The distinct roles of T cell-derived cytokines and a novel follicular dendritic cell-signaling molecule 8D6 in germinal center-B cell differentiation

From neglect to spotlight: the underappreciated role of B cells in cutaneous inflammatory diseases

The skin, covering our entire body as its largest organ, manifests enormous complexities and a profound interplay of systemic and local responses. In this heterogeneous domain, B cells were considered strangers. Yet, recent studies have highlighted their existence in the skin and their distinct role in modulating cutaneous immunity across various immune contexts. Accumulating evidence is progressively shedding light on the significance of B cells in maintaining skin health and in skin disorders. Herein, we integrate current insights on the systemic and local contributions of B cells in three prevalent inflammatory skin conditions: Pemphigus Vulgaris (PV), Systemic Lupus Erythematosus (SLE), and Atopic Dermatitis (AD), underscoring the previously underappreciated importance of B cells within skin immunity. Moreover, we address the potential adverse effects of current treatments used for skin diseases, emphasizing their unintentional consequences on B cells. These comprehensive approaches may pave the way for innovative therapeutic strategies that effectively address the intricate nature of skin disorders.

Requirements of IL-4 during the Generation of B Cell Memory

IL-4 has long been established as a key regulator of Th cells and for promoting effective B cell survival and isotype class switching. Yet, despite having been extensively studied, the specific role of IL-4 in generating humoral memory in vivo is unclear. In this review, we explore the recent studies that unravel the cellular sources and spatiotemporal production of IL-4, the relationship between IL-4 and IL-21 during germinal center responses and the formation of Ab-secreting cells, and the current understanding of whether IL-4 promotes or suppresses memory B cell generation in vitro and in vivo.

Differential Spleen miRNA Expression Profile of Beagle Dogs Infected with Toxocara canis

Toxocara canis is an unnoticed zoonotic helminth that causes severe disease in animals and humans. The spleen has a wide range of immunological functions in protecting the host against infection by many pathogens, but the function of the spleen in T. canis infection is still to be clarified, especially for the role of spleen microRNAs (miRNAs). In this study, deep sequencing of spleen RNA samples of 18 Beagle puppies was conducted to uncover the miRNAs expression profiling at 24 h post-infection (hpi), 96 hpi, and 36 days post infection (dpi). A total of 20, 34, and 19 differentially expressed miRNAs (DEmiRNAs) were identified at 24 hpi, 96 hpi, and 36 dpi, respectively. These DEmiRNAs (e.g., cfa-miR-206, cfa-miR-331, and cfa-miR-339) could play critical roles in Beagle puppies against T. canis infection, such as influencing inflammatory and immune-related cells and cytokines, by regulating target genes that are tightly associated with host immune function and enriched in immune response and immune pathways based on GO annotation and KEGG enrichment analysis. The current study discovered marked alterations of spleen miRNAs after T. canis infection, with potential effects on the pathogenesis of toxocariasis.

The avian retroviral receptor Tva mediates the uptake of transcobalamin bound vitamin B12 (cobalamin)

The Avian sarcoma and leukosis viruses (ASLVs) are important chicken pathogens. Some of the virus subgroups, including ASLV-A and K, utilize the Tva receptor for cell entrance. Though Tva was identified three decades ago, its physiological function remains unknown. Previously, we have noted an intriguing resemblance and orthology between the chicken gene coding for Tva and the human gene coding for CD320, a receptor involved in cellular uptake of transcobalamin (TC) in complex with vitamin B12/cobalamin (Cbl).Here we show that both the transmembrane and the glycosylphosphatidylinositol (GPI)-anchored form of Tva in the chicken cell line DF-1 promotes the uptake of Cbl with help of expressed and purified chicken TC. The uptake of TC-Cbl complex was monitored using an isotope- or fluorophore-labeled Cbl. We show that (i) TC-Cbl is internalized in chicken cells; and (ii) the uptake is lower in the Tva-knockout cells and higher in Tva-overexpressing cells when compared with wild type chicken cells. The relation between physiological function of Tva and its role in infection was elaborated by showing that infection with ASLV subgroups (targeting Tva) impairs the uptake of TC-Cbl, while this is not the case for cells infected with ASLV-B (not recognized by Tva). In addition, exposure of the cells to a high concentration of TC-Cbl alleviates the infection with Tva-dependent ASLV.IMPORTANCE: We demonstrate that the ASLV receptor Tva participates in the physiological uptake of TC-Cbl, because the viral infection suppresses the uptake of Cbl and vice versa. Our results pave the road for future studies addressing the issues: (i) whether a virus infection can be inhibited by TC-Cbl complexes in vivo; and (ii) whether any human virus employs the human TC-Cbl receptor CD320. In broader terms, our study sheds light on the intricate interplay between physiological roles of cellular receptors and their involvement in virus infection.

Synaptic Interactions in Germinal Centers

The germinal center (GC) is a complex, highly dynamic microanatomical niche that allows the generation of high-affinity antibody-producing plasma cells and memory B cells. These cells constitute the basis of long-lived highly protective antibody responses. For affinity maturation to occur, B cells undergo multiple rounds of proliferation and mutation of the genes that encode the immunoglobulin V region followed by selection by specialized T cells called follicular helper T (T_FH) cells. In order to achieve this result, the GC requires spatially and temporally coordinated interactions between the different cell types, including B and T lymphocytes and follicular dendritic cells. Cognate interactions between T_FH and GC B cells resemble cellular connections and synaptic communication within the nervous system, which allow signals to be transduced rapidly and effectively across the synaptic cleft. Such immunological synapses are particularly critical in the GC where the speed of T–B cell interactions is faster and their duration shorter than at other sites. In addition, the antigen-based specificity of cognate interactions in GCs is critical for affinity-based selection in which B cells compete for T cell help so that rapid modulation of the signaling threshold determines the outcome of the interaction. In the context of GCs, which contain large numbers of cells in a highly compacted structure, focused delivery of signals across the interacting cells becomes particularly important. Promiscuous or bystander delivery of positive selection signals could potentially lead to the appearance of long-lived self-reactive B cell clones. Cytokines, cytotoxic granules, and more recently neurotransmitters have been shown to be transferred from T_FH to B cells upon cognate interactions. This review describes the current knowledge on immunological synapses occurring during GC responses including the type of granules, their content, and function in T_FH-mediated help to B cells.

