B cell receptor and Toll-like receptor signaling coordinate to control distinct B-1 responses to both self and the microbiota

A B cell screen against endogenous retroviruses identifies glycan-reactive IgM that recognizes a broad array of enveloped viruses

Endogenous retroviruses (ERVs), comprising a substantial portion of the vertebrate genome, are remnants of ancient genetic invaders. ERVs with near-intact coding potential reactivate in B cell-deficient mice. To study how B cells contribute to host anti-ERV immunity, we used an antigen-baiting strategy to enrich B cells reactive to ERV surface antigens. We identified ERV-reactive B-1 cells expressing germline-encoded natural IgM antibodies in naïve mice, the level of which further increases upon innate immune sensor stimulation. B cell receptor repertoire profiling of ERV-reactive B-1 cells revealed increased usage of the Igh VH gene that gives rise to glycan-specific antibodies targeting terminal N-acetylglucosamine moieties on ERV glycoproteins, which further engage the complement pathway to mediate anti-ERV responses. These same antibodies also recognize glycoproteins of other enveloped viruses but not self-proteins. These results reveal an innate antiviral mechanism of germline-encoded antibodies with broad reactivity to enveloped viruses, which constitutes a natural antibody repertoire capable of preventing the emergence of infectious ERVs.

Toll-like receptors in health and disease

Toll-like receptors (TLRs) are inflammatory triggers and belong to a family of pattern recognition receptors (PRRs) that are central to the regulation of host protective adaptive immune responses. Activation of TLRs in innate immune myeloid cells directs lymphocytes to produce the most appropriate effector responses to eliminate infection and maintain homeostasis of the body's internal environment. Inappropriate TLR stimulation can lead to the development of general autoimmune diseases as well as chronic and acute inflammation, and even cancer. Therefore, TLRs are expected to be targets for therapeutic treatment of inflammation-related diseases, autoimmune diseases, microbial infections, and human cancers. This review summarizes the recent discoveries in the molecular and structural biology of TLRs. The role of different TLR signaling pathways in inflammatory diseases, autoimmune diseases such as diabetes, cardiovascular diseases, respiratory diseases, digestive diseases, and even cancers (oral, gastric, breast, colorectal) is highlighted and summarizes new drugs and related clinical treatments in clinical trials, providing an overview of the potential and prospects of TLRs for the treatment of TLR-related diseases.

Aortic Cellular Heterogeneity in Health and Disease: Novel Insights Into Aortic Diseases From Single-Cell RNA Transcriptomic Data Sets

Aortic diseases such as atherosclerosis, aortic aneurysms, and aortic stiffening are significant complications that can have significant impact on end-stage cardiovascular disease. With limited pharmacological therapeutic strategies that target the structural changes in the aorta, surgical intervention remains the only option for some patients with these diseases. Although there have been significant contributions to our understanding of the cellular architecture of the diseased aorta, particularly in the context of atherosclerosis, furthering our insight into the cellular drivers of disease is required. The major cell types of the aorta are well defined; however, the advent of single-cell RNA sequencing provides unrivaled insights into the cellular heterogeneity of each aortic cell type and the inferred biological processes associated with each cell in health and disease. This review discusses previous concepts that have now been enhanced with recent advances made by single-cell RNA sequencing with a focus on aortic cellular heterogeneity.

Intergenerational protective anti-gut commensal immunoglobulin G originates in early life

Maternal immunoglobulins of the class G (IgGs) protect offspring from enteric infection, but when, where, and how these antibodies are physiologically generated and confer protection remains enigmatic. We found that circulating IgGs in adult mice preferentially bind early-life gut commensal bacteria over their own adult gut commensal bacteria. IgG-secreting plasma cells specific for early-life gut bacteria appear in the intestine soon after weaning, where they remain into adulthood. Manipulating exposure to gut bacteria or plasma cell development before, but not after, weaning reduced IgG-secreting plasma cells targeting early-life gut bacteria throughout life. Further, the development of this anti-gut commensal IgG response coincides with the early-life interval in which goblet cell-associated antigen passages (GAPs) are present in the colon. Offspring of dams "perturbed" by B cell ablation or reduced bacterial exposure in early life were more susceptible to enteric pathogen challenge. In contrast to current concepts, protective maternal IgGs targeted translocating gut commensals in the offspring, not the enteric pathogen. These early-life events affecting anti-commensal IgG production have intergenerational effects for protection of the offspring.

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

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

Loss of B1 and marginal zone B cells during ovarian cancer

Recent advances in immunotherapy have not addressed the challenge presented by ovarian cancer. Although the peritoneum is an "accessible" locus for this disease there has been limited characterization of the immunobiology therein. We investigated the ID8-C57BL/6J ovarian cancer model and found marked depletion of B1 cells from the ascites of the peritoneal cavity. There was also selective loss of the B1 and marginal zone B cell subsets from the spleen. Immunity to antigens that activate these subsets validated their loss rather than relocation. A marked influx of myeloid-derived suppressor cells correlated with B cell subset depletion. These observations are discussed in the context of the housekeeping burden placed on innate B cells during ovarian cancer and to foster consideration of B cell biology in therapeutic strategies to address this challenge.

B-1a Cells, but Not Marginal Zone B Cells, Are Implicated in the Accumulation of Autoreactive Plasma Cells in Lyn-/- Mice

Mice deficient in Lyn, a tyrosine kinase that limits B cell activation, develop a lupus-like autoimmune disease characterized by the accumulation of splenic plasma cells and the production of autoantibodies. Lyn-/- mice have reduced numbers of marginal zone (MZ) B cells, a B cell subset that is enriched in autoreactivity and prone to plasma cell differentiation. We hypothesized that this is due to unchecked terminal differentiation of this potentially pathogenic B cell subpopulation. However, impairing MZ B cell development in Lyn-/- mice did not reduce plasma cell accumulation or autoantibodies, and preventing plasma cell differentiation did not restore MZ B cell numbers. Instead, Lyn-/- mice accumulated B-1a cells when plasma cell differentiation was impaired. Similar to MZ B cells, B-1a cells tend to be polyreactive or weakly autoreactive and are primed for terminal differentiation. Our results implicate B-1a cells, but not MZ B cells, as contributors to the autoreactive plasma cell pool in Lyn-/- mice.

