Reproductive immunology

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Reproductive immunology refers to a field of medicine that studies interactions (or the absence of them) between the immune system and components related to the reproductive system, such as maternal immune tolerance towards the fetus, or immunological interactions across the blood-testis barrier. The concept has been used by fertility clinics to explain fertility problems, recurrent miscarriages and pregnancy complications observed when this state of immunological tolerance is not successfully achieved. Immunological therapy is a method for treating many cases of previously "unexplained infertility" or recurrent miscarriage. [1]

Contents

The immune system and pregnancy

The immunological system of the mother plays an important role in pregnancy considering the embryo's tissue is half foreign and unlike mismatched organ transplant, is not normally rejected. During pregnancy, immunological events that take place within the body of the mother are crucial in determining the healthiness of both the mother and the fetus. In order to provide protection and immunity for both the mother and her fetus without developing rejection reactions, the mother must develop immunotolerance to her fetus since both organisms live in an intimate symbiotic situation. Progesterone-induced-blocking factor 1 (PIBF1) is one of the several known contributing immunomodulatory factors to play a role in immunotolerance during pregnancy. [2]

The placenta also plays an important part in protecting the embryo for the immune attack from the mother's system. Secretory molecules produced by placental trophoblast cells and maternal uterine immune cells, within the decidua, work together to develop a functioning placenta. [3] Studies have proposed that proteins in semen may help a person's immune system prepare for conception and pregnancy. For example, there is substantial evidence for exposure to partner's semen as prevention for pre-eclampsia, a pregnancy disorder, largely due to the absorption of several immune modulating factors present in seminal fluid, such as transforming growth factor beta (TGFβ). [4] [5]

Insufficient immune tolerance

An insufficiency in the maternal immune system where the fetus is treated as a foreign substance in the body can lead to many pregnancy-related complications.

Microbiology

Uterine natural killer (uNK) cells

The maternal immune system, specifically within the uterus, makes some changes in order to allow for implantation and protect a pregnancy from attack. One of these changes are to the uterine natural killer cells (uNK). NK cells, part of the innate immune system, are cytotoxic and responsible for attacking pathogens and infected cells. However, the number and type of receptors the uNK cells contain during a healthy pregnancy differs compared to an abnormal pregnancy. [12] In the first trimester of pregnancy, uNK cells are among the most abundant leukocytes present, but the number of uNK cells present slowly declines up until term. [13] Despite the fetus containing foreign paternal antigens, uNK cells do not recognize it as "non-self". [12] Therefore, the cytotoxic effects of the uNK cells do not target the developing fetus. [12] It has even been proposed that uNK contributes to the protection of extravillous trophoblast (EVT), important cells that contribute to the growth and development of a fetus. [14] [15] uNK cells secrete transforming growth factor-β (TGF-β) which is believed to have an immunosuppressive effect through modulation of leukocyte response to trophoblasts. [14]

Killer immunoglobulin-like receptors (KIRs) and human leukocyte antigen (HLA)

KIRs are expressed by the uNK cells of the mother. Both polymorphic maternal KIRs and fetal HLA-C molecules are variable and specific to a particular pregnancy. In any pregnancy, the maternal KIR genotype could be AA (no activating KIRs), AB, or BB (1–10 activating KIRs) and the HLA-C ligands for KIRs are divided into two groups: HLA-C1 and HLA-C2. Studies have shown that there is a bad compatibility between specifically maternal KIR AA and fetal HLA-C2 which leads to recurrent miscarriage, preeclampsia and implantation failures. In assisted reproduction, these new insights could have an impact on the selection of single embryo transfer, oocyte, and/or sperm donor selection according to KIRs and HLA in patients with recurrent miscarriages. [16]

Medication exposure during pregnancy

Pharmacokinetics

Pregnant-related anatomical and physiological changes affect pharmacokinetics (absorption, distribution, metabolism, and excretion) of many drugs, which may require drug regimen adjustment. Gastrointestinal motility is affected by delayed gastric emptying and increase gastric pH during pregnancy, which may alter drug absorption. [17] [18] Changes in body composition during pregnancy may change drugs volume of distribution due to increased body weight and fat, increased total plasma volume, and decreased albumin. [18] For drugs susceptible to hepatic elimination are influenced by increased production of estrogen and progesterone. [17] In addition, change in hepatic enzyme activity may increase or decrease drug metabolism based on drug composition, however most hepatic enzymes increase both metabolism and elimination during pregnancy. [17] Also, pregnancy increase glomerular filtration, renal plasma flow, and the activity of transporters, which may require increased drug dosage. [17]

