A Pan-Cancer Single-Cell Transcriptomic Atlas of Natural Killer (NK) Cells Reveals Intrinsic and Extrinsic Mediators of NK Cell Anti-Tumor Functions

Background: Natural killer (NK) cells are critical mediators of anti-tumor immunity, engaging in direct cytotoxic responses and promoting pro-inflammatory immune microenvironments. The molecular mechanisms underlying these functions, however, have yet to be fully elucidated. This is due in part to a lack of understanding of the phenotypic and functional diversity of NK cells across cancer types. Here, we present a single-cell transcriptomic atlas of >250,000 NK cells spanning 340 patients across 17 cancer types to advance our understanding of tumor-infiltrating NK cells. Methods: We re-analyzed publicly available single-cell RNA-sequencing (scRNA-seq) datasets of primary, therapy-naïve tumors, as well as matching peripheral blood samples and healthy donor blood samples. We employed systematic computational analyses to deconvolute the phenotypic and functional diversity of tumor-infiltrating NK cells and define key regulators of NK cell functions in both a cancer type-specific and pan-cancer manner. Feature expression module scoring was used to infer phenotypic and functional characteristics of NK cell clusters in each cancer type. Transcription factor regulon and cell-cell communication prediction algorithms were applied to understand the mechanistic drivers of NK cell populations within tumors. Results: Tumor-infiltrating NK cells display distinct patterns of phenotypic and functional features across cancer types. Nonetheless, we identify consensus NK cell populations across cancers that demonstrate the utility and performance of our gene module-based phenotypic and functional annotations. Interestingly, we observe the enrichment of adaptive-like and CD56 bright NK cell populations in tumors that exhibit features of tissue residency, including the upregulation of ITGA1, ITGAE, and CD69 alongside the downregulation of FCG3RA. Notably, these tissue-resident populations display enhanced chemokine and cytokine activity compared to their recruited counterparts, suggesting the significance of tissue-resident NK cell populations in immune recruitment to tumors. We find the phenotypic composition of intra-tumoral NK cells is vastly different than that of circulating NK cells which harbor predominantly CD56 dim populations, as well as a significant proportion of adaptive-like NK cells in several cancer types. Leveraging the paired nature of several tumor and peripheral blood samples, we performed temporal comparative analyses that revealed the robust expression of KLF2 and S1PR5 in circulation which is then lost in paired tumor tissues. While KLF2 and S1PR5 have been previously implicated in the homing and migration of immune cells, their roles in the trafficking of NK cells to tumors have yet to be defined. In modeling transitions from canonical to stressed NK cell states, we not only identify well-established NK cell checkpoints ( TIM-3, LAG3, and CISH) but also identify novel putative checkpoints yet to be investigated in NK cells, such as the AP-1 family of transcription factors. Additionally, we examine extrinsic factors contributing to intra-tumoral NK cell phenotypes, detecting significant crosstalk between NK cells and immunomodulatory macrophages and CD4+ T cells mediated by galectin-9 and TGFβ interactions. Conclusions: We define the phenotypic and functional pan-cancer landscape of NK cells and uncover both intrinsic and extrinsic factors influencing their anti-tumor functions. We envision our NK cell atlas will facilitate the development of strategies to enhance NK cell-based immunotherapies against cancer.