Functional reconstitution of human FcRn in Madin-Darby canine kidney cells requires co-expressed human beta 2-microglobulin

The neonatal Fc receptor (FcRn) is required for porcine reproductive and respiratory syndrome virus uncoating

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to cause substantial economic losses to the pig industry worldwide. Previous studies from other groups showed that CD163 is required for PRRSV uncoating and genome release. However, CD163 does not interact with nucleocapsid (N) protein. In this study, the neonatal Fc receptor (FcRn) was demonstrated to be irreplaceable for PRRSV infection by knockdown, overexpression, antibodies or IgG blocking, knockout, and replenishment assays. FcRn was further revealed to be involved in PRRSV uncoating for the first time rather than viral attachment and internalization. In detail, FcRn was determined to colocalize with CD163 and PRRSV virions in early endosomes and specially interact with N protein in early endosomes. Taken together, these results provide evidence that FcRn is an essential cellular factor for PRRSV uncoating, which will be a promising target to interfere with the viral infection.IMPORTANCEPRRSV infection results in a severe swine disease affecting pig farming in the world. Although CD163 has been implicated as the uncoating receptor for PRRSV but the uncoating mechanism of PRRSV remains unclear. Here, we identified that FcRn facilitated virion uncoating via interaction with viral N protein in early endosomes. Our work actually expands the knowledge of PRRSV infection and provides an attractive therapeutic target for the prevention and control of PRRS.

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.

Endoplasmic reticulum stress in the intestinal epithelium initiates purine metabolite synthesis and promotes Th17 cell differentiation in the gut

Intestinal IL-17-producing T helper (Th17) cells are dependent on adherent microbes in the gut for their development. However, how microbial adherence to intestinal epithelial cells (IECs) promotes Th17 cell differentiation remains enigmatic. Here, we found that Th17 cell-inducing gut bacteria generated an unfolded protein response (UPR) in IECs. Furthermore, subtilase cytotoxin expression or genetic removal of X-box binding protein 1 (Xbp1) in IECs caused a UPR and increased Th17 cells, even in antibiotic-treated or germ-free conditions. Mechanistically, UPR activation in IECs enhanced their production of both reactive oxygen species (ROS) and purine metabolites. Treating mice with N-acetyl-cysteine or allopurinol to reduce ROS production and xanthine, respectively, decreased Th17 cells that were associated with an elevated UPR. Th17-related genes also correlated with ER stress and the UPR in humans with inflammatory bowel disease. Overall, we identify a mechanism of intestinal Th17 cell differentiation that emerges from an IEC-associated UPR.

Improving the efficacy of plant-made anti-HIV monoclonal antibodies for clinical use

Introduction

Broadly neutralising antibodies are promising candidates for preventing and treating Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome (HIV/AIDS), as an alternative to or in combination with antiretroviral therapy (ART). These mAbs bind to sites on the virus essential for virus attachment and entry, thereby inhibiting entry into the host cell. However, the cost and availability of monoclonal antibodies, especially combinations of antibodies, hampers implementation of anti-HIV bNAb therapies in low- to middle- income countries (LMICs) where HIV-1 prevalence is highest.

Methods

We have produced three HIV broadly neutralizing antibodies (bNAbs), 10-1074, VRC01 and 3BNC117 in the Nicotiana benthamiana transient expression system. The impact of specific modifications to enhance potency and efficacy were assessed. To prolong half-life and increase bioavailability, a M252Y/S254T/T256E (YTE) or M428L/N434S (LS) mutation was introduced. To increase antibody dependent cellular cytotoxicity (ADCC), we expressed an afucosylated version of each antibody using a glycoengineered plant line.

Results

The majority of bNAbs and their variants could be expressed at yields of up to 47 mg/kg. Neither the expression system nor the modifications impacted the neutralization potential of the bNAbs. Afucosylated bNAbs exhibit enhanced ability to bind to FcγRIIIa and trigger ADCC, regardless of the presence of Fc amino acid mutations. Lastly, we demonstrated that Fc-modified variants expressed in plants show enhanced binding to FcRn, which results in a favourable in vivo pharmacokinetic profile compared to their unmodified counterparts.

Conclusion

Tobacco plants are suitable expression hosts for anti-HIV bNAbs with increased efficacy and an improved pharmacokinetic profile.

Assessing developability early in the discovery process for novel biologics

Beyond potency, a good developability profile is a key attribute of a biological drug. Selecting and screening for such attributes early in the drug development process can save resources and avoid costly late-stage failures. Here, we review some of the most important developability properties that can be assessed early on for biologics. These include the influence of the source of the biologic, its biophysical and pharmacokinetic properties, and how well it can be expressed recombinantly. We furthermore present in silico, in vitro, and in vivo methods and techniques that can be exploited at different stages of the discovery process to identify molecules with liabilities and thereby facilitate the selection of the most optimal drug leads. Finally, we reflect on the most relevant developability parameters for injectable versus orally delivered biologics and provide an outlook toward what general trends are expected to rise in the development of biologics.

Depletion of the apical endosome in response to viruses and bacterial toxins provides cell-autonomous host defense at mucosal surfaces

Polarized epithelial cells form an essential barrier against infection at mucosal surfaces. Many pathogens breach this barrier to cause disease, often by co-opting cellular endocytosis mechanisms to enter the cell through the lumenal (apical) cell surface. We recently discovered that the loss of the cell polarity gene PARD6B selectively diminishes apical endosome function. Here, we find that in response to the entry of certain viruses and bacterial toxins into the epithelial cells via the apical membrane, PARD6B and aPKC, two components of the PARD6B-aPKC-Cdc42 apical polarity complex, undergo rapid proteasome-dependent degradation. The perturbation of apical membrane glycosphingolipids by toxin- or virus-binding initiates degradation of PARD6B. The loss of PARD6B causes the depletion of apical endosome function and renders the cell resistant to further infection from the lumenal cell surface, thus enabling a form of cell-autonomous host defense.

Colostrogenesis: Role and Mechanism of the Bovine Fc Receptor of the Neonate (FcRn)

Colostrogenesis is a separate and unique phase of mammary epithelial cell activity occurring in the weeks before parturition and rather abruptly ending after birth in the bovine. It has been the focus of research to define what controls this process and how it produces high concentrations of specific biologically active components important for the neonate. In this review we consider colostrum composition and focus upon components that appear in first milked colostrum in concentrations exceeding that in blood serum. The Fc Receptor of the Neonate (FcRn) is recognized as the major immunoglobulin G (IgG) and albumin binding protein that accounts for the proteins' long half-lives. We integrate the action of the pinocytotic (fluid phase) uptake of extracellular components and merge them with FcRn in sorting endosomes. We define and explore the means of binding, sorting, and the transcytotic delivery of IgG1 while recycling IgG2 and albumin. We consider the means of releasing the ligands from the receptor within the endosome and describe a new secretion mechanism of cargo release into colostrum without the appearance of FcRn itself in colostrum. We integrate the insulin-like growth factor family, some of which are highly concentrated bioactive components of colostrum, with the mechanisms related to FcRn endosome action. In addition to secretion, we highlight the recent findings of a role of the FcRn in phagocytosis and antigen presentation and relate its significant and abrupt change in cellular location after parturition to a role in the prevention and resistance to mastitis infections.

