The activation of Toll-like receptors (TLRs) in dendritic cells (DCs) triggers a rapid inflammatory response to pathogens. However, this response must be tightly regulated because unrestrained TLR signaling generates a chronic inflammatory milieu that often leads to autoimmunity. We have found that the TAM receptor tyrosine kinases-Tyro3, Axl, and Mer-broadly inhibit both TLR and TLR-induced cytokine-receptor cascades. Remarkably, TAM inhibition of inflammation is transduced through an essential stimulator of inflammation-the type I interferon receptor (IFNAR)-and its associated transcription factor STAT1. TLR induction of IFNAR-STAT1 signaling upregulates the TAM system, which in turn usurps the IFNAR-STAT1 cassette to induce the cytokine and TLR suppressors SOCS1 and SOCS3. These results illuminate a self-regulating cycle of inflammation, in which the obligatory, cytokine-dependent activation of TAM signaling hijacks a proinflammatory pathway to provide an intrinsic feedback inhibitor of both TLR- and cytokine-driven immune responses.

In adaptive antibody responses, B cells are induced to grow either in follicles where they form germinal centers or in extrafollicular foci as plasmablasts. Extrafollicular growth typically occurs in the medullary cords of lymph nodes and in foci in the red pulp of the spleen. It is not a feature of secondary lymphoid tissue associated with the internal epithelia of the body. All types of naïve and memory B cells can be recruited into extrafollicular responses. These responses are associated with immunoglobulin class switching but, at the most, only low-level hypermutation.

Multiple inhibitory molecules create a profoundly immunuosuppressive environment during chronic viral infections in humans and mice. Therefore, eliciting effective immunity in this context represents a challenge. Here we report that during a murine chronic viral infection, interleukin-6 (IL-6) was produced by irradiation resistant cells in a biphasic manner, with late IL-6 being absolutely essential for viral control. The underlying mechanism involved IL-6 signaling on virus-specific CD4 T cells that caused up-regulation of the transcription factor Bcl6 and enhanced T follicular helper (Tfh) cell responses at late, but not early, stages of chronic viral infection. This resulted in escalation of germinal center reactions and improved antibody responses. Our results uncover an antiviral strategy that helps to safely resolve a persistent infection in vivo.

SUMMARY During microbial infection, responding CD8+ T lymphocytes differentiate into heterogeneous subsets that together provide immediate and durable protection. To elucidate the dynamic transcriptional changes that underlie this process, we applied a single-cell RNA sequencing approach and analyzed individual CD8+ T lymphocytes sequentially throughout the course of a viral infection in vivo. Our analyses revealed a striking transcriptional divergence among cells that had undergone their first division and identified previously unknown molecular determinants controlling CD8+ T lymphocyte fate specification. These findings suggest a model of terminal effector cell differentiation initiated by an early burst of transcriptional activity and subsequently refined by epigenetic silencing of transcripts associated with memory lymphocytes, highlighting the power and necessity of single-cell approaches.

The prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a formidable battle horse for the study of viral immunology, as well as viral persistence and associated diseases. Investigations with LCMV have uncovered basic mechanisms by which viruses avoid elimination by the host adaptive immune response. In this study we show that LCMV also disables the host innate defense by interfering with beta interferon (IFN-␤) production in response to different stimuli, including infection with Sendai virus and liposome-mediated DNA transfection. Inhibition of IFN production in LCMV-infected cells was caused by an early block in the IFN regulatory factor 3 (IRF-3) activation pathway. This defect was restored in cells cured of LCMV, indicating that one or more LCMV products are responsible for the inhibition of IRF-3 activation. Using expression plasmids encoding individual LCMV proteins, we found that expression of the LCMV nucleoprotein (NP) was sufficient to inhibit both IFN production and nuclear translocation of IRF-3. To our knowledge, this is the first evidence of an IFN-counteracting viral protein in the Arenaviridae family. Inhibition of IFN production by the arenavirus NP is likely to be a determinant of virulence in vivo.Arenaviruses merit significant attention both as tractable model systems to study acute and persistent viral infections (28, 51) and as clinically important human pathogens, including several causative agents of severe hemorrhagic fever, chiefly Lassa fever virus (LFV) (8,14). In addition, the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) has proven to be a Rosetta stone in the fields of viral immunology and pathogenesis (28,51).LCMV is an enveloped virus with a bisegmented RNA genome (8,25). Each segment, designated L (ca 7.2 kb) and S (ca 3.4 kb), expresses two viral gene products using an ambisense coding strategy. The S RNA directs the synthesis of the nucleoprotein NP and the glycoprotein precursor GPC. The NP, the most abundant viral protein, encapsidates viral genomes and antigenomic replicative intermediates. GPC is posttranslationally cleaved by the cellular subtilase S1P into mature viral glycoproteins, GP-1 and GP-2 (5, 34). Noncovalently associated GP-1/GP-2 complexes make up the spikes on the virion envelope and mediate virus interaction with the host cell receptor (20). The L segment codes for the virus RNA-dependent RNA polymerase (L) and a small (11-kDa) RING finger protein called Z that functions as the arenavirus counterpart of the matrix protein found in many negative-strand RNA viruses (32, 42). Additional roles of Z in the arenavirus life cycle have been proposed on the basis of its interaction with several host cell proteins (8, 25) and its ability to inhibit RNA synthesis mediated by the virus polymerase (17, 25).The mechanisms underlying arenavirus hemorrhagic fever disease are not understood. Individuals succumbing to LF generate only minimal or no anti-LFV immune response, while those recovering from LF disease show evidence of both Tand B-ce...

SUMMARY Although deficient CD8 T cell responses have long been associated with chronic viral infections, the underlying mechanisms are still unclear. Here we report that enhanced and sustained TGF-β/Smad signaling is a distinctive feature of virus-specific CD8 T cells during chronic versus acute viral infections in vivo. The result is TGF-β-dependent up-regulation of the pro-apoptotic protein Bim that relates to cell-intrinsic apoptosis and significantly reduced numbers of virus-specific CD8 T cells. Moreover, selective attenuation of TGF-β signaling on T cells increases the function of CD8 T cells in an indirect fashion, rapidly eradicates the persistence-prone virus and enables the generation of an effective memory response. Our findings reveal persisting TGF-β/Smad signaling as a hallmark and key regulator of virus-specific CD8 T cell responses during chronic viral infections in vivo.

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Understanding, treating, and preventing diseases caused by immunosuppression and/or persistent infections remain both a major challenge in biomedical research and an important health goal. For a virus or any infectious agent to persist, it must utilize strategies to suppress or evade the host's immune response. Here, we report that two dissimilar viruses employ a common maneuver to cause a profound immunosuppression. Measles virus (MV) and lymphocytic choriomeningitis virus (LCMV) interfere with dendritic cell (DC) development and expansion in vivo and in vitro. The underlying mechanism for this is through the generation of type I interferon (IFN) that acts via a signal transducer and activator of a transcription (STAT)2-dependent, but STAT1-independent, pathway. Thus, viruses subvert the known antiviral effect of type I IFN through STAT2-specific signaling to benefit their survival. These observations have implications for understanding and developing therapies to treat diseases caused by immunosuppression and/or persistent infections.