CD21/CD19 Coreceptor Signaling Promotes B Cell Survival during Primary Immune Responses

Role of complement in humoral immunity

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) after solid organ transplantation remains an unsolved problem and leads to poor early and late patient outcomes. The complement system is a well recognized pathogenic mediator of AMR. Herein, we review the known molecular mechanisms of disease and results from ongoing clinical testing of complement inhibitors after solid organ transplant.

RECENT FINDINGS

Activation and regulation of the complement cascade is critical not only for the terminal effector function of donor-specific antibodies, but also for the regulation of T and B cell subsets to generate the antidonor humoral response. Donor-specific antibodies (DSA) have heterogenous features, as are their interactions with the complement system. Clinical testing of complement inhibitors in transplant patients have shown good safety profiles but mixed efficacy to date.

SUMMARY

The complement cascade is a critical mediator of AMR and clinical trials have shown early promising results. With the steady emergence of novel complement inhibitors and our greater understanding of the molecular mechanisms linking complement and AMR, there is greater optimism now for new prognostic and therapeutic tools to deploy in transplant patients with AMR.

Reversal of splicing infidelity is a pre-activation step in B cell differentiation

Introduction

B cell activation and differentiation is central to the adaptive immune response. Changes in exon usage can have major impacts on cellular signaling and differentiation but have not been systematically explored in differentiating B cells.

Methods

We analyzed exon usage and intron retention in RNA-Seq data from subsets of human B cells at various stages of differentiation, and in an in vitro laboratory model of B cell activation and differentiation (Epstein Barr virus infection).

Results

Blood naïve B cells were found to have an unusual splicing profile, with unannotated splicing events in over 30% of expressed genes. Splicing changed substantially upon naïve B cell entry into secondary lymphoid tissue and before activation, involving significant increases in exon commitment and reductions in intron retention. These changes preferentially involved short introns with weak splice sites and were likely mediated by an overall increase in splicing efficiency induced by the lymphoid environment. The majority of transcripts affected by splicing changes showed restoration of encoded conserved protein domains and/or reduced targeting to the nonsense-mediated decay pathway. Affected genes were enriched in functionally important immune cell activation pathways such as antigen-mediated signaling, cell cycle control and mRNA processing and splicing.

Discussion

Functional observations from donor B cell subsets in progressive states of differentiation and from timecourse experiments using the in vitro model suggest that these widespread changes in mRNA splicing play a role in preparing naïve B cells for the decisive step of antigen-mediated activation and differentiation.

The validity of animal models to explore the pathogenic role of the complement system in multiple sclerosis: A review

Animal models of multiple sclerosis (MS) have been extensively used to characterize the disease mechanisms in MS, as well as to identify potential pharmacologic targets for this condition. In recent years, the immune complement system has gained increased attention as an important effector in the pathogenesis of MS. Evidence from histological, serum, and CSF studies of patients supports an involvement of complement in both relapsing-remitting and progressive MS. In this review, we discuss the history and advances made on the use of MS animal models to profile the effects of the complement system in this condition. The first studies that explored the complement system in the context of MS used cobra venom factor (CVF) as a complement depleting agent in experimental autoimmune encephalomyelitis (EAE) Lewis rats. Since then, multiple mice and rat models of MS have revealed a role of C3 and the alternative complement cascade in the opsonization and phagocytosis of myelin by microglia and myeloid cells. Studies using viral vectors, genetic knockouts and pharmacologic complement inhibitors have also shown an effect of complement in synaptic loss. Antibody-mediated EAE models have revealed an involvement of the C1 complex and the classical complement as an effector of the humoral response in this disease. C1q itself may also be involved in modulating microglia activation and oligodendrocyte differentiation in these animals. In addition, animal and in vitro models have revealed that multiple complement factors may act as modulators of both the innate and adaptive immune responses. Finally, evidence gathered from mice models suggests that the membrane attack complex (MAC) may even exert protective roles in the chronic stages of EAE. Overall, this review summarizes the importance of MS animal models to better characterize the role of the complement system and guide future therapeutic approaches in this condition.

New Insights into the Role of the Complement System in Human Viral Diseases

The complement system (CS) is part of the human immune system, consisting of more than 30 proteins that play a vital role in the protection against various pathogens and diseases, including viral diseases. Activated via three pathways, the classical pathway (CP), the lectin pathway (LP), and the alternative pathway (AP), the complement system leads to the formation of a membrane attack complex (MAC) that disrupts the membrane of target cells, leading to cell lysis and death. Due to the increasing number of reports on its role in viral diseases, which may have implications for research on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), this review aims to highlight significant progress in understanding and defining the role of the complement system in four groups of diseases of viral etiology: (1) respiratory diseases; (2) acute liver failure (ALF); (3) disseminated intravascular coagulation (DIC); and (4) vector-borne diseases (VBDs). Some of these diseases already present a serious global health problem, while others are a matter of concern and require the collaboration of relevant national services and scientists with the World Health Organization (WHO) to avoid their spread.

