Interference by human and bovine serum and serum protein fractions with the absorption of antibodies by suckling rats and mice

Immune system: development and acquisition of immunological competence

OBJECTIVES

To describe the ontogeny of the immune system and the adaptive mechanisms of the immune system in the neonatal period, with an emphasis on transplacental antibody transport and breastfeeding.

SOURCE OF DATA

Non-systematic literature review in the PubMed database.

SUMMARY OF THE FINDINGS

The last two decades have witnessed a great advance in the knowledge of the immune system since conception. Several investigation tools have provided insight on phenomena that were previously inadequately understood. Still expanding, the functional and molecular investigation of various aspects of the immune system will make it possible to understand how intra-uterus maternal-fetal exchanges, the maternal microbiota interacting with the fetus and newborn, and the acquisition of immunological competence occur in healthy and disease scenarios.

CONCLUSIONS

In-depth knowledge of the development of the immune system and of the adaptive mechanisms that allow a safer transition to the extrauterine environment are fundamental components of optimizing maternal and young infant vaccination, as well as the strategies associated with full postnatal development, and the early diagnosis and treatment of innate errors of immunity.

TGEV infection up-regulates FcRn expression via activation of NF-κB signaling

It has been well characterized that the neonatal Fc receptor (FcRn) transports maternal IgG to a fetus or newborn and protects IgG from degradation. We previously reported that FcRn is expressed in a model of normal porcine intestinal epithelial cells (IPEC-J2). Transmissible gastroenteritis is an acute enteric disease of swine that is caused by transmissible gastroenteritis virus (TGEV). How porcine FcRn (pFcRn) expression is regulated by pathogenic infection remains unknown. Our research shows that IPEC-J2 cells infected with TGEV had up-regulated pFcRn expression. In addition, the NF-κB signaling pathway was activated in IPEC-J2 cells by TGEV infection. Furthermore, treatment of TGEV-infected IPEC-J2 cells with the NF-κB-specific inhibitor BAY 11-7082 resulted in down-regulation of pFcRn expression. Transient transfection of pFcRn promoter luciferase report plasmids with overexpression of NF-κB p65 transcription factor enhanced the activation of the luciferase report plasmids. We identified four NF-κB transcription factor binding sites in the promoter region of this gene using luciferase reporter system, chromatin immunoprecipitation, electromobility shift assay, and supershift analysis. Together, the data provide the first evidence that TGEV infection up-regulates pFcRn expression via activation of NF-κB signaling.

Synthetic Cystine-Knot Miniproteins - Valuable Scaffolds for Polypeptide Engineering

Peptides with the cystine-knot architecture, often termed knottins, are promising scaffolds for biomolecular engineering. These unique molecules combine diverse bioactivities with excellent structural, thermal, and proteolytical stability. Being different in the composition and structure of their amino acid backbone, knottins share the same core element, namely cystine knot, which is built by six cysteine residues forming three disulfides upon oxidative folding. This motif ensures a notably rigid framework that highly tolerates both rational and combinatorial changes in the primary structure. Being accessible through recombinant production and total chemical synthesis, cystine-knot miniproteins can be endowed with novel bioactivities by variation of surface-exposed loops and incorporation of non-natural elements within their non-conserved regions towards the generation of tailor-made peptidic compounds. In this chapter the topology of cystine-knot peptides, their synthesis and applications for diagnostics and therapy is discussed.

Cystine-knot peptides targeting cancer-relevant human cytotoxic T lymphocyte-associated antigen 4 (CTLA-4)

Cystine-knot peptides sharing a common fold but displaying a notably large diversity within the primary structure of flanking loops have shown great potential as scaffolds for the development of therapeutic and diagnostic agents. In this study, we demonstrated that the cystine-knot peptide MCoTI-II, a trypsin inhibitor from Momordica cochinchinensis, can be engineered to bind to cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), an inhibitory receptor expressed by T lymphocytes, that has emerged as a target for the treatment of metastatic melanoma. Directed evolution was used to convert a cystine-knot trypsin inhibitor into a CTLA-4 binder by screening a library of variants using yeast surface display. A set of cystine-knot peptides possessing dissociation constants in the micromolar range was obtained; the most potent variant was synthesized chemically. Successive conjugation with neutravidin, fusion to antibody Fc domain or the oligomerization domain of C4b binding protein resulted in oligovalent variants that possessed enhanced (up to 400-fold) dissociation constants in the nanomolar range. Our data indicate that display of multiple knottin peptides on an oligomeric scaffold protein is a valid strategy to improve their functional affinity with ramifications for applications in diagnostics and therapy.

