Maternal administration of busulfan before in utero transplantation of human hematopoietic stem cells enhances engraftments in sheep

Immune modulation permits tolerance and engraftment in a murine model of late-gestation transplantation

ABSTRACT

In utero hematopoietic cell transplantation is an experimental nonmyeloablative therapy with potential applications in hematologic disorders, including sickle cell disease (SCD). Its clinical utility has been limited due to the early acquisition of T-cell immunity beginning at ∼14 weeks gestation, posing significant technical challenges and excluding treatment fetuses evaluated after the first trimester. Using murine neonatal transplantation at 20 days postcoitum (DPC) as a model for late-gestation transplantation (LGT) in humans, we investigated whether immune modulation with anti-CD3 monoclonal antibody (mAb) could achieve donor-specific tolerance and sustained allogeneic engraftment comparable with that of the early-gestation fetal recipient at 14 DPC. In allogeneic wild-type strain combinations, administration of anti-CD3 mAb with transplantation resulted in transient T-cell depletion followed by central tolerance induction confirmed by donor-specific clonal deletion and skin graft tolerance. Normal immune responses to third-party major histocompatibility complex and viral pathogens were preserved, and graft-versus-host disease did not occur. We further demonstrated the successful application of this approach in the Townes mouse model of SCD. These findings confirm the developing fetal T-cell response as a barrier to LGT and support transient T-cell depletion as a safe and effective immunomodulatory strategy to overcome it.

Advancements in medical research using fetal sheep: Implications for human health and treatment methods

Sheep are typically considered as industrial animals that provide wool and meals. However, they play a significant role in medical research in addition to their conventional use. Notably, sheep fetuses are resistant to surgical invasions and can endure numerous manipulations, such as needle puncture and cell transplantation, and surgical operations requiring exposure beyond the uterus. Based on these distinguishing characteristics, we established a chimeric sheep model capable of producing human/monkey pluripotent cell-derived blood cells via the fetal liver. Furthermore, sheep have become crucial as human fetal models, acting as platforms for developing and improving techniques for intrauterine surgery to address congenital disorders and clarifying the complex pharmacokinetic interactions between mothers and their fetuses. This study emphasizes the significant contributions of fetal sheep to advancing human disease understanding and treatment strategies, highlighting their unique characteristics that are not present in other animals.

Experimental and clinical progress of in utero hematopoietic cell transplantation therapy for congenital disorders

In utero hematopoietic cell transplantation (IUHCT) is considered a potentially efficient therapeutic approach with relatively few side effects, compared to adult hematopoietic cell transplantation, for various hematological genetic disorders. The principle of IUHCT has been extensively studied in rodent models and in some large animals with close evolutionary similarities to human beings. However, IUHCT has only been used to rebuild human T cell immunity in certain patients with inherent immunodeficiencies. This review will first summarize the animal models utilized for IUHCT investigations and describe the associated outcomes. Recent advances and potential barriers for successful IUHCT are discussed, followed by possible strategies to overcome these barriers experimentally. Lastly, we will outline the progress made towards utilizing IUHCT to treat inherent disorders for patients, list out associated limitations and propose feasible means to promote the efficacy of IUHCT clinically.

In Utero Gene Therapy for Primary Immunodeficiencies

Primary immunodeficiencies (PIDs) have become a prime target for gene therapy given the morbidity, mortality, and the single gene etiology. Given that outcomes are better the earlier gene therapy is implemented, it is possible that fetal gene therapy may be an important future direction for the treatment of PIDs. In this chapter, the current treatments available for several PIDs will be reviewed, as well as the history and current status of gene therapy for PIDs. The possibility of in utero gene therapy as a possibility will then be discussed.

Fetal sheep support the development of hematopoietic cells in vivo from human induced pluripotent stem cells

We report that a sheep fetal liver provides a microenvironment for generating hematopoietic cells with long-term engrafting capacity and multilineage differentiation potential from human induced pluripotent stem cell (iPSC)-derived hemogenic endothelial cells (HEs). Despite the promise of iPSCs for making any cell types, generating hematopoietic stem and progenitor cells (HSPCs) is still a challenge. We hypothesized that the hematopoietic microenvironment, which exists in fetal liver but is lacking in vitro, turns iPSC-HEs into HSPCs. To test this, we transplanted CD45-negative iPSC-HEs into fetal sheep liver, in which HSPCs first grow. Within 2 months, the transplanted cells became CD45 positive and differentiated into multilineage blood cells in the fetal liver. Then, CD45-positive cells translocated to the bone marrow and were maintained there for 3 years with the capability of multilineage differentiation, indicating that hematopoietic cells with long-term engraftment potential were generated. Moreover, human hematopoietic cells were temporally enriched by xenogeneic donor-lymphocyte infusion into the sheep. This study could serve as a foundation to generate HSPCs from iPSCs.

