Research progress of ICOSL/ICOS pathway in maternal-fetal immune tolerance

MEI Jiao-qi, YANG Xiao-hui, LIMENG Yong-wei, MA Yan-lin, HUANG Yuan-hua

Hainan Provincial Key Laboratory for Human Reproductive Medicine and Genetic Reseach; Department of Reproductive; The First Affiliated Hospital of Hainan Medical University; Hainan Provincial Clinical Research Center for Thalassemia; Key Laboratory of Tropical Translational Medicine of Ministry of Education, Hainan Medical University; Haikou Key Laboratory for Preservation of Human Genetic Resource, The First Affiliated Hospital of Hainan Medical University, Haikou 571199, China

Keywords:

ABSTRACT Co-signaling molecules are molecules whose ligands on the surface of cells interact with receptors on the surface of T cells to convey stimulatory or inhibitory signals to regulate immune responses.Co-signaling molecules play an important role in tumor and autoimmune diseases.Lately, studies have shown that co-signaling molecules are also involved in the regulation of maternal-fetal immune tolerance, and abnormalities of co-signaling molecules may lead to the imbalance of maternal-fetal immune tolerance, resulting in recurrent abortion,eclampsia and other pregnancy complications.ICOSL/ICOS is a ligand and receptor of costimulatory signals, which regulates maternal and fetal immune tolerance by participating in T cell differentiation and Th1 and Th2 cytokine secretion.Therefore, this article reviews the structure of ICOSL/ICOS, the distribution of ICOSL/ICOS at the maternal-fetal interface and its immune regulation during pregnancy, in order to provide new ideas for the future study of immunotherapy of pregnancy complications caused by abnormal co-signaling molecules.

During pregnancy, the maternal immune system not only recognize and tolerate the fetus carrying paternal antigen, but also resist the invasion of foreign pathogens.Therefore, the mother maintains the balance between them by establishing immune tolerance[1].Abnormal maternal fetal immune tolerance will lead to early pregnancy failure and the occurrence of late pregnancy complications, such as repeated pregnancy loss, preeclampsia,premature delivery, etc.[2].Co signaling molecules are receptors expressed on the surface of a variety of immune cells.They specifically combine with corresponding ligands to transmit stimulus or inhibition signals to cells, thereby regulating immune response[3].Immunotherapy targeting co signal molecules has been actively studied in autoimmune diseases, tumors, organ transplantation and other fields[4].The process of fetal escape from maternal immune surveillance is similar to that of tumor or graft escape from immune surveillance[5].Therefore, in order to better explain the role of maternal fetal immune tolerance in pregnancy and solve clinical problems, research on co signaling molecules in maternal fetal immunity has gradually attracted people’s attention,but its role in maternal fetal immunity has not been clarified.Inducible costimulatory molecule ligand (ICOSL) and its receptor ICOS (inducible costimulatory molecule, ICOS) are one of the costimulatory molecules.They activate T cells by activating costimulatory signals, induce T cell differentiation, and participate in the regulation of maternal fetal immune tolerance by secreting cytokines.This article reviews the distribution of ICOS/ICOSL at the maternal fetal interface and its role in maternal fetal immunity,with a view to providing a new direction for the study of maternal fetal immunity.

1.CO stimulation and T cell activation

T cell activation requires cooperation between different cells.When foreign antigens contact antigen presenting cells (APCs), the major histocompatibility complex (MHC), a cell surface specific polypeptide, specifically recognizes antigens and activates T cell receptor (TCR).However, TCR signals do not activate T cells, At this time, APC secretes co signaling molecules that interact with T cell surface receptors to enhance TCR induced signal activation of T cells [6].For example, the co-signal molecule receptors CD28,CTLA-4 and ICOS on T cells can regulate the proliferation,differentiation and cytokine secretion of T cells under the co stimulation of the co signal ligands B7-1, B7-2 and ICOSL on the surface of APC [7].In recent years, the co signaling molecule ICOSL/ICOS has gradually attracted attention in maternal fetal immune regulation.During pregnancy, ICOSL, as a transmembrane protein,is expressed in professional antigen presenting cells, including macrophages, dendritic cells, B cells, etc.[8].ICOS is expressed in T cells of reaction cells.After contacting with ligand ICOSL, ICOS transmits stimulus signals to T cells, promotes T cells to differentiate into effector T cells, regulatory T cells (Treg), and secretes Th1 and Th2 cell molecules[9], which play an important role in maternal fetal immunity.

