Identification of the earliest B lineage stage in mouse bone marrow

BAFF induces CXCR5 expression during B cell differentiation in bone marrow

B cell activating factor (BAFF) plays an important role in antibody production through differentiation and maturation of B cells mainly in secondary lymphoid organs. On the other hand, the role of BAFF in the bone marrow, the primary lymphoid organ of B cell development, has not been well elucidated. Here, effects of BAFF in bone marrow B cell development were examined by using BAFF-deficient mice. When mRNA expression levels of B cell differentiation markers including Cd19, Bcl2, Igμ, Il7r and Cxcr5 were compared between bone marrow of wild-type and BAFF-KO mice, a lower level of Cxcr5 expression was found in the KO mice. Additionally, protein expression of CXCR5 on IgM⁺ cells in the bone marrow was decreased by BAFF deficiency. In vitro studies also confirmed the effect of BAFF on CXCR5 by IgM⁺ cells; culturing bone marrow cells from BAFF-KO mice with BAFF in vitro increased the proportion of CXCR5⁺ cells in IgM⁺ cells compared with non-treated bone marrow cells. In addition, BAFF synergized with TNF-α and IL-6 to increase the expression of CXCR5⁺ on IgM⁺ cells. The BAFF-mediated up-regulation of CXCR5 expression was reproduced by using CD19⁺ cells purified from BAFF-KO bone marrow cells, suggesting that BAFF directly affects B-lineage cells in bone marrow to promote CXCR5 expression. Together, this study suggests that BAFF has an important role in B cell differentiation in bone marrow by directly inducing CXCR5 expression which affect their migration to secondary lymphoid organs.

Large-scale antibody immune response mapping of splenic B cells and bone marrow plasma cells in a transgenic mouse model

Molecular characterization of antibody immunity and human antibody discovery is mainly carried out using peripheral memory B cells, and occasionally plasmablasts, that express B cell receptors (BCRs) on their cell surface. Despite the importance of plasma cells (PCs) as the dominant source of circulating antibodies in serum, PCs are rarely utilized because they do not express surface BCRs and cannot be analyzed using antigen-based fluorescence-activated cell sorting. Here, we studied the antibodies encoded by the entire mature B cell populations, including PCs, and compared the antibody repertoires of bone marrow and spleen compartments elicited by immunization in a human immunoglobulin transgenic mouse strain. To circumvent prior technical limitations for analysis of plasma cells, we applied single-cell antibody heavy and light chain gene capture from the entire mature B cell repertoires followed by yeast display functional analysis using a cytokine as a model immunogen. We performed affinity-based sorting of antibody yeast display libraries and large-scale next-generation sequencing analyses to follow antibody lineage performance, with experimental validation of 76 monoclonal antibodies against the cytokine antigen that identified three antibodies with exquisite double-digit picomolar binding affinity. We observed that spleen B cell populations generated higher affinity antibodies compared to bone marrow PCs and that antigen-specific splenic B cells had higher average levels of somatic hypermutation. A degree of clonal overlap was also observed between bone marrow and spleen antibody repertoires, indicating common origins of certain clones across lymphoid compartments. These data demonstrate a new capacity to functionally analyze antigen-specific B cell populations of different lymphoid organs, including PCs, for high-affinity antibody discovery and detailed fundamental studies of antibody immunity.

Exploring the reprogramming potential of B cells and comprehending its clinical and therapeutic perspective

Initiating from multipotent progenitors, the lineages extrapolated from hematopoietic stem cells are determined by transcription factors specific to each of them. The commitment factors assist in the differentiation of progenitor cells into terminally differentiated cells. B lymphocytes constitute a population of cells that expresses clonally diverse cell surface immunoglobulin (Ig) receptors specific to antigenic epitopes. B cells are a significant facet of the adaptive immune system. The secreted antibodies corresponding to the B cell recognize the antigens via the B cell receptor (BCR). Following antigen recognition, the B cell is activated and thereafter undergoes clonal expansion and proliferation to become memory B cells. The essence of 'cellular reprogramming' has aided in reliably altering the cells to desired tissue type. The potential of reprogramming has been harnessed to decipher and find solutions for various genetically inherited diseases and degenerative disorders. B lymphocytes can be reprogrammed to their initial naive state from where they get differentiated into any lineage or cell type similar to a pluripotent stem cell which can be accomplished by the deletion of master regulators of the B cell lineage. B cells can be reprogrammed into pluripotent stem cells and also can undergo transdifferentiation at the midway of cell differentiation to other cell types. Mandated expression of C/EBP in specialized B cells corresponds to their fast and effective reprogramming into macrophages, reversing the cell fate of these lymphocytes and allowing them to differentiate freshly into other types of cells. The co-expression of C/EBPα and OKSM (Oct4, Sox2, Klf4, c-Myc) amplified the reprogramming efficiency of B lymphocytes. Various human somatic cells including the immune cells are compliant to reprogramming which paves a path for opportunities like autologous tissue grafts, blood transfusion, and cancer immunotherapy. The ability to reprogram B cells offers an unprecedented opportunity for developing a therapeutic approach for several human diseases. Here, we will focus on all the proteins and transcription factors responsible for the developmental commitment of B lymphocytes and how it is harnessed in various applications.

Single-cell genomics identifies distinct B1 cell developmental pathways and reveals aging-related changes in the B-cell receptor repertoire

Background

B1 cells are self-renewing innate-like B lymphocytes that provide the first line of defense against pathogens. B1 cells primarily reside in the peritoneal cavity and are known to originate from various fetal tissues, yet their developmental pathways and the mechanisms underlying maintenance of B1 cells throughout adulthood remain unclear.

Results

We performed high-throughput single-cell analysis of the transcriptomes and B-cell receptor repertoires of peritoneal B cells of neonates, young adults, and elderly mice. Gene expression analysis of 31,718 peritoneal B cells showed that the neonate peritoneal cavity contained many B1 progenitors, and neonate B cell specific clustering revealed two trajectories of peritoneal B1 cell development, including pre-BCR dependent and pre-BCR independent pathways. We also detected profound age-related changes in B1 cell transcriptomes: clear difference in senescence genetic program was evident in differentially aged B1 cells, and we found an example that a B1 subset only present in the oldest mice was marked by expression of the fatty-acid receptor CD36. We also performed antibody gene sequencing of 15,967 peritoneal B cells from the three age groups and discovered that B1 cell aging was associated with clonal expansion and two B1 cell clones expanded in the aged mice had the same CDR-H3 sequence (AGDYDGYWYFDV) as a pathogenically linked cell type from a recent study of an atherosclerosis mouse model.

