Life and death in germinal centers (redux)

PPAR agonists as effective adjuvants for COVID-19 vaccines, by modifying immunogenetics: a review of literature

BACKGROUND

Several coronavirus vaccine have been fast-tracked to halt the pandemic, the usage of immune adjuvants that can boost immunological memory has come up to the surface. This is particularly of importance in view of the rates of failure of seroconversion and re-infection after COVID-19 infection, which could make the vaccine role and response debatable. Peroxisome proliferator-activated receptors (PPARs) have an established immune-modulatory role, but their effects as adjuvants to vaccination have not been explored to date. It is increasingly recognized that PPAR agonists can upregulate the levels of anti-apoptotic factors such as MCL-1. Such effect can improve the results of vaccination by enhancing the longevity of long-lived plasma cells (LLPCs). The interaction between PPAR agonists and the immune system does not halt here, as T cell memory is also stimulated through enhanced T regulatory cells, antagonizing PD-L1 and switching the metabolism of T cells to fatty acid oxidation, which has a remarkable effect on the persistence of T memory cells. What is even of a more significant value is the effect of PPAR gamma on ensuring a profound secretion of antibodies upon re-exposure to the offending antigen through upregulating lipoxin B4, therefore potentially assisting the vaccine response and deterring re-infection.

SHORT CONCLUSION

In view of the above, we suggest the use of PPAR as adjuvants to vaccines in general especially the emerging COVID-19 vaccine due to their role in enhancing immunologic memory through DNA-dependent mechanisms.

Cutting Edge: T Cell-Dependent Plasmablasts Form in the Absence of Single Differentiated CD4+ T Cell Subsets

The T follicular helper (Tfh) cell subset of CD4+ Th cells promotes affinity maturation by B cells in germinal centers. The contribution of other Th cell subsets to B cell responses has not been fully explored in vivo. We addressed this issue by analyzing the T cell-dependent B cell response to the protein Ag PE in mice lacking specific Th cell subsets. As expected, PE-specific germinal center B cell production required Tfh cells. However, Tfh, Th1, or Th17 cell-deficient mice produced as many PE-specific, isotype-switched plasmablasts as wild-type mice. This response depended on Th cell expression of CD154 and Ag presentation by B cells. These results indicate that many Th cell subsets can promote plasmablast formation by providing CD40 signals to naive B cells.

Deficient for endoplasmic reticulum calcium sensors Stim1 and Stim2 affects aberrant antibody affinity maturation in B cells

Antigen specific B cells undergo a process termed affinity maturation in the germinal centers of secondary lymphoid organs where B cells with high affinity receptors are selected to mature into antibody-producing cells or to the memory B cell pool. It is known that B cell antigen receptor (BCR) signaling plays pivotal role in this selection process. Calcium influx is an essential component of BCR signaling. The current report is to determine the effect of calcium influx on antibody affinity maturation. In our studies, mice deficient for both endoplasmic reticulum calciumsensor Stim1 and Stim2 was immunized with T-cell dependent and independent antigens. Antibody affinity was measured by ELISA. We demonstrated that Stim1 &Stim2 deficient B cells exhibit accelerated pace of affinity maturation compared to wild type controls while the overall antibody production was not dramatically impaired to T-independent antigen immunization. In conclusion, calcium influx plays an important role in antibody affinity maturation in humoral immune responses. The knowledge can be used in manipulate humoral immune response for the design of effective vaccines.

Mathematical modeling of lymphocytes selection in the germinal center

Lymphocyte selection is a fundamental process of adaptive immunity. In order to produce B-lymphocytes with a target antigenic profile, mutation selection and division occur in the germinal center, a specific part of lymph nodes. We introduce in this article a simplified mathematical model of this phenomenon, taking into account the main mechanisms. This model is written as a non-linear, non-local, inhomogeneous second order partial differential equation, for which we develop a mathematical analysis. We assess, mathematically and numerically, in the case of piecewise-constant coefficients, the performance of the biological function by evaluating the duration of this production process as a function of several parameters such as the mutation rate or the selection profile, in various asymptotic regimes.

Cellular senescence impact on immune cell fate and function

Cellular senescence occurs not only in cultured fibroblasts, but also in undifferentiated and specialized cells from various tissues of all ages, in vitro and in vivo. Here, we review recent findings on the role of cellular senescence in immune cell fate decisions in macrophage polarization, natural killer cell phenotype, and following T-lymphocyte activation. We also introduce the involvement of the onset of cellular senescence in some immune responses including T-helper lymphocyte-dependent tissue homeostatic functions and T-regulatory cell-dependent suppressive mechanisms. Altogether, these data propose that cellular senescence plays a wide-reaching role as a homeostatic orchestrator.

Chronic lymphocytic leukemia cells diversify and differentiate in vivo via a nonclassical Th1-dependent, Bcl-6-deficient process

Xenografting primary tumor cells allows modeling of the heterogeneous natures of malignant diseases and the influences of the tissue microenvironment. Here, we demonstrate that xenografting primary chronic lymphocytic leukemia (CLL) B lymphocytes with activated autologous T cells into alymphoid mice results in considerable CLL B cell division and sizable T cell expansion. Nevertheless, most/all CD5⁺CD19⁺ cells are eventually lost, due in part to differentiation into antibody-secreting plasmablasts/plasma cells. CLL B cell differentiation is associated with isotype class switching and development of new IGHV-D-J mutations and occurs via an activation-induced deaminase-dependent pathway that upregulates IRF4 and Blimp-1 without appreciable levels of the expected Bcl-6. These processes were induced in IGHV-unmutated and IGHV-mutated clones by Th1-polarized T-bet⁺ T cells, not classical T follicular helper (Tfh) cells. Thus, the block in B cell maturation, defects in T cell action, and absence of antigen-receptor diversification, which are often cardinal characteristics of CLL, are not inherent but imposed by external signals and the microenvironment. Although these activities are not dominant features in human CLL, each occurs in tissue proliferation centers where the mechanisms responsible for clonal evolution operate. Thus, in this setting, CLL B cell diversification and differentiation develop by a nonclassical germinal center-like reaction that might reflect the cell of origin of this leukemia.

