Human antibody–Fc receptor interactions illuminated by crystal structures

Impact of epidermal growth factor receptor (EGFR) cell surface expression levels on effector mechanisms of EGFR antibodies

The epidermal growth factor receptor (EGFR) is a widely expressed Ag that is successfully targeted in tumor patients by mAbs or tyrosine kinase inhibitors. A clinical study in non-small cell lung cancer patients demonstrated a positive correlation between EGFR expression levels and the therapeutic efficacy of the EGFR mAb cetuximab. However, the impact of EGFR expression on the different mechanisms of action (MoAs) triggered by the EGFR mAb has not been defined. In this study, BHK-21 cells were stably transfected to express different EGFR levels, which were quantified by immunofluorescence and immunohistochemistry and compared with EGFR levels of clinical non-small cell lung cancer samples. These cells were used to systematically investigate the impact of target Ag expression levels on Fab- or Fc-mediated MoAs of EGFR mAb. A negative correlation between EGFR levels and potency of Fab-mediated MoA was observed. Interestingly, Ab-dependent cell-mediated cytotoxicity (ADCC) by NK cells, monocytes, or polymorphonuclear cells as well as complement-dependent cytotoxicity positively correlated with the number of EGFR molecules. In comparison with ADCC by mononuclear cells, polymorphonuclear cell-mediated ADCC and complement-dependent cytotoxicity required higher EGFR expression levels and higher mAb concentrations to trigger significant tumor cell killing. This correlation between EGFR expression levels and Fc-mediated MoA was confirmed in an independent panel of human tumor cell lines carrying diverse genetic alterations. Furthermore, RNA interference-induced knockdown experiments reinforced the impact of EGFR expression on tumor cell killing by EGFR mAb. In conclusion, these results suggest that EGFR expression levels may determine distinct patterns of MoAs that contribute to the therapeutic efficacy of EGFR mAb.

Harnessing human plasmacytoid dendritic cells as professional APCs

The plasmacytoid dendritic cell (pDC) constitutes a unique DC subset that links the innate and adaptive arm of the immune system. Whereas the unique capability of pDCs to produce large amounts of type I IFNs in response to pathogen recognition is generally accepted, their antigen-presenting function is often neglected since most studies on antigen presentation are aimed at other DC subsets. Recently, pDCs were demonstrated capable to present antigen leading to protective tumor immunity. In this review, we discuss how pDCs could be exploited in the fight against cancer by analyzing their capacity to capture, process and (cross-) present antigen.

Immunogenicity to biologics: mechanisms, prediction and reduction

Currently, there is a significant rise in the development and clinical use of a unique class of pharmaceuticals termed as Biopharmaceuticals or Biologics, in the management of a range of disease conditions with, remarkable therapeutic benefits. However, there is an equally growing concern regarding development of adverse effects like immunogenicity in the form of anti-drug antibodies (ADA) production and hypersensitivity. Immunogenicity to biologics represents a significant hurdle in the continuing therapy of patients in a number of disease settings. Efforts focussed on the identification of factors that contribute towards the onset of immunogenic response to biologics have led to reductions in the incidence of immunogenicity. An in-depth understanding of the cellular and molecular mechanism underpinning immunogenic responses will likely improve the safety profile of biologics. This review addresses the mechanistic basis of ADA generation to biologics, with emphasis on the role of antigen processing and presentation in this process. The article also addresses the potential contribution of complement system in augmenting or modulating this response. Identifying specific factors that influences processing and presentation of biologic-derived antigens in different genotype and disease background may offer additional options for intervention in the immunogenic process and consequently, the management of immunogenicity to biologics.

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.

Selective deactivation of serum IgG: a general strategy for the enhancement of monoclonal antibody receptor interactions

Serum IgG is a potent inhibitor of monoclonal antibody (mAb) binding to the cell-surface Fcγ receptors (FcγRs), which mediate cytotoxic and phagocytic effector functions. Here, we show that this competition can be eliminated, selectively, by the introduction to serum of (i) an enzyme that displaces Fc from FcγRs and (ii) a modification present in the therapeutic mAb that renders it resistant to that enzyme. Specifically, we show that (i) EndoS (endoglycosidase S) cleaves only complex-type glycans of the type found on IgG but (ii) is inactive against an engineered IgG Fc with oligomannose-type glycans. EndoS thus reduces FcγR binding of serum IgG, but not that of engineered mAb. Introduction of both the engineered mAb and endoglycosidase in serum leads to a dramatic increase in FcγR binding compared to the introduction of mAb in serum alone. Antibody receptor refocusing is a general technique for boosting the effector signal of therapeutic antibodies.

Increased survival and reduced renal injury in MRL/lpr mice treated with a human Fcγ receptor II (CD32) peptide

Systemic lupus erythematosus (SLE) is a multisystem chronic inflammatory disease affecting many organs. The deposition in kidney tissue of immune complexes and their interaction with macrophages is thought to trigger the inflammatory response leading to glomerulonephritis. It has been demonstrated that inhibition of this interaction in murine models can alleviate the disease. Six synthetic peptides were derived from the membrane-proximal extracellular domain (EC2) of human Fcγ receptor II (huFcγRII). Of these, one peptide, huRII6, was shown to be a potent competitive inhibitor of IgG binding to recombinant FcγRII in vitro. To examine the possible therapeutic impact of huRII6 in vivo, this peptide, or a control, was given by subcutaneous injection to female MRL/lpr mice from weeks 7 to 36, resulting in an enhanced survival rate compared with control-treated animals and a reduction of proteinuria. Histopathological examination of the kidneys showed a reduction in deposition of immune complexes and preservation of structure. Such a functional peptide should prove useful for examining the role of IgG-FcγR interactions in experimental models of disease and may provide for the development of FcR-targeting drugs to treat autoimmune disorders.

