Only the CD62L+ subpopulation of CD4+CD25+ regulatory T cells protects from lethal acute GVHD

Graft-versus-host disease

Allogeneic haematopoietic stem-cell transplantation (SCT) is a curative therapy for haematological malignancies and inherited disorders of blood cells, such as sickle-cell anaemia. Mature alphabeta T cells that are contained in the allografts reconstitute T-cell immunity and can eradicate malignant cells in the recipient. Unfortunately, these T cells recognize the recipient as 'non-self' and employ a wide range of immune mechanisms to attack recipient tissues in a process known as graft-versus-host disease (GVHD). The full therapeutic potential of allogeneic haematopoietic SCT will not be realized until approaches to minimize GVHD, while maintaining the positive contributions of donor T cells, are developed. This Review focuses on research in mouse models pursued to achieve this goal.

Immunobiology of allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) has evolved into an effective adoptive cellular immunotherapy for the treatment of a number of cancers. The immunobiology of allogeneic HSCT is unique in transplantation in that it involves potential immune recognition and attack between both donor and host. Much of the immunobiology of allogeneic HSCT has been gleaned from preclinical models and correlation with clinical observations. We review our current understanding of some of the issues that affect the success of this therapy, including host-versus-graft (HVG) reactions, graft-versus-host disease (GVHD), graft-versus-tumor (GVT) activity, and restoration of functional immunity to prevent transplant-related opportunistic infections. We also review new strategies to optimize the GVT and improve overall immune function while reducing GVHD and graft rejection.

Invariant NKT cells and tolerance

Invariant natural killer T (iNKT) cells are innate cells that can bias an immune response toward inflammation or toward a negative regulatory response. iNKT cells can produce cytokines immediately on exposure to activating signals, but the role of iNKT cells in the differentiation of T regulatory (Treg) cells and peripheral tolerance was elucidated only within the past decade. The purpose of this review is to outline the current knowledge of how iNKT cells function in various tolerance paradigms. The roles of iNKT cell in anterior chamber-associated immune deviation (ACAID), oral tolerance, other tolerance systems, and autoimmune diseases is discussed.

CCR4-dependent regulatory T cell function in inflammatory bowel disease

Inflammatory bowel disease (IBD) is an idiopathic inflammatory disease of the intestine. CD4⁺ T lymphocytes play an important role in both initiating and regulating intestinal inflammatory immune responses. CD4⁺CD25⁺CD45RB^low regulatory T (T reg) cells are capable of preventing the development of colitis in a mouse model of IBD. The precise mechanism of T reg cell–mediated prevention of colitis in this model is unclear, and the role of chemokine receptors in the trafficking and function of T reg cells in this model has not been determined. We examined the role of the chemokine receptor CCR4 in in vivo trafficking and suppressive function of T reg cells in a mouse adoptive transfer model of IBD. CCR4-deficient T reg cells failed to accumulate in the mesenteric lymph nodes (MLNs) at early time points (2–5 d) after adoptive transfer, resulting in a failure to suppress the generation of pathogenic T cells and the development of colitis. Moreover, although CCR4-deficent T cells had equivalent in vitro suppressive activity and accumulated in MLNs at later time points (42–56 d), they were unable to suppress colitis. Our study demonstrates that CCR4 plays an important role in T reg cell trafficking in LNs and that this is critical for T reg cell suppressive function in vivo.

Distinctive role of CCR7 in migration and functional activity of naive- and effector/memory-like Treg subsets

Foxp3+CD25+CD4+ Treg play a fundamental role in the maintenance of self tolerance and the control of inflammatory reactions. Previous data demonstrated a division of labor between naive- and effector/memory-like Treg subsets, which is largely based on their lymph node-recirculating and inflammation-seeking migration behavior, respectively. The chemokine receptor CCR7 is expressed on both types of Treg subsets, albeit at different levels. Whether it fulfills similar or distinct roles in these subsets has not been studied so far. We here show that the recirculation of naive-like Treg through LN and, to some extent, the gut is dependent on CCR7. Lack of CCR7 not only prevents recirculation, but also almost completely abolishes the ability of naive-like Treg to control the priming phase of an immune response. In contrast, CCR7 deficiency in effector/memory-like Treg promotes their accumulation in inflamed sites, compatible with a role of CCR7 for exit from the tissue. Local Treg accumulation was accompanied by an enhanced suppression of inflammation. Together, our findings provide conclusive evidence that CCR7 expression on Treg differentially controls in vivo function of the naive- and effector/memory-like subsets.

Inability of memory T cells to induce graft-versus-host disease is a result of an abortive alloresponse

Several groups, including our own, have independently demonstrated that effector memory T cells from non-alloantigen-primed donors do not cause graft-versus-host disease (GVHD). In the current study, we further investigated whether this approach could be extended to all memory T cells, and we studied the underlying mechanisms. Neither total memory T cells nor purified central memory T cells were able to induce GVHD. Memory T cells were at least 3-log less potent than bulk T cells in mediating GVHD. As expected, memory T cells failed to elicit cytotoxicity and proliferated poorly against alloantigens in standard 5-day mixed-lymphocyte cultures. However, the proliferative responses of memory T cells were more comparable with those of bulk and naive T cells when the culture time was shortened. Moreover, the frequencies of IL-2-secreting cells measured by 42-hour enzyme-linked immunosorbent spot (ELISPOT) assay were similar among naive, memory, and bulk T cells. These data indicated that memory T cells are able to respond to alloantigens initially but fail to develop to full potential. The abortive immune response, which was mediated by non-alloantigen-specific memory T cells in response to alloantigens, may explain why memory T cells from unprimed and non-alloantigen-primed donors could not induce GVHD.

Regulatory T-Cells in the Graft and the Risk of Acute Graft-Versus-Host Disease After Allogeneic Stem Cell Transplantation

BACKGROUND

FOXP3+ regulatory T-cells (Treg) are important regulators of allo-reactivity and may therefore represent an important predictor for the risk of graft versus-host disease (GVHD) after allogeneic stem cell transplantation.

METHODS

To determine the clinical significance of Treg-content in stem cell grafts, we analyzed 58 human leukocyte antigen (HLA)-identical sibling donors (34 patients received myeloablative and 24 patients reduced intense conditioning regimens) and correlated the Treg frequency with clinical outcome after stem cell transplantation (SCT).

RESULTS

A mean value of 9.1 x 10(6) CD4+ FOXP3+ Treg per kg body weight (bw) of the recipient was transplanted (ranging from 0.7 to 33.7 x 10(6) Treg/kg bw). Graft content of Treg correlated with mononuclear cells and CD3+ T-cells. Patients receiving low numbers of Treg (Treg(low)) after myeloablative conditioning for SCT had a significantly increased cumulative incidence of 76% for acute GVHD when compared with 23% for individuals receiving high numbers of Treg (Treg(high)). This observation, however, was not made in patients after reduced intense conditioning-SCT. Notably, relapse rate was not significantly different between Treg(low) and Treg(high) patients in either patient group and overall survival was even increased in Treg(high) patients after myeloablative SCT. Finally, low Treg graft levels represent an independent prognostic factor in multivariate analysis for the appearance of acute GHVD.

