Selective Treg Research Articles

Selective IL-2 responsiveness of regulatory T cells through multiple intrinsic mechanisms supports the use of low-dose IL-2 therapy in type 1 diabetes.

Low-dose interleukin-2 (IL-2) inhibited unwanted immune responses in several clinical settings and is currently being tested in patients with type 1 diabetes (T1D). Low-dose IL-2 selectively targets regulatory T cells (Tregs), but the mechanisms underlying this selectivity are poorly understood. We show that IL-2-dependent STAT5 activation in Tregs from healthy individuals and patients with T1D occurred at an ∼10-fold lower concentration of IL-2 than that required by T memory (TM) cells or by in vitro-activated T cells. This selective Treg responsiveness is explained by their higher expression of IL-2 receptor subunit α (IL-2Rα) and γ chain and also endogenous serine/threonine phosphatase protein phosphates 1 and/or 2A activity. Genome-wide profiling identified an IL-2-dependent transcriptome in human Tregs. Quantitative assessment of selected targets indicated that most were optimally activated by a 100-fold lower concentration of IL-2 in Tregs versus CD4(+) TM cells. Two such targets were selectively increased in Tregs from T1D patients undergoing low-dose IL-2 therapy. Thus, human Tregs possess an IL-2-dependent transcriptional amplification mechanism that widens their selective responses to low IL-2. Our findings support a model where low-dose IL-2 selectively activates Tregs to broadly induce their IL-2/IL-2R gene program and provide a molecular underpinning for low-dose IL-2 therapy to enhance Tregs for immune tolerance in T1D.

Abstract LB-259: Interferon-α enhances clinical benefits of regulatory T cell depletion in ovarian cancer through direct T cell effects and by inducing bystander IL-6

Abstract Tregs hinder anti-tumor immunity, and depleting them treats cancers effectively in mice. By contrast, Treg depletion in human trials has limited efficacy. In a phase I trial of advanced stage carcinomas (breast, lung, melanoma, ovarian, bladder), we showed that the IL-2/diphtheria fusion toxin denileukin diftitox (DT) significantly depleted functional CD4+CD25hiFoxp3+ Tregs in blood with improved T cell function (increased Ki-67 and interferon-γ) with minimal effects on other blood mononuclear cells. One patient with stage IV ovarian carcinoma experienced significant reduction of metastatic tumor burden with denileukin diftitox at 12 μg/kg. Additional work showed that Treg depletion effects could last up to 4 weeks, and that weekly DT at 12 μg/kg eventually depleted anti-tumor and tumor-specific CD8+ T cells. Based on these data, we conducted a phase II clinical trial of epithelial ovarian carcinomas FIGO stage III-IV failing standard treatments, using DT as a single agent at 12 μg/kg by intravenous infusion every 3-4 weeks. In this trial, DT significantly depleted blood and tumor microenvironmental Tregs with only grade I-II toxicities, but with minimal clinical efficacy in 28 consecutive patients. This trial was halted for futility according to the Simon 2-stage design. Interferon-α alone does not treat ovarian cancer, but we now show that it significantly improves clinical and immune DT-mediated Treg depletion efficacy in human ovarian cancer. In the ID8 mouse ovarian carcinoma model, DT modestly increased survival and anti-tumor immunity. Interferon-α alone did not affect Treg numbers or function, but enhanced CD8+ T cell anti-tumor immunity. Interferon-α plus DT increased mouse survival significantly over either individual drug. In type I IFNR-/- mice unable to mediate interferon-α signals, interferon-α directly increased adoptively transferred IFNR+CD8+ T cell function independent of CD4+ T cell help. When combined with DT, IFN-α reduced Treg function without further numerical Treg reduction by indirect effects on tumor microenvironmental dendritic cells. In vitro studies identified IFN-α-driven dendritic cell IL-6 as a mechanism for reducing Treg function. When three ovarian cancer patients failed DT alone in the phase II trial, two had clinical and immune benefit by adding pegylated interferon-α2a in a separate trial, with acceptable toxicities. This trial was halted due to a lack of further DT. These studies demonstrate that DT depletes Tregs in distinct human carcinomas but is unlikely to be clinically effective as a single agent. We identified novel IFN-α mechanisms that improve Treg depletion effects using FDA-approved agents that can be rapidly translated. More selective Treg depletion agents and rationale combinations with other agents could improve clinical efficacy further. Citation Format: Suzanne Thibodeaux, Vincent Hurez, Shawna Wall, Srilakshmi Pandeswara, Benjamin Daniel, Aijie Liu, Lishi Sun, Leslie Wood, Weiping Zou, Tyler Curiel. Interferon-α enhances clinical benefits of regulatory T cell depletion in ovarian cancer through direct T cell effects and by inducing bystander IL-6. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-259. doi:10.1158/1538-7445.AM2014-LB-259

Antigen-specific expansion of human regulatory T cells as a major tolerance mechanism against mucosal fungi.