ICOS-LICOS interaction is critically involved in TGN1412-mediated T-cell activation

TGN1412, a superagonistic CD28-specific antibody, was shown to require Fc-cross-linking or immobilization as a prerequisite to mediate T-cell proliferation and cytokine release in vitro. We used primary human umbilical vein endothelial cells (HUVECs) to study their ability to induce activation of TGN1412-treated T cells. We confirmed that peripheral primary human T cells do not show activation upon stimulation with soluble TGN1412 alone. Nevertheless, cocultivation of TGN1412-treated T cells with HUVECs induced T-cell activation that was further enhanced using cytokine prestimulated HUVECs. Unexpectedly, Fc-FcγR interaction was dispensable for endothelial cell-mediated proliferation of TGN1412-treated T cells. Transwell-culture assays showed that TGN1412-treated T cells need direct cell-to-cell contact to HUVECs to induce proliferation. We found that costimulatory ICOS-LICOS interaction between T cells and endothelial cells is critically involved in TGN1412-mediated effects. Blocking LICOS reduced TGN1412-mediated T-cell proliferation significantly, whereas recombinant LICOS fully conferred TGN1412-mediated T-cell proliferation. Of note, cytokine stimulation enhanced LICOS expression on HUVECs and ICOS-LICOS interaction up-regulated ICOS expression on TGN1412-treated T cells. Hence, we provide a model of positive feedback conferred by ICOS-LICOS interaction between TGN1412-treated T cells and endothelial cells.

Cutting edge: human FcRL4 and FcRL5 are receptors for IgA and IgG

Fc receptor-like (FcRL) proteins are a family of cellular receptors homologous to FcγRI and are predominantly expressed by B cells. They function to costimulate or inhibit BCR signaling through consensus ITAMs and ITIMs; however, the extracellular ligands of these receptors remain unknown or controversial. In this study, we tested the ability of human FcRL proteins to bind Igs and found FcRL4 and FcRL5 to be bona fide Fc receptors. In cellular binding assays, FcRL4 bound efficiently to IgA and FcRL5 binds all IgG isotypes with varied efficiency. Additionally, we generated mAbs capable of specifically blocking these interactions. Given their expression on activated B cells and potential for inhibitory signaling, FcRL4 and FcRL5 are likely to be important for immune complex-dependent human B cell regulation, and they represent novel therapeutic targets for receptor blockade therapies.

Isotype and glycoform selection for antibody therapeutics

We live in a hostile environment but are protected by the innate and adaptive immune system. A major component of the latter is mediated by antibody molecules that bind to pathogens, with exquisite specificity, and the immune complex formed activates cellular mechanisms leading to the removal and destruction of the complex. Five classes of antibody are identified; however, the IgG class predominates in serum and a majority of monoclonal antibody (mAb) therapeutics are based on the IgG format. Selection within the antibody repertoire allows the generation of (mAb) having specificity for any selected target, including human antigens. This review focuses on the structure and function of the Fc region of IgG molecules that mediates biologic functions, within immune complexes, by interactions with cellular Fc receptors (FcγR) and/or the C1q component of complement. A property of IgG that is suited to its use as a therapeutic is the long catabolic half life of ~21 days, mediated through the structurally distinct neonatal Fc receptor (FcRn). Our understanding of structure/function relationships is such that we can contemplate engineering the IgG-Fc to enhance or eliminate biologic activities to generate therapeutics considered optimal for a given disease indication. There are four subclasses of human IgG that exhibit high sequence homology but a unique profile of biologic activities. The FcγR and the C1q binding functions are dependent on glycosylation of the IgG-Fc. Normal human serum IgG is comprised of multiple glycoforms and biologic activities, other than catabolism, varies between glycoforms.

A striking example of convergent evolution observed for the ggFcR:IgY interaction closely resembling that of mammalian FcR:IgG

We have recently identified a novel IgY specific chicken FcR (ggFcR) on chromosome 20, a region where no FcR gene is present in mammals. Serially deleted IgY fusion proteins were tested in a reporter assay to identify C(H) domains involved in ggFcR binding. Single C(H) domains did not bind to ggFcR, whereas Fcυ2 to Fcυ4 induced good and the Fcυ3 to Fcυ4 domains moderate activity. When IgY from diverse birds were assayed, only IgY from gallinaceous birds showed binding, which enabled us to pinpoint several potential contact sites by a sequence comparison and molecular modelling. Point mutations of critical residues at these sites revealed the Fcυ2 and Fcυ3 domains as major ggFcR:IgY binding sites similar to mammalian IgG. These results demonstrate that ggFcR has a contact site to IgY which closely resembles that of human IgG bound to FcR.

IgG1 and IVIg induce inhibitory ITAM signaling through FcγRIII controlling inflammatory responses

Intravenous immunoglobulin (IVIg) has been used in the treatment of several autoimmune and inflammatory diseases. However, its mechanism of action remains incompletely understood. Here, we investigated the possibility that IVIg induces its anti-inflammatory effects through activating Fcγ receptors bearing an immunoreceptor tyrosine-based activation motif (ITAM) in the FcRγ signaling adaptor. Recently, the concept of inhibitory ITAM (ITAMi) has emerged as a new means to negatively control the immune response. We found that interaction of FcRγ-associated mouse or human FcγRIII with uncomplexed IgG1 or IVIg, or with bivalent anti-FcγRIII F(ab')(2) reduced calcium responses, reactive oxygen species production, endocytosis, and phagocytosis, induced by heterologous activating receptors on monocyte/macrophages and FcγRIII(+) transfectants. Inhibition required the ITAMi configuration of the FcγRIII-associated FcRγ subunit and SHP-1 recruitment involving formation of intracellular "inhibisome" clusters containing FcγRIII, and the targeted heterologous activating receptor. IVIg as well as anti-FcγRIII treatments controlled the development of nonimmune mediated inflammation in vivo independently of FcγRIIB. These results demonstrate that circulating immunoglobulins (Ig)Gs are not functionally inert but act through continuous interaction with FcγRIII-inducing ITAMi signaling to maintain immune homeostasis. These data support a new mechanism of action for IVIg and demonstrate the therapeutic potential of FcγRIIIA targeting in inflammation.

