A disquisition on suppressor T cells

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

Dysfunction of peripheral regulatory T cells predicts lung injury after cardiopulmonary bypass

Lung injury caused by cardiopulmonary bypass (CPB) increases the mortality after cardiac surgery. Previous studies have shown that regulatory T cells (Tregs) play a protective role during CPB, but the correlation between Tregs and CPB-induced lung injury remains unclear. Here, we conducted a prospective study about Treg cells in patient receiving CPB. Treg cells were collected from patients before the CPB operation (pre-CPB Tregs), and the effect of pre-CPB Tregs on the occurrence of CPB-induced lung injury was evaluated. Data showed that the baseline level of Treg cells in peripheral blood were lower in patients who developed lung injury after CPB, compared to those who did not develop lung injury after CPB. Function analyses revealed that pre-CPB Tregs from CPB-induced lung injury patients presented decreased ability in suppressing the proliferation and IFN-γ production of CD4 and CD8 T cell. Also, pre-surgery levels of TGF- β and IL-10 were markedly lower in lung injury patients than in non-lung injury patients. In addition, PD-1 and Tim-3 expression on pre-CPB Tregs were significantly lower in CPB-induced lung injury patients than the CPB patients without lung injury. Above all, we found impaired peripheral Treg responses in CPB-induced lung injury patients, indicating a potential role of Treg cells in the early diagnosis of CPB-induced lung injury.

Progranulin Improves Acute Lung Injury through Regulating the Differentiation of Regulatory T Cells and Interleukin-10 Immunomodulation to Promote Macrophage Polarization

Progranulin (PGRN), which plays an anti-inflammatory role in acute lung injury (ALI), is promising as a potential drug. Studies have shown that regulatory T cells (Tregs) and interleukin- (IL-) 10 can repress inflammation and alleviate tissue damage during ALI. In this study, we built a lipopolysaccharide- (LPS-) induced ALI mouse model to illustrate the effect of PGRN on regulation of Treg differentiation and modulation of IL-10 promoting macrophage polarization. We found that the proportion of Tregs in splenic mononuclear cells and peripheral blood mononuclear cells was higher after treatment with PGRN. The increased proportion of Tregs after PGRN intratracheal instillation was consistent with the decreased severity of lung injury, the reduction of proinflammatory cytokines, and the increase of anti-inflammatory cytokines. In vitro, the percentages of CD4⁺CD25⁺FOXP3⁺ Tregs from splenic naïve CD4⁺ T cells increased after PGRN treatment. In further research, it was found that PGRN can regulate the anti-inflammatory factor IL-10 and affect the polarization of M1/M2 macrophages by upregulating IL-10. These findings show that PGRN likely plays a protective role in ALI by promoting Treg differentiation and activating IL-10 immunomodulation.

Ubiquitin-specific peptidase 18 regulates the differentiation and function of Treg cells

Ubiquitin-specific peptidase 18 (USP18) plays an important role in the development of CD11b⁺ dendritic cells (DCs) and Th17 cells, however, its role in the differentiation of other T cell subsets, especially in regulatory T (Treg) cells, is unknown. In our study, we used Usp18 KO mice to study the loss of USP18 on the impact of Treg cell differentiation and function. We found that USP18 deficiency upregulates the differentiation of Treg cells, which may lead to disrupted homeostasis of peripheral T cells, and downregulates INF-γ, IL-2, IL-17A producing CD4⁺ T cells and INF-γ producing CD8⁺ T cells. Mechanistically, we also found that the upregulation of Tregs is due to elevated expression of CD25 in Usp18 KO mice. Finally, we found that the suppressive function of Usp18 KO Tregs is downregulated. Altogether, our study was the first to identify the role of USP18 in Tregs differentiation and its suppressive function, which may provide a new reference for the treatment of Treg function in many autoimmune diseases, and USP18 can be used as a new therapeutic target for precise medical treatment.

Shikonin Controls the Differentiation of CD4+CD25+ Regulatory T Cells by Inhibiting AKT/mTOR Pathway

CD4⁺CD25⁺ regulatory T (Treg) cells maintain the function of immune tolerance and the balance of immune cells. Defects in the number and function of Treg cells can induce the development and progression of inflammatory disease. Shikonin, the main active ingredient of Lithospermum, has anti-inflammatory and anti-tumor effects. Shikonin is also an effective drug for the treatment of psoriasis, which is a chronic inflammatory skin disease. However, the underlying mechanism is not yet clear. To evaluate the role of shikonin on the induction of Treg cells, we tested the number and function of Treg cells in vivo and in vitro. Shikonin can effectively promote the differentiation of iTreg cells by inhibiting the AKT/mTOR pathway in vitro. Moreover, in vivo, intragastrically administered shikonin effectively improved lesions in mice with imiquimod-induced psoriasis and increased the number of iTreg cells in the spleen and their secretion. Shikonin significantly increases the expression of Foxp3mRNA in skin of the psorisic mice. Therefore, we expect that shikonin can prevent the development of inflammation and treat psoriasis by regulating iTreg cells. Novel ideas for the treatment of psoriasis are also proposed.

The Therapeutic Strategies of Regulatory T Cells in Malignancies and Stem Cell Transplantations

Regulatory T cells (Treg cells) are considered one of the main dynamic cell types within the immune system. Because Treg cells suppress immune responses, they have potential roles in immunological self-tolerance and may help to maintain immune homeostasis. Promoting Treg cell function and increasing their numbers might be useful in treating autoimmune disorders, as well as preventing allograft rejection. However, studies of mice and humans demonstrate that Treg cells promote cancer progression and suppress antitumor immunity. Therefore, suppressing Treg cell function or reducing their numbers could support the immune system's response to pathogenic microorganisms and tumors. As a result, there is great interest in investigating the Treg cells role in the treatment of hematological and nonhematological malignancies. Consequently, Treg cells could be a fundamentally important target for pathologies of the immune system. Targeting effector Treg cells could help to distinguish and selectively decrease these cells while preserving other Treg cells needed to suppress autoimmunity. Currently, a promising way to treat malignancies and other autoimmune disorders is stem cell transplantation. Stem cell transplants (SCT) can help to manage the production of Treg cells and also may produce more efficient Treg cells, thereby suppressing clinical disease progression. Specifically, mature T cells within the engrafted stem cells mediate this SCT beneficial effect. During SCT, the recipient's immune system is replaced with a donor, which allows for improved immune system function. In addition, SCT can protect from disease relapse, as graft-versus-host disease (GvHD) in transplant patients can be protective against cancer recurrence. The current review will define the role of regulatory T cells in treatment of malignancy. Additionally, it will summarize current promising research regarding the utility of regulatory T cells in stem cell transplantation.

