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

Abstract

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.

Lipopolysaccharide‐primed heterotolerant dendritic cells suppress experimental autoimmune uveoretinitis by multiple mechanisms

“…Here, we show that TLR4‐specific ultrapure (up)LPS‐primed phenotypically activated BMDC, rather than worsening the severity of EAU, significantly prevent the development of EAU. We show that upLPS‐primed BMDC are endotoxin homotolerant (ET‐BMDC) and further show that they are heterotolerant to M. tuberculosis protein extract in that they are (i) susceptible to apoptosis45, 46, 47 (confirmed here) through a CD14/nuclear factor of activated T cells (NFATc)‐associated mechanism, and (ii) fail to secrete IL‐1 β on exposure to M. tuberculosis extract. As M. tuberculosis mediated C‐type lectin receptor signalling via the Syk/CARD‐9 complex,48 a major route for inflammasome activation, has been shown to be an essential mediator of IRBP‐CFA‐induced EAU,48, 49 we propose that inhibition of IL‐1 β secretion is one mechanism whereby heterotolerant LPS‐primed BMDC promote immunological tolerance.…”

“…Zanoni et al 30, 47, 77. have shown that LPS‐primed BMDC are programmed for apoptosis, unlike LPS‐primed macrophages which, although refractory to a second challenge of LPS, survive in culture for prolonged periods.…”

“…3). 47 Apoptotic cells are known to promote immune tolerance when administered in vivo . In particular, the clearance of apoptotic cells has been identified as an event that is directly responsible for tolerance induction.…”

Transcriptional control of macrophage diversity and specialization

“…Macrophages develop from hematopoietic stem cells through common myeloid progenitors in the BM, and repopulate in peripheral tissues [2]. Currently, macrophages can be classified into several different subtypes, based on their reactions to different stimuli [3][4][5]. Macrophages involved in inflammatory responses to bacterial and viral infection are called M1 macrophages.…”

“…Regardless of classification controversies about cells of the myelomonocytic lineage, an example of their functional peculiarities is the non-overlapping responsiveness of macrophages and DCs to the prototypical toll-like receptor 4 (TLR4) stimulator lipopolysaccharide (LPS), with features that can only be partially explained by differences in the expression of responder proteins [3]. Adding complexity to the picture is the action of tissue-specific microenvironmental cues, which likely control specialized behaviors and fine-tune cellular identities, in fact representing developmental modifiers operating on differentiated cells [4].…”