Regulatory T cells: the physiology of autoreactivity in dominant tolerance and "quality control" of immune responses

The effects of oral administration of Aureobasidium pullulans-cultured fluid containing β-glucan on concanavalin A injected mice

A black yeast, Aureobasidium pullulans, extracellularly produces β-(1,3), (1,6)-D-glucan (β-glucan) under certain conditions. The β-glucan is known to be an immunomodulatory agent, and β-glucan enriched A. pullulans cultured fluid (AP-CF) is used in supplements to maintain human health. Concanavalin A (ConA) is a lectin, and when injected it is known to cause T cell mediated autoimmune hepatitis in mice. The present study investigated the effects of oral administration of AP-CF on ConA injection in mice. The results demonstrated that increases in serum alanine transaminase (ALT) levels after ConA injection were significantly suppressed in an AP-CF administered group of mice. To understand the mechanism of the ALT lowering effects of AP-CF, we used Foxp3 (forkhead box P3) knock-in mice which express the green fluorescent protein (GFP) in Foxp3 induced cells, and the effects of AP-CF on the regulatory T cell (T_reg) populations were investigated. The results show that the basal level of Foxp3⁺ T_reg populations in peripheral blood lymphocytes, liver infiltrating lymphocytes, and splenocytes was decreased after 7 days of administration of AP-CF. These findings suggest that oral administration of AP-CF suppresses the basal level of inflammation, and that it may be postulated to be involved in the ALT lowering effects of AP-CF.

An evolutionary perspective on the systems of adaptive immunity

We propose an evolutionary perspective to classify and characterize the diverse systems of adaptive immunity that have been discovered across all major domains of life. We put forward a new function-based classification according to the way information is acquired by the immune systems: Darwinian immunity (currently known from, but not necessarily limited to, vertebrates) relies on the Darwinian process of clonal selection to 'learn' by cumulative trial-and-error feedback; Lamarckian immunity uses templated targeting (guided adaptation) to internalize heritable information on potential threats; finally, shotgun immunity operates through somatic mechanisms of variable targeting without feedback. We argue that the origin of Darwinian (but not Lamarckian or shotgun) immunity represents a radical innovation in the evolution of individuality and complexity, and propose to add it to the list of major evolutionary transitions. While transitions to higher-level units entail the suppression of selection at lower levels, Darwinian immunity re-opens cell-level selection within the multicellular organism, under the control of mechanisms that direct, rather than suppress, cell-level evolution for the benefit of the individual. From a conceptual point of view, the origin of Darwinian immunity can be regarded as the most radical transition in the history of life, in which evolution by natural selection has literally re-invented itself. Furthermore, the combination of clonal selection and somatic receptor diversity enabled a transition from limited to practically unlimited capacity to store information about the antigenic environment. The origin of Darwinian immunity therefore comprises both a transition in individuality and the emergence of a new information system - the two hallmarks of major evolutionary transitions. Finally, we present an evolutionary scenario for the origin of Darwinian immunity in vertebrates. We propose a revival of the concept of the 'Big Bang' of vertebrate immunity, arguing that its origin involved a 'difficult' (i.e. low-probability) evolutionary transition that might have occurred only once, in a common ancestor of all vertebrates. In contrast to the original concept, we argue that the limiting innovation was not the generation of somatic diversity, but the regulatory circuitry needed for the safe operation of amplifiable immune responses with somatically acquired targeting. Regulatory complexity increased abruptly by genomic duplications at the root of the vertebrate lineage, creating a rare opportunity to establish such circuitry. We discuss the selection forces that might have acted at the origin of the transition, and in the subsequent stepwise evolution leading to the modern immune systems of extant vertebrates.

Thoughts engendered by Bretscher's Two-step, Two-signal model for a peripheral self-non-self discrimination and the origin of primer effector T helpers

There are three questions under re-examination here that have been inspired by Bretscher's 'Two-step, Two-signal' model. First, what is the nature of the steps required in order for antigen-responsive cells to become effectors? Second, how does the immune system get started? Third and the most troublesome, what is the mechanism that relates the delivery of the two signals? To answer the first question, Bretscher proposes a pathway that I will place in another context by comparing it with what had been envisaged under the Associative Recognition of Antigen (ARA) model. The second question, how does the immune system gets started, is crucial to our understanding of the self-non-self discrimination. This problem boils down to, what is the origin of the first effector T helper (eTh) cells required to activate all antigen-responsive cells including the T helpers themselves (the primer problem)? To deal with this question, I proposed an antigen-independent pathway to primer eTh. Bretscher presents us with an antigen-dependent pathway to primer eTh. As competing models are precious in clarifying thinking and in guiding experimentation, I felt it important to reanalyse the two models and look for ways to decide between them. The third question deals with the requirement for and the mechanism of associative (linked) recognition of antigen (ARA). The concept of ARA is so compelling at both the experimental and theoretical levels that to save it, a new perspective will be introduced.

The HER2 peptide nelipepimut-S (E75) vaccine (NeuVax™) in breast cancer patients at risk for recurrence: correlation of immunologic data with clinical response

Nelipepimut-S (formerly known as E75) is an immunogenic peptide from the HER2 protein that is highly expressed in breast cancer. The NeuVax™ (Galena, OR, USA) vaccine, nelipepimut-S plus granulocyte-macrophage colony-stimulating factor, is designed for the prevention of clinical recurrences in high risk, disease-free breast cancer patients. Although cancer vaccines such as NeuVax represent promising approaches to cancer immunotherapy, much remains to be elucidated regarding their mechanisms of action: particularly given that multiple cancer vaccine trials have failed to demonstrate a correlation between immunologic data and clinical outcome. Here, we briefly discuss our clinical trial experience with NeuVax focusing on immunologic response data and its implication on how the immune system may be affected by this peptide vaccine. Most importantly, we demonstrate the potential capability of certain immunologic assays to predict clinical benefit in our trials.

CAR/FoxP3-engineered T regulatory cells target the CNS and suppress EAE upon intranasal delivery

BACKGROUND

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS). In the murine experimental autoimmune encephalomyelitis (EAE) model of MS, T regulatory (Treg) cell therapy has proved to be beneficial, but generation of stable CNS-targeting Tregs needs further development. Here, we propose gene engineering to achieve CNS-targeting Tregs from naïve CD4 cells and demonstrate their efficacy in the EAE model.

METHODS

CD4+ T cells were modified utilizing a lentiviral vector system to express a chimeric antigen receptor (CAR) targeting myelin oligodendrocyte glycoprotein (MOG) in trans with the murine FoxP3 gene that drives Treg differentiation. The cells were evaluated in vitro for suppressive capacity and in C57BL/6 mice to treat EAE. Cells were administered by intranasal (i.n.) cell delivery.

RESULTS

The engineered Tregs demonstrated suppressive capacity in vitro and could efficiently access various regions in the brain via i.n cell delivery. Clinical score 3 EAE mice were treated and the engineered Tregs suppressed ongoing encephalomyelitis as demonstrated by reduced disease symptoms as well as decreased IL-12 and IFNgamma mRNAs in brain tissue. Immunohistochemical markers for myelination (MBP) and reactive astrogliosis (GFAP) confirmed recovery in mice treated with engineered Tregs compared to controls. Symptom-free mice were rechallenged with a second EAE-inducing inoculum but remained healthy, demonstrating the sustained effect of engineered Tregs.

