Introduction
Successful pregnancy is an immunological paradox (18).
Immune tolerance in pregnancy is defined as a state where the immune system of the mother does not show any immunological response towards the fetus and placenta (87). The concept of immune tolerance was first put forward by Medawar in 1953 (10). The success of a pregnancy depends upon maternal immune response (regulatory and inhibitory); vigilantly balanced modification in order to accept the fetus; semi allogeneic having paternal antigens (11). Immune tolerance of fetal/ placental unit depends upon the placenta (immune regulator), maternal immune system and seminal fluid (plays an important role in the immunology of pregnancy) (53).
Placenta(fetal origin) acts as an interface forming a complex microenvironment comprising of regulatory factors(cellular and soluble) between maternal and fetal circulation (uteroplacental fetoplacental) acting as an immunological barrier and conduit (4). Activated macrophages, activated Tregs and CD8 are found abundantly in the decidua of this interface (22). The placental barrier in early pregnancy is formed by syncytiotrophoblast, cytotrophoblast (produce IL 10- immunosuppressive), basement membrane, stromal tissue, and endothelium of fetal capillary wall with its basement membrane (35, 50).
Major Histocompatibility Complex (MHC)-located on chromosome 6; encodes membrane proteins Human Leukocyte Antigen (HLA) Class I and HLA Class II. Host T lymphocytes recognise foreign MHC molecules which is responsible for graft rejection (21). The trophoblast cells (placenta) are devoid of HLA antigens (MHC Class I isotypes) which prevent fetal rejection (destruction by cytotoxic T cells of the mother) (29, 30). HLA- E and HLA- G; atypical MHC class I isotypes are expressed by trophoblasts and they prevent knocking down by maternal NK cells (13).
The site of implantation is immune privileged due to:
• Fas Ligand (FasL) mediated clonal deletion of immune cells that identify paternal antigens (placenta) (32, 54, 61).
• Allospecific maternal Tcell deletion in pregnancy by FasL (fetal) (83).
However, some studies show contrasting data saying FasL promotes allograft rejection (1, 24).
Decidua at implantation site has a specific type of NK cells (uterine NK cells- dependent on progesterone for survival; progesterone- immunomodulation). uNK cells possess receptors, NKR-Killer Ig like receptor; KIR, Ig like transcripts; ILT, C type lectin family which distinguish between the target (having ligand) and non-target cells (45). KIRs(mother) respond to HLA-C specific to fetus thus maintaining a fine balance in each pregnancy (17). uNKs are responsible for the proliferation of trophoblast, invasion due to interaction with trophoblast HLA Class I molecule, maternal tolerance of the fetus by decreasing the inflammation mediated by Th17 cells via IFN gamma and thus maintaining the pregnancy (12, 77). They do not kill the trophoblast cells as they are weakly cytotoxic; lack CD16 expression but high CD56 concentrations (37). uNKs are a prominent source of angiogenic factors Angiopoietin1 (Ang 1), Ang2, Transforming Growth Factor Beta 1(TGF Beta 1), Vascular Endothelial Growth Factor C (VEGF C) (40) and metalloproteinases (MMPs-proteolytic zinc-requiring enzymes) especially MMP-2 (55). All these factors are responsible for trophoblast invasion, remodelling of extracellular matrix and angiogenesis which play a crucial role in decidualisation, placentation, and initiation of early pregnancy (88). Apart from uNK cells, decidual macrophages (M2 mainly- immunomodulatory) are found abundantly in the first trimester prior to Extravillous trophoblast (EVT) and aid in the remodelling of the spiral artery (23, 74).
Decidual NK cells interact with trophoblast HLA class I molecules and cytokines thus derived regulate the invasion of EVT cells (by Interleukin-8 and Interferon- inducible protein- 10 chemokines) into the spiral arteries (27). Spiral arteries are then converted into high conductance, low resistance uteroplacental arteries (84). Invasion of trophoblast and inner third of myometrium is vital for positive pregnancy outcome. Some studies conflict saying human uNK cells (contain Interferon-gamma in early pregnancy) inhibit the invasion of trophoblast by their apoptosis leading to decreased secretion of MMP-2 by trophoblast cells (39).
