George Gabriel Stokes as a biologist

It is universally acknowledged that George Gabriel Stokes (1819-1903) was a polymath who made major contributions to the fields of mathematics, chemistry, physics, fluidics and optics. However, his contributions to biology have received far less attention and this brief communication examines two of Stokes' major biological contributions, namely his description of the phenomenon of fluorescence and his studies on the changes in the colour of blood following oxidation and reduction. The paper on fluorescence is discussed because in it, Stokes demonstrates his wide-ranging biological knowledge and because the use of fluorescence is an invaluable experimental tool in biology. It was by developing the experimental approaches and equipment used to investigate fluorescence that Stokes then applied these to other investigations, including that of blood. From what we now know, what Stokes was describing in his paper on blood were the changes in the configuration of the haemoglobin molecule upon the acquisition and release of oxygen. This article is part of the theme issue 'Stokes at 200 (part 2)'.

Modeling the Hematopoietic Landscape

Some time ago, we proposed a continuum-like view of the lineages open to hematopoietic stem cells (HSCs); each HSC self-renews or chooses from the spectrum of all end-cell options and can then "merely" differentiate. Having selected a cell lineage, an individual HSC may still "step sideways" to an alternative, albeit closely related, fate: HSC and their progeny therefore remain versatile. The hematopoietic cytokines erythropoietin, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, granulocyte/macrophage colony-stimulating factor and ligand for the fms-like tyrosine kinase 3 instruct cell lineage. Sub-populations of HSCs express each of the cytokine receptors that are positively auto-regulated upon cytokine binding. Many years ago, Waddington proposed that the epigenetic landscape played an important role in cell lineage choice. This landscape is dynamic and unstable especially regarding DNA methylation patterns across genomic DNA. This may underlie the receptor diversity of HSC and their decision-making.

Accumulation of Multipotent Hematopoietic Progenitors in Peripheral Lymphoid Organs of Mice Over-expressing Interleukin-7 and Flt3-Ligand

Interleukin-7 (IL-7) and Flt3-ligand (FL) are two cytokines important for the generation of B cells, as manifested by the impaired B cell development in mice deficient for either cytokine or their respective receptors and by the complete block in B cell differentiation in the absence of both cytokines. IL-7 is an important survival and proliferation factor for B cell progenitors, whereas FL acts on several early developmental stages, prior to B cell commitment. We have generated mice constitutively over-expressing both IL-7 and FL. These double transgenic mice develop splenomegaly and lymphadenopathy characterized by tremendously enlarged lymph nodes even in young animals. Lymphoid, myeloid and dendritic cell numbers are increased compared to mice over-expressing either of the two cytokines alone and the effect on their expansion is synergistic, rather than additive. B cell progenitors, early progenitors with myeloid and lymphoid potential (EPLM), common lymphoid progenitors (CLP) and lineage⁻, Sca1⁺, kit⁺ (LSK) cells are all increased not only in the bone marrow but also in peripheral blood, spleen and even lymph nodes. When transplanted into irradiated wild-type mice, lymph node cells show long-term multilineage reconstitution, further confirming the presence of functional hematopoietic progenitors therein. Our double transgenic mouse model shows that sustained and combined over-expression of IL-7 and FL leads to a massive expansion of most bone marrow hematopoietic progenitors and to their associated presence in peripheral lymphoid organs where they reside and potentially differentiate further, thus leading to the synergistic increase in mature lymphoid and myeloid cell numbers. The present study provides further in vivo evidence for the concerted action of IL-7 and FL on lymphopoiesis and suggests that extramedullary niches, including those in lymph nodes, can support the survival and maintenance of hematopoietic progenitors that under physiological conditions develop exclusively in the bone marrow.

DN2 Thymocytes Activate a Specific Robust DNA Damage Response to Ionizing Radiation-Induced DNA Double-Strand Breaks

For successful bone marrow transplantation (BMT), a preconditioning regime involving chemo and radiotherapy is used that results in DNA damage to both hematopoietic and stromal elements. Following radiation exposure, it is well recognized that a single wave of host-derived thymocytes reconstitutes the irradiated thymus, with donor-derived thymocytes appearing about 7 days post BMT. Our previous studies have demonstrated that, in the presence of donor hematopoietic cells lacking T lineage potential, these host-derived thymocytes are able to generate a polyclonal cohort of functionally mature peripheral T cells numerically comprising ~25% of the peripheral T cell pool of euthymic mice. Importantly, we demonstrated that radioresistant CD44⁺ CD25⁺ CD117+ DN2 progenitors were responsible for this thymic auto-reconstitution. Until recently, the mechanisms underlying the radioresistance of DN2 progenitors were unknown. Herein, we have used the in vitro “Plastic Thymus” culture system to perform a detailed investigation of the mechanisms responsible for the high radioresistance of DN2 cells compared with radiosensitive hematopoietic stem cells. Our results indicate that several aspects of DN2 biology, such as (i) rapid DNA damage response (DDR) activation in response to ionizing radiation-induced DNA damage, (ii) efficient repair of DNA double-strand breaks, and (iii) induction of a protective G₁/S checkpoint contribute to promoting DN2 cell survival post-irradiation. We have previously shown that hypoxia increases the radioresistance of bone marrow stromal cells in vitro, at least in part by enhancing their DNA double-strand break (DNA DSB) repair capacity. Since the thymus is also a hypoxic environment, we investigated the potential effects of hypoxia on the DDR of DN2 thymocytes. Finally, we demonstrate for the first time that de novo DN2 thymocytes are able to rapidly repair DNA DSBs following thymic irradiation in vivo.