Effect of variable domain charge on in vitro and in vivo disposition of monoclonal antibodies

A growing body of evidence supports the important role of molecular charge on antibody pharmacokinetics (PK), yet a quantitative description of the effect of charge on systemic and tissue disposition of antibodies is still lacking. Consequently, we have systematically engineered complementarity-determining regions (CDRs) of trastuzumab to create a series of variants with an isoelectric point (pI) range of 6.3–8.9 and a variable region (Fv) charge range of −8.9 to +10.9 (at pH 5.5), and have investigated in vitro and in vivo disposition of these molecules. These monoclonal antibodies (mAbs) exhibited incrementally enhanced binding to cell surfaces and cellular uptake with increased positive charge in antigen-negative cells. After single intravenous dosing in mice, a bell-shaped relationship between systemic exposure and Fv charge was observed, with both extended negative and positive charge patches leading to more rapid nonspecific clearance. Whole-body PK experiments revealed that, although overall exposures of most variants in the tissues were very similar, positive charge of mAbs led to significantly enhanced tissue:plasma concentration ratios for most tissues. In well-perfused organs such as liver, spleen, and kidney, the positive charge variants show superior accumulation. In tissues with continuous capillaries such as fat, muscle, skin, and bone, plasma concentrations governed tissue exposures. The in vitro and in vivo disposition data presented here facilitate better understanding of the impact of charge modifications on antibody PK, and suggest that alteration in the charge may help to improve tissue:plasma concentration ratios for mAbs in certain tissues. The data presented here also paves the way for the development of physiologically based pharmacokinetic models of mAbs that incorporate charge variations.

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 in vitro FcRn- dependent transcytosis assay as a screening tool for predictive assessment of nonspecific clearance of antibody therapeutics in humans

A cell-based assay employing Madin–Darby canine kidney cells stably expressing human neonatal Fc receptor (FcRn) heavy chain and β2-microglobulin genes was developed to measure transcytosis of monoclonal antibodies (mAbs) under conditions relevant to the FcRn-mediated immunoglobulin G (IgG) salvage pathway. The FcRn-dependent transcytosis assay is modeled to reflect combined effects of nonspecific interactions between mAbs and cells, cellular uptake via pinocytosis, pH-dependent interactions with FcRn, and dynamics of intracellular trafficking and sorting mechanisms. Evaluation of 53 mAbs, including 30 marketed mAb drugs, revealed a notable correlation between the transcytosis readouts and clearance in humans. FcRn was required to promote efficient transcytosis of mAbs and contributed directly to the observed correlation. Furthermore, the transcytosis assay correctly predicted rank order of clearance of glycosylation and Fv charge variants of Fc-containing proteins. These results strongly support the utility of this assay as a cost-effective and animal-sparing screening tool for evaluation of mAb-based drug candidates during lead selection, optimization, and process development for desired pharmacokinetic properties.

Phosphatidylethanolamine made in the inner mitochondrial membrane is essential for yeast cytochrome bc1 complex function

Of the four separate PE biosynthetic pathways in eukaryotes, one occurs in the mitochondrial inner membrane (IM) and is executed by phosphatidylserine decarboxylase (Psd1). Deletion of Psd1 is lethal in mice and compromises mitochondrial function. We hypothesize that this reflects inefficient import of non-mitochondrial PE into the IM. Here, we test this by re-wiring PE metabolism in yeast by re-directing Psd1 to the outer mitochondrial membrane or the endomembrane system and show that PE can cross the IMS in both directions. Nonetheless, PE synthesis in the IM is critical for cytochrome bc1 complex (III) function and mutations predicted to disrupt a conserved PE-binding site in the complex III subunit, Qcr7, impair complex III activity similar to PSD1 deletion. Collectively, these data challenge the current dogma of PE trafficking and demonstrate that PE made in the IM by Psd1 support the intrinsic functionality of complex III.

Targeting FcRn to Generate Antibody-Based Therapeutics

The MHC class I-related receptor FcRn serves multiple roles ranging from the regulation of levels of IgG isotype antibodies and albumin throughout the body to the delivery of antigen into antigen loading compartments in specialized antigen-presenting cells. In parallel with studies directed towards understanding FcRn at the molecular and cellular levels, there has been an enormous expansion in the development of engineering strategies involving FcRn to modulate the dynamic behavior of antibodies, antigens, and albumin. In this review article, we focus on a discussion of FcRn-targeted approaches that have resulted in the production of novel antibody-based platforms with considerable potential for use in the clinic.

Cardiomyopathy-associated mutation in the ADP/ATP carrier reveals translation-dependent regulation of cytochrome c oxidase activity

How the absence of the major mitochondrial ADP/ATP carrier in yeast, Aac2p, results in a specific defect in cytochrome c oxidase (COX; complex IV) activity is a long-standing mystery. Aac2p physically associates with respiratory supercomplexes, which include complex IV, raising the possibility that its activity is dependent on its association with Aac2p. Here, we have leveraged a transport-dead pathogenic AAC2 point mutant to determine the basis for the reduced COX activity in the absence of Aac2p. The steady-state levels of complex IV subunits encoded by the mitochondrial genome are significantly reduced in the absence of Aac2p function, whether its association with respiratory supercomplexes is preserved or not. This diminution in COX amounts is not caused by a reduction in the mitochondrial genome copy number or the steady-state level of its transcripts, and does not reflect a defect in complex IV assembly. Instead, the absence of Aac2p activity, genetically or pharmacologically, results in an aberrant pattern of mitochondrial translation. Interestingly, compared with the complete absence of Aac2p, the complex IV-related defects are greater in mitochondria expressing the transport-inactive Aac2p mutant. Our results highlight a critical role for Aac2p transport in mitochondrial translation whose disturbance uniquely impacts cytochrome c oxidase.

Human IgG lacking effector functions demonstrate lower FcRn-binding and reduced transplacental transport

We have previously generated human IgG1 antibodies that were engineered for reduced binding to the classical Fcγ receptors (FcγRI-III) and C1q, thereby eliminating their destructive effector functions (constant region G1Δnab). In their potential use as blocking agents, favorable binding to the neonatal Fc receptor (FcRn) is important to preserve the long half-life typical of IgG. An ability to cross the placenta, which is also mediated, at least in part, by FcRn is desirable in some indications, such as feto-maternal alloimmune disorders. Here, we show that G1Δnab mutants retain pH-dependent binding to human FcRn but that the amino acid alterations reduce the affinity of the IgG1:FcRn interaction by 2.0-fold and 1.6-fold for the two antibodies investigated. The transport of the modified G1Δnab mutants across monolayers of human cell lines expressing FcRn was approximately 75% of the wild-type, except that no difference was observed with human umbilical vein endothelial cells. G1Δnab mutation also reduced transport in an ex vivo placenta model. In conclusion, we demonstrate that, although the G1Δnab mutations are away from the FcRn-binding site, they have long-distance effects, modulating FcRn binding and transcellular transport. Our findings have implications for the design of therapeutic human IgG with tailored effector functions.

