Endothelial cell support of hematopoiesis is differentially altered by IL-1 and glucocorticoids

The potency of hematopoietic stem cell reprogramming for changing immune tone

Innate immune memory endows innate immune cells with antigen independent heightened responsiveness to subsequent challenges. The durability of this response can be mediated by inflammation induced epigenetic and metabolic reprogramming in hematopoietic stem and progenitor cells (HSPCs) that are maintained through differentiation to mature immune progeny. Understanding the mechanisms and extent of trained immunity induction by pathogens and vaccines, such as BCG, in HSPC remains a critical area of exploration with important implications for health and disease. Here we review these concepts and present new analysis to highlight how inflammatory reprogramming of HSPC can potently alter immune tone, including to enhance specific anti-tumor responses. New findings in the field pave the way for novel HSPC targeting therapeutic strategies in cancer and other contexts of immune modulation. Future studies are expected to unravel diverse and extensive effects of infections, vaccines, microbiota, and sterile inflammation on hematopoietic progenitor cells and begin to illuminate the broad spectrum of immunologic tuning that can be established through altering HSPC phenotypes. The purpose of this review is to draw attention to emerging and speculative topics in this field where we posit that focused study of HSPC in the framework of trained immunity holds significant promise.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

Tumor-conditioned mesenchymal stem cells display hematopoietic differentiation and diminished influx of Ca2+

Mesenchymal stem cells (MSCs) that are present in many adult tissues can generate new cells either continuously or in response to injury/cancer. An increasing number of studies demonstrated that MSCs have the ability to differentiate into cells of mesodermal origin and transdifferentiate into cells such as hepatocytes, neural cells. There has been growing interest in the application of MSCs to cancer therapy. The relationship between MSCs and cancer cells remains highly controversial. In this study, we analyzed the interaction of bone marrow-derived MSCs and cancer cells by cell-cell contact and transwell culture system. The flow cytometry and real-time polymerase chain reaction showed that after coculture of MSCs and cancer cells, MSCs displayed the hematopoietic cell markers such as CD34, CD45, and CD11b. The CD68, MRCI, and CSF1R were dramatically upregulated after coculture. The cytokine array showed that MSCs after coculture secreted monokines and chemokines much more than that of intact MSCs. The MSCs under tumor conditions were responsive to stimulation with lipopolysaccharide by cytokines release. The tumor-conditioned MSCs showed phagocytic ability and enhanced release of nitric oxide, which are the characteristics of macrophages. Calcium ion is an important intracellular messenger responsible for differentiation and gene expression regulations. The influx of Ca(2+) into MSCs was obviously reduced after coculture. The blocking of calcium channel with verapamil obviously increased the expression of CD34, CD45, and CD11b, thus indicating that the diminished calcium ion influx is coupled with the hematopoietic differentiation of MSCs under tumor conditions. Taken together, in a cancer environment, MSCs could effectively differentiate into immune hematopoietic cells, precisely macrophages. Diminished transient influx of Ca(2+) may mediate the hematopoietic differentiation of MSCs.

The vascular niche: home for normal and malignant hematopoietic stem cells

Hematopoietic stem cells (HSCs) are uniquely capable of self-renewal and provision of all of the mature elements of the blood and immune system throughout the lifetime of an individual. HSC self-renewal is regulated by both intrinsic mechanisms and extrinsic signals mediated via specialized microenvironments or 'niches' wherein HSCs reside. HSCs have been shown to reside in close association with bone marrow (BM) osteoblasts in the endosteal niche and also in proximity to BM sinusoidal vessels. An unresolved question surrounds whether the endosteal and vascular niches provide synchronous or redundant regulation of HSC fate or whether these niches provide wholly unique regulatory functions. Furthermore, while some aspects of the mechanisms through which osteoblasts regulate HSC fate have been defined, the mechanisms through which the vascular niche regulates HSC fate remain obscure. Here, we summarize the anatomic and functional basis supporting the concept of an HSC vascular niche as well as the precise function of endothelial cells, perivascular cells and stromal cells within the niche in regulating HSC fate. Lastly, we will highlight the role of the vascular niche in regulating leukemic stem cell fate in vivo.

