Commensal Microbiota Modulation of Natural Resistance to Virus Infection.

Interferon (IFN)-Is are crucial mediators of antiviral immunity and homeostatic immune system regulation. However, the source of IFN-I signaling under homeostatic conditions is unclear. We discovered that commensal microbes regulate the IFN-I response through induction of IFN-ß by colonic DCs. Moreover, the mechanism by which a specific commensal microbe induces IFN-ß was identified. Outer membrane (OM)-associated glycolipids of gut commensal microbes belonging to the Bacteroidetes phylum induce expression of IFN-ß. Using Bacteroides fragilis and its OM-associated polysaccharide A, we determined that IFN-ß expression was induced via TLR4-TRIF signaling. Antiviral activity of this purified microbial molecule against infection with either vesicular stomatitis virus (VSV) or influenza was demonstrated to be dependent on the induction of IFN-ß. In a murine VSV infection model, commensal-induced IFN-ß regulated natural resistance to virus infection. Due to the physiological importance of IFN-Is, discovery of an IFN-ß-inducing microbial molecule represents a potential approach for the treatment of some human diseases.

Sensing Self and Foreign Circular RNAs by Intron Identity.

Circular RNAs (circRNAs) are single-stranded RNAs that are joined head to tail with largely unknown functions. Here we show that transfection of purified in vitro generated circRNA into mammalian cells led to potent induction of innate immunity genes and confers protection against viral infection. The nucleic acid sensor RIG-I is necessary to sense foreign circRNA, and RIG-I and foreign circRNA co-aggregate in cytoplasmic foci. CircRNA activation of innate immunity is independent of a 5' triphosphate, double-stranded RNA structure, or the primary sequence of the foreign circRNA. Instead, self-nonself discrimination depends on the intron that programs the circRNA. Use of a human intron to express a foreign circRNA sequence abrogates immune activation, and mature human circRNA is associated with diverse RNA binding proteins reflecting its endogenous splicing and biogenesis. These results reveal innate immune sensing of circRNA and highlight introns-the predominant output of mammalian transcription-as arbiters of self-nonself identity.

The transcription factor Foxo1 controls central-memory CD8+ T cell responses to infection.

Memory T cells protect hosts from pathogen reinfection, but how these cells emerge from a pool of antigen-experienced T cells is unclear. Here, we show that mice lacking the transcription factor Foxo1 in activated CD8+ T cells have defective secondary, but not primary, responses to Listeria monocytogenes infection. Compared to short-lived effector T cells, memory-precursor T cells expressed higher amounts of Foxo1, which promoted their generation and maintenance. Chromatin immunoprecipitation sequencing revealed the transcription factor Tcf7 and the chemokine receptor Ccr7 as Foxo1-bound target genes, which have critical functions in central-memory T cell differentiation and trafficking. These findings demonstrate that Foxo1 is selectively incorporated into the genetic program that regulates memory CD8+ T cell responses to infection.

Novel Foxo1-dependent transcriptional programs control T(reg) cell function.

Regulatory T (T(reg)) cells, characterized by expression of the transcription factor forkhead box P3 (Foxp3), maintain immune homeostasis by suppressing self-destructive immune responses. Foxp3 operates as a late-acting differentiation factor controlling T(reg) cell homeostasis and function, whereas the early T(reg)-cell-lineage commitment is regulated by the Akt kinase and the forkhead box O (Foxo) family of transcription factors. However, whether Foxo proteins act beyond the T(reg)-cell-commitment stage to control T(reg) cell homeostasis and function remains largely unexplored. Here we show that Foxo1 is a pivotal regulator of T(reg )cell function. T(reg) cells express high amounts of Foxo1 and display reduced T-cell-receptor-induced Akt activation, Foxo1 phosphorylation and Foxo1 nuclear exclusion. Mice with T(reg)-cell-specific deletion of Foxo1 develop a fatal inflammatory disorder similar in severity to that seen in Foxp3-deficient mice, but without the loss of T(reg) cells. Genome-wide analysis of Foxo1 binding sites reveals ~300 Foxo1-bound target genes, including the pro-inflammatory cytokine Ifng, that do not seem to be directly regulated by Foxp3. These findings show that the evolutionarily ancient Akt-Foxo1 signalling module controls a novel genetic program indispensable for T(reg) cell function.

miR-182 is largely dispensable for adaptive immunity: lack of correlation between expression and function.

