Positive selection of IgG+ over IgM+ B cells in the germinal center reaction

Highly sensitive ELISA for determination of nifursol metabolite in food samples without derivatization: From rational hapten design to molecular recognition mechanism

High-affinity antibodies to nitrofuran metabolites could avoid derivatization, but existing nitrofuran haptens yield low-affinity antibodies due to poor immunogenicity. Herein, we selected 3,5-dinitrosalicylic acid hydrazide (DNSH), the metabolite of nifursol, as model analytes to produce high-affinity antibodies against metabolites. Three novel DNSH haptens were rationally designed, with aromatic haptens specially incorporating nitro group to enhance immunogenicity. After rationality evaluation by computational chemistry, antisera results indicated that nitro-modified haptens, DNSH-1 and DNSH-2, induced a stronger immune response. Furthermore, high-affinity mAb 2H6 was obtained from DNSH-1 with IC50 of 0.16 ng mL-1 for DNSH. Molecular simulations demonstrated that abundant electrostatic characteristics and hydrogen bonds of nitro group contributed to its immune-enhancing effect. Finally, a rapid and sensitive icELISA was developed with limit of detections of 0.1-0.5 μg kg-1 in pork, chicken and fish meat. The total assay time was less than 3 h, eliminating prolonged 16-h derivatization and significantly improves detection efficiency.

From immunobiology to intervention: Pathophysiology of autoimmune encephalitis

Autoimmune encephalitides (AEs) are neurological disorders caused by autoantibodies against neuronal and glial surface proteins. Nearly 20 years after their discovery, AE have evolved from being frequently misdiagnosed and untreated to a growing group of increasingly well-characterized conditions where patients benefit from targeted therapeutic strategies. This narrative review provides an immunological perspective on AE, focusing on NMDAR, CASPR2 and LGI1 encephalitis as the three most common forms of AE associated with anti-neuronal surface autoantibodies. We examine the autoreactive B cell subsets, the tolerance checkpoints that may fail, and the known triggers and predispositions contributing to disease. In addition, we discuss the roles of other immune cells, including T cells and microglia, in the pathogenesis of AE. By analyzing therapeutic strategies and treatment responses we draw insights into AE pathophysiology. Written at a time of transformative therapeutic advancements through cell therapies this work underscores the synergy between detailed immunological research and the development of innovative therapies.

Estrogen influences class-switched memory B cell frequency only in humans with two X chromosomes

Sex differences in immunity are well-documented, though mechanisms underpinning these differences remain ill-defined. Here, in a human-only ex vivo study, we demonstrate that postpubertal cisgender females have higher levels of CD19+CD27+IgD- class-switched memory B cells compared with age-matched cisgender males. This increase is only observed after puberty and before menopause, suggesting a strong influence for sex hormones. Accordingly, B cells express high levels of estrogen receptor 2 (ESR2), and class-switch-regulating genes are enriched for ESR2-binding sites. In a gender-diverse cohort, blockade of natal estrogen in transgender males (XX karyotype) reduced class-switched memory B cell frequency, while gender-affirming estradiol treatment in transgender females (XY karyotype) did not increase these levels. In postmenopausal cis-females, class-switched memory B cells were increased in those taking hormone replacement therapy (HRT) compared with those who were not. These data demonstrate that sex hormones and chromosomes work in tandem to impact immune responses, with estrogen only influencing the frequency of class-switched memory B cells in individuals with an XX chromosomal background.

IL-21 shapes the B cell response in a context-dependent manner

The T-cell-derived cytokine IL-21 is crucial for germinal center (GC) responses, but its precise role in B cell function has remained elusive. Using IL-21 receptor (Il21r) conditional knockout mice and ex vivo culture systems, we demonstrate that IL-21 has dual effects on B cells. While IL-21 induced apoptosis in a STAT3-dependent manner in naive B cells, it promoted the robust proliferation of pre-activated B cells, particularly IgG1+ B cells. In vivo, B-cell-specific Il21r deletion impaired IgG1 responses post-immunization and disrupted progression from pre-GC to GC states. Although Il21r deficiency did not affect the proportion of IgG1+ cells among GC B cells, it greatly diminished the proportion of IgG1+ cells among the plasmablast/plasma cell population. Collectively, our findings suggest that IL-21 serves as a critical regulator of B cell fates, influencing B cell apoptosis and proliferation in a context-dependent manner.

B cell adapter for PI 3-kinase (BCAP) coordinates antigen internalization and trafficking through the B cell receptor

B cell adapter for PI 3-kinase (BCAP) is an adaptor molecule associated with signaling through multiple immune receptors, including the B cell receptor (BCR). However, B cell-intrinsic role of BCAP in antibody responses is unclear. We investigated the role of BCAP in B cell response to viral particles and found a previously unidentified mechanism by which BCAP regulates antigen-specific responses. B cell-specific deletion of BCAP in mice leads to decreases in antigen-specific responses through defects in BCR-antigen endocytosis. BCAP is necessary to orchestrate actin reorganization around the antigen for efficient endocytosis through BCR and intracellular processing of antigens. Therefore, loss of BCAP from B cells leads to defects in antigen endocytosis, hampering the propagation of antigen-derived signals and decreasing the ability of B cells to present antigens to T cells. Thus, our study clarifies how BCAP regulates B cell responses to complex antigens and elucidates that antigen positioning inside B cells determines different B cell activation outcomes.

