Natural killer (NK) cells have a dual role in human reproduction for maternal-fetal tolerance and protection from infection. During the ovarian cycle and pregnancy, peripheral NK (pNK) and uterine NK (uNK) cells dynamically change their proportions and cytotoxicities to prepare and accommodate invading trophoblast and maintain pregnancy. However, dysregulated pNK and uNK cell proportions and cytotoxic activities have been associated with aberrant spiral artery remodeling and trophoblast invasion, leading to implantation failures and recurrent pregnancy losses (RPLs). This review will focus on the role of NK cells in RPLs reviewing the ontogeny of NK cells, changes in pNK and uNK cell levels, and activities during the ovarian cycle, normal pregnancy, and RPL. In addition, the immunopathological role of NK cells in endometrial/decidual vascular development and killer immunoglobin-like receptor (KIR) and human leukocyte antigen (HLA)-C interactions are discussed.

Menopause signals the end of the reproductive period in women. However, fertility and fecundity decrease with increasing age prior to menopause demonstrating that changes in the premenopausal female reproductive tract (FRT) are already occurring that negatively impact reproductive success. The effects of age on the endometrium are poorly understood, in contrast to the ovary where changes occur with increasing age that negatively affect successful reproduction. The endometrial immune system is essential for generating a receptive endometrium, but the link between the immune and reproductive systems in the endometrium in the years prior to menopause has not been well-defined. Since the endometrial immune system is tightly regulated to maximize reproductive success and pathogen protection, changes in immune function with increasing premenopausal age have the potential to impact reproduction.

Complement component 1q (C1q) is the recognition molecule of the classical pathway of the complement system that can bind to an array of closely spaced antigen-bound immunoglobulin G (IgG) and IgM antibodies. In addition to its involvement in defence against a range of pathogens and clearance of apoptotic and necrotic cells, C1q has also been implicated in immune and non-immune homeostasis. C1q is locally produced by immune cells such as monocytes, macrophages, and dendritic cells. C1q is also synthesized by decidual endothelial cells, thus acting as a link between decidual cells and trophoblasts, as well as contributing to the remodelling of spiral arteries. Furthermore, C1q is produced by the extravillous trophoblasts (EVTs) invading the decidua. As a pro-angiogenic molecule, C1q is also important for normal placentation processes as it favors the active angiogenesis in the developing decidua. These observations have been validated by C1q gene knock-out mice which showed pre-eclampsia (PE)-like symptoms, characterized by hypertension, proteinuria, glomerular endotheliosis, and increased soluble fms-like tyrosine kinase-1 (sFlt-1)/placental growth factor (PlGF) ratio, and increased oxidative stress. The role of C1q in normal and adverse human pregnancy is being studied extensively due to its absence or low level as a likely precipitating factor for the development of PE.

Uterine natural killer (uNK) cells, a specific type of natural killer (NK) cells, are important cells at the foeto-maternal interface in humans as well as in mice. uNK cells are part of the innate lymphoid cells group 1. Especially in the mouse, but also in the rat, many in vivo studies have been performed to evaluate the role of uNK cells in placental development. These studies have shown that uNK cells are not indispensable to pregnancy, but that they play an important role in optimal decidual angiogenesis in early pregnancy, trophoblast invasion and spiral artery remodelling in the mouse placenta. Based on the mouse studies, various in vitro studies, as well as immunohistological studies of the human placenta from elective abortions, have shown that uNK cells have similar functions in the human placenta. In the present narrative review, the role of the uNK cells in the development of the mouse and rat placenta will be discussed first. Thereafter, studies on the role of human uNK cells in the human placenta will be reviewed and these studies will be discussed in the light of the knowledge on mouse uNK cells.

Infertility affects millions of people of reproductive age. The failure of a blastocyst to implant is a leading cause of psychological distress. It became increasingly evident that an effective immune dialogue occurs at each step in the fluids surrounding the oocyte, the spermatozoa, the embryo, or the endometrium. Exploring and deciphering this dialogue could potentially help understand why 50% of healthy euploid blastocysts fail to implant. Introducing immunology into reproductive medicine requires a change of mindset to bring immune hypothesis to clinical applications. Implantation of an embryo requires a prepared uterus in order to dialogue with the embryo, which is able to express and repair itself. Exploring the uterine immune profile of patients with previous implantation failures (IF) or recurrent miscarriages (RM) has already been developed and is under evaluation as a precision tool to equilibrate the uterine environment before implantation to increase the subsequent live birth rate after the embryo transfer. Immunology may also be fundamental in the future to identify through non-invasive procedure the competence of oocytes or embryos through reliable immune biomarkers quantified in follicular fluids or embryo supernatants during the in vitro fertilization (IVF) process. Non-invasive biomarkers would allow physicians to identify competent oocytes or embryos based on their ability to communicate with the mother and their energetic potential for all the self-repair processes that should occur during the preimplantation and the implantation period. This area of research is only beginning.

