Interactions of Immunoglobulins Outside the Antigen-Combining Site

Evidence for the Role of a Second Fc-Binding Receptor in Placental IgG Transfer in Nonhuman Primates

Transplacental transfer of maternal antibodies provides the fetus and newborn with passive protection against infectious diseases. While the role of the highly conserved neonatal Fc receptor (FcRn) in transfer of IgG in mammals is undisputed, recent reports have suggested that a second receptor may contribute to transport in humans. We report poor transfer efficiency of plant-expressed recombinant HIV-specific antibodies, including engineered variants with high FcRn affinity, following subcutaneous infusion into rhesus macaques close to parturition. Unexpectedly, unlike those derived from mammalian tissue culture, plant-derived antibodies were essentially unable to cross macaque placentas. This defect was associated with poor Fcγ receptor binding and altered Fc glycans and was not recapitulated in mice. These results suggest that maternal-fetal transfer of IgG across the three-layer primate placenta may require a second receptor and suggest a means of providing maternal antibody treatments during pregnancy while avoiding fetal harm. IMPORTANCE This study compared the ability of several human HIV envelope-directed monoclonal antibodies produced in plants with the same antibodies produced in mammalian cells for their ability to cross monkey and mouse placentas. We found that the two types of antibodies have comparable transfer efficiencies in mice, but they are differentially transferred across macaque placentas, consistent with a two-receptor IgG transport model in primates. Importantly, plant-produced monoclonal antibodies have excellent binding characteristics for human FcRn receptors, permitting desirable pharmacokinetics in humans. The lack of efficient transfer across the primate placenta suggests that therapeutic plant-based antibody treatments against autoimmune diseases and cancer could be provided to the mother while avoiding transfer and preventing harm to the fetus.

Enhanced Immunomodulatory Effect of Intravenous Immunoglobulin by Fc Galactosylation and Nonfucosylation

Intravenous immunoglobulin (IVIG) is used as an immunomodulatory agent in the treatment of various autoimmune/inflammatory diseases although its mechanism of action remains elusive. Recently, nonfucosylated IgG has been shown to be preferentially bound to Fcγ receptor IIIa (FcγRIIIa) on circulating natural killer cells; therefore, we hypothesized that nonfucosylated IVIG may modulate immune responses through FcγRIIIa blockade. Here, homogeneous fucosylated or nonfucosylated glycoforms of normal polyclonal IgG bearing sialylated, galactosylated or nongalactosylated Fc oligosaccharides were generated by chemoenzymatic glycoengineering to investigate whether the IgG glycoforms can inhibit antibody-dependent cellular cytotoxicity (ADCC). Among the six IgG glycoforms, galactosylated, nonfucosylated IgG [(G2)₂] had the highest affinity to FcγRIIIa and 20 times higher potency to inhibit ADCC than native IgG. A pilot study of IVIG treatment in mice with collagen antibody-induced arthritis highlighted the low-dose (G2)₂ glycoform of IVIG (0.1 g/kg) as an effective immunomodulatory agent as the 10-fold higher dose of native IVIG. These preliminary results suggest that the anti-inflammatory activity of IVIG is in part mediated via activating FcγR blockade by galactosylated, nonfucosylated IgG and that such nonfucosylated IgG glycoforms bound to FcγRs on immune cells play immunomodulatory roles in health and disease. This study provides insights into improved therapeutic strategies for autoimmune/inflammatory diseases using glycoengineered IVIG and recombinant Fc.

Immunoglobulin G Subclass-Specific Glycosylation Changes in Primary Epithelial Ovarian Cancer

Epithelial ovarian cancer (EOC) was previously shown to be associated with glycosylation changes of total serum and total IgG proteins. However, as a majority of previous studies analyzed released glycan profiles, still little is known about IgG subclass-specific alterations in ovarian cancer. Hence, in this study, we investigated EOC-related glycosylation changes of the three most abundant IgG subclasses, namely, IgG₁, IgG₂ and IgG₃ isolated from sera of 87 EOC patients and 74 age-matched healthy controls. In order to separate IgG₂ and IgG₃, we performed a two-step affinity purification employing Protein A and Protein G Sepharose. After tryptic digestion, IgG glycopeptides were enriched and measured by MALDI-TOF-MS. Finally, EOC-related glycosylation changes were monitored at the level of total agalactosylation, monogalactosylation, digalactosylation, sialylation, bisection and fucosylation, which were calculated separately for each IgG subclass. Interestingly, aside from an EOC-related increase in agalactosylation/decrease in monogalactosylation and digalactosylation observed in all IgG subclasses, some subclass-specific trends were detected. Glycosylation of IgG₁ was found to be most strongly affected in EOC, as it exhibited the highest number of significant differences between healthy controls and EOC patients. Specifically, IgG₁ was the only subclass that showed a significant decrease in sialylation and a significant increase in fucosylation in EOC patients. Interestingly, IgG₂ and IgG₃ that were often investigated collectively in previous studies, were found to have distinct glycosylation patterns. IgG₃ displayed stronger EOC-related increase in agalactosylation/decrease in digalactosylation and was characterized by notably higher sialylation, which consequently decreased in EOC patients. In conclusion, our study indicates that IgG subclasses exhibit subtly distinct glycosylation patterns of EOC-related alterations and that IgG₁ and IgG₃ agalactosylation show the strongest association with CA125, the routine diagnostic marker. Additionally, our results show that simultaneous analyses of IgG₂ and IgG₃ might lead to wrong conclusions as these two subclasses exhibit noticeably different glycosylation phenotypes.

