Postnatal regulation of B-1a cell development and survival by the CIC-PER2-BHLHE41 axis.

B-1 cell development mainly occurs via fetal and neonatal hematopoiesis and is suppressed in adult bone marrow hematopoiesis. However, little is known about the factors inhibiting B-1 cell development at the adult stage. We report that capicua (CIC) suppresses postnatal B-1a cell development and survival. CIC levels are high in B-1a cells and gradually increase in transitional B-1a (TrB-1a) cells with age. B-cell-specific Cic-null mice exhibit expansion of the B-1a cell population and a gradual increase in TrB-1a cell frequency with age but attenuated B-2 cell development. CIC deficiency enhances B cell receptor (BCR) signaling in transitional B cells and B-1a cell viability. Mechanistically, CIC-deficiency-mediated Per2 derepression upregulates Bhlhe41 levels by inhibiting CRY-mediated transcriptional repression for Bhlhe41, consequently promoting B-1a cell formation in Cic-null mice. Taken together, CIC is a key transcription factor that limits the B-1a cell population at the adult stage and balances B-1 versus B-2 cell formation.

Prenatal development of human immunity.

The blood and immune systems develop in parallel during early prenatal life. Waves of hematopoiesis separated in anatomical space and time give rise to circulating and tissue-resident immune cells. Previous observations have relied on animal models, which differ from humans in both their developmental timeline and exposure to microorganisms. Decoding the composition of the human immune system is now tractable using single-cell multi-omics approaches. Large-scale single-cell genomics, imaging technologies, and the Human Cell Atlas initiative have together enabled a systems-level mapping of the developing human immune system and its emergent properties. Although the precise roles of specific immune cells during development require further investigation, the system as a whole displays malleable and responsive properties according to developmental need and environmental challenge.

Discovery of a CD10-negative B-progenitor in human fetal life identifies unique ontogeny-related developmental programs.

Human lymphopoiesis is a dynamic lifelong process that starts in utero 6 weeks postconception. Although fetal B-lymphopoiesis remains poorly defined, it is key to understanding leukemia initiation in early life. Here, we provide a comprehensive analysis of the human fetal B-cell developmental hierarchy. We report the presence in fetal tissues of 2 distinct CD19+ B-progenitors, an adult-type CD10+ve ProB-progenitor and a new CD10-ve PreProB-progenitor, and describe their molecular and functional characteristics. PreProB-progenitors and ProB-progenitors appear early in the first trimester in embryonic liver, followed by a sustained second wave of B-progenitor development in fetal bone marrow (BM), where together they form >40% of the total hematopoietic stem cell/progenitor pool. Almost one-third of fetal B-progenitors are CD10-ve PreProB-progenitors, whereas, by contrast, PreProB-progenitors are almost undetectable (0.53% ± 0.24%) in adult BM. Single-cell transcriptomics and functional assays place fetal PreProB-progenitors upstream of ProB-progenitors, identifying them as the first B-lymphoid-restricted progenitor in human fetal life. Although fetal BM PreProB-progenitors and ProB-progenitors both give rise solely to B-lineage cells, they are transcriptionally distinct. As with their fetal counterparts, adult BM PreProB-progenitors give rise only to B-lineage cells in vitro and express the expected B-lineage gene expression program. However, fetal PreProB-progenitors display a distinct, ontogeny-related gene expression pattern that is not seen in adult PreProB-progenitors, and they share transcriptomic signatures with CD10-ve B-progenitor infant acute lymphoblastic leukemia blast cells. These data identify PreProB-progenitors as the earliest B-lymphoid-restricted progenitor in human fetal life and suggest that this fetal-restricted committed B-progenitor might provide a permissive cellular context for prenatal B-progenitor leukemia initiation.

A novel prospective isolation of murine fetal liver progenitors to study in utero hematopoietic defects.