Regulatory T Cells and Acute Lung Injury: Cytokines, Uncontrolled Inflammation, and Therapeutic Implications

Acute respiratory distress syndrome/acute lung injury (ALI) was described in 1967. The uncontrolled inflammation is a central issue of the syndrome. The regulatory T cells (Tregs), formerly known as suppressor T cells, are a subpopulation of T cells. Tregs indirectly limits immune inflammation-inflicted tissue damage by employing multiple mechanisms and creating the appropriate immune environment for successful tissue repair. And it plays a central role in the resolution of ALI. Accordingly, for this review, we will focus on Treg populations which are critical for inflammatory immunity of ALI, and the effect of interaction between Treg subsets and cytokines on ALI. And then explore the possibility of cytokines as beneficial factors in inflammation resolution of ALI.

Cell Therapy in Kidney Transplantation: Focus on Regulatory T Cells

Renal transplantation is the renal replacement modality of choice for suitable candidates with advanced CKD or ESRD. Prevention of rejection, however, requires treatment with nonspecific pharmacologic immunosuppressants that carry both systemic and nephrologic toxicities. Use of a patient's own suppressive regulatory T cells (Tregs) is an attractive biologic approach to reduce this burden. Here, we review the immunologic underpinnings of Treg therapy and technical challenges to developing successful cell therapy. These issues include the selection of appropriate Treg subsets, ex vivo Treg expansion approaches, how many Tregs to administer and when, and how to care for patients after Treg administration.

Selective Sparing of Human Tregs by Pharmacologic Inhibitors of the Phosphatidylinositol 3-Kinase and MEK Pathways

Phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase/extracellular signal-regulated (MEK) signaling are central to the survival and proliferation of many cell types. Multiple lines of investigation in murine models have shown that control of the PI3K pathway is particularly important for regulatory T cell (Treg) stability and function. PI3K and MEK inhibitors are being introduced into the clinic, and we hypothesized that pharmacologic inhibition of PI3K, and possibly MEK, in mixed cultures of human mononuclear cells would preferentially affect CD4(+) and CD8(+) lymphocytes compared with Tregs. We tested this hypothesis using four readouts: proliferation, activation, functional suppression, and signaling. Results showed that Tregs were less susceptible to inhibition by both δ and α isoform-specific PI3K inhibitors and by an MEK inhibitor compared with their conventional CD4(+) and CD8(+) counterparts. These studies suggest less functional reliance on PI3K and MEK signaling in Tregs compared with conventional CD4(+) and CD8(+) lymphocytes. Therefore, the PI3K and MEK pathways are attractive pharmacologic targets for transplantation and treatment of autoimmunity.

Immunity by equilibrium

The classical model of immunity posits that the immune system reacts to pathogens and injury and restores homeostasis. Indeed, a century of research has uncovered the means and mechanisms by which the immune system recognizes danger and regulates its own activity. However, this classical model does not fully explain complex phenomena, such as tolerance, allergy, the increased prevalence of inflammatory pathologies in industrialized nations and immunity to multiple infections. In this Essay, I propose a model of immunity that is based on equilibrium, in which the healthy immune system is always active and in a state of dynamic equilibrium between antagonistic types of response. This equilibrium is regulated both by the internal milieu and by the microbial environment. As a result, alteration of the internal milieu or microbial environment leads to immune disequilibrium, which determines tolerance, protective immunity and inflammatory pathology.

IL-12 and IL-4 activate a CD39-dependent intrinsic peripheral tolerance mechanism in CD8(+) T cells

Immune responses to protein antigens involve CD4(+) and CD8(+) T cells, which follow distinct programs of differentiation. Naïve CD8 T cells rapidly develop cytotoxic T-cell (CTL) activity after T-cell receptor stimulation, and we have previously shown that this is accompanied by suppressive activity in the presence of specific cytokines, i.e. IL-12 and IL-4. Cytokine-induced CD8(+) regulatory T (Treg) cells are one of several Treg-cell phenotypes and are Foxp3(-) IL-10(+) with contact-dependent suppressive capacity. Here, we show they also express high level CD39, an ecto-nucleotidase that degrades extracellular ATP, and this contributes to their suppressive activity. CD39 expression was found to be upregulated on CD8(+) T cells during peripheral tolerance induction in vivo, accompanied by release of IL-12 and IL-10. CD39 was also upregulated during respiratory tolerance induction to inhaled allergen and on tumor-infiltrating CD8(+) T cells. Production of IL-10 and expression of CD39 by CD8(+) T cells was independently regulated, being respectively blocked by extracellular ATP and enhanced by an A2A adenosine receptor agonist. Our results suggest that any CTL can develop suppressive activity when exposed to specific cytokines in the absence of alarmins. Thus negative feedback controls CTL expansion under regulation from both nucleotide and cytokine environment within tissues.

Dynamic ocular surface and lacrimal gland changes induced in experimental murine dry eye

Dry eye disease can be a consequence of lacrimal gland insufficiency in Sjögren’s Syndrome or increased tear film evaporation despite normal lacrimal gland function. To determine if there is a correlation between severity effects in these models and underlying pathophysiological responses, we compared the time dependent changes in each of these parameters that occur during a 6 week period. Dry eye was induced in 6-week-old female C57BL/6 mice by exposing them to an Intelligently Controlled Environmental System (ICES). Sixty mice were housed in ICES for 1, 2, 4 and 6 weeks respectively. Twelve were raised in normal environment and received subcutaneous injections of scopolamine hydrobromide (SCOP) 3 times daily for 5 days. Another sixty mice were housed in a normal environment and received no treatment. Corneal fluorescein staining along with corneal MMP-9 and caspase-3 level measurements were performed in parallel with the TUNEL assay. Interleukin-17(IL-17), IL-23, IL-6, IL-1, TNF-α, IFN-γ and TGF-β2 levels were estimated by real-time PCR measurements of conjunctival and lacrimal gland samples (LGs). Immunohistochemistry of excised LGs along with flow cytometry in cervical lymph nodes evaluated immune cell infiltration. Light and transmission electron microscopy studies evaluated LGs cytoarchitectural changes. ICES induced corneal epithelial destruction and apoptosis peaked at 2 weeks and kept stable in the following 4 weeks. In the ICES group, lacrimal gland proinflammatory cytokine level increases were much lower than those in the SCOP group. In accord with the lower proinflammatory cytokine levels, in the ICES group, lacrimal gland cytosolic vesicular density and size exceeded that in the SCOP group. ICES and SCOP induced murine dry eye effects became progressively more severe over a two week period. Subsequently, the disease process stabilized for the next four weeks. ICES induced local effects in the ocular surface, but failed to elicit lacrimal gland inflammation and cytoarchitectural changes, which accounts for less dry eye severity in the ICES model than that in the SCOP model.

