Bidirectional FcRn-dependent IgG transport in a polarized human intestinal epithelial cell line

Modeling Pharmacokinetics and Pharmacodynamics of Therapeutic Antibodies: Progress, Challenges, and Future Directions

With more than 90 approved drugs by 2020, therapeutic antibodies have played a central role in shifting the treatment landscape of many diseases, including autoimmune disorders and cancers. While showing many therapeutic advantages such as long half-life and highly selective actions, therapeutic antibodies still face many outstanding issues associated with their pharmacokinetics (PK) and pharmacodynamics (PD), including high variabilities, low tissue distributions, poorly-defined PK/PD characteristics for novel antibody formats, and high rates of treatment resistance. We have witnessed many successful cases applying PK/PD modeling to answer critical questions in therapeutic antibodies’ development and regulations. These models have yielded substantial insights into antibody PK/PD properties. This review summarized the progress, challenges, and future directions in modeling antibody PK/PD and highlighted the potential of applying mechanistic models addressing the development questions.

In Translation: FcRn across the Therapeutic Spectrum

As an essential modulator of IgG disposition, the neonatal Fc receptor (FcRn) governs the pharmacokinetics and functions many therapeutic modalities. In this review, we thoroughly reexamine the hitherto elucidated biological and thermodynamic properties of FcRn to provide context for our assessment of more recent advances, which covers antigen-binding fragment (Fab) determinants of FcRn affinity, transgenic preclinical models, and FcRn targeting as an immune-complex (IC)-clearing strategy. We further comment on therapeutic antibodies authorized for treating SARS-CoV-2 (bamlanivimab, casirivimab, and imdevimab) and evaluate their potential to saturate FcRn-mediated recycling. Finally, we discuss modeling and simulation studies that probe the quantitative relationship between in vivo IgG persistence and in vitro FcRn binding, emphasizing the importance of endosomal transit parameters.

Neonatal Fc receptor in human immunity: Function and role in therapeutic intervention

The humoral immune response provides specific, long-lived protection against invading pathogens, via immunoglobulin production and other memory functions. IgG, the most abundant immunoglobulin isotype, has the longest half-life and protects against bacterial and viral infections. The neonatal Fc receptor (FcRn) transports IgG across barriers, for example, the placenta, enhancing fetal humoral immunity to levels similar to their mothers'. Importantly, FcRn, by protecting IgG from intracellular degradation, results in an approximately 21-day circulating IgG half-life and high plasma levels; similarly, FcRn recycles albumin and is the portal of entry for enteric cytopathic human orphan (echo) virus infection. Dysregulated immune responses may lead to antibodies against self-antigens (autoantibodies), resulting in organ-specific or systemic autoimmune diseases. Autoantibody-mediated diseases have been treated by nonspecific immunoglobulin-lowering/modulating therapies, including immunoadsorption, plasma exchange, and high-dose intravenous immunoglobulin. However, targeting FcRn with specific inhibitors results in reduction in only IgG levels. The effectiveness of FcRn inhibitors in autoimmune diseases, including myasthenia gravis and immune thrombocytopenia, provides further evidence that IgG is a primary driver in these autoantibody-mediated diseases. We describe the role of FcRn in human biology, including insights that clinical testing of FcRn inhibitors have provided into FcRn biology and autoimmune disease mechanisms, allowing fact-based speculation on their therapeutic potential.

Influence of FcRn binding properties on the gastrointestinal absorption and exposure profile of Fc molecules

The neonatal Fc receptor (FcRn) represents a transport system with the potential to facilitate absorption of biologics across the gastrointestinal barrier. How biologics interact with FcRn to enable their gastrointestinal absorption, and how these interactions might be optimized in a biological therapeutic are not well understood. Thus, we studied the absorption of Fc molecules from the intestine using three IgG₄-derived Fc variants with different, pH-dependent FcRn binding and release profiles. Using several different intestinal models, we consistently observed that FcRn binding affinity correlated with transcytosis. Our findings support targeting FcRn to enable intestinal absorption of biologics and highlight additional strategic considerations for future work.

Immunological perspectives on the pathogenesis, diagnosis, prevention and treatment of COVID-19

Coronavirus disease 2019 (COVID-19) is an acute respiratory disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-COV-2). COVID-19 can spread to the entire body and cause multiple organ failure. It is a daunting challenge to control the fast growing worldwide pandemic because effective prevention and treatment strategies are unavailable currently. Generally, the immune response of the human body triggered by viral infection is essential for the elimination of the virus. However, severe COVID-19 patients may manifest dysregulated immune responses, such as lymphopenia, lymphocyte exhaustion, exacerbated antibody response, cytokine release syndrome (CRS), etc. Understanding of these immunological characteristics may help identify better approaches for diagnosis, prognosis and treatment of COVID-19 patients. As specific anti-viral agents are notoriously difficult to develop, strategies for modulating the immune responses by either developing novel vaccines or using immunotherapy hold great promise to improve the management of SARS-CoV-2 infection.

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.

Development of Coronavirus Treatments Using Neutralizing Antibodies

The Coronavirus disease 2019 (COVID-19), caused by the novel coronavirus SARS-CoV-2, was first reported in December 2019 in Wuhan, Hubei province, China. This virus has led to 61.8 million cases worldwide being reported as of December 1st, 2020. Currently, there are no definite approved therapies endorsed by the World Health Organization for COVID-19, focusing only on supportive care. Treatment centers around symptom management, including oxygen therapy or invasive mechanical ventilation. Immunotherapy has the potential to play a role in the treatment of SARS-CoV-2. Monoclonal antibodies (mAbs), in particular, is a relatively new approach in the world of infectious diseases and has the benefit of overcoming challenges with serum therapy and intravenous immunoglobulins preparations. Here, we reviewed the articles published in PubMed with the purpose of summarizing the currently available evidence for the use of neutralizing antibodies as a potential treatment for coronaviruses. Studies reporting in vivo results were summarized and analyzed. Despite promising data from some studies, none of them progressed to clinical trials. It is expected that neutralizing antibodies might offer an alternative for COVID-19 treatment. Thus, there is a need for randomized trials to understand the potential use of this treatment.

Human FcRn expression and Type I Interferon signaling control Echovirus 11 pathogenesis in mice

Neonatal echovirus infections are characterized by severe hepatitis and neurological complications that can be fatal. Here, we show that expression of the human homologue of the neonatal Fc receptor (hFcRn), the primary receptor for echoviruses, and ablation of type I interferon (IFN) signaling are key host determinants involved in echovirus pathogenesis. We show that expression of hFcRn alone is insufficient to confer susceptibility to echovirus infections in mice. However, expression of hFcRn in mice deficient in type I interferon (IFN) signaling, hFcRn-IFNAR-/-, recapitulate the echovirus pathogenesis observed in humans. Luminex-based multianalyte profiling from E11 infected hFcRn-IFNAR-/- mice revealed a robust systemic immune response to infection, including the induction of type I IFNs. Furthermore, similar to the severe hepatitis observed in humans, E11 infection in hFcRn-IFNAR-/- mice caused profound liver damage. Our findings define the host factors involved in echovirus pathogenesis and establish in vivo models that recapitulate echovirus disease in humans.

