Beyond Uterine Natural Killer Cell Numbers in Unexplained Recurrent Pregnancy Loss: Combined Analysis of CD45, CD56, CD16, CD57, and CD138

Chronic endometritis and recurrent reproductive failure: a systematic review and meta-analysis

Background

The endometrium holds a crucial role in reproduction by supporting blastocyst adhesion, cytotrophoblast invasion and fetal development. Among the various uterine disorders, endometritis, particularly chronic endometritis (CE), has gained attention due to its association with adverse reproductive outcomes (recurrent pregnancy loss (RPL), recurrent implantation failure (RIF), and infertility). The association between CE and adverse reproductive outcomes stresses the necessity for comprehensive diagnostic and therapeutic strategies to optimize fertility outcomes and support individuals in their journey towards parenthood.

Aim

To explore the relationship between CE and reproductive disorders.

Methods

Following PRISMA guidelines, a systematic review and meta-analysis using published data from 1990 to 2024 were carried out.

Results

A population of 1,038 women was included. Regarding CE-infertility association, a positive correlation was found, with 19.46% CE rate in infertile women compared to 7.7% in controls (OR: 2.96, 95% CI 1.53-5.72, p 0.001). No significant association was observed between RIF and CE (OR: 1.10, 95% CI 0.26-4.61, p 0.90), CE rates in both groups were relatively comparable, with 6.35% in women with RIF and 5.8% in controls. On the opposite, a strong association between CE and RPL was found, reporting a CE rate of 37.6% in RPL cases compared to 16.4% in controls (OR: 3.59, 95% CI 2.46-5.24, p < 0.00001).

Conclusions

CE appears to be associated to infertility and RPL, while no significant association was noted in cases of RIF.

Systematic review registration

https://www.crd.york.ac.uk/prospero/#recordDetails PROSPERO, identifier CRD42024541879.

When Less Is More – Pipelle Endometrial Sampling for Quantification of Uterine Natural Killer Cells in Patients With Recurrent Implantation Failure or Habitual Abortion

Despite recent advancements in reproductive medicine, recurrent implantation failure and habitual abortion remain ongoing issues. One of the most important aspects of successful implantation is the intricate immune response and regulation necessary for the acceptance of the hemiallogenic embryo. The most numerous immune cells in the decidua are uterine natural killer cells (uNK). Studies suggest that changes in the uNK count and physiology may be responsible for the aforementioned pathological conditions. Thus, testing for uNK may provide valuable insights into their pathogenesis. The study compared Pipelle endometrial sampling with conventional curettage to find out whether the less invasive Pipelle method is a viable alternative of tissue collection. Tissue samples from 14 patients obtained by both methods were examined. The average size of tissue samples obtained with Pipelle was 17 mm2, samples obtained with curettage had on average 34 mm2. Using immunohistochemical visualization of CD56 (NK cells) and granzyme B antigens (serine protease-expressing activation state of NK cells), it was found that the average total count of CD56 / mm2 was 115 for Pipelle and 120 for curettage, respectively. The study also proved a correlation between granzyme B positivity and identification of NK cells clusters. The results indicated that Pipelle endometrial sampling seems a suitable method of tissue harvesting for the purpose of uNK cells examination. Pipelle endometrial sampling is safe, cost-effective and can be performed on an outpatient basis without the need of anesthesia or analgesia. Several issues remain yet to be solved: how to standardize the subsequent uNK testing, how to interpret the results and finally yet importantly, how to use this knowledge in personalized treatment protocols.

A blinded multicenter investigation: Accentuated NK lymphocyte CD335 (NKp46) expression predicts reproductive failures after IVF

The peripheral blood NK cell diversity is highly complex. Recent studies have described more than a thousand phenotypes sharing NK cell receptors (NKRs), across the leukocyte lineages. Previously, we have found that accentuated NK p46 phenotype has prognostic value for NK cytotoxicity status, and is characteristic for patients with recurrent implantation failure (RIF). In a blinded investigation we studied blood samples from IVF women before embryo transfer (pre-implantation genetic tested [PGT] embryos n = 116; not tested embryos n = 219). We studied NKp46 expression by flow cytometry and anti-cardiolipin antibody (aCL) levels. aCL results were transmitted to the clinic but NKp46 expression was blinded (for us and for the clinic) and not analyzed before termination of the study (end of last pregnancy). Association of NKp46 phenotype with clinical pregnancy rate (CPR), pregnancy failure (PF) rate and life birth rate (LBR) were analyzed. aCL positive and IvIg treated cases were excluded. IVF success was dependent on p46 NK phenotype in patients with PGT embryos. Elevated p46 expression on NK (>93%) as well as decreased (<66%) significantly reduce CPR (OR 12.7 and 3.8) without affecting pregnancy failure frequency. Both accentuations (taken together) resulted in a significant reduction of LBR (OR 3.9 p = 0.019) compared with non-accentuated phenotypes (p46 levels 66-93%). Elevated NK cell levels (>14.5% weakly) were associated with PF (OR 3.1 p = 0.069), but not significantly with reduced LBR. In contrast, numbers of NKCD335+ lymphocytes (>11.5%) were a significant predictor of PF (OR-4.0 p<0.05) and decreased LBR (OR 2.1 p = 0.06). At the same time, accentuated numbers of NKCD335neg lymphocytes (<0.7 and >4%) were also associated with decreased LBR (OR 2,65 p = 0.05). In patients with NKCD335++ numbers (<5 and >21%), we found a weakly association with IVF failure. We found similar associations in IVF patients without PGT -A but at lower significance levels regardless the higher number of patients. Impact of NKp46 phenotype for IVF success was significant in patients with donor's ET and almost imperceptible in patients > 35y.o. with own embryo transfer. Accentuated increased or decreased CD335 expression on NK was associated with embryo implantation failure. Balanced CD335 levels form a condition favorable for implantation. Elevated numbers of p46+NK (CD3-CD56+CD335+) predicts pregnancy failures at higher significance levels than elevated NK cell numbers. Elevated numbers of p46negNK (CD3-CD56+CD335-) indicate reduced LBR. Accentuation of p46 expression on NK cells is associated with reproductive failures. In combination with PGD it provides a powerful prediction algorithm and treatment option.

