Ehrlichiosis: Understanding immune mechanisms that lead to the development of fatal disease

The clinical manifestations of HME caused by Ehrlichia chaffeensis, a Gram-negative, LPS-lacking bacterium, including toxic shock-like syndrome and liver injury in the absence of overwhelming bacterial burdens, has led to the hypothesis that severe disease is immune-mediated. Our previous studies showed that intraperitoneal infection of wild-type mice with virulent Ehrlichia (IOE) results in CD8+ T cell-mediated fatal toxic shock-like syndrome marked by apoptosis of CD4+ T cells, a weak CD4 Th1 response, and overproduction of TNF-alpha and IL-10. C57BL/6 mice were inoculated intradermally or intraperitoneally with IOE, models that closely reproduce the pathology and immunity associated with tick-transmitted HME, or intraperitoneally with mildly virulent Ehrlichia muris. Both fatal IOE models resulted in increased numbers of hepatic NK cells and CD11c+ dendritic cells, and decreased numbers of hepatic NKT cells. Mice infected intraperitoneally with IOE had increased splenic and hepatic numbers of CD11b+ cells, increased hepatic CD4+ and CD8+ T cells, and decreased splenic CD4+ T cells. Because of the observed differences in NKT cells, I investigated the role of CD1d-restricted NKT cells in induction of toxic shock-like syndrome. Absence of NKT cells restored the absolute numbers of CD4+ and CD8+ T cells and CD11b+ cells in the spleen, decreased expression of Fas on splenic CD4+ lymphocytes and granzyme B in hepatic CD8+ lymphocytes, and decreased the expression of CD40 on APCs. Surprisingly, absence of CD40L did not result in decreased pathology as indicated by similar serum TNF-alpha concentrations, organ bacterial burdens, and liver injury in both knockout and wild-type mice. Because not only NKT cells were activated, but NK cells were also, and both these cell types expressed high levels of intracellular IFN-gamma, TNF-alpha, and granzyme B during severe infection, I depleted NK cells the mouse model of fatal ehrlichiosis. Most strikingly, depletion of NK cells resulted in much less pathology including significantly lower systemic levels of cytokines, lower ehrlichial burdens, and minimal liver injury when compared to control mice. My data provide evidence that unlike NKT cells, which play both beneficial and detrimental roles during severe ehrlichiosis, NK cells are strongly pathogenic and are responsible for induction of Ehrlichia-mediated immunopathology.