Fc Engineering Strategies to Advance IgA Antibodies as Therapeutic Agents

In the past three decades, a great interest has arisen in the use of immunoglobulins as therapeutic agents. In particular, since the approval of the first monoclonal antibody Rituximab for B cell malignancies, the progress in the antibody-related therapeutic agents has been incremental. Therapeutic antibodies can be applied in a variety of diseases, ranging from cancer to autoimmunity and allergy. All current therapeutic monoclonal antibodies used in the clinic are of the IgG isotype. IgG antibodies can induce the killing of cancer cells by growth inhibition, apoptosis induction, complement activation (CDC) or antibody-dependent cellular cytotoxicity (ADCC) by NK cells, antibody-dependent cellular phagocytosis (ADCP) by monocytes/macrophages, or trogoptosis by granulocytes. To enhance these effector mechanisms of IgG, protein and glyco-engineering has been successfully applied. As an alternative to IgG, antibodies of the IgA isotype have been shown to be very effective in tumor eradication. Using the IgA-specific receptor FcαRI expressed on myeloid cells, IgA antibodies show superior tumor-killing compared to IgG when granulocytes are employed. However, reasons why IgA has not been introduced in the clinic yet can be found in the intrinsic properties of IgA posing several technical limitations: (1) IgA is challenging to produce and purify, (2) IgA shows a very heterogeneous glycosylation profile, and (3) IgA has a relatively short serum half-life. Next to the technical challenges, pre-clinical evaluation of IgA efficacy in vivo is not straightforward as mice do not naturally express the FcαR. Here, we provide a concise overview of the latest insights in these engineering strategies overcoming technical limitations of IgA as a therapeutic antibody: developability, heterogeneity, and short half-life. In addition, alternative approaches using IgA/IgG hybrid and FcαR-engagers and the impact of engineering on the clinical application of IgA will be discussed.

In Vitro Induction of Trained Innate Immunity by bIgG and Whey Protein Extracts

Bovine immunoglobulin G (bIgG) was previously shown to enhance innate immune responses to toll-like receptor (TLR) stimulation, via induction of trained immunity. In this study, we investigated whether minimally processed dairy streams with high levels of whey proteins as potential infant nutrition ingredients could also induce trained immunity, and to what extent this can be explained by the presence of bIgG. The minimally processed whey ingredients serum protein concentrate (SPC) and whey protein concentrate (WPC) were tested for their ability to induce trained immunity in human peripheral blood monocytes. Both ingredients induced trained immunity as evidenced by an increased production of TNF-α and, to a lesser extent, of IL-6 upon stimulation with TLR ligands. This was comparable to isolated bovine immunoglobulin G (bIgG) that served as positive control. Depletion of bIgG from both whey protein-containing ingredients did not significantly inhibit the induction of trained immunity, suggesting that the streams contain other components in addition to bIgG that are able to induce trained immunity. These results indicate that minimally processed whey ingredients may contribute to protection against infections through enhancing innate immune responsiveness to pathogens.

Impact of Glycosylation and Species Origin on the Uptake and Permeation of IgGs through the Nasal Airway Mucosa

Although we have recently reported the involvement of neonatal Fc receptor (FcRn) in intranasal transport, the transport mechanisms are far from being elucidated. Ex vivo porcine olfactory tissue, primary cells from porcine olfactory epithelium (OEPC) and the human cell line RPMI 2650 were used to evaluate the permeation of porcine and human IgG antibodies through the nasal mucosa. IgGs were used in their wild type and deglycosylated form to investigate the impact of glycosylation. Further, the expression of FcRn and Fc-gamma receptor (FCGR) and their interaction with IgG were analyzed. Comparable permeation rates for human and porcine IgG were observed in OEPC, which display the highest expression of FcRn. Only traces of porcine IgGs could be recovered at the basolateral compartment in ex vivo olfactory tissue, while human IgGs reached far higher levels. Deglycosylated human IgG showed significantly higher permeation in comparison to the wild type in RPMI 2650 and OEPC, but insignificantly elevated in the ex vivo model. An immunoprecipitation with porcine primary cells and tissue identified FCGR2 as a potential interaction partner in the nasal mucosa. Glycosylation sensitive receptors appear to be involved in the uptake, transport, but also degradation of therapeutic IgGs in the airway epithelial layer.

Limited Neonatal Carbohydrate-Specific Antibody Repertoire Consecutive to Partial Prenatal Transfer of Maternal Antibodies

Despite the prominence of carbohydrate-specific antibodies in human sera, data on their emergence and antigen specificities are limited. Whereas maternal IgG are transferred prenatally to the fetal circulation, IgM present in cord blood originate from fetal B lymphocytes. Considering the limited exposure of the fetus to foreign antigens, we assessed the repertoire of carbohydrate-specific antibodies in human cord blood and matched maternal blood samples using glycan arrays. Carbohydrate-specific IgM was absent in cord blood, whereas low cord blood IgG reactivity to glycans was detectable. Comparing IgG reactivities of matched pairs, we observed a general lack of correlation in the antigen specificity of IgG from cord blood and maternal blood due to a selective exclusion of most carbohydrate-specific IgG from maternofetal transfer. Given the importance of intestinal bacteria in inducing carbohydrate-specific antibodies, we analyzed global antibody specificities toward commensal bacteria. Similar IgG reactivities to specific Bacteroides species were detected in matched cord and maternal blood samples, thus pointing to an efficient maternal transfer of anti-microbial IgG. Due to the observed selectivity in maternofetal IgG transfer, the lack of fetal antibodies to carbohydrate epitopes is only partially compensated by maternal IgG, thus resulting in a weak response to carbohydrate antigens in neonates.

FcRn is a CD32a coreceptor that determines susceptibility to IgG immune complex-driven autoimmunity

IgG immune complexes (ICs) promote autoimmunity through binding fragment crystallizable (Fc) γ-receptors (FcγRs). Of these, the highly prevalent FcγRIIa (CD32a) histidine (H)-131 variant (CD32aH) is strongly linked to human autoimmune diseases through unclear mechanisms. We show that, relative to the CD32a arginine (R)-131 (CD32aR) variant, CD32aH more avidly bound human (h) IgG1 IC and formed a ternary complex with the neonatal Fc receptor (FcRn) under acidic conditions. In primary human and mouse cells, both CD32a variants required FcRn to induce innate and adaptive immune responses to hIgG1 ICs, which were augmented in the setting of CD32aH. Conversely, FcRn induced responses to IgG IC independently of classical FcγR, but optimal responses required FcRn and FcγR. Finally, FcRn blockade decreased inflammation in a rheumatoid arthritis model without reducing circulating autoantibody levels, providing support for FcRn's direct role in IgG IC-associated inflammation. Thus, CD32a and FcRn coregulate IgG IC-mediated immunity in a manner favoring the CD32aH variant, providing a novel mechanism for its disease association.

