In situ studies of the primary immune response to (4-hydroxy-3-nitrophenyl)acetyl. I. The architecture and dynamics of responding cell populations

Coordinated Regulation of Extrafollicular B Cell Responses by IL-12 and IFNγ

Upon activation, B cells undergo either the germinal center (GC) or extrafollicular (EF) response. While GC are known to generate high-affinity memory B cells and long-lived plasma cells, the role of the EF response is less well understood. Initially, it was thought to be limited to that of a source of fast but lower-quality antibodies until the GC can form. However, recent evidence strongly supports the EF response as an important component of the humoral response to infection. EF responses are now also recognized as a source of pathogenic B cells in autoimmune diseases. The EF response itself is dynamic and regulated by pathways that are only recently being uncovered. We have identified that the cytokine IL-12 acts as a molecular switch, enhancing the EF response and suppressing GC through multiple mechanisms. These include direct effects on both B cells themselves and the coordinated differentiation of helper CD4 T cells. Here, we explore this pathway in relation to other recent advancements in our understanding of the EF response's role and highlight areas for future research. A better understanding of how the EF response forms and is regulated is essential for advancing treatments for many disease states.

Mechanism for evolution of diverse autologous antibodies upon broadly neutralizing antibody therapy of people with HIV

Antiretroviral therapy (ART) inhibits Human Immunodeficiency Virus (HIV) replication to maintain undetectable viral loads in people living with HIV, but does not result in a cure. Due to the significant challenges of lifelong ART for many, there is strong interest in therapeutic strategies that result in cure. Recent clinical trials have shown that administration of broadly neutralizing antibodies (bnAbs) when there is some viremia can lead to ART-free viral control in some people; however, the underlying mechanisms are unclear. Our computational modeling shows that bnAbs administered in the presence of some viremia promote the evolution of autologous antibodies (aAbs) that target diverse epitopes of HIV spike proteins. This "net" of polyclonal aAbs could confer control since evasion of this response would require developing mutations in multiple epitopes. Our results provide a common mechanistic framework underlying recent clinical observations upon bnAb/ART therapy, and they should also motivate and inform new trials.

Interferon activation in bone marrow long-lived plasma cells in systemic lupus erythematosus

While durable antibody responses from long-lived plasma cell (LLPC) populations are important for protection against pathogens, LLPC may be harmful if they produce antibodies against self-proteins or self-nuclear antigens as occurs in autoimmune diseases such as systemic lupus erythematosus (SLE). Thus, the elimination of autoreactive LLPC may improve the treatment of antibody-driven autoimmune diseases. However, LLPC remain a challenging therapeutic target. Here, we compare the matched bone marrow (BM) and peripheral blood (PBL) plasma cell (PC) compartments of SLE and healthy donors (HD). We show a similar distribution of CD138- and CD138+ PC, including putative LLPC (CD19- CD138+ CD38+), between SLE and HD BM. For both SLE and HD, CD138+ PC are at a higher frequency in BM than PBL. Expression of Ki-67 associates with the PBL compartment where it is found on all PC subsets regardless of CD19 or CD138 expression. Transcriptomic analysis identifies an interferon (IFN) gene signature in transitional B cells in the SLE BM, but surprisingly also in the BM PC derived from SLE. BM PC and B cells phosphorylate STAT1 in response to type I IFN stimulation in vitro, but with decreased fold change compared to those from the PBL. While BM PC bind type I IFN receptor-blocking antibody anifrolumab, it is to a lesser degree than circulating B cells. Anti-nuclear autoantibodies (ANA) are found in the BM supernatant and PBL serum of SLE patients. Both SLE and HD BM-derived PC have increased survival compared to their PBL counterparts when treated with verdinexor. In summary, these findings show evidence of IFN activation in BM PC from SLE.

Aberrant zonal recycling of germinal center B cells impairs appropriate selection in lupus

Autoimmune diseases such as lupus are characterized by polyclonal B cell activation, leading to the production of autoantibodies. The mechanism leading to B cell dysregulation is unclear; however, the defect may lie in selection within germinal centers (GCs). GC B cells cycle between proliferation and mutation in the dark zone and selection in the light zone (LZ). Temporal assessment of GCs from mice with either persistent infection or lupus showed an accumulation of LZ B cells. Yet, only in lupus, GC B cells exhibited reduced proliferation and progressive loss of MYC and FOXO1, which regulate zonal recycling and differentiation. As lupus progressed, decreased mutational frequency and repertoire diversity were associated with reduced responsiveness to CD40 signaling, despite accumulation of plasma cells. Collectively, these findings suggest that lupus disease progression coincides with an intrinsic defect in LZ B cell signaling, altering the zonal recycling, selection, and differentiation of autoreactive B cells.

Maturation of germinal center B cells after influenza virus vaccination in humans

Germinal centers (GC) are microanatomical lymphoid structures where affinity-matured memory B cells and long-lived bone marrow plasma cells are primarily generated. It is unclear how the maturation of B cells within the GC impacts the breadth and durability of B cell responses to influenza vaccination in humans. We used fine needle aspiration of draining lymph nodes to longitudinally track antigen-specific GC B cell responses to seasonal influenza vaccination. Antigen-specific GC B cells persisted for at least 13 wk after vaccination in two out of seven individuals. Monoclonal antibodies (mAbs) derived from persisting GC B cell clones exhibit enhanced binding affinity and breadth to influenza hemagglutinin (HA) antigens compared with related GC clonotypes isolated earlier in the response. Structural studies of early and late GC-derived mAbs from one clonal lineage in complex with H1 and H5 HAs revealed an altered binding footprint. Our study shows that inducing sustained GC reactions after influenza vaccination in humans supports the maturation of responding B cells.

