Cell cycle distribution of primitive haematopoietic cells stimulated in vitro and in vivo

Towards predictive models of stem cell fate

Quantitative approaches are essential for the advancement of strategies to manipulate stem cells or their derivatives for therapeutic applications. Predictive models of stem cell systems would provide the means to pose and validate non-intuitive hypotheses and could thus serve as an important tool for discerning underlying regulatory mechanisms governing stem cell fate decisions. In this paper we review the development of computational models that attempt to describe mammalian adult and embryonic stem (ES) cell responses. Early stochastic models that relied exclusively on statistical distributions to describe the in vitro or in vivo output of stem cells are being revised to incorporate the contributions of exogenous and endogenous parameters on specific stem cell fate processes. Recent models utilize cell specific data (for example, cell-surface receptor distributions, transcription factor half-lives, cell-cycle status, etc.) to provide mechanistic descriptions that are consistent with biologically observed phenomena. Ultimately, the goal of these computational models is to, a priori, predict stem cell output given an initial set of conditions. Our efforts to develop a predictive model of ES cell fate are discussed. The quantitative studies presented in this review represent an important step in developing bioengineering approaches to characterize and predict stem cell behavior. Ongoing efforts to incorporate genetic and signaling network data into computational models should accelerate our understanding of fundamental principles governing stem cell fate decisions.

Clinical utility of flow cytometry in the study of erythropoiesis and nonclonal red cell disorders

Erythropoiesis involves proliferation and differentiation of small population of hematopoietic stem cells resident in the bone marrow into mature red blood cells. The determination of the cellular composition of the blood is a valuable tool in the diagnosis of diseases and monitoring of therapy. Flow cytometric analysis is increasingly being used to characterize the heterogeneous cell populations present in the blood and the hematopoietic cell differentiation and maturation pathways of the bone marrow. Here we discuss the role of flow cytometry in the study of erythropoiesis and nonclonal red blood cell disorders. First, we discuss flow cytometric analysis of reticulocytes. Next, we review salient quantitative methods that can be used for detection of fetal-maternal hemorrhage (FMH). We also discuss flow cytometric analysis of high hemoglobin F (HbF) in Sickle Cell Disease (SCD), hereditary spherocytosis (HS), red cell survival and red cell volume. We conclude by discussing cell cycle of erythroid cells.

The potential of human peripheral blood derived CD34+ cells for ex vivo red blood cell production

The potential of peripheral blood derived CD34+ cells for ex vivo erythropoiesis was investigated in a stroma-free culture system using a novel strategy of daily passaging. By expanding PB-derived CD34+ cells up to 1.5 x 10(6)-fold this method achieved expansion factors previously only reported for CD34+ cells derived from more potent stem cell sources such as cord blood, bone marrow and mobilized peripheral blood. Analysis of cell surface markers showed differentiation of immature CD34+ cells to populations with 80% CD71-/GpA+ cells and up to 45% enucleated cells, indicating a significant amount of terminal maturation. Cell crowdedness was found to have decisive effects on in vitro erythropoiesis. Cell density per surface area rather than cell concentration per media volume determined cell expansion during exponential growth where more crowded cells showed reduced overall expansion. In late stage erythropoiesis, however, when cells no longer proliferating, increased cell density was seen to enhance cell viability. These results indicate that peripheral blood derived haematopoietic stem cells can be an alternative to cells sourced from bone marrow, cord blood or leukapheresis in terms of expansion potential. This provides distinct advantages in terms of availability for studies of conditions for scale-up and maturation, and may have particular clinical applications in the future.

Bone marrow recovery following use of systemic (153)Sm-lexidronam and (89)Sr-chloride for bone pain palliation after myelosuppressive therapy

PURPOSE

Using plasma flt3 ((FMS (Friend murine strain))-like tyrosine kinase 3)-ligand cytokine (FL) as a biomarker, the purpose of this study was to determine whether patients receiving palliative radionuclide treatment following chemotherapy experienced enhanced myelosuppression.