Intracellular neutralisation of rotavirus by VP6-specific IgG

Rotavirus is a major cause of gastroenteritis in children, with infection typically inducing high levels of protective antibodies. Antibodies targeting the middle capsid protein VP6 are particularly abundant, and as VP6 is only exposed inside cells, neutralisation must be post-entry. However, while a system of poly immune globulin receptor (pIgR) transcytosis has been proposed for anti-VP6 IgAs, the mechanism by which VP6-specific IgG mediates protection remains less clear. We have developed an intracellular neutralisation assay to examine how antibodies neutralise rotavirus inside cells, enabling comparison between IgG and IgA isotypes. Unexpectedly we found that neutralisation by VP6-specific IgG was much more efficient than by VP6-specific IgA. This observation was highly dependent on the activity of the cytosolic antibody receptor TRIM21 and was confirmed using an in vivo model of murine rotavirus infection. Furthermore, mice deficient in only IgG and not other antibody isotypes had a serious deficit in intracellular antibody-mediated protection. The finding that VP6-specific IgG protect mice against rotavirus infection has important implications for rotavirus vaccination. Current assays determine protection in humans predominantly by measuring rotavirus-specific IgA titres. Measurements of VP6-specific IgG may add to existing mechanistic correlates of protection.

An IgA mimicry of IgG that binds Polymeric Immunoglobulin Receptor for mucosa transcytosis

Most pathogens establish infection through mucosa, where secretary IgA (sIgA) plays an "immune exclusion" role in humoral defense. Extravasation of intravenously administrated therapeutic IgG mainly relies on convection and/or FcRn-mediated transcytosis from circulation into interstitial space. Active transport of interstitial IgG further across epithelium into mucosa, like sIgA, is a much desired feature for the next generation of therapeutic antibodies, especially for anti-infection purposes. For the first time, we report the engineering of an IgA mimicry of IgG, with its Fc portion in fusion with the 18-aa tail piece (tp) of sIgA and the J chain, possessing sIgA's full binding activity towards Polymeric Immunoglobulin Receptor (pIgR) that mediates mucosa transcytosis. In a Diphtheria toxin receptor (DTR) knockin mouse model, i.v. injected anti-DT IgG(tp)J protected DTR+ cells from deletion upon DT injection. The compact design of IgG(tp)J opens new revenues for more effective therapeutic IgG mimicking some of the important biological functions of IgA.

The biodistribution of therapeutic proteins: Mechanism, implications for pharmacokinetics, and methods of evaluation

Therapeutic proteins (TPs) are a diverse drug class that include monoclonal antibodies (mAbs), recombinantly expressed enzymes, hormones and growth factors, cytokines (e.g. chemokines, interleukins, interferons), as well as a wide range of engineered fusion scaffolds containing IgG1 Fc domain for half-life extension. As the pharmaceutical industry advances more potent and selective protein-based medicines through discovery and into the clinical stages of development, it has become widely appreciated that a comprehensive understanding of the mechanisms of TP biodistribution can aid this endeavor. This review aims to highlight the literature that has advanced our understanding of the determinants of TP biodistribution. A particular emphasis is placed on the multi-faceted role of the neonatal Fc receptor (FcRn) in mAb and Fc-fusion protein disposition. In addition, characterization of the TP-target interaction at the cell-level is discussed as an essential strategy to establish pharmacokinetic-pharmacodynamic (PK/PD) relationships that may lead to more informed human dose projections during clinical development. Methods for incorporation of tissue and cell-level parameters defining these characteristics into higher-order mechanistic and semi-mechanistic PK models will also be presented.

Relevance of the Materno-Fetal Interface for the Induction of Antigen-Specific Immune Tolerance

In humans, maternal IgGs are transferred to the fetus from the second trimester of pregnancy onwards. The transplacental delivery of maternal IgG is mediated by its binding to the neonatal Fc receptor (FcRn) after endocytosis by the syncytiotrophoblast. IgGs present in the maternal milk are also transferred to the newborn through the digestive epithelium upon binding to the FcRn. Importantly, the binding of IgGs to the FcRn is also responsible for the recycling of circulating IgGs that confers them with a long half-life. Maternally delivered IgG provides passive immunity to the newborn, for instance by conferring protective anti-flu or anti-pertussis toxin IgGs. It may, however, lead to the development of autoimmune manifestations when pathological autoantibodies from the mother cross the placenta and reach the circulation of the fetus. In recent years, strategies that exploit the transplacental delivery of antigen/IgG complexes or of Fc-fused proteins have been validated in mouse models of human diseases to impose antigen-specific tolerance, particularly in the case of Fc-fused factor VIII (FVIII) domains in hemophilia A mice or pre-pro-insulin (PPI) in the case of preclinical models of type 1 diabetes (T1D). The present review summarizes the mechanisms underlying the FcRn-mediated transcytosis of IgGs, the physiopathological relevance of this phenomenon, and the repercussion for drug delivery and shaping of the immune system during its ontogeny.

Efficient mucosal vaccination of a novel classical swine fever virus E2-Fc fusion protein mediated by neonatal Fc receptor

Classical swine fever (CSF) remains one of the most important highly contagious and fatal viral disease of swine with high morbidity and mortality. CSF is caused by classical swine fever virus (CSFV), a small, enveloped RNA virus of the genus Pestivirus. The aim of this study was to construct the a novel CSFV Fc-fusion recombinant protein and evaluate the efficacy as a vaccine against CSFV. Here, we obtained a novel subunit vaccine expressing CSFV E2 recombinant fusion protein in CHO-S cells. Functional analysis revealed that CSFV Fc-fusion recombinant protein (CSFV-E2-Fc) could bind to FcγRI on antigen-presenting cells (APCs) and significantly increase IgA levels in serum and feces, inducing stronger mucosal immune response in swine. Additionally, CSFV-E2-Fc immunization enhanced CSFV-specific T cell immune response with a Th1-like pattern of cytokine secretion, remarkably stimulated the Th1-biased cellular immune response and humoral immune response. Further, the protective effects of CSFV-E2-Fc subunit vaccines were confirmed. The data suggest that CSFV E2-Fc recombinant fusion protein may be a promising candidate subunit vaccine to elicit immune response and protect against CSFV.

