Insight Into TLR4-Mediated Immunomodulation in Normal Pregnancy and Related Disorders

Abstract

Unlike organ transplants where an immunosuppressive environment is required, a successful pregnancy involves an extremely robust, dynamic, and responsive maternal immune system to maintain the development of the fetus. A specific set of hormones and cytokines are associated with a particular stage of pregnancy. Any disturbance that alters this fine balance could compromise the development and function of the placenta. Although there are numerous underlying causes of pregnancy-related complications, untimely activation of Toll-like receptors (TLR), primarily TLR4, by intrauterine microbes poses the greatest risk. TLR4 is an important Pattern Recognition Receptor (PRR), which activates both innate and adaptive immune cells. TLR4 activation by LPS or DAMPs leads to the production of pro-inflammatory cytokines via the MyD88 dependent or independent pathway. Immune cells modulate the materno-fetal interface by TLR4-mediated cytokine production, which changes at different stages of pregnancy. In most pregnancy disorders, such as PTB, PE, or placental malaria, the TLR4 expression is upregulated in immune cells or in maternal derived cells, leading to the aberrant production of pro-inflammatory cytokines at the materno-fetal interface. Lack of functional TLR4 in mice has reduced the pro-inflammatory responses, leading to an improved pregnancy, which further strengthens the fact that abnormal TLR4 activation creates a hostile environment for the developing fetus. A recent study proposed that endothelial and perivascular stromal cells should interact with each other in order to maintain a homeostatic balance during TLR4-mediated inflammation. It has been reported that depleting immune cells or supplying anti-inflammatory cytokines can prevent PTB, PE, or fetal death. Blocking TLR4 signaling or its downstream molecule by inhibitors or antagonists has proven to improve pregnancy-related complications to some extent in clinical and animal models. To date, there has been a lack of knowledge regarding whether TLR4 accessories such as CD14 and MD-2 are important in pregnancy and whether these accessory molecules could be promising drug targets for combinatorial treatment of various pregnancy disorders. This review mainly focuses on the activation of TLR4 during pregnancy, its immunomodulatory functions, and the upcoming advancement in this field regarding the improvement of pregnancy-related issues by various therapeutic approaches.

Keywords: TLR4; innate immunity; pregnancy; preterm birth; pro-inflammatory.

Figure 1

TLR4 Signaling pathway. The LPS Binding Protein (LBP) binds to LPS and transfers it to CD14 or MD-2, which are the accessory proteins involved in the ligand recognition, dimerization, and endocytosis of TLR4. TLR4, upon dimerization, can signal via two separate pathways, the MyD88-dependent and the MyD88-independent pathway. The MyD88-dependent pathway involves the activation of IRAKs and TRAF6, which results in the phosphorylation of transcription factors, such as NF-κB and AP-1. These transcription factors upon phosphorylation translocate to the nucleus and are involved in triggering the transcription of pro-inflammatory cytokine genes. The MyD88-independent pathway, or the TRIF-dependent pathway, however, involves TRAF3 for the activation of transcription factor IRF-3, which favors the production of Type I interferons, such as IFN α, β.

Figure 2

Schematic showing distribution of immune cells across materno–fetal interface during early pregnancy. Macrophages, Treg cells, and dendritic cells are present in the myometrium as well as decidua, while uterine natural killer (uNK) cells are restricted to decidua. The effective crosstalk between various immune cells and extravillous trophoblast cells creates an immunosuppressive environment and helps in the formation of various pregnancy-related tissues, both of which are essential for a successful pregnancy. Extravillous cytotrophoblasts enter the decidua to reach out to maternal spiral arteries for obtaining required nutrient to support developing embryo. Other nutrients, gas, and waste exchange happens via placental villus, which interacts with the maternal blood directly. The villus has double layer of cells consisting of syncytiotrophoblasts and cytotrophoblasts. It encloses the fetal blood vessels along with fibroblasts and fetal macrophages (Hofbauer cells). Immune cells as well as placental cells protect the fetus by expressing PRRs, such as Toll-like receptors, on their surface throughout the pregnancy.

Figure 3

Immunological stages of pregnancy: during first trimester of pregnancy, the inflammatory response is required for blastocyst implantation. (A) The second trimester is described by an anti-inflammatory and T-helper 2 (Th2)-type immune microenvironment that is necessary for fetal growth. In the third trimester, switching from anti-inflammatory to an inflammatory response happens, and this is essential for labor and delivery. (B) Different stages of pregnancy have altered the level of TLR4 activation the first and third trimester have more TLR4 activation in immune cells and trophoblast cells, which results in inflammation that is required for blastocyst implantation and term labor and delivery. In the second trimester, the lowered TLR4 activation supports the anti-inflammatory response for fetal growth. (C) Trophoblast-mediated immune regulation. (i) Trophoblast cells secrete number of cytokines and chemokines, such as CC-chemokine ligand 2 (CCL2), CXC-chemokine ligand 12 (CXCL12), CXCL8, and transforming growth factor-β (TGFβ), which are responsible for recruitment of immune cells to the materno–fetal interface. Immune cells provide support for invasion and implantation of trophoblast. (ii) Trophoblast cells secrete cytokines that help in training of uterine natural killer (uNK) cells and M2-like macrophages; in turn, these immune cells support the vascular and tissue remodeling that is necessary for trophoblast invasion and differentiation. TGFβ secreted by trophoblast cells induces the polarization of regulatory T (T_reg) cells, and these cells provide a feto-tolerant environment at the materno–fetal interface. (iii) Trophoblast cells express PRRs, such as TLRs, that allow them to sense and respond to DAMPs and PAMPs produced during tissue damage and infection. IL, interleukin; M-CSF, macrophage colony-stimulating factor.

Figure 4

Alteration of TLR4 signaling in pregnancy disorders. (A) Bacterial products, such as lipopolysaccharide, activate TLR4 and the signal TRIF-TBK1-IRF-3 pathway to induce the baseline expression of IFN-β (which is encoded by IFNB) in the placenta. The production of IFN-β by the placenta modulates the maternal immune system and promotes tolerance, while providing protection against viral and bacterial infections. (B) Bacterial products stimulate the production of type I IFNs by trophoblast cells and maintain an anti-inflammatory environment as well as active surveillance and protection against infections. However, in case higher antigen load or if pathogenic infection reaches the placenta, TLR4 expression increases and modulate the activation signals through MyD88-TRAF6-NF-κB leading to inhibition of type I IFNs, and promotion of pro-inflammatory response that is responsible for preterm labor. TRAM is the TRIF-related adapter molecule (also known as TICAM2). Role of macrophages and TLR4 in Pregnancy: (C) In normal pregnancy, M2-like macrophages are available around spiral arteries and the endothelium, which helps in the remodeling of these arteries by producing various factors associated with angiogenesis and tissue remodeling. They also play a role in immunomodulation, for instance by producing IL-10. (D) During preeclampsia, increased numbers of M1-like macrophages are found in the materno–fetal interface. These M1 type decidual macrophages have more TLR4 expression and signal via NF-κB pathway to produce pro-inflammatory cytokines, such as TNF-α and IFN-γ, which induces apoptosis of the trophoblast cells.

Figure 5

Various Drugs that target TLR4 pathway in pregnancy disorders: drugs and anti-inflammatory agents that target TLR4 pathway and its downstream molecules during infection induced preterm birth. Hormones and drugs targeting TLR4 expression help in switching the pro-inflammatory environment to anti-inflammatory in various pregnancy disorders. TNF inhibitors reduce the increased TNF production during altered TLR4 activation in preeclampsia.