Dendritic cell maturation: functional specialization through signaling specificity and transcriptional programming

Abstract

Dendritic cells (DC) are key regulators of both protective immune responses and tolerance to self-antigens. Soon after their discovery in lymphoid tissues by Steinman and Cohn, as cells with the unique ability to prime naïve antigen-specific T cells, it was realized that DC can exist in at least two distinctive states characterized by morphological, phenotypic and functional changes-this led to the description of DC maturation. It is now well appreciated that there are several subsets of DC in both lymphoid and non-lymphoid tissues of mammals, and these cells show remarkable functional specialization and specificity in their roles in tolerance and immunity. This review will focus on the specific characteristics of DC subsets and how their functional specialization may be regulated by distinctive gene expression programs and signaling responses in both steady-state and in the context of inflammation. In particular, we will highlight the common and distinctive genes and signaling pathways that are associated with the functional maturation of DC subsets.

Keywords: dendritic cells; homeostasis; immunity; tolerance.

Figure 1. Major subsets of DC in mouse and human

Human and mouse DC subsets can be aligned into four major subsets irrespective of their location in secondary lymphoid tissues or in the parenchyma of non-lymphoid organs. They correspond to Xcr1⁺ cDC, CD11b⁺ cDC, pDC and moDC. The precursors that are found in the blood and give rise to the four major DC subsets are shown. Alternative markers or names used to identify those subsets are also indicated, as well as the proposed conserved functional specialization of these subsets.

Figure 2. Comparison of the expression patterns of TLRs across DC and monocyte subsets in mice

The bar graphs show relative gene expression (mean ± SD) for individual TLRs across blood monocyte subsets (gray bars), spleen LT-DC subsets from untreated animals (plain color bars) and cutaneous LN (CLN) mig-NLT-DCs from DNFB skin-painted animals (hatched color bars). These data are compiled from our own publically available datasets and those of the Immgen consortium (Heng et al, ; Tamoutounour et al, 2013). Key: c monocytes, classical blood monocytes characterized as CD11b⁺ Ly6c^hi MHCII⁻ cells; nc monocytes, non-classical monocytes characterized as CD11b⁺ Ly6C^lo MHCII⁻ cells. For each gene, expression values are normalized to maximal expression across all samples and the mean of 2 to 5 replicates for each cell subset is shown.

Figure 3. Comparison of homeostatic versus PRR-induced DC maturation

(A) Converging changes in gene expression between homeostatic and PRR-induced DC maturation. Heatmap showing the fold change between immature and mature DC for two sets of genes previously reported to be, respectively, decreased (“CORE DOWN”) or increased (“CORE UP”) upon TLR-induced maturation; irrespective of DC subset, stimuli and species of origin. For homeostatic maturation, fold change in gene expression levels was computed by comparing mature DC having migrating in cutaneous or lung lymph nodes under steady-state conditions (CLN CD11b⁺ mig-NLT-DC and LULN CD103⁺ mig-NLT-DC) to their immature counterparts from skin or lung. For TLR-induced maturation, fold change in gene expression levels was computed by comparing TLR-stimulated DC (lung CD103⁺ DC isolated from PolyI:C-treated animals, spleen CD11b⁺ DC and CD8α⁺ DC isolated from MCMV-infected mice, and spleen CD8α⁺ DC isolated from PolyI:C-treated animals) to their immature, unstimulated, counterparts from the same tissue. Genes that showed a statistically significant and similar regulation in their expression in homeostatic maturation and in PolyI:C-induced maturation of lung CD103⁺ DC are shown in bold, black font. Genes that showed a statistically significant regulation in these conditions but with a reciprocal change between homeostatic maturation versus PolyI:C-induced maturation of lung CD103⁺ DC are highlighted in bold, red font. (B-D) Overlapping instructive signals drive homeostatic and PRR-induced DC maturation. Venn diagrams were drawn for comparing the sets of genes significantly induced (UP) or repressed (DOWN) upon TLR-induced maturation of lung CD03⁺ DC isolated from PolyI:C-treated animals, and upon homeostatic maturation of CLN CD11b⁺ mig-NLT-DC or in LULN CD103⁺ mig-NLT-DC. Ingenuity pathway analysis was used to search whether resulting gene lists were enriched for targets of known transcription factors, activation receptors or cytokines for repressed (C) genes or induced (D) genes. The results of the most significant enrichments obtained are shown as bar graphs, with regulators regrouped by functional network according to IPA classification (➊, IRF/IFN-I network; ➋, NF-κB/TNF/IL-1β network; ➌, CD40/CD40LG network; ➍, IFN-γ network). Red bars indicate p-values for TLR-induced maturation (red circle of the Venn diagrams) and blue bars for homeostatic maturation (specifically for the genes commonly regulated upon both conditions of homeostatic maturation, area circled by a blue line on the Venn Diagrams). The corresponding gene lists are given in Supplementary Table S1. (E) Changes in the expression of genes involved in activation versus inhibition of immune responses upon homeostatic or PRR-induced DC maturation. The heatmap shows the relative expression of individual genes in immature versus mature DC, normalized to the mean expression levels in immature DC. Genes that showed a statistically significant and similar induction in their expression in homeostatic maturation and in TLR-induced maturation are shown in bold, black font. Genes that showed a statistically significant induction in TLR-induced maturation reaching levels higher than those observed in homeostatic maturation are highlighted in bold, red font. Conversely, genes that showed a statistically significant induction upon homeostatic maturation reaching levels higher than those observed in TLR-induced maturation are highlighted in bold, blue font.

Figure 4. Key components of homeostatic versus PRR-induced DC maturation

The scheme illustrates commonalities and differences in the signaling pathways and downstream regulation of gene expression triggered in DC during homeostatic (tolerogenic) versus TLR-induced (immunogenic) maturation. The stronger and broader activation of signaling by the IRF/IFN-I and NFκB/TNF/IL-1β networks in TLR-induced maturation might not only result from activating receptors on DC but also from differences in the kinetics of the activation of negative regulators including A20 and p105, with a very early induction during homeostatic DC maturation contrasting with a delayed, although even stronger induction occurring during TLR-induced activation, as a late negative feedback loop in response to autocrine/paracrine inflammatory cytokine signaling.