Dendritic cells as controllers of antigen-specific Foxp3+ regulatory T cells

Abstract

Regulatory T cells (Treg) are a subpopulation of CD4(+) lymphocytes that maintain immunological self-tolerance in the periphery. Treg also regulate or suppress other classes of immune response such as allograft rejection, allergy, tumor immunity, and responses to microbes. Treg express the Foxp3 transcription factor and CD25, the high affinity interleukin-2 receptor (IL-2R). Treg are divided into two types: naturally occurring Treg derived from thymus (natural Treg) and Treg induced from Foxp3(-) CD4(+) T cells in the periphery (induced Treg). It would be valuable to understand how to control the generation of antigen-specific Treg, which could also provide a new approach to treat autoimmunity, allergy or allograft rejection without suppressing immune responses to tumor and microbes. In this review, we will discuss the role of dendritic cells (DCs) in controlling antigen-specific natural Treg and induced Treg. Natural Treg are anergic upon T cell receptor stimulation generally, however, we found that the antigen-specific natural Treg can be expanded by antigen-presenting mature bone marrow-derived dendritic cells (BM-DCs). Furthermore, recent studies showed that antigen-specific Treg can be induced from Foxp3(-) CD25(-) CD4(+) T cells by antigen-presenting DCs, particularly select subsets of DCs in the periphery. These findings need to be pursued to develop novel immune suppressive therapies using antigen-specific Treg educated by DCs.

Fig. 1. Natural Treg vs induced Treg

Natural Treg are derived from thymus. The development and function of Treg is determined by the transcription factor, Foxp3. Treg are also induced from Foxp3⁻ cells in the periphery by TGF-β (induced Treg). Natural Treg and induced Treg help maintain immunological self-tolerance in the periphery.

Fig. 2. Induction of Foxp3⁺ Treg from Foxp3⁻ CD25⁻ CD4⁺ T cells by DC plus TGF-β

Foxp3⁻ CD25⁻ CD4⁺ T cells (2×10⁴) from DO11.10 RAG^−/− mice were cultured for 7 days with spleen CD11c⁺ DCs (2×10⁴) and 0.3 μg/ml OVA peptide in the presence of TGF-β (2 ng/ml). Cells were stained with anti-CD4, KJ1.26 clonotype, CD25, and Foxp3 or isotype control Abs, and analyzed by fluorescence-activated cell sorting (FACS) (Left). Absolute numbers of Foxp3⁺CD4⁺KJ1.26 clonotype⁺ T cells per culture at day 7 are also shown (Right).

Fig. 3. Lower doses of DCs induce greater number of induced Treg with lower doses of antigen than DC-depleted APCs

Foxp3⁻ CD25⁻ CD4⁺ T cells from DO11.10 RAG^−/− mice were cultured as in Fig. 2 in the presence of TGF-β. Lower numbers of spleen CD11c⁺ DCs induced more Foxp3⁺ Treg and with lower doses of antigen than DC-depleted spleen APCs. The DCs did not require supplementation by IL-2, whereas DC-depleted spleen APCs required IL-2 supplementation to induce Foxp3⁺ Treg.

Fig. 4. CD8⁺ spleen DCs induce Foxp3⁺ T reg from Foxp3⁻ CD25⁻ CD4⁺ T cells in the absence of exogenous TGF-β in vitro

As in Fig. 1, but Foxp3⁻ CD25⁻ CD4⁺ T cells from DO11 RAG^−/− mice were cultured with CD8⁻ or CD8⁺ spleen DCs plus peptide in the absence of added TGF-β. After 5-days of culture, cells were analyzed by FACS. Cells were gated on CD4⁺ CD11c⁻ T cells.

Fig. 5. Antigen targeting to DC subsets by anti-DC subset mAb and Foxp3⁺ Treg

OVA was engineered into anti-DEC-205 mAb (DEC-OVA) or anti-DCIR-2 33D1 mAb (33D1-OVA). In vivo injection of these chimera Abs delivers antigen to each DC subset selectively [36]. We found that antigen targeting via DEC-205 led to the induction of Foxp3⁺ Treg from Foxp3⁻ precursors, whereas antigen targeting within 33D1 mAb expanded preformed Foxp3⁺ Treg (natural Treg) in vivo [17].