Macrophages in mouse type 2 diabetic nephropathy: Correlation with diabetic state and progressive renal injury. Background Macrophage-mediated renal injury has been implicated in progressive forms of glomerulonephritis; however, a role for macrophages in type 2 diabetic nephropathy, the major cause of end-stage renal failure, has not been established. Therefore, we examined whether macrophages may promote the progression of type 2 diabetic nephropathy in db/db mice. Methods The incidence of renal injury was examined in db/db mice with varying blood sugar and lipid levels at 8 months of age. The association of renal injury with the accumulation of kidney macrophages was analyzed in normal db/+ and diabetic db/db mice at 2, 4, 6, and 8 months of age. Results In db/db mice, albuminuria and increased plasma creatinine correlated with elevated blood glucose and hemoglobin A_1c (HbA_1c) levels but not with obesity or hyperlipidemia. Progressive diabetic nephropathy in db/db mice was associated with increased kidney macrophages. Macrophage accumulation and macrophage activation in db/db mice correlated with hyperglycemia, HbA_1c levels, albuminuria, elevated plasma creatinine, glomerular and tubular damage, renal fibrosis, and kidney expression of macrophage chemokines [monocyte chemoattractant protein-1 (MCP-1), osteopontin, migration inhibitory factor (MIF), monocyte-colony-stimulating factor (M-CSF)]. The accrual and activation of glomerular macrophages also correlated with increased glomerular IgG and C3 deposition, which was itself dependent on hyperglycemia. Conclusion Kidney macrophage accumulation is associated with the progression of type 2 diabetic nephropathy in db/db mice. Macrophage accumulation and activation in diabetic db/db kidneys is associated with prolonged hyperglycemia, glomerular immune complex deposition, and increased kidney chemokine production, and raises the possibility of specific therapies for targeting macrophage-mediated injury in diabetic nephropathy.

Myofibroblasts play a central role in renal fibrosis although the origin of these cells remains controversial. We recently reported that bone marrow-derived macrophages can give rise to myofibroblasts through macrophage to myofibroblast transition (MMT). However, several important issues remain to be addressed, including whether MMT occurs in human kidney disease and verification of the MMT process through lineage tracing. Biopsies from a cohort of 58 patients with various forms of kidney disease were examined for MMT cells that co-express macrophage (CD68) and myofibroblast (α-smooth muscle actin, α-SMA) markers. MMT cells were evident in active fibrotic lesions, but were largely absent in acute inflammatory or sclerotic lesions, suggesting that MMT cells contribute to progressive renal fibrosis. Fate-mapping studies in LysM^CreTomato mice identified substantial numbers of Tomato⁺ myeloid cells with F4/80⁺ macrophage phenotype expressing α-SMA and collagen I in the unilateral ureteral obstructive model of renal fibrosis, providing direct evidence for the MMT process during the development of renal fibrosis. In addition, MMT cells had a predominant M2 phenotype in both human and mouse renal fibrosis. Finally, selective depletion of myeloid cells via diphtheria toxin in LysM^CreiDTR mice largely abolished macrophage infiltration and MMT cells in the obstructed kidney and substantially reduced accumulation of α-SMA⁺ myofibroblasts and collagen deposition, revealing a pathogenic role for inflammatory macrophages in MMT and tissue fibrosis. In conclusion, these findings provide substantial new data to support the postulate that macrophages can directly transdifferentiate into collagen-producing myofibroblasts in human and experimental kidney disease.