FDA regulations

FDA established labeling request for drugs and biological products with medication risks, allowing informed decision making for pregnant and breastfeeding women and their health care providers. [19] Pregnancy category was required on the drug label for systemically absorbed medications with the risk of fetal injury, which is now replaced with pregnancy and lactation labeling rule (PLLR). [20] In addition to pregnancy category requirements on information of pregnancy, labor and delivery, and nursing mothers, PLLR also includes information on females animals of reproductive potential. [20] The labeling change were effective starting June 30, 2015. [20] The labeling requirements of over-the-counter (OTC) medicines we not affected. [20]

Pharmacologic consideration

The change in medication exposure during pregnancy should concern both mother and fetus independently. For example, within antibiotics, penicillin may be used during pregnancy, whereas tetracycline is not recommended due to potential risk of fetus for a wide range of adverse effects. [21]

Drugs

Medications to reduce risk of miscarriage

Progesterone is a medication often used to prevent threatened miscarriage. A threatened miscarriage is signs or symptoms of miscarriage, most often including bleeding that occurs in the first 20-weeks of a pregnancy. [22] Research has shown that supplementation of progesterone can lower the rate of miscarriage, however, it did not have an effect on lowering the rate of pre-term births and live births. [23] In reference to micronized vaginal progesterone, these results were more prominent for people who were at high risk of miscarriage, including people who have had three or more miscarriages and are currently experiencing bleeding. [24]

The use of low dose aspirin may be linked to increased rates of live births and fewer pregnancy losses for people who have had one or two miscarriages. [25] The National Institute of Health has recently changed their stance on using low dose aspirin, stating "low-dose aspirin therapy before conception and during early pregnancy may increase pregnancy chances and live births among a person who has experienced one or two prior miscarriages." [26] This is a change from the previous stance on aspirin preventing pregnancy loss from the National Institute of Health. [27] The reasoning behind the change was the determination that adherence to the medication and not discontinuing low dose aspirin due to side effects "could improve the odds for pregnancy and live birth in this group of people." [26]

Sulfonamides and their risk of congenital malformations

Some studies have shown that maternal exposure to sulfonamides during pregnancy may have an increased risk of congenital malformations. [28] There has been no evidence that certain types of sulfonamides or doses administered may increase or decrease the risk. Exposure to sulfonamides has been the only direct connection. [28]

Medications to increase live birth rate for persons with antiphospholipid syndrome

Some studies have found that using both aspirin and heparin can increase the rate of live birth in a person with antiphospholipid syndrome. [29] It was also found to increase birth weight and gestation age when using heparin and aspirin together. [29] It was also found that people with antiphospholipid syndrome had an increased live birth rate when low molecular weight heparin was substituted in for heparin and co-administered with aspirin. [30]

Sperm cells within a male

The presence of anti-sperm antibodies in infertile men was first reported in 1954 by Rumke and Wilson. It has been noticed that the number of cases of sperm autoimmunity is higher in the infertile population leading to the idea that autoimmunity could be a cause of infertility. Anti sperm antigen has been described as three immunoglobulin isotopes (IgG, IgA, IgM) each of which targets different part of the spermatozoa. If more than 10% of the sperm are bound to anti-sperm antibodies (ASA), then infertility is suspected. The blood-testis barrier separates the immune system and the developing spermatozoa. The tight junction between the Sertoli cells form the blood-testis barrier but it is usually breached by physiological leakage. Not all sperms are protected by the barrier because spermatogonia and early spermatocytes are located below the junction. They are protected by other means like immunologic tolerance and immunomodulation.

Infertility after anti-sperm antibody binding can be caused by autoagglutination, sperm cytotoxicity, blockage of sperm-ovum interaction, and inadequate motility. Each presents itself depending on the binding site of ASA.

