic provirus sequences were detected in 10 of 13 long-term repopulated animals, while amphotropic proviral sequences were detected in only one recipient. A second distinct population of murine HSC were isolated that express 3-fold higher levels of amphoR mRNA. When these HSC were analyzed simultaneously for ecotropic and amphotropic retrovirus transduction, 11 of 11 repopulated mice contained ecotropic provirus and 6 of 11 contained amphotropic provirus sequences, a significant increase in the amphotropic retrovirus transduction (P = 0.018). These results indicate that, among the heterogeneous populations of HSC present in adult mouse bone marrow, the subpopulation with the highest level of amphoR mRNA is more efficiently transduced by amphotropic retrovirus. In a related study, we found low levels of human amphoR mRNA in purified populations of human HSC (CD34+ CD38-) and higher levels in committed progenitor cells (CD34+ CD38+). We conclude that the amphoR mRNA level in HSC correlates with amphotropic retrovirus transduction efficiency.

High-resolution cell cycle analysis of defined phenotypic subsets within primitive human hematopoietic cell populations

We have developed a novel protocol for analysis of cell cycle status within specific subsets of primitive human hematopoietic cells. The technique, referred to as SID (surface, intracellular, and DNA) analysis, allows for the simultaneous characterization of cell surface and intracellular antigens, as well as quantitation of DNA content. To evaluate the technique, early human hematopoietic cells were examined using surface staining for the CD34 and CD38 antigens to identify primitive cells. Relative cell cycle status within defined phenotypic subsets (CD34+ and CD34+/CD38-) was determined by simultaneous two-parameter analysis using DNA content vs. antibody staining for the Ki-67 antigen. Ki-67 is not expressed in quiescent cells, but is quickly up-regulated as cells are induced to cycle. Consequently, expression of Ki-67, in combination with DNA content can be used to delineate all phases of the cell cycle (G0, G1, S, and GZ/M). We demonstrate that cycle induction of CD34+ cells, using IL-3, IL-6, and stem cell factor (SCF), does not correlate with activation of the CD34+CD38- subpopulation during ex vivo culture. Rather, CD34+/CD38- cells are much more refractory to cycle activation, requiring at least 72 hours to show significant levels of induction. In addition, primitive cells derived from bone marrow (BM) vs. mobilized peripheral blood (PB) show differing degrees of responsiveness to conventional ex vivo culture conditions. Finally, the effect of IL-3, IL-6, SCF, and Flt3 ligand (FL) on cycle induction was examined. It was observed that IL-3 synergized strongly with IL6+SCF to activate quiescent CD34+/CD38- cells. Moreover, when FL was combined with IL-3+IL-6+SCF, there was a small but reproducible increase in activation of CD34+/CD38- cells from G0 to G1. These data suggest that ex vivo behavior of primitive human stem cell populations is amenable to comprehensive flow cytometric analysis, and that such studies can provide detailed information on the biological response of stem cells to ex vivo culture and manipulation.

Transduction of primitive human hematopoietic cells with recombinant adenovirus vectors

We have examined the ability of recombinant adenoviral vectors to transduce human hematopoietic cells. Our findings indicate that adenovirus readily infects a large proportion of CD34+ cells. Using adenovirus vectors that transduce either a lacZ or an alkaline phosphatase reporter gene, we observed up to 45% of total CD34+ cells infected. Upon more detailed analysis, we observed comparable levels of transduction for CD34+/CD38- cells and for CD34+ cells in G(zero) phase of the cell cycle. Importantly, exposure to adenovirus resulted in negligible levels of toxicity as assayed by propidium iodide staining and colony-forming ability. Using adenovirus vectors, we also describe a model system for regulated gene expression in early hematopoietic tissues. CD34+ cells were simultaneously infected with two viruses, one carrying a TetR/VP16 transactivator (tTA) and the second carrying a tTA-dependent lacZ reporter gene. Using this approach, beta-gal expression was only observed upon coinfection with the transactivator vector. In addition, as shown previously (Gossen and Bujard, Proc Natl Acad Sci USA 89:5547, 1992), tetracycline was able to inhibit tTA mediated induction, thereby providing an effective means to regulate expression of the reporter gene. We conclude that recombinant adenovirus is an effective vehicle for transiently expressing genes in primitive human hematopoietic cells.

Long-term repopulating abilities of enriched fetal liver stem cells measured by competitive repopulation

To characterize hematopoietic cell biology, many investigators have used protocols that enrich for primitive hematopoietic stem cells (PHSC). In this study, we quantified the long-term repopulating ability (LTRA) of enriched and discarded fractions of PHSC from day-14 murine fetal liver using the competitive repopulation assay. We fractionated populations of fetal cells using the antigenic markers AA4.1+, AA4.1+/Sca+, and AA4.1+/Linlow/Sca+. Differentiating and repopulating abilities of each of these populations were directly compared using competitive repopulation. Adult bone marrow was mixed with fetal cell fractions from congenic donors having genetically distinguishable markers, and mixtures were given to irradiated recipients. Differentiating and repopulating abilities of the enriched donor cells were measured by the proportions of myeloid and lymphoid cells having donor markers that repopulated the recipients. LTRA was found primarily in the AA4.1+ and AA4.1+/Sca+ subpopulations. Further fractionation of the AA4.1+ cells to derive an AA4.1+/Linlow/Sca+ fraction showed that virtually all of the long-term stem cell activity was found in this subpopulation. These cells were 1400- to 1600-fold enriched in long-term functional ability compared to fresh marrow. This very high multilineage repopulating ability per cell was directly measured using a long-term functional assay in vivo. Importantly, the measured repopulating ability for AA4.1+/Linlow/Sca+ cells was about five-fold less than expected from the fraction of cells enriched and remained two- to three-fold less even after compensating for repopulating ability in discarded fractions. This illustrates that long-term functional abilities of enriched PHSC cannot be estimated from fractions enriched but should be quantitatively assayed.

