Nrf2 Deficiency Enhances the Immunosuppressive Function of Monocytic Myeloid-derived Suppressor Cells in a PPARγ- Dependent Manner.

Background: Myeloid-derived suppressor cells (MDSCs) comprise monocytic MDSCs (M-MDSCs) and granulocytic MDSCs (G-MDSCs), both of which are effective for controlling T cell responses. Although Nrf2 participates in the expansion of MDSCs and MDSC-mediated immunosuppression, the underlying mechanisms of Nrf2 in MDSC differentiation remain poorly understood.

Methods: In this study, G-MDSCs or M-MDSCs were induced and sorted from the bone marrow (BM) of wild-type (WT) or Nrf2-/- mice using flow cytometry in vitro, with or without GW9662 treatment. Mouse models of tumorigenesis, acute kidney injury (AKI), and chronic kidney disease (CKD) were used to evaluate the immunosuppressive function of WT or Nrf2-/- M-MDSCs treated with or without GW9662. Histological analysis was performed to evaluate tumor angiogenesis or kidney injury. Immunohistochemical staining was used to evaluate lymphocyte infiltration in kidney. Masson's trichrome and Sirius red staining were performed to evaluate kidney fibrosis. Additionally, RNA sequencing (RNA-seq) was conducted to identify gene expression difference between WT and Nrf2-/- M-MDSCs, with real-time PCR or western blot used to validate key findings.

Results: The immunosuppressive function of M-MDSCs generated from BM of Nrf2-/- mice and sorted from Nrf2-/- tumor-bearing mice was dramatically enhanced compared to the G-MDSC counterparts. Moreover, M-MDSCs with an Nrf2 deficiency could effectively ameliorate the inflammatory disorders in mice with AKI or CKD. Intriguingly, the efficacy of Nrf2-/- M-MDSCs could be attributed to increased expression of inducible nitric oxide synthase but decreased levels of arginase 1, indicating an unconventional mechanism of activation. Further mechanistic studies using RNA-seq and bioinformatics analyses identified an essential role of peroxisome proliferator-activated receptor-γ (PPARγ) in the differentiation and suppressive ability of Nrf2-/- M-MDSCs with which the effect could be markedly blocked with GW9662, a specific PPARγ inhibitor.

Conclusions: Our findings elucidate an immunoregulatory mechanism of Nrf2 deficiency related to M-MDSC function and provides a foundation for the application of Nrf2-/- M-MDSCs in inflammatory diseases.