The repertoire of serum IgM in normal mice is largely independent of external antigenic contact

Functions of IgM fc receptor (FcµR) related to autoimmunity

Unlike Fc receptors for switched immunoglobulin (Ig) isotypes, Fc receptor for IgM (FcµR) is selectively expressed by lymphocytes. The ablation of the FcµR gene in mice impairs B cell tolerance as evidenced by concomitant production of autoantibodies of IgM and IgG isotypes. In this essay, we reiterate the autoimmune phenotypes observed in mutant mice, ie IgM homeostasis, dysregulated humoral immune responses including autoantibodies, and Mott cell formation. We also propose the potential phenotypes in individuals with FCMR deficiency and the model for FcµR-mediated regulation of self-reactive B cells.

Glycan-Reactive Innate-like B Cells and Developmental Checkpoints

Using an Ig H chain conferring specificity for N-acetyl-d-glucosamine (GlcNAc), we developed transgenic (VHHGAC39 TG) mice to study the role of self-antigens in GlcNAc-reactive B-1 B cell development. In VHHGAC39 TG mice, GlcNAc-reactive B-1 B cell development during ontogeny and in adult bone marrow was normal. However, adult TG mice exhibited a block at transitional-2 immature B cell stages, resulting in impaired allelic exclusion and accumulation of a B cell subset coexpressing endogenous Ig gene rearrangements. Similarly, VHHGAC39 B cell fitness was impeded compared with non-self-reactive VHJ558 B TG cells in competitive mixed bone marrow chimeras. Nonetheless, adult VHHGAC39 mice immunized with Streptococcus pyogenes produce anti-GlcNAc Abs. Peritoneal cavity B cells transferred from VHHGAC39 TG mice into RAG-/- mice also exhibited robust expansion and anti-GlcNAc Ab production. However, chronic treatment of young VHHGAC39 mice with GlcNAc-specific mAbs leads to lower GlcNAc-binding B cell frequencies while increasing the proportion of GlcNAc-binding B1-a cells, suggesting that Ag masking or clearance of GlcNAc Ags impedes maturation of newly formed GlcNAc-reactive B cells. Finally, BCR H chain editing promotes expression of endogenous nontransgenic BCR alleles, allowing potentially self-reactive TG B cells to escape anergy or deletion at the transitional stage of precursor B cell development. Collectively, these observations indicate that GlcNAc-reactive B cell development is sensitive to the access of autologous Ags.

Natural antibodies targeting LPS in pleural effusions of various etiologies

Respiratory infection, cancer, and heart failure can cause abnormal accumulation of fluid in the pleural cavity. The immune responses within the cavity are orchestrated by leucocytes that reside in the serosal-associated lymphoid tissue. Natural antibodies (NAbs) are abundant in the serum (S) having a major role in systemic and mucosal immunity; however, their occurrence in pleural fluid (PF) remains an open question. Our aim herein was to detect and measure the levels of NAbs (IgM, IgG, IgA) targeting lipopolysaccharides (LPS) in both the pleural fluid and the serum of 78 patients with pleural effusions (PEs) of various etiologies. The values of anti-LPS NAb activity were extracted through a normalization step regarding the total IgM, IgG, and IgA levels, all determined by in-house ELISA. In addition, the ratios of PF/S values were analyzed further with other critical biochemical parameters from pleural fluids. Anti-LPS NAbs of all Ig classes were detected in most of the samples, while a significant increase of anti-LPS activity was observed in infectious and noninfectious compared with malignant PEs. Multivariate linear regression confirmed a negative correlation of IgM and IgA anti-LPS PF/S ratio with malignancy. Moreover, anti-LPS NAbs PF/S measurements led to increased positive and negative predictive power in ROC curves generated for the discrimination between benign and malignant PEs. Our results highlight the role of anti-LPS NAbs in the pleural cavity and demonstrate the potential translational impact that should be further explored.NEW & NOTEWORTHY Here we describe the detection and quantification of natural antibodies (NAbs) in the human pleural cavity. We show for the first time that IgM, IgG, and IgA anti-LPS natural antibodies are detected and measured in pleural effusions of infectious, noninfectious, and malignant etiologies and provide clinical correlates to demonstrate the translational impact of our findings.

Galectin-4 Antimicrobial Activity Primarily Occurs Through its C-Terminal Domain

Although immune tolerance evolved to reduce reactivity with self, it creates a gap in the adaptive immune response against microbes that decorate themselves in self-like antigens. This is particularly apparent with carbohydrate-based blood group antigens, wherein microbes can envelope themselves in blood group structures similar to human cells. In this study, we demonstrate that the innate immune lectin, galectin-4 (Gal-4), exhibits strain-specific binding and killing behavior towards microbes that display blood group-like antigens. Examination of binding preferences using a combination of microarrays populated with ABO(H) glycans and a variety of microbial strains, including those that express blood group-like antigens, demonstrated that Gal-4 binds mammalian and microbial antigens that have features of blood group and mammalian-like structures. Although Gal-4 was thought to exist as a monomer that achieves functional bivalency through its two linked carbohydrate recognition domains, our data demonstrate that Gal-4 forms dimers and that differences in the intrinsic ability of each domain to dimerize likely influences binding affinity. While each Gal-4 domain exhibited blood group-binding activity, the C-terminal domain (Gal-4C) exhibited dimeric properties, while the N-terminal domain (Gal-4N) failed to similarly display dimeric activity. Gal-4C not only exhibited the ability to dimerize but also possessed higher affinity toward ABO(H) blood group antigens and microbes expressing glycans with blood group-like features. Furthermore, when compared to Gal-4N, Gal-4C exhibited more potent antimicrobial activity. Even in the context of the full-length protein, where Gal-4N is functionally bivalent by virtue of Gal-4C dimerization, Gal-4C continued to display higher antimicrobial activity. These results demonstrate that Gal-4 exists as a dimer and exhibits its antimicrobial activity primarily through its C-terminal domain. In doing so, these data provide important insight into key features of Gal-4 responsible for its innate immune activity against molecular mimicry.

The role of B-1 cells in cancer progression and anti-tumor immunity

In recent years, in addition to the well-established role of T cells in controlling or promoting tumor growth, a new wave of research has demonstrated the active involvement of B cells in tumor immunity. B-cell subsets with distinct phenotypes and functions play various roles in tumor progression. Plasma cells and activated B cells have been linked to improved clinical outcomes in several types of cancer, whereas regulatory B cells have been associated with disease progression. However, we are only beginning to understand the role of a particular innate subset of B cells, referred to as B-1 cells, in cancer. Here, we summarize the characteristics of B-1 cells and review their ability to infiltrate tumors. We also describe the potential mechanisms through which B-1 cells suppress anti-tumor immune responses and promote tumor progression. Additionally, we highlight recent studies on the protective anti-tumor function of B-1 cells in both mouse models and humans. Understanding the functions of B-1 cells in tumor immunity could pave the way for designing more effective cancer immunotherapies.

Sex, T Cells, and the Microbiome in Natural ABO Antibody Production in Mice

BACKGROUND

"Natural" ABO antibodies (Abs) are produced without known exposure to A/B carbohydrate antigens, posing significant risks for hyperacute rejection during ABO-incompatible transplantation. We investigated anti-A "natural" ABO antibodies versus intentionally induced Abs with regard to the need for T-cell help, the impact of sex, and stimulation by the microbiome.