Reconstructing and mining the B cell repertoire with ImmunediveRsity

The B cell antigen receptor repertoire is highly diverse and constantly modified by clonal selection. High-throughput DNA sequencing (HTS) of the lymphocyte repertoire (Rep-Seq) represents a promising technology to explore such diversity ex-vivo and assist in the identification of antigen-specific antibodies based on molecular signatures of clonal selection. Therefore, integrative tools for repertoire reconstruction and analysis from antibody sequences are needed. We developed ImmunediveRity, a stand-alone pipeline primarily based in R programming for the integral analysis of B cell repertoire data generated by HTS. The pipeline integrates GNU software and in house scripts to perform quality filtering, sequencing noise correction and repertoire reconstruction based on V, D and J segment assignment, clonal origin and unique heavy chain identification. Post-analysis scripts generate a wealth of repertoire metrics that in conjunction with a rich graphical output facilitates sample comparison and repertoire mining. Its performance was tested with raw and curated human and mouse 454-Roche sequencing benchmarks providing good approximations of repertoire structure. Furthermore, ImmunediveRsity was used to mine the B cell repertoire of immunized mice with a model antigen, allowing the identification of previously validated antigen-specific antibodies, and revealing different and unexpected clonal diversity patterns in the post-immunization IgM and IgG compartments. Although ImmunediveRsity is similar to other recently developed tools, it offers significant advantages that facilitate repertoire analysis and repertoire mining. ImmunediveRsity is open source and free for academic purposes and it runs on 64 bit GNU/Linux and MacOS. Available at: https://bitbucket.org/ImmunediveRsity/immunediversity/

Innate response activator B cells: origins and functions

Innate response activator (IRA) B cells are a subset of B-1a derived B cells that produce the growth factors granulocyte macrophage colony stimulating factor and IL-3. In mouse models of sepsis and pneumonia, B-1a B cells residing in serosal sites recognize bacteria, migrate to the spleen or lung, and differentiate to IRA B cells that then contribute to the host response by amplifying inflammation and producing polyreactive IgM. In atherosclerosis, IRA B cells accumulate in the spleen, where they promote extramedullary hematopoiesis and activate classical dendritic cells. In this review, we focus on the ontogeny and function of IRA B cells in acute and chronic inflammation.

Massive parallel IGHV gene sequencing reveals a germinal center pathway in origins of human multiple myeloma

Human multiple myeloma (MM) is characterized by accumulation of malignant terminally differentiated plasma cells (PCs) in the bone marrow (BM), raising the question when during maturation neoplastic transformation begins. Immunoglobulin IGHV genes carry imprints of clonal tumor history, delineating somatic hypermutation (SHM) events that generally occur in the germinal center (GC). Here, we examine MM-derived IGHV genes using massive parallel deep sequencing, comparing them with profiles in normal BM PCs. In 4/4 presentation IgG MM, monoclonal tumor-derived IGHV sequences revealed significant evidence for intraclonal variation (ICV) in mutation patterns. IGHV sequences of 2/2 normal PC IgG populations revealed dominant oligoclonal expansions, each expansion also displaying mutational ICV. Clonal expansions in MM and in normal BM PCs reveal common IGHV features. In such MM, the data fit a model of tumor origins in which neoplastic transformation is initiated in a GC B-cell committed to terminal differentiation but still targeted by on-going SHM. Strikingly, the data parallel IGHV clonal sequences in some monoclonal gammopathy of undetermined significance (MGUS) known to display on-going SHM imprints. Since MGUS generally precedes MM, these data suggest origins of MGUS and MM with IGHV gene mutational ICV from the same GC B-cell, arising via a distinctive pathway.

Follicular dendritic cells: dynamic antigen libraries

Follicular dendritic cells (FDCs) are essential for high-affinity antibody production and for the development of B cell memory. Historically, FDCs have been characterized as 'accessory' cells that passively support germinal centre (GC) responses. However, recent observations suggest that FDCs actively shape humoral immunity. In this Review, we discuss recent findings concerning the antigen acquisition and retention functions of FDCs, and relevant implications for protective immunity. Furthermore, we describe the roles of FDCs within GCs in secondary lymphoid organs and discuss FDC development within this dynamic environment. Finally, we discuss how a better understanding of FDCs could facilitate the design of next-generation vaccines.

Surface plasmon resonance measurements of plasma antibody avidity during primary and secondary responses to anthrax protective antigen

Establishment of humoral immunity against pathogens is dependent on events that occur in the germinal center and the subsequent induction of high-affinity neutralizing antibodies. Quantitative assays that allow monitoring of affinity maturation and duration of antibody responses can provide useful information regarding the efficacy of vaccines and adjuvants. Using an anthrax protective antigen (rPA) and alum model antigen/adjuvant system, we describe a methodology for monitoring antigen-specific serum antibody concentration and avidity by surface plasmon resonance during primary and secondary immune responses. Our analyses showed that following a priming dose in mice, rPA-specific antibody concentration and avidity increases over time and reaches a maximal response in about six weeks, but gradually declines in the absence of antigenic boost. Germinal center reactions were observed early with maximal development achieved during the primary response, which coincided with peak antibody avidity responses to primary immunization. Boosting with antigen resulted in a rapid increase in rPA-specific antibody concentration and five-fold increase in avidity, which was not dependent on sustained GC development. The described methodology couples surface plasmon resonance-based plasma avidity measurements with germinal center analysis and provides a novel way to monitor humoral responses that can play a role in facilitating vaccine and adjuvant development.

Endocytosis and recycling of immune complexes by follicular dendritic cells enhances B cell antigen binding and activation

Stromal-derived follicular dendritic cells (FDCs) are a major reservoir for antigen that are essential for formation of germinal centers, the site where memory and effector B cells differentiate. A long-standing question is how FDCs retain antigen in its native form for extended periods and how they display it to specific B cells. Here we found that FDCs acquired complement-coated immune complexes (ICs) from noncognate B cells via complement receptors 1 and 2 (CD35 and CD21, respectively) and rapidly internalized them by an actin-dependent pathway. ICs were retained intact within a nondegradative cycling compartment and were displayed periodically on the cell surface where they were accessible to antigen-specific B cells. This would explain how antigens are protected from damage and retained over long periods of time, while remaining accessible for B cells.

