FcRn: an IgG receptor on phagocytes with a novel role in phagocytosis

Lack of FcRn Impairs Natural Killer Cell Development and Functions in the Tumor Microenvironment

The neonatal Fc receptor (FcRn) is responsible for the recycling and transcytosis of IgG and albumin. FcRn level was found altered in cancer tissues and implicated in tumor immunosurveillance and neoplastic cell growth. However, the consequences of FcRn down-regulation in the anti-tumor immune response are not fully elucidated. By using the B16F10 experimental lung metastasis model in an FcRn-deficient microenvironment (FcRn-/- mice), we found lung metastasis associated with an abnormal natural killer (NK) cell phenotype. In FcRn-/- mice, NK cells were immature, as shown by their surface marker profile and their decreased ability to degranulate and synthesize interferon γ after chemical and IL-2 or IL-12, IL-15 and IL-18 activation. These new findings support the critical role of FcRn downregulation in the tumor microenvironment in anti-tumor immunity, via NK cell maturation and activation.

Hematopoietic cells as site of first-pass catabolism after subcutaneous dosing and contributors to systemic clearance of a monoclonal antibody in mice

The neonatal Fc receptor (FcRn) has been demonstrated to contribute to a high bioavailability of monoclonal antibodies (mAbs). In this study, we explored the cellular sites of FcRn-mediated protection after subcutaneous (SC) and intravenous (IV) administration. SC absorption and IV disposition kinetics of a mAb were studied in hFcRn transgenic (Tg) bone marrow chimeric mice in which hFcRn was restricted to radioresistant cells or hematopoietic cells. SC bioavailabilities close to 90% were observed in hFcRn Tg mice and chimeric mice with hFcRn expression in hematopoietic cells, whereas SC bioavailabilities were markedly lower when FcRn was missing in hematopoietic cells. Our study demonstrates: 1) FcRn in radiosensitive hematopoietic cells is required for high SC bioavailability, indicating first-pass catabolism after SC administration by hematopoietic cells; 2) FcRn-mediated transcytosis or recycling by radioresistent cells is not required for high SC bioavailability; and 3) after IV administration hematopoietic and radioresistent cells contribute about equally to clearance of the mAb. A pharmacokinetic model was devised to describe a mixed elimination via radioresistent and hematopoietic cells from vascular and extravascular compartments, respectively. Overall, the study indicates a relevant role of hematopoietic cells for first-pass clearance of mAbs after SC administration and confirms their role in the overall clearance of mAbs.

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.

Downregulation of the neonatal Fc receptor expression in non-small cell lung cancer tissue is associated with a poor prognosis

Lung cancer is the leading cause of cancer-related death worldwide. Although the recommended tumor, node and metastasis (TNM) classification and stage determination are important to select therapeutic options for patients with non-small cell lung carcinoma (NSCLC), additional molecular markers are required to indicate the prognosis, in particular within a specific stage, and help with the management of patients.

Because neonatal Fc receptor (FcRn) has recently been involved in colon cancer immunosurveillance, we measured its expression in non-cancerous and NSCLC lung tissues and evaluated its prognostic value in overall survival for patient with NSCLC. FcRn expression was determined at both mRNA and protein levels on cancerous and adjacent non-cancerous tissues from 80 NSCLC patients. In NSCLC, FcRn was mainly found in resident and tumor infiltrating immune cells. The corresponding mRNA and protein were significantly less abundant in lung tumor than non-cancerous tissue. Moreover, analysis of our cohort and datasets from the public data bases show that FCGRT mRNA down-regulation is a robust and independent, unfavorable predictive factor of NSCLC patient survival. We conclude that FCGRT mRNA expression may be a useful additional marker for immunoscoring, reflecting tumor immune system, and help in the decision-making process for NSCLC patients.

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

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

The Impact of IgG transplacental transfer on early life immunity

Pediatric vaccines have significantly reduced infectious disease-related infant mortality, but as protective immunity often require several infant vaccine doses; maternally-acquired antibodies are critical to protect infants during the first months of life. Consequently, immunization of pregnant women is an important strategy not only to protect mothers from infection, but also to provide immunity to young infants. Nevertheless, maternal immunization can also negatively impact early life immunity. In fact, maternal antibodies can interfere with the development of infant immune responses, though it is unclear if such interference is clinically significant. Moreover, the transplacental transfer of maternal immunoglobulin therapeutics can be harmful to the fetus. Thus, the risk/benefit of maternal immunization for both the mother and the fetus should be carefully weighed. In addition, it is critical to fully understand the mechanisms by which IgG is transferred across the placenta in order to develop optimal maternal and infant immunization strategies.

Factors Affecting the FcRn-Mediated Transplacental Transfer of Antibodies and Implications for Vaccination in Pregnancy

At birth, neonates are particularly vulnerable to infection and transplacental transfer of immunoglobulin G (IgG) from mother to fetus provides crucial protection in the first weeks of life. Transcytosis of IgG occurs via binding with the neonatal Fc receptor (FcRn) in the placental synctiotrophoblast. As maternal vaccination becomes an increasingly important strategy for the protection of young infants, improving our understanding of transplacental transfer and the factors that may affect this will become increasingly important, especially in low-income countries where the burden of morbidity and mortality is highest. This review highlights factors of relevance to maternal vaccination that may modulate placental transfer—IgG subclass, glycosylation of antibody, total maternal IgG concentration, maternal disease, infant gestational age, and birthweight—and outlines the conflicting evidence and questions that remain regarding the complexities of these relationships. Furthermore, the intricacies of the Ab–FcRn interaction remain poorly understood and models that may help address future research questions are described.