Multiple Factors Determine the Oncolytic or Carcinogenic Effects of TLRs Activation in Cancer

Toll‐like receptors (TLRs) belong to a germline‐encoded protein family. These are pattern recognition receptors. They sense pathogen‐associated molecular patterns (PAMPs). When this occurs, activation of the NF‐ĸB pathway follows. This triggers the innate immune response of the host. The consequent inflammatory cytokine response usually contributes to the elimination of the pathogen. Activation of TLRs also induces an adaptive immune response by a cross‐prime mechanism. This mechanism is employed in cancer immunotherapy. Using TLR ligands as adjuvants induces upregulation of costimulatory signals which in turn activates a cytotoxic leukocyte response against cancer cells. However, TLRs are also overexpressed in human cancer cells resulting in increased cell proliferation, migration, invasion, and angiogenesis. An intracellular adaptor, myeloid differentiation factor 88 (MyD88) probably mediates this process. MyD88 is intimately involved with all TLRs except TLR3. One consequence of the interaction between a TLR and MyD88 is activation of NF‐ĸB. In this context of a variety of proinflammtory cytokines being produced, chronic inflammation may result. Inflammation is an important protective mechanism. However, chronic inflammation is also involved in carcinogenesis. Activation of NF‐ĸB inhibits apoptosis and under certain circumstances, tumor cell survival. In this review, the potential therapeutic value of TLRs in immunotherapy and its role in oncogenesis are explored. The emerging use of artificial intelligence is mentioned.

Noncanonical B Cells: Characteristics of Uncharacteristic B Cells

B lymphocytes were originally described as a cell type uniquely capable of secreting Abs. The importance of T cell help in Ab production was revealed soon afterward. Following these seminal findings, investigators made great strides in delineating steps in the conventional pathway that B cells follow to produce high-affinity Abs. These studies revealed generalized, or canonical, features of B cells that include their developmental origin and paths to maturation, activation, and differentiation into Ab-producing and memory cells. However, along the way, examples of nonconventional B cell populations with unique origins, age-dependent development, tissue localization, and effector functions have been revealed. In this brief review, features of B-1a, B-1b, marginal zone, regulatory, killer, NK-like, age-associated, and atypical B cells are discussed. Emerging work on these noncanonical B cells and functions, along with the study of their significance for human health and disease, represents an exciting frontier in B cell biology.

B cells and the intestinal microbiome in time, space and place

The gut immune system is shaped by the continuous interaction with the microbiota. Here we dissect temporal, spatial and contextual layers of gut B cell responses. The microbiota impacts on the selection of the developing pool of pre-immune B cells that serves as substrate for B cell activation, expansion and differentiation. However, various aspects of the gut B cell response display unique features. In particular, occurrence of somatically mutated B cells, chronic gut germinal centers in T cell-deficient settings and polyreactive binding of gut IgA to the microbiota questioned the nature and microbiota-specificity of gut germinal centers. We propose a model to reconcile these observations incorporating recent work demonstrating microbiota-specificity of gut germinal centers. We speculate that adjuvant effects of the microbiota might modify permissiveness for B cell to enter and exit gut germinal centers. We propose that separating aspects of time, space and place facilitate the occasionally puzzling discussion of gut B cell responses to the microbiota.

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.

B-1 cells in immunotoxicology: Mechanisms underlying their response to chemicals and particles

Since their discovery nearly 40 years ago, B-1 cells have continued to challenge the boundaries between innate and adaptive immunity, as well as myeloid and lymphoid functions. This B-cell subset ensures early immunity in neonates before the development of conventional B (B-2) cells and respond to immune injuries throughout life. B-1 cells are multifaceted and serve as natural- and induced-antibody-producing cells, phagocytic cells, antigen-presenting cells, and anti-/pro-inflammatory cytokine-releasing cells. This review retraces the origin of B-1 cells and their different roles in homeostatic and infectious conditions before focusing on pollutants comprising contact-sensitivity-inducing chemicals, endocrine disruptors, aryl hydrocarbon receptor (AHR) ligands, and reactive particles.

B-1 plasma cells require non-cognate CD4 T cell help to generate a unique repertoire of natural IgM

Evolutionarily conserved, "natural" (n)IgM is broadly reactive to both self and foreign antigens. Its selective deficiency leads to increases in autoimmune diseases and infections. In mice, nIgM is secreted independent of microbial exposure to bone marrow (BM) and spleen B-1 cell-derived plasma cells (B-1PC), generating the majority of nIgM, or by B-1 cells that remain non-terminally differentiated (B-1sec). Thus, it has been assumed that the nIgM repertoire is broadly reflective of the repertoire of body cavity B-1 cells. Studies here reveal, however, that B-1PC generate a distinct, oligoclonal nIgM repertoire, characterized by short CDR3 variable immunoglobulin heavy chain regions, 7-8 amino acids in length, some public, many arising from convergent rearrangements, while specificities previously associated with nIgM were generated by a population of IgM-secreting B-1 (B-1sec). BM, but not spleen B-1PC, or B-1sec also required the presence of TCRαβ CD4 T cells for their development from fetal precursors. Together, the studies identify important previously unknown characteristics of the nIgM pool.

Virus-associated fungal infections and lost immune resistance

Invasive fungal infections are an increasing threat to human health. Of recent concern is the emergence of influenza- or SARS-CoV-2-virus-associated invasive fungal infections. Understanding acquired susceptibilities to fungi requires consideration of the collective and newly explored roles of adaptive, innate, and natural immunity. Neutrophils are known to provide host resistance, but new concepts are emerging that implicate innate antibodies, the actions of specialized B1 B cell subsets, and B cell-neutrophil crosstalk in mediating antifungal host resistance. Based on emerging evidence, we propose that virus infections impact on neutrophil and innate B cell resistance against fungi, leading to invasive infections. These concepts provide novel approaches to developing candidate therapeutics with the aim of restoring natural and humoral immunity and boosting neutrophil resistance against fungi.

