Chapter 3. 

 

Immune Protection of Vitally Important Organs and Cells in Human   Embryos     and Early Fetuses

 

Whether early human embryos have immune protection remains an open question because they do not have immune competent cells (except monocytes) or immune (lymphoid) organs. The common immune system only starts to form at the very beginning of the fetal period. However,  protein components of the  SIS, such as SC, J chain and Igs, are already present in the epithelial structures of 3.5- to 4-week-old embryos (1). During organogenesis, the  primary structure  of some organs changes dramatically, and they lose their connection with the superficial epithelium and the organ lumen. As a result, such organs do not secrete Igs, and  the SC disappears (2). However,  in some of these organs, especially endocrine glands, neurons of the brain and spinal ganglions, gonads, the myocardium and adrenals, Igs and J chain have been described despite the absence from the start of the SC (3,4).

 

 3.1) Immune components in fetal endocrine glands and their precursors

 

Endocrine gland function during intrauterine life is very important for fetal survival and development. Inadequate immune protection of the endocrine system during this period can result in serious disruption of fetal development (5,6). The presence of SIS components in endocrine gland cells and their ontological precursors has been described from as early as week 3.5- to 4 of embryonic development (3). In 3.5-4- to 7-week-old embryos which had not been exposed to massive antigenic influences, SC, J chain and IgG were observed in 82% to 98% of the cells from the epithelia of the oral cavity, pharyngeal gut, thyroglossal duct, Rathke’s and pharyngeal pouches, and pancreas, while the concentration of IgA and IgM was weak or nonexistent. The adrenal cells were positive for J chain, weakly positive for IgG and negative for SC, IgA and IgM. Lymphocytes were not present in the stroma of all these organs, except for the occasional single cells (Table II).

 

The anterior portion of the pituitary body (hypophysis) of 8-week-old embryos and 9-week-old fetuses has a tubular structure and is lined with multiple layers of epithelium (3). Of these cells, which are derived from Rathke’s pouch, 53% to 68% were found be strongly positive for SC. In second-trimester fetuses,  the SC was seen in 15% to 23% of these cells, and in the third trimester - in 5% to 8% of them. In glandular cells of the pituitary pars intermedia during the second and third trimesters of gestation, SC was seen in 68% to 88% of the cells, but no SC was present in the neurohypophysis. J chain, IgG and IgA were observed in all epithelial cells of the pars anterior and intermedia of the hypophysis, and only weakly in the neurohypophysis. In a few instances, a weak positive reaction to IgM was observed.

The functioning of the neuroendocrine system in fetuses is mediated by cytokines (7). Different interleukins (IL), tumor necrosis factor (TNF-ą) and interferon (INF-γ) affect the secretion of hypothalamic and anterior pituitary hormones, and specific high-affinity receptors for IL-1, IL-2, and IL-6 have been identified in neuroendocrine tissues. IL-1 and  IL-6  are present in the hypothalamus as well as in the anterior and intermediate lobes of the hypophysis indicating their paracrine role in the regulation of neuroendocrine functions. 

System ontogenesis of endocrine organs begins with the appearance and histological development of the thyroid and pituitary glands followed by the development of the hypothalamus and the pituitary portal vascular system (8). The thyroid gland starts to develop during weeks 3 to 4 of gestation, and  at weeks 7 to 9, consists of trabecular and alveolar groups of epithelial cells (3), 97%   to 85%  of which  contain  SC and J chain  and are weakly positive for IgG, IgA and IgM. When its follicular structure becomes defined at 10 to 11 weeks, the follicular epithelium is positive for SC, J chain and Igs. The follicular colloid, however, is more immunopositive for IgG, IgA and IgM than the follicular epithelium, and negative for SC and J chain. Hypothalamic-pituitary control of thyroid function matures during the last half of human fetal development. The parathyroid glands,   formed during weeks 4 to 7 of gestation from the endoderm of III and IV pharyngeal pouches,   contain J chain and Igs in their chief cells, and a weak positive reaction was observed for SC  in 43% of the cases studied (3).