CONCLUSION

Donor-derived Treg might be of particular significance for the development of acute GVHD after myeloablative SCT using HLA-identical sibling donors.

Induced SHIP deficiency expands myeloid regulatory cells and abrogates graft-versus-host disease

Graft-vs-host disease (GVHD) is the leading cause of treatment-related death in allogeneic bone marrow (BM) transplantation. Immunosuppressive strategies to control GVHD are only partially effective and often lead to life-threatening infections. We previously showed that engraftment of MHC-mismatched BM is enhanced and GVHD abrogated in recipients homozygous for a germline SHIP mutation. In this study, we report the development of a genetic model in which SHIP deficiency can be induced in adult mice. Using this model, we show that the induction of SHIP deficiency in adult mice leads to a rapid and significant expansion of myeloid suppressor cells in peripheral lymphoid tissues. Consistent with expansion of myeloid suppressor cells, splenocytes and lymph node cells from adult mice with induced SHIP deficiency are significantly compromised in their ability to prime allogeneic T cell responses. These results demonstrate that SHIP regulates homeostatic signals for these immunoregulatory cells in adult physiology. Consistent with these findings, induction of SHIP deficiency before receiving a T cell-replete BM graft abrogates acute GVHD. These findings indicate strategies that target SHIP could increase the efficacy and utility of allogeneic BM transplantation, and thereby provide a curative therapy for a wide spectrum of human diseases.

An early age-related increase in the frequency of CD4+ Foxp3+ cells in BDC2.5NOD mice

The role of regulatory T cells (Treg) in maintaining tolerance to self has been intensively scrutinized, particularly since the discovery of Foxp3 as a Treg-specific transcription factor. The BDC2.5NOD transgenic mouse is an excellent model of immunoregulation because it has a very low incidence of diabetes despite a highly autoreactive T-cell repertoire. It has previously been shown that reactivity against islets decreases with age in BDC2.5NOD mice. Here we show that there is a markedly higher frequency of Foxp3(+) Treg in the CD4(+) subset of 16-20-week-old mice compared with 4- or 8-week-old mice. This phenomenon can be observed in the spleen, thymus, pancreatic draining lymph nodes and the pancreas itself. We show that this early age-related increase in the frequency of Foxp3(+) cells does not occur in wild-type NOD, BALB/c or C57BL/6 mice. Further, we show that, in contrast to some reports on Treg in wild-type NOD mice, the suppressive function of BDC2.5NOD Treg from 16- to 20-week-old mice is intact and comparable to that from 4- to 8-week-old mice both in vitro and in vivo. Our data offer insights into the long-term protection of BDC2.5NOD mice from diabetes and an explanation for the age-related decrease in anti-islet responses seen in BDC2.5NOD mice.

Naive B cells generate regulatory T cells in the presence of a mature immunologic synapse

Naive B cells are ineffective antigen-presenting cells and are considered unable to activate naive T cells. However, antigen-specific contact of these cells leads to stable cell pairs that remain associated over hours in vivo. The physiologic role of such pairs has not been evaluated. We show here that antigen-specific conjugates between naive B cells and naive T cells display a mature immunologic synapse in the contact zone that is absent in T-cell-dendritic-cell (DC) pairs. B cells induce substantial proliferation but, contrary to DCs, no loss of L-selectin in T cells. Surprisingly, while DC-triggered T cells develop into normal effector cells, B-cell stimulation over 72 hours induces regulatory T cells inhibiting priming of fresh T cells in a contact-dependent manner in vitro. In vivo, the regulatory T cells home to lymph nodes where they potently suppress immune responses such as in cutaneous hypersensitivity and ectopic allogeneic heart transplant rejection. Our finding might help to explain old observations on tolerance induction by B cells, identify the mature immunologic synapse as a central functional module of this process, and suggest the use of naive B-cell-primed regulatory T cells, "bTregs," as a useful approach for therapeutic intervention in adverse adaptive immune responses.

CD25+CD4+ regulatory T cell migration requires L-selectin expression: L-selectin transcriptional regulation balances constitutive receptor turnover

The molecular mechanisms controlling regulatory CD25(+)Foxp3(+)CD4(+) T cell (T(reg)) migration are central to in vivo immune responses. T(reg) cell subsets differentially express L-selectin, an adhesion molecule mediating lymphocyte migration to peripheral LNs (PLNs) and leukocyte rolling during inflammation. In this study, L-selectin was essential for T(reg) cell migration and normal tissue distribution. Specifically, there was a 90% reduction in PLN T(reg) cells in L-selectin(-/-) mice with a compensatory increase in spleen T(reg) cell numbers. Unexpectedly, however, 40% of the CD4(+) T cells remaining within PLNs of L-selectin(-/-) mice were T(reg) cells. The migratory properties of T(reg) cells were nonetheless markedly different from those of naive CD4(+) T cells, with 3- to 9-fold lower migration of T(reg) cells into PLNs and approximately 2-fold lower migration into the spleen. T(reg) cells also turned over cell surface L-selectin at a faster rate than CD25(-)CD4(+) T cells, but maintained physiologically appropriate L-selectin densities for optimal migration. Specifically, T(reg) cells expressed 30-40% more cell surface L-selectin when its endoproteolytic cleavage was blocked genetically, which resulted in a 2-fold increase in T(reg) cell migration into PLNs. However, increased L-selectin cleavage by T(reg) cells in wild-type mice was accompanied by 2-fold higher L-selectin mRNA levels, which resulted in equivalent cell surface L-selectin densities on T(reg) and naive T cells. Thus, T(reg) cells and CD25(-)CD4(+) T cells share similar requirements for L-selectin expression during migration, although additional molecular mechanisms constrain T(reg) cell migration beyond what is required for naive CD4(+) T cell migration.

An essential role for IFN-gamma in regulation of alloreactive CD8 T cells following allogeneic hematopoietic cell transplantation

We previously found that CD8 T cells from IFN-gamma gene knockout (GKO) donors induce more severe lethal GVHD compared with CD8 T cells from wild-type (WT) donors in fully MHC-mismatched strain combinations. In this study, we investigated the mechanisms by which IFN-gamma inhibits GVHD in a parent --> F1 (B6 --> B6D2F1) allogeneic HCT (allo-HCT) model. IFN-gamma was strongly protective against GVHD in this parent --> F1 haplotype-mismatched allo-HCT model. Irradiated B6D2F1 mice that received GKO B6 CD4-depleted splenocytes developed lethal GVHD with severe lung and liver injury, whereas those receiving a similar cell population from WT B6 donors survived long term. Donor CD8 cells showed rapid activation, accelerated cell division, and reduced/delayed activation-induced cell death in allogeneic recipients in which donor cells were incapable of producing IFN-gamma. In consequence, the numbers of activated/effector (ie, CD25+, CD62L-, and CD44(high)) donor CD8 T cells in the recipients of GKO allo-HCT significantly exceeded those in mice receiving WT allo-HCT. These data show that IFN-gamma negatively regulates the CD8 T cell response by inhibiting cell division and promoting cell death and suggest that blockade of IFN-gamma could augment the severity of GVHD in patients undergoing allo-HCT.

