Renal fibrosis

<b>Introduction and Aims:</b> Fibrocytes are bone marrow-derived and type I collagen producing spindle-shaped cells which express markers of hematopoietic and stromal cells. Fibrocytes are associated with fibrosis (e.g. pulmonary and renal fibrosis) and inflammation. Our previous work showed <it>in vitro</it> that CD115+CD11b+Gr-1+ monocytes are the precursor cells of fibrocytes. Using depleting monoclonal antibodies against CCR2 (MC-21) and Gr-1 (RB6-8C5), and CCR2-/- mice we examined <it>in vivo</it> whether fibrocyte development in the kidney and renal fibrosis is dependent on CCR2+Gr-1+ monocytes. <b>Methods:</b> Unilateral ureteral obstruction (UUO) is a well-described model of renal fibrosis in mice. After 1 week of UUO blood, spleen and kidneys were harvested and tissues were analyzed for expression of CD45, CD11b and collagen type I by flow cytometry. Collagen type I was also quantified by RT-PCR and mmunostaining. <b>Results:</b> Daily application of anti-CCR2 mAb (MC-21) resulted in depletion of CCR2+ monocytes and significantly reduced the number of these monocytes in the peripheral blood on day 0 and 3 compared to controls. However, on day 7 the monocyte frequency in blood, spleen or kidneys was no longer reduced in MC-21 treated mice. The development of fibrocytes measured on day 7 in the obstructed kidney was not affected by MC-21 mediated short term depletion of CCR2+ monocytes. In contrast, CCR2-/- mice showed markedly reduced numbers of fibrocytes and expression of collagen I mRNA in the obstructed kidneys. In the contralateral kidneys the number of fibrocytes was low and unchanged. The number of infiltrating monocytes was significantly decreased in CCR2-/- mice by 81 % in the contralateral and 67 % in the UUO-kidney. CCR2- /- mice also displayed reduced numbers of Gr1+ monocytes in the peripheral blood (10 % of controls) and the spleen (60% of controls) throughout the course of UUO. Furthermore, the daily application of anti-Gr-1 reduced the number of Gr-1+ monocytes in the peripheral blood on day 3 but not on day 7. However, the application from day 3 until day 6 after UUO showed significantly lower numbers of monocytes in the peripheral blood, the spleen and the kidneys on day 7 along with the significant decrease in the number of fibrocytes by 91 % in the UUO-kidney. <b>Conclusions:</b> In conclusion, these results strongly demonstrate that CCR2+Gr-1+ monocytes are precursor cells of fibrocytes in renal fibrosis. Reduced numbers of renal monocytes in CCR2-/- mice and anti-Gr-1 treated mice results in reduced numbers of fibrocytes and diminished fibrosis. However, transient depletion of CCR2+ monocytes in the peripheral blood and spleen but not in the kidney with a monoclonal antibody has no influence on fibrocyte development and fibrosis. These findings are concordant with those of our previous <it>in vitro</it> studies.