Functional analysis of highly defined, FACS-isolated populations of human regulatory CD4+ CD25+ T cells

An optimized method for the functional analysis of human regulatory T cells

Naturally occurring regulatory T cells (Treg) suppress the activation of antigen-responsive T cells in a cell contact-dependent manner. In order to investigate the impact of soluble mediators and receptor-ligand interactions on the interplay between naive T cells and Treg, a reproducible suppressor cell assay which functions in the absence of additional feeder cells or antigen-presenting cells is mandatory. Here, we describe such a method which is suited to study the modulation of responder T cell/Treg interactions in vitro. Treg were isolated from negatively purified total human CD4+ T cells by positive selection using anti-CD25 monoclonal antibody (MoAb)-coated Dynabeads followed by a detachment step. The remaining CD4+ CD25- responder T cells were cocultured with CD4+ CD25+ Treg in the presence of T-cell Activation/Expansion Beads from Miltenyi Biotec pre-coated with anti-CD3 plus anti-CD28 monoclonal antibody (MoAb). The optimal concentration for coating was 5 microg/ml for both MoAb. At this concentration, strong proliferation of responder T cells was elicited which was almost completely suppressed by Treg at 1:1 cell ratios. When higher concentrations of anti-CD3/anti-CD28 MoAb were used for coating, Treg also showed some degree of proliferation. The optimized suppressor assay proved to be highly reproducible and was used here to confirm the partial or complete reversal of Treg-mediated T-cell suppression by some cytokines (IL-2, IL-15), soluble IL-6 receptor/IL-6 fusion protein and recombinant GITR-ligand. Furthermore, our data confirm that Treg do not need other cell types to suppress proliferation of CD4+ CD25- responder T cells.

Transforming growth factor-beta and T-cell-mediated immunoregulation in the control of autoimmune diabetes

It is now well-established that CD4+ regulatory T cells are instrumental in controlling immune responses both to self-antigens and to non-self-antigens. However, the precise modalities involved in their differentiation and survival, their mode of action and their antigen specificity are only partially understood. We have been particularly interested in the study of regulatory T cells controlling autoimmune insulin-dependent diabetes. Here, we provide evidence to support the phenotypic and functional diversity of regulatory T cells mediating transferable 'active' or 'dominant' peripheral tolerance in the non-obese diabetic mouse model (NOD). They include natural and adaptive regulatory T cells that are operational both in unmanipulated NOD mice and in animals undergoing treatments aimed at inducing/restoring tolerance to self-beta-cell antigens. At least in our hands, the differential cytokine-dependency appears as a major distinctive feature of regulatory T cells subsets. Among immunoregulatory cytokines, transforming growth factor-beta(TGF-beta) appeared to play a key role. Herein we discuss these results and the working hypothesis they evoke in the context of the present literature, where the role of TGF-beta-dependent T-cell-mediated immunoregulation is still debated.

Cutting edge: Decreased accumulation and regulatory function of CD4+ CD25(high) T cells in human STAT5b deficiency

We show that STAT5b is important for the in vivo accumulation of CD4+ CD25(high) T cells with regulatory cell function. A patient homozygous for a missense A630P STAT5b mutation displayed immune dysregulation and decreased numbers of CD4+ CD25(high) T cells. STAT5b(A630P/A630P) CD4+ CD25(high) T cells had low expression of forkhead box P3 and an impaired ability to suppress the proliferation of or to kill CD4+ CD25- T cells. Expression of CD25, a component of the high-affinity IL-2R, was also reduced in response to IL-2 or after in vitro propagation. The impact of the STAT5b mutation was selective in that IL-2-mediated up-regulation of the common gamma-chain cytokine receptor and perforin, and activation-induced expressions of CD154 and IFN-gamma were normal. These results indicate that STAT5b propagates an important IL-2-mediated signal for the in vivo accumulation of functional regulatory T cells.

CD4(+)CD25high regulatory T cells increase with tumor stage in patients with gastric and esophageal cancers

PURPOSE

Regulatory T cells (T regs) can inhibit immune responses mediated by T cells. It has been shown that there is an increased proportion of T regs in several different human malignancies, although the actual mechanism remains unclear. In the present study, we evaluated the prevalence of CD4(+)CD25high T regs in PBMCs from patients with gastric and esophageal cancers in relation to the clinical outcome.