NFκB induces overexpression of bovine FcRn: a novel mechanism that further contributes to the enhanced immune response in genetically modified animals carrying extra copies of FcRn

Among the many functions of the neonatal Fc receptor (FcRn) for IgG, it binds to IgG-opsonized antigen complexes and propagates their traffic into lysosomes where antigen processing occurs. We previously reported that transgenic (Tg) mice and rabbits that carry multiple copies and overexpress FcRn have augmented humoral immune responses. Nuclear factor-kappa B (NFκB) is a critical molecule in the signaling cascade in the immune response. NFκB induces human FcRn expression and our previous in silico analysis suggested NFκB binding sites in the promoter region of the bovine (b) FcRn α-chain gene (FCGRT). Here, we report the identification of three NFκB transcription binding sites in the promoter region of this gene using luciferase reporter gene technology, electromobility shift assay and supershift analysis. Stimulation of primary bovine endothelial cells with the Toll-like receptor-4 ligand lipopolysaccharide (LPS), which mediates its effect via NFκB, resulted in rapid upregulation of the bFcRn expression and a control gene, bovine E-selectin. This rapid bFcRn gene induction was also observed in the spleen of bFcRn Tg mice treated with intraperitoneally injected LPS, analyzed by northern blot analysis. Finally, NFκB-mediated bFcRn upregulation was confirmed at the protein level in macrophages isolated from the bFcRn Tg mice using flow cytometry with a newly developed FcRn specific monoclonal antibody that does not cross-react with the mouse FcRn. We conclude that NFκB regulates bFcRn expression and thus optimizes its functions, e.g., in the professional antigen presenting cells, and contributes to the much augmented humoral immune response in the bFcRn Tg mice.

Bovine FcRn-mediated human immunoglobulin G transfer across the milk-blood barrier in transgenic mice

Maternal-fetal IgGs transport occurs either prenatally or postnatally, which confers the newborns with passive immunity before their own immune system has matured. However, little is known about the mechanisms of postnatal IgGs passage in the mammary gland. To investigate how FcRn mediates the IgGs transport in the mammary gland, we first generated bFcRn and anti-HAV mAb transgenic mice, and then obtained HF transgenic mice expressing both transgenes by mating the above two strains. Transgene expression of bFcRn in the four lines was determined by qRT-PCR and western blot. We then localized the expression of bFcRn to the acinar epithelial cells in the mammary gland, and anti-HAV mAb was mainly detected in the acini with weak staining in the acinar epithelial cells. Human IgGs could be detected in both milk and serum of HF transgenic mice by western blot and ELISA. A significantly lower milk to serum ratio of human IgGs in HF mice compared with that of anti-HAV mAb mice, indicating that bFcRn could transport human IgGs across the milk-blood barrier from milk to serum during lactation in HF mice. While, there were no transport of murine IgGs, IgAs, or IgMs. These results provide understandings about the mechanisms of maternal-fetal immunity transfer in the mammary gland.

Simplification of complex physiologically based pharmacokinetic models of monoclonal antibodies

Monoclonal antibodies (mAbs) exhibit biexponential profiles in plasma that are commonly described with a standard two-compartment model with elimination from the central compartment. These models adequately describe mAb plasma PK. However, these models ignore elimination from the peripheral compartment. This may lead to underestimation of the volume of distribution of the peripheral compartment and thus over-predicts concentration in the peripheral compartment. We developed a simple and physiologically relevant model that incorporates information on binding and dissociation rates between mAb and FcRn receptor, mAb uptake, reflection, and catabolic degradation. We employed a previously published PBPK model and, with assumptions regarding rates of processes controlling mAb disposition, reduced the complex PBPK model to a simpler circular model with central, peripheral, and lymph compartments specifying elimination from both central and peripheral. We successfully applied the model to describe the PK of an investigational mAb. Our model presents an improvement over standard two-compartmental models in predicting whole-body average tissue concentrations while adequately describing plasma PK with minimal complexity and physiologically more meaningful parameters.

