Toll-like receptors and B-cell receptors synergize to induce immunoglobulin class-switch DNA recombination: relevance to microbial antibody responses

Unravelling the Impact of Outer Membrane Protein, OmpA, From S. Typhimurium on Aberrant AID Expression and IgM to IgA Class Switching in Human B-Cells

Salmonella enterica serovar Typhimurium is a Gram-negative bacterium that causes gastrointestinal infection and poses significant public health risks worldwide. This study aims to explore how S. Typhimurium manipulates B-cell function through outer membrane protein A (OmpA). We investigate the effect of OmpA on Raji human B-cells, leading to the induction of activation-induced cytidine deaminase (AID) protein, which plays an important role in generating antibody diversity in B-cells, via initiating the process of somatic hypermutation (SHM) and class switch recombination (CSR). Our key findings demonstrate that OmpA is crucial for inducing aberrant AID expression in B-cells, leading to increased CSR. Interestingly, the increased AID expression was likely due to overexpression of cMYC, an activator for AID expression. Not only was the expression of cMYC elevated, but its occupancy on the aicda locus was raised. Furthermore, increased AID expression induced CSR events, specifically switching to IgA. In summary, our study suggests that OmpA plays a potential role in modulating B-cell regulation and controlling the adaptive immune system. These functional attributes of OmpA implicate its potential as a therapeutic target for combating S. Typhimurium pathogenesis.

NOD1 Agonist Induces Proliferation and Plasma Cell Differentiation of Mouse B Cells Especially CD23high B Cells

BACKGROUND

Like innate cells, B cells also express Pattern Recognition Receptors (PRRs) to detect danger signal such as tissue damage or pathogen intrusion. Production of specific antibodies by plasma cells results from the activation and differentiation of B cells following three signals: (i) antigen recognition by B Cell Receptors, (ii) recognition of danger and (iii) T-cell help. However, it is unclear whether T-cell help is dispensable for B cell activation and differentiation or not. Few studies have investigated the role of cytosolic PRRs such as NOD1 in B cell differentiation.

METHODS

We used splenic C57BL6J B cells to evaluate NOD1 expression and then assessed the effect of stimulation with C12-iE-DAP, a NOD1 ligand, with or without CD40L as a T-cell help signal on B-cell responses globally or according to their CD23 expression level.

RESULTS

We showed that murine B cells express NOD1 and that the presence of C12-iE-DAP induces activation, proliferation and initiates differentiation in plasma cells even in the absence of a T-dependent signal. Surprisingly, CD23high B cells are more sensitive than CD23low B cells to stimulation.

CONCLUSION

Our results suggest that the NLR pathway could induce antibody development during infections and be exploited to develop more effective vaccination.

Lipid Antigens: Revealing the Hidden Players in Adaptive Immune Responses

Traditionally, research on the adaptive immune system has focused on protein antigens, but emerging evidence has underscored the essential role of lipid antigens in immune modulation. Lipid antigens are presented by CD1 molecules and activate invariant natural killer T (iNKT) cells and group 1 CD1-restricted T cells, whereby they impact immune responses to pathogens and tumors. Recent advances in mass spectrometry, imaging techniques, and lipidomics have revolutionized the identification and characterization of lipid antigens and enhanced our understanding of their structural diversity and functional significance. These advancements have paved the way for lipid-based vaccines and immunotherapies through the application of nanoparticles and synthetic lipid antigens designed to boost immune responses against cancers and infectious diseases. Lipid trafficking, CD1 molecule interactions, and the immune system's response to lipid antigens are yet to be completely understood, particularly in the context of autoimmunity and microbial infections. In the years to come, continued research efforts are needed to uncover its underlying biological mechanisms and to exploit the full potential of therapies directed against lipid antigens.

Pim1 is Critical in T-cell-independent B-cell Response and MAPK Activation in B Cells

The Pim family consists of three members that encode a distinct class of highly conserved serine/threonine kinases. In this study, we generated and examined mice with hematopoiesis-specific deletion of Pim1 and bone marrow (BM) chimeric mice with B-cell-specific targeted deletion of Pim1. Pim1 was expressed at all stages of B-cell development and hematopoietic-specific deletion of Pim1 altered B-cell development in BM, spleen and peritoneal. However, Pim1 deficiency did not affect T-cell development. Studies of BM chimeric mice showed that Pim1 is required in a cell-intrinsic manner to maintain normal B-cell development. Pim1 deficiency led to significant changes in B cell antibody responses. Additionally, Pim1 deficiency resulted in reduced B cell receptor (BCR)-induced cell proliferation and cell cycle progression. Examination of the various BCR-activated signaling pathways in Pim1-deficient B cells reveals defective activation of mitogen-activated protein kinases (MAPKs), which are known to regulate genes involved in cell proliferation and survival. qRT-PCR analysis of BCR-engaged B cells from Pim1-deficient B cells revealed reduced expression of cyclin-dependent kinase (CDK) and cyclin genes, including CDK2, CCNB1 and CCNE1. In conclusion, Pim1 plays a crucial role in B-cell development and B cell activation.

Humoral responses are enhanced by facilitating B cell viability by Fcrl5 overexpression in B cells

B cell initial activity is regulated through a balance of activation and suppression mediated by regulatory molecules expressed in B cells; however, the molecular mechanisms underlying this process remain incompletely understood. In this study, we investigated the function of the Fc receptor-like (Fcrl) family molecule Fcrl5, which is constitutively expressed in naive B cells, in humoral immune responses. Our study demonstrated that B cell-specific overexpression of Fcrl5 enhanced antibody (Ab) production in both T cell-independent type 1 (TI1) and T cell-dependent (TD) responses. Additionally, it promoted effector B cell formation under competitive conditions in TD responses. Mechanistically, in vitro ligation of Fcrl5 by agonistic Abs reduced cell death and enhanced proliferation in lipopolysaccharide-stimulated B cells. In the presence of anti-CD40 Abs and IL-5, the Fcrl5 ligation not only suppressed cell death but also enhanced differentiation into plasma cells. These findings reveal a novel role of Fcrl5 in promoting humoral immune responses by enhancing B cell viability and plasma cell differentiation.

Cracking the type 1 diabetes code: Genes, microbes, immunity, and the early life environment

Type 1 diabetes (T1D) results from a complex interplay of genetic predisposition, immunological dysregulation, and environmental triggers, that culminate in the destruction of insulin-secreting pancreatic β cells. This review provides a comprehensive examination of the multiple factors underpinning T1D pathogenesis, to elucidate key mechanisms and potential therapeutic targets. Beginning with an exploration of genetic risk factors, we dissect the roles of human leukocyte antigen (HLA) haplotypes and non-HLA gene variants associated with T1D susceptibility. Mechanistic insights gleaned from the NOD mouse model provide valuable parallels to the human disease, particularly immunological intricacies underlying β cell-directed autoimmunity. Immunological drivers of T1D pathogenesis are examined, highlighting the pivotal contributions of both effector and regulatory T cells and the multiple functions of B cells and autoantibodies in β-cell destruction. Furthermore, the impact of environmental risk factors, notably modulation of host immune development by the intestinal microbiome, is examined. Lastly, the review probes human longitudinal studies, unveiling the dynamic interplay between mucosal immunity, systemic antimicrobial antibody responses, and the trajectories of T1D development. Insights garnered from these interconnected factors pave the way for targeted interventions and the identification of biomarkers to enhance T1D management and prevention strategies.

Majority of human circulating IgG plasmablasts stop blasting in a cell-free pro-survival culture

Following infection or vaccination, early-minted antibody secreting cells (ASC) or plasmablasts appear in circulation transiently, and a small fraction migrates to the spleen or bone marrow (BM) to mature into long-lived plasma cells (LLPC). While LLPC, by definition, are quiescent or non-dividing, the majority of blood ASC are thought to be "blasting" or proliferative. In this study, we find > 95% nascent blood ASC in culture express Ki-67 but only 6-12% incorporate BrdU after 4 h or 24 h labeling. In contrast, < 5% BM LLPC in culture are Ki-67+ with no BrdU uptake. Due to limitations of traditional flow cytometry, we utilized a novel optofluidic technology to evaluate cell division with simultaneous functional IgG secretion. We find 11% early-minted blood ASC undergo division, and none of the terminally differentiated BM LLPC (CD19-CD38hiCD138+) divide during the 7-21 days in culture. While BM LLPC undergo complete cell cycle arrest, the process of differentiation into an ASC or plasmablasts also discourages entry into S phase. Since the majority of Ki-67+ nascent blood ASC have exited cell cycle and are no longer actively "blasting", the term "plasmablast", which traditionally refers to an ASC that still has the capacity to divide, may probably be a misnomer.