METHODS

Anti-A was measured by hemagglutination assay of sera from untreated C57BL/6 wild-type (WT) or T cell-deficient mice of both sexes. Human ABO-A reagent blood cell membranes were injected intraperitoneally to induce anti-A Abs. The gut microbiome was eliminated by maintenance of mice in germ-free housing.

RESULTS

Compared with WT mice, CD4 + T-cell knockout (KO), major histocompability complex-II KO, and αβ/γδ T-cell receptor KO mice produced much higher levels of anti-A nAbs; females produced dramatically more anti-A nAbs than males, rising substantially with puberty. Sensitization with human ABO-A reagent blood cell membranes did not induce additional anti-A in KO mice, unlike WT. Sex-matched CD4 + T-cell transfer significantly suppressed anti-A nAbs in KO mice and rendered mice responsive to A-sensitization. Even under germ-free conditions, WT mice of several strains produced anti-A nAbs, with significantly higher anti-A nAbs levels in females than males.

CONCLUSIONS

Anti-A nAbs were produced without T-cell help, without microbiome stimulation, in a sex- and age-dependent manner, suggestive of a role for sex hormones in regulating anti-A nAbs. Although CD4 + T cells were not required for anti-A nAbs, our findings indicate that T cells regulate anti-A nAb production. In contrast to anti-A nAbs, induced anti-A production was T-cell dependent without a sex bias.

Leishmania genetic exchange is mediated by IgM natural antibodies

Host factors that mediate Leishmania genetic exchange are not well defined. Here we demonstrate that natural IgM (IgMn)1-4 antibodies mediate parasite genetic exchange by inducing the transient formation of a spherical parasite clump that promotes parasite fusion and hybrid formation. We establish that IgMn from Leishmania-free animals binds to the surface of Leishmania parasites to induce significant changes in the expression of parasite transcripts and proteins. Leishmania binding to IgMn is partially lost after glycosidase treatment, although parasite surface phosphoglycans, including lipophosphoglycan, are not required for IgMn-induced parasite clumping. Notably, the transient formation of parasite clumps is essential for Leishmania hybridization in vitro. In vivo, we observed a 12-fold increase in hybrid formation in sand flies provided a second blood meal containing IgMn compared with controls. Furthermore, the generation of recombinant progeny from mating hybrids and parental lines were only observed in sand flies provided with IgMn. Both in vitro and in vivo IgM-induced Leishmania crosses resulted in full genome hybrids that show equal patterns of biparental contribution. Leishmania co-option of a host natural antibody to facilitate mating in the insect vector establishes a new paradigm of parasite-host-vector interdependence that contributes to parasite diversity and fitness by promoting genetic exchange.

Mechanisms of Formation of Antibodies against Blood Group Antigens That Do Not Exist in the Body

The system of the four different human blood groups is based on the oligosaccharide antigens A or B, which are located on the surface of blood cells and other cells including endothelial cells, attached to the membrane proteins or lipids. After transfusion, the presence of these antigens on the apical surface of endothelial cells could induce an immunological reaction against the host. The final oligosaccharide sequence of AgA consists of Gal-GlcNAc-Gal (GalNAc)-Fuc. AgB contains Gal-GlcNAc-Gal (Gal)-Fuc. These antigens are synthesised in the Golgi complex (GC) using unique Golgi glycosylation enzymes (GGEs). People with AgA also synthesise antibodies against AgB (group A [II]). People with AgB synthesise antibodies against AgA (group B [III]). People expressing AgA together with AgB (group AB [IV]) do not have these antibodies, while people who do not express these antigens (group O [0; I]) synthesise antibodies against both antigens. Consequently, the antibodies are synthesised against antigens that apparently do not exist in the body. Here, we compared the prediction power of the main hypotheses explaining the formation of these antibodies, namely, the concept of natural antibodies, the gut bacteria-derived antibody hypothesis, and the antibodies formed as a result of glycosylation mistakes or de-sialylation of polysaccharide chains. We assume that when the GC is overloaded with lipids, other less specialised GGEs could make mistakes and synthesise the antigens of these blood groups. Alternatively, under these conditions, the chylomicrons formed in the enterocytes may, under this overload, linger in the post-Golgi compartment, which is temporarily connected to the endosomes. These compartments contain neuraminidases that can cleave off sialic acid, unmasking these blood antigens located below the acid and inducing the production of antibodies.

Autoantibodies to Interferons in Infectious Diseases

Anti-cytokine autoantibodies and, in particular, anti-type I interferons are increasingly described in association with immunodeficient, autoimmune, and immune-dysregulated conditions. Their presence in otherwise healthy individuals may result in a phenotype characterized by a predisposition to infections with several agents. For instance, anti-type I interferon autoantibodies are implicated in Coronavirus Disease 19 (COVID-19) pathogenesis and found preferentially in patients with critical disease. However, autoantibodies were also described in the serum of patients with viral, bacterial, and fungal infections not associated with COVID-19. In this review, we provide an overview of anti-cytokine autoantibodies identified to date and their clinical associations; we also discuss whether they can act as enemies or friends, i.e., are capable of acting in a beneficial or harmful way, and if they may be linked to gender or immunosenescence. Understanding the mechanisms underlying the production of autoantibodies could improve the approach to treating some infections, focusing not only on pathogens, but also on the possibility of a low degree of autoimmunity in patients.

Highly pathogenic natural monoclonal antibody B4-IgM recognizes a post-translational modification comprised of acetylated N-terminal methionine followed by aspartic or glutamic acid

The natural monoclonal antibody B4-IgM recognizes murine annexin 4 (mAn4) and exacerbates ischemia-reperfusion injury in many mouse models. During apoptosis, the intracellular mAn4 protein translocates to the membrane surface, remaining attached to the outer membrane leaflet where it is recognized by the anti-mAn4 B4-IgM antibody. B4-IgM does not recognize human annexin 4 (hAn4). However, the B4-IgM antibody epitope was detected by Western blot of unknown human proteins and by flow cytometry on all studied human cell lines undergoing apoptosis and on a minor subset of healthy cells. The B4-IgM antibody also recognizes the epitope on necrotic cells in cytoplasmic proteins, apparently entering through pores large enough to allow natural antibodies to penetrate the cells and bind to the epitope expressed on self-proteins. Using proteomics and site-directed mutagenesis, we found that B4-IgM binds to an epitope with post-translationally modified acetylated N-terminal methionine, followed by either glutamic or aspartic acid. The epitope is not induced by apoptosis or injury because this modification can also occur during protein translation. This finding reveals an additional novel mechanism whereby injured cells are detected by natural antibodies that initiate pathogenic complement activation through the recognition of epitopes that are shared across multiple proteins found in variable cell lines.

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.

