title: Natural killer cells and reproductive failure.
abstract: OBJECTIVE
  The purpose of this review is to provide an update of what is known about the role of natural killer cells in reproduction.
  
  
  RESULTS
  In humans, natural killer cells are present in abundance in the uterus and appear to play an important role in early pregnancy. Women with reproductive failure have been shown to have higher levels of natural killer cell numbers and activity in peripheral blood. The evidence relating to uterine natural killer cells, however, is contradictory. While earlier studies suggested an increase of uterine natural killer cells in women with recurrent pregnancy loss, more recent investigations have not confirmed this trend. Uterine natural killer cell number or activity appear to be higher in spontaneous abortions with normal chromosomes compared with chromosomally abnormal pregnancies. Furthermore, a rise of the cytotoxic CD56(dim)CD16+ natural killer cell subset is seen in the follicular fluid of patients with idiopathic infertility and it is postulated that this increase may decrease fertilization rates in this group.
  
  
  CONCLUSIONS
  Natural killer cell number and activity appear to be altered in women with reproductive failure although it is unclear if the difference is a cause or effect of reproductive failure. Whether natural killer cells are increased or decreased depends on whether peripheral blood, endometrium or first-trimester decidua is analyzed. Until more is known about the role of natural killer cells in normal pregnancy, there does not appear to be any benefit in offering natural killer cell testing to women with reproductive failure outside of research protocols.
authors.affiliation: Division of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Connecticut 06520, USA. anne.wold@yale.edu
authors.forename: Anne S Devi
authors.initials: AS
authors.lastname: Wold
authors.forename: Aydin
authors.initials: A
authors.lastname: Arici
mesh_terms: D000026:Abortion, Habitual / Q000378:metabolism*; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D007167:Immunotherapy; D007247:Infertility, Female / Q000378:metabolism* / Q000628:therapy; D007694:Killer Cells, Natural / Q000378:metabolism*; D017211:Treatment Failure; D014599:Uterus / Q000378:metabolism