Human placenta and chorion: potential additional sources of hematopoietic stem cells for transplantation

BACKGROUND

Hematopoietic stem cell (HSC) transplantation is an essential element of medical therapy, leading to cures of previously incurable hematological and nonhematological diseases. Many patients do not find matched donors in a timely manner, which has driven efforts to find alternative pools of transplantable HSCs. The use of umbilical cord blood (UCB) as a source of transplantable HSCs began more than two decades ago. However, the use of UCB as a reliable source of HSCs for transplantation still faces crucial challenges: the number of HSCs present in a unit of UCB is usually sufficient for younger children but not for adults, and the persistent delayed engraftment often seen can result in high rates of infection and mortality.

STUDY DESIGN AND METHODS

We propose a new approach to a solution of these problems: a potential increase of the limited number of UCB-HSCs available by harvesting HSCs contained in the placenta and the fetal chorionic membrane available at birth.

RESULTS

We investigated the presence of hematopoietic progenitors and HSCs in human placenta and chorion at different gestational ages. The characterization of these cells was performed by flow cytometry and immunolocalization, and their functional status was investigated by transplanting them into immunodeficient mice.

CONCLUSION

HSCs are present in extraembryonic tissues and could be banked in conjunction to the UCB-HSCs. This novel approach could have a large impact on the field of HSC banking and, more crucially, on the outcome of patients undergoing this treatment by greatly improving the use of life-saving hematopoietic transplants.

Cellular therapies supplement: the peritoneum as an ectopic site of hematopoiesis following in utero transplantation

BACKGROUND

In utero transplantation (IUT) has the potential to treat birth defects early before full development of the immune system. Relatively small grafts, which are not matched for major histocompatibility antigens, can be delivered even before onset of disease symptoms. IUT of hematopoietic stem cells is usually performed via intraperitoneal injection, yet the fate of donor cells in the peritoneal cavity is not fully understood. We review our recent work and present new data demonstrating that the peritoneum can be a site of ectopic hematopoiesis with implications for IUT and immune tolerance induction.

STUDY DESIGN AND METHODS

Haplogeneic and allogeneic fetal transplants were performed in mice and engraftment tracked by flow cytometry. Immune tolerance was studied by mixed lymphocyte reactions and skin transplantation. Adult syngeneic murine transplants and xenogeneic human into immunodeficient mouse transplants were performed to follow hematopoietic retention in the peritoneum and engraftment of the marrow.

RESULTS

Although most transplanted cells rapidly clear the peritoneum, hematopoietic cells and cells with the phenotype of hematopoietic precursors can remain in the peritoneal cavity for months after transplant. The presence of donor cells in the peritoneum can contribute to donor-specific tolerance, but sufficient peripheral blood chimerism is required to ensure acceptance of donor skin grafts.

CONCLUSION

Ectopic hematopoiesis and the survival of stem cells in the peritoneum offer the possibility of better using the peritoneal cavity to delivery stem cells and foster the development of immune tolerance to alloantigens or other foreign antigens.

Coexpression of CD14 and CD326 discriminate hepatic precursors in the human fetal liver

The molecular and cellular profile of liver cells during early human development is incomplete, complicating the isolation and study of hepatocytes, cholangiocytes, and hepatic stem cells from the complex amalgam of hepatic and hematopoietic cells, that is, the fetal liver. Epithelial cell adhesion molecule, CD326, has emerged as a marker of hepatic stem cells, and lipopolysaccharide receptor CD14 is known to be expressed on adult hepatocytes. Using flow cytometry, we studied the breadth of CD326 and CD14 expression in midgestation liver. Both CD45(+) hematopoietic and CD45(-) nonhematopoietic cells expressed CD326. Moreover, diverse cell types expressing CD326 were revealed among CD45(-) cells by costaining for CD14. Fluorescence-activated cell sorting was used to isolate nonhematopoietic cells distinguished by expression of high levels of CD326 and low CD14 (CD326(++)CD14(lo)), which were characterized for gene expression associated with liver development. CD326(++)CD14(lo) cells expressed the genes albumin, α-fetoprotein, hepatic nuclear factor 3α, prospero-related homeobox 1, cytochrome P450 3A7, α(1)-antitrypsin, and transferrin. Proteins expressed included cell-surface CD24, CD26, CD29, CD34, CD49f, CD243, and CD324 and, in the cytoplasm, cytokeratins-7/8 (CAM 5.2 antigen) and some cytokeratin-19. Cultured CD326(++)CD14(lo) cells yielded albumin(+) hepatocytes, cytokeratin-19(+) cholangiocytes, and hepatoblasts expressing both markers. Using epifluorescence microscopy we observed CD326 and CD14 expression on fetal hepatocytes comprising the liver parenchyma, as well as on cells associated with ductal plates and surrounding large vessels. These findings indicate that expression of CD14 and CD326 can be used to identify functionally distinct subsets of fetal liver cells, including CD326(++)CD14(lo) cells, representing a mixture of parenchymal cells, cholangiocytes, and hepatoblasts.

Characterization of tolerance induction through prenatal marrow transplantation: the requirement for a threshold level of chimerism to establish rather than maintain postnatal skin tolerance

Hematopoietic chimerism resulting from prenatal marrow transplantation does not consistently result in allotolerance for unidentified causes. In a C57BL/6-into-FVB/N murine model, we transplanted T-cell-depleted adult marrow on gestational day 14 to elucidate the immunological significance of chimerism towards postnatal tolerance. Postnatally, chimerism was examined by flow cytometry, and tolerance by skin transplantation and mixed lymphocyte reaction. Regulatory T cells were quantified by FoxP3 expression. Peripheral chimerism linearly related to thymic chimerism, and predicted the degree of graft acceptance with levels >3% at skin placement, yielding consistent skin tolerance. Low- and high-level chimeras had lower intrathymic CD3(high) expression than microchimeras or untransplanted mice. Regardless of the skin tolerance status in mixed chimeras, donor-specific alloreactivity by lymphocytes was suppressed but could be partially restored by exogenous interleukin-2. Recipients that lost peripheral chimerism did not accept donor skin unless prior donor skin had engrafted at sufficient chimerism levels, suggesting that complete tolerance can develop as a consequence of chimerism-related immunosuppression of host lymphocytes and the tolerogenic effects of donor skin. Thus, hematopoietic chimerism exerted immunomodulatory effects on the induction phase of allograft tolerance. Once established, skin tolerance did not fade away along with spontaneous regression of peripheral and tissue chimerism, as well as removal of engrafted donor skin. Neither did it break following in vivo depletion of increased regulatory T cells, and subcutaneous interleukin-2 injection beneath the engrafted donor skin. Those observations indicate that the maintenance of skin tolerance is multifaceted, neither solely dependent upon hematopoietic chimerism and engrafted donor skin nor on the effects of regulatory T cells or clonal anergy. We conclude that hematopoietic chimerism generated by in utero hematopoietic stem cell transplantation is critical to establish rather than maintain postnatal skin tolerance. Therefore, the diminution of hematopoietic chimerism below a threshold level does not nullify an existing tolerance state, but lessens the chance of enabling complete tolerance.

Requirement of retinoids for the expression of CD38 on human hematopoietic progenitors in vitro

BACKGROUND

Cells expressing high levels of CD34 and little or no CD38 comprise a primitive compartment of progenitors, thought to include hematopoietic stem cells. In this study we sought to determine the feasibility of using CD34 and CD38 as markers of hematopoietic differentiation in vitro, using retinoids to induce the expression of CD38.

METHODS

The effects over time of culture, sera and retinoids on the expression of CD34 and CD38 were determined using a base-medium lacking serum. Two early progenitor populations, isolated by FACS from human fetal liver, were studied: CD38(-)CD34(++) and CD38(+)CD34(++) cells. Additionally, HL-60 cells were adapted to grow in serum-deprived medium to study factors that control CD38 expression. Colony forming cell (CFC) assays and short-term expansion cultures were used to measure the effects of all-trans-retinoic acid (ATRA) oil the growth of fetal progenitors.

RESULTS

Fetal progenitors and HL-60 cells grown under serum-deprived conditions exhibited almost no CD38 expression. However, CD34 expression was observed on fetal progenitors and declined slowly over time. Addition of FBS or human serum restored CD38 expression to cultured cells, but at levels below those found on progenitors in vivo. Addition of ATRA or 9-cis-retinoic acid (9CRA) to cultures of fetal progenitors or HL-60 cells, resulted in a time- and dose-dependent increase in CD38 expression, ATRA being the more potent of the two retinoids. However, ATRA inhibited colony formation, reduced the expansion of CFC and accelerated the loss of CD34 expression at doses required for the induction of CD38 expression.

DISCUSSION

ATRA-induced CD38 expression on cells to levels comparable to those found on progenitors in vivo. ATRA also inhibited the growth of early progenitors, which was partly due to ATRA accelerating the differentiation of the progenitors. These findings indicate that CD34 and CD38 expression may be followed as markers of hematopoietic differentiation in vitro, but at the cost of culture conditions that are less than optimal for maintaining early progenitors.