The lupus-prone NZM2410/NZW strain-derived Sle1b sublocus alters the germinal center checkpoint in female mice in a B cell-intrinsic manner

C57BL/6 (B6) mice carrying the Sle1b sublocus (named B6.Sle1b), which harbors the lupus-associated NZM2410/NZW SLAM family genes, produce antinuclear Abs (ANAs). However, the role and mechanism(s) involved in the alteration of the germinal center (GC) tolerance checkpoint in the development of ANAs in these mice is not defined. In this study, we show significantly higher spontaneously formed GCs (Spt-GCs) in B6.Sle1b female mice compared with B6 controls. We also found a significant increase in CD4(+)CXCR5(hi)PD-1(hi) spontaneously activated follicular Th cells in B6.Sle1b female mice. Compared with B6 controls, B6.Sle1b female mice had increased numbers of proliferating B cells predominantly located in Spt-GCs. The elevated Spt-GCs in B6.Sle1b female mice were strongly associated with increased ANA-specific Ab-forming cells and ANA titers. The increased numbers of Spt-GCs and spontaneously activated follicular Th cells in B6.Sle1b mice were not the result of a generalized defect in B cells expressing Sle1b. Consistent with the elevated spontaneous response in B6.Sle1b mice, the attenuated GC response characteristic of DNA and p-azophenylarsonate reactive B cells from Ig V(H) knock-in mice (termed HKIR) were relieved in adoptively transferred recipients in the presence of Sle1b. Finally, by generating mixed bone marrow chimeras, we showed that the effect of Sle1b on Spt-GC, follicular Th cell, and autoantibody responses in B6.Sle1b mice was B cell autonomous. These data indicate that the NZM2410/NZW-derived Sle1b sublocus in conjunction with the female sex primarily affects B cells, leading to the alteration of the GC tolerance checkpoint and the generation of ANA-specific Ab-forming cells.

Impaired clearance of apoptotic cells in germinal centers: implications for loss of B cell tolerance and induction of autoimmunity

Germinal centers (GCs) comprise lymphoid microenvironments where antigen-stimulated B cells undergo rapid proliferation and somatic hypermutation (SHM), resulting in the generation of B cells with high affinity for antigen. However, this process also generates B cell clones with low antigen affinity and with the potential for autoreactivity. It has been suggested that GC B cells with low antigen affinity and autoreactivity are eliminated via apoptosis and are rapidly cleared by tingible body macrophages (TBMφs). Inefficient clearance of apoptotic cells (ACs) results in autoimmunity that is thought to be mediated by various intracellular molecules possessing danger-associated molecular patterns (DAMPs), including nuclear self-Ags. DAMPs can be released from ACs undergoing "secondary necrosis" due to a disruption in AC clearance within GCs. This review discusses the role and mechanisms associated with impaired clearance of ACs in GCs in loss of B cell tolerance leading to autoantibody production and the development of autoimmunity.

Mass spectrometry analysis and quantitation of peptides presented on the MHC II molecules of mouse spleen dendritic cells

Major histocompatibility complex class II (MHC II) molecules are expressed on the surface of antigen-presenting cells and display short bound peptide fragments derived from self- and nonself antigens. These peptide-MHC complexes function to maintain immunological tolerance in the case of self-antigens and initiate the CD4(+) T cell response in the case of foreign proteins. Here we report the application of LC-MS/MS analysis to identify MHC II peptides derived from endogenous proteins expressed in freshly isolated murine splenic DCs. The cell number was enriched in vivo upon treatment with Flt3L-B16 melanoma cells. In a typical experiment, starting with about 5 × 10(8) splenic DCs, we were able to reliably identify a repertoire of over 100 MHC II peptides originating from about 55 proteins localized in membrane (23%), intracellular (26%), endolysosomal (12%), nuclear (14%), and extracellular (25%) compartments. Using synthetic isotopically labeled peptides corresponding to the sequences of representative bound MHC II peptides, we quantified by LC-MS relative peptide abundance. In a single experiment, peptides were detected in a wide concentration range spanning from 2.5 fmol/μL to 12 pmol/μL or from approximately 13 to 2 × 10(5) copies per DC. These peptides were found in similar amounts on B cells where we detected about 80 peptides originating from 55 proteins distributed homogenously within the same cellular compartments as in DCs. About 90 different binding motifs predicted by the epitope prediction algorithm were found within the sequences of the identified MHC II peptides. These results set a foundation for future studies to quantitatively investigate the MHC II repertoire on DCs generated under different immunization conditions.

AID expression during B-cell development: searching for answers

Expression of activation-induced cytidine deaminase (AID) by germinal center (GC) B cells drives the processes of immunoglobulin (Ig) somatic hypermutation (SHM) and class switch recombination (CSR) necessary for the generation of high affinity IgG serum antibody and the memory B-cell compartment. Increasing evidence indicates that AID is also expressed at low levels in developing B cells but to date, this early, developmentally regulated AID expression has no known function. Does the timing and extent of AID expression in developmentally immature, non-GC B cells provide clues to reveal its physiologic role?

Plexin-D1 is a novel regulator of germinal centers and humoral immune responses

Long-lived humoral immune responses depend upon the generation of memory B cells and long-lived plasma cells during the germinal center (GC) reaction. These memory compartments, characterized by class-switched IgG and high-affinity Abs, are the basis for successful vaccination. We report that a new member of the plexin family of molecules, plexin-D1, controls the GC reaction and is required for secondary humoral immune responses. Plexin-D1 was not required for B cell maturation, marginal zone precursor development, dark and light zone formation, Igλ(+) and Igκ(+) B cell skewing, B1/B2 development, and the initial extrafollicular response. Plexin-D1 expression was increased following B cell activation, and PlxnD1(-/-) mice exhibited defective GC reactions during T-dependent immune activation. PlxnD1(-/-) B cells showed a defect in migration toward the GC chemokines, CXCL12, CXCL13, and CCL19. Accordingly, PlxnD1(-/-) mice exhibited defective production of IgG1 and IgG2b, but not IgG3 serum Ab, accompanied by reductions in long-lived bone marrow plasmacytes and recall humoral memory responses. These data show a new role for immune plexins in the GC reaction and generation of immunologic memory.

