Mechanism of activation of human peripheral blood NK cells at the single cell level by Echinacea water soluble extracts: recruitment of lymphocyte–target conjugates and killer cells and activation of programming for lysis

Echinacea purpurea, a plant originally used by native Americans to treat respiratory infections, has also been shown to exert immunomodulatory activities both in vivo and in vitro. However, the mechanism underlying Echinacea-induced immunomodulation remains largely unknown. This study examined in vitro the effects of soluble extracts of E. purpurea on natural killer (NK) cells present in human peripheral blood mononuclear cells (PBMC). Flow cytometric methods were used to examine activation, cytotoxicity, NK–target binding, and killer cell frequency. Treatment of PBMC with Echinacea overnight resulted in the activation of CD69 expression and increase in mean fluorescence intensity in both the CD16+ and CD16+CD56+ NK subsets. However, the frequency of CD16+ cells was decreased as well as the mean fluorescence intensity was down-regulated. NK cytotoxicity was augmented 100% at the concentration of 0.1 μg/ml of Echinacea in a short time (4-h) assay. Examination at the single cell level revealed augmentation of the frequency of CD56+ NK–target conjugates and a plateau was reached after 30–60 min of incubation. Likewise, the frequency of CD56+ killer cells in the conjugates was also significantly increased by Echinacea. There was recruitment of non-conjugated CD56+ cells into CD16+ NK–target conjugates and activation of the NK–target non-killer conjugates into killer cells. These findings demonstrate that Echinacea extracts are potent activators of NK cytotoxicity. Echinacea augments the frequency of NK target conjugates and activates the programming for lysis of NK cells.