Chapter 7, Part1

 

                                          Chapter 7. 

      The Secretory Immune System and the Placental Barrier

 

    7.1) The placental barrier: its morphology, function and pathology 

 

The placental barrier between the mother and her fetus is the main structure providing normal functioning and development of these two immunologically different organisms (1). The placenta contains fetal and maternal parts. The fetal part (villous and membranous) is composed of fetal tissues such as the trophoblast, amnion, mesenchyme and blood vessels carrying fetal blood. Aside from the  lymphocytes, the fetal part of the placenta contains many macrophages which are responsible for immune and non-immune phagocytosis, fixation of maternal antibodies  and production of cytokines (2).

 

The maternal part includes decidual tissues and intervillous spaces filled with maternal blood. The border between the maternal and fetal tissues passes not only between the trophoblast and the maternal blood but also at the contact surfaces of the trophoblast and decidual tissue (3). In the decidual tissues, about 10% of the cells are lymphocytes and about 20% are macrophages. These cells of maternal origin   are immunocompetent and participate together with the decidual cells in various immune reactions.

The concept of placental barrier was evaluated about 20 years ago using  advances in ultrastructural analysis and in transport physiology (4). Structurally, the barrier effect is grounded by the syncytiotrophoblast's continuity,   basal and plasma membrane  electrical charges and by basement membrane porosity. The continuity of the aqueous phase for diffusion operates through intercellular gaps, fenestrations (rat, rabbit) or transcellular channels (guinea pig). However, these connections are not apparent in the human syncytiotrophobast. In terms of molecular size selectivity, the human hemochorial placenta  with a pore radius of 10 nm appears much less selective than the epitheliochorial one in animals. The main structural and functional units of the placenta are the chorionic (trophoblastic) villi within which the fetal blood is separated by only three or four cell layers (placental membrane or barrier) from the maternal blood in the surrounding intervillous spaces (5). The metabolic capacity of the placental cells (trophoblast, macrophages) participates in the barrier effect by metabolizing or by converting some substrates.

Trophoblast specificity in the location of enzymes, carriers and receptors on the outer (maternal side) and on the basal (fetal side) plasma membranes, and in the release of secretory products, contributes to maintaining separate fetal and maternal compartments. Trophoblastic cells, macrophages, and perhaps also fetal blood cells can form a sequential barrier blocking maternally activated defense cells bearing CD95 molecules and thereby blocking anti-fetus CD95⁺ T lymphocytes from entering apoptosis (6).

Trophoblastic cells participate in the establishment  of specific interactions between the maternal and fetal circulation in the placenta. Mammalian embryos have an intimate relationship with the placental vasculature from which they obtain essential nutrients for growth.  The vascular bed exhibits interesting specificity because number and diameter of maternal vessels change dramatically during gestation and, in rodents and primates, the terminal blood space becomes lined with placental trophoblast cells rather than endothelial ones (7). The transition from endothelium-lined artery to trophoblast-lined (hemochorial) blood space is associated with giant trophoblast cells. The shaping of the maternal blood spaces within the labyrinth is dependent on chorioallantoic morphogenesis. 

The placenta is the highly specialized organ of pregnancy that supports the normal growth and development of the fetus. Growth and function of the placenta are precisely regulated and coordinated to ensure that the exchange of oxygen and nutrients between the maternal and fetal circulatory systems as well as removal carbon dioxide and other waste products operate at maximal efficiency (5,8,9).   Oxygen transport is limited by placental blood flow, but the transport of glucose and amino acids is determined by the abundance and activity of specific transport proteins. Glucose in the maternal blood passes freely through pores of the cytotrophoblast, transported into the cytoplasm of syncytiotrophoblast I via GLUT1, an isoform of facilitated-diffusion glucose transporters, then enters syncytiotrophoblast II through the gap junctions, and finally leaves syncytiotrophoblast II via GLUT1 and enters the fetal blood through pores of the endothelial cells (10).

Placental metabolism of glucose has major effects on both the quantity of carbon and nitrogen delivered to the fetus, and on the composition of the involved substrates. The placenta's capacity for glucose transport under moderate glucose deprivation is up regulated in part  by increased expression of the GLUT3 transport protein (8). During severe glucose deprivation however, placental transfer and fetal uptake of glucose are constrained in proportion with the maternal supply, leading to fetal growth retardation.

