Germinal Centre Shutdown

The unusual metabolism of germinal center B cells

In the germinal center (GC), B cells undergo rounds of somatic hypermutation (SHM), proliferation, and positive selection to develop into high-affinity, long-lived plasma cells and memory B cells. It is well established that, upon activation, B cells significantly alter their metabolism, but until recently little was understood about their metabolism within the GC. In this review we discuss novel in vivo models in which GC B cell (GCBC) metabolism is disrupted; these have greatly increased our understanding of B cell metabolic phenotype. GCBCs are unusual in that, unlike almost all other rapidly proliferating immune cells, they use little glycolysis but prefer fatty acid oxidation (FAO) to fuel ATP synthesis, whilst preferentially utilizing glucose and amino acids as carbon and nitrogen sources for biosynthetic pathways.

Altered immune signatures in breast cancer lymph nodes with metastases revealed by spatial proteome analyses

BACKGROUND

Metastasis to lymph nodes is strongly associated with reduced survival in breast cancer patients. To increase the understanding on how lymph node metastasis impairs the local immune response in affected lymph nodes, we here studied spatial proteomic changes of critical lymph node immune populations in uninvolved lymph nodes (UnLN) and paired lymph nodes with metastases (LNM) from five breast cancer patients.

METHODS

The proteome was analyzed for cortical lymphocyte compartments, subcapsular sinus (SCS) and medullary sinus (MS) CD169+ macrophages, using the Digital Spatial Profiling (DSP) platform from NanoString.

RESULTS

Our results identified a stable proteome of SCS CD169+ macrophages in LNM, with the exception for downregulation of the anti-apoptotic protein Bcl-xL and FAPα, but a clear reduction in numbers of SCS CD169+ macrophages in LNM. In contrast, the proteome of MS CD169+ macrophages, B-cell compartments and interfollicular T-cells showed altered immune signatures in LNM, indicating that the decline in SCS CD169+ macrophages coincide with a malfunction in the local, anti-tumor immune responses.

CONCLUSIONS

The findings from our study support the notion that metastasis to lymph nodes in breast cancer patients modifies local immune responses. These changes may contribute to explain unsuccessful therapeutic responses, and thereby worsened prognosis, for breast cancer patients with LNM.

Maturation of Tertiary Lymphoid Structures

Tertiary lymphoid structures (TLS) are organized collections of B and T lymphocytes that arise in nonlymphoid tissue in response to chronic, unresolved inflammation. TLS have structural and functional similarities to germinal centers found in lymph nodes and are believed to support the establishment of lymph node-like adaptive immune responses at local sites of inflammation. However, understanding of the underlying biology of these structures remains limited, particularly the different stages of TLS life cycle and the signals governing the initiation, maturation, and termination of TLS. Here, we review current understanding of the maturation of TLS and the signals and cell types involved in various stages of development with particular emphasis on recent studies of TLS in cancer, where evidence suggests that TLS may play an important role in supporting antitumor immune responses in solid tumors.

Differential response of IgM and IgG memory B cell populations to CD40L: insights of T cell - memory B cell interactions

Memory B cells (mBCs) are characterized by their long-term stability, fast reactivation, and capability to rapidly differentiate into antibody-secreting cells (ASCs). However, the role of T cells in the differentiation of mBCs, in contrast to naive B cells, remains to be delineated. We study the role of T cells in mBC responses, using CD40L stimulation and autologous T-B co-cultures. Our results showed that increased CD40L levels led to a selective increased proliferation of IgM+ mBC, which did not class-switched, resulting in higher frequencies of IgM+ ASCs and a lower frequency of IgG+ ASCs. The IgG+/IgA+ mBCs were unaffected. We further compared the transcription of immune-related genes in IgM+ and IgG+ pre-plasmablasts cultured at high (500 ng/mL) and low (50 ng/mL) CD40L levels. In response to increased CD40L levels, both populations exhibited a core response to genes related to activation (TRAF1, AKT3, CD69, and CD80). However, they differed in genes related to cytokine/chemokine/homing interactions (CCL3/4/17, LTA, NKX2-3, BCL2 and IL21R) and cell-cell interactions (HLADR, CD40, and ICOSL), which were largely confined to IgG+ cells. Our findings revealed that in co-cultures with a high T-ratio, the response was similar to that found in cultures with high CD40L levels. These results suggest that IgG+ mBCs have a greater capacity for proliferation and T cell interaction, and weaker migration capabilities, leading to a preference for an IgG response over IgM in the short term. This adaptable response could fine-tune the memory repertoire with different functions of IgG versus IgM mBCs.

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.

