The spleen is a major site of megakaryopoiesis following transplantation of murine hematopoietic stem cells

Absence of Nkx2-3 induces ectopic lymphatic endothelial differentiation associated with impaired extramedullary stress hematopoiesis in the spleen

The red and white pulps as two main parts of the spleen are arranged around distinct types of vasculature, and perform significantly different functions in both humans and mice. Previous observations indicated a profound alteration of the local vessel specialization in mice lacking Nkx2-3 homeodomain transcription factor, including contradictory results suggesting presence of an ectopic lymphatic vascular structure. Furthermore, how the absence of Nkx2-3 and the consequential changes in endothelial components affect the extramedullary hematopoietic activity restricted to the splenic red pulp is unknown. In this work, we investigated the role of Nkx2-3 homeodomain transcription factor as a major morphogenic determinant for vascular specification, and its effect in the extramedullary hematopoiesis following acute blood loss and pharmacological stimulation of megakaryocyte differentiation after treatment with thrombopoietin-receptor mimetic Romiplostim. We found that, in mice lacking Nkx2-3, Prox1-positive lymphatic capillaries containing gp38/CD31 double positive lymphatic endothelial cells develop, arranged into an extensive meshwork, while the Clever1-positive venous segments of red pulp blood vasculature are absent. This lymphatic endothelial shift is coupled with a severely compromised splenic erythropoiesis and a significantly reduced splenic megakaryocyte colony formation following Romiplostim treatment in mice lacking Nkx2-3. These findings indicate that the shift of microvascular patterning in the absence of Nkx2-3 includes the emergence of ectopic Prox1-positive lymphatic vessels, and that this pivoting towards lymph node-like vascular patterning is associated with an impaired reserve hematopoietic capacity of the splenic red pulp.

Occurrence and role of lung megakaryocytes in infection and inflammation

Megakaryocytes (MKs) are large cells giving rise to platelets. It is well established that in adults, MKs develop from hematopoietic stem cells and reside in the bone marrow. MKs are also rare but normal constituents of the venous blood returning to the lungs, and MKs are found in the lung vasculature (MK_circ), suggesting that these cells are migrants from the bone marrow and get trapped in lung capillaries where the final steps of platelet production can occur. An unprecedented increase in the number of lung and circulating MKs was described in coronavirus disease 2019 (COVID-19) patients, suggesting that lung thrombopoiesis may be increased during lung infection and/or thromboinflammation. In addition to the population of platelet-producing intravascular MKs in the lung, a population of lung-resident megakaryocytes (MK_L) has been identified and presents a specific immune signature compared to its bone marrow counterparts. Recent single-cell analysis and intravital imaging have helped us gain a better understanding of these populations in mouse and human. This review aims at summarizing the recent data on increased occurrence of lung MKs and discusses their origin, specificities, and potential role in homeostasis and inflammatory and infectious lung diseases. Here, we address remaining questions, controversies, and methodologic challenges for further studies of both MK_circ and MK_L.

Time-dependent biodistribution profiles and reaction of polyethylene glycol-coated iron oxide nanoclusters in the spleen after intravenous injection in the mice

Polyethylene glycol (PEG) is widely used polymer in the field of pharmaceutics, particularly in which related to drug delivery systems (DDS). Surface coating of the nanoparticles (NPs) with PEG (i.e. pegylation) adds novel characteristics that make their use in vivo more effective with lower cytotoxicity. The biodistribution profiles, reaction, and fate of PEG-coated NPs in vivo still unclear and need more detailed studies. Here in this study, we prepared PEG-coated iron oxide nanoclusters (PEG-coated IONCs) to investigate their biodistribution profiles and reactions in spleen after intravenous injection time-dependently. Using Prussian blue staining method as specific histochemical reaction for iron detection in the tissues, the PEG-coated IONCs were observed in a higher ratio in spleen red pulp after 1 day of injection but decreased time-dependently after 10 days and 20 days. Interestingly, PEG-coated IONCs moved from red pulp into the white pulp specially after 20 days of injection. After long time exposure (20 days), higher amount of PEG-coated IONCs was observed in the center of spleen white pulp follicle. Using histological staining, the reaction of PEG-coated IONCs with splenocytes or immune cells induced cellular abnormalities such as, nucleic acid damages, induction of megakaryocytes number, and sever vacuolation in the white pulp area specially after 20 days of injection. Histochemically, the localization of PEG-coated IONCs in the splenic parenchyma induced the level of the collagen fibers particularly after 1 day and 10 days of injection. Interestingly, cellular abnormalities in the splenic red pulp as well as collagen levels decreased after 20 days of injection due to the clearance of PEG-coated IONCs from this area. This data indicated that cytotoxicity produced by the reaction of PEG-coated IONCs in the spleen are reversible specially after 20 days of in the intravenous injection. Understanding the detailed mechanism of the fate and reaction of the coated nanomaterials after intravenous injection is important to design effective and safe DDS based NPs.

New insights into cytoskeletal remodeling during platelet production

The past decade has brought unprecedented advances in our understanding of megakaryocyte (MK) biology and platelet production, processes that are strongly dependent on the cytoskeleton. Facilitated by technological innovations, such as new high-resolution imaging techniques (in vitro and in vivo) and lineage-specific gene knockout and reporter mouse strains, we are now able to visualize and characterize the molecular machinery required for MK development and proplatelet formation in live mice. Whole genome and RNA sequencing analysis of patients with rare platelet disorders, combined with targeted genetic interventions in mice, has led to the identification and characterization of numerous new genes important for MK development. Many of the genes important for proplatelet formation code for proteins that control cytoskeletal dynamics in cells, such as Rho GTPases and their downstream targets. In this review, we discuss how the final stages of MK development are controlled by the cellular cytoskeletons, and we compare changes in MK biology observed in patients and mice with mutations in cytoskeleton regulatory genes.

