bcl-2 transgene expression inhibits apoptosis in the germinal center and reveals differences in the selection of memory B cells and bone marrow antibody-forming cells

IgE plasma cells are transcriptionally and functionally distinct from other isotypes

Understanding the phenotypic and transcriptional signature of immunoglobulin E (IgE)-producing cells is fundamental to plasma cell (PC) biology and development of therapeutic interventions for allergy. Here, using a mouse model of intranasal house dust mite (HDM) exposure, we showed that short-lived IgE PCs emerge in lung draining lymph nodes (dLNs) during early exposure (<3 weeks) and long-lived IgE PCs accumulate in the bone marrow (BM) with prolonged exposure (>7 weeks). IgE PCs had distinct surface and gene expression profiles in these different tissues compared with other Ig isotypes. IgE BMPCs up-regulated genes associated with prosurvival and BM homing, whereas IgE dLN PCs expressed genes associated with recent class switching and differentiation. IgE PCs also exhibited higher expression of endoplasmic reticulum (ER) stress and protein coding genes and higher antibody secretion rate when compared with IgG1. Overall, this study highlights the unique developmental path and transcriptional signature of short-lived and long-lived IgE PCs.

An immunohistochemical atlas of necroptotic pathway expression

Necroptosis is a lytic form of regulated cell death reported to contribute to inflammatory diseases of the gut, skin and lung, as well as ischemic-reperfusion injuries of the kidney, heart and brain. However, precise identification of the cells and tissues that undergo necroptotic cell death in vivo has proven challenging in the absence of robust protocols for immunohistochemical detection. Here, we provide automated immunohistochemistry protocols to detect core necroptosis regulators - Caspase-8, RIPK1, RIPK3 and MLKL - in formalin-fixed mouse and human tissues. We observed surprising heterogeneity in protein expression within tissues, whereby short-lived immune barrier cells were replete with necroptotic effectors, whereas long-lived cells lacked RIPK3 or MLKL expression. Local changes in the expression of necroptotic effectors occurred in response to insults such as inflammation, dysbiosis or immune challenge, consistent with necroptosis being dysregulated in disease contexts. These methods will facilitate the precise localisation and evaluation of necroptotic signaling in vivo.

Interleukin-4 downregulates transcription factor BCL6 to promote memory B cell selection in germinal centers

Germinal center (GC)-derived memory B cells (MBCs) are critical for humoral immunity as they differentiate into protective antibody-secreting cells during re-infection. GC formation and cellular interactions within the GC have been studied in detail, yet the exact signals that allow for the selection and exit of MBCs are not understood. Here, we showed that IL-4 cytokine signaling in GC B cells directly downregulated the transcription factor BCL6 via negative autoregulation to release cells from the GC program and to promote MBC formation. This selection event required additional survival cues and could therefore result in either GC exit or death. We demonstrate that both increasing IL-4 bioavailability or limiting IL-4 signaling disrupted MBC selection stringency. In this way, IL-4 control of BCL6 expression serves as a tunable switch within the GC to tightly regulate MBC selection and affinity maturation.

B cell memory: from generation to reactivation: a multipronged defense wall against pathogens

Development of B cell memory is a conundrum that scientists are still exploring. Studies have been conducted in vitro and using advanced animal models to elucidate the mechanism underlying the generation of memory B cells (MBCs), the precise roles of MBCs against pathogens, and their protective functions against repeated infections throughout life. Lifelong immunity against invading diseases is mainly the result of overcoming a single infection. This protection is largely mediated by the two main components of B cell memory-MBCs and long-lived plasma cells (PCs). The chemical and cellular mechanisms that encourage fat selection for MBCs or long-lived PCs are an area of active research. Despite the fact that nearly all available vaccinations rely on the capacity to elicit B-cell memory, we have yet to develop successful vaccines that can induce broad-scale protective MBCs against some of the deadliest diseases, including malaria and AIDS. A deeper understanding of the specific cellular and molecular pathways that govern the generation, function, and reactivation of MBCs is critical for overcoming the challenges associated with vaccine development. Here, we reviewed literature on the development of MBCs and their reactivation, interaction with other cell types, strategies against invading pathogens, and function throughout life and discussed the recent advances regarding the key signals and transcription factors which regulate B cell memory and their relevance to the quest for vaccine development.

Germinal center versus extrafollicular responses in systemic autoimmunity: Who turns the blade on self?

Spontaneously formed germinal centers (GCs) have been reported in most mouse models of human autoimmune disease and autoimmune patients, and have long been considered a source of somatically-mutated and thus high affinity autoantibodies, but their role in autoimmunity is becoming increasingly controversial, particularly in the context of systemic autoimmune diseases like lupus. On the one hand, there is good evidence that some pathogenic lupus antibodies have acquired somatic mutations that increase affinity for self-antigens. On the other hand, recent studies that have genetically prevented GC formation, suggest that GCs are dispensable for systemic autoimmunity, pointing instead to pathogenic extrafollicular (EF) B-cell responses. Furthermore, several lines of evidence suggest germinal centers may in fact be somewhat protective in the context of autoimmunity. Here we review how some of the conflicting evidence arose, and current views on the role of GCs in autoimmunity, outlining mechanisms by which GC may eliminate self-reactivity. We also discuss recent advances in understanding extrafollicular B cell subsets that participate in autoimmunity.

Role of affinity in plasma cell development in the germinal center light zone

Protective immune responses to many pathogens depend on the development of high-affinity antibody-producing plasma cells (PC) in germinal centers (GCs). Transgenic models suggest that there is a stringent affinity-based barrier to PC development. Whether a similar high-affinity barrier regulates PC development under physiologic circumstances and the nature of the PC fate decision has not been defined precisely. Here, we use a fate-mapping approach to examine the relationship between GC B cells selected to undergo additional rounds of affinity maturation, GC pre-PC, and PC. The data show that initial PC selection overlaps with GC B cell selection, but that the PC compartment accumulates a less diverse and higher affinity collection of antibodies over time. Thus, whereas the GC continues to diversify over time, affinity-based pre-PC selection sieves the GC to enable the accumulation of a more restricted group of high-affinity antibody-secreting PC.

