More than 30 years ago, Brambell postulated that the mechanisms by which the catabolic rate of IgG is controlled and by which IgG is transported from mother to young were remarkably alike and were mediated by similar Fc receptors (1). Assessing today the data available to him we would say the putative receptors were virtually identical. The receptors, present in the walls of intracellular vesicles, he envisioned as binding pinocytosed IgG and transporting it either back to the surface or across the cell, thereby protecting it from the usual fate of catabolic degradation and regulating its concentration in blood and tissues. Such a mechanism accounted for all that was known about these two processes, including the relatively long lifespan of IgG and the paradoxically inverse relationship of IgG concentration to lifespan (2). It has recently become clear that the Brambell receptor, responsible for both IgG transport and protection from degradation, is in fact the pH-dependent IgG transporter (Fc receptor neonatal, FcRn)1 initially described as the molecule that moves IgG across the neonatal rat gut (3–5).
The structure of FcRn is now known in great detail. It is a heterodimer of β2-microglobulin (β2m) and a 45-kD
chain closely related to MHC class I (6) that is expressed in virtually all tissues of the body (7, 8) (Sedmak, D.D., manuscript submitted) and at least in mammals and birds (9). Its crystal structure shows that the peptide groove is too narrow to bind ligand; rather, Fc fragments interact with an adjacent surface of the molecule in a manner that may, under appropriate circumstances, permit two receptors to bind a single IgG ligand (10–12). The 100-fold gain in affinity between pH 7 and 6 is accounted for by critical histidines near the site of ligand–receptor interaction (13).
The crucial link between Brambell's receptor and FcRn has been provided by recent studies of β2m-deficient mice. These mice are FcRn deficient as well (14), and because of this deficiency are unable to absorb IgG from milk as neonates (15), are IgG deficient as adults (15–19), and catabolize IgG several times the normal rate (20–22) (Roopenian, D.C., manuscript submitted), but apparently have normal concentrations of the other Ig classes and normal rates of IgG synthesis (21). The broad outline and many details of IgG catabolism and FcRn-mediated transport have recently been reviewed (23). An important point that needs now to be established is how this receptor might participate in certain diseases.
It was noted recently that the severity of experimental systemic lupus erythematosus (SLE) is greatly attenuated in β2m-deficient mice. In the genetically determined lpr/lpr model, wherein the affected mice develop both marked lymphoproliferation and an SLE-like syndrome consisting of hypergammaglobulinemia, autoantibody production, and glomerulonephritis, the lack of β2m appears to protect against both the SLE syndrome and the lymphoproliferative response (18, 19, 24–26). Although the absence of MHC class I molecules sufficiently explains abrogation of the lymphoproliferative response, it has never satisfactorily accounted for protection from the SLE syndrome (19). Similarly, in a second model of SLE, induced by the infusion of a specific anti-idiotype antibody, the absence of β2m protects against the disease (27). Noting that β2m-deficient mice are IgG deficient (15–19), we propose that they are protected against the SLE syndrome because, lacking FcRn, they rapidly catabolize their pathogenic IgG autoantibodies.
A more direct test of the hypothesis, that the absence of FcRn protects against autoantibody-mediated disease, would be to determine whether β2m-deficient mice are resistant to experimental bullous pemphigoid (BP). BP is an autoimmune skin disorder characterized by subepidermal blisters and autoantibodies directed against two hemidesmosomal antigens, BP230 and BP180 (28). The experimental mouse model of BP that involves the passive transfer of anti-mBP180 antibodies into neonatal BALB/c mice reproduces all key immunopathological features of human BP; namely, IgG and complement deposition at the basement membrane zone (BMZ), inflammatory infiltration of the upper dermis, and subepidermal blister formation (29). The subepidermal blistering in experimental BP is initiated by anti-mBP180 IgG and is dependent on complement activation and neutrophil recruitment (30, 31, 31a).
We subjected β2m-deficient and normal mice to experimental BP and compared the clinical, histological, and immunological manifestations. Herein, we describe those experiments.
 |
Materials and Methods
|
|---|
Animals.