Reprogramming mouse fibroblasts into engraftable myeloerythroid and lymphoid progenitors

Recent efforts have attempted to convert non-blood cells into hematopoietic stem cells (HSCs) with the goal of generating blood lineages de novo. Here we show that hematopoietic transcription factors Scl, Lmo2, Runx1 and Bmi1 can convert a developmentally distant lineage (fibroblasts) into ‘induced hematopoietic progenitors' (iHPs). Functionally, iHPs generate acetylcholinesterase⁺ megakaryocytes and phagocytic myeloid cells in vitro and can also engraft immunodeficient mice, generating myeloerythoid and B-lymphoid cells for up to 4 months in vivo. Molecularly, iHPs transcriptionally resemble native Kit⁺ hematopoietic progenitors. Mechanistically, reprogramming factor Lmo2 implements a hematopoietic programme in fibroblasts by rapidly binding to and upregulating the Hhex and Gfi1 genes within days. Moreover the reprogramming transcription factors also require extracellular BMP and MEK signalling to cooperatively effectuate reprogramming. Thus, the transcription factors that orchestrate embryonic hematopoiesis can artificially reconstitute this programme in developmentally distant fibroblasts, converting them into engraftable blood progenitors.

Direct reprogramming of closely-related lineages can generate hematopoietic stem cells. Here, the authors show hematopoietic transcription factors Scl, Lmo2, Runx1 and Bmi1 can reprogram fibroblasts into induced hematopoietic progenitors (iHPs), which are engraftable blood progenitors.

P-Selectin Sustains Extramedullary Hematopoiesis in the Gata1 low Model of Myelofibrosis

Splenomegaly is a major manifestation of primary myelofibrosis (PMF) contributing to clinical symptoms and hematologic abnormalities. The spleen from PMF patients contains increased numbers of hematopoietic stem cells (HSC) and megakaryocytes (MK). These MK express high levels of P-selectin (P-sel) that, by triggering neutrophil emperipolesis, may cause TGF-β release and disease progression. This hypothesis was tested by deleting the P-sel gene in the myelofibrosis mouse model carrying the hypomorphic Gata1(low) mutation that induces megakaryocyte abnormalities that recapitulate those observed in PMF. P-sel(null) Gata1(low) mice survived splenectomy and lived 3 months longer than P-sel(WT) Gata1(low) littermates and expressed limited fibrosis and osteosclerosis in the marrow or splenomegaly. Furthermore, deletion of P-sel disrupted megakaryocyte/neutrophil interactions in spleen, reduced TGF-β content, and corrected the HSC distribution that in Gata1(low) mice, as in PMF patients, is abnormally expanded in spleen. Conversely, pharmacological inhibition of TGF-β reduced P-sel expression in MK and corrected HSC distribution. Spleens, but not marrow, of Gata1(low) mice contained numerous cKIT(pos) activated fibrocytes, probably of dendritic cell origin, whose membrane protrusions interacted with MK establishing niches hosting immature cKIT(pos) hematopoietic cells. These activated fibrocytes were not detected in spleens from P-sel(null) Gata1(low) or TGF-β-inhibited Gata1(low) littermates and were observed in spleen, but not in marrow, from PMF patients. Therefore, in Gata1(low) mice, and possibly in PMF, abnormal P-sel expression in MK may mediate the pathological cell interactions that increase TGF-β content in MK and favor establishment of a microenvironment that supports myelofibrosis-related HSC in spleen.

Angptl4 is upregulated under inflammatory conditions in the bone marrow of mice, expands myeloid progenitors, and accelerates reconstitution of platelets after myelosuppressive therapy

BACKGROUND

Upon inflammation, myeloid cell generation in the bone marrow (BM) is broadly enhanced by the action of induced cytokines which are produced locally and at multiple sites throughout the body.

METHODS

Using microarray studies, we found that Angptl4 is upregulated in the BM during systemic inflammation.

RESULTS

Recombinant murine Angptl4 (rmAngptl4) stimulated the proliferation of myeloid colony-forming units (CFUs) in vitro. Upon repeated in vivo injections, rmAngptl4 increased BM progenitor cell frequency and this was paralleled by a relative increase in phenotypically defined granulocyte-macrophage progenitors (GMPs). Furthermore, in vivo treatment with rmAngptl4 resulted in elevated platelet counts in steady-state mice while allowing a significant acceleration of reconstitution of platelets after myelosuppressive therapy. The administration of rmAngptl4 increased the number of CD61(+)CD41(low)-expressing megakaryocytes (MK) in the BM of steady-state and in the spleen of transplanted mice. Furthermore, rmAngptl4 improved the in vitro differentiation of immature MKs from hematopoietic stem and progenitor cells. Mechanistically, using a signal transducer and activator of transcription 3 (STAT3) reporter knockin model, we show that rmAngptl4 induces de novo STAT3 expression in immature MK which could be important for the effective expansion of MKs after myelosuppressive therapy.

CONCLUSION

Whereas the definitive role of Angptl4 in mediating the effects of lipopolysaccharide (LPS) on the BM has to be demonstrated by further studies involving multiple cytokine knockouts, our data suggest that Angptl4 plays a critical role during hematopoietic, especially megakaryopoietic, reconstitution following stem cell transplantation.

