Chlamydia trachomatisis the most common sexually transmitted bacterial pathogen and the etiologic agent of blinding trachoma. Intracellular signaling pathways leading to host cell inflammation and innate immunity toChlamydiainclude those mediated by Toll-like receptors (TLRs) and nucleotide binding oligomerization domain 1 (Nod1) protein. In epithelial cells, TLR-dependent signaling contributes to local immune responses via induction of inflammatory mediators. There is evidence that TLR3, TLR4, and, particularly, TLR2 are critical forChlamydia-mediated host cell activation and pathology. Despite the importance of TLR2, major chlamydial TLR2 antigens have not been identified so far. Numerous bacterial porins are known TLR2 agonists, i.e., porins fromNeisseriae,Shigella,Salmonella,Haemophilus influenzae, andFusobacterium nucleatum, which share structural and functional similarities with the chlamydial major outer membrane protein (MOMP), a strong antigen candidate for a potential vaccine againstC. trachomatis. We describe the ability of purified, detergent-free MOMP to signal via TLR2in vitroin TLR-overexpressing cells and TLR2-competent human reproductive tract epithelial cell lines. Using MOMP formed in pure protein micelles (proteosomes), we show the induction of TLR2-dependent interleukin-8 (IL-8) and IL-6 secretionin vitro, the involvement of TLR1 as a TLR2 coreceptor, and the activation of both NF-κB and mitogen-activated protein (MAP) kinase intracellular pathways. Interestingly, MOMP proteosomes induce cytokine secretion in endocervical epithelial cells (End/E6E7) but not in urethral epithelial cells (THUECs). A detailed understanding of the TLR2-dependent molecular mechanisms that characterize the effect of MOMP proteosomes on host cells may provide new insights for its successful development as an immunotherapeutic target againstChlamydia.
A recombinant Chlamydia trachomatis major outer membrane protein binds to heparan sulfate receptors on epithelial cells.