Regulatory T cells: present facts and future hopes

Abstract Naturally occurring CD4+CD25+Foxp3+

regulatory T cells and several subsets of induced sup-

pressor T cells are key players of the immune tolerance

network and control the induction and effector phase

of our immunological defense system. These T cell

populations actively control the properties of other

immune cells by suppressing their functional activity to

prevent autoimmunity and transplant rejection but also

influence the immune response to allergens as well as

against tumor cells and pathogens. Even though we are

far from completely understanding the molecular and

cellular mechanisms that manage the different regula-

tory T cell populations, increasing evidence exists

about their functional importance. The knowledge on

their induction and activation opens the possibility for

their selective manipulation in vivo as an attractive

approach for an immunotherapy of unwanted immune

responses. This review summarizes this knowledge and

discusses the potential of regulatory T cells for novel

immunointervention strategies in the future.

Introduction

Immune tolerance to self and innocuous foreign anti-

gens is a basic process to prevent non-essential and self-

destructive immune responses. The process of T cell

receptor (TCR) generation, based on random rear-

rangements and promiscuity of the resulting receptors,

inevitably bears the risk for the development of auto-

reactive T cells. Tolerance induction based on clonal

deletion of such self-reactive T cells in the thymus upon

interaction with dendritic cells (DC) is a well-known

mechanism to prevent autoaggressive immune reac-

tions in the periphery, and is defined as central toler-

ance [1]. However, since clonal deletion in the thymus is

not perfect, autoreactive T cells do escape into the

periphery [2]. A still growing body of evidence reveals

that specific T cell populations with suppressive/regu-

latory properties tightly control autoaggressive T cells

[3–6]. Many investigators have shown the enormous

potential of these regulatory T cells (Tregs) to prevent

pathological immune responses in autoimmune dis-

eases [3], transplantation [7], graft-versus-host diseases

[8] and allergy [9]. Indeed, Treg-deficiency is associated

with multiple sclerosis, type-1 diabetes, rheumatoid

arthritis as well as several allergic responses [10–12]. On

the other hand, increased frequencies of Tregs that

prevent an effector cell response have been demon-

strated in patients with cancer and persistent microbial

diseases [13, 14]. Characterization of mechanisms that

control the balance between immune tolerance and

effective immunity are one of the most connotatively

investigations in immunology today to develop new

concepts for immunotherapy: boost immune responses

in cancer and microbial diseases or suppress those

unwanted in autoimmunity and transplantation.

C. Becker Æ S. Stoll Æ H. Jonuleit (&)Department of Dermatology,Johannes Gutenberg-University,Langenbeckstr. 1, 55101 Mainz, Germanye-mail: [email protected]

T. Bopp Æ E. SchmittInstitute of Immunology of the JohannesGutenberg-University, Mainz, Germany

Med Microbiol Immunol (2006) 195:113–124

DOI 10.1007/s00430-006-0017-y

123

REVIEW

Regulatory T cells: present facts and future hopes

Christian Becker Æ Sabine Stoll Æ Tobias Bopp ÆEdgar Schmitt Æ Helmut Jonuleit

Received: 28 March 2006 / Published online: 20 May 2006� Springer-Verlag 2006

Naturally occurring CD4+CD25+ regulatory T cells

Various types of Tregs with different origins, multiple

functions and distinct modes of action have been de-

scribed. One type, the co-called naturally occurring

CD4+CD25+ Tregs (nTregs), arises within the thymus

early in development [15], and constitutively express

the a-chain of the interleukin-2 (IL-2) receptor

(CD25). Mice, thymectomized a few days after birth,

lack this population of resident nTregs resulting in the

development of various autoimmune diseases charac-

terized by inflammatory tissue destruction through

autoaggressive T cells [16], and a loss of immune

homeostasis [17]. These nTregs are produced by the

normal thymus as a distinct and mature population of

T cells with a broad repertoire of self-reactive TCRs

[18]. In the periphery, nTregs comprise 5–10% of

CD4+ T cells in the lymphoid organs [4]. Although

nTregs do not have a unique phenotype, a combination

of several surface markers, including CD25, CTLA-4,

GITR, OX40 and L-selectin (CD62L), enables the

isolation of nTregs to demonstrate their functional

properties in vitro [6] and—after transfer—in vivo [5].

Nevertheless, the functional importance of these sur-

face markers for the suppressive properties of nTregs

remains unclear since all of them are also expressed on

conventional T helper cells after activation.

To date, there is only one lineage specific marker,

the transcription repressor Foxp3, which serves as a

master regulator of nTreg development and function

[19, 20]. Foxp3-defective mice (scurfy mice) lack

nTregs and develop a fatal severe lymphoproliferative

autoimmune syndrome, which leads to their death

4–5 weeks after birth [21]. Similarly, IPEX (immuno-

dysregulation, polyendocrinopathy, enteropathy,

X-linked syndrome) is a rare, X-linked human disease in

boys, characterized by an aggressive autoimmunity and

is caused by mutations in the FOXP3 gene [22]. De-

tailed analyses of the CD4+CD25+ T cell compartment

revealed that both, scurfy mice and IPEX patients, lack

nTregs leading to a complete loss of T cell-derived

suppressive activity. In addition, scurfy mice could be

rescued by transferring wild type CD4+CD25+FoxP3+

nTregs [23]. In mice, Foxp3 is exclusively expressed in

nTregs and its ectopic expression in conventional

CD4+CD25– T cells confers suppressor function onto

these T cells [19], although not to an extend observed in

nTregs. These data clearly indicate that in mice Foxp3

is critical for both, the development and function of

nTregs. These findings discriminate Foxp3 from other

nTreg-associated markers such as CD25 and GITR,

which are generally expressed on activated T cells.

However, examinations of Foxp3 expression in human

T cells disclosed several differences between mice and

man: In contrast to murine T cells, Foxp3 is also

upregulated after TCR-dependent stimulation in con-

ventional human CD4+ T helper cells and expressed in

several non-Treg clones [24], suggesting that in humans

Foxp3 is rather an activation marker in CD4+ T cells.

Mechanisms of action in vitro

All hitherto performed in vitro studies of murine and

human nTregs clearly vote for a cell contact-dependent

mechanism that requires the activation of nTregs via

their TCR (Fig. 1). Hence, it is common sense that

signaling through the TCR of nTregs leads to the

expression of a surface molecule directly associated

with their suppressor function. However, although a

great effort has been invested to identify molecules

that are exclusively expressed on nTregs, the respon-

sible molecules are still elusive. Because nTregs sup-

press the transcription of the IL-2 gene in target cells

[25], one worthwhile possibility to identify the sup-

pressive mechanism of nTregs would be to search for

known signals that mediate an inhibition of IL-2 gene

transcription.

Surface molecules

Notwithstanding several proposed mechanisms such as

competition for antigen-presenting cells (APC) or

consumption of IL-2, all in vitro and in vivo studies

point to a cell contact-dependent mechanism of nTreg-

mediated suppression and thus to membrane-bound or

membrane-associated molecules on the surface of

nTregs. In fact, most in vitro studies advert that the

presence of APC does not seem essential for the sup-

pressive properties of nTregs [26]. Even though con-

troversially discussed, it was reported that cytotoxic T

lymphocyte antigen 4 (CTLA-4) plays a distinct role in

nTreg-mediated suppression [27]. In this context, the

addition of huge amounts of antibodies against CTLA-

4 abrogated suppression [27]. Moreover, Paust et al.