METHODS

PBMCs in 72 patients with gastric cancer and 42 patients with esophageal cancer were evaluated for the proportion of CD4(+)CD25high T cells, as a percentage of the total CD4(+) cells, by flow cytometric analysis with triple-color staining. Actuarial overall survival rates of the patients were analyzed by the Kaplan-Meier method.

RESULTS

The percentages of CD4(+)CD25high T cells for cases of gastric cancer (4.9+/-1.2%) and esophageal cancer (5.2+/-2.1%) were significantly higher than those for healthy donors (1.9+/-1.1%, P<0.01). There were significant differences in the prevalence of CD4(+)CD25high T cells between the early and advanced disease stages, both in gastric cancer (stage I vs. III, P<0.05; stage I vs. IV, P<0.05) and esophageal cancer (stage I vs. IV, P<0.05). The patients with a high proportion of CD4(+)CD25high T cells showed poorer survival rates in comparison to those with a low proportion, in both gastric and esophageal cancers. After patients received curative resections of gastric cancers (n=57), the increased proportions of CD4(+)CD25high T cells were significantly reduced, and the levels were almost equal to those in normal healthy donors. In addition, studies of gastric cancer patients with postoperative recurrent tumors (n=6) revealed that the prevalence of CD4(+)CD25high T cells individually increased compared to 2 months after the operations. CD4(+)CD25high T cells expressed FOXP3 mRNA and had abundant CD45RO and intracellular CTLA-4 molecules.

CONCLUSIONS

These results strongly suggest that tumor-related factors induce and expand CD4(+)CD25high T regs.

Naive regulatory T cells: a novel subpopulation defined by resistance toward CD95L-mediated cell death

Most CD4(+)CD25(hi)FOXP3(+) regulatory T cells (T(regs)) from adult peripheral blood express high levels of CD45RO and CD95 and are prone to CD95L-mediated apoptosis in contrast to conventional T cells (T(convs)). However, a T(reg) subpopulation remained consistently apoptosis resistant. Gene microarray and 6-color flow cytometry analysis including FOXP3 revealed an increase in naive T-cell markers on the CD95L-resistant T(regs) compared with most T(regs). In contrast to T(regs) found in adult humans, most CD4(+)CD25(+)FOXP3(+) T cells found in cord blood are naive and exhibit low CD95 expression. Furthermore, most of these newborn T(regs) are not sensitive toward CD95L similar to naive T(regs) from adult individuals. After short stimulation with anti-CD3/CD28 monoclonal antibodies (mAbs), cord blood T(regs) strongly up-regulated CD95 and were sensitized toward CD95L. This functional change was paralleled by a rapid up-regulation of memory T-cell markers on cord blood T(regs) that are frequently found on adult memory T(regs). In summary, we show a clear functional difference between naive and memory T(regs) that could result in different survival rates of those 2 cell populations in vivo. This new observation could be crucial for the planning of therapeutic application of T(regs).

CD127 expression inversely correlates with FoxP3 and suppressive function of human CD4+ T reg cells

Regulatory T (T reg) cells are critical regulators of immune tolerance. Most T reg cells are defined based on expression of CD4, CD25, and the transcription factor, FoxP3. However, these markers have proven problematic for uniquely defining this specialized T cell subset in humans. We found that the IL-7 receptor (CD127) is down-regulated on a subset of CD4⁺ T cells in peripheral blood. We demonstrate that the majority of these cells are FoxP3⁺, including those that express low levels or no CD25. A combination of CD4, CD25, and CD127 resulted in a highly purified population of T reg cells accounting for significantly more cells that previously identified based on other cell surface markers. These cells were highly suppressive in functional suppressor assays. In fact, cells separated based solely on CD4 and CD127 expression were anergic and, although representing at least three times the number of cells (including both CD25⁺CD4⁺ and CD25⁻CD4⁺ T cell subsets), were as suppressive as the “classic” CD4⁺CD25^hi T reg cell subset. Finally, we show that CD127 can be used to quantitate T reg cell subsets in individuals with type 1 diabetes supporting the use of CD127 as a biomarker for human T reg cells.