EVTs coordinate bi-directional communication (providing structural and biochemical barrier) and are expressed by the layer of syncytiotrophoblast which is in direct contact with the decidua (8).
Functions (80):
• Support and regulate the development of fetus and placenta.
• Harmonise maternal immune responses (innate and adaptive).
• Release HLA-G; decidual antigen presenting cells become tolerogenic dendritic cells which secrete IL-10 and trigger Treg cells.
Immunoglobulin G (IgG) are transferred from the mother to the fetus via the placenta. They protect the neonate from infections before the development of an active immune system in the fetus (57).
Maternal immune responses (Table 1);
INNATE
ADAPTIVE
It is non-specific.
Specific, distinguishes between self and non self.
The first line of host defence to infections.
Second line.
Acts fast once the pathogen is recognised.
Delayed immune response.
No immunological memory.
Reinfection is prevented by the adaptive immune response (immunological memory).
Consists of the complement system.
Consists of- Antigen presenting cells (APC), Lymphocytes (B and T) and MHC.
Involves various immune cells-
i)Phagocytes(monocytes, neutrophils and macrophages)
ii) Basophils and Eosinophils
iii) Natural killer cells.
Phagocytes contain lysosomal enzymes- ingestion and digestion of microorganism.
T Lymphocytes (T helper and T cytotoxic cells).
T helper have two types Th1 and Th2.
Th1 (cell-mediated immunity)- activate the macrophages.
Th2 (humoral immunity)- B cell differentiation
Cytokines (TNF alpha, IL-1, IL-6) and chemokines trigger an immune response. Tolerance of the fetus by maternal immune response is mainly regulated by the presence of immunomodulatory molecules on the placental surface and MHC molecules which exhibit restricted expression (58).
Fetal programming; development of an immune system in fetus occurs as a result of an increase in levels of cytokines in amniotic fluid and blood (IL-1,6,12, granulocyte-macrophage colony stimulating factor and Tumor Necrosis Factor alpha) (46).
During pregnancy, there is a shift (mainly at the maternal-fetal interface) of maternal immune response (immunomodulation) from Th1 to Th2 (81). This switch occurs because of:
• Th2 cell migration.
• Th2 cells eduction at maternal-fetal interface.
• Th2 accumulate mainly in the decidua and drive native T cells in decidua of the uterus to become Th2.
Production of PGD2 by placenta plays an important role in chemoattraction of Th2 cells (express CRTH2 chemoreceptors) towards the maternal-fetal interface (48). The production of anti-inflammatory cytokines is stimulated by Th2 (76, 86). Increase in Th2 cytokines (IL-4,10, monocyte colony stimulating factor) is associated with pregnancy outcome. This Th2 cytokine environment is seen due to the generation of IL4, IL-6, IL-10, IL-13 by decidua, trophoblast, and amnion (9). Th2 profile is promoted by the production of leukemic inhibitory factor, estradiol, progesterone and prostaglandin D2 in normal pregnancy. IL10 is immunosuppressive in nature and is produced in the decidua (by macrophages and Tregs) and cytotrophoblasts. This immunosuppression is responsible for immune tolerance of the mother towards the fetal antigens (68).
IL-4 and IL-10 play important role in preventing the following:
• Fetal allograft rejection (by inhibition of macrophages and Th1
• The onset of labor by inhibiting PG E2, Cyclooxygenase-2 and TNF alpha (25, 26, 36, 59).
Treg cells
They are of two types (6):
• Thymic T regulatory cells (tTreg-occur naturally)- CD4+CD25+Foxp3 and they express antigen 4 (associated with cytotoxic T lymphocyte). Foxp3 regulates the development and functioning of Treg cells (28) (71).
• Extrathymic/ peripheral T regulatory cells (pTreg- induced; develop from native T cells in the periphery as a result of exposure to antigens).