A holistic view of the dynamisms of teleost IgM: a case study of Streptococcus iniae vaccinated rainbow trout (Oncorhynchus mykiss)

To date, little is known about how trout IgM, the primary antibody of fish, varies in titer, specificity, disulfide cross-linking, and affinity following immunization with a pathogen. Work using defined antigens has demonstrated that the disulfide cross-linking structure of IgM becomes increasingly more polymerized during an immune response, coinciding with an increase in affinity, but it is unknown if this has relevance to aquatic pathogens. Understanding how IgM varies following vaccination with an aquatic pathogen is of considerable importance as effector functions allocated to multiple antibody isotypes in mammals are essentially relegated to this single molecule. To gain insights into the dynamism of IgM, rainbow trout were immunized with Streptococcus iniae and individual serum titers, their specificity and affinity to S. iniae, and the disulfide cross-linking pattern of both total-serum and specific Ig were analyzed over a period of 37 weeks. We found that in vaccinated animals titer increased by a factor of ≈100 from starting levels, affinity increased 10-fold, and diversity of S. iniae proteins recognized by trout antibody increased at least 5-fold. Most intriguing, though less cross-linked IgM predominated early in response, by week 5, the fully tetramerized antibody comprised 50% of total specific protein. We propose that this is a mechanism to optimize efficacy of carrying out effector functions and recognizing a wide array of epitopes with higher affinity.

Extensive polymorphism in the porcine Toll-like receptor 10 gene

The great importance of the Toll-like receptors (TLRs) in innate immunity is well established, but one family member--TLR10--remains elusive. TLR10 is expressed in various tissues in several species, but its ligand is not known and its function is still poorly understood. The open reading frame of TLR10 was sequenced in 15 wild boars, representing three populations, and in 15 unrelated domestic pigs of Hampshire, Landrace and Large White origin. Amino acid positions corresponding to detected nonsynonymous single nucleotide polymorphisms (SNPs) were analysed in the crystal structures determined for the human TLR1-TLR2-lipopeptide complex and the human TLR10 Toll/Interleukin 1 receptor (TIR) dimer. SNP occurrence in wild boars and domestic pigs was compared, and haplotypes for the TLR10 gene and the TLR6-1-10 gene cluster were reconstructed. Despite the limited number of animals sequenced in the present study (N = 30), a larger number of SNPs were found in TLR10 than recently reported for TLR1, TLR6 and TLR2. Thirty-three SNPs were detected, of which 20 were nonsynonymous. The relative frequency of nonsynonymous (d(N) ) and synonymous (d(S) ) SNPs between wild boars and domestic pigs was higher in TLR10 than recently reported for TLR1, TLR6 and TLR2. However, the polymorphism reported in the present study seems to leave the function of the TLR10 molecule unaffected. Furthermore, no nonsynonymous SNPs were detected in the part of the gene corresponding to the hinge region of the receptor, probably reflecting rigorously acting functional constraint. The total number of SNPs and the number of nonsynonymous SNPs were significantly lower (P < 0.05) in the wild boars than in the domestic pigs, and fewer TLR10 haplotypes were present in the wild boars. The majority of the TLR6-1-10 haplotypes were specific for either wild boars or domestic pigs, probably reflecting differences in microbial environment and population history.

Structure and function relationships in IgA

Immunoglobulin A (IgA) has a critical role in immune defense particularly at the mucosal surfaces, and is equipped to do so by the unique structural attributes of its heavy chain and by its ability to polymerize. Here, we provide an overview of human IgA structure, describing the distinguishing features of the IgA1 and IgA2 subclasses and mapping the sites of interaction with host receptors important for IgA's functional repertoire. Remarkably, these same interaction sites are targeted by binding proteins and proteases produced by various pathogens as a means to subvert the protective IgA response. As interest in the prospect of therapeutic IgA-based monoclonal antibodies grows, the emerging understanding of the relationship between IgA structure and function will be invaluable for maximizing the potential of these novel reagents.

The human immunoglobulin A Fc receptor FcαRI: a multifaceted regulator of mucosal immunity

Immunoglobulin A (IgA) is commonly recognized as the most prevalent antibody (Ab) at mucosal sites with an important role in defense by shielding mucosal surfaces from invasion by pathogens. However, its potential to both actively dampen excessive immune responses or to initiate potent proinflammatory cellular processes is less well known. Interestingly, either functional outcome is mediated through interaction with the myeloid IgA Fc receptor FcαRI (CD89). Monomeric interaction of IgA with FcαRI triggers inhibitory signals that block activation via other receptors, whereas multimeric FcαRI crosslinking induces phagocytosis, reactive oxygen species production, antigen presentation, Ab-dependent cellular cytotoxicity, and cytokine release. Thus, FcαRI acts as a regulator between anti- and proinflammatory responses of IgA. As such, the biology of FcαRI, and its multifaceted role in immunity will be the focus of this review.

