Tchavdar L. Vassilev

Ferrous Ions and Reactive Oxygen Species Increase Antigen-binding and Anti-inflammatory Activities of Immunoglobulin G

Polyspecific antibodies represent a first line of defense against infection and regulate inflammation, properties hypothesized to rely on their ability to interact with multiple antigens. We demonstrated that IgG exposure to pro-oxidative ferrous ions or to reactive oxygen species enhances paratope flexibility and hydrophobicity, leading to expansion of the spectrum of recognized antigens, regulation of cell proliferation, and protection in experimental sepsis. We propose that ferrous ions, released from transferrin and ferritin at sites of inflammation, synergize with reactive oxygen species to modify the immunoglobulins present in the surrounding microenvironment, thus quenching pro-inflammatory signals, while facilitating neutralization of pathogens.

IVIG pluripotency and the concept of Fc-sialylation: challenges to the scientist

, Schwab and Nimmerjahn discuss current insights into the immunomodulatory mechanisms of IVIG preparations. The authors promote a recently challenged concept that the sialylation of IgG Fc fragments is likely to be responsible for the therapeutic activity of IVIG, however, they fall short in their discussion of the experimental evidence that does not support this concept. Schwab and Nimmerjahn discuss a hypothetical sialic acid-dependent pathway that involves the binding of Fc-sialylated IgG to mouse SIGNR1 and its human orthologue DC-SIGN (DC-specific ICAM3-grabbing non-integrin) that ultimately leads to the upregulation of the inhibitory Fc receptor (FcR) FcγRIIB on macrophages and dendritic cells (DCs) 1

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 IVIgs 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. IVIgs with high and low self‐binding may have different immunomodulating activities when infused to autoimmune patients.

Natural polyreactive IgA and IgM autoantibodies in human colostrum.

Secretory antibodies against bacteria and viruses in human colostrum and milk are known to be important protective factors for the breast‐fed infant. The authors have shown by enzyme immunoassay that colostrum contains IgA and IgM antibodies to a number of autoantigens: native DNA, actin, myosin, myoglobin, laminin, transferrin and thyroglobulin. These antibodies were polyspecific—those with anti‐DNA reactivity immunopurified on a DNA‐cellulose affinity column bound to a panel of self‐ and environmental antigens. The levels of natural autoantibodies in the immunoglobulin fraction of human colostrum were 3–10 times lower (when presented as antibody activity per μg of immunoglobulin) than in the immunoglobulin fraction of serum. The biological significance of the presence of B cells with autoantibody specificity in the mammary gland and of natural autoantibodies in colostrum and milk is not clear. It has been suggested that self‐reacting autoantibodies in serum play a major role in the selection of the pre‐immune B‐cell repertoire and in the maintenance of the immune homeostasis. The authors hypothesize that the natural autoantibodies in colostrum and milk may contribute to the selection process of physiological repertoire during the early postnatal period in breast‐fed infants. This could explain the lower frequency of allergic, inflammatory and autoimmune diseases and lymphomas which is seen in their later life when compared with that observed in children who have been formula‐fed after birth.

Normal human immunoglobulin suppresses experimental myasthenia gravis in SCID mice

Serum IgM has been shown to participate in the control of IgG autoreactivity in healthy subjects. We have recently shown that an immunoglobulin preparation of pooled normal human IgM (IVIgM) contains anti‐idiotypic antibodies against disease‐associated IgG autoantibodies in autoimmune patients and protects rats from experimental autoimmunity. The aim of the present study was to asses the in vitro and in vivo immunomodulatory effects of IVIgM in comparison with IgG, in SCID mice reconstituted with thymic cells from a myasthenia gravis patient. Non‐leaky SCID mice were injected i.p. with 60 × 106 thymic cells from a patient with myasthenia gravis and 1 day later boosted with 106 irradiated acetylcholine receptor (AchR)‐expressing TE671 cells. On days 14, 21 and 28, mice were treated with IVIgM or with equimolar amounts of human serum albumin. The level of anti‐AchR antibodies in the sera of three out of four IgM‐treated animals was less than 1 nM. Further, there was a significant decrease in the loss of endplate AchR on the diaphragms of IgM‐treated SCID mice. These findings indicate that pooled normal IgM exerts an immunoregulatory role in experimental myasthenia gravis, and suggests that IgM may be considered as an alternative approach in the therapy of autommune diseases.