CD47 deficiency ameliorates autoimmune nephritis in Fas(lpr) mice by suppressing IgG autoantibody production

CD47, a self-recognition marker, plays an important role in both innate and adaptive immune responses. To explore the potential role of CD47 in activation of autoreactive T and B cells and the production of autoantibodies in autoimmune disease, especially systemic lupus erythematosus (SLE), we have generated CD47 knockout Fas(lpr) (CD47(-/-) -Fas(lpr) ) mice and examined histopathological changes in the kidneys, cumulative survival rates, proteinuria, extent of splenomegaly and autoantibodies, serum chemistry and immunological parameters. In comparison with Fas(lpr) mice, CD47(-/-) -Fas(lpr) mice exhibit a prolonged lifespan and delayed autoimmune nephritis, including glomerular cell proliferation, basement membrane thickening, acute tubular atrophy and vacuolization. CD47(-/-) -Fas(lpr) mice have lower levels of proteinuria, associated with reduced deposition of complement C3 and C1q, and IgG but not IgM in the glomeruli, compared to age-matched Fas(lpr) mice. Serum levels of antinuclear antibodies and anti-double-stranded DNA antibodies are significantly lower in CD47(-/-) -Fas(lpr) than in Fas(lpr) mice. CD47(-/-) -Fas(lpr) mice also display less pronounced splenomegaly than Fas(lpr) mice. The mechanistic studies further suggest that CD47 deficiency impairs the antigenic challenge-induced production of IgG but not IgM, and that this effect is associated with reduction of T follicular cells and impairment of germinal centre development in lymphoid tissues. In conclusion, our results demonstrate that CD47 deficiency ameliorates lupus nephritis in Fas(lpr) mice via suppression of IgG autoantibody production.

Factors regulating immunoglobulin production by normal and disease-associated plasma cells

Immunoglobulins are molecules produced by activated B cells and plasma cells in response to exposure to antigens. Upon antigen exposure, these molecules are secreted allowing the immune system to recognize and effectively respond to a myriad of pathogens. Immunoglobulin or antibody secreting cells are the mature form of B lymphocytes, which during their development undergo gene rearrangements and selection in the bone marrow ultimately leading to the generation of B cells, each expressing a single antigen-specific receptor/immunoglobulin molecule. Each individual immunoglobulin molecule has an affinity for a unique motif, or epitope, found on a given antigen. When presented with an antigen, activated B cells differentiate into either plasma cells (which secrete large amounts of antibody that is specific for the inducing antigen), or memory B cells (which are long-lived and elicit a stronger and faster response if the host is re-exposed to the same antigen). The secreted form of immunoglobulin, when bound to an antigen, serves as an effector molecule that directs other cells of the immune system to facilitate the neutralization of soluble antigen or the eradication of the antigen-expressing pathogen. This review will focus on the regulation of secreted immunoglobulin by long-lived normal or disease-associated plasma cells. Specifically, the focus will be on signaling and transcriptional events that regulate the development and homeostasis of long-lived immunoglobulin secreting plasma cells.

Apoptosis and other immune biomarkers predict influenza vaccine responsiveness

Despite the importance of the immune system in many diseases, there are currently no objective benchmarks of immunological health. In an effort to identifying such markers, we used influenza vaccination in 30 young (20-30 years) and 59 older subjects (60 to >89 years) as models for strong and weak immune responses, respectively, and assayed their serological responses to influenza strains as well as a wide variety of other parameters, including gene expression, antibodies to hemagglutinin peptides, serum cytokines, cell subset phenotypes and in vitro cytokine stimulation. Using machine learning, we identified nine variables that predict the antibody response with 84% accuracy. Two of these variables are involved in apoptosis, which positively associated with the response to vaccination and was confirmed to be a contributor to vaccine responsiveness in mice. The identification of these biomarkers provides new insights into what immune features may be most important for immune health.

Follicular dendritic cells in health and disease

Follicular dendritic cells (FDCs) are unique immune cells that contribute to the regulation of humoral immune responses. These cells are located in the B-cell follicles of secondary lymphoid tissues where they trap and retain antigens (Ags) in the form of highly immunogenic immune complexes (ICs) consisting of Ag plus specific antibody (Ab) and/or complement proteins. FDCs multimerize Ags and present them polyvalently to B-cells in periodically arranged arrays that extensively crosslink the B-cell receptors for Ag (BCRs). FDC-FcγRIIB mediates IC periodicity, and FDC-Ag presentation combined with other soluble and membrane bound signals contributed by FDCs, like FDC-BAFF, -IL-6, and -C4bBP, are essential for the induction of the germinal center (GC) reaction, the maintenance of serological memory, and the remarkable ability of FDC-Ags to induce specific Ab responses in the absence of cognate T-cell help. On the other hand, FDCs play a negative role in several disease conditions including chronic inflammatory diseases, autoimmune diseases, HIV/AIDS, prion diseases, and follicular lymphomas. Compared to other accessory immune cells, FDCs have received little attention, and their functions have not been fully elucidated. This review gives an overview of FDC structure, and recapitulates our current knowledge on the immunoregulatory functions of FDCs in health and disease. A better understanding of FDCs should permit better regulation of Ab responses to suit the therapeutic manipulation of regulated and dysregulated immune responses.

