Review

Types and functions of regulatory T cells

T_REG cells can be classified according to their surface phenotype or their cytokine secretion profile. There are two main subsets of T_REG cells: natural T_REG (nT_REG) cells and inducible T_REG (iT_REG) cells.

The best-characterized population of nT_REG cells consists of CD4⁺CD25⁺ T_REG cells. These cells develop in the thymus and are distinguished by their expression of interleukin 2 receptor (CD25) and the transcription factor forkhead box protein P3 (FOXP3). They display a T-cell receptor repertoire that is skewed towards the recognition of self antigens.⁸ There are doubts regarding whether a unique marker for human nT_REG cells exists.⁹ FOXP3 expression, for example, has been shown to be a normal consequence of CD4⁺ T-cell activation in humans and cannot, therefore, be considered an exclusive marker of nT_REG cells.^{10, 11}

A novel population of nT_REG cells has recently been identified in human peripheral blood. These cells can be either CD4⁺ or CD8⁺, they lack FOXP3 expression, and they are distinguished from other nT_REG cells by the expression of human leukocyte antigen G (HLA-G; Table 1 and Figure 1).^{12, 13}

Table 1 Major subsets of human regulatory T cells.

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The iT_REG cells include T-helper 3 (T_H3) cells, which originate from naive T cells that are either CD4⁺ or CD8⁺, and type 1 T_REG (T_R1) cells, which are derived from CD4⁺ precursor cells. The iT_REG cells are induced in the periphery from nonregulatory T cells or by autoantigens during an autoimmune response, and they may or may not express FOXP3.⁸ The activities of T_EFF cells and antigen-presenting cells are reduced by immunosuppressive cytokines, such as interleukin 10 (IL-10) and transforming growth factor (TGF-), that are produced by iT_REG cells (Table 1 and Figure 1). Certain CD8⁺ iT_REG cells have been shown to have suppressive capabilities,¹⁴ and CD8⁺ iT_REG cells that lack expression of CD28 were recently identified as a distinct population with the capacity to suppress immune responses by directly interacting with antigen-presenting cells and rendering them tolerogenic.¹⁵

Regulatory T cells and autoimmunity

Experiments involving targeted deletions or mutations of molecules that are relevant for T_REG-cell function have substantiated the importance of T_REG cells for tolerance and protection against autoimmunity. In addition, T_REG cells have been shown to be important in the maintenance of peripheral tolerance and for protection against numerous experimental autoimmune diseases,^{16, 17, 18} including experimental autoimmune encephalomyelitis (EAE), the animal model of MS.^{19, 20, 21, 22, 23} FOXP3 has been identified as a key regulatory molecule in the development of thymic nT_REG cells, and a genetic defect in or inducible ablation of the Foxp3 gene causes a scurfy autoimmune phenotype in mice.²⁴ Similarly, mutations in the FOXP3 gene in humans can result in an aggressive and fatal autoimmune disorder known as immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome, or IPEX.²⁵ Finally, the deletion of Foxp3 in mice leads to reduced expression of putative suppressor genes, the expression of which is a characteristic of T_REG cells, suggesting that T_REG-cell development requires continuous expression of Foxp3_REG.^{26, 27} Loss of the T_REG cell markers cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), interleukin 2, IL-10 and glucocorticoid-induced tumor-necrosis factor receptor-related protein (GITR) has also been found to correlate with immunopathology.⁷ In one study, CD39^- T_REG cells failed to block allograft rejection, and both CD39 and CD73 were suggested to be potential markers for FOXP3⁺ T_REG cells.²⁸

Lessons from animal studies

The theory that T_REG cells have a key role in the prevention of T-cell autoaggression against CNS structures has been substantiated by a number of pivotal studies in rodent EAE models. Table 2 summarizes the most important findings from investigations of T_REG cells in experimental CNS inflammation.

Table 2 Studies that have investigated the role of regulatory T cells in mouse models of CNS autoimmunity.