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.Print ISSN: 0022-1767 Online ISSN: 1550-6606. Immunologists, Inc. All rights reserved. Copyright

Inflammation-induced enhancement of IgG immunoreactivity

Abstract.Natural polyreactive IgG antibodies are found in the sera of all healthy individuals. The in vitro exposure of pooled human IgG to protein-destabilizing chemical or physical factors has been previously shown to result in the exposure of their “hidden” polyspecificity. We hypothesize that such an enhancement of their pre-existing immunoreactivity may occur in vivo in the aggressive microenvironment of inflammation sites. An increase in the antigen binding intensity as well as of the number of recognized antigens was observed in the sera of IgG-infused immunodeficient SCID mice with induced local inflammation. The expansion of the IgG pathogen-binding repertoire may have important biological consequences.

Antibodies use heme as a cofactor to extend their pathogen elimination activity and to acquire new effector functions.

Various pathological processes are accompanied by release of high amounts of free heme into the circulation. We demonstrated by kinetic, thermodynamic, and spectroscopic analyses that antibodies have an intrinsic ability to bind heme. This binding resulted in a decrease in the conformational freedom of the antibody paratopes and in a change in the nature of the noncovalent forces responsible for the antigen binding. The antibodies use the molecular imprint of the heme molecule to interact with an enlarged panel of structurally unrelated epitopes. Upon heme binding, monoclonal as well as pooled immunoglobulin G gained an ability to interact with previously unrecognized bacterial antigens and intact bacteria. IgG-heme complexes had an enhanced ability to trigger complement-mediated bacterial killing. It was also shown that heme, bound to immunoglobulins, acted as a cofactor in redox reactions. The potentiation of the antibacterial activity of IgG after contact with heme may represent a novel and inducible innate-type defense mechanism against invading pathogens.

Modulation of autoimmune responses by intravenous immunoglobulin (IVIg)

Significant progresss has been made in understanding the mechanisms by which intravenous immunoglobulins (IVIg) exert immunomodulatory effects in the treatment of autoimmune diseases. A unique property of immunoglobulins is the diversity of variable (V) regions. The evidence discussed in this communication supports our notion that the diversity of V regions in IVIg preparations is a determining factor for the antiinflammatory substitutive and immunomodulatory functions of IVIg therapy. We have demonstrated the presence in IVIg, of anti-idiotypic antibodies directed against various autoantibodies. The ability of IVIg to interact through V regions with complementary V regions of antibodies and antigen receptors as well as with relevant soluble and surface molecules provides the basis for inducing the selection of immune repertoires. The study of the mechanisms by which IVIg mediates selection of autoreactive repertoires is essential for our understanding of the mechanisms underlying the emergence of pathological autoimmunity and of the physiological role of natural antibodies in the establishment and maintenance of tolerance to self and homeostasis of autoreactivity in healthy individuals.

The human IgG anti-carbohydrate repertoire exhibits a universal architecture and contains specificity for microbial attachment sites

The IgG anti-carbohydrate repertoire reveals potential glycan antigen determinants relevant to vaccine design. Antibodies: Looking Past the Trees Understanding how individual antibodies affect specific targets has led to successful antibody-based therapies for many diseases. Yet, these advances may have obscured the big picture. Now, Schneider et al. use a systems biology approach to examine the repertoire of carbohydrate-specific IgG antibodies. They find a universal architecture of this repertoire, with an association between immunogenicity or tolerance and particular glycan structure. Indeed, anti-glycan antibodies could differentiate microbial antigens, host glycans, and even tumor-associated glycans. Further understanding of how these glycan determinants affect the immune response may serve to improve vaccine targets, diagnostics, and antibody-based therapies. Despite the paradigm that carbohydrates are T cell–independent antigens, isotype-switched glycan-specific immunoglobulin G (IgG) antibodies and polysaccharide-specific T cells are found in humans. We used a systems-level approach combined with glycan array technology to decipher the repertoire of carbohydrate-specific IgG antibodies in intravenous and subcutaneous immunoglobulin preparations. A strikingly universal architecture of this repertoire with modular organization among different donor populations revealed an association between immunogenicity or tolerance and particular structural features of glycans. Antibodies were identified with specificity not only for microbial antigens but also for a broad spectrum of host glycans that serve as attachment sites for viral and bacterial pathogens and/or exotoxins. Tumor-associated carbohydrate antigens were differentially detected by IgG antibodies, whereas non-IgG2 reactivity was predominantly absent. Our study highlights the power of systems biology approaches to analyze immune responses and reveals potential glycan antigen determinants that are relevant to vaccine design, diagnostic assays, and antibody-based therapies.