Enlarge / After fecal transplants from responding humans, the gut cells of mice (blue) were flooded with cancer-fighting immune cells (red, green) (credit: Dr. Luigi Nezi])
New, potent cancer therapies can act like daggers pressed into the hindquarters of the immune system, prodding it to lunge at any cancerous cells in the body. When the drugs work, the immune system tramples tumors into oblivion.

But they don’t always work—in fact, cancer drugs can fail 60 to 70 percent of the time.

The drugs might not give the immune system a sharp enough sticking in every patient.

But according to a pair of new studies, it may not be the immune system that needs a swift kick—it may be the gut.
Some intestinal-dwelling bacteria appear to corral and train immune cells to fight off cancer cells—prior to any spurring from cancer immunotherapies. Without such microbial priming, the drugs may only offer a futile prod.
In both studies, published this week in Science, researchers found that the cancer patients who saw no benefit from the drugs (non-responders) were the ones who lacked certain beneficial gut bugs, particularly after taking antibiotics. Meanwhile, cancer patients who did respond to the drugs had bacteria that could prompt the immune system to release chemicals that get cancer-killing immune cells—T cells—to chomp at the bit.
When the researchers transferred the gut microbes from their human cancer patients into germ-free mice with cancer, the rodents mirrored the patients’ fates.

That is, mice that got gut microbes from non-responding humans also did not respond to immunotherapies.

But, the mice that got microbes from responders responded.

And when researchers swapped responder gut microbes into non-responding mice, the mice converted and fought back the cancer.
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