Selective inhibition of matrix metalloproteinase 10 (MMP10) with a single-domain antibody

Since their discovery, the matrix metalloproteinase (MMP) family proteases have been considered as therapeutic targets in numerous diseases and disorders. Unfortunately, clinical trials with MMP inhibitors have failed to yield any clinical benefits of these inhibitors. These failures were largely due to a lack of MMP-selective agents; accordingly, it has become important to identify a platform with which high selectivity can be achieved. To this end, we propose using MMP-targeting antibodies that can achieve high specificity in interactions with their targets. Using a scaffold of single-domain antibodies, here we raised a panel of MMP10-selective antibodies through immunization of llamas, a member of the camelid family, whose members generate conventional heavy/light-chain antibodies and also smaller antibodies lacking light-chain and CH1 domains. We report the generation of a highly selective and tightly binding MMP10 inhibitor (K_i < 2 nm). Using bio-layer interferometry-based binding assays, we found that this antibody interacts with the MMP10 active site. Activity assays demonstrated that the antibody selectively inhibits MMP10 over its closest relative, MMP3. The ability of a single-domain antibody to discriminate between the most conserved MMP pair via an active site-directed mechanism of inhibition reported here supports the potential of this antibody as a broadly applicable scaffold for the development of selective, tightly binding MMP inhibitors.

Microbiota-targeted maternal antibodies protect neonates from enteric infection

Although maternal antibodies protect newborn babies from infection1,2, little is known about how protective antibodies are induced without prior pathogen exposure. Here we show that neonatal mice that lack the capacity to produce IgG are protected from infection with the enteric pathogen enterotoxigenic Escherichia coli by maternal natural IgG antibodies against the maternal microbiota when antibodies are delivered either across the placenta or through breast milk. By challenging pups that were fostered by either maternal antibody-sufficient or antibody-deficient dams, we found that IgG derived from breast milk was crucial for protection against mucosal disease induced by enterotoxigenic E. coli. IgG also provides protection against systemic infection by E. coli. Pups used the neonatal Fc receptor to transfer IgG from milk into serum. The maternal commensal microbiota can induce antibodies that recognize antigens expressed by enterotoxigenic E. coli and other Enterobacteriaceae species. Induction of maternal antibodies against a commensal Pantoea species confers protection against enterotoxigenic E. coli in pups. This role of the microbiota in eliciting protective antibodies to a specific neonatal pathogen represents an important host defence mechanism against infection in neonates.

Expressing the immunodominant projection domain of infectious bursal disease virus fused to the fragment crystallizable of chicken IgY in yellow maize for a prospective edible vaccine

Control of Infectious bursal disease virus (IBDV) in endemic countries has been based on early immunization of chicks using conventional live or inactivated vaccines that became not fully effectual and have biosafety concerns. This endeavor seeks generating a recombinant chimeric protein merging the projection domain (PD) of IBDV VP2 capsid with the fragment crystallizable (Fc) of avian IgY (FcIgY), in maize as a prospective poultry edible vaccine. The PD sequence was built on the basis of very virulent IBDV isolates circulating in Egypt. After optimization of codon-usage in maize, sequences of PD and FcIgY were effectively expressed in two elites of yellow maize via bombardment transformation in immature embryos. Chimeric protein amount in stable transgenic samples ranged from1.36% to 3.03% of the total soluble protein based on tissue age and maize cultivar. IBDV VP2 coding sequence was amplified from viral RNA, cloned, and expressed in E. coli. A group of Balb/C mice were hyper-immunized with purified recombinant VP2 protein for raising anti- recombinant VP2 antibodies (anti-rVP2 Ab). Proper expression in maize and immunoreactivity of the chimeric protein (PD-FcIgY) to chicken anti- IBDV and anti-rVP2 Ab were confirmed by both direct and indirect double antibody sandwich (DAS)-ELISAs as well as western blotting. Seeds of regenerated transgenic maize will be validated for chickens as edible vaccination in further studies.

Orally Administrable Therapeutic Synthetic Nanoparticle for Zika Virus

The spread of Zika virus (ZIKV) infection across the USA and various countries in the last three years will not only have a direct impact on the U.S. health care system but has caused international concerns as well. The ultimate impact of ZIKV infection remains to be understood. Currently, there are no therapeutic or vaccine options available to protect those infected by ZIKV. The drug ivermectin (IVM) was found to be a viable agent for the prevention of transmission of ZIKV. Ivermectin is unstable in the presence of water and does not remain in adequate concentration in the human bloodstream to be effective in treatment for ZIKV. Biodegradable nanoparticles would aid in the delivery of ivermectin by providing a high enough concentration of drug and ensuring the drug is gradually released to maintain an appropriate level in the body. The overall goal of this study was to develop and optimize an orally administrable nanoformulation of IVM which can circulate in the blood for a long period for efficient delivery. To achieve the goal, we synthesized and optimized a synthetic nanoformulation of IVM for oral use which can cross the intestinal epithelial barrier to enter the bloodstream. Our studies documented that when delivered with the synthetic nanoparticle (NP), IVM can be accumulated in the blood at a higher concentration and preliminary studies highlighted that NP delivered IVM has the ability to target nonstructural 1 protein of ZIKV. For potential clinical relevance, long-term storable formulation of IVM-nanoparticle in dry powder state for inclusion in a capsule form and cryoprotectant containing frozen forms revealed promising findings. Further, our preliminary in vitro studies documented that ivermectin crosses the placental barrier, thus making it unsafe for the pregnant ZIKV population, whereas the ivermectin-loaded nanoparticle did not show any significant placental barrier crossing, thus indicating its potential suitability for such population. We envision that this work will fill a great unmet need by developing safer and more effective therapies for the treatment of viral infections, including ZIKV.

Orally Administrable Therapeutic Synthetic Nanoparticle for Zika Virus

The spread of Zika virus (ZIKV) infection across the USA and various countries in the last three years will not only have a direct impact on the U.S. health care system but has caused international concerns as well. The ultimate impact of ZIKV infection remains to be understood. Currently, there are no therapeutic or vaccine options available to protect those infected by ZIKV. The drug ivermectin (IVM) was found to be a viable agent for the prevention of transmission of ZIKV. Ivermectin is unstable in the presence of water and does not remain in adequate concentration in the human bloodstream to be effective in treatment for ZIKV. Biodegradable nanoparticles would aid in the delivery of ivermectin by providing a high enough concentration of drug and ensuring the drug is gradually released to maintain an appropriate level in the body. The overall goal of this study was to develop and optimize an orally administrable nanoformulation of IVM which can circulate in the blood for a long period for efficient delivery. To achieve the goal, we synthesized and optimized a synthetic nanoformulation of IVM for oral use which can cross the intestinal epithelial barrier to enter the bloodstream. Our studies documented that when delivered with the synthetic nanoparticle (NP), IVM can be accumulated in the blood at a higher concentration and preliminary studies highlighted that NP delivered IVM has the ability to target nonstructural 1 protein of ZIKV. For potential clinical relevance, long-term storable formulation of IVM-nanoparticle in dry powder state for inclusion in a capsule form and cryoprotectant containing frozen forms revealed promising findings. Further, our preliminary in vitro studies documented that ivermectin crosses the placental barrier, thus making it unsafe for the pregnant ZIKV population, whereas the ivermectin-loaded nanoparticle did not show any significant placental barrier crossing, thus indicating its potential suitability for such population. We envision that this work will fill a great unmet need by developing safer and more effective therapies for the treatment of viral infections, including ZIKV.

