Eric M. Pietras

IL-17 is essential for host defense against cutaneous Staphylococcus aureus infection in mice

Staphylococcus aureus is the most common cause of skin and soft tissue infections, and rapidly emerging antibiotic-resistant strains are creating a serious public health concern. If immune-based therapies are to be an alternative to antibiotics, greater understanding is needed of the protective immune response against S. aureus infection in the skin. Although neutrophil recruitment is required for immunity against S. aureus, a role for T cells has been suggested. Here, we used a mouse model of S. aureus cutaneous infection to investigate the contribution of T cells to host defense. We found that mice deficient in gammadelta but not alphabeta T cells had substantially larger skin lesions with higher bacterial counts and impaired neutrophil recruitment compared with WT mice. This neutrophil recruitment was dependent upon epidermal Vgamma5+ gammadelta T cell production of IL-17, but not IL-21 and IL-22. Furthermore, IL-17 induction required IL-1, TLR2, and IL-23 and was critical for host defense, since IL-17R-deficient mice had a phenotype similar to that of gammadelta T cell-deficient mice. Importantly, gammadelta T cell-deficient mice inoculated with S. aureus and treated with a single dose of recombinant IL-17 had lesion sizes and bacterial counts resembling those of WT mice, demonstrating that IL-17 could restore the impaired immunity in these mice. Our study defines what we believe to be a novel role for IL-17-producing epidermal gammadelta T cells in innate immunity against S. aureus cutaneous infection.

DUBA: A Deubiquitinase That Regulates Type I Interferon Production

Production of type I interferon (IFN-I) is a critical host defense triggered by pattern-recognition receptors (PRRs) of the innate immune system. Deubiquitinating enzyme A (DUBA), an ovarian tumor domain-containing deubiquitinating enzyme, was discovered in a small interfering RNA–based screen as a regulator of IFN-I production. Reduction of DUBA augmented the PRR-induced IFN-I response, whereas ectopic expression of DUBA had the converse effect. DUBA bound tumor necrosis factor receptor–associated factor 3 (TRAF3), an adaptor protein essential for the IFN-I response. TRAF3 is an E3 ubiquitin ligase that preferentially assembled lysine-63–linked polyubiquitin chains. DUBA selectively cleaved the lysine-63–linked polyubiquitin chains on TRAF3, resulting in its dissociation from the downstream signaling complex containing TANK-binding kinase 1. A discrete ubiquitin interaction motif within DUBA was required for efficient deubiquitination of TRAF3 and optimal suppression of IFN-I. Our data identify DUBA as a negative regulator of innate immune responses.

Regulation of antiviral responses by a direct and specific interaction between TRAF3 and Cardif

Upon recognition of viral infection, RIG‐I and Helicard recruit a newly identified adapter termed Cardif, which induces type I interferon (IFN)‐mediated antiviral responses through an unknown mechanism. Here, we demonstrate that TRAF3, like Cardif, is required for type I interferon production in response to intracellular double‐stranded RNA. Cardif‐mediated IFNα induction occurs through a direct interaction between the TRAF domain of TRAF3 and a TRAF‐interaction motif (TIM) within Cardif. Interestingly, while the entire N‐terminus of TRAF3 was functionally interchangeable with that of TRAF5, the TRAF domain of TRAF3 was not. Our data suggest that this distinction is due to an inability of the TRAF domain of TRAF5 to bind the TIM of Cardif. Finally, we show that preventing association of TRAF3 with this TIM by mutating two critical amino acids in the TRAF domain also abolishes TRAF3‐dependent IFN production following viral infection. Thus, our findings suggest that the direct and specific interaction between the TRAF domain of TRAF3 and the TIM of Cardif is required for optimal Cardif‐mediated antiviral responses.

Inflammasome-Mediated Production of IL-1β Is Required for Neutrophil Recruitment against Staphylococcus aureus In Vivo

IL-1R activation is required for neutrophil recruitment in an effective innate immune response against Staphylococcus aureus infection. In this study, we investigated the mechanism of IL-1R activation in vivo in a model of S. aureus infection. In response to a S. aureus cutaneous challenge, mice deficient in IL-1β, IL-1α/IL-1β, but not IL-1α, developed larger lesions with higher bacterial counts and had decreased neutrophil recruitment compared with wild-type mice. Neutrophil recruitment and bacterial clearance required IL-1β expression by bone marrow (BM)-derived cells and not by non-BM-derived resident cells. In addition, mice deficient in the inflammasome component apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) had the same defects in neutrophil recruitment and host defense as IL-1β-deficient mice, demonstrating an essential role for the inflammasome in mediating the production of active IL-1β to promote neutrophil recruitment in host defense against S. aureus. This finding was further supported by the ability of recombinant active IL-1β to control the infection and promote bacterial clearance in IL-1β-deficient mice. These studies define a key host defense circuit where inflammasome-mediated IL-1β production by BM-derived cells signals IL-1R on non-BM-derived resident cells to activate neutrophil recruitment in the innate immune response against S. aureus in vivo.

