Interleukin‐12 is involved in the enhancement of human natural killer cell activity by Lactobacillus casei Shirota

Summary We conducted a placebo‐controlled, cross‐over trial to examine the effect of Lactobacillus casei Shirota (LcS) on natural killer (NK) cell activity in humans. NK cell activity exhibited a declining trend during the period of placebo ingestion, but NK cell activity increased after intake for 3 weeks of fermented milk containing 4 × 10 10 live LcS. When human peripheral blood mononuclear cells were cultured in the presence of heat‐killed LcS, NK cell activity was enhanced. The ability of LcS to enhance NK cell activity and induce interleukin (IL)‐12 production was correlated, and the addition of anti‐IL‐12 monoclonal antibody reduced the enhancement of NK cell activity triggered by LcS. In addition, separation of NK cells from LcS‐stimulated monocytes with membrane filter reduced NK cell activity to the intermediate level and almost deprived monocytes of the ability to produce IL‐12. These results demonstrate that LcS can enhance NK cell activity in vivo and in vitro in humans, and IL‐12 may be responsible for enhancement of NK cell activity triggered by LcS.