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J Med Microbiol 56 (2007), 896-906; DOI: 10.1099/jmm.0.47050-0
© 2007 Society for General Microbiology
ISSN 1473-5644

Comparative analysis of host-cell signalling mechanisms activated in response to infection with Rickettsia conorii and Rickettsia typhi

Elena Rydkina1, Abha Sahni1, David J. Silverman2 and Sanjeev K. Sahni1

1 Hematology-Oncology Unit, Department of Medicine, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA

2 Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, MD 21201, USA

Correspondence
Sanjeev K. Sahni
Sanjeev_Sahni{at}urmc.rochester.edu

Received 3 November 2006
Accepted 15 March 2007

The Gram-negative intracellular bacteria Rickettsia conorii and Rickettsia typhi are the aetiological agents of Mediterranean spotted fever and endemic typhus, respectively, in humans. Infection of endothelial cells (ECs) lining vessel walls, and the resultant vascular inflammation and haemostatic alterations are salient pathogenetic features of both of these rickettsial diseases. An important consideration, however, is that dramatic differences in the intracellular motility and accumulation patterns for spotted fever versus typhus group rickettsiae have been documented, suggesting the possibility of unique and potentially different interactions with host cells. This study characterized and compared R. conorii- and R. typhi-mediated effects on cultured human ECs. The DNA-binding activity of nuclear transcription factor-{kappa}B (NF-{kappa}B) and the phosphorylation status of stress-activated p38 kinase were determined as indicators of NF-{kappa}B and p38 activation. R. conorii infection resulted in a biphasic activation of NF-{kappa}B, with an early increase in DNA-binding activity at 3 h, followed by a later peak at 24 h. The activated NF-{kappa}B species were composed mainly of RelA p65–p50 heterodimers and p50 homodimers. R. typhi infection of ECs resulted in only early activation of NF-{kappa}B at 3 h, composed primarily of p65–p50 heterodimers. Whilst R. conorii infection induced increased phosphorylation of p38 kinase (threefold mean induction) with the maximal response at 3 h, a considerably less-intense response peaking at about 6 h post-infection was found with R. typhi. Furthermore, mRNA expression of the chemokines interleukin (IL)-8 and monocyte chemoattractant protein-1 in ECs infected with either Rickettsia species was higher than the corresponding controls, but there were distinct differences in the secretion patterns for IL-8, suggesting the possibility of involvement of post-transcriptional control mechanisms or differences in the release from intracellular storage sites. Thus, the intensity and kinetics of host-cell responses triggered by spotted fever and typhus species exhibit distinct variations that could subsequently lead to differences in the extent of endothelial activation and inflammation and serve as important determinants of pathogenesis.

Abbreviations: EC, endothelial cell; EMSA, electrophoretic mobility shift assay; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; I{kappa}B, inhibitor of {kappa}B protein; IL, interleukin; MAPK, mitogen-activated protein kinase; MCP, monocyte chemoattractant protein; NF-{kappa}B, nuclear transcription factor-{kappa}B; p.i., post-infection; SFG, spotted fever group; TG, typhus group; TNF, tumour necrosis factor.






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