Foxp3(+) regulatory T cells (Treg) have a central role for keeping the balance between pro- and anti-inflammatory immune responses against chronically encountered antigens at mucosal sites. However, their antigen specificity especially in humans is largely unknown. Here we used a sensitive enrichment technology for antigen-reactive T cells to directly compare the conventional vs. regulatory CD4(+) T-cell response directed against two ubiquitous mucosal fungi, Aspergillus fumigatus and Candida albicans. In healthy humans, fungus-specific CD4(+)CD25(+)CD127(-)Foxp3(+) Treg are strongly expanded in peripheral blood and possess phenotypic, epigenetic and functional features of thymus-derived Treg. Intriguingly, for A. fumigatus, the strong Treg response contrasts with minimal conventional T-cell memory, indicating selective Treg expansion as an effective mechanism to prevent inappropriate immune activation in healthy individuals. By contrast, in subjects with A. fumigatus allergies, specific Th2 cells were strongly expanded despite the presence of specific Treg. Taken together, we demonstrate a largely expanded Treg population specific for mucosal fungi as part of the physiological human T-cell repertoire and identify a unique capacity of A. fumigatus to selectively generate Treg responses as a potentially important mechanism for the prevention of allergic reactions.

Therapeutic Potential of Regulatory T cells in Autoimmune Disorders

Regulatory T cells (Tregs) play a dominant role in the regulation of immune responses. Quantitative and/or qualitative abnormalities of Tregs were observed in patients with autoimmune diseases and therapeutic interventions focusing Tregs are an attractive new target with the potential to cure these disorders. Biological agents approved for treatment of inflammatory rheumatic diseases transiently influence Treg prevalences and function and experimental therapies including novel biological agents, gene therapy, activation and ex vivo expansion of purified Tregs as well as substances influencing tolerogenic dendritic cells will be developed for selective Treg therapy. Although many of these interventions are effective in vitro, in animal models as well as in early clinical trials, significant concerns exist regarding the stability of Treg modifications as well as the long-term safety of Treg-based therapies.

Differential Effects of TCR Stimulation and IL-2 On Apoptotic Pathways in CD4 Regulatory T Cells Compared to Conventional CD4 T Cells and CD8 T Cells

