Differential expression of CD21 identifies developmentally and functionally distinct subsets of human transitional B cells

Pediatric lymphoproliferative disorders associated with inborn errors of immunity

Both non-malignant and malignant lymphoproliferative disorders (LPD) are commonly seen in patients with inborn errors of immunity (IEI), which may be the presenting manifestations or may develop during the IEI disease course. Here we review the clinical, histopathological, and molecular features of benign and malignant LPD associated with IEI and recognize the diagnostic challenges.

B-cell depletion in autoimmune diseases

B cells have a pivotal function in the pathogenesis of autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis and systemic lupus erythematosus. In autoimmune disease, B cells orchestrate antigen presentation, cytokine production and autoantibody production, the latter via their differentiation into antibody-secreting plasmablasts and plasma cells. This article addresses the current therapeutic strategies to deplete B cells in order to ameliorate or potentially even cure autoimmune disease. It addresses the main target antigens in the B-cell lineage that are used for therapeutic approaches. Furthermore, it summarises the current evidence for successful treatment of autoimmune disease with monoclonal antibodies targeting B cells and the limitations and challenges of these approaches. Finally, the concept of deep B-cell depletion and immunological reset by chimeric antigen receptor T cells is discussed, as well as the lessons from this approach for better understanding the role of B cells in autoimmune disease.

scMHNN: a novel hypergraph neural network for integrative analysis of single-cell epigenomic, transcriptomic and proteomic data

Technological advances have now made it possible to simultaneously profile the changes of epigenomic, transcriptomic and proteomic at the single cell level, allowing a more unified view of cellular phenotypes and heterogeneities. However, current computational tools for single-cell multi-omics data integration are mainly tailored for bi-modality data, so new tools are urgently needed to integrate tri-modality data with complex associations. To this end, we develop scMHNN to integrate single-cell multi-omics data based on hypergraph neural network. After modeling the complex data associations among various modalities, scMHNN performs message passing process on the multi-omics hypergraph, which can capture the high-order data relationships and integrate the multiple heterogeneous features. Followingly, scMHNN learns discriminative cell representation via a dual-contrastive loss in self-supervised manner. Based on the pretrained hypergraph encoder, we further introduce the pre-training and fine-tuning paradigm, which allows more accurate cell-type annotation with only a small number of labeled cells as reference. Benchmarking results on real and simulated single-cell tri-modality datasets indicate that scMHNN outperforms other competing methods on both cell clustering and cell-type annotation tasks. In addition, we also demonstrate scMHNN facilitates various downstream tasks, such as cell marker detection and enrichment analysis.

Cluster-independent marker feature identification from single-cell omics data using SEMITONES

Identification of cell identity markers is an essential step in single-cell omics data analysis. Current marker identification strategies typically rely on cluster assignments of cells. However, cluster assignment, particularly for developmental data, is nontrivial, potentially arbitrary, and commonly relies on prior knowledge. In response, we present SEMITONES, a principled method for cluster-free marker identification. We showcase and evaluate its application for marker gene and regulatory region identification from single-cell data of the human haematopoietic system. Additionally, we illustrate its application to spatial transcriptomics data and show how SEMITONES can be used for the annotation of cells given known marker genes. Using several simulated and curated data sets, we demonstrate that SEMITONES qualitatively and quantitatively outperforms existing methods for the retrieval of cell identity markers from single-cell omics data.

STAT5B restrains human B-cell differentiation to maintain humoral immune homeostasis

BACKGROUND

Lymphocyte differentiation is regulated by coordinated actions of cytokines and signaling pathways. IL-21 activates STAT1, STAT3, and STAT5 and is fundamental for the differentiation of human B cells into memory cells and antibody-secreting cells. While STAT1 is largely nonessential and STAT3 is critical for this process, the role of STAT5 is unknown.

OBJECTIVES

This study sought to delineate unique roles of STAT5 in activation and differentiation of human naive and memory B cells.

METHODS

STAT activation was assessed by phospho-flow cytometry cell sorting. Differential gene expression was determined by RNA-sequencing and quantitative PCR. The requirement for STAT5B in B-cell and CD4+ T-cell differentiation was assessed using CRISPR-mediated STAT5B deletion from B-cell lines and investigating primary lymphocytes from individuals with germline STAT5B mutations.

RESULTS

IL-21 activated STAT5 and strongly induced SOCS3 in human naive, but not memory, B cells. Deletion of STAT5B in B-cell lines diminished IL-21-mediated SOCS3 induction. PBMCs from STAT5B-null individuals contained expanded populations of immunoglobulin class-switched B cells, CD21loTbet+ B cells, and follicular T helper cells. IL-21 induced greater differentiation of STAT5B-deficient B cells into plasmablasts in vitro than B cells from healthy donors, correlating with higher expression levels of transcription factors promoting plasma cell formation.

CONCLUSIONS

These findings reveal novel roles for STAT5B in regulating IL-21-induced human B-cell differentiation. This is achieved by inducing SOCS3 to attenuate IL-21 signaling, and BCL6 to repress class switching and plasma cell generation. Thus, STAT5B is critical for restraining IL-21-mediated B-cell differentiation. These findings provide insights into mechanisms underpinning B-cell responses during primary and subsequent antigen encounter and explain autoimmunity and dysfunctional humoral immunity in STAT5B deficiency.

A Novel Targeted Amplicon Next-Generation Sequencing Gene Panel for the Diagnosis of Common Variable Immunodeficiency Has a High Diagnostic Yield: Results from the Perth CVID Cohort Study

With the advent of next-generation sequencing (NGS), monogenic forms of common variable immunodeficiency (CVID) have been increasingly described. Our study aimed to identify disease-causing variants in a Western Australian CVID cohort using a novel targeted NGS panel. Targeted amplicon NGS was performed on 22 unrelated subjects who met the formal European Society for Immunodeficiencies-Pan-American Group for Immunodeficiency diagnostic criteria for CVID and had at least one of the following additional criteria: disease onset at age <18 years, autoimmunity, low memory B lymphocytes, family history, and/or history of lymphoproliferation. Candidate variants were assessed by in silico predictions of deleteriousness, comparison to the literature, and classified according to the American College of Medical Genetics and Genomics-Association for Molecular Pathology criteria. All detected genetic variants were verified independently by an external laboratory, and additional functional studies were performed if required. Pathogenic or likely pathogenic variants were detected in 6 of 22 (27%) patients. Monoallelic variants of uncertain significance were also identified in a further 4 of 22 patients (18%). Pathogenic variants, likely pathogenic variants, or variants of uncertain significance were found in TNFRSF13B, TNFRSF13C, ICOS, AICDA, IL21R, NFKB2, and CD40LG, including novel variants and variants with unexpected inheritance pattern. Targeted amplicon NGS is an effective tool to identify monogenic disease-causing variants in CVID, and is comparable or superior to other NGS methods. Moreover, targeted amplicon NGS identified patients who may benefit from targeted therapeutic strategies and had important implications for family members.

Concomitant loss of regulatory T and B cells is a distinguishing immune feature of antibody-mediated rejection in kidney transplantation

Although considerable advances have been made in understanding the cellular effector mechanisms responsible for donor-specific antibody generation leading to antibody-mediated rejection (ABMR), the identification of cellular regulators of such immune responses is lacking. To clarify this, we used high dimensional flow cytometry to concomitantly profile and track the two major subsets of regulatory lymphocytes in blood: T regulatory (T_REG) and transitional B cells in a cohort of 96 kidney transplant recipients. Additionally, we established co-culture assays to address their respective capacity to suppress antibody responses in vitro. TREG and transitional B cells were found to be potent suppressors of T follicular helper-mediated B-cell differentiation into plasmablast and antibody generation. TREG and transitional B cells were both durably expanded in patients who did not develop donor-specific antibody post-transplant. However, patients who manifested donor-specific antibody and progressed to ABMR displayed a marked and persistent numerical reduction in TREG and transitional B cells. Strikingly, specific cell clusters expressing the transcription factor T-bet were selectively depleted in both TREG and transitional B-cell compartments in patients with ABMR. Importantly, the coordinated loss of these T-bet⁺CXCR5⁺TREG and T-bet⁺CD21^- transitional B-cell clusters was correlated with increased and inflammatory donor specific antibody responses, more extensive microvascular inflammation and a higher rate of kidney allograft loss. Thus, our study identified coordinated and persistent defects in regulatory T- and B-cell responses in patients undergoing ABMR, which may contribute to their loss of humoral immune regulation, and warrant timely therapeutic interventions to replenish and sustain TREG and transitional B cells in these patients.

