Background Blockade of the PD-1/L1 axis has emerged as an effective treatment for various cancers, resulting in remarkable clinical efficacy, but most patients have no response. Gut microbiota is a critical factor in determining the efficacy of PD-1/L1 blockade. We aimed to explore the role and mechanism of Sutterella wadsworthensis (S.wadsworthensis) in improving the responsiveness of anti-PD-1 immunotherapy.

Methods The effects of S.wadsworthensis on PD-1 blockade efficacy were evaluated in murine models of colorectal cancer. Flow cytometry was performed to evaluate the anti-tumor immunity. RNA sequencing (RNA-seq) and peptidomics were used to identify potential immunomodulatory pathways and peptides, respectively.

Results We first reported that live S.wadsworthensis supplementation remarkably improved anti-PD-1 efficacy in the MC38 and CT26 murine model, with increased tumor-infiltrating CD4⁺ICOS⁺ T cells, and activated Th1 (CD4⁺IFN-γ⁺TNF-α⁺) and cytotoxic T cells (CD8⁺IFN-γ⁺TNF-α⁺) response. Colonic RNA-seq results showed that S.wadsworthensis up-regulated the expression of genes related to major histocompatibility complex II (MHC-II), and induced the enrichment of antigen processing and presentation signals. Moreover, tumor RNA-seq results suggested that enhanced T cell receptor (TCR) signal mediated by MHC-II restricted antigen presentation process was required for the activation of T cell response in the combination of S.wadsworthensis and PD-1 mAb, as evidenced by abolished synergistic effect after the specific block of MHC-II signal. Furthermore, two peptides stably bound to MHC-II were identified from S.wadsworthensis, which enhanced the killing activity of mouse splenocytes against MC38 cells in vitro and augmented the antitumour activity of PD-1 mAb in vivo by increasing CD4⁺IFN-γ⁺ and CD8⁺IFN-γ⁺ T cells.

Conclusions Our study showed that S.wadsworthensis and S.wadsworthensis-derived peptides induced the crosstalk between the gut and distal tumor via the MHC-II/TCR-ICOS signal to remodel anti-tumor immunity, achieving synergistic effect with PD-1 mAb. S.wadsworthensis and S.wadsworthensis-derived peptides may serve as potential adjuvants to enhance PD-1 mAb efficacy.