Most patients with head and neck cancer see little benefit from immunotherapy, even as these drugs revolutionize care for others. The pattern has frustrated oncologists for years: same diagnosis, vastly different outcomes, with few explanations for why the immune system rallies in some cases and stays dormant in others.
Two studies from Cleveland Clinic, published January 2 in Nature Cancer, suggest the answer may lie not in tumor genetics but in microscopic passengers hiding within the cancer itself. High levels of bacteria living inside head and neck tumors can effectively blind the immune system, preventing it from recognizing cancer cells as threats and rendering immunotherapy useless.
When Bacteria Redirect the Immune Response
The research analyzed tumor samples, preclinical models, and data from the Phase III JAVELIN Head and Neck 100 trial, which tested whether adding the immunotherapy drug avelumab to standard chemoradiotherapy improved survival. Patients with high bacterial counts fared significantly worse on immunotherapy than those receiving chemoradiotherapy alone, a counterintuitive result that pointed toward active interference rather than simple treatment failure.
The mechanism appears straightforward but consequential. When bacteria colonize a tumor, they attract neutrophils, white blood cells that normally fight infection. Inside cancer tissue, these cells suppress the adaptive immune responses immunotherapy depends on. The immune system’s attention gets pulled toward the bacteria, leaving cancer cells room to grow unchecked. This creates what researchers call a “myeloid-rich” environment, essentially a shield that makes tumors invisible to T cells.
It wasn’t specific bacterial species driving resistance; overall bacterial burden mattered most. Tumors with low bacterial levels often contained tertiary lymphoid structures, organized immune hubs associated with better survival, while bacteria-heavy tumors lacked these defensive formations entirely.
“By identifying bacteria as a key barrier to treatment, we’re opening the door to new strategies for patient selection and targeted antibiotic therapies,” Timothy Chan, chair of Cleveland Clinic’s Department of Cancer Sciences, explains.
Testing Antibiotics as an Immunotherapy Booster
Preclinical models validated the pattern in controlled conditions. When researchers used antibiotics to reduce tumor bacteria, immune responses improved and tumors shrank. Adding bacteria back rendered the same tumors resistant again. The work suggests a testable clinical strategy: clear bacterial colonization before or during immunotherapy to restore the immune system’s focus on cancer.
Cleveland Clinic has launched a trial testing whether antibiotics can improve immunotherapy outcomes in patients whose tumors harbor high bacterial levels. The approach moves cancer treatment beyond targeting tumor mutations alone, acknowledging that the entire ecosystem surrounding cancer cells, including uninvited microbial residents, shapes whether treatments succeed or fail.
Other questions remain open. Why do some tumors accumulate more bacteria than others? Can microbes directly influence cancer mutations, or do they merely create environments where resistant cells thrive? The research teams are now investigating whether anatomical factors, immune deficiencies, or treatment history predispose certain tumors to bacterial colonization.
For patients facing immunotherapy failure, the findings offer a concrete explanation and a potential path forward. The difference between response and resistance may come down to clearing microscopic passengers most oncologists never thought to look for.
Nature Cancer: 10.1038/s43018-025-01068-0
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