Breeding pairs of BALB/ByJ and C57BL/6J mice were obtained from The Jackson Laboratory (Bar Harbor, ME). β2m–/– MRL/MpJ–B2mtm1Unc/Dcr and C57BL/6J–B2mtm1Unc/ Dcr mice were produced as described (19). β2m–/– neonates were produced from an F1 cross of MRL/MpJ–B2mtm1Unc x C57BL/6J–B2mtm1Unc mice. Genetically matched β2m+/– (wild-type) mice were produced from an F1 cross of MRL/MpJ– B2mtm1Unc x C57BL/6J–B2m+/+ mice. The animals were bred at The Jackson Laboratory and litters were delivered at the Medical College of Wisconsin Animal Resource Center. Neonatal mice (24–36 h old with body weights between 1.4 and 1.6 g) were used for passive transfer experiments.
Preparation of Pathogenic Rabbit Anti-murine BP180 IgG.
The preparation of recombinant mBP180 and the immunization of rabbits were performed as previously described (29, 32). In brief, a segment of the mBP180 antigen containing the pathogenic epitope was expressed, purified to homogeneity by affinity chromatography (33), and used to immunize New Zealand White rabbits. The IgG fraction from the sera (referred to as R621) was purified as previously described (29). The titer of rabbit anti-mBP180 antibodies was assayed by indirect IF using mouse skin cryosections as substrate (29). The pathogenicity of the IgG preparations was tested by passive transfer experiments as described below.
Induction of Experimental BP and Animal Evaluation.
A 50 µl dose of sterile IgG in PBS was administered to neonatal mice by intraperitoneal injection (2.5 mg IgG/g body weight) or intradermal (2.5 mg IgG/g body weight). The skin of neonatal mice from the test and control groups was examined 12 or 24 h after the IgG injection. The extent of cutaneous disease was scored as follows: minus, no detectable skin disease; 1+, mild erythematous reaction with no evidence of the epidermal detachment sign elicited by gentle friction of the mouse skin, which, when positive, produced fine, persistent wrinkling of the epidermis; 2+, intense erythema and epidermal detachment involving 10–50% of the epidermis in localized areas; and 3+, intense erythema with frank epidermal detachment involving more than 50% of the epidermis. The animals were then sacrificed and skin sections were taken for light microscopy (hematoxylin and eosin, H/E) and for direct immunofluorescence (IF) to detect rabbit IgG and mouse C3 deposition at the BMZ. Sera of injected animals were assayed by indirect IF to determine the circulating titers of anti-mBP180 IgG (29). Monospecific FITC-conjugated goat anti–rabbit IgG was obtained commercially (Kirkeggard & Perry Laboratories, Inc., Gaithersburg, MD). Monospecific goat anti–mouse C3 was purchased from Cappel Laboratories (Durham, NC). Both anti-rabbit IgG and anti-C3 antibodies were used at a 1:100 dilution.
Quantitation of Skin Site PMN Accumulation.
Tissue MPO activity in skin sites of the injected animals was assayed as described (34, 35). A standard reference curve was first established using known concentrations of purified myeloperoxidase (MPO). The skin samples were extracted by homogenization in an extraction buffer containing 0.1 M Tris–HCl, pH 7.6, 0.1 M NaCl, 0.5% hexadecyltrimethylammonium bromide. MPO activity in the supernatant fraction was measured by the change in optical density at OD 460nm resulting from decomposition of H2O2 in the presence of O-dianisidine. MPO content was expressed as units of MPO activity/mg protein. These numbers were subtracted from background MPO activity of skin of mice injected with PBS alone and sacrificed at the same timepoints as the test mice. Protein concentrations were determined by the Bio-Rad dye binding assay using BSA as a standard.
Determination of Serum Rabbit IgG Levels.
The concentration of serum rabbit IgG was measured by ELISA relative to purified rabbit IgG (Sigma Chemical Co., St. Louis, MO). Microtiter plates were coated with polyclonal goat anti–rabbit IgG Fc antibody (Cappel Laboratories, Durham, NC), incubated with dilutions of serum, and then developed with horseradish peroxidase– conjugated goat antibodies specific for rabbit IgG F(ab')2 (Cappel Laboratories, Durham, NC) and read at OD492nm against a standard curve (Bio-Rad EIA reader, model 2550).
