Figure 1. Intrahepatic NK cells in humans.

The cell composition of lymphocytes in the human liver is shown. Human liver NK cells include the CD56^bright and CD56^dim subsets. The two subsets exhibit significant differences in their proliferative responses to IL-2, intrinsic cytotoxic capacity, cytokine production, NKR repertoire, and adhesion molecule expression. (1) CD56^bright NK cells expand in response to low doses of IL-2, whereas CD56^dim NK cells respond poorly to IL-2 stimulation. (2) CD56^bright NK cells have a high level of expression of the CD94/ NKG2 C-type lectin receptors and less than 10% of them express KIR. In contrast, more than 85% of the CD56^dim NK cells are KIR⁺ and have a low level of expression of CD94/NKG2. (3) CD56^dim NK cells are more cytotoxic against NK-sensitive targets but produce lower amounts of cytokines than CD56^bright NK cells. (4) CD56^bright NK cells express high levels of CCR7 and CXCR3. (5) Finally, CD56^bright NK cells can be induced from NK cell precursors by IL-15 and may then differentiate into CD56^dim NK cells.