Barley as a green factory for the production of functional Flt3 ligand

Biologically active recombinant human Flt3 ligand was expressed and isolated from transgenic barley seeds. Its expression is controlled by a tissue specific promoter that confines accumulation of the recombinant protein to the endosperm tissue of the seed. The recombinant Flt3 ligand variant expressed in the seeds contains an HQ-tag for affinity purification on immobilized metal ion affinity chromatography (IMAC) resin. The tagged protein was purified from seed extracts to near homogeneity using sequential chromatography on IMAC affinity resin and cation exchange resin. We also show that the recombinant Flt3 ligand protein undergoes posttranslational modifications: it is a glycoprotein containing alpha-1,3-fucose and alpha-1,2-xylose. The HQ-tagged Flt3 ligand variant exhibits comparable biological activity to commercial Flt3 ligand. This is the first report showing expression and accumulation of recombinant human growth factor in barley seeds with a yield of active protein similar to a bacterial expression system. The present results demonstrate that plant molecular farming is a viable approach for the bioproduction of human-derived growth factors.

Characterization of Chikungunya pseudotyped viruses: Identification of refractory cell lines and demonstration of cellular tropism differences mediated by mutations in E1 glycoprotein

Chikungunya virus (CHIKV) is an alphavirus responsible for a number of large outbreaks. Here we describe the efficient incorporation of CHIKV envelope glycoproteins into lentiviral and rhabdoviral particles. Vectors pseudotyped with CHIKV envelope proteins efficiently transduced many cell types from different species. However, hematopoietic cell types were either partially or completely refractory. A mutation in E1 (A226V) has been linked with expansion of tropism for mosquito species, although differences in in vitro infection of mosquito cell lines have not been noted. However, pseudovirion infectivity assays detected subtle differences in infection of mosquito cells, suggesting an explanation for the changes in mosquito tropism. The presence of C-type lectins increased CHIKV pseudotyped vector infectivity, but not infection of refractory cells, suggesting that they act as attachment factors rather than primary receptors. CHIKV pseudotypes will serve as an important tool for the study of neutralizing antibodies and the analysis of envelope glycoprotein functions.

A new role for the human placenta as a hematopoietic site throughout gestation

We investigated whether the human placenta contributes to embryonic and fetal hematopoietic development. Two cell populations--CD34(++)CD45(low) and CD34( +)CD45(low)--were found in chorionic villi. CD34(++) CD45(low) cells display many markers that are characteristic of multipotent primitive hematopoietic progenitors and hematopoietic stem cells. Clonogenic in vitro assays showed that CD34(++)CD45( low) cells contained colony-forming units-culture with myeloid and erythroid potential and differentiated into CD56(+) natural killer cells and CD19(+) B cells in culture. CD34(+)CD45(low) cells were mostly enriched in erythroid- and myeloid-committed progenitors. While the number of CD34(++)CD45(low) cells increased throughout gestation in parallel with placental mass. However, their density (cells per gram of tissue) reached its peak at 5 to 8 weeks, decreasing more than 7-fold from the ninth week onward. In addition to multipotent progenitors, the placenta contained intermediate progenitors, indicative of active hematopoiesis. Together, these data suggest that the human placenta is potentially an important hematopoietic organ, opening the possibility of banking placental hematopoietic stem cells along with cord blood for transplantation.

Prenatal tolerance induction: relationship between cell dose, marrow T-cells, chimerism, and tolerance

It was reported that the dose of self-antigens can determine the consequence of deletional tolerance and donor T cells are critical for tolerance induction in mixed chimeras. This study aimed at assessing the effect of cell doses and marrow T cells on engraftment and tolerance induction after prenatal bone marrow transplantation. Intraperitoneal cell transplantation was performed in FVB/N (H-2K(q)) mice at gestational day 14 with escalating doses of adult C57BL/6 (H-2K(b)) marrows. Peripheral chimerism was examined postnatally by flow cytometry and tolerance was tested by skin transplantation. Transplantation of light-density marrow cells showed a dose response. High-level chimerism emerged with a threshold dose of 5.0 x 10(6) and host leukocytes could be nearly replaced at a dose of 7.5-10.0 x 10(6). High-dose transplants conferred a steady long-lasting donor-specific tolerance but were accompanied by >50% incidence of graft-versus-host disease. Depletion of marrow T cells lessened graft-versus-host disease to the detriment of engraftment. With low-level chimerism, tolerance was a graded phenomenon dependent upon the level of chimerism. Durable chimerism within 6 months required a threshold of > or = 2% chimerism at 1 month of age and predicted a 50% chance of long-term tolerance, whereas transient chimerism (<2%) only caused hyporesponsiveness to the donor. Tolerance induction did not succeed without peripheral chimerism even if a large amount of injected donor cells persisted in the peritoneum. Neither did an increase in cell doses or donor T-cell contents benefit skin graft survivals unless it had substantially improved peripheral chimerism. Thus, peripheral chimerism level can be a simple and straightforward test to predict the degree of prenatal immune tolerance.

Maternal alloantigens promote the development of tolerogenic fetal regulatory T cells in utero

As the immune system develops, T cells are selected or regulated to become tolerant of self antigens and reactive against foreign antigens. In mice, the induction of such tolerance is thought to be attributable to the deletion of self-reactive cells. Here, we show that the human fetal immune system takes advantage of an additional mechanism: the generation of regulatory T cells (Tregs) that suppress fetal immune responses. We find that substantial numbers of maternal cells cross the placenta to reside in fetal lymph nodes, inducing the development of CD4+CD25highFoxP3+ Tregs that suppress fetal antimaternal immunity and persist at least until early adulthood. These findings reveal a form of antigen-specific tolerance in humans, induced in utero and probably active in regulating immune responses after birth.

Polysialic acid, a glycan with highly restricted expression, is found on human and murine leukocytes and modulates immune responses

Polysialic acid (polySia) is a large glycan with restricted expression, typically found attached to the protein scaffold neural cell adhesion molecule (NCAM). PolySia is best known for its proposed role in modulating neuronal development. Its presence and potential functions outside the nervous systems are essentially unexplored. Herein we show the expression of polySia on hematopoietic progenitor cells, and demonstrate a role for this glycan in immune response using both acute inflammatory and tumor models. Specifically, we found that human NK cells modulate expression of NCAM and the degree of polymerization of its polySia glycans according to activation state. This contrasts with the mouse, where polySia and NCAM expression are restricted to multipotent hematopoietic progenitors and cells developing along a myeloid lineage. Sialyltransferase 8Sia IV(-/-) mice, which lacked polySia expression in the immune compartment, demonstrated an increased contact hypersensitivity response and decreased control of tumor growth as compared with wild-type animals. This is the first demonstration of polySia expression and regulation on myeloid cells, and the results in animal models suggest a role for polySia in immune regulation.

The human placenta is a hematopoietic organ during the embryonic and fetal periods of development

We studied the potential role of the human placenta as a hematopoietic organ during embryonic and fetal development. Placental samples contained two cell populations-CD34(++)CD45(low) and CD34(+)CD45(low)-that were found in chorionic villi and in the chorioamniotic membrane. CD34(++)CD45(low) cells express many cell surface antigens found on multipotent primitive hematopoietic progenitors and hematopoietic stem cells. CD34(++)CD45(low) cells contained colony-forming units culture (CFU-C) with myeloid and erythroid potential in clonogenic in vitro assays, and they generated CD56(+) natural killer cells and CD19(+)CD20(+)sIgM(+) B cells in polyclonal liquid cultures. CD34(+)CD45(low) cells mostly comprised erythroid- and myeloid-committed progenitors, while CD34(-) cells lacked CFU-C. The placenta-derived precursors were fetal in origin, as demonstrated by FISH using repeat-sequence chromosome-specific probes for X and Y. The number of CD34(++)CD45(low) cells increased with gestational age, but their density (cells per gram of tissue) peaked at 5-8 wk, decreasing more than sevenfold at the onset of the fetal phase (9 wk of gestation). In addition to multipotent progenitors, the placenta contained myeloid- and erythroid-committed progenitors indicative of active in situ hematopoiesis. These data suggest that the human placenta is an important hematopoietic organ, raising the possibility of banking placental hematopoietic stem cells along with cord blood for transplantation.

Cytokine networks involved in the regulation of haemopoietic stem cell proliferation and differentiation

Additive and synergistic interactions between haemopoietic growth factors and cytokines can be demonstrated in vitro in clonogenic and suspension cultures of murine and human bone marrow. Purification of early stem cells by combinations of purging with cytotoxic agents (5-fluorouracil, 4-hydroperoxy-cyclophosphamide) and selection with monoclonal antibodies for CD34+, CD33- cells permits analysis of factor interactions at the level of the primitive pluripotential stem cell. Interactions between interleukins, tumour necrosis factor and the colony-stimulating factors can be monitored. In vivo, IL-1 alone, or in combination with G-CSF produces accelerated reconstitution of haemopoiesis after chemotherapy, irradiation and bone marrow transplantation in murine and primate systems. IL-1 elicits cytokine cascades that may have positive or negative actions on lymphohaemopoiesis. Induction of products of the cyclooxygenase and lipooxygenase pathways, as well as tumour necrosis factor and TGF-beta, modulate haemopoiesis.

