The role of cell-surface receptors in the transport and catabolism of immunoglobulins

A new era for migraine: Pharmacokinetic and pharmacodynamic insights into monoclonal antibodies with a focus on galcanezumab, an anti-CGRP antibody

PURPOSE

To review pharmacokinetic and pharmacodynamic characteristics of antibodies that bind to soluble ligands within the framework of calcitonin gene-related peptide antibodies.

OVERVIEW

Calcitonin gene-related peptide has been implicated in the pathophysiology of migraine. Galcanezumab is an antibody that binds to the ligand calcitonin gene-related peptide. Other antibodies that target calcitonin gene-related peptide include eptinezumab and fremanezumab. To understand how antibodies can affect the extent and duration of free ligand concentrations, it is important to consider the dose and pharmacokinetics of an antibody, and the kinetics of the ligand and antibody-ligand complex. Insights regarding the pharmacokinetic/pharmacodynamic properties of galcanezumab as a probe antibody drug and calcitonin gene-related peptide as its binding ligand regarding its clinical outcomes are provided.

DISCUSSION

Antibodies are administered parenterally because oral absorption is limited by gastrointestinal degradation and inefficient diffusion through the epithelium. The systemic absorption of antibodies following intramuscular or subcutaneous administration most likely occurs via convective transport through lymphatic vessels into blood. The majority of antibody elimination occurs via intracellular catabolism into peptides and amino acids following endocytosis. Binding of ligand to an antibody reduces the free ligand that is available to interact with the receptor and efficacy is driven by the magnitude and duration of the reduction in free ligand concentration. A galcanezumab pharmacokinetic/pharmacodynamic model shows that galcanezumab decreases free calcitonin gene-related peptide concentrations in a dose- and time-dependent manner and continues to suppress free calcitonin gene-related peptide with repeated dosing. The model provides evidence for a mechanistic linkage to galcanezumab therapeutic effects for the preventive treatment of migraine.

Bovine FcRn-mediated human immunoglobulin G transfer across the milk-blood barrier in transgenic mice

Maternal-fetal IgGs transport occurs either prenatally or postnatally, which confers the newborns with passive immunity before their own immune system has matured. However, little is known about the mechanisms of postnatal IgGs passage in the mammary gland. To investigate how FcRn mediates the IgGs transport in the mammary gland, we first generated bFcRn and anti-HAV mAb transgenic mice, and then obtained HF transgenic mice expressing both transgenes by mating the above two strains. Transgene expression of bFcRn in the four lines was determined by qRT-PCR and western blot. We then localized the expression of bFcRn to the acinar epithelial cells in the mammary gland, and anti-HAV mAb was mainly detected in the acini with weak staining in the acinar epithelial cells. Human IgGs could be detected in both milk and serum of HF transgenic mice by western blot and ELISA. A significantly lower milk to serum ratio of human IgGs in HF mice compared with that of anti-HAV mAb mice, indicating that bFcRn could transport human IgGs across the milk-blood barrier from milk to serum during lactation in HF mice. While, there were no transport of murine IgGs, IgAs, or IgMs. These results provide understandings about the mechanisms of maternal-fetal immunity transfer in the mammary gland.

Abundant intracellular IgG in enterocytes and endoderm lacking FcRn

FcRn, a non-classical MHCI molecule, transports IgG from mother to young and regulates the rate of IgG degradation throughout life. Brambell proposed a mechanism that unified these two functions, saying that IgG was pinocytosed nonspecifically by the cell into an FcRn-expressing endosome, where, at low pH, it bound to FcRn and was exocytosed. This theory was immediately challenged by claims that FcRn specificity for ligand could be conferred at the cell surface in neonatal jejunum. Assessing Brambell's hypothesis we found abundant nonspecifically endocytosed IgG present in the cytoplasm of FcRn(-/-) enterocytes. Further, IgG was present in the intercellular clefts and the cores of FcRn(+/+) but not FcRn(-/-) jejunum. FcRn specificity for ligand could be determined within the cell.