CONCLUSION

CNS-targeting Tregs delivered i.n. localized to the CNS and efficiently suppressed ongoing inflammation leading to diminished disease symptoms.

Environment and T regulatory cells in allergy

The central role of T regulatory cells in the responses against harmless environmental antigens has been confirmed by many studies. Impaired T regulatory cell function is implicated in many pathological conditions, particularly allergic diseases. The "hygiene hypothesis" suggests that infections and infestations may play a protective role for allergy, whereas environmental pollutants favor the development of allergic diseases. Developing countries suffer from a variety of infections and are also facing an increasing diffusion of environmental pollutants. In these countries allergies increase in relation to the spreading use of xenobiotics (pesticides, herbicides, pollution, etc.) with a rate similar to those of developed countries, overcoming the protective effects of infections. We review here the main mechanisms of non-self tolerance, with particular regard to relations between T regulatory cell activity, infections and infestations such as helminthiasis, and exposure to environmental xenobiotics with relevant diffusion in developing countries.

Inhibition of tumor angiogenesis in lung cancer by T4 phage surface displaying mVEGFR2 vaccine

Vascular endothelial growth factor (VEGF) has been known as a potential vasculogenic and angiogenic factor and its receptor (VEGFR2) is a major receptor to response to the angiogenic activity of VEGF. The technique that to break the immune tolerance of "self-antigens" associated with angiogenesis is an attractive approach for cancer therapy with T4 phage display system. In this experiment, mouse VEGFR2 was constructed on T4 phage nanometer-particle surface as a recombinant vaccine. T4-mVEGFR2 recombinant vaccine was identified by PCR and western blot assay. Immunotherapy with T4-mVEGFR2 was confirmed by protective immunity against Lewis lung carcinoma (LLC) in mice. The antibody against mVEGFR2 was detected by ELISPOT, ELISA and Dot ELISA. The inhibitive effects against angiogenesis were studied using CD31 and CD105 via histological analysis. VEGF-mediated endothelial cells proliferation and tube formation were inhibited in vitro by immunoglobulin induced by T4-mVEGFR2. The antitumor activity was substantiated from the adoptive transfer of the purified immunoglobulin. Antitumor activity and autoantibody production of mVEGFR2 could be neutralized by the depletion of CD4+T lymphocytes. These studies strongly suggest that T4-mVEGFR2 recombinant vaccine might be a promising antitumor approach.

Study of liver-targeted regulatory T cells in hepatitis B and C virus in chronically infected patients

INTRODUCTION

Regulatory T cells (Tregs) play a critical role in chronic viral infections. The role of Tregs in chronic hepatitis B (CHB) and chronic hepatitis C (CHC) is unknown. This study examined the distribution and frequency of forkhead box p3(+) (Foxp3(+)) Tregs in the liver tissue and compared the clinicopathological characteristics of CHB and CHC patients.

METHODS

Liver needle biopsies were obtained from 26 patients who were hepatitis B surface antigen positive and 27 patients who were hepatitis C virus antibody positive.

RESULTS

The ratio of Foxp3(+) Tregs in CD3(+) T cells was similar in HBV and in HCV cases. In HBV cases, the variables that were positively associated with the ratio of Foxp3(+) Tregs in CD3(+) T cells included the serum alanine aminotransferase level (R=0.402, P=0.025) and the ratio of CD8(+) T cell plus CD56(+) NK cell against CD4(+) T cell (R=0.53, P=0.005). The ratio of Foxp3(+) Tregs in CD3(+) T cells increased more in the severe activity group than in the mild activity group (P=0.04). In HCV cases, the ratio of Foxp3(+) Tregs in CD3(+) T cells increased significantly in terms of the genotype2 (P=0.0002) and male gender (P=0.04). In addition, the ratio of Foxp3(+) Tregs in CD3(+) T cells showed a negative correlation with the ratio of CD8(+) T cell plus CD56(+) NK cell against CD4(+) T cell (R=-0.508, P=0.005) and HCV viral load (R=-0.482, P=0.001).

CONCLUSIONS

Liver-targeted regulatory T cells present similarly in CHB and CHC, but their relationship with the effector cell population, the inflammation grade or the viral load is different between CHB and CHC.

Development of regulatory T cells requires IL-7Ralpha stimulation by IL-7 or TSLP

Interleukin-7 (IL-7), a cytokine produced by stromal cells, is required for thymic development and peripheral homeostasis of most major subsets of T cells. We examined whether regulatory T (Treg) cells also required the IL-7 pathway by analyzing IL-7Ralpha(-/-) mice. We observed a striking reduction in cells with the Treg surface phenotype (CD4, CD25, GITR (glucocorticoid-induced tumor necrosis factor [TNF]-like receptor), CD45RB, CD62L, CD103) or intracellular markers (cytotoxic T-lymphocyte-associated antigen-4, CTLA-4, and forkhead box transcription factor 3, Foxp3). Foxp3 transcripts were virtually absent in IL-7Ralpha(-/-) lymphoid tissues, and no Treg cell suppressive activity could be detected. There are 2 known ligands for IL-7Ralpha: IL-7 itself and thymic stromal lymphopoietin (TSLP). Surprisingly, mice deficient in IL-7 or the other chain of the TSLP receptor (TSLPR) developed relatively normal numbers of Treg cells. Combined deletion of IL-7 and TSLP receptor greatly reduced Treg cell development in the thymus but was not required for survival of mature peripheral Treg cells. We conclude that Treg cells, like other T cells, require signals from the IL-7 receptor, but unlike other T cells, do not require IL-7 itself because of at least partially overlapping actions of IL-7 and TSLP for development of Treg cells.

The E75 HER2/neu peptide vaccine

E75 (HER2/neu 369-377) is an immunogenic peptide from the HER2/neu protein which is overexpressed in many breast cancer patients. A large amount of preclinical work and a small number of Phase I trials have been completed evaluating the vaccine potential of the E75 peptide mixed with an immunoadjuvant. Our group has performed two concurrent E75 + GM-CSF Phase II trials in node-positive and node-negative disease-free breast cancer patients. These trials, totaling 186 patients, were designed to assess the ability of the E75 vaccine to prevent disease recurrence in these high risk patients. In this review article, we discuss the safety of the vaccine, the immunologic response to the peptide, and most importantly, the potential clinical benefit of the vaccine. The recurrence rate, mortality associated with recurrence, and the distribution of recurrences are presented and discussed. Additionally, the lessons learned from these trials to include optimal dosing and the need for booster inoculations are addressed. We also present data exploring possible explanations and mechanisms behind the potential clinical utility of this simple single epitope vaccine. Finally, we present some of the future directions for our Cancer Vaccine Development Program assessing multi-epitope peptide vaccines and combination immunotherapies.

