How the Signaling Crosstalk of B Cell Receptor (BCR) and Co-Receptors Regulates Antibody Class Switch Recombination: A New Perspective of Checkpoints of BCR Signaling

Immunoglobulin class-switch recombination: Mechanism, regulation, and related diseases

Maturation of the secondary antibody repertoire requires class-switch recombination (CSR), which switches IgM to other immunoglobulins (Igs), and somatic hypermutation, which promotes the production of high-affinity antibodies. Following immune response or infection within the body, activation of T cell-dependent and T cell-independent antigens triggers the activation of activation-induced cytidine deaminase, initiating the CSR process. CSR has the capacity to modify the functional properties of antibodies, thereby contributing to the adaptive immune response in the organism. Ig CSR defects, characterized by an abnormal relative frequency of Ig isotypes, represent a rare form of primary immunodeficiency. Elucidating the molecular basis of Ig diversification is essential for a better understanding of diseases related to Ig CSR defects and could provide clues for clinical diagnosis and therapeutic approaches. Here, we review the most recent insights on the diversification of five Ig isotypes and choose several classic diseases, including hyper-IgM syndrome, Waldenström macroglobulinemia, hyper-IgD syndrome, selective IgA deficiency, hyper-IgE syndrome, multiple myeloma, and Burkitt lymphoma, to illustrate the mechanism of Ig CSR deficiency. The investigation into the underlying mechanism of Ig CSR holds significant potential for the advancement of increasingly precise diagnostic and therapeutic approaches.

B cells mediate lung ischemia/reperfusion injury by recruiting classical monocytes via synergistic B cell receptor/TLR4 signaling

Ischemia/reperfusion injury-mediated (IRI-mediated) primary graft dysfunction (PGD) adversely affects both short- and long-term outcomes after lung transplantation, a procedure that remains the only treatment option for patients suffering from end-stage respiratory failure. While B cells are known to regulate adaptive immune responses, their role in lung IRI is not well understood. Here, we demonstrated by intravital imaging that B cells are rapidly recruited to injured lungs, where they extravasate into the parenchyma. Using hilar clamping and transplant models, we observed that lung-infiltrating B cells produce the monocyte chemokine CCL7 in a TLR4-TRIF-dependent fashion, a critical step contributing to classical monocyte (CM) recruitment and subsequent neutrophil extravasation, resulting in worse lung function. We found that synergistic BCR-TLR4 activation on B cells is required for the recruitment of CMs to the injured lung. Finally, we corroborated our findings in reperfused human lungs, in which we observed a correlation between B cell infiltration and CM recruitment after transplantation. This study describes a role for B cells as critical orchestrators of lung IRI. As B cells can be depleted with currently available agents, our study provides a rationale for clinical trials investigating B cell-targeting therapies.

B-Cell Receptor Signaling and Beyond: The Role of Igα (CD79a)/Igβ (CD79b) in Normal and Malignant B Cells

B-cell receptor (BCR) is a B cell hallmark surface complex regulating multiple cellular processes in normal as well as malignant B cells. Igα (CD79a)/Igβ (CD79b) are essential components of BCR that are indispensable for its functionality, signal initiation, and signal transduction. CD79a/CD79b-mediated BCR signaling is required for the survival of normal as well as malignant B cells via a wide signaling network. Recent studies identified the great complexity of this signaling network and revealed the emerging role of CD79a/CD79b in signal integration. In this review, we have focused on functional features of CD79a/CD79b, summarized signaling consequences of CD79a/CD79b post-translational modifications, and highlighted specifics of CD79a/CD79b interactions within BCR and related signaling cascades. We have reviewed the complex role of CD79a/CD79b in multiple aspects of normal B cell biology and how is the normal BCR signaling affected by lymphoid neoplasms associated CD79A/CD79B mutations. We have also summarized important unresolved questions and highlighted issues that remain to be explored for better understanding of CD79a/CD79b-mediated signal transduction and the eventual identification of additional therapeutically targetable BCR signaling vulnerabilities.

B cells in tumor metastasis: friend or foe?