Isolation, expansion, and characterization of human natural and adaptive regulatory T cells

Regulatory T cells play a central role in controlling homeostasis, and in inducing and maintaining tolerance to both foreign and self-antigens. Several types of T cells with regulatory activity have been described both in mice and humans, and those within the CD4+ subset have been extensively studied. Among them, the best characterized are the naturally occurring CD4+CD25+ regulatory T (Treg) cells, and the adaptive type 1 regulatory T (Tr1) cells. Natural Treg cells can arise directly from the thymus, are characterized by the constitutive expression of the transcription factor Foxp3, and suppress T cell responses in a cell-cell contact mediated mechanism. On the contrary, adaptive Tr1 cells arise in the periphery upon encountering antigen in a tolerogenic environment, produce high levels of interleukin (IL)-10 and mediate suppression via IL-10. During the last decade, much effort has been placed on developing protocols to generate regulatory T-cell lines and clones, to further define the similarities and differences between various regulatory T-cell subsets. In this chapter, we will outline protocols to expand naturally occurring Treg cells, to differentiate homogeneous population of Tr1 cells in vitro, and to generate natural Treg and Tr1 cell clones and cell lines.

Low number of regulatory T cells in skin lesions of patients with cutaneous lupus erythematosus

OBJECTIVE

To define the phenotype and function of CD4+,CD25+ regulatory T cells (Treg) in patients with cutaneous lupus erythematosus (CLE), a heterogeneous autoimmune disease characterized primarily by inflammatory skin lesions.

METHODS

The number of Treg in skin specimens obtained from patients with various subtypes of CLE was investigated by immunohistochemical analysis, using anti-Foxp3 and anti-CD4 monoclonal antibodies. Furthermore, characterization of peripheral blood CD4+,CD25+ Treg from normal healthy donors and patients with CLE was carried out by flow cytometry, analyzing the expression of Foxp3 and Treg subpopulations. We also purified CD4+,CD25(high) Treg obtained from patients with CLE and tested the sensitivity of these cells to CD95L-mediated apoptosis.

RESULTS

Quantitative analysis of CD4+ T cells in skin lesions from patients with CLE revealed that the number was similar to that in lesions from patients with other chronic inflammatory diseases, but the number of Foxp3+ Treg in CLE was significantly reduced. There was no correlation between disease subtype and the frequency of Foxp3+ Treg in the skin of patients with CLE. In peripheral blood, no significant differences were observed in the number and phenotype of CD4+,CD25+ Treg or in the sensitivity to apoptosis of CD4+,CD25(high) Treg derived from patients with CLE and those derived from normal healthy donors.

CONCLUSION

These data suggest that an organ-specific abnormality of Treg in the skin underscores the importance of analyzing Treg in the affected tissue. Such a local process might give insight into the pathogenic mechanisms of CLE and differs from a global peripheral dysfunction as reported for patients with a systemic manifestation of the disease.

Targeting Acute Allograft Rejection by Immunotherapy With Ex Vivo-Expanded Natural CD4+CD25+ Regulatory T Cells

BACKGROUND

Natural CD4CD25 regulatory T (Treg) cells have been implicated in suppressing alloreactivity in vitro and in vivo. We hypothesized that immunotherapy using ex vivo-expanded natural Treg could prevent acute allograft rejection in mice.

METHODS

Natural CD4+ CD25+ Treg were freshly purified from naive mice via automated magnetic cell sorter and expanded ex vivo by anti-CD3/CD28 monoclonal antibody (mAb)-coated Dynabeads. Suppression was assayed in vitro by mixed lymphocyte reaction and in vivo by targeting cardiac allograft rejection. Survival of Treg or effector T (Teff) cells after adoptive transfer in vivo was tracked by flow cytometry and all allografts were examined by histology and immunohistochemistry.

RESULTS

By day nine in culture, 26.6+/-5.3-fold of expansion was achieved by co-culture of fresh natural Treg with anti-CD3/CD28 mAb-coated Dynabeads and interleukin-2. Ex vivo-expanded Treg exerted stronger suppression than fresh ones towards alloantigens in vitro and prevented CD4 Teff-mediated but only delayed CD4+/CD8+ Teff-mediated heart allograft rejection in Rag-/- mice. Long-term surviving allografts showed no signs of acute or chronic rejection with graft-infiltrating Treg expressing CD25 and FoxP3. Infused Treg persisted and expanded long-term in vivo and trafficked through the peripheral lymphoid tissues. CD25 expression was dynamic in vivo: maintained CD25 expression on Treg was indicative for the preservation of allosuppression, while significantly enhanced CD25 expression on CD4+ effector T cells was most likely associated with T-cell expansion and graft rejection.

CONCLUSIONS

Therapeutic use of ex vivo-expanded natural CD4+ CD25+ Treg may be a feasible and nontoxic modality for controlling allograft rejection or perhaps inducing allograft tolerance.

A Potential Side Effect of Cyclosporin A: Inhibition of CD4+CD25+ Regulatory T Cells in Mice

BACKGROUND

CD4CD25 regulatory T (Treg) cells are essential for the induction and maintenance of immunologic self-tolerance as well as transplant tolerance. The effects of cyclosporin A (CsA), a widely used immunosuppressive agent, on CD4CD25Treg cells in mice were investigated.

METHODS

Balb/c mice were injected with CsA or control solution for one month. The levels, phenotype, and function of CD4CD25Treg cells in these mice were then assayed.

RESULTS

The percentages and total cell numbers of CD4CD25Treg cells in the peripheral blood and spleen were significantly reduced after the treatment with CsA. The total numbers of CD4CD25Treg cells in the thymus of CsA-treated mice were markedly reduced as compared to the control mice. However, the percentage of CD4CD25Treg cells in the thymus of CsA-treated mice was markedly enhanced. More CD4CD25Treg cells expressing high levels of CD44 and CD45RB, and less CD4CD25Treg cells expressing CD62L were observed in CsA-treated mice, compared with the control mice. CD4CD25Treg cells expressed slightly lower levels of Foxp3 in CsA-treated mice. Furthermore, CsA markedly impaired the immunosuppressive function of CD4CD25Treg cells.

CONCLUSIONS

CsA significantly impaired the development and function of CD4CD25Treg cells. The present studies suggest that CsA may block the potential induction of immune tolerance and increase the susceptibility to develop autoimmune diseases while preventing graft rejection.

