The MHC class II-associated invariant chain interacts with the neonatal Fc gamma receptor and modulates its trafficking to endosomal/lysosomal compartments

The therapeutic age of the neonatal Fc receptor

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

Roles of FcRn in Antigen-Presenting Cells during Autoimmunity and a Clinical Evaluation of Efgartigimod as an FcRn Blocker

The immune system is a complex network of multiple cells, tissues, and organs that protects the body against foreign pathogenic invaders. However, the immune system may mistakenly attack healthy cells and tissues due to the cross-reactivity of anti-pathogen immunity, leading to autoimmunity by autoreactive T cells and/or autoantibody-secreting B cells. Autoantibodies can accumulate, resulting in tissue or organ damage. The neonatal crystallizable fragment receptor (FcRn) is an important factor in immune regulation through controlling the trafficking and recycling of immunoglobulin G (IgG) molecules, the most abundant antibody in humoral immunity. In addition to its role in IgG trafficking and recycling, FcRn is also involved in antigen presentation, which is a crucial step in the activation of the adaptive immune response via directing the internalization and trafficking of antigen-bound IgG immune complexes into compartments of degradation and presentation in antigen-presenting cells. Efgartigimod, an FcRn inhibitor, has shown promise in reducing the levels of autoantibodies and alleviating the autoimmune severity of myasthenia gravis, primary immune thrombocytopenia, and pemphigus vulgaris/foliaceus. This article aims to provide an overview of the importance of FcRn in antigen-presenting cells and its potential as a therapeutic target in autoimmune diseases, using efgartigimod as an example.

Localization of Chicken Rab22a in Cells and Its Relationship to BF or Ii Molecules and Genes

Rab22a is an important small GTPase protein the molecule that is involved in intracellular transportation and regulation of proteins. It also plays an important role in antigens uptake, transportation, regulation of endosome morphology, and also regulates the transport of antigens to MHC (Major Histocompatibility Complex) molecules. To investigate the role of Rab22a, the intracellular co-localization of chicken Rab22a (cRab22a) molecule and its relationship to BF and chicken invariant chain (cIi) molecules was studied. A 3D protein structure of Rab22a was constructed by using informatics tools (DNASTAR 4.0 and DNAMAN). Based on the model, the corresponding recombinant eukaryotic plasmids were constructed by point mutations in the protein's structural domains. HEK 293T cells were co-transfected with plasmids pEGFP-C1-cIi to observe the intracellular co-localization. Secondly, the DC2.4 Mouse Dendritic Cell and Murine RAW 264.7 cells were transfected with recombinant plasmids of pmCherry-cRab22a and pmCherry-mRab22a respectively. Subsequently, the intracellular localization of cRab22a in early and late endosomes was observed with specific antibodies against EEA1 and LAMP1 respectively. For gene expression-based studies, the cRab22a gene was down-regulated and up-regulated in HD11 cells, following the detection of transcription levels of the BFa (MHCIa) and cIi genes by real-time quantitative PCR (RT-qPCR). The interactions of the cRab22a gene with BFa and cIi were detected by co-immunoprecipitation (Co-IP) and Western blot. The results showed that the protein structures of chicken and mouse Rab22a were highly homologous (95.4%), and both localize to the early and late endosomes. Ser41 and Tyr74 are key amino acids in the Switch regions of Rab22a which maintain its intracellular localization. The down-regulation of cRab22a gene expression significantly reduced (p < 0.01) the transcription of BFa (MHCIa) and cIi in HD11 cells. However, when the expression of the cRab22a gene was increased 55 times as compared to control cells, the expression of the BFa (MHCIa) gene was increased 1.7 times compared to the control cells (p < 0.01), while the expression of the cIi gene did not significantly differ from control (p > 0.05). Western blot results showed that cRab22a could not directly bind to BFa and cIi. So, cRab22a can regulate BFa and cIi protein molecules indirectly. It is concluded that cRab22a was localized with cIi in the endosome. The Switch regions of cRab22a are the key domains that affect intracellular localization and colocalization of the cIi molecule.

Regulation of dendritic cell function by Fc-γ-receptors and the neonatal Fc receptor

Dendritic cells (DCs) express receptors to sense pathogens and/or tissue damage and to communicate with other immune cells. Among those receptors, Fc receptors (FcRs) are triggered by the Fc region of antibodies produced during adaptive immunity. In this review, the role of FcγR and neonatal Fc receptor (FcRn) in DC immunity will be discussed. Their expression in DC subsets and impact on antigen uptake and presentation, DC maturation and polarisation of T cell responses will be described. Lastly, we will discuss the importance of FcR-mediated DC function in the context of immunity during viral infection, inflammatory disease, cancer and immunotherapy.

