CCR2-positive monocytes contribute to the pathogenesis of early diabetic retinopathy in mice

Accumulating evidence suggests that leucocytes play a critical role in diabetes-induced vascular lesions and other abnormalities that characterise the early stages of diabetic retinopathy. However, the role of monocytes has yet to be fully investigated; therefore, we used Ccr2^-/- mice to study the role of CCR2⁺ inflammatory monocytes in the pathogenesis of diabetes-induced degeneration of retinal capillaries.

Experimental diabetes was induced in wild-type and Ccr2^-/- mice using streptozotocin. After 2 months, superoxide levels, expression of inflammatory genes, leucostasis, leucocyte- and monocyte-mediated cytotoxicity against retinal endothelial cell death, retinal thickness and visual function were evaluated. Retinal capillary degeneration was determined after 8 months of diabetes. Flow cytometry of peripheral blood for differential expression of CCR2 in monocytes was assessed.

In nondiabetic mice, CCR2 was highly expressed on monocytes, and Ccr2^-/- mice lack CCR2⁺ monocytes in the peripheral blood. Diabetes-induced retinal superoxide, expression of proinflammatory genes Inos and Icam1, leucostasis and leucocyte-mediated cytotoxicity against retinal endothelial cells were inhibited in diabetic Ccr2-deficient mice and in chimeric mice lacking Ccr2 only from myeloid cells. In order to focus on monocytes, these cells were immuno-isolated after 2 months of diabetes, and they significantly increased monocyte-mediated endothelial cell cytotoxicity ex vivo. Monocytes from Ccr2-deficient mice caused significantly less endothelial cell death. The diabetes-induced retinal capillary degeneration was inhibited in Ccr2^-/- mice and in chimeric mice lacking Ccr2 only from myeloid cells.

CCR2⁺ inflammatory monocytes contribute to the pathogenesis of early lesions of diabetic retinopathy.