Unveiling the roles of unconventional subsets and other Innate Lymphoid Cells in human Natural Killer cell ontogenesis

While Natural Killer(NK) cells characterization is evolving, the definition of the maturation intermediates is uncertain. Literature hints the Bone Marrow(BM)-Niche as a major site for NK cell ontogenesis. Committed precursors migrate then to secondary lymphoid tissues to differentiate in cytokine-secreting CD56brightCD16neg(CD56bright) that later give rise to cytotoxic CD56dimCD16pos(CD56dim) NK cells. Additionally, unconventional subsets have been described, I.e., CD56dimCD16neg(unCD56dim) NK cells, whose role in the ontogenesis is unknown. The BM-niche perturbation can cause hematologic diseases as in Myelodysplastic Syndromes(MDS), clonal hematopoietic disorders characterized by an aberrant BM, ineffective hematopoiesis and peripheral cytopenia. The general immune dysregulation favors the progression to Acute Myeloid Leukemia(AML). We demonstrated an impairment of the NK compartment in MDS patients, where undifferentiated stages increase at the expense of terminally differentiated NK cells. A differentiation assay done co-culturing FACS-sorted CD3neg/CD14neg/CD19neg/CD16neg lymphocytes, with OP9 cells as feeder-layer, confirmed that patients-derived cells fail to generate mature NK cells. Rather, they led to stromal cell disruption, suggesting an intrinsic interplay during the pathological switch. Attempting thus to define the physiological NK cell differentiation, we designed a BM-niche mimicking system by co-culturing BM-derived stromal cells with FACS-sorted precursors and less differentiated NK cells from healthy donors. We observed precursors developing to CD56bright and CD56bright differentiating in unCD56dim NK cells, without providing CD56dim NK cells, probably due to missing stimuli, and unCD56dim remaining stable, hinting an alternative, still differentiated nature. Supported by in silico analysis, our data suggest alternative fates for developing NK cells, and a mutual, intimate relationship with the developmental microenvironment.