How antibodies act as natural adjuvants

Complement C3 and marginal zone B cells promote IgG-mediated enhancement of RBC alloimmunization in mice

Administration of anti-RhD immunoglobulin (Ig) to decrease maternal alloimmunization (antibody-mediated immune suppression [AMIS]) was a landmark clinical development. However, IgG has potent immune-stimulatory effects in other settings (antibody-mediated immune enhancement [AMIE]). The dominant thinking has been that IgG causes AMIS for antigens on RBCs but AMIE for soluble antigens. However, we have recently reported that IgG against RBC antigens can cause either AMIS or AMIE as a function of an IgG subclass. Recent advances in mechanistic understanding have demonstrated that RBC alloimmunization requires the IFN-α/-β receptor (IFNAR) and is inhibited by the complement C3 protein. Here, we demonstrate the opposite for AMIE of an RBC alloantigen (IFNAR is not required and C3 enhances). RBC clearance, C3 deposition, and antigen modulation all preceded AMIE, and both CD4+ T cells and marginal zone B cells were required. We detected no significant increase in antigen-specific germinal center B cells, consistent with other studies of RBC alloimmunization that show extrafollicular-like responses. To the best of our knowledge, these findings provide the first evidence of an RBC alloimmunization pathway which is IFNAR independent and C3 dependent, thus further advancing our understanding of RBCs as an immunogen and AMIE as a phenomenon.

The Immune Correlates of Orthohantavirus Vaccine

Zoonotic transmission of orthohantaviruses from rodent reservoirs to humans has been the cause of severe fatalities. Human infections are reported worldwide, but vaccines have been approved only in China and Korea. Orthohantavirus vaccine development has been pursued with no sense of urgency due to the relative paucity of cases in countries outside China and Korea. However, the orthohantaviruses continuously evolve in hosts and thus the current vaccine may not work as well against some variants. Therefore, a more effective vaccine should be prepared against the orthohantaviruses. In this review, we discuss the issues caused by the orthohantavirus vaccine. Given the pros and cons of the orthohantavirus vaccine, we suggest strategies for the development of better vaccines in terms of pandemic preparedness.

A Highly Expressing, Soluble, and Stable Plant-Made IgG Fusion Vaccine Strategy Enhances Antigen Immunogenicity in Mice Without Adjuvant

Therapeutics based on fusing a protein of interest to the IgG Fc domain have been enormously successful, though fewer studies have investigated the vaccine potential of IgG fusions. In this study, we systematically compared the key properties of seven different plant-made human IgG1 fusion vaccine candidates using Zika virus (ZIKV) envelope domain III (ZE3) as a model antigen. Complement protein C1q binding of the IgG fusions was enhanced by: 1) antigen fusion to the IgG N-terminus; 2) removal of the IgG light chain or Fab regions; 3) addition of hexamer-inducing mutations in the IgG Fc; 4) adding a self-binding epitope tag to create recombinant immune complexes (RIC); or 5) producing IgG fusions in plants that lack plant-specific β1,2-linked xylose and α1,3-linked fucose N-linked glycans. We also characterized the expression, solubility, and stability of the IgG fusions. By optimizing immune complex formation, a potently immunogenic vaccine candidate with improved solubility and high stability was produced at 1.5 mg IgG fusion per g leaf fresh weight. In mice, the IgG fusions elicited high titers of Zika-specific antibodies which neutralized ZIKV using only two doses without adjuvant, reaching up to 150-fold higher antibody titers than ZE3 antigen alone. We anticipate these findings will be broadly applicable to the creation of other vaccines and antibody-based therapeutics.

Anti-bacterial Monoclonal Antibodies

The failing efficacy of antibiotics and the high mortality rate among high-risk patients calls for new treatment modalities for bacterial infections. Due to the vastly divergent pathogenesis of human pathogens, each microbe requires a tailored approach. The main modes of action of anti-bacterial antibodies are virulence factor neutralization, complement-mediated bacterial lysis and enhancement of opsonophagocytic uptake and killing (OPK). Gram-positive bacteria cannot be lysed by complement and their pathogenesis often involves secreted toxins, therefore typically toxin-neutralization and OPK activity are required to prevent and ameliorate disease. In fact, the success stories in terms of approved products, in the anti-bacterial mAb field are based on toxin neutralization (Bacillus anthracis, Clostridium difficile). In contrast, Gram-negative bacteria are vulnerable to antibody-dependent complement-mediated lysis, while their pathogenesis rarely relies on secreted exotoxins, and involves the pro-inflammatory endotoxin (lipopolysaccharide). Given the complexity of bacterial pathogenesis, antibody therapeutics are expected to be most efficient upon targeting more than one virulence factor and/or combining different modes of action. The improved understanding of bacterial pathogenesis combined with the versatility and maturity of antibody discovery technologies available today are pivotal for the design of novel anti-bacterial therapeutics. The intensified research generating promising proof-of-concept data, and the increasing number of clinical programs with anti-bacterial mAbs, indicate that the field is ready to fulfill its promise in the coming years.

Formation of Immune Complexes with a Tetanus-Derived B Cell Epitope Boosts Human T Cell Responses to Covalently Linked Peptides in an Ex Vivo Blood Loop System

Enhancing T cell responses against both viral and tumor Ags requires efficient costimulation and directed delivery of peptide Ags into APCs. Long peptide vaccines are considered favorable vaccine moieties from a clinical perspective, as they can harbor more than one immunogenic epitope enabling treatment of a broader target population. In addition, longer peptides are not extracellularly loaded on MHC class I; rather, they require intracellular processing and will thereby be presented to T cells mainly by professional APCs, thereby avoiding the risk of tolerance induction. The drawback of peptide vaccines regardless of peptide length is that naked peptides are not actively targeted to and taken up by APCs, and the standard nonconjugated adjuvant-peptide mixtures do not ensure cotargeting of the two to the same APC. We have identified a tetanus toxin-derived B cell epitope that can mediate the formation of immune complexes in the presence of circulating Abs. In this study, we show that these immune complexes improve both Ag uptake by APCs (blood monocytes and CD1c⁺ dendritic cells) and consequently improve CD8⁺ T cell recall responses in a human ex vivo blood loop system. The uptake of the peptide conjugate by blood monocytes is dependent on Abs and the complement component C1q. We envision that this strategy can be used to facilitate active uptake of Ags into APCs to improve T cell responses against pathogens or cancer.

