Sustained interleukin-6 and interleukin-8 expression following infection with Chlamydia trachomatis serovar L2 in a HeLa/THP-1 cell co-culture model.

Chlamydia trachomatis, an intracellular obligate bacterium, remains responsible for a large spectrum of disorders that can progress to chronic diseases, resulting in severe sequelae, such as tubal infertility and blindness. These sequelae may be due to deleterious immune responses induced by repeated or persistent infections. By initiating and regulating inflammation as well as immune responses, pro-inflammatory cytokines secreted by local infected epithelial and immune cells, such as monocytes, may play an essential role in immunity and in the immunopathogenesis of chlamydial diseases. In this study, we mimicked the in vivo interaction between epithelial cells and monocytes by co-culturing epithelial-like HeLa cells with monocyte-like THP-1 cells. Pro-inflammatory cytokines [interleukin-beta (IL-1beta), IL-6, IL-8, IL-10, IL-12p70 and tumour necrosis factor-alpha (TNF-alpha)] were measured by multiplexed cytometric bead array assay over a period of 18 days. We observed that pro-inflammatory cytokine secretion was augmented after C. trachomatis infection in HeLa and THP-1 cells. However, this heightened secretion was subsequently reduced. When infected HeLa cells were co-cultured with THP-1 cells, IL-6 and IL-8 secretion was sustained, IL-1beta expression followed a bell-shaped curve and IL-10, IL-12p70 and TNF-alpha synthesis was down regulated. IL-6 and IL-8 may be involved in the immunopathogenesis of chronic chlamydial infections. We also observed that throughout C. trachomatis persistence induced by doxycycline (Dox) treatment, IL-1beta, IL-6, IL-8 and TNF-alpha expression was reduced, whereas the synthesis of IL-10 and IL-12p70 remained unchanged but not sustained. Thus, during chlamydial persistence infection evoked by treatment with Dox, none of the tested cytokines showed sustained expression.