Cellular niches controlling B lymphocyte behavior within bone marrow during development

Imaging Cytokine Concentration Fields Using PlaneView Imaging Devices

We describe here a method to visualize concentration fields of cytokines around cytokine-secreting cells. The main challenge is that physiological cytokine concentrations can be very low, in the pico-molar range. Since it is currently impossible to measure such concentrations directly, we rely on cell's response to the cytokines-the phosphorylation of a transcription factor-that can be visualized through antibody staining. Our devices aim at mimicking conditions in dense tissues, such as lymph nodes. A small number of secreting cells is deposited on a polylysine-coated glass and covered by multiple layers of cytokine-consuming. The cells are left to communicate for 1 h, after which the top layers are removed and the bottom layer of cells is antibody labeled for the response to cytokines. Then a cross-section of cytokine fields can be visualized by standard fluorescence microscopy. This manuscript summarized our method to quantify the extent of cytokine-mediated cell-to-cell communications in dense collection of cells in vitro.

CXCR4-CXCL12 interaction is important for plasma cell homing and survival in NZB/W mice

Antibody-secreting cells (ASCs), including short-lived plasmablasts and long-lived memory plasma cells (LLPCs), contribute to autoimmune pathology. ASCs, particularly LLPCs, refractory to conventional immunosuppressive drugs pose a major therapeutic challenge. Since stromal cells expressing C-X-C motif chemokine-12 (CXCL12) organize survival niches for LLPCs in the bone marrow, we investigated the effects of CXCL12 and its ligand CXCR4 (C-X-C chemokine receptor 4) on ASCs in lupus mice (NZB/W). Fewer adoptively transferred splenic ASCs were retrieved from the bone marrow of recipient immunodeficient Rag1^-/- mice when the ASCs were pretreated with the CXCR4 blocker AMD3100. CXCR4 blockade also significantly reduced anti-OVA ASCs in the bone marrow after secondary immunization with OVA. In this study, AMD3100 efficiently depleted ASCs, including LLPCs. After two weeks, it decreased the total number of ASCs in the spleen and bone marrow by more than 60%. Combination with the proteasome inhibitor bortezomib significantly enhanced the depletion effect of AMD3100. Continuous long-term (five-month) CXCR4 blockade with AMD3100 after effective short-term LLPCs depletion kept the number of LLPCs in the bone marrow low, delayed proteinuria development and prolonged the survival of the mice. These findings identify the CXCR4-CXCL12 axis as a potential therapeutic target likely due to its importance for ASC homing and survival.

MiR221 promotes precursor B-cell retention in the bone marrow by amplifying the PI3K-signaling pathway in mice

Hematopoietic stem cells and lineage-uncommitted progenitors are able to home to the bone marrow upon transplantation and reconstitute the host with hematopoietic progeny. Expression of miR221 in B-lineage committed preBI-cells induces their capacity to home to the bone marrow. However, the molecular mechanisms underlying miR221-controlled bone marrow homing and retention remain poorly understood. Here, we demonstrate, that miR221 regulates bone marrow retention of such B-cell precursors by targeting PTEN, thus enhancing PI3K signaling in response to the chemokine CXCL12. MiR221-enhanced PI3K signaling leads to increased expression of the anti-apoptotic protein Bcl2 and VLA4 integrin-mediated adhesion to VCAM1 in response to CXCL12 in vitro. Ablation of elevated PI3K activity abolishes the retention of miR221 expressing preBI-cells in the bone marrow. These results suggest that amplification of PI3K signaling by miR221 could be a general mechanism for bone marrow residence, shared by miR221-expressing hematopoietic cells.

Plasma Cell Differentiation Pathways in Systemic Lupus Erythematosus

Plasma cells (PCs) are responsible for the production of protective antibodies against infectious agents but they also produce pathogenic antibodies in autoimmune diseases, such as systemic lupus erythematosus (SLE). Traditionally, high affinity IgG autoantibodies are thought to arise through germinal center (GC) responses. However, class switching and somatic hypermutation can occur in extrafollicular (EF) locations, and this pathway has also been implicated in SLE. The pathway from which PCs originate may determine several characteristics, such as PC lifespan and sensitivity to therapeutics. Although both GC and EF responses have been implicated in SLE, we hypothesize that one of these pathways dominates in each individual patient and genetic risk factors may drive this predominance. While it will be important to distinguish polymorphisms that contribute to a GC-driven or EF B cell response to develop targeted treatments, the challenge will be not only to identify the differentiation pathway but the molecular mechanisms involved. In B cells, this task is complicated by the cross-talk between the B cell receptor, toll-like receptors (TLR), and cytokine signaling molecules, which contribute to both GC and EF responses. While risk variants that affect the function of dendritic cells and T follicular helper cells are likely to primarily influence GC responses, it will be important to discover whether some risk variants in the interferon and TLR pathways preferentially influence EF responses. Identifying the pathways of autoreactive PC differentiation in SLE may help us to understand patient heterogeneity and thereby guide precision therapy.

Bone Marrow Micro-Environment in Normal and Deranged Hematopoiesis: Opportunities for Regenerative Medicine and Therapies

Various cell types cooperate to create a highly organized and dynamic micro-environmental niche in the bone marrow. Over the past several years, the field has increasingly recognized the critical roles of the interplay between bone marrow environment and hematopoietic cells in normal and deranged hematopoiesis. These advances rely on several new technologies that have allowed us to characterize the identity and roles of these niches in great detail. Here, we review the progress of the last several years, list some of the outstanding questions in the field and propose ways to target the diseased environment to better treat hematologic diseases. Understanding the extrinsic regulation by the niche will help boost hematopoiesis for regenerative medicine. Based on natural development of hematologic malignancies, we propose that combinatory targeting the niche and hematopoietic intrinsic mechanisms in early stages of hematopoietic malignancies may help eliminate minimal residual disease and have the highest efficacy.

Lack of galectin-3 modifies differentially Notch ligands in bone marrow and spleen stromal cells interfering with B cell differentiation

Galectin-3 (Gal-3) is a β-galactoside binding protein that controls cell-cell and cell-extracellular matrix interactions. In lymphoid organs, gal-3 inhibits B cell differentiation by mechanisms poorly understood. The B cell development is dependent on tissue organization and stromal cell signaling, including IL-7 and Notch pathways. Here, we investigate possible mechanisms that gal-3 interferes during B lymphocyte differentiation in the bone marrow (BM) and spleen. The BM of gal-3-deficient mice (Lgals3^-/- mice) was evidenced by elevated numbers of B220⁺CD19⁺c-Kit⁺IL-7R⁺ progenitor B cells. In parallel, CD45^- bone marrow stromal cells expressed high levels of mRNA IL-7, Notch ligands (Jagged-1 and Delta-like 4), and transcription factors (Hes-1, Hey-1, Hey-2 and Hey-L). The spleen of Lgals3^-/- mice was hallmarked by marginal zone disorganization, high number of IgM⁺IgD⁺ B cells and CD138⁺ plasma cells, overexpression of Notch ligands (Jagged-1, Delta-like 1 and Delta-like 4) by stromal cells and Hey-1. Morever, IgM⁺IgD⁺ B cells and B220⁺CD138⁺ CXCR4⁺ plasmablasts were significantly increased in the BM and blood of Lgals3^-/- mice. For the first time, we demonstrated that gal-3 inhibits Notch signaling activation in lymphoid organs regulating earlier and terminal events of B cell differentiation.