Intrinsic Defects in B Cell Development and Differentiation, T Cell Exhaustion and Altered Unconventional T Cell Generation Characterize Human Adenosine Deaminase Type 2 Deficiency

PURPOSE

Deficiency of adenosine deaminase type 2 (ADA2) (DADA2) is a rare inborn error of immunity caused by deleterious biallelic mutations in ADA2. Clinical manifestations are diverse, ranging from severe vasculopathy with lacunar strokes to immunodeficiency with viral infections, hypogammaglobulinemia and bone marrow failure. Limited data are available on the phenotype and function of leukocytes from DADA2 patients. The aim of this study was to perform in-depth immunophenotyping and functional analysis of the impact of DADA2 on human lymphocytes.

METHODS

In-depth immunophenotyping and functional analyses were performed on ten patients with confirmed DADA2 and compared to heterozygous carriers of pathogenic ADA2 mutations and normal healthy controls.

RESULTS

The median age of the patients was 10 years (mean 20.7 years, range 1-44 years). Four out of ten patients were on treatment with steroids and/or etanercept or other immunosuppressives. We confirmed a defect in terminal B cell differentiation in DADA2 and reveal a block in B cell development in the bone marrow at the pro-B to pre-B cell stage. We also show impaired differentiation of CD4⁺ and CD8⁺ memory T cells, accelerated exhaustion/senescence, and impaired survival and granzyme production by ADA2 deficient CD8⁺ T cells. Unconventional T cells (i.e. iNKT, MAIT, Vδ2⁺ γδT) were diminished whereas pro-inflammatory monocytes and CD56^bright immature NK cells were increased. Expression of the IFN-induced lectin SIGLEC1 was increased on all monocyte subsets in DADA2 patients compared to healthy donors. Interestingly, the phenotype and function of lymphocytes from healthy heterozygous carriers were often intermediate to that of healthy donors and ADA2-deficient patients.

CONCLUSION

Extended immunophenotyping in DADA2 patients shows a complex immunophenotype. Our findings provide insight into the cellular mechanisms underlying some of the complex and heterogenous clinical features of DADA2. More research is needed to design targeted therapy to prevent viral infections in these patients with excessive inflammation as the overarching phenotype.

Mechanistic understanding of the combined immunodeficiency in complete human CARD11 deficiency

BACKGROUND

Germline pathogenic variants impairing the caspase recruitment domain family member 11 (CARD11)-B cell chronic lymphocytic leukemia/lymphoma 10 (BCL10)-MALT1 paracaspase (MALT1) (CBM) complex are associated with diverse human diseases including combined immunodeficiency (CID), atopy, and lymphoproliferation. However, the impact of CARD11 deficiency on human B-cell development, signaling, and function is incompletely understood.

OBJECTIVES

This study sought to determine the cellular, immunological, and biochemical basis of disease for 2 unrelated patients who presented with profound CID associated with viral and fungal respiratory infections, interstitial lung disease, and severe colitis.

METHODS

Patients underwent next-generation sequencing, immunophenotyping by flow cytometry, signaling assays by immunoblot, and transcriptome profiling by RNA-sequencing.

RESULTS

Both patients carried identical novel pathogenic biallelic loss-of-function variants in CARD11 (c.2509C>T; p.Arg837∗) leading to undetectable protein expression. This variant prevented CBM complex formation, severely impairing the activation of nuclear factor-κB, c-Jun N-terminal kinase, and MALT1 paracaspase activity in B and T cells. This functional defect resulted in a developmental block in B cells at the naive and type 1 transitional B-cell stage and impaired circulating T follicular helper cell (cT_FH) development, which was associated with impaired antibody responses and absent germinal center structures on lymph node histology. Transcriptomics indicated that CARD11-dependent signaling is essential for immune signaling pathways involved in the development of these cells. Both patients underwent hematopoietic stem cell transplantations, which led to functional normalization.

CONCLUSIONS

Complete human CARD11 deficiency causes profound CID by impairing naive/type 1 B-cell and cT_FH cell development and abolishing activation of MALT1 paracaspase, NF-κB, and JNK activity. Hematopoietic stem cell transplantation functionally restores impaired signaling pathways.

Human marginal zone B cell development from early T2 progenitors

B cells emerge from the bone marrow as transitional (TS) B cells that differentiate through T1, T2, and T3 stages to become naive B cells. We have identified a bifurcation of human B cell maturation from the T1 stage forming IgMhi and IgMlo developmental trajectories. IgMhi T2 cells have higher expression of α4β7 integrin and lower expression of IL-4 receptor (IL4R) compared with the IgMlo branch and are selectively recruited into gut-associated lymphoid tissue. IgMhi T2 cells also share transcriptomic features with marginal zone B cells (MZBs). Lineage progression from T1 cells to MZBs via an IgMhi trajectory is identified by pseudotime analysis of scRNA-sequencing data. Reduced frequency of IgMhi gut-homing T2 cells is observed in severe SLE and is associated with reduction of MZBs and their putative IgMhi precursors. The collapse of the gut-associated MZB maturational axis in severe SLE affirms its existence in health.

Dissection of the Pre-Germinal Center B-Cell Maturation Pathway in Common Variable Immunodeficiency Based on Standardized Flow Cytometric EuroFlow Tools

Introduction

Common Variable Immunodeficiency (CVID) is characterized by defective antibody production and hypogammaglobulinemia. Flow cytometry immunophenotyping of blood lymphocytes has become of great relevance for the diagnosis and classification of CVID, due to an impaired differentiation of mature post-germinal-center (GC) class-switched memory B-cells (MBC) and severely decreased plasmablast/plasma cell (Pb) counts. Here, we investigated in detail the pre-GC B-cell maturation compartment in blood of CVID patients.

Methods

In this collaborative multicentric study the EuroFlow PID 8-color Pre-GC B-cell tube, standardized sample preparation procedures (SOPs) and innovative data analysis tools, were used to characterize the maturation profile of pre-GC B-cells in 100 CVID patients, vs 62 age-matched healthy donors (HD).

Results

The Pre-GC B-cell tube allowed identification within pre-GC B-cells of three subsets of maturation associated immature B-cells and three subpopulations of mature naïve B-lymphocytes. CVID patients showed overall reduced median absolute counts (vs HD) of the two more advanced stages of maturation of both CD5⁺ CD38^+/++ CD21^het CD24⁺⁺ (2.7 vs 5.6 cells/µl, p=0.0004) and CD5⁺ CD38^het CD21⁺ CD24⁺ (6.5 vs 17 cells/µl, p<0.0001) immature B cells (below normal HD levels in 22% and 37% of CVID patients). This was associated with an expansion of CD21^-CD24^- (6.1 vs 0.74 cells/µl, p<0.0001) and CD21^-CD24⁺⁺ (1.8 vs 0.4 cells/µl, p<0.0001) naïve B-cell counts above normal values in 73% and 94% cases, respectively. Additionally, reduced IgMD⁺ (21 vs 32 cells/µl, p=0.03) and IgMD^- (4 vs 35 cells/µl, p<0.0001) MBC counts were found to be below normal values in 25% and 77% of CVID patients, respectively, always together with severely reduced/undetectable circulating blood pb. Comparison of the maturation pathway profile of pre-GC B cells in blood of CVID patients vs HD using EuroFlow software tools showed systematically altered patterns in CVID. These consisted of: i) a normally-appearing maturation pathway with altered levels of expression of >1 (CD38, CD5, CD19, CD21, CD24, and/or smIgM) phenotypic marker (57/88 patients; 65%) for a total of 3 distinct CVID patient profiles (group 1: 42/88 patients, 48%; group 2: 8/88, 9%; and group 3: 7/88, 8%) and ii) CVID patients with a clearly altered pre-GC B cell maturation pathway in blood (group 4: 31/88 cases, 35%).