2.Biological functions of ICOSL/ICOS

ICSOL is the third member of the B7 family, also known as B7-H2/B7RP-1 /CD275/GL50.It is a 281-amino-acid protein encoded by a full-length cDNA isolated from a mouse lymphocyte, there were 235、21 and 25 amino acids in extracellular, transmembrane and cytoplasmic regions, respectively.The extracellular domain of ICOSL has IgG-IgV domain, which is similar to B7-1 and B7-2.ICOSL can recognize and bind specific receptors.The receptor ICOS is a member of the CD28 family, consisting of two 27 kDa and 29 kDa polypeptide chains linked by disulfide bonds to form 55-60 kda homodimer acids, first discovered in 1999 by Hutloff with McAb F44 in human activated T cells[10].The mRNA Open reading frame of the ICOS receptor encodes a 199-amino-acid transmembrane protein.The extracellular, transmembrane and cytoplasmic regions were 141,23 and 35 amino acids, respectively.The amino acid sequence of ICOS shares 24% and 17% homology with CD28 and CTLA4, respectively[11].The structure of ICOS is very similar to that of CD28 and CTLA4.The extracellular region contains a v-like IG functional region in which the FDPPF motif interacts with its ligand and triggers intracellular signaling responses via the conserved motifs YMFM, IProx, KKKY in the ICOS cytoplasmic tail[12].The YMFM motif activates intracellular Phosphatidylinositol kinase (phosphoinositide 3-kinase, P13K) signaling to promote the flow of calcium ions, 13 and the YMFM motif, upon icoslicos binding, recruitment of PI3K regulates the binding of P85 to T cell surface receptor tyrosine for tyrosine phosphorylation, prompting PI3K to recruit the catalytic subunit p110 to the cytoplasm and activate, generating phosphatidylinositol-3-phosphate (Pi3p) , pI3P can activate AKT to phosphorylate FOXO transcription factor(which can inhibit the production of growth factors and promote the expression of apoptotic molecules) , and promote the differentiation of TFH cells and the release of IL-2 and IL-17A.Activation of phospholipase Cγ1(phospholipase C γ1, PLCγ1) by IProx motif activation causes actin remodeling and thereby induces the release of intracellular calcium ions, 14 which are closely associated with the activation of T cells.ICOSL/ICOS is critical for the development and function of follicular helper cells (Tfh).At present, the specific pathway of ICOSL/ICOS is not clear, and some studies have found that ICOSL/ICOS can significantly enhance the activation of AKT mediated by TCR, and to some extent enhance the activation of MAPK signaling (ERK, JNK, P38)[15,16].