Antiphospholipid syndrome (APS) is defined by recurrent pregnancy morbidity and/or vascular thrombosis associated with the persistent presence of antibodies against anionic phospholipid-binding proteins. Beta 2 glycoprotein I (β2GPI) and prothrombin (PT) are the major antigens for antiphospholipid antibodies (aPL) detectable by functional coagulation [lupus anticoagulant (LA)] or solid-phase assays [anti-β2GPI-dependent cardiolipin (aCL) and anti-β2GPI]. β2GPI-dependent aPL are responsible for the positivity of the three classification laboratory criteria. While medium/high titers of antibodies against β2GPI are risk factors for both the vascular and the obstetric manifestations of APS, persistent low titers are also associated with pregnancy complications. There is evidence from animal models of aPL-dependent fetal loss and from in vitro systems that β2GPI-dependent aPL can be pathogenic. β2GPI is physiologically found in large quantities at the placental level being available for the specific antibodies circulating in the maternal blood. Once bound to the protein, the antibodies trigger a local inflammation via the activation of the complement cascade and affect trophoblast and decidual function. The final result is represented by defective placentation, while thrombotic events are apparently less important. β2GPI is a pleiotropic molecule with scavenging properties towards several molecules including apoptotic material and displays anti-oxidant activity. These functions may explain the β2GPI placental localization in an area of intensive tissue remodeling and low oxygen tension. Since β2GPI interacts also with the complement and the coagulation cascade, its binding with specific antibodies may affect the physiology of placentation in several ways.

Endometriosis (EMS) is an inflammatory, gynaecologic disease characterized by the growth of endometrial tissues outside the uterus. With no satisfactory therapies or non-invasive diagnostics available, a shift in perspectives on EMS pathophysiology is overdue. The implication of immune dysregulation in EMS pathogenesis and disease progression has been an evolving area of research, with numerous immune and inflammatory pathways identified. Traditional theories regarding the establishment of endometriotic lesions have lacked mechanistic explanations for their proliferation and survival until recent research unearthed the involvement of mesenchymal stem cell (MSC) and myeloid-derived suppressor cells (MDSCs) in a complex network of immune-endocrine signaling. The unique immunology of EMS is likely owing to estrogen dominance, as endocrine imbalance reliably cultivates immune dysregulation. Many of the phenomena observed in EMS parallel immune biology seen in various cancers, including accelerated somatic mutations in endometrial epithelial cells. Here, the high mutational load leads to EMS neoantigen development which potentially contributes to the lesion immune microenvironment. As well, EMS manifests comorbidity with several chronic inflammatory diseases that share common dysregulation of the interleukin-23 (IL-23)/IL-17 pathway (as seen in inflammatory bowel disease, psoriasis, and rheumatoid arthritis). EMS is especially relevant to the study of chronic pelvic pain (CPP) as 60% of EMS patients experience this symptom and chronic inflammation is believed to be central to the process of pain sensitization. Since the onset of the disease usually occurs in adolescence, and diagnosis only occurs years later once moderate to severe symptoms have developed, it is vital to innovate non-invasive diagnostic tools for earlier detection. Several potential biomarkers are being studied, including some cytokines, gene signatures, and extracellular vesicle (EV) signatures. By incorporating the immune perspectives of EMS into our research, approaches to diagnosis, and treatment solutions, the field has more promising avenues to clearly define EMS and offer patients relief.

“There is many a slip twist the cup and the lip” is a proverb that dates back to the 3rd century. This proverb comes to mind while writing a review on pregnancy loss; so many complications can occur between fertilization and development of the embryo through the long period of gestation until successful delivery of the baby. These include failure of implantation of the embryo, spontaneous miscarriage in the first trimester, pre-eclampsia in the second trimester, premature rupture of fetal membranes, pre-term labour, and pre-term delivery. The maternal immune system which does a phenomenal job of protecting the host from a daunting variety of infections, sometimes also mounts adverse reactions that complicate pregnancy and endanger the fetus. Maternal immune reactions that can adversely affect pregnancy have been shown to be mediated by lymphocytes, macrophages and natural killer cells, and by cytokines secreted by these cellular effectors. This review summarizes the deleterious effects of cytokines leading to recurrent spontaneous miscarriage, pre-eclampsia and pre-term delivery, which are the major complications of pregnancy. It then goes on to discuss the potential use of progesterone and dydrogesterone, an orally-administered progestogen, as immunomodulatory molecules that can be considered for the prevention and/or treatment of these complications.