Conclusion

Our results show that maturation of pre-GC B-cells in blood of CVID is systematically altered with up to four distinctly altered maturation profiles. Further studies, are necessary to better understand the impact of such alterations on the post-GC defects and the clinical heterogeneity of CVID.

Molecular requirements for human lymphopoiesis as defined by inborn errors of immunity

Hematopoietic stem cells (HSCs) are the progenitor cells that give rise to the diverse repertoire of all immune cells. As they differentiate, HSCs yield a series of cell states that undergo gradual commitment to become mature blood cells. Studies of hematopoiesis in murine models have provided critical insights about the lineage relationships among stem cells, progenitors, and mature cells, and these have guided investigations of the molecular basis for these distinct developmental stages. Primary immune deficiencies are caused by inborn errors of immunity that result in immune dysfunction and subsequent susceptibility to severe and recurrent infection(s). Over the last decade there has been a dramatic increase in the number and depth of the molecular, cellular, and clinical characterization of such genetically defined causes of immune dysfunction. Patients harboring inborn errors of immunity thus represent a unique resource to improve our understanding of the multilayered and complex mechanisms underlying lymphocyte development in humans. These breakthrough discoveries not only enable significant advances in the diagnosis of such rare and complex conditions but also provide substantial improvement in the development of personalized treatments. Here, we will discuss the clinical, cellular, and molecular phenotypes, and treatments of selected inborn errors of immunity that impede, either intrinsically or extrinsically, the development of B- or T-cells at different stages.

High proportion of anergic B cells in the bone marrow defined phenotypically by CD21(-/low)/CD38- expression predicts poor survival in diffuse large B cell lymphoma

Background

Diffuse large B cell lymphoma (DLBCL) is the commonest lymphoma that is highly aggressive where one-third of the patients relapse despite effective treatment. Interaction between the lymphoma cells and the non-clonal immune cells within the bone marrow microenvironment is thought to play a critical role in the pathogenesis of DLBCL.

Methods

We used flow cytometry to characterize the proportion of B cell subpopulations in the bone marrow (N = 47) and peripheral blood (N = 54) of 75 DLBCL patients at diagnosis and study their impact on survival.

Results

Anergic B cells in the bone marrow (BM), characterized as having CD21(−/low)/CD38- expression, influenced survival with high numbers (defined as > 13.9%) being associated with significantly shorter overall survival (59.7 months vs 113.6 months, p = 0.0038). Interestingly, low numbers of anergic B cells in the BM (defined as ≤13.9%) was associated with germinal center B cell type of DLBCL (p = 0.0354) that is known to have superior rates of survival when compared to activated B cell type. Finally, Cox regression analysis in our cohort of patients established that the inferior prognosis of having high numbers of anergic B cells in the bone marrow was independent of the established Revised International Prognostic Index (R-IPI) score.

Conclusions

High proportion of anergic B cells in the BM characterized by CD21(−/low)/CD38- expression predicts poor survival outcomes in DLBCL.

Regulation of the germinal center and humoral immunity by interleukin-21

Here we review the critical and non-redundant functions of IL-21 in regulating humoral immune responses. We particularly focus on studies in natura—from individuals from inborn errors of immunity that impact on IL-21 production and/or function.

Cytokines play critical roles in regulating the development, survival, differentiation, and function of immune cells. Cytokines exert their function by binding specific receptor complexes on the surface of immune cells and activating intracellular signaling pathways, thereby resulting in induction of specific transcription factors and regulated expression of target genes. While the function of cytokines is often fundamental for the generation of robust and effective immunity following infection or vaccination, aberrant production or function of cytokines can underpin immunopathology. IL-21 is a pleiotropic cytokine produced predominantly by CD4⁺ T cells. Gene-targeting studies in mice, in vitro analyses of human and murine lymphocytes, and the recent discoveries and analyses of humans with germline loss-of-function mutations in IL21 or IL21R have revealed diverse roles of IL-21 in immune regulation and effector function. This review will focus on recent advances in IL-21 biology that have highlighted its critical role in T cell–dependent B cell activation, germinal center reactions, and humoral immunity and how impaired responses to, or production of, IL-21 can lead to immune dysregulation.

Transitional immature regulatory B cells and regulatory cytokines can discriminate chronic antibody-mediated rejection from stable graft function

BACKGROUND

The balance between inflammatory and anti-inflammatory responses of the immune system has been demonstrated to determine the fate of transplanted allografts. Here we analyzed CD19⁺CD24^hiCD38^hi immature transitional regulatory B (TRB) cells, as well as the gene and protein levels of interleukin (IL)-10 and transforming growth factor (TGF)-β in the three separate groups, include of stable transplanted subjects, chronic antibody-mediated rejection (cAMR) patients, and healthy individuals.

METHOD

Peripheral blood mononuclear cells (PBMCs) from stable subjects (n = 36), cAMR patients (n = 36) and healthy controls (n = 18) were isolated. Flowcytometry was performed for CD19, CD24, and CD38 surface markers. ELISA and quantitative real-time PCR were performed for IL-10 and TGF-β cytokines.

RESULT

The percentages of immature TRB cells were significantly decrease in cAMR patients (0.98%) versus stable recipients (2.81%) and healthy subjects (4.03%) (P = 0.001 and P < 0.001, respectively). Total lymphocytes, circulating B cells, memory and mature subsets of B cells did not show any significant difference between the groups. TGF-β mRNA was 3-fold upregulated in the cAMR group compared to stable patients (P < 0.001.), but without significant alteration at the protein level. Also, long-term survival renal transplant recipients had a higher protein but not mRNA levels of IL-10 than short-term survival renal transplant recipients.

CONCLUSION

It seems that immature TRB cell subpopulation might be a crucial regulator of immune system response and plays an important role in determining the transplantation outcome. Furthermore, immunosuppressive IL-10 and TGF-β cytokines might act as a double sword and can exhibit either pathogenic or protective effects against allograft.

DOCK2 couples with LEF-1 to regulate B cell metabolism and memory response

Dedicator of cytokinesis 2 (DOCK2) is essential for the B cell differentiation, BCR signaling and humoral immune response. However, the role of DOCK2 in the memory response of B cell is unknown. By using two DOCK2 deficient patients, we found that the memory B cells were decreased and the early activation of DOCK2 deficient memory B cells was abolished to the degree of naïve B cells due to the decreased expression of CD19 and CD21 mechanistically. Interestingly the expression of LEF-1, a negative regulator of CD21, was increased in DOCK2 deficient B cells. This was linked to the increased expression of HIF-1α and cell metabolism, which in turn affected the ER structure. Finally, the reduction of memory B cells in DOCK2 patients was due to the increased apoptosis, which might be related with the increased metabolism.

The Plasticity of Newly Formed B Cells

Newly formed B cells (NF-B cells) that emerge from the bone marrow to the periphery have often been referred to as immature or transitional B cells. However, NF-B cells have several striking characteristics, including a distinct BCR repertoire, high expression of AID, high sensitivity to PAMPs, and the ability to produce cytokines. A number of findings do not support their designation as immature because NF-B cells have the potential to become Ab-producing cells and to undergo class-switch recombination. In this review, we provide a fresh perspective on NF-B cell functions and describe some of the signals driving their activation. We summarize growing evidence supporting a role for NF-B cells in protection against infections and as a potential source of autoantibody-producing cells in autoimmune diseases such as systemic lupus erythematosus.

Transitional B cells involved in autoimmunity and their impact on neuroimmunological diseases

Transitional B cells (TrB cells) represent a crucial link between immature B cells in the bone marrow and mature peripheral B cells. Although TrB cells represent one of the regulatory B cell subpopulations in healthy individuals, the frequency of CD24^hiCD38^hi TrB cells in circulation may be altered in individuals with autoimmune diseases, such as multiple sclerosis, neuromyelitisoptica spectrum disorders, systemic lupus erythematosus, Sjögren’s syndrome, rheumatoid arthritis, systemic sclerosis, and juvenile dermatomyositis. Although TrB cells play regulatory roles under inflammatory conditions, consequences of their functional impairment vary across autoimmune diseases. Since the origin, development, and function of TrB cells, especially in humans, remain unclear and controversial, this review aimed to discuss the characteristics of TrB cells at steady state and explore their role in various immune diseases, including autoimmune rheumatic diseases and neuroimmunological diseases.