Ex vivo expansion of human CD4+ CD25high regulatory T cells from transplant recipients permits functional analysis of small blood samples

Regulatory T cell (Treg) function is associated with immune tolerance and could serve as a biomarker for optimization of immunosuppressive regimens. This is hampered by the limited number of Treg in the blood circulation; functional Treg analysis requires large volumes of blood or is dependent on indirect analysis. A more attractive strategy is the ex vivo expansion of Treg, provided the original T cell pool remains unaltered. Here, we show that it is possible to ex vivo expand Treg from limited amounts of blood, preserving the original TCR Vbeta repertoire and suppressive capacity. The protocol proved successful in selected renal transplant recipients and leukopenic patients. Sampling of 10-20 ml blood sufficed; Treg numbers increased over 100-fold during the 2-3 week expansion period, easily reaching cell numbers required for functional analysis (>10(6) cells). This protocol will facilitate the monitoring of Treg function in patients in order to address the role of Treg in transplantation tolerance.

A role for CD4+CD25+ T cells in regulation of the immune response during human tuberculosis

Active tuberculosis (TB) is associated with prolonged suppression of Mycobacterium tuberculosis (MTB)-specific immune responses, but mechanisms involved are understood incompletely. We investigated a potential role for CD4+CD25+ regulatory T cells in depressed anti-MTB immunity by evaluating serially CD4 cell phenotype and interferon (IFN)-gamma production by mononuclear cells from patients with TB. At diagnosis, frequencies of CD4+CD25+ T cells were increased in blood from TB patients compared to healthy purified protein derivative (PPD)-positive controls (with a history of prior TB exposure), and remained elevated at completion of therapy (6 months). By contrast, expression of another activation marker, CD69, by CD4 T cells was increased at diagnosis, but declined rapidly to control levels with treatment. Among CD4+CD25+ T cells from TB patients at diagnosis those expressing high levels of CD25, probably representing regulatory T cells, were increased 2.9-fold when compared to control subjects, while MTB-stimulated IFN-gamma levels in whole blood supernatants were depressed. A role for CD4+CD25+ T cells in depressed IFN-gamma production during TB was substantiated in depletion experiments, where CD25+-depleted CD4 T cells produced increased amounts of IFN-gamma upon MTB stimulation compared to unseparated T cells. At follow-up, IFN-gamma production improved most significantly in blood from TB patients with high baseline frequencies of CD4+CD25+ T cells (more than threefold higher than controls for both total and CD25hi+ CD4 T cells), who also had a significant drop in frequencies of both total and 'regulatory' CD4+CD25+ T cells in response to treatment. Expansion of CD4+CD25+ regulatory T cells during active TB may play a role in depressed T cell IFN-gamma production.

Effect of medical castration on CD4+ CD25+ T cells, CD8+ T cell IFN-gamma expression, and NK cells: a physiological role for testosterone and/or its metabolites

The higher prevalence of autoimmune disease among women compared with men suggests that steroids impact immune regulation. To investigate how sex steroids modulate cellular immune function, we conducted a randomized trial in 12 healthy men aged 35-55 yr treated for 28 days with placebo, a GnRH antagonist, acyline to induce medical castration, or acyline plus daily testosterone (T) gel to replace serum T, followed by a 28-day recovery period. Serum hormones were measured weekly and peripheral blood lymphocytes (PBLs) were collected biweekly for analyses of thymus-derived lymphocyte (T cell) subtypes and natural killer (NK) cells. Compared with the other groups and to baseline throughout the drug exposure period, men receiving acyline alone had significant reductions in serum T (near or below castrate levels), dihydrotestosterone, and estradiol (P < 0.05). Medical castration significantly reduced the percentage of CD4+ CD25+ T cells (P < 0.05), decreased mitogen-induced CD8+ T cell IFN-gamma expression, and increased the percentage of NK cells without affecting the ratio of CD4+ to CD8+ T cells and the expression of NK cell-activating receptor NKG2D or homing receptor CXCR1. No changes in immune composition were observed in subjects receiving placebo or acyline with replacement T. These data suggest that T and/or its metabolites may help maintain the physiological balance of autoimmunity and protective immunity by preserving the number of regulatory T cells and the activation of CD8+ T cells. In addition, sex steroids suppress NK cell proliferation. This study supports a complex physiological role for T and/or its metabolites in immune regulation.