Ex Vivo Fluorescent Labeling of HBV-Specific B Cells in Chronic Hepatitis B Patients

Fluorescently conjugated antigen-bait systems have been extensively used to identify antigen-specific B cells and probe humoral immunity across different settings. Following this approach, we used HBV antigens to bind the B cell receptor (BCR), permitting antigen-specific B cell detection by flow cytometry. Fluorochromes can either be attached covalently via chemical conjugation to the antigen or attached non-covalently by biotinylating the antigen. Dual-staining antigen-baits (where an antigen is directly conjugated to two distinct fluorochromes) have now been used to identify HBsAg- and HBcAg-specific B cells with a high degree of reliability and specificity. This system can be used to detect and characterize cells ex vivo or adapted to isolate antigen-specific cells using fluorescence-activated cell sorting.

Oral Pathogenic Bacteria and the Oral-Gut-Liver Axis: A New Understanding of Chronic Liver Diseases

Liver diseases have long been a prevalent cause of morbidity and mortality, and their development and progression involve multiple vital organs throughout the body. Recent studies on the oral-gut-liver axis have revealed that the oral microbiota is associated with the pathophysiology of chronic liver diseases. Since interventions aimed at regulating oral biological disorders may delay the progress of liver disease, it is crucial to better comprehend this process. Oral bacteria with potential pathogenicity have been extensively studied and are closely related to several types of chronic liver diseases. Therefore, this review will systemically describe the emerging role of oral pathogenic bacteria in common liver diseases, including alcoholic liver disease (ALD), non-alcoholic steatohepatitis (NASH), non-alcoholic fatty liver disease (NAFLD), cirrhosis, autoimmune liver diseases (AILD), and liver cancer, and bring in new perspectives for future research.

Majority of human circulating plasmablasts stop blasting: A probable misnomer

Following infection or vaccination, early-minted antibody secreting cells (ASC) or plasmablasts appear in circulation transiently, and a small fraction migrates to the spleen or bone marrow (BM) to mature into long-lived plasma cells (LLPC). While LLPC, by definition, are quiescent or non-dividing, the majority of blood ASC are thought to be "blasting" or proliferative. In this study, we find >95% nascent blood ASC in culture express Ki-67 but only 6-12% incorporate BrdU after 4h or 24h labeling. In contrast, <5% BM LLPC in culture are Ki-67 + with no BrdU uptake. Due to limitations of traditional flow cytometry, we utilized a novel optofluidic technology to evaluate cell division with simultaneous functional Ig secretion. We find 11% early-minted blood ASC undergo division, and none of the terminally differentiated BM LLPC (CD19 - CD38 hi CD138 + ) divide during the 7-21 days in culture. While BM LLPC undergo complete cell cycle arrest, the process of differentiation into an ASC of plasmablasts discourages entry into S phase. Since the majority of Ki-67 + nascent blood ASC have exited cell cycle and are no longer actively "blasting", the term "plasmablast", which traditionally refers to an ASC that still has the capacity to divide, may probably be a misnomer.

Elevated levels of enteric IgA in an unimmunised mouse model of Hyper IgM syndrome derived from gut-associated secondary lymph organs even in the absence of germinal centres

Introduction

Patients with Human Hyper IgM syndromes (HIGM) developed pulmonary and gastrointestinal infections since infancy and most patients have mutations in the CD40 ligand (CD40L) gene. Most HIGM patients compared to healthy subjects have higher/similar IgM and lower IgG, and IgA serum concentrations but gut antibody concentrations are unknown. CD40L on activated T-cells interacts with CD40 on B-cells, essential for the formation of germinal centres (GCs) inside secondary lymphoid organs (SLOs), where high-affinity antibodies, long-lived antibody-secreting plasma cells, and memory B-cells, are produced. C57BL6-CD40 ligand deficient mice (C57BL6-cd40l ^-/-), are a model of HIGM, because serum immunoglobulin concentrations parallel levels observed in HIGM patients and have higher faecal IgA concentrations. In mice, TGFβ and other cytokines induce IgA production.

Aims

To compare and evaluate B-cell populations and IgA-producing plasma cells in peritoneal lavage, non-gut-associated SLOs, spleen/inguinal lymph nodes (ILN), and gut-associated SLOs, mesenteric lymph nodes (MLN)/Peyer´s patches (PP) of unimmunised C57BL6-cd40l ^-/- and C57BL6-wild-type (WT) mice.

Material and methods

Peritoneal lavages, spleens, ILN, MLN, and PP from 8-10 weeks old C57BL6-cd40l ^-/- and WT mice, were obtained. Organ cryosections were analysed by immunofluorescence and B-cell populations and IgA-positive plasma cell suspensions by flow cytometry.

Results

In unimmunised WT mice, GCs were only observed in the gut-associated SLOs, but GCs were absent in all C57BL6-cd40l ^-/- SLOs. PP and MLN of C57BL6-cd40l ^-/- mice exhibited a significantly higher number of IgA-producing cells than WT mice. In the spleen and ILN of C57BL6-cd40l^{- /-} mice IgA-producing cells significantly decreased, while IgM-positive plasma cells increased. C57BL6-cd40l ^-/- B-1 cells were more abundant in all analysed SLOs, whereas in WT mice most B-1 cells were contained within the peritoneal cavity. C57BL6-cd40l ^-/- B-cells in MLN expressed a higher TGFβ receptor-1 than WT mice. Mouse strains small intestine microvilli (MV), have a similar frequency of IgA-positive cells.

Discussion

Together our results confirm the role of PP and MLN as gut inductive sites, whose characteristic features are to initiate an IgA preferential immune response production in these anatomical sites even in the absence of GCs. IgA antibodies play a pivotal role in neutralising, eliminating, and regulating potential pathogens and microorganisms in the gut.

Overview of human B-cell development and antibody deficiencies

B cells are a key component of the humoral (antibody-mediated) immune response which is responsible for defense against a variety of pathogens. Here we provide an overview of the current understanding of B cell development and function and briefly describe inborn errors of immunity associated with B cell development defects which can manifest as immune deficiency, malignancy, autoimmunity, or allergy. The knowledge and application of B cell biology are essential for laboratory evaluation and clinical assessment of these B cell disorders.

Intrinsic B cell TLR-BCR linked coengagement induces class-switched, hypermutated, neutralizing antibody responses in absence of T cells

Maturation of antibody responses entails somatic hypermutation (SHM), class-switch DNA recombination (CSR), plasma cell differentiation, and generation of memory B cells, and it is thought to require T cell help. We showed that B cell Toll-like receptor 4 (TLR4)-B cell receptor (BCR) (receptor for antigen) coengagement by 4-hydroxy-3-nitrophenyl acetyl (NP)-lipopolysaccharide (LPS) (Escherichia coli lipid A polysaccharide O-antigen) or TLR5-BCR coengagement by Salmonella flagellin induces mature antibody responses to NP and flagellin in Tcrβ-/-Tcrδ-/- and NSG/B mice. TLR-BCR coengagement required linkage of TLR and BCR ligands, "linked coengagement." This induced B cell CSR/SHM, germinal center-like differentiation, clonal expansion, intraconal diversification, plasma cell differentiation, and an anamnestic antibody response. In Tcrβ-/-Tcrδ-/- mice, linked coengagement of TLR4-BCR by LPS or TLR5-BCR by flagellin induced protective antibodies against E. coli or Salmonella Typhimurium. Our findings unveiled a critical role of B cell TLRs in inducing neutralizing antibody responses, including those to microbial pathogens, without T cell help.