The Complement System in Ovarian Cancer: An Underexplored Old Path

Ovarian cancer is one of the most lethal gynecological cancers. Current therapeutic strategies allow temporary control of the disease, but most patients develop resistance to treatment. Moreover, although successful in a range of solid tumors, immunotherapy has yielded only modest results in ovarian cancer. Emerging evidence underscores the relevance of the components of innate and adaptive immunity in ovarian cancer progression and response to treatment. Particularly, over the last decade, the complement system, a pillar of innate immunity, has emerged as a major regulator of the tumor microenvironment in cancer immunity. Tumor-associated complement activation may support chronic inflammation, promote an immunosuppressive microenvironment, induce angiogenesis, and activate cancer-related signaling pathways. Recent insights suggest an important role of complement effectors, such as C1q or anaphylatoxins C3a and C5a, and their receptors C3aR and C5aR1 in ovarian cancer progression. Nevertheless, the implication of these factors in different clinical contexts is still poorly understood. Detailed knowledge of the interplay between ovarian cancer cells and complement is required to develop new immunotherapy combinations and biomarkers. In this context, we discuss the possibility of targeting complement to overcome some of the hurdles encountered in the treatment of ovarian cancer.

RNA-binding protein Ptbp1 is essential for BCR-mediated antibody production

The RNA-binding protein polypyrimidine tract-binding protein-1 (Ptbp1) binds to the pyrimidine-rich sequence of target RNA and controls gene expression via post-transcriptional regulation such as alternative splicing. Although Ptbp1 is highly expressed in B lymphocytes, its role to date is largely unknown. To clarify the role of Ptbp1 in B-cell development and function, we generated B-cell-specific Ptbp1-deficient (P1BKO) mice. B-cell development in the bone marrow, spleen and peritoneal cavity of the P1BKO mice was nearly normal. However, the P1BKO mice had significantly lower levels of natural antibodies in serum compared with those of the control mice. To investigate the effect of Ptbp1 deficiency on the immune response in vivo, we immunized the P1BKO mice with T-cell-independent type-2 (TI-2) antigen NP-Ficoll and T-cell-dependent (TD) antigen NP-CGG. We found that B-cell-specific Ptbp1 deficiency causes an immunodeficiency phenotype due to defective production of antibody against both TI-2 and TD antigen. This immunodeficiency was accompanied by impaired B-cell receptor (BCR)-mediated B-cell activation and plasmablast generation. These findings demonstrate that Ptbp1 is essential for the humoral immune response.

Adjuvants: What a Difference 15 Years Makes!

Vaccination is a critical tool in modern animal production and key to maintaining animal health. Adjuvants affect the immune response by increasing the rate, quantity, or quality of the protective response generated by the target antigens. Although adjuvant technology dates back to the nineteenth century, there was relatively little improvement in adjuvant technology before the late twentieth century. With the discovery of molecular pathways that regulate the timing, quantity, and quality of the immune response, new technologies are focused on bringing safer, more effective, and inexpensive adjuvants to commercial use.

Carbohydrate-dependent B cell activation by fucose-binding bacterial lectins

Bacterial lectins are typically multivalent and bind noncovalently to specific carbohydrates on host tissues to facilitate bacterial adhesion. Here, we analyzed the effects of two fucose-binding lectins, BambL from Burkholderia ambifaria and LecB from Pseudomonas aeruginosa, on specific signaling pathways in B cells. We found that these bacterial lectins induced B cell activation, which, in vitro, was dependent on the cell surface expression of the B cell antigen receptor (BCR) and its co-receptor CD19, as well as on spleen tyrosine kinase (Syk) activity. The resulting release of intracellular Ca²⁺ was followed by an increase in the cell surface abundance of the activation marker CD86, augmented cytokine secretion, and subsequent cell death, replicating all of the events that are observed in vitro upon canonical and antigen-mediated B cell activation. Moreover, injection of BambL in mice resulted in a substantial, BCR-independent loss of B cells in the bone marrow with simultaneous, transient enlargement of the spleen (splenomegaly), as well as an increase in the numbers of splenic B cells and myeloid cells. Together, these data suggest that bacterial lectins can initiate polyclonal activation of B cells through their sole capacity to bind to fucose.

The Role of Allograft Inflammatory Factor-1 in the Effects of Experimental Diabetes on B Cell Functions in the Heart

Diabetes mellitus (DM) often causes chronic inflammation, hypertrophy, apoptosis and fibrosis in the heart and subsequently leads to myocardial remodeling, deteriorated cardiac function and heart failure. However, the etiology of the cardiac disease is unknown. Therefore, we assessed the gene expression in the left ventricle of diabetic and non-diabetic mice using Affymetrix microarray analysis. Allograft inflammatory factor-1 (AIF-1), one of the top downregulated B cell inflammatory genes, is associated with B cell functions in inflammatory responses. Real-time reverse transcriptase-polymerase chain reaction confirmed the Affymetrix data. The expression of CD19 and AIF-1 were downregulated in diabetic hearts as compared to control hearts. Using in vitro migration assay, we showed for the first time that AIF-1 is responsible for B cell migration as B cells migrated to GFP-AIF-1-transfected H9C2 cells compared to empty vector-transfected cells. Interestingly, overexpression of AIF-1 in diabetic mice prevented streptozotocin-induced cardiac dysfunction, inflammation and promoted B cell homing into the heart. Our results suggest that AIF-1 downregulation inhibited B cell homing into diabetic hearts, thus promoting inflammation that leads to the development of diabetic cardiomyopathy, and that overexpression of AIF-1 could be a novel treatment for this condition.