Single-cell analysis of normal and FOXP3-mutant human T cells: FOXP3 expression without regulatory T cell development

Forkhead winged-helix transcription factor Foxp3 serves as the dedicated mediator of the genetic program governing CD25+CD4+ regulatory T cell (T(R)) development and function in mice. In humans, its role in mediating T(R) development has been controversial. Furthermore, the fate of T(R) precursors in FOXP3 deficiency has yet to be described. Making use of flow cytometric detection of human FOXP3, we have addressed the relationship between FOXP3 expression and human T(R) development. Unlike murine Foxp3- T cells, a small subset of human CD4+ and CD8+ T cells transiently up-regulated FOXP3 upon in vitro stimulation. Induced FOXP3, however, did not alter cell-surface phenotype or suppress T helper 1 cytokine expression. Furthermore, only ex vivo FOXP3+ T(R) cells persisted after prolonged culture, suggesting that induced FOXP3 did not activate a T(r) developmental program in a significant number of cells. FOXP3 flow cytometry was also used to further characterize several patients exhibiting symptoms of immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) with or without FOXP3 mutations. Most patients lacked FOXP3-expressing cells, further solidifying the association between FOXP3 deficiency and immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome. Interestingly, one patient bearing a FOXP3 mutation enabling expression of stable FOXP3(mut) protein exhibited FOXP3(mut)-expressing cells among a subset of highly activated CD4+ T cells. This observation raises the possibility that the severe autoimmunity in FOXP3 deficiency can be attributed, in part, to aggressive T helper cells that have developed from T(R) precursors.

Are regulatory T-cells linked with aging?

There is increasing evidence for an active and 'dominant' tolerance mediated by regulatory T-cells. Out of these CD4+ 'naturally occurring' regulatory T-cells (TREGs) are currently the main research focus in this field. TREGs exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and the forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3). Age-related increment of the prevalences of CD4+ CD25(hi) TREGs were described controversially, and whether such changes explain immune dysfunction in the elderly is still unclear. During aging thymic TREG output may decrease with significant loss of thymic capacity to generate new T-cells, and TREG homeostasis has been shown to be sustained by alternative pathways like peripheral generation of TREGs. An imbalance of TREG homeostasis would then predispose to immune dysfunction in aged individuals explaining their higher risk of immune-mediated diseases, cancer or infections.

Imbalance of regulatory T cells in human autoimmune diseases

The breakdown of mechanisms assuring the recognition of self and non-self is a hallmark feature of autoimmune diseases. In the past 10 years, there has been a steadily increasing interest in a subpopulation of regulatory T cells, which exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3) (TREGs). Recent findings of changed prevalences and functional efficiencies indicate that these TREGs play a unique role in autoimmune diseases. Clinical findings in patients with mutated FOXP3 genes and a specific polymorphism in the promotor region of FOXP3 also support the role of FOXP3 as a 'master control gene' in the development and functioning of TREGs. Both altered generation of TREGs and insufficient suppression of inflammation in autoimmune diseases are considered to be crucial for the initiation and perpetuation of disease. TREG-related somatic cell therapy is considered as an intriguing new intervention to approach autoimmune diseases.

Paucity of FOXP3+ cells in skin and peripheral blood distinguishes Sézary syndrome from other cutaneous T-cell lymphomas

Cutaneous T-cell lymphomas (CTCL) are mainly comprised of two variants: mycosis fungoides (MF) with CD4(+) tumor cells confined to the skin and the leukemic Sézary syndrome with tumor cell spread to the blood. In this study, we investigated cutaneous expression of the regulatory T-cell (T(reg)) marker FOXP3 in 30 CTCL patients. Immunohistochemical analysis revealed significantly lower numbers of CD4(+)FOXP3(+) cells within the dermal lymphomononuclear infiltrate of Sézary patients (16% FOXP3(+) cells of CD4(+) cells) in contrast to MF (43% FOXP3(+) cells (P<0.05)) and rare types of CTCL (45% FOXP3(+) cells). Furthermore, CD4(+)FOXP3(+) T cells were also markedly reduced in the CD4(+) population within the peripheral blood of Sézary patients compared to controls as determined by fluorescence-activated cell sorter, quantitative PCR and functional analyses. The data support the conclusion that the neoplastic cells in CTCL do not express the T(reg) marker FOXP3. Our data also identify Sézary syndrome as, to our knowledge, the first reported neoplastic disease with a clear reduction in T(reg) numbers within the CD4(+) population. This lack of T(reg) might account for the more aggressive nature of Sézary syndrome compared with other CTCL.

