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

Nipocalimab, an immunoselective FcRn blocker that lowers IgG and has unique molecular properties

Nipocalimab is a human immunoglobulin G (IgG)1 monoclonal antibody that binds to the neonatal Fc receptor (FcRn) with high specificity and high affinity at both neutral (extracellular) and acidic (intracellular) pH, resulting in the reduction of circulating IgG levels, including those of pathogenic IgG antibodies. Here, we present the molecular, cellular, and nonclinical characteristics of nipocalimab that support the reported clinical pharmacology and potential clinical application in IgG-driven, autoantibody- and alloantibody-mediated diseases. The crystal structure of the nipocalimab antigen binding fragment (Fab)/FcRn complex reveals its binding to a unique epitope on the IgG binding site of FcRn that supports the observed pH-independent high-binding affinity to FcRn. Cell-based and in vivo studies demonstrate concentration/dose- and time-dependent FcRn occupancy and IgG reduction. Nipocalimab selectively reduces circulating IgG levels without detectable effects on other adaptive and innate immune functions. In vitro experiments and in vivo studies in mice and cynomolgus monkeys generated data that align with observations from clinical studies of nipocalimab in IgG autoantibody- and alloantibody-mediated diseases.

An intranasal subunit vaccine induces protective systemic and mucosal antibody immunity against respiratory viruses in mouse models

Although vaccines are usually given intramuscularly, the intranasal delivery route may lead to better mucosal protection and limit the spread of respiratory virus while easing administration and improving vaccine acceptance. The challenge, however, is to achieve delivery across the selective epithelial cell barrier. Here we report on a subunit vaccine platform, in which the antigen is genetically fused to albumin to facilitate FcRn-mediated transport across the mucosal barrier in the presence of adjuvant. Intranasal delivery in conventional and transgenic mouse models induces both systemic and mucosal antigen-specific antibody responses that protect against challenge with SARS-CoV-2 or influenza A. When benchmarked against an intramuscularly administered mRNA vaccine or an intranasally administered antigen fused to an alternative carrier of similar size, only the albumin-based intranasal vaccine yields robust mucosal IgA antibody responses. Our results thus suggest that this needle-free, albumin-based vaccine platform may be suited for vaccination against respiratory pathogens.

Differential effects of FcRn antagonists on the subcellular trafficking of FcRn and albumin

The homeostasis of IgG is maintained by the neonatal Fc receptor, FcRn. Consequently, antagonism of FcRn to reduce endogenous IgG levels is an emerging strategy for treating antibody-mediated autoimmune disorders using either FcRn-specific antibodies or an engineered Fc fragment. For certain FcRn-specific antibodies, this approach has resulted in reductions in the levels of serum albumin, the other major ligand transported by FcRn. Cellular and molecular analyses of a panel of FcRn antagonists have been carried out to elucidate the mechanisms leading to their differential effects on albumin homeostasis. These analyses have identified 2 processes underlying decreases in albumin levels during FcRn blockade: increased degradation of FcRn and competition between antagonist and albumin for FcRn binding. These findings have potential implications for the design of drugs to modulate FcRn function.

The therapeutic age of the neonatal Fc receptor

IgGs are essential soluble components of the adaptive immune response that evolved to protect the body from infection. Compared with other immunoglobulins, the role of IgGs is distinguished and enhanced by their high circulating levels, long half-life and ability to transfer from mother to offspring, properties that are conferred by interactions with neonatal Fc receptor (FcRn). FcRn binds to the Fc portion of IgGs in a pH-dependent manner and protects them from intracellular degradation. It also allows their transport across polarized cells that separate tissue compartments, such as the endothelium and epithelium. Further, it is becoming apparent that FcRn functions to potentiate cellular immune responses when IgGs, bound to their antigens, form IgG immune complexes. Besides the protective role of IgG, IgG autoantibodies are associated with numerous pathological conditions. As such, FcRn blockade is a novel and effective strategy to reduce circulating levels of pathogenic IgG autoantibodies and curtail IgG-mediated diseases, with several FcRn-blocking strategies on the path to therapeutic use. Here, we describe the current state of knowledge of FcRn-IgG immunobiology, with an emphasis on the functional and pathological aspects, and an overview of FcRn-targeted therapy development.

Does Risk of Hyperhomocysteinemia Depend on Thiol-Disulfide Exchange Reactions of Albumin and Homocysteine?

Significance: Increased plasma concentrations of total homocysteine (tHcy; mild-moderate hyperhomocysteinemia: 15-50 μM tHcy) are considered an independent risk factor for the onset/progression of various diseases, but it is not known about how the increase in tHcy causes pathological conditions. Recent Advances: Reduced homocysteine (HSH ∼1% of tHcy) is presumed to be toxic, unlike homocystine (∼9%) and mixed disulfide between homocysteine and albumin (HSS-ALB; homocysteine [Hcy]-albumin mixed disulfide, ∼90%). This and other notions make it difficult to explain the pathogenicity of Hcy because: (i) lowering tHcy does not improve pathological outcomes; (ii) damage due to HSH usually emerges at supraphysiological doses; and (iii) it is not known why tiny increments in plasma concentrations of HSH can be pathological. Critical Issues: Albumin may have a role in Hcy toxicity, because HSS-ALB could release toxic HSH via thiol-disulfide (SH/SS) exchange reactions in cells. Similarly, thiol-disulfide exchange processes of reduced albumin (albumin with free SH group of Cys34 [HS-ALB]) or N-homocysteinylated albumin are plausible alternatives for initiating Hcy pathological events. Adverse effects of albumin and other data reviewed here suggest the hypothesis of a role of albumin in Hcy toxicity. Future Directions: HSS-ALB might be involved in disruption of the antioxidant/oxidant balance in critical tissues (brain, liver, kidney). Since homocysteine-albumin mixed disulfide is a possible intermediate of thiol-disulfide exchange reactions, we suggest that homocysteinylated albumin could be a new pathological factor, and that studies on the redox role of albumin and mixed disulfide production via thiol-disulfide exchange reactions could offer new therapeutic insights for reducing Hcy toxicity.

New insight into strategies used to develop long-acting G-CSF biologics for neutropenia therapy

Over the last 20 years, granulocyte colony-stimulating factors (G-CSFs) have become the major therapeutic option for the treatment of patients with neutropenia. Most of the current G-CSFs require daily injections, which are inconvenient and expensive for patients. Increased understanding of G-CSFs' structure, expression, and mechanism of clearance has been very instrumental in the development of new generations of long-acting G-CSFs with improved efficacy. Several approaches to reducing G-CSF clearance via conjugation techniques have been investigated. PEGylation, glycosylation, polysialylation, or conjugation with immunoglobulins or albumins have successfully increased G-CSFs' half-lives. Pegfilgrastim (Neulasta) has been successfully approved and marketed for the treatment of patients with neutropenia. The rapidly expanding market for G-CSFs has increased demand for G-CSF biosimilars. Therefore, the importance of this review is to highlight the principle, elimination's route, half-life, clearance, safety, benefits, and limitations of different strategies and techniques used to increase the half-life of biotherapeutic G-CSFs. Understanding these strategies will allow for a new treatment with more competitive manufacturing and lower unit costs compared with that of Neulasta.

