With the war on cancer at full throttle, new avenues of investigation are constantly coming to the fore. A novel study into the gut flora of mice has yielded surprisingly positive anti-cancer effects. Share on Pinterest Natural gut flora appears to strengthen the immune system’s response to tumors. Research carried out at the University of Chicago and published recently in Science found that adding certain bacteria into the digestive tracts of mice increased their immune system’s ability to attack tumor cells. The positive effect of the added bacterial flora was similar in strength to those of checkpoint inhibitors, a type of successful anti-tumor drug. When oral doses of bacteria were given in conjunction with checkpoint inhibitors, tumor outgrowth was almost abolished. Study Director Dr. Thomas Gajewski said: “Our results clearly demonstrate a significant, although unexpected, role for specific gut bacteria in enhancing the immune system’s response to melanoma and possibly many other tumor types.”

Checkpoint inhibitors T cells, a type of white blood cell, are central characters in the immune response. They hunt down infected or cancerous cells and destroy them. Tumor cells, however, can attach to specific T cell receptors, turning them off and rendering them inert. Checkpoint inhibitors, such as anti-PD-L1 antibodies, block the tumor cell’s ability to attach to the T cells. This, in turn, keeps the T cells active and on the hunt for faulty or errant cells. Although checkpoint inhibitors have proven very successful in treating a number of cancer types, only around 1 in 3 patients have a vigorous response. This has perplexed researchers. Dr. Gajewski and his team noticed a similar reaction in the mice that they used. Mice bought from the Jackson Laboratory (JAX) showed a much more robust, natural immune response to tumors than mice purchased from Taconic Biosciences (TAC). This marked difference, however, completely disappeared once the two strains of mice had spent 3 weeks together in the same cage. The research team suspected that the mice might be sharing microbes, giving the TAC mice an enhanced anti-tumor response.

Fecal transplant Following their hunch, they transferred JAX fecal matter into the stomachs of TAC mice. The results were as expected. The TAC mice’s response to the tumor was now as robust as the JAX mice. The reverse process of transferring TAC feces into JAX mice gave no effect. To test the strength of the bacterial response, the team compared the effect of bacterial transfer with anti-PD-L1 antibodies (a checkpoint inhibitor). The responses were equivalent, and when the methods were combined, the results were even more positive. The team’s next step was to hunt down the specific bacteria that was mediating the response. After testing the more prominent species of bacteria, one stood out: bifidobacteria. In the next phase of tests, bifidobacteria was given to TAC mice orally. After just 2 weeks, the mice showed a marked improvement in their spontaneous T cell reaction to the insertion of a tumor. This response was just as strong as when the fecal matter was transferred wholesale. Impressively, the positive effects of the bifidobacteria were relatively long lasting. TAC mice exposed to tumors as long as 6 weeks after the bacterial influx still mounted a strong immune response.