Conclusions

Beyond offering an unprecedent data resource to explore the cell-to-cell variation in B cells, our study has revealed that B1 precursor subsets are present in the neonate peritoneal cavity and dissected the developmental pathway of the precursor cells. Besides, this study has found the expression of CD36 on the B1 cells in the aged mice. And the single-cell B-cell receptor sequencing reveals B1 cell aging is associated with clonal expansion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00795-6.

A self-sustaining layer of early-life-origin B cells drives steady-state IgA responses in the adult gut

The adult immune system consists of cells that emerged at various times during ontogeny. We aimed to define the relationship between developmental origin and composition of the adult B cell pool during unperturbed hematopoiesis. Lineage tracing stratified murine adult B cells based on the timing of output, revealing that a substantial portion originated within a restricted neonatal window. In addition to B-1a cells, early-life time-stamped B cells included clonally interrelated IgA plasma cells in the gut and bone marrow. These were actively maintained by B cell memory within gut chronic germinal centers and contained commensal microbiota reactivity. Neonatal rotavirus infection recruited recurrent IgA clones that were distinct from those arising by infection with the same antigen in adults. Finally, gut IgA plasma cells arose from the same hematopoietic progenitors as B-1a cells during ontogeny. Thus, a complex layer of neonatally imprinted B cells confer unique antibody responses later in life.

Role of microRNAs in B-Cell Compartment: Development, Proliferation and Hematological Diseases

B-cell development is a very orchestrated pathway that involves several molecules, such as transcription factors, cytokines, microRNAs, and also different cells. All these components maintain the ideal microenvironment and control B-cell differentiation. MicroRNAs are small non-coding RNAs that bind to target mRNA to control gene expression. These molecules could circulate in the body in a free form, protein-bounded, or encapsulated into extracellular vesicles, such as exosomes. The comprehension of the role of microRNAs in the B-cell development was possible based on microRNA profile of each B-cell stage and functional studies. Herein, we report the knowledge about microRNAs in the B-cell the differentiation, proliferation, and also in hematological malignancies.

The Role of B Lymphocyte Subsets in Adipose Tissue Development, Metabolism, and Aging

Adipose tissue contains resident B lymphocytes (B cells) with varying immune functions and mechanisms, depending on the adipose depot type and location. The heterogeneity of B cells and their functions affect the immunometabolism of the adipose tissue in aging and age-associated metabolic disorders. B cells exist in categorizations of subsets that have developmental or phenotypic differences with varying functionalities. Subsets can be categorized as either protective or pathogenic depending on their secretion profile or involvement in metabolic maintenance. In this article, we summarized recent finding on the B cell heterogeneity and discuss how we can utilize our current knowledge of adipose resident B lymphocytes for potential treatment for age-associated metabolic disorders. © 2022 American Physiological Society. Compr Physiol 12: 1-13, 2022.

MicroRNA-regulated B cells in obesity

Obesity is a prevalent health risk by inducing chronic, low-grade inflammation and insulin resistance, in part from adipose tissue inflammation perpetuated by activated B cells and other resident immune cells. However, regulatory mechanisms controlling B-cell actions in adipose tissue remain poorly understood, limiting therapeutic innovations. MicroRNAs are potent regulators of immune cell dynamics through fine-tuning a network of downstream genes in multiple signaling pathways. In particular, miR-150 is crucial to B-cell development and suppresses obesity-associated inflammation via regulating adipose tissue B-cell function. Herein, we review the effect of microRNAs on B-cell development, activation, and function and highlight miR-150-regulated B-cell actions during obesity which modulate systemic inflammation and insulin resistance. In this way, we hope to promote translational discoveries that mitigate obesity-induced health risks by targeting microRNA-regulated B-cell actions.

Efficient Isolation of Lymphocytes and Myogenic Cells from the Tissue of Muscle Regeneration

Isolation of both lymphocytes and myogenic cells from muscle tissue is required for elucidating the cellular and molecular mechanisms of muscle regeneration. Here, we aimed to establish an optimal method obtaining a high yield of lymphocytes during muscle regeneration. After the muscle injury, we observed higher infiltration of lymphocytic cells in the muscle on day 3 after injury. Then, we compared two different white blood cell isolation methods, the Percoll gradient and CD45-magnetic bead methods, to assess the percentage and number of T and B cells. Flow cytometry analysis showed that the CD45-magnetic bead method has a better efficiency in isolating CD4⁺, CD8⁺ T cells, and B cells from injured muscle tissues of wild-type and mdx mice than that by the Percoll gradient method. Moreover, we found that the CD45-negative fraction from wild-type and mdx mice includes myogenic cells. In conclusion, we report that the CD45-magnetic bead method is suitable to isolate T and B cells during muscle regeneration with higher purity and yield and can also isolate myogenic cells within the same sample. This method provides a technical basis for further studies on muscle regeneration, involving lymphocytes and muscle cells, with a wide range of clinical applications.

Regeneration of immunocompetent B lymphopoiesis from pluripotent stem cells guided by transcription factors

Regeneration of functional B lymphopoiesis from pluripotent stem cells (PSCs) is challenging, and reliable methods have not been developed. Here, we unveiled the guiding role of three essential factors, Lhx2, Hoxa9, and Runx1, the simultaneous expression of which preferentially drives B lineage fate commitment and in vivo B lymphopoiesis using PSCs as a cell source. In the presence of Lhx2, Hoxa9, and Runx1 expression, PSC-derived induced hematopoietic progenitors (iHPCs) immediately gave rise to pro/pre-B cells in recipient bone marrow, which were able to further differentiate into entire B cell lineages, including innate B-1a, B-1b, and marginal zone B cells, as well as adaptive follicular B cells. In particular, the regenerative B cells produced adaptive humoral immune responses, sustained antigen-specific antibody production, and formed immune memory in response to antigen challenges. The regenerative B cells showed natural B cell development patterns of immunoglobulin chain switching and hypermutation via cross-talk with host T follicular helper cells, which eventually formed T cell-dependent humoral responses. This study exhibits de novo evidence that B lymphopoiesis can be regenerated from PSCs via an HSC-independent approach, which provides insights into treating B cell-related deficiencies using PSCs as an unlimited cell resource.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

Single-cell analysis identifies dynamic gene expression networks that govern B cell development and transformation

Integration of external signals and B-lymphoid transcription factor activities organise B cell lineage commitment through alternating cycles of proliferation and differentiation, producing a diverse repertoire of mature B cells. We use single-cell transcriptomics/proteomics to identify differentially expressed gene networks across B cell development and correlate these networks with subtypes of B cell leukemia. Here we show unique transcriptional signatures that refine the pre-B cell expansion stages into pre-BCR-dependent and pre-BCR-independent proliferative phases. These changes correlate with reciprocal changes in expression of the transcription factor EBF1 and the RNA binding protein YBX3, that are defining features of the pre-BCR-dependent stage. Using pseudotime analysis, we further characterize the expression kinetics of different biological modalities across B cell development, including transcription factors, cytokines, chemokines, and their associated receptors. Our findings demonstrate the underlying heterogeneity of developing B cells and characterise developmental nodes linked to B cell transformation.