Unique properties of memory B cells of different isotypes

Memory antibody responses are typically seen to T-cell-dependent antigens and are characterized by the rapid production of high titers of high-affinity antigen-specific antibody. The hallmark of T-cell-dependent memory B cells is their expression of a somatically mutated, isotype-switched B-cell antigen receptor, features that are mainly generated in germinal centers. Classical studies have focused on isotype-switched memory B cells (mainly IgG isotype) and demonstrated their unique intrinsic properties in terms of localization and responsiveness to antigen re-exposure. However, recent advances in monitoring antigen-experienced B cells have revealed the considerable heterogeneity of memory B cells, which include unswitched IgM(+) and/or unmutated memory B cells. The IgM and IgG type memory B cells reside in distinct locations and appear to possess distinct origins and effector functions, together orchestrating humoral memory responses.

IL-21 and IL-10 have redundant roles but differential capacities at different stages of Plasma Cell generation from human Germinal Center B cells

The GC is the anatomical site where antigen-activated B cells differentiate into PC, producing high-affinity antibodies in physiological and pathological states. PC differentiation is regulated by multiple factors within the GC microenvironment, including cytokines. IL-21, a recently identified type I cytokine produced by GC-Th cells, promotes differentiation of human B cells into ISC. In this study, we investigated in detail the functional role of IL-21 in the course of GC-B cell differentiation into terminally differentiated PC compared with that of IL-10, a well-known PC differentiation factor. IL-21 had a greater capacity to initiate PC differentiation from CD77(+) centroblasts than IL-10 by strongly inducing PC transcription factors through activation of STAT3; however, IL-10 was more potent than IL-21 in generating CD138(+) PC from CD20(-)CD38(++) plasmablasts in the terminal stage of GC-B cell differentiation. This differential effect of IL-21 and IL-10 was reflected in receptor expression on B cell subsets emerging in the course of differentiation. Our studies have revealed that IL-21 is a critical decision-maker for driving initial PC differentiation at the stage of CD77(+) centroblasts, yet IL-10 is more effective in producing IgG by generating terminally differentiated CD138(+) PC at the later stage of PC differentiation in the GC.

Characterization of a Novel Gene in the Extended MHC Region of Mouse, NG29/Cd320, a Homolog of the Human CD320

Background

The MHC region of the chromosome contains a lot of genes involved in immune responses. Here we have investigated the mouse NG29/Cd320 gene in the centrometrically extended MHC region of chromosome 17.

Methods

We cloned the NG29 gene by RT-PCR and confirmed the tissue distribution of its gene expression by northern blot hybridization. We generated the NG29 gene expression constructs and polyclonal antibody against the NG29 protein to perform the immunofluorescence, immunoprecipitation and flow cytometric analysis.

Results

The murine NG29 gene and its human homologue, the CD320/8D6 gene, were similar in the gene structure and tissue expression patterns. We cloned the NG29 gene and confirmed its expression in plasma membrane and intracellular compartments by transfecting its expresssion constructs into HEK 293T cells. The immunoprecipitation studies with rabbit polyclonal antibody raised against the NG29-NusA fusion protein indicated that NG29 protein was a glycoprotein of about 45 kDa size. A flow cytometric analysis also showed the NG29 expression on the surface of Raw 264.7 macrophage cell line.

Conclusion

These findings suggested that NG29 gene in mouse extended MHC class II region was the orthologue of human CD320 gene even though human CD320/8D6 gene was located in non-MHC region, chromosome 19p13.

The protein and the gene encoding the receptor for the cellular uptake of transcobalamin-bound cobalamin

The transcobalamin (TC, TCII) receptor (TCblR) on the plasma membrane binds TC- cobalamin (Cbl) and internalizes the complex by endocytosis. This receptor was purified from human placental membranes by affinity chromatography. Tryptic digest of the protein extracted from a sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel and subjected to liquid chromatography/mass spectrometry identified 4 peptides that matched with a membrane protein in the data bank. TCblR belongs to the low-density lipoprotein receptor family, with 2 low-density lipoprotein receptor type A domains separated by a complement-like cysteine-rich region. The 282-amino acid sequence includes a signal peptide of 31 residues, extracellular domain of 198 residues, a transmembrane region of 21 residues, and a cytoplasmic domain of 32 residues. The binding of TC-Cbl does not require the cytoplasmic domain or its orientation in the plasma membrane because the recombinant extracellular domain binds TC-Cbl with high affinity and specificity. The protein is heavily glycosylated and accounts for the 58-kDa size by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The human gene first identified as 8D6A and more recently as CD 320 encoding TCblR is located at p13.2 on the short arm of chromosome 19, spans a length of 6.224 kb, and is composed of 5 exons and 4 introns.

Follicular dendritic cell networks of primary follicles and germinal centers: phenotype and function

Follicular dendritic cells (FDCs) were identified decades ago by their ability to retain immune complexes and more recent findings indicate that they are a source of B cell attractants and trophic factors. New imaging studies have shown that B cells closely associate with their dendritic processes during migration. Here we will review the properties of these specialized follicular stromal cells and provide an update on the requirements for their maturation into phenotypically distinct cells within germinal center light and dark zones. We will then discuss current understanding of how they help support the B cell immune response.

Splenic CD19-CD35+B220+ cells function as an inducer of follicular dendritic cell network formation

Follicular dendritic cells (FDCs) form a reticular FDC network in the lymphoid follicle that is essential for the retention and presentation of native antigens in the form of antigen-antibody immune complexes (ICs) to B cells during secondary immune response. Although the presence of migrating precursors of FDCs has been hypothesized, their entity has not been elucidated. Here we report the identification of murine splenic CD19(-)CD11c(-)CD35(+)B220(+) cells as an inducer of FDC network formation. We demonstrated that CD19(-)-CD11c(-)CD35(+)B220(+) cells, together with stromal cells, had the remarkable ability to form lymphoid-follicle-like structures that contained B220(+)FDC-M1(+) reticular cells originally derived from CD19(-)-CD11c(-)CD35(+)B220(+) cells in the CD35(+) reticulum. Our results indicate that CD19(-)CD11c(-)CD35(+)B220(+) cells function as an inducer of FDC network formation and that the interaction between CD19(-)CD11c(-)CD35(+)B220(+) cells and stromal cells is required to initiate lymphoid follicle formation.

