Beta 2-microglobulin deficient mice catabolize IgG more rapidly than FcRn- alpha-chain deficient mice

Transport of antibody into the skin is only partially dependent upon the neonatal Fc-receptor

The dermis is the portal of entry for most vector-transmitted pathogens, making the host's immune response at this site critical in mitigating the magnitude of infection. For malaria, antibody-mediated neutralization of Plasmodium parasites in the dermis was recently demonstrated. However, surprisingly little is known about the mechanisms that govern antibody transport into the skin. Since the neonatal Fc receptor (FcRn) has been shown to transcytose IgG into various tissues, we sought to understand its contribution to IgG transport into the skin and antibody-mediated inhibition of Plasmodium parasites following mosquito bite inoculation. Using confocal imaging, we show that the transport of an anti-Langerin mAb into the skin occurs but is only partially reduced in mice lacking FcRn. To understand the relevance of FcRn in the context of malaria infection, we use the rodent parasite Plasmodium berghei and show that passively-administered anti-malarial antibody in FcRn deficient mice, does not reduce parasite burden to the same extent as previously observed in wildtype mice. Overall, our data suggest that FcRn plays a role in the transport of IgG into the skin but is not the major driver of IgG transport into this tissue. These findings have implications for the rational design of antibody-based therapeutics for malaria as well as other vector-transmitted pathogens.

Murine cancer cachexia models replicate elevated catabolic pembrolizumab clearance in humans

BACKGROUND

Monoclonal antibody (mAb) immune checkpoint inhibitor (ICI) therapies have dramatically impacted oncology this past decade. However, only about one-third of patients respond to treatment, and biomarkers to predict responders are lacking. Recent ICI clinical pharmacology data demonstrate high baseline drug clearance (CL0) significantly associates with shorter overall survival, independent of ICI exposure, in patients receiving ICI mAb therapies. This suggests CL0 may predict outcomes from ICI therapy, and cachectic signalling may link elevated CL0 and poor response. Our aim was to determine if mouse models of cancer cachexia will be useful for studying these phenomena and their underlying mechanisms.

METHODS

We evaluated pembrolizumab CL in the C26 and Lewis lung carcinoma mouse models of cancer cachexia. A single treatment of vehicle or pembrolizumab, at a dose of 2 or 10 mg/kg, was administered intravenously by tail vein injection. Pembrolizumab was quantified by an ELISA in serial plasma samples, and FcRn gene (Fcgrt) expression was assessed in liver using real-time quantitative reverse transcription PCR. Non-compartmental and mixed-effects pharmacokinetics analyses were performed.

RESULTS

We observed higher pembrolizumab CL0 and decreased Fcgrt expression in whole liver tissue from tumour-bearing vs. tumour-free mice. In multivariate analysis, presence of tumour, total murine IgG, muscle weight and Fcgrt expression were significant covariates on CL, and total murine IgG was a significant covariate on V1 and Q.

CONCLUSIONS

These data demonstrate increases in catabolic clearance of monoclonal antibodies observed in humans can be replicated in cachectic mice, in which Fcgrt expression is also reduced. Notably, FcRn activity is essential for proper antigen presentation and antitumour immunity, which may permit the study of cachexia's impact on FcRn-mediated clearance and efficacy of ICI therapies.

Boosting therapeutic potency of antibodies by taming Fc domain functions

Monoclonal antibodies (mAbs) are one of the most widely used drug platforms for infectious diseases or cancer therapeutics because they selectively target pathogens, infectious cells, cancerous cells, and even immune cells. In this way, they mediate the elimination of target molecules and cells with fewer side effects than other therapeutic modalities. In particular, cancer therapeutic mAbs can recognize cell-surface proteins on target cells and then kill the targeted cells by multiple mechanisms that are dependent upon a fragment crystallizable (Fc) domain interacting with effector Fc gamma receptors, including antibody-dependent cell-mediated cytotoxicity and antibody-dependent cell-mediated phagocytosis. Extensive engineering efforts have been made toward tuning Fc functions by either reinforcing (e.g. for targeted therapy) or disabling (e.g. for immune checkpoint blockade therapy) effector functions and prolonging the serum half-lives of antibodies, as necessary. In this report, we review Fc engineering efforts to improve therapeutic potency, and propose future antibody engineering directions that can fulfill unmet medical needs.