Assessment of Short-Term Engraftment Potential of Ex Vivo Expanded Hematopoietic Stem Cells Using Normal Fetal Mouse in Utero Transplantation Model

Objective

Ex vivo expansion is a promising strategy to overcome the low number of human umbilical cord blood hematopoietic stem cells (hUCB-HSCs). Although based on the obtained results in unnatural physiological condition of irradiated genetically immune-deficient mouse models, there has always been concern that the expanded cells have less engraftment potential. The purpose of this study was to investigate effect of common ex vivo expansion method on engraftment potential of hUCB-mononuclear cells (MNCs), using normal fetal mouse, as a model with more similarity to human physiological conditions.

Materials and Methods

In this experimental study, briefly, isolated hUCB-MNCs were cultured in common expansion medium containing stem cell factor, Flt3 ligand and thrombopoietin. The unexpanded and expanded cells were transplanted to the fetal mice on gestational days of 11.5-13.5. After administration of human hematopoiesis growth factors (hHGFs), presence of human CD45⁺ cells, in the peripheral blood of recipients, was assessed at various time points after transplantation.

Results

The expanded MNCs showed 32-fold increase in the expression of CD34⁺38^- phenotype and about 3-fold higher clonogenic potential as compared to the uncultured cells. Four weeks after transplantation, 73% (19/26) of expanded-cell recipients and 35% (7/20) of unexpanded-cell recipients were found to be successfully engrafted with human CD45⁺ cells. The engraftment level of expanded MNCs was significantly (1.8-fold) higher than unexpanded cells. After hHGFs administration, the level was increased to 3.2, 3.8 and 2.6-fold at respectively 8, 12, and 16 weeks of post transplantation. The increased expression of CXCR4 protein in expanded MNCs is a likely explanation for the present findings.

Conclusion

The presented data showed that expanded MNCs compared to unexpended cells are capable of more rapid and higher short-term engraftment in normal fetal mouse. It could also be suggested that in utero transplantation (IUT) of normal fetal mice could be an appropriate substitute for NOD/SCID mice in xenotransplantation studies.

CD34-negative hematopoietic stem cells show distinct expression profiles of homing molecules that limit engraftment in mice and sheep

We and others have reported that human hematopoietic stem cells (HSCs) are also present in the CD34-negative (CD34^-) fraction of human cord blood (CB). Here, we examined the hematopoietic engraftment potential of 13 or 18 lineage-negative (13Lin^- or 18Lin^-) CD34^+/- cells from human CB in mice and sheep. Both 13Lin^- and 18Lin^- CD34⁺ cells efficiently engrafted in mice irrespective of transplantation route, be it by tail-vein injection (TVI) or by intra-bone marrow injection (IBMI). These cells also engrafted in sheep after in utero fetal intra-hepatic injection (IHI). In contrast, neither 13Lin^- nor 18Lin^- CD34^- cells engrafted in either mice or sheep when transplanted by regular routes (i.e., TVI and fetal IHI, respectively), although both 13Lin^- and 18Lin^- CD34^- cells engrafted in mice when transplanted by IBMI and exhibited multilineage reconstitution ability. Thus, the homing ability of CD34^- HSCs is significantly more limited than that of CD34⁺ HSCs. As for 18Lin^-, CD34^- HSCs are characterized by low expression of the tetraspanin CD9, which promotes homing, and high expression of the peptidase CD26, which inhibits homing. This unique expression pattern homing-related molecules on CD34^- HSCs could thus explain in part their reduced ability to home to the BM niche.