Expression and Function of Tetraspanins and Their Interacting Partners in B Cells

Tetraspanins are transmembrane proteins that modulate multiple diverse biological processes, including signal transduction, cell–cell communication, immunoregulation, tumorigenesis, cell adhesion, migration, and growth and differentiation. Here, we provide a systematic review of the involvement of tetraspanins and their partners in the regulation and function of B cells, including mechanisms associated with antigen presentation, antibody production, cytokine secretion, co-stimulator expression, and immunosuppression. Finally, we direct our focus to the signaling mechanisms, evolutionary conservation aspects, expression, and potential therapeutic strategies that could be based on tetraspanins and their interacting partners.

Targeting complement-mediated immunoregulation for cancer immunotherapy

Complement was initially discovered as an assembly of plasma proteins "complementing" the cytolytic activity of antibodies. However, our current knowledge places this complex system of several plasma proteins, receptors, and regulators in the center of innate immunity as a bridge between the initial innate responses and adaptive immune reactions. Consequently, complement appears to be pivotal for elimination of pathogens, not only as an early response defense, but by directing the subsequent adaptive immune response. The discovery of functional intracellular complement and its roles in cellular metabolism opened novel avenues for research and potential therapeutic implications. The recent studies demonstrating immunoregulatory functions of complement in the tumor microenvironment and the premetastatic niche shifted the paradigm on our understanding of functions of the complement system in regulating immunity. Several complement proteins, through their interaction with cells in the tumor microenvironment and in metastasis-targeted organs, contribute to modulating tumor growth, antitumor immunity, angiogenesis, and therefore, the overall progression of malignancy and, perhaps, responsiveness of cancer to different therapies. Here, we focus on recent progress in our understanding of immunostimulatory vs. immunoregulatory functions of complement and potential applications of these findings to the design of novel therapies for cancer patients.

Evidence for B Cell Exhaustion in Chronic Graft-versus-Host Disease

Chronic graft-versus-host disease (cGvHD) remains a major complication of allogeneic hematopoietic stem cell transplantation (HSCT). A number of studies support a role for B cells in the pathogenesis of cGvHD. In this study, we report the presence of an expanded population of CD19+CD21− B cells with features of exhaustion in the peripheral blood of patients with cGvHD. CD21− B cells were significantly increased in patients with active cGvHD compared to patients without cGvHD and healthy controls (median 12.2 versus 2.12 versus 3%, respectively; p < 0.01). Compared with naïve (CD27−CD21+) and classical memory (CD27+CD21+) B cells, CD19+CD21− B cells in cGvHD were CD10 negative, CD27 negative and CD20hi, and exhibited features of exhaustion, including increased expression of multiple inhibitory receptors such as FCRL4, CD22, CD85J, and altered expression of chemokine and adhesion molecules such as CD11c, CXCR3, CCR7, and CD62L. Moreover, CD21− B cells in cGvHD patients were functionally exhausted and displayed poor proliferative response and calcium mobilization in response to B-cell receptor triggering and CD40 ligation. Finally, the frequencies of circulating CD21− B cells correlated with cGvHD severity in patients after HSCT. Our study further characterizes B cells in chronic cGVHD and supports the use of CD21−CD27−CD10− B cell frequencies as a biomarker of disease severity.

Transcriptome Analysis of B Cell Immune Functions in Periodontitis: Mucosal Tissue Responses to the Oral Microbiome in Aging

Evidence has shown activation of T and B cells in gingival tissues in experimental models and in humans diagnosed with periodontitis. The results of this adaptive immune response are noted both locally and systemically with antigenic specificity for an array of oral bacteria, including periodontopathic species, e.g., Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans. It has been recognized through epidemiological studies and clinical observations that the prevalence of periodontitis increases with age. This report describes our studies evaluating gingival tissue transcriptomes in humans and specifically exploiting the use of a non-human primate model of naturally occurring periodontitis to delineate gingival mucosal tissue gene expression profiles focusing on cells/genes critical for the development of humoral adaptive immune responses. Patterns of B cell and plasmacyte genes were altered in aging healthy gingival tissues. Substantial increases in a large number of genes reflecting antigen-dependent activation, B cell activation, B cell proliferation, and B cell differentiation/maturation were observed in periodontitis in adults and aged animals. Finally, evaluation of the relationship of these gene expression patterns with those of various tissue destructive molecules (MMP2, MMP9, CTSK, TNFα, and RANKL) showed a greater frequency of positive correlations in healthy tissues versus periodontitis tissues, with only MMP9 correlations similar between the two tissue types. These results are consistent with B cell response activities in healthy tissues potentially contributing to muting the effects of the tissue destructive biomolecules, whereas with periodontitis this relationship is adversely affected and enabling a progression of tissue destructive events.

Bacillus anthracis Spore Surface Protein BclA Mediates Complement Factor H Binding to Spores and Promotes Spore Persistence

Spores of Bacillus anthracis, the causative agent of anthrax, are known to persist in the host lungs for prolonged periods of time, however the underlying mechanism is poorly understood. In this study, we demonstrated that BclA, a major surface protein of B. anthracis spores, mediated direct binding of complement factor H (CFH) to spores. The surface bound CFH retained its regulatory cofactor activity resulting in C3 degradation and inhibition of downstream complement activation. By comparing results from wild type C57BL/6 mice and complement deficient mice, we further showed that BclA significantly contributed to spore persistence in the mouse lungs and dampened antibody responses to spores in a complement C3-dependent manner. In addition, prior exposure to BclA deletion spores (ΔbclA) provided significant protection against lethal challenges by B. anthracis, whereas the isogenic parent spores did not, indicating that BclA may also impair protective immunity. These results describe for the first time an immune inhibition mechanism of B. anthracis mediated by BclA and CFH that promotes spore persistence in vivo. The findings also suggested an important role of complement in persistent infections and thus have broad implications.

