The protection receptor for IgG catabolism is the beta2-microglobulin-containing neonatal intestinal transport receptor

Cloning and analysis of the gene encoding the human neonatal Fc receptor

The neonatal Fc receptor, FcRn, is expressed in human placental syncytiotrophoblast, capillary endothelium, intestinal epithelium, and other tissues. By analogy with its role in the mouse, human FcRn is expected to transport maternal IgG to the foetus, and protect circulating IgG from catabolism. The larger subunit of FcRn is homologous to the alpha chains of the major histocompatibility complex (MHC) class I proteins, but is encoded outside the MHC on chromosome 19. We report the isolation of clones encoding the alpha chain of human FcRn from chromosome 19-specific libraries. The sequence revealed a similar organization to classical and non-classical MHC, and MHC-related genes. Compared with classical MHC class I genes, the human FcRn alpha chain gene has expanded by acquiring many repetitive sequences in its introns, including multiple Alu elements in the fourth intron. Primer extension analysis showed that there are two transcription initiation sites in the upstream flanking sequence.

Regulation of antibody responses via antibodies, complement, and Fc receptors

Antibodies can completely suppress or enhance the antibody response to their specific antigen by several hundredfold. Immunoglobulin M (IgM) enhances antibody responses via the complement system, and complement activation by IgM probably starts the chain of events leading to antibody responses to suboptimal antigen doses. IgG can enhance primary antibody responses in the absence of the complement system and seems to be dependent on Fc receptors for IgG (FcgammaRs). IgE enhances antibody responses via the low-affinity receptor for IgE (FcepsilonRII/CD23). The precise effector mechanisms that cause enhancement are not known, but direct B-cell signaling, antigen presentation, and increased follicular localization are all possibilities. IgG, IgE, and IgM may also suppress antibody responses when used in certain immunization regimes, and it seems reasonable that an important mechanism behind suppression is the masking of antigenic epitopes by antibodies. In addition, FcgammaRIIB, which contains a cytoplasmic inhibitory motif, acts as a negative regulator of antibody responses. This receptor, however, may prevent the antibody responses from exceeding a certain level rather than causing complete suppression.

Multiple roles for the major histocompatibility complex class I- related receptor FcRn

Multiple functions have recently been identified for the neonatal Fc receptor FcRn. In addition, a human homolog of the rodent forms of FcRn has been identified and characterized. This major histocompatibility complex class I-related receptor plays a role in the passive delivery of immunoglobulin (Ig)Gs from mother to young and the regulation of serum IgG levels. In addition, FcRn expression in tissues such as liver, mammary gland, and adult intestine suggests that it may modulate IgG transport at these sites. These diverse functions are apparently brought about by the ability of FcRn to bind IgGs and transport them within and across cells. However, the molecular details as to how FcRn traffics within cells have yet to be fully understood, although in vitro systems have been developed for this purpose. The molecular nature of the FcRn-IgG interaction has been studied extensively and encompasses residues located at the CH2-CH3 domain interface of the Fc region of IgG. These Fc amino acids are highly conserved in rodents and man and interact with residues primarily located on the alpha2 domain of FcRn. Thus, it is now possible to engineer IgGs with altered affinities for FcRn, and this has relevance to the modulation of IgG serum half-life and maternofetal IgG transport for therapeutic applications.

Intravenous immunoglobulin in autoimmune and inflammatory dermatoses. A review of proposed mechanisms of action and therapeutic applications

Off-label use of intravenous immunoglobulin (IVIG) at high doses has resulted in numerous anecdotal reports of its effectiveness in a variety of autoimmune and inflammatory conditions. Despite its growing acceptance as a viable therapeutic option in the management of several such disorders, the poorly defined mechanism of action of IVIG has stifled its rational therapeutic application. The lack of carefully designed prospective randomized clinical trials has further fueled controversy and mitigates against optimal application of this burgeoning therapy. Nevertheless, some standardization of IVIG therapy is slowly advancing that promises to support the use of this treatment for a growing number of autoimmune and inflammatory dermatoses.

Analysis of the promoter region of the human FcRn gene

The 5'-flanking region of the human FcRn alpha-chain gene was analyzed for its ability to directly express the chloramphenicol acetyltransferase (CAT) reporter gene in NIH3T3 and Lu106 cells. Transient transfection of the CAT constructs revealed that there was promoter activity in the region -660 to +300 of the 5'-flanking sequence. Electrophoretic mobility-shift assays showed that there are functional binding sites for Sp1 or Sp1-like factors, AP1 or a related factor, and additional unidentified proteins in the promoter region.

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 beta-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 beta-2 microglobulin mRNA in the mammary gland increased during the switch phase when milk IgG concentration also increases. This correlation between beta-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 beta-2 microglobulin and that its expression is upregulated during the period of highest IgG-transfer into milk.

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.

Bidirectional transcytosis of IgG by the rat neonatal Fc receptor expressed in a rat kidney cell line: a system to study protein transport across epithelia

The neonatal Fc receptor, FcRn, transports immunoglobulin G (IgG) across cellular barriers between mother and offspring. FcRn also protects circulating IgG from catabolism, probably during transport across the capillary endothelium. Only one cell culture model of transcytosis has been used extensively, the transport of IgA from the basolateral to the apical surface of Madin-Darby canine kidney cells by the polymeric immunoglobulin receptor (pIgR). We report that rat inner medullary collecting duct (IMCD) cells transfected with DNA encoding the (alpha) subunit of rat FcRn specifically and saturably transport Fc when grown as polarized monolayers. Using this system, we have found that transcytosis by FcRn, like transcytosis by the pIgR, depends upon an intact microtubule system. FcRn differs most strikingly from the pIgR in its ability to transport its ligand in both the apical to basolateral and basolateral to apical directions. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 inhibited basolateral to apical transport by FcRn more than apical to basolateral transport, suggesting that there are differences in the mechanisms of transport in the two directions. Lastly, we found that transcytosis by FcRn depends upon vesicular acidification. We anticipate that the IMCD cell culture model will allow further elucidation of the mechanism of IgG transport by FcRn.

