FcRn: The architect behind the immune and non-immune functions of IgG and albumin

Abstract

The neonatal Fc receptor (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 antigens. 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 important role in immunity at mucosal and systemic sites through both its ability to affect the lifespan of IgG as well as its participation in innate and adaptive immune responses. Even though the details of its biology are still emerging, the property of FcRn to rescue albumin and IgG from early degradation represents an attractive approach to alter the plasma half-life of pharmaceuticals. Here, we will review some of the most novel aspects of FcRn biology, both immune as well as non-immune, and provide some examples of FcRn-based therapies.

From Ehrlich passive immunity and Brambell Receptor to FcRn

It was more than one hundred and twenty years ago that Paul Ehrlich described the ability of maternal antibodies to cross to offspring and protect them from infections in early life (1). From the available studies, it was then recognized that the acquisition of this passive immunity varied depending on species. For instance, in humans and rabbits antibody transfer occurred mainly before birth (antenatally) either transplacentally or via the yolk sac, respectively. In ruminants, horses and pigs, however, postnatal transmission took place, where antibodies were transferred in colostrum or milk, and were then absorbed trans-intestinally. In mice, rats, dogs and cats, antibody transfer occurred both before and after birth, being more predominant in the neonatal period (2). Nonetheless, it was unknown whether all types of globulins were transferred, how they were transported, and whether identical, equivalent or diverse transfer systems were operating in different species. In the 1950s and 60s, by studying these phenomena, F. W. Rogers Brambell and colleagues showed that γ-globulins (consisting mostly of immunoglobulins, Ig) in particular were selected for transmission while most other proteins from maternal circulation were not (3). Further, it was recognized that such transfer was completely dependent on the Fragment crystallisable (Fc) region of Ig and not on the Fragment antigen-binding (Fab) region (4). Brambell also investigated the fate of γ-globulins outside of pregnancy, reflecting on their long persistence in adult circulation (~20 versus ~five days for most of other plasma proteins) and the characteristics of their elimination. He recognized that the long half-life of Ig required the Fc region and that very rapid elimination occurred upon high dose administration, indicating that a saturable rescue process was involved (5, 6). Thus, Brambell postulated that a singular receptor may control both the transport of IgG during early life and the protection of IgG from catabolism in later life. Concurrently, Hermann E. Schultze and Joseph F. Heremans, observing fractional catabolic rates of different serum proteins, made similar predictions about the existence of an albumin specific protection receptor (7).

Next, in parallel to the demonstration of Fc dependent in utero rabbit Ab transfer, neonatal transmission of only γ-G globulin (IgG) across the intestinal mucosa in neonatal rats was defined at the functional and biochemical levels (8). In the 1970s, the proximal intestine was identified as the main site of transmission of passive immunity in neonatal rats (9). Subsequently, it was shown that this Fc receptor of the neonatal (FcRn) rat intestine or human intestine preferentially bound IgG under acidic conditions (8, 10). In the acidic environment typical for the duodenum and jejunum during early life, IgG was bound by FcRn on enterocyte surface, endocytosed and transcytosed, travelling from the lumen of the intestine, to the basolateral side where it was released at physiological pH (Figure 1B) (11, 12). Following the purification of the heterodimeric receptor consisting of heavy (p51) and light (p14) chains from rat enterocytes, the cloning the FcRn was achieved in 1989 (13, 14). During the same period, the initial observations on cellular and temporal expression of FcRn were extended. Crystallization of the receptor and the identification of physical domains and amino acid residues responsible for Fc binding were accomplished rapidly thereafter (15–17). In the 1990s, the demonstration that animals deficient in the gene encoding the p14 light chain did not express the p51 heavy chain, were unable to acquire IgG from maternal milk, and had low IgG circulating levels, undeniably demonstrated that the receptor responsible for IgG protection and fetal/neonate transfer were the same and only receptor; that is FcRn (18–20).

Figure 1.

