Mediation of human NK-activity by components in extracts of Viscum album

Viscum album extracts (Iscador) were investigated for their potency to influence NK cytotoxicity in vitro. In vitro short term cytotoxicity assays (4 h) with human peripheral mononuclear cells (PMNC) and human K 562 tumor cells showed a drastic enhancement of NK cytotoxicity in the presence of V. album extracts. The presence of the V. album components during tumor cell lysis was essential since preincubation of PMNC with V. album extract followed by thorough washing did not lead to enhancement of NK cytotoxicity. One responding effector cell was identified as a member of the large granular lymphocyte (LGL) family carrying both Leu 7 and Leu 11 surface markers. Furthermore, monocytes depleted of LGL, but not differentiated macrophages, showed a weak enhancement of their cytolytic activity in the presence of V. album extract. Fractionation of V. album extracts revealed two active fractions one (C1) with about 3-4000 D and the other (C2) less than 1000 D. Both components enhanced NK cytotoxicity of LGL (Leu 7+, Leu 11+) as well as of monocytes showing enhancing effects also against moderately NK-sensitive tumor cell lines.