Crystal structure at 2.8 A of an FcRn/heterodimeric Fc complex: mechanism of pH-dependent binding

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.

Engineered CH2 domains (nanoantibodies)

Currently, almost all FDA approved therapeutic antibodies (except ReoPro, Lucentis and Cimzia which are Fabs), and the vast majority of those in clinical trials are full-size antibodies mostly in IgG1 format of about 150 kDa size. A fundamental problem for such large molecules is their poor penetration into tissues (e.g., solid tumors) and poor or absent binding to regions on the surface of some molecules (e.g., on the HIV envelope glycoprotein) which are fully accessible only by molecules of smaller size. Therefore, much work especially during the last decade has been aimed at developing novel scaffolds of much smaller size and high stability. Here I briefly describe a proposition to use the immunoglobulin (Ig) constant CH2 domain (CH3 for IgE and IgM) as a scaffold. CH2 is critical for the Ig effector functions. Isolated CH2 is stable monomer in contrast to all other constant domains and most of the variable domains. CH2 and engineered CH2 domains with improved stability can be used as scaffolds for construction of libraries containing diverse binders to various antigens. Such binders based on a CH2 scaffold could also confer some effector functions. Because the CH2 domains are the smallest independently folded antibody domains that can be engineered to contain simultaneously antigen-binding sites and binding sites mediating effector and stability functions, and to distinguish them from domain antibodies which are used to denote engineered VH or VL domains or nanobodies which are used to denote camelid VHH, I termed them nanoantibodies (nAbs).

A therapeutic anti-VEGF antibody with increased potency independent of pharmacokinetic half-life

Bevacizumab [Avastin; anti-vascular endothelial growth factor (VEGF) antibody] is an antiangiogenic IgG approved for treating patients with certain types of colon, breast, and lung cancer. In these indications, bevacizumab is administered every 2 to 3 weeks, prompting us to study ways to reduce the frequency of administration. Increasing affinity to neonatal Fc receptor (FcRn) may extend the pharmacokinetic half-life of an antibody, but the quantitative effect of FcRn affinity on clearance has not been clearly elucidated. To gain further insight into this relationship, we engineered a series of anti-VEGF antibody variants with minimal amino acid substitutions and showed a range of half-life improvements in primates. These results suggest that, if proven clinically safe and effective, a modified version of bevacizumab could potentially provide clinical benefit to patients on long-term anti-VEGF therapy through less-frequent dosing and improved compliance with drug therapy. Moreover, despite having half-life similar to that of wild-type in mice due to the species-specific FcRn binding effects, the variant T307Q/N434A exhibited superior in vivo potency in slowing the growth of certain human tumor lines in mouse xenograft models. These results further suggest that FcRn variants may achieve increased potency through unidentified mechanisms in addition to increased systemic exposure.

An intracellular traffic jam: Fc receptor-mediated transport of immunoglobulin G

Recent advances in imaging techniques along with more powerful in vitro and in vivo models of receptor-mediated ligand transport are facilitating advances in our understanding of how cells efficiently direct receptors and their cargo to target destinations within the cytoplasm and at the plasma membrane. Specifically, light and 3D electron microscopy studies examining the trafficking behavior of the neonatal Fc receptor (FcRn), a transport receptor for immunoglobulin G (IgG), have given us new insights into the dynamic interplay between the structural components of the cytosolic trafficking machinery, its protein regulators, and the receptors it directs to various locations within the cell. These studies build upon previous biochemical characterizations of FcRn transport and are allowing us to begin formulation of a more complete model for the intracellular trafficking of receptor-ligand complexes.

Importance of neonatal FcR in regulating the serum half-life of therapeutic proteins containing the Fc domain of human IgG1: a comparative study of the affinity of monoclonal antibodies and Fc-fusion proteins to human neonatal FcR

The neonatal FcR (FcRn) binds to the Fc domain of IgG at acidic pH in the endosome and protects IgG from degradation, thereby contributing to the long serum half-life of IgG. To date, more than 20 mAb products and 5 Fc-fusion protein products have received marketing authorization approval in the United States, the European Union, or Japan. Many of these therapeutic proteins have the Fc domain of human IgG1; however, the serum half-lives differ in each protein. To elucidate the role of FcRn in the pharmacokinetics of Fc domain-containing therapeutic proteins, we evaluated the affinity of the clinically used human, humanized, chimeric, or mouse mAbs and Fc-fusion proteins to recombinant human FcRn by surface plasmon resonance analysis. The affinities of these therapeutic proteins to FcRn were found to be closely correlated with the serum half-lives reported from clinical studies, suggesting the important role of FcRn in regulating their serum half-lives. The relatively short serum half-life of Fc-fusion proteins was thought to arise from the low affinity to FcRn. The existence of some mAbs having high affinity to FcRn and a short serum half-life, however, suggested the involvement of other critical factor(s) in determining the serum half-life of such Abs. We further investigated the reason for the relatively low affinity of Fc-fusion proteins to FcRn and suggested the possibility that the receptor domain of Fc-fusion protein influences the structural environment of the FcRn binding region but not of the FcgammaRI binding region of the Fc domain.

[Neonatal Fc receptor, key control of immunoglobulins biodistribution]

In 1969, Brambell, while studying the long serum half-life of IgG and their ability to cross the materno-foetal barrier, attributed these two properties to the existence of a specific Fc receptor, which was later denominated FcRn for neonatal Fc receptor. The resolution of its structure revealed that it is a MHC class-I-like molecule. FcRn is able to load IgG and albumin in a pH-dependent manner. It acts as an intracellular transport protein and as such is controling the serum half-life of these proteins (apical recycling of IgG and albumin in endothelial cells), IgG biodistribution (apical to basolateral and basolateral to apical transport of IgG in epithelial and endothelial cells) and it may also contribute to phagocytosis. FcRn is thus a key partner in the pharmacokinetics of therapeutic antibodies, opening interesting prospects for optimisation of their use.