Experimental myocardial infarction induces altered regulatory T cell hemostasis, and adoptive transfer attenuates subsequent remodeling

Background

Ischemic cardiac damage is associated with upregulation of cardiac pro-inflammatory cytokines, as well as invasion of lymphocytes into the heart. Regulatory T cells (Tregs) are known to exert a suppressive effect on several immune cell types. We sought to determine whether the Treg pool is influenced by myocardial damage and whether Tregs transfer and deletion affect cardiac remodeling.

Methods and Results

The number and functional suppressive activity of Tregs were assayed in mice subjected to experimental myocardial infarction. The numbers of splenocyte-derived Tregs in the ischemic mice were significantly higher after the injury than in the controls, and their suppressive properties were significantly compromised. Compared with PBS, adoptive Treg transfer to mice with experimental infarction reduced infarct size and improved LV remodeling and functional performance by echocardiography. Treg deletion with blocking anti-CD25 antibodies did not influence infarct size or echocardiographic features of cardiac remodeling.

Conclusion

Treg numbers are increased whereas their function is compromised in mice with that underwent experimental infarction. Transfer of exogeneous Tregs results in attenuation of myocardial remodeling whereas their ablation has no effect. Thus, Tregs may serve as interesting potential interventional targets for attenuating left ventricular remodeling.

Regulatory T cells as immunotherapy

Regulatory T cells (Tregs) suppress exuberant immune system activation and promote immunologic tolerance. Because Tregs modulate both innate and adaptive immunity, the biomedical community has developed an intense interest in using Tregs for immunotherapy. Conditions that require clinical tolerance to improve outcomes – autoimmune disease, solid organ transplantation, and hematopoietic stem cell transplantation – may benefit from Treg immunotherapy. Investigators have designed ex vivo strategies to isolate, preserve, expand, and infuse Tregs. Protocols to manipulate Treg populations in vivo have also been considered. Barriers to clinically feasible Treg immunotherapy include Treg stability, off-cell effects, and demonstration of cell preparation purity and potency. Clinical trials involving Treg adoptive transfer to treat graft versus host disease preliminarily demonstrated the safety and efficacy of Treg immunotherapy in humans. Future work will need to confirm the safety of Treg immunotherapy and establish the efficacy of specific Treg subsets for the treatment of immune-mediated disease.

Regulation generation: the suppressive functions of human regulatory T cells

Proper regulation of immune homeostasis is necessary to limit inflammation and prevent autoimmune and chronic inflammatory diseases. Many autoimmune diseases, such as psoriasis, are driven by vicious cycles of activated T cells that are unable to be suppressed by regulatory T cells. Effective suppression of auto-reactive T cells by regulatory T cells (Treg) is critical for the prevention of spontaneous autoimmune disease. Psoriatic Treg cells have been observed to a defect in their capacity to regulate, which clearly contributes to psoriasis pathogenesis. A challenge for translational research is the development of novel therapeutic interventions for autoimmune diseases that will result in durable remissions. Understanding the mechanism(s) of dysregulated T cell responses in autoimmune disease will allow for the development of future therapeutic strategies that may be employed to specifically target pathogenic, proinflammatory cells. Several reports have demonstrated a pathogenic role for Thl and Thl7 cells in psoriasis as well as other autoimmune diseases. Similarly, several laboratories have independently demonstrated functional defects in regulatory T cells isolated from patients with numerous divergent autoimmune diseases. One primary challenge of research in autoimmune diseases is therefore to restore the balance between chronic T cell activation and impairment of Treg suppressor mechanisms. To this end, it is critical to develop an understanding of the many suppressive mechanisms employed by Treg cells in hopes of developing more targeted therapeutic strategies for Treg-mediated autoimmune diseases.

Oral tolerance

The gut-associated lymphoid tissue is the largest immune organ in the body and is the primary route by which we are exposed to antigens. Tolerance induction is the default immune pathway in the gut, and the type of tolerance induced relates to the dose of antigen fed: anergy/deletion (high dose) or regulatory T-cell (Treg) induction (low dose). Conditioning of gut dendritic cells (DCs) by gut epithelial cells and the gut flora, which itself has a major influence on gut immunity, induces CD103(+) retinoic acid-dependent DC that induces Tregs. A number of Tregs are induced at mucosal surfaces. Th3 type Tregs are transforming growth factor-β dependent and express latency-associated peptide (LAP) on their surface and were discovered in the context of oral tolerance. Tr1 type Tregs (interleukin-10 dependent) are induced by nasal antigen and forkhead box protein 3(+) iTregs are induced by oral antigen and by oral administration of aryl hydrocarbon receptor ligands. Oral or nasal antigen ameliorates autoimmune and inflammatory diseases in animal models by inducing Tregs. Furthermore, anti-CD3 monoclonal antibody is active at mucosal surfaces and oral or nasal anti-CD3 monoclonal antibody induces LAP(+) Tregs that suppresses animal models (experimental autoimmune encephalitis, type 1 and type 2 diabetes, lupus, arthritis, atherosclerosis) and is being tested in humans. Although there is a large literature on treatment of animal models by mucosal tolerance and some positive results in humans, this approach has yet to be translated to the clinic. The successful translation will require defining responsive patient populations, validating biomarkers to measure immunologic effects, and using combination therapy and immune adjuvants to enhance Treg induction. A major avenue being investigated for the treatment of autoimmunity is the induction of Tregs and mucosal tolerance represents a non-toxic, physiologic approach to reach this goal.

Transplantation tolerance: Clinical potential of regulatory T cells

The major challenge in transplantation medicine remains long-term allograft acceptance, with preserved allograft function under minimal chronic immunosuppression. To safely achieve the goal of sustained donor-specific T and B cell non-responsiveness, research efforts are now focusing on therapies based on cell subsets with regulatory properties. In particular the transfusion of human regulatory T cells (Treg) is currently being evaluated in phase I/II clinical trials for the treatment of graft versus host disease following hematopoietic stem cell transplantation, and is also under consideration for solid organ transplantation. The purpose of this review is to recapitulate current knowledge on naturally occurring as well as induced human Treg, with emphasis on their specific phenotype, suppressive function and how these cells can be manipulated in vitro and/or in vivo for therapeutic purposes in transplantation medicine. We highlight the potential but also possible limitations of Treg-based strategies to promote long-term allograft survival. It is evident that the bench-to-beside translation of these protocols still requires further understanding of Treg biology. Nevertheless, current data already suggest that Treg therapy alone will not be sufficient and needs to be combined with other immunomodulatory approaches in order to induce allograft tolerance.

Regulatory T cells: immune suppression and beyond

Foxp3-expressing regulatory T cells (Tregs) were originally identified as critical in maintaining self-tolerance and immune homeostasis. The immunosuppressive functions of Tregs are widely acknowledged and have been extensively studied. Recent studies have revealed many diverse roles of Tregs in shaping the immune system and the inflammatory response. This review will discuss our efforts as well as the efforts of others towards understanding the multifaceted function of Tregs in immune regulation.