FcRider: a recombinant Fc nanoparticle with endogenous adjuvant activities for hybrid immunization

Active immunization (vaccination) induces long-lasting immunity with memory, which takes weeks to months to develop. Passive immunization (transfer of neutralizing antibodies) provides immediate protection, yet with high cost and effects being comparatively short-lived. No currently approved adjuvants are compatible with formulations to combine active and passive immunizations, not to mention their huge disparities in administration routes and dosage. To solve this, we engineered the Fc fragment of human IgG1 into a hexamer nanoparticle and expressed its afucosylated form in Fut8-/- CHO cells, naming it "FcRider." FcRider is highly soluble with long-term stability, easily produced at high levels equivalent to those of therapeutic antibodies, and is amenable to conventional antibody purification schemes. Most importantly, FcRider possesses endogenous adjuvant activities. Using SWHEL B cell receptor (BCR) transgenic mice, we found that HEL-FcRider induced GL7+ germinal center B cells and HEL-specific IgG. Similarly, immunizing mice with UFO-BG-FcRider, a fusion containing the stabilized human immunodeficiency virus-1 (HIV-1) Env protein as immunogen, promoted somatic hypermutation and generation of long CDR3 of the IgG heavy chains. Intramuscular injection of (Fba + Met6)3-FcRider, a fusion with two peptide epitopes from Candida albicans cell surface, stimulated strong antigen-specific IgG titers. In three different models, we showed that afucosylated FcRider functions as a multivalent immunogen displayer and stimulates antigen-specific B cells without any exogenous adjuvant. As an antibody derivative, afucosylated FcRider could be a novel platform combining vaccines and therapeutic antibodies, integrating active and passive immunizations into single-modality "hybrid immunization" to provide complete and long-lasting protection against infections, and may open new avenues in cancer immunotherapy as well.

BCR signaling in germinal center B cell selection

When mature B cells are activated by antigens, the selection of these activated B cells takes place particularly during T cell-dependent immune responses in which an improved antibody repertoire is generated through somatic hypermutation in germinal centers (GCs). In this process the importance of antigen presentation by GC B cells, and subsequent T follicular helper (Tfh) cell help in positive selection of GC B cells, has been well appreciated. By contrast, the role of B cell receptor (BCR) signaling per se remains unclear. Strong experimental support for the involvement of BCR signaling in GC B cell selection has now been provided. Interestingly, these studies suggest that several checkpoints operating through the BCR ensure affinity maturation.

Regulation of B-cell function and expression of CD11c, T-bet, and FcRL5 in response to different activation signals

CD11c, FcRL5, or T-bet are commonly expressed by B cells expanding during inflammation, where they can make up >30% of mature B cells. However, the association between the proteins and differentiation and function in the host response remains largely unclear. We have assessed the co-expression of CD11c, T-bet, and FcRL5 in an in vitro B-cell culture system to determine how stimulation via the BCR, toll-like receptor 9 (TLR9), and different cytokines influence CD11c, T-bet, and FcRL5 expression. We observed different expression dynamics for all markers, but a largely overlapping regulation of CD11c and FcRL5 in response to BCR and TLR9 activation, while T-bet was strongly dependent on IFN-γ signaling. Investigating plasma cell differentiation and APC functions, there was no association between marker expression and antibody secretion or T-cell help. Rather the functions were associated with TLR9-signalling and B-cell-derived IL-6 production, respectively. These results suggest that the expression of CD11c, FcRL5, and T-bet and plasma cell differentiation and improved APC functions occur in parallel and are regulated by similar activation signals, but they are not interdependent.

Open-ST: High-resolution spatial transcriptomics in 3D

Spatial transcriptomics (ST) methods unlock molecular mechanisms underlying tissue development, homeostasis, or disease. However, there is a need for easy-to-use, high-resolution, cost-efficient, and 3D-scalable methods. Here, we report Open-ST, a sequencing-based, open-source experimental and computational resource to address these challenges and to study the molecular organization of tissues in 2D and 3D. In mouse brain, Open-ST captured transcripts at subcellular resolution and reconstructed cell types. In primary head-and-neck tumors and patient-matched healthy/metastatic lymph nodes, Open-ST captured the diversity of immune, stromal, and tumor populations in space, validated by imaging-based ST. Distinct cell states were organized around cell-cell communication hotspots in the tumor but not the metastasis. Strikingly, the 3D reconstruction and multimodal analysis of the metastatic lymph node revealed spatially contiguous structures not visible in 2D and potential biomarkers precisely at the 3D tumor/lymph node boundary. All protocols and software are available at https://rajewsky-lab.github.io/openst.

IL-21 Shapes the B Cell Response in a Context-Dependent Manner

The cytokine interleukin-21 (IL-21) is a pivotal T cell-derived signal crucial for germinal center (GC) responses, but the precise mechanisms by which IL-21 influences B cell function remain elusive. Here, we investigated the B cell-intrinsic role of IL-21 signaling by employing a novel IL-21 receptor ( Il21r ) conditional knock-out mouse model and ex vivo culture systems and uncovered a surprising duality of IL-21 signaling in B cells. While IL-21 stimulation of naïve B cells led to Bim-dependent apoptosis, it promoted robust proliferation of pre-activated B cells, particularly class-switched IgG1 + B cells ex vivo . Consistent with this, B cell-specific deletion of Il21r led to a severe defect in IgG1 responses in vivo following immunization. Intriguingly, Il21r -deleted B cells are significantly impaired in their ability to transition from a pre-GC to a GC state following immunization. Although Il21r -deficiency did not affect the proportion of IgG1 + B cells among GC B cells, it greatly diminished the proportion of IgG1 + B cells among the plasmablast/plasma cell population. Collectively, our data suggest that IL-21 serves as a critical regulator of B cell fates, influencing B cell apoptosis and proliferation in a context-dependent manner.

Antibody modulation of B cell responses-Incorporating positive and negative feedback

Antibodies are powerful modulators of ongoing and future B cell responses. While the concept of antibody feedback has been appreciated for over a century, the topic has seen a surge in interest due to the evidence that the broadening of antibody responses to SARS-CoV-2 after a third mRNA vaccination is a consequence of antibody feedback. Moreover, the discovery that slow antigen delivery can lead to more robust humoral immunity has put a spotlight on the capacity for early antibodies to augment B cell responses. Here, we review the mechanisms whereby antibody feedback shapes B cell responses, integrating findings in humans and in mouse models. We consider the major influence of epitope masking and the diverse actions of complement and Fc receptors and provide a framework for conceptualizing the ways antigen-specific antibodies may influence B cell responses to any form of antigen, in conditions as diverse as infectious disease, autoimmunity, and cancer.