Targeting Leukemia Stem Cells in the Bone Marrow Niche

Abstract: The bone marrow (BM) niche encompasses multiple cells of mesenchymal and hematopoietic origin and represents a unique microenvironment that is poised to maintain hematopoietic stem cells. In addition to its role as a primary lymphoid organ through the support of lymphoid development, the BM hosts various mature lymphoid cell types, including naïve T cells, memory T cells and plasma cells, as well as mature myeloid elements such as monocyte/macrophages and neutrophils, all of which are crucially important to control leukemia initiation and progression. The BM niche provides an attractive milieu for tumor cell colonization given its ability to provide signals which accelerate tumor cell proliferation and facilitate tumor cell survival. Cancer stem cells (CSCs) share phenotypic and functional features with normal counterparts from the tissue of origin of the tumor and can self-renew, differentiate and initiate tumor formation. CSCs possess a distinct immunological profile compared with the bulk population of tumor cells and have evolved complex strategies to suppress immune responses through multiple mechanisms, including the release of soluble factors and the over-expression of molecules implicated in cancer immune evasion. This chapter discusses the latest advancements in understanding of the immunological BM niche and highlights current and future immunotherapeutic strategies to target leukemia CSCs and overcome therapeutic resistance in the clinic.

Tyrosine kinase inhibitors and mesenchymal stromal cells: effects on self-renewal, commitment and functions

The hope of selectively targeting cancer cells by therapy and eradicating definitively malignancies is based on the identification of pathways or metabolisms that clearly distinguish “normal” from “transformed” phenotypes. Some tyrosine kinase activities, specifically unregulated and potently activated in malignant cells, might represent important targets of therapy. Consequently, tyrosine kinase inhibitors (TKIs) might be thought as the “vanguard” of molecularly targeted therapy for human neoplasias. Imatinib and the successive generations of inhibitors of Bcr-Abl1 kinase, represent the major successful examples of TKI use in cancer treatment. Other tyrosine kinases have been selected as targets of therapy, but the efficacy of their inhibition, although evident, is less definite. Two major negative effects exist in this therapeutic strategy and are linked to the specificity of the drugs and to the role of the targeted kinase in non-malignant cells. In this review, we will discuss the data available on the TKIs effects on the metabolism and functions of mesenchymal stromal cells (MSCs). MSCs are widely distributed in human tissues and play key physiological roles; nevertheless, they might be responsible for important pathologies. At present, bone marrow (BM) MSCs have been studied in greater detail, for both embryological origins and functions. The available data are evocative of an unexpected degree of complexity and heterogeneity of BM-MSCs. It is conceivable that this grade of intricacy occurs also in MSCs of other organs. Therefore, in perspective, the negative effects of TKIs on MSCs might represent a critical problem in long-term cancer therapies based on such inhibitors.

Glucocorticoids Drive Diurnal Oscillations in T Cell Distribution and Responses by Inducing Interleukin-7 Receptor and CXCR4

Glucocorticoids are steroid hormones with strong anti-inflammatory and immunosuppressive effects that are produced in a diurnal fashion. Although glucocorticoids have the potential to induce interleukin-7 receptor (IL-7R) expression in T cells, whether they control T cell homeostasis and responses at physiological concentrations remains unclear. We found that glucocorticoid receptor signaling induces IL-7R expression in mouse T cells by binding to an enhancer of the IL-7Rα locus, with a peak at midnight and a trough at midday. This diurnal induction of IL-7R supported the survival of T cells and their redistribution between lymph nodes, spleen, and blood by controlling expression of the chemokine receptor CXCR4. In mice, T cell accumulation in the spleen at night enhanced immune responses against soluble antigens and systemic bacterial infection. Our results reveal the immunoenhancing role of glucocorticoids in adaptive immunity and provide insight into how immune function is regulated by the diurnal rhythm.

Quantification of plasma cell dynamics using mathematical modelling

Plasma cells (PCs) are the main antibody-producing cells in humans. They are long-lived so that specific antibodies against either pathogens or vaccines are produced for decades. PC longevity is attributed to specific areas within the bone marrow micro-environment, the so-called 'niche', providing the cells with required growth and survival factors. With antigen encounters, e.g. infection or vaccination, new PCs are generated and home to the bone marrow where they compete with resident PCs for the niche. We propose a parametrized mathematical model describing healthy PC dynamics in the bone marrow. The model accounts for competition for the niche between newly produced PCs owing to vaccination and resident PCs. Mathematical analysis and numerical simulations of the model allow explanation of the recovery of PC homoeostasis after a vaccine-induced perturbation, and the fraction of vaccine-specific PCs inside the niche. The model enables quantification of the niche-related dynamics of PCs, i.e. the duration of PC transition into the niche and the impact of different rates for PC transitions into and out of the niche on the observed cell dynamics. Ultimately, it provides a potential basis for further investigations in health and disease.

The good and bad faces of the CXCR4 chemokine receptor

Chemokines are chemotactic cytokines that promote cell migration and activation under homeostatic and inflammatory conditions. Chemokines bind to seven transmembrane-spanning receptors that are coupled to heterotrimeric guanine nucleotide-binding (G) proteins, which are the responsible for intracellularly transmitting the activating signals for cell migration. Hematopoiesis, vascular development, lymphoid organ morphogenesis, cardiogenesis and neural differentiation are amongst the processes involving chemokine function. In addition, immune cell trafficking from bone marrow to blood circulation, and from blood and lymph to lymphoid and inflamed tissues, is tightly regulated by chemokines both under physiological conditions and also in autoimmune diseases. Furthermore, chemokine binding to their receptors stimulate trafficking to and positioning of cancer cells into target tissues and organs during tumour dissemination. The CXCL12 chemokine (also known as stromal-cell derived factor-1α, SDF-1α) plays key roles in hematopoiesis and lymphoid tissue architecture, in cardiogenesis, vascular formation and neurogenesis, as well as in the trafficking of solid and hematological cancer cell types. CXCL12 binds to the CXCR4 receptor, a multi-facetted molecule which tightly mirrors CXCL12 functions in homeostasis and disease. This review addresses the important roles of the CXCR4-CXCL12 axis in homeostasis, specially focusing in hematopoiesis, as well as it provides a picture of CXCR4 as mediator of cancer cell spreading, and a view of the available CXCR4 antagonists in different cancer types.