Truncation of the Murine Neonatal Fc Receptor Cytoplasmic Tail Does Not Alter IgG Metabolism or Transport In Vivo

The neonatal Fc receptor (FcRn) is involved in IgG metabolism and transport in placental mammals. However, whether FcRn is responsible for IgG transfer from maternal serum to colostrum/milk is controversial. Interestingly, large domestic animals, such as cows, pigs, sheep, and horses, in which passive IgG transfer is exclusively completed via colostrum/milk, all express an FcRn α-chain that is shorter in the cytoplasmic tail (CYT) than its counterparts in humans and rodents. To address whether the length variation has any functional significance, we performed in vitro experiments using the Transwell system with the MDCK cell line stably transfected with various FcRn constructs; these clearly suggested that truncation of the CYT tail caused a polar change in IgG transfer. However, we observed no evidence supporting functional changes in IgG in vivo using mice in which the FcRn CYT was precisely truncated. These data suggest that the length variation in FcRn is not functionally associated with passive IgG transfer routes in mammals.

Maternal IgG immune complexes induce food allergen-specific tolerance in offspring

The role of maternal immune responses in tolerance induction is poorly understood. To study whether maternal allergen sensitization affects offspring susceptibility to food allergy, we epicutaneously sensitized female mice with ovalbumin (OVA) followed by epicutaneous sensitization and oral challenge of their offspring with OVA. Maternal OVA sensitization prevented food anaphylaxis, OVA-specific IgE production, and intestinal mast cell expansion in offspring. This protection was mediated by neonatal crystallizable fragment receptor (FcRn)-dependent transfer of maternal IgG and OVA immune complexes (IgG-IC) via breast milk and induction of allergen-specific regulatory T (T reg) cells in offspring. Breastfeeding by OVA-sensitized mothers or maternal supplementation with IgG-IC was sufficient to induce neonatal tolerance. FcRn-dependent antigen presentation by CD11c⁺ dendritic cells (DCs) in offspring was required for oral tolerance. Human breast milk containing OVA-IgG-IC induced tolerance in humanized FcRn mice. Collectively, we demonstrate that interactions of maternal IgG-IC and offspring FcRn are critical for induction of T reg cell responses and control of food-specific tolerance in neonates.

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

Multitiered and Cooperative Surveillance of Mitochondrial Phosphatidylserine Decarboxylase 1

Phosphatidylserine decarboxylase 1 (Psd1p), an ancient enzyme that converts phosphatidylserine to phosphatidylethanolamine in the inner mitochondrial membrane, must undergo an autocatalytic self-processing event to gain activity. Autocatalysis severs the protein into a large membrane-anchored β subunit that noncovalently associates with the small α subunit on the intermembrane space side of the inner membrane. Here, we determined that a temperature sensitive (ts) PSD1 allele is autocatalytically impaired and that its fidelity is closely monitored throughout its life cycle by multiple mitochondrial quality control proteases. Interestingly, the proteases involved in resolving misfolded Psd1^ts vary depending on its autocatalytic status. Specifically, the degradation of a Psd1^ts precursor unable to undergo autocatalysis requires the unprecedented cooperative and sequential actions of two inner membrane proteases, Oma1p and Yme1p. In contrast, upon heat exposure postautocatalysis, Psd1^ts β subunits accumulate in protein aggregates that are resolved by Yme1p acting alone, while the released α subunit is degraded in parallel by an unidentified protease. Importantly, the stability of endogenous Psd1p is also influenced by Yme1p. We conclude that Psd1p, the key enzyme required for the mitochondrial pathway of phosphatidylethanolamine production, is closely monitored at several levels and by multiple mitochondrial quality control mechanisms present in the intermembrane space.

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.

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.

A targeted RNAi screen identifies factors affecting diverse stages of receptor-mediated transcytosis

Transcytosis plays an important role in establishing cell polarity and in mediating transport of large cargo across epithelial barriers, but its molecular basis is unclear. Nelms et al. present a new dataset of genes involved in receptor-mediated transcytosis and show that the apical and basolateral recycling and transcytotic pathways are genetically separable.

Endosome transport by transcytosis is the primary mechanism by which proteins and other large cargo traverse epithelial barriers in normal tissue. Transcytosis is also essential for establishing and maintaining membrane polarity in epithelia and other polarized cells. To identify novel components of this pathway, we conducted a high-throughput RNA interference screen for factors necessary for the bidirectional transcytosis of IgG by the Fcγ receptor FcRn. This screen identified 23 genes whose suppression resulted in a reproducible decrease in FcRn-mediated transcytosis. Pulse-chase kinetic transport assays on four of the top-ranking genes (EXOC2, EXOC7, PARD6B, and LEPROT) revealed distinct effects on the apical and basolateral recycling and transcytotic pathways, demonstrating that these pathways are genetically separable. We also found a strong dependence on PARD6B for apical, but not basolateral, recycling, implicating this cell polarity gene in assembly or maintenance of the apical endosomal system. This dataset yields insights into how vesicular transport is adapted to the specialized functions of differentiated cell types and opens new research avenues into epithelial trafficking.

Localization of the human neonatal Fc receptor (FcRn) in human nasal epithelium

The airway epithelium is a central player in the defense against pathogens including efficient mucociliary clearance and secretion of immunoglobulins, mainly polymeric IgA, but also IgG. Pulmonary administration of therapeutic antibodies on one hand, and intranasal immunization on the other, are powerful tools to treat airway infections. In either case, the airway epithelium is the primary site of antibody transfer. In various epithelia, bi-polar transcytosis of IgG and IgG immune complexes is mediated by the human neonatal Fc receptor, FcRn, but FcRn expression in the nasal epithelium had not been demonstrated, so far. We prepared affinity-purified antibodies against FcRn α-chain and confirmed their specificity by Western blotting and immunofluorescence microscopy. These antibodies were used to study the localization of FcRn α-chain in fixed nasal tissue. We here demonstrate for the first time that ciliated epithelial cells, basal cells, gland cells, and endothelial cells in the underlying connective tissue express the receptor. A predominant basolateral steady state distribution of the receptor was observed in ciliated epithelial as well as in gland cells. Co-localization of FcRn α-chain with IgG or with early sorting endosomes (EEA1-positive) but not with late endosomes/lysosomes (LAMP-2-positive) in ciliated cells was observed. This is indicative for the presence of the receptor in the recycling/transcytotic pathway but not in compartments involved in lysosomal degradation supporting the role of FcRn in IgG transcytosis in the nasal epithelium.

