Monocytes in healthy individuals express the NKG2D ligand ULBP-4. Monocytes or NK cells were purified from the peripheral blood of healthy volunteers and analyzed for expression of NKG2D ligands. (A-D) Example flow cytometry histograms (A-B) and contour plots (C-D) of NKG2D ligand staining of monocytes (4′,6-diamidino-2-phenylindole-negative ⁻CD14⁺ cells) (A,C) or NK cells (4′,6-diamidino-2-phenylindole–negative CD3⁻CD56⁺ cells) (B,D) from 1 individual. The numbers shown are the percent of cells with staining with the given NKG2D ligand-specific antibody above control antibody staining. (E) The percent (average [AVG] ± standard error of the mean [SEM]) of monocytes that expressed ULBP-4 on the surface in all healthy individuals tested (n = 9). (F) Immunoblotting with ULBP-4–specific antibody with HeLa cells, monocytes purified from a healthy individual, and rULBP-Fc. These results are representative of 4 individual experiments. (G) Monocytes were cultured for 20 hours, and ULBP-4 was measured in the supernatant by ELISA. These results are representative of 4 individual experiments. ***P < .001 in 2-tailed Mann-Whitney U test. Ctrl, control; M, marker; mAb, monoclonal antibody; mono, monocytes; ND, not detected.