A swine model of acute thrombocytopenia with prolonged bleeding time produced by busulfan

Animal models of thrombocytopenia are indispensable for evaluating the in vivo efficacy of hemostatic agents, cryopreserved platelets, and artificial platelets, but no large animal models are available. In this study, we generated a swine model of acute thrombocytopenia with prolonged bleeding times by administering the chemotherapeutic drug busulfan. First, we tested multiple doses of busulfan (4, 6, and 8 mg/kg) in pigs, and found that 6 mg/kg of busulfan is an optimal dose for producing a safe and moderate thrombocytopenia, with a platelet count of less than 30,000/µl. The pigs administered 6 mg/kg of busulfan (n=8) reached half their initial counts at day 7, counts below 30,000/µl at day 12, and their nadirs at day 15 (on average). The minimal platelet count was 14,000/µl. With this dose of busulfan (6 mg/kg), bleeding times were significantly prolonged in addition to the decrease in platelet counts (r=−0.63, P<0.01), while there were no cases of apparent hemorrhage. White blood cell counts were maintained at over 5,000/µl, and there were no infections or other adverse events including anemia or appetite or body weight loss. All pigs were sacrificed on day 16, with subsequent examination showing a significant reduction in cellularity and colony-forming units in the bone marrow, indicating that thrombocytopenia was the result of myelosuppression. In summary, administration with 6 mg/kg of busulfan induces safe and moderate thrombocytopenia with a prolonged bleeding time in swine.

In utero stem cell transplantation and gene therapy: rationale, history, and recent advances toward clinical application

Recent advances in high-throughput molecular testing have made it possible to diagnose most genetic disorders relatively early in gestation with minimal risk to the fetus. These advances should soon allow widespread prenatal screening for the majority of human genetic diseases, opening the door to the possibility of treatment/correction prior to birth. In addition to the obvious psychological and financial benefits of curing a disease in utero, and thereby enabling the birth of a healthy infant, there are multiple biological advantages unique to fetal development, which provide compelling rationale for performing potentially curative treatments, such as stem cell transplantation or gene therapy, prior to birth. Herein, we briefly review the fields of in utero transplantation (IUTx) and in utero gene therapy and discuss the biological hurdles that have thus far restricted success of IUTx to patients with immunodeficiencies. We then highlight several recent experimental breakthroughs in immunology, hematopoietic/marrow ontogeny, and in utero cell delivery, which have collectively provided means of overcoming these barriers, thus setting the stage for clinical application of these highly promising therapies in the near future.

Xenotransplantation of human adipose-derived stem cells in zebrafish embryos

Zebrafish is a widely used animal model with well-characterized background in developmental biology. The fate of human adipose-derived stem cells (ADSCs) after their xenotransplantation into the developing embryos of zebrafish is unknown. Therefore, human ADSCs were firstly isolated, and then transduced with lentiviral vector system carrying a green fluorescent protein (GFP) reporter gene, and followed by detection of their cell viability and the expression of cell surface antigens. These GFP-expressing human ADSCs were transplanted into the zebrafish embryos at 3.3–4.3 hour post-fertilization (hpf). Green fluorescent signal, the proliferation and differentiation of human ADSCs in recipient embryos were respectively examined using fluorescent microscopy and immunohistochemical staining. The results indicated that human ADSCs did not change their cell viability and the expression levels of cell surface antigens after GFP transduction. Microscopic examination demonstrated that green fluorescent signals of GFP expressed in the transplanted cells were observed in the embryos and larva fish at post-transplantation. The positive staining of Ki-67 revealed the survival and proliferation of human ADSCs in fish larvae after transplantation. The expression of CD105 was observable in the xenotransplanted ADSCs, but CD31 expression was undetectable. Therefore, our results indicate that human ADSCs xenotransplanted in the zebrafish embryos not only can survive and proliferate at across-species circumstance, but also seem to maintain their undifferentiation status in a short term. This xenograft model of zebrafish embryos may provide a promising and useful technical platform for the investigation of biology and physiology of stem cells in vivo.

Influence of a dual-injection regimen, plerixafor and CXCR4 on in utero hematopoietic stem cell transplantation and engraftment with use of the sheep model

BACKGROUND AIMS

Inadequate engraftment of hematopoietic stem cells (HSCs) after in utero HSC transplantation (IUHSCT) remains a major obstacle for the prenatal correction of numerous hereditary disorders. HSCs express CXCR4 receptors that allow homing and engraftment in response to stromal-derived factor 1 (SDF-1) ligand present in the bone marrow stromal niche. Plerixafor, a mobilization drug, works through the interruption of the CXCR4-SDF-1 axis.

