Achille Broggi

An endogenous caspase-11 ligand elicits interleukin-1 release from living dendritic cells

Immune activation in context Dendritic cells (DCs) initiate protective immunity upon binding molecules derived from microbes or released from dying cells. Zanoni et al. examined how microbial and endogenous signals interact to shape the course of the ensuing immune response (see the Perspective by Napier and Monack). They found that oxPAPC, an oxidized phospholipid released from dying cells, binds to a protein called caspase-11 in DCs, activating an inflammatory program in these cells. Whereas caspase-11 binding to oxPAPC and bacterial lipopolysaccharide causes DCs to produce the cytokine interleukin-1 (IL-1) and undergo cell death, binding to oxPAPC alone triggers DCs to secrete IL-1 and induce strong adaptive immunity. Thus, context-dependent signals can shape the ensuing immune response. Science, this issue p. 1232; see also p. 1173 Oxidized phospholipids released from dying cells shape dendritic cell immunity. Dendritic cells (DCs) use pattern recognition receptors to detect microorganisms and activate protective immunity. These cells and receptors are thought to operate in an all-or-nothing manner, existing in an immunologically active or inactive state. Here, we report that encounters with microbial products and self-encoded oxidized phospholipids (oxPAPC) induce an enhanced DC activation state, which we call “hyperactive.” Hyperactive DCs induce potent adaptive immune responses and are elicited by caspase-11, an enzyme that binds oxPAPC and bacterial lipopolysaccharide (LPS). oxPAPC and LPS bind caspase-11 via distinct domains and elicit different inflammasome-dependent activities. Both lipids induce caspase-11–dependent interleukin-1 release, but only LPS induces pyroptosis. The cells and receptors of the innate immune system can therefore achieve different activation states, which may permit context-dependent responses to infection.

Migratory, and not lymphoid-resident, dendritic cells maintain peripheral self-tolerance and prevent autoimmunity via induction of iTreg cells

There is evidence that dendritic cells (DCs) induce peripheral tolerance. Nevertheless, it is not known whether immature DCs in general are able to tolerize CD4(+) T cells or if this is a prerogative of specialized subtypes. Here we show that, when autoantigen presentation is extended to all conventional mouse DCs, immature lymphoid tissue resident DCs are unable to induce autoantigen-specific regulatory T (iTreg) cell conversion. In contrast, this is an exclusive prerogative of steady-state migratory DCs. Because only lymph nodes host migratory DCs, iTreg cells develop and are retained solely in lymph nodes, and not in the spleen. Mechanistically, in cutaneous lymph nodes, DC-derived CCL22 contributes to the retention of iTreg cells. The importance of the local generation of iTreg cells is emphasized by their essential role in preventing autoimmunity.

IL-15 cis Presentation Is Required for Optimal NK Cell Activation in Lipopolysaccharide-Mediated Inflammatory Conditions

Natural killer (NK) cells have antitumor, antiviral, and antibacterial functions, and efforts are being made to manipulate them in immunotherapeutic approaches. However, their activation mechanisms remain poorly defined, particularly during bacterial infections. Here, we show that upon lipopolysaccharide or E. coli exposure, dendritic cells (DCs) produce three cytokines-interleukin 2 (IL-2), IL-18, and interferon β (IFN-β)-necessary and sufficient for NK cell activation. IFN-β enhances NK cell activation by inducing IL-15 and IL-15 receptor α not only in DCs but, surprisingly, also in NK cells. This process allows the transfer of IL-15 on NK cell surface and its cis presentation. cis-presented NK cell-derived and trans-presented DC-derived IL-15 contribute equally to optimal NK cell activation.

Microbe- and danger-induced inflammation.

The ability of the immune system to give rise to an effective response against pathogens while maintaining tolerance towards self-tissues has always been an object of keen interest for immunologist. Over the years, different theories have been proposed to explain if and how the immune system is able to discriminate between self and non-self, including the Infectious Non-self theory from Charles Janeway and Polly Matzingers Danger theory. Nowadays we know Janeways theory is largely true, however the immune system does respond to injured, stressed and necrotic cells releasing danger signals (DAMPs) with a potent inflammatory response. To avoid unwanted prolonged autoimmune reactions, though, danger-induced inflammation should be tightly regulated. In the present review we discuss how prototypic DAMPs are able to induce inflammation and the peculiarity of danger-induced inflammation, as opposed to a complete immune response to fight pathogen invasions.