Polycystic ovary syndrome (PCOS) is one of the most frequently observed endocrinopathies among women of reproductive age that redound to subfertility. The specific etiology of this heterogenic syndrome remains ambiguous. Metabolic complications, hormonal imbalance, deregulation in the immune system and their interrelationship make PCOS more complex. Hyperandrogenism and chronic low-grade inflammation modulate each other and enhance the self-perpetuation of PCOS. Even though there are many literature studies on PCOS and immune deregulation, this review focuses on the endocrine-immune nexus and how the altered endocrine system is embroiled in the immunopathology of PCOS.

Among decidual immune cells, regulatory T cells (Tregs) have been unanimously recognized as central contributors to tolerance and maintenance of healthy pregnancy. Numerical and functional downregulation of Tregs or disturbed interaction of Tregs with trophoblasts and other immune cells have been linked to early pregnancy loss such as idiopathic infertility and miscarriage and later-onset adverse pregnancy outcomes including preeclampsia. This review focuses on the mechanisms for regulating the generation, expansion, and function of Tregs, the roles of Tregs in maintaining maternal immune tolerance through crosstalk with trophoblasts and other decidual regulatory immune cells, and how Tregs may play foes to pregnancy and contribute to the programming of pregnancy-related complications. Therapeutic options for implantation failure and adverse pregnancy outcomes are now part of the emerging significance of Tregs in pregnancy tolerance and maintenance.

Humans are afflicted by a wide spectrum of autoimmune disorders, ranging from those affecting just one or a few organs to those associated with more systemic effects. In most instances, the etiology of such disorders remains unknown; a consequence of this lack of knowledge is a lack of specific treatment options. Systemic lupus erythematosus (SLE) is the prototypic systemic autoimmune disorder; pathology is believed to be antibody-mediated, and multiple organs are targeted. Periods of disease “flares” are often followed by long periods of remission. The fact that SLE is more commonly observed in females, and also that it more particularly manifests in females in the reproductive age group, has quite naturally drawn attention to the potential roles that hormones play in disease onset and progression. This review attempts to shed light on the influences that key hormones might have on disease indicators and pathology. Databases (Google Scholar, PubMed) were searched for the following keywords (sometimes in certain combinations), in conjunction with the term “lupus” or “SLE”: autoantibodies, recurrent abortion, polycystic ovarian syndrome (PCOS), preeclampsia, pre-term delivery, estrogens, progesterone, androgens, prolactin, leptin, human chorionic gonadotropin (hCG). Cited publications included both research articles and reviews.

Endometriosis is an inflammatory oestrogen-dependent chronic disease and is mainly expressed by pain and increased infertility. Several studies showed an increased prevalence of autoimmune systemic diseases and various autoantibodies in endometriosis. The association of these autoimmune markers and diseases could raise the fact that endometriosis is an authentic autoimmune or inflammatory disease and thus could argue for the use of immunomodulatory therapies. Usually, it is considered that the autoantibodies did not directly act in endometrium implants growth, and could be rather implicated in endometriosis-related infertility. The use of immunomodulatory strategies could be an important alternative or additional strategy to the use of hormones and surgery but need prospective well-designed trials.

It is well recognized that immune tolerance is important to prevent semiallografted fetuses from rejection by maternal immunocompetent cells; however, immune activation also plays an important role in placental development and fetal growth. Basic and clinical studies have shown that an imbalance between immune activation and regulation can lead to implantation failure, miscarriage, and preeclampsia. Here, the balance between immunostimulation and immunoregulation in reproduction will be reviewed.

