MMP9 and Stroke: In view of the importance of MMP-9 in early BBB disruption after stroke (Asahi et al., 2001; Rosell et al., 2006), we further showed that PD-L1 expression on Tregs mediates Treg-afforded neuroprotection against experimental stroke through the inhibition of peripheral neutrophil-derived MMP-9 and through subsequent preservation of BBB integrity (Li et al., 2014).