Immunoregulation Of G Protein-coupled Receptor GPR84 On NK Cells And The Underlying Mechanisms

Natural killer?NK?cells,a major component of innate immunity,serve as the first line of defence against transformed tumours and virus-infected cells.NK cells can directly lyse target cells by releasing granzymes.Another mechanism by which NK cells eliminate their target cells is to induce apoptosis throuth death receptor such as Fas,and the killing ability can also be fulfilled by secretion of cytokines such as interferon-??IFN-??.NK cells derive from hematopoietic stem cells?HSCs?in the bone marrow?BM?.Each step of NK cell development is finely regulated via signalling pathway of various cytokines and transcription factors.Common lymphoid progenitors?CLPs?derived from multipotent progenitors can differentiate into NK progenitors?NKps?.NKps express IL-15 receptor?chain?CD122?that allows them to respond to IL-15.Under IL-15 signalling,NKps there after develop into immature NK?iNKs?and mature NK cells?mNKs?.In addition,transcription factors are needed for the development of NK cells.E4BP4 directs the transition of iNK to mNK.Eomes and T-bet are required for the maturation of NK cells.Activation of NK cells is mainly through IL-15-mediated mTOR signaling pathway.mTOR kinase has been reported to play a crucial role as a key metabolic checkpoint in NK cell proliferation and activation.The signalling mediated by mTOR regulates early NK cell development mainly via the induction of E4BP4,an NK cell-specific transcription factor,which maintains IL-15responsiveness by upregulating CD122 expression.G protein-coupled receptors?GPCRs?are seven-transmembrane domain receptors.GPCRs play important roles in various physiological processes,including the regulation of metabolism,development,hormonal homeostasis,inflammatory response,and feeding behavior.Therefore,GPCRs are considered attractive targets for the development of drugs for many human diseases.GPR84 is a member of the G protein-coupled receptor family and belongs to the medium chain fatty acid receptor.GPR84 is closely related to inflammation in immune cells,but little is known about its association with NK cells.Therefore,it is very important to study the immunoregulatory effect of G protein-coupled receptor GPR84 on NK cells.In this study,we first found that GPR84 receptor expression is up-regulated after NK cell activation,indicating that GPR84 receptor might have some association with the development and function of NK cells.Therefore,we first detected the ratio of various immune cells in the spleen of GPR84^-/-and WT mice.The results showed that there was no significant difference in the ratio of spleen T cells and B cells between GPR84^-/-and WT mice.But the proportion and number of NK cells in GPR84^-/-mice decreased comparing to WT mice.Next,we studied the development and maturation of NK cells.We analyzed NK precursor cells?NKp?,immature NK cells?iNK?and mature NK cells?mNK?in the spleen of GPR84^-/-mice.The results showed that GPR84receptor promoted development and maturation of NK cells.To further investigate the effects of GPR84 receptor on NK cell function,we studied the killing ability and anti-tumor ability of NK cells in vitro and in vivo,repectively.The results showed that the killing ability of GPR84^-/-NK cells was weakened,and the melanoma metastasis to the lungs was faster in GPR84^-/-mice,indicating GPR84 receptor promote the killing ability of NK cells and inhibit the occurrence and development of tumors.Finally,we found that GPR84 receptor promoted the expression of T-bet and Eomes in NK cells to promote NK cell development and maturation,and that GPR84 receptor also promoted the activation of mTOR signaling pathway in NK cells to regulate mature and function of NK cells.In summary,the GPR84 receptor is an activating receptor for NK cells,which promotes the expression of T-bet and Eomes by promoting the activation of IL-15-mediated mTOR signaling pathway,thereby promoting the development,maturity and function of NK cells.This study suggests that the GPR84 receptor may provide a potential target for activating NK cell fuction.