Midkine, a heparin-binding growth factor, selectively stimulates proliferation of definitive zone cells of the human fetal adrenal gland

CONTEXT

In the human fetal adrenal gland (HFA), the inner fetal zone (FZ) secretes dehydroepiandrosterone sulfate. The function of the outer definitive zone (DZ) is less clear; however, the DZ phenotype is that of a reservoir of progenitor cells, many of which are mitotically active. Midkine (MK) is a heparin-binding growth factor with various bioactivities.

OBJECTIVE

The objective of this study was to investigate expression, proliferative effects, and ACTH regulation of MK in the HFA.

DESIGN AND SETTING

RNA, cryosections, and primary cell cultures from HFAs (14-24 wk) and adult adrenal RNA were used.

MAIN OUTCOME MEASURES

The main outcome measures were MK mRNA levels (measured by quantitative real-time RT-PCR); MK localization (measured by immunostaining); MK proliferative effects and mechanism (measured by proliferation assays, flow cytometry, pharmacological interventions); and ACTH regulation (measured by quantitative real-time RT-PCR).

RESULTS

HFA MK mRNA levels were 4-fold higher than in adult adrenals (P < 0.05) and were comparable to levels in fetal and adult brains (positive controls). MK immunoreactivity was abundant throughout the HFA. Exogenous MK caused proliferation of isolated DZ cells but not FZ cells (72 h, P < 0.05). In contrast, basic fibroblast growth factor induced proliferation of cells from both zones. Pharmacological interventions indicated that MK-induced DZ cell proliferation may be mediated by phosphatidylinositol 3-kinase, MAPK kinase, and Src family kinases. ACTH (1 nm) increased MK mRNA by 3.5-fold (48 h, P < 0.01) in isolated FZ cells.

CONCLUSIONS

MK likely plays a key role in HFA development. MK's selective in vitro mitotic effects on DZ cells may provide insights into the mechanism underlying the distinct in vivo differences in mitotic activity between the DZ and FZ.

Maintenance of proliferative capacity and retroviral transduction efficiency of human fetal CD38(-)/CD34(++) stem cells

Methods for the efficient transduction and expansion of fetal hematopoietic stem cells could lead to novel in utero therapies for blood cell disorders and enzymatic deficiencies. Here we describe a new assay to measure rapidly the effects of cytokines on the differentiation or expansion of primitive progenitors and stem cells found among CD38(-)CD34(++) lineage() cells isolated from human midgestation liver. Importantly, conditions that otherwise supported the expansion of clonogenic progenitors reduced their proliferative capacity. A combination of megakaryocyte growth and development factor and granulocyte-macrophage colony-stimulating factor maintained proliferative potential while also yielding an intermediate level of progenitor expansion. Retroviral transduction was achieved using Moloney murine leukemia virus-based vectors. Freshly isolated candidate stem cells could be transduced at almost 17% efficiency by a 1-h exposure to virus with centrifugation to aid transduction. This was increased to a mean 35.5% transduction efficiency after 1 day of culture. Additionally, the transduction efficiency of candidate stem cells isolated from fetal placental blood was 33.0%. These findings encourage further investigation into the feasibility of ex utero gene therapy whereby fetal cells are isolated from the circulation, transduced, and expanded ex utero before being returned to the fetus.

In utero transplantation: baby steps towards an effective therapy

In utero transplantation (IUT) offers the potential to treat a large number of diseases by transplantation of healthy cells into a fetus with a birth defect. Prenatal diagnosis is feasible for many diseases prior to the full development of the fetal immune system offering the opportunity to introduce foreign cells and antigens into the developing fetus. At least 45 cases of IUT have been performed for a variety of diseases. IUT has successfully treated severe combined immunodeficiency and there are indications that it may be effective in treating some nonhematopoietic diseases. However, many diseases remain resistant to fetal therapy owing to the low levels of chimerism that can be achieved. Promising efforts to improve the levels of engraftment are focusing on optimizing the graft and developing donor-specific tolerance in the fetal recipient. Mounting evidence suggests that donor T cells can aid in achieving clinically significant levels of chimerism. The use of fetal donor cells may also offer some benefit. Animal experiments suggest that even low-level chimerism can lead to tolerance, which can be exploited by booster transplants in the neonate. Continued research appears likely to succeed in developing IUT into an effective form of therapy for a variety of diseases.

Prevention of graft rejection by donor type II CD8(+) T cells (Tc2 cells) is not sufficient to improve engraftment in fetal transplantation

OBJECTIVES

Tc2 cells, a subset of CD8(+) T cells, are able to facilitate engraftment in a murine model of postnatal allogeneic bone marrow transplantation. The purpose of this study was to evaluate whether Tc2 cells could improve engraftment in fetal transplantation.

METHODS

Gestational day 13 C57BL/6 (H-2(b)) fetal mice were used as recipients, adult B6D2F(1) mice (C57BL/6 x DBA/2, H-2(b/d)) as donors, and splenocytes from B6C3F(1) (C57BL/6 x C3H/He, H-2(b/k)) mice were used as stimulators in cultures used to generate the Tc2 cells from B6D2F(1) mice. Peripheral blood chimerism was examined monthly for 3 months. Thereafter, recipients were sacrificed to evaluate the levels of peritoneal, splenic and bone marrow chimerism. The T-cell responses of recipient splenocytes to cells of host origin were measured as a proliferative response in mixed lymphocyte cultures.

RESULTS

Low levels of peripheral blood cell chimerism (<0.3%) were observed at 1 month of age, which declined further by 3 months of age. The levels of donor cells in the spleen, bone marrow and peritoneal cavity were usually not more than 0.05%. The peritoneal cavity tended to have higher levels of donor cells with 1 recipient sustaining as high as 25.03% at the age of 3 months. Higher peritoneal chimerism correlated with a lower donor-specific T-cell response.

CONCLUSIONS

Transplantation of Tc2 cells was insufficient to improve bone marrow engraftment in utero, suggesting that graft rejection is not the major barrier to successful in utero transplantation. Donor cells can persist in the peritoneal cavity and might play an important role in inducing immune tolerance in fetuses.

Haploidentical donor T cells fail to facilitate engraftment but lessen the immune response of host T cells in murine fetal transplantation

The effects of donor T cells, or their CD8+ subset, on engraftment and tolerance induction in fetal transplantation were evaluated using an F1-into-parent mouse-model that does not permit a graft-versus-host effect. Gestational day 13 C57BL/6 (H-2Kb) fetuses were transplanted with B6D2F1 (H-2Kb/d) light density bone marrow cells (LDBMC) containing 1-2% T cells, T-cell depleted bone marrow cells (TDBMC, < 0.1% T cells), or TDBMC with enriched CD8+ T cells (CD8). Chimaerism levels in the peripheral blood, spleen and bone marrow were usually below 0.2% in all groups, indicating that T cells do not improve engraftment without a graft-versus-host effect. A significant, but transient, wave of donor cells was seen in the peripheral blood at 1 month of age in the CD8 and LDBMC groups. Relatively high levels of chimaerism (< 17%) were sometimes detected in the peritoneal cavities of recipients. T-cell tolerance specific to donor cells was evaluated in mixed lymphocyte cultures. The CD8 and LDBMC groups had significantly lower T-cell responses than untransplanted controls. These findings indicate that in utero transplantation of haploidentical donor CD8+ or CD3+ cells can help to lessen the immune response of host T cells towards donor cells. The persistence of donor cells in the peritoneal cavity also correlated with tolerance induction.

Maternal microchimerism in the livers of patients with biliary atresia

Background

Biliary atresia (BA) is a neonatal cholestatic disease of unknown etiology. It is the leading cause of liver transplantation in children. Many similarities exist between BA and graft versus host disease suggesting engraftment of maternal cells during gestation could result in immune responses that lead to BA. The aim of this study was to determine the presence and extent of maternal microchimerism (MM) in the livers of infants with BA.

Methods

Using fluorescent in situ hybridization (FISH), 11 male BA & 4 male neonatal hepatitis (NH) livers, which served as controls, were analyzed for X and Y-chromosomes. To further investigate MM in BA, 3 patients with BA, and their mothers, were HLA typed. Using immunohistochemical stains, the BA livers were examined for MM. Four additional BA livers underwent analysis by polymerase chain reaction (PCR) for evidence of MM.

Results

By FISH, 8 BA and 2 NH livers were interpretable. Seven of eight BA specimens showed evidence of MM. The number of maternal cells ranged from 2–4 maternal cells per biopsy slide. Neither NH specimen showed evidence of MM. In addition, immunohistochemical stains confirmed evidence of MM. Using PCR, a range of 1–142 copies of maternal DNA per 25,000 copies of patients DNA was found.

Conclusions

Maternal microchimerism is present in the livers of patients with BA and may contribute to the pathogenesis of BA.

