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The complement system plays a critical role in the innate immune response, acting as a first line of defense against invading pathogens. However, dysregulation of the complement system is implicated in the pathogenesis of numerous diseases, ranging from Alzheimer's to age-related macular degeneration and rare blood disorders. As such, complement inhibitors have enormous potential to alleviate disease burden. While a few complement inhibitors are in clinical use, there is still a significant unmet medical need for the discovery and development of novel inhibitors to treat patients suffering from disorders of the complement system. A key hurdle in the development of complement inhibitors has been the determination of their mechanism of action. Progression along the complement cascade involves the formation of numerous multimeric protein complexes, creating the potential for inhibitors to act at multiple nodes in the pathway. This is especially true for molecules that target the central component C3 and its fragment C3b, which serve a dual role as a substrate for the C3 convertases and as a scaffolding protein in both the C3 and C5 convertases. Here, we report a step-by-step in vitro reconstitution of the complement alternative pathway using bio-layer interferometry. By physically uncoupling each step in the pathway, we were able to determine the kinetic signature of inhibitors that act at single steps in the pathway and delineate the full mechanism of action of known and novel C3 inhibitors. The method could have utility in drug discovery and further elucidating the biochemistry of the complement system., Competing Interests: Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Chlamydia trachomatis is an obligate intracellular pathogen that causes sexually transmitted disease. In women, chlamydial infections may cause pelvic inflammatory disease (PID), ectopic pregnancy, and infertility. The role of antibodies in protection against a primary Chlamydia infection is unclear and was a focus of this work. Using the C. muridarum mouse infection model, we show that intestinal mucosa is infected via intranasal (i.n.) or per-oral (p.o.) Chlamydia inoculation and that unlike the female reproductive tract (FRT) mucosa, it halts systemic Chlamydia dissemination. Moreover, p.o. immunization or infection with Chlamydia confers protection against per-vaginal (p.v.) challenge, resulting in significantly decreased bacterial burden in the FRT, accelerated Chlamydia clearance, and reduced hydrosalpinx pathology. In contrast, subcutaneous (s.c.) immunization conferred no protection against the p.v. challenge. Both p.o. and s.c. immunizations induced Chlamydia -specific serum IgA. However, IgA was found only in the vaginal washes and fecal extracts of p.o.-immunized animals. Following a p.v. challenge, unimmunized control and s.c.-s.c.-immunized animals developed Chlamydia -specific intestinal IgA yet failed to develop IgA in the FRT, indicating that IgA response in the FRT relies on the FRT to gastrointestinal tract (GIT) antigen transport. Vaginal secretions of p.o.-immunized animals neutralize Chlamydia in vivo , resulting in significantly lower Chlamydia burden in the FRT and Chlamydia transport to the GIT. We also show that infection of the GIT is not necessary for induction of protective immunity in the FRT, a finding that is important for the development of p.o. subunit vaccines to target Chlamydia and possibly other sexually transmitted pathogens., (Copyright © 2020 American Society for Microbiology.)

Chlamydia trachomatis is a Gram-negative bacterial pathogen and a major cause of sexually transmitted disease and preventable blindness. In women, infections with C. trachomatis may lead to pelvic inflammatory disease (PID), ectopic pregnancy, chronic pelvic pain, and infertility. In addition to infecting the female reproductive tract (FRT), Chlamydia spp. are routinely found in the gastro-intestinal (GI) tract of animals and humans and can be a reservoir for reinfection of the FRT. Whether Chlamydia disseminates from the FRT to the GI tract via internal routes remains unknown. Using mouse-specific C. muridarum as a model pathogen we show that Chlamydia disseminates from the FRT to the GI tract in a stepwise manner, by first infecting the FRT-draining iliac lymph nodes (ILNs), then the spleen, then the GI tract. Tissue CD11c+ DCs mediate the first step: FRT to ILN Chlamydia transport, which relies on CCR7:CCL21/CCL19 signaling. The second step, Chlamydia transport from ILN to the spleen, also relies on cell transport. However, this step is dependent on cell migration mediated by sphingosine 1-phosphate (S1P) signaling. Finally, spleen to GI tract Chlamydia spread is the third critical step, and is significantly hindered in splenectomized mice. Inhibition of Chlamydia dissemination significantly reduces or precludes the induction of Chlamydia-specific serum IgG antibodies, presence of which is correlated with FRT pathology in women. This study reveals important insights in context of Chlamydia spp. pathogenesis and will inform the development of therapeutic targets and vaccines to combat this pathogen., Competing Interests: The authors have declared that no competing interests exist.