Characterization and localization of the neonatal Fc receptor in adult human kidney

The binding of Fc fragments of Ig on glomerular epithelial cells (GEC) was observed previously, but the receptor could not be identified. In immunofluorescence and immunohistochemical studies using normal adult human kidney sections, the presence of the so-called neonatal Fc receptor (FcRn) was demonstrated on GEC as well as in the brush border of proximal tubular cells. FcRn transcripts were also detected on isolated glomeruli by reverse transcription-PCR. Using an immortalized GEC line, the presence of the FcRn was confirmed by flow cytometry, reverse transcription-PCR, Western blotting, and by the pH dependence of the binding of heat-aggregated IgG. Because it is well established that the FcRn is involved in IgG transcytosis, it is hypothesized that the FcRn in the kidney may play a role in the reabsorption of IgG. Ongoing studies should clarify the role of the FcRn as a potential target for immune complexes on GEC and should assess its relevance in physiology and pathology.

Viral persistence in vivo through selection of neutralizing antibody-escape variants

Despite initial virus control by CD8(+) cytotoxic T lymphocytes (CTLs), noncytopathic or variably cytopathic viruses (e.g., hepatitis B and C viruses, HIV) are able to establish persistent infections. The role of neutralizing antibodies (nAbs) in controlling disease progression is unclear. Therefore, the phenomenon of viral evasion from the nAb response and its implications for virus persistence remain controversial. Here we demonstrate nAb-mediated viral clearance in CTL-deficient mice infected with the prototypic noncytopathic lymphocytic choriomeningitis virus (strain WE). During prolonged CTL absence, neutralization-resistant virus mutants were selected in individual mice within 70-90 days. In naive animals infected with these virus variants only low nAb responses were induced, resulting in an increased tendency of virus to persist.

Monocyte-macrophage system as targets for immunomodulation by intravenous immunoglobulin

Pooled human intravenous immunoglobulin (IVIg) has been used successfully to treat or ameliorate the clinical manifestations of humoral immune deficiencies, haematological disorders, HIV infection and many other diseases states. However, the mechanism of action of IVIg remains unclear. Several mechanisms of action of IVIg have been proposed. These include Fcy receptor blockade, accelerated clearance of endogenous pathogenic auto-antibodies, inhibition of components of the complement cascade, neutralization of super-antigens and bacterial toxins as well as anti-cytokine and anti-idiotype effects. A major contributor to host immunity and immune surveillance against infection, tissue or cell damage and malignancy is the monocyte/macrophage system. Monocyte-directed inflammation is a desirable consequence of microbiological or malignant challenge. However, monocyte hyperactivity may contribute to certain pathological conditions. These include the systemic inflammatory response syndrome (SIRS), septic shock, other dysregulated inflammatory disorders and auto-immunity. Novel therapies that can suppress the hyperactive state or correct monocyte/macrophage dysfunction without compromising normal host cell-mediated immunity are desirable. In this review, we discuss the immunomodulatory effects of IVIg focussing particularly upon the monocyte/macrophage system in pertinent disease states.

Cloning and characterization of the bovine MHC class I-like Fc receptor

In the cow, maternal immunity is exclusively mediated by colostral Igs, but the receptor responsible for the IgG transport has not yet been identified. The role of an IgG-Fc receptor (FcRn) that resembles a class I MHC Ag in transporting IgGs through epithelial cells was recently shown in selected species. We now report the cloning and characterization of the bovine FcRn (bFcRn). The cDNA and deduced amino acid sequences show high similarity to the FcRn in other species, and it consists of three extracellular domains, a hydrophobic transmembrane region, and a cytoplasmic tail. Despite the high similarity of the extracellular domains with other species, the bovine cytoplasmic tail is the shortest thus far analyzed. Aligning the known FcRn sequences, we noted that the bovine protein shows a 3-aa deletion compared to the rat and mouse sequences in the alpha1 loop. Furthermore, we found a shorter transcript of the bFcRn reflecting an exon 6-deleted mRNA, which results from an inadequate splice acceptor site in intron 5 and produces a transmembrane-deficient molecule, as was previously demonstrated in the related MHC class I gene family in mouse and humans. The presence of bFcRn transcripts in multiple tissues, including the mammary gland, suggests their involvement both in IgG catabolism and transcytosis.

Preparation of fcgamma for addition to sulfhydryl-expressing ligands with minimal disturbance of the hinge

We described previously a scheme for linking functionally intact human normal Fcgamma1, via a thioether linkage emerging from its hinge, to any molecule expressing a free sulfhydryl group (SH). The scheme entails reducing the Fc to release four SH from the two inter-gamma disulfide bonds (SS) in the hinge, blocking one SH by a stochastic alkylation, restoring by SS-interchange the inter-gamma SS whose two SH are still available, and attaching a bismaleimide linker to the one remaining SH. One thereby obtains Fc with a single maleimide group (Fc-maleimide) for attachment to the SH-displaying partner. Restoration of the inter-gamma SS is necessary if the final chimeric construct is to be able to activate the classical complement pathway. However, during this preparation of Fc-maleimide, there is apparently some SS-formation between non-homologous SH, so that not all hinges emerge with a reconstituted inter-gamma SS. To reduce this error we have modified the preparative procedure after investigating an initial partial reduction of the hinge, and reviewing the conditions for stochastic alkylation. During partial reduction by dithiothreitol, the two hinge SS were cleaved apparently randomly: there was no evidence for one bond being more susceptible to reduction than the other, and little indication that the reduction of one bond enhanced the susceptibility of the other. By limiting reduction to an average of one SS per molecule, and alkylation to 0.8 SH per molecule, a final Fc-maleimide product is obtained in which most of the molecules have passed through the entire preparation with one of their hinge SS, and by inference much of the hinge conformation, remaining intact.

Long-term remission of refractory stiff-man syndrome after treatment with intravenous immunoglobulin

The stiff-man syndrome is a rare neuromuscular disorder characterized by progressive rigidity, stiffness, and intermittent spasm of axial and extremity muscles. Its etiology is unknown. Different therapeutic regimens have been used with variable success. We present a case of refractory stiff-man syndrome, in which the symptoms were successfully controlled by the administration of intravenous immunoglobulin (IVIg). This case gives evidence that IVIg can be a safe and an efficient treatment of refractory stiff-man syndrome. The exact indication for and the cost-effectiveness of IVIg in the treatment of this rare entity remain to be determined.

Sedimentation equilibrium analysis of recombinant mouse FcRn with murine IgG1

The interaction of mouse IgG1 or IgG1-derived Fc fragment with recombinant, insect cell expressed mouse FcRn has been analyzed using sedimentation equilibrium. This results in a model for the interaction in which the two binding sites for FcRn on Fc or IgG1 have significantly different affinities with macroscopic binding constants of < 130 nM and 6 microM. This data indicates the formation of an asymmetric FcRn:Fc (or IgG1):FcRn complex which is consistent with earlier suggestions that for this form of recombinant FcRn, binding to IgG1 or Fc does not result in a symmetric 2:1 complex in which both binding sites are equivalent.

