Genetic control of natural antibody repertoires: I. IgH, MHC and TCR beta loci

Marginal zone B cells: From housekeeping function to autoimmunity?

Marginal zone (MZ) B cells comprise a subset of innate-like B cells found predominantly in the spleen, but also in lymph nodes and blood. Their principal functions are participation in quick responses to blood-borne pathogens and secretion of natural antibodies. The latter is important for housekeeping functions such as clearance of apoptotic cell debris. MZ B cells have B cell receptors with low poly-/self-reactivity, but they are not pathogenic at steady state. However, if simultaneously stimulated with self-antigen and pathogen- and/or damage-associated molecular patterns (PAMPs/DAMPs), MZ B cells may participate in the initial steps towards breakage of immunological tolerance. This review summarizes what is known about the role of MZ B cells in autoimmunity, both in mouse models and human disease. We cover factors important for shaping the MZ B cell compartment, how the functional properties of MZ B cells may contribute to breaking tolerance, and how MZ B cells are being regulated.

Repertoire of BALB/c Mice Natural Anti-Carbohydrate Antibodies: Mice vs. Humans Difference, and Otherness of Individual Animals

One of the most common genetic backgrounds for mice used as a model to investigate human diseases is the inbred BALB/c strain. This work is aimed to characterize the pattern of natural anti-carbohydrate antibodies present in the serum of 20 BALB/c mice by printed glycan array technology and to compare their binding specificities with that of human natural anti-carbohydrate antibodies. Natural antibodies (NAbs) from the serum of BALB/c mice interacted with 71 glycans from a library of 419 different carbohydrate structures. However, only seven of these glycans were recognized by the serum of all the animals studied, and other five glycans by at least 80% of mice. The pattern of the 12 glycans mostly recognized by the circulating antibodies of BALB/c mice differed significantly from that observed with natural anti-carbohydrate antibodies in humans. This lack of identical repertoires of natural anti-carbohydrate antibodies between individual inbred mice, and between mice and humans, should be taken into consideration when mouse models are intended to be used for investigation of NAbs in biomedical research.

Natural anti-intestinal goblet cell autoantibody production from marginal zone B cells

Expression of a germline VH3609/D/JH2 IgH in mice results in the generation of B1 B cells with anti-thymocyte/Thy-1 glycoprotein autoreactivity by coexpression of Vk21-5/Jk2 L chain leading to production of serum IgM natural autoantibody. In these same mice, the marginal zone (MZ) B cell subset in spleen shows biased usage of a set of Ig L chains different from B1 B cells, with 30% having an identical Vk19-17/Jk1 L chain rearrangement. This VH3609/Vk19-17 IgM is reactive with intestinal goblet cell granules, binding to the intact large polymatrix form of mucin 2 glycoprotein secreted by goblet cells. Analysis of a μκ B cell AgR (BCR) transgenic (Tg) mouse with this anti-goblet cell/mucin2 autoreactive (AGcA) specificity demonstrates that immature B cells expressing the Tg BCR become MZ B cells in spleen by T cell-independent BCR signaling. These Tg B cells produce AGcA as the predominant serum IgM, but without enteropathy. Without the transgene, AGcA autoreactivity is low but detectable in the serum of BALB/c and C.B17 mice, and this autoantibody is specifically produced by the MZ B cell subset. Thus, our findings reveal that AGcA is a natural autoantibody associated with MZ B cells.

Protective Roles of Natural IgM Antibodies

Antibodies are a vital part of the armamentarium of the adaptive immune system for the fine-tuning of the recognition and response to foreign threats. However, in health there are some types of antibodies that instead recognize self-antigens and these contribute to the enhancement of primitive innate functions. This repertoire of natural IgM antibodies is postulated to have been selected during immune evolution for their contributions to critical immunoregulatory and housekeeping properties. The clearance of dying cells is one of the most essential responsibilities of the immune system, which is required to prevent uncontrolled inflammation and autoimmunity. In the murine immune system, natural IgM antibodies that recognize apoptotic cells have been shown to enhance the phagocytic clearance of dead and dying cells and to suppress innate immune signaling pathways. In the mouse, natural IgM are often the products of B-1 cell clones that arise during immune development without an absolute requirement for exogenous antigenic stimulation. In patients with systemic lupus erythematosus, IgM autoantibodies, which bind to neo-epitopes on apoptotic cells, have been demonstrated to be present at significantly higher levels in patients with lower disease activity and with less severe organ damage. While certain specificities of IgM autoantibodies correlate with protection from lupus renal disease, others may convey protective properties from lupus-associated atherosclerotic cardiovascular disease. New and unexpected insights into the functional roles of IgM antibodies are still emerging, especially regarding the functions of natural antibodies. Herein, we review recent progress in our understanding of the potential roles of natural IgM autoantibodies in the regulation of immune homeostasis and for protection from autoimmune and inflammatory diseases.