Surface displaying of swine IgG1 Fc enhances baculovirus-vectored vaccine efficacy by facilitating viral complement escape and mammalian cell transduction

Baculovirus-mediated gene transfer has been developed as a vaccine design strategy against a number of diseases without apparent viral replication. However, it has been hampered by complement-dependent inactivation, thus hindering the in vivo application of baculovirus. A variety of approaches have been exploited to bypass the complement system in the serum. In this study, we constructed and screened a series of baculovirus vectors displaying complement interfering factors, of which a baculovirus vector displaying swine IgG1 Fc (pFc) showed the highest complement antagonism (75.6%). Flow cytometry analysis of transduced cells demonstrated that the baculovirus display of pFc had a significant increase in transduction efficiency and transgene expression of reporter genes. On this basis, a VSV-G-pseudotyped with swine IgG1 Fc surface displayed baculovirus vector was developed to express the classical swine fever virus (CSFV) E2 gene. The translational enhancers Syn21 and P10UTR were incorporated to improve the antigen expression. The E2 gene was efficiently expressed in both insect and mammalian cells. Pigs immunized with this recombinant baculovirus developed high levels of E2-specific antibody, CSFV-specific neutralizing antibody and IFN-γ-secreting cellular immune responses. These results demonstrate that the strategy of surface-displaying swine IgG1 Fc has a great potential to improve the efficiency of baculovirus-vectored vaccine for CSFV and other swine pathogens.

Host- and pathogen-derived adjuvant coatings on protein nanoparticle vaccines

Nanoparticulate and molecular adjuvants have shown great efficacy in enhancing immune responses, and the immunogenic vaccines of the future will most likely contain both. To investigate the immunostimulatory effects of molecular adjuvants on nanoparticle vaccines, we have designed ovalbumin (OVA) protein nanoparticles coated with two different adjuvants-flagellin (FliC) and immunoglobulin M (IgM). These proteins, derived from Salmonella and mice, respectively, are representatives of pathogen- and host-derived molecules that can enhance immune responses. FliC-coated OVA nanoparticles, soluble FliC (sFliC) admixed with OVA nanoparticles, IgM-coated nanoparticles, and OVA-coated nanoparticles were assessed for immunogenicity in an in vivo mouse immunization study. IgM coatings on nanoparticles significantly enhanced both antibody and T cell responses, and promoted IgG2a class switching but not affinity maturation. FliC-coated nanoparticles and FliC-admixed with nanoparticles both triggered IgG2a class switching, but only FliC-coated nanoparticles enhanced antibody affinity maturation. Our findings that affinity maturation and class switching can be directed independently of one another suggest that adjuvant coatings on nanoparticles can be tailored to generate specific vaccine effector responses against different classes of pathogens.

Selective Memory to Apoptotic Cell-Derived Self-Antigens with Implications for Systemic Lupus Erythematosus Development

Autoimmune diseases are characterized by pathogenic immune responses to self-antigens. In systemic lupus erythematosus (SLE), many self-antigens are found in apoptotic cells (ACs), and defects in removal of ACs from the body are linked to a risk for developing SLE. This includes pathological memory that gives rise to disease flares. In this study, we investigated how memory to AC-derived self-antigens develops and the contribution of self-memory to the development of lupus-related pathology. Multiple injections of ACs without adjuvant into wild-type mice induce a transient primary autoimmune response without apparent anti-nuclear Ab reactivity or kidney pathology. Interestingly, as the transient Ab response reached baseline, a single boost injection fully recalled the immune response to ACs, and this memory response was furthermore transferable into naive mice. Additionally, the memory response contains elements of pathogenicity, accompanied by selective memory to selective Ags. Thus, we provide evidence for a selective self-memory that underlies progression of the response to self-antigens with implications for SLE development therapy.

Converting monoclonal antibody-based immunotherapies from passive to active: bringing immune complexes into play

Monoclonal antibodies (mAbs), which currently constitute the main class of biotherapeutics, are now recognized as major medical tools that are increasingly being considered to fight severe viral infections. Indeed, the number of antiviral mAbs developed in recent years has grown exponentially. Although their direct effects on viral blunting have been studied in detail, their potential immunomodulatory actions have been overlooked until recently. The ability of antiviral mAbs to modulate antiviral immune responses in infected organisms has recently been revealed. More specifically, upon recognition of their cognate antigens, mAbs form immune complexes (ICs) that can be recognized by the Fc receptors expressed on different immune cells of infected individuals. This binding may be followed by the modulation of the host immune responses. Harnessing this immunomodulatory property may facilitate improvements in the therapeutic potential of antiviral mAbs. This review focuses on the role of ICs formed with different viral determinants and mAbs in the induction of antiviral immune responses in the context of both passive immunotherapies and vaccination strategies. Potential deleterious effects of ICs on the host immune response are also discussed.

IgA directly inhibits antigen-dependent B cell activation following distinctive distribution of the antigen in mice

CONTEXT

Serum IgA suppresses immune responses when exposed to antigens recognized by the antibody; however, the underlying mechanism remains unclear.

OBJECTIVE

We herein clarified the relationships between changes in antigen distribution and antigen-dependent B cell activation in the presence or absence of IgA against the antigen in mice.