The MHC class I-like Fc receptor promotes humorally mediated autoimmune disease

The MHC class I family-like Fc receptor, FcRn, is normally responsible for extending the life span of serum IgG Ab's, but whether this molecule contributes to autoimmune pathogenesis remains speculative. To determine directly whether this function contributes to humoral autoimmune disease, we examined whether a deficiency in the FcRn heavy chain influences autoimmune arthritis in the K/BxN mouse model. FcRn deficiency conferred either partial or complete protection in the arthritogenic serum transfer and the more aggressive genetically determined K/BxN autoimmune arthritis models. The protective effects of an FcRn deficiency could be overridden with excessive amounts of pathogenic IgG Ab's. The therapeutic saturation of FcRn by high-dose intravenous IgG (IVIg) also ameliorated arthritis, directly implicating FcRn blockade as a significant mechanism of IVIg's anti-inflammatory action. The results suggest that FcRn is a potential therapeutic target that links the initiation and effector phases of humoral autoimmune disease.

The role of the Brambell receptor (FcRB) in liver: protection of endocytosed immunoglobulin G (IgG) from catabolism in hepatocytes rather than transport of IgG to bile

The Brambell receptor (FcRB) mediates functions of both immunoglobulin G (IgG) transport, transmitting immunity from mother to young, and IgG protection, making IgG the longest surviving of all plasma proteins. Reflecting its role as transport receptor (termed FcRn, for neonatal rat intestine, the tissue from which it was first cloned), FcRB is expressed antenatally in the rabbit, mouse and rat fetal yolk sac and in human placental syncytiotrophoblasts, and neonatally in the intestinal epithelium of mice and rats. Reflecting its role as protection receptor (FcRp), FcRB is expressed in the vascular endothelium throughout life, where it protects IgG from the on-going catabolic activities of this tissue. FcRB detected in hepatocytes was hypothesized to mediate transport of IgG from serum to bile, thus potentially extending the transport expression (FcRn) of this receptor beyond the perinatal period. Our results show serum-to-bile transport of IgG to be unaffected in mice functionally deleted for FcRB. Accordingly, the hypothesis is rejected that FcRB functions as transport receptor (FcRn) in liver. The default conclusion is that FcRB in hepatocytes functions as FcRp, serving to protect IgG from catabolism in hepatocytes that accompanies the endocytic activity of these cells. We conclude that there remains to date no evidence of an FcRn-like transport function of the Brambell receptor beyond the perinatal period, after which the FcRp function of the receptor predominates, paralleling the endocytic activities of the associated tissues.

Postendocytotic sorting of the ligand for the polymeric immunoglobulin receptor in Madin-Darby canine kidney cells

The polymeric immunoglobulin receptor (pIg-R) is responsible for the receptor-mediated transcytosis of polymeric immunoglobulins (IgA and IgM) across various epithelia. We have expressed the cDNA for the pIg-R in Madin-Darby canine kidney (MDCK) cells and found that this system mimics that found in vivo (Mostov, K. E., and D. L. Deitcher. 1986. Cell. 46:613-621). We have now investigated the postendocytotic pathway of the ligand for the pIg-R. After a 5-min internalization at the basolateral surface, approximately 45% of internalized ligand recycles to the basolateral medium and 30% is transcytosed to the apical medium. We have also examined why transcytosis of ligand is unidirectional, going only from basolateral to apical, but not from apical to basolateral. Several factors could explain this, such as proteolytic cleavage of the pIg-R at the apical surface, decreased apical endocytosis of ligand, or an intracellular sorting event. In this report, we show that the protease inhibitor, leupeptin, inhibits the cleavage of the pIg-R but does not alter the unidirectionality of transcytosis. In addition, we demonstrate that there is a significant amount of apical endocytosis of ligand (70% of that observed basolaterally). Finally, we demonstrate that apically endocytosed ligand can return only to the apical surface. Thus, once ligand reaches the apical surface, it is "trapped" and cannot return to the basolateral surface. We propose that the unidirectionality of transcytosis is the result of intracellular sorting, and that this results from a signal(s) present on the pIg-R.