Megakaryocyte growth and development factor is a potent growth factor for primitive hematopoietic progenitors in the human fetus

Megakaryocyte growth and development factor (MGDF), or thrombopoietin, has received considerable attention as a therapeutic agent for treating thrombocytopenia or for its use in the ex vivo culture of hematopoietic stem cells. MGDF is known to support the growth of a broad spectrum of hematopoietic precursors obtained from adult or neonatal tissues, but its effects on the growth of fetal progenitors and stem cells has not been studied. Human CD38(+)CD34(2+) progenitors and CD38(-)CD34(2+) cells, a population that contains stem cells, were isolated from midgestation liver and grown under defined conditions with MGDF and various cytokines known to support the growth of primitive hematopoietic precursors. In clonal assays of colony-forming cells (CFCs), MGDF supported the growth of 15-25% of candidate stem cells when combined with granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor (GM-CSF), flk-2/flt3 ligand, or stem cell factor. MGDF was observed to strongly support the early stages of hematopoiesis and expansion of high proliferative potential CFCs. More mature progenitors were expanded nearly 78-fold in 1 wk of culture with MGDF+SCF+GM-CSF. MGDF alone was also found to support the short-term (2 d) survival of CD38(-)CD34(2+) high proliferative potential CFCs. The effects of MGDF were more modest on CD38(+)CD34(2+) progenitors with only additive increases in colony formation being observed. These findings suggest that MGDF administration in fetuses and neonates may strongly affect the growth and mobilization of primitive hematopoietic progenitors and that MGDF may find use in the ex vivo growth and expansion of fetal stem cells.

Searching for common stem cells of the hepatic and hematopoietic systems in the human fetal liver: CD34+ cytokeratin 7/8+ cells express markers for stellate cells

BACKGROUND/AIMS

The hematopoietic and hepatic systems are intertwined in the liver during fetal life. Cells expressing the hematopoietic stem cell marker CD34 and cytokeratin 7/8 (CK7/8) are hypothesized to be common stem cells for the hematopoietic and hepatic systems. Our aim was to determine if human fetal liver cells expressing CD34 and CK7/8 represent a common stem cell for both the hematopoietic and hepatic systems.

METHODS

CD34+CK7/8+ cells from midgestation livers were analyzed for the expression of various markers by flow cytometry and isolated based on their expression of CD34, nerve growth factor receptor (NGFR) and lack of CD45 expression. CD34+CD38- hematopoietic stem cells were also isolated and cultured in the presence of various hepatopoietins.

RESULTS

CD34+CK7/8+ cells comprised 3.4-8.5% of the erythrocyte-depleted liver. CD34+CK7/8+ cells had unique light-scatter properties compared to hematopoietic precursors and did not express most markers associated with hematopoietic cells. They did stain with CD13, CD59, NGFR, desmin and alpha-smooth muscle actin. In culture, these cells had a stellate appearance. Cultured hematopoietic stem cells failed to generate hepatocytes.

CONCLUSIONS

CD34+CK7/8+ cells are not common stem cells but rather appear to be hepatic stellate cells. A link between the hematopoietic and hepatic systems during fetal life requires further investigation.

Ontogenic changes in CD95 expression on human leukocytes: prevalence of T-cells expressing activation markers and identification of CD95-CD45RO+ T-cells in the fetus

The ontogeny of the human immune system was studied by analyzing fetal and adult tissues for the presence of various lymphocyte populations and activation/maturation markers. CD95 (fas) was expressed in hematopoietic tissues during the final stages of development of monocytes, granulocytes, NK cells and T cells, but to a much lesser extent on B cells. In the periphery, CD95 expression declined on granulocytes and NK cells. CD95 was expressed at a higher level on CD45RA+ peripheral T-cells in the fetus than in the adult. Contrary to the belief that most fetal T-cells are naïve or resting, a notable number of CD45RO+ T-cells were observed as well as an unique CD95-CD45RO+ population. Activation markers CD25, CD122, CD69 and CD80 were also present on fetal T-cells. These findings indicate that in the initial weeks following thymic maturation, a high frequency of T-cells is activated in the periphery of the fetus.

A kinetic study of the murine mixed lymphocyte reaction by 5,6-carboxyfluorescein diacetate succinimidyl ester labeling

Alternatives to the use of radioisotopes to measure cell proliferation in mixed lymphocyte reactions (MLR) are desirable to avoid the hazards and costs associated with radioisotope use. The versatile fluorochrome 5,6-carboxyfluorescein diacetate succinimidyl ester (CFSE) has been used to measure MLR and provides the opportunity to measure several different growth parameters. This study was aimed at determining which growth parameter is most practical and suitable for measuring murine MLR. The parameters measured were: the relative number of daughter T-cells, the relative number and frequency of reactive T-cell precursors and the relative number of mitotic events. Responder cells were CFSE-labeled unfractionated splenocytes from C57BL/6 mice. Stimulator cells included irradiated splenocytes from C57BL/6 (control), B6D2F(1) (haplo-allogeneic) or FVB/N (allogeneic) mice. Cultures were harvested daily for 1 week. Stimulator T-cells rapidly declined to less than 0.2-0.3% of the mixed population by day 2 of culture. Experimental groups had a significantly higher number of daughter T-cells and mitotic events after 2 days of culture with the number of daughter T-cells climbing exponentially after 5 days of culture. The number and frequency of reactive T-cell precursors were significantly higher in experimental groups on days 2-3, but this difference became insignificant by day 4. Among all the parameters, the relative number of daughter T-cells was the most practical for measuring MLR, after 5 days of culture, based upon the growth kinetics of responder T-cells and the survival of the stimulator cells.

Isolation of definitive zone and chromaffin cells based upon expression of CD56 (neural cell adhesion molecule) in the human fetal adrenal gland

The cortex of the human midgestation adrenals comprises a thin layer of cells, the definitive zone (DZ) that surrounds the larger, inner fetal zone (FZ). CD56 expression was observed by immunohistochemistry on DZ cells and isolated groups of cells within the FZ. CD56 mRNA expression also was detected among DZ cells but not selected sections of FZ cells isolated by laser capture microdissection. Flow cytometric analysis of dispersed adrenal cells indicated CD56 expression on a subset of adrenal cells lacking expression of hematopoietic (CD45(+) and CD235a(+)) and endothelial (CD31(+)) cell markers. The CD56(+)CD31(-)CD45(-)CD235a(-) cells were isolated by discontinuous-gradient centrifugation and fluorescence-activated cell sorting. The purified cells were enriched for DZ cells based on expression of mRNA for metallopanstimulin-1, nephroblastoma overexpressed gene, and 3-beta-hydroxysteroid dehydrogenase. P450c17 mRNA expression also was detected among a subset of CD56(+) cells consistent with expression of low levels of this protein in some DZ cells. The presence of a subpopulation of chromaffin cells among the CD56(+) population also was shown by the expression of chromogranin A mRNA. These findings indicate that CD56 expression can be used to isolate DZ and chromaffin cells to further study their functional and developmental properties.

Isolation, growth and identification of colony-forming cells with erythroid, myeloid, dendritic cell and NK-cell potential from human fetal liver

The study of hematopoietic stem cells (HSCs) and the process by which they differentiate into committed progenitors has been hampered by the lack of in vitro clonal assays that can support erythroid, myeloid and lymphoid differentiation. We describe a method for the isolation from human fetal liver of highly purified candidate HSCs and progenitors based on the phenotypes CD38(-)CD34(++) and CD38(+)CD34(++), respectively. We also describe a method for the growth of colony-forming cells (CFCs) from these cell populations, under defined culture conditions, that supports the differentiation of erythroid, CD14/CD15(+) myeloid, CD1a(+) dendritic cell and CD56(+) NK cell lineages. Flow cytometric analyses of individual colonies demonstrate that CFCs with erythroid, myeloid and lymphoid potential are distributed among both the CD38(-) and CD38(+) populations of CD34(++) progenitors.

Broad distribution of colony-forming cells with erythroid, myeloid, dendritic cell, and NK cell potential among CD34(++) fetal liver cells

The generation of erythroid, myeloid, and lymphoid cells from human fetal liver progenitors was studied in colony-forming cell (CFC) assays. CD38(-) and CD38(+) progenitors that expressed high levels of CD34 were grown in serum-deprived medium supplemented with kit ligand, flk2/flt3 ligand, GM-CSF, c-mpl ligand, erythropoietin, and IL-15. The resulting colonies were individually analyzed by flow cytometry. CD56(+) NK cells were detected in 21.9 and 9.9% of colonies grown from CD38(-) and CD38(+) progenitors, respectively. NK cells were detected in mostly large CD14(+)/CD15(+) myeloid colonies that also, in some cases, contained red cells. NK cells were rarely detected in erythroid colonies, suggesting an early split between the erythroid and the NK cell lineages. CD1a(+) dendritic cells were also present in three-quarters of the colonies grown from CD38(-) and CD38(+) progenitors. Multilineage colonies containing erythrocytes, myeloid cells, and NK cells were present in 13.7 and 2.7% of colonies grown from CD38(-) and CD38(+) progenitors, respectively. High proliferative-potential CFCs that generated multilineage colonies were also detected among both populations of progenitors. The total number of high proliferative-potential CFCs with erythroid, myeloid, and NK cell potential was estimated to be 2-fold higher in the CD38(+) fraction compared with the CD38(-) fraction because of the higher frequency of CD38(+) cells among CD34(++) cells. The broad distribution of multipotent CFCs among CD38(-) and CD38(+) progenitors suggests that the segregation of the erythroid, myeloid, and lymphoid lineages may not always be an early event in hemopoiesis. Alternatively, some stem cells may be present among CD38(+) cells.