Antiviral Functions of Human Immunodeficiency Virus Type 1 (HIV-1)-Specific IgG Antibodies: Effects of Antiretroviral Therapy and Implications for Therapeutic HIV-1 Vaccine Design

Contemporary antiretroviral therapy (ART) is effective and tolerable for long periods of time but cannot eradicate human immunodeficiency virus type 1 (HIV-1) infection by either elimination of viral reservoirs or enhancement of HIV-1-specific immune responses. Boosting "protective" HIV-1-specific immune responses by active or passive immunization will therefore be necessary to control or eradicate HIV-1 infection and is currently the topic of intense investigation. Recently reported studies conducted in HIV patients and non-human primate (NHP) models of HIV-1 infection suggest that HIV-1-specific IgG antibody responses may contribute to the control of HIV-1 infection. However, production of IgG antibodies with virus neutralizing activity by vaccination remains problematic and while vaccine-induced natural killer cell-activating IgG antibodies have been shown to prevent the acquisition of HIV-1 infection, they may not be sufficient to control or eradicate established HIV-1 infection. It is, therefore, important to consider other functional characteristics of IgG antibody responses. IgG antibodies to viruses also mediate opsonophagocytic antibody responses against virions and capsids that enhance the function of phagocytic cells playing critical roles in antiviral immune responses, particularly conventional dendritic cells and plasmacytoid dendritic cells. Emerging evidence suggests that these antibody functions might contribute to the control of HIV-1 infection. In addition, IgG antibodies contribute to the intracellular degradation of viruses via binding to the cytosolic fragment crystallizable (Fc) receptor tripartite motif containing-21 (TRIM21). The functional activity of an IgG antibody response is influenced by the IgG subclass content, which affects binding to antigens and to Fcγ receptors on phagocytic cells and to TRIM21. The IgG subclass content and avidity of IgG antibodies is determined by germinal center (GC) reactions in follicles of lymphoid tissue. As HIV-1 infects cells in GCs and induces GC dysfunction, which may persist during ART, strategies for boosting HIV-1-specific IgG antibody responses should include early commencement of ART and possibly the use of particular antiretroviral drugs to optimize drug levels in lymphoid follicles. Finally, enhancing particular functions of HIV-1-specific IgG antibody responses by using adjuvants or cytokines to modulate the IgG subclass content of the antibody response might be investigated in NHP models of HIV-1 infection and during trials of therapeutic vaccines in HIV patients.

Crucial Role for Immune Complexes but Not FcRn in Immunization against Anti-TNF-α Antibodies after a Single Injection in Mice

The immunogenicity of infliximab and adalimumab is a major concern because patients may develop Abs also called antidrug Abs (ADA), directed against these anti-TNF-α Abs after just a few weeks of treatment. These ADAs can lead to a decrease in biologic concentration, which is associated with lower treatment efficacy. Our aim was to study the involvement of immune complexes and neonatal Fc receptor (FcRn) in the emergence of ADAs in the case of anti-TNF-α Abs. Wild type and FcRn knockout mice were injected once with either infliximab or adalimumab, alone or preincubated with TNF-α. Adalimumab cross-reacts with murine TNF-α whereas infliximab is species specific. When injected alone, only adalimumab elicited a humoral response. By preforming immune complexes with TNF-α, an anti-infliximab response was elicited. Surprisingly, both wild type and FcRn knockout mice were able to mount an immune response against anti-TNF-α Abs, suggesting that immune complexes are a major determinant of this immunization.

Fcγ Receptor Heterogeneity in Leukocyte Functional Responses

Antibodies participate in defense of the organism from all types of pathogens, including viruses, bacteria, fungi, and protozoa. IgG antibodies recognize their associated antigen via their two Fab portions and are in turn recognized though their Fc portion by specific Fcγ receptors (FcγRs) on the membrane of immune cells. Multiple types and polymorphic variants of FcγR exist. These receptors are expressed in many cells types and are also redundant in inducing cell responses. Crosslinking of FcγR on the surface of leukocytes activates several effector functions aimed toward the destruction of pathogens and the induction of an inflammatory response. In the past few years, new evidence on how the particular IgG subclass and the glycosylation pattern of the antibody modulate the IgG-FcγR interaction has been presented. Despite these advances, our knowledge of what particular effector function is activated in a certain cell and in response to a specific type of FcγR remains very limited today. On one hand, each immune cell could be programmed to perform a particular cell function after FcγR crosslinking. On the other, each FcγR could activate a particular signaling pathway leading to a unique cell response. In this review, I describe the main types of FcγRs and our current view of how particular FcγRs activate various signaling pathways to promote unique leukocyte functions.

IgG Fc variant cross-reactivity between human and rhesus macaque FcγRs

Non-human primate (NHP) studies are often an essential component of antibody development efforts before human trials. Because the efficacy or toxicity of candidate antibodies may depend on their interactions with Fcγ receptors (FcγR) and their resulting ability to induce FcγR-mediated effector functions such as antibody-dependent cell-meditated cytotoxicity and phagocytosis (ADCP), the evaluation of human IgG variants with modulated affinity toward human FcγR is becoming more prevalent in both infectious disease and oncology studies in NHP. Reliable translation of these results necessitates analysis of the cross-reactivity of these human Fc variants with NHP FcγR. We report evaluation of the binding affinities of a panel of human IgG subclasses, Fc amino acid point mutants and Fc glycosylation variants against the common allotypes of human and rhesus macaque FcγR by applying a high-throughput array-based surface plasmon resonance platform. The resulting data indicate that amino acid variation present in rhesus FcγRs can result in disrupted, matched, or even increased affinity of IgG Fc variants compared with human FcγR orthologs. These observations emphasize the importance of evaluating species cross-reactivity and developing an understanding of the potential limitations or suitability of representative in vitro and in vivo models before human clinical studies when either efficacy or toxicity may be associated with FcγR engagement.

