Tolerance through Education: How Tolerogenic Dendritic Cells Shape Immunity

Abstract

Dendritic cells (DCs) are central players in the initiation and control of responses, regulating the balance between tolerance and immunity. Tolerogenic DCs are essential in the maintenance of central and peripheral tolerance by induction of clonal T cell deletion and T cell anergy, inhibition of memory and effector T cell responses, and generation and activation of regulatory T cells. Therefore, tolerogenic DCs are promising candidates for specific cellular therapy of allergic and autoimmune diseases and for treatment of transplant rejection. Studies performed in rodents have demonstrated the efficacy and feasibility of tolerogenic DCs for tolerance induction in various inflammatory diseases. In the last years, numerous protocols for the generation of human monocyte-derived tolerogenic DCs have been established and some first phase I trials have been conducted in patients suffering from autoimmune disorders, demonstrating the safety and efficiency of this cell-based immunotherapy. This review gives an overview about methods and protocols for the generation of human tolerogenic DCs and their mechanisms of tolerance induction with the focus on interleukin-10-modulated DCs. In addition, we will discuss the prerequisites for optimal clinical grade tolerogenic DC subsets and results of clinical trials with tolerogenic DCs in autoimmune diseases.

Keywords: immunotherapy; nanoparticles; regulatory T cells; tolerance; tolerogenic dendritic cells.

Figure 1

Flow cytometric phenotyping of dendritic cells (DCs) aligned across tissues. Surface marker expression of human and mouse DCs in a variety of tissues was defined previously by Guilliams et al. (22) for cDC1s and conventional type 2 DCs (cDC2s). To identify DC subpopulations, a multi-color FACS staining, FSC/SSC pre-gating, and linage (lymphocytes and NK cells) as well as macrophage exclusion has to be performed. If applicable for the desired tissue, afterward, CD45 immune cells are gated for CD1αhighCD11cint Langerhans cells (LCs). LCs excluded cells are then identified by the expression of either CADM1^highC172a^lowCD141^highCD26^highCD11c^mid-high as cDC1s or CADM1^highC172a^highCD1c^highCD11c^high cDC2s. In humans and mice, DC cell fate can be additionally identified on the level of transcription factors: DCs in general are dependent on flt-3. cDC1 development depends on BTAF3 and high levels of IRF8, whereas cDC2 evolution is dependent on IRF4 but independent of BATF3.

Figure 2

Immunosuppressive mechanisms of tolerogenic dendritic cells (DCs). Immunosuppressive mechanisms of tolerogenic DCs include secretion of immunomodulatory mediators, like interleukin (IL)-10 and TGF-β, or retinoic acid, resulting in induction of tolerogenic DCs, inhibition of effector T cell function, and Treg generation. In addition, absence or reduction of major histocompatibility and co-stimulatory molecules is involved in induction of anergic T cells with regulatory capacity. Furthermore, expression of immune-modulatory/-inhibitory molecules like PDL-1/-2, CTLA-4, and ILT-3/4 or expression of death receptors like TRAIL or FAS represent mechanisms to inhibit efficient T cell responses by tolerogenic DCs. In addition, deprivation of nutrition factors by the expression of indoleamine 2,3-dioxygenase (IDO) and heme oxygenase-1 (HO-1) results in reduced T cell proliferation and Treg induction, respectively. In a similar way, shedding of soluble CD25 leads to IL-2 deprivation and reduced T cell proliferation.

Figure 3

Human tolerogenic dendritic cells (DCs) are induced by various immunosuppressive drugs and mediators. Immuno-activating and -inhibitory surface molecules as well as secreted signaling molecules are demonstrated. Arrows indicate up/downregulation or unchanged expression or secretion by human tolerogenic DCs compared to either mature DCs (activating surface molecules + secretion) or immature DCs (iDCs) (inhibitory surface molecules), respectively. As an exception, expression of activation molecules, and secretion of immune mediators marked with * are compared to iDCs. Note: in some protocols, tolerance-inducing agent is added at the beginning and during the entire culture of tolerogenic DCs, whereas others are added at the end of the culture for 1–3 days either with or without a maturation stimulus. Protocols that involve a maturation stimulus are marked with an orange flash. *Compared to iDCs.

Figure 4

Phenotype of monocyte-derived interleukin (IL)-10 dendritic cells (DCs) obtained by different protocols. Immuno-activating and -inhibitory surface molecules as well as secreted signaling molecules and the T cell response are depicted. Arrows indicate up/downregulated or unchanged expression or secretion by human IL-10-modulated tolerogenic DCs compared to mature DC. IL-10 DCs are generated by addition of the immunosuppressive cytokine during the maturation step at the end of the culture, whereas DC10 are obtained by incubation with IL-10 during the entire culture period.

Figure 5

Tolerogenic dendritic cells (DCs) in clinical application. For clinical applications, CD14⁺ monocytes are isolated from apheresis products to generate tolerogenic DCs, which are loaded with (auto-) antigens or allergens. Subsequently, antigen-specific tolerogenic DCs are reinjected into the patients to affect the inflammatory immune response of autoimmune or allergic diseases.