The resistance of human monocyte-derived macrophages to Chlamydia pneumoniae infection is enhanced by interferon-gamma

Chlamydia pneumoniae is an intracellular bacterium which commonly causes re spiratory infections. Chronic infections have been associated with atherosc lerosis and the organism has been detected in macrophages in the disease le sions. Growth of chlamydiae in different epithelial cell lines is restricte d by interferon-gamma (IFN-gamma), a monocyte activator produced by T cells . We have studied the influence of IFN-gamma on the growth and infectivity of C. pneumoniae in HL-cells and human monocyte-derived macrophages. Low co ncentrations of the cytokine significantly restricted the growth and produc tivity of C. pneumoniae in epithelial cells in vitro. In macrophages, howev er, no effect on the growth of the bacteria in infected cells was found, bu t high doses clearly restricted the production of infectious progeny. The r esults suggest that IFN-gamma participates in the resistance to C. pneumoni ae. The bacterium is, however, still capable of infecting macrophages that are important in the pathogenesis of atherosclerosis and it may thus partic ipate in the inflammatory process associated with the disease.