Single-Cell Profiling Reveals Functional Heterogeneity and Serial Killing in Human Peripheral and Ex Vivo-Generated CD34+ Progenitor-Derived Natural Killer Cells.

Increasing evidence suggests that natural killer (NK) cells are composed of distinct functional subsets. This multifunctional role has made them an attractive choice for anticancer immunotherapy. A functional NK cell repertoire is generated through cellular education, resulting in a heterogeneous NK cell population with distinct capabilities responding to different stimuli. The application of a high-throughput droplet-based microfluidic platform allows monitoring of NK cell-target cell interactions at the single-cell level and in real-time. A variable response of single NK cells toward different target cells is observed, and a distinct population of NK cells (serial killers) capable of inducing multiple target lysis is identified. By assessing the cytotoxic dynamics, it is shown that single umbilical cord blood-derived CD34+ hematopoietic progenitor (HPC)-NK cells display superior antitumor cytotoxicity. With an integrated analysis of cytotoxicity and cytokine secretion, it is shown that target cell interactions augment cytotoxic as well as secretory behavior of NK cells. By providing an integrated assessment of NK cell functions by microfluidics, this study paves the way to further functionally characterize NK cells ultimately aimed to improve cancer immunotherapy.

Understanding natural killer cell biology from a single cell perspective.

During the last decade, advances in single cell technologies have ignited increased understanding of natural killer cells (NK cells), which turned out to be far more complex than originally thought. Ample studies have established tissue-specific phenotypic variation within this cell population; however, the functional implication of this vast variation is still unclear. At single-cell level, the function of a NK cell is tightly regulated by several checkpoints however upon proper recognition the cell can deliver a lytic hit as early as 10 min or could take hours before they can kill their target cells. Moreover, only a fraction of NK cells appears to kill target cells while the larger portion of NK cells appear to be non-cytotoxic. All these studies showed that the NK cell compartment is composed of cells with different functional strengths and efficacies, thereby highlighting the necessity of analytical platforms that allow the study of these important innate immune cells at single-cell level. In this review, we discuss and provide an overview on phenotypical and functional heterogeneity within the NK cell population and subsequently provide information regarding emerging technologies that highlight the importance of single-cell studies to understand the biology of these cells.