Global transcriptional analysis delineates the differential inflammatory response interleukin-15 elicits from cultured human T cells

OBJECTIVE

Interleukin 15 (IL)-15 controls proliferation and survival of T cells, but its effects and the underlying cellular regulation are not well understood. Previous studies have focused on its effects on short-term T-cell cultures. In view of the potential problems associated with using IL-2 alone in adoptive immunotherapy protocols, we investigated the impact of IL-15 on T-cell cultures and the global transcriptional effects it elicits in such cultures.

MATERIALS AND METHODS

DNA microarrays and flow cytometry were used to examine the differential effect of 20 ng/mL IL-15 on primary serum-free T-cell cultures activated and cultured in the presence of IL-2. Quantitative reverse transcriptase polymerase chain reaction confirmed select microarray data.

RESULTS

IL-15 significantly increased ex vivo expansion of primary human T cells over the entire 11-day expansions without affecting viability. The 1133 genes were consistently differentially expressed among three donor samples. Ontological analysis demonstrated that IL-15 increases expression of genes involved in inflammatory response (e.g., tumor necrosis factor [TNF]-alpha, Oncostatin M, CD40L, and CD33) and apoptosis (e.g., TNF-related apoptosis-inducing ligand). IL-15 also induced expression of four suppressors of cytokine signaling (SOCS) family genes (SOCS1-3, cytokine-inducible SH2-containing protein), which are classical negative regulators of cytokine signaling. IL-15 strongly suppressed the expression of inhibitory natural killer cell receptor genes, including three C-type lectins (KLRB1, KLRC1, and KLRD1), as well as IL-7Ra and Granzyme H. Finally, IL-15 induced differential expression of TNF receptor superfamily members (CD27 and CD30).

CONCLUSION

These findings suggest that exogenous IL-15 may have a potential role in adoptive immunotherapy by both enhancing proliferation and modulating functionality during ex vivo T-cell expansion.

Follicular dendritic cell (FDC)-FcgammaRIIB engagement via immune complexes induces the activated FDC phenotype associated with secondary follicle development

Follicular dendritic cell (FDC)-FcgammaRIIB levels are up-regulated 1-3 days after challenge of actively immunized mice with Ag. This kinetics suggested that memory cells are not driving this response, prompting the hypothesis that immune complex (IC)-FDC interactions lead to FDC activation. To test this, mice passively immunized with anti-OVA Ab were OVA challenged to produce IC. After 3 days, levels of IC, FcgammaRIIB, ICAM-1, and VCAM-1 on FDC were analyzed. FDC were also stimulated with IC in vitro, and mRNA for FcgammaRIIB, ICAM-1, and VCAM-1 was quantified by quantitative RT-PCR. IC labeling in passively immunized WT and FcgammaRIIB-/- mice revealed five to six FDC-reticula per LN midsagittal section. In WT mice, these IC-bearing FDC-reticula corresponded with FDC-reticula labeling for FcgammaRIIB, ICAM-1, and VCAM-1. Increases in these molecules on IC-stimulated FDC were confirmed by flow cytometry. In marked contrast, in FcgammaRIIB-/- mice, no increased VCAM-1 or ICAM-1 was seen on IC-bearing FDC-reticula or on purified FDC. Addition of IC in vitro resulted in dramatic increases in mRNA for FcgammaRIIB, ICAM-1 and VCAM-1 in WT FDC, but not in FDC from FcgammaRIIB-/- mice, 2.4G2-pretreated WT FDC, B cells, or macrophages. Thus, although FDC-FcgammaRIIB was not essential for IC trapping, engagement of FDC-FcgammaRIIB with IC initiated an FDC activation pathway.

Establishment of Lymphotoxin β Receptor Signaling-Dependent Cell Lines with Follicular Dendritic Cell Phenotypes from Mouse Lymph Nodes

Follicular dendritic cells (FDCs) have been shown to play a crucial role in the positive selection of high-affinity B cells that are generated by somatic hypermutation in germinal center (GC). Because of technical difficulties in preparing and maintaining pure FDCs, a role for FDCs in this complicated process has not been fully elucidated. In this study, we established a cell line designated as pFL that retained major FDC phenotypes from a three-dimensional culture of mouse lymph node cells. pFL cells proliferated slowly in response to an agonistic anti-lymphotoxin beta receptor mAb and TNF-alpha. A more rapidly growing clone, named FL-Y, with similar requirements for growth was isolated from a long-term culture of pFL. Analysis of surface markers in these two cell lines by immunostaining, flow cytometry, and DNA microarray revealed the expression of genes, including those of CD21, FcgammaRIIB, lymphotoxin beta receptor, ICAM-1, VCAM-1, IL-6, and C4, which have been shown to be characteristic of FDCs. In addition, B cell-activating factor was expressed in these two cell lines. At the pFL or FL-Y:B cell ratio of 1:100, the cell lines markedly sustained B cell survival and Ab production during 2 wk of culture, while most B cells collapsed within 1 wk in the absence of the FDC-like cells. Interestingly, expression of typical GC markers, Fas and GL-7, was notably augmented in B cells that were cocultured with Th cells on these two cell lines. Thus, pFL and FL-Y cells may be useful for providing insight into the functional role for FDCs in GC.

