Isoflavones impair anti-PD1 efficacy in breast cancer, regardless of dietary fiber or fecal short-chain fatty acid levels

Background Fermentable dietary fibers, or microbiota-accessible carbohydrates (MACs), are hypothesized to enhance responsiveness to immune checkpoint blockade (ICB) therapy in breast cancer (BC) by increasing fecal short-chain fatty acid (SCFA) production. However, existing research findings have been inconsistent. Given that hormone-sensitive breast cancer is highly influenced by estrogen levels, the presence of estrogenic isoflavones in certain MAC sources may partially account for these discrepancies. Consequently, investigating the roles of isoflavones versus MACs in BC models is warranted. Methods C57BL/6Tac mice were fed low-MAC (AIN93G), low-MAC supplemented with isoflavone genistein, high-MAC (5V5M), or high-MAC isoflavone (high-MACi; 5058D) diet to assess anti-PD1 efficacy against E0771 triple-negative breast cancer (TNBC) and 7,12-dimethylbenz[a]anthracene (DMBA)-initiated estrogen receptor α-positive (ERα + ) mammary tumors. Effects of blocking ERα with tamoxifen (TAM) and dietary impact on the gut microbiome and immune signaling (NanoString) were also evaluated. Results High-MAC diets increased fecal microbial diversity, the abundances of SCFA-producing families, and fecal SCFA levels, compared with the low-MAC diet. Anti-PD1 was effective in TNBC models with high-MAC or low-MAC diets, but responsiveness was eliminated by the inclusion of isoflavones (high MACi) or genistein (low MAC). Anti-PD1 reduced exhausted CD8 + T cells in high-MAC-fed mice but increased them in high-MACi-fed mice. ERα + tumors were resistant to anti-PD1. TAM induced sensitivity to anti-PD1 in both TNBC and ERα + models possibly by modulating TH17 pathways. Conclusions Our results highlight the role of diet in impacting the effectiveness of ICB therapies. Increased SCFA alone is not predictive of response to anti-PD1, but if the tumor expresses ERα or if the diet contains ERα-activating compounds, such as isoflavones, blocking ERα + might convert unresponsive tumors responsive to anti-PD1.