[28] suggested that engagement of molecules of the B7-

family on the responder cell are responsible for an

inhibitory signal leading to the suppression of these

cells. In support of this hypothesis, the authors showed

that responder cells from mice deficient for CD80 and

CD86 were not only resistant to nTreg-mediated sup-

pression in vitro but also induced multiple severe

and rapid-developing autoimmune diseases when

transferred into lymphopenic mice. Interestingly, this

autoimmune phenotype was resistant against the

co-transfer of nTregs. The most suitable molecule on

nTregs interacting with CD80/CD86 on the responder

114 Med Microbiol Immunol (2006) 195:113–124

123

cell would be again CTLA-4. Albeit interference with

CTLA-4-signaling seems to be prosperous in breaking

immunosuppression, many reports favor a model in

which CTLA-4 is dispensable for cell contact-depen-

dent mechanism of nTreg-mediated suppression since

nTregs from CTLA-4-deficient mice showed no

impairment in their suppressive capacity [29]. Interac-

tion of CTLA-4 with CD80/CD86 seems to be at least

partially responsible for the activation of the indole-

amine-2,3-dioxygenase (IDO) in APC. To this end, an

activation of IDO leads to a heightened tryptophan

metabolism in APC, resulting in reduced amounts of

accessible, free tryptophan for naı̈ve CD4+ T cells,

which seems to be important for their activation [30].

But as the suppressor function of nTregs appears to be,

at least in vitro, independent of APC, the IDO-

dependent mechanism is—if at all—only one facet of

mechanism of nTreg-mediated suppression.

Soluble factors

Although most in vitro studies clearly failed to identify

a soluble factor that mediates the suppressive function

of nTregs, there is still a controversial discussion

about the role of the T cell inhibitory cytokine TGF-b[31–33]. Anyhow, nTregs express high amounts of

membrane-bound TGF-b and also co-express throm-

bospondin, a factor capable of transforming latent

TGF-b into its active form [31, 34]. However, it

was undoubtedly shown by using T cells from Smad3-

deficient and dominant-negative TGF-b RII transgenic

mice, which obviously cannot react on TGF-b are still

susceptible to nTreg-mediated suppression. Moreover,

TGF-b-deficient nTregs are as suppressive as wild type

nTregs [33].

Another promising molecule seemed to be gran-

zyme-B. Grossman et al. [35] showed that human

nTregs express granzyme-B upon activation via anti-

bodies against CD3 and CD46 and these nTregs kill co-

cultured CD4+ and CD8+ T cells. This killing was not

mediated by Fas/FasL interactions but dependent on

CD18 as antibodies against CD18 interfered with kill-

ing. Similarly, Gondek et al. [36] suggested a gran-

zyme-B-dependent suppressive mechanism for nTregs

in the murine system. Moreover, Zhao et al. [37]

emphasized the idea that nTregs suppress miscella-

neous immune cells, including those from the innate

and adaptive immune system and particularly antigen-

presenting B cells via a granzyme-B and perforin-

dependent mechanism.

Taken together, despite great endeavor to find an

inhibitory molecule either on the surface or secreted by

nTregs, the results are controversial and the discussion is

so far open.

Fig. 1 Development and function of naturally occurringCD4+CD25+ FoxP3+ regulatory T cells (nTregs). Development:bone marrow-derived CD4+ T cell precursors develop naturallyinto nTregs upon beneficial TCR engagement by self-peptide–MHC complexes and Foxp3 induction in the thymus. Uponinstruction in the thymus, nTregs emigrate into the periphery asfunctionally fully competent cells. Mode of action: upon TCR

cross-linking, peripheral nTregs suppress the proliferation andIL-2 production by responder CD25–CD4+ or CD8+ T cells in acontact-dependent manner either (a) directly or (b) indirectly viathe APC. In addition: nTregs may (c) condition DC to becometolerogenic and turn down the response of conventional T cellson her part

Med Microbiol Immunol (2006) 195:113–124 115

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In vivo mechanisms of action

Whereas in vitro the suppressive activity of nTregs is

clearly cell contact-dependent, the in vivo mechanism

of nTreg function remains controversial [27, 32, 38].

Particularly, the contact-dependent suppressive mech-

anism suggests an appropriate co-localization of sup-

pressor and effector T cells and the formation of stable

cell contacts. However, recent studies using two-pho-

ton laser-scanning microscopy to directly visualize

nTregs and effector T cells within the same lymph node

disproved stable associations between nTregs and

effector T cells [39]. In contrast, nTregs formed per-

sistent cell contacts with DC that preceded the effector

T cell inhibition by the latter, assuming that a cell

contact-dependent mechanism of suppression by

nTregs might preferentially target DC (Fig. 1). In

addition to the local activity of nTregs, the in vivo

mechanism of suppression by nTregs might also in-

clude far-ranging activities such as the production of

TGF-b or IL-10 [32]. Moreover, as discussed below in

more detail, nTregs provide the fundamental basis of

peripheral tolerance not only by directly inhibiting

other cells but also by building up a suppressive net-

work via conveying cell contact-independent suppres-

sive properties to the suppressed responder cells in a

mechanism first described as infectious tolerance

[40–42]. By virtue of all recent data, the possible levels

and rules of nTreg-mediated suppression in vivo re-

quire further investigation.

Adaptive regulatory T cells

In addition to nTregs, adaptive regulatory T cells

(aTregs) can develop extrathymically from conven-

tional T helper cells. The development of aTregs in

general is driven by tolerizing conditions such as sub-

optimal antigenic stimulation [43], or the presence of

immunosuppressive cytokines like IL-10 and TGF-b[44]. Likewise, immunosuppressive and anti-inflam-

matory drugs also promote the development of T cell

populations with suppressive activities [45–47]. In

accordance with these in vivo conditions IL-10-pro-

ducing Tr1 cells are generated by activation of resting

or naı̈ve CD4+ T cells in the presence of IL-10 and by

IL-10-sensitive APC [48], or anti-CD3 and anti-CD46

antibodies [49] and Th3-like TGF-b-secreting T cells

can be induced by culture of conventional T cells with

TGF-b, IL-4 and IL-10, in the absence of IL-12 [43].

Phenotypically, aTregs are heterogeneous with var-

iable levels of CD25 and Foxp3 expression, supposedly

depending on the condition of their induction.

Whereas Tr1 cells seem to evolve independent of

Foxp3 [50], Foxp3 expression in Th3 cells is not char-

acterized. In terms of function, different mechanisms

of suppression by aTregs have been described, includ-

ing cell contact-dependent and cytokine-dependent

suppressive activities [43, 48] (Fig. 2).

Suppression mediated by sensitive antigen-

presenting cells

Antigen-presenting cells can become tolerogenic under

the influence of cytokines such as IL-4, IL-10, IL-13

and TGF-b [50, 51]. Considerable evidence suggests

that aTregs downregulate the T cell stimulatory

capacity of APC through IL-10 and TGF-b. In partic-

ular, IL-10 modulates the expression of costimulatory

molecules such as CD80 and CD86, and MHC class II

and thus profoundly affects the ability of APC to

activate T cells [50]. In contrast to T cells that down-

regulate the IL-10 receptor upon activation, activated

monocytes upregulate this molecule and become sus-

ceptible to a negative feedback pathway [52]. As a

result, once rendered inhibitory, the suppressive APC

phenotype is persistent and the APC can act as a

central regulator of the immune response by further

interacting with antigen-specific T cells mediating T

cell apoptosis, T cell anergy and the production of

suppressive cytokines [53]. In addition, the production

and function of IL-10 and TGF-b may be interdepen-

dent as IL-10 enhances the production of TGF-b, and

controls the ability of cells to respond to the latter.

