B cell dissemination patterns during the germinal center reaction revealed by whole-organ imaging

T cell help induces Myc transcriptional bursts in germinal center B cells during positive selection

Antibody affinity maturation occurs in secondary lymphoid organs within germinal centers (GCs). At these sites, B cells mutate their antibody-encoding genes in the dark zone, followed by preferential selection of the high-affinity variants in the light zone by T cells. The strength of the T cell-derived selection signals is proportional to the B cell receptor affinity and to the magnitude of subsequent Myc expression. However, because the lifetime of Myc mRNA and its corresponding protein is very short, it remains unclear how T cells induce sustained Myc levels in positively selected B cells. Here, by direct visualization of mRNA and active transcription sites in situ, we found that an increase in transcriptional bursts promotes Myc expression during B cell positive selection in GCs. Elevated T cell help signals predominantly enhance the percentage of cells expressing Myc in GCs as opposed to augmenting the quantity of Myc transcripts per individual cell. Visualization of transcription start sites in situ revealed that T cell help promotes an increase in the frequency of transcriptional bursts at the Myc locus in GC B cells located primarily in the LZ apical rim. Thus, the rise in Myc, which governs positive selection of B cells in GCs, reflects an integration of transcriptional activity over time rather than an accumulation of transcripts at a specific time point.

Follicular regulatory T cell subsets in mice and humans: origins, antigen specificity and function

Follicular regulatory T (Tfr) cells play various roles in immune responses, contributing to both positive and negative regulation of foreign antigen-specific B cell responses, control over autoreactive antibody responses and autoimmunity, and B cell class-switching to IgE and allergy development. Studies conducted on mice uncovered various subsets of CXCR5+FoxP3+CD4+ Tfr cells that could differently contribute to immune regulation. Moreover, recent studies of human Tfr cells revealed similar complexity with various subsets of follicular T cells of different origins and immunosuppressive and/or immunostimulatory characteristics. In this review we will overview and compare Tfr subsets currently identified in mice and humans and will discuss their origins and antigen specificity, as well as potential modes of action and contribution to the control of the autoimmune and allergic reactions.

Microbiota-dependent and -independent postnatal development of salivary immunity

While saliva regulates the interplay between the microbiota and the oral immune system, the mechanisms establishing postnatal salivary immunity are ill-defined. Here, we show that high levels of neutrophils and neonatal Fc receptor (FcRn)-transferred maternal IgG are temporarily present in the neonatal murine salivary glands in a microbiota-independent manner. During weaning, neutrophils, FcRn, and IgG decrease in the salivary glands, while the polymeric immunoglobulin receptor (pIgR) is upregulated in a growth arrest-specific 6 (GAS6)-dependent manner independent of the microbiota. Production of salivary IgA begins following weaning and relies on CD4-help, IL-17, and the microbiota. The weaning phase is characterized by a transient accumulation of dendritic cells capable of migrating from the oral mucosa to the salivary glands upon exposure to microbial challenges and activating T cells. This study reveals the postnatal mechanisms developed in the salivary glands to induce immunity and proposes the salivary glands as an immune inductive site.

LigA formulated in AS04 or Montanide ISA720VG induced superior immune response compared to alum, which correlated to protective efficacy in a hamster model of leptospirosis

Leptospirosis is a zoonotic disease of global importance. The current vaccine provides serovar-specific and short-term immunity and does not prevent bacterial shedding in infected animals. Subunit vaccines based on surface proteins have shown to induce protection in an animal model. However, these proteins were tested with non-clinical adjuvants and induced low to moderate protective efficacy. We formulated a variable region of Leptospira immunoglobulin-like protein A (LAV) in clinical adjuvants, AS04 and Montanide ISA720VG, and then evaluated the immune response in mice and protective efficacy in a hamster model. Our results show that animals immunized with LAV-AS04 and LAV-Montanide ISA720VG (LAV-M) induced significantly higher levels of LAV-specific antibodies than LAV-Alum. While LAV-Alum induced Th2 response with the induction of IgG1 and IL-4, AS04 and LAV-M induced a mixed Th1/Th2 response with significant levels of both IgG1/IL-4 and IgG2c/IFN-γ. Both LAV-AS04 and LAV-M induced the generation of a significantly higher number of cytotoxic T cells (CTLs). The immune response in LAV-AS04- and LAV-M-immunized animals was maintained for a long period (>180 days) with the generation of a significant level of B- and T-cell memory. The strong immune response by both vaccines correlated to enhanced recruitment and activation of innate immune cells particularly DCs at draining lymph nodes and the formation of germinal centers (GCs). Furthermore, the immune response generated in mice correlated to protective efficacy in the hamster model of leptospirosis. These results indicate that LAV-AS04 and LAV-M are promising vaccines and can be further evaluated in clinical trials.

