The Mechanism Of The Non-Immune Checkpoint Function Of PD-L1 And CD47

Cancer is one of the lethal disorders that poses serious threats to human health.In recent years,cancer immunotherapy has achieved impressive success in clinical,and has created a revolutionary new era of cancer therapy.Cancer cells can evade immune surveillance of immune system by modulating immune checkpoint molecules,including the adaptive and innate immune systems.Among which,programmed cell death ligand 1(PD-L1)sends a critical “don’t find me” signal to the adaptive immune system,and CD47(cluster of differentiation 47)transmits a “don’t eat me” signal to the innate immunity system.Immune checkpoint blocking drugs have achieved great success in tumor therapy.However,these drugs still face many challenges such as low overall response rate and immune tolerance in clinical application.Therefore,it is important to comprehensively explore the mechanism and function of immune checkpoint molecules.In adaptive immune system,PD-L1 binds to programmed cell death protein 1(PD-1)on T cells and inhibits anti-tumor immunity by counteracting T cell-activating signals.The immunosuppressive function and regulatory mechanism of plasma membrane anchored PD-L1 has been well studied,but the functions of PD-L1 inside tumor cells are poorly understood.Here,we report a novel function of PD-L1 in the nucleus in maintaining the genomic stability.We found that PD-L1 deletion significantly increased chromosomal defects,including abnormal telomere cohesion and aneuploidy in cancer cells.Furthermore,we demonstrated that PD-L1 directly binds to stromal antigen 1(SA1)via its cytosolic tail,which is one of the key components of cohesin complex involving the maintenance of chromatid cohesion.Depletion of PD-L1 reduced the nuclear distribution of SA1,resulting in the abnormal chromatid cohesion,which could be reversed by expression of wild-type PD-L1 rather than by its mutant of cytosolic tail deletion.In addition,we found that KPNA2 and KPNA6,members of the importin-α family,play an important role in the nuclear importing of PD-L1.In summary,PD-L1 nuclear localization is regulated in an importin-dependent manner,and the nuclear PD-L1 regulates the function of cohesin in chromatid cohesion by interacting with SA1.CD47 is another type of immune checkpoint molecule expressed on cancer cells,which binds to SIRPα on the surface of macrophages to trigger immunosuppressive signals to inhibit phagocytosis and shield tumor cells from immunosurveillance.Therefore,blocking the interaction between CD47 and SIRPα enables the enhancement of anti-tumor effect,and preclinical studies and early clinical data showed the promising applications for tumor therapy.Most of the previous studies investigating CD47 focused on the immunosuppressive effects,only a few studies explored the tumor intrinsic function of CD47.In this study,we found that CD47 can not only localize on the plasma membrane but also on the lysosome,and the lysosomal localized CD47 can bind to p18,which is an important component of the ragulator complex in the mTORC1 signaling pathway.To sum up,we found that CD47 is not only localized on plasma membrane but also on lysosome,and lysosome-localized CD47 affects the signal of mTORC1 activation through p18,thereby regulating cell proliferation.In summary,we uncovered the non-plasma membrane localization of the immune checkpoints PD-L1 and CD47 and revealed novel intrinsic functions of them.The nuclear PD-L1 regulates the function of cohesin through SA1 and thus affects chromosomal adhesion;the lysosomal localized CD47 promotes the activity of mTORC1 through p18 and affects cell proliferation.Based on the subcellular localization of PD-L1 and CD47,this project clarified the underlying mechanisms of their non-immune checkpoint function which can provide new ideas for tumor immunotherapy.