The Vast Majority of CLA+ T Cells Are Resident in Normal Skin

There are T cells within normal, noninflamed skin that most likely conduct immunosurveillance and are implicated in the development of psoriasis. We isolated T cells from normal human skin using both established and novel methods. Skin resident T cells expressed high levels of CLA, CCR4, and CCR6, and a subset expressed CCR8 and CXCR6. Skin T cells had a remarkably diverse TCR repertoire and were mostly Th1 memory effector cells with smaller subsets of central memory, Th2, and functional T regulatory cells. We isolated a surprising number of nonexpanded T cells from normal skin. To validate this finding, we counted T cells in sections of normal skin and determined that there are approximately 1 x 10(6) T cells/cm(2) normal skin and an estimated 2 x 10(10) T cells in the entire skin surface, nearly twice the number of T cells in the circulation. Moreover, we estimate that 98% of CLA(+) effector memory T cells are resident in normal skin under resting conditions. These findings demonstrate that there is a large pool of memory T cells in normal skin that can initiate and perpetuate immune reactions in the absence of T cell recruitment from the blood.

The role of regulatory T lymphocytes in the induced immune response mediated by biological vaccines

Immunotherapy has become a novel therapeutic alternative for various kinds of tumours. Recently, we have finalized the first phase I clinical study in Chile for the treatment of advanced malignant melanoma, using dendritic cells (DCs) loaded with allogeneic melanoma cell lysate. This study included 20 patients and the obtained results, pioneer in Latin America, showed that DC-based immunotherapy is innocuous, even provided in combination with IL-2. In addition, immunological responses were detected in 50% of the treated patients, establishing a positive correlation between the delayed type hypersensitivity (DTH) reaction, which indicates induction of in vivo immunological memory, and patients surviving. Nevertheless, objective clinical responses in vaccinated patients are still insufficient. Only sporadic objective metastasis regressions have been registered and an important proportion of the treated patients did not respond, or their responses were weak. Several strategies have been described to be used by tumours to escape from the immune response. Actually, we have demonstrated that IL-10 inhibits antigen presentation in melanoma, reducing tumour sensitivity to melanoma-specific cytotoxic T lymphocytes (CTLs). Regulation of the immunological response by inhibitory cells could be another possible cause of clinical unresponsiveness. Lately, the existence of subpopulations of regulatory T lymphocytes (RTL) able to limit the immune response in a specific form has been established, specially inhibiting the proliferation and activity of CD4+ and CD8+ effector T lymphocytes. These cellular subpopulations, mostly CD4+/CD25+/Foxp3+ T lymphocytes (Treg) of thymic origin, or TR1 lymphocytes able to release IL-10, and tumour growth factor beta (TGF-beta) producing TH3 lymphocytes, would be accumulated in the body during tumour growth, inhibiting the immune response. In relation to RTL and cancer, evidence indicates that Treg cell numbers are increased in blood and other tissues in different types of cancer. Additionally, it has been demonstrated that in patients with refractory metastatic melanoma, the adoptive transference of anti-tumour CD8+ T lymphocytes after non-myeloablative chemotherapy was able to induce important tumour regressions that would be due to elimination of RTL populations. Additionally, chemotherapeutical drugs like decarbazine, besides their effect on tumour proliferation, also have an immunosuppressive effect on T lymphocyte populations, as well as on accumulated RTL. In this article, a novel strategy for the study of RTL is proposed, including potential therapeutic innovations, which is being pioneered in current clinical trials.

Regulatory T cells: which role in the pathogenesis and treatment of allergic disorders?

Several recent data suggest the importance of different types of cells in the regulation of T-cell effector-mediated immune responses. However, a unique specific marker for these cells has not yet been identified. Moreover, in addition to a dedicated functional lineage, even a 'plastic' phenotype of regulatory T cells seems to exist. The lack of a unique specific marker for regulatory T cells, as well as their heterogeneity, make it difficult to determine whether a defect of regulatory T cells plays a role in the pathogenesis of common allergic disorders. Novel therapeutic strategies based on the induction or potentiation of regulatory T cells able to hamper allergic inflammatory reactions are desirable, but their possible efficacy and safety are not yet known. At present, therapeutic strategies able to induce an immune deviation of allergen-specific type 2 T-helper responses to a less polarized phenotype rather than a general immune suppression appear more realistic.