NONO regulates B-cell development and B-cell receptor signaling

The paraspeckle protein NONO is a multifunctional nuclear protein participating in the regulation of transcriptional regulation, mRNA splicing and DNA repair. However, whether NONO plays a role in lymphopoiesis is not known. In this study, we generated mice with global deletion of NONO and bone marrow (BM) chimeric mice in which NONO is deleted in all of mature B cells. We found that the global deletion of NONO in mice did not affect T-cell development but impaired early B-cell development in BM at pro- to pre-B-cell transition stage and B-cell maturation in the spleen. Studies of BM chimeric mice demonstrated that the impaired B-cell development in NONO-deficient mice is B-cell-intrinsic. NONO-deficient B cells displayed normal BCR-induced cell proliferation but increased BCR-induced cell apoptosis. Moreover, we found that NONO deficiency impaired BCR-induced activation of ERK, AKT, and NF-κB pathways in B cells, and altered BCR-induced gene expression profile. Thus, NONO plays a critical role in B-cell development and BCR-induced B-cell activation.

Effect of curcumin on regulatory B cells in chronic colitis mice involving TLR/MyD88 signaling pathway

Curcumin (Cur) is a natural active phenolic compound extracted from the root of Curcuma Longa L. It has anti-inflammatory, anti-tumor and other pharmacological activities, and is commonly used to treat ulcerative colitis (UC). However, it is not clear whether curcumin regulates the function and differentiation of Breg cells to treat UC. In this study, mice with chronic colitis were induced by dextran sulfate sodium (DSS), and treated with curcumin for 12 days. Curcumin effectively improved the body weight, colonic weight, colonic length, decreased colonic weight index and pathological injury score under colonoscopy in mice with chronic colitis, and significantly inhibited the production of IL-1β, IL-6, IL-33, CCL-2, IFN-γ, TNF-α, and promoted the secretion of IL-4, IL-10, IL-13 and IgA. Importantly, curcumin markedly upregulated CD3- CD19+ CD1d+ , CD3- CD19+ CD25+ , CD3- CD19+ Foxp3+ Breg cells level and significantly down-regulated CD3- CD19+ PD-L1+ , CD3- CD19+ tim-1+ , CD3- CD19+ CD27+ Breg cells level. In addition, our results also showed that curcumin observably inhibited TLR2, TLR4, TLR5, MyD88, IRAK4, p-IRAK4, NF-κB P65, IRAK1, TRAF6, TAB1, TAB2, TAK1, MKK3, MKK6, p38MAPK, p-p38MAPK and CREB expression in TLR/MyD88 signaling pathway. These results suggest that curcumin can regulate the differentiation and function of Breg cell to alleviate DSS-induced colitis, which may be realized by inhibiting TLR/MyD88 pathway.

Unwinding the mechanism of macrophage repolarization potential of Oceanimonas sp. BPMS22-derived protein protease inhibitor through Toll-like receptor 4 against experimental visceral leishmaniasis

The Oceanimonas sp. BPMS22-derived protein protease inhibitor (PPI) has been proven to shift macrophages towards an inflammatory state and reduce Leishmania donovani infection in vitro and in vivo. The current study explored and validated the mechanistic aspects of the PPI and Toll-like receptor (TLR) interaction. The PPI exhibited the upregulation of TLR2, TLR4, and TLR6 during treatment which was proven to orchestrate parasite clearance effectively. An in silico study confirmed the high interaction with TLR4 and PPI. Immune blotting confirmed the significant upregulation of TLR4 in macrophages irrespective of L. donovani infection. Pharmacological inhibition and immune blot study confirmed the involvement of the PPI in TLR4-mediated phosphorylation of p38 MAPK and dephosphorylation of ERK1/2, repolarizing to pro-inflammatory macrophage state against experimental visceral leishmaniasis. In addition, in TLR4 knockdown condition, PPI treatment failed to diminish M2 phenotypical markers (CD68, Fizz1, Ym1, CD206, and MSR-2) and anti-inflammatory cytokines (IL-4, IL-10, and TGF-β). Simultaneously, the PPI failed to upregulate the M1 phenotypical markers and pro-inflammatory cytokines (IL-1β, IL-6, IL-12, and IFN-γ) (p < 0.001) during the TLR4 knockdown condition. In the absence of TLR4, the PPI also failed to reduce the parasite load and T-cell proliferation and impaired the delayed-type hypersensitivity response. The absence of pro-inflammatory cytokines was observed during a co-culture study with PPI-treated macrophages (in the TLR4 knockdown condition) with day 10 T-cell obtained from L. donovani-infected mice. This study supports the immunotherapeutic potential of the PPI as it interacted with TLR4 and promoted macrophage repolarization (M2-M1) to restrict the L. donovani parasite burden and helps in the mounting immune response against experimental visceral leishmaniasis.

Chickens, more than humans, focus the diversity of their immunoglobulin genes on the complementarity-determining region but utilise amino acids, indicative of a more cross-reactive antibody repertoire

The mechanisms of B-cell diversification differ greatly between aves and mammals, but both produce B cells and antibodies capable of supporting an effective immune response. To see how differences in the generation of diversity might affect overall repertoire diversity, we have compared the diversity characteristics of immunoglobulin genes from domestic chickens to those from humans. Both use V(D)J gene rearrangement and somatic hypermutation, but only chickens use somatic gene conversion. A range of diversity analysis tools were used to investigate multiple aspects of amino acid diversity at both the germline and repertoire levels. The effect of differing amino acid usages on antibody characteristics was assessed. At both the germline and repertoire levels, chickens exhibited lower amino acid diversity in comparison to the human immunoglobulin genes, especially outside of the complementarity-determining region (CDR). Chickens were also found to possess much larger and more hydrophilic CDR3s with a higher predicted protein binding potential, suggesting that the antigen-binding site in chicken antibodies is more flexible and more polyreactive than that seen in human antibodies.

Third-Generation Vaccines: Features of Nucleic Acid Vaccines and Strategies to Improve Their Efficiency

Gene immunization comprises mRNA and DNA vaccines, which stand out due to their simple design, maintenance, and high efficacy. Several studies indicate promising results in preclinical and clinical trials regarding immunization against ebola, human immunodeficiency virus (HIV), influenza, and human papillomavirus (HPV). The efficiency of nucleic acid vaccines has been highlighted in the fight against COVID-19 with unprecedented approval of their use in humans. However, their low intrinsic immunogenicity points to the need to use strategies capable of overcoming this characteristic and increasing the efficiency of vaccine campaigns. These strategies include the improvement of the epitopes' presentation to the system via MHC, the evaluation of immunodominant epitopes with high coverage against emerging viral subtypes, the use of adjuvants that enhance immunogenicity, and the increase in the efficiency of vaccine transfection. In this review, we provide updates regarding some characteristics, construction, and improvement of such vaccines, especially about the production of synthetic multi-epitope genes, widely employed in the current gene-based vaccines.

Autoantibodies and Cardiomyopathy: Focus on Beta-1 Adrenergic Receptor Autoantibodies

ABSTRACT

Antibody response to self-antigens leads to autoimmune response that plays a determinant role in cardiovascular disease outcomes including dilated cardiomyopathy (DCM). Although the origins of the self-reactive endogenous autoantibodies are not well-characterized, it is believed to be triggered by tissue injury or dysregulated humoral response. Autoantibodies that recognize G protein-coupled receptors are considered consequential because they act as modulators of downstream receptor signaling displaying a wide range of unique pharmacological properties. These wide range of pharmacological properties exhibited by autoantibodies has cellular consequences that is associated with progression of disease including DCM. Increase in autoantibodies recognizing beta-1 adrenergic receptor (β1AR), a G protein-coupled receptor critical for cardiac function, is observed in patients with DCM. Cellular and animal model studies have indicated pathological roles for the β1AR autoantibodies but less is understood about the molecular basis of their modulatory effects. Despite the recognition that β1AR autoantibodies could mediate deleterious outcomes, emerging evidence suggests that not all β1AR autoantibodies are deleterious. Recent clinical studies show that β1AR autoantibodies belonging to the IgG3 subclass is associated with beneficial cardiac outcomes in patients. This suggests that our understanding on the roles the β1AR autoantibodies play in mediating outcomes is not well-understood. Technological advances including structural determinants of antibody binding could provide insights on the modulatory capabilities of β1AR autoantibodies in turn, reflecting their diversity in mediating β1AR signaling response. In this study, we discuss the significance of the diversity in signaling and its implications in pathology.