Phenotype of CD4+ T cell subsets that develop following mouse facial nerve axotomy

We have previously shown that CD4(+) T helper (Th) 2 cells, but not Th1 cells, participate in the rescue of mouse facial motoneurons (FMN) from axotomy-induced cell death. Recently, a number of other CD4(+) T cell subsets have been identified in addition to the Th1 and Th2 effector subsets, including Th17, inducible T regulatory type 1 (Tr1), and naturally thymus-born Foxp3(+) regulatory (Foxp3(+) Treg) cells. These subsets regulate the nature of a T cell-mediated immune response. Th1 and Th17 cells are pro-inflammatory subsets, while Th2, Tr1, and Foxp3(+) Treg cells are anti-inflammatory subsets. Pro-inflammatory responses in the central nervous system are thought to be neurodestructive, while anti-inflammatory responses are considered neuroprotective. However, it remains to be determined if another CD4(+) T cell subset, other than the Th2 cell, develops after peripheral nerve injury and participates in FMN survival. In the present study, we used FACS analysis to determine the temporal frequency of Th1, Th17, Th2, Tr1 and Foxp3(+) Treg CD4(+) T cell subset development in C57BL/6 wild type mice after facial nerve transection at the stylomastoid foramen in the mouse. The results indicate that all of the known CD4(+) T cell subsets develop and expand in number within the draining lymph node, with a peak in number primarily at 7 days postoperative (dpo), followed by a decline at 9 dpo. In addition to the increase in subset frequency over time, FACS analysis of individual cells showed that the level of cytokine expressed per cell also increased for interferon-gamma (IFN-gamma), interleukin (IL)-10 and IL-17, but not IL-4. Additional control double-cytokine labeling experiments were done which indicate that, at 7dpo, the majority of cells indeed have committed to a specific phenotype and express only 1 cytokine. Collectively, our findings indicate for the first time that there is no preferential activation and expansion of any single CD4(+) T cell subset after peripheral nerve injury but, rather, that both pro-inflammatory and anti-inflammatory CD4(+) T cells develop.

Naturally occurring B-cell autoreactivity: A critical overview

In over one century of research in immunology marked progress in the scientific knowledge and the implications derived from it has been made. At the same time several contradictory and seemingly opposing results have been obtained. The term autoimmunity is still conceived by many as a term directly related to an immunopathological state. However, strong evidence exist that not only the immune system is able to recognize self-constituents, but it appears also that this property is essential for homeostasis. Direct or indirect alterations of such self-recognition properties of the immune system may contribute to pathology. In this review, the most recent advances in the field of naturally occurring B-cell autoreactivity in health as well as in disease are presented and discussed.

Interleukin-2 Mastering Regulation in Cancer and Autoimmunity

Autoimmunity and tumor immunity evolved as two distinct arenas in immunological research. However, the identification of self-antigens as the major components of malignant cells may define a central role for autoimmunity in cancer control tuned by peripheral immunoregulatory mechanisms avoiding self-aggression. Emerging evidence documents a triple antagonistic role of interleukin-2 (IL-2) in vivo promoting survival, apoptosis, and the generation of regulatory T cells. We have found that IL-2 administration reduces the clinical course of experimental autoimmune encephalomyelitis and enhances immunoregulation in tumor-bearing mice. However, actively induced anti-IL-2 antibodies restore the response to nominal antigens in tumor-induced immunosuppressed host and induced therapeutic effect in transplantable and chemically induced tumors. It is suggested that IL-2 may contribute dynamically to the maintenance of natural immunological tolerance, preventing pathological autoimmunity, but may affect antitumor immunity. Cancer research has gained from autoimmunity understanding that tumor escape strategies include the natural mechanisms of immune tolerance and the ways to imbalance the peripheral regulatory mechanisms. Interestingly, therapeutic manipulations of immunoregulation have limited antitumor effects, although promoting collaterally infrequent autoimmune diseases. It may suggest that tumors may reinforce tolerance to protect themselves from the immune attack, a process that may involve dynamically various mechanisms including IL-2. Understanding this acquired experience from tumors may help utilize them to revert the immunopathology in autoimmune diseases.

Full restoration of peripheral Foxp3+ regulatory T cell pool by radioresistant host cells in scurfy bone marrow chimeras

Mutations in the gene encoding the transcription factor Foxp3 lead to fatal autoimmune pathology in mice and humans, which is associated with a deficiency in Foxp3(+) regulatory T cells (T(reg)). It has also been proposed that Foxp3 inactivation in nonhematopoietic tissues, particularly in thymic epithelium, is required for the pathogenesis, because Foxp3 mutant scurfy bone marrow cells fail to transmit the disease to lethally irradiated WT hosts. We demonstrate here that the lack of pathology in these radiation chimeras is due to the presence of radioresistant endogenous Foxp3(+) T(reg) of the host. In addition, chimeras carrying the scurfy mutation only in nonhematopoietic cells exhibit no evidence of autoimmune pathology. Thus, Foxp3 deficiency in nonhematopoietic cells does not contribute to the scurfy disease. Furthermore, our analyses of radiation chimeras revealed that the peripheral T(reg) pool is fully and specifically restored and maintained by radioresistant endogenous T(reg) or adoptively transferred exogenous T(reg) through "homeostatic" proliferation in the absence of T(reg) production from scurfy donor bone marrow cells. These results thus provide evidence that the autoimmune pathology in scurfy mice results indeed from a T(reg) deficiency and illustrate a robust homeostatic mechanism that strictly controls the size of peripheral T(reg) pool by fine-tuning of homeostatic proliferation.

Differential recognition of heat-shock protein dnaJ–derived epitopes by effector and Treg cells leads to modulation of inflammation in juvenile idiopathic arthritis

OBJECTIVE

To identify epitopes on Escherichia coli heat-shock protein (HSP) dnaJ or on homologous human HSP dnaJ involved in the induction/modulation of autoimmune inflammation in patients with oligoarticular juvenile idiopathic arthritis (JIA).

METHODS

We used a proliferation assay and cytokine production to evaluate the immune responses of synovial fluid mononuclear cells (SFMCs) to pan-HLA-DR binder peptides derived from either homologous or nonhomologous regions on bacterial and human HSP dnaJ. Cytofluorometric analysis was performed in order to phenotype and sort Treg cells. Sorted cells were then analyzed for the expression of the forkhead box P3 (FoxP3) transcription factor, and their regulatory capacity was tested in coculture assays.

RESULTS

T cell responses to E coli HSP dnaJ-derived peptides were eminently proinflammatory. Conversely, peptides derived from human HSP dnaJ induced interleukin-10 (IL-10) production from SFMCs of patients with oligoarticular JIA. A positive correlation was found between disease with a better prognosis (persistent oligoarticular JIA) and recognition of 3 human HSP dnaJ-derived peptides. The recognition of the human peptide H134-148 also induced a significantly greater amount of IL-10 in patients with persistent oligoarticular JIA than in those with extended oligoarticular JIA (P = 0.0012). Incubation of SFMCs from patients with persistent oligoarticular JIA with this human epitope increased the percentage of Treg cells and FoxP3 expression. It also induced the recovery of suppressor activity by Treg cells.

CONCLUSION

This is the first description of a self-regulating immune modulator circuit active during autoimmune inflammation through recognition of HSP epitopes with different functional properties. These epitopes induce T cells with regulatory function. Such induction correlates with disease severity and prognosis.

