Figure 2.

The immunosuppressive function of MDSCs. MDSCs produce arginase, which decomposes l-arginine into urea and l-ornithine (163, 164). MDSCs express iNOS2, which converts l-arginine into NO and l-citrulline (165). L-Arginine deficiency inhibits T cell proliferation. NO interferes with JAK/STAT signalling proteins, inhibits the transcription of MHC class II genes, and induces T cell apoptosis (166–168). MDSCs produce high levels of peroxynitrite and ROS when in direct contact with T cells to induce TCR and CD8 molecular modification, resulting in CD8⁺ T cells losing the ability to bind to the pMHC complex; this results in nonresponse of peripheral blood tumour-specific CD8⁺ T cells (169).