METHODS

We used the fetal sheep large-animal model to test our hypotheses that (i) by administering plerixafor in utero before performing IUHSCT to release fetal HSCs and thus vacating recipient HSC niches, (ii) by using human mesenchymal stromal/stem cells (MSCs) to immunomodulate and humanize the fetal BM niches and (iii) by increasing the CXCR4(+) fraction of CD34(+) HSCs, we could improve engraftment. Human cord blood-derived CD34(+) cells and human bone marrow-derived MSCs were used for these studies.

RESULTS

When MSCs were transplanted 1 week before CD34(+) cells with plerixafor treatment, we observed 2.80% donor hematopoietic engraftment. Combination of this regimen with additional CD34(+) cells at the time of MSC infusion increased engraftment levels to 8.77%. Next, increasing the fraction of CXCR4(+) cells in the CD34(+) population albeit transplanting at a late gestation age was not beneficial. Our results show engraftment of both lymphoid and myeloid lineages.

CONCLUSIONS

Prior MSC and HSC cotransplantation followed by manipulation of the CXCR4-SDF-1 axis in IUHSCT provides an innovative conceptual approach for conferring competitive advantage to donor HSCs. Our novel approach could provide a clinically relevant approach for enhancing engraftment early in the fetus.

Long-term follow-up study on the engraftment of human hematopoietic stem cells in sheep

Xenograft models of human hematopoiesis are essential to the study of the engraftment and proliferative potential of human hematopoietic stem cells (HSCs) in vivo. Immunodeficient mice and fetal sheep are often used as xenogeneic recipients because they are immunologically naive. In this study, we transplanted human HSCs into fetal sheep and assessed the long-term engraftment of transplanted human HSCs after birth. Fourteen sheep were used in this study. In 4 fetal sheep, HSCs were transduced with homeo-box B4 (HOXB4) gene before transplantation, which promoted the expansion of HSCs. Another 4 fetal sheep were subjected to non-myeloablative conditioning with busulfan. Seven of these 8 sheep showed successful engraftment of human HSCs (1–3% of colony-forming units) as assessed after the birth of fetal sheep (5 months post-transplantation), although HOXB4-transduced HSCs showed sustained engraftment for up to 40 months. Intact HSCs were transplanted into six non-conditioned fetal sheep, and human colony-forming units were not detected in the sheep after birth. These results suggest that, as compared with mouse models, where the short lifespan of mice limits long-term follow-up of HSC engraftment, the fetal sheep model provides a unique perspective for evaluating long-term engraftment and proliferation of human HSCs.

In utero depletion of fetal hematopoietic stem cells improves engraftment after neonatal transplantation in mice

Although in utero hematopoietic cell transplantation is a promising strategy to treat congenital hematopoietic disorders, levels of engraftment have not been therapeutic for diseases in which donor cells have no survival advantage. We used an antibody against the murine c-Kit receptor (ACK2) to deplete fetal host hematopoietic stem cells (HSCs) and increase space within the hematopoietic niche for donor cell engraftment. Fetal mice were injected with ACK2 on embryonic days 13.5 to 14.5 and surviving pups were transplanted with congenic hematopoietic cells on day of life 1. Low-dose ACK2 treatment effectively depleted HSCs within the bone marrow with minimal toxicity and the antibody was cleared from the serum before the neonatal transplantation. Chimerism levels were significantly higher in treated pups than in controls; both myeloid and lymphoid cell chimerism increased because of higher engraftment of HSCs in the bone marrow. To test the strategy of repeated HSC depletion and transplantation, some mice were treated with ACK2 postnatally, but the increase in engraftment was lower than that seen with prenatal treatment. We demonstrate a successful fetal conditioning strategy associated with minimal toxicity. Such strategies could be used to achieve clinically relevant levels of engraftment to treat congenital stem cell disorders.

In utero hematopoietic cell transplantation--recent progress and the potential for clinical application

In utero hematopoietic stem cell transplantation (IUHCT) is a potential therapeutic alternative to postnatal hematopoietic stem cell transplantation (HSCT) for congenital hematologic disorders that can be diagnosed early in gestation and can be cured by HSCT. The rationale is to take advantage of normal events during hematopoietic and immunologic ontogeny to facilitate allogeneic hematopoietic engraftment. Although the rationale remains compelling, IUHCT has not yet achieved its clinical potential. This review will discuss recent experimental progress toward overcoming the barriers to allogeneic engraftment and new therapeutic strategies that may hasten clinical application.