From the time of their appearance during week 5 of gestation, epithelial cells of the pancreatic acini and ducts, are strongly positive for SC, J chain   and Igs (3). Islets cells, which develop from acinar cells at the beginning of the second trimester of gestation,  are negative for SC  and positive for J chain and Igs. Both epithelial and mesenchymal pancreatic cells express chemokine receptors, suggesting their role in leukocyte recruitment and perhaps in early pancreatic development. Mature macrophages have been demonstrated in the pancreas of  6-week-old human embryos and 12-week-old fetuses (9,10).

The adrenal glands appear  during week 6 of gestation in the form of clumps  of coelomic epithelial cells of mesodermal origin. At week 8, these cells multiply intensively and form two zones of the  cortex: a definite zone under the capsule and a more deeply located embryonic or fetal zone. J chain and Igs are seen in the embryonic zone.  In the second and third trimesters, a weak  positive reaction to  J chain, IgA, IgG, and sometimes for IgM is seen (3). Reaction  to SC is always negative. 

 

An increasing number  of immunocompetent cells can be seen in the various endocrine glands as development progresses.  Lymphocytes secreting IgA and IgM appear after weeks 9  to 10 of pregnancy (Table II). Massive antigenic exposure  causes little change  in the distribution and immunoreactivity  of SC and J chain in the endocrine cells. However,  Ig immunoreactivity declines, especially if infection occurrs during the second or third trimesters. A parallel decrease is observed in the number of Ig-positive endocrine cells. In the thyroid, for example, Ig-positive follicular cells amounted to less than 12% in fetuses exposed to infections, compared to 85% to 97% in unexposed fetuses (3). In the pancreas of infected fetuses, Igs were found in 45% to 59% of the islet cells and in 2% to 5% of the acinar cells, as compared 69% to 88% and 53% to 62%, respectively,  in unaffected fetuses. Chorioamnionitis results in a reduced number of different  subsets  of lymphocytes in the stroma of the endocrine glands. In contrast, a higher number of lymphocytes that secrete IgA and especially IgM are present in the regional (cervical and retroperitoneal) lymph nodes in infected fetuses than in their uninfected counterparts  (2.8-3.9/50,000 µm²   and 0.1-0.4/50,000 µm², respectively).

 

Not all SIS components appear to be at the same extent  in different endocrine organs. J chain  is constantly present in all endocrine cells of developing fetuses and in the precursor cells of embryos. The content and reactivity of Igs are changed with fetal maturation and with the establishment of the endocrine glands' functional status. Four to 7-week-old embryos do not have their own Ig-producing lymphocytes, and all  endocrine gland precursors contain maternal IgG or IgA which has passed through the placental barrier (11),  i.e. the embryonic SIS functions via maternal antibodies.  In fetuses of the second and third trimesters, Ig-immunoreactivity decreases, especially in the adrenal glands. 

 

The third protein component, SC, is present in some of the endocrine glands (thyroid, pars intermedia and sometimes the anterior lobe of the hypophysis, and the pancreatic islets)  and consistently in cells of the endocrine gland precursors (3). The various manifestations of SC in different endocrine glands are closely related to changes in the organs’ cellular activity during intrauterine life. Cells of the pancreatic acini and ducts that perform excretory functions, including excretion of various Igs, consistently  contain SC. Pancreatic islet cells perform only endocrine functions and contain no, or only trace amounts of SC. These cells have lost the ability to secrete Igs,   storing them instead in the cytoplasm. This is very pronounced in cases in  which massive antigenic stimulation causes large secretion of Igs from acinar and ductal cells containing SC, as witnessed by their negative reaction to Igs; however this event has no influence on the Ig  content of the islet cells.

 

A similar SIS-component pattern is found during the transformation of the Rathke’s pouch epithelium into cells of the adenohypophysis. During this process, loss of cellular excretory function is accompanied by the loss of SC. Thus, the absence of SC and the storage of Igs in pancreatic islet and adenohypophysis cells are related events. However, other endocrine-organ  structures, such as the epithelial cells of the pars intermedia of the hypophysis and of thyroid follicular cells, contain SC during their entire intrauterine life and still preserve the ability to secrete Igs. After secretion, these Igs are stored in the follicular colloid, which contains more of them than the follicular epithelium. These data reflect the close relationship between the presence of SC in endocrine cells and their capacity for exocrine secretion of Igs.   In adrenals, the absence of SC is, perhaps, connected with the mesenchymal origin of the cortical cells.

