Figure 1.

Possible linear and branched models of human NK cell development. In the linear model of human NK cell development, hematopoietic stem cells differentiate into lymphoid-primed multipotential progenitors, which then become common lymphoid progenitors. Lineage commitment occurs at the NK precursor stage. These cells then mature first into CD56^bright NK cells and then CD56^dim NK cells. Differentiation into adaptive NK cells could subsequently occur in response to viral infection. In the branched model of human NK cell development, hematopoietic stem cells differentiate into lymphoid-primed multipotent progenitors, which then differentiate toward common lymphoid or myeloid progenitors. Either of these progenitors could give rise to NK cell progenitors. These NK cell progenitors could then differentiate into CD56^bright, CD56^dim, or adaptive NK cells. Dashed arrows represent hypothetical routes of development/differentiation.