FDC-Specific Functions of p55TNFR and IKK2 in the Development of FDC Networks and of Antibody Responses

The FDC-specific molecular signals required in the formation of FDC networks, B cell follicles, and germinal centers (GCs) have remained poorly understood. We used FDC-specific gene targeting to investigate the function of p55TNFR and IKK2 in lymphoid organ structure and function. Here we show that FDC-specific expression of p55TNFR is necessary and sufficient to promote FDC network and B cell follicle formation, restore the expression of CXCL13 and VCAM-1/ICAM-1 in FDCs, and lead to productive GCs. Notably, FDC-specific disruption of IKK2 does not affect formation of FDC networks. Yet, after antigen engagement or immune complex (IC) deposition, FDCs lacking IKK2 fail to upregulate VCAM-1 and ICAM-1, and GCs remain sterile. These findings demonstrate that IKK2-independent function of p55TNFR on FDCs is sufficient to support the development of FDC networks and GCs, while FDC-specific IKK2 is indispensable for the generation of efficient humoral immune responses.

Lymphotoxin-beta receptor-dependent genes in lymph node and follicular dendritic cell transcriptomes

Affinity maturation and Ab class switches occur in lymphoid germinal centers (GCs), in which differentiation and maintenance depend on lymphotoxin (LT) signaling and include differentiation of follicular dendritic cells (FDCs). The events leading to FDC and GC maturation are poorly defined. Using several approaches of functional genomics, we enumerated transcripts affected in mice by suppressing LT beta receptor (LTbetaR) signaling and/or overrepresented in FDC-enriched GC isolates. Protein expression analysis of 3 of 12 genes both enriched in FDCs and down-regulated by LTbetaR signaling suppression validated them as FDC markers. Functional analysis of one of these three, clusterin, suggests a role as an FDC-derived trophic factor for GC B cells. Hence, the set of genes presented in this study includes markers emanating from LTbetaR signaling and transcripts relevant to GC and FDC function.

BAFF supports human B cell differentiation in the lymphoid follicles through distinct receptors

B cell-activating factor of the tumor necrosis factor family (BAFF/BLys) plays a critical role in B cell survival and immune responses through its three receptors: BAFF receptor (BAFF-R/BR3), transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B cell maturation antigen (BCMA). Using specific antibodies, we have investigated the expression of BAFF-R on human tonsillar B cells and their functional roles in naive and germinal center (GC) B cell differentiation. Our studies show that BAFF-R is the dominant receptor on naive B cells. However, three receptors are differentially modulated during in vitro GC-B cell differentiation. BAFF-R expression increased initially and then decreased with a corresponding induction of TACI and BCMA expression during differentiation to plasma cells (PCs). Consistently, blocking of BAFF-R alone with specific mAb inhibited GC-B cell proliferation and PC generation in the early period of their differentiation, whereas depletion of BAFF with TACI-Ig exhibited consistent inhibition throughout the differentiation. Finally, histological and molecular analyses of human tonsil tissue revealed that follicular dendritic cells produce BAFF. In conclusion, BAFF in the GC plays an important role through more than one receptor, and the three known receptors are differentially modulated as GC-B cells differentiate to PCs.

Complement receptors regulate differentiation of bone marrow plasma cell precursors expressing transcription factors Blimp-1 and XBP-1

Humoral immune responses are thought to be enhanced by complement-mediated recruitment of the CD21-CD19-CD81 coreceptor complex into the B cell antigen receptor (BCR) complex, which lowers the threshold of B cell activation and increases the survival and proliferative capacity of responding B cells. To investigate the role of the CD21-CD35 complement receptors in the generation of B cell memory, we analyzed the response against viral particles derived from the bacteriophage Qbeta in mice deficient in CD21-CD35 (Cr2(-/-)). Despite highly efficient induction of early antibody responses and germinal center (GC) reactions to immunization with Qbeta, Cr2(-/-) mice exhibited impaired antibody persistence paralleled by a strongly reduced development of bone marrow plasma cells. Surprisingly, antigen-specific memory B cells were essentially normal in these mice. In the absence of CD21-mediated costimulation, Qbeta-specific post-GC B cells failed to induce the transcriptional regulators Blimp-1 and XBP-1 driving plasma cell differentiation, and the antiapoptotic protein Bcl-2, which resulted in failure to generate the precursor population of long-lived plasma cells residing in the bone marrow. These results suggest that complement receptors maintain antibody responses by delivery of differentiation and survival signals to precursors of bone marrow plasma cells.

How do follicular dendritic cells interact intimately with B cells in the germinal centre?

The germinal centre is a dynamic microenvironment where antigen-activated B cells rapidly expand and differentiate, generating plasma cells and memory B cells. These cellular events are accompanied by dramatic changes in the antibody molecules that undergo somatic hypermutation and isotype switching. Follicular dendritic cells (FDCs) are the stromal cells located in the germinal centre. Although the capacity of FDCs to present antigen to B cells through antigen-antibody complexes has been recognized for many years, additional critical functions of FDCs have only recently been recognized. FDCs prevent apoptosis of germinal centre B cells and stimulate cellular interaction and proliferation. Here, we review the FDC signalling molecules that have recently been identified, some of which offer potential therapeutic targets for autoimmune diseases and B-cell lymphomas.

Follicular dendritic cells produce IL-15 that enhances germinal center B cell proliferation in membrane-bound form

Factors that control the survival and proliferation of Ag-stimulated B cells within the germinal center (GC) are crucial for humoral immune responses with high affinity Abs against infectious agents. The follicular dendritic cell (FDC) is known as a key cellular component of the GC microenvironment for GC-B cell survival and proliferation. In this study, we report that IL-15 is produced by human FDC in vivo and by an FDC cell line, FDC/HK cells, in vitro. IL-15 is captured by IL-15Ralpha on the surface of FDC/HK cells. The surface IL-15 is functionally active and augments GC-B cell proliferation. Because GC-B cells have the signal-transducing components (IL-2/15Rbetagamma), but not a receptor for binding of soluble IL-15 (IL-15Ralpha), IL-15 signaling is possibly transduced by transpresentation from FDCs to GC-B cells via cell-cell contact. Together, these results suggest that IL-15 from FDC, in membrane-bound form, plays an important role in supporting GC-B cell proliferation, proposing a new target for immune modulation as well as treatment of B cell tumors of GC origin.

