Dendritic cells (DCs) are antigen-presenting cells with a unique ability to induce primary immune responses. DCs capture and transfer information from the outside world to the cells of the adaptive immune system. DCs are not only critical for the induction of primary immune responses, but may also be important for the induction of immunological tolerance, as well as for the regulation of the type of T cell-mediated immune response. Although our understanding of DC biology is still in its infancy, we are now beginning to use DC-based immunotherapy protocols to elicit immunity against cancer and infectious diseases.

Immunobiology of Dendritic Cells

Interleukin-10 (IL-10), first recognized for its ability to inhibit activation and effector function of T cells, monocytes, and macrophages, is a multifunctional cytokine with diverse effects on most hemopoietic cell types. The principal routine function of IL-10 appears to be to limit and ultimately terminate inflammatory responses. In addition to these activities, IL-10 regulates growth and/or differentiation of B cells, NK cells, cytotoxic and helper T cells, mast cells, granulocytes, dendritic cells, keratinocytes, and endothelial cells. IL-10 plays a key role in differentiation and function of a newly appreciated type of T cell, the T regulatory cell, which may figure prominently in control of immune responses and tolerance in vivo. Uniquely among hemopoietic cytokines, IL-10 has closely related homologs in several virus genomes, which testify to its crucial role in regulating immune and inflammatory responses. This review highlights findings that have advanced our understanding of IL-10 and its receptor, as well as its in vivo function in health and disease.

Interleukin-10 and the interleukin-10 receptor.

Regulatory T Cells and Immune Tolerance

The expanding universe of T-cell subsets: Th1, Th2 and more

Induction and maintenance of peripheral tolerance are important mechanisms to maintain the balance of the immune system. In addition to the deletion of T cells and their failure to respond in certain circumstances, active suppression mediated by T cells or T-cell factors has been proposed as a mechanism for maintaining peripheral tolerance. However, the inability to isolate and clone regulatory T cells involved in antigen-specific inhibition of immune responses has made it difficult to understand the mechanisms underlying such active suppression. Here we show that chronic activation of both human and murine CD4+ T cells in the presence of interleukin (IL)-10 gives rise to CD4+ T-cell clones with low proliferative capacity, producing high levels of IL-10, low levels of IL-2 and no IL-4. These antigen-specific T-cell clones suppress the proliferation of CD4+ T cells in response to antigen, and prevent colitis induced in SCID mice by pathogenic CD4+CD45RB(high) splenic T cells. Thus IL-10 drives the generation of a CD4+ T-cell subset, designated T regulatory cells 1 (Tr1), which suppresses antigen-specific immune responses and actively downregulates a pathological immune response in vivo.

A CD4 + T-cell subset inhibits antigen-specific T-cell responses and prevents colitis

Experimental autoimmune encephalomyelitis (EAE) is a cell-mediated autoimmune disease that serves as an animal model for multiple sclerosis. Oral administration of myelin basic protein (MBP) suppresses EAE by inducing peripheral tolerance. T cell clones were isolated from the mesenteric lymph nodes of SJL mice that had been orally tolerized to MBP. These clones were CD4+ and were structurally identical to T helper cell type 1 (TH1) encephalitogenic CD4+ clones in T cell receptor usage, major histocompatibility complex restriction, and epitope recognition. However, they produced transforming growth factor-beta with various amounts of interleukin-4 and interleukin-10 and suppressed EAE induced with either MBP or proteolipid protein. Thus, mucosally derived TH2-like clones induced by oral antigen can actively regulate immune responses in vivo and may represent a different subset of T cells.

https://science.sciencemag.org/content/sci/265/5176/1237.full.pdf

Regulatory T cell clones induced by oral tolerance: suppression of autoimmune encephalomyelitis