Ex vivo rapamycin generates Th1/Tc1 or Th2/Tc2 Effector T cells with enhanced in vivo function and differential sensitivity to post-transplant rapamycin therapy

Rapamycin prevention of murine graft-versus-host disease (GVHD) is associated with a shift toward Th2- and Tc2-type cytokines. Recently, we found that use of rapamycin during ex vivo donor Th2 cell generation enhances the ability of adoptively transferred Th2 cells to prevent murine GVHD. In this study, using a method, without antigen-presenting cells, of T-cell expansion based on CD3,CD28 costimulation, we evaluated whether (1) rapamycin preferentially promotes the generation of Th2/Tc2 cells relative to Th1/Tc1 cells, (2) rapamycin-generated T-cell subsets induce cytokine skewing after allogeneic bone marrow transplantation (BMT), and (3) such in vivo cytokine skewing is sensitive to post-BMT rapamycin therapy. Contrary to our hypothesis, rapamycin did not preferentially promote Th2/Tc2 cell polarity, because rapamycin-generated Th1/Tc1 cells secreted type I cytokines (interleukin [IL]-2 and interferon-gamma) did not secrete type II cytokines (IL-4, IL-5, IL-10, or IL-13) and mediated fasL-based cytolysis. Rapamycin influenced T-cell differentiation, because each of the Th1, Th2, Tc1, and Tc2 subsets generated in rapamycin had increased expression of the central-memory T-cell marker, L-selectin (CD62L). Rapamycin-generated Th1/Tc1 and Th2/Tc2 cells were not anergic but instead had increased expansion after costimulation in vitro, increased expansion in vivo after BMT, and maintained full capacity to skew toward type I or II cytokines after BMT, respectively; further, rapamycin-generated Th1/Tc1 cells mediated increased lethal GVHD relative to control Th1/Tc1 cells. Rapamycin therapy after BMT in recipients of rapamycin-generated Th1/Tc1 cells greatly reduced Th1/Tc1 cell number, greatly reduced type I cytokines, and reduced lethal GVHD; in marked contrast, rapamycin therapy in recipients of rapamycin-generated Th2/Tc2 cells nominally influenced the number of Th2/Tc2 cells in vivo and did not abrogate post-BMT type II cytokine skewing. In conclusion, ex vivo and in vivo usage of rapamycin may be used to modulate the post-BMT balance of Th1/Tc1 and Th2/Tc2 cell subsets.

CCR6 regulates the function of alloreactive and regulatory CD4+ T cells during acute graft-versus-host disease

The chemokine receptor CCR6 is expressed by CD4+ T cell effector/memory and regulatory effector/memory (TREM) subsets. Here we show that CCR6 modulates graft-versus-host-disease (GVHD) responses in both alloreactive CD4+ T effector cells and regulatory T (Treg) cells. Mortality and morbidity due to acute GVHD were drastically reduced and delayed when naïve T cells were derived from CCR6-deficient donor mice. This deficiency also affected the suppressive ability of Treg cells in GVHD. CCR6-/- Treg cells were able to suppress T cell proliferation in vitro, but their in vivo capacity to downregulate target tissue damage induced by naïve wild type (WT) T cells was impaired. The data demonstrate a requirement for CCR6 in CD4+ T cell function in GVHD, in both effector and regulatory cell subsets.

In vivo dynamics of regulatory T-cell trafficking and survival predict effective strategies to control graft-versus-host disease following allogeneic transplantation

CD4(+)CD25(+) regulatory T cells (Tregs) suppress immune responses to alloantigens. The in vivo circulation and tissue localization of Tregs during an adaptive immune response remain unclear. We noninvasively tracked luciferase-expressing Tregs over time in an allogeneic bone marrow transplant model and demonstrated colocalization with effector T cells and initial expansion in secondary lymphoid organs before migration into inflamed tissues. Inflammation induced by irradiation and the allogeneic setting provided crucial stimuli for early Treg expansion and migration, leading to parallel reduction of effector T-cell proliferation in lymphoid organs and peripheral tissues. Treg transplants conferred long-term protection from systemic inflammatory challenge consistent with Treg in vivo survival. Suppression occurred during multiple phases of inflammation, but is optimal in the initial phase, providing protection from graft-versus-host disease while maintaining the graft-versus-tumor effect even at physiologic doses of Tregs due to their in vivo expansion, hence overcoming a major barrier to potential clinical applications of Tregs given their rarity.

Early CD30 signaling is critical for adoptively transferred CD4+CD25+ regulatory T cells in prevention of acute graft-versus-host disease

Murine CD4+CD25+ regulatory T cells (Treg cells) reduce acute graft-versus-host disease (aGvHD). However, surface molecules critical for suppression are unclear. Deficiency of CD30 (CD30-/-) leads to impaired thymic negative selection and augmented T-cell autoreactivity. Therefore, we investigated the role of CD30 signaling in Treg-cell function during aGvHD. Treg cells derived from CD30-/- animals were significantly less effective in preventing aGvHD lethality. Early blockade of the CD30/CD153 pathway with a neutralizing anti-CD153 mAb reduced Treg-mediated protection from proinflammatory cytokine accumulation and donor-type T-cell apoptosis. In vivo bioluminescence imaging demonstrated intact homing but reduced expansion of luciferase-expressing Treg cells when CD153 was blocked during the early phase after adoptive transfer. CD30 surface expression on Treg cells increased with alloantigen exposure, and CD153 expression on recipient-type dendritic cells increased in the presence of a proinflammatory environment. These data demonstrate that early CD30 signaling is critical for Treg-mediated aGvHD protection after major MHC-mismatch bone marrow transplantation.

Regulatory T-cell physiology and application to treat autoimmunity

Endowed with the ability to actively suppress an immune response, regulatory T cells (Tregs) hold the promise of halting ongoing pathogenic autoimmunity and restoring self-tolerance in patients suffering from autoimmune diseases. Through many in vitro and in vivo studies, we have learned that Tregs can function in the lymph nodes as well as in the peripheral tissues. In vivo, Tregs act through dendritic cells to limit autoreactive T-cell activation, thus preventing their differentiation and acquisition of effector functions. By limiting the supply of activated pathogenic cells, Tregs prevent or slow down the progression of autoimmune diseases. However, this protective mechanism appears insufficient in autoimmune individuals, likely because of a shortage of Tregs cells and/or the development and accumulation of Treg-resistant pathogenic T cells over the long disease course. Thus, restoration of self-tolerance in these patients will likely require purging of pathogenic T cells along with infusion of Tregs with increased ability to control ongoing tissue injury. In this review, we highlight advances in dissecting Treg function in vivo in autoimmune settings and summarize multiple studies that have overcome the limitations of the low abundance of Tregs and their hypoproliferative phenotype to develop Treg-based therapies.