The placenta metabolizes a number of substances and can release metabolic products into the maternal and/or fetal circulation (11). The placenta can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. In addition, it releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many changes in placental function occur to accommodate the increasing metabolic demands of the developing fetus dueing gestation.  Some drugs are pumped across the placenta by various active transporters located on both the fetal and maternal sides of the trophoblast, and pinocytosis and phagocytosis are considered too slow to have any significant effect on fetal drug concentrations (11).

Placental inflammatory disorders represent a diverse and important category of pathological processes leading to fetal and neonatal morbidity and mortality. These processes can be divided into two broad subcategories, those caused by microorganisms and those caused by host immune responses to non-replicating antigens. The mechanisms by which these inflammatory processes cause death and disability are diverse and can be separated into four distinct classes: i) placental damage with loss of function, ii) induction of premature labor and subsequent preterm birth, iii) release of inflammatory mediators leading to fetal organ damage, and iv) transplacental infection of the fetus (12). Each specific inflammatory process can be modulated by properties of the specific organism, the route and timing of the infection and variations in the host's genetic background and immune responsiveness.

The main pathology of the placenta is the inflammation of the chorionic (trophoblastic) villi termed villitis. The inflammatory response in placental villitis of unknown etiology is characterized by the invasion of fetal villi by maternal T cells and associated with focal destruction of the syncytiotrophoblast  (13). Placentas with villitis exhibit significantly higher syncytiotrophoblast intercellular adhesion molecule-1 expression than placentas without villitis (14).

 

Villitis caused by Toxoplasma gondii,   Trypanosoma cruzi, or  Paracoccidioides brasiliensis is characterized by rupture of the trophoblastic barrier and influx of immune cells into the villi  (15). In some cases, placental toxoplasmosis is accompanied by granulomatous villitis (16). The immune response in  villitis  consists of maternal lymphocytes as the predominant intravillous population: mainly T cells (CD3, CD8 and CD4) and only rarely B lymphocytes (17). CD68+ macrophages and CD8+ T lymphocytes make up the major portion of the cell population in villitis caused by T. cruzi (18).

Placental mesenchymal dysplasia is found in association with intrauterine fetal death. In  severe cases,  the placenta is markedly enlarged (up to 1,050 g), and approximately 80% of it is occupied by extraordinarily enlarged villous structures with a myxoid appearance (19). Histologically, the dysplastic villi have myxoid stroma and a decreased number of fetal vessels, which are occasionally seen to be obliterated. There is no abnormal trophoblastic proliferation. Large-sized fetal vessels in the chorionic plate frequently contain  organized thrombi.

Placental hemorrhagic endovasculitis is found in association with stillbirth and with abnormal growth and development in live births (20). Lesions occurring with significant frequency in such placentas include villitis of unknown etiology, chorionic thrombi, villous fibrosis, erythroblastosis, and meconium staining. The segmental pattern of villous fibrosis and the high incidence of growth restriction, erythroblastosis, and meconium suggest a chronicity of adverse intrauterine events that may precede fetal loss.

 

There is a direct link between parasitic malarial infection of the mother and syncytiotrophoblast damage (21). Placentas with active malaria infection showed erythrocyte adhesion of infected cells to the syncytiotrophoblast, syncytial degradation, increased syncytial knotting and  localized destruction of the villi. Past malarial infection is characterized by syncytiotrophoblast disruption and fibrin-type fibrinoid (FTF) deposition. Perivillous FTF deposition is consistent with increased syncytial lesions, and both increased lesions and syncytial knots have been associated with reductions in birth weight. Syncytial destruction could have serious implications, impairing fetal growth and, in some   cases,  providing a previously unrecognized pathway for congenital infection.

 

Massive chronic intervillositis is a  placental lesion associated with malarial infection.  It is characterized by a prominent inflammatory infiltrate in the intervillous space, composed mainly of monocytes and macrophages which can simulate a maternal malignant disorder involving the placenta (22).