Challenges and Opportunities for Consistent Classification of Human B Cell and Plasma Cell Populations

The increasingly recognized role of different types of B cells and plasma cells in protective and pathogenic immune responses combined with technological advances have generated a plethora of information regarding the heterogeneity of this human immune compartment. Unfortunately, the lack of a consistent classification of human B cells also creates significant imprecision on the adjudication of different phenotypes to well-defined populations. Additional confusion in the field stems from: the use of non-discriminatory, overlapping markers to define some populations, the extrapolation of mouse concepts to humans, and the assignation of functional significance to populations often defined by insufficient surface markers. In this review, we shall discuss the current understanding of human B cell heterogeneity and define major parental populations and associated subsets while discussing their functional significance. We shall also identify current challenges and opportunities. It stands to reason that a unified approach will not only permit comparison of separate studies but also improve our ability to define deviations from normative values and to create a clean framework for the identification, functional significance, and disease association with new populations.

Flow Cytometric-Based Analysis of Defects in Lymphocyte Differentiation and Function Due to Inborn Errors of Immunity

The advent of flow cytometry has revolutionized the way we approach our research and answer specific scientific questions. The flow cytometer has also become a mainstream diagnostic tool in most hospital and pathology laboratories around the world. In particular the application of flow cytometry has been instrumental to the diagnosis of primary immunodeficiencies (PIDs) that result from monogenic mutations in key genes of the hematopoietic, and occasionally non-hematopoietic, systems. The far-reaching applicability of flow cytometry is in part due to the remarkable sensitivity, down to the single-cell level, of flow-based assays and the extremely user-friendly platforms that enable comprehensive analysis, data interpretation, and importantly, robust and rapid methods for diagnosing PIDs. A prime example is the absence of peripheral blood B cells in patients with agammaglobulinemia due to mutations in BTK or related genes in the BCR signaling pathway. Similarly, the development of intracellular staining protocols to detect expression of SAP, XIAP, or DOCK8 expedites the rapid diagnosis of the X-linked lymphoproliferative diseases or an autosomal recessive form of hyper-IgE syndrome (HIES), respectively. It has also become evident that distinct cohorts of PID patients exhibit unique “lymphocyte phenotypic signatures” that are often diagnostic even prior to identifying the genetic lesion. Flow cytometry-based sorting provides a technique for separating specific subsets of immune cells such that they can be studied in isolation. Thus, flow-based assays can be utilized to measure immune cell function in patients with PIDs, such as degranulation by cytotoxic cells, cytokine expression by many immune cells (i.e., CD4⁺ and CD8⁺ T cells, macrophages etc.), B-cell differentiation, and phagocyte respiratory burst in vitro. These assays can also be performed using unfractionated PBMCs, provided the caveat that the composition of lymphocytes between healthy donors and the PID patients under investigation is recognized. These functional deficits can assist not only in the clinical diagnosis of PIDs, but also reveal mechanisms of disease pathogenesis. As we move into the next generation of multiparameter flow cytometers, here we review some of our experiences in the use of flow cytometry in the study, diagnosis, and unraveling the pathophysiology of PIDs.

National Institutes of Health-Defined Chronic Graft-vs.-Host Disease in Pediatric Hematopoietic Stem Cell Transplantation Patients Correlates With Parameters of Long-Term Immune Reconstitution

Recent data revealed the importance of immune reconstitution (IR) for the evaluation of possible biomarkers in National Institutes of Health (NIH)–defined chronic graft-vs.-host disease (cGVHD) and its clinical aspects. In this large pediatric study (n = 146), we have analyzed whether cellular and humoral parameters of IR in the long-term follow-up (FU) with a special emphasis on B-cell reconstitution correlate with NIH-defined cGVHD criteria. HYPOTHESIS: we were especially interested in whether meaningful cGVHD biomarkers could be defined in a large pediatric cohort. We here demonstrate for the first time in a highly homogenous pediatric patient cohort that both cGVHD (n = 38) and its activity were associated with the perturbation of the B-cell compartment, including low frequencies of CD19⁺CD27⁺ memory B-cells and increased frequencies of circulating CD19⁺CD21^low B-cells, a well-known hyperactivated B-cell subset frequently found elevated in chronic infection and autoimmunity. Notably, resolution of cGVHD correlated with expansion of CD19⁺CD27⁺ memory B-cells and normalization of CD19⁺CD21^low B-cell frequencies. Moreover, we found that the severity of cGVHD had an impact on parameters of IR and that severe cGVHD was associated with increased CD19⁺CD21^low B-cell frequencies. When comparing the clinical characteristics of the active and non-active cGVHD patients (in detail at time of analyses), we found a correlation between activity and a higher overall severity of cGVHD, which means that in the active cGVHD patient group were more patients with a higher disease burden of cGVHD—despite similar risk profiles for cGVHD. Our data also provide solid evidence that the time point of analysis regarding both hematopoietic stem cell transplantation (HSCT) FU and cGVHD disease activity may be of critical importance for the detailed investigation of pediatric cohorts. Finally, we have proven that the differences in risk factors and patterns of IR, with cGVHD as its main confounding factor, between malignant and non-malignant diseases, are important to be considered in future studies aiming at identification of novel biomarkers for cGVHD.

Belimumab alters transitional B-cell subset proportions in patients with stable systemic lupus erythematosus

OBJECTIVE

We evaluated the effects of the B-cell activating factor (BAFF)-targeting antibody Belimumab on human nonmemory B-cell pools. Human B-cell pools were identified using surface markers adapted from mouse studies that specifically assessed reductions in immature B cells due to BAFF depletion. Patients with systemic lupus erythematosus (SLE) have high levels of both BAFF and immature B cells. Mechanistic mouse studies provide a framework for understanding human responses to therapies that target B cells.

METHODS

Peripheral blood mononuclear cells were isolated from healthy donors and SLE patients on Belimumab or standard-of-care therapy (SCT). Cells were stained for flow cytometry to identify B-cell subsets based on CD21/CD24. Differences in subset proportions were determined by one-way ANOVA and Tukey's post hoc test.

RESULTS

Patients treated with Belimumab show alterations in the nonmemory B-cell pool characterized by a decrease in the Transitional 2 (T2) subset (p = 0.002), and an increase in the proportion of Transitional 1 (T1) cells (p = 0.005) as compared with healthy donors and SCT patients. The naïve B-cell compartment showed no significant differences between the groups (p = 0.293).

CONCLUSION

Using a translational approach, we show that Belimumab-mediated BAFF depletion reduces the T2 subset in patients, similar to observations in mouse models with BAFF depletion.

Hematopoietic stem cell transplant effectively rescues lymphocyte differentiation and function in DOCK8-deficient patients

Bi-allelic inactivating mutations in DOCK8 cause a combined immunodeficiency characterised by severe pathogen infections, eczema, allergies, malignancy and impaired humoral responses. These clinical features result from functional defects in most lymphocyte lineages. Thus, DOCK8 plays a key role in immune cell function. Hematopoietic stem cell transplantation (HSCT) is curative for DOCK8 deficiency. While previous reports have described clinical outcomes for DOCK8 deficiency following HSCT, the effect on lymphocyte reconstitution and function has not been investigated. Our study determined whether defects in lymphocyte differentiation and function in DOCK8-deficient patients were restored following HSCT. DOCK8-deficient T and B lymphocytes exhibited aberrant activation and effector function in vivo and in vitro. Frequencies of αβ T and MAIT cells were reduced while γδT cells were increased in DOCK8-deficient patients. HSCT improved, abnormal lymphocyte function in DOCK8-deficient patients. Elevated total and allergen-specific IgE in DOCK8-deficient patients decreased over time following HSCT. Our results document the extensive catalogue of cellular defects in DOCK8-deficient patients, and the efficacy of HSCT to correct these defects, concurrent with improvements in clinical phenotypes. Overall, our findings provide mechanisms at a functional cellular level for improvements in clinical features of DOCK8 deficiency post-HSCT, identify biomarkers that correlate with improved clinical outcomes, and inform the general dynamics of immune reconstitution in patients with monogenic immune disorders following HSCT.

