In vitro and in vivo NK-cell-DC interactions. (A) In vitro NK cell-DC interactions. Three types of NK-cell-DC interactions have been described. They are all dependent on cell-cell contact and cytokine signals.¹  Activated DCs mediate NK-cell activation. In particular, DC-derived IL-15, IL-12/IL-18, and IFN-α/β could specifically enhance NK-cell proliferation, IFN-γ secretion, and cytotoxic function, respectively.²  Activated NK cells promote DC maturation. In human, NKp30 appears to play a central role in this interaction.³  Immature DCs, but not mature DCs, are killed by NK cells. This phenomenon appears to be dependent on NKp30 as well. iDC indicates immature DCs; mDC, mature DCs; rNK, resting NK cells; aNK, activated NK cells. (Ai-Aiii) Examples of L (ligand)-R (receptor) interactions. In panel Ai, L1 indicates MICA/B; R1, NKG2D. In panel Aii, R2 indicates TREM2 (mouse) or NKp30 (human). In panel Aiii, R3 indicates NKp30; and L4, HLA-E. (B) In vivo NK-cell-DC interactions during MCMV infection. (Bi) pDCs promote early activation of NK cells through IL-12 and IFN-α/β production following activation by viral CpG motifs. (Bii) CD8α DCs sustain later activation of Ly49H⁺ NK cells. Although Ly49H is known to bind to m157, it should be stressed that m157-Ly49H interaction between NK cells and DCs has not been formally demonstrated.