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

In Vitro and In Vivo studies of monoclonal antibodies with prominent bactericidal activity against Burkholderia pseudomallei and Burkholderia mallei

Our laboratory has developed more than a hundred mouse monoclonal antibodies (MAbs) against Burkholderia pseudomallei and Burkholderia mallei. These antibodies have been categorized into different groups based on their specificities and the biochemical natures of their target antigens. The current study first examined the bactericidal activities of a number of these MAbs by an in vitro opsonic assay. Then, the in vivo protective efficacy of selected MAbs was evaluated using BALB/c mice challenged intranasally with a lethal dose of the bacteria. The opsonic assay using dimethyl sulfoxide-treated human HL-60 cells as phagocytes revealed that 19 out of 47 tested MAbs (40%) have prominent bactericidal activities against B. pseudomallei and/or B. mallei. Interestingly, all MAbs with strong opsonic activities are those with specificity against either the capsular polysaccharides (PS) or the lipopolysaccharides (LPS) of the bacteria. On the other hand, none of the MAbs reacting to bacterial proteins or glycoproteins showed prominent bactericidal activity. Further study revealed that the antigenic epitopes on either the capsular PS or LPS molecules were readily available for binding in intact bacteria, while the epitopes on proteins/glycoproteins were less accessible to the MAbs. Our in vivo study showed that four MAbs reactive to either the capsular PS or LPS were highly effective in protecting mice against lethal bacterial challenge. The result is compatible with that of our in vitro study. The MAbs with the highest protective efficacy are those reactive to either the capsular PS or LPS of the Burkholderia bacteria.

Intravenous immunoglobulin-based immunotherapy: an arsenal of possibilities for patients and science

The use of intravenous immunoglobulin (IVIG) concentrated from pooled healthy donors' plasma has gained increasing popularity. IVIG therapy has become important as a replacement therapy in primary and acquired humoral immunodeficiencies, and it has been extended to autoimmune, neurodegenerative and inflammatory conditions and transplantation therapy. Recurrent pregnancy failure and cancer are rather new platforms, where IVIG has shown its beneficial effects. This manuscript is focused on these two off-labelled usages. The immunomodulatory mechanisms of IVIG therapy appear as a coordinated orchestration of different functions, resulting in a synergistic effect. Treatment monitoring and detailed molecular analyses reveal how such treatments may interfere with disease pathogenesis. These finding may foster the development of novel therapeutic and/or preventive strategies. Studying this field with bidirectional bench-to-bedside and bedside-to-bench approaches fit well into 'the two-way road' paradigm of translational medicine.

The double life of a B-1 cell: self-reactivity selects for protective effector functions

During their development, B and T cells with self-reactive antigen receptors are generally deleted from the repertoire to avoid autoimmune diseases. Paradoxically, innate-like B-1 cells in mice are positively selected for self-reactivity and form a pool of long-lived, self-renewing B cells that produce most of the circulating natural IgM antibodies. This Review provides an overview of the developmental processes that shape the B-1 cell pool in mice, outlines the functions of B-1 cells in both the steady state and during host defence, and discusses possible functional B-1 cell homologues that exist in humans.

The auto-antigen repertoire in myasthenia gravis

Myasthenia Gravis (MG) is an antibody-mediated autoimmune disorder affecting the postsynaptic membrane of the neuromuscular junction (NMJ). MG is characterized by an impaired signal transmission between the motor neuron and the skeletal muscle cell, caused by auto-antibodies directed against NMJ proteins. The auto-antibodies target the nicotinic acetylcholine receptor (nAChR) in about 90% of MG patients. In approximately 5% of MG patients, the muscle specific kinase (MuSK) is the auto-antigen. In the remaining 5% of MG patients, however, antibodies against the nAChR or MuSK are not detectable (idiopathic MG, iMG). Although only the anti-nAChR and anti-MuSK auto-antibodies have been demonstrated to be pathogenic, several other antibodies recognizing self-antigens can also be found in MG patients. Various auto-antibodies associated with thymic abnormalities have been reported, as well as many non-MG-specific auto-antibodies. However, their contribution to the cause, pathology and severity of the disease is still poorly understood. Here, we comprehensively review the reported auto-antibodies in MG patients and discuss their role in the pathology of this autoimmune disease.

