Dendritic cells (DCs) play variable roles in immune activation and tolerance, in a manner dependent on their highly heterogenic phenotypes or functions [1,2]. Regulatory or tolerogenic DCs induce immune tolerance towards harmless components, which are critical for the maintenance of immune homeostasis [3]. Regulatory DCs are characterized by downregulated expression of co‐stimulatory molecules such as CD40, CD80, MHC class II and proinflammatory cytokines such as IL-12 and CCL5, but increased expression of inhibitory molecules and anti‐inflammatory cytokines such as IL-10 and TGF-β [4]. In addition, regulatory DCs play important roles in inducing regulatory T cell generation or T cell apoptosis, and in inhibiting T cell proliferation and activation [5,6]. A breakdown of DCs-mediated tolerance or dysfunction of regulatory DCs is closely linked with the pathogenesis of inflammatory and autoimmune diseases. New immunotherapies have taken advantage of the tolerogenic potential of regulatory DCs for the treatment of related diseases [[7], [8], [9]]. Understanding the function and internal modulatory mechanism of regulatory DCs is of great significance for understanding the pathogenesis of autoimmune and inflammatory diseases and developing therapeutic strategies [10].

The generation and function of regulatory DCs are controlled by a complex network of environmental signals and intrinsic cellular processes, with the underlying mechanisms remaining elusive [4]. Emerging findings indicate that tissue stroma plays an active role in orchestrating the generation and function of immune cells including regulatory DCs [[11], [12], [13], [14]]. These findings highlight the important role of region-specific tissue microenvironment in determining the function and fate of immune cells. In our previous studies, we showed that various stromal microenvironments including those in the spleen, lung, liver, and tumor can program the generation of CD11bhigh Ialow regulatory DCs with potent immunosuppressive function [12,[15], [16], [17], [18]]. In particular, we demonstrate splenic stroma can drive the differentiation of immune-activating mature DCs (maDCs) toward a unique tolerogenic DC subset (herein namely diffDCs) in a manner dependent on cell-cell interactions and anti-inflammatory cytokine transforming growth factor-beta (TGF-β) [12]. However, what internal adaptions occur in tissue stroma-induced regulatory DCs and the molecular control of regulatory DCs generation, function, and tolerogenicity maintenance needs to be further investigated.

Metabolic enzymes and metabolites play important roles in modulating cellular identity and function [[19], [20], [21], [22]]. A crosstalk between metabolism and immunity is essential for DCs development and function [23]. Various metabolites have been found to either enhance or inhibit DC-mediated immune responses. For example, glycolysis is sustained for the synthesis of ATP for survival in inflammatory DCs, while oxidative phosphorylation (OXPHOS) is progressively inhibited by nitric oxide at a late time after activation [24,25]. The glycolytic metabolites such as NAD+ promote DCs migration through stabilizing F-actin polarization and polymerization [26,27]. On the other side, the Irg1/itaconate pathway plays a direct regulatory role in DCs for attenuating type 2 airway inflammation in allergic asthma [28]. The tryptophan metabolic enzyme indoleamine 2,3-dioxygenase 1 (IDO1)-catalyzed tryptophan metabolite l-kynurenine in cDC1 can promote the tolerogenic function of inflammatory cDC2 [29]. However, the metabolic and transcriptional profiles during differentiation and functional transition of splenic stroma-induced regulatory DCs, and the role of metabolic enzymes or metabolites in endowing DCs with tolerogenic properties need to be addressed. In particular, whether metabolites regulate regulatory DCs function via modulating unconventional PTMs such as succinylation and the function of metabolic enzymes selectively involved in the process remain poorly understood.

To characterize the metabolic feature of regulatory DCs and its correlation with regulatory DCs’ function, we profiled the metabolic and transcriptional changes during diffDCs differentiation through metabolomics and transcriptomics analysis with mature DCs as a control. We identified succinate-CoA ligase β subunit Suclg2 as a metabolic regulator, which is upregulated in diffDCs, in maintaining the immunosuppressive function of diffDCs. We also identified Lactb as a new positive regulator of NF-κB signaling whose succinylation at the lysine 288 residue was inhibited by Suclg2. Therefore, we demonstrate that Suclg2 is required for the metabolic maintenance of tolerogenic properties of diffDCs.