A Surprising Benefit of Gut Bacteria: Opening the Door to Treatments That Strengthen Cancer Immunotherapy

A study published last month found that certain gut bacteria strengthen immune responses against cancer cells and improve tumor control.

Our intestines contain trillions of bacteria, and those called good bacteria, normal gut flora, provide many benefits. They are especially important for healthy clotting and digestion. Increased harmful or reduced beneficial bacteria can trigger inflammation extending to colorectal cancer.

Studies Have Found Some Bacterial Metabolites Promote Bowel Cancer While Others Prevent It

A mouse study compared germ-free animals with normal gut bacteria-bearing mice. With similar large bowel injury, germ-free mice healed poorly and developed more cancer.

Recent attention has focused on bacterial relationships with immunity. A study in cancer-bearing mice published last month showed that adding certain bacteria to the digestive tract increases anticancer immune responses.

T cells, a white blood cell type central to immune responses, identify and eliminate faulty cells. Cancer cells, the most faulty state, evade defense by binding and disabling T cells. Called immune checkpoint blockers, immunotherapy drugs such as Ipilimumab and Nivolumab prevent this escape by blocking cancer's braking mechanism on T cells.

Although checkpoint drugs succeed in many cancers, strong responses occur in only one third of patients. Why remains unclear, but Dr. Gajewski's observation may explain it. Mice with melanoma obtained from 2 laboratories showed major differences in immune responses and tumor growth. After 3 weeks together in one cage, these differences vanished. He proposed that different gut bacteria explained this, with shared surroundings eventually producing similar bacteria and responses. To verify it, stool from strong responders was transferred into weaker responders' intestines. As expected, comparable strong responses developed.

Bacterial Transfer Slowed Tumor Growth as Effectively as Checkpoint Drugs

Combining transfer and immunotherapy produced much better tumor control. Researchers particularly identified bifidobacteriaas prominent in strengthening immunity. Giving them orally to low-response mice produced, after 2 weeks, increases comparable to stool transfer. Effects persisted, with strong immunity even 6 weeks later.

In conclusion, gut microbes and immunity clearly have a strong relationship. Using it through bacterial regulation seems highly reasonable against cancer. Animal and clinical studies must confirm these findings, identifying other organisms that influence immunity positively or negatively so this knowledge can best benefit patients.