Recycling of memory B cells between germinal center and lymph node subcapsular sinus supports affinity maturation to antigenic drift

Infection or vaccination leads to the development of germinal centers (GC) where B cells evolve high affinity antigen receptors, eventually producing antibody-forming plasma cells or memory B cells. Here we follow the migratory pathways of B cells emerging from germinal centers (BEM) and find that many BEM cells migrate into the lymph node subcapsular sinus (SCS) guided by sphingosine-1-phosphate (S1P). From the SCS, BEM cells may exit the lymph node to enter distant tissues, while some BEM cells interact with and take up antigen from SCS macrophages, followed by CCL21-guided return towards the GC. Disruption of local CCL21 gradients inhibits the recycling of BEM cells and results in less efficient adaption to antigenic variation. Our findings thus suggest that the recycling of antigen variant-specific BEM cells and transport of antigen back to GC may support affinity maturation to antigenic drift.

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.

Intravital three-photon microscopy allows visualization over the entire depth of mouse lymph nodes

Intravital confocal microscopy and two-photon microscopy are powerful tools to explore the dynamic behavior of immune cells in mouse lymph nodes (LNs), with penetration depth of ~100 and ~300 μm, respectively. Here, we used intravital three-photon microscopy to visualize the popliteal LN through its entire depth (600-900 μm). We determined the laser average power and pulse energy that caused measurable perturbation in lymphocyte migration. Long-wavelength three-photon imaging within permissible parameters was able to image the entire LN vasculature in vivo and measure CD8⁺ T cells and CD4⁺ T cell motility in the T cell zone over the entire depth of the LN. We observed that the motility of naive CD4⁺ T cells in the T cell zone during lipopolysaccharide-induced inflammation was dependent on depth. As such, intravital three-photon microscopy had the potential to examine immune cell behavior in the deeper regions of the LN in vivo.

Complete Visualization of T Follicular Helper Cells in Germinal Centers by Light Sheet Fluorescence Microscopy

Long-lasting immunity depends on generation of antibody forming cells in germinal centers (GCs). Conventional methods such as immunohistology and intravital live imaging have been used extensively to investigate the location of cellular assemblies within tissues as well as their dynamic motility and cellular interactions. Two photon laser scanning microscopy (TPLSM) intravital imaging allows scanning of large areas within tissues and reveals multiple immune cell niches. Nonetheless, this type of imaging is limited by the depth of penetration and cannot capture effectively all of the GC niches within lymphoid organs. Here we describe a method to visualize antigen-specific T and B cells in multiple microanatomical locations and niches at the level of a whole organ. This large-scale imaging approach can greatly increase our understanding of the spatial distribution of immune cells and help obtain detailed 3D maps of their locations and quantities.

Spatial determinates of effector and memory CD8+ T cell fates

The lymph node plays a critical role in mounting an adaptive immune response to infection, clearance of foreign pathogens, and cancer immunosurveillance. Within this complex structure, intranodal migration is vital for CD8⁺ T cell activation and differentiation. Combining tissue clearing and volumetric light sheet fluorescent microscopy of intact lymph nodes has allowed us to explore the spatial regulation of T cell fates. This has determined that short-lived effector (T_SLEC ) are imprinted in peripheral lymph node interfollicular regions, due to CXCR3 migration. In contrast, stem-like memory cell (T_SCM ) differentiation is determined in the T cell paracortex. Here, we detail the inflammatory and chemokine regulators of spatially restricted T cell differentiation, with a focus on how to promote T_SCM . We propose a default pathway for T_SCM differentiation due to CCR7-directed segregation of precursors away from the inflammatory effector niche. Although volumetric imaging has revealed the consequences of intranodal migration, we still lack knowledge of how this is orchestrated within a complex chemokine environment. Toward this goal, we highlight the potential of combining microfluidic chambers with pre-determined complexity and subcellular resolution microscopy.

A molecular theory of germinal center B cell selection and division

The selection of B cells (BCs) in germinal centers (GCs) is pivotal to the generation of high-affinity antibodies and memory BCs, but it lacks global understanding. Based on the idea of a single Tfh-cell signal that controls BC selection and division, experiments appear contradictory. Here, we use the current knowledge on the molecular pathways of GC BCs to develop a theory of GC BC selection and division based on the dynamics of molecular factors. This theory explains the seemingly contradictory experiments by the separation of signals for BC fate decision from signals controlling the number of BC divisions. Three model variants are proposed and experiments are predicted that allow one to distinguish those. Understanding information processing in molecular BC states is critical for targeted immune interventions, and the proposed theory implies that selection and division can be controlled independently in GC reactions.

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.

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.

Getting toGether in Germinal centers

In this issue of Science Immunology, Gallman et al. reveal how S-geranylgeranyl-l-glutathione cleavage and transport support P2RY8-driven B cell confinement to the germinal centers and its role in lymphocyte homing to the bone marrow.

