The complexity of immune regulation at the interface between the mother and fetus is that it serves contradictory immunological purposes. On the one hand, immunity protects the fetus from maternal pathogens, and on the other hand, it prevents the rejection of the fetus and placenta that are genetically different from the mother.
The tissues of the mother and fetus are in direct contact where the fetus’s placenta invades and is implanted into the layer of the maternal endometrial tissue-the decidual membrane. There are many maternal leukocytes in the decidual membrane, especially natural killer cells (NK) and regulatory T cells (Treg). These cells provide protective immunity and prevent the rejection of the fetus.
In addition to preventing the transmission of maternal pathogens to the fetus, proper regulation of cytokine signaling, including interferons (IFN), is vital for a healthy pregnancy.
Interferon signaling
Interferons are divided into three families: type I, II, and III. Each type uses its cellular receptors to transmit a signal, and each has its functional properties.
In humans, mice, and inhuman primates, IFN type I includes IFN-alpha, IFN-beta, IFN-epsilon, IFN-kappa, in primates – IFN-omega, in mice – IFN-zeta. Additional IFN I was found in other mammals, for example, IFN-tau in ruminants and IFN-delta in pigs. All type I IFNs transmit a signal through the same IFNAR receptor.
The IFN II family consists of one member-IFN-gamma, which transmits a signal through the IFNGR receptor.
Type III interferons are subtypes of IFN-lambda. All IFN III transmit a signal through the IFNLR receptor.
Since each interferon family uses its receptors to transmit signals, targeting the receptors makes it possible to suppress the activity of the entire IFN family selectively. In the laboratory, genetically modified mice are used for this purpose, which does not have an interferon receptor globally or in specific cell types: for example, Ifnar1 – / -, Ifngr1 – / – and Ifnlr1 – / -mice. In addition, monoclonal antibodies blocking interferon receptors are also used or targeting interferon receptors in cell culture using knockdown or gene editing approaches.
Despite different receptors, interferons of types I and III are functionally similar in many respects. Both kinds of IFN are released when the pattern recognition receptor (PRR) recognizes a viral infection. Interferons bind to the receptor and activate IFN-stimulated genes (ISG), limiting the penetration, replication, and spread of viruses. IFN I and III also perform immunomodulatory functions, affecting the adaptive immune response.
Although IFN I and III trigger similar antiviral responses, they have different physiological effects, determined mainly by the expression of the interferon receptor. IFNAR is expressed everywhere, but IFNLR expression is highest on epithelial cells and some subsets of immune cells, including neutrophils and NK cells. Moreover, although similar PRR signaling pathways activate IFN I and III production, some stimuli contribute only to the production of IFN III. Such triggers include infections of respiratory epithelial cells, the transmission of PRR MAVS signals from peroxisomes rather than from mitochondria, and transmission of PRR TLR4 signals from the plasma membrane rather than from endosomes. IFN III signaling is less powerful and less inflammatory than IFN I. IFN III signaling predominates at anatomical barriers. Hence, IFN III provides the first line of defense against viruses in the epithelium and minimizes the activation of the IFN I systemic response and immune pathologies.
While type I and type III interferons are best known for their antiviral effect, type II interferon has pro-inflammatory and immunomodulatory functions. In addition, IFN II is produced by T cells and NK cells and acts at later stages of infection, clearing infected tissues of viruses. Thus, IFN II also suppresses viral infections.
The intersection of IFN I and II responses can affect the outcome of infection. For example, suppression of the type II interferon receptor by IFN I signaling leads to more severe Listeria monocytogenes infection in mice without the IFN I receptor (Ifnar1 – / -). Although Ifnar1 – / – mice usually have an increased susceptibility to most viral infections, these mice are protected from disease by many bacterial and protozoan pathogens, the immune defense against which strongly depends on type II IFN.
Transmission of interferon signals during pregnancy
Pregnancy has several stages of development, including implantation, fetal growth, and childbirth. Each stage has its unique immunological requirements. For example, early pregnancy events, such as the attachment of the embryo to the uterine wall and the subsequent ingrowth of the placenta, are inflammatory processes that physically destroy and modify the maternal tissue at the implantation site. On the contrary, fetal growth occurs in the environment of type 2 anti-inflammatory T-helpers (Th2). This environment is typical for most of the pregnancy. Finally, childbirth is an inflammatory process. Since implantation, fetal growth, and delivery have different immunological features, IFN plays different roles at each stage of pregnancy. Accordingly, IFN can have other effects in viral infection at various stages of pregnancy.
