Cell membrane Complex Enzyme Eukaryotic cell or cell component Microbe-host cell complex Microorganism or its component Other Other -- ion Pathway or action Protein Protein or gene Protein or gene complex Protein or protein complex Bacterial membrane or virus envelope Cell membrane Cytoplasm Eukaryotic cell or cell component Extracellular Golgi Golgi membrane Intercellular Intracellular Mitochondria Nucleocapsid/Cytoplasm Organelle Organelle -- Cell membrane Organelle -- Endoplasmic reticulum Organelle -- ER Organelle -- Golgi Organelle -- Golgi membrane Organelle -- Nucleus Organelle -- Phagolysosome Organelle -- Phagosome Organelle -- Ribosome Other Phagolysosome Phagosome Chaperone Defense, immunity protein Enzyme Enzyme activator Enzyme inhibitor Genomic S segment Infection Ligand binding or carrier Motor Nucleic acid binding Other Signal transducer Toxicity Transcription factor binding Transcription regulation Transporter Unknown IC Inferred by Curator IDA Inferred from Direct Assay IEA Inferred from Electronic Annotation IEP Inferred from Expression Pattern IGI Inferred from Genetic Interaction IMP Inferred from Mutant Phenotype IPI Inferred from Physical Interaction ISS Inferred from Sequence or Structural Similarity NAS Non-traceable Author Statement ND No biological Data available RCA inferred from Reviewed Computational Analysis TAS Traceable Author Statement NR Not Recorded Heinzen RA, Grieshaber SS, Van Kirk LS, Devin CJ Infection and immunity Dynamics of actin-based movement by Rickettsia rickettsii in vero cells 1999 10417192 PubMed Wisseman CL Jr, Edlinger EA, Waddell AD, Jones MR Infection and immunity Infection cycle of Rickettsia rickettsii in chicken embryo and L-929 cells in culture 1976 825463 PubMed Joshi SG, Francis CW, Silverman DJ, Sahni SK FEMS microbiology letters NF-kappaB activation suppresses host cell apoptosis during Rickettsia rickettsii infection via regulatory effects on intracellular localization or levels of apoptogenic and anti-apoptotic proteins 200415 15135541 PubMed Hackstadt T Infectious agents and disease The biology of rickettsiae 1996 8805076 PubMed Eremeeva ME, Silverman DJ Infection and immunity Effects of the antioxidant alpha-lipoic acid on human umbilical vein endothelial cells infected with Rickettsia rickettsii 1998 9573120 PubMed Teysseire N, Boudier JA, Raoult D Infection and immunity Rickettsia conorii entry into Vero cells 1995 7806381 PubMed Silverman DJ, Santucci LA, Meyers N, Sekeyova Z Infection and immunity Penetration of host cells by Rickettsia rickettsii appears to be mediated by a phospholipase of rickettsial origin 1992 1612741 PubMed Walker DH, Valbuena GA, Olano JP Annals of the New York Academy of Sciences Pathogenic mechanisms of diseases caused by Rickettsia 2003 12860594 PubMed Eremeeva ME, Silverman DJ Microbiology (Reading, England) Rickettsia rickettsii infection of the EA.hy 926 endothelial cell line: morphological response to infection and evidence for oxidative injury 1998 9720025 PubMed Li H, Walker DH Microbial pathogenesis rOmpA is a critical protein for the adhesion of Rickettsia rickettsii to host cells 1998 9600861 PubMed Adhesin, rOmpA Function: Ligand binding or carrier. With monoclonal antibodies and the technological approach of flow cytometry, rOmpA was demonstrated to play a critical role in the attachment of R. rickettsii to host cells (Li and Walker, 1998). rOmpA and rOmpB are immunodominant, surface-exposed proteins of Rickettsia rickettsii. Prior evidence suggests that adhesion of R. rickettsii to the host cell is mediated by a rickettsial protein. Five monoclonal antibodies to rOmpA, five to rOmpB, and one to the rickettsial lipopolysaccharide (LPS) were tested for inhibition of rickettsial attachment. All the monoclonal antibodies to rOmpA inhibited adhesion of rickettsiae to the L-929 cells with some inhibition rates as high as 90%. In contrast, monoclonal antibodies to rOmpB and LPS did not block attachment. When Fab fragments of monoclonal antibodies against rOmpA and rOmpB were used, similar results were observed as for the intact monoclonals, non-adhesion and adhesion, respectively. Purified rOmpA showed a competitive inhibitive effect on the attachment of R. rickettsii to host cells. Trypsin completely digested rOmpA but not rOmpB from the surface of intact R. rickettsii, resulting in loss of the ability of the rickettsiae to attach to the host cell (Li and Walker, 1998).Endothelial cells of small- and medium-sized vessels are the primary target cells for Rickettsia rickettsii during infection of vertebrate hosts (Eremeeva and Silverman, 1998).