Repression of the B cell identity factor Pax5 is not required for plasma cell development

DNA methylation regulates B cell activation via repressing Pax5 expression in teleost

In mammals, the transcription factor Pax5 is a key regulator of B cell development and maturation and specifically expressed in naive/mature B cells but repressed upon B cell activation. Despite the long-standing proposal that Pax5 repression is essential for proper B cell activation, the underlying mechanisms remain largely elusive. In this study, we used a teleost model to elucidate the mechanisms governing Pax5 repression during B cell activation. Treatment with lipopolysaccharide (LPS) and chitosan oligosaccharide (COS) significantly enhanced the antibody secreting ability and phagocytic capacity of IgM+ B cells in large yellow croaker (Larimichthys crocea), coinciding with upregulated expression of activation-related genes, such as Bcl6, Blimp1, and sIgM, and downregulated expression of Pax5. Intriguingly, two CpG islands were identified within the promoter region of Pax5. Both CpG islands exhibited hypomethylation in naive/mature B cells, while CpG island1 was specifically transited into hypermethylation upon B cell activation. Furthermore, treatment with DNA methylation inhibitor 5-aza-2'-deoxycytidine (AZA) prevented the hypermethylation of CpG island1, and concomitantly impaired the downregulation of Pax5 and activation of B cells. Finally, through in vitro methylation experiments, we demonstrated that DNA methylation exerts an inhibitory effect on promoter activities of Pax5. Taken together, our findings unveil a novel mechanism underlying Pax5 repression during B cell activation, thus promoting the understanding of B cell activation process.

Transcriptomic Analysis Reveals Sixteen Potential Genes Associated with the Successful Differentiation of Antibody-Secreting Cells through the Utilization of Unfolded Protein Response Mechanisms in Robust Responders to the Influenza Vaccine

The seasonal influenza vaccine remains one of the vital recommended infection control measures for the elderly with chronic illnesses. We investigated the immunogenicity of a single dose of influenza vaccine in 123 seronegative participants and classified them into four distinct groups, determined by the promptness of vaccine response, the longevity of humoral immunity, and the likelihood of exhibiting cross-reactivity. Subsequently, we used transcriptional profiling and differential gene expression analysis to identify potential genes directly associated with the robust response to the vaccine. The group of exemplary vaccine responders differentially expressed 16 genes, namely: MZB1, MYDGF, TXNDC5, TXNDC11, HSP90B1, FKBP11, PDIA5, PRDX4, CD38, SDC1, TNFRSF17, TNFRSF13B, PAX5, POU2AF1, IRF4, and XBP1. Our findings point out a list of expressed proteins that are related to B cell proliferation, unfolded protein response, and cellular haemostasis, as well as a linkage of these expressions to the survival of long-lived plasma cells.

Deep immunophenotyping reveals circulating activated lymphocytes in individuals at risk for rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease with currently no universally highly effective prevention strategies. Identifying pathogenic immune phenotypes in 'At-Risk' populations prior to clinical disease onset is crucial to establishing effective prevention strategies. Here, we applied mass cytometry to deeply characterize the immunophenotypes in blood from At-Risk individuals identified through the presence of serum antibodies to citrullinated protein antigens (ACPA) and/or first-degree relative (FDR) status (n=52), as compared to established RA (n=67), and healthy controls (n=48). We identified significant cell expansions in At-Risk individuals compared with controls, including CCR2+CD4+ T cells, T peripheral helper (Tph) cells, type 1 T helper cells, and CXCR5+CD8+ T cells. We also found that CD15+ classical monocytes were specifically expanded in ACPA-negative FDRs, and an activated PAX5 low naïve B cell population was expanded in ACPA-positive FDRs. Further, we developed an "RA immunophenotype score" classification method based on the degree of enrichment of cell states relevant to established RA patients. This score significantly distinguished At-Risk individuals from controls. In all, we systematically identified activated lymphocyte phenotypes in At-Risk individuals, along with immunophenotypic differences among both ACPA+ and ACPA-FDR At-Risk subpopulations. Our classification model provides a promising approach for understanding RA pathogenesis with the goal to further improve prevention strategies and identify novel therapeutic targets.