Fusion of pseudorabies virus glycoproteins to IgG Fc enhances protective immunity against pseudorabies virus

Molecular adjuvants are vaccine delivery vehicle to increase specific antigens effectiveness. Herein, we concentrated on IgG Fc, an effective molecular adjuvant, to develop novel pseudorabies virus (PRV) subunit vaccines. Two major protective antigen genes of PRV were constructed and linked into the mouse IgG Fc fragment. The gD, gD-IgG2aFc, gB and gB-IgG2aFc proteins were expressed using a baculovirus system. Mice intranasally immunized with gD-IgG2aFc or gB-IgG2aFc subunit vaccine exhibited significantly higher PRV-specific antibodies, neutralizing antibodies and intracellular cytokines than the mice intranasally immunized with gD or gB subunit vaccine. Moreover, no histopathological lesions were observed in mice immunized with gB-IgG2aFc subunit vaccine via histopathology examination. Further, the gB-IgG2aFc subunit vaccine was efficient for PRV infection compared with live attenuated vaccine. Overall, these results suggest that IgG2a Fc fragment, as a potential molecular adjuvant, fused with PRV antigen might be a promising and efficient PRV vaccine candidate.

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 cytomegalovirus evades antibody-mediated immunity through endoplasmic reticulum-associated degradation of the FcRn receptor

Human cytomegalovirus (HCMV) can persistently infect humans, but how HCMV avoids humoral immunity is not clear. The neonatal Fc receptor (FcRn) controls IgG transport from the mother to the fetus and prolongs IgG half-life. Here we show that US11 inhibits the assembly of FcRn with β₂m and retains FcRn in the endoplasmic reticulum (ER), consequently blocking FcRn trafficking to the endosome. Furthermore, US11 recruits the ubiquitin enzymes Derlin-1, TMEM129 and UbE2J2 to engage FcRn, consequently initiating the dislocation of FcRn from the ER to the cytosol and facilitating its degradation. Importantly, US11 inhibits IgG-FcRn binding, resulting in a reduction of IgG transcytosis across intestinal or placental epithelial cells and IgG degradation in endothelial cells. Hence, these results identify the mechanism by which HCMV infection exploits an ER-associated degradation pathway through US11 to disable FcRn functions. These results have implications for vaccine development and immune surveillance.

Human cytomegalovirus (HCMV) can persist for the life of a host in the face of robust immune responses owing to a wide range of immune evasion strategies. Here Liu and colleagues show that HCMV evades the IgG-mediated response by the endoplasmic reticulum-associated degradation of the neonatal Fc receptor for IgG.

Contribution of DNA methylation to the expression of FCGRT in human liver and myocardium

FcRn mediates recycling and transcytosis of IgG and albumin in various cell types. The MHC-class-I-like protein of the FcRn heterodimer is encoded by FCGRT. Few determinants of variable FCGRT expression in humans have been identified so far. In this study, we investigated the presence of DNA methylation in regulatory regions of FCGRT in samples of human liver and myocardium tissue, and we examined the impact of FCGRT methylation on FcRn expression in model cell lines. Quantitative DNA methylation analysis of the FCGRT locus revealed differentially methylated regions in DNA from liver and myocardium. Methylation status in individual CpG sites correlated with FCGRT mRNA expression. Data from model cell lines suggest that differential methylation in the -1058 to -587 bp regulatory region of FCGRT contributes to FcRn expression. Chromatin immunoprecipitation assays indicate that CpG site methylation impacts the binding of the methylation sensitive transcription factors Zbtb7a and Sp1. This study provides a foundation to further define the contribution of epigenetic factors during the control of FcRn expression and IgG traffic in human tissues.

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.

FcRn-Dependent Transcytosis of Monoclonal Antibody in Human Nasal Epithelial Cells In Vitro: A Prerequisite for a New Delivery Route for Therapy?

Monoclonal antibodies (mAbs) are promising therapies to treat airway chronic inflammatory disease (asthma or nasal polyps). To date, no study has specifically assessed, in vitro, the potential function of neonatal Fc receptor (FcRn) in IgG transcytosis through the human nasal airway epithelium. The objective of this study was to report the in vitro expression and function of FcRn in nasal human epithelium. FcRn expression was studied in an air–liquid interface (ALI) primary culture model of human nasal epithelial cells (HNEC) from polyps. FcRn expression was characterized by quantitative RT-PCR, western blot, and immunolabeling. The ability of HNECs to support mAb transcytosis via FcRn was assessed by transcytosis assay. This study demonstrates the expression of FcRn mRNA and protein in HNEC. We report a high expression of FcRn in the cytosol of ciliated, mucus, and basal cells by immunohistochemistry with a higher level of FcRn proteins in differentiated HNEC. We also proved in vitro transepithelial delivery of an IgG1 therapeutic mAb with a dose–response curve. This is the first time that FcRn expression and mAb transcytosis has been shown in a model of human nasal respiratory epithelium in vitro. This study is a prerequisite for FcRn-dependent nasal administration of mAbs.

The neonatal Fc receptor is a pan-echovirus receptor

Echoviruses are amongst the most common causative agents of aseptic meningitis worldwide and are particularly devastating in the neonatal population, where they are associated with severe hepatitis, neurological disease, including meningitis and encephalitis, and even death. Here, we identify the neonatal Fc receptor (FcRn) as a pan-echovirus receptor. We show that loss of expression of FcRn or its binding partner beta 2 microglobulin (β2M) renders cells resistant to infection by a panel of echoviruses at the stage of virus attachment, and that a blocking antibody to β2M inhibits echovirus infection in cell lines and in primary human intestinal epithelial cells. We also show that expression of human, but not mouse, FcRn renders nonpermissive human and mouse cells sensitive to echovirus infection and that the extracellular domain of human FcRn directly binds echovirus particles and neutralizes infection. Lastly, we show that neonatal mice expressing human FcRn are more susceptible to echovirus infection by the enteral route. Our findings thus identify FcRn as a pan-echovirus receptor, which may explain the enhanced susceptibility of neonates to echovirus infections.