Before implantation, the embryo is surrounded by a shell-the trophectoderm, which attaches to the maternal endometrium and becomes the layers of the trophoblast that make up the placenta. In mice, IFN I is expressed in the trophectoderm before implantation of the embryo, in the decidual shell after implantation, and in several layers of the trophoblast in the middle of pregnancy. Accordingly, ISGS is also activated in the post-implantation decidual membrane.
Human trophoblasts also produce IFN-gamma in the early stages of pregnancy and by giant mouse trophoblast cells. Decidual NK cells with reduced cytotoxic potential make up most white blood cells in the decidual membrane of the mother and secrete IFN-gamma during pregnancy. The transmission of IFN-gamma signals at the interface between the mother and fetus organisms contributes to the acquisition of specific properties by decidual NK cells and contributes to forming the placenta and maintaining the decidual envelope.
In addition to the fact that IFN-gamma provides a healthy pregnancy development, it is associated with adverse fetal outcomes in models of congenital infection in mice:
- Mice without the type II interferon receptor (Ifngr1 – / –) are less likely to lose a fetus when infected with Toxoplasma gondii and Plasmodium berghei.
- The introduction of an antibody against IFN-gamma protects females from fetal loss due to Brucella abortus
Since IFN-gamma is a critical component of pro-inflammatory immune responses, a fetal loss may be a typical response to infection and inflammation at the interface between maternal and fetal organisms in mice.
The placenta is the place of contact between the blood supply of the fetus and the mother. It ensures the exchange of nutrients and waste between the mother and the fetus. Maternal blood is delivered to the placenta through spiral arteries, reconstructed in the early stages of pregnancy. This process involves the transmission of IFN signals. Mice that lack IFN I or II signaling have incomplete remodeling of the spiral artery. This suggests that type I and type II IFNs contribute to this process.
IFN I perform various functions during pregnancy in other mammals:
- Ruminants express several subtypes of IFN-tau, which transmits a signal through IFNAR and triggers the expression of ISG but does not protect against viruses. Unlike IFN-alpha and beta, IFN-tau expression is triggered not by a viral infection but by the development of trophoblasts. In trophoblasts, IFN-tau serves as a pregnancy recognition factor that affects the hormonal status of the mother before implantation and triggers the expression of ISG in the mother’s endometrium.
- In pigs, another type I interferon (IFN-delta) and IFN-gamma affect the expression of maternal endometrial genes before the attachment of the trophoblast.
- In macaques, IFN-epsilon is secreted by epithelial cells of the mucous membrane of the vagina and cervix and the lungs, foreskin, small and large intestines.
- IFN-beta is produced in mice, humans, and inhuman primates and is constantly secreted in the female genital tract. In mice, IFN-beta is expressed mainly in the endometrium of the uterus, ovaries, and cervix and protects against sexually transmitted infections, including herpes simplex virus type 2 and Chlamydia muridarum. IFN-beta levels fluctuate depending on the estrus cycle and are not determined by the transmission of PRR signals. The role of IFN-beta during pregnancy is not described, but since IFN-beta is secreted in the female reproductive tract, it can potentially protect the fetus from ascending infections.
Interferon signaling in a mouse model of congenital Zika virus
Infection with the Zika virus (ZIKV) during pregnancy can lead to adverse outcomes for the fetus and the newborn, including a decrease in the size of the brain, intrauterine development delay (IVD), loss of vision and hearing, as well as miscarriage.
To study the Zika virus in pregnant women, scientists use mice with a deficiency of IFN / IFN receptors/defects in transmitting type I or II IFN signals/mice receiving antibodies to the IFN I receptor. It is necessary because, in mice, the transmission of type I IFN signals restricts the replication of ZIKV. Congenital ZIKV infection in IFN-deficient mice leads to the transmission of the virus to the fetus, damage to the placenta, and adverse pregnancy outcomes, including ZSD and fetal death.
Recombinant IFN-lambda 2, administered to pregnant females, restricts the transplacental transmission of ZIKV and triggers the ISG response in both placental and decidual cells. That is consistent with the antiviral effect of type III IFN at the interface between maternal and fetal organisms.
The crossing of the mother Ifnar1 – / – and the father Ifnar1 – / – leads to pregnancy, in which 50% of the fetuses and their placentas located inside the Ifnar1 – / – queens do not transmit IFN I signals (Ifnar1 – / –), and in the remaining 50% of fetuses, IFN I signal transmission is not damaged (Ifnar1 – / –).