Antibody-Based Immunotherapy To Treat and Prevent Infection with Hypervirulent Klebsiella pneumoniae

Hypervirulent Klebsiella pneumoniae (hvKp) strains are predicted to become a major threat in Asia if antibiotic resistance continues to spread. Anticapsular antibodies (Abs) were developed because disseminated infections caused by hvKp are associated with significant morbidity and mortality, even with antibiotic-sensitive strains. K1-serotype polysaccharide capsules (K1-CPS) are expressed by the majority of hvKp strains. In this study, K1-CPS-specific IgG Abs were generated by conjugation of K1-CPS to immunogenic anthrax protective antigen (PA) protein. Opsonophagocytic efficacy was measured in vitro and in vivo by intravital microscopy in murine livers. In vivo protection was tested in murine models, including a novel model for dissemination in hvKp-colonized mice. Protective efficacy of monoclonal antibodies (MAbs) 4C5 (IgG1) and 19A10 (IgG3) was demonstrated both in murine sepsis and pulmonary infection. In hvKp-colonized mice, MAb treatment significantly decreased dissemination of hvKp from the gut to mesenteric lymph nodes and organs. Intravital microscopy confirmed efficient opsonophagocytosis and clearance of bacteria from the liver. In vitro studies demonstrate that MAbs work predominantly by promoting FcR-mediated phagocytosis but also indicate that MAbs enhance the release of neutrophil extracellular traps (NETs). In anticipation of increasing antibiotic resistance, we propose further development of these and other Klebsiella-specific MAbs for therapeutic use.

Neonatal Fc Receptor Regulation of Lung Immunoglobulin and CD103+ Dendritic Cells Confers Transient Susceptibility to Tuberculosis

The neonatal Fc receptor (FcRn) extends the systemic half-life of IgG antibodies by chaperoning bound Fc away from lysosomal degradation inside stromal and hematopoietic cells. FcRn also transports IgG across mucosal barriers into the lumen, and yet little is known about how FcRn modulates immunity in the lung during homeostasis or infection. We infected wild-type (WT) and FcRn-deficient (fcgrt^−/−) mice with Pseudomonas aeruginosa or Mycobacterium tuberculosis to investigate whether recycling and transport of IgG via FcRn influences innate and adaptive immunity in the lung in response to bacterial infection. We found that FcRn expression maintains homeostatic IgG levels in lung and leads to preferential secretion of low-affinity IgG ligands into the lumen. Fcgrt^−/− animals exhibited no evidence of developmental impairment of innate immunity in the lung and were able to efficiently recruit neutrophils in a model of acute bacterial pneumonia. Although local humoral immunity in lung increased independently of the presence of FcRn during tuberculosis, there was nonetheless a strong impact of FcRn deficiency on local adaptive immunity. We show that the quantity and quality of IgG in airways, as well as the abundance of dendritic cells in the lung, are maintained by FcRn. FcRn ablation transiently enhanced local T cell immunity and neutrophil recruitment during tuberculosis, leading to a lower bacterial burden in lung. This novel understanding of tissue-specific modulation of mucosal IgG isotypes in the lung by FcRn sheds light on the role of mucosal IgG in immune responses in the lung during homeostasis and bacterial disease.

Tyrosine kinase signaling pathways in neutrophils

Neutrophils play a critical role in antimicrobial host defense, but their improper activation also contributes to inflammation-induced tissue damage. Therefore, understanding neutrophil biology is important for the understanding, diagnosis, and therapy of both infectious and inflammatory diseases. Neutrophils express a large number of cell-surface receptors that sense extracellular cues and trigger various functional responses through complex intracellular signaling pathways. During the last several years, we and others have shown that tyrosine kinases play a critical role in those processes. In particular, Src-family and Syk tyrosine kinases couple Fc-receptors and adhesion receptors (integrins and selectins) to various neutrophil effector functions. This pathway shows surprising similarity to lymphocyte antigen receptor signaling and involves various other enzymes (e.g. PLCγ2), exchange factors (e.g. Vav-family members) and adapter proteins (such as ITAM-containing adapters, SLP-76, and CARD9). Those mediators trigger various antimicrobial functions and play a critical role in coordinating the inflammatory response through the release of inflammatory mediators, such as chemokines and LTB4 . Interestingly, however, tyrosine kinases have a limited direct role in the migration of neutrophils to the site of inflammation. Here, we review the role of tyrosine kinase signaling pathways in neutrophils and how those pathways contribute to neutrophil activation in health and disease.

The role of Fc receptors in HIV prevention and therapy

Over the past decade, a wealth of experimental evidence has accumulated supporting the importance of Fc receptor (FcR) ligation in antibody-mediated pathology and protection in many disease states. Here we present the diverse evidence base that has accumulated as to the importance of antibody effector functions in the setting of HIV prevention and therapy, including clinical correlates, genetic associations, viral evasion strategies, and a rapidly growing number of compelling animal model experiments. Collectively, this work identifies antibody interactions with FcR as important to both therapeutic and prophylactic strategies involving both passive and active immunity. These findings mirror those in other fields as investigators continue to work toward identifying the right antibodies and the right effectors to be present at the right sites at the right time.

The multiple facets of FcRn in immunity

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

Cross-talk between pathogen recognizing Toll-like receptors and immunoglobulin Fc receptors in immunity

The individual role of pathogen-binding Toll-like receptors (TLRs) and antibody-binding Fc receptors (FcRs) during pathogenic infections has been studied extensively. However, combined activation of these different receptor classes has received little attention, even though they are triggered simultaneously when immune cells bind antibody-opsonized pathogens. In the last few years, it has become evident that joined activation of TLRs and FcRs substantially tailors inflammatory immune responses, which is an efficient and controlled mechanism of the host to act upon invading pathogens. In this review, we discuss the mechanisms of cross-talk between different TLRs and FcRs and the resulting inflammatory immune responses. Furthermore, we propose how chronic activation via this cross-talk might be detrimental in inflammatory (auto) immune diseases. We conclude with the potential exploitation of the interplay between TLRs and FcRs for monoclonal antibody therapy to target tumors. Future interests in this field of research include establishing a more detailed and mechanistic understanding of the mode of action of TLR and FcR cross-talk and exploration of its physiological importance in health and disease. This may furthermore open up novel therapeutic options for intervention in inflammatory diseases or cancer.

