Toll-like receptors (TLRs) are the best characterized pattern recognition receptors. Individual TLRs recruit diverse combinations of adaptor proteins, triggering signal transduction pathways and leading to the activation of various transcription factors, including nuclear factor kappaB, activation protein 1 and interferon regulatory factors. Interleukin-2 is one of the molecules produced by mouse dendritic cells after stimulation by different pattern recognition receptor agonists. By analogy with the events after T-cell receptor engagement leading to interleukin-2 production, it is therefore plausible that the stimulation of TLRs on dendritic cells may lead to activation of the Ca(2+)/calcineurin and NFAT (nuclear factor of activated T cells) pathway. Here we show that mouse dendritic cell stimulation with lipopolysaccharide (LPS) induces Src-family kinase and phospholipase Cgamma2 activation, influx of extracellular Ca(2+) and calcineurin-dependent nuclear NFAT translocation. The initiation of this pathway is independent of TLR4 engagement, and dependent exclusively on CD14. We also show that LPS-induced NFAT activation via CD14 is necessary to cause the apoptotic death of terminally differentiated dendritic cells, an event that is essential for maintaining self-tolerance and preventing autoimmunity. Consequently, blocking this pathway in vivo causes prolonged dendritic cell survival and an increase in T-cell priming capability. Our findings reveal novel aspects of molecular signalling triggered by LPS in dendritic cells, and identify a new role for CD14: the regulation of the dendritic cell life cycle through NFAT activation. Given the involvement of CD14 in disease, including sepsis and chronic heart failure, the discovery of signal transduction pathways activated exclusively via CD14 is an important step towards the development of potential treatments involving interference with CD14 functions.

Chronic infection with hepatitis B virus (HBV) is a major risk factor for the development of hepatocellular carcinoma (HCC). The pathogenesis of HBV-associated HCC involves both viral and host factors. The latter include a functionally inefficient CD8 + T-cell response that fails to clear the infection from the liver but sustains a chronic necroinflammatory process that contributes to the development of HCC. According to this scenario, amelioration of immune-mediated chronic liver injury may prevent HCC. Because platelets facilitate immune-mediated liver injury by promoting the hepatic accumulation of virus-specific CD8 + T cells, we evaluated the long-term consequences of antiplatelet therapy in an HBV transgenic mouse model of chronic immune-mediated necroinflammatory liver disease that progresses to HCC. Treatment with aspirin and clopidogrel during the chronic phase of the disease diminished the number of intrahepatic HBV-specific CD8 + T cells and HBV-nonspecific inflammatory cells, the severity of liver fibrosis, and the development of HCC. Antiplatelet therapy improved overall survival without causing significant side effects. In contrast, the same antiplatelet regimen had no antitumor effect when HCC was induced nonimmunologically by chronic exposure to a hepatotoxic chemical. The unprecedented observation that antiplatelet therapy inhibits or delays immune-mediated hepatocarcinogenesis suggests that platelets may be key players in the pathogenesis of HBV-associated liver cancer and supports the notion that immune-mediated necroinflammatory reactions are an important cause of hepatocellular transformation during chronic hepatitis.

IntroductionInduction and maintenance of T-cell tolerance toward self-antigens is vital to prevent autoimmunity. To this purpose, many different overlapping and nonoverlapping mechanisms of T-cell tolerization exist, both at central and peripheral levels. [1][2][3][4][5] A common vision of how dendritic cells (DCs) contribute to the induction and maintenance of peripheral CD4 ϩ T-cell tolerance is that, in resting conditions, immature DCs, expressing low levels of signal 1 (specificity) and low or no levels of signal 2 (costimulation), are able to induce T-cell unresponsiveness.However, the effective knowledge concerning the contribution of DCs in inducing and maintaining CD4 ϩ T-cell tolerance in peripheral lymphoid organs derives from the selective analysis of specific DC subpopulations. In particular, CD205 ϩ DCs are able to induce CD4 ϩ T-cell tolerance in conditions of suboptimal activation. [6][7][8][9] Moreover, steady-state migratory DC (ssmDC) subpopulations from the gut and the skin, phenotypically CD103 ϩ and CD103 Ϫ , respectively, mediate the conversion of naive T cells into Foxp3 ϩ regulatory T cells (iTreg) in a retinoic acid (RA)-dependent manner. [10][11][12][13] Because the lymph constantly carries peptides for loading on both migratory and lymphoid tissue resident immature DCs in a dose range suitable for tolerization, 14 it cannot be excluded that, in addition to the analyzed populations and in agreement with the common vision, all conventional immature DCs can induce autoantigen specific CD4 ϩ T-cell tolerance in the periphery. Therefore, it was relevant to know whether DCs in general are able to induce T-cell tolerance at the steady state or whether this is a prerogative of specialized subsets. We investigated this question using an experimental system where antigen presentation, in contrast to previous studies, is not a priori confined to a specific DC subpopulation but is extended to all conventional DC subtypes. Specifically, we adopted the 2a T transgenic animal model. 15 In this experimental system, T-cell receptor (TCR) transgenic T cells (2a T cells) recognize a portion of the CH3 region (435-451) of the IgG2a b , the Bpep, in association with the MHC molecule, I-A d . We also generated a mouse model in which the Bpep was presented by CD11c ϩ cells that include all conventional DC subtypes. By performing a systematic study of the behavior of naive autoantigen-specific T cells after interaction with all conventional CD11c ϩ DCs in homeostatic conditions, we found that DCs are able to induce CD4 ϩ T-cell tolerance by promoting the conversion of autoantigen-specific naive T cells into iTreg cells. Among the different DC subtypes, ssmDCs possess the unique ability to induce antigen-specific iTreg cells in an RA-dependent manner. Diversely, lymphoid tissue resident DCs do not show this capacity. Therefore, iTreg cells develop solely in lymph nodes and not in the spleen, which does not host the migratory DC subtype. We also show that iTreg cells that are newly generated in lymph nodes do n...

Natural killer (NK) cells are critical players against tumors. The outcome of anti-tumor vaccination protocols depends on the efficiency of NK-cell activation, and efforts are constantly made to manipulate them for immunotherapeutic approaches. Thus, a better understanding of NK-cell activation dynamics is needed. NK-cell interactions with accessory cells and trafficking between secondary lymphoid organs and tumoral tissues remain poorly characterized. Here, we show that upon triggering innate immunity with lipopolysaccharide (LPS), NK cells are transiently activated, leave the lymph node, and infiltrate the tumor, delaying its growth. Interestingly, NK cells are not actively recruited at the draining lymph node early after LPS administration, but continue their regular homeostatic turnover. Therefore, NK cells resident in the lymph node at the time of LPS administration become activated and exert anti-tumor functions. NK-cell activation correlates with the establishment of prolonged interactions with dendritic cells (DCs) in lymph nodes, as observed by two-photon microscopy. Close DC and NK-cell contacts are essential for the localized delivery of DC-derived IL-18 to NK cells, a strict requirement in NK-cell activation.

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.