Clinical Significance of Serum Albumin and Implications of FcRn Inhibitor Treatment in IgG-Mediated Autoimmune Disorders

Serum albumin (SA), the most abundant soluble protein in the body, maintains plasma oncotic pressure and regulates the distribution of vascular fluid and has a range of other important functions. The goals of this review are to expand clinical knowledge regarding the functions of SA, elucidate effects of dysregulated SA concentration, and discuss the clinical relevance of hypoalbuminemia resulting from various diseases. We discuss potential repercussions of SA dysregulation on cholesterol levels, liver function, and other processes that rely on its homeostasis, as decreased SA concentration has been shown to be associated with increased risk for cardiovascular disease, hyperlipidemia, and mortality. We describe the anti-inflammatory and antioxidant properties of SA, as well as its ability to bind and transport a plethora of endogenous and exogenous molecules. SA is the primary serum protein involved in binding and transport of drugs and as such has the potential to affect, or be affected by, certain medications. Of current relevance are antibody-based inhibitors of the neonatal Fc receptor (FcRn), several of which are under clinical development to treat immunoglobulin G (IgG)-mediated autoimmune disorders; some have been shown to decrease SA concentration. FcRn acts as a homeostatic regulator of SA by rescuing it, as well as IgG, from intracellular degradation via a common cellular recycling mechanism. Greater clinical understanding of the multifunctional nature of SA and the potential clinical impact of decreased SA are needed; in particular, the potential for certain treatments to reduce SA concentration, which may affect efficacy and toxicity of medications and disease progression.

Stabilization of Keratinocyte Monolayer Integrity in the Presence of Anti-Desmoglein-3 Antibodies through FcRn Blockade with Efgartigimod: Novel Treatment Paradigm for Pemphigus?

Pemphigus vulgaris is an autoimmune blistering disease of the epidermis, caused by autoantibodies against desmosomal proteins, mainly desmogleins 1 and 3, which induce an impairment of desmosomal adhesion and blister formation. Recent findings have shown that inhibition of immunoglobulin G binding on the neonatal Fc receptor, FcRn, results in reduced autoantibody recycling and shortens their half-life, providing a valid treatment option for PV. We have here analyzed the role of FcRn in human keratinocytes treated with antibodies isolated from pemphigus vulgaris patient or with recombinant anti-desmoglein-3 antibodies that induce pathogenic changes in desmosomes, such as loss of monolayer integrity, aberrant desmoglein-3 localization and degradation of desmoglein-3. We show that blocking IgG binding on FcRn by efgartigimod, a recombinant Fc fragment undergoing clinical studies for pemphigus, stabilizes the keratinocyte monolayer, whereas the loss of desmoglein-3 is not prevented by efgartigimod. Our data show that FcRn may play a direct role in the pathogenesis of pemphigus at the level of the autoantibody target cells, the epidermal keratinocytes. Our data suggest that in keratinocytes, FcRn may have functions different from its known function in IgG recycling. Therefore, stabilization of keratinocyte adhesion by FcRn blocking entities may provide a novel treatment paradigm for pemphigus.

IgG regulation through FcRn blocking: A novel mechanism for the treatment of myasthenia gravis

The neonatal Fc receptor (FcRn) is an MHC class I-like molecule that is widely distributed in mammalian organs, tissues, and cells. FcRn is critical to maintaining immunoglobulin G (IgG) and albumin levels through rescuing these molecules from lysosomal degradation. IgG autoantibodies are associated with many autoimmune diseases, including myasthenia gravis (MG), a rare neuromuscular autoimmune disease that causes debilitating and, in its generalized form (gMG), potentially life-threatening muscle weakness. IgG autoantibodies are directly pathogenic in MG and target neuromuscular junction proteins, causing neuromuscular transmission failure. Treatment approaches that reduce autoantibody levels, such as therapeutic plasma exchange and intravenous immunoglobulin, have been shown to be effective for gMG patients but are not indicated as ongoing maintenance therapies and can be associated with burdensome side effects. Agents that block FcRn-mediated recycling of IgG represent a rational and promising approach for the treatment of gMG. Blocking FcRn allows targeted reduction of all IgG subtypes without decreasing concentrations of other Ig isotypes; therefore, FcRn blocking could be a safe and effective treatment strategy for a broad population of gMG patients. Several FcRn-blocking antibodies and one antibody Fc fragment have been developed and are currently in various stages of clinical development. This article describes the mechanism of FcRn blockade as a novel approach for IgG-mediated disease therapy and reviews promising clinical data using such FcRn blockers for the treatment of gMG.

Prevention of diabetes-associated fibrosis: Strategies in FcRn-targeted nanosystems for oral drug delivery

Diabetes mellitus is a chronic disease with an elevated risk of micro- and macrovascular complications, such as fibrosis. To prevent diabetes-associated fibrosis, the symptomatology of diabetes must be controlled, which is commonly done by subcutaneous injection of antidiabetic peptides. To minimize the pain and distress associated with such injections, there is an urgent need for non-invasive oral transmucosal drug delivery strategies. However, orally administered peptide-based drugs are exposed to harsh conditions in the gastrointestinal tract and poorly cross the selective intestinal epithelium. Thus, targeting of drugs to receptors expressed in epithelial cells, such as the neonatal Fc receptor (FcRn), may therefore enhance uptake and transport through mucosal barriers. This review compiles how in-depth studies of FcRn biology and engineering of receptor-binding molecules may pave the way for design of new classes of FcRn-targeted nanosystems. Tailored strategies may open new avenues for oral drug delivery and provide better treatment options for diabetes and, consequently, fibrosis prevention.

An engineered human albumin enhances half-life and transmucosal delivery when fused to protein-based biologics

Needle-free uptake across mucosal barriers is a preferred route for delivery of biologics, but the efficiency of unassisted transmucosal transport is poor. To make administration and therapy efficient and convenient, strategies for the delivery of biologics must enhance both transcellular delivery and plasma half-life. We found that human albumin was transcytosed efficiently across polarized human epithelial cells by a mechanism that depends on the neonatal Fc receptor (FcRn). FcRn also transported immunoglobulin G, but twofold less than albumin. We therefore designed a human albumin variant, E505Q/T527M/K573P (QMP), with improved FcRn binding, resulting in enhanced transcellular transport upon intranasal delivery and extended plasma half-life of albumin in transgenic mice expressing human FcRn. When QMP was fused to recombinant activated coagulation factor VII, the half-life of the fusion molecule increased 3.6-fold compared with the wild-type human albumin fusion, without compromising the therapeutic properties of activated factor VII. Our findings highlight QMP as a suitable carrier of protein-based biologics that may enhance plasma half-life and delivery across mucosal barriers.