ABO blood group antigens and differential glycan expression: Perspective on the evolution of common human enzyme deficiencies

Enzymes catalyze biochemical reactions and play critical roles in human health and disease. Enzyme variants and deficiencies can lead to variable expression of glycans, which can affect physiology, influence predilection for disease, and/or directly contribute to disease pathogenesis. Although certain well-characterized enzyme deficiencies result in overt disease, some of the most common enzyme deficiencies in humans form the basis of blood groups. These carbohydrate blood groups impact fundamental areas of clinical medicine, including the risk of infection and severity of infectious disease, bleeding risk, transfusion medicine, and tissue/organ transplantation. In this review, we examine the enzymes responsible for carbohydrate-based blood group antigen biosynthesis and their expression within the human population. We also consider the evolutionary selective pressures, e.g. malaria, that may account for the variation in carbohydrate structures and the implications of this biology for human disease.

Antibodies in action: the role of humoral immunity in the fight against atherosclerosis

The sequestering of oxidation-modified low-density lipoprotein by macrophages results in the accumulation of fatty deposits within the walls of arteries. Necrosis of these cells causes a release of intercellular epitopes and the activation of the adaptive immune system, which we predict leads to robust autoantibody production. T cells produce cytokines that act in the plaque environment and further stimulate B cell antibody production. B cells in atherosclerosis meanwhile have a mixed role based on subclass. The current model is that B-1 cells produce protective IgM antibodies in response to oxidation-specific epitopes that work to control plaque formation, while follicular B-2 cells produce class-switched antibodies (IgG, IgA, and IgE) which exacerbate the disease. Over the course of this review, we discuss further the validation of these protective antibodies while evaluating the current dogma regarding class-switched antibodies in atherosclerosis. There are several contradictory findings regarding the involvement of class-switched antibodies in the disease. We hypothesize that this is due to antigen-specificity, and not simply isotype, being important, and that a closer evaluation of these antibodies' targets should be conducted. We propose that specific antibodies may have therapeutical potential in preventing and controlling plaque development within a clinical setting.

Sialylation as an Important Regulator of Antibody Function

Antibodies play a critical role in linking the adaptive immune response to the innate immune system. In humans, antibodies are categorized into five classes, IgG, IgM, IgA, IgE, and IgD, based on constant region sequence, structure, and tropism. In serum, IgG is the most abundant antibody, comprising 75% of antibodies in circulation, followed by IgA at 15%, IgM at 10%, and IgD and IgE are the least abundant. All human antibody classes are post-translationally modified by sugars. The resulting glycans take on many divergent structures and can be attached in an N-linked or O-linked manner, and are distinct by antibody class, and by position on each antibody. Many of these glycan structures on antibodies are capped by sialic acid. It is well established that the composition of the N-linked glycans on IgG exert a profound influence on its effector functions. However, recent studies have described the influence of glycans, particularly sialic acid for other antibody classes. Here, we discuss the role of glycosylation, with a focus on terminal sialylation, in the biology and function across all antibody classes. Sialylation has been shown to influence not only IgG, but IgE, IgM, and IgA biology, making it an important and unappreciated regulator of antibody function.

Role of Programmed Cell Death Protein-1 and Lymphocyte Specific Protein Tyrosine Kinase in the Aryl Hydrocarbon Receptor- Mediated Impairment of the IgM Response in Human CD5+ Innate-Like B Cells

Innate-like B cells (ILBs) are a heterogeneous population B cells which participate in innate and adaptive immune responses. This diverse subset of B cells is characterized by the expression of CD5 and has been shown to secrete high levels of immunoglobulin M (IgM) in the absence of infection or vaccination. Further, CD5+ ILBs have been shown to express high basal levels of lymphocyte specific protein tyrosine kinase (LCK) and programmed cell death protein-1 (PD-1), which are particularly sensitive to stimulation by interferon gamma (IFNγ). Previous studies have demonstrated that activation of the aryl hydrocarbon receptor (AHR), a cytosolic ligand-activated transcription factor, results in suppressed IgM responses and is dependent on LCK. A recent study showed that CD5+ ILBs are particularly sensitive to AHR activation as evidenced by a significant suppression of the IgM response compared to CD5- B cells, which were refractory. Therefore, the objective of this study was to further investigate the role of LCK and PD-1 signaling in AHR-mediated suppression of CD5+ ILBs. In addition, studies were conducted to establish whether IFNγ alters the levels of LCK and PD-1 in CD5+ ILBs. We found that AHR activation led to a significant upregulation of total LCK and PD-1 proteins in CD5+ ILBs, which correlated with suppression of IgM. Interestingly, treatment with recombinant IFNγ reduced LCK protein levels and reversed AHR-mediated IgM suppression in CD5+ ILBs in a similar manner as LCK inhibitors. Collectively, these results support a critical role for LCK and PD-1 in AHR-mediated suppression of the IgM response in human CD5+ ILBs.

Mini Review: Structure and Function of Nematode Phosphorylcholine-Containing Glycoconjugates

An unusual aspect of the biology of nematodes is the covalent attachment of phosphorylcholine (PC) to carbohydrate in glycoconjugates. Investigation of the structure of these molecules by ever-increasingly sophisticated analytical procedures has revealed that PC is generally in phosphodiester linkage with C6 of N-acetylglucosamine (GlcNAc) in both N-type glycans and glycosphingolipids. Up to five PC groups have been detected in the former, being located on both antenna and core GlcNAc. The PC donor for transfer to carbohydrate appears to be phosphatidylcholine but the enzyme responsible for transfer remains to be identified. Work primarily involving the PC-containing Acanthocheilonema viteae secreted product ES-62, has shown that the PC attached to nematode N-glycans possesses a range of immunomodulatory properties, subverting for example, pro-inflammatory signalling in various immune system cell-types including lymphocytes, mast cells, dendritic cells and macrophages. This has led to the generation of PC-based ES-62 small molecule analogues (SMAs), which mirror the parent molecule in preventing the initiation or progression of disease in mouse models of a number of human conditions associated with aberrant inflammatory responses. These include rheumatoid arthritis, systemic lupus erythematosus and lung and skin allergy such that the SMAs are considered to have widespread therapeutic potential.

How Children Are Protected From COVID-19? A Historical, Clinical, and Pathophysiological Approach to Address COVID-19 Susceptibility

The origin and the global spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing coronavirus disease 2019 (COVID-19) in early 2020 was accompanied by high rates of mortality in regions belonging to the ancient silk road, such as the south of China, Iran, Turkey and the northern parts of Italy. However, children seem to be spared in the epidemic as very small percentage worldwide being ill. The protection of children and neonates suggests the involvement of a specific component of adaptive immunity present at early development. Native immunoglobulin belonging to the class of IgM is abundantly present in neonates and children and is known for its recognition of self- and altered self-antigens. Native IgM may be able to neutralize virus by the recognition of endogenous "danger signal" encoded in the viral envelope and originally imprinted in the membranes of infected and stressed cells. Noteworthy, thrombosis and vasculitis, two symptoms in severely affected adult and pediatric patients are shared between COVID-19 and patients with Behcet's disease, an autoimmune disorder exhibiting a region-specific prevalence in countries of the former silk road. Molecular mechanisms and clinical indicators suggest reactive oxygen species as trigger factor for severe progression of COVID-19 and establish a link to the innate immune defense against bacteria. The selective pressure exerted by bacterial pathogens may have shaped the genetics of inhabitants at this ancient trade route in favor of bacterial defense, to the detriment of severe COVID-19 progression in the 21th century.