In recent years, highly detailed characterization of adult bone marrow (BM) myeloid progenitors has been achieved and, as a result, the impact of somatic defects on different hematopoietic lineage fate decisions can be precisely determined. Fetal liver (FL) hematopoietic progenitor cells (HPCs) are poorly characterized in comparison, potentially hindering the study of the impact of genetic alterations on midgestation hematopoiesis. Numerous disorders, for example infant acute leukemias, have in utero origins and their study would therefore benefit from the ability to isolate highly purified progenitor subsets. We previously demonstrated that a Runx1 distal promoter (P1)-GFP::proximal promoter (P2)-hCD4 dual-reporter mouse (Mus musculus) model can be used to identify adult BM progenitor subsets with distinct lineage preferences. In this study, we undertook the characterization of the expression of Runx1-P1-GFP and P2-hCD4 in FL. Expression of P2-hCD4 in the FL immunophenotypic Megakaryocyte-Erythroid Progenitor (MEP) and Common Myeloid Progenitor (CMP) compartments corresponded to increased granulocytic/monocytic/megakaryocytic and decreased erythroid specification. Moreover, Runx1-P2-hCD4 expression correlated with several endogenous cell surface markers' expression, including CD31 and CD45, providing a new strategy for prospective identification of highly purified fetal myeloid progenitors in transgenic mouse models. We utilized this methodology to compare the impact of the deletion of either total RUNX1 or RUNX1C alone and to determine the fetal HPCs lineages most substantially affected. This new prospective identification of FL progenitors therefore raises the prospect of identifying the underlying gene networks responsible with greater precision than previously possible.

Myogenic Differentiation Potential of Mesenchymal Stem Cells Derived from Fetal Bovine Bone Marrow.

The myogenic potential of bovine fetal MSC (bfMSC) derived from bone marrow (BM) remains unknown; despite its potential application for the study of myogenesis and its implications for livestock production. In the present study, three protocols for in vitro myogenic differentiation of bfMSC based on the use of DNA methyltransferase inhibitor 5-Aza-2'-deoxycytidine (5-Aza), myoblast-secreted factor Galectin-1 (Gal-1), and myoblast culture medium SkGM-2 BulletKit were used. Plastic-adherent bfMSC were isolated from fetal BM collected from abattoir-derived fetuses. Post-thaw viability analyses detected 85.6% bfMSC negative for propidium iodine (PI). Levels of muscle regulatory factors (MRF) MYF5, MYF6, MYOD, and DES mRNA were higher (P < 0.05) in bfMSC cultured under 100 µM of 5-Aza compared to 1 and 10 µM. Treatment of bfMSC with 10 µM of 5-Aza resulted in down-regulation of MYOD mRNA (Days 7 to 21) and up-regulation of MYF6 (Day 7), MYF5, and DES mRNA (Day 21). Gal-1 and SkGM-2 BulletKit induced sequential down-regulation of early MRF (MYF5) and up-regulation of intermediate (MYOD) and late MRF (DES) mRNA. Moreover, DES and MYF5 were immunodetected in differentiated bfMSC. In conclusion, protocols evaluated in bfMSC induced progress into myogenic differentiation until certain extent evidenced by changes in MRF gene expression.

Embryonic Hematopoietic Progenitor Cells Reside in Muscle before Bone Marrow Hematopoiesis.

In mice, hematopoietic cells home to bone marrow from fetal liver prenatally. To elucidate mechanisms underlying homing, we performed immunohistochemistry with the hematopoietic cell marker c-Kit, and observed c-Kit(+) cells localized inside muscle surrounding bone after 14.5 days post coitum. Flow cytometric analysis showed that CD45(+) c-Kit(+) hematopoietic cells were more abundant in muscle than in bone marrow between 14.5 and 17.5 days post coitum, peaking at 16.5 days post coitum. CD45(+) c-Kit(+) cells in muscle at 16.5 days post coitum exhibited higher expression of Gata2, among several hematopoietic genes, than did fetal liver or bone marrow cells. Colony formation assays revealed that muscle hematopoietic cells possess hematopoietic progenitor activity. Furthermore, exo utero transplantation revealed that fetal liver hematopoietic progenitor cells home to muscle and then to BM. Our findings demonstrate that hematopoietic progenitor cell homing occurs earlier than previously reported and that hematopoietic progenitor cells reside in muscle tissue before bone marrow hematopoiesis occurs during mouse embryogenesis.