Similarly, in vitro, both interleukin-10 and TGF-b can

convert immature DC into tolerizing APC [54–56].

Thus, the contribution of aTregs to the functional

tuning of APC offers an explanation for the induction

of aTregs in the periphery, and for their role in

immunoregulation. However, the downregulation of

immune responses by suppression of APC is also a

well-known mechanism of cross-regulation of Th1

versus Th2 responses and therefore in part indepen-

dent of specific tolerogenic Treg populations [57].

Induction by TGF-b

Both murine and human naı̈ve peripheral T cells can

acquire suppressive activity when stimulated in the

presence of TGF-b [58, 59] and murine TGF-b-induced

aTregs showed regulatory activity in adoptive transfer

models in vivo [60]. However, as most of these data

were obtained in lymphopenic animals, it is not clear

whether they might be relevant under physiological

conditions. A role for TGF-b-induced aTregs has

furthermore been suggested in murine models of


123

tolerance induction [61]. Due to the lack of exclusive

markers, it is difficult to discriminate nTregs from

aTregs in vivo, it seems that TGF-b signaling in T cells

is important for the induction of tolerance in the ab-

sence of nTregs [62]. However, in vitro, TGF-b-in-

duced aTregs may not represent a functionally stable

population (C. Becker, personal communication). It is

therefore not clear, whether TGF-b-induced aTregs

hold promise for therapeutic intervention in humans.

Infectious tolerance

Activated nTregs can directly turn suppressed T cells

into aTregs that modulate immune responses mainly

via TGF-b and IL-10 [41, 42, 63]. This observation

suggested a hierarchic model of tolerance induction

by Tregs starting from nTregs and spreading to

secondary aTregs by functional conversion of con-

ventional T helper cells. This mechanism has been

termed ‘‘infectious tolerance,’’ in analogy to the

dominant and transferable tolerance that can be

induced by short-term treatment of adult mice with

non-depleting monoclonal antibodies to CD4

and CD8 [64]. Recently, the phenomenon of anti-

CD4-mediated tolerance induction has been reeval-

uated in respect of nTregs, and TGF-b-induced [61]

and IL-10-producing [65] aTregs may be involved.

Additionally, our preliminary data showed that

human nTregs can be functionally activated by CD4

cross-linking in vitro (C. Becker et al., unpublished

data), suggesting that a direct activation of nTregs by

CD4-stimulation could be a prerequisite for infec-

tious tolerance induction as described before by

Waldmann’s group in vivo [64]. Of note, aTregs

induced by infectious tolerance and nTregs cannot be

reliably distinguished once the immune system has

been perturbed.

Crosstalk with dendritic cells

Although Tregs are mainly thymus derived [15], T cells

with suppressive properties can also be induced under

non-inflammatory conditions in the periphery after

crosstalk with DC [66, 67].

So far, the functional properties of DC were asso-

ciated with the efficient induction of effector T cells.

However, increasing evidence exist that DC are not

only controlling the activation, but also the regulation

of the immune response by induction and maintenance

of peripheral T cell tolerance depending on their type

and maturation stage, respectively [67]. For example,

repetitive stimulation of human naı̈ve CD4+ T cells

with allogeneic immature DC in vitro results in the

differentiation of naı̈ve T cells into alloantigen-specific

aTregs [68]. These suppressive T cell subsets prolifer-

ate poorly but produce high amounts of IL-10, similar

to Tr1 cells. On the other hand, their suppressive

Fig. 2 Extrathymic induction and function of adaptive regula-tory T cells. Adaptive regulatory T cells (aTregs) differentiatefrom naive conventional CD4+ T cells either as a result ofsuboptimal antigenic stimulation by resting/immature DC, theinfluence of suppressive cytokines like IL-10, TGF-b or cell

contact-dependent interaction with activated nTregs (infectioustolerance). Their mode of action involves both cell contact-dependent (Tr1 cells) and contact-independent suppressiveactivities (Th3 cells). Through the production of IL-10 andTGF-b they convert immature DC into tolerizing APC


123

activity is strictly cell contact-dependent. So a ‘‘quiet’’

state of DC may not only cause simple ignorance, but

also promote the expansion and function of aTregs [69,

70] (Fig. 3).

The Treg-inducing ability of DC is the result of

their differentiation state but can also be mediated by

specialized subsets of DC. Groux et al. described a

special subset of DC which produces IL-10 when

stimulated by specific Toll-like receptors (TLR)-li-

gands. These cells express a stable immature pheno-

type and induce Tr1 cells that can prevent colitis,

induced by injection of CD45RBhi T cells into SCID

mice [71]. Also, organs like liver or gut which are

intrinsically tolerogenic could contain high numbers

of tolerogenic DC [72]. A model concerning organ-

specific Tregs even describes the acquisition of sup-

pressive activity through activation by DC-expressing

organ-derived antigens to expand rare, antigen-

specific Tregs from diverse polyclonal populations.

Still, simply the environment of these organs could be

responsible for the phenotype of these DC rather than

a specialized subset of DC.

Taken together, DC, which are involved in the

induction and maintenance of peripheral tolerance,

show a rather immature phenotype by expression of

low amounts of MHC class II, CD40, CD80, CD86 and

low levels of IL-12, while expressing high levels of IL-

10 which drive naı̈ve T cells towards a regulatory

phenotype rather than an effector one [67]. There is

growing evidence that not only the low expression of

costimulatory molecules and the secretion of IL-10 is

sufficient to induce aTregs, but an upregulation of

molecules that act as dominant tolerogenic factors is

required. For instance, bronchial DC induced during

nasal tolerance promote the differentiation of Tregs

via a mechanism that requires ICOS-L expression [73].

Likewise, ICOS-deficient CD4+ T cells are unable to

become anergic and to adopt a regulatory phenotype

after stimulation with tolerogenic DC (own unpub-

lished data). Particularly, the balance of interaction

between CD80/CD86 and ICOS-L on DC with the

corresponding CD28 and ICOS molecules on inter-

acting CD4+ T cells significantly swayed the resulting

T cell response either into IFN-c-producing effector

Fig. 3 Role of Tregs in earlyand late stages of microbialinfections. In the stages of animmune response against amicrobial infection Tregsbehave differently. aThroughout the early phaseof the response thesuppressive activity of Tregsis turned down by effector Tcell-derived IL-2 andmicrobial components such asTLR-ligands. Tregs respondto the stimulation by matureDC and proliferate. b At thelate stage of the response,when the invading organism iscleared from the host, Tregsregain their suppressivefunction and participate in thesilencing of the T cellresponse by acting on effectorT cells and DC. Possibly, thislate activity is also for theproper development ofmemory T cells


123

T cells or IL-10+ aTregs (S. Stoll et al., unpublished

data). In vitro, monocyte-derived DC can be turned to

tolerogenic DC by treatment with IL-10, since these

cells induce anergic regulatory CD4+ as well as CD8+ T

cells which suppress the activation of naı̈ve and effec-

tor T cells in an antigen-specific, IL-10 and TGF-b-

independent manner [54, 56]. In contrast, T cells that

secret cytokines like TGF-b and IL-10 can promote the

generation of DC with an immature phenotype by

influencing IL-10-sensitive DC progenitors. Thus, a

feedback loop between the phenotype of DC and the

generation of DC-modulating T cells exists that favors

the induction of tolerance [67].