High-efficiency retroviral transduction of fetal liver CD38-CD34++ cells: implications for in utero and ex utero gene therapy

OBJECTIVES

Defining methods for the efficient transduction of fetal stem cells could lead to novel fetal therapies for blood cell disorders and other birth defects. In this study, we analyzed the effects of various parameters on the retroviral transduction of primitive hematopoietic progenitors/stem cells isolated from fetal liver.

METHODS

Candidate stem cells were isolated by fluorescence-activated cell sorting from midtrimester human livers based on the phenotype CD38-CD34++lineage- (lineage = glycophorin A, CD3, CD14, CD19, CD20 and CD56). A murine retroviral vector with a truncated human low-affinity nerve growth factor receptor (Delta NGFR) gene was used to transduce the candidate stem cells. Marker gene expression was monitored by flow cytometry using an anti-NGFR mAb. Candidate stem cells were transduced immediately after isolation or after up to 4 days of culture in serum-deprived medium containing the growth factors kit ligand and granulocyte-macrophage colony-stimulating factor. The effects on transduction efficiency of the addition of 4 microg/ml protamine sulfate and/or centrifugation to concentrate the candidate stem cells and virus were tested. After transduction, the cells were expanded for 10-21 days before determining the frequency of NGFR+ cells among the different hematopoietic progeny.

RESULTS

Efficient transduction of candidate stem cells, at an average rate of 46%, was achieved after 3 days of culture with a single exposure to virus. Longer than 3 days of culture or repeated exposure to viral supernatant did not significantly improve the rate of transduction. The use of centrifugation at 1,200 g for 1 h and the addition of protamine sulfate during the transduction procedure were critical to achieving a high rate of transduction. Marker gene expression was observed on the progeny of the transduced cells in conjunction with CD34 (progenitors), glycophorin A (erythrocytes), CD14 (monocytes), CD15 (granulocytes) and CD41 (megakaryocytes).

CONCLUSIONS

This study demonstrates that the efficient transduction of fetal candidate stem cells can be achieved under defined culture conditions using a retroviral vector. These results encourage further examination of in utero and ex utero gene therapy as a means of treating birth defects.

Disparate regulation of human fetal erythropoiesis by the microenvironments of the liver and bone marrow

The liver and the bone marrow (BM) are the major organs that support hematopoiesis in the human fetus. Although both tissues contain the spectrum of hematopoietic cells, erythropoiesis dominates the liver. Previous studies suggested that a unique responsiveness of fetal burst-forming units erythroid (BFU-E) to erythropoietin (EPO) obviates the need for cytokines with burst-promoting activity (BPA) in fetal erythropoiesis. This potential regulatory mechanism whereby fetal erythropoiesis is enhanced was further investigated. Fluorescence-activated cell sorting was used to isolate liver and BM progenitors based on their levels of CD34 and CD38 expression. The most mature population of CD34+ lineage (Lin-) cells was also the most prevalent of the three subpopulations and contained BFU-E responsive to EPO alone under serum-deprived conditions. Kit ligand (KL) also strongly synergized with EPO in stimulating the growth of these BFU-E. An intermediate subset of CD34++CD38+Lin- cells contained erythroid progenitors responsive to EPO alone, but also displayed synergism between EPO and KL, granulocyte-macrophage colony-stimulating factor (GM-CSF), or interleukin (IL)-3, demonstrating that erythroid progenitors that respond to cytokines with BPA do exist in fetal tissues as in the adult BM. Candidate stem cells (CD34++CD38-Lin- cells) did not respond to EPO. Synergisms among KL, GM-CSF, and IL-3, and to a lesser extent granulocyte colony-stimulating factor (G-CSF) and FLK-2/FLT-3 ligand (FL), supported the growth of primitive multipotent progenitors that became responsive to EPO. These data define the limits of EPO activity in fetal erythropoiesis to cells that express CD38 and demonstrate the potential for various cytokine interactions to be involved in regulating fetal erythropoiesis. Furthermore, a comparison of the responses of liver and BM erythroid progenitors revealed similarity in their responses to cytokines but a difference in the frequency of BFU-E among the three subpopulations examined. A higher frequency of BFU-E among the intermediate and late progenitor subsets in the liver indicates that regulatory factors acting on stem cells and their immediate progeny are partially responsible for the high content of erythropoiesis in the liver. These data implicate a critical role for the microenvironments of the liver and BM in regulating the disparate levels of erythropoiesis in these tissues.

Antibodies to human fetal erythroid cells from a nonimmune phage antibody library

The ability to isolate fetal nucleated red blood cells (NRBCs) from the maternal circulation makes possible prenatal genetic analysis without the need for diagnostic procedures that are invasive for the fetus. Such isolation requires antibodies specific to fetal NRBCs. To generate a panel of antibodies to antigens present on fetal NRBCs, a new type of nonimmune phage antibody library was generated in which multiple copies of antibody fragments are displayed on each phage. Antibody fragments specific for fetal NRBCs were isolated by extensive predepletion of the phage library on adult RBCs and white blood cells (WBCs) followed by positive selection and amplification on fetal liver erythroid cells. After two rounds of selection, 44% of the antibodies analyzed bound fetal NRBCs, with two-thirds of these showing no binding of WBCs. DNA fingerprint analysis revealed the presence of at least 16 unique antibodies. Antibody specificity was confirmed by flow cytometry, immunohistochemistry, and immunofluorescence of total fetal liver and adult RBCs and WBCs. Antibody profiling suggested the generation of antibodies to previously unknown fetal RBC antigens. We conclude that multivalent display of antibodies on phage leads to efficient selection of panels of specific antibodies to cell surface antigens. The antibodies generated to fetal RBC antigens may have clinical utility for isolating fetal NRBCs from maternal circulation for noninvasive prenatal genetic diagnosis. Some of the antibodies may also have possible therapeutic utility for erythroleukemia.

Transplantation of a fetus with paternal Thy-1(+)CD34(+)cells for chronic granulomatous disease

A fetus diagnosed with X-linked chronic granulomatous disease was transplanted with Thy-1(+)CD34(+) cells of paternal origin. The transplant was performed at 14 weeks gestation by ultrasound guided injection into the peritoneal cavity. The fetus was delivered at 38 weeks gestation after an otherwise uneventful pregnancy. Umbilical cord blood was collected and used to determine the level of peripheral blood chimerism as well as levels of functional engrafted cells. Flow cytometry was used to detect donor leukocytes identified as HLA-A2(-)B7(+) cells, whereas recipient cells were identified as HLA-A2(+)B7(-) cells. No evidence of donor cell engraftment above a level of 0.01% was found. PCR was used to detect HLA-DRB1*15(+) donor cells among the recipient's HLA-DRB1*15(-) cells, but no engraftment was seen with a sensitivity of 1:1000. The presence of functional, donor-derived neutrophils was assessed by flow cytometry using two different fluorescent dyes that measure reactive oxygen species generated by the phagocyte NADPH oxidase. No evidence of paternal-derived functional neutrophils above a level of 0.15% was observed. Peripheral blood and bone marrow samples were collected at 6 months of age. Neither sample showed engraftment by HLA typing using both flow cytometry and PCR. Functional phagocytes were also not observed. Furthermore, no indication of immunological tolerance specific for the donor cells was indicated by a mixed lymphocyte reaction assay performed at 6 months of age. While there appears to be no engraftment of the donor stem cells, the transplant caused no harm to the fetus and the child was healthy at 6 months of age. Analyses of fetal tissues, obtained from elective abortions, revealed that CD3(+) T cells and CD56(+)CD3(-) NK cells are present in the liver at 8 weeks gestation and in the blood by 9 weeks gestation. The presence of these lymphocytes may contribute to the lack of donor cell engraftment in the human fetus.

Hematopoietic Stem Cell Transplantation in Utero Produces Sheep–Goat Chimeras

Both allogeneic and xenogeneic hematopoietic chimera models have been developed, including fetal sheep models that demonstrated high levels of stable, multilineage engraftment created by in utero hematopoietic stem cell transplantation. The aim of this study was to test the efficacy of in utero transplantation to create xenogeneic sheep-goat hematopoietic chimeras. Fetal liver cells and T-cell-depleted adult bone marrow were tested as sources of hematopoietic stem cells. Donor cells were injected intraperitoneally into 130 recipient fetuses between 49 and 62 days of gestation. Groups 1 and 2 received crude fetal liver cell preparations. Group 3 received fetal liver cells that were incubated overnight in a phytohemagglutinin-stimulated lymphocyte-conditioned medium (PHA-LCM). In Group 4, hematopoietic stem cells were concentrated by using additional density separations. Group 5 fetal recipients received low-density, T-cell-depleted adult bone marrow cells. In Group 1, fetuses were accessed via hysterotomy. Hematopoietic stem cells were injected into Groups 2, 3, 4, and 5 without cutting through the uterine wall. Fetal survival in the five groups ranged from 56 to 100%. The percentage of chimeras from injected fetuses ranged from 43 to 92% by FACS and PCR analyses; however, levels of chimerism were low (<1%). The highest rates of chimerism were found among recipients of low-density fetal liver cells. Despite the pre-immunocompetent status of the fetal recipients and the genetic similarities between sheep and goats, high levels of engraftment were not observed. The consistently low levels of chimerism observed in this study, as well as the poor results recently reported by others using these procedures, indicate that significant barriers exist to transplanting hematopoietic stem cells in utero.