Oxidation-specific epitopes are dominant targets of innate natural antibodies in mice and humans

Atherosclerosis is a chronic inflammatory disease characterized by the accumulation of oxidized lipoproteins and apoptotic cells. Adaptive immune responses to various oxidation-specific epitopes play an important role in atherogenesis. However, accumulating evidence suggests that these epitopes are also recognized by innate receptors, such as scavenger receptors on macrophages, and plasma proteins, such as C-reactive protein (CRP). Here, we provide multiple lines of evidence that oxidation-specific epitopes constitute a dominant, previously unrecognized target of natural Abs (NAbs) in both mice and humans. Using reconstituted mice expressing solely IgM NAbs, we have shown that approximately 30% of all NAbs bound to model oxidation-specific epitopes, as well as to atherosclerotic lesions and apoptotic cells. Because oxidative processes are ubiquitous, we hypothesized that these epitopes exert selective pressure to expand NAbs, which in turn play an important role in mediating homeostatic functions consequent to inflammation and cell death, as demonstrated by their ability to facilitate apoptotic cell clearance. These findings provide novel insights into the functions of NAbs in mediating host homeostasis and into their roles in health and diseases, such as chronic inflammatory diseases and atherosclerosis.

HLA-association of serum levels of natural antibodies

Natural antibodies of IgM or IgG types are present in sera of most healthy individuals and are important participants of the immune response. Little is known, however, about the genetic regulation of their plasma levels in humans. We determined the concentrations of three IgM type natural autoantibodies (NAAbs) reactive to certain conserved self-antigens (citrate synthase (A-CIT), chondroitin sulphate C (A-COS) and 60 kDa heat shock proteins (A-HSP) in the sera of 78 healthy individuals and in their 86 children. In case of all the 164 individuals alleles of several polymorphisms were determined in class II (HLA-DQ, -DR), class III (AGER-429T>C, HSP70-2 1267A>G, TNF-308G>A, CFB S/F, copy number of the C4A and C4B genes), and class I (HLA-A, -B) regions of the major histocompatibility complex (MHC). Since the samples originated from a family study, extended MHC haplotypes were also determined for each study participant. Our results show that children of parents with low NAAb concentration have significantly lower serum concentrations of all the three NAAbs, as compared to offsprings of parents without reduced serum concentration. This indicates that the serum levels of these NAAbs were partly regulated by factors which are inherited from the parents to offsprings. In further studies performed only in genetically independent parents, we found significant differences in the serum levels of the IgM type A-CIT and A-COS antibodies (Abs) between carriers and non-carriers of the HLA-DR2 (15 and 16) antigens. In both cases the Ab concentrations were higher in the HLA-DR15 carriers (p=0.002 and p=0.008, respectively) and lower in DR16 carriers (p=0.029 and p=0.049, respectively) than in the non-carriers. Even more significant differences were found when the levels of two Abs were evaluated together. Frequency of the DR15 carriers was significantly lower among subjects with one or two low (in the lowest quartile) titers of A-CIT/A-COS Abs (p=0.014), A-CIT/A-HSP Abs (p=0.016) and A-COS/A-HSP Abs (p=0.013) as compared to those with normal Ab titers for both antigens. By contrast, frequency of the DR16 carriers was significantly higher among subjects with one or two low A-CIT/A-COS Abs (p=0.001), A-CIT/A-HSP Abs (p=0.002) and A-COS/A-HSP Abs (p=0.021) as compared to those with normal Ab titers for both antigens. Similar differences were found for both IgM type antibodies when carriers and non-carriers of the HLA-DR15-DQ6 and HLA-DR16-DQ5 haplotypes were considered. These novel observations indicate that not only adaptive immune response but also natural autoantibody pattern, as a part of innate immune response, is influenced by the MHC allele composition.