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Transgenic (Tg) mice bearing a T-cell receptor against the autoantigen myelin basic protein (MBP) and crossed to a recombination-activating gene 1 (Rag1)-deficient background (Tg MBP/Rag^-/-) developed spontaneous EAE, whereas Tg MBP/Rag^+/+ mice were protected.²⁹ This finding was explained by the presence of regulatory cells in Rag^+/+ mice but not in Rag^-/- mice. Hori et al. conducted experiments that showed that T_REG cells were indeed crucially contributing to this phenomenon; the investigators demonstrated that the adoptive transfer of CD4⁺CD25⁺ T cells from wild-type animals to Tg MBP/Rag^-/- mice could prevent the development of spontaneous EAE.³⁰ In addition, adoptive transfer experiments have shown that large quantities of CD4⁺CD25⁺ T cells purified from peripheral lymph nodes of naive mice can reduce the incidence and severity of EAE in both C57Bl/6³¹ and SJL²³ recipient mouse strains, which experience chronic and relapsing–remitting forms of EAE, respectively (Table 2). In a study conducted by Matsumoto et al., peripheral CD4⁺CD25⁺ T cells from mice with EAE suppressed the development of chronic EAE in recipient rats.³²

The epitope-specific activation of T_REG cells was initially tested in SJL mice, and proteolipid protein (PLP)-specific expanded T_REG cells that were passively transferred into these host mice were able to protect the mice from PLP-induced EAE.²² Similarly, McGeachy et al. showed that myelin-oligodendrocyte glycoprotein (MOG)-induced EAE was markedly alleviated in mice that received passive transfer of low numbers of CNS-derived T_REG cells that were isolated from mice in the recovery phase of MOG-induced EAE.²¹ Remarkably, the same numbers of CD4⁺CD25⁺ T cells isolated from lymph nodes had no such effect.²¹ These observations indicate that antigen-specific CD4⁺CD25⁺ T cells that have been isolated from the target tissue (i.e. the CNS) have a higher potency with respect to the downregulation of ongoing CNS inflammation than those isolated from other tissues (i.e. the lymph nodes).

McGeachy et al. also reported that the inactivation or depletion of T_REG cells in C57Bl/6 mice by use of an anti-CD25 monoclonal antibody resulted in increased susceptibility to EAE and removed the resistance to re-induction of EAE.²¹ The depletion of T_REG cells also prevented secondary EAE remissions in SJL mice.^{33, 34}

Myelin-specific T_REG cells are able to migrate to and accumulate in the CNS in animals with EAE,^{21, 35, 36} and T_REG-cell accumulation and frequency in the CNS has consistently been shown to correlate with recovery from EAE.^{21, 35} These cells were not sufficient, however, to reduce the function of encephalitogenic T_EFF cells during the peak of EAE.³⁵

Several attempts have been made to elevate the numbers of nT_REG cells in vivo to suppress ongoing autoimmunity in EAE models.^{37, 38, 39, 40, 41} A CD28-specific superagonistic monoclonal antibody, injected at very low doses into Lewis rats, directly expanded and activated T_REG cells, resulting in a reduction in disease severity.⁴² In addition, anti-CD28-expanded T_REG cells were shown to interfere with T_EFF-cell migration within secondary lymphoid organs, thereby inhibiting the infiltration of pathogenic T cells into the CNS.⁴³

Regulatory T cells in multiple sclerosis

Over the past few years, there has been an increase in the number of studies that have investigated the role of T_REG cells in patients with MS. These studies included the assessment of frequency and function of various T_REG-cell populations in patients with MS and in control individuals, the relationship between these cell populations and different disease courses, the status of disease activity, and the effects of different immune-oriented therapies.

The frequency of CD4⁺CD25^high T_REG cells does not differ between patients with MS and healthy controls (Table 3).^{44, 45, 46, 47} Several groups have shown, however, that CD4⁺CD25^high T_REG cells are functionally impaired or have deficits in their maturation or in their thymic emigration in patients with MS.^{44, 45, 47, 48, 49} In several of these experiments, lymphocytes from patients with MS were stimulated with mitogen, polyclonally with anti-CD3 and anti-CD28 antibodies,⁴⁷ or antigen-specifically with MOG⁴⁵ or MBP,⁴⁹ and were analyzed for their suppressive capacity. CD4⁺CD25^high T_REG cells from patients with MS exhibited functional impairment. The primary defect in T_REG-cell function in patients with MS was shown to be intrinsic to T_REG cells and could not be attributed to a higher activation status or to resistance to inhibition of autoreactive T cells.^{47, 50} Single-cell cloning experiments revealed a reduced cloning frequency of CD4⁺CD25^high T cells in patients with MS compared with healthy controls.⁴⁷ Huan et al. found that patients with MS have lower levels of FOXP3 messenger RNA and protein expression than do healthy individuals, suggesting an involvement of diminished FOXP3 expression in impaired T_REG-cell immunoregulation in MS.⁵¹ Venken et al. found an impairment of T_REG-cell function accompanied by decreased FOXP3 expression in patients with relapsing–remitting MS, but the FOXP3 level and suppressive function were normalized during secondary progressive MS.⁵² Impaired CD4⁺CD25⁺FOXP3⁺ T_REG-cell function could serve as a general explanation for why tolerance against autoantigens becomes imbalanced, leading to disease susceptibility and influencing the course of autoimmunity. It is not yet clear, however, whether T_REG-cell dysfunction has a causal role in MS or whether this cell dysfunction represents a more general defect within the immunoregulatory network that can be associated with any autoimmune disorder.⁵³