Metastasis is an important cause of cancer-related death. Immunotherapy may be an effective way to prevent and treat tumor metastasis in the future. Currently, many studies have focused on T cells, whereas fewer have focused on B cells and their subsets. B cells play an important role in tumor metastasis. They not only secrete antibodies and various cytokines but also function in antigen presentation to directly or indirectly participate in tumor immunity. Furthermore, B cells are involved in both inhibiting and promoting tumor metastasis, which demonstrates the complexity of B cells in tumor immunity. Moreover, different subgroups of B cells have distinct functions. The functions of B cells are also affected by the tumor microenvironment, and the metabolic homeostasis of B cells is also closely related to their function. In this review, we summarize the role of B cells in tumor metastasis, analyze the mechanisms of B cells, and discuss the current status and prospects of B cells in immunotherapy.

Upregulated Expression of the IL-9 Receptor on TRAF3-Deficient B Lymphocytes Confers Ig Isotype Switching Responsiveness to IL-9 in the Presence of Antigen Receptor Engagement and IL-4

The pleiotropic cytokine IL-9 signals to target cells by binding to a heterodimeric receptor consisting of the unique subunit IL-9R and the common subunit γ-chain shared by multiple cytokines of the γ-chain family. In the current study, we found that the expression of IL-9R was strikingly upregulated in mouse naive follicular B cells genetically deficient in TNFR-associated factor 3 (TRAF3), a critical regulator of B cell survival and function. The highly upregulated IL-9R on Traf3-/- follicular B cells conferred responsiveness to IL-9, including IgM production and STAT3 phosphorylation. Interestingly, IL-9 significantly enhanced class switch recombination to IgG1 induced by BCR crosslinking plus IL-4 in Traf3-/- B cells, which was not observed in littermate control B cells. We further demonstrated that blocking the JAK-STAT3 signaling pathway abrogated the enhancing effect of IL-9 on class switch recombination to IgG1 induced by BCR crosslinking plus IL-4 in Traf3-/- B cells. Our study thus revealed, to our knowledge, a novel pathway that TRAF3 suppresses B cell activation and Ig isotype switching by inhibiting IL-9R-JAK-STAT3 signaling. Taken together, our findings provide (to our knowledge) new insights into the TRAF3-IL-9R axis in B cell function and have significant implications for the understanding and treatment of a variety of human diseases involving aberrant B cell activation such as autoimmune disorders.

How do nuclear factor kappa B (NF-κB)1 and NF-κB2 defects lead to the incidence of clinical and immunological manifestations of inborn errors of immunity?

INTRODUCTION

Genetic defects affect the manner of the immune system's development, activation, and function. Nuclear factor-kappa B subunit 1 (NF-κB1) and NF-κB2 are involved in different biological processes, and deficiency in these transcription factors may reveal clinical and immunological difficulties.

AREAS COVERED

This review article gathers the most frequent clinical and immunological remarkable characteristics of NF-κB1 and NF-κB2 deficiencies. Afterward, an effort is made to describe the biological mechanism, which is likely to be the cause of these clinical and immunological abnormalities.

EXPERT OPINION

The present review article has explained the mechanism of contributions of the NF-κB1 and NF-κB2 deficiency in revealing immunodeficiency symptoms, specifically immunological and clinical manifestations. These mechanisms demonstrate the importance of NF-κB1 and NF-κB2 signaling pathways for B and T cell development, activation, antibody production, and immunotolerance. The manifestation of a mutation can range from no symptoms to severe complications in a family.

Molecular Bases of Protein Antigenicity and Determinants of Immunogenicity, Anergy, and Mitogenicity

BACKGROUND

The immune system is able to recognize substances that originate from inside or outside the body and are potentially harmful. Foreign substances that bind to immune system components exhibit antigenicity and are defined as antigens. The antigens exhibiting immunogenicity can induce innate or adaptive immune responses and give rise to humoral or cell-mediated immunity. The antigens exhibiting mitogenicity can cross-link cell membrane receptors on B and T lymphocytes leading to cell proliferation. All antigens vary greatly in physicochemical features such as biochemical nature, structural complexity, molecular size, foreignness, solubility, and so on.

