Direct evidence for pH-dependent Fc receptors on proximal enterocytes of suckling rat gut

Receptor-mediated transport of IgG across the intestinal epithelium of the neonatal rat

The absorptive epithelium of the neonatal rat is developmentally specialized to transfer maternal immunoglobulin G (IgG) intact to the circulation while other milk protein are digested. The epithelial cells of the duodenum and proximal jejunum which are responsible for IgG transfer represent a particular striking experimental model for study of receptor-mediated intracellular transport. Receptors located on the luminal plasma membrane selectively bind the Fc region of IgG. The IgG enters the cell by constitutive endocytosis within coated vesicles and is then released at the basolateral plasma membrane. Morphological evidence supports a model in which IgG crosses the cell as a ligand-receptor complex that dissociates only on exposure to a pH 7.4 environment found at the basolateral cell surface. Although uptake of IgG at the luminal plasma membrane is highly selective, small but significant amounts of other proteins enter the cell apparently non-selectively. Nevertheless, these latter proteins are not transferred across the cell. Double-tracer experiments indicate that IgG and these other proteins enter the cell simultaneously within the same endocytic vesicles, but that non-membrane-bound proteins are removed from the IgG transport pathway by an as yet poorly defined mechanism and sequestered within small apical vacuoles and lysosomes.

Immunoglobulin G receptors of intestinal brush borders from neonatal rats

Intestinal absorptive cells of the neonatal rat display on their brush border membranes receptors for immunoglobulin G (IgG) which function in selective transfer of maternal IgG. Our Scatchard analysis of [125I]IgG binding to isolated brush borders has corroborated the presence of two classes of specific binding sites (KA1 = 2.4 X 10(7) M-1 and KA2 = 3.7 X 10(5) M-1) and the increase in overall binding with decreased buffer concentration, as shown by Wallace & Rees 1980. However, our Scatchard analysis of binding at different buffer concentrations indicates that the only significant effect of lowered buffer concentration is to increase the number of low-affinity sites. Neither the number nor the affinity of the high-affinity sites is affected. Furthermore, brush borders from rats at 21 days have only the low-affinity sites and at this age the selective transfer of IgG has ceased. Morphological experiments with tracers for both light and electron microscopy suggest that the high-affinity sites correspond to the specific IgG receptors on the apical membrane. The majority of the low-affinity sites are found within the terminal web and are likely not to be involved in selective transport of IgG.

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.

IgG transport across the gut of the suckling opossum (Monodelphis domestica)

We investigated IgG transport across the gut of suckling opossums to see whether it is likely to be Fc gamma R-mediated. Enterocytes isolated from the proximal and distal regions of the small intestine of suckling aged 12-52 days, and reacted with indicator SRBC at pH 6.0 or 7.2, bound opossum IgG in rosette assays. Considerable overall variation was observed in the numbers of enterocytes forming rosettes. No binding was seen with rabbit IgG at these ages, or with opossum and rabbit IgG when enterocytes were obtained from opossums aged 55-73 days. Opossum anti-SRBC antibody (IgG) fed to sucklings at 52 days and earlier (but not later) could subsequently be detected in the serum. However, rabbit anti-SRBC antibody (IgG) could not be detected in the blood serum when fed to sucklings of any age. Fluorescent tracing of FITC-labelled opossum and rabbit IgG fed to suckling opossums, and of endogenous opossum IgG, pointed to transport of the homologous IgG occurring across gut enterocytes of the proximal region. These results suggest that IgG is recognised and transcytosed by specific Fc gamma Rs present on opossum enterocytes prior to weaning.

Effect of dexamethasone on Fc gamma receptor expression in foetal and neonatal rat gut

When injected into 12-day-old suckling rats, dexamethasone caused a precocious disappearance of Fc gamma receptors from enterocytes of the proximal small intestine. However, dexamethasone appeared to be necessary for the maintenance or production of such receptors in foetal rat gut cultured in vitro.

Ontogeny of the Fc gamma receptor in rat small intestine

The time course of Fc gamma receptor expression on isolated enterocytes of the small intestine of rat fetuses and sucklings has been studied. This was achieved principally using indicator sheep red blood cells (SRBC) sensitized with rabbit IgG in an erythrocyte-antibody rosette assay which detects receptors located mainly on the lateral and basal plasma membrane, and in a more limited way using binding of rabbit IgG to metabolically inhibited gut as detected by immunofluorescence and which detects receptors located on the apical brush border. From the time they were first detectable in the rosette assay (20-day-old fetuses) to the time they disappeared (22-day-old sucklings) such receptors were found always to be acid pH dependent and restricted to enterocytes from the proximal region. Acid pH, Fc-dependent binding of rabbit IgG to metabolically inhibited gut was first detectable at 21 days gestation and there were indications that receptors differentiate on enterocytes in a proximal to distal direction. This was also indicated by electron microscope studies using rabbit PAP injected into the duodenum of 21-day-old fetuses. Such studies also provided evidence for the receptor-mediated translocation of IgG across the duodenum of the fetal rat in a manner similar to that described for older sucklings.