Abstract 3280CD4+CD25+Foxp3+ regulatory T cells (Treg) play an important role in the maintenance of self-tolerance and immune homeostasis and Treg deficiency contributes to the development of autoimmune diseases. CD4Treg, conventional CD4 T cells (Tcon) and CD8 T cells are derived from lymphocyte progenitor cells that differentiate into distinct functional subsets in the thymus before export to the peripheral circulation. As T cells differentiate and expand in the periphery, each T cell subset is differentially regulated and subjected to distinct homeostatic signals. For example, interleukin-2 (IL-2) is a critical regulator of Treg development, expansion and survival and lack of IL-2 results in selective Treg deficiency. In regulating Treg homeostasis, IL-2 has multiple and distinct effects on Treg differentiation, proliferation and susceptibility to apoptosis. To determine the mechanism whereby IL-2 affects susceptibility of Treg to apoptosis, we used a new flow cytometry-based assay (BH3 profiling) to measure the mitochondrial membrane depolarization in response to a panel of pro-apoptotic BH3 peptides (BIM, BID, BAD, NOXA, PUMA, BMF, HRK). This assay allowed us to compare “priming” which we define as susceptibility to BH3 peptide-induced mitochondrial membrane depolarization in different T cell subsets, including CD4 Treg, CD4 Tcon and CD8 T cells. We also examined cell surface expression of CD95 death receptor (Fas) and cytoplasmic expression of Bcl-2 and Ki67 as additional measures of susceptibility to apoptosis and proliferation in each subset. In resting blood obtained from healthy donors (n=10), CD4 Treg were more “primed” than either CD4 Tcon or CD8 T cells when exposed to several BH3 peptides (PUMA, BMF and the combination of BAD+NOXA). CD4 Treg were also found to have decreased expression of Bcl-2 and increased expression of CD95 and Ki67 compared to CD4 Tcon or CD8 T cells. Thus, Treg in healthy individuals have higher proliferative activity and are more susceptible to apoptosis than other major T cell subsets through both mitochondrial and death receptor pathways.To establish the functional effects of TCR stimulation and IL-2, CD4 Treg, CD4 Tcon and CD8 T cells were purified by cell sorting and cultured for 5–6 days with or without TCR stimulation (1μg/ml anti-CD3 + 1μg/ml anti-CD28) and IL-2 (100 IU/ml). Results were compared to cells cultured in media alone. Results are summarized in the table below. CD4 Tcon and CD8 T cells responded in a similar fashion to either TCR stimulation alone or TCR plus IL-2. This response included increased BH3 priming, reduced expression of Bcl-2, increased expression of CD95 and increased proliferation (Ki-67). IL-2 alone had no effect on CD4 Tcon or CD8 T cells. In contrast, TCR stimulation alone had no effect on CD4 Treg but IL-2 alone reduced BH3 priming and increased expression of Bcl-2. Combined TCR stimulation plus IL-2 in Treg increased BH3 priming, reduced expression of Bcl-2, increased expression of CD95 and increased proliferation. Thus, TCR stimulation reversed the anti-apoptotic effects of IL-2 alone and markedly increased susceptibility of Treg to apoptosis. When compared with CD4 Tcon and CD8 T cells, these studies demonstrate distinct effects of TCR stimulation and IL-2 on both mitochondrial and death receptor pathways of apoptosis in CD4 Treg and define mechanisms whereby TCR stimulation and IL-2 interact to regulate Treg homeostasis.Table 1Effects of in vitro TCR stimulation and IL-2 on apoptotic pathways of T cell subsetsTCR StimulationIL-2TCR + IL2BH3 primingBcl-2CD95Ki67BH3 primingBcl-2CD95Ki67BH3 primingBcl-2CD95Ki67CD4 Treg––––↓↑––↑↓↑↑CD4 Tcon↑↓↑↑––––↑↓↑↑CD8↑↓↑↑––––↑↓↑↑ Disclosures:No relevant conflicts of interest to declare.

Effects of Daily Low Dose IL-2 Therapy on Homeostatic Regulation of CD4+Foxp3+ Regulatory T Cells In Patients with Chronic Graft-Versus-Host Disease