The cellular biology of plasma cells: Unmet challenges and opportunities

Plasma cells and the antibodies they secrete are paramount for protection against infection but can also be implicated in diseases including autoantibody-mediated disease and multiple myeloma. Plasma cell terminal differentiation relies on a transcriptional switch and on important morphological changes. The cellular and molecular mechanisms underlying these processes are partly understood and how plasma cells manage to survive for long periods of time while secreting large quantities of antibodies remains unclear. In this review we aim to put in perspective what is known about plasma cell cellular biology to highlight the challenges faced by this field of research but also to illustrate how new opportunities may arise from the study of the fundamental mechanisms sustaining plasma cell survival and function.

An arrayed CRISPR screen of primary B cells reveals the essential elements of the antibody secretion pathway

Background

Humoral immunity depends on the differentiation of B cells into antibody secreting cells (ASCs). Excess or inappropriate ASC differentiation can lead to antibody-mediated autoimmune diseases, while impaired differentiation results in immunodeficiency.

Methods

We have used CRISPR/Cas9 technology in primary B cells to screen for regulators of terminal differentiation and antibody production.

Results

We identified several new positive (Sec61a1, Hspa5) and negative (Arhgef18, Pold1, Pax5, Ets1) regulators that impacted on the differentiation process. Other genes limited the proliferative capacity of activated B cells (Sumo2, Vcp, Selk). The largest number of genes identified in this screen (35) were required for antibody secretion. These included genes involved in endoplasmic reticulum-associated degradation and the unfolded protein response, as well as post-translational protein modifications.

Discussion

The genes identified in this study represent weak links in the antibody-secretion pathway that are potential drug targets for antibody-mediated diseases, as well as candidates for genes whose mutation results in primary immune deficiency.

Maintenance of Lineage Identity: Lessons from a B Cell

The maintenance of B cell identity requires active transcriptional control that enforces a B cell-specific program and suppresses alternative lineage genes. Accordingly, disrupting the B cell identity regulatory network compromises B cell function and induces cell fate plasticity by allowing derepression of alternative lineage-specific transcriptional programs. Although the B lineage is incredibly resistant to most differentiating factors, loss of just a single B lineage-specific transcription factor or the forced expression of individual non-B cell lineage transcription factors can radically disrupt B cell maintenance and allow dedifferentiation or transdifferentiation into entirely distinct lineages. B lymphocytes thereby offer an insightful and useful case study of how a specific cell lineage can maintain a stable identity throughout life and how perturbations of a single master regulator can induce cellular plasticity. In this article, we review the regulatory mechanisms that safeguard B cell identity, and we discuss how dysregulation of the B cell maintenance program can drive malignant transformation and enable therapeutic resistance.

PAX5 alterations in B-cell acute lymphoblastic leukemia

PAX5, a master regulator of B cell development and maintenance, is one of the most common targets of genetic alterations in B-cell acute lymphoblastic leukemia (B-ALL). PAX5 alterations consist of copy number variations (whole gene, partial, or intragenic), translocations, and point mutations, with distinct distribution across B-ALL subtypes. The multifaceted functional impacts such as haploinsufficiency and gain-of-function of PAX5 depending on specific variants have been described, thereby the connection between the blockage of B cell development and the malignant transformation of normal B cells has been established. In this review, we provide the recent advances in understanding the function of PAX5 in orchestrating the development of both normal and malignant B cells over the past decade, with a focus on the PAX5 alterations shown as the initiating or driver events in B-ALL. Recent large-scale genomic analyses of B-ALL have identified multiple novel subtypes driven by PAX5 genetic lesions, such as the one defined by a distinct gene expression profile and PAX5 P80R mutation, which is an exemplar leukemia entity driven by a missense mutation. Although altered PAX5 is shared as a driver in B-ALL, disparate disease phenotypes and clinical outcomes among the patients indicate further heterogeneity of the underlying mechanisms and disturbed gene regulation networks along the disease development. In-depth mechanistic studies in human B-ALL and animal models have demonstrated high penetrance of PAX5 variants alone or concomitant with other genetic lesions in driving B-cell malignancy, indicating the altered PAX5 and deregulated genes may serve as potential therapeutic targets in certain B-ALL cases.

The Pleiotropy of PAX5 Gene Products and Function

PAX5, a member of the Paired Box (PAX) transcription factor family, is an essential factor for B-lineage identity during lymphoid differentiation. Mechanistically, PAX5 controls gene expression profiles, which are pivotal to cellular processes such as viability, proliferation, and differentiation. Given its crucial function in B-cell development, PAX5 aberrant expression also correlates with hallmark cancer processes leading to hematological and other types of cancer lesions. Despite the well-established association of PAX5 in the development, maintenance, and progression of cancer disease, the use of PAX5 as a cancer biomarker or therapeutic target has yet to be implemented. This may be partly due to the assortment of PAX5 expressed products, which layers the complexity of their function and role in various regulatory networks and biological processes. In this review, we provide an overview of the reported data describing PAX5 products, their regulation, and function in cellular processes, cellular biology, and neoplasm.