Depletion of preexisting B-cell lymphoma 2-expressing senescent cells before vaccination impacts antigen-specific antitumor immune responses in old mice

The age-related decline in immunity reduces the effectiveness of vaccines in older adults. Immunosenescence is associated with chronic, low-grade inflammation, and the accumulation of senescent cells. The latter express Bcl-2 family members (providing resistance to cell death) and exhibit a pro-inflammatory, senescence-associated secretory phenotype (SASP). Preexisting senescent cells cause many aging-related disorders and therapeutic means of eliminating these cells have recently gained attention. The potential consequences of senescent cell removal on vaccine efficacy in older individuals are still ignored. We used the Bcl-2 family inhibitor ABT-263 to investigate the effects of pre-vaccination senolysis on immune responses in old mice. Two different ovalbumin (OVA)-containing vaccines (containing a saponin-based or a CpG oligodeoxynucleotide adjuvant) were tested. ABT-263 depleted senescent cells (apoptosis) and ablated the basal and lipopolysaccharide-induced production of SASP-related factors in old mice. Depletion of senescent cells prior to vaccination (prime/boost) had little effect on OVA-specific antibody and T-cell responses (slightly reduced and augmented, respectively). We then used a preclinical melanoma model to test the antitumor potential of senolysis before vaccination (prime with the vaccine and OVA boost by tumor cells). Surprisingly, ABT-263 treatment abrogated the vaccine's ability to protect against B16 melanoma growth in old animals, an effect associated with reduced antigen-specific T-cell responses. Some, but not all, of the effects were age-specific, which suggests that preexisting senescent cells were partly involved. Hence, depletion of senescent cells modifies immune responses to vaccines in some settings and caution should be taken when incorporating senolytics into vaccine-based cancer therapies.

SpiB regulates the expression of B-cell-related genes and increases the longevity of memory B cells

Generation of memory B cells is one of the key features of adaptive immunity as they respond rapidly to re-exposure to the antigen and generate functional antibodies. Although the functions of memory B cells are becoming clearer, the regulation of memory B cell generation and maintenance is still not well understood. Here we found that transcription factor SpiB is expressed in some germinal center (GC) B cells and memory B cells and participates in the maintenance of memory B cells. Overexpression and knockdown analyses revealed that SpiB suppresses plasma cell differentiation by suppressing the expression of Blimp1 while inducing Bach2 in the in-vitro-induced germinal center B (iGB) cell culture system, and that SpiB facilitates in-vivo appearance of memory-like B cells derived from the iGB cells. Further analysis in IgG1+ cell-specific SpiB conditional knockout (cKO) mice showed that function of SpiB is critical for the generation of late memory B cells but not early memory B cells or GC B cells. Gene expression analysis suggested that SpiB-dependent suppression of plasma cell differentiation is independent of the expression of Bach2. We further revealed that SpiB upregulates anti-apoptosis and autophagy genes to control the survival of memory B cells. These findings indicate the function of SpiB in the generation of long-lasting memory B cells to maintain humoral memory.

IL-4 downregulates BCL6 to promote memory B cell selection in germinal centers

Germinal center (GC)-derived memory B cells (MBCs) are critical for humoral immunity as they differentiate into protective antibody-secreting cells during re-infection. GC formation and cellular interactions within the GC have been studied in detail, yet the exact signals that allow for the selection and exit of MBCs are not understood. Here, we show that IL-4 signaling in GC B cells directly downregulates BCL6 via negative autoregulation to release cells from the GC program and promote MBC formation. This selection event requires additional survival cues and can therefore result in either GC exit or death. We demonstrate that both increasing IL-4 bioavailability or limiting IL-4 signaling disrupt MBC selection stringency. In this way, IL-4 control of BCL6 expression serves as a tunable switch within the GC to tightly regulate MBC selection and affinity maturation.

Insights into the relationship between persistent antibody secretion and metabolic programming - A question for single-cell analysis

Vaccination aims to generate a protective and persisting antibody response. Indeed, humoral vaccine-mediated protection depends on the quality and quantity of the produced antigen-specific antibodies for its initial magnitude and the persistence of the plasma cells for its duration. Therefore, understanding the mechanisms behind the generation, selection and maintenance of long-lived plasma cells secreting protective antibodies is of fundamental importance for understanding long-term immunity, vaccine responses, therapeutical approaches for autoimmune disease and multiple myeloma. Recent studies have observed correlations between the generation, function and lifespan of plasma cells and their metabolism, with metabolism being both a main driver and primary consequence of changes in cellular behavior. This review introduces how metabolic programs influence and drive immune cell functions in general and plasma cell differentiation and longevity more specifically, summarizing the current knowledge on metabolic pathways and their influences on cellular fate. In addition, available technologies to profile metabolism and their limitations are discussed, leading to the unique and open technological challenges for further advancement of this research field.

Memory B-cell diversity: From early generation to tissue residency and reactivation

Memory B cells (MBCs) have a crucial function in providing an enhanced response to repeated infections. Upon antigen encounter, MBC can either rapidly differentiate to antibody secreting cells or enter germinal centers (GC) to further diversify and affinity mature. Understanding how and when MBC are formed, where they reside and how they select their fate upon reactivation has profound implications for designing strategies to improve targeted, next-generation vaccines. Recent studies have crystallized much of our knowledge on MBC but also reported several surprising discoveries and gaps in our current understanding. Here, we review the latest advancements in the field and highlight current unknowns. In particular, we focus on timing and cues leading to MBC generation before and during the GC reaction, discuss how MBC become resident in mucosal tissues, and finally, provide an overview of factors shaping MBC fate-decision upon reactivation in mucosal and lymphoid tissues.

Epigenetic modulators of B cell fate identified through coupled phenotype-transcriptome analysis

High-throughput methodologies are the cornerstone of screening approaches to identify novel compounds that regulate immune cell function. To identify novel targeted therapeutics to treat immune disorders and haematological malignancies, there is a need to integrate functional cellular information with the molecular mechanisms that regulate changes in immune cell phenotype. We facilitate this goal by combining quantitative methods for dissecting complex simultaneous cell phenotypic effects with genomic analysis. This combination strategy we term Multiplexed Analysis of Cells sequencing (MAC-seq), a modified version of Digital RNA with perturbation of Genes (DRUGseq). We applied MAC-seq to screen compounds that target the epigenetic machinery of B cells and assess altered humoral immunity by measuring changes in proliferation, survival, differentiation and transcription. This approach revealed that polycomb repressive complex 2 (PRC2) inhibitors promote antibody secreting cell (ASC) differentiation in both murine and human B cells in vitro. This is further validated using T cell-dependent immunization in mice. Functional dissection of downstream effectors of PRC2 using arrayed CRISPR screening uncovered novel regulators of B cell differentiation, including Mybl1, Myof, Gas7 and Atoh8. Together, our findings demonstrate that integrated phenotype-transcriptome analyses can be effectively combined with drug screening approaches to uncover the molecular circuitry that drives lymphocyte fate decisions.