Afucosylated IgG responses in humans - structural clues to the regulation of humoral immunity

Healthy immune responses require efficient protection without excessive inflammation. Recent discoveries on the degree of fucosylation of a human N-linked glycan at a conserved site in the immunoglobulin IgG-Fc domain might add an additional regulatory layer to adaptive humoral immunity. Specifically, afucosylation of IgG-Fc enhances the interaction of IgG with FcγRIII and thereby its activity. Although plasma IgG is generally fucosylated, afucosylated IgG is raised in responses to enveloped viruses and Plasmodium falciparum proteins expressed on infected erythrocytes, as well as during alloimmune responses. Moreover, while afucosylation can exacerbate some infectious diseases (e.g., COVID-19), it also correlates with traits of protective immunity against malaria and HIV-1. Herein we discuss the implications of IgG afucosylation for health and disease, as well as for vaccination.

Toll-like receptor (TLR) agonists as a driving force behind next-generation vaccine adjuvants and cancer therapeutics

Until recently, the development of new human adjuvants was held back by a poor understanding of their mechanisms of action. The field was revolutionized by the discovery of the toll-like receptors (TLRs), innate immune receptors that directly or indirectly are responsible for detecting pathogen-associated molecular patterns (PAMPs) and respond to them by activating innate and adaptive immune pathways. Hundreds of ligands targeting various TLRs have since been identified and characterized as vaccine adjuvants. This work has important implications not only for the development of vaccines against infectious diseases but also for immuno-therapies against cancer, allergy, Alzheimer's disease, drug addiction and other diseases. Each TLR has its own specific tissue localization and downstream gene signalling pathways, providing researchers the opportunity to precisely tailor adjuvants with specific immune effects. TLR agonists can be combined with other TLR or alternative adjuvants to create combination adjuvants with synergistic or modulatory effects. This review provides an introduction to the various classes of TLR adjuvants and their respective signalling pathways. It provides an overview of recent advancements in the TLR field in the past 2-3 years and discusses criteria for selecting specific TLR adjuvants based on considerations, such as disease mechanisms and correlates of protection, TLR immune biasing capabilities, route of administration, antigen compatibility, new vaccine technology platforms, and age- and species-specific effects.

Optimized Protocols for In-Vitro T-Cell-Dependent and T-Cell-Independent Activation for B-Cell Differentiation Studies Using Limited Cells

Background/Methods

For mechanistic studies, in-vitro human B-cell differentiation and generation of plasma cells are invaluable techniques. However, the heterogeneity of both T-cell-dependent (TD) and T-cell-independent (TI) stimuli and the disparity of culture conditions used in existing protocols make the interpretation of results challenging. The aim of the present study was to achieve the most optimal B-cell differentiation conditions using isolated CD19⁺ B cells and peripheral blood mononuclear cell (PBMC) cultures. We addressed multiple seeding densities, different durations of culturing, and various combinations of TD and TI stimuli including B-cell receptor (BCR) triggering. B-cell expansion, proliferation, and differentiation were analyzed after 6 and 9 days by measuring B-cell proliferation and expansion, plasmablast and plasma cell formation, and immunoglobulin (Ig) secretion. In addition, these conditions were extrapolated using cryopreserved cells and differentiation potential was compared.

Results

This study demonstrates improved differentiation efficiency after 9 days of culturing for both B-cells and PBMC cultures using CD40L and IL-21 as TD stimuli and 6 days for CpG and IL-2 as TI stimuli. We arrived at optimized protocols requiring 2,500 and 25,000 B–cells per culture well for the TD and TI assays, respectively. The results of the PBMC cultures were highly comparable to the B-cell cultures, which allows dismissal of additional B-cell isolation steps prior to culturing. In these optimized TD conditions, the addition of anti-BCR showed a little effect on phenotypic B-cell differentiation; however, it interferes with Ig secretion measurements. The addition of IL-4 to the TD stimuli showed significantly lower Ig secretion. The addition of BAFF to optimized TI conditions showed enhanced B-cell differentiation and Ig secretion in B-cell but not in PBMC cultures. With this approach, efficient B-cell differentiation and Ig secretion were accomplished when starting from fresh or cryopreserved samples.

Conclusion

Our methodology demonstrates optimized TD and TI stimulation protocols for more in-depth analysis of B-cell differentiation in primary human B-cell and PBMC cultures while requiring low amounts of B cells, making them ideally suited for future clinical and research studies on B-cell differentiation of patient samples from different cohorts of B-cell-mediated diseases.

Cecal Patches Generate Abundant IgG2b-Bearing B Cells That Are Reactive to Commensal Microbiota

Gut-associated lymphoid tissue (GALT), such as Peyer's patches (PPs), are key inductive sites that generate IgA⁺ B cells, mainly through germinal center (GC) responses. The generation of IgA⁺ B cells is promoted by the presence of gut microbiota and dietary antigens. However, the function of GALT in the large intestine, such as cecal patches (CePs) and colonic patches (CoPs), and their regulatory mechanisms remain largely unknown. In this study, we demonstrate that the CePs possess more IgG2b⁺ B cells and have fewer IgA⁺ B cells than those in PPs from BALB/c mice with normal gut microbiota. Gene expression analysis of postswitched transcripts supported the differential expression of dominant antibody isotypes in B cells in GALT. Germ-free (GF) mice showed diminished GC B cells and had few IgA⁺ or IgG2b⁺ switched B cells in both the small and large intestinal GALT. In contrast, myeloid differentiation factor 88- (MyD88-) deficient mice exhibited decreased GC B cells and presented with reduced numbers of IgG2b⁺ B cells in CePs but not in PPs. Using ex vivo cell culture, we showed that CePs have a greater capacity to produce total and microbiota-reactive IgG2b, in addition to microbiota-reactive IgA, than the PPs. In line with the frequency of GC B cells and IgG2b⁺ B cells in CePs, there was a decrease in the levels of microbiota-reactive IgG2b and IgA in the serum of GF and MyD88-deficient mice. These data suggest that CePs have a different antibody production profile compared to PPs. Furthermore, the innate immune signals derived from gut microbiota are crucial for generating the IgG2b antibodies in CePs.

B cell class switching in intestinal immunity in health and disease

The gastrointestinal tract is colonized by trillions of commensal microorganisms that collectively form the microbiome and make essential contributions to organism homeostasis. The intestinal immune system must tolerate these beneficial commensals, whilst preventing pathogenic organisms from systemic spread. Humoral immunity plays a key role in this process, with large quantities of immunoglobulin (Ig)A secreted into the lumen on a daily basis, regulating the microbiome and preventing bacteria from encroaching on the epithelium. However, there is an increasing appreciation of the role of IgG antibodies in intestinal immunity, including beneficial effects in neonatal immune development, pathogen and tumour resistance, but also of pathological effects in driving chronic inflammation in inflammatory bowel disease (IBD). These antibody isotypes differ in effector function, with IgG exhibiting more proinflammatory capabilities compared with IgA. Therefore, the process that leads to the generation of different antibody isotypes, class-switch recombination (CSR), requires careful regulation and is orchestrated by the immunological cues generated by the prevalent local challenge. In general, an initiating signal such as CD40 ligation on B cells leads to the induction of activation-induced cytidine deaminase (AID), but a second cytokine-mediated signal determines which Ig heavy chain is expressed. Whilst the cytokines driving intestinal IgA responses are well-studied, there is less clarity on how IgG responses are generated in the intestine, and how these cues might become dysfunctional in IBD. Here, we review the key mechanisms regulating class switching to IgA vs IgG in the intestine, processes that could be therapeutically manipulated in infection and IBD.