Dynamic stability in random and scale-free B-lymphocyte networks

One of the most intriguing features of the immune system is regulation: a limited response when perturbed repeatedly. We propose a minimal network model for immune regulation in a lymphocyte network containing two types of elements: B lymphocytes and ligands that bind to their receptors. Effective interactions between B cells, mediated by other components of the immune system can be excitatory or inhibitory. In our model, B cell clones and ligand species are represented by nodes, and interactions by links. We expect that, as in many complex systems, the connectivity distribution is broad, motivating study of the model on a scale-free network; for comparison we study the same dynamics on a random graph. We characterize the dynamics of the model and its response to perturbations. Our model reproduces several key features of immune system dynamics: regulation (saturation of response), and more rapid response upon repeated perturbation with the same agents. Our results suggest that a scale-free network of interactions contributes to the regulation and dynamics of the immune system.

Diversified serum IgG response involving non-myelin CNS proteins during experimental autoimmune encephalomyelitis

We sequentially analyzed the serum IgG response against normal mouse brain during experimental autoimmune encephalomyelitis in SJL/J mice injected with CFA, Bordetella pertussis toxin (BPT) and proteolipid protein 139-151 peptide, compared with mice that received CFA and BPT or were uninjected. Dynamic changes were observed from day 0 to day 28 in the 3 groups. Six highly discriminant antigenic bands (kappa=0.974) were identified. Three non-myelin proteins were characterized (mitochondrial aconitase hydratase 2, phosphoglycerate mutase 1, brain specific pyruvate deshydrogenase). The IgG response against two of them was less frequent in EAE whereas it was associated with multiple sclerosis in our previous work.

Heritable factors shape natural human IgM reactivity to Ro60/SS-A and may predispose for SLE-associated IgG anti-Ro and anti-La autoantibody production

Systemic lupus erythematosus (SLE) is characterized by various IgG autoreactivities, among which anti-Ro/SS-A is particularly pathology-associated and early detectable. SLE also shows significant familial aggregation, but genetic factors are not well understood and remain controversial for disease-associated IgG. Here we report that IgM anti-Ro showed a uniquely high degree of heritability in a study of SLE-affected families. Unlike IgM anti-La or anti-dsDNA, IgM anti-Ro was also significantly correlated to IgG anti-Ro among SLE patients, as well as to IgG anti-La and anti-dsDNA. We conclude that largely genetically determined, thus natural IgM anti-Ro-bearing precursor B-cells, may be an important factor for class switching and determinant spreading in early phases of SLE pathogenesis. Furthermore, we found unexpected sex differences in isotype/specificity correlations among SLE-unaffected relatives and control subjects, which could help understand the strong gender bias associated with SLE. We propose that the study of such correlation structures may reveal characteristic spreading pathways relevant for human SLE.

Alterations of self-reactive antibody repertoires in HIV disease: An insight into the role of T cells in the selection of autoreactive B cells

Infection with human immunodeficiency virus (HIV) is characterized by a progressive depletion of CD4(+) T cells that parallels a dysfunction of the B cell compartment and a disturbed recognition of self-antigens. The relationship between T lymphocyte homeostasis and abnormalities in the selection of self-reactive B cells is not clear as yet. We have therefore compared repertoires of natural antibodies of healthy donors and of patients at various stages of HIV infection. The reactivity of IgM and IgG antibodies in plasma of healthy blood donors and of HIV-positive patients with high and low CD4(+) T cell counts was assessed by semi-quantitative immunoblotting using self-antigens extracted from normal human tissues. Repertoires of reactivites were compared between groups of individuals by means of multiparametric statistical analysis. We observed that repertoires of self-reactive IgM and IgG from HIV-seropositive patients exhibited significantly altered patterns of reactivity, as compared to those of healthy controls. Further, self-reactive repertoires of IgM and IgG of patients with high CD4(+) T cell counts differed significantly from those of patients with low CD4(+) T cell counts. A longitudinal analysis of self-reactive antibody repertoires of progressor and non-progressor patients suggested an influence of CD4(+) T cell counts on immunoglobulin reactivity toward self-antigens. These observations support the hypothesis that altered T cell/B cell interactions due to altered CD4(+) T cell help severely impact on the selection of self-reactive antibody repertoires and may contribute to the onset of pathological autoimmunity in HIV disease.

