The respiratory tract pathogen Chlamydia pneumoniae has been associated with atherosclerosis. Monocytes are supposed to serve as a vehicle for systemic dissemination of intracellular C. pneumoniae from the lung to the artery vessel wall. We were therefore interested in pathogen-induced cellular events associated with NF-kappaB, a crucial transcription factor for both inflammatory cytokines and antiapoptotic molecules. In this study we demonstrate by electrophoretic mobility shift assay that C. pneumoniae infection of the human monocytic cell line Mono Mac 6 induces activation of NF-kappaB over 48 h, with a maximum level at 1 h postinfection. As shown by supershift assay, the activated NF-kappaB complex consists of the subunits RelA (p65) and NF-kappaB1 (p50). Apoptotic host cells were not detected during the early stages of the infection when maximal activation of NF-kappaB was detected. Pretreatment of Mono Mac 6 with the antioxidant and NF-kappaB inhibitor PDTC (pyrrolidine dithiocarbamate) induced activation of caspase-3 and led to apoptotic cell death. The C. pneumoniae-induced activation of the NF-kappaB complex was reduced by PDTC, which in parallel resulted in an increased apoptosis, as quantified by annexin V labeling and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling reaction. In the complete absence of activated NF-kappaB, when Mono Mac 6 cells were pretreated with the more potent NF-kappaB inhibitors MG-132 and parthenolide a C. pneumoniae-mediated rescue of cells from induced apoptosis could not be achieved. Our results indicate that activation of NF-kappaB in C. pneumoniae-infected Mono Mac 6 cells is associated with protection of Mono Mac 6 cells against apoptosis and might thereby contribute to systemic spread of the pathogen.