Transplantation studies in C3-deficient animals reveal a novel role of the third complement component (C3) in engraftment of bone marrow cells

Hematopoiesis Revolves Around the Primordial Evolutional Rhythm of Purinergic Signaling and Innate Immunity - A Journey to the Developmental Roots

A cell's most significant existential task is to survive by ensuring proper metabolism, avoiding harmful stimuli, and adapting to changing environments. It explains why early evolutionary primordial signals and pathways remained active and regulate cell and tissue integrity. This requires energy supply and a balanced redox state. To meet these requirements, the universal intracellular energy transporter purine nucleotide-adenosine triphosphate (ATP) became an important signaling molecule and precursor of purinergic signaling after being released into extracellular space. Similarly, ancient proteins involved in intracellular metabolism gave rise to the third protein component (C3) of the complement cascade (ComC), a soluble arm of innate immunity. These pathways induce cytosol reactive oxygen (ROS) and reactive nitrogen species (RNS) that regulate the redox state of the cells. While low levels of ROS and RNS promote cell growth and differentiation, supra-physiological concentrations can lead to cell damage by pyroptosis. This balance explains the impact of purinergic signaling and innate immunity on cell metabolism, organogenesis, and tissue development. Subsequently, along with evolution, new regulatory cues emerge in the form of growth factors, cytokines, chemokines, and bioactive lipids. However, their expression is still modulated by both primordial signaling pathways. This review will focus on the data that purinergic signaling and innate immunity carry on their ancient developmental task in hematopoiesis and specification of hematopoietic stem/progenitor cells (HSPCs). Moreover, recent evidence shows both these regulatory pathways operate in a paracrine manner and inside HSPCs at the autocrine level.

Hematopoietic stem cells on the crossroad between purinergic signaling and innate immunity

Hematopoiesis is regulated by several mediators such as peptide-based growth factors, cytokines, and chemokines, whose biological effects have been studied for many years. However, several other mediators have been identified recently that affect the fate of hematopoietic stem/progenitor cells (HSPC) as well as non-hematopoietic cells in the bone marrow microenvironment. These new mediators comprise members of purinergic signaling pathways and are active mediators of the soluble arm of innate immunity, the complement cascade (ComC). In this review, we will discuss the coordinated effects of these pathways in regulating the biology of HSPC. Importantly, both purinergic signaling and the ComC are activated in stress situations and interact with specific receptors expressed on HSPC. Evidence has accumulated indicating that some of the purinergic as well as ComC receptors could also be activated intracellularly by intrinsically expressed ligands. To support this recent evidence, our work indicates that the major mediator of purinergic signaling, adenosine triphosphate, and the cleavage product of the fifth component of the ComC (C5), C5a anaphylatoxin, can activate their corresponding receptors expressed on the outer mitochondrial membrane in an autocrine manner. We will also discuss recent evidence that these responses, mediated by purinergic signaling and the ComC network, are coordinated by activation of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 - reactive oxygen species - NLR family pyrin domain containing 3 (NLRP3) inflammasome (Nox2-ROS-NLRP3) axis.

External Liver-Derived Complement and Intrinsic Present in Hematopoietic Stem/Progenitor Cells Complosome Modulate Cell Metabolism and Response to Stress

Hematopoietic stem/progenitor cells (HSPCs) express receptors for complement cascade (ComC) cleavage fragments C3a and C5a and may respond to inflammation-related cues by sensing pathogen-associated molecular pattern molecules (PAMPs) released by pathogens as well as non-infectious danger associated molecular pattern molecules (DAMPs) or alarmin generated during stress/tissue damage sterile inflammation. To facilitate this HSPCs are equipped with C3a and C5a receptors, C3aR and C5aR, respectively, and express on the outer cell membrane and in cytosol pattern recognition receptors (PPRs) that sense PAMPs and DAMPs. Overall, danger-sensing mechanisms in HSPCs mimic those seen in immune cells, which should not surprise as hematopoiesis and the immune system develop from the same common stem cell precursor. This review will focus on the role of ComC-derived C3a and C5a that trigger nitric oxide synthetase-2 (Nox2) complex to release reactive oxygen species (ROS) that activate important cytosolic PRRs-Nlrp3 inflammasome, which orchestrates responsiveness of HSPCs to stress. Moreover, recent data indicate that in addition to circulating in peripheral blood (PB) activated liver-derived ComC proteins, a similar role plays ComC expressed and intrinsically activated in HSPCs known as "complosome". We postulate that ComC triggered Nox2-ROS-Nlrp3 inflammasome responses, if they occur within non-toxic to cells' "hormetic range of activation", positively regulate HSCs migration, metabolism, and proliferation. This sheds a new light on the immune-metabolic regulation of hematopoiesis.

Myeloablative Conditioning for Transplantation Induces State of Sterile Inflammation in the Bone Marrow: Implications for Optimizing Homing and Engraftment of Hematopoietic Stem Cells

Significance: The success rate of hematopoietic stem cell transplantation depends mainly on the number of transplanted hematopoietic stem/progenitor cells (HSPCs) followed by the speed of their engraftment in the myeloablated transplant recipient. Therefore, clinical outcomes will significantly benefit from accelerating the homing and engraftment of these cells. This is, in particular, important when the number of cells available for the transplantation of HSPCs is limited. Recent Advances: We postulated that myeloablative conditioning for hematopoietic transplantation by radio- or chemotherapy induces a state of sterile inflammation in transplant recipient peripheral blood (PB) and bone marrow (BM). This state is mediated by activation of the BM stromal and innate immunity cells that survive myeloablative conditioning and respond to danger-associated molecular patterns released from the cells damaged by myeloablative conditioning. As a result of this, several factors are released that promote proper navigation of HSPCs infused into PB of transplant recipient and prime recipient BM to receive transplanted cells. Critical Issues: We will present data that cellular innate immunity arm and soluble arm comprised complement cascade proteins, promoting the induction of the BM sterile inflammation state that facilitates the navigation, homing, and engraftment of HSPCs. Future Directions: Deciphering these mechanisms would allow us to better understand the mechanisms that govern hematopoietic recovery after transplantation and, in parallel, provide important information on how to optimize this process in the clinic by employing small molecular modifiers of innate immunity and purinergic signaling. Antioxid. Redox Signal. 37, 1254-1265.

Systematic Modification and Evaluation of Enzyme-Loaded Chitosan Nanoparticles

Polymeric-based nano drug delivery systems have been widely exploited to overcome protein instability during formulation. Presently, a diverse range of polymeric agents can be used, among which polysaccharides, such as chitosan (CS), hyaluronic acid (HA) and cyclodextrins (CDs), are included. Due to its unique biological and physicochemical properties, CS is one of the most used polysaccharides for development of protein delivery systems. However, CS has been described as potentially immunogenic. By envisaging a biosafe cytocompatible and haemocompatible profile, this paper reports the systematic development of a delivery system based on CS and derived with HA and CDs to nanoencapsulate the model human phenylalanine hydroxylase (hPAH) through ionotropic gelation with tripolyphosphate (TPP), while maintaining protein stability and enzyme activity. By merging the combined set of biopolymers, we were able to effectively entrap hPAH within CS nanoparticles with improvements in hPAH stability and the maintenance of functional activity, while simultaneously achieving strict control of the formulation process. Detailed characterization of the developed nanoparticulate systems showed that the lead formulations were internalized by hepatocytes (HepG2 cell line), did not reveal cell toxicity and presented a safe haemocompatible profile.