MATERIALS AND METHODS

A total of 48 patients with solid tumors who failed multi-agent chemotherapy were investigated; they previously received 1-3 cycles of combination chemotherapy over 4-10 months. Patients were divided into four cohorts including 10 patients with early stage solid tumors prior to chemotherapy induction (naive group), 10 patients with (non-metastatic) malignancy actively undergoing chemotherapy without radionuclide follow-up (chemotherapy-alone group), 13 patients who underwent standard (1.0 mCi/kg) dose (153)Sm ((153)samarium)-lexidronam therapy following chemotherapy ((153)Sm group), and 15 patients who underwent standard (4 mCi) dose (89)Sr ((89)strontium)-chloride therapy following chemotherapy ((89)Sr group). Plasma FL was measured using a quantitative sandwich enzyme immunoassay and CBC (complete blood count), measuring WBC (white blood cell) and PLT (platelet), was performed.

RESULTS

Plasma FL concentration demonstrated a gradual decrease after chemotherapy. In patients who received (153)Sm within two weeks of completing chemotherapy, there is a distinguishable spike in FL concentration at approximately three weeks after dose administration, which precedes a decrease in WBC and PLT counts. On the other hand, a spike in FL levels in patients who received (89)Sr therapy is noted at approximately 10 weeks (p < 0.034).

CONCLUSIONS

Increases in FL concentration associated with (153)Sm-lexidronam therapy following combination chemotherapy occurred earlier and returned to control levels more rapidly than did those in patients similarly treated with (89)Sr. These differences might be associated with the shorter decay half-life and lower particle emission energies of (153)Sm.

The systematic production of cells for cell therapies

Stem cells have emerged as the starting material of choice for bioprocesses to produce cells and tissues to treat degenerative, genetic, and immunological disease. Translating the biological properties and potential of stem cells into therapies will require overcoming significant cell-manufacturing and regulatory challenges. Bioprocess engineering fundamentals, including bioreactor design and process control, need to be combined with cellular systems biology principles to guide the development of next-generation technologies capable of producing cell-based products in a safe, robust, and cost-effective manner. The step-wise implementation of these bioengineering strategies will enhance cell therapy product quality and safety, expediting clinical development.

Analysis of cell differentiation by division tracking cytometry

We propose a quantitative method to characterize growth and differentiation dynamics of multipotent cells from time series carboxyfluorescein diacetate, succinimidyl ester (CFDA-SE) division tracking data. The dynamics of cell proliferation and differentiation was measured by combining (CFDA-SE) division tracking with phenotypic analysis. We define division tracking population statistics such as precursor cell frequency, generation time and renewal rate that characterize growth of various phenotypes in a heterogeneous culture system. This method is illustrated by study of the divisional recruitment of cord blood CD34(+) cells by hematopoietic growth factors. The technical issue of assigning the correct generation number to cells was addressed by employing high-resolution division tracking methodology and daily histogram analysis. We also quantified division-tracking artifacts such as CFDA-SE degeneration and cellular auto-fluorescence. Mitotic activation of cord blood CD34(+) cells by cytokines commenced after 2 days of cytokine stimulation. Mean generation number increased linearly thereafter, and it was conclusively shown that CD34(+) cells cycle slower than CD34(-) cells. Generation times for CD34(+) and CD34(-) cells were 24.7 +/- 0.8 h and 15.1 +/- 0.9 h (+/-SD, n = 5), respectively. The 20-fold increase in CD34(+) cell numbers at Day 6 could be attributed to a high CD34(+) cell renewal rate (91% +/- 2% per division). Although cultures were initiated with highly purified CD34(+) cells (approximately 96%), CD34(-) numbers had expanded rapidly by Day 6. This rapid expansion could be explained by their short generation time as well as a small fraction of CD34(+) cells (approximately 5%) that differentiated into CD34(-) cells. Multitype division tracking provides a detailed analysis of multipotent cell differentiation dynamics.

Expression of Fas and Fas-ligand in donor hematopoietic stem and progenitor cells is dissociated from the sensitivity to apoptosis

OBJECTIVE

The interaction between the Fas receptor and its cognate ligand (FasL) has been implicated in the mutual suppression of donor and host hematopoietic cells after transplantation. Following the observation of deficient early engraftment of Fas and FasL-defective donor cells and recipients, we determined the role of the Fas-FasL interaction.

METHODS

Donor cells were recovered after syngeneic (CD45.1-->CD45.2) transplants from various organs and assessed for expression of Fas/FasL in reference to lineage markers, carboxyfluorescein succinimidyl ester dilution, Sca-1 and c-kit expression. Naïve and bone marrow-homed cells were challenged for apoptosis ex vivo.

