Dynamics of TIGIT and PD-1 expression on NK cells during the course of normal pregnancy

Immune checkpoint for pregnancy

A successful pregnancy relies on the precise regulation of the maternal immune system to recognize and tolerate the allogeneic fetus, while simultaneously preventing infection. Immune checkpoint molecules (ICMs), such as programmed death receptor 1 (PD-1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), T cell immunoglobulin, and mucin-domain containing-3 (Tim-3), play critical roles in regulating the immune response during pregnancy. Emerging research highlights the therapeutic potential of targeting these molecules to restore the immune balance in complicated pregnancies. Understanding the dynamic regulation of ICMs during pregnancy may provide new insights into the pathogenesis of these conditions and offer novel approaches for clinical interventions. Here, we review the expression patterns and functions of key ICMs at the maternal-fetal interface, and their involvement in maintaining immune tolerance throughout gestation. Additionally, we describe the current understanding of immune checkpoint pathways in the pathogenesis of complicated pregnancies and discuss the potential for therapeutic targeting of these pathways in this setting.

Inhibition of TIGIT on NK cells improves their cytotoxicity and HIV reservoir eradication potential

The latent human immunodeficiency virus (HIV) reservoir presents the biggest obstacle to curing HIV chronic infection. Consequently, finding novel strategies to control the HIV reservoir is critical. Natural killer (NK) cells are essential for antiviral immunity. However, the influence of NK cell subsets and their associated inhibitory or activating receptors on their cytotoxicity toward the HIV reservoir has not been fully studied. We investigated the relationship between the percentage of NK cells or NK cell subsets and the HIV reservoir. Our results indicated that the percentage of CD56-CD16+ NK cells was positively associated with HIV reservoir size (i.e., HIV DNA, HIV msRNA, or HIV usRNA). Additionally, we observed that the percentage of IFN-γ+ NK cells was inversely related to the HIV reservoir. Furthermore, the expression of TIGIT on NK cells, particularly CD56-CD16+ and CD56dim NK cell subsets, positively correlated with the HIV reservoir. Notably, individuals with higher percentage of TIGIT+ NK and lower percentage of CD226+ NK cells exhibited larger HIV reservoir. Mechanistically, we discovered that TIGIT could inhibit the PI3K-Akt-mTOR-mTORC1 (s6k) signaling pathway to decrease the production of IFN-γ in NK cells. Importantly, inhibiting TIGIT in NK cells enhanced their ability to eliminate reactivated latently infected CD4+ T cells. Our experiments underscored the crucial role of NK cells in controlling the HIV reservoir and suggested that TIGIT serves as a promising target for enhancing the NK cell-mediated clearance of the HIV reservoir.

IMPORTANCE

As a major barrier to human immunodeficiency virus (HIV) cure, HIV reservoir persist in viremia-suppressed infected individuals. NK cells are important antiviral cells, and their impact on reservoir has rarely been reported. We analyzed the relationship between the size of reservoir and NK cell subsets, inhibitory receptor TIGIT expression. Our analysis found that the percentage of CD56-CD16+ NK cells was positively associated with HIV reservoir size. Furthermore, TIGIT expression on NK cells and CD56-CD16+ NK cells or CD56dim NK cells has a positive correlation with the HIV reservoir. TIGIT can inhibit the PI3K-Akt-mTOR-mTORC1 (s6k) signaling pathway to decrease the production of IFN-γ on NK cells. Blocking TIGIT in NK cells can enhance their ability to eliminate reactivated latently infected CD4+ T cells. Our study indicated that NK cells are critical to the control of the reservoir size, and TIGIT may be a target for enhancing the NK cell-mediated elimination of the reservoir.