The role of CD4+CD25+ regulatory T cells in patients with Rheumatoid Arthritis

CD4(+)CD25(+) regulatory T cells play an important role in preventing autoimmunity. We investigated the presence of CD4(+)CD25(+) regulatory T cells in the peripheral blood of patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and systemic sclerosis (SSc), using flow cytometry. The percentage of CD4(+)CD25(+) regulatory T cells was significantly decreased in RA, especially in patients with high serum levels of either CRP or MMP-3. In SSc and SLE, the percentage of CD4(+)CD25(+) regulatory T cells was higher in patients than in controls, but not significant. We also investigated the serum levels of IL-10, which influences the function of CD4(+)CD25(+) regulatory T cells and other regulatory T cells. In RA, on contrast to CD4(+)CD25(+) regulatory T cells, the serum levels of IL-10 increased in patients with higher serum levels of CRP, or MMP-3. In SLE and SSc, the serum level of IL-10 increased significantly in patients than in controls. These data thus indicated that CD4(+)CD25(+) regulatory T cells contributes to occurrence and progression of RA, and other regulatory T cells or cytokines contribute to occurrence and progression of SSc and SLE.

CTLA-4 blockade by a human MAb enhances the capacity of AML-derived DC to induce T-cell responses against AML cells in an autologous culture system

BACKGROUND

Cells from AML patients can differentiate into the phenotype of DC when cultured with GM-CSF and IL-4. Such cytokine-treated AML-derived DC (AML-DC) can stimulate autologous T cells. In this study we examined whether an anti-CTLA-4 MAb (MDX-010) could enhance the generation of autologous anti-AML T cells.

METHODS

MAb MDX-010 was added to AML PBMC cultures in the presence of GM-CSF and IL-4, a previously reported AML-DC culture method of generating anti-AML T cells. T-cell activation and proliferation were examined thereafter.

RESULTS

Addition of MDX-010 to GM-CSF/IL-4-conditioned AML-DC cultures induced a mean seven-fold increase in the numbers of autologous T cells compared with cultures without MDX-010 (P < 0.007). T cells stimulated by AML-DC with CTLA-4 blockade were significantly more cytotoxic towards autologous AML cells than those without MDX-010 (42 +/- 23% vs. 26 +/- 15%, E:T ratio of 20). T cells stimulated by AML-DC with CTLA-4 blockade had significantly greater proportions of T cells producing IFN-gamma in response to autologous AML cells than those cultured with AML-DC alone (10.7 +/- 4.7% vs. 4.5 +/- 2.4% for CD4+ IFN-gamma+ CD69+ and 9.8 +/- 4.1% vs. 4 +/- 2.1% for CD8+ IFN-gamma+ CD69+ with or without MDX-010; n = 7; P < 0.007, P < 0.003, respectively).

DISCUSSION

CTLA-4 blockade enhances the activity and numbers of AML-reactive T cells that can be stimulated by autologous AML-DC and may enhance the efficacy of adoptive immunotherapy of AML.

Upregulation of TGF-β, FOXP3, and CD4+CD25+ Regulatory T Cells Correlates with More Rapid Parasite Growth in Human Malaria Infection

Understanding the regulation of immune responses is central for control of autoimmune and infectious disease. In murine models of autoimmunity and chronic inflammatory disease, potent regulatory T lymphocytes have recently been characterized. Despite an explosion of interest in these cells, their relevance to human disease has been uncertain. In a longitudinal study of malaria sporozoite infection via the natural route, we provide evidence that regulatory T cells have modifying effects on blood-stage infection in vivo in humans. Cells with the characteristics of regulatory T cells are rapidly induced following blood-stage infection and are associated with a burst of TGF-beta production, decreased proinflammatory cytokine production, and decreased antigen-specific immune responses. Both the production of TGF-beta and the presence of CD4+CD25+FOXP3+ regulatory T cells are associated with higher rates of parasite growth in vivo. P. falciparum-mediated induction of regulatory T cells may represent a parasite-specific virulence factor.