Tolerogenic Dendritic Cell

A tolerogenic dendritic cell (tolDC) is an immature, steady-state migratory, or pharmacologically conditioned dendritic cell subset that presents cognate peptide–major histocompatibility complex (pMHC) complexes to T lymphocytes in the absence of pro-inflammatory costimulation while delivering coinhibitory receptor engagement and immunosuppressive cytokines, thereby enforcing peripheral tolerance and central tolerance. Conceptualized in the late 1990s and early 2000s by Ralph M. Steinman, Michel C. Nussenzweig, Daniel Hawiger, Angus W. Thomson, and Manfred B. Lutz, the discovery of tolerogenic dendritic cells overturned the classical view that immature dendritic cells are merely quiescent precursors awaiting pathogen activation. By coupling constitutive self-antigen capture with low expression of CD80, CD86, and CD40, elevated expression of PD-L1, ILT3 (LILRB4), and ILT4 (LILRB2), and secretion of interleukin-10 (IL-10), transforming growth factor-beta (TGF-β), all-trans retinoic acid (ATRA), and indoleamine 2,3-dioxygenase 1 (IDO1), tolerogenic dendritic cells drive T-cell clonal deletion, clonal anergy, and conversion into Foxp3⁺ regulatory T cells (pT_reg) and type 1 regulatory T cells (Tr1).

Steinman and Nussenzweig's Steady-State DEC-205 Targeting Experiments (2001)

To test how dendritic cells process and present self-antigens in the absence of infection or adjuvant, Daniel Hawiger, Kayo Inaba, Michel Nussenzweig, and Ralph Steinman (2001) engineered a recombinant chimeric monoclonal antibody that delivered hen egg lysozyme (HEL) or ovalbumin (OVA) directly to the endocytic C-type lectin receptor DEC-205 (CD205) on steady-state lymphoid-resident dendritic cells in vivo:

1. Steady-State Presentation Without Adjuvant: When antigen was targeted to DEC-205⁺ dendritic cells in the steady state (without agonistic anti-CD40 antibody or Toll-like receptor ligands), antigen-specific CD4⁺ and CD8⁺ T cells underwent a brief burst of abortive proliferation followed within 7 to 14 days by Bim-dependent clonal deletion and functional anergy of the surviving clones, rendering the mice tolerant to subsequent challenge with antigen in complete Freund's adjuvant.
2. Low-Dose Conversion Into Foxp3⁺ pT_reg Cells (Kretschmer et al., 2005): When Harald von Boehmer and Karsten Kretschmer (2005) delivered sub-immunogenic doses of DEC-205-targeted peptide in the presence of steady-state TGF-β signaling, naive CD4⁺Foxp3⁻ T cells were converted de novo into stable, suppressive Foxp3⁺ peripheral regulatory T cells (pT_reg).
3. Semi-Mature Tolerogenic Dendritic Cells (Lutz & Schuler, 2002): Manfred B. Lutz and Gerold Schuler subsequently demonstrated that tolerogenicity is not restricted to immature dendritic cells: semi-mature dendritic cells (stimulated by TNF-α, apoptotic bodies, or Wnt/β-catenin signaling without microbial TLR ligands) upregulate MHC class II and CCR7 (enabling migration to draining lymph nodes via high endothelial venules and afferent lymphatics) and moderate levels of costimulatory molecules, but completely lack IL-12p70, IL-6, IL-1β, and IL-23 secretion, driving IL-10-producing regulatory T cells rather than effector T_H1/T_H17 immunity.

1. CD103⁺ Mucosal Dendritic Cells, Retinoic Acid (RALDH2), and TGF-β Activation

Within the intestinal lamina propria and mesenteric lymph nodes, CD103⁺ (α_Eβ₇ integrin) conventional dendritic cells—conditioned by colonic short-chain fatty acids (butyrate), thymic stromal lymphopoietin (TSLP), and mucin-2 signaling—act as the master physiological architects of oral tolerance to dietary proteins and commensal bacteria. As discovered by Yasmine Belkaid, Fiona Powrie, and Makoto Iwata, mucosal CD103⁺ tolDCs express two cooperating molecular modules:

• Integrin α_vβ₈-Mediated Latent TGF-β Cleavage: High surface expression of integrin α_vβ₈ mechanically liberates active TGF-β1 from its latency-associated peptide (LAP) directly across the immunological synapse.
• Retinaldehyde Dehydrogenase 2 (RALDH2 / ALDH1A2): Mucosal CD103⁺ DCs oxidize dietary vitamin A (retinol → retinal) into all-trans retinoic acid (ATRA). Diffusing into docked naive CD4⁺ T cells, ATRA ligates nuclear RARα/RXR heterodimers, which synergize with TGF-β-activated SMAD2/3 at the FOXP3 CNS1 enhancer to drive Foxp3⁺ pT_reg differentiation while imprinting gut-homing receptors integrin α₄β₇ and CCR9 and suppressing RORγt-driven T_H17 differentiation.