Surface Immune Checkpoints as Potential Biomarkers in Physiological Pregnancy and Recurrent Pregnancy Loss

Due to the genetic diversity between the mother and the fetus, heightened control over the immune system during pregnancy is crucial. Immunological parameters determined by clinicians in women with idiopathic recurrent spontaneous abortion (RSA) include the quantity and activity of Natural Killer (NK) and Natural Killer T (NKT) cells, the quantity of regulatory T lymphocytes, and the ratio of pro-inflammatory cytokines, which indicate imbalances in Th1 and Th2 cell response. The processes are controlled by immune checkpoint proteins (ICPs) expressed on the surface of immune cells. We aim to investigate differences in the expression of ICPs on T cells, T regulatory lymphocytes, NK cells, and NKT cells in peripheral blood samples collected from RSA women, pregnant women, and healthy multiparous women. We aim to discover new insights into the role of ICPs involved in recurrent pregnancy loss. Peripheral blood mononuclear cells (PBMCs) were isolated by gradient centrifugation from blood samples obtained from 10 multiparous women, 20 pregnant women (11-14th week of pregnancy), and 20 RSA women, at maximum of 72 h after miscarriage. The PBMCs were stained for flow cytometry analysis. Standard flow cytometry immunophenotyping of PBMCs was performed using antibodies against classical lymphocyte markers, including CD3, CD4, CD8, CD56, CD25, and CD127. Additionally, ICPs were investigated using antibodies against Programmed Death Protein-1 (PD-1, CD279), T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3, CD366), V-domain Ig suppressor of T cell activation (VISTA), T cell immunoglobulin and ITIM domain (TIGIT), and Lymphocyte activation gene 3 (LAG-3). We observed differences in the surface expression of ICPs in the analyzed subpopulations of lymphocytes between early pregnancy and RSA, after miscarriage, and in women. We noted diminished expression of PD-1 on T lymphocytes (p = 0.0046), T helper cells (CD3CD4 positive cells, p = 0.0165), T cytotoxic cells (CD3CD8 positive cells, p = 0.0046), T regulatory lymphocytes (CD3CD4CD25CD127 low positive cells, p = 0.0106), and NKT cells (CD3CD56/CD16 positive cells, p = 0.0438), as well as LAG-3 on lymphocytes T (p = 0.0225) T helper, p = 0.0426), T cytotoxic cells (p = 0.0458) and Treg (p = 0.0293), and cells from RSA women. Impaired expression of TIM-3 (p = 0.0226) and VISTA (p = 0.0039) on CD8 cytotoxic T and NK (TIM3 p = 0.0482; VISTA p = 0.0118) cells was shown, with an accompanying increased expression of TIGIT (p = 0.0211) on NKT cells. The changes in the expression of surface immune checkpoints indicate their involvement in the regulation of pregnancy. The data might be utilized to develop specific therapies for RSA women based on the modulation of ICP expression.

PD-1: A critical player and target for immune normalization

Immune system imbalances contribute to the pathogenesis of several different diseases, and immunotherapy shows great therapeutic efficacy against tumours and infectious diseases with immune-mediated derivations. In recent years, molecules targeting the programmed cell death protein 1 (PD-1) immune checkpoint have attracted much attention, and related signalling pathways have been studied clearly. At present, several inhibitors and antibodies targeting PD-1 have been utilized as anti-tumour therapies. However, increasing evidence indicates that PD-1 blockade also has different degrees of adverse side effects, and these new explorations into the therapeutic safety of PD-1 inhibitors contribute to the emerging concept that immune normalization, rather than immune enhancement, is the ultimate goal of disease treatment. In this review, we summarize recent advancements in PD-1 research with regard to immune normalization and targeted therapy.

Exploring Natural Killer Cell Testing in Embryo Implantation and Reproductive Failure: An Overview of Techniques and Controversies

The blastocyst nidation is the most crucial stage to a successful pregnancy, as the white cells work to promote a favorable endometrial microenvironment for this process. Intriguingly, this implantation window lasts, on average, 6 days in most regular women, and its quality is affected by many pathological conditions. Since the grounds of reproductive failure in healthy couples are still uncharted, studies have widely suggested a potential hostile role of the immune system in the equilibrium of the maternal-fetal interface. In recent years, natural killer cells have been the highlight as they represent the greatest lymphocyte in the uterus and have immune surveillance through cytotoxicity during the implantation window. This review explored the main techniques used for natural killer (NK) cell testing in the implantation window over the last 13 years on the PubMed® database. Of 2167 published articles potentially relevant for the review, only thirty-three were about cell evaluation in healthy women, met the inclusion criteria, and had their methodology critically analyzed. Here, we bring a summary from the study group and sample collection to evidence comments about their findings and correlations. Meanwhile, we also summarize the current relationship between NK cells and endometrial receptivity with reproductive failure to help enhance the possibilities for future research. In conclusion, our overview points out that restricted and unstandardized methods support the controversy between the NK population and unsuccessful embryo implantation, which is an obstacle to studying why healthy eggs do not thrive and finding a solution for one of the most controversial topics in human reproduction.