The foetus expressing paternal antigens ought to be “rejected” by the maternal immune system. However, the immunological relationship of the mother and the foetus does not follow the rules of transplantation immunology. Maternal immune functions are re-adjusted during pregnancy, to create a tolerant environment for the developing foetus. Progesterone and its downstream mediator; the progesterone induced blocking factor (PIBF) are important in this process. The mRNA transcribed from the PIBF1 gene contains 18 exons, and codes for a 90 kDa protein. The 90 kDa form is associated with the centrosome and plays a role in cell cycle regulation, while smaller isoforms produced by alternative spicing are secreted, and bind to the glycosylphosphatidylinositol (GPI) anchored PIBF receptor. Upon ligation, the former forms a heterodimer with the alpha chain of the interleukin-4 (IL-4) receptor and activates the Janus kinase/signal transducers and activators of transcription (Jak/STAT) pathway, via which, PIBF induces increased production of T helper2 (Th2) cytokines. PIBF regulates natural killer (NK) cytotoxicity, by inhibiting perforin release from the cytoplasmic granules of NK cells. During normal human pregnancy, the serum concentrations of PIBF increase with gestational age, and lower than normal serum levels predict spontaneous pregnancy termination. Depletion of PIBF during the peri-implantation period in mice, results in lower implantation and increased resorption rates, together with increased decidual and peripheral NK activity, downregulation of the genes implicated in T cell activation in CD4⁺ cells, and Th1 differentiation of the T cells. PIBF is expressed in rapidly proliferating immature cells as well as several tumours, and regulates invasion. The PIBF gene has been identified in the chromosomal region 13q21-q22—which is a common site for somatic deletions in a variety of malignant tumours. These data suggest that PIBF might be involved in tumorigenesis.

This article is a tribute and homage to Gerard Chaouat who invited me to contribute this article. My years in France have remained very memorable to me. Reviewed briefly is the vaccine that was made against human chorionic gonadotropin (hCG) to prevent unwanted pregnancy in sexually active women. It has now been developed as a genetically engineered recombinant vaccine and passed onto industry for its production under good manufacturing practices (GMP) conditions for confirmatory trials. The trials have received the approval of the Drugs Controller General of India. The trials have started but have been interrupted by the coronavirus disease 2019 (COVID-19) pandemic. This vaccine is likely to have another highly beneficial application in the treatment of cancers expressing ectopically hCG.

The maternal syndrome preeclampsia is triggered by syncytiotrophoblast (STB) stress; the heterogeneity of the syndrome is caused by the different pathways leading to this STB stress. Inflammation plays a pivotal role in the pathogenesis of preeclampsia. While, the immune system at large is therefore intimately involved in the causation of this heterogeneous syndrome, the role of the adaptive immune system is more controversial. The classic paradigm placed preeclampsia as the disease of the nulliparous pregnant women. Up to the later part of the 20th century, human reproduction, particularly in Western societies, was characterised by a low rate of pre-marital sex, and the great majority of children being born within one stable sexual relationship. More prolonged periods of regular sexual intercourse within a stable relationship have been demonstrated to reduce the risk of preeclampsia and fetal growth restriction. Primarily animal studies have indeed shown that repetitive sperm exposure leads to partner specific mucosal tolerance. Societal changes made partner change over the reproductive period of individual women extremely common. For the adaptive immune system of multiparous women, being pregnant in a new sexual relationship (primipaternity) would represent being faced with a new “hemi-allograft”. In these pregnancies, potential couple-specific immune “maladaptation” could lead to the superficial cytotrophoblast invasion of the spiral arteries, known to be associated with early-onset preeclampsia. Having a new pregnancy in a different relationship does indeed increase the risk for this type of preeclampsia. Large epidemiologic population studies identified prolonged birth interval but not “primipaternity” as a risk factor for preeclampsia in multiparous women. This apparent contradiction is explained by the fact that the great majority of preeclampsia cases in these population studies involve term preeclampsia. In late-onset preeclampsia, the far more common phenotype of the syndrome, STB stress is not caused by lack of proper spiral artery modification, but involves maternal genetic predisposition to cardiovascular and metabolic disease, with in particular obesity/metabolic syndrome representing major players. Partner or couple specific issues are not detectable in this disease phenotype.

The testis is designated as one of the immune previleged sites in the body and harbours a unique immunoregulatory environment, which is important for preventing an immune response against sperm antigens which otherwise are recognized as “foreign” by the immune system. The blood-testis barrier along with the unique immune cells repertoire and various immunoregulatory & immunosuppressive factors secreted by the Leydig cells, Sertoli cells and peritubular cells act in concert to maintain the tolerogenic environment in the testis. Abberations in immunotolerant mechanisms in the testis can lead to generation of anti-sperm antibodies that have an association with male infertility. It can also lead to inflammatory conditions of the male reproductive tract manifested as epididymitis and orchitis, generally due to bacterial or viral infections. In addition, non-infectious epididymitis and orchitis, having autoimmune origin have also been reported in males. While the immune privilege status of human testis protects the germ cells from an immune attack, it can also make the testis a succeptible reservoir for viruses such as human immunodeficiency virus-1, Zika virus and severe acute respiratory syndrome coronavirus-2, all of which have adverse consequences on male reproduction.