(Li et al., 1998)(Eremeeva et al., 1998) Protein-dependent Receptor Function: Ligand binding or carrier. Rickettsia attach to a protein dependent receptor on the host cell membrane and induce focal host cell cytoskeletal rearrangements at the site of attachment, resulting in their entry into the host cell even in nonprofessional phagocytes by a mechanism requiring rickettsial metabolic activity (Walker et al., 2003).Important unidentified elements of the rickettsia-host cell interaction include the host cell membrane receptor for the rickettsial adhesin(s) (Walker et al., 2003).(Walker et al., 2003) Rickettsia in Phagocyte Function: . Internalization of obligate intracellular bacteria belonging to the genus Rickettsia by eukaryotic cells requires participation of both the parasitized host and the microorganism. The term"induced phagocytosis" has been used specifically to describe the entry of Rickettsia prowazekii, although a similar mechanism is likely for R. rickettsii. A role for a phospholipase in the internalization process has been proposed for both of these organisms, with the strongest supporting evidence provided for R. prowazekii. Despite general acceptance of the notion that phospholipase activity is involved in the internalization process of these bacteria, the origin of the enzyme is not known. The results of the study presented here, which used R. rickettsii and Vero cells, suggest that a rickettsial phospholipase, rather than a host cell phospholipase, mediates internalization of the organism. This conclusion is based upon results which show that pretreatment of R. rickettsii, but not of host cells, with a specific chemical inhibitor of phospholipase, and also antiserum to this enzyme, significantly reduces uptake of the organism and its ability to cause plaque formation (Silverman et al., 1992).The entry of rickettsiae into eukaryotic cells is mediated by an induced phagocytosis, but rickettsiae have never been observed in a closed phagocytic vacuole. In this study, Rickettsia conorii entry into Vero cells was observed by transmission electron microscopy during a period of 3 to 20 min after bacterium-cell contact. The entry occurred within 3 min after bacterium-cell contact, and R. conorii was observed in the process of engulfment, within a phagocytic vacuole, or free in the cytosol. Escape from the phagosome is a very rapid step since phagosome lysis was only occasionally observed. By 12 min, 90% of bacteria were internalized and half were free in the cytosol. This report confirms that rickettsiae penetrate nonphagocytic cells by induced phagocytosis and is the first demonstration of rickettsiae within a complete phagocytic vacuole (Teysseire et al., 1995).(Silverman et al., 1992)(Teysseire et al., 1995) Rickettsia in Cytoplasm Function: . Rickettsia rapidly lyse the phagosomal membrane and escape into the cytosol prior to phagolysomal fusion avoiding exposure to the lysosomal enzymes. In the cytosol they acquire their nutrients (e.g., glutamate), a part of their energy requirements (ADP/ATP transporter), and many components required for growth (e.g., amino acids) (Walker et al., 2003).The ultrastructural changes that occur during parasitism by R. rickettsii include a marked dilation of internal membranes, particularly the rough endoplasmic reticulum. The explicit mechanism of rickettsia-induced cellular injury has not been defined, but hypothetical mechanisms include trypsin-like protease activity, free-radical-induced lipid peroxidation, and phospholipase A (PLA) activity directly or through the generation of diacylglycerol and arachidonic acid metabolites (Hackstadt, 1996).Rickettsia rickettsii infection of endothelial cells is manifested in very distinctive changes in cell