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Stem Cells 2004;22:135-143 www.StemCells.com
© 2004 AlphaMed Press

Fast But Durable Megakaryocyte Repopulation and Platelet Production in NOD/SCID Mice Transplanted with Ex-Vivo Expanded Human Cord Blood CD34+ Cells

Stefania Brunoa,b, Monica Gunettia,b, Loretta Gammaitonia,b, Eliana Perissinottoa,b, Luisa Caionea,b, Fiorella Sanavioa, Franca Fagiolic, Massimo Agliettaa,b, Wanda Piacibelloa,b

a Department of Oncological Sciences, University of Torino Medical School,
b Division of Clinical Oncology, Institute for Cancer Research and Treatment, Candiolo, Torino, Italy;
c The Pediatric Department, University of Torino Medical School, Torino, Italy

Key Words. Cord blood • CD34+ cell expansion • NOD/SCID • Megakaryocyte engraftment

Wanda Piacibello, M.D., University of Torino Medical School, Department of Oncological Sciences, Institute for Cancer Research and Treatment, Laboratory of Clinical Oncology, Prov. 142, 10060 Candiolo, Torino, Italy. Telephone: 39-011-9933349; Fax: 39-011-9933522; e-mail: wanda.piacibello{at}ircc.it


   ABSTRACT
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We have previously established a stroma-free culture with Flt-3 ligand (FL), stem cell factor (SCF), and thrombopoietin (TPO) that allows the maintenance and the expansion for several weeks of a cord blood (CB) CD34+ cell population capable of multilineage and long-lasting hematopoietic repopulation in non-obese diabetic/ severe combined immunodeficient (NOD/SCID) mice.

In this work the kinetics of megakarocyte (Mk)-engraftment that is often poor and delayed in CB transplantation, and human platelet (HuPlt) generation in NOD/SCID mice of baseline CD34+ cells (b34+), and of CD34+ cells reisolated after a 4-week expansion with FL+SCF+TPO (4w34+) were compared.

With b34+ cells Mk-engraftment was first seen at week 3 (CD41+: 0.4%); 4w34+ cells allowed a more rapid Mk-engraftment (at weeks 2 and 3 the CD41+ cells were 0.3% and 0.8%). Circulating HuPlts were first seen at weeks 2 and 1, respectively.

Mk-engraftment levels of b34+ and 4w34+ cells 6–8 weeks after transplantation were similar (12 ± 3.5 versus 15 ± 5% CD45+; 1.3 ± 0.5 versus 1.8 ± 0.5% CD41+ cells). Also serial transplant experiments were performed with expanded and reselected CB cells. In secondary and tertiary recipients the Mk population was detected with bone marrow fluorescence-activated cell sorter analysis; these experiments indicate the effective long-term repopulation of expanded cells. Selected CD34+ cells after a 4-week expansion with FL+SCF+TPO are more efficient in Mk engraftment than the same number of unmanipulated cells.


   INTRODUCTION
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The numbers of hematopoietic progenitors and stem cells in cord blood (CB) may be enough to support engraftment in children, but their ex-vivo expansion might be required to successfully engraft an adult. Moreover, a long-lasting severe post-transplant thrombocytopenia is often observed even in pediatric patients [1–8].

Therefore, two important aspects of the biology of ex-vivo expanded cells relate to cultured cells: either maintaining their self-renewal capacity and multilineage differentiation potential, or improving their short-term engraftment ability when transplanted into myeloablated recipients. Several growth factor combinations have been tested to identify suitable culture conditions to induce expansion of primitive stem cells (SCs). So far, only a few studies have shown that primitive non-obese diabetic severe combined immunodeficient (NOD/SCID) mouse repopulating stem cells from CB can be expanded (a few or several-fold) after in vitro culture [9–13].

In CB transplants, the megakaryocyte (Mk) lineage takes the longest time to engraft. However, to date, if only a few experimental studies have addressed the issue of the short-term engraftment ability of fresh CB SCs, even fewer have addressed that of ex-vivo expanded SCs [14–16]. Using the NOD/SCID mouse model, the short-term as well as the long-term repopulating ability and the differentiation and maturation potential of human hematopoietic lineages in an in vivo experimental model can be analyzed [17, 18].