The interferon response to bacterial and viral infections

Type I interferons (IFNs) were first described several decades ago as soluble factors that were capable of `interfering with viral replication when added to infected cells. Type I IFNs have been shown to be induced by recognition of viral DNA and RNA via three distinct pathways: (i) a TRIFdependent pathway in macrophages via TLRs 3 and 4; (ii) a MyD88-dependent pathway in plasmacytoid dendritic cells (pDCs) via TLRs 7/8 and 9; and (iii) an intracellular recognition pathway utilizing the cytoplasmic receptors RIG-I/MDA5. Interestingly, these viral recognition pathways converge on TRAF3, which induces interferon through the activation of IRF3 or IRF7 by the TBK-1 and IKKi complexes. While type I IFN has been traditionally associated with antiviral responses, recent studies have demonstrated that many bacteria also induce type I interferon responses. The mechanisms of type I IFN induction and its role in host defense, however, are largely unclear. Studies with the Gram-positive intracellular bacterium Listeria monocytogenes indicated that it may trigger type I IFN induction through novel TLR-independent intracellular receptors and type I IFN may play a detrimental role to host response against listerial infection. In this article, we summarize some of these findings and discuss the functional differences of type I IFNs in bacterial and viral infections.

IRF3-dependent Type I Interferon Response in B Cells Regulates CpG-mediated Antibody Production

Hypomethylated CpG oligonucleotides (CpG) are not only potent adjuvants for enhancing adaptive immune responses but may also play a critical role in the development of autoimmune diseases such as Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE). Here we provide evidence that, in addition to dendritic cells, murine B lymphocytes also exhibit a type I IFN response to CpG-B. Unlike dendritic cells, B cell-mediated type I IFN induction depended on the transcription factor IRF3, but similar to dendritic cells this pathway was independent of the IRF3 kinase TBK1. Utilizing type I IFN receptor-deficient mice, we were able to demonstrate that this IFN pathway enhanced Syndecan-1 expression and IgM production and was required for IgG2a production following CpG-B stimulation. Overall, our findings identify a unique IFN pathway in B cells that may play a central role in mediating B cell biology in response to CpG, potentially implicating this pathway in autoantibody production and the pathogenesis of certain autoimmune diseases.

A MyD88-dependent IFNγR-CCR2 signaling circuit is required for mobilization of monocytes and host defense against systemic bacterial challenge

Monocytes are mobilized to sites of infection via interaction between the chemokine MCP-1 and its receptor, CCR2, at which point they differentiate into macrophages that mediate potent antimicrobial effects. In this study, we investigated the mechanisms by which monocytes are mobilized in response to systemic challenge with the intracellular bacterium Francisella tularensis. We found that mice deficient in MyD88, interferon-γ (IFNγ)R or CCR2 all had defects in the expansion of splenic monocyte populations upon F. tularensis challenge, and in control of F. tularensis infection. Interestingly, MyD88-deficient mice were defective in production of IFNγ, and IFNγR-deficient mice exhibited defective production of MCP-1, the ligand for CCR2. Transplantation of IFNγR-deficient bone marrow (BM) into wild-type mice further suggested that mobilization of monocytes in response to F. tularensis challenge required IFNγR expression on BM-derived cells. These studies define a critical host defense circuit wherein MyD88-dependent IFNγ production signals via IFNγR expressed on BM-derived cells, resulting in MCP-1 production and activation of CCR2-dependent mobilization of monocytes in the innate immune response to systemic F. tularensis challenge.

A New TRADDition in Intracellular Antiviral Signaling

To effectively defeat viral infection, a mammalian host must detect the presence of viruses and activate a robust antiviral response, which is mediated by cytokines known as type I interferons (IFNs). Much excitement has been generated by the discovery of the retinoic acid–induced gene 1–like helicase (RLH) pathway, which detects RNA-containing viruses in the cytoplasm and activates the type I IFN response. A study now describes a role for the tumor necrosis factor receptor (TNFR)–associated death domain (TRADD) in the antiviral response and sheds new light on how the RLH signaling complex is organized upon viral infection.

New tricks for old NODs

Recent work has identified the human NOD-like receptor NLRX1 as a negative regulator of intracellular signaling leading to type I interferon production. Here we discuss these findings and the questions and implications they raise regarding the function of NOD-like receptors in the antiviral response.