Natural and Induced Mitochondrial Phosphate Carrier Loss: DIFFERENTIAL DEPENDENCE OF MITOCHONDRIAL METABOLISM AND DYNAMICS AND CELL SURVIVAL ON THE EXTENT OF DEPLETION

The relevance of mitochondrial phosphate carrier (PiC), encoded by SLC25A3, in bioenergetics is well accepted. However, little is known about the mechanisms mediating the cellular impairments induced by pathological SLC25A3 variants. To this end, we investigated the pathogenicity of a novel compound heterozygous mutation in SLC25A3 First, each variant was modeled in yeast, revealing that substituting GSSAS for QIP within the fifth matrix loop is incompatible with survival on non-fermentable substrate, whereas the L200W variant is functionally neutral. Next, using skin fibroblasts from an individual expressing these variants and HeLa cells with varying degrees of PiC depletion, PiC loss of ∼60% was still compatible with uncompromised maximal oxidative phosphorylation (oxphos), whereas lower maximal oxphos was evident at ∼85% PiC depletion. Furthermore, intact mutant fibroblasts displayed suppressed mitochondrial bioenergetics consistent with a lower substrate availability rather than phosphate limitation. This was accompanied by slowed proliferation in glucose-replete medium; however, proliferation ceased when only mitochondrial substrate was provided. Both mutant fibroblasts and HeLa cells with 60% PiC loss showed a less interconnected mitochondrial network and a mitochondrial fusion defect that is not explained by altered abundance of OPA1 or MFN1/2 or relative amount of different OPA1 forms. Altogether these results indicate that PiC depletion may need to be profound (>85%) to substantially affect maximal oxphos and that pathogenesis associated with PiC depletion or loss of function may be independent of phosphate limitation when ATP requirements are not high.

Defining functional classes of Barth syndrome mutation in humans

The X-linked disease Barth syndrome (BTHS) is caused by mutations in TAZ; TAZ is the main determinant of the final acyl chain composition of the mitochondrial-specific phospholipid, cardiolipin. To date, a detailed characterization of endogenous TAZ has only been performed in yeast. Further, why a given BTHS-associated missense mutation impairs TAZ function has only been determined in a yeast model of this human disease. Presently, the detailed characterization of yeast tafazzin harboring individual BTHS mutations at evolutionarily conserved residues has identified seven distinct loss-of-function mechanisms caused by patient-associated missense alleles. However, whether the biochemical consequences associated with individual mutations also occur in the context of human TAZ in a validated mammalian model has not been demonstrated. Here, utilizing newly established monoclonal antibodies capable of detecting endogenous TAZ, we demonstrate that mammalian TAZ, like its yeast counterpart, is localized to the mitochondrion where it adopts an extremely protease-resistant fold, associates non-integrally with intermembrane space-facing membranes and assembles in a range of complexes. Even though multiple isoforms are expressed at the mRNA level, only a single polypeptide that co-migrates with the human isoform lacking exon 5 is expressed in human skin fibroblasts, HEK293 cells, and murine heart and liver mitochondria. Finally, using a new genome-edited mammalian BTHS cell culture model, we demonstrate that the loss-of-function mechanisms for two BTHS alleles that represent two of the seven functional classes of BTHS mutation as originally defined in yeast, are the same when modeled in human TAZ.

FcRn binding is not sufficient for achieving systemic therapeutic levels of immunoglobulin G after oral delivery of enteric-coated capsules in cynomolgus macaques

Although much speculation has surrounded intestinally expressed FcRn as a means for systemic uptake of orally administered immunoglobulin G (IgG), this has not been validated in translational models beyond neonates or in FcRn-expressing cells in vitro. Recently, IgG1 intestinal infusion acutely in anesthetized cynomolgus resulted in detectable serum monoclonal antibody (mAb) levels. In this study, we show that IgG2 has greater protease resistance to intestinal enzymes in vitro and mice in vivo, due to protease resistance in the hinge region. An IgG2 mAb engineered for FcRn binding, was optimally formulated, lyophilized, and loaded into enteric-coated capsules for oral dosing in cynomolgus. Small intestinal pH 7.5 was selected for enteric delivery based on gastrointestinal pH profiling of cynomolgus by operator-assisted IntelliCap System(®). Milling of the lyophilized IgG2 M428L FcRn-binding variant after formulation in 10 mmol/L histidine, pH 5.7, 8.5% sucrose, 0.04% PS80 did not alter the physicochemical properties nor the molecular integrity compared to the batch released in PBS. Size 3 hard gel capsules (23.2 mg IgG2 M428L ~3 mg/kg) were coated with hydroxypropyl methylcellulose acetate succinate for rapid dissolution at pH 7.5 in small intestine and FcRn binding of encapsulated mAb confirmed. Initial capsule dosing by endoscopic delivery into the small intestine achieved 0.2 + 0.1 ng/mL (n = 5) peak at 24 h. Weekly oral capsule dosing for 6 weeks achieved levels of 0.4 + 0.2 ng/mL and, despite increasing the dose and frequency, remained below 1 ng/mL. In conclusion, lyophilized milled mAb retains FcRn binding and molecular integrity for small intestinal delivery. The low systemic exposure has demonstrated the limitations of intestinal FcRn in non-human primates and the unfeasibility of employing this for therapeutic levels of mAb. Local mAb delivery with limited systemic exposure may be sufficient as a therapeutic for intestinal diseases.

Certolizumab pegol does not bind the neonatal Fc receptor (FcRn): Consequences for FcRn-mediated in vitro transcytosis and ex vivo human placental transfer

Antibodies to tumor necrosis factor (anti-TNF) are used to treat inflammatory diseases, which often affect women of childbearing age. The active transfer of these antibodies across the placenta by binding of the Fc-region to the neonatal Fc receptor (FcRn) may result in adverse fetal or neonatal effects. In contrast to other anti-TNFs, certolizumab pegol lacks an Fc-region. The objective of this study was to determine whether the structure of certolizumab pegol limits active placental transfer. Binding affinities of certolizumab pegol, infliximab, adalimumab and etanercept to human FcRn and FcRn-mediated transcytosis were determined using in vitro assays. Human placentas were perfused ex vivo to measure transfer of certolizumab pegol and positive control anti-D IgG from the maternal to fetal circulation. FcRn binding affinity (KD) was 132nM, 225nM and 1500nM for infliximab, adalimumab and etanercept, respectively. There was no measurable certolizumab pegol binding affinity, similar to that of the negative control. FcRn-mediated transcytosis across a cell layer (mean±SD; n=3) was 249.6±25.0 (infliximab), 159.0±20.2 (adalimumab) and 81.3±13.1ng/mL (etanercept). Certolizumab pegol transcytosis (3.2±3.4ng/mL) was less than the negative control antibody (5.9±4.6ng/mL). No measurable transfer of certolizumab pegol from the maternal to the fetal circulation was observed in 5 out of 6 placentas that demonstrated positive-control IgG transport in the ex vivo perfusion model. Together these results support the hypothesis that the unique structure of certolizumab pegol limits its transfer through the placenta to the fetus and may be responsible for previously reported differences in transfer of other anti-TNFs from mother to fetus.

