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.

FcγRIIb on liver sinusoidal endothelium clears small immune complexes.

It has long been known that the ITIM-bearing IgG Fc receptor (FcγRIIb, RIIb) is expressed on liver sinusoidal endothelial cells (LSEC) and that the liver is the major site of small immune complex (SIC) clearance. Thus, we proposed that RIIb of LSEC eliminates blood-borne SIC, thereby controlling immune complex-mediated autoimmune disease. Testing this hypothesis, we found most RIIb of the mouse, fully three-quarters, to be expressed in liver.

Rapid and efficient clearance of blood-borne virus by liver sinusoidal endothelium.

The liver removes quickly the great bulk of virus circulating in blood, leaving only a small fraction to infect the host, in a manner characteristic of each virus. The scavenger cells of the liver sinusoids are implicated, but the mechanism is entirely unknown. Here we show, borrowing a mouse model of adenovirus clearance, that nearly all infused adenovirus is cleared by the liver sinusoidal endothelial cell (LSEC).

IgG is transported across the mouse yolk sac independently of FcgammaRIIb.

While generally accepted that FcRn of the human syncytiotrophoblast and the mouse yolk sac endoderm is the major IgG transporter, the finding of a different Fc receptor FcgammaRIIb (RIIb) in the human placental endothelium has suggested the existence of an additional IgG transporter. Testing our hypothesis in mouse, we found that while RIIb is expressed in the yolk sac vasculature, IgG concentrations in fetuses of wild-type mice (RIIb(+/+)) and mice with a null mutation in the gene encoding RIIb (RIIb(-/-) mice) are not different, and we thus reject our hypothesis that yolk sac RIIb transports IgG in utero in the mouse. However, the capillary bed in the mouse yolk sac is structurally more complex than in human placenta, consisting of three types of cells: an RIIb-negative endothelium, a unique RIIb-bearing cell that also expresses 2 out of 4 macrophage markers but not endothelial cell or pericyte markers, and pericytes.

FcRn in the yolk sac endoderm of mouse is required for IgG transport to fetus.

In adults, the nonclassical MHC class I molecule, FcRn, binds both IgG and albumin and rescues both from a degradative fate, endowing both proteins with high plasma concentrations. FcRn also transports IgG from mother to young during gestation. Anticipating that a detailed understanding of gestational IgG transport in the mouse may give us a useful model to understand FcRn function in the human placenta, we have studied FcRn in the mouse yolk sac placenta in detail.

Beta 2-microglobulin deficient mice catabolize IgG more rapidly than FcRn- alpha-chain deficient mice.

FcRn, a nonclassical MHC-I protein bound to beta 2-microglobulin (beta 2m), diverts IgG and albumin from an intracellular degradative fate, prolonging the half-lives of both. While knockout mouse strains lacking either FcRn-alpha-chain (AK) or beta 2m (BK) show much shorter half-lives of IgG and albumin than normal mice, the plasma IgG half-life in the BK and AK strains is different, being shorter in the BK strain. Since beta 2m does not affect the IgG production rate, we tested whether an additional beta 2m-associated mechanism protects IgG from catabolism.

Accelerated transferrin degradation in HFE-deficient mice is associated with increased transferrin saturation.

HFE, a major histocompatibility complex class I-related protein, is implicated in the iron overload disease, hereditary hemochromatosis. Whereas patients with hereditary hemochromatosis have low serum transferrin levels, little is known about transferrin turnover in HFE deficiency states. We injected mice intravenously with radioiodinated transferrin and compared plasma transferrin decay and steady-state endogenous transferrin concentration in the plasma between HFE-deficient and wild-type C57BL/6 mouse strains.

Perspective-- FcRn transports albumin: relevance to immunology and medicine.

Recent evidence validates a forgotten 40-year-old hypothesis: the MHC-related Fc receptor for IgG (FcRn) protects albumin from intracellular catabolic degradation, as it does for IgG, accounting for the uniquely long half-lives of both molecules and explaining their direct concentration-catabolism relationships. Albumin and IgG bind to FcRn at low pH but not at physiological pH. These two ligands bind independently of one another by distinctive mechanisms and to different surfaces of the receptor.

Familial hypercatabolic hypoproteinemia caused by deficiency of the neonatal Fc receptor, FcRn, due to a mutant beta2-microglobulin gene.

Two siblings, products of a consanguineous marriage, were markedly deficient in both albumin and IgG because of rapid degradation of these proteins, suggesting a lack of the neonatal Fc receptor, FcRn. FcRn is a heterodimeric receptor composed of a nonclassical MHC class I alpha-chain and beta(2)-microglobulin (beta(2)m) that binds two ligands, IgG and albumin, and extends the catabolic half-lives of both. Eight relatives of the siblings were moderately IgG-deficient.