Characterization of the rabbit neonatal Fc receptor (FcRn) and analyzing the immunophenotype of the transgenic rabbits that overexpresses FcRn

The neonatal Fc receptor (FcRn) regulates IgG and albumin homeostasis, mediates maternal IgG transport, takes an active role in phagocytosis, and delivers antigen for presentation. We have previously shown that overexpression of FcRn in transgenic mice significantly improves the humoral immune response. Because rabbits are an important source of polyclonal and monoclonal antibodies, adaptation of our FcRn overexpression technology in this species would bring significant advantages. We cloned the full length cDNA of the rabbit FcRn alpha-chain and found that it is similar to its orthologous analyzed so far. The rabbit FcRn - IgG contact residues are highly conserved, and based on this we predicted pH dependent interaction, which we confirmed by analyzing the pH dependent binding of FcRn to rabbit IgG using yolk sac lysates of rabbit fetuses by Western blot. Using immunohistochemistry, we detected strong FcRn staining in the endodermal cells of the rabbit yolk sac membrane, while the placental trophoblast cells and amnion showed no FcRn staining. Then, using BAC transgenesis we generated transgenic rabbits carrying and overexpressing a 110 kb rabbit genomic fragment encoding the FcRn. These transgenic rabbits--having one extra copy of the FcRn when hemizygous and two extra copies when homozygous--showed improved IgG protection and an augmented humoral immune response when immunized with a variety of different antigens. Our results in these transgenic rabbits demonstrate an increased immune response, similar to what we described in mice, indicating that FcRn overexpression brings significant advantages for the production of polyclonal and monoclonal antibodies.

Recent advances using FcRn overexpression in transgenic animals to overcome impediments of standard antibody technologies to improve the generation of specific antibodies

This review illustrates the salutary effects of neonatal Fc receptor (FcRn) overexpression in significantly improving humoral immune responses in the generation of antibodies for immunotherapy and diagnostics. These include: (1) improved IgG protection; (2) augmented antigen-specific humoral immune response with larger numbers of antigen specific B cells, thus offering a wider spectrum of clones; (3) generation of antibodies against weakly immunogenic antigens; (4) significant improvements in the number and substantial developments in the diversity of hybridomas. FcRn transgenesis thus confers a number of practical benefits, including faster antibody production, higher antibody yields and improved generation of hybridomas for monoclonal antibody production. Notably, these efficiencies in polyclonal antibody production were also demonstrated in FcRn transgenic rabbits. Overall, FcRn transgenic animals yield more antibodies and provide a route to the generation of antibodies against antigens of low immunogenicity that are difficult to obtain using currently available methods.

Preterm and term neonates transplacentally acquire IgG antibodies specific to LPS from Klebsiella pneumoniae, Escherichia coli and Pseudomonas aeruginosa

High incidences of Gram-negative bacteria are found in neonatal nosocomial infections. Our aim was to investigate placental transmission of immunoglobulin G (IgG) reactive with lipopolysaccharide from Klebsiella pneumoniae, Pseudomonas aeruginosa and Escherichia coli O111, O6 and O26. The total and lipopolysaccharide-specific IgM and IgG were determined in 11 maternal/umbilical-cord sera aged ≤33 weeks (GI); 21 aged >33 and <37 weeks (GII); and 32 term newborns (GIII). The total and lipopolysaccharide-specific IgM concentrations were equivalent in maternal sera. The total IgG concentrations were equivalent in maternal and newborn sera, with the exception of GIII newborns as compared with their mothers (P<0.0001) and with neonates from GI and GII (P<0.05). Lipopolysaccharide-specific IgG concentrations were lower in GI neonates than in their mothers (P<0.01) and lower in GII (P<0.05). Lower lipopolysaccharide-specific IgG levels were observed among neonates only for O111 in GI (P<0.05) and for O26 and Pseudomonas in GII, both as compared with GIII (P<0.05). The anti-lipopolysaccharide IgG transfer ratios were lower in GI (except for O26) and in GII (except for Klebsiella and O111) as compared with GIII (P<0.05). Our results suggest that the greater susceptibility to infections in preterm infants is influenced (besides the humoral response) by factors intrinsic and extrinsic to the condition of prematurity.