MATERIALS AND METHODS

DBA/1J and HR-1 mice were intravenously injected with ovalbumin (OVA) and anti-OVA monoclonal IgA OA-4. The distribution of the antigen and B cell responses were measured.

RESULTS

B cell activation by injected OVA, namely, increases in anti-OVA IgG production and the populations of B220(+)GL7(+) and B220(+)CD69(high) splenocytes, was diminished by the co-injection of OA-4. Co-injected OA-4 increased OVA in the serum as well as in the bile and gut. This was coincident with its decrease in the urine due to the inhibition of OVA monomer secretion through the formation of immune complexes. The apparent similarities in the association between fluorescein isothiocyanate (FITC)-OVA and splenic B cells in the presence and absence of OA-4 in vivo appeared to be attributed to compensation between the two effects of OA-4; an increase in serum OVA in vivo and inhibition of the association between OVA and B cells, as suggested by in vitro experiments.

DISCUSSION

Based on these results, the stimulation of B cells by OVA may be directly reduced, at least partly, by the neutralization of OVA by OA-4.

CONCLUSION

IgA may be an effective drug for the treatment of immune disorders due to its ability to blunt antigen-specific B cell activation.

The Split Virus Influenza Vaccine rapidly activates immune cells through Fcγ receptors

Seasonal influenza vaccination is one of the most common medical procedures and yet the extent to which it activates the immune system beyond inducing antibody production is not well understood. In the United States, the most prevalent formulations of the vaccine consist of degraded or "split" viral particles distributed without any adjuvants. Based on previous reports we sought to determine whether the split influenza vaccine activates innate immune receptors-specifically Toll-like receptors. High-dimensional proteomic profiling of human whole-blood using Cytometry by Time-of-Flight (CyTOF) was used to compare signaling pathway activation and cytokine production between the split influenza vaccine and a prototypical TLR response ex vivo. This analysis revealed that the split vaccine rapidly and potently activates multiple immune cell types but yields a proteomic signature quite distinct from TLR activation. Importantly, vaccine induced activity was dependent upon the presence of human sera indicating that a serum factor was necessary for vaccine-dependent immune activation. We found this serum factor to be human antibodies specific for influenza proteins and therefore immediate immune activation by the split vaccine is immune-complex dependent. These studies demonstrate that influenza virus "splitting" inactivates any potential adjuvants endogenous to influenza, such as RNA, but in previously exposed individuals can elicit a potent immune response by facilitating the rapid formation of immune complexes.

The ABCs (Antibody, B Cells, and Carbohydrate Epitopes) of Cholera Immunity: Considerations for an Improved Vaccine

Cholera, a diarrheal disease, is known for explosive epidemics that can quickly kill thousands. Endemic cholera is a seasonal torment that also has a significant mortality. Not all nations with extensive rural communities can achieve the required infrastructure or behavioral changes to prevent epidemic or endemic cholera. For some communities, a single-dose cholera vaccine that protects those at risk is the most efficacious means to reduce morbidity and mortality. It is clear that our understanding of what a protective cholera immune response is has not progressed at the rate our understanding of the pathogenesis and molecular biology of cholera infection has. This review addresses V. cholerae lipopolysaccharide (LPS)-based immunogens because LPS is the only immunogen proven to induce protective antibody in humans. We discuss the role of anti-LPS antibodies in protection from cholera, the importance and the potential role of B cell subsets in protection that is based on their anatomical location and the intrinsic antigen-receptor specificity of various subsets is introduced.

Fc-fusion proteins: new developments and future perspectives

Since the first description in 1989 of CD4-Fc-fusion antagonists that inhibit human immune deficiency virus entry into T cells, Fc-fusion proteins have been intensely investigated for their effectiveness to curb a range of pathologies, with several notable recent successes coming to market. These promising outcomes have stimulated the development of novel approaches to improve their efficacy and safety, while also broadening their clinical remit to other uses such as vaccines and intravenous immunoglobulin therapy. This increased attention has also led to non-clinical applications of Fc-fusions, such as affinity reagents in microarray devices. Here we discuss recent results and more generally applicable strategies to improve Fc-fusion proteins for each application, with particular attention to the newer, less charted areas.

Mucosal antibodies in the regulation of tolerance and allergy to foods

The intestinal mucosa is densely packed with antibody-secreting B cells, the majority of which produce IgA. Mucosal antibodies have traditionally been thought of as neutralizing antibodies that exclude antigens, but they also function in antigen sampling, allowing for selective transcytosis of antigens from the intestinal lumen. IgE-mediated antigen uptake can facilitate the development of allergic reactions to foods, but emerging evidence indicates that IgG-mediated antigen uptake may also play an important role in the development of immune tolerance to foods, particularly in the neonate. This review will focus on the role of intestinal immunoglobulins in the development of clinical tolerance and allergy to food antigens.

FcRn overexpression in transgenic mice results in augmented APC activity and robust immune response with increased diversity of induced antibodies

Our previous studies have shown that overexpression of bovine FcRn (bFcRn) in transgenic (Tg) mice leads to an increase in the humoral immune response, characterized by larger numbers of Ag-specific B cells and other immune cells in secondary lymphoid organs and higher levels of circulating Ag-specific antibodies (Abs). To gain additional insights into the mechanisms underlying this increase in humoral immune response, we further characterized the bFcRn Tg mice. Our Western blot analysis showed strong expression of the bFcRn transgene in peritoneal macrophages and bone marrow derived dendritic cells; and a quantitative PCR analysis demonstrated that the expression ratios of the bFcRn to mFcRn were 2.6- and 10-fold in these cells, respectively. We also found that overexpression of bFcRn enhances the phagocytosis of Ag-IgG immune complexes (ICs) by both macrophages and dendritic cells and significantly improves Ag presentation by dendritic cells. Finally, we determined that immunized bFcRn mice produce a much greater diversity of Ag-specific IgM, whereas only the levels, but not the diversity, of IgG is increased by overexpression of bFcRn. We suggest that the increase in diversity of IgG in Tg mice is prevented by a selective bias towards immunodominant epitopes of ovalbumin, which was used in this study as a model antigen. These results are also in line with our previous reports describing a substantial increase in the levels of Ag-specific IgG in FcRn Tg mice immunized with Ags that are weakly immunogenic and, therefore, not affected by immunodominance.