Aglycosylated IgG variants expressed in bacteria that selectively bind FcgammaRI potentiate tumor cell killing by monocyte-dendritic cells

The N-linked glycan of immunoglobulin G (IgG) is indispensable for the interaction of the Fc domain with Fcgamma receptors on effector cells and the clearance of target cells via antibody dependent cell-mediated cytotoxicity (ADCC). Escherichia coli expressed, aglycosylated Fc domains bind effector FcgammaRs poorly and cannot elicit ADCC. Using a novel bacterial display/flow cytometric library screening system we isolated Fc variants that bind to FcgammaRI (CD64) with nanomolar affinity. Binding was critically dependent on amino acid substitutions (E382V, and to a lesser extent, M428I) distal to the putative FcgammaRI binding epitope within the CH3 domain. These mutations did not adversely affect its pH-dependent interaction with FcRn in vitro nor its serum persistence in vivo. Remarkably, the anti-Her2 IgG trastuzumab containing the E382V, M428I substitutions and expressed in E. coli exhibited highly selective binding to FcgammaRI but not to the other activating receptors (FcgammaRIIa, FcgammaRIIIa) nor to the inhibitory receptor, FcgammaRIIb. In contrast, the glycosylated version of trastuzumab (E382V, M428I) purified from HEK293T cells bound to all Fcgamma receptors in a manner similar to that of clinical grade trastuzumab. E. coli-purified trastuzumab (E382V, M428I), but not glycosylated trastuzumab (E382V, M428I) or clinical grade trastuzumab, was capable of potentiating the killing of Her2 overexpressing tumor cells with dendritic cells (DCs) as effectors. These results indicate that aglycosylated IgGs can be engineered to display unique FcgammaR selectivity profiles that, in turn, mediate ADCC via mechanisms that are not normally displayed by glycosylated monoclonal antibodies.

Cross-species binding analyses of mouse and human neonatal Fc receptor show dramatic differences in immunoglobulin G and albumin binding

The neonatal Fc receptor (FcRn) regulates the serum half-life of both IgG and albumin through a pH-dependent mechanism that involves salvage from intracellular degradation. Therapeutics and diagnostics built on IgG, Fc, and albumin fusions are frequently evaluated in rodents regarding biodistribution and pharmacokinetics. Thus, it is important to address cross-species ligand reactivity with FcRn, because in vivo testing of such molecules is done in the presence of competing murine ligands, both in wild type (WT) and human FcRn (hFcRn) transgenic mice. Here, binding studies were performed in vitro using enzyme-linked immunosorbent assay and surface plasmon resonance with recombinant soluble forms of human (shFcRn(WT)) and mouse (smFcRn(WT)) receptors. No binding of albumin from either species was observed at physiological pH to either receptor. At acidic pH, a 100-fold difference in binding affinity was observed. Specifically, smFcRn(WT) bound human serum albumin with a K(D) of approximately 90 microM, whereas shFcRn(WT) bound mouse serum albumin with a K(D) of 0.8 microM. shFcRn(WT) ignored mouse IgG1, and smFcRn(WT) bound strongly to human IgG1. The latter pair also interacted at physiological pH with calculated affinity in the micromolar range. In all cases, binding of albumin and IgG from either species to both receptors were additive. Cross-species albumin binding differences could partly be explained by non-conserved amino acids found within the alpha2-domain of the receptor. Such distinct cross-species FcRn binding differences must be taken into consideration when IgG- and albumin-based therapeutics and diagnostics are evaluated in rodents for their pharmacokinetics.

Methionine oxidation in human IgG2 Fc decreases binding affinities to protein A and FcRn

Susceptibility of methionine residues to oxidation is a significant issue of protein therapeutics. Methionine oxidation may limit the product's clinical efficacy or stability. We have studied kinetics of methionine oxidation in the Fc portion of the human IgG2 and its impact on the interaction with FcRn and Protein A. Our results confirm previously published observations for IgG1 that two analogous solvent-exposed methionine residues in IgG2, Met 252 and Met 428, oxidize more readily than the other methionine residue, Met 358, which is buried inside the Fc. Met 397, which is not present in IgG1 but in IgG2, oxidizes at similar rate as Met 358. Oxidation of two labile methionines, Met 252 and Met 428, weakens the binding of the intact antibody with Protein A and FcRn, two natural protein binding partners. Both of these binding partners share the same binding site on the Fc. Additionally, our results shows that Protein A may serve as a convenient and inexpensive surrogate for FcRn binding measurements.

Immune and non-immune functions of the (not so) neonatal Fc receptor, FcRn

Careful regulation of the body's immunoglobulin-G (IgG) and albumin concentrations is necessitated by the importance of their respective functions. As such, the neonatal Fc receptor (FcRn) which, as a single receptor, is capable of regulating both of these molecules, has become an important focus of investigation. In addition to these essential protection functions, FcRn possesses a host of other functions that are equally as critical. During the very first stages of life, FcRn mediates the passive transfer of IgG from mother to offspring both before and after birth. In the adult, FcRn regulates the persistence of both IgG and albumin in the serum as well as the movement of IgG, and any bound cargo, between different compartments of the body. This shuttling allows for the movement not only of monomeric ligand but also of antigen/antibody complexes from one cell type to another in such a way as to facilitate the efficient initiation of immune responses towards opsonized pathogens. As such, FcRn continues to play the role of an immunological sensor throughout adult life, particularly in regions such as the gut which are exposed to a large number of infectious antigens. Increasing appreciation for the contributions of FcRn to both homeostatic and pathological states is generating an intense interest in the potential for therapeutic modulation of FcRn binding. A greater understanding of FcRn's pleiotropic roles is thus imperative for a variety of therapeutic purposes.