The Human GATA1 Gene Retains a 5' Insulator That Maintains Chromosomal Architecture and GATA1 Expression Levels in Splenic Erythroblasts

GATA1 is a key transcription factor for erythropoiesis. GATA1 gene expression is strictly regulated at the transcriptional level. While the regulatory mechanisms governing mouse Gata1 (mGata1) gene expression have been studied extensively, how expression of the human GATA1 (hGATA1) gene is regulated remains to be elucidated. To address this issue, we generated hGATA1 bacterial artificial chromosome (BAC) transgenic mouse lines harboring a 183-kb hGATA1 locus covering the hGATA1 exons and distal flanking sequences. Transgenic hGATA1 expression coincides with endogenous mGata1 expression and fully rescues hematopoietic deficiency in mGata1 knockdown mice. The transgene exhibited copy number-dependent and integration position-independent expression of hGATA1, indicating the presence of chromatin insulator activity within the transgene. We found a novel insulator element at 29 kb 5' to the hGATA1 gene and refer to this element as the 5' CCCTC-binding factor (CTCF) site. Substitution mutation of the 5' CTCF site in the hGATA1 BAC disrupted the chromatin architecture and led to a reduction of hGATA1 expression in splenic erythroblasts under conditions of stress erythropoiesis. Our results demonstrate that expression of the hGATA1 gene is regulated through the chromatin architecture organized by 5' CTCF site-mediated intrachromosomal interactions in the hGATA1 locus.

Regulator of G-protein signaling 18 controls both platelet generation and function

RGS18 is a myeloerythroid lineage-specific regulator of G-protein signaling, highly expressed in megakaryocytes (MKs) and platelets. In the present study, we describe the first generation of a RGS18 knockout mouse model (RGS18-/-). Interesting phenotypic differences between RGS18-/- and wild-type (WT) mice were identified, and show that RGS18 plays a significant role in both platelet generation and function. RGS18 deficiency produced a gain of function phenotype in platelets. In resting platelets, the level of CD62P expression was increased in RGS18-/- mice. This increase correlated with a higher level of plasmatic serotonin concentration. RGS18-/- platelets displayed a higher sensitivity to activation in vitro. RGS18 deficiency markedly increased thrombus formation in vivo. In addition, RGS18-/- mice presented a mild thrombocytopenia, accompanied with a marked deficit in MK number in the bone marrow. Analysis of MK maturation in vitro and in vivo revealed a defective megakaryopoiesis in RGS18-/- mice, with a lower bone marrow content of only the most committed MK precursors. Finally, RGS18 deficiency was correlated to a defect of platelet recovery in vivo under acute conditions of thrombocytopenia. Thus, we highlight a role for RGS18 in platelet generation and function, and provide additional insights into the physiology of RGS18.

Expansion of the neonatal platelet mass is achieved via an extension of platelet lifespan

The fetal/neonatal hematopoietic system must generate enough blood cells to meet the demands of rapid growth. This unique challenge might underlie the high incidence of thrombocytopenia among preterm neonates. In this study, neonatal platelet production and turnover were investigated in newborn mice. Based on a combination of blood volume expansion and increasing platelet counts, the platelet mass increased sevenfold during the first 2 weeks of murine life, a time during which thrombopoiesis shifted from liver to bone marrow. Studies applying in vivo biotinylation and mathematical modeling showed that newborn and adult mice had similar platelet production rates, but neonatal platelets survived 1 day longer in circulation. This prolonged lifespan fully accounted for the rise in platelet counts observed during the second week of murine postnatal life. A study of pro-apoptotic and anti-apoptotic Bcl-2 family proteins showed that neonatal platelets had higher levels of the anti-apoptotic protein Bcl-2 and were more resistant to apoptosis induced by the Bcl-2/Bcl-xL inhibitor ABT-737 than adult platelets. However, genetic ablation or pharmacologic inhibition of Bcl-2 alone did not shorten neonatal platelet survival or reduce platelet counts in newborn mice, indicating the existence of redundant or alternative mechanisms mediating the prolonged lifespan of neonatal platelets.

Ação da melatonina sobre a dinâmica sanguínea de ratas prenhes e sobre a histogênese do baço e do timo da prole

Investigou-se a influência da melatonina sobre o hemograma de ratas prenhes e dos filhotes e sobre a histogênese e morfometria do baço e do timo dos filhotes. A melatonina foi administrada na dose 0,5mg/kg de peso corporal, dissolvida em 0,1mL de etanol e diluída em 0,3mL de solução salina. Para análise do hematócrito, contagem total de hemácias e contagem total e diferencial dos leucócitos, amostras de sangue foram coletadas no sétimo, 14ºe 21ºdias de prenhez e aos 10 dias de nascimento dos filhotes. Cortes histológicos do baço e do timo da prole foram utilizados para histoquímica e morfometria. A ausência da melatonina promoveu alterações no hemograma apenas no terço final da gestação, sem interferir no hemograma dos filhotes, e induziu modificações morfológicas e morfométricas no timo e no baço nos primeiros dias de vida dos filhotes. Concluiu-se que a melatonina materna é importante para a modulação do hemograma em ratas prenhes e para o desenvolvimento normal do baço e do timo dos filhotes.