Activation of the JNK/MAPK Signaling Pathway by TGF-β1 Enhances Neonatal Fc Receptor Expression and IgG Transcytosis

The neonatal Fc receptor (FcRn) transports maternal immunoglobulin G (IgG) to the foetus or newborn and protects the IgG from degradation. FcRn is expressed in several porcine tissues and cell types and its expression levels are regulated by immune and inflammatory events. IPEC-J2 cells are porcine intestinal columnar epithelial cells that were isolated from neonatal piglet mid-jejunum. We hypothesized that transforming growth factor β1 (TGF-β1) upregulated pFcRn expression in IPEC-J2 cells. To test this hypothesis, we treated IPEC-J2 cells with TGF-β1 and demonstrated that porcine FcRn (pFcRn) expression was significantly increased. SP600125, a specific mitogen-activated protein kinase (MAPK) inhibitor, reduced TGF-β1-induced pFcRn expression in IPEC-J2 cells. We performed luciferase reporter assays and showed that the c-JUN sensitive region of the pFcRn promoter gene was located between positions -1215 and -140. The c-JUN sequence, in combination with the pFcRn promoter, regulated luciferase reporter activity in response to TGF-β1 stimulation. Chromatin immunoprecipitation confirmed that there were three c-JUN binding sites in the pFcRn promoter. Furthermore, in addition to increased pFcRn expression, TGF-β1 also enhanced IgG transcytosis in IPEC-J2 cells. In summary, our data showed that the modulation of JNK/MAPK signaling by TGF-β1 was sufficient to upregulate pFcRn expression.

Immunogenicity Challenges Associated with Subcutaneous Delivery of Therapeutic Proteins

The subcutaneous route of administration has provided convenient and non-inferior delivery of therapeutic proteins compared to intravenous infusion, but there is potential for enhanced immunogenicity toward subcutaneously administered proteins in a subset of patients. Unwanted anti-drug antibody response toward proteins or monoclonal antibodies upon repeated administration is shown to impact the pharmacokinetics and efficacy of multiple biologics. Unique immunogenicity challenges of the subcutaneous route have been realized through various preclinical and clinical examples, although subcutaneous delivery has often demonstrated comparable immunogenicity to intravenous administration. Beyond route of administration as a treatment-related factor of immunogenicity, certain product-related risk factors are particularly relevant to subcutaneously administered proteins. This review attempts to provide an overview of the mechanism of immune response toward proteins administered subcutaneously (subcutaneous proteins) and comments on product-related risk factors related to protein structure and stability, dosage form, and aggregation. A two-wave mechanism of antigen presentation in the immune response toward subcutaneous proteins is described, and interaction with dynamic antigen-presenting cells possessing high antigen processing efficiency and migratory activity may drive immunogenicity. Mitigation strategies for immunogenicity are discussed, including those in general use clinically and those currently in development. Mechanistic insights along with consideration of risk factors involved inspire theoretical strategies to provide antigen-specific, long-lasting effects for maintaining the safety and efficacy of therapeutic proteins.

Invariant chain regulates endosomal fusion and maturation through an interaction with the SNARE Vti1b

The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.

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.

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.

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.

Human IgGs induce synthesis and secretion of IgGs and neonatal Fc receptor in human umbilical vein endothelial cells

Human IgGs are increasingly used in the therapy of many different immune and inflammatory diseases, however their mechanism of action still remains unclear in most diseases. To gain insight into the mechanism by which IgGs might also exert their effects on endothelial cells, we tested human IgGs on human umbilical vein endothelial cells (HUVECs). IgGs induced a time-dependent increase in the synthesis and secretion of IgGs, together with a marked angiogenic-like transformation of HUVECs that was maximal after a 20-h incubation. IgGs stimulated IG gene transcription without affecting the process of gene rearrangement, already present in control HUVECs. The mechanism involved the activation of transcription factors with the increased expression of HSP90, HSP70 and inactive MMP-9 responsible for the phenotypic differentiation associated with the most intense IgG synthesis and secretion. However, even a short incubation with IgGs followed by recovery of cells was sufficient to trigger and sustain in time the synthesis and secretion of new IgGs, independently of the angiogenic-like transformation visible only when cells were continuously exposed to IgGs. Under the stimulus of IgGs, specific secretory pathways were also activated in HUVECs together with the expression of FcRn, which was always associated with IgGs of new synthesis, forming complexes that were also secreted. Our results disclose a so far unknown and unexpected mechanism of IgGs on HUVECs that behave as Ig-producing immune cells. Results might have relevance for the effects that IgGs also exert in vivo in physiological conditions.

The multifaceted roles of the invariant chain CD74--More than just a chaperone

The invariant chain (CD74) is well known for its essential role in antigen presentation by mediating assembly and subcellular trafficking of the MHCII complex. Beyond this, CD74 has also been implicated in a number of processes independent of MHCII. These include the regulation of endosomal trafficking, cell migration and cellular signalling as surface receptor of the pro-inflammatory cytokine macrophage migration inhibitory factor (MIF). In several forms of cancer, CD74 is up-regulated and associated with enhanced proliferation and metastatic potential. In this review, an overview of the diverse biological functions of the CD74 protein is provided with a particular focus on how these may be regulated. In particular, proteolysis of CD74 will be discussed as a central mechanism to control the actions of this important protein at different levels.