Differential effects of interleukin-3, interleukin-7, interleukin 15, and granulocyte-macrophage colony-stimulating factor in the generation of natural killer and B cells from primitive human fetal liver progenitors

The regulatory roles of a number of early-acting growth factors on the generation of natural killer (NK) cells and B cells from primitive progenitors were studied. Experiments focused on the contributions of granulocyte-macrophage colony-stimulates factor (GM-CSF) and interleukin-3 (IL-3) to the regulation of the early events of lymphopoiesis.Two progenitor populations isolated from human fetal liver were studied, CD38(-)CD34(++)lineage(-) (Lin(-)) cells (candidate hematopoietic stem cells [HSCs]) and the more mature CD38(+)CD34(++)Lin(-) cells. The effects of different cytokines on the generation of CD56(+)CD3(-) NK cells and CD19(+) B cells were studied in serum-deprived cultures in the absence of stroma.NK cells generated in vitro were able to kill NK-sensitive target cells, expressed NK-associated marker CD161 (NKR-P1A), but exhibited little or no expression of CD2, CD8, CD16, CD94/NKG2A, or killer cell inhibitory receptors (KIRs). Among the cytokine combinations tested, kit ligand (KL) and IL-15 provided the best conditions for generating CD56(+) NK cells from CD38(+)CD34(++)Lin(-) cells. However, either flk-2/flt3 ligand (FL), GM-CSF, IL-3, or IL-7 could partially substitute KL. All of these cytokines also supported the growth of NK-cell progenitors from candidate HSC, with the combination of IL-15, KL, GM-CSF, and FL generating the greatest number of CD56(+) cells. B cells were generated from both progenitor populations in response to the combined effects of KL, FL, and IL-7. Both B and NK cells were generated with the further addition of IL-15 to these cultures. The in vitro generated B cells were CD10(+), CD19(+), HLA-DR(+), HLA-DQ(+), and some were CD20(+), but no cytoplasmic or surface immunoglobulin M expression was observed. In contrast with NK lymphopoiesis, GM-CSF, IL-3, and IL-15 had no effect on the generation of B cells from CD38(-)CD34(++)Lin(-) cells, and GM-CSF inhibited B-cell generation from CD38(+)CD34(++)Lin(-) progenitors. These findings indicate a differential regulation of NK and B lymphopoiesis beginning in the early stages of hematopoiesis as exemplified by the distinctive roles of IL-7, IL-15, GM-CSF, and IL-3.

Fetal bone marrow as a source of stem cells for in utero or postnatal transplantation

We examined the potential of human fetal bone marrow (FBM) as a source of haematopoietic stem cells for transplantation. The median number of cells obtained between 20 and 24 weeks' gestation was 1.9 x 109 and a median 1.17 x 108 of these cells expressed CD34. Flow cytometry was also used to estimate the content of three different candidate stem cell populations in the tissues older than 20 weeks' gestation. A median 8.8 x 105 CD34++CD38- cells, 1.37 x 106 CD34++CD4+ cells and 2.20 x 106 CD34++CD90+ cells were detected. The content of colony-forming units culture (CFU-C) in the FBM ranged from 2.8 x 104 to 6.0 x 106 per fetus. The CFU-C content could be expanded 50-fold by culture for 1 week in serum-deprived medium and the growth factors kit ligand and granulocyte-macrophage colony-stimulating factor. Positive selection of FBM CD34+/++ cells was achieved using the Baxter Isolex 50 device. An average purity of 82% and yield of up to 19% of CD34+/++ cells was achieved. T cells were depleted by 99.84%. Analysis of candidate stem cell populations and primitive CFU-C suggested a preferential enrichment of these cells over the total population of CD34+/++ cells. All FBM samples were found to be free of microbial contamination at the time of harvest and after selection of CD34+/++ cells. Thus, FBM is a safe source of stem cells. The large number of progenitors and candidate stem cells that can be obtained from FBM makes it suitable for in utero and possibly postnatal transplantation.

Role of CD95/Fas and its ligand in the regulation of the growth of human CD34(++)CD38(-) fetal liver cells

The functional significance of CD95/Fas expressed by candidate hematopoietic stem cells (HSCs) from human fetal liver was studied by testing the effect of agonistic anti-CD95 monoclonal antibody (mAb) CH-11 and soluble CD95 ligand (sCD95L) on the growth of CD34(++)CD38(-)lineage cells in vitro. Candidate fetal HSCs exhibited a dose-dependent proliferative response to CH-11 as well as to sCD95L when combined with kit ligand (KL) + interleukin 3 (IL-3) under serum-deprived culture conditions. CH-11 mAb increased, in a synergistic fashion, the number of myeloid colony-forming unit culture (CFU-C) generated by candidate HSCs in liquid cultures with the cytokine combinations KL + IL-3, KL + granulocytemacrophage colony-stimulating factor, and KL + IL-6. CH-11 mAb and sCD95L also enhanced erythropoiesis supported by KL + IL-3 + erythropoietin (Epo). Furthermore, sCD95L was able to increase the number of megakaryocytes, granulocytes, and CD34- cells generated in the presence of KL + IL-3 + Epo + thrombopoietin. An analysis performed using Western blotting revealed that the membrane-bound CD95L (mCD95L) was expressed by both immature (total CD34+/++) and mature (CD34-) hematopoietic lin(-) FL cells. Among the CD34(++)lin(-)cells, both the freshly isolated CD38+ and CD38 subsets as well as CD95+ and CD95- cells constitutively expressed mCD95L, demonstrating that the CD95/CD95L system represents a paracrine and potentially autocrine regulator of early hematopoiesis. To study the role of the endogenously produced CD95L, we determined the effects of a neutralizing anti-CD95L NOK-1 on the growth of candidate HSCs. By blocking the endogenous CD95L with NOK-1 mAb, we observed an increase in CFU-C generated by candidate HSCs. We conclude that the endogenous CD95L has an inhibitory effect on fetal candidate HSCs, which can be blocked by sCD95L and CH-11 mAb.

Administration of Flk2/Flt3 ligand induces expansion of human high-proliferative potential colony-forming cells in the SCID-hu mouse

The effects of Flk2/Flt3 ligand (FL) administration on human hematopoiesis were investigated using SCID-hu mice transplanted with human fetal bone fragments. Treatment with recombinant human FL induced significant increases in the frequencies of the high-proliferative potential colony-forming cells and low-proliferative potential colony-forming cells in steady-state human bone marrow. FL also promoted the expansion of high-proliferative potential colony-forming cells and low-proliferative potential colony-forming cells in the human bone marrow during the recovery phase after irradiation, which was evident in increases in the frequencies as well as in the absolute numbers of colony-forming cells. Furthermore, higher percentages of CD33+ CD15- cells were found in the marrows treated with FL as compared to that of controls, indicating that FL hastened the recovery of at least some aspect of myelopoiesis after irradiation. These results indicate that FL induces the expansion of primitive hematopoietic progenitor cells in vivo and, therefore, may be useful in treating patients to promote an early hematopoietic recovery after cytoablative therapies.

The biology and ethics of banking fetal liver hematopoietic stem cells for in utero transplantation

BACKGROUND/PURPOSE

Transplantation of fetal liver hematopoietic stem cells (HSCs) in utero has the potential to treat a variety of hematologic, immunologic, and metabolic diseases. One prerequisite for broad clinical application is the establishment of a bank of fetal liver HSC tissue. The authors describe their methods for processing fetal liver free of known human pathogens while maximizing HSC activity after cryopreservation.

METHODS

The authors developed a protocol that separates the abortion decision from the donation decision and preserves confidentiality between donor and recipient. Human fetal livers (12 to 14 weeks' gestation) were procured from aborted specimens and the light-density hematopoietic cells isolated by density centrifugation. Total viable cell count increased with gestational age and averaged from 4.36 x 10(7) cells for 12-week livers to 2.0 x 10(8) cells for 14-week livers.

RESULTS

Flow cytometric analysis demonstrated the presence of early progenitors in fresh and thawed specimens and a low number of T cells in each group. The functional capacity of fetal liver progenitors was assessed with colony-forming assays before and after cryopreservation. Thawed specimens showed an average 63% recovery rate for the high-proliferative potential colony-forming cells, a primitive subset of progenitors thought to include HSC. However, the more mature fraction of low-proliferative potential colony-forming cells had a recovery rate of only 35%. These data suggest that fetal liver HSC maybe more resistant to the detrimental effects of cryopreservation than mature progenitors. The fetal liver was screened for bacterial, fungal, and viral contaminates and the serum from donor mothers was screened for human immunodeficiency virus (HIV), hepatitis A, B, and C, human T-cell lymphoma virus (HTLV I/II), rapid plasma reagent (RPR), cytomegalovirus (CMV), and toxoplasmosis IgM. The bacterial contamination rate was 14% (n = 28). The maternal serum was positive for CMV in 78% of cases, and positive for hepatitis C in 0.7% of cases (n = 28). However, all fetal liver specimens were culture negative for CMV.