Mouse and human FcR effector functions

Mouse and human FcRs have been a major focus of attention not only of the scientific community, through the cloning and characterization of novel receptors, and of the medical community, through the identification of polymorphisms and linkage to disease but also of the pharmaceutical community, through the identification of FcRs as targets for therapy or engineering of Fc domains for the generation of enhanced therapeutic antibodies. The availability of knockout mouse lines for every single mouse FcR, of multiple or cell-specific--'à la carte'--FcR knockouts and the increasing generation of hFcR transgenics enable powerful in vivo approaches for the study of mouse and human FcR biology. This review will present the landscape of the current FcR family, their effector functions and the in vivo models at hand to study them. These in vivo models were recently instrumental in re-defining the properties and effector functions of FcRs that had been overlooked or discarded from previous analyses. A particular focus will be made on the (mis)concepts on the role of high-affinity IgG receptors in vivo and on results from antibody engineering to enhance or abrogate antibody effector functions mediated by FcRs.

IgG1 Allotypes Influence the Pharmacokinetics of Therapeutic Monoclonal Antibodies through FcRn Binding

Because IgG1 allotypes might have different half-lives, their influence on infliximab (G1m17,1 allotype) pharmacokinetics was investigated in a group of spondyloarthritis patients. Infliximab was found to have a shorter half-life in patients homozygous for the G1m17,1 allotypes than in those carrying the G1m3 with no G1m1 (G1m3,-1) allotype. Because the neonatal FcR (FcRn) is involved in the pharmacokinetics of mAbs, the interaction of different IgG1 allotypes with FcRn was examined using cellular assays and surface plasmon resonance. G1m17,1 mAbs, such as infliximab and rituximab, were shown to bind more efficiently to FcRn and to be transcytosed better than the G1m3,-1 mAb cetuximab, which explains why infliximab is a better competitor for endogenous IgG1 in G1m3,-1 allotype-bearing patients. A set of four allotype variants of adalimumab (G1m17,1; G1m17,-1; G1m3,1; and G1m3,-1) was also tested for its binding to FcRn, revealing that the G1m3,1 variant, not present in commercial mAbs, binds more efficiently to FcRn and is transcytosed better than the other three variants, all of which are found in therapeutic mAbs.

Improved in vivo anti-tumor effects of IgA-Her2 antibodies through half-life extension and serum exposure enhancement by FcRn targeting

Antibody therapy is a validated treatment approach for several malignancies. All currently clinically applied therapeutic antibodies (Abs) are of the IgG isotype. However, not all patients respond to this therapy and relapses can occur. IgA represents an alternative isotype for antibody therapy that engages FcαRI expressing myeloid effector cells, such as neutrophils and monocytes. IgA Abs have been shown to effectively kill tumor cells both in vitro and in vivo. However, due to the short half-life of IgA Abs in mice, daily injections are required to reach an effect comparable to IgG Abs. The relatively long half-life of IgG Abs and serum albumin arises from their capability of interacting with the neonatal Fc receptor (FcRn). As IgA Abs lack a binding site for FcRn, we generated IgA Abs with the variable regions of the Her2-specific Ab trastuzumab and attached an albumin-binding domain (ABD) to the heavy or light chain (HCABD/LCABD) to extend their serum half-life. These modified Abs were able to bind albumin from different species in vitro. Furthermore, tumor cell lysis of IgA-Her2-LCABD Abs in vitro was similar to unmodified IgA-Her2 Abs. Pharmacokinetic studies in mice revealed that the serum exposure and half-life of the modified IgA-Her2 Abs was extended. In a xenograft mouse model, the modified IgA1 Abs exhibited a slightly, but significantly, improved anti-tumor response compared to the unmodified Ab. In conclusion, empowering IgA Abs with albumin-binding capacity results in in vitro and in vivo functional Abs with an enhanced exposure and prolonged half-life.

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.

Analysis of Response Elements Involved in the Regulation of the Human Neonatal Fc Receptor Gene (FCGRT)

Human epithelial, endothelial and PMA-differentiated THP-1 cell lines were used as model systems to study the transcriptional regulation of the human FCGRT gene encoding the alpha chain of hFcRn. The data obtained from site-directed mutagenesis in transient transfection experiments indicate that the Sp1 sites at positions -641, -635, and -313, CF1/YY1 elements at positions -586 and -357, and the AP-1 motif at -276 within the-660/-233 fragment of the human FCGRT promoter (hFCGRT) participate in the regulation of human FCGRT in all selected cell lines. However, their individual contribution to promoter activity is not equivalent. The Sp1 binding site at -313 and the AP-1 site at -276 are critical for the activity of the hFCGRT promoter in epithelial and endothelial cells. Moreover, the CF1/YY1 site at -586 in differentiated THP-1 cells, plays an essential role in the transcriptional activity of the promoter. In addition, the C/EBPbeta binding site at -497 of the hFCGRT promoter in epithelial and endothelial cells, and the C/EBPbeta motif located at -497 and -233 within the hFCGRT promoter in differentiated THP-1 cells may function as positive regulatory sequences in response to LPS or PMA stimulation. EMSA and supershift analyses showed that the functionally identified binding motifs in the hFCGRT promoter were able to specifically interact with their corresponding (Sp1, Sp2, Sp3, c-Fos, c-Jun, YY1, and C/EBPbeta or C/EBPdelta) transcription factors (TFs), suggesting their possible involvement in the regulation of the human FCGRT gene expression.

FcRn: The Architect Behind the Immune and Nonimmune Functions of IgG and Albumin

The neonatal FcR (FcRn) belongs to the extensive and functionally divergent family of MHC molecules. Contrary to classical MHC family members, FcRn possesses little diversity and is unable to present Ags. Instead, through its capacity to bind IgG and albumin with high affinity at low pH, it regulates the serum half-lives of both of these proteins. In addition, FcRn plays an important role in immunity at mucosal and systemic sites through its ability to affect the lifespan of IgG, as well as its participation in innate and adaptive immune responses. Although the details of its biology are still emerging, the ability of FcRn to rescue albumin and IgG from early degradation represents an attractive approach to alter the plasma half-life of pharmaceuticals. We review some of the most novel aspects of FcRn biology, immune as well as nonimmune, and provide some examples of FcRn-based therapies.