We discovered an immune modulatory mechanism of Bacillus anthracis mediated by the spore surface protein BclA. We showed for the first time that BclA mediated the binding of complement factor H, a major negative regulator of complement, to the surface of spores. The binding led to the down-regulation of complement activities in vitro and in an animal model. Using mice deficient in complement components, we further showed that BclA promoted spore persistence in the mouse lungs and impaired antibody responses against spores in a complement-dependent manner. We further provided evidence suggesting a role of BclA in the development of protective immunity against lethal B. anthracis challenges. These findings draw attention to a previously understudied aspect of the complement system. They suggest that in addition to conferring resistance to complement-mediated killing and phagocytosis, complement inhibition by pathogens have long-term consequences with respect to persistent infections and development of protective immunity. Considering a growing list of microbial pathogens capable of modulating complement activities, our findings have broad implications.

Mesenchymal stem cells promote leukaemic cells aberrant phenotype from B-cell acute lymphoblastic leukaemia

BACKGROUND AND OBJECTIVES

The role of bone marrow-mesenchymal stem cells (BM-MSC) in leukaemic cell control is controversial. The purpose of this work was to evaluate BM-MSC role regarding the viability, proliferation and immunophenotype of normal B-cell precursors from control (Ct) patients and leukaemic cells from B-acute lymphoblastic leukaemia (B-ALL) patients.

PATIENTS AND METHODS

BM-MSC were isolated and characterised from voluntary donors. Mononuclear cells isolated from Ct and B-ALL bone marrow samples were cultured in the presence or absence of BM-MSC for 7days. Cell viability was determined with LIVE/DEAD and proliferation index evaluated by CFSE labelling. Cell population immunophenotypes were characterised by estimating CD19, CD10, CD20 and CD45 antigens by flow cytometry.

RESULTS

After co-culture, B-ALL cells exhibited higher viability (20-40%) as compared to just cells (3-10%). Ct and B-ALL absolute cell counts were higher in the presence of BM-MSC (Ct: 25/mm(3)cf8/mm(3), B-ALL: 15/mm(3)cf3/mm(3)). Normal B-cell subpopulations in co-culture had increased expression of CD19 and CD10 (Pre-pre B) and CD45 and CD20 antigens (Pre-B). B-ALL cells co-cultured with BM-MSC showed an increase in CD19 and CD20, although the greatest increase was observed in the CD10 antigen.

CONCLUSIONS

Lymphoid cell maintenance, at early stages of differentiation, was significantly promoted by BM-MSC in normal and leukaemic cells. Co-cultures also modulated the expression of antigens associated with the B-ALL asynchronous phenotype as CD10 co-expressed with CD19 and CD20. To our knowledge, this is the first time that CD10, CD19 and CD20 leukaemic antigens have been reported as being regulated by BM-MSC.

Multiple checkpoint breach of B cell tolerance in Rasgrp1-deficient mice

Lymphopenic hosts offer propitious microenvironments for expansion of autoreactive B and T cells. Despite this, many lymphopenic hosts do not develop autoimmune disease, suggesting that additional factors are required for breaching self-tolerance in the setting of lymphopenia. Mice deficient in guanine nucleotide exchange factor Rasgrp1 develop a lymphoproliferative disorder with features of human systemic lupus erythematosus. Early in life, Rasgrp1-deficient mice have normal B cell numbers but are T lymphopenic, leading to defective homeostatic expansion of CD4 T cells. To investigate whether B cell-intrinsic mechanisms also contribute to autoimmunity, Rasgrp1-deficient mice were bred to mice containing a knockin autoreactive BCR transgene (564Igi), thereby allowing the fate of autoreactive B cells to be assessed. During B cell development, the frequency of receptor-edited 564Igi B cells was reduced in Rasrp1-deficient mice compared with Rasgrp1-sufficient littermate control mice, suggesting that tolerance was impaired. In addition, the number of 564Igi transitional B cells was increased in Rasgrp1-deficient mice compared with control mice. Immature 564Igi B cells in bone marrow and spleen lacking RasGRP1 expressed lower levels of Bim mRNA and protein, suggesting that autoreactive B cells elude clonal deletion during development. Concomitant with increased serum autoantibodies, Rasgrp1-deficient mice developed spontaneous germinal centers at 8-10 wk of age. The frequency and number of 564Igi B cells within these germinal centers were significantly increased in Rasgrp1-deficient mice relative to control mice. Taken together, these studies suggest that autoreactive B cells lacking Rasgrp1 break central and peripheral tolerance through both T cell-independent and -dependent mechanisms.