Thus, by means of this in vivo model we set up experiments to evaluate the Mk lineage reconstitution ability and functional platelet release by baseline CB CD34+ cells (b34+) and CB CD34+ cells reisolated after a 4-week expansion (4w34+) in the presence of Flt-3 ligand (FL), thrombopoietin (TPO), and stem cell factor (SCF).


   MATERIALS AND METHODS
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 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Recombinant Human Cytokines
The following recombinant purified human cytokines were used in these studies: recombinant human (rh) TPO, a generous gift from Kirin Brewery (Tokyo, Japan; http://www.kirin.co.jp/english/), rh interleukin-6 (IL-6) (PeproTech Inc.; Rocky Hill, NJ; http://www.peprotech.com), rhIL-3 (Sandoz; Basel, Switzerland; http://www.sandoz.com), rhFL kindly provided by S.D. Lyman (Immunex Corp; Seattle, WA; http://www.immunex.com), and rhSCF (a gift from Amgen; Thousand Oaks, CA; http://www.amgen.com).

Human Cells
Umbilical CB was obtained following informed written consent at the end of full-term deliveries, by clamping and cutting the cord and draining blood into sterile collection tubes containing the anticoagulant citrate-phosphate dextrose.

CD34+ Cell Purification and Culture
Mononuclear cells were isolated from CB using Ficoll Hypaque (density 1.077 g/cm3; [Nyegaard; Oslo, Norway; http://www.amershamhealth.com]) density centrifugation. The CD34+ fraction was isolated with superparamagnetic microbead selection using high-gradient magnetic field and MiniMACS column (Miltenyi Biotech; Gladbach, Germany; http://www.miltenyibiotech.com). The efficiency of the purification was verified by flow cytometry counter staining with a CD34-phycoerythrin (PE) HPCA-2 antibody (Becton Dickinson; San Jose, CA; http://www.bd.com). In the cell fraction containing purified cells, the percentage of CD34+ cells ranged from 90%-98%. These cells were denominated b34+.

Long-Term Expansion Cultures for Primitive Repopulating Cells
Stroma-free expansion cultures were performed as previously described [11, 19, 20]. Briefly, CB CD34+ cells at 5 x 104/ml in Iscove’s modified Dulbecco’s medium (GIBCO) with 10% fetal calf serum (HyClone; Logan, UT; http://www.hyclone.com) were inoculated into tissue culture T75 flasks with FL (50 ng/ml), SCF (50 ng/ml), and TPO (10 ng/ml). Growth factors were added at the start of cultures and then twice a week. Each week the cells were counted and the same volume of fresh medium plus growth factors was added. After 4 weeks all cells were harvested, washed, counted, and then subjected to immunoselection with MiniMACS columns as described above, to obtain CD34+ populations (denominated 4w34+) to be injected into sublethally irradiated NOD/SCID mice. 4w34+ cells were 96%-98.7% pure; only 1.5%-2.8% of these cells were CD34+/CD41+.

Injection of Cells in NOD/SCID Mice
NOD/LtSz scid/scid NOD/SCID mice were purchased from The Jackson Laboratory (Bar Harbor, ME; http://www.jax.org) and maintained in the Centro di Immunologia ed Oncologia Sperimentale animal facilities (Torino, Italy). All animals were handled under sterile conditions and maintained in cage microisolators. Sublethally irradiated (350 cGy of total body irradiation from a 137Cs source) 6- to 8-week-old mice were injected in the tail vein with 2.5 x 105 b34+ or 2.5 x 105 4w34+ cells. The mice were sacrificed 1, 2, 3, and 4 weeks (short-term reconstitution) post-transplant or after a longer period of time (6 to 8 weeks post injection), and bone marrow (BM) cells were flushed from femurs and tibias using a syringe and 26-gauge needle to assess the number and types of human cells.

The appearance of human platelets (HuPlts) in murine peripheral blood (PB) 7, 14, 21, and 28 days after injection was also assessed as described [21]. No growth factors were administered to the animals.