CD14 and NFAT mediate lipopolysaccharide-induced skin edema formation in mice

Inflammation is a multistep process triggered when innate immune cells - for example, DCs - sense a pathogen or injured cell or tissue. Edema formation is one of the first steps in the inflammatory response; it is fundamental for the local accumulation of inflammatory mediators. Injection of LPS into the skin provides a model for studying the mechanisms of inflammation and edema formation. While it is known that innate immune recognition of LPS leads to activation of numerous transcriptional activators, including nuclear factor of activated T cells (NFAT) isoforms, the molecular pathways that lead to edema formation have not been determined. As PGE2 regulates many proinflammatory processes, including swelling and pain, and it is induced by LPS, we hypothesized that PGE2 mediates the local generation of edema following LPS exposure. Here, we show that tissue-resident DCs are the main source of PGE2 and the main controllers of tissue edema formation in a mouse model of LPS-induced inflammation. LPS exposure induced expression of microsomal PGE synthase-1 (mPGES-1), a key enzyme in PGE2 biosynthesis. mPGES-1 activation, PGE2 production, and edema formation required CD14 (a component of the LPS receptor) and NFAT. Therefore, tissue edema formation induced by LPS is DC and CD14/NFAT dependent. Moreover, DCs can regulate free antigen arrival at the draining lymph nodes by controlling edema formation and interstitial fluid pressure in the presence of LPS. We therefore suggest that the CD14/NFAT/mPGES-1 pathway represents a possible target for antiinflammatory therapies.

Similarities and differences of innate immune responses elicited by smooth and rough LPS.

The lipopolysaccharide is the major component of Gram-negative bacteria outer membrane. LPS comprises three covalently linked regions: the lipid A, the rough core oligosaccharide, and the O-antigenic side chain determining serotype specificity. Wild-type LPS (sLPS) contains the O-antigenic side chain and is referred to as smooth. Rough LPS (rLPS) does not contain the O-side chain. Most wt bacteria and especially wt Enterobacteriaceae express prevalently the sLPS form although some truncated rLPS molecules always reach the external membrane. The two sLPS and rLPS forms are used almost indistinctly to study the effects on innate immune cells. Nevertheless, there is evidence that their mechanism of action may be different. For instance, while sLPS requires CD14 for the initiation of both MyD88-dependent and independent signal transduction pathways at least at low doses, rLPS leads to MyD88-dependent responses in the absence of CD14 even at low doses. Here we have identified additional differences in the signaling capacity of the two LPS species in the mouse. We have found that rLPS, diversely from sLPS, is capable of activating in dendritic cells (DCs) the Ca(2+)/calcineurin and NFAT pathway in a CD14-independent manner, moreover it is also capable per se of activating the inflammasome and eliciting IL-1β secretion independent of the presence of additional stimuli required instead for sLPS. The ability of rLPS of activating the inflammasome in vitro has as a direct consequence a higher efficiency of rLPS-exposed DCs in activating natural killer (NK) cells compared to sLPS-exposed DCs. However, diversely from possible predictions, we found that the different efficiencies of the two LPS species in eliciting innate responses are almost nullified in vivo. Therefore, sLPS and rLPS induce nearly similar in vivo innate responses but with different mechanisms of signaling.

IFN-[lambda] suppresses intestinal inflammation by non-translational regulation of neutrophil function

Interferon-λ (IFN-λ) is a central regulator of mucosal immunity; however, its signaling specificity relative to that of type I interferons is poorly defined. IFN-λ can induce antiviral interferon-stimulated genes (ISGs) in epithelia, while the effect of IFN-λ in non-epithelial cells remains unclear. Here we report that neutrophils responded to IFN-λ. We found that in addition to inducing ISG transcription, IFN-λ (but not IFN-β) specifically activated a translation-independent signaling pathway that diminished the production of reactive oxygen species and degranulation in neutrophils. In mice, IFN-λ was elicited by enteric viruses and acted on neutrophils to decrease oxidative stress and intestinal damage. Thus, IFN-λ acted as a unique immunomodulatory agent by modifying transcriptional and non-translational neutrophil responses, which might permit a controlled development of the inflammatory process.