Neonatal FcR overexpression boosts humoral immune response in transgenic mice

The neonatal FcR (FcRn) regulates IgG and albumin homeostasis, mediates maternal IgG transport, takes active part in phagocytosis, and delivers Ag for presentation. We have previously shown that overexpression of FcRn in transgenic (Tg) mice extends the half-life of mouse IgG by reducing its clearance. In this paper, we demonstrate that immunization of these mice with OVA and trinitrophenyl-conjugated human IgG results in a 3- to 10-fold increase of Ag-specific IgM and IgG in serum. The IgM increase was unexpected because FcRn does not bind IgM. Our results showed that the affinity of the Ag-specific IgG was at least as good in Tg mice as in the wild-type (wt) controls, implying appropriate affinity maturation in both groups. Influenza vaccination produced a 2-fold increase in the amount of virus-specific Ab in Tg animals, which proved twice as efficient in a hemagglutination inhibition assay as was the case in wt controls. After immunization, Tg mice displayed significantly larger spleens containing a higher number of Ag-specific B cells and plasma cells, as well as many more granulocytes and dendritic cells, analyzed by ELISPOT and flow cytometric studies. The neutrophils from these Tg mice expressed the Tg FcRn and phagocytosed IgG immune complexes more efficiently than did those from wt mice. These results show that FcRn overexpression not only extends the IgG half-life but also enhances the expansion of Ag-specific B cells and plasma cells. Although both effects increase the level of Ag-specific IgG, the increase in immune response and IgG production seems to be more prominent compared with the reduced IgG clearance.

An intracellular traffic jam: Fc receptor-mediated transport of immunoglobulin G

Recent advances in imaging techniques along with more powerful in vitro and in vivo models of receptor-mediated ligand transport are facilitating advances in our understanding of how cells efficiently direct receptors and their cargo to target destinations within the cytoplasm and at the plasma membrane. Specifically, light and 3D electron microscopy studies examining the trafficking behavior of the neonatal Fc receptor (FcRn), a transport receptor for immunoglobulin G (IgG), have given us new insights into the dynamic interplay between the structural components of the cytosolic trafficking machinery, its protein regulators, and the receptors it directs to various locations within the cell. These studies build upon previous biochemical characterizations of FcRn transport and are allowing us to begin formulation of a more complete model for the intracellular trafficking of receptor-ligand complexes.

Immune and non-immune functions of the (not so) neonatal Fc receptor, FcRn

Careful regulation of the body's immunoglobulin-G (IgG) and albumin concentrations is necessitated by the importance of their respective functions. As such, the neonatal Fc receptor (FcRn) which, as a single receptor, is capable of regulating both of these molecules, has become an important focus of investigation. In addition to these essential protection functions, FcRn possesses a host of other functions that are equally as critical. During the very first stages of life, FcRn mediates the passive transfer of IgG from mother to offspring both before and after birth. In the adult, FcRn regulates the persistence of both IgG and albumin in the serum as well as the movement of IgG, and any bound cargo, between different compartments of the body. This shuttling allows for the movement not only of monomeric ligand but also of antigen/antibody complexes from one cell type to another in such a way as to facilitate the efficient initiation of immune responses towards opsonized pathogens. As such, FcRn continues to play the role of an immunological sensor throughout adult life, particularly in regions such as the gut which are exposed to a large number of infectious antigens. Increasing appreciation for the contributions of FcRn to both homeostatic and pathological states is generating an intense interest in the potential for therapeutic modulation of FcRn binding. A greater understanding of FcRn's pleiotropic roles is thus imperative for a variety of therapeutic purposes.

The neonatal Fc receptor plays a crucial role in the metabolism of IgG in livestock animals

The role of the FcRn is fundamental in IgG metabolism. It is involved in transporting maternal immunity and protects IgG from fast degradation throughout life. While the acquisition of the humoral immunity through the transfer of IgG from mother to offspring shows species-specific differences, the mechanism how FcRn protects IgG from degradation is highly similar in all species analyzed so far. This review summarizes the current understanding of the FcRn-mediated IgG metabolism in livestock animals (cattle, sheep and pig) and point out those aspects that remain to be exposed for better understanding the function of this system in these species and also to take advantages of it for economical purposes.

Tailoring the Pharmacokinetics and Positron Emission Tomography Imaging Properties of Anti–Carcinoembryonic Antigen Single-Chain Fv-Fc Antibody Fragments

Antibody fragments are recognized as promising vehicles for delivery of imaging and therapeutic agents to tumor sites in vivo. The serum persistence of IgG1 and fragments with intact Fc region is controlled by the protective neonatal Fc receptor (FcRn) receptor. To modulate the half-life of engineered antibodies, we have mutated the Fc-FcRn binding site of chimeric anti–carcinoembryonic antigen (CEA) antibodies produced in a single-chain Fv-Fc format. The anti-CEA T84.66 single-chain Fv-Fc format wild-type and five mutants (I253A, H310A, H435Q, H435R, and H310A/H435Q, Kabat numbering system) expressed well in mammalian cell culture. After purification and characterization, effective in vitro antigen binding was shown by competition ELISA. Biodistribution studies in BALB/c mice using 125I- and 131I-labeled fragments revealed blood clearance rates from slowest to fastest as follows: wild-type &gt; H435R &gt; H435Q &gt; I253A &gt; H310A &gt; H310A/H435Q. The terminal half-lives of the mutants ranged from 83.4 to 7.96 hours, whereas that of the wild-type was ∼12 days. Additionally, 124I-labeled wild-type, H435Q, I253A, H310A, and H310A/H435Q variants were evaluated in LS174T xenografted athymic mice by small animal positron emission tomography imaging, revealing localization to the CEA-positive xenografts. The slow clearing wild-type and H435Q constructs required longer to localize to the tumor and clear from the circulation. The I253A and H310A fragments showed intermediate behavior, whereas the H310A/H435Q variant quickly localized to the tumor site, rapidly cleared from the animal circulation and produced clear images. Thus, attenuating the Fc-FcRn interaction provides a way of controlling the antibody fragment serum half-life without compromising expression and tumor targeting.