Serial transplant experiments were performed as previously described [22]. Briefly, 20 to 40 x 106 unseparated BM cells from a primary or a secondary mouse were injected i.v. into a single sublethally irradiated secondary or tertiary NOD/SCID mouse. Secondary and tertiary mice were sacrificed 6 weeks post injection, and BM cells were harvested and processed as described.

Analysis of Murine BM
Flow cytometry was used to analyze the levels of human cells in the BM of the mice; the cells were resuspended at 1 to 2 x 106 cells/ml and incubated with mouse immunoglobulin G (Fluka Chemika Biochemika; Buchs, Switzerland; http://www.wiz.uni-kassel.de) to block non-specific binding to the Fc receptor. Cells were then incubated with fluorescein isothiocyanate (FITC) or PE-labeled monoclonal antibody (mAb) specific for human CD for 30 minutes at 4°C to quantify human hematopoietic cells. Some cells from each suspension were similarly incubated with isotype control mAbs labeled with FITC and PE (CALTAG Laboratory; Burlingame, CA; http://www.caltag.com). After staining the cells were washed once in phosphate-buffered saline with 0.1% bovine serum albumin and 0.01% sodium azide. Contaminating RBCs were lysed with EDTA 10-4 mol/l, KHCO3 10-3 mol/l, and NH4CL 0.17 mol/l. Flow cytometric analysis was performed using a FACSVantage cytometer (Becton Dickinson). At least 20,000 events were acquired for each analysis. Analysis was performed with CellQuest software (Becton Dickinson).

The antibodies used were FITC-labeled antihuman CD41, CD42 (DAKO; Glostrup, Denmark; http://www.dakocytomation.com), and PE-labeled antihuman CD14 (DAKO), CD19 (CALTAG Laboratory), CD34 (Becton Dickinson), CD71 (DAKO), and anti-GpA (DAKO). CD45 TRI-COLOR conjugated (CALTAG Laboratory) was also used.

For human colony assays, 1 to 5 x 105 BM cells, according to the levels of human engraftment, were plated in plasma clot assays as previously reported [11] by substituting bovine plasma with pooled human AB plasma. Human GM and erythroid colonies were enumerated after 14 days of incubation at 37°C in a fully humidified atmosphere at 5% CO2 from triplicate dishes containing erythropoietin (3 U/ml), SCF (50 ng/ml), GM-CSF (20 ng/ml), and IL-3 (5 ng/ml). Colony-forming units (CFU-Mk) were enumerated after 12 days of cultures from triplicate dishes at the immunofluorescence microscope after staining with an FITC-conjugated mAb recognizing human GP IIb/IIIa (CD41) [23, 24]. In the described culture conditions only human colonies could be detected. At the various time points BM cells from irradiated and not transplanted NOD/SCID mice were cultured, but in no case could human colonies be detected.

Human Platelet Detection in NOD/SCID Mouse Peripheral Blood
Platelet appearance in murine PB 7, 14, 21, and 28 days after injection was also assessed as described [21]. Briefly, murine PB (10 µl) was incubated at room temperature with FITC-conjugated anti-mouse CD41 (Becton Dickinson) and PE-conjugated anti-human CD41 (DAKO) or isotype control for 5 minutes and analyzed immediately by flow cytometry. Fifty-thousand events were acquired with a primary gate set on a dual parameter histogram of log forward light scatter and log side light scatter. Background fluorescence was assessed with platelets labeled with the FITC- and PE-conjugated isotype control antibody. PB from untransplanted mice and from a human donor was analyzed as additional controls. A FACSVantage flow cytometry (Becton Dickinson) was used for acquisition of platelet data, and analysis was performed using CellQuest software (Becton Dickinson).

Activation of Human Platelets by Thrombin
Aliquots of mouse PB (10 µl) were incubated with thrombin (at a final concentration of 50 U/ml) for 10 minutes. After this incubation the platelet CD62P (CALTAG Laboratory) expression was assessed by flow cytometry. Live acquisition of 1,000 to 2,000 HuPlts events was performed by gating human CD41PE+ events in the platelet size range.

Statistical Analysis
Results of experimental points obtained from multiple experiments are reported as the mean ± standard deviation (SD). The significance of differences in mean value was determined by using the Student’s t-test.