VEGFR1 stimulates a CXCR4-dependent translocation of megakaryocytes to the vascular niche, enhancing platelet production in mice

It has previously been reported that VEGF-A stimulates megakaryocyte (MK) maturation in vitro. Here we show that treatment of mice with the isoform VEGF-A165 resulted in a significant increase in circulating numbers of platelets. Using specific VEGFR1 and VEGFR2 blocking mAbs and selective VEGFR1 and 2 agonists, PlGF-2 and VEGF-E, respectively, we show directly that stimulation of VEGFR1, but not VEGFR2, increases circulating platelet numbers in vivo. Using flow cytometric analysis of harvested MKs, we show that while PlGF does not change the absolute numbers of MKs present in the bone marrow and the spleen, it increases both their maturation and cell-surface expression of CXCR4 in the bone marrow. Histology of the bone marrow revealed a redistribution of MKs from the endosteal to the vascular niche in response to both VEGF-A165 and PlGF-2 treatment in vivo. Antagonism of CXCR4 suppressed both the VEGFR1-stimulated redistribution of megakyocytes within the bone marrow compartment and the VEGF-A165–induced thrombocytosis. In conclusion, we define a novel proinflammatory VEGFR1-mediated pathway that stimulates the maturation and up-regulation of CXCR4 on megakaryocytes, leading to their redistribution within the bone marrow environment, thereby enhancing platelet production in vivo.

Differences between newborn and adult mice in their response to immune thrombocytopenia

BACKGROUND

Sick neonates frequently develop severe thrombocytopenia.

OBJECTIVE AND METHODS

In order to test the ability of fetal mice to increase their megakaryocyte size and ploidy in response to thrombocytopenia, we injected an antiplatelet antibody (MWReg30) into pregnant mice daily for 7 days, and into nonpregnant adult mice to serve as controls. After that time, platelet counts were obtained and megakaryocytes in the bone marrow, liver, and spleen were stained with anti-von Willebrand factor antibody, individually measured, and quantified.

RESULTS

Our study demonstrated that megakaryocytopoiesis in newborn mice shares many features of human fetal/neonatal megakaryocytopoiesis, including the small size of megakaryocytes. In response to thrombocytopenia, adult mice increased megakaryocyte volume and concentration, primarily in the spleen. Newborn mice, in contrast, increased the megakaryocyte concentration in the spleen, but exhibited no increase in megakaryocyte volume in any of the organs studied. In fact, the megakaryocyte mass was significantly lower in the bone marrow of thrombocytopenic neonates than in age-matched controls.

CONCLUSIONS

We concluded that fetuses have a limited ability to increase their megakaryocyte mass in response to consumptive thrombocytopenia, compared to adult mice. These observations provide further evidence for the existence of biological differences between fetal/neonatal and adult megakaryocytopoiesis.

Evidence for organ-specific stem cell microenvironments

The X-linked Gata1(low) mutation in mice induces strain-restricted myeloproliferative disorders characterized by extramedullary hematopoiesis in spleen (CD1 and DBA/2) and liver (CD1 only). To assess the role of the microenvironment in establishing this myeloproliferative trait, progenitor cell compartments of spleen and marrow from wild-type and Gata1(low) mice were compared. Phenotype and clonal assay of non-fractionated cells indicated that Gata1(low) mice contain progenitor cell numbers 4-fold lower and 10-fold higher than normal in marrow and spleen, respectively. However, progenitor cells prospectively isolated from spleen, but not from marrow, of Gata1(low) mice expressed colony-forming function in vitro. Therefore, calculation of cloning activity of purified cells demonstrated that the total number of Gata1(low) progenitor cells was 10- to 100-fold lower than normal in marrow and >1,000 times higher than normal in spleen. This observation indicates that Gata1(low) hematopoiesis is favored by the spleen and is in agreement with our previous report that removal of this organ induces wild-type hematopoiesis in heterozygous Gata1(low/+) females (Migliaccio et al., 2009, Blood 114:2107). To clarify if rescue of wild-type hematopoiesis by splenectomy prevented extramedullary hematopoiesis in liver, marrow cytokine expression profile and liver histopathology of splenectomized Gata1(low/+) females were investigated. After splenectomy, the marrow expression levels of TGF-beta, VEGF, osteocalcin, PDGF-alpha, and SDF-1 remained abnormally high while Gata1(low) hematopoiesis was detectable in liver of both CD1 and DBA/2 mutants. Therefore, in the absence of the spleen, Gata1(low) hematopoiesis is supported by the liver suggesting that treatment of myelofibrosis in these animals requires the rescue of both stem cell and microenvironmental functions.

Removal of the spleen in mice alters the cytokine expression profile of the marrow micro-environment and increases bone formation

Splenectomized mice express progressively increased numbers of platelets in the blood and reduced numbers of megakaryocytes in the marrow with age. The megakaryocytes in the marrow of these animals express reduced levels of Gata1, a transcription factor necessary for their maturation. In addition, the marrow from these animals expresses greater levels of cytokines (TGF-beta, PDGF-alpha, and VEGF) known to be produced at high levels by megakaryocytes expressing reduced levels of Gata1. This high level of cytokine expression is in turn associated with active osteoblast proliferation localized to areas of the femur, where megakaryocytes expressing reduced Gata1 levels are also found. These results confirm the role of megakaryocytes as regulator of bone formation in mice and suggest that a cross-talk between the spleen and marrow may regulate the total numbers of hemopoietic niches present in an animal.

Gata1 expression driven by the alternative HS2 enhancer in the spleen rescues the hematopoietic failure induced by the hypomorphic Gata1low mutation

Rigorously defined reconstitution assays developed in recent years have allowed recognition of the delicate relationship that exists between hematopoietic stem cells and their niches. This balance ensures that hematopoiesis occurs in the marrow under steady-state conditions. However, during development, recovery from hematopoietic stress and in myeloproliferative disorders, hematopoiesis occurs in extramedullary sites whose microenvironments are still poorly defined. The hypomorphic Gata1(low) mutation deletes the regulatory sequences of the gene necessary for its expression in hematopoietic cells generated in the marrow. By analyzing the mechanism that rescues hematopoiesis in mice carrying this mutation, we provide evidence that extramedullary microenvironments sustain maturation of stem cells that would be otherwise incapable of maturing in the marrow.