Statistical Analysis.
The data were expressed as mean ± SEM and were analyzed using the Student's t-test. A P value <0.05 was considered significant.
 |
Results
|
|---|
Pathogenic anti-mBP180 antibodies administered intraperitoneally do not induce experimental BP in β2m–/– mice. When neonatal BALB/c (n = 5), C57BL/6J (n = 5), and β2m+/– (n = 5) mice were injected intraperitoneally with pathogenic anti-mBP180 IgG (2.5 mg/g body weight), as expected, these animals developed extensive blisters 24 h after injection (Fig. 1 A; Table 1). The skin of these animals was markedly erythematous and, upon gentle friction, developed persistent epidermal wrinkling due to epidermal detachment from underlying dermis. Direct IF of cryosections of lesional and peri-lesional skin showed in vivo deposition of rabbit IgG and mouse C3 at the dermal– epidermal junction (Fig. 1 B). H/E stained sections from these mice showed dermal–epidermal separation with neutrophilic infiltration (Fig. 1 C). In contrast, β2m–/– mice (n = 5) exhibited no blisters 24 h after injection with an identical dose of pathogenic anti-mBP180 IgG (Fig. 1 D). Direct IF also showed a significant reduction in in situ deposition of rabbit IgG and mouse C3 at the BMZ (Fig. 1 E). H/E staining of the skin sections from injected mice exhibited no epidermal detachment from the dermis and a neutrophilic infiltrate milder (Fig. 1 F) than positive control mice. Quantitation of neutrophilic infiltration with the MPO assay showed significant changes in the extractable enzyme activity at the injected site at 24 h after injection, with 0.06 ± 0.01 in β2m–/– mice versus 0.41 ± 0.04 in BALB/c, 0.38 ± 0.04 in C57BL/6J, and 0.38 ± 0.03 in β2m+/– mice (P <0.001) (Fig. 2). However, there was no difference in tissue MPO activity in both β2m–/– and control mice 4 h after intraperitoneal administration of anti-mBP180 IgG (Fig. 2), suggesting that lack of β2m expression did not interfere with neutrophil migration from circulation into the site of tissue inflammation.
1 Brambell FWR. The transmission of immunity from mother to young and catabolism of immunoglobulins, Lancet, 1966, 2, 1087–1093.[Medline]
2 Fahey JL & Robinson AG. Factors controlling serum gammaglobulin concentration, J Exp Med, 1963, 118, 845–868.[Abstract]
3 Jones EA & Waldman TA. The mechanism of intestinal uptake and transcellular transport of IgG in the neonatal rat, J Clin Invest, 1972, 51, 2916–2927.[Medline]
4 Rodewald R. Intestinal transport of antibodies in the newborn rat, J Cell Biol, 1973, 58, 198–214.
5 Rodewald R & Kraehenbuhl J-P. Receptor-mediated transport of IgG, J Cell Biol, 1984, 99, 159s–164s.[Abstract/Free Full Text]
6 Simister NE & Mostov KE. An Fc receptor structurally related to MHC class I antigens, Nature (Lond), 1989, 337, 184–187.[Medline]
7 Story CM, Mikulska JE & Simister NE. A major histocompatibility complex class I–like Fc receptor cloned from human placenta: possible role in transfer of immunoglobulin G from mother to fetus, J Exp Med, 1994, 180, 2377–2381.[Abstract/Free Full Text]
8 Leach JL, Sedmak DD, Osborne JM, Rahill B, Lairmore MD & Anderson CL. Isolation from human placenta of the IgG transporter, FcRn, and localization to the syncytiotrophoblast: implications for maternal–fetal antibody transport, J Immunol, 1996, 157, 3317–3322.[Abstract]
9 Kandil E, Noguchi M, Ishibashi T & Kasahara M. Structural and phylogenetic analysis of the MHC class I–like Fc receptor gene, J Immunol, 1995, 154, 5907–5918.[Abstract]
10 Burmeister WP, Gastinel LN, Simister NE, Blum ML & Bjorkman PJ. Crystal structure at 2.2 Å resolution of the MHC-related neonatal Fc receptor, Nature (Lond), 1994, 372, 336–343.[Medline]
11 Burmeister WP, Huber AH & Bjorkman PJ. Crystal structure of the complex of rat neonatal Fc receptor with Fc, Nature (Lond), 1994, 372, 379–383.[Medline]
12 Popov S, Hubbard JG, Kim J-K, Ober B, Ghetie V & Ward ES. The stochiometry and affinity of the interaction of murine Fc fragments with the MHC class I–related receptor, FcRn, Mol Immunol, 1996, 33, 521–530.[Medline]
13 Raghavan M, Bonagura VR, Morrison SL & Bjorkman PJ. Analysis of the pH dependence of the neonatal Fc receptor/immunoglobulin G interaction using antibody and receptor variants, Biochemistry, 1995, 34, 1649–1657.
