How the controller is controlled – neonatal Fc receptor expression and immunoglobulin G homeostasis

With the discovery of immunoglobulins, immunoglobulin G (IgG) was recognized to possess two unique properties: selective pre- or postnatal transepithelial transport across the placenta in humans or the intestinal epithelium in rodents, respectively, and a prolonged half-life relative to other serum proteins, suggesting protection from catabolism in adults. These observations were noted by Francis Brambell, who further predicted the presence of a saturable receptor responsible for both biological functions.¹ Two decades after these predictions, biochemical and ultimately molecular biological evidence was obtained, in the late 1980s, for the presence of a receptor that was physiologically active in neonatal rodent epithelium.^2,3 However, not until recently with the generation of X-ray crystallographic structures, subsequent structure–function analyses and the creation of a knock-out animal, has it become clear that the so-called neonatal Fc receptor for IgG (FcRn) is responsible for both of the aforementioned functional attributes of IgG physiology: its transport across the neonatal epithelium of rodents and the avoidance of catabolism.^4–8

FcRn consists of a glycosylated heavy chain that is closely related to major histocompatibility complex (MHC) class I molecules in non-covalent association with β2-microglobulin (β2m).⁹ FcRn binds IgG in a strictly pH-dependent manner, in which efficient binding is seen only at acidic pH (< 6·5) and not at neutral pH (> 7·0). X-ray crystallography has shown that FcRn binds IgG with a 2 : 1 stoichiometry, with FcRn contacting IgG at the CH2–CH3 domain interface.^4,10 Site-directed mutagenesis has shown that critical histidine residues (H310, H433 and H435) on IgG play a critical role and account for the pH dependence of binding.^11,12 FcRn homologues have been identified in rodents (mice and rats), humans, cows, pigs, sheep and monkeys.

In mice and rats, FcRn is expressed at high levels in the intestinal epithelial cells of suckling pups, where it is responsible for the transport of IgG in maternal milk across the epithelial cells into the digestive circulation of the newborn animals.⁶ At the time of weaning (approximately 14 days of age), FcRn expression is down-regulated approximately 1000-fold within the epithelium at the time of epithelial closure and simultaneously with the cessation of IgG transport.¹³ This phenomenon accounts for the ascription of ‘neonatal’ for this particular Fc receptor. It is believed that FcRn in the intestinal epithelium of the neonatal rodent binds IgG at the acidic pH of the neonatal lumen along the apical surface of the enterocyte, whereupon IgG is transported to the opposite (basolateral) surface of the epithelium in a process termed ‘transcytosis’, where IgG is released at the neutral pH of the interstitium.¹⁴ In humans, FcRn is expressed in placental syncytiotrophoblast cells, wherein it mediates the selective transport of maternal IgG to the fetus, giving the full-term fetus IgG levels above maternal levels and providing protective immunity to the newborn.¹⁵ It is believed that, in this case, IgG is internalized by fluid-phase endocytosis, whereupon receptor (FcRn) and ligand (IgG) interact at the acidic pH of endosomes, whereupon transcytosis takes place.¹⁶

The second important role of FcRn is in the protection of IgG from catabolism and the maintenance of serum IgG levels.^7,8 FcRn within endosomes binds endocytosed IgG and diverts IgG from a degradative fate within lysosomes and instead transports the IgG back to the cell surface for release into the plasma fluid. Endothelial cells in skin, muscle and liver express FcRn and are thought to be the primary sites of serum IgG homeostasis in adult mice and presumably humans.¹⁷ Recently, FcRn has also been implicated in prolonging the half-life of plasma albumin by a similar mechanism.¹⁸ In a recent human case report, two siblings with a β2m gene mutation and therefore reduced expression of functional FcRn showed marked deficiency in both serum IgG and albumin as a result of rapid degradation of these proteins.¹⁹

As noted above, FcRn is developmentally down-regulated at the time of weaning in the rodent intestine. However, it has recently been appreciated that FcRn continues to be expressed in adult life in humans, pigs, cows, monkeys and even rodents.^20–23 Human FcRn continues to be expressed in many adult human cell types, including intestinal, kidney and bronchial epithelial cells,^20,24,25 endothelial cells, small intestinal macrophages, peripheral blood monocytes and monocyte-derived dendritic cells.²⁶ Similarly, FcRn is expressed in adult mouse bone marrow derived dendritic cells, peritoneal exudate macrophages and macrophage cell lines (S.-W. Qiao and R. S. Blumberg, unpublished observation). In pigs, for example, FcRn is expressed in the adult intestinal epithelium, where it is associated with the transport of IgG from the lumen into the circulation.²⁷ In an animal model in which the human FcRn was expressed as a transgene in an FcRn-deficient mouse, human FcRn was observed to be expressed in intestinal epithelial cells and was shown to be involved in the transport of serum IgG to the apical region of the epithelium, allowing subsequent retrieval of luminal antigens and transport into the lamina propria, and hence allowing antigen capture by local dendritic cells and antigen presentation.²⁸

This high degree of recently appreciated complexity in FcRn expression and consequently function focuses significant attention on the manner in which FcRn expression is regulated. However, very little is known about how FcRn is transcriptionally regulated.

Characterization of human, mouse and rat FcRn promoter regions has revealed several putative promoter binding sites, and some of these have been shown to be functional binding sites for stimulating protein 1 (Sp1) or Sp1-like factors.^29–32 However, the crucial regulatory element that drives FcRn gene expression remains unknown. Administration of either corticosteroids or thyroxine to suckling rat pups abolishes both FcRn mRNA expression and IgG absorption in a dose-dependent manner,³³ and our laboratory has previously shown that PMA stimulation increases the total FcRn protein expression level in human monocytes.²⁶ However, it is unknown how FcRn is transcriptionally regulated in these cases.

In a recent issue of this journal, Sachs et al. describe for the first time a polymorphism in the promoter region of the human FcRn gene that is associated with variable FcRn binding capacity. The polymorphism consists of variable numbers of 37-bp long tandem repeats. In Caucasians, most individuals carry alleles with three tandem repeats (allele frequency 92%), with a minority being heterozygous for two and three tandem repeats. Sachs et al. showed that there are significantly fewer FcRn transcripts in monocytes from heterozygous individuals than in those from homozygotes. Moreover, monocytes derived from carriers of two tandem repeats with lower FcRn transcript levels displayed a diminished IgG binding capacity compared with those derived from homozygous individuals carrying three tandem repeats. The authors interestingly suggest that lower FcRn expression may lead to a less efficient transfer of IgG from the mother to the fetal circulation, thus reducing the disease severity of haemolytic diseases and neonatal alloimmune thrombocytopenia of the newborn caused by maternal pathogenic IgG.

It will be interesting to see whether IgG homeostasis is different in individuals carrying the low-activity transcriptional polymorphism. However, studies on the FcRn binding affinities and serum half-lives of different murine IgG isotypes failed to demonstrate an obvious correlation between FcRn affinity and serum half-life, suggesting some other factors, such as susceptibility to different catabolic enzymes, are also contributing to the half-lives of different IgG isotypes.^34,35 More studies on the relationship between FcRn expression and IgG homeostasis, preferably in human subjects, are needed to further elucidate the role of FcRn in IgG-mediated diseases. However, studies such as those performed by Sachs et al. are an important contribution to understanding how FcRn, which has been firmly established as the catabolic protection receptor as proposed by Brambell,¹ determines the specific levels of IgG within the circulation of a particular individual in health and disease.