3.ICSOL/ICOS characteristics of costimulatory signals in normal pregnancy

Embryo implantation refers to the invasion of embryonic trophoblast cells into the maternal uterine decidua.The maternal-fetal interface formed between trophoblast and uterine decidual cells is a critical part of the development of maternal-fetal immune tolerance[17].The costimulatory molecule ICOSL/ICOS is a B7/CD28 family of ligands and receptors that affect maternal-fetal immune tolerance by regulating the cellular functions of immune cells such as NK cells, T cells, macrophages and dendritic cell, to optimize the immune response[18].It is suggested that the expression of costimulatory molecule ICOSL/ICOS is different in placental tissues and immune cells.In the first trimester placenta, ICOSL is expressed in villous trophoblast cells and syncytiotrophoblast cells, but at a lower level, and later, as trophoblast cells invade the uterine decidua,the elevated expression of ICOSL in extra villous trophoblast cells and villous terminals suggests that ICOSL expression may be correlated with the invasive ability of trophoblast [19].On decidual T cells, expression of ICOS is higher in the CD45RO+phenotype(memory T cells) than in the CD45RA+phenotype (naive T cells) ,which may modulate the immune response of T lymphocyte[20].In addition, the secretion of Th1 and Th2 cytokines is also crucial in the maintenance of pregnancy in terms of the regulation of immune cells by ICOSL/ICSO.ICOSL transcription is present in the yolk sac of mouse embryos and may skew cytokines secreted by maternal T cells towards Th2-type and thus play a role during pregnancy[21].In the ICOS-deficient mouse model, Treg cells and effector T cells were significantly reduced, leading to the production of Th1 cytokines INF-γ and TNF-α, which enhanced Th1-type immunity.Intraperitoneal injection of B7h Mab in a mouse model of allogeneic pregnancy inhibited ICOSL expression and found a significant increase in fetal absorption and a significant decrease in litter size[10].ICOSL/ICOS could promote the proliferation and differentiation of CD4+T and CD8+T cells.CD4+T cells can further differentiate into Th cells (Th1, Th2, Th17) , Treg cells and Tfh.And the production of Th2 cytokines preferentially in the CD4+T proliferation state induces maternal-fetal immune tolerance[10].Nagamatsu[22] blockade of the ICOSL/ICOS pathway with B7H mabs (ICOSL blockers)inhibited the secretion of Th2 cytokines by decidual T cells.Wang s et al.23 also found that after ICOSL/ICOS interaction was blocked,CD4+T cells produced more Th1 cytokines (IFN-γ) and less Th2 cytokines (IL-4, IL-10).In a dendritic cell (dendritic cell, DC), costimulatory signals of ICOS can promote T cell activation through the P38-MAPK pathway by promoting the secretion of the Th2 cytokine IL-6 and enhancing the phagocytic antigen and antigenpresenting functions of DCs[24].In addition, the ICOSL/ICOS pathway may assist in the activation of B cells by promoting the differentiation of Tfh cells, facilitating the formation of germinal centers and conversion of immunoglobulin types, B cells also act as Antigen-presenting cell to activate T cells, and some studies have shown that B cells secrete the neurotransmitter Gamma Gaba, which induces monocyte polarization toward M2 macrophages, producing the TH2 cytokine Il-10, suppression of the immune response is beneficial to pregnancy[25].

4.The characteristics of co-stimulation in abnormal pregnancy

4.1 ICOSL/ICOS and recurrent spontaneous abortion

Recurrent spontaneous abortion (RSA) is one of the most common complications caused by immune tolerance imbalance in the maternal-fetal interface[26].Co-signaling molecules have been shown to be used as biomarkers for the diagnosis of RSA.As the fetus develops during pregnancy, the maternal immune system adapts[27].Co-signaling molecules expressed at different levels under normal and disease conditions may lead to failure of maternal immune system adaptation, resulting in abnormal pregnancy.Wei Jia[28]found that the expression level of ICOS receptor in deciduous tissue of patients with recurrent miscarriage was significantly higher than that in women with induced abortion, and flow cytometry showed that ICOS+CD3+T and ICOS+CD4+T lymphocyte subsets were also significantly higher than those in the control group.Therefore, the abnormal increase in the expression of ICOS co-stimulation signals may cause abnormal activation of T cells leading to imbalance of cytokine expression, which is associated with recurrent miscarriage.In addition, Luan X[29] found that CD4+CXCR5+PD-1+ICOS+Tfh cells in deciduous tissues of patients with recurrent miscarriage were also significantly higher than those in women with abortion in the first trimester, and CD4+CXCR5+PD-1+ICOS+Tfh cells as subsets of Tfh cells could enhance the immune response by promoting the production of B cells, leading to maternal immune rejection of the fetus.