A Novel Organized Nasopharynx-Associated Lymphoid Tissue in Teleosts That Expresses Molecular Markers Characteristic of Mammalian Germinal Centers

Nasal immunity is an ancient and conserved arm of the mucosal immune system in vertebrates. In teleost fish, we previously reported the presence of a nasopharynx-associated lymphoid tissue (NALT) characterized by scattered immune cells located in the trout olfactory lamellae. This diffuse NALT mounts innate and adaptive immune responses to nasal infection or vaccination. In mammals, lymphoid structures such as adenoids and tonsils support affinity maturation of the adaptive immune response in the nasopharyngeal cavity. These structures, known as organized NALT (O-NALT), have not been identified in teleost fish to date, but their evolutionary forerunners exist in sarcopterygian fish. In this study, we report that the rainbow trout nasal cavity is lined with a lymphoepithelium that extends from the most dorsal opening of the nares to the ventral nasal cavity. Within the nasal lymphoepithelium we found lymphocyte aggregates called O-NALT in this study that are composed of ∼ 56% CD4+, 24% IgM+, 16% CD8α+, and 4% IgT+ lymphocytes and that have high constitutive aicda mRNA expression. Intranasal (i.n.) vaccination with live attenuated infectious hematopoietic necrosis virus triggers expansions of B and T cells and aicda expression in response to primary i.n. vaccination. IgM+ B cells undergo proliferation and apoptosis within O-NALT upon prime but not boost i.n. vaccination. Our results suggest that novel mucosal microenvironments such as O-NALT may be involved in the affinity maturation of the adaptive immune response in early vertebrates.

Long-lived plasma cells accumulate in the bone marrow at a constant rate from early in an immune response

Vaccines work largely by generating long-lived plasma cells (LLPCs), but knowledge of how such cells are recruited is sparse. Although it is clear that LLPCs preferentially originate in germinal centers (GCs) and relocate to survival niches in bone marrow where they can persist for decades, the issues of the timing of LLPC recruitment and the basis of their retention remain uncertain. Here, using a genetic timestamping system in mice, we show that persistent PCs accrue in bone marrow at an approximately constant rate of one cell per hour over a period spanning several weeks after a single immunization with a model antigen. Affinity-based selection was evident in persisting PCs, reflecting a relative and dynamic rather than absolute affinity threshold as evidenced by the changing pattern of V H gene somatic mutations conveying increased affinity for antigen. We conclude that the life span of persistent, antigen-specific PCs is in part intrinsic, preprogrammed, and varied and that their final number is related to the duration of the response in a predictable way. This implies that modulating vaccines to extend the duration of the GC reaction will enhance antibody-mediated protective immunity.

The Clonal Diversity of Peripheral B Cell Receptor Immune Repertoire Impaired by Residual Malignant B Cells Predicts Treatment Efficacy in B Cell Lymphoma Patients

Germinal center (GC) is the vital locus for the evolution of naïve B cells into memory B and plasma cells, but also a hotbed for the proliferation of malignant B cells. We hypothesized that malignant B cells may locally or globally impact GCs to produce peripheral B cell receptor immune repertoire (BCR IR) with reduced clonal diversity. In this study, we first validated our hypothesis in a novel human in-vitro GC (hiGC) model. The addition of the diffuse large B cell lymphoma (DLBCL) cells to the hiGC culture attenuated the rate of diversity growth. For clinical validation, we collected samples from 17 DLBCL patients at various points during high-dose therapy and autologous stem cell rescue. The elimination and reestablishment of the patients’ lymphatic pool allowed us to unambiguously monitor the impact of tumor cells on the replenishment of the peripheral BCR IR. Compared to the nine patients who did not relapse after treatment, relapsed patients tended to have a slower rate of recovery regarding the clonal diversity of their peripheral BCR IR. Our results suggest a mechanistic and clinical connection between residual tumor cells and abnormal peripheral BCR IR, which may corelate with treatment efficacy in B cell lymphomas.

Kinetic of the Antibody Response Following AddaVax-Adjuvanted Immunization with Recombinant Influenza Antigens

Notwithstanding the current SARS-CoV-2 pandemic, influenza virus infection still represents a global health concern in terms of hospitalizations and possible pandemic threats. The objective of next-generation influenza vaccines is not only to increase the breadth of response but also to improve the elicitation of an effective and robust immune response, especially in high-risk populations. To achieve this second objective, the administration of adjuvanted influenza vaccines has been considered. In this regard, the monitoring and characterization of the antibody response associated with the administration of adjuvanted vaccines has been evaluated in this study in order to shed light on the kinetic, magnitude and subclass usage of antibody secreting cells (ASCs) as well as of circulating antigen-specific serum antibodies. Specifically, we utilized the DBA/2J mouse model to assess the kinetic, magnitude and IgG subclass usage of the antibody response following an intramuscular (IM) or intraperitoneal (IP) immunization regimen with AddaVax-adjuvanted bivalent H1N1 and H3N2 computationally optimized broadly reactive antigen (COBRA) influenza recombinant hemagglutinins (rHAs). While the serological evaluation revealed a homogeneous kinetic of the antibody response, the detection of the ASCs through a FluoroSpot platform revealed a different magnitude, subclass usage and kinetic of the antigen-specific IgG secreting cells peaking at day 5 and day 9 following the IP and IM immunization, respectively.

Skeletal muscle provides the immunological micro-milieu for specific plasma cells in anti-synthetase syndrome-associated myositis

Anti-synthetase syndrome (ASyS)-associated myositis is a major subgroup of the idiopathic inflammatory myopathies (IIM) and is characterized by disease chronicity with musculoskeletal, dermatological and pulmonary manifestations. One of eight autoantibodies against the aminoacyl-transferase RNA synthetases (ARS) is detectable in the serum of affected patients. However, disease-specific therapeutic approaches have not yet been established.To obtain a deeper understanding of the underlying pathogenesis and to identify putative therapeutic targets, we comparatively investigated the most common forms of ASyS associated with anti-PL-7, anti-PL-12 and anti-Jo-1. Our cohort consisted of 80 ASyS patients as well as healthy controls (n = 40), diseased controls (n = 40) and non-diseased controls (n = 20). We detected a reduced extent of necrosis and regeneration in muscle biopsies from PL-12⁺ patients compared to Jo-1⁺ patients, while PL-7⁺ patients had higher capillary dropout in biopsies of skeletal muscle. Aside from these subtle alterations, no significant differences between ASyS subgroups were observed. Interestingly, a tissue-specific subpopulation of CD138⁺ plasma cells and CXCL12⁺/CXCL13⁺CD20⁺ B cells common to ASyS myositis were identified. These cells were localized in the endomysium associated with alkaline phosphatase⁺ activated mesenchymal fibroblasts and CD68⁺MHC-II⁺CD169⁺ macrophages. An MHC-I⁺ and MHC-II⁺ MxA negative type II interferon-driven milieu of myofiber activation, topographically restricted to the perifascicular area and the adjacent perimysium, as well as perimysial clusters of T follicular helper cells defined an extra-medullary immunological niche for plasma cells and activated B cells. Consistent with this, proteomic analyses of muscle tissues from ASyS patients demonstrated alterations in antigen processing and presentation. In-depth immunological analyses of peripheral blood supported a B-cell/plasma-cell-driven pathology with a shift towards immature B cells, an increase of B-cell-related cytokines and chemokines, and activation of the complement system. We hypothesize that a B-cell-driven pathology with the presence and persistence of a specific subtype of plasma cells in the skeletal muscle is crucially involved in the self-perpetuating chronicity of ASyS myositis. This work provides the conceptual framework for the application of plasma-cell-targeting therapies in ASyS myositis.