Dendritic cells expand antigen-specific Foxp3+ CD25+ CD4+ regulatory T cells including suppressors of alloreactivity

Thymic derived naturally occurring CD25+ CD4+ T regulatory cells (Tregs) suppress immune responses, including transplantation. Here we discuss the capacity of dendritic cells (DCs) to expand antigen-specific Tregs, particularly polyclonal Tregs directed to alloantigens. Initial studies have shown that mature DCs are specialized antigen-presenting cells (APCs) for expanding antigen-specific CD25+ CD4+ Tregs from TCR transgenic mice. When triggered by specific antigen, these Tregs act back on immature DCs to block the upregulation of CD80 and CD86 costimulatory molecules. More recently, DCs have been used to expand alloantigen-specific CD25+ CD4+ Tregs from the polyclonal repertoire in the presence of interleukin-2 (IL-2). Allogeneic DCs are much more effective than allogeneic spleen cells for expanding CD25+ CD4+ Tregs. The DC-expanded Tregs continue to express high levels of Foxp3, even without supplemental IL-2, whereas spleen cells poorly sustain Foxp3 expression. When suppressive activity is tested, relatively small numbers of DC-expanded CD25+ CD4+ Tregs exert antigen-specific suppression in the mixed leukocyte reaction (MLR), blocking immune responses to the original stimulating strain 10 times more effectively than to third party stimulating cells. DC-expanded Tregs also retard graft versus host disease (GVHD) across full major histocompatibility complex (MHC) barriers. In vitro and in vivo, the alloantigen-specific CD25+ CD4+ Tregs are much more effective suppressors of transplantation reactions than polyclonal populations. We suggest that the expansion of Tregs from a polyclonal repertoire via antigen-presenting DCs will provide a means for antigen-specific control of unwanted immune reactions.

Role of Naturally Arising Regulatory T Cells in Hematopoietic Cell Transplantation

Naturally arising CD4(+)CD25(+) regulatory T cells (Tregs) have the potential to suppress aberrant immune responses and to regulate peripheral T-cell homeostasis. In murine models of bone marrow transplantation, Tregs promote donor bone marrow engraftment and decrease the incidence and severity of graft-versus-host-disease without abrogating the beneficial graft-versus-tumor immunologic effect. These findings, in concert with observations that Tregs in mice and humans share phenotypic and functional characteristics, have led to active investigations into the use of these cells to decrease complications associated with human hematopoietic cell transplantation. Early human studies suggest that an imbalance of Tregs and effector T cells may contribute to the development of graft-versus-host-disease. However, the mechanisms of immunoregulation, in particular the allorecognition properties of Tregs, their effects on and interaction with other immune cells, and their sites of suppressive activity, are not well understood. In this review, we discuss the current knowledge of Treg biology and the potential therapeutic strategies and barriers of Treg immunotherapy in human hematopoietic cell transplantation.

Physiologic regulation of central and peripheral T cell tolerance: lessons for therapeutic applications

Immunologic tolerance is a state of unresponsiveness that is specific for a particular antigen. The immune system has an extraordinary potential for making T cell and B cell that recognize and neutralize any chemical entity and microbe entering the body. Certainly, some of these T cells and B cells recognize self-components; therefore, cellular mechanisms have evolved to control the activity of these self-reactive cells and achieve immunological self-tolerance. The most important in vivo biological significance of mechanisms regulating self-tolerance is to prevent the immune system from mounting an attack against the host's own tissues resulting in autoimmunity. This review summarizes recent developments in our understanding of T-helper cell tolerance and discusses how the new findings can be exploited to prevent and treat autoimmune diseases, allergy, cancer, and chronic infection, or establish donor-specific transplantation tolerance.

Manipulating immunity against Aspergillus fumigatus

Efficient response to Aspergillusfumigatus requires different mechanisms. Polymorphonuclear neutrophils (PMNs) are the predominant immune cells in the acute stage of most fungal infections and play a crucial role in determining the type of pathology associated with fungal infections in different clinical settings. Dendritic cells (DC) are able to decode the fungus-associated information and translate it into different T helper (Th) and regulatory (Treg) cell responses. Functionally distinct Treg cells are activated after exposure to Aspergillus conidia. Early in infection, inflammation/Th1 reactivity is controlled by Treg cells suppressing PMNs and the immunogenic program of DC. The levels of IFN-γ produced in this phase set the subsequent adaptive stage by conditioning the indoleamine 2, 3-dioxygenase (IDO)-dependent tolerogenic program of DC and the subsequent activation of tolerogenic Treg cells, which inhibit Th2 cells and prevent allergy to the fungus. Knowledge of the immunopathogenesis of Aspergillus infections may pave the way to promising strategies for immunotherapy.

Migration and function of FoxP3+ regulatory T cells in the hematolymphoid system

FoxP3+ T cells play critical roles in regulation of the hematolymphoid system and prevention of autoimmunity. Many FoxP3+ T cells, generated in thymus as the result of T cell receptor (TCR) recognition of self antigens, preferentially migrate to secondary lymphoid tissues such as lymph nodes and spleen in a manner similar to conventional naïve T cells. FoxP3+ T cells differentiated in the periphery acquire homing phenotype to bone marrow and nonlymphoid tissues. Consistently, lymphoid- and nonlymphoid-tissue-homing FoxP3+ T cell subsets express different trafficking and chemokine receptors. FoxP3+ T cells regulate hematopoiesis by limiting the activation of immune cells and their production of hematopoietic cytokines available for stem and progenitor cells. In mice deficient in FoxP3+ T cells, aberrant regulation of hematopoiesis including excessive myelopoiesis occurs. In transplantation of allogenic hematopoietic cells, FoxP3+ T cells selectively suppress harmful graft-vs-host disease (GVHD) but leave beneficial graft-vs-leukemia (GVL) activity intact. Therefore, FoxP3+ T cells play essential roles in regulation of the hematolymphoid system in health and diseases, and are likely to be utilized as effective therapeutics for many diseases in the hematolymphoid system in the future.

Function and recruitment of mucosal regulatory T cells in human chronic Helicobacter pylori infection and gastric adenocarcinoma

CD4(+)CD25(high) FOXP3-expressing regulatory T cells (Treg) can suppress immune responses to infections and tumors, thereby promoting microbial persistence and tumor progression. However, little is known about the phenotype and function of human mucosal Treg. Therefore, we analyzed the suppressive activity and homing phenotype of Treg in gastric mucosa of Helicobacter pylori-infected gastric adenocarcinoma patients. We found increased numbers of CD4(+)FOXP3(+) Treg in the tumor compared to tumor-free gastric mucosa. Gastric Treg cells were able to suppress H. pylori-induced T cell proliferation and IFN-gamma production. Furthermore, gastric Treg expressed increased levels of l-selectin and CCR4, compared to non-Treg cells, suggesting that these receptors contribute to Treg recruitment. The presence of functional antigen-specific Treg in H. pylori-infected gastric mucosa supports an important role for these cells in suppression of mucosal effector T cell responses, which probably contribute to bacterial persistence and possibly also to gastric tumor progression.

Only the CD45RA+ subpopulation of CD4+CD25high T cells gives rise to homogeneous regulatory T-cell lines upon in vitro expansion

Thymus-derived CD4+ CD25+ regulatory T cells suppress autoreactive CD4+ and CD8+ T cells and thereby protect from autoimmunity. In animal models, adoptive transfer of CD4+ CD25+ regulatory T cells has been shown to prevent and even cure autoimmune diseases as well as pathogenic alloresponses after solid organ and stem-cell transplantations. We recently described methods for the efficient in vitro expansion of human regulatory T cells for clinical applications. We now demonstrate that only CCR7- and L-selectin (CD62L)-coexpressing cells within expanded CD4+ CD25high T cells maintain phenotypic and functional characteristics of regulatory T cells. Further analysis revealed that these cells originate from CD45RA+ naive cells within the CD4+ CD25high T-cell compartment, as only this subpopulation homogeneously expressed CD62L, CCR7, cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), and forkhead box P3 (FOXP3), produced no inflammatory cytokines and maintained robust suppressive activity after expansion. In contrast, cell lines derived from CD45RA- memory-type CD4+ CD25high T cells lost expression of lymph node homing receptors CCR7 and CD62L, contained interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) as well as IL-10-secreting cells, showed only moderate suppression and, most importantly, did not maintain FOXP3 expression. Based on these unexpected findings, we suggest that isolation and expansion of CD45RA+ naive CD4+ CD25high T cells is the best strategy for adoptive regulatory T (Treg)-cell therapies.