2. Indoleamine 2,3-Dioxygenase 1 (IDO1) Catalytic and Non-Catalytic Signaling

Induced by CTLA-4 (CD152) engagement of CD80/CD86, IFN-γ, or TGF-β, the heme-containing enzyme indoleamine 2,3-dioxygenase 1 (IDO1) enforces tolerogenicity via two distinct mechanisms (Grohmann, Puccetti, & Mellor):

• Catalytic Tryptophan Starvation and Kynurenine–AhR Signaling: IDO1 cleaves the pyrrole ring of the essential amino acid L-tryptophan into N-formylkynurenine and L-kynurenine. Local tryptophan depletion activates the uncharged-tRNA sensor GCN2 kinase and inhibits mTORC1 in neighboring T cells (arresting effector proliferation in the G₁ phase), while secreted L-kynurenine binds the cytosolic aryl hydrocarbon receptor (AhR) in naive CD4⁺ T cells, driving nuclear translocation of AhR/ARNT to transcribe FOXP3 and IL10.
• Non-Catalytic ITIM Moonlighting Signaling (Pallotta et al., 2011): Independently of its enzymatic activity, human and murine IDO1 contains two immunoreceptor tyrosine-based inhibitory motifs (ITIM1 and ITIM2). When phosphorylated by Fyn kinase in response to TGF-β, the IDO1 ITIM motifs recruit the tyrosine phosphatases SHP-1 and SHP-2, activating the non-canonical NF-κB (RelB/p52) pathway to drive sustained, self-amplifying transcription of IDO1 and TGFB1 for weeks—underpinning infectious tolerance.

3. Immunoglobulin-Like Transcripts (ILT3 / ILT4) and HLA-G

Human tolerogenic dendritic cells—especially the specialized DC-10 subset characterized by Maria Grazia Roncarolo and Silvia Gregori (CD14⁺CD16⁺CD141⁺CD163⁺ DCs induced by IL-10)—express high densities of the inhibitory receptors ILT3 (LILRB4) and ILT4 (LILRB2) alongside the non-classical MHC class I molecule HLA-G. Concurrent binding of dendritic-cell ILT4 and HLA-G to ligand partners on naive CD4⁺ T cells in the presence of DC-derived IL-10 and IL-27 activates c-Maf, AhR, and STAT3, programming the naive T cells into Foxp3-negative, IL-10-hypersecreting type 1 regulatory T cells (Tr1).

Because pharmacologically conditioned tolDCs remain maturation-resistant when re-challenged with lipopolysaccharide or pro-inflammatory cytokines, autologous monocyte-derived tolerogenic dendritic cells (mo-tolDCs) are manufactured ex vivo from patient leukapheresis monocytes using GM-CSF + IL-4 combined with tolerizing agents—including dexamethasone, 1α,25-dihydroxyvitamin D₃ (calcitriol), rapamycin, IL-10, NF-κB oligonucleotide decoys, or antisense oligonucleotides targeting CD40/CD80/CD86—pulsed with disease-specific autoantigens or donor alloantigens (or allowed to acquire donor MHC via cross-dressing), and infused into patients:

• Rheumatoid Arthritis (Rheumavax & AuToDeCRA Trials — Thomas et al., 2015; Isaacs et al., 2017): Intra-articular or intradermal injection of autologous NF-κB-inhibited (BAY 11-7082) or dexamethasone/vitamin D₃-conditioned tolDCs loaded with citrullinated filaggrin, vimentin, aggrecan, and type II collagen peptides safely reduced DAS28 disease activity scores and increased the systemic ratio of regulatory T cells to effector T cells in HLA-DRB1 shared-epitope-positive rheumatoid arthritis patients.
• Type 1 Diabetes (Giannoukakis et al., 2011; Roep et al., 2015): Intradermal administration of antisense-oligonucleotide-silenced (CD40/CD80/CD86-knockdown) or vitamin D₃/dexamethasone-modulated tolDCs pulsed with proinsulin peptide (C19-A3) preserved endogenous C-peptide secretion and induced antigen-specific IL-10⁺ Tr1 cells without systemic immunosuppression.
• Multiple Sclerosis and Neuromyelitis Optica (Zubizarreta et al., 2019): Phase Ib trials of dexamethasone-conditioned tolDCs loaded with seven myelin peptides (MBP, MOG, PLP) or aquaporin-4 (AQP4) peptides demonstrated clinical safety and expansion of IL-10-producing PBMC subsets.
• Solid-Organ Transplantation (The ONE Study — Moreau et al., 2020; Thomson et al., 2018): Infusion of autologous or donor-derived regulatory dendritic cells (ATDCs / DC_reg) 1 to 7 days prior to living-donor kidney or liver transplantation enabled weaning of mycophenolate mofetil and reduction of tacrolimus trough requirements while preventing acute allograft rejection.