Current Trends in the Gene Therapy of Hematologic Disorders

Recent advances in molecular genetics and the invention of new technologies led to an advance in the development of gene therapy. Gene therapy is used to correct defective genes in order to cure a disease or help the body better fight a disease. It works by restoring or modifying cellular functions through the introduction of a functional gene into the target cell. The concept of gene therapy is simple, but introducing it to routine clinical practice is not. The main concerns are related to some safety issues as well as to the problem that maintaining a stable and prolonged expression in target cells may not be easily achieved. In spite of the difficulties, gene therapy remains a hope for many hematological disorders that cannot be effectively treated so far. This article reviews the current status of gene therapy with a focus on hematological disorders. In addition, clinically applied approaches are presented through particular examples of approved gene therapy drugs.

TGF-β/IL-7 Chimeric Switch Receptor-Expressing CAR-T Cells Inhibit Recurrence of CD19-Positive B Cell Lymphoma

Chimeric antigen receptor (CAR)-T cells are effective in the treatment of hematologic malignancies but have shown limited efficacy against solid tumors. Here, we demonstrated an approach to inhibit recurrence of B cell lymphoma by co-expressing both a human anti-CD19-specific single-chain variable fragment (scFv) CAR (CD19 CAR) and a TGF-β/IL-7 chimeric switch receptor (tTRII-I7R) in T cells (CD19 CAR-tTRII-I7R-T cells). The tTRII-I7R was designed to convert immunosuppressive TGF-β signaling into immune-activating IL-7 signaling. The effect of TGF-β on CD19 CAR-tTRII-I7R-T cells was assessed by western blotting. Target-specific killing by CD19 CAR-tTRII-I7R-T cells was evaluated by Eu-TDA assay. Daudi tumor-bearing NSG (NOD/SCID/IL2Rγ^-/-) mice were treated with CD19 CAR-tTRII-I7R-T cells to analyze the in vivo anti-tumor effect. In vitro, CD19 CAR-tTRII-I7R-T cells had a lower level of phosphorylated SMAD2 and a higher level of target-specific cytotoxicity than controls in the presence of rhTGF-β1. In the animal model, the overall survival and recurrence-free survival of mice that received CD19 CAR-tTRII-I7R-T cells were significantly longer than in control mice. These findings strongly suggest that CD19 CAR-tTRII-I7R-T cell therapy provides a new strategy for long-lasting, TGF-β-resistant anti-tumor effects against B cell lymphoma, which may lead ultimately to increased clinical efficacy.

Notch signaling promotes disease initiation and progression in murine chronic lymphocytic leukemia

NOTCH1 gain-of-function mutations are recurrent in B-cell chronic lymphocytic leukemia (B-CLL), where they are associated with accelerated disease progression and refractoriness to chemotherapy. The specific role of NOTCH1 in the development and progression of this malignancy is unclear. Here, we assess the impact of loss of Notch signaling and pathway hyperactivation in an in vivo mouse model of CLL (IgH.TEμ) that faithfully replicates many features of the human pathology. Ablation of canonical Notch signaling using conditional gene inactivation of RBP-J in immature hematopoietic or B-cell progenitors delayed CLL induction and reduced incidence of mice developing disease. In contrast, forced expression of a dominant active form of Notch resulted in more animals developing CLL with early disease onset. Comparative analysis of gene expression and epigenetic features of Notch gain-of-function and control CLL cells revealed direct and indirect regulation of cell cycle-associated genes, which led to increased proliferation of Notch gain-of-function CLL cells in vivo. These results demonstrate that Notch signaling facilitates disease initiation and promotes CLL cell proliferation and disease progression.

Pax5 Negatively Regulates Osteoclastogenesis through Downregulation of Blimp1

Paired box protein 5 (Pax5) is a crucial transcription factor responsible for B-cell lineage specification and commitment. In this study, we identified a negative role of Pax5 in osteoclastogenesis. The expression of Pax5 was time-dependently downregulated by receptor activator of nuclear factor kappa B (RANK) ligand (RANKL) stimulation in osteoclastogenesis. Osteoclast (OC) differentiation and bone resorption were inhibited (68.9% and 48% reductions, respectively) by forced expression of Pax5 in OC lineage cells. Pax5 led to the induction of antiosteoclastogenic factors through downregulation of B lymphocyte-induced maturation protein 1 (Blimp1). To examine the negative role of Pax5 in vivo, we generated Pax5 transgenic (Pax5^Tg) mice expressing the human Pax5 transgene under the control of the tartrate-resistant acid phosphatase (TRAP) promoter, which is expressed mainly in OC lineage cells. OC differentiation and bone resorption were inhibited (54.2–76.9% and 24.0–26.2% reductions, respectively) in Pax5^Tg mice, thereby contributing to the osteopetrotic-like bone phenotype characterized by increased bone mineral density (13.0–13.6% higher), trabecular bone volume fraction (32.5–38.1% higher), trabecular thickness (8.4–9.0% higher), and trabecular number (25.5–26.7% higher) and decreased trabecular spacing (9.3–10.4% lower) compared to wild-type control mice. Furthermore, the number of OCs was decreased (48.8–65.3% reduction) in Pax5^Tg mice. These findings indicate that Pax5 plays a negative role in OC lineage specification and commitment through Blimp1 downregulation. Thus, our data suggest that the Pax5–Blimp1 axis is crucial for the regulation of RANKL-induced osteoclastogenesis.

How do CARs work?: Early insights from recent clinical studies targeting CD19

Second-generation chimeric antigen receptors (CARs) are powerful tools to redirect antigen-specific T cells independently of HLA-restriction. Recent clinical studies evaluating CD19-targeted T cells in patients with B-cell malignancies demonstrate the potency of CAR-engineered T cells. With results from 28 subjects enrolled by five centers conducting studies in patients with chronic lymphocytic leukemia (CLL) or lymphoma, some insights into the parameters that determine T-cell function and clinical outcome of CAR-based approaches are emerging. These parameters involve CAR design, T-cell production methods, conditioning chemotherapy as well as patient selection. Here, we discuss the potential relevance of these findings and in particular the interplay between the adoptive transfer of T cells and pre-transfer patient conditioning.