AbstractAbstract 895CD4+FoxP3+ regulatory T cells (Treg) play a central role in the maintenance of tolerance after allogeneic hematopoietic stem cell transplantation (HSCT) and recent studies have demonstrated that Treg deficiency leads to the development of chronic GHVD (cGVHD). Interleukin-2 (IL-2) is known to promote thymic generation and maintenance of peripheral Treg and IL-2 deficiency results in a profound deficiency of Treg in vivo. Based on these findings we initiated a clinical trial to evaluate the safety, clinical efficacy and immunologic effects of low dose recombinant IL-2 in patients with steroid-refractory cGVHD. We recently reported the clinical outcomes of this trial demonstrating that IL-2 administration preferentially increased Treg in patients with active cGVHD and resulted in clinical improvement with only minor toxicities (Koreth et al, ASBMT 2010). However, the mechanisms responsible for Treg expansion in patients during IL-2 administration have not been characterized. To elucidate these mechanisms, we examined phenotypic and functional characteristics of Treg in 14 patients who received daily subcutaneous IL-2 (3×105-3×106IU/m2/day) for 8 weeks. Peripheral blood samples were obtained before and at 1, 2, 4, 6, 8, 10 and 12 weeks after starting IL-2. Treg were compared to conventional CD4+FoxP3- T cells (Tcon) within individual patient samples and examined for expression of Ki-67, PD-1 and BCL-2. In some experiments, Treg and Tcon were further divided into subpopulations by the expression of CD45RA and CD31. Absolute numbers of functionally suppressive Treg increased 5-fold in the first 4 weeks of therapy. Treg numbers then slowly decreased despite continued IL-2 therapy, but remained 2-fold higher than baseline at 8 weeks. Absolute numbers of Tcon increased 2-fold in the first 4 weeks and then returned to baseline levels at 8 weeks. This resulted in a sustained increase of Treg/Tcon ratio for the entire duration of therapy, which persisted for at least 4 weeks after treatment was completed. Plasma IL-2 levels peaked at 1 week and gradually declined despite continued treatment at the same dose. Nevertheless, IL-2 levels remained significantly higher than baseline throughout treatment (median 1.4pg/ml at baseline and 18.1pg/ml at 8 weeks, p<0.05). The proliferative response to IL-2 was examined by measuring expression of Ki-67 in each subset. Initially, Ki-67 expression in Treg rapidly increased in an IL-2 dose-dependent manner. Ki-67 also increased in Tcon but at a significantly lower level (median 20.0% Treg vs 6.7% Tcon, p=0.0001). Increased Ki-67 was seen in both CD45RA+CD31+ recent thymic emigrant Treg (RTE-Treg) and CD45RA- activated/memory Treg (MEM-Treg) at similar levels (median 20.1% and 18.5%, respectively, p=0.54). Treg proliferation peaked in the first week of IL-2, and rapidly returned to baseline levels in weeks 2–3. Despite changes in proliferation, the absolute number of RTE-Treg remained significantly elevated (median 1.05/ul at baseline, 9.43/ul at 4 weeks, p=0.001). In contrast, the absolute number of RTE-Tcon did not change. Phenotypic analysis of Treg showed that expression of both PD-1 and BCL-2 increased during IL-2 therapy (%PD-1+ Treg; median 15.7% at baseline and 38.3% at 8 weeks, p=0.02: relative BCL-2 MFI; median 1.00 at baseline and 1.59 at 8 weeks, p=0.04). To determine the functional effects of these changes on susceptibility to apoptosis, Treg and Tcon were purified and cultured with or without agonistic FAS antibody, and apoptosis was measured using Annexin-V staining. Remarkably, Treg obtained during IL-2 therapy were relatively resistant to apoptosis compared to baseline. In summary, these results indicate that the selective expansion of Treg during prolonged IL-2 administration is characterized by a series of homeostatic changes. Initial high levels of IL-2 lead to selective and rapid Treg proliferation. Treg proliferation is not maintained as numbers of Treg increase and IL-2 levels decrease. Subsequent maintenance of increased Treg appears to be mediated primarily by increased resistance to apoptosis and prolonged survival. Increased thymic output of Treg also appears to support this peripheral homeostatic process. These findings demonstrate the complex effects of IL-2 on Treg homeostasis and provide important information for developing strategies to promote immune tolerance. Disclosures:No relevant conflicts of interest to declare.

Selective Depletion of Foxp3+ Regulatory T Cells Improves Effective Therapeutic Vaccination against Established Melanoma

Tumor-bearing individuals have been reported to harbor increased numbers of Foxp3(+) regulatory T cells (Treg), which prevent the development of efficient antitumor immune responses. Thus, Treg depletion has already been tested as a promising therapeutic approach in various animal models and entered clinical trials. However, the use of nonspecific Treg targeting agents such as CD25 depleting antibodies, which in addition to CD25(+) Tregs also deplete recently activated CD25(+) effector T cells, potentially masked the tremendous potential of this therapeutic strategy. To avoid such nonspecific effects, we used transgenic DEREG (depletion of regulatory T cells) mice, which express a diphtheria toxin receptor under control of the Foxp3 locus, allowing selective depletion of Foxp3(+) Tregs even during ongoing immune responses. We showed that Foxp3(+) Treg depletion induced partial regression of established ovalbumin (OVA)-expressing B16 melanoma, which was associated with an increased intratumoral accumulation of activated CD8(+) cytotoxic T cells. The antitumor effect could be significantly enhanced when Treg depletion was combined with vaccination against OVA. To further assess whether this therapeutic approach would break self-tolerance, we crossed DEREG mice with RipOVA(low) mice, expressing OVA as neo-self-antigen under control of the rat insulin promoter. In these mice, combined Treg depletion and vaccination also induced tumor regression without the onset of diabetes. Together, our data suggest that selective Treg targeting strategies combined with vaccinations against tumor-associated (self) antigens have the potential to evoke efficient antitumor responses without inducing overt autoimmunity. These findings might have implications for future therapeutic interventions in cancer patients.