The Implication and Significance of Beta 2 Microglobulin: A Conservative Multifunctional Regulator

OBJECTIVE

This review focuses on the current knowledge on the implication and significance of beta 2 microglobulin (β2M), a conservative immune molecule in vertebrate.

DATA SOURCES

The data used in this review were obtained from PubMed up to October 2015. Terms of β2M, immune response, and infection were used in the search.

STUDY SELECTIONS

Articles related to β2M were retrieved and reviewed. Articles focusing on the characteristic and function of β2M were selected. The exclusion criteria of articles were that the studies on β2M-related molecules.

RESULTS

β2M is critical for the immune surveillance and modulation in vertebrate animals. The dysregulation of β2M is associated with multiple diseases, including endogenous and infectious diseases. β2M could directly participate in the development of cancer cells, and the level of β2M is deemed as a prognostic marker for several malignancies. It also involves in forming major histocompatibility complex (MHC class I or MHC I) or like heterodimers, covering from antigen presentation to immune homeostasis.

CONCLUSIONS

Based on the characteristic of β2M, it or its signaling pathway has been targeted as biomedical or therapeutic tools. Moreover, β2M is highly conserved among different species, and overall structures are virtually identical, implying the versatility of β2M on applications.

Evidence that FcRn mediates the transplacental passage of maternal IgE in the form of IgG anti-IgE/IgE immune complexes

BACKGROUND

The mechanism(s) responsible for acquisition of maternal antibody isotypes other than IgG are not fully understood. This uncertainty is a major reason underlying the continued controversy regarding whether cord blood (CB) IgE originates in the mother or fetus.

OBJECTIVE

To investigate the capacity of maternal IgE to be transported across the placenta in the form of IgG anti-IgE/IgE immune complexes (ICs) and to determine the role of the neonatal Fc receptor (FcRn) in mediating this process.

METHODS

Maternal and CB serum concentrations of IgE, IgG anti-IgE, and IgG anti-IgE/IgE ICs were determined in a cohort of allergic and non-allergic mother/infant dyads. Madin-Darby canine kidney (MDCK) cells stably transfected with human FcRn were used to study the binding and transcytosis of IgE in the form of IgG anti-IgE/IgE ICs.

RESULTS

Maternal and CB serum concentrations of IgG anti-IgE/IgE ICs were highly correlated, regardless of maternal allergic status. IgG anti-IgE/IgE ICs generated in vitro bound strongly to FcRn-expressing MDCK cells and were transcytosed in an FcRn-dependent manner. Conversely, monomeric IgE did not bind to FcRn and was not transcytosed. IgE was detected in solutions of transcytosed IgG anti-IgE/IgE ICs, even though essentially all the IgE remained in complex form. Similarly, the majority of IgE in CB sera was found to be complexed to IgG.

CONCLUSIONS AND CLINICAL RELEVANCE

These data indicate that human FcRn facilitates the transepithelial transport of IgE in the form of IgG anti-IgE/IgE ICs. They also strongly suggest that the majority of IgE in CB sera is the result of FcRn-mediated transcytosis of maternal-derived IgG anti-IgE/IgE ICs. These findings challenge the widespread perception that maternal IgE does not cross the placenta. Measuring maternal or CB levels of IgG anti-IgE/IgE ICs may be a more accurate predictor of allergic risk.

Enhanced FcRn-dependent transepithelial delivery of IgG by Fc-engineering and polymerization

Monoclonal IgG antibodies (Abs) are used extensively in the clinic to treat cancer and autoimmune diseases. In addition, therapeutic proteins are genetically fused to the constant Fc part of IgG. In both cases, the Fc secures a long serum half-life and favourable pharmacokinetics due to its pH-dependent interaction with the neonatal Fc receptor (FcRn). FcRn also mediates transport of intact IgG across polarized epithelial barriers, a pathway that is attractive for delivery of Fc-containing therapeutics. So far, no study has thoroughly compared side-by-side how IgG and different Fc-fusion formats are transported across human polarizing epithelial cells. Here, we used an in vitro cellular transport assay based on the human polarizing epithelial cell line (T84) in which both IgG1 and Fc-fusions were transported in an FcRn-dependent manner. Furthermore, we found that the efficacy of transport was dependent on the format. We demonstrate that transepithelial delivery could be enhanced by Fc-engineering for improved FcRn binding as well as by Fc-polymerization. In both cases, transport was driven by pH-dependent binding kinetics and the pH at the luminal side. Hence, efficient transcellular delivery of IgG-based drugs across human epithelial cells requires optimal pH-dependent FcRn binding that can be manipulated by avidity and Fc-engineering, factors that should inspire the design of future therapeutics targeted for transmucosal delivery.

The neonatal Fc receptor, FcRn, as a target for drug delivery and therapy

Immunoglobulin G (IgG)-based drugs are arguably the most successful class of protein therapeutics due in part to their remarkably long blood circulation. This arises from IgG interaction with the neonatal Fc receptor, FcRn. FcRn is the central regulator of IgG and albumin homeostasis throughout life and is increasingly being recognized as an important player in autoimmune disease, mucosal immunity, and tumor immune surveillance. Various engineering approaches that hijack or disrupt the FcRn-mediated transport pathway have been devised to develop long-lasting and non-invasive protein therapeutics, protein subunit vaccines, and therapeutics for treatment of autoimmune and infectious disease. In this review, we highlight the diverse biological functions of FcRn, emerging therapeutic opportunities, as well as the associated challenges of targeting FcRn for drug delivery and disease therapy.

Phosphatidylserine decarboxylase 1 autocatalysis and function does not require a mitochondrial-specific factor

Phosphatidylethanolamine (PE) is a major cellular phospholipid that can be made by four separate pathways, one of which resides in the mitochondrion. The mitochondrial enzyme that generates PE is phosphatidylserine decarboxylase 1 (Psd1p). The pool of PE produced by Psd1p, which cannot be compensated for by the other cellular PE metabolic pathways, is important for numerous mitochondrial functions, including oxidative phosphorylation and mitochondrial dynamics and morphology, and is essential for murine development. To become catalytically active, Psd1p undergoes an autocatalytic processing step involving a conserved LGST motif that separates the enzyme into α and β subunits that remain non-covalently attached and are anchored to the inner membrane by virtue of the membrane-embedded β subunit. It was speculated that Psd1p autocatalysis requires a mitochondrial-specific factor and that for Psd1p to function in vivo, it had to be embedded with the correct topology in the mitochondrial inner membrane. However, the identity of the mitochondrial factor required for Psd1p autocatalysis has not been identified. With the goal of defining molecular requirements for Psd1p autocatalysis, we demonstrate that: 1) despite the conservation of the LGST motif from bacteria to humans, only the serine residue is absolutely required for Psd1p autocatalysis and function; 2) yeast Psd1p does not require its substrate phosphatidylserine for autocatalysis; and 3) contrary to a prior report, yeast Psd1p autocatalysis does not require mitochondrial-specific phospholipids, proteins, or co-factors, because Psd1p re-directed to the secretory pathway undergoes autocatalysis normally and is fully functional in vivo.