The Transcription Factor Hif-1 Enhances the Radio-Resistance of Mouse MSCs

Mesenchymal stromal cells (MSCs) are multipotent progenitors supporting bone marrow hematopoiesis. MSCs have an efficient DNA damage response (DDR) and are consequently relatively radio-resistant cells. Therefore, MSCs are key to hematopoietic reconstitution following total body irradiation (TBI) and bone marrow transplantation (BMT). The bone marrow niche is hypoxic and via the heterodimeric transcription factor Hypoxia-inducible factor-1 (Hif-1), hypoxia enhances the DDR. Using gene knock-down, we have previously shown that the Hif-1α subunit of Hif-1 is involved in mouse MSC radio-resistance, however its exact mechanism of action remains unknown. In order to dissect the involvement of Hif-1α in the DDR, we used CRISPR/Cas9 technology to generate a stable mutant of the mouse MSC cell line MS5 lacking Hif-1α expression. Herein, we show that it is the whole Hif-1 transcription factor, and not only the Hif-1α subunit, that modulates the DDR of mouse MSCs. This effect is dependent upon the presence of a Hif-1α protein capable of binding to both DNA and its heterodimeric partner Arnt (Hif-1β). Detailed transcriptomic and proteomic analysis of Hif1a KO MS5 cells leads us to conclude that Hif-1α may be acting indirectly on the DNA repair process. These findings have important implications for the modulation of MSC radio-resistance in the context of BMT and cancer.

Anti-donor antibody induction following intramuscular injections of allogeneic mesenchymal stromal cells

Allogeneic mesenchymal stromal cells (allo-MSC) are a promising "off-the-shelf" therapy with anti-inflammatory and pro-repair properties. This study investigated humoral immune responses to intramuscular (IM) injections of allo-MSC. Total and isotype-specific anti-donor IgG and donor-specific complement-mediated lysis were determined in sera from healthy mice 2 weeks after single or repeated IM injections of fully mismatched-MHC allo-MSC with comparison to mice receiving syngeneic MSC, allogeneic splenocytes or saline. In mice subjected to hind limb ischemia (HLI), anti-donor IgG was analyzed following IM allo-MSC injection with and without administration of the T-cell immunosuppressant tacrolimus. Recipients of single and repeated IM allo-MSC developed readily-detectable anti-donor IgG. Serum anti-donor IgG levels were similar to those of allo-splenocyte recipients but had higher IgG1/IgG2a ratio and variable capacity for complement-mediated lysis of donor cells. The induced anti-donor IgG bound readily to allo-MSC and this binding was increased following allo-MSC pretreatment with interferon gamma. In mice with HLI, IM injection of allo-MSC into the ischemic limb was also associated with induction of anti-donor IgG but this was abrogated by tacrolimus (FK-506). The results indicate that allo-MSC are inherently immunogenic when delivered intramuscularly to healthy and ischemic mouse hind limb, but induce an IgG1-skewed humoral response that is suppressed by tacrolimus.

Phenotypic and functional heterogeneity of human intermediate monocytes based on HLA-DR expression

Human blood monocytes are subclassified as classical, intermediate and nonclassical. In this study, it was shown that conventionally defined human intermediate monocytes can be divided into two distinct subpopulations with mid- and high-level surface expression of HLA-DR (referred to as DRmid and DRhi intermediate monocytes). These IM subpopulations were phenotypically and functionally characterized in healthy adult blood by flow cytometry, migration assays and lipoprotein uptake assays. Their absolute numbers and proportions were then compared in blood samples from obese and nonobese adults. DRmid and DRhi intermediate monocytes differentially expressed several proteins including CD62L, CD11a, CX3CR1 and CCR2. Overall, the DRmid intermediate monocytes surface profile more closely resembled that of classical monocytes while DRhi intermediate monocytes were more similar to nonclassical. However, in contrast to classical monocytes, DRmid intermediate monocytes migrated weakly to CCL2, had reduced intracellular calcium flux following CCR2 ligation and favored adherence to TNFα-activated endothelium over transmigration. In lipid uptake assays, DRmid intermediate monocytes demonstrated greater internalization of oxidized and acetylated low-density lipoprotein than DRhi intermediate monocytes. In obese compared to nonobese adults, proportions and absolute numbers of DRmid , but not DRhi intermediate monocytes, were increased in blood. The results are consistent with phenotypic and functional heterogeneity within the intermediate monocytes subset that may be of specific relevance to lipoprotein scavenging and metabolic health.

Single-cell RNA sequencing reveals developmental heterogeneity among early lymphoid progenitors

Single-cell RNA sequencing is a powerful technology for assessing heterogeneity within defined cell populations. Here, we describe the heterogeneity of a B220⁺CD117^intCD19^-NK1.1^- uncommitted hematopoietic progenitor having combined lymphoid and myeloid potential. Phenotypic and functional assays revealed four subpopulations within the progenitor with distinct lineage developmental potentials. Among them, the Ly6D⁺SiglecH^-CD11c^- fraction was lymphoid-restricted exhibiting strong B-cell potential, whereas the Ly6D^-SiglecH^-CD11c^- fraction showed mixed lympho-myeloid potential. Single-cell RNA sequencing of these subsets revealed that the latter population comprised a mixture of cells with distinct lymphoid and myeloid transcriptional signatures and identified a subgroup as the potential precursor of Ly6D⁺SiglecH^-CD11c^- Subsequent functional assays confirmed that B220⁺CD117^intCD19^-NK1.1^- single cells are, with rare exceptions, not bipotent for lymphoid and myeloid lineages. A B-cell priming gradient was observed within the Ly6D⁺SiglecH^-CD11c^- subset and we propose a herein newly identified subgroup as the direct precursor of the first B-cell committed stage. Therefore, the apparent multipotency of B220⁺CD117^intCD19^-NK1.1^- progenitors results from underlying heterogeneity at the single-cell level and highlights the validity of single-cell transcriptomics for resolving cellular heterogeneity and developmental relationships among hematopoietic progenitors.

