Qingfei mixture mitigates immunosuppression of tumor microenvironment in non-small cell lung cancer by blocking stat1/Ido1-mediated tryptophan-kynurenine pathway

Programmed death-1 (PD-1) acts as a T cell checkpoint and is important in controlling T cell exhaustion. Blocking the intercommunication across PD-1 and PD-L1 is promising for advanced lung cancer treatment. However, the response rate requires being strengthened. This study aimed to determine whether the combination treatment of Qingfei mixture (QFM) and PD-1 inhibitor could improve the sensitivity of monoclonal antibody by regulating STAT1/IDO1-mediated tryptophan (Trp)-kynurenine (Kyn) pathway. The in vivo imaging system, immunofluorescence, hematoxylin-eosin staining, TUNEL, flow cytometry, HPLC, and ELISA were used to estimate the anti-tumor effects in LLC-luc tumor-bearing C57BL/6 mice treated with QFM, PD-1 inhibitor, 2-NP (enhancer of STAT1 transcription), and FICZ (AhR agonist) alone or in combination. IFN-γ-mediated A549 and LLC cells were treated with QFM-containing serum and fludarabine (FLU, STAT1 inhibitor), and cell viability, apoptosis, and Kyn content were then evaluated using CCK-8 assays, flow cytometry, and HPLC assays, respectively. Additionally, the expressions of STAT1, IDO1, AhR, NFATc1, TRIP12, PD-1, and PD-L1 were measured in vivo and in vitro. We found QFM increased the anti-cancer actions of PD-1 inhibitors by increasing the CD8+IFNγ+ T cells infiltration and decreasing the ratio of Kyn/Trp. Besides, QFM-containing serum suppressed the proliferation and promoted apoptosis in A549 and LLC cells, meanwhile, FLU boosted the effects of QFM-containing serum. Moreover, the suppression of tumor growth in the combination therapy was attenuated in the mice receiving 2-NP or FICZ. The occurrence of the above results was accompanied by a decrease in STAT1, IDO1, AhR, PD-1, and PD-L1 expressions. Collectively, the findings suggested that QFM may increase the influences of PD-1 inhibitors at least partially by blocking the STAT1/IDO1-mediated tryptophan-kynurenine pathway in lung cancer.