Toward Therapeutic Targeting of Bone Marrow Leukemic Niche Protective Signals in B-Cell Acute Lymphoblastic Leukemia

B-cell acute lymphoblastic leukemia (B-ALL) represents the malignant counterpart of bone marrow (BM) differentiating B cells and occurs most frequently in children. While new combinations of chemotherapeutic agents have dramatically improved the prognosis for young patients, disease outcome remains poor after relapse or in adult patients. This is likely due to heterogeneity of B-ALL response to treatment which relies not only on intrinsic properties of leukemic cells, but also on extrinsic protective cues transmitted by the tumor cell microenvironment. Alternatively, leukemic cells have the capacity to shape their microenvironment towards their needs. Most knowledge on the role of protective niches has emerged from the identification of mesenchymal and endothelial cells controlling hematopoietic stem cell self-renewal or B cell differentiation. In this review, we discuss the current knowledge about B-ALL protective niches and the development of therapies targeting the crosstalk between leukemic cells and their microenvironment.

Conditional antibody expression to avoid central B cell deletion in humanized HIV-1 vaccine mouse models

HIV-1 vaccine development aims to elicit broadly neutralizing antibodies (bnAbs) against diverse viral strains. In some HIV-1-infected individuals, bnAbs evolved from precursor antibodies through affinity maturation. To induce bnAbs, a vaccine must mediate a similar antibody maturation process. One way to test a vaccine is to immunize mouse models that express human bnAb precursors and assess whether the vaccine can convert precursor antibodies into bnAbs. A major problem with such mouse models is that bnAb expression often hinders B cell development. Such developmental blocks may be attributed to the unusual properties of bnAb variable regions, such as poly-reactivity and long antigen-binding loops, which are usually under negative selection during primary B cell development. To address this problem, we devised a method to circumvent such B cell developmental blocks by expressing bnAbs conditionally in mature B cells. We validated this method by expressing the unmutated common ancestor (UCA) of the human VRC26 bnAb in transgenic mice. Constitutive expression of the VRC26UCA led to developmental arrest of B cell progenitors in bone marrow; poly-reactivity of the VRC26UCA and poor pairing of the VRC26UCA heavy chain with the mouse surrogate light chain may contribute to this phenotype. The conditional expression strategy bypassed the impediment to VRC26UCA B cell development, enabling the expression of VRC26UCA in mature B cells. This approach should be generally applicable for expressing other bnAbs that are under negative selection during B cell development.

Glucocorticoids Regulate Bone Marrow B Lymphopoiesis After Stroke

RATIONALE

After a stroke, patients frequently experience altered systemic immunity resulting in peripheral immunosuppression and higher susceptibility to infections, which is at least partly attributed to lymphopenia. The mechanisms that profoundly change the systemic leukocyte repertoire after stroke are incompletely understood. Emerging evidence indicates that stroke alters hematopoietic output of the bone marrow.

OBJECTIVE

To explore the mechanisms that lead to defects of B lymphopoiesis after ischemic stroke.

METHODS AND RESULTS

We here report that ischemic stroke triggers brain-bone marrow communication via hormonal long-range signals that regulate hematopoietic B lineage decisions. Bone marrow fluorescence-activated cell sorter analyses and serial intravital microscopy indicate that transient middle cerebral artery occlusion in mice arrests B-cell development beginning at the pro-B-cell stage. This phenotype was not rescued in Myd88^-/- and TLR4^-/- mice with disrupted TLR (Toll-like receptor) signaling or after blockage of peripheral sympathetic nerves. Mechanistically, we identified stroke-induced glucocorticoid release as the main instigator of B lymphopoiesis defects. B-cell lineage-specific deletion of the GR (glucocorticoid receptor) in CD19-Cre loxP Nr3c1 mice attenuated lymphocytopenia after transient middle cerebral artery. In 20 patients with acute stroke, increased cortisol levels inversely correlated with blood lymphocyte numbers.

CONCLUSIONS

Our data demonstrate that the hypothalamic-pituitary-adrenal axis mediates B lymphopoiesis defects after ischemic stroke.

Coexpression of CCR7 and CXCR4 During B Cell Development Controls CXCR4 Responsiveness and Bone Marrow Homing

The CXCL12-CXCR4 axis plays a key role in the retention of stem cells and progenitors in dedicated bone marrow niches. It is well-known that CXCR4 responsiveness in B lymphocytes decreases dramatically during the final stages of their development in the bone marrow. However, the molecular mechanism underlying this regulation and whether it plays a role in B-cell homeostasis remain unknown. In the present study, we show that the differentiation of pre-B cells into immature and mature B cells is accompanied by modifications to the relative expression of chemokine receptors, with a two-fold downregulation of CXCR4 and upregulation of CCR7. We demonstrate that expression of CCR7 in B cells is involved in the selective inactivation of CXCR4, and that mature B cells from CCR7-/- mice display higher responsiveness to CXCL12 and improved retention in the bone marrow. We also provide molecular evidence supporting a model in which upregulation of CCR7 favors the formation of CXCR4-CCR7 heteromers, wherein CXCR4 is selectively impaired in its ability to activate certain G-protein complexes. Collectively, our results demonstrate that CCR7 behaves as a novel selective endogenous allosteric modulator of CXCR4.