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.

Finally! The Brambell receptor (FcRB). Mediator of transmission of immunity and protection from catabolism for IgG

F. W. Rogers Brambell was the father of the field of transmission of immunity, which he entered 50 years before the present era. As part of his quantitative and temporal studies on transmission, he defined the first Fc receptor system for IgG, and furthermore recognized the link between transmission of passive immunity from mother to young and protection from catabolism for IgG. This article provides a historical overview of the efforts of Professor Brambell and summarizes the subsequent elaboration of the details of the physiology and molecular biology of this remarkable receptor system.

THE TRANSMISSION OF ANTIBODIES AND NORMAL GAMMA-GLOBULINS ACROSS THE YOUNG MOUSE GUT

When 11-day mice are fed with homologous or heterologous antibodies or with heterologous γ-globulin, the maximum concentration of these proteins in their sera is reached 2 h later, irrespective of the amount or specificity of the proteins administered, or of the final concentration achieved. The relation between y ( = relative titre of antibody, or concentration of heterologous γ-globulin, attained in the serum) and x ( = dose of γ-globulin administered) is best represented by ( a/y — b/x ) = 1, a rectangular hyperbola with a theoretical maximum of y max . = a when x is very large. For any x , the proportion of y max . attained depends inversely on the value of b which is greatest for rat γ-globulin and then falls progressively for mouse anti- Salmonella pullorum agglutinins, bovine γ-globulin, guinea-pig γ-globulin, rabbit anti- Brucella abortus incomplete agglutinins, rat anti- Salm . pullorum agglutinins, rabbit γ-globulin and guinea-pig anti- Salm . pullorum agglutinins. The lower the value of b for any protein the less is the transmission of that protein affected by admixture with other γ -globulins, and if the protein is a γ-globulin, the more it reduces the transmission of antibodies or of other γ-globulins mixed with it. The value of b for rat anti- Salm . pullorum agglutinins is less than that for the corresponding normal γ-globulin. Fragment III of rabbit γ-globulin, prepared by the digestion method of Porter, is transmitted across the gut less readily than the intact molecule, but its ability to reduce the transmission of antibody is greater. These observations are consistent with the hypothesis of competitive absorption of ingested γ-globulin by a specific receptor within the absorptive cells of the gut. The receptor protects the attached protein from proteolysis and conveys it to the blood or lymph, possibly altered to a form more acceptable to the host.

Isolation and characterization of the Fc receptor from the fetal yolk sac of the rat

The yolk sac of the fetal rat and the proximal small intestine of the neonatal rat selectively transport maternal IgG. IgG-Fc receptors are thought to mediate transport across the epithelium of both tissues. We used a mouse mAb (MC-39) against the 45-54-kD component of the Fc receptor of the neonatal intestine to find an antigenically related protein that might function as an Fc receptor in fetal yolk sac. In immunoblots of yolk sac, MC-39 recognized a protein band with apparent molecular mass of 54-58 kD. MC-39 bound to the endoderm of yolk sac in immunofluorescence studies. In immunogold-labeling experiments MC-39 was associated mainly with small vesicles in the apical cytoplasm and in the region near the basolateral membrane of endodermal cells. The MC-39 cross-reactive protein and beta 2-microglobulin, a component of the intestinal Fc receptor, were copurified from detergent-solubilized yolk sac by an affinity purification that selected for proteins which, like the intestinal receptor, bound to IgG at pH 6.0 and eluted at pH 8.0. In summary, the data suggest that we have isolated the Fc receptor of the yolk sac and that this receptor is structurally and functionally related to the Fc receptor of the neonatal intestine. An unexpected finding is that, unlike the intestinal receptor which binds maternal IgG on the apical cell surface, the yolk sac receptor appears to bind IgG only within apical compartments which we suggest represent the endosomal complex.