Dysregulated germinal center reaction with expanded T follicular helper cells in autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy lymph nodes

BACKGROUND

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED, also called APS-1) is an inborn error of immunity with clear signs of B-cell autoimmunity such as neutralizing anti-IFN antibodies. In APECED, mutations in the AIRE gene impair thymic negative selection of T cells. The resulting T-cell alterations may then cause dysregulation of B-cell responses. However, no analysis of interactions of T and B cells in the germinal centers (GCs) in patients' secondary lymphatic tissues has been reported.

OBJECTIVE

This study examined the relationship between B cells and follicular T helper cells (TfH) in peripheral blood and lymph node (LN) GCs in patients with APECED.

METHODS

Immunophenotyping of peripheral blood B cells and TfH was performed for 24 patients with APECED. Highly multiplexed fluorescent immunohistochemical staining was performed on 7 LN biopsy samples from the patients to study spatial interactions of lymphocytes in the GCs at the single-cell level.

RESULTS

The patients' peripheral B-cell phenotype revealed skewing toward a mature B-cell phenotype with marked loss of transitional and naive B cells. The frequency of circulating TfH cells was diminished in the patients, while in the LNs the TfH population was expanded. In LNs the overall frequency of Treg cells and interactions of Treg cells with nonfollicular T cells were reduced, suggesting that aberrant Treg cell function might fail to restrain TfH differentiation.

CONCLUSIONS

GC reactions are disrupted in APECED as a result of defective T-cell control.

Biomaterial engineering strategies for B cell immunity modulations

B cell immunity has a penetrating effect on human health and diseases. Therapeutics aiming to modulate B cell immunity have achieved remarkable success in combating infections, autoimmunity, and malignancies. However, current treatments still face significant limitations in generating effective long-lasting therapeutic B cell responses for many conditions. As the understanding of B cell biology has deepened in recent years, clearer regulation networks for B cell differentiation and antibody production have emerged, presenting opportunities to overcome current difficulties and realize the full therapeutic potential of B cell immunity. Biomaterial platforms have been developed to leverage these emerging concepts to augment therapeutic humoral immunity by facilitating immunogenic reagent trafficking, regulating T cell responses, and modulating the immune microenvironment. Moreover, biomaterial engineering tools have also advanced our understanding of B cell biology, further expediting the development of novel therapeutics. In this review, we will introduce the general concept of B cell immunobiology and highlight key biomaterial engineering strategies in the areas including B cell targeted antigen delivery, sustained B cell antigen delivery, antigen engineering, T cell help optimization, and B cell suppression. We will also discuss our perspective on future biomaterial engineering opportunities to leverage humoral immunity for therapeutics.

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 lymph 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 mechanic and applied research questions in future.

Unmutated but T cell dependent IgM antibodies targeting Streptococcus suis play an essential role in bacterial clearance

Streptococcus suis serotype 2 is an important encapsulated bacterial swine pathogen and zoonotic agent for which no effective vaccine exists. The interaction with B cells and the humoral response against S. suis are poorly understood despite their likely relevance for a potential vaccine. We evaluated germinal center (GC) B cell kinetics, as well as the production and role of S. suis-specific antibodies following infections in a mouse model. We found that mice infected with S. suis developed GC that peaked 13-21 days post-infection. GC further increased and persisted upon periodic reinfection that mimics real life conditions in swine farms. Anti-S. suis IgM and several IgG subclasses were produced, but antibodies against the S. suis capsular polysaccharide (CPS) were largely IgM. Interestingly, depletion of total IgG from the wild-type mice sera had no effect on bacterial killing by opsonophagocytosis in vitro. Somatic hypermutation and isotype switching were dispensable for controlling the infection or anti-CPS IgM production. However, T cell-deficient (Tcrb-/-) mice were unable to control bacteremia, produce optimal anti-CPS IgM titers, or elicit antibodies with opsonophagocytic activity. SAP deficiency, which prevents GC formation but not extrafollicular B cell responses, ablated anti S. suis-IgG production but maintained IgM production and eliminated the infection. In contrast, B cell deficient mice were unable to control bacteremia. Collectively, our results indicate that the antibody response plays a large role in immunity against S. suis, with GC-independent but T cell-dependent germline IgM being the major effective antibody specificities. Our results further highlight the importance IgM, and potentially anti-CPS antibodies, in clearing S. suis infections and provide insight for future development of S. suis vaccines.