14 Zijlstra M, Bix M, Simister NE, Loring JM, Raulet DH & Jaenisch R. β2-microglobulin deficient mice lack CD4–8+ cytolytic cells, Nature (Lond), 1990, 344, 742–746.[Medline]
15 Israel EJ, Patel VK, Taylor SF, Marshak-Rothstein A & Simister NE. Requirement for a β2-microglobulin–associated Fc receptor for acquisition of maternal IgG by fetal and neonatal mice, J Immunol, 1995, 154, 6246–6251.[Abstract]
16 Spriggs MK, Koller BH, Sato T, Morrissey PJ, Fanslow WC, Smithies O, Voice RF, Widmer MB & Maliszewski CR. Beta 2-microglobulin–, CD8+ T-cell– deficient mice survive inoculation with high doses of vaccinia virus and exhibit altered IgG responses, Proc Natl Acad Sci USA, 1992, 89, 6070–6074.[Abstract/Free Full Text]
17 Raulet DH. MHC class I–deficient mice, Adv Immunol, 1994, 55, 381–421.[Medline]
18 Maldonado MA, Eisenberg RA, Roper E, Cohen PL & Kotzin BL. Greatly reduced lymphoproliferation in lpr mice lacking major histocompatibility complex class I, J Exp Med, 1995, 181, 641–648.[Abstract/Free Full Text]
19 Christianson GJ, Blankenburg RL, Duffy TM, Panka D, Marshak-Rothstein A, Roths JB & Roopenian DC. MHC class I dependence of the lupus-like autoimmune syndrome of MRL-lpr mice, J Immunol, 1996, 176, 4933–4939.
20 Ghetie V, Hubbard JG, Kim J-K, Tsen M-F, Lee Y & Ward ES. Abnormally short serum half-lives of IgG in β2-microglobulin–deficient mice, Eur J Immunol, 1996, 26, 690–696.[Medline]
21 Junghans RP & Anderson CL. The protection receptor for IgG catabolism is the β2-microglobulin–containing neonatal intestinal transport receptor, Proc Natl Acad Sci USA, 1996, 93, 5512–5516.[Abstract/Free Full Text]
22 Israel EJ, Wilsker DF, Hayes KC, Schoenfeld D & Simister NE. Increased clearance of IgG in mice that lack β2-microglobulin: possible protective role of FcRn, Immunology, 1997, 89, 573–578.[Medline]
23 Junghans RP. The Brambell receptor (FcRB): mediator of transmission of immunity and protection from catabolism for IgG, Immunol Res, 1997, 16, 29–59.[Medline]
24 Giese T & Davidson WF. In CD8+ T cell–deficient lpr/lpr mice, CD4+B220+ and CD4+B220– T cells replace B220+ double-negative T cells as the predominant populations in enlarged lymph nodes, J Immunol, 1995, 154, 4986–4995.[Abstract]
25 Ohteki T, Iwamoto M, Izui S & MacDonald HR. Reduced development of CD4–8–B220+ T cells but normal autoantibody production in lpr/lpr mice lacking major histocompatibility complex class I molecules, Eur J Immunol, 1995, 25, 37–41.[Medline]
26 Mixter PF, Russell JQ, Durie FH & Budd RC. Decreased CD4–CD8– TCR-alpha beta + cells in lpr/lpr mice lacking beta 2-microglobulin, J Immunol, 1995, 154, 2063–2074.[Abstract]
27 Mozes E, Kohn LD, Hakim F & Singer DS. Resistance of MHC class I–deficient mice to experimental systemic lupus erythematosus, Science (Wash DC), 1993, 261, 91–93.[Abstract/Free Full Text]
28 Korman NJ. Bullous pemphigoid, Dermatologic Clin, 1993, 11, 483–498.[Medline]