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

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

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

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

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.

Overexpression of Bovine FcRn in Mice Enhances T-Dependent Immune Responses by Amplifying T Helper Cell Frequency and Germinal Center Enlargement in the Spleen

The neonatal Fc receptor (FcRn) plays key roles in IgG and albumin homeostasis, maternal IgG transport, and antigen presentation of IgG-opsonized antigens. Previously, we reported that transgenic (Tg) mice that overexpress the bovine FcRn (bFcRn) have augmented T-dependent humoral immune response with increased IgG protection, higher level of antigen-specific antibodies, greater number of antigen-specific B cells, and effective immune response even against weakly immunogenic epitopes. In the current study, we analyzed the localization of the bFcRn in secondary lymphoid organs, and focused to demonstrate the in vivo impact of its overexpression in the spleen on the course of antibody production. bFcRn was highly expressed by red pulp macrophages and marginal zone macrophages in the spleen and by subcapsular sinus macrophages and macrophage-like cells in the interfollicular areas in the lymph node cortex. We also demonstrated that splenic dendritic cells of Tg mice express bFcRn and intraperitoneal immunization of these mice with T-dependent antigens led to more than threefold increase in the number of antigen-specific activated T helper cells with increased size and numbers of germinal centers compared to wild-type controls. bFcRn expression in splenic B cells was also detected and that may also contribute to the enhanced B cell activation. Finally, we demonstrated that these Tg mice developed efficient immune response against very low dose of antigen, reflecting another important practical benefit of these Tg mice.

The intracellular localization and association of porcine Ia-associated invariant chain with the MHC class I-related porcine neonatal Fc receptor for IgG

The neonatal Fc receptor (FcRn) transports IgG from mother to young and is involved in antigen presentation. FcRn is structurally similar to MHC class I, but its intracellular trafficking pathway is much more analogous to that of MHC class II. Ia-associated invariant chain (Ii) molecules play an additional role in directing MHC class II trafficking within the endocytic compartments by physical association with MHC class II. This study addresses the question of whether pig Ii chain plays this important role in FcRn trafficking to the endoplasmic reticulum. Red or green fluorescent protein-fused Ii or FcRn was constructed, and the intracellular localization of pig Ii with FcRn was detected using confocal microscopy. Immunoprecipitation and western blotting were used to test for their association. The results indicate that pig Ii chain specifically interacts with both FcRn H chain alone and FcRn-β2m complex, and the CLIP in Ii was required for FcRn-Ii association. A truncated FcRn deletion in the cytoplasmic tail changed the intracellular localization of FcRn. However, the truncated FcRn can still combine Ii. This indicated that the cytoplasmic tail of FcRn fails to affect FcRn association with Ii. These results suggest that association of FcRn with Ii chain is relevant, and appreciation of this process is important to the understanding of how IgG is transported.

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

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

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.

Suppressed expression of homotypic multinucleation, extracellular domains of CD172α (SIRP-α) and CD47 (IAP) receptors in TAMs upregulated by Hsp70-peptide complex in Dalton's lymphoma

CD172α and CD47 are members of glycoprotein expressed on macrophages and various immune cells, promote immune recognition and T cell stimulation that priming phagocytosis of pathogens and apoptotic bodies and malignant cell. Tumour-releasing immunosuppressive factor promotes tumour growth and transforms the tumour resident M1 phenotype of macrophage to M2 phenotype (TAMs) that promotes tumour progression by downregulating the expression of different surface receptor including CD172α and CD47. Recent studies have reported that CD172α and CD47 are involved in the pathogenesis and promote malignancies such as lymphoma, leukaemia, melanoma, lung cancer and multiple myeloma, and their expression varies during infection and malignancies. Autologous Hsp70 is well recognized for its role in activating macrophages leading to enhance production of inflammatory cytokines. It has been observed that Hsp70 derived from normal tissues do not elicit tumour immunity, while Hsp70 preparation from tumour cell was able to elicit tumour immunity. However, the role of exogenous autologous hsp70 on the formation of giant cells is completely unknown. Therefore, in the present study, we sought to investigate the effect of Hsp70-peptide complex on the expression of CD172α and CD47 receptors in normal peritoneal macrophages (NMO) and TAMs. Finding shows that the expression of CD172α and CD47 enhances in TAMs and it reverts back the suppressed function of TAMs into M1 state of immunoregulatory phenotype that promotes tumour regression by enhanced multinucleation and phagocytosis of malignant cells and significantly enhances the homotypic fusion of macrophages and polykaryon formation in vitro by enhancing the expression of SIRPα and IAP.

