Improved efficacy and safety of in utero cell transplantation in sheep using an ultrasound-guided method

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.

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.

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.

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.

Generation of CD34+ cells from human embryonic stem cells using a clinically applicable methodology and engraftment in the fetal sheep model

Until now, ex vivo generation of CD34(+) hematopoietic stem cells (HSCs) from human embryonic stem cells (hESCs) mostly involved use of feeder cells of nonhuman origin. Although they provided invaluable models to study hematopoiesis, in vivo engraftment of hESC-derived HSCs remains a challenging task. In this study, we used a novel coculture system composed of human bone marrow-derived mesenchymal stromal/stem cells (MSCs) and peripheral blood CD14(+) monocyte-derived macrophages to generate CD34(+) cells from hESCs in vitro. Human ESC-derived CD34(+) cells generated using this method expressed surface makers associated with adult human HSCs and upregulated hematopoietic stem cell genes comparable to human bone marrow-derived CD34(+) cells. Finally, transplantation of purified hESC-derived CD34(+) cells into the preimmune fetal sheep, primed with transplantation of MSCs derived from the same hESC line, demonstrated multilineage hematopoietic activity with graft presence up to 16 weeks after transplantation. This in vivo demonstration of engraftment and robust multilineage hematopoietic activity by hESC-derived CD34(+) cells lends credence to the translational value and potential clinical utility of this novel differentiation and transplantation protocol.

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

In utero transplantation (IUT) of human hematopoietic stem cells has been conducted in sheep, which are used as large animal models of human hematopoietic reconstitution and models for clinical IUT; however, the levels of engraftment have generally been low. Busulfan (BU), a myeloablative agent, is often administered to patients before hematopoietic stem cells transplantation to improve the engraftment. In this study, hematopoietic activity was evaluated in adult sheep after administering BU at different doses. Next, pregnant ewes were administered BU, and dams as well as their fetuses were evaluated, as BU readily crosses the sheep placenta. Then, the BU dose with the desired outcomes was selected and administered to pregnant ewes at 2 or 6 days before performing IUT using human cord blood CD34(+) cells. The engraftment was evaluated in recipients that underwent IUT in the presence or absence of BU. As a result, hematopoietic activity was safely and transiently suppressed in adult sheep treated with 5 to 7.5 mg/kg BU. BU crossed the sheep placenta, and fetal sheep were indeed conditioned by administering 3 mg/kg BU to pregnant ewes. Engraftment of human CD34(+) cells in fetal recipients was enhanced when IUT was carried out 6 days post-BU. Up to 3.3% engraftment levels (in terms of bone marrow colony-forming units) were achieved with the IUT of 0.72 to 2.4 million CD34(+) cells when BU was used. BU can be administered to pregnant ewes to effectively condition the fetal recipient for IUT with enhanced engraftment of donor cells.

Fetal liver-derived mesenchymal stem cell engraftment after allogeneic in utero transplantation into rabbits

Prenatal transplantation of genetically engineered mesenchymal stem cells (MSCs) might benefit prevention or treatment of early-onset genetic disorders due to the cells' intrinsic regenerative potential plus the acquired advantage from therapeutic transgene expression. However, a thorough assessment of the safety, accessibility, and behavior of these MSCs in the fetal environment using appropriate animal models is required before we can advance toward a clinical application. We have recently shown that fetal rabbit liver MSCs (fl-MSCs) have superior growth rate, clonogenic capability, and in vitro adherence and differentiation abilities compared with adult rabbit bone marrow MSCs. In this follow-up study, we report safe and widespread distribution of recombinant pSF-EGFP retrovirus-transduced fl-MSCs (EGFP(+)-fl-MSCs) in neonatal rabbit tissues at 10 days after fetal allogeneic transplantation through both intrahepatic and intra-amniotic administration. Conversely, a more restricted biodistribution pattern according to the route of administration was apparent in the young rabbits intervened at 16 weeks after fetal EGFP(+)-fl-MSC transplantation. Furthermore, the presence of these cells in the recipients' tissues, tracked with the reporter provirus, was inversely related to the developmental stage of the fetuses at the time of intervention. Long-term engraftment was confirmed both by fluorescence in situ hybridization analysis on touch tissue imprints using a chromosome Y-specific BAC probe, and by immunohistochemical localization of EGFP expression. Finally, there was no evidence of immune responses against the transplanted EGFP(+)-fl-MSCs or the EGFP transgenic product in the treated young rabbits. Thus, cell transplantation approaches using genetically engineered fetal MSCs may prove particularly valuable to frontier medical treatments for congenital birth defects in perinatology.

Migration of cells from the yolk sac to hematopoietic tissues after in utero transplantation of early and mid gestation canine fetuses

BACKGROUND

In utero hematopoietic cell transplantation offers a means of early intervention for the treatment of diseases before birth. Delivery of cells to the yolk sac is a minimally invasive approach that results in low levels of chimerism. However, there is little information on the optimal doses, timing of delivery, and migration of transplanted cells from the yolk sac into the fetus.