4.2 ICOSL/ICOS and preeclampsia

Pre-eclampsia (PE) is a disease endemic to pregnancy, and aberrant activation of the immune system, as well as oxidative stress, plays an important role in the etiological development[30].Treg cells are immunosuppressive cells differentiated by CD4 + T cells, which express many kinds of marker molecules to regulate the activation of Treg.The ICOS-labeled Treg cells are closely related to the immunosuppressive molecules IL-10, TGF-β, Fas-FasL and granzyme B, may enable ICOS-labeled Tregs to have stronger immunosuppressive functions[31].Yu Qing[32] used Flow cytometry analysis of ICOS-labeled Treg cells in the peripheral blood of PE patients and women with normal late pregnancy at the same time and found that the expression level in PE patients was significantly lower than that in normal pregnant women, it is suggested that the decrease of ICOS-labeled Treg cells may lead to the decrease of immunosuppressive function, which may be related to the imbalance of maternal-fetal immunity and lead to the occurrence of PE.In addition, the expression of ICOS has been linked to the production of the placental marker indoleamine 2.3-dioxygenase (IDO), which has been shown to scavenge free radicals and inhibit the activation of T cells[33].In an allogeneic pregnancy mouse model, the expression of IDO was markedly downregulated in placental tissue of B7-mabtreated mice, which may cause oxidative stress as well as aberrant t-cell activation, leading to an enhanced immune response; Killing placental trophoblast, the invasion of trophoblast weakened, and the occurrence of PE related[34].At the genetic level, ICOS gene is associated with PE susceptibility.By including 130 PE patients and 261 control women for Case-control study, Pendeloski K P [35]found that patients with PE had an ICOS genotype (TT vs Tc vs CC), the frequency of TT genotype was significantly lower than that of the control group (41.9% vs 59.3% , P < 0.01) , but the biological significance of ICOS polymorphism was still unclear due to the small sample size.

5.Summary and Prospect

ICOSL/ICOS as a costimulatory signal molecule, is beneficial to maternal-fetal immune tolerance by promoting the secretion of Th2 cytokines by T cells.However, the abnormal expression of ICOSL/ICOS may lead to the over-activation and abnormal differentiation of T cells, which may lead to immune imbalance and pathological pregnancy.In the maternal-fetal interface, the Th1 and Th2 immune responses are in the same Dynamic equilibrium, and the predominant immune responses are different at different stages of pregnancy.Therefore, it is suggested that the costimulatory signaling molecule ICOSL/ICOS in the regulation of maternal-fetal immune tolerance should also be in a dynamic equilibrium.Whether ICOSL is inhibited or abnormally activated, it will upset the maternal-fetal immune balance and be disadvantageous to pregnancy.

In recent years, it has been found that co-signaling molecules play an important role in maternal-fetal immunomodulation.Cosignaling molecules participate in the initiation and development of maternal-fetal immune tolerance through differential expression at the maternal-fetal interface and regulation of the biological functions of immune cells.In the process of immunomodulation,the classical co-signaling ligand CD80/CD86 and the receptor CD28/CTLA -4 compete with each other and inhibit each other,therefore, it is not clear whether the interaction between ICSOL/Icos and these co-signaling molecules also exists.ICOSL/Icos, as one of the costimulatory signaling molecules, is mainly involved in the regulation of T cells and the secretion of cytokines to induce maternal-fetal immune tolerance, there are few studies on the regulation of other immune cells in the maternal-fetal interface,and the mechanism is not clear.However, in the treatment of autoimmune diseases and tumors, ICOS is an immune checkpoint expressed on activated T cells, and its targeted molecular therapeutic drugs have entered clinical trials and achieved good results.However, in reproductive immunity, targeted drug therapy for cosignaling molecules is rarely reported in the treatment of pregnancy complications.On the one hand, it may consider the special group of pregnant women, on the other hand also reflects the current research is not clear enough.Therefore, more studies are needed to explore the role of ICOSL/Icos in maternal-fetal immunity, and to provide new ideas for the study of co-signaling molecules in maternal-fetal immunity so as to better solve clinical problems.

Authors’ contribution

Meijiaoqi: put forward research direction, research topic, thesis writing; Huang yuanhua, Mayanlin: thesis review; Limengyongwei:Literature Collection and collation; Yang xiaohui: thesis revision.