Promoting effect of the new chymotrypsin inhibitor FK-448 on the intestinal absorption of insulin in rats and dogs

FK-448 is a potent and specific inhibitor of chymotrypsin, which enhances the intestinal absorption of insulin in rats and dogs resulting in a decrease in blood glucose levels in these animals. In dogs, the immunoreactive insulin (IRI) level of plasma rose proportionally to the decrease in blood glucose level. From in-vitro data, insulin was inactivated by pancreatic enzymes or the supernatants of intestine or liver homogenates. FK-448 suppressed the digestion of insulin by pancreatic enzymes and its enhancement of the intestinal absorption of insulin was found to be related to its inhibition of digestive enzymes, especially chymotrypsin.

Studies on the immunoglobulin-G Fc-fragment receptor from neonatal rat small intestine

1. A method for preparing the small-intestinal brush-border membrane of neonatal rats is described in which enzymic methods are used to remove associated polysaccharide and cell nuclei. 2. 125I-labelled IgG (immunoglobulin G) and 125I-labelled IgG Fc fragment have high specific binding and low non-specific binding to brush borders prepared in this way. F(ab)'2 fragment however, does not bind, indicating the existence of a specific receptor for the Fc fragment of IgG. The receptor system is saturable, and the affinity (KA) for the binding of rat IgG was determined by both equilibrium and kinetic methods. 3. The binding of heterologous IgG species (human and bovine) was compared and demonstrated a close similarity between human IgG and rat IgG in their receptor affinities. 4. Kinetic results are presented that are consistent with previously proposed models of ligand-induced receptor aggregation.

Intestinal uptake and transport of proteins in the adult rat

The transport of immunoglobulin and ferritin across the intestinal mucosa of adult rats provides an excellent model for transcellular protein transport study. Intestinal uptake and transcellular transport have been extensively studied in the neonatal rat, but not to such an extent in the adult rat. The transport of 125I labelled bovine immunoglobulin G and ferritin was studied in 100 days old rats using intestinally administered proteins. Antigen was estimated in the tissues by reacting extracts against specific immune antiserum prepared in rats, and visualization studies were carried out by fluorescence microscopy and direct deposition autoradiography at electron microscopic level. From these studies, it can be seen that these proteins are taken up by the intestinal cells and transported, antigenically intact, across the barriers to the body organs.

The human yolk sac and yolk sac carcinoma. An ultrastructural study

The ultrastructure of the yolk sac of a 39 day old human embryo was studied. The subcellular organization was suggestive of a highly specialized absorptive function proceeding in an exocelomic-viteline direction. These findings, compatible with intense metabolic activity, are at variance with the concept of rapid involution of the yolk sac following completion of its hemopoietic and angiogenetic functions. The speculation is advanced that a potential avenue exists in the yolk sac whereby maternally derived products encounter fetal endoderm. Ultrastructural features in the normal yolk sac were compared to those existing in a tumor showing the "endodermal sinus" pattern, and reviewed in the light of the pertinent literature. These findings support the concept that attributes an endomesoblastic derivation to such neoplasms.

Role of the cell surface in selection during transport of proteins from mother to foetus and newly born

The transport of immunoglobulins from mother to foetus and newly born mammal involves selective events which are independent of molecular size, related to immunoglobulin class, structure, and species of origin, and involve considerable protein degradation. Such events are briefly described as background information to a discussion of how selection of proteins might take place during transport across the cellular barriers concerned, namely the yolk sac splanchnopleur, chorio-allantoic placenta, and small intesting. Until recently the Brambell hypothesis has been the most favoured explanation. This implies that selection occurs intracellularly, within endodermal cells of the yolk sac splanchnopleur and small intestine, and within the syncytiotrophoblast of the chorio-allantoic placenta, of certain species. It also suggests that specific receptors are present which give attached proteins protection from degradation when the vesicles containing them fuse with lysosomes; such protected proteins are then liberated from the vesicle by exocytosis. This hypothesis is examined in the light of what is now known about the mechanism of uptake and transport of proteins by the endodermal cells and syncytiotrophoblast. It is suggested that rather than being an intracellular event, involving protection from proteolytic degradation, selection takes place at the cell surface. Evidence is presented, some direct and some circumstantial, that proteins may be selectively endocytosed by coated micropinocytotic vesicles, and non-selectively endocytosed through a complex apical canalicular system leading to macropinocytotic vesicle formation. In the small intesting of the suckling rat these two processes appear to be segregated, selective uptake occurring in the proximal half and non-selective uptake occurring in the distal half. In the endodermal cells of the rabbit yolk sac splanchnopleur, and by implication in the syncytiotrophoblast of man and monkey, it is suggested that both selective, and non-selective, uptake of protein occurs. Non-selective uptake into macropinocytotic vesicles is regarded as an event leading to complete degradation of all contained protein and functioning so as to supply the foetus and newly born mammal with essential amino acids. Selective uptake into coated micropinocytotic vesicles is regarded as an event leading to the transport of immunoglobulins across the cell without any contact with lysosomes, and functioning so as to supply the newly born mammal with protection against invasive organism. Specific receptors are still required but only for the initial uptake and segregation of proteins into coated micropinocytotic vesicles. The role which the glycocalyx might have in such selective binding of proteins is considered and possible difficulties in characterization of specific receptors brought to light in view of the likely overwhelming need for non-specific binding to effect non-selective uptake.