 

Accumulation of Igs in cells of the main endocrine glands may act as a local protective mechanism against foreign antigens. The decreased   Ig-immunoreactivity in endocrine cells following chorioamnionitis supports this assumption. Protein components of the SIS are detected as early as one month into intrauterine embryonic development, and are present within the endocrine organs for the remainder of the gestational period. Thus, the SIS in the endocrine organs appears and acquires  functional activity much earlier than the common (systemic) immune system in its organs (thymus, spleen, lymph nodes) (12).

 

To summarize,     two main types of the immune protection of the endocrine glands can be recognized  during their intrauterine development. The first is the SIS, the components of which are present at week 4 of gestation in the precursors of the endocrine glands. During subsequent development, this system remains only in the thyroid and in the intermediate lobe of the hypophysis. In the thyroid, SIS components may be related to the synthesis of hormones in this gland: they exocytose into the colloid, then by endocytosis (reabsorbtion) they re-enter the thyrocytes, and finally, via a second exocytotic event, enter into the inter-tissue fluid and progress in the capillaries through the basal parts of the thyrocytes.

 

The SIS does not produce its own Igs. Its functions during the embryonic period are performed by maternal Igs in the blood, and after weeks 9 to 11, by a small amount of B lymphocytes-synthesized IgM that appears in the stroma (Table II).  One of the signs of functional SIS activity can be a decrease in the amount of intracellular Igs in endocrine cells under inflammation and antigenic attacks, as has been observed in the hypophysis at meningitis, and in the thyroid, pancreas and adrenals in common infections  (3).

 

During organogenesis from weeks 5 to 8, changes in the SIS prepare it for the protection of parenchyma cells in the endocrine glands. The main changes in the SIS are as follows:  SC is not produced and exocytosis of Igs does not occur, as they are stored instead in the parenchyma cells of the different glands. In the presence of infection, the concentration of Igs in these cells remains at high levels, while in cells of the SIS area in the same organs, Ig concentration decreases sharply as a result of exocytosis. This is especially clearly seen in the pancreas where, upon infection, Igs practically disappear in the epithelial cells of the acini and ducts (an area of the SIS), while in islets (an area of the changing SIS) the intracellular concentration of Igs remains unchanged .

 

The common immune system begins to form at weeks 9 to 10 of gestation when the first lymphocytes appear, particularly B lymphocyte-synthesized Igs. The functional activity of this system is manifested in the endocrine glands  in the form of weak infiltration of immunocompetent cells, such as T lymphocytes, helpers and suppressors,  and B lymphocytes (Table II).  Monocytes appear earlier when of the yolk sac begins to function. It seems that the regional lymph nodes, such as the cervical for glands in this area or peritoneal for the pancreas and adrenals, are important organs of the common immune system.

 

 

 

 

 

 

 

 

 

 

 

Table II.

The number of immunocompetent cells in 50,000 µm² of the endocrine glands stroma in human fetuses (mean±SE)

(After ref. 3)

 

 

^a Significantly different from the same group of trimester I, p<0.05-0.001.

^b Significantly different from the group I of the same trimester, p<0.05.

 

Note that lymphocytes appear in an unaffected group at weeks 9 to 10 of gestation, and that their amount increases significantly during the second and third trimesters. Fetuses show a distinct reaction to antigen effects. Monocyte activity,   including a clear response to antigen effects, is already seen in the first trimester of gestation, and this response increases to the end of pregnancy. In both compared groups, the clearest reaction to antigen effects was found in the pancreas, perhaps because of its location near the intestine, in an area with high pathogen effect.  The reaction of T helpers is very weak in both groups of fetuses.

