Review
A review of immune transfer by the placenta
Introduction
The placenta is a transient organ that is present only during pregnancy. Its functions are responsible for protection, nutrition, respiration and endocrine control (King, 1982). Moreover, it is of great immunological importance in the transfer of antibodies, tolerance and regulation of fetal development, release of cytokines, and in the helper and cytotoxic subpopulation of lymphocytes (Junqueira and Carneiro, 1999, Michelon et al., 2006).
The term “placental barrier” or “placental interface” refers to the number of tissue layers interposed between maternal and fetal circulation systems (Leiser and Kaufmann, 1994). Using this information, one can classify placentas in accordance with the criteria shown in Table 1.
Currently, the placenta is being used extensively as a model in the study of transplant immunology. This is because the fetus is regarded as semi-allogeneic tissue; i.e., it possesses parts of both the maternal and paternal genetics. However, genetic expression leading to placental development and function in mammals is epigenetically influenced (Bressan et al., 2009).
The placenta can transfer various substances from maternal circulation to the fetal circulation. These include, for example, oxygen, water, electrolytes, carbohydrates, lipids, proteins, vitamins, hormones, and some drugs. In addition, carbon dioxide, water, hormones and residual metabolism products can pass from the fetoplacental circulation to the maternal circulation (Junqueira and Carneiro, 1999).
In addition to the type of placenta, the transfer of maternal antibodies during gestation also varies according to species (Enders and Blankenship, 1999, Marques et al., 2007b). As reported by Porter (1976), immune transfer and placental permeability are inversely proportional to the number of tissue layers interposed between the maternal and fetoplacental circulation systems.
Brambell et al. (1949) described the first evidence of immune transfer in the hemochorial placenta approximately 60 years ago. They suggested that the transfer of antibodies from mother to fetus occurred via the vitelline sac (Brambell et al., 1949).
Passive immunity acquired by the fetus via the placenta is essential for adaptation of the neonate to the extrauterine environment. The maternal antibodies transferred via the placenta grant the newborn protection against infectious agents during the first months of life (Tizard, 2009).
For animal species whose placenta functions as a barrier for antibody transfer to the fetus during gestation, passive immune transfer is dependent on the ingestion and absorption of colostrum (Giguère and Polkes, 2005, Crisman and Scarratt, 2008). Failure to accomplish this transfer can bring about a high infection risk in neonatal mammals. This increases morbidity and mortality, especially with respect to neonatal infectious diseases (Giguère and Polkes, 2005).
Based on the aforementioned factors, the objective of the present study was to carry out an extensive literature review of the different types of placentations and placental barriers. The mechanism of antibody transfer associated with each is described, as are factors that can influence the immune transfer process.
Section snippets
Epitheliochorial and synepitheliochorial placentation: immune transfer
In epitheliochorial placentation, there are minimal changes in the stroma of the uterine mucosa during pregnancy, apart from local angiogenesis, which is needed to increase the blood flow and deliver nutrients to the uterine surface. Trophoblast cells can attach (and even fuse with) the surface epithelium of the uterus but there is no invasion by the trophoblast cells (Moffett and Loke, 2006).
Fetal and maternal tissues interdigitate extensively, yielding a large surface contact area. The
Endotheliochorial and hemochorial placentation: immune transfer
In these two types of placentation, trophoblast cells are always the outer most layers of fetal cells and overlie an inner core of mesenchyme and fetal capillaries. Trophoblast-cell infiltration through the surface epithelium of the uterus is characteristic. For example, trophoblast cells can migrate to abut maternal blood vessels (in endotheliochorial placentation). The most invasive form is seen when trophoblast cells infiltrate through the maternal vessels to come into direct contact with
Placental transport of antibodies in humans
The active transport of immunoglobin G through the placenta starts early, during the first 12 weeks of pregnancy and it increases proportionally until the end of pregnancy. Pregnancy age has influence on the levels of total IgG of the umbilical cord, being present a linear correlation between them. Around 32 weeks of pregnancy, the detectable levels in the newly born are approximately 400 mg/dl, with possible values greater than 1.000 mg/dl in the newborn, being sometimes higher than the maternal
Factors influencing the transplacental transport of antibodies in humans
Apart from Fc receptor function and density, several other variables may influence IgG transplacental transport. Transplacental transfer of antibodies is facilitated, as demonstrated by Kohler and Farr (1966), but, as noted above, the process is more efficient later in gestation, such that prematurity is one of the situations in which low levels of IgG antibodies are observed at birth (Carvalho et al., 1988). However, some, but not all investigators also have noted reduced circulating IgG
Conclusion
The hemochorial and endotheliochorial placentations allow the passage of antibodies by the placenta, though this passage can be influenced by numerous factors. In contrast, the complex barrier between the maternal and fetal circulation in epitheliochorial and synepitheliochorial placentas limit or obviated antibody transfer. Thus, in species with these types of placentation, immune transfer occurs exclusively via the colostrum and nourishment that must be given to a newborn within the first 48 h
Acknowledgements
To FAPESP and Surgery Department and Post-graduate Program of Anatomy of Domestic and Wild Animals, FMVZ-USP, São Paulo, SP.
To Phelipe Favaron for his contribution in the making of diagrams which show the different types of placentation.
To Rose Eli Ricci for the correction and help in the wording of the article.
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