Mature tertiary lymphoid structures: important contributors to anti-tumor immune efficacy

Tertiary lymphoid structures (TLS) represent the ectopic aggregations of immune cells arising during chronic inflammation or tumor progression. In cancer, TLS are often associated with beneficial clinical outcomes in patients undergoing immunotherapy, underscoring their prognostic and predictive significance. Mature TLS, characterized by germinal centers and areas of T-cell and B-cell aggregation, are considered primary locations for activating and maintaining both humoral and cellular anti-tumor immune effects. Despite their recognized importance, the mechanisms driving the formation of mature TLS in cancer and their influence on the immune response within tumors remain insufficiently understood. Therefore, this review aims to comprehensively explore the structural composition, development mechanisms, maturity impact factors, immunological function, and innovative therapeutic strategies of mature TLS within the tumor microenvironment. The research summarized herein offers novel insights and considerations for therapeutic approaches to promote TLS generation and maturation in patients with cancer, representing a promising avenue for future cancer therapies.

Regulation of BCR-mediated Ca2+ mobilization by MIZ1-TMBIM4 safeguards IgG1+ GC B cell-positive selection

The transition from immunoglobulin M (IgM) to affinity-matured IgG antibodies is vital for effective humoral immunity. This is facilitated by germinal centers (GCs) through affinity maturation and preferential maintenance of IgG+ B cells over IgM+ B cells. However, it is not known whether the positive selection of the different Ig isotypes within GCs is dependent on specific transcriptional mechanisms. Here, we explored IgG1+ GC B cell transcription factor dependency using a CRISPR-Cas9 screen and conditional mouse genetics. We found that MIZ1 was specifically required for IgG1+ GC B cell survival during positive selection, whereas IgM+ GC B cells were largely independent. Mechanistically, MIZ1 induced TMBIM4, an ancestral anti-apoptotic protein that regulated inositol trisphosphate receptor (IP3R)-mediated calcium (Ca2+) mobilization downstream of B cell receptor (BCR) signaling in IgG1+ B cells. The MIZ1-TMBIM4 axis prevented mitochondrial dysfunction-induced IgG1+ GC cell death caused by excessive Ca2+ accumulation. This study uncovers a unique Ig isotype-specific dependency on a hitherto unidentified mechanism in GC-positive selection.

Mapping and modelling human B cell maturation in the germinal centre

The maturation of B cells within the germinal centre (GC) is necessary for antigen-specific immune responses and memory. Dysfunction in the GC can lead to immunodeficiencies, autoimmune diseases, or lymphomas. Here we describe how recent advances in single-cell and spatial genomics have enabled new discoveries about the diversity of human GC B cell states. However, with the advent of these hypothesis-generating technologies, the field should now transition towards testing bioinformatic predictions using experimental models of the human GC. We review available experimental culture systems for modelling human B cell responses and discuss the potential limitations of different methods in capturing bona fide GC B cell states. Together, the combination of cell atlas-based mapping with experimental modelling of lymphoid tissues holds great promise to better understand the maturation of human B cells in the GC response and generate new insights into human immune health and disease.

Notch2 controls developmental fate choices between germinal center and marginal zone B cells upon immunization

Sustained Notch2 signals induce trans-differentiation of Follicular B (FoB) cells into Marginal Zone B (MZB) cells in mice, but the physiology underlying this differentiation pathway is still elusive. Here, we demonstrate that most B cells receive a basal Notch signal, which is intensified in pre-MZB and MZB cells. Ablation or constitutive activation of Notch2 upon T-cell-dependent immunization reveals an interplay between antigen-induced activation and Notch2 signaling, in which FoB cells that turn off Notch2 signaling enter germinal centers (GC), while high Notch2 signaling leads to generation of MZB cells or to initiation of plasmablast differentiation. Notch2 signaling is dispensable for GC dynamics but appears to be re-induced in some centrocytes to govern expansion of IgG1+ GCB cells. Mathematical modelling suggests that antigen-activated FoB cells make a Notch2 dependent binary fate-decision to differentiate into either GCB or MZB cells. This bifurcation might serve as a mechanism to archive antigen-specific clones into functionally and spatially diverse B cell states to generate robust antibody and memory responses.

Germinal centers output clonally diverse plasma cell populations expressing high- and low-affinity antibodies

Germinal centers (GCs) form in lymph nodes after immunization or infection to facilitate antibody affinity maturation and memory and plasma cell (PC) development. PC differentiation is thought to involve stringent selection for GC B cells expressing the highest-affinity antigen receptors, but how this plays out during complex polyclonal responses is unclear. We combine temporal lineage tracing with antibody characterization to gain a snapshot of PCs developing during influenza infection. GCs co-mature B cell clones with antibody affinities spanning multiple orders of magnitude; however, each generates PCs with similar efficiencies, including weak binders. Within lineages, PC selection is not restricted to variants with the highest-affinity antibodies. Differentiation is commonly associated with proliferative expansion to produce "nodes" of identical PCs. Immunization-induced GCs generate fewer PCs but still of low- and high-antibody affinities. We propose that generating low-affinity antibody PCs reflects an evolutionary compromise to facilitate diverse serum antibody responses.

Antigen receptor signaling and cell death resistance controls intestinal humoral response zonation

Immunoglobulin A (IgA) maintains commensal communities in the intestine while preventing dysbiosis. IgA generated against intestinal microbes assures the simultaneous binding to multiple, diverse commensal-derived antigens. However, the exact mechanisms by which B cells mount broadly reactive IgA to the gut microbiome remains elusive. Here, we have shown that IgA B cell receptor (BCR) is required for B cell fitness during the germinal center (GC) reaction in Peyer's patches (PPs) and for generation of gut-homing plasma cells (PCs). We demonstrate that IgA BCR drove heightened intracellular signaling in mouse and human B cells, and as a consequence, IgA+ B cells received stronger positive selection cues. Mechanistically, IgA BCR signaling offset Fas-mediated death, possibly rescuing low-affinity B cells to promote a broad humoral response to commensals. Our findings reveal an additional mechanism linking BCR signaling, B cell fate, and antibody production location, which have implications for how intestinal antigen recognition shapes humoral immunity.