Nanogold as a specific marker for electron cryotomography

While electron cryotomography (ECT) provides "molecular" resolution, three-dimensional images of unique biological specimens, sample crowdedness, and/or resolution limitations can make it difficult to identify specific macromolecular components. Here we used a 1.4 nm Nanogold cluster specifically attached to the Fc fragment of IgG to monitor its interaction with the neonatal Fc receptor (FcRn), a membrane-bound receptor that transports IgG across cells in acidic intracellular vesicles. ECT was used to image complexes formed by Nanogold-labeled Fc bound to FcRn attached to the outer surface of synthetic liposomes. In the resulting three-dimensional reconstructions, 1.4 nm Nanogold particles were distributed predominantly along the interfaces where 2:1 FcRn-Fc complexes bridged adjacent lipid bilayers. These results demonstrate that the 1.4 nm Nanogold cluster is visible in tomograms of typically thick samples (approximately 250 nm) recorded with defocuses appropriate for large macromolecules and is thus an effective marker.

Optimizing pH response of affinity between protein G and IgG Fc: how electrostatic modulations affect protein-protein interactions

Protein-protein interaction in response to environmental conditions enables sophisticated biological and biotechnological processes. Aiming toward the rational design of a pH-sensitive protein-protein interaction, we engineered pH-sensitive mutants of streptococcal protein G B1, a binder to the IgG constant region. We systematically introduced histidine residues into the binding interface to cause electrostatic repulsion on the basis of a rigid body model. Exquisite pH sensitivity of this interaction was confirmed by surface plasmon resonance and affinity chromatography employing a clinically used human IgG. The pH-sensitive mechanism of the interaction was analyzed and evaluated from kinetic, thermodynamic, and structural viewpoints. Histidine-mediated electrostatic repulsion resulted in significant loss of exothermic heat of the binding that decreased the affinity only at acidic conditions, thereby improving the pH sensitivity. The reduced binding energy was partly recovered by "enthalpy-entropy compensation." Crystal structures of the designed mutants confirmed the validity of the rigid body model on which the effective electrostatic repulsion was based. Moreover, our data suggested that the entropy gain involved exclusion of water molecules solvated in a space formed by the introduced histidine and adjacent tryptophan residue. Our findings concerning the mechanism of histidine-introduced interactions will provide a guideline for the rational design of pH-sensitive protein-protein recognition.

Impact of methionine oxidation on the binding of human IgG1 to Fc Rn and Fc gamma receptors

Methionine oxidation commonly occurs in the Fc fragment of therapeutic monoclonal antibodies; however, its impact on antibody function has not been addressed. Using surface plasmon resonance and cell binding assays, we examined the impact of methionine oxidation on the binding of two humanized IgG1 antibodies to Fc gamma receptors (Fc gammaR) and to the neonatal Fc receptor (Fc Rn). A panel of Fc gammaRs, including Fc gammaRI, Fc gammaRIIa-131H, Fc gammaRIIa-131R, Fc gammaRIIb/c, Fc gammaRIII ALF, Fc gammaRIII ALV, and Fc gammaRIIIb was evaluated. The binding of oxidized IgG1 molecules to individual receptors remained the same with the exception of Fc gammaRIIa where a subtle decrease in binding to the 131H allele was observed. In contrast, but in agreement with recently reported structural changes associated with Met oxidation, binding to Fc Rn was significantly affected. An increase in K(D) values at pH 6.0 was observed with increasing degree of oxidation, reaching several-fold greater value in highly oxidized samples. To our knowledge this is the first report demonstrating that chemical degradations in the constant region of monoclonal antibodies can impact their function and it highlights the importance of avoiding oxidation in therapeutic antibodies.

New insights into intra- and intermolecular interactions of immunoglobulins: crystal structure of mouse IgG2b-Fc at 2.1-A resolution

The structure of the Fc fragment of monoclonal antibody IgG2b from hybridom M75 of Mus musculus has been determined by single crystal X-ray diffraction. This is the first report of the structure of the murine immunoglobulin isotype IgG2b. The structure refined at 2.1 A resolution provides more detailed structural information about native oligosaccharides than was previously available. High-quality Fourier maps provide a clear identification of alpha-l-fucose with partial occupancy in the first branch of the antennary oligosaccharides. A unique Fc:Fc interaction was observed at the C(H)2-C(H)3 interface.

Structural characterization of a human Fc fragment engineered for extended serum half-life

The first three-dimensional structure of a human Fc fragment genetically engineered for improved pharmacokinetics properties is reported. When introduced into the C(H)2 domain of human immunoglobulin G (IgG) molecules, the triple mutation M252Y/S254T/T256E ('YTE') causes an about 10-fold increase in their binding to the human neonatal Fc receptor (FcRn). This translates into an almost 4-fold increase in the serum half-life of YTE-containing human IgGs in cynomolgus monkeys. A recombinantly produced human Fc/YTE fragment was crystallized and its structure solved at a resolution of 2.5A using molecular replacement. This revealed that Fc/YTE three-dimensional structure is very similar to that of other human Fc fragments in the experimentally visible region spanning residues 236-444. We propose that the enhanced interaction between Fc/YTE and human FcRn is likely mediated by local effects at the substitutions sites. Molecular modeling suggested that potential favorable hydrogen bonds along with an increase in the surface of contact between the two partners may account in part for the corresponding increase in affinity.

Engineering therapeutic monoclonal antibodies

During last two decades, the chimerization and humanization of monoclonal antibodies (mAbs) have led to the approval of several for the treatment of cancer, autoimmune diseases, and transplant rejection. Additional approaches have been used to further improve their in vivo activity. These include combining them with other modalities such as chemotherapy and redesigning them for improved pharmacokinetics, effector function, and signaling activity. The latter has taken advantage of new insights emerging from an increased understanding of the cellular and molecular mechanisms that are involved in the interaction of immunoglobulin G with Fc receptors and complement as well as the negative signaling resulting from the hypercrosslinking of their target antigens. Hence, mAbs have been redesigned to include mutations in their Fc portions, thereby endowing them with enhanced or decreased effector functions and more desirable pharmacokinetic properties. Their valency has been increased to decrease their dissociation rate from cells and enhance their ability to induce apoptosis and cell cycle arrest. In this review we discuss these redesigned mAbs and current data concerning their evaluation both in vitro and in vivo.