Table 3 Studies that have investigated the role of regulatory T cells in human multiple sclerosis.

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The mechanisms that underlie T_REG-cell dysfunction are currently poorly understood. Work conducted by Haas et al. has provided evidence of altered generation of T_REG cells in the thymus in patients with MS.⁵⁴ This group defined naive CD4⁺CD25⁺FOXP3⁺ T cells that coexpress CD31 (PECAM-1) as recent thymic emigrants (RTEs), and the investigators found a reduced number of RTEs in the blood of patients with MS. The reduced number of RTEs was compensated for by increased amounts of memory T_REG cells, resulting in a stable cell count of the total T_REG-cell population. Disequilibrium in the homeostatic composition and an altered thymic release of T_REG cells was suggested to be responsible for the diminished suppressive properties of T_REG cells in the blood of patients with MS.⁵⁴ In such patients, the reduced level of RTE-T_REG cells coincided with a contracted T-cell receptor V repertoire⁵⁴ and was consistent with impaired clonal expansion of T_REG cells, which was demonstrated previously.⁴⁷ By use of CD39 as a marker that is coexpressed by FOXP3⁺ T_REG cells, Borsellino et al. demonstrated strikingly reduced numbers of CD39⁺ T_REG cells in the blood of patients with relapsing–remitting MS.⁵⁵ CD4⁺CD25⁺ T_REG cells are not the only population of T_REG cells that show dysfunctional behavior in MS; T_R1 cells that lack IL-10 expression also have an altered suppressive function in MS.^{56, 57, 58}

The role of T_REG cells in the control of CNS parenchymal inflammation in response to both self and non-self antigens is important to address. Conceptually, T_REG cells within the CNS could combat destructive inflammatory components, thereby providing anti-inflammatory or CNS-protective properties. This idea would be in accordance with findings in both rheumatoid arthritis and juvenile diabetes, in which an accumulation of suppressive nT_REG cells in the target organ has been demonstrated.⁵³ Similar to data that show high T_REG-cell numbers in the CNS of mice in the recovery phase of EAE,²¹ CD4⁺CD25⁺ T_REG cells were found in the cerebrospinal fluid (CSF) of patients with MS.^{44, 45} Early work suggested an equal T_REG-cell distribution between the CSF and blood of patients with MS,⁴⁵ but a later study by Feger et al. revealed that there are elevated numbers of CD4⁺CD25⁺FOXP3⁺ T_REG cells in the CSF compared with the peripheral blood in these patients (Table 3).⁴⁴ An increase in T_REG cells in the CSF was accompanied by a modest decrease in the percentage of T_REG cells in the blood, which could suggest that T_REG cells are recruited to the site of inflammation. Alternatively, increased frequencies of CD4⁺CD25⁺ T_REG cells could reflect de novo generation in the inflamed CNS. No data are yet available on the functional properties of T_REG cells that have been isolated from the human CNS.