Glycoprotein biomarkers and analysis in chronic obstructive pulmonary disease and lung cancer with special focus on serum immunoglobulin G

Chronic obstructive pulmonary disease (COPD) and lung cancer are two major diseases of the lung with high rate of mortality, mostly among tobacco smokers. The glycosylation patterns of various plasma proteins show significant changes in COPD and subsequent hypoxia, inflammation and lung cancer, providing promising opportunities for screening aberrant glycan structures contribute to early detection of both diseases. Glycoproteins associated with COPD and lung cancer consist of highly sialylated N-glycans, which play an important role in inflammation whereby hypoxia leads to accumulation of sialyl Lewis A and X glycans. Although COPD is an inflammatory disease, it is an independent risk factor for lung cancer. Marked decrease in galactosylation of plasma immunoglobulin G (IgG) together with increased presence of sialic acids and more complex highly branched N-glycan structures are characteristic for COPD and lung cancer. Numerous glycan biomarkers have been discovered, and analysis of glycovariants associated with COPD and lung cancer has been carried out. In this paper we review fundamental glycosylation changes in COPD and lung cancer glycoproteins, focusing on IgG to provide an opportunity to distinguish between the two diseases at the glycoprotein level with diagnostic value.

Engineering the interactions between a plant-produced HIV antibody and human Fc receptors

Plants can provide a cost-effective and scalable technology for production of therapeutic monoclonal antibodies, with the potential for precise engineering of glycosylation. Glycan structures in the antibody Fc region influence binding properties to Fc receptors, which opens opportunities for modulation of antibody effector functions. To test the impact of glycosylation in detail, on binding to human Fc receptors, different glycovariants of VRC01, a broadly neutralizing HIV monoclonal antibody, were generated in Nicotiana benthamiana and characterized. These include glycovariants lacking plant characteristic α1,3-fucose and β1,2-xylose residues and glycans extended with terminal β1,4-galactose. Surface plasmon resonance-based assays were established for kinetic/affinity evaluation of antibody-FcγR interactions, and revealed that antibodies with typical plant glycosylation have a limited capacity to engage FcγRI, FcγRIIa, FcγRIIb and FcγRIIIa; however, the binding characteristics can be restored and even improved with targeted glycoengineering. All plant-made glycovariants had a slightly reduced affinity to the neonatal Fc receptor (FcRn) compared with HEK cell-derived antibody. However, this was independent of plant glycosylation, but related to the oxidation status of two methionine residues in the Fc region. This points towards a need for process optimization to control oxidation levels and improve the quality of plant-produced antibodies.

Single-step Protein A and Protein G avidity purification methods to support bispecific antibody discovery and development

Heavy chain (Hc) heterodimers represent a majority of bispecific antibodies (bsAbs) under clinical development. Although recent technologies achieve high levels of Hc heterodimerization (HD), traces of homodimer contaminants are often present, and as a consequence robust purification techniques for generating highly pure heterodimers in a single step are needed. Here, we describe two different purification methods that exploit differences in Protein A (PA) or Protein G (PG) avidity between homo- and heterodimers. Differential elution between species was enabled by removing PA or PG binding in one of the Hcs of the bsAb. The PA method allowed the avidity purification of heterodimers based on the VH3 subclass, which naturally binds PA and interferes with separation, by using a combination of IgG3 Fc and a single amino acid change in VH3, N82aS. The PG method relied on a combination of three mutations that completely disrupts PG binding, M428G/N434A in IgG1 Fc and K213V in IgG1 CH1. Both methods achieved a high level of heterodimer purity as single-step techniques without Hc HD (93–98%). Since PA and PG have overlapping binding sites with the neonatal Fc receptor (FcRn), we investigated the effects of our engineering both in vitro and in vivo. Mild to moderate differences in FcRn binding and Fc thermal stability were observed, but these did not significantly change the serum half-lives of engineered control antibodies and heterodimers. The methods are conceptually compatible with various Hc HD platforms such as BEAT® (Bispecific Engagement by Antibodies based on the T cell receptor), in which the PA method has already been successfully implemented.

