Blastocyst Implantation & Development

Blastocyst Implantation Basics

• Inner Cell Mass (ICM) develops into the embryo
• Trophoblast develops into the placenta

Key Terms

• Prenatal: Occurs before birth, relating to pregnancy
• Antenatal: Before birth
• Parturition: The process of birth and offspring delivery
• Labor: Childbirth, especially from uterine contractions to delivery
• Placenta: Flattened circular organ in the uterus of pregnant eutherian mammals that nourishes, and maintains the fetus via the umbilical cord
• Eutherian: Mammal subgroup Eutheria, including placentals and excluding marsupials/monotremes
• Trophoblast cells: Supports embryo attachment to the uterus lining, protects the embryo, and forms part of the placenta
• Blastocyst: An early-stage ball of cells formed in pregnancy, about 5-6 days after sperm fertilization
• Embryo: Initial stage of a multicellular organism's development
• Decidua: Thick modified mucous membrane that lines the uterus during pregnancy; shed with afterbirth
• Fetal-placental unit: A term for the developing fetus and placenta as a single unit
• Allogenic: Genetically dissimilar tissues or cells; immunologically incompatible, from individuals of the same species
• Semi-allogeneic: Sharing of some, but not all genes, like parent-offspring
• Allograft: Tissue graft from a genetically non-identical donor of the same species
• Semi-allograft: A transplant with half of genes from another individual (paternal, from the father)
• Autograft: Tissue moved from one location to another on the same body
• Xenograft: Tissue from a donor of a different species
• MHC molecule: Cell surface molecules displaying self-antigens, prevent the immune system from targeting its own cells.
• Myometrium: Middle layer of uterine wall; smooth muscle cells induce uterine contractions during labor
• Conceptus: Products of conception, including the embryo/fetus, associated membranes, placenta, and umbilical cord

Immunology of Pregnancy: Initial Theory

• Initial theory: pregnancy is generally immunosuppressive so the maternal system won't reject the fetus

Why the Initial Theory Was Incorrect

• It was proven incorrect dueto the dynamic and tightly regulated nature of the immune system
• dNK cells, macrophages, dendritic cells, and regulatory T cells (Tregs) are important at the maternal-fetal interface
• Communication and regulation are demonstrated through cell and molecule interactions like HLA-G
• Immune environment molded by trophoblast interaction with maternal immune cells
• To support different stages, the immune system's activity changes

New Model for Immunology of Pregnancy

• Immune regulation is highly dynamic during pregnancy to promote implantation, placental growth, and fetal development
• Angiogenesis: Generation of new blood vessels
• Metastatic: Spread of cancer cells from the original site to another within the host

Immunological Phases of Pregnancy

• First trimester: Associated with inflammation, which is required for blastocyst implantation
• Second trimester: Characterized by an anti-inflammatory and T helper 2 (Th2) immune microenvironment for fetal growth
• Third trimester: Switch to inflammatory and Th1-type immune state for labor and delivery

Gestational Immune Dynamics and Importance

• First Trimester (Implantation and Placentation):
  • Inflammation is necessary; implantation involves trophoblast invasion of the uterine lining (decidua)
  • Decidualization: Transformation of the uterine lining to support the developing embryo
  • Trophoblast invasion: Controlled inflammation aids trophoblast invasion and blood supply establishment
  • Tolerance initiation: Tolerance begins to prevent fetal rejection
  • Uterine natural killer cells (uNK) are abundant in the decidua to regulate blood vessel development/trophoblast invasion
  • Macrophages remodel tissue and contribute to tolerance
• Second Trimester (Maintenance and Growth):
  • As the placenta is established, the immune response becomes more anti-inflammatory to defending from attack
  • Hormone production and nutrient/waste exchange taken over by the placenta
  • Rapid fetal development
  • Maternal immune system remains tolerant but combats infections
  • Regulatory T cells (Tregs) increasingly important for suppressing harmful immune responses
  • Balance of signaling molecules (cytokines) favors tolerance promotion (IL-10)
• Third Trimester (Preparation for Parturition):
  • There is a switch back towards a pro-inflammatory state
  • Cervical ripening: Cervix begins to contract, expelling the fetus
  • Contractions of the uterus start to expel the fetus
  • Amniotic sac breaks
  • Macrophages produce factors that contribute to cervical ripening/uterine contractions
  • Pro-inflammatory cytokine levels rise to help initiate labor

Main Placental Functions

• Extravillous trophoblast (EVT) cells invade uterine spiral arteries to transform them into low-resistance vessels
• Maternal cells and placenta cells have direct contact; maternal and fetal cells never do
• The placenta is the unique endocrine organ of eutherian mammals that self-sustaining and transient
• Supplies nutrients and gas exchange (oxygen) for the fetus
• Hormone production to maintain a healthy pregnancy/aid in fetal development
• Act as a physical and immune barrier

