Chlamydia pneumoniae Induces Interleukin-10 Production that Down-Regulates Major Histocompatibility Complex Class I Expression Sylvie Caspar-Bauguil,1 BeÂneÂdicte Puissant,1

Recently, it was demonstrated that CD8+ T cells are important for the response 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 function. 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 signi®cant alteration of tumor growth factor±b secretion or MHC class II expression. Recombinant IL-10 added to uninfected U937 cells decreased the expression of MHC class I, whereas blocking antibodies to IL-10 and its receptor abolished the C. pneumoniae±induced inhibition of MHC class I expression. Analysis of our data pro-vides evidence that IL-10 secretion induced by C. pneumoniae infection of monocytic cells down-regulates the expression of MHC class I molecules and thereby might reduce the pre-sentation of bacterial epitopes by MHC. This would decrease the ability of CD8+ T cells to eliminate infected cells. Chlamydia pneumoniae is a gram-negative bacterium that has recently emerged as a pathogen with a potential causal role i