BACKGROUND: Parabacteroides distasonis (P. distasonis) could regulate inflammatory markers, promote intestinal barrier integrity, and block tumor formation in colon. However, the regulatory effect of P. distasonis on non-small cell lung cancer (NSCLC) remains unknown. This study aimed to investigate the regulatory effect of P. distasonis on NSCLC and its impact on tumor immunity. METHODS: We first established a mouse model of Lewis lung cancer, and administered P. distasonis and intrabitoneal injection of anti-mouse PD-1 monoclonal antibody to assess the impact of P. distasonis on tumor immunity, and mouse intestinal barrier. Then, we explored the effect of P. distasonis on CD8(+)T cells and CXCL9 secretion mediated by tumor-associated macrophages (TAM). We used the TLR1/2 complex inhibitor CPT22 to evaluate its effect on macrophage activation. Finally, we explored the effect of P. distasonis on CD8(+)T cells and CXCL9 secreted by TAM in vivo. RESULTS: In vivo, P. distasonis enhanced anti-tumor effects of anti-PD-1 in NSCLC mice, improved intestinal barrier integrity, recruited macrophages, and promoted M1 polarization. In vitro, CD86 and iNOS levels in BMDM were elevated and CD206 and Arg1 levels were suppressed in membrane fraction of P. distasonis (PdMb) group in comparison to Control group. With additional CPT22 pre-treatment, the levels of CD86 and iNOS in BMDM were reduced, and the levels of CD206 and Arg1 were increased. Compared to PBS group, P. distasonis group exhibited higher proportion of CD8(+)T cells in tumor tissues, along with increased positive proportion of GZMB and IFN-γ in CD8(+)T cells. Additionally, in comparison to Control group, PdMb group showed an elevated proportion of GZMB(+)T and IFN-γ(+)T cells within CD8(+)T cells, and secretion of IFN-γ, TNF-α, perforin, and GZMB in CD8(+)T cell supernatant increased. Moreover, the proportion of CXCL9(+)F4/80(+) macrophages in tumor tissues was higher in P. distasonis group compared to PBS group. In comparison to Control group, CXCL9 protein level in BMDM and CXCL9 secretion level in BMDM supernatant were increased in PdMb group. Finally, P. distasonis enhanced CD8(+)T cell activity by secreting CXCL9 from macrophages in vivo. CONCLUSIONS: P. distasonis promoted CXCL9 secretion of TAM and enhanced CD8(+)T cell activity to trigger anti-tumor immunity against anti-PD-1 treatment in NSCLC mice.