Active antimetastatic immunotherapy in Lewis lung carcinoma with self EGFR extracellular domain protein in VSSP adjuvant

The epidermal growth factor receptor (EGFR) plays a central role in regulating neoplastic processes. The EGFR overexpression in many human epithelial tumors has been correlated with disease progression and bad prognosis. Passive EGFR-directed immunotherapy, but not active specific approaches, has already been introduced in medical oncology practice. Then we wonder if mice immunization with the extracellular domain of murine EGFR (mEGFR-ECD) in adjuvants can circumvent tolerance to self EGFR, by inducing an immune response with consequent antitumor effect. The present study demonstrated that despite mEGFR expression in thymus, strong DTH response was induced by inoculation of mice with the mEGFR-ECD. This self-immunization, using both CFA and very small sized proteoliposomes from Neisseria meningitidis (VSSP), promoted highly specific IgG titers, predominantly IgG2a and IgG2b. Sera from mice immunized with mEGFR-ECD/VSSP not only recognized EGFR+ tumor cell lines by FACS, but also inhibited their in vitro growth, even in the absence of complement. Noteworthy, vaccination of mice with mEGFR-ECD/VSSP stimulated a potent antimetastatic effect in the EGFR+ Lewis lung carcinoma model, while reproduction-associated side effects were absent. Curiously, mice immunized with the human EGFR-ECD (Her1-ECD) in VSSP though induced highly specific IgG antibodies with strong in vitro cytotoxic effect over EGFR+ human cell lines, showed low cross-reactivity with the mEGFR-ECD. These results further encouraged the development of the Her1-ECD/VSSP vaccine project for patients with EGFR+ tumors.

Levels of circulating regulatory CD4+CD25+ T cells are decreased in breast cancer patients after vaccination with a HER2/neu peptide (E75) and GM-CSF vaccine

PURPOSE

We are conducting clinical trials in breast cancer (BrCa) patients to test the HER2/neu peptide vaccine (E75). We have investigated the impact of this vaccine on circulating levels of regulatory T cells (Treg) and the resulting effects on antitumor responses.

EXPERIMENTAL DESIGN

Twenty-two blood samples from healthy individuals and from 22 BrCa patients including pre- and post-vaccination samples from seven vaccinated HLA-A2+ patients were stained for CD4, CD25, and CD69 as well as CD8 and E75:HLA-A2 Ig dimer and quantified by flow cytometry. Cytotoxic activity against HER2/neu+ tumors was measured by 51Cr-release. Serum from BrCa patients and normal subjects were analyzed for TGF-beta levels.

RESULTS

BrCa patients have a greater percentage of circulating Treg (CD4+CD25+, 4.45% versus 2.96%; p=0.007) than normal subjects. HLA-A2+ BrCa patients had more Treg compared to the HLA-A2- BrCa patients (CD4+CD25+, 5.63% versus 3.28%; p=0.001). E75 vaccination increased circulating activated CD4+ T cells post-vaccination (CD4+CD69+, 1.23 versus 3.81%; p=0.03). However, T(reg) were significantly reduced after vaccination (CD4+CD25+, 5.31-1.81%; p<0.0001). Furthermore, activated Treg also decreased (CD4+CD25+CD69+, 0.23% versus 0.08%; p=0.06). Importantly, post-vaccination decreases in Treg were temporally associated with increased E75 vaccine-specific CD8+ T cells and corresponding HER2/neu+ tumor cytotoxicity. Serum TGF-beta levels were significantly elevated in BrCa patients compared to normals (3548 pg/ml versus 1007 pg/ml; p=0.007). Four of seven vaccinated patients showed decreased serum TGF-beta levels post-vaccination.

CONCLUSIONS

Treg, are increased in BrCa patients along with serum levels of TGF-beta. E75 vaccination resulted in CD4+ recruitment but was associated with a significant decrease in circulating Treg and TGF-beta levels in the majority of the vaccinated patients. Successful cancer vaccination strategies may require the alteration of complex immune interactions.

On the cross-fertilization between biotechnology and immunology: current situation in Cuba

Biotechnology has created opportunities for new therapeutic products. More than half of biotechnological products being developed intend some manipulation of the immune system. Vaccines are 29% of the worldwide biotechnology pipeline and more than 50% of Cuban biotechnology pipeline. Current understanding of antigen presentation and the maturation of dendritic cells has opened the way for a more rational design of new adjuvants, intended not only to deliver the antigen to antigen presenting cells (APC) and to induce APC maturation, but also to direct lymphocyte differentiation towards either Th1 or Th2 phenotypes, and to deal with disease-induced immunodepression. Therapeutic cancer vaccines are a field in which new adjuvants and new immunization schedules will be tested. This knowledge will be pertinent for other vaccines, so fertilizing biotechnology with the indication of the new molecules that need to be manufactured.

Liver-targeted and peripheral blood alterations of regulatory T cells in primary biliary cirrhosis

CD4+CD25high regulatory T cells (Tregs) play a critical role in self-tolerance, as seen in murine autoimmunity. Studies on Tregs in human autoimmunity have focused primarily on peripheral blood samples. A study targeting diseased tissue should identify direct relationships between Tregs and autoimmunity. Peripheral blood samples were collected from 91 patients with primary biliary cirrhosis (PBC), 28 immediate relatives, and 41 healthy controls, and Treg frequencies were determined as a percentage of CD4+CD25high T cells in CD4+TCR-alphabeta+ T cells. A tissue-targeted determination of frequency and distribution of FoxP3+ Tregs was also performed on 90 different liver tissue specimens exhibiting PBC (n = 52), chronic hepatitis C (CHC) (n = 30), and autoimmune hepatitis (AIH) (n = 8). Treg suppression studies were performed on 50 PBC patients and 27 controls. Patients with PBC demonstrated a relative reduction of Tregs compared with controls (P < .0002). Interestingly, a deficiency in CD4+CD25+ Tregs was also found in the daughters and sisters of PBC patients compared with controls (P < .0007). However, functional studies did not reveal a global PBC Treg defect. The level of FoxP3-expressing Tregs was markedly lower in affected PBC portal tracts compared with CHC and AIH (P < .001). In addition, the CD8+T cell/FoxP3+ Treg ratio was significantly higher in livers of late-stage PBC compared with those of CHC (P < .001) and early-stage AIH (P < .001). In conclusion, these data provide support for a genetic modulation of Treg frequency and illustrate the role Tregs play in the loss of tolerance in PBC.