A) Transport of IgG across the placenta in humans or fetal yolk sac of rabbits. In the endoderm of the fetal yolk sac IgG is internalized by fluid-phase endocytosis and encounters FcRn in early endosome; B) Transport of IgG across the intestine in rodents or ruminants. In new-born rats IgG is taken up by fluid-phase endocytosis or through binding to FcRn at the apical cell surface of enterocytes. This enables active FcRn-mediated transport of IgG across the cell and its subsequent release on the opposite, extracellular side at neutral pH. In such circumstances, FcRn transcytosis exhibits a dominant vector of transport from apical-to-basolateral directions which likely reflects concentration and pH gradients as well as cell-dependent factors; C) The catabolism of IgG (and potentially albumin) by FcRn in endothelial cells or hemaotpoietic cells (not shown here). Serum IgG is internalized by fluid-phase endocytosis and binds to FcRn in an acidic endosomal compartment. FcRn then recycles IgG back into neutral pH milieu of the circulation, thus extending its serum half-life. IgG not bound to FcRn due to levels that exceed FcRn capacity or other serum proteins are destined for lysosomal degradation.

Albumin possesses an equally long half-life (~20 days) in circulation and the existence of an albumin protection receptor had also been hypothesized. Although several receptors able to bind albumin exist such as cubulin and megalin, none of them could account for the long albumin persistence (21). As it was subsequently shown that FcRn binds albumin at acidic pH prolonging its half-life in the circulation, FcRn was recognized as the receptor responsible for rescuing albumin from destruction (22). In support of this conclusion, the identification of patients with defective FcRn function due to p14 light chain deficiency or the genetic deletion of the p51 heavy chain in animals clearly illustrated that this resulted in the reduction of both albumin and IgG levels in the circulation (23, 24).

Recent years have brought forth several important advances in the study of FcRn biology. These include the recognition that FcRn expression continues after the neonatal period and is more widespread than initially suspected (25). Equally, in polarized epithelial cells FcRn is uniquely responsible for the bidirectional transcytosis of IgG, in contrast to the polymeric Ig receptor (pIgR) that transports polymeric IgA and IgM unidirectionally (26). This is critically important to the immune responses at barrier surfaces that contain IgG. Further, FcRn is now recognized to exhibit significant expression throughout the hematopoietic system, notably in myeloid cells (27). Such targeted expression endows FcRn⁺ cells with particular function: to distinguish between monomeric IgG and multimeric IgG-immune complexes (IC). The former recognition results in protection from degradation and the maintenance of serum IgG levels, while the latter initiates routing of the IgG-IC for breakdown in antigen presentation compartments that leads to improved MHC class I and II presentation and the production of inflammatory cytokines (18, 28–30). The insights into the biology of FcRn over the last 60 years are also now being converted into new therapeutic approaches for several diseases. This review summarizes these recent advances and their implications.

General aspects of FcRn: From gene to protein

The FCGRT (Fcgrt) gene is encoded on chromosome 19q13 in humans, and 7 in mice (31–33). Orthologues have been identified broadly in numerous mammalian and marsupial species (34, 35). The human FCGRT encoded receptor shares overall 66% homology with mouse FcRn while Fcgrt is highly conserved between mice and rats. The allelic diversity of FCGRT/Fcgrt gene is low, with few polymorphisms identified (31–33). Of note, five different alleles of variable number of tandem repeats (VNTR1–5) in the 5′-flanking region of hFCGRT have been described, with VNTR2 and VNTR3 being the most common (7.5% and 92% in Caucasians, and 3.2% and 96.8% in Japanese, respectively) (36, 37). Presence of the VNTR3 variant results in higher FCGRT transcriptional activity when compared with the VNTR2 allele, yet no significant effect on materno-fetal IgG transfer, or on the elimination of therapeutic antibodies has been observed (38, 39). FCGRT encodes MHC class I-like α, heavy chain (p51) that non-covalently associates with light chain (p14), also known as β₂-microglobulin (β₂m), forming a heterodimer (13). The reported molecular mass of the α chain varies from 45 to 55 kDa depending on the species or cell type it is isolated from, and relates mostly to variability in glycosylation with a single N-glycosylation site in human FcRn compared with three in rodent FcRn (40). FcRn shares high sequence and structural homology with MHC class I molecules, however it is encoded outside of the HLA or H-2 loci and is nonpolymorphic (31, 41). Thus FcRn is a type I glycoprotein consisting of α1, α2, α3 extracellular domains, a transmembrane region and cytoplasmic tail. The α1, α2 domains form a narrow groove that is unable to present peptides but nonetheless in conjunction with the α3 domain and β₂m form the ligand binding regions (13, 16).