Some approaches were successful in inducing

transplantation tolerance using agents that target both

cell types involved in tolerance induction in the

periphery: T cells and DC [74]. Since agents

approaching only T cells did not show a reliable tol-

erance induction as well as agents only approaching

DC, the combination of targeting both cell populations

showed promising synergistical effects. Thomas et al.

[75] succeeded to induce a synergistical tolerance by

simultaneously targeting T cells with immunotoxin and

DC with deoxyspergualine, a drug that potentially

inhibits the differentiation and maturation of DC.

Deoxyspergualine-treated DC expressed low levels of

costimulatory molecules and showed an impaired

secretion of pro-inflammatory cytokines like IL-12.

Additionally, Vitamin D3-treated DC both fail to

activate conventional T cells and simultaneously cause

a hyporesponsiveness of T cells towards further stim-

ulation with mature DC.

Dendritic cells not only influence the immunological

balance by inducing aTregs like Tr1 cells, but also

generate nTregs. Apostolou and von Boehmer [76]

showed that CD4+CD25+Foxp3+ Tregs matching all

properties of nTregs can develop in adult thymectom-

ized TCR-transgenic mice on a Rag–/– background

upon continuous delivery of sub-immunogenic

amounts of peptides to DC. In addition, DC were ob-

served to form clusters with nTregs in vivo in the

draining lymph node of pre-diabetic NOD mice and

drive nTregs to proliferate in vitro [77]. Taken to-

gether, for the perpetuation of peripheral tolerance in

the periphery, the crosstalk of DC as sentinels of the

immune system with a self-maintaining feedback loop

of tolerogenic DC and their influence to the homeo-

stasis of nTregs as well as aTregs renders DC impor-

tant players not only for immunity but also for

tolerance induction. This crosstalk of T cells and DC

could be a promising approach for tolerance induction

in autoimmune diseases or the prevention of transplant

rejection.

Clinical potential of regulatory T cells

Regulatory T cells are of central importance for the

immune tolerance network and, as mentioned, mal-

function of this T cell population can either lead to

impaired or increased suppression obviously resulting

in an array of distinct diseases.

In murine tumor models, transient depletion of

nTregs by anti-CD25 mAb in vivo can elicit a potent

immune response that leads to the eradication of a

syngenic tumor through tumorantigen-specific CTL

and tumorantigen-nonspecific CD4–CD8– cytotoxic

cells with NK-like activity [78, 79]. These findings

strongly suggest that tumor cells exploit the suppres-

sive capacity of nTregs to escape an effective anti-tu-

mor immune response. Tumor-derived IL-10/TGF-b as

well as tumor-induced endogenous IL-10/TGF-b lo-

cally favor the development of aTregs [80, 81]. In

addition, solid tumors may produce chemokines

(CCL22) that attract nTregs, which express CCR4 [13].

Both mechanisms—induction of aTregs and/or attrac-

tion of nTregs—will prevent tumor-specific T effector

cells from successfully attacking and eradicating the

tumor cells. Thus, the transient depletion of Treg

populations is a promising therapeutic objective to

enable a curative anti-tumor immune response. How-

ever, the abrogation of tumor-mediated suppression by

depletion of CD25+ nTregs is a potentially risky strat-

egy since tumor-specific T effector cells also express

CD25. In fact, both T cell populations will be elimi-

nated by depleting anti-CD25 mAb so that sophisti-

cated kinetics and dosage studies are required in the

course of such an anti-tumor therapy. A more appli-

cable therapeutic strategy was published recently that

suggests the application of non-depleting anti-GITR

mAb [82]. Injection of anti-GITR mAb into tumor-

bearing mice led to a strong anti-tumor response that

was further improved by the simultaneous application

of non-depleting anti-CTLA-4 mAb. Regarding the

mode of action, the authors suggested that treatment

with anti-GITR mAb neutralizes the suppressive

properties of nTregs and in parallel co-activates tu-

morantigen-specific T effector cells so that these cells

escape from Treg-mediated suppression. Especially the

later interpretation is strongly corroborated by a study

that demonstrates that the engagement of GITR on T

effector cells by its ligand mediates resistance to sup-

pression by nTregs [83]. However, this concept is not

easily transferable into the human system, since the

function of GITR is not well characterized in man and

cross-linking of GITR poses the risk of a polyclonal

effector T cell stimulation potentially causing fatal

autoimmunity.


123

As alternative to direct manipulation of nTregs and

T effector cell activity, immunologic suppression can

be broken by conditioning of mature DC with specific

TLR-ligands. Several approaches clearly demonstrate

that such a treatment, based on the usage of in vitro-

conditioned DC or through direct application of TLR-

ligands in vivo, can neutralize or overcome the sup-

pressive effects of nTregs leading to strong cytolytic

activities and anti-tumor responses [84–86]. Using a

model of established tumor tolerance, it was found that

a persistent TLR-signal is needed in combination with

a DC-based vaccine for an effective anti-tumor re-

sponse in consequence of a considerable abolishment

of suppression [87].

While anti-tumor therapy can be improved pro-

foundly by the ablation of nTreg-mediated suppres-

sion, many other diseases are at least partially caused

by missing or insufficient suppression. Especially, the

etiology of autoimmune and allergic diseases is asso-

ciated with insufficient suppressive mechanisms.

Moreover, the successful induction of persistent

transplantation tolerance and the prevention of

GVHD in the course of bone marrow transplantation

depend also on the induction of aTregs [61, 88, 89].

Corresponding to these data, the acceptance of a semi-

allogeneic fetus during pregnancy by the maternal

immune system is also favored by T regulatory phe-

nomena [90, 91].

In general, many studies indicate that the enhance-

ment of Treg-mediated suppression especially via

expansion of nTregs can be used for the prevention or

treatment of autoimmune and allergic diseases, the

induction of transplantation tolerance and the main-

tenance of feto-maternal tolerance. Principally, nTregs

can be expanded antigen-specifically in vitro or in vivo

by using antigen-pulsed mature DC in combination

with TLR-ligands [69]. Recently, it was shown that

nTregs can also be selectively expanded in vivo by a

combination of murine IL-2 and certain anti-IL-2 mAb

[92]. Regarding aTregs, it was shown that the stimu-

lation of conventional CD4+ T cells in combination

with TGF-b leads to the development of Foxp3+ T cells

that have a profound suppressive potency [44, 93]. The

fact that such aTregs inhibit the induction of colitis in a

murine IBD model indicates their therapeutic potency

[60]. Similarly, conventional T cells can be differenti-

ated in the presence of IL-10 to Tr1 cells that them-

selves produce IL-10 and prevent colitis induced in

SCID mice by pathogenic CD4+CD45RBhigh splenic T

cells [94]. In addition, IL-10 in combination with

dexamethasone and vitamin D3 strongly induces the

production of IL-10 by aTregs from asthma patients

[95]. These aTregs inhibit cytokine production of

allergen-specific Th2 cells in an IL-10-dependent

manner. Our increasing knowledge about the immune

tolerance network will certainly lead to an improve-

ment of this elegant treatment for allergic and auto-

immune diseases [96].