Targeting a Neutralizing Epitope of HIV Envelope Gp120 by Immune Complex Vaccine

There are formidable challenges in developing HIV vaccines that elicit potent neutralizing antibodies against a broad array of HIV-1 isolates. The key targets for these neutralizing antibodies are the viral envelope antigens gp120 and gp41. Although broadly reactive neutralizing epitopes on gp120 and gp41 have been mapped and studied extensively, these epitopes are poorly immunogenic. Indeed, various vaccine candidates tested in preclinical and clinical trials do not generate antibodies against these epitopes. Hence, novel immunogen designs to augment the immunogenicity of these neutralizing epitopes are wanted. In this review, a unique immunogen design strategy that exploits immune complexes of gp120 and selected anti-gp120 monoclonal antibodies (mAb) to elicit neutralizing antibodies against cross-reactive V3 epitopes is discussed. The ability of these complexes to stimulate neutralizing antibodies is dictated by fine specificity and affinity of mAbs used to form the complexes, indicating the contribution of Fab-mediated activity, rather than conventional Fc-mediated enhancement. Further improvement of V3 immunogenicity is attainable by forming immune complexes with gp120 mutants lacking site-specific N-linked glycans. The increased V3 immunogenicity on the mutated gp120/mAb complexes correlates with enhancement of in vitro antibody recognition (antigenicity) and proteolytic resistance of V3 epitopes when presented on the complexes. These insights should provide guidelines for the development of more potent immunogens that target not only the prototypic V3 epitopes but also other broadly reactive epitopes on the HIV envelope.

HIV-1 and influenza antigens synthetically linked to IgG2a Fc elicit superior humoral responses compared to unmodified antigens in mice

Using murine IgG subclass molecules (IgG1 or IgG2a) synthetically fused to HIV-1 or influenza test antigens, we explored the potential for IgG Fc scaffolds to augment immunogenicity. Each antigen (Ag) was grafted onto a hinge-Fc scaffold containing all critical residues necessary for interaction with effector cells, thus retaining effector functions of the native IgG subclass. We hypothesized that the differential affinity of FcγRs for specific IgG subclasses would influence the magnitude of immune responses elicited by immunization with an Ag-IgG Fc fusion vaccine. We demonstrate here that the antigen-specific humoral response elicited by Ag-IgG2a fusion vaccines is at least tenfold greater than that elicited by native antigen, that this response is superior to that elicited by Ag-IgG1, and that the augmented antigen-specific humoral response elicited is Fcγ receptor-dependent.

Improving immunogenicity of HIV-1 envelope gp120 by glycan removal and immune complex formation

HIV-1 envelope (Env) gp120 is an important target for neutralizing antibody (Ab) responses against the virus; however, developing gp120 vaccines that elicit potent and broad neutralizing Abs has proven to be a formidable challenge. Previously, removal of an N-linked glycan at residue 448 by an N to Q mutation (N448Q) has been found to enhance the in vitro antigenicity of neutralizing epitopes in the V3 loop. In this study the mutated gp120 was first compared with wild type gp120 for immunogenicity in mice using a DNA prime and protein boost immunization regimen. The N448Q mutant did not elicit higher titers of anti-gp120 serum Abs and failed to generate anti-V3 Abs. The sera also had no virus-neutralizing activity, even though the mutant induced higher levels of lymphoproliferation and cytokine production. Subsequently, the N448Q mutant was used to construct an immune complex vaccine with the anti-CD4 binding site monoclonal antibody (mAb) 654. The N448Q/654 complex stimulated comparably high levels of serum Abs to gp120 and V3 as the wild type complex. However, Abs against the C1 and C2 regions in the gp120 core were more elevated. Importantly, the mutant complex also elicited higher titers of neutralizing Abs activity than the wild type counterpart. Similar results were achieved with a complex made with gp120 bearing an N448E mutation, confirming the importance of the N448-linked glycan in modulating gp120 immunogenicity. Neutralizing activity was directed to V3 and other undefined neutralizing epitopes. Improved immunogenicity of the immune complexes correlated with alterations in exposure of V3 and other Ab epitopes and their stability against proteases. These data demonstrate the advantage of combining site-specific N-glycan removal and immune complex formation as a novel vaccine strategy to improve immunogenicity of targeted Ab epitopes on critical regions of HIV-1 gp120.

IgM and its receptors: structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cµ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/µR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcµR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.

Recent advances using FcRn overexpression in transgenic animals to overcome impediments of standard antibody technologies to improve the generation of specific antibodies

This review illustrates the salutary effects of neonatal Fc receptor (FcRn) overexpression in significantly improving humoral immune responses in the generation of antibodies for immunotherapy and diagnostics. These include: (1) improved IgG protection; (2) augmented antigen-specific humoral immune response with larger numbers of antigen specific B cells, thus offering a wider spectrum of clones; (3) generation of antibodies against weakly immunogenic antigens; (4) significant improvements in the number and substantial developments in the diversity of hybridomas. FcRn transgenesis thus confers a number of practical benefits, including faster antibody production, higher antibody yields and improved generation of hybridomas for monoclonal antibody production. Notably, these efficiencies in polyclonal antibody production were also demonstrated in FcRn transgenic rabbits. Overall, FcRn transgenic animals yield more antibodies and provide a route to the generation of antibodies against antigens of low immunogenicity that are difficult to obtain using currently available methods.

