Human macrophages trigger NK cell proliferation, IFN-γ secretion, and prime NK cell cytotoxicity to susceptible target cells. Autologous NK cells were coincubated for 48 hours with monocytes, DCs, macrophages, or macrophages in the presence of 100 ng/mL (macrophage^{low LPS}) or 200 ng/mL (macrophage^{high LPS}) LPS at the ratios shown. (A) ³H-TdR incorporation by NK cells. The first data point shows the extent of ³H-TdR incorporation in NK cells cultured in media alone. (B) Viable NK cells, unstained by trypan blue, after 48 hours of coincubation with APCs at the ratios indicated. Cells were counted using a hemocytometer. The first bar indicates the number of viable NK cells after culture in media alone. (C) IFN-γ secretion from NK cells cocultured with the indicated APCs, at the ratios shown, measured by ELISA. NK cells were isolated after coincubation with indicated APCs and tested for cytolytic activity against (D) the EBV-transformed B-cell line 721.221 and (E) the myeloid leukemia cell line K562. *P < .05, **P < .01 by Student t test for unpaired values. The results are expressed as the mean ± SE of at least 3 independent experiments using cells from different donors.