Advancement in preoperative desensitization therapy for ABO incompatible kidney transplantation recipients

ABO incompatibility has long been considered an absolute contraindication for kidney transplantation. However, with the increasing number of patients with ESRD in recent years, ABO-incompatible kidney transplantation (ABOi-KT) has expanded the types of donors by crossing the blood group barrier through preoperative desensitization therapy. At present, the desensitization protocols consist of removal of preexisting ABO blood group antibody titers and prevention of ABO blood group antibody return. Studies have suggested similar patient and graft survival among ABOi-KT and ABOc-KT recipients. In this review, we will summarize the effective desensitization regimens of ABOi-KT, aiming to explore effective ways to improve the success rate and the long-term survival rate of ABOi-KT recipients.

Isolation, cloning and analysis of parvovirus-specific canine antibodies from peripheral blood B cells

B-cell cloning methods enable the analysis of antibody responses against target antigens and can be used to reveal the host antibody repertoire, antigenic sites (epitopes), and details of protective immunity against pathogens. Here, we describe improved methods for isolation of canine peripheral blood B cells producing antibodies against canine parvovirus (CPV) capsids by fluorescence-activated cell sorting, followed by cell cloning. We cultured sorted B cells from an immunized dog in vitro and screened for CPV-specific antibody production. Updated canine-specific primer sets were used to amplify and clone the heavy and light chain immunoglobulin sequences directly from the B cells by reverse transcription and PCR. Monoclonal canine IgGs were produced by cloning heavy and light chain sequences into antibody expression vectors, which were screened for CPV binding. Three different canine monoclonal antibodies were analyzed, including two that shared the same heavy chain, and one that had distinct heavy and light chains. The antibodies showed broad binding to CPV variants, and epitopes were mapped to antigenic sites on the capsid. The methods described here are applicable for the isolation of canine B cells and monoclonal antibodies against many antigens.

Germinal center-dependent and -independent immune responses of tumor-infiltrating B cells in human cancers

B cells play essential roles in immunity, mainly through the production of high affinity plasma cells (PCs) and memory B (Bmem) cells. The affinity maturation and differentiation of B cells rely on the integration of B-cell receptor (BCR) intrinsic and extrinsic signals provided by antigen binding and the microenvironment, respectively. In recent years, tumor infiltrating B (TIL-B) cells and PCs (TIL-PCs) have been revealed as important players in antitumor responses in human cancers, but their interplay and dynamics remain largely unknown. In lymphoid organs, B-cell responses involve both germinal center (GC)-dependent and GC-independent pathways for Bmem cell and PC production. Affinity maturation of BCR repertoires occurs in GC reactions with specific spatiotemporal dynamics of signal integration by B cells. In general, the reactivation of high-affinity Bmem cells by antigens triggers GC-independent production of large numbers of PC without BCR rediversification. Understanding B-cell dynamics in immune responses requires the integration of multiple tools and readouts such as single-cell phenotyping and RNA-seq, in situ analyses, BCR repertoire analysis, BCR specificity and affinity assays, and functional tests. Here, we review how those tools have recently been applied to study TIL-B cells and TIL-PC in different types of solid tumors. We assessed the published evidence for different models of TIL-B-cell dynamics involving GC-dependent or GC-independent local responses and the resulting production of antigen-specific PCs. Altogether, we highlight the need for more integrative B-cell immunology studies to rationally investigate TIL-B cells as a leverage for antitumor therapies.

B cells: The many facets of B cells in allergic diseases

B cells play a key role in our immune system through their ability to produce antibodies, suppress a proinflammatory state, and contribute to central immune tolerance. We aim to provide an in-depth knowledge of the molecular biology of B cells, including their origin, developmental process, types and subsets, and functions. In allergic diseases, B cells are well known to induce and maintain immune tolerance through the production of suppressor cytokines such as IL-10. Similarly, B cells protect against viral infections such as severe acute respiratory syndrome coronavirus 2 that caused the recent coronavirus disease 2019 pandemic. Considering the unique and multifaceted functions of B cells, we hereby provide a comprehensive overview of the current knowledge of B-cell biology and its clinical applications in allergic diseases, organ transplantation, and cancer.

Plasma autoantibodies IgG and IgM to PD1/PDL1 as potential biomarkers and risk factors of lung cancer

Antibodies targeting programmed cell death-1 (PD1) and its ligand (PDL1) have transformed current cancer therapy while little is known about the expression of anti-PD1/PDL1 autoantibodies between lung cancer (LC) patients and normal controls (NC). The expression level of anti-PD1/PDL1 IgG and IgM was detected in plasma of 325 LC and 324 NC by indirect enzyme-linked immune sorbent assay (ELISA). Western blot and indirect immunofluorescence (IIF) were used to verify the ELISA results. The association analysis was used to evaluate the odds ratio (OR) of LC. The expression of anti-PD1/PDL1 IgG in LC samples was significantly higher than NC (P < 0.001 and P < 0.05, respectively). The positive rate of anti-PD1/PDL1 IgG in LC was significantly higher than NC and significant difference was also shown in LC samples of different clinical characteristics, such as clinical stage, nodules diameter, lymph node metastasis and distant metastasis (P < 0.001). Moreover, PD1/PDL1 expression in tissues showed no significant relation with that in plasma (P > 0.05). Anti-PD1/PDL1 IgG were the risk factors related to LC (OR (95% CI): 22.433 (5.426-92.745) and 5.051 (1.316-19.386)), while anti-PD1/PDL1 IgM were the risk factors for LC with ≤ 60 years (OR (95% CI): 6.122 (1.365-27.455) and 7.664 (1.715-34.251)) and anti-PD1 IgM was also the risk factor for male LC cases(OR (95% CI): 6.948 (1.076-44.868)). Plasma anti-PD1/PDL1 IgG and IgM might serve as potential biomarkers and risk predictors for LC.