Intestinal transport of antibodies in the newborn rat

Evidence has been reported that the proximal small intestine of the neonatal rat selectively transports antibodies into the circulation. This study describes the morphology of the absorptive epithelial cells in this region of the intestine and their transport of several immunoglobulin tracers: ferritin-conjugated immunoglobulins (IgG-Ft) and antiperoxidase antibodies. Cells exposed to rat IgG-Ft bound the tracer on the membrane of tubular invaginations of the apical cell surface. Tubular and coated vesicles within the cell also contained the tracer, as did the intercellular spaces. Uptake of tracer was highly selective and occurred only with rat or cow IgG-Ft; when cells were exposed to chicken IgG-Ft, ferritin-conjugated bovine serum albumin, or free ferritin, tracer did not enter the cell or appear in the intercellular spaces. Experiments with rat and chicken antiperoxidase showed a similar selective uptake and transport of only the homologous antibody. When cells from the distal small intestine were exposed to the tracers, all tracers were absorbed nonselectively but none were released from the cells. Cells from the proximal small intestine of the 22-day-old rat failed to absorb even rat IgG-Ft. A model is presented for selective antibody transport in proximal cells of the neonatal rat in which antibodies are selectively absorbed at the apical cell surface by pinocytosis within tubular vesicles. The antibodies are then transferred to the intercellular space within coated vesicles. Distal cells function only to digest proteins nonselectively.

The mechanism of intestinal uptake and transcellular transport of IgG in the neonatal rat

The transport of immunoglobulins across the intestinal mucosa of neonatal rats provides an excellent model for the study of transcellular protein transport. The mechanism of intestinal uptake and transcellular transport of plasma proteins has been studied in 12-14-day old rats using intraduodenally administered radioiodinated proteins. Appreciable quantities of rat IgG, mouse IgG, rabbit IgG, and all four subclasses of human IgG were taken up by the intestinal wall (19-54% of administered dose at 4 hr) and transported to the animal (10-35% of administered dose at 4 hr). In contrast there was little or no uptake of human IgM, IgA, and IgE and little or no transport of human IgM, IgA, IgD, IgE, albumin, transferrin, and ceruloplasmin. Both the uptake and transport of labeled IgG were significantly inhibited by unlabeled IgG. Further insight into the transport process was obtained from the observation that an appreciable proportion of the label of IgG in intestinal wall homogenates, but not in plasma or intestinal washings, migrated in a sucrose ultracentrifugation gradient much more rapidly than did the administered 7S molecules. This pattern was not observed with other proteins studied. This apparent binding of labeled IgG was also markedly inhibited by unlabeled IgG. In subcellular fractionation studies of intestinal homogenates the complexed labeled IgG was shown to be associated predominantly with cell membrane rather than cell sap fractions. In addition IgG could be shown to bind to purified enterocyte microvillous membranes in vitro. IT IS CONCLUDED THAT IN THE NEONATAL RAT: (a) the major processes involved in both intestinal uptake and transport of IgG are specific and saturable; (b) intestinal transport is associated with complexing of IgG molecules with membranes, most probably with enterocyte microvillous membranes; and (c) the part of the IgG structure involved in this process is probably similar to that involved in the concentration-catabolism effect but is not identical to that mediating other non-antigen combining functions of IgG. Our data are consistent with the existence of specific receptors for IgG on enterocyte microvillous membranes of the neonatal rat. Such receptors would be necessary for the specific uptake and transport of these molecules.