Older adults have a low capacity to opsonize pneumococci due to low IgM antibody response to pneumococcal vaccinations

Since the 23-valent pneumococcal polysaccharide vaccine (PPV23) is less effective for older adults than for young adults, it is important to investigate the immunologic basis for the reduced efficacy of PPV23 among older adults. We determined the effectiveness of PPV23 among young (n = 55) and older (n = 44) adults by measuring the serum IgG, IgM, and IgA concentrations and opsonic capacities against serotypes 14, 18C, and 23F. While young and older adults showed no difference in levels of IgG antibodies against pneumococcal polysaccharide (PPS), older adults had lower IgA and IgM antibody levels than young adults for all three serotypes. In both age groups, anti-PPS IgA or IgM antibody levels were much lower than anti-PPS IgG antibody levels. Young adults showed higher opsonic capacities than older adults for serotypes 14 and 23F. In order to determine the effects of anti-PPS IgA or IgM antibodies on the functional difference between young and older adults, anti-PPS IgA or IgM antibodies were removed from immune sera by affinity chromatography. The difference in opsonic capacity between young and older adults disappeared for serotypes 14 and 23F (but not for serotype 18C) when IgM antibody was removed. However, there was no significant difference between the two age groups when IgA antibody was removed. In conclusion, even though anti-PPS IgG antibody levels are high compared with anti-PPS IgM antibody levels, the low levels of anti-PPS IgM antibody alone can explain the functional difference observed between young and older adults immunized with PPV23 with regard to some pneumococcal serotypes.

A novel subpopulation of B-1 cells is enriched with autoreactivity in normal and lupus-prone mice

OBJECTIVE

B-1 cells have long been suggested to play an important role in lupus. However, reports to date have been controversial regarding their pathogenic or protective roles in different animal models. We undertook this study to investigate a novel subpopulation of B-1 cells and its roles in murine lupus.

METHODS

Lymphocyte phenotypes were assessed by flow cytometry. Autoantibody secretion was analyzed by enzyme-linked immunosorbent assay, autoantigen proteome array, and antinuclear antibody assay. Cell proliferation was measured by thymidine incorporation and 5,6-carboxyfluorescein succinimidyl ester dilution. B cell Ig isotype switching was measured by enzyme-linked immunospot assay.

RESULTS

Anti-double-stranded DNA (anti-dsDNA) autoantibodies were preferentially secreted by a subpopulation of CD5+ B-1 cells that expressed programmed death ligand 2 (termed L2pB1 cells). A substantial proportion of hybridoma clones generated from L2pB1 cells reacted to dsDNA. Moreover, these clones were highly cross-reactive with other lupus-related autoantigens. L2pB1 cells were potent antigen-presenting cells and promoted Th17 cell differentiation in vitro. A dramatic increase of circulating L2pB1 cells in lupus-prone BXSB mice was correlated with elevated serum titers of anti-dsDNA antibodies. A significant number of L2pB1 cells preferentially switched to IgG1 and IgG2b when stimulated with interleukin-21.

CONCLUSION

Our findings identify a novel subpopulation of B-1 cells that is enriched for autoreactive specificities, undergoes isotype switch, manifests enhanced antigen presentation, promotes Th17 cell differentiation, and is preferentially associated with the development of lupus in a murine model. Together, these findings suggest that L2pB1 cells have the potential to initiate autoimmunity through serologic and T cell-mediated mechanisms.

Changes of circulating antibody levels induced by ABO antibody adsorption for ABO-incompatible kidney transplantation

ABO-incompatible kidney transplantation using immunoadsorption to remove anti-A/B antibodies has become a successful clinical practice. Since the data on the specificity of the ABO columns are controversial, the present study assessed the efficiency and specificity of the ABO immunoadsorption, the effect on total immunoglobulins and antibodies previously induced by vaccination. Anti-A/B antibodies were measured by agglutination and ABO flow cytometry, total IgG/IgM, carbohydrate- and protein-specific antibodies by nephelometry and ELISA. The first immunoadsorption not only efficiently reduced donor-specific anti-A/B IgM (81%) and IgG (56%) but also reduced compatible anti-A/B IgM (59%) and IgG (34%). The measurements of antidonor A/B antibodies by direct agglutination (IgM) or flow cytometry better represented the effective antibody levels than the indirect agglutination test (IgG). The median reduction of total IgM and total IgG levels after a single immunoadsorption was 34% and 18%, respectively. Antibodies against pneumococcus and haemophilus polysaccharide antigens were significantly reduced, whereas antitetanus and antidiphtheria protein antibodies were not affected. Intravenous immunoglobulin administration restored the protective anticarbohydrate antibody levels. In summary, immunoadsorption efficiently removed antidonor A/B antibodies, but was not specific for A/B antigens. Anti-A/B antibody levels as determined by ABO flow cytometry are useful to establish the minimal number of immunoadsorptions needed for successful ABO-incompatible transplantation.