Preferentially immunoglobulin (IgG) subclasses production in primary Sjögren's syndrome patients

Background: Patients with primary Sjögren's syndrome (pSS) mostly have high levels of polyclonal immunoglobulin (IgG) and selective increase of IgG subclasses. As IgG subclasses profiles represent specific immunological processes and may be indicative of the underlying pathological pattern in pSS, it would be of great clinical importance to investigate the distribution of IgG subclasses in pSS patients. Methods: In archived sera from 155 patients with pSS, 50 patients with systemic lupus erythematosus (SLE) and 50 normal controls were analyzed by immunonephelometric assay. Results: The median levels of IgG1, IgG2, IgG3 and IgG4 in pSS patients were 11300 mg/L, 6130 mg/L, 961 mg/L and 142 mg/L, respectively. When compared with the normal controls, significant increases of IgG1 (p<0.001), IgG2 (p<0.001) and IgG3 (p<0.001) was observed in pSS patients, while the IgG4 level was significantly decreased (p=0.002). The ratio of IgG1/IgG2 in pSS patients was 2.3±1.4, which was significantly higher than that in the normal controls (p<0.001). When the pSS patients were divided into different groups, a higher level of IgG3 was observed in pSS with systematic disorders (p=0.008). In addition, the level of IgG1 was statistically higher in the anti-Ro or anti-La positive group than that in the negative group (p<0.001), but the IgG4 concentration was lower than that in the negative group (p=0.046). A significant positive correlation between the concentration of IgG3 and the disease duration (p<0.001) was also identified. Conclusions: The data supported the pathogenic role of IgG1 and IgG3 subclasses in the process of pSS. Further investigations to elucidate the reasons for the increase of IgG2 and whether IgG2 has any contribution to the pathogenesis of pSS are needed.

Cyclosporine A suppresses immunoglobulin G biosynthesis via inhibition of cyclophilin B in murine hybridomas and B cells

Immunoglubulin G (IgG) is a major isotype of antibody, which is predominantly involved in immune response. The complete tetramer is needed to fold and assemble in endoplasmic reticulum (ER) prior to secretion from cells. Protein quality control guided by ER chaperons is most essential for full biological activity. Cyclophilin B (CypB) was initially identified as a high-affinity binding protein for the immunosuppressive drug Cyclosporine A (CsA). CsA suppresses organ rejection by halting productions of pro-inflammatory molecules in T cell and abolishes the enzymatic property of CypB that accelerates the folding of proteins by catalysing the isomerization of peptidyl-proline bonds in ER. Here, we reported that CsA significantly inhibited IgG biosynthesis at posttranslational level in antibody secreting cells. Moreover, CsA stimulated the extracellular secretion of CypB and induced ROS generation, leading to expressions of ER stress markers. In addition, the absence of intracellular CypB impaired the formation of ER multiprotein complex, which is most important for resisting ER stress. Interestingly, CsA interrupted IgG folding via occupying the PPIase domain of CypB in ER. Eventually, unfolded IgG is degraded via Herp-dependent ERAD pathway. Furthermore, IgG biosynthesis was really abrogated by inhibition of CypB in primary B cells. We established for the first time the immunosuppressive effect of CsA on B cells. Conclusively, the combined results of the current study suggest that CypB is a pivotal molecule for IgG biosynthesis in ER quality control.

Polymeric human Fc-fusion proteins with modified effector functions

The success of Fc-fusion bio-therapeutics has spurred the development of other Fc-fusion products for treating and/or vaccinating against a range of diseases. We describe a method to modulate their function by converting them into well-defined stable polymers. This strategy resulted in cylindrical hexameric structures revealed by tapping mode atomic force microscopy (AFM). Polymeric Fc-fusions were significantly less immunogenic than their dimeric or monomeric counterparts, a result partly owing to their reduced ability to interact with critical Fc-receptors. However, in the absence of the fusion partner, polymeric IgG1-Fc molecules were capable of binding selectively to FcγRs, with significantly increased affinity owing to their increased valency, suggesting that these reagents may prove of immediate utility in the development of well-defined replacements for intravenous immunoglobulin (IVIG) therapy. Overall, these findings establish an effective IgG Fc-fusion based polymeric platform with which the therapeutic and vaccination applications of Fc-fusion immune-complexes can now be explored.

Structure of FcRY, an avian immunoglobulin receptor related to mammalian mannose receptors, and its complex with IgY

Fc receptors transport maternal antibodies across epithelial cell barriers to passively immunize newborns. FcRY, the functional counterpart of mammalian FcRn (a major histocompatibility complex homolog), transfers IgY across the avian yolk sac, and represents a new class of Fc receptor related to the mammalian mannose receptor family. FcRY and FcRn bind immunoglobulins at pH ≤6.5, but not pH ≥7, allowing receptor-ligand association inside intracellular vesicles and release at the pH of blood. We obtained structures of monomeric and dimeric FcRY and an FcRY-IgY complex and explored FcRY's pH-dependent binding mechanism using electron cryomicroscopy (cryoEM) and small-angle X-ray scattering. The cryoEM structure of FcRY at pH 6 revealed a compact double-ring "head," in which the N-terminal cysteine-rich and fibronectin II domains were folded back to contact C-type lectin-like domains 1-6, and a "tail" comprising C-type lectin-like domains 7-8. Conformational changes at pH 8 created a more elongated structure that cannot bind IgY. CryoEM reconstruction of FcRY dimers at pH 6 and small-angle X-ray scattering analysis at both pH values confirmed both structures. The cryoEM structure of the FcRY-IgY revealed symmetric binding of two FcRY heads to the dimeric FcY, each head contacting the C(H)4 domain of one FcY chain. FcRY shares structural properties with mannose receptor family members, including a head and tail domain organization, multimerization that may regulate ligand binding, and pH-dependent conformational changes. Our results facilitate understanding of immune recognition by the structurally related mannose receptor family and comparison of diverse methods of Ig transport across evolution.

The chicken leukocyte receptor cluster

Receptors of the immunoglobulin-like superfamily are critically involved in virtually every aspect of immune responses. One large chromosomal area encoding such immunoregulatory receptors is the leukocyte receptor cluster. Here we review various aspects of the chicken Ig-like receptor (CHIR) family, located on microchromosome 31, an orthologous position to the mammalian leukocyte receptor cluster. The CHIR family has been massively expanded with over hundred CHIR genes that are further distinguished into activating, inhibitory and bifunctional receptors. Comparisons of various features such as amino acid motifs, genomic structure, expression and associated adaptor molecules reveal the homology of CHIR to both the killer Ig-like and the leukocyte Ig-like receptor families, with most pronounced correlation of certain CHIR to the NK cell receptor KIR2DL4. To date the CHIR ligands remain largely obscure with the exception of CHIR-AB1 that binds to chicken IgY. Detailed analyses of CHIR-AB1, its crystal structure, the interaction to IgY and functional capabilities allow us to draw conclusions regarding Fc receptor phylogeny and function.

