Expression and secretion of chemotactic cytokines IL-8 and MCP-1 by human endothelial cells after Rickettsia rickettsii infection: Regulation by nuclear transcription factor NF-κB

Infection of endothelial cells (EC) with Rickettsia rickettsii results in Rocky Mountain spotted fever, an acute illness characterized by systemic inflammation. Interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) are important chemokines for activating neutrophils and monocytes, respectively, and recruiting these circulating immune cells to the sites of inflammation. In this study, we have measured the expression and secretion of these chemokines during R. rickettsii infection of cultured human EC. In comparison to uninfected controls, increased mRNA expression of IL-8 and MCP-1 in R. rickettsii-infected EC was evident as early as 3 h and was sustained up to 21 h. Subsequent analysis of culture supernatants revealed significantly enhanced secretion of both chemokines at 3, 8, and 18 h post-infection (5-28-fold increase in IL-8 and 4-16-fold increase in MCP-1). The presence of peptide-aldehyde compound MG132 to inhibit proteasome-mediated degradation of the inhibitory protein IκBα and synthetic peptide SN-50 to inhibit the nuclear translocation of nuclear factor-kappa B (NF-κB) resulted in significant inhibition of the chemokine response. Also, T24 cells expressing a super-repressor mutant of IκBα (to render NF-κB inactivatable) secreted significantly lower quantities of IL-8 than mock-transfected cells. A neutralizing antibody against IL-1α or an IL-1 specific receptor antagonist had no effect on the early phase of R. rickettsii-induced NF-κB activation and IL-8/MCP-1 secretion at 3 h. Both of these treatments, however, diminished late-phase NF-κB activation by about 33% and only partially suppressed the infection-induced chemokine release at 21 h. Thus, while chemokine response early during the infection likely depends on the direct activation of NF-κB, subtle autocrine effects of newly synthesized IL-1α may contribute, in part, to the control of NF-κB activation and chemokine production at later times. These findings implicate a prominent role for host EC in recruiting immune cells to the site of inflammation during Rickettsia infection and provide important insights to further our understanding of the pathogenesis of spotted fever group rickettsioses.