With regard to infectious diseases, it was found that

Tregs can play an ambivalent role. The inhibition of

anti-microbial T effector cells by nTregs may lead on

the one hand to severe and chronic infections, but on

the other hand nTregs and aTregs can prevent collat-

eral damage of host tissue caused by vigorous anti-

microbial immune responses [97, 98]. In addition, in an

infectious model using L. major it was found that these

parasites can be totally eradicated in the absence of

nTregs [99]. However, these mice did not develop a

protective memory response. Obviously, in this infec-

tious model long lasting immunity of the host is based

on a compromise that depends on a persisting low-level

parasitic infection, which is enabled by a limited Treg-

mediated suppression. Thus, in the course of an anti-

microbial immune response, Tregs play a very delicate

and sophisticated role. During the initial phase, nTregs

should ideally not inhibit an anti-microbial immune

response in order to immediately allow for a full-blown

response. Recent data strongly suggest that this is

accomplished by two mechanisms, induced by the

invading microbes themselves. DC especially condi-

tioned by microbial TLR-ligands penetrate the intrinsic

suppressive shield of nTregs by strongly co-stimulating

the proliferation of T effector cells and nTregs as well

[100–102]. Simultaneously, TLR2-ligands directly in-

duce the proliferation of nTregs thereby transiently

neutralizing their suppressive potency [103] (see

Fig. 1). In the late phase of the response—after clear-

ance of the microbial stimulators—DC remain in a non-

activated stage and the expanded nTregs regain their

suppressive properties for effector T cells and DC as

well (see Fig. 3). In addition, according to results from

in vitro studies, demonstrating that pre-activated

nTregs have a much higher suppressive capacity than

freshly isolated nTregs, the expanded nTregs in the late

phase have even stronger suppressive properties as

compared to the nTregs during the initiation of the

microbial infection [104, 105]. Thus, the successful

combat of a microbial infection is a result of a dynamic

regulation and modulation of the properties of nTregs,

effector T cells and DC under the influence of microbial

components in particular TLR-ligands.

Regulatory T cells were initially identified as central

players for maintenance of self-tolerance in the

periphery, a decade later, the specific manipulation of

Treg activity becomes an attractive approach for treat-

ment of unwanted immune responses in man. However,


123

antigen-specific induction and regulation of immune

responses is a complex process significantly influenced

by the local milieu of inflammatory and anti-inflamma-

tory factors. Furthermore, most data about Treg func-

tion are obtained in mice and differences between

humans and mice can represent considerable obstacles

with regard to the exploitation of findings from animal

studies for the therapy of human diseases. Without de-

tailed knowledge of the interacting molecules, injection

of modulating agents such as anti-CD25 mAb that binds

to Tregs and conventional T cells includes the risk of

accidental enhancement of the immunological imbal-

ance in autoimmune or allergic diseases. Nevertheless,

systemic modulation of imbalanced immunity by direc-

ted manipulation of Treg activity has the potential to

work therapeutically with the immunological causes of

these diseases (Fig. 4).

Acknowledgments The authors are grateful to Jan Kubach, DrE. von Stebut, and Dr J. Knop for critical reading of this man-uscript and helpful discussions. This work was supported by theDeutsche Forschungsgemeinschaft grant SFB548-A6 (to E.Schmitt) and grant SFB548-A8 (to H. Jonuleit).

References

1. Sprent J (1995) Central tolerance of T cells. Int RevImmunol 13:95–105

2. Mason D (1998) A very high level of crossreactivity is anessential feature of the T-cell receptor. Immunol Today19:395–404

3. Sakaguchi S, Fukuma K, Kuribayashi K, Masuda T (1985)Organ-specific autoimmune diseases induced in mice byelimination of T cell subset. I. Evidence for the activeparticipation of T cells in natural self-tolerance; deficit of aT cell subset as a possible cause of autoimmune disease. JExp Med 161:72–87

4. Sakaguchi S, Sakaguchi N, Asano M, Itoh M, Toda M(1995) Immunologic self-tolerance maintained by activatedT cells expressing IL-2 receptor alpha-chains (CD25).Breakdown of a single mechanism of self-tolerance causesvarious autoimmune diseases. J Immunol 155:1151–1164

5. Suri-Payer E, Amar AZ, Thornton AM, Shevach EM(1998) CD4+CD25+ T cells inhibit both the induction andeffector function of autoreactive T cells and represent aunique lineage of immunoregulatory cells. J Immunol160:1212–1218

6. Jonuleit H, Schmitt E, Stassen M, Tuettenberg A, Knop J,Enk AH (2001) Identification and functional characteriza-tion of human CD4(+)CD25(+) T cells with regulatoryproperties isolated from peripheral blood. J Exp Med193:1285–1294

7. Chai JG, Xue SA, Coe D, Addey C, Bartok I, Scott D,Simpson E, Stauss HJ, Hori S, Sakaguchi S, Dyson J (2005)Regulatory T cells, derived from naive CD4+CD25– T cellsby in vitro Foxp3 gene transfer, can induce transplantationtolerance. Transplantation 79:1310–1316

8. Taylor PA, Lees CJ, Blazar BR (2002) The infusion of exvivo activated and expanded CD4(+)CD25(+) immuneregulatory cells inhibits graft-versus-host disease lethality.Blood 99:3493–3499

9. Akbari O, Stock P, DeKruyff RH, Umetsu DT (2003) Roleof regulatory T cells in allergy and asthma. Curr OpinImmunol 15:627–633

10. Viglietta V, Baecher-Allan C, Weiner HL, Hafler DA(2004) Loss of functional suppression by CD4+CD25+regulatory T cells in patients with multiple sclerosis. J ExpMed 199:971–979

Treg-based immune intervention strategiesTumor immunity

Microbial infection

Autoimmunity

Allergy

Transplantation

Pregnancy disorders

+/-

+

+

+

++/-

+

Transient depletion

Curtailing

Enhancement

of Treg function

Treg

Fig. 4 Treg-based immune intervention strategies. Selectivemanipulation of Treg function is an emerging target for immuneintervention strategies to either boost responses in cancer andmicrobial diseases or suppress those unwanted in autoimmunity,allergy, transplantation and pregnancy disorders. The transient

depletion of Tregs as well as their modulation by microbialagents may allow a transient reduction of Treg activity andenforce anti-tumor responses and immunity against viral infec-tions. On the other hand their selective activation could diminishchronic pathological immune responses


123

11. Lindley S, Dayan CM, Bishop A, Roep BO, Peakman M,Tree TI (2005) Defective suppressor function inCD4(+)CD25(+) T-cells from patients with type 1 diabetes.Diabetes 54:92–99

12. Ehrenstein MR, Evans JG, Singh A, Moore S, Warnes G,Isenberg DA, Mauri C (2004) Compromised function ofregulatory T cells in rheumatoid arthritis and reversal byanti-TNFalpha therapy. J Exp Med 200:277–285

13. Curiel TJ, Coukos G, Zou L, Alvarez X, Cheng P, MottramP, Evdemon-Hogan M, Conejo-Garcia JR, Zhang L, BurowM, Zhu Y, Wei S, Kryczek I, Daniel B, Gordon A, Myers L,Lackner A, Disis ML, Knutson KL, Chen L, Zou W (2004)Specific recruitment of regulatory T cells in ovarian carci-noma fosters immune privilege and predicts reduced sur-vival. Nat Med 10:942–949

14. Mendez S, Reckling SK, Piccirillo CA, Sacks D, Belkaid Y(2004) Role for CD4(+) CD25(+) regulatory T cells inreactivation of persistent leishmaniasis and control of con-comitant immunity. J Exp Med 200:201–210

15. Itoh M, Takahashi T, Sakaguchi N, Kuniyasu Y, Shimizu J,Otsuka F, Sakaguchi S (1999) Thymus and autoimmunity:production of CD25+CD4+ naturally anergic and suppres-sive T cells as a key function of the thymus in maintainingimmunologic self-tolerance. J Immunol 162:5317–5326