The immunoglobulin, IgG Fc receptor and complement triangle in autoimmune diseases

Immunoglobulin G (IgG)-mediated activation of complement and IgG Fc receptors (FcγRs) are important defense mechanisms of the innate immune system to ward off infections. However, the same mechanisms can drive severe and harmful inflammation, when IgG antibodies react with self-antigens in solution or tissues, as described for several autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, and immune vasculitis. More specifically, IgG immune complexes (ICs) can activate all three pathways of the complement system resulting in the generation of C3 and C5 cleavage products that can activate a panel of different complement receptors on innate and adaptive immune cells. Importantly, complement and FcγRs are often co-expressed on inflammatory immune cells such as neutrophils, monocytes, macrophages or dendritic cells and act in concert to mediate the inflammatory response in autoimmune diseases. In this context, the cross-talk between the receptor for the anaphylatoxin C5a, i.e. C5ar1 (CD88) and FcγRs is of major importance. Recent data suggest a model of bidirectional regulation, in which CD88 acts upstream of FcγRs and sets the threshold for FcγR-dependent effector responses by regulating the ratio between activating and inhibitory FcγRs. Vice versa, FcγR ligation can either amplify or block C5aR-mediated effector functions, depending on whether IgG IC aggregate activating or inhibitory FcγRs. Further, complement and FcγRs cooperate on B cells and on follicular dendritic cells to regulate the development of autoreactive B cells, their differentiation into plasma cells and, eventually, the production of autoantibodies. Here, we will give an update on recent findings regarding this complex regulatory network between complement and FcγRs, which may also regulate the inflammatory response in allergy, cancer and infection.

The Fas/CD95 Receptor Regulates the Death of Autoreactive B Cells and the Selection of Antigen-Specific B Cells

Cell death receptors have crucial roles in the regulation of immune responses. Here we review recent in vivo data confirming that the Fas death receptor (TNFSR6) on B cells is important for the regulation of autoimmunity since the impairment of only Fas function on B cells results in uncontrolled autoantibody production and autoimmunity. Fas plays a role in the elimination of the non-specific and autoreactive B cells in germinal center, while during the selection of antigen-specific B cells different escape signals ensure the resistance to Fas-mediated apoptosis. Antigen-specific survival such as BCR or MHCII signal or coreceptors (CD19) cooperating with BCR inhibits the formation of death inducing signaling complex. Antigen-specific survival can be reinforced by antigen-independent signals of IL-4 or CD40 overproducing the anti-apoptotic members of the Bcl-2 family proteins.

The autoreactivity of B cells in hereditary angioedema due to C1 inhibitor deficiency

Patients with hereditary angioedema (HAE) tend to produce autoantibodies and have a propensity to develop immunoregulatory disorders. We characterize the profile of autoantibodies in a group of HAE patients and investigate their memory B cells' phenotype and activation status. We studied the activity status phenotype, Toll-like receptor (TLR)-9 expression and total phosphotyrosine in B cells isolated from HAE patients. Additionally, the following autoantibodies were assessed in the serum of 61 HAE patients: anti-nuclear, rheumatoid factor, anti-cardiolipin, anti-tissue transglutaminase, anti-endomysial, anti-Saccharomyces cerevisiae, anti-thyroid and anti-neutrophil cytoplasmic antibodies. In 47·5% of HAE patients we detected at least one of the tested autoantibodies. Expression of CD69, CD5 and CD21 was found to be significantly higher on memory B cells from HAE patients compared to healthy controls (4·59 ± 4·41 versus 2·06 ± 1·81, P = 0·04, 8·22 ± 7·17 versus 3·65 ± 3·78, P = 0·05, 2·43 ± 0·54 versus 1·92 ± 0·41, P = 0·01, respectively). Total phosphotyrosine in B cells from HAE patients was significantly higher compared to healthy controls (4·8 ± 1·1 versus 2·7 ± 1·3, P = 0·0003). Memory B cells isolated from the HAE group contained higher amounts of TLR-9 compared to healthy controls (8·17 ± 4·1 versus 4·56 ± 1·6, P = 0·0027). Furthermore, the expression of TLR-9 in memory B cells from HAE patients with autoantibodies was significantly higher than the control group (10 ± 4·7 versus 4·56 ± 1·6, P = 0·0002) and from that in HAE patients without autoantibodies (10 ± 4·7 versus 5·8 ± 0·9, P = 0·036). HAE patients have enhanced production of autoantibodies due most probably to the increased activation of B cells, which was found to be in association with a high expression of TLR-9.

Vaccine-induced antibody responses in patients with carcinoma

Cancer vaccines based on defined antigens are capable of inducing antibodies that recognize and kill tumor cells. Antibodies are ideally suited to address minimal residual disease, and vaccination in an adjuvant setting may favorably influence the outcome of a disease. The present article gives a short summary of antibody production by B cells, and the mechanism of action of antibodies, as well as a description of the current methods for measuring antibody responses and for assessing their antitumor efficacy in the context of clinical trials. It concludes with an overview of antibody responses induced by vaccines based on structurally defined tumor-associated antigens tested in patients with carcinomas. Correlation between antibody responses, T-cell responses and clinical outcome has been noted in a few studies, signaling the importance of vaccine design and adjuvants to exploit the interactions of the innate and adaptive immune system. However, humoral responses, which may provide a surrogate marker for T-helper responses and simplify monitoring of large Phase III trials, are still not or incompletely explored in many vaccination trials.

Complement and its role in innate and adaptive immune responses

The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges. As such, a study of its functions allows insight into the molecular underpinnings of host-pathogen interactions as well as the organization and orchestration of the host immune response. This review attempts to summarize the roles that complement plays in both innate and adaptive immune responses and the consequences of these interactions on host defense.

