Probiotics and the Immune System
Gut-associated lymphoid tissue interacts with bacterial or dietary antigens inducing a humoral and a cellular immune response in the gut (8). Probiotics stimulate lymphocytes to produce cytokine IFN-y, and prompt nonspecific phagocytic and lymphocytic activity. For example, subjects taking milk with B. lactis for 6 weeks significantly enhanced the levels of IFN-y, and polymorphonuclear cell phagocytic capacity upon stimulation of their peripheral blood mononuclear cells in culture, in comparison to the placebo control group, which received milk alone (76).
Newberry et al. (77), observed in transgenic mice with a single lymphocyte population specific for dietary antigen, an obligatory input from the normal flora in the generation of regulatory lymphocytes that maintained oral tolerance, through the induction of macrophage prostaglandin E2 production via IL-10 and TGF-~.
In this model, hen eggwhite lysozyme is fed to mice expressing a transgenic T-cell receptor that recognizes hen egg-white lysozyme peptide 46-6. This did not result in intestinal pathology; however, the simultaneous administration of cyclooxygenase-2 inhibitors and dietary hen egg-white lysozyme increased the proliferation of lamina propria mononuclear cells and crypt epithelial cells, crypt expansion and villous blunting.
Lamina propria mononuclear cells produce high levels of cyclooxygenase-2-dependent arachidonic acid metabolites, which act as immunomodulators in the immune response to dietary antigen. The results showed that cyclooxygenase-2 dependent arachidonic acid metabolites were essential in the development and maintenance of intestinal immune homeostasis (77).