FcRn expression is now recognized to be widespread, occurring throughout life. FcRn is expressed by a wide variety of parenchymal cell types in many different species. These include vascular endothelium (including the central nervous system), most epithelial cell types such as placental (syncytiotrophoblasts), epidermal (keratinocytes), intestinal (enterocytes), renal glomerular (podocytes), bronchial, mammary gland (ductal and acinar), retinal pigment epithelial cells, renal proximal tubular cells (PTC), hepatocytes, melanocytes, as well as cells of the choroid, ciliary body and iris in the eye (42, 43). FcRn is also widely expressed by hematopoietic cells including monocytes, macrophages, dendritic cells (DC), neutrophils and B cells where, in contrast to polarized epithelial cells, it is detected in significant quantities on the cell surface (27). In epithelial cells, which possess distinct apical and basal membranes, FcRn is predominantly intracellular with a variable distribution along the vesicular network and cell surface in a cell and species specific manner (44). For instance, a greater quantity of FcRn has been observed in the apical regions of syncytiotropoblasts and rodent enterocytes or primary renal PTC, whereas in model polarized cells such as Madin-Darby canine kidney (MDCK), human FcRn is expressed predominantly basally including expression at the basolateral surface (45). This differential distribution has been attributed to the presence of dileucine and tryptophan motifs, and several serine phosphorylation sites within the cytoplasmic tail, as well as the glycosylation status of the extracellular portion of the receptor (40, 45–47). The variable cellular localization of the receptor also affects the direction of IgG transport: in transfected model systems and cell lines apical-to-basal transcytosis is greater with rodent FcRn in comparison to human FcRn, where basal-to-apical transport dominates (45, 47).

FcRn expression is also temporally and developmentally modulated. During fetal life, FcRn expression in the syncytiotrophoblasts of the placenta is responsible for passive IgG transfer (Figure 1A). In rodents during the first weeks of life, the receptor is present at high levels in proximal intestinal epithelium coinciding with ingestion of mother’s milk (Figure 1B), and is rapidly down regulated after weaning (12, 48). In contrast, FcRn expression in human enterocytes is lower than observed in neonatal rodents but does not decrease with age (26, 49). Further, exposure of epithelial cells, human THP-1 (macrophage-like) cells, or human PBMC to TNF-α, LPS or CpG induces a significant increase in FcRn expression in an NF-κB dependent manner, while IFN-γ priming results in FcRn down-regulation (50, 51). These results suggest that FcRn expression can be modulated by the immunological context of the surrounding milieu. Whether additional modes of spatiotemporal regulation of FcRn expression, in humans or rodents, occur is presently unknown.

The ligands of FcRn

FcRn binds IgG and albumin at acidic pH, which is characteristic of early and late endosomes, the proximal intestine during neonatal life and, potentially, the extracellular milieu associated with inflamed tissues (52). Albumin is the most abundant protein in mammalian plasma (~65% of the circulating proteins) and transports many endogenous and exogenous molecules, maintains colloidal osmotic pressure, and exerts antioxidant functions (53). Albumin is a 65–70 kDa protein consisting of three globular domains (DI, DII, and DIII). More than 60 variants of human albumin have been identified (53). Some, like the Casebrook variant, contains a single point mutation that decreases its affinity to FcRn (54). Likewise, IgGs represent the second most abundant proteins in serum, and are the most frequent antibody isotype found in the circulation. Of the four IgG subclasses in humans (IgG1, IgG2, IgG3 and IgG4), binding affinity to FcRn ranges from 20 nM (IgG1) to 80 nM (IgG4) (55). IgG3, with a longer hinge region relative to other IgG subclasses has the lowest potential to bind FcRn and possesses a half-life of only nine days in circulation (56). In mice, a similar range of affinities among the IgG subclasses (IgG1, IgG2a, IgG2b, IgG3) to bind FcRn has been demonstrated (57). Several polymorphisms and allotypes of the human IgG heavy chain exist but whether these have differential binding to FcRn is unknown (58). Changes in the complementarity-determining region (CDR) of IgG have also been reported to affect FcRn binding even though these are distant from the FcRn binding site (59, 60). Furthermore, variation in binding specificity is seen between FcRn orthologs. Whereas rat and mouse FcRn are more liberal in their ability to bind IgG from different species (including human, rabbit, mouse and bovine), human FcRn mainly interacts with human, rabbit and guinea pig IgG (61). Such disparity has also been illustrated for albumin binding, where rodent FcRn binds weakly to rhesus monkey or human albumin as compared with strong binding to mouse and rat albumin, whereas human or rhesus monkey FcRn binds more strongly mouse and rat albumin than to the human or rhesus orthologue (62).

