A pro-inflammatory role of Fcα/μR on marginal zone B cells in sepsis

Distribution characteristics of FcμR positive cells in small intestinal lymph nodes of Bactrian camel

Exploring the expression characteristics of FcμR in small intestinal lymph nodes of bactrian camels can lay the foundation for further revealing the function of FcμR. The FcμR expression characteristics were systematically analysed by using prokaryotic expression, antibody preparation, immunohistochemical staining and statistical analysis. FcμR positive cells were mainly located in the lymphoid follicles and their numbers decreased in the order of duodenal lymph nodes, jejunal lymph nodes and ileal lymph nodes, and the number of positive cells was statistically significant between different intestinal segments (P<0.05). The FcμR is expressed in lymphoid follicular B cells, which not only facilitates the body's ability to regulate secretory IgM levels, but also acts as a local immune defence barrier. The small intestine has dual functions of immune tolerance and immune response, the proximal part mainly focuses on immune tolerance, and the distal part mainly focuses on immune response. This distribution ensures the unity of the duodenal absorption and immune defence, and also significantly increases the efficiency of the entire small intestine, which is why the number of FcμR positive cells decreases in the order of duodenal lymph nodes, jejunal lymph nodes and ileal lymph nodes.

Insights into the Roles of B Cells in Patients with Sepsis

Sepsis is a life-threatening yet common disease, still posing high mortality worldwide. Sepsis-related deaths primarily occur during immunosuppression; the disease can hamper the numbers and function of B cells, which mediate innate and adaptive immune responses to maintain immune homeostasis. Dysfunction of B cells, along with aggravated immunosuppression, are closely related to poor prognosis. However, B cells in patients with sepsis have garnered little attention. This article focuses on the significance of B-cell subsets, including regulatory B cells, in sepsis and how the counts and function of circulating B cells are affected in patients with sepsis. Finally, potential B-cell-related immunotherapies for sepsis are explored.

The emerging roles and therapeutic potential of B cells in sepsis

Sepsis is a life-threatening syndrome caused by anomalous host response to infection. The pathogenesis of sepsis is complex, and immune dysfunction is the central link in its occurrence and development. The sepsis immune response is not a local and transient process but a complex and continuous process involving all major cell types of innate and adaptive immunity. B cells are traditionally studied for their ability to produce antibodies in the context of mediating humoral immunity. However, over the past few years, B cells have been increasingly recognized as key modulators of adaptive and innate immunity, and they can participate in immune responses by presenting antigens, producing cytokines, and modulating other immune cells. Recently, increasing evidence links B-cell dysfunction to mechanisms of immune derangement in sepsis, which has drawn attention to the powerful properties of this unique immune cell type in sepsis. Here, we reviewed the dynamic alterations of B cells and their novel roles in animal models and patients with sepsis, and provided new perspectives for therapeutic strategies targeting B cells in sepsis.

Immunoglobulin M in Health and Diseases: How Far Have We Come and What Next?

B lymphocytes are important in secreting antibodies that protect against invading pathogens such as viruses, bacteria, parasites, and also in mediating pathogenesis of allergic diseases and autoimmunity. B lymphocytes develop in the bone marrow and contain heavy and light chains, which upon ligation form an immunoglobulin M (IgM) B cell receptor (BCR) expressed on the surface of naïve immature B cells. Naïve B cells expressing either IgM or IgD isotypes are thought to play interchangeable functions in antibody responses to T cell-dependent and T cell-independent antigens. IgM short-lived plasma cells (SLPCs) and antigen-specific IgM memory B cells (MBCs-M) are critical in the first few days of infection, as well as long-term memory induced by vaccination, respectively. At mucosal surfaces, IgM is thought to play a critical part in promoting mucosal tolerance and shaping microbiota together with IgA. In this review, we explore how IgM structure and BCR signaling shapes B cell development, self and non-self-antigen-specific antibody responses, responses to infectious (such as viruses, parasites, and fungal) and non-communicable diseases (such as autoimmunity and allergic asthma). We also explore how metabolism could influence other B cell functions such as mucosal tolerance and class switching. Finally, we discuss some of the outstanding critical research questions in both experimental and clinical settings targeting IgM.