 

 

 

7.2)            The role of SC and J chain in maternal immunoglobulin transport  

                                         through the placental barrier 

 

Transfer of Igs through the placenta is considered here as an example of the presence of the barrier portion of the SIS. Maternal Igs are important   in the immune protection of the embryo/fetus, especially in the first trimester, when the embryo's immune system   has not yet developed. Ig transport  is already seen in early embryos, at 3.5-4  to 5 weeks (23). SC, J chain and Igs are located in  both layers of the trophoblast in a very similar manner, which may help them unite into one complex, that is unique to the trophoblast (Fig. 10).

Although it is accepted that maternal Igs are able to pass through the decidua (maternal part of the placenta) to the amnion and  can reach the fetus' lungs and intestines via the amniotic fluid, it has been claimed that only IgG passes through the placenta and that all other types of Igs (IgA, IgM, IgD) do not  (24,25).  However in some pathological cases, such as cytomegalovirus, Toxoplasma gondii and rubella, transport of IgA from mother to   fetus has been reported (25,26). Prematurity and LBW are associated with impaired placental transfer of IgG subclasses 1 and 2 (27). Maternal IgE has also been found in the umbilical blood of  newborns whose mothers had allergies  (28). Fetuses begin synthesizing their own Igs  after the ninth week  of intrauterine development   (29,30).

 

Igs have to pass three different parts of the fetal portion of the human placental barrier during their transport to embryos/fetuses (31-34). The first part, the trophoblast (the fetal part of the placenta) with its two layers, the cyto- and syncytiotrophoblast, contains SC. In embryos without the effect of foreign antigens, SC was found either in both layers of the trophoblast  or accumulated in one of them (Fig. 10).   J chain, IgG and IgA were found in both layers of the trophoblast, corresponding with changes in SC accumulation. IgM was sometimes found in the apical microvilli of the syncytiotrophoblast immersed in the maternal blood lacunae. The second part of the placental barrier is the stroma of the  trophoblastic villi.  In non-infectious aborts, about 60% of  the villi contained   IgG, 25% contained   IgA, and about 12% contained  IgM. Fcγ receptors were not seen in half of the embryos but were found in the fetuses. Capillaries,  the third part of the placental barrier, were already detected in embryos in each of the trophoblastic tertiary villi, 3 to 22 µm from the basal membrane of the trophoblast. In second-trimester fetuses,  they often contacted  the basal membrane. In third-trimester fetuses, such contact was rare but the  number of capillaries rose. At all stages studied, a weak reaction to SC receptors was sometimes noted in the endothelium of the capillaries.

 

Igs can be transported from the stroma of the trophoblastic villi through the endothelium of the blood capillaries, which contains the receptors FcγRII (35) and FcRn  (neonatal Fc receptors) (24).  The close contact between the basal membrane of the cytotrophoblast and the capillaries (35,36) aids in transporting Igs into the fetal blood. Embryonic erythroblasts, along with the other nucleus-containing blood cells (monocytes, lymphocytes, leucocytes, etc.), are able to catch Fc  receptors (37,38),  and  a  loss of this ability  parallels the loss of nuclei in the erythrocytes after week 9 of intrauterine development.  The presence of the Ig-containing erythroblasts allows us to consider these cells as   part of the cellular Ig-transport system in embryos. The finding of Igs in the  blood plasma in the fetal capillaries (39) indicates the manner in which  Igs are transported in the bloodstream.   

           

Our observations have shown that in normal pregnancy,  mainly IgG passes through the placental barrier (40). Transport of IgA was seen in 78% of the samples, whereas no transport of IgM  was detected.  In cases with moderate inflammation of the birth canal, transport of IgG and IgA, and to a lesser extent  IgM,  increased. In cases with severe inflammation, transport of all types of Igs increased, with  IgG showing the  highest level.