Immunophenotyping of Murine Precursor B-Cell Leukemia/Lymphoma: A Comparison of Immunohistochemistry and Flow Cytometry

In humans and in mouse models, precursor B-cell lymphoblastic leukemia (B-ALL)/lymphoblastic lymphoma (B-LBL) can be classified as either the pro-B or pre-B subtype. This is based on the expression of antigens associated with the pro-B and pre-B stages of B-cell development. Antigenic markers can be detected by flow cytometry or immunohistochemistry (IHC), but no comparison of results from these techniques has been reported for murine B-ALL/LBL. In our analysis of 30 cases induced by chemical or viral mutagenesis on a WT or Pax5+/- background, 18 (60%) were diagnosed as pro-B by both flow cytometry and IHC. Discordant results were found for 12 (40%); 6 were designated pro-B by IHC and pre-B by flow cytometry and the reverse for the remaining 6 cases. Discordance occurred because different markers were used to define the pro-B-to-pre-B transition by IHC vs flow cytometry. IHC expression of cytoplasmic IgM (μIgM) defined the pre-B stage, whereas the common practice of using CD25 as a surrogate marker in flow cytometry was employed here. These results show that CD25 and μIgM are not always concurrently expressed in B-ALL/LBL, in contrast to normal B-cell development. Therefore, when subtyping B-ALL/LBL in mice, an IHC panel of B220, PAX5, TdT, c-Kit/CD117, CD43, IgM, and ΚLC should be considered. For flow cytometry, cytoplasmic IgM may be an appropriate marker in conjunction with the surface markers B220, CD19, CD43, c-Kit/CD117, BP-1, and CD25.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (second edition)

These guidelines are a consensus work of a considerable number of members of the immunology and flow cytometry community. They provide the theory and key practical aspects of flow cytometry enabling immunologists to avoid the common errors that often undermine immunological data. Notably, there are comprehensive sections of all major immune cell types with helpful Tables detailing phenotypes in murine and human cells. The latest flow cytometry techniques and applications are also described, featuring examples of the data that can be generated and, importantly, how the data can be analysed. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid, all written and peer-reviewed by leading experts in the field, making this an essential research companion.

CAR T-cell bioengineering: Single variable domain of heavy chain antibody targeted CARs

Redirecting the recognition specificity of T lymphocytes to designated tumour cell surface antigens by transferring chimeric antigen receptor (CAR) genes is becoming an effective strategy to combat cancer. Today, CAR T-cell therapy has proven successful in the treatment of haematological malignancies and the first CD19 CAR T-cell products has already entered the market. This success is expanding CAR design for broader malignancies including solid tumours. Nevertheless, CARs such as those built on antigen-specific single chain antibody variable fragment (scFv) may induce some adverse effects. Here, we briefly review CAR T-cell bioengineering and discuss selected important initiatives for improved T-cell reprogramming, function and safety. In this respect, we further elaborate on unconventional CARs structured on single variable domain of heavy chain (VHH) antibodies (single-domain antibodies) as an alternative to scFv, because of their interesting immunological and physicochemical characteristics and unique structure, which shows a high degree of homology with human VH3 gene family.

Mesenchymal lineage cells and their importance in B lymphocyte niches

Early B lymphopoiesis occurs in the bone marrow and is reliant on interactions with numerous cell types in the bone marrow microenvironment, particularly those of the mesenchymal lineage. Each cellular niche that supports the distinct stages of B lymphopoiesis is unique. Different cell types and signaling molecules are important for the progressive stages of B lymphocyte differentiation. Cells expressing CXCL12 and IL-7 have long been recognized as having essential roles in facilitating progression through stages of B lymphopoiesis. Recently, a number of other factors that extrinsically mediate B lymphopoiesis (positively or negatively) have been identified. In addition, the use of transgenic mouse models to delete specific genes in mesenchymal lineage cells has further contributed to our understanding of how B lymphopoiesis is regulated in the bone marrow. This review will cover the current understanding of B lymphocyte niches in the bone marrow and key extrinsic molecules and signaling pathways involved in these niches, with a focus on how mesenchymal lineage cells regulate B lymphopoiesis.

Integrin alpha11 is an Osteolectin receptor and is required for the maintenance of adult skeletal bone mass

We previously discovered a new osteogenic growth factor that is required to maintain adult skeletal bone mass, Osteolectin/Clec11a. Osteolectin acts on Leptin Receptor⁺ (LepR⁺) skeletal stem cells and other osteogenic progenitors in bone marrow to promote their differentiation into osteoblasts. Here we identify a receptor for Osteolectin, integrin α11, which is expressed by LepR⁺ cells and osteoblasts. α11β1 integrin binds Osteolectin with nanomolar affinity and is required for the osteogenic response to Osteolectin. Deletion of Itga11 (which encodes α11) from mouse and human bone marrow stromal cells impaired osteogenic differentiation and blocked their response to Osteolectin. Like Osteolectin deficient mice, Lepr-cre; Itga11^fl/fl mice appeared grossly normal but exhibited reduced osteogenesis and accelerated bone loss during adulthood. Osteolectin binding to α11β1 promoted Wnt pathway activation, which was necessary for the osteogenic response to Osteolectin. This reveals a new mechanism for maintenance of adult bone mass: Wnt pathway activation by Osteolectin/α11β1 signaling.

Throughout our lives, our bones undergo constant remodeling. Cells called osteoclasts break down old bone and cells called osteoblasts lay down new. Normally, the two cell types work in balance but if the rate of breakdown outpaces new bone formation the skeleton can become weak. This weakness leads to a condition called osteoporosis, in which people suffer from fragile bones. Osteoporosis is hard to reverse, in part because our ability to encourage new bone to form is limited.

In 2016, researchers discovered a protein called osteolectin, which promotes new bone formation during adulthood by helping skeletal stem cells transform into bone cells. But so far, it has been unclear how osteolectin achieves this. To investigate this further, Shen et al. – including some researchers involved in the 2016 study – marked osteolectin with a molecular tag and tested what it bound on the surface of mouse and human bone marrow cells.

The experiments revealed that osteolectin binds to a specific receptor protein called α11 integrin, which can only be found on skeletal stem cells and the osteoblasts they give rise to. Once osteolectin binds to the receptor, it activates a signaling pathway that induces the stem cells to develop into osteoblasts. Mice that lacked either osteolectin or α11 integrin produced less bone and lost bone tissue faster as adults.

Osteolectin could potentially be useful in the treatment of osteoporosis or broken bones. Since only skeletal stem cells and osteoblasts cells produce α11 integrin, osteolectin would specifically target these cells without affecting cells that do not form bones. A next step will be to assess how well osteolectin compares to existing treatments for fragile bones.