B Cell Reconstitution and Influencing Factors After Hematopoietic Stem Cell Transplantation in Children

B cell reconstitution after hematopoietic stem cell transplantation (HSCT) is variable and influenced by different patient, donor, and treatment related factors. In this review we describe B cell reconstitution after pediatric allogeneic HST, including the kinetics of reconstitution of the different B cell subsets and the development of the B cell repertoire, and discuss the influencing factors. Observational studies show important roles for stem cell source, conditioning regimen, and graft vs. host disease in B cell reconstitution. In addition, B cell recovery can play an important role in post-transplant infections and vaccine responses to encapsulated bacteria, such as pneumococcus. A substantial number of patients experience impaired B cell function and/or dependency on Ig substitution after allogeneic HSCT. The underlying mechanisms are largely unresolved. The integrated aspects of B cell recovery after HSCT, especially BCR repertoire reconstitution, are awaiting further investigation using modern techniques in order to gain more insight into B cell reconstitution and to develop strategies to improve humoral immunity after allogeneic HSCT.

Effective B cell activation in vitro during viremic HIV-1 infection with surrogate T cell stimulation

Identifying HIV-1-associated B cell defects and responses to activation may direct interventions to circumvent their impaired antibody responses to infection and vaccines. Among 34 viremic HIV-1-infected and 20 seronegative control adults, we measured baseline frequencies and activation of B and T cell subsets, expression of activation-induced cytidine deaminase (AID), potential determinants of B cell activation in vivo and B and T cell responses in vitro. At baseline, HIV-1 infection was associated with increased IgM memory and decreased anergic cell frequencies, as well as increased activation in all 10 B cell subsets compared with controls. HIV-1 status, TFH activation, and BAFF were significant potential drivers of B cell activation. Despite high baseline activation among HIV-1-infected subjects, stimulation in vitro with combined surrogates for antigen (anti-IgM), cognate (CD40 ligand) and soluble T cell factors (IL-4) elicited comparable B cell activation, transitions from naïve to class-switched memory cells and AID expression in both groups. In summary, viremic HIV-1 infection perturbs circulating B cell subsets and activation at each stage of B cell maturation. However, that appropriate stimulation of B cells elicits effective activation and maturation provides impetus for advancing vaccine development to prevent secondary infections by circumventing early B cell defects.

Comprehensive B Cell Phenotyping Profile for Chronic Graft-versus-Host Disease Diagnosis

Previous studies have reported single B cell-related chronic graft-versus-host disease diagnostic (cGVHD) biomarkers, such as B cell-activating factor (BAFF), CD21^low, and immature B cells, but research on the performance of biomarker combinations and the covariate effect of steroids is lacking. The primary objective of this study was to determine the most accurate combination of B cell populations using cell surface staining flow cytometry in an independent cohort of patients with cGVHD. Secondary objectives included assessing the effect of corticosteroid use at sample collection on the makeup and accuracy of the diagnostic panel and identifying the mechanism underlying low surface expression of BAFF receptor (BAFF-R) on B cells in cGVHD. Flow cytometry analysis was performed in an adult cohort of post-HCT patients with cGVHD onset (n = 44) and time-matched recipients without cGVHD (n = 63). We confirmed that the onset of cGVHD was associated with higher soluble BAFF (sBAFF) levels, elevated CD27^-CD10^-CD21^low CD19⁺ B cell and classical switched memory B cell counts, and reduced transitional and naïve B cell counts. The highest single B cell population area under the receiver operating characteristic (ROC) curve (AUC) was .72 for transitional type 1 CD21^low B cells. We also showed a significant inverse relationship between sBAFF and surface BAFF-R expression caused by sBAFF modulation of BAFF-R. Steroid use at sample collection influenced the significance of the sBAFF:B cell ratio, naïve and marginal zone-like B cells. The optimal combination of B cell subsets most significantly associated with cGVHD onset with or without concurrent corticosteroid use resulted in ROC AUCs of .87 and .84, respectively. Transitional and CD21^low B cells were the only populations present in both panels; however, analyzing only these populations resulted in ROC AUCs of .79 and .78, respectively. This suggests that the inclusion of other populations and use of different panels depending on steroid use is necessary to achieve better accuracy. sBAFF was not a component of either panel. These novel B cell profiles could be tested prospectively in patients post-HSCT and could lead to focused mechanistic studies.

Deficient humoral responses and disrupted B-cell immunity are associated with fatal SFTSV infection

Severe Fever with Thrombocytopenia Syndrome (SFTS), an emerging infectious disease caused by a novel phlebovirus, is associated with high fatality. Therapeutic interventions are lacking and disease pathogenesis is yet to be fully elucidated. The anti-viral immune response has been reported, but humoral involvement in viral pathogenesis is poorly understood. Here we show defective serological responses to SFTSV is associated with disease fatality and a combination of B-cell and T-cell impairment contribute to disruption of anti-viral immunity. The serological profile in deceased patients is characterized by absence of specific IgG to viral nucleocapsid and glycoprotein due to failure of B-cell class switching. Expansion and impairment of antibody secretion is a signature of fatal SFTSV infection. Apoptosis of monocytes in the early stage of infection diminishes antigen-presentation by dendritic cells, impedes differentiation and function of T follicular helper cells, and contributes to failure of the virus-specific humoral response.

SFTSV is a novel phlebovirus associated with high fatality, but understanding of pathogenesis is lacking. Here the authors show defective cellular immunity, deficient antibody production and defunct humoral immunity is associated with fatal infection in human cases of infection.

Germline-activating mutations in PIK3CD compromise B cell development and function

Gain-of-function (GOF) mutations in PIK3CD, encoding the p110δ subunit of phosphatidylinositide 3-kinase (PI3K), cause a primary immunodeficiency. Affected individuals display impaired humoral immune responses following infection or immunization. To establish mechanisms underlying these immune defects, we studied a large cohort of patients with PIK3CD GOF mutations and established a novel mouse model using CRISPR/Cas9-mediated gene editing to introduce a common pathogenic mutation in Pik3cd In both species, hyperactive PI3K severely affected B cell development and differentiation in the bone marrow and the periphery. Furthermore, PI3K GOF B cells exhibited intrinsic defects in class-switch recombination (CSR) due to impaired induction of activation-induced cytidine deaminase (AID) and failure to acquire a plasmablast gene signature and phenotype. Importantly, defects in CSR, AID expression, and Ig secretion were restored by leniolisib, a specific p110δ inhibitor. Our findings reveal key roles for balanced PI3K signaling in B cell development and long-lived humoral immunity and memory and establish the validity of treating affected individuals with p110δ inhibitors.

Identification of early B cell precursors (stage 1 and 2 hematogones) in the peripheral blood

Differentiating malignant B-lymphoblasts from early benign B cell precursors (hematogones) is a vital component of the diagnosis of B-lymphoblastic leukaemia. It has been previously reported that only late-stage B cell precursors circulate in the peripheral blood. Consequently, flow cytometric detection of cells with immunophenotypic findings similar to earlier stage precursors in the peripheral blood justifiably raises concern for involvement by B-lymphoblastic leukaemia. We report here, however, that benign early B cell precursors can indeed be detected in the peripheral blood, thus complicating the interpretation of flow cytometric findings derived from these sample types. A retrospective search of our collective databases identified 13 cases containing circulating early stage B cell precursors. The patients ranged in age from 15 days to 85 years old. All positive cases demonstrated that the earlier B cell precursors were associated with later stage precursors, a finding that could help differentiate these cells from B-lymphoblastic leukaemia.

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

Association of Defective Regulation of Autoreactive Interleukin-6-Producing Transitional B Lymphocytes With Disease in Patients With Systemic Sclerosis

OBJECTIVE

Systemic sclerosis (SSc) has the highest case-specific mortality of any rheumatic disease, and no effective therapy is available. A clear manifestation of SSc is the presence of autoantibodies. However, the origin of autoantibody-producing B lymphocytes, their mechanisms of activation and autoantibody production, and their role remain unclear. This study was undertaken to identify mechanisms that contribute to pathogenic B cell generation and involvement in SSc and to assess the altered distribution and function of B cells in SSc patients.