Cellular and molecular characterization of the role of the flk-2/flt-3 receptor tyrosine kinase in hematopoietic stem cells

The flk-2/flt-3 receptor tyrosine kinase was cloned from a hematopoietic stem cell population and is considered to play a potential role in the developmental fate of the stem cell. Using antibodies derived against the extracellular domain of the receptor, we show that stem cells from both murine fetal liver and bone marrow can express flk-2/flt-3. However, in both these tissues, there are stem cell populations that do not express the receptor. Cell cycle analysis shows that stem cells that do not express the receptor have a greater percentage of the population in G0 when compared with the flk-2/flt-3-positive population. Development of agonist antibodies to the receptor shows a proliferative role for the receptor in stem cell populations. Stimulation with an agonist antibody gives rise to an expansion of both myeloid and lymphoid cells and this effect is enhanced by the addition of kit ligand. These studies serve to further illustrate the importance of the flk-2/flt-3 receptor in the regulation of the hematopoietic stem cell.

Primitive hemopoietic stem cells: direct assay of most productive populations by competitive repopulation with simple binomial, correlation and covariance calculations

Quantitative analyses of primitive hemopoietic stem cell (PHSC) populations are important both for basic biology and for clinical applications. Unfortunately, many conventional assays fail to measure long-term repopulating ability and maximal differentiating ability, the most important characteristics of the PHSC. The competitive repopulation assay described here focuses on this characteristic, assaying the precursors from which most differentiated cells are descended over large fractions of the life span in laboratory mice. Thus long-term repopulating ability and the ability to differentiate into both myeloid and lymphoid lineages are measured directly from 2.5 to 12.5 months after transplantation. This technique also has found high correlations between granulocytes, macrophages, and T and B lymphocytes as early as 3 weeks after transplantation. All or most differentiated cells of these widely disparate types appear to be descended from a common precursor cell, while myeloid-specific or lymphoid-specific precursors produce few or no descendants. However, large increases in variances between 3 and 6 weeks and 12 weeks after transplantation suggest that most of the initially active multilineage precursors are exhausted. Thus the ability to differentiate into widely disparate lineages does not establish long-term repopulating ability.

A receptor tyrosine kinase cDNA isolated from a population of enriched primitive hematopoietic cells and exhibiting close genetic linkage to c-kit

We have cloned a receptor tyrosine kinase cDNA, designated fetal liver kinase 1 (Flk-1), from mouse cell populations enriched for hematopoietic stem and progenitor cells. Sequence analysis of this clone reveals strong homology to the c-Kit subfamily of receptor kinases, and in particular to the Flt gene product. Chromosomal mapping shows that the Flk-1, Kit, and Pdgfra genes are closely linked. Flk-1 mRNA is expressed in primitive and more mature hematopoietic cells as well as in a wide variety of nonhematopoietic tissues.

A receptor tyrosine kinase specific to hematopoietic stem and progenitor cell-enriched populations

To elucidate the molecular biology of the hematopoietic stem cell, we have begun to isolate genes from murine cell populations enriched in stem cell activity. One such cDNA encodes a novel receptor tyrosine kinase, designated fetal liver kinase-2 or flk-2, which is related to the W locus gene product c-kit. Expression analyses suggest an extremely restricted distribution of flk-2. It is expressed in populations enriched for stem cells and primitive uncommitted progenitors, and is absent in populations containing more mature cells. Therefore, this receptor may be a key signal transducing component in the totipotent hematopoietic stem cell and its immediate self-renewing progeny.

Cellular and developmental properties of fetal hematopoietic stem cells

We have characterized the fetal totipotent hematopoietic stem cell using a novel strategy that integrates physical analysis of cell properties and genetic analysis of in vivo developmental behavior. This approach allows the simultaneous isolation and in vivo characterization of any stem cell population. Using this procedure we demonstrate that a cell surface marker, recognized by monoclonal antibody AA4.1, defines 0.5%-1.0% of fetal liver tissue that contains the entire hierarchy of primitive hematopoietic cells. The AA4.1+ subpopulation includes multipotential in vitro progenitors, CFU-S cells, and lymphoid-myeloid stem cells that function to yield permanent and oligoclonal blood systems. Further fractionation of these cells by analysis of density, fibronectin binding, and surface antigen distribution has defined 0.1%-0.2% of fetal liver that contains the totipotent stem cell.

Clonal and systemic analysis of long-term hematopoiesis in the mouse

We have analyzed the temporal in vivo fate of 142 individual stem cell clones in 63 reconstituted mice. Long-term sequential analyses of the four major peripheral blood lineages, obtained from animals engrafted with genetically marked stem cells, indicate that developmental behavior is primarily a function of time. As such, the first 4-6 months post-engraftment is characterized by frequent fluctuations in stem cell proliferation and differentiation behavior. Gradually, a stable hematopoietic system emerges, dominated by a small number of totipotent clones. We demonstrate that single stem cell clones are sufficient to maintain hematopoiesis over the lifetime of an animal and suggest that mono- or oligoclonality may be a hallmark of long-term reconstituted systems. A model is proposed, wherein lineage-restricted differentiation and dramatic clonal flux are consequences of mechanisms acting on an expanding pool of totipotent cells and are not indicative of intrinsically distinct stem cell classes.