Two different molecular defects in the Tva receptor gene explain the resistance of two tvar lines of chickens to infection by subgroup A avian sarcoma and leukosis viruses

The subgroup A to E avian sarcoma and leukosis viruses (ASLVs) are highly related and are thought to have evolved from a common ancestor. These viruses use distinct cell surface proteins as receptors to gain entry into avian cells. Chickens have evolved resistance to infection by the ASLVs. We have identified the mutations responsible for the block to virus entry in chicken lines resistant to infection by subgroup A ASLVs [ASLV(A)]. The tva genetic locus determines the susceptibility of chicken cells to ASLV(A) viruses. In quail, the ASLV(A) susceptibility allele tva(s) encodes two forms of the Tva receptor; these proteins are translated from alternatively spliced mRNAs. The normal cellular function of the Tva receptor is unknown; however, the extracellular domain contains a 40-amino-acid, cysteine-rich region that is homologous to the ligand binding region of the low-density lipoprotein receptor (LDLR) proteins. The chicken tva(s) cDNAs had not yet been fully characterized; we cloned the chicken tva cDNAs from two lines of subgroup A-susceptible chickens, line H6 and line 0. Two types of chicken tva(s) cDNAs were obtained. These cDNAs encode a longer and shorter form of the Tva receptor homologous to the Tva forms in quail. Two different defects were identified in cDNAs cloned from two different ASLV(A)-resistant inbred chickens, line C and line 7(2). Line C tva(r) contains a single base pair substitution, resulting in a cysteine-to-tryptophan change in the LDLR-like region of Tva. This mutation drastically reduces the binding affinity of Tva(R) for the ASLV(A) envelope glycoproteins. Line 7(2) tva(r2) contains a 4-bp insertion in exon 1 that causes a change in the reading frame, which blocks expression of the Tva receptor.

Novel follicular dendritic cell molecule, 8D6, collaborates with CD44 in supporting lymphomagenesis by a Burkitt lymphoma cell line, L3055

The lymphoid follicle is a specialized microenvironment for the differentiation of antigen (Ag)–activated B cells; the major stromal cell components in lymphoid follicle are the follicular dendritic cells (FDCs). At the same time, most of the B-cell lymphomas originate from the germinal center, and the generation and blast transformation of B-cell lymphoma occurs in close association with FDCs in the early stage of tumorigenesis. To study the functional roles of FDCs in lymphomagenesis, we established an inducible tumor model. The human B-cell lymphoma cell line, L3055, formed solid tumors only when inoculated with an FDC line, HK. In addition, 2 FDC-signaling molecules (FDC-SMs), a novel protein 8D6 and 4G10/CD44, are required for tumor formation in vivo, because monoclonal antibodies (mAbs) specific to these 2 proteins inhibited lymphomagenesis completely when they were inoculated with L3055 and HK cells. However, these 2 FDC-SMs have distinct functional roles in tumor formation. FDC-SM-8D6 enhances L3055 cell proliferation, whereas FDC-SM-4G10/CD44 inhibits its apoptosis. Identification of the functional roles of these critical FDC-SMs may lead to the discovery of therapeutic drugs that suppress the survival and growth of lymphoma cells.

Identification and characterization of a novel nasopharyngeal carcinoma-associated peptide: NAP-1

Nasopharyngeal carcinoma (NPC) is one of the most commons cancers in Southeast Asia and Southern China. Several NPC-associated genes have been so far described and here we describe the identification and the characterization of a novel nasopharyngeal carcinoma-associated peptide: NAP-1. NAP-1 was identified with the human genome draft searching method combined with nested PCR mapping of the chromosome 4q13 region. NAP-1 encodes an 85 amino acid alkaline peptide with a calculated isoelectric point of 9.3, three phosphorilation sites and a proline-rich region. Northern blot analysis revealed that NAP-1 is expressed as a 0.6 kb transcript in normal lymph nodes and trachea. In addition, reverse transcription (RT)-PCR showed that NAP-1 is expressed not only in NPC but in normal nasopharynx (NP) and various other tumors and tissues of the head and neck including: tonsils, lymph nodes, carcinoma of the tonsil, T cell lymphomas, squamous cell carcinoma of the hard palate, papilloma of the nasopharynx, nasopharyngitis, lymphoma of the tongue root and follicular dendritic cells (FDC). In addition, NAP-1 is not expressed in normal tissues or tumors from other anatomical regions and was not expressed by NPC cell lines. Surprisingly, differential RT-PCR demonstrated decreased expression of NAP-1 in NPC compared with paired NP biopsies in 42.5 % of cases (17 out of 40). In addition, in situ hybridization and immunohistochemistry demonstrated that NAP-1 is expressed by S100+ CD35+ FDCs of the germinal center and not in other normal immune cells infiltrating NP or NPC. Therefore, it is likely that NAP-1 is secreted by FDC in the NP and may play an immune modulatory role in NPC.

Aging and innate immune cells

The innate immune system serves an important role in preventing microbial invasion. However, it experiences significant changes with advancing age. Among the age-associated changes are: Aged macrophages and neutrophils have impaired respiratory burst and reactive nitrogen intermediates as a result of altered intracellular signaling, rendering them less able to destroy bacteria. Aged neutrophils are also less able to respond to rescue from apoptosis. Aged dendritic cells (DC) are less able to stimulate T and B cells. The altered T cell stimulation is a result of changes in human leukocyte antigen expression and cytokine production, and lower B cell stimulation is a result of changes in DC immune complex binding. Natural killer (NK) cells from the elderly are less capable of destroying tumor cells. NK T cells increase in number and have greater interleukin-4 production with age. Levels of various complement components are also altered with advancing age.