Mapping the site on human IgG for binding of the MHC class I-related receptor, FcRn

The analysis of the pharmacokinetics of wild-type and mutated Fc fragments derived from human IgG1 indicates that Ile253, His310 and His435 play a central role in regulating serum half-life in mice. Reduced serum half-life of the recombinant, mutated fragments correlates with decreased binding to the MHC class I-related neonatal Fc receptor, FcRn. In addition, the analysis of an Fc fragment in which His435 is mutated to Arg435 demonstrates that the sequence difference at this position between human IgG1 (His435) and IgG3 (Arg435) most likely accounts for the shorter serum half-life of IgG3 relative to IgG1. In contrast to His310 and His435, the data indicate that His433 does not play a role in regulating the serum half-life of human IgG1. Thus, the interaction site of mouse FcRn on human and mouse IgG1 involves the same conserved amino acids located at the CH2-CH3 domain interface of the IgG molecule. The sequence similarities between mouse and human FcRn suggest that these studies have direct relevance to understanding the factors that govern the pharmacokinetics of therapeutic IgG.

Different responses of human anti-HLA and anti-alphagal antibody to long-term intravenous immunoglobulin therapy

Concentrated human immunoglobulin (IVIG) has been administered intravenously in the treatment of autoimmune disorders and to reduce anti-HLA antibodies in highly sensitized patients awaiting organ transplantation. It has also been shown, in experimental animals, to prevent the hyperacute rejection of discordant xenografts, possibly by anticomplement activity. The aim of the present study was to assess the effect of IVIG therapy on both acquired anti-HLA antibodies and natural antigalactose alpha1-3 galactose (alphaGal) antibodies in five patients awaiting heart transplantation. Five patients placed on mechanical circulatory support who had developed high HLA panel-reactive antibodies (PRA) or in whom the percentage of PRA was increasing rapidly were treated weekly with 500 mg/kg IVIG, which contained 1% of anti-alphaGal IgG. Levels of PRA, anti-alphaGal IgG and IgM, and serum cytotoxicity to pig cells were measured before, during, and after therapy. PRA percentages in the five patients were initially 85%, 53%, 23%, 19% and 19% (mean 39%). Mean PRA fell by 66% after 3 months of therapy (to a mean PRA of 14%), and by 96% after 6 months therapy (to a mean PRA of 2%). Anti-alphaGal antibody levels and serum cytotoxicity to pig aortic endothelial cells did not change significantly. These results confirm the effectiveness of IVIG therapy in reducing PRA in HLA highly sensitized patients. It is likely that IVIG does not contain the relevant anti-HLA antibody, resulting in an accelerated catabolism of native alloantibodies. However, as IVIG contains a normal level of anti-alphaGal IgG, catabolism of anti-alphaGal IgG is not modified, as it is being continuously replaced. To achieve a decrease in the anti-alphaGal IgG level it would be necessary to use IVIG depleted of this antibody.

IgG-Mediated Enhancement of Antibody Responses Is Low in Fc Receptor γ Chain-Deficient Mice and Increased in FcγRII-Deficient Mice

Immunization with IgG/Ag or IgE/Ag complexes leads to a higher production of specific Abs than immunization with Ag alone. The enhancing effect of IgE is exclusively dependent upon the low-affinity receptor for IgE, FcεRII, whereas the mechanism behind IgG-mediated enhancement is unknown. We have investigated whether receptors for the Fc part of IgG are required for responses to IgG/Ag. Mice lacking the γ subunit of Fc receptors (FcRs) (FcRγ−/−), FcγRII (FcγRII−/−), or FcγRIII (FcγRIII−/−) were immunized with BSA-2,4,6-trinitrophenyl (TNP) alone or BSA-TNP complexed to monoclonal TNP-specific IgG1, IgG2a, or IgG2b. As expected, all subclasses enhanced the Ab-response to BSA in wild-type mice. Enhancement was in the same order of magnitude in FcγRIII−/− mice (≤177-fold of controls administered Ag alone), whereas it was abrogated in FcRγ−/− mice and augmented in FcγRII−/− mice (≤5147-fold of controls). The response to IgE/Ag complexes in FcRγ−/− and FcγRII−/− mice was similar to that seen for wild-type mice, demonstrating that non-FcγR-dependent responses were normal. Our observations suggest that IgG/Ag complexes enhance Ab responses via FcγRs. Moreover, they reveal a strong negative regulation of Ab responses to IgG/Ag exerted by FcγRII.

Intracellular traffic of the MHC class I-like IgG Fc receptor, FcRn, expressed in epithelial MDCK cells

Transfer of passive immunity from mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is thought to be mediated by FcRn, an IgG Fc receptor with homology to MHC class I antigens. We have now expressed the human FcRn in polarized MDCK cells and analyzed the intracellular routing of the receptor. FcRn showed a predominant intracellular localization at steady state. Newly synthesized FcRn was delivered in a non-vectorial fashion to both the apical and basolateral surfaces of MDCK cell monolayers. Following internalization from the apical or basolateral domain, the receptor transcytosed to the opposite surface. These findings provide direct evidence for the transepithelial transport function of FcRn and indicate that the receptor undergoes multiple rounds of transcytosis.

Differing MHC class I requirements for induction and propagation of experimental systemic lupus erythematosus

Mice deficient in beta2-microglobulin expression are resistant to the induction of experimental systemic lupus erythematosus (SLE). The present studies were designed to identify the beta2-microglobulin-dependent cell surface molecule(s) that confers sensitivity to experimental SLE, and to determine its role in disease development. We report hat mice lacking the transporter associated with antigen presentation (TAP-/-) were also resistant to disease, whereas CD1-/- and CD8-/- mice were susceptible; susceptibility also did not correlate with neonatal Fc receptor or HEPH expression. These data indicate that disease susceptibility is determined by expression of MHC class I. Furthermore, by analyzing both adoptive transfer and radiation bone marrow chimeras, we demonstrate that MHC class I expression is necessary for propagation of disease, but not for induction of pathogenic cells.