CONCLUSIONS

These findings demonstrate that human fetal liver HSCs can be procured ethically and processed to ensure a safe graft with a small number of T-cells, and a high yield of progenitors after cryopreservation. A bank of fetal liver HSC will prove useful in treating a variety of genetic diseases before birth by in utero HSC transplantation.

Colony-forming cells expressing high levels of CD34 are the main targets for granulocyte colony-stimulating factor and macrophage colony-stimulating factor in the human fetal liver

The effects of the granulocyte (G) and macrophage (M) colony-stimulating factors (CSFs) on the growth of purified subpopulations of human fetal liver progenitors were investigated. In contradiction to the characterization of these cytokines as CSFs acting late in the course of hematopoiesis, both G-CSF and M-CSF were most potent in promoting the growth of fetal liver colony-forming cells (CFCs) that express high levels of CD34 and CD38 (CD34++CD38+) and are depleted of cells expressing a panel of lineage markers (Lin-). Cultures of these cells in serum-deprived conditions generated a mean of 11.2 and 39.1 low-proliferative potential (LPP)-CFCs per 1.0 x 10(3) CD34++CD38+Lin- cells grown in G-CSF and M-CSF, respectively. Cultures of more mature progenitors, isolated based on a lower level of CD34 expression (CD34+ Lin-), generated few LPP-CFCs and 6.3 and 4.7 clusters per 1.0 x 10(3) CD34+Lin- cells in response to G-CSFs and M-CSF, respectively. G-CSF was also found to synergistically enhance colony growth by either kit-ligand (KL) or fit-3/flk-2 ligand (FL) in cultures of CD34++CD38+Lin- cells as well as the more primitive compartment of CD34++CD38-Lin- cells. Synergism between G-CSF and KL or FL was also observed in liquid cultures of CD34++CD38-Lin- cells. The effects of G-CSF on CD342++CD38-Lin- cells were further demonstrated by the ability of G-CSF to support the short-term survival of these cells in clonal cultures. In contrast, M-CSF did not affect the growth or survival of CD34++CD38-Lin- cells, a finding that was also supported by the observation that the receptor for M-CSF (CD115 or fms) was only expressed on CD34++CD38+Lin- cells. G-CSF receptor expression and flt-3/flk-2 expression were detected by flow cytometry on both the CD38- and CD38+ subpopulations of CD34++Lin- cells, but these receptors were not detected on CD34+ cells. Receptors for KL (CD117) and interleukin-3 (CD123), for which the ligands are active on a broad range of fetal liver progenitors, were detected on cells expressing both high and low levels of CD34. These data help to define the potential roles of cytokines in human fetal hematopoiesis.

Phenotypic and functional evidence for the expression of CD4 by hematopoietic stem cells isolated from human fetal liver

Expression of the CD4 antigen was observed on human fetal liver, fetal bone marrow (BM), and umbilical cord blood progenitors expressing high levels of CD34. Using clonal and liquid-culture assays, CD4+ CD34++ Lin- (lineage = CD3, CD8, CD10, CD14, CD15, CD16, CD19, CD20, and glycophorin A) fetal liver progenitors were found to have a greater proliferative potential than CD4- CD34++ Lin- progenitors, whereas the CD4- fraction was more enriched for erythroid progenitors. Both the CD4+ and the CD4- progenitor subpopulations also gave rise to multilineage engraftment upon transplantation into human fetal bone fragments, supportive of B-lymphoid and myeloid growth, or into human fetal thymic fragments, supportive of T-cell growth, implanted in scid/scid (SCID) mice. However, in SCID-hu mice transplanted with graded doses of donor cells ranging from 2.0 x 10(2) to 2.0 x 10(4) cells, BM reconstitution by the CD4+ fraction of CD34++ Lin- cells was more frequent than by the CD4- fraction when low numbers of cells were injected. These functional data strongly suggest that stem cells reside among CD4+ CD34++ Lin- fetal liver cells. This hypothesis was further supported by the observations that CD4+ CD34++ Lin- fetal liver cells were enriched for CDw90+ (Thy-1), CD117+ (kit), CD123+, HLA-DR+, CD7-, CD38-, CD45RA-, CD71-, CD115- (fms), and rhodamine 123(dull) cells, a phenotypic profile believed to represent fetal stem cells. Furthermore, all CD4+ CD34++ Lin- fetal liver cells also expressed CD13 and CD33.

Expression of Fas/CD95 and Bcl-2 by primitive hematopoietic progenitors freshly isolated from human fetal liver

The cell-surface expression and the functional status of the CD95/Fas antigen on primitive hematopoietic progenitors (PHPs) freshly isolated from human fetal liver (FL) were studied. PHPs were phenotypically defined as CD34++ CD38 -/+ cells. The most immature subfractions of PHPs, CD34++CD38- and CD34+2CD38+ FL cells, expressed CD95, whereas the more mature CD34++CD38++ and CD34+CD38++2 FL cells displayed low CD95 expression. Combinations of cytokines, such as kit ligand (KL) + interleukin-3 or KL + granulocyte-macrophage colony-stimulating factor (GM-CSF) upregulated the expression of CD95 on PHPs upon in vitro culture. Tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) further increased the CD95 expression induced by KL+GM-CSF. The hematopoietic potential of sorted CD34++lineage (lin)- CD95+ versus CD34++ lin-CD95-FL cells was compared by colony-forming unit-culture (CFU-C) assays performed in serum-deprived medium. Lin+ cells were composed of erythrocytes, monocytes, T cells, B cells, and natural killer cells. Our results indicated that both CD95- and CD95+ subsets contained pluripotent progenitors, generating myeloid and erythroid progenitors. The functional status of CD95 and the effects of TNF-alpha and IFN-gamma, cytokines known to induce CD95-mediated apoptosis, were analyzed by incubation of PHPs in the presence of anti-CD95 monoclonal antibodies (MoAbs). The effect of anti-CD95 MoAbs was measured by viable cell counting, flow cytometry, and CFU-C assays. A decrease of CFU-C numbers was observed in the presence of anti-CD95 MoAbs and TNF-alpha and/or IFN-gamma. However, whereas growth factor deprivation induced apoptosis of PHPs, cross-linking of CD95 did not lead to apoptosis of PHPs measured by flow cytometry and viable cell counting. The correlation of increased intracytoplasmic levels of bcl-2 with high levels of cell-surface CD34 and the presence of CD95 on fresh FL cells suggests that bcl-2 may be involved in protecting against CD95-mediated apoptosis of FL PHPs.

Regulatory roles of the ligand for Flk2/Flt3 tyrosine kinase receptor on human hematopoiesis

The biological activities of the ligand for the Flk2/Flt3 receptor tyrosine kinase (FL) on human hematopoietic cells are reviewed. In in vitro studies, FL shows relatively few effects by itself on the proliferation and differentiation of hematopoietic cells, but exhibits a potent costimulatory activity in enhancing the proliferation of progenitor cells of multiple lineages. FL promotes the growth of clonogenic myeloid progenitor cells in the presence of other cytokines known to be active on myeloid progenitors, including GM-CSF, interleukin 3 (IL-3), kit ligand (KL), M-CSF and G-CSF. In addition, FL synergizes with IL-7 in inducing the proliferation of pro-B cells, whereas FL has little effect on the growth of clonogenic erythroid progenitors. Furthermore, FL induces the in vitro expansion of the high proliferative potential colony-forming cells (HPP-CFC) and stimulates the proliferation of long-term culture-initiating cells (LTC-IC), suggesting an activity on the proliferation of putative stem cells. Thus, FL plays important roles in regulating the proliferation of hematopoietic progenitor cells and, therefore, may have therapeutic applications.

The FLK2/FLT3 ligand synergizes with interleukin-7 in promoting stromal-cell-independent expansion and differentiation of human fetal pro-B cells in vitro

The effects of a novel cytokine FLK2/FLT3 ligand (FL) on human fetal bone marrow-derived CD34+CD19+ pro-B cells were analyzed in a stromal-cell-independent, serum-deprived culture system. FL, like interleukin-3 (IL-3), synergized with IL-7 in promoting pro-B cell growth, and differentiation of these cells into CD34-CD19+clgM+slgM- pre-B cells, whereas a small proportion of these cells even differentiate into more mature slgM+ B cells. In contrast, KIT ligand (KL) and granulocyte-macrophage colony-stimulating factor (GM-CSF) were ineffective in promoting IL-7-dependent pro-B cell growth and differentiation. Maximal levels of pro-B cell expansion, generally resulting in 15- to 30-fold increases in cellularity, were obtained in cultures supplemented with optimal doses of FL + IL-7 + IL-3. The addition of mouse bone marrow stromal cells further enhanced the proliferation and differentiation of pro-B cells obtained in the presence of these three cytokines. Under these conditions, cultures could be maintained for more than 4 weeks, and in general 40- to 50-fold increases in cell numbers were observed by 3 weeks of culture. The percentages of clgM+ and slgM+ B cells increased 1.5- to 3-fold and 2-fold, respectively, suggesting that stromal cells may provide additional costimulatory signals for human B-cell growth and differentiation that are different from IL-7, IL-3, and FL. Collectively, our results indicate that FL, in contrast to KL, strongly promotes long-term expansion and differentiation of human pro-B cells in the presence of IL-7 or in combination of IL-7 and IL-3, which is a novel property of this hematopoietic growth factor.

