Activation of MMP-2 and/or MMP-9 has been found to play an important role in the pathogenesis of diabetic retinopathy [60, 61], nephropathy [62], neuropathy [63], vascular complications [64], skin complications [65], etc. In this study, MMP-9 expression and enzymatic activity were found to be increased upon PM2.5 exposure in mouse alveolar macrophages and this effect was enhanced in cells with high glucose pretreatment. Here, MMP9 is linked to diabetic retinopathy.