Fusobacterium nucleatum Induces PD-L1 Expression by Tumor-Associated Macrophages and Inhibits Anti-Tumor Immunity in Colorectal Cancer.

Fusobacterium nucleatum (Fn) infection has been linked to the initiation and development of colorectal cancer (CRC). However, the underlying mechanisms remains unexplored. Tissues from Fn-infected mice were first collected for single-cell RNA sequencing. we investigated the correlation between Fn infection and accumulations of tumor associated macrophages (TAMs) in tissues from human CRC or AOM/DSS treated mice. The impact of Fn on biological function of TAMs were interrogated. We deciphered the mechanisms for interaction between Fn and TAMs in CRC. We explored the impact of Fn infection on immune-checkpoint blockade (ICB) treatment in a multicohort study.The analysis of single-cell RNA sequencing revealed an enrichment of TAMs in Fn positive tissues. A significant correlation between Fn DNA level and TAMs was observed. Functionally, Fn could recruit TAMs in CRC tissues and promote their biological functions by enhancing PD-L1 expression and thus suppressing CD8+ T cells. Mechanistically, Fn infection led to increased secreted IL-6 via TLR4/MyD88/NF-κB pathways, and the activated IL-6/STAT3 pathway enhanced PD-L1 expression in TAMs. By using several mice models, we found IL-6 inhibition, loss of MyD88 or anti-PD-L1 treatment could significantly inhibit tumorigenesis. Clinically, Fn positive patients tended to be more sensitive to ICB treatment, with a better prognosis than Fn-negative patients.