Mechanistic Basis of ex Vivo Umbilical Cord Blood Stem Progenitor Cell Expansion

Umbilical cord blood (CB) transplantation has been used successfully in humans for three decades due to its rapid availability for patients lacking a suitable allogeneic donor, less stringent HLA matching requirements, and low rates of relapse and chronic graft-versus-host disease (GVHD). However, CB transplantation is associated with complications, such as delayed hematopoietic engraftment, graft failure, which increases infection and bleeding and causes longer hospital stays, and transplant-related mortality. The majority of these biological limitations are due to the unforeseeable functional potency of multipotent hematopoietic stem cells (HSCs), which reduce the predictability of successful transplantation; however, several strategies have been developed to increase the number of hematopoietic stem progenitor cells (HSPCs) infused during CB transplantation. This review primarily addresses the methods that promote ex vivo CB expansion within the context of symmetrical and asymmetrical HSC division and those that rely on epigenetic mechanisms, along with the reportedly most successful cytokine combinations. We also review recent clinical research on small molecules (StemRegenin-1, UM171, and nicotinamide) in ex vivo expanded CB and discuss yet unvalidated preclinical strategies. Expanding and transplanting CB graft enriched in HSPCs in a single CB unit is a particularly exciting prospect with the potential to improve the use and availability of CB grafts. Greater knowledge of optimal ex vivo expansion strategies, cell longevity, and graft potency will expand the scope of cellular therapies. Also the development of adequate ex vivo HSPC expansion strategies could bring expanded cord blood grafts to the forefront of transplant therapy and regenerative medicine.

C3a and C5a facilitates the metastasis of myeloma cells by activating Nrf2

Multiple myeloma (MM) is still an incurable hematological malignancy, with even poorer prognosis in MM patients with distant invasion. The present study was designed to explore the effects of C3a and C5a on the migration, invasion, and adhesion of MM tumor cells and to investigate the underlying mechanisms. As a result, the levels of C3a and C5a in plasma of MM patients were significantly higher than those of healthy donors. Consistently, the expression of C3a and C5a receptors on myeloma cells of MM patients was also significantly higher than that on sorted plasma cells of normal donors. C3a and C5a have been confirmed to increase the migration, invasion and adhesion of MM cell lines by activating the MEK/ERK pathway and increasing the nuclear transfer of Nrf2 in vitro. Moreover, the MM cell line U266 with Nrf2 downregulation was incubated with C3a and C5a, followed by injection into the tail vein of NOD-SCID mice. We found that Nrf2 downregulation attenuated the migration of anaphylatoxin C3a and C5a to MM tumor cells in bone marrow, liver and lung in vivo. In conclusion, our results indicate that activation of the complement cascade in MM patients may contribute to the migration, invasion and adhesion of MM cells, and this type of tumor cells dissemination in MM is, at least partially, regulated by Nrf2. Thereby, complement suppression or Nrf2 downregulation might offer a novel therapeutic opportunity for MM.

Components of the Lectin Pathway of Complement in Haematologic Malignancies

The complement system is activated cascadically via three distinct major routes: classical pathway (CP), alternative pathway (AP) or lectin pathway (LP). The unique factors associated with the latter are collectins (mannose-binding lectin, collectin-10, collectin-11), ficolins (ficolin-1, ficolin-2, ficolin-3) and proteins of the mannose-binding lectin-associated serine protease (MASP) family (MASP-1, MASP-2, MASP-3, MAp19, MAp44). Collectins and ficolins are both pattern-recognising molecules (PRM), reactive against pathogen-associated molecular patterns (PAMP) or danger-associated molecular patterns (DAMP). The MASP family proteins were first discovered as complexes with mannose-binding lectin (MBL) and therefore named MBL-associated serine proteases, but later, they were found to interact with ficolins, and later still, collectin-10 and collectin-11. As well as proteolytic enzymes (MASP-1, MASP-2, MASP-3), the group includes non-enzymatic factors (MAp19, MAp44). In this review, the association-specific factors of the lectin pathway with haematologic malignancies and related infections are discussed.

Innate immunity orchestrates the mobilization and homing of hematopoietic stem/progenitor cells by engaging purinergic signaling-an update

Bone marrow (BM) as an active hematopoietic organ is highly sensitive to changes in body microenvironments and responds to external physical stimuli from the surrounding environment. In particular, BM tissue responds to several cues related to infections, strenuous exercise, tissue/organ damage, circadian rhythms, and physical challenges such as irradiation. These multiple stimuli affect BM cells to a large degree through a coordinated response of the innate immunity network as an important guardian for maintaining homeostasis of the body. In this review, we will foc++us on the role of purinergic signaling and innate immunity in the trafficking of hematopoietic stem/progenitor cells (HSPCs) during their egression from the BM into peripheral blood (PB), as seen along pharmacological mobilization, and in the process of homing and subsequent engraftment into BM after hematopoietic transplantation. Innate immunity mediates these processes by engaging, in addition to certain peptide-based factors, other important non-peptide mediators, including bioactive phosphosphingolipids and extracellular nucleotides, as the main topic of this review. Elucidation of these mechanisms will allow development of more efficient stem cell mobilization protocols to harvest the required number of HSPCs for transplantation and to accelerate hematopoietic reconstitution in transplanted patients.

Strategies for elevating hematopoietic stem cells expansion and engraftment capacity

Hematopoietic stem cells (HSCs) are a rare cell population in adult bone marrow, mobilized peripheral blood, and umbilical cord blood possessing self-renewal and differentiation capability into a full spectrum of blood cells. Bone marrow HSC transplantation has been considered as an ideal option for certain disorders treatment including hematologic diseases, leukemia, immunodeficiency, bone marrow failure syndrome, genetic defects such as thalassemia, sickle cell anemia, autoimmune disease, and certain solid cancers. Ex vivo proliferation of these cells prior to transplantation has been proposed as a potential solution against limited number of stem cells. In such culture process, MSCs have also been shown to exhibit high capacity for secretion of soluble mediators contributing to the principle biological and therapeutic activities of HSCs. In addition, endothelial cells have been introduced to bridge the blood and sub tissues in the bone marrow, as well as, HSCs regeneration induction and survival. Cell culture in the laboratory environment requires cell growth strict control to protect against contamination, symmetrical cell division and optimal conditions for maximum yield. In this regard, microfluidic systems provide culture and analysis capabilities in micro volume scales. Moreover, two-dimensional cultures cannot fully demonstrate extracellular matrix found in different tissues and organs as an abstract representation of three dimensional cell structure. Microfluidic systems can also strongly describe the effects of physical factors such as temperature and pressure on cell behavior.

Novel View on Umbilical Cord Blood and Maternal Peripheral Blood-an Evidence for an Increase in the Number of Circulating Stem Cells on Both Sides of the Fetal-Maternal Circulation Barrier

Umbilical cord blood (UCB) is a rich source of stem cells, including hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), endothelial progenitors cells (EPCs), and very small embryonic-like stem cells (VSELs). These cells most likely are mobilized into UCB in response to hypoxia and delivery stress. We have hypothesized that they may play a role in repairing certain tissue/organ injuries that occur in the newborn child after delivery. Here we asked whether delivery also mobilizes stem cells into maternal blood, as the mother also experiences hypoxia and several types of internal tissue injuries, particularly in the reproductive tract. We observed that the number of HSCs, MSCs, EPCs, and VSELs increases in maternal blood at 24 h after physiological delivery (n = 17). Based on this observation, we propose that delivery stress is associated with an increase in the number of circulating stem cells, not only on the fetal side but also on the maternal side of the fetal–maternal circulatory barrier.

