NOS2 and pulmonary emphysema: To test the biological functions of migrated MDSCs in the emphysema-induced lung–gut cross-talk, PCR analysis was conducted to clarify the expression of NADPH oxidase 2 (NOX2), enzyme ARG1, and INOS in the intestinal tract, which are reported to be produced in MDSCs and featured by this immune subset to exert their immunosuppressive function [28].