METHODS

Multicolor flow cytometry was performed to determine B cell subset distribution, cytokine production, and tolerance induction in SSc patients and healthy controls. Cytokine production following stimulation of the cells ex vivo was determined by multiplex assay.

RESULTS

A range of defects in B lymphocyte tolerance and cytokine production in SSc were noted. There was evidence of altered distribution of transitional B cell subsets, increased production of interleukin-6 (IL-6) and IL-8, and defective tolerance induction in SSc B cells. In addition, B cells from SSc patients had a reduced ability to produce IL-10 when stimulated through innate immune pathways. In contrast to healthy individuals, tolerance checkpoints in SSc patients failed to suppress the emergence of B cells that produce autoantibodies with specificity to the Scl-70 antigen, which is strongly associated with SSc. These defects were paralleled by altered intracellular signaling and apoptosis following B cell receptor engagement.

CONCLUSION

Our findings provide new insights into mechanisms underlying defective B lymphocyte responses in patients with SSc and their contribution to disease.

Evidence for B Cell Exhaustion in Chronic Graft-versus-Host Disease

Chronic graft-versus-host disease (cGvHD) remains a major complication of allogeneic hematopoietic stem cell transplantation (HSCT). A number of studies support a role for B cells in the pathogenesis of cGvHD. In this study, we report the presence of an expanded population of CD19+CD21- B cells with features of exhaustion in the peripheral blood of patients with cGvHD. CD21- B cells were significantly increased in patients with active cGvHD compared to patients without cGvHD and healthy controls (median 12.2 versus 2.12 versus 3%, respectively; p < 0.01). Compared with naïve (CD27-CD21+) and classical memory (CD27+CD21+) B cells, CD19+CD21- B cells in cGvHD were CD10 negative, CD27 negative and CD20hi, and exhibited features of exhaustion, including increased expression of multiple inhibitory receptors such as FCRL4, CD22, CD85J, and altered expression of chemokine and adhesion molecules such as CD11c, CXCR3, CCR7, and CD62L. Moreover, CD21- B cells in cGvHD patients were functionally exhausted and displayed poor proliferative response and calcium mobilization in response to B-cell receptor triggering and CD40 ligation. Finally, the frequencies of circulating CD21- B cells correlated with cGvHD severity in patients after HSCT. Our study further characterizes B cells in chronic cGVHD and supports the use of CD21-CD27-CD10- B cell frequencies as a biomarker of disease severity.

Early recovery of circulating immature B cells in B-lymphoblastic leukemia patients after CD19 targeted CAR T cell therapy: A pitfall for minimal residual disease detection

BACKGROUND

CD19-targeted chimeric-antigen receptor-modified T-cells (CAR-T) are promising in the treatment of refractory B-lymphoblastic leukemia (B-ALL). Minimal residual disease (MRD) detection by multicolor flow cytometry (FCM) is critical to distinguish B-ALL MRD from regenerating, non-neoplastic B-cell populations.

METHODS

FCM was performed on samples from 9 patients with B-ALL treated with CAR-T.

RESULTS

All 9 patients showed response to CAR-T. Additionally, FCM revealed circulating CD10 + B cells, potentially mimicking MRD. Circulating CD10+ B-cells were detected in blood from 3 days to 3 months after CAR-T, comprising 73% (median) of B-cells (52-83%, 95%CI). They expressed CD19, CD10, CD20, bright CD9, CD22, CD24, moderate CD38 and dim CD58, but were CD34 (-), with bright CD45 and polyclonal surface light chain immunoglobulin (sIg) expression. A similar CD10 + B-cell subpopulation was detected by marrow FCM, amidst abundant B-cell precursors.

CONCLUSIONS

These circulating CD10 + B-cells are compatible with immature B-cells, and are a reflection of B-cell recovery within the marrow. They are immunophenotypically distinguishable from residual B-ALL. Expression of light chain sIg and key surface antigens characterizing regenerating B-cell precursors can distinguish immature B-cells from B-ALL MRD and prevent misdiagnosis. © 2017 International Clinical Cytometry Society.

Aiolos Overexpression in Systemic Lupus Erythematosus B Cell Subtypes and BAFF-Induced Memory B Cell Differentiation Are Reduced by CC-220 Modulation of Cereblon Activity

BAFF is a B cell survival and maturation factor implicated in the pathogenesis of systemic lupus erythematosus (SLE). In this in vitro study, we describe that soluble BAFF in combination with IL-2 and IL-21 is a T cell contact-independent inducer of human B cell proliferation, plasmablast differentiation, and IgG secretion from circulating CD27⁺ memory and memory-like CD27^-IgD^- double-negative (DN) B cells, but not CD27^-IgD⁺ naive B cells. In contrast, soluble CD40L in combination with IL-2 and IL-21 induces these activities in both memory and naive B cells. Blood from healthy donors and SLE patients have similar circulating levels of IL-2, whereas SLE patients exhibit elevated BAFF and DN B cells and reduced IL-21. B cell differentiation transcription factors in memory, DN, and naive B cells in SLE show elevated levels of Aiolos, whereas Ikaros levels are unchanged. Treatment with CC-220, a modulator of the cullin ring ligase 4-cereblon E3 ubiquitin ligase complex, reduces Aiolos and Ikaros protein levels and BAFF- and CD40L-induced proliferation, plasmablast differentiation, and IgG secretion. The observation that the soluble factors BAFF, IL-2, and IL-21 induce memory and DN B cell activation and differentiation has implications for extrafollicular plasmablast development within inflamed tissue. Inhibition of B cell plasmablast differentiation by reduction of Aiolos and Ikaros may have utility in the treatment of SLE, where elevated levels of BAFF and Aiolos may prime CD27⁺ memory and DN memory-like B cells to become Ab-producing plasmablasts in the presence of BAFF and proinflammatory cytokines.

B-Cell-Based and Soluble Biomarkers in Body Liquids for Predicting Acute/Chronic Graft-versus-Host Disease after Allogeneic Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the main curative therapy for hematological malignancy such as leukemias, lymphomas, or multiple myelomas and some other hematological disorders. In this therapy, cure of hematological diseases relies on graft-versus-malignancy effects by allogenic immune cells. However, severe posttransplant treatment-associated complications such as acute graft-versus-host disease (aGvHD) and chronic graft-versus-host disease (cGvHD) limit this approach. Most research into GvHD has concentrated on the aGvHD, while the more complex and multifaceted chronic form has been largely poorly investigated. cGvHD is a multi-organ autoimmune disorder and is the major cause of non-relapse morbidity and mortality following allo-HSCT, occurring in about 50% of patients, or 13,000–15,000 patients per year worldwide. Therefore, there is a high medical need for an early prediction of these therapy-associated toxicities. Biomarkers have gained importance over the last decade in diagnosis, in prognosis, and in prediction of pending diseases or side effects. Biomarkers can be cells, factors isolated from target tissues, or soluble factors that can be detected in body fluids. In this review, we aim to summarize some of the recent developments of biomarkers in the field of allo-HSCT. We will focus on cell-based biomarkers (B-cell subsets) for cGvHD and soluble factors including microRNA (miRNA), which are excreted into serum/plasma and urine. We also discuss the potential role of cytosolic and extracellular 70 kDa heat shock proteins (HSP70) as potential biomarkers for aGvHD and their role in preclinical models. Proteomic biomarkers in the blood have been used as predictors of treatment responses in patients with aGvHD for many years. More recently, miRNAs have been found to serve as a biomarker to diagnose aGvHD in the plasma. Another development relates to urine-based biomarkers that are usually detected by capillary electrophoresis and mass spectrometry. These biomarkers have the potential to predict the development of severe aGvHD (grades III–IV), overall mortality, and the pending development of cGvHD in patients posttransplant.