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.

Regulation of immune responses by the neonatal fc receptor and its therapeutic implications

As a single receptor, the neonatal Fc receptor (FcRn) is critically involved in regulating albumin and IgG serum concentrations by protecting these two ligands from degradation. In addition to these essential homeostatic functions, FcRn possesses important functions in regulating immune responses that are equally as critical and are increasingly coming to attention. During the first stages of life, FcRn mediates the passive transfer of IgG across the maternal placenta or neonatal intestinal walls of mammals, thereby conferring passive immunity to the offspring before and after birth. In fact, FcRn is one of the very few molecules that are known to move from luminal to serosal membranes of polarized cells that form epithelial barriers of the lung and intestines. Together with FcRn's recently explored critical role in eliciting MHC II presentation and MHC class I cross-presentation of IgG-complexed antigen, this renders FcRn capable of exerting broad and potent functions in regulating immune responses and immunosurveillance at mucosal sites. Further, it is now clear that FcRn dependent mucosal absorption of therapeutic molecules is a clinically feasible and potent novel route of non-invasive drug delivery, and the interaction between FcRn and IgG has also been utilized for the acquisition of humoral immunity at mucosal sites. In this review, we begin by briefly summarizing the basic knowledge on FcRn expression and IgG binding, then describe more recent discoveries pertaining to the mechanisms by which FcRn orchestrates IgG related mucosal immune responses and immunosurveillance at host-environment interfaces within the adult organism. Finally, we outline how the knowledge of actions of FcRn at mucosal boundaries can be capitalized for the development and engineering of powerful mucosal vaccination strategies and novel routes for the non-invasive delivery of Fc-based therapeutics.

Toxin-mediated paracellular transport of antitoxin antibodies facilitates protection against Clostridium difficile infection

The exotoxins TcdA and TcdB are the major virulence factors of Clostridium difficile. Circulating neutralizing antitoxin antibodies are protective in C. difficile infection (CDI), as demonstrated, in part, by the protective effects of actoxumab and bezlotoxumab, which bind to and neutralize TcdA and TcdB, respectively. The question of how systemic IgG antibodies neutralize toxins in the gut lumen remains unresolved, although it has been suggested that the Fc receptor FcRn may be involved in active antibody transport across the gut epithelium. In this study, we demonstrated that genetic ablation of FcRn and excess irrelevant human IgG have no impact on actoxumab-bezlotoxumab-mediated protection in murine and hamster models of CDI, suggesting that Fc-dependent transport of antibodies across the gut wall is not required for efficacy. Tissue distribution studies in hamsters suggest, rather, that the transport of antibodies depends on toxin-induced damage to the gut lining. In an in vitro two-dimensional culture system that mimics the architecture of the intestinal mucosal epithelium, toxins on the apical side of epithelial cell monolayers are neutralized by basolateral antibodies, and antibody transport across the cell layer is dramatically increased upon addition of toxin to the apical side. Similar data were obtained with F(ab')2 fragments, which lack an Fc domain, consistent with FcRn-independent paracellular, rather than transcellular, transport of antibodies. Kinetic studies show that initial damage caused by apical toxin is required for efficient neutralization by basolateral antibodies. These data may represent a general mechanism of humoral response-mediated protection against enteric pathogens.

Enhanced neonatal Fc receptor function improves protection against primate SHIV infection

To protect against human immunodeficiency virus (HIV-1) infection, broadly neutralizing antibodies (bnAbs) must be active at the portals of viral entry in the gastrointestinal or cervicovaginal tracts. The localization and persistence of antibodies at these sites is influenced by the neonatal Fc receptor (FcRn), whose role in protecting against infection in vivo has not been defined. Here, we show that a bnAb with enhanced FcRn binding has increased gut mucosal tissue localization, which improves protection against lentiviral infection in non-human primates. A bnAb directed to the CD4-binding site of the HIV-1 envelope (Env) protein (denoted VRC01) was modified by site-directed mutagenesis to increase its binding affinity for FcRn. This enhanced FcRn-binding mutant bnAb, denoted VRC01-LS, displayed increased transcytosis across human FcRn-expressing cellular monolayers in vitro while retaining FcγRIIIa binding and function, including antibody-dependent cell-mediated cytotoxicity (ADCC) activity, at levels similar to VRC01 (the wild type). VRC01-LS had a threefold longer serum half-life than VRC01 in non-human primates and persisted in the rectal mucosa even when it was no longer detectable in the serum. Notably, VRC01-LS mediated protection superior to that afforded by VRC01 against intrarectal infection with simian-human immunodeficiency virus (SHIV). These findings suggest that modification of FcRn binding provides a mechanism not only to increase serum half-life but also to enhance mucosal localization that confers immune protection. Mutations that enhance FcRn function could therefore increase the potency and durability of passive immunization strategies to prevent HIV-1 infection.

The Role of FcRn in Antigen Presentation

Immunoglobulins are unique molecules capable of simultaneously recognizing a diverse array of antigens and themselves being recognized by a broad array of receptors. The abundance specifically of the IgG subclass and the variety of signaling receptors to which it binds render this an important immunomodulatory molecule. In addition to the classical Fcγ receptors that bind IgG at the cell surface, the neonatal Fc receptor (FcRn) is a lifelong resident of the endolysosomal system of most hematopoietic cells where it determines the intracellular fate of both IgG and IgG-containing immune complexes (IgG IC). Cross-linking of FcRn by multivalent IgG IC within antigen presenting cells such as dendritic cells initiates specific mechanisms that result in trafficking of the antigen-bearing IgG IC into compartments from which the antigen can successfully be processed into peptide epitopes compatible with loading onto both major histocompatibility complex class I and II molecules. In turn, this enables the synchronous activation of both CD4(+) and CD8(+) T cell responses against the cognate antigen, thereby bridging the gap between the humoral and cellular branches of the adaptive immune response. Critically, FcRn-driven T cell priming is efficient at very low doses of antigen due to the exquisite sensitivity of the IgG-mediated antigen delivery system through which it operates. FcRn-mediated antigen presentation has important consequences in tissue compartments replete with IgG and serves not only to determine homeostatic immune activation at a variety of sites but also to induce inflammatory responses upon exposure to antigens perceived as foreign. Therapeutically targeting the pathway by which FcRn enables T cell activation in response to IgG IC is thus a highly attractive prospect not only for the treatment of diseases that are driven by immune complexes but also for manipulating local immune responses against defined antigens such as those present during infections and cancer.