Tracing the expression of CD7 and other antigens during T- and myeloid-cell differentiation in the human fetal liver and thymus

During the last decade, the function/s of the cell membrane CD7 antigen have been investigated in human mature T and NK cells, showing the direct involvement of this molecule in multiple effector functions related with activation, proliferation, production of cytokines and modification of adhesion properties. The CD7 glycoprotein is not only expressed by mature lymphoid cells, but also by early hematopoietic progenitors and several types of leukemias, suggesting a role of CD7 during hematopoiesis. However, the function of CD7 in the early stages of hematopoietic development has not yet been elucidated. CD7 has been classically considered the earliest T-cell specific marker. This assumption was based on data indicating the presence of CD45+CD7+CD3-CD4-CD8- cells in the human embryonic/fetal liver at the gestational age at which the thymic rudiment is colonized by T-cell progenitors. In the present article, we review recent results obtained by several groups concerning the expression of CD7 and various other cell surface antigens by T-, B- and myeloid-cell progenitors generated in the adult bone marrow and fetal liver. In addition, we present an hypothetical model of hematopoiesis in the fetal liver and thymus.

FLK-2/FLT-3 ligand regulates the growth of early myeloid progenitors isolated from human fetal liver

The effects of the recently identified FLK-2/FLT-3 ligand (FL) on the growth of purified human fetal liver progenitors were investigated under serum-deprived culture conditions. FL alone was found to stimulate modest proliferation in short-term cultures of CD34++ CD38+ lineage (Lin)- light-density fetal liver (LDFL) cells and the more primitive CD34++ CD38- Lin- LDFL cells. However, the low levels of growth induced by FL were insufficient for colony formation in clonal cultures. Synergism between FL and either granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) or KIT ligand (KL) was observed in promoting the growth of high-proliferative potential (HPP) colony-forming cells (CF) and/or low-proliferative potential (LPP)-CFC in cultures of CD34++ CD38+ Lin- and CD34++ CD38- Lin- LDFL-cells. FL, alone or in combination with other cytokines, was not found to affect the growth of CD34+ Lin- LDFL cells, the most mature subpopulation of fetal liver progenitors investigated. The growth of the most primitive subset of progenitors studied, CD34++ CD38- Lin- LDFL cells, required the interactions of at least two cytokines, because only very low levels of growth were observed in response to either FL, GM-CSF, IL-3 or KL alone. However, the results of delayed cytokine-addition experiments suggested that individually these cytokines did promote the survival of this early population of progenitors. Although two-factor combinations of FL, KL, and GM-CSF were observed to promote the growth of early progenitors in a synergistic manner, neither of these factors was found to make fetal liver progenitors more responsive to suboptimal concentrations of a second cytokine. Only myeloid cells were recovered from liquid cultures of CD34++ CD38- Lin- LDFL cells grown in the presence of combinations of FL, KL, and GM-CSF. These results indicate that FL is part of a network of growth factors that regulate the growth and survival of early hematopoietic progenitors.

Progress in the ex vivo expansion of hematopoietic progenitors

In this review we describe how studies on the cytokine-stimulated growth of murine bone marrow (BM) progenitors have lead to the observations that large increases in progenitor numbers can be achieved in short-term cytokine-stimulated liquid cultures. Transplantation of these ex vivo expanded murine BM cells was shown to decrease the number of BM cells required to confer radioprotection and to increase the recovery rate of both myeloid and erythroid peripheral blood cells. The ex vivo expansion of murine BM cells does not however, markedly diminish stem cells capable of long-term hematopoietic reconstitution. Investigations on the expansion of human BM, peripheral blood, umbilical cord blood and fetal hematopoietic progenitors have demonstrated that clinically useful increases in progenitor numbers from these tissues are possible. Thus, ex vivo progenitor expansion may soon be of use in transplantation protocols to accelerate hematopoietic reconstitution and in gene therapy protocols if hematopoietic stem cells can be maintained during ex vivo culture.

Identification of a common T/natural killer cell progenitor in human fetal thymus

The phenotypic similarities between natural killer (NK) and T cells have led to the hypothesis that these distinctive lymphocyte subsets may be developmentally related and thus may share a common progenitor (Lanier, L. L., H. Spits, and J. H. Phillips, 1992. Immunol. Today. 13:392; Rodewald, H.-R., P. Moingeon, J. L. Lurich, C. Dosiou, P. Lopez, and E. L. Reinherz. 1992. Cell. 69:139). In this report, we have investigated the potential of human CD34+ triple negative thymocytes ([TN] CD3-, CD4-, CD8-) to generate both T cells and NK cells in murine fetal thymic organ cultures (mFTOC) and in vitro clonogenic assays. CD34+ TN thymocytes, the majority of which express prominent cytoplasmic CD3 epsilon (cytoCD3 epsilon) protein, can be divided into high (CD34Bright) and low (CD34Dim) surface expressing populations. CD34Bright TN thymocytes were capable of differentiating into T and NK cells when transferred into mFTOC, and demonstrated high NK cell clonogenic capabilities when cultured in interleukin (IL)-2, IL-7, and stem cell factor (SCF). Likewise, CD34Bright TN thymocyte clones after 5 d in culture were capable of generating NK and T cells when transferred into mFTOC but demonstrated clonogenic NK cell differentiation capabilities when maintained in culture with IL-2. CD34Dim TN thymocytes, however, possessed only T cell differentiation capabilities in mFTOC but were not expandable in clonogenic conditions containing IL-2, IL-7, and SCF. No significant differentiation of other cell lineage was detected in either mFTOC or in clonogenic assays from CD34+ TN thymocytes. These results represent the first definitive evidence of a common T/NK cell progenitor in the human fetal thymus and delineate the point in thymocyte differentiation where T and NK cells diverge.

Lymphoid and myeloid differentiation of fetal liver CD34+lineage- cells in human thymic organ culture

In this article, we report that the human fetal thymus contains CD34bright cells (< 0.01% of total thymocytes) with a phenotype that resembles that of multipotent hematopoietic progenitors in the fetal bone marrow. CD34bright thymocytes were CD33-/dull and were negative for CD38, CD2, and CD5 as well as for the lineage markers CD3, CD4, and CD8 (T cells), CD19 and CD20 (B cells), CD56 (NK cells), glycophorin (erythrocytes), and CD14 (monocytes). In addition, total CD34+ lineage negative (lin-) thymocytes contained a low number of primitive myeloid progenitor cells, thus suggesting that the different hematopoietic lineages present in the thymus may be derived from primitive hematopoietic progenitor cells seeding the thymus. To investigate whether the thymus is permissive for the development of non-T cells, human fetal organ culture (FTOC) assays were performed by microinjecting sorted CD34+lin- fetal liver cells into fragments of HLA-mismatched fetal thymus. Sequential phenotypic analysis of the FTOC-derived progeny of CD34+lin- cells indicated that the differentiation into T cells was preceded by a wave of myeloid differentiation into CD14+CD11b+CD4dull cells. Donor-derived B cells (CD19+CD20+) were also generated, which produced immunoglobulins (IgG and IgM) when cultured under appropriate conditions, as well as functional CD56+CD3- NK cells, which efficiently killed K562 target cells in cytotoxicity assays. These results demonstrate that the microinjection of fetal liver hematopoietic progenitors into fetal thymic organ fragments results in multilineage differentiation in vitro.

Expression of CD33, CD38, and HLA-DR on CD34+ human fetal liver progenitors with a high proliferative potential

High proliferative-potential colony-forming cells (HPP-CFC) have been identified in the bone marrow of mice and adult humans, and have been characterized as a compartment of primitive progenitors possibly including stem cells. In this report we describe the human fetal liver (FL) as a source of HPP-CFC. These FL HPP-CFC develop in clonal cultures in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) within 3 to 4 weeks. The median frequency of HPP-CFC in FL tissues between 16 and 21 weeks of gestational age was 1 in 3,000 total FL cells. After 4 weeks of growth, FL HPP-CFC grew to a median colony size of 8.3 x 10(4) cells/colony. Using cell-sorting techniques FL HPP-CFC were shown to be predominantly contained in the CD34+ CD33+ CD38- fraction of FL cells. FL HPP-CFC were heterogeneous for HLA-DR expression, and no differences in proliferative capacities were observed between HLA-DR+ and HLA-DR- HPP-CFC. The CD34+ CD33-HLA-DR- CD38- population, previously suggested to contain stem cells, was observed to be very rare in the FL, representing approximately 1 in 1.7 x 10(5) light-density FL cells and containing almost no CFC. Therefore, it is possible that stem cells are contained in the CD33+ fraction of FL cells. Phenotypic characterization of CD34+ CD33+ CD38- lin -LDFL cells showed that these cells are also CD13+, predominantly Thy-1+, CD45RA-, CD45RO-, CD71-, and heterogenoeous for c-kit expression. These data suggest that FL HPP-CFC represent a heterogeneous compartment of primitive myeloid progenitors that may include stem cells.