Fc Engineering of Human IgG1 for Altered Binding to the Neonatal Fc Receptor Affects Fc Effector Functions

Engineering of the constant Fc part of monoclonal human IgG1 (hIgG1) Abs is an approach to improve effector functions and clinical efficacy of next-generation IgG1-based therapeutics. A main focus in such development is tailoring of in vivo half-life and transport properties by engineering the pH-dependent interaction between IgG and the neonatal Fc receptor (FcRn), as FcRn is the main homeostatic regulator of hIgG1 half-life. However, whether such engineering affects binding to other Fc-binding molecules, such as the classical FcγRs and complement factor C1q, has not been studied in detail. These effector molecules bind to IgG1 in the lower hinge-CH2 region, structurally distant from the binding site for FcRn at the CH2-CH3 elbow region. However, alterations of the structural composition of the Fc may have long-distance effects. Indeed, in this study we show that Fc engineering of hIgG1 for altered binding to FcRn also influences binding to both the classical FcγRs and complement factor C1q, which ultimately results in alterations of cellular mechanisms such as Ab-dependent cell-mediated cytotoxicity, Ab-dependent cellular phagocytosis, and Ab-dependent complement-mediated cell lysis. Thus, engineering of the FcRn-IgG1 interaction may greatly influence effector functions, which has implications for the therapeutic efficacy and use of Fc-engineered hIgG1 variants.

Targeting FcRn for the modulation of antibody dynamics

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

Selection of IgG Variants with Increased FcRn Binding Using Random and Directed Mutagenesis: Impact on Effector Functions

Despite the reasonably long half-life of immunoglogulin G (IgGs), market pressure for higher patient convenience while conserving efficacy continues to drive IgG half-life improvement. IgG half-life is dependent on the neonatal Fc receptor (FcRn), which among other functions, protects IgG from catabolism. FcRn binds the Fc domain of IgG at an acidic pH ensuring that endocytosed IgG will not be degraded in lysosomal compartments and will then be released into the bloodstream. Consistent with this mechanism of action, several Fc-engineered IgG with increased FcRn affinity and conserved pH dependency were designed and resulted in longer half-life in vivo in human FcRn-transgenic mice (hFcRn), cynomolgus monkeys, and recently in healthy humans. These IgG variants were usually obtained by in silico approaches or directed mutagenesis in the FcRn-binding site. Using random mutagenesis, combined with a pH-dependent phage display selection process, we isolated IgG variants with improved FcRn-binding, which exhibited longer in vivo half-life in hFcRn mice. Interestingly, many mutations enhancing Fc/FcRn interaction were located at a distance from the FcRn-binding site validating our random molecular approach. Directed mutagenesis was then applied to generate new variants to further characterize our IgG variants and the effect of the mutations selected. Since these mutations are distributed over the whole Fc sequence, binding to other Fc effectors, such as complement C1q and FcγRs, was dramatically modified, even by mutations distant from these effectors’ binding sites. Hence, we obtained numerous IgG variants with increased FcRn-binding and different binding patterns to other Fc effectors, including variants without any effector function, providing distinct “fit-for-purpose” Fc molecules. We therefore provide evidence that half-life and effector functions should be optimized simultaneously as mutations can have unexpected effects on all Fc receptors that are critical for IgG therapeutic efficacy.

NFκB induces overexpression of bovine FcRn: a novel mechanism that further contributes to the enhanced immune response in genetically modified animals carrying extra copies of FcRn

Among the many functions of the neonatal Fc receptor (FcRn) for IgG, it binds to IgG-opsonized antigen complexes and propagates their traffic into lysosomes where antigen processing occurs. We previously reported that transgenic (Tg) mice and rabbits that carry multiple copies and overexpress FcRn have augmented humoral immune responses. Nuclear factor-kappa B (NFκB) is a critical molecule in the signaling cascade in the immune response. NFκB induces human FcRn expression and our previous in silico analysis suggested NFκB binding sites in the promoter region of the bovine (b) FcRn α-chain gene (FCGRT). Here, we report the identification of three NFκB transcription binding sites in the promoter region of this gene using luciferase reporter gene technology, electromobility shift assay and supershift analysis. Stimulation of primary bovine endothelial cells with the Toll-like receptor-4 ligand lipopolysaccharide (LPS), which mediates its effect via NFκB, resulted in rapid upregulation of the bFcRn expression and a control gene, bovine E-selectin. This rapid bFcRn gene induction was also observed in the spleen of bFcRn Tg mice treated with intraperitoneally injected LPS, analyzed by northern blot analysis. Finally, NFκB-mediated bFcRn upregulation was confirmed at the protein level in macrophages isolated from the bFcRn Tg mice using flow cytometry with a newly developed FcRn specific monoclonal antibody that does not cross-react with the mouse FcRn. We conclude that NFκB regulates bFcRn expression and thus optimizes its functions, e.g., in the professional antigen presenting cells, and contributes to the much augmented humoral immune response in the bFcRn Tg mice.

IgG subclasses and allotypes: from structure to effector functions

Of the five immunoglobulin isotypes, immunoglobulin G (IgG) is most abundant in human serum. The four subclasses, IgG1, IgG2, IgG3, and IgG4, which are highly conserved, differ in their constant region, particularly in their hinges and upper CH2 domains. These regions are involved in binding to both IgG-Fc receptors (FcγR) and C1q. As a result, the different subclasses have different effector functions, both in terms of triggering FcγR-expressing cells, resulting in phagocytosis or antibody-dependent cell-mediated cytotoxicity, and activating complement. The Fc-regions also contain a binding epitope for the neonatal Fc receptor (FcRn), responsible for the extended half-life, placental transport, and bidirectional transport of IgG to mucosal surfaces. However, FcRn is also expressed in myeloid cells, where it participates in both phagocytosis and antigen presentation together with classical FcγR and complement. How these properties, IgG-polymorphisms and post-translational modification of the antibodies in the form of glycosylation, affect IgG-function will be the focus of the current review.

