O509 2-hour Oral Session Frontiers in tuberculosis The role of microRNAs in tuning the immune response in Mycobacterium tuberculosis infection Joyoti Basu*1 1Bose Institute, Chemistry, Kolkata, India Background: The role of post-transcriptional regulatory mechanisms in calibrating the immune response of macrophages infected with Mycobacterium tuberculosis (Mtb), remains poorly understood. We have performed global miRNA and transcriptome profiling to build an miRNA – mRNA network in Mtb- infected macrophages, and to understand its impact on the immune response to infection. Material/methods: We have performed global qRT-PCR profiling of miRNAs and microarray analysis of mRNAs to profile the transcriptome of macrophages infected with Mtb. Bioinformatic analyses has been performed using the tool MAGIA, to build an miRNA-mRNA network. We have then focused on selected miRNAs to analyze their effects on the immune response by analyzing pathways associated with inflammation and autophagy. Results: During Mtb infection, the let-7 miRNAs appeared as highly connected nodes of the miRNA- mRNA network. We establish that A 20 (TNFAIP3), a negative regulator of NF- B signaling is a target of let- 7. Augmented release of TNF-alfa, IL-1beta, CCL2, CXCL1, IL-6 and NO occurs in Mtb- infected macrophages in the absence of A20. During infection, downregulation of let -7f leads to concomitant rise in A20, likely tuning down inflammatory mediators. Transfection with a let -7f mimic, leads to increased cytokine release after infection. In addition to let-7f, miR-17 family members are also downregulated during Mtb infection of macrophages. We show that forced expression of miR-17 enhances autophagy during Mtb infection. We confirm that miR-17 enhances autophagy by targeting two negative regulators, namely Mcl-1 which sequesters Beclin-1 and its transcriptional activator, STAT3. Both let-7f and miR-17 levels decrease during progression of infection in vivo in mice. Conclusions: We propose that the early decrease in let-7f following infection, offers an advantage to the pathogen as assessed by CFU counts in the absence or presence of let-7f mimic. We hypothesize that by downregulating miR-17, Mtb suppresses autophagy during infection. Our results encourage further investigation into the promise of miRNA mimics or antagonists in adjunctive therapy of tuberculosis. .