   RESULTS
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Megakaryocyte Reconstitution in NOD/SCID Mice by Baseline and Expanded CD34+ Cells (b34+ and 4w34+)
To evaluate short-term Mk-engraftment, 2.5 x 105 b34+ cells were injected into cohorts of sublethally irradiated NOD/SCID mice that were sacrificed 1, 2, and 3 weeks after inoculation. BM cells of the sacrificed animals were obtained from both femurs and tibias and assessed for the presence of human hematopoietic cells. Human cells in the murine BM were quantified for myeloid, lymphoid, erythroid, and Mk lineage antigen expression. Fluorescence-activated cell sorter (FACS) analysis of the BM of the sacrificed animals showed that at week 1 post-transplant levels of human CD45+ cells were low but clearly detectable (3.7 ± 2.8). Within the human cells, the most represented were the CD34+ and the CD19+ subpopulations (Table 1Go). Erythroid cells, identified by CD71 and GpA expression were also present. Surprisingly, no cells belonging to the Mk lineage could be found. At week 2 post injection, the levels of human CD45+ cells were quite similar. At week 3, human cell engraftment further increased. Only at this time point the Mk subpopulation, although at very low percentages, became detectable (Table 1Go).


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  		Table 1. Short-term engraftment of NOD/SCID mice transplanted with fresh (b34+) or ex vivo expanded (4w34+) cells
 
To evaluate the short-term Mk-engraftment capacity of more expanded primitive cells, in five different experiments, 2.5 x 105 CD34+ cells were cultured in triplicate stroma-free liquid cultures in the presence of FL, SCF, and TPO as described [11]. After 4 weeks of expansion they yielded a mean of 48.5 ± 2.4 x 106 total cells that included 1.9 ± 0.43 x 106 CD34+ cells. Following immunoselection, only 2.5 x 105 CD34+ (4w34+) cells were injected in each mouse. In these mice the levels of human engraftment at week 1 were similar to those found in b34+-transplanted mice and increased with time. Here the growth of some CFU-Mk colonies could be achieved. CFU-Mk number increased at weeks 2 and 3 (Table 1Go). By contrast, in mice injected with b34+ cells, no CFU-Mk colonies could be found at week 1. Only at week 3 were there as many Mk colonies as those found much earlier (week 1) in 4w34+ transplanted mice.

Long-term Mk-engraftment was evaluated in NOD/SCID mice sacrificed 6–8 weeks after inoculation. Table 2Go shows the mean engraftment level of ten mice injected with 2.5 x 105 b34+ cells. Flow cytometry analysis showed that the human cells belonged to all hematopoietic lineages; cells of the Mk lineage were found in all mice. CFU-Mk colonies were detected in plasma clot cultures seeded with the BM cells of the transplanted animals (Table 2Go).


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  		Table 2. Six- to 8-week engraftment of NOD/SCID mice transplanted with b34+ and 4w34+ CB cells
 
The results of injection of 2.5 x 105 4w34+ cells are reported in Table 2Go. All the mice, after 6–8 weeks from the injection, were successfully engrafted (10/10). The mean engraftment level of the ten mice injected with this cell dose was 15 ± 5%. FACS analysis showed that human cells belonged to all hematopoietic lineages (not shown).

Moreover, five mice were transplanted with all of the CD34+ cells generated by initial 2.5 x 105 CD34+ cells during a 4-week expansion. At 6–8 weeks post-transplant the BM engraftment levels were very high (79 ± 11.4) and the Mk population was well-represented (3.6 ± 0.4% of the total BM).

The presence of human CFU-Mk in the BM of mice was evaluated. Overall, 1,900 ± 302, 2,200 ± 159 and 8,259 ± 1,102 human Mk colonies were detected in mice transplanted with b34+ and the two concentrations of 4w34+ cells, respectively.

To evaluate the effective long-term Mk-engraftment of expanded cells, the unseparated BM cells of three primary mice, harvested 6 weeks after injection of 2.5 x 105 4w34+ cells, were transplanted in three secondary and subsequently in three tertiary sublethally irradiated recipients. In these experiments of serial transplants, mice were successfully engrafted, and the Mk population was well-represented [22] (Fig. 1Go).