The transmission of IFN signals I and III has an antiviral effect at the interface between the mother and fetus organisms and restricts the transplacental transmission of ZIKV in mice. However, Ifnar1 – / – fetuses inside Ifnar1 – / – queens die from ZIKV infection. It indicates that the transmission of fetal and placental IFN I signals is insufficient to limit congenital ZIKV infection.
Ifnar1 – / – fetuses have fewer pathologies caused by ZIKV inside Ifnar1 – / – queens than Ifnar1 + /- fetuses . That indicates the detrimental effect of IFN I signaling on the developing placenta and fetus and highlights the delicate balance necessary for maternal antiviral immunity in the context of congenital infection.
Uncontrolled viral replication and impaired regulation of cytokine signaling in Ifnar1 – / – mice lead to high serum concentrations of IFN I. This may contribute to the occurrence of severe infection-induced pathologies in Ifnar1 + /- fetuses inside Ifnar1 – / – queens. Moreover, pathologies caused by type I IFN are not a specific effect of ZIKV since pathologies in Ifnar1 + /- fetuses can also be caused by other flaviviruses and the introduction of imitation viral RNA poly (I:C).
On the contrary, the transmission of fetal and placental IFN I signals can reduce the severity of viral infection in the mother. IFN I signaling is also associated with premature birth in mice after infection with lymphocytic choriomeningitis virus or L. monocytogenes and after administration of poly (I:C) or lipopolysaccharide.
One of the mechanisms of placental pathology caused by IFN may include the ISG response, which affects the physiology of the placenta. For example, IFITM proteins suppress viral infection by preventing membrane fusion, but IFITM expression in the placenta can disrupt cell fusion necessary for the formation of syncytiotrophoblasts – a layer of cells between the blood circulation of the mother and fetus. It proves the fact that mice deprived of IFITM are protected from pregnancy pathology caused by IFN.
Although the outcome of the disease is worse in Ifnar1 – / – mice with a viral infection, with a bacterial infection of L. monocytogenes or C. muridarum, the bacterial load in the placenta and spleen decreases faster in these mice.
Congenital infection of the Zika virus on a model of inhuman primates
Compared to non-pregnant women, the Zika virus has been present in the blood of pregnant rhesus monkeys for a long time. The same is observed in ZIKV-infected pregnant women. Congenital ZIKV infection in rhesus monkeys leads to disease outcomes similar to human ones, including fetal infection, placental pathology, neuropathology, eye diseases, and fetal loss.
ZIKV infection causes a stable systemic adaptive immune response in both pregnant and non-pregnant inhuman primates, characterized by the production of IFN-gamma, an increase in white blood cells, and IgG and IgM antibodies specific to ZIKV. During acute ZIKV infection in non-pregnant inhuman primates, the response of IFN I and II and ISG and pro-inflammatory cytokines and chemokines is rapidly activated in peripheral blood mononuclear cells (PBMC). The levels of IFN I and III in PBMC correlate with the level of ZIKV viral load in the blood. ZIKV infection at the interface between maternal and fetal organisms also leads to stable immune responses in the decidual membrane of the mother and fetal tissues. After infection, the levels of IFN-gamma and other pro-inflammatory cytokines increase in the fetal blood.
During a healthy pregnancy, the immunity of the decidual membrane is strictly regulated to allow trophoblasts to penetrate the decidual tissue and prevent the rejection of the placenta that is genetically different from the maternal tissues. After the end of placentation (the end of the first trimester, ~ 3 weeks in rhesus monkeys), an anti-inflammatory immune environment is preserved in the decidual shell of the mother. Decidual leukocytes are dominated by unique subpopulations of NK cells with reduced cytotoxic potential and regulatory T cells and M2-like macrophages.
In late pregnancy (135 days), rhesus monkeys infected with ZIKV had an increased level of inflammatory leukocytes in the placenta’s decidual membrane and villous tree, including non-classical subpopulations of monocytes and CD4+ T cells, compared with the uninfected control group.
Although the classical molecules of the main histocompatibility complex are not expressed on trophoblasts, Treg and Th2 immune responses are necessary to prevent fetal rejection. The IFN-gamma response and the pro-inflammatory immune response of type 1 T-helper cells (Th1) at the interface between the mother and fetus, such as a viral infection, can damage the placenta and fetal death during congenital ZIKV disease.
