deep-dermal-injection-as-model-candida-albicans-skin-infection-for

Deep Dermal Injection As a Model of <em>Candida albicans</em> Skin Infection for Histological Analyses

The skin is an extremely extended organ of the body and, due to this large surface, it is continuously exposed to microorganisms. Skin damage can easily ...

University of Milano-Bicocca

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.

CD14 regulates the dendritic cell life cycle after LPS exposure through NFAT activation.

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.

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

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.

Prolonged contact with dendritic cells turns lymph node-resident NK cells into anti-tumor effectors.

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  skin infections: the protective containment (abscess) and the elimination (expulsion) phases. During the early containment phase, transforming growth factor- (TGF-) induces the deposit of collagen around newly recruited polymorphonuclear cells to prevent microbial spreading. During the elimination phase, interferon- (IFN-) blocks differentiation of fibroblasts into myofibroblasts by antagonizing TGF- signaling. IFN- 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  and is a protective response to minimize tissue damage and optimize pathogen elimination.

Article	Year
CD14 regulates the dendritic cell life cycle after LPS exposure through NFAT activation. Nature\| PubMed ID: 19525933	2009
Migratory, and not lymphoid-resident, dendritic cells maintain peripheral self-tolerance and prevent autoimmunity via induction of iTreg cells. Blood\| PubMed ID: 22760781	2012
Prolonged contact with dendritic cells turns lymph node-resident NK cells into anti-tumor effectors. EMBO molecular medicine\| PubMed ID: 27406819	2016
Skin infections are eliminated by cooperation of the fibrinolytic and innate immune systems. Science immunology\| PubMed ID: 28939652	2017

Francesca Mingozzi

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