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Figure 3 of this Letter contains an inadvertently duplicated panel: the PBS 30 panel is identical to the aGalCer panel (top right). The corrected panels are shown here. Our results and conclusions are unaffected by this oversight. CORRIGENDUM

No abstract

Human monocytotropic ehrlichiosis (HME) is an emerging, life-threatening, infectious disease caused by Ehrlichia chaffeensis, an obligate intracellular bacterium that lacks cell wall LPS. We have previously developed an animal model of severe HME using a strain of Ehrlichia isolated from Ixodes ovatus ticks (IOE). To understand the basis of susceptibility to severe monocytotropic ehrlichiosis, we compared low and high doses of the highly virulent IOE strain and the less virulent Ehrlichia muris strain that are closely related to E. chaffeensis in C57BL/6 mice. Lethal infections caused by high or low doses of IOE were accompanied by extensive liver damage, extremely elevated levels of TNF-α in the serum, high frequency of Ehrlichia-specific, TNF-α-producing CD8+ T cells in the spleen, decreased Ehrlicha-specific CD4+ T cell proliferation, low IL-12 levels in the spleen, and a 40-fold decrease in the number of IFN-γ-producing CD4+ Th1 cells. All groups contained negligible numbers of IL-4-producing cells in the spleen. Transfer of Ehrlichia-specific polyclonal Abs and IFN-γ-producing Ehrlichia-specific CD4+ and CD8+ type 1 cells protected naive mice against lethal IOE challenge. Interestingly, infection with high dose E. muris provided protection against rechallenge with a lethal dose of IOE. Cross-protection was associated with substantial expansion of IFN-γ-producing CD4+ and CD8+ cells, but not TNF-α-producing CD8+ T cells, a high titer of IgG2a, and a low serum level of TNF-α. In conclusion, uncontrolled TNF-α production by CD8+ T cells together with a weak CD4+ Th1 cell response are associated with immunopathology and failure to clear IOE in the fatal model of HME.

Rickettsiae cause some of the most severe human infections, including epidemic typhus and Rocky Mountain spotted fever. Substantial progress has been made in research into the genomics, vector relationships, pathogenesis and immunity of these obligate, intracellular, arthropod-transmitted bacteria. This Review summarizes our understanding of the early and late events in pathogenesis and immunity, modulation of the host response to rickettsial infection by the vector, host defence, virulence mechanisms and rickettsial manipulation of host cells.