Germinal center expansion but not plasmablast differentiation is proportional to peptide-MHCII density via CD40-CD40L signaling strength

T follicular helper (TFH) cells promote expansion of germinal center (GC) B cells and plasma cell differentiation. Whether cognate peptide-MHCII (pMHCII) density instructs selection and cell fate decisions in a quantitative manner remains unclear. Using αDEC205-OVA to differentially deliver OVA peptides to GC B cells on the basis of DEC205 allelic copy number, we find DEC205+/+ B cells take up 2-fold more antigen than DEC205+/- cells, leading to proportional TFH cell help and B cell expansion. To validate these results, we establish a caged OVA peptide, which is readily detected by OVA-specific TFH cells after photo-uncaging. In situ uncaging of peptides leads to multiple serial B-T contacts and cell activation. Differential CD40 signaling, is both necessary and sufficient to mediate 2-fold differences in B cell expansion. While plasmablast numbers are increased, pMHCII density does not directly control the output or quality of plasma cells. Thus, we distinguish the roles TFH cells play in expansion versus differentiation.

Generation of High Quality Memory B Cells

Protection against pathogen re-infection is mediated, in large part, by two humoral cellular compartments, namely, long-lived plasma cells and memory B cells. Recent data have reinforced the importance of memory B cells, particularly in response to re-infection of different viral subtypes or in response with viral escape mutants. In regard to memory B cell generation, considerable advancements have been made in recent years in elucidating its basic mechanism, which seems to well explain why the memory B cells pool can deal with variant viruses. Despite such progress, efforts to develop vaccines that induce broadly protective memory B cells to fight against rapidly mutating pathogens such as influenza virus and HIV have not yet been successful. Here, we discuss recent advances regarding the key signals and factors regulating germinal center-derived memory B cell development and activation and highlight the challenges for successful vaccine development.

Germinal Centers

Germinal centers (GCs) are microanatomical sites of B cell clonal expansion and antibody affinity maturation. Therein, B cells undergo the Darwinian process of somatic diversification and affinity-driven selection of immunoglobulins that produces the high-affinity antibodies essential for effective humoral immunity. Here, we review recent developments in the field of GC biology, primarily as it pertains to GCs induced by infection or immunization. First, we summarize the phenotype and function of the different cell types that compose the GC, focusing on GC B cells. Then, we review the cellular and molecular bases of affinity-dependent selection within the GC and the export of memory and plasma cells. Finally, we present an overview of the emerging field of GC clonal dynamics, focusing on how GC and post-GC selection shapes the diversity of antibodies secreted into serum. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

Pathogenesis of follicular lymphoma: genetics to the microenvironment to clinical translation

Follicular lymphoma (FL) represents a heterogeneous disease both clinically and biologically. The pathognomonic t(14;18) translocation can no longer be thought of as the primary genetic driver, with increasing recognition of the biological relevance of recurrent genetic alterations in epigenetic regulators that now feature as a pivotal hallmark of this lymphoma subtype. Furthermore, sequencing studies have provided a near complete catalogue of additional genetic aberrations. Longitudinal and spatial genetic studies add an additional layer to the biological heterogeneity, providing preliminary molecular insights into high-risk phenotypes such as early progressors and transformation, and also supporting evidence for the existence of persisting re-populating cells that act as lymphoma reservoirs and harbingers for FL recurrence. Simultaneously, understanding of the tumour microenvironmental cues promoting lymphomagenesis and disease progression continue to broaden. More recently, studies are beginning to unravel the convergence and co-operation between the genetics, epigenetics and microenvironment. There is a pressing need to marry biology with therapeutics, especially with the burgeoning treatment landscape in FL, to aid in optimising patient selection and guiding the 'right drug to the right patient'.

Context-dependent regulation of immunoglobulin mutagenesis by p53

p53 plays a major role in genome maintenance. In addition to multiple p53 functions in the control of DNA repair, a regulation of DNA damage bypass via translesion synthesis has been implied in vitro. Somatic hypermutation of immunoglobulin genes for affinity maturation of antibody responses is based on aberrant translesion polymerase action and must be subject to stringent control to prevent genetic alterations and lymphomagenesis. When studying the role of p53 in somatic hypermutation in vivo, we found altered translesion polymerase-mediated A:T mutagenesis in mice lacking p53 in all organs, but notably not in mice with B cell-specific p53 inactivation, implying that p53 functions in non-B cells may alter mutagenesis in B cells. During class switch recombination, when p53 prevents formation of chromosomal translocations, we in addition detected a B cell-intrinsic role for p53 in altering G:C and A:T mutagenesis. Thus, p53 regulates translesion polymerase activity and shows differential activity during somatic hypermutation versus class switch recombination in vivo. Finally, p53 inhibition leads to increased somatic hypermutation in human B lymphoma cells. We conclude that loss of p53 function may promote genetic instability via multiple routes during antibody diversification in vivo.

Germinal center-dependent and -independent memory B cells produced throughout the immune response

Memory B cells comprise a heterogenous group of cells that differ in origin and phenotype. During the early phases of the immune response, activated B cells can differentiate into IgM-expressing memory cells, short-lived plasma cells, or seed germinal centers (GCs). The memory compartment is subsequently enriched by B cells that have been through several rounds of division and selection in the GC. Here, we report on the use of an unbiased lineage-tracking approach to explore the origins and properties of memory B cell subsets in mice with an intact immune system. We find that activated B cells continue to differentiate into memory B cells throughout the immune response. When defined on the basis of their origins, the memory B cells originating from activated B cells or GCs differ in isotype and overall gene expression, somatic hypermutation, and their affinity for antigen.

