Macrophage-myofibroblast transition-a mechanism contributing to myofibroblast accumulation in human and experimental renal fibrosis.

Abstract

Background: Macrophage infiltration is common feature in active fibrosis and can promote these lesions through production of pro-fibrotic molecules. However, it is unclear whether macrophages can directly contribute to fibrosis by transitioning into myofibroblasts. Aim(s): To determine whether macrophage-myofibroblast transition (MMT) is involved in renal fibrosis. Method(s): Macrophages in transition to myofibroblasts (MMT cells) can be detected by co-expression of markers of macrophages (CD11b, F4/80, CD68) and myofibroblasts (alpha-SMA).We examined biopsies of human kidney disease and the mouse unilateral ureteric obstruction (UUO) model of renal fibrosis. Result(s): Analysis of biopsies of various types of kidney disease identified CD68+alpha-SMA+ co-expressing MMT cells in active fibrotic lesions, but not in acute inflammatory or late-stage fibrotic lesions. Confocal analysis identified MMT cells on day 7 UUO on the basis of co-expression of F4/80/alpha-SMA or CD11b/alpha-SMA. MMT cells were also identified in enzyme digested day 7 UUO kidney using flow cytometric analysis of single cell suspensions, with F4/80 or CD11b expressed by 10-30% of alpha-SMA+ cells. CD11b+ cells isolated from the UUO kidney showed significantly increased mRNA levels of a panel of profibrotic molecules (alpha-SMA, col I and III, fibronectin, HSP47, PAI-1) compared to CD11b+ isolated from the contralateral control kidney. Next, CD11b+ cells isolated from bone marrow were dye-labeled and injected into mice on day 2 UUO and then isolated from the day 7 UUO kidney. These transferred macrophages showed marked up-regulation of the panel of pro-fibrotic molecules compared to the freshly isolated cells prior to injection. Finally, TGF-beta1 was shown to induce MMT in bone marrow-derived macrophages in vitro. Conclusion(s): We have identified macrophage-myofibroblast transition as a significant source of myofibroblasts in human and experimental fibrosis.