Mouse monocytes promote NK-cell differentiation. Mouse Txb21^+/+ and Txb21^−/− monocytes were cocultured with Txb21^−/− (A) or Txb21^+/+ (B) NKp46⁺CD3⁻CD27⁺ NK cells, and the appearance of CD27⁻ NK cells was evaluated 24 hours later. Shown is a representative experiment of 3. (C) KLRG-1 expression in Txb21^−/− CD27^hi (solid line) and CD27^low (dotted line) NK cells cocultured with monocytes as in panel D. (D) Intracellular T-bet expression in enriched mouse monocytes cultured in the presence (dotted line) or the absence (solid line) of 100 ng/mL mouse IFN-γ. (E) Intracellular T-bet expression in CD14-enriched human peripheral blood mononuclear cell monocytes cultured in the presence (dotted line) or the absence (solid line) of 100 ng/mL human IFN-γ. (F) Surface expression of IL-15Rα (dotted lines) on mouse Txb21^+/+ (left panel) and Txb21^−/− (right panel) mouse monocytes enriched and cultured as before; solid lines indicate isotype controls. (G) Enriched mouse monocytes and Txb21^−/− NK cells were cocultured as before in the presence of blocking anti-IL-15Rα antibodies or isotype as controls. Shown is the frequency (mean ± SD) of CD27^low NK cells 24 hours after culture, of 3 independent experiments. (H) Txb21^−/− NK cells were cultured with IL-15, IL-15Rα-Fc, or IL-15/IL-15Rα-Fc complexes, in the presence or the absence of Txb21^−/− monocytes. In the latter, monocytes were preincubated with cytokines or complexes before coculture with NK cells. Shown is the frequency of CD27^low NK cells 24 hours after culture. (Inset) The expression of IL-15Rα in the surface of Txb21^−/− monoyctes that were preincubated with IL-15/IL-15Rα-Fc complexes (dotted line); solid line indicates the staining of monocytes preincubated with phosphate-buffered saline.