Divergent outcomes following transcytosis of IgG targeting intracellular and extracellular chlamydial antigens

Antibodies can have a protective but non-essential role in natural chlamydial infections dependent on antigen specificity and antibody isotype. IgG is the dominant antibody in both male and female reproductive tract mucosal secretions, and is bi-directionally trafficked across epithelia by the neonatal Fc receptor (FcRn). Using pH-polarized epididymal epithelia grown on Transwells, IgG specifically targeted at an extracellular chlamydial antigen; the major outer membrane protein (MOMP), enhanced uptake and translocation of infection at pH 6-6.5 but not at neutral pH. This was dependent on FcRn expression. Conversely, FcRn-mediated transport of IgG targeting the intracellular chlamydial inclusion membrane protein A (IncA), induced aberrant inclusion morphology, recruited autophagic proteins independent of lysosomes and significantly reduced infection. Challenge of female mice with MOMP-specific IgG-opsonized Chlamydia muridarum delayed infection clearance but exacerbated oviduct occlusion. In male mice, MOMP-IgG elicited by immunization afforded no protection against testicular chlamydial infection, whereas the transcytosis of IncA-IgG significantly reduced testicular chlamydial burden. Together these data show that the protective and pathological effects of IgG are dependent on FcRn-mediated transport as well as the specificity of IgG for intracellular or extracellular antigens.

FcRn: from molecular interactions to regulation of IgG pharmacokinetics and functions

The neonatal Fc receptor, FcRn, is related to MHC class I with respect to its structure and association with β2microglobulin (β2m). However, by contrast with MHC class I molecules, FcRn does not bind to peptides, but interacts with the Fc portion of IgGs and belongs to the Fc receptor family. Unlike the 'classical' Fc receptors, however, the primary functions of FcRn include salvage of IgG (and albumin) from lysosomal degradation through the recycling and transcytosis of IgG within cells. The characteristic feature of FcRn is pH-dependent binding to IgG, with relatively strong binding at acidic pH (<6.5) and negligible binding at physiological pH (7.3-7.4). FcRn is expressed in many different cell types, and endothelial and hematopoietic cells are the dominant cell types involved in IgG homeostasis in vivo. FcRn also delivers IgG across cellular barriers to sites of pathogen encounter and consequently plays a role in protection against infections, in addition to regulating renal filtration and immune complex-mediated antigen presentation. Further, FcRn has been targeted to develop both IgGs with extended half-lives and FcRn inhibitors that can lower endogenous antibody levels. These approaches have implications for the development of longer lived therapeutics and the removal of pathogenic or deleterious antibodies.

Human and non-human primate intestinal FcRn expression and immunoglobulin G transcytosis

Purpose

To evaluate transcytosis of immunoglobulin G (IgG) by the neonatal Fc receptor (FcRn) in adult primate intestine to determine whether this is a means for oral delivery of monoclonal antibodies (mAbs).

Methods

Relative regional expression of FcRn and localization in human intestinal mucosa by RT-PCR, ELISA & immunohistochemistry. Transcytosis of full-length mAbs (sandwich ELISA-based detection) across human intestinal segments mounted in Ussing-type chambers, human intestinal (caco-2) cell monolayers grown in transwells, and serum levels after regional intestinal delivery in isoflurane-anesthetized cynomolgus monkeys.

Results

In human intestine, there was an increasing proximal-distal gradient of mucosal FcRn mRNA and protein expression. In cynomolgus, serum mAb levels were greater after ileum-proximal colon infusion than after administration to stomach or proximal small intestine (1–5 mg/kg). Serum levels of wild-type mAb dosed into ileum/proximal colon (2 mg/kg) were 124 ± 104 ng/ml (n = 3) compared to 48 ± 48 ng/ml (n = 2) after a non-FcRn binding variant. In vitro, mAb transcytosis in polarized caco-2 cell monolayers and was not enhanced by increased apical cell surface IgG binding to FcRn. An unexpected finding in primate small intestine, was intense FcRn expression in enteroendocrine cells (chromagranin A, GLP-1 and GLP-2 containing).

Conclusions

In adult primates, FcRn is expressed more highly in distal intestinal epithelial cells. However, mAb delivery to that region results in low serum levels, in part because apical surface FcRn binding does not influence mAb transcytosis. High FcRn expression in enteroendocrine cells could provide a novel means to target mAbs for metabolic diseases after systemic administration.

Abundant intracellular IgG in enterocytes and endoderm lacking FcRn

FcRn, a non-classical MHCI molecule, transports IgG from mother to young and regulates the rate of IgG degradation throughout life. Brambell proposed a mechanism that unified these two functions, saying that IgG was pinocytosed nonspecifically by the cell into an FcRn-expressing endosome, where, at low pH, it bound to FcRn and was exocytosed. This theory was immediately challenged by claims that FcRn specificity for ligand could be conferred at the cell surface in neonatal jejunum. Assessing Brambell's hypothesis we found abundant nonspecifically endocytosed IgG present in the cytoplasm of FcRn(-/-) enterocytes. Further, IgG was present in the intercellular clefts and the cores of FcRn(+/+) but not FcRn(-/-) jejunum. FcRn specificity for ligand could be determined within the cell.

Humanized FcRn mouse models for evaluating pharmacokinetics of human IgG antibodies

A key element for the successful development of novel therapeutic antibodies is to fully understand their pharmacokinetic and pharmacodynamic behavior before performing clinical trials. While many in vitro modeling approaches exist, these simply cannot substitute for data obtained from appropriate animal models. It was established quite early that the unusual long serum half-life of immunoglobulin G's (IgGs) and Fc domains are due to their rescue and recycling by the neonatal Fc receptor (FcRn). The diverse roles of FcRn became apparent after isolation and cloning. Interesting are the significant species differences between rodent and human FcRn reactivity, rendering wild type rodents an inadequate model for studying IgG serum half-life. With the advance of genetic engineering mouse models have been established expressing human FcRn, and lacking mouse FcRn protein. These models have become highly relevant tools for serum half-life analysis of Fc-containing compounds.

Multivalent immune complexes divert FcRn to lysosomes by exclusion from recycling sorting tubules

Study of receptor sorting between recycling and degradative pathways shows that sorting into the recycling pathway depends not only on recognition of sorting motifs by cytosolic adaptors, but also on the physical properties of the endosomal luminal complexes, as shown by the neonatal receptor for IgG FcRn.

The neonatal receptor for immunoglobulin G (IgG; FcRn) prevents IgG degradation by efficiently sorting IgG into recycling endosomes and away from lysosomes. When bound to IgG-opsonized antigen complexes, however, FcRn traffics cargo into lysosomes, where antigen processing can occur. Here we address the mechanism of sorting when FcRn is bound to multivalent IgG-opsonized antigens. We find that only the unbound receptor or FcRn bound to monomeric IgG is sorted into recycling tubules emerging from early endosomes. Cross-linked FcRn is never visualized in tubules containing the unbound receptor. Similar results are found for transferrin receptor, suggesting a general mechanism of action. Deletion or replacement of the FcRn cytoplasmic tail does not prevent diversion of trafficking to lysosomes upon cross-linking. Thus physical properties of the lumenal ligand–receptor complex appear to act as key determinants for sorting between the recycling and lysosomal pathways by regulating FcRn entry into recycling tubules.