A therapeutic anti-Streptococcus mutans monoclonal antibody used in human passive protection trials influences the adaptive immune response

The adhesin known as Antigen I/II, P1 or PAc of the cariogenic dental pathogen Streptococcus mutans is a target of protective immunity and candidate vaccine antigen. Previously we demonstrated that immunization of mice with S. mutans complexed with anti-AgI/II monoclonal antibodies (MAbs) resulted in changes in the specificity, isotype and functionality of elicited anti-AgI/II antibodies in the serum of immunized mice compared to administration of bacteria alone. In the current study, an anti-AgI/II MAb reported in the literature to confer unexplained long term protection against S. mutans re-colonization following passive immunization in human clinical trials (MAb Guy's 13), and expressed in tobacco plants (MAb Guy's 13 plantibody), was evaluated for its potential immunomodulatory properties. Immunization of BALB/c mice with immune complexes of Guy's 13 plantibody bound to S. mutans whole cells resulted in a similar change in specificity, isotype, and functionality of elicited anti-AgI/II antibodies as had been observed for other immunomodulatory MAbs. This new information, coupled with the recently solved crystal structure of the adhesin, now provides a rational explanation and plausible mechanism of action of passively administered Guy's 13/Guy's 13 plantibody in human clinical trials, and how long-term prevention of S. mutans carriage well past the application period of the therapeutic antibody could have been achieved.

IgE immune complexes stimulate an increase in lung mast cell progenitors in a mouse model of allergic airway inflammation

Mast cell numbers and allergen specific IgE are increased in the lungs of patients with allergic asthma and this can be reproduced in mouse models. The increased number of mast cells is likely due to recruitment of mast cell progenitors that mature in situ. We hypothesized that formation of IgE immune complexes in the lungs of sensitized mice increase the migration of mast cell progenitors to this organ. To study this, a model of allergic airway inflammation where mice were immunized with ovalbumin (OVA) in alum twice followed by three daily intranasal challenges of either OVA coupled to trinitrophenyl (TNP) alone or as immune complexes with IgE-anti-TNP, was used. Mast cell progenitors were quantified by a limiting dilution assay. IgE immune complex challenge of sensitized mice elicited three times more mast cell progenitors per lung than challenge with the same dose of antigen alone. This dose of antigen challenge alone did not increase the levels of mast cell progenitors compared to unchallenged mice. IgE immune complex challenge of sensitized mice also enhanced the frequency of mast cell progenitors per 10(6) mononuclear cells by 2.1-fold. The enhancement of lung mast cell progenitors by IgE immune complex challenge was lost in FcRγ deficient mice but not in CD23 deficient mice. Our data show that IgE immune complex challenge enhances the number of mast cell progenitors in the lung through activation of an Fc receptor associated with the FcRγ chain. This most likely takes place via activation of FcεRI, although activation via FcγRIV or a combination of the two receptors cannot be excluded. IgE immune complex-mediated enhancement of lung MCp numbers is a new reason to target IgE in therapies against allergic asthma.

Mast cells contribute to the mucosal adjuvant effect of CTA1-DD after IgG-complex formation

Mast cell activation is one of the most dramatic immune-mediated responses the body can encounter. In the worst scenario (i.e., anaphylaxis), this response is fatal. However, the importance of mast cells as initiators and effectors of both innate and adaptive immunity in healthy individuals has recently been appreciated. It was reported that mast cell activation can be used as an adjuvant to promote Ag-specific humoral immune responses upon vaccination. In this study, we have used a clinically relevant mucosal adjuvant, cholera toxin A1 subunit (CTA1)-DD, which is a fusion protein composed of CTA1, the ADP-ribosylating part of cholera toxin, and DD, two Ig-binding domains derived from Staphylococcus aureus protein A. CTA1-DD in combination with polyclonal IgG induced degranulation and production of TNF-alpha from mouse mast cells. Furthermore, CTA1-DD and polyclonal IgG complex induced mast cell degranulation in mouse skin tissue and nasal mucosa. We also found that intranasal immunization with hapten (4-hydroxy-3-nitrophenyl) acetyl (NP) coupled to chicken gammaglobulin admixed with CTA1-DD complexed with polyclonal IgG greatly enhanced serum IgG anti-NP Ab responses and stimulated higher numbers of NP-specific plasma cells in the bone marrow as compared with that observed in mice immunized with NP-chicken gammaglobulin with CTA1-DD alone. This CTA1-DD/IgG complex-mediated enhancement was mast cell dependent because it was absent in mast cell-deficient Kit(W-sh/W-sh) mice. In conclusion, our data suggest that a clinically relevant adjuvant, CTA1-DD, exerts additional augmenting effects through activation of mucosal mast cells, clearly demonstrating that mast cells could be further exploited for improving the efficacy of mucosal vaccines.

The yin and yang of B cells in graft rejection and tolerance

Various lineages of B cells are being increasingly recognized as important players in the etiology and prognosis of both acute and chronic graft rejection. The role of immature, chronically activated B cells, as efficient antigen-presenting cells, supporting recalcitrant cell-mediated graft rejection and late lineage B cells driving humoral rejections, is being increasingly recognized. This review captures the recent literature on this subject and discusses the various roles of the B cell in renal graft rejection and conversely, also in graft tolerance, both in animal and human studies. In addition, novel therapies targeting specific B-cell lineages in graft rejection are also discussed, with a view to developing more targeted therapies for graft rejection.

