M. Kazim Panjwani ^1* Simon Grassmann ² Rosa Sottile ¹ Jean-Benoit Le Luduec ¹ Theodota Kontopoulos ¹ Kattria van der Ploeg ¹ Joseph C Sun ² Katharine C. Hsu ^1,3*

• ¹ Human Oncology and Pathogenesis Program, Memorial Sloan Kettering Cancer Center, New York, United States
• ² Department of Immunology, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, United States
• ³ Weill Cornell Medicine, Cornell University, New York, New York, United States

Development of antigen-specific memory upon pathogen exposure is a hallmark of the adaptive immune system. While natural killer (NK) cells are considered part of the innate immune system, humans exposed to the chronic viral pathogen cytomegalovirus (CMV) often possess a distinct NK cell population lacking in individuals who have not been exposed, termed "adaptive" NK cells. To identify the "naïve" population from which this "memory" population derives, we performed phenotypic, transcriptional, and functional profiling of NK cell subsets. We identified immature precursors to the Adaptive NK cells that are equally present in both CMV+ and CMVindividuals, resolved an Adaptive transcriptional state distinct from most mature NK cells and sharing a common gene program with the immature CD56 bright population, and demonstrated retention of proliferative capacity and acquisition of superior IFNγ production in the Adaptive population. Furthermore, we distinguish the CD56 bright and Adaptive NK populations by expression of the transcription factor CXXC5, positioning these memory NK cells at the inflection point between innate and adaptive lymphocytes.Natural killer (NK) cell biology straddles the border between innate and adaptive immunity, as the cells lack the germline-rearranged antigen-specific receptor of adaptive lymphocytes such as T and B cells, but still develop a population primed to respond to human cytomegalovirus following exposure. These "memory" or "adaptive" NK cells have been the subject of separate phenotypic, trancriptomic, and functional investigations without consistent definition and outside the context of broader lymphocyte biology. Using an integrated multi-modal approach, our results phenotypically, transcriptionally, and functionally identify three major human NK populations, and align them to the naïve-memory-effector paradigm of adaptive immunity.Reconciling this framework enables meaningful comparisons across immune cell types for the improved understanding of shared mechanisms and features of immunological memory.