Distinctive Surface Glycosylation Patterns Associated With Mouse and Human CD4+ Regulatory T Cells and Their Suppressive Function

Regulatory T-cells (Treg) are essential for maintaining immune homeostasis and tolerance. Surface glycosylation is ubiquitous on mammalian cells and regulates diverse biological processes. While it is currently well accepted that surface glycan expression influences multiple aspects of T-cell function, little is known about the relevance of glycosylation to Treg biology. This study aimed to profile the surface glycosylation characteristics of Treg in various lymphoid compartments of mouse and in human peripheral blood with comparison to non-regulatory, conventional CD4⁺ T-cells (Tconv). It also sought to determine the relationship between the surface glycosylation characteristics and suppressive potency of Treg. Lectin-based flow cytometric profiling demonstrated that Treg surface glycosylation differs significantly from that of Tconv in the resting state and is further modified by activation stimuli. In mouse, the surface glycosylation profiles of FoxP3⁺ Treg from spleen and lymph nodes were closely comparable but greater variability was observed for Treg in thymus, bone marrow, and blood. Surface levels of tri/tetra-antennary N-glycans correlated with expression of proteins known to be involved in Treg suppressive functions, including GITR, PD-1, PD-L1, CD73, CTLA-4, and ICOS. In coculture experiments involving purified Treg subpopulations and CD4⁺ or CD8⁺ Tconv, higher surface tri/tetra-antennary N-glycans was associated with greater Treg suppressive potency. Enzymatic manipulation of mouse Treg surface glycosylation resulting in a temporary reduction of surface N-glycans significantly reduced Treg capacity to suppress Tconv activation through contact-dependent mechanisms. Overall, these findings demonstrate that Treg have distinctive surface glycan characteristics that show variability across anatomical locations and are modulated by activation events. They also provide evidence of an important role for surface glycosylation in determining Treg phenotype and suppressive potency. These insights may prove relevant to the analysis of Treg in disease settings and to the further development of Treg-based immunotherapies.

Differential Response of Mouse Thymic Epithelial Cell Types to Ionizing Radiation-Induced DNA Damage

Thymic epithelial cells (TECs) are the main components of the thymic stroma that support and control T-cell development. Preparative regimens using DNA-damaging agents, such as total body irradiation and/or chemotherapeutic drugs, that are necessary prior to bone marrow transplantation (BMT) have profound deleterious effects on the hematopoietic system, including the thymic stroma, which may be one of the main causes for the prolonged periods of T-cell deficiency and the inefficient T cell reconstitution that are common following BMT. The DNA damage response (DDR) is a complex signaling network that allows cells to respond to all sorts of genotoxic insults. Hypoxia is known to modulate the DDR and play a role affecting the survival capacity of different cell types. In this study, we have characterized in detail the DDR of cortical and medullary TEC lines and their response to ionizing radiation, as well as the effects of hypoxia on their DDR. Although both mTECs and cTECs display relatively high radio-resistance, mTEC cells have an increased survival capacity to ionizing radiation (IR)-induced DNA damage, and hypoxia specifically decreases the radio-resistance of mTECs by upregulating the expression of the pro-apoptotic factor Bim. Analysis of the expression of TEC functional factors by primary mouse TECs showed a marked decrease of highly important genes for TEC function and confirmed cTECs as the most affected cell type by IR. These findings have important implications for improving the outcomes of BMT and promoting successful T cell reconstitution.

Pro-B cells propagated in stromal cell-free cultures reconstitute functional B-cell compartments in immunodeficient mice

Up to now long-term in vitro growth of pro-B cells was thought to require stromal cells. However, here we show that fetal liver (FL) and bone marrow (BM) derived pro-B cells can be propagated long-term in stromal cell-free cultures supplemented with IL-7, stem cell factor and FLT3 ligand. Within a week, most cells expressed surface CD19, CD79A, λ5, and VpreB antigens and had rearranged immunoglobulin D-J heavy chain genes. Both FL and BM pro-B cells reconstituted the B-cell compartments of immuno-incompetent Rag2-deficient mice, with FL pro-B cells generating follicular, marginal zone (MZB) and B1a B cells, and BM pro-B cells giving rise mainly to MZB cells. Reconstituted Rag2-deficient mice generated significant levels of IgM and IgG antibodies to a type II T-independent antigen; mice reconstituted with FL pro-B cells generated surprisingly high IgG₁ titers. Finally, we show for the first time that mice reconstituted with mixtures of pro-B and pro-T cells propagated in stromal cell-free in vitro cultures mounted a T-cell-dependent antibody response. This novel stromal cell-free culture system facilitates our understanding of B-cell development and might be applied clinically.

