Circulating extracellular vesicles carrying Firmicutes reflective of the local immune status may predict clinical response to pembrolizumab in urothelial carcinoma patients.

Bacterial flora has clinical significance for the host. The metabolic environment created by this flora influences immunotherapy in urothelial carcinoma. However, there are no reports on the clinical significance of bacterial flora in the host bloodstream. We aimed to clarify the correlation between extracellular vesicle (EV)-derived blood microflora information and tumor immunological status in urothelial carcinoma (UC) patients. Serum samples were collected from 20 healthy donors, 50 patients with localized UC, and 31 patients with metastatic UC (mUC) who had undergone pembrolizumab treatment. Bacterial DNA in EVs was extracted from each sample. Metagenomic sequencing was performed after amplification of the V1-V2 region of the bacterial 16S rRNA gene. Using the matched tumor tissue and serum samples, we revealed that the smaller amount of peripheral EVs carrying Firmicutes DNA was significantly correlated with the higher number of infiltrating T cells within tumor tissues (CD3; p = 0.015, CD4; p = 0.039, CD8; p = 0.0084) and the higher expression of activation markers on their surface (ICOS on both CD4; p = 0.0013 and CD8 T cells; p = 0.016 and 4-1BB on CD4 T cells; p = 0.016). In terms of circulating metabolic information, L-Ser and L-Pro levels, which play important roles in T cell expansion and proliferation, were significantly higher in the Firmicutes-low group (p = 0.010). All of the patients with higher Firmicutes abundance had disease progression without any clinical response (p = 0.026) and significantly inferior prognosis for pembrolizumab therapy (p = 0.035). This is the first study on the importance of peripheral bacterial EVs in cancer patients treated with cancer immunotherapy.