Glycosylation-dependent antitumor therapeutic monoclonal antibodies

The therapeutic market for monoclonal antibodies (MAbs) has grown exponentially since 2000. It is expected that the world-wide market for MAbs could reach $125 billion in 2020. For cancer treatment alone, more than 30 MAbs have been approved by the US Food and Drug Administration since 1997. Unlike structure-defined small molecule-based anti-cancer drugs, the expensive MAb is a mixture of heterogeneously glycosylated proteins. All MAbs typically have a single N-glycosylation site on each of the Fc region. The clinical efficacy of the MAbs depends on the N-glycan structures. Loss of N-glycosylation on the MAbs leads to the loss of the ability to activate complement, to bind to Fc receptors, and to induce antibody-dependent cellular cytotoxicity (ADCC). Moreover, antigen-antibody complexes produced from N-glycan-deficient MAbs are failed to be eliminated rapidly from the blood circulation. Even in certain cases, the N-glycan heterogeneity does not significantly influence pharmacokinetics or half-life of MAbs, reduced terminal galactosylation decreases complement-dependent cytotoxicity, the absence of core fucosylation enhances ADCC due to the increased affinities for the FcγRIIIа receptor, and high sialylation levels reduce ADCC activity and impact inflammatory responses. Furthermore, only mammalian cell lines that make human-like N-glycan structures can be used for MAbs production since certain mammalian cell lines can produce non-human glycan epitopes such as galactose-α-1,3-galactose and N-glycolylneuraminic acid (NGNA), which can trigger unwanted immune response. Therefore, mastering the knowledge of N-glycan structures and glycobiology is the key to produce and provide patients with reliable MAbs with consistent glycosylation profile and expected clinical efficacy.

IgG3 regulates tissue-like memory B cells in HIV-infected individuals

Immunoglobulin G3 (IgG3) has an uncertain role in the response to infection with and vaccination against human immunodeficiency virus (HIV). Here we describe a regulatory role for IgG3 in dampening the immune system-activating effects of chronic HIV viremia on B cells. Secreted IgG3 was bound to IgM-expressing B cells in vivo in HIV-infected chronically viremic individuals but not in early-viremic or aviremic individuals. Tissue-like memory (TLM) B cells, a population expanded by persistent HIV viremia, bound large amounts of IgG3. IgG3 induced clustering of B cell antigen receptors (BCRs) on the IgM⁺ B cells, which was mediated by direct interactions between soluble IgG3 and membrane IgM of the BCR (IgM-BCR). The inhibitory IgG receptor CD32b (FcγRIIb), complement component C1q and inflammatory biomarker CRP contributed to the binding of secreted IgG3 onto IgM-expressing B cells of HIV-infected individuals. Notably, IgG3-bound TLM B cells were refractory to IgM-BCR stimulation, thus demonstrating that IgG3 can regulate B cells during chronic activation of the immune system.

Global conformational changes in IgG-Fc upon mutation of the FcRn-binding site are not associated with altered antibody-dependent effector functions

Antibody engineering is important for many diagnostic and clinical applications of monoclonal antibodies. We recently reported a series of fragment crystallizable (Fc) mutations targeting the neonatal Fc receptor (FcRn) site on a Lewis Y (Le^y) binding IgG1, hu3S193. The hu3S193 variants displayed shortened in vivo half-lives and may have potential for radioimaging or radiotherapy of Le^y-positive tumors. Here, we report Fc crystal structures of wild-type hu3S193, seven FcRn-binding site variants, and a variant lacking C1q binding or complement-dependent cytotoxicity (CDC) activity. The Fc conformation of the FcRn-binding sites was similar for wild-type and all mutants of hu3S193 Fc, which suggests that FcRn interactions were directly affected by the amino acid substitutions. The C1q-binding site mutant Fc was nearly identical with the wild-type Fc. Surprisingly, several hu3S193 Fc variants showed large changes in global structure compared with wild-type Fc. All hu3S193 Fc mutants had similar antibody-dependent cellular cytotoxicity, despite some with conformations expected to diminish Fc gamma receptor binding. Several hu3S193 variants displayed altered CDC, but there was no correlation with the different Fc conformations. All versions of hu3S193, except the C1q-binding site mutant, bound C1q, suggesting that the altered CDC of some variants could result from different propensities to form IgG hexamers after engaging Le^y on target cells. Overall, our findings support the concept that the antibody Fc is both flexible and mobile in solution. Structure-based design approaches should take into account the conformational plasticity of the Fc when engineering antibodies with optimal effector properties.

The characterization of 1-(4-bromophenyl)-5-phenyl-1H-1,2,3-triazole on acute toxicity, antimicrobial activities, photophysical property, and binding to two globular proteins

