Rickettsia rickettsii infection of cultured human endothelial cells induces NF-κB activation

Rickettsia rickettsii, the etiologic agent of Rocky Mountain spotted fever, is an obligate intracellular bacterial organism that infects primarily the vascular endothelial cells (EC). A component of the EC response to infection is transcriptional activation, which may contribute to the thrombotic and inflammatory consequences of disease. In this study, we explore R. rickettsii-induced activation of the nuclear factor-κB/Rel (NF- κB) family of transcription factors involved in early transcriptional responses to injurious stimuli. Two NF-κB species were activated by infection and reacted with a double-stranded oligonucleotide probe corresponding to the κB binding domain of the murine kappa light-chain gene enhancer. Gel supershift analysis demonstrated the reactivity of these complexes with antibodies against p65 and p50, and the induced species were tentatively identified as p50-p50 homodimers and p50-p65 heterodimers. Semiquantitative reverse transcription-PCR analysis revealed dramatic increases in the steady-state levels of mRNA coding for the inhibitory subunit of NF-κB (IκBα), transcription of which is enhanced by the binding of NF-κB within the IκBα promoter region. NF-κB activation was first detected 1.5 h following infection and was biphasic, with an early peak of activation at approximately 3 h, a return to baseline levels at 14 h, and even higher levels of activation at 24 h. It is likely that NF-κB activation requires cellular uptake of R. rickettsii, since treatment of EC with cytochalasin B during infection to block entry inhibited activation by only 70% at 3 h. R. rickettsii-induced activation of NF-κB may be an important controlling factor in the transcriptional responses of EC to infection with this obligate intracellular organism.