OBJECTIVE

Thus, this review aims to describe the molecular bases of protein-antigenicity and those molecular bases that lead to an immune response, lymphocyte proliferation, or unresponsiveness.

CONCLUSION

The epitopes of an antigen are located in surface areas; they are about 880-3,300 Da in size. They are protein, carbohydrate, or lipid in nature. Soluble antigens are smaller than 1 nm and are endocytosed less efficiently than particulate antigens. The more the structural complexity of an antigen increases, the more the antigenicity increases due to the number and variety of epitopes. The smallest immunogens are about 4,000-10,000 Da in size. The more phylogenetically distant immunogens are from the immunogen-recipient, the more immunogenicity increases. Antigens that are immunogens can trigger an innate or adaptive immune response. The innate response is induced by antigens that are pathogen-associated molecular patterns. Exogenous antigens, T Dependent or T Independent, induce humoral immunogenicity. TD protein-antigens require two epitopes, one sequential and one conformational to induce antibodies, whereas, TI non-protein-antigens require only one conformational epitope to induce low-affinity antibodies. Endogenous protein antigens require only one sequential epitope to induce cell-mediated immunogenicity.

Chronic CD40L blockade is required for long-term cardiac allograft survival with a clinically relevant CTLA4-Ig dosing regimen

Introduction

In de-novo kidney transplantation, the CTLA4-Ig fusion protein belatacept is associated with improved graft function but also an increased risk of acute rejection compared to calcineurin inhibitor therapy. The combination with a second costimulation blocker could potentially improve outcome while avoiding calcineurin inhibitor toxicity. The aim of this study was to define the conditions under which the combination of CTLA4-Ig and CD40L blockade leads to rejection-free permanent graft survival in a stringent murine heart transplantation model.

Methods

Naïve wild-type or CD40L (CD154) knock-out mice received a fully mismatched BALB/c cardiac allograft. Selected induction and maintenance protocols for CTLA4-Ig and blocking αCD40L monoclonal antibodies (mAB) were investigated. Graft survival, rejection severity and donor-specific antibody (DSA) formation were assessed during a 100-day follow-up period.

Results and Discussion

Administering αCD40L mAb as monotherapy at the time of transplantation significantly prolonged heart allograft survival but did not further improve the outcome when given in addition to chronic CTLA4-Ig therapy (which prolongs graft survival to a median of 22 days). Likewise, chronic αCD40L mAb therapy (0.5mg) combined with perioperative CTLA4-Ig led to rejection in a proportion of mice and extensive histological damage, despite abrogating DSA formation. Only the permanent interruption of CD40-CD40L signaling by using CD40L-/- recipient mice or by chronic αCD40L administration synergized with chronic CTLA4-Ig to achieve long-term allograft survival with preserved histological graft integrity in all recipients without DSA formation. The combination of α-CD40L and CTLA4-Ig works most effectively when both therapeutics are administered chronically.

Contribution of T- and B-cell intrinsic toll-like receptors to the adaptive immune response in viral infectious diseases

Toll-like receptors (TLRs) comprise a class of highly conserved molecules that recognize pathogen-associated molecular patterns and play a vital role in host defense against multiple viral infectious diseases. Although TLRs are highly expressed on innate immune cells and play indirect roles in regulating antiviral adaptive immune responses, intrinsic expression of TLRs in adaptive immune cells, including T cells and B cells, cannot be ignored. TLRs expressed in CD4 + and CD8 + T cells play roles in enhancing TCR signal-induced T-cell activation, proliferation, function, and survival, serving as costimulatory molecules. Gene knockout of TLR signaling molecules has been shown to diminish antiviral adaptive immune responses and affect viral clearance in multiple viral infectious animal models. These results have highlighted the critical role of TLRs in the long-term immunological control of viral infection. This review summarizes the expression and function of TLR signaling pathways in T and B cells, focusing on the in vitro and vivo mechanisms and effects of intrinsic TLR signaling in regulating T- and B-cell responses during viral infection. The potential clinical use of TLR-based immune regulatory drugs for viral infectious diseases is also explored.