As mentioned above, HLA-G-expressing T_REG cells have recently been described as a novel nT_REG cell population in humans.¹² These cells are found in small but appreciable quantities (0.1–4.0% of CD4⁺ T cells) in the peripheral blood of healthy adults and were shown to be hypoproliferative, CD25^- and FOXP3^-, and to exhibit potent suppressive properties that are partially mediated by HLA-G and immunoglobulin-like transcript 2. It is hypothesized that HLA-G forms part of the endogenous tolerogenic mechanism employed in the CNS to counteract inflammation.^{13, 59} In contrast to CD4⁺CD25⁺ T_REG cells, HLA-G-expressing T_REG cells from patients with MS do not exhibit impaired function (Huang YH et al., personal communication). HLA-G-expressing T_REG cells accumulate at sites of inflammation; for example, patients with acute neuroinflammation (e.g. encephalitis or acute relapses in MS) uniformly show higher frequencies of HLA-G-expressing cells in the CSF than in peripheral blood.¹³ HLA-G-expressing T_REG cells might, therefore, migrate to or be selectively attracted to the sites of inflammation in an effort to combat destructive inflammation. A putative role for HLA-G in the immunoregulatory processes of MS has been suggested by previous studies.⁶⁰ Interestingly, a drop in numbers of CD4⁺ and CD8⁺ HLA-G-expressing cells increases the risk of postpartum relapse in women with MS.⁶¹

Any theory proposing that a general defect of T_REG cells could cause an 'organ-specific' autoimmune disorder must explain how such a general defect could possibly result in selective immunopathology. In MS, the antigens that trigger autoimmune responses are largely unknown. These antigens probably differ between patients and vary according to disease status and activity. It is possible to speculate that CNS vulnerability factors such as blood–brain-barrier disruption, infection or trauma, in conjunction with defects in the immunoregulatory network, could eventually lead to an organ-specific immunopathology in the CNS. Any attempts to exploit the T_REG-cell system for therapeutic purposes must take into account the potential adverse effects of nonselective immune manipulation, for example an increased risk of malignancies and infection. Equally, the unintended triggering of iT_REG cells and of cytokine release by these cells could theoretically result in unexpected side effects.

Dendritic-cell subsets

Two general subsets of DCs can be distinguished in humans and mice: myeloid DCs (mDCs), which, as the name implies, are of myeloid origin; and plasmacytoid DCs (pDCs), which are of lymphoid origin.⁶⁴ These two cell types express different repertoires of pattern recognition receptors and exhibit different cytokine production profiles. Generally, both types of DCs link innate and adaptive immunity, resulting in different immune responses depending on environmental factors (Figure 2).

Figure 2 The influence of human dendritic cell subsets on the generation of effector and regulatory T cells.

(A) Myeloid dendritic cells. imDCs exhibit tolerogenic properties and stimulate the generation of cells.⁹⁶ This process can occur under the influence of steroids (ciclosporin, sirolimus [rapamycin]), hormones (1,25-dihydroxyvitamin D₃), and the immunosuppressive cytokine IL-10. In response to bacteria or intracellular parasites, imDCs undergo maturation to become mmDCs and produce IL-12, which stimulates CD4⁺ T cells to differentiate into IFN--producing T_H1 cells.⁹⁷ mmDCs that show reduced expression of IL-12 can induce the development of T_H2 cells, which release IL-4, IL-5 and IL-13. OX40L, which is expressed by mmDCs, has been identified as a key molecule in this process.⁹⁸ In mice, mmDCs in the CNS were shown to produce IL-23, IL-6, and TGF-, which induce the generation of T_H17 cells.^{99, 100} (B) Plasmacytoid dendritic cells. Under steady-state conditions, ipDCs induce T-cell anergy, a process that requires engagement between T-cell receptors and major histocompatibility complex antigens.¹⁰¹ When activated by viruses or synthetic CpG oligodeoxynucleotides, ipDCs undergo maturation through activation of TLR-7 and TLR-9. mpDC, the maturation of which has been induced by CD40L and IL-3, upregulate ILT3 and ILT4 and prime naive CD8⁺ T cells to differentiate into IL-10-producing regulatory T cells.^{102, 103} Furthermore, CD4⁺ T cells differentiate into CD4⁺CD25⁺ FOXP3⁺ T_REG cells through CpG-mediated mechanisms.¹⁰⁴ The molecular pathway of mpDC-driven generation of IL-10-producing CD4⁺ T_REG cells is considered to be mediated by a rapid and strong upregulation of ICOSL on maturing pDCs.¹⁰⁵ Virus-stimulated mpDCs drive naive CD4⁺ T cells to differentiate into cytotoxic CTL_REG cells, which produce IFN-, IL-10, perforin and granzymes.¹⁰⁶ Abbreviations: CTL_REG, cytotoxic regulatory T cell; FOXP3, forkhead box protein P3; ICOSL, inducible costimulatory molecule ligand; IFN-, interferon ; IL-, interleukin; ILT, immunoglobulin-like transcript; imDC, immature myeloid dendritic cell; ipDC, immature plasmacytoid dendritic cell; mmDC; mature myeloid dendritic cell; mpDC, mature plasmacytoid dendritic cell; TGF-, transforming growth factor ; T_H, T-helper cell; TLR, Toll-like receptor; T_REG, regulatory T cell.