On the perplexingly low rate of transport of IgG2 across the human placenta

The neonatal receptor, FcRn, mediates both serum half–life extension as well as active transport of maternal IgG to the fetus during pregnancy. Therefore, transport efficiency and half-life go hand-in-hand. However, while the half-life of the human IgG2 subclass is comparable to IgG1, the placental transport of IgG2 is not, with the neonatal IgG1 levels generally exceeding maternal levels at birth, but not for IgG2. We hypothesized that the unique short-hinged structure of IgG2, which enables its κ-, but not λ-isotype to form at least three different structural isoforms, might be a contributing factor to these differences. To investigate whether there was any preference for either light chain, we measured placental transport of IgG subclasses as well as κ/λ-light chain isotypes of IgG1 and IgG2 in 27 matched mother-child pairs. We also studied the half-life of IgG1 and IgG2 light chain isotypes in mice, as well as that of synthesized IgG2 structural isotypes κA and κB. In order to investigate serum clearance of IgG1 and IgG2 light-chain isotypes in humans, we quantified the relative proportions of IgG1 and IgG2 light chains in hypogammaglobulinemia patients four weeks after IVIg infusion and compared to the original IVIg isotype composition. None of our results indicate any light chain preference in either of the FcRn mediated mechanisms; half-life extension or maternal transport.

Prophylactic anti-D preparations display variable decreases in Fc-fucosylation of anti-D

BACKGROUND

RhIG is obtained from hyperimmunized healthy anti-D donors (HIDs) boosted with D+ red blood cells (RBCs). One hypothesis for its mechanism of action is fast clearance of opsonized D+ RBCs through Fcγ receptor (FcγR)III. Levels of immunoglobulin (Ig)G Fc-fucosylation influence interactions with FcγRIII, with less Fc-fucosylation strengthening the interaction.

STUDY DESIGN AND METHODS

Anti-D IgG1 Fc-glycosylation patterns in 93 plasma samples from 28 male and 28 female Dutch HIDs and RhIG were analyzed with mass spectrometry. The Fc-glycosylation profiles of HIDs were evaluated with regard to their immunization history.

RESULTS

HID sera demonstrated clearly lowered anti-D Fc-fucosylation compared to normal IgG fucosylation (93%); this was more pronounced for female than for male HIDs (47% vs. 65%, p = 0.001). RhIG preparations from seven manufacturers varied greatly in the level of Fc-fucosylation (56%-91%). The level of fucosylation slightly increased upon repeated immunization, although it remained fairly constant over time. The RhIG from the different manufacturers all demonstrated increased Fc-galactosylation (64%-82%) compared to total IgG (38%-51%).

CONCLUSION

RhIG preparations vary in Fc-fucosylation and all demonstrate increased galactosylation. Despite not knowing the exact working mechanism, immunoprophylaxis could perhaps be optimized by selection of donors whose anti-D have low amounts of Fc-fucose, to increase the clearance activity of anti-D preparations, as well as high amounts of galactosylation, for anti-inflammatory effects. Implementing a biologic assay in the standardization of RhIG preparations might be considered.

The Role of FcRn in Antigen Presentation

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

Low anti-RhD IgG-Fc-fucosylation in pregnancy: a new variable predicting severity in haemolytic disease of the fetus and newborn

Haemolytic disease of the fetus and newborn (HDFN) may occur when maternal IgG antibodies against red blood cells (RBCs), often anti-RhD (anti-D) antibodies, cross the placenta and mediate the destruction of RBCs via phagocytic IgG-Fc-receptors (FcγR). Clinical severity is not strictly related to titre and is more accurately predicted by the diagnostically-applied monocyte-based antibody-dependent cellular cytotoxicity (ADCC), a sensitive test with relatively low specificity. This suggests that other factors are involved in the pathogenesis of HDFN. Binding of IgG to FcγR requires the N-linked glycan at position 297 in the IgG-Fc-region, consisting of several different glycoforms. We therefore systematically analysed IgG-derived glycopeptides by mass spectrometry from 70 anti-D IgG1 antibodies purified from the plasma of alloimmunized pregnant women. This revealed a variable decrease in Fc-fucosylation in the majority of anti-D IgG1 (even down to 12%), whereas the total IgG of these patients remained highly fucosylated, like in healthy individuals (>90%). The degree of anti-D fucosylation correlated significantly with CD16 (FcγRIIIa)-mediated ADCC, in agreement with increased affinity of defucosylated IgG to human FcγRIIIa. Additionally, low anti-D fucosylation correlated significantly with low fetal-neonatal haemoglobin levels, thus with increased haemolysis, suggesting IgG-fucosylation to be an important pathological feature in HDFN with diagnostic potential.

Contribution of Human FcγRs to Disease with Evidence from Human Polymorphisms and Transgenic Animal Studies

The biological activities of human IgG antibodies predominantly rely on a family of receptors for the Fc portion of IgG, FcγRs: FcγRI, FcγRIIA, FcγRIIB, FcγRIIC, FcγRIIIA, FcγRIIIB, FcRL5, FcRn, and TRIM21. All FcγRs bind IgG at the cell surface, except FcRn and TRIM21 that bind IgG once internalized. The affinity of FcγRs for IgG is determined by polymorphisms of human FcγRs and ranges from 2 × 10⁴ to 8 × 10⁷ M⁻¹. The biological functions of FcγRs extend from cellular activation or inhibition, IgG-internalization/endocytosis/phagocytosis to IgG transport and recycling. This review focuses on human FcγRs and intends to present an overview of the current understanding of how these receptors may contribute to various pathologies. It will define FcγRs and their polymorphic variants, their affinity for human IgG subclasses, and review the associations found between FcγR polymorphisms and human pathologies. It will also describe the human FcγR-transgenic mice that have been used to study the role of these receptors in autoimmune, inflammatory, and allergic disease models.

Opportunities to exploit non-neutralizing HIV-specific antibody activity

Antibodies act as a nexus between innate and adaptive immunity: they provide a means to engage a spectrum of innate immune effector cells in order to clear viral particles and infected cells and prime antigen presentation. This functional landscape is remarkably complex, and depends on antibody isotype, subclass, and glycosylation; the expression levels and patterns of a suite of Fc receptors with both complementary and opposing activities; and a host of innate immune cells capable of differential responses to opsonized particles and present at different sites. In vivo, even neutralizing antibodies rely on their ability to act as molecular beacons and recruit innate immune effector cells in order to provide protection, and results from both human and macaque studies have implicated these effector functions in vaccinemediated protection. Thus, while enhancing effector function is a tractable handle for potentiating antibody-mediated protection from HIV infection, success will depend critically on leveraging understanding of the means by which antibodies with specific functional profiles could be elicited, which effector functions could provide optimal protection, and perhaps most critically, how to efficiently recruit the innate effector cells present at sites of infection.