16. Sakaguchi S, Sakaguchi N (1994) Thymus, T cells and au-toimmunity: various causes but a common mechanism ofautoimmune disease. In: Coutinho A, Kazatchkine MD(eds) Autoimmunity: physiology and disease. Wiley-Liss,New York, pp 203–227

17. Gavin MA, Clarke SR, Negrou E, Gallegos A, Rudensky A(2002) Homeostasis and anergy of CD4(+)CD25(+) sup-pressor T cells in vivo. Nat Immunol 3:33–41

18. Hsieh CS, Liang Y, Tyznik AJ, Self SG, Liggitt D, RudenskyAY (2004) Recognition of the peripheral self by naturallyarising CD25+ CD4+ T cell receptors. Immunity 21:267–277

19. Hori S, Nomura T, Sakaguchi S (2003) Control of regula-tory T cell development by the transcription factor Foxp3.Science 299:1057–1061

20. Fontenot JD, Gavin MA, Rudensky AY (2003) Foxp3programs the development and function of CD4+CD25+regulatory T cells. Nat Immunol 4:330–336

21. Brunkow ME, Jeffery EW, Hjerrild KA, Paeper B, ClarkLB, Yasayko SA, Wilkinson JE, Galas D, Ziegler SF,Ramsdell F (2001) Disruption of a new forkhead/winged-helix protein, scurfin, results in the fatal lymphoproliferativedisorder of the scurfy mouse. Nat Genet 27:68–73

22. Wildin RS, Ramsdell F, Peake J, Faravelli F, Casanova JL,Buist N, Levy-Lahad E, Mazzella M, Goulet O, Perroni L,Bricarelli FD, Byrne G, McEuen M, Proll S, Appleby M,Brunkow ME (2001) X-linked neonatal diabetes mellitus,enteropathy and endocrinopathy syndrome is the humanequivalent of mouse scurfy. Nat Genet 27:18–20

23. Nishimura E, Sakihama T, Setoguchi R, Tanaka K, Sakag-uchi S (2004) Induction of antigen-specific immunologictolerance by in vivo and in vitro antigen-specific expansionof naturally arising Foxp3+CD25+CD4+ regulatory T cells.Int Immunol 16:1189–1201

24. Ziegler SF (2005) FOXP3: of mice and men. Annu RevImmunol

25. Thornton AM, Shevach EM (1998) CD4+CD25+ immu-noregulatory T cells suppress polyclonal T cell activation invitro by inhibiting interleukin 2 production. J Exp Med188:287–296

26. Shevach EM, McHugh RS, Piccirillo CA, Thornton AM(2001) Control of T-cell activation by CD4+ CD25+suppressor T cells. Immunol Rev 182:58–67

27. Takahashi T, Tagami T, Yamazaki S, Uede T, Shimizu J,Sakaguchi N, Mak TW, Sakaguchi S (2000) Immunologicself-tolerance maintained by CD25(+)CD4(+) regulatory Tcells constitutively expressing cytotoxic T lymphocyte-associated antigen 4. J Exp Med 192:303–310

28. Paust S, Lu L, McCarty N, Cantor H (2004) Engagement ofB7 on effector T cells by regulatory T cells prevents auto-immune disease. Proc Natl Acad Sci USA 101:10398–10403

29. Tang Q, Boden EK, Henriksen KJ, Bour-Jordan H, Bi M,Bluestone JA (2004) Distinct roles of CTLA-4 and TGF-beta in CD4+CD25+ regulatory T cell function. Eur JImmunol 34:2996–3005

30. Mellor AL, Munn DH (2004) IDO expression by dendriticcells: tolerance and tryptophan catabolism. Nat RevImmunol 4:762–774

31. Nakamura K, Kitani A, Strober W (2001) Cell contact-dependent immunosuppression by CD4(+)CD25(+) regu-latory T cells is mediated by cell surface-bound transform-ing growth factor beta. J Exp Med 194:629–644

32. Maloy KJ, Salaun L, Cahill R, Dougan G, Saunders NJ,Powrie F (2003) CD4+CD25+ T(R) cells suppress innateimmune pathology through cytokine-dependent mecha-nisms. J Exp Med 197:111–119

33. Piccirillo CA, Letterio JJ, Thornton AM, McHugh RS,Mamura M, Mizuhara H, Shevach EM (2002)CD4(+)CD25(+) regulatory T cells can mediate suppressorfunction in the absence of transforming growth factor beta1production and responsiveness. J Exp Med 196:237–246

34. Oida T, Zhang X, Goto M, Hachimura S, Totsuka M, Ka-minogawa S, Weiner HL (2003) CD4+CD25– T cells thatexpress latency-associated peptide on the surface suppressCD4+CD45RBhigh-induced colitis by a TGF-beta-depen-dent mechanism. J Immunol 170:2516–2522

35. Grossman WJ, Verbsky JW, Barchet W, Colonna M,Atkinson JP, Ley TJ (2004) Human T regulatory cells canuse the perforin pathway to cause autologous target celldeath. Immunity 21:589–601

36. Gondek DC, Lu LF, Quezada SA, Sakaguchi S, Noelle RJ(2005) Cutting edge: contact-mediated suppression byCD4+CD25+ regulatory cells involves a granzyme B-dependent, perforin-independent mechanism. J Immunol174:1783–1786

37. Zhao DM, Thornton AM, Dipaolo RJ, Shevach EM (2006)Activated CD4+CD25+ T cells selectively kill B lympho-cytes. Blood

38. Shevach EM (2002) CD4+ CD25+ suppressor T cells: morequestions than answers. Nat Rev Immunol 2:389–400

39. Tang Q, Adams JY, Tooley AJ, Bi M, Fife BT, Serra P,Santamaria P, Locksley RM, Krummel MF, Bluestone JA(2006) Visualizing regulatory T cell control of autoim-mune responses in nonobese diabetic mice. Nat Immunol7:83–92

40. Cobbold S, Waldmann H (1998) Infectious tolerance. CurrOpin Immunol 10:518–524

41. Jonuleit H, Schmitt E, Kakirman H, Stassen M, Knop J,Enk AH (2002) Infectious tolerance: human CD25(+) reg-ulatory T cells convey suppressor activity to conventionalCD4(+) T helper cells. J Exp Med 196:255–260

42. Dieckmann D, Bruett CH, Ploettner H, Lutz MB, SchulerG (2002) Human CD4(+)CD25(+) regulatory, contact-dependent T cells induce interleukin 10-producing, contact-independent type 1-like regulatory T cells. J Exp Med196:247–253

43. Weiner HL (2001) Oral tolerance: immune mechanisms andthe generation of Th3-type TGF-beta-secreting regulatorycells. Microbes Infect 3:947–954


123

44. Chen W, Jin W, Hardegen N, Lei KJ, Li L, Marinos N,McGrady G, Wahl SM (2003) Conversion of peripheralCD4+CD25– naive T cells to CD4+CD25+ regulatory Tcells by TGF-beta induction of transcription factor Foxp3. JExp Med 198:1875–1886

45. Battaglia M, Stabilini A, Roncarolo MG (2005) Rapamycinselectively expands CD4+CD25+FoxP3+ regulatory T cells.Blood 105:4743–4748

46. Barrat FJ, Cua DJ, Boonstra A, Richards DF, Crain C,Savelkoul HF, de Waal-Malefyt R, Coffman RL, Haw-rylowicz CM, O’Garra A (2002) In vitro generation ofinterleukin 10-producing regulatory CD4(+) T cells is in-duced by immunosuppressive drugs and inhibited by Thelper type 1 (Th1)- and Th2-inducing cytokines. J ExpMed 195:603–616