Balance between immunoregulatory B cells and plasma cells drives pancreatic tumor immunity

Plasma cell responses are associated with anti-tumor immunity and favorable response to immunotherapy. B cells can amplify anti-tumor immune responses through antibody production; yet B cells in patients and tumor-bearing mice often fail to support this effector function. We identify dysregulated transcriptional program in B cells that disrupts differentiation of naive B cells into anti-tumor plasma cells. The signaling network contributing to this dysfunction is driven by interleukin (IL) 35 stimulation of a STAT3-PAX5 complex that upregulates the transcriptional regulator BCL6 in naive B cells. Transient inhibition of BCL6 in tumor-educated naive B cells is sufficient to reverse the dysfunction in B cell differentiation, stimulating the intra-tumoral accumulation of plasma cells and effector T cells and rendering pancreatic tumors sensitive to anti-programmed cell death protein 1 (PD-1) blockade. Our findings argue that B cell effector dysfunction in cancer can be due to an active systemic suppression program that can be targeted to synergize with T cell-directed immunotherapy.

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Balance between regulatory B cells and plasma cells shapes pancreatic tumor growth

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Cancer primes naive B cells toward regulatory B cell differentiation

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IL-35 drives B cell reprogramming via formation of a pSTAT3-Pax5 complex

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IL-35/BCL6 blockade in naive B cells enhances αPD1 efficacy

Molecular mechanisms governing circulating immune cell heterogeneity across different species revealed by single-cell sequencing

BACKGROUND

Immune cells play important roles in mediating immune response and host defense against invading pathogens. However, insights into the molecular mechanisms governing circulating immune cell diversity among multiple species are limited.

METHODS

In this study, we compared the single-cell transcriptomes of immune cells from 12 species. Distinct molecular profiles were characterized for different immune cell types, including T cells, B cells, natural killer cells, monocytes, and dendritic cells.

RESULTS

Our data revealed the heterogeneity and compositions of circulating immune cells among 12 different species. Additionally, we explored the conserved and divergent cellular crosstalks and genetic regulatory networks among vertebrate immune cells. Notably, the ligand and receptor pair VIM-CD44 was highly conserved among the immune cells.

CONCLUSIONS

This study is the first to provide a comprehensive analysis of the cross-species single-cell transcriptome atlas for peripheral blood mononuclear cells (PBMCs). This research should advance our understanding of the cellular taxonomy and fundamental functions of PBMCs, with important implications in evolutionary biology, developmental biology, and immune system disorders.

Maturation trajectories and transcriptional landscape of plasmablasts and autoreactive B cells in COVID-19

In parasite and viral infections, aberrant B cell responses can suppress germinal center reactions thereby blunting long-lived memory and may provoke immunopathology including autoimmunity. Using COVID-19 as model, we set out to identify serological, cellular, and transcriptomic imprints of pathological responses linked to autoreactive B cells at single-cell resolution. We show that excessive plasmablast expansions are prognostically adverse and correlate with autoantibody production but do not hinder the formation of neutralizing antibodies. Although plasmablasts followed interleukin-4 (IL-4) and BAFF-driven developmental trajectories, were polyclonal, and not enriched in autoreactive B cells, we identified two memory populations (CD80⁺/ISG15⁺ and CD11c⁺/SOX5⁺/T-bet^+/−) with immunogenetic and transcriptional signs of autoreactivity that may be the cellular source of autoantibodies in COVID-19 and that may persist beyond recovery. Immunomodulatory interventions discouraging such adverse responses may be useful in selected patients to shift the balance from autoreactivity toward long-term memory.

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Plasmablast expansions correlate with disease severity and autoantibodies in COVID-19

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Patients with high plasmablast levels exhibit IGHV4-34 skewing

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Autoreactive BCRs are enriched in atypical memory, not plasmablast populations

Plasma cell survival: The intrinsic drivers, migratory signals, and extrinsic regulators

Antibody-secreting cells (ASC) are the effectors of protective humoral immunity and the only cell type that produces antibodies or immunoglobulins in mammals. In addition to their formidable capacity to secrete massive quantities of proteins, ASC are terminally differentiated and have unique features to become long-lived plasma cells (LLPC). Upon antigen encounter, B cells are activated through a complex multistep process to undergo fundamental morphological, subcellular, and molecular transformation to become an efficient protein factory with lifelong potential. The ASC survival potential is determined by factors at the time of induction, capacity to migration from induction to survival sites, and ability to mature in the specialized bone marrow microenvironments. In the past decade, considerable progress has been made in identifying factors regulating ASC longevity. Here, we review the intrinsic drivers, trafficking signals, and extrinsic regulators with particular focus on how they impact the survival potential to become a LLPC.