Transcriptional profiling of the hematopoietic support of interleukin-stimulated human umbilical vein endothelial cells (HUVECs)

Endothelial cells can be successfully used to maintain or increase the number of hematopoietic stem cells in vitro. Previously we identified hematopoietic progenitor cell (HPC) expansion or survival benefit induced by IL-1β-, IL-3-, and IL-6-stimulated human umbilical vein endothelial cell (HUVEC) supernatants. In order to identify molecular mechanisms that support hematopoiesis, we examined the time-dependent expression profiles of IL-1β-, IL-3-, and IL-6-stimulated HUVECs via microarray. Here, we present 24 common upregulated elements and three common downregulated elements of IL-1β- and IL-3-stimulated HUVECs, with these factors exhibiting great potential for the observed HPC expansion. Furthermore, metabolic pathway analysis resulted in the identification of nonproteinogenic factors such as prostaglandin E(2) (PGE(2)) and nitric oxide (NO) and determined their HPC expansion potential via delta, methylcellulose, and cobblestone assays. We confirmed PGE(2) and spermine as hematopoietic expansion factors. Furthermore, we identified several factors such as SSAT, extracellular matrix components, microRNA21, and a microvesicle-mediated cross-talk between the endothelium and HPCs that may play a crucial role in determining stem cell fate. Our results suggest that microarray in combination with functional annotations is a convenient method to identify novel factors with great impact on HPC proliferation and differentiation.

Endothelial cell biology: role in the inflammatory response

Human endothelial cells are multifunctional cells that line blood vessels and are capable of secreting a variety of biologically active mediators. They normally maintain vascular hemostasis and prevent thrombotic complications. When affected by infection, stress, hypertension, dyslipidemia, or high homocysteine levels, endothelial cells undergo changes resulting in "dysfunction," characterized typically by decreased endothelial expression of nitric oxide, enhanced expression of cell adhesion molecules, and associated increased binding of circulating leukocytes to these cells. There is accompanying cytokine and chemokine elaboration, resulting in cellular recruitment and the orchestration of an acute inflammatory response that can culminate in chronic inflammation if reparative mechanisms are not operative. This review will address the basic biology of endothelial cells; the expression and regulation of endothelial-derived cytokines, chemokines, and growth factors; the transcriptional regulation of these genes in endothelial cells; and the role played by these fascinating cells in human disease.

Primary cells as feeder cells for coculture expansion of human hematopoietic stem cells from umbilical cord blood--a comparative study

Although umbilical cord blood (UCB) has been widely accepted as an alternative source of hematopoietic stem cells (HSC) for transplantation, its use in adults is restricted because of low absolute HSC numbers. To overcome this obstacle, expansion of HSC in coculture with feeder cells is a promising possibility. In this study, we compared the potential of three human primary cell types, namely, mesenchymal stem cells (MSC), human umbilical cord vein endothelial cells (HUVEC), and Wharton's jelly cells (WJC), for use as feeder cells in a potentially clinically applicable coculture system. In first experiments, we evaluated procedures needed to obtain feeder cells, the possibility to separate them from cells derived from CD34(+) cells after coculture, their ability to activate allogeneic T cells, and their survival in CD34(+)-adapted medium. Finally, we compared their support for UCB-derived CD34(+) expansion. MSC and WJC were superior to HUVEC in terms of ease and reliability of isolation procedures needed. None of the potential feeder cells expressed CD34 or CD45, thus providing markers for cell sorting after coculture. Other markers (CD31, CD90, CD105, CD166) were expressed differently on feeder cell types. While MSC in higher concentrations did not activate allogeneic T cells, those were stimulated by lower concentrations of MSC as shown by CD25, CD69, and CD71 expression. In contrast, HUVEC and WJC were proven to activate T cells at all ratios tested. Feeder cells survived a 7-day culture in CD34(+)-adapted medium. In cocultures of UCB CD34(+)cells with primary feeder cells, mononuclear cell expansion was 30- to 60-fold, colony-forming cell expansion 20- to 40-fold, and cobblestone area-forming cell expansion 10- to 50-fold. We conclude that after a careful further evaluation especially of their immunological properties, all three primary cell types might possibly be suitable for use in a potentially clinically applicable system for expansion from UCB CD34(+)cells, with WJC being best choice and MSC still superior to HUVEC.