A multiscale spatial modeling framework for the germinal center response

The germinal center response or reaction (GCR) is a hallmark event of adaptive humoral immunity. Unfolding in the B cell follicles of the secondary lymphoid organs, a GC culminates in the production of high-affinity antibody-secreting plasma cells along with memory B cells. By interacting with follicular dendritic cells (FDC) and T follicular helper (Tfh) cells, GC B cells exhibit complex spatiotemporal dynamics. Driving the B cell dynamics are the intracellular signal transduction and gene regulatory network that responds to cell surface signaling molecules, cytokines, and chemokines. As our knowledge of the GC continues to expand in depth and in scope, mathematical modeling has become an important tool to help disentangle the intricacy of the GCR and inform novel mechanistic and clinical insights. While the GC has been modeled at different granularities, a multiscale spatial simulation framework - integrating molecular, cellular, and tissue-level responses - is still rare. Here, we report our recent progress toward this end with a hybrid stochastic GC framework developed on the Cellular Potts Model-based CompuCell3D platform. Tellurium is used to simulate the B cell intracellular molecular network comprising NF-κB, FOXO1, MYC, AP4, CXCR4, and BLIMP1 that responds to B cell receptor (BCR) and CD40-mediated signaling. The molecular outputs of the network drive the spatiotemporal behaviors of B cells, including cyclic migration between the dark zone (DZ) and light zone (LZ) via chemotaxis; clonal proliferative bursts, somatic hypermutation, and DNA damage-induced apoptosis in the DZ; and positive selection, apoptosis via a death timer, and emergence of plasma cells in the LZ. Our simulations are able to recapitulate key molecular, cellular, and morphological GC events, including B cell population growth, affinity maturation, and clonal dominance. This novel modeling framework provides an open-source, customizable, and multiscale virtual GC simulation platform that enables qualitative and quantitative in silico investigations of a range of mechanistic and applied research questions on the adaptive humoral immune response in the future.

'Persistent germinal center responses: slow-growing trees bear the best fruits'

Germinal centers (GCs) are key microanatomical sites in lymphoid organs where responding B cells mature and undergo affinity-based selection. The duration of the GC reaction has long been assumed to be relatively brief, but recent studies in humans, nonhuman primates, and mice indicate that GCs can last for weeks to months after initial antigen exposure. This review examines recent studies investigating the factors that influence GC duration, including antigen persistence, T-follicular helper cells, and mode of immunization. Potential mechanisms for how persistent GCs influence the B-cell repertoire are considered. Overall, these studies provide a blueprint for how to design better vaccines that elicit persistent GC responses.

Antigen presentation dynamics shape the antibody response to variants like SARS-CoV-2 Omicron after multiple vaccinations with the original strain

The Omicron variant of SARS-CoV-2 is not effectively neutralized by most antibodies elicited by two doses of mRNA vaccines, but a third dose increases anti-Omicron neutralizing antibodies. We reveal mechanisms underlying this observation by combining computational modeling with data from vaccinated humans. After the first dose, limited antigen availability in germinal centers (GCs) results in a response dominated by B cells that target immunodominant epitopes that are mutated in an Omicron-like variant. After the second dose, these memory cells expand and differentiate into plasma cells that secrete antibodies that are thus ineffective for such variants. However, these pre-existing antigen-specific antibodies transport antigen efficiently to secondary GCs. They also partially mask immunodominant epitopes. Enhanced antigen availability and epitope masking in secondary GCs together result in generation of memory B cells that target subdominant epitopes that are less mutated in Omicron. The third dose expands these cells and boosts anti-variant neutralizing antibodies.

Mechanisms that promote the evolution of cross-reactive antibodies upon vaccination with designed influenza immunogens

Immunogens that elicit broadly neutralizing antibodies targeting the conserved receptor-binding site (RBS) on influenza hemagglutinin may serve as candidates for a universal influenza vaccine. Here, we develop a computational model to interrogate antibody evolution by affinity maturation after immunization with two types of immunogens: a heterotrimeric "chimera" hemagglutinin that is enriched for the RBS epitope relative to other B cell epitopes and a cocktail composed of three non-epitope-enriched homotrimers of the monomers that comprise the chimera. Experiments in mice find that the chimera outperforms the cocktail for eliciting RBS-directed antibodies. We show that this result follows from an interplay between how B cells engage these antigens and interact with diverse helper T cells and requires T cell-mediated selection of germinal center B cells to be a stringent constraint. Our results shed light on antibody evolution and highlight how immunogen design and T cells modulate vaccination outcomes.

Continuous germinal center invasion contributes to the diversity of the immune response

Antibody responses are characterized by increasing affinity and diversity over time. Affinity maturation occurs in germinal centers by a mechanism that involves repeated cycles of somatic mutation and selection. How antibody responses diversify while also undergoing affinity maturation is not as well understood. Here, we examined germinal center (GC) dynamics by tracking B cell entry, division, somatic mutation, and specificity. Our experiments show that naive B cells continuously enter GCs where they compete for T cell help and undergo clonal expansion. Consistent with late entry, invaders carry fewer mutations but can contribute up to 30% or more of the cells in late-stage germinal centers. Notably, cells entering the germinal center at later stages of the reaction diversify the immune response by expressing receptors that show low affinity to the immunogen. Paradoxically, the affinity threshold for late GC entry is lowered in the presence of high-affinity antibodies.

Instructing durable humoral immunity for COVID-19 and other vaccinable diseases

Many aspects of SARS-CoV-2 have fully conformed with the principles established by decades of viral immunology research, ultimately leading to the crowning achievement of highly effective COVID-19 vaccines. Nonetheless, the pandemic has also exposed areas where our fundamental knowledge is thinner. Some key unknowns are the duration of humoral immunity post-primary infection or vaccination and how long booster shots confer protection. As a corollary, if protection does not last as long as desired, what are some ways it can be improved? Here, I discuss lessons from other infections and vaccines that point to several key features that influence durable antibody production and the perseverance of immunity. These include (1) the specific innate sensors that are initially triggered, (2) the kinetics of antigen delivery and persistence, (3) the starting B cell receptor (BCR) avidity and antigen valency, and (4) the memory B cell subsets that are recalled by boosters. I further highlight the fundamental B cell-intrinsic and B cell-extrinsic pathways that, if understood better, would provide a rational framework for vaccines to reliably provide durable immunity.