Suppressing Immune Responses Using Siglec Ligand-Decorated Anti-receptor Antibodies

The sialic acid-binding immunoglobulin-type lectins (Siglecs) are expressed predominantly on white blood cells and participate in immune cell recognition of self. Most Siglecs contain cytoplasmic inhibitory immunoreceptor tyrosine-based inhibitory motifs characteristic of inhibitory checkpoint co-receptors that suppress cell signaling when they are recruited to the immunological synapse of an activating receptor. Antibodies to activatory receptors typically activate immune cells by ligating the receptors on the cell surface. Here, we report that the conjugation of high affinity ligands of Siglecs to antibodies targeting activatory immune receptors can suppress receptor-mediated activation of immune cells. Indeed, B-cell activation by antibodies to the B-cell receptor IgD is dramatically suppressed by conjugation of anti-IgD with high affinity ligands of a B-cell Siglec CD22/Siglec-2. Similarly, degranulation of mast cells induced by antibodies to IgE, which ligate the IgE/FcεR1 receptor complex, is suppressed by conjugation of anti-IgE to high affinity ligands of a mast cell Siglec, CD33/Siglec-3 (CD33L). Moreover, the anti-IgE-CD33L suppresses anti-IgE-mediated systemic anaphylaxis of sensitized humanized mice and prevents anaphylaxis upon subsequent challenge with anti-IgE. The results demonstrate that attachment of ligands of inhibitory Siglecs to anti-receptor antibodies can suppress the activation of immune cells and modulate unwanted immune responses.

Multimericity Amplifies the Synergy of BCR and TLR4 for B Cell Activation and Antibody Class Switching

Sustained signaling through the B cell antigen receptor (BCR) is thought to occur only when antigen(s) crosslink or disperse multiple BCR units, such as by multimeric antigens found on the surfaces of viruses or bacteria. B cell-intrinsic Toll-like receptor (TLR) signaling synergizes with the BCR to induce and shape antibody production, hallmarked by immunoglobulin (Ig) class switch recombination (CSR) of constant heavy chains from IgM/IgD to IgG, IgA or IgE isotypes, and somatic hypermutation (SHM) of variable heavy and light chains. Full B cell differentiation is essential for protective immunity, where class switched high affinity antibodies neutralize present pathogens, memory B cells are held in reserve for future encounters, and activated B cells also serve as semi-professional APCs for T cells. But the rules that fine-tune B cell differentiation remain partially understood, despite their being essential for naturally acquired immunity and for guiding vaccine development. To address this in part, we have developed a cell culture system using splenic B cells from naive mice stimulated with several biotinylated ligands and antibodies crosslinked by streptavidin reagents. In particular, biotinylated lipopolysaccharide (LPS), a Toll-like receptor 4 (TLR4) agonist, and biotinylated anti-IgM were pre-assembled (multimerized) using streptavidin, or immobilized on nanoparticles coated with streptavidin, and used to active B cells in this precisely controlled, high throughput assay. Using B cell proliferation and Ig class switching as metrics for successful B cell activation, we show that the stimuli are both synergistic and dose-dependent. Crucially, the multimerized immunoconjugates are most active over a narrow concentration range. These data suggest that multimericity is an essential requirement for B cell BCR/TLRs ligands, and clarify basic rules for B cell activation. Such studies highlight the importance in determining the choice of single vs multimeric formats of antigen and PAMP agonists during vaccine design and development.

Characterization of Definitive Regulatory B Cell Subsets by Cell Surface Phenotype, Function and Context

Regulatory B cell or “Breg” is a broad term that represents the anti-inflammatory activity of B cells, but does not describe their individual phenotypes, specific mechanisms of regulation or relevant disease contexts. Thus, given the variety of B cell regulatory mechanisms reported in human disease and their animal models, a more thorough and comprehensive identification strategy is needed for tracking and comparing B cell subsets between research groups and in clinical settings. This review summarizes the discovery process and mechanism of action for well-defined regulatory B cell subsets with an emphasis on the mouse model of multiple sclerosis experimental autoimmune encephalomyelitis. We discuss the importance of conducting thorough B cell phenotyping along with mechanistic studies prior to defining a particular subset of B cells as Breg. Since virtually all B cell subsets can exert regulatory activity, it is timely for their definitive identification across studies.