Effect of interleukin-15 on effector and regulatory function of anti-CD3/anti-CD28-stimulated CD4(+) T cells

Autologous transfer of anti-CD3/anti-CD28 (CD3/CD28)-activated CD4(+) T cells may benefit patients receiving autologous stem cell transplant with severe CD4 lymphopenia. Interleukin (IL)-15, an IL-2-like cytokine that promotes T cell survival may enhance immune reconstitution in conjunction with adoptive immunotherapy. We investigated the effect of IL-15 on effector and regulatory function of CD3/CD28-activated CD4(+) T cells. IL-15 upregulated CD45RO and CD25 whereas it down regulated CD62L expression of CD3/CD28-stimulated CD4(+) T cells. Both type 1 (IFN-gamma, tumor necrosis factor (TNF)-alpha) and type 2 (IL-5 and IL-10) production by CD3/CD28-activated CD4(+) T cells was further enhanced by IL-15. Co-culture experiments revealed that CD3/CD28-activated CD4(+) T cells down regulated proliferation of autologous peripheral blood lymphocytes (PBLs) and CD8(+) PBL subsets upon TCR ligation, a contact-dependent effect that was further enhanced by pretreatment with IL-15. Flow cytometric analysis of cell mixture with carboxyfluorescein diacetate succinimidyl ester and Annexin-V-PE staining revealed that CD3/CD28+IL-15-activated CD4(+) T cells showed increased apoptosis over CD4(+) T cells stimulated with CD3/CD28 alone. Taken together, pretreatment of CD3/CD28-activated CD4(+) T cells with IL-15 may increase regulatory function but may aggravate activation-induced apoptosis of CD3/CD28 CD4(+) T cells.

Assessing the potential role of photopheresis in hematopoietic stem cell transplant

The First International Symposium on Photopheresis in Hematopoietic Stem Cell Transplantation was held in Vienna, Austria with an educational grant from Therakos Inc. from 25 May to 27 May 2005. Three general issues were addressed: (1) pathophysiology of graft-versus-host disease (GvHD), (2) induction of immune tolerance and the immunology of phototherapy and (3) current standard treatment and prevention strategies of acute and chronic GvHD and the use of extracorporeal photopheresis (ECP). The objectives of the meeting were to open a dialogue among leading researchers in photobiology, immunology, and hematopoietic stem cell transplantation; foster discussions and suggestions for future studies of the mechanism of action of ECP in acute and chronic GvHD; and promote collaboration between basic scientists and clinicians. As can be seen from the summaries of the individual presentations, important advances have been made in our understanding of GvHD, including the use of photoimmunology interventions and the development of robust model systems. It is our expectation that data from photoimmunology studies can be used to generate hypotheses in animal models that can further define the mechanism of action of ECP and help translate the findings to clinical trials of ECP for the prophylaxis and treatment of both chronic and acute GvHD.

The role of CD4+CD25hi regulatory T cells in the physiopathogeny of graft-versus-host disease

Donor T cells present within hematopoietic stem cell transplants promote engraftment, contribute to T-cell reconstitution and provide an antileukemic effect. However, they are also responsible for the life-threatening graft-versus-host disease. The subpopulation of CD4+CD25hi regulatory T cells, initially identified as crucial players in the regulation of autoimmune processes, might also play a role in the control of alloreactivity. Experimental studies in mice indicate that donor regulatory T cells indeed control alloreactive responses and reduce graft-versus-host disease. Recent clinical reports also suggest that higher numbers of CD4+CD25hi cells within the transplant or in the blood of grafted patients might be associated with reduced graft-versus-host disease. Hence, these cells are attractive immunoregulatory candidates to prevent graft-versus-host disease in humans.

Human regulatory T cells and their role in autoimmune disease

As self-recognition is fundamental to the efficient operation of the immune system, a number of mechanisms have evolved to keep this potential pathologic self-reactivity in check. Thus, even though the majority of strongly self-reactive T cells are deleted in the thymus during T-cell maturation, a number of mature T cells that recognize self-antigens can be found in the peripheral circulation in healthy individuals as well as in patients with autoimmune disease. These self-reactive cells are kept in a non-responsive state in healthy individuals while they appear to be involved in the etiology of a number of autoimmune diseases in patients. The primary role of a relatively recently identified T-cell population, referred to as natural CD4+ CD25+ regulatory T cells, is to modulate the activity of these self-reactive cells. Although it is still unclear how these regulatory cells function, they can inhibit the activation of other potentially pathologic T cells in in vitro assays. Using such assays, regulatory T cells isolated from patients with a number of autoimmune diseases have been shown to exhibit reduced inhibitory function as compared with those isolated from healthy individuals. In this review, we discuss human natural regulatory T cells, what is known about their function, and their associations with specific autoimmune diseases.

In vivo imaging using bioluminescence: a tool for probing graft-versus-host disease

Immunological reactions have a key role in health and disease and are complex events characterized by coordinated cell trafficking to specific locations throughout the body. Clarification of these cell-trafficking events is crucial for improving our understanding of how immune reactions are initiated, controlled and recalled. As we discuss here, an emerging modality for revealing cell trafficking is bioluminescence imaging, which harnesses the light-emitting properties of enzymes such as luciferase for quantification of cells and uses low-light imaging systems. This strategy could be useful for the study of a wide range of biological processes, such as the pathophysiology of graft-versus-host and graft-versus-leukaemia reactions.

Current therapeutic strategies for acute myeloid leukaemia

Improvements in survival in adult acute myeloid leukaemia (AML) have yet to be gleaned from either refinements in the understanding of the pathophysiology of the disease or from the expanding pool of targeted therapies. Outcomes have remained particularly dismal in older patients. Ongoing and planned trials will assess the effects of drugs targeting biological pathways whose clinical importance may vary as a function of the unique genotype and phenotype of each case of AML. The success of these ventures will ultimately require well-designed clinical trials in subsets of patients with risk being dependent not only on age and cytogenetics, but on additional, increasingly quantifiable biological variables. Inhibitors of fms-like tyrosine kinase-3, farnesyl transferase, apoptotic and angiogenic pathways are being studied alone and in combination with chemotherapy. Biological therapies, including monoclonal antibodies, peptide vaccines and interleukin-2, are undergoing evaluation. The role of autologous as well as allogeneic myeloablative and reduced-intensity transplantation continues to be defined. Several potentially useful new cytotoxic agents are being introduced. Critically important to advancing the field in light of such an increasing number of choices is a reassessment of traditional phase II trial designs so that more efficient evaluation of new therapies may take place, even as well-designed phase III trials continue to be performed.