Cross-presentation of IgG-containing immune complexes

IgG is a molecule that functionally combines facets of both innate and adaptive immunity and therefore bridges both arms of the immune system. On the one hand, IgG is created by adaptive immune cells, but can be generated by B cells independently of T cell help. On the other hand, once secreted, IgG can rapidly deliver antigens into intracellular processing pathways, which enable efficient priming of T cell responses towards epitopes from the cognate antigen initially bound by the IgG. While this process has long been known to participate in CD4(+) T cell activation, IgG-mediated delivery of exogenous antigens into a major histocompatibility complex (MHC) class I processing pathway has received less attention. The coordinated engagement of IgG with IgG receptors expressed on the cell-surface (FcγR) and within the endolysosomal system (FcRn) is a highly potent means to deliver antigen into processing pathways that promote cross-presentation of MHC class I and presentation of MHC class II-restricted epitopes within the same dendritic cell. This review focuses on the mechanisms by which IgG-containing immune complexes mediate such cross-presentation and the implications that this understanding has for manipulation of immune-mediated diseases that depend upon or are due to the activities of CD8(+) T cells.

Contribution of FcRn binding to intestinal uptake of IgG in suckling rat pups and human FcRn-transgenic mice

Immunoglobulin G (IgG) is transcytosed across intestinal epithelial cells of suckling mammals by the neonatal Fc receptor (FcRn); however, the contribution of FcRn vs. FcRn-independent uptake to serum IgG levels had not been determined in either rat pups or human (h)FcRn-expressing mice (Tg276 and Tg32). In isoflurane-anesthetized rodents, serum levels were determined after regional intestinal delivery of human monoclonal antibodies (hIgG) with either wild-type (WT) Fc sequences or variants engineered for different FcRn binding affinities. Detection of full-length hIgG was by immunoassay; intestinal hFcRn and hIgG localization was by immunocytochemistry. High (μg/ml) serum levels of hIgG were detected after proximal intestinal delivery (0.1-10 mg/kg) in 2-wk-old rats. Human FcRn was visualized in epithelial cells of Tg276 mice, but low serum hIgG levels (<10 ng/ml) were obtained. In rat pups, intraintestinal hIgG1 WT administration resulted in dose-related and saturable uptake, whereas uptake of a low FcRn-binding affinity variant was nonsaturable. There were no differences in hIgG levels from systemic and hepatic portal vein serum samples, and intense hIgG immunostaining was noted in villi enterocytes and within lymphatic lacteal-like vessels. This study demonstrated that FcRn-mediated uptake in rat pups accounted for ~80% of serum hIgG levels and that IgG enters the circulation via the lymph and not the hepatic portal vein. The remaining uptake though the immature intestine is nonreceptor mediated. Intestinal epithelial cell hFcRn expression occurred in Tg276 mice, but receptor-mediated transport of IgG was not observed. The suckling rat pup intestine is a mechanistic model of FcRn-IgG-mediated transcytosis.

Selection of nanobodies that target human neonatal Fc receptor

FcRn is a key player in several immunological and non-immunological processes, as it mediates maternal-fetal transfer of IgG, regulates the serum persistence of IgG and albumin, and transports both ligands between different cellular compartments. In addition, FcRn enhances antigen presentation. Thus, there is an intense interest in studies of how FcRn binds and transports its cargo within and across several types of cells, and FcRn detection reagents are in high demand. Here we report on phage display-selected Nanobodies that target human FcRn. The Nanobodies were obtained from a variable-domain repertoire library isolated from a llama immunized with recombinant human FcRn. One candidate, Nb218-H4, was shown to bind FcRn with high affinity at both acidic and neutral pH, without competing ligand binding and interfering with FcRn functions, such as transcytosis of IgG. Thus, Nb218-H4 can be used as a detection probe and as a tracker for visualization of FcRn-mediated cellular transport.

The immunologic functions of the neonatal Fc receptor for IgG

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), as a single receptor, is capable of regulating both of these molecules and has become an important focus of investigation. In addition to these essential protection functions, FcRn possesses a number of other functions that are equally as critical and are increasingly coming to attention. 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 via transcytosis across polarized cells. FcRn is also expressed by hematopoietic cells; consistent with this, FcRn regulates MHC class II presentation and MHC class I cross-presentation by dendritic cells. As such, FcRn plays an important role in immune surveillance throughout adult life. The increasing appreciation for FcRn in both homeostatic and pathological conditions is generating an intense interest in the potential for therapeutic modulation of FcRn binding to IgG and albumin.

Human antibodies can cross guinea pig placenta and bind its neonatal Fc Receptor: implications for studying immune prophylaxis and therapy during pregnancy

Despite increased use of monoclonal and polyclonal antibody therapies, including during pregnancy, there is little data on appropriate animal models that could humanely be used to understand determinants of protection and to evaluate safety of these biologics in the mother and the developing fetus. Here, we demonstrate that pregnant guinea pigs can transport human IgG transplacentally at the end of pregnancy. We also observe that human IgG binds to an engineered soluble variant of the guinea pig neonatal Fc receptor in vitro in a manner similar to that demonstrated for the human variant, suggesting that this transplacental transport mirrors the receptor-based mechanism seen in humans. Using an intravenous antihepatitis B-specific immune globulin preparation as an example, we show that this transport results in neutralizing activity in the mother and the newborn that would potentially be prophylactic against hepatitis B viral infection. These preliminary data lay the groundwork for introducing pregnant guinea pigs as an appropriate model for the evaluation of antibody therapies and advancing the health of women and neonates.

Neonatal Fc receptor for IgG (FcRn) expressed in the gastric epithelium regulates bacterial infection in mice

Neonatal Fc receptors for immunoglobulin (Ig)G (FcRn) assume a central role in regulating host IgG levels and IgG transport across polarized epithelial barriers. We have attempted to elucidate the contribution of FcRn in controlling Helicobacter infection in the stomach. C57BL/6J wild-type or FcRn(-/-) mice were infected with Helicobacter heilmannii, and gastric lesions, bacterial load and the levels of antigen-specific IgG in serum and gastric juice were analyzed. The elevated levels of anti-H. heimannii IgG in gastric juice were observed exclusively in wild-type mice but not in FcRn(-/-) mice. In contrast, an increase in lymphoid follicles and bacterial loads along with deeper gastric epithelium invasion were noted in FcRn(-/-) mice. C57BL/6J wild-type or FcRn(-/-) mice were also infected with Helicobacter pylori SS1, and the results of the bacterial load in stomachs of these mice and the anti-H. pylori IgG levels in serum and gastric juice were similar to those from H. heilmannii infection. Our data suggest that FcRn can be functionally expressed in the stomach, which is involved in transcytosis of IgG, and prevent colonization by H. heilmannii and the associated pathological consequences of infection.