29 Li K, Tamai K, Tan EML & Uitto J. Cloning of type XVII collagen. Complementary and genomic dna sequences of mouse 180-Kilodalton bullous pemphigoid antigen (BPAG2) predict an interrupted collagenous domain, a transmembrane segment, and unusual features in the 5'-end of the gene and the 3'-untranslated region of the mRNA, J Biol Chem, 1993, 268, 8825–8834.[Abstract/Free Full Text]
30 Liu Z, Giudice GJ, Swartz SJ, Fairley JA, Till GO, Troy JL & Diaz LA. The role of complement in experimental bulous pemphigoid, J Clin Invest, 1995, 95, 1539–1544.[Medline]
31 Liu Z, Diaz LA, Swartz SJ, Troy JL, Fairley JA & Giudice GJ. Molecular mapping of a pathogenically relevant BP180 epitope associated with experimentally induced murine bullous pemphigoid, J Immunol, 1995, 155, 5449–5454.[Abstract]
31 Liu, Z., G.J. Giudice, X. Zhou, S.J. Swartz, J.L. Troy, J.A. Fairley, G.O. Till, and L.A. Diaz. 1997. A major role for neutrophils in experimental bullous pemphigoid. J. Clin. Invest. In press.
32 Liu Z, Diaz LA, Troy JL, Taylor AF, Emery DJ, Fairley JA & Giudice GJ. A passive transfer model of the organ-specific autoimmune disease, bullous pemphigoid, using antibodies generated against the hemidesmosomal antigen, BP180, J Clin Invest, 1993, 92, 2480–2488.[Medline]
33 Liu Z, Diaz LA, Haas AL & Giudice GJ. cDNA cloning of a novel human ubiquitin carrier protein. An antigenic domain specifically recognized by endemic pemphigus foliaceus autoantibodies is encoded in a secondary reading frame of this human epidermal transcript, J Biol Chem, 1992, 267, 15829–15835.[Abstract/Free Full Text]
34 Bradley PP, Priebat DA, Christensen RD & Rothstein G. Measurement of cutaneous inflammation: estimation of neutrophil content with an enzyme marker, J Invest Dermatol, 1982, 78, 206–209.[Medline]
35 Mulligan MS, Jones ML, Bolanoswki MA, Baganoff MP, Deppeler CL, Meyers DM, Ryan US & Ward PA. Inhibition of lung inflammatory reactions in rats by an anti-human IL-8 antibody, J Immunol, 1993, 150, 5585–5595.[Abstract]
36 Simister NE, Story CM, Chen H-L & Hunt S. An IgG transporting Fc receptor expressed in the syncytiotrophoblast of human placenta, J Immunol, 1996, 26, 1527–1596.
37 Brutkiewicz RR, Bennink JR, Yewdell JW & Bendelac A. TAP-independent, beta 2-microglobulin– dependent surface expression of functional mouse CD1.1, J Exp Med, 1995, 182, 1913–1919.[Abstract/Free Full Text]
38 deSousa M, Reimao R, Lacerda R, Hugo P, Kaufmann SHE & Porto G. Iron overload in β2-microglobulin–deficient mice, Immunol Lett, 1994, 39, 105–111.[Medline]
39 Rothenburg BE & Voland JR. B2 Knockout mice develop parenchymal iron overload: a putative role for class I genes of the major histocompatability complex in iron metabolism, Proc Natl Acad Sci USA, 1996, 93, 1529–1534.[Abstract/Free Full Text]
40 Feder JN, Gnirke A, Thomas W, Tsuchihashi Z, Ruddy DA, Basava A, Dormishian F, Domingo RJ, Ellis MD, Fullan A et al.. A novel MHC class I–like gene is mutated in patients with hereditary haemochromatosis, Nature Genet, 1996, 13, 399–408.[Medline]