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.     

The insensitivity of the developing rat foetus to the toxic effects of acrylamide.

Acrylamide was given to pregnant rats either in a single dose or by feeding acrylamide in the diet. No adverse effects were seen in the offspring, even at doses which produced neuropathy in the mothers. This insensitivity is not due to failure of acrylamide to cross the placenta.     

Lactoferrin-binding proteins of Tritrichomonas foetus.

Tritrichomonas foetus is a common, sexually transmitted, protozoan parasite of cattle. It has an essential requirement for iron, which it obtains from host lactoferrin. However, specific lactoferrin-binding protein receptors have not yet been identified in T. foetus. To differentiate specific and nonspecific binding of lactoferrin, lactoferrin affinity chromatography and Western blotting was used to identify metabolically or surface-labeled T. foetus lactoferrin-binding proteins. Bovine lactoferrin was shown to bind more efficiently than human lactoferrin, and each of these bound much better than bovine transferrin. This is relevant because T. foetus is both species-specific and only infects the mucosal surface of the reproductive tract, which has little transferrin. Whereas the majority of lactoferrin binding was specific, competitive inhibition studies showed that nonspecific, charge-related binding of lactoferrin to T. foetus may also be involved. In the presence of bovine cervical mucus, binding of lactoferrin to T. foetus was diminished, suggesting that mucus has an effect on lactoferrin binding. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of surface biotinylated proteins affinity-purified on lactoferrin-Sepharose showed biotinylated bands at Mr values of 22, 49, 55, 72, and 155 kDa. Because lactoferrin-binding proteins may be susceptible to digestion by T. foetus extracellular cysteine proteinases, it is suspected that the 155-kDa protein is the specific lactoferrin-binding protein and that the lower-Mr lactoferrin-binding molecules may be fragmentation products that contain the lactoferrin-binding site; however, other interpretations are clearly feasible. It is possible that there may be multiple proteins or multimers of the same protein. In summary, the data showed that binding of lactoferrin to T. foetus may be regulated by an interplay of specific receptor interactions as well as by hydrophobic and charge-related interactions.     

Dynamics of antibody transfer from mother to fetus through the yolk-sac cells in the rat

In rodents, maternal immunoglobulins are transported intact by the yolk-sac visceral epithelium from mother to fetus. The main purpose of the present paper is to study the dynamics of the uptake and transport of immunoglobulins by the rat yolk-sac using a new experimental design. The results show the rapid binding of IgG to the cell membrane microvilli since only 30 sec were sufficient for this attachment to occur. The endocytic process also appears to be very fast as localization of IgG in clusters, pits and microvesicles were observed after 5 min of contact between the yolk-sac and the IgG solution. Moreover, the antibodies were detected in the intracellular spaces within 15 min of incubation.     

Immune relations between mother and foetus in the viviparous poeciliid fish Xiphophorus hellerii Haeckel

Advanced embryos of the viviparous poeciliid fish Xiphophorus hellerii (swordtail) were removed from the site of gestation within the ovary and re-implanted into the peritoneal cavities of unrelated immunologically competent adults of the same species. Most of those transplanted with intact fertilization membranes were normal in appearance, and many were clearly still alive. Those transplanted without a fertilization membrane were all dead, with lymphocyte infiltration and other evidence of rejection as allografts. The integrity of the membrane in late pregnancy was indicated by swelling of follicles in hypoosmotic liquid. It was concluded that the fertilization membrane is capable of protecting an antigenically foreign embryo in an immunologically hostile environment, and that it probably serves this function during normal pregnancy.     

Immune relations between mother and foetus in the viviparous poeciliid fish Xiphophorus hellerii Haeckel

Ectopic implantation of Xiphophorus causes the accelerated rejection of subsequent grafts of antigenically similar adult tissues; conversely, prior transplantation of adult tissues causes the accelerated rejection of subsequent grafts of related embryos. Xiphophorus embryos carry adult histocompatibility antigens before birth; the failure of the embryo to elicit a maternal immune response during gestation is not due to antigenic immaturity.     

Transplacental transfer of a growth hormone-releasing hormone peptide from mother to fetus in the rat

Previous studies showed that when growth hormone-releasing hormone (GHRH) was administered to either pregnant rats or pigs as a plasmid-mediated therapy, pituitary weight, somatotroph and lactotroph numbers, and postnatal growth rate of the offspring increased. To determine if these responses resulted from direct effects of GHRH on the fetus or were secondary to effects incurred in the mother, we studied in the rat the transplacental transfer of a GHRH analog (HV-GHRH) to the fetus from the maternal circulation. For the in vivo study, HV-GHRH was labeled with 125I and purified by reverse-phase high-performance liquid chromatography (HPLC). At 18 days of gestation, pregnant dams were administered a priming intravenous dose followed by a constant infusion of the labeled peptide. Approximately 2 days later, intact [125I]-HV-GHRH was isolated from the fetal liver, stomach contents, and brain. The amounts of tracer were positively correlated with those present in the corresponding dam's plasma. These data suggest that a GHRH analog of nonplacental origin, even at physiologic concentrations, can cross the placenta and, therefore, has the potential to influence fetal pituitary development directly.     

Turnover of liver glycogen in the rat foetus.

Incorporation and release of the radioactivity in the liver glycogen of 18.5- and 19.5-day-old rat foetuses were studied after intravenous injection of E11-14C]glycerol. Incorporation occurred during 1 h after injection of the radioactive tracer to the foetus; then, the incorporated radioactivity decreased. Glycogen content in the liver, and glycogen phosphorylase and glycogen synthase were not modified during the experiment. It is therefore postulated that a physiological turnover of glycogen exists in the liver of the rat foetus.     

The passage of radioactive lanthanum from the biliary to the vascular system

Summary Radioactive lanthanum nitrate, an electron opaque tracer, was injected into the common bile duct of rats. Two minutes following the end of injection, samples of blood for radioactivity counts, and of liver and kidney for electron microscopic studies were taken. High levels of radioactivity found in the blood, and demonstration of lanthanum in the kidney by electron microscopy, indicated that this tracer entered the blood stream in vivo. No lanthanum was seen in the cytoplasm of liver cells, and there was no evidence of rupture in bile ducts, junctional ducts, or liver cells. Tight junctions connecting parenchymal cells and cholangiolar cells appeared well preserved. Lanthanum was seen in bile canaliculi, interspaces between liver cells, portions of the extracellular space of the liver, lumina of cholangioles and lumina of portal venules and sinusoids. It is postulated that lanthanum passed from the biliary tract, the site of injection, through the tight junction between liver cells and cholangiolar cells. It is suggested that such passage may represent a physiologic pathway, but the possibility of a chemical action of lanthanum on the tight junction can not be ruled out.This study was supported by the A. D. Williams Fund of Virginia Commonwealth University and the U. S. Veterans Administration. Radiation Counting Equipment was loaned by Dr. F. O'Foghludha, Department of Radiation Physics, Virginia Commonwealth University.     

The passage of macrophages and lymphocytes from the interstitium across the lymphatic endothelium of rat lacteals

Summary The passage of cells across the lymphatic endothelium of rat lacteals in both normal and non-pathological experimental conditions (fasting, lymphatic stasis) was studied by means of serial thin sections and three-dimensional models. Two different pathways of transendothelial migration were observed: (1) macrophages enter the lymphatic lumen via the cytoplasm of endothelial cells, without involvement of intercellular junctions, whereas (2) lymphocytes migrate through intraendothelial channels, dynamic structures organized by the lymphatic endothelium under physiological conditions.