Increased polyclonal immunoglobulin reactivity toward human and bacterial proteins is associated with clinical protection in human Plasmodium infection

Background

Polyclonal B-cell activation is well known to occur in Plasmodium infections, but its role in pathogenesis or protection remains unclear. However, protective properties of natural antibodies have previously been demonstrated in other contexts.

Methods

Sera from asymptomatic and symptomatic Plasmodium-infected subjects locally detected in a survey study in the Brazilian Amazon, and from unexposed and exposed but presently uninfected control subjects, were assayed by a standardized quantitative immunoblot method allowing simultaneous detection of IgG or IgM reactivity to a large number of parasite-unrelated proteins.

Results

In subjects free of coinfection with hepatitis B virus, IgG reactivity to human brain antigens and Escherichia coli proteins was strikingly enhanced in asymptomatic Plasmodium-infected individuals when compared to such with clinical malaria symptoms, or to uninfected control subjects. This difference was most characteristic for limited exposure times (less than ten years locally, or 20 years in endemic areas). It was more significant than a similar trend found for IgG to Plasmodium falciparum antigens, and unrelated to parasitaemia levels. Asymptomatic subjects with comparatively short exposure characteristically showed relatively elevated IgG versus IgM reactivity. Polyclonal IgG reactivity appears triggered by previous P. falciparum but not Plasmodium vivax malaria.

Conclusion

The observed difference in polyclonal antibody production seems related to intrinsic activation states of infected individuals, rather than to parasite-antigen specific immune responses. However, it appears influenced by preceding stimuli. This supports the idea that acquired clinical immunity may not exclusively depend on antigen-specific responses, but also on the individual polyclonal reaction.

The natural autoantibody repertoire and autoimmune disease

The incidence of autoimmune diseases has shown a significant increase in developed countries during the last 40 years. The cause of this increase is still unknown, and reliable methods for the detection of individuals at risk of developing autoimmune disease are not available yet. To explore new methods for the diagnosis and monitoring of autoimmune disease, we have studied the repertoire of natural autoantibodies (NA) and its relationship with autoimmune disease using large arrays of defined antigens. NA are found in healthy humans and mice, apparently in the absence of immunization with their target antigens. We used knock-out mice to demonstrate that the repertoire of NA is influenced by factors not directly related to antigenic stimulation such as endogenous levels of histamine. By studying strains of mice known to differ in their susceptibility to autoimmune disease, we could conclude that the repertoire of NA reflects the susceptibility to develop autoimmune disease. The study of the human repertoire of NA required the development of bio-informatic tools to overcome the variation introduced by individual differences in the genetic background and immune history. We found that human NA are organized in clusters that can differentiate healthy subjects from patients with type 1 diabetes mellitus, type 2 diabetes mellitus or Behçet's disease patients. The development of new tools to undertake large-scale NA analysis could also enhance our understanding of the immune system, and leave us in a better position to face the up-coming epidemics of autoimmune disorders.

Knock-out of the histidine decarboxylase gene modifies the repertoire of natural autoantibodies

Natural antibodies (NA) are antibodies produced in the absence of known immunization with specific antigens. NA are found in the blood of healthy humans and mice. Histamine influences many aspects of the immune response, including antibody production. However, the role of histamine in the generation of NA has not yet been studied. In this work, we used an ELISA assay to characterize the self-antigen binding repertoires of NA in wild type (WT) mice and in histidine decarboxylase knock-out (HDC-KO) mice, unable to synthesize histamine. We now report that HDC-KO and WT mice differed in the patterns of autoreactivity of their IgM and IgG NA. The NA in HDC-KO sera manifested a larger repertoire of IgM autoantibodies than did the WT sera. The self-antigens bound by IgM from HDC-KO mice included structural proteins, enzymes associated with cellular metabolism, double-stranded and single-stranded DNA, and tissue-specific antigens like insulin. There were relatively fewer differences in the NA repertoire of IgG autoantibodies of the mice: notably, the HDC-KO sera reacted with glutamic acid decarboxylase (GAD), an antigen associated with autoimmune diabetes. These results demonstrate that endogenous histamine can influence the self-reactivity of the NA repertoire.