Análisis de la importancia del programa de donación de sangre de cordón umbilical y su relación con la información ofrecida por el personal sanitario / Analysis of the importance of the blood donation program of umbilical cord and its relation with the infomation given by the health workers

La sangre de cordón umbilical (SCU) ofrece una fuente rica de células progenitoras hematopoyéticas, caracterizadas por su capacidad de proliferación, diferenciación y renovación celular de los tejidos sobre los que se encuentran. Los beneficiarios de esta donación son todas aquellas personas con enfermedades de la médula ósea. Desde que en el año 1988 se realizara con éxito el primer trasplante de sangre de cordón umbilical entre hermanos HLA idénticos, se han llevado a cabo numerosas donaciones, de manera que, actualmente, cualquier mujer que dé a luz en alguno de los centros autorizados para ello de nuestro país podrá donar, de forma voluntaria, este material hematopoyético al Banco de Cordón Umbilical más cercano del cual dependa su comunidad. La puesta en marcha del protocolo de recogida de muestras ha sido y es una tarea difícil que ha precisado de formación, motivación y colaboración tanto interprofesional como entre distintos niveles asistenciales, además de suponer un esfuerzo que consideramos aún poco difundido (AU)

Umbilical Cord Blood has a rich source of haematopoietic progenitor cells (HSC) characterized by their capacity for proliferation, differentiation and cell renewal of tissues on which they are located. The beneficiaries of this donation are all these people with diseases of the bone marrow. Since the year 1988 will be held the first successful transplantation of umbilical cord blood from HLA-identical siblings, have conducted numerous donations, so that, today, any woman who gives birth in any of the centers authorized may donate voluntarily to this material hematopoietic to the Cord Blood Bank which depends their community. Sample collection, has been and is a difficult task that has required training, motivation and collaboration both interprofessional and between different levels of care, while making an effort to consider an information still little known (AU)

MRTF-SRF signaling is required for seeding of HSC/Ps in bone marrow during development.

Chemokine signaling is important for the seeding of different sites by hematopoietic stem cells (HSCs) during development. Serum response factor (SRF) controls multiple genes governing adhesion and migration, mainly by recruiting members of the myocardin-related transcription factor (MRTF) family of G-actin-regulated cofactors. We used vav-iCre to inactivate MRTF-SRF signaling early during hematopoietic development. In both Srf- and Mrtf-deleted animals, hematopoiesis in fetal liver and spleen is intact but does not become established in fetal bone marrow. Srf-null HSC progenitor cells (HSC/Ps) fail to effectively engraft in transplantation experiments, exhibiting normal proximal signaling responses to SDF-1, but reduced adhesiveness, F-actin assembly, and reduced motility. Srf-null HSC/Ps fail to polarize in response to SDF-1 and cannot migrate through restrictive membrane pores to SDF-1 or Scf in vitro. Mrtf-null HSC/Ps were also defective in chemotactic responses to SDF-1. Srf-null HSC/Ps exhibit substantial deficits in cytoskeletal gene expression. MRTF-SRF signaling is thus critical for expression of genes required for the response to chemokine signaling during hematopoietic development.

Development of the fetal bone marrow niche and regulation of HSC quiescence and homing ability by emerging osteolineage cells.

Hematopoietic stem cells (HSCs) reside within a specialized niche where interactions with vasculature, osteoblasts, and stromal components regulate their self-renewal and differentiation. Little is known about bone marrow niche formation or the role of its cellular components in HSC development; therefore, we established the timing of murine fetal long bone vascularization and ossification relative to the onset of HSC activity. Adult-repopulating HSCs emerged at embryonic day 16.5 (E16.5), coincident with marrow vascularization, and were contained within the c-Kit(+)Sca-1(+)Lin(-) (KSL) population. We used Osterix-null (Osx(-/-)) mice that form vascularized marrow but lack osteolineage cells to dissect the role(s) of these cellular components in HSC development. Osx(-/-) fetal bone marrow cells formed multilineage colonies in vitro but were hyperproliferative and failed to home to and/or engraft transplant recipients. Thus, in developing bone marrow, the vasculature can sustain multilineage progenitors, but interactions with osteolineage cells are needed to regulate long-term HSC proliferation and potential.

