Chlamydia pneumoniae induces interleukin-10 production that down-regulatesmajor histocompatibility complex class I expression

Recently, it was demonstrated that CD8(+) T cells are important for the res ponse against Chlamydia pneumoniae. By use of the human monocytic cell line U937 and human monocytes taken from peripheral blood, we investigated the effect of infection on various molecules critical for CD8(+) T cell functio n. A strong secretion of interleukin (IL)-10 by infected cells was observed , together with an inhibited expression of major histocompatibility complex (MHC) class I antigens, but without significant alteration of tumor growth factor-beta secretion or MHC class II expression. Recombinant IL-10 added to uninfected U937 cells decreased the expression of MHC class I, whereas b locking antibodies to IL-10 and its receptor abolished the C. pneumoniae-in duced inhibition of MHC class I expression. Analysis of our data provides e vidence that IL-10 secretion induced by C. pneumoniae infection of monocyti c cells down-regulates the expression of MHC class I molecules and thereby might reduce the presentation of bacterial epitopes by MHC. This would decr ease the ability of CD8(+) T cells to eliminate infected cells.