FcRn as a Transporter for Nasal Delivery of Biologics: A Systematic Review

FcRn plays a major role in regulating immune homeostasis, but it is also able to transport biologics across cellular barriers. The question of whether FcRn could be an efficient transporter of biologics across the nasal epithelial barrier is of particular interest, as it would allow a less invasive strategy for the administration of biologics in comparison to subcutaneous, intramuscular or intravenous administrations, which are often used in clinical practice. A focused systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. It was registered on the international prospective register of systematic reviews PROSPERO, which helped in identifying articles that met the inclusion criteria. Clinical and preclinical studies involving FcRn and the nasal delivery of biologics were screened, and the risk of bias was assessed across studies using the Oral Health Assessment Tool (OHAT). Among the 12 studies finally included in this systematic review (out of the 758 studies screened), 11 demonstrated efficient transcytosis of biologics through the nasal epithelium. Only three studies evaluated the potential toxicity of biologics' intranasal delivery, and they all showed that it was safe. This systematic review confirmed that FcRn is expressed in the nasal airway and the olfactory epithelium, and that FcRn may play a role in IgG and/or IgG-derived molecule-transcytosis across the airway epithelium. However, additional research is needed to better characterize the pharmacokinetic and pharmacodynamic properties of biologics after their intranasal delivery.

Fusion of engineered albumin with factor IX Padua extends half-life and improves coagulant activity

The short half‐life of coagulation factor IX (FIX) for haemophilia B (HB) therapy has been prolonged through fusion with human serum albumin (HSA), which drives the neonatal Fc receptor (FcRn)‐mediated recycling of the chimera. However, patients would greatly benefit from further FIX‐HSA half‐life extension. In the present study, we designed a FIX‐HSA variant through the engineering of both fusion partners. First, we developed a novel cleavable linker combining the two FIX activation sites, which resulted in improved HSA release. Second, insertion of the FIX R338L (Padua) substitution conferred hyperactive features (sevenfold higher specific activity) as for FIX Padua alone. Furthermore, we exploited an engineered HSA (QMP), which conferred enhanced human (h)FcRn binding [dissociation constant (K_D) 0·5 nM] over wild‐type FIX‐HSA (K_D 164·4 nM). In hFcRn transgenic mice, Padua‐QMP displayed a significantly prolonged half‐life (2·7 days, P < 0·0001) versus FIX‐HSA (1 day). Overall, we developed a novel FIX‐HSA protein with improved activity and extended half‐life. These combined properties may result in a prolonged functional profile above the therapeutic threshold, and thus in a potentially widened therapeutic window able to improve HB therapy. This rational engineering of both partners may pave the way for new fusion strategies for the design of engineered biotherapeutics.

The neonatal Fc receptor in mucosal immune regulation

The neonatal Fc receptor (FcRn) was first recognized for its role in transfer of maternal IgG to the foetus or newborn, providing passive immunity early in life. However, it has become clear that the receptor is versatile, widely expressed and plays an indispensable role in both immunological and non-immunological processes throughout life. The receptor rescues immunoglobulin G (IgG) and albumin from intracellular degradation and shuttles the ligands across polarized cell barriers, in all cases via a pH-dependent binding-and-release mechanism. These processes secure distribution and high levels of both IgG and albumin throughout the body. At mucosal sites, FcRn transports IgG across polarized epithelial cells where it retrieves IgG in complex with luminal antigens that is delivered to tissue-localized immune cells. In dendritic cells (DCs), FcRn orchestrates processing of IgG-opsonized immune complexes (ICs) in concert with classical Fcγ receptors, which results in antigen presentation and cross-presentation of antigenic peptides on MHC class II and I to CD4+ and CD8+ T cells, respectively. Hence, FcRn regulates transport of the ligands within and across different types of cells, but also processing of IgG-ICs by immune cells. As such, the receptor is involved in immune surveillance and protection against infections. In this brief review, we highlight how FcRn expressed by hematopoietic and non-hematopoietic cells contributes to immune regulation at mucosal barriers-biology that can be utilized in development of biologics and subunit vaccines for non-invasive delivery.

Albumin fusion at the N-terminus or C-terminus of human lactoferrin leads to improved pharmacokinetics and anti-proliferative effects on cancer cell lines

Human lactoferrin (hLF), a soluble factor of the innate immune system, exhibits various biological functions and therefore has potential as a therapeutic protein. However, the clinical applications of hLF are limited by its low stability in blood. We therefore attempted to resolve this by producing recombinant hLF fused to human serum albumin (HSA). Two HSA-fused hLFs with different fusion orientations (hLF-HSA and HSA-hLF) were produced in Chinese hamster ovary (CHO) DG44 cells. hLF-HSA revealed higher thermal stability, resistance to peptic degradation, and stability during the process of cellular uptake and release in an intestinal enterocyte model (Caco-2 cells) than HSA-hLF. The lower stability of HSA-hLF is presumably due to the steric hindrance imposed by HSA fusion to the N-terminus of hLF. Both HSA fusion proteins, especially HSA-hLF, displayed improved pharmacokinetic properties despite the lower protein stability of HSA-hLF. hLF-HSA and HSA-hLF exhibited approximately 3.3- and 20.7-fold longer half-lives (64.0 and 403.6 min), respectively, than holo-rhLF (19.5 min). Both HSA fusion proteins were found to exert enhanced growth inhibition effects on cancer cells in vitro, but not normal cells. Their enhanced growth inhibitory activities were considered to be due to the synergetic effects of hLF and HSA because hLF alone or HSA alone failed to exert such an effect. Altogether, Fusion of HSA to hLF yielded superior pharmacokinetics and anti-proliferative activities against cancer cells. HSA-fused hLF is a novel candidate for further application of hLF as biopharmaceuticals for intravenous administration.