Immunosuppression in Experimental Chagas Disease Is Mediated by an Alteration of Bone Marrow Stromal Cell Function During the Acute Phase of Infection

After infection with Trypanosoma cruzi, the etiologic agent of Chagas disease, immunosuppression, and apoptosis of mature lymphocytes contribute to the establishment of the parasite in the host and thereby to persistence and pathology in the chronic stage of infection. In a systemic mouse model of experimental Chagas disease, we have demonstrated a strong depletion of mature B cells in the spleen during the first 2 weeks of infection. Remarkably, the decrease in this cell population commenced already in the bone marrow from infected mice and was a concomitant of an increased apoptosis in pro- and pre-B cell populations. Pro- and pre-B cells in the bone marrow showed a significant reduction accompanied by a functional disturbance of bone marrow-derived stromal cells resulting in diminished levels of IL-7, an essential factor for the development of B cell precursors. Ex vivo, stromal cells isolated from the bone marrow of infected mice had a strikingly impaired capacity to maintain the development of pro- and pre-B cells obtained from uninfected animals. Together, the reduction of an active humoral immune response during acute Chagas disease suggests to be an initial immune evasion mechanism of the parasite to establish persistent infection. Therefore, prevention of B cell depletion by rescuing the stromal cells during this early phase, could give rise to new therapeutic approaches.

mTORC1 plays an important role in osteoblastic regulation of B-lymphopoiesis

Skeletal osteoblasts are important regulators of B-lymphopoiesis, serving as a rich source of factors such as CXCL12 and IL-7 which are crucial for B-cell development. Recent studies from our laboratory and others have shown that deletion of Rptor, a unique component of the mTORC1 nutrient-sensing complex, early in the osteoblast lineage development results in defective bone development in mice. In this study, we now demonstrate that mTORC1 signalling in pre-osteoblasts is required for normal B-lymphocyte development in mice. Targeted deletion of Rptor in osterix-expressing pre-osteoblasts (Rptor_ob^-/-) leads to a significant reduction in the number of B-cells in the bone marrow, peripheral blood and spleen at 4 and 12 weeks of age. Rptor_ob^-/- mice also exhibit a significant reduction in pre-B and immature B-cells in the BM, indicative of a block in B-cell development from the pro-B to pre-B cell stage. Circulating levels of IL-7 and CXCL12 are also significantly reduced in Rptor_ob^-/- mice. Importantly, whilst Rptor-deficient osteoblasts are unable to support HSC differentiation to B-cells in co-culture, this can be rescued by the addition of exogenous IL-7 and CXCL12. Collectively, these findings demonstrate that mTORC1 plays an important role in extrinsic osteoblastic regulation of B-cell development.

Immune Cell Hacking: Challenges and Clinical Approaches to Create Smarter Generations of Chimeric Antigen Receptor T Cells

T cells equipped with chimeric antigen receptors (CAR T cells) have recently provided promising advances as a novel immunotherapeutic approach for cancer treatment. CAR T cell therapy has shown stunning results especially in B-cell malignancies; however, it has shown less success against solid tumors, which is more supposed to be related to the specific characteristics of the tumor microenvironment. In this review, we discuss the structure of the CAR, current clinical advantages from finished and ongoing trials, adverse effects, challenges and controversies, new engineering methods of CAR, and clinical considerations that are associated with CAR T cell therapy both in hematological malignancies and solid tumors. Also, we provide a comprehensive description of recently introduced modifications for designing smarter models of CAR T cells. Specific hurdles and problems that limit the optimal function of CAR T cells, especially on solid tumors, and possible solutions according to new modifications and generations of CAR T cells have been introduced here. We also provide information of the future directions on how to enhance engineering the next smarter generations of CAR T cells in order to decrease the adverse effects and increase the potency and efficacy of CAR T cells against cancer.

Molecular Regulation of Differentiation in Early B-Lymphocyte Development

B-lymphocyte differentiation is one of the best understood developmental pathways in the hematopoietic system. Our understanding of the developmental trajectories linking the multipotent hematopoietic stem cell to the mature functional B-lymphocyte is extensive as a result of efforts to identify and prospectively isolate progenitors at defined maturation stages. The identification of defined progenitor compartments has been instrumental for the resolution of the molecular features that defines given developmental stages as well as for our understanding of the mechanisms that drive the progressive maturation process. Over the last years it has become increasingly clear that the regulatory networks that control normal B-cell differentiation are targeted by mutations in human B-lineage malignancies. This generates a most interesting link between development and disease that can be explored to improve diagnosis and treatment protocols in lymphoid malignancies. The aim of this review is to provide an overview of our current understanding of molecular regulation in normal and malignant B-cell development.

Overcoming Resistance of Human Non-Hodgkin's Lymphoma to CD19-CAR CTL Therapy by Celecoxib and Histone Deacetylase Inhibitors

Patients with B-cell non-Hodgkin’s lymphoma (B-NHL) who fail to respond to first-line treatment regimens or develop resistance, exhibit poor prognosis. This signifies the need to develop alternative treatment strategies. CD19-chimeric antigen receptor (CAR) T cell-redirected immunotherapy is an attractive and novel option, which has shown encouraging outcomes in phase I clinical trials of relapsed/refractory NHL. However, the underlying mechanisms of, and approaches to overcome, acquired anti-CD19CAR CD8⁺ T cells (CTL)-resistance in NHL remain elusive. CD19CAR transduced primary human CTLs kill CD19⁺ human NHLs in a CD19- and caspase-dependent manner, mainly via the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) apoptotic pathway. To understand the dynamics of the development of resistance, we analyzed several anti-CD19CAR CTL-resistant NHL sublines (R-NHL) derived by serial exposure of sensitive parental lines to excessive numbers of anti-CD19CAR CTLs followed by a limiting dilution analysis. The R-NHLs retained surface CD19 expression and were efficiently recognized by CD19CAR CTLs. However, R-NHLs developed cross-resistance to CD19CAR transduced human primary CTLs and the Jurkat human T cell line, activated Jurkat, and lymphokine activated killer (LAK) cells, suggesting the acquisition of resistance is independent of CD19-loss and might be due to aberrant apoptotic machinery. We hypothesize that the R-NHL refractoriness to CD19CAR CTL killing could be partially rescued by small molecule sensitizers with apoptotic-gene regulatory effects. Chromatin modifiers and Celecoxib partially reversed the resistance of R-NHL cells to the cytotoxic effects of anti-CD19CAR CTLs and rhTRAIL. These in vitro results, though they require further examination, may provide a rational biological basis for combination treatment in the management of CD19CAR CTL-based therapy of NHL.

Dissection of progenitor compartments resolves developmental trajectories in B-lymphopoiesis

Jensen et al. report the identification and characterization of novel lymphoid progenitor populations in the mouse bone marrow. The work resolves the complexity of the BLP/pre-pro–B/Fraction A compartments and provides a developmental trajectory for early B cell development.