Germinal center reaction and output: recent advances

The germinal center (GC) reaction is unique in that it incorporates clonal expansion, somatic mutagenesis, affinity-based selection, and differentiation events all in one tightly packed but highly dynamic microenvironment to produce affinity-matured plasma cells (PCs) or memory B cells (MBCs). Here, we review recent advances in our understanding of how cyclic expansion and selection are orchestrated, how stringency and efficiency of selection are maintained, and how external signals are integrated in B cells to promote post-GC development of PCs and MBCs.

Adenoviral-vectored epigraph vaccine elicits robust, durable, and protective immunity against H3 influenza A virus in swine

Current methods of vaccination against swine Influenza A Virus (IAV-S) in pigs are infrequently updated, induce strain-specific responses, and have a limited duration of protection. Here, we characterize the onset and duration of adaptive immune responses after vaccination with an adenoviral-vectored Epigraph vaccine. In this longitudinal study we observed robust and durable antibody responses that remained above protective titers six months after vaccination. We further identified stable levels of antigen-specific T cell responses that remained detectable in the absence of antigen stimulation. Antibody isotyping revealed robust class switching from IgM to IgG induced by Epigraph vaccination, while the commercial comparator vaccine failed to induce strong antibody class switching. Swine were challenged six months after initial vaccination, and Epigraph-vaccinated animals demonstrated significant protection from microscopic lesion development in the trachea and lungs, reduced duration of viral shedding, lower presence of infectious virus and viral antigens in the lungs, and significant recall of antigen-specific T cell responses following challenge. The results obtained from this study are useful in determining the kinetics of adaptive immune responses after vaccination with adjuvanted whole inactivated virus vaccines compared to adenoviral vectored vaccines and contribute to the continued efforts of creating a universal IAV-S vaccine.

Safety and immunogenicity following co-administration of Yellow fever vaccine with Tick-borne encephalitis or Japanese encephalitis vaccines: Results from an open label, non-randomized clinical trial

BACKGROUND

Flavivirus infections pose a significant global health burden underscoring the need for the development of safe and effective vaccination strategies. Available flavivirus vaccines are from time to time concomitantly delivered to individuals. Co-administration of different vaccines saves time and visits to health care units and vaccine clinics. It serves to provide protection against multiple pathogens in a shorter time-span; e.g., for individuals travelling to different endemic areas. However, safety and immunogenicity-related responses have not been appropriately evaluated upon concomitant delivery of these vaccines. Therefore, we performed an open label, non-randomized clinical trial studying the safety and immunogenicity following concomitant delivery of the yellow fever virus (YFV) vaccine with tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus (JE) virus vaccines.

METHODS AND FINDINGS

Following screening, healthy study participants were enrolled into different cohorts receiving either TBEV and YFV vaccines, JEV and YFV vaccines, or in control groups receiving only the TBEV, JEV, or YFV vaccine. Concomitant delivery was given in the same or different upper arms for comparison in the co-vaccination cohorts. Adverse effects were recorded throughout the study period and blood samples were taken before and at multiple time-points following vaccination to evaluate immunological responses to the vaccines. Adverse events were predominantly mild in the study groups. Four serious adverse events (SAE) were reported, none of them deemed related to vaccination. The development of neutralizing antibodies (nAbs) against TBEV, JEV, or YFV was not affected by the concomitant vaccination strategy. Concomitant vaccination in the same or different upper arms did not significantly affect safety or immunogenicity-related outcomes. Exploratory studies on immunological effects were additionally performed and included studies of lymphocyte activation, correlates associated with germinal center activation, and plasmablast expansion.

CONCLUSIONS

Inactivated TBEV or JEV vaccines can be co-administered with the live attenuated YFV vaccine without an increased risk of adverse events and without reduced development of nAbs to the respective viruses. The vaccines can be delivered in the same upper arm without negative outcome. In a broader perspective, the results add valuable information for simultaneous administration of live and inactivated flavivirus vaccines in general.

TRIAL REGISTRATION

Eudra CT 2017-002137-32.

Modeling and predicting the overlap of B- and T-cell receptor repertoires in healthy and SARS-CoV-2 infected individuals

Adaptive immunity's success relies on the extraordinary diversity of protein receptors on B and T cell membranes. Despite this diversity, the existence of public receptors shared by many individuals gives hope for developing population-wide vaccines and therapeutics. Using probabilistic modeling, we show many of these public receptors are shared by chance in healthy individuals. This predictable overlap is driven not only by biases in the random generation process of receptors, as previously reported, but also by their common functional selection. However, the model underestimates sharing between repertoires of individuals infected with SARS-CoV-2, suggesting strong specific antigen-driven convergent selection. We exploit this discrepancy to identify COVID-associated receptors, which we validate against datasets of receptors with known viral specificity. We study their properties in terms of sequence features and network organization, and use them to design an accurate diagnostic tool for predicting SARS-CoV-2 status from repertoire data.

Increased B Cell Understanding Puts Improved Vaccine Platforms Just Over the Horizon

In the face of an endlessly expanding repertoire of Ags, vaccines are constantly being tested, each more effective than the last. As viruses and other pathogens evolve to become more infectious, the need for efficient and effective vaccines grows daily, which is especially obvious in an era that is still attempting to remove itself from the clutches of the severe acute respiratory syndrome coronavirus 2, the cause of coronavirus pandemic. To continue evolving alongside these pathogens, it is proving increasingly essential to consider one of the main effector cells of the immune system. As one of the chief orchestrators of the humoral immune response, the B cell and other lymphocytes are essential to not only achieving immunity, but also maintaining it, which is the vital objective of every vaccine.