ATP-degrading ENPP1 is required for survival (or persistence) of long-lived plasma cells

Survival of antibody-secreting plasma cells (PCs) is vital for sustained antibody production. However, it remains poorly understood how long-lived PCs (LLPCs) are generated and maintained. Here we report that ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is preferentially upregulated in bone marrow LLPCs compared with their splenic short-lived counterparts (SLPCs). We studied ENPP1-deficient mice (Enpp1^−/−) to determine how the enzyme affects PC biology. Although Enpp1^−/− mice generated normal levels of germinal center B cells and plasmablasts in periphery, they produced significantly reduced numbers of LLPCs following immunization with T-dependent antigens or infection with plasmodium C. chabaudi. Bone marrow chimeric mice showed B cell intrinsic effect of ENPP1 selectively on generation of bone marrow as well as splenic LLPCs. Moreover, Enpp1^−/− PCs took up less glucose and had lower levels of glycolysis than those of wild-type controls. Thus, ENPP1 deficiency confers an energetic disadvantage to PCs for long-term survival and antibody production.

Longitudinal intravital imaging of the femoral bone marrow reveals plasticity within marrow vasculature

The bone marrow is a central organ of the immune system, which hosts complex interactions of bone and immune compartments critical for hematopoiesis, immunological memory, and bone regeneration. Although these processes take place over months, most existing imaging techniques allow us to follow snapshots of only a few hours, at subcellular resolution. Here, we develop a microendoscopic multi-photon imaging approach called LIMB (longitudinal intravital imaging of the bone marrow) to analyze cellular dynamics within the deep marrow. The approach consists of a biocompatible plate surgically fixated to the mouse femur containing a gradient refractive index lens. This microendoscope allows highly resolved imaging, repeatedly at the same regions within marrow tissue, over months. LIMB reveals extensive vascular plasticity during bone healing and steady-state homeostasis. To our knowledge, this vascular plasticity is unique among mammalian tissues, and we expect this insight will decisively change our understanding of essential phenomena occurring within the bone marrow.

Longitudinal imaging of bone marrow would shed insight into long-term cellular dynamics within this compartment. Here, the authors develop a multi-photon imaging approach for the mouse femur and reveal extensive vascular plasticity within the bone marrow during bone healing and steady-state homeostasis.

The chronically inflamed central nervous system provides niches for long-lived plasma cells

Although oligoclonal bands in the cerebrospinal fluid have been a hallmark of multiple sclerosis diagnosis for over three decades, the role of antibody-secreting cells in multiple sclerosis remains unclear. T and B cells are critical for multiple sclerosis pathogenesis, but increasing evidence suggests that plasma cells also contribute, through secretion of autoantibodies. Long-lived plasma cells are known to drive various chronic inflammatory conditions as e.g. systemic lupus erythematosus, however, to what extent they are present in autoimmune central nervous system inflammation has not yet been investigated. In brain biopsies from multiple sclerosis patients and other neurological diseases, we could detect non-proliferating plasma cells (CD138⁺Ki67⁻) in the parenchyma. Based on this finding, we hypothesized that long-lived plasma cells can persist in the central nervous system (CNS). In order to test this hypothesis, we adapted the multiple sclerosis mouse model experimental autoimmune encephalomyelitis to generate a B cell memory response. Plasma cells were found in the meninges and the parenchyma of the inflamed spinal cord, surrounded by tissue areas resembling survival niches for these cells, characterized by an up-regulation of chemokines (CXCL12), adhesion molecules (VCAM-1) and survival factors (APRIL and BAFF). In order to determine the lifetime of plasma cells in the chronically inflamed CNS, we labeled the DNA of proliferating cells with 5-ethynyl-2′-deoxyuridine (EdU). Up to five weeks later, we could detect EdU⁺ long-lived plasma cells in the murine CNS. To our knowledge, this is the first study describing non-proliferating plasma cells directly in the target tissue of a chronic inflammation in humans, as well as the first evidence demonstrating the ability of plasma cells to persist in the CNS, and the ability of the chronically inflamed CNS tissue to promote this persistence. Hence, our results suggest that the CNS provides survival niches for long-lived plasma cells, similar to the niches found in other organs. Targeting these cells in the CNS offers new perspectives for treatment of chronic autoimmune neuroinflammatory diseases, especially in patients who do not respond to conventional therapies.

Generation of an osteoblast-based artificial niche that supports in vitro B lymphopoiesis

B lymphocytes are produced from hematopoietic stem cells (HSCs) through the highly ordered process of B lymphopoiesis, which is regulated by a complex network of cytokines, chemokines and cell adhesion molecules derived from the hematopoietic niche. Primary osteoblasts function as an osteoblastic niche (OBN) that supports in vitro B lymphopoiesis. However, there are significant limitations to the use of primary osteoblasts, including their relative scarcity and the consistency and efficiency of the limited purification and proliferation of these cells. Thus, development of a stable osteoblast cell line that can function as a biomimetic or artificial OBN is necessary. In this study, we developed a stable osteoblastic cell line, designated OBN4, which functions as an osteoblast-based artificial niche that supports in vitro B lymphopoiesis. We demonstrated that the production of a B220⁺ cell population from Lineage⁻ (Lin⁻) Sca-1⁺ c-Kit⁺ hematopoietic stem and progenitor cells (HSPCs) was increased ~1.7-fold by OBN4 cells relative to production by primary osteoblasts and OP9 cells in coculture experiments. Consistently, OBN4 cells exhibited the highest production of B220⁺ IgM⁺ cell populations (6.7±0.6–13.6±0.6%) in an IL-7- and stromal cell-derived factor 1-dependent manner, with higher production than primary osteoblasts (3.7±0.5–6.4±0.6%) and OP9 cells (1.8±0.6–3.9±0.5%). In addition, the production of B220⁺ IgM⁺ IgD⁺ cell populations was significantly enhanced by OBN4 cells (15.4±1.1–18.9±3.2%) relative to production by primary osteoblasts (9.5±0.6–14.6±1.6%) and OP9 cells (9.1±0.5–10.3±1.8%). We conclude that OBN4 cells support in vitro B lymphopoiesis of Lin⁻ Sca-1⁺ c-Kit⁺ HSPCs more efficiently than primary osteoblasts or OP9 stromal cells.

One Niche to Rule Both Maintenance and Loss of Stemness in HSCs

Hemato-lymphopoiesis initiated by hematopoietic stem cells (HSCs) is tightly regulated by factors present in the bone marrow (BM) niche. Using genetically modified mice, Gomes et al. (2016) show that IL-7 is produced by HSC niche-forming cells and find that the same niche that controls HSC self-renewal can also support differentiation.