Affinity Tag Coating Enables Reliable Detection of Antigen-Specific B Cells in Immunospot Assays

Assessment of humoral immunity to SARS-CoV-2 and other infectious agents is typically restricted to detecting antigen-specific antibodies in the serum. Rarely does immune monitoring entail assessment of the memory B-cell compartment itself, although it is these cells that engage in secondary antibody responses capable of mediating immune protection when pre-existing antibodies fail to prevent re-infection. There are few techniques that are capable of detecting rare antigen-specific B cells while also providing information regarding their relative abundance, class/subclass usage and functional affinity. In theory, the ELISPOT/FluoroSpot (collectively ImmunoSpot) assay platform is ideally suited for antigen-specific B-cell assessments since it provides this information at single-cell resolution for individual antibody-secreting cells (ASC). Here, we tested the hypothesis that antigen-coating efficiency could be universally improved across a diverse set of viral antigens if the standard direct (non-specific, low affinity) antigen absorption to the membrane was substituted by high-affinity capture. Specifically, we report an enhancement in assay sensitivity and a reduction in required protein concentrations through the capture of recombinant proteins via their encoded hexahistidine (6XHis) affinity tag. Affinity tag antigen coating enabled detection of SARS-CoV-2 Spike receptor binding domain (RBD)-reactive ASC, and also significantly improved assay performance using additional control antigens. Collectively, establishment of a universal antigen-coating approach streamlines characterization of the memory B-cell compartment after SARS-CoV-2 infection or COVID-19 vaccinations, and facilitates high-throughput immune-monitoring efforts of large donor cohorts in general.

Plasma cell biology: Foundations for targeted therapeutic development in transplantation

Solid organ transplantation is a life-saving procedure for patients with end-stage organ disease. Over the past 70 years, tremendous progress has been made in solid organ transplantation, particularly in T-cell-targeted immunosuppression and organ allocation systems. However, humoral alloimmune responses remain a major challenge to progress. Patients with preexisting antibodies to human leukocyte antigen (HLA) are at significant disadvantages in regard to receiving a well-matched organ, moreover, those who develop anti-HLA antibodies after transplantation face a significant foreshortening of renal allograft survival. Historical therapies to desensitize patients prior to transplantation or to treat posttransplant AMR have had limited effectiveness, likely because they do not significantly reduce antibody levels, as plasma cells, the source of antibody production, remain largely unaffected. Herein, we will discuss the significance of plasma cells in transplantation, aspects of their biology as potential therapeutic targets, clinical challenges in developing strategies to target plasma cells in transplantation, and lastly, novel approaches that have potential to advance the field.

Germinal Centre Shutdown

Germinal Centres (GCs) are transient structures in secondary lymphoid organs, where affinity maturation of B cells takes place following an infection. While GCs are responsible for protective antibody responses, dysregulated GC reactions are associated with autoimmune disease and B cell lymphoma. Typically, ‘normal’ GCs persist for a limited period of time and eventually undergo shutdown. In this review, we focus on an important but unanswered question – what causes the natural termination of the GC reaction? In murine experiments, lack of antigen, absence or constitutive T cell help leads to premature termination of the GC reaction. Consequently, our present understanding is limited to the idea that GCs are terminated due to a decrease in antigen access or changes in the nature of T cell help. However, there is no direct evidence on which biological signals are primarily responsible for natural termination of GCs and a mechanistic understanding is clearly lacking. We discuss the present understanding of the GC shutdown, from factors impacting GC dynamics to changes in cellular interactions/dynamics during the GC lifetime. We also address potential missing links and remaining questions in GC biology, to facilitate further studies to promote a better understanding of GC shutdown in infection and immune dysregulation.

Single-cell BCR and transcriptome analysis after influenza infection reveals spatiotemporal dynamics of antigen-specific B cells

B cell responses are critical for antiviral immunity. However, a comprehensive picture of antigen-specific B cell differentiation, clonal proliferation, and dynamics in different organs after infection is lacking. Here, by combining single-cell RNA and B cell receptor (BCR) sequencing of antigen-specific cells in lymph nodes, spleen, and lungs after influenza infection in mice, we identify several germinal center (GC) B cell subpopulations and organ-specific differences that persist over the course of the response. We discover transcriptional differences between memory cells in lungs and lymphoid organs and organ-restricted clonal expansion. Remarkably, we find significant clonal overlap between GC-derived memory and plasma cells. By combining BCR-mutational analyses with monoclonal antibody (mAb) expression and affinity measurements, we find that memory B cells are highly diverse and can be selected from both low- and high-affinity precursors. By linking antigen recognition with transcriptional programming, clonal proliferation, and differentiation, these finding provide important advances in our understanding of antiviral immunity.

Follicular lymphoma dynamics

Follicular lymphoma (FL) is an indolent yet challenging disease. Despite a generally favorable response to immunochemotherapy regimens, a fraction of patients does not respond or relapses early with unfavorable prognosis. For the vast majority of those who initially respond, relapses will repeatedly occur with increasing refractoriness to available treatments. Addressing the clinical challenges in FL warrants deep understanding of the nature of treatment-resistant FL cells seeding relapses, and of the biological basis of early disease progression. Great progress has been made in the last decade in the description and interrogation of the (epi)genomic landscape of FL cells, of their major dependency to the tumor microenvironment (TME), and of the stepwise lymphomagenesis process, from healthy to subclinical disease and to overt FL. A new picture is emerging, in which an ever-evolving tumor-TME duo sparks a complex and multilayered clonal and functional heterogeneity, blurring the discovery of prognostic biomarkers, patient stratification and reliable designs of risk-adapted treatments. Novel technological approaches allowing to decipher both tumor and TME heterogeneity at the single-cell level are beginning to unravel unsuspected cell dynamics and plasticity of FL cells. The upcoming drawing of a comprehensive functional picture of FL within its ecosystem holds great promise to address the unmet medical needs of this complex lymphoma.

miR-15a/16-1 deletion in activated B cells promotes plasma cell and mature B-cell neoplasms

Chromosome 13q deletion [del(13q)], harboring the miR-15a/16-1 cluster, is one of the most common genetic alterations in mature B-cell malignancies, which originate from germinal center (GC) and post-GC B cells. Moreover, miR-15a/16 expression is frequently reduced in lymphoma and multiple myeloma (MM) cells without del(13q), suggesting important tumor-suppressor activity. However, the role of miR-15a/16-1 in B-cell activation and initiation of mature B-cell neoplasms remains to be determined. We show that conditional deletion of the miR-15a/16-1 cluster in murine GC B cells induces moderate but widespread molecular and functional changes including an increased number of GC B cells, percentage of dark zone B cells, and maturation into plasma cells. With time, this leads to development of mature B-cell neoplasms resembling human extramedullary plasmacytoma (EP) as well as follicular and diffuse large B-cell lymphomas. The indolent nature and lack of bone marrow involvement of EP in our murine model resembles human primary EP rather than MM that has progressed to extramedullary disease. We corroborate human primary EP having low levels of miR-15a/16 expression, with del(13q) being the most common genetic loss. Additionally, we show that, although the mutational profile of human EP is similar to MM, there are some exceptions such as the low frequency of hyperdiploidy in EP, which could account for different disease presentation. Taken together, our studies highlight the significant role of the miR-15a/16-1 cluster in the regulation of the GC reaction and its fundamental context-dependent tumor-suppression function in plasma cell and B-cell malignancies.