Immune responses to malignancies

Immune responses to tumor-associated antigens (TAs) are often detectable in tumor-bearing hosts, but they fail to eliminate malignant cells or prevent the development of metastases. Patients with cancer generate robust immune responses to infectious agents (bacteria and viruses) perceived as a "danger signal" but only ineffective weak responses to TAs, which are considered as "self." This fundamental difference in responses to self versus nonself is further magnified by the ability of tumors to subvert the host immune system. Tumors induce dysfunction and apoptosis in CD8(+) antitumor effector cells and promote expansion of regulatory T cells, myeloid-derived suppressor cells, or both, which downregulate antitumor immunity, allowing tumors to escape from the host immune system. The tumor escape is mediated by several distinct molecular mechanisms. Recent insights into these mechanisms encourage expectations that a more effective control of tumor-induced immune dysfunction will be developed in the near future. Novel strategies for immunotherapy of cancer are aimed at the protection and survival of antitumor effector cells and also of central memory T cells in the tumor microenvironment.

The development and function of regulatory T cells

Regulatory T cells (Tregs) are a critical subset of T cells that mediate peripheral tolerance. There are two types of Tregs: natural Tregs, which develop in the thymus, and induced Tregs, which are derived from naive CD4(+) T cells in the periphery. Tregs utilize a variety of mechanisms to suppress the immune response. While Tregs are critical for the peripheral maintenance of potential autoreactive T cells, they can also be detrimental by preventing effective anti-tumor responses and sterilizing immunity against pathogens. In this review, we will discuss the development of natural and induced Tregs as well as the role of Tregs in a variety of disease settings and the mechanisms they utilize for suppression.

Human T regulatory cell therapy: take a billion or so and call me in the morning

Immune system regulation is of paramount importance to host survival. In settings of autoimmunity and alloimmunity, control is lost, resulting in injury to vital organs and tissues. Naturally occurring, thymic-derived T regulatory (Treg) cells that express CD4, CD25, and the forkhead box protein 3 (FoxP3) are potent suppressors of these adverse immune responses. Preclinical studies have shown that either freshly isolated or ex vivo expanded Treg cells can prevent both local and systemic organ and tissue destruction. Although promising, human Treg cell infusion therapy has heretofore been difficult to implement in the clinic, and relatively few clinical trials have been initiated. This review will focus on the preclinical models that provide the rationale for current trials and it will address both the challenges and opportunities in human Treg cell therapy.

The expression profile of functional regulatory T cells, CD4+CD25high+/forkhead box protein P3+, in patients with ulcerative colitis during active and quiescent disease

Regulatory T cells (T(reg)) have an essential role in maintaining immune tolerance in the gut. The functional CD4(+) T(reg) express the transcription factor forkhead box protein 3 (FoxP3) or a CD25(high) in humans. Further, depletion of elevated granulocytes/monocytes by extracorporeal adsorption (GMA) induces immunomodulation in patients with ulcerative colitis (UC). We investigated the impact of GMA on T(reg). Thirty-one UC patients, clinical activity index (CAI) 12.1 +/- 2.97, refractory to conventional medications including intravenous corticosteroid and 13 healthy controls (HC), were included. Patients received five GMA sessions over 5 weeks. Biopsies from the rectal mucosa and blood samples at baseline and post-GMA were immunostained with anti-CD4/FoxP3 and anti-CD4/CD25 antibodies for immunohistochemistry and flow cytometry. Following GMA, 22 of 31 patients achieved remission (CAI <or= 4, P < 0.01) and their endoscopic activity index decreased from 10.6 +/- 2.32 to 4.75 +/- 1.48 (P = 0.003). The circulating CD4(+)CD25(high+) T(reg) level was low and increased markedly in responders (P < 0.02). In the nine non-responders, the baseline CD4(+)CD25(high+) T(reg) level was about 50% of the level in the responders (P < 0.03) or in the HC (P < 0.01), and all nine had to undergo colectomy. Conversely, the number of CD4(+)/FoxP3(+) mucosal T(reg) in GMA responders decreased significantly after the fifth GMA session compared with the baseline level (P < 0.05). It is believed that the CD4(+) T(reg) has an essential role in the control of immune pathology in UC patients and a net influx of these cells from the circulation into the mucosa may proceed to suppress inflammation. GMA can impact the circulating as well as the mucosal levels of T(reg).

TGF-beta and regulatory T cell in immunity and autoimmunity

INTRODUCTION

The immune response is controlled by several inhibitory mechanisms. These mechanisms include regulatory T cells, which exist in multiple classes. Notable among these are Foxp3-expressing regulatory T cells (Treg), NKT cells, and Tr1 cells. Common to these mechanisms are inhibitory cytokines such as interleukin-10 and transforming growth factor-beta (TGF-beta). TGF-beta and Foxp3-expressing Treg cells are critical in maintaining self-tolerance and immune homeostasis.

DISCUSSIONS

The immune suppressive functions of TGF-beta and Treg cells are widely acknowledged and extensively studied. Nonetheless, recent studies revealed the positive roles for TGF-beta and Treg cells in shaping the immune system and the inflammatory responses. In this paper, we will discuss the role of these mechanisms in the control of immunity and autoimmunity and the mechanisms that underlie how these molecules control these responses.

Special regulatory T-cell review: Regulatory T cells and the intestinal tract--patrolling the frontier

Tolerance to self and harmless antigens is one of the central features of the immune system, and it is obtained through a combination of multiple mechanisms. Discriminating between pathogens and non-pathogenic antigens is especially important in the intestine, which constitutes the main contact surface between the body and the outside environment. Recently, the role of Foxp3+ regulatory T cells (Treg) in the establishment and maintenance of tolerance has been the focus of numerous studies. In this review, we briefly discuss the historical background leading to the identification of Foxp3+ Treg and give an overview of their role in controlling systemic and mucosal immune responses.

'Yin-Yang' functions of transforming growth factor-beta and T regulatory cells in immune regulation

Transforming growth factor-beta (TGF-beta) and forkhead box p3-expressing T-regulatory (Treg) cells are critical in maintaining self-tolerance and immune homeostasis. The immune suppressive functions of TGF-beta and Treg cells are widely acknowledged and extensively studied. Nonetheless, recent studies revealed the positive roles of TGF-beta and Treg cells in shaping the immune system and the inflammatory responses. This review discusses our and other's efforts in understanding the negative (Yin) as well as the positive (Yang) roles for TGF-beta and Treg cells in immune regulation.