Immunocontraceptive vaccine

Immunocontraceptive vaccines with a variety of proposed intervention strategies have been in development and under investigation since the 1970s. [31] One approach is a vaccine designed to inhibit the fusing of spermatozoa to the zona pellucida. This vaccine has been tested in animals with a view to use as effective contraceptive for humans. Normally, spermatozoa fuse with the zona pellucida surrounding the mature oocyte; the resulting acrosome reaction breaks down the egg's tough coating so that the sperm can fertilize the ovum. The mechanism of the vaccine is injection with cloned ZP cDNA, therefore this vaccine is a DNA based vaccine. This results in the production of antibodies against the ZP, which stop the sperm from binding to the zona pellucida and ultimately from fertilizing the ovum. [32]

Another vaccine that has been investigated is one against human chorionic gonadotropin (hCG). In phase I and early phase II human clinical trials, an experimental vaccine consisting of a dimer of β-hCG, with the tetanus toxoid (TT) as an adjuvant, produced antibodies against hCG in the small group of women immunized. The anti-hCG antibodies generated were capable of neutralizing the biological activity of hCG. Without active hCG, maintenance of the uterus in a condition receptive for implantation is not possible, thereby forestalling pregnancy. As only 80% of the women in the study had a level of circulating anti-hCG sufficient to prevent pregnancy, further development of this approach will be to enhance the immunogenicity of the vaccine, in order that it produces a reliable and consistent immune response in a higher proportion of women. Towards this goal, vaccine variations using a peptide of β-hCG that is uniquely specific to hCG, while absent in other hormones – luteinizing hormone (LH), follicle-stimulating hormone (FSH), and fhyroid-stimulating hormone (TSH) – are under investigation in animal models, for their possible enhancement of responses. [32]

Research

Studying the female reproductive tract, especially in humans, allows for a better understanding of the immune system, including during pregnancy. However, studying the female reproductive tract has been a challenging area of research due to existing limitations in the in vitro and in vivo tools available. Ethical concerns is another contributing factor in hindering the study of reproductive immunology. Given such limitations, research in this field relies on stem cell culture and technological advancements by allowing scientists to conduct research on organoids instead of living human subjects. In 2018, a Review study concluded that organoids can be used to model organ development and disease. [33] Other studies have concluded that with further technological advancements, it is possible to create a detailed 3D organoid model of the female reproductive tract which introduces a more efficient method to conduct research and collect data in the fields of drug discovery, basic research and essentially reproductive immunology. [34]

Single-cell technologies

The maternal-fetal interface has the ability to protect against pathogens by providing reproductive immunity. Simultaneously, it is remodeling the tissues needed for placentation. This unique feature of the maternal-fetal interface suggests that the decidual immunome, or the immune function of the female reproductive tract, is not fully understood, yet. [34] [35]

In order to have a better understanding of Reproductive Immunology, more data needs to be collected and analyzed. Technological advances allow reproductive immunologists to collect increasingly complex data at a cellular resolution. Polychromatic flow cytometry allows for greater resolution in the identifying novel cell types by surface and intracellular protein. [35] Two examples of methods in data acquisition include:

Reproductive immunology remains an open area of research as not enough data is available to introduce a significant finding. [34]

Cytokine profiling

Maternal immune activation can be assessed by measuring multiple cytokines (cytokine profiling) in serum or plasma. This method is safe for the fetus since it only requires a peripheral blood sample from the mother and has been used to map maternal immune development throughout normal pregnancies as well as studying the relationship between immune activation and pregnancy complications or abnormal development of the fetus. Unfortunately, the method itself is unable to determine the sources and the targets of the cytokines and only shows systemic immune activation (as long as peripheral blood is analyzed), and the cytokine profile may vary rapidly as cytokines are short-lived proteins. It is also difficult to establish the exact relation between a cytokine profile and the underlying immunological processes.

The impact of unfavorable immune activation on fetal development and the risk of pregnancy complications is an active field of research. Many studies have reported an association between cytokine levels, especially for inflammatory cytokines, and the risk of developing preeclampsia, although the findings are mixed. [36] However, decreased cytokine levels in early pregnancy has been associated to impaired fetal growth. [37] Increased maternal cytokine levels have also been found to increase the risk of neurodevelopmental disorders such as autism spectrum disorders [38] and depression [39] in the offspring. However, more research is needed before these associations are fully understood.

See also

Related Research Articles

<span class="mw-page-title-main">Placenta</span> Organ that connects the fetus to the uterine wall

The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.

<span class="mw-page-title-main">Amniocentesis</span> Sampling of amniotic fluid done mainly to detect fetal chromosomal abnormalities

Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation.

<span class="mw-page-title-main">Natural killer cell</span> Type of cytotoxic lymphocyte

Natural killer cells, also known as NK cells or large granular lymphocytes (LGL), are a type of cytotoxic lymphocyte critical to the innate immune system. They belong to the rapidly expanding family of known innate lymphoid cells (ILC) and represent 5–20% of all circulating lymphocytes in humans. The role of NK cells is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. NK cells provide rapid responses to virus-infected cell and other intracellular pathogens acting at around 3 days after infection, and respond to tumor formation. Most immune cells detect the antigen presented on major histocompatibility complex (MHC) on infected cell surfaces, but NK cells can recognize and kill stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction. They were named "natural killers" because of the notion that they do not require activation to kill cells that are missing "self" markers of MHC class I. This role is especially important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells.

