Abstract
Background: Probiotics are known for their ability to enhance cellular immunity, which includes activation of macrophages, natural killer cells, and cytotoxic T lymphocytes. Natto is a Japanese traditional probiotic food made by fermenting soybean with the bacteria, Bacillus subtilis natto. Components of natto, which include spores of B. subtilis natto, poly-γ-glutamic acid, and levan, have demonstrated their immunoadjuvant and anti-allergic effects through various in vitro and in vivo studies. However, it remains unclear whether oral administration of natto can modulate the immune activity in animals.
Aim: The aim of this study was to investigate the effects of oral administration of natto on the immune system of dogs.
Methods: Eight dogs were randomly divided into 2 groups: a natto-treated group and an untreated group. The dogs in the natto-treated group were fed with 10 g/head/day of a freeze-dried natto product in addition to a usual amount of a regular dry food for 14 days, whereas the dogs in the untreated group were fed with the regular dry food alone. For determination of cellular immune activity, the cell surface antigen analysis of peripheral blood lymphocytes and cytotoxicity analysis of peripheral blood mononuclear cells were performed before and after the natto administration period. Additionally, relative expression of inflammatory cytokines in peripheral blood monocytes after an antigen-stimulation was also examined.
Results: At the end of the administration period, a proportion of natural killer cells (CD3-CD5-CD21- cells and CD3+CD5dimCD8+ cells) in peripheral blood lymphocytes was found to be significantly increased and the cytotoxic effect of the peripheral blood mononuclear cells on canine tumor cells were significantly enhanced in the natto-treated group, but not in the untreated group. The expression of TNF-α in peripheral blood mononuclear cells following an antigen-stimulation was significantly increased in the dogs after administration of natto.
Conclusion: We concluded that oral administration of natto activated the cytotoxic activity of peripheral natural killer cells in dogs and a daily intake of natto might be useful to augment cellular immune activity.

Key words: Bacillus subtilis natto, cellular immune, dog, fermented soy bean, natural killer cell