CD4+CD25+ regulatory T-cell frequency in HER-2/neu (HER)-positive and HER-negative advanced-stage breast cancer patients

PURPOSE

CD4(+)CD25(bright) regulatory T cells (Tregs) are increased in patients with several malignancies and correlate with disease stage and prognosis. Breast cancer patients represent a heterogeneous population with unpredictable disease progression even at advanced stages. Circulating Tregs in correlation with HER-2/neu (HER) status and treatment with chemotherapy, either alone or in combination with trastuzumab therapy, were monitored in advanced-stage breast cancer patients.

EXPERIMENTAL DESIGN

Circulating Treg frequency and absolute counts of 46 HER(+) and 28 HER(-), stage III and IV, breast cancer patients before therapy and during trastuzumab therapy and/or chemotherapy have been compared with 24 healthy donors and correlated with plasma HER extracellular domain concentration and clinical outcome.

RESULTS

Treg frequency in HER(+) patients was significantly increased compared with both HER(-) patients and healthy donors. Trastuzumab therapy, with or without combined chemotherapy, resulted in a progressive decrease of circulating Tregs. Percentage change in Tregs statistically correlated with percentage change in plasma HER extracellular domain. Furthermore, decrease in Tregs correlated with either objective clinical response or stable disease, whereas increased Treg frequency during trastuzumab therapy coincided with disease progression. No statistically significant change in Treg frequency following chemotherapy was observed in HER(-) patients.

CONCLUSIONS

Treg cell frequency does not directly correlate with clinical stage in breast cancer, as stage III and IV HER(+) and HER(-) patients exhibit significantly different Treg profiles. Trastuzumab therapy, either alone or combined with chemotherapy, results in decreased Treg frequency in HER(+) advanced patients with an objective clinical response.

Transduction of an HLA-DP4-restricted NY-ESO-1-specific TCR into primary human CD4+ lymphocytes

cDNAs encoding functional T cell receptor (TCR) alpha and beta chains from a CD4+ T cell line (SG6) generated by repeated stimulation of a melanoma patient's peripheral blood mononuclear cells with HLA-DP4-restricted, NY-ESO-1-specific peptide p161-180 were cloned using a 5'rapid amplification of cDNA end method. Three different TCR alpha chains and 7 TCR beta chains were found among the 84 alpha and 162 beta cDNA clones tested. By screening different combination of the alpha/beta chains using RNA electroporation, TRAV9-1 (Valpha22.1) and TRBV20-1 (Vbeta2) were found to be the functional pair in line SG6. Antibody blocking experiments confirmed that the specificity of TRAV9-1/TRBV20-1 mRNA-transfected T cells were CD4 dependent and HLA-DP4 restricted. A retroviral vector expressing both TRAV9-1 and TRBV20-1 was constructed and used for transduction of OKT3-stimulated peripheral blood lymphocytes from melanoma patients. TCR-transduced CD4 T cells were capable of recognizing peptide-pulsed antigen-presenting cells (Epstein-Barr virus transformed B-cells, dendritic cells, and peripheral blood mononuclear cells), and protein-pulsed dendritic cells. Transduced cells were also capable of proliferation upon peptide stimulation and recognized peptide concentrations that were recognized by the parental line (0.2 microM). In contrast to SG6, which could not recognize human tumors, TCR-transduced CD4 T cells could specifically recognize NY-ESO-1/HLA-DP4-expressing melanoma cells. Major histocompatibility complex class II TCR-transduced CD4 T cells provides an alternative source of tumor antigen-specific T cells for adoptive immunotherapy of cancer patients.

Regulatory T cell-mediated transplantation tolerance

The existence of naturally occurring regulatory T cells in normal hosts and their pivotal role in maintaining both auto- and allo-tolerance have direct implications on the therapy of autoimmune disorders and for achieving immunosuppression-free allotransplantation. Among the various forms of regulatory T cells described, CD4(+)CD25(+) T cells have emerged as one of the most potent tolerogenic subsets. In this review, we discuss the molecular basis of development and function of these regulatory T cells and their potential role in the context of chronic lung allograft rejection.

Cytokine-induced IL-10-secreting CD8 T cells represent a phenotypically distinct suppressor T-cell lineage

Populations of regulatory T cells (Tregs) control autoimmune and allergic immunopathology induced by self or foreign antigens. Several types of CD4(+) MHC class II-restricted Treg populations have been characterized, but the biology of CD8(+), MHC class I-restricted Tregs is less understood. We show here that CD8(+) Tregs are rapidly generated in the presence of IL-4 and IL-12, produce IL-10, and exhibit a unique cell-surface phenotype with coexpression of activation and naive cell-associated markers. They block activation of naive or effector T cells and suppress IgG/IgE antibody responses and graft-versus-host disease in vivo. Suppression is dependent on cell contact and mediated by direct T-cell-T-cell interaction that antagonizes T-cell-receptor (TCR) signals. The data establish the existence of a CD8 T-cell suppressor effector subset distinct in both phenotype and function from T cytotoxic 1 (Tc1) and Tc2 cells. Production of such CD8 Tregs has potential for cell-based therapy of CD4 or CD8 T-cell-mediated disease.

Oral tolerance

Multiple mechanisms of tolerance are induced by oral antigen. Low doses favor active suppression, whereas higher doses favor clonal anergy/deletion. Oral antigen induces T-helper 2 [interleukin (IL)-4/IL-10] and Th3 [transforming growth factor (TGF)-beta] T cells plus CD4+CD25+ regulatory cells and latency-associated peptide+ T cells. Induction of oral tolerance is enhanced by IL-4, IL-10, anti-IL-12, TGF-beta, cholera toxin B subunit, Flt-3 ligand, and anti-CD40 ligand. Oral (and nasal) antigen administration suppresses animal models of autoimmune diseases including experimental autoimmune encephalitis, uveitis, thyroiditis, myasthenia, arthritis, and diabetes in the non-obese diabetic (NOD) mouse, plus non-autoimmune diseases such as asthma, atherosclerosis, graft rejection, allergy, colitis, stroke, and models of Alzheimer's disease. Oral tolerance has been tested in human autoimmune diseases including multiple sclerosis (MS), arthritis, uveitis, and diabetes and in allergy, contact sensitivity to dinitrochlorobenzene (DNCB), and nickel allergy. Although positive results have been observed in phase II trials, no effect was observed in phase III trials of CII in rheumatoid arthritis or oral myelin and glatiramer acetate (GA) in MS. Large placebo effects were observed, and new trials of oral GA are underway. Oral insulin has recently been shown to delay onset of diabetes in at-risk populations, and confirmatory trials of oral insulin are being planned. Mucosal tolerance is an attractive approach for treatment of autoimmune and inflammatory diseases because of lack of toxicity, ease of administration over time, and antigen-specific mechanisms of action. The successful application of oral tolerance for the treatment of human diseases will depend on dose, developing immune markers to assess immunologic effects, route (nasal versus oral), formulation, mucosal adjuvants, combination therapy, and early therapy.