Long-term hematopoietic reconstitution by transplantation of kidney hematopoietic stem cells in lethally irradiated clonal ginbuna crucian carp (Carassius auratus langsdorfii)

The study of hematopoietic stem cells (HSCs) has been facilitated by the transplantation of bone marrow cell populations into lethally irradiated mice. It is widely known that HSCs have the capacity for long-term and multilineage hematopoietic reconstitution in lethally irradiated hosts. Here, we developed a transplantation model system using clonal ginbuna crucian carp (Carassius auratus langsdorfii) that were exposed to a lethal dose of X-rays. The minimum lethal dose (MLD) of ginbuna was approximately 25Gy, which is lethal due to hematopoietic failure. The transplantation of kidney hematopoietic cells into lethally irradiated ginbuna resulted in the rescue of recipient fish for more than 180 days. We examined the reconstitution activity of head kidney (HK), body kidney (BK), and spleen cells. Transplantation experiments showed that only HK and BK cells had the long-term and multilineage reconstitution activity. These results indicate that teleost HSCs have the ability to fully reconstitute the hematopoietic system in lethally irradiated hosts, and that they are present in HK and BK, but not in spleen. This transplantation model system using clonal ginbuna is useful for studies of in vivo kinetics and functions of HSCs in teleosts.

Primitive hematopoietic cell populations reside in the spleen: Studies in the pig, baboon, and human

OBJECTIVE

We previously observed high levels (>40%) of multilineage hematopoietic cell chimerism following spleen transplantation across full MHC barriers in immunosuppressed miniature swine. We therefore investigated the spleen as a source of hematopoietic progenitor cells (HPCs).

MATERIALS AND METHODS

Specific cell-surface markers were used to identify HPCs in the spleen and bone marrow (BM) of young adult (n = 15) and fetal (n = 9) miniature swine by flow cytometry. Hoechst dye-effluxing side population (SP) cells were analyzed in adult spleen, BM, and blood for their expression of c-kit. Functional HPC activity of varying repopulation potential in vitro was investigated by the ability of spleens and BM to give rise to colony-forming units (CFUs) and cobblestone area-forming cells (CAFCs) in long-term stromal cultures. Studies were also carried out on baboon and human spleens and BM.

RESULTS

Spleen c-kit+ cells co-expressed more lymphoid markers, but equal myeloid markers, when compared with BM c-kit+ cells. BM and spleen both contained significant percentages of c-kit+ SP cells. Although the frequency of early-forming CFUs in the spleen was only 0.1 to 1.3% of that in the BM, the frequency of CAFCs developing after 8 weeks in culture was comparable to that of BM. Secondary CFUs in long-term culture-initiating cell assays confirmed the presence of long-term repopulating cells at comparable frequencies in spleen and BM. Similar findings were found with regard to baboon and human spleen cells.

CONCLUSION

The adult spleen is a relatively rich source of very primitive HPCs, possibly hematopoietic stem cells (HSCs), and may be of therapeutic value.

Sites and kinetics of donor thrombopoiesis following transplantation of whole bone marrow and progenitor subsets

INTRODUCTION

Little is known about the sites and kinetics of thrombopoiesis following bone marrow transplant. The spleen is a site of hematopoiesis in a healthy mouse, and hematopoietic activity increases in response to stress. We hypothesized that the spleen is a major site of early post-transplant thrombopoiesis.

METHODS

We transplanted whole bone marrow (WBM) or lineage depleted progenitor subsets fractionated based on expression of c-kit and Sca-1 from transgenic mice expressing green fluorescent protein into lethally irradiated C57BL/6 recipients. We also transplanted whole bone marrow cells into healthy and splenectomized mice. Post-transplant megakaryopoiesis was assessed by measuring circulating platelet number, percent donor-derived platelets, bone marrow cellularity, splenic weight, megakaryocyte size, and megakaryocyte concentration from hour 3 to day 28 post transplant.

RESULTS

Following transplant, circulating donor-derived platelets were derived only from c-kit expressing subsets. Donor-derived platelets first appeared on post-transplant day five. Splenectomy reduced the number of these earliest circulating platelets. Splenic megakaryopoiesis increased dramatically from day 7-14 post-transplant. However, splenectomy accelerated platelet engraftment during this time frame.

CONCLUSION

Overall, these results demonstrate that the first platelets are produced by c-kit expressing megakaryocyte progenitors in the bone marrow and spleen. After post-transplant day 5, the net effect of the spleen on thrombopoiesis is to slow engraftment due to immune effects or hypersplenism.

