Natural killer cells in Hodgkin Lymphoma

Programmed cell death protein 1 (PD-1) blockade has proven clinical utility inrelapsed/refractory Hodgkin Lymphoma (HL). Interestingly, a recent study in HL from mylaboratory demonstrated that PD-1, aside from T cells, is also expressed on NK cells. Inaddition, the “missing-self” hypothesis dictates that NK cells have capacity to lyse targetcells with lost expression of Major Histocompatibility Complex molecule I (MHC-I). Hence,the frequent absence of MHC-I on Hodgkin-Reed-Sternberg (HRS)-cells, the malignant cellsin HL, suggests that manipulation of host NK cells might be a viable strategy to enhancePD-1 blockade therapy outcomes.The Inositol-requiring enzyme-1-X-box binding protein 1 (IRE1-XBP1) pathway has beendemonstrated to play an important role in a range of immune cells, and notably its role variesacross tested settings. For instance, in macrophages it is required for the optimal secretionof TNFα, whereas in B cells and eosinophils it is a key factor involved in cell differentiation.The role of this pathway in Natural Killer (NK) cells and in NK cells from patients with HL hasnot yet been explored.Here, I outline new data identifying a hitherto unrecognized mechanism that is pivotal to howNK cell function, including the relatively poorly understood processes of NK cell migration,and NK cells Immune Synapse (NKIS) formation.Firstly, I demonstrate that splicing of the unfolded protein response (UPR)-relatedtranscription factor XBP1 to its active form XBP1s is induced during NK cell-mediated directcytotoxicity against various blood cancer cell lines. Notably, the induction of the IRE1-XBP1pathway was not associated with up-regulation of known XBP1s target molecules implicatedin ER stress alleviation, indicating the XBP1 splicing in NK cells occurs independently fromthe canonical UPR system.Then, utilising a small molecule inhibitor, 4-methyl umbelliferone 8-carbaldehyde (4µ8c), Iinterrogated the role of the IRE1-XBP1 pathway in NK cell function. Inhibition of the IRE1-mediated splicing led to significant decrease in XBP1s expression, which was associatedwith impaired migration, NKIS formation and secretion of IFNγ and TNFα. These findingsfurther demonstrate the importance of the IRE1-XBP1 pathway in NK cell function.Next, I show that in pre-therapy HL patient blood samples, there is a ~2-fold reduction inXBP1s in response to activation by a HL line, compared to blood from healthy donors. Notably, this observation was restricted to the CD56brightCD16- subset of cells that mylaboratory has previously shown to be expanded and to display high expression of PD-1 inpatients with HL. The cells with suppressed XBP1s activation displayed impaired NKISformation, degranulation, and secretion of IFNγ and TNFα. Not only did addition of PD-1blockade enhance IFNγ secretion and NKIS formation to a PD-L1/PD-L2 gene amplifiedHRS-line, but critically the beneficial impact of PD-1 blockade on IFNγ and NKIS formationwas partially reversed by inhibition of the XBP1s-pathway. Manipulation of the IRE1-XBP1pathway to enhance anti-tumour responses should be further explored as a novel lymphomatreatment strategy to enhance host NK cell immunity.