Structural studies have shown that FcRn binds to IgG with 1:1 or 2:1 stoichiometry under non-equilibrium or equilibrium conditions, respectively (63, 64). In contrast, one FcRn receptor binds to one albumin molecule (65). FcRn interacts with each of its two ligands through contacts associated with opposite surfaces, such that FcRn can bind IgG and albumin simultaneously without either competition or cooperation occurring between each other (65). Biochemical and crystallographic data indicate that upon binding at pH 6.0, neither FcRn nor IgG undergo major conformational changes. Instead, it is the protonation of histidine residues (H310, H435, H436) in the C_H2-C_H3 hinge region of IgG1 which enable binding (66). This allows for the formation of salt bridges at the FcRn-Fc interface, specifically the acidic residues on the C-terminal portion of the α2 domain (E117, E132 and D137) in FcRn, and the first residue of β₂m (55, 66, 67). Notably, the sites within IgG that bind to FcRn competitively overlap with IgG Fc binding to Staphylococcal protein A (68). In contrast, FcRn binding to albumin is mostly hydrophobic in nature and thought to be stabilized by a pH dependent hydrogen bonding network internal to each protein. This interaction involves two tryptophan (W53 and W59) residues of FcRn and three histidine (H464, H510 and H535) residues within albumin third domain (DIII), which are fundamental for pH-dependent FcRn binding, as well as some contribution of the first domain (DI) (54, 69, 70). Contrary to FcRn-IgG interactions, no pH-dependent intermolecular salt bridges exist in hydrophobic FcRn-albumin binding. Available data suggest that FcRn possesses higher affinity for IgG than for albumin (54, 65). These unique interaction properties of FcRn and its ligands form the basis for a wide variety of physiologically important FcRn-driven functions.

FcRn functions: Not just bi-directional transporter

FcRn-mediated recycling and the protection of monomeric IgG and albumin from degradation

FcRn plays a major role in the maintenance of serum IgG levels. Early studies with FcRn suggested that the major cell type involved in this process was vascular endothelium (71). Yet, chimerism of WT mice with Fcgrt^−/− bone marrow Tie-2-Cre driven, deletion of Fcgrt, have revealed that cells of the hematopoietic origin participate in the IgG salvage pathway (28, 72, 73). Given that only small amounts of FcRn receptor expression has been detected on the surface of endothelial cells, IgG uptake is believed to occur mostly via non-specific, fluid-phase pinocytosis rather than by receptor-mediated endocytosis (18). Once inside the cell, IgG binding to FcRn is thought to occur as the early endosome becomes increasingly acidic and permissive for pH-dependent interaction of FcRn with IgG. FcRn bound IgG is then sorted into common recycling endosomes which recycle IgG away from lysosomes and back to the cell surface via rapid and slow release routes where IgG is extruded into the extracellular milieu due to the neutral pH in that locale (Figure 1C) (74).

Less is known about the FcRn dependent salvage of albumin, which is partly extrapolated from insights derived from IgG-FcRn interactions. Based upon this, an equivalent pathway for FcRn receptor-mediated salvage of albumin is thought to exist (22). However, the efficiency of these two processes is vastly different and it is estimated that for every six FcRn-recycled albumin molecules in humans, only one IgG molecule is rescued, while the ratio in mice is even lower at 30:1 (75). Nonetheless, this level of recycling is sufficient for sustaining high IgG plasma concentrations with a relatively minor contribution provided by IgG production. In contrast, high serum albumin levels depend more on a high rate of hepatic synthesis rather than on FcRn-dependent salvage (75). Thus the manner in which FcRn maintains IgG and albumin in the circulation may share some but not all mechanistic features. Furthermore, it remains unknown whether cells of hematopeitic and parenchymal origin participate equivalently in IgG and albumin retention.

