The Mechanism Of NK Cells Maintaining Infected Trophoblasts Survival

Natural Killer(NK)cells are capable of killing a variety of tumor cells and microbe-infected cells without prior sensitization.During the first trimester of pregnancy,decidual NK(dNK)cells are the most abundant immune cells in the decidua microenvironment and play multiple roles in protecting against microbial infections,maintaining immune tolerance,and promoting embryonic growth and development.It has been reported that NK cells can secrete granulysin and granzyme A to selectively kill intracellular bacteria and protect the infected host cells.The protective mechanism of NK cells against host cells infected with intracellular bacteria is still unclear.We therefore sought to investigate how NK cells maintain the survival of host cells after infection,and obtained the following results:1.Decidual NK cells promote the survival of intracellular bacteria-infected trophoblast organoids.Primary trophoblast cells from 3-6 weeks of human gestation were infected with Listeria monocytogenes(Lm),then co-incubated with homologous dNK cells and established trophoblast organoids culture model.We found that dNK cells could not only effectively remove bacteria from trophoblasts,but also alleviate the apoptosis of trophoblasts caused by intracellular bacterial infection and promote the formation and differentiation of trophoblast organoids.2.Decidual NK cells promote cellular response to lipids in intracellular bacteria-infected trophoblast cells.To investigate the protective effect of dNK cells on trophoblast cells encountering infection,transcriptome sequencing of trophoblast cell line JEG3 was performed.The analysis revealed that dNK cells inhibited the expression of genes related to apoptosis and DNA damage in JEG3 caused by intracellular bacterial infection,such as CASP and BCL family members,and induced JEG3 to upregulate the expression of lipid metabolism-related genes such as DGAT1 and DGKQ,enriching the cellular response to lipids,lipid biosynthesis,and lipid storage biological processes in JEG3.3.Decidual NK cells transport lipids via APOD to alleviate lipid reduction in trophoblast cells.To detect changes in lipid content in trophoblast cells,lipid staining was used to find that Lm infection resulted in a significant reduction in lipid droplet content in trophoblast cells,while dNK cells were able to mitigate their reduction in lipid droplet content.In combination with transcriptome sequencing,dNK cells were found to have elevated gene expression levels of Apolipoprotein D(APOD)after co-incubation with intracellular bacteria-infected trophoblast cells.The expression level of APOD gene was significantly higher in dNK cells than in peripheral blood NK cells(pNK).By flow cytometry and immunofluorescence techniques,we further found that dNK cells could synthesize more lipids after co-incubation with intracellular bacteria-infected trophoblast cells and may deliver lipids to trophoblast cells via APOD.4.Blocking the APOD-LRP1 axis inhibits the protective effect of decidual NK cells on trophoblast cells.To verify that the protective effect of dNK cells on intracellular bacterial-infected trophoblast cells is dependent on the lipid transport process,the antagonist LRPAP1 was used to block the receptor of APOD,low-density lipoprotein receptor-related protein 1(LRP1).As a result,the lipid transport from dNK cells to trophoblast cells was blocked,and the reduction of lipid droplets and disruption of lipid metabolism triggered by bacterial infection could not be restored,which eventually led to cell apoptosis.In conclusion,this study demonstrates that NK cells can initiate a protective mechanism against host cells while selectively killing intracellular bacteria,and maintain the energy supply and lipid metabolism homeostasis of the infected cells by transporting lipids through APOD.This study provides new evidence for the immunosurveillance and protective role of NK cells against embryos in the first trimester of pregnancy.