Fine-tuning the function of monoclonal antibodies (mAbs) holds promise for developing new therapeutic agents. Antibodies bind to pathogens or cancer cells, flagging them with Fc (fragment crystallizable) domain for destruction by the immune system. mAbs attached only to specific target cells enable lower side effect than other conventional drugs. Sang Taek Jung at Korea University and Tae Hyun Kang at Kookmin University, both in Seoul, reviewed recent developments in engineering therapeutic potency of mAbs. They report that mAbs can be engineered to activate effective immune cell types to treat a particular disease. Engineering can also increase mAbs’ persistence in the blood, enabling less frequent administration. Antibodies engineered to bind to two different antigens at once can also improve therapeutic efficacy. Applying these techniques could help developing new treatments against cancer, and infectious and autoimmune diseases.

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.

Rediscovering Beta-2 Microglobulin As a Biomarker across the Spectrum of Kidney Diseases

There is currently an unmet need for better biomarkers across the spectrum of renal diseases. In this paper, we revisit the role of beta-2 microglobulin (β₂M) as a biomarker in patients with chronic kidney disease and end-stage renal disease. Prior to reviewing the numerous clinical studies in the area, we describe the basic biology of β₂M, focusing in particular on its role in maintaining the serum albumin levels and reclaiming the albumin in tubular fluid through the actions of the neonatal Fc receptor. Disorders of abnormal β₂M function arise as a result of altered binding of β₂M to its protein cofactors and the clinical manifestations are exemplified by rare human genetic conditions and mice knockouts. We highlight the utility of β₂M as a predictor of renal function and clinical outcomes in recent large database studies against predictions made by recently developed whole body population kinetic models. Furthermore, we discuss recent animal data suggesting that contrary to textbook dogma urinary β₂M may be a marker for glomerular rather than tubular pathology. We review the existing literature about β₂M as a biomarker in patients receiving renal replacement therapy, with particular emphasis on large outcome trials. We note emerging proteomic data suggesting that β₂M is a promising marker of chronic allograft nephropathy. Finally, we present data about the role of β₂M as a biomarker in a number of non-renal diseases. The goal of this comprehensive review is to direct attention to the multifaceted role of β₂M as a biomarker, and its exciting biology in order to propose the next steps required to bring this recently rediscovered biomarker into the twenty-first century.

Immunoglobulin Transporting Receptors Are Potential Targets for the Immunity Enhancement and Generation of Mammary Gland Bioreactor

The functions of immunoglobulin transporting receptors (Ig transporting receptors) in immune system encompass from passive immunity to adaptive immunity by transporting immunoglobulins (Igs) and prolonging their half-life as well as enhancing immunosurveillance. Prior to the weaning, Ig transportations from mother to offspring confer the immediate passive immunity for neonates. After the weaning, FcRn and polymeric immunoglobulin receptor on infant intestinal epithelial cells retrieve Ig in intestinal lamina propria into the gut lumen for preventing pathogen invasion. This is not only improving the pathological consequences of infection but also helping the neonates for developing their own immune response; besides it would be the guidance for designing novel vaccines. Moreover, the investigations on Ig transporting receptors over-expressed transgenic animals have been carried out to improve Ig concentrations in serum and milk; thus, it would be a sustainable method to produce antibody-enriched milk-derived colostrum replacer for neonates. In order to generate mammary gland bioreactor, a series of methods have been developed for enhanced regulation of Ig transporting receptors expression and Ig transportation.

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.

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.

Fc-fusion proteins and FcRn: structural insights for longer-lasting and more effective therapeutics

Nearly 350 IgG-based therapeutics are approved for clinical use or are under development for many diseases lacking adequate treatment options. These include molecularly engineered biologicals comprising the IgG Fc-domain fused to various effector molecules (so-called Fc-fusion proteins) that confer the advantages of IgG, including binding to the neonatal Fc receptor (FcRn) to facilitate in vivo stability, and the therapeutic benefit of the specific effector functions. Advances in IgG structure-function relationships and an understanding of FcRn biology have provided therapeutic opportunities for previously unapproachable diseases. This article discusses approved Fc-fusion therapeutics, novel Fc-fusion proteins and FcRn-dependent delivery approaches in development, and how engineering of the FcRn-Fc interaction can generate longer-lasting and more effective therapeutics.