Negative signaling in health and disease

Signal transduction induced by receptors can elicit intracellular biochemical events that either support or inhibit cell activation. Induction of the latter has been termed "negative signaling" and can be triggered by receptors on immune cells that are distinct from activating receptors while other growth-promoting receptors induce both positive and negative signaling events. Here, the biochemistry leading to cell activation or inhibition and induced by receptors on immune cells are reviewed. Furthermore, recent experimental evidence is reviewed that indicates an important contribution of negative signaling to the intracellular survival of infectious pathogens.

CD4(+) T cells and the proinflammatory cytokines gamma interferon and interleukin-6 contribute to alveolar bone loss in mice

In this study, we used a mouse model to examine the role of the adaptive immune response in alveolar bone loss induced by oral infection with the human gram-negative anaerobic bacterium Porphyromonas gingivalis. Severe combined immunodeficient mice, which lack B and T lymphocytes, exhibited considerably less bone loss than did immunocompetent mice after oral infection, suggesting that lymphocytes contribute to this process. Bone loss after oral infection was decreased in mice deficient in major histocompatibility complex (MHC) class II-responsive CD4(+) T cells, but no change in bone loss was observed in mice deficient in MHC class I-responsive CD8(+) T cells or NK1(+) T cells. Mice lacking the cytokine gamma interferon or interleukin-6 also demonstrated decreased bone loss. These results suggest that the adaptive immune response, and in particular CD4(+) T cells and the proinflammatory cytokines that they secrete, are important effectors of bone loss consequent to P. gingivalis oral infection. The studies also reinforce the utility of the mouse oral infection model in dissecting the pathobiology of periodontal disease.

Lipid antigen presentation in the immune system: lessons learned from CD1d knockout mice

CD1 molecules represent a distinct lineage of antigen-presenting molecules that are evolutionarily related to the classical major histocompatibility complex (MHC) class I and class II molecules. Unlike the classical MHC products that bind peptides, CD1 molecules have evolved to bind lipids and glycolipids. Murine and human CD1d molecules can present glycolipid antigens such as alpha-galactosylceramide (alpha-GalCer) to CD1d-restricted natural killer (NK) T cells. Using CD1d knockout mice we demonstrated that CD1d expression is required for the development of NK T cells. These animals were also deficient in the rapid production of interleukin-4 and interferon-gamma in response to stimulation by anti-CD3 antibodies. Despite these defects, CD1d knockout animals were able to generate strong T-helper type 1 (TH1) and TH2 responses. Spleen cells from these animals neither proliferated nor produced cytokines in response to stimulation by alpha-GalCer. Repeated injection of alpha-GalCer into wild-type but not CD1d mutant mice was able to clear metastatic tumors. We further showed that alpha-GalCer can inhibit disease in diabetes-prone non-obese diabetic mice. Collectively, these findings with CD1d knockout animals indicate a critical role for CD1d-dependent T cells in various disease conditions, and suggest that alpha-GalCer may be useful for therapeutic intervention in these diseases.

Antibody feedback suppression: towards a unifying concept?

IgG antibodies can negatively regulate antibody responses. When IgG anti-sheep erythrocytes (SRBC) is administered to an animal together with SRBC, the response against SRBC will frequently be less than 1% of the response in animals immunized with SRBC alone. The mechanism behind this phenomenon is not fully understood. It has been suggested that suppression is caused by masking of epitopes by IgG, thus preventing B cells from recognizing the antigen. Other possible explanations are that IgG/antigen complexes are eliminated via Fc-receptor dependent phagocytosis or that the complexes inhibit B cell activation by co-crosslinking the B cell receptor for antigen and the inhibitory Fc-receptor, FcgammaRIIB, expressed by B cells. Whereas the first mechanism would operate independently of the Fc-portion of IgG, the two latter would be Fc-dependent. In the literature data has been presented supporting both Fc-dependence and Fc-independence of suppression. It has recently been shown that IgG suppresses more than 90% of the antibody response in gene targeted mice lacking the known Fc-receptors for IgG and that F(ab')2 fragments as well as IgE are efficient suppressors. These findings strongly suggest that IgG is able to efficiently suppress antibody responses independently of the Fc-part and favor the model of epitope masking. Here, a way of interpreting available experimental data which can explain many of the discrepancies in the literature, is presented.

Nonvectorial surface transport, endocytosis via a Di-leucine-based motif, and bidirectional transcytosis of chimera encoding the cytosolic tail of rat FcRn expressed in Madin-Darby canine kidney cells

Transfer of passive immunity from the mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is thought to be mediated by FcRn, an IgG Fc receptor with homology to major histocompatibility class I antigens. Here, we analyzed the intracellular routing of chimera encoding the rat FcRn tail fused to the ecto- and transmembrane domain of the macrophage FcgammaRIIb. Newly synthesized chimera were delivered in a nonvectorial manner to the apical and basolateral cell surface, from where the chimera were able to internalize and transcytose. Apical to basolateral and basolateral to apical transcytosis were differently regulated. This intracellular routing of the chimera is similar to that of the native FcRn, indicating that the cytosolic tail of the receptor is necessary and sufficient to endow an unrelated FcR with the intracellular transport behavior of FcRn. Furthermore, the di-leucine motif in the cytosolic domain of FcRn was required for rapid and efficient endocytosis but not for basolateral sorting of the chimera.

Vaccination with a recombinant vaccinia virus encoding a "self" antigen induces autoimmune vitiligo and tumor cell destruction in mice: requirement for CD4(+) T lymphocytes

Many human and mouse tumor antigens are normal, nonmutated tissue differentiation antigens. Consequently, immunization with these "self" antigens could induce autoimmunity. When we tried to induce immune responses to five mouse melanocyte differentiation antigens, gp100, MART-1, tyrosinase, and tyrosinase-related proteins (TRP) 1 and TRP-2, we observed striking depigmentation and melanocyte destruction only in the skin of mice inoculated with a vaccinia virus encoding mouse TRP-1. These mice rejected a lethal challenge of B16 melanoma, indicating the immune response against TRP-1 could destroy both normal and malignant melanocytes. Cytotoxic T lymphocytes specific for TRP-1 could not be detected in depigmented mice, but high titers of IgG anti-TRP-1 antibodies were present. Experiments with knockout mice revealed an absolute dependence on major histocompatibility complex class II, but not major histocompatibility complex class I, for the induction of both vitiligo and tumor protection. Together, these results suggest that the deliberate induction of self-reactivity using a recombinant viral vector can lead to tumor destruction, and that in this model, CD4(+) T lymphocytes are an integral part of this process. Vaccine strategies targeting tissue differentiation antigens may be valuable in cancers arising from nonessential cells and organs such as melanocytes, prostate, testis, breast, and ovary.