1-(4-Bromophenyl)-5-phenyl-1H-1,2,3-triazole (BPT) was a newly synthesized compound. The acute toxicities of BPT to mice by intragastric administration have been determined and the result indicates that the intragastric administration of BPT did not produce any significant toxic effect on Kunming strain mice. It is also evaluated for the antimicrobial activity of BPT against three kinds of plant mycoplasma, Fusarium Wilt (race 4), Colletotrichum gloeosporioides Penz. and Xanthomonas oryzae by different method in vitro. The compound exhibited distinct inhibitory activities against Fusarium Wilt (race 4) and Colletotrichum gloeosporioides Penz. by mycelium growth rate test and the values of EC₅₀ were 29.34 and 12.53μg/mL respectively. And BPT had also the most potent inhibitory activities against Xanthomonas oryzae when compared with that of control drugs by the agar well diffusion method. In addition, the structural and photophysical properties of BPT including ionization energy, electron affinities, and theoretical spectrum was studied by quantum-chemical methods. Then the interaction of BPT with two kinds of globular proteins, human immunoglobulin (HIg) and bovine hemoglobin (BHg) was investigated by using UV-vis absorption spectra, synchronous fluorescence, 3D fluorescence spectra, and fluorescence titration in combination with molecular modeling. UV-vis absorption, 3D and synchronous fluorescence measurements show that BPT has influence on the microenvironment surrounding HIg or BHg in aqueous solution and the fluorescence experiments show that BPT quenches the fluorescence intensity of HIg or BHg through a static mechanism. The binding parameters including the binding constants, the number of binding site and average binding distance between BPT and HIg or BHg at different temperatures were calculated. The thermodynamic parameters suggest that the hydrophobic interaction is the predominant intermolecular forces in stabilizing the BPT-HIg or BPT-BHg complex. Molecular docking was performed to reveal that the BPT moiety binds to the hydrophobic cavity of HIg or BHg and they are in good agreement with the spectroscopic measurements.

WITHDRAWN: Diversity of immunoglobulin G functions

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Antibody Aggregation: Insights from Sequence and Structure

Monoclonal antibodies (mAbs) are the fastest-growing biological therapeutics with important applications ranging from cancers, autoimmunity diseases and metabolic disorders to emerging infectious diseases. Aggregation of mAbs continues to be a major problem in their developability. Antibody aggregation could be triggered by partial unfolding of its domains, leading to monomer-monomer association followed by nucleation and growth. Although the aggregation propensities of antibodies and antibody-based proteins can be affected by the external experimental conditions, they are strongly dependent on the intrinsic antibody properties as determined by their sequences and structures. In this review, we describe how the unfolding and aggregation susceptibilities of IgG could be related to their cognate sequences and structures. The impact of antibody domain structures on thermostability and aggregation propensities, and effective strategies to reduce aggregation are discussed. Finally, the aggregation of antibody-drug conjugates (ADCs) as related to their sequence/structure, linker payload, conjugation chemistry and drug-antibody ratio (DAR) is reviewed.

Prebiotics as functional food ingredients preventing diet-related diseases

This paper reviews the potential of prebiotic-containing foods in the prevention or postponement of certain diet-related diseases, such as cardiovascular diseases with hypercholesterolemia, osteoporosis, diabetes, gastrointestinal infections and gut inflammation.

Characterization of the binding of shikonin to human immunoglobulin using scanning electron microscope, molecular modeling and multi-spectroscopic methods

Shikonin, one of the active components isolated from the root of Arnebia euchroma (Royle) Johnst, have anti-tumor, anti-bacterial and anti-inflammatory activities and has been used clinically in phlebitis and vascular purpura. In the present work, the interaction of human immunoglobulin (HIg) with shikonin has been investigated by using scanning electron microscope (SEM), Fourier transform infrared (FT-IR) spectroscopy, fluorescence polarization, synchronous and 3D fluorescence spectroscopy in combination with molecular modeling techniques under physiological conditions with drug concentrations of 3.33-36.67 μM. The results of SEM exhibited visually the special effect on aggregation behavior of the complex formed between HIg and shikonin. The fluorescence polarization values indicated that shikonin molecules were found in a motionally unrestricted environment introduced by HIg. Molecular docking showed the shikonin moiety bound to the hydrophobic cavity of HIg, and there are four hydrogen-bonding interactions between shikonin and the residues of protein. The synchronous and 3D fluorescence spectra confirmed that shikonin could quench the intrinsic fluorescence of HIg and has an effect on the microenvironment around HIg in aqueous solution. The changes in the secondary structure of HIg were estimated by qualitative and quantitative FT-IR spectroscopic analysis. The binding constants and thermodynamic parameters for shikonin-HIg systems were obtained under different temperatures (300 K, 310 K and 320 K). The above results revealed the binding mechanism of shikonin and HIg at the ultrastructure and molecular level.

Potent anti-tumor response by targeting B cell maturation antigen (BCMA) in a mouse model of multiple myeloma

Multiple myeloma (MM) is an aggressive incurable plasma cell malignancy with a median life expectancy of less than seven years. Antibody-based therapies have demonstrated substantial clinical benefit for patients with hematological malignancies, particular in B cell Non-Hodgkin's lymphoma. The lack of immunotherapies specifically targeting MM cells led us to develop a human-mouse chimeric antibody directed against the B cell maturation antigen (BCMA), which is almost exclusively expressed on plasma cells and multiple myeloma cells. The high affinity antibody blocks the binding of the native ligands APRIL and BAFF to BCMA. This finding is rationalized by the high resolution crystal structure of the Fab fragment in complex with the extracellular domain of BCMA. Most importantly, the antibody effectively depletes MM cells in vitro and in vivo and substantially prolongs tumor-free survival under therapeutic conditions in a xenograft mouse model. A BCMA-antibody-based therapy is therefore a promising option for the effective treatment of multiple myeloma and autoimmune diseases.