Uncoupling CD21 and CD19 of the B-cell coreceptor

Complement receptors (CRs) CD21 and CD35 form a coreceptor with CD19 and CD81 on murine B cells that when coligated with the B-cell receptor lowers the threshold of activation by several orders of magnitude. This intrinsic signaling role is thought to explain the impaired humoral immunity of mice bearing deficiency in CRs. However, CRs have additional roles on B cells independent of CD19, such as transport of C3-coated immune complexes and regulation of C4 and C3 convertase. To test whether association of CR with CD19 is necessary for their intrinsic activation-enhancing role, knockin mice expressing mutant receptors, Cr2(Delta/Deltagfp), that bind C3 ligands but do not signal through CD19 were constructed. We found that uncoupling of CR and CD19 significantly diminishes survival of germinal center B cells and secondary antibody titers. However, B memory is less impaired relative to mice bearing a complete deficiency in CRs on B cells. These findings confirm the importance of interaction of CR and CD19 for coreceptor activity in humoral immunity but identify a role for CR in B-cell memory independent of CD19.

Complement and humoral immunity

The complement system was discovered almost a century ago as an important effector in antibody-dependent killing of microorganisms. Since this early period much was learned aboutthe biochemistry and structure of complement proteins and their function in mediating inflammation. More recently, a prominent role for complement was identified in linkage of innate and adaptive immunity. In this review, I will discuss our current understanding of the importance of complement in enhancing the humoral immune response to both model antigens and pathogens. As discussed below, it is evident that the complement system participates in marking of "foreign" pathogens and "presenting" them to B cells in a manner that enhances both antibody production and long-term memory. In this special issue of Vaccine, we see examples of how complement is critical in the immune response to bacterial and viral pathogens. Moreover, the finding that most organisms have co-evolved proteins to evade complement detection underscores its importance in host protection.

Effects of Diesel Exhaust Particles on Antigen-Presenting Cells and Antigen-Specific Th Immunity in Mice

Diesel exhaust particles (DEP) exacerbate antigen-related airway inflammation and hyperresponsiveness in mice; however, the mechanisms remain undefined. The present study characterized more precisely which pathways and cellular events of the allergic response are amplified by DEP in view of the maturation/activation/function of antigen-presenting cells (APC) and the antigen-specific Th response. We evaluated the effects of DEP on the phenotype and function of bone marrow-derived dendritic cells (BMDC) in vitro and on the expression pattern of APC-related molecules in the murine lung in the presence or absence of antigen in vivo. Also, we tested the effects of in vivo DEP co-exposure with antigen on the splenic antigen-specific Th response in the context of cytokine production. DEP significantly increased both allogeneic and antigen (ovalbumin: OVA)-specific syngeneic T-cell proliferation in vitro. In addition, an in vivo experiment showed that repetitive pulmonary exposure to DEP plus antigen (OVA) increased the numbers of MHC class II+cells and those expressing CD11c, DEC205 (DC markers), CD80, CD86 (co-stimulatory molecules), F4/80 (a macrophage marker), and CD19 (a B-cell differentiation antigen) in the lung as compared to that of others (vehicle, DEP, or OVA). Furthermore, an ex vivo assay system demonstrated that splenic mononuclear cells primed by DEP plus OVA produced a greater amount of interleukin (IL)-4, IL-5, and IL-13 after in vitro antigen stimulation compared to those primed by the other treatments. In conclusion, enhancement of allergic responses by DEP can be explained via two novel mechanisms, i.e., enhancement effects on APC including DC and on antigen-specific Th response, which culminate in the promotion of local and systemic dysregulated Th immunity.

Complement receptors 1 and 2 influence the immune environment in a B cell receptor-independent manner

The CD21/35 proteins are complement receptors implicated in controlling and interpreting activation states of the innate and acquired immune responses. One defect of CD21/35(-/-) animals is depressed production of Ag-specific IgG3 which we show is evident in vivo but not in vitro. Gene expression profiles obtained from naive wild-type and CD21/35(-/-) splenocytes demonstrated enhanced expression of inflammatory mediators from CD11b(+) splenocytes in the CD21/35(-/-) animals. Splenocyte populations between wild-type and CD21/35(-/-) mice were similar except for a moderate increase in GR1(low)CD31(+) immature myeloid cells. Furthermore, depletion of neutrophils and other GR1-expressing cells alleviates elevated inflammatory gene expression in the CD21/35(-/-) spleen. Complement activation also plays a key role in the differential gene expression observed in the CD21/35-deficient mouse as depletion of C3 or inhibition of C3a receptor signaling within the animal returned inflammatory gene expression within the spleen to wild-type levels. Finally, C3 depletion before immunization allowed for the enhanced production of Ag-specific IgG3 production in the CD21/35(-/-) mouse compared with mock-depleted animals. These data suggest that the overall environment of the CD21/35(-/-) spleen is quite different from that of the wild-type animal perhaps due to altered complement convertase activity. This difference may be responsible for a number of the phenotypes ascribed to the deficiency of CD21/35 proteins on B cells and follicular dendritic cells.