Elimination of the Immunogenicity of Therapeutic Antibodies

The immunogenicity of therapeutic Abs limits their long-term use. The processes of complementarity-determining region grafting, resurfacing, and hyperchimerization diminish mAb immunogenicity by reducing the number of foreign residues. However, this does not prevent anti-idiotypic and anti-allotypic responses following repeated administration of cell-binding Abs. Classical studies have demonstrated that monomeric human IgG is profoundly tolerogenic in a number of species. If cell-binding Abs could be converted into monomeric non-cell-binding tolerogens, then it should be possible to pretolerize patients to the therapeutic cell-binding form. We demonstrate that non-cell-binding minimal mutants of the anti-CD52 Ab CAMPATH-1H lose immunogenicity and can tolerize to the “wild-type” Ab in CD52-expressing transgenic mice. This finding could have utility in the long-term administration of therapeutic proteins to humans.

Efficient IgG-mediated suppression of primary antibody responses in Fcgamma receptor-deficient mice

IgG antibodies can suppress more than 99% of the antibody response against the antigen to which they bind. This is used clinically to prevent rhesus-negative (Rh-) women from becoming immunized against Rh+ erythrocytes from their fetuses. The suppressive mechanism is poorly understood, but it has been proposed that IgG/erythrocyte complexes bind to the inhibitory Fc receptor for IgG (FcgammaRIIB) on the B cell surface, thereby triggering negative signals that turn off the B cell. We show that IgG induces the same degree of suppression of the response to sheep erythrocytes in animals lacking the known IgG-binding receptors FcgammaRIIB, FcgammaRI + III, FcgammaRI + IIB + III, and FcRn (the neonatal Fc receptor) as in wild-type animals. Reinvestigation of the ability of F(ab')2 fragments to suppress antibody responses demonstrated that they were nearly as efficient as intact IgG. In addition, monoclonal IgE also was shown to be suppressive. These findings suggest that IgG inhibits antibody responses through Fc-independent mechanisms, most likely by masking of antigenic epitopes, thereby preventing B cells from binding and responding to antigen. In agreement with this, we show that T cell priming is not abolished by passively administered IgG. The results have implications for the understanding of in vivo regulation of antibody responses and Rh prophylaxis.

Absence of spontaneous central nervous system remyelination in class II-deficient mice infected with Theiler's virus

We previously showed that Theiler's murine encephalomyelitis virus (TMEV)-infected major histocompatibility complex (MHC) class II-deficient mice develop both demyelination and neurologic deficits, whereas MHC class I-deficient mice develop demyelination but no neurologic deficits. The absence of neurologic deficits in the class I-deficient mice was associated with preserved sodium channel densities in demyelinated lesions, a relative preservation of axons, and extensive spontaneous remyelination. In this study, we investigated whether TMEV-infected class II-deficient mice, which have an identical genetic background (C57BL/6 x 129) as the class I-deficient mice, have preserved axons and spontaneous myelin repair following chronic TMEV-infection. Both class I- and class II-deficient mice showed similar extents of demyelination of the spinal cord white matter 4 months after TMEV infection. However, the class I-deficient mice demonstrated remyelination by oligodendrocytes, whereas class II-deficient mice showed minimal if any myelin repair. Demyelinated lesions, characterized by inflammatory infiltrates in both mutants, revealed disruption of axons in class II- but not class I-deficient mice. Further characterization revealed that even though class II-deficient mice lacked TMEV-specific IgG, they had virus-specific IgM, which, however, did not neutralize TMEV in vitro. In addition, class II-deficient mice developed TMEV-specific cytotoxic T-lymphocytes in the CNS during the acute (7 days) disease, but these cytotoxic lymphocytes were not present in the chronic stage of disease, despite a high titer of infectious virus throughout the disease. We envision that the presence of demyelination, high virus titer, absence of remyelination, and axonal disruption in chronically infected class II-deficient mice contributes to the development of paralytic disease.

Computational determination of the structure of rat Fc bound to the neonatal Fc receptor

The available crystal structure for the complex between the Fc fragment of immunoglobulin G (IgG) and the neonatal Fc receptor (FcRn) was determined at low resolution and has no electron density for a large portion of the CH2 domain of the Fc. Here, we use a well validated computational docking algorithm in conjunction with known crystallographic data to predict the orientation of CH2 when bound to FcRn, and validate the predicted structure with data from site-specific mutagenesis experiments. The predicted Fc structure indicates that the CH2 domain moves upon binding FcRn , such that the end-to-end distance of the bound Fc fragment is greater than it is in the crystal structure of isolated Fc. The calculated orientation of the bound CH2 domain is displaced by an average of 6 A from the CH2 orientation in the structure of Fc alone, and shows improved charge complementarity with FcRn. The predicted effects of 11 specific mutations in Fc and FcRn are calculated and the results are compared with experimental measurements. The predicted structure is consistent with all reported mutagenesis data, some of which are explicable only on the basis of our model. The current study predicts that FcRn-bound Fc is asymmetric due to reorientation of the CH2 domain upon FcRn binding, a rearrangement that would be likely to interfere with optimal binding of FcRn at the second binding site of the Fc homodimer.

Structural bases of Fc gamma R functions

This review describes structures which determine the biological activities triggered by Fc gamma R and account for the cell-mediated functions of IgG antibodies in physiology and pathology. The binding specificity and affinity of Fc gamma R depend primarily on IgG-binding structures, in their immunoglobulin-like extracellular domains. Binding is however also influenced by subunits that associate to multichain Fc gamma R. Effector and regulatory intracytoplasmic sequences that are unique to molecules of the Fc gamma RIIB family determine the internalization properties of these receptors. Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) are intracytoplasmic effector sequences shared by Fc gamma R and other receptors involved in the recognition of antigen, which trigger cell activation and internalization. Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs) are intracytoplasmic sequences, shared by Fc gamma RIIB and a growing number of negative coreceptors which negatively regulate cell activation via ITAM-bearing receptors. Altogether, these structures enable IgG antibodies to exert a variety of finely tuned biological effects during the immune response.