A multi-DNA preventive vaccine for p53/Neu-driven cancer syndrome

The highly aggressive cancer syndrome of female mice carrying a p53 knockout allele and a rat HER-2/neu (Neu) transgene (BALB-p53Neu) can be prevented by a cell vaccine presenting three components: Neu, interleukin (IL)-12 production, and allogeneic major histocompatibility complex (MHC) alleles (Triplex cell vaccine). Here we tested a second-generation Triplex DNA-based vaccine (Tri-DNA), consisting of the combination of three gene components (a transmembrane-extracellular domain fragment of the Neu gene, IL-12 genes, and the H-2D(q) allogeneic MHC gene), carried by separate plasmids. The Tri-DNA vaccine was at least as effective as the Triplex cell vaccine for cancer immunoprevention, giving a similar delay in the onset of mammary cancer and complete protection from salivary cancer. Both vaccines induced anti-Neu antibodies of the murine IgG2a isotype at similar levels. The Tri-DNA vaccine gave more restricted immunostimulation, consisting of a fully helper T cell type 1 (Th1)-polarized response, with effective production of interferon (IFN)-gamma in response to the vaccine but no spontaneous production, and no induction of anti-Neu IgG3 antibodies. On the other hand, the Triplex cell vaccine induced both Th1 and Th2 cytokines, a strong increase in spontaneous IFN-gamma production, and high levels of IgG3 antibodies recognizing Neu-positive syngeneic cells. In conclusion, the Tri-DNA vaccine is as effective as Triplex cell vaccine, exploiting a more restricted immune stimulation.

Targeting of Escherichia coli F4 fimbriae to Fcgamma receptors enhances the maturation of porcine dendritic cells

F4(+) enterotoxigenic Escherichia coli (ETEC) infections are an important cause of postweaning diarrhoea in piglets and an oral immunization of piglets with purified F4 fimbriae protects them from a subsequent F4(+) ETEC infection. However, oral immunization of suckling piglets is hampered due to the immature status of their immune system. Targeting of antigens to Fcgamma receptors (FcgammaR) on human and murine dendritic cells (DC) has been shown to enhance DC maturation and both humoral and cellular immune responses. To investigate the effect of F4 fimbriae incorporated in immune complexes (F4-IC) on porcine DC, we used porcine monocytic-derived DC (MoDC) as a model system. The results in this study demonstrate that FcgammaRI, II and III mRNA is expressed by porcine MoDC. Furthermore, we show that FcgammaRII and III are expressed on the cell surface and that F4-IC are internalized by MoDC via FcgammaR. This FcgammaR ligation induced a significantly enhanced expression of Major Histocompatibility complex (MHCII) class II and the costimulatory molecules CD80/86 and CD40 by MoDC compared with immature MoDC. Furthermore, the phagocytic capacity of F4-IC stimulated MoDC was reduced as evidenced by a reduced uptake of DQ-ovalbumin and FITC-dextran. In an allogenic and autologous mixed lymphocyte reaction, these F4-IC-activated MoDC showed an improved T cell stimulatory capacity in comparison with immature MoDC. The F4-IC induced DC maturation correlated with significant higher expression levels of several pro-inflammatory cytokines such as interleukine (IL) 1beta, IL-6 and Tumor necrosis factor alpha, the chemokine IL-8 and IL-12p40 in comparison with immature MoDC. Altogether, these results clearly demonstrate that FcgammaR engagement enhances the maturation of porcine MoDC, which may suggest that antigen targeting to FcgammaR on DC could improve vaccine design against infections.

The use of immune complex vaccines to enhance antibody responses against neutralizing epitopes on HIV-1 envelope gp120

The capacity of immune complexes to augment antibody (Ab) responses is well established. The enhancing effects of immune complexes have been attributed mainly to Fc-mediated adjuvant activity, while the ability of Abs to induce antigenic alterations of specific epitopes as a result of immune complex formation has been less well studied. Previously we have shown that the interaction of anti-CD4-binding site (CD4bs) Abs with HIV-1 gp120 induces conformation changes that lead to enhanced antigenicity and immunogenicity of neutralizing epitopes in the V3 loop. The present study shows that significant increases in the antigenicity of the V3 and C1 regions of gp120 were attained for several subtype B gp120s and a subtype C gp120 upon immune complex formation with the anti-CD4bs monoclonal Ab (mAb) 654-D. Such enhancement was observed with immune complexes made with other anti-CD4bs mAbs and anti-V2 mAbs, but not with anti-C2 mAbs, indicating this activity is determined by antigen specificity of the mAb that formed the immune complex. When immune complexes of gp120(LAI)/654-D and gp120(JRFL)/654-D were tested as immunogens in mice, serum Abs to gp120 and V3 were generated at significantly higher titers than those induced by the respective uncomplexed gp120s. Notably, the anti-V3 Ab responses had distinct fine specificities; gp120(JRFL)/654-D stimulated more cross-reactive anti-V3 Abs than gp120(LAI)/654-D. Neutralizing activities against viruses with heterologous envelope were also detected in sera of mice immunized with gp120(JRFL)/654-D, although the neutralization breadth was still limited. Overall this study shows the potential use of gp120/Ab complexes to augment the immunogenicity of HIV-1 envelope gp120, but further improvements are needed to elicit virus-neutralizing Ab responses with higher potency and breadth.

Fc-receptors and immunity to malaria: from models to vaccines

The complexity and number of antigens (Ags) seen during an immune response has hampered the development of malaria vaccines. Antibodies (Abs) play an important role in immunity to malaria and their passive administration is effective at controlling the disease. Abs represent approximately 25% of all proteins undergoing clinical trials, and these 'smart biologicals' have undergone a major revival with the realization that Abs lie at the interface between innate and adaptive immunity. At least 18 Abs have FDA approval for clinical use and approximately 150 are in clinical trials, the majority for the treatment of cancer, allograft rejection or autoimmune disease. Despite these triumphs none are in development for malaria, principally because they are perceived as being too expensive for a disease mainly afflicting poor and marginalized populations. Although unlikely, at least in the foreseeable future, that Ab-based prophylaxis will be made available to the millions of people at risk from malaria, they may be incorporated into current vaccine approaches, since Abs act as correlates of protection in studies aimed at defining the best Ags to include in vaccines. Abs may also form the basis for novel vaccination strategies by targeting Ags to appropriate antigen presenting cells. Therefore, to develop the most efficacious vaccines it will be necessary to fully understand which Abs and Fc-receptors (FcRs) are best engaged for a positive outcome.