Dynamic Intracellular Metabolic Cell Signaling Profiles During Ag-Dependent B-Cell Differentiation

Human B-cell differentiation has been extensively investigated on genomic and transcriptomic grounds; however, no studies have accomplished so far detailed analysis of antigen-dependent maturation-associated human B-cell populations from a proteomic perspective. Here, we investigate for the first time the quantitative proteomic profiles of B-cells undergoing antigen-dependent maturation using a label-free LC-MS/MS approach applied on 5 purified B-cell subpopulations (naive, centroblasts, centrocytes, memory and plasma B-cells) from human tonsils (data are available via ProteomeXchange with identifier PXD006191). Our results revealed that the actual differences among these B-cell subpopulations are a combination of expression of a few maturation stage-specific proteins within each B-cell subset and maturation-associated changes in relative protein expression levels, which are related with metabolic regulation. The considerable overlap of the proteome of the 5 studied B-cell subsets strengthens the key role of the regulation of the stoichiometry of molecules associated with metabolic regulation and programming, among other signaling cascades (such as antigen recognition and presentation and cell survival) crucial for the transition between each B-cell maturation stage.

Antibody Affinity Shapes the Choice between Memory and Germinal Center B Cell Fates

Immunological memory is required for protection against repeated infections and is the basis of all effective vaccines. Antibodies produced by memory B cells play an essential role in many of these responses. We have combined lineage tracing with antibody cloning from single B cells to examine the role of affinity in B cell selection into germinal centers (GCs) and the memory B cell compartment in mice immunized with an HIV-1 antigen. We find that contemporaneously developing memory and GC B cells differ in their affinity for antigen throughout the immune response. Whereas GC cells and their precursors are enriched in antigen binding, memory B cells are not. Thus, the polyclonal memory B cell compartment is composed of B cells that were activated during the immune response but whose antigen binding affinity failed to support further clonal expansion in the GC.

Chronic allergen exposure drives accumulation of long-lived IgE plasma cells in the bone marrow, giving rise to serological memory

Immunoglobulin E (IgE) plays an important role in allergic diseases. Nevertheless, the source of IgE serological memory remains controversial. We reexamined the mechanism of serological memory in allergy using a dual reporter system to track IgE⁺ plasma cells in mice. Short-term allergen exposure resulted in the generation of IgE⁺ plasma cells that resided mainly in secondary lymphoid organs and produced IgE that was unable to degranulate mast cells. In contrast, chronic allergen exposure led to the generation of long-lived IgE⁺ plasma cells that were primarily derived from sequential class switching of IgG1, accumulated in the bone marrow, and produced IgE capable of inducing anaphylaxis. IgE⁺ plasma cells were found in the bone marrow of human allergic, but not nonallergic donors, and allergen-specific IgE produced by these cells was able to induce mast cell degranulation when transferred to mice. These data demonstrate that long-lived IgE⁺ bone marrow plasma cells arise during chronic allergen exposure and establish serological memory in both mice and humans.

The transcription factor Hhex cooperates with the corepressor Tle3 to promote memory B cell development

Memory B cells (MBCs) are essential for long-lived humoral immunity. However, the transcription factors involved in MBC differentiation are poorly defined. Here, using single-cell RNA sequencing analysis, we identified a population of germinal center (GC) B cells in the process of differentiating into MBCs. Using an inducible CRISPR-Cas9 screening approach, we identified the hematopoietically expressed homeobox protein Hhex as a transcription factor regulating MBC differentiation. The corepressor Tle3 was also identified in the screen and was found to interact with Hhex to promote MBC development. Bcl-6 directly repressed Hhex in GC B cells. Reciprocally, Hhex-deficient MBCs exhibited increased Bcl6 expression and reduced expression of the Bcl-6 target gene Bcl2. Overexpression of Bcl-2 was able to rescue MBC differentiation in Hhex-deficient cells. We also identified Ski as an Hhex-induced transcription factor involved in MBC differentiation. These findings establish an important role for Hhex-Tle3 in regulating the transcriptional circuitry governing MBC differentiation.

Genetic evolution of in situ follicular neoplasia to aggressive B-cell lymphoma of germinal center subtype

In situ follicular neoplasia (ISFN) is the earliest morphologically identifiable precursor of follicular lymphoma (FL). Although it is genetically less complex than FL and has low risk for progression, ISFN already harbors secondary genetic alterations, in addition to the defining t(14;18)(q32;q21) translocation. FL, in turn, frequently progresses to diffuse large B-cell lymphoma (DLBCL) or high-grade B-cell lymphoma (HGBL). By BCL2 staining of available reactive lymphoid tissue obtained at any time point in patients with aggressive B-cell lymphoma (BCL), we identified ten paired cases of ISFN and DLBCL/HGBL, including six de novo tumors and four tumors transformed from FL as an intermediate step, and investigated their clonal evolution using microdissection and next-generation sequencing. A clonal relationship between ISFN and aggressive BCL was established by immunoglobulin and/or BCL2 rearrangements and/or the demonstration of shared somatic mutations for all ten cases. Targeted sequencing revealed CREBBP, KMT2D, EZH2, TNFRSF14 and BCL2 as the genes most frequently mutated already in ISFN. Based on the distribution of private and shared mutations, two patterns of clonal evolution were evident. In most cases, the aggressive lymphoma, ISFN and, when present, FL revealed divergent evolution from a common progenitor, whereas linear evolution with sequential accumulation of mutations was less frequent. In conclusion, we demonstrate for the first time that t(14;18)+ aggressive BCL can arise from ISFN without clinically evident FL as an intermediate step and that during this progression, branched evolution is common.

Restriction of memory B cell differentiation at the germinal center B cell positive selection stage

Memory B cells (MBCs) are key for protection from reinfection. However, it is mechanistically unclear how germinal center (GC) B cells differentiate into MBCs. MYC is transiently induced in cells fated for GC expansion and plasma cell (PC) formation, so-called positively selected GC B cells. We found that these cells coexpressed MYC and MIZ1 (MYC-interacting zinc-finger protein 1 [ZBTB17]). MYC and MIZ1 are transcriptional activators; however, they form a transcriptional repressor complex that represses MIZ1 target genes. Mice lacking MYC-MIZ1 complexes displayed impaired cell cycle entry of positively selected GC B cells and reduced GC B cell expansion and PC formation. Notably, absence of MYC-MIZ1 complexes in positively selected GC B cells led to a gene expression profile alike that of MBCs and increased MBC differentiation. Thus, at the GC positive selection stage, MYC-MIZ1 complexes are required for effective GC expansion and PC formation and to restrict MBC differentiation. We propose that MYC and MIZ1 form a module that regulates GC B cell fate.