Dissection of the neonatal Fc receptor (FcRn)-albumin interface using mutagenesis and anti-FcRn albumin-blocking antibodies

Albumin is the most abundant protein in blood and plays a pivotal role as a multitransporter of a wide range of molecules such as fatty acids, metabolites, hormones, and toxins. In addition, it binds a variety of drugs. Its role as distributor is supported by its extraordinary serum half-life of 3 weeks. This is related to its size and binding to the cellular receptor FcRn, which rescues albumin from intracellular degradation. Furthermore, the long half-life has fostered a great and increasing interest in utilization of albumin as a carrier of protein therapeutics and chemical drugs. However, to fully understand how FcRn acts as a regulator of albumin homeostasis and to take advantage of the FcRn-albumin interaction in drug design, the interaction interface needs to be dissected. Here, we used a panel of monoclonal antibodies directed towards human FcRn in combination with site-directed mutagenesis and structural modeling to unmask the binding sites for albumin blocking antibodies and albumin on the receptor, which revealed that the interaction is not only strictly pH-dependent, but predominantly hydrophobic in nature. Specifically, we provide mechanistic evidence for a crucial role of a cluster of conserved tryptophan residues that expose a pH-sensitive loop of FcRn, and identify structural differences in proximity to these hot spot residues that explain divergent cross-species binding properties of FcRn. Our findings expand our knowledge of how FcRn is controlling albumin homeostasis at a molecular level, which will guide design and engineering of novel albumin variants with altered transport properties.

IgG-effector functions: "the good, the bad and the ugly"

IgG-antibodies are potent and versatile mediators of host protection. They elicit their biological effects through specific interaction of the Fc-part with complement, specific cellular receptors, or both. Several factors should be taken into consideration when analyzing the nature and intensity of the immunological response elicited via IgG-effector functions, especially for the family of IgG-Fc receptors (FcγRs) exclusively expressed on immune cells. These include the various classes of leukocyte FcγR, expressed variably on different immune cells, each with distinct affinity for every IgG subclass, as well as genetic FcγR-polymorphisms affecting expression and affinity for IgG. Furthermore, various aspects of the IgG itself are also crucial for the outcome of the biological response. These include endogenously encoded IgG-polymorphisms, such as IgG3 polymorphisms, and post-transcriptional IgG-modifications, in particular IgG-Fc-glycosylation, affecting IgG effector functions through modified binding affinity to FcγR. These latter aspects concerning the variability in IgG3 on its half-life and placental transport and the clinical consequences of altered IgG-quality through glycosylation, will be the focus of this review.

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.

The function of Fcγ receptors in dendritic cells and macrophages

Dendritic cells (DCs) and macrophages use various receptors to recognize foreign antigens and to receive feedback control from adaptive immune cells. Although it was long believed that all immunoglobulin Fc receptors are universally expressed by phagocytes, recent findings indicate that only monocyte-derived DCs and macrophages express high levels of activating Fc receptors for IgG (FcγRs), whereas conventional and plasmacytoid DCs express the inhibitory FcγR. In this Review, we discuss how the uptake, processing and presentation of antigens by DCs and macrophages is influenced by FcγR recognition of immunoglobulins and immune complexes in the steady state and during inflammation.

Antibody Engineering

Advanced molecular biology techniques developed during the past few decades have allowed the industry to exploit and commercialize the natural defense mechanisms that antibodies provide. This review discusses the latest advances in antibody-engineering technologies to enhance clinical efficacy and outcomes. For the constant regions, the choice of the antibody class and isotype has to be made carefully to suit the therapeutic applications. Engineering of the Fc region, either by direct targeted mutagenesis or by modifying the nature of its N -glycan, has played an important role in recent years in increasing half-life or controlling effector functions. The variable regions of the antibody are responsible for binding affinity and exquisite specificity to the target molecule, which together with the Fc determine the drug's efficacy and influence the drug dose required to obtain the desired effectiveness. A key requirement during antibody development is therefore to affinity mature the variable regions when necessary, so that they bind the therapeutic target with sufficiently high affinity to guarantee effective occupancy over prolonged periods. If the antibody was obtained from a non-human source, such as rodents, a humanization process has to be applied to minimize immunogenicity while maintaining the desired binding affinity and selectivity. Finally, we discuss the next next-generation antibodies, such as antibody-drug conjugates, bispecific antibodies, and immunocytokines, which are being developed to meet future challenges.

Combined glyco- and protein-Fc engineering simultaneously enhance cytotoxicity and half-life of a therapeutic antibody

While glyco-engineered monoclonal antibodies (mAbs) with improved antibody-dependent cell-mediated cytotoxicity (ADCC) are reaching the market, extensive efforts have also been made to improve their pharmacokinetic properties to generate biologically superior molecules. Most therapeutic mAbs are human or humanized IgG molecules whose half-life is dependent on the neonatal Fc receptor FcRn. FcRn reduces IgG catabolism by binding to the Fc domain of endocytosed IgG in acidic lysosomal compartments, allowing them to be recycled into the blood. Fc-engineered mAbs with increased FcRn affinity resulted in longer in vivo half-life in animal models, but also in healthy humans. These Fc-engineered mAbs were obtained by alanine scanning, directed mutagenesis or in silico approach of the FcRn binding site. In our approach, we applied a random mutagenesis technology (MutaGen™) to generate mutations evenly distributed over the whole Fc sequence of human IgG1. IgG variants with improved FcRn-binding were then isolated from these Fc-libraries using a pH-dependent phage display selection process. Two successive rounds of mutagenesis and selection were performed to identify several mutations that dramatically improve FcRn binding. Notably, many of these mutations were unpredictable by rational design as they were located distantly from the FcRn binding site, validating our random molecular approach. When produced on the EMABling(®) platform allowing effector function increase, our IgG variants retained both higher ADCC and higher FcRn binding. Moreover, these IgG variants exhibited longer half-life in human FcRn transgenic mice. These results clearly demonstrate that glyco-engineering to improve cytotoxicity and protein-engineering to increase half-life can be combined to further optimize therapeutic mAbs.