Effects of autologous stromal cells and cytokines on differentiation of equine bone marrow-derived progenitor cells

OBJECTIVE To develop an in vitro system for differentiation of equine B cells from bone marrow hematopoietic progenitor cells on the basis of protocols for other species. SAMPLE Bone marrow aspirates aseptically obtained from 12 research horses. PROCEDURES Equine bone marrow CD34⁺ cells were sorted by use of magnetic beads and cultured in medium supplemented with cytokines (recombinant human interleukin-7, equine interleukin-7, stem cell factor, and Fms-like tyrosine kinase-3), murine OP9 stromal cell preconditioned medium, and equine fetal bone marrow mesenchymal stromal cell preconditioned medium. Cells in culture were characterized by use of flow cytometry, immunocytofluorescence microscopy, and quantitative reverse-transcriptase PCR assay. RESULTS For these culture conditions, bone marrow-derived equine CD34⁺ cells differentiated into CD19⁺IgM⁺ B cells that expressed the signature transcription factors early B-cell factor and transcription factor 3. These conditions also supported the concomitant development of autologous stromal cells, and their presence was supportive of B-cell development. CONCLUSIONS AND CLINICAL RELEVANCE Equine B cells were generated from bone marrow aspirates by use of supportive culture conditions. In vitro generation of equine autologous B cells should be of use in studies on regulation of cell differentiation and therapeutic transplantation.

Ephrin ligands and Eph receptors contribution to hematopoiesis

Hematopoietic stem and progenitor cells reside predominantly in the bone marrow. They supply billions of mature blood cells every day during life through maturation into multilineage progenitors and self-renewal. Newly produced mature cells serve to replenish the pool of circulating blood cells at the end of their life-span. These mature blood cells and a few hematopoietic progenitors normally exit the bone marrow through the sinusoidal vessels, a specialized venous vascular system that spreads throughout the bone marrow. Many signals regulate the coordinated mobilization of hematopoietic cells from the bone marrow to the circulation. In this review, we present recent advances on hematopoiesis and hematopoietic cell mobilization with a focus on the role of Ephrin ligands and their Eph receptors. These constitute a large family of transmembrane ligands and receptors that play critical roles in development and postnatally. New insights point to distinct roles of ephrin and Eph in different aspects of hematopoiesis.

IL-17 Promotes Differentiation of Splenic LSK- Lymphoid Progenitors into B Cells following Plasmodium yoelii Infection

Lineage^-Sca-1⁺c-Kit^- (LSK^-) cells are a lymphoid progenitor population that expands in the spleen and preferentially differentiates into mature B cells in response to Plasmodium yoelii infection in mice. Furthermore, LSK^- derived B cells can subsequently contribute to the ongoing immune response through the generation of parasite-specific Ab-secreting cells, as well as germinal center and memory B cells. However, the factors that promote their differentiation into B cells in the spleen postinfection are not defined. In this article, we show that LSK^- cells produce the cytokine IL-17 in response to Plasmodium infection. Using Il-17ra^-/- mice, IL-17R signaling in cells other than LSK^- cells was found to support their differentiation into B cells. Moreover, primary splenic stromal cells grown in the presence of IL-17 enhanced the production of CXCL12, a chemokine associated with B cell development in the bone marrow, by a population of IL-17RA-expressing podoplanin⁺CD31^- stromal cells, a profile associated with fibroblastic reticular cells. Subsequent blockade of CXCL12 in vitro reduced differentiation of LSK^- cells into B cells, supporting a direct role for this chemokine in this process. Immunofluorescence indicated that podoplanin⁺ stromal cells in the red pulp were the primary producers of CXCL12 after P. yoelii infection. Furthermore, podoplanin staining on stromal cells was more diffuse, and CXCL12 staining was dramatically reduced in Il-17ra^-/- mice postinfection. Together, these results identify a distinct pathway that supports lymphoid development in the spleen during acute Plasmodium infection.

V(D)J Recombination Exploits DNA Damage Responses to Promote Immunity

It has been recognized for 40 years that the variable (diversity) joining [V(D)J] recombination-mediated assembly of diverse B and T lymphocyte antigen receptor (AgR) genes is not only essential for adaptive immunity, but also a risk for autoimmunity and lymphoid malignancies. Over the past few years, several studies have revealed that recombination-activating gene (RAG) endonuclease-induced DNA double-strand breaks (DSBs) transcend hazardous intermediates during antigen receptor gene assembly. RAG cleavage within the genomes of lymphocyte progenitors and immature lymphocytes regulates the expression of ubiquitous and lymphocyte-specific gene transcripts to control the differentiation and function of both adaptive and innate immune cell lineages. These unexpected discoveries raise important new questions that have broad implications for basic immunology research and the screening, diagnosis, and treatment of human immunological disease.

Effects of Mesenchymal Stem Cell Derivatives on Hematopoiesis and Hematopoietic Stem Cells

Hematopoiesis is a balance among quiescence, self-renewal, proliferation, and differentiation, which is believed to be firmly adjusted through interactions between hematopoietic stem and progenitor cells (HSPCs) with the microenvironment. This microenvironment is derived from a common progenitor of mesenchymal origin and its signals should be capable of regulating the cellular memory of transcriptional situation and lead to an exchange of stem cell genes expression. Mesenchymal stem cells (MSCs) have self-renewal and differentiation capacity into tissues of mesodermal origin, and these cells can support hematopoiesis through release various molecules that play a crucial role in migration, homing, self-renewal, proliferation, and differentiation of HSPCs. Studies on the effects of MSCs on HSPC differentiation can develop modern solutions in the treatment of patients with hematologic disorders for more effective Bone Marrow (BM) transplantation in the near future. However, considerable challenges remain on realization of how paracrine mechanisms of MSCs act on the target tissues, and how to design a therapeutic regimen with various paracrine factors in order to achieve optimal results for tissue conservation and regeneration. The aim of this review is to characterize and consider the related aspects of the ability of MSCs secretome in protection of hematopoiesis.

Early Emergence of CD19-Negative Human Antibody-Secreting Cells at the Plasmablast to Plasma Cell Transition

Long-lived human plasma cells (PCs) play central roles in immunity and autoimmunity and are enriched among the subpopulation of CD19neg human PCs. However, whether human CD19neg PCs are necessarily aged cells that have gradually lost CD19 expression is not known. Assessing peripheral blood samples at steady-state and during the acute response to influenza vaccination in healthy donors, we identify the presence of phenotypic CD19neg plasmablasts, the proliferative precursor state to mature PCs, and demonstrate by ELISPOT that these are Ab-secreting cells (ASCs). During the acute response to influenza vaccination, CD19pos, CD19low, and CD19neg ASCs secrete vaccine-specific Abs and show linked IGHV repertoires. To address precursor/product relationships, we use in vitro models that mimic T-dependent and T-independent differentiation, finding that the CD19neg state can be established at the plasmablast to PC transition, that CD19neg PCs increase as a percentage of surviving PCs in vitro, and that CD19neg and CD19pos PCs can be maintained independently. These data provide proof-of-principle for the view that newly generated ASCs can acquire a mature PC phenotype that is accompanied by loss of CD19 expression at an early stage of differentiation and that aging is not an obligate requirement for a CD19neg state to be established.