Mathematical Modeling of Proliferative Immune Response Initiated by Interactions Between Classical Antigen-Presenting Cells Under Joint Antagonistic IL-2 and IL-4 Signaling

During an adaptive immune response from pathogen invasion, multiple cytokines are produced by various immune cells interacting jointly at the cellular level to mediate several processes. For example, studies have shown that regulation of interleukin-4 (IL-4) correlates with interleukin-2 (IL-2) induced lymphocyte proliferation. This motivates the need to better understand and model the mechanisms driving the dynamic interplay of proliferation of lymphocytes with the complex interaction effects of cytokines during an immune response. To address this challenge, we adopt a hybrid computational approach comprising of continuous, discrete and stochastic non-linear model formulations to predict a system-level immune response as a function of multiple dependent signals and interacting agents including cytokines and targeted immune cells. We propose a hybrid ordinary differential equation-based (ODE) multicellular model system with a stochastic component of antigen microscopic states denoted as Multiscale Multicellular Quantitative Evaluator (MMQE) implemented using MATLAB. MMQE combines well-defined immune response network-based rules and ODE models to capture the complex dynamic interactions between the proliferation levels of different types of communicating lymphocyte agents mediated by joint regulation of IL-2 and IL-4 to predict the emergent global behavior of the system during an immune response. We model the activation of the immune system in terms of different activation protocols of helper T cells by the interplay of independent biological agents of classic antigen-presenting cells (APCs) and their joint activation which is confounded by the exposure time to external pathogens. MMQE quantifies the dynamics of lymphocyte proliferation during pathogen invasion as bivariate distributions of IL-2 and IL-4 concentration levels. Specifically, by varying activation agents such as dendritic cells (DC), B cells and their joint mechanism of activation, we quantify how lymphocyte activation and differentiation protocols boost the immune response against pathogen invasion mediated by a joint downregulation of IL-4 and upregulation of IL-2. We further compare our in-silico results to in-vivo and in-vitro experimental studies for validation. In general, MMQE combines intracellular and extracellular effects from multiple interacting systems into simpler dynamic behaviors for better interpretability. It can be used to aid engineering of anti-infection drugs or optimizing drug combination therapies against several diseases.

Therapeutic Antibodies: An Overview

Polyclonal immunoglobulin (Ig) preparations have been used for several decades for treatment of primary and secondary immunodeficiencies and for treatment of some infections and intoxications. This has demonstrated the importance of Igs, also called antibodies (Abs) for prevention and elimination of infections. Moreover, elucidation of the structure and functions of Abs has suggested that they might be useful for targeted treatment of several diseases, including cancers and autoimmune diseases. The development of technologies for production of specific monoclonal Abs (MAbs) in large amounts has led to the production of highly effective therapeutic antibodies (TAbs), a collective term for MAbs (MAbs) with demonstrated clinical efficacy in one or more diseases. The number of approved TAbs is currently around hundred, and an even larger number is under development, including several engineered and modified Ab formats. The use of TAbs has provided new treatment options for many severe diseases, but prediction of clinical effect is difficult, and many patients eventually lose effect, possibly due to development of Abs to the TAbs or to other reasons. The therapeutic efficacy of TAbs can be ascribed to one or more effects, including binding and neutralization of targets, direct cytotoxicity, Ab-dependent complement-dependent cytotoxicity, Ab-dependent cellular cytotoxicity or others. The therapeutic options for TAbs have been expanded by development of several new formats of TAbs, including bispecific Abs, single domain Abs, TAb-drug conjugates, and the use of TAbs for targeted activation of immune cells. Most promisingly, current research and development can be expected to increase the number of clinical conditions, which may benefit from TAbs.

HomA and HomB, outer membrane proteins of Helicobacter pylori down-regulate activation-induced cytidine deaminase (AID) and Ig switch germline transcription and thereby affect class switch recombination (CSR) of Ig genes in human B-cells

H. pylori is one of the major causes of chronic gastritis, peptic ulcer disease (PUD), gastric mucosa-associated lymphoid tissue lymphoma (MALT) and gastric carcinoma. H. pylori toxin VacA is responsible for host cell apoptosis, whereas CagA is known to aberrantly induce expression of activation-induced cytidine deaminase (AID) in gastric epithelial cells that causes mutations in oncogenes and tumour suppressor genes, leading to the transformation of normal cells into cancerous cells. Although, a significant amount of research has been conducted to understand the role of bacterial factors modulating deregulated host cell pathways, the interaction between H. pylori and immune cells of the marginal zone and its consequences are still not well understood. HomB and HomA, outer membrane proteins (OMPs) from H. pylori, which assist in the adhesion of bacteria to host cells, are found to be associated with H. pylori virulent strains and promote inflammation. Interestingly, we observed that the interaction of HomB/HomA OMPs with B-cells transiently downregulates AID expression and Ig switch germline transcription. Downregulation of AID leads to impairment of class switch recombination (CSR), resulting in significantly reduced switching to IgG and IgA antibodies. Besides, we examined the immune-suppressive response of B-cells and observed that the cells stimulated with HomA/B show upregulation in the levels of IL10, IL35, as well as PDL1, a T-cell inhibition marker. Our study suggests the potential role of OMPs in immune response modulation strategies used by the pathogen to evade the immune response. These results provide a better understanding of H. pylori pathogenesis and assist in identifying novel targets for therapy.

Immunological drivers of IgA nephropathy: Exploring the mucosa-kidney link

IgA nephropathy (IgAN) is the most common pattern of primary glomerular disease reported worldwide. Up to 40% of those with IgAN progress to end-stage kidney disease within 20 years of diagnosis, with no currently available disease-specific treatment. This is likely to change rapidly, with evolving insights into the mechanisms driving this disease. IgAN is an immune-complex-mediated disease, and its pathophysiology has been framed by the 'four-hit hypothesis', which necessitates four events to occur for clinically significant disease to develop. However, this hypothesis does not explain the wide variability observed in its presentation or clinical progression. Recently, there has been great interest in exploring the role of the mucosal immune system in IgAN, especially given the well-established link between mucosal infections and disease flares. Knowledge of antigen-mucosal interactions is now being successfully leveraged for therapeutic purposes; the gut-directed drug Nefecon (targeted release formulation-budesonide) is on track to become the first medication to be approved specifically for the treatment of IgAN. In this review, we examine established immunological paradigms in IgAN, explore how antigen-mucosal immune responses drive disease, and discuss how this knowledge is being used to develop new treatments.

Key Considerations for the Development of Safe and Effective SARS-CoV-2 Subunit Vaccine: A Peptide-Based Vaccine Alternative

COVID-19 is disastrous to global health and the economy. SARS-CoV-2 infection exhibits similar clinical symptoms and immunopathological sequelae to SARS-CoV infection. Therefore, much of the developmental progress on SARS-CoV vaccines can be utilized for the development of SARS-CoV-2 vaccines. Careful antigen selection during development is always of utmost importance for the production of effective vaccines that do not compromise recipient safety. This holds especially true for SARS-CoV vaccines, as several immunopathological disorders are associated with the activity of structural and nonstructural proteins encoded in the virus's genetic material. Whole viral protein and RNA-encoding full-length proteins contain both protective and "dangerous" sequences, unless pathological fragments are deleted. In light of recent advances, peptide vaccines may present a very safe and effective alternative. Peptide vaccines can avoid immunopathological pro-inflammatory sequences, focus immune responses on neutralizing immunogenic epitopes, avoid off-target antigen loss, combine antigens with different protective roles or mechanisms, even from different viral proteins, and avoid mutant escape by employing highly conserved cryptic epitopes. In this review, an attempt is made to exploit the similarities between SARS-CoV and SARS-CoV-2 in vaccine antigen screening, with particular attention to the pathological and immunogenic properties of SARS proteins.

Intravital quantification reveals dynamic calcium concentration changes across B cell differentiation stages

Calcium is a universal second messenger present in all eukaryotic cells. The mobilization and storage of Ca²⁺ ions drives a number of signaling-related processes, stress-responses, or metabolic changes, all of which are relevant for the development of immune cells and their adaption to pathogens. Here, we introduce the Förster resonance energy transfer (FRET)-reporter mouse YellowCaB expressing the genetically encoded calcium indicator TN-XXL in B lymphocytes. Calcium-induced conformation change of TN-XXL results in FRET-donor quenching measurable by two-photon fluorescence lifetime imaging. For the first time, using our novel numerical analysis, we extract absolute cytoplasmic calcium concentrations in activated B cells during affinity maturation in vivo. We show that calcium in activated B cells is highly dynamic and that activation introduces a persistent calcium heterogeneity to the lineage. A characterization of absolute calcium concentrations present at any time within the cytosol is therefore of great value for the understanding of long-lived beneficial immune responses and detrimental autoimmunity.

