Researchers Uncover Bacterial Link to Immunotherapy Resistance in Cancer

Cleveland Clinic researchers have revealed a significant connection between bacteria present in tumors and the effectiveness of immunotherapy for patients with head and neck squamous cell carcinoma. Two studies published in the journal Nature Cancer indicate that elevated bacterial levels within the tumor microenvironment may suppress the immune response, leading to treatment resistance in some patients.

“This research shifts the focus of immunotherapy resistance away from tumor genetics to unexpected factors like the tumor microbiome,” stated Timothy Chan, M.D., Ph.D., chair of the Department of Cancer Sciences at Cleveland Clinic and lead author of one of the studies. Identifying bacteria as a crucial barrier to treatment opens the door to new strategies for patient selection and targeted antibiotic therapies.

The research team, which included Daniel McGrail, Ph.D., an assistant staff member in the Center for Immunotherapy & Precision Immuno-Oncology, and Natalie Silver, M.D., M.S., director of Head and Neck Cancer Research, validated their findings through a combination of patient samples, preclinical models, and clinical trial data.

In the first study, Dr. McGrail analyzed genetic data from patient tumor samples and found that higher bacterial levels—not specific strains—correlated with a weakened immune response. Dr. Silver’s preclinical models confirmed these findings, showing that antibiotics reduced tumor size and enhanced immune response. Conversely, introducing bacteria made tumors more resistant to immunotherapy. Collaborating with Renata Ferrarotto, M.D., from the University of Texas MD Anderson Cancer Center, they further examined the relationship between bacterial levels and treatment responses in clinical trial samples.

“Immunotherapy holds promise for patients with head and neck cancer, yet many do not respond to it,” Dr. Silver remarked. “Our research investigates how bacteria contribute to treatment failures, enabling us to identify patients who are more likely to benefit from immunotherapy and avoid unnecessary risks. Ultimately, we aim to develop targeted interventions that can enhance the effectiveness of immunotherapy for those who initially do not respond.”

In the second study, Dr. Chan led an analysis of the Javelin HN100 Phase III clinical trial, which assessed the impact of combining anti-PD-L1 immunotherapy with standard chemoradiotherapy. The analysis confirmed that patients with high tumor bacteria levels experienced poorer outcomes with immunotherapy compared to those receiving standard treatment. This trial involved collaboration with Memorial Sloan Kettering Cancer Center and Dana-Farber Cancer Institute.

Both studies highlighted that elevated bacterial levels in tumors attract neutrophils, a type of white blood cell that fights infections. While essential for combating bacterial infections, neutrophils can inhibit the immune response necessary for effective immunotherapy.

These findings provide a foundation for future research into why bacteria are attracted to tumors and how modifying this relationship may improve treatment outcomes. Building on these discoveries, Dr. Silver has initiated a clinical trial to evaluate whether antibiotics can reduce tumor microbiome levels and enhance immunotherapy responses in patients with head and neck squamous cell carcinoma.

Meanwhile, Dr. McGrail is investigating how bacteria influence cancer development and why certain tumors contain higher bacterial levels. Dr. Chan is exploring the potential for bacteria to induce DNA mutations in tumors.

“Uncovering the tumor microbiome’s role in immunotherapy resistance represents a significant advancement in understanding the complex interactions between cancer and the immune system,” Dr. McGrail noted. “This research broadens our perspective on cancer treatment and paves the way for developing personalized therapies that could improve patient outcomes.”

These studies exemplify the evolving landscape of cancer research, where unexpected factors like the tumor microbiome are becoming central to understanding treatment efficacy.