CD4 and acute respiratory distress syndrome: Figure 8A shows the representative FACS analysis of CD39+ Tregs. We found that the percentage of CD39+ Tregs in the CD4+ T cell population was significantly decreased compared to that of the healthy donors (Figure 8B). We classified the ARDS patients as either mild, moderate, and severe according to PaO2/FiO2. A significant difference was observed between the three groups (P = 0.002) (Figure 8C). As the disease worsened, the proportion of CD39+ Tregs gradually decreased.