Sialic Acid Protects Nontypeable Haemophilus influenzae from Natural IgM and Promotes Survival in Murine Respiratory Tract

Nontypeable Haemophilus influenzae (NTHi), a common inhabitant of the human nasopharynx and upper airways, causes opportunistic respiratory tract infections that are frequently recurring and chronic. NTHi utilizes sialic acid from the host to evade antibacterial defenses and persist in mucosal tissues; however, the role of sialic acid scavenged by NTHi during infection is not fully understood. We previously showed that sialylation protects specific epitopes on NTHi lipooligosaccharide (LOS) targeted by bactericidal IgM in normal human serum. Here, we evaluated the importance of immune evasion mediated by LOS sialylation in the mouse respiratory tract using wild-type H. influenzae and an isogenic siaB mutant incapable of sialylating the LOS. Sialylation protected common NTHi glycan structures recognized by human and murine IgM and protected NTHi from complement-mediated killing directed by IgM against these structures. Protection from IgM binding by sialylated LOS correlated with decreased survival of the siaB mutant versus the wild type in the murine lung. Complement depletion with cobra venom factor increased survival of the siaB mutant in the nasopharynx but not in the lungs, suggesting differing roles of sialylation at these sites. Prior infection increased IgM against H. influenzae but not against sialic acid-protected epitopes, consistent with sialic acid-mediated immune evasion during infection. These results provide mechanistic insight into an NTHi evasive strategy against an immune defense conserved across host species, highlighting the potential of the mouse model for development of anti-infective strategies targeting LOS antigens of NTHi.

Purification and Immune Phenotyping of B-1 Cells from Body Cavities of Mice

B-1 cells are fetal-origin B lymphocytes with unique developmental and functional characteristics that can generate natural, polyreactive antibodies with important functions in tissue homeostasis and immune defense. While B-1 cell frequencies in bone marrow and secondary lymphoid tissues are low, relative high frequencies exist within peritoneal and pleural cavities of mice, including both CD5⁺ and CD5^- B-1 cells. These cells represent B-1 reservoirs that, when activated, migrate to lymphoid tissues to secrete antibodies and/or cytokines. Here, we outline efficient methods for the extraction and magnetic isolation of CD5⁺ B-1 cells from the peritoneal and pleural cavities as well as the separation and phenotypic characterization of CD5⁺ and CD5^- B-1 cells by flow cytometry.

The effect of influenza vaccine immunization on natural antibodies

Natural, polyreactive, low-affinity antibodies are known to play an important role not only in the immediate defense against pathogens, but also in shaping the acquired immune response. On the other hand, antigen specific, high-affinity antibodies can affect the balance of natural antibodies and lead to autoimmune diseases. In this study, we have analyzed the changes that occur in the IgM and IgG pool of natural antibodies after immunization with split or whole virion influenza vaccine. For this purpose, "in-house" developed ELISAs were used. The subjects were divided, according to the vaccination status, into those who had been immunized with the influenza vaccine in previous years and those who had been immunized for the first time. The analysis indicated that the pool of natural antibodies was not impaired by the immunization, evidenced by the lack of changes in any of the groups, and that certain fluctuations were induced in order to maintain the homeostasis of the immune system.

Immunoglobulin M: An Ancient Antiviral Weapon - Rediscovered

Recent discoveries have shed new light onto immunoglobulin M (IgM), an ancient antibody class preserved throughout evolution in all vertebrates. First, IgM – long thought to be a perfect pentamer – was shown to be asymmetric, resembling a quasi-hexamer missing one monomer and containing a gap. Second, this gap allows IgM to serve as carrier of a specific host protein, apoptosis inhibitor of macrophages (AIM), which is released to promote removal of dead-cell debris, cancer cells, or pathogens. Third, recombinant IgM delivered mucosally by passive immunization gave proof-of-concept that this antibody class can prevent mucosal simian-human immunodeficiency virus transmission in non-human primates. Finally, IgM’s role in adaptive immunity goes beyond being only a first defender to respond to pathogen invasion, as long-lived IgM plasma cells have been observed predominantly residing in the spleen. In fact, IgM produced by such cells contained somatic hypermutations and was linked to protection against lethal influenza virus challenge in murine models. Importantly, such long-lived IgM plasma cells had been induced by immunization 1 year before challenge. Together, new data on IgM function raise the possibility that vaccine strategies aimed at preventing virus acquisition could include this ancient weapon.

Neonatal Exposure to Commensal-Bacteria-Derived Antigens Directs Polysaccharide-Specific B-1 B Cell Repertoire Development

B-1 B cells derive from a developmental program distinct from that of conventional B cells, through B cell receptor (BCR)-dependent positive selection of fetally derived precursors. Here, we used direct labeling of B cells reactive with the N-acetyl-D-glucosamine (GlcNAc)-containing Lancefield group A carbohydrate of Streptococcus pyogenes to study the effects of bacterial antigens on the emergent B-1 B cell clonal repertoire. The number, phenotype, and BCR clonotypes of GlcNAc-reactive B-1 B cells were modulated by neonatal exposure to heat-killed S. pyogenes bacteria. GlcNAc-reactive B-1 clonotypes and serum antibodies were reduced in germ-free mice compared with conventionally raised mice. Colonization of germ-free mice with a conventional microbiota promoted GlcNAc-reactive B-1 B cell development and concomitantly elicited clonally related IgA⁺ plasma cells in the small intestine. Thus, exposure to microbial antigens in early life determines the clonality of the mature B-1 B cell repertoire and ensuing antibody responses, with implications for vaccination approaches and schedules.

Antibody Complexes

Monoclonal based therapeutics have always been looked at as a futuristic natural way we could take care of pathogens and many diseases. However, in order to develop, establish and realize monoclonal based therapy we need to understand how the immune system contains or kill pathogens. Antibody complexes serve the means to decode this black box. We have discussed examples of antibody complexes both at biochemical and structural levels to understand and appreciate how discoveries in the field of antibody complexes have started to decoded mechanism of viral invasion and create potential vaccine targets against many pathogens. Antibody complexes have made advancement in our knowledge about the molecular interaction between antibody and antigen. It has also led to identification of potent protective monoclonal antibodies. Further use of selective combination of monoclonal antibodies have provided improved protection against deadly diseases. The administration of newly designed and improved immunogen has been used as potential vaccine. Therefore, antibody complexes are important tools to develop new vaccine targets and design an improved combination of monoclonal antibodies for passive immunization or protection with very little or no side effects.

B Cell-Mediated Autoimmune Diseases

Extensive studies have suggested a central role of B cells in the autoimmune pathogenesis, as loss of B cell tolerance results in increased serum levels of autoantibodies, enhanced effector T cell response and tissue damages. Here, we provide an overview of dysregulated B cell responses in the development of autoimmunity. In addition to their presence in the target organs, autoreactive B cells can promote the formation of ectopic lymphoid structures and differentiate into plasma cells that produce large amounts of autoantibodies and cytokines. In animal models that recapitulate the key features of human autoimmune disease, mechanistic studies have indicated two categories of autoantibodies: (1) serological markers for disease diagnosis and prognosis; (2) effector molecules that induce organ hypofunction or damage directly in an epitope-specific manner, or indirectly by activating other immune cell subsets. Moreover, B cell-derived cytokines usually promote the autoreactive T cell response during autoimmune development, but there is compelling evidence that a subpopulation of B cells negatively regulates immune responses, also known as regulatory B cells via secreting anti-inflammatory cytokines (IL-10, IL-35, etc.) or a contact-dependent fashion. Although B cell depletion could eliminate most circulating B cells in the periphery, the clinical outcomes of B cell depletion therapy for autoimmune diseases vary among individuals due to differential activation or survival signals for B cells provided by tissue microenvironment. Thus, therapeutic combinations that target immune checkpoints and B cell activation may represent a promising strategy for the effective treatment of human autoimmune diseases.

