SMOX and pneumonia: Since we observed a significant increase in the number of PD-L1high neutrophils as early as 3 days following PM2.5 exposure (Fig. 2D), we test this hypothesis by utilizing a mouse pneumonia model via intratracheally injecting PAO1 (5 × 106 CFU) into mice after continuous 3 days of PM2.5 exposure (Fig. 4A).