METHODS

Varying cell doses of mesenchymal stromal cells or bone marrow mononuclear cells labeled with fluorescent supraparamagnetic iron oxide nanoparticles and a fluorescent intracellular dye, 5- and 6-([(4-chloromethyl)benzoyl]-amino) tetramethylrhodamine, were transplanted under ultrasound guidance to the yolk sacs of day 25 or day 35 canine fetuses. Ex vivo whole body fluorescence imaging and microscopy of tissue sections were correlated with the presence of iron oxide in injected and control fetuses.

RESULTS

Day 25 and day 35 recipients showed similar survival rates after injection of cells into yolk sacs, although increased fetal morality was associated with cell doses greater than 10 cells/kg to day 25 fetuses. The fluorescence and iron oxide signals were predominantly localized to the abdominal regions, with no fluorescence visible in yolk sacs. Microscopy of tissues revealed colocalization of fluorophore with iron oxide in donor cells detected in the fetal livers and bone marrow of recipients 7 and 17 days after receiving mesenchymal stromal cells or bone marrow mononuclear cells.

CONCLUSIONS

These studies demonstrated that cells injected into the yolk sacs of early gestation canine fetuses migrate to recipient hematopoietic tissues. Thus, yolk sac injection offers a safe and effective approach for engraftment of cells to fetal hematopoietic tissues.

Autologous transplantation of amniotic fluid-derived mesenchymal stem cells into sheep fetuses

Long-term engraftment and phenotype correction has been difficult to achieve in humans after in utero stem cell transplantation mainly because of allogeneic rejection. Autologous cells could be obtained during gestation from the amniotic fluid with minimal risk for the fetus and the mother. Using a sheep model, we explored the possibility of using amniotic fluid mesenchymal stem cells (AFMSCs) for autologous in utero stem cell/gene therapy. We collected amniotic fluid (AF) under ultrasound-guided amniocentesis in early gestation pregnant sheep (n = 9, 58 days of gestation, term = 145 days). AFMSCs were isolated and expanded in all sampled fetal sheep. Those cells were transduced using an HIV vector encoding enhanced green fluorescent protein (GFP) with 63.2% (range 38.3-96.2%) transduction efficiency rate. After expansion, transduced AFMSCs were injected into the peritoneal cavity of each donor fetal sheep at 76 days under ultrasound guidance. One ewe miscarried twin fetuses after amniocentesis. Intraperitoneal injection was successful in the remaining 7 fetal sheep giving a 78% survival for the full procedure. Tissues were sampled at postmortem examination 2 weeks later. PCR analysis detected GFP-positive cells in fetal tissues including liver, heart, placenta, membrane, umbilical cord, adrenal gland, and muscle. GFP protein was detected in these tissues by Western blotting and further confirmed by cytofluorimetric and immunofluorescence analyses. This is the first demonstration of autologous stem cell transplantation in the fetus using AFMSCs. Autologous cells derived from AF showed widespread organ migration and could offer an alternative way to ameliorate prenatal congenital disease.

Ex vivo expansion of human HSCs with Sendai virus vector expressing HoxB4 assessed by sheep in utero transplantation

OBJECTIVE

The homeobox B4 (HoxB4) gene promotes expansion of hematopoietic stem cells (HSCs). However, frequent development of leukemia in large animals due to retrovirally transduced HoxB4 gene has been reported. To prevent tumorigenesis, we developed a nonintegrating and nonreplicating Sendai virus vector that did not contain the phosphoprotein gene (SeV/ΔP), which enabled clearance of the vector and transgene shortly after transduction. We tested the SeV/ΔP vector expressing the HoxB4 gene (SeV/ΔP/HoxB4) for the ex vivo expansion of human cord blood CD34(+) cells (HSCs) using a sheep in utero transplantation assay.

MATERIALS AND METHODS

Human HSCs were ex vivo-expanded by transduction with SeV/ΔP/HoxB4 vector and transplanted into the abdominal cavity of fetal sheep. The engraftment of human HSCs in the lambs was quantitatively evaluated by hematopoietic colony-forming unit assays.

RESULTS

After transplantation, the HoxB4-transduced HSCs contributed to longer-period (up to 20 months) repopulation in sheep, and human hematopoietic progenitors were detected more frequently in the bone marrow of the HoxB4 group as compared with the control untreated group (p < 0.05). The expansion of human HSCs with the SeV/ΔP/HoxB4 vector was comparable with previously reported retroviral vectors expressing HoxB4. The SeV/ΔP/HoxB4 vector and the transgene were cleared from the recipient sheep and leukemia was not detected at 20 months post-transplantation.

CONCLUSIONS

The SeV/ΔP vector would be suitable for transient expression of HoxB4 in human CD34(+) cells. In addition, the SeV/ΔP vector is free of concern about transgene-related and insertional leukemogenesis and should be safer than retroviral vectors.