Several analogues of beat-stable enterotoxins (ST, and ST,) produced by enterotoxigenic Escherichia cdi were synthesized. Peptides (ST&-181 and ST&171) consisting of 13 amino acid residues from the Cys residue near the N-terminus to the Cys residue near the C-terminus and linked by three disubide bonds had the same biological and immunological properties as native ST, and ST,, respectively. The results indicated that the sequence with the 13 amino acid residues and three disulfide linkages is essential for full biological activity of ST.

Lactobacillus casei strain Shirota (LcS) has been demonstrated to have beneficial effects in numerous murine disease models via host immune modulation. It has also been reported that LcS induced recovery of host immune responses that were decreased by treatment with carcinogens and augmented the natural killer activity and T-cell functions of host immune cells. After LcS is ingested by the host, it is incorporated into M cells in Peyer's patches (PP) and digested to form active components. In PP, macrophages or dendritic cells that phagocytosed LcS gained the ability to produce several cytokines, especially tumor necrosis factor-alpha. The components of LcS digested in PP were then recognized through toll-like receptor 2 in antigen-presenting cells, resulting in the production of several cytokines that elicited varied responses in host immune cells. Also, it was observed by 2D-PAGE analyses that the expression level and/or the phosphorylation of some proteins in PP and mesenteric lymph nodes were definitely altered by the ingestion of LcS, providing more evidence of cellular responses. These results suggest that some probiotic bacteria have the potential to augment or modify the host immune function through the regulation of host immune cells.

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