Kidney Transplantation: The Challenge of Human Leukocyte Antigen and Its Therapeutic Strategies

Kidney transplantation remains the treatment of choice for end-stage renal failure. When the immune system of the recipient recognizes the transplanted kidney as a foreign object, graft rejection occurs. As part of the host immune defense mechanism, human leukocyte antigen (HLA) is a major challenge for graft rejection in transplantation therapy. The impact of HLA mismatches between the donor and the potential recipient prolongs the time for renal transplantation therapy, tethered to dialysis, latter reduces graft survival, and increases mortality. The formation of pretransplant alloantibodies against HLA class I and II molecules can be sensitized through exposures to blood transfusions, prior transplants, and pregnancy. These preformed HLA antibodies are associated with rejection in kidney transplantation. On the other hand, the development of de novo antibodies may increase the risk for acute and chronic rejections. Allograft rejection results from a complex interplay involving both the innate and the adaptive immune systems. Thus, further insights into the mechanisms of tissue rejection and the risk of HLA sensitization is crucial in developing new therapies that may blunt the immune system against transplanted organs. Therefore, the purpose of this review is to highlight facts about HLA and its sensitization, various mechanisms of allograft rejection, the current immunosuppressive approaches, and the directions for future therapy.

Innate Immunity and Mobilization of Hematopoietic Stem Cells

PURPOSE OF REVIEW

Several mechanisms have been postulated to orchestrate mobilization of hematopoietic stem/progenitor cells (HSPCs), and still more work is needed to better understand this process and to gain better mechanistic insight.

RECENT FINDINGS

Evidence accumulated that mobilization of HSPCs is a part of innate immunity response to tissue organ injury, stress, and infection. This evolutionary ancient process is orchestrated by granulocytes and monocytes that trigger activation of complement cascade and the coagulation cascade.

SUMMARY

We will present data from our laboratory that initiation of complement cascade activation and subsequently activation of the coagulation cascade during mobilization process are dependent on mannan-binding lectin (MBL). The mannan-binding pathway activates MBL-associated serine proteases (MASP-1 and MASP-2) that cleave the third complement component C3 and prothrombin. Cleavage of C3 leads to formation of classical C5 convertase and cleavage of prothrombin generates thrombin, which has "C5-like convertase" activity. Finally, both C5 convertase and thrombin cleave the fifth complement component C5, and activate distal part of the complement cascade that is crucial for egress of HSCPs from bone marrow niches into peripheral blood.

Activation of the complement cascade enhances motility of leukemic cells by downregulating expression of HO-1

As a crucial arm of innate immunity, the complement cascade (ComC) is involved both in mobilization of normal hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood and in their homing to BM. Despite the fact that ComC cleavage fragments alone do not chemoattract normal HSPCs, we found that leukemia cell lines as well as clonogenic blasts from chronic myeloid leukemia and acute myeloid leukemia patients respond robustly to C3 and C5 cleavage fragments by chemotaxis and increased adhesion. This finding was supported by the detection of C3a and C5a receptors in cells from human malignant hematopoietic cell lines and patient blasts at the mRNA (reverse transcriptase-polymerase chain reaction) and protein level (fluorescence-activated cell sorting), and by the demonstration that these receptors respond to stimulation by C3a and C5a by phosphorylation of p42/44 and p38 mitogen-activated protein kinases (MAPK), and protein kinase B (PKB/AKT). We also found that inducible heme oxygenase 1 (HO-1) is a negative regulator of ComC-mediated trafficking of leukemic cells, and that stimulation of leukemic cells by C3 or C5 cleavage fragments activates p38 MAPK, which downregulates HO-1 expression, rendering cells more mobile. We conclude that activation of the ComC in leukemia/lymphoma patients (for example, as a result of accompanying infections) enhances the motility of malignant cells and contributes to their spread in a p38 MAPK-HO-1-dependent manner. Therefore, inhibition of p38 MAPK or upregulation of HO-1 by small-molecule modulators would have a beneficial effect on ameliorating cell migration-mediated expansion of leukemia/lymphoma cells when the ComC becomes activated.

Identification of heme oxygenase 1 (HO-1) as a novel negative regulator of mobilization of hematopoietic stem/progenitor cells

Activation of complement cascade (ComC) play and important role in mobilization of hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB). While there are vast experimental data on the mechanisms and factors that induce or promote mobilization of HSPCs, there is relatively less data on negative regulators of this process. We demonstrate for the first time that heme oxygenase-1 (HO-1) that has a well-documented anti-inflammatory potential plays an important and heretofore unrecognized role in retention of HSPCs in BM niches by i) modulating negatively activation of mobilization promoting ComC, ii) maintaining stromal derived factor-1 (SDF-1) level in the BM microenvironment and iii) attenuating chemotactic responsiveness of HSPCs to SDF-1 and sphingosine-1 phosphate (S1P) gradients in PB. Furthermore, our data showing a positive mobilizing effect by a non-toxic small-molecule inhibitor of HO-1 (SnPP) suggest that blockade of HO-1 would be a promising strategy to facilitate mobilization of HSPCs. Further studies are also needed to evaluate better the molecular mechanisms responsible for the potential effect of HO-1 in homing of HSPCs after transplantation.

Complement activation in the context of stem cells and tissue repair

The complement pathway is best known for its role in immune surveillance and inflammation. However, its ability of opsonizing and removing not only pathogens, but also necrotic and apoptotic cells, is a phylogenetically ancient means of initiating tissue repair. The means and mechanisms of complement-mediated tissue repair are discussed in this review. There is increasing evidence that complement activation contributes to tissue repair at several levels. These range from the chemo-attraction of stem and progenitor cells to areas of complement activation, to increased survival of various cell types in the presence of split products of complement, and to the production of trophic factors by cells activated by the anaphylatoxins C3a and C5a. This repair aspect of complement biology has not found sufficient appreciation until recently. The following will examine this aspect of complement biology with an emphasis on the anaphylatoxins C3a and C5a.

Facilitated Neural Differentiation of Adipose Tissue-Derived Stem Cells by Electrical Stimulation and Nurr-1 Gene Transduction

Neuron-like cells derived from adipose tissue-derived stem cells (ADSCs) have been considered one of the most promising cells for the treatment of neurodegenerative diseases and neurotrauma in the central nervous system (CNS). Thus far, extensive efforts have been made to facilitate neuronal differentiation of ADSCs, but limited progress has been achieved. In the present study, we tested the possibility of using a combination of electrical stimulation (ES) with Nurr-1 gene transduction to promote neuronal differentiation of ADSCs. The tolerance of ADSCs to ES was first examined by a cell apoptosis assay. The proliferation of cells was characterized using a CCK-8 assay. The morphology of cells was examined by scanning electron microscopy (SEM). The differentiation of ADSCs into neuron-like cells was examined by immunocytochemistry (ICC)-immunofluorescence staining, quantitative real-time polymerase chain reaction (qRT-PCR), Western blotting, and enzyme linked immunosorbent assay (ELISA). The gene expression of microtubule-associated protein 2 (MAP-2), β-tubulin, neurofilament 200 (NF-200), octamer binding transcription factor 4 (OCT-4), and glial fibrillary acidic protein (GFAP) after stimulation was examined by qRT-PCR. We found that the optimal intensity of ES for neuronal differentiation of ADSCs was 1 V/cm. In addition, ES combined with Nurr-1 gene transduction increased the neuronal differentiation rate of ADSCs, the length of neurite-like processes, and the secretion of dopamine. Further studies showed that a combination of ES with Nurr-1 gene transduction was capable of promoting the expression of MAP-2, β-tubulin, and NF-200 but decreased the expression of OCT-4 and GFAP. All of these findings indicate that a combination of ES with Nurr-1 gene transduction could facilitate neuronal differentiation of ADSCs, which raises the possibility of its application in the treatment of neurodegenerative diseases and neurotrauma in the CNS.