A novel humanized mouse model with significant improvement of class-switched, antigen-specific antibody production

Humanized mice are a powerful tool for the study of human hematopoiesis and immune function in vivo. However, the existing models cannot support robust adaptive immune responses, especially the generation of class-switched, antigen-specific antibody responses. Here we describe a new mouse strain, in which human interleukin 6 (IL-6) gene encoding the cytokine that is important for B- and T-cell differentiation was knocked into its respective mouse locus. The provision of human IL-6 not only enhanced thymopoiesis and periphery T-cell engraftment, but also significantly increased class switched memory B cells and serum immunoglobulin G (IgG). In addition, immunization with ovalbumin (OVA) induced OVA-specific B cells only in human IL-6 knock-in mice. These OVA-specific antibodies displayed the highest frequency of somatic mutation, further suggesting that human IL-6 is important for efficient B-cell activation and selection. We conclude that human IL-6 knock-in mice represent a novel and improved model for human adaptive immunity without relying on complex surgery to transplant human fetal thymus and liver. These mice can therefore be used to exploit or evaluate immunization regimes that would be unethical or untenable in humans.

Polyspecificity of Anti-lipid A Antibodies and Its Relevance to the Development of Autoimmunity

The process of natural selection favours germ-line gene segments that encode CDRs that have the ability to recognize a range of structurally related antigens. This presents an immunological advantage to the host, as it can confer protection against a common pathogen and still cope with new or changing antigens. Cross-reactive and polyspecific antibodies also play a central role in autoimmune responses, and a link has been shown to exist between auto-reactive B cells and certain bacterial infections. Bacterial DNA, lipids, and carbohydrates have been implicated in the progression of autoimmune diseases such as systemic lupus erythematosus. As well, reports of anti-lipid A antibody polyspecificity towards single-stranded DNA together with the observed sequence homology amongst isolated auto- and anti-lipid A antibodies has prompted further study of this phenomenon. Though the lipid A epitope appears cryptic during Gram-negative bacterial infection, there have been several reported instances of lipid A-specific antibodies isolated from human sera, some of which have exhibited polyspecificity for single stranded DNA. In such cases, the breakdown of negative selection through polyspecificity can have the unfortunate consequence of autoimmune disease. This review summarizes current knowledge regarding such antibodies and emphasizes the features of S1-15, A6, and S55-5, anti-lipid A antibodies whose structures were recently determined by X-ray crystallography.

Immune Reconstitution after Allogeneic Hematopoietic Stem Cell Transplantation

The timely reconstitution and regain of function of a donor-derived immune system is of utmost importance for the recovery and long-term survival of patients after allogeneic hematopoietic stem cell transplantation (HSCT). Of note, new developments such as umbilical cord blood or haploidentical grafts were associated with prolonged immunodeficiency due to delayed immune reconstitution, raising the need for better understanding and enhancing the process of immune reconstitution and finding strategies to further optimize these transplant procedures. Immune reconstitution post-HSCT occurs in several phases, innate immunity being the first to regain function. The slow T cell reconstitution is regarded as primarily responsible for deleterious infections with latent viruses or fungi, occurrence of graft-versus-host disease, and relapse. Here we aim to summarize the major steps of the adaptive immune reconstitution and will discuss the importance of immune balance in patients after HSCT.

Origin of Waldenstrom's macroglobulinaemia

Waldenstrom's macroglobulinaemia (WM) is an MYD88^L265P-mutated lymphoplasmacytic lymphoma that invades bone marrow and secretes monoclonal immunoglobulin M (IgM). WM cells are usually unable to undergo class switch recombination, and have mutated IGHV, with a typical immunophenotype CD19⁺/CD22^low+/CD23^-/CD25⁺/CD27⁺/CD45⁺/CD38^low+/SmIgM⁺ (negative for CD5, CD10, CD11c, CD103). This immunophenotype matches memory B cells (smIgM^-/+/CD10^-/CD19⁺/CD20⁺/CD27⁺/CD38^low+/CD45⁺), representing 30% of B cells in the blood. Fifty percent of them have not undergone class switch recombination and are IgM⁺. These cells have suffered somatic hypermutation as WM cells. Genetic abnormalities do not abrogate the capacity to progress to plasma cells that usually belong to the clonal WM compartment, with a normal immunophenotype and functional characteristics. However, some WM cells are CD27^-, MYD88^WT, without somatic hypermutation, or with class switch recombination capable of reactivation. Thus, most data support a B-memory-cell origin for WM, but a small fraction of cases may have a different origin.

Novel Strategy for Phenotypic Characterization of Human B Lymphocytes from Precursors to Effector Cells by Flow Cytometry

A precise identification and phenotypic characterization of human B-cell subsets is of crucial importance in both basic research and medicine. In the literature, flow cytometry studies for the phenotypic characterization of B-lymphocytes are mainly focused on the description of a particular cell stage, or of specific cell stages observed in a single type of sample. In the present work, we propose a backbone of 6 antibodies (CD38, CD27, CD10, CD19, CD5 and CD45) and an efficient gating strategy to identify, in a single analysis tube, a large number of B-cell subsets covering the whole B-cell differentiation from precursors to memory and plasma cells. Furthermore, by adding two antibodies in an 8-color combination, our approach allows the analysis of the modulation of any cell surface marker of interest along B-cell differentiation. We thus developed a panel of seven 8-colour antibody combinations to phenotypically characterize B-cell subpopulations in bone marrow, peripheral blood, lymph node and cord blood samples. Beyond qualitative information provided by biparametric representations, we also quantified antigen expression on each of the identified B-cell subsets and we proposed a series of informative curves showing the modulation of seventeen cell surface markers along B-cell differentiation. Our approach by flow cytometry provides an efficient tool to obtain quantitative data on B-cell surface markers expression with a relative easy-to-handle technique that can be applied in routine explorations.

Dysfunctions in the migratory phenotype and properties of circulating immature transitional B cells during HIV-1 infection

OBJECTIVE

The frequency of immature transitional B cells is increased in blood of HIV-1-infected individuals. We investigated whether HIV-1 infection affects expression and function of chemokine receptors important for egress of immature transitional B cells from bone marrow and migration to lymphoid organs.

DESIGN

This is a cross-sectional study analysing the migratory phenotype and function of immature transitional B cells in HIV-1-infected individuals, in relation to antiretroviral treatment and age.

METHODS

Frequency of blood immature transitional B cells and their phenotypic characteristics, including chemokine receptors and a maturation marker, were determined by immunostainings. Migratory capacities were studied in a migration assay.

RESULTS

The increased frequency of immature transitional B cells in untreated HIV-1 infection was normalized in patients receiving antiretroviral treatment; in our cohorts, age did not have an impact on the frequency of circulating immature transitional B cells. Immature transitional B cells from nontreated patients expressed low levels of CD21 molecule. We found an elevated frequency of CXCR3 and CXCR4 expressing immature transitional B cells in treated and nontreated patients. CXCR4 receptor was unresponsive to CXCL12 ligand in in-vitro migration and internalization assays. In addition, CXCR5 expression was downregulated on immature transitional B cells from infected patients, and these cells migrated poorly in response to CXCR5 ligand.

CONCLUSION

Circulating immature transitional B cells from HIV-1-infected patients are not fully mature, probably due to premature egress from bone marrow; these cells showed a phenotype which could impair entry into secondary lymphoid organs. Changes in migratory capacity of immature transitional B cells may affect B-cell maturation during HIV-1 infection.

The B-cell compartment in antibody-deficient infants and young children - developing common variable immunodeficiency or transient immune maturation?

Background

Hypogammaglobulinemia in early childhood is a common feature characterized by distinct intrinsic and extrinsic factors leading to disturbed peripheral blood lymphocyte homeostasis. Detailed flow cytometric immunophenotyping of the peripheral blood B cell compartment is an informative tool for delineating disturbed generation of B cell subpopulations crucial for the diagnosis of hypogammaglobulinemia in young children.

Methods

We analyzed by flow cytometry the proportions and absolute values of total, naïve, memory - non-switched and switched, transitional and immature B lymph cells as well as plasmablasts in the peripheral blood of 50 hypogammaglobulinemic children aged from 3 to 50 months.

