Effects of the non-commensal Methylococcus capsulatus Bath on mammalian immune cells

Dietary inclusions of a bacterial meal consisting mainly of the non-commensal, methanotrophic bacteria Methylococcus capsulatus Bath have been shown to ameliorate symptoms of intestinal inflammation in different animal models. In order to investigate the molecular mechanisms causing these effects, we have studied the influence of this strain on different immune cells central for the regulation of inflammatory responses. Effects were compared to those induced by the closely related strain M. capsulatus Texas and the well-described probiotic strain Escherichia coli Nissle 1917. M. capsulatus Bath induced macrophage polarization toward a pro-inflammatory phenotype, but not to the extent observed after exposure to E. coli Nissle 1917. Likewise, dose-dependent abilities to activate NF-κB transcription in U937 cells were observed, with E. coli Nissle 1917 being most potent. High levels of CD141 on human primary monocyte-derived dendritic cells (moDCs) were only detected after exposure to E. coli Nissle 1917, which collectively indicate a superior capacity to induce Th1 cell responses for this strain. On the other hand, the M. capsulatus strains were more potent in increasing the expression of the maturation markers CD80, CD83 and CD86 than E. coli Nissle 1917. M. capsulatus Bath induced the highest levels of IL-6, IL-10 and IL-12 secretion from dendritic cells, suggesting that this strain generally the post potent inducer of cytokine secretion. These results show that M. capsulatus Bath exhibit immunogenic properties in mammalian in vitro systems which diverge from that of E. coli Nissle 1917. This may provide clues to how M. capsulatus Bath influence the adaptive immune system in vivo. However, further in vivo experiments are required for a complete understanding of how this strain ameliorates intestinal inflammation in animal models.