Adult haematopoietic stem cell niches

Stem cell niches are specialized microenvironments that promote the maintenance of stem cells and regulate their function. Recent advances have improved our understanding of the niches that maintain adult haematopoietic stem cells (HSCs). These advances include new markers for HSCs and niche cells, systematic analyses of the expression patterns of niche factors, genetic tools for functionally identifying niche cells in vivo, and improved imaging techniques. Together, they have shown that HSC niches are perivascular in the bone marrow and spleen. Endothelial cells and mesenchymal stromal cells secrete factors that promote HSC maintenance in these niches, but other cell types also directly or indirectly regulate HSC niches.

Impact of CXCR4/CXCL12 Blockade on Normal Plasma Cells In Vivo

Plasma cells (PCs) are a major source of alloantibody in transplant patients and are resistant to current therapy. Because receptor-ligand interactions in stromal microenvironments play important roles in the localization, development, and survival of normal PCs, we hypothesized that interfering with CXCR4/CXCL12 interactions with plerixafor might cause PC depletion and enhance the efficacy of the proteasome inhibitor bortezomib. PCs in mouse spleen, bone marrow, and peripheral blood demonstrated CXCR4 expression. We then treated with plerixafor in doses ranging from 240 μg/kg in a single dose to a 1-mg/kg daily dose for 10 days. CXCR4/CXCL12 blockade with plerixafor resulted in increased mobilization of PCs into the peripheral blood. Splenectomy completely abrogated this effect, suggesting that all plerixafor-mobilized cells were from the spleen. The total number of PCs in the spleen and marrow remained constant despite treatment with plerixafor. Bortezomib caused a reduction in PCs, but adding plerixafor did not increase killing. We conclude that CXCR4/CXCL12 interactions are important for the retention of a subpopulation of PCs in the spleen, but this interaction has minimal effect on PCs in the marrow. The lack of enhancement of bortezomib-mediated depletion suggests that factors other than CXCR4/CXCL12 interactions are responsible for drug resistance.

The osteoblastic niche in hematopoiesis and hematological myeloid malignancies

PURPOSE OF REVIEW

This review focuses on evidence highlighting the bidirectional crosstalk between the hematopoietic stem cell (HSC) and their surrounding stromal cells, with a particular emphasis on cells of the osteoblast lineage. The role and molecular functions of osteoblasts in normal hematopoiesis and in myeloid hematological malignancies is discussed.

RECENT FINDINGS

Cells of the osteoblast lineage have emerged as potent regulators of HSC expansion that regulate their recruitment and, depending on their stage of differentiation, their activity, proliferation and differentiation along the lymphoid, myeloid and erythroid lineages. In addition, mutations in mature osteoblasts or their progenitors induce myeloid malignancies. Conversely, signals from myelodysplastic cells can remodel the osteoblastic niche to favor self-perpetuation.

SUMMARY

Understanding cellular crosstalk between osteoblastic cells and HSCs in the bone marrow microenvironment is of fundamental importance for developing therapies against benign and malignant hematological diseases.

Bone Marrow Graft-Versus-Host Disease in Major Histocompatibility Complex-Matched Murine Reduced-Intensity Allogeneic Hemopoietic Cell Transplantation

BACKGROUND

Most clinical allogeneic hemopoietic cell transplants (alloHCT) are now performed using reduced-intensity conditioning (RIC) instead of myeloablative conditioning (MAC); however, the biology underlying this treatment remains incompletely understood.

METHODS

We investigated a murine model of major histocompatibility complex-matched multiple minor histocompatibility antigen-mismatched alloHCT using bone marrow (BM) cells and splenocytes from B6 (H-2) donor mice transplanted into BALB.B (H-2) recipients after RIC with fludarabine of 100 mg/kg per day for 5 days, cyclophosphamide of 60 mg/kg per day for 2 days, and total body irradiation (TBI).

RESULTS

The lowest TBI dose capable of achieving complete donor chimerism in this mouse strain combination was 325 cGy given as a single fraction. Mice that underwent RIC had a reduced incidence and delayed onset of graft-versus-host disease (GVHD) and significantly prolonged survival compared with MAC-transplanted recipients (TBI of 850 cGy plus cyclophosphamide of 60 mg/kg per day for 2 days). Compared with syngeneic controls, RIC mice with GVHD showed evidence of BM suppression, have anemia, reduced BM cellularity, and showed profound reduction in BM B cell lymphopoiesis associated with damage to the endosteal BM niche. This was associated with an increase in BM CD8 effector T cells in RIC mice and elevated blood and BM plasma levels of T helper1 cytokines. Increasing doses of splenocytes resulted in increased incidence of GVHD in RIC mice.

CONCLUSIONS

We demonstrate that the BM is a major target organ of GVHD in an informative clinically relevant RIC mouse major histocompatibility complex-matched alloHCT model by a process that seems to be driven by CD8 effector T cells.

A Chemoattractant-Guided Walk Through Lymphopoiesis: From Hematopoietic Stem Cells to Mature B Lymphocytes

B lymphocytes develop from hematopoietic stem cells (HSCs) in specialized bone marrow niches composed of rare mesenchymal lineage stem/progenitor cells (MSPCs) and sinusoidal endothelial cells. These niches are defined by function and location: MSPCs are mostly perisinusoidal cells that together with a small subset of sinusoidal endothelial cells express stem cell factor, interleukin-7 (IL-7), IL-15, and the highest amounts of CXCL12 in bone marrow. Though rare, MSPCs are morphologically heterogeneous, highly reticular, and form a vast cellular network in the bone marrow parenchyma capable of interacting with large numbers of hematopoietic cells. HSCs, downstream multipotent progenitor cells, and common lymphoid progenitor cells utilize CXCR4 to fine-tune access to critical short-range growth factors provided by MSPCs for their long-term maintenance and/or multilineage differentiation. In later stages, developing B lymphocytes use CXCR4 to navigate the bone marrow parenchyma, and predominantly cannabinoid receptor-2 for positioning within bone marrow sinusoids, prior to being released into peripheral blood circulation. In the final stages of differentiation, transitional B cells migrate to the spleen where they preferentially undergo further rounds of differentiation until selection into the mature B cell pool occurs. This bottleneck purges up to 97% of all developing B cells in a peripheral selection process that is heavily controlled not only by the intensity of BCR signaling and access to BAFF but also by the proper functioning of the B cell motility machinery.

T Regulatory Cells Support Plasma Cell Populations in the Bone Marrow

Long-lived plasma cells (PCs) in the bone marrow (BM) are a critical source of antibodies after infection or vaccination, but questions remain about the factors that control PCs. We found that systemic infection alters the BM, greatly reducing PCs and regulatory T (Treg) cells, a population that contributes to immune privilege in the BM. The use of intravital imaging revealed that BM Treg cells display a distinct behavior characterized by sustained co-localization with PCs and CD11c-YFP+ cells. Gene expression profiling indicated that BM Treg cells express high levels of Treg effector molecules, and CTLA-4 deletion in these cells resulted in elevated PCs. Furthermore, preservation of Treg cells during systemic infection prevents PC loss, while Treg cell depletion in uninfected mice reduced PC populations. These studies suggest a role for Treg cells in PC biology and provide a potential target for the modulation of PCs during vaccine-induced humoral responses or autoimmunity.