<span class="mw-page-title-main">Antiphospholipid syndrome</span> Medical condition

Antiphospholipid syndrome, or antiphospholipid antibody syndrome, is an autoimmune, hypercoagulable state caused by antiphospholipid antibodies. APS can lead to blood clots (thrombosis) in both arteries and veins, pregnancy-related complications, and other symptoms like low platelets, kidney disease, heart disease, and rash. Although the exact etiology of APS is still not clear, genetics is believed to play a key role in the development of the disease. Diagnosis is made based on symptoms and testing, but sometimes research criteria are used to aid in diagnosis. The research criteria for definite APS requires one clinical event and two positive blood test results spaced at least three months apart that detect lupus anticoagulant, anti-apolipoprotein antibodies, and/or anti-cardiolipin antibodies.

<span class="mw-page-title-main">Pre-eclampsia</span> Hypertension occurring during pregnancy

Pre-eclampsia is a multi-system disorder specific to pregnancy, characterized by the onset of high blood pressure and often a significant amount of protein in the urine. When it arises, the condition begins after 20 weeks of pregnancy. In severe cases of the disease there may be red blood cell breakdown, a low blood platelet count, impaired liver function, kidney dysfunction, swelling, shortness of breath due to fluid in the lungs, or visual disturbances. Pre-eclampsia increases the risk of undesirable as well as lethal outcomes for both the mother and the fetus including preterm labor. If left untreated, it may result in seizures at which point it is known as eclampsia.

Rh disease is a type of hemolytic disease of the fetus and newborn (HDFN). HDFN due to anti-D antibodies is the proper and currently used name for this disease as the Rh blood group system actually has more than 50 antigens and not only the D-antigen. The term "Rh Disease" is commonly used to refer to HDFN due to anti-D antibodies, and prior to the discovery of anti-Rho(D) immune globulin, it was the most common type of HDFN. The disease ranges from mild to severe, and occurs in the second or subsequent pregnancies of Rh-D negative women when the biologic father is Rh-D positive.

Recurrent miscarriage or recurrent pregnancy loss (RPL) is the spontaneous loss of 2-3 pregnancies that is estimated to affect up to 5% of women. The exact number of pregnancy losses and gestational weeks used to define RPL differs among medical societies. In the majority of cases, the exact cause of pregnancy loss is unexplained despite genetic testing and a thorough evaluation. When a cause for RPL is identified, almost half are attributed to a chromosomal abnormality. RPL has been associated with several risk factors including parental and genetic factors, congenital and acquired anatomical conditions, lifestyle factors, endocrine disorders, thrombophila, immunological factors, and infections. The American Society of Reproductive Medicine recommends a thorough evaluation after 2 consecutive pregnancy losses, however, this can differ from recommendations by other medical societies. RPL evaluation be evaluated by numerous tests and imaging studies depending on the risk factors. These range from cytogenetic studies, blood tests for clotting disorders, hormone levels, diabetes screening, thyroid function tests, sperm analysis, antibody testing, and imaging studies. Treatment is typically tailored to the relevant risk factors and test findings. RPL can have a significant impact on the psychological well-being of couples and has been associated with higher levels of depression, anxiety, and stress. Therefore, it is recommended that appropriate screening and management be considered by medical providers.  

Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.

<span class="mw-page-title-main">Microchimerism</span> Presence of cells originating from another individual

Microchimerism is the presence of a small number of cells in an individual that have originated from another individual and are therefore genetically distinct. This phenomenon may be related to certain types of autoimmune diseases although the responsible mechanisms are unclear. The term comes from the prefix "micro" + "chimerism" based on the hybrid Chimera of Greek mythology. The concept was first discovered in the 1960s with the term gaining usage in the 1970s.

Rho(D) immune globulin (RhIG) is a medication used to prevent RhD isoimmunization in mothers who are RhD negative and to treat idiopathic thrombocytopenic purpura (ITP) in people who are Rh positive. It is often given both during and following pregnancy. It may also be used when RhD-negative people are given RhD-positive blood. It is given by injection into muscle or a vein. A single dose lasts 12 weeks. It is made from human blood plasma.