IgG binding sites on human Fc gamma receptors

The structure for the three human Fc gamma receptors classes Fc gamma RI (CD64), Fc gamma RII (CD32) and Fc gamma RIII (CD16) has been well characterized. Here the IgG binding sites on Fc gamma RII and Fc gamma RII with their responsive FG, BC and C'/E loops on the membrane proximal domains are described in detail. For Fc gamma RI the second extracellular domain is suggested as a key structure of IgG binding. The lower hinge regions of human and murine IgG binding to these Fc receptors and their structural relationship in Fc gamma R-IgG interactions are discussed. The potential of inhibiting the pathophysiological effects of Fc gamma receptors by blocking studies are considered for future therapeutic modalities.

Epithelial transcytosis of immunoglobulins

Transcytosis plays a central role in the immunological functions of epithelia, including the sampling of antigens that enter the body via the digestive, respiratory and urogenital tracts and their presentation to underlying lymphoid tissues, the secretion of specific immunoglobulins required for the immune protection of mucosal surfaces and the transfer of maternal immunoglobulins to the fetus or newborn, providing the latter with passive immunity for the first weeks of independent life.

Comparative studies of rat IgG to further delineate the Fc:FcRn interaction site

Recent data have indicated that the MHC class I-related receptor, FcRn, regulates the half-lives of serum IgG in addition to its known role in transferring IgG from mother to young. In the current study, the activity of rat IgG (rIgG) isotypes in FcRn-mediated functions has been analyzed. The serum half-life and maternofetal transfer in mice decreased in the order rIgG2a > rIgG1 > rIgG2c > rIgG2b. This decrease in activity correlates well with reduced binding affinity for soluble mouse FcRn, and site-directed mutagenesis of a recombinant Fc-hinge fragment has been used to investigate the molecular basis for the differences in activities of the rIgG. Analysis of the serum half-lives of the mutated Fc-hinge fragments demonstrated that, in addition to Ile253, His310, His435 and His436 that were identified in earlier studies, amino acids at positions 257, 307 and 309 play a role in building the FcRn interaction site of IgG. The study also excludes the involvement of amino acids in a fourth loop located at the CH2-CH3 domain interface that encompasses residues 386-387 in FcRn binding. Sequence differences at positions 257, 307 and 309 between rIgG most likely account for the reduced affinity of rIgG2b and IgG2c relative to rIgG1 and rIgG2a for binding to FcRn.

IgG-Fc-mediated effector functions: molecular definition of interaction sites for effector ligands and the role of glycosylation

The Fc region of human IgG expresses interaction sites for many effector ligands. In this review the topographical distributions of ten of these sites are discussed in relation to functional requirement. It is apparent that interaction sites localised to the inter-CH2-CH3 domain region of the Fc allow for functional divalency, whereas sites localised to the hinge proximal region of the CH2 domain are functionally monovalent, with expression of the latter sites being particularly dependent on glycosylation. All x-ray crystal structures for Fc and Fc-ligand complexes report that the protein structure of the hinge proximal region of the CH2 domain is "disordered", suggesting "internal mobility". We propose a model in which such "internal mobility" results in the generation of a dynamic equilibrium between multiple conformers, certain of which express interaction sites specific to individual ligands. The emerging understanding of the influence of oligosaccharide/protein interactions on protein conformation and biological function of IgG antibodies suggests a potential to generate novel glycoforms of antibody molecules having unique profiles of effector functions.

Divergent roles for Fc receptors and complement in vivo

Recent results obtained in mice deficient in either FcRs or complement have revealed distinct functions for these two classes of molecules. While each is capable of interacting with antibodies or immune complexes, the two systems mediate distinct biological effector responses. Complement-deficient mice are unable to mediate innate immune responses to several bacterial pathogens and bacterial toxins, yet respond normally to the presence of cytotoxic antibodies and pathogenic immune complexes. In contrast, FcR-deficient mice display no defects in innate immunity or susceptibility to a variety of pathogens, yet they are unable to mediate inflammatory responses to cytotoxic IgG antibodies or IgG immune complexes, despite the presence of a normal complement system. These results lead to the surprising conclusion that these two systems have evolved distinct functions in host immunity, with complement and its receptors mediating the interaction of natural antibodies (IgM) with pathogens to effect protection, while FcRs couple the interaction of IgG antibodies to effector cells to trigger inflammatory sequelae. These results necessitate a fundamental revision of the role of these antibody-binding systems in the immune response.

CD4+ and CD8+ T Cell Interactions in IFN-γ and IL-4 Responses to Viral Infections: Requirements for IL-2

Cytokine responses to lymphocytic choriomeningitis virus infections were evaluated, and CD8+ T cell, CD4+ T cell, and IL-2 contributions delineated. In immunocompetent mice, lymphocytic choriomeningitis virus induced both IFN-γ and IL-4 as well as IL-2. Experiments in mice either β2-microglobulin-deficient, lacking MHC class I molecules and CD8+ T cells, or Aβb-deficient, lacking MHC class II molecules and CD4+ T cells, demonstrated that mixtures of T cell responses were required for optimal ex vivo cytokine productions. Intracellular cytokine expression analyses of cells from immunocompetent and immunodeficient mice showed that CD8+ T cells were predominant IFN-γ producers, and that expansion of CD8+ T cells primed to make IFN-γ was independent of CD4+ T cells in vivo. Studies in IL-2-deficient mice demonstrated that this cytokine promoted IFN-γ and IL-4 responses, and ex vivo experiments showed that exogenous IL-2 was required to maintain high-level IFN-γ production by in vivo-primed CD8+ T cells. Conditions associated with cytokine decreases were accompanied by reduced detectable plasma Ab responses. The results indicate that, although IL-2-dependent CD8+ T cell proliferation does not require endogenous CD4+ T cells, IL-2 production by the CD4+ T cells may promote continued cytokine release from activated CD8+ T cells. By defining these critical steps in cellular and cytokine interactions for shaping endogenous immune responses, the studies advance understanding of the unique conditions regulating CD8+ T cell responses to viral challenges.