Migratory conventional dendritic cells in the induction of peripheral T cell tolerance

Conventional DCs are an extremely heterogeneous cell population that comprises several different subsets. A major distinction can be made between lymphoid‐resident DCs that are present in the lymphoid tissues and the blood and migratory DCs that reside in the nonlymphoid tissues and migrate to the lymph nodes, both in homeostatic conditions and during the course of an infection. Migratory DCs differ from tissue to tissue but share the unique ability to transport antigens to the draining lymph nodes—in particular, tissue‐restricted antigens in homeostatic conditions and microbial antigens after an infection. Recently, steady‐state migratory DCs have gained much attention after the discovery of their high tolerogenic potential. The purpose of this review is to give a picture of the recent finding regarding steady‐state migratory DCs with particular interest in their role in inducing T cell tolerance.

Systemically administered DNA and fowlpox recombinants expressing four vaccinia virus genes although immunogenic do not protect mice against the highly pathogenic IHD-J vaccinia strain.

The first-generation smallpox vaccine was based on live vaccinia virus (VV) and it successfully eradicated the disease worldwide. Therefore, it was not administered any more after 1980, as smallpox no longer existed as a natural infection. However, emerging threats by terrorist organisations has prompted new programmes for second-generation vaccine development based on attenuated VV strains, which have been shown to cause rare but serious adverse events in immunocompromised patients. Considering the closely related animal poxviruses that might also be used as bioweapons, and the increasing number of unvaccinated young people and AIDS-affected immunocompromised subjects, a safer and more effective smallpox vaccine is still required. New avipoxvirus-based vectors should improve the safety of conventional vaccines, and protect from newly emerging zoonotic orthopoxvirus diseases and from the threat of deliberate release of variola or monkeypox virus in a bioterrorist attack. In this study, DNA and fowlpox recombinants expressing the L1R, A27L, A33R and B5R genes were constructed and evaluated in a pre-clinical trial in mouse, following six prime/boost immunisation regimens, to compare their immunogenicity and protective efficacy against a challenge with the lethal VV IHD-J strain. Although higher numbers of VV-specific IFNγ-producing T lymphocytes were observed in the protected mice, the cytotoxic T-lymphocyte response and the presence of neutralising antibodies did not always correlate with protection. In spite of previous successful results in mice, rabbits and monkeys, where SIV/HIV transgenes were expressed by the fowlpox vector, the immune response elicited by these recombinants was low, and most of the mice were not protected.

Skin infections are eliminated by cooperation of the fibrinolytic and innate immune systems

A balanced TGF-β–mediated fibrotic response drives the control and elimination of fungal and bacterial skin infections. There’s more than one way to skin an infection There are two phases of innate immune control of skin infection by Candida albicans—protective containment and elimination. Now, Santus et al. report that early activation of the transcript factor nuclear factor of activated T cells (NFAT) balances these two phases. During the containment phase, NFAT regulates active TGF-β expression, which induces collagen deposition and traps the microbe. During the elimination phase, NFAT induces IFN-γ that promotes skin ulceration and microbial expulsion. These functions are not restricted to Candida because cross-talk between the innate immune and fibrinolytic responses also contributed to defense against Staphylococcus aureus. This cooperation is critical to minimizing tissue damage while fighting infection. Nuclear factor of activated T cells (NFAT) is activated in innate immune cells downstream of pattern recognition receptors, but little is known about NFAT’s functions in innate immunity compared with adaptive immunity. We show that early activation of NFAT balances the two major phases of the innate response to Candida albicans skin infections: the protective containment (abscess) and the elimination (expulsion) phases. During the early containment phase, transforming growth factor–β (TGF-β) induces the deposit of collagen around newly recruited polymorphonuclear cells to prevent microbial spreading. During the elimination phase, interferon-γ (IFN-γ) blocks differentiation of fibroblasts into myofibroblasts by antagonizing TGF-β signaling. IFN-γ also induces the formation of plasmin that, in turn, promotes abscess capsule digestion and skin ulceration for microbial discharge. NFAT controls innate IFN-γ production and microbial expulsion. This cross-talk between the innate immune and the fibrinolytic systems also occurs during infection with Staphylococcus aureus and is a protective response to minimize tissue damage and optimize pathogen elimination.