Epigenetic gene regulation in plasma cells

Humoral immunity provides protection from pathogenic infection and is mediated by antibodies following the differentiation of naive B cells (nBs) to antibody-secreting cells (ASCs). This process requires substantial epigenetic and transcriptional rewiring to ultimately repress the nB program and replace it with one conducive to ASC physiology and function. Notably, these reprogramming events occur within the framework of cell division. Efforts to understand the relationship of cell division with reprogramming and ASC differentiation in vivo have uncovered the timing and scope of reprogramming, as well as key factors that influence these events. Herein, we discuss the unique physiology of ASC and how nBs undergo epigenetic and genome architectural reorganization to acquire the necessary functions to support antibody production. We also discuss the stage-wise manner in which reprogramming occurs across cell divisions and how key molecular determinants can influence B cell fate outcomes.

Spatiotemporal Analysis of B Cell- and Antibody Secreting Cell-Subsets in Human Melanoma Reveals Metastasis-, Tumor Stage-, and Age-Associated Dynamics

Background: The role of tumor-associated B cells in human cancer is only starting to emerge. B cells typically undergo a series of developmental changes in phenotype and function, however, data on the composition of the B cell population in human melanoma are largely absent including changes during tumor progression and their potential clinical significance.

Methods: In this study, we compared the number and distribution of six major B cell and antibody secreting cell subpopulations outside tertiary lymphoid structures in whole tumor sections of 154 human cutaneous melanoma samples (53 primary tumors without subsequent metastasis, 44 primary tumors with metastasis, 57 metastatic samples) obtained by seven color multiplex immunohistochemistry and automated tissue imaging and analysis.

Results: In primary melanomas, we observed the highest numbers for plasmablast-like, memory-like, and activated B cell subtypes. These cells showed a patchy, predominant paratumoral distribution at the invasive tumor-stroma margin. Plasma cell-like cells were hardly detected, germinal center- and transitional/regulatory-like B cells not at all. Of the major clinicopathologic prognostic factors for primary melanomas, metastasis was associated with decreased memory-like B cell numbers and a higher age associated with higher plasmablast-like cell numbers. When we compared the composition of B cell subpopulations in primary melanomas and metastatic samples, we found a significantly higher proportion of plasma cell-like cells at distant metastatic sites and a higher proportion of memory-like B cells at locoregional than distant metastatic sites. Both cell types were detected mainly in the para- and intratumoral stroma.

Conclusion: These data provide a first comprehensive and comparative spatiotemporal analysis of major B cell and antibody secreting cell subpopulations in human melanoma and describe metastasis-, tumor stage-, and age-associated dynamics, an important premise for B cell-related biomarker and therapy studies.

Multiscale Modeling of Germinal Center Recapitulates the Temporal Transition From Memory B Cells to Plasma Cells Differentiation as Regulated by Antigen Affinity-Based Tfh Cell Help

Germinal centers play a key role in the adaptive immune system since they are able to produce memory B cells and plasma cells that produce high affinity antibodies for an effective immune protection. The mechanisms underlying cell-fate decisions are not well understood but asymmetric division of antigen, B-cell receptor affinity, interactions between B-cells and T follicular helper cells (triggering CD40 signaling), and regulatory interactions of transcription factors have all been proposed to play a role. In addition, a temporal switch from memory B-cell to plasma cell differentiation during the germinal center reaction has been shown. To investigate if antigen affinity-based Tfh cell help recapitulates the temporal switch we implemented a multiscale model that integrates cellular interactions with a core gene regulatory network comprising BCL6, IRF4, and BLIMP1. Using this model we show that affinity-based CD40 signaling in combination with asymmetric division of B-cells result in switch from memory B-cell to plasma cell generation during the course of the germinal center reaction. We also show that cell fate division is unlikely to be (solely) based on asymmetric division of Ag but that BLIMP1 is a more important factor. Altogether, our model enables to test the influence of molecular modulations of the CD40 signaling pathway on the production of germinal center output cells.