Regulatory T-cell Trafficking: From Thymic Development to Tumor-Induced Immune Suppression

Regulatory T cells (Tregs) have become a priority for many investigators in immunology due to their potent immunosuppressive and tolerogenic effects. While Treg activity is required for normal immune homeostasis, dysregulation of their numbers can induce autoimmunity or aid in the pathogenesis of disease. Therefore, great effort has been made to understand the mechanisms by which Tregs accumulate in different areas of the body. Like other lymphocytes, Tregs migrate in response to a network of chemotactic stimuli involving chemokines, chemokine receptors, integrins, and their corresponding ligands. However, many of these stimuli are exclusive to Tregs, inducing their migration while leaving conventional populations unaffected. It is these selective stimuli that result in increased ratios of Tregs among conventional effector populations, leading to changes in immune suppression and homeostasis. This review explores selective Treg trafficking during thymic Treg development, migration to secondary lymphoid tissues and emigration into the periphery during homeostatic conditions, inflammation, and the tumor microenvironment, placing emphasis on stimuli that selectively recruits Tregs to target locations.

Differences in Tumor Regulatory T-Cell Localization and Activation Status Impact Patient Outcome

The presence of regulatory T cells (Treg) has been described in a large panel of solid tumors. However, their impact on tumor progression differs according to the tumor type analyzed. We recently obtained evidence in breast carcinoma that Treg localized within lymphoid aggregates, but not in the tumor bed, have a negative impact on patients' survival. Moreover, we showed selective Treg recruitment through CCR4/CCL22 in the lymphoid aggregates upon contact with dendritic cells (DC), where they became strongly and selectively activated (ICOS(high)) and block conventional T-cell response. Here, we discuss the meaning and potential implication of these novel findings.

Regulatory T Cells Recruited through CCL22/CCR4 Are Selectively Activated in Lymphoid Infiltrates Surrounding Primary Breast Tumors and Lead to an Adverse Clinical Outcome

Immunohistochemical analysis of FOXP3 in primary breast tumors showed that a high number of tumor-infiltrating regulatory T cells (Ti-Treg) within lymphoid infiltrates surrounding the tumor was predictive of relapse and death, in contrast to those present within the tumor bed. Ex vivo analysis showed that these tumor-infiltrating FOXP3(+) T cells are typical Treg based on their CD4(+)CD25(high)CD127(low)FOXP3(+) phenotype, their anergic state on in vitro stimulation, and their suppressive functions. These Ti-Treg could be selectively recruited through CCR4 as illustrated by (a) selective blood Treg CCR4 expression and migration to CCR4 ligands, (b) CCR4 down-regulation on Ti-Treg, and (c) correlation between Ti-Treg in lymphoid infiltrates and intratumoral CCL22 expression. Importantly, in contrast to other T cells, Ti-Treg are selectively activated locally and proliferate in situ, showing T-cell receptor engagement and suggesting specific recognition of tumor-associated antigens (TAA). Immunohistochemical stainings for ICOS, Ki67, and DC-LAMP show that Ti-Treg were close to mature DC-LAMP(+) dendritic cells (DC) in lymphoid infiltrates but not in tumor bed and were activated and proliferating. Furthermore, proximity between Ti-Treg, CD3(+), and CD8(+) T cells was documented within lymphoid infiltrates. Altogether, these results show that Treg are selectively recruited within lymphoid infiltrates and activated by mature DC likely through TAA presentation, resulting in the prevention of effector T-cell activation, immune escape, and ultimately tumor progression. This study sheds new light on Treg physiology and validates CCR4/CCL22 and ICOS as therapeutic targets in breast tumors, which represent a major health problem.

Myeloid‐derived suppressor cells (MDSCs) and tumor‐associated macrophages (TAMs) produce CCL22 which selectively recruits regulatory T‐cells (Tregs) to the tumor microenvironment

Tumor‐induced immune suppression involves the accumulation of immune suppressive infiltrates in the microenvironment. We demonstrated an increased number of Tregs in Lewis Lung Carcinoma (LLC). High levels of CCR4 on Tregs lead us to hypothesize that CCR4‐chemokines are secreted by LLC and selectively recruit Tregs. ELISA shows a large increase in CCL22 secretion by LLC bearing tissue. Lack of CCL22 in primary cultures suggests that CCL22 is NOT produced by the tumor, but rather by some other component of the microenvironment. Migration assays showed that normal, and to a greater extent, LLC‐bearing tissue selectively recruits Tregs. Neutralization of CCL22 significantly reduced the selective Treg migration. Fractionated normal and LLC bearing tissue shows that while NK, CD31+, and CD11c+ cells all produce CCL22, only the CD11b+ population secretes higher levels in LLC bearing tissue. This is magnified by the increased number of CD11b+ cells found in LLC bearing tissue. Of the CD11b+ cells, two populations emerge. One population is absent in normal lung tissue and expresses F4/80, suggesting that these are TAMs. The other population expresses Gr‐1, which is consistent with MDSCs. Together, our data suggests that other immune suppressive phenotypes such as TAMS and MDSCs play an active role in Treg recruitment to the microenvironment through the production of CCL22.Supported by the Dept. of Veteran's Affairs &amp; NIH

