FcRn mRNA Research Articles

P‐96 Fetal endothelial cells in full‐term placenta, but not in first trimester placenta, express FcγRIIb2 mRNA

In the human placenta, IgG‐transport from maternal to fetal blood starts in the second trimester and continues through the third trimester. There are two cellular barriers for IgG‐transport: the first and second barriers are syncytiotrophoblast and fetal endothelial cells (ECs), respectively. Sorting via FcRn is considered the IgG‐transporter in the first barrier. The mechanism of IgG‐transport in the second barrier is not fully understood. Recently, we reported a novel FcγRIIb‐containing compartment that may serve as an IgG‐transporter in the second barrier (Mol Biol Cell 13 (suppl) 548a, 2002). To further investigate the feasibility of the FcγRIIb‐vesicle involvement in IgG‐transport in placental ECs, we have studied FcR‐expression in the developing human placenta by RT‐PCR and direct sequencing analysis. First trimester and full‐term placentas were used. We proved that the FcγRII expressed in villus ECs in full‐term placenta was the b2 isoform. FcγRIIb2 mRNA expression could not be detected in first trimester placenta, while it was expressed in full‐term placenta. In contrast, FcRn mRNA expression was detectable in first trimester placenta as well as in full‐term placenta. These results seem consistent with our hypothesis that in first trimester placenta IgG is transcytosed via FcRn across the first barrier but IgG cannot be transported in the second barrier that does not express FcγRIIb2 and that the FcγRIIb2‐positive compartment is critical for IgG‐transport in the human placenta.

Net absorption of IgG via FcRn-mediated transcytosis across rat alveolar epithelial cell monolayers

We characterized immunoglobulin G (IgG) transport across rat alveolar epithelial cell monolayers cultured on permeable supports. Unidirectional fluxes of biotin-labeled rat IgG (biot-rIgG) were measured in the apical-to-basolateral (ab) and opposite (ba) directions as functions of [rIgG] in upstream fluids at 37 and 4 degrees C. We explored specificity of IgG transport by measuring fluxes in the presence of excess Fc, Fab, F(ab')2, or chicken Ig (IgY). Expression of the IgG receptor FcRn and the effects of dexamethasone on FcRn expression and biot-rIgG fluxes were determined. Results show that ab flux of biot-rIgG is about fivefold greater than ba flux at an upstream concentration of 25 nM biot-rIgG at 37 degrees C. Both ab and ba fluxes of rIgG saturate, resulting in net absorption with half-maximal concentration and maximal flow of 7.1 nM and 1.3 fmol.cm(-2).h(-1). At 4 degrees C, both ab and ba fluxes significantly decrease, nearly collapsing net absorption. The presence of excess unlabeled Fc [but not Fab, F(ab')2, or IgY] significantly reduces biot-rIgG fluxes. RT-PCR demonstrates expression of alpha- and beta-subunits of rat FcRn. Northern analysis further confirms the presence of alpha-subunit of rat FcRn mRNA of approximately 1.6 kb. Dexamethasone exposure for 72 h decreases the steady-state level of mRNA for rat FcRn alpha-subunit and the ab (but not ba) flux of biot-rIgG. These data indicate that IgG transport across alveolar epithelium takes place via regulable FcRn-mediated transcytosis, which may play an important role in alveolar homeostasis in health and disease.

Sequence and expression of the FcRn in the porcine mammary gland

Transport of immunoglobulin G across epithelial cell barriers is thought to occur by a system involving the Fcγ receptor called the neonatal Fc receptor (FcRn). The FcRn may also play a role in IgG transport in the mammary gland. To determine the presence of FcRn in the porcine mammary gland, biopsies were taken from glands 3 days prepartum and on the day of farrowing. The full length porcine FcRn cDNA sequence was obtained by rapid amplification of cDNA ends and determined to be 1557 base pairs in length that codes for a 359 amino acid peptide. Expression of FcRn mRNA in the porcine mammary gland was determined by reverse transcriptase-PCR and revealed that the mRNA is present prepartum and on the day of farrowing. These results indicate that the FcRn is expressed in porcine mammary tissue and are consistent with the hypothesis that FcRn may have a role in mammary gland immunoglobulin transport during colostrogenesis.

MHC class I-related neonatal Fc receptor for IgG is functionally expressed in monocytes, intestinal macrophages, and dendritic cells.