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

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

Data mining of supersecondary structure homology between light chains of immunogloblins and MHC molecules: absence of the common conformational fragment in the human IgM rheumatoid factor

It is shown that fuzzy search and data mining techniques of supersecondary structure homology for subunits of proteins using conformational code patterns of α-helix-type (3β5α4β) and β-sheet-type (6α4β4β) fragments can be used to extract correlations between fragments of MHC class I molecules and the light chain of immunoglobulins. The new method of conformational pattern analysis with fuzzy search of structural code homology reflects well the shape of main chain rather than secondary structure in comparison with the DSSP method. Further, the data mining technique using the combination of h- and s-fragment patterns can quantify the supersecondary structure homology between any subunits of proteins with different amino acid sequences. Characteristic fragment patterns (string "shhshss"), which were sandwiched between two identical amino acid sequences His and Pro, were found in light chains of various types of immunogloblins, α-chain and β-2 microglobulin of MHC class I and α-chain and β-chain of MHC class II, but not in heavy chains of Fab immunoglobulin fragments and T cell receptors (TCR). Leukocyte immunoglobulin-like receptors (LILR) are related by the conformational fragment (string "shhshss") to β-2 microglobulins as a type of pair forms (string "sohsss"). Further, human IgM rheumatoid factor, one of the immunogloblins, did not strongly exhibit the conformational fragment pattern. Nonclassic MHC class I molecules CD1D, MIC-A, and MIC-B, which have functions to activate NKT, NK, and T cells, did not also clearly show the patterns. These code-driven mining techniques can be utilized as a metadata-generating tool for systems biology to elucidate the biological function of such conformational fragments of MHC I and II molecules, which come in contact with various signal ligands on the surface of T cells and natural killer cells.

Pathways of antigen processing

T cell recognition of antigen-presenting cells depends on their expression of a spectrum of peptides bound to major histocompatibility complex class I (MHC-I) and class II (MHC-II) molecules. Conversion of antigens from pathogens or transformed cells into MHC-I- and MHC-II-bound peptides is critical for mounting protective T cell responses, and similar processing of self proteins is necessary to establish and maintain tolerance. Cells use a variety of mechanisms to acquire protein antigens, from translation in the cytosol to variations on the theme of endocytosis, and to degrade them once acquired. In this review, we highlight the aspects of MHC-I and MHC-II biosynthesis and assembly that have evolved to intersect these pathways and sample the peptides that are produced.

The p35 human invariant chain in transgenic mice restores mature B cells in the absence of endogenous CD74

The invariant chain (Ii; CD74) has pleiotropic functions and Ii-deficient mice show defects in MHC class II (MHC II) transport and B cell maturation. In humans, but not in mice, a minor Iip35 isoform of unknown function includes an endoplasmic reticulum-retention motif that is masked upon binding of MHC II molecules. To gain further insight into the roles of Ii in B cell homeostasis, we generated Iip35 transgenic mice (Tgp35) and bred these with mice deficient for Ii (Tgp35/mIiKO). Iip35 was shown to compete with mIi for the binding to I-A(b) . In addition, classical endosomal degradation products (p20/p10) and the class II-associated invariant chain peptide (CLIP) fragment were detected. Moreover, Iip35 favored the formation of compact peptide-MHC II complexes in the Tgp35/mIiKO mice. I-A(b) levels were restored at the plasma membrane of mature B cells but Iip35 affected the fine conformation of MHC II molecules as judged by the increased reactivity of the AF6-120.1 antibody in permeabilized cells. However, the human Iip35 cannot fully replace the endogenous Ii. Indeed, most immature B cells in the bone marrow and spleen of transgenic mice had reduced surface expression of MHC II molecules, demonstrating a dominant-negative effect of Iip35 in Tgp35 mice. Interestingly, while maturation to follicular B cells was normal, Iip35 expression appeared to reduce the proportions of marginal zone B cells. These results emphasize the importance of Ii in B cell homeostasis and suggest that Iip35 could have regulatory functions.

Differential regulation of MHC II and invariant chain expression during maturation of monocyte-derived dendritic cells

DCs are potent initiators of adaptive immune responses toward invading pathogens. Upon reception of pathogenic stimuli, DCs initiate a complex differentiation program, culminating in mature DCs with an extreme capacity to activate naïve T cells. During this maturation, DCs reduce the synthesis and turnover of MHC II molecules. This allows for a stable population of MHC II, presenting peptides captured at the time and place of activation, thus provoking specific immune responses toward the activating pathogen. The efficient loading of antigenic peptides onto MHC II is vitally dependent on the accessory molecule Ii, which aids in the assembly of the MHC II α- and β-chains in the ER and directs their trafficking to the endocytic compartments, where they encounter endocytosed antigen. However, Ii plays additional roles in DC function by influencing migration, antigen uptake, and processing. To examine the biosynthetic background for diverse Ii functions in DCs, we investigated mRNA and protein levels of Ii compared with MHC II in human moDCs during maturation using various stimuli. We find that the production of Ii did not correlate with that of MHC II and that mature DCs maintain abundant levels of Ii despite a reduced production of new MHC II.