A population of CD19highCD45R-/lowCD21low B lymphocytes poised for spontaneous secretion of IgG and IgA antibodies

Ab responses to selected Ags are produced by discrete B cell populations whose presence and functional relevance vary along the ontogeny. The earliest B lineage-restricted precursors in gestational day 11 mouse embryos display the CD19(+)CD45R/B220(-) phenotype. Phenotypically identical cells persist throughout gestation and in postnatal life, in parallel to the later-arising, CD19(+)CD45R(+) B cells. Very early after birth, the CD19(+)CD45R(-) B cell subset included high frequencies of spontaneously Ig-secreting cells. In the adult spleen, a small subset of CD19(high)CD45R(-/low)IgM(+/-)IgD(-)CD21/Cr2(-/low) cells, which was detected in perifollicular areas, displayed genetic and phenotypical traits of highly differentiated B cells, and was enriched in IgG- and IgA-secreting plasma cells. In vitro differentiation and in vivo adoptive transfer experiments of multipotent hemopoietic progenitors revealed that these CD19(high)CD45R(-/low) B cells were preferentially regenerated by embryo-, but not by adult bone marrow-, derived progenitors, except when the latter were inoculated into newborn mice. Both the early ontogenical emergence and the natural production of serum Igs, are shared features of this CD19(high)CD45R(-/low) B cell population with innate-like B lymphocytes such as B1 and marginal zone B cells, and suggest that the new population might be related to that category.

Protective DNA vaccination against myelin oligodendrocyte glycoprotein is overcome by C3d in experimental autoimmune encephalomyelitis

Complement receptor 2 (CR2) and its physiological ligand, C3d, known for its molecular adjuvant property on the immune response, exhibit opposite effects with regard to autoimmunity. Although CR2 has been implicated in maintaining self-tolerance, recent studies reported a role for C3d signaling to CR2 in tolerance breakdown to self-antigens and the initiation of inflammatory autoimmune pathologies. In the present study, we have investigated the effect of C3d in a model of tolerogenic DNA vaccination encoding the myelin oligodendrocyte glycoprotein (MOG-DNA) which protected mice from the induction of an experimental autoimmune encephalomyelitis (EAE). We show that fusing two or three copies of C3d to MOG overcomes the protective effect of DNA vaccination. Multimeric C3d was able to revert the unresponsiveness state of specific T cells induced by MOG-DNA, independently of a modification in the Th1/Th2 cytokine pattern. Interestingly, the adjuvant effect of C3d was not sufficient to boost the anti-MOG antibody response after DNA vaccination. These findings suggest that C3d might be involved in self-tolerance breakdown and could contribute to the pathogenesis of central nervous system autoimmune disorders.

Possible therapeutic applications of single-chain antibodies in systemic autoimmune diseases

B cells participate in the induction and maintenance of systemic autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus, via production of pathogenic autoantibodies, contributing to the formation of immune complexes. Immune complex deposition in the kidney and joints causes inflammation and organ destruction, and chemokine production enhances T cell activation and tissue damage. The development of the disorder depends on several factors, for example, genetic susceptibility, environmental factors or immune dysregulation. Traditional therapies, which aimed at the alleviation of symptoms, are giving way to biological therapies with the potential of disrupting disease progression. This article focuses on antibody therapies, especially on the applications of single-chain antibodies, as new biological agents for the treatment of systemic autoimmune disorders.

The Many Effects of Complement C3- and C5-Binding Proteins in Renal Injury

The complement system is an important component of the innate immune system and a modulator of adaptive immunity. The entire complement system is focused on C3 and C5. Thus, there are proteins that activate C3 and C5, those that regulate this activation, and those that transduce the effects of C3 and C5 activation products; each can affect the kidney in renal injury. The normal kidney has the inherent capacity to protect itself from complement activation through cellular expression of decay-accelerating factor, membrane cofactor protein (in human beings), and Crry (in rodents). In addition, plasma factor H protects vascular spaces in the kidney. Although the main function of these proteins is to limit complement activation, there is now considerable evidence that they can transduce signals on engagement in immune cells. The G-protein-coupled 7-span transmembrane receptors for C3a and C5a, and the integral membrane complement receptors (CR) for C3b, iC3b, and C3dg, are expressed outside the kidney, particularly in cells of hematopoietic and immune lineage. These are important in renal injury through their infiltration of the kidney and/or by affecting kidney-directed immune responses. There is mounting evidence that intrinsic glomerular and tubular cell C3aR and C5aR expression and activation also can affect renal injury. CR1 on podocytes and the beta2 integrins CR3 and CR4 in kidney dendritic cells have functions that remain poorly defined. Cells of the kidney also have the capacity to produce and activate their own complement proteins. Thus, intrinsic renal cells express decay-accelerating factor, membrane cofactor protein, Crry, C3aR, C5aR, CR1, CR3, and CR4. These can be engaged by C3 and C5 activation products derived from systemic and local pools in renal injury. Given their capacity to provide signals that influence kidney cellular behavior, their activation can have substantial effects in renal injury. Defining these in a cell- and disease-specific fashion is an exciting challenge for future research.

Polymorphism of immunoglobulin enhancer element HS1,2A: allele *2 associates with systemic sclerosis. Comparison with HLA-DR and DQ allele frequency

OBJECTIVE

To investigate the relationship of the polymorphic enhancer HS1,2 central to the 3' enhancer complex regulatory region (IgH3'EC) of the immunoglobulin heavy chain genes with systemic sclerosis (SSc) disease and compare it with HLA-DR and DQ associations.