Inhibition of terminal differentiation of B cells mediated by CD27 and CD40 involves signaling through JNK

B cells responding to cognate Ag in vivo undergo clonal expansion that is followed by differentiation into Ab-secreting plasma cells or into quiescent restimulable memory. Both these events occur in the germinal center and require that cells exit from proliferation, but the signals that lead to one or the other of these mutually exclusive differentiation pathways have not been definitively characterized. Previous experiments have shown that signals transduced through the TNFRs CD27 and CD40 at the time of B cell stimulation in vitro or in vivo can influence this cell fate decision by inhibiting terminal differentiation and promoting memory. In this study, we show that the PIQED domain of the cytoplasmic tail of murine CD27 and the adapter molecule TNFR-associated factor 2 are involved in this effect. Using pharmacological inhibitors of signaling intermediates, we identify JNK as being necessary and sufficient for the observed inhibition of terminal differentiation. While JNK is involved downstream of CD40, inhibition of the MEK pathway can also partially restore plasma cell generation, indicating that both signaling intermediates may be involved. We also show that inhibition of induction of IFN regulatory factor 4 and B lymphocyte induced maturation protein 1 are downstream events common to both receptors.

Impaired apoptotic cell clearance in the germinal center by Mer-deficient tingible body macrophages leads to enhanced antibody-forming cell and germinal center responses

Germinal centers (GCs) are specialized microenvironments that generate high-affinity Ab-forming cells (AFCs) and memory B cells. Many B cells undergo apoptosis during B cell clonal selection in GCs. Although the factors that regulate the AFC and GC responses are not precisely understood, it is widely believed that dysregulated AFCs and GCs contribute to autoimmunity. The Mer receptor tyrosine kinase (Mer) facilitates macrophage clearance of apoptotic cells. The Tyro-3, Axl, and Mer receptors, including Mer, suppress TLRs and cytokine-mediated inflammatory responses. We report in this study that tingible body macrophages (TBMφs) in GCs express Mer. Compared to C57BL/6 (B6) controls, Mer-deficient (Mer(-/-)) mice had significantly higher AFC, GC, and Th1-skewed IgG2 Ab (especially IgG2c) responses against the T cell-dependent Ag (4-hydroxy-3-nitrophenyl) acetyl-chicken γ globulin. Mer(-/-) mice had a significantly higher percentage of GC B cells on days 9, 14, and 21 postimmunization compared with B6 controls. Significantly increased numbers of apoptotic cells accumulated in Mer(-/-) GCs than in B6 GCs, whereas the number of TBMφs remained similar in both strains. Our data are the first, to our knowledge, to demonstrate a critical role for Mer in GC apoptotic cell clearance by TBMφs and have interesting implications for Mer in the regulation of B cell tolerance operative in the AFC and GC pathways.

Cellular source and molecular form of TNF specify its distinct functions in organization of secondary lymphoid organs

Secondary lymphoid organs provide a unique microenvironment for generation of immune responses. Using a cell type-specific conditional knockout approach, we have dissected contributions of tumor necrosis factor (TNF) produced by B cells (B-TNF) or T cells (T-TNF) to the genesis and homeostatic organization of secondary lymphoid organs. In spleen, lymph nodes and Peyer patches, the cellular source of TNF, and its molecular form (soluble versus membrane-bound) appeared distinct. In spleen, in addition to major B-TNF signal, a complementary T-TNF signal contributed to the microstructure. In contrast, B-TNF predominantly controlled the development of follicular dendritic cells and B-cell follicles in Peyer patches. In lymph nodes, cooperation between TNF expressed by B and T cells was necessary for the maintenance of microarchitecture and for generation of an efficient humoral immune response. Unexpectedly, soluble but not membrane TNF expressed by B cells was essential for the organization of the secondary lymphoid organs. Thus, the maintenance of each type of secondary lymphoid organ is orchestrated by distinct contributions of membrane-bound and soluble TNF produced by B and T lymphocytes.

Novel atypical nucleotide insertions specifically at VH-DH junction generate exceptionally long CDR3H in cattle antibodies

Some IgM cattle antibodies are amongst the largest known to exist in jawed vertebrates where CDR3H size may extend up to 61 amino acids. To understand the origin of such an exceptionally long CDR3H, bovine D(H) gene locus was completely characterized from Holstein cattle that revealed the presence of a total of eight D(H) genes, including D(H)Q52, with a distinct organization in sub-clusters. However, a total of 10 D(H) genes are identified at the polymorphic D-gene locus in cattle that are classified into four families, designated as BovD(H)A, BovD(H)B, BovD(H)C and BovD(H)D. In fetal B-cells, VDJ recombinations encoding long CDR3H (>50 codons) are directly encoded by the single germline V(H) gl.110.20, the longest D(H)2 and the J(H)1 genes, apart from few N- and P-nucleotide additions at the junctions. Further, non J-proximal D(H)7 gene is preferentially expressed in fetal B cells. The adult VDJ recombinations, however, are distinctly remarkable for 'conserved short nucleotide sequence' ('CSNS'; 13-18 nucleotides), of non-V(H) or D(H) gene origin, inserted specifically at V(H)-D(H) junctions resulting in extension of CDR3H size up to 61 codons. Together with P-nucleotides, N-additions (1-9 nucleotides) are noted at both the V(H)-D(H) and D(H)-J(H) junctions. Such 'CSNS' insertions at V(H)-D(H) junction of adult VDJ recombinations encoding exceptionally long CDR3H provide novel mechanism of antibody diversification in cattle, not yet observed in other species. Further, analysis of V(H)-D(H)-J(H) recombinations originating from fetal B-cells reveals the presence of substitution, deletion or addition mutations without prior exposure to external antigen. Thus, somatic hypermutations may contribute towards diversification of the developing nascent antibody repertoire in cattle. In conclusion, the outlined experiments provide novel antibody diversification mechanism via 'CSNS' insertions, specifically at the V(H)-D(H) junction, in generating exceptionally long CDR3H extending up to 61 codons in cattle antibodies.

