Ly49 receptors activate angiogenic mouse DBA⁺ uterine natural killer cells

In humans, specific patterns of killer immunoglobulin-like receptors (KIRs) expressed by uterine natural killer (uNK) cells are linked through HLA-C with pregnancy complications (infertility, recurrent spontaneous abortion, intrauterine growth restriction and preeclampsia). To identify mechanisms underpinning the associations between NK cell activation and pregnancy success, pregnancies were studied in mice with genetic knockdown (KD) of the MHC-activated Ly49 receptor gene family. B6.Ly49 KD pregnancies were compared to normal control B6.Ly49 129 and C57BL/6 (B6) pregnancies. At mid-pregnancy (gestation day (gd9.5)), overall uNK cell (TCRβ^- CD122 ⁺ DBA⁺ DX5^- (DBA⁺ DX5^-)) and TCRβ^- CD122⁺ DBA^- DX5⁺ (DBA^- DX5⁺)) frequencies in pregnant uterus were similar between genotypes. Ly49 KD lowered the normal frequencies of Ly49⁺ uNK cells from 90.3% to 47.8% in DBA^- DX5⁺ and 78.8% to 6.3% in DBA⁺ DX5^- uNK cell subtypes. B6.Ly49 KD matings frequently resulted in expanded blastocysts that did not implant (subfertility). B6.Ly49 KD mice that established pregnancy had gestational lengths and litter sizes similar to controls. B6.Ly49 KD neonates, however, were heavier than controls. B6.Ly49 KD implantation sites lagged in early (gd6.5) decidual angiogenesis and were deficient in mid-pregnancy (gd10.5) spiral arterial remodelling. Ultrastructural analyses revealed that B6.Ly49 KD uNK cells had impaired granulogenesis, while immunocytochemistry revealed deficient vascular endothelial cell growth factor (VEGFA) production. Perforin and IFNG expression were normal in B6.Ly49 KD uNK cells. Thus, in normal mouse pregnancies, Ly49 receptor signaling must promote implantation, early decidual angiogenesis and mid-pregnancy vascular remodelling. Disturbances in these functions may underlie the reported genetic associations between human pregnancy complications and the inability of specific conceptus MHCs to engage activating KIR on uNK cells.