Galectin-3 Mediates IL-4-Induced Survival and Differentiation of B Cells: Functional Cross-Talk and Implications during Trypanosoma cruzi Infection

The role of transcription factors in B cell survival and differentiation has been delineated during the last years. However, little is known about the intermediate signals and the intracellular pathways that control these events. In this study, we provide evidence both in vitro and in vivo, showing that galectin-3 (Gal-3), a beta-galactoside-binding protein, is a critical mediator of B cell differentiation and survival. Although Gal-3 is not expressed in resting B cells from normal mice, its expression is markedly induced after activation with stimuli such as IL-4 and CD40 cross-linking. These signals promote survival and block the final differentiation of these cells, thus allowing the rising of a memory B cell phenotype. In addition, Gal-3 is expressed in B cells from Trypanosoma cruzi-infected mice, which received signals for activation and differentiation in vivo. By using an antisense strategy, we determined that Gal-3 is a critical signal mediating the effects of IL-4 on B cell fate. Blockade of intracellular Gal-3 in vitro abrogated IL-4-induced survival of activated B cells, favoring the differentiation toward a plasma cell pathway. Moreover, B cells with restrained endogenous Gal-3 expression failed to down-regulate the Blimp-1 transcription factor after IL-4 stimulation. Finally, inhibition of Gal-3 in vivo skewed the balance toward plasma cell differentiation, which resulted in increased Ig production and parasite clearance during T. cruzi infection. Thus, the present study provides evidence of a novel role for Gal-3 as an intracellular mediator of B cell survival and a checkpoint in IL-4-induced B cell commitment toward a memory phenotype.

Are follicular dendritic cells really good for nothing?

Follicular dendritic cells (FDCs), which reside in the primary B-cell follicles and germinal centres of lymphoid tissues, can sequester antigen in the form of immune complexes and are thought to be pivotal to the germinal-centre reaction and the maintenance of immunological memory. But, many recent studies question the importance of FDCs and their bound immune complexes in B-cell responses. This article asks whether we can truly rule out a requirement for these cells in host defence.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

Impaired germinal center formation and recall T-cell-dependent immune responses in mice lacking the costimulatory ligand B7-H2

B7-H2, which is expressed constitutively on B cells and binds the inducible costimulator (ICOS) on antigen-activated T cells, is a member of the B7 family of costimulatory ligands. We have inactivated B7-H2 in the mouse. B7-H2-/- mice generate normal populations of B and T cells in their various lymphoid organs but have lower basal levels of heavy chain class-switched antibodies in their sera. These mice are able to mount normal immune responses to both type I and type II T-cell-independent antigens. However, their pattern of responses to a T-cell-dependent antigen is altered, with greatly reduced production of antigen-specific heavy chain class-switched antibodies, the levels of which could not be elevated even with repeated immunizations. This suggests a critical role for B7-H2 in the recall phases of the immune response. Germinal center formation is also impaired in the mutant mice. While B cells from the mutant mice could response normally to anti-IgM, anti-CD40, and lipopolysaccharide stimulation, the production of T-helper-type II cytokines such as interleukin-4 (IL-4) and IL-10 by primed CD4+ T cells from mutant mice were reduced. This indicated that the defects in humoral responses and germinal center formation in B7-H2-deficient mice are due to the lack of T-cell-mediated help to the B cells. Hence, B7-H2 on B cells is important for recruiting T-cell help via its interaction with ICOS and plays a critical role in costimulating humoral immune responses.

Transcriptional regulatory cascades controlling plasma cell differentiation

Plasma cells are the terminally differentiated effector cells of the B lymphocyte lineage. Recently, studies using genetically altered mice and analyses of global gene expression programs have significantly expanded our understanding of the molecular mechanisms regulating plasmacytic differentiation. Specific molecular components of a multistep cascade of transcriptional regulators have been identified. Furthermore, two transcriptional regulators, X box binding protein-1 (XBP-1) and B lymphocyte induced maturation protein-1 (Blimp-1), have been shown to be necessary for plasmacytic differentiation. In addition to providing a mechanistic basis for the induction of genes necessary for immunoglobulin secretion, cessation of cell cycle and other phenotypic changes characteristic of terminally differentiated plasma cells, these studies have led to the important concept that plasmacytic differentiation involves repression of regulators, such as Bcl-6 and Pax5, that are necessary to maintain the earlier developmental phenotype of activated, germinal center B cells. This review describes our current understanding of the transcriptional cascades regulating terminal differentiation of B cells.

3C8 antigen is a novel protein expressed by human follicular dendritic cells

Although the pivotal role of follicular dendritic cells (FDCs) in humoral immune responses has been demonstrated, little is known at the molecular level of how FDCs contribute to the organogenesis, B cell differentiation, and regulation of T cell functions in the germinal center. By immunizing with FDC-like cells, we have developed a monoclonal antibody (MAb), which stains the germinal centers in tonsil section. In the current study, the target cell of MAb 3C8 was identified as FDC by confocal scanning fluorescence microscopy. Unlike other MAbs against FDC, MAb 3C8 does not cross-react with bone marrow-derived blood cells. Amino acid sequencing of NH(2)-terminal region of immunoprecipitated 3C8 Ag reveals that 3C8 is a novel FDC protein. Further studies of 3C8 molecule will shed light on the cellular origin of FDC and reveal unknown functions of FDC.

