Objective To evaluate the influential factors of iron acquisition during Francisella tularensis LVS infection of mouse macrophages.Methods F.tularensis LVS expressing green fluorescent protein was used to infect murine macrophage J774A.1 cells.Transferrin receptor 1(Tfr1)was detected with mono-antibody and visualized with a goat-anti mouse IgG conjugated to Alexa 594.The expression profile of 5 iron metabolism related genes of J774A.1 murine macrophages uninfected or infected with F.tularensis LVS was determined with real-time PCR.Immunoblot analysis was used to compare the Tfr1 expression of live Francisella infected macrophage with dead bacteria.Tfr1 knock-off in J774A.1 cells was performed with siRNA.The transfected cells were infected with Francisella for immunoblotting and microscopy and infection assay.Results It was revealed that the live vaccine strain of F.tularensis induced the expression of Tfr1 in host macrophages.Gene expression analysis indicated that F.tularensis LVS drove an active iron acquisition program with induction of Tfr1 and iron regulatory proteins(Irp1 and Irp2).It was shown by Western-blotting that the siRNA-Tfrc-1 could knock off about 75% of Tfr1 in J774A.1 cells.It was determined by infection assay that,Tfr1 was knocked off,the bacteria number at 1h infection with Francisella was not different from that of control(F=1.06,P=0.326 5),while it was decreased significantly after 24h of infection(F=24.12,P=0.000 6).Conclusions It is demonstrated that upregulation of the Tfr1 may be mediated by post-transcriptional regulation during early infection,but sustained later through increased expression of Irp 1 and Irp 2.Increased expression of Tfr1 expands the intracellular iron pool through transferrin-mediated delivery and may thus be readily available for uptaking by Francisella.Knocking off the expression of Tfr1 does not affect bacterial invasion.Francisella,however,may fail to proliferate in macrophages in which the expression of transferrin receptor has been suppressed.