Fc-mediated transport of nanoparticles across airway epithelial cell layers

In a study directed towards non-invasive delivery of therapeutic biomacromolecules, we examined whether surface modification of sub-200 nm model nanoparticles with the Fc portion of IgG promotes their cell uptake and transport across the airway epithelial cells. The study initially confirms the expression of the relevant receptor, namely neonatal Fc receptor (FcRn), by Calu-3 cell layers simulating the airway epithelium and demonstrates FcRn-mediated cell association, internalization and transcellular transport of molecular IgG. Surface decoration of nanoparticles with the Fc portion of IgG enhanced both cell uptake and translocation of the particulate system across the cell layers, in a manner strongly suggesting FcRn involvement in these processes. The study further demonstrates the potential of Fc-modified nanoparticles to 'shuttle' a model therapeutic antibody fragment across the epithelial cell layers. Fc-modified nanoparticles are transported in the μg/h/cm(2) range, presenting a substantial increase in transport capacity in comparison to molecular IgG (ng/h/cm(2) range), therefore warranting consideration of the FcRn transcytotic pathway for further investigation as a means to achieve transmucosal delivery of nanoparticulate systems that could act as carriers of a range of biotherapeutics.

Recent advances using FcRn overexpression in transgenic animals to overcome impediments of standard antibody technologies to improve the generation of specific antibodies

This review illustrates the salutary effects of neonatal Fc receptor (FcRn) overexpression in significantly improving humoral immune responses in the generation of antibodies for immunotherapy and diagnostics. These include: (1) improved IgG protection; (2) augmented antigen-specific humoral immune response with larger numbers of antigen specific B cells, thus offering a wider spectrum of clones; (3) generation of antibodies against weakly immunogenic antigens; (4) significant improvements in the number and substantial developments in the diversity of hybridomas. FcRn transgenesis thus confers a number of practical benefits, including faster antibody production, higher antibody yields and improved generation of hybridomas for monoclonal antibody production. Notably, these efficiencies in polyclonal antibody production were also demonstrated in FcRn transgenic rabbits. Overall, FcRn transgenic animals yield more antibodies and provide a route to the generation of antibodies against antigens of low immunogenicity that are difficult to obtain using currently available methods.

A neonatal Fc receptor-targeted mucosal vaccine strategy effectively induces HIV-1 antigen-specific immunity to genital infection

Strategies to prevent the sexual transmission of HIV include vaccines that elicit durable, protective mucosal immune responses. A key to effective mucosal immunity is the capacity for antigens administered locally to cross epithelial barriers. Given the role of neonatal Fc receptor (FcRn) in transferring IgG across polarized epithelial cells which line mucosal surfaces, FcRn might be useful for delivering HIV vaccine antigens across mucosal epithelial barriers to the underlying antigen-presenting cells. Chimeric proteins composed of HIV Gag (p24) fused to the Fc region of IgG (Gag-Fc) bind efficiently to airway mucosa and are transported across this epithelial surface. Mice immunized intranasally with Gag-Fc plus CpG adjuvant developed local and systemic immunity, including durable B and T cell memory. Gag-specific immunity was sufficiently potent to protect against an intravaginal challenge with recombinant vaccinia virus expressing the HIV Gag protein. Intranasal administration of a Gag-Fc/CpG vaccine protected at a distal mucosal site. Our data suggest that targeting of FcRn with chimeric immunogens may be an important strategy for mucosal immunization and should be considered a new approach for preventive HIV vaccines.

Hypoxia increases membranal and secreted HLA-DR in endothelial cells, rendering them T-cell activators

Transplantation involves preoperative ischemic periods that contribute to endothelial cell (EC) dysfunction and T-cell activation, leading to graft rejection. As hypoxia is a major constituent of ischemia, we evaluated its effect on the ability of ECs to express HLA-DR, which is required for presentation of antigens to T cells, and by itself serves as an important target for allogeneic T cells. Primary human umbilical vein ECs (HUVEC) and the human endothelial cell line EaHy926 were incubated in normoxia or hypoxia (PO(2) < 0.3%). Hypoxia increased the membranal expression (by 4-6 fold, P < 0.01) and secretion (by sixfold, P < 0.05) of HLA-DR protein, without influencing the accumulation of its mRNA. Alternative splicing, attenuated trafficking, or shedding from the plasma membrane were not observed, but the lysosomal inhibitor bafilomycin A1 reduced HLA-DR secretion. Hypoxia-induced endothelial HLA-DR elevated and diminished the secretion of IL-2 and IL-10, respectively, from co-cultured allogeneic CD4(+) T cells in a HLA-DR-dependent manner, as demonstrated by the use of monoclonal anti-HLA-DR. Our results indicate a yet not fully understood post-translational mechanism(s), which elevate both membranal and soluble HLA-DR expression. This elevation is involved in allogeneic T-cell activation, highlighting the pivotal role of ECs in ischemia/hypoxia-associated injury and graft rejection.