Astaxanthin Improves Human Sperm Capacitation by Inducing Lyn Displacement and Activation

Astaxanthin (Asta), a photo-protective red pigment of the carotenoid family, is known for its multiple beneficial properties. In this study, the effects of Asta on isolated human sperm were evaluated. Capacitation involves a series of transformations to let sperm acquire the correct features for potential oocyte fertilization, including the generation of a controlled amount of reactive oxygen species (ROS), cholesterol depletion of the sperm outer membrane, and protein tyrosine phosphorylation (Tyr-P) process in the head region. Volunteers, with normal spermiogram values, were divided in two separate groups on the basis of their ability to generate the correct content of endogenous ROS. Both patient group (PG) and control group (CG) were analysed for Tyr-phosphorylation (Tyr-P) pattern and percentages of acrosome-reacted cells (ARC) and non-viable cells (NVC), in the presence or absence of Asta. In addition, the involvement of ROS on membrane reorganization and the presence of Lyn, a Src family kinase associated with lipid rafts, were investigated. Results show that Lyn is present in the membranes of human sperm, mainly confined in midpiece in resting conditions. Following capacitation, Lyn translocated to the head concomitantly with raft relocation, thus allowing the Tyr-P of head proteins. Asta succeeded to trigger Lyn translocation in PG sperm thus bypassing the impaired ROS-related mechanism for rafts and Lyn translocation. In this study, we showed an interdependence between ROS generation and lipid rafts and Lyn relocation leading the cells to undergo the successive acrosome reaction (AR). Asta, by ameliorating PG sperm functioning, may be utilised to decrease male idiopathic infertility.

Ex vivo expansion of hematopoietic stem cells

Ex vivo expansion of hematopoietic stem cells (HSCs) would benefit clinical applications in several aspects, to improve patient survival, utilize cord blood stem cells for adult applications, and selectively propagate stem cell populations after genetic manipulation. In this review we summarize and discuss recent advances in the culture systems of mouse and human HSCs, which include stroma/HSC co-culture, continuous perfusion and fed-batch cultures, and those supplemented with extrinsic ligands, membrane transportable transcription factors, complement components, protein modification enzymes, metabolites, or small molecule chemicals. Some of the expansion systems have been tested in clinical trials. The optimal condition for ex vivo expansion of the primitive and functional human HSCs is still under development. An improved understanding of the mechanisms for HSC cell fate determination and the HSC culture characteristics will guide development of new strategies to overcome difficulties. In the future, development of a combination treatment regimen with agents that enhance self-renewal, block differentiation, and improve homing will be critical. Methods to enhance yields and lower cost during collection and processing should be employed. The employment of an efficient system for ex vivo expansion of HSCs will facilitate the further development of novel strategies for cell and gene therapies including genome editing.

CD44, Hyaluronan, the Hematopoietic Stem Cell, and Leukemia-Initiating Cells

CD44 is an adhesion molecule that varies in size due to glycosylation and insertion of so-called variant exon products. The CD44 standard isoform (CD44s) is highly expressed in many cells and most abundantly in cells of the hematopoietic system, whereas expression of CD44 variant isoforms (CD44v) is more restricted. CD44s and CD44v are known as stem cell markers, first described for hematopoietic stem cells and later on confirmed for cancer- and leukemia-initiating cells. Importantly, both abundantly expressed CD44s as well as CD44v actively contribute to the maintenance of stem cell features, like generating and embedding in a niche, homing into the niche, maintenance of quiescence, and relative apoptosis resistance. This is surprising, as CD44 is not a master stem cell gene. I here will discuss that the functional contribution of CD44 relies on its particular communication skills with neighboring molecules, adjacent cells and, last not least, the surrounding matrix. In fact, it is the interaction of the hyaluronan receptor CD44 with its prime ligand, which strongly assists stem cells to fulfill their special and demanding tasks. Recent fundamental progress in support of this “old” hypothesis, which may soon pave the way for most promising new therapeutics, is presented for both hematopoietic stem cell and leukemia-initiating cell. The contribution of CD44 to the generation of a stem cell niche, to homing of stem cells in their niche, to stem cell quiescence and apoptosis resistance will be in focus.

Membrane lipid rafts, master regulators of hematopoietic stem cell retention in bone marrow and their trafficking

Cell outer membranes contain glycosphingolipids and protein receptors, which are integrated into glycoprotein microdomains, known as lipid rafts, which float freely in the membrane bilayer. These structures have an important role in assembling signaling molecules (e.g., Rac-1, RhoH and Lyn) together with surface receptors, such as the CXCR4 receptor for α-chemokine stromal-derived factor-1, the α4β1-integrin receptor (VLA-4) for vascular cell adhesion molecule-1 and the c-kit receptor for stem cell factor, which together regulate several aspects of hematopoietic stem/progenitor cell (HSPC) biology. Here, we discuss the role of lipid raft integrity in the retention and quiescence of normal HSPCs in bone marrow niches as well as in regulating HSPC mobilization and homing. We will also discuss the pathological consequences of the defect in lipid raft integrity seen in paroxysmal nocturnal hemoglobinuria and the emerging evidence for the involvement of lipid rafts in hematological malignancies.

Expansion and homing of umbilical cord blood hematopoietic stem and progenitor cells for clinical transplantation

The successful expansion of hematopoietic stem and progenitor cells (HSPCs) from umbilical cord blood (UCB) for transplantation could revolutionize clinical practice by improving transplantation-related outcomes and making available UCB units that have suboptimal cell doses for transplantation. New cytokine combinations appear able to promote HSPC growth with minimal differentiation into mature precursors and new agents, such as insulin-like growth factor-binding protein 2, are being used in clinical trials. Molecules that simulate the HSPC niche, such as Notch ligand, have also shown promise. Further improvements have been made with the use of mesenchymal stromal cells, which have made possible UCB expansion without a potentially deleterious prior CD34/CD133 cell selection step. Chemical molecules, such as copper chelators, nicotinamide, and aryl hydrocarbon antagonists, have shown excellent outcomes in clinical studies. The use of bioreactors could further add to HSPC studies in future. Drugs that could improve HSPC homing also appear to have potential in improving engraftment times in UCB transplantation. Technologies to expand HSPC from UCB and to enhance the homing of these cells appear to have attained the goal of accelerating hematopoietic recovery. Further discoveries and clinical studies are likely to make the goal of true HSPC expansion a reality for many applications in future.

Mixed chimerism and transplant tolerance are not effectively induced in C3a-deficient mice

Mixed chimerism, a phenomenon involved in the development of specific alloantigen tolerance, could be achieved through the transplantation of hematopoietic stem cells into properly prepared recipients. Because the C3a complement component modulates hematopoietic cell trafficking after transplantation, in the present study, we investigated the influence of the C3a deficiency on mixed chimerism and alloantigen tolerance induction. To induce mixed chimerism, C57BL/6J (wild-type strain; H-2K(b); I-E(-)) and B6.129S4-C3(tm1Crr)/J (C3a-deficient) mice were exposed to 3 G total body irradiation (day -1). Subsequently, these mice were treated with CD8-blocking (day -2) and CD40L-blocking (days 0 and 4) antibodies, followed by transplantation with 20 × 10(6) Balb/c (H-2K(d); I-E(+)) bone marrow cells (day 0). The degree of mixed chimerism in peripheral blood leukocytes was measured several times during the 20-week experiment. The tolerance to Balb/c mouse antigens was assessed based on the number of lymphocytes expressing Vβ5 and Vβ11 T-cell receptor and on skin-graft (day 0) acceptance. Applying our experimental model, mixed chimerism and alloantigen tolerance were effectively induced in C57BL/6J (wild-type) mice, but not in C3a(-/-) animals. The present study is, to our knowledge, the first to demonstrate that C3a is vital for achieving stable mixed chimerism and related to this induction of transplant tolerance.