Regulatory T cells in graft-versus-host disease

Alloreactive T cells present in a bone marrow transplant are responsible for graft-vs-host disease, but their depletion is associated with impaired engraftment, immunosuppression, and loss of the graft-vs-leukemia effect. The subpopulation of CD4(+)CD25(+) immunoregulatory T cells was first identified based on its crucial role in the control of autoimmune processes, but they also play a role in alloreactive responses. Moreover, these cells could be used to develop innovative strategies in the field of transplantation and particularly to prevent graft-vs-host disease. Indeed, high numbers of CD4(+)CD25(+) immunoregulatory T cells can modulate graft-vs-host disease if administered at the same time as allogeneic hematopoietic stem cell transplantation in mice. This review discusses various important issues regarding the possible use of CD4(+)CD25(+) immunoregulatory T cells to modulate alloreactivity in hematopoietic stem cell transplantation.

Host B cells produce IL-10 following TBI and attenuate acute GVHD after allogeneic bone marrow transplantation

Host antigen-presenting cells (APCs) are known to be critical for the induction of graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT), but the relative contribution of specific APC subsets remains unclear. We have studied the role of host B cells in GVHD by using B-cell-deficient microMT mice as BMT recipients in a model of CD4-dependent GVHD to major histocompatibility complex antigens. We demonstrate that acute GVHD is initially augmented in microMT recipients relative to wild-type recipients (mortality: 85% vs 44%, P < .01), and this is the result of an increase in donor T-cell proliferation, expansion, and inflammatory cytokine production early after BMT. Recipient B cells were depleted 28-fold at the time of BMT by total body irradiation (TBI) administered 24 hours earlier, and we demonstrate that TBI rapidly induces sustained interleukin-10 (IL-10) generation from B cells but not dendritic cells (DCs) or other cellular populations within the spleen. Finally, recipient mice in which B cells are unable to produce IL-10 due to homologous gene deletion develop more severe acute GVHD than recipient mice in which B cells are wild type. Thus, the induction of IL-10 in host B cells during conditioning attenuates experimental acute GVHD.

T regulatory cells as an immunotherapy for transplantation

Advances in immunosuppressive therapies have made tissue and organ transplantation a common procedure in clinical medicine. However, true donor and recipient tolerance is not regularly achieved and almost all transplant recipients continue to require immunosuppressants throughout life, which is associated with side effects of the drugs. The identification and characterisation of regulatory T cells (Tregs) has recently opened up exciting opportunities for new ways of adoptive immunotherapy in transplantation. CD4+CD25+ Tregs of thymic origin have been shown to be key regulators of unseasoned immune responses in mice and in humans, preventing graft-versus-host disease and organ graft rejection in the transplantation setting. Although these cells can be found in the peripheral blood of healthy individuals, their isolation to a satisfying degree of purity is time-consuming and ineffective. Therefore, a variety of different methods to expand or induce regulatory T cells ex vivo have been advocated. Antigen-specific activation of Tregs is a prerequisite for their optimal function, making the design of new strategies to create and expand antigen-specific Tregs highly desirable. This review will focus on recent advances achieved in the field of transplantation tolerance using naturally occurring Tregs (CD4+CD25+), as well as other Tregs, and will discuss future applications of these cells in immunotherapy.

Tumor-induced expansion of regulatory T cells by conversion of CD4+CD25- lymphocytes is thymus and proliferation independent

The CD25- and CD25+ CD4 T-lymphocyte compartments are tightly regulated. We show here that tumors break such balance, increasing the number of CD4+CD25+ T cells in draining lymph node and spleen but not contralateral node of tumor-bearing mice. Tumor injection in thymectomized and CD25-depleted mice shows that CD4+CD25+ T-cell expansion occurs even in the absence of the thymus and independently from proliferation of preexisting CD25+ T cells. These newly generated cells are bona fide regulatory T cells (T reg) in terms of Foxp3 expression and suppression of CD3-stimulated or allogeneic effector cell proliferation. Transfer of congenic Thy1.1 CD4+CD25- T cells, from mice treated or not with vinblastine, into tumor-bearing or tumor-free mice and analysis of recovered donor lymphocytes indicate that conversion is the main mechanism for acquiring the expression of CD25 and Foxp3 through a process that does not require proliferation. Although conversion of CD4+CD25- T cells for generation of T regs has been described as a natural process that maintains peripheral T-reg population, this process is used by the tumor for immune escape. The prompt recovery of T regs from monoclonal antibody-mediated CD25 depletion in tumor-bearing mice suggests attempts able to inactivate rather than deplete them when treating existing tumors.

CD4+CD25+ regulatory T lymphocytes in bone marrow transplantation

Induction of immunological tolerance to alloantigens would be the treatment of choice to prevent graft-versus-host disease (GvHD) and allograft rejection in transplantation medicine. Organisms use a variety of mechanisms to avoid potentially deadly immunity to self-antigens. The most potent self-tolerance mechanism is probably dominant tolerance assured by regulatory and suppressor T lymphocytes. It appears therefore attractive to use the same mechanism to induce transplantation-tolerance. We here review and discuss recent advances in the use of one of the best-characterized regulatory T lymphocyte populations, CD4(+)CD25(+) T cells, to prevent graft-versus-host disease and bone marrow allograft rejection.

Regulatory T cells: How do they find their space in the immunological arena?

After more of a decade of controversy, the concept of suppressor T cells initially promoted by Gershon, has now become an established paradigm. Current knowledge withholds that populations of suppressor or regulatory T cells (Treg cells) constitute a pivotal mechanism of immunological tolerance. Diverse types of Treg cells, with distinct origins, disparate and multiple functions have been described. However, despite the recognized importance of Treg cells in regulating immune responses, our understanding of their significance, mechanism of action and interplay with other means of immunological tolerance is still in its infancy. Increased comprehension of how Treg cells exert their function holds the promise for therapeutic intervention by manipulating one of the most sophisticated features of immunity to either boost responses in cancer and microbial diseases or suppresses those unwanted in autoimmunity and transplantation. Here, I will discuss new findings and open questions related to CD25(+)Treg cells in an attempt to cover some of the most puzzling aspects in the functioning of these cells.

Regulatory T-cell compartmentalization and trafficking

CD4(+)CD25(+)FOXP3(+) regulatory T cells (CD4(+) Treg cells) are thought to differentiate in the thymus and immigrate from the thymus to the periphery. Treg cells can regulate both acquired and innate immunity through multiple modes of suppression. The cross-talk between Treg cells and targeted cells, such as antigen-presenting cells (APCs) and T cells, is crucial for ensuring suppression by Treg cells in the appropriate microenvironment. Emerging evidence suggests that Treg compartmentalization and trafficking may be tissue or/and organ specific and that distinct chemokine receptor and integrin expression may contribute to selective retention and trafficking of Treg cells at sites where regulation is required. In this review, the cellular and molecular signals that control specialized migration and retention of Treg cells are discussed.