The FcRn-dependent diversion of IgG and albumin away from lysosomal degradation is the basis for designing new or modified drugs with enhanced FcRn binding capacity and, thus, prolonged serum half-lives and potentially improved pharmacokinetics. A large number of modifications within the C_H2-C_H3 domains of IgG spanning the FcRn contact surface (such as L253, H410 and H435 residues) have been described to significantly enhance pH dependent binding of IgG to FcRn (reviewed in (76)). Although they have not yet entered clinical use, a number of preclinical studies in non-human primates have shown increased efficacy of such engineered therapeutics in anti-cancer and anti-infection therapeutic approaches (77–79). Alternatively, pharmacologic FcRn inhibition using peptide mimetics, anti-FcRn monoclonal antibodies or antibodies modified to possess Fc-dependent and low pH-independent binding (so-called Abdegs) have been shown to decrease circulating levels of pathogenic IgG and confer protection in models of myasthenia gravis (MG), systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and experimental autoimmune encephalomyelitis (EAE)(77). Similarly, engineering IgGs to lack the ability to bind to FcRn, (such as the mutation of the I253, H410 and H435 residues (IgG_IHH)), enables accelerated catabolism of IgG-radioimmuno conjugates during molecular imaging, endowing them with fewer toxic side effects than conventional chemotherapeutic drugs (80). Similar approaches aiming to develop long-lived albumin-based therapies demonstrate the enormous potential derived from understanding of mechanisms associated with FcRn-mediated recycling.

Transcytosis of IgG and its implications for drug delivery and vaccine development

In polarized cells such as the epithelium, FcRn is able to transport IgG bi-directionally via transcytosis which allows for the transfer of IgG from mother to young (Figure 1A, B), and the delivery of IgG-complexed environmental antigens and microbial products into the host resulting in the induction of tolerance or immunity as discussed further below (81, 82). Similar to the recycling pathway, IgG is likely internalized by fluid phase pinocytosis into polarized epithelial cells. Still, the trafficking of FcRn is highly dynamic, and while most of the receptor occupies endosomal compartments at any single time, there is always some FcRn located at the cell surface with a flux of FcRn moving through the plasma membrane over time. Thus, receptor-mediated endocytosis is also thought to occur in light of early observations wherein FcRn expression was detected on the apical enterocyte surface membrane and in the context of an acidic microenvironment in the neonate proximal intestine (74, 83, 84). That said, there is a paucity of data in support of the existence of an acidic pH environment in the neonate intestine which is further complicated by the fact that the uptake of IgG by neonatal rodent enterocytes has been observed to take place in the absence of FcRn raising the possibility that the initial internalization of IgG in the neonatal enterocyte might be FcRn and pH independent (85). “Whether fluid phase and receptor mediated pathways of IgG uptake operate concurrently and whether they escort IgG to the same intracellular compartments still needs to be assessed.

Subsequent to internalization, IgG is firstly diverted into early sorting endosomes and then the common recycling endosome (86). The acidification of endosomes allows for FcRn to bind IgG in these compartments thereby enabling active FcRn-mediated transport of IgG across the cell and its subsequent release on the opposite side, once exposed to the neutral extracellular pH (Figure 1B). The intracellular mechanisms that determine whether apical or basal recycling versus apical-to-basal or basal-to-apical transcytosis occurs have only recently begun to come to light. Apical to basolateral transport, for example, depends on motifs present in the cytoplasmic tail, as well as on the ability of FcRn to bind calmodulin in polarized epithelial model systems (46, 87, 88).