MicroRNA (miR)-mediated regulation of protein abundance is a pervasive mechanism of directing cellular processes. The well-studied and abundant miR-182 has previously been implicated in many aspects of T cell function, DNA repair, and cancer. In this study, we show that miR-182 is the most highly induced miR in B cells undergoing class-switch recombination. To elucidate the requirement of miR-182 in lymphocyte function, we extensively characterized mice with a targeted deletion of Mir182. We show that despite its dramatic induction, loss of miR-182 has minimal impact on B cell development, the ability of B cells to undergo class-switch recombination ex vivo and to undergo Ag-driven affinity maturation in vivo. Furthermore, in striking contrast to knockdown studies that demonstrated the requirement of miR-182 in T cell function, miR-182-deficient mice display no defect in T cell development and activation. Finally, we show that T cell-dependent immune response to experimental Listeria monocytogenes infection is intact in miR-182-deficient mice. We conclude that, contrary to previous studies, miR-182 does not play a significant role in all measured aspects of mouse adaptive immunity. This striking absence of a phenotype highlights the lack of correlation between expression pattern and functional requirement, underscores the limitations of using knockdown approaches to assess miR requirements, and suggests that miR networks may compensate for the chronic loss of specific miRs.

Topical application of aminoglycoside antibiotics enhances host resistance to viral infections in a microbiota-independent manner.

Antibiotics are widely used to treat infections in humans. However, the impact of antibiotic use on host cells is understudied. Here we identify an antiviral effect of commonly used aminoglycoside antibiotics. We show that topical mucosal application of aminoglycosides prophylactically increased host resistance to a broad range of viral infections including herpes simplex viruses, influenza A virus and Zika virus. Aminoglycoside treatment also reduced viral replication in primary human cells. This antiviral activity was independent of the microbiota, because aminoglycoside treatment protected germ-free mice. Microarray analysis uncovered a marked upregulation of transcripts for interferon-stimulated genes (ISGs) following aminoglycoside application. ISG induction was mediated by Toll-like receptor 3, and required Toll/interleukin-1-receptor-domain-containing adapter-inducing interferon-ß signalling adaptor, and Interferon regulatory factors 3 and 7, transcription factors that promote ISG expression. XCR1+ dendritic cells, which uniquely express Toll-like receptor 3, were recruited to the vaginal mucosa upon aminoglycoside treatment and were required for ISG induction. These results highlight an unexpected ability of aminoglycoside antibiotics to confer broad antiviral resistance in vivo.

Murine In vitro Memory T Cell Differentiation.

Upon pathogen encounter, naïve CD8+ T cells are primed and undergo massive clonal expansion. A fraction of effector CD8+ T cells remains during the contraction phase and differentiate into memory T cells critical for mounting robust recall responses in response to secondary infection. Low frequency of memory T cells in vivo is a major obstacle to investigate their functional aspects including migration capacity and genetic regulation. Here, we describe detailed protocol for memory T cell differentiation developed by von Andrian's group to generate large number of CD44hiCD62Lhi antigen-specific memory T cells in vitro.

Murine in vivo CD8+ T Cell Killing Assay.

Antigen-specific killing ability of effector CD8+ T cells is critical for protective immunity against infection. Here, we describe in vivo cytotoxic T cell assay to examine effector function of antigen-specific CD8+ T cells. Mice infected with Listeria monocytogenes (L. monocytogenes) expressing chicken ovalbumin as a model antigen mount ovalbumin-specific CD8+ T cell responses. Effector CD8+ T cell function in vivo is determined by mixed transfer of OVA peptide-pulsed target cells with control target cells into the previously immunized mice. Difference in CFSE expression levels clearly marks two distinct populations: Antigen-pulsed target cells-CFSElow vs. unpulsed target cells-CFSEhi. The frequencies between antigen-pulsed target cells and control target cells are used as readouts of antigen-specific killing.

The cellular and molecular origin of tumor-associated macrophages.

Long recognized as an evolutionarily ancient cell type involved in tissue homeostasis and immune defense against pathogens, macrophages are being rediscovered as regulators of several diseases, including cancer. Here we show that in mice, mammary tumor growth induces the accumulation of tumor-associated macrophages (TAMs) that are phenotypically and functionally distinct from mammary tissue macrophages (MTMs). TAMs express the adhesion molecule Vcam1 and proliferate upon their differentiation from inflammatory monocytes, but do not exhibit an "alternatively activated" phenotype. TAM terminal differentiation depends on the transcriptional regulator of Notch signaling, RBPJ; and TAM, but not MTM, depletion restores tumor-infiltrating cytotoxic T cell responses and suppresses tumor growth. These findings reveal the ontogeny of TAMs and a discrete tumor-elicited inflammatory response, which may provide new opportunities for cancer immunotherapy.