Hematopoietic progenitor cells and interleukin-stimulated endothelium: expansion and differentiation of myeloid precursors

BACKGROUND

Cytokine-stimulated endothelial cells (EC) propagate hematopoietic progenitor cell (HPC) expansion. However, the effects on the functional capacities of cultured progenitors have not been evaluated. HPC were assessed by flow cytometry, colony and cobblestone assays and long-term cultures (LTC) after culturing in the supernatant of EC stimulated by IL-1beta, IL-3 or IL-6.

RESULTS

EC incubation with IL-6 did not improve cell expansion in comparison to non-stimulated EC supernatant, while the HPCs' phenotype and functional capacities were retained. In contrast, IL-1beta and IL-3 stimulation resulted in a 10- and 100-fold increase in cell numbers with more than 90% of these cells being CD33(+). Plating efficiencies and LTC initiating cells were greatest in IL-6 supernatants, whereas the highest numbers of burst-forming units were observed using IL-3. IL-1beta supernatants diminished the number of 5-week cobblestone-areas, whereas the number of 2-week cobblestone areas remained equal to freshly isolated HPC. Fewer 2-week cobblestones and greater amounts of 5-week cobblestones were observed with IL-6 and IL-3. Expanded progenitors from all interleukin conditions were further matured into functional granulocytes.

CONCLUSION

IL-1beta and IL-3 stimulated endothelium induces proliferation and differentiation of myeloid precursors, while IL-6 treatment induced a benefit of HPC survival.

A human endothelial cell feeder system that efficiently supports the undifferentiated growth of mouse embryonic stem cells

Feeder cells are commonly used to culture embryonic stem cells to maintain their undifferentiated and pluripotent status. Conventionally, mouse embryonic fibroblasts (MEFs), supplemented with leukemia inhibitory factor (LIF), are used as feeder cells to support the growth of mouse embryonic stem cells (mESCs) in culture. To prepare for fresh MEF feeder or for MEF-conditioned medium, sacrifice of mouse fetuses repeatedly is unavoidable in these tedious culture systems. Here we report the discovery of a human endothelial cell line (ECV-304 cell line) that efficiently supports growth of mESCs LIF-free conditions. mESCs that were successfully cultured for eight to 20 passages on ECV-304 feeders showed morphological characteristics similar to cells cultured in traditional feeder cell systems. These cells expressed the stem cell markers Oct3/4, Nanog, Sox2, and SSEA-1. Furthermore, cells cultured on the ECV-304 cell line were able to differentiate into three germ layers and were able to generate chimeric mice. Compared with traditional culture systems, there is no requirement for mouse fetuses and exogenous LIF does not need to be added to the culture system. As a stable cell line, the ECV-304 cell line efficiently replaces MEFs as an effective feeder system and allows the efficient expansion of mESCs.

Human umbilical vein endothelial cells increase ex vivo expansion of human CD34(+) PBPC through IL-6 secretion

BACKGROUND

Ex vivo expansion of hematopoietic stem cells (HSC) can help reduce cytopenia following transplantation, especially in NHL patients whose BM is deficient because of extensive chemotherapy. We have previously reported that human umbilical vein endothelial cells (HUVEC) can contribute to improved PBPC expansion when used in co-culture with CD34(+) cells.

METHODS

We evaluated the roles of direct HUVEC CD34(+) contact and HUVEC-produced soluble factors. We cultured CD34(+) PBPC harvested from NHL patients in four different conditions: (1) liquid culture without HUVEC; (2) co-culture in contact with HUVEC; (3) co-culture with HUVEC but without direct contact; (4) liquid culture with HUVEC-conditioned medium (CM). Thrombopoietin (Tpo), Flk2Flt3 ligand (FL) and c-kit ligand (KL) with or without rhIL-6 were added to these four culture conditions.

RESULTS AND DISCUSSION

Our results showed that HUVEC co-culture or addition of HUVEC-CM to Tpo, FL and KL (TFK) improved CD34(+) PBPC expansion compared with liquid culture, as determined by total viable nucleated cells (TNC), colony-forming cell assay (CFC) and week-6 cobblestone area-forming cells (Wk-6 CAFC) expansions. Non-contact culture led to similar PBPC expansion as contact co-culture; moreover, HUVEC-CM improved PBPC expansion. However, when rhIL-6 was added to HUVEC-CM with TFK, no significant difference was observed. Finally, high quantities of IL-6 were detected in HUVEC-CM and addition of anti-IL-6 Ab inhibited the positive effect of HUVEC on PBPC expansion. Our results thus suggest that HUVEC may improve PBPC expansion, at least through IL-6 secretion.