The role of geometric features in a germinal center

The germinal center (GC) is a self-organizing structure produced in the lymphoid follicle during the T-dependent immune response and is an important component of the humoral immune system. However, the impact of the special structure of GC on antibody production is not clear. According to the latest biological experiments, we establish a spatiotemporal stochastic model to simulate the whole self-organization process of the GC including the appearance of two specific zones: the dark zone (DZ) and the light zone (LZ), the development of which serves to maintain an effective competition among different cells and promote affinity maturation. A phase transition is discovered in this process, which determines the critical GC volume for a successful growth in both the stochastic and the deterministic model. Further increase of the volume does not make much improvement on the performance. It is found that the critical volume is determined by the distance between the activated B cell receptor (BCR) and the target epitope of the antigen in the shape space. The observation is confirmed in both 2D and 3D simulations and explains partly the variability of the observed GC size.

Small molecule inhibitors of nuclear export ameliorate lupus by modulating plasma cell generation and survival

OBJECTIVE

To investigate the hypothesis that selective inhibitors of nuclear export (SINE), recently approved for the treatment of refractory plasma cell (PC) malignancy, may have potential in the treatment of lupus.

METHODS

NZB/NZW female mice were treated with SINE or vehicle control. Tissue was harvested and analyzed by flow cytometry using standard markers. Nephritis was monitored by evaluation for proteinuria and by histologic analysis of kidneys. Serum anti- double-stranded DNA (anti-dsDNA) levels were measured by enzyme-linked immunosorbent assay (ELISA) and total IgG and dsDNA antibody-secreting cells (ASC) by enzyme-linked immunospot assay.

RESULTS

SINE abrogated murine lupus nephritis at both early and late stages of the disease and rapidly impaired generation of autoreactive PC in germinal centers (GC). SINE inhibited the production of the NF-κB-driven homeostatic chemokines by stromal cells, altering splenic B and T cell strategic positioning and significantly reducing T follicular helper cells (TFH), GC B cells, and autoreactive PC. SINE also decreased cytokines/chemokines involved in PC survival and recruitment in the kidney of lupus-prone mice. Exportin 1, the SINE target, was detected in GC of human tonsils, splenic B cells of lupus patients, and multiple B cell subsets in the kidney of patients with lupus nephritis.

CONCLUSION

Our collective results support the therapeutic potential of SINE via targeting several molecular and cellular pathways critical in lupus pathogenesis, including autoantibody production by plasma cells.

Imaging the different timescales of germinal center selection

Germinal centers (GCs) are the site of antibody affinity maturation, a fundamental immunological process that increases the potency of antibodies and thereby their ability to protect against infection. GC biology is highly dynamic in both time and space, making it ideally suited for intravital imaging. Using multiphoton laser scanning microscopy (MPLSM), the field has gained insight into the molecular, cellular, and structural changes and movements that coordinate affinity maturation in real time in their native environment. On the other hand, several limitations of MPLSM have had to be overcome to allow full appreciation of GC events taking place across different timescales. Here, we review the technical advances afforded by intravital imaging and their contributions to our understanding of GC biology.

Long-Range Control of Class Switch Recombination by Transcriptional Regulatory Elements

Immunoglobulin class switch recombination (CSR) plays a crucial role in adaptive immune responses through a change of the effector functions of antibodies and is triggered by T-cell-dependent as well as T-cell-independent antigens. Signals generated following encounter with each type of antigen direct CSR to different isotypes. At the genomic level, CSR occurs between highly repetitive switch sequences located upstream of the constant gene exons of the immunoglobulin heavy chain locus. Transcription of switch sequences is mandatory for CSR and is induced in a stimulation-dependent manner. Switch transcription takes place within dynamic chromatin domains and is regulated by long-range regulatory elements which promote alignment of partner switch regions in CSR centers. Here, we review recent work and models that account for the function of long-range transcriptional regulatory elements and the chromatin-based mechanisms involved in the control of CSR.

Critical roles of the E3 ubiquitin ligase FBW7 in B-cell response and the pathogenesis of experimental autoimmune arthritis

Proper regulation of B-cell function is essential for effective humoral immunity and maintenance of immune tolerance. Here, we found that FBW7 (F-box/WD40 repeat-containing protein 7) is highly expressed in germinal centre B and B1 cells, and confirmed that it has an intrinsic role in maintaining homeostasis of mature B cells and B-1 cells. FBW7 deletion led to an impairment of antibody response, and although germinal centre formation was not affected, antibody class-switch recombination and affinity maturation processes were defective. Likewise, memory immune response was severely impaired. Moreover, FBW7 ablation ameliorated the pathogenesis of an autoimmune disease model, collagen-induced arthritis, by reducing the production of anti-collagen II autoantibodies. Taken together, these data suggest that FBW7 may be an attractive target for developing new therapeutics for the treatment of autoimmune diseases.

A fluorescent probe for STED microscopy to study NIP-specific B cells

We have developed a series of monovalent fluorophore-conjugated affinity probes based on the hapten 3-nitro-4-hydroxy-5-iodophenylacetyl (NIP), which is widely used as a model antigen to study B lymphocytes and the functional principles of B cell antigen receptors (BCRs). We successfully used them in flow-cytometry, confocal and super-resolution microscopy techniques.

In Silico Analysis of the Longevity and Timeline of Individual Germinal Center Reactions in a Primary Immune Response

Germinal centers (GCs) are transient structures in the secondary lymphoid organs, where B cells undergo affinity maturation to produce high affinity memory and plasma cells. The lifetime of GC responses is a critical factor limiting the extent of affinity maturation and efficiency of antibody responses. While the average lifetime of overall GC reactions in a lymphoid organ is determined experimentally, the lifetime of individual GCs has not been monitored due to technical difficulties in longitudinal analysis. In silico analysis of the contraction phase of GC responses towards primary immunization with sheep red blood cells suggested that if individual GCs had similar lifetimes, the data would be consistent only when new GCs were formed until a very late phase after immunization. Alternatively, there could be a large variation in the lifetime of individual GCs suggesting that both long and short-lived GCs might exist in the same lymphoid organ. Simulations predicted that such differences in the lifetime of GCs could arise due to variations in antigen availability and founder cell composition. These findings identify the potential factors limiting GC lifetime and contribute to an understanding of overall GC responses from the perspective of individual GCs in a primary immune response.

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.