Preferential Migration of T Regulatory Cells Induced by IL-16

As a natural ligand for CD4, IL-16 has been shown to preferentially induce migration in Th1 cells, and, in long-term cultures with IL-2, IL-16 facilitates the expansion of CD4(+)CD25(+) cells. In addition, IL-16 has an immunomodulatory role in asthmatic inflammation, as exogenous administration significantly reduces inflammation and airway hyperreactivity. The mechanism for this, however, is not clear. Based on its functional characteristics and potential immunomodulatory role, we investigated the ability of IL-16 to recruit and influence the development of T regulatory (Treg) cells. We now demonstrate that IL-16 preferentially induces migration in a CD25(+)CTLA-4(+) human T cell subset and that responding cells produce IFNgamma and TGFbeta but not IL-10. These cells are relatively unresponsive to antigenic stimulation and can suppress proliferation and IL-5, but not IFNgamma, production by autologous T cells. We further demonstrate that IL-16-recruited cells are enriched for Forkhead box P3 (Foxp3). In addition, we find that IL-16 stimulation may facilitate de novo induction of Foxp3(+) Treg cells, because the stimulation of FoxP3-negative T cells for 48 h results in the expression of FoxP3 mRNA and protein. These data indicate that at sites of inflammation IL-16 may contribute to selective Treg cell expansion through the preferential induction of a migratory response from existing Treg cells, as well as by the induction of de novo generation of FoxP3(+) cells. These findings offer a potential mechanism for the immunosuppressive effects of IL-16 seen in Th2-mediated inflammation.

Lewis Lung Carcinoma-derived CCL22 recruitment of T regulatory cells (B148)

Abstract Two aspects of cancer progression that contribute to malignancy are the ability of the tumor to metastasize to remote locations in the body, and the ability to evade effective host anti-tumor immune responses. It has been shown that T regulatory cells (Treg) can inhibit immune effector cell function and aid in anti-tumor immune evasion. Several types of tumors have elevated Treg numbers, and it is believed that tumors may actively recruit these cell types. Treg migration is induced by several chemokines and their receptors such as CCL22 and CCR4. Here, we use flow cytometry to analyze CCR4 expression on T cell populations and show that Tregs express more CCR4 than any other T-cell type. We also used ELISA to compare CCL22 secretion levels in normal and Lewis Lung Carcinoma (LLC) bearing lung tissue. While high basal levels of CCL22 are secreted from normal lung tissue, even higher levels are secreted by LLC bearing lung tissue. In addition, we used media conditioned on these tissues in transwell migration assays to measure preferential Treg chemoattraction. Normal lung tissue induced high levels of selective Treg chemoattraction, while LLC bearing lung selectively attracted even greater numbers of Tregs. These data suggest that normal lung tissue may be able to recruit elevated levels of immune inhibitory Tregs through the secretion of CCL22 prior to metastasis, and that once LLC metastasis occurs, both CCL22 and Treg levels increase even further.

Radioimmunoassay of human serum thyrotrophin.

The double antibody radioimmunoassay of serum thyroid-stimulating hormone (TSH) allows measurement of circulating levels of the hormone in most normal subjects. The serum TSH level in normal subjects is 1.6 +/- 0.8muU/ml. Patients with non-toxic goitre and acromegaly have normal TSH levels. Values are always raised in hypothyroid patients (with primary thyroid disease) and are significantly lowered in those with hyperthyroidism. Of the many stimuli used in an attempt to raise TSH levels in normal adult subjects only three-synthetic thyrotrophin-releasing hormone, ethinyloestradiol, and carbimazole plus iodides-have been effective. The major clinical application of the TSH immunoassay lies in the diagnosis of minor degrees of hypothyroidism. An impaired response of serum TSH to synthetic thyrotrophin-releasing hormone should also help in the diagnosis of hypopituitarism affecting TSH production.