N-Glycan Moieties in Neonatal Fc Receptor Determine Steady-state Membrane Distribution and Directional Transport of IgG

The neonatal Fc receptor (FcRn) is a major histocompatibility complex class I-related molecule known to protect IgG and albumin from catabolism and transport IgG across polarized epithelial cells in a bidirectional manner. Previous studies have shown species-specific differences in ligand binding, IgG transport direction, and steady-state membrane distribution when expressed in polarized epithelial cells. We hypothesized that these differences may be due to the additional N-glycans expressed on the rat FcRn, because N-glycans have been proposed to function as apical targeting signals, and that two of the N-glycan moieties have been shown to contribute to the IgG binding of rat FcRn. A panel of mutant human FcRn variants was generated to resemble the N-glycan expression of rat FcRn in various combinations and subsequently transfected into Madin-Darby canine kidney II cells together with human beta2-microglobulin. Mutant human FcRn clones that contained additional N-glycan side-chain modifications, including that which was fully rodentized, still exhibited specificity for human IgG and failed to bind to mouse IgG. At steady state, the mutant human FcRn with additional N-glycans redistributed to the apical cell surface similar to that of rat FcRn. Furthermore, the rodentized human FcRn exhibited a reversal of IgG transport with predominant transcytosis from an apical-to-basolateral direction, which resembled that of the rat FcRn isoform. These studies show that the N-glycans in FcRn contribute significantly to the steady-state membrane distribution and direction of IgG transport in polarized epithelia.

Chapter 4: Multitasking by exploitation of intracellular transport functions the many faces of FcRn

The MHC Class I-related receptor, FcRn, transports antibodies of the immunoglobulin G (IgG) class within and across a diverse array of different cell types. Through this transport, FcRn serves multiple roles throughout adult life that extend well beyond its earlier defined function of transcytosing IgGs from mother to offspring. These roles include the maintenance of IgG levels and the delivery of antigen in the form of immune complexes to degradative compartments within cells. Recent studies have led to significant advances in knowledge of the intracellular trafficking of FcRn and (engineered) IgGs at both the molecular and cellular levels. The engineering of FcRn-IgG (or Fc) interactions to generate antibodies of increased longevity represents an area of active interest, particularly in the light of the expanding use of antibodies in therapy. The strict pH dependence of FcRn-IgG interactions, with binding at pH 6 that becomes essentially undetectable as near neutral pH is approached, is essential for efficient transport. The requirement for retention of low affinity at near neutral pH increases the complexity of engineering antibodies for increased half-life. Conversely, engineered IgGs that have gained significant binding for FcRn at this pH can be potent inhibitors of FcRn that lower endogenous IgG levels and have multiple potential uses as therapeutics. In addition, molecular studies of FcRn-IgG interactions indicate that mice have limitations as preclinical models for FcRn function, primarily due to cross-species differences in FcRn-binding specificity.

Targeting the neonatal fc receptor for antigen delivery using engineered fc fragments

The development of approaches for Ag delivery to the appropriate subcellular compartments of APCs and the optimization of Ag persistence are both of central relevance for the induction of protective immunity or tolerance. The expression of the neonatal Fc receptor, FcRn, in APCs and its localization to the endosomal system suggest that it might serve as a target for Ag delivery using engineered Fc fragment-epitope fusions. The impact of FcRn binding characteristics of an Fc fragment on in vivo persistence allows this property to also be modulated. We have therefore generated recombinant Fc (mouse IgG1-derived) fusions containing the N-terminal epitope of myelin basic protein that is associated with experimental autoimmune encephalomyelitis in H-2(u) mice. The Fc fragments have distinct binding properties for FcRn that result in differences in intracellular trafficking and in vivo half-lives, allowing the impact of these characteristics on CD4(+) T cell responses to be evaluated. To dissect the relative roles of FcRn and the "classical" FcgammaRs in Ag delivery, analogous aglycosylated Fc-MBP fusions have been generated. We show that engineered Fc fragments with increased affinities for FcRn at pH 6.0-7.4 are more effective in delivering Ag to FcRn-expressing APCs in vitro relative to their lower affinity counterparts. However, higher affinity of the FcRn-Fc interaction at near neutral pH results in decreased in vivo persistence. The trade-off between improved FcRn targeting efficiency and lower half-life becomes apparent during analyses of T cell proliferative responses in mice, particularly when Fc-MBP fusions with both FcRn and FcgammaR binding activity are used.

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.

Blockade of maternal anti-HPA-1a-mediated platelet clearance by an HPA-1a epitope-specific F(ab') in an in vivo mouse model of alloimmune thrombocytopenia

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT) is most commonly caused by transplacental passage of maternal human platelet-specific alloantigen (HPA)-1a antibodies that bind to fetal platelets (PLTs) and mediate their clearance. SZ21, a monoclonal antibody (MoAb) directed against PLT glycoprotein IIIa, competitively inhibits the binding of anti-HPA-1a alloantibodies to PLTs in vitro. The purpose of this investigation was to determine whether SZ21 F(ab')(2) fragments might be therapeutically effective in inhibiting or displacing maternal HPA-1a antibodies from the fetal PLT surface and preventing their clearance from circulation.

STUDY DESIGN AND METHODS

Resting human PLTs from HPA-1ab heterozygous donors were injected into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Purified F(ab')(2) fragments of SZ21 or control immunoglobulin G (IgG) were injected intraperitoneally 30 minutes before introduction of HPA-1a antibodies. Blood samples were taken periodically and analyzed by flow cytometry to determine the percentage of circulating human PLTs.

RESULTS

Anti-HPA-1a IgG from NAIT cases were able to efficiently clear HPA-1a-positive PLTs from murine circulation. Administration of SZ21 F(ab')(2) fragments not only inhibited binding of HPA-1a antibodies to circulating human PLTs, preventing their clearance, but also displaced bound HPA-1a antibodies from the PLT surface.

CONCLUSION

F(ab')(2) fragments of HPA-1a-selective MoAb SZ21 effectively inhibit anti-HPA-1a-mediated clearance of human PLT circulating in an in vivo NOD/SCID mouse model. These results suggest that agents that inhibit binding of anti-HPA-1a to PLTs may have therapeutic potential in the treatment of NAIT.