Studies on the mechanism by which antigen-specific IgG suppresses primary antibody responses: evidence for epitope masking and decreased localization of antigen in the spleen

Immunoglobulin (IgG) has the ability to suppress the Ab response against the Ag to which it binds. Although the mechanism remains unclear, this phenomenon has physiological relevance and is used clinically in Rh prophylaxis. As suppression works well in mice lacking the inhibitory FcgammaRIIB, the two most likely explanations are that IgG masks epitopes and/or that IgG increases the clearance of Ag. In the present study, mice were immunized with sheep red blood cells (SRBC) to which the hapten 5-iodo-4-hydroxyl-3-nitrophenacetyl (NIP) was conjugated at high or low density and the ability of IgG anti-NIP to suppress the Ab response to NIP and SRBC was assayed. Only the NIP-specific response was suppressed when mice were immunized with SRBC-NIP(low), whereas both NIP- and SRBC-specific responses were suppressed when SRBC-NIP(high) was used. This is best explained by epitope masking; at high epitope density, IgG also blocks neighbouring epitopes from recognition by B cells. We also examined the effects of IgG-mediated suppression on T-cell responses directly in vivo. While IgG anti-SRBC administered with sheep red blood cells ovalbumin (SRBC-OVA) almost completely suppressed the anti-SRBC and anti-OVA Ab responses, the OVA-specific T-cell response was still 50% of that observed in control mice. This is probably the result of decreased Ag exposure as IgG-bound SRBC were cleared faster from the bloodstream and were found at lower concentration in the spleen than unbound SRBC. These results suggest that both Ag clearance and epitope masking occurs during IgG-mediated suppression, but that under physiological circumstances epitope masking is the predominant mechanism.

Immune complex relay by subcapsular sinus macrophages and noncognate B cells drives antibody affinity maturation

Subcapsular sinus (SCS) macrophages capture antigens from lymph and present them intact for B cell encounter and follicular delivery. However, the properties of SCS macrophages are poorly defined. Here we show SCS macrophage development depended on lymphotoxin-alpha1beta2, and the cells had low lysosomal enzyme expression and retained opsonized antigens on their surface. Intravital imaging revealed immune complexes moving along macrophage processes into the follicle. Moreover, noncognate B cells relayed antigen opsonized by newly produced antibodies from the subcapsular region to the germinal center, and affinity maturation was impaired when this transport process was disrupted. Thus, we characterize SCS macrophages as specialized antigen-presenting cells functioning at the apex of an antigen transport chain that promotes humoral immunity.

B cell intrinsic MyD88 signals drive IFN-gamma production from T cells and control switching to IgG2c

The question of whether Ab responses to T-dependent Ags require B cell intrinsic signaling via the main TLR adaptor (MyD88) has become embroiled in confusion. In part this may be related to the methods used to analyze B cell intrinsic signaling. We have used a mixed bone marrow chimera model to generate mice in which the B cell compartment is completely deficient in MyD88 expression, while the other hematopoietic lineages are largely normal. These mice were immunized with T-dependent Ags or infected with Salmonella. We found that the Ag-specific IgG2c primary response was absolutely dependent on MyD88 signaling to B cells, while other Ig classes were not (IgG1 and IgG3) or much less so (IgG2b, IgA). The MyD88(B-/-) chimeric mice exhibited an impairment of development of IFN-gamma effector T cells, a likely contributory factor in the lack of IgG2c. We also found that B cell intrinsic MyD88 signals are required for the production of natural Abs. The data emphasize the nonredundant role of B cells as programmers of T cell differentiation in vivo.

High levels of IgG4 antibodies to foods during infancy are associated with tolerance to corresponding foods later in life

Children with eczema and sensitization to foods are recommended skin care and, if food allergy is proven by challenge, an elimination diet. For most children the diet period is transient, but the process behind tolerance development and the influence of decreased allergen exposure is not fully known. The aim of the study was to investigate the effect of elimination diet on serum and salivary antibodies and to identify immunological parameters related to the ability to tolerate foods. Eighty-nine children, below 2 yr of age, with eczema and suspected food allergy were included. Recommended treatment was skin care to all children, and 60 children had a period of elimination diet. At 4(1/2) yr of age, the children were divided into two groups, based on if they had been able to introduce the eliminated foods, or not. Serum and salivary antibodies were analyzed with enzyme-linked immunosorbent assay and UniCAP before and after a 6-wk treatment period and at 4(1/2) yr of age. Children sensitized to egg and/or milk that could eat and drink the offending foods at 4(1/2) yr of age, had higher levels of Immunoglobulin G(4) antibodies to ovalbumin and beta-lactoglobulin and also higher IgG(4)/Immunoglobulin E ratios on inclusion in the study, than those who had to eliminate egg and/or milk from their diet, beyond 4(1/2) yr of age. The highest IgG(4)/IgE ratios were found in children with circulating IgE antibodies to egg and/or milk but negative skin prick test on inclusion. The 6-wk treatment period did not significantly affect the levels of serum and salivary antibodies. In conclusion, eczematous, food sensitized infants with high levels of IgG(4) and high ratios of IgG(4)/IgE antibodies to food allergens are more likely to consume these foods at 4(1/2) yr than infants with low levels and ratios.

