Natural Killer Cells and Their Role in Obstetric Antiphospholipid Syndrome

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Natural Killer Cells and Their Role in Obstetric Antiphospholipid Syndrome

Pregnancy is a delicate balance of immune tolerance and growth—but for women with antiphospholipid syndrome (APS), this balance can go wrong. APS is an autoimmune disease where persistent antiphospholipid antibodies (aPLs) trigger blood clots or pregnancy complications like recurrent pregnancy loss (RPL) or preeclampsia. While researchers have long linked aPLs to these issues, new studies highlight a key player in the drama: natural killer (NK) cells.

NK cells are immune cells best known for fighting viruses and tumors, but during pregnancy, they take on a softer role. Uterine NK (uNK) cells—found in the lining of the uterus (decidua)—help the embryo implant by supporting trophoblast (placental cell) invasion and remodeling spiral arteries (blood vessels that feed the placenta). They also secrete cytokines like transforming growth factor-beta (TGF-β) and interleukin-10 (IL-10) to calm inflammation and promote blood vessel growth. In short, uNK cells are “pregnancy helpers”—until something throws them off balance.

What Are NK Cells, and How Do They Work?

NK cells come in two main flavors: peripheral blood NK (pNK) cells (circulating in the blood) and uNK cells (residing in the uterus). Scientists think uNK cells develop in three ways:

  1. Local transformation: Signals from the decidua (like galectin-9/TIM-3) turn pNK cells into uNK-like cells.
  2. Migration: CD16+ pNK cells move into the uterus and change phenotype under TGF-β’s influence.
  3. Progenitor differentiation: IL-15 (a growth factor) pushes CD34+ stem cells to become uNK cells when co-cultured with decidual stromal cells.

To do their job, NK cells use activating killer receptors (AKRs) (which turn them “on”) and inhibiting killer receptors (IKRs) (which turn them “off”). For example:

  • IKRs like KIR2DL1/2/3: Bind to HLA-C (a protein on trophoblasts) to keep uNK cells from attacking the fetus.
  • AKRs like KIR2DS1 or NKG2D: Activate NK cells via adaptor proteins (e.g., DAP12) to produce pro-inflammatory cytokines.

When AKRs and IKRs are balanced, uNK cells support pregnancy. When unbalanced? They turn harmful.

NK Cells in Normal Pregnancy: The “Peacekeeper” Role

During a healthy pregnancy, uNK cells:

  • Support implantation: Help trophoblasts burrow into the uterus.
  • Remodel blood vessels: Turn narrow spiral arteries into wide, low-pressure vessels to feed the placenta.
  • Secrete growth factors: CD49a+ Eomes+ uNK cells produce factors that help the embryo grow before the placenta forms.

The number of pNK and uNK cells also matters. Too many (or too few) can disrupt pregnancy. For example, Zhang et al. linked higher uNK cell counts to preeclampsia and fetal growth restriction. Conflicting studies on pNK cells (some show no difference in RPL, others show higher counts) likely reflect differences in research methods or pregnancy stages.

How NK Cells Go Wrong in APS

In APS, aPLs—especially anti-β2 glycoprotein I (anti-β2GPI) antibodies—throw NK cells out of balance. Here’s how:

  1. Inflammation overload: Anti-β2GPI antibodies stop trophoblasts from “cleaning house” (autophagy), which activates inflammasomes (inflammatory protein complexes) and boosts IL-1β (a pro-inflammatory cytokine). This triggers excessive inflammation at the maternal-fetal interface.
  2. Cytotoxicity: β2GPI pushes helper T cells (Th cells) to become Th17 cells (pro-inflammatory) and activates uNK cells to produce interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α). These cytokines make uNK cells more cytotoxic—they start attacking placental cells, damaging blood vessels, and causing microthrombi (tiny clots).
  3. Uncontrolled growth: Excess TNF-α makes uNK cells multiply and turn into “killer” cells that target the fetus, leading to RPL.

Animal studies back this up: Mice exposed to bacterial endotoxins (which cause inflammation) had abnormally activated uNK cells that attacked embryos. Inhibiting uNK cells (with antibodies) or using low-dose rapamycin (which adjusts uNK cell numbers and receptor levels) reduced embryo loss.

Treatment Insights: Can We Fix NK Cell Balance?

Researchers are testing ways to reset NK cell function in APS-related pregnancy loss:

  • Prednisone: Reduces uNK cell counts but doesn’t improve pregnancy outcomes (study by Cooper et al.).
  • Intravenous immunoglobulin (IVIG): Lowers NK cell cytotoxicity and adjusts receptor levels (increasing IKRs like KIR2DL1/2/3, decreasing AKRs like NKG2A/KIR2DS1). Ahmadi et al. found IVIG boosted live birth rates in RPL patients.

The Big Picture: Balance Is Key

Healthy pregnancy depends on NK cells staying “calmed” by IKRs and “focused” on supporting growth—not attacking. In APS, aPLs disrupt this balance, turning uNK cells from helpers to harmers. While treatments like IVIG show promise, researchers still need to unravel the exact signaling pathways and cytokines involved.

This review, led by Rongxiu Huo, Qianyu Guo, and Ke Xu from the Department of Rheumatology and Immunology at the Third Hospital of Shanxi Medical University (and colleagues), was published in the Chinese Medical Journal in 2022. It builds on work funded by the National Natural Science Foundation of China (No. 81871292) and Shanxi Province’s Key R&D Projects (No. 201803D31136).

For more details, you can access the original study here: doi.org/10.1097/CM9.0000000000001908

References (from the original study):

  1. Ge YY, Duan HJ, Deng XL. Chinese Medical Journal (2021).
  2. Papuchová H et al. Frontiers in Immunology (2019).
  3. Barrow AD et al. Frontiers in Immunology (2019).
  4. Kuon RJ et al. Journal of Reproductive Immunology (2017).
  5. Zhang J et al. EBioMedicine (2019).
  6. Zhou F et al. Chinese Medical Journal (2020).
  7. Shields CA et al. American Journal of Physiology (2018).
  8. Lu H et al. Autophagy (2021).
  9. Cooper S et al. Journal of Reproductive Immunology (2019).
  10. Ahmadi M et al. Journal of Cellular Physiology (2019).

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