Development of a flow cytometry-based potency assay for measuring the in vitro immunomodulatory properties of mesenchymal stromal cells

Human bone marrow-derived mesenchymal stromal/stem cells (MSC) have well-documented modulatory effects on multiple immune cell types. Although these effects are linked to their therapeutic benefit in diverse diseases, a reliable, quantitative assay of the immunomodulatory potency of individual human MSC preparations is lacking. The aims of this study were to develop an optimised rapid turnaround, flow cytometry-based whole-blood assay to monitor MSC potency and to validate its application to MSC immunomodulation. A protocol for short-term LPS stimulation of anti-coagulated whole blood samples followed by combined surface CD45/CD14 and intracellular TNF-α staining was initially developed for analysis on a 4 colour desktop cytometer. Optimal monocyte activation was dependent on the presence of extracellular calcium ions thereby precluding the use of EDTA and sodium citrate as anticoagulants. Optimal assay conditions proved to be 1ng/mL ultrapure-LPS added to 10-fold diluted, heparin anti-coagulated whole blood incubated for 6h at 37°C. Under these conditions, addition of human bone marrow-derived MSC (hBM-MSC) from multiple donors resulted in a reproducible, dose-dependent inhibition of LPS-stimulated monocyte TNF-α expression. We conclude that this protocol represents a practical, quantitative assay of a clinically relevant functional effect of hBM-MSCs as well as other immunomodulatory agents.

Hypoxia enhances the radioresistance of mouse mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) are radioresistant bone marrow progenitors that support hematopoiesis and its reconstitution following total body irradiation. MSCs reside in hypoxic niches within the bone marrow and tumor microenvironments. The DNA damage response (DDR) represents a network of signaling pathways that enable cells to activate biological responses to DNA damaging agents. Hypoxia-mediated alterations in the DDR contribute to the increased radioresistance of hypoxic cancer cells, limiting therapeutic efficacy. The DDR is important in mediating mouse MSC radioresistance. However, the effects of hypoxia on MSC radioresistance are currently unknown. In this report, hypoxia was found to (a) increase MSC proliferation rate and colony size; (b) increase long-term survival post-irradiation (IR), and (c) improve MSC recovery from IR-induced cell cycle arrest. DNA double-strand break (DSB) repair in MSCs was upregulated in hypoxia, accelerating the resolution of highly genotoxic IR-induced DNA DSBs. In addition, HIF-1α was found to contribute to this enhanced DSB repair by regulating (a) the expression of DNA ligase IV and DNA-PKcs and (b) Rad51 foci formation in response to DNA DSBs in hypoxic MSCs. We have demonstrated, for the first time, that hypoxia enhances mouse MSC radioresistance in vitro. These findings have important implications for our understanding of MSC functions in supporting allogeneic bone marrow transplantation and in tumorigenesis.

LymAnalyzer: a tool for comprehensive analysis of next generation sequencing data of T cell receptors and immunoglobulins

The adaptive immune system includes populations of B and T cells capable of binding foreign epitopes via antigen specific receptors, called immunoglobulin (IG) for B cells and the T cell receptor (TCR) for T cells. In order to provide protection from a wide range of pathogens, these cells display highly diverse repertoires of IGs and TCRs. This is achieved through combinatorial rearrangement of multiple gene segments in addition, for B cells, to somatic hypermutation. Deep sequencing technologies have revolutionized analysis of the diversity of these repertoires; however, accurate TCR/IG diversity profiling requires specialist bioinformatics tools. Here we present LymAnalzyer, a software package that significantly improves the completeness and accuracy of TCR/IG profiling from deep sequence data and includes procedures to identify novel alleles of gene segments. On real and simulated data sets LymAnalyzer produces highly accurate and complete results. Although, to date we have applied it to TCR/IG data from human and mouse, it can be applied to data from any species for which an appropriate database of reference genes is available. Implemented in Java, it includes both a command line version and a graphical user interface and is freely available at https://sourceforge.net/projects/lymanalyzer/.

Isolation, selection and culture methods to enhance clonogenicity of mouse bone marrow derived mesenchymal stromal cell precursors

Introduction

Conventionally cultured mouse bone marrow mesenchymal stromal cells (mBM-MSC) are a heterogeneous population that often initially contain contaminating haematopoietic cells. Variability in isolation methods, culture protocols and the lack of specific mBM MSC markers might explain this heterogeneity. The aim of this study is to optimise the isolation, culture conditions and selection of mBM-MSC.

Methods

Mouse BM-MSCs were isolated from crushed long bones (cBM) or flushed bone marrow (fBM) from 6–8 week old C57Bl/6 mice. These subpopulations were analysed by flow cytometry using commonly used mBM-MSC cell surface marker, e.g. Sca-1, CD29 and CD44. Cells were cultured and expanded in vitro in hypoxic conditions of either 2 % or 5 % oxygen. Cell sorting and qRT-PCR was used to determine transcript levels of stem cell and lineage related genes in individual subpopulations.

Results

During early passaging not only do contaminating haematopoietic cells disappear, but there is a change in the phenotype of mBM-MSC affecting particularly CD44 and Sca-1 expression. By fluorescence activated cell sorting of CD45⁻/Ter119⁻ mBM stroma based on Sca-1 expression and expansion in hypoxic conditions, we show that Sca-1⁺ cells had higher CFU-F frequencies and showed enhanced proliferation compared with Sca-1⁻ cells. As evaluated by in vitro assays and qRT-PCR, these cells presented in vitro tri-lineage differentiation along osteocyte, chondrocyte, and adipocyte lineages. Finally, by prospective isolation of Sca-1⁺PDGFRα⁺CD90⁺ cells we have isolated mBM-MSC on a single cell level, achieving a CFU-F frequency of 1/4. Functional investigations demonstrated that these MSC clones inhibited T-lymphocyte proliferation.