Molecular Profile and Partial Functional Analysis of Novel Endothelial Cell-Derived Growth Factors that Regulate Hematopoiesis

Recent progress has been made in the identification of the osteoblastic cellular niche for hematopoietic stem cells (HSCs) within the bone marrow (BM). Attempts to identify the soluble factors that regulate HSC self-renewal have been less successful. We have demonstrated that primary human brain endothelial cells (HUBECs) support the ex vivo amplification of primitive human BM and cord blood cells capable of repopulating non-obese diabetic/severe combined immunodeficient repopulating (SCID) mice (SCID repopulating cells [SRCs]). In this study, we sought to characterize the soluble hematopoietic activity produced by HUBECs and to identify the growth factors secreted by HUBECs that contribute to this HSC-supportive effect. Extended noncontact HUBEC cultures supported an eight-fold increase in SRCs when combined with thrombopoietin, stem cell factor, and Flt-3 ligand compared with input CD34(+) cells or cytokines alone. Gene expression analysis of HUBEC biological replicates identified 65 differentially expressed, nonredundant transcripts without annotated hematopoietic activity. Gene ontology studies of the HUBEC transcriptome revealed a high concentration of genes encoding extracellular proteins with cell-cell signaling function. Functional analyses demonstrated that adrenomedullin, a vasodilatory hormone, synergized with stem cell factor and Flt-3 ligand to induce the proliferation of primitive human CD34(+)CD38(-)lin(-) cells and promoted the expansion of CD34(+) progenitors in culture. These data demonstrate the potential of primary HUBECs as a reservoir for the discovery of novel secreted proteins that regulate human hematopoiesis.

Oxygen concentration influences mRNA processing and expression of the cd34 gene

CD34 is a cell surface glycoprotein expressed on hematopoietic stem and progenitor cells that disappears with their maturation. This gene is transcribed in two alternatively spliced mRNAs that encode full length and truncated form of CD34 cell surface antigen. Some publications suggested that CD34 full length plays a role in the maintenance of their self renewal capacity. An examination of CD34 regulation by a low O2 concentration that ensures a better maintenance of stem cells may provide important insights into the molecular control of hematopoiesis. Using human cord blood CD34+ cells, we first compared the effect of short term (24 h) culture in hypoxia (1% O2) and normoxia (20% O2) on the expression of full length and truncated form of cd34 transcripts and on the expression of the CD34 antigen. Hypoxia maintained a larger quantity of cd34 full length transcripts and a higher cd34 full length/cd34 truncated form ratio than normoxia. After 72 h of culture at 1% and 20% O2, sorted CD34low sub-population from 1% O2 primary culture still contained more cd34 full length mRNAs than those from 20% O2, maintained better CD34 antigen expression during secondary culture at 20% O2 and contained more undifferentiated cells. This work provides the first evidence of the regulation of the cd34 gene by hypoxia resulting in a delayed higher and longer antigen expression by cord blood cells. We suggest that this phenomenon is related to the better maintenance of primitive stem cells in hypoxia.

Platelet-derived growth factor enhances expansion of umbilical cord blood CD34+ cells in contact with hematopoietic stroma

Stem cell expansion remains an elusive but highly desirable goal. Here we show that platelet-derived growth factor (PDGF), along with cultured endothelial or stromal cells, significantly enhances expansion of human CD34+ cells in vitro. In media supplemented with thrombopoietin, stem cell factor, flt-3 ligand, and granulocyte-colony stimulating factor, CD34+ cells, as well as CFU-GM, BFU-CFU-E, CFU-GEMM, and CFU-MK, increased by 34.3-, 138-, 59.7-, 38.4-, and 86.0-fold, respectively. Co-culturing of CD34+ cells with cultured stromal cells or human umbilical cord vein endothelial cells (HUVECs) greatly enhanced expansion efficiency. The presence of PDGF (50 ng/ml) further augmented expansion, such that increases of 77.0-, 262-, 90.0-, 93.0-, and 200-fold, respectively, were achieved. Six weeks after infusion of expanded cells into NOD/SCID mice, human CD45+ cells were detected in recipients' bone marrow, spleen, and peripheral blood. Our results provide a rationale for development of a stem cell expansion protocol for clinical applications.