Age-related changes in antigen-specific natural antibodies are influenced by sex

Introduction

Natural antibody (NAb) derived from CD5+ B-1 cells maintains tissue homeostasis, controls inflammation, aids in establishing long-term protective responses against pathogens, and provides immediate protection from infection. CD5+ B-1 cell NAbs recognize evolutionarily fixed epitopes, such as phosphatidylcholine (PtC), found on bacteria and senescent red blood cells. Anti-PtC antibodies are essential in protection against bacterial sepsis. CD5+ B-1 cell-derived NAbs have a unique germline-like structure that lacks N-additions, a feature critical for providing protection against infection. Previously, we demonstrated the repertoire and germline status of PtC+CD5+ B-1 cell IgM obtained from male mice changes with age depending on the anatomical location of the B-1 cells. More recently, we demonstrated serum antibody from aged female mice maintains protection against pneumococcal infection, whereas serum antibody from male mice does not provide protection.

Results

Here, we show that aged female mice have significantly more splenic PtC+CD5+ B-1 cells and more PtC specific serum IgM than aged male mice. Furthermore, we find both age and biological sex related repertoire differences when comparing B cell receptor (BCR) sequencing results of PtC+CD5+ B-1 cells. While BCR germline status of PtC+CD5+ B-1 cells from aged male and female mice is similar in the peritoneal cavity, it differs significantly in the spleen, where aged females retain germline configuration and aged males do not. Nucleic acid sensing toll-like receptors are critical in the maintenance of PtC+ B-1 cells; therefore, to begin to understand the mechanism of differences observed between the male and female PtC+CD5+ B-1 cell repertoire, we analyzed levels of cell-free nucleic acids and found increases in aged females.

Conclusion

Our results suggest the antigenic milieu differs between aged males and females, leading to differential selection of antigen-specific B-1 cells over time. Further elucidation of how biological sex differences influence the maintenance of B-1 cells within the aging environment will be essential to understand sex and age-related disparities in the susceptibility to bacterial infection and will aid in the development of more effective vaccination and/or therapeutic strategies specific for males and females.

A B1a-natural IgG-neutrophil axis is impaired in viral- and steroid-associated aspergillosis

The lung naturally resists Aspergillus fumigatus (Af) in healthy individuals, but multiple conditions can disrupt this resistance, leading to lethal invasive infections. Core processes of natural resistance and its breakdown are undefined. We investigated three distinct conditions predisposing to lethal aspergillosis-severe SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) infection, influenza A viral pneumonia, and systemic corticosteroid use-in human patients and murine models. We found a conserved and essential coupling of innate B1a lymphocytes, Af-binding natural immunoglobulin G antibodies, and lung neutrophils. Failure of this axis concealed Af from neutrophils, allowing rapid fungal invasion and disease. Reconstituting the axis with immunoglobulin therapy reestablished resistance, thus representing a realistic pathway to repurpose currently available therapies. Together, we report a vital host resistance pathway that is responsible for protecting against life-threatening aspergillosis in the context of distinct susceptibilities.

Do B-1 cells play a role in response to Mycobacterium tuberculosis Beijing lineages?

We highlight the need to include an analysis of the B-1 B cell subset to complement the characterization of the cell-mediated immune response to the Mycobacterium tuberculosis Beijing lineage. The literature describes the B-1 cell repertoire's involvement in the cell-mediated response within granulomas, which is different from the classic antibody response B cells are generally associated with. Specifically, the B-1 B cell subset migrates from other compartments along with other cells to the infection site. We provide details to complement the reported results from Cerezo-Cortes et al.

E6020, a TLR4 Agonist Adjuvant, Enhances Both Antibody Titers and Isotype Switching in Response to Immunization with Hapten-Protein Antigens and Is Diminished in Mice with TLR4 Signaling Insufficiency

The mechanisms by which TLR4-based adjuvants enhance immunogenicity are not fully understood. We have taken advantage of a novel knock-in mouse strain that homozygously expresses two single-nucleotide polymorphisms (SNPs) that are homologous to human TLR4 (rs4986790 and rs4986791) and have been associated with LPS hyporesponsiveness in vivo and in vitro. TLR4-SNP (coexpressing mutations D298G/N397I in TLR4) mice that recapitulate the human phenotype were compared with wild-type (WT) mice for their hapten-specific Ab responses after immunization with hapten 4-hydroxy-3-nitrophenyl acetyl (NP) NP-Ficoll or NP-OVA in the absence or presence of a water-soluble TLR4 analog adjuvant, E6020. IgM and IgG anti-NP responses were comparable in WT and TLR4-SNP mice after immunization with either NP-Ficoll or NP-OVA only. E6020 significantly yet transiently improved the IgM and IgG anti-NP responses of both WT and TLR4-SNP mice to NP-Ficoll (T-independent), with modestly enhanced Ab production in WT mice. In contrast, T-dependent (NP-OVA), adjuvant-enhanced responses showed sustained elevation of NP-specific Ab titers in WT mice, intermediate responses in TLR4-SNP mice, and negligible enhancement in TLR4-/- mice. E6020-enhanced early humoral responses in WT and TLR4-SNP mice to NP-OVA favored an IgG1 response. After a second immunization, however, the immune responses of TLR4-SNP mice remained IgG1 dominant, whereas WT mice reimmunized with NP-OVA and E6020 exhibited increased anti-NP IgG2c titers and a sustained increase in the IgG1 and IgG2c production by splenocytes. These findings indicate that E6020 increases and sustains Ab titers and promotes isotype class switching, as evidenced by reduced titers and IgG1-dominant immune responses in mice with TLR4 insufficiency.