 

           3.2)    Immune components in the developing myocardium

 

The primordial heart tube is seen at week 3 of gestation as a part of a large  cardio-craniofacial morphogenetic field, and at week 4 its wall consists of three layers: the epicardium, myocardium  and pericardium (13,14). Heart differentiation is accompanied by programmed cell death (apoptosis) of the myocytes   that proliferate rapidly during fetal life, mediate  remodeling of the bulbus cordis  and accompany the transition from a fetal to an adult circulatory system (15-17). These processes are mediated by a few specific proteins, such as the Fas death receptor CD95 and the TNF-type II membrane family protein, which can induce both cell death and proliferation/differentiation of the receptor-bearing cells (18). Shc  proteins, participating in the control of apoptosis, are primarily expressed in the cardiovascular system during early embryogenesis and regulate heart and blood vessels development (19).  Myocytes of the fetal heart undergo mitosis/apoptosis during gestation (16).

 

Some immune components are present in the developing heart. J chain, IgG and IgA are found in the endocardium  and myocardium during the entire period of intrauterine development. On the other hand, SC is not observed. In the pericardium, all protein components of the SIS are present. In fetuses affected by antigenic attacks, there are no essential concentration changes in these components. In the myocardium, single CD3+ and CD20+ lymphocytes and a moderate number of monocytes (1.02±0.02/50,000  μm²) are observed (Gurevich, unpublished data).  In infants with congenital heart disorders,    reduced percentages of total T lymphocytes and T helper cells,   and low levels of IgG and IgA, as well as of complements C3 and C4, have been found (21,22). 

 

Some components of the common immune system are present in the  fetal myocardium in the form of a small amount of lymphocytes and macrophages as well as of Igs entering with the blood. The presence of J chain and Igs in cardiomyocytes can be considered indications of immune protection of the heart. The presence of all protein components of the SIS (SC, J chain and Igs) in the epicardium and pericardium can be interpreted as a barrier type of the SIS in these areas.

 

 

        3.3)  Immune protection of the developing brain

 

The anatomical development of the central nervous system (CNS – the brain and its various parts) extends over a protracted period from week 4  of gestation to 20 months  after birth (22).  Its cellular elements arise from the primary and secondary neuroepithelium (23). Different proteins participate in the normal process of brain development. Apoptosis and bcl-2 proteins are important for CNS development  at weeks 14 to 32 of gestation (24). The neural cell adhesion molecule  L1  participates in the neural cell migration, axon elongation and axonal fasciculation (25). The microtubule-associated protein-5 is essential for the elongation and maturation as well as the function maintenance of axons and dendrites in the developing human brain (26).

 

Components of the common immune system, such as T and B lymphocytes, macrophages, etc.,  participate in protecting the developing brain.  It is known that  MHC class II molecules are known to be related to an early phase of immunological response. These molecules are responsible for the binding, transport, and presentation of  foreign antigens to T helper lymphocytes and determine the type of antibodies produced. They also stimulate the multiplication of specific B lymphocytes and participate in the elimination of autoreactive lymphocytes and the maturation of T lymphocytes. Cells expressing MHC II molecules on their surface have been observed in the frontal and temporal lobes of  human fetal brain  between gestational weeks 11 and 22 (27,28). MHC II expression   was noted on the surface of the cerebral meninges cells, in the choroid plexuses of the lateral cerebral ventricles and blood vessel lumens, and in the microglia of the both cerebral hemispheres of human fetuses. The expression of MHC II on cells of the CNS   already at as early as week 11 of gestation may constitute evidence not only of a capacity for  early immune protection of the fetal nervous system, but also of a significant role that is potentially played by this system in normal embryogenesis.

 

Choroid plexus macrophages may contribute to an inflammatory cascade in the brain  (29). Cytokines are signaling proteins that can be produced as a part of the inflammatory response to both ischemia and infection (30). Preterm infants with cerebral white matter injury had high levels of IL-6, IL-10, and TNF-ą in the cerebrospinal fluid.  The intrauterine environment can significantly affect fetal brain development, and a range of hemodynamic and metabolic compensations  protects  the fetal brain from the effects of different factors, such as acute hypoxia  (31-33).

