Targeting tumour-associated microbiota: a new strategy to hit tumour cells in the lung and breast

In recent years, increasing evidence has revealed that tissues once considered sterile har-bour distinct microbial communities. It has also been observed that the tissue microbiota is modified by the presence of a growing tumour. Several studies indicate that each tumour type is characterized by a specific microbiome, and that microorganisms located within or around tumours can influence cancer progression by reshaping the local immune microenvironment. This thesis investigates the role of tumour-resident bacteria in shaping the immune micro-environment of lung and breast cancers, to explore their potential as therapeutic targets and biomarkers. In preliminary studies, we observed that the perturbation of pulmonary microbiota with aerosolized antibiotics reduced immunosuppressive regulatory T cells (Tregs), enhanced immunosurveillance, and prevented the implantation of B16 lung metastases. Based on this evidence, in this thesis I showed that the growth of experimental lung metastases from different murine tumours alters the pulmonary microbiota, fostering bacteria such as Klebsiella oxytoca that release metabolites capable of inducing Tregs. An aerosolized antibiotic therapy disrupted these communities, reduced Treg accumulation, and enhanced cytotoxic immune activity by reshaping the T cell landscape, resulting in a reduced metastatic bur-den. In human non-small cell lung cancer (NSCLC), I found bacterial sequences similar to those observed in mouse tumours, and their presence was associated with high-er Treg infiltration and lower overall T cell abundance in the tumour. In a large NSCLC cohort, microbiota profiling revealed distinct microbial community compositions be-tween tumour and adjacent normal tissues, and that each of them was associated with patient outcome. Based on these bacteria, we constructed a bacterial score that showed a good prognostic performance, and that was associated with a tumour-specific subtype and Treg infiltration. In a murine triple-negative breast cancer model, we previously observed an abundant presence of Staphylococcus epidermidis within tumours, found to be in charge of a po-tent inflammation and increasing Treg accumulation, and an antibiotic treatment was able to reduce its load and reduce tumour burden. Based on this, we extended this analysis to breast cancer patients. Real-time PCR demonstrated variable Staphylococcus abundance across tumours. Higher Staphylococcus levels were associated with reduced relapse-free survival. These findings were validated in a second independent breast cancer patient cohort, where Staphylococcus was found to be an independent prognostic marked beyond established clinicopathological variables, corroborating the link be-tween Staphylococcus abundance and poorer prognosis. Together, these findings demonstrate that tumour-associated bacteria actively contribute to immune skewing and tumour progression in both lung and breast cancers. They also highlight the translational potential of studying the tumour microbiota, either as a biomarker for risk stratification or as therapeutic targets to restore anti-tumour immunity.