Cold-blooded vertebrates evolved organized germinal center-like structures

Germinal centers (GCs) or analogous secondary lymphoid microstructures (SLMs) are thought to have evolved in endothermic species. However, living representatives of their ectothermic ancestors can mount potent secondary antibody responses upon infection or immunization, despite the apparent lack of SLMs in these cold-blooded vertebrates. How and where adaptive immune responses are induced in ectothermic species in the absence of GCs or analogous SLMs remain poorly understood. Here, we infected a teleost fish (trout) with the parasite Ichthyophthirius multifiliis (Ich) and identified the formation of large aggregates of highly proliferating IgM+ B cells and CD4+ T cells, contiguous to splenic melanomacrophage centers (MMCs). Most of these MMC-associated lymphoid aggregates (M-LAs) contained numerous antigen (Ag)-specific B cells. Analysis of the IgM heavy chain CDR3 repertoire of microdissected splenic M-LAs and non-M-LA areas revealed that the most frequent B cell clones induced after Ich infection were highly shared only within the M-LAs of infected animals. These M-LAs represented highly polyclonal SLMs in which Ag-specific B cell clonal expansion occurred. M-LA-associated B cells expressed high levels of activation-induced cytidine deaminase and underwent significant apoptosis, and somatic hypermutation of Igμ genes occurred prevalently in these cells. Our findings demonstrate that ectotherms evolved organized SLMs with GC-like roles. Moreover, our results also point to primordially conserved mechanisms by which M-LAs and mammalian polyclonal GCs develop and function.

Continually recruited naïve T cells contribute to the follicular helper and regulatory T cell pools in germinal centers

Follicular helper T cells (TFH) mediate B cell selection and clonal expansion in germinal centers (GCs), and follicular regulatory T cells (TFR) prevent the emergence of self-reactive B cells and help to extinguish the reaction. Here we show that GC reactions continually recruit T cells from both the naïve conventional and naive thymic regulatory T cell (Treg) repertoires. In the early GC, newly recruited T cells develop into TFH, whereas cells entering during the contraction phase develop into TFR cells that contribute to GC dissolution. The TFR fate decision is associated with decreased antigen availability and is modulated by slow antigen delivery or mRNA vaccination. Thus, invasion of ongoing GCs by newly developing TFH and TFR helps remodel the GC based on antigen availability.

Immune landscape of tertiary lymphoid structures in hepatocellular carcinoma (HCC) treated with neoadjuvant immune checkpoint blockade

Neoadjuvant immunotherapy is thought to produce long-term remissions through induction of antitumor immune responses before removal of the primary tumor. Tertiary lymphoid structures (TLS), germinal center-like structures that can arise within tumors, may contribute to the establishment of immunological memory in this setting, but understanding of their role remains limited. Here, we investigated the contribution of TLS to antitumor immunity in hepatocellular carcinoma (HCC) treated with neoadjuvant immunotherapy. We found that neoadjuvant immunotherapy induced the formation of TLS, which were associated with superior pathologic response, improved relapse free survival, and expansion of the intratumoral T and B cell repertoire. While TLS in viable tumor displayed a highly active mature morphology, in areas of tumor regression we identified an involuted TLS morphology, which was characterized by dispersion of the B cell follicle and persistence of a T cell zone enriched for ongoing antigen presentation and T cell-mature dendritic cell interactions. Involuted TLS showed increased expression of T cell memory markers and expansion of CD8+ cytotoxic and tissue resident memory clonotypes. Collectively, these data reveal the circumstances of TLS dissolution and suggest a functional role for late-stage TLS as sites of T cell memory formation after elimination of viable tumor.

50 years on and still very much alive: 'Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics'

Cell death is part of the lifecycle of every multicellular organism. Nineteenth-century pathologists already recognised that organised forms of cell death must exist to explain the demise and turnover of cells during metamorphosis (of insects), embryogenesis and normal tissue homoeostasis [1]. Nevertheless, Kerr, Wyllie and Currie in their seminal paper of 1972, were the first to collate and define the distinct morphological features of controlled cell death in different contexts [2]. To describe the processes of cell deletion observed under both physiological and pathological conditions, they coined the term 'Apoptosis' (derived from the Greek word 'ἀπόπτωσις', meaning 'dropping off or falling off' of petals from flowers). Kerr, Wyllie and Currie defined apoptosis as a mechanism 'complementary to mitosis in the regulation of animal cell populations'. In addition, they already recognised the potential to use this programmed form of cell death for cancer therapy, but they also emphasised the occurrence of apoptosis during cancer development. In this article, some 50 years after its initial publication in The British Journal of Cancer, we revaluate and put the authors initial assumptions and general concepts about apoptosis into the context of modern-day biology.

