Isolation of IFN-γ CD8 T-cell-inducing human strains
Although the relationship between commensal gut bacteria and the transition of CD 4 T cells (T cells generally thought of as related to the innate immune system) to T17 helper cells had been previously explored, the relationship between CD 8 T cells and the gut biome had not despite previous results that suggested there might be a connection. After determining that there was a reduction of CD 8 INF- γ-releasing T cells in both germ-free and antibiotic-treated mice and a subsequent recovery of this cell population when a fecal suspension (To replace the missing microbiome) was orally administered to either mouse population, the authors decided to conduct experiments to better characterize the microbe-CD 8 INF- γ-releasing T cell relationship (Tanoue et al. 2019)
To do so, fecal suspensions were created from the waste of six human donors and used to inoculate a series of germ-free mine. All thus inoculated mice exhibited an increase in CD 8 INF- γ-releasing T cell populations in the colon, although there was a great deal of variability in the amount of induction (Tanoue et al. 2019). The mice with the strongest induced response were selected, and after transferring their fecal biome to another set of germ-free mice via oral gavage, the mice were treated with various antibiotics to help determine the constituents of the biome that were eliciting the response. Further experimentation and eventually sequencing and analysis of 16s-ribosomal RNA discovered 26 strains of bacteria potentially related to the T-cell induction (Tanoue et al. 2019). Of these, an 11-member subgroup was eventually selected for further testing. The proliferation of CD 8 INF- γ-releasing T cells was deemed to be independent of any inflammatory activity, did not seem to be accompanied by a release of any toxins from the constituent bacteria, and although the selected bacteria had a number of resistance genes such as the production of various β-lactamases, none were multi-drug resistant. In other words, the 11-member population seemed to be relatively benign in all measurable ways (Tanoue et al. 2019).
Mechanisms underlying IFN-γ CD8 T cell accumulation
The authors next attempted to determine the mechanism by which these bacteria induced the accumulation of CD 8 INF- γ-releasing T cells. First, the 11-member population of bacteria was heat-killed and inoculated into germ-free mice, failing to produce an immune response indicating that an active process was in progress (likely colonization of the gut mucus layer) and not a systemic response to a residual antigen (Tanoue et al. 2019). The authors then used a variety of murine hosts with various knock-outs and deletions to detect which receptors and cellular messengers were required for the proliferation of CD 8 INF- γ-releasing T cells as mediated by the 11-member bacterial population (Tanoue et al. 2019). The authors reached two conclusions; the increase in CD 8 INF- γ-releasing T cells was due to recruitment to the colon, differentiation triggered by the combination of bacterial antigens presented by the constituent bacteria, and cellular expansion, and that the mechanism bypassed the signaling pathways of the innate immune system, and conditioned CD103+ dendritic cells to activate the CD 8 INF- γ-releasing T cells (Tanoue et al. 2019).
Systemic effects of 11-mix colonization
The authors next evaluated the inoculated mice for evidence of the spread of the bacterial mix beyond the colon. Although they did not find bacterial colonization outside of the colon, they did find system-wide activation of CD 8 INF- γ-releasing T cells. The authors suggested that circulating molecules such as mevalonate and dimethylglycine, which were seen in greater serum concentrations in the inoculated mice might be responsible for the increase (Tanoue et al. 2019).
Enhancement of anti-microbial immunity
Since the purpose of examining the immunomodulatory capabilities of the gut biome is to suggest possible therapeutics, the impact of the increase in CD 8 INF- γ-releasing T cells elicited by the 11-member bacterial population was measured by quantifying resistance to caecum infection by Listeria monocytogenes. Inoculated mice had less weight loss, less CFU of L. monocytogenes, and healthier colon histology post-infection with L. monocytogenes than controls (Tanoue et al. 2019). Depleting the supply of CD 8 INF- γ-releasing T cells using a monoclonal antibody resulted in the loss of this resistance, thus suggesting that the CD 8 INF- γ-releasing T cells were responsible for the increased immunity (Tanoue et al. 2019).
Enhancement of anti-tumor immunity
Since CD 8 INF- γ-releasing T cells are known to play an important part in cell-cycle mediated tumor control, the authors examined the role the 11-member bacterial population might have in enhancing immune checkpoint inhibitor (ICI) therapies designed to treat cancers. The authors found that with or without the addition of anti-cancer compounds such as PD1 inhibitors, the immunomodulation triggered by the 11-member population significantly suppressed the growth of tumor cells transplanted into the gut of inoculated mice (Tanoue et al. 2019).
Overall, the authors made clear that a subset of the human gut microbiome can have significant and positive impacts on the function of the immune system both in the gut and systemically. They further demonstrated that these impacts are not limited to the suppression of invasion by pathogens but also extend to the prevention of tumor growth. They do admit, however, that there may be many combinations of microbes that can produce the same effect in the human gut; the 11 selected were simply an example of what might be possible.
Tanoue T, Morita S, Plichta DR, Skelly AS, Suda W, Seiko S, Narushima S, Vlamakis H, Motoo I, Sugita K et al. 2019. A defined commensal consortium elicits CD8 T cells and anti-cancer immunity. Nature.com [Online]. [Cited 2019 February 3] Available from: doi.org/10.1038/s41586-019-0878-z