Antibody responses and opsonic activity in sera of preterm neonates with coagulase-negative staphylococcal septicemia and the effect of the administration of fresh frozen plasma

Coagulase-negative staphylococcal septicemia is the most prominent nosocomial infection in neonatal intensive care units. Immaturity of host defenses in premature neonates is assumed to constitute an important risk factor. Opsonophagocytosis is considered to be the key host defense system against staphylococci with IgG antibodies as a major opsonin. For this reason we have studied serum IgG antibody titers and opsonic activity to coagulase-negative staphylococci in 20 infants with septicemia and 40 matched control subjects. In addition, we assessed the effect of administration of fresh frozen plasma (FFP) on IgG antibody titer and serum opsonic activity in 12 patients with septicemia. IgG antibodies, quantified by ELISA and opsonic activity, determined by flow cytometry, were expressed as a percentage of the value of pooled normal human reference serum. Both patients and control subjects showed low IgG titers (median, 21%; range, 1-192%) and a low opsonic activity (median, 33%; range, 8-484%) at birth. During the first 2 postnatal wk IgG titers decreased significantly in septicemia patients (from a median of 30 to 17%, p = 0.025), but not in control subjects, whereas opsonic activity remained unchanged. The titer of IgG antibodies increased significantly in septicemia patients after FFP administration (from a median of 17 to 41%, p = 0.002), whereas the effect on opsonic activity was unpredictable, showing a moderate increase in 10 out of 12 infants, and in 2 patients even a substantial decrease (>50%), despite adequate opsonic activity in the corresponding FFP batches. Immunoblotting of sepsis isolates with the corresponding patient sera demonstrated that septicemic infants may generate IgG antibodies against their blood isolate. Neonates who acquire coagulase-negative staphylococcal septicemia cannot be distinguished from control subjects on the basis of IgG antibodies and opsonic activity to staphylococci either at birth or during the first 2 postnatal wk. The administration of FFP to septicemia neonates has an unpredictable effect on opsonic activity and therefore does not seem to be a useful addition to antibiotic therapy.

Down-regulation of Fc alpha receptors on blood cells of IgA nephropathy patients: evidence for a negative regulatory role of serum IgA

IgA nephropathy (IgAN) is associated with increased serum IgA1 and IgA1-immune complexes (IC). As Fc alpha receptors (Fc alpha R) are candidate molecules to regulate IgA levels, increased receptor occupation by IgA1 prompted us to study the expression of Fc alpha R on blood cells of IgAN patients. Surface and cytoplasmic Fc alpha R expression were markedly decreased on monocytes, despite normal levels of transcripts. Fc alpha R expression on patients' neutrophils was slightly decreased, exclusively at the cell surface. However, when autologous plasma was removed from the cells Fc alpha R was up-regulated. This observation led us to search for circulating regulatory factors. In vitro experiments revealed that Fc alpha R was down-regulated on normal monocytes following long-term culture with control or patient purified serum IgA at high concentrations (5 mg/ml). Moreover, polymeric myeloma IgA1 induced stronger down-regulation than monomeric IgA1. These results point to a negative regulatory role of serum IgA on surface Fc alpha R expression. This is also supported by a negative correlation between levels of Fc alpha F on blood cells and serum IgA. On the other hand, endogenous IgA bound to IgAN cells was significantly higher than IgA bound to control cells pre-incubated with patients' plasma, suggesting abnormalities in the receptor-ligand interaction. Patient Fc alpha R had a higher Mr (60 to 85 kDa) than those of controls (55 to 75 kDa) and a decreased binding to a sialic acid-specific lectin on blots, indicating post-translational modifications with impaired sialylation of surface Fc alpha R molecules that might be involved in enhanced IgA binding. Continuous Fc alpha R occupation by IgA, associated with receptor down-regulation, might contribute to the enhancement of circulating IgA1 and IgA1-IC by impairing their binding and degradation. Finally, increased receptor occupation by IgA on monocytes was linked to mesangial proliferation and glomerular sclerosis, suggesting a role for IgA-bound cells in the pathogenesis of mesangial damage.

Borna disease virus-induced neurological disorder in mice: infection of neonates results in immunopathology

Borna disease virus (BDV) is a neurotropic nonsegmented negative-stranded RNA virus that persistently infects warm-blooded animals. In horses and other natural animal hosts, infections with BDV cause meningoencephalitis and behavioral disturbances. Experimental infection of adult mice takes a nonsymptomatic course, an observation previously believed to indicate that this animal species is not suitable for pathogenesis studies. We now demonstrate that BDV frequently induces severe neurological disease in infected newborn mice. Signs of neurological disease were first observed 4 to 6 weeks after intracerebral infection. They included a characteristic nonphysiological position of the hind limbs at an early stage of the disease and paraparesis at a later stage. Histological examination revealed large numbers of perivascular and meningeal inflammatory cells in brains of diseased mice and, unexpectedly, no increase in immunoreactivity to glial fibrillar acidic protein. The incidence and severity of BDV-induced disease varied dramatically among mouse strains. While only 13% of the infected C57BL/6 mice showed disease symptoms, which were mostly transient, more than 80% of the infected MRL mice developed severe neurological disorder. In spite of these differences in susceptibility to disease, BDV replicated to comparable levels in the brains of mice of the various strains used. Intracerebral infections of newborn beta2-microglobulin-deficient C57BL/6 and MRL mice, which both lack CD8+ T cells, did not result in meningoencephalitis or neurological disease, indicating that the BDV-induced neurological disorder in mice is a cytotoxic T-cell-mediated immunopathological process. With this new animal model it should now be possible to characterize the disease-inducing immune response to BDV in more detail.

In Vivo IL-4 Responses to Anti-IgD Antibody Are MHC Class II Dependent and β2-Microglobulin Independent and Develop Normally in the Absence of IL-4 Priming of T Cells

A crucial role for CD1-responsive, MHC class II-unrestricted T cells in the generation of T cell IL-4 responses is suggested by the: 1) requirement for IL-4 to prime in vitro IL-4 responses by naive CD4+ T cells; 2) ability of TCR cross-linking to induce CD1-responsive T cells, but not conventional naive T cells, to produce IL-4; 3) failure of anti-IgD Ab to induce an IL-4-dependent IgE response in β2-microglobulin-deficient mice, which lack CD1; and 4) reported ability of MHC class II-deficient mice to make IgE responses to anti-IgD Ab. In contrast, the Ag specificity of cytokine and Ab responses in anti-IgD-injected mice and the normal IgE responses made by anti-IgD-treated CD1-deficient mice are difficult to reconcile with this view. We now find that the failure of β2-microglobulin-deficient mice to make an IgE response to anti-IgD Ab is caused by their rapid degradation of anti-IgD; sustained anti-IgD treatment induces them to make relatively normal IL-4 and IgE responses. Furthermore, in our study, MHC class II-deficient mice make little or no IL-4 or IgE responses to anti-IgD Ab and β2-microglobulin-deficient mice make large in vivo IL-4 responses to anti-CD3 mAb. Finally, although IL-4 priming of T cells for IL-4 production is Stat6 dependent, Stat6-deficient mice make normal IL-4 responses to anti-IgD. Thus, CD1-responsive T cells and other β2-microglobulin-dependent T cells are not required to prime conventional CD4+ T cells to make IL-4 responses to anti-IgD in vivo; in fact, the large IL-4 response made in this system does not require IL-4 priming.