The conservative physiology of the immune system

Current immunological opinion disdains the necessity to define global interconnections between lymphocytes and regards natural autoantibodies and autoreactive T cells as intrinsically pathogenic. Immunological theories address the recognition of foreignness by independent clones of lymphocytes, not the relations among lymphocytes or between lymphocytes and the organism. However, although extremely variable in cellular/molecular composition, the immune system preserves as invariant a set of essential relations among its components and constantly enacts contacts with the organism of which it is a component. These invariant relations are reflected, for example, in the life-long stability of profiles of reactivity of immunoglobulins formed by normal organisms (natural antibodies). Oral contacts with dietary proteins and the intestinal microbiota also result in steady states that lack the progressive quality of secondary-type reactivity. Autoreactivity (natural autoantibody and autoreactive T cell formation) is also stable and lacks the progressive quality of clonal expansion. Specific immune responses, currently regarded as the fundament of the operation of the immune system, may actually result from transient interruptions in this stable connectivity among lymphocytes. More permanent deficits in interconnectivity result in oligoclonal expansions of T lymphocytes, as seen in Omenn's syndrome and in the experimental transplantation of a suboptimal diversity of syngeneic T cells to immunodeficient hosts, which also have pathogenic consequences. Contrary to theories that forbid autoreactivity as potentially pathogenic, the physiology of the immune system is conservative and autoreactive. Pathology derives from failures of these conservative mechanisms.

Analysis of human self-reactive antibody repertoires by quantitative immunoblotting

We review the use of a quantitative immunoblotting technique to characterize human self-reactive antibody repertoires in health and disease. The interactions of plasma IgM and IgG with tissue extracts as sources of self-antigens were analyzed by quantitative immunoblotting. Data were compared by means of multiparametric statistical analysis. The data summarized here demonstrate that natural self-reactive antibody repertoires of healthy individuals are restricted to a limited subset of immunodominant autoantigens that is selected early in development, and remains conserved between individuals through ageing. The selection of human natural self-reactive IgG antibody repertoires requires normal T-/B-cell interactions. The immunoblotting assay has the potential to distinguish between autoimmune diseases with organ-related oligoclonal expansion of self-reactive clones and those characterized by broad alterations of immunoregulation. However, organ-specific autoimmune diseases may be characterized by altered patterns of antibody repertoires unrelated to the target organ. The assay also revealed an unexpected defect in the regulatory function of self-reactive IgM on the expression of self-reactive IgG repertoires in several systemic and organ-specific autoimmune diseases. The results are discussed in the light of our current understanding of the processes of selection of self-reactive B-cells and the pathophysiology of autoimmunity.

Germ-line selection ensures embryonic autoreactivity and a positive discrimination of self mediated by supraclonal mechanisms

It is necessary to clarify principles and mechanisms of natural tolerance to body tissues, in order to derive appropriate diagnostics, therapeutics and prognostics of autoimmune diseases (AID). I will argue that AIDs result from deficits in autoreactive regulatory T cell generation and/or function, and propose a model that explains why relatively few prototypes of AID exist, as well as their organ-specificity or systemic nature. The model suggests that natural tolerance is achieved through evolutionarily selected developmental genetic programs: (i) for patterns of V-region expression early in life that ensure auto(multi)reactivity at the outset of the system; (ii) for a cellular composition of thymic stroma that 'breeds' and activates regulatory (autoreactive) T cells in early development; (iii) for lymphocyte differentiation and population dynamics, that results in peripheral 'education' of regulatory tissue-specific cells, while allowing for 'unregulated' clonal responses to nonself. In the present model, S/NS discrimination is 'supraclonal' and 'dominant', related to other 'systemic' properties such as the regulation of total lymphocyte numbers, the 'open-endedness' of repertoires, and their differences in health and disease. Dominant tolerance models in general, also solve the paradox that pathogenic autoreactivity is rare, in spite of the extensive V-region degeneracy of lymphocyte recognition and the high frequency of cross-reactivity between S/NS; in short, it is astonishing that we are not autoimmune every time we get infected. As in other areas of biomedical science, time is perhaps ripe to move from component (clonal) analysis to system's biology, as some have proned for years.

