Natural killer cells in Hodgkin Lymphoma

Abstract

Programmed cell death protein 1 (PD-1) blockade has proven clinical utility in relapsed/refractory Hodgkin Lymphoma (HL). Interestingly, a recent study in HL from my laboratory demonstrated that PD-1, aside from T cells, is also expressed on NK cells. In addition, the "missing-self" hypothesis dictates that NK cells have capacity to lyse target cells 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 cells in HL, suggests that manipulation of host NK cells might be a viable strategy to enhance PD-1 blockade therapy outcomes. The Inositol-requiring enzyme-1-X-box binding protein 1 (IRE1-XBP1) pathway has been demonstrated to play an important role in a range of immune cells, and notably its role varies across tested settings. For instance, in macrophages it is required for the optimal secretion of 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 has not yet been explored. Here, I outline new data identifying a hitherto unrecognized mechanism that is pivotal to how NK 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)-related transcription factor XBP1 to its active form XBP1s is induced during NK cell-mediated direct cytotoxicity against various blood cancer cell lines. Notably, the induction of the IRE1-XBP1 pathway was not associated with up-regulation of known XBP1s target molecules implicated in ER stress alleviation, indicating the XBP1 splicing in NK cells occurs independently from the canonical UPR system. Then, utilising a small molecule inhibitor, 4-methyl umbelliferone 8-carbaldehyde (4µ8c), I interrogated the role of the IRE1-XBP1 pathway in NK cell function. Inhibition of the IRE1mediated splicing led to significant decrease in XBP1s expression, which was associated with impaired migration, NKIS formation and secretion of IFNγ and TNFα. These findings further 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 in XBP1s in response to activation by a HL line, compared to blood from healthy donors.