Components of the common immune system exhibit high activity during antigenic effects on the developing brain. Fetal T lymphocytes can be activated during fetal exposure to infection (34). These include specific recognition of bacterial antigens  and autoantigens, polyclonal activation by Toll-like receptors, and bystander activation by cytokines. High concentrations of cytokines   (TNF-ą, and bIL1, IL6, and IL10) and CD45RO+ T lymphocytes in the umbilical blood were found in pre-term infants with cerebral lesions who were born at 23 to 29 weeks of gestation (35).  Functional chemokine receptors and chemokines are expressed by microglial cells which may influence cellular function within the CNS (36). The CXCR3 chemokine receptor, expressed on activated T lymphocytes, is seen in the  CNS  in inflammatory conditions with a prominent T-cell response (37). 

Fetal inflammatory response syndrome appears to be  seems crucial to the association between intrauterine infection and a brain white matter disease in human preterm infants (38).  Chronic exposure to intra-amniotic lipopolysaccharide-caused chorioamnionitis affects the ovine fetal brain,   manifesting itself as a moderate to extensive infiltration of activated microglial macrophages in the subcortical white matter (39).  The fetal inflammatory response that develops in response to intrauterine infection may contribute to the occurrence of brain damage (40,41). Components of the common immune system, such as  cytokine or IL-6 elevation in the fetal plasma and neutrophil infiltrates in the umbilical cord, are   significant risk factors  for brain damage and/or cerebral palsy  (42-45).

 

 

 

         3.3.1)  Secretory immune components in the developing brain

The SIS constitutes  the first in the immune protection  for the developing brain and it has been observed at week 4 of gestation (Gurevich, unpublished observations). At this stage, its function involves the transport of maternal Igs into fetus's cerebrospinal fluid (CSF). Protein components of the SIS, such as SC, J chain and IgG and to a lesser extent IgA, are located alongside the entire length of the neural tube in all cellular layers, from basal neuroepithelial cells to the superficial neuroblasts. IgG, IgM and IgA    have been found in different parts of the rat fetal brain, such as the corpus callosum, cingulum and habenula   (46). 

Capillaries in the choroid plexuses form one of the hemo-encephalic barrier interfaces that control the brain's internal environment  (47).    The  secretory function of capillaries consists of their ability to transfer Igs into the CSF. The state of the CSF has great significance in the protection of the developing brain. The brain fluid contains different salts, very little proteins, and starting from the second trimester of gestation, immunocompetent cells, including different lymphocyte subtypes. These cells pass into the CSF from the choroid plexuses of the brain ventricles and to a lesser extent from the capillaries and blood vessels in the brain membranes. A connection between the common blood stream and brain capillaries and choroids plexuses should be considered part of the immune system.

The number and distribution of choroid plexus epithelial cells, containing alpha-fetoprotein (AFP), IgG, IgA, and IgM,  were recorded in the human fetal brain at different developmental stages (48). AFP  and IgG  were found in less than 40% of the cells, and this proportion declined later in gestation to only a few percents. In human fetuses at  the end of the first trimester and the beginning of the second trimester of gestation, the concentration of  SC in epithelial cells of the brain decreased and then disappeared,  remaining only in narrow (undifferentiated) areas of the neural tube. The other protein components of the SIS, such as J chain, IgG and IgA, were still present in the neuroblasts alongside the neural tube. There were regional differences in the distribution of the SIS components. In the choroid  plexus of the fourth brain ventricle, all mentioned protein components of the SIS were seen: SC, J chain, IgG and IgA. In neurons  of the  spinal ganglions from the caudal equine, J chain and Igs were found, but SC was not. In the second and third trimesters of gestation, neurons and dendrites in the brain and spinal cord, as well as neurons of the spinal ganglions,  contained  J chain, IgG, IgA, and sometimes IgM, but not SC. The whole protein complex of the SIS, including SC,  was present in the choroid plexus epithelium and in the ependyma of the brain ventricles. Cells of the microglia did not contain any components of the SIS.

Pathological changes in the brain without inflammation have not been shown to develop  in embryos that died as a result  of birth canal infection and/or chorioamnionitis.  The embryonic death in these cases comes very fast, and inflammation has no time to develop in the brain. At immune fetal-mother conflict during weeks 3 to 8 of gestation, the antigenic effect of maternal antibodies on the embryonic brain causes its early death during week 4. This is accompanied by massive apoptosis in the brain  seen  in 20% to 50% of the cells.