Conclusion

By positive selection using a combination of antibodies to Sca-1, CD90 and PDGFRα and culturing in hypoxia, we have found a subpopulation of BM cells from C57Bl/6 mice with a CFU-F cloning efficiency of 1/4. To our knowledge these results represent the highest frequencies of mouse MSC cloning from C57Bl/6 mice yet reported.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0139-5) contains supplementary material, which is available to authorized users.

Versatility of stem and progenitor cells and the instructive actions of cytokines on hematopoiesis

For many years, developing hematopoietic cells have been strictly compartmentalized into a rare population of multi-potent self-renewing hematopoietic stem cells (HSC), multi-potent hematopoietic progenitor cells (MPP) that are undergoing commitment to particular lineage fates, and recognizable precursor cells that mature towards functional blood and immune cells. A single route to each end-cell type is prescribed in the "classical" model for the architecture of hematopoiesis. Recent findings have led to the viewpoint that HSCs and MPPs are more versatile than previously thought. Underlying this are multiple routes to a particular fate and cells having clandestine fate options even when they have progressed some way along a pathway. The primary role of cytokines during hematopoiesis has long been seen to be regulation of the survival and proliferation of developing hematopoietic cells. Some cytokines now clearly have instructive actions on cell-fate decisions. All this leads to a new way of viewing hematopoiesis whereby versatile HSC and MPP are directed towards lineage outcomes via cytokine regulated cell-fate decisions. This means greater flexibility to the shaping of hematopoiesis.

Promiscuous mRNA splicing under the control of AIRE in medullary thymic epithelial cells

MOTIVATION

The expression of tissue-restricted antigens (TRAs) in the thymus is required to ensure efficient negative selection of potentially auto-reactive T lymphocytes and avoid autoimmune disease. This promiscuous expression is under the control of the autoimmune regulator (AIRE), a transcription factor expressed in medullary thymic epithelial cells (mTECs). Tissue-specific alternative splicing may also produce TRAs but the extent to which splice isoforms that are restricted to specific tissues are expressed in mTECs is yet to be investigated.

RESULTS

We reanalyzed microarray and RNA-Seq datasets from mouse mTECs and other epithelial and non-epithelial cell types and found that the diversity of splice isoforms in mTECs was greater than in any of the other cell types or tissues studied. We identified tissue-specific isoforms from a panel of mouse tissues and found several examples of such isoforms that are expressed in mTECs. The number of isoforms with restricted expression found in mTECs was significantly higher than for comparable cell types. Furthermore, we found evidence that AIRE influences the increased splicing diversity observed in mTECs as the genes for which tissue restricted isoforms are produced in mTECs were significantly more likely than other genes to be differentially spliced between AIRE knock-out and wild-type samples. Our results suggest that developing T lymphocytes are exposed to diverse tissue-restricted splice isoforms in the thymus and that AIRE has a direct or indirect role in this process, representing a novel aspect of its role in the maintenance of immune self-tolerance.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Post-traumatic immunosuppression is reversed by anti-coagulated salvaged blood transfusion: deductions from studying immune status after knee arthroplasty

Major trauma increases vulnerability to systemic infections due to poorly defined immunosuppressive mechanisms. It confers no evolutionary advantage. Our objective was to develop better biomarkers of post-traumatic immunosuppression (PTI) and to extend our observation that PTI was reversed by anti-coagulated salvaged blood transfusion, in the knowledge that others have shown that non-anti-coagulated (fibrinolysed) salvaged blood was immunosuppressive. A prospective non-randomized cohort study of patients undergoing primary total knee arthroplasty included 25 who received salvaged blood transfusions collected post-operatively into acid-citrate-dextrose anti-coagulant (ASBT cohort), and 18 non-transfused patients (NSBT cohort). Biomarkers of sterile trauma included haematological values, damage-associated molecular patterns (DAMPs), cytokines and chemokines. Salvaged blood was analysed within 1 and 6 h after commencing collection. Biomarkers were expressed as fold-changes over preoperative values. Certain biomarkers of sterile trauma were common to all 43 patients, including supranormal levels of: interleukin (IL)-6, IL-1-receptor-antagonist, IL-8, heat shock protein-70 and calgranulin-S100-A8/9. Other proinflammatory biomarkers which were subnormal in NSBT became supranormal in ASBT patients, including IL-1β, IL-2, IL-17A, interferon (IFN)-γ, tumour necrosis factor (TNF)-α and annexin-A2. Furthermore, ASBT exhibited subnormal levels of anti-inflammatory biomarkers: IL-4, IL-5, IL-10 and IL-13. Salvaged blood analyses revealed sustained high levels of IL-9, IL-10 and certain DAMPs, including calgranulin-S100-A8/9, alpha-defensin and heat shock proteins 27, 60 and 70. Active synthesis during salvaged blood collection yielded increasingly elevated levels of annexin-A2, IL-1β, Il-1-receptor-antagonist, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12p70, IL-17A, IFN-γ, TNF-α, transforming growth factor (TGF)-β1, monocyte chemotactic protein-1 and macrophage inflammatory protein-1α. Elevated levels of high-mobility group-box protein-1 decreased. In conclusion, we demonstrated that anti-coagulated salvaged blood reversed PTI, and was attributed to immune stimulants generated during salvaged blood collection.

Concise review: adult mesenchymal stromal cell therapy for inflammatory diseases: how well are we joining the dots?

