The Development Of Human NK Cells And The Function Of Decidual NK Cells During Pregnancy Tolerance

Natural killer (NK) cell is one of the cellular mediators of innate defence. NK cell is also the lymphoid cell that, without the need for pre-activation or immunization, can recognize and kill aberrant cells and rapidly produce soluble factors, including chemokines and cytokines that have antimicrobial effects and/or prime other cells of the immune system. More and more recent researches have found that except classic killing ability towards tumours, NK cells can also regulate immune system and maintain tissue homeostasis. Especially in the early pregnancy, more than 70% lymphocytes at the maternal-fetal are NK cells. It is still unknown about the immune function of such NK cells. Classical theories about immune tolerance or tumour immune escape cannot explain why the allograft embryo can survive. Thus, Researches on the regulation function of NK cells, especially the mechanism about embryo tolerance are very important, not only for the pregnancy immunity itself, but also for the transplantation immunity and clinic cell therapy.By contrast to T cells and B cells, the knowledge about the development and the functions of NK cells is still in its infancy. In order to have researches on the functions about NK cell in the embryo tolerance, first we should know better about the characteristic of these cells. Decidual NK cells have many differences in the gene expression, development stages and functions compared to periphery NK cells. To have researches on the development of human NK cells, we first select three kinds of NK cells: pNK, cNK and dNK. Here we characterise four novel populations defined by CD11b and CD27, which can represent the distinct stages of human NK cells from different tissues. And dNK are more immature compare to the other two kinds of NK cells. More than 90% of NK cells from peripheral blood are CD11b+CD27- population whereas NK cells from cord blood have 80% CD11b+CD27- and 20% CD11b+CD27+ populations.Interestingly, we have found that there are more than 60% CD11b-CD27- population of NK cells from deciduas, which are differ from the other kinds of NK cells. These CD11b-CD27- NK cells display an immature phenotype, expressing high percentage of NKG2A and having potential for differentiation, which further verify that dNK cells are immature. We also demonstrate that each population could be characterised by unique functional and phenotypic attributes. CD11b-CD27+ and CD11b+CD27+ NK cells show the best ability to secrete cytokines. CD11b+CD27- NK cells exhibit high cytolytic function. Thus, we demonstrate that human NK cells at different developmental stages have special functions and describe a new model of human NK cell differentiation. Meanwhile,we have verified that decidual NK cells have different characteristics compared to periphery NK cells, including a lot of CD11b-CD27- NK cells with developmental potential and CD11b-CD27+ and CD11b+CD27+NK cells with cytokine expressions.Given that the distinct characteristic of decidual NK cells and large numbers of accumulation at the maternal-fetal interface during pregnancy, we are curious about the exact role of NK cells in successful pregnancy and the possible mechanisms involved. Here we found that more than 70% decidual lymphocytes are NK cells in the first trimester of pregnancy with less than 10% T cells. And more than 20% of these decidual NK cells are CD56brightCD27+ NK cells, which are much higher than the percentage of this subset in the periphery blood. Furthermore, when the pregnancy is over, CD56brightCD27+NK decrease as well as the increase of T cells. These CD56brightCD27+NK cells are the main source of IFN-γamong NK cells. So what kind of role do these CD56brightCD27+NK cells play when they appear at the key point of embryonic implantation?In order to implant and grow, the trophoblast cells from the allograft fetus need to invade the mother tissues and inevitably bring certain inflammatory reaction. How to control these inflammations and maintain tissue homeostasis is the key problem of embryo tolerance. TH17 cells, as the key inflammation cells, have been proved to be crucial in many diseases. Here we analysis the decidual tissues of normal pregnancy and found out less than 2% TH17 cells existed, which shows the mild inflammation. Given that IFN-γcan inhibit the polarization of TH17 cells, we verified that normal decidual NK cells can inhibit TH17 cells by IFN-γand maintain immune balance at maternal-fetal interface.We also found severe inflammations and significantly increased TH17 cells at maternal-fetal interface from patients of recurrent spontaneous abortion. The percentage of NK cells decreased and so as the inhibitory cytokines. Meanwhile, many inflammation cytokines, such as IL-6 and IL-1βincrease, which make the inflammation worse.To verify whether the failure of pregnancy has connected with the increased TH17 cells, we select and have research on the classic CBA/J×DBA/2 mice model. The results show that The CBA/J×DBA/2 pregnancy mice have more TH17 cells at day10 compared to the control. Furthermore, after adoptive transfer TH17 cells, the normal CBA/J×Balb/c mice also have fetal loss, which indicate that large amount of TH17 cells can induce pregnancy loss directly.Thus, our research first provides evidence that decidual NK cells are more immature and including large amount of DN (CD11b-CD27-) NK cells. We also demonstrate that CD11b/CD27 markers can separate human NK cells into different subsets with different developmental stages and special functions, which describe a new model of human NK cell differentiation. Second, our research provide evidence that decidual NK cells play a pivotal role in ensuring successful pregnancy by antagonizing inflammatory TH17 cells. The anti-inflammatory and immune-regulatory role of decidual NK cells is confined to the CD56brightCD27+ subset, such NK cells readily secret IFN-γand IL-1RA, which suppresses the differentiation of TH17 cells. We also found recurrent spontaneous abortion in humans is associated with a prominent TH17 response and increased inflammatory cytokines, especially IL-6 and IL-1β, during which the regulatory function of decidual NK cells is disrupted in spontaneous abortion patients. Our study identifies a subset of decidual NK cells as key regulatory cells by suppressing TH17-mediated inflammation during successful pregnancy and substantiates the immune tolerance model at the maternal-fetal interface in human.