A phase I study of the antibody drug conjugate ASG-5ME, an SLC44A4-targeting antibody carrying auristatin E, in metastatic castration-resistant prostate cancer

Background Antibody drug conjugates (ADC) offer the potential of maximizing efficacy while minimizing systemic toxicity. ASG-5ME, an SLC44A4-targeting antibody carrying monomethyl auristatin E (MMAE), a microtubule-disrupting agent, was investigated in men with metastatic castration resistant prostate cancer. Methods The primary objective of this phase I study was to determine maximum tolerated dose (MTD) and recommended phase II dose. Secondary objectives were safety, antitumor activity, pharmacokinetic properties, immunogenicity, and the detection of SLC44A4 on circulating tumor cells. Patients (pts) were treated among 7 dose levels every 21 days. A dose expansion phase enrolled 20 additional pts. at the MTD. Results Twenty-six and 20 pts. were treated in dose escalation and dose expansion cohorts respectively. The MTD was 2.7 mg/kg. Dose-limiting toxicities occurred in 4 pts.: grade 3 fatigue (n = 1); grade 3 abdominal pain, diarrhea and fatigue (n = 1); grade 4 neutropenia and hyponatremia and grade 3 maculopapular rash, constipation and hypoxia (n = 1); grade 3 troponin elevation without cardiac sequelae (n = 1). Fatigue and diarrhea were the most prevalent adverse events (AEs) across all cycles. Two grade 5 AEs occurred in the dose expansion cohort, each after 1 dose: 1 pt. developed grade 3 hyperglycemia, renal insufficiency and leukopenia; 1 pt. developed grade 3 hyperglycemia complicated by bacteremia. Free MMAE levels did not accumulate with repeat dosing. Of evaluable pts., 52% had either stable disease or a partial response. Conclusions Further development of ASG-5ME is not being pursued due to its narrow therapeutic index. Some toxicities were potentially related to on-target effects on normal tissue expressing the SLC44A4 protein. However, other toxicities were consistent with studies of previous MMAE-containing ADCs. Unconjugated MMAE is a less likely etiology based on prior data.

Microfluidic Nanoassembly of Bioengineered Chitosan-Modified FcRn-Targeted Porous Silicon Nanoparticles @ Hypromellose Acetate Succinate for Oral Delivery of Antidiabetic Peptides

Microfluidics technology is emerging as a promising strategy in improving the oral delivery of proteins and peptides. Herein, a multistage drug delivery system is proposed as a step forward in the development of noninvasive therapies. Undecylenic acid-modified thermally hydrocarbonized porous silicon (UnPSi) nanoparticles (NPs) were functionalized with the Fc fragment of immunoglobulin G for targeting purposes. Glucagon-like peptide-1 (GLP-1) was loaded into the NPs as a model antidiabetic drug. Fc-UnPSi NPs were coated with mucoadhesive chitosan and ultimately entrapped into a polymeric matrix with pH-responsive properties by microfluidic nanoprecipitation. The final formulation showed a controlled and narrow size distribution. The pH-responsive matrix remained intact in acidic conditions, dissolving only in intestinal pH, resulting in a sustained release of the payload. The NPs presented high cytocompatibility and increased levels of interaction with intestinal cells when functionalized with the Fc fragment, which was supported by the validation of the Fc-fragment integrity after conjugation to the NPs. Finally, the Fc-conjugated NPs showed augmented GLP-1 permeability in an intestinal in vitro model. These results highlight the potential of microfluidics as an advanced technique for the preparation of multistage platforms for oral administration. Moreover, this study provides new insights on the potential of the Fc receptor transcytotic capacity for the development of targeted therapies.

Unidirectional transport of IgG by neonatal Fc receptor in human thyrocytes varies across different IgG subclasses

Neonatal Fc receptor (FcRn) is down-regulated in Hashimoto's thyroiditis (HT) thyrocytes and mediates IgG endocytosis in thyrocytes. The serum distribution of IgG subclasses (of TgAb and TPOAb) differs between HT patients and normal individuals. We aimed to explore the direction and regulation of FcRn-mediated IgG transport in thyrocyte monolayers and the difference between various IgG subclass transport. IgG was transported by FcRn from the basolateral to apical side in the thyrocyte monolayers grown on Transwell filters and the transport was inhibited by IFN-γ and TNF-α. Stimulation by T3 and TSH down-regulated FcRn expression in thyrocytes. IgG1 was transported preferentially over IgG2 and IgG4, which might be related to the differences in FcRn-binding affinities as shown by SPR. FcRn mediates unidirectional IgG transport in thyrocytes in a tissue-specific manner. Down-regulation of FcRn is speculated to play a protective role in HT pathogenesis by mainly reducing IgG1 transport in thyrocytes.

FcRn Expression in Wildtype Mice, Transgenic Mice, and in Human Tissues

Quantitative real-time PCR and Western blot methods were developed to assess neonatal Fc-receptor (FcRn) mRNA and protein expression in human FcRn transgenic mice, Swiss Webster mice, and in select human tissues. Additionally, FcRn turnover was evaluated via pulse-chase. FcRn mRNA expression was significantly higher in transgenic mice when compared to mouse FcRn mRNA in Swiss Webster mice and it ranged from 184-fold higher in the kidney to 109,000-fold higher in the skin. FcRn protein expression was found to be 13-fold lower in kidney to 5.6-fold higher in lung obtained from transgenic mice compared to FcRn protein expression in lung samples obtained from Swiss Webster mice. FcRn protein expression in human liver and small intestine tissues matched more closely with FcRn expression in Swiss Webster mice but were significantly lower when compared to values found from Swiss Webster and transgenic mice. Although FcRn mRNA expression correlated significantly with protein expression (p < 0.0005), the correlation coefficient was only 0.113. As such, the measurement of FcRn protein may be preferred to FcRn mRNA for quantitative applications. Significant differences were found in FcRn expression in transgenic mice, Swiss Webster mice, and human tissues, which may have implications for the use of mouse models in the assessment of monoclonal antibody disposition, efficacy, and safety.

Limited Innovations After More Than 65 Years of Immunoglobulin Replacement Therapy: Potential of IgA- and IgM-Enriched Formulations to Prevent Bacterial Respiratory Tract Infections

Patients with primary immunoglobulin deficiency have lower immunoglobulin levels or decreased immunoglobulin function, which makes these patients more susceptible to bacterial infection. Most prevalent are the selective IgA deficiencies (~1:3,000), followed by common variable immune deficiency (~1:25,000). Agammaglobulinemia is less common (~1:400,000) and is characterized by very low or no immunoglobulin production resulting in a more severe disease phenotype. Therapy for patients with agammaglobulinemia mainly relies on prophylactic antibiotics and the use of IgG replacement therapy, which successfully reduces the frequency of invasive bacterial infections. Currently used immunoglobulin preparations contain only IgG. As a result, concurrent IgA and IgM deficiency persist in a large proportion of agammaglobulinemia patients. Especially patients with IgM deficiency remain at risk for recurrent infections at mucosal surfaces, which includes the respiratory tract. IgA and IgM have multiple functions in the protection against bacterial infections at the mucosal surface. Because of their multimeric structure, both IgA and IgM are able to agglutinate bacteria efficiently. Agglutination allows for entrapment of bacteria in mucus that increases clearance from the respiratory tract. IgA is also important for blocking bacterial adhesion by interfering with bacterial adhesion receptors. IgM in its place is very well capable of activating complement, therefore, it is thought to be important in complement-mediated protection at the mucosal surface. The purpose of this Mini Review is to highlight the latest advances regarding IgA- and IgM-enriched immunoglobulin replacement therapy. We describe the different IgA- and IgM-enriched IgG formulations, their possible modes of action and potential to protect against respiratory tract infections in patients with primary immunoglobulin deficiencies.