Preclinical screening for acute toxicity of therapeutic monoclonal antibodies in a hu-SCID model

Monoclonal antibodies (mAbs) have been a spectacular clinical and commercial success in the treatment of cancer and autoimmune diseases. Many of these mAbs (for example, OKT3, Campath-1H, rituximab and infliximab) are against surface or secreted products of lymphocytes. However, mAbs can have a variety of adverse effects including fever, chills and nausea. This is probably a result of cytokine release, which is most seriously manifested as a ‘cytokine storm' as highlighted by the TGN1412 (anti-CD28) trial. Prediction of adverse effects of mAbs would be clinically advantageous and numerous in vitro assays attempting to predict adverse effects have been reported. Here, we report an in vivo humanized mouse model to detect adverse effects in response to OKT3, Campath-1H or the polyclonal Ab preparation anti-thymocyte globulin. We found that the administration of each of these Abs to humanized mice led to acute clinical symptoms such as piloerection, hypomotility and hypothermia, particularly when delivered via the intravenous route. A cytokine storm occurred in the humanized mice receiving OKT3. This model system is a potentially useful tool to predict adverse effects and select initial doses for first-in-human trials. We would advocate this in vivo model, in addition to current in vitro preclinical testing, as a more representative and robust means of assessing potential adverse effects of mAb before their human use.

Aptamers in immunological research

In recent years aptamers, synthetic DNA or RNA single-chain oligonucleotides, have been used in various immunological studies to bind specific ligands. Detailed data on the interactions of an RNA aptamer with a human Fc fragment were obtained by X-ray crystallography. The complex formation involves multiple weak interactions that resemble protein-protein interactions. Aptamers specific to cell surface receptors may serve as antagonists or agonists blocking or stimulating cell activities. As aptamers can modify T-cell reactions, they could be useful in the treatment of chronic diseases such as autoimmune and oncological pathologies. In chimeras constructed for the delivery of active substances to defined targets, aptamers specific to surface proteins may be used to transport constructs directed to targets such as tumor cells. Aptamers are also employed as highly specific reagents in immunological assays after being labeled with reporter groups such as fluorescent dyes or following immobilization on insoluble carriers such as membranes or microspheres.

IgG Fc N-glycosylation in Guillain-Barré syndrome treated with immunoglobulins

Intravenous immunoglobulin (IVIg) is the treatment of choice for Guillain-Barré syndrome (GBS), an immune-mediated peripheral neuropathy causing rapidly progressive limb weakness and respiratory failure. The working mechanism of IVIg in autoimmune diseases has not been elucidated, but previous studies indicate that some anti-inflammatory effects may be mediated by the N-glycosylation of the Fc-portion of IgG. GBS is a model disease to investigate these effects because GBS is an acute and monophasic disorder usually affecting healthy persons, which is treated with a standard course of IVIg, although the clinical response is highly variable. In the current study, the N-glycosylation of the Fc-portion of serum IgG was investigated in patients with GBS before and after treatment with IVIg in relation to clinical course and outcome. Glycoforms of serum IgG1 and IgG2 were determined separately by liquid chromatography mass spectrometry. These IgG subclasses were purified from the serum of 174 GBS patients before and in 150 patients 2 weeks after standard IVIg treatment regimen. Treatment-naive GBS patients compared with age- and sex-matched controls had lower levels of galactosylation of IgG1 and IgG2. IVIg preparations contained relatively high levels of galactosylated and sialylated IgG Fc glycoforms compared with serum IgG in patients. Treatment with IVIg resulted in an increase in serum of the Fc-galactosylation and -sialylation of both IgG1 and IgG2. The extent of normalization in serum IgG Fc glycosylation varied between patients. Multiple logistic regression analysis showed that patients with persistent low IgG galactosylation and sialylation despite IVIg treatment had the most severe forms of GBS and needed ventilator support more often. Kaplan-Meier analysis showed that these patients also needed more time to be able to walk again compared with patients with a normalized IgG Fc glycosylation profile. In conclusion, our results suggest that serum IgG Fc glycosylation in GBS is related to disease severity and clinical recovery after IVIg and may help to develop new measures to monitor the efficacy of treatment.

