Research Article: Survival and death of intestinal cells infected by Chlamydia trachomatis

Date Published: April 26, 2019

Publisher: Public Library of Science

Author(s): Claudio Foschi, Massimo Bortolotti, Giacomo Marziali, Letizia Polito, Antonella Marangoni, Andrea Bolognesi, David M. Ojcius.


The sexually transmitted pathogen Chlamydia trachomatis (CT) is able to replicate and survive in human intestinal epithelial cells, being the gastro-intestinal tract a suitable site of residence for this microorganism. In this context, no detailed information about the mechanisms of cell death in intestinal cell lines after a chlamydial infection is available. The aim of this study was to compare the effect of two different CT serovars (D and L2) on the survival/death of different intestinal cell lines (Caco-2 and COLO-205), using endocervical cells (HeLa) as a reference model of genital infection. Seventy two hours after chlamydial infection at different multiplicity of infection (MOI) levels, the viability of HeLa, Caco-2 and COLO 205 cells was evaluated through dose-response experiments by means of a MTS-based assay. To get deeper insights in the mechanisms of cell death induced by CT, cell viability was assessed in presence of different inhibitors (i.e. pan-caspase inhibitor Z-VAD, necroptosis inhibitor Necrostatin-1, hydrogen peroxide scavenger catalase, caspase-1 inhibitor Ac-YVAD-cmk). Moreover, the activation of effector caspases and the presence of cellular apoptotic/necrotic changes were evaluated at different time points after CT infection. Our results demonstrated that, for both chlamydial serovars, intestinal cell lines are more resistant to CT-induced cell death compared to HeLa, thus representing a suitable ‘niche’ for chlamydial residence and replication. In literature, apoptosis has been widely described to be the main cell death mechanism elicited by chlamydia infection. However, our data demonstrate that necroptosis plays a relevant role, proceeding in parallel with apoptosis. The protective effect of catalase suggests the involvement of oxidative stress in triggering both cell death pathways. Moreover, we demonstrated that caspase-1 is involved in CT-induced cell death, potentially contributing to host inflammatory response and tissue damage. Cells infected by L2 serovar displayed a higher activation of effector caspases compared to cells infected with serovar D, suggesting a serovar-specific activation of apoptotic pathways and potentially explaining the greater virulence of L serovars. Finally, we found that Chlamydia elicits the early externalization of phosphatidylserine on the external leaflet of plasma membrane independently of caspase activation.

Partial Text

Chlamydia trachomatis (CT) is the causative agent of the most common bacterial sexually transmitted infection (STI), worldwide, with a relevant clinical and economic impact [1]. CT serovars from D to K are responsible of common uro-genital infections (i.e. urethritis and cervicitis) and can potentially lead to several sequelae and complications, including pelvic inflammatory disease (PID), tubal infertility and epididymo-orchitis [2]. Notably, CT can be found also at extra-genital sites, as pharyngeal and rectal mucosa, especially in women and ‘men having sex with men’ (MSM) [3]. Specific distinct CT serovars (L1-L3) are associated with lymphogranuloma venereum (LGV), emerging in Europe and North America as a leading cause of proctitis and proctocolitis in MSM, in particular in HIV-positive patients [4].

In the last years, much attention has been paid to the interaction between Chlamydia trachomatis and the gastro-intestinal tract. Animal models of infection and epidemiological studies suggest that the gastrointestinal tract can act as a reservoir of chlamydiae, that can lead to repeat urogenital infections [16–18]. Indeed, it has been shown that human intestinal epithelial cells (Caco-2 cells) support C. trachomatis replication with a significantly high infectivity [17, 19]. Nevertheless, no detailed information about the mechanisms of cell survival and death in intestinal cell lines after chlamydial infection are available. Thus, we investigated the cytotoxic effect of C. trachomatis (serovars D and L2) on different human intestinal cell lines (Caco-2 and COLO-205), using endocervical cells (HeLa cells) as a reference model of genital infection.