Trophoblast Cells and Immune Profile

• Some placental trophoblasts lack MHC molecules, making them invisible to the maternal immune system
• Other placental trophoblasts (EVTs) have specific MHC molecules that inhibit dNK cell activity
• HLA-G (expressed on trophoblasts) controls dNK cell cytotoxicity
• Trophoblast cells transform uterine arteries into spiral arteries and interact with maternal immune cells

Maternal Tolerance

• Maternal tolerance permits a mother to carry the fetus to term despite the presence of a foreign fetal antigen
• Maternal tolerance must be established for a successful pregnancy
• Failure is linked to preeclampsia and miscarriage
• Restriction and modulation of leukocytes mediates tolerance

Mechanisms of Maternal Tolerance

• dNK cell cytotoxicity controlled by HLA-G binding to Killer cell immunoglobulin-like receptors (KIR)
  • Expression of HLA by trophoblast defends against NK cell-mediated death
  • HLA-G: Histocompatibility antigen, class I, G
  • Is a nonclassical class I heavy chain paralogue; acts as an immune checkpoint
  • Major Histocompatibility complex (MHC) proteins: Cell surface molecules that display self-antigens
• Certain placental trophoblasts have specific MHC molecules to inhibit dNK cell activity
• Some placental trophoblasts have no MHC molecules, making them essentially invisible to the maternal immune system
• T cells are suppressed by by Regulatory T cells (Tregs)
• Decidual macrophages hinder T cell activation

T-Cell Recognition

• Trophoblast (fetal or paternal) antigens are not directly recognized by T-cells in the decidua
• Instead, antigen presentation likely occurs in the lymph

Topic 2: Microbes and Pregnancy Complications

Microbes at the Maternal-Fetal Interface

• TORCH pathogens, congenital, hematogenous, neonatal Fc receptor, miscarriage, tropism, vertical transmission.
  • TORCH pathogens:
    • Toxoplasma gondii
    • Listeria monocytogenes, varicella zoster virus (VZV), HIVm Zika virus
    • Rubella virus (CMV)
    • Herpes simplex virus (HSV)

Infections and Preterm Birth

• 1 in 4 preterm births linked to subclinical intravenous-amniotic infection
• Microorganisms ascend from the lower genital tract; disseminate via blood
• Microbial-induced preterm labor via inflammation; induction of chemokines, cytokines, prostaglandins and proteases leading to pathway activation.

Routes of Infection

• Ascending infection via the cervix: HSV and Treponema pallidum and ZIKV
• Extravillous trophoblast infection: T.gondii and Listeria monocytogenes
• Transport across trophoblast layers (transcytosis): rubella virus and CMV
  • Routes of ZIKV is it unknown
    • Breaks in the SYN layer
    • Direct infection of the SYN layer
    • Bypassing the SYN layer (transcytosis)
  • Infection of EVTs

Infections in Early Pregnancy and Miscarriage

• 1 in 5 pregnancies result in miscarriage
  • Can occur as a result of infection where
    • placental trophoblast invasion/function is altered
    • the maternal immune response shifts
    • placental apoptosis/damage happens

Placental Defence Mechanisms

• The syncytiotrophoblast layer forms a dense physical barrier made of actin cytoskeleton which lacks cellular junctions
• Trophoblasts secrete antiviral molecules (IFN-λ)and antimicrobial peptides
• Active transport of protective antibodies (immunoglobulins) from maternal blood to fetal blood via neonatal FC receptor

Pathogens Overcoming Placental Barriers

• Viruses: CMV, rubella virus, and Zika virus
• Bacteria: Listeria monocytogenes and Treponema palladium (syphilis)
• Parasites: Toxoplasma gondii

Adverse Outcomes of Placental Barrier Breach

• Congenital malformations
• Fetal growth restriction
• Stillbirth
• Neurological damage
• Prematurity
• Long-term health issues

Vertically Transmitted Pathogens

• Know the possible outcomes of infection by vertically transmitted and non-vertically transmitted pathogens during pregnancy

• Definition of prenatal programming

Fetal Development

• Understand the difficulties or limitations of modeling some of these outcomes in animals
• Understand the arguments for and against the presence of an intrauterine microbiome
• Know an alternate way that microbes can still be impacting fetal development even if they are not in the intrauterine environment. - Treating pregnant mice with antibiotics, leads to abnormal development of the embryonic brain - Addition of particular microbial metabolites can rescue this abnormality - Maternal gut microbiota supports placental development in mice. - Short-chain fatty acids (SCFAs) stimulate culture endothelial cell tube formation and prevent abnormalities in placenta vascularization

Key Papers

• "The unique immunological and microbial aspects of pregnancy” (2017) Gil Mor, P. Aldo, A. Alvero Nature Reviews Immunology
• “Killing the pathogen & sparing the placenta" by I. Mysorekar (2020) N Engl J Med
• “Stealth killing by uterine NK cells" by P. Dogra & D. Farber (2020) Cell
• “Mum’s bacteria linked to baby’s behavior" by C. Powell (2017) Nature