Follicular dendritic cell dysfunction contributes to impaired antigen-specific humoral responses in sepsis-surviving mice

Sepsis survivors exhibit impaired responsiveness to antigen (Ag) challenge associated with increased mortality from infection. The contribution of follicular dendritic cells (FDCs) in the impaired humoral response in sepsis-surviving mice is investigated in this study. We demonstrated that mice subjected to sepsis from cecal ligation and puncture (CLP mice) have reduced NP-specific high-affinity class-switched Ig antibodies (Abs) compared with sham-operated control mice following immunization with the T cell-dependent Ag, NP-CGG. NP-specific germinal center (GC) B cells in CLP mice exhibited reduced TNF-α and AID mRNA expression compared with sham-operated mice. CLP mice showed a reduction in FDC clusters, a reduced binding of immune complexes on FDCs, and reduced mRNA expression of CR2, ICAM-1, VCAM-1, FcγRIIB, TNFR1, IKK2, and LTβR compared with sham-operated mice. Adoptive transfer studies showed that there was no B cell-intrinsic defect. In summary, our data suggest that the reduced Ag-specific Ab response in CLP mice is secondary to a disruption in FDC and GC B cell function.

A Posttranscriptional Pathway of CD40 Ligand mRNA Stability Is Required for the Development of an Optimal Humoral Immune Response

CD40 ligand (CD40L) mRNA stability is dependent on an activation-induced pathway that is mediated by the binding complexes containing the multifunctional RNA-binding protein, polypyrimidine tract-binding protein 1 (PTBP1) to a 3' untranslated region of the transcript. To understand the relationship between regulated CD40L and the requirement for variegated expression during a T-dependent response, we engineered a mouse lacking the CD40L stability element (CD40LΔ5) and asked how this mutation altered multiple aspects of the humoral immunity. We found that CD40LΔ5 mice expressed CD40L at 60% wildtype levels, and lowered expression corresponded to significantly decreased levels of T-dependent Abs, loss of germinal center (GC) B cells and a disorganized GC structure. Gene expression analysis of B cells from CD40LΔ5 mice revealed that genes associated with cell cycle and DNA replication were significantly downregulated and genes linked to apoptosis upregulated. Importantly, somatic hypermutation was relatively unaffected although the number of cells expressing high-affinity Abs was greatly reduced. Additionally, a significant loss of plasmablasts and early memory B cell precursors as a percentage of total GL7⁺ B cells was observed, indicating that differentiation cues leading to the development of post-GC subsets was highly dependent on a threshold level of CD40L. Thus, regulated mRNA stability plays an integral role in the optimization of humoral immunity by allowing for a dynamic level of CD40L expression on CD4 T cells that results in the proliferation and differentiation of pre-GC and GC B cells into functional subsets.

Pseudocell Tracer-A method for inferring dynamic trajectories using scRNAseq and its application to B cells undergoing immunoglobulin class switch recombination

Single cell RNA sequencing (scRNAseq) can be used to infer a temporal ordering of cellular states. Current methods for the inference of cellular trajectories rely on unbiased dimensionality reduction techniques. However, such biologically agnostic ordering can prove difficult for modeling complex developmental or differentiation processes. The cellular heterogeneity of dynamic biological compartments can result in sparse sampling of key intermediate cell states. To overcome these limitations, we develop a supervised machine learning framework, called Pseudocell Tracer, which infers trajectories in pseudospace rather than in pseudotime. The method uses a supervised encoder, trained with adjacent biological information, to project scRNAseq data into a low-dimensional manifold that maps the transcriptional states a cell can occupy. Then a generative adversarial network (GAN) is used to simulate pesudocells at regular intervals along a virtual cell-state axis. We demonstrate the utility of Pseudocell Tracer by modeling B cells undergoing immunoglobulin class switch recombination (CSR) during a prototypic antigen-induced antibody response. Our results revealed an ordering of key transcription factors regulating CSR to the IgG1 isotype, including the concomitant expression of Nfkb1 and Stat6 prior to the upregulation of Bach2 expression. Furthermore, the expression dynamics of genes encoding cytokine receptors suggest a poised IL-4 signaling state that preceeds CSR to the IgG1 isotype.

Preliminary study on serum immunoglobulin G responses following intramuscular inoculation of adjuvanted polyhydroxyalkanoate microparticles with Pasteurella multocida vaccine in white rats

A natural biodegradable polymer, polyhydroxyalkanoate (PHA), was adjuvanted with a vaccine seed to observe the biomaterial's ability in enhancing an immune response in rats. The adjuvant potential of PHA was tested using the whole-killed Pasteurella multocida B:2 (PMB2) vaccine in Sprague Dawley (SD) rats to detect changes in serum immunoglobulin G (IgG) and immunoglobulin M (IgM) responses. A common PHA, poly(3-hydroxybutyrate) [P(3HB)], from Bacillus megaterium UMTKB-1 was constructed into microparticles using the solvent evaporation method. Twelve SD rats were divided into four treatment groups: 1) non-treatment as negative control, 2) P(3HB) adjuvant, 3) PMB2 vaccine, and 4) adjuvanted-P(3HB)/PMB2 vaccine groups, which were intramuscularly vaccinated twice. Immunoglobulins IgG and IgM levels were used as markers of the immune response induced by the adjuvanted-P(3HB)/PMB2 vaccine and analysed over an eight-week study period. The group vaccinated specifically with adjuvanted-P(3HB)/PMB2 vaccine had higher concentrations of immunoglobulins compared to other treatment groups, hence demonstrating the potential of the adjuvant to enhance immune response. Findings showed a need to delay the delivery of the second booster dose to determine the appropriate regime for the adjuvanted-P(3HB)/PMB2 vaccine.

JEM career launchpad

JEM has been a launching pad for scientific careers since its inception. Here is a collection of testimonials attesting to the diversity of the scientific community it serves.

The Shaping of a B Cell Pool Maximally Responsive to Infections

B cell subsets differ in development, tissue distribution, and mechanisms of activation. In response to infections, however, all can differentiate into extrafollicular plasmablasts that rapidly provide highly protective antibodies, indicating that these plasmablasts are the main humoral immune response effectors. Yet, the effectiveness of this response type depends on the presence of antigen-specific precursors in the circulating mature B cell pool, a pool that is generated initially through the stochastic processes of B cell receptor assembly. Importantly, germinal centers then mold the repertoire of this B cell pool to be increasingly responsive to pathogens by generating a broad array of antimicrobial memory B cells that act as highly effective precursors of extrafollicular plasmablasts. Such B cell repertoire molding occurs in two ways: continuously via the chronic germinal centers of mucosal lymphoid tissues, driven by the presence of the microbiome, and via de novo generated germinal centers following acute infections. For effectively evaluating humoral immunity as a correlate of immune protection, it might be critical to measure memory B cell pools in addition to antibody titers.

