Characterization of murine polyreactive antigen-binding B cells: presentation of antigens to T cells

Stimulation of Toll-Like Receptors profoundly influences the titer of polyreactive antibodies in the circulation

Polyreactive antibodies are a major component of the natural antibody repertoire and bind to a variety of structurally unrelated molecules. These antibodies are thought to provide a first line of defense against bacterial infections and play a major role in the clearance of apoptotic cells. What triggers the secretion of these antibodies has remained an enigma. Using a surrogate assay for measuring polyreactive antibodies, we found that about 50% of serum IgM is polyreactive and that stimulation of TLR4^+/+, but not TLR4^−/−, mice resulted in a 40 fold increase in polyreactive antibodies. Stimulation of TLRs 3, 7, 9 also increased the secretion of polyreactive antibodies. Infection with a virus or tissue damage induced by a toxin similarly led to an increase in polyreactive antibodies in MyD88^+/+, but not MyD88^−/− mice. We conclude that stimulation of TLRs is a key link in the mechanism of polyreactive antibody secretion into the circulation.

Antibody polyreactivity in health and disease: statu variabilis

An Ab molecule or a BCR that is able to bind multiple structurally unrelated Ags is defined as polyreactive. Polyreactive Abs and BCRs constitute an important part of immune repertoires under physiological conditions and may play essential roles in immune defense and in the maintenance of immune homeostasis. In this review, we integrate and discuss different findings that reveal the indispensable role of Ag-binding polyreactivity in the immune system. First, we describe the functional and molecular characteristics of polyreactive Abs. The following part of the review concentrates on the biological roles attributed to polyreactive Abs and to polyreactive BCRs. Finally, we discuss recent studies that link Ig polyreactivity with distinct pathological conditions.

Polyreactive antibodies in adaptive immune responses to viruses

B cells express immunoglobulins on their surface where they serve as antigen receptors. When secreted as antibodies, the same molecules are key elements of the humoral immune response against pathogens such as viruses. Although most antibodies are restricted to binding a specific antigen, some are polyreactive and have the ability to bind to several different ligands, usually with low affinity. Highly polyreactive antibodies are removed from the repertoire during B-cell development by physiologic tolerance mechanisms including deletion and receptor editing. However, a low level of antibody polyreactivity is tolerated and can confer additional binding properties to pathogen-specific antibodies. For example, high-affinity human antibodies to HIV are frequently polyreactive. Here we review the evidence suggesting that in the case of some pathogens like HIV, polyreactivity may confer a selective advantage to pathogen-specific antibodies.

B-cell activating factor (BAFF) promotes CpG ODN-induced B cell activation and proliferation

It is controversial whether naïve B cells are directly activated in response to TLR9 ligand, CpG ODN. Although bovine blood-derived CD21(+) B cells express TLR9 and proliferate in response to CpG in mixed-cell populations, purified bovine B cells do not proliferate significantly in response to CpG ODN, even when the B cell receptor is engaged. When co-cultured with CD14(+) myeloid cells and/or B-cell activating factor (BAFF), a cytokine produced by activated myeloid cells, there was a significant increase in CpG-specific B cell proliferation, and the number of large B cells in general or positive for CD25, all of which are markers for B cell activation. These data suggest that activated myeloid cells and BAFF prime B cells for significant CpG-specific activation. Understanding the signals required to mediate efficient CpG-induced, antigen-independent and T-cell independent activation of B cells has implications for polyclonal B cell activation and the development of autoimmune diseases.

Antigen-specific enhancement of natural human IgG antibodies to phosphatidylcholine, phosphatidylglycerol, phosphatidylinositol-4-phosphate, cholesterol, and lipid A by a liposomal vaccine containing lipid A

Natural IgG antibodies (NA) to lipids are ubiquitously distributed in sera of healthy humans and are believed to serve beneficial functions. Although NA to lipids generally exhibit germ line or near germ line binding specificities, the antibodies commonly increase transiently in the acute phases of most, if not all, infectious diseases and may serve as a first line of defense. In order to determine whether similar anti-lipid antibodies can be induced by a vaccine in humans, we examined stored sera obtained from volunteers who had previously received a candidate vaccine to Plasmodium falciparum. The vaccine had consisted of liposomes that contained both the recombinant protein antigen and also contained monophosphoryl lipid A (MPLA) as an adjuvant. All of the pre-immune sera contained NA to one or more of the liposomal lipids in the vaccine: dimyristol phosphatidylcholine (DMPC), dimyristoyl phosphatidylglycerol (DMPG), cholesterol, and MPLA. After initial immunization, followed by a boost, increased levels of IgG antibodies to all of the liposomal lipids, especially DMPG and MPLA, were observed by ELISA. Antibodies to phosphatidylinositol-4-phosphate (PIP) above the normal pre-immune NA to PIP were also observed. Although PIP was not present in the immunizing liposomes, based on the adsorption of anti-PIP antibodies by DMPG the anti-PIP antibodies were thought to represent cross-reacting anti-DMPG antibodies. The immune response was apparently antigen-specific in that NA to unrelated lipids, other than PIP, that were not present in the liposomes, galactosyl ceramide and ganglioside GM1, were not increased by the immunization. We conclude that antibodies to DMPC, DMPG, PIP, cholesterol, and MPLA can be induced in humans by immunization with liposomes containing MPLA.