Statistical mechanical concepts in immunology

Higher organisms, such as humans, have an adaptive immune system that usually enables them to successfully combat diverse (and evolving) microbial pathogens. The adaptive immune system is not preprogrammed to respond to prescribed pathogens. Yet it mounts pathogen-specific responses against diverse microbes and establishes memory of past infections (the basis of vaccination). Although major advances have been made in understanding pertinent molecular and cellular phenomena, the mechanistic principles that govern many aspects of an immune response are not known. We illustrate how complementary approaches from the physical and life sciences can help confront this challenge. Specifically, we describe work that brings together statistical mechanics and cell biology to shed light on how key molecular/cellular components of the adaptive immune system are selected to enable pathogen-specific responses. We hope these examples encourage physical chemists to work at this crossroad of disciplines where fundamental discoveries with implications for human health might be made.

A protease-dependent mechanism for initiating T-dependent B cell responses to large particulate antigens

Ab production is critical for antimicrobial immunity, and the initial step in this process is the binding of Ag to the BCR. It has been shown that small soluble proteins can directly access the lymph node follicles to reach naive B cells, but virus particles must be translocated into follicles via subcapsular sinus macrophages. In this article, we explore how large particulate Ags generate humoral immune responses. Ag-specific follicular B cells rapidly acquired Ag, presented peptide:MHC class II ligands, and produced T-dependent Ab responses following s.c. injection of 1-mum, Ag-linked microspheres, despite the microspheres being confined to the subcapsular sinus. The mechanism of Ag acquisition did not require dendritic cells, subcapsular sinus macrophages, or B cell movement to the subcapsular sinus. Rather, B cell Ag acquisition was protease-dependent, suggesting that some protein Ags are cleaved from the surface of particles to directly initiate humoral immune responses.

Signals that influence T follicular helper cell differentiation and function

Follicular helper T cells have recently emerged as a separate CD4(+) T helper lineage specialised in provision of help to B cells. They develop independently from Th1, Th2 and Th17 cells and are critical for humoral immunity, including the generation of long-lived and high affinity plasma cells and memory cells crucial for long-term protection against infections. A stepwise differentiation programme has emerged in which T cell receptor (TCR) signalling strength, CD28-mediated costimulation, B cell-derived inducible costimulator ligand signals, induction of c-maf and actions of cytokines, including interleukin (IL)-6 and IL-21, lead to upregulation of the transcriptional repressor B cell lymphoma 6 (Bcl-6) that drives T follicular helper (Tfh) cell differentiation. Bcl-6 turns on a repression programme that targets Blimp-1, transcriptional regulators of other helper lineages and microRNAs. Their concerted actions modulate expression of chemokine receptors, surface molecules and cytokines critical for follicular homing and B cell helper functions. Here, we review the nature of Tfh cells providing help to B cells during the two phases of B cell activation that occur in the outer T zone and, for some B cells, in germinal centres (GC). Recent insights into the signalling events that drive terminal differentiation of Tfh cells critical for selecting somatically mutated GC B cells and the consequences of Tfh dysregulation for immunodeficiency and autoimmune pathology are discussed.

T follicular helper cells during immunity and tolerance

Helper T cells are required for the generation of a potent immune response to foreign antigens. Amongst them, T follicular helper (Tfh) cells are specialized in promoting protective, long-lived antibody responses that arise from germinal centers. Within these structures, the specificity of B cell receptors may change, due to the process of random somatic hypermutation aimed at increasing the overall affinity of the antibody response. The danger of emerging self-reactive specificities is offset by a stringent selection mechanism delegated in great part to Tfh cells. Only those B cells receiving survival signals from Tfh cells can exit the germinal centers to join the long-lived pools of memory B cells and bone marrow-homing plasma cells. Thus, a crucial immune tolerance checkpoint to prevent long-term autoantibody production lies in the ability to tolerize Tfh cells and to control positive and negative selection signals delivered by this subset. This review tackles the known mechanisms that ensure Tfh tolerance, many of them shared by other T helper subsets during thymic development and priming, but others unique to Tfh cells. Amongst the latter are checkpoints at the stages of Tfh differentiation, follicular migration, growth, longevity, and quality control of selection signals. Finally, we also discuss the consequences of a breakdown in Tfh tolerance.

Germinal center B cells latently infected with Epstein-Barr virus proliferate extensively but do not increase in number

In this study we show that in long-term persistent infection, Epstein-Barr virus (EBV)-infected cells undergoing a germinal center (GC) reaction in the tonsils are limited to the follicles and proliferate extensively. Despite this, the absolute number of infected cells per GC remains small (average of 3 to 4 cells per germinal center; range, 1 to 9 cells), and only about 38 to 55% (average, 45%) of all GCs carry infected cells. The data fit a model where, on average, cells in the GC divide approximately three times; however, only one progeny cell survives to undergo a further three divisions. Thus, a fraction of cells undergo multiple rounds of division without increasing in numbers; i.e., they die at the same rate that they are dividing. We conclude that EBV-infected cells in the GC undergo the extensive proliferation characteristic of GC cells but that the absolute number is limited either by the immune response or by the availability of an essential survival factor. We suggest that this behavior is a relic of the mechanism by which EBV establishes persistence during acute infection. Lastly, the expression of the viral latent protein LMP1 in GC B cells, unlike in vitro, does not correlate directly with the expression of bcl-2 or bcl-6. This emphasizes our claim that observations made regarding the functions of EBV proteins in cell lines or in transgenic mice should be treated with skepticism unless verified in vivo.

Id2-, RORgammat-, and LTbetaR-independent initiation of lymphoid organogenesis in ocular immunity

The eye is protected by the ocular immunosurveillance system. We show that tear duct–associated lymphoid tissue (TALT) is located in the mouse lacrimal sac and shares immunological characteristics with mucosa-associated lymphoid tissues (MALTs), including the presence of M cells and immunocompetent cells for antigen uptake and subsequent generation of mucosal immune responses against ocularly encountered antigens and bacteria such as Pseudomonas aeruginosa. Initiation of TALT genesis began postnatally; it occurred even in germ-free conditions and was independent of signaling through organogenesis regulators, including inhibitor of DNA binding/differentiation 2, retinoic acid–related orphan receptor γt, lymphotoxin (LT) α1β2–LTβR, and lymphoid chemokines (CCL19, CCL21, and CXCL13). Thus, TALT shares immunological features with MALT but has a distinct tissue genesis mechanism and plays a key role in ocular immunity.