Complement component 3 is necessary to preserve myocardium and myocardial function in chronic myocardial infarction

Activation of the complement cascade (CC) with myocardial infarction (MI) acutely initiates immune cell infiltration, membrane attack complex formation on injured myocytes, and exacerbates myocardial injury. Recent studies implicate the CC in mobilization of stem/progenitor cells and tissue regeneration. Its role in chronic MI is unknown. Here, we consider complement component C3, in the chronic response to MI. C3 knockout (KO) mice were studied after permanent coronary artery ligation. C3 deficiency exacerbated myocardial dysfunction 28 days after MI compared to WT with further impaired systolic function and LV dilation despite similar infarct size 24 hours post-MI. Morphometric analysis 28 days post-MI showed C3 KO mice had more scar tissue with less viable myocardium within the infarct zone which correlated with decreased c-kit(pos) cardiac stem/progenitor cells (CPSC), decreased proliferating Ki67(pos) CSPCs and decreased formation of new BrdU(pos) /α-sarcomeric actin(pos) myocytes, and increased apoptosis compared to WT. Decreased CSPCs and increased apoptosis were evident 7 days post-MI in C3 KO hearts. The inflammatory response with MI was attenuated in the C3 KO and was accompanied by attenuated hematopoietic, pluripotent, and cardiac stem/progenitor cell mobilization into the peripheral blood 72 hours post-MI. These results are the first to demonstrate that CC, through C3, contributes to myocardial preservation and regeneration in response to chronic MI. Responses in the C3 KO infer that C3 activation in response to MI expands the resident CSPC population, increases new myocyte formation, increases and preserves myocardium, inflammatory response, and bone marrow stem/progenitor cell mobilization to preserve myocardial function.

Biomechanical force in blood development: extrinsic physical cues drive pro-hematopoietic signaling

The hematopoietic system is dynamic during development and in adulthood, undergoing countless spatial and temporal transitions during the course of one's life. Microenvironmental cues in the many unique hematopoietic niches differ, characterized by distinct soluble molecules, membrane-bound factors, and biophysical features that meet the changing needs of the blood system. Research from the last decade has revealed the importance of substrate elasticity and biomechanical force in determination of stem cell fate. Our understanding of the role of these factors in hematopoiesis is still relatively poor; however, the developmental origin of blood cells from the endothelium provides a model for comparison. Many endothelial mechanical sensors and second messenger systems may also determine hematopoietic stem cell fate, self renewal, and homing behaviors. Further, the intimate contact of hematopoietic cells with mechanosensitive cell types, including osteoblasts, endothelial cells, mesenchymal stem cells, and pericytes, places them in close proximity to paracrine signaling downstream of mechanical signals. The objective of this review is to present an overview of the sensors and intracellular signaling pathways activated by mechanical cues and highlight the role of mechanotransductive pathways in hematopoiesis.

Bioactive lipids and cationic antimicrobial peptides as new potential regulators for trafficking of bone marrow-derived stem cells in patients with acute myocardial infarction

Acute myocardial infarction (AMI) triggers mobilization of stem cells from bone marrow (BM) into peripheral blood (PB). Based on our observation that the bioactive sphingophospholipids, sphingosine-1 phosphate (S1P), and ceramide-1 phosphate (C1P) regulate trafficking of hematopoietic stem cells (HSCs), we explored whether they also direct trafficking of non-hematopoietic stem cells (non-HSCs). We detected a 3-6-fold increase in circulating CD34+, CD133+, and CXCR4+ lineage-negative (Lin-)/CD45- cells that are enriched in non-HSCs [including endothelial progenitors (EPCs) and very small embryonic-like stem cells (VSELs)] in PB from AMI patients (P<0.05 vs. controls). Concurrently, we measured a ∼3-fold increase in S1P and C1P levels in plasma from AMI patients. At the same time, plasma obtained at hospital admission and 6 h after AMI strongly chemoattracted human BM-derived CD34+/Lin- and CXCR4+/Lin- cells in Transwell chemotaxis assays. This effect of plasma was blunted after depletion of S1P level by charcoal stripping and was further inhibited by the specific S1P1 receptor antagonist such as W146 and VPC23019. We also noted that the expression of S1P receptor 1 (S1P1), which is dominant in naïve BM, is reduced after the exposure to S1P at concentrations similar to the plasma S1P levels in patients with AMI, thus influencing the role of S1P in homing to the injured myocardium. Therefore, we examined mechanisms, other than bioactive lipids, that may contribute to the homing of BM non-HSCs to the infarcted myocardium. Hypoxic cardiac tissue increases the expression of cathelicidin and β-2 defensin, which could explain why PB cells isolated from patients with AMI migrated more efficiently to a low, yet physiological, gradient of stromal-derived factor-1 in Transwell migration assays. Together, these observations suggest that while elevated S1P and C1P levels early in the course of AMI may trigger mobilization of non-HSCs into PB, cathelicidin and β-2 defensin could play an important role in their homing to damaged myocardium.

Complement-triggered pathways orchestrate regenerative responses throughout phylogenesis

Adult tissue plasticity, cell reprogramming, and organ regeneration are major challenges in the field of modern regenerative medicine. Devising strategies to increase the regenerative capacity of tissues holds great promise for dealing with donor organ shortages and low transplantation outcomes and also provides essential impetus to tissue bioengineering approaches for organ repair and replacement. The inherent ability of cells to reprogram their fate by switching into an embryonic-like, pluripotent progenitor state is an evolutionary vestige that in mammals has been retained mostly in fetal tissues and persists only in a few organs of the adult body. Tissue regeneration reflects the capacity of terminally differentiated cells to re-enter the cell cycle and proliferate in response to acute injury or environmental stress signals. In lower vertebrates, this regenerative capacity extends to several organs and remarkably culminates in precise tissue patterning, through cellular transdifferentiation and complex morphogenetic processes that can faithfully reconstruct entire body parts. Many lessons have been learned from robust regeneration models in amphibians such as the newt and axolotl. However, the dynamic interactions between the regenerating tissue, the surrounding stroma, and the host immune response, as it adapts to the actively proliferating tissue, remain ill-defined. The regenerating zone, through a sequence of distinct molecular events, adopts phenotypic plasticity and undergoes rigorous tissue remodeling that, in turn, evokes a significant inflammatory response. Complement is a primordial sentinel of the innate immune response that engages in multiple inflammatory cascades as it becomes activated during tissue injury and remodeling. In this respect, complement proteins have been implicated in tissue and organ regeneration in both urodeles and mammals. Distinct complement-triggered pathways have been shown to modulate critical responses that promote tissue reprogramming, pattern formation, and regeneration across phylogenesis. This article will discuss the mechanistic insights underlying the crosstalk of complement with cytokine and growth factor signaling pathways that drive tissue regeneration and will provide a unified conceptual framework for considering complement modulation as a novel target for regenerative therapeutics.