Immunization of preterm infants: current evidence and future strategies to individualized approaches

Preterm infants are at particularly high risk for infectious diseases. As this vulnerability extends beyond the neonatal period into childhood and adolescence, preterm infants benefit greatly from infection-preventive measures such as immunizations. However, there is an ongoing discussion about vaccine safety and efficacy due to preterm infants' distinct immunological features. A significant proportion of infants remains un- or under-immunized when discharged from primary hospital stay. Educating health care professionals and parents, promoting maternal immunization and evaluating the potential of new vaccination tools are important means to reduce the overall burden from infectious diseases in preterm infants. In this narrative review, we summarize the current knowledge about vaccinations in premature infants. We discuss the specificities of early life immunity and memory function, including the role of polyreactive B cells, restricted B cell receptor diversity and heterologous immunity mediated by a cross-reactive T cell repertoire. Recently, mechanistic studies indicated that tissue-resident memory (Trm) cell populations including T cells, B cells and macrophages are already established in the fetus. Their role in human early life immunity, however, is not yet understood. Tissue-resident memory T cells, for example, are diminished in airway tissues in neonates as compared to older children or adults. Hence, the ability to make specific recall responses after secondary infectious stimulus is hampered, a phenomenon that is transcriptionally regulated by enhanced expression of T-bet. Furthermore, the microbiome establishment is a dominant factor to shape resident immunity at mucosal surfaces, but it is often disturbed in the context of preterm birth. The proposed function of Trm T cells to remember benign interactions with the microbiome might therefore be reduced which would contribute to an increased risk for sustained inflammation. An improved understanding of Trm interactions may determine novel targets of vaccination, e.g., modulation of T-bet responses and facilitate more individualized approaches to protect preterm babies in the future.

Anti-Toxoplasma gondii IgM Long Persistence: What Are the Underlying Mechanisms?

Diagnosis of Toxoplasma gondii acute infection was first attempted by detection of specific IgM antibodies, as for other infectious diseases. However, it was noted that this immunoglobulin declines slowly and may last for months or even years. Apart from the diagnostic problem imposed on clinical management, this phenomenon called our attention due to the underlying phenomena that may be causing it. We performed a systematic comparison of reports studying IgM antibody kinetics, and the data from the papers were used to construct comparative plots and other graph types. It became clear that this phenomenon is quite generalized, and it may also occur in animals. Moreover, this is not a technical issue, although some tests make more evident the prolonged IgM decay than others. We further investigated biological reasons for its occurrence, i.e., infection dynamics (micro-reactivation–encystment, reinfection and reactivation), parasite strain relevance, as well as host innate, natural B cell responses and Ig class-switch problems inflicted by the parasite. The outcomes of these inquiries are presented and discussed herein.

T-bet+ B cells Dominate the Peritoneal Cavity B Cell Response during Murine Intracellular Bacterial Infection

T-bet⁺ B cells have emerged as a major B cell subset associated with both protective immunity and immunopathogenesis. T-bet is a transcription factor associated with the type I adaptive immune response to intracellular pathogens, driving an effector program characterized by the production of IFN-γ. Murine infection with the intracellular bacterium, Ehrlichia muris, generates protective extrafollicular T cell-independent T-bet⁺ IgM-secreting plasmablasts, as well as T-bet⁺ IgM memory cells. Although T-bet is a signature transcription factor for this subset, it is dispensable for splenic CD11c⁺ memory B cell development, but not for class switching to IgG2c. In addition to the T-bet⁺ plasmablasts found in the spleen, we show that Ab-secreting cells can also be found within the mouse peritoneal cavity; these cells, as well as their CD138^- counterparts, also expressed T-bet. A large fraction of the T-bet⁺ peritoneal B cells detected during early infection were highly proliferative and expressed CXCR3 and CD11b, but, unlike in the spleen, they did not express CD11c. T-bet⁺ CD11b⁺ memory B cells were the dominant B cell population in the peritoneal cavity at 30 d postinfection, and although they expressed high levels of T-bet, they did not require B cell-intrinsic T-bet expression for their generation. Our data uncover a niche for T-bet⁺ B cells within the peritoneal cavity during intracellular bacterial infection, and they identify this site as a reservoir for T-bet⁺ B cell memory.

Relapse of Hepatitis C Virus Cryoglobulinemic Vasculitis After Sustained Viral Response After Interferon-Free Direct-Acting Antivirals

INTRODUCTION

Direct-acting antiviral agents (DAAs) have modified the management of chronic hepatitis C virus (HCV) infection, including HCV-related cryoglobulinemic vasculitis (CryoVas). However, patients might experience vasculitis relapse, and no reliable predictors of CryoVas relapse after sustained virologic response (SVR) have been established. We aimed to describe HCV-CryoVas relapse rates and factors associated with it.

METHODS

An international multicenter cohort where patients with HCV-CryoVas from Egypt, France, and Italy treated with DAA were analyzed retrospectively. Factors associated with relapse-free survival were evaluated in a multivariate-adjusted model.

RESULTS

Of 913 patients, 911 (99.8%) obtained SVR. After 35 months of the median follow-up, 798 patients (87.4%) had sustained remission of vasculitis, while 115 (12.6%) experienced CryoVas relapse. By the time of relapse, skin involvement was present in 100%, renal involvement in 85.2%, and peripheral neuropathy in 81.7%. Relapses were treated with glucocorticoids in 90.9%, associated with plasma exchange, cyclophosphamide, or rituximab in 50%, 37.3%, and 6.4%, respectively. The cumulative incidence of CryoVas relapse was 0.7% (95% CI 0.3-1.4), 12.3% (95% CI 10.2-14.6), and 13.1% (95% CI 11.0-15.5) at 12, 24, and 36 months after DAA treatment, respectively. Independent baseline risk factors associated with CryoVas relapse were male sex, skin ulcers, kidney involvement at baseline, and peripheral neuropathy at the end of DAA treatment. Death occurred in 11 relapsers, mainly due to infections.

DISCUSSION

A substantial proportion of patients with CryoVas experience relapse after DAA-induced SVR. Relapses are moderate-to-severe and affect survival after 24 months, mainly due to infections. Independent risk factors for relapse or death were found.