Placental infection in ex vivo models and cell cultures
Ex vivo studies are carried out on tissues taken from a living organism and placed in an artificial environment. Using primary placental and decidual tissues cultured ex vivo, the scientists modeled ZIKV infection and IFN responses and production. Specimens from the average human gestation and full-term placenta cultured ex vivo secrete IFN type III. IFN III protects syncytiotrophoblasts from viral infection. After ZIKV infection, decidual cells produce IFN I and III in the middle of gestation, and placental macrophages have IFN-alpha (but not IFN-beta or IFN-lambda). That indicates additional sources of interferons I and III at the interface between the mother and fetus organisms.
Since decidual cells of maternal origin react to IFN I and III, interferons originating from the placenta can cause an antiviral state on both sides of the interface between the mother and fetus organisms. Furthermore, since IFN III causes a less powerful antiviral response than IFN I, the transmission of IFN III signals at the interface between maternal and fetal organisms can cause an antiviral state without the immune pathology caused by type I IFN. The observation confirms this: IFN-beta caused pathological morphological changes (syncytial nodes and processes) of chorionic villi in the middle of human pregnancy, while IFN-lambda 3 did not cause pathology.
Violation of the regulation of interferon signal transmission in autoimmune diseases and adverse pregnancy outcomes
Women with impaired regulation of IFN I signaling – stable IFN production or impaired regulation of interferon receptors – have poor pregnancy outcomes. These include preeclampsia, placental damage, and defects in the development of the nervous system, similar to defects caused by congenital infection.
An increased level of IFN I during pregnancy may be pathogenic since diseases in which the production of IFN I is increased are associated with miscarriage. This relationship is most evident in patients with systemic lupus erythematosus, a rheumatological illness characterized by activation of the type I IFN response. The PBMC analysis of pregnant patients with lupus showed that preeclampsia and other fetal complications are associated with high ISG expression, in contrast to suppressing the IFN I response observed during healthy pregnancy and uncomplicated pregnancy with lupus. In addition, some patients with lupus develop antiphospholipid antibody syndrome – a disease associated with miscarriage, in which patients produce antibodies that increase blood clotting time in laboratory tests but cause the formation of blood clots in vivo.
Although patients with lupus produce autoantibodies, the potentially pathogenic role of some autoantibodies remains uncertain. However, one of the most convincing arguments in favor of the pathogenicity of autoantibodies in lupus is the link between autoantibodies against SSA and SSB and the development of neonatal lupus – a disease characterized by a rash and other signs of lupus in the first few months of life – until maternal antibodies are removed from the bloodstream of newborns. Neonatal lupus can also lead to irreversible damage to the conductive system of the developing heart, which will require the installation of a pacemaker.
Although maternal autoantibodies to SSA and SSB are closely associated with the risk of neonatal lupus, the expression of high levels of IFN I may also contribute to the development of this disease. IFN-alpha can inhibit the formation of new blood vessels and affect the pathogenesis of preeclampsia during pregnancy with lupus. A case confirms that IFN-alpha can contribute to neonatal lupus: treatment of IFN-alpha during pregnancy was associated with ZUR and a facial rash characteristic of neonatal lupus.
Interferonopathies associated with increased production of type I interferon also give an idea of the contribution of IFN I to the pathogenesis of human diseases. Examples of interferonopathies are Aicardi-Gutierrez syndrome and STING-associated vasculopathy with onset in infancy (SAVI). Both of these diseases are caused by mutations that activate the IFN I response.
Although it is believed that these interferonopathies are caused by increased production of IFN I, this has not been definitively demonstrated in humans. In mice, SAVI is associated with mortality in pregnant females and ISG activation in adults. Autoimmunity in this model of interferonopathy may also reflect an effect on adaptive immunity. For example, in addition to an increased level of IFN I, mice with SAVI have severe lung disease caused by T cells. It highlights the difficulty of determining the exact contribution of IFN I to interferonopathy, even if there is a type I IFN signature in the patient’s cells.
Conclusion
Interferons of types I, II, and III play an essential role during pregnancy, both in maintaining normal physiology at the interface between the mother and fetus and preventing the transmission of maternal pathogens to the fetus. However, excessive or unregulated IFN signaling may be pathogenic during pregnancy, complicated by a viral infection or systemic interferonopathies associated with autoimmunity. Understanding the balance between the protective and pathogenic effects of IFN during pregnancy can help create new treatments to improve outcomes in complicated pregnancies related to infections or excessive IFN production.
Source
Protective and Pathogenic Effects of Interferon Signaling During Pregnancy