Innate immunity derived factors as external modulators of the CXCL12-CXCR4 axis and their role in stem cell homing and mobilization

The α-chemokine CXCL12 (stromal derived factor-1; SDF-1) and its corresponding G_αI protein-coupled CXCR4 receptor axis play an important role in retention of hematopoietic stem progenitor cells (HSPCs) in bone marrow (BM) stem cell niches. CXCL12 has also been identified as a strong chemoattractant for HSPCs and implicated both in homing of HSPCs to BM after transplantation and in egress of these cells from BM into peripheral blood (PB). However, since CXCL12, as a peptide, is highly susceptible to degradation by proteolytic enzymes, its real biological availability in biological fluids may be somewhat limited. In this review, we will present data demonstrating that the CXCL12-CXCR4 axis is positively modulated by innate immunity-derived several external factors, ensuring that even low (near threshold) doses of CXCL12 still exert a robust chemotactic influence on HSPCs.

Innate immunity as orchestrator of bone marrow homing for hematopoietic stem/progenitor cells

The first step that precedes hematopoietic transplantation is elimination of pathological hematopoiesis by administration of myeloablative doses of radiochemotherapy. This eliminates hematolymphopoietic cells and at the same time damages hematopoietic microenvironment in bone marrow (BM). The damage of BM tissue leads to activation of complement cascade (CC), and bioactive CC cleavage fragments modulate several steps of BM recovery after transplantation of hematopoietic stem progenitor cells (HSPCs). Accordingly, C3 cleavage fragments (soluble C3a/desArgC3a and solid phase iC3b) and generation of soluble form of C5b-C9 also known as membrane attack complex (MAC) as well as release of antimicrobial cationic peptides from stromal cells (cathelicidin or LL-37 and beta-2 defensin) promote homing of HSPCs. To support this, C3 cleavage fragments and antimicrobial cationic peptides increase homing responsiveness of transplanted HSPCs to stroma-derived factor-1 (SDF-1) gradient. Furthermore, damaged BM cells release several other chemoattractants for HSPCs such as bioactive lipids sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P) and chemotactic purines (ATP and UTP). In this chapter, we will discuss the current view on homing of transplanted HSPCs into BM that in addition to SDF-1 is orchestrated by CC, antimicrobial cationic peptides, and several other prohoming factors. We also propose modulation of CC as a novel strategy to optimize/accelerate homing of HSPCs.

An emerging link in stem cell mobilization between activation of the complement cascade and the chemotactic gradient of sphingosine-1-phosphate

Under steady-state conditions, hematopoietic stem/progenitor cells (HSPCs) egress from bone marrow (BM) and enter peripheral blood (PB) where they circulate at low levels. Their number in PB, however, increases significantly in several stress situations related to infection, organ/tissue damage, or strenuous exercise. Pharmacologically mediated enforced egress of HSPCs from the BM microenvironment into PB is called "mobilization", and this phenomenon has been exploited in hematological transplantology as a means to obtain HSPCs for hematopoietic reconstitution. In this review we will present the accumulated evidence that innate immunity, including the complement cascade and the granulocyte/monocyte lineage, and the PB plasma level of the bioactive lipid sphingosine-1-phosphate (S1P) together orchestrate this evolutionarily conserved mechanism that directs trafficking of HSPCs.

Umbilical cord blood graft enhancement strategies: has the time come to move these into the clinic?

Umbilical cord blood (UCB) is an attractive stem cell graft option for patients who need allogeneic hematopoietic stem cell support, but lack a suitable HLA-matched donor. However, the limited number of hematopoietic progenitor cells in a single cord blood unit can lead to an increased risk of graft failure, delayed hematological recovery and prolonged immunosuppression, particularly in adult patients. Several strategies to overcome these potential limitations are being evaluated. In this review, we discuss promising ex vivo manipulations to enhance cord blood engraftment capacity such as culture of UCB cells with stimulatory cytokines and growth factors, mesenchymal cells, Notch ligand, copper chelators, prostaglandins, complement components, nicotinamide and CD26/DPPIV inhibitors. All these approaches are now in early clinical trials. However, despite the fact that several cord blood enhancement strategies have resulted in increased numbers of progenitor cells and faster neutrophil recovery, the ability of these techniques to significantly shorten engraftment time and permit the use of cord units with low numbers of total nucleated cells, or accomplish reliable engraftment with a single cord, have yet to be convincingly demonstrated. The ultimate clinical value of ex vivo cord blood expansion or manipulation has not been defined yet, and the current data do not permit predicting which technology will prove to be the optimal strategy. Nevertheless, expectations remain high that eventually ex vivo enhancement will be able to improve clinical outcomes and significantly extend the applicability of UCB transplantation.

The expanding family of bone marrow homing factors for hematopoietic stem cells: stromal derived factor 1 is not the only player in the game

The α-chemokine stromal derived factor 1 (SDF-1), which binds to the CXCR4 and CXCR7 receptors, directs migration and homing of CXCR4⁺ hematopoietic stem/progenitor cells (HSPCs) to bone marrow (BM) and plays a crucial role in retention of these cells in stem cell niches. However, this unique role of SDF-1 has been recently challenged by several observations supporting SDF-1-CXCR4-independent BM homing. Specifically, it has been demonstrated that HSPCs respond robustly to some bioactive lipids, such as sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P), and migrate in response to gradients of certain extracellular nucleotides, including uridine triphosphate (UTP) and adenosine triphosphate (ATP). Moreover, the responsiveness of HSPCs to an SDF-1 gradient is enhanced by some elements of innate immunity (e.g., C3 complement cascade cleavage fragments and antimicrobial cationic peptides, such as cathelicidin/LL-37 or β2-defensin) as well as prostaglandin E2 (PGE2). Since all these factors are upregulated in BM after myeloblative conditioning for transplantation, a more complex picture of homing emerges that involves several factors supporting, and in some situations even replacing, the SDF-1-CXCR4 axis.

Stromal-Cell-Derived Factor-1 (SDF-1)/CXCL12 as Potential Target of Therapeutic Angiogenesis in Critical Leg Ischaemia

In the Western world, peripheral vascular disease (PVD) has a high prevalence with high morbidity and mortality. In a large percentage of these patients, lower limb amputation is still required. Studies of ischaemic skeletal muscle disclosed evidence of endogenous angiogenesis and adaptive skeletal muscle metabolic changes in response to hypoxia. Chemokines are potent chemoattractant cytokines that regulate leukocyte trafficking in homeostatic and inflammatory processes. More than 50 different chemokines and 20 different chemokine receptors have been cloned. The chemokine stromal-cell-derived factor-1 (SDF-1 aka CXCL12) is a constitutively expressed and inducible chemokine that regulates multiple physiological processes, including embryonic development and organ homeostasis. The biologic effects of SDF-1 are mediated by chemokine receptor CXCR4, a 352 amino acid rhodopsin-like transmembrane-specific G protein-coupled receptor (GPCR). There is evidence that the administration of SDF-1 increases blood flow and perfusion via recruitment of endothelial progenitor cells (EPCs). This review will focus on the role of the SDF-1/CXCR4 system in the pathophysiology of PVD and discuss their potential as therapeutic targets for PVD.