The role of the donor in the repair of the marrow vascular niche following hematopoietic stem cell transplant

Bone marrow sinusoids maintain homeostasis between developing hematopoietic cells and the circulation, and they provide niches for hematopoietic progenitors. Sinusoids are damaged by chemotherapy and radiation. Hematopoietic stem cells (HSCs) have been shown to produce endothelial progenitor cells that contribute to the repair of damaged blood vessels. Because HSCs home to the marrow during bone marrow transplant, these cells may play a role in repair of marrow sinusoids. Here, we explore the role of donor HSCs in the repair of damaged sinusoids following hematopoietic stem cell transplant. We used three methods to test this role: (a) expression of platelet endothelial cell adhesion molecule to identify endothelial progenitors and the presence of the Y chromosome to identify male donor cells in female recipients; (b) presence of the Y chromosome to identify male donor cells in female recipients, and expression of the panendothelial marker mouse endothelial cell antigen-32 to identify sinusoidal endothelium; and (c) use of Tie-2/green fluorescent protein mice as donors or recipients and presence of Dil-Ac-LDL to identify sinusoids. We found that sinusoids were predominantly host-derived posttransplant. Donor cells spread along the marrow vasculature early post-transplant in a pattern that matched stromal-derived factor-1 expression. Furthermore, these engrafting progenitors were positioned to provide physical support, as well as growth and survival signals in the form of vascular-endothelial growth factor-A. Occasionally, donor cells provide cellular "patches" in the damaged sinusoids, although this occurred at a low level compared with hematopoietic engraftment. Donor support for the repair of the marrow vascular niche may be a critical first step of hematopoietic engraftment.

Thrombospondins deployed by thrombopoietic cells determine angiogenic switch and extent of revascularization

Thrombopoietic cells may differentially promote or inhibit tissue vascularization by releasing both pro- and antiangiogenic factors. However, the molecular determinants controlling the angiogenic phenotype of thrombopoietic cells remain unknown. Here, we show that expression and release of thrombospondins (TSPs) by megakaryocytes and platelets function as a major antiangiogenic switch. TSPs inhibited thrombopoiesis, diminished bone marrow microvascular reconstruction following myelosuppression, and limited the extent of revascularization in a model of hind limb ischemia. We demonstrate that thrombopoietic recovery following myelosuppression was significantly enhanced in mice deficient in both TSP1 and TSP2 (TSP-DKO mice) in comparison with WT mice. Megakaryocyte and platelet levels in TSP-DKO mice were rapidly restored, thereby accelerating revascularization of myelosuppressed bone marrow and ischemic hind limbs. In addition, thrombopoietic cells derived from TSP-DKO mice were more effective in supporting neoangiogenesis in Matrigel plugs. The proangiogenic activity of TSP-DKO thrombopoietic cells was mediated through activation of MMP-9 and enhanced release of stromal cell-derived factor 1. Thus, TSP-deficient thrombopoietic cells function as proangiogenic agents, accelerating hemangiogenesis within the marrow and revascularization of ischemic hind limbs. As such, interference with the release of cellular stores of TSPs may be clinically effective in augmenting neoangiogenesis.

MiHA reactive CD4 and CD8 T-cells effect resistance to hematopoietic engraftment following reduced intensity conditioning

Reduced intensity conditioning (RIC) prior to allogeneic hematopoietic cell transplantation (HCT) has shown promise in lowering the incidence of post-transplant complications including infection and graft-versus-host disease. T-cell-mediated graft rejection, however, remains a crucial factor in determining how 'mild' a level of immunosuppression can be administered. Understanding the kinetics of resistance responses as well as the role of CD4+ and CD8+ T cells underlies the development of protocols to circumvent resistance and support hematopoietic engraftment. In these studies, a major histocompatibility complex (MHC)-matched/minor histocompatibility antigen (MiHA) disparate RIC HCT model was developed in which resistance against donor hematopoietic progenitors as well as mature peripheral blood cells could be assessed. Interestingly, resistance was diminished in the absence of either host CD4+ or CD8+ T cells. However, its impairment was more severe in CD4-/- mice where resistance was not detected. Host CD4+ T cells were required for optimal expansion of specific (H60) T-cell receptor (TCR) expressing host anti-donor MiHA reactive CD8+ T cells following HCT. These observations demonstrate a critical role for host CD4+ T cells in resistance against MiHA disparate HCT. This RIC HCT resistance model will be useful for the analysis of the barrier to engraftment mediated by host T cells and the development of strategies to support engraftment.

Developmental abnormalities in multiple proliferative tissues of Apc(Min/+) mice

Germ-line mutation of the Apc gene has been linked to familial adenomatous polyposis (FAP) that predisposes to colon cancer. Apc(Min/+) mice, heterozygous for the Apc gene mutation, progressively develop small intestinal tumours in a manner that is analogous to that observed in the colon of patients with FAP (Su et al. 1992; Fodde et al. 1994; Moser et al. 1995). We have studied the effects of Apc gene mutation on murine intestinal and extra-intestinal, proliferatively active tissues. We have contrasted the histology to that of the age- and sex-matched wild-type C57BL/6 mice. Histological assessment of the normal appearing intestinal mucosa demonstrates minimal change in size of crypts. In contrast, villi are longer in the ileum of Apc(Min/+) mice relative to C57BL/6 mice at 12 and 15 weeks of age. Vigorous splenic haematopoiesis in Apc(Min/+) mice was seen at 12 and 15 weeks of age, as reflected by marked splenomegaly, increased splenic haematopoietic cells and megakaryocytes. Peripheral blood counts, however, did not differ between C57BL/6 and Apc(Min/+) mice at 15 weeks of age. Lymphoid depletion in Apc(Min/+) mice was characterized by diminished numbers of splenic lymphoid follicles and small intestinal Peyer's patches. The ovaries of 12- and 15-week-old Apc(Min/+) mice exhibited increased numbers of atretic follicles, and estrous cycling by serial vaginal smears showed tendency of elongation in the mutant mice during these age ranges. The testicles of 10-week-old Apc(Min/+) mice showed increased numbers of underdeveloped seminiferous tubules. Collectively, these data suggest that, in addition to its obvious effects upon intestinal adenoma formation, Apc gene mutation causes impairment of developmental and apparent differentiation blockade in proliferative tissues, including those of the haematopoietic system, lymphoid and reproductive tract.