Human serum albumin presents isoform variants with altered neonatal Fc receptor interactions

Human serum albumin (HSA) is the most abundant protein in plasma and presents the particularity, with IgG, to have an extraordinary long serum half-life conferred by its interaction with the neonatal Fc receptor (FcRn). If the impact of IgG post-translational modifications (PTMs) on FcRn binding is well documented, it is far less reported for HSA despite numerous PTMs occurring on the protein in plasma. HSA is susceptible to numerous degradation reactions in plasma, because of aging, oxidative stress or liver and pancreas related pathologies. In the present study, we combined FcRn affinity chromatography and mass spectrometry to investigate the impact of HSA PTMs upon FcRn binding. This methodology presents the advantage to distinguish the effect of a single modification from a plasma HSA preparation made of a mixture of different isoforms. Cys₃₄ oxidation, Lys₅₂₅ glycation, and Leu₅₈₅ C-terminal truncation, which are modifications related to several pathological conditions, were demonstrated to act negatively on HSA-FcRn interaction. The HSA-FcRn binding alteration generated by these modifications is consistent with their vicinity with the interaction interface of the two proteins. Results were discussed regarding altered half-life of HSA observed in several disease states and pave the way toward new understandings of the hypoalbuminemia pathogenesis. SIGNIFICANCE STATEMENT: In this study, we investigated the impact of several post-translational modifications of HSA toward its ability to bind to the neonatal Fc receptor using in vitro affinity chromatography, mass spectrometry, and surface plasmon resonance. Cys34 oxidation, Lys525 glycation, and Leu585 C-terminal truncation were demonstrated to decrease HSA-FcRn binding. These modifications occurring in circulating HSA were discussed in relation to several pathologies as well as for the use of HSA as a therapeutic protein.

An intact C-terminal end of albumin is required for its long half-life in humans

Albumin has an average plasma half-life of three weeks and is thus an attractive carrier to improve the pharmacokinetics of fused therapeutics. The half-life is regulated by FcRn, a cellular receptor that protects against intracellular degradation. To tailor-design the therapeutic use of albumin, it is crucial to understand how structural alterations in albumin affect FcRn binding and transport properties. In the blood, the last C-terminal residue (L585) of albumin may be enzymatically cleaved. Here we demonstrate that removal of the L585 residue causes structural stabilization in regions of the principal FcRn binding domain and reduces receptor binding. In line with this, a short half-life of only 3.5 days was measured for cleaved albumin lacking L585 in a patient with acute pancreatitis. Thus, we reveal the structural requirement of an intact C-terminal end of albumin for a long plasma half-life, which has implications for design of albumin-based therapeutics.

Nilsen et al. show that structural alterations in the last C-terminal α-helix of albumin strongly reduce its binding to the neonatal Fc receptor, decreasing the half-life of albumin in humans. This study suggests the structural requirement of the C-terminal of albumin for its long plasma half-life, providing insights into the design of albumin used to carry drugs.

An overview of albumin and alpha-1-acid glycoprotein main characteristics: highlighting the roles of amino acids in binding kinetics and molecular interactions

Although Albumin (ALB) and alpha-1-acid glycoprotein (AGP) have distinctive structural and functional characteristics, they both play a key role in binding a large variety of endogenous and exogenous ligands. An extensive binding to these plasma proteins could have a potential impact on drugs disposition (e.g. bioavailability, distribution and clearance), on their innocuity and their efficacy. This review summarizes the common knowledge about the structural and molecular characteristics of both ALB and AGP in humans, and about the most involved amino acids in their high-affinity binding pockets. However, the variability in residues found in binding pockets, for the same species, allows each plasma protein to interact differently with the ligands. The protein-ligand interaction influences differently the disposition of drugs that bind to either of these plasma proteins. The content of this review is useful for the design of new drug entities with high-binding characteristics, in qualitative and quantitative modelling (e.g. in vitro-in vivo extrapolations, 3D molecular docking, interspecies extrapolations), and for other interdisciplinary research.

Mechanisms Influencing the Pharmacokinetics and Disposition of Monoclonal Antibodies and Peptides

Monoclonal antibodies (mAbs) and peptides are an important class of therapeutic modalities that have brought improved health outcomes in areas with limited therapeutic optionality. Presently, there more than 90 mAb and peptide therapeutics on the United States market, with over 600 more in various clinical stages of development in a broad array of therapeutic areas, including diabetes, autoimmune disorders, oncology, neuroscience, and cardiovascular and infectious diseases. Notwithstanding this potential, there is high clinical rate of attrition, with approximately 10% reaching patients. A major contributor to the failure of the molecules is often times an incomplete or poor understanding of the pharmacokinetics (PK) and disposition profiles leading to limited or diminished efficacy. Increased and thorough characterization efforts directed at disseminating mechanisms influencing the PK and disposition of mAbs and peptides can aid in improving the design for their intended pharmacological activity, and thereby their clinical success. The PK and disposition factors for mAbs and peptides are broadly influenced by target-mediated drug disposition and nontarget-related clearance mechanisms related to the interplay between the relationship of the structure and physiochemical properties of mAbs and peptides with physiologic processes. This review focuses on nontarget-related factors influencing the disposition and PK of mAbs and peptides. Contemporary considerations around the increasing in silico approaches to identify nontarget-related molecule limitations and enhancing the druggability of mAbs and peptides, including parenteral and nonparenteral delivery strategies that are geared toward improving patient experience and compliance, are also discussed.

The Neonatal Fc Receptor (FcRn): A Misnomer?

Antibodies are essential components of an adaptive immune response. Immunoglobulin G (IgG) is the most common type of antibody found in circulation and extracellular fluids. Although IgG alone can directly protect the body from infection through the activities of its antigen binding region, the majority of IgG immune functions are mediated via proteins and receptors expressed by specialized cell subsets that bind to the fragment crystallizable (Fc) region of IgG. Fc gamma (γ) receptors (FcγR) belong to a broad family of proteins that presently include classical membrane-bound surface receptors as well as atypical intracellular receptors and cytoplasmic glycoproteins. Among the atypical FcγRs, the neonatal Fc receptor (FcRn) has increasingly gained notoriety given its intimate influence on IgG biology and its ability to also bind to albumin. FcRn functions as a recycling or transcytosis receptor that is responsible for maintaining IgG and albumin in the circulation, and bidirectionally transporting these two ligands across polarized cellular barriers. More recently, it has been appreciated that FcRn acts as an immune receptor by interacting with and facilitating antigen presentation of peptides derived from IgG immune complexes (IC). Here we review FcRn biology and focus on newer advances including how emerging FcRn-targeted therapies may affect the immune responses to IgG and IgG IC.