The role of mucosal T lymphocytes in regulating intestinal inflammation

Suppression of chronic intestinal inflammation by different subtypes of T cells has been described in recent years. In particular, naturally arising CD4(+)CD25(+) regulatory T cells and IL-10-producing regulatory T cell type 1 CD4(+) T lymphocytes have been implicated in the regulation of intestinal inflammation. Here we focus on the ability of CD4(+)CD25(+) regulatory T cells to suppress innate and T-cell responses and discuss implications for immunoregulation in human inflammatory bowel disease. Besides the modulation of lymphoproliferation, a role for CD4(+)CD25(+) T cells in down-modulation of innate immune responses is emerging and the immunoregulatory activities of regulatory T cells in vivo may be mediated via effects on dendritic cells. Considering the extraordinary regenerative potential of the intestinal mucosa, the ability to impede pathogenic T-cell responses by active regulation might be of particular therapeutic benefit for the treatment of chronic intestinal inflammatory diseases such as Crohn's disease and ulcerative colitis.

CD4 T cells in tumor immunity

T cell immunity is the key to protective immune responses against tumors. Traditionally, this function has been ascribed to CD8 T lymphocytes with cytotoxic activity, which are restricted by MHC class I molecules. In recent years the realization that CD4 T cells can also play a relevant role in protective anti-tumor responses has received growing attention. Here we will discuss the role of MHC class II-restricted T cells in response to, and in the regulation of, tumor antigens. Emphasis will be placed on four areas: (1) the role of CD4 T cell immunity in tumor protection in animal models and putative mode of action, (2) tumor antigens recognized by human CD4 T cells, (3) the cooperation between two CD4 T cells of different specificity as a new way to jump start the response against sub-immunogenic determinants of tumor antigens in a tolerant environment, and (4) the negative impact of regulatory CD4 T cells on anti-tumor T cell responses. By drawing attention to these four areas, it is our intention to provide the reader with a comprehensive view of issues of contemporary importance for this field, in the expectation that the information will help a better design of therapeutic cancer vaccines.

TGF-beta regulates in vivo expansion of Foxp3-expressing CD4+CD25+ regulatory T cells responsible for protection against diabetes

CD4+CD25+ regulatory T cells are essential in the protection from organ-specific autoimmune diseases. In the pancreas, they inhibit actions of autoreactive T cells and thereby prevent diabetes progression. The signals that control the generation, the maintenance, or the expansion of regulatory T cell pool in vivo remain poorly understood. Here we show that a transient pulse of transforming growth factor beta (TGF-beta) in the islets during the priming phase of diabetes is sufficient to inhibit disease onset by promoting the expansion of intraislet CD4+CD25+ T cell pool. Approximately 40-50% of intraislet CD4+ T cells expressed the CD25 marker and exhibited characteristics of regulatory T cells including small size, high level of intracellular CTLA-4, expression of Foxp3, and transfer of protection against diabetes. Results from in vivo incorporation of BrdUrd revealed that the generation of a high frequency of regulatory T cells in the islets is due to in situ expansion upon TGF-beta expression. Thus, these findings demonstrate a previously uncharacterized mechanism by which TGF-beta inhibits autoimmune diseases via regulation of the size of the CD4+CD25+ regulatory T cell pool in vivo.

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The role of the combination of IL-2 and TGF-beta or IL-10 in the generation and function of CD4+ CD25+ and CD8+ regulatory T cell subsets

Recently, considerable attention has been focused on thymus‐derived CD4⁺ regulatory T cells that constitutively express CD25 and have a contact‐dependent, cytokine‐independent mechanism in vitro. However, peripheral CD4⁺ and CD8⁺ T cells can also be induced to become regulatory T cells. Here we review our studies using the combination of IL‐2 and transforming growth factor β (TGF‐β) to generate regulatory T cell subsets ex vivo, and the work of others using IL‐10 to induce suppressive activity. Under certain conditions, the autocrine effects of TGF‐β and IL‐10 induce peripheral T cells to produce immunosuppressive levels of each of these cytokines. This effect of TGF‐β is IL‐2 dependent. Under other conditions IL‐2 and TGF‐β can induce CD4⁺ cells to develop potent contact‐dependent, cytokine‐independent regulatory activity. At present, there is considerable confusion concerning the mechanism of action of CD4⁺ CD25⁺ cells because cytokine‐producing regulatory T cells generated in the periphery can express CD25 and other markers displayed by naturally occurring, thymus‐derived regulatory T cells. We, therefore, propose a nomenclature that identifies thymus‐derived and peripheral regulatory cells, and that also differentiates T regulatory cells from T helper cells. Because T regulatory cells broadly control T helper cell reactivity, the mechanisms that control regulatory cell function are also reviewed. Finally, the potential use of regulatory T cells generated ex vivo as an adoptive immunotherapy for certain autoimmune diseases, to prevent organ graft rejection, or to prevent pathologic host responses to infectious agents is discussed.

The potential of human regulatory T cells generated ex vivo as a treatment for lupus and other chronic inflammatory diseases

Regulatory T cells prevent autoimmunity by suppressing the reactivity of potentially aggressive self-reactive T cells. Contact-dependent CD4+ CD25+ 'professional' suppressor cells and other cytokine-producing CD4+ and CD8+ T-cell subsets mediate this protective function. Evidence will be reviewed that T cells primed with transforming growth factor (TGF)-beta expand rapidly following restimulation. Certain CD4+ T cells become contact-dependent suppressor cells and other CD4+ and CD8+ cells become cytokine-producing regulatory cells. This effect is dependent upon a sufficient amount of IL-2 in the microenvironment to overcome the suppressive effects of TGF-beta. The adoptive transfer of these suppressor cells generated ex vivo can protect mice from developing chronic graft-versus-host disease with a lupus-like syndrome and alter the course of established disease. These data suggest that autologous T cells primed and expanded with TGF-beta have the potential to be used as a therapy for patients with systemic lupus erythematosus and other chronic inflammatory diseases. This novel adoptive immunotherapy also has the potential to prevent the rejection of allogeneic transplants.

Conditions that induce tolerance in mature CD4+ T cells

Establishment of antigen-specific tolerance among mature T cells has been a long debated, yet poorly understood issue. In this study we have used transgenic mice bearing a class II--restricted TCR specific for the hemmagglutinin of the influenza virus in order to test the behavior of CD4+ T cells upon exposure to antigen in different forms and doses. We first studied the fate of T cells expressing the transgenic TCR (6.5) in double transgenic mice where HA was expressed as a self antigen by hemapoietic cells. In these mice, we found some mature T cells in periphery that had escaped thymic deletion and that showed signs of activation but which were anergic. Mature CD4+6.5+ cells that were transferred into antigen-containing recipients went through an initial phase of expansion after which most cells were deleted and those remaining became unresponsive, as previously described for CD8+ cells. Inducing tolerance in CD4+6.5+ cells in situ in single transgenic mice proved a difficult task: classical protocols using single doses of soluble or deaggregated antigen as well as feeding antigen all failed to induce antigen-specific unresponsiveness. It was only after decreasing cell numbers by CD4 antibody treatment and by repeatedly reintroducing antigen thereafter that unresponsiveness of 6.5+ cells was achieved and maintained. In no case could we observe the appearance of antigen-specific T cells with a Th2 cytokine profile among the remaining cells and therefore conclude that deletion and anergy represent the major mechanisms of tolerance in our studies.