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  		Figure 1. Human megakaryocyte engraftment in serial transplant NOD/SCID mice. FACS profile of marrow cells from a representative NOD/SCID mouse that 6 weeks earlier was transplanted with 2.5 x 105 4wCD34+ (CD34+cells immunoselected after 4 weeks of expansion in presence of FL, SCF, and TPO). The BM of this primary mouse was injected into a secondary sublethally irradiated NOD/SCID mouse sacrificed 6 weeks after transplantation; the BM of this mouse was injected in a tertiary mouse also sacrificed 6 weeks after transplantation. Human CD45+ cells represented 22% of the BM cells of the primary mouse, 7% of the secondary recipient that had received 25 x 106 unseparated BM cells of the primary mouse, and 2.5% of the tertiary mouse that received 30 x 106 total BM cells of the secondary recipient. FACS analyses of human CD41 expression in the BM of primary, secondary, and tertiary mice were performed on total BM: the percentages of CD41+ cells were respectively: 1.3%, 0.6%, and 0.1%. The bottom panels represent the analysis of the CD41+ population within the CD45+ cell gate in each of the three recipients.

 
Platelet Production in NOD/SCID Mice
The appearance of HuPlts in the mouse PB after CB injection was monitored from week 1 by FACS analyses. After total body irradiation murine platelets decreased from a mean of 1.45 ± 0.3 x 1012/l to a mean of 0.45 ± 0.1 x 1012/l at week 1 and increased to a mean of 0.8 ± 0.2 x 1012/l at week 3.

HuPlts were detected by staining PB cells with an anti-CD41 mAb against HuPlts surface GP IIb/IIIa. After transplantation of b34+ cells, a maximum of 0.3% HuPlts was detected only at week 3 (Fig. 2Go). At week 4 HuPlt count was a mean of 4.5%; the percentage of the HuPlts was similar at 6–8 weeks after transplant in some transplanted mice. By contrast in the PB of mice injected with 2.5 x 105 4w34+ cells, 0.5% of HuPlts were seen as early as week 1, even if the human CD41+ cells in the murine BM were below the FACS detection limit (<0.1%). Plts were a mean of 0.7% at week 2 and 12% at week 3 (Fig. 2Go and Fig. 3AGo). The HuPlts persisted in murine PB even 6–8 weeks after transplant. When the mice were transplanted with all of the 4w34+ cells generated by initial 2.5 x 105 CD34+ cells, platelet levels at week 1 were very high (6.8%) (Fig. 2Go).



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  		Figure 2. Human platelet appearance. Kinetics of HuPlt appearance in the PB of NOD/SCID mice injected with 2.5 x 105 baseline CD34+ cells, 2.5 x 105 CD34+ cells immunoselected from 4-week expanded cultures and with all the CD34+ progeny of 2.5 x 105 initial CD34+ cells expanded for 4 weeks. Results show the mean ± SD of the percentage of HuPlts detected by FACS analysis in the murine PB at the indicated time points after transplantation (4 mice per experimental point, 3 separate experiments).

 


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  		Figure 3. Flow cytometric analysis of human platelets in peripheral blood of NOD/SCID mice. A) PB samples (from a representative untransplanted mouse, a normal human donor, and a mouse injected 3 weeks previously with 2.5 x 105 4w34+ cells) were labeled with a mAb against human CD41a. Analysis was performed within the platelet population gate based upon forward and side scatter. B) Flow cytometric analysis of HuPlts from a mouse transplanted 3 weeks previously with 2.5 x 105 4w34+ cells before and after thrombin stimulation. On the y-axis PE-anti-human CD41a, on the x-axis FITC-anti-human CD62P. CD62P is expressed only on activated platelets. FACS analysis is carried out within the gate of HuPlts.

 
HuPlt activation in response to in vitro challenge with thrombin was tested. Thrombin induces granule secretion resulting in CD62P expression on the platelet membrane. After thrombin stimulation, the expression of CD62P on human CD41+ platelets was increased (Fig. 3BGo).