Human and mouse albumin bind their respective neonatal Fc receptors differently

Albumin has a serum half-life of three weeks in humans and is utilized to extend the serum persistence of drugs that are genetically fused or conjugated directly to albumin or albumin-binding molecules. Responsible for the long half-life is FcRn that protects albumin from intracellular degradation. An in-depth understanding of how FcRn binds albumin across species is of importance for design and evaluation of albumin-based therapeutics. Albumin consists of three homologous domains where domain I and domain III of human albumin are crucial for binding to human FcRn. Here, we show that swapping of two loops in domain I or the whole domain with the corresponding sequence in mouse albumin results in reduced binding to human FcRn. In contrast, humanizing domain I of mouse albumin improves binding. We reveal that domain I of mouse albumin plays a minor role in the interaction with the mouse and human receptors, as domain III on its own binds with similar affinity as full-length mouse albumin. Further, we show that P573 in domain III of mouse albumin is required for strong receptor binding. Our study highlights distinct differences in structural requirements for the interactions between mouse and human albumin with their respective receptor, which should be taken into consideration in design of albumin-based drugs and evaluation in mouse models.

Loss of expression of the recycling receptor, FcRn, promotes tumor cell growth by increasing albumin consumption

Tumor cells rely on high concentrations of amino acids to support their growth and proliferation. Although increased macropinocytic uptake and lysosomal degradation of the most abundant serum protein, albumin, in Ras-transformed cells can meet these demands, it is not understood how the majority of tumor cells that express wild type Ras achieve this. In the current study we reveal that the neonatal Fc receptor, FcRn, regulates tumor cell proliferation through the ability to recycle its ligand, albumin. By contrast with normal epithelial cells, we show that human FcRn is present at very low or undetectable levels in the majority of tumor cell lines analyzed. Remarkably, shRNA-mediated ablation of FcRn expression in an FcRn-positive tumor cell line results in a substantial growth increase of tumor xenografts, whereas enforced expression of this receptor by lentiviral transduction has the reverse effect. Moreover, intracellular albumin and glutamate levels are increased by the loss of FcRn-mediated recycling of albumin, combined with hypoalbuminemia in tumor-bearing mice. These studies identify a novel role for FcRn as a suppressor of tumor growth and have implications for the use of this receptor as a prognostic indicator and therapeutic target.

A human endothelial cell-based recycling assay for screening of FcRn targeted molecules

Albumin and IgG have remarkably long serum half-lives due to pH-dependent FcRn-mediated cellular recycling that rescues both ligands from intracellular degradation. Furthermore, increase in half-lives of IgG and albumin-based therapeutics has the potential to improve their efficacies, but there is a great need for robust methods for screening of relative FcRn-dependent recycling ability. Here, we report on a novel human endothelial cell-based recycling assay (HERA) that can be used for such pre-clinical screening. In HERA, rescue from degradation depends on FcRn, and engineered ligands are recycled in a manner that correlates with their half-lives in human FcRn transgenic mice. Thus, HERA is a novel cellular assay that can be used to predict how FcRn-binding proteins are rescued from intracellular degradation.

Chemoselective Installation of Amine Bonds on Proteins through Aza-Michael Ligation

Chemical modification of proteins is essential for a variety of important diagnostic and therapeutic applications. Many strategies developed to date lack chemo- and regioselectivity as well as result in non-native linkages that may suffer from instability in vivo and adversely affect the protein's structure and function. We describe here the reaction of N-nucleophiles with the amino acid dehydroalanine (Dha) in a protein context. When Dha is chemically installed in proteins, the addition of a wide-range N-nucleophiles enables the rapid formation of amine linkages (secondary and tertiary) in a chemoselective manner under mild, biocompatible conditions. These new linkages are stable at a wide range of pH values (pH 2.8 to 12.8), under reducing conditions (biological thiols such as glutathione) and in human plasma. This method is demonstrated for three proteins and is shown to be fully compatible with disulfide bridges, as evidenced by the selective modification of recombinant albumin that displays 17 structurally relevant disulfides. The practicability and utility of our approach is further demonstrated by the construction of a chemically modified C2A domain of Synaptotagmin-I protein that retains its ability to preferentially bind to apoptotic cells at a level comparable to the native protein. Importantly, the method was useful for building a homogeneous antibody-drug conjugate with a precise drug-to-antibody ratio of 2. The kinase inhibitor crizotinib was directly conjugated to Dha through its piperidine motif, and its antibody-mediated intracellular delivery results in 10-fold improvement of its cancer cell-killing efficacy. The simplicity and exquisite site-selectivity of the aza-Michael ligation described herein allows the construction of stable secondary and tertiary amine-linked protein conjugates without affecting the structure and function of biologically relevant proteins.

Characterization of the interactions of rabbit neonatal Fc receptor (FcRn) with rabbit and human IgG isotypes

Despite the increasing importance of rabbit as an animal model in pharmacological studies like investigating placental transfer of therapeutic IgGs, little is known about the molecular interaction of the rabbit neonatal Fc receptor (FcRn) with rabbit and human IgG molecules. We analyzed the interactions of the rabbit and human FcRn with rabbit and human IgG isotypes using surface plasmon resonance assay. Similar to FcRn of other species, rabbit FcRn functions in pH-dependent manner, as it binds IgGs at pH 6.0, but no binding occurs at pH 7.4. We also showed that rabbit FcRn binds rabbit IgG and human IgG1 with nearly identical affinity, whereas it has stronger interactions with the other human IgG isotypes. The similar affinity of rabbit IgG and human IgG1 for rabbit FcRn was confirmed by in vitro FcRn-mediated recycling assay. These data verify that rabbit is an appropriate animal model for analyzing the pharmacokinetics of human therapeutic monoclonal antibodies.

Glucagon-like Peptide 1 Conjugated to Recombinant Human Serum Albumin Variants with Modified Neonatal Fc Receptor Binding Properties. Impact on Molecular Structure and Half-Life

Glucagon-like peptide 1 (GLP-1) is a small incretin hormone stimulated by food intake, resulting in an amplification of the insulin response. Though GLP-1 is interesting as a drug candidate for the treatment of type 2 diabetes mellitus, its short plasma half-life of <3 min limits its clinical use. A strategy for extending the half-life of GLP-1 utilizes the long half-life of human serum albumin (HSA) by combining the two via chemical conjugation or genetic fusion. HSA has a plasma half-life of around 21 days because of its interaction with the neonatal Fc receptor (FcRn) expressed in endothelial cells of blood vessels, which rescues circulating HSA from lysosomal degradation. We have conjugated GLP-1 to C34 of native sequence recombinant HSA (rHSA) and two rHSA variants, one with increased and one with decreased binding affinity for human FcRn. We have investigated the impact of conjugation on FcRn binding affinities, GLP-1 potency, and pharmacokinetics, combined with the solution structure of the rHSA variants and GLP-1-albumin conjugates. The solution structures, determined by small-angle X-ray scattering, show the GLP-1 pointing away from the surface of rHSA. Combining the solution structures with the available structural information about the FcRn and GLP-1 receptor obtained from X-ray crystallography, we can explain the observed in vitro and in vivo behavior. We conclude that the conjugation of GLP-1 to rHSA does not affect the interaction between rHSA and FcRn, while the observed decrease in the potency of GLP-1 can be explained by a steric hindrance of binding of GLP-1 to its receptor.