The Forgotten Brother: The Innate-like B1 Cell in Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease of the central nervous system (CNS), traditionally considered a chronic autoimmune attack against the insulating myelin sheaths around axons. However, the exact etiology has not been identified and is likely multi-factorial. Recently, evidence has been accumulating that implies that autoimmune processes underlying MS may, in fact, be triggered by pathological processes initiated within the CNS. This review focuses on a relatively unexplored immune cell-the "innate-like" B1 lymphocyte. The B1 cell is a primary-natural-antibody- and anti-inflammatory-cytokine-producing cell present in the healthy brain. It has been recently shown that its frequency and function may differ between MS patients and healthy controls, but its exact involvement in the MS pathogenic process remains obscure. In this review, we propose that this enigmatic cell may play a more prominent role in MS pathology than ever imagined. We aim to shed light on the human B1 cell in health and disease, and how dysregulation in its delicate homeostatic role could impact MS. Furthermore, novel therapeutic avenues to restore B1 cells' beneficial functions will be proposed.

nnResearch progress on B cells and thoracic aortic aneurysm/dissection

Thoracic aortic aneurysm/dissection (TAAD) is a rare cardiovascular disease characterized by acute onset, rapid progression and high morbidity and mortality. One of the crucial factors leading to TAAD is the inflammatory response, which is regulated by many immune cell subgroups, including B cells. Compared with normal aortic tissue, the number of B cells in the aortic tissue of TAAD patients is significantly higher. Activated B cells participate in the vascular immune inflammatory response by producing antibodies and inflammatory factors and activating the complement system. These effects can lead to collagen degradation and aortic wall remodeling, both of which are the main pathologic characteristics of TAAD. Therefore, B cells play a key role in the occurrence and development of TAAD. B cells can be divided into B1 cells, B2 cells and regulatory B cells, which have different mechanisms of action in TAAD. This article will review the role of B cells in TAAD from the perspective of three different subtypes of B cells.

Genetic mapping reveals Nfkbid as a central regulator of humoral immunity to Toxoplasma gondii

Protective immunity to parasitic infections has been difficult to elicit by vaccines. Among parasites that evade vaccine-induced immunity is Toxoplasma gondii, which causes lethal secondary infections in chronically infected mice. Here we report that unlike susceptible C57BL/6J mice, A/J mice were highly resistant to secondary infection. To identify correlates of immunity, we utilized forward genetics to identify Nfkbid, a nuclear regulator of NF-κB that is required for B cell activation and B-1 cell development. Nfkbid-null mice (“bumble”) did not generate parasite-specific IgM and lacked robust parasite-specific IgG, which correlated with defects in B-2 cell maturation and class-switch recombination. Though high-affinity antibodies were B-2 derived, transfer of B-1 cells partially rescued the immunity defects observed in bumble mice and were required for 100% vaccine efficacy in bone marrow chimeric mice. Immunity in resistant mice correlated with robust isotype class-switching in both B cell lineages, which can be fine-tuned by Nfkbid gene expression. We propose a model whereby humoral immunity to T. gondii is regulated by Nfkbid and requires B-1 and B-2 cells for full protection.

Eukaryotic parasitic diseases account for approximately one fifth of all childhood deaths, yet no highly protective vaccine exists for any human parasite. More research must be done to discover how to elicit protective vaccine-induced immunity to parasitic pathogens. We used an unbiased genetic screen to find key genes responsible for immunity to the eukaryotic parasite Toxoplasma gondii. Our screen found Nfkbid, a transcription factor regulator, which controls B cell activation and innate-like B-1 cell development. Mice without Nfkbid were not protected against T. gondii and were deficient at making antibodies against the parasite. Our survival studies of vaccinated mice with and without B-1 compartments found that B-1 cells improved survival, suggesting that B-1 cells act in conjunction with B-2 cells to provide vaccine-induced immunity. Nfkbid and other loci identified in our unbiased screen represent potential targets for vaccines to elicit protective immune responses against parasitic pathogens.

Microbial-Driven Immunological Memory and Its Potential Role in Microbiome Editing for the Prevention of Colorectal Cancer

Over the last several years, many advances have been made in understanding the role of bacteria in the pathogenesis of gastrointestinal cancers. Beginning with Helicobacter pylori being recognized as the first bacterial carcinogen and the causative agent of most gastric cancers, more recent studies have examined the role of enteric microbes in colorectal cancer. In the digestive tract, these communities are numerous and have a complex interrelationship with local immune/inflammatory responses that impact the health of the host. As modifying the microbiome in the stomach has decreased the risk of gastric cancer, modifying the distal microbiome may decrease the risk of colorectal cancers. To date, very few studies have considered the notion that mucosal lymphocyte-dependent immune memory may confound attempts to change the microbial components in these communities. The goal of this review is to consider some of the factors impacting host-microbial interactions that affect colorectal cancer and raise questions about how immune memory responses to the local microbial consortium affect any attempt to modify the composition of the intestinal microbiome.

Role of Toll-Like Receptor 4 in Diabetic Retinopathy

Diabetic retinopathy (DR) is the most frequent microvascular complication of diabetes mellitus (DM) and a leading cause of blindness worldwide. Evidence has shown that DR is an inflammatory disease with hyperglycemia playing a causative role in the development of its main features, including inflammation, cellular apoptosis, neurodegeneration, oxidative stress, and neovascularization. Toll-like receptors (TLRs) are a well-known family of pattern recognition receptors (PRRs) responsible for the initiation of inflammatory and immune responses. TLR4 identifies both endogenous and exogenous ligands and is associated with various physiological and pathological pathways in the body. While the detailed pathophysiology of DR is still unclear, increasing data suggests a crucial role for TLR4 in the development of DR. Due to hyperglycemia, TLR4 expression increases in diabetic retina, which activates various pathways leading to DR. Considering the role of TLR4 in DR, several studies have focused on the association of TLR4 polymorphisms and risk of DR development. Moreover, evidence concerning the effect of microRNAs in the pathogenesis of DR, through their interaction with TLR4, indicates the determinant role of TLR4 in this disease. Of note, several agents have proven as effective in alleviating DR through the inhibition of the TLR4 pathway, suggesting new avenues in DR treatment. In this review, we provided a brief overview of the TLR4 structure and biological function and a more comprehensive discussion about the mechanisms of TLR4 activation in DR. Furthermore, we summarized the relationship between TLR4 polymorphisms and risk of DR and the relationship between microRNAs and TLR4 in DR. Finally, we discussed the current progress in designing TLR4 inhibitors, which could be helpful in DR clinical management.