RESULTS

The Fas receptor and ligand were markedly upregulated to 40% to 60% (p < 0.001 vs 5-10% in naïve cells) within 2 days after syngeneic transplantation, while residual host cells displayed modest and delayed upregulation of these molecules ( approximately 10%). All lin(-)Sca(+)c-kit(+) cells were Fas(+)FasL(+), including 95% of Sca-1(+) and 30% of c-kit(+) cells. Fas and FasL expression varied in donor cells that homed to bone marrow, spleen, liver and lung, and was induced by interaction with the stroma, irradiation, cell cycling, and differentiation. Bone marrow-homed donor cells challenged with supralethal doses of FasL were insensitive to apoptosis (3.2% +/- 1% vs 38% +/- 5% in naïve bone marrow cells), and engraftment was not affected by pretransplantation exposure of donor cells to an apoptotic challenge with FasL.

CONCLUSION

There was no evidence of Fas-mediated suppression of donor and host cell activity after transplantation. Resistance to Fas-mediated apoptosis evolves as a functional characteristic of hematopoietic reconstituting stem and progenitor cells, providing them competitive engraftment advantage over committed progenitors.

Simulating mouse mammary gland development: cell ageing and its relation to stem and progenitor activity

BACKGROUND

Somatic stem and progenitor cell division is likely to be an important determinant of tumor development. Each division is accompanied by a risk of fixing genetic mutations, and/or generating innately immortal cells that escape normal physiological controls.

AIM

Using biological information, we aimed to devise a theoretical model for mammary gland development that described the effect of various stem/progenitor cells activities on the demographics of adult mammary epithelial cell populations.

RESULTS

We found that mammary ductal trees should develop in juvenile mice despite widely variant levels of activity in the progenitor compartment. Sequestration (inactivation) of progenitor cells dramatically affected the aging-maturation of the population without affecting the total regenerative capacity of the gland. Our results showed that if stem and progenitor cells can be demonstrated in glands regenerated by serial transplantation, they originated in a canonical primary stem cell (providing a functional definition of mammary stem cells). Finally, when the probability of symmetric division of stem cells increased above a threshold, the mammary epithelial population overall was immortal during serial transplantation.

CONCLUSIONS

This model provides, (1) a theoretical framework for testing whether the phenotypes of genetically modified mice (many of which are breast cancer models) derive from changes of stem and progenitor activity, and (2) a means to evaluate the resolving power of functional assays of regenerative capacity in mammary epithelial cell populations.

Estimating Lymphocyte Division and Death Rates from CFSE Data

The division tracking dye, carboxyfluorescin diacetate succinimidyl ester (CFSE) is currently the most informative labeling technique for characterizing the division history of cells in the immune system. Gett and Hodgkin [Nat. Immunol. 1:239-244, 2000] have pioneered the quantitative analysis of CFSE data. We confirm and extend their data analysis approach using simple mathematical models. We employ the extended Gett and Hodgkin [Nat. Immunol. 1:239-244, 2000] method to estimate the time to first division, the fraction of cells recruited into division, the cell cycle time, and the average death rate from CFSE data on T cells stimulated under different concentrations of IL-2. The same data is also fitted with a simple mathematical model that we derived by reformulating the numerical model of Deenick et al. [J. Immunol. 170:4963-4972, 2003]. By a non-linear fitting procedure we estimate parameter values and confidence intervals to identify the parameters that are influenced by the IL-2 concentration. We obtain a significantly better fit to the data when we assume that the T cell death rate depends on the number of divisions cells have completed. We provide an outlook on future work that involves extending the Deenick et al. [J. Immunol. 170:4963-4972, 2003] model into the classical smith-martin model, and into a model with arbitrary probability distributions for death and division through subsequent divisions.