Direct demonstration of a neonatal Fc receptor (FcRn)-driven endosomal sorting pathway for cellular recycling of albumin

Albumin is the most abundant plasma protein involved in the transport of many compounds, such as fatty acids, bilirubin, and heme. The endothelial cellular neonatal Fc receptor (FcRn) has been suggested to play a central role in maintaining high albumin plasma levels through a cellular recycling pathway. However, direct mapping of this process is still lacking. This work presents the use of wild-type and engineered recombinant albumins with either decreased or increased FcRn affinity in combination with a low or high FcRn-expressing endothelium cell line to clearly define the FcRn involvement, intracellular pathway, and kinetics of albumin trafficking by flow cytometry, quantitative confocal microscopy, and an albumin-recycling assay. We found that cellular albumin internalization was proportional to FcRn expression and albumin-binding affinity. Albumin accumulation in early endosomes was independent of FcRn-binding affinity, but differences in FcRn-binding affinities significantly affected the albumin distribution between late endosomes and lysosomes. Unlike albumin with low FcRn-binding affinity, albumin with high FcRn-binding affinity was directed less to the lysosomes, suggestive of FcRn-directed albumin salvage from lysosomal degradation. Furthermore, the amount of recycled albumin in cell culture media corresponded to FcRn-binding affinity, with a ∼3.3-fold increase after 1 h for the high FcRn-binding albumin variant compared with wild-type albumin. Together, these findings uncover an FcRn-dependent endosomal cellular-sorting pathway that has great importance in describing fundamental mechanisms of intracellular albumin recycling and the possibility to tune albumin-based therapeutic effects by FcRn-binding affinity.

Site-selective conjugation of an anticoagulant aptamer to recombinant albumins and maintenance of neonatal Fc receptor binding

Aptamers are an attractive molecular medicine that offers high target specificity. Nucleic acid-based aptamers, however, are prone to nuclease degradation and rapid renal excretion that require blood circulatory half-life extension enabling technologies. The long circulatory half-life, predominately facilitated by engagement with the cellular recycling neonatal Fc receptor (FcRn), and ligand transport properties of albumin promote it as an attractive candidate to improve the pharmacokinetic profile of aptamers. This study investigates the effect of Cys34 site-selective covalent attachment of a factor IXa anticoagulant aptamer on aptamer functionality and human FcRn (hFcRn) engagement using recombinant human albumin (rHA) of either a wild type (WT) or an engineered human FcRn high binding variant (HB). Albumin-aptamer conjugates, connected covalently through a heterobifunctional succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate linker, were successfully prepared and purified by high performance liquid chromatography as confirmed by gel electrophoresis band-shift analysis and matrix-assisted laser desorption/ionization time of flight. Minimal reduction (∼25%) in activity of WT-linked aptamer to that of aptamer alone was found using an anticoagulant activity assay measuring temporal levels of activated partial thrombin. Covalent albumin-aptamer conjugation, however, substantially compromized binding to hFcRn, to 10% affinity of that of non-conjugated WT, determined by biolayer interferometry. Binding could be rescued by aptamer conjugation to recombinant albumin engineered for higher FcRn affinity (HB) that exhibited an 8-fold affinity compared to WT alone. This work describes a novel albumin-based aptamer delivery system whose hFcRn binding can be increased using a HB engineered albumin.

Hepatic FcRn regulates albumin homeostasis and susceptibility to liver injury

The neonatal crystallizable fragment receptor (FcRn) is responsible for maintaining the long half-life and high levels of the two most abundant circulating proteins, albumin and IgG. In the latter case, the protective mechanism derives from FcRn binding to IgG in the weakly acidic environment contained within endosomes of hematopoietic and parenchymal cells, whereupon IgG is diverted from degradation in lysosomes and is recycled. The cellular location and mechanism by which FcRn protects albumin are partially understood. Here we demonstrate that mice with global or liver-specific FcRn deletion exhibit hypoalbuminemia, albumin loss into the bile, and increased albumin levels in the hepatocyte. In vitro models with polarized cells illustrate that FcRn mediates basal recycling and bidirectional transcytosis of albumin and uniquely determines the physiologic release of newly synthesized albumin into the basal milieu. These properties allow hepatic FcRn to mediate albumin delivery and maintenance in the circulation, but they also enhance sensitivity to the albumin-bound hepatotoxin, acetaminophen (APAP). As such, global or liver-specific deletion of FcRn results in resistance to APAP-induced liver injury through increased albumin loss into the bile and increased intracellular albumin scavenging of reactive oxygen species. Further, protection from injury is achieved by pharmacologic blockade of FcRn-albumin interactions with monoclonal antibodies or peptide mimetics, which cause hypoalbuminemia, biliary loss of albumin, and increased intracellular accumulation of albumin in the hepatocyte. Together, these studies demonstrate that the main function of hepatic FcRn is to direct albumin into the circulation, thereby also increasing hepatocyte sensitivity to toxicity.

The multiple facets of FcRn in immunity

The neonatal Fc receptor, FcRn, is best known for its role in transporting IgG in various tissues, providing newborns with humoral immunity, and for prolonging the half-life of IgG. Recent findings implicate the involvement of FcRn in a far wider range of biological and immunological processes, as FcRn has been found to bind and extend the half-life of albumin; to be involved in IgG transport and antigen sampling at mucosal surfaces; and to be crucial for efficient IgG-mediated phagocytosis. Herein, the function of FcRn will be reviewed, with emphasis on its recently documented significance for IgG polymorphisms affecting the half-life and biodistribution of IgG3, on its role in phagocyte biology, and the subsequent role for the presentation of antigens to lymphocytes.

Expression of soluble human Neonatal Fc-receptor (FcRn) in Escherichia coli through modification of growth environment

Neonatal Fc-receptor (FcRn) with its affinity to immunoglobulin G (IgG) has been the subject of many pharmacokinetic studies in the past century. This protein is well known for its unique feature in maintaining the circulating IgG from degradation in blood plasma. FcRn is formed by non-covalent association between the α-chain with the β-2-microglobulin (β2m). Many studies have been conducted to produce FcRn in the laboratory, mainly using mammalian tissue culture as host for recombinant protein expression. In this study, we demonstrate a novel strategy to express the α-chain of FcRn using Escherichia coli as the expression host. The expression vector that carries the cDNA of the α-chain was transformed into expression host, Rosetta-gami 2 strain for inducible expression. The bacterial culture was grown in a modified growth medium which constitutes of terrific broth, sodium chloride (NaCl), glucose and betaine. A brief heat shock at 45 °C was carried out after induction, before the temperature for expression was reduced to 22 °C and grown for 16 h. The soluble form of the α-chain of FcRn expressed was tested in the ELISA and dot blot immunoassay to confirm its native functionality. The results implied that the α-chain of FcRn expressed using this method is functional and retains its pH-dependent affinity to IgG. Our study significantly suggests that the activity of human FcRn remain active and functional in the absence of β2m.

