Introduction: Chronic allograft dysfunction (CAD) is the most limiting factor of long term graft survival and characterized by fibrotic remodeling, renal injury and chronic inflammation. Recent studies identified several microRNAs (miRs), small non-coding RNAs involved in gene regulation, that are enriched in kidneys in response to injury and have pro-fibrotic potential [1]. Thus, miR antagonists (antagomiRs) could serve as a promising treatment strategy since they have been shown to be a powerful tool to reduce or even prevent fibrotic remodeling. The aim of this study is to investigate the inhibition of pro-fibrotic miR-21 in an evaluated murine kidney transplantation model. Methods and Results: Allogenic kidney transplantation (KTx) was performed at day 0 from male C57BL/6 mice into female Balb/c mice (ca 25g, n=6) under isoflurane anaestesia (3-4% induction, 2-3% maintenance, in 100% O2) and analgesia (buprenorphin, 1mg/kg BW, s.c.) [2]. Recipient mice were treated at day -1 and day 7 either with LNA-SCR (control) or LNA-21 (inhibitor of miR-21) (20mg/kg BW, i.p.). Kidneys were harvested and analyzed six weeks after KTx. Following data are compared by ANOVA. Using RT-PCR, we determined increased expression levels of markers for fibrosis (e.g. collagen 3 (Col3), fibroblast secretory protein-1 (FSP-1); p=0.002 and p=0.01 respectively) and inflammation (e.g. interleukin-6 (IL-6), macrophage inflammatory protein-1 (MIP-1); p=0.02 and p=0.006 respectively) in transplanted kidneys which were rescued by miR-21 inhibition. Moreover, allografts of LNA-21 treated mice showed significantly less fibrosis development (Sirius Red, p<0.001) and had a lower BANFF chronic rejection score (p=0.02). The miR-21 promoter region harbors a putative binding site of transcription factor STAT3, which is activated by IL-6. We identified upregulated IL-6 expression in inflammatory cells (primary peritoneal macrophages and cell line RAW264.7 (p<0.0001 respectively)) upon activation with lipopolysaccharide (10ng/mL, 24h) and hypothesized, that infiltrating immune cells produce and secrete cytokines that might affect resident renal cells causing fibrosis and injury development. Co-culture assays confirmed a crosstalk between RAW264.7 and renal fibroblasts NRK49F with increased expression levels of IL-6 (p=0.005), connective tissue growth factor (CTGF, p=0.02) and miR-21 (p=0.003) in NRK49F. Similar results were observed due to IL-6 treatment (10ng/mL, 12h) of NRK49F. Conclusion: In our murine model of CAD allograft rejection is preserved by inhibition of miR-21 due to less inflammation and fibrosis. IL-6 was identified as a crucial signal mediator that increases miR-21 expression level in renal fibroblasts. Thus, our study suggests an essentially needed new treatment strategy based on inhibition of pro-fibrotic miR-21 in transplantation medicine.