Background Colorectal cancer (CRC) remains one of the deadliest cancers in the US, with a concerning increase in the incidence of early-onset cases. Patients who develop metastatic disease tend to have a poor prognosis. Cancer treatments targeting the immune system (immunotherapy) have resulted in long-term durable responses for many across tumor types. However, CRC patients with metastatic disease have an extremely poor prognosis, and only 10% of colorectal tumors (classified as MSI-H) respond to anti-PD1 therapy. Previous work has shown that the microbiome composition plays a large role in immunotherapy success, but the underlying mechanisms remain unknown. Given that CRC patients undergo numerous treatments prior to immunotherapy, we hypothesized that previous treatment with chemotherapy shift the microbiome and reduce anti-PD1 efficacy.
Methods We used a subcutaneous MC38 CRC model to measure the impact of broad-spectrum antibiotic use and chemotherapy on anti-PD1 effectiveness. Tumors were measured every 2–3 days to track growth and analyze tumor clearance rates. Lymphocytes were isolated from tumors and lymph nodes of mice 15 days post tumor injection to assess the changes in tumor infiltrating T cells through immunofluorescence imaging and flow cytometry. Fecal microbiome transplants (FMT) were performed to directly test the impact of microbiome changes in immunotherapy success.
Results Mice who received prior chemotherapy (5-flurouracil, 5-FU) had lower tumor clearance rates with anti-PD1 compared to mice who received anti-PD1 treatment alone. Tumor infiltrating CD8+ T cells were more exhausted, as evidenced by a loss of polyfunctionality. In addition, 5-FU treatment was sufficient to cause a durable shift in the gut microbiome. Most notably, lack of response could be completely recapitulated in 5-FU naïve mice through an FMT from a 5-FU donor mouse.
Conclusions Here, we have demonstrated that prior traditional cancer treatments, such as chemotherapy, significantly alter the microbiome and decreases the efficacy of anti-PD1 in a CRC mouse model. Interestingly, we have shown that chemotherapy-induced shifts in the microbiome are both necessary and sufficient to drive this phenotype. Treating mice with 5-FU leads to increased numbers of exhausted CD8+ T cells found in tumors and when this microbiome is transplanted into naïve mice, we can replicate these results. This helps to support the hypothesis that the gut microbiome composition is involved in immune regulation that can increase or decrease anti-PD1 efficacy to CRC tumors. We can use this information to better inform healthcare providers on how to better treat patients.
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