Acute Lymph Node Slices Are a Functional Model System to Study Immunity Ex Vivo

The lymph node is a highly organized and dynamic structure that is critical for facilitating the intercellular interactions that constitute adaptive immunity. Most ex vivo studies of the lymph node begin by reducing it to a cell suspension, thus losing the spatial organization, or fixing it, thus losing the ability to make repeated measurements. Live murine lymph node tissue slices offer the potential to retain spatial complexity and dynamic accessibility, but their viability, level of immune activation, and retention of antigen-specific functions have not been validated. Here we systematically characterized live murine lymph node slices as a platform to study immunity. Live lymph node slices maintained the expected spatial organization and cell populations while reflecting the 3D spatial complexity of the organ. Slices collected under optimized conditions were comparable to cell suspensions in terms of both 24-h viability and inflammation. Slices responded to T cell receptor cross-linking with increased surface marker expression and cytokine secretion, in some cases more strongly than matched lymphocyte cultures. Furthermore, slices processed protein antigens, and slices from vaccinated animals responded to ex vivo challenge with antigen-specific cytokine secretion. In summary, lymph node slices provide a versatile platform to investigate immune functions in spatially organized tissue, enabling well-defined stimulation, time-course analysis, and parallel read-outs.

Potential Application of T-Follicular Regulatory Cell Therapy in Transplantation

Regulatory T cells (Tregs) constitute a small proportion of circulating CD4⁺ T cells that function to maintain homeostasis and prevent autoimmunity. In light of their powerful immunosuppressive and tolerance-promoting properties, Tregs have become an interesting potential candidate for therapeutic use in conditions such as solid organ transplant or to treat autoimmune and inflammatory conditions. Clinical studies have demonstrated the safety of polyclonally expanded Tregs in graft-versus-host disease, type 1 diabetes, and more recently in renal and liver transplantation. However, Tregs are heterogenous. Recent insights indicate that only a small proportion of Tregs, called T follicular regulatory cells (Tfr) regulate interactions between B cells and T follicular helper (Tfh) cells within the germinal center. Tfr have been mainly described in mouse models due to the challenges of sampling secondary lymphoid organs in humans. However, emerging human studies, characterize Tfr as being CD4⁺CD25⁺FOXP3⁺CXCR5⁺ cells with different levels of PD-1 and ICOS expression depending on their localization, in the blood or the germinal center. The exact role they play in transplantation remains to be elucidated. However, given the potential ability of these cells to modulate antibody responses to allo-antigens, there is great interest in exploring translational applications in situations where B cell responses need to be regulated. Here, we review the current knowledge of Tfr and the role they play focusing on human diseases and transplantation. We also discuss the potential future applications of Tfr therapy in transplantation and examine the evidence for a role of Tfr in antibody production, acute and chronic rejection and tertiary lymphoid organs. Furthermore, the potential impact of immunosuppression on Tfr will be explored. Based on preclinical research, we will analyse the rationale of Tfr therapy in solid organ transplantation and summarize the different challenges to be overcome before Tfr therapy can be implemented into clinical practice.

Characterization of Immunological Niches within Peyer's Patches by ex vivo Photoactivation and Flow Cytometry Analysis

T follicular helper (Tfh) cells regulate B cell selection for entry into the germinal center (GC) reaction or for differentiation into antibody forming cells. This process takes place at the border between the T and B zones in lymphoid organs and involves physical contacts between T and B cells. During these interactions, T cells endow the B cells with selection signals that promote GC seeding or plasmablast differentiation based on their B cell receptor affinity. In Peyer's patches (PPs), T cells promote B cell colonization of the subepithelial dome (SED) without effective affinity-based clonal selection. To specifically characterize the T cell population that resides within the SED niche, we performed ex vivo photoactivation of the SED compartment followed by flow cytometry analysis of the labeled cells, as described in this protocol. This technique integrates both spatial and cellular information in studies of immunological niches and can be adapted to various experimental systems.

Systematic Evaluation of Chemically Distinct Tissue Optical Clearing Techniques in Murine Lymph Nodes

Activation of adaptive immunity is a complex process coordinated at multiple levels in both time and the three-dimensional context of reactive lymph nodes (LNs). Although microscopy-based visualization of its spatiotemporal dynamics unravels complexities of developing immune response, such approach is highly limited by light-obstructing nature of tissue components. Recently, tissue optical clearing (TOC) techniques were established to bypass this obstacle and now allow to image and quantify the entire murine organs with cellular resolution. However, the spectrum of TOC is represented by wide variety of chemically distinct methods, each having certain advantages and disadvantages that were unsatisfactorily compared for suitability to LNs clearing. In this study, we have systematically tested 13 typical TOC techniques and assessed their impact on a number of critical factors such as LN transparency, imaging depth, change in size, compatibility with proteinaceous fluorophores, immunostaining, H&E staining, and light-sheet fluorescence microscopy. Based on the detailed data specific to TOC process of murine LNs, we provide a reliable reference for most suitable methods in an application-dependent manner.