Gut T cell-independent IgA responses to commensal bacteria require engagement of the TACI receptor on B cells

The gut mounts secretory immunoglobulin A (SIgA) responses to commensal bacteria through nonredundant T cell-dependent (TD) and T cell-independent (TI) pathways that promote the establishment of mutualistic host-microbiota interactions. SIgAs from the TD pathway target penetrant bacteria, and their induction requires engagement of CD40 on B cells by CD40 ligand on T follicular helper cells. In contrast, SIgAs from the TI pathway bind a larger spectrum of bacteria, but the mechanism underpinning their production remains elusive. Here, we show that the intestinal TI pathway required CD40-independent B cell-activating signals from TACI, a receptor for the innate CD40 ligand-like factors BAFF and APRIL. TACI-induced SIgA responses targeted a fraction of the gut microbiota without shaping its overall composition. Of note, TACI was dispensable for TD induction of IgA in gut-associated lymphoid organs. Thus, BAFF/APRIL signals acting on TACI orchestrate commensal bacteria-specific SIgA responses through an intestinal TI program.

The increased marginal zone B cells attenuates early inflammatory responses during sepsis in Gpr174 deficient mice

GPR174 plays a crucial role in immune responses, but the role of GPR174 in the pathological progress of sepsis remains incompletely understood. In this study, we generated a sepsis model by cecal ligation and puncture (CLP) to investigate the role of GPR174 in regulating functions and underlying mechanism of marginal zone B (MZ B) cells in sepsis. We found that in Gpr174 deficient mice, the number of splenic MZ B cells was increased. Moreover, Gpr174^-/- MZ B cells exhibited an enhanced response to LPS stimulation in vitro. By using the CLP-induced sepsis model, we demonstrated that the increased MZ B cells attenuated early inflammatory responses during sepsis. RNA sequencing results revealed that the expression of c-fos in splenic B lymphocytes was upregulated in Gpr174 deficient mice. However, the protective role of increased MZ B cells in Gpr174 deficient mice was weakened by a c-fos-specific inhibitor. Collectively, these findings suggested that GPR174 plays an immunomodulatory role in early immune responses during sepsis through the regulation of MZ B cells.

Considerations for the Design of Antibody-Based Therapeutics

Antibody-based proteins have become an important class of biologic therapeutics, due in large part to the stability, specificity, and adaptability of the antibody framework. Indeed, antibodies not only have the inherent ability to bind both antigens and endogenous immune receptors but also have proven extremely amenable to protein engineering. Thus, several derivatives of the monoclonal antibody format, including bispecific antibodies, antibody-drug conjugates, and antibody fragments, have demonstrated efficacy for treating human disease, particularly in the fields of immunology and oncology. Reviewed here are considerations for the design of antibody-based therapeutics, including immunological context, therapeutic mechanisms, and engineering strategies. First, characteristics of antibodies are introduced, with emphasis on structural domains, functionally important receptors, isotypic and allotypic differences, and modifications such as glycosylation. Then, aspects of therapeutic antibody design are discussed, including identification of antigen-specific variable regions, choice of expression system, use of multispecific formats, and design of antibody derivatives based on fragmentation, oligomerization, or conjugation to other functional moieties. Finally, strategies to enhance antibody function through protein engineering are reviewed while highlighting the impact of fundamental biophysical properties on protein developability.

Role of the IgM Fc Receptor in Immunity and Tolerance

Immunoglobulin (Ig) M is the first antibody isotype to appear during evolution, ontogeny and immune responses. IgM not only serves as the first line of host defense against infections but also plays an important role in immune regulation and immunological tolerance. For many years, IgM is thought to function by binding to antigen and activating complement system. With the discovery of the IgM Fc receptor (FcμR), it is now clear that IgM can also elicit its function through FcμR. In this review, we will describe the molecular characteristics of FcμR, its role in B cell development, maturation and activation, humoral immune responses, host defense, and immunological tolerance. We will also discuss the functional relationship between IgM-complement and IgM-FcμR pathways in regulating immunity and tolerance. Finally, we will discuss the potential involvement of FcμR in human diseases.