Investigating the interaction between hematopoietic stem cells and their niche during embryonic development: optimizing the isolation of fetal and newborn stem cells from liver, spleen, and bone marrow.

Hematopoietic stem cells (HSCs) are maintained in a particular microenvironment termed a "niche." Within the niche, a number of critical molecules and supportive cell types have been identified to play key roles in modulating adult HSC quiescence, proliferation, differentiation, and reconstitution. However, unlike in the adult bone marrow (BM), the components of stem cell niches, as well as their interactions with fetal HSC during different stages of embryonic development, are poorly understood. During embryogenesis, hematopoietic development migrates through multiple organs, each with different cellular and molecular components and hence each with a potentially unique HSC niche. As a consequence, isolating fetal HSC from each organ at the time of hematopoietic colonization is fundamental for assessing and understanding both HSC function and their interactions with specific microenvironments. Herein, we describe methodologies for harvesting cells as well as the purification of stem and progenitors from fetal and newborn liver, spleen, and BM at various developmental stages following the expansion of hematopoiesis in the fetal liver at E14.5.

Osterix marks distinct waves of primitive and definitive stromal progenitors during bone marrow development.

Mesenchymal stem and progenitor cells (MSPCs) contribute to bone marrow (BM) homeostasis by generating multiple types of stromal cells. MSPCs can be labeled in the adult BM by Nestin-GFP, whereas committed osteoblast progenitors are marked by Osterix expression. However, the developmental origin and hierarchical relationship of stromal cells remain largely unknown. Here, by using a lineage-tracing system, we describe three distinct waves of contributions of Osterix(+) cells in the BM. First, Osterix(+) progenitors in the fetal BM contribute to nascent bone tissues and transient stromal cells that are replaced in the adult marrow. Second, Osterix-expressing cells perinatally contribute to osteolineages and long-lived BM stroma, which have characteristics of Nestin-GFP(+) MSPCs. Third, Osterix labeling in the adult marrow is osteolineage-restricted, devoid of stromal contribution. These results uncover a broad expression profile of Osterix and raise the intriguing possibility that distinct waves of stromal cells, primitive and definitive, may organize the developing BM.

Erythroid nuclear irregularities: a review of fetal and neonatal autopsies and correlation with clinical features.

In our autopsy experience, abnormal erythroblast nuclear contours are frequently observed in the stillborn fetus or neonate without marrow failure disorders. Bone marrow and liver slides from autopsies of fetuses and infants less than six months of age were analyzed for the percent erythroblasts with nuclear irregularities, and correlated with gestational age at birth, days of life, cause of death, postmortem interval, presence of hydrops, and hematocrit at death. In total, 77 cases had sufficient marrow or liver erythroblasts for review, including 37 stillborns and 40 liveborns. Erythroid nuclear irregularities in >10% of erythroid precursors were present in either the liver or marrow in 54% of stillborns and 68% of liveborns, more commonly seen in the liver. Cases with <1% abnormal erythroblasts were rare. Fetuses with >10% abnormal erythroblasts in the liver were more likely to have died in utero, whereas those with less were more commonly terminations (p=0.008). No significant association between the extent of abnormal erythroblasts and the presence of anemia or hydrops was observed. While the finding of erythroblasts with nuclear irregularities is common in stillborns and liveborns and could be solely a postmortem artifact, we cannot exclude a potential fetal erythropoietic response to hypoxic stimuli. Dyspoietic-appearing erythroblasts alone should not be used as the basis for the diagnosis of a marrow failure disorder at autopsy.

Temporal definition of haematopoietic stem cell niches in a large animal model of in utero stem cell transplantation.

