Probiotic Lactobacillus casei Shirota supplementation does not modulate immunity in healthy men with reduced natural killer cell activity.
ABSTRACT Oral intake of probiotic bacteria may beneficially modulate functions of NK cells. In healthy individuals, contradictory results exist as to whether NK cell functions can be modulated by probiotic bacteria. Therefore, the primary objective of our randomized, double-blind, placebo-controlled trial was to determine the effects of the probiotic strain Lactobacillus casei Shirota (LcS) on the activity of NK cells in healthy men who had been preselected for a reduced lytic function of their NK cells. Study participants (n = 68) were supplemented for 4 wk with a probiotic drink providing 1.95 × 10(10) CFU LcS/d or with a similar milk drink without probiotic additive. A run-in period of 2 wk preceded the probiotic supplementation followed by a 2-wk follow-up phase without the probiotic or control drink. Changes in the relative proportions of NK cells and other leukocytes as well as multiple functional measurements were determined longitudinally at baseline, after the 4-wk supplementation, and at the end of the follow-up. The probiotic supplementation had no significant effect on NK cell numbers and function or on phagocytosis, respiratory burst, or cytokine secretion of peripheral blood mononuclear cells. In conclusion, 4 wk of supplementation with LcS does not increase NK cell activity in healthy men with a reduced NK cell lytic activity. However, other doses of LcS, time of intervention, or differences, e.g. in the background diet, may result in a different outcome.