Colostrogenesis: IgG1 transcytosis mechanisms

Biological transport of intact proteins across epithelial cells has been documented for many absorptive and secretory tissues. Immunoglobulins were some of the earliest studied proteins in this category. The transcellular transport (transcytosis) of immunoglobulins in neonatal health and development has been recognized; the process is especially significant with ungulates because they do not transcytose immunoglobulins across the placenta to the neonate. Rather, they depend upon mammary secretion of colostrum and intestinal absorption of immunoglobulins in order to provide intestinal and systemic defense until the young ungulate develops its own humoral defense mechanisms. The neonatal dairy calf's ability to absorb immunoglobulins from colostrum is assisted by a ~24 h "open gut" phenomenon where large proteins pass the intestinal epithelial cells and enter the systemic system. However, a critical problem recognized for newborn dairy calves is that an optimum mass of colostrum Immunoglobulin G (IgG) needs to be absorbed within that 24 h window in order to provide maximal resistance to disease. Many calves do not achieve the optimum because of poor quality colostrum. While many studies have focused on calf absorption, the principal cause of the problem resides with the extreme variation (g to kg) in the mammary gland's capacity to transfer blood IgG1 into colostrum. Colostrum is a unique mammary secretory product that is formed during late pregnancy when mammary cells are proliferating and differentiating in preparation for lactation. In addition to the transcytosis of immunoglobulins, the mammary gland also concentrates a number of circulating hormones into colostrum. Remarkably, the mechanisms in the formation of colostrum in ungulates have been rather modestly studied. The mechanisms and causes of this variation in mammary gland transcytosis of IgG1 are examined, evaluated, and in some cases, explained.

Mechanism of the 24-hour rhythm of tumor necrosis factor-alpha formed by onset of rheumatoid arthritis

OBJECTIVE

Morning stiffness and plasma cytokine levels in rheumatoid arthritis (RA) patients exhibit 24-hour variations. Tumor necrosis factor-α (TNF-α) plays a central role in RA clinical conditions, including the invasion of inflammatory cells, destruction of cartilage, systemic inflammatory response and its levels show a 24-hour rhythm after the onset of RA. In this study, we investigated what cytokines and/or transcriptional factors are involved in the formation of 24-hour variations in TNF-α levels after the onset of RA using MRL/Mpj-Tnfrsf6(lpr) (MRL/lpr) mice.

METHOD

Blood was drawn at six different times from MRL/lpr mice to measure cytokines, serum amyloid A (SAA), IgG rheumatoid factor (IgG-RF) and corticosterone levels. Cytokine and transcriptional factor levels at the different times were measured in 10- and/or 15-week-old MRL/lpr mice. The promoter activity of TNF-α by lymphotoxins (LTs) was investigated using a dual-luciferase assay.

RESULTS

SAA and TNF-α concentrations clearly exhibited 24-hour rhythms with higher levels at the light phase and lower levels at the dark phase after RA crisis. The expression of LT-α and LT-β showed significant 24-hour rhythms in 15-week-old MRL/lpr mice and the phases of LT-α and LT-β levels were antiphase compared with that of TNF-α. AP-1 binding sites were found in LT-α and LT-β promoter regions, and jun mRNA expression corresponded to LT-α and LT-β levels. TNF-α promoter activity was decreased due to the co-transfection of LT-α and LT-β.

CONCLUSION

LT-α and LT-β controls the 24-hour rhythm in TNF-α levels after the onset of RA in order to suppress TNF-α promoter activity.

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.

ADME of monoclonal antibody biotherapeutics: knowledge gaps and emerging tools

Absorption, distribution, metabolism and excretion (ADME) data are pivotal for small-molecule drug development, with well-developed in vitro and in vivo correlation tools and guidances from regulatory agencies. In the past two decades, monoclonal antibody (mAb) biotherapeutics have been successfully approved, including derived novel conjugates of active molecules (toxins or bioactive peptides) for specific target delivery or half-life extension. However, ADME information of mAb therapeutics lags behind that of small molecules due to the complex nature of the molecules and lack of appropriate tools to study drug exposure, biotransformation, and target engagement in the vascular and tissue spaces. In this perspective, the current knowledge gaps on ADME of mAb-related therapeutics are reviewed with potential solutions from emerging analytical technologies.

