The Role of PD-1 Signalling in NK Cells: Implications in Physiological and Pathological Conditions

Programmed death-1 (PD-1) is a critical immune checkpoint receptor that limits immune responses through engagement with its ligands (PD-L1/PD-L2). While therapeutic PD-1 blockade is established in T-cell-centered oncology, the expression and functional significance of PD-1 on Natural Killer (NK) cells remains poorly understood. This thesis explores the dual nature of PD-1+ NK cells, moving from their physiological role in immune regulation to their clinical exploitation as therapeutic targets in malignancy. Part I: The Biological Spectrum of PD-1+ NK Cells Firstly, PD-1 expression was identified on NK cells across diverse biological contexts, each associated with distinct functional implications. In cord blood, PD-1+ NK cells likely reflect an early tolerogenic program during perinatal adaptation. In healthy adults with prior HCMV infection, PD-1 marks a subset of terminally differentiated NK cells, potentially acting as a "brake" to prevent excessive immune activation. However, in high-grade serous ovarian carcinoma (HGSC), this axis is co-opted by the tumor: PD-1+ NK cells become functionally impaired, co-expressing the inhibitory receptor NKG2A and localizing within PD-L1+ tumor regions. This state of tumor-driven exhaustion leads to reduced cytotoxicity, which we demonstrated can be reversed through dual checkpoint blockade (PD-1 and NKG2A). Part II: Clinical Translation in Hodgkin Lymphoma (HL) Building on this mechanistic understanding, these insights were translated into the context of Relapsed or Refractory Hodgkin Lymphoma (R/R HL). HL represents a unique therapeutic challenge where Reed-Sternberg (RS) cells frequently lack HLA class I expression, rendering them "invisible" to T cells but highly susceptible to NK cell-mediated killing. Reported are the results of a clinical trial investigating early postautologous hematopoietic stem cell transplantation (ASCT), administration of Immune Checkpoint Inhibitors (ICIs) supported by Autologous Lymphocyte Infusion (ALI). Results: In this cohort (n=30), 15 patients received the ALI + ICIs combination. Remarkably, all patients in this group achieved a Complete Response (CR), with Median Overall and Disease-Free Survival (DFS) not reached after 55 months of follow-up. In contrast, the control group (ALI alone) showed a 36% relapse rate. Summary and Conclusion Correlative biological studies confirmed that ALI + ICIs restored the activating receptor expression and cytotoxic function of NK cells, which were crucial in targeting HLAdeficient RS cells. Furthermore, spatial profiling revealed that high expression of the PD-1/PD-L1 axis in the tumor microenvironment (TME) serves as a positive predictive marker for clinical response. In conclusion, this work demonstrates that the shift of PD- 1 from a physiological regulator to a marker of exhaustion can be therapeutically exploited. Our findings underscore the efficacy of early post-ASCT ALI + ICIs in R/R HL and highlight the restoration of NK cell function as a cornerstone for future immunotherapeutic strategies in tumors enriched in NK and/or T cells that are potentially reactivatable within the TME.