Brucella and Osteoarticular Cell Activation: Partners in Crime

Osteoarticular brucellosis is the most common presentation of human active disease although its prevalence varies widely. The three most common forms of osteoarticular involvement are sacroiliitis, spondylitis, and peripheral arthritis. The molecular mechanisms implicated in bone damage have been recently elucidated. B. abortus induces bone damage through diverse mechanisms in which TNF-α and the receptor activator of nuclear factor kappa-B ligand (RANKL)-the natural modulator of bone homeostasis are involved. These processes are driven by inflammatory cells, like monocytes/macrophages, neutrophils, Th17 CD4⁺ T, and B cells. In addition, Brucella abortus has a direct effect on osteoarticular cells and tilts homeostatic bone remodeling. These bacteria inhibit bone matrix deposition by osteoblasts (the only bone cells involved in bone deposition), and modify the phenotype of these cells to produce matrix metalloproteinases (MMPs) and cytokine secretion, contributing to bone matrix degradation. B. abortus also affects osteoclasts (cells naturally involved in bone resorption) by inducing an increase in osteoclastogenesis and osteoclast activation; thus, increasing mineral and organic bone matrix resorption, contributing to bone damage. Given that the pathology induced by Brucella species involved joint tissue, experiments conducted on synoviocytes revealed that besides inducing the activation of these cells to secrete chemokines, proinflammatory cytokines and MMPS, the infection also inhibits synoviocyte apoptosis. Brucella is an intracellular bacterium that replicates preferentially in the endoplasmic reticulum of macrophages. The analysis of B. abortus-infected synoviocytes indicated that bacteria also replicate in their reticulum suggesting that they could use this cell type for intracellular replication during the osteoarticular localization of the disease. Finally, the molecular mechanisms of osteoarticular brucellosis discovered recently shed light on how the interaction between B. abortus and immune and osteoarticular cells may play an important role in producing damage in joint and bone.

Regulating dynamic signaling between hematopoietic stem cells and niche cells via a hydrogel matrix

Hematopoietic stem cells (HSC) reside in unique bone marrow niches and are influenced by signals from surrounding cells, the extracellular matrix (ECM), ECM-bound or diffusible biomolecules. Here we describe the use of a three-dimensional hydrogel to alter the balance of HSC-generated autocrine feedback and paracrine signals generated by co-cultured niche-associated cells. We report shifts in HSC proliferation rate and fate specification in the presence of lineage positive (Lin⁺) niche cells. Hydrogels promoting autocrine feedback enhanced expansion of early hematopoietic progenitors while paracrine signals from Lin⁺ cells increased myeloid differentiation. We report thresholds where autocrine vs. paracrine cues alter HSC fate transitions, and were able to selectively abrogate the effects of matrix diffusivity and niche cell co-culture via the use of inhibitory cocktails of autocrine or paracrine signals. Together, these results suggest diffusive biotransport in three-dimensional biomaterials are a critical design element for the development of a synthetic stem cell niche.

The hematopoietic stem-cell niche in health and leukemia

Research in the last decade has shown that hematopoietic stem cells (HSCs) interact with and are modulated by a complex multicellular microenvironment in the bone marrow, which includes both the HSC progeny and multiple non-hematopoietic cell types. Intense work is gradually throwing light on the composition of the HSC niche and the molecular cues exchanged between its components, which has implications for HSC production, maintenance and expansion. In addition, it has become apparent that bidirectional interactions between leukemic cells and their niche play a previously unrecognized role in the initiation and development of hematological malignancies. Consequently, targeting of the malignant niche holds considerable promise for more specific antileukemic therapies. Here we summarize the latest insights into HSC niche biology and recent work showing multiple connections between hematological malignancy and alterations in the bone marrow microenvironment.

The Role of the Bone Marrow Stromal Compartment in the Hematopoietic Response to Microbial Infections

Continuous production of blood cells unfolds within a complex three-dimensional tissue scaffold established by highly organized stromal cell networks of mesenchymal, neural, and vascular origin inside bone marrow (BM) cavities. Collectively, stromal cells have been shown to serve two principal roles; first as primary participants of bone remodeling and metabolism and second as master regulators of different stages of blood cell development and production. Indeed, ample evidence demonstrates that stromal cells can sense and integrate systemic signals to shape hematopoietic responses and that these regulatory mechanisms are subverted in multiple pathologic conditions. Microbial infections are stressors that elicit potent inflammatory reactions and induce substantial alterations of hematopoietic output. Whether the cellular components of the BM stromal microenvironment are targeted by infections and participate in infection-induced hematopoiesis has not been investigated in sufficient detail to date. In this manuscript, we provide a succinct updated overview of the different cell populations that are currently known to form BM stroma. We discuss experimental evidence demonstrating that different stromal components are actively damaged or functionally altered by pathogens and/or ensuing inflammatory signals and review how these effects are known to contribute to the hematologic manifestations observed during infections.

"Inflamm-aging" influences immune cell survival factors in human bone marrow

The bone marrow (BM) plays a key role in the long-term maintenance of immunological memory. However, the impact of aging on the production of survival factors for effector/memory T cells and plasma cells in the human BM has not been studied. We now show that the expression of molecules involved in the maintenance of immunological memory in the human BM changes with age. While IL-15, which protects potentially harmful CD8⁺ CD28^- senescent T cells, increases, IL-7 decreases. IL-6, which may synergize with IL-15, is also overexpressed. In contrast, a proliferation-inducing ligand, a plasma cell survival factor, is reduced. IFN-y, TNF, and ROS accumulate in the BM in old age. IL-15 and IL-6 expression are stimulated by IFN-y and correlate with ROS levels in BM mononuclear cells. Both cytokines are reduced by incubation with the ROS scavengers N-acetylcysteine and vitamin C. IL-15 and IL-6 are also overexpressed in the BM of superoxide dismutase 1 knockout mice compared to their WT counterparts. In summary, our results demonstrate the role of inflammation and oxidative stress in age-related changes of immune cell survival factors in the BM, suggesting that antioxidants may be beneficial in counteracting immunosenescence by improving immunological memory in old age.