Variegation of the phenotype induced by the Gata1low mutation in mice of different genetic backgrounds

All mice harboring the X-linked Gata1low mutation in a predominantly CD1 background are born anemic and thrombocytopenic. They recover from anemia at 1 month of age but remain thrombocytopenic all their life and develop myelofibrosis, a syndrome similar to human idiopathic myelofibrosis, at 12 months. The effects of the genetic background on the myelofibrosis developed by Gata1low mice was assessed by introducing the mutation, by standard genetic approaches, in the C57BL/6 and DBA/2 backgrounds and by analyzing the phenotype of the different mutants at 12 to 13 (by histology) and 16 to 20 (by cytofluorimetry) months of age. Although all the Gata1low mice developed fibrosis at 12 to 13 months, variegations were observed in the severity of the phenotype expressed by mutants of different backgrounds. In C57BL/6 mice, the mutation was no longer inherited in a Mendelian fashion, and fibrosis was associated with massive osteosclerosis. Instead, DBA/2 mutants, although severely anemic, expressed limited fibrosis and osteosclerosis and did not present tear-drop poikilocytes in blood or extramedullary hemopoiesis in liver up to 20 months of age. We propose that the variegation in myelofibrosis expressed by Gata1low mutants of different strains might represent a model to study the variability of the clinical picture of the human disease.

Engraftment of Splenic Tissue as a Method to Investigate Repopulation by Hematopoietic Cells from Host and Donor Marrow

The lymphohematopoietic function of the spleen in mice varies dependent on age and hematopoietic requirements. A method was developed to study splenic repopulation of mature and progenitor cell populations by grafting neonatal or adult spleen tissue under the renal capsule of splenectomized mice. Two weeks following implant of irradiated syngeneic neonatal spleens into B6-Ly 5.1 or B6-gfp recipients, host lymphoid (B220(+), CD4/8(+)) and myeloid cells (CD11b(+)) had repopulated the splenic grafts and constituted the majority of cells contained in these heterotopic implants. Notably, the percentage of lymphoid and myeloid cells approximated adult levels in contrast to preimplant neonatal spleen levels. This observation indicated relatively rapid repopulation of the grafted tissue by adult host cells and suggests that the repopulation patterns were regulated by the host. Three months post-implantation, the cell composition in the graft remained comparable to adult levels. Microscopic examination demonstrated normal splenic architecture including follicles and red pulp. Lymphocytes within the graft were functional as indicated by their proliferation in response to lipopolysaccharide (LPS) and concanavalin A (ConA) stimulation. Progenitor cell activity determined by colony-forming unit interleukin-3 (CFU-IL-3) levels was also present in these grafts. Splenic implants were then assessed in transplant models following lethal irradiation and syngeneic or allogeneic bone marrow transplantation (BMT). Two weeks post-BMT, adult splenic tissue implants contained donor-derived B cells, T cells, and myeloid cell populations. As typically detected in the host spleen post-BMT, the grafted tissue also contained elevated levels of donor progenitor cells. By 3 months post-BMT, CFU-IL3 levels in the graft reflected the decreased levels characteristic of adult levels. The functional integrity of post-transplant splenocytes in the implants was also demonstrated by mitogenic responsiveness. In summary, this method should provide a useful model for the transfer of the splenic microenvironment to study the biology of the spleen in non-transplant and BMT settings.

PECAM-1 is expressed on hematopoietic stem cells throughout ontogeny and identifies a population of erythroid progenitors

Platelet endothelial cell adhesion molecule-1 (PECAM-1) (CD31) is an adhesion molecule expressed on endothelial cells and subsets of leukocytes. Analysis of phenotypically defined hematopoietic stem cells (HSCs) from the yolk sac, fetal liver, and adult bone marrow demonstrates CD31 expression on these cells throughout development. CD31+ c-kit+ cells, but not CD31- c-kit+ cells, isolated from day-9.5 yolk sac give rise to multilineage hematopoiesis in vivo. Further evaluation of the CD31+ lineage marker-negative fraction of adult bone marrow reveals functionally distinct cell subsets. Transplantation of CD31+ Lin- c-kit- cells fails to protect lethally irradiated recipients, while CD31+ Lin- c-kit+ Sca-1- cells (CD31+ Sca-1-) provide radioprotection in the absence of long-term donor-derived hematopoiesis. Although donor-derived leukocytes were not detected in CD31+ Sca-1- recipients, donor-derived erythroid cells were transiently produced during the initial phases of bone marrow recovery. These results demonstrate CD31 expression on hematopoietic stem cells throughout ontogeny and identify a population of CD31+ short-term erythroid progenitors cells that confer protection from lethal doses of radiation.

The Effect of the Spleen on Compartmental Levels and Distribution of Donor Progenitor Cells after Syngeneic and Allogeneic Bone Marrow Transplants