Enhanced humoral immune responses against T-independent antigens in Fc alpha/muR-deficient mice

IgM is an antibody class common to all vertebrates that plays a primary role in host defenses against infection. Binding of IgM with an antigen initiates the complement cascade, accelerating cellular and humoral immune responses. However, the functional role of the Fc receptor for IgM in such immune responses remains obscure. Here we show that mice deficient in Fc alpha/muR, an Fc receptor for IgM expressed on B cells and follicular dendritic cells (FDCs), have enhanced germinal center formation and affinity maturation and memory induction of IgG3(+) B cells after immunization with T-independent (TI) antigens. Moreover, Fc alpha/muR-deficient mice show prolonged antigen retention by marginal zone B (MZB) cells and FDCs. In vitro studies demonstrate that interaction of the IgM immune complex with Fc alpha/muR partly suppress TI antigen retention by MZB cells. We further show that downregulation of complement receptor (CR)1 and CR2 or complement deprivation by in vivo injection with anti-CR1/2 antibody or cobra venom factor attenuates antigen retention by MZB cells and germinal center formation after immunization with TI antigens in Fc alpha/muR(-/-) mice. Taken together, these results suggest that Fc alpha/muR negatively regulates TI antigen retention by MZB cells and FDCs, leading to suppression of humoral immune responses against T-independent antigens.

The tetraspanin protein CD37 regulates IgA responses and anti-fungal immunity

Immunoglobulin A (IgA) secretion by plasma cells in the immune system is critical for protecting the host from environmental and microbial infections. However, the molecular mechanisms underlying the generation of IgA⁺ plasma cells remain poorly understood. Here, we report that the B cell–expressed tetraspanin CD37 inhibits IgA immune responses in vivo. CD37-deficient (CD37−/−) mice exhibit a 15-fold increased level of IgA in serum and significantly elevated numbers of IgA⁺ plasma cells in spleen, mucosal-associated lymphoid tissue, as well as bone marrow. Analyses of bone marrow chimeric mice revealed that CD37–deficiency on B cells was directly responsible for the increased IgA production. We identified high local interleukin-6 (IL-6) production in germinal centers of CD37−/− mice after immunization. Notably, neutralizing IL-6 in vivo reversed the increased IgA response in CD37−/− mice. To demonstrate the importance of CD37—which can associate with the pattern-recognition receptor dectin-1—in immunity to infection, CD37−/− mice were exposed to Candida albicans. We report that CD37−/− mice are evidently better protected from infection than wild-type (WT) mice, which was accompanied by increased IL-6 levels and C. albicans–specific IgA antibodies. Importantly, adoptive transfer of CD37−/− serum mediated protection in WT mice and the underlying mechanism involved direct neutralization of fungal cells by IgA. Taken together, tetraspanin protein CD37 inhibits IgA responses and regulates the anti-fungal immune response.

Antibody, or immunoglobulin (Ig), production by plasma cells in the immune system is important for protecting the host from microbial infections. IgA is the most abundant antibody isotype produced in the body. However, the molecular mechanisms underlying the generation of IgA–producing plasma cells remain poorly understood. We now report that the B cell–expressed protein CD37 regulates IgA immune responses, both in steady-state conditions and during infection. We found highly increased levels of IgA in serum and elevated numbers of IgA⁺ plasma cells in lymphoid tissue of mice that are deficient for CD37 (CD37−/− mice). To demonstrate the importance of CD37 in immunity to infection, CD37−/− mice were exposed to the fungus Candida albicans. C. albicans can cause systemic infection with high mortality in immunocompromised patients. We demonstrate that CD37−/− mice are evidently better protected from infection than wild-type mice, which was dependent on C. albicans–specific IgA antibodies. The underlying mechanism involved direct neutralization of fungal cells by IgA. In summary, the B cell protein CD37 inhibits IgA responses and anti-fungal immunity. This study may contribute to the development of novel immunotherapeutic approaches for invasive fungal disease.

Cytokine-secreting follicular T cells shape the antibody repertoire

High-affinity antibodies are critical for host protection and underlie successful vaccines. The generation of such antibodies requires T cell-dependent help, which mediates germinal center reactions in which mutation and selection of B cells occurs. Using an interleukin 4-reporter system, we show here that CD4(+) follicular helper T cells constituted essentially all of the cytokine-secreting T cells in lymph nodes and were functionally distinct from T cells secreting the same cytokine in peripheral tissues. Follicular helper T cells with different cytokine profiles could be isolated as conjugates with B cells undergoing cytokine-specific immunoglobulin class switching with evidence of somatic hypermutation. Our findings support a model in which B cells compete for cytokines produced by follicular helper T cells that shape the affinity and isotype of the antibody response.

Prevention of autoimmunity and control of recall response to exogenous antigen by Fas death receptor ligand expression on T cells

Mice with mutations in the gene encoding Fas ligand (FasL) develop lymphoproliferation and systemic autoimmune diseases. However, the cellular subset responsible for the prevention of autoimmunity in FasL-deficient mice remains undetermined. Here, we show that mice with FasL loss on either B or T cells had identical life span as littermates, and both genotypes developed signs of autoimmunity. In addition, we show that T cell-dependent death was vital for the elimination of aberrant T cells and for controlling the numbers of B cells and dendritic cells that dampen autoimmune responses. Furthermore, we show that the loss of FasL on T cells affected the follicular dentritic cell network in the germinal centers, leading to an impaired recall response to exogenous antigen. These results disclose the distinct roles of cellular subsets in FasL-dependent control of autoimmunity and provide further insight into the role of FasL in humoral immunity.