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Dendritic cells in experimental autoimmune encephalomyelitis

Poised between the CNS and the immune system, DCs have been proposed to be pivotal in the development and maintenance of CNS autoimmunity and inflammation.^{62, 65} Experimental evidence regarding the use of DCs to modulate EAE is summarized in Table 4. In the context of CNS autoimmunity, DCs can exhibit tolerogenic^{66, 67, 68} or immunogenic^{69, 70, 71} properties, depending on the route of administration or means of differentiation. Conceptually, CNS-derived DCs could be linked to T_REG-cell expansion, thereby contributing to the resolution of CNS inflammation. This concept has not yet been proven experimentally, however, and the mechanism through which CNS-derived DCs might induce suppressive properties in T_REG cells remains elusive at present. Moreover, no cell type responsible for the regulation of CD4⁺CD25⁺ T cells in the inflamed CNS has yet been identified.

Table 4 Studies that have investigated the effects of in vitro-manipulated dendritic cells in experimental autoimmune encephalomyelitis.

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A direct influx of antigen-reactive T_REG cells from the peripheral compartments into the inflamed CNS during natural resolution of autoimmunity has been suggested to occur.³⁶ These CNS-antigen-specific T_REG cells might be able to clonally expand in the CNS in a similar manner to emigrating T_EFF cells.³⁶ One study indicated that the conversion of encephalitogenic T cells into T_REG cells could potentially be mediated by neurons,⁷² although the direct induction or in situ expansion of T_REG cells via parenchymal CNS cell interaction is still a subject for debate.³² Brain-derived DCs were shown to induce antigen-specific T-cell activation⁷³ and tolerance⁷⁴ in vitro, and DCs can efficiently promote the proliferation of CD4⁺CD25⁺ T_REG cells.⁷⁵ DCs are able to traffic from the CNS into the periphery,⁷⁶ and they readily cross the blood–brain barrier to return to the brain.⁷⁷ We have shown that intracerebral MOG-presenting DCs initiate neuroantigen-specific T-cell responses and are capable of inducing an accumulation of MOG-specific T_REG cells in the CNS (Zozulya AL et al., unpublished data). These observations imply that the peripheral presentation of CNS-derived antigens (e.g. in cervical lymph nodes) could be essential for the outcome of CNS autoaggression. Embryonic-stem-cell-derived DCs (ES-DCs) that present MOG and simultaneously express tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) were consistently shown to protect mice from EAE.⁶⁶ The tolerogenic action of DCs on T_REG cells seemed to occur at the target organ (the CNS) rather than at the level of peripheral priming. ES-DC-TRAIL/MOG-protected mice had elevated numbers of neuroantigen-specific FOXP3⁺ cells in their spinal cords.

There is still some controversy regarding whether activated T_REG cells need to be antigen-specific to confer protection from autoimmune CNS disease. Weber et al. reported that glatiramer acetate exerts its beneficial mode of action via the modulation of type II monocytes, which, when derived from the bone marrow, can serve as precursor cells for mDCs.⁷⁸ Type II monocytes that were generated under glatiramer acetate treatment directed the differentiation of T_H2 and T_REG cells independently of antigen specificity. Adoptive transfer of type II monocyte-induced T_REG cells that are specific for a foreign antigen can confer protection from EAE,⁷⁸ which suggests that a central role exists for type II monocytes in T-cell-mediated modulation of autoimmunity.

Interactions between dendritic cells and regulatory T cells in multiple sclerosis

To date, only a few studies have assessed the role of DCs in MS. Huang et al. reported elevated numbers of peripheral blood DCs in patients with MS,⁷⁹ and both mDCs and pDCs have been found in the CSF of such patients.⁸⁰ Serafini et al. showed that DCs are present in CNS specimens from patients with MS and that these cells are preferentially localized in the perivascular space.⁸¹ The same group provided a detailed immunohistochemical characterization of different DC subsets found in early active and chronic MS lesions.⁸² These data clearly support a theory in which DCs are instrumental in maintaining a balance between the activities of T_EFF and T_REG cells in the inflamed CNS (Figure 3). It is important to note, however, that many aspects of this model remain speculative at present.