Structure of an isolated unglycosylated antibody C(H)2 domain

The C(H)2 (C(H)3 for IgM and IgE) domain of an antibody plays an important role in mediating effector functions and preserving antibody stability. It is the only domain in human immunoglobulins (Igs) which is involved in weak interchain protein-protein interactions with another C(H)2 domain solely through sugar moieties. The N-linked glycosylation at Asn297 is conserved in mammalian IgGs as well as in homologous regions of other antibody isotypes. To examine the structural details of the C(H)2 domain in the absence of glycosylation and other antibody domains, the crystal structure of an isolated unglycosylated antibody gamma1 C(H)2 domain was determined at 1.7 A resolution and compared with corresponding C(H)2 structures from intact Fc, IgG and Fc receptor complexes. Furthermore, the oligomeric state of the protein in solution was studied using size-exclusion chromatography. The results suggested that the unglycosylated human antibody C(H)2 domain is a monomer and that its structure is similar to that found in the intact Fc, IgG and Fc receptor complex structures. However, certain structural variations were observed in the Fc receptor-binding sites. Owing to its small size, stability and non-immunogenic Ig template, the C(H)2-domain structure could be useful for the development by protein design of antibody domains exerting effector functions and/or antigen specificity and as a robust scaffold in protein-engineering applications.

Structural and molecular basis for hyperspecificity of RNA aptamer to human immunoglobulin G

Potential applications for functional RNAs are rapidly expanding, not only to address functions based on primary nucleotide sequences, but also by RNA aptamer, which can suppress the activity of any target molecule. Aptamers are short DNA or RNA folded molecules that can be selected in vitro on the basis of their high affinity for a target molecule. Here, we demonstrate the ability of RNA aptamers to recognize--and bind to--human IgG with high specificity and affinity. An optimized 23-nucleotide aptamer, Apt8-2, was prepared, and was shown to bind to the Fc domain of human IgG, but not to other IgG's, with high affinity. Apt8-2 was observed to compete with protein A, but not with the Fcgamma receptor, for IgG binding. NMR chemical-shift analyses localized the aptamer-binding sites on the Fc subdomain, which partially overlaps the protein A binding site but not the Fcgamma receptor binding site. The tertiary structures of the predicted recognition sites on the Fc domain differ significantly between human IgG and other species of IgGs; this, in part, accounts for the high specificity of the selected aptamer. Apt8-2 can therefore be used as a protein A alternative for affinity purification of human IgG and therapeutic antibodies. Using Apt8-2 would have several potential advantages, raising the possibility of developing new applications based on aptamer design.

TRIM21 is an IgG receptor that is structurally, thermodynamically, and kinetically conserved

The newly identified tripartite motif (TRIM) family of proteins mediate innate immunity and other critical cellular functions. Here we show that TRIM21, which mediates the autoimmune diseases rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome, is a previously undescribed IgG receptor with a binding mechanism unlike known mammalian Fcgamma receptors. TRIM21 simultaneously targets conserved hot-spot residues on both Ig domains of the Fc fragment using a PRYSPRY domain with a preformed multisite interface. The binding sites on both TRIM21 and Fc are highly conserved to the extent that the proteins are functionally interchangeable through murine, canine, primate, and human species. Pre-steady-state analysis exposes mechanistic conservation at the level of individual residues, which make the same energetic and kinetic contributions to binding despite varying in sequence. Together, our results reveal that TRIM21 is a previously undescribed type of IgG receptor based on a non-Ig scaffold whose interaction at the fundamental level-structural, thermodynamic, and kinetic-is evolutionarily conserved.

The chicken yolk sac IgY receptor, a mammalian mannose receptor family member, transcytoses IgY across polarized epithelial cells

In mammals the transfer of passive immunity from mother to young is mediated by the MHC-related receptor FcRn, which transports maternal IgG across epithelial cell barriers. In birds, maternal IgY in egg yolk is transferred across the yolk sac to passively immunize chicks during gestation and early independent life. The chicken yolk sac IgY receptor (FcRY) is the ortholog of the mammalian phospholipase A2 receptor, a mannose receptor family member, rather than an FcRn or MHC homolog. FcRn and FcRY both exhibit ligand binding at the acidic pH of endosomes and ligand release at the slightly basic pH of blood. Here we show that FcRY expressed in polarized mammalian epithelial cells functioned in endocytosis, bidirectional transcytosis, and recycling of chicken FcY/IgY. Confocal immunofluorescence studies demonstrated that IgY binding and endocytosis occurred at acidic but not basic pH, mimicking pH-dependent uptake of IgG by FcRn. Colocalization studies showed FcRY-mediated internalization via clathrin-coated pits and transport involving early and recycling endosomes. Disruption of microtubules partially inhibited apical-to-basolateral and basolateral-to-apical transcytosis, but not recycling, suggesting the use of different trafficking machinery. Our results represent the first cell biological evidence of functional equivalence between FcRY and FcRn and provide an intriguing example of how evolution can give rise to systems in which similar biological requirements in different species are satisfied utilizing distinct protein folds.

Fcgamma receptors as regulators of immune responses

In addition to their role in binding antigen, antibodies can regulate immune responses through interacting with Fc receptors (FcRs). In recent years, significant progress has been made in understanding the mechanisms that regulate the activity of IgG antibodies in vivo. In this Review, we discuss recent studies addressing the multifaceted roles of FcRs for IgG (FcgammaRs) in the immune system and how this knowledge could be translated into novel therapeutic strategies to treat human autoimmune, infectious or malignant diseases.