Gut microbiota shape B cell in health and disease settings

Recent accumulating evidence supports the hypothesis that the intricate interaction between gut microbiota and the immune system profoundly affects health and disease in humans and mice. In this context, microbiota plays an important role in educating and shaping the host immune system which, in turn, regulates gut microbiota diversity and function to maintain homeostasis. Studies have demonstrated that intestinal microbiota participates in shaping B cells in health and disease settings. Herein, we review the recent progress in understanding how microbiota regulates B-cell development, focusing on early-life B-cell repertoire generation in GALT and how microbial products, including microbial antigens and metabolites, affect B-cell activation and differentiation to ultimately regulate B-cell function. We also discuss the interaction between gut microbiota and B cells under pathogenic conditions and highlight new approaches that can be applied to treat various diseases.

TLR-MyD88 signaling blockades inhibit refractory B-1b cell immune responses to transplant-related glycan antigens

Refractory B cell responses to T cell-independent (TI) carbohydrate antigens (Ags) are critical drivers of rejection reactions to ABO-incompatible allogeneic grafts and xenogeneic grafts from other species. To explore the biological significance of crosstalk between Toll-like receptors (TLRs) and B cell receptors (BCRs) in the TI B cell immunity, we here used MyD88-, TRIF-, and α-galactosyltransferase-deficient mice to study B cell phenotypes and functional properties during TI transplant-related glycan Ag exposure. BCR stimulation alone induced differentiation into CD5^high (B-1a) cells, which were highly sensitive to a calcineurin inhibitor (CNI), while co-stimulation of TLRs and BCRs induced differentiation into CD5^dim (B-1b) cells in MyD88-dependent and CNI-resistant manner. MyD88-dependent TLR stimulation in B-1b cells enhanced downstream factors in the BCR-calcineurin pathway, including a nuclear factor of activated T cells, cytoplasmic 1 (NFATc1). TLR inhibitor together with CNI abrogated refractory B-1b cell immune responses against the ABO-blood group Ags, while blocking both BCRs and TLR-MyD88 by using Bruton's tyrosine kinase inhibitor and histone deacetylase inhibitor abrogated refractory B-1b cell immune responses against Gal-glycan Ags. Thus, this study provides a rationale for a novel therapeutic approach to overcome refractory transplant-related anti-glycan Ab production by blocking both BCR and TLR-MyD88 signals.

Effects of early nutrition and sanitary conditions on antibody levels in early and later life of broiler chickens

Immune maturation of broiler chickens may be affected by management, such as early life feeding strategy (early versus delayed nutrition) or by low or high sanitary conditions (LSC versus HSC). We compared systemic maternal (MAb), natural (NAb), natural auto- (NAAb), and antigen specific antibody (SpAb) levels (IgM, IgY) between broilers (n = 48 per treatment) that received early (EN) or delayed nutrition for 72 h (DN) housed in either low (LSC) or high sanitary conditions (HSC) between 7 and 35 d of age. We found minimal interactions between feeding strategy and sanitary conditions. At 7 d of age, broilers receiving EN compared with DN, had elevated levels of IgM binding keyhole limpet hemocyanin (KLH), phosphoryl-conjugated ovalbumin (PC-OVA), and muramyl dipeptide (MDP), whereas effects of feeding strategy diminished at later ages. In LSC compared with HSC broilers, levels of NAb agglutinating RRBC and sheep red blood cells (SRBC) were already elevated from 14 d of age onwards. At 33 d of age, antibody levels (NAb, NAAb, anti-LPS, anti-MDP) were all elevated in LSC, compared with HSC broilers, for both IgM and IgY, but not IgM against KLH. Western blotting revealed different binding patterns of NAAb against chicken liver homogenate, which may indicate that the NAAb repertoire is affected by antigenic pressure. Our data suggest that antibody levels are affected for an important part by environmental conditions (feeding strategy and sanitary conditions), but minimally by their interaction. However, it remains to be further studied whether the enhanced levels of antibodies as initiated by EN and LSC contribute to enhanced resistance to infectious diseases.

B-1 Cells Carry the Memory of Neonatal Immune Imprinting

Group A Streptococcus is a common pathogen that elicits a protective humoral response against the cell wall component GlcNAc. In this issue of Immunity, New et al. demonstrate the ability of long-lived B-1 cells to be programmed by microbial colonization and early life immunization to uniquely incorporate GlcNAc reactivity in mice, establishing their critical role in mediating neonatal immune imprinting.

Production and Function of Immunoglobulin A

Among antibodies, IgA is unique because it has evolved to be secreted onto mucosal surfaces. The structure of IgA and the associated secretory component allow IgA to survive the highly proteolytic environment of mucosal surfaces but also substantially limit IgA's ability to activate effector functions on immune cells. Despite these characteristics, IgA is critical for both preventing enteric infections and shaping the local microbiome. IgA's function is determined by a distinct antigen-binding repertoire, composed of antibodies with a variety of specificities, from permissive polyspecificity to cross-reactivity to exquisite specificity to a single epitope, which act together to regulate intestinal bacteria. Development of the unique function and specificities of IgA is shaped by local cues provided by the gut-associated lymphoid tissue, driven by the constantly changing environment of the intestine and microbiota.

Developmental changes in the rules for B cell selection

The autoimmune checkpoint during B cell maturation eliminates self-antigen reactive specificities from the mature B cell repertoire. However, an exception to this rule is illustrated by B-1 cells, an innate-like self-reactive B cell subset that is positively selected into the mature B cell pool in a self-antigen-driven fashion. The mechanisms by which B-1 cells escape central tolerance have puzzled the field for decades. A key clue comes from their restricted developmental window during fetal and neonatal life. Here we use B-1 cells as a prototypic early life derived B cell subset to explore developmental changes in the constraints of B cell selection. We discuss recent advancements in the understanding of the molecular program, centered around the RNA binding protein Lin28b, that licenses self-reactive B-1 cell output during ontogeny. Finally, we speculate on the possible link between the unique rules of early life B cell tolerance and the establishment of B cell - microbial mutualism to propose an integrated model for how developmental and environmental cues come together to create a protective layer of B cell memory involved in neonatal immune imprinting.