The broad antibacterial activity of the natural antibody repertoire is due to polyreactive antibodies

Polyreactive antibodies bind to a variety of structurally unrelated antigens. The function of these antibodies, however, has remained an enigma, and because of their low binding affinity their biological relevance has been questioned. Using a panel of monoclonal polyreactive antibodies, we showed that these antibodies can bind to both Gram-negative and Gram-positive bacteria and acting through the classical complement pathway can inhibit bacterial growth by lysis, generate anaphylatoxin C5a, enhance phagocytosis, and neutralize the functional activity of endotoxin. Polyreactive antibody-enriched, but not polyreactive antibody-reduced, IgM prepared from normal human serum displays antibacterial activity similar to that of monoclonal polyreactive IgM. We conclude that polyreactive antibodies are a major contributor to the broad antibacterial activity of the natural antibody repertoire.

The immunoglobulin constant region contributes to affinity and specificity

A central dogma in immunology is that antibody specificity is solely the result of variable (V)-region interactions with an antigen. However, this view is not tenable in light of numerous reports that constant heavy (C(H)) domains can affect binding affinity and specificity and V-region structure. Kinetic and thermodynamic proof for the occurrence of this phenomenon is now available. C(H)-region effects on affinity and specificity suggest new mechanisms for generating antibody diversity and polyreactivity (multispecificity) that impact current views on idiotype regulation, autoimmunity, and B cell selection and change our understanding of vaccine responses.

Properties and function of polyreactive antibodies and polyreactive antigen-binding B cells

The advent of hybridoma technology has made it possible to study in-depth individual antibody molecules. These studies have revealed a number of surprises that have and are continuing to change our view of the immune system. None of these was more surprising than the demonstration that many antibody molecules are polyreactive - that is they can bind to a variety of different and structurally unrelated self- and non-self-foreign antigens. These findings make it clear that self-reactivity is a common and not necessarily forbidden or pathogenic feature of the immune system and that the well-known broad antibacterial activity of natural antibodies is largely due to polyreactive antibodies. In this brief review we will discuss these insights and their impact on basic and clinical immunology.

The potential for induction of autoimmune disease by a randomly-mutated self-antigen

The pathology of most autoimmune diseases is well described. However, the exact event that triggers the onset of the inflammatory cascade leading to disease is less certain and most autoimmune diseases are complex idiopathic diseases with no single gene known to be causative. In many cases, a relation to an infectious disease is described, and it is thought that microbes can play a direct role in induction of autoimmunity, for instance by molecular mimicry or bystander activation of autoreactive T cells. In contrast, less attention has been given to the possibility that modified self-antigens can be immunogenic and lead to autoimmunity against wildtype self-antigens. In theory, modified self-antigens can arise by random errors and mutations during protein synthesis and would be recognized as foreign antigens by naïve B and T lymphocytes. Here, it is postulated that the initial auto-antigen is not a germline self-antigen, but rather a mutated self-antigen. This mutated self-antigen might interfere with peripheral tolerance if presented to the immune system during an infection. The infection lead to bystander activation of naïve T and B cells with specificity for mutated self-antigen and this can lead to epitopespreading in which T and B cells with specificity for wildtype self-antigens are activated as a result of general inflammation.

The cytotoxic T lymphocyte response against a protein antigen does not decrease the antibody response to that antigen although antigen-pulsed B cells can be targets

The role of activated CD8+ T cells in shaping the dynamics of in vivo antigen presentation and immune responses is a subject receiving more attention. We studied whether cytotoxic T lymphocyte (CTL) would limit antibody responses by targeting antigen-specific B cells. A modified in vivo CTL assay was developed and used herein to demonstrate cytotoxicity in vivo, and to show that antigen-specific B cells that process exogenous antigen and present peptide in association with MHC class I can be the targets of CD8+ T cells. B cells from C57BL/6 mice immunized with ovalbumin (OVA)/alum were pulsed with OVA in vitro, and transferred into C57BL/6 recipient mice that had been immunized with vaccinia virus expressing SIINFEKL minigene to generate CD8+ CTL against K(b)/SIINFEKL. OVA-pulsed B220+ B cells from OVA-immunized mice were killed to a greater extent than B220+ B cells from naïve mice (28+/-20% versus 12+/-16%, p=0.0042). However, mice receiving vaccinia-SIINFEKL and generating CTL, did not appear to target endogenous B cells, since both primary and secondary antibody responses to OVA were unaffected. Our findings indicate that CTL responses to the protein antigen do not interfere with endogenous B cell responses, even though exogenous B cells expressing the CTL epitope can be efficiently lysed.