FcRn dependent transcytosis of IgG and likely albumin also occurs in vivo. In rodents, forced FcRn expression restricted to intestinal epithelial cells results in IgG secretion into the gut lumen (82). In addition, Fc-dependent absorption of IgG has been described in the lungs and intestines of humanized mice that only express human FcRn (FCGRT/B2M Tg/Fcgrt^−/− mice) (Figure 2A) (89). Similarly, Fc-fusion proteins, but not Fc-fusion proteins disabled in FcRn binding, can be absorbed in the lungs of mice and non-human primates (90, 91). Further, consistent with the near absence of serum proteins in the urine, albumin is re-absorbed in the renal PTC through an active process involving FcRn, suggestive of transcytosis (92). In this pathway, albumin in the renal ultrafiltrate is internalized by binding to the megalin-cubulin complex via receptor-mediated endocytosis on the apical surface of PTCs. Subsequently, this complex is trafficked to late endosomes where, at acidic pH, albumin is captured by FcRn and directed back into the circulation (86, 93, 94). The involvement of FcRn in albumin re-absorbtion by PTC requires additional substantiation and consideration as FcRn-deficient mice, for instance, may not display significant albuminuria (95). This may be due to low albumin levels in the circulation of these animals (hypoalbuminemia) which results in reduced amounts of albumin in the glomerular ultrafiltrate. As such, the loss of albumin in the urine of FcRn-deficient mice should be investigated in animals with normal circulating albumin levels. FcRn-mediated IgG handling in the kidneys differs from albumin. In order to prevent potential accumulation of protein complexes that could obstruct glomerular filtration, IgG is removed from the glomerular basement membrane in an FcRn-mediated process that allows for the movement of IgG across the podocyte and its excretion into the glomerular capsular space and thus away from the circulation (96). As IgG is not subsequently re-absorbed by the PTC, it is lost in the urine (93). The latter process may be the basis for immune protection within the urinary excretory system.

Figure 2.

A) In the adult human gut, both enterocytes and antigen-presenting cells (APCs) in the lamina propria express FcRn. Enterocytes transcytose IgG into the gut lumen where it binds to antigens. The IgG–immune complexes (IC, IgG-IC) are then delivered to dendritic cells (DC) in the lamina propria. Antigen-loaded DCs then migrate to the draining lymph nodes to prime T-cell responses. B) The IgG-IC can bind to FcγR on the surface of DC at neutral pH, initiating receptor-mediated endocytosis. This delivers the IgG-IC into the endolysosomal compartments. As these vesicles mature they become more acidic. Acidification allows IgG-IC to dissociate from FcγR and favours binding to FcRn. Such a “hand-off” enables efficient trafficking of the IgG-IC and the delivery of antigen into antigen processing pathways that promote the loading onto MHC class I and MHC class II molecules. Ligation of FcRn by IgG-IC also induces the production of IL-12 by the DC. The peptide loaded MHC molecules derived from IgG-IC, are then able to prime CD8⁺ and CD4⁺ T cells. While the secreted IL-12 acts upon the primed CD4⁺ T cells to induce T_H1 polarization, upon CD8⁺ T cells it promotes activation, cytotoxicity and a T_C1 phenotype. For simplification, although a monomeric IgG immune complex is shown in panels A and B, multimeric IgG-IC are the types responsible for antigen processing and induction of IL-12 secretion.

The transcytosis of IgG or IgG-IC by FcRn at mucosal barriers such as the lung, intestines or genitourinary tract can have important consequences for the host immune response. In addition to conferring passive immunity, mouse dams tolerized to antigens are able to confer tolerance to their offspring during the period of suckling by transfer of specific IgG or IgG-IC (97, 98). Intriguingly, IgG-IC that formed in the context of antibody excess have been found to be immunosuppressive in contrast to IC formation in setting of antigen excess wherein they are immunogenic (99). More recently, using the prototype antigen OVA in an allergic airway disease model, airborne antigen exposure of lactating mice resulted in decreased airway hyper-reactivity only in breastfed offspring (100). It was subsequently shown that the milk from OVA sensitized mothers contained TGF-β as well as IgG-IC, which was transferred to the newborn via FcRn and that both are required for the induction of tolerance (101). Moreover, due the production of IgG with anti-IgE specificities, FcRn can mediate the transfer of IgE from the lumen into the circulation in form of IgG anti-IgE-IC, which may also play a role in pathways of early life tolerance (102).

In vivo studies with FcRn humanized mice have demonstrated that circulating IgG can be delivered into the intestinal lumen where they bind orally administered antigens and form IgG-IC which are then transported back into the lamina propria in an FcRn-dependent manner (Figure 2A) (82). Since the IgG-IC were shown to subsequently be taken up by CD11c⁺ DCs that induced antigen specific CD4⁺ T cell responses, the FcRn mediated transcytosis of IgG and IgG-IC across mucosal barriers in adult life confers a sensing capacity on IgG which scavenges luminal antigens and permits their efficient delivery to the immune system. Indeed, the presence of anti-pathogen specific IgG reduced disease severity only in FcRn competent mice upon challenge with pathogens such as Helicobacter pylori, Citrobacter rodentium or Chlamydia muridarum (89, 103, 104).