In cases with brain inflammation during the second and third trimesters of gestation, no essential changes were seen in the cellular distribution of SC,  J chain or Igs, in the choroid  plexus epithelium, ependyma of the brain ventricles and neurons of various ganglions. In cases with inflammation of the brain membranes and ventricles, a large accumulation  of neutrophil leukocytes, monocytes, and different lymphocyte subtypes was seen in the CSF and the stroma of the choroid  plexuses (Gurevich, unpublished data). Intracellular concentration of SC and J chain decreased, IgG and IgM disappeared, and IgA was sometimes present at weak concentration. In ganglion neurons of the medulla oblongata and cerebellum, intracellular concentration of the J chain did not change, while Ig concentration decreased.

 

                3.3.2)  Types of barrier protection of the developing brain

 

The mechanisms of barrier protection in the developing brain are distinguished from the hemo-encephalic barrier of the adult brain, and they can be non-immune (mechanical) or immune. An example of a non-immune barrier  is  the internal membrane of the brain (ependyma). In the adult brain, this is not merely an inert lining but may regulate the transport of ions, small molecules, and water between the CSF and neuropil (a feltwork of naked nerve fibers and neuroglial cells processes) and serve as an important barrier that protects neural tissue from potentially harmful substances. The fetal ependyma is believed to be secretory and plays a role in neurogenesis, neuronal differentiation/ axonal guidance, transport, and support (49,50). Differentiation of the ependymal cells proceeds along particular regional and temporal gradients as does the expression of various cytoskeletal and secretory proteins (51).

 

The growth of the capillary net in the brain is another example of a non-immune barrier. Vessel endothelium growth factor (VEGF) affects capillary growth (47).  This protein is expressed in high amounts during embryonic angiogenesis and is not produced in adults. VEGF is well correlated with the factor of vessel penetration, and both reflect the activity of the hemo-encephalic barrier.   The immature brain contains  a few non- immune barriers (52,53).  The  hemo-encephalic barrier comprises mechanisms that control the exchange of molecules between the internal environment of the brain and the rest of the body.   The underlying morphological feature of this barrier is the presence of tight junctions between cerebral endothelial cells and choroid plexus epithelial cells (54,55). These junctions are present in the fetal brain's blood vessels and are effective in restricting the entry of proteins from the blood into the brain and CSF.   In the immature brain, there are additional morphological barriers at the interface between the CSF and the brain tissue, such as strap junctions at the inner neuro-ependymal surface and intercellular membrane specializations at the outer  pia-arachnoid  surface. These barriers disappear later in development and are absent in adults.

  

The hemo-encephalic barrier and choroid  plexuses are good examples of the immune barrier type of brain protection. SC, a protein component of the SIS, is detected in the ependyma of choroid plexuses and some other structures which participate in the formation of the blood-tissue and tissue-tissue barriers (47,56,57). The presence of SC in these structures suggest the possibility of  Ig secretion by the same mechanism  described in the mucosal membranes and endocrine glands.  Moreover, the existence in the barrier system of IgG as well as of IgA and IgM secretion may be considered additional proof of the presence in the developing brain of an immune-type protective barrier system.

In rat fetuses,  choroid plexus tight junctions are impermeable to small molecules at day 15 of gestation or earlier, indicating that the blood-CSF barrier is morphologically and functionally mature early in the embryonic period of  development (58). The transfer of proteins from the fetal blood to the CSF is selective:   transfer of albumin is four- to fivefold greater in humans than in bovines. The number of choroid  plexus epithelial cells that are immunopositive for endogenous plasma protein increases in parallel with an increases in the total protein content of the expanding ventricular system. These results suggest that different transcellular mechanisms are operating in the transfer of proteins and small molecules across the embryonic blood-CSF interface.