Frequency of Natural Regulatory CD4+CD25+ T Lymphocytes Determines the Outcome of Tolerance across Fully Mismatched MHC Barrier through Linked Recognition of Self and Allogeneic Stimuli

We show in this study that long-term tolerance to allogeneic skin grafts can be established in the absence of immunosuppression by the combination of the following elements: 1) augmenting the frequency of regulatory CD4(+)CD25(+) T cells (Treg) and 2) presentation of the allogeneic stimuli through linked recognition of allo- and self-epitopes on semiallogeneic F(1) APCs. BALB/c spleen cells enriched for CD4(+)CD25(+) T lymphocytes were transferred either to BALB/c nu/nu mice or to BALB/c nu/nu previously injected with F(1)(BALB/c x B6.Ba) spleen cells, or else grafted with F(1)(BALB/c x B6.Ba) skin (chimeric BALB/c nu/nu-F(1)). Chimeric BALB/c nu/nu-F(1) reconstituted with syngeneic CD25(+)-enriched spleen cells were unable to reject the previously transferred F(1)(BALB/c x B6.Ba) spleen cells or F(1)(BALB/c x B6.Ba) skin grafts, and a specific tolerance to a secondary B6 graft was obtained, with rejection of third-party CBA grafts. BALB/c nu/nu mice reconstituted only with syngeneic CD25(+)-enriched spleen cells rejected both B6 and CBA skin grafts. In contrast, when chimeric BALB/c nu/nu-F(1) were reconstituted with spleen populations comprising normal frequencies of Treg cells, the linked recognition of allo and self resulted in breaking of self tolerance and rejection of syngeneic grafts, strongly suggesting that linked recognition works in both directions, either to establish tolerance to allo, or to break tolerance to self, the critical parameter being the relative number of Treg cells.

Thymic commitment of regulatory T cells is a pathway of TCR-dependent selection that isolates repertoires undergoing positive or negative selection

The seminal work of Le Douarin and colleagues (Ohki et al. 1987; Ohki et al. 1988; Salaun et al. 1990; Coutinho et al. 1993) first demonstrated that peripheral tissue-specific tolerance is centrally established in the thymus, by epithelial stromal cells (TEC). Subsequent experiments have shown that TEC-tolerance is dominant and mediated by CD4 regulatory T cells (Treg) that are generated intrathymically by recognition of antigens expressed on TECs (Modigliani et al. 1995; Modigliani et al. 1996a). From these and other observations, in 1996 Modigliani and colleagues derived a general model for the establishment and maintenance of natural tolerance (MM96) (Modigliani et al. 1996b), with two central propositions: (1) T cell receptor (TCR)-dependent sorting of emergent repertoires generates TEC-specific Treg displaying the highest TCR self-affinities below deletion thresholds, thus isolating repertoires undergoing positive and negative selection; (2) Treg are intrathymically committed (and activated) for a unique differentiative pathway with regulatory effector functions. The model explained the embryonic/perinatal time window of natural tolerance acquisition, by developmental programs determining (1) TCR multireactivity, (2) the cellular composition in the thymic stroma (relative abundance of epithelial vs hemopoietic cells), and (3) the dynamics of peripheral lymphocyte pools, built by accumulation of recent thymic emigrants (RTE) that remain recruitable to regulatory functions. We discuss here the MM96 in the light of recent results demonstrating the promiscuous expression of tissue-specific antigens by medullary TECs (Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004) and indicating that Treg represent a unique differentiative pathway (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003), which is adopted by CD4 T cells with high avidity for TEC-antigens (Bensinger et al. 2001; Jordan et al. 2001; Apostolou et al. 2002). In the likelihood that autoimmune diseases (AID) result from Treg deficits, some of which might have a thymic origin, we also speculate on therapeutic strategies aiming at selectively stimulating their de novo production or peripheral function, within recent findings on Treg responses to inflammation (Caramalho et al. 2003; Lopes-Carvalho et al., submitted, Caramalho et al., submitted). In short, the MM96 argued that natural tolerance is dominant, established and maintained by the activity of Treg, which are selected upon high-affinity recognition of self-ligands on TECs, and committed intrathymically to a unique differentiative pathway geared to anti-inflammatory and antiproliferative effector functions. By postulating the intrathymic deletion of self-reactivities on hemopoietic stromal cells (THC), together with the inability of peripheral resident lymphocytes to engage in the regulatory pathway, the MM96 simultaneously explained the maintenance of responsiveness to non-self in a context of suppression mediating dominant self-tolerance. The major difficulty of the MM96 is related to the apparent tissue specificity of Treg repertoires generated intrathymically. This difficulty has now been principally solved by the work of Hanahan, Kyewski and others (Jolicoeur et al. 1994; Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004), demonstrating the selective expression of a variety of tissue-specific antigens by TECs, in topological patterns that are compatible with the MM96, but difficult to conciliate with recessive tolerance models (Kappler et al. 1987; Kisielow et al. 1988). While the developmentally regulated multireactivity of TCR repertoires (Gavin and Bevan 1995), as well as the peripheral recruitment of Treg among RTE (Modigliani et al. 1996a) might add to this process, it would seem that the establishment of tissue-specific tolerance essentially stems from the "promiscuous expression of tissue antigens" by TEC. The findings of AID resulting from natural mutations (reviewed in Pitkanen and Peterson 2003) or the targeted inactivation (Anderson et al. 2002; Ramsey et al. 2002) of the AIRE transcription factor that regulates promiscuous gene expression on TECs support this conclusion. The observations on the correlation of natural or forced expression of the Foxp3 transcription factor in CD4 T cells with Treg phenotype and function (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003) provided support for the MM96 contention that Treg represent a unique differentiative pathway that is naturally established inside the thymus. Furthermore, Caton and colleagues (Jordan et al. 2001), as well as several other groups (Bensinger et al. 2001; Apostolou et al. 2002), have provided direct evidence for our postulate that Treg are selected among differentiating CD4 T cells with high affinity for ligands expressed on TECs (Modigliani et al. 1996b). Finally, the demonstration by Caramalho et al. that Treg express innate immunity receptors (Caramalho et al. 2003) and respond to pro-inflammatory signals and products of inflammation (Caramalho et al., submitted) brought about a new understanding on the peripheral regulation of Treg function. Together with the observation that Treg also respond to ongoing activities of "naïve/effector" T cells--possibly through the IL-2 produced in these conditions--these findings explain the participation of Treg in all immune responses (Onizuka et al. 1999; Shimizu et al. 1999; Annacker et al. 2001; Curotto de Lafaille et al. 2001; Almeida et al. 2002; Shevach 2002; Bach and Francois Bach 2003; Wood and Sakaguchi 2003; Mittrucker and Kaufmann 2004; Sakaguchi 2004), beyond their fundamental role in ensuring self-tolerance (e.g., Modigliani et al. 1996a; Shevach 2000; Hori et al. 2003; Sakaguchi 2004; Thompson and Powrie 2004). Thus, anti-inflammatory and anti-proliferative Treg are amplified by signals that promote or mediate inflammation and proliferation, accounting for the quality control of responses (Coutinho et al. 2001). In turn, such natural regulation of Treg by immune responses to non-self may well explain the alarming epidemiology of allergic and AID in wealthy societies (Wills-Karp et al. 2001; Bach 2002; Yazdanbakhsh et al. 2002), where a variety of childhood infections have become rare or absent. Thus, it is plausible that Treg were evolutionarily set by a given density of infectious agents in the environment. With hindsight, it is not too surprising that natural Treg performance falls once hygiene, vaccination, and antibiotics suddenly (i.e., 100 years) plunged infectious density to below some critical physiological threshold. As the immune system is not adapted to modern clean conditions of postnatal development, clinical immunologists must now deal with frequent Treg deficiencies (allergies and AID) for which they have no curative or rational treatments. It is essential, therefore, that basic immunologists concentrate on strategies to selectively stimulate the production, survival, and activity of this set of lymphocytes that is instrumental in preventing immune pathology. We have argued that the culprit of this inability of basic research to solve major clinical problems has been the self-righteousness of recessive tolerance champions, from Ehrlich to some of our contemporaries. It is ironical, however, that none of us--including the heretic opponents of horror autotoxicus--had understood that self-tolerance, or its robustness at least, is in part determined by the frequency and intensity of the responses to non-self. In the evolution of ideas on immunological tolerance, the time might be ripe for some kinds of synthesis. First, conventional theory reduced self-tolerance to negative selection and microbial defense to positive selection, while the MM96 solution was the precise opposite: positive selection of autoreactivities for self-tolerance (Treg) and negative selection (of Treg) for ridding responses. In contrast, it would now appear that positive and negative selection of autoreactive T cells are both necessary to establish either self-tolerance or competence to eliminate microbes, two processes that actually reinforce each other in the maintenance of self-integrity. Second, V-region recognition has generally been held responsible for specific discrimination between what should be either tolerated or eliminated from the organism. In contrast again, it would now seem that both processes of self-tolerance and microbial defense (self/non-self discrimination) also operate on the basis of evolutionarily ancient, germ-line-encoded innate, nonspecific receptors (Medzhitov and Janeway 2000) capable of a coarse level of self/non-self discrimination (Coutinho 1975). It could thus be interesting to revisit notions of cooperativity between V-regions and such mitogen receptors, both in single cell functions (Coutinho et al. 1974) and in the system's evolution (Coutinho 1975, 1980) as well. After all, major transitions in evolution were cooperative (Maynard-Smith and Szathmary 1995).