Tumor necrosis factor alpha-stimulated endothelium: an inducer of dendritic cell development from hematopoietic progenitors and myeloid leukemic cells

Especially when exposed to inflammatory stimuli, endothelial cells (EC) have been shown to promote the maturation of monocytes into dendritic cells (DC) and the long-term proliferation of CD34+ cells by constitutive cytokine production and direct cellular contact. We therefore hypothesized that cytokine-stimulated EC would induce hematopoietic progenitor cells to develop into mature dendritic cells. To test this theory, human CD34+ cells derived from cord blood or leukapheresis products were cultured with a monolayer of either interleukin (IL)-1beta, IL-4, or tumor necrosis factor (TNF)-alpha-stimulated human umbilical cord EC. The cells in suspension were analyzed weekly over a period of 6 weeks. IL-1beta supported cell expansion, whereas IL-4 had no effect on cell expansion or DC differentiation. Only TNF-alpha-stimulated EC induced the development of mature, allostimulatory DC with a high expression of CD83, HLA-DR, CD1a, and costimulatory molecules like CD80 and CD86. Acute myeloid leukemia cells from the cell line Kasumi-1 also developed DC-like features when cocultured with TNF-alpha-stimulated EC. Direct contact between endothelial and progenitor cells increased the number of developing DC. Cell cycle analysis and apoptosis studies demonstrated a reduced G2M fraction, an increased S fraction, and a decrease in TNF-alpha-dependent apoptosis of DC developing in the presence of endothelial cells. As shown by electron and confocal microscopic studies, intimate interactions between EC and DC occurred, resulting in the internalization of the developing DC within the EC monolayer and a bidirectional exchange of proteins. We conclude that, via the action of TNF-alpha, inflamed human endothelium can induce CD34+ and leukemic cells to differentiate into dendritic cells.

Flt3-ligand induces adhesion of haematopoietic progenitor cells via a very late antigen (VLA)-4- and VLA-5-dependent mechanism

The adhesion of haematopoietic progenitor cells (HPC) to the bone marrow microenvironment is a process regulated by cytokines. In this study, we have shown that flt3-ligand (FL), a growth factor that controls early haematopoiesis, regulated the function and expression of the beta-1 integrins, very late antigen (VLA)-4 and VLA-5 on HPC. The modulation of the adhesiveness of HPC by FL was studied by adhesion assays on umbilical vein endothelial cells (HUVEC). Stimulation by FL induced two peaks of increased adhesiveness of HPC. The first peak was at around 30 min and was mechanistically related to an activation of the beta-1 integrins, mainly VLA-4 and VLA-5. The second peak was at around 12 h and was related to increased expression of VLA-4 and VLA-5. The control of HPC adhesiveness by FL is a previously unreported property of FL that may be important for the homing and the retention of flt3-expressing HPC within the bone marrow microenvironment.

Ex vivo expansion of stem and progenitor cells in co-culture of mobilized peripheral blood CD34+ cells on human endothelium transfected with adenovectors expressing thrombopoietin, c-kit ligand, and Flt-3 ligand

To optimize conditions for ex vivo expansion of adult hematopoietic stem cells, we evaluated the co-culture of G-CSF mobilized human peripheral blood (PB) CD34(+) cells with endothelial cells engineered to overexpress various hematopoietic growth factors. Immortalized human bone marrow endothelial cells (BMEC) transfected with an expression vector carrying cDNA encoding the human telomerase reverse transcriptase (hTERT) and human umbilical vein endothelial cells (HUVEC) were transfected with combinations of adenovectors expressing murine c-kit ligand (KL), human thrombopoietin (TPO), human Flt3 ligand (FL), and human granulocyte-macrophage colony-stimulating factor (GM-CSF). Ex vivo expansion of PB CD34(+) cells from normal donors and non-Hodgkin lymphoma (NHL) patients in endothelial co-culture was evaluated weekly for total cell production, progenitor (CFU-GM, BFU-E) cell production, and stem cell production as measured by Week-5 Cobblestone Area Forming Cell assay (Wk-5 CAFC). HUVEC transfected with adenovectors expressing TPO, KL, and FL provided the best co-culture system for expanding CD34(+) cells. Maximal total nuclear cell, CFU-GM, and Wk-5 CAFC production occurred between weeks 2 and 3 with 113-fold, 25-fold, and 2.2-5.5-fold expansions, respectively. We did not detect significant differences when GM-CSF was added to the co-culture system. Expansion was also obtained using recombinant human cytokines, but was not maintained beyond 3 weeks. We demonstrated that continuous generation of high levels of TPO, FL, and KL as well as other factors secreted by endothelium provided a clinically relevant co-culture method for ex vivo expansion of stem and progenitor cells from cryopreserved CD34(+) populations.