 

In all embryos and fetuses  with moderate inflammation of the birth canal,  the permeability of the placental barrier was seen to increase (41).   More than 82% of the villi contained IgG, 34% of them contained  IgA, and  18% contained  IgM (Fig. 11). The number of CD68+ monocytes increased sharply (Fig. 12, 13).  The concentration of Fcγ receptors  increased  from 5% to 25% in embryonic monocytes  to 90% in fetal cells after week 13.  J chain was seen in 63% of all monocytes; some of them contained  IgG.  In embryos  with acute infections and severe inflammation of the birth canal,   distinct reactions to SC, J chain, IgG and IgA, and a weaker one to IgM were seen in all cases in the cytotrophoblast and in the apical microvilli of the syncytiotrophoblast.  In erythroblasts, SC was not seen but J chain, IgG, IgA and, rarely, IgM were found. 

           

Transport of Igs from the placenta into the fetal blood is also performed by monocytes till week 9. Monocytes  are located close to  the  basal membrane of the cytotrophoblast, in the Ig-impregnated stroma, and near capillaries  (42,43). This location allows them to catch Igs from the basal membrane of the trophoblast or from the tissue fluid in the stroma, and to transfer them  to the capillaries. Monocytes contain a complex of Fcγ receptors (35,39), Fcą  receptors (35,43) and, possibly, the Fc receptor for IgM, because this last receptor has been found in the cytoplasm of monocytes. Fc receptors located on monocytes catch Igs with their Fc fragments and build the immune complexes  (39). The SC and FcRn are absent in monocytes (23,35). 

           

 

 

 

 

 

 

 

 

 

 

(Color Fig.)

 

 

Fig. 10.

A. The   placenta of a 13-week-old fetus with  low antigenic effect.   Note the fetal SIS contents of SC (brown staining) in syncytio- and cytotrophoblast, and in arterial myocytes (an arrow). Macrophages (heads of arrows) do not contain SC. x400.

B.  The   placenta of a 23-week-old fetus without an antigenic effect.  Note IgA in the cyto- and syncytiotrophoblast, in the endothelium of blood capillaries, in the stroma of chorionic villi and monocytes. This indicate on transport of IgA throughout the placental barrier. x200.

C. The placenta of a 21-week-old fetus with   moderate antigenic effect.  Note IgM in the chorionic villi (v), in the invasive trophoblast (it), and in the deciduas (d). In the villous stroma IgM is absent. x200.

D. The placenta of a 20-week-old fetus. Chorioamnionitis. Fetal part of the SIS: note a large amount of CD68⁺ macrophages (brown staining) in chorionic villi. x200. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 11. The number of tertiary chorionic villi which stroma contains Igs (% of the total number of tertiary villi). Material contained embryos of 3,5 to 8 weeks of gestation and fetuses of 9 to 12 weeks. Groups of patients studied: I, without inflammatory; II, with moderate inflammatory; III, with severe infection of the birth canal. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 12. The number of macrophages containing Igs (% of the average number of CD68+ monocytes). Groups of material studied, see footnotes to Fig. 11. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 13. The average number of monocytes in villi per 50,000 nm² in embryos (E, 3.5 to 8 weeks of gestation) and fetuses (F, 9 to 12 weeks). Groups of material studied, see footnotes to Fig. 11. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In early human embryogenesis, at 3.5-4 to 5 weeks, Fcγ receptors were seen in 57% of the monocytes (23). The role of monocytes in the transport of Igs appears to be quite important.  The number  of monocytes in the trophoblastic villi,   even in normal embryos, is significantly higher than the  number of macrophages in the decidua. At term, the number  of monocytes  reaches 81% of the total number of CD68+ cells, and they show  a strong reaction to FcγRIII receptors  and Igs.   In cases with infections, the number  of monocytes increases sharply. Monocytes, together with Igs, pass  between the endothelial cells into the capillaries and via the bloodstream, reaching all of the organs and tissues of the embryo/fetuses. 

 

Although many reviews have been devoted, in recent years, to the study of  transcellular  Igs  transport   (44-46),  opinions   differ with respect to the role of J chain in this process.  Some think it plays a key role in  involving  polymeric (p)Igs in  transepithelial transport and exocrine secretion (45). Others have written about the non-essential role of J chain in Ig transport (47), and still others do not even mention J chain in their description of this process (48). The mechanism of interaction between J chain and pIgR/SC, and the biological function of the former remain unknown.