Structured reactions of serum IgM repertoires to immunization are dependent on major histocompatibility complex genes

In normal animals, responses to immunization include alterations in the serum IgM antibody repertoire, as scored on autologous tissue antigens with no respect for the immunizing antigen. These immunogen-nonspecific antibody reactions were found previously to display specific structures dependent on strain and immunization protocols. Using major histocompatability complex (MHC)-congenic Lewis rats, we show that such IgM repertoire reactions are under the control of MHC-linked genes, including a class I locus. This strongly suggests the involvement of T cells restricted by both class I and class II MHC, in regulating serum IgM repertoires. Immunogen-nonspecific repertoire reactions to immunization may, therefore, represent degenerate, but prototypical, reactions or regulatory mechanisms embodying the natural repertoires of T- and B cells connected to autoantigens. Natural (auto)immunity could so serve to regulate the effector class of adaptive immune responses, particularly in order to avoid pathogenic autoreactivity following specific immunization with self-cross-reacting antigens. Appropriate analysis of nonspecific repertoire reactions could therefore contribute to the understanding of general structures of immune regulation and natural tolerance.

Type I IFN sets the stringency of B cell repertoire selection in the bone marrow

Locally produced type I interferon (IFN-I) enhances the sensitivity of bone marrow B cell to IgM receptor ligation. The establishment of B cell repertoires, on the other hand, seems to involve selective processes that are critically dependent on B cell receptor (BCR) ligation. In order to assess the importance of BCR triggering thresholds on the selection of polyclonal unmanipulated B cell populations, we compared VH gene expression and reactivity repertoires in various B cell compartments of wild-type and IFN-I receptor-deficient mice (IFN-I-R-/-). These analyses demonstrate that increased B cell sensitivity to BCR ligation mediated by IFN-I in the bone marrow (BM) has consequences on the stringency of B cell repertoire selection. Thus, the normal counter-selection of both VH7183 gene family expression and multireactivity was impaired among immature BM B cells from mutant mice. Furthermore, as a result of reduced efficiency of BCR ligation-dependent inhibition of terminal differentiation, IFN-I-R-/- animals produce, in BM and thymus, higher numbers of plasma cells secreting antibodies that are more multireactive than wild-type animals. Finally, mutant serum IgM natural antibodies display a more reactive repertoire than controls, a likely reflection of the BM resident plasma cell repertoire. The present observations demonstrate, therefore, that local modulation of BCR triggering thresholds leads to important modifications in the generation and/or selection of normal B cell populations.

Dynamics of serum IgM autoreactive repertoires following immunization: strain specificity, inheritance and association with autoimmune disease susceptibility

Immunization of Lewis rats with myelin basic protein (MBP) in complete Freud's adjuvant (CFA) provokes experimental allergic encephalomyelitis (EAE). Here we compare, irrespective of antigen specificity, the structure and dynamics of serum IgM autoreactive repertoires following immunization with MBP/CFA in EAE-susceptible Lewis and relatively resistant Fischer rats. Prior to the appearance of clinical symptoms, Lewis rats developed a specific modification of serum IgM autoreactivities that, scored on other determinants than MBP itself, showed a prognostic association with EAE symptoms. Although comparable in their production of MBP-specific serum IgM and IgG antibodies, Fischer rats did not share these MBP/CFA-induced IgM autoreactivities of Lewis rats when immunized in the same manner. Moreover, while the Lewis-type repertoire reaction was specific for MBP/CFA alone, the respective Fischer reaction was not qualitatively different from that observed in this strain upon non-pathogenic immunization with self-related or -unrelated antigens. In general, the repertoire reactions differed qualitatively between the strains, consisting of components with typical behavior and strain preferences. The EAE-associated, as well as the other components of both Lewis- and Fischer-type repertoire reactions were usually co-dominantly inherited in F1 animals. These results indicate that a global antibody repertoire analysis may serve as a tool to describe prototypical response structures, possibly involved in immune regulation and susceptibility to pathogenic autoimmunity.