Glycan Reactive Natural Antibodies and Viral Immunity

Adaptive antibody responses provide a crucial means of host defense against viral infections by mediating the neutralization and killing infectious pathogens. At the forefront of humoral defense against viruses lie a subset of innate-like serum antibodies known as natural antibodies (NAbs). NAbs serve multifaceted functions in host defense and play an essential role in early immune responses against viruses. However, there remain many unanswered questions with regard to both the breadth of viral antigens recognized by NAbs, and how B cell ontology and individual antigenic histories intersect to control the development and function of antiviral human NAbs. In the following article we briefly review the current understanding of the functions and source of NAbs in the immune repertoire, their role during antiviral immune responses, the factors influencing the maturation of the NAb repertoire, and finally, the gaps and future research needed to advance our understanding of innate-like B cell biology for the purpose of harnessing NAbs for host defense against viral infections.

Interaction of adenovirus with antibodies, complement, and coagulation factors

Adenovirus (AdV) is one of the most widely used vectors for gene therapy and vaccine studies due to its excellent transduction efficiency, capacity for large transgenes, and high levels of gene expression. When administered intravascularly, the fate of AdV vectors is heavily influenced by interactions with host plasma proteins. Some plasma proteins can neutralize AdV, but AdV can also specifically bind plasma proteins that protect against neutralization and preserve activity. This review summarizes the plasma proteins that interact with AdV, including antibodies, complement, and vitamin K-dependent coagulation factors. We will also review the complex interactions of these plasma proteins with each other and with cellular proteins, as well as strategies for developing better AdV vectors that evade or manipulate plasma proteins.

Secreted IgM: New tricks for an old molecule

Secreted IgM (sIgM) is a multifunctional evolutionary conserved antibody that is critical for the maintenance of tissue homeostasis as well as the development of fully protective humoral responses to pathogens. Constitutive secretion of self- and polyreactive natural IgM, produced mainly by B-1 cells, provides a circulating antibody that engages with autoantigens as well as invading pathogens, removing apoptotic and other cell debris and initiating strong immune responses. Pathogen-induced IgM production by B-1 and conventional B-2 cells strengthens this early, passive layer of IgM-mediated immune defense and regulates subsequent IgG production. The varied effects of secreted IgM on immune homeostasis and immune defense are facilitated through its binding to numerous different cell types via different receptors. Recent studies identified a novel function for pentameric IgM, namely as a transporter for the effector protein ″apoptosis-inhibitor of macrophages″ (AIM/CD5L). This review aims to provide a summary of the known functions and effects of sIgM on immune homeostasis and immune defense, and its interaction with its various receptors, and to highlight the many critical immune regulatory functions of this ancient and fascinating immunoglobulin.

Streptococcal Enzymes as Precision Tools Against Pathogenic IgG Autoantibodies in Small Vessel Vasculitis

In primary systemic small vessel vasculitis autoantibodies are common and seem to play an important role in the pathogenesis. Autoantibodies in vasculitis are preferentially directed against components of the immune system or directly against components of the vessel wall. Plasmapheresis is often applied in emergency situationists when the function of vital organs is jeopardized, the level of clinical evidence to apply such therapy, however, varies between low and non-existing. Plasmapheresis is a blunt and unspecific instrument that requires several sessions to achieve a substantial reduction of autoantibody levels. IdeS and EndoS are two relatively recently discovered enzymes produced by S. pyogenes, that have a remarkable capacity to degrade and disarm IgG. They have shown positive results in several in vivo models of autoimmunity, and treatment with IdeS has successfully been used to inactivate HLA alloantibodies in patients undergoing renal transplantation. Both IdeS and EndoS have the potential to become precision tools to replace plasmapheresis in the treatment of vasculitic emergencies and a clinical trial of IdeS in anti-GBM vasculitis is now ongoing.

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

B-1a cells play an important role in mediating tissue homeostasis and protecting against infections. They are the main producers of 'natural' IgM, spontaneously secreted serum antibodies predominately reactive to self antigens, like phosphatidylcholine (PtC), or antigens expressed by the intestinal microbiota. The mechanisms that regulate the B-1a immunoglobulin (Ig) repertoire and their antibody secretion remain poorly understood. Here, we use a novel reporter mouse to demonstrate that production of self- and microbiota-reactive antibodies is linked to BCR signaling in B-1a cells. Moreover, we show that Toll-like receptors (TLRs) are critical for shaping the Ig repertoire of B-1a cells as well as regulating their antibody production. Strikingly, we find that both the colonization of a microbiota as well as microbial-sensing TLRs are required for anti-microbiota B-1a responses, whereas nucleic-acid sensing TLRs are required for anti-PtC responses, demonstrating that linked activation of BCR and TLRs controls steady state B-1a responses to both self and microbiota-derived antigens.

Immune Surveillance by Natural IgM Is Required for Early Neoantigen Recognition and Initiation of Adaptive Immunity

Early recognition of neoantigen-expressing cells is complex, involving multiple immune cell types. In this study, in vivo, we examined how antigen-presenting cell subtypes coordinate and induce an immunological response against neoantigen-expressing cells, particularly in the absence of a pathogen-associated molecular pattern, which is normally required to license antigen-presenting cells to present foreign or self-antigens as immunogens. Using two reductionist models of neoantigen-expressing cells and two cancer models, we demonstrated that natural IgM is essential for the recognition and initiation of adaptive immunity against neoantigen-expressing cells. Natural IgM antibodies form a cellular immune complex with the neoantigen-expressing cells. This immune complex licenses surveying monocytes to present neoantigens as immunogens to CD4⁺ T cells. CD4⁺ T helper cells, in turn, use CD40L to license cross-presenting CD40⁺ Batf3⁺ dendritic cells to elicit a cytotoxic T cell response against neoantigen-expressing cells. Any break along this immunological chain reaction results in the escape of neoantigen-expressing cells. This study demonstrates the surprising, essential role of natural IgM as the initiator of a sequential signaling cascade involving multiple immune cell subtypes. This sequence is required to coordinate an adaptive immune response against neoantigen-expressing cells.