Receptor-mediated transport of IgG

The intestinal epithelium of the neonatal rat is a model system for the study of receptor-mediated endocytosis in which large amounts of IgG are transferred intact across polarized cells. This review summarizes the ultrastructural pathway followed by IgG during cellular transit and several important properties of the membrane receptor that recognizes the IgG.

The uptake and transmission of protein by neonatal rat enterocytes

1. Proximal enterocytes transmitted intact immunoglobulin G (IgG) preferentially in the order rat, human, sheep and bovine; the removal from the vascular compartment of these transmitted molecules occurred at about the same rate. 2. Heterologous IgGs are processed similarly to rat IgG: they are either transmitted intact or broken down to less than 1000 mol. wt. fragments. 3. All of the human transferrin removed from the intestine was broken down to less than 1000 mol. wt. fragments, but a small amount of bovine serum albumin (BSA) was transmitted intact. 4. The IgGs and BSA are relatively indigestible molecules whereas human transferrin is relatively digestible. 5. These observations are discussed in the context of receptor-mediated transmission.

Distribution of immunoglobulin G receptors in the small intestine of the young rat

Conjugates of horseradish peroxidase (HRP) and immunoglobulin G (IgG) were used to map the distribution of cell surface receptors that can bind IgG at 0 degrees C within the small intestine of 10-12-d-old rats. Luminal receptors are present only within the duodenum and proximal jejunum. In these locations, receptors are limited to absorptive cells that line the upper portion of individual villi. Near villus tips, receptors are relatively evenly distributed over the entire luminal plasmalemma. In the midregion of villi, receptors are unevenly distributed over the luminal surface. Receptors (a) specifically bind rat and rabbit IgG, (b) recognize the Fc portion of the immunoglobulins, and (c) bind at pH 6.0 but not pH 7.4. To determine whether IgG receptors are confined to the luminal portion of the plasmalemma, intact epithelial cells were isolated from the proximal intestine of 10-12-d-old rats and incubated with HRP conjugates at 0 degree C. The specific binding of rat IgG-HRP to cells at pH 6.0 indicates that IgG receptors, which are functionally similar to those found on the luminal surface, are also present over the entire abluminal surface of absorptive cells. These results are consistent with the transport of IgG to the abluminal plasma membrane in the form of IgG-receptor complexes on the surface of vesicles. Exposure of these complexes to the serosal plasma, which is presumably at pH 7.4, would cause release of IgG from the receptors. To assess possible inward movement of vesicles from the abluminal surface after discharge of IgG, intravenously injected HRP was used as a space-filling tracer in the serosal plasma. HRP could be visualized within the coated and tubular vesicles responsible for transport of IgG in the opposite direction. These vesicles may, therefore, provide a pathway whereby receptors shuttle between the luminal and abluminal surfaces of cells.

The digestion and transmission of labelled immunoglobulin G by enterocytes of the proximal distal regions of the small intestine of young rats

Density gradient centrifugation of samples prepared from proximal gut homogenates after intra-lumenal injection of 125I-labelled IgG, was used to prepare batches of IgG fragments according to sedimentation coefficients. 2. Ultrafiltration was employed to partition the radioactivity in the vascular compartments, viscera and carcasses of rats aged 14-15 days, 2 hr after the injection of standard doses of labelled IgG into the proximal and distal regions of the small intestine. 3. Radioactive samples prepared by these methods were re-introduced into young rats by intra-cardiac injection, and the rate at which they were removed from the vascular compartment was assessed. 4. Proximal enterocytes transmitted about 39% of the IgG which had been removed from the intestine in intact form. Most of this was retained in the vascular compartment; they degraded up to about 57% of the total removed into fragments less than 1000 mol. wt. and about 4% into intermediate sized fragments. 5. Distal enterocytes degraded almost 90% of the IgG processed into fragments less than 1000 mol. wt., about 8% as fragments greater than 100,000 mol. wt. 6. Fragments, of all sizes, were cleared rapidly from the circulation into the viscera and carcass. 7. The relevance of these results to protein transmission and digestion by the rat small intestine is discussed.

