Modulating immune responses

Probiotics – A viable concept

The use of probiotics to maintain health and to prevent or to treat disease is a topic that has a long record and that continues to receive substantial research interest. Recent work on the properties and functionality of living microorganisms in food (supplements) has suggested that probiotics play an important role in digestive and immunological functions, and that they could have a significant effect in the prevention or alleviation of infectious diseases. “But how probiotics achieve these effects remains largely unknown,” says Professor Michiel Kleerebezem, who works at NIZO food research and also holds an academic chair in Bacterial Metagenomics at Wageningen University. “Elucidation of the molecular mechanisms is needed accordingly.”

Kleerebezem is studying the effect of probiotics in relation to gut health using a multidisciplinary approach, which combines the efforts of food technologists, gastroenterologists, cell biologists, and microbiologists. Powerful genomics tools offer new opportunities to study, in a detailed manner, the endogenous gut microbiota, and the diet-derived transient microbes such as probiotics, in relation to health. “Step by step, we are gathering insights into the ways probiotic bacteria sense and adapt to the gastrointestinal tract environment,” Kleerebezem explains. Complementary approaches using molecular techniques, in vitro cell systems, and in vivo human volunteer trials, are revealing specific intestinal responses to probiotics. “By understanding the underlying mechanisms, it should be possible to modulate human health in a better, more specific way,” Kleerebezem predicts. “By proceeding along these lines, we will, in the end, be able to compose or design improved probiotics and food compositions with specific health functionalities.”

Scanning Electro-Micrograph of Lactobacillus plantarum on a standard grid

Two placebo-controlled human intervention studies have indicated that intestinal exposure to a Lactobacillus strain induced time-dependent, transcriptional responses in healthy intestinal mucosa (BMC Genomics 2008;9:374-388). Expression of human genes involved in lipid metabolism, cellular growth or development, and immune response was affected. Furthermore, Kleerebezem and his colleagues from the Top Institute Food and Nutrition were the first to publish scientific evidence obtained in humans regarding the mechanism by which probiotics can affect the immune response (PNAS 2009;106:2371-2376). In a randomized, double blind, placebo-controlled, crossover study, eight healthy volunteers ingested preparations of a living or heat-killed model-bacterium (Lactobacillus plantarum WSFS1), which is a common food bacterium, and a representative of a probiotic species. Living L. plantarum bacteria were harvested from two distinct growth phases: exponential (fast-growing) or stationary. After a six-hour intake regime, biopsies were taken from the intestinal duodenum mucosa of the participating volunteers. Gene expression profiles were determined in these biopsy tissues using whole-genome microarrays, and interpreted by biological pathway reconstructions.

Interaction of Lactobacillus plantarum (LP), with intestinal mucosa comprising Intestinal Epithelial Cells (IEC), but also dedicated immune cells like Immature Dendritic Cells (IEC) and Naive T-Cells (nTC)

In addition, histochemical and microscopic evaluation of the tissue sections was performed. “Interpretation of the mucosal gene transcriptome took most of our time” Kleerebezem says. “Expression profiles of the human mucosa displayed striking differences in modulation of NF-κB dependent pathways; notably, after consumption of living L. plantarum bacteria obtained from different growth phases. The identified gene expression pattern elicited by consumption of stationary phase bacteria appears to reflect a ‘code orange alert,’ in which the mucosal immune system is brought to attentiveness, but does not launch an inflammatory response.” Overall analysis of these mucosal expression profiles after consumption of L. plantarum WCFS1 suggests that these profiles do not reflect a proinflammatory immune response, but rather, indicate a balanced response to commensal bacteria that can be associated with immune tolerance.

In a subsequent study of similar design, three commercially available Lactobacillus strains were tested for their effects on the human mucosal transcriptome. Striking differences in response to the different strains were obtained (to be published).

In order to obtain a more robust scientific knowledge base, which will facilitate the development of probiotic strategies in relation to various health topics, Kleerebezem and his colleagues will continue to explore the host/microbe interactions by gaining molecular support for the probiotic activities of strains. Kleerebezem already presented the complete genome sequence of L. plantarum WCFS1 in 2003 (PNAS 2003;100:1990-1995). He also identified various characteristics of this strain, including, among other things, the large collection of surface-anchored proteins that indicate that L. plantarum has the potential to associate with a large variety of surfaces and potential substrates for growth. In addition, its relatively high number of regulatory functions implies that this strain can effectively adapt to many conditions. The molecular makeup of bacterial cells most likely provides, at least, a partial explanation for the differential responses observed in human mucosa. So, Kleerebezem now focuses on differences in cell wall composition of different bacteria, including different species and strains as well as in the same bacterial strain harvested from different growth phases.

Kleerebezem predicts, “There will be a role for probiotics in health, especially with respect to disease prevention, and alleviation of relatively mild cases of certain diseases. Probiotics represent a viable concept – literally.”

Michiel Kleerebezem

Contact details:
email: michiel.kleerebezem@nizo.nl
websites: www.nizo.com ; www.mib.wur.nl/UK/; www.tifn.nl