The fetal sheep model has served as a biologically relevant and translational model to study in utero haematopoietic stem cell transplantation (IUHSCT), yet little is known about the ontogeny of the bone marrow (BM) niches in this model. Because the BMmicroenvironment plays a critical role in the outcome of haematopoietic engraftment, we have established the correlation between the fetal-sheep and fetal-human BM niche ontogeny, so that studies addressing the role of niche development at the time of IUHSCT could be accurately performed. Immunofluorescence confocal microscopic analysis of sheep fetal bone from gestational days (gd) 25-68 showed that the BM microenvironment commences development with formation of the vascular niche between 25 and 36 gd in sheep; correlating with the events at 10-11 gestational weeks (gw) in humans. Subsequently, between 45 and 51 gd in sheep (c. 14 gw in humans), the osteoblastic/endosteal niche started developing, the presence of CD34(+)  CD45(+) cells were promptly detected, and their number increased with gestational age. IUHSCT, performed in sheep at 45 and 65 gd, showed significant haematopoietic engraftment only at the later time point, indicating that a fully functional BM microenvironment improved engraftment. These studies show that sheep niche ontogeny closely parallels human, validating this model for investigating niche influence/manipulation in IUHSCT engraftment.

Concise review: From greenhouse to garden: the changing soil of the hematopoietic stem cell microenvironment during development.

The hematopoietic system has been intensely studied for many decades. For this reason, it has become the best understood stem cell-derived system that serves as a paradigm for stem cell biology and has found numerous applications in the clinics. While a lot of progress has recently been made in describing the bone marrow components that maintain and control blood stem cell function in the adult, very little is currently known about the regulatory microenvironment in which the first adult-repopulating hematopoietic stem cells are formed during development. Knowledge of these processes is crucial for understanding the basic regulation of hematopoietic stem cell production and behavior and to allow their in vitro expansion and generation from embryonic stem cells or iPS cells for clinical and research purposes. This review summarizes the recent advances that have been made in defining the cellular components, as well as the soluble and physical factors, that are part of the niche involved in regulating hematopoietic stem cell generation in the embryo. The findings are compared with what is known about the adult bone marrow niche to find common pathways for stem cell regulation, but also to highlight processes uniquely required for de novo hematopoietic stem cell generation, as these are the conditions that will need to be recreated for the successful production of blood stem cells in culture.

Zinc finger protein 148 is dispensable for primitive and definitive hematopoiesis in mice.

Hematopoiesis is regulated by transcription factors that induce cell fate and differentiation in hematopoietic stem cells into fully differentiated hematopoietic cell types. The transcription factor zinc finger protein 148 (Zfp148) interacts with the hematopoietic transcription factor Gata1 and has been implicated to play an important role in primitive and definitive hematopoiesis in zebra fish and mouse chimeras. We have recently created a gene-trap knockout mouse model deficient for Zfp148, opening up for analyses of hematopoiesis in a conventional loss-of-function model in vivo. Here, we show that Zfp148-deficient neonatal and adult mice have normal or slightly increased levels of hemoglobin, hematocrit, platelets and white blood cells, compared to wild type controls. Hematopoietic lineages in bone marrow, thymus and spleen from Zfp148 (gt/gt) mice were further investigated by flow cytometry. There were no differences in T-cells (CD4 and CD8 single positive cells, CD4 and CD8 double negative/positive cells) in either organ. However, the fraction of CD69- and B220-positive cells among lymphocytes in spleen was slightly lower at postnatal day 14 in Zfp148 (gt/gt) mice compared to wild type mice. Our results demonstrate that Zfp148-deficient mice generate normal mature hematopoietic populations thus challenging earlier studies indicating that Zfp148 plays a critical role during hematopoietic development.

The migration of hematopoietic progenitors from the fetal liver to the fetal bone marrow: lessons learned and possible clinical applications.

The ontogeny of hematopoietic stem cells (HSCs) is complex, with multiple sites of embryonic origin as well as several locations of expansion and maturation in the embryo and the adult. Hematopoietic progenitors (HPs) with diverse developmental potential are first found in the yolk sac, aorta-gonad-mesonephros region and placenta. These progenitors then colonize the fetal liver (FL), where they undergo expansion and maturation. HSCs from the FL colonize the fetal bone marrow (FBM), governed by a complex orchestration of transcription programs including migratory molecules with chemotactic activity, adhesion molecules, and molecules that modulate the extracellular matrix. Understanding the mechanisms that regulate the patterns of HSC migration between FL and FBM could improve the engraftment potential of embryonic stem cell-derived HPs, because these cells might display a migratory behavior more similar to early HPs than to adult HSCs. Understanding the changes in migratory behavior in the context of FL to FBM HSC migration could lead to new approaches in the treatment of blood malignancies. We will review the current knowledge in the field of FL to the FBM HSCs migration during development, focusing on changes in expression of molecules important for this process and exploring its clinical applications.

