Population balance model of in vivo neutrophil formation following bone marrow rescue therapy

Population Balance Modeling: Current Status and Future Prospects

Population balance modeling is undergoing phenomenal growth in its applications, and this growth is accompanied by multifarious reviews. This review aims to fortify the model's fundamental base, as well as point to a variety of new applications, including modeling of crystal morphology, cell growth and differentiation, gene regulatory processes, and transfer of drug resistance. This is accomplished by presenting the many faces of population balance equations that arise in the foregoing applications.

Deconstructing stem cell population heterogeneity: single-cell analysis and modeling approaches

Isogenic stem cell populations display cell-to-cell variations in a multitude of attributes including gene or protein expression, epigenetic state, morphology, proliferation and proclivity for differentiation. The origins of the observed heterogeneity and its roles in the maintenance of pluripotency and the lineage specification of stem cells remain unclear. Addressing pertinent questions will require the employment of single-cell analysis methods as traditional cell biochemical and biomolecular assays yield mostly population-average data. In addition to time-lapse microscopy and flow cytometry, recent advances in single-cell genomic, transcriptomic and proteomic profiling are reviewed. The application of multiple displacement amplification, next generation sequencing, mass cytometry and spectrometry to stem cell systems is expected to provide a wealth of information affording unprecedented levels of multiparametric characterization of cell ensembles under defined conditions promoting pluripotency or commitment. Establishing connections between single-cell analysis information and the observed phenotypes will also require suitable mathematical models. Stem cell self-renewal and differentiation are orchestrated by the coordinated regulation of subcellular, intercellular and niche-wide processes spanning multiple time scales. Here, we discuss different modeling approaches and challenges arising from their application to stem cell populations. Integrating single-cell analysis with computational methods will fill gaps in our knowledge about the functions of heterogeneity in stem cell physiology. This combination will also aid the rational design of efficient differentiation and reprogramming strategies as well as bioprocesses for the production of clinically valuable stem cell derivatives.

Stem cell modeling: From gene networks to cell populations

Despite rapid advances in the field of stem/progenitor cells through experimental studies, relevant modeling approaches have not progressed with a similar pace. Various models have focused on particular aspects of stem cell physiology including gene regulatory networks, gene expression noise and signaling cascades activated by exogenous factors. However, the self-renewal and differentiation of stem cells is driven by the coordinated regulation of events at the subcellular, intercellular and milieu levels. Such events also span multiple time domains from the fast molecular reactions governing gene expression to the slower cell cycle and division. Thus, the development of multiscale computational frameworks for stem cell populations is highly desirable. Multiscale models are expected to aid the design of efficient differentiation strategies and bioprocesses for the generation of therapeutically useful stem cell progeny. Yet, challenges in making these models tractable and pairing those to sufficient experimental data prevent their wide adoption by the stem cell community. Here, we review modeling approaches reported for stem cell populations and associated hurdles.

Scalable stirred-suspension bioreactor culture of human pluripotent stem cells

Advances in stem cell biology have afforded promising results for the generation of various cell types for therapies against devastating diseases. However, a prerequisite for realizing the therapeutic potential of stem cells is the development of bioprocesses for the production of stem cell progeny in quantities that satisfy clinical demands. Recent reports on the expansion and directed differentiation of human embryonic stem cells (hESCs) in scalable stirred-suspension bioreactors (SSBs) demonstrated that large-scale production of therapeutically useful hESC progeny is feasible with current state-of-the-art culture technologies. Stem cells have been cultured in SSBs as aggregates, in microcarrier suspension and after encapsulation. The various modes in which SSBs can be employed for the cultivation of hESCs and human induced pluripotent stem cells (hiPSCs) are described. To that end, this is the first account of hiPSC cultivation in a microcarrier stirred-suspension system. Given that cultured stem cells and their differentiated progeny are the actual products used in tissue engineering and cell therapies, the impact of bioreactor's operating conditions on stem cell self-renewal and commitment should be considered. The effects of variables specific to SSB operation on stem cell physiology are discussed. Finally, major challenges are presented which remain to be addressed before the mainstream use of SSBs for the large-scale culture of hESCs and hiPSCs.