Interleukin-4 biases differentiation of B cells from Trypanosoma cruzi-infected mice and restrains their fratricide: role of Fas ligand down-regulation and MHC class II-transactivator up-regulation

In the present work, we demonstrate that interleukin (IL)-4 is able to rescue B cells from Trypanosoma cruzi-infected mice, counteracting the strong apoptotic signals that these cells received in vivo. We have observed that IL-4 restrains the apoptosis of immunoglobulin (Ig)M(+) and IgG(+) B cells from infected and normal mice without inducing them to proliferate. In addition, IL-4 does not modify the quantity or quality of the antibodies secreted by B cells from infected mice, as it blocks their terminal differentiation to plasma cells and favors memory pathway. It is interesting that the protective effect of IL-4 over B cells from infected mice is mediated, at least partly, by the down-regulation of Fas ligand (FasL) expression, which leads to interference in the apoptosis executed by these B cells through the Fas/FasL death pathway. Accordingly, a marked up-regulation of the "FasL gene repressor" class II transactivator was observed, suggesting that this would be one mechanism underlying the IL-4-mediated FasL down-regulation.

Distinct response of human B cell subpopulations in recognition of an innate immune signal, CpG DNA

Innate immunity has recently gained renewed interest in its ability to regulate adaptive immunity. Among the innate immune signals, CpG DNA has revealed its potential as a vaccine adjuvant. However, the cellular mechanism for the effect of CpG DNA on the humoral immune response is not well understood. Here, we investigated the effects of CpG DNA on human B cell differentiation using highly purified B cell subsets: naive, germinal center (GC), and memory B cells. In the in vitro culture system that mimics the primary or secondary immune response in vivo, CpG DNA markedly augmented the proliferation and generation of plasma cells from naive and memory B cells. CpG DNA dramatically increased plasma cell generation from GC B cells. However, CpG DNA did not have effect on memory B cell generation from GC B cells. These results suggest that CpG DNA potentiates the B cell adaptive immune response by enhancing terminal differentiation, but does not affect the generation of memory B cells.

FDC-SP, a novel secreted protein expressed by follicular dendritic cells

To define better the molecular basis for follicular dendritic cell (FDC) function, we used PCR-based cDNA subtraction to identify genes specifically expressed in primary FDC isolated from human tonsils. In this work we report the discovery of a novel gene encoding a small secreted protein, which we term FDC-SP (FDC secreted protein). The FDC-SP gene lies on chromosome 4q13 adjacent to clusters of proline-rich salivary peptides and C-X-C chemokines. Human and mouse FDC-SP proteins are structurally unique and contain a conserved N-terminal charged region adjacent to the leader peptide. FDC-SP has a very restricted tissue distribution and is expressed by activated FDCs from tonsils and TNF-alpha-activated FDC-like cell lines, but not by B cell lines, primary germinal center B cells, or anti-CD40 plus IL-4-activated B cells. Strikingly, FDC-SP is highly expressed in germinal center light zone, a pattern consistent with expression by FDC. In addition, FDC-SP is expressed in leukocyte-infiltrated tonsil crypts and by LPS- or Staphylococcus aureus Cowan strain 1-activated leukocytes, suggesting that FDC-SP can also be produced in response to innate immunity signals. We provide evidence that FDC-SP is posttranslationally modified and secreted and can bind to the surface of B lymphoma cells, but not T lymphoma cells, consistent with a function as a secreted mediator acting upon B cells. Furthermore, we find that binding of FDC-SP to primary human B cells is markedly enhanced upon activation with the T-dependent activation signals such as anti-CD40 plus IL-4. Together our data identify FDC-SP as a unique secreted peptide with a distinctive expression pattern within the immune system and the ability to specifically bind to activated B cells.

Germinal center B cells constitute a predominant physiological source of IL-4: implication for Th2 development in vivo

Protective immunity depends upon the capability of the immune system to properly adapt the response to the nature of an infectious agent. CD4(+) Th cells are implicated in this orchestration by secreting a polarized pattern of cytokines. Although Th2 development in animal models and in human cells in vitro to a large extent depends on IL-4, the nature of the cells that provide the initial IL-4 in vivo is still elusive. In this report, we describe the anatomical localization as well as the identity of IL-4-producing cells in human tonsil, a representative secondary lymphoid organ. We demonstrate that IL-4 production is a normal and intrinsic feature of germinal center (GC) B cells. We also show that expression of IL-4 is highly confined to the GCs, in which the B cells constitute the prevalent cellular source. Furthermore, immunofluorescence analysis of colon mucosa reveals a strikingly similar pattern of IL-4-expressing cells compared with tonsils, demonstrating that IL-4 production from GC B cells is not a unique feature of the upper respiratory tract. Our results show that GCs provide the most appropriate microenvironment for IL-4-dependent Th2 polarization in vivo and imply a critical role for GC B cells in this differentiation process.

Disturbances in peripheral blood B cell subpopulations in autoimmune patients

A variety of cell surface markers are being used to identify B cell subpopulations in peripheral blood. Currently at least eight subpopulations have been identified. Analyses of healthy individuals indicate that in general the various B cell subpopulations exist in relatively similar ratios in unrelated individuals. It has been demonstrated that B lymphocyte homeostasis is disturbed during infection and autoimmune disease. In this review we compare the distribution of B cell subpopulations in the peripheral blood of patients with systemic lupus erythematosus, rheumatoid arthritis and primary Sjogren's syndrome with each other, and with healthy individuals. The different autoimmune diseases have distinct changes in the B cell subpopulations. Understanding the nature of these B subpopulation signatures will potentially impact understanding the mechanisms of disease, diagnosis and therapy.

Plasma cells: finding new light at the end of B cell development

Plasma cells are cellular factories devoted entirely to the manufacture and export of a single product: soluble immunoglobulin (Ig). As the final mediators of a humoral response, plasma cells play a critical role in adaptive immunity. Although intense effort has been devoted to studying the regulation and requirements for early B cell development, little information has been available on plasma cells. However, more recent work-including studies on genetically altered mice and data from microarray analyses-has begun to identify the regulatory cascades that initiate and maintain the plasma cell phenotype. This review will summarize our current understanding of the molecules that regulate commitment to a plasma cell fate and those that mediate plasma cell function.