Biologic roles of the ABH and Lewis histo-blood group antigens Part I: infection and immunity

The ABH and Lewis antigens were among the first of the human red blood cell polymorphisms to be identified and, in the case of the former, play a dominant role in transfusion and transplantation. But these two therapies are largely twentieth century innovations, and the ABH and related carbohydrate antigens are not only expressed on a very wide range of human tissues, but were present in primates long before modern humans evolved. Although we have learned a great deal about the biochemistry and genetics of these structures, the biological roles that they play in human health and disease are incompletely understood. This review and its companion, to appear in a later issue of Vox Sanguinis, will focus on a few of the biologic and pathologic processes which appear to be affected by histo-blood group phenotype. The first of the two reviews will explore the interactions of two bacteria with the ABH and Lewis glycoconjugates of their human host cells, and describe the possible connections between the immune response of the human host to infection and the development of the AB-isoagglutinins. The second review will describe the relationship between ABO phenotype and thromboembolic disease, cardio-vascular disease states, and general metabolism.

Association between anti-α-1,4-D-polygalacturonic acid antibodies and Henoch-Schönlein purpura in children

Objective

To investigate the relationship between anti-α-1,4-D-polygalacturonic acid (PGA) antibodies, particularly immunoglobulin (Ig)A, and Henoch-Schönlein purpura (HSP) in children.

Methods

This observational case–control study investigated PGA-IgA, PGA-IgG, and PGA/PGA-IgA circulating immune complex (PGA/PGA-IgA CIC) in paediatric patients with HSP versus controls. Children with HSP were also evaluated for food specific IgG and food intolerance. Between-group differences in anti-PGA antibodies were analysed.

Results

Serum PGA-IgA and PGA-IgG levels were significantly increased in patients with acute HSP (n = 251) versus those with urticaria (n = 48), acute respiratory infections (n = 95), surgical controls (n = 53) and neonates (n = 92). PGA/PGA-IgA CIC levels were also significantly higher in the acute HSP group versus surgical control and neonate groups. Levels of PGA/PGA-IgA CIC and PGA-IgA were significantly correlated (r = 0.997), and PGA-IgA showed high diagnostic specificity for HSP. No statistically significant differences were observed in PGA-IgA and PGA-IgG between various degrees of food intolerance in children with HSP.

Conclusion

Increased anti-PGA antibodies, particularly PGA-IgA and PGA/PGA-IgA CIC, were significantly associated with acute HSP in children. Food intolerance was not found to be associated with increased anti-PGA antibodies in children with HSP.

Impact of a Heat Shock Protein Impurity on the Immunogenicity of Biotherapeutic Monoclonal Antibodies

Purpose

Anti-drug antibodies can impair the efficacy of therapeutic proteins and, in some circumstances, induce adverse health effects. Immunogenicity can be promoted by aggregation; here we examined the ability of recombinant mouse heat shock protein 70 (rmHSP70) - a common host cell impurity - to modulate the immune responses to aggregates of two therapeutic mAbs in mice.

Methods

Heat and shaking stress methods were used to generate aggregates in the sub-micron size range from two human mAbs, and immunogenicity assessed by intraperitoneal exposure in BALB/c mice.

Results

rmHSP70 was shown to bind preferentially to aggregates of both mAbs, but not to the native, monomeric proteins. Aggregates supplemented with 0.1% rmHSP70 induced significantly enhanced IgG2a antibody responses compared with aggregates alone but the effect was not observed for monomeric mAbs. Dendritic cells pulsed with mAb aggregate showed enhanced IFNγ production on co-culture with T cells in the presence of rmHSP70.

Conclusion

The results indicate a Th1-skewing of the immune response by aggregates and show that murine rmHSP70 selectively modulates the immune response to mAb aggregates, but not monomer. These data suggest that heat shock protein impurities can selectively accumulate by binding to mAb aggregates and thus influence immunogenic responses to therapeutic proteins.

Electronic supplementary material

The online version of this article (10.1007/s11095-019-2586-7) contains supplementary material, which is available to authorized users.

IgM Natural Autoantibodies in Physiology and the Treatment of Disease

Antibodies are vital components of the adaptive immune system for the recognition and response to foreign antigens. However, some antibodies recognize self-antigens in healthy individuals. These autoreactive antibodies may modulate innate immune functions. IgM natural autoantibodies (IgM-NAAs) are a class of primarily polyreactive immunoglobulins encoded by germline V-gene segments which exhibit low affinity but broad specificity to both foreign and self-antigens. Historically, these autoantibodies were closely associated with autoimmune disease. Nevertheless, not all human autoantibodies are pathogenic and compelling evidence indicates that IgM-NAAs may exert a spectrum of effects from injurious to protective depending upon cellular and molecular context. In this chapter, we review the current state of knowledge regarding the potential physiological and therapeutic roles of IgM-NAAs in different disease conditions such as atherosclerosis, cancer, and autoimmune disease. We also describe the discovery of two reparative IgM-NAAs by our laboratory and delineate their proposed mechanisms of action in central nervous system (CNS) disease.

Polyreactive IgM initiates complement activation by PF4/heparin complexes through the classical pathway

The mechanisms by which exposure to heparin initiates antibody responses in many, if not most, recipients are poorly understood. We recently demonstrated that antigenic platelet factor 4 (PF4)/heparin complexes activate complement in plasma and bind to B cells. Here, we describe how this process is initiated. We observed wide stable variation in complement activation when PF4/heparin was added to plasma of healthy donors, indicating a responder "phenotype" (high, intermediate, or low). Proteomic analysis of plasma from these healthy donors showed a strong correlation between complement activation and plasma immunoglobulin M (IgM) levels (r = 0.898; P < .005), but not other Ig isotypes. Complement activation response to PF4/heparin in plasma displaying the low donor phenotype was enhanced by adding pooled IgM from healthy donors, but not monoclonal IgM. Depletion of IgM from plasma abrogated C3c generation by PF4/heparin. The complement-activating features of IgM are likely mediated by nonimmune, or natural, IgM, as cord blood and a monoclonal polyreactive IgM generate C3c in the presence of PF4/heparin. IgM facilitates complement and antigen deposition on B cells in vitro and in patients receiving heparin. Anti-C1q antibody prevents IgM-mediated complement activation by PF4/heparin complexes, indicating classical pathway involvement. These studies demonstrate that variability in plasma IgM levels correlates with functional complement responses to PF4/heparin. Polyreactive IgM binds PF4/heparin, triggers activation of the classical complement pathway, and promotes antigen and complement deposition on B cells. These studies provide new insights into the evolution of the heparin-induced thrombocytopenia immune response and may provide a biomarker of risk.

Repertoire of Abs primed by bacteria in gnotobiotic mice

Innate immunity natural Abs (NAbs) execute a number of functions, including protection and surveillance. Despite active research, the stimuli that induce the formation of NAbs are still described only hypothetically. Here, we compared repertoires of anti-glycan Abs in the peripheral blood of mice that received per os various bacteria. The repertoires of Abs of mice primed in this way were compared using a microarray that included about 350 glycans, as well as 150 bacterial polysaccharides. Sterile mice did not possess anti-glycan Abs. Oral inoculation of a single strain or combination of two to four strains of bacteria, as well as putting the animals on short-term nutrition with non-sterile food, did not contribute significantly to the formation of Abs, whereas a single gavage of digested food of non-sterile mice induced the formation of a repertoire close to the natural ones. Interestingly, the priming with polysaccharide Ags (in a composition of the bacterial cell envelope), that is, dominant Ags of bacteria, led to the induction of Abs against typical glycans of mammalian glycoproteins and glycolipids (e.g. Abs of the ABH blood group system) that do not have a structural similarity to the polysaccharides. The results support the importance of early contact with a naïve immune system with microorganisms of the environment to form a normal NAbs repertoire.