Unraveling the mysteries of plasma cells

Antibody-secreting plasma cells are the central pillars of humoral immunity. They are generated in a fundamental cellular restructuring process from naive B cells upon contact with antigen. This outstanding process is guided and controlled by a complex transcriptional network accompanied by a fascinating morphological metamorphosis, governed by the combined action of Blimp-1, Xbp-1 and IRF-4. The survival of plasma cells requires the intimate interaction with a specific microenvironment, consisting of stromal cells and cells of hematopoietic origin. Cell-cell contacts, cytokines and availability of metabolites such as glucose and amino acids modulate the survival abilities of plasma cells in their niches. Moreover, plasma cells have been shown to regulate immune responses by releasing cytokines. Furthermore, plasma cells are central players in autoimmune diseases and malignant transformation of plasma cells can result in the generation of multiple myeloma. Hence, the development of sophisticated strategies to deplete autoreactive plasma cells and myeloma cells represents a challenge for current and future research.

Survival of Long-Lived Plasma Cells (LLPC): Piecing Together the Puzzle

Durable humoral immunity is dependent upon the generation of antigen-specific antibody titers, produced by non-proliferating bone marrow resident long-lived plasma cells (LLPC). Longevity is the hallmark of LLPC, but why and how they survive and function for years after antigen exposure is only beginning to be understood. LLPC are not intrinsically long-lived; they require continuous signals from the LLPC niche to survive. Signals unique to LLPC survival (vs. PC survival in general) most notably include those that upregulate the anti-apoptotic factor Mcl-1 and activation of the CD28 receptor expressed on LLPC. Other potential factors include expression of BCMA, upregulation of the transcription factor ZBTB20, and upregulation of the enzyme ENPP1. Metabolic fitness is another key component of LLPC longevity, facilitating the diversion of glucose to generate pyruvate during times of stress to facilitate long term survival. A third major component of LLPC survival is the microenvironment/LLPC niche itself. Cellular partners such as stromal cells, dendritic cells, and T regulatory cells establish a niche for LLPC and drive survival signaling by expressing ligands such as CD80/CD86 for CD28 and producing soluble and stromal factors that contribute to LLPC longevity. These findings have led to the current paradigm wherein both intrinsic and extrinsic mechanisms are required for the survival of LLPC. Here we outline this diverse network of signals and highlight the mechanisms thought to regulate and promote the survival of LLPC. Understanding this network of signals has direct implications in increasing our basic understanding of plasma cell biology, but also in vaccine and therapeutic drug development to address the pathologies that can arise from this subset.

Non-classical B Cell Memory of Allergic IgE Responses

The long-term effectiveness of antibody responses relies on the development of humoral immune memory. Humoral immunity is maintained by long-lived plasma cells that secrete antigen-specific antibodies, and memory B cells that rapidly respond to antigen re-exposure by generating new plasma cells and memory B cells. Developing effective immunological memory is essential for protection against pathogens, and is the basis of successful vaccinations. IgE responses have evolved for protection against helminth parasites infections and against toxins, but IgE is also a potent mediator of allergic diseases. There has been a dramatic increase in the incidence of allergic diseases in recent decades and this has provided the impetus to study the nature of IgE antibody responses. As will be discussed in depth in this review, the IgE memory response has unique features that distinguish it from classical B cell memory.

Germinal Center B Cells Replace Their Antigen Receptors in Dark Zones and Fail Light Zone Entry when Immunoglobulin Gene Mutations are Damaging

Adaptive immunity involves the development of bespoke antibodies in germinal centers (GCs) through immunoglobulin somatic hypermutation (SHM) in GC dark zones (DZs) and clonal selection in light zones (LZs). Accurate selection requires that cells fully replace surface B cell receptors (BCRs) following SHM, but whether this happens before LZ entry is not clear. We found that most GC B cells degrade pre-SHM receptors before leaving the DZ, and that B cells acquiring crippling mutations during SHM rarely reached the LZ. Instead, apoptosis was triggered preferentially in late G1, a stage wherein cells with functional BCRs re-entered cell cycle or reduced surface expression of the chemokine receptor CXCR4 to enable LZ migration. Ectopic expression of the anti-apoptotic gene Bcl2 was not sufficient for cells with damaging mutations to reach the LZ, suggesting that BCR-dependent cues may actively facilitate the transition. Thus, BCR replacement and pre-screening in DZs prevents the accumulation of clones with non-functional receptors and facilitates selection in the LZ.

The Maintenance of Memory Plasma Cells

It is now well accepted that plasma cells can become long-lived (memory) plasma cells and secrete antibodies for months, years or a lifetime. However, the mechanisms involved in this process of humoral memory, which is crucial for both protective immunity and autoimmunity, still are not fully understood. This article will address a number of open questions. For example: Is longevity of plasma cells due to their intrinsic competence, extrinsic factors, or a combination of both? Which internal signals are involved in this process? What factors provide external support? What survival factors play a part in inflammation and autoreactive disease? Internal and external factors that contribute to the maintenance of memory long-lived plasma cells will be discussed. The aim is to provide useful additional information about the maintenance of protective and autoreactive memory plasma cells that will help researchers design effective vaccines for the induction of life-long protection against infectious diseases and to efficiently target pathogenic memory plasma cells.

Generation of memory B cells and their reactivation

The successful establishment of humoral memory response depends on at least two layers of defense. Pre-existing protective antibodies secreted by long-lived plasma cells act as a first line of defense against reinfection ("constitutive humoral memory"). Previously, a second line of defense in which pathogen-experienced memory B cells are rapidly reactivated to produce antibodies ("reactive humoral memory"), was considered as simply a back-up system for the first line (particularly for re-infection with homologous viruses). However, in the case of re-infection with similar but different strains of viruses, or in response to viral escape mutants, the reactive humoral memory plays a crucial role. Here, we review recent progress in our understanding of how memory B cells are generated in the pre-GC stage and during the GC reaction, and how these memory B cells are robustly reactivated with the help of memory Tfh cells to generate the secondary antibody response. In addition, we discuss how these advances may be relevant to the quest for a vaccine that can induce broadly reactive antibodies against influenza and HIV.

