| Summary: | Several pathogens, such as Rickettsia, have developed different tactics to evade immune responses in order to subsist and proliferate within the host. Rickettsial species are resistant to the serum bactericidal effects and can evade complement-mediated killing, suggesting that they developed mechanisms to inhibit recognition by host serum components. Interestingly, many pathogenic bacteria have also been shown to use non-immune immunoglobulin (Ig)-binding proteins to avoid recognition by innate immune serum components. With this work, we provide evidence for a novel Ig-binding protein from Rickettsia, anticipating additional immune-evasion tactics for this obligate pathogen. We demonstrate that the rickettsial retropepsin APRc is capable of binding to immunoglobulins from different species and classes. Also, our results suggest that the interaction of APRc with human IgG promotes the oligomeric stabilization of APRc. Mapping the APRc region responsible for binding revealed the segment between amino acids 150-166 as one of the interacting regions. Moreover, we demonstrate that the interaction occurs in the Fab domain but does not entail IgG cleavage. We also demonstrate IgG-APRc binding in serum samples. Finally, we demonstrate IgG-binding at the surface of Rickettsia. APRc-IgG binding and its localization at Rickettsia's outer membrane anticipate APRc as one of the proteins contributing to that activity. Altogether, we propose that APRc may act as a novel evasin by playing a role in protecting Rickettsia from complement-mediated killing through this non immune IgG-binding activity. Since Rickettsia are arthropod-borne pathogensthat can originate severe diseases in humans, there is a growing concern about the increasing incidence of rickettsioses and their impact on global health. Therefore, with the lack of reliable protective vaccines for rickettsial diseases, the development of alternative therapeutics is critical. With this work, we provide valuable information for a deeper understanding of the molecular mechanisms underlying rickettsial pathogenesis, contributing to the future development of alternative therapeutics.
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