Mathematical modelling of skeletal repair

Tissue engineering offers significant promise as a viable alternative to current clinical strategies for replacement of damaged tissue as a consequence of disease or trauma. Since mathematical modelling is a valuable tool in the analysis of complex systems, appropriate use of mathematical models has tremendous potential for advancing the understanding of the physical processes involved in such tissue reconstruction. In this review, the potential benefits, and limitations, of theoretical modelling in tissue engineering applications are examined with specific emphasis on tissue engineering of bone. A central tissue engineering approach is the in vivo implantation of a biomimetic scaffold seeded with an appropriate population of stem or progenitor cells. This review will therefore consider the theory behind a number of key factors affecting the success of such a strategy including: stem cell or progenitor population expansion and differentiation ex vivo; cell adhesion and migration, and the effective design of scaffolds; and delivery of nutrient to avascular structures. The focus will be on current work in this area, as well as on highlighting limitations and suggesting possible directions for future work to advance health-care for all.

Human reconstituting hematopoietic stem cells up-regulate Fas expression upon active cell cycling but remain resistant to Fas-induced suppression

The Fas receptor and its ligand have been implicated in mediating the bone marrow (BM) suppression observed in graft-versus-host disease and a number of other BM-failure syndromes. However, previous studies have suggested that Fas is probably not expressed on human hematopoietic stem cells (HSCs), but up-regulated as a consequence of their commitment and differentiation, suggesting that progenitors or differentiated blood cells, rather than HSCs, are the targets of Fas-mediated suppression. The present studies confirm that candidate HSCs in human cord blood and BM lack constitutive expression of Fas, but demonstrate that Fas expression on CD34+ progenitor and stem cells is correlated to their cell cycle and activation status. With the use of recently developed in vitro conditions promoting HSC self-renewing divisions, Fas was up-regulated on virtually all HSCs capable of multilineage reconstituting nonobese diabetic/severe combined immunodeficiency (NOD-SCID) mice in vivo, as well as on long-term culture-initiating cells (LTC-ICs). Similarly, in vivo cycling of NOD-SCID repopulating cells upon transplantation, resulted in up-regulation of Fas expression. However, repopulating HSCs expressing high levels of Fas remained highly resistant to Fas-mediated suppression, and HSC function was compromised only upon coactivation with tumor necrosis factor. Thus, reconstituting human HSCs up-regulate Fas expression upon active cycling, demonstrating that HSCs could be targets for Fas-mediated BM suppression.

Analysis of cell kinetics using a cell division marker: mathematical modeling of experimental data

We consider an age-maturity structured model arising from a blood cell proliferation problem. This model is "hybrid", i.e., continuous in time and age but the maturity variable is discrete. This is due to the fact that we include the cell division marker carboxyfluorescein diacetate succinimidyl ester. We use our mathematical analysis in conjunction with experimental data taken from the division analysis of primitive murine bone marrow cells to characterize the maturation/proliferation process. Cell cycle parameters such as proliferative rate beta, cell cycle duration tau, apoptosis rate gamma, and loss rate micro can be evaluated from CarboxyFluorescein diacetate Succinimidyl Ester + cell tracking experiments. Our results indicate that after three days in vitro, primitive murine bone marrow cells have parameters beta = 2.2 day(-1), tau = 0.3 day, gamma = 0.3 day(-1), and micro = 0.05 day(-1).

Cell cycle modulation of gene targeting by a triple helix-forming oligonucleotide

Successful gene-targeting reagents must be functional under physiological conditions and must bind chromosomal target sequences embedded in chromatin. Triple helix-forming oligonucleotides (TFOs) recognize and bind specific sequences via the major groove of duplex DNA and may have potential for gene targeting in vivo. We have constructed chemically modified, psoralen-linked TFOs that mediate site-specific mutagenesis of a chromosomal gene in living cells. Here we show that targeting efficiency is sensitive to the biology of the cell, specifically, cell cycle status. Targeted mutagenesis was variable across the cycle with the greatest activity in S phase. This was the result of differential TFO binding as measured by cross-link formation. Targeted cross-linking was low in quiescent cells but substantially enhanced in S phase cells with adducts in approximately 20-30% of target sequences. 75-80% of adducts were repaired faithfully, whereas the remaining adducts were converted into mutations (>5% mutation frequency). Clones with mutations could be recovered by direct screening of colonies chosen at random. These results demonstrate high frequency target binding and target mutagenesis by TFOs in living cells. Successful protocols for TFO-mediated manipulation of chromosomal sequences are likely to reflect a combination of appropriate oligonucleotide chemistry and manipulation of the cell biology.