Bruton's Tyrosine Kinase Is Not Essential for B Cell Survival beyond Early Developmental Stages

Bruton's tyrosine kinase (Btk) is a crucial regulator of B cell signaling and is a therapeutic target for lymphoma and autoimmune disease. BTK-deficient patients suffer from humoral immunodeficiency, as their B cells fail to progress beyond the bone marrow. However, the role of Btk in fully developed, mature peripheral B cells is not well understood. Analysis using BTK inhibitors is complicated by suboptimal inhibition, off-target effects, or failure to eliminate BTK's adaptor function. Therefore a Btk^flox/Cre-ER^T2 mouse model was developed and used to excise Btk after B cell populations were established. Mice lacking Btk from birth are known to have reduced follicular (FO) compartments, with expanded transitional populations, suggesting a block in development. In adult Btk^flox/Cre-ER^T2 mice, Btk excision did not reduce FO B cells, which persisted for weeks. Autoimmune-prone B1 cells also survived conditional Btk excision, contrasting their near absence in global Btk-deficient mice. Therefore, Btk supports BCR signaling during selection into the FO and B1 compartments, but is not needed to maintain these cell populations. B1-related natural IgM levels remained normal, contrasting global Btk deficiency, but B cell proliferation and T-independent type II immunization responses were blunted. Thus, B cells have nuanced signaling responses that are differentially regulated by Btk for development, survival, and function. These findings raise the possibility that Btk may also be expendable for survival of mature human B cells, therefore requiring prolonged dosing to be effective, and that success of BTK inhibitors may depend in part on off-target effects.

A Novel Role for C5a in B-1 Cell Homeostasis

B-1 cells constitute a unique subpopulation of lymphocytes residing mainly in body cavities like the peritoneal cavity (PerC) but are also found in spleen and bone marrow (BM). As innate-like B cells, they mediate first line immune defense through low-affinity natural IgM (nIgM) antibodies. PerC B-1 cells can egress to the spleen and differentiate into nIgM antibody-secreting plasma cells that recognize conserved exogenous and endogenous cellular structures. Homing to and homeostasis within the PerC are regulated by the chemokine CXCL13 released by PerC macrophages and stroma cells. However, the exact mechanisms underlying the regulation of CXCL13 and B-1 homeostasis are not fully explored. B-1 cells play important roles in the inflammatory response to infection, autoimmunity, ischemia/reperfusion injury, obesity, and atherosclerosis. Remarkably, this list of inflammatory entities has a strong overlap with diseases that are regulated by complement suggesting a link between B-1 cells and the complement system. Interestingly, up to now, no data exist regarding the role of complement in B-1 cell biology. Here, we demonstrate for the first time that C5a regulates B-1 cell steady-state dynamics within the peritoneum, the spleen, and the BM. We found decreased B-1a cell numbers in the peritoneum and the spleen of C5aR1^−/− mice associated with increased B1-a and B1-b numbers in the spleen and high serum titers of nIgM antibodies directed against phosphorylcholine and several pneumococcal polysaccharides. Similarly, peritoneal B-1a cells were decreased in the peritoneum and splenic B-1a and B-1b cells were increased in C5aR2^−/− mice. The decrease in peritoneal B-1 cell numbers was associated with decreased peritoneal CXCL13 levels in C5aR1^−/− and C5aR2^−/− mice. In search for mechanisms, we found that combined TLR2 and IL-10 receptor activation in PerC macrophages induced strong CXCL13 production, which was significantly reduced in cells from C5aR1- and C5aR2-deficient mice and after combined C5aR-targeting. Such stimulation also induced marked local C5 production by PerC macrophages and C5a generation. Importantly, peritoneal in vivo administration of C5a increased CXCL13 production. Taken together, our findings suggest that local non-canonical C5 activation in PerC macrophages fuels CXCL13 production as a novel mechanism to control B-1 cell homeostasis.

Linking autoimmunity to the origin of the adaptive immune system

In jawed vertebrates, the adaptive immune system (AIS) cooperates with the innate immune system (IIS) to protect hosts from infections. Although targeting non-self-components, the AIS also generates self-reactive antibodies which, when inadequately counter-selected, can give rise to autoimmune diseases (ADs). ADs are on the rise in western countries. Why haven’t ADs been eliminated during the evolution of a ∼500 million-year old system? And why have they become more frequent in recent decades? Self-recognition is an attribute of the phylogenetically more ancient IIS and empirical data compellingly show that some self-reactive antibodies, which are classifiable as elements of the IIS rather then the AIS, may protect from (rather than cause) ADs. Here, we propose that the IIS’s self-recognition system originally fathered the AIS and, as a consequence of this relationship, its activity is dampened in hygienic environments. Rather than a mere breakdown or failure of the mechanisms of self-tolerance, ADs might thus arise from architectural constraints.

Natural IgM and the Development of B Cell-Mediated Autoimmune Diseases

Most serum immunoglobulin M (IgM) is "natural IgM", which is produced apparently spontaneously by a distinct subset of B cells requiring no exogenous antigenic or microbial stimuli. Natural IgM is an evolutionarily conserved molecule and reacts with a variety of epitopes expressed on both self- and non-self antigens. It has long been understood that secreted (s) IgM contributes to the removal of altered self-antigens, such as apoptotic and dying cells. As we outline in this review, it is thought that this sIgM housekeeping function removes potential triggers of autoresponse induction. However, we recently demonstrated an unexpected and distinct role for sIgM in the control of autoreactive B cells: the regulation of bone marrow B cell development. The absence of sIgM blocked pro- to pre- B-cell transition and greatly altered the BCR repertoire of the developing B cells and the peripheral B-cell pools in genetically engineered mice. This finding strongly suggests that IgM is critical for B-cell central tolerance induction. Given that treatment of sIgM-deficient mice with polyclonal IgM corrected these developmental defects, therapeutic application of IgM could be of clinical relevance in the treatment of some B-cell-mediated autoimmune diseases.

Genetic aspects of auto-immune profiles of healthy chickens

Auto-antibody profiles binding liver antigens differed between chicken lines divergently selected for specific antibody responses to SRBC, and were affected by ageing suggesting both genetic and environmental effects. Presence and levels of IgM and IgG antibodies binding chicken liver cell lysate (CLL) fragments in plasma at 5 weeks of age from 10 individual full sibs and their parents from 5 H_srbc and 5 L_srbc line families was studied to reveal genetic relations. Non-genetic maternal effects were studied by comparing auto-antibody profiles of 36 weeks old hens from 2 other unrelated lines with the profiles from their chicks at hatch. IgM and IgG antibodies from parents and progeny from both H_srbc and L_srbc lines bound CLL fragments. Significant line and generation differences and their interactions were found for both isotypes. Higher staining of CLL fragments was usually found for H_srbc line birds. Lines were clustered by auto-antibody profiles, but staining by birds of both lines in both generations was very individual for IgG and IgM. The current data with full sibs therefore not supported a genetic basis for auto-antibody profiles. IgG but not IgM auto-antibody profiles of chicks correlated with maternal auto-antibody profiles. The results suggest that the auto-antibody repertoire of healthy chickens is largely stochastically initiated and may be affected by environmental challenges during ageing, but genetic mechanisms may underlie staining intensity of individual bound CLL fragments. The present results suggest that identification of fragments or profiles to be used at early age for genetic selection for health traits is not feasible yet. Secondly, the IgM profile of neonatal chickens seems non-organised independent of the maternal profile, but the neonatal IgG profile is much more related with the maternal profile. Consequences of these findings for disease susceptibility or breeding for optimal health are discussed.