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 conserved host and pathogen recognition site on immunoglobulins: structural and functional aspects

A common site in the constant region (Fc) of immunoglobulins is recognized by host receptors and is a frequent target of proteins expressed by pathogens. This site is located at the junction of two constant domains in the antibody heavy chains and produces a large shallow cavity formed by loops of the CH2 and CH3 domains in IgG and IgA (CH3 and CH4 domains in IgM). Crystal structures have been determined for complexes of IgG-Fc and IgA-Fc with a structurally diverse set of host, pathogen and in vitro selected ligands. While pathogen proteins may directly block interactions with the immunoglobulins thereby evading host immunity, it is likely that the same pathogen molecules also interact with other host factors to carry out their primary biological function. Herein we review the structural and functional aspects of host and pathogen molecular recognition of the common site on the Fc of immunoglobulins. We also propose that some pathogen proteins may promote virulence by affecting the bridging between innate and adaptive immunity.

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.

Multiplexed analytical glycomics: rapid and confident IgG N-glycan structural elucidation

N-glycans attached to the C(H)2 domains of the Fc or the antigen binding regions of IgG play an important role in stabilizing and modulating antibody activity. Exhaustive elucidation of 32 IgG N-glycans using a combination of weak anion exchange enrichment and exoglycosidase array digestion with subsequent profiling exceeded 48 h. Pursuing increased throughput and associated structural annotation confidence, we compared the 1.7 μm hydrophilic interaction phase for UPLC with CE-LIF for the rapid and comprehensive characterization of N-glycans released from healthy human serum polyclonal IgG. Combination of the data individually generated using each technique demonstrated that complete structural annotation was possible within a total analysis time of 20 min due to the advantageous orthogonality of the separation mechanisms. The parallel use of both analytical techniques provides a powerful platform for rapid and comprehensive analysis of IgG N-glycosylation present on therapeutic antibodies or on antibodies of biomedical or pathological significance.

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.

Chimeric, divalent and tetravalent anti-CD19 monoclonal antibodies with potent in vitro and in vivo antitumor activity against human B-cell lymphoma and pre-B acute lymphoblastic leukemia cell lines

CD19 is an attractive therapeutic target for treating human B-cell tumors. In our study, chimeric (c) divalent (cHD37) and tetravalent (cHD37-DcVV) anti-CD19 monoclonal antibodies (MAbs) were constructed, expressed and evaluated for their binding to human 19-positive (CD19(+)) tumor cell lines. They were also tested for proapoptotic activity and the ability to mediate effector functions. The antitumor activity of these MAbs was further tested in mice xenografted with the CD19(+) Burkitt's lymphoma cell line, Daudi or the pre-B acute lymphoblastic leukemia (ALL) cell line, NALM-6. The cHD37 and cHD37-DcVV MAbs exhibited specific binding and comparable proapoptotic activity on CD19(+) tumor cell lines in vitro. In addition, the cHD37 and cHD37-DcVV MAbs were similar in their ability to mediate antibody-dependent cell-mediated phagocytosis (ADCP). However, the tetravalent cHD37-DcVV MAb bound more avidly, had a slower dissociation rate, and did not internalize as well. It also had enhanced antibody-dependent cellular cytotoxicity (ADCC) with human but not murine effector cells. The cHD37 and cHD37-DcVV MAbs exhibited comparable affinity for the human neonatal Fc receptor (FcRn) and similar pharmacokinetics (PKs) in mice. Moreover, all the HD37 constructs were similar in extending the survival of mice xenografted with Daudi or NALM-6 tumor cells. Therefore, the cHD37 and cHD37-DcVV MAbs have potent antitumor activity and should be further developed for use in humans. Although not evident in mice, due to its increased ability to mediate ADCC with human but not mouse effector cells, the cHD37-DcVV MAb should have superior therapeutic efficacy in humans.

Interactions between immunoglobulin G molecules

Immune globulin preparations such as intravenous immunoglobulin (IVIG) and monoclonal antibodies are widely used in clinics as effective therapeutic agents for the treatment of a number of autoimmune diseases, cancer, inflammations and other pathologies. Significant amounts of IgG aggregates have been found in the highly concentrated solutions of therapeutic immune proteins. The IgG self-aggregation that appears especially after prolonged storage increases the immunogenicity of the preparations and also modifies their physical properties, first of all producing the high viscosity. The attractive IgG-IgG interactions pose a significant problem for the clinical usage of the immune proteins. During last decades intensive studies of the IgG self-association were performed. The presence of IgG dimers was demonstrated in pooled preparations. These complexes are the result of idiotype-anti-idiotype interactions. In concentrated solutions of immune globulins and monoclonal antibodies self-associated IgG molecules formed a network, increasing the viscosity. The forces responsible for the IgG association are characteristic of the protein-protein interactions in general. The amino acid residues of the Fab and Fc portions participate in the IgG-IgG contacts. Recently contact residues were modified by the site-directed mutagenesis in order to decrease the formation of the IgG self-aggregates. The mutant IgG antibodies were characterized by enhanced stability as compared with the non-modified antibody molecules. Peptic pFc' fragment and the C(H)3 domain were shown to be capable of interacting with Fc regions, thus preventing IgG aggregation. In perspective both approaches could improve the formulation of immune globulin preparations. Removal of IgG aggregates could be achieved by chromatography on hydroxyapatite.