Next generation of immune checkpoint molecules in maternal-fetal immunity

Successful pregnancy is a unique situation requires the maternal immune system to recognize and tolerate a semi-identical fetus and allow normal invasion of trophoblast cells. Although efforts have been made, the deep mechanisms of the maternal-fetal crosstalk have not yet been fully deciphered. Immune checkpoint molecules (ICMs) are a group of negative modulators of the immune response that avoid immune damage. They have been extensively studied in the fields of oncology and transplantation, while the latest evidence suggests that they are closely associated with pregnancy outcomes via multiple inhibitory mechanisms. Although studies have mostly demonstrated the regulatory role of the well-known PD-1, CTLA-4 at the maternal-fetal interface, what is unique about the newly discovered multiple ICMs remains a mystery. Here, we review the latest knowledge on ICMs, focusing on the first generation of checkpoints (PD-1, CTLA-4) and the next generation (Tim-3, Tigit, Lag-3, VISTA) highlighting their immunoregulatory roles in maternal-fetal tolerance and decidual vascular remodeling, and their involvement in pathological pregnancies. The content covers three aspects: the characteristics they possess, the dynamic expression profile of their expression at the maternal-fetal interface, and their involvement in pathological pregnancy. In immunotherapy strategies for pregnancy complications, upregulation of immune checkpoints may play a role. Meanwhile, the impact on pregnancy outcomes when using ICMs in clinical cancer treatment during pregnancy is a topic worth exploring. These may serve as a guide for future basic research and clinical applications of maternal-fetal immunity.

PD-1, CTLA-4, LAG-3, and TIGIT: The roles of immune checkpoint receptors on the regulation of human NK cell phenotype and functions

The roles of immune checkpoint receptors were defined in many cancers and autoimmune diseases, while there is limited information on their functional roles in the NK cells of healthy individuals. Immune checkpoint receptor expression of NK cell subsets and their association with NK cell functions (cytotoxic capacity and cytokine production) in healthy population were investigated. PD-1, CTLA-4, LAG-3 and TIGIT expression of peripheral blood NK cells, cytokine levels (TNF-α, IFN-γ, IL-10) and cytotoxic functions (granzyme A, perforin, CD107a; with/without K562 target cell stimulation) were evaluated by flow cytometry. CD56^dimCD16^dim NK cells had the highest expression of TIGIT, while CD56^dimCD16^- NK cells had highest expression of PD-1, CTLA-4 and LAG-3. PD-1⁺ NK cells, CTLA-4⁺ NK cells and LAG-3⁺ NK cells had increased amount of IL-10 however, reduced IFN-γ and TNF-α levels. Cytotoxic granule expressions (perforin and granzyme A) were reduced in PD-1⁺ NK cells, CTLA-4⁺ NK cells and LAG-3⁺ NK cells. However, TIGIT expression did not alter perforin and granzyme A expressions. Degranulation capacity was reduced in three groups of NK cells (PD-1⁺ or LAG-3⁺ or TIGIT⁺). TIGIT⁺ NK cells responded strongly to target cell stimulation, while NK cells in the other groups (PD-1⁺ or CTLA-4⁺ or LAG-3⁺) were resistant. PD-1⁺ NK cells, CTLA-4⁺ NK cells and LAG-3⁺ NK cells had a regulatory phenotype, impaired cytotoxic functions, and response to target cell stimulation. In contrast, TIGIT⁺ NK cells had strong baseline cytotoxic activity that further increased in response to target cell stimulation.