Role of Peripheral Immune Cells in Multiple Sclerosis and Experimental Autoimmune Encephalomyelitis

Multiple sclerosis (MS) is a chronic autoimmune disease that affects the myelination of the neurons present in the central nervous system (CNS). The exact etiology of MS development is unclear, but various environmental and genetic factors might play a role in initiating the disease. Experimental autoimmune encephalomyelitis (EAE) is a mouse model that is used to study the pathophysiology of MS disease as well as the effects of possible therapeutic agents. In addition, autoreactive immune cells trigger an inflammatory process upon the recognition of CNS antigens, which leads to destruction of the neurons. These include innate immune cells such as macrophages, dendritic cells, and natural killer cells. Additionally, the activation and extravasation of adaptive immune cells such as CD4+ T cells into the CNS may lead to further exacerbation of the disease. However, many studies revealed that immune cells could have either a protective or pathological role in MS. In this review, we highlight the roles of innate and adaptive immune cellular and soluble players that contribute to the pathogenesis of MS and EAE, which may be used as potential targets for therapy.

The Role of APOBECs in Viral Replication

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) proteins are a diverse and evolutionarily conserved family of cytidine deaminases that provide a variety of functions from tissue-specific gene expression and immunoglobulin diversity to control of viruses and retrotransposons. APOBEC family expansion has been documented among mammalian species, suggesting a powerful selection for their activity. Enzymes with a duplicated zinc-binding domain often have catalytically active and inactive domains, yet both have antiviral function. Although APOBEC antiviral function was discovered through hypermutation of HIV-1 genomes lacking an active Vif protein, much evidence indicates that APOBECs also inhibit virus replication through mechanisms other than mutagenesis. Multiple steps of the viral replication cycle may be affected, although nucleic acid replication is a primary target. Packaging of APOBECs into virions was first noted with HIV-1, yet is not a prerequisite for viral inhibition. APOBEC antagonism may occur in viral producer and recipient cells. Signatures of APOBEC activity include G-to-A and C-to-T mutations in a particular sequence context. The importance of APOBEC activity for viral inhibition is reflected in the identification of numerous viral factors, including HIV-1 Vif, which are dedicated to antagonism of these deaminases. Such viral antagonists often are only partially successful, leading to APOBEC selection for viral variants that enhance replication or avoid immune elimination.

NF-κB signaling induces inductive expression of the downstream molecules and IgD gene in the freshwater carp, Catla catla

Toll-like receptors (TLRs) in innate immune system act as primary sensors in detecting the microbial components and activate their signaling cascades to induce NF-κB (nuclear factor NF-κB) towards the augmentation of immunoglobulin (Ig) synthesis. To gain insights into the efficacy of NF-κB pathway in immunoglobulin D (IgD) synthesis in the Indian Major Carp Catla catla, cloning and sequencing of TLR-signaling downstream molecules [TRAF3 (TNF receptor-associated factor 3), NEMO (nuclear factor-kappa B essential modulator), NF-κB and BAFF (B cell activating factor)] were performed by infecting the fish with pathogens. mRNA expression analysis of the downstream molecules and IgD showed significant up-regulation of these genes in kidney (P ≤ 0.001) as compared to spleen (P ≤ 0.05). To ascertain the role of NF-κB pathway in IgD synthesis, the primary cell culture of kidney and spleen in monolayer cell suspension was treated with NF-κB inhibitor (BAY 11-7082) and down-regulation of BAFF, NEMO, NF-κB, and IgD gene was observed. These results highlight the importance of NF-κB signaling pathway in augmenting the IgD gene expression in the freshwater carp, Catla catla.

A cutting-edge immunoinformatics approach for design of multi-epitope oral vaccine against dreadful human malaria

Human malaria is a pathogenic disease mainly caused by Plasmodium falciparum, which was responsible for about 405,000 deaths globally in the year 2018. To date, several vaccine candidates have been evaluated for prevention, which failed to produce optimal output at various preclinical/clinical stages. This study is based on designing of polypeptide vaccines (PVs) against human malaria that cover almost all stages of life-cycle of Plasmodium and for the same 5 genome derived predicted antigenic proteins (GDPAP) have been used. For the development of a multi-immune inducer, 15 PVs were initially designed using T-cell epitope ensemble, which covered >99% human population as well as linear B-cell epitopes with or without adjuvants. The immune simulation of PVs showed higher levels of T-cell and B-cell activities compared to positive and negative vaccine controls. Furthermore, in silico cloning of PVs and codon optimization followed by enhanced expression within Lactococcus lactis host system was also explored. Although, the study has sound theoretical and in silico findings, the in vitro/in vivo evaluation seems imperative to warrant the immunogenicity and safety of PVs towards management of P. falciparum infection in the future.

Integrated Gene Expression Profiling Analysis Reveals Probable Molecular Mechanism and Candidate Biomarker in Anti-TNFα Non-Response IBD Patients

Purpose

To explore the molecular mechanism and search for candidate biomarkers in the gene expression profile of IBD patients associated with the response to anti-TNFα agents.

Methods

Differentially expressed genes (DEGs) of response vs non-response IBD patients in datasets GSE12251, GSE16879, and GSE23597 were integrated using NetworkAnalyst. We conducted functional enrichment analysis of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and extracted hub genes from the protein–protein interaction network. The proportion of immune cell types was estimated via CIBERSORT. ROC curve analysis and binomial Lasso regression were applied to assess the expression level of hub genes in datasets GSE12251, GSE16879, and GSE23597, and another two datasets GSE107865 and GSE42296.

Results

A total of 287 DEGs were obtained from the integrated dataset. They were enriched in 14 Gene Ontology terms and 11 KEGG pathways. Polarization from M2 to M1 macrophages was relatively high in non-response individuals. We found nine hub genes (TLR4, TLR1, TLR8, CCR1, CD86, CCL4, HCK, and FCGR2A), mainly related to the interaction between Toll-like Receptor (TLR) pathway and FcγR signaling in non-response anti-TNFα individuals. FCGR2A, HCK, TLR1, TLR4, TLR8, and CCL4 show great value for prediction in intestinal tissue. Besides, FCGR2A, HCK, and TLR8 might be candidate blood biomarkers of anti-TNFα non-response IBD patients.

Conclusion

Over-activated interaction between FcγR-TLR axis in the innate immune cells of IBD patients might be used to identify non-response individuals and increased our understanding of resistance to anti-TNFα therapy.

An Inflammatory Story: Antibodies in Tuberculosis Comorbidities

Mycobacterium tuberculosis (Mtb) resides in a quarter of the world's population and is the causative agent for tuberculosis (TB), the most common infectious reason of death in humans today. Although cellular immunity has been firmly established in the control of Mtb, there is growing evidence that antibodies may also modulate the infection. More specifically, certain antibody features are associated with inflammation and are divergent in different states of human infection and disease. Importantly, TB impacts not just the healthy but also those with chronic conditions. While HIV represents the quintessential comorbid condition for TB, recent epidemiological evidence shows that additional chronic conditions such as diabetes and kidney disease are rising. In fact, the prevalence of diabetes as a comorbid TB condition is now higher than that of HIV. These chronic diseases are themselves independently associated with pro-inflammatory immune states that encompass antibody profiles. This review discusses isotypes, subclasses, post-translational modifications and Fc-mediated functions of antibodies in TB infection and in the comorbid chronic conditions of HIV, diabetes, and kidney diseases. We propose that inflammatory antibody profiles, which are a marker of active TB, may be an important biomarker for detection of TB disease progression within comorbid individuals. We highlight the need for future studies to determine which inflammatory antibody profiles are the consequences of comorbidities and which may potentially contribute to TB reactivation.