These studies investigate the involvement of the spleen in progenitor (PC) cell numbers and "cross-talk" with the marrow compartment following syngeneic or allogeneic bone marrow transplantation (BMT) in sham or fully splenectomized mice. Intact recipient B6 mice were lethally irradiated prior to transplant with T cell-depleted bone marrow (BM-TCD). The kinetics of PC reconstitution following i.v. transplant consistently revealed a dramatic increase in splenic colony-forming unit interleukin-3 (CFU IL-3) and CFU (high proliferative potential-(HPP) levels between days 5 and 12 post-BMT. Direct injection of TCD-BM into the recipient marrow cavity did not alter this pattern of reconstitution in the splenic compartment. In contrast to spleens from normal adult B6 mice containing 0.9% and 0.6% of the total combined splenic and marrow committed (CFU IL-3) and primitive (CFU-HPP) progenitors, respectively, spleens of syngeneic BMT recipients at day 12 contained a 10-fold increase (p < 0.001) over the progenitor levels in normal spleens. These splenic numbers decreased to normal, homeostatic levels by day 28 post-BMT. In contrast, the level of marrow CFU IL-3 progenitors continued to increase post-transplant, reaching near homeostatic levels by day 28 post-BMT. Interestingly, early seeding of 5- (and -6)carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled or green fluorescent protein (GFP) donor bone marrow cells (BMC) to the marrow compartment was not different in sham splenectomies or recipients splenectomized 14 days earlier. However, recipient splenectomy consistently resulted in significantly higher numbers of CFU IL-3 in the bone marrow during the first 2 weeks post-transplant compared to sham controls. These elevated levels exceeded the combined progenitor numbers of the splenic and marrow compartments of intact recipients. Notably, this increase in marrow progenitor activity in splenectomized recipients was observed after syngeneic as well as allogeneic BMT. Allogeneic transplants across major, or those limited to minor, histocompatibility antigen differences exhibited this increased marrow progenitor activity. Splenectomy performed 2 h post-transplant to assure "normal" marrow seeding also resulted in higher marrow progenitor activity. Thus, this "marrow response" to splenectomy is not induced by early "shunting" of infused BM cells to the marrow compartment. These results suggest that communication between the splenic and marrow compartments following syngeneic and allogeneic BMT exists during early hematopoietic reconstitution, one effect of which is to impact the compartmental distribution of donor progenitor cells. The role of the spleen on engraftment, chimerism, and tolerance in allogeneic BMT models are now under investigation.

L-selectin defines a bone marrow analog to the thymic early T-lineage progenitor

The recent description of an early T-lineage progenitor (ETP) population in adult mouse thymus implies the presence of a bone marrow predecessor that has not yet been identified. Here we describe a Lin(Neg) Sca-1(Pos) c-kit(Hi) Thy-1.1(Neg) L-selectin(Pos) adult mouse bone marrow population that resembles the thymic ETP in both antigen expression phenotype and posttransplantation lineage potential. These cells produce wavelike kinetics of thymic seeding and reconstitute the irradiated thymus with kinetics comparable to a thymocyte graft after intravenous transplantation. Transient B-lineage reconstitution is also observed, but little myeloid potential can be detected in transplant experiments. A second subset of progenitors is L-selectin(Neg) and is highly enriched for rapid and persistent T- and B-lineage potential, as well as some myeloid potential. L-selectin (CD62L) is therefore an effective marker for separating lymphoid progenitors from myeloid progenitors and hematopoietic stem cells in mouse bone marrow.

Early stages of hematopoietic differentiation

Mouse bone marrow contains hematopoietic stem cells as well as progenitor cells, which are partially differentiated offspring of stem cells. We have utilized several approaches to separate progenitors from stem cells in order to characterize essential differences between these two stages of development. As a first approach, we utilized the supravital fluorescent dye rhodamine-123 (Rh-123) to distinguish quiescent stem cells (Rh-123(low)) from metabolically active progenitor cells (Rh-123(hi)). Analysis of megakaryocyte potential in a tissue culture assay demonstrated that Rh-123(hi) progenitor cells were capable of robust megakaryocyte differentiation, while Rh-123(low) stem cells produced fewer colonies containing megakaryocytes. Transplantation of the two cell populations into irradiated recipients revealed the opposite outcome, suggesting that the tissue culture assay failed to predict behavior in a transplant setting. We also evaluated functional potential of lymphoid progenitors isolated by selecting for differential expression of Thy-1.1 and c-kit. The potential of defined cell populations to differentiate as T or B lymphocytes in vivo was dependent upon the time post transplant at which animals were evaluated. These studies underscore the need for caution in the interpretation of lineage potentials evaluated by both in vitro and in vivo assays.

Major age-related changes of mouse hematopoietic stem/progenitor cells

To study age-related changes of mouse bone marrow (BM) cells and hematopoietic stem cells (HSCs), we isolated rhodamine-123(low) (Rh(low)) Thy1.1(low) Lin(-)Sca-1(+) (TLS) HSCs from the BM of old mice and compared their functional characteristics to cells of the same phenotype isolated from young mice. We observed impaired recovery of B lymphocytes and decreased self-renewal in recipients of old Rh(low) cells compared to young Rh(low) cells. Blockade of Rh efflux using verapamil improved lymphoid reconstitution by enriched HSCs, and isolation of aged HSCs based on efflux of a fluorescent multi-drug resistance (MDR) substrate (Bodipy-verapamil) resulted in enrichment of HSC activity equivalent to that obtained with Rh. These observations suggest a complex relationship between MDR activity and HSC function during aging. To address whether the difference between young and aged donors was intrinsic to the HSC compartment or was due to a shift in HSC phenotype, we co-transplanted normal BM derived from young or old donors and followed repopulation simultaneously in the same recipient animals. In a parallel experiment, we co-transplanted HSCs purified from old donors with BM derived from young donors. In both experiments, transplants were given to both young and old recipients. The results show a clear defect in B-cell engraftment from either BM or HSCs of old donors, irrespective of the age of the recipient. In contrast, myeloid engraftment was predominantly derived from BM or HSCs derived from aged donors, again irrespective of recipient age. These data suggest a stem cell basis for B-cell immuno-senescence and the increased incidence of myelocytic leukemia in elderly people.