Abrogation of gamma interferon-induced inhibition of Ehrlichia chaffeensis infection in human monocytes with iron transferrin

Abstract

Ehrlichia chaffeensis is an obligate intracellular bacterium which infects cells of the macrophage/monocyte lineage. To test whether gamma interferon (IFN-γ) inhibits infection of monocytes with E. chaffeensis, human peripheral blood monocytes were incubated with recombinant human IFN-γ for 3 h and then exposed to E. chaffeensis. With 2,000 U of IFN-γ per ml, maximal inhibition of infection by E. chaffeensis was observed. THP-1 cells, a human monocyte cell line, pretreated with phorbol myristic acetate or not pretreated, were incubated with various concentrations of IFN-γ. Maximum inhibition was obtained at 1,000 U of IFN-γ per ml with phorbol myristic acetate-treated THP-1 cells. However, nontreated cells did not achieve a similar level of anti-ehrlichial activity even with 10,000 U of IFN-γ per ml. IFN-γ given within 6 h postinfection was effective in inhibiting E. chaffeensis. Nitric oxide production was not demonstrated in the monocyte medium incubated with IFN-γ and E. chaffeensis. None of the reactive oxygen intermediate scavengers tested blocked the IFN-γ-induced anti-ehrlichial activity. Deferoxamine, an intracellular iron chelator, at 15 μM completely inhibited the survival of E. chaffeensis. Iron-saturated transferrin at 1.67 mg/ml completely reversed the IFN-γ-induced ehrlichial killing. These results indicate that (i) E. chaffeensis is sensitive to intracytoplasmic iron depletion, (ii) E. chaffeensis is sensitive to IFN-γ-induced killing, and (iii) the anti-ehrlichial activity induced in human monocytes by IFN-γ is mediated by limitation of available cytoplasmic iron and is not due to the generation of reactive oxygen intermediates or nitric oxide.