Fractionation studies on the absorption of labelled immunoglobulin G by the gut of young rats

1. Centrifugation in density gradients was used to study the fragments produced during intraluminal and intracellular digestion, after the injection of 125I-labelled immunoglobulin G (IgG) into different regions of the small intestine of 14 to 15-day-old (pre-closure) and 24-day-old (post-closure/ rats. 2. After injection into the proximal small intestine and into the ileum of pre-closure animals, the bulk of the radioactivity recorded for gut washes and gut homogenates was located at 4S-7S. The serum from animals which had received injections into the proximal small intestine had high radioactivity and one peak at 7S; the serum from animals which had received injections into the ileum had low radioactivity and no activity in the 7S region. 3. After injection into the proximal small intestine of post-closure animals, the bulk of the radioactivity recorded for gut wash samples was located at 3-5S--5S. Gut homogenates had peak activity at 2-5S--4S. Thus large molecular weight products can be absorbed by the proximal enterocytes of post-closure rats and degraded. The sera of these animals had low radioactivity. 4. After injection into the distal small intestine of post-closure animals, the bulk of the radioactivity recorded for gut wash and gut homogenate samples was located at 4S-7S and in this respect the radioactivity plots resembled those for (2) above. Serum radioactivity was low. 5. The effect of precipitation with trichloroacetic acid and incubation with specific antiserum upon the radioactivity of gut washes, gut homogenates and serum samples was recorded. 6. The relevance of these findings to studies on the transmission of protein by the rat small intestine is discussed.

Quantitative assessment of the transmission of labelled protein by the proximal and distal regions of the small intestine of young rats

1. The plasma volume in rats aged 15-16 days was measured by dilution analysis using homologous, 125I-labelled immunoglobulin G. A mean plasma volume of 5-53 ml./100 g and a mean blood volume of 8-01 ml./100 g were obtained.2. After the injection of labelled immunoglobulin G into the heart, homogenates of various abdominal organs and of the carcass were prepared. Labelled immunoglobulin G left the vascular compartment at a rate of about 9-10%/hr over a 3 hr period. About 11% of the labelled immunoglobulin G was catabolized in 2 hr. 3. The data obtained from these studies was used to make quantitative estimates of the amount of intact immunoglobulin G transmitted from the proximal intestine and from the ileum after the intra-intestinal injection of 1000 mug of labelled immunoglobulin G. Homogenates of the experimental animals were prepared and it was estimated that over 40% of the labelled immunoglobulin G was transmitted as intact protein from the proximal intestine. The results suggest that no intact immunoglobulin G was transmitted from the ileum, but about 15% of the protein removed from the ileum was recovered in the whole body as degraded fragments precipitable with trichloroacetic acid. 4. These observations are discussed in the context of the transmission of antibodies, and their relevance to the receptor hypothesis is considered.

The effects of corticosterone and cortisone on the uptake of polyvinyl pyrrolidone and the transmission of immunoglobulin G by the small intestine in young rats

1. The distribution of polyvinyl pyrrolidone along the intestinal lumen and in the intestinal wall, following oral administration to normal and corticosterone treated rats, was found to be extremely variable. Valid comparisons between the two groups of animals could not be made using this technique. 2. Three, 4 and 5 days after corticosterone treatment there was no significant change in the uptake of 125I-labelled polyvinyl pyrrolidone from standard doses injected into ligated segments of the distal small intestine; nor did the treatment induce precocious replacement of the absorptive cells in this region. Cortisone induced precocious cell replacement, a process which took up to 4 days to complete, and also led to a marked reduction in the uptake of 125I-labelled polyvinyl pyrrolidone from ligated segments of the distal intestine. 3. Three days after treatment with corticosterone (5 mg I.P. at 12 days) there was a marked reduction of labelled immunoglobulin G transport into the blood. Four and 5 days after treatment there was some recovery of the immunoglobulin G transport function. Three days after treatment with cortisone (5 mg I.P. at 12 days) there was closure of the gut to labelled immunoglobulin G. 4. The relevance of these results to antibody transmission and the termination of immunoglobulin transport is discussed.