Naturally arising Foxp3-expressing CD25+CD4+ regulatory T cells in immunological tolerance to self and non-self

Naturally arising CD25(+)CD4(+) regulatory T cells actively maintain immunological self-tolerance. Deficiency in or dysfunction of these cells can be a cause of autoimmune disease. A reduction in their number or function can also elicit tumor immunity, whereas their antigen-specific population expansion can establish transplantation tolerance. They are therefore a good target for designing ways to induce or abrogate immunological tolerance to self and non-self antigens.

Role for suppressor T cells in the pathogenesis of autoimmune diseases (including rheumatoid arthritis). Facts and hypotheses

Although uncontrolled clones of autoreactive T cells play a central role in the pathogenesis of autoimmunity, another mechanism potentially involved in many autoimmune diseases is deficiency of suppressor T cells, most notably those belonging to the antiidiopeptide TH3/Tr1 TCD4+CD25+(high) subset. Failure of suppressor mechanisms may be in part primary, due to defective positive selection of suppressor T cells in the thymus, and in part acquired, secondary to chronic infections promoted by deficiencies in innate immunity. Renewed interest in suppressor TCD4+ cells has generated plausible explanations for many events including paradoxical induction of autoimmune disorders by immunosuppressive agents or thymectomy. Insights into the physiology of these regulatory T-cell clones might suggest new treatment options, although many currently used drugs (including anti-TNF alpha agents) enhance the activity of several suppressor T-cell clones. Investigation of these suppressor clones in rheumatoid arthritis is still in its infancy and faces obstacles such as the need for identifying key clones in each individual patient and the presence of T-cell repertoire contraction. This last phenomenon exists at disease onset and may stem from early thymus dysfunction, which may also lead to a reduction in suppressor TCD4+ cell counts. Thus, although restoring deficient suppressor clones may provide a full recovery in animals, the high prevalence of T-cell repertoire contraction in humans with rheumatoid arthritis may severely limit the beneficial effects of this therapeutic approach.

Foxp3: a critical regulator of the development and function of regulatory T cells

Regulatory T cells are capable of suppressing deleterious immune responses against self- or non-self-antigens. Their essential role in tolerance and immune regulation has been illustrated by recent findings that mutations in the Foxp3 gene leads to the defective development of regulatory T cells and the emergence of a fatal autoimmune, inflammatory and allergic disease. This review discusses the critical role for this transcription factor in the development and function of regulatory T cells.

Innate immunity: sensing the environment and regulating the regulators

PURPOSE OF REVIEW

Innate immune cells act as sensors for environmental pathogens and key regulators of pathogen-specific T-cell effector responses. The expression of Toll-like receptors is essential for these functions. Innate immunity is a rapidly evolving field. Great progress has been made in the past year in characterizing the signaling pathways downstream of Toll-like receptors, and the role of Toll-like receptors in the regulation of pathogen responses.

RECENT FINDINGS

The ability to control the activation of regulatory T cells has emerged as a key function of innate immunity. Regulatory T cells are involved in the induction of tolerance and the prevention of harmful immune pathology, particularly T-cell responses directed against self-antigens. Toll-like receptor-mediated interactions between pathogen-stimulated innate immune cells and regulatory T cells result in the release of suppression by regulatory T cells, thus allowing pathogen-specific responses. However, pathogenic microorganisms may exploit this interaction to evade the host's response. In addition, recent studies raise the possibility that regulatory T cells may express some Toll-like receptor family members, thereby responding directly to pathogens.

SUMMARY

A deeper understanding of the complex role of innate immune cells as sensors of the environment and regulators of pathogen responses will probably influence the current models of immune regulation, particularly those centered on the role of the environment in shaping immune responses.

NF-kappa B-inducing kinase establishes self-tolerance in a thymic stroma-dependent manner

Physical contact between thymocytes and the thymic stroma is essential for T cell maturation and shapes the T cell repertoire in the periphery. Stromal elements that control these processes still remain elusive. We used a mouse strain with mutant NF-kappaB-inducing kinase (NIK) to examine the mechanisms underlying the breakdown of self-tolerance. This NIK-mutant strain manifests autoimmunity and disorganized thymic structure with abnormal expression of Rel proteins in the stroma. Production of immunoregulatory T cells that control autoreactive T cells was impaired in NIK-mutant mice. The autoimmune disease seen in NIK-mutant mice was reproduced in athymic nude mice by grafting embryonic thymus from NIK-mutant mice, and this was rescued by supply of exogenous immunoregulatory T cells. Impaired production of immunoregulatory T cells by thymic stroma without normal NIK was associated with altered expression of peripheral tissue-restricted Ags, suggesting an essential role of NIK in the thymic microenvironment in the establishment of central tolerance.

Control of autoimmunity by naturally arising regulatory CD4+ T cells

Naturally acquired immunological self-tolerance is not entirely accounted for by clonal deletion, anergy, and ignorance. It is now well established that the T cell-repertoire of healthy individuals harbors self-reactive lymphocytes with a potential to cause autoimmune disease and these lymphocytes are under dominant control by a unique subpopulation of CD4+ T cells now called regulatory T cells. Efforts to delineate these Treg cells naturally present in normal individuals have revealed that they are enriched in the CD25+ CD4+ population. The identification of the CD25 molecule as a useful marker for naturally arising CD4+ regulatory T cells has made it possible to investigate many key aspects of their immunobiology, including their antigen specificities and the cellular/molecular pathways involved in their development and their mechanisms of action. Furthermore, reduction or dysfunction of the CD25+ CD4+ regulatory T cell population can be responsible for certain autoimmune diseases in humans.