Spectrotypic analysis of passively acquired and newly synthesised IgG antibodies in the neonatal and young mouse

Isoelectric focussing with autoradiography has been used to analyse the selective nature of passive transmission of specific anti-Ig allotypic antibodies from the mother to the young mouse, and to study the generation of spectrotypic (clonal) diversity of autologous IgG2a (carrying the paternal inherited Igh-1b allotype) in BALB/c X SJL/J F1 (Igh- 1ab heterozygote) mice. Transmission of anti-allotypic antibody to neonatal mice was found to be pI restricted, with selection favouring electrophoretically fast IgG. Comparison of the antibody spectrotype in maternal serum, milk and neonatal serum revealed that the pI restriction in transmission operates at the level of the neonatal gut. Analysis of the paternally inherited Igh-1b IgG2a molecules as they are first synthesised and secreted into the neonatal serum revealed an extensive polyclonality on first detection by the very sensitive focussing assay. It can be deduced that IgV region diversity is generated by IgG2a-synthesising cells prior to, or at the time of, the first secretion of this class of antibody into the serum.

Distribution of Fc gamma receptors on isolated gut enterocytes of the suckling rat

An erythrocyte-antibody rosette assay has been used to study the presence and distribution of Fc gamma receptors on enterocytes isolated from 12-day-old suckling rat gut by means of a buffer medium containing EGTA. Such receptors were found to be restricted to enterocytes in the proximal region (duodenum and jejunum) of the small intestine and to be acid-pH dependent. For the majority of enterocytes indicator red cells bound in high density to the abluminal plasmalemma but not to the apical microvillous brush border. Since immunofluorescence studies revealed strong binding of added IgG to the microvillous region, a likely explanation is that there is a paucity of Fc gamma receptors from the tips of microvilli (at least under the conditions of the rosette assay) and that receptors more deeply situated as inaccessible to indicator red cells. Binding of indicator red cells was readily inhibited by rabbit, human, guinea pig and rat IgG but less so by bovine IgG, and of the two sub-classes, bovine IgG2 inhibited much more readily than bovine IgG1. Cortisone acetate injection virtually abolished Fc gamma receptor expression on isolated enterocytes within three days. These findings correlate both with selective transport of IgG of different species in vivo and the known effect of cortisone acetate to terminate such transport.

Evidence for the sorting of endocytic vesicle contents during the receptor-mediated transport of IgG across the newborn rat intestine

Fc receptors on the luminal membranes of intestinal epithelial cells in the neonatal rat mediate the vesicular transfer of functionally intact IgG from the intestinal lumen to the circulation. In addition, there is a low level of nonselective protein uptake, but in this case transfer does not occur. To determine whether a specialized class of endocytic vesicles could account for the selective transfer of IgG, mixtures of IgG conjugated to ferritin (IgG-Ft) and unconjugated horseradish peroxidase (HRP) were injected together into the proximal intestine of 10-d-old rats, and the cellular distribution of these two different tracers was determined by electron microscopy. Virtually all apical endocytic vesicles contained both tracers, indicating simultaneous uptake of both proteins within the same vesicle. However, only IgG-Ft bound to the apical plasma membrane, appeared within coated vesicles at the lateral cell surface, and was released from cells. HRP did not bind to the luminal membrane and was not transferred across cells but was confined to apical lysosomes as identified by acid phosphatase and aryl sulfatase activities. To test the possibility that the binding of IgG to its receptor stimulated endocytosis, HRP was used as a fluid volume tracer, and the amount of HRP taken up by cells in the presence and absence of IgG was measured morphologically and biochemically. The results demonstrate that endocytosis in these cells is constitutive and occurs at the same level in the absence of IgG. The evidence presented indicates that the principal selective mechanism for IgG transfer is the binding of IgG to its receptor during endocytosis. Continued binding to vesicle membranes appears to be required for successful transfer because unbound proteins are removed from the transport pathway before exocytosis. These results favor the proposal that IgG is transferred across cells as an IgG-receptor complex.