Neutralizing activity and protective immunity to ricin toxin conferred by B subunit (RTB)-specific Fab fragments

SylH3 and 24B11 are murine monoclonal antibodies directed against different epitopes on ricin toxin's binding (RTB) subunit that have been shown to passively protect mice against ricin challenge. Here we report that Fab fragments of SylH3 and 24B11 neutralize ricin in a cell based assay, and in a mouse challenge model as effectively as their respective full length parental IgGs. These data demonstrate that immunity to ricin can occur independent of Fc-mediated clearance.

Crucial role of gamma interferon-producing CD4+ Th1 cells but dispensable function of CD8+ T cell, B cell, Th2, and Th17 responses in the control of Brucella melitensis infection in mice

Brucella spp. are facultative intracellular bacterial pathogens responsible for brucellosis, a worldwide zoonosis that causes abortion in domestic animals and chronic febrile disease associated with serious complications in humans. There is currently no approved vaccine against human brucellosis, and antibiotic therapy is long and costly. Development of a safe protective vaccine requires a better understanding of the roles played by components of adaptive immunity in the control of Brucella infection. The importance of lymphocyte subsets in the control of Brucella growth has been investigated separately by various research groups and remains unclear or controversial. Here, we used a large panel of genetically deficient mice to compare the importance of B cells, transporter associated with antigen processing (TAP-1), and major histocompatibility complex class II-dependent pathways of antigen presentation as well as T helper 1 (Th1), Th2, and Th17-mediated responses on the immune control of Brucella melitensis 16 M infection. We clearly confirmed the key function played by gamma interferon (IFN-γ)-producing Th1 CD4(+) T cells in the control of B. melitensis infection, whereas IFN-γ-producing CD8(+) T cells or B cell-mediated humoral immunity plays only a modest role in the clearance of bacteria during primary infection. In the presence of a Th1 response, Th2 or Th17 responses do not really develop or play a positive or negative role during the course of B. melitensis infection. On the whole, these results could improve our ability to develop protective vaccines or therapeutic treatments against brucellosis.

Antibody-dependent cell-mediated cytotoxicity- and complement-dependent cytotoxicity-independent bactericidal activity of an IgG against Pseudomonas aeruginosa O6ad

In addition to Ag recognition, some Abs are capable of killing target organisms in the absence of phagocytes and complement. In this study, we report that an anti-Pseudomonas aeruginosa O6ad LPS IgG(1), tobacco-expressed human S20 IgG(1) (te-hS20), as well as its recombinant Fab and single-chain variable fragment (scFv) fragments have cellular- and complement-independent bactericidal activity. te-hS20 and its Fab and scFv significantly reduced viability of P. aeruginosa O6ad in dose- and time-dependent manners in vitro and also showed lower levels of bactericidal activity against P. aeruginosa PAO1, but had no activity against P. aeruginosa O10, Escherichia coli TG1, and Streptococcus agalactiae. The H chain and its Fd fragment both had significant Ag-binding and bactericidal activities against P. aeruginosa O6ad. Bactericidal activity was completely inhibited with specific LPS Ag, suggesting that Ag binding is involved in the bactericidal mechanism. Live/dead cell staining and electron microscopic observations indicate that the bactericidal effect was due to disruption of the cell wall and suggest inhibition of cell division. In addition to te-hS20, the Fab and scFv were also protective in vivo, as leukopenic mice had prolonged and improved survival after administration of these Ab fragments followed by challenge with P. aeruginosa O6ad cells at 80-90% lethal dose, supporting a bactericidal mechanism independent of phagocytes and complement. Understanding of the bactericidal mechanism will allow assessment of the potential for therapeutic application of these Abs.

Functional and biological determinants affecting the duration of action and efficacy of anti-(+)-methamphetamine monoclonal antibodies in rats

These studies examined the in vivo pharmacokinetics and efficacy of five anti-methamphetamine monoclonal antibodies (mAbs, K(D) values from 11 to 250 nM) in rats. While no substantive differences in mAb systemic clearance (t(1/2)=6.1-6.9 days) were found, in vivo function was significantly reduced within 1-3 days for four of the five mAbs. Only mAb4G9 was capable of prolonged efficacy, as judged by prolonged high methamphetamine serum concentrations. MAb4G9 also maintained high amphetamine serum concentrations, along with reductions in methamphetamine and amphetamine brain concentrations, indicating neuroprotection. The combination of broad specificity for methamphetamine-like drugs, high affinity, and prolonged action in vivo suggests mAb4G9 is a potentially efficacious medication for treating human methamphetamine-related medical diseases.

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

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