Vaccination to gain humoral immune memory

The concept of immune memory forms the biological basis for vaccination programs. Despite advancements in the field of immune memory and vaccination, most current vaccines are evaluated by magnitude of antigen-specific antibody titers in serum or mucosa after vaccination. It has been shown, however, that antibody-mediated humoral immune memory is established regardless of the magnitude and duration of immune reactions, suggesting that assessment of vaccine efficacy should be performed for several years after vaccination. This long-term investigation is disadvantageous for prevalent and pandemic infections. Long-lived memory plasma cells and memory helper T cells which contribute to humoral immune memory are generated in the bone marrow after migration of memory cell precursors through bloodstream. Thus, it may be a novel evaluation strategy to assess the precursors of memory cells in the blood in the early phase of the immune reaction(s). We here review recent advances on the generation and maintenance of immune memory cells involved in humoral immunity and introduce a current concept of direct and short-term assessment of humoral immune memory formation upon vaccination as a correlate of protection.

Cellular players of hematopoietic stem cell mobilization in the bone marrow niche

Hematopoietic stem cells (HSC) reside in perivascular regions of the bone marrow (BM) embedded within a complex regulatory unit called the niche. Cellular components of HSC niches include vascular endothelial cells, mesenchymal stromal progenitor cells and a variety of mature hematopoietic cells such as macrophages, neutrophils, and megakaryocytes-further regulated by sympathetic nerves and complement components as described in this review. Three decades ago the discovery that cytokines induce a large number of HSC to mobilize from the BM into the blood where they are easily harvested, revolutionised the field of HSC transplantation-curative for immune-deficiencies and some malignancies. However, despite now routine use of granulocyte-colony stimulating factor (G-CSF) to mobilise HSC for transplant, only in last 15 years has research on the mechanisms behind why and how HSC can be induced to move into the blood began. These studies have revealed the complexity of the niche that retains HSC in the BM. This review describes how BM niches and HSC themselves change during administration of G-CSF-or in the recovery phase of chemotherapy-to facilitate movement of HSC into the blood, and research now leading to development of novel therapeutics to further boost HSC mobilization and transplant success.

Hematopoietic Stem Cell Niches Produce Lineage-Instructive Signals to Control Multipotent Progenitor Differentiation

Hematopoietic stem cells (HSCs) self-renew in bone marrow niches formed by mesenchymal progenitors and endothelial cells expressing the chemokine CXCL12, but whether a separate niche instructs multipotent progenitor (MPP) differentiation remains unclear. We show that MPPs resided in HSC niches, where they encountered lineage-instructive differentiation signals. Conditional deletion of the chemokine receptor CXCR4 in MPPs reduced differentiation into common lymphoid progenitors (CLPs), which decreased lymphopoiesis. CXCR4 was required for CLP positioning near Interleukin-7⁺ (IL-7) cells and for optimal IL-7 receptor signaling. IL-7⁺ cells expressed CXCL12 and the cytokine SCF, were mesenchymal progenitors capable of differentiation into osteoblasts and adipocytes, and comprised a minor subset of sinusoidal endothelial cells. Conditional Il7 deletion in mesenchymal progenitors reduced B-lineage committed CLPs, while conditional Cxcl12 or Scf deletion from IL-7⁺ cells reduced HSC and MPP numbers. Thus, HSC maintenance and multilineage differentiation are distinct cell lineage decisions that are both controlled by HSC niches.

Targeting of Mesenchymal Stromal Cells by Cre-Recombinase Transgenes Commonly Used to Target Osteoblast Lineage Cells

The targeting specificity of tissue-specific Cre-recombinase transgenes is a key to interpreting phenotypes associated with their use. The Ocn-Cre and Dmp1-Cre transgenes are widely used to target osteoblasts and osteocytes, respectively. Here, we used high-resolution microscopy of bone sections and flow cytometry to carefully define the targeting specificity of these transgenes. These transgenes were crossed with Cxcl12gfp mice to identify Cxcl12-abundant reticular (CAR) cells, which are a perivascular mesenchymal stromal population implicated in hematopoietic stem/progenitor cell maintenance. We show that in addition to osteoblasts, Ocn-Cre targets a majority of CAR cells and arteriolar pericytes. Surprisingly, Dmp1-Cre also targets a subset of CAR cells, in which expression of osteoblast-lineage genes is enriched. Finally, we introduce a new tissue-specific Cre-recombinase, Tagln-Cre, which efficiently targets osteoblasts, a majority of CAR cells, and both venous sinusoidal and arteriolar pericytes. These data show that Ocn-Cre and Dmp1-Cre target broader stromal cell populations than previously appreciated and may aid in the design of future studies. Moreover, these data highlight the heterogeneity of mesenchymal stromal cells in the bone marrow and provide tools to interrogate this heterogeneity. © 2016 American Society for Bone and Mineral Research.

Bone Marrow Mesenchymal Stem Cells Enhance the Differentiation of Human Switched Memory B Lymphocytes into Plasma Cells in Serum-Free Medium

The differentiation of human B lymphocytes into plasma cells is one of the most stirring questions with regard to adaptive immunity. However, the terminal differentiation and survival of plasma cells are still topics with much to be discovered, especially when targeting switched memory B lymphocytes. Plasma cells can migrate to the bone marrow in response to a CXCL12 gradient and survive for several years while secreting antibodies. In this study, we aimed to get closer to niches favoring plasma cell survival. We tested low oxygen concentrations and coculture with mesenchymal stem cells (MSC) from human bone marrow. Besides, all cultures were performed using an animal protein-free medium. Overall, our model enables the generation of high proportions of CD38⁺CD138⁺CD31⁺ plasma cells (≥50%) when CD40-activated switched memory B lymphocytes were cultured in direct contact with mesenchymal stem cells. In these cultures, the secretion of CXCL12 and TGF-β, usually found in the bone marrow, was linked to the presence of MSC. The level of oxygen appeared less impactful than the contact with MSC. This study shows for the first time that expanded switched memory B lymphocytes can be differentiated into plasma cells using exclusively a serum-free medium.

Eosinophils in Helminth Infection: Defenders and Dupes

Eosinophilia is a central feature of the host response to helminth infection. Larval stages of parasitic worms are killed in vitro by eosinophils in the presence of specific antibodies or complement. These findings established host defense as the paradigm for eosinophil function. Recently, studies in eosinophil-ablated mouse strains have revealed an expanded repertoire of immunoregulatory functions for this cell. Other reports document crucial roles for eosinophils in tissue homeostasis and metabolism, processes that are central to the establishment and maintenance of parasitic worms in their hosts. In this review, we summarize current understanding of the significance of eosinophils at the host-parasite interface, highlighting their distinct functions during primary and secondary exposure.

Concise Review: The Malignant Hematopoietic Stem Cell Niche

Hematopoietic stem cell (HSC) proliferation, self-renewal, and trafficking are dependent, in part, upon signals generated by stromal cells in the bone marrow. Stromal cells are organized into niches that support specific subsets of hematopoietic progenitors. There is emerging evidence that malignant hematopoietic cells may generate signals that alter the number and/or function of specific stromal cell populations in the bone marrow. At least in some cases, the resulting alterations in the bone marrow microenvironment confer a competitive advantage to the malignant HSC and progenitor cells and/or render them less sensitive to chemotherapy. Targeting these signals represents a promising therapeutic strategy for selected hematopoietic malignancies. In this review, we focus on two questions. How do alterations in bone marrow stromal cells arise in hematopoietic malignancies, and how do they contribute to disease pathogenesis? Stem Cells 2017;35:3-8.