The neonatal FcR-mediated presentation of immune-complexed antigen is associated with endosomal and phagosomal pH and antigen stability in macrophages and dendritic cells

The FcγRs found on macrophages (Ms) and dendritic cells (DCs) efficiently facilitate the presentation or cross-presentation of immune-complexed Ags to T cells. We found that the MHC class I-related neonatal FcR for IgG (FcRn) in both Ms and DCs failed to have a strong effect on the cross-presentation of immune complex (IC) OVA Ag to CD8(+) T cells. Interestingly, endosomal FcRn enhanced the presentation of the monomeric OVA-IC to CD4(+) T cells robustly, whereas FcRn in phagosomes exerted distinctive effects on Ag presentation between Ms and DCs. The presentation of phagocytosed OVA-ICs to CD4(+) T cells was considerably enhanced on wild-type versus FcRn-deficient Ms, but was not affected in FcRn-deficient DCs. This functional discrepancy was associated with the dependence of IgG-FcRn binding in an acidic pH. Following phagocytosis, the phagosomal pH dropped rapidly to <6.5 in Ms but remained in the neutral range in DCs. This disparity in pH determined the rate of degradation of phagocytosed ICs. Thus, our findings reveal that FcRn expression has a different effect on Ag processing and presentation of ICs to CD4(+) T cells in the endosomal versus phagosomal compartments of Ms versus DCs.

Stoichiometry of HLA class II-invariant chain oligomers

BACKGROUND

The HLA gene complex encodes three class II isotypes, DR, DQ, and DP. HLA class II molecules are peptide receptors that present antigens for recognition by T lymphocytes. In antigen presenting cells, the assembly of matched α and β subunits to heterodimers is chaperoned by invariant chain (Ii). Ii forms a homotrimer with three binding sites for class II heterodimers. The current model of class II and Ii structure states that three αβ heterodimers bind to an Ii trimer.

METHODOLOGY/PRINCIPAL FINDINGS

[corrected] We have now analyzed the composition and size of the complexes of class II and Ii using epitope tagged class II subunits and density gradient experiments. We show here that class II-Ii oligomers consist of one class II heterodimer associated with one Ii trimer, such that the DR, DQ and DP isotypes are contained within separate complexes with Ii.

CONCLUSION/SIGNIFICANCE

We propose a structural model of the class II-Ii oligomer and speculate that the pentameric class II-Ii complex is bent towards the cell membrane, inhibiting the binding of additional class II heterodimers to Ii.

CD23-dependent transcytosis of IgE and immune complex across the polarized human respiratory epithelial cells

IgE-mediated allergic inflammation occurs when allergens cross-link IgE on the surface of immune cells, thereby triggering the release of inflammatory mediators as well as enhancing Ag presentations. IgE is frequently present in airway secretions, and its level can be enhanced in human patients with allergic rhinitis and bronchial asthma. However, it remains completely unknown how IgE appears in the airway secretions. In this study, we show that CD23 (FcεRII) is constitutively expressed in established or primary human airway epithelial cells, and its expression is significantly upregulated when airway epithelial cells were subjected to IL-4 stimulation. In a transcytosis assay, human IgE or IgE-derived immune complex (IC) was transported across a polarized Calu-3 monolayer. Exposure of the Calu-3 monolayer to IL-4 stimulation also enhanced the transcytosis of either human IgE or the IC. A CD23-specific Ab or soluble CD23 significantly reduced the efficiency of IgE or IC transcytosis, suggesting a specific receptor-mediated transport by CD23. Transcytosis of both IgE and the IC was further verified in primary human airway epithelial cell monolayers. Furthermore, the transcytosed Ag-IgE complexes were competent in inducing degranulation of the cultured human mast cells. Because airway epithelial cells are the first cell layer to come into contact with inhaled allergens, our study implies CD23-mediated IgE transcytosis in human airway epithelial cells may play a critical role in initiating and contributing to the perpetuation of airway allergic inflammation.

Efficient mucosal vaccination mediated by the neonatal Fc receptor

Vaccine strategies to prevent invasive mucosal pathogens are being sought because 80–90% of infectious diseases are initiated at mucosal surfaces. However, our ability to deliver an intact vaccine antigen across a mucosal barrier for induction of effective immunity is limited. The neonatal Fc receptor (FcRn) mediates the transport of IgG across polarized epithelial cells lining mucosal surfaces. By mimicking IgG transfer at mucosal surfaces, intranasal immunization with a model antigen, herpes simplex virus type-2 (HSV-2) glycoprotein gD fused with an IgG Fc fragment, in combination with the adjuvant CpG, resulted in complete protection of wild type, but not FcRn knockout, mice that were intravaginally challenged with virulent HSV-2 186. This immunization strategy induced efficient mucosal and systemic antibody, B and T cell immune responses, including memory immune responses, which remained stable at least 6 months post-vaccination and conferred protection for a majority of animals. These results demonstrate that the FcRn-IgG transcellular transport pathway may represent a novel mucosal vaccine delivery route for a subunit vaccine against abundant mucosal pathogens.