A quantitative assessment of primary and secondary immune responses in cattle using a B cell ELISPOT assay

The aim of the study was to build a comprehensive picture of the appearance in the blood stream of Ag-specific plasma cells and memory B cells in the bovine model. For this purpose, we have developed a method allowing the detection and quantification of both cell types within individual calves immunised with ovalbumin. During the primary response, we detected a burst of ovalbumin-specific plasma cells at days 6 and 7 post-immunisation, which was followed by the production of specific Ab, whereas a gradual increase of memory B cells was only detected from day 15. As expected, a boost immunisation performed 7 weeks later induced a quicker and stronger secondary response. Indeed, a burst of plasma cells was detected in the blood at days 3 and 4, which was followed by a strong increase in Ab titres. Furthermore, a burst of memory B cells, and not a gradual increase, was detected at days 5 and 6 post-boost immunisation. Importantly, we showed a strong correlation between the anti-ovalbumin-specific IgG titres detected 5 months after secondary immunisation and the plasma cell numbers detected in the blood at the peak response after secondary immunisation. The detection and quantification of plasma cells following an immunisation/vaccination strategy could constitute a very effective means for predicting the magnitude and longevity of an Ab response.

B lymphocytes: how they develop and function

The discovery that lymphocyte subpopulations participate in distinct components of the immune response focused attention onto the origins and function of lymphocytes more than 40 years ago. Studies in the 1960s and 1970s demonstrated that B and T lymphocytes were responsible primarily for the basic functions of antibody production and cell-mediated immune responses, respectively. The decades that followed have witnessed a continuum of unfolding complexities in B-cell development, subsets, and function that could not have been predicted. Some of the landmark discoveries that led to our current understanding of B lymphocytes as the source of protective innate and adaptive antibodies are highlighted in this essay. The phenotypic and functional diversity of B lymphocytes, their regulatory roles independent of antibody production, and the molecular events that make this lineage unique are also considered. Finally, perturbations in B-cell development that give rise to certain types of congenital immunodeficiency, leukemia/lymphoma, and autoimmune disease are discussed in the context of normal B-cell development and selection. Despite the significant advances that have been made at the cellular and molecular levels, there is much more to learn, and cross-disciplinary studies in hematology and immunology will continue to pave the way for new discoveries.

Lymph node architecture collapse and consequent modulation of FOXO3a pathway on memory T- and B-cells during HIV infection

Lymph nodes (LNs) represent the principal site where antigen-specific memory T- and B-cell responses are primed and differentiated into memory and effector cells. During chronic viral infections such as HIV, these lymphoid tissues undergo substantial structural changes. These changes are mostly caused by an imbalanced cytokine milieu, hyper-immune activation and collagen deposition leading to fibrotic LNs. The structural integrity of the LNs is essential to prime and maintain memory responses. Because cellular signalling events both up- and down-stream of FOXO3a are critical to the generation and the maintenance of lymphocyte memory, this review will focus on the interplay between the deregulation of the immune system caused by the virus and its impact on FOXO3a.

A role for DRAK2 in the germinal center reaction and the antibody response

DAP-related apoptotic kinase-2 (DRAK2), a death-associated protein kinase family member, is highly expressed in B and T lymphocytes in the human and the mouse. To determine whether DRAK2 plays a role in B-cell activation and differentiation, we analyzed germinal centers (GCs) and the specific antibody response to NP in drak2-/- mice immunized with the thymus-dependent (TD) conjugated hapten NP16-CGG. In drak2-/- mice, spleen GCs were normal in size and morphology, but their number was reduced by as much as 5-fold, as compared to their wild-type littermates. This was not due to a defect in B-cell proliferation, as the BrdU uptake was comparable in DRAK2-deficient and wild-type B cells. Rather, the proportion of apoptotic GC B and T cells in drak2-/- mice was significantly higher than that in wild-type control mice, as shown by 7-AAD and terminal deoxynucleotide transferase dUTP nick end labeling (TUNEL) staining. In drak2-/- mice, the generation high affinity IgG antibodies was impaired in spite of the seemingly normal somatic hypermutation and class switch DNA recombination machineries in drak2-/- B cells. In NP16-CGG-immunized drak2-/- mice, T-cell-intrinsic Bcl-xL transgene expression increased the number of GCs and rescued the high affinity IgG response to NP. These findings suggest a novel role for DRAK2 in regulating the GC reaction and the response to TD antigens, perhaps through increased survival of T cells and enhanced B-cell positive selection. They also suggest that DRAK2-deficiency is not involved in regulating intrinsic B-cell apoptosis.

B cells in Sjögren's syndrome: indications for disturbed selection and differentiation in ectopic lymphoid tissue

Primary Sjögren's syndrome (pSS) is an autoimmune disorder characterized by specific pathological features. A hallmark of pSS is B-cell hyperactivity as manifested by the production of autoantibodies, hypergammaglobulinemia, formation of ectopic lymphoid structures within the inflamed tissues, and enhanced risk of B-cell lymphoma. Changes in the distribution of peripheral B-cell subsets and differences in post-recombination processes of immunoglobulin variable region (IgV) gene usage are also characteristic features of pSS. Comparison of B cells from the peripheral blood and salivary glands of patients with pSS with regard to their expression of the chemokine receptors CXCR4 and CXCR5, and their migratory capacity towards the corresponding ligands, CXCL12 and CXCL13, provide a mechanism for the prominent accumulation of CXCR4⁺CXCR5⁺ memory B cells in the inflamed glands. Glandular B cells expressing distinct features of IgV light and heavy chain rearrangements, (re)circulating B cells with increased mutations of cμ transcripts in both CD27^- and CD27⁺ memory B-cell subsets, and enhanced frequencies of individual peripheral B cells containing IgV heavy chain transcripts of multiple isotypes indicate disordered selection and incomplete differentiation processes of B cells in the inflamed tissues in pSS. This may possibly be related to a lack of appropriate censoring mechanisms or different B-cell activation pathways within the ectopic lymphoid structures of the inflamed tissues. These findings add to our understanding of the pathogenesis of this autoimmune inflammatory disorder and may result in new therapeutic approaches.