The Shaping of a B Cell Pool Maximally Responsive to Infections

B cell subsets differ in development, tissue distribution, and mechanisms of activation. In response to infections, however, all can differentiate into extrafollicular plasmablasts that rapidly provide highly protective antibodies, indicating that these plasmablasts are the main humoral immune response effectors. Yet, the effectiveness of this response type depends on the presence of antigen-specific precursors in the circulating mature B cell pool, a pool that is generated initially through the stochastic processes of B cell receptor assembly. Importantly, germinal centers then mold the repertoire of this B cell pool to be increasingly responsive to pathogens by generating a broad array of antimicrobial memory B cells that act as highly effective precursors of extrafollicular plasmablasts. Such B cell repertoire molding occurs in two ways: continuously via the chronic germinal centers of mucosal lymphoid tissues, driven by the presence of the microbiome, and via de novo generated germinal centers following acute infections. For effectively evaluating humoral immunity as a correlate of immune protection, it might be critical to measure memory B cell pools in addition to antibody titers.

A TLR4-TRIF-dependent signaling pathway is required for protective natural tumor-reactive IgM production by B1 cells

Metastatic cancer involving spread to the peritoneal cavity is referred to as peritoneal carcinomatosis and has a very poor prognosis. Our previous studies demonstrated a toll-like receptor 4 (TLR4) and C-type lectin receptor (CLR; Mincle/MCL) agonist pairing of monophosphoryl lipid A (MPL) and trehalose-6,6'-dicorynomycolate (TDCM) effectively inhibits peritoneal tumor growth and ascites development through a mechanism dependent upon B1a cell-produced natural IgM, complement, and phagocytes. In the current study, we investigated the requirement for TLR4 and Fc receptor common γ chain (FcRγ), required for Mincle/MCL signaling, in the MPL/TDCM-elicited response. MPL/TDCM significantly increased macrophages and Ly6Chi monocytes in the peritoneal cavity of both TLR4-/- and FcRγ-/- mice, suggesting redundancy in the signals required for monocyte/macrophage recruitment. However, B1 cell activation, antibody secreting cell differentiation, and tumor-reactive IgM production were defective in TLR4-/-, but not FcRγ-/- mice. TRIF was required for production of IgM reactive against tumor- and mucin-related antigens, but not phosphorylcholine, whereas TLR4 was required for production of both types of reactivities. Consistent with this, B1 cells lacking TLR4 or TRIF did not proliferate or differentiate into tumor-reactive IgM-producing cells in vitro and did not reconstitute MPL/TDCM-dependent protection against peritoneal carcinomatosis in CD19-/- mice. Our results indicate a TLR4/TRIF-dependent pathway is required by B1 cells for MPL/TDCM-elicited production of protective tumor-reactive natural IgM. The dependency on TRIF signaling for tumor-reactive, but not phosphorylcholine-reactive, IgM production reveals unexpected heterogeneity in TLR4-dependent regulation of natural IgM production, thereby highlighting important differences to consider when designing vaccines or therapies targeting these specificities.

Current Understanding of Natural Antibodies and Exploring the Possibilities of Modulation Using Veterinary Models. A Review

Natural antibodies (NAb) are defined as germline encoded immunoglobulins found in individuals without (known) prior antigenic experience. NAb bind exogenous (e.g., bacterial) and self-components and have been found in every vertebrate species tested. NAb likely act as a first-line immune defense against infections. A large part of NAb, so called natural autoantibodies (NAAb) bind to and clear (self) neo-epitopes, apoptotic, and necrotic cells. Such self-binding antibodies cannot, however, be considered as pathogenic autoantibodies in the classical sense. IgM and IgG NAb and NAAb and their implications in health and disease are relatively well-described in humans and mice. NAb are present in veterinary (and wildlife) species, but their relation with diseases and disorders in veterinary species are much less known. Also, there is little known of IgA NAb. IgA is the most abundant immunoglobulin with essential pro-inflammatory and homeostatic properties urging for more research on the importance of IgA NAb. Since NAb in humans were indicated to fulfill important functions in health and disease, their role in health of veterinary species should be investigated more often. Furthermore, it is unknown whether levels of NAb-isotypes and/or idiotypes can and should be modulated. Veterinary species as models of choice fill in a niche between mice and (non-human) primates, and the study of NAb in veterinary species may provide valuable new insights that will likely improve health management. Below, examples of the involvement of NAb in several diseases in mostly humans are shown. Possibilities of intravenous immunoglobulin administration, targeted immunotherapy, immunization, diet, and genetic modulation are discussed, all of which could be well-studied using animal models. Arguments are given why veterinary immunology should obtain inspiration from human studies and why human immunology would benefit from veterinary models. Within the One Health concept, findings from veterinary (and wildlife) studies can be related to human studies and vice versa so that both fields will mutually benefit. This will lead to a better understanding of NAb: their origin, activation mechanisms, and their implications in health and disease, and will lead to novel health management strategies for both human and veterinary species.

Immunoglobulins with Non-Canonical Functions in Inflammatory and Autoimmune Disease States

Immunoglobulins are known to combine various effector mechanisms of the adaptive and the innate immune system. Classical immunoglobulin functions are associated with antigen recognition and the initiation of innate immune responses. However, in addition to classical functions, antibodies exhibit a variety of non-canonical functions related to the destruction of various pathogens due to catalytic activity and cofactor effects, the action of antibodies as agonists/antagonists of various receptors, the control of bacterial diversity of the intestine, etc. Canonical and non-canonical functions reflect the extreme human antibody repertoire and the variety of antibody types generated in the organism: antigen-specific, natural, polyreactive, broadly neutralizing, homophilic, bispecific and catalytic. The therapeutic effects of intravenous immunoglobulins (IVIg) are associated with both the canonical and non-canonical functions of antibodies. In this review, catalytic antibodies will be considered in more detail, since their formation is associated with inflammatory and autoimmune diseases. We will systematically summarize the diversity of catalytic antibodies in normal and pathological conditions. Translational perspectives of knowledge about natural antibodies for IVIg therapy will be also discussed.

