Regulation of human dendritic cell activation

Division Of Basic Sciences - Nci

Investigators

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Abstract

To induce an effective immune response, microorganisms must stimulate complex sets of pattern-recognition receptors, both within and outside of the TLR family. The combined activation of these different receptors can result in complementary, synergistic or antagonistic effects that modulate innate and adaptive immunity. Our past work has uncovered the significant synergy in dendritic cell activation between ligands of different T cell receptors and the role of type I interferon in regulating TLR downstream signaling in both dendritic cells and tissue and tumor cells. The cellular response to TLR ligands is not only production of pro-inflammatory mediators, but they are also involved in control of tissue homeostasis and regulate cellular differentiation, proliferation, and apoptosis. The balance between MyD88 and TRIF signaling and the production of type I IFN determine proliferation versus apoptosis in tissue and tumor cells and activation versus survival in dendritic cells. We also have found that the stimulation of the dendritic cells by beta-glucan (a component of yeast and fungi) through the dectin 1 receptor facilitate the induction of a Th17 response in human and synergize with TLR receptor stimulation for activation of dendritic cells and pro-inflammatory cytokine production. Signaling through dectin 1, a receptor with an ITAM-like motif in its cytoplasmic portion, results in the induction of only a small number of early responding cytokine such as IL-1beta, IL-6, and TNF and in only a very modest activation of NF-kappaB. However, IL-1beta induced by beta-glucan stimulation exerts a potent positive feedback mechanism that it is necessary for optimal NF-kappaB activation and production of late responsive cytokines such as IL-12 and IL-23. Mechansisms of differentiation of human Th22 cells: T-helper cells represent a functionally diverse group of regulatory cells that orchestrate the immune response by controlling activation or inhibition of a wide range of immune and non-immune cells. Th22 cells are a relatively novel addition to the T-helper. Using analysis of T-helper cells polarized under different conditions we showed that Th22 cells are plastic and capable of becoming IL-17 producing cells upon stimulation with Th17-polarizing cytokines. Transcriptome analysis showed that majority of genes expressed by Th22 cells contained signatures of Th17 and Th0 cells, with only small contribution of their own. These results are now been extended to studies using single cell RNAseq as well as analysis of the transcriptional regulation of T cell subset genome by ATAC-seq. On the basis of our results, given the high degree of plasticity and the intermediate/transitional gene expression signature, we suggest that Th22 cells do not represent a completely independent T-helper subset, but a quasi-T-helper cell subset closely related to Th17 cells, which together rather comprise a single Th17/Th22 regulatory axis with a primary function to control immune responses of barrier and connective tissues. Therefore, we propose that Th22 cells represent a transcriptionally novel mode of T-helper cell differentiation and speculate of a possibility of other T-helper cells falling into a similar quasi-T-helper subset category. Human NK cells prime inflammatory DC precursors to induce Tc17 differentiation: Adaptive immune responses are acknowledged to evolve from innate immunity. However, limited information exists regarding whether encounters between innate cells direct the generation of specialized T-cell subsets. We aim to understand how natural killer (NK) cells modulate cell-mediated immunity in humans. We found that human CD14+CD16- monocytes that differentiate into inflammatory dendritic cells (DCs) are shaped at the early stages of differentiation by cell-to-cell interactions with NK cells. Although a fraction of monocytes is eliminated by NK-cell-mediated cytotoxicity, the polarization of interferon-g (IFN-g) at the NKp30-stabilized synapses triggers a stable IFN-g signature in surviving monocytes that persists after their differentiation into DCs. Notably, NK-cell-instructed DCs drive the priming of type 17 CD8+ T cells (Tc17) with the capacity to produce IFN-g and interleukin-17A. Compared with healthy donors, this cellular network is impaired in patients with classical NK-cell deficiency driven by mutations in the GATA2 gene. Our findings reveal a previously unrecognized connection by which Tc17-mediated immunity might be regulated by NK-cell-mediated tuning of antigen-presenting cells. Requirements for the differentiation of innate T-bet(high) memory-phenotype CD4(+) T lymphocytes under steady state: In parallel mouse studies, we studied CD4+ T lymphocytes that consist of naive, conventional antigen-specific memory, and memory-phenotype (MP) cell compartments in steady state. MP cells exert significant, innate-like effector function in host defense against pathogens controlled by Th1-type immunity. However, the mechanisms determining the differentiation of MP cells under homeostatic conditions remain to be defined. We first characterized MP lymphocytes as consisting of T-bet(high), T-bet(int), and T-bet(low) subsets, with innate Th1-like effector activity exclusively associated with T-bet(high) cells. We further showed that steady-state differentiation of T-bet(high) MP cells is promoted by IL-12-producing CD8+ DCs localized in the T cell zone. Tonic IL-12 production depended on microbial-independent TLR-MyD88 signaling and was further enhanced by CD40-CD40L interactions with MP CD4+ T lymphocytes. The latter response was positively associated with the TCR affinity of these cells to self. Taken together, our findings reveal that the optimal differentiation of host-protective T-bet(high) MP lymphocytes in steady state requires IL-12 tonically produced by CD8+ DCs in a MyD88 and CD40 signaling-dependent fashion. In a related mouse study, we characterize the role of IL-27 as an immunological checkpoint for tumor growth. While inflammatory mechanisms driving hepatocellular carcinoma (HCC) had been proposed, the regulators of anti-cancer immunity in HCC remain poorly understood. We found that IL-27 receptor (IL-27R) signaling promotes HCC development in vivo. High IL-27EBI3 cytokine or IL-27RA expression correlated with poor prognosis for patients with HCC. Loss of IL-27R suppressed HCC in vivo in two different models of hepatocarcinogenesis. Mechanistically, IL-27R signaling within the tumor microenvironment restrains the cytotoxicity of innate cytotoxic lymphocytes. IL-27R ablation enhanced their accumulation and activation, while depletion or functional impairment of innate cytotoxic cells abrogated the effect of IL-27R disruption. Pharmacological neutralization of IL-27 signaling increased infiltration of innate cytotoxic lymphocytes with upregulated cytotoxic molecules and reduced HCC development. Our data reveal an unexpected role of IL-27R signaling as an immunological checkpoint regulating innate cytotoxic lymphocytes and promoting HCC of different etiologies, thus indicating a therapeutic potential for IL-27 pathway blockade in HCC. In a recent Comment article, Ziegler-Heitbrock and colleagues challenge the current definition of plasmacytoid dendritic cells (pDCs) as a sublineage of dendritic cells (DCs), and instead propose to reclassify them as innate lymphocytes. That proposal represents an important discussion regarding the complex nature and lineage affiliation of pDCs in view of the emerging new data in the field. At the same time, we believe that the proposed reclassification of pDCs is not warranted by the currently available information.