Infection with Chlamydia pneumoniae, a causative agent of acute and chronic respiratory diseases, has recently been implicated as a potential risk factor in atherosclerosis. Atherosclerotic lesions are characterized by monocyte infiltration, which may be regulated by the chemokine monocyte chemotactic protein 1 (MCP-1). We have previously shown that C. pneumoniae infection stimulates MCP-1 production in human endothelial cells, an event which may be specific to this species of Chlamydia, sinceChlamydia trachomatis infection fails to induce this response. To examine the underlying mechanisms by which C. pneumoniae infection induces MCP-1 production in endothelial cells, the present study investigated the role of transcription factor NF-κB in MCP-1 mRNA expression. Human umbilical vein endothelial cells (HUVEC) were infected with the coronary isolate C. pneumoniae A-03 or with C. trachomatis L2, and MCP-1 mRNA expression was assessed after different periods of infection by reverse transcription-PCR. Expression of MCP-1 mRNA in C. pneumoniae-infected HUVEC was significantly elevated as early as 1 h postinfection and increased dramatically by 12 and 24 h compared to baseline controls. Nuclear translocation of NF-κB occurred by 30 min of infection, as determined by electrophoretic mobility shift assays and immunofluorescence staining. Treatment ofC. pneumoniae-infected HUVEC with parthenolide, a specific inhibitor of NF-κB activation, suppressed MCP-1 mRNA expression. In contrast, infection with C. trachomatis L2 did not induce MCP-1 mRNA in infected HUVEC and failed to activate NF-κB. Results from this study demonstrate a requirement for NF-κB activation in stimulation of MCP-1 gene expression by C. pneumoniae in human endothelial cells. Furthermore, the data suggest that, within the genus Chlamydia, functionally distinct signaling pathways leading to NF-κB activation are utilized by C. pneumoniaein endothelial cells during infection.