Whither T-suppressors: if they didn’t exist would we have to invent them?

Arriving at an understanding of the role of suppressor T-cells (regulatory T-cells, CD4(+)CD25+) depends on whether their functional repertoire is somatically selected to be anti-Self or anti-Nonself. Immunologists are ambivalent; often publications espousing opposite views share an author. Here the arguments are detailed that the suppressor repertoire is not somatically selected to be anti-Self, but rather it is anti-Nonself. Therefore, suppression cannot regulate the Self-Nonself discrimination; its function is to regulate the magnitude and class of the anti-Nonself effector response.

Toward rigorous comprehension of biological complexity: modeling, execution, and visualization of thymic T-cell maturation

One of the problems biologists face is a data set too large to comprehend in full. Experimenters generate data at an ever-growing pace, each from their own niche of interest. Current theories are each able, at best, to capture and model only a small part of the data. We aim to develop a general approach to modeling that will help broaden biological understanding. T-cell maturation in the thymus is a telling example of the accumulation of experimental data into a large disconnected data set. The thymus is responsible for the maturation of stem cells into mature T cells, and its complexity divides research into different fields, for example, cell migration, cell differentiation, histology, electron microscopy, biochemistry, molecular biology, and more. Each field forms its own viewpoint and its own set of data. In this study we present the results of a comprehensive integration of large parts of this data set. The integration is performed in a two-tiered visual manner. First, we use the visual language of Statecharts, which makes specification precise, legible, and executable on computers. We then set up a moving graphical interface that dynamically animates the cells, their receptors, the different gradients, and the interactions that constitute thymic maturation. This interface also provides a means for interacting with the simulation.

Tolerance and immunity in a mathematical model of T-cell mediated suppression

Regulatory CD4+CD25+ T cells play a major role in natural tolerance to body components and therefore are relevant to understand the self-non-self discrimination by the immune system. The most pressing theoretical question, regarding the fact that these regulatory cells perform their function through linked recognition of the APCs, is how this "non-specific" mechanism permits a proper balance between tolerance and immunity that is compatible with an effective self-non-self discrimination. To tackle this issue, we develop a numerical simulation, which extends a previous mathematical model of T-cell-mediated suppression to include the thymic generation and the peripheral dynamics of many T cell clones. This simulation can mimic the capacity of the immune system to establish natural tolerance to self-antigens and reliably mount immune responses to foreign antigens. Natural tolerance is based on ubiquitous and constitutive self-antigens, which select and sustain clones of specific regulatory cells. Immune responses to foreign antigens are only achieved if they displace the self-antigens from the APCs, leading to a loss of the regulatory cells, and/or if the foreign antigen introduction entails a sharp increase in the total number of APCs. Meaningful behavior is obtained even if differentiation of regulatory cells in the thymus is antigen non-specific, but requires that a minimum number of new T cells enter the periphery per unit of time, and that the repertoire is selected so that anti-self-affinities are within a proper interval. We conclude that positive selection is required to generate a sufficiently high frequency of self-antigen specific regulatory cells that reliably mediate natural tolerance. Negative selection is required to avoid the emergence at the periphery of very high affinity anti-self-regulatory cells that will make the tolerant state so robust that it could no be broken by the introduction of a foreign antigen. This result highlights the importance of repertoire selection in dominant tolerance proposing a novel role for the processes of positive and negative selection within this framework.

Toll receptors modulate allergic responses: interaction with dendritic cells, T cells and mast cells

PURPOSE OF REVIEW

The discovery of Toll-like receptors has generated much interest in understanding the impact of innate immunity on adaptive immune responses, including allergic diseases.

RECENT FINDINGS

Recent studies suggest that Toll-like receptor pathways may mediate interactions between dendritic cells, T lymphocytes and mast cells, thus modulating allergic immune responses. Toll-like receptor signaling triggers dendritic cell maturation, which primes naive T lymphocytes towards specific T helper cell types 1 and 2 immune responses. Although a T helper cell type 1/2 balance may be important in modulating allergic responses, T regulatory cells that suppress certain immune responses may be critical in immune regulation.

SUMMARY

With the identification of different subsets of dendritic cells and the discovery of Toll-like receptors on T regulatory cells and mast cells, the manipulation of Toll-like receptor signaling may lead to novel therapeutic options in allergic diseases.

Regulatory T cells selectively express toll-like receptors and are activated by lipopolysaccharide

Regulatory CD4 T cells (Treg) control inflammatory reactions to commensal bacteria and opportunist pathogens. Activation of Treg functions during these processes might be mediated by host-derived proinflammatory molecules or directly by bacterial products. We tested the hypothesis that engagement of germline-encoded receptors expressed by Treg participate in the triggering of their function. We report that the subset of CD4 cells known to exert regulatory functions in vivo (CD45RB(low) CD25(+)) selectively express Toll-like receptors (TLR)-4, -5, -7, and -8. Exposure of CD4(+) CD25(+) cells to the TLR-4 ligand lipopolysaccharide (LPS) induces up-regulation of several activation markers and enhances their survival/proliferation. This proliferative response does not require antigen-presenting cells and is augmented by T cell receptor triggering and interleukin 2 stimulation. Most importantly, LPS treatment increases CD4(+) CD25(+) cell suppressor efficiency by 10-fold and reveals suppressive activity in the CD4(+) CD45RB(low) CD25(-) subset that when tested ex-vivo, scores negative. Moreover, LPS-activated Treg efficiently control naive CD4 T cell-dependent wasting disease. These findings provide the first evidence that Treg respond directly to proinflammatory bacterial products, a mechanism that likely contributes to the control of inflammatory responses.

Control of regulatory T cell development by the transcription factor Foxp3

Regulatory T cells engage in the maintenance of immunological self-tolerance by actively suppressing self-reactive lymphocytes. Little is known, however, about the molecular mechanism of their development. Here we show that Foxp3, which encodes a transcription factor that is genetically defective in an autoimmune and inflammatory syndrome in humans and mice, is specifically expressed in naturally arising CD4+ regulatory T cells. Furthermore, retroviral gene transfer of Foxp3 converts naïve T cells toward a regulatory T cell phenotype similar to that of naturally occurring CD4+ regulatory T cells. Thus, Foxp3 is a key regulatory gene for the development of regulatory T cells.

The conservative physiology of the immune system

Current immunological opinion disdains the necessity to define global interconnections between lymphocytes and regards natural autoantibodies and autoreactive T cells as intrinsically pathogenic. Immunological theories address the recognition of foreignness by independent clones of lymphocytes, not the relations among lymphocytes or between lymphocytes and the organism. However, although extremely variable in cellular/molecular composition, the immune system preserves as invariant a set of essential relations among its components and constantly enacts contacts with the organism of which it is a component. These invariant relations are reflected, for example, in the life-long stability of profiles of reactivity of immunoglobulins formed by normal organisms (natural antibodies). Oral contacts with dietary proteins and the intestinal microbiota also result in steady states that lack the progressive quality of secondary-type reactivity. Autoreactivity (natural autoantibody and autoreactive T cell formation) is also stable and lacks the progressive quality of clonal expansion. Specific immune responses, currently regarded as the fundament of the operation of the immune system, may actually result from transient interruptions in this stable connectivity among lymphocytes. More permanent deficits in interconnectivity result in oligoclonal expansions of T lymphocytes, as seen in Omenn's syndrome and in the experimental transplantation of a suboptimal diversity of syngeneic T cells to immunodeficient hosts, which also have pathogenic consequences. Contrary to theories that forbid autoreactivity as potentially pathogenic, the physiology of the immune system is conservative and autoreactive. Pathology derives from failures of these conservative mechanisms.