How can the same dendritic cell lineage induce immunity during infection but tolerance in the steady state?

Dendritic cell function is governed by the integration of Signal 1 (pMHC engagement of the TCR), Signal 2 (costimulatory vs. coinhibitory receptor balance), and Signal 3 (polarizing cytokines and metabolites). During infection, microbial pathogen-associated molecular patterns (PAMPs) activate Toll-like receptors, RIG-I, or cGAS–STING, upregulating CD80/CD86/CD40 and IL-12/IL-23 to drive effector T-cell clonal expansion. In the absence of PAMPs (or when conditioned by apoptotic cells, TGF-β, IL-10, vitamin D₃, or retinoic acid), steady-state and semi-mature tolerogenic dendritic cells present Signal 1 while withholding CD80/CD86 costimulation, engaging PD-1 and CTLA-4, depleting tryptophan via IDO1, and secreting IL-10 and TGF-β—converting Signal 1 into an instruction for T-cell deletion, anergy, or T_reg differentiation.

Why must clinical-grade tolerogenic dendritic cells be tested for "maturation resistance" before infusion?

If an immature dendritic cell loaded with autoantigens (such as proinsulin or myelin peptides) or donor transplant antigens is injected into an inflamed patient joint, pancreas, or allograft without being epigenetically and metabolically locked into a tolerogenic state, local pro-inflammatory cytokines (TNF-α, IL-1β, IFN-γ) or damage-associated molecular patterns (DAMPs) could trigger full immunogenic maturation—upregulating CD80/CD86 and IL-12 and paradoxically worsening autoimmune destruction or graft rejection. Conditioning with 1α,25-dihydroxyvitamin D₃ + dexamethasone or IL-10 epigenetically silences the IL12B and CD40 promoters and locks the cell into oxidative phosphorylation so that even lipopolysaccharide (LPS) or CD40L challenge fails to induce IL-12p70.

What is the difference between DC-10 cells and conventional CD103⁺ mucosal tolerogenic dendritic cells?

CD103⁺ mucosal dendritic cells reside in the gut lamina propria and mesenteric lymph nodes and rely primarily on RALDH2-generated retinoic acid and α_vβ₈-activated TGF-β1 to convert naive CD4⁺ T cells into Foxp3⁺ peripheral T_reg (pT_reg) cells. By contrast, DC-10 cells are a distinct human CD14⁺CD16⁺CD141⁺CD163⁺ subset present in spleen, decidual tissue, and peripheral blood that express membrane HLA-G and ILT4 and secrete high levels of IL-10, driving naive CD4⁺ T cells to differentiate into Foxp3-negative type 1 regulatory T cells (Tr1).

1. Hawiger, D., Inaba, K., Dorsett, Y., Guo, M., Mahnke, K., Rivera, M., Ravetch, J. V., Steinman, R. M., & Nussenzweig, M. C. (2001). "Dendritic cells induce peripheral T cell unresponsiveness under steady state conditions in vivo." Journal of Experimental Medicine, 194(6), 769–779. https://doi.org/10.1084/jem.194.6.769
2. Steinman, R. M., Hawiger, D., & Nussenzweig, M. C. (2003). "Tolerogenic dendritic cells." Annual Review of Immunology, 21(1), 685–711. https://doi.org/10.1146/annurev.immunol.21.120601.141040
3. Gregori, S., Tomasoni, D., Pacciani, V., Scirpoli, M., Battaglia, M., Magnani, C. F., Hauben, E., & Roncarolo, M. G. (2010). "Differentiation of type 1 T regulatory cells (Tr1) by tolerogenic DC-10 requires the IL-10–dependent ILT4/HLA-G pathway." Blood, 116(6), 935–944. https://doi.org/10.1182/blood-2009-07-234872
4. Pallotta, M. T., Orabona, C., Volpi, C., Vacca, C., Belladonna, M. L., Bianchi, R., Servillo, G., Brunacci, C., Calvitti, M., Bicciato, S., Mazza, E. M. C., Boon, L., Grassi, F., Fioretti, M. C., Fallarino, F., Puccetti, P., & Grohmann, U. (2011). "Indoleamine 2,3-dioxygenase is a signaling protein in long-term tolerance by dendritic cells." Nature Immunology, 12(9), 870–878. https://doi.org/10.1038/ni.2077