47. Adorini L, Giarratana N, Penna G (2004) Pharmacologicalinduction of tolerogenic dendritic cells and regulatory Tcells. Semin Immunol 16:127–134

48. Roncarolo MG, Bacchetta R, Bordignon C, Narula S,Levings MK (2001) Type 1 T regulatory cells. Immunol Rev182:68–79

49. Kemper C, Chan AC, Green JM, Brett KA, Murphy KM,Atkinson JP (2003) Activation of human CD4+ cells withCD3 and CD46 induces a T-regulatory cell 1 phenotype.Nature 421:388–392

50. Levings MK, Gregori S, Tresoldi E, Cazzaniga S, Bonini C,Roncarolo MG (2004) Differentiation of Tr1 cells byimmature dendritic cells requires IL-10 but notCD25+CD4+ Treg cells. Blood 105:1162–1169

51. Jonuleit H, Schmitt E, Steinbrink K, Enk AH (2001) Den-dritic cells as a tool to induce anergic and regulatory T cells.Trends Immunol 22:394–400

52. Moore KW, de Waal MR, Coffman RL, O’Garra A (2001)Interleukin-10 and the interleukin-10 receptor. Annu RevImmunol 19:683–765

53. Coates PT, Colvin BL, Hackstein H, Thomson AW (2002)Manipulation of dendritic cells as an approach to improvedoutcomes in transplantation. Expert Rev Mol Med 2002:1–21

54. Steinbrink K, Wolfl M, Jonuleit H, Knop J, Enk AH (1997)Induction of tolerance by IL-10-treated dendritic cells. JImmunol 159:4772–4780

55. Chen ZM, O’Shaughnessy MJ, Gramaglia I, Panoskaltsis-Mortari A, Murphy WJ, Narula S, Roncarolo MG, BlazarBR (2003) IL-10 and TGF-beta induce alloreactive CD4+.Blood 101:5076–5083

56. Steinbrink K, Jonuleit H, Muller G, Schuler G, Knop J, EnkAH (1999) Interleukin-10-treated human dendritic cells in-duce a melanoma-antigen-specific anergy in CD8(+) T cellsresulting in a failure to lyse tumor cells. Blood 93:1634–1642

57. Fiorentino DF, Bond MW, Mosmann TR (1989) Two typesof mouse T helper cell. IV. Th2 clones secrete a factor thatinhibits cytokine production by Th1 clones. J Exp Med170:2081–2095

58. Rao PE, Petrone AL, Ponath PD (2005) Differentiationand expansion of T cells with regulatory function fromhuman peripheral lymphocytes by stimulation in the pres-ence of TGF-b. J Immunol 174:1446–1455

59. Zheng SG, Gray JD, Ohtsuka K, Yamagiwa S, Horwitz DA(2002) Generation ex vivo of TGF-beta-producing regula-tory T cells from CD4+CD25– precursors. J Immunol169:4183–4189

60. Fantini MC, Becker C, Tubbe I, Nikolaev A, Lehr HA,Galle PR, Neurath MF (2005) TGF-{beta} induced Foxp3+regulatory T cells suppress Th1-mediated experimentalcolitis. Gut (in press)

61. Cobbold SP, Castejon R, Adams E, Zelenika D, Graca L,Humm S, Waldmann H (2004) Induction of foxP3+ regu-latory T cells in the periphery of T cell receptor transgenicmice tolerized to transplants. J Immunol 172:6003–6010

62. Kretschmer K, Apostolou I, Hawiger D, Khazaie K, Nus-senzweig MC, von Boehmer H (2005) Inducing andexpanding regulatory T cell populations by foreign antigen.Nat Immunol 6:1219–1227

63. Stassen M, Fondel S, Bopp T, Richter C, Muller C, KubachJ, Becker C, Knop J, Enk AH, Schmitt S, Schmitt E,Jonuleit H (2004) Human CD25+ regulatory T cells: twosubsets defined by the integrins alpha 4 beta 7 or alpha 4beta 1 confer distinct suppressive properties upon CD4+ Thelper cells. Eur J Immunol 34:1303–1311

64. Qin S, Cobbold SP, Pope H, Elliott J, Kioussis D, Davies J,Waldmann H (1993) ‘‘Infectious’’ transplantation tolerance.Science 259:974–977

65. Kingsley CI, Karim M, Bushell AR, Wood KJ (2002)CD25+CD4+ regulatory T cells prevent graft rejection:CTLA-4- and IL-10-dependent immunoregulation of al-loresponses. J Immunol 168:1080–1086

66. Coates PT, Duncan FJ, Colvin BL, Wang Z, Zahorchak AF,Shufesky WJ, Morelli AE, Thomson AW (2004) In vivo-mobilized kidney dendritic cells are functionally immature,subvert alloreactive T-cell responses, and prolong organallograft survival. Transplantation 77:1080–1089

67. Mahnke K, Schmitt E, Bonifaz L, Enk AH, Jonuleit H(2002) Immature, but not inactive: the tolerogenicfunction of immature dendritic cells. Immunol Cell Biol80:477–483

68. Jonuleit H, Schmitt E, Schuler G, Knop J, Enk AH (2000)Induction of interleukin 10-producing, nonproliferatingCD4(+) T cells with regulatory properties by repetitivestimulation with allogeneic immature human dendritic cells.J Exp Med 192:1213–1222

69. Yamazaki S, Iyoda T, Tarbell K, Olson K, Velinzon K,Inaba K, Steinman RM (2003) Direct expansion offunctional CD25+ CD4+ regulatory T cells by antigen-processing dendritic cells. J Exp Med 198:235–247

70. Fisson S, Darrasse-Jeze G, Litvinova E, Septier F, Klatz-mann D, Liblau R, Salomon BL (2003) Continuous acti-vation of autoreactive CD4(+) CD25(+) regulatory T cellsin the steady state. J Exp Med 198:737–746

71. Groux H, O’Garra A, Bigler M, Rouleau M, Antonenko S,de Vries JE, Roncarolo MG (1997) A CD4+ T-cell subsetinhibits antigen-specific T-cell responses and prevents coli-tis. Nature 389:737–742

72. Pillarisetty VG, Shah AB, Miller G, Bleier JI, DeMatteoRP (2004) Liver dendritic cells are less immunogenic thanspleen dendritic cells because of differences in subtypecomposition. J Immunol 172:1009–1077

73. Akbari O, Freeman GJ, Meyer EH, Greenfield EA, ChangTT, Sharpe AH, Berry G, DeKruyff RH, Umetsu DT(2002) Antigen-specific regulatory T cells develop via theICOS–ICOS-ligand pathway and inhibit allergen-inducedairway hyperreactivity. Nat Med 8:1024–1032

74. Jonuleit H, Adema G, Schmitt E (2003) Immune regulationby regulatory T cells: implications for transplantation.Transpl Immunol 11:267–276

75. Thomas JM, Contreras JL, Jiang XL, Eckhoff DE, WangPX, Hubbard WJ, Lobashevsky AL, Wang W, Asiedu C,Stavrou S, Cook WJ, Robbin ML, Thomas FT, NevilleDM Jr (1999) Peritransplant tolerance induction inmacaques: early events reflecting the unique synergybetween immunotoxin and deoxyspergualin. Transplanta-tion 68:1660–1673


123

76. Apostolou I, von Boehmer H (2004) In vivo instruction ofsuppressor commitment in naive T cells. J Exp Med199:1401–1408