morphology, consisting of extensive dilatation of the membranes of the endoplasmic reticulum and outer nuclear envelope and blebbing of the plasma membrane, as seen by transmission electron microscopy. These changes in cellular architecture are thought to be due to oxidant-mediated cell injury, since their occurrence correlates with dramatic alterations in cellular metabolism, particularly with regard to antioxidant systems (Eremeeva and Silverman, 1998).Rickettsia rickettsii, a gram-negative and obligate intracellular bacterium, is the causative agent of Rocky Mountain spotted fever. In human infections, the primary target of R. rickettsii infection is vascular endothelium. Our laboratory has shown that activation of nuclear transcription factor-kappa B (NF-kappaB) during R. rickettsii infection of cultured human endothelial cells protects against apoptosis by preventing the activation of apical caspases-8 and -9, and the effector caspase-3 (Joshi et al., 2004).(Walker et al., 2003)(Hackstadt, 1996)(Eremeeva et al., 1998)(Joshi et al., 2004) Rickettsia in Nucleus Function: . Intranuclear growth of R. rickettsii occurred, but it appeared to be a relatively rare phenomenon in both CE and L cells. Intranuclear growth could be identified with reasonable certainty only after compact masses of organisms appeared within the borders of the nuclear membrane. It was impossible to classify single or a few dispersed organisms within the nuclear membrane border into intranuclear and superimposed intracytoplasmic populations. Hence, it was not possible to determine how early in the growth cycle rickettsiae entered the nucleus or to measure early intranuclear growth kinetics. However, by 45 to 48 h of incubation, compact intranuclear clumps of rickettsiae were rarely but readily identified. In contrast to the dispersed distribution of rickettsial body count (RLB) in the cytoplasm, the intranuclear rickettsiae remained in compact masses and tended to increase in numbers, which at times almost completely filled the nucleus with a dense mass of organisms (Wisseman et al., 1976).The low frequency of intranuclear rickettsiae suggests that R. rickettsii does not enter the nucleus from the cytoplasm as readily as it enters through the plasma membrane into the cytoplasm from the extracellular environment. Moreover, the accumulation of large numbers of intranuclear rickettsiae in those nuclei that are identifiably infected suggests that, if the organisms escape from the nucleus, the rate of passage out of the nucleus is probably much slower than that out of the cytoplasm into the extracellular space, at least relative to the rates of multiplication in the respective cellular compartment (Wisseman et al., 1976).(Wisseman et al., 1976) Extracellular Rickettsia Function: . Insight into the virulence of spotted fever group rickettsiae, such as R. rickettsii, was achieved with the discovery that these organisms utilize an intracellular actin-based motility (ABM) system to promote direct cell-to-cell spread. This mechanism of pathogenesis is also exploited by the facultative intracellular bacteria Listeria monocytogenes and Shigella flexneri. By using the propulsive force supplied by parasite-directed polymerization of host cell actin, motile bacteria move into pseudopodia that can be subsequently engulfed by neighboring cells. Escape from the double membrane vacuole allows infection of the new cytoplasm. The ability of spotted fever group rickettsiae to spread via ABM within the endothelium (the target host tissue of rickettsia) by directly passing from one cell to another allows evasion of the host humoral immune response, minimizes exposure to cell-impermeant antibiotics, and maintains rickettsiae within their required intracellular niche (Heinzen et al., 1999).At 96 to 120 h of incubation, the host cells showed progressive degeneration and destruction. However, light microscopy showed that very large numbers of rickettsiae were extracellular at this time. Many were free, as single organisms or clumps, unassociated with cell fragments, and intact extracellular doughnuts were occasionally seen (Wisseman et al., 1976).(Heinzen et al., 1999)(Wisseman et al., 1976)