The role of innate immunity in trafficking of hematopoietic stem cells-an emerging link between activation of complement cascade and chemotactic gradients of bioactive sphingolipids

Hematopoietic stem and progenitor cells (HSPCs) circulate under steady-state conditions at detectable levels in peripheral blood (PB). The phenomenon of enforced release of HSPCs from BM into PB is called mobilization and may be envisioned as a danger-sensing response mechanism triggered by hypoxia or mechanical- or infection-induced tissue damage and is a part of stress response. It is unquestionable that the a-chemokine stromal derived factor-1 (SDF-1)-CXCR4 axis plays crucial role in retention of HSPCs in BM. However, all factors that direct mobilization of HSPCs into PB and homing back to the BM or their allocation to damaged organs are not characterized very well. In this chapter we will present mounting evidence that elements of innate immunity such as complement cascade (CC) cleavage fragments (e.g., C3a and C5a), granulocytes, generation of membrane attack complex (MAC) together with sphingosine-1 phosphate (S1P) orchestrate HSPC mobilization. On other hand some other bioactive lipids e.g., ceramide-1-phosphate (C1P) that is released from damaged/"leaky" cells in BM after myeloablative conditioning for transplant may play an opposite important role in homing of HSPCs to BM. Finally, the chemotactic activity of all chemoattractants for HSPCs including SDF-1, S1P and C1P is enhanced in presence of CC cleavage fragments (e.g., C3a) and MAC that is a final product of CC activation.

Cord blood stem cells for hematopoietic transplantation

Cord blood (CB) is an important alternative source of hematopoietic stem cells (HSCs) for transplantation today. The principal drawbacks of cord blood transplantation are the limited number of hematopoietic stem cells and a long time to engraftment. Several promising approaches for engraftment enhancement are under intensive investigation. Such are transplantation with two cord blood units, co transplantation of cord blood and haploidentical HSCs and different methods for expansion of cord blood hematopoietic stem cells. In addition there are several ways for improving of homing of HSCs such as co- infusion of CB hematopoietic stem cells and mesenchymal stem cells, administration of parathyroid hormone (PTH), intra- bone transplantation and targeting the CXCR4/SDF1 system. These strategies are expected to increase the availability of transplantation to adults, for whom the chance to find a cord blood suitable for a single unit transplant is small. Recent advances in elucidation of the molecular mechanisms responsible for the proliferation and self-renewal of hematopoietic stem cells may bring further improvement of the outcomes of cord blood transplantation. This review summarizes the recent progress in the field of cord blood derived hematopoietic stem cells. It presents the strategies applied and points out directions for the future.

The bone marrow-expressed antimicrobial cationic peptide LL-37 enhances the responsiveness of hematopoietic stem progenitor cells to an SDF-1 gradient and accelerates their engraftment after transplantation

We report that the bone marrow (BM) stroma-released LL-37, a member of the cathelicidin family of antimicrobial peptides, primes/increases the responsiveness of murine and human hematopoietic stem/progenitor cells (HSPCs) to an α-chemokine stromal-derived factor-1 (SDF-1) gradient. Accordingly, LL-37 is upregulated in irradiated BM cells and enhances the chemotactic responsiveness of hematopoietic progenitors from all lineages to a low physiological SDF-1 gradient as well as increasing their (i) adhesiveness, (ii) SDF-1-mediated actin polymerization and (iii) MAPK(p42/44) phosphorylation. Mice transplanted with BM cells ex vivo primed by LL-37 showed accelerated recovery of platelet and neutrophil counts by ∼3-5 days compared with mice transplanted with unprimed control cells. These priming effects were not mediated by LL-37 binding to its receptor and depended instead on the incorporation of the CXCR4 receptor into membrane lipid rafts. We propose that LL-37, which has primarily antimicrobial functions and is harmless to mammalian cells, could be clinically applied to accelerate engraftment as an ex vivo priming agent for transplanted human HSPCs. This novel approach would be particularly important in cord blood transplantations, where the number of HSCs available is usually limited.

A novel perspective on stem cell homing and mobilization: review on bioactive lipids as potent chemoattractants and cationic peptides as underappreciated modulators of responsiveness to SDF-1 gradients

Hematopoietic stem progenitor cells (HSPCs) respond robustly to α-chemokine stromal-derived factor-1 (SDF-1) gradients, and blockage of CXCR4, a seven-transmembrane-spanning G(αI)-protein-coupled SDF-1 receptor, mobilizes HSPCs into peripheral blood. Although the SDF-1-CXCR4 axis has an unquestionably important role in the retention of HSPCs in bone marrow (BM), new evidence shows that, in addition to SDF-1, the migration of HSPCs is directed by gradients of the bioactive lipids sphingosine-1 phosphate and ceramide-1 phosphate. Furthermore, the SDF-1 gradient may be positively primed/modulated by cationic peptides (C3a anaphylatoxin and cathelicidin) and, as previously demonstrated, HSPCs respond robustly even to very low SDF-1 gradients in the presence of priming factors. In this review, we discuss the role of bioactive lipids in stem cell trafficking and the consequences of HSPC priming by cationic peptides. Together, these phenomena support a picture in which the SDF-1-CXCR4 axis modulates homing, BM retention and mobilization of HSPCs in a more complex way than previously envisioned.

Conditioning for hematopoietic transplantation activates the complement cascade and induces a proteolytic environment in bone marrow: a novel role for bioactive lipids and soluble C5b-C9 as homing factors

We have observed that conditioning for hematopoietic transplantation by lethal irradiation induces a proteolytic microenvironment in the bone marrow (BM) that activates the complement cascade (CC). As a result, BM is enriched for proteolytic enzymes and the soluble form of the terminal product of CC activation, the membrane attack complex C5b-C9 (MAC). At the same time, proteolytic enzymes induced in irradiated BM impair the chemotactic activity of α-chemokine stromal-derived factor-1 (SDF-1). As SDF-1 is considered a crucial BM chemoattractant for transplanted hematopoietic stem/progenitor cells (HSPCs), we sought to determine whether other factors that are resistant to proteolytic enzymes have a role in this process, focusing on proteolysis-resistant bioactive lipids. We found that the concentrations of sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P) increase in the BM after conditioning for transplantation and that both S1P and, as we show here for the first time, C1P are potent chemoattractants for HSPCs. Next, we observed that C5-deficient mice that do not generate MAC show impaired engraftment of HSPCs. In support of a role for MAC in homing and engraftment, we found that soluble MAC enhances in a CR3 (CD11b/CD18)-dependent manner the adhesion of HSPCs to BM stromal cells and increases the secretion of SDF-1 by BM stroma. We conclude that an increase in BM levels of proteolytic enzyme-resistant S1P and C1P and activation of CC, which leads to the generation of MAC, has an important and previously underappreciated role in the homing of transplanted HSPCs.

Granulocyte-derived cationic Peptide enhances homing and engraftment of bone marrow stem cells after transplantation

Current strategies to accelerate hematopoietic reconstitution after transplantation include transplantation of greater numbers of hematopoietic stem/progenitor cells (HSPCs) or ex vivo expansion of harvested HSPCs before transplant. However, the number of cells available for transplantation is usually low, and strategies to expand HSPCs and maintain equivalent engraftment capability ex vivo are limited. We noted that activated granulocyte-derived cationic peptides positively primed responsiveness of HSPCs to a CXCL12 gradient. Accordingly, we noted that accelerated homing/engraftment of β-defensin-2, a well-known antimicrobial cationic peptide, primed bone marrow nucleated cells (BMNCs) compared to normal BMNCs after transplantation into lethally irradiated recipients. We envision that small cationic peptides, which primarily possess antimicrobial functions and are harmless to mammalian cells, could be applied to prime HSPCs before transplantation. This novel approach would be particularly important in cord blood transplantation, where the number of HSPCs available for transplantation is usually limited.

Umbilical cord blood: current status and future directions

Once considered biological waste, umbilical cord blood (UCB) has become an accepted source of haematopoietic stem cells (HSCs). With initial success in the pediatric setting, UCB transplantation continues to gain favor in the adult patient population. Novel approaches to UCB transplantation include use of two units and a variety of graft manipulations. Additional uses for UCB are currently being explored and include applications in regenerative medicine and immunotherapy.