Preferential Presentation of High-Affinity Immune Complexes in Germinal Centers Can Explain How Passive Immunization Improves the Humoral Response

Passive immunization (PI) with external antibodies has been used classically for rapid but temporary alleviation of disease. Transcending this role, recent studies have shown PI to induce lasting improvements in natural antibody production, suggesting that PI could become a powerful tool to engineer humoral responses. We propose a mechanism with which PI can alter the humoral response. Antigen-specific B cells evolve and get selected in germinal centers (GCs) on the basis of their ability to acquire antigen from antibody-antigen complexes presented in GCs. When external antibodies of high affinity for antigen are used, they form the majority of the complexes in GCs, letting only B cells with even higher affinities be selected. Using an in silico GC reaction model, we show that this mechanism explains the improved humoral responses following PI. The model also synthesizes several independent experimental observations, indicating the robustness of the mechanism, and proposes tunable handles to optimize PI.

Multifaceted Effects of Antigen Valency on B Cell Response Composition and Differentiation In Vivo

How antigen valency affects B cells in vivo during immune responses is not well understood. Here, using HIV immunogens with defined valencies ranging from 1 to 60, we investigated the role of antigen valency during different phases of B cell responses in vivo. Highly multimerized immunogens preferentially rapidly activated cognate B cells, with little affinity discrimination. This led to strong early induction of the transcription factors IRF4 (interferon regulatory factor 4) and Bcl6, driving both early extrafollicular plasma cell and germinal center responses, in a CD4+ T-cell-dependent manner, involving B cells with a broad range of affinities. Low-valency antigens induced smaller effector B cell responses, with preferential recruitment of high-affinity B cells. Thus, antigen valency has multifaceted effects on B cell responses and can dictate affinity thresholds and competitive landscapes for B cells in vivo, with implications for vaccine design.

Differential Roles of IDO1 and IDO2 in T and B Cell Inflammatory Immune Responses

Indoleamine-2,3-dioxygenase (IDO)1 and IDO2 are two closely related tryptophan catabolizing enzymes encoded by linked genes. The IDO pathway is also immunomodulatory, with IDO1 well-characterized as a mediator of tumor immune evasion. Due to its homology with IDO1, IDO2 has been proposed to have a similar immunoregulatory function. Indeed, IDO2, like IDO1, is necessary for the differentiation of regulatory T cells in vitro. However, compared to IDO1, in vivo studies demonstrated a contrasting role for IDO2, with experiments in preclinical models of autoimmune arthritis establishing a proinflammatory role for IDO2 in mediating B and T cell activation driving autoimmune disease. Given their potentially opposing roles in inflammatory responses, interpretation of results obtained using IDO1 or IDO2 single knockout mice could be complicated by the expression of the other enzyme. Here we use IDO1 and IDO2 single and double knockout (dko) mice to define the differential roles of IDO1 and IDO2 in B cell-mediated immune responses. Autoreactive T and B cell responses and severity of joint inflammation were decreased in IDO2 ko, but not IDO1 ko arthritic mice. Dko mice had a reduction in the number of autoantibody secreting cells and severity of arthritis: however, percentages of differentiated T cells and their associated cytokines were not reduced compared to IDO1 ko or wild-type mice. These data suggest that autoreactive B cell responses are mediated by IDO2, while autoreactive T cell responses are indirectly affected by IDO1 expression in the IDO2 ko mice. IDO2 also influenced antibody responses in models of influenza infection and immunization with T cell-independent type II antigens. Taken together, these studies provide evidence for the contrasting roles IDO1 and IDO2 play in immune responses, with IDO1 mediating T cell suppressive effects and IDO2 working directly in B cells as a proinflammatory mediator of B cell responses.

Mitochondrial Pyruvate Carrier Function in Health and Disease across the Lifespan

As a nodal mediator of pyruvate metabolism, the mitochondrial pyruvate carrier (MPC) plays a pivotal role in many physiological and pathological processes across the human lifespan, from embryonic development to aging-associated neurodegeneration. Emerging research highlights the importance of the MPC in diverse conditions, such as immune cell activation, cancer cell stemness, and dopamine production in Parkinson’s disease models. Whether MPC function ameliorates or contributes to disease is highly specific to tissue and cell type. Cell- and tissue-specific differences in MPC content and activity suggest that MPC function is tightly regulated as a mechanism of metabolic, cellular, and organismal control. Accordingly, recent studies on cancer and diabetes have identified protein–protein interactions, post-translational processes, and transcriptional factors that modulate MPC function. This growing body of literature demonstrates that the MPC and other mitochondrial carriers comprise a versatile and dynamic network undergirding the metabolism of health and disease.

Distinct Requirements of CHD4 during B Cell Development and Antibody Response

The immunoglobulin heavy chain (Igh) locus features a dynamic chromatin landscape to promote class switch recombination (CSR), yet the mechanisms that regulate this landscape remain poorly understood. CHD4, a component of the chromatin remodeling NuRD complex, directly binds H3K9me3, an epigenetic mark present at the Igh locus during CSR. We find that CHD4 is essential for early B cell development but is dispensable for the homeostatic maintenance of mature, naive B cells. However, loss of CHD4 in mature B cells impairs CSR because of suboptimal targeting of AID to the Igh locus. Additionally, we find that CHD4 represses p53 expression to promote B cell proliferation. This work reveals distinct roles for CHD4 in B cell development and CSR and links the H3K9me3 epigenetic mark with AID recruitment to the Igh locus.

Yen et al. demonstrate that CHD4, a component of the NuRD remodeling complex, is essential for early B cell development, represses p53 expression in mature B cells, and influences the recruitment of AID to DNA during class switch recombination.