Immunoglobulin A: A next generation of therapeutic antibodies?

Although immunoglobulin (Ig) A is commonly recognized as the most prevalent antibody subclass at mucosal sites with an important role in mucosal defense, its potential as a therapeutic monoclonal antibody is less well known. However, IgA has multifaceted anti-, non-, and pro-inflammatory functions that can be exploited for different immunotherapeutical strategies, which will be the focus of this review.

Basis of the intrinsic flexibility of the Cε3 domain of IgE

Allergic reactions are triggered by the interaction between IgE and its high-affinity receptor, FcεRI. Various studies have mapped the interaction surface between IgE and its cellular receptors to the third constant domain of IgE (Cε3). The isolated Cε3 domain has been shown to exist as a molten globule, and the domain retains significant flexibility within the context of the IgE protein. Here we have analyzed the structural basis of the intrinsic flexibility of this domain. We have compared the sequence of the Cε3 domain to the sequences of other members of the C1 subset of the immunoglobulin superfamily and observed that Cε3 has an unusually high electrostatic charge and an unusually low content of hydrophobic residues. Mutations restoring Cε3 to a more canonical sequence were introduced in an attempt to derive a more structured domain, and several mutants display decreased levels of disorder. Engineered domains of Cε3 with a range of structural rigidities could serve as important tools for the elucidation of the role of flexibility of the Cε3 domain in IgE's biological functions.

The antibody paradigm: present and future development as a scaffold for biopharmaceutical drugs

Early studies of the humoral immune response revealed an apparent paradox: an infinite diversity of antibody specificities encoded within a finite genome. In consequence antibodies became a focus of interest for biochemists and geneticists. It resulted in the elucidation of the basic structural unit, the immunoglobulin (Ig) domain, comprised of ~ 100 amino acid residues that generate the characteristic "immunoglobulin (Ig) fold". The Ig fold has an anti-parallel ß-pleated sheet (barrel) structure that affords structural stability whilst the ß-bends allow for essentially infinite structural variation and functional diversity. This versatility is reflected in the Ig domain being the most widely utilised structural unit within the proteome. Human antibodies are comprised of multiple Ig domains and their structural diversity may be enhanced through the attachment of oligosaccharides. This review summarizes our current understanding of the immunoglobulin structure/function relationships and the application of protein and oligosaccharide engineering to further develop the Ig domain as a scaffold for the generation of new and novel antibody based therapeutics.

Recombinant proteins in rheumatology - recent advances

New targeted anti-inflammatory drugs have revolutionized the therapeutic strategies in rheumatology. Recombinant DNA selection technologies have enabled the isolation and humanization of specific antibody fragments of any specificity that can be 'armed' to deliver effective anti-inflammatory 'payloads'. Antibodies and other targeted provide the opportunity to block key soluble mediators of inflammation in their milieu, or alternatively to block intracellular inflammation-triggering pathways by binding to an upstream cell-surface receptor. Designed proteins can be improved with respect to desired pharmacokinetic and pharmacodynamic effects. They facilitate the delivery of the required immunosuppressive effect. However, the individual extent of desired and undesired effects of a particular biologic therapy can be broader than initially predicted and requires careful evaluation during clinical trials. This review highlights advances in the application of recombinant antibody technology for novel biologic therapies in rheumatology.

Immunological methods for nursing research: from cells to systems

Scientists and clinicians frequently use immunological methods (IMs) to investigate complex biological phenomena. Commonly used IMs include immunocytochemistry (IC), enzyme-linked immunosorbent assays (ELISA) and flow cytometry. Each of these methodologies exploits a common principle in IMs -the binding of an antibody to its antigen. Scientists continue to develop new methodologies, such as high-throughput immunohistochemistry (IHC) and in vivo imaging techniques, which exploit antibody-antigen binding, to more accurately answer complex research questions involving single cells up to whole organ systems. The purpose of this paper is to discuss established and evolving IMs and to illustrate the application of these methods to nursing research.

Anti-tumor necrosis factor-α antibodies induce regulatory macrophages in an Fc region-dependent manner

BACKGROUND & AIMS

Anti-tumor necrosis factor (TNF)α antibodies are effective in treating patients with Crohn's disease whereas soluble TNFα receptors have not shown clinical efficacy; the mechanism that underlies these different effects is not clear. We examined the immunosuppressive effects of different anti-TNFα reagents on activated T cells.

METHODS

We studied the effects of anti-TNFα antibodies infliximab and adalimumab, the soluble TNFα receptor etanercept, the pegylated F(ab') fragment certolizumab, and certolizumab-immunoglobulin (Ig)G on primary activated T cells. T cells were grown in isolation or in a mixed lymphocyte reaction (MLR). Proliferation was measured by (3)H thymidine incorporation and apoptosis was examined using Annexin V labeling and a colorimetric assay for activated caspase-3. Macrophage phenotypes were assayed by flow cytometry and cytokine secretion.

RESULTS

Infliximab and adalimumab reduced T-cell proliferation in an MLR whereas etanercept and certolizumab did not; this effect was lost after Fc receptors were blocked. The infliximab F(ab')2 fragment did not inhibit proliferation whereas certolizumab-IgG did inhibit proliferation. In the MLR, the antibodies against TNF induced formation of a new population of macrophages in an Fc region-dependent manner; these macrophages had an immunosuppressive phenotype because they inhibit proliferation of activated T cells, produce anti-inflammatory cytokines, and express the regulatory macrophage marker CD206.