In summary, it should be emphasized that components designed to protect  the brain and the entire CNS are present at the very beginning of the embryonic period. Protein components of the SIS, such as  the SC, J chain, and maternal Igs, are seen in neuroepithelial cells and neuroblasts of the developing neural tube. These components remain in the choroid  plexuses and ependyma of the brain ventricles. Neuroblasts that transform into microglia lose all of their SIS protein components, while the same  neuroblasts transformed into neurons   lose only   their SC and retain their J chain, IgG, IgA and sometimes IgM, similar to that which occurs in the endocrine glands.  These data suggest that during the brain's development, the SIS undergoes some transformation that protects neurons of the brain and ganglions.  Components of the common immune system are also seen in the fetal brain, and immunocompetent cells are present in the CSF. The developing brain has also protective barrier mechanisms that can be non-immune (mechanical) or immune. 

 

             3.4)  Immune protection of gonads and genital tracts

 

     3.4.1)   Immune protection of gonads and genital tracts in adults

Protection of the sexual cells' genome and thus of future generations starts long before fertilization and the appearance of the embryo  itself.  The immune system creates the protective background for the whole organism, and its protective elements have been found in the gonads, germ cells and genital tracts of both females  and males.  

 

 

  3.4.1-1)  Immunoprotective components in the female genital tract, ovaries and   oocytes

Different types of ILs and cytokines have been described as major components of the protective system in these organs. Factors produced by activated immune cells, including IL-1 and IL-2, play a role in the down regulation of these responses,  and stimulation of T- and B-lymphocyte functions in females  (59,60), and participate in the inflammation response (61).  In the female reproductive tract, the immune system represents a defense mechanism that can act directly against pathogens and mediate an inflammatory response (62). The complement regulatory proteins are expressed throughout the female genital tract, and play an important role in protecting the traversing sperm and implanting blastocyst from a complement-mediated damage (63). Endometrial cells  are protected from complement attacks by membrane-bound complement regulatory proteins. The survival of these cells with some biochemical modifications enables the immune response.

The human female reproductive tract is an inductive site for immune responses, and cell-mediated immunity with all effector components of the SIS  is present throughout the entire tract (64.65). B lymphocytes and plasma cells that secreted IgA  (and in lower amounts also  IgM and IgG) are found in the uterine endocervix and ectocervix, oviducts, and vagina. Epithelial cells lining the oviducts and endocervix express  SC, which is required for the transepithelial transport of polymeric (p)IgA into external secretions. Secretory IgA (sIgA), which provides the first line of defense against invading pathogens, is produced locally in the female reproductive tract (64).  Approximately two-thirds of the Ig-positive cells contain sIgA and J chain, indicating that they produce (p)IgA.  A local immune system functions in the human oviducts and may provide a first line of defense against ascending infection: T-suppressor cells, which participate in the induction of the immune tolerance, are found in  the human oviducts (63). 

Paracellular diffusion of serum-derived and locally produced IgG through the epithelia is an important part of humoral immunity in the female genital tract (66).    The endometrium can  perform external translocation of pIgA. Uterine and cervical epithelial cells play a key regulatory role in the control of IgA transcytosis from the tissue into secretions, a manifestation of SIS functions (67). SC production by uterine epithelial cells is correlated with increased transepithelial resistance.

The lower reproductive tract in women is also immunocompetent as judged by the presence of different subtypes of T lymphocytes, macrophages, and dendritic cells in the endocervix and ectocervix, and in the vagina (68). The quantity and subclass distribution of IgA produced by the human uterine cervix   have a significant impact on the defense against sexually transmitted diseases and even against  oncological disorders (64,69). The female genital tract protects the host from pathogen challenges induced by such infections as herpes simplex viruses or Trichomonas fetus (70, 71).

The SIS appears to play an important role, additional to that of the common immune system in the protecting the female genital tract and gonads in relation to the peculiarities of the antigenic effects and the structure and function of target organs. As in the other organs, a protective mechanism consists first of Ig secretion s on the surface of the mucous epithelium with of SC and J chain participation in this process  (69). The presence of complex proteins characteristic of the SIS, such as SC, J chain and Igs, as well as of immunocompetent cells (macrophages, T and B lymphocytes, plasma cells), that participate in the synthesis of Igs, is highly specific to the SIS (72). Some of these cells  (B lymphocytes and  plasma cells) that participate in the synthesis of IgA and to a lesser extent IgG and IgM, are located in the subepithelial layer of the endocervix and ectocervix, as well as that of  the oviducts, uterus, and vagina. The presence in the plasma cells of J chain and IgA indicates the synthesis of pIgA which is typical of the SIS. Eighty percent of the IgA in the mucous  membrane of the uterine cervix was found to be polymeric, while in the vaginal fluid  only 55% was (73). Infection of the genital tract causes a strong increase in the secretory activity of all components  of the SIS (70,71).