OMV Vaccines and the Role of TLR Agonists in Immune Response

Outer Membrane Vesicles (OMVs) are bacterial nanoparticles that are spontaneously released during growth both in vitro and in vivo by Gram-negative bacteria. They are spherical, bilayered membrane nanostructures that contain many components found within the external surface of the parent bacterium. Naturally, OMVs serve the bacteria as a mechanism to deliver DNA, RNA, proteins, and toxins, as well as to promote biofilm formation and remodel the outer membrane during growth. On the other hand, as OMVs possess the optimal size to be uptaken by immune cells, and present a range of surface-exposed antigens in native conformation and Toll-like receptor (TLR) activating components, they represent an attractive and powerful vaccine platform able to induce both humoral and cell-mediated immune responses. This work reviews the TLR-agonists expressed on OMVs and their capability to trigger individual TLRs expressed on different cell types of the immune system, and then focuses on their impact on the immune responses elicited by OMVs compared to traditional vaccines.

Drivers and regulators of humoral innate immune responses to infection and cancer

The complement cascade consists of cell bound and serum proteins acting together to protect the host from pathogens, remove cancerous cells and effectively links innate and adaptive immune responses. Despite its usefulness in microbial neutralization and clearance of cancerous cells, excessive complement activation causes an immune imbalance and tissue damage in the host. Hence, a series of complement regulatory proteins present at a higher concentration in blood plasma and on cell surfaces tightly regulate the cascade. The complement cascade can be initiated by B-1 B cell production of natural antibodies. Natural antibodies arise spontaneously without any known exogenous antigenic or microbial stimulus and protect against invading pathogens, clear apoptotic cells, provide tissue homeostasis, and modulate adaptive immune functions. Natural IgM antibodies recognize microbial and cancer antigens and serve as an activator of complement mediated lysis. This review will discuss advances in complement activation and regulation in bacterial and viral infections, and cancer. We will also explore the crosstalk of natural antibodies with bacterial populations and cancer.

Nfkbie-deficiency leads to increased susceptibility to develop B-cell lymphoproliferative disorders in aged mice

Aberrant NF-κB activation is a hallmark of most B-cell malignancies. Recurrent inactivating somatic mutations in the NFKBIE gene, which encodes IκBε, an inhibitor of NF-κB-inducible activity, are reported in several B-cell malignancies with highest frequencies in chronic lymphocytic leukemia and primary mediastinal B-cell lymphoma, and account for a fraction of NF-κB pathway activation. The impact of NFKBIE deficiency on B-cell development and function remains, however, largely unknown. Here, we show that Nfkbie-deficient mice exhibit an amplification of marginal zone B cells and an expansion of B1 B-cell subsets. In germinal center (GC)-dependent immune response, Nfkbie deficiency triggers expansion of GC B-cells through increasing cell proliferation in a B-cell autonomous manner. We also show that Nfkbie deficiency results in hyperproliferation of a B1 B-cell subset and leads to increased NF-κB activation in these cells upon Toll-like receptor stimulation. Nfkbie deficiency cooperates with mutant MYD88 signaling and enhances B-cell proliferation in vitro. In aged mice, Nfkbie absence drives the development of an oligoclonal indolent B-cell lymphoproliferative disorders, resembling monoclonal B-cell lymphocytosis. Collectively, these findings shed light on an essential role of IκBε in finely tuning B-cell development and function.

Fc Receptor-Like 6 (FCRL6) Discloses Progenitor B Cell Heterogeneity That Correlates With Pre-BCR Dependent and Independent Pathways of Natural Antibody Selection

B-1a cells produce "natural" antibodies (Abs) to neutralize pathogens and clear neo self-antigens, but the fundamental selection mechanisms that shape their polyreactive repertoires are poorly understood. Here, we identified a B cell progenitor subset defined by Fc receptor-like 6 (FCRL6) expression, harboring innate-like defense, migration, and differentiation properties conducive for natural Ab generation. Compared to FCRL6- pro B cells, the repressed mitotic, DNA damage repair, and signaling activity of FCRL6+ progenitors, yielded VH repertoires with biased distal Ighv segment accessibility, constrained diversity, and hydrophobic and charged CDR-H3 sequences. Beyond nascent autoreactivity, VH11 productivity, which predominates phosphatidylcholine-specific B-1a B cell receptors (BCRs), was higher for FCRL6+ cells as was pre-BCR formation, which was required for Myc induction and VH11, but not VH12, B-1a development. Thus, FCRL6 revealed unexpected heterogeneity in the developmental origins, regulation, and selection of natural Abs at the pre-BCR checkpoint with implications for autoimmunity and lymphoproliferative disorders.

TLR induces reorganization of the IgM-BCR complex regulating murine B-1 cell responses to infections

In mice, neonatally-developing, self-reactive B-1 cells generate steady levels of natural antibodies throughout life. B-1 cells can, however, also rapidly respond to infections with increased local antibody production. The mechanisms regulating these two seemingly very distinct functions are poorly understood, but have been linked to expression of CD5, an inhibitor of BCR-signaling. Here we demonstrate that TLR-mediated activation of CD5+ B-1 cells induced the rapid reorganization of the IgM-BCR complex, leading to the eventual loss of CD5 expression, and a concomitant increase in BCR-downstream signaling, both in vitro and in vivo after infections of mice with influenza virus and Salmonella typhimurium. Both, initial CD5 expression and TLR-mediated stimulation, were required for the differentiation of B-1 cells to IgM-producing plasmablasts after infections. Thus, TLR-mediated signals support participation of B-1 cells in immune defense via BCR-complex reorganization.