Impact of antigenemia on the bioactivity of infused anti-Tac antibody: implications for dose selection in antibody immunotherapies

In patients with malignancies and immune disorders expressing Tac (alpha chain of the interleukin 2 receptor; CD25), physiologic shedding of this receptor may lead to high blood levels of the soluble form (sTac). This system was used to model the interaction of soluble antigen with antibody in therapeutic settings and to develop rational principles to optimize the delivery of antibody to tumor target cells. First, we confirmed that sTac in vivo can block anti-Tac binding sites and diminish antibody binding to Tac+ cells. Second, the bioactivity of antibody in vivo correlated directly with the amount of antibody infused and inversely with the sTac concentration. Third, bindability of antibody declined in the hours and days after anti-Tac infusion in patients. Finally, tumor targeting was achieved even in the presence of excess sTac, demonstrating a partition of antibody between soluble and cell-bound antigen. A role is proposed for the Brambell receptor (FcRB) to delay saturation of human or chimeric antibodies via differential catabolism of antigen-antibody complexes. Principles are developed for predicting activity of administered antibody in the presence of soluble antigen to assist in dose selection in passive, radioimmuno and immunotoxin therapies.

MHC-II but not MHC-I responses are required for vaccine-induced protection against ocular challenge with HSV-1

PURPOSE

To determine the importance of major histocompatibility complex (MHC) class-I versus MHC class-II immune responses in protecting naive versus vaccinated mice against an ocular HSV-1 challenge.

METHODS

Class-II deficient A beta o/o (CD4-CD8+ T cells) knockout mice, which are effectively CD4+ T cells-negative, and class-I deficient beta(2)mo/o (CD4+CD8- T cells) knockout mice, which are effectively CD8+ T cells negative, were either vaccinated or mock-vaccinated and examined for their ability to withstand HSV-1 ocular challenge.

RESULTS

Unvaccinated A beta o/o and beta(2)mo/o mice were both more susceptible to lethal ocular HSV-1 infection than the parental wild type C57BL/6J mice, indicating that both MHC-I and MHC-II were required for optimal protection of naive mice against ocular HSV-1 challenge. Vaccinated beta(2)mo/o mice produced significant neutralizing antibody titers, and following ocular challenge, these mice were completely protected against death and corneal scarring. In contrast, vaccinated A beta o/o mice developed no neutralizing antibody titers and vaccination did not provide these mice with any protection against death or corneal scarring. Passive transfer of anti-HSV-1 antibody into A beta o/o mice up to 6 days post ocular challenge resulted in complete protection against death and corneal scarring.

CONCLUSIONS

Passive antibody transfer, but not vaccination, protected A beta o/o mice against ocular challenge. In contrast, vaccination completely protected beta(2)mo/o mice. These results suggest for a vaccine to provide optimal protection against ocular HSV-1 challenge in this system, it is not only sufficient, but it is also required, that the vaccine induce an effective neutralizing antibody response.

Expression of the neonatal Fc receptor, FcRn, on human intestinal epithelial cells

Maternal IgG is transferred to the suckling mouse and rat through a major histocompatibility complex (MHC) class I-related Fc receptor (FcRn) on the brush border of the proximal small intestine. We have previously described a site on the epithelial surface of the human fetal intestine with IgG binding characteristics similar to FcRn. We report here the identification by reverse transcriptase polymerase chain reaction amplification and sequencing of the human orthologue of rat and mouse FcRn in tissue obtained from human fetal and adult intestine. FcRn protein was detected in adult human intestine by western blot. Immunohistochemical studies of sections of human intestine show that the FcRn is localized mostly to the epithelial cells, where it is in the apical region. These data suggest that the binding of IgG previously seen in the fetal intestine is due to the presence of FcRn. Potential roles for this MHC class I-like Fc receptor in the human intestine include the transfer of passive immunity, induction of oral tolerance, and immunosurveillance.

Beta2-microglobulin-deficient mice are resistant to bullous pemphigoid

Recent understanding of the mechanism of immunoglobulin G (IgG) catabolism has yielded new insight into antibody-mediated diseases. We proposed that beta2-microglobulin (beta2m)-deficient mice have been protected from systemic lupus erythematosis (SLE)-like syndromes because they lack the beta2m-associated IgG protection receptor (FcRn) and therefore catabolize IgG, including pathogenic IgG autoantibodies, considerably more rapidly than normal mice. Such an hypothesis would predict that beta2m-deficient mice would also be resistant to experimental bullous pemphigoid, a disease with a pathogenesis thought to be much simpler than SLE, being the result of antibody directed toward a pathogenic epitope on the epidermal hemidesmosome that anchors basal keratinocytes to the basement membrane. To test this hypothesis, we administered pathogenic rabbit antibody directed toward the hemidesmosome to beta2m-deficient mice and to normal control mice, both intraperitoneally and intradermally, and assessed the mice clinically, histologically, and immunologically for manifestations of skin disease. We found that the beta2m-deficient mice were protected when the antibody was given intraperitoneally whereas intradermal administration resulted in blisters only slightly less severe than those seen in normal mice. These data would indicate that autoantibody-mediated inflammation might be prevented or controlled by appropriate modulation of FcRn function.

Increasing the serum persistence of an IgG fragment by random mutagenesis

The major histocompatibility complex (MHC) class I-related receptor FcRn is involved in regulating serum gammaglobulin (IgG) levels in mice. With the aim of increasing the serum half-life of a recombinant murine Fc gamma 1 fragment, the affinity for binding to FcRn at pH 6.0 has been increased by random mutagenesis of Thr252, Thr254, and Thr256 followed by selection using bacteriophage display. These residues were chosen as they are in proximity to the FcRn-IgG (Fc) interaction site. Two mutants with higher affinity (due to lower off-rates) than the wild-type Fc have been isolated and analyzed in pharmacokinetic studies in mice. The mutant with the highest affinity has a significantly longer serum half-life than the wild type fragment, despite its lower off-rate from FcRn at pH 7.4. The results provide support for the involvement of FcRn in the homeostasis of serum IgGs in mice. The indications that a homologous FcRn regulates IgG levels in humans suggest that this approach has implications for increasing the serum persistence of therapeutic antibodies.