Upon recognition of viral components by pattern recognition receptors, including TLRs and retinoic acid-inducible gene I (RIG-I)- like helicases, cells are activated to produce type I IFN and proinflammatory cytokines. These pathways are tightly regulated by host to prevent inappropriate cellular response, but viruses can down-regulate these pathways for their survival. Recently, identification of negative regulators for cytoplasmic RNA-mediated antiviral signaling, especially the RIG-I pathway, attract much attention. However, there is no report about negative regulation of RIG-I antiviral pathway by microRNAs (miRNA) to date. We found that vesicular stomatitis virus (VSV) infection up-regulated miR-146a expression in mouse macrophages in TLR-myeloid differentiation factor 88-independent but RIG-I-NF-κB-dependent manner. In turn, miR-146a negatively regulated VSV-triggered type I IFN production, thus promoting VSV replication in macrophages. In addition to two known miR-146a targets, TRAF6 and IRAK1, we proved that IRAK2 was another target of miR-146a, which also participated in VSV-induced type I IFN production. Furthermore, IRAK1 and IRAK2 participated in VSV-induced type I IFN production by associating with Fas-associated death domain protein, an important adaptor in RIG-I signaling, in a VSV infection-inducible manner. Therefore, we demonstrate that miR-146a, up-regulated during viral infection, is a negative regulator of the RIG-I-dependent antiviral pathway by targeting TRAF6, IRAK1, and IRAK2.

Interferon-γ (IFN-γ) has a critical role in immune responses to intracellular bacterial infection. MicroRNAs (miRNAs) are important in the regulation of innate and adaptive immunity. However, whether miRNAs can directly target IFN-γ and regulate IFN-γ production post-transcriptionally remains unknown. Here we show that infection of mice with Listeria monocytogenes or Mycobacterium bovis bacillus Calmette-Guérin (BCG) downregulated miR-29 expression in IFN-γ-producing natural killer cells, CD4(+) T cells and CD8(+) T cells. Moreover, miR-29 suppressed IFN-γ production by directly targeting IFN-γ mRNA. We developed mice with transgenic expression of a 'sponge' target to compete with endogenous miR-29 targets (GS29 mice). We found higher serum concentrations of IFN-γ and lower L. monocytogenes burdens in L. monocytogenes-infected GS29 mice than in their littermates. GS29 mice had enhanced T helper type 1 (T(H)1) responses and greater resistance to infection with BCG or Mycobacterium tuberculosis. Therefore, miR-29 suppresses immune responses to intracellular pathogens by targeting IFN-γ.

Effective recognition of viral infection and subsequent triggering of antiviral innate immune responses are essential for the host antiviral defense, which is tightly regulated by multiple regulators, including microRNAs. Our previous study showed that a panel of microRNAs, including miR-155, was markedly upregulated in macrophages upon vesicular stomatitis virus infection; however, the biological function of miR-155 during viral infection remains unknown. In this paper, we show that RNA virus infection induces miR-155 expression in macrophages via TLR/MyD88-independent but retinoic acid-inducible gene I/JNK/NF-κB–dependent pathway. And the inducible miR-155 feedback promotes type I IFN signaling, thus suppressing viral replication. Furthermore, suppressor of cytokine signaling 1 (SOCS1), a canonical negative regulator of type I IFN signaling, is targeted by miR-155 in macrophages, and SOCS1 knockdown mediates the enhancing effect of miR-155 on type I IFN-mediated antiviral response. Therefore, we demonstrate that inducible miR-155 feedback positively regulates host antiviral innate immune response by promoting type I IFN signaling via targeting SOCS1.

SUMMARY Zika virus (ZIKV) has become a public health threat due to its global transmission and link to severe congenital disorders. The host immune responses to ZIKV infection have not been fully elucidated, and effective therapeutics are not currently available. Herein, we demonstrated that cholesterol-25-hydroxylase (CH25H) was induced in response to ZIKV infection and that its enzymatic product, 25-hydroxycholesterol (25HC), was a critical mediator of host protection against ZIKV. Synthetic 25HC addition inhibited ZIKV infection in vitro by blocking viral entry, and treatment with 25HC reduced viremia and conferred protection against ZIKV in mice and rhesus macaques. 25HC suppressed ZIKV infection and reduced tissue damage in human cortical organoids and the embryonic brain of the ZIKV-induced mouse microcephaly model. Our findings highlight the protective role of CH25H during ZIKV infection and the potential use of 25HC as a natural antiviral agent to combat ZIKV infection and prevent ZIKV-associated outcomes, such as microcephaly.