To understand the developmental trajectories in early lymphocyte differentiation, we identified differentially expressed surface markers on lineage-negative lymphoid progenitors (LPs). Single-cell polymerase chain reaction experiments allowed us to link surface marker expression to that of lineage-associated transcription factors (TFs) and identify GFRA2 and BST1 as markers of early B cells. Functional analyses in vitro and in vivo as well as single-cell gene expression analyses supported that surface expression of these proteins defined distinct subpopulations that include cells from both the classical common LPs (CLPs) and Fraction A compartments. The formation of the GFRA2-expressing stages of development depended on the TF EBF1, critical both for the activation of stage-specific target genes and modulation of the epigenetic landscape. Our data show that consecutive expression of Ly6D, GFRA2, and BST1 defines a developmental trajectory linking the CLP to the CD19⁺ progenitor compartment.

Role of B-cell receptors for B-cell development and antigen-induced differentiation

B-cell development is characterized by a number of tightly regulated selection processes. Signals through the B-cell receptor (BCR) guide and are required for B-cell maturation, survival, and fate decision. Here, we review the role of the BCR during B-cell development, leading to the emergence of B1, marginal zone, and peripheral follicular B cells. Furthermore, we discuss BCR-derived signals on activated B cells that lead to germinal center and plasma cell differentiation.

CAR T-cells for cancer therapy

Chimeric antigen receptor (CAR) T-cells are redirected T-cells that can recognize cancer antigens in a major histocompatibility complex (MHC)-independent fashion. A typical CAR is comprised of two main functional domains: an extracellular antigen recognition domain, called a single-chain variable fragment (scFv), and an intracellular signaling domain. Based on the number of intracellular signaling molecules, CARs are categorized into four generations. CAR T-cell therapy has become a promising treatment for hematologic malignancies. However, results of its clinical trials on solid tumors have not been encouraging. Here, we described the structure of CARs and summarized the clinical trials of CD19-targeted CAR T-cells. The side effects, safety management, challenges, and future prospects of CAR T-cells for the treatment of cancer, particularly for solid tumors, were also discussed.

A Hard(y) Look at B-1 Cell Development and Function

A small population of B cells exists in lymphoid tissues and body cavities of mice that is distinct in development, phenotype, and function from the majority (B-2) B cell population. This population, originally termed "Ly-1" and now "B-1," has received renewed interest as an innate-like B cell population of fetal-derived hematopoiesis, responsible for natural Ab production and rapid immune responses. Molecular analyses have begun to define fetal and adult hematopoiesis, while cell-fate mapping studies have revealed complex developmental origins of B-1 cells. Together the studies provide a more detailed understanding of B-1 cell regulation and function. This review outlines studies that defined B-1 cells as natural Ab- and cytokine-producing B cells of fetal origin, with a focus on work conducted by R.R. Hardy, an early pioneer and codiscoverer of B-1 cells, whose seminal contributions enhanced our understanding of this enigmatic B cell population.

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.

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.

MiR-128-2 inhibits common lymphoid progenitors from developing into progenitor B cells

A considerable number of studies revealed that B cell development is finely regulated by transcription factors (TFs). Recent studies suggested that TFs are coordinated with microRNAs to control the development of B cells in numerous checkpoints. In the present study, we first found that miR-128-2 was differentially expressed in various immune organs and immunocytes. B cell development was inhibited in miR-128-2-overexpressed chimera and transgenic (TG) mice in bone marrow with decreased preproB, preB, proB, immature B, and recirculating B cells, as well as increased common lymphoid progenitors (CLPs). Further experiments showed that the apoptosis of CLP decreased, but proliferation was not altered in miR-128-2-overexpressed mice. Extensive studies suggested that the inhibition of apoptosis of CLP may be caused by miR-128-2 targeting A2B and MALT1, thereby increasing the phosphorylation of ERK and P38 MAPK. Such findings have prompted future investigations on the function of miR-128-2 in lymph genesis.

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.

An Atypical Splenic B Cell Progenitor Population Supports Antibody Production during Plasmodium Infection in Mice

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions and in response to inflammation due to infection or stress. Whereas the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occur in the spleen during acute Plasmodium infection, a critical step in the host immune response. In this study, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a lineage(-)Sca-1(+)c-Kit(-) (LSK(-)) phenotype that proliferates in response to infection with nonlethal Plasmodium yoelii 17X. Infection-derived LSK(-) cells upon transfer into naive congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK(-) cells gave rise to B cells capable of entering into a germinal center reaction, and they developed into memory B cells and Ab-secreting cells that were capable of producing parasite-specific Abs. Differentiation of LSK(-) cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK(-) cells expand and differentiate in direct response to the parasite. However, the ability of LSK(-) cells to differentiate into B cells was not dependent on MyD88, as myd88(-/-) LSK(-) cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B lymphocytes in the spleen and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.

Generation and characterization of RAG2 knockout pigs as animal model for severe combined immunodeficiency

Pigs with severe combined immunodeficiency (SCID) are versatile animal models for human medical research because of their biological similarities to humans, suitable body size, and longevity for practical research. SCID pigs with defined mutation(s) can be an invaluable tool for research on porcine immunity. In this study, we produced RAG2-knockout pigs via somatic cell nuclear transfer and analyzed their phenotype. The V(D)J recombination processes were confirmed as being inactivated. They consistently lacked mature T and B cells but had substantial numbers of cells considered to be T- or B-cell progenitors as well as NK cells. They also lacked thymic medulla and lymphoid aggregations in the spleen, mesenteric lymph nodes, and ileal Peyer's patches. We showed more severe immunological defects in the RAG2 and IL2RG double-knockout pig through this study. Thus, SCID pigs could be promising animal models not only for translational medical research but also for immunological studies of pigs themselves.

Chimeric Antigen Receptors Modified T-Cells for Cancer Therapy

The genetic modification and characterization of T-cells with chimeric antigen receptors (CARs) allow functionally distinct T-cell subsets to recognize specific tumor cells. The incorporation of costimulatory molecules or cytokines can enable engineered T-cells to eliminate tumor cells. CARs are generated by fusing the antigen-binding region of a monoclonal antibody (mAb) or other ligand to membrane-spanning and intracellular-signaling domains. They have recently shown clinical benefit in patients treated with CD19-directed autologous T-cells. Recent successes suggest that the modification of T-cells with CARs could be a powerful approach for developing safe and effective cancer therapeutics. Here, we briefly review early studies, consider strategies to improve the therapeutic potential and safety, and discuss the challenges and future prospects for CAR T-cells in cancer therapy.