In the context of viral infection, the protective role of FcRn can be variable. Analysis of herpes simplex virus 2 (HSV-2) infection has shown that HSV-2-specific antibodies are transcytosed from the systemic circulation into the genital lumen in wild type (WT) mice, but not in FcRn-deficient mice, and that this confers protection against intra-vaginal viral challenge (105, 106). Similar protective FcRn-mediated effects were noticeable after administration of anti-influenza hemagglutinin-specific monoclonal antibody upon influenza infection (107). In contrast, during cytomegalovirus (CMV) infection, infectious virions were able to disseminate to the placenta via their associations with poorly, but not strongly, neutralizing anti-CMV antibodies when transported by FcRn (108). FcRn transcytosis of IgG-virion complexes across mucosal genital surfaces has also been illustrated for HIV infections (109, 110). In a study designed to test the ability of neutralizing and non-neutralizing antibodies to protect macaques from vaginal simian HIV (SHIV) challenge, the strongly neutralizing IgG provided sterilizing immunity while non-neutralizing IgG did not (111). Indeed, passive administration of a highly neutralizing human anti-HIV antibody (VRCO1) engineered to enhance pH dependent binding to FcRn protected from the mucosal transmission of SHIV virus infection (78). Thus, depending on antibody neutralizing capacities, site and tissue pH, FcRn may be responsible for shuttling infectious agents which either facilitate or prevent infection.

FcRn dependent regulation of IgG-IC by haematopoietic cells and the role played in innate and adaptive immunity

Both mouse and human hematopoietic cells have been demonstrated to express FcRn (27). Notably, these include all subsets of DC, macrophages and monocytes, as well as neutrophils and B cells (27, 72, 73). Most FcRn carrying hematopoietic cells are thus proficient in antigen presentation or phagocytosis. Indeed, in addition to being a major site of monomeric IgG protection from degradation, FcRn expression by DC has been shown to play an important role in the ability of an antigen carried by IgG to be processed and displayed by MHC class I molecules via cross-presentation and MHC class II molecules for presentation to CD8⁺ and CD4⁺ T cells, respectively (29, 30). This is consistent with the importance of FcRn in facilitating the degradation of IgG-IC in vivo (28, 73, 112). In neutrophils such functions of FcRn have been linked to the uptake of IgG-opsonized bacteria and their delivery into phagolysosomes (113). The size of the IC also determines the degree to which IgG and antigen are diverted to lysosomes as large IgG-IC are more likely to exhibit this fate in comparison to small IgG-IC (114). In the latter case, small IgG-IC’s are handled similarly to monomeric IgG and are protected from degradation (28). In epithelial cells, IgG-IC are also diverted to lysosomes as shown by the behavior of neutralizing anti-influenza monoclonal antibodies (107), making such observations a consequence of FcRn interactions with multimeric IgG rather than that of the cell type per se. These findings clearly indicate a fundamental functional difference in how FcRn traffics monomeric IgG and multimeric IgG in form of IgG-IC.

The expression of classical Fcγ receptors (FcγR) on the cell surface enables APCs to capture and internalize IgG-ICs (115). In doing so, APC primed with IgG-IC effectively activate CD4⁺ and CD8⁺ T cells (116–119). It has recently been demonstrated, that FcγR and FcRn cooperate in the processes of MHC class II presentation and MHC class I cross-presentation. Both of these receptors possess distinct pH dependent binding to IgG: FcγR at neutral pH and the FcRn at acidic pH. This is consistent with the concept of IgG-IC “hand-off” within acidified endosomes where FcγR enables IgG-IC internalization initially, and FcRn shapes the subsequent fate of the IgG-IC (Figure 2B). Consistent with this, DCs exposed to IgG-IC, but not to the FcRn non-binding IgG_IHH-IC, induce greater CD4⁺ T cell proliferation both in vitro and in vivo when compared to FcRn-deficient DCs (28, 29). Therefore, FcRn cooperates with FcγR in antigen presentation and cross-presentation initiated by the uptake of IgG captured antigens.