Neonatal FcR overexpression boosts humoral immune response in transgenic mice

The neonatal FcR (FcRn) regulates IgG and albumin homeostasis, mediates maternal IgG transport, takes active part in phagocytosis, and delivers Ag for presentation. We have previously shown that overexpression of FcRn in transgenic (Tg) mice extends the half-life of mouse IgG by reducing its clearance. In this paper, we demonstrate that immunization of these mice with OVA and trinitrophenyl-conjugated human IgG results in a 3- to 10-fold increase of Ag-specific IgM and IgG in serum. The IgM increase was unexpected because FcRn does not bind IgM. Our results showed that the affinity of the Ag-specific IgG was at least as good in Tg mice as in the wild-type (wt) controls, implying appropriate affinity maturation in both groups. Influenza vaccination produced a 2-fold increase in the amount of virus-specific Ab in Tg animals, which proved twice as efficient in a hemagglutination inhibition assay as was the case in wt controls. After immunization, Tg mice displayed significantly larger spleens containing a higher number of Ag-specific B cells and plasma cells, as well as many more granulocytes and dendritic cells, analyzed by ELISPOT and flow cytometric studies. The neutrophils from these Tg mice expressed the Tg FcRn and phagocytosed IgG immune complexes more efficiently than did those from wt mice. These results show that FcRn overexpression not only extends the IgG half-life but also enhances the expansion of Ag-specific B cells and plasma cells. Although both effects increase the level of Ag-specific IgG, the increase in immune response and IgG production seems to be more prominent compared with the reduced IgG clearance.

The invariant chain transports TNF family member CD70 to MHC class II compartments in dendritic cells

CD70 is a TNF-related transmembrane molecule expressed by mature dendritic cells (DCs), which present antigens to T cells via major histocompatibility complex (MHC) molecules. In DCs, CD70 localizes with MHC class II molecules in late endosomal vesicles, known as MHC class II compartments (MIICs). MIICs are transported to the immune synapse when a DC contacts an antigen-specific CD4+ T cell. Consequently, MHC class II and CD70 are simultaneously exposed to the T cell. Thereby, T-cell activation via the antigen receptor and CD70-mediated co-stimulation are synchronized, apparently to optimize the proliferative response. We report here that the invariant chain (Ii), a chaperone known to transport MHC class II to MIICs, performs a similar function for CD70. CD70 was found to travel by default to the plasma membrane, whereas Ii coexpression directed it to late endosomes and/or lysosomes. In cells containing the MHC class II presentation pathway, CD70 localized to MIICs. This localization relied on Ii, since transport of CD70 from the Golgi to MIICs was impeded in Ii-deficient DCs. Biophysical and biochemical studies revealed that CD70 and Ii participate in an MHC-class-II-independent complex. Thus, Ii supports transport of both MHC class II and CD70 to MIICs and thereby coordinates their delivery to CD4+ T cells.

MHC class II-associated invariant chain (Ii) modulates dendritic cells-derived microvesicles (DCMV)-mediated activation of microglia

Dendritic cells (DC) - the sentinels of the immune system - play an important role in the maintenance of tolerance and induction of immunity. However, in autoimmune diseases, DC initiate the diseases by presenting self antigens to autoreactive T cells, causing the immune system to mount a response against the body. An example is multiple sclerosis (MS) and its corresponding animal model, experimental autoimmune encephalomyelitis (EAE). During inflammation of the central nervous system (CNS), DC are recruited to activate autoreactive T cells. Microglia - resident mononuclear phagocytes of the brain - also play a role in the pathogenesis of the disease. In this study, we demonstrated that microvesicles derived from DC (DCMV) induced the activation of NF-κB in microglia. Furthermore, MHC class II-associated invariant chain (Ii), also known as CD74, was specifically recruited to DCMV and interestingly, was able to enhance the DCMV-mediated activation of NF-κB in microglia. Thus, this study emphasizes the role of microvesicles and Ii in the communication between DC and microglia.

An FcRn-dependent role for anti-flagellin immunoglobulin G in pathogenesis of colitis in mice

BACKGROUND & AIMS

The neonatal Fc receptor for immunoglobulin (Ig)G (FcRn) protects monomeric IgG from catabolism in parenchymal and hematopoietic cells during adult life. In dendritic cells, FcRn also promotes presentation of antigens in association with IgG. Because IgGs with anti-bacterial specificity are a hallmark of inflammatory bowel disease, we sought to determine their significance and relationship to FcRn expression in antigen-presenting cells, focusing on IgGs specific for flagellin.

