Abstract
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.
Highlights
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]
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 [16,17,18]
Summary
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]. 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]. CT is an obligate intracellular pathogen, able to enter and replicate into different cellular targets, as endocervical and intestinal epithelial cells. EBs enter the mucosal cells and differentiate into RBs in a membrane bound compartment, called inclusion. After several rounds of replication, RBs start to re-differentiate into EBs and are released from the host cell, ready to infect neighbouring cells [5, 6]
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