The Notch pathway in hematopoietic stem cells.

Hematopoiesis is the process that generates all the cell types of the blood, which are responsible for oxygen transport and immune defense. It has been now more than 50 years from the demonstration that blood cells derive from a common ancestor called Hematopoietic Stem Cell (HSC) McCulloch and Till (1960). Thus, the hematopoietic process relies on the unlimited and distinctive self-renewal ability of HSC, which in the adult mammalian organisms reside in the bone marrow, but their generation occurs during embryonic life. Questions still remain about how HSCs acquire and maintain the features of self-renewal and pluripotency that define stem-cell populations. Notch is a crucial signaling pathway involved in the generation of cell diversity and stem-cell maintenance in different systems. In some cases, Notch prevents differentiation, while in other contexts Notch directly participates in promoting cell differentiation. In the following sections, we will review what is known about the role of Notch in HSC establishment and hematopoietic cell lineage specification.

Angiotensin-converting enzyme (CD143) specifies emerging lympho-hematopoietic progenitors in the human embryo.

Adult-type lympho-myeloid hematopoietic progenitors are first generated in the aorta-gonad-mesonephros region between days 27 and 40 of human embryonic development, but an elusive blood forming potential is present earlier in the underlying splanchnopleura. In the present study, we show that angiotensin-converting enzyme (ACE, also known as CD143), a recently identified cell-surface marker of adult human hematopoietic stem cells, is already expressed in all presumptive and developing blood-forming tissues of the human embryo and fetus: para-aortic splanchnopleura, yolk sac, aorta-gonad-mesonephros, liver, and bone marrow (BM). Fetal liver and BM-derived CD34(+)ACE(+) cells, but not CD34(+)ACE(-) cells, are endowed with long-term culture-initiating cell potential and sustain multilineage hematopoietic cell engraftment when transplanted into NOD/SCID mice. Furthermore, from 23-26 days of development, ACE expression characterizes rare CD34(-)CD45(-) cells concentrated in the hemogenic portion of the para-aortic splanchnopleura. ACE(+) cells sorted from the splanchnopleura generated colonies of hematopoietic cells more than 40 times more frequently than ACE(-) cells. These data suggest that, in addition to being a marker of adult human hematopoietic stem cells, ACE identifies embryonic mesodermal precursors responsible for definitive hematopoiesis, and we propose that this enzyme is involved in the regulation of human blood formation.

A rapid and simple assay for human blood malignancy engraftment, homing and chemotherapy treatment using fluorescent imaging of avian embryos.

Detection of grafted human cells in mice using fluorescence is a rapid and simple technique whose use is continually expanding. Robust engraftment of human hematological malignancy (HHM) lines and patient cells into the naturally immunodeficient turkey embryo has recently been demonstrated by polymerase chain reaction (PCR), fluorescence activated cell sorting (FACS) and histology. We demonstrate here that fluorescence imaging is a rapid and simple technique for detecting engraftment and homing of cells derived from HHM in turkey embryos. Raji lymphoma cells expressing a far-red fluorescent protein were injected intravascularly into turkey embryos and fluorescence was detected 8 days later in their limbs and skulls. Much stronger signals were obtained after removal of the bones from the limbs. Unlabeled Raji cells did not give a fluorescent signal. Treatment with doxorubicin dramatically reduced the fluorescent signal. Intravenously injected HL-60 leukemia cells labeled with infrared-fluorescing dye were detected in the bone marrow after 16 h. Homing was active, although some non-specific fluorescence was present. Use of fluorescence imaging of HHM in turkey embryos is therefore feasible and reduces the time, effort and expense for detecting engraftment. This technique has potential to become a high-throughput xenograft system for hematological chemotherapy development and testing, and for study of hematological cell homing.

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.