Ehrlichia chaffeensis and E. sennetsu are genetically divergent obligatory intracellular bacteria of human monocytes and macrophages, and the human granulocytic
ehrlichiosis (HGE) agent is an obligatory intracellular bacterium of granulocytes.
Infection with both E. chaffeensis and E. sennetsu, but not HGE agent, in the
acute monocytic leukemia cell line THP-1 almost completely inhibited by treatment with
deferoxamine, a cell-permeable
iron chelator.
Transferrin receptors (TfRs) accumulated on both E. chaffeensis and E. sennetsu, but not HGE agent, inclusions in THP-1 cells or the cells of the promyelocytic
leukemia cell line HL-60. Reverse transcription-PCR showed an increase in the level of TfR
mRNA 6 h postinfection which peaked at 24 h postinfection with both E. chaffeensis and E. sennetsu
infection in THP-1 or HL-60 cells. In contrast, HGE agent in THP-1 or HL-60 cells induced no increase in TfR
mRNA levels. Heat treatment of E. chaffeensis or the addition of
monodansylcadaverine, a
transglutaminase inhibitor, 3 h prior to
infection inhibited the up-regulation of TfR
mRNA. The addition of
oxytetracycline 6 h after E. chaffeensis
infection caused a decrease in TfR
mRNA which returned to the basal level by 24 h postinfection. These results indicate that both internalization and continuous proliferation of ehrlichial organisms or the production of ehrlichial
proteins are required for the up-regulation of TfR
mRNA. Results of electrophoretic mobility shift assays showed that both E. chaffeensis and E. sennetsu
infection increased the binding activity of
iron-responsive
protein 1 (IRP-1) to the
iron-responsive
element at 6 h postinfection and remained elevated at 24 h postinfection. However, HGE agent
infection had no effect on
IRP-1 binding activity. This result suggests that activation of
IRP-1 and subsequent stabilization of TfR
mRNA comprise the mechanism of TfR
mRNA up-regulation by E. chaffeensis and E. sennetsu
infection.