The role of suppressor T cells in regulation of immune responses

Suppressor T cells play important roles in the regulation of immune responses and the mediation of dominant immunologic tolerance. Studies of suppressor T-cell function have been hampered until their recent identification as a minor fraction (approximately 10%) of CD4 ( +) T cells that coexpress CD25. CD4(+)CD25(+ ) T cells have been shown to play a critical role in the prevention of organ- specific autoimmunity and allograft rejection. Because tumor antigens are self- antigens, it is not surprising that CD4(+)CD25(+) T cells also inhibit the induction of tumor immunity. The spectrum of activity of CD4(+ ) CD25(+) cells extends to non-self-antigens, including infectious agents. Indeed, T cell-mediated suppression might be responsible for the low level of chronic infection seen with many pathogens. Interestingly, however, this persistent level of infection might be beneficial to the host and needed for maintenance of immunologic memory. Although CD4(+ ) CD25(+) T cells are capable of inhibiting T(H)2 responses, their role in the suppression of allergic responses has not been firmly established. Depending on the desired immune response, enhancement or restraint of suppressor T-cell function might be required. Therefore immunologic or pharmacologic manipulation of regulatory T-cell populations represents an important future approach to immunotherapy of a wide range of immune responses.

Stem cell medicine encounters the immune system

Recent progress in deriving human embryonic stem (hES) cells and defining their capacity to differentiate has inspired hope that they could become a source of replacement cells for damaged or diseased tissues. We review the immunological barriers to transplanting hES cells and consider several potential solutions, including stem-cell banking, modification of the immunogenicity of donor cells and induction of tolerance to the graft. We evaluate the probable efficacy of these approaches with a view to facilitating the use of hES cells in clinical practice.

Induction of central deletional T cell tolerance by gene therapy

Transgenic mice expressing an alloreactive TCR specific for the MHC class I Ag K(b) were used to examine the mechanism by which genetic engineering of bone marrow induces T cell tolerance. Reconstitution of lethally irradiated mice with bone marrow infected with retroviruses carrying the MHC class I gene H-2K(b) resulted in lifelong expression of K(b) on bone marrow-derived cells. While CD8 T cells expressing the transgenic TCR developed in control mice reconstituted with mock-transduced bone marrow, CD8 T cells expressing the transgenic TCR failed to develop in mice reconstituted with H-2K(b) transduced bone marrow. Analysis of transgene-expressing CD8 T cells in the thymus and periphery of reconstituted mice revealed that CD8 T cells expressing the transgenic TCR underwent negative selection in the thymus of mice reconstituted with K(b) transduced bone marrow. Negative selection induced by gene therapy resulted in tolerance to K(b). Thus, genetic engineering of bone marrow can be used to alter T cell education in the thymus by inducing negative selection.

Specificity requirements for selection and effector functions of CD25+4+ regulatory T cells in anti-myelin basic protein T cell receptor transgenic mice

CD25(+)4(+) regulatory T cells (T(reg)) play an indispensable role in preventing autoimmunity. Little is known, however, about the antigen specificities required for their development and effector functions. Mice transgenic for an anti-myelin basic protein (MBP) T cell antigen receptor (TCR) spontaneously develop experimental autoimmune encephalomyelitis (EAE) when deficient for the RAG-1 gene (T/R(-)), whereas RAG-1-competent transgenic animals (T/R(+)) remain healthy, protected by CD4(+) T(reg)-expressing endogenous TCRs. We have now investigated the role and specificity of CD25(+)4(+) T(reg) in this system. The results show that T/R(+) animals contain MBP-specific suppressive CD25(+)4(+) cells, whereas T/R(-) do not. Adoptive transfer of CD25(+)4(+) cells from nontransgenic or T/R(+) donors into T/R(-) mice prevented the development of EAE. Surprisingly, transfer of nontransgenic CD25(+)4(+) cells purified from T/R(+) donors conferred only a limited protection, possibly because of their restricted repertoire diversity that we demonstrate here. Absence of transgenic CD25(+)4(+) cells in animals deficient for endogenous TCRalpha chains and analyses of endogenous TCR gene expression in subsets of CD4(+) cells from T/R(+) mice demonstrate that development of transgenic MBP-specific CD25(+)4(+) T(reg) depends on the coexpression of endogenous TCRalpha chains. Taken together, these results indicate that specificity to MBP is required for effector functions but is not sufficient for thymic selection/commitment of CD25(+)4(+) T(reg) preventing EAE.

CD25+CD4+ regulatory T cells suppress CD4+ T cell-mediated pulmonary hyperinflammation driven by Pneumocystis carinii in immunodeficient mice

The CD4(+) T cell-mediated inflammatory response to Pneumocystis carinii (PC) critically contributes to the clinical severity of PC pneumonia. It has been suggested that lymphopenic conditions predispose individuals to this immunopathology, although the mechanisms remain poorly understood. Another set of evidence indicates that a subpopulation of CD4(+) T cells constitutively expressing the CD25 molecule prevent lymphopenia-induced autoimmunity and inflammatory bowel disease. We tested the ability of this CD25(+)CD4(+) population to regulate CD4(+) T cell-mediated inflammatory response to PC. Adoptive transfer of CD25(-)CD4(+) cells into PC-infected recombination-activating gene-2-deficient mice led to lethal pneumonia within 13 days post-transfer. PC infection appeared to trigger CD25(-)CD4(+) cells, since recipients with reduced PC load survived up to 5 weeks after transfer. In contrast, transfer of CD25(+)CD4(+) cells did not induce lethal pneumonia and prevented the development of the disease induced by CD25(-)CD4(+) cells. Furthermore, CD25(-)CD4(+) cells reduced the PC load in the lung, while CD25(+)CD4(+) cells suppressed this immune response. Our results indicate an essential role for CD25(+)CD4(+) T cells in the control of PC-driven immunopathology, and suggest that in immunocompromised hosts PC pneumonia may result from a deficiency in regulatory T cells.

Autoreactivity, dynamic tuning and selectivity

The immune system adjusts its response to the context in which antigens, including self-antigens, are recognized. Recent observations support a conceptual framework for understanding how this may be achieved at the cellular and cell-population levels. At both levels, 'perturbations' elicit competition between excitation and de-excitation, resulting either in adaptation or in various responses. The responsiveness of individual cells is dynamically tuned, reflecting their recent experience. The tuning of T-cell activation thresholds by self-ligands facilitates positive selection and continuously regulates the level of autoreactivity in the periphery. Autoreactivity appears to be involved in regulation of the immune response, homeostasis, maintaining of the functional integrity of naïve and memory cells, and in other physiological functions.