Modeling the Pro-inflammatory Tumor Microenvironment in Acute Lymphoblastic Leukemia Predicts a Breakdown of Hematopoietic-Mesenchymal Communication Networks

Lineage fate decisions of hematopoietic cells depend on intrinsic factors and extrinsic signals provided by the bone marrow microenvironment, where they reside. Abnormalities in composition and function of hematopoietic niches have been proposed as key contributors of acute lymphoblastic leukemia (ALL) progression. Our previous experimental findings strongly suggest that pro-inflammatory cues contribute to mesenchymal niche abnormalities that result in maintenance of ALL precursor cells at the expense of normal hematopoiesis. Here, we propose a molecular regulatory network interconnecting the major communication pathways between hematopoietic stem and progenitor cells (HSPCs) and mesenchymal stromal cells (MSCs) within the BM. Dynamical analysis of the network as a Boolean model reveals two stationary states that can be interpreted as the intercellular contact status. Furthermore, simulations describe the molecular patterns observed during experimental proliferation and activation. Importantly, our model predicts instability in the CXCR4/CXCL12 and VLA4/VCAM1 interactions following microenvironmental perturbation due by temporal signaling from Toll like receptors (TLRs) ligation. Therefore, aberrant expression of NF-κB induced by intrinsic or extrinsic factors may contribute to create a tumor microenvironment where a negative feedback loop inhibiting CXCR4/CXCL12 and VLA4/VCAM1 cellular communication axes allows for the maintenance of malignant cells.

MEDI-551 Treatment Effectively Depletes B Cells and Reduces Serum Titers of Autoantibodies in Mice Transgenic for Sle1 and Human CD19

OBJECTIVE

To evaluate treatment with MEDI-551, a humanized anti-human CD19 monoclonal antibody, in a model of autoimmunity involving mice transgenic (Tg) for Sle1 and human CD19 (hCD19).

METHODS

Sle1.hCD19-Tg mice were given either a single intravenous dose of MEDI-551 or repeated doses of MEDI-551 biweekly for up to 12 weeks. The numbers of B cells in the blood, spleen, and bone marrow were determined by flow cytometry assay. In the spleen and bone marrow, the number of IgM- and IgG-specific antibody-secreting cells (ASCs) and the number of ASCs specific for anti-double-stranded DNA (anti-dsDNA) were determined by enzyme-linked immunospot assay. Serum autoantibody and total immunoglobulin levels were determined by enzyme-linked immunosorbent assay, and levels of inflammatory proteins were tested using a multianalyte profiling platform.

RESULTS

MEDI-551 treatment of Sle1.hCD19-Tg mice resulted in effective and sustained B cell depletion throughout the duration of the experiment. The frequency of IgM and IgG ASCs in the spleen was reduced by ≥90%, whereas in the bone marrow, the total ASC frequency was not changed. Levels of autoantibodies specific for dsDNA as well as antihistone and antinuclear antibodies were each reduced by 40-80%, but total serum immunoglobulin levels were largely unchanged at the end of 12 weeks of treatment.

CONCLUSION

These findings highlight the ability of MEDI-551 to deplete B cells and ASCs in autoimmune Sle1.hCD19-Tg mice. MEDI-551 treatment resulted in a robust reduction of autoantibodies but had minimal effect on total serum immunoglobulins. Thus, the novel ability of MEDI-551 to remove a broad range of B cells as well as to lower most disease-driving autoantibodies in an autoimmune disease mouse model warrants continued research. Several clinical studies to explore the safety and activity of MEDI-551 in autoantibody-associated autoimmune diseases are ongoing.

Identification of Stage-Specific Surface Markers in Early B Cell Development Provides Novel Tools for Identification of Progenitor Populations

Whereas the characterization of B lymphoid progenitors has been facilitated by the identification of lineage- and stage-specific surface markers, the continued identification of differentially expressed proteins increases our capacity to explore normal and malignant B cell development. To identify novel surface markers with stage-specific expression patterns, we explored the reactivity of CD19(+) B cell progenitor cells to Abs targeted to 176 surface proteins. Markers with stage-specific expression were identified using a transgenic reporter gene system subdividing the B cell progenitors into four surface IgM(-) stages. This approach affirmed the utility of known stage-specific markers, as well as identifying additional proteins that selectively marked defined stages of B cell development. Among the stage-specific markers were the cell adhesion proteins CD49E, CD11A, and CD54 that are highly expressed selectively on the most immature progenitors. This work identifies a set of novel stage-specific surface markers that can be used as a complement to the classical staining protocols to explore B lymphocyte development.

Distinctive Mesenchymal-Parenchymal Cell Pairings Govern B Cell Differentiation in the Bone Marrow

Bone marrow niches for hematopoietic progenitor cells are not well defined despite their critical role in blood homeostasis. We previously found that cells expressing osteocalcin, a marker of mature osteolineage cells, regulate the production of thymic-seeding T lymphoid progenitors. Here, using a selective cell deletion strategy, we demonstrate that a subset of mesenchymal cells expressing osterix, a marker of bone precursors in the adult, serve to regulate the maturation of early B lymphoid precursors by promoting pro-B to pre-B cell transition through insulin-like growth factor 1 (IGF-1) production. Loss of Osx⁺ cells or Osx-specific deletion of IGF-1 led to a failure of B cell maturation and the impaired adaptive immune response. These data highlight the notion that bone marrow is a composite of specialized niches formed by pairings of specific mesenchymal cells with parenchymal stem or lineage committed progenitor cells, thereby providing distinctive functional units to regulate hematopoiesis.

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Loss of Osx⁺ osteolineage cells halted B cell maturation and caused immune failure

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Mice with Osx⁺ cell-specific deletion of IGF-1 phenocopied Osx⁺ cell ablated mice

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Osx⁺ cell promotes pro-B to pre-B cell transition through IGF-1 production

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Specialized niche cell-hematopoietic progenitor pairings regulate hematopoiesis

Two Niches in the Bone Marrow: A Hypothesis on Life-long T Cell Memory

The concept is emerging that the bone marrow (BM) sustains life-long persistence of memory T cells, as it does for plasma cells. Recent studies revived the debate on how this is achieved: is the BM essentially a nest for the proliferation of recirculating memory T cells, or a storage depot for resting memory T cells? Learning from division of labor in hematopoietic stem cells, this article proposes that two distinct BM niches support memory T cell cycling and quiescence, thereby enabling memory T cells to maintain all their distinguishing features. This framework might be instrumental to interpret some puzzling findings and conceptualize the mechanisms preserving either stability of memory T cell numbers or the capacity to mount secondary responses.