METHODS

Levels of circulating anti-flagellin IgG were induced in wild-type and FcRn(-/-) mice, followed by induction of colitis with dextran sodium sulfate (DSS). Bone marrow chimera models were used to localize the site of FcRn action.

RESULTS

Wild-type mice that received anti-flagellin IgG exhibited more severe colitis following administration of DSS, compared with mice that received control IgG. Wild-type mice immunized with flagellin exhibited significantly more severe colitis in response to DSS administration than that observed in similarly treated FcRn(-/-) mice. In chimera studies, FcRn(-/-) mice given wild-type bone marrow and immunized with flagellin exhibited significantly more colitis than wild-type mice given FcRn(-/-) bone marrow and immunized with flagellin. Serum anti-flagellin IgG levels were similar in both sets of chimeric mice, consistent with the equal participation of hematopoietic and nonhematopoeitic cells in FcRn-mediated IgG protection.

CONCLUSIONS

Anti-bacterial IgG antibodies are involved in the pathogenesis of colitis; this pathway requires FcRn in antigen presenting cells, the major subset of hematopoietic cells that express FcRn.

MHC II and the endocytic pathway: regulation by invariant chain

The major histocompatibility complex (MHC) class I and II molecules perform vital functions in innate and adaptive immune responses towards invading pathogens. MHC class I molecules load peptides in the endoplasmatic reticulum (ER) and display them to the T cell receptors (TcR) on CD8(+) T lymphocytes. MHC class II molecules (MHC II) acquire their peptides in endosomes and present these to the TcR on CD4+ T lymphocytes. They are vital for the generation of humoral immune responses. MHC II assembly in the ER and trafficking to endosomes is guided by a specialized MHC II chaperone termed the invariant chain (Ii). Ii self-associates into a trimer in the ER, this provides a scaffold for the assembly of three MHC II heterodimers and blocks their peptide binding grooves, thereby avoiding premature peptide binding. Ii then transports the nascent MHC II to more or less specialized compartment where they can load peptides derived from internalized pathogens.

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.

Chapter 4: Multitasking by exploitation of intracellular transport functions the many faces of FcRn

The MHC Class I-related receptor, FcRn, transports antibodies of the immunoglobulin G (IgG) class within and across a diverse array of different cell types. Through this transport, FcRn serves multiple roles throughout adult life that extend well beyond its earlier defined function of transcytosing IgGs from mother to offspring. These roles include the maintenance of IgG levels and the delivery of antigen in the form of immune complexes to degradative compartments within cells. Recent studies have led to significant advances in knowledge of the intracellular trafficking of FcRn and (engineered) IgGs at both the molecular and cellular levels. The engineering of FcRn-IgG (or Fc) interactions to generate antibodies of increased longevity represents an area of active interest, particularly in the light of the expanding use of antibodies in therapy. The strict pH dependence of FcRn-IgG interactions, with binding at pH 6 that becomes essentially undetectable as near neutral pH is approached, is essential for efficient transport. The requirement for retention of low affinity at near neutral pH increases the complexity of engineering antibodies for increased half-life. Conversely, engineered IgGs that have gained significant binding for FcRn at this pH can be potent inhibitors of FcRn that lower endogenous IgG levels and have multiple potential uses as therapeutics. In addition, molecular studies of FcRn-IgG interactions indicate that mice have limitations as preclinical models for FcRn function, primarily due to cross-species differences in FcRn-binding specificity.

Targeting the neonatal fc receptor for antigen delivery using engineered fc fragments

The development of approaches for Ag delivery to the appropriate subcellular compartments of APCs and the optimization of Ag persistence are both of central relevance for the induction of protective immunity or tolerance. The expression of the neonatal Fc receptor, FcRn, in APCs and its localization to the endosomal system suggest that it might serve as a target for Ag delivery using engineered Fc fragment-epitope fusions. The impact of FcRn binding characteristics of an Fc fragment on in vivo persistence allows this property to also be modulated. We have therefore generated recombinant Fc (mouse IgG1-derived) fusions containing the N-terminal epitope of myelin basic protein that is associated with experimental autoimmune encephalomyelitis in H-2(u) mice. The Fc fragments have distinct binding properties for FcRn that result in differences in intracellular trafficking and in vivo half-lives, allowing the impact of these characteristics on CD4(+) T cell responses to be evaluated. To dissect the relative roles of FcRn and the "classical" FcgammaRs in Ag delivery, analogous aglycosylated Fc-MBP fusions have been generated. We show that engineered Fc fragments with increased affinities for FcRn at pH 6.0-7.4 are more effective in delivering Ag to FcRn-expressing APCs in vitro relative to their lower affinity counterparts. However, higher affinity of the FcRn-Fc interaction at near neutral pH results in decreased in vivo persistence. The trade-off between improved FcRn targeting efficiency and lower half-life becomes apparent during analyses of T cell proliferative responses in mice, particularly when Fc-MBP fusions with both FcRn and FcgammaR binding activity are used.