The human cytomegalovirus Fc receptor gp68 binds the Fc CH2-CH3 interface of immunoglobulin G

Recognition of immunoglobulin G (IgG) by surface receptors for the Fc domain of immunoglobulin G (Fcgamma), FcgammaRs, can trigger both humoral and cellular immune responses. Two human cytomegalovirus (HCMV)-encoded type I transmembrane receptors with Fcgamma-binding properties (vFcgammaRs), gp34 and gp68, have been identified on the surface of HCMV-infected cells and are assumed to confer protection against IgG-mediated immunity. Here we show that Fcgamma recognition by both vFcgammaRs occurs independently of N-linked glycosylation of Fcgamma, in contrast with the properties of host FcgammaRs. To gain further insight into the interaction with Fcgamma, truncation mutants of the vFcgammaR gp68 ectodomain were probed for Fcgamma binding, resulting in localization of the Fcgamma binding site on gp68 to residues 71 to 289, a region including an immunoglobulin-like domain. Gel filtration and biosensor binding experiments revealed that, unlike host FcgammaRs but similar to the herpes simplex virus type 1 (HSV-1) Fc receptor gE-gI, gp68 binds to the C(H)2-C(H)3 interdomain interface of the Fcgamma dimer with a nanomolar affinity and a 2:1 stoichiometry. Unlike gE-gI, which binds Fcgamma at the slightly basic pH of the extracellular milieu but not at the acidic pH of endosomes, the gp68/Fcgamma complex is stable at pH values from 5.6 to pH 8.1. These data indicate that the mechanistic details of Fc binding by HCMV gp68 differ from those of host FcgammaRs and from that of HSV-1 gE-gI, suggesting distinct functional and recognition properties.

Changes in major proteins in the embryonic capsule during immobilization (fixation) of the conceptus in the third week of pregnancy in the mare

During the third week of pregnancy, the equine conceptus is enclosed within a capsule, the glycan composition of which changes at around day 16 (ovulation = day 0) when the conceptus becomes immobilized (fixed) in the uterine lumen. Our objective was to characterize the process of fixation by identifying changes in major capsule-associated proteins. Individual equine conceptuses (n = 55) were collected transcervically by uterine lavage between days 13.5 and 26.5. Major proteins extracted from capsules were compared with those in fluids from the uterus and yolk sac by SDS-PAGE. Until day 14, a major capsule-associated protein that migrated at approximately 10 kDa was identified by N-terminal sequencing as equine beta2 microglobulin (beta2M). During fixation, beta2M in the capsule underwent limited proteolysis to an approximately 8 kDa form lacking nine amino acids from the N terminus, and was subsequently degraded. Expression of beta2M mRNA was detected in the yolk-sac wall tissues and endometrium between days 13.5 and 17.5. During this period, beta2M in the capsule was evidently not part of a complex with major histocompatibility complex class 1 heavy alpha chain bands because these were undetectable in the capsule and uterine lavage. Uterocalin (p19) was detected in uterine lavage and capsule throughout fixation, but in yolk-sac fluid only before fixation. These studies indicate that intact beta2M is a major protein associated with the embryonic capsule before fixation, after which it undergoes limited proteolysis to a truncated approximately 8 kDa form that remains in the capsule after the conceptus is immobilized.

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.

FcRn: the neonatal Fc receptor comes of age

The neonatal Fc receptor for IgG (FcRn) has been well characterized in the transfer of passive humoral immunity from a mother to her fetus. In addition, throughout life, FcRn protects IgG from degradation, thereby explaining the long half-life of this class of antibody in the serum. In recent years, it has become clear that FcRn is expressed in various sites in adults, where its potential function is now beginning to emerge. In addition, recent studies have examined the interaction between FcRn and the Fc portion of IgG with the aim of either improving the serum half-life of therapeutic monoclonal antibodies or reducing the half-life of pathogenic antibodies. This Review summarizes these two areas of FcRn biology.

A freeze substitution fixation-based gold enlarging technique for EM studies of endocytosed Nanogold-labeled molecules

We have developed methods to locate individual ligands that can be used for electron microscopy studies of dynamic events during endocytosis and subsequent intracellular trafficking. The methods are based on enlargement of 1.4 nm Nanogold attached to an endocytosed ligand. Nanogold, a small label that does not induce misdirection of ligand-receptor complexes, is ideal for labeling ligands endocytosed by live cells, but is too small to be routinely located in cells by electron microscopy. Traditional pre-embedding enhancement protocols to enlarge Nanogold are not compatible with high pressure freezing/freeze substitution fixation (HPF/FSF), the most accurate method to preserve ultrastructure and dynamic events during trafficking. We have developed an improved enhancement procedure for chemically fixed samples that reduced auto-nucleation, and a new pre-embedding gold enlarging technique for HPF/FSF samples that preserved contrast and ultrastructure and can be used for high-resolution tomography. We evaluated our methods using labeled Fc as a ligand for the neonatal Fc receptor. Attachment of Nanogold to Fc did not interfere with receptor binding or uptake, and gold-labeled Fc could be specifically enlarged to allow identification in 2D projections and in tomograms. These methods should be broadly applicable to many endocytosis and transcytosis studies.

Why are proteins charged? Networks of charge-charge interactions in proteins measured by charge ladders and capillary electrophoresis

Almost all proteins contain charged amino acids. While the function in catalysis or binding of individual charges in the active site can often be identified, it is less clear how to assign function to charges beyond this region. Are they necessary for solubility? For reasons other than solubility? Can manipulating these charges change the properties of proteins? A combination of capillary electrophoresis (CE) and protein charge ladders makes it possible to study the roles of charged residues on the surface of proteins outside the active site. This method involves chemical modification of those residues to generate a large number of derivatives of the protein that differ in charge. CE separates those derivatives into groups with the same number of modified charged groups. By studying the influence of charge on the properties of proteins using charge ladders, it is possible to estimate the net charge and hydrodynamic radius and to infer the role of charged residues in ligand binding and protein folding.

Structural Comparison of Fucosylated and Nonfucosylated Fc Fragments of Human Immunoglobulin G1

Removal of the fucose residue from the oligosaccharides attached to Asn297 of human immunoglobulin G1 (IgG1) results in a significant enhancement of antibody-dependent cellular cytotoxicity (ADCC) via improved IgG1 binding to Fcgamma receptor IIIa. To provide structural insight into the mechanisms of affinity enhancement, we determined the crystal structure of the nonfucosylated Fc fragment and compared it with that of fucosylated Fc. The overall conformations of the fucosylated and nonfucosylated Fc fragments were similar except for hydration mode around Tyr296. Stable-isotope-assisted NMR analyses confirmed the similarity of the overall structures between fucosylated and nonfucosylated Fc fragments in solution. These data suggest that the glycoform-dependent ADCC enhancement is attributed to a subtle conformational alteration in a limited region of IgG1-Fc. Furthermore, the electron density maps revealed that the traces between Asp280 and Asn297 of our fucosylated and nonfucosylated Fc crystals were both different from that in previously reported isomorphous Fc crystals.