Mechanism of increased clearance of glycated albumin by proximal tubule cells

Serum albumin is the most abundant plasma protein and has a long half-life due to neonatal Fc receptor (FcRn)-mediated transcytosis by many cell types, including proximal tubule cells of the kidney. Albumin also interacts with, and is modified by, many small and large molecules. Therefore, the focus of the present study was to address the impact of specific known biological albumin modifications on albumin-FcRn binding and cellular handling. Binding at pH 6.0 and 7.4 was performed since FcRn binds albumin strongly at acidic pH and releases it after transcytosis at physiological pH. Equilibrium dissociation constants were measured using microscale thermophoresis. Since studies have shown that glycated albumin is excreted in the urine at a higher rate than unmodified albumin, we studied glucose and methylgloxal modified albumins (21 days). All had reduced affinity to FcRn at pH 6.0, suggesting these albumins would not be returned to the circulation via the transcytotic pathway. To address why modified albumin has reduced affinity, we analyzed the structure of the modified albumins using small-angle X-ray scattering. This analysis showed significant structural changes occurring to albumin with glycation, particularly in the FcRn-binding region, which could explain the reduced affinity to FcRn. These results offer an explanation for enhanced proximal tubule-mediated sorting and clearance of abnormal albumins.

A comprehensive review of the neonatal Fc receptor and its application in drug delivery

Advances in the understanding of neonatal Fc receptor (FcRn) biology and function have demonstrated that this receptor, primarily identified for the transfer of passive immunity from mother infant, is involved in several biological and immunological processes. In fact, FcRn is responsible for the long half-life of IgG and albumin in the serum, by creating an intracellular protein reservoir, which is protected from lysosomal degradation and, importantly, trafficked across the cell. Such discovery has led researchers to hypothesize the role for this unique receptor in the controlled delivery of therapeutic agents. A great amount of FcRn-based strategies are already under extensive investigation, in which FcRn reveals to have profound impact on the biodistribution and half-life extension of therapeutic agents. This review summarizes the main findings on FcRn biology, function and distribution throughout different tissues, together with the main advances on the FcRn-based therapeutic opportunities and model systems, which indicate that this receptor is a potential target for therapeutic regimen modification.

Albumin-based drug delivery: harnessing nature to cure disease

The effectiveness of a drug is dependent on accumulation at the site of action at therapeutic levels, however, challenges such as rapid renal clearance, degradation or non-specific accumulation requires drug delivery enabling technologies. Albumin is a natural transport protein with multiple ligand binding sites, cellular receptor engagement, and a long circulatory half-life due to interaction with the recycling neonatal Fc receptor. Exploitation of these properties promotes albumin as an attractive candidate for half-life extension and targeted intracellular delivery of drugs attached by covalent conjugation, genetic fusions, association or ligand-mediated association. This review will give an overview of albumin-based products with focus on the natural biological properties and molecular interactions that can be harnessed for the design of a next-generation drug delivery platform.

Albumin-based drug delivery: harnessing nature to cure disease

The effectiveness of a drug is dependent on accumulation at the site of action at therapeutic levels, however, challenges such as rapid renal clearance, degradation or non-specific accumulation requires drug delivery enabling technologies. Albumin is a natural transport protein with multiple ligand binding sites, cellular receptor engagement, and a long circulatory half-life due to interaction with the recycling neonatal Fc receptor. Exploitation of these properties promotes albumin as an attractive candidate for half-life extension and targeted intracellular delivery of drugs attached by covalent conjugation, genetic fusions, association or ligand-mediated association. This review will give an overview of albumin-based products with focus on the natural biological properties and molecular interactions that can be harnessed for the design of a next-generation drug delivery platform.

Proximal Tubules Have the Capacity to Regulate Uptake of Albumin

Evidence from multiple studies supports the concept that both glomerular filtration and proximal tubule (PT) reclamation affect urinary albumin excretion rate. To better understand these roles of glomerular filtration and PT uptake, we investigated these processes in two distinct animal models. In a rat model of acute exogenous albumin overload, we quantified glomerular sieving coefficients (GSC) and PT uptake of Texas Red-labeled rat serum albumin using two-photon intravital microscopy. No change in GSC was observed, but a significant decrease in PT albumin uptake was quantified. In a second model, loss of endogenous albumin was induced in rats by podocyte-specific transgenic expression of diphtheria toxin receptor. In these albumin-deficient rats, exposure to diphtheria toxin induced an increase in albumin GSC and albumin filtration, resulting in increased exposure of the PTs to endogenous albumin. In this case, PT albumin reabsorption was markedly increased. Analysis of known albumin receptors and assessment of cortical protein expression in the albumin overload model, conducted to identify potential proteins and pathways affected by acute protein overload, revealed changes in the expression levels of calreticulin, disabled homolog 2, NRF2, angiopoietin-2, and proteins involved in ATP synthesis. Taken together, these results suggest that a regulated PT cell albumin uptake system can respond rapidly to different physiologic conditions to minimize alterations in serum albumin level.

The role of albumin receptors in regulation of albumin homeostasis: Implications for drug delivery

Albumin is the most abundant protein in blood and acts as a molecular taxi for a plethora of small insoluble substances such as nutrients, hormones, metals and toxins. In addition, it binds a range of medical drugs. It has an unusually long serum half-life of almost 3weeks, and although the structure and function of albumin has been studied for decades, a biological explanation for the long half-life has been lacking. Now, recent research has unravelled that albumin-binding cellular receptors play key roles in the homeostatic regulation of albumin. Here, we review our current understanding of albumin homeostasis with a particular focus on the impact of the cellular receptors, namely the neonatal Fc receptor (FcRn) and the cubilin-megalin complex, and we discuss their importance on uses of albumin in drug delivery.