Inhibition of B Lymphopoiesis by Adipocytes and IL-1-Producing Myeloid-Derived Suppressor Cells

B lymphopoiesis declines with age, and this decline correlates with increased adipose tissue in the bone marrow (BM). Also, adipocyte-derived factors are known to inhibit B lymphopoiesis. Using cocultures of mouse BM cells with OP9 stromal cells, we found that adipocyte-conditioned medium induces the generation of CD11b(+)Gr1(+) myeloid cells, which inhibit B cell development in vitro. Adipocyte-conditioned medium-induced CD11b(+)Gr1(+) cells express Arg1 (arginase) and Nos2 (inducible NO synthase) and suppress CD4(+) T cell proliferation, indicating that these cells are myeloid-derived suppressor cells (MDSCs). Blocking arginase and inducible NO synthase did not restore B lymphopoiesis, indicating that inhibition is not mediated by these molecules. Transwell and conditioned-medium experiments showed that MDSCs inhibit B lymphopoiesis via soluble factors, and by cytokine array we identified IL-1 as an important factor. Addition of anti-IL-1 Abs restored B lymphopoiesis in BM cultures containing MDSCs, showing that MDSC inhibition of B lymphopoiesis is mediated by IL-1. By treating hematopoietic precursors with IL-1, we found that multipotent progenitors are targets of IL-1. This study uncovers a novel function for MDSCs to inhibit B lymphopoiesis through IL-1. We suggest that inflammaging contributes to a decline of B lymphopoiesis in aged individuals, and furthermore, that MDSCs and IL-1 provide therapeutic targets for restoration of B lymphopoiesis in aged and obese individuals.

The pharmacology of second-generation chimeric antigen receptors

Second-generation chimeric antigen receptors (CARs) retarget and reprogramme T cells to augment their antitumour efficacy. The combined activating and co-stimulatory domains incorporated in these CARs critically determine the function, differentiation, metabolism and persistence of engineered T cells. CD19-targeted CARs that incorporate CD28 or 4-1BB signalling domains are the best known to date. Both have shown remarkable complete remission rates in patients with refractory B cell malignancies. Recent data indicate that CD28-based CARs direct a brisk proliferative response and boost effector functions, whereas 4-1BB-based CARs induce a more progressive T cell accumulation that may compensate for less immediate potency. These distinct kinetic features can be exploited to further develop CAR-based T cell therapies for a variety of cancers. A new field of immunopharmacology is emerging.

B cells in the aging immune system: time to consider B-1 cells

The investigation of immune senescence has uncovered many changes in B cell development, maintenance, and function with increasing age. However, most of these studies have focused on conventional B cell subsets in the spleen. The B-1 cell subset is an essential arm of the innate immune system, which in general has been understudied in terms of immune senescence. Here, we review what is currently known about B cells during aging and go on to describe why B-1 cell biology is an important component of the aging immune system in the context of diseases that most affect the aged population.

SSC(high)CD11b(high)Ly-6C(high)Ly-6G(low) myeloid cells curtail CD4 T cell response by inducible nitric oxide synthase in murine hepatitis

Myeloid-derived suppressor cells (MDSCs) play an important role in maintaining immune tolerance in response to tumors and inflammatory diseases. Several liver MDSCs have been described in hepatitis in humans and mouse models. Although all the murine MDSCs are CD11b(+)Gr-1(+), their true phenotype and mechanism of suppression remain elusive. This study revealed that SSC(high)CD11b(high)Ly-6C(high)Ly-6G(low) monocytic cells but not the other liver-infiltrating, CD11b(+)Gr-1(+) subsets could suppress CD4 T cell responses. Their suppressive activity was remarkably effective even at a ratio of 1:50 when co-cultured with CD4 T cells. Mechanistically, the suppression was dependent on nitric oxide production by inducible nitric oxide synthase (iNOS). Furthermore, the suppressive function by these liver MDSCs was found to require direct contact with activated CD4 T cells. Adoptive transfer experiments demonstrate that these liver MDSCs can dramatically ameliorate concanavalin A (Con A)-induced fulminant hepatitis in mice. Finally, MDSC-mediated suppression in vivo was dependent on iNOS expression. Altogether, SSC(high)CD11b(high)Ly-6C(high)Ly-6G(low) cells represent authentic MDSCs in the inflammatory liver and may function to minimize collateral damage caused by an overzealous CD4 T cell response following hepatitis infection.

The prenatal origins of cancer

The concept that some childhood malignancies arise from postnatally persistent embryonal cells has a long history. Recent research has strengthened the links between driver mutations and embryonal and early postnatal development. This evidence, coupled with much greater detail on the cell of origin and the initial steps in embryonal cancer initiation, has identified important therapeutic targets and provided renewed interest in strategies for the early detection and prevention of childhood cancer.

B-1a cell diversity: nontemplated addition in B-1a cell Ig is determined by progenitor population and developmental location

Natural Abs produced by B-1a cells are required for immediate protection against infection. The protective capacity of natural Abs is attributed to germline-like structure, which includes the relative absence of N-region addition. Previous studies have shown B-1a cell Ig from aged mice contains abundant nontemplated (N)-additions. B-1a cells have been shown to derive from a specific lineage-negative (Lin(-))CD45R(low/-)CD19(+) progenitor found both in fetal liver and adult bone marrow. In this study, we report identification of a fetal liver population characterized phenotypically as Lin(-)CD45R(-)CD19(-), which gives rise to IgM(+)IgD(low)CD45R(low)CD5(+)Mac-1(+)CD19(high)CD43(+)CD23(low) B-1a cells upon adoptive transfer to SCID recipients. These B-1a cells derived from the Lin(-)CD45R(-)CD19(-) fetal liver population produce natural Ab that binds pneumococcal Ags, but this Ig contains substantial N-addition despite initial absence of TdT. Furthermore, we show extensive N-addition is also present in B-1a cells derived from the Lin(-)CD45R(low/-)CD19(+) B-1 progenitor found in the bone marrow. Together these results demonstrate B-1a cell N-addition depends on the type of progenitor and the location of the progenitor during its development. These findings have implications for how regulation of different progenitors from fetal liver and bone marrow may play a role in the age-related increase in N-region addition by B-1a cells in normal animals.

Genetic and epigenetic control of early lymphocyte development

T, B, and NK lymphocytes are generated from pluripotent hematopoietic stem cells through a successive series of lineage restriction processes. Many regulatory components, such as transcription factors, cytokines/cytokine receptors, and signal transduction molecules orchestrate cell fate specification and determination. In particular, transcription factors play a key role in regulating lineage-associated gene programs. Recent findings suggest the involvement of epigenetic factors in the maintenance of cell fate. Here, we review the early developmental events during lymphocyte lineage determination, focusing on the transcriptional networks and epigenetic regulation. Finally, we also discuss the developmental relationship between acquired and innate lymphoid cells.