During infection, TLR agonists are released and trigger mature as well as differentiating innate immune cells. Early encounter with TLR agonists (R848; LPS) blocks conventional differentiation of CD14 1 monocytes into immature dendritic cells (iDCs) resulting in a deviated phenotype. We and others characterized these APCs (TLR-APC) by a retained expression of CD14 and a lack of CD1a. Here, we show in addition, expression of programmed death ligand-1 (PD-L1). TLR-APCs failed to induce T-cell proliferation and furthermore were able to induce CD25 1 Foxp3 1 T regulatory cells (Tregs). Since PD-L1 is described as a key negative regulator and inducer of tolerance, we further analyzed its regulation. PD-L1 expression was regulated in a MAPK/cytokine/STAT-3-dependent manner: high levels of IL-6 and IL-10 that signal via STAT-3 were produced by TLR-APCs. Blocking of STAT-3 activation prevented PD-L1 expression. Moreover, chromatin immunoprecipitation revealed direct binding of STAT-3 to the PD-L1 promoter. Those findings indicate a pivotal role of STAT-3 in regulating PD-L1 expression. MAPKs were indirectly engaged, as blocking of p38 and p44/42 MAPKs decreased IL-6 and IL-10 thus reducing STAT-3 activation and subsequent PD-L1 expression. Hence, during DC differentiation TLR agonists induce a STAT-3-mediated expression of PD-L1 and favor the development of tolerogenic APCs.Keywords: DC . PD-L1 . STAT-3 . Tolerance . TLR See accompanying Commentary by Sumpter and ThomsonSupporting Information available online IntroductionDC are initiators and modulators of the adaptive immune response [1]. They are able to induce T-cell activation as well as T-cell tolerance. During infection, DCs are confronted with pathogen-associated molecular patterns (PAMP), which in turn trigger effector functions in innate immune cells. For example, immature DCs (iDCs) generated from monocytes by in vitro culture with GM-CSF and IL-4 (G4) mature and become fully activated upon stimulation with TLR agonists. Mature DCs (mDCs) in turn activate most efficiently naïve T cells [2]. However, during infection induction of inhibitory immune pathways can also be observed [3,4]. Here, we investigate an alternative TLR-induced APC phenotype, which inhibits immune reactivity. It has been shown that encounter of monocytes with LPS during the very beginning of the differentiation process blocks conventional differentiation to iDCs. A phenotypically distinct APC type (TLR-APC) is generated, characterized by a [5][6][7]. Activation of p38 MAPK, the secretion of IL-10 and the inactivation of ERK and NF-kB [7] have been correlated with the generation of TLR-APCs. LPS-treated cells showed in addition an intense STAT-3 phosphorylation. Differentiation processes of DCs are plastic and can be influenced by various factors, e.g. cytokines. Many cytokines mediate their cellular response via the JAK/STAT signaling pathway thereby controlling the status of transcription and cellular differentiation. For instance, during the maturation of DCs, a switch occurs from constit...
The upper airways are prone to contact with pathogenic as well as non-pathogenic microbes, therefore immune recognition principles have to be tightly controlled. Here we show that human BEAS-2B bronchial epithelial cells inhibited secretion of the proinflammatory cytokines TNF-a and IL-12 by monocytes, macrophages and dendritic cells. This inhibitory effect could be transferred by supernatant of resting BEAS-2B cells and was also observed when primary murine tracheal epithelial cells were prepared. In contrast to inhibition of pro-inflammatory cytokine secretion epithelial cell-conditioned dendritic cells showed increased expression of IL-10 and arginase-1, thus displaying properties of alternative activation. Accordingly, Toll-like receptor-mediated up-regulation of CD40, CD86 and PD-L2 (CD273) on murine dendritic cells was reduced in the presence of bronchial epithelial cell supernatant. However, expression of negative regulatory PD-L1 (CD274) was increased and dendritic cell induced proliferation of T lymphocytes was diminished. Epithelial cells also showed a direct inhibitory effect on T lymphocyte proliferation and this was due to the constitutive secretion of TGF-b by bronchial epithelial cells. Moreover, epithelial cell-conditioned T lymphocytes showed increased differentiation towards IL-10-producing Tr1 cells. The results indicate that bronchial epithelial cells induce a noninflammatory microenvironment that regulates local immune homeostasis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.