Expression of the Plasma Cell Transcriptional Regulator Blimp-1 by Dark Zone Germinal Center B Cells During Periods of Proliferation

Long-lived plasma cells (PCs) develop in germinal centers (GCs) by the differentiation of affinity matured B cells. Antibody affinity maturation involves iterative rounds of somatic hypermutation in dark zones (DZs) and selection in light zones (LZs), however the details of where, when and how PC commitment occurs are not well-understood. Fate bifurcation at the time of selection is one possibility, with the very highest affinity GC B cells differentiating as an alternative to DZ re-entry. However, how this model fits with a need to also retain these clones in the response is not clear. Here, we show that subsets of bona fide DZ cells express the plasma cell master regulator Blimp-1 at low levels during periods of proliferation. Ex vivo culture experiments demonstrate that these cells are not yet committed to plasma cell differentiation but that they may be sensitized to go down that route. Contrary to models in which T cells directly select GC B cells to begin expressing Blimp-1, we found that expression of this transcriptional regulator occurred even when follicular helper T cells were ablated. We speculate that Blimp-1 may be induced during proliferation in the DZ, and that as such single selected cells might give rise to both GC and post-GC progeny.

Role and Regulation of Pro-survival BCL-2 Proteins in Multiple Myeloma

Apoptosis plays a key role in protection against genomic instability and maintaining tissue homeostasis, and also shapes humoral immune responses. During generation of an antibody response, multiple rounds of B-cell expansion and selection take place in germinal centers (GC) before high antigen affinity memory B-cells and long-lived plasma cells (PC) are produced. These processes are tightly regulated by the intrinsic apoptosis pathway, and malignant transformation throughout and following the GC reaction is often characterized by apoptosis resistance. Expression of pro-survival BCL-2 family protein MCL-1 is essential for survival of malignant PC in multiple myeloma (MM). In addition, BCL-2 and BCL-XL contribute to apoptosis resistance. MCL-1, BCL-2, and BCL-XL expression is induced and maintained by signals from the bone marrow microenvironment, but overexpression can also result from genetic lesions. Since MM PC depend on these proteins for survival, inhibiting pro-survival BCL-2 proteins using novel and highly specific BH3-mimetic inhibitors is a promising strategy for treatment. This review addresses the role and regulation of pro-survival BCL-2 family proteins during healthy PC differentiation and in MM, as well as their potential as therapeutic targets.

The Lupus-Associated Fcγ Receptor IIb-I232T Polymorphism Results in Impairment in the Negative Selection of Low-Affinity Germinal Center B Cells Via c-Abl in Mice

Objective

Fcγ receptor IIb (FcγRIIb) is an essential negative regulator of B cells that blocks B cell receptor (BCR) signaling and triggers c‐Abl–dependent apoptosis of B cells. FcγRIIb‐deficient mice display splenomegaly with expansion of B cells, leading to lupus. FcγRIIb‐I232T is a hypofunctional polymorphism associated with lupus susceptibility in humans, an autoimmune disease linked to diminished deletion of autoreactive B cells. In the context of the FcγRIIb‐I232T polymorphism, we investigated the role of FcγRIIb in the deletion of low‐affinity germinal center (GC) B cells, an important mechanism for preventing autoimmunity.

Methods

We generated FcγRIIb^232T/T mice to mimic human FcγRIIb‐I232T carriers and immunized mice with chicken gamma globulin (CGG)–conjugated NP, a T cell–dependent antigen, to examine the response of GC B cells.

Results

Compared to wild‐type (WT) mice, FcγRIIb^232T/T mice showed increased numbers of low‐affinity NP‐specific IgG and NP‐specific B cells and plasma cells; additionally, the expression of a somatic mutation (W33L) in their V_H186.2 genes encoding high‐affinity BCR was reduced. Notably, FcγRIIb^232T/T mice had a higher number of GC light zone B cells and showed less apoptosis than WT mice, despite having equivalent follicular helper T cell numbers and function. Moreover, phosphorylation of c‐Abl was reduced in FcγRIIb^232T/T mice, and treatment of WT mice with the c‐Abl inhibitor nilotinib during the peak of GC response resulted in reduced affinity maturation reminiscent of FcγRIIb^232T/T mice.

Conclusion

Our findings provide evidence of a critical role of FcγRIIb/c‐Abl in the negative selection of GC B cells in FcγRIIb^232T/T mice. Importantly, our findings indicate potential benefits of up‐regulating FcγRIIb expression in B cells for treatment of systemic lupus erythematosus.

Critical influences on the pathogenesis of follicular lymphoma

The development of follicular lymphoma (FL) from a founder B cell with an upregulation of B-cell lymphoma 2 (BCL2), via the t(14;18) translocation, to a proliferating clone, poised to undergo further transformation to an aggressive lymphoma, illustrates the opportunistic Darwinian process of tumorigenesis. Protection against apoptosis allows an innocent cell to persist and divide, with dangerous accumulation of further mutational changes, commonly involving inactivation of chromatin-modifying genes. But this is not all. FL cells reflect normal B cells in relying on expression of surface immunoglobulin. In doing so, they add another supportive mechanism by exploiting the natural process of somatic hypermutation of the IGV genes. Positive selection of motifs for addition of glycan into the antigen-binding sites of virtually all cases, and the placement of unusual mannoses in those sites, reveals a posttranslational strategy to engage the microenvironment. A bridge between mannosylated surface immunoglobulin of FL cells and macrophage-expressed dendritic cell-specific ICAM-3-grabbing nonintegrin produces a persistent low-level signal that appears essential for life in the hostile germinal center. Early-stage FL therefore requires a triad of changes: protection from apoptosis, mutations in chromatin modifiers, and an ability to interact with lectin-expressing macrophages. These changes are common and persistent. Genetic/epigenetic analysis is providing important data but investigation of the posttranslational landscape is the next challenge. We have one glimpse of its operation via the influence of added glycan on the B-cell receptor of FL. The consequential interaction with environmental lectins illustrates how posttranslational modifications can be exploited by tumor cells, and could lead to new approaches to therapy.

Plasma Cell Differentiation Pathways in Systemic Lupus Erythematosus

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

Atypical chemokine receptor 4 shapes activated B cell fate

Activated B cells can initially differentiate into three functionally distinct fates-early plasmablasts (PBs), germinal center (GC) B cells, or early memory B cells-by mechanisms that remain poorly understood. Here, we identify atypical chemokine receptor 4 (ACKR4), a decoy receptor that binds and degrades CCR7 ligands CCL19/CCL21, as a regulator of early activated B cell differentiation. By restricting initial access to splenic interfollicular zones (IFZs), ACKR4 limits the early proliferation of activated B cells, reducing the numbers available for subsequent differentiation. Consequently, ACKR4 deficiency enhanced early PB and GC B cell responses in a CCL19/CCL21-dependent and B cell-intrinsic manner. Conversely, aberrant localization of ACKR4-deficient activated B cells to the IFZ was associated with their preferential commitment to the early PB linage. Our results reveal a regulatory mechanism of B cell trafficking via an atypical chemokine receptor that shapes activated B cell fate.