Neonatal Fc receptor (FcRn): a novel target for therapeutic antibodies and antibody engineering

The biomedical applications of antibodies as prophylactics, therapeutics and diagnostics are developing rapidly. Neonatal Fc receptor (FcRn) is a major IgG Fc receptor capable of facilitating the translocation of IgG. FcRn can protect IgG from intracellular catabolism, thereby increasing its half-life. In recent decade, the interaction between FcRn and the Fc region has been reported with the focuses on either prolonging the plasma half-life of therapeutic IgG or shortening the half-life of pathogenic IgG. The FcRn-IgG interaction can be altered by modifying the Fc region to change their affinity (increase or decrease), and/or by reducing the Fc fragments of IgG to enhance its penetration into tissues or cells. By over expression of FcRn, the exogenous catabolism can be reduced, meanwhile the circulating IgG level could be enhanced. It has been confirmed in different FcRn over-expressed transgenic mice models, substantial humoral responses against antigens with weak immunogenicity can be mounted. In addition, designing inhibitors for FcRn-IgG interaction is another application prospect for treating IgG-mediated autoimmune diseases. Recent research advancements strengthen the understanding that FcRn is a key and promising drugable target in IgG intervention in the field of antibody engineering.

The proximal tubule and albuminuria: really!

Recent data highlight the role of the proximal tubule (PT) in reabsorbing, processing, and transcytosing urinary albumin from the glomerular filtrate. Innovative techniques and approaches have provided exciting insights into these processes, and numerous investigators have shown that selective PT cell defects lead to significant albuminuria, even reaching nephrotic range in animal models. Thus, the mechanisms of albumin reabsorption and transcytosis are undergoing intense study. Working in concert with megalin and cubilin, a nonselective multireceptor complex that predominantly directs proteins for lysosomal degradation, the neonatal Fc receptor (FcRn) located at the brush border of the apical membrane has been implicated as the "receptor" mediating albumin transcytosis. The FcRn pathway facilitates reabsorption and mediates transcytosis by its pH-dependent binding affinity in endosomal compartments. This also allows for selective albumin sorting within the PT cell. This reclamation pathway minimizes urinary losses and catabolism of albumin, thus prolonging its serum half-life. It may also serve as a molecular sorter to preserve and reclaim normal albumin while allowing "altered" albumin to be catabolized via lysosomal pathways. Here, we critically review the data supporting this novel mechanism.

Using 2-photon microscopy to understand albuminuria

Intravital 2-photon microscopy, along with the development of fluorescent probes and innovative software, has rapidly advanced the study of intracellular and intercellular processes at the organ level. Researchers can quantify the distribution, behavior, and dynamic interactions of up to four labeled chemical probes and proteins simultaneously and repeatedly in four dimensions (3D + time) with subcellular resolution in real time. Transgenic fluorescently labeled proteins, delivery of plasmids, and photo-activatable probes enhance these possibilities. Thus, multi-photon microscopy has greatly extended our ability to understand cell biology intra-vitally at cellular and subcellular levels. For example, evaluation of rat surface glomeruli and accompanying proximal tubules has shown the long held paradigm regarding limited albumin filtration under physiologic conditions is to be questioned. Furthermore, the role of proximal tubules in determining albuminuria under physiologic and disease conditions was supported by direct visualization and quantitative analysis.

Transgenic rabbits that overexpress the neonatal Fc receptor (FcRn) generate higher quantities and improved qualities of anti-thymocyte globulin (ATG)

Immune suppression with rabbit anti-thymocyte globulin (rATG) is a well-established therapeutic concept for preventing host rejection of transplanted organs and graft versus host disease. Increasing the efficiency of rATG production by reducing the number of animals would be highly beneficial to lower cost and to improve quality standards. We have developed transgenic (Tg) mice and rabbits that overexpress the neonatal Fc receptor (FcRn) and have shown an augmented humoral immune response in these animals. To test whether our FcRn Tg rabbits produced rATG more efficiently, we immunized them and their New Zealand White controls with live Jurkat cells. By day 21 after immunization, Tg animals produced significantly, 1.5 times higher amount of total IgG compared to their wt littermates. Also, the binding efficiency of Tg sera to Jurkat cells and their complement-mediated cytotoxicity was significantly higher. The purified Tg IgG preparation contained 2.6 the amount of Jurkat specific IgG as the wt preparation analyzed by complement-mediated lysis, suggesting greater antigen-specific B cell activation in the Tg rabbits. To test this hypothesis, immunization with ovalbumin and human α₁-antitrypsin was performed, resulting in significantly greater numbers of antigen-specific B-cells in the FcRn Tg rabbits as compared with wt controls. The shift towards significantly larger populations of antigen-specific B cells relative to the non-specific B cell pool is further corroborated by our previous findings in FcRn Tg mice. Consequently, our FcRn Tg rabbits have the potential to offer substantial qualitative and quantitative improvements for the production of rATG and other polyclonal or monoclonal antibodies.

Antibodies and their receptors: different potential roles in mucosal defense

Over recent years it has become increasingly apparent that mucosal antibodies are not only restricted to the IgM and IgA isotypes, but that also other isotypes and particularly IgG can be found in significant quantities at some mucosal surfaces, such as in the genital tract. Their role is more complex than traditionally believed with, among other things, the discovery of novel function of mucosal immunoglobulin receptors. A thorough knowledge in the source and function and mucosal immunoglobulins is particularly important in development of vaccines providing mucosal immunity, and also in the current climate of microbicide development, to combat major world health issues such as HIV. We present here a comprehensive review of human antibody mediated mucosal immunity.