Binding site on human immunoglobulin G for the affinity ligand HWRGWV

Affinity ligand HWRGWV has demonstrated the ability to isolate human immunoglobulin G (hIgG) from mammalian cell culture media. The ligand specifically binds hIgG through its Fc portion. This work shows that deglycosylation of hIgG has no influence on its binding to the HWRGWV ligand and the ligand does not compete with Protein A or Protein G in binding hIgG. It is suggested by the mass spectrometry (MS) data and docking simulation that HWRGWV binds to the pFc portion of hIgG and interacts with the amino acids in the loop Ser383-Asn389 (SNGQPEN) located in the C(H)3 domain. Subsequent modeling has suggested a possible three-dimensional minimized solution structure for the interaction of hIgG and the HWRGWV ligand. The results support the fact that a peptide as small as a hexamer can have specific interactions with large proteins such as hIgG.

IgM, Fc mu Rs, and malarial immune evasion

IgM is an ancestral Ab class found in all jawed vertebrates, from sharks to mammals. This ancient ancestry is shared by malaria parasites (genus Plasmodium) that infect all classes of terrestrial vertebrates with whom they coevolved. IgM, the least studied and most enigmatic of the vertebrate Igs, was recently shown to form an intimate relationship with the malaria parasite Plasmodium falciparum. In this article, we discuss how this association might have come about, building on the recently determined structure of the human IgM pentamer, and how this interaction could affect parasite survival, particularly in light of the just-discovered Fc mu R localized to B and T cell surfaces. Because this parasite may exploit an interaction with IgM to limit immune detection, as well as to manipulate the immune response when detected, a better understanding of this association may prove critical for the development of improved vaccines or vaccination strategies.

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.

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.

Circulating non-immune IgG complexes in health and disease

IgG molecules possess a well-defined ability to form complexes with various proteins at interaction sites composed of residues of the constant domains. Such non-immune complexes (non-ICs) were recently identified in the circulatory system of healthy people, as well as patients suffering from various pathologies. By forming non-ICs, attached proteins that are harmful to the organism (anaphylatoxins, for example) are removed from the circulation. Non-immune IgG complexes can react simultaneously with two cell receptors-one specific for IgG, and another specific for an associated protein. Such double reactions augment cellular responses. The attachment of a protein to an IgG site may induce structural changes in neighboring areas of IgG molecules. The formation of non-ICs helps proteins with low molecular mass to escape glomerular filtration, as well as enzymatic degradation and cell uptake. Non-immune IgG complexes have been found in commercial immune globulin preparations used for the treatment of various diseases. Among the IgG-attached proteins, there are specific disease biomarkers used for clinical diagnostics and understanding disease processes. Therefore, in order to identify potential biomarkers, not only proteins that are free in the liquid phase of serum but also proteins associated with abundant proteins such as IgG must be investigated.

Glycosylation of antibody therapeutics: optimisation for purpose

Recombinant antibody therapeutics represent a significant success story in terms of clinical benefit delivered and revenue (profit) generated within the biopharmaceutical industry. Additionally, it is estimated that 30% of new drugs likely to be licensed during the next decade will be based on antibody products. High volume production with the maintenance of structural and functional fidelity of these large biological molecules results in high "cost of goods" that can limit their availability to patients, due to the strain it puts on national and private health budgets. The challenge in reducing cost of goods is that each antibody is unique, both in structure and function. Optimal clinical efficacy will require engineering of antibody genes to deliver products with enhanced activities produced by cell lines engineered to deliver antibody homogeneous for pre-selected post-translational modifications, that is, protein structures and glycoforms. A "universal" production vehicle cannot meet these demands and several production mammalian cells are now available, alternatives to mammalian cell lines are also reaching maturity. Advances in downstream processing also need to be realised whilst chemical changes during processing and storage must be minimised.

Porcine IgG: structure, genetics, and evolution

Eleven genomic porcine Cgamma gene sequences are described that represent six putative subclasses that appear to have originated by gene duplication and exon shuffle. The genes previously described as encoding porcine IgG1 and IgG3 were shown to be the IgG1(a) and IgG1(b) allelic variants of the IGHG1 gene, IgG2a and IgG2b are allelic variants of the IGHG2 gene, while "new" IgG3 is monomorphic, has an extended hinge, is structurally unique, and appears to encode the most evolutionarily conserved porcine IgG. IgG5(b) differs most from its putative allele, and its C(H)1 domain shares sequence homology with the C(H)1 of IgG3. Four animals were identified that lacked either IgG4 or IgG6. Alternative splice variants were also recovered, some lacking the C(H)1 domain and potentially encoding heavy chain only antibodies. Potentially, swine can transcribe >20 different Cgamma chains. A comparison of mammalian Cgamma gene sequences revealed that IgG diversified into subclasses after speciation. Thus, the effector functions for the IgG subclasses of each species should not be extrapolated from "same name subclasses" in other species. Sequence analysis identified motifs likely to interact with Fcgamma receptors, FcRn, protein A, protein G, and C1q. These revealed IgG3 to be most likely to activate complement and bind FcgammaRs. All except IgG5(a) and IgG6(a) should bind to FcgammaRs, while all except IgG6(a) and the putative IgG5 subclass proteins should bind well to porcine FcRn, protein A, and protein G.