Factors in B cell competition and immunodominance

The majority of all vaccines work by inducing protective antibody responses. The mechanisms by which the B cells responsible for producing protective antibodies are elicited to respond are not well understood. Interclonal B cell competition to complex antigens, particularly in germinal centers, has emerged as an important hurdle in designing effective vaccines. This review will focus on recent advances in understanding the roles of B cell precursor frequency, B cell receptor affinity for antigen, antigen avidity, and other factors that can substantially alter the outcomes of B cell responses to complex antigens. Understanding the interdependence of these fundamental factors that affect B cell responses can inform current vaccine design efforts for pathogens with complex proteins as candidate immunogens such as HIV, influenza, and coronaviruses.

Spleen Oxylipin and Polyunsaturated Fatty Acid Profiles are Altered by Dietary Source of Polyunsaturated Fatty Acid and by Sex

As the largest secondary lymphoid organ, the spleen plays an important role in immune responses. The role of arachidonic acid (ARA) and its 20-carbon eicosanoids in modulating immune function has long been of interest. However, recent advances have enabled the identification of numerous other n-6 and n-3 polyunsaturated fatty acid (PUFA)-derived oxylipins. Here, we investigate the effects of diet and sex on the spleen nonesterified oxylipin profiles and phospholipid and neutral lipid PUFA composition in Sprague-Dawley rats supplemented with oils rich in α-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or linoleic acid. Dietary ALA, EPA, and DHA resulted in lower levels of ARA and ARA oxylipins. Oxylipins derived from other n-6 PUFA were also reduced despite no or opposite effect on their PUFA levels. Each diet also resulted in higher levels of oxylipins almost exclusively derived from the supplemented PUFA, despite PUFA in the same biosynthetic pathway also often being increased. Further, while oxylipin differences often reflected changes to phospholipid PUFA, there were instances where they corresponded more closely to changes in neutral lipid PUFA. With respect to sex effects, >50% of lipoxygenase ARA-derived oxylipins were higher in males in at least one diet group, while multiple DHA oxylipins were lower in males only in rats provided the DHA diet. This fundamental description of oxylipin composition in the spleen, including the influence of diet and sex and the relationship to PUFA composition, will help inform future studies examining the functions of these oxylipins under physiological and pathological conditions.

Synthetic Liposomal Mimics of Biological Viruses for the Study of Immune Responses to Infection and Vaccination

Human viruses possess very complex supramolecular structures. Both icosahedral and enveloped viruses typically display an array of viral-encoded protein antigens at varied spatial densities on the viral particle surface. The viral nucleic acid genome, on the other hand, is encapsulated inside the viral particle. Although both the surface antigen and the interior nucleic acids could independently produce immunological responses, how B cells integrate these two types of signals and respond to a typical virus particle to initiate activation is not well understood at a molecular level. The study of these fundamental biological processes would benefit from the development of viral structural mimics that are well constructed to incorporate both quantitative and qualitative viral features for presentation to B cells. These novel tools would enable researchers to systematically dissect the underlying processes. Here we report the development of such particulate antigens based on liposomes engineered to display a model protein antigen, hen egg lysozyme (HEL). We developed methods to overexpress and purify various affinity mutants of HEL from E. coli. We conjugated the purified recombinant HEL proteins onto the surface of a virion-sized liposome in an orientation-specific manner at defined spatial densities and also encapsulated nucleic acid molecules into the interior of the liposome. Both the chemical conjugation of the HEL antigen on liposome surfaces and the encapsulation of nucleic acids were stable under physiologically relevant conditions. These liposomes elicited antigen-specific B-cell responses in vitro, which validate these supramolecular structures as a novel and effective approach to mimic and systematically isolate the role of essential viral features in directing the B-cell response to particulate antigens.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

Factors Affecting Early Antibody Secreting Cell Maturation Into Long-Lived Plasma Cells

Antibody secreting cells (ASCs) are terminally differentiated cells of the humoral immune response and must adapt morphologically, transcriptionally, and metabolically to maintain high-rates of antibody (Ab) secretion. ASCs differentiate from activated B cells in lymph nodes and transiently circulate in the blood. Most of the circulating ASCs undergo apoptosis, but a small fraction of early ASCs migrate to the bone marrow (BM) and eventually mature into long-lived plasma cells (LLPCs). LLPC survival is controlled both intrinsically and extrinsically. Their differentiation and maintenance programs are governed by many intrinsic mechanisms involving anti-apoptosis, autophagy, and metabolism. The extrinsic factors involved in LLPC generation include BM stromal cells, cytokines, and chemokines, such as APRIL, IL-6, and CXCL12. In humans, the BM CD19⁻CD38^hiCD138⁺ ASC subset is the main repository of LLPCs, and our recent development of an in vitro BM mimic provides essential tools to study environmental cues that support LLPC survival and the critical molecular mechanisms of maturation from early minted blood ASCs to LLPCs. In this review, we summarize the evidence of LLPC generation and maintenance and provide novel paradigms of LLPC maturation.

Class-Switch Recombination Occurs Infrequently in Germinal Centers

Class-switch recombination (CSR) is a DNA recombination process that replaces the immunoglobulin (Ig) constant region for the isotype that can best protect against the pathogen. Dysregulation of CSR can cause self-reactive BCRs and B cell lymphomas; understanding the timing and location of CSR is therefore important. Although CSR commences upon T cell priming, it is generally considered a hallmark of germinal centers (GCs). Here, we have used multiple approaches to show that CSR is triggered prior to differentiation into GC B cells or plasmablasts and is greatly diminished in GCs. Despite finding a small percentage of GC B cells expressing germline transcripts, phylogenetic trees of GC BCRs from secondary lymphoid organs revealed that the vast majority of CSR events occurred prior to the onset of somatic hypermutation. As such, we have demonstrated the existence of IgM-dominated GCs, which are unlikely to occur under the assumption of ongoing switching.

Regulatory roles of IL-10-producing human follicular T cells

Uncontrolled IgE responses drive allergies and anaphylaxis. Here, Cañete et al. describe a human follicular regulatory T cell population that does not express FOXP3 and produces abundant IL-10, which limits IgE switching. These cells appear to be key regulators of atopy.