A mathematical model for reconstitution of granulopoiesis after high dose chemotherapy with autologous stem cell transplantation

High dose chemotherapy supported with hematopoietic progenitor cells gives a characteristic neutropenic period (blood neutrophils < 0.5 x 10(9) c/l) ranging from 10 to 16 days. The question of a correlation between the CFU-GM content of the transplanted CD34+ cells and time to neutrophil recovery by patients having been given high-dose chemotherapy (HD-CT) with stem cell support was addressed by means of a mathematical model of granulopoiesis. The model utilizes a convection-reaction partial differential equation (PDE) with feedback from a cytokine compartment on proliferation, maturation, and mobilization of granulocytes from bone marrow to blood. The observed number of CFU-GM cells in the transplanted CD34+ cell autograft was used as input to the model. Using this approach, the observed gross relationship between CFU-GM content in the reinfused blood product and engraftment time could be reproduced. At the same time, the effects of assumed physiological mechanisms, especially some of the effects of G-CSF on proliferation rate, maturation rate, mobilization, and cell death, could be investigated and discussed relative to observed engraftment. The model makes it possible to explain how cytokines interfere with progenitor cell mobilization from bone marrow to blood, and it points out the implications of a regulating mechanism for the granulocyte maturation rate.

Cell density-dependent proliferation in frequently-fed peripheral blood mononuclear cell cultures

BACKGROUND

Our goal was to produce granulocyte progenitor (CFU-G) and post-progenitor (CD15(+)CD11b(+/-)) cells for subsequent transplantation. We hypothesized that increasing the feeding frequency and maintaining constant densities may overcome inhibitory growth conditions (i.e. low pH) in high-density cultures.

METHODS

To study the effect of cell density on total cell expansion, differentiation and lactate production, 50% daily medium exchanges were used in cultures of peripheral blood mononuclear cells (PB MNC) maintained at constant densities (ranging from 5 x 10(4)cells/mL to 2.5 x 10(6)cells/mL).

RESULTS

We observed a significant increase in total cell expansion when the density was increased from 5 x 10(4) cells/mL to 1 x 10(6) cells/mL, but a further increase to 2.5 x 10(6)cells/mL resulted in a decline in cell expansion. Increasing feeding to 90% daily exchange in cultures with 2.5 x 10(6) cells/mL did not enhance cell expansion; nor did reducing the extent of feeding in cultures with 5 x 10(4) cells/mL to 10% daily exchange. We did not observe a relationship between cell density and the percentage of granulocyte progenitor and post-progenitor (CD15(+)CD11b(-/+)) cells. While specific lactate production (q(lac)) in cultures with 2.5 x 10(6) cells/mL was approximately 60% of those observed in lower density cultures by Day 13, this difference was largely eliminated by increasing the extent of feeding in cultures with 2.5 x 10(6) cells/mL.

DISCUSSION

Our results suggest that feeding rates must be adjusted according to cell density to maximize culture performance. They also suggest that cellular crowding on the culture surface can limit expansion in suspension (nonadherent) cultures.

Clinical-scale production of granulocyte progenitor and post-progenitor cells using daniplestim, leridistim, Progenipoietin, Promegapoietin and autologous plasma

BACKGROUND

Supplementation of PBPC autografts with ex vivo expanded PBMC may significantly reduce or eliminate the period of neutropenia associated with high-dose chemotherapy.

METHODS

Unmanipulated growth-factor mobilized PBMC were expanded in media containing daniplestim, leridistim, Promegapoietin, and Progenipoietin (DLPP) and 2% autologous plasma at 4 x 10(5) PBMC/mL, first in 25 cm(2) T-flasks, with sampling on Days 7, 10, 13 and 15, and then in 1264 cm(2) Nunclon Cell Factories, with sampling on Days 7 and 13.

RESULTS

In T25-flasks, maximal CFU-GM expansion ([38.2 +/- 9.5]-fold) occurred on Day 10, whereas maximal total cell expansion ([6.7 +/- 1.1]-fold) occurred on Day 15. Production of CD15(+)CD11b(-) and CD15(+)CD11b(+) granulocytic post-progenitors (3.0 +/- 0.4 x 10(6) and 3.7 +/- 0.9 x 10(6), respectively) was also maximal at Day 15. Compared with the previously studied combination of Flt3L, PIXY321, G-CSF, GM-CSF and Epo, the DLPP cocktail performed similarly, with the exception of yielding larger GM colonies at Day 10 and fewer granulocyte post-progenitors on Day 15. In Cell Factories, CFU-GM were expanded (31.6 +/- 14.5)-fold, while total nonadherent cells were expanded (2.6 +/- 0.5)-fold. The two stack Cell Factory cultures seeded with 1.0 x 10(8) unselected PBMC produced approximately 3.3 x 10(6) CFU-GM and 1.3 x 10(8) myeloid post-progenitors.