Shared biology of GVHD and GVT effects: Potential methods of separation

The difficult separation of clinical graft-versus-tumor (GVT) effects from graft-versus-host disease (GVHD) reflects their shared biology. Experimental approaches to mediate GVT effects while limiting GVHD include: (1) allograft T cell depletion followed by immune enhancement; (2) modulation of T cell dose or T cell subset composition; (3) donor lymphocyte infusion; (4) reduced-intensity host preparation; (5) modulation of Th1/Th2 and Tc1/Tc2 cell balance; (6) cytokine therapy or neutralization; (7) T regulatory cell therapy; (8) co-stimulatory pathway modulation; (9) chemokine pathway modulation; (10) induction of antigen-specific T cells; (11) alloreactive NK cell therapy; and (12) targeted pharmaceutical inhibition of proteosome, mammalian target of rapamycin, and histone deacetylase pathways. Clearly, a multitude of approaches exist that hold promise for separating GVT effects from GVHD. Future success in this endeavor will require a strong commitment towards translational research and continued advances in cell, vaccine, cytokine, monoclonal antibody, and targeted molecular therapy.

Isolation of CD4+CD25+ Regulatory T Cells for Clinical Trials

The adoptive transfer of donor CD4+CD25+ regulatory T cells has been shown to protect from lethal graft-versus-host disease after allogeneic bone marrow transplantation in murine disease models. Efficient isolation strategies that comply with good manufacturing practice (GMP) guidelines are prerequisites for the clinical application of human CD4+CD25+ regulatory T cells. Here we describe the isolation of CD4+CD25+ T cells with regulatory function from standard leukapheresis products by using a 2-step magnetic cell-separation protocol performed under GMP conditions. The generated cell products contained on average 49.5% CD4+CD25high T cells that phenotypically and functionally represented natural CD4+CD25+ regulatory T cells and showed a suppressive activity comparable to that of CD4+CD25+ regulatory T-cell preparations purified by non-GMP-approved fluorescence-activated cell sorting.

Clinical application of expanded CD4+25+ cells

A central goal immunologists has been to develop targeted therapies that will induce or maintain immunologic tolerance in the absence of potentially harmful immunosuppression. The ability to isolate and expand regulatory T-cell populations with immune suppressive activity will enable new forms of adoptive immunotherapy that may achieve this long held dream. Assuming that certain technical challenges regarding the manufacturing of regulatory T cells can be overcome, a wide variety of clinical applications can be envisioned using adoptively transferred CD4(+)CD25(+) regulatory T cells. It is likely that suppressor T cells will first be tested for their ability to prevent or treat graft-versus-host disease (GVHD) following allogeneic bone marrow or stem cell transplantation. A related approach will be clinical studies to induce allogeneic or xenogeneic tolerance using regulatory T cells in solid organ transplantation. A more technically challenging approach will be the use of regulatory T-cell therapy for autoimmune disorders. Finally on the horizon are approaches that will use genetically engineered lymphocytes to replace regulatory T cells in the immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome, and potentially to create more potent regulatory T (Treg) cells with enhanced suppressive activity.

Immunity and Tolerance to Aspergillus Involve Functionally Distinct Regulatory T Cells and Tryptophan Catabolism

The inherent resistance to diseases caused by Aspergillus fumigatus suggests the occurrence of regulatory mechanisms that provide the host with adequate defense without necessarily eliminating the fungus or causing unacceptable levels of host damage. In this study, we show that a division of labor occurs between functionally distinct regulatory T cells (Treg) that are coordinately activated by a CD28/B-7-dependent costimulatory pathway after exposure of mice to Aspergillus conidia. Early in infection, inflammation is controlled by the expansion, activation and local recruitment of CD4+CD25+ Treg capable of suppressing neutrophils through the combined actions of IL-10 and CTLA-4 on indoleamine 2,3-dioxygenase. The levels of IFN-gamma produced in this early phase set the subsequent adaptive stage by conditioning the indoleamine 2,3-dioxygenase-dependent tolerogenic program of dendritic cells and the subsequent activation and expansion of tolerogenic Treg, which produce IL-10 and TGF-beta, inhibit Th2 cells, and prevent allergy to the fungus. The coordinate activation of Treg may, however, be subverted by the fungus, as germinating conidia are capable of interfering with anti-inflammatory and tolerogenic Treg programs. Thus, regulation is an essential component of the host response in infection and allergy to the fungus, and its manipulation may allow the pathogen to overcome host resistance and promote disease.

A Revision of Billingham’s Tenets: The Central Role of Lymphocyte Migration in Acute Graft-versus-Host Disease

The migration of cells from vascular to extravascular compartments effects a sequential cascade of events, involving an interplay between adhesion molecules and chemokines. All T cell-mediated immune responses, of which acute graft-versus-host disease (GVHD) is an example, require that effector cells reach their target tissues. Lymphocytes do not enter specific tissues because they "see" a given antigen; they enter because they possess the requisite combination of homing receptors and chemokine receptors to engage the endothelium at the target tissue(s). Billingham's tenets on the immunobiology of GVHD must be expanded to accommodate this obligatory homing component. Because GVHD is relatively organ specific--principally affecting the skin, gut, and liver-our increasing knowledge of the pertinent adhesion molecules and chemokines directing effector cell trafficking to these sites offers novel therapeutic approaches for prevention or treatment of GVHD. The potential efficiency of this form of therapy could eliminate the use of graft manipulations (eg, T-cell depletion) and broad immunosuppressives, thereby lessening infectious complications and preserving the potent graft-versus-malignancy effect of allogeneic hematopoietic stem cell transplantation.

Functional Dynamics of Naturally Occurring Regulatory T Cells in Health and Autoimmunity

A network of regulatory T (Treg) cells exists to downregulate immune responses in various inflammatory circumstances and ultimately assure peripheral T cell tolerance. Naturally occurring CD4(+)CD25(+) Treg cell represents a major lymphocyte population engaged in the dominant control of self-reactive T responses and maintenance of tolerance within this network. CD4(+)CD25(+) Treg cells differentiate in the normal thymus as a functionally distinct subpopulation of T cells bearing a broad T cell receptor repertoire endowing these cells with the capacity to recognize a wide spectrum of self-Ag and non-self-Ag specificities. The development of CD4(+)CD25(+) Treg cells is genetically determined, influenced by Ag-specific and nonspecific signals, costimulation, and cytokines that control their activation, expansion, and suppressive activity. Functional abrogation of these cells in vivo, or genetic defects that affect their development or function, unequivocally predisposes animals and humans to the onset of autoimmune and other inflammatory diseases. Studies have shed light in our understanding of the cellular and molecular basis of CD4(+)CD25(+) Treg cell-mediated immune regulation. In this chapter, we discuss the contribution of naturally occurring CD4(+)CD25(+) Treg cells in the induction of immunologic self-tolerance in animal models and humans and attempt to provide a comprehensive overview of recent findings regarding the phenotype, functional dynamics, and effector mechanism of these cells in autoimmune diseases.