Soluble antigen abrogates the appearance of anti-protein IgG1-forming cell precursors during primary immunization

The anti-human serum albumin (HSA) B-cell repertoire of C57BL/6 mice was examined by culturing splenocytes at limiting dilution following polyclonal stimulation with Escherichia coli lipopolysaccharide and a lymphokine mixture. The frequency of anti-HSA precursors was determined before and after immunization with alum-precipitated HSA and 10(9) killed Bordetella pertussis organisms, by submitting clonal supernatants to an ELISA. Anti-HSA IgG1-forming precursors were rare in unimmunized spleens, representing approximately equal to 1 in 500,000 splenocytes or only approximately equal to 100 cells per spleen. Between day 5 and day 7 after immunization, this figure increased to approximately equal to 20,000 cells per spleen. Over the following 3 weeks, there was a progressive increase in the mean optical density generated in the clonal ELISA, presumably due to affinity maturation of the B-cell population. When freshly deaggregated HSA was injected before or even up to 4 days after challenge immunization, the appearance of anti-HSA IgG1-forming cell precursors was largely prevented. The effect was most marked with 5 mg or 1 mg of soluble HSA, but impressive partial effects could be seen with as little as 10 micrograms of HSA if administered before challenge immunization. Most of the few clones seen after the higher doses of the toleragen appeared to make antibody of low affinity. The capacity to influence the B-cell pool by soluble antigen administered just 1-2 days before the sudden appearance of IgG1 precursors argues against the totality of the effect being due to T-cell-mediated suppression and in favor of a direct effect on B cells.

Microbe-induced lymphocyte blastogenesis enhancement after preculture

The in vitro blastogenic response of human peripheral blood mononuclear cells to Fusobacterium nucleatum and other oral microorganisms was enhanced if the peripheral blood mononuclear cells were cultured for 24 h at 37 degrees C prior to the addition of stimulant. The enhancement which occurred at optimal and supraoptimal concentrations of F. nucleatum (10 to 100 micrograms/ml) was detected after a preculture period of as little as 2 h. The blastogenic response was a result of T-cell proliferation, and enhancement occurred independently of monocytes. Suppressor activity was induced by culturing fresh lymphocytes for 24 h in the presence of supraoptimal concentrations of F. nucleatum. The enhancement phenomenon occurred independently of the prostaglandin effects on lymphocyte blastogenesis and was not abrogated by treatment with indomethacin.

Mechanisms of idiotype suppression. IV. Functional neutralization in mixtures of idiotype-specific suppressor and hapten-specific suppressor T cells

Specific tolerance to phosphorylcholine (PC) can be induced in BALB/c mice by neonatal injection with either pneumococcal C-polysaccharide (PnC) or anti-TEPC 15 idiotype (T15Id) antibody specific for the major idiotype (Id) of anti-PC antibody. Spleen cells from these tolerant mice exhibited T cell-mediated active suppression of anti-PC response when they were co-cultured with normal spleen cells. Suppressor cells from the PnC-injected mice appeared to bear either Lyt-1 or Lyt-2 antigens, whereas suppressor cells from anti-Id-treated mice expressed Lyt-2 antigens. Analyses of the specific receptors of these suppressor T cells, based on either adherence to PC and T15-coated petri dishes or cytolysis by rabbit anti-T15Id and monoclonal IgM anti-PC antibody with complement, revealed that receptors of PnC-induced suppressor T cells recognize PC, whereas receptors of anti-Id-induced suppressor T cells react with the T15Id. The possible interaction of the two different types of suppressor T cells was examined by co-culturing normal spleen cells with mixtures of the different suppressor cell types in various cell ratios in the presence of the T-independent PC-antigen, R36a. A brief incubation of anti-Id-induced, T15Id-specific suppressor T cells with PnC-induced, hapten-specific, and T15Id-bearing suppressor T cells resulted in complete cancellation of their suppressor function. These results suggest that idiotype network regulation may also occur among suppressor T cell population.

Blocking effect of lyt-2 antibodies on T cell functions

Monoclonal anti-Lyt-2 antibodies blocked effector function of cytotoxic thymus-derived (T) cells in the absence of added complement. Cytolysis of both allogeneic cells and syngeneic lymphoma or sarcoma target cells was inhibited at the level of the effector lymphocytes. Anti-Lyt-1 and anti-Thy-1 antibodies did not block killer cells. Proliferation of T cells in mixed lymphocyte culture was also inhibited by anti-Lyt-2, but not affected by anti-Lyt-1 or anti-Thy-1 antibodies. Although Lyt-1+ lymphocytes were required in the mixed lymphocyte reaction as helper cells for proliferation of Lyt-2+ lymphocytes, their helper function was not affected by the presence of Lyt-1 antibodies. Thus, although anti-Lyt-1, anti-Lyt-2 and anti-Thy-1 were of the same gamma 2A immunoglobulin class, had high titers, and interacted with T cells to the same extent, only anti-Lyt-2 blocked T cell functions. Polyclonal activation of T lymphocytes by concanavalin A, in contrast to activation by alloantigens, was not inhibited by Lyt-2 antibodies, suggesting that Lyt-2 antibodies interfere with T cell function at the level of the T cell antigen-receptor. The role which Lyt-2 molecules may play in T cell function is discussed.

Immunoregulatory properties of serum from patients with different stages of syphilis

The response of lymphocytes from 17 patients with primary, secondary, and tertiary syphilis to phytohaemagglutinin (PHA) and to allogeneic lymphocytes was normal in heterologous serum; however, the responsiveness of cells from some patients with primary and secondary disease was significantly reduced in the presence of autologous serum. As cells from healthy controls invariably responded better to these stimuli in autologous serum, the sera from 81 patients with syphilis were screened for immunosuppressive properties. Sera from 25 primary, 32 secondary, two tertiary, six congenital, and 16 latent cases of syphilis were examined for their ability to reduce the responsiveness of normal cells to PHA. These experiments were performed with test sera as the sole source of serum for the cultures or with test sera added to cultures containing optimally supportive amounts of pooled human plasma.Stimulation of normal cells from one control in human plasma and 20% test serum showed that only in sera from congenital cases of syphilis was the mean response significantly different from the response seen in control sera; a significant increase in the response to stimulation occurred. The range of response to PHA with sera from cases of primary and secondary syphilis was wider than with normal sera. Sera from five (20%) cases of primary and 14 (44%) cases of secondary syphilis appeared to be immunosuppressive. When retested on another sample of normal cells, these sera were consistently immunosuppressive even in the presence of 15% pooled human plasma. Thus, in early syphilis antigenic stimulation may result in the release from suppressor cells of non-specific immunoregulators of cell-mediated immunity. Such phenomena may be a prelude to the development of tolerance to treponemal antigens. In congenital syphilis the development of suppressor cells may be impaired, resulting in the apparent immunostimulatory properties of serum from such cases.