   DISCUSSION
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The xenogenic NOD/SCID mouse model offers one of the best approaches for investigating the in vivo repopulating ability of human hematopoietic tissues [14–16, 25–28]. In such a model we previously demonstrated that CB CD34+ cells could be expanded for several weeks in stroma-free cultures containing FL, TPO, and SCF and that expanded cells maintained and increased their in vivo repopulating capacity [11, 12, 22]. Moreover, we have previously demonstrated that basal and expanded CD34+ cells, subsequently "committed" in vitro toward the Mk lineage, gave a rapid and transient Mk engraftment [29].

In this work CB CD34+ cells, isolated after 4 weeks of expansion (4w34+ cells), were injected in NOD/SCID mouse recipients. These experiments show that purified 4w34+ cells retain their capacity to provide long-term support of the megakaryocytopoiesis in the BM of several generations of sublethally irradiated NOD/SCID mice. The levels of human hematopoietic reconstitution were similar in mice transplanted with same numbers of b34+ and 4w34+ CB cells (or slightly higher with the expanded cells). As already reported, all hematopoietic lineages including the Mk, were found in the NOD/SCID mice BM at least 6 to 8 weeks post-transplant of expanded cells [11]. The Mk lineage was represented by CD41+ cells and by more immature CFU-Mk.

Although previous studies reported that human CB, BM, and PB could generate human CD41+ cells in NOD/SCID mice [16, 29], only a few papers have provided evidence for human Mk development and terminal differentiation into functional HuPlts with human CD34+ PB and CB cells [14, 15]. To our knowledge this research is the first that shows expanded CB cells are better than unmanipulated cells in terms of Mk short-term engraftment and terminal Mk maturation (platelets production was already found 1 week after the injection of ex vivo expanded CD34+ cells).

In fact, these studies indicate that short-term engraftment is achieved with both baseline and expanded CD34+ cells. Furthermore, while Mk reconstitution is slower with b34+ cells (a few Mk cells and CFU-Mk appear in the BM, and platelets are found in the PB only at week 3 and 2 post-transplant, respectively), Mk-engraftment by purified 4w34+ cells is detectable earlier. The speed and degree of Mk-engraftment by expanded cells are higher in the latter case. Terminal differentiation of Mk progenitors and precursors is also achieved in this case, as HuPlts are well detected in the PB from week 1 on. Probably at 1 week post-injection in the BM of 4w34+ transplanted mice and at week 2 post-injection in the BM of b34+ transplanted mice, there were a few human CD41+ cells that were able to produce platelets, but the level in total BM (murine and human cells) is below the FACS detection limits.

Our findings on b34+ cells are consistent with those reported by Verstegen et al. who found peak, but low levels, of HuPlts at week 2 (0.1% to 0.2%) in macrophage-depleted SCID mice injected with CB CD34+ cells [15]. A previous study by Güenechea et al. reported that day 7-expanded CB cells engraft in the NOD/SCID mouse more slowly than the unmanipulated counterpart [30]. Our data indicate that even equal numbers of expanded and reisolated CD34+ cells, compared to baseline CB cells, provide similar engraftment at 1, 2, and 3 weeks post-transplant in terms of total CD45+ cells and a faster Mk reconstitution. If baseline cells and the entire expansion equivalents obtained in the present setting are compared, then marrow engraftment at both early time points (25% CD45+ at week 1) and at the standard 6–8 weeks is much higher with 4w34+-expanded cells. Culture conditions, in particular growth factor combinations employed in the two studies, are quite different, and this explains the opposite findings. This suggests that several aspects of cell expansion and manipulation must be carefully studied, especially the growth factor combinations to be adopted, before a clinical protocol is implemented.

In conclusion, by means of serial transplants we show that ex-vivo expanded cells are capable of sustained long-term Mk-engraftment. Short-term engraftment by ex vivo expanded cells seems even more efficient than with unmanipulated cells.


   ACKNOWLEDGMENT
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Support for this work was provided by grants from the Associazione Italiana per la Ricerca sul Cancro (AIRC), Milan, Italy and from the Ministero dell’Università e della Ricerca Scientifica e Tecnologica (MURST), Rome to W.P. and to M.A., and from CNR (Progetto Finalizzato Oncologia). The authors wish to thank Mrs. L. Ramini for invaluable secretarial assistance.


   REFERENCES
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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Received July 1, 2003; accepted for publication October 7, 2003.



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