Bifidobacterium adolescentis modulates the specific immune response to another human gut bacterium, Bacteroides thetaiotaomicron, in gnotobiotic rats

In order to investigate the capability of an autochthonous bacterium to mod ulate the host's immune response against the indigenous microflora, the imm unogenicity of two selected bacterial species of the human gut was investig ated in a gnotobiotic rat model. Germ-free (GF) rats were monoassociated wi th either Bifidobacterium (B.) adolescentis or Bacteroides (B.) thetaiotaom icron and the development of bacteria-specific IgG and IgA in serum and spe cific secretory IgA (sIgA) in feces of the animals were measured. Knowing t he antibody levels in gnotobiotic rats induced by monoassociation, we subse quently diassociated two groups of rats in order to investigate the impact of B. adolescentis on the immune reaction against B. thetaiotaomicron. One group was diassociated simultaneously with B. adolescentis and B. thetaiota omicron, the second group was diassociated with these bacteria in sequence. In contrast to B. thetaiotaomicron, B. adolescentis was not able to induce a systemic immune response in monoassociated animals as evident from serum IgG and IgA. However, both bacterial species challenged the mucosal immune system as indicated by an increase in sIgA in the feces. The specific immun e response to B. thetaiotaomicron was significantly lower in diassociated a nimals than in animals monoassociated with B. thetaiotaomicron. This effect was more pronounced in the rats, that had been associated sequentially. Th e presence of B. adolescentis down-regulated the humoral immunity to B. the tatiotaomicron.