Human anti-smallpox long-lived memory B cells are defined by dynamic interactions in the splenic niche and long-lasting germinal center imprinting

Memory B cells (MBCs) can persist for a lifetime, but the mechanisms that allow their long-term survival remain poorly understood. Here, we isolated and analyzed human splenic smallpox/vaccinia protein B5-specific MBCs in individuals who were vaccinated more than 40 years ago. Only a handful of clones persisted over such an extended period, and they displayed limited intra-clonal diversity with signs of extensive affinity-based selection. These long-lived MBCs appeared enriched in a CD21hiCD20hi IgG+ splenic B cell subset displaying a marginal-zone-like NOTCH/MYC-driven signature, but they did not harbor a unique longevity-associated transcriptional or metabolic profile. Finally, the telomeres of B5-specific, long-lived MBCs were longer than those in patient-paired naive B cells in all the samples analyzed. Overall, these results imply that separate mechanisms such as early telomere elongation, affinity selection during the contraction phase, and access to a specific niche contribute to ensuring the functional longevity of MBCs.

The dark side of Tregs during aging

In the last century, we have seen a dramatic rise in the number of older persons globally, a trend known as the grey (or silver) tsunami. People live markedly longer than their predecessors worldwide, due to remarkable changes in their lifestyle and in progresses made by modern medicine. However, the older we become, the more susceptible we are to a series of age-related pathologies, including infections, cancers, autoimmune diseases, and multi-morbidities. Therefore, a key challenge for our modern societies is how to cope with this fragile portion of the population, so that everybody could have the opportunity to live a long and healthy life. From a holistic point of view, aging results from the progressive decline of various systems. Among them, the distinctive age-dependent changes in the immune system contribute to the enhanced frailty of the elderly. One of these affects a population of lymphocytes, known as regulatory T cells (Tregs), as accumulating evidence suggest that there is a significant increase in the frequency of these cells in secondary lymphoid organs (SLOs) of aged animals. Although there are still discrepancies in the literature about modifications to their functional properties during aging, mounting evidence suggests a detrimental role for Tregs in the elderly in the context of bacterial and viral infections by suppressing immune responses against non-self-antigens. Interestingly, Tregs seem to also contribute to the reduced effectiveness of immunizations against many pathogens by limiting the production of vaccine-induced protective antibodies. In this review, we will analyze the current state of understandings about the role of Tregs in acute and chronic infections as well as in vaccination response in both humans and mice. Lastly, we provide an overview of current strategies for Treg modulation with potential future applications to improve the effectiveness of vaccines in older individuals.

B cell intrinsic and extrinsic factors impacting memory recall responses to SRBC challenge

MBCs (MBCs) generated in T-dependent immune responses can persist for a lifetime and rapidly react upon secondary antigen exposure to differentiate into plasma cells (PCs) and/or to improve the affinity of their BCR through new rounds of hypermutation in germinal centers (GCs). The fate of a MBC in secondary immune reactions appears to depend upon multiple parameters, whose understanding is mandatory for the design of efficient vaccine strategies. We followed the behavior of MBCs in recall responses to SRBCs using an inducible AID fate mapping mouse model in which B cells engaged in a germinal center (GC) response are irreversibly labeled upon simultaneous tamoxifen ingestion and immunization. We used different schemes of mouse immunization and tamoxifen feeding in adoptive-transfer experiments of total splenic B cells into congenic mice that have been pre-immunized or not, to assess the contribution of the different effector subsets in a physiological competitive context. We were able to show that naive B cells can differentiate into GC B cells with kinetics similar to MBCs in the presence of previously activated T follicular helper (TFH) cells and a primed microenvironment. We also showed that MBCs are recruited into secondary GCs, together with naive B cells. In contrast, PC differentiation, which dominated secondary MBC responses, was not dependent upon a previous TFH activation. We observed that the presence of persisting germinal centers and circulating antibody levels are key factors determining the germinal center versus plasma cell fate in a recall response. Notably, disruption of persistent germinal center structures by a lymphotoxin beta-receptor fusion protein or a longer timing between the prime and the boost, which correlated with reduced antigen-specific immunoglobulin levels in serum, were two conditions with an opposite impact, respectively inhibiting or promoting a GC fate for MBCs. Altogether, these studies highlight the complexity of recall responses, whose outcome varies according to immunization contexts.

B cells in the balance: Offsetting self-reactivity avoidance with protection against foreign

Antibodies are theoretically limitless in their diversity and specificity to foreign antigens; however they are constrained by the need to avoid binding to self. Germinal centers (GC) allow diversification and maturation of the antibody response towards the foreign antigen. While self-tolerance mechanisms controlling self-reactivity during B cell maturation are well recognized, the mechanisms by which GCs balance self-tolerance and foreign binding especially in the face of cross-reactivity between self and foreign, remain much less well defined. In this review we explore the extent to which GC self-tolerance restricts affinity maturation. We present studies suggesting that the outcome is situationally dependent, affected by affinity and avidity to self-antigen, and the extent to which self-binding and foreign-binding are interdependent. While auto-reactive GC B cells can mutate away from self while maturing towards the foreign antigen, if no mutational trajectories allow for self-reactive redemption, self-tolerance prevails and GC responses to the foreign pathogen are restricted, except when self-tolerance checkpoints are relaxed. Finally, we consider whether polyreactivity is subject to the same level of restriction in GC responses, especially if polyreactivity is linked to an increase in foreign protection, as occurs in certain broadly neutralizing antibodies. Overall, the outcomes for GC B cells that bind self-antigen can range from redemption, transient relaxation in self-tolerance or restriction of the antibody response to the foreign pathogen.

Alternative B Cell Differentiation During Infection and Inflammation

Long-term protective immunity to infectious disease depends on cell-mediated and humoral immune responses. Induction of a strong humoral response relies on efficient B cell activation and differentiation to long-lived plasma cells and memory B cells. For many viral or bacterial infections, a single encounter is sufficient to induce such responses. In malaria, the induction of long-term immunity can take years of pathogen exposure to develop, if it occurs at all. This repeated pathogen exposure and suboptimal immune response coincide with the expansion of a subset of B cells, often termed atypical memory B cells. This subset is present at low levels in healthy individuals as well but it is observed to expand in an inflammatory context during acute and chronic infection, autoimmune diseases or certain immunodeficiencies. Therefore, it has been proposed that this subset is exhausted, dysfunctional, or potentially autoreactive, but its actual role has remained elusive. Recent reports have provided new information regarding both heterogeneity and expansion of these cells, in addition to indications on their potential role during normal immune responses to infection or vaccination. These new insights encourage us to rethink how and why they are generated and better understand their role in our complex immune system. In this review, we will focus on recent advances in our understanding of these enigmatic cells and highlight the remaining gaps that need to be filled.

