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

The mechanism of killing of obligately intracellular Rickettsia conorii wit hin human target cells, mainly endothelium and, to a lesser extent, macroph ages 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 (h uman hepatocytes) stimulated by gamma interferon, tumor necrosis factor alp ha, interleukin 1 beta, and RANTES (regulated by activation, normal T-cell- expressed and -secreted chemokine) killed intracellular rickettsiae by a ni tric oxide-dependent mechanism. Human umbilical vein endothelial cells (HUV ECs), when stimulated,vith 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 peroxi de-dependent intracellular killing of R. conorii was demonstrated in HUVECs , THP-1 cells (human macrophages), and human peripheral blood monocytes act ivated with the cytokines. Rickettsial killing in the human macrophage cell line was also mediated by a limitation of the availability of tryptophan i n association with the expression of the tryptophan-degrading enzyme indole amine 2,3-dioxygenase. The rates of survival of all of the cell types inves tigated under the conditions of activation and infection in these experimen ts indicated that death of the host cells was not the explanation for the c ontrol of rickettsial infection. This finding represents the first demonstr ation that activated human hepatocytes and, in some eases, endothelium can kill intracellular pathogens via nitric oxide and that RANTES plays a role in immunity to rickettsiae. Human cells are capable of controlling ricketts ial infections intracellularly, the most relevant location in these infecti ons, by one or a combination of three mechanisms involving nitric oxide syn thesis, hydrogen peroxide production, and tryptophan degradation.