Production of IL10 and TGF 2 by Treg cells is responsible for maintenance of maternal-fetal tolerance by suppressing self-reactive lymphocytes (38, 72).TReg cells also play a vital role in the implantation of an embryo. They are paternal alloantigen specific and thus prevent fetal rejection (3, 19, 51). During pregnancy activity of maternal Th1/Th17 on the fetus is suppressed by alloantigenic- independent expansion of Tregs in decidua (under the influence of estradiol) (41, 70, 78). A study by Robertson suggested that this expansion can also be regulated by seminal fluid. Thus Treg cell expansion is both antigen-dependant and independent (64). They induce tolerance by inhibiting production of cytokines and proliferation in CD4+, CD8+T cells, dendritic cell maturation, cytotoxic property of NK cells and production of Ig by B cells (2).
Ligands of chemokine receptors (CCR) expressed on Tregs are present at the maternal-fetal interface which brings about the migration of Tregs to the decidua (41). Chemokine ligands (CCL1,4,17) produced by immune cells also attract Treg by expressing CCR 4, CCR 8 (15, 20, 33, 79). Studies also show that Human Chorionic Gonadotropin (hCG) is the key factor which attracts Treg cells (73).
Seminal plasma:
• Plays a crucial role in triggering specific Treg cells(paternal alloantigenic) (69).
• Maternal immune response priming by seminal fluid results in expansion of immune tolerant cells (paternal Ag-specific) (52) and this immune tolerance is triggered by cytokine composition due to TGFB (63).
• There is an increase in fetal loss and abnormality in the absence of exposure of embryos to the male fluids (85, 14).
• The uterine inflammatory response is principally triggered by TGFB majority of which is activated by plasmin and other enzymes in the female reproductive tract after insemination (65, 82). This response results in activation of the immune response in females specific to antigens and proteins (paternal transplantation) in semen, endometrial receptivity due to tissue remodelling, regulation of cytokines and growth factors responsible for preimplantation and development of embryo.
• Activation of immune response leads to increase in size of the lymph nodes that drain the uterus and activation of lymphocytes (7, 34). Paternal Ag rejection by this activation is prevented by PGE and TGFB (immune-regulators) in the seminal fluids (43).
• Exposure to semen results in immune tolerance (functional) of female immune response to paternal antigens (66).
• Macrophages trigger remodelling of the endometrial environment in response to semen which helps in implantation and development of placenta (5, 31).
• Semen regulates the synthesis of cytokines (embryotrophic); GMCSF leading to an increase in number of feasible blastomeres (67).
Table 2
IMMUNOLOGICAL CONTROL OF FERTILITY:
– Placental Hormones
– Zona pellucida
– Human Sperm antigen
– Antisperm antibodies
Clinical significance
1) First trimester abortions due to immune factors
Autoimmunity-
• Anti-DNA antibodies
• Anti-nuclear antibodies
• Anti-phospholipid antibodies- anticardiolipin, lupus anticoagulant, anti-beta glycoprotein
Parental HLA sharing
Leucocytotoxic antibodies
Absence of maternal blocking
Results in rejection of early pregnancy:
• inhibition of proliferation of the trophoblast and its function
• Thrombosis of the spiral artery and placental intervillous.
• Release of cytokines by the complement pathway
• Decidual vasculopathy with fibrinoid necrosis
Clinical studies/ trials
• Greater pregnancy outcomes were seen in patients with low antiphospholipid antibody titres (75).
• Overexpression of microRNA-30a-3p effects IGF-1 and impairs invasion of trophoblast and apoptosis (56).
• Indoleamine-2,3-dioygenase regulates fetal invasion, circulation and maternal immune suppression (16).
• In unexplained miscarriages (recurrent)- High C4d levels at maternal-fetal interface (47).
• Msh Homeobox 2 may be responsible for the invasion of trophoblast cells and its dysregulation can lead to pre-eclampsia (44).
• Th17/Treg ratio alterations can lead to recurrent miscarriage (62).
• Higher plasma TNF alpha levels seen in patients having secondary recurrent miscarriage (60).
• Decrease CRTH2+ expression in women with recurrent loss of pregnancy (48).
Conclusion
Recent scientific data suggest that study of reproductive immunology may improve the pregnancy outcomes in the field of assisted reproduction in patients with recurrent abortions but this approach needs caution.