CONCLUSIONS

Regulatory macrophages have immunosuppressive properties and an important role in wound healing. Antibodies against TNF induce regulatory macrophages in an Fc region-dependent manner. These functions of anti-TNFs might contribute to the resolution of inflammation.

Novel GLP-1 fusion chimera as potent long acting GLP-1 receptor agonist

GLP-1 has a variety of anti-diabetic effects. However, native GLP-1 is not suitable for therapy of diabetes due to its short half-life (t1/2<2 min). To circumvent this, we developed a long-lasting GLP-1 receptor agonist by the fusion of GLP-1 with human IgG2 Fc (GLP-1/hIgG2). ELISA-based receptor binding assay demonstrated that GLP-1/hIgG2 had high binding affinity to the GLP-1R in INS-1 cells (Kd = 13.90±1.52 nM). Upon binding, GLP-1/hIgG2 was rapidly internalized by INS-1 cells in a dynamin-dependent manner. Insulin RIA showed that GLP-1/IgG2 dose-dependently stimulated insulin secretion from INS-1 cells. Pharmacokinetic studies in CD1 mice showed that with intraperitoneal injection (i.p.), the GLP-1/hIgG2 peaked at 30 minutes in circulation and maintained a plateau for >168 h. Intraperitoneal glucose tolerance test (IPGTT) in mice showed that GLP-1/hIgG2 significantly decreased glucose excursion. Furthermore, IPGTT performed on mice one week after a single drug-injection also displayed significantly reduced glucose excursion, indicating that GLP-1/hIgG2 fusion protein has long-lasting effects on the modulation of glucose homeostasis. GLP-1/hIgG2 was found to be effective in reducing the incidence of diabetes in multiple-low-dose streptozotocin-induced type 1 diabetes in mice. Together, the long-lasting bioactive GLP-1/hIgG2 retains native GLP-1 activities and thus may serve as a potent GLP-1 receptor agonist.

Identification of a linear epitope for Fc-binding in the mouse FcγRIII

Fc receptors are transmembrane proteins, found on the surfaces of immune cells, that aid in the removal of foreign pathogens by binding to antibody-coated targets via the Fc regions of the antibodies. To identify sites on mouse FcgammaRIII (moFcgammaRIII) alpha-chain that bind to the Fc region, peptides derived from the proximal extracellular domain (EC2) of moFcgammaRIII alpha-chain corresponding to the homologous region of human FcgammaRIIIB alpha-chain were synthesized. Binding of mouse IgG to the different peptides was tested by Dot-blot assay. The effective peptide (119)SFFHNEKSVRYH(130) located in the putative C-C' loop of the EC2 domain was found to bind mouse IgG specifically with an affinity of approximately 5.58 x 10(-5) M and inhibit the binding of mouse IgG to the receptor. Such a functional peptide should be very useful for further understanding the IgG-FcgammaR interaction and development of FcR-targeting drugs.

Stability of IgG isotypes in serum

Drug development from early discovery to late stage commercialization is a long arduous process where a number of factors are taken into consideration when deciding on a particular immunoglobulin isotype for a therapeutic purpose. There are no general rules for which isotype is selected; however, prior experiences, effector function and the specific therapy targeted, as well as extensive testing early in development help in pairing the number of candidates. Over 20 monoclonal antibodies are FDA-approved, and most are IgG1 isotype, although a number of non-IgG1 molecules have been approved recently and the number in development is on the rise. Analytical techniques that examine the physicochemical properties of a molecule provide vital information on the stability and efficacy of candidate antibody therapeutics, but most of these studies are conducted using standard buffers and under well defined storage conditions. It has recently become apparent that analysis of antibody therapeutics recovered after circulation in blood show altered physicochemical characteristics, and in many instances therapeutic molecules recovered from serum show lower potency. This review examines some of these studies, with a focus on the physicochemical changes observed in the molecules. Technologies that can facilitate rapid screening of candidate antibody therapeutics directly from blood are highlighted. The facts indicate that antibody therapeutic development programs must incorporate understanding of the basic biology of the isotype and its stability in serum, which is the intended environment of the therapeutic.

Generation of a high-affinity Fcgamma receptor by Ig-domain swapping between human CD64A and CD16A

A recombinant soluble version of the human high-affinity receptor for IgG, rh-FcgammaRIA or CD64A, was expressed in mammalian cells and purified from their conditioned media. As assessed by circular dichroism, size exclusion chromatography and dynamic light scattering, incubation of rh-FcgammaRIA at 37 degrees C resulted in time-dependent formation of soluble aggregates caused by protein unfolding and loss of native structure. Aggregate formation was irreversible, temperature-dependent and was independent of rh-FcgammaRIA concentration. Aggregated rh-FcgammaRIA lost its ability to inhibit immune complex precipitation and failed to bind to IgG-Sepharose. Addition of human IgG1 to rh-FcgammaRIA prior to incubation at 37 degrees C blocked the formation of rh-FcgammaRIA aggregates. Production of soluble monomeric rh-FcgammaRIA was limited by aggregate formation during cell culture. Substitution of the membrane distal D1 Ig domain of FcgammaRIA with the D1 Ig domain of FcgammaRIIIA or CD16A resulted in a chimeric receptor, FcgammaR3A1A, with enhanced temperature stability. Relative to native rh-FcgammaRIA, FcgammaR3A1A exhibited less aggregation in Chinese hamster ovary cell-conditioned media or when purified receptor was incubated for up to 24 h at 37 degrees C. Both receptors bound to immobilized human IgG1 with high affinity and were equipotent at blockade of immune complex-mediated cytokine production from cultured mast cells. Equivalent dose-dependent reductions in edema and neutrophil infiltration in the cutaneous Arthus reaction in mice were noted for rh-FcgammaRIA and FcgammaR3A1A. These data demonstrate that the D1 Ig domains of FcgammaRIA and FcgammaRIIIA are functionally interchangeable and further suggest that the chimeric receptor FcgammaR3A1A is an effective inhibitor of type III hypersensitivity in mice.