Mesenchymal stromal (stem) cells (MSCs) continue to be a strong area of focus for academic- and industry-based researchers who share the goal of expanding their therapeutic use for diverse inflammatory and immune-mediated diseases. Recently, there has been an accelerated rate of scientific publication, clinical trial activity, and commercialisation in the field. This has included the reporting of exciting new developments in four areas that will be of key importance to future successful use of MSC-based therapies in large numbers of patients: (a) fundamental biology of the primary cells in bone marrow and other tissues that give rise to MSCs in culture. (b) Mechanisms by which MSCs modulate immune and inflammatory responses in vivo. (c) Insights into MSC kinetics, safety, and efficacy in relevant animal disease models. (d) Isolation, definition, and clinical trial-based testing of human MSCs by biomedical companies and academic medical centers. Despite this progress, it remains unclear whether MSCs will enter mainstream therapeutic practice as a frequently used alternative to pharmacotherapy or surgical/radiological procedures in the foreseeable future. In this review, we summarize some of the most significant new developments for each of the four areas that contribute to the process of translating MSC research to the clinical arena. In the context of this recent progress, we discuss key challenges and specific knowledge gaps which, if not addressed in a coordinated fashion, may hinder the creation of robust "translational pipelines" for consolidating the status of MSC-based therapies.

The influence of hypoxia on the differentiation capacities and immunosuppressive properties of clonal mouse mesenchymal stromal cell lines

Multipotent mesenchymal stromal cells are multipotent cells capable of differentiating into different mesodermal cell types. Enigmatically, mesenchymal stromal cells present in the bone marrow support early lymphopoiesis yet can inhibit mature lymphocyte growth. Critical features of the bone marrow microenvironment, such as the level of oxygen, play an important role in mesenchymal stromal cell biology. Herein, we show that a panel of continuously growing mouse mesenchymal stromal cell lines, namely OP9, MS5, PA6, ST2 and B16-14, exhibit mesenchymal stromal cell characteristic phenotypes and respond physiologically to oxygen deprivation. Culturing freshly isolated bone marrow-derived mesenchymal stromal cells or cell lines at 5% O2 resulted in a dramatic increase in expression of hypoxia-inducible factor family members and of key genes involved in their differentiation. Phenotypically, their osteogenic and adipogenic differentiation capacity was generally improved in hypoxia, whereas their inhibitory effects on in vitro T-cell proliferation were preserved. Taken together, we conclude that these continuously growing mouse cell lines behave as canonical mesenchymal stromal cells and respond physiologically to hypoxia, thereby providing a potent tool for the study of different aspects of mesenchymal stromal cell biology.

In vivo evidence for an instructive role of fms-like tyrosine kinase-3 (FLT3) ligand in hematopoietic development

Cytokines are essential regulators of hematopoiesis, acting in an instructive or permissive way. Fms-like tyrosine kinase 3 ligand (FLT3L) is an important cytokine for the development of several hematopoietic populations. Its receptor (FLT3) is expressed on both myeloid and lymphoid progenitors and deletion of either the receptor or its ligand leads to defective developmental potential of hematopoietic progenitors. In vivo administration of FLT3L promotes expansion of progenitors with combined myeloid and lymphoid potential. To investigate further the role of this cytokine in hematopoietic development, we generated transgenic mice expressing high levels of human FLT3L. These transgenic mice displayed a dramatic expansion of dendritic and myeloid cells, leading to splenomegaly and blood leukocytosis. Bone marrow myeloid and lymphoid progenitors were significantly increased in numbers but retained their developmental potential. Furthermore, the transgenic mice developed anemia together with a reduction in platelet numbers. FLT3L was shown to rapidly reduce the earliest erythroid progenitors when injected into wild-type mice, indicating a direct negative role of the cytokine on erythropoiesis. We conclude that FLT3L acts on multipotent progenitors in an instructive way, inducing their development into myeloid/lymphoid lineages while suppressing their megakaryocyte/erythrocyte potential.

The versatile landscape of haematopoiesis: are leukaemia stem cells as versatile?

Since the early 1980s, developing haematopoietic cells have been categorised into three well-defined compartments: multi-potent haematopoietic stem cells (HSC), which are able to self-renew, followed by haematopoietic progenitor cells (HPC), which undergo decision-making and age as they divide rather than self-renew, and the final compartment of functional blood and immune cells. The classic model of haematopoiesis divides cells into two families, myeloid and lymphoid, and dictates a route to a particular cell fate. New discoveries question these long-held principles, including: (i) the identification of lineage-biased cells that self-renew; (ii) a strict myeloid/lymphoid dichotomy is refuted by the existence of progenitors with lymphoid potential and an incomplete set of myeloid potentials; (iii) there are multiple routes to some end cell types; and (iv) thymocyte progenitor cells that have progressed some way along this pathway retain clandestine myeloid options. In essence, the progeny of HSC are more versatile and the process of haematopoiesis is more flexible than previously thought. Here we examine this new way of viewing haematopoiesis and the impact of rewriting an account of haematopoiesis on our understanding of what goes awry in leukaemia.