Fully human monoclonal antibody inhibitors of the neonatal fc receptor reduce circulating IgG in non-human primates

The therapeutic management of antibody-mediated autoimmune disease typically involves immunosuppressant and immunomodulatory strategies. However, perturbing the fundamental role of the neonatal Fc receptor (FcRn) in salvaging IgG from lysosomal degradation provides a novel approach - depleting the body of pathogenic immunoglobulin by preventing IgG binding to FcRn and thereby increasing the rate of IgG catabolism. Herein, we describe the discovery and preclinical evaluation of fully human monoclonal IgG antibody inhibitors of FcRn. Using phage display, we identified several potent inhibitors of human-FcRn in which binding to FcRn is pH-independent, with over 1000-fold higher affinity for human-FcRn than human IgG-Fc at pH 7.4. FcRn antagonism in vivo using a human-FcRn knock-in transgenic mouse model caused enhanced catabolism of exogenously administered human IgG. In non-human primates, we observed reductions in endogenous circulating IgG of >60% with no changes in albumin, IgM, or IgA. FcRn antagonism did not disrupt the ability of non-human primates to mount IgM/IgG primary and secondary immune responses. Interestingly, the therapeutic anti-FcRn antibodies had a short serum half-life but caused a prolonged reduction in IgG levels. This may be explained by the high affinity of the antibodies to FcRn at both acidic and neutral pH. These results provide important preclinical proof of concept data in support of FcRn antagonism as a novel approach to the treatment of antibody-mediated autoimmune diseases.

Targeting FcRn for the modulation of antibody dynamics

The MHC class I-related receptor, FcRn, is a multitasking protein that transports its IgG ligand within and across cells of diverse origins. The role of this receptor as a global regulator of IgG homeostasis and transport, combined with knowledge of the molecular details of FcRn-IgG interactions, has led to opportunities to modulate the in vivo dynamics of antibodies and their antigens through protein engineering. Consequently, the generation of half-life extended antibodies has shown a rapid expansion over the past decade. Further, FcRn itself can be targeted by inhibitors to induce decreased levels of circulating IgGs, which could have applications in multiple clinical settings. The engineering of antibody-antigen interactions to reduce antibody-mediated buffering of soluble ligand has also developed into an active area of investigation, leading to novel antibody platforms designed to result in more effective antigen clearance. Similarly, the target-mediated elimination of antibodies by internalizing, membrane bound antigens (receptors) can be decreased using novel engineering approaches. These strategies, combined with subcellular trafficking analyses of antibody/antigen/FcRn behavior in cells to predict in vivo behavior, have considerable promise for the production of next generation therapeutics and diagnostics.

Unraveling the Interaction between FcRn and Albumin: Opportunities for Design of Albumin-Based Therapeutics

The neonatal Fc receptor (FcRn) was first found to be responsible for transporting antibodies of the immunoglobulin G (IgG) class from the mother to the fetus or neonate as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin, as FcRn is expressed in many different cell types and organs at diverse body sites. Thus, to gain a complete understanding of the biological function of each ligand, and also their distribution in the body, an in-depth characterization of how FcRn binds and regulates the transport of both ligands is necessary. Importantly, such knowledge is also relevant when developing new drugs, as IgG and albumin are increasingly utilized in therapy. This review discusses our current structural and biological understanding of the relationship between FcRn and its ligands, with a particular focus on albumin and design of albumin-based therapeutics.

Bovine FcRn-mediated human immunoglobulin G transfer across the milk-blood barrier in transgenic mice

Maternal-fetal IgGs transport occurs either prenatally or postnatally, which confers the newborns with passive immunity before their own immune system has matured. However, little is known about the mechanisms of postnatal IgGs passage in the mammary gland. To investigate how FcRn mediates the IgGs transport in the mammary gland, we first generated bFcRn and anti-HAV mAb transgenic mice, and then obtained HF transgenic mice expressing both transgenes by mating the above two strains. Transgene expression of bFcRn in the four lines was determined by qRT-PCR and western blot. We then localized the expression of bFcRn to the acinar epithelial cells in the mammary gland, and anti-HAV mAb was mainly detected in the acini with weak staining in the acinar epithelial cells. Human IgGs could be detected in both milk and serum of HF transgenic mice by western blot and ELISA. A significantly lower milk to serum ratio of human IgGs in HF mice compared with that of anti-HAV mAb mice, indicating that bFcRn could transport human IgGs across the milk-blood barrier from milk to serum during lactation in HF mice. While, there were no transport of murine IgGs, IgAs, or IgMs. These results provide understandings about the mechanisms of maternal-fetal immunity transfer in the mammary gland.

An engineered affibody molecule with pH-dependent binding to FcRn mediates extended circulatory half-life of a fusion protein

Proteins endocytosed from serum are degraded in the lysosomes. However, serum albumin (SA) and IgG, through its Fc part, bind to the neonatal Fc receptor (FcRn) at low pH in the endosome after endocytosis, and are transported back to the cellular surface, where they are released into the bloodstream, resulting in an extended serum circulation time. Association with Fc or SA has been used to prolong the in vivo half-life of biopharmaceuticals, using the interaction with FcRn to improve treatment regimens. This has been achieved either directly, by fusion or conjugation to Fc or SA, or indirectly, using SA-binding proteins. The present work takes this principle one step further, presenting small affinity proteins that bind directly to FcRn, mediating extension of the serum half-life of fused biomolecules. Phage display technology was used to select affibody molecules that can bind to FcRn in the pH-dependent manner required for rescue by FcRn. The biophysical and binding properties were characterized in vitro, and the affibody molecules were found to bind to FcRn more strongly at low pH than at neutral pH. Attachment of the affibody molecules to a recombinant protein, already engineered for increased half-life, resulted in a nearly threefold longer half-life in mice. These tags should have general use as fusion partners to biopharmaceuticals to extend their half-lives in vivo.

Interaction with both domain I and III of albumin is required for optimal pH-dependent binding to the neonatal Fc receptor (FcRn)

Albumin is an abundant blood protein that acts as a transporter of a plethora of small molecules like fatty acids, hormones, toxins, and drugs. In addition, it has an unusual long serum half-life in humans of nearly 3 weeks, which is attributed to its interaction with the neonatal Fc receptor (FcRn). FcRn protects albumin from intracellular degradation via a pH-dependent cellular recycling mechanism. To understand how FcRn impacts the role of albumin as a distributor, it is of importance to unravel the structural mechanism that determines pH-dependent binding. Here, we show that although the C-terminal domain III (DIII) of human serum albumin (HSA) contains the principal binding site, the N-terminal domain I (DI) is important for optimal FcRn binding. Specifically, structural inspection of human FcRn (hFcRn) in complex with HSA revealed that two exposed loops of DI were in proximity with the receptor. To investigate to what extent these contacts affected hFcRn binding, we targeted selected amino acid residues of the loops by mutagenesis. Screening by in vitro interaction assays revealed that several of the engineered HSA variants showed decreased binding to hFcRn, which was also the case for two missense variants with mutations within these loops. In addition, four of the variants showed improved binding. Our findings demonstrate that both DI and DIII are required for optimal binding to FcRn, which has implications for our understanding of the FcRn-albumin relationship and how albumin acts as a distributor. Such knowledge may inspire development of novel HSA-based diagnostics and therapeutics.