TLR4/CD14/MD2 Revealed as the Limited Toll-like Receptor Complex for Chlamydia trachomatis -Induced NF-κB Signaling.

Chlamydia trachomatis (Ct) is the most common cause of genital tract infections as well as preventable blindness worldwide. Pattern recognition receptors such as toll-like receptors (TLRs) represent the initial step in recognizing pathogenic microorganisms and are crucial for the initiation of an appropriate immune response. However, our understanding of TLR-signaling in Chlamydia -infected immune cells is incomplete. For a better comprehension of pathological inflammatory responses, robust models for interrogating TLR-signaling upon chlamydial infections are needed. To analyze the TLR response, we developed and utilized a highly sensitive and selective fluorescent transcriptional cellular reporter system to measure the activity of the transcription factor NF-κB. Upon incubation of the reporter cells with different preparations of Ct , we were able to pinpoint which components of TLRs are involved in the recognition of Ct . We identified CD14 associated with unique characteristics of different serovars as the crucial factor of the TLR4/CD14/MD2 complex for Ct -mediated activation of the NF-κB pathway. Furthermore, we found the TLR4/CD14/MD2 complex to be decisive for the uptake of Ct -derived lipopolysaccharides but not for infection and replication of Ct . Imaging flow cytometry provided information about inclusion formation in myeloid- as well as lymphocytic cells and was highest for Ct L2 with at least 25% of inclusion forming cells. Ct E inclusion formation was eminent in Jurkat cells without CD14 expression (11.1%). Thus, our model enables to determine Ct uptake and signal induction by pinpointing individual components of the recognition and signaling pathways to better understand the immune response towards infectious pathogens.