Figure 3 Balance between immunogenic and tolerogenic mechanisms in multiple sclerosis: a hypothesis.

Autoimmune neuroinflammation is considered to result from a disrupted balance between immune cells that cause damage (T_EFF cells) and cells with suppressive capabilities (T_REG cells and tolerogenic DCs). (1) In response to environmental stimuli, DCs (red) can prime and activate naive CD4⁺ T cells to produce IFN- (T_H1 cells), IL-4 (T_H2 cells), or IL-17 (T_H17 cells). (2) Activated myelin-specific T cells travel through the peripheral circulation and across the blood–brain barrier into the perivascular space of the brain, where they are presumably reactivated by myelin-antigen-presenting DCs that reside in the brain or have infiltrated into the CNS from the periphery. The T cells subsequently become pathogenic. (3) Activated encephalitogenic CD4⁺ T cells exert effector function by releasing proinflammatory cytokines (e.g. IFN-, IL-6, IL-23), which activate microglia and macrophages, thereby contributing to a cascade of pathological events that result in demyelination and neuronal damage. The presentation of myelin antigen epitopes by resident CNS DCs causes further myelin breakdown, leading to epitope spreading.⁶⁵ (4) Tolerogenic DCs (blue) cause naive T cells to become regulatory (T_H3, T_R1 or CD8⁺CD28^-). The resulting iT_REG cell populations can enter the CNS. In addition, nT_REG cells (HLA-G⁺ or CD4⁺CD25⁺) that are circulating in the blood can enter the perivascular space of the CNS. The mechanisms and triggers that cause nT_REG and iT_REG cells to traffic across the blood–brain barrier into the brain are poorly understood. (5) T_REG cells in the CNS can either directly suppress encephalitogenic T cells through cytokine secretion (IL-10, TGF-, soluble HLA-G) and T cell–T cell interaction, or act on local DCs, thereby rendering them tolerogenic. The tolerogenic DCs in turn could generate or propagate T_REG-cell expansion or suppressive function (iT_REG) that contributes to the suppression of ongoing autoinflammation. (6) T_REG-cell populations that become expanded in response to specific CNS antigens suppress ongoing autoimmune inflammation in the CNS. It is not known whether or how T_REG cells act directly within the CNS and whether T_REG-cell populations can be generated within the brain parenchyma. Abbreviations: DC, dendritic cell; DCtol, tolerogenic dendritic cell; HLA-G, human leukocyte antigen G; M, macrophage; Mc, microglial cell; ICOSL, inducible costimulatory molecule ligand; IFN-, interferon ; IL-, interleukin; ILT, immunoglobulin-like transcript; iT_REG, inducible regulatory T_REG cell; MBP, myelin basic protein; MCP, monocyte chemotactic protein; MOG, myelin oligodendrocyte glycoprotein; nT_REG, natural regulatory T_REG cell; PLP, proteolipid protein; T_EFF, T effector cell; TGF-, transforming growth factor ; T_H, T-helper cell; T_R1, T-regulatory 1 cell; T_REG, regulatory T cell.

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DCs with an altered phenotype, as well as dysfunctional interactions between DCs and T_REG cells, have been described in patients with MS.^{83, 84} The frequency and phenotype of circulating mDCs and pDCs in the blood of patients with primary progressive MS suggests that DCs are in an impaired maturation state in this condition.⁸⁵ The effects of treatment with a high dose of intravenous methylprednisolone during MS relapses suggest a correlation between increased numbers of T_REG cells and reduced numbers of mDCs and pDCs during short-term treatment.⁸³ Interferon -1a (IFN-1a) treatment upregulates the tolerogenic molecule B7-H1 on DCs, thereby changing their inhibitory properties and contributing to immunoregulatory mechanisms that are relevant to MS.⁸⁶ Other studies have claimed that IFN-1a has a role in inducing T_REG cells via DC–T-cell interactions.^{87, 88} These results underline the importance of understanding the interactions between DC and T_REG cells to identify the mechanisms that underlie effective immunotherapy for patients with MS.