Immunogenetic regulation of Natural Killer cell function in pregnancy
Uterine NK (uNK) cells are a distinct subset of NK cells in the decidua of humans and rodents during pregnancy, which are essential for remodelling of the spiral arteries supplying the feto-placental unit. Similarly to peripheral NK cells, uNK cells express Natural Killer receptors (NKRs) that engage MHC class I molecules. Evidence from human genetic association studies suggests that, in the presence of allogeneic cognate paternal MHC class I ligands, inhibitory uterine NKRs are associated with disorders of pregnancy arising from impaired decidual vascular remodelling. Conversely, enhancement of human uNK cell activity through activating NKRs is associated with high birth weight. Evidence from mouse models corroborates that uNK cell activity is modulated by interactions between NKRs and MHC class I, but has largely focussed on the effect of paternal MHC.
In this study, the contribution of maternal immunogenetic regulation of NK cell function to reproductive outcome was assessed independently of parental MHC disparity in mice. To evaluate the role of NKR genes in isolation, I used congenic B6.BALB-TC1 (TC1) mice that differ from C57BL/6 (B6) mice only within the region of chromosome six encoding NKRs that recognise MHC class I. Absence of a major inhibitory NKR for self-MHC, Ly49I, in TC1 mice causes a compensatory shift in the NKR repertoire expressed and preserves a majority subpopulation of educated NK cells. B6 and TC1 splenic and uterine NK cells are similarly functionally reactive and mature, and no significant differences could be detected in spiral arterial remodelling or fetal growth between these strains in MHC-syngeneic matings. This supports data from human immunogenetic studies showing that maternal uterine NKRs are not associated with differences in pregnancy outcome in the absence of novel paternal MHC class I ligands, and highlights the importance of maternal and paternal co-regulation of uNK cell activity during pregnancy.
No mouse models of uNK cell activation are currently available with which to corroborate human immunogenetic associations between activating uterine NKRs and high birth weight. Male m157-transgenic (m157-Tg) mice, which ubiquitously express viral m157 glycoprotein ligands for the activating NKR Ly49H, were mated with B6 females. Exclusive expression of m157 glycoprotein by trophoblast improved placental efficiency, but did not enhance fetal growth. Some fertility clinics surmise that uNK cell activation initiates the pathogenesis of spontaneous abortion. It has been suggested that this may occur due to reduced expression by human uNK cells of miR-483-3p, which stimulates endogenous insulin-like growth factor (IGF)-1 production and uNK cell cytotoxicity in vitro. It is demonstrated here that neither miR-483-3p nor IGF-1 regulate murine NK cell development, maturation or function. No discernible reproductive phenotype is evident in miR-483 deficient females. It can be inferred that post-transcriptional control by miR-483 is not biologically relevant to murine NK cell function. Although m157-Tg mice may provide an interesting model to further study uNK cell-mediated placental adaptations, it remains important to identify a murine model of enhanced uNK cell function to corroborate human immunogenetic associations with high birth weight and to challenge the supposition that uNK cell activation is harmful to pregnancy.