Strategies to enhance umbilical cord blood stem cell engraftment in adult patients

Umbilical cord blood (UCB) has been used successfully as a source of hematopoietic stem cells (HSCs) for allogeneic transplantation in children and adults in the treatment of hematologic diseases. However, compared with marrow or mobilized peripheral blood stem cell grafts from adult donors, significant delays in the rates and kinetics of neutrophil and platelet engraftment are noted after UCB transplant. These differences relate in part to the reduced numbers of HSCs in UCB grafts. To improve the rates and kinetics of engraftment of UCB HSC, several strategies have been proposed, including ex vivo expansion of UCB HSCs, addition of third-party mesenchymal cells, intrabone delivery of HSCs, modulation of CD26 expression, and infusion of two UCB grafts. This article will focus on ex vivo expansion of UCB HSCs and strategies to enhance UCB homing as potential solutions to overcome the problem of low stem cell numbers in a UCB graft.

Bioactive Sphingolipids and Complement Cascade as New Emerging Regulators of Stem Cell Mobilization and Homing

The α-chemokine stromal derived factor-1 (SDF-1) - seven transmembrane span receptor CXCR4 axis plays a crucial role in retention of hematopoietic stem progenitor cells (HSPCs) in BM. However, the mechanisms that govern mobilization/release of HSPCs from bone marrow (BM) into peripheral blood (PB) and direct a reverse process of their homing back into BM microenvironment after transplantation are still poorly understood. Augmenting evidence demonstrates that during both mobilization and myeloablative conditioning for transplantation a proteolytic microenvironment is induced in BM and complement cascade (CC) becomes activated. In this review we will present augmenting evidence that as result of induction of proteolytis microenvironment as well as CC activation bioactive sphingolipids - sphingosine - 1 phosphate (S1P) and ceramide-1-phosphate (C1P) together with CC cleavage fragments (C3a, C5a and C5b-C9) orchestrate both homing and mobilization of HSCPs.

Complement C1q enhances homing-related responses of hematopoietic stem/progenitor cells

BACKGROUND

Previously, we reported that the complement cleavage fragments C3a and C5a are important modulators of trafficking of hematopoietic stem/progenitor cells (HSPCs). The aim of this study was to examine a possible role for complement component 1, subcomponent q (C1q) in HSPC migration.

STUDY DESIGN AND METHODS

CD34+ HSPCs isolated from cord blood (CB), bone marrow (BM), and granulocyte-colony-stimulating factor (G-CSF)-mobilized peripheral blood (mPB) were evaluated for the expression of C1q and its receptor for phagocytosis (C1qRp) using reverse transcription-polymerase chain reaction, Western blotting, and fluorescence-activated cell sorting. Chemotactic responses and chemoinvasiveness toward stromal cell-derived factor (SDF)-1 and expression of matrix metalloproteinase (MMP)-9 were also examined after C1q stimulation. Moreover, G-CSF- and zymosan-induced mobilization was evaluated in C1q-deficient mice.

RESULTS

C1q was expressed in CD34+ cells from mPB, but not from CB or steady-state BM; however, stimulation of the latter with G-CSF induced C1q expression. C1qRp receptor was found on BM, CB, and mPB CD34+ cells and more mature ex vivo expanded myeloid and megakaryocytic precursors. Although C1q itself was not a chemoattractant for HSPCs, it primed/enhanced the chemotactic response of CD34+ cells to a low SDF-1 gradient and their chemoinvasion across the reconstituted basement membrane Matrigel and increased secretion of MMP-9 by these cells. Moreover, in in vivo studies C1q-deficient mice were found to be easy G-CSF mobilizers compared to wild-type mice and normal zymosan mobilizers.

CONCLUSION

We demonstrated that C1q primes the responses of CD34+ HSPCs to an SDF-1 gradient, which may enhance their ability to stay within BM niches, suggesting that the C1q/C1qRp axis contributes to HSPC homing/retention in BM.

Innate immunity as orchestrator of stem cell mobilization

Hematopoietic stem and progenitor cells (HSPCs), as well as other types of stem cells, circulate under steady-state conditions at detectable levels in peripheral blood (PB), with their numbers increasing in response to stress, inflammation and tissue/organ injury. This mobilization process may be envisioned as a danger-sensing response mechanism triggered by hypoxia or mechanical or infection-induced tissue damage that recruits into PB different types of stem cells that have a role in immune surveillance and organ/tissue regeneration. Mobilization is also significantly enhanced by the administration of pharmacological agents, which has been exploited in hematological transplantology as a means to obtain HSPCs for hematopoietic reconstitution. In this review we will present mounting evidence that innate immunity orchestrates this evolutionarily conserved mechanism of HSPC mobilization.

A novel composition for the culture of human adipose stem cells which includes complement C3

Adipose tissue is an easily accessible and abundant source of stem cells. Adipose stem cells (ASCs) are currently being researched as treatment options for repair and regeneration of damaged tissues. The standard culture conditions used for expansion of ASCs contain fetal bovine serum (FBS) which is undefined, could transmit known and unknown adventitious agents, and may cause adverse immune reactions. We have described a novel culture condition which excludes the use of FBS and characterised the resulting culture. Human ASCs were cultured in the novel culture medium, which included complement protein C3. These cultures, called C-ASCs, were compared with ASCs cultured in medium supplemented with FBS. Analysis of ASCs for surface marker profile, proliferation characteristics and differentiation potential indicated that the C-ASCs were similar to ASCs cultured in medium containing FBS. Using a specific inhibitor, we show that C3 is required for the survival of C-ASCs. This novel composition lends itself to being developed into a defined condition for the routine culture of ASCs for basic and clinical applications.

The complement cascade as a mediator of tissue growth and regeneration

Recent evidence has demonstrated that the complement cascade is involved in a variety of physiologic and pathophysiologic processes in addition to its role as an immune effector. Research in a variety of organ systems has shown that complement proteins are direct participants in maintenance of cellular turnover, healing, proliferation and regeneration. As a physiologic housekeeper, complement proteins maintain tissue integrity in the absence of inflammation by disposing of cellular debris and waste, a process critical to the prevention of autoimmune disease. Developmentally, complement proteins influence pathways including hematopoietic stem cell engraftment, bone growth, and angiogenesis. They also provide a potent stimulus for cellular proliferation including regeneration of the limb and eye in animal models, and liver proliferation following injury. Here, we describe the complement cascade as a mediator of tissue growth and regeneration.

Complement and the central nervous system: emerging roles in development, protection and regeneration

As expanding research reveals the novel ability of complement proteins to promote proliferation and regeneration of tissues throughout the body, the concept of the complement cascade as an innate immune effector has changed rapidly. In particular, its interactions with the central nervous system have provided a wealth of information regarding the ability of complement proteins to mediate neurogenesis, synaptogenesis, cell migration, neuroprotection, proliferation and regeneration. At numerous phases of the neuronal and glial cell cycle, complement proteins exert direct or indirect influence over their behavior and fate. Neuronal stem cells differentiate and migrate in response to complement, and it prevents injury and death in adult cells in response to toxic agents. Furthermore, complement proteins promote survival via anti-apoptotic actions, and can facilitate clearance and regeneration of injured tissues in various models of CNS disease. In summary, we highlight the protean abilities of complement proteins in the central nervous system, underscoring an exciting avenue of research that has yielded greater understanding of complement's role in central nervous system health and disease.