ORAL administration of antigen is used to induce antigen-specific peripheral immune tolerance1,2. As well as preventing systemic immune responses to ingested proteins3, oral tolerance to autoanti-gens has also been used to suppress autoimmune diseases in animals4-10and humans11,12. Both active suppression and clonal anergy are suggested to be mechanisms of oral tolerance, depending on the dose of antigen fed13,14. Here we report that oral antigen can delete antigen-reactive T cells in Peyer's patches, in mice transgenic for the ovalbumin-specific T-cell receptor genes. The deletion was mediated by apoptosis, and was dependent on dosage and frequency of feeding. At lower doses deletion was not observed; instead there was induction of antigen-specific cells that produced transforming growth factor (TGF)-β and interleukin (IL)-4 and IL-10 cytokines. At higher doses, both Thl and Th2 cells were deleted following their initial activation, whereas cells which secrete TGF-β were resistant to deletion. These findings demonstrate that orally administered antigen can induce tolerance not only by active suppression and clonal anergy but by extrathymic deletion of antigen-reactive Th1 and Th2 cells.

Peripheral deletion of antigen-reactive T cells in oral tolerance.

Orally administered antigens induce a state of immunologic hyporesponsiveness termed oral tolerance. Different mechanisms are involved in mediating oral tolerance depending on the dose fed. Low doses of antigen generate cytokine-secreting regulatory cells, whereas high doses induce anergy or deletion. We used mice transgenic for a T-cell receptor (TCR) derived from an encephalitogenic T-cell clone specific for the acetylated N-terminal peptide of myelin basic protein (MBP) Ac-1-11 plus I-Au to test whether a regulatory T cell could be generated from the same precursor cell as that of an encephalitogenic Th1 cell and whether the induction was dose dependent. The MBP TCR transgenic mice primarily have T cells of a precursor phenotype that produce interleukin 2 (IL-2) with little interferon gamma (IFN-gamma), IL-4, or transforming growth factor beta (TGF-beta). We fed transgenic animals a low-dose (1 mg x 5) or high-dose (25 mg x 1) regimen of mouse MBP and without further immunization spleen cells were tested for cytokine production. Low-dose feeding induced prominent secretion of IL-4, IL-10, and TGF-beta, whereas minimal secretion of these cytokines was observed with high-dose feeding. Little or no change was seen in proliferation or IL-2/IFN-gamma secretion in fed animals irrespective of the dose. To demonstrate in vivo functional activity of the cytokine-secreting cells generated by oral antigen, spleen cells from low-dose-fed animals were adoptively transferred into naive (PLJ x SJL)F1 mice that were then immunized for the development of experimental autoimmune encephalomyelitis (EAE). Marked suppression of EAE was observed when T cells were transferred from MBP-fed transgenic animals but not from animals that were not fed. In contrast to oral tolerization, s.c. immunization of transgenic animals with MBP in complete Freund's adjuvant induced IFN-gamma-secreting Th1 cells in vitro and experimental encephalomyelitis in vivo. Despite the large number of cells reactive to MBP in the transgenic animals, EAE was also suppressed by low-dose feeding of MBP prior to immunization. These results demonstrate that MBP-specific T cells can differentiate in vivo into encephalitogenic or regulatory T cells depending upon the context by which they are exposed to antigen.

https://www.pnas.org/content/pnas/93/1/388.full.pdf

Oral tolerance in myelin basic protein T-cell receptor transgenic mice: suppression of autoimmune encephalomyelitis and dose-dependent induction of regulatory cells.