Irradiation enhances the support of haemopoietic cell transmigration, proliferation and differentiation by endothelial cells

Endothelial cells (ECs) are a critical component of the bone marrow stroma in the regulation of haemopoiesis. Recovery of bone marrow aplasia after radiation exposure depends, in part, on the repair of radiation-induced endothelial damage. Therefore, we assessed the ability of an irradiated human bone marrow EC line (TrHBMEC) to support transmigration, proliferation and differentiation of CD34+ bone marrow cells either irradiated or not in transendothelial migration or co-culture models. Radiation-induced EC damage was reflected by an increased release of soluble intercellular adhesion molecule (sICAM)-1 and platelet endothelial cell adhesion molecule (PECAM)-1. Irradiation of TrHBMECs with a 10 Gy dose strongly enhanced the transmigration of CD34+ cells, granulo-monocytic progenitors (CFU-GM) and erythroid progenitors (BFU-E). While ICAM-1 and PECAM-1 expression on irradiated TrHBMECs was increased, only antibodies against PECAM-1 inhibited the radiation-induced enhanced transmigration of haemopoietic cells. Irradiation of TrHBMECs (5-15 Gy) also increased proliferation and differentiation towards the granulo-monocytic lineage of co-cultured CD34+ cells, as well as colony formation by those cells and the production of interleukin 6 (IL-6), IL-8, granulocyte colony-stimulating factor (CSF) and granulocyte-macrophage CSF. Irradiated TrHBMECs were more capable of stimulating irradiated (1,2 Gy) CD34+ cells and haemopoietic progenitors than non-irradiated TrHBMECs. Together, these results suggest that, despite the radiation-induced damage, irradiated ECs may favour haemopoietic reconstitution after radiation exposure.

Umbilical cord blood for allogeneic transplantation in children and adults

Early clinical reports outlining outcomes for primarily pediatric patients undergoing UCB transplantation, point to delayed time to hematopoietic recovery, and favorable incidence and severity of graft-versus-host disease. Intensive clinical and laboratory research is ongoing focused on strategies to foster UCB allogeneic donor engraftment, thereby allowing wider application of this stem cell source for patients requiring allogeneic transplantation.

CD40-ligand stimulates myelopoiesis by regulating flt3-ligand and thrombopoietin production in bone marrow stromal cells

CD40 ligand (CD40L)/CD40 interactions play a central role in T-cell–dependent B-cell activation as previously shown by in vitro studies, the phenotype of CD40L knockout mice and the defective expression of CD40L in patients who have X-linked immunodeficiency with hyper-IgM. The distribution of CD40 in cells other than of myeloid and lymphoid lineages has suggested additional functions for this receptor/ligand couple. Here we show that CD40L stimulates myelopoiesis with a noticeable effect on megakaryocytopoiesis in cocultures of hematopoietic progenitor cells and bone marrow stromal cells. These results suggest a mechanism by which T-cell or platelet-associated or soluble CD40L may regulate myelopoiesis.

CD40-ligand stimulates myelopoiesis by regulating flt3-ligand and thrombopoietin production in bone marrow stromal cells

CD40 ligand (CD40L)/CD40 interactions play a central role in T-cell–dependent B-cell activation as previously shown by in vitro studies, the phenotype of CD40L knockout mice and the defective expression of CD40L in patients who have X-linked immunodeficiency with hyper-IgM. The distribution of CD40 in cells other than of myeloid and lymphoid lineages has suggested additional functions for this receptor/ligand couple. Here we show that CD40L stimulates myelopoiesis with a noticeable effect on megakaryocytopoiesis in cocultures of hematopoietic progenitor cells and bone marrow stromal cells. These results suggest a mechanism by which T-cell or platelet-associated or soluble CD40L may regulate myelopoiesis.