Mucosal lymphoid tissues such as human tonsil are colonized by bacteria and exposed to ingested and inhaled antigens, requiring tight regulation of immune responses. Antibody responses are regulated by follicular helper T (T_FH) cells and FOXP3⁺ follicular regulatory T (T_FR) cells. Here we describe a subset of human tonsillar follicular T cells identified by expression of T_FH markers and CD25 that are the main source of follicular T (T_F) cell–derived IL-10. Despite lack of FOXP3 expression, CD25⁺ T_F cells resemble T reg cells in high CTLA4 expression, low IL-2 production, and their ability to repress T cell proliferation. CD25⁺ T_F cell–derived IL-10 dampens induction of B cell class-switching to IgE. In children, circulating total IgE titers were inversely correlated with the frequencies of tonsil CD25⁺ T_F cells and IL-10–producing T_F cells but not with total T reg cells, T_FR, or IL-10–producing T cells. Thus, CD25⁺ T_F cells emerge as a subset with unique T and B cell regulatory activities that may help prevent atopy.

Optimal Development of Mature B Cells Requires Recognition of Endogenous Antigens

It has long been appreciated that highly autoreactive BCRs are actively removed from the developing B cell repertoire by Ag-dependent receptor editing and deletion. However, there is persistent debate about whether mild autoreactivity is simply tolerated or positively selected into the mature B cell repertoire as well as at what stage, to what extent, under what conditions, and into which compartments this occurs. In this study, we describe two minor, trackable populations of B cells in B1-8i Ig transgenic mice that express the VH186.2 H chain and recognize a common foreign Ag (the hapten 4-hydroxy-3-nitrophenylacetyl) but differ in L chain expression. We use the Nur77-eGFP reporter of BCR signaling to define their reactivity toward endogenous Ags. The less autoreactive of these two populations is strongly counterselected during the development of mature B1a, follicular, and marginal zone B cells. By genetically manipulating the strength of BCR signal transduction via the titration of surface CD45 expression, we demonstrate that this B cell population is not negatively selected but instead displays characteristics of impaired positive selection. We demonstrate that mild self-reactivity improves the developmental fitness of B cell clones in the context of a diverse population of B cells, and positive selection by endogenous Ags shapes the mature B cell repertoire.

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.

Recurrent group A Streptococcus tonsillitis is an immunosusceptibility disease involving antibody deficiency and aberrant TFH cells

"Strep throat" is highly prevalent among children, yet it is unknown why only some children develop recurrent tonsillitis (RT), a common indication for tonsillectomy. To gain insights into this classic childhood disease, we performed phenotypic, genotypic, and functional studies on pediatric group A Streptococcus (GAS) RT and non-RT tonsils from two independent cohorts. GAS RT tonsils had smaller germinal centers, with an underrepresentation of GAS-specific CD4+ germinal center T follicular helper (GC-TFH) cells. RT children exhibited reduced antibody responses to an important GAS virulence factor, streptococcal pyrogenic exotoxin A (SpeA). Risk and protective human leukocyte antigen (HLA) class II alleles for RT were identified. Lastly, SpeA induced granzyme B production in GC-TFH cells from RT tonsils with the capacity to kill B cells and the potential to hobble the germinal center response. These observations suggest that RT is a multifactorial disease and that contributors to RT susceptibility include HLA class II differences, aberrant SpeA-activated GC-TFH cells, and lower SpeA antibody titers.

The B Cell Activation-Induced miR-183 Cluster Plays a Minimal Role in Canonical Primary Humoral Responses

Although primary humoral responses are vital to durable immunity, fine-tuning is critical to preventing catastrophes such as autoimmunity, chronic inflammation, and lymphomagenesis. MicroRNA (miRNA)-mediated regulation is particularly well suited for fine-tuning roles in physiology. Expression of clustered paralogous miR-182, miR-96, and miR-183 (collectively, 183c) is robustly induced upon B cell activation, entry into the germinal center, and plasmablast differentiation. 183c^GT/GT mice lacking 183c miRNA expression exhibit largely normal primary humoral responses, encompassing class switch recombination, affinity maturation, and germinal center reaction, as well as plasmablast differentiation. Our rigorous analysis included ex vivo class switch recombination and plasmablast differentiation models as well as in vivo immunization with thymus-dependent and thymus-independent Ags. Our work sways the debate concerning the role of miR-182 in plasmablast differentiation, strongly suggesting that 183c miRNAs are dispensable. In the process, we present a valuable framework for systematic evaluation of primary humoral responses. Finally, our work bolsters the notion of robustness in miRNA:target interaction networks and advocates a paradigm shift in miRNA studies.

Dynamics of Virus-Specific Memory B Cells and Plasmablasts following Viral Infection of the Central Nervous System

The prevalence and role of antigen-specific Bmem in the CNS during viral encephalomyelitis is largely undefined. A lack of reliable markers identifying murine Bmem has made it difficult to assess their contribution to local antiviral protection via antigen presentation or conversion to ASC. Using reporter mice infected with neurotropic coronavirus to track virus-specific Bmem and ASC, this report demonstrates that both subsets only emerge in the CNS following peripheral GC formation and subsequently prevail. While early GC reactions supported preferential Bmem accumulation in the CNS, late GC reactions favored ASC accumulation, although Bmem outnumbered ASC in draining lymph nodes throughout infection. Importantly, virus-specific B cells undergoing sustained GC selection were continually recruited to the persistently infected CNS. Elucidating the factors governing temporal events within GCs, as well as regional CNS cues during viral persistence, will aid intervention to modulate CNS humoral responses in the context of infection and associated autoimmune pathologies.

