Intracellular bacteria modulate the immune microenvironment of oral squamous cell carcinoma.

Neoadjuvant immune checkpoint blockade (NICB) therapy has shown significant efficacy in oral squamous cell carcinoma (OSCC); however, the mechanisms by which intracellular microbiota influence immune function within the tumor microenvironment remain unclear. In this study, we employ invasion-adhesion-directed expression sequencing (INVADEseq) technology to simultaneously capture host single-cell RNA sequencing data and bacterial signatures, revealing that intracellular bacteria inhibit PDCD1 expression, enhance infection responses, promote antigen presentation and macrophage activation, and reduce T cell-related immune gene expression. The key genera, Fusobacterium, Streptococcus, and Capnocytophaga, are associated with increased risks and adverse outcomes in immunotherapy across multiple tumor types. We identify GZMK+ CD8+ T cells and ZBTB16+ TAMs as markers of complete response to NICB. However, intracellular bacteria weaken the communication between cDC1 and these immune cells, potentially reducing therapeutic efficacy. Our study provides a foundation for further investigation into the mechanisms by which intracellular bacteria mediate effective responses to NICB in OSCC.