Reactive oxygen species reprogram macrophages to suppress antitumor immune response through the exosomal miR-155-5p/PD-L1 pathway

Background: Cancer cells have an imbalance in oxidation-reduction (redox) homeostasis. Understanding the precise mechanisms and the impact of the altered redox microenvironment on the immunologic reaction to tumors is limited.

Methods: We isolated exosomes from ovarian cancer cells through ultracentrifuge and characterized by Western-blots and Nanoparticle Tracking Analysis. 2D, 3D-coculture tumor model, and 3D live cell imaging were used to study the interactions between tumor cells, macrophages and CD3 T cells in vitro. The role of exosomal miR-155-5p in tumor growth was evaluated in xenograft nude mice models and immune-competent mice models. Flow cytometry and flow sorting were used to determine the expression levels of miR-155-5p and PD-L1 in ascites and splenic macrophages, and the percentages of CD3 T cells subpopulations.

Results: The elevation of reactive oxygen species (ROS) greatly downregulated exosomal miR-155-5p expression in tumor cells. Neutralization of ROS with N-acetyl-L-cysteine (NAC) increased the levels of miR-155-5p in tumor exosomes that were taken up by macrophages, leading to reduction of macrophage migration and tumor spheroid infiltration. We further found that programmed death ligand 1 (PD-L1) is a functional target of miR-155-5p. Co-culture of macrophages pre-treated with NAC-derived tumor exosomes or exosomal miR-155-5p with T-lymphocytes leading to an increased percentage of CD8⁺ T-lymphocyte and a decreased CD3⁺ T cell apoptosis through PD-L1 downregulation. Tumor growth in nude mice was delayed by treatment with NAC-derived tumor exosomes. Delivery of tumor exo-miR-155-5p in immune-intact mice suppressed ovarian cancer progression and macrophage infiltration, and activated CD8⁺ T cell function. It is of note that exo-miR-155-5p inhibited tumor growth more potently than the PD-L1 antibody, suggesting that in addition to PD-L1, other pathways may also be targeted by this approach.

Conclusions: Our findings demonstrate a novel mechanism, ROS-induced down-regulation of miR-155-5p, by which tumors modulate the microenvironment that favors tumor growth. Understanding of the negative impact of ROS on the tumor immune response will improve current therapeutic strategies. Targeting miR-155-5p can be an alternative approach to prevent formation of an immunosuppressive TME through downregulation of PD-L1 and other immunosuppressive factors.