Structural basis for PRYSPRY-mediated tripartite motif (TRIM) protein function

The human tripartite motif (TRIM) family comprises 70 members, including HIV restriction factor TRIM5alpha and disease-associated proteins TRIM20 (pyrin) and TRIM21. TRIM proteins have conserved domain architecture but diverse cellular roles. Here, we describe how the C-terminal PRYSPRY domain mediates diverse TRIM functions. The crystal structure of TRIM21 PRYSPRY in complex with its target IgG Fc reveals a canonical binding interface comprised of two discrete pockets formed by antibody-like variable loops. Alanine scanning of this interface has identified the hot-spot residues that control TRIM21 binding to Fc; the same hot-spots control HIV/murine leukemia virus restriction by TRIM5alpha and mediate severe familial Mediterranean fever in TRIM20/pyrin. Characterization of the IgG binding site for TRIM21 PRYSPRY reveals TRIM21 as a superantigen analogous to bacterial protein A and suggests that an antibody bipolar bridging mechanism may contribute to the pathogenic accumulation of anti-TRIM21 autoantibody immune complex in autoimmune disease.

Fusion antibody for Alzheimer's disease with bidirectional transport across the blood-brain barrier and abeta fibril disaggregation

Delivery of monoclonal antibody therapeutics across the blood-brain barrier is an obstacle to the diagnosis or therapy of CNS disease with antibody drugs. The immune therapy of Alzheimer's disease attempts to disaggregate the amyloid plaque of Alzheimer's disease with an anti-Abeta monoclonal antibody. The present work is based on a three-step model of immune therapy of Alzheimer's disease: (1) influx of the anti-Abeta monoclonal antibody across the blood-brain barrier in the blood to brain direction, (2) binding and disaggregation of Abeta fibrils in brain, and (3) efflux of the anti-Abeta monoclonal antibody across the blood-brain barrier in the brain to blood direction. This is accomplished with the genetic engineering of a trifunctional fusion antibody that binds (1) the human insulin receptor, which mediates the influx from blood to brain across the blood-brain barrier, (2) the Abeta fibril to disaggregate amyloid plaque, and (3) the Fc receptor, which mediates the efflux from brain to blood across the blood-brain barrier. This fusion protein is a new antibody-based therapeutic for Alzheimer's disease that is specifically engineered to cross the human blood-brain barrier in both directions.

Radioiodinated versus radiometal-labeled anti-carcinoembryonic antigen single-chain Fv-Fc antibody fragments: optimal pharmacokinetics for therapy

Antibody fragments with optimized pharmacokinetic profiles hold potential for detection and therapy of tumor malignancies. We studied the behavior of three anti-carcinoembryonic antigen (CEA) single-chain Fv-Fc (scFv-Fc) variants (I253A, H310A, and H310A/H435Q; Kabat numbering system) that exhibited differential serum persistence. Biodistribution studies done on CEA-positive tumor xenografted mice revealed that the 111In-labeled I253A fragment with the slowest clearance kinetics (T1/2beta, 27.7 h) achieved the highest tumor uptake (44.6% ID/g at 24 h), whereas the radiometal-labeled H310A/H435Q fragment with the most rapid elimination (T1/2beta, 7.05 h) reached a maximum of 28.0% ID/g at 12 h postinjection. The H310A protein was characterized by both intermediate serum half-life and tumor uptake. The 111In-based biodistribution studies showed that all three fragments were eliminated primarily through the liver, and hepatic radiometal activity correlated with the rate of fragment clearance. The 111In-labeled H310A/H435Q protein exhibited the highest liver uptake (23.5% ID/g at 24 h). Metabolism of the 125I-labeled scFv-Fc proteins resulted in low normal organ activity. Finally, the 125I/111In biodistribution data allowed for dose estimations, which suggest the 131I-labeled scFv-Fc H310A/H435Q as a promising candidate for radioimmunotherapy.

The conserved histidine 166 residue of the human neonatal Fc receptor heavy chain is critical for the pH-dependent binding to albumin

The MHC class I-related neonatal Fc receptor (FcRn) serves in the homeostatic regulation of IgG and albumin by increasing their half-lives. FcRn may bind IgG and albumin simultaneously, and in a pH-dependent manner, with ligand binding at pH 6.0-6.5 and release at pH 7.0-7.4. The FcRn-IgG interaction has been extensively characterized at the amino acid level and shown to depend on conserved histidine residues in the IgG-Fc part that interact with negatively charged residues in the alpha-2 domain of FcRn. The recently discovered FcRn-albumin interaction remains to be elucidated. Guided by the pH dependence of the FcRn-albumin interaction, we compared the sequence of the FcRn alpha-2 domain from eleven different species, and identified histidine residues that were conserved in all (H166) or seven (H161) of these. Both residues are located directly opposite to the IgG interaction site in the folded molecule. We did in vitro mutagenesis (H161A or H166A) in combination with interaction studies (ELISA and surface plasmon resonance) with recombinant, soluble, purified receptors and IgG and albumin to investigate the role of the two histidine residues. Our results show clear evidence that the conserved H166 is a key player in the FcRn-albumin interaction.