Germinal center structure and function: lessons from CD19

The germinal center is a critical locus in the production of protective immunity, but its function is poorly understood. Studies of mutant forms of CD19 revealed differences in signaling in different compartments inside the germinal center, and structural findings indicate a selective role in the interaction with follicular dendritic cells in the GC. Loss of these signals leads to surprising changes in germinal center B cells that challenge previous models of GC function.

Analysis of B cell selection in the germinal center reaction during a T-dependent antibody response at a single cell level

The quasimonoclonal mouse is useful to examine B cell selection during T-dependent antibody (Ab) responses because of its limited B cell populations mainly expressing the knockin 17.2.25 V(H)-encoded H chain (V(H)T) paired with the lambda1 or lambda2 L chain. It has been reported that both two V(H)T/lambda1 and V(H)T/lambda2 B cell populations responded to a T-dependent antigen conjugated with a hapten p-nitrophenylacetyl (pNP), but only V(H)T/lambda2 B cells differentiated to secrete high affinity anti-pNP IgG Abs by acquiring a critical mutation (T313A) in the V(H)T. The V(H)T/lambda2 B cells may be more potent in migrating to the germinal centers (GCs) due to about 50-fold higher affinity for pNP than V(H)T/lambda1 B cells. Here, to uncover how V(H)T/lambda2 B cells were preferentially recruited for affinity maturation during the anti-pNP Ab response, we examined the L chain usage and mutation frequency of V(H)T(+) GC B cells at a single cell level. V(H)T/lambda2 B cells bearing the unmutated V(H)T gene were found in the GCs more frequently than V(H)T/lambda1 and mutated V(H)T/lambda2 counterparts in an early phase of the Ab response. In the course of the GC reaction, the number of V(H)T/lambda2 B cells that mutated their V(H)T genes preferentially expanded, and finally V(H)T/lambda2 B cells bearing the T313A mutation occupied V(H)T(+) GC B cell population. Thus, it is suggested that B cells with a higher affinity were selected not only for entry to the GCs but also in the affinity maturation process during a T-dependent Ab response.

Immunology

The concept of forbidden foods that should not be eaten goes back to the Garden of Eden and apart from its religious meanings it may also have foreshadowed the concept of foods that can provoke adverse reactions. Thus we could say that allergic diseases have plagued mankind since the beginning of life on earth. The prophet Job was affected by a condition that following the rare symptoms described by the Holy Bible might be identified as a severe form of atopic dermatitis (AD). The earliest record of an apparently allergic reaction is 2621 B.C., when death from stinging insects was first described by hieroglyphics carved into the walls of the tomb of Pharaoh Menes depicting his death following the sting of a wasp. In 79 A.D., the death of the Roman admiral Pliny the Elder was ascribed to the SO₂-rich gases emanating from the eruption of Mount Vesuvius. Hippocrates (460–377 B.C.) was probably the first to describe how cow’s milk (CM) could cause gastric upset and hives, proposing dietetic measures including both treatment and prevention for CM allergy.

Reciprocal generation of Th1/Th17 and Treg cells by B1 and B2 B cells

Regulatory T (T(reg)) cells are indispensable for maintaining peripheral tolerance, whereas T helper (Th)1 and Th17 cells induce inflammation and tissue destruction. Using Foxp3-GFP knock-in mice, we report a novel regulatory role for B cell subsets in influencing the differentiation of T(reg) versus Th1/Th17 cells. Peritoneal B1 cells strongly promoted T cell proliferation and cytokine secretion when presenting nominal or allogeneic antigens, as compared to conventional follicular B (B2) cells. However, peritoneal B1 cells largely failed to convert naive Foxp3(-)CD4(+) T cells into Foxp3(+) T(reg) cells in the presence of TGF-beta and IL-2, in marked contrast to conventional B2 cells, which excelled in T(reg) conversion. Interestingly, under the same T(reg) conversion conditions, peritoneal B1 cells preferentially promoted Th1 and Th17 cell differentiation. Blockade of CD86 but not CD80 costimulation markedly enhanced T(reg) cell induction by B1 cells. Thus, B cell antigen presentation function is inversely correlated with de novo T(reg) cell induction for these B cell subsets. Our findings suggest that B1 and B2 cell subsets play distinct roles in immune regulation by promoting reciprocal differentiation of T cell lineages.

Antibodies and B Cell Memory in Viral Immunity

Humoral immunity, in particular secreted neutralizing antibodies, is of central importance to protect the body against acutely cytopathic viruses, whereas noncytopathic viruses have found ways of balanced coexistence with the immune system to avoid antibody-mediated elimination. There is evidence that polyspecific "natural" antibodies provide early protection, independent of T cell help. If that line of defense is crossed, T cell-dependent immune responses then generate a humoral memory provided by long-lived plasma cells secreting specific antibodies of adapted avidity and function, i.e., isotype, even in the absence of virus. Secreted protective antibodies of humoral memory provide an efficient line of defense against reinfection and are backed up by specific B and T memory cells of reactive memory. Whereas humoral memory has developed effective antiviral protection, some viruses (i.e., HIV) have managed to develop specific evasion strategies to escape it. Thus, coevolution provides us with some insight into just how substantial antiviral antibodies and memory B cell are in protecting the host from virus infection.

Proapoptotic BH3-only protein Bim is essential for developmentally programmed death of germinal center-derived memory B cells and antibody-forming cells

T cell-dependent B-cell immune responses induce germinal centers that are sites for expansion, diversification, and selection of antigen-specific B cells. During the immune response, antigen-specific B cells are removed in a process that favors the retention of cells with improved affinity for antigen, a cell death process inhibited by excess Bcl-2. In this study, we examined the role of the BH3-only protein Bim, an initiator of apoptosis in the Bcl-2-regulated pathway, in the programmed cell death accompanying an immune response. After immunization, Bim-deficient mice showed persistence of both memory B cells lacking affinity-enhancing mutations in their immunoglobulin genes and antibody-forming cells secreting low-affinity antibodies. This was accompanied by enhanced survival of both cell types in culture. We have identified for the first time the physiologic mechanisms for killing low-affinity antibody-expressing B cells in an immune response and have shown this to be dependent on the BH3-only protein Bim.