Plasma and the soluble plasma fraction of CLL patients inhibit NK cell function. (A) Cytotoxicity assays using NK cells of healthy donors that were left untreated (control) or incubated overnight with 25% plasma of healthy donors (n = 3) or CLL patients (n = 3). The ratio of effector NK cells to target cells (Raji, B lymphocyte cell line) is indicated. (B) NK cell markers and activating receptors expressed on control NK cells (incubated in medium) and NK cells that were incubated with healthy donor or CLL patient plasma were analyzed by flow cytometry. A significant down-regulation was observed for CD16 and CD56 upon incubation with CLL plasma (n = 3 for donor NK cells; n = 3 or 6 for plasma from healthy donors or CLL patients). (C) NK cells were left untreated (control), treated with plasma samples (pl), the soluble plasma fraction (so), or the exosomal/vesicular fraction (exo) derived from healthy donors or CLL patients and then used for a cytotoxicity assay. One representative example is shown (left); the bar diagram (D) summarizes data from 6 CLL samples. The inhibition of NK cell cytotoxicity by CLL plasma or the soluble plasma fraction was significant (n = 6).