 

Only some immunoprotective components have been described in the ovaries and oocytes in only a few publications. Oocytes, and follicular and thecal ovarian cells could represent sources and targets of ILs (74)  which  participate in local regulation of many reproductive functions (75).  Different subclasses of lymphocytes that participate in the synthesis of Igs are found in the human ovaries (76). Ovulation has been found to be accompanied by the migrating macrophages into the follicle  and the production of two macrophage-specific chemokines   (77). 

 

 

 3.4.1-2)  Immunoprotective components in the testes and male genital tract

In the epithelial lining of the testis, epididymis, and vas deferens,   lymphoid cells are represented predominantly by T suppressor/cytotoxic cells.   Elements of the SIS, such as SC and IgA, have been described in  the male rodent's urogenital tract (78,79). Protective IgG antibody-mediated immunity in the male genital tract is provided by IgG and IgA derived from the systemic immunoglobulin pool and from local synthesis (73).   Because genital tract tissues lack inductive mucosal sites analogous to the intestinal Peyer's patches, the local humoral and cellular immune responses stimulated by infections are weak or absent, and repeat local immunizations result in minimal responses (80,81). 

The production of autoantibodies to spermatozoa in males is inhibited by both the physical isolation of the spermatozoa from the systemic immune system and by active immunosuppression mechanisms (63).   In the testes, physiological protection from autoimmune attacks  is provided by different ILs, particularly IL1 and IL18 (82-84). Testicular cytokines and growth factors,  such as IL-1, IL-6, TNF, IFN-γ, etc., have been shown to affect germ-cell proliferation (85) and participate in pathologies such as orchitis, acute lymphoblastic leukemia of the testis, systemic inflammation and infertility disorders in men (86,87). 

 

    3.4.2)  The secretory immune system in developing gonads and genital tracts

 

Development and differentiation of gonads and genital tracts are accompanied by the early appearance and functional activity of different component of the SIS.

 

 

3.4.2-1) Secretory immune components in developing genital tracts

Both male and female embryos initially have two pair of genital ducts: i) mesonephric and ii) paramesonephric ducts. The mesonephric ducts represent a continuation of the mesonephros, whereas the paramesonephric duct arises on each side of the early embryo as a longitudinal invagination of the coelomic epithelium on the anterior-lateral surface of the primordial urogenital ridge. The mesonephric duct forms the genital tract in males while the  paramesonephric duct develops into the genital tract in females.

 

In 3.5-4- to 6-week-old healthy embryos, the epithelium of the mesonephros tubules, the mesonephric ducts and the paramesonephric ducts, the proliferating coelomic epithelium and cells of the primitive sex cords of the genital ridge show high immunoreactivity to SC, J chain and IgG (4). Igs are secreted on the mucosa of these organs and have been observed in the lumen of the genital organs, fetal urine and amniotic fluid (88,89).  IgA is present in embryos at weeks 5 to 6 of pregnancy, while IgM has been found in some of 6- to 7-week-old embryos. The epithelial cells of the uterine tubes and endometrium were found positive for  SC, J chain and all Igs from week 9 until the end of pregnancy (4). In parts of the male genital tract, such as the   deferent duct, epithelial cells are strongly reactive for SC, J chain, IgG and IgA, and weakly reactive for IgM. Massive antigenic stimulation in embryos with chorioamnionitis has no  essential effect on SIS components,  except for a decrease in Ig reactivity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Color Figure)

 

Fig. 3.

The ovary of a 22-week-old human fetus.

A. SC in the follicular  cells (brown staining). Note that in oogonia SC is not present. x400.

B. J chain in oogonia and  follicular cells. x400.

C. IgA in oogonia and follicular cells. x400.

            The testis of a 22-week-old human fetus.

            D. J chain in Leydig cells and spermatogonia.  x1000.