Mutations in an avian IgY-Fc fragment reveal the locations of monocyte Fc receptor binding sites

The avian IgY antibody isotype shares a common ancestor with both mammalian IgG and IgE and so provides a means to study the evolution of their structural and functional specialisations. Although both IgG and IgE bind to their leukocyte Fc receptors with 1:1 stoichiometry, IgY binds to CHIR-AB1, a receptor expressed in avian monocytes, with 2:1 stoichiometry. The mutagenesis data reported here explain the structural basis for this difference, mapping the CHIR-AB1 binding site to the Cupsilon3/Cupsilon4 interface and not the N-terminal region of Cupsilon3 where, at equivalent locations, the IgG and IgE leukocyte Fc receptor binding sites lie. This finding, together with the phylogenetic relationship of the antibodies and their receptors, indicates that a substantial shift in the nature of Fc receptor binding occurred during the evolution of mammalian IgG and IgE.

FcγRs in health and disease

Genetic defects affecting the humoral immune response and especially the production of antibodies of the immunoglobulin G (IgG) isotype result in a heightened susceptibility to infections. Studies over the last years have demonstrated the crucial role of Fc-receptors for IgG (FcγRs) widely expressed on innate immune effector cells in mediating the protective function of IgG. During the last years, additional ligands interacting with FcγRs as well as additional receptors binding to IgG glycosylation variants have been identified. In this review, we discuss how the interaction of these different ligands with classical and novel Fcγ-receptors influences the immune response and which strategies microorganisms have developed to prevent them.

Biodistribution mechanisms of therapeutic monoclonal antibodies in health and disease

The monoclonal antibody market continues to witness an impressive rate of growth and has become the leading source of expansion in the biologic segment within the pharmaceutical industry. Currently marketed monoclonal antibodies target a diverse array of antigens. These antigens are distributed in a variety of tissues such as tumors, lungs, synovial fluid, psoriatic plaques, and lymph nodes. As the concentration of drug at the proximity of the biological receptor determines the magnitude of the observed pharmacological responses, a significant consideration in effective therapeutic application of monoclonal antibodies is a thorough understanding of the processes that regulate antibody biodistribution. Monoclonal antibody distribution is affected by factors such as molecular weight, blood flow, tissue and tumor heterogeneity, structure and porosity, target antigen density, turnover rate, and the target antigen expression profile.

Chicken IgY binds its receptor at the CH3/CH4 interface similarly as the human IgA: Fc alpha RI interaction

Chicken IgY, the ancestral form of mammalian IgE and IgG, is recognized by the high-affinity FcY receptor CHIR-AB1, a member of the leukocyte receptor family. In this study, we have characterized the receptor ligand interaction site by consecutive truncations of the Fcv IgY domains and mutational analyses of selected residues. Using several fusion proteins that linked the human Cgamma2 and Cgamma3 domains with the Fcv IgY domains, a binding assay revealed that both the Fcv3 and Fcv4 domains were essential for the IgY CHIR-AB1 interaction. Sequence comparisons of chicken IgY with human IgA demonstrated that 11 of the 19 contact residues important for the IgA FcalphaRI interaction have been conserved in chicken IgY, although the overall amino acid identity is only 34%. Among the 19 amino acids at respective positions in IgY, the mutation of two residues in the Fcv3 and two in the Fcv4 domain completely abolished the IgY to CHIR-AB1 binding revealed by two independent assays. Three further mutations substantially altered the interaction. Molecular modeling on the Cv3 to Cv4 crystal structure revealed that all critical residues, although on two domains, are in close proximity. The importance of N-linked carbohydrates was demonstrated by the failure of the CHIR-AB1 interaction after mutation of the glycosylation site. The identification of the IgY Cv3/Cv4 interdomain region as critical for binding to CHIR-AB1 significantly enhances our understanding of the IgY receptor interaction and allows further conclusions regarding the FcR phylogeny.

Aglycosylated antibodies and the methods of making and using them: WO2008030564

More than 20 recombinant IgG antibody therapeutics are now licensed for the treatment of a variety of diseases and there are, literally, hundreds under development. Human IgG is a glycoprotein and the presence of oligosaccharides, attached at a single site, can decisively influence the mode of action of recombinant antibody therapeutics (rMAbs) and efficacy can vary depending on the particular oligosaccharide attached. This represents a considerable challenge to the biopharmaceutical industry; however, production vehicles are becoming available that allow for the manufacture of rMAbs bearing preselected oligosaccharides. The patent under review offers a radical alternative approach through the application of protein engineering to generate aglycosylated IgG molecules with restored and/or enhanced effector activities. Removal of the need for glycosylation has the potential to widen, and simplify, the production vehicles that could be used. It remains to be demonstrated that such radical structural changes do not have a negative impact on the stability and immunogenicity of these constructs.

A monomeric chicken IgY receptor binds IgY with 2:1 stoichiometry

IgY is the principal serum antibody in birds and reptiles, and an IgY-like molecule was the evolutionary precursor of both mammalian IgG and IgE. A receptor for IgY on chicken monocytes, chicken leukocyte receptor AB1 (CHIR-AB1), lies in the avian leukocyte receptor cluster rather than the classical Fc receptor cluster where the genes for mammalian IgE and IgG receptors are found. IgG and IgE receptors bind to the lower hinge region of their respective antibodies with 1:1 stoichiometry, whereas the myeloid receptor for IgA, FcalphaRI, and the IgG homeostasis receptor, FcRn, which are found in the mammalian leukocyte receptor cluster, bind with 2:1 stoichiometry between the heavy chain constant domains 2 and 3 of each heavy chain. In this paper, the extracellular domain of CHIR-AB1 was expressed in a soluble form and shown to be a monomer that binds to IgY-Fc with 2:1 stoichiometry. The two binding sites have similar affinities: K(a)(1) = 7.22 +/- 0.22 x 10(5) m(-1) and K(a)(2) = 3.63 +/- 1.03 x 10(6) m(-1) (comparable with the values reported for IgA binding to its receptor). The affinity constants for IgY and IgY-Fc binding to immobilized CHIR-AB1 are 9.07 +/- 0.07 x 10(7) and 6.11 +/- 0.02 x 10(8) m(-1), respectively, in agreement with values obtained for IgY binding to chicken monocyte cells and comparable with reported values for human IgA binding to neutrophils. Although the binding site for CHIR-AB1 on IgY is not known, the data reported here with a monomeric receptor binding to IgY at two sites with low affinity suggest an IgA-like interaction.