The Autoreactivity of Therapeutic Intravenous Immunoglobulin (IVIg) Preparations Depends on the Fractionation Methods Used

Natural immunoglobulin G (IgG) autoantibodies are present in the plasma of healthy individuals and, as a result, in pooled therapeutic intravenous immunoglobulin (IVIg) preparations. The production processes of commercial IVIg preparations involve different fractionation and virus-inactivation steps that include in some cases treatments at extreme conditions. Different physical and chemical treatments are known to augment greatly the reactivity of natural autoantibodies to self-antigens. It is not clear to what extent the self-reactivity of IVIg preparations is due to the presence of natural IgG antibodies in the plasma pools used for fractionation, and to what extent it is due to the treatments that the IgG molecules have been subjected to during the fractionation process. We compared the binding of seven different commercial IVIg preparations to human liver antigens. All studied IVIg's could be clearly separated into two distinct groups: those that possess significant self-reactivity and those with low binding to self-antigens. Increased self-binding was seen in the preparations produced using a fractionation step at low pH. The treatment of IVIg at low pH resulted in increasing the inhibitory effect of the pooled IgG on PHA-induced proliferation of human peripheral blood mononuclear cells. IVIg's with high and low self-binding may have different immunomodulating activities when infused to autoimmune patients.

Serum IgM, IgG and IgA block by F(ab')-dependent mechanism the binding of natural IgG autoantibodies from therapeutic immunoglobulin preparations to self-antigens

Natural polyreactive IgG autoantibodies are present in the plasma of healthy individuals and as a result in pooled therapeutic intravenous immunoglobulin (i.v.Ig) preparations. The spectrum of self-antigens to which these autoantibodies bind, their fate after intravenous infusion and their biological activity are not well understood. The identity of serum proteins that mask binding of natural autoantibodies to self-proteins is a matter of controversy. The spectrum of native serum proteins bound by i.v.Ig was analyzed by two-dimensional electrophoresis. The reactivity of i.v.Ig was directed mainly to circulating immunoglobulins. The binding of the IgG autoantibodies from i.v.Ig to native human liver antigens was blocked not only by a F(ab')2-dependent mechanism by circulating IgM and IgG (as has been previously suggested), but also by serum IgA. This control of anti-self reactivity may be inefficient in some autoimmune diseases.

The VP6 protein of rotavirus interacts with a large fraction of human naive B cells via surface immunoglobulins

Immunity to human group A rotavirus (RV), a major cause of viral gastroenteritis in infants, involves B lymphocytes that provide RV-specific antibodies. Additionally, some arguments suggest that naive B cells could be implicated in the first steps of the immune response against RV. The aim of our study was to analyze the interaction of VP6 and VP7 RV capsid proteins with human B cells depending on the immune status of the individual, i.e., naive or RV experienced. For this purpose, a two-color virus-like particle flow cytometry assay was devised to evaluate the blood B-lymphocyte reactivity to VP6 and VP7 proteins from healthy RV-exposed adults, recently infected infants, and neonates at birth. Both VP6 and VP7 interactions with B cells were mediated by surface immunoglobulins and probably by their Fab portions. VP7-reactive B lymphocytes were mainly detected from RV-experienced patients and almost exclusively in the CD27-positive memory cell fraction. Conversely, VP6-reactive B lymphocytes were detected at similar and high frequencies in adult, infant, and neonate samples. In adult samples, VP6 reacted with about 2% of the CD27-negative (CD27(neg)) naive B cells. These results demonstrated that the VP6 RV protein interacted with a large fraction of naive B lymphocytes from both adults and neonates. We propose that naive B cell-VP6 interaction might influence the strength and quality of the acquired immune response and should be considered for elaborating RV vaccine strategies.

Polyreactive antigen-binding B (PAB-) cells are widely distributed and the PAB population consists of both B-1+ and B-1- phenotypes

B cells that make polyreactive antibodies (PAB+ cells) express polyreactive Ig receptors on their surface and can bind a variety of different antigens. The present study shows that PAB+ cells are widely distributed, are present in varying numbers in different lymphoid organs and that their phenotype varies depending on the organs from which they are isolated. Up to 10 times more cells in PAB+ enriched populations bind antigens as compared to PAB- populations. Comparison of PAB+ with B-1+ cells showed that a high percentage of PAB+ cells are B-1+, but that many PAB+ cells do not express B-1 cell surface markers and, in fact, are B-1-. It is concluded that the B cell population consists of PAB+/B-1+, PAB+/B-1-, PAB-/B-1+, and PAB-/B-1- cells. The presence of PAB+ cells in the thymus points to the possibility that PAB+ cells may carry endogenous host antigens from peripheral tissues to the thymus where they may contribute to immunological tolerance.