Immunity to Vibrio cholerae in the mouse. I. Passive protection of newborn mice

Mice were immunized subcutaneously with either killed cells or a ribosome-containing fraction (RF) obtained from Vibrio cholerae Ogawa 41. At appropriate time intervals, these mice or their progeny were challenged with uniformly lethal doses of Ogawa or Inaba serotype. Half of the offspring born to mice immunized with 20 mug of RF were protected against homologous challenge at 7.5 weeks of age, and significant protection was observed up to 15 weeks of age. Similar protection was observed with heterologous challenge, but the duration of protection was reduced. The duration of protection obtained in newborns was related to the quantity of RF given to the mother. Protection was transferred from mother to young via colostrum or milk. Protection was not due to transfer of antigen, as active immunity could not be induced in newborn mice immunized with RF.

The use of [125-I] polyvinyl pyrrolidone K. 60 in the quantitative assessment of the uptake of macromolecular substances by the intestine of the young rat

1. A method has been developed which allows the quantitative estimation of the uptake of labelled polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) by the wall of the small intestine of young rats.2. Four hours after feeding a standard dose of [(125)I]PVP by stomach tube, the small intestine was thoroughly washed out, and the radioactivity of the intestinal wall measured. Under these conditions, the small intestine of animals less than 18 days old took up more than 50% of the radioactivity which had left the stomach. There was no increase in PVP uptake if the duration of absorption exceeded 4 hr. The PVP was taken up by the epithelial cells of the villus, and its intracellular localization has been demonstrated by fluorescence microscopy and can be related to vacuolation in the cells.3. In animals between 18 and 20 days old the uptake of PVP declined progressively, until, in animals more than 20 days old, less than 5% of the radioactivity was taken up by the intestinal wall.4. There is good agreement between the reported age of termination of antibody absorption in young rats and the age at which PVP uptake ceased in the present experiments. It is suggested that the loss of ability of the intestine to take up substances of high mol. wt. may be the factor which limits the duration of the period of antibody absorption in this species.

The intestinal absorption of pig and bovine immune lactoglobulin and human serum albumin by the new-born pig

1. Homologous and heterologous colostral immune globulins and human serum albumin were fed to new-born pigs and an attempt was made to estimate the amounts appearing subsequently in serum.2. All three proteins, fed separately in large amounts to different pigs, appeared in the serum in low concentration about 45 min after feeding, and then rose quickly to a high level. No difference could be detected between the amounts absorbed when equal amounts had been fed but there was a wide variation between pigs. Previous dialysis of pig colostrum against bicarbonate saline did not affect the rate or amount of pig immune globulin absorbed after feeding.3. When pig and bovine colostral IgG were fed together at equal concentrations in bovine colostrum, the absorption of pig IgG was greater than that of bovine IgG. Human serum albumin, added to bovine colostral IgG in bovine colostrum, was absorbed readily and this did not interfere significantly with the absorption of bovine colostral IgG.4. The efficiency with which the pig intestine absorbed bovine colostral IgG depended on the dose and/or concentration fed, increasing as the dose fed was increased to 2 g and remaining constant for higher doses.5. Some of the absorbed immune globulin was shown to exist in a partly degraded form.6. The process of protein transfer across the intestine of the new-born pig may select, to a limited degree, between different proteins, but the digestion of protein shown to take place and the large variation between individual pigs makes interpretation of these results uncertain.

The in vitro transfer of bovine immune lactoglobulin across the intestine of new-born pigs

1. Everted sacs of pig intestine, used soon after birth, maintained transmural potentials and transferred water and glucose to the serosal surface.2. Immune globulin, fed as bovine colostrum to the new-born pig, appeared in the serosal fluid of everted sacs during incubation in bicarbonate saline. The particular segment showing maximum transferring ability varied between limits and appeared to depend on the amount or concentration of colostrum fed to the pig. Sacs from unfed pigs incubated in bovine colostrum also transferred colostral IgG to the serosal fluid. This transfer was dependent on the concentration of colostral IgG in the incubation medium and became more pronounced in the middle third of the small intestine.3. Human serum albumin inhibited the transfer of colostral IgG and about twenty molecules of albumin were transferred for every molecule of colostral IgG, when both were presented together in equal concentration on the basis of weight, to the middle segment of the small intestine.4. Some of the immune globulin collected in vitro after feeding bovine colostrum was found in a degraded form, but the amounts present could not be estimated. There was no apparent degradation of immune globulin in the purely in vitro experiments.5. The in vitro transfer of bovine colostral IgG showed selectivity between molecules of albumin and colostral IgG, the nature of which warrants further study.