New mechanism of immune evasion by Chlamydia trachomatis in mesenchymal stem cells

Chlamydia trachomatis is a Gram-negative obligate intracellular bacterial pathogen. It is associated with significant human illness, including preventable blindness and arthritis. It is considered the most common agent of sexually transmitted diseases worldwide with serious consequences in women that include pelvic inflammatory disease, ectopic pregnancy and sterility. As an obligate intracellular pathogen, Chlamydia relies on the host for its survival, starting from attachment to and invasion into the host cell, establishing a niche for replication, hijacking cellular pathways, and evading host innate defense mechanisms. It is well known that production of toxic nitric oxide (NO) by host cells is considered an effective antimicrobial defense mechanism. However, how NO induces chlamydial growth arrest is still largely unknown. Similarly, little is known about the mechanisms underlying subversion of cellular innate immunity by C. trachomatis. Therefore, the aim of this study was to unveil mechanisms by which Chlamydia subverts the NO innate immune response. Work presented in this thesis demonstrates that chlamydial infection led to downregulated expression of inducible nitric oxide synthase (iNOS) in mesenchymal stem cells (MSCs). Further, infection with Chlamydia upregulated the expression of the rate-limiting enzyme in the polyamine biosynthetic pathway, ornithine decarboxylase (ODC). This upregulation subsequently enhanced the synthesis of polyamines which paved the way for establishing a successful intracellular bacterial niche. Chemical inhibition of ODC activity using the competitive inhibitor difluoromethylornithine (DFMO) restored iNOS protein expression in infected cells. Further, iNOS activity increased in infected cells upon DFMO treatment as determined by detectable intracellular nitric oxide (NO) and the extracellular NO stable end product, nitrite. NO production significantly inhibited chlamydial growth in DFMO treated cells. This inhibition was mediated through the process of tyrosine nitration of chlamydial proteins by the action of peroxynitrite, an NO metabolite. In this context, the increased level of tyrosine nitration was specifically detected in chlamydial Hsp60 in response to DFMO treatment. Based on the present findings, activation of the polyamine synthetic pathway strongly prevented the antimicrobial effect of NO by inhibiting iNOS protein expression in C. trachomatis-infected MSCs. These findings have revealed a novel strategy of Chlamydia to avoid an innate immune mechanism in MSCs, via induction of the polyamine biosynthetic pathway, which inhibits NO production permitting intracellular survival.