When Less Is More - Pipelle Endometrial Sampling for Quantification of Uterine Natural Killer Cells in Patients With Recurrent Implantation Failure or Habitual Abortion

Despite recent advancements in reproductive medicine, recurrent implantation failure and habitual abortion remain ongoing issues. One of the most important aspects of successful implantation is the intricate immune response and regulation necessary for the acceptance of the hemiallogenic embryo. The most numerous immune cells in the decidua are uterine natural killer cells (uNK). Studies suggest that changes in the uNK count and physiology may be responsible for the aforementioned pathological conditions. Thus, testing for uNK may provide valuable insights into their pathogenesis. The study compared Pipelle endometrial sampling with conventional curettage to find out whether the less invasive Pipelle method is a viable alternative of tissue collection. Tissue samples from 14 patients obtained by both methods were examined. The average size of tissue samples obtained with Pipelle was 17 mm2, samples obtained with curettage had on average 34 mm2. Using immunohistochemical visualization of CD56 (NK cells) and granzyme B antigens (serine protease-expressing activation state of NK cells), it was found that the average total count of CD56 / mm2 was for Pipelle 115 and 120 for curettage, respectively. The study also proved a correlation between granzyme B positivity and identification of NK cells clusters. The results indicated that Pipelle endometrial sampling seems a suitable method of tissue harvesting for the purpose of uNK cells examination. Pipelle endometrial sampling is safe, cost-effective and can be performed on an outpatient basis without the need of anesthesia or analgesia. Several issues remain yet to be solved: how to standardize the subsequent uNK testing, how to interpret the results and finally yet importantly, how to use this knowledge in personalized treatment protocols.

Emerging Concepts in Innate Lymphoid Cells, Memory, and Reproduction

Members of the innate immune system, innate lymphoid cells (ILCs), encompass five major populations (Natural Killer (NK) cells, ILC1s, ILC2s, ILC3s, and lymphoid tissue inducer cells) whose functions include defense against pathogens, surveillance of tumorigenesis, and regulation of tissue homeostasis and remodeling. ILCs are present in the uterine environment of humans and mice and are dynamically regulated during the reproductive cycle and pregnancy. These cells have been repurposed to support pregnancy promoting maternal immune tolerance and placental development. To accomplish their tasks, immune cells employ several cellular and molecular mechanisms. They have the capacity to remember a previously encountered antigen and mount a more effective response to succeeding events. Memory responses are not an exclusive feature of the adaptive immune system, but also occur in innate immune cells. Innate immune memory has already been demonstrated in monocytes/macrophages, neutrophils, dendritic cells, and ILCs. A population of decidual NK cells characterized by elevated expression of NKG2C and LILRB1 as well as a distinctive transcriptional and epigenetic profile was found to expand during subsequent pregnancies in humans. These cells secrete high amounts of interferon-γ and vascular endothelial growth factor likely favoring placentation. Similarly, uterine ILC1s in mice upregulate CXCR6 and expand in second pregnancies. These data provide evidence on the development of immunological memory of pregnancy. In this article, the characteristics, functions, and localization of ILCs are reviewed, emphasizing available data on the uterine environment. Following, the concept of innate immune memory and its mechanisms, which include epigenetic changes and metabolic rewiring, are presented. Finally, the emerging role of innate immune memory on reproduction is discussed. Advances in the comprehension of ILC functions and innate immune memory may contribute to uncovering the immunological mechanisms underlying female fertility/infertility, placental development, and distinct outcomes in second pregnancies related to higher birth weight and lower incidence of complications.