Monoclonal Antibody Clearance

The neonatal Fc receptor (FcRn) plays a critical role in regulating IgG homeostasis in vivo. There are mixed reports on whether modification of the interaction with FcRn can be used as an engineering strategy to improve the pharmacokinetic and pharmacodynamic properties of monoclonal antibodies. We tested whether the T250Q/M428L mutations, which improved the pharmacokinetics of humanized IgGs in the rhesus monkey, would translate to a pharmacokinetic benefit in both cynomolgus monkeys and mice when constructed on a different humanized IgG framework (anti-tumor necrosis factor-alpha (TNFalpha)). The T250Q/M428L anti-TNFalpha variant displayed an approximately 40-fold increase in binding affinity to cynomolgus monkey FcRn (C-FcRn) at pH 6.0, with maintenance of the pH binding dependence. We also constructed another anti-TNFalpha variant (P257I/Q311I) whose binding kinetics with the C-FcRn was similar to that of the T250Q/M428L variant. The binding affinity of the T250Q/M428L variant for murine FcRn was increased approximately 500-fold, with maintenance of pH dependence. In contrast to the interaction with C-FcRn, this interaction was driven mainly by a decrease in the rate of dissociation. Despite the improved in vitro binding properties of the anti-TNFalpha T250Q/M428L and P257I/Q311I variants to C-FcRn, the pharmacokinetic profiles of these molecules were not differentiated from the wild-type antibody in cynomolgus monkeys after intravenous administration. When administered intravenously to mice, the T250Q/M428L anti-TNFalpha variant displayed improved pharmacokinetics, characterized by an approximately 2-fold slower clearance than the wild-type antibody. The discrepancy between these data and previously reported benefits in rhesus monkeys and the inability of these mutations to translate to improved kinetics across species may be related to a number of factors. We propose extending consideration to differences in the absolute IgG-FcRn affinity, the kinetics of the IgG/FcRn interaction, and differences in the relative involvement of this pathway in the context of other factors influencing the disposition or elimination of monoclonal antibodies.

Tunable pharmacokinetics: modifying the in vivo half-life of antibodies by directed mutagenesis of the Fc fragment

Immunoglobulins (Igs) are large proteins of 150 kDa with prolonged residence time in blood. Their half-life is controlled by their ability to interact with the protective neonatal Fc receptor (FcRn, Brambell receptor) present on endothelial cells. Here, we describe a protocol using site-specific mutagenesis of individual residues responsible for this interaction, resulting in engineered antibodies with distinct half-lives. The method is a powerful tool that enables manipulation of half-lives and is applicable to all antibodies and Fc fusion proteins for the development of agents with controlled pharmacokinetic properties. Moreover, the protocol is applicable to any situation where the structure and/or function of engineered proteins are to be studied. The protocol begins with the mutagenesis reaction at the DNA level and proceeds to describe mammalian expression and purification of recombinant proteins, radiolabeling and evaluation in vivo. The time frame for completing the procedure is about 6 months, provided that no complications are encountered.

Divergent activities of an engineered antibody in murine and human systems have implications for therapeutic antibodies

The MHC class I-related receptor, neonatal Fc receptor (FcRn), plays a central role in regulating the transport and in vivo persistence of immunoglobulin G (IgG). IgG-FcRn interactions can be targeted for engineering to modulate the in vivo longevity and transport of an antibody, and this has implications for the successful application of therapeutic IgGs. Although mice are widely used to preclinically test antibodies, human and mouse FcRn have significant differences in binding specificity. Here we show that an engineered human IgG1 has disparate properties in murine and human systems. The mutant shows improved transport relative to wild-type human IgG1 in assays of human FcRn function but has short in vivo persistence and competitively inhibits FcRn activity in mice. These studies indicate potential limitations of using mice as preclinical models for the analysis of engineered antibodies. Alternative assays are proposed that serve as indicators of the properties of IgGs in humans.

Pulmonary administration of therapeutic proteins using an immunoglobulin transport pathway

We have applied a "physiologic" approach to the pulmonary delivery of therapeutic proteins, utilizing an immunoglobulin (antibody) transport pathway recently shown to be present predominantly in the conducting airways of the human respiratory tract. Therapeutic proteins are fused to the Fc-domain of an IgG1, allowing them to bind with high affinity to the antibody transport receptor, FcRn. Liquid aerosols are administered into the lung using normal breathing maneuvers and efficient delivery of several different Fc-fusion proteins has been achieved with retention of biological activity and an increase in circulating half-life. A new paradigm for the pulmonary delivery of therapeutic proteins and a fundamental advance in the construction of Fc-fusion proteins for this purpose will be described.

Humanized IgG1 variants with differential binding properties to the neonatal Fc receptor: relationship to pharmacokinetics in mice and primates

It is well established that the neonatal Fc receptor (FcRn) plays a critical role in regulating IgG homeostasis in vivo. As such, modification of the interaction of IgG with FcRn has been the focus of protein-engineering strategies designed to generate therapeutic antibodies with improved pharmacokinetic properties. In the current work, we characterized differences in interaction of IgG between mouse and primate receptors using three humanized anti-tumor necrosis factor alpha antibodies with variant IgG(1) Fc regions. The wild-type and variant IgG showed a differential combination of improved affinity, modified dissociation kinetics, and altered pH-dependent complex dissociation when evaluated on the primate and murine receptors. The observed in vitro binding differences within and between species allowed us to more completely relate these parameters to their influence on the in vivo pharmacokinetics in mice and cynomolgus monkeys. The variant antibodies have different pharmacokinetic behavior in cynomolgus monkeys and mice, which appears to be related to the unique binding characteristics observed with the murine receptor. However, we did not observe a direct relationship between increased binding affinity to the receptor and improved pharmacokinetic properties for these molecules in either species. This work provides further insights into how the FcRn/IgG interaction may be modulated to develop monoclonal antibodies with improved therapeutic properties.

Structural insights into antibody-mediated mucosal immunity

The mucosal regions of the body are responsible for defense against environmental pathogens. Particularly in the lumen of the gut, antibody-mediated immune responses are critical for preventing invasion by pathogens. In this chapter, we review structural studies that have illuminated various aspects of mucosal immunity. Crystal structures of IgA1-Fc and IgA-binding fragments of the polymeric immunoglobulin receptor and Fc alphaRI, combined with models of intact IgA and IgM from solution scattering studies, reveal potential mechanisms for immune exclusion and induction of inflammatory responses. Other recent structures yield insights into bacterial mechanisms for evasion of the host immune response.