Recent advances in transcriptome analyses revealed that thousands of non- coding RNAs, especially long non-coding (lnc) RNAs, are transcribed in mammalian genomes and are involved in various aspects of biological processes. In innate immune cells such as macrophages, lipopolysaccharide (LPS)—ligand for Toll-like receptor (TLR)-4—can strongly and rapidly induce lncRNA transcription as well as inflammatory cytokine transcription, such as tumor necrosis factor-a (TNFa) and interleukin (IL)-6. In this study, we identified a novel lncRNA, lncRNA activator for secondary response cytokine (LASC), which is essential for inflammatory cytokine expression. LASC is composed of 3 exons and is 1.8 kbp in length. Ligands for TLRs such as LPS, R848, and CpG, but not inflammatory cytokines such as TNFa and IL-1b, can strongly induce LASC expression within 3 hours after stimulation by LPS in mouse macrophages. RNA fluorescence in situ hybridization revealed that LASC largely localizes in the cytoplasm. Silencing LASC by shRNA showed decreased expression of IL-6 and IL-1b mRNAs and proteins upon LPS stimulation in the mouse macrophage cell lines RAW264.7 and J774.1. To elucidate global change in mRNA expression by LASC knockdown, we revealed that approximately 200 genes, including IL-6, GM- CSF, Lcn2, and MMP13, were reduced in LASC-knockdown cells, using a microarray analysis. TNFa and IFNb expression was also modestly decreased. C/EBPb and IkBz, which are essential inducible co-activators for IL-6 and Lcn2, were normal in both control and LASC-knockdown cells after exposure to LPS. In addition, a restriction enzyme accessibility assay showed that LPS-induced chromatin remodeling was retained at the IL-6 locus in both control and LASC- knockdown cells. However, mobilization of RNA polymerase II at the transcriptional start sites of IL-6 and GM-CSF was severely impaired by LASC silencing. These results suggest that LASC regulates inflammatory cytokine expression at the transcriptional level. To investigate its in vivo function, we generated LASC-knockout mice harboring the SV40 late poly A signal sequence at the LASC locus, using CRISPR/Cas9, and confirmed that LASC expression is abrogated in antigen-presenting cells derived from LASC-knockout mice. Furthermore, LASC-knockout mice showed significantly prolonged survival after administration of a lethal dose of LPS. In contrast, LASC KO mice showed highly susceptibility to DSS-induced colitis. In conclusion, LASC, a cytoplasmic lncRNA induced by TLR ligands, plays a crucial role in inflammatory cytokine expression and involves systemic and local inflammation in vivo.