Differential effect of interferon alpha on chronic myelogenous leukaemia and normal haematopoietic progenitors in a stromal cell co-culture context: role of the flt3 ligand

Interferon alpha (IFN-alpha) is used to treat chronic myelogenous leukaemia (CML) patients. However, its target(s) remain(s) unknown. One possibility is that there is a differing sensitivity of the leukaemic from the normal colony-forming cell (CFC) compartments to IFN-alpha. Co-cultures of progenitors with stromal cells provide a valuable tool to dissect direct and indirect activities of IFN-alpha. In this study, we have used endothelial cells (EC) as a source of stromal cells. In co-cultures of normal progenitors with EC, IFN-alpha increased the generation of clonogenic cells, mainly via an increased production of flt3 ligand (FL) by EC. In contrast, in co-cultures of CML progenitors with EC, IFN-alpha inhibited the generation of clonogenic cells, mainly by direct inhibition on the progenitors, the up-regulation of FL production by stromal cells being unable to compensate for the direct inhibitory effects of IFN-alpha. These data provide evidence for a differential effect of IFN-alpha on the growth of CML and normal CFC cells in a stromal context and suggest that an alteration in the response of CML progenitor cells to FL is important in the explanation of this differential effect.

CD39 modulates IL-1 release from activated endothelial cells

The activation of endothelial cells (EC) and monocyte-macrophages (Mφ) by lipopolysaccharide (LPS) is considered an important element of the vascular injury observed in endotoxemia. Interleukin-1 (IL-1) beta release from Mφ in response to LPS, appears to be mediated by the autocrine/paracrine release of ATP via P2X7 receptor activation. In EC, similar nucleotide-mediated signaling pathways may be influenced by high levels of expression of CD39, the vascular nucleoside triphosphate diphosphohydrolase (NTPDase; ENTPD I). To determine whether CD39 modulates ATP-mediated release of IL-1 from EC, we stimulated human EC with LPS and measured levels of ATP secretion and IL-1 release. LPS triggered ATP secretion from EC that was soon followed by IL-1alpha release. Overexpression of CD39 following infection with recombinant CD39 adenoviral vectors (AdCD39) abrogated the initial phase of ATP secretion and inhibited IL-1alpha release; comparable results were obtained with soluble NTPDase. These data demonstrate that CD39/NTPDase modulates IL-1alpha release from LPS stimulated human EC.

Interleukin 8 (IL-8) and the release of leukocytes from the bone marrow

Interleukin 8 (IL-8) is produced by various cells upon stimulation and influences a variety of functions of leukocytes in particular neutrophils. Systemic administration of IL-8 induces a rapid neutropenia associated by sequestration of neutrophils in the lung that is followed by a neutrophilia characterized by the rapid release of neutrophils from the bone marrow. These cells are released predominantly from the bone marrow venous sinusoids. In addition, several studies have shown the potential role of IL-8 in hematopoiesis and trafficking of hematopoietic stem cells. Systemic administration of IL-8 induces a rapid mobilization of progenitors from the bone marrow with long-term myelo-lymphoid repopulation capacity. It has been employed clinically to mobilize hematopoietic progenitor cells into the peripheral blood and used for autologous or allogeneic bone marrow transplantation. The mechanism for these effects of IL-8 is largely speculative. This report summarizes current ideas on the possible mechanisms how IL-8 influences cell trafficking in and from the bone marrow.

Expression of Flt3-ligand by the endothelial cell

Flt3-ligand (FL) is a cytokine that is of paramount importance in the proliferation of primitive hematopoietic progenitors. In this study, we show that endothelial cells (EC) produce large amounts of soluble FL and express a membrane-bound form of the molecule. Bone marrow microvascular EC also produce FL, suggesting that EC are an important source of FL in the bone marrow. High concentrations of FL in EC supernatants contrast with its undetectable levels in long-term bone marrow cultures. A single mRNA for FL is detected, suggesting that soluble FL derives from the membrane-bound species by proteolytic release. FL mRNA is stable with a half-life of about 3 h. II-1alpha increases FL mRNA levels and membrane and soluble FL expression. Glucocorticoids, known inhibitors for many hematopoietic growth factors do not down-regulate the expression of FL. On the contrary, GC increase the expression of both species of FL. The neutralization of FL in cocultures EC/ hematopoietic progenitors results in an acceleration of the maturation of the progenitors. IFN-alpha, MIP-1 alpha and TGF-beta stimulate production of membrane-bound and soluble FL. This stimulation is essential to explain their modulatory effect on the generation of clonogenic cells in cocultures EC/hematopoietic progenitors. Leukemia (2000) 14, 153-162.