Mice homozygous for a beta 2-microglobulin gene disruption do not express any detectable beta 2-m protein. They express little if any functional major histocompatibility complex (MHC) class I antigen on the cell surface yet are fertile and apparently healthy. They show a normal distribution of gamma delta, CD4+8+ and CD4+8- T cells, but have no mature CD4-8+ T cells and are defective in CD4-8+ T cell-mediated cytotoxicity. Our results strongly support earlier evidence that MHC class I molecules are crucial for positive selection of T cell antigen receptor alpha beta+ CD4-8+ T cells in the thymus and call into question the non-immune functions that have been ascribed to MHC class I molecules.

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Maternal immunoglobulin G transmitted to the fetus or newborn provides humoral immunity for the first weeks of mammalian life. Fc receptors on intestinal epithelial cells of the neonatal rat (FcRn) mediate the uptake of IgG from milk. Affinity-purified FcRn is resolved by SDS-PAGE into components of relative molecular masses 45,000-53,000 (p51) and about 14,000 (p14). We report the identification of the smaller component as beta 2-microglobulin. Association of beta 2-microglobulin with p51 was confirmed by crosslinking in intestinal epithelial cell brush borders. We have cloned a cDNA encoding the presumptive Fc-binding subunit, p51, and its predicted primary structure has three extracellular domains and a transmembrane region which are all homologous to the corresponding domains of class I major histocompatibility complex (MHC) antigens. This is the first time a function has been assigned to an MHC antigen-related molecule.

IntroductionSteady-state levels of IgG in the blood of adult mice, and likely all mammals, depend on IgG catabolism mediated in part by the MHC class I-related Fc receptor, FcRn (1). FcRn also mediates vectorial transport of IgG across certain epithelial barriers. In suckling mice and rats, intestinal absorption of maternal IgG from breast milk into the systemic circulation depends on FcRn (2). In humans, maternofetal transfer of IgG across the placenta also likely depends on FcRn (3). Thus, FcRn plays critical and well-documented roles in the regulation of IgG metabolism in adults and in the acquisition of humoral immunity in early life. These effects on the physiology of IgG in vivo result from the action of FcRn as an intracellular trafficking receptor (4).FcRn has been cloned from the rat, mouse, and human. The molecule is expressed as a heterodimer composed of a glycosylated heavy (α) chain (51 kDa in rodents and 40-45 kDa in humans) associated noncovalently with β2-microglobulin (β2M) (5). Binding of IgG to FcRn requires contact between solvent-exposed peptide sequences in the CH2 and CH3 domains of IgG and the α1 and α2 domains of FcRn, together with a single contact site in β2M (6-11). A hallmark of FcRn interaction with IgG is its pH dependence, showing high-affinity binding at acidic pH (pH ≤ 6.5) and weak or no binding at neutral pH (pH ≥ 7.0) (12, 13). FcRn is the only Fcγ receptor that exhibits MHC class I structure, and the only Fcγ receptor to exhibit pH dependency in ligand binding.The function of FcRn in the intestine of suckling mice and rats has been well documented (14). In neonatal mice and rats, FcRn is expressed at high levels by intestinal epithelial cells and mediates absorption of IgG by receptor-mediated transcytosis. FcRn expression in the neonatal rodent is developmentally downregulated, resulting in nearly complete loss of intestinal FcRn at the time of weaning (15)(16)(17) The MHC class I-related Fc receptor, FcRn, mediates the intestinal absorption of maternal IgG in neonatal rodents and the transplacental transport of maternal IgG in humans by receptor-mediated transcytosis. In mice and rats, expression of FcRn in intestinal epithelial cells is limited to the suckling period. We have recently observed, however, clear expression of FcRn in the adult human intestine, suggesting a function for FcRn in intestinal IgG transport beyond neonatal life in humans. We tested this hypothesis using the polarized human intestinal T84 cell line as a model epithelium. Immunocytochemical data show that FcRn is present in T84 cells in a punctate apical pattern similar to that found in human small intestinal enterocytes. Solute flux studies show that FcRn transports IgG across T84 monolayers by receptor-mediated transcytosis. Transport is bidirectional, specific for FcRn, and dependent upon endosomal acidification. These data define a novel bidirectional mechanism of IgG transport across epithelial barriers that predicts an important effect of FcRn on IgG function in immune surveillance and host de...