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.

Tracking Immunoglobulin Repertoire and Transcriptomic Changes in Germinal Center B Cells by Single-Cell Analysis

In response to T-cell-dependent antigens, mature B cells in the secondary lymphoid organs are stimulated to form germinal centers (GCs), which are histological structures deputed to antibody affinity maturation, a process associated with immunoglobulin gene editing by somatic hypermutation (SHM) and class switch recombination (CSR). GC B cells are heterogeneous and transition across multiple stages before being eliminated by apoptosis or committing to post-GC differentiation as memory B cells or plasma cells. In order to explore the dynamics of SHM and CSR during the GC reaction, we identified GC subpopulations by single-cell (sc) transcriptomics and analyzed the load of immunoglobulin variable (V) region mutations as well as the isotype class distribution in each subpopulation. The results showed that the large majority of GC B cells display a quantitatively similar mutational load in the V regions and analogous IGH isotype class distribution, except for the precursors of memory B cells (PreM) and plasma cells (PBL). PreM showed a bimodal pattern with about half of the cells displaying high V region germline identity and enrichment for unswitched IGH, while the rest of the cells carried a mutational load similar to the bulk of GC B cells and showed a switched isotype. PBL displayed a bias toward expression of IGHG and higher V region germline identity compared to the bulk of GC B cells. Genes implicated in SHM and CSR were significantly induced in specific GC subpopulations, consistent with the occurrence of SHM in dark zone cells and suggesting that CSR can occur within the GC.

Transcriptomics and metabolomics analysis reveal the anti-oxidation and immune boosting effects of mulberry leaves in growing mutton sheep

Introduction

Currently, the anti-oxidation of active ingredients in mulberry leaves (MLs) and their forage utilization is receiving increasing attention. Here, we propose that MLs supplementation improves oxidative resistance and immunity.

Methods

We conducted a trial including three groups of growing mutton sheep, each receiving fermented mulberry leaves (FMLs) feeding, dried mulberry leaves (DMLs) feeding or normal control feeding without MLs.

Results

Transcriptomic and metabolomic analyses revealed that promoting anti-oxidation and enhancing disease resistance of MLs is attributed to improved tryptophan metabolic pathways and reduced peroxidation of polyunsaturated fatty acids (PUFAs). Furthermore, immunity was markedly increased after FMLs treatment by regulating glycolysis and mannose-6-phosphate pathways. Additionally, there was better average daily gain in the MLs treatment groups.

Conclusion

These findings provide new insights for understanding the beneficial effects of MLs in animal husbandry and provide a theoretical support for extensive application of MLs in improving nutrition and health care values.

Follicular dendritic cells restrict interleukin-4 availability in germinal centers and foster memory B cell generation

B cells within germinal centers (GCs) enter cycles of antibody affinity maturation or exit the GC as memory cells or plasma cells. Here, we examined the contribution of interleukin (IL)-4 on B cell fate decisions in the GC. Single-cell RNA-sequencing identified a subset of light zone GC B cells expressing high IL-4 receptor-a (IL4Ra) and CD23 and lacking a Myc-associated signature. These cells could differentiate into pre-memory cells. B cell-specific deletion of IL4Ra or STAT6 favored the pre-memory cell trajectory, and provision of exogenous IL-4 in a wild-type context reduced pre-memory cell frequencies. IL-4 acted during antigen-specific interactions but also influenced bystander cells. Deletion of IL4Ra from follicular dendritic cells (FDCs) increased the availability of IL-4 in the GC, impaired the selection of affinity-matured B cells, and reduced memory cell generation. We propose that GC FDCs establish a niche that limits bystander IL-4 activity, focusing IL-4 action on B cells undergoing selection and enhancing memory cell differentiation.

Limited access to antigen drives generation of early B cell memory while restraining the plasmablast response

Cell fate decisions during early B cell activation determine the outcome of responses to pathogens and vaccines. We examined the early B cell response to T-dependent antigen in mice by single-cell RNA sequencing. Early after immunization, a homogeneous population of activated precursors (APs) gave rise to a transient wave of plasmablasts (PBs), followed a day later by the emergence of germinal center B cells (GCBCs). Most APs rapidly exited the cell cycle, giving rise to non-GC-derived early memory B cells (eMBCs) that retained an AP-like transcriptional profile. Rapid decline of antigen availability controlled these events; provision of excess antigen precluded cell cycle exit and induced a new wave of PBs. Fate mapping revealed a prominent contribution of eMBCs to the MBC pool. Quiescent cells with an MBC phenotype dominated the early response to immunization in primates. A reservoir of APs/eMBCs may enable rapid readjustment of the immune response when failure to contain a threat is manifested by increased antigen availability.

Advances in understanding the formation and fate of B-cell memory in response to immunization or infection

Immunological memory has the potential to provide lifelong protection against recurrent infections. As such, it has been crucial to the success of vaccines. Yet, the recent pandemic has illuminated key gaps in our knowledge related to the factors influencing effective memory formation and the inability to predict the longevity of immune protection. In recent decades, researchers have acquired a number of novel and powerful tools with which to study the factors underpinning humoral memory. These tools have been used to study the B-cell fate decisions that occur within the germinal centre (GC), a site where responding B cells undergo affinity maturation and are one of the major routes for memory B cell and high-affinity long-lived plasma cell formation. The advent of single-cell sequencing technology has provided an enhanced resolution for studying fate decisions within the GC and cutting-edge techniques have enabled researchers to model this reaction with more accuracy both in vitro and in silico. Moreover, modern approaches to studying memory B cells have allowed us to gain a better appreciation for the heterogeneity and adaptability of this vital class of B cells. Together, these studies have facilitated important breakthroughs in our understanding of how these systems operate to ensure a successful immune response. In this review, we describe recent advances in the field of GC and memory B-cell biology in order to provide insight into how humoral memory is formed, as well as the potential for generating lasting immunity to novel pathogens such as severe acute respiratory syndrome coronavirus 2.