Human immunoglobulin allotypes: possible implications for immunogenicity

More than twenty recombinant monoclonal antibodies are approved as therapeutics. Almost all of these are based on the whole IgG isotype format, but vary in the origin of the variable regions between mouse (chimeric), humanized mouse and fully human sequences; all of those with whole IgG format employ human constant region sequences. Currently, the opposing merits of the four IgG subclasses are considered with respect to the in vivo biological activities considered to be appropriate to the disease indication being treated. Human heavy chain genes also exhibit extensive structural polymorphism(s) and, being closely linked, are inherited as a haplotype. Polymorphisms (allotypes) within the IgG isotype were originally discovered and described using serological reagents derived from humans; demonstrating that allotypic variants can be immunogenic and provoke antibody responses as a result of allo-immunization. The serologically defined allotypes differ widely within and between population groups; therefore, a mAb of a given allotype will, inevitably, be delivered to a cohort of patients homozygous for the alternative allotype. This publication reviews the serologically defined human IgG allotypes and considers the potential for allotype differences to contribute to or potentiate immunogenicity.

Utilization of immunoglobulin G Fc receptors by human immunodeficiency virus type 1: a specific role for antibodies against the membrane-proximal external region of gp41

Receptors (FcgammaRs) for the constant region of immunoglobulin G (IgG) are an important link between humoral immunity and cellular immunity. To help define the role of FcgammaRs in determining the fate of human immunodeficiency virus type 1 (HIV-1) immune complexes, cDNAs for the four major human Fcgamma receptors (FcgammaRI, FcgammaRIIa, FcgammaRIIb, and FcgammaRIIIa) were stably expressed by lentiviral transduction in a cell line (TZM-bl) commonly used for standardized assessments of HIV-1 neutralization. Individual cell lines, each expressing a different FcgammaR, bound human IgG, as evidence that the physical properties of the receptors were preserved. In assays with a HIV-1 multisubtype panel, the neutralizing activities of two monoclonal antibodies (2F5 and 4E10) that target the membrane-proximal external region (MPER) of gp41 were potentiated by FcgammaRI and, to a lesser extent, by FcgammaRIIb. Moreover, the neutralizing activity of an HIV-1-positive plasma sample known to contain gp41 MPER-specific antibodies was potentiated by FcgammaRI. The neutralizing activities of monoclonal antibodies b12 and 2G12 and other HIV-1-positive plasma samples were rarely affected by any of the four FcgammaRs. Effects with gp41 MPER-specific antibodies were moderately stronger for IgG1 than for IgG3 and were ineffective for Fab. We conclude that FcgammaRI and FcgammaRIIb facilitate antibody-mediated neutralization of HIV-1 by a mechanism that is dependent on the Fc region, IgG subclass, and epitope specificity of antibody. The FcgammaR effects seen here suggests that the MPER of gp41 could have greater value for vaccines than previously recognized.

Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses

Distinct genes encode 6 human receptors for IgG (hFcγRs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFcγRs for the 4 human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to FcγRI; FcγRIIA, IIB, and IIC; FcγRIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFcγRs; (2) IgG2 bind not only to FcγRIIAH131, but also, with a lower affinity, to FcγRIIAR131 and FcγRIIIAV158; (3) IgG4 bind to FcγRI, FcγRIIA, IIB and IIC and FcγRIIIAV158; and (4) the inhibitory receptor FcγRIIB has a lower affinity for IgG1, IgG2, and IgG3 than all other hFcγRs. We also identified parameters that determine the specificity and affinity of hFcγRs for IgG subclasses. These results document how hFcγR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific hFcγRs in the therapeutic and pathogenic effects of antibodies in disease.

Single nucleotide changes in the human Igamma1 and Igamma4 promoters underlie different transcriptional responses to CD40

Analysis of subclass-specific germline transcription in activated peripheral B cells revealed a highly biased expression pattern of the four Igamma transcripts to signals through CD40 and IL-4. This difference was most pronounced when comparing the profile of Igamma1 and Igamma4 transcripts and was not expected given the very high degree of sequence conservation between promoters. In this report, the influence of sequence differences on the regulation of the Igamma1 and Igamma4 promoters has been investigated given the highly muted transcriptional activity of the Igamma4 promoter. Two regions were analyzed where single nucleotide differences corresponded to major changes in transcriptional activity. These regions were the previously defined CD40 response region containing three putative NF-kappaB-binding sites and the downstream 36-bp region containing CREB/activating transcription factor and kappaB6 sites. Mutation of a single nucleotide at position 6 within the Igamma4 kappaB6 site increased promoter activity to approximately 50% of the activity of the Igamma1 promoter. Furthermore, elevated promoter strength corresponded with increased binding of p50, p65, c-Rel, RelB, and p300 proteins to a level comparable with that of Igamma1. Minor nucleotide changes to both the Igamma4 CD40 response region and the 36-bp element resulted in a response undistinguishable from an Igamma1 response, suggesting cooperation between the two regulatory regions for optimal transcriptional activity. Collectively, these mutational analyses suggest that minor sequence differences contribute to the composition and affinity of transcriptional protein complexes regulating subclass-specific germline transcription, which in part impacts the overall level of class switch recombination to targeted C(H) regions.