Intra- Versus Extracellular Bacteria Dictate Tumor Immunity and Metastatic Recurrence

An increasing body of literature suggests that the presence of bacteria within tumors may play a role in tumor progression and metastatic initiation. However, less is known regarding bacterial effects on subsequent metastatic outgrowth. Yao and colleagues demonstrate a role for bacteria in eliciting immune responses that impact tumor progression at metastatic sites. To mimic circulating tumor cell clusters that migrate to form metastases, mice were injected with breast cancer organoids that contained intracellular Staphylococcus xylosus. Doxycycline-induced elimination of bacteria resulted in a lower rate of recurrence (6.7%) compared to bacteria-containing controls (65%), dependent on an intact immune system. Analysis of lung immune cells following tumor colonization revealed increased neutrophil infiltration, which was reversed with antibiotic treatment. Single-cell RNA-sequencing (RNA-seq) analysis showed upregulation of immunosuppressive pathways, including PD-1, in the presence of bacteria, while the abolishment of bacteria led to enhanced cytotoxic programs. Interestingly, while intracellular bacteria induced immunosuppressive pathways, extracellular bacteria elicited an opposite response, activating phagosome and antigen-presenting immunogenic programs. Cluster analysis of the neutrophil population supported this spatially dependent heterogeneity. RNA-seq data from bacteria-infiltrated tumor cells highlighted a role for IL17B in triggering T-cell suppression and pro-tumor cell survival. This was corroborated by enhanced metastatic outgrowth and relapse in mice with Il17b overexpression. Transcriptomic data additionally identified upregulation of the STING pathway in bacteria-containing tumors. Accumulation of intratumoral cytosolic DNA following bacterial invasion suggested activation of cGAS–STING signaling, which was found to be necessary for IL17B expression. To assess effects in a clinically relevant setting, a mouse model of post-surgery recurrence was generated. Mice that received doxycycline to eliminate intracellular bacteria displayed lower recurrence when compared with bacteria-harboring mice. In contrast, mice exposed to cell wall components, mimicking extracellular bacteria, demonstrated reduced recurrence and improved overall survival, again highlighting the location-dependent divergent impacts of bacteria. Furthermore, crosstalk between bacteria, tumor cells, and neutrophils was conserved in human breast cancer models. Overall, this study establishes a spatially dependent role for intratumor bacteria in affecting metastatic progression through induction of an IL17B-driven, neutrophil-mediated immunosuppressive state.Yao B, Liu X, Ruan K, Fang X, Jiang C, Bian W, et al. Divergent tumor immunity determined by bacteria-cancer cell engagement. Cell 2026 Feb 4 [Epub ahead of print].Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at https://aacrjournals.org/cdnews.