Development and optimization of an EGFP-based reporter for measuring the general stress response in Listeria monocytogenes

A characteristic of the food-borne pathogen Listeria monocytogenes is its tolerance to the harsh conditions found both in minimally processed foods and the human gastrointestinal tract. This trait is partly under the control of the alternative sigma factor sigma B (σ(B)). To study the mechanisms that trigger the activation of σ(B) , and hence the development of stress tolerance, we have developed a fluorescent reporter fusion that allows the real-time activity of σ(B) to be monitored. The reporter, designated Plmo2230::egfp, fuses the strong σ(B)-dependent promoter from the lmo2230 gene (which encodes a putative arsenate reductase) to a gene encoding enhanced green fluorescence protein (EGFP). The reporter was integrated into the genomes of the wild-type strain L. monocytogenes EGD-e as well as two mutant derivatives lacking either sigB or rsbV. The resulting strains were used to study σ(B) activation in response to growth phase and hyperosmotic stress. The wild-type was strongly fluorescent in stationary phase or in cultures with added NaCl and this fluorescence was abolished in both the sigB and rsbV backgrounds, consistent with the σ(B)-dependency of the lmo2230 promoter. During sudden osmotic upshock (addition of 0.5 M NaCl during growth) a real-time increase in fluorescence was observed microscopically, reaching maximal activation after 30 min. Flow cytometry was used to study the activation of σ(B) at a population level by hyperosmotic stress during exponential growth. A strong and proportional increase in fluorescence was observed as the salt concentration increased from 0 to 0.9 M NaCl. Interestingly, there was considerable heterogeneity within the population and a significant proportion of cells failed to induce a high level of fluorescence, suggesting that σ(B) activation occurs stochastically in response to hyperosmotic stress. Thus the Plmo2230::egfp is a powerful tool that will allow the stress response to be better studied in this important human pathogen.

When one cell is enough

For many years, the hematopoietic stem cell (HSC) has been well characterized in mice as a cell that can singly reconstitute the whole hematopoietic system of primary recipient animals as well as that of secondary hosts. That clinical bone marrow transplantation is a successful treatment strategy is indirect evidence that such a cell exists in humans. To date, similar criteria have not been applied to human HSCs. However, using a humanized mouse model of xenotransplantation, a recent paper shows that single human cells can fully reconstitute the lymphomyeloid system of primary recipient animals and, in some cases, that of secondary hosts.

Interleukin-1 accounts for intrarenal Th17 cell activation during ureteral obstruction

Interleukin 17A-secreting T-helper 17 (Th17) cells are pathogenic in inflammatory kidney diseases, but their intrarenal regulation is poorly understood. In order to better define Th17 cell dynamics during interstitial inflammation, we utilized the mouse unilateral ureteral obstruction model to analyze inflammatory cell subtypes by multicolor flow cytometry and cell sorting and by effects on in vitro-generated Th17 cells. Interleukin 17A expression localized to CCR6(+)CCR4(+/-)CD4(+) T-cells and progressively increased in obstructed kidneys. The number of CCR6(+)CD4(+) T-cells increased over 10-fold by 72 h, were enriched for interleukin 17A production, and were highly proliferative based on in vivo bromodeoxyuridine incorporation. Secreted products of leukocytes isolated from obstructed kidneys enhanced the interleukin 17A production of in vitro-generated Th17 cells. This Th17-enhancing activity was identified as interleukin-1 produced by renal dendritic cells and monocytes. The in vivo validity of these findings was confirmed in mice lacking the interleulin-1 receptor and in mice treated with a recombinant interleukin-1 receptor antagonist, each of which exhibited reduced intrarenal Th17 activity compared with control mice. Thus, the inflamed kidney accumulates CCR6(+) Th17 cells that undergo activation and proliferation. Production of interleukin 1 family cytokines by resident dendritic cells and infiltrating monocytes enhances intrarenal Th17 activation in acute kidney injury.

Mesenchymal stem cell inhibition of T-helper 17 cell- differentiation is triggered by cell-cell contact and mediated by prostaglandin E2 via the EP4 receptor

Mesenchymal stem cells (MSCs) inhibit T-cell activation and proliferation but their effects on individual T-cell-effector pathways and on memory versus naïve T cells remain unclear. MSC influence on the differentiation of naïve and memory CD4(+) T cells toward the Th17 phenotype was examined. CD4(+) T cells exposed to Th17-skewing conditions exhibited reduced CD25 and IL-17A expression following MSC co-culture. Inhibition of IL-17A production persisted upon re-stimulation in the absence of MSCs. These effects were attenuated when cell-cell contact was prevented. Th17 cultures from highly purified naïve- and memory-phenotype responders were similarly inhibited. Th17 inhibition by MSCs was reversed by indomethacin and a selective COX-2 inhibitor. Media from MSC/Th17 co-cultures contained increased prostaglandin E2 (PGE2) levels and potently suppressed Th17 differentiation in fresh cultures. MSC-mediated Th17 inhibition was reversed by a selective EP4 antagonist and was mimicked by synthetic PGE2 and a selective EP4 agonist. Activation-induced IL-17A secretion by naturally occurring, effector-memory Th17 cells from a urinary obstruction model was also inhibited by MSC co-culture in a COX-dependent manner. Overall, MSCs potently inhibit Th17 differentiation from naïve and memory T-cell precursors and inhibit naturally-occurring Th17 cells derived from a site of inflammation. Suppression entails cell-contact-dependent COX-2 induction resulting in direct Th17 inhibition by PGE2 via EP4.