Although FcRn is expressed by all types of APC, an analysis of the subsets of APC involved in MHC class I cross-presentation have shown that FcRn enables IgG-dependent cross-presentation most strongly in the CD8⁻CD11b⁺ DC subset. The CD8⁻CD11b⁺ DC, in contrast to the CD8⁺ DC, possess acidic endolysosomal compartments conducive to IgG-FcRn binding (29). In a model of chronic colitis with high levels of anti-bacterial IgG antibodies, inflammatory CD8⁻CD11b⁺ DCs expanded and, in WT but not Fcgrt^−/− mice, enabled efficient IgG-dependent activation of CD8⁺ T cells (29). Similar mechanisms presumably account for the improved expansion of antigen-specific CD8⁺ T cells after vaccination with haptenated Ag (120). Therefore, this suggests that FcRn couples IgG-IC responses with the efficient induction and activation of CD4⁺ and CD8⁺ T cells depending upon the DC subset involved.

This is physiologically relevant in steady state settings given that conditional deletion of FcRn within the CD11c⁺ cell fraction results in decreased quantities of CD8⁺ T cells at mucosal sites (30). Further, deficient CD8⁺ T cell cytokine production and an inability to block the growth of induced or spontaneous colorectal or lung cancers was also observed (30). This defect in CD8⁺ T cell numbers was dependent on FcRn expressing CD8⁻CD11b⁺ DC fraction, as adoptive transfer of WT DC conferred protection to Fcgrt^−/− recipients. Interestingly, these cellular defects of Fcgrt^−/− mice were not accompanied by decreased IgG levels within the intestine despite the quasi absence of IgG in the serum (30). This supports the notion that a major function of FcRn in tissues is the maintenance of cellular immunity through the processing of antigen-antibody complexes.

Studies of multimeric, IgG-IC interactions with FcRn have further revealed that upon cross-linking, FcRn is capable of orchestrating a signaling cascade that is associated with secretion of cytokines skewed towards T helper 1 (T_H1) and T cytotoxicity 1 (T_C1) responses, particularly IL-12 (Figure 2B). In humans, immunohistochemical staining demonstrated that FcRn⁺CD11c⁺ DCs were present in situ within colorectal cancers and adjacent colon in close juxtaposition to CD8⁺ T cells. Most importantly, colorectal cancer patients with higher frequency of FcRn⁺CD11c⁺ DCs at or near cancer sites had significantly longer survival times than did those with fewer FcRn⁺CD11c⁺ cells (30). These observations support the notion that FcRn⁺ APC regulate CD8⁺ T cells and their function in anti-tumor and presumably other similar types of immunity. It may be surmised that a major function of FcRn in tissues is in its direct effects on innate and adaptive immunity through regulation of cytokine tone and antigen presentation that supports T_H1 and T_c1 responses.

Conclusions

The name of FcRn largely originates from the initial historical observations focusing on the neonatal IgG transmission in rodents (10). With the rediscovery and confirmation of Brambell’s, Hereman’s and Schultze’s original hypotheses that FcRn regulates systemic IgG and albumin homeostasis and documentation that FcRn is broadly expressed throughout life, this neonate nomenclature, although archaic, still applies.

FcRn’s functions have recently expanded from passive immunity and IgG protection to an active IgG and antigen trafficking receptor. The strategic presence at mucosal barrier sites takes advantage of FcRn’s ability to bi-directionally transport IgG and to deliver antigens to the mucosal immune system. Then again FcRn expression in APC and specific transfer of multimeric IgG-IC to antigen processing vesicles allows for efficient MHC class I and II presentation. The ensuing effect is more potent elicitation of CD4⁺ and CD8⁺ T cell dependent responses and enhanced protection against both bacterial and viral pathogens. This further suggests that in tissues FcRn directed degradation of IgG-IC might be its major function. FcRn ability to bi-directionally move IgG, to exploit IgG’s abundance in particular at mucosal sites and its specificity towards diverse soluble antigens, endows this receptor to convey IgG-IC for efficient T cell priming within APCs reuniting ideally the humoral and cellular arms of immunity.