Mechanisms of intracellular killing of Rickettsia conorii in infected human endothelial cells, hepatocytes, and macrophages

The mechanism of killing of obligately intracellular Rickettsia conorii within human target cells, mainly endothelium and, to a lesser extent, macrophages and hepatocytes, has not been determined. It has been a controversial issue as to whether or not human cells produce nitric oxide. AKN-1 cells (human hepatocytes) stimulated by gamma interferon, tumor necrosis factor alpha, interleukin 1beta, and RANTES (regulated by activation, normal T-cell-expressed and -secreted chemokine) killed intracellular rickettsiae by a nitric oxide-dependent mechanism. Human umbilical vein endothelial cells (HUVECs), when stimulated with the same concentrations of cytokines and RANTES, differed in their capacity to kill rickettsiae by a nitric oxide-dependent mechanism and in the quantity of nitric oxide synthesized. Hydrogen peroxide-dependent intracellular killing of R. conorii was demonstrated in HUVECs, THP-1 cells (human macrophages), and human peripheral blood monocytes activated with the cytokines. Rickettsial killing in the human macrophage cell line was also mediated by a limitation of the availability of tryptophan in association with the expression of the tryptophan-degrading enzyme indoleamine-2,3-dioxygenase. The rates of survival of all of the cell types investigated under the conditions of activation and infection in these experiments indicated that death of the host cells was not the explanation for the control of rickettsial infection. This finding represents the first demonstration that activated human hepatocytes and, in some cases, endothelium can kill intracellular pathogens via nitric oxide and that RANTES plays a role in immunity to rickettsiae. Human cells are capable of controlling rickettsial infections intracellularly, the most relevant location in these infections, by one or a combination of three mechanisms involving nitric oxide synthesis, hydrogen peroxide production, and tryptophan degradation.