Outsmarting the host: bacteria modulating the immune response

Pathogenic bacteria and their hosts have had a two-way conversation for millions of years. This interaction has led to many measure/counter-measure responses by the host and bacteria. The host immune response has developed many mechanisms to neutralize and remove pathogen bacteria. In turn pathogenic bacteria have developed mechanisms to alter and evade the host immune response. We will review some of the mechanisms utilized by bacteria to accomplish this goal. We will also examine the current state of understanding of Francisella tularensis mediated immune evasion.

Nature and functions of autoantibodies

Antibodies that react with self-molecules occur in healthy individuals and are referred to as natural antibodies or autoantibodies. Natural autoantibodies are mainly IgM, are encoded by unmutated V(D)J genes and display a moderate affinity for self-antigens. They provide a first line of defense against infections, probably serve housekeeping functions and contribute to the homeostasis of the immune system. By contrast, high-affinity, somatically mutated IgG autoantibodies reflect a pathologic process whereby homeostatic pathways related to cell clearance, antigen-receptor signaling or cell effector functions are disturbed. In some autoimmune disorders, autoantibodies might be present before disease onset, show remarkable specificity and serve as biomarkers providing an opportunity for diagnosis and therapeutic intervention. In organ-specific autoimmune diseases, such as myasthenia gravis or pemphigus, autoantibodies directly bind to and injure target organs. In systemic autoimmune diseases, autoantibodies react with free molecules, such as phospholipids, as well as cell surface and nucleoprotein antigens, forming pathogenic antigen-antibody (immune) complexes. These autoantibodies injure tissues and organs through engagement of Fc gammaR activation of complement as well as internalization and activation of Toll-like receptors. Activation of intracellular Toll-like receptors in plasmacytoid dendritic cells leads to the production of type I interferon, whereas engagement of intracellular Toll-like receptors on antigen-presenting cells stimulates cell activation and the production of other inflammatory cytokines. Thus, immune complexes might perpetuate a positive feedback loop amplifying inflammatory responses.

Functional variability of antibodies upon oxidative processes

Reactive oxygen species (ROS) released from activated phagocytes are involved in the innate immune defense against pathogens. However, when released in excess and when the antioxidant systems are impaired, ROS may induce cellular and tissue damage and dissociation of iron ions or iron containing compounds (heme) from protein-bound state. Free iron ions and free heme are prooxidative. Immunoglobulins usually perform their biological functions at sites of inflammation, where they may encounter reactive oxygen species and/or redox active compounds. It has been demonstrated that the exposure of some antibodies to heme, to transition metal ions or to reactive oxygen species induces an appearance of new binding specificities for various autoantigens. This review article is devoted to the interplay between redox active agents and antibodies. The biological significance of the appearance of new antigen binding specificities on antibodies after exposure to redox-active agents is discussed.

Integrating immunity: the immunculus and self-reactivity

The most important theoretical contribution to autoimmunity was the clonal selection theory of Burnet. This thesis, including the unique terminology of the forbidden clone, laid the foundation for discussions and research regarding self-reactivity and remains the foundation for discussions of thymic education. There were, however, additional works which suggested that autoantibodies, coined natural autoantibodies, exist even in healthy individuals. The ontogenic appearance of the immune system was a series of events that were developed in evolution to recognize the biologic essence of complementary antigen recognition and elimination of foreign pathogens. Interestingly, microbial and host epitopes are often highly conserved in evolution and this homology has functional significance for both the microbe and the host. Further, while the immune system is a series of interconnecting pathways, in reality it is also composed of promiscuous and sometimes independent pathways. The interaction between antibodies and epitopes is determined not necessarily by a primary chemical structure, but more likely by the stereochemistry of the antigen. As such, we increasingly recognize a large repertoire of natural autoantibodies and the development of autoimmune disease requires more than an innocent bystander mimicry pathway, but rather expansion of lymphocytic populations that are either already naturally present, or derived from genetic/acquired defects in immune regulation. We suggest that discussion of loss of tolerance should not include focus on the classic terminology of peripheral versus central tolerance but must consider the integrative activity of the immune system and the balance between well-balanced self-reactivity and the processing of foreign antigens. These events, while primarily theoretical, provide a framework to understand the elements involved in genetic susceptibility to autoimmunity.

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.

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.