METHODS

A total of 116 patients with SSc were classified as diffuse (dSSc) or limited (lSSc), and as carriers of antitopoisomerase I (anti-Scl70) or anticentromere (ACA) antibodies. Allele and genotype frequencies were assessed in the population as a whole and in the two major subsets, dSSc and lSSc. The concentration of peripheral blood immunoglobulin levels was also determined and analysed according to the genotypes.

RESULTS

The analysis of genotypes for the four alleles of the HS1,2A enhancer showed an increased frequency of allele *2 in the SSc cohort highly significant versus controls (57% vs. 40%, p<0.0001). Considering the autoantibody pattern, we found that the frequency of the 2/2 genotype was increased in ACA+ patients (42%) and anti-Scl70+ patients (31%) compared with the control group (15%). The differences of allelic frequencies among dSSc versus lSSc or ACA+ versus anti-Scl70+ patients were not significant, although highly significant when comparing each subgroup with the control group. HLA-DRB1*11 and DQB1*03 associated with SSc. No association was seen between HS1,2A enhancer polymorphism and HLA alleles.

CONCLUSIONS

These data confirm there was an increased risk of having SSc in carriers of allele *2, suggesting an intriguing function of this polymorphism for B-cell regulation.

Control of B lymphocyte apoptosis by the transcription factor NF-kappaB

B cells maintain homeostasis by balancing cell viability and cell death. B lymphocytes are susceptible to mitochondria- and receptor-initiated cell death at various stages of peripheral differentiation and during immune responses. The inducible transcription factor NF-kappaB enhances cell viability by activating genes that counteract both cell-death pathways. This review uses characteristic features of NF-kappaB activation and downregulation to provide insight into the regulation of B cell apoptosis in the periphery. In particular, the temporal patterns of NF-kappaB induction, differences between Rel family members, and the intersection between canonical and noncanonical signaling pathways in keeping B cells alive are discussed.

Notch activity synergizes with B-cell-receptor and CD40 signaling to enhance B-cell activation

How diverse environmental cues are integrated to regulate B-cell activation and development remains poorly understood. Here we show that Notch activity synergizes with B-cell receptor (BCR) and/or CD40 signaling to enhance several aspects of B-cell activation and function. We find that costimulation of follicular B cells with the Notch ligand Delta-like-1 leads to significant increases in BCR- and CD40-mediated proliferation and enhances production of IgG1(+) cells in vitro and in vivo. We further find that coengagement of Notch and the BCR results in increased activation of the MAPK pathway, and MAPK and Notch inhibitors prevent B-cell activation events mediated by coengagement of Notch and the BCR. These data suggest that the BCR and CD40 signaling pathways collaborate with the Notch pathway to optimize B-cell activation.

The role of CR2 in autoimmunity

Complement activation is one of the most powerful mechanisms taking place during inflammation and immune responses. Over the last 30 years increasing evidence has proven the role of C3 and receptors for its activation fragments in the initiation and regulation of immune responses. Since complement also has a basic importance in the maintenance of immune homeostasis, abnormalities affecting complement proteins and their receptors may lead to pathological conditions. Autoimmune conditions develop as a result of a range of genetic and environmental factors. Findings obtained from animal models support the notion that malfunctioning of complement receptors, particularly CR2, might be involved in the breakdown of tolerance and excessive antibody production by auto reactive B-cell clones. In addition to B cells, activated, CR2-bearing T cells may also contribute to the pathogenesis of autoimmunity as they can receive activating/survival signals in the inflamed tissue. Results obtained from mouse experiments however, should be extended to the human system with great care, since there are basic differences between the structure and function of human and murine CR1 and CR2.

An antibody-deficiency syndrome due to mutations in the CD19 gene

BACKGROUND

The CD19 protein forms a complex with CD21, CD81, and CD225 in the membrane of mature B cells. Together with the B-cell antigen receptor, this complex signals the B cell to decrease its threshold for activation by the antigen.

METHODS

We evaluated four patients from two unrelated families who had increased susceptibility to infection, hypogammaglobulinemia, and normal numbers of mature B cells in blood. We found a mutation in the CD19 gene in all four patients. The CD19 gene in the patients and their first-degree relatives was sequenced, and flow-cytometric immunophenotyping of B cells, immunohistochemical staining of lymphoid tissues, and DNA and messenger RNA analysis were performed. B-cell responses on the triggering of the B-cell receptor were investigated by in vitro stimulation; the antibody response after vaccination with rabies vaccine was also studied.

RESULTS

All four patients had homozygous mutations in the CD19 gene. Levels of CD19 were undetectable in one patient and substantially decreased in the other three. Levels of CD21 were decreased, whereas levels of CD81 and CD225 were normal, in all four patients. The composition of the precursor B-cell compartment in bone marrow and the total numbers of B cells in blood were normal. However, the numbers of CD27+ memory B cells and CD5+ B cells were decreased. Secondary follicles in lymphoid tissues were small to normal in size and had a normal cellular composition. The few B cells that showed molecular signs of switching from one immunoglobulin class to another contained V(H)-C(alpha) and V(H)-C(gamma) transcripts with somatic mutations. The response of the patients' B cells to in vitro stimulation through the B-cell receptor was impaired, and in all four patients, the antibody response to rabies vaccination was poor.

CONCLUSIONS

Mutation of the CD19 gene causes a type of hypogammaglobulinemia in which the response of mature B cells to antigenic stimulation is defective.