Current Desensitization Strategies in Heart Transplantation

Heart transplant candidates sensitized to HLA antigens wait longer for transplant, are at increased risk of dying while waiting, and may not be listed at all. The increasing prevalence of HLA sensitization and limitations of current desensitization strategies underscore the urgent need for a more effective approach. In addition to pregnancy, prior transplant, and transfusions, patients with end-stage heart failure are burdened with unique factors placing them at risk for HLA sensitization. These include homograft material used for congenital heart disease repair and left ventricular assist devices (LVADs). Moreover, these risks are often stacked, forming a seemingly insurmountable barrier in some cases. While desensitization protocols are typically implemented uniformly, irrespective of the mode of sensitization, the heterogeneity in success and post-transplant outcomes argues for a more tailored approach. Achieving this will require progress in our understanding of the immunobiology underlying the innate and adaptive immune response to these varied allosensitizing exposures. Further attention to B cell activation, memory, and plasma cell differentiation is required to establish methods that durably abrogate the anti-HLA antibody response before and after transplant. The contribution of non-HLA antibodies to the net state of sensitization and the potential implications for graft longevity also remain to be comprehensively defined. The aim of this review is to first bring forth select issues unique to the sensitized heart transplant candidate. The current literature on desensitization in heart transplantation will then be summarized providing context within the immune response. Building on this, newer approaches with therapeutic potential will be discussed emphasizing the importance of not only addressing the short-term pathogenic consequences of circulating HLA antibodies, but also the need to modulate alloimmune memory.

The unique biology of germinal center B cells

Germinal center (GC) B cells are the source of the high-affinity, class-switched antibodies required for protective immunity. The unique biology of GC B cells involves iterative rounds of antibody gene somatic hypermutation coupled to multiple selection and differentiation pathways. Recent advances in areas such as single cell and gene editing technologies have shed new light upon these complex and dynamic processes. We review these findings here and integrate them into the current understanding of GC B cell replication and death, the retention of high-affinity and class-switched B cells in the GC, and differentiation into plasma and memory cell effectors. We also discuss how the biology of GC responses relates to vaccine effectiveness and outline current and future challenges in the field.

Heterogeneity of germinal center B cells: New insights from single-cell studies

Upon antigen exposure, activated B cells in antigen-draining lymphoid organs form microanatomical structures, called germinal centers (GCs), where affinity maturation occurs. Within the GC microenvironment, GC B cells undergo proliferation and B cell receptor (BCR) genes somatic hypermutation in the dark zone (DZ), and affinity-based selection in the light zone (LZ). In the current paradigm of GC dynamics, high-affinity LZ B cells may be selected by cognate T- follicular helper cells to either differentiate into plasma cells or memory B cells, or re-enter the DZ and initiate a new round of proliferation and BCR diversification, before migrating back to the LZ. Given the diversity of cell states and potential cell fates that GC B cells may adopt, the two-state DZ-LZ paradigm has been challenged by studies that explored GC B-cell heterogeneity with a variety of single-cell technologies. Here, we review studies and single-cell technologies which have allowed to refine the working model of GC B-cell cellular and molecular heterogeneity during affinity maturation. This review also covers the use of single-cell quantitative data for mathematical modeling of GC reactions, and the application of single-cell genomics to the study of GC-derived malignancies.

BCR Affinity Influences T-B Interactions and B Cell Development in Secondary Lymphoid Organs

B cells produce high-affinity immunoglobulins (Igs), or antibodies, to eliminate foreign pathogens. Mature, naïve B cells expressing an antigen-specific cell surface Ig, or B cell receptor (BCR), are directed toward either an extrafollicular (EF) or germinal center (GC) response upon antigen binding. B cell interactions with CD4⁺ pre-T follicular helper (pre-Tfh) cells at the T-B border and effector Tfh cells in the B cell follicle and GC control B cell development in response to antigen. Here, we review recent studies demonstrating the role of B cell receptor (BCR) affinity in modulating T-B interactions and the subsequent differentiation of B cells in the EF and GC response. Overall, these studies demonstrate that B cells expressing high affinity BCRs preferentially differentiate into antibody secreting cells (ASCs) while those expressing low affinity BCRs undergo further affinity maturation or differentiate into memory B cells (MBCs).

SARS-CoV-2-specific humoral and cellular immunity persists through 9 months irrespective of COVID-19 severity at hospitalisation

OBJECTIVES

Humoral and cellular immunity to SARS-CoV-2 following COVID-19 will likely contribute to protection from reinfection or severe disease. It is therefore important to characterise the initiation and persistence of adaptive immunity to SARS-CoV-2 amidst the ongoing pandemic.

METHODS

Here, we conducted a longitudinal study on hospitalised moderate and severe COVID-19 patients from the acute phase of disease into convalescence at 5 and 9 months post-symptom onset. Utilising flow cytometry, serological assays as well as B cell and T cell FluoroSpot assays, we assessed the magnitude and specificity of humoral and cellular immune responses during and after human SARS-CoV-2 infection.

RESULTS

During acute COVID-19, we observed an increase in germinal centre activity, a substantial expansion of antibody-secreting cells and the generation of SARS-CoV-2-neutralising antibodies. Despite gradually decreasing antibody levels, we show persistent, neutralising antibody titres as well as robust specific memory B cell responses and polyfunctional T cell responses at 5 and 9 months after symptom onset in both moderate and severe COVID-19 patients.

CONCLUSION

Our findings describe the initiation and, importantly, persistence of cellular and humoral SARS-CoV-2-specific immunological memory in hospitalised COVID-19 patients long after recovery, likely contributing towards protection against reinfection.