PD-L2 Regulates B-1 Cell Antibody Production against Phosphorylcholine through an IL-5-Dependent Mechanism

B-1 cells produce natural Abs which provide an integral first line of defense against pathogens while also performing important homeostatic housekeeping functions. In this study, we demonstrate that programmed cell death 1 ligand 2 (PD-L2) regulates the production of natural Abs against phosphorylcholine (PC). Naive PD-L2-deficient (PD-L2^-/-) mice produced significantly more PC-reactive IgM and IgA. This afforded PD-L2^-/- mice with selectively enhanced protection against PC-expressing nontypeable Haemophilus influenzae, but not PC-negative nontypeable Haemophilus influenzae, relative to wild-type mice. PD-L2^-/- mice had significantly increased PC-specific CD138⁺ splenic plasmablasts bearing a B-1a phenotype, and produced PC-reactive Abs largely of the T15 Id. Importantly, PC-reactive B-1 cells expressed PD-L2 and irradiated chimeras demonstrated that B cell-intrinsic PD-L2 expression regulated PC-specific Ab production. In addition to increased PC-specific IgM, naive PD-L2^-/- mice and irradiated chimeras reconstituted with PD-L2^-/- B cells had significantly higher levels of IL-5, a potent stimulator of B-1 cell Ab production. PD-L2 mAb blockade of wild-type B-1 cells in culture significantly increased CD138 and Blimp1 expression and PC-specific IgM, but did not affect proliferation. PD-L2 mAb blockade significantly increased IL-5⁺ T cells in culture. Both IL-5 neutralization and STAT5 inhibition blunted the effects of PD-L2 mAb blockade on B-1 cells. Thus, B-1 cell-intrinsic PD-L2 expression inhibits IL-5 production by T cells and thereby limits natural Ab production by B-1 cells. These findings have broad implications for the development of therapeutic strategies aimed at altering natural Ab levels critical for protection against infectious disease, autoimmunity, allergy, cancer, and atherosclerosis.

sIgM-FcμR Interactions Regulate Early B Cell Activation and Plasma Cell Development after Influenza Virus Infection

Previous studies with mice lacking secreted IgM (sIgM) due to a deletion of the μs splice region (μs^-/- ) had shown sIgM involvement in normal B cell development and in support of maximal Ag-specific IgG responses. Because of the changes to B cell development, it remains unclear to which extent and how sIgM directly affects B cell responses. In this study, we aimed to explore the underlying mechanisms of sIgM-mediated IgG response regulation during influenza virus infection. Generating mice with normally developed μs-deficient B cells, we demonstrate that sIgM supports IgG responses by enhancing early Ag-specific B cell expansion, not by altering B cell development. Lack of FcμR expression on B cells, but not lack of Fcα/μR expression or complement activation, reduced antiviral IgG responses to the same extent as observed in μs^-/- mice. B cell-specific Fcmr^-/- mice lacked robust clonal expansion of influenza hemagglutinin-specific B cells early after infection and developed fewer spleen and bone marrow IgG plasma cells and memory B cells, compared with controls. However, germinal center responses appeared unaffected. Provision of sIgM rescued plasma cell development from μs^-/- but not Fcmr^-/- B cells, as demonstrated with mixed bone marrow chimeric mice. Taken together, the data suggest that sIgM interacts with FcμR on B cells to support early B cell activation and the development of long-lived humoral immunity.

Blimp-1-dependent and -independent natural antibody production by B-1 and B-1-derived plasma cells

Natural antibodies contribute to tissue homeostasis and protect against infections. They are secreted constitutively without external antigenic stimulation. The differentiation state and regulatory pathways that enable continuous natural antibody production by B-1 cells, the main cellular source in mice, remain incompletely understood. Here we demonstrate that natural IgM-secreting B-1 cells in the spleen and bone marrow are heterogeneous, consisting of (a) terminally differentiated B-1-derived plasma cells expressing the transcriptional regulator of differentiation, Blimp-1, (b) Blimp-1⁺, and (c) Blimp-1^neg phenotypic B-1 cells. Blimp-1^neg IgM-secreting B-1 cells are not simply intermediates of cellular differentiation. Instead, they secrete similar amounts of IgM in wild-type and Blimp-1-deficient (PRDM-1^ΔEx1A) mice. Blimp-1^neg B-1 cells are also a major source of IgG3. Consequently, deletion of Blimp-1 changes neither serum IgG3 levels nor the amount of IgG3 secreted per cell. Thus, the pool of natural antibody-secreting B-1 cells is heterogeneous and contains a distinct subset of cells that do not use Blimp-1 for initiation or maximal antibody secretion.

Authentic IgM Fc Receptor (FcμR)

Since the bona fide Fc receptor for IgM antibody (FcµR) was identified eight years ago, much progress has been made in defining its biochemical nature, cellular distribution, and effector function. However, there are clearly conflicting results, especially about the cellular distribution and function of murine FcµR. In this short article, we will discuss recent findings from us and other investigators along with our interpretations and comments that may help to resolve the existing puzzles and should open new avenues of investigation.

FcμR in human B cell subsets in primary selective IgM deficiency, and regulation of FcμR and production of natural IgM antibodies by IGIV

IgMFcR (FcμR) are expressed on B cell and B cell subsets. Mice deficient in secreted IgM and FcμR share properties of impaired specific antibody response and autoimmunity with patient with selective IgM deficiency (SIGMD). Intravenous immunoglobulin (IGIV) regulates immune response, including modulation of IgGFc receptors. However, there are no data on the expression of FcμR in patients with SIGMD, and the effects of IGIV on FcμR. In this study, we investigated FcμR expression in naïve marginal zone (MZ), IgM memory, and class-switched memory B cells in patients with selective IgM deficiency and healthy controls. Furthermore, we examined the direct effect of IGIV on FcμR expression and on the upregulation of FcμR by TLR2 agonist (Pam3). Finally, we examined the effect of IVIG on spontaneously produced IgM and natural IgM anti-phosphorylcholine (PC) antibodies by B cells and B1 cells. FcμR expression is significantly decreased in MZ B cells in patients with SIGMD as compared to control. IGIV, at immunomodulatory concentrations, inhibited FcμR upregulation by Pam3 in MZ B cells, and IgM-depleted IGIV inhibited spontaneous secretion of natural IgM anti-PC antibodies and not total IgM by B1 cells. These data suggest that decreased FcμR expression on MZ B cells may play a role in the pathogenesis of SIGMD, and an inhibition of TLR-2-induced upregulation of FcμR by IGIV may be one of the mechanisms of its anti-inflammatory action. IGIV-induced inhibition of natural IgM antibodies may be one of the mechanisms of IGIV-induced immunoregulation.