Investigating the Mechanism of Germinal Center Shutdown

Germinal centers (GCs) are transient structures where affinity maturation of B cells gives rise to high affinity plasma and memory cells. The mechanism of GC shutdown is unclear, despite being an important phenomenon maintaining immune homeostasis. In this study, we used a mathematical model to identify mechanisms that can independently promote contraction of GCs leading to shutdown. We show that GC shutdown can be promoted by antigen consumption by B cells, antigen masking by soluble antibodies, alterations in follicular dendritic cell (FDC) network area, modulation of immune complex cycling rate constants, alterations in T follicular helper signaling, increased terminal differentiation and reduced B cell division capacity. Proposed mechanisms promoted GC contraction by ultimately decreasing the number of B cell divisions and recycling cells. Based on the in-silico predictions, we suggest a combination of experiments that can be potentially employed by future studies to unravel the mechanistic basis of GC shutdown such as measurements of the density of pMHC presentation of B cells, FDC network size per B cell, fraction of cells expressing differentiation markers. We also show that the identified mechanisms differentially affect the efficiency of GC reaction estimated based on the quantity and quality of resulting antibodies.

Affinity maturation for an optimal balance between long-term immune coverage and short-term resource constraints

Humoral immunity relies on the mutation and selection of B cells to better recognize pathogens. This affinity maturation process produces cells with diverse recognition capabilities. Examining optimal immune strategies that maximize the long-term immune coverage at a minimal metabolic cost, we show when the immune system should mount a de novo response rather than rely on existing memory cells. Our theory recapitulates known modes of the B cell response, predicts the empirical form of the distribution of clone sizes, and rationalizes as a trade-off between metabolic and immune costs the antigenic imprinting effects that limit the efficacy of vaccines (original antigenic sin). Our predictions provide a framework to interpret experimental results that could be used to inform vaccination strategies.

In order to target threatening pathogens, the adaptive immune system performs a continuous reorganization of its lymphocyte repertoire. Following an immune challenge, the B cell repertoire can evolve cells of increased specificity for the encountered strain. This process of affinity maturation generates a memory pool whose diversity and size remain difficult to predict. We assume that the immune system follows a strategy that maximizes the long-term immune coverage and minimizes the short-term metabolic costs associated with affinity maturation. This strategy is defined as an optimal decision process on a finite dimensional phenotypic space, where a preexisting population of cells is sequentially challenged with a neutrally evolving strain. We show that the low specificity and high diversity of memory B cells—a key experimental result—can be explained as a strategy to protect against pathogens that evolve fast enough to escape highly potent but narrow memory. This plasticity of the repertoire drives the emergence of distinct regimes for the size and diversity of the memory pool, depending on the density of de novo responding cells and on the mutation rate of the strain. The model predicts power-law distributions of clonotype sizes observed in data and rationalizes antigenic imprinting as a strategy to minimize metabolic costs while keeping good immune protection against future strains.

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.

Aging-Related Cellular, Structural and Functional Changes in the Lymph Nodes: A Significant Component of Immunosenescence? An Overview

Aging affects all tissues and organs. Aging of the immune system results in the severe disruption of its functions, leading to an increased susceptibility to infections, an increase in autoimmune disorders and cancer incidence, and a decreased response to vaccines. Lymph nodes are precisely organized structures of the peripheral lymphoid organs and are the key sites coordinating innate and long-term adaptive immune responses to external antigens and vaccines. They are also involved in immune tolerance. The aging of lymph nodes results in decreased cell transport to and within the nodes, a disturbance in the structure and organization of nodal zones, incorrect location of individual immune cell types and impaired intercellular interactions, as well as changes in the production of adequate amounts of chemokines and cytokines necessary for immune cell proliferation, survival and function, impaired naïve T- and B-cell homeostasis, and a diminished long-term humoral response. Understanding the causes of these stromal and lymphoid microenvironment changes in the lymph nodes that cause the aging-related dysfunction of the immune system can help to improve long-term immune responses and the effectiveness of vaccines in the elderly.

The Germinal Center Milieu in Rheumatoid Arthritis: The Immunological Drummer or Dancer?

Rheumatoid Arthritis (RA) is a chronic autoimmune disease characterized by joint inflammation, affecting approximately 1% of the general population. To alleviate symptoms and ameliorate joint damage, chronic use of immunosuppressives is needed. However, these treatments are only partially effective and may lead to unwanted side effects. Therefore, a more profound understanding of the pathophysiology might lead to more effective therapies, or better still, a cure. The presence of autoantibodies in RA indicates that B cells might have a pivotal role in the disease. This concept is further supported by the fact that a diverse antibody response to various arthritis-related epitopes is associated with arthritis development. In this context, attention has focused in recent years on the role of Germinal Centers (GCs) in RA. Since GCs act as the main anatomic location of somatic hypermutations, and, thus, contributing to the diversity and specificity of (auto) antibodies, it has been speculated that defects in germinal center reactions might be crucial in the initiation and maintenance of auto-immune events. In this paper, we discuss current evidence that various processes within GCs can result in the aberrant production of B cells that possess autoreactive properties and might result in the production of RA related autoantibodies. Secondly, we discuss various (pre-)clinical studies that have targeted various GC processes as novel therapies for RA treatment.