Antibody-mediated oral delivery of therapeutic DNA for type 2 diabetes mellitus

Background

Diabetes mellitus (DM) is a chronic progressive metabolic disease that involves uncontrolled elevation of blood glucose levels. Among various therapeutic approaches, GLP-1 prevents type 2 diabetes mellitus (T2DM) patients from experiencing hyperglycemic episodes. However, the short half-life (< 5 min) and rapid clearance of GLP-1 often limits its therapeutic use. Here, we developed an oral GLP-1 gene delivery system to achieve an extended antidiabetic effect.

Methods

Human IgG1 (hIgG1)-Fc-Arg/pDNA complexes were prepared by an electrostatic complexation of the expression plasmid with various ratios of the positively modified Fc fragments of an antibody (hIgG1-Fc-Arg) having a targeting ability to FcRn receptor. The shape and size of the complexes were examined by atomic force and field emission electron microscope. The stability of the complexes was tested in simulated gastrointestinal pH and physiological serum condition. Cellular uptake, transport, and toxicity of the complexes were tested in the Caco-2 cells. Biodistribution and antidiabetic effect of the complexes were observed in either Balb/c mice or Lep_db/db mice.

Results

A 50/1 ratio of the hIgG1-Fc-Arg/pDNA produced a complex structure having approximately 40 ~ 60 nm size and also demonstrated protection of pDNA in the complex from the physiological pH and serum conditions. Cellular uptake and transport of the complex were demonstrated in Caco-2 cells having FcRn receptor expression and forming the monolayer-polarized structure. The cellular toxicity of both delivery vehicle and the complex revealed their minimal toxicity comparable with nontoxicity of a commercial transfection reagent. Biodistribution of the complex showed the detectable distribution of the complex in the most parts of gastrointestinal tract due to ubiquitous expression of the FcRn receptors. An in vivo type 2 diabetes treatment study of oral administration of hIgG1-Fc-9Arg/pGLP-1 complexes showed absorption and expression in GI tract of either Balb/c mice or Lep_db/db mice.

Conclusion

In this study, we developed an oral GLP-1 gene delivery system on the platform of cationic hIgG1-Fc-9Arg. Prolonged t1/2, less immunoactivity, and better bioactivities of hIgG-Fc-9Arg/pGLP-1 complexes appeared to be a promising approach to achieve potent treatment of type 2 diabetes treatment.

Electronic supplementary material

The online version of this article (10.1186/s40824-018-0129-7) contains supplementary material, which is available to authorized users.

Allogenic Fc Domain-Facilitated Uptake of IgG in Nasal Lamina Propria: Friend or Foe for Intranasal CNS Delivery?

Background: The use of therapeutic antibodies for the treatment of neurological diseases is of increasing interest. Nose-to-brain drug delivery is one strategy to bypass the blood brain barrier. The neonatal Fc receptor (FcRn) plays an important role in transepithelial transcytosis of immunoglobulin G (IgG). Recently, the presence of the FcRn was observed in nasal respiratory mucosa. The aim of the present study was to determine the presence of functional FcRn in olfactory mucosa and to evaluate its role in drug delivery. Methods: Immunoreactivity and messenger RNA (mRNA) expression of FcRn was determined in ex vivo porcine olfactory mucosa. Uptake of IgG was performed in a side-by-side cell and analysed by immunofluorescence. Results: FcRn was found in epithelial and basal cells of the olfactory epithelium as well as in glands, cavernous bodies and blood vessels. Allogenic porcine IgGs were found time-dependently in the lamina propria and along axonal bundles, while only small amounts of xenogenic human IgGs were detected. Interestingly, lymphoid follicles were spared from allogenic IgGs. Conclusion: Fc-mediated transport of IgG across the nasal epithelial barrier may have significant potential for intranasal delivery, but the relevance of immune interaction in lymphoid follicles must be clarified to avoid immunogenicity.

Polymeric Immunoglobulin Receptor Mediates Immune Excretion of Mucosal IgM-Antigen Complexes Across Intestinal Epithelium in Flounder (Paralichthys olivaceus)

Polymeric immunoglobulin receptor (pIgR) is one important player of mucosal defenses, but very little is known on pIgR-mediated immune excretion of the antigens that penetrate mucosal surface in fish. Previously, we cloned the pIgR of flounder (Paralichthys olivaceus) and developed anti-pIgR antibody. In this study, the flounders were immunized intraperitoneally with the chicken ovalbumin (OVA) and the control protein bovine serum albumin (BSA) to elicit mucosal IgM antibody and pIgR response, and then challenged with OVA via caudal vein injection after the immunized OVA was absent from fish body at the fourth week after immunization. After OVA challenge, strong OVA-positive fluorescence signals were observed in lamina propria (LP) submucosa and epithelial cells of the hindgut at 30 min, increased proceeding toward the distal portion of intestinal folds, reached a peak at 2–3 h, and then weakened and disappeared at 12 h, indicating that the OVA rapidly diffused from bloodstream into LP submucosa and excreted across intestinal epithelium. Whereas in BSA-immunized and OVA-challenged control fish, the OVA was detected in LP submucosa but not in intestinal epithelium due to the lack of OVA-specific antibody. Accordingly, in intestinal epithelium, the transepithelial transport of OVA was confirmed by immunogold electron microscopy, and co-localization of OVA, IgM, and pIgR was illuminated by multiple-label immunofluorescence confocal microscopy and analyzed using Image J software. Furthermore, in gut mucus but not in serum, an ~800-kDa protein band showed IgM-positive, OVA-positive, and pIgR-positive simultaneously, and the OVA, together with IgM and secretory component (SC) of pIgR, could be immunoprecipitated by anti-OVA antibody, demonstrating the existence of SC–polymeric IgM–OVA complexes. All these results collectively revealed that the pIgR could transport mucosal IgM–OVA complexes from LP across intestinal epithelium into gut mucus via the transcytosis in flounder. These new findings provided direct evidences for pIgR-mediated immune excretion of IgM–antigen complexes, and better understanding the role of pIgR in mucosal immunity in teleost fish.

Engineered Multifunctional Albumin-Decorated Porous Silicon Nanoparticles for FcRn Translocation of Insulin

The last decade has seen remarkable advances in the development of drug delivery systems as alternative to parenteral injection-based delivery of insulin. Neonatal Fc receptor (FcRn)-mediated transcytosis has been recently proposed as a strategy to increase the transport of drugs across the intestinal epithelium. FcRn-targeted nanoparticles (NPs) could hijack the FcRn transcytotic pathway and cross the epithelial cell layer. In this study, a novel nanoparticulate system for insulin delivery based on porous silicon NPs is proposed. After surface conjugation with albumin and loading with insulin, the NPs are encapsulated into a pH-responsive polymeric particle by nanoprecipitation. The developed NP formulation shows controlled size and homogeneous size distribution. Transmission electron microscopy (TEM) images show successful encapsulation of the NPs into pH-sensitive polymeric particles. No insulin release is detected at acidic conditions, but a controlled release profile is observed at intestinal pH. Toxicity studies show high compatibility of the NPs with intestinal cells. In vitro insulin permeation across the intestinal epithelium shows approximately fivefold increase when insulin is loaded into FcRn-targeted NPs. Overall, these FcRn-targeted NPs offer a toolbox in the development of targeted therapies for oral delivery of insulin.