Accentuated Peripheral Blood NK Cytotoxicity Forms an Unfavorable Background for Embryo Implantation and Gestation

Problem Many studies have demonstrated the negative impact of high rates of NK cytotoxicity (NKc) on reproductive processes, but there is no agreement as to the appropriateness and feasibility of using the NKc for routine diagnostic in IVF patients. This study conducted a retrospective analysis of embryo transfer (ET) success rates and live birth rates (LBR) in patients with different NKc values. Method of study 1854 cycles of ET were selected and divided into three groups according to NKc levels, and randomized by anamnesis and age: normal (nNKc, n = 871), elevated (eNKc, n = 759), and reduced NKc (rNKc, n = 123). ET with donors’ embryos (n = 101) were analyzed separately. NKc-to-K562 was measured in PBMC (peripheral blood mononuclear cells) by flow cytometry before ET. The patients did not obtain any additional treatments. Results Patients with eNKc, in addition to having reduced clinical pregnancy rates (OR1.59, p < 0.0001), had increased levels of subsequent pregnancy failures (OR2.545, p < 0.0001) when compared to nNKc patients. As a result, patients with eNKc had almost half the LBR than patients with nNKc (OR2.2, p < 0.0001). In patients with rNKc, LBR was also lowered. eNKc was equally unfavorable for implantation and delivery in cryo- or fresh cycles. Markedly, eNKc was much more unfavorable for reproduction than slightly elevated NKc. The donor’s embryos were implanted irrespective of the recipient’s NKc levels, but the later stages of pregnancy were worse in patients with eNKc. Conclusions Our findings highlighted the negative impact of high levels of NK cytotoxicity on pregnancy outcomes.

Bone Marrow Mesenchymal Stem Cells (BMSCs)-Derived miR-31 in the Pathogenesis of Recurrent Abortion by Regulating Kisspeptins 1 (KISS1) Expression

Bone marrow mesenchymal stem cells (BMSCs) are potentially efficacious in treating recurrent pregnancy disorders and endometrial injury. Uterine parenchymal cells interact with BMSCs to promote functional recovery. Our research aimed to explore the effect of BMSCs-derived miR-31 on recurrent pregnancy loss. A recurrent pregnancy loss mouse model was constructed followed by nanoparticle analysis of BMSC and miR-31 expressing by RT-PCR. The levels of miR-31 in BMSCs (miR-31+BMSCs or BMSCs) and their counterpart exosomes were up- or down-regulated to explore the effects of aberrant expression of miR-31 on endometrial damage in recurrent pregnancy loss. The analysis of BMSC nanoparticles showed that miR-31 was derived from BMSC. We found increased levels of miR-31 in miR-340 + BMSCs after incubation with endometrial stromal cells (ESCs) compared to controls. Labeling of exosomes by red fluorescent protein indicated that exosomes were liberated out of BMSCs and translocated into neighboring ESCs, and mice treated with miR-340 + BMSCs had improved functional recovery from recurrent pregnancy loss. BMSC-derived miR-31 mediates functional recovery induced in recurrent pregnancy miscarriage mice by regulating KISS1 expression and fibrosis gene expression.

Clinical consequences of defective decidualization

Decidualization is characterized by a series of genetic, metabolic, morphological, biochemical, vascular and immune changes occurring in the endometrial stroma in response to the implanting embryo or even before conception and involves the stromal cells of the endometrium. It is a fundamental reproductive event occurring in mammalian species with hemochorial placentation. A growing body of experimental and clinical evidence strongly suggests that defective or disrupted decidualization contributes to the establishment of an inappropriate maternal-fetal interface. This has relevant clinical consequences, ranging from recurrent implantation failure and recurrent pregnancy loss in early pregnancy to several significant complications of advanced gestation. Moreover, recent evidence indicates that selected diseases of the endometrium, such as chronic endometritis and endometriosis, can have a detrimental impact on the decidualization response in the endometrium and may help explain some aspects of the reduced reproductive outcome associated with these conditions. Further research efforts are needed to fully understand the biomolecular mechanisms ans events underlying an abnormal decidualization response. This will permit the development of new diagnostic and therapeutic strategies aimed to improve the likelihood of achieveing a successful pregnancy.

Immunomodulatory properties of extracellular vesicles in the dialogue between placental and immune cells

Extracellular vesicle (EV)-mediated communication has been implicated in the cooperative alliance between trophoblast and immune cells toward maternal tolerance and placentation. Syncytiotrophoblast cells secrete EVs directly into the maternal circulation, which are taken up by immune cells, endothelial cells, and other cell types. Initial evidence also shows that EVs produced by immune cells are, in turn, incorporated by trophoblast cells and modulate placental responses. Non-coding RNAs (ncRNAs), proteins, and lipid mediators transported in EVs are able to influence proliferation, differentiation, cytokine production, and immunological responses of recipient cells. The molecular alphabet and cellular targets involved in this dialogue are being revealed. Nevertheless, several questions regarding the whole content, surface markers, and biological functions of EVs still remain to be investigated in both physiological and pathological conditions. Analysis of circulating EVs in maternal blood has the potential to serve as a minimally invasive approach to monitoring placental functions and immunological features of pregnancy, aiding in the diagnostics of complications. This review addresses the immunomodulatory properties of EVs and their tasks in the communication between placental and immune cells.