The neonatal Fc receptor (FcRn) for IgG, an MHC class I-related molecule, functions to transport IgG across polarized epithelial cells and protect IgG from degradation. However, little is known about whether FcRn is functionally expressed in immune cells. We show here that FcRn mRNA was identifiable in human monocytes, macrophages, and dendritic cells. FcRn heavy chain was detectable as a 45-kDa protein in monocytic U937 and THP-1 cells and in purified human intestinal macrophages, peripheral blood monocytes, and dendritic cells by Western blot analysis. FcRn colocalized in vivo with macrosialin (CD68) and Ncl-Macro, two macrophage markers, in the lamina propria of human small intestine. The heavy chain of FcRn was associated with the beta(2)-microglobulin (beta(2)m) light chain in U937 and THP-1 cells. FcRn bound human IgG at pH 6.0, but not at pH 7.5. This binding could be inhibited by human IgG Fc, but not Fab. FcRn could be detected on the cell surface of activated, but not resting, THP-1 cells. Furthermore, FcRn was uniformly present intracellularly in all blood monocytes and intestinal macrophages. FcRn was detectable on the cell surface of a significant fraction of monocytes at lower levels and on a small subset of tissue macrophages that expressed high levels of FcRn on the cell surface. These data show that FcRn is functionally expressed and its cellular distribution is regulated in monocytes, macrophages, and dendritic cells, suggesting that it may confer novel IgG binding functions upon these cell types relative to typical Fc gamma Rs: Fc gamma RI, Fc gamma RII, and Fc gamma RIII.

Expression of the Fc receptor in the mammary gland during lactation in the marsupial Trichosurus vulpecula (brushtail possum)

One of several functions described for the Fc receptor is regulation of IgG isotype transport into milk. The first marsupial homologues of the Fc receptor heavy and light chains, FcRn and β-2 microglobulin, from the brushtail possum have been cloned and characterised. The level of FcRn mRNA in the possum mammary gland was highest at the start of lactation, and decreased slowly thereafter. Expression of FcRn mRNA did not increase during the switch phase when the concentration of IgG in milk is highest. In contrast, the level of β-2 microglobulin mRNA in the mammary gland increased during the switch phase when milk IgG concentration also increases. This correlation between β-2 microglobulin mRNA expression in the mammary gland with the time of active IgG-transfer into milk was also observed in the bovine and murine mammary gland. This suggests that expression of the Fc receptor in the mammary gland is controlled by the expression of β-2 microglobulin and that its expression is upregulated during the period of highest IgG-transfer into milk.

New functions of the MHC class I-related Fc receptor, FcRn.

FcRn was originally identified as the receptor responsible for IgG binding to the intestinal epithelium of neonatal rats and mice. FcRn transports IgG from milk across the intestinal epithelial cells of the suckling animal. Subsequently, FcRn was detected in tissues involved in the transmission of IgG from mother to fetus: rat fetal yolk sac, mouse fetal yolk sac and human placental syncytiotrophoblast. In addition, FcRn mRNA has been detected in many tissues of adult rats, mice and humans, and FcRn is present in several adult tissues and in cell lines. The selective protection from catabolism of IgG compared with other immunoglobulins is lost in mice that lack functional FcRn. One function of FcRn in tissues that do not transport IgG, and beyond the perinatal period, is thus to rescue circulating IgG from degradation. These recent observations reveal a more widespread use of FcRn than had been supposed.

Ontogenetic development and distribution of antibody transport and Fc receptor mRNA expression in rat intestine.

The intestine of the suckling rat has the unique capacity of absorbing immunoglobulins from maternal milk. We investigated intestinal Fc receptor mRNA expression and the absorption of orally administered antibodies to delineate the ontogeny and tissue specificity of this transport system. Duodenal expression of Fc receptor mRNA was at maximum levels between 1 and 19 days of age, but was not detectable during fetal life and in animals after weaning. Along the horizontal axis of the intestine, FcRn mRNA expression was maximum in the proximal duodenum and declined gradually in distal bowel. Similarly, absorption of orally administered antibody was low shortly after birth, but reached maximum levels at 14 days of age. By the time of weaning, antibody uptake had almost completely ceased. These data further delineate the temporal and spatial nature of the intestinal immunoglobulin transport system, and represent additional examples of how the intestinal Fc receptor is transcriptionally regulated.

An IgG-transporting Fc receptor expressed in the syncytiotrophoblast of human placenta.

During normal human pregnancy, maternal IgG crosses the placenta and provides passive immunity for the fetus. In so doing, IgG passes through two cellular barriers: the syncytiotrophoblast and the fetal capillary endothelium. The Fc region of IgG is required for its transport across the placenta, but the Fc receptors responsible have not been identified definitively. We recently reported the isolation from a placental cDNA library of clones encoding the alpha chain of a human homologue of the major histocompatibility complex class I-related Fc receptor, the neonatal Fc receptor (FcRn). In mice, FcRn is essential for the transport of maternal IgG to the fetus and the neonate. We report here the localization of human FcRn mRNA within the placenta by in situ hybridization, and of human FcRn protein by immunohistochemistry. Both methods show that human FcRn is expressed in syncytiotrophoblast, and is, thus, appropriately located to transport maternal IgG across the first barrier. We confirm previous findings that specific binding of IgG to placental membranes is greater at pH 6.0 than pH 7.5. This corresponds with the pH dependence of IgG binding to FcRn and is consistent with the presence of FcRn in syncytiotrophoblast. We propose a transport model in which maternal IgG binds FcRn at low pH in endosomes within the syncytiotrophoblast. FcRn is not expressed in fetal capillary endothelia, and the mechanism of IgG transport across the second barrier remains unknown.