Results

Beyond physiological, age-related changes within the B cell pool, a proportion of children manifested defective differentiation into switched memory and accumulation of CD21lo immature B cells.

Conclusions

Dynamic shifts within B cell subpopulations of the immature immune system being most prominent during the first two years of life contribute to the age-related developmental abnormalities of the B cell compartment. Therefore, a reliable diagnosis of common variable immunodeficiency (CVID) in young hypogammaglobulinemic children cannot yet be established despite their clinical and immunological phenotypes sharing common features with this primary immunodeficiency.

CD21(-/low) B cells in human blood are memory cells

The complement receptor 2 (CR2, CD21) is part of a complex (CD21/CD19/CD81) acting as a co-receptor to the B cell receptor (BCR). Simultaneous triggering of the BCR and CD21 lowers the threshold for B cell activation. Although CD21 is important, B cells that express low amounts or lack surface CD21 (CD21(-/low) ) are increased in conditions with chronic inflammation, e.g. autoimmune diseases. However, little is known about the CD21(-/low) B cell subset in peripheral blood from healthy donors. Here, we show that CD21(-/low) cells represent approximately 5% of B cells in peripheral blood from adults but are barely detectable in cord blood, after excluding transitional B cells. The CD21(-/low) subset can be divided into CD38(-) 24(+) and CD38(-) 24(low) cells, where most of the CD38(-) 24(+) are CD27(+) immunoglobulin (Ig)M(+) IgD(+) and the CD38(-) 24(low) are switched CD27(-) . Expression levels of additional markers, e.g. CD95 and CD62L, are similar to those on classical memory B cells. In contrast to naive cells, the majority of CD21(-/low) cells lack expression of the ABCB1 transporter. Stimulation with a combination of BCR, Toll-like receptor (TLR)-7/8 and interleukin (IL)-2 induces proliferation and differentiation of the CD21(-/low) B cells comparable to CD21(+) CD27(+) memory B cells. The response excluding BCR agonist is not on par with that of classical memory B cells, although clearly above that of naive B cells. This is ascribed to a weaker response by the CD38(-) 24(low) subset, implying that some memory B cells require not only TLR but also BCR triggering. We conclude that the CD21(-/low) cells in healthy donors are memory B cells.

Naïve and memory B cells exhibit distinct biochemical responses following BCR engagement

Immunological memory is characterized by the rapid reactivation of memory B cells that produce large quantities of high-affinity antigen-specific antibodies. This contrasts the response of naïve B cells, and the primary immune response, which is much slower and of lower affinity. Memory responses are critical for protection against infectious diseases and form the basis of most currently available vaccines. Although we have known about the phenomenon of long-lived memory for centuries, the biochemical differences underlying these diverse responses of naïve and memory B cells is incompletely resolved. Here we investigated the nature of B-cell receptor (BCR) signaling in human splenic naïve, IgM(+) memory and isotype-switched memory B cells following multivalent BCR crosslinking. We observed comparable rapid and transient phosphorylation kinetics for proximal (phosphotyrosine and spleen tyrosine kinase) and propagation (B-cell linker, phospholipase Cγ2) signaling components in these different B-cell subsets. However, the magnitude of activation of downstream components of the BCR signaling pathway were greater in memory compared with naïve cells. Although no differences were observed in the magnitude of Ca(2+) mobilization between subsets, IgM(+) memory B cells exhibited a more rapid Ca(2+) mobilization and a greater depletion of the Ca(2+) endoplasmic reticulum stores, while IgG(+) memory B cells had a prolonged Ca(2+) uptake. Collectively, our findings show that intrinsic signaling features of B-cell subsets contribute to the robust response of human memory B cells over naïve B cells. This has implications for our understanding of memory B-cell responses and provides a framework to modulate these responses in the setting of vaccination and immunopathologies, such as immunodeficiency and autoimmunity.

Lentiviral hematopoietic stem cell gene therapy for X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency (SCID-X1) is a profound deficiency of T, B, and natural killer (NK) cell immunity caused by mutations inIL2RGencoding the common chain (γc) of several interleukin receptors. Gamma-retroviral (γRV) gene therapy of SCID-X1 infants without conditioning restores T cell immunity without B or NK cell correction, but similar treatment fails in older SCID-X1 children. We used a lentiviral gene therapy approach to treat five SCID-X1 patients with persistent immune dysfunction despite haploidentical hematopoietic stem cell (HSC) transplant in infancy. Follow-up data from two older patients demonstrate that lentiviral vector γc transduced autologous HSC gene therapy after nonmyeloablative busulfan conditioning achieves selective expansion of gene-marked T, NK, and B cells, which is associated with sustained restoration of humoral responses to immunization and clinical improvement at 2 to 3 years after treatment. Similar gene marking levels have been achieved in three younger patients, albeit with only 6 to 9 months of follow-up. Lentiviral gene therapy with reduced-intensity conditioning appears safe and can restore humoral immune function to posthaploidentical transplant older patients with SCID-X1.

Presence of Rheumatoid Factor during Chronic HCV Infection Is Associated with Expansion of Mature Activated Memory B-Cells that Are Hypo-Responsive to B-Cell Receptor Stimulation and Persist during the Early Stage of IFN Free Therapy

Approximately half of those with chronic hepatitis C virus (HCV) infection have circulating rheumatoid factor (RF), and a portion of these individuals develop cryoglobulinemic vasculitis. B cell phenotype/function in relation to RF in serum has been unclear. We examined B cell subset distribution, activation state (CD86), cell cycle state (Ki67), and ex-vivo response to BCR, TLR9 and TLR7/8 stimulation, in chronic HCV-infected donors with or without RF, and uninfected donors. Mature-activated B-cells of HCV-infected donors had lower CD86 expression compared to uninfected donors, and in the presence of RF they also showed reduced CD86 expression in response to BCR and TLR9 stimulation. Additionally, mature activated memory B cells of HCV RF+ donors less commonly expressed Ki67+ than HCV RF- donors, and did not proliferate as well in response to BCR stimulation. Proportions of mature-activated B cells were enhanced, while naïve B-cells were lower in the peripheral blood of HCV-RF+ compared to RF- and uninfected donors. None of these parameters normalize by week 8 of IFN free direct acting antiviral (DAA) therapy in HCV RF+ donors, while in RF- donors, mature activated B cell proportions did normalize. These data indicate that while chronic HCV infection alone results in a lower state of activation in mature activated memory B cells, the presence of RF in serum is associated with a more pronounced state of unresponsiveness and an overrepresentation of these B cells in the blood. This phenotype persists at least during the early time window after removal of HCV from the host.

B cells as multi-functional players during Mycobacterium tuberculosis infection and disease

Immunity to tuberculosis is still understood to be driven and maintained by T-cell derived immune responses. With a steady influx of data, it is becoming clear that B cells, the mediators of humoral immunity, have the capacity to function in roles not previously appreciated within the traditional B cell dogma. In this review we aim to discuss B cells, from its generation through to its functioning as effectors in both the innate and adaptive immune response, within the tuberculosis domain.

CD21 and CD19 deficiency: Two defects in the same complex leading to different disease modalities

PURPOSE

Deficiencies in CD19 and CD81 (forming the CD19-complex with CD21 and CD225) cause a severe clinical phenotype. One CD21 deficient patient has been described. We present a second CD21 deficient patient, with a mild clinical phenotype and compared the immunobiological characteristics of CD21 and CD19 deficiency.

METHODS

CD21 deficiency was characterized by flowcytometric immunophenotyping and sequencing. Real-time PCR, in vitro stimulation and next generation sequencing were used to characterize B-cell responses and affinity maturation in CD21(-/-) and CD19(-/-) B cells.

RESULTS

A compound heterozygous mutation in CD21 caused CD21 deficiency. CD21(-/-) B cells responded normally to in vitro stimulation and AID was transcribed. Affinity maturation was less affected by CD21 than by CD19 deficiency.

CONCLUSIONS

Both CD21 and CD19 deficiencies cause hypogammaglobulinemia and reduced memory B cells. CD19 deficiency causes a more severe clinical phenotype. B-cell characteristics reflect this, both after in vitro stimulation as in affinity maturation.