Mesenchymal stem cell effects on T-cell effector pathways

Mesenchymal stem (stromal) cells (MSCs) are rare, multipotent progenitor cells that can be isolated and expanded from bone marrow and other tissues. Strikingly, MSCs modulate the functions of immune cells, including T cells, B cells, natural killer cells, monocyte/macrophages, dendritic cells, and neutrophils. T cells, activated to perform a range of different effector functions, are the primary mediators of many autoimmune and inflammatory diseases as well as of transplant rejection and graft-versus-host disease. Well-defined T-cell effector phenotypes include the CD4⁺ (T helper cell) subsets Th1, Th2, and Th17 cells and cytotoxic T lymphocytes derived from antigen-specific activation of naïve CD8⁺ precursors. In addition, naturally occurring and induced regulatory T cells (T_reg) represent CD4⁺ and CD8⁺ T-cell phenotypes that potently suppress effector T cells to prevent autoimmunity, maintain self-tolerance, and limit inflammatory tissue injury. Many immune-mediated diseases entail an imbalance between T_reg and effector T cells of one or more phenotypes. MSCs broadly suppress T-cell activation and proliferation in vitro via a plethora of soluble and cell contact-dependent mediators. These mediators may act directly upon T cells or indirectly via modulation of antigen-presenting cells and other accessory cells. MSC administration has also been shown to be variably associated with beneficial effects in autoimmune and transplant models as well as in several human clinical trials. In a small number of studies, however, MSC administration has been found to aggravate T cell-mediated tissue injury. The multiple effects of MSCs on cellular immunity may reflect their diverse influences on the different T-cell effector subpopulations and their capacity to specifically protect or induce T_reg populations. In this review, we focus on findings from the recent literature in which specific modulatory effects of MSCs on one or more individual effector T-cell subsets and T_reg phenotypes have been examined in vitro, in relevant animal models of in vivo immunological disease, and in human subjects. We conclude that MSCs have the potential to directly or indirectly inhibit disease-associated Th1, Th2, and Th17 cells as well as cytotoxic T lymphocytes but that many key questions regarding the potency, specificity, mechanistic basis, and predictable therapeutic value of these modulatory effects remain unanswered.

Tolerance checkpoints in B-cell development: Johnny B good

B-cell development up to the immature B-cell stage takes place in the bone marrow, while final maturation into mature B cells occurs in the spleen. During differentiation, the precursor and immature B cells have to pass several checkpoints, including those in which they are censored for being auto-reactive, and therefore being potentially dangerous. Numerous studies have shown that the immature B-cell stage in the bone marrow and the transitional B-cell stages in the spleen comprise distinct checkpoints at which auto-reactivity is censored. Recently, evidence has been provided that the large pre-BII stage in the bone marrow, at which the pre-BCR is expressed, is yet another B-cell tolerance checkpoint. Here, we review these findings and speculate on directions for possible further experimentation.

Cutting edge: IL-7 regulates the peripheral pool of adult ROR gamma+ lymphoid tissue inducer cells

During fetal life, CD4(+)CD3(-) lymphoid tissue inducer (LTi) cells are required for lymph node and Peyer's patch development in mice. In adult animals, CD4(+)CD3(-) cells are found in low numbers in lymphoid organs. Whether adult CD4(+)CD3(-) cells are LTi cells and are generated and maintained through cytokine signals has not been directly addressed. In this study we show that adult CD4(+)CD3(-) cells adoptively transferred into neonatal CXCR5(-/-) mice induced the formation of intestinal lymphoid tissues, demonstrating for the first time their bona fide LTi function. Increasing IL-7 availability in wild-type mice either by IL-7 transgene expression or treatment with IL-7/anti-IL-7 complexes increased adult LTi cell numbers through de novo generation from bone marrow cells and increased the survival and proliferation of LTi cells. Our observations demonstrate that adult CD4(+)lineage(-) cells are LTi cells and that the availability of IL-7 determines the size of the adult LTi cell pool.

TCR-beta chains derived from peripheral gammadelta T cells can take part in alphabeta T-cell development

Between 10 and 20% of the peripheral gammadelta T cells express cytoplasmic TCR-beta proteins, but whether such TCR-beta chains can partake in alphabeta T-cell development has never been systematically investigated. Therefore, we reconstituted the T-cell compartment of CD3epsilon-deficient mice with Pax5-TCR-beta deficient proB cells expressing, via a retroviral vector, TCR-beta chains from either peripheral gammadelta or alphabeta T cells. Recipient thymi reconstituted with proB cells containing empty vector were small (<15x10(6) cells), contained few gammadelta T but no alphabeta T cells. In contrast, thymi from mice receiving proB cells containing gammadelta or alphabeta T-cell-derived TCR-beta chains contained 80-130x10(6) cells, and showed a normal CD4, CD8 and alphabeta TCR expression pattern. However, regardless of the source of TCR-beta chain, reconstituted mice rapidly showed signs of autoimmunity dying 5-15 wk following reconstitution. Autoimmune disease induction could be prevented by co-transfer of Treg cells thereby allowing the functionality of the generated T cells to be assessed. Results obtained show that TCR-beta chains from gammadelta T cells can efficiently take part in alphabeta T-cell development. The implications of these findings for gammadelta T-cell development will be discussed.

Ectopic lymphoid-organ development occurs through interleukin 7-mediated enhanced survival of lymphoid-tissue-inducer cells

Development of Peyer's patches and lymph nodes requires the interaction between CD4+ CD3- IL-7Ralpha+ lymphoid-tissue inducer (LTi) and VCAM-1+ organizer cells. Here we showed that by promoting their survival, enhanced expression of interleukin-7 (IL-7) in transgenic mice resulted in accumulation of LTi cells. With increased IL-7 availability, de novo formation of VCAM-1+ Peyer's patch anlagen occurred along the entire fetal gut resulting in a 5-fold increase in Peyer's patch numbers. IL-7 overexpression also led to formation of multiple organized ectopic lymph nodes and cecal patches. After immunization, ectopic lymph nodes developed normal T cell-dependent B cell responses and germinal centers. Mice overexpressing IL-7 but lacking either RORgamma, a factor required for LTi cell generation, or lymphotoxin alpha1beta2 had neither Peyer's patches nor ectopic lymph nodes. Therefore, by controlling LTi cell numbers, IL-7 can regulate the formation of both normal and ectopic lymphoid organs.