The Nod2 Agonist Muramyl Dipeptide Cooperates with the TLR4 Agonist Lipopolysaccharide to Enhance IgG2b Production in Mouse B Cells

Many studies have shown that Toll-like receptors (TLRs) and Nod-like receptors (NLRs) were expressed in B cells and their signaling affects B cell functions. Nonetheless, the roles played by these receptors in B cell antibody (Ab) production have not been completely elucidated. In the present study, we examined the effect of the Nod2 agonist muramyl dipeptide (MDP) in combination with the TLR4 agonist lipopolysaccharide (LPS), a well-known B cell mitogen, on B cell viability, proliferation, and activation, and Ab production by in vitro culture of purified mouse spleen resting B cells. MDP combined with LPS to reinforce B cell viability, proliferation, and activation. Moreover, MDP enhanced LPS-induced IgG2b production, germline γ2b transcript (GLTγ2b) expression, and surface IgG2b expression. In an experiment with Nod2- and TLR4-deficient mouse B cells, we observed that the combined effect of MDP and LPS is dependent on Nod2 and TLR4 receptors. Furthermore, the combined effect on B cell viability and IgG2b switching was not observed in Rip2-deficient mouse cells. Collectively, this study suggests that Nod2 signaling enhances TLR4-activated B cell proliferation, IgG2b switching, and IgG2b production.

Immunoglobulin G structure and rheumatoid factor epitopes

Antibodies are important for immunity and exist in several classes (IgM, IgD, IgA, IgG, IgE). They are composed of symmetric dimeric molecules with two antigen binding regions (Fab) and a constant part (Fc), usually depicted as Y-shaped molecules. Rheumatoid factors found in patients with rheumatoid arthritis are autoantibodies binding to IgG and paradoxically appear to circulate in blood alongside with their antigen (IgG) without reacting with it. Here, it is shown that rheumatoid factors do not react with native IgG in solution, and that their epitopes only become accessible upon certain physico-chemical treatments (e.g. heat treatment at 57 °C), by physical adsorption on a hydrophobic surface or by antigen binding. Moreover, chemical cross-linking in combination with mass spectrometry showed that the native state of IgG is a compact (closed) form and that the Fab parts of IgG shield the Fc region and thereby control access of rheumatoid factors and presumably also some effector functions. It can be inferred that antibody binding to pathogen surfaces induces a conformational change, which exposes the Fc part with its effector sites and rheumatoid factor epitopes. This has strong implications for understanding antibody structure and physiology and necessitates a conceptual reformulation of IgG models.

IgG and Fcγ Receptors in Intestinal Immunity and Inflammation

Fcγ receptors (FcγR) are cell surface glycoproteins that mediate cellular effector functions of immunoglobulin G (IgG) antibodies. Genetic variation in FcγR genes can influence susceptibility to a variety of antibody-mediated autoimmune and inflammatory disorders, including systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). More recently, however, genetic studies have implicated altered FcγR signaling in the pathogenesis of inflammatory bowel disease (IBD), a condition classically associated with dysregulated innate and T cell immunity. Specifically, a variant of the activating receptor, FcγRIIA, with low affinity for IgG, confers protection against the development of ulcerative colitis, a subset of IBD, leading to a re-evaluation of the role of IgG and FcγRs in gastrointestinal tract immunity, an organ system traditionally associated with IgA. In this review, we summarize our current understanding of IgG and FcγR function at this unique host-environment interface, from the pathogenesis of colitis and defense against enteropathogens, its contribution to maternal-fetal cross-talk and susceptibility to cancer. Finally, we discuss the therapeutic implications of this information, both in terms of how FcγR signaling pathways may be targeted for the treatment of IBD and how FcγR engagement may influence the efficacy of therapeutic monoclonal antibodies in IBD.

Diversity of Immunoglobulin (Ig) Isotypes and the Role of Activation-Induced Cytidine Deaminase (AID) in Fish

The disparate diversity in immunoglobulin (Ig) repertoire has been a subject of fascination since the emergence of prototypic adaptive immune system in vertebrates. The carboxy terminus region of activation-induced cytidine deaminase (AID) has been well established in tetrapod lineage and is crucial for its function in class switch recombination (CSR) event of Ig diversification. The absence of CSR in the paraphyletic group of fish is probably due to changes in catalytic domain of AID and lack of cis-elements in IgH locus. Therefore, understanding the arrangement of Ig genes in IgH locus and functional facets of fish AID opens up new realms of unravelling the alternative mechanisms of isotype switching and antibody diversity. Further, the teleost AID has been recently reported to have potential of catalyzing CSR in mammalian B cells by complementing AID deficiency in them. In that context, the present review focuses on the recent advances regarding the generation of diversity in Ig repertoire in the absence of AID-regulated class switching in teleosts and the possible role of T cell-independent pathway involving B cell activating factor and a proliferation-inducing ligand in activation of CSR machinery.

ETS1 and PAX5 transcription factors recruit AID to Igh DNA

B lymphocytes optimize antibody responses by class switch recombination (CSR), which changes the expressed constant region exon of the immunoglobulin heavy chain (IgH), and by somatic hypermutation (SH) that introduces point mutations in the variable regions of the antibody genes. Activation-induced cytidine deaminase (AID) is the key mutagenic enzyme that initiates both these antibody diversification processes by deaminating cytosine to uracil. Here we asked the question if transcription factors can mediate the specific targeting of the antibody diversification by recruiting AID. We have recently reported that AID is together with the transcription factors E2A, PAX5 and IRF4 in a complex on key sequences of the Igh locus. Here we report that also ETS1 is together with AID in this complex on key sequences of the Igh locus in splenic B cells of mice. Furthermore, we show that both ETS1 and PAX5 can directly recruit AID to DNA sequences from the Igh locus with the specific binding site for the transcription factor. Taken together, our findings support the notion of a targeting mechanism for the selective diversification of antibody genes with limited genome wide mutagenesis by recruitment of AID by PAX5 and ETS1 in a transcription factor complex.

Predictable Irreversible Switching Between Acute and Chronic Inflammation

Many a disease associates with inflammation. Upon binding of antigen-antibody complexes to immunoglobulin-like receptors, mast cells release tumor necrosis factor-α and proteases, causing fibroblasts to release endogenous antigens that may be cross reactive with exogenous antigens. We made a predictive dynamic map of the corresponding extracellular network. In silico, this map cleared bacterial infections, via acute inflammation, but could also cause chronic inflammation. In the calculations, limited inflammation flipped to strong inflammation when cross-reacting antigen exceeded an “On threshold.” Subsequent reduction of the antigen load to below this “On threshold” did not remove the strong inflammation phenotype unless the antigen load dropped below a much lower and subtler “Off” threshold. In between both thresholds, the network appeared caught either in a “low” or a “high” inflammatory state. This was not simply a matter of bi-stability, however, the transition to the “high” state was temporarily revertible but ultimately irreversible: removing antigen after high exposure reduced the inflammatory phenotype back to “low” levels but if then the antigen dosage was increased only a little, the high inflammation state was already re-attained. This property may explain why the high inflammation state is indeed “chronic,” whereas only the naive low-inflammation state is “acute.” The model demonstrates that therapies of chronic inflammation such as with anti-IgLC should require fibroblast implantation (or corresponding stem cell activation) for permanence in order to redress the irreversible transition.

Regulation of the DNA Repair Complex during Somatic Hypermutation and Class-Switch Recombination

B lymphocytes optimize Ab responses by somatic hypermutation (SH), which introduces point mutations in the variable regions of the Ab genes and by class-switch recombination (CSR), which changes the expressed C region exon of the IgH. These Ab diversification processes are initiated by the deaminating enzyme activation-induced cytidine deaminase followed by many DNA repair enzymes, ultimately leading to deletions and a high mutation rate in the Ab genes, whereas DNA lesions made by activation-induced cytidine deaminase are repaired with low error rate on most other genes. This indicates an advanced regulation of DNA repair. In this study, we show that initiation of Ab diversification in B lymphocytes of mouse spleen leads to formation of a complex between many proteins in DNA repair. We show also that BCR activation, which signals the end of successful SH, reduces interactions between some proteins in the complex and increases other interactions in the complex with varying kinetics. Furthermore, we show increased localization of SH- and CSR-coupled proteins on switch regions of the Igh locus upon initiation of SH/CSR and differential changes in the localization upon BCR signaling, which terminates SH. These findings provide early evidence for a DNA repair complex or complexes that may be of functional significance for carrying out essential roles in SH and/or CSR in B cells.