Delayed-type hypersensitivity to influenza virus. Induction of antigen-specific suppressor T cells for delayed-type hypersensitivity to hemagglutinin during influenza virus infection in mice

Mice infected with A/England/939/69 X A/Puerto Rico/8/34 (Rec 31) influenza virus by aerosol develop significantly lower levels of delayed-type hypersensitivity (DTH) to A/Hong Kong/1/68 X A/Puerto Rico/8/34/ (X31) virus compared to uninfected mice. The suppression of DTH to the hemagglutinin appears to be mediated by suppressor T cells which carry Lyt-1 membrane antigen marker, and not by sy serum antibody. The suppressor T cells for DTH induced by Rec 31 virus (H3N1) infection suppress the DTH response to the variants of the H3 subtype of influenza viruses, but have no effect on the DTH responses to A/Puerto Rico/8/34 virus (H0N1), a B influenza virus or the matrix protein of type A influenza virus. Suppressor T cells for DTH appear 2 wk after infection and are detectable in the spleen for at least 40 d thereafter. T-helper cells for antibody response to hemagglutinin are induced concomitantly with the T-suppressor cells for DTH. Possible implications of the present findings on the regulation of the immune response to viral infection are discussed.

T-cell-mediated suppression of anti-tumor immunity. An explanation for progressive growth of an immunogenic tumor

The results of this paper are consistent with the hypothesis that progressive growth of the Meth A fibrosarcoma evokes the generation of a T-cell-mediated mechanism of immunosuppression that prevents this highly immunogenic tumor from being rejected by its immunocompetent host. It was shown that it is possible to cause the regression of large, established Meth A tumors by intravenous infusion of tumor-sensitized T cells from immune donors, but only if the tumors are growing in T-cell-deficient recipients. It was also shown that the adoptive T-cell-mediated regression of tumors in such recipients can be prevented by prior infusion of splenic T cells from T-cell-intact, tumor-bearing donors. The results leave little doubt that the presence of suppressor T cells in T-cell-intact, tumor-bearing mice is responsible for the loss of an earlier generated state of concomitant immunity, and for the inability of intravenously infused, sensitized T cells to cause tumor regression. Because the presence of suppressor T cells generated in response to the Meth A did not suppress the capacity of Meth A-bearing mice to generate and express immunity against a tumor allograft, it is obvious that they were not in a state of generalized immunosuppression.

Regulatory mechanisms in cell-mediated immune responses. IV. Expression of a receptor for mixed lymphocyte reaction suppressor factor on activated T lymphocytes

Suppression of the mixed lymphocyte reaction (MLR) by a soluble factor produced by alloantigen-activated spleen cells requires genetic homology between the factor-producing cells and responder cells in MLR. The ability of lymphocytes used as MLR responder cells to adsorb MLR suppressor factor was tested to investigate the expression of a receptor structure for suppressor molecules. Normal spleen or thymus cells had no effect on suppressor activity. Concanavalin A (Con A)-activated thymocytes, however, effectively removed suppressor activity, suggesting that the receptor is expressed only after activation and is not present or not functional on resting cells. Significantly neither phytohemagglutinin- nor lipopolysaccharide-activated lymphoid cells absorbed the factor. Furthermore, only Con A-activated thymocytes demonstrating genetic homology with the cell producing suppressor factor for H-2 regions to the right of I-E were effective absorbants. Alloantigen-stimulated spleen cells syngeneic to the suppressor cell also removed suppressor activity. These data support an hypothesis that subsequent to stimulation in MLR, T lymphocytes express a receptor, either through synthesis or alteration of an existing molecular structure, which then provides the appropriate site for interaction with suppressor molecules.

Heterogeneity of murine regulatory T cells. I. Subpopulations of amplifier and suppressor T cells

Immunization of C3H/HeJ mice with 4 X 10(9) SRBC yields a whole splenic T-cell population which can, upon transfer, specifically suppress recipient direct and indirect plaque-forming cells (PFC) responses to sheep erythrocytes (SRBC). Discontinuous bovine serum albumin density gradient fractionation of these T cells demonstrated a population of low density T cells which augmented and a population of high density T cells which suppressed recipient responses irrespective of the number of T cells transferred. Moreover, infusion of admixtures of low and high density cells resulted in intermediate regulatory functions which could be predicted by knowing the regulatory capacity of each population alone. In addition to heterogeneity existing among regulatory T cells as regards amplification and suppression, it appeared that heterogeneity existed within the suppressor T population. Thus, T cells capable of inhibiting direct PFC could be distinguished from those suppressing indirect PFC by their differential localization in peripheral lymphoid tissue, differences in the dissipation of suppressive influences during incubation at 37 degrees C, and by differences in the possible requirement for adherent cell populations. While the relative frequency of both low density amplifier and high density suppressor cells increased with the dose of SRBC used for their induction, it appeared that suppressor cells might be generated in response to feedback signals from amplifier cells. These studies indicate that further delineation of heterogeneity existing within suppressor populations may be helpful in defining mechanisms required for the induction and manifestation of suppressive regulatory forces.

Suppressor cells: dependence on assay conditions for functional activity

Spleen cells educated in vitro with sheep red blood cells (SRBC) suppressed the plaque-forming cell response of Mishell-Dutton assay cultures challenged with optimal doses of SRBC. Changing conditions in the assay cultures changed the effect educated cells had on the assay culture responses. For example, educated cells helped rather than suppressed assay cultures of suboptimal numbers of spleen cells. Similarly, augmentation resulted upon addition of educated cells to assay cultures challenged with suboptimal doses of SRBC. Such a reversal of regulatory effects was not observed when assay cultures were challenged with supraoptimal antigen doses. Educated cells helped assay cultures of B spleen cells, and the addition of normal T cells reinstated suppression. Furthermore, maintenance of assay cultures under stationary rather than the usual rocking conditions allowed educated cells to help rather than suppress the antibody response of assay cultures. These results show that when the response of the target population (assay cultures) is low, the regulator (educated) cells augment the response, and vice versa, supporting the hypothesis that the effect regulator cells produce depends on the activity of the cells they regulate.