Humoral responses within the central nervous system (CNS) are common to many neurotropic viral infections, with antibody (Ab)-secreting cells (ASC) contributing to local protection. However, a role for virus-specific memory B cells (Bmem) within the CNS is poorly explored due to lack of robust phenotypic or functional identification in mice. This study takes advantage of the progeny of mice expressing tamoxifen-inducible Cre recombinase (Cre-ERT2) under the Aicda promoter crossed with Rosa26-loxP-tdTomato reporter mice (AID^Cre-Rosa26^tdTomato) to monitor B cells having undergone activation-induced cytidine deaminase (AID)-mediated somatic hypermutation (SHM) following neurotropic coronavirus infection. AID detection via tdTomato expression allowed tracking of virus-specific ASC and Bmem in priming and effector sites throughout infection. In draining lymph nodes, tdTomato-positive (tdTomato⁺) ASC were most prevalent prior to germinal center (GC) formation, but total tdTomato⁺ B cells only peaked with robust GC formation at day 14 p.i. Moreover, their proportion of Bmem dominated over the proportion of ASC throughout infection. In the CNS, tdTomato⁺ cells started emerging at day 14 p.i. While they initially comprised mainly Bmem, the proportions of ASC and Bmem became similar as tdTomato⁺ B cells increased throughout viral persistence. Delayed tamoxifen treatment demonstrated ongoing CNS recruitment of tdTomato⁺ B cells, mainly ASC, primed late during GC reactions. Overall, the data support the idea that virus-induced B cells exhibiting SHM require peripheral GC formation to emerge in the CNS. Ongoing GC reactions and regional signals further regulate dynamics within the CNS, with preferential maintenance of tdTomato⁺ B cells in spinal cords relative to that in brains during viral persistence.

IMPORTANCE The prevalence and role of antigen-specific Bmem in the CNS during viral encephalomyelitis is largely undefined. A lack of reliable markers identifying murine Bmem has made it difficult to assess their contribution to local antiviral protection via antigen presentation or conversion to ASC. Using reporter mice infected with neurotropic coronavirus to track virus-specific Bmem and ASC, this report demonstrates that both subsets only emerge in the CNS following peripheral GC formation and subsequently prevail. While early GC reactions supported preferential Bmem accumulation in the CNS, late GC reactions favored ASC accumulation, although Bmem outnumbered ASC in draining lymph nodes throughout infection. Importantly, virus-specific B cells undergoing sustained GC selection were continually recruited to the persistently infected CNS. Elucidating the factors governing temporal events within GCs, as well as regional CNS cues during viral persistence, will aid intervention to modulate CNS humoral responses in the context of infection and associated autoimmune pathologies.

Subcapsular Sinus Macrophages: The Seat of Innate and Adaptive Memory in Murine Lymph Nodes

Subcapsular sinus (SCS) macrophages are strategically positioned at the lymph-tissue interface in the lymph node to trap and present antigen to B cells. Recent murine data has shown that SCS macrophages also prevent the systemic spread of lymph-borne pathogens and are capable of activating a diverse range of innate effector and adaptive memory cells, including follicular memory T cells and memory B cells (B_mems), that are either pre-positioned or rapidly recruited to the subcapsular niche following infection and inflammation. Furthermore, B_mems are rapidly reactivated to differentiate into plasma cells in subcapsular proliferative foci (SPF). Thus, understanding how SCS macrophages coordinate both innate and adaptive memory responses in the subcapsular niche can provide new opportunities to bolster immunity against pathogens and cancer.

High-affinity IgM+ memory B cells are defective in differentiation into IgM antibody-secreting cells by re-stimulation with a T cell-dependent antigen

IgM antibodies (Abs) are thought to play a major role in humoral immunity but only at the early stage of the primary immune response. However, two subsets of IgM⁺ memory B cells (MBCs), one with high affinity gained by means of multiple somatic hypermutation (SHM) and the other with low affinity and no SHMs, are generated through the germinal center (GC)-dependent and GC-independent (non-GC) pathway, respectively, after immunization with (4-hydroxy-3-nitrophenyl)acetyl (NP)-chicken γ-globulin. Surprisingly, an analysis of antibody-secreting cells reveals that a large amount of anti-NP IgM Ab with few SHMs is secreted during the recall response, indicating that only non-GC MBCs have terminal differentiation potential. Since secondary IgM Abs are capable of binding to dinitrophenyl ligands, they likely provide broad cross-reactivity in defense against microbial infection.

Understanding and Manipulating Viral Immunity: Antibody Immunodominance Enters Center Stage

Adaptive immune responses against antigenically variable viruses and cellular pathogens are efficient in many cases, but largely limited to the infecting or immunizing strain. A major factor that limits immunity is immunodominance (ID), the hierarchical focusing of adaptive immune responses on a subset of antigenic determinants. While CD8⁺ T cell ID has been extensively studied, studies of basic mechanisms of B cell ID are limited, despite the importance of antibodies (Abs) for durable protection against pathogens. Here, we review recent progress in understanding the basic rules and mechanisms of B cell ID, compare B and CD8⁺ T cell ID, and outline challenges to overcoming ID to develop Ab-based 'universal' vaccines for influenza A and other highly variable viruses.

SYK Inhibition Induces Apoptosis in Germinal Center-Like B Cells by Modulating the Antiapoptotic Protein Myeloid Cell Leukemia-1, Affecting B-Cell Activation and Antibody Production

B cells play a major role in the antibody-mediated rejection (AMR) of solid organ transplants, a major public health concern. The germinal center (GC) is involved in the generation of donor-specific antibody-producing plasma cells and memory B cells, which are often poorly controlled by current treatments. Myeloid cell leukemia-1 (Mcl-1), an antiapoptotic member of the B-cell lymphoma-2 family, is essential for maintenance of the GC reaction and B-cell differentiation. During chronic AMR (cAMR), tertiary lymphoid structures resembling GCs appear in the rejected organ, suggesting local lymphoid neogenesis. We report the infiltration of the kidneys with B cells expressing Mcl-1 in patients with cAMR. We modulated GC viability by impairing B-cell receptor signaling, by spleen tyrosine kinase (SYK) inhibition. SYK inhibition lowers viability and Mcl-1 protein levels in Burkitt's lymphoma cell lines. This downregulation of Mcl-1 is coordinated at the transcriptional level, possibly by signal transducer and activator of transcription 3 (STAT3), as shown by (1) the impaired translocation of STAT3 to the nucleus following SYK inhibition, and (2) the lower levels of Mcl-1 transcription upon STAT3 inhibition. Mcl-1 overproduction prevented cells from entering apoptosis following SYK inhibition. In vitro studies with primary tonsillar B cells confirmed that SYK inhibition impaired cell survival and decreased Mcl-1 protein levels. It also impaired B-cell activation and immunoglobulin G secretion by tonsillar B cells. These findings suggest that the SYK-Mcl-1 pathway could be targeted, to improve graft survival by manipulating the humoral immune response.