The strength of the epidemiologic and clinical associations of Chlamydia pneumoniae with atherosclerosis can be increased by the demonstration that C pneumoniae can initiate and sustain growth in human vascular cells as well as in animal models. To investigate the biological basis for the dissemination and proliferation of this organism in vascular cells, the in vitro growth of C pneumoniae was studied in 2 macrophage cell lines, peripheral blood monocyte (PBMC)-derived macrophages, human bronchoalveolar lavage (BAL) macrophages, several endothelial cell lines, and aortic artery smooth muscle cells. Five of 5 strains of C pneumoniae were capable of 3 passages in human U-937 macrophages and in murine RAW 246.7 macrophages. Titers were suppressed in both macrophage types with each passage as compared with growth in HEp-2 cells. Both human BAL macrophages and PBMC-derived macrophages were able to inhibit C pneumonia eafter 96 hours' growth. Eleven C pneumoniae strains were capable of replicating in normal human aortic artery-derived endothelial cells, umbilical vein-derived endothelial cells, and pulmonary artery endothelial cells. Infection in human aortic artery smooth muscle cells was also established for 13 strains of C pneumoniae. C pneumoniae was also capable of growing in endothelial cells derived from human cadaver coronary artery endothelial cells (CAEC). U-937 human macrophages that were infected with C pneumoniae were capable of transmitting the infection to CAEC when they were brought into contact with the endothelial cells by centrifugation, rocking overnight, and direct layering overnight, with and without using artificial laboratory tissue culture enhancements, such as centrifugation of the inoculum and cycloheximide in the growth media. The in vitro ability of C pneumoniae to maintain infections in macrophages, endothelial cells, and aortic smooth muscle cells may provide support for the hypothesis that C pneumoniae can infect such cells, which when followed by an immune response may contribute to atheroma formation in vivo. Stimulation of cytokine responses by infection with C pneumoniae has indicated that this organism is capable of interacting with the immune system. In vitro infection by C pneumoniae of U-937 macrophages stimulated the production of IL-1beta, IFN-gamma, and TNF-alpha in tissue culture. Human CAEC that are infected with C pneumoniae produce more IL-8 compared with those inoculated with killed C pneumoniae or negative control cells, indicating a chemokine response to infection that may play a role in recruitment of inflammatory cells to sites of infection in vascular cells. When IFN-gamma was used to up regulate HEp-2 and U-937 cells before infection by C pneumoniae, inhibition of a lytic growth cycle occurred in a dose related response. However, removal of the IFN-gamma after 24 to 48 hours' exposure allowed subsequent productive growth in the cells, perhaps indicating the prior induction of a persistent infection. More studies are needed to study the complex relationship between lytic infection and persistence, the ability of C pneumoniae to affect the immune response of vascular cells, and the potential for C pneumoniae to influence the initiation of or progression of atheromatous lesions.