Killer cells in pregnancy, primarily uterine natural killer (uNK) cells, are a crucial part of the maternal immune system. They are the most abundant leukocytes (white blood cells) in the endometrium (uterine lining) during implantation. Contrary to their name, these cells don't simply kill; their role is far more nuanced and vital for a successful pregnancy.
The Role of uNK Cells in Pregnancy
Instead of attacking the fetus (which is genetically different from the mother and would normally be treated as a foreign body by the immune system), uNK cells play several key roles:
- Implantation: They help the embryo attach to the uterine wall, a process essential for pregnancy establishment. [Source: Uterine natural killer (uNK) cells form the major leucocyte population in the endometrium at the time of implantation1 and have received considerable attention in relation to their role in normal implantation and early placental development.]
- Placental Development: uNK cells contribute significantly to the development of blood vessels in the placenta, enabling nutrient and oxygen transfer to the fetus. [Source: Additionally, NK cells have immune-regulatory roles and participate in angiogenesis and vascular remodeling during early pregnancy [6, 7].]
- Immune Regulation: They regulate the immune response to prevent the mother's immune system from rejecting the fetus. This is particularly important because the fetus carries paternal genes, making it partially "foreign" to the mother. [Source: The fetus is considered to be an allograft that, paradoxically, survives pregnancy despite the laws of classical transplantation immunology.]
- Angiogenesis: uNK cells are involved in the formation of new blood vessels (angiogenesis) in the uterine lining, which is critical for supporting the growing fetus. [Source: Additionally, NK cells have immune-regulatory roles and participate in angiogenesis and vascular remodeling during early pregnancy [6, 7].]
uNK Cells and Pregnancy Complications
While crucial for normal pregnancy, imbalances in uNK cell activity have been linked to complications like:
- Recurrent Miscarriage: Abnormal uNK cell numbers or function may contribute to repeated pregnancy loss. [Source: Intense research efforts have begun to shed light on the roles of NK cells and Tregs in early pregnancy loss, although much remains to be unraveled.]
- Preeclampsia: This pregnancy-related disorder, characterized by high blood pressure and potential organ damage, may involve altered uNK cell activity. [Source: This review is focused on the role of dNK cells in normal pregnancy and pathological pregnancy, including preeclampsia, recurrent spontaneous abortion, ...]
Beyond uNK Cells: Other NK Cells in Pregnancy
While uNK cells are the main focus, other natural killer cells also play roles in pregnancy, though their functions are less well-understood. Research is ongoing to fully elucidate their contributions to successful pregnancy and potential complications. [Source: Intense research efforts have begun to shed light on the roles of NK cells and Tregs in early pregnancy loss, although much remains to be unraveled.]