Catastrophic antiphospholipid syndrome (CAPS), also known as Asherson's syndrome, is a rare autoimmune disease in which widespread, intravascular clotting causes multi-organ failure. The syndrome is caused by antiphospholipid antibodies that target a group of proteins in the body that are associated with phospholipids. These antibodies activate endothelial cells, platelets, and immune cells, ultimately causing a large inflammatory immune response and widespread clotting. CAPS was first described by Ronald Asherson in 1992. The syndrome exhibits thrombotic microangiopathy, multiple organ thromboses, and in some cases tissue necrosis and is considered an extreme or catastrophic variant of the antiphospholipid syndrome.

The Eutherian Fetoembryonic Defense System(eu-FEDS) is a hypothetical model describing a method by which immune systems are capable of recognizing additional states of relatedness like "own species" such as is observed in maternal immune tolerance in pregnancy. The model includes descriptions of the proposed signaling mechanism and several proposed examples of exploitation of this signaling in disease states.

<span class="mw-page-title-main">Decidualization</span>

Decidualization is a process that results in significant changes to cells of the endometrium in preparation for, and during, pregnancy. This includes morphological and functional changes to endometrial stromal cells (ESCs), the presence of decidual white blood cells (leukocytes), and vascular changes to maternal arteries. The sum of these changes results in the endometrium changing into a structure called the decidua. In humans, the decidua is shed during childbirth.

<span class="mw-page-title-main">Minor histocompatibility antigen</span>

Minor histocompatibility antigen are peptides presented on the cellular surface of donated organs that are known to give an immunological response in some organ transplants. They cause problems of rejection less frequently than those of the major histocompatibility complex (MHC). Minor histocompatibility antigens (MiHAs) are diverse, short segments of proteins and are referred to as peptides. These peptides are normally around 9-12 amino acids in length and are bound to both the major histocompatibility complex (MHC) class I and class II proteins. Peptide sequences can differ among individuals and these differences arise from SNPs in the coding region of genes, gene deletions, frameshift mutations, or insertions. About a third of the characterized MiHAs come from the Y chromosome. Prior to becoming a short peptide sequence, the proteins expressed by these polymorphic or diverse genes need to be digested in the proteasome into shorter peptides. These endogenous or self peptides are then transported into the endoplasmic reticulum with a peptide transporter pump called TAP where they encounter and bind to the MHC class I molecule. This contrasts with MHC class II molecules's antigens which are peptides derived from phagocytosis/endocytosis and molecular degradation of non-self entities' proteins, usually by antigen-presenting cells. MiHA antigens are either ubiquitously expressed in most tissue like skin and intestines or restrictively expressed in the immune cells.

Hypercoagulability in pregnancy is the propensity of pregnant women to develop thrombosis. Pregnancy itself is a factor of hypercoagulability, as a physiologically adaptive mechanism to prevent post partum bleeding. However, when combined with an additional underlying hypercoagulable states, the risk of thrombosis or embolism may become substantial.

Certain sites of the mammalian body have immune privilege, meaning they are able to tolerate the introduction of antigens without eliciting an inflammatory immune response. Tissue grafts are normally recognised as foreign antigens by the body and attacked by the immune system. However, in immune privileged sites, tissue grafts can survive for extended periods of time without rejection occurring. Immunologically privileged sites include:

Immune tolerance in pregnancy or maternal immune tolerance is the immune tolerance shown towards the fetus and placenta during pregnancy. This tolerance counters the immune response that would normally result in the rejection of something foreign in the body, as can happen in cases of spontaneous abortion. It is studied within the field of reproductive immunology.

Lupus and pregnancy can present some particular challenges for both mother and child.

<span class="mw-page-title-main">Adaptive NK cell</span> Specialized subtype of cytotoxic lymphocyte

An adaptive natural killer (NK) cell or memory-like NK cell is a specialized natural killer cell that has the potential to form immunological memory. They can be distinguished from cytotoxic NK (cNK) cells by their receptor expression profile and epigenome. Adaptive NK cells are so named for properties which they share with the adaptive immune system. Though adaptive NK cells do not possess antigen specificity, they exhibit dynamic expansions of defined cell subsets, increased proliferation and long-term persistence for up to 3 months in vivo, high IFN-γ production, potent cytotoxic activity upon ex vivo restimulation, and protective memory responses.

Vikki Martyne Abrahams is an English–American reproductive immunologist. She is a full professor of obstetrics, gynecology and reproductive sciences at the Yale School of Medicine. Her research focuses on understanding the role of innate immune toll-like receptor and NOD-like receptor family members in placental and maternal-fetal immune responses.

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