Identification of residues in the Cmu4 domain of polymeric IgM essential for interaction with Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1)

The binding of nonspecific human IgM to the surface of infected erythrocytes is important in rosetting, a major virulence factor in the pathogenesis of severe malaria due to Plasmodium falciparum, and IgM binding has also been implicated in placental malaria. Herein we have identified the IgM-binding parasite ligand from a virulent P. falciparum strain as PfEMP1 (TM284var1 variant), and localized the region within this PfEMP1 variant that binds IgM (DBL4beta domain). We have used this parasite IgM-binding protein to investigate the interaction with human IgM. Interaction studies with domain-swapped Abs, IgM mutants, and anti-IgM mAbs showed that PfEMP1 binds to the Fc portion of the human IgM H chain and requires the IgM Cmu4 domain. Polymerization of IgM was shown to be crucial for the interaction because PfEMP1 binding did not occur with mutant monomeric IgM molecules. These results with PfEMP1 protein have physiological relevance because infected erythrocytes from strain TM284 and four other IgM-binding P. falciparum strains showed analogous results to those seen with the DBL4beta domain. Detailed investigation of the PfEMP1 binding site on IgM showed that some of the critical amino acids in the IgM Cmu4 domain are equivalent to those regions of IgG and IgA recognized by Fc-binding proteins from bacteria, suggesting that this region of Ig molecules may be of major functional significance in host-microbe interactions. We have therefore shown that PfEMP1 is an Fc-binding protein of malaria parasites specific for polymeric human IgM, and that it shows functional similarities with Fc-binding proteins from pathogenic bacteria.

IgG4 autoantibodies induce dermal-epidermal separation

Bullous pemphigoid (BP) is a sub-epidermal autoimmune blistering disease associated with autoantibodies to the dermal-epidermal junction (DEJ). Patients' autoantibodies induce dermal-epidermal separation when co-incubated with cryosections of human skin and leucocytes from healthy volunteers. IgG autoantibodies trigger complement and/or leucocyte activation resulting in specific pathology in several autoimmune conditions. In these diseases, IgG1 and IgG3 isotypes, but not the IgG4 subclass, are thought to trigger inflammatory pathways resulting in tissue damage. The capacity of IgG4 autoantibodies to mediate tissue damage has not yet been demonstrated. In this study, we isolated IgG1 and IgG4 autoantibodies from bullous pemhigoid patients'serum and analysed their blister-inducing potential in our cryosection assay. As expected, complement-fixing IgG1 autoantibodies induced sub-epidermal splits in this experimental model. Purified IgG4 did not fix complement, but, interestingly, like IgG1, activated leucocytes and induced dermal-epidermal separation. The potential of IgG4 autoantibodies to induce Fc-dependent dermal-epidermal separation was significantly lower compared to IgG1. Our results demonstrate that IgG4 autoantibodies are able to activate leucocytes and point to a hitherto less recognized function of IgG4. Moreover, for the first time, we clearly demonstrate that BP IgG4 autoantibodies have the capacity to induce leucocyte-dependent tissue damage.

Antibody therapeutics:

Recombinant monoclonal antibody (rMAb) therapy may be instituted to achieve one of two broad outcomes: i) killing of cells or organisms (e.g., cancer cells, bacteria); and ii) neutralisation of soluble molecules (e.g., cytokines in chronic disease or toxins in infection). The choice of rMAb isotype is a critical decision in the development of a therapeutic antibody as it will determine the biological activities triggered in vivo. It is not possible, however, to accurately predict the in vivo activity because multiple parameters impact on the functional outcome, for example, IgG subclass, IgG-Fc glycoform, epitope density, cellular Fc receptors polymorphisms and so on. The present understanding of the molecular interactions between IgG-Fc and effector ligands in vitro has allowed the generation of new antibody structures with altered/improved effector function profiles that may prove optimal for given disease indications. Thus, when maximal antibody-dependent cell-mediated cytotoxicity activity is indicated a non-fucosylated IgG1 format may be optimal; when minimal activity is indicated an aglycosylated IgG2 may be the form of choice.

Post-translational modifications in the context of therapeutic proteins

The majority of protein-based biopharmaceuticals approved or in clinical trials bear some form of post-translational modification (PTM), which can profoundly affect protein properties relevant to their therapeutic application. Whereas glycosylation represents the most common modification, additional PTMs, including carboxylation, hydroxylation, sulfation and amidation, are characteristic of some products. The relationship between structure and function is understood for many PTMs but remains incomplete for others, particularly in the case of complex PTMs, such as glycosylation. A better understanding of such structural-functional relationships will facilitate the development of second-generation products displaying a PTM profile engineered to optimize therapeutic usefulness.