Background Hepatitis B virus (HBV) replication has been reported to be involved in many extrahepatic viral disorders;however,the mechanism by which HBV is transinfected into extrahepatic tissues such as myocardium and causes HBV associated myocarditis remains largely unknown.Methods In this study,endothelial progenitor cells (EPCs) were infected by HBV and then transfused into ischemic model of mice.HBV surface and core antigen as well as mutation of HBV particles were detected by immunonistochemistry,fluorescent activated cell sorter and transmission electron microscopy in vitro and in vivo.Results Human cord blood EPCs, but not human umbilical vein endothelial cells (HUVECs) could be effectively infected by taking up HBV in vitro.HBV envelope surface and core antigen expressions were first detectable in EPCs at day 3 after virus challenge,sustained for up to 11 days,and decreased thereafter.Similarly,the virus particles were the most abundant in EPCs ln the first week observed by a transmission electron microscope,and declined in 3 weeks after HBV infection.HBV DNA but not HBV cccDNA in EPCs were detectable even 3 weeks after virus challenge,as shown by PCR analysis.Furthermore,intravenous transplantation of HBV-treated EPCs into myocardial infarction Sprague & Dawley rats model resulted in incorporation of both EPCs and HBV into injured endothelial tissues of capillaries in the ischemic border zone.Conclusions These results strongly support that EPCs serve as virus carrier mediating HBV trans-infection into the injured myocardial tissues.The findings might suggest a novel mechanism for HBV-associated myocarditis.