DISCUSSION

Whereas expansion of cell numbers, CFU-GM and granulocytic post-progenitors in Cell Factories mirrored that achieved in T25-flasks, future preclinical studies with the DLPP cytokine combination may be performed in small volumes, with subsequent translation to the larger volume Cell Factories. Sufficient expansion can be achieved using the DLPP cytokine combination in the Cell Factories to provide the numbers of progenitors required for clinical trials.

Dynamic model of ex vivo granulocytic kinetics to examine the effects of oxygen tension, pH, and interleukin-3

OBJECTIVE

Evaluating kinetics in hematopoietic cultures is complicated by the distribution of cells over various stages of differentiation and by the presence of cells from different lineages. Thus, an observed response is an integral response from distributed cell populations. Growth factors and other parameters can greatly affect the lineage and maturation stage of the culture outcome. To resolve the kinetics and more clearly define the differential effects of O(2) tension (pO(2)), pH, and interleukin-3 (IL-3) on granulopoiesis, a mathematical model-based approach was undertaken.

MATERIALS AND METHODS

Granulocytic differentiation is described within a continuous, deterministic framework in which cells develop from primitive granulocytic progenitors to mature neutrophils. The model predicts two distributed populations-quiescent and cycling cells-by incorporating rates of growth, death, differentiation, and transition between quiescence and active cycling. The response of these four model processes to changes in the culture environment was examined.

RESULTS

Model simulations of experimental data revealed the following: 1) pO(2) effects are exerted only on the growth rate but not maturation times. 2) pH effects between pH 7.25 and 7.4 on growth and differentiation are coupled; however, with increasing pH values, especially at pH 7. 6, the death rate for cells in the early stages of differentiation becomes increasingly significant. 3) The absence of IL-3 increases the death rate for primitive cells only minimally but markedly enhances the rate of differentiation through the myeloblast window in the differentiation pathway. The combined effects of these environmental factors can be predicted based on changes in the model parameters derived from the individual effects.

CONCLUSIONS

Experimental data combined with mathematical modeling can elucidate the mechanisms underlying the regulation of granulopoiesis by pO(2), pH, and IL-3. The model also can be readily adapted to evaluate the effects of other culture conditions. The increased understanding of experimental results gained with this approach can be used to modify culture conditions to optimize ex vivo production of neutrophil precursors.

Physiologically significant effects of pH and oxygen tension on granulopoiesis

OBJECTIVE

Granulocyte differentiation in the bone marrow (BM) takes place in regions with lower pH and O(2) tension (pO(2)) than those in the BM sinuses. This suggests that granulopoiesis will be enhanced at subvascular pH and pO(2).

MATERIALS AND METHODS

The effects of pH AND pO2 on granulocyte proliferation, differentiation, and granulocyte colony-stimulating factor receptor (G-CSFR) expression were evaluated using mobilized peripheral blood CD34(+) cells directed down the granulocytic pathway with stem cell factor, interleukin 3, interleukin 6, and G-CSF.

RESULTS

Cell expansion was enhanced at subvascular pH, with twice as many total cells and CD15(bright)/CD11b(+) late neutrophil precursors (myelocytes, metamyelocytes, bands) produced at pH 7.07 to 7.21 as was produced at pH 7.38. Low pH accelerated the rate of differentiation concomitant with this increase in proliferation. Also, total, CD15(bright)/CD11b(-) (promyelocytes, early myelocytes), and CD15(bright)/CD11b(+) cell expansion was enhanced at lower pO(2), with twice as many of each cell type produced at 5% O(2) as at 20% O(2). The effects of low pH and low pO(2) were additive, such that generation of total, CD15(bright)/CD11b(-), and CD15(bright)/CD11b(+) cells was 3.5-, 2.4-, and 4.0-fold greater at pH 7.21 and 5% O(2) than at the standard hematopoietic culture conditions of pH 7.38 and 20% O(2). Low pH resulted in faster upregulation of G-CSFR surface expression, whereas pO(2) had no effect on G-CSFR expression.

CONCLUSION

These data provide compelling evidence that pH and pO(2) gradients within the BM play significant roles in regulating hematopoiesis. More rapid granulocytic cell proliferation and differentiation at low pH may be explained in part by more rapid G-CSFR expression. The ability to alter cell development by manipulating pH and pO(2) has important implications for optimizing ex vivo production of neutrophil precursors.