Decreased specific antibody responses to alpha-Gal-conjugated antigen in animals with preexisting high levels of natural antibodies binding alpha-Gal residues

High levels of natural antibodies (NAb) binding the alpha-Gal residue (Galalpha1-3Galbeta1-4GlcNAc) are found in poultry (and humans), which is probably reflected by high levels of natural agglutinating antibodies (Ab) to rabbit red blood cells (RRBC) in plasma from chickens (and humans). Recently, it was shown that alpha-Gal conjugation of proteins induced higher antiprotein Ab responses in alpha-Gal knockout mice, suggesting immune-enhancing features of preexisting Ab binding carbohydrate-protein conjugates. We challenged chickens s.c. with either alpha-Gal-conjugated human serum albumin (HuSA), beta-Gal-conjugated HuSA, or unconjugated ("native") HuSA, respectively, and measured primary and secondary Ab responses to HuSA, including isotype IgM and IgG responses, and cellular immune responses in vitro (lymphoproliferation) to HuSA or concanavalin A. alpha-Gal conjugation, but not beta-Gal conjugation, of HuSA resulted in significantly decreased primary and secondary Ab responses to HuSA, especially IgG isotype responses, as compared with Ab responses to native HuSA. Lymphoproliferation in vitro was also decreased, although not significantly, in birds challenged with alpha-Gal-conjugated HuSA. High levels of agglutinating Ab levels to RRBC and NAb binding porcine thyroglobulin were detected in all birds, as was true for (natural) Ab levels binding alpha-Gal-conjugated HuSA before immunization, whereas low levels of preexisting (natural) antibodies directed to native HuSA were present in plasma before immunization. Levels of RRBC agglutinins and Ab binding thyroglobulin were not affected by immunization with HuSA, alpha-Gal-conjugated HuSA, or beta-Gal-conjugated HuSA. Our data confirm the presence of high levels of (preexisting) NAb in the plasma of chickens directed to the alpha-Gal residue. The decreased responsiveness to alpha-Gal-bearing antigens in the current study shows that, in addition to immune-enhancing features, NAb may also have suppressive effects on specific immune responses, which substantiates the regulatory role of innate immunity (NAb) in mounting specific immune responses.

Analyzing antibody-Fc-receptor interactions

Cellular receptors for immunoglobulins (Fc-receptors; FcR) are central mediators of antibody-triggered effector functions. Immune complex (IC) binding to FcRs results in a variety of reactions such as the release of inflammatory mediators, antibody dependent cellular cytotoxicity (ADCC) and phagocytosis of ICs. Analyzing antibody-FcR (Ab-FcR) interactions in vitro is essential to determine the effector mechanisms, binding characteristics and affinity parameters that will impact and predict antibody activity in vivo. The methods described in this chapter include the generation of ICs and soluble FcR variants, as well as ELISA and FACS-based assays to study Ab-FcR interactions.

Monoclonal antibodies for prophylaxis and therapy of infectious diseases

Monoclonal antibodies (mAb) are attractive biologic drugs due to their exquisite specificity and well understood mechanisms of action, which results in a higher predictability and lower attrition rate compared with other drugs. Therefore, it may seem surprising that only a single mAb is presently marketed for an infectious disease indication. However, the antibiotic resistance crisis, emerging viral diseases and bioterroristic threats have recently spurred the development of anti-infective mAbs, of which more than a dozen are being tested in clinical trials. Conceptually, and validated in many preclinical models, mAbs will be most effective when used prophylactically against acute viral infections and bacterial toxins. The acute bacterial and chronic viral infections, which are medically and economically far more important, are much more difficult to control by antibodies, as the recent clinical failure of some polyclonal antibody products has shown. In these situations, the synergistic action of two or more mAbs together with a small molecule drug will most likely be required for therapeutic efficacy. This review aims to highlight the scientific and economic opportunities and obstacles that are encountered in the quest to add mAbs to the armament of anti-infective drugs.

IL-18 deficiency inhibits both Th1 and Th2 cytokine production but not the clinical symptoms in experimental autoimmune neuritis

IL-18 deficient (IL-18-/-) mice were used to investigate the role of IL-18 in the pathogenesis of experimental autoimmune neuritis (EAN) which was induced by immunization of the mice with P0 protein peptide 180-199. The clinical course was not different between IL-18-/- and wild-type mice. The splenic mononuclear cell (MNC) proliferation was also similar in both animal groups. However, the percentages of IFN-gamma, IL-10 and IL-12 positive cells were decreased among infiltrating MNC of cauda equine in IL-18-/- mice. This indicates that IL-18 deficiency inhibits the production of both Th1 and Th2 cytokines in the target organ of mice with EAN.

Effects of protein aggregates: an immunologic perspective

The capacity of protein aggregates to enhance immune responses to the monomeric form of the protein has been known for over a half-century. Despite the clear connection between protein aggregates and antibody mediated adverse events in treatment with early therapeutic protein products such as intravenous immune globulin (IVIG) and human growth hormone, surprisingly little is known about the nature of the aggregate species responsible for such effects. This review focuses on a framework for understanding how aggregate species potentially interact with the immune system to enhance immune responses, garnered from basic immunologic research. Thus, protein antigens presented in a highly arrayed structure, such as might be found in large nondenatured aggregate species, are highly potent in inducing antibody responses even in the absence of T-cell help. Their potency may relate to the ability of multivalent protein species to extensively cross-link B-cell receptor, which (1) activates B cells via Bt kinases to proliferate, and (2) targets protein to class II major histocompatibility complex (MHC)-loading compartments, efficiently eliciting T-cell help for antibody responses. The review further focuses on protein aggregates as they affect an immunogenicity risk assessment, the use of animal models and studies in uncovering effects of protein aggregates, and changes in product manufacture and packaging that may affect generation of protein aggregates.