Immunization Elicits Antigen-Specific Antibody Sequestration in Dorsal Root Ganglia Sensory Neurons

The immune and nervous systems are two major organ systems responsible for host defense and memory. Both systems achieve memory and learning that can be retained, retrieved, and utilized for decades. Here, we report the surprising discovery that peripheral sensory neurons of the dorsal root ganglia (DRGs) of immunized mice contain antigen-specific antibodies. Using a combination of rigorous molecular genetic analyses, transgenic mice, and adoptive transfer experiments, we demonstrate that DRGs do not synthesize these antigen-specific antibodies, but rather sequester primarily IgG1 subtype antibodies. As revealed by RNA-seq and targeted quantitative PCR (qPCR), dorsal root ganglion (DRG) sensory neurons harvested from either naïve or immunized mice lack enzymes (i.e., RAG1, RAG2, AID, or UNG) required for generating antibody diversity and, therefore, cannot make antibodies. Additionally, transgenic mice that express a reporter fluorescent protein under the control of Igγ1 constant region fail to express Ighg1 transcripts in DRG sensory neurons. Furthermore, neural sequestration of antibodies occurs in mice rendered deficient in neuronal Rag2, but antibody sequestration is not observed in DRG sensory neurons isolated from mice that lack mature B cells [e.g., Rag1 knock out (KO) or μMT mice]. Finally, adoptive transfer of Rag1-deficient bone marrow (BM) into wild-type (WT) mice or WT BM into Rag1 KO mice revealed that antibody sequestration was observed in DRG sensory neurons of chimeric mice with WT BM but not with Rag1-deficient BM. Together, these results indicate that DRG sensory neurons sequester and retain antigen-specific antibodies released by antibody-secreting plasma cells. Coupling this work with previous studies implicating DRG sensory neurons in regulating antigen trafficking during immunization raises the interesting possibility that the nervous system collaborates with the immune system to regulate antigen-mediated responses.

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.

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.

Oral delivery of anti-diabetes therapeutics using cell penetrating and transcytosing peptide strategies

Oral delivery of insulin and other anti-diabetic peptides is inhibited by low intestinal absorption caused by the poor permeability across cellular membranes and the susceptibility to enzymatic degradation in the gastrointestinal tract. Cell-penetrating peptides (CPPs) have been investigated for a number of years as oral absorption enhancers for hydrophilic macromolecules by electrostatic or covalent conjugation on in conjunction with nanotechnology. Endogenous cellular uptake mechanisms present in the intestine can be exploited by engineering peptide conjugates that transcytose; entering cells by endocytosis and leaving by exocytosis. Efficiently delivering hydrophilic and sensitive peptide drugs to safely transverse the digestive barrier with no effect on gut physiology using remains a key driver for formulation research. Here we review the use of CPP and transcytosis peptide approaches, their modification and use in delivering anti-diabetic peptides (with the primary example of Insulin and engineered homologues) by direct oral administration to treat diabetes and associated metabolic disorders.

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.

Equine Arteritis Virus Elicits a Mucosal Antibody Response in the Reproductive Tract of Persistently Infected Stallions

Equine arteritis virus (EAV) has the ability to establish persistent infection in the reproductive tract of the stallion (carrier) and is continuously shed in its semen. We have recently demonstrated that EAV persists within stromal cells and a subset of lymphocytes in the stallion accessory sex glands in the presence of a significant local inflammatory response. In the present study, we demonstrated that EAV elicits a mucosal antibody response in the reproductive tract during persistent infection with homing of plasma cells into accessory sex glands. The EAV-specific immunoglobulin isotypes in seminal plasma included IgA, IgG1, IgG3/5, and IgG4/7. Interestingly, seminal plasma IgG1 and IgG4/7 possessed virus-neutralizing activity, while seminal plasma IgA and IgG3/5 did not. However, virus-neutralizing IgG1 and IgG4/7 in seminal plasma were not effective in preventing viral infectivity. In addition, the serological response was primarily mediated by virus-specific IgM and IgG1, while virus-specific serum IgA, IgG3/5, IgG4/7, and IgG6 isotype responses were not detected. This is the first report characterizing the immunoglobulin isotypes in equine serum and seminal plasma in response to EAV infection. The findings presented herein suggest that while a broader immunoglobulin isotype diversity is elicited in seminal plasma, EAV has the ability to persist in the reproductive tract, in spite of local mucosal antibody and inflammatory responses. This study provides further evidence that EAV employs complex immune evasion mechanisms during persistence in the reproductive tract that warrant further investigation.

The Neonatal Fc Receptor and Complement Fixation Facilitate Prophylactic Vaccine-Mediated Humoral Protection against Viral Infection in the Ocular Mucosa

The capacity of licensed vaccines to protect the ocular surface against infection is limited. Common ocular pathogens, such as HSV-1, are increasingly recognized as major contributors to visual morbidity worldwide. Humoral immunity is an essential correlate of protection against HSV-1 pathogenesis and ocular pathology, yet the ability of Ab to protect against HSV-1 is deemed limited due to the slow IgG diffusion rate in the healthy cornea. We show that a live-attenuated HSV-1 vaccine elicits humoral immune responses that are unparalleled by a glycoprotein subunit vaccine vis-à-vis Ab persistence and host protection. The live-attenuated vaccine was used to assess the impact of the immunization route on vaccine efficacy. The hierarchical rankings of primary immunization route with respect to efficacy were s.c. ≥ mucosal > i.m. Prime-boost vaccination via sequential s.c. and i.m. administration yielded greater efficacy than any other primary immunization route alone. Moreover, our data support a role for complement in prophylactic protection, as evidenced by intracellular deposition of C3d in the corneal epithelium of vaccinated animals following challenge and delayed viral clearance in C3-deficient mice. We also identify that the neonatal Fc receptor (FcRn) is upregulated in the cornea following infection or injury concomitant with increased Ab perfusion. Lastly, selective small interfering RNA-mediated knockdown of FcRn in the cornea impeded protection against ocular HSV-1 challenge in vaccinated mice. Collectively, these findings establish a novel mechanism of humoral protection in the eye involving FcRn and may facilitate vaccine and therapeutic development for other ocular surface diseases.

Pharmacokinetics of Monoclonal Antibodies

Monoclonal antibodies (mAbs) have developed in the last two decades into the backbone of pharmacotherapeutic interventions in a variety of indications, with currently more than 40 mAbs approved by the US Food and Drug Administration, and several dozens more in clinical development. This tutorial will review major drug disposition processes relevant for mAbs, and will highlight product‐specific and patient‐specific factors that modulate their pharmacokinetic (PK) behavior and need to be considered for successful clinical therapy.