J-chain participation in transcellular transport of Igs should be considered in terms of  the three phases of this process: endocytosis, transcytosis and exocytosis (23).  The heart, most of the endocrine glands, gonads, brain, the epithelium of the renal canals and capillary endothelium, etc., were found to be among the organs in human embryos and fetuses whose parenchyma cells contain J chain and Igs and are free of SC. The presence of Igs in the cytoplasm of these cells, which are not able to synthesize Igs themselves, indicates the presence of endocytosis, i.e. Ig internalization, without the participation of SC.

Fc receptors can carry out the SC function in endocytosis by joining to the Fc fragment of Igs.  Fc-ąR, Fc-γR, and others have been described in this respect (49-51). FcRn participates in the transport of IgG through the cyto- and syncytiotrophoblast  of the placenta (52) and through some types of the epithelium   (53-55). The role of J chain in endocytosis with the participation  of Fc receptors has not been described, and the mechanism underlying their interaction as well as the relationship between  J chain and SC remain elusive (45).

Our morphological observations indicate that in cells containing J chain and free of SC the process of exocytosis does not occur. Igs exocytosis in SIS organs  containing SC and J chain, such as the epithelium of the stomach and intestine, acinis and pancreatic ducts,   is accompanied by a sharp decrease in the immune reactivity of both Igs and SC (30). In contrast, the immune reactivity of J chain does not decrease.

 

        7.3) The secretory immune system as  part of the placental barrier

 

The placenta contains the SC, IgA and  IgM (56,57), components of the SIS which are characteristic for adults and protects the organism against invading microorganisms and foreign antigens. In human fetuses, the SIS has been found in the third trimester of gestation (58,59) and  involved in immune disturbances in fetuses of ill mothers (56,60). The role of the SIS in the placental barrier has been described elsewhere (2).

In our studies of the placentas of 32 human fetuses who died during the second trimester of pregnancy (weeks 13 to 25) of different causes, such as abruptio placenta, placenta praevia, chorioamnionitis, or abortion for medical or social reasons, the material was divided into three groups (2). Group I consisted of seven placentas with little lymphoid-macrophageal infiltration into the tissues.  Group II consisted of 12 placentas with moderate infiltration that reflected weak or moderate immune processes in the placenta. Group III included 13 placentas with acute chorioamnionitis and deciduitis, complicated by severe foreign antigenic effects such as fetal sepsis  or pneumonia (61).

In the fetal part of the placentas with low and moderate lymphoid-macrophageal infiltration (groups I and II),  SC and J chain were consistently found in the cytoplasm of the cyto- and syncytiotrophoblast of the chorionic villi, in the epithelium of the amnion, and in the arterial myocytes of the umbilical cord  and stem villi. IgA and IgM were also detected in the cytoplasm of trophoblast  but their immunoreactivity in the myocytes was weaker than in the other structures. IgG was found in the cyto- and syncytiotrophoblast. There were few lymphocytes, and about half of the B lymphocytes were positive to IgA or IgM  (Table XIV). Macrophages were abundant in placentas with low infiltration, and even more so in those with moderate infiltration (Table XIV). They contained J chain but not SC. From 50% to 55% of macrophages contained IgG, but no IgA or  IgM were found.

 

In the placentas with acute chorioamnionitis (group III), the rate of immunoreactivity and the distribution of SC  and J chain were similar to those found in the other two groups. However, the reactivity of IgA, IgM and IgG in the chorionic and amniotic epithelia was low. Whereas the number of lymphocytes was  similar to that in the other  groups, the number of macrophages increased and the amount of IgG-positive macrophages decreased sharply (Table XIV). Inflammation was seen in the fetal membranes.

Table XIV.  Cellular composure of infiltrates in the fetal part of the placenta

(No. of cells/50,000 µm²) (mean ± SE). (After ref.  2)

 

^a Significantly different from group I, p<0.01

^b Significantly different from group II, p<0.01

 

 

 

Table XV. Cellular composure of infiltrates in the maternal part of the placenta

(No. of cells/50,000 µm²) (mean ± SE).  (After ref.  2) 

 

^a Significantly different from group I, p<0.01

^b Significantly different from group II, p<0.01