Role of RNA-binding protein in immune evasion of Mtb in macrophages

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Project Summary/Abstract Tuberculosis (TB) remains a leading global health problem with about 9 million new cases and nearly 1.5 million TB-related deaths worldwide each year. Mycobacterium tuberculosis (Mtb) infection persists in macrophages when the host immune response is suppressed or evaded. However, mechanisms of TB immune evasion remain incomplete. Better understanding the mechanisms will facilitate development of host-directed therapy (HDT), especially against deadly drug-resistant Mtb. Our preliminary data indicate that an RNA-binding protein, tristetraprolin (TTP) facilitates evasion and can be a target for host-directed TB therapy. TTP is an RNA -binding protein involved in the regulation of inflammatory responses at the post-transcriptional level. TTP binds to adenine-uridine-rich elements (AREs) within the 3’ untranslated region (3'UTR) causing destabilization of mRNAs encoding several cytokines important for Mtb clearance, and has been shown by us and others to be an important regulator maintaining homeostasis. Whether TTP affects Mtb replication or reactivation, however, remains unknown. We found for the first time a significant TTP induction in lungs of Mtb-infected wild type mice and in macrophages infected with mycobacteria, and deleting TTP resulted in suppression of mycobacterial growth in macrophages. Further, mice deficient in TTP challenged with live Mtb had significantly less bacteria in lungs than infected wild type littermates. TTP mRNA levels were significantly higher in caseous granulomas of TB patients than in normal lung parenchyma. Reactivation and transmission of TB involve developing caseous granulomas that cavitate and release Mtb. In addition, our data show for the first time that inhibition of mTOR signaling by rapamycin almost completely abolishes TTP induction in Mtb-infected macrophages. Based on our findings, we propose a novel role for mTOR/TTP axis in TB immune evasion: Virulent Mtb activates mTOR pathway that induces TTP, resulting in suppression of anti-TB immunity, Mtb replicates and primary and/or latent reactivation diseases. TTP may serve as a target to reverse immune evasion of Mtb. We propose two aims to test this novel hypothesis: Aim 1. Determine the role of TTP in macrophages for mTOR-mediated Mtb growth and the effects of targeting TTP on Mtb growth for potential host-directed therapy. Aim 2. Explore the molecular mechanisms of TTP induction by Mtb in macrophages. Identification of the mechanisms of TTP induction will not only enhance our understanding of Mtb evasion mechanisms, but also provide targets that can be used to develop host-directed therapy breaking TB evasion. Results of these exploratory and mechanistic studies will provide a better understanding of TB immune evasion mechanisms and potentially lead to TTP-targeted therapy, especially with drug-resistant Mtb.