We have previously shown that following oral administration of myelin basic protein (MBP), regulatory T cells are generated from gut-associated lymphoid tissue and that these cells suppress experimental allergic encephalomyelitis (EAE). These regulatory T cells produce transforming growth factor-β (TGF-β) with various amounts of IL-4 and IL-10 and these TGF-β-secreting T cells have been termed Th3 cells. T cells in lymphoid organs drained by mucosal sites secrete IL-4 as a primary T cell growth factor. In the present study, we examined the role of IL-4 on oral tolerance and in the generation of TGF-β secreting cells. Treatment of (PLJ × SJL)F1 mice with intraperitoneal (i.  p.) IL-4 and low-dose oral MBP (0.5 mg) given three times reduced the severity of EAE, whereas i.  p. injection of IL-4 alone or oral MBP alone given in these suboptimal doses, showed no protection. Spleen cells from protected mice produced increased amounts of TGF-β and reduced IFN-γ upon stimulation with MBP in vitro. Mucosal MBP-specific IgA production was significantly increased in IL-4 plus MBP fed animals. Moreover, oral administration of IL-4 (1 μg per feeding) also enhanced the suppression of EAE by oral MBP and this protective effect was reversed by administration of anti-TGF-β antibody in vivo. Reverse transcription-PCR showed enhanced suppression of IFN-γ in Peyer's patch in animals fed MBP and IL-4 versus those fed MBP alone. We then investigated the role of IL-4 in the generation of TGF-β-secreting cells using MBP Ac1-11 TCR transgenic animals. Cells were cultured with IL-2, IL-4, or IFN-γ in the presence of MBP and limiting dilution analysis for cytokine-secreting cells performed. We found that IL-4, but not IL-2 or IFN-γ, generated TGF-β-secreting T cells from naive splenic T cells and that these cells provided help for IgA production. These findings demonstrate that IL-4 is a differentiation factor for TGF-β-secreting Th3 cells and oral IL-4 has a synergistic effect on low-dose oral tolerance that is associated with increased TGF-β secretion.

IL‐4 is a differentiation factor for transforming growth factor‐β secreting Th3 cells and oral administration of IL‐4 enhances oral tolerance in experimental allergic encephalomyelitis

We have previously shown that orally administered myelin basic protein (MBP) suppresses experimental autoimmune encephalomyelitis in both the Lewis rat and the SJL mouse. In the Lewis rat fed low doses of MBP, we found that protection can be adoptively transferred by CD8+ cells and that these cells inhibit immune responses via the secretion of TGF-beta after Ag-specific triggering. In the present study, we investigated the cellular requirements for the generation of active suppression following oral administration of MBP in SJL and (PLJ x SJL)F1 mice. We first determined the frequency of MBP cells secreting Th1 (IFN-gamma) and Th2 (IL-4/IL-10) cytokines or TGF-beta after oral administration of MBP. We found that in SJL mice, orally administered MBP (0.5 mg/feeding) led to an increased frequency of TGF-beta-, IL-4-, and IL-10-secreting cells and a decreased frequency of IFN-gamma-producing cells. This pattern was observed in both CD4+ and CD8+ populations; adoptive transfer of either CD4+ or CD8+ cells from orally tolerized mice suppressed autoimmune encephalomyelitis in recipient animals. We then studied the role of CD8+ cells on the generation of oral tolerance to MBP by depleting CD8+ cells in vivo with anti-CD8 mAb. Oral tolerance was successfully induced in such animals, as demonstrated by a decrease in clinical disease and T cell proliferative responses, although there was less TGF-beta production in vitro and less disease protection on days 20 to 22 in CD8-depleted animals. These studies demonstrate that CD4+ cells in the absence of CD8+ cells can mediate the active suppression component of oral tolerance in mice and that there is a reciprocal relationship between Th1- and Th2-type cytokine production associated with oral tolerization.

Jun-ichi Inobe

Papers

Regulatory T cell clones induced by oral tolerance: suppression of autoimmune encephalomyelitis

Peripheral deletion of antigen-reactive T cells in oral tolerance.

Oral tolerance in myelin basic protein T-cell receptor transgenic mice: suppression of autoimmune encephalomyelitis and dose-dependent induction of regulatory cells.

IL‐4 is a differentiation factor for transforming growth factor‐β secreting Th3 cells and oral administration of IL‐4 enhances oral tolerance in experimental allergic encephalomyelitis

Induction of oral tolerance to myelin basic protein in CD8-depleted mice: both CD4+ and CD8+ cells mediate active suppression.