Leisure activities in old age may protect against cognitive decline for both women and men, and different types of activities seem to benefit different cognitive domains.

Myeloid-derived suppressor cells (MDSCs) potently suppress the anti-tumor immune responses and also orchestrate the tumor microenvironment that favors tumor angiogenesis and metastasis. The molecular networks regulating the accumulation and functions of tumor-expanded MDSCs are largely unknown. In this study, we identified microRNA-494 (miR-494), whose expression was dramatically induced by tumor-derived factors, as an essential player in regulating the accumulation and activity of MDSCs by targeting of phosphatase and tensin homolog (PTEN) and activation of the Akt pathway. TGF-β1 was found to be the main tumor-derived factor responsible for the upregulation of miR-494 in MDSCs. Expression of miR-494 not only enhanced CXCR4-mediated MDSC chemotaxis but also altered the intrinsic apoptotic/survival signal by targeting of PTEN, thus contributing to the accumulation of MDSCs in tumor tissues. Consequently, downregulation of PTEN resulted in increased activity of the Akt pathway and the subsequent upregulation of MMPs for facilitation of tumor cell invasion and metastasis. Knockdown of miR-494 significantly reversed the activity of MDSCs and inhibited the tumor growth and metastasis of 4T1 murine breast cancer in vivo. Collectively, our findings reveal that TGF-β1–induced miR-494 expression in MDSCs plays a critical role in the molecular events governing the accumulation and functions of tumor-expanded MDSCs and might be identified as a potential target in cancer therapy.

The molecular mechanisms involved in the full activation of innate immunity achieved through Toll-like receptors (TLRs) remain to be fully elucidated. In addition to their classical antigen-presenting function, major histocompatibility complex (MHC) class II molecules might mediate reverse signaling. Here we report that deficiency in MHC class II attenuated the TLR-triggered production of proinflammatory cytokines and type I interferon in macrophages and dendritic cells, which protected mice from endotoxin shock. Intracellular MHC class II molecules interacted with the tyrosine kinase Btk via the costimulatory molecule CD40 and maintained Btk activation, but cell surface MHC class II molecules did not. Then, Btk interacted with the adaptor molecules MyD88 and TRIF and thereby promoted TLR signaling. Therefore, intracellular MHC class II molecules can act as adaptors, promoting full activation of TLR-triggered innate immune responses.

MicroRNAs (miRNAs) are involved in the regulation of immunity, including the lymphocyte development and differentiation, and inflammatory cytokine production. Dendritic cells (DCs) play important roles in linking innate and adaptive immune responses. However, few miRNAs have been found to regulate the innate response and APC function of DCs to date. Calcium/calmodulin-dependent protein kinase II (CaMKII), a major downstream effector of calcium (Ca2+), has been shown to be an important regulator of the maturation and function of DCs. Our previous study showed that CaMKIIα could promote TLR-triggered production of proinflammatory cytokines and type I IFN. Inspired by the observations that dicer mutant Drosophila display defect in endogenous miRNA generation and higher CaMKII expression, we wondered whether miRNAs can regulate the innate response and APC function of DCs by targeting CaMKIIα. By predicting with software and confirming with functional experiments, we demonstrate that three members of the miRNA (miR)-148 family, miR-148a, miR-148b, and miR-152, are negative regulators of the innate response and Ag-presenting capacity of DCs. miR-148/152 expression was upregulated, whereas CaMKIIα expression was downregulated in DCs on maturation and activation induced by TLR3, TLR4, and TLR9 agonists. We showed that miR-148/152 in turn inhibited the production of cytokines including IL-12, IL-6, TNF-α, and IFN-β upregulation of MHC class II expression and DC-initiated Ag-specific T cell proliferation by targeting CaMKIIα. Therefore, miRNA-148/152 can act as fine-tuner in regulating the innate response and Ag-presenting capacity of DCs, which may contribute to the immune homeostasis and immune regulation.