77. Bruder D, Westendorf AM, Hansen W, Prettin S, GruberAD, Qian Y, von Boehmer H, Mahnke K, Buer J (2005) Onthe edge of autoimmunity: T-cell stimulation by steady-statedendritic cells prevents autoimmune diabetes. Diabetes54:3395–3407

78. Onizuka S, Tawara I, Shimizu J, Sakaguchi S, Fujita T,Nakayama E (1999) Tumor rejection by in vivo adminis-tration of anti-CD25 (interleukin-2 receptor alpha) mono-clonal antibody. Cancer Res 59:3128–3133

79. Sutmuller RP, van Duivenvoorde LM, van Elsas A,Schumacher TN, Wildenberg ME, Allison JP, Toes RE,Offringa R, Melief CJ (2001) Synergism of cytotoxic Tlymphocyte-associated antigen 4 blockade and depletion ofCD25(+) regulatory T cells in antitumor therapy revealsalternative pathways for suppression of autoreactive cyto-toxic T lymphocyte responses. J Exp Med 194:823–832

80. Gerlini G, Tun-Kyi A, Dudli C, Burg G, Pimpinelli N,Nestle FO (2004) Metastatic melanoma secreted IL-10down-regulates CD1 molecules on dendritic cells in meta-static tumor lesions. Am J Pathol 165:1853–1863

81. Van Belle P, Rodeck U, Nuamah I, Halpern AC, Elder DE(1996) Melanoma-associated expression of transforminggrowth factor-beta isoforms. Am J Pathol 148:1887–1894

82. Ko K, Yamazaki S, Nakamura K, Nishioka T, Hirota K,Yamaguchi T, Shimizu J, Nomura T, Chiba T, Sakaguchi S(2005) Treatment of advanced tumors with agonistic anti-GITR mAb and its effects on tumor-infiltratingFoxp3+CD25+CD4+ regulatory T cells. J Exp Med202:885–891

83. Stephens GL, McHugh RS, Whitters MJ, Young DA,Luxenberg D, Carreno BM, Collins M, Shevach EM (2004)Engagement of glucocorticoid-induced TNFR family-re-lated receptor on effector T cells by its ligand mediatesresistance to suppression by CD4+CD25+ T cells. JImmunol 173:5008–5020

84. Goldszmid RS, Idoyaga J, Bravo AI, Steinman R, MordohJ, Wainstok R (2003) Dendritic cells charged with apoptotictumor cells induce long-lived protective CD4+ and CD8+ Tcell immunity against B16 melanoma. J Immunol 171:5940–5947

85. Rechtsteiner G, Warger T, Osterloh P, Schild H, RadsakMP (2005) Cutting edge: priming of CTL by transcutaneouspeptide immunization with imiquimod. J Immunol174:2476–2480

86. Warger T, Osterloh P, Rechtsteiner G, Fassbender M, HeibV, Schmid B, Schmitt E, Schild H, Radsak MP (2006)Synergistic activation of dendritic cells by combined Toll-like receptor ligation induces superior CTL responses invivo. Blood (in press)

87. Yang Y, Huang CT, Huang X, Pardoll DM (2004) Persis-tent Toll-like receptor signals are required for reversal ofregulatory T cell-mediated CD8 tolerance. Nat Immunol5:508–515

88. Graca L, Le Moine A, Lin CY, Fairchild PJ, Cobbold SP,Waldmann H (2004) Donor-specific transplantation toler-ance: the paradoxical behavior of CD4+CD25+ T cells. ProcNatl Acad Sci USA 101:10122–10126

89. Taylor PA, Panoskaltsis-Mortari A, Swedin JM, Lucas PJ,Gress RE, Levine BL, June CH, Serody JS, Blazar BR(2004) L-Selectin(hi) but not the L-selectin(lo) CD4+25+

T-regulatory cells are potent inhibitors of GVHD and BMgraft rejection. Blood 104:3804–3812

90. Zenclussen AC, Gerlof K, Zenclussen ML, Sollwedel A,Bertoja AZ, Ritter T, Kotsch K, Leber J, Volk HD (2005)Abnormal T-cell reactivity against paternal antigens inspontaneous abortion: adoptive transfer of pregnancy-in-duced CD4+CD25+ T regulatory cells prevents fetal rejec-tion in a murine abortion model. Am J Pathol 166:811–822

91. Saito S, Sasaki Y, Sakai M (2005) CD4(+)CD25high regu-latory T cells in human pregnancy. J Reprod Immunol65:111–120

92. Boyman O, Kovar M, Rubinstein MP, Surh CD, Sprent J(2006) Selective stimulation of T cell subsets with antibody–cytokine immune complexes. Science 311:1924–1927

93. Huber S, Schramm C, Lehr HA, Mann A, Schmitt S, BeckerC, Protschka M, Galle PR, Neurath MF, Blessing M (2004)Cutting edge: TGF-beta signaling is required for the in vivoexpansion and immunosuppressive capacity of regulatoryCD4+CD25+ T cells. J Immunol 173:6526–6531

94. Groux H, O’Garra A, Bigler M, Rouleau M, Antonenko S,de Vries JE, Roncarolo MG (1997) A CD4+ T-cell subsetinhibits antigen-specific T-cell responses and prevents coli-tis. Nature 389:737–742

95. Xystrakis E, Kusumakar S, Boswell S, Peek E, Urry Z,Richards DF, Adikibi T, Pridgeon C, Dallman M, Loke TK,Robinson DS, Barrat FJ, O’Garra A, Lavender P, Lee TH,Corrigan C, Hawrylowicz CM (2006) Reversing the defec-tive induction of IL-10-secreting regulatory T cells in glu-cocorticoid-resistant asthma patients. J Clin Invest 116:146–155

96. Weiner HL, Myer L, Strober W (2004) Oral tolerance: newinsights and prospects for clinical application. Ann NYAcad Sci 1029:1–425

97. Rouse BT, Suvas S (2004) Regulatory cells and infectiousagents: detentes cordiale and contraire. J Immunol173:2211–2215

98. Suvas S, Azkur AK, Kim BS, Kumaraguru U, Rouse BT(2004) CD4+CD25+ regulatory T cells control the severityof viral immunoinflammatory lesions. J Immunol 172:4123–4132

99. Belkaid Y, Piccirillo CA, Mendez S, Shevach EM, Sacks DL(2002) CD4+CD25+ regulatory T cells control Leishmaniamajor persistence and immunity. Nature 420:502–507

100. Iwasaki A, Medzhitov R (2004) Toll-like receptor control ofthe adaptive immune responses. Nat Immunol 5:987–995

101. Pasare C, Medzhitov R (2003) Toll pathway-dependentblockade of CD4+CD25+ T cell-mediated suppression bydendritic cells. Science 299:1033–1036

102. Pasare C, Medzhitov R (2004) Toll-dependent controlmechanisms of CD4 T cell activation. Immunity 21:733–741

103. Sutmuller RP, den Brok MH, Kramer M, Bennink EJ,Toonen LW, Kullberg BJ, Joosten LA, Akira S, Netea MG,Adema GJ (2006) Toll-like receptor 2 controls expansionand function of regulatory T cells. J Clin Invest 116:485–494

104. Thornton AM, Shevach EM (2000) Suppressor effectorfunction of CD4+CD25+ immunoregulatory T cells isantigen nonspecific. J Immunol 164:183–190

105. Stassen M, Jonuleit H, Muller C, Klein M, Richter C, BoppT, Schmitt S, Schmitt E (2004) Differential regulatorycapacity of CD25+ T regulatory cells and preactivatedCD25+ T regulatory cells on development, functionalactivation, and proliferation of Th2 cells. J Immunol173:267–274


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