B Cell-Intrinsic mTORC1 Promotes Germinal Center-Defining Transcription Factor Gene Expression, Somatic Hypermutation, and Memory B Cell Generation in Humoral Immunity

B lymphocytes migrate among varied microenvironmental niches during diversification, selection, and conversion to memory or Ab-secreting plasma cells. Aspects of the nutrient milieu differ within these lymphoid microenvironments and can influence signaling molecules such as the mechanistic target of rapamycin (mTOR). However, much remains to be elucidated as to the B cell-intrinsic functions of nutrient-sensing signal transducers that modulate B cell differentiation or Ab affinity. We now show that the amino acid-sensing mTOR complex 1 (mTORC1) is vital for induction of Bcl6-a key transcriptional regulator of the germinal center (GC) fate-in activated B lymphocytes. Accordingly, disruption of mTORC1 after B cell development and activation led to reduced populations of Ag-specific memory B cells as well as plasma cells and GC B cells. In addition, induction of the germ line transcript that guides activation-induced deaminase in selection of the IgG1 H chain region during class switching required mTORC1. Expression of the somatic mutator activation-induced deaminase was reduced by a lack of mTORC1 in B cells, whereas point mutation frequencies in Ag-specific GC-phenotype B cells were only halved. These effects culminated in a B cell-intrinsic defect that impacted an antiviral Ab response and drastically impaired generation of high-affinity IgG1. Collectively, these data establish that mTORC1 governs critical B cell-intrinsic mechanisms essential for establishment of GC differentiation and effective Ab production.

Profiling Atlantic salmon B cell populations: CpG-mediated TLR-ligation enhances IgM secretion and modulates immune gene expression

While TLR-activated pathways are key regulators of B cell responses in mammals, their impact on teleost B cells are scarcely addressed. Here, the potential of Atlantic salmon B cells to respond to TLR ligands was shown by demonstrating a constitutive expression of nucleic-acid sensing TLRs in magnetic sorted IgM+ cells. Of the two receptors recognizing CpG in teleosts, tlr9 was the dominating receptor with over ten-fold higher expression than tlr21. Upon CpG-stimulation, IgM secretion increased for head kidney (HK) and splenic IgM+ cells, while blood B cells were marginally affected. The results suggest that CpG directly affects salmon B cells to differentiate into antibody secreting cells (ASCs). IgM secretion was also detected in the non-treated controls, again with the highest levels in the HK derived population, signifying that persisting ASCs are present in this tissue. In all tissues, the IgM+ cells expressed high MHCII levels, suggesting antigen-presenting functions. Upon CpG-treatment the co-stimulatory molecules cd83 and cd40 were upregulated, while cd86 was down-regulated under the same conditions. Finally, ifna1 was upregulated upon CpG-stimulation in all tissues, while a restricted upregulation was evident for ifnb, proposing that salmon IgM+ B cells exhibit a type I IFN-response.

Toll-like Receptor 1/2 Agonist Pam3CSK4 Suppresses Lipopolysaccharide-driven IgG1 Production while Enhancing IgG2a Production by B Cells

Interaction between pathogen-associated molecular patterns and pattern recognition receptors triggers innate and adaptive immune responses. Several studies have reported that toll-like receptors (TLRs) are involved in B cell proliferation, differentiation, and Ig class switch recombination (CSR). However, roles of TLRs in B cell activation and differentiation are not completely understood. In this study, we investigated the direct effect of stimulation of TLR1/2 agonist Pam3CSK4 on mouse B cell viability, proliferation, activation, Ig production, and Ig CSR in vitro. Treatment with 0.5 µg/ml of Pam3CSK4 only barely induced IgG1 production although it enhanced B cell viability. In addition, high-dosage Pam3CSK4 diminished IgG1 production in a dose-dependent manner, whereas the production of other Igs, cell viability, and proliferation increased. Pam3CSK4 additively increased TLR4 agonist lipopolysaccharide (LPS)-induced mouse B cell growth and activation. However, interestingly, Pam3CSK4 abrogated LPS-induced IgG1 production but enhanced LPS-induced IgG2a production. Further, Pam3CSK4 decreased LPS-induced germline γ1 transcripts (GLTγ1)/GLTε expression but increased GLTγ2a expression. On the other hand, Pam3CSK4 had no effect on LPS-induced plasma cell differentiation. Taken together, these results suggest that TLR1/2 agonist Pam3CSK4 acts as a potent mouse B cell mitogen in combination with TLR4 agonist LPS, but these 2 different TLR agonists play diverse roles in regulating the Ig CSR of each isotype, particularly IgG1/IgE and IgG2a.

Enhanced B-cell differentiation driven by advanced cirrhosis resulting in hyperglobulinemia

BACKGROUND AND AIM

The mechanism underlying hyperglobulinemia in cirrhosis, a long appreciated phenomenon, has never been clearly understood. The aim of this study is to investigate the basis for changes in humoral immunity observed in cirrhosis.

METHODS

We retrospectively reviewed our medical record to analyze serum immunoglobulin (Ig) levels in patients with liver disease. We also prospectively analyzed peripheral blood mononuclear cells and sera from liver disease patients. Peripheral blood mononuclear cell surface marker expressions were measured by flow cytometry and serum B-cell-activating factor was measured by enzyme-linked immunosorbent assay. Expression of specific gene expression in magnetically separated B cells was also analyzed by real-time polymerase chain reaction.

RESULTS

In retrospective analysis, we found that advancing cirrhosis, irrespective of underlying etiology or hepatocellular carcinoma, resulted in progressively increasing levels of serum IgG and IgA. In prospective analysis using clinical samples, we demonstrated that advancing cirrhosis stage was associated with increased toll-like-receptor (TLR)9 expression in CD27+ B cell and serum B-cell-activating factor levels but decreased CD27+ memory B-cell frequency. The remaining CD27+ B cells in peripheral blood exhibited increased activation-induced cytidine deaminase mRNA expression. Finally, we also demonstrated isolated B cells from advanced cirrhosis were more reactive to TLR9 stimulation that drove antibody secreting cells differentiation leading to hyperimmunoglobulinemia in vitro.

CONCLUSIONS

Enhanced TLR9-induced differentiation into antibody secreting cell may explain peripheral reductions of circulating CD27+ memory B cells as well as increased serum Ig levels in cirrhosis.

Generation of precursor, immature, and mature murine B1-cell lines from c-myc/bcl-xL-overexpressing pre-BI cells

Deregulated expression of c-myc and bcl-xL is long known to generate transformed B cells in humans and mice. We overexpressed these genes to induce in vitro and in vivo differentiation of fetal liver-derived mouse pre-BI cells to B1-lineage pre-BII-like, immature and mature B-cell lines, and to Ig-secreting cells. In vitro, doxycycline-controlled c-myc/bcl-xL-overexpressing CD19⁺ CD93⁺ c-kikt⁺ IgM^- pre-BI cells differentiate to and survive as CD19⁺ CD93⁺ c-kit^- IgM⁺ immature B1 cells. Timed CpG stimulation of these oncogene-overexpressing pre-B or immature B1 cells generates either CD19⁺ CD93^low c-kit^- IgM^- SLC^- pre-BII-like or IgM⁺ MHCII⁺ CD73⁺ CD80⁺ CD40⁺ mature B1-cell lines and IgM-secreting B1 cells in vitro and fixes their state of differentiation. All cell lines are clonable, but a majority of immature and mature B1-cell clones eventually reach a nonproliferating, surviving G₀ -state. Transplanted in vivo, c-myc/bcl-xL-overexpressing pre-B cells expand to mature B1 cells, and to IgM- and IgA-secreting plasmablasts and plasma cells. Within 2 months, plasmablasts have expanded most prominently in BM and spleen, indicating that the host selectively expanded development of these transformed plasma cells. The sIgM⁺ B1-cell lines and clones offer the possibility to study their roles in the development of B1-Ab repertoires, of B1-cell-mediated autoimmune diseases and of B1-cell malignancies.