Characterization of immunodeficiency in a patient with growth hormone insensitivity secondary to a novel STAT5b gene mutation

Relevance of laboratory testing for the diagnosis of primary immunodeficiencies: a review of case-based examples of selected immunodeficiencies

The field of primary immunodeficiencies (PIDs) is one of several in the area of clinical immunology that has not been static, but rather has shown exponential growth due to enhanced physician, scientist and patient education and awareness, leading to identification of new diseases, new molecular diagnoses of existing clinical phenotypes, broadening of the spectrum of clinical and phenotypic presentations associated with a single or related gene defects, increased bioinformatics resources, and utilization of advanced diagnostic technology and methodology for disease diagnosis and management resulting in improved outcomes and survival. There are currently over 200 PIDs with at least 170 associated genetic defects identified, with several of these being reported in recent years. The enormous clinical and immunological heterogeneity in the PIDs makes diagnosis challenging, but there is no doubt that early and accurate diagnosis facilitates prompt intervention leading to decreased morbidity and mortality. Diagnosis of PIDs often requires correlation of data obtained from clinical and radiological findings with laboratory immunological analyses and genetic testing. The field of laboratory diagnostic immunology is also rapidly burgeoning, both in terms of novel technologies and applications, and knowledge of human immunology. Over the years, the classification of PIDs has been primarily based on the immunological defect(s) ("immunophenotype") with the relatively recent addition of genotype, though there are clinical classifications as well. There can be substantial overlap in terms of the broad immunophenotype and clinical features between PIDs, and therefore, it is relevant to refine, at a cellular and molecular level, unique immunological defects that allow for a specific and accurate diagnosis. The diagnostic testing armamentarium for PID includes flow cytometry - phenotyping and functional, cellular and molecular assays, protein analysis, and mutation identification by gene sequencing. The complexity and diversity of the laboratory diagnosis of PIDs necessitates many of the above-mentioned tests being performed in highly specialized reference laboratories. Despite these restrictions, there remains an urgent need for improved standardization and optimization of phenotypic and functional flow cytometry and protein-specific assays. A key component in the interpretation of immunological assays is the comparison of patient data to that obtained in a statistically-robust manner from age and gender-matched healthy donors. This review highlights a few of the laboratory assays available for the diagnostic work-up of broad categories of PIDs, based on immunophenotyping, followed by examples of disease-specific testing.

STAT5b deficiency: lessons from STAT5b gene mutations

Growth hormone (GH) regulates insulin-like growth factor (IGF)-I production primarily through activation of the GH receptor (GHR)-signal transducer and activator of transcription (STAT)-5b signaling cascade. One of four STAT proteins (STAT1, -3, -5a and -5b) activated by the GH-GHR system, the critical importance of STAT5b in IGF-I production became evident with the identification of homozygous, autosomal recessive STAT5b mutations in patients who presented with severe postnatal growth failure, growth hormone insensitivity syndrome (GHIS) and marked IGF-I deficiency. Unlike GHIS due to GHR mutations, patients carrying STAT5b mutations also presented with chronic pulmonary disease and evidence of perturbations of T-cell homeostasis. At present, no single treatment(s) is available to improve both poor statural growth and immune deficiency. Continued clinical evaluations of patients with STAT5b mutations and elucidating the impact of the mutation on STAT5b structure and function, are important to understanding the pathophysiology of this rare, complex, disease (MIM 245590).

IL-2 as a therapeutic target for the restoration of Foxp3+ regulatory T cell function in organ-specific autoimmunity: implications in pathophysiology and translation to human disease

Peripheral immune tolerance requires a finely controlled balance between tolerance to self-antigens and protective immunity against enteric and invading pathogens. Self-reactive T cells sometimes escape thymic clonal deletion, and can subsequently provoke autoimmune diseases such as type 1 diabetes (T1D) unless they are controlled by a network of tolerance mechanisms in the periphery, including CD4+ regulatory T cells (Treg) cells. CD4+ Treg cells are characterized by the constitutive expression of the IL-2Rα chain (CD25) and preferentially express the forkhead winged helix transcriptional regulator Foxp3. These cells have been shown to possess immunosuppressive properties towards various immune cell subsets and their defects are thought to contribute to many autoimmune disorders. Strong evidence shows that IL-2 is one of the important stimulatory signals for the development, function and fitness of Treg cells. The non-obese diabetic (NOD) mouse model, a prototypic model of spontaneous autoimmunity, mimics many features of human T1 D. Using this model, the contribution of the IL-2-IL-2R pathway to the development of T1 D and other autoimmune disorders has been extensively studied. In the past years, strong genetic and molecular evidence has indicated an essential role for the IL-2/IL-2R pathway in autoimmune disorders. Thus, the major role of IL-2 is to maintain immune tolerance by promoting Treg cell development, functional fitness and stability. Here we first summarize the genetic and experimental evidence demonstrating a role for IL-2 in autoimmunity, mainly through the study of the NOD mouse model, and analyze the cellular and molecular mechanisms of its action on Treg cells. We then move on to describe how this data can be translated to applications for human autoimmune diseases by using IL-2 as a therapeutic agent to restore Treg cell fitness, numbers and functions.

Structural and thermodynamic characterization of the TYK2 and JAK3 kinase domains in complex with CP-690550 and CMP-6

Janus kinases (JAKs) are critical regulators of cytokine pathways and attractive targets of therapeutic value in both inflammatory and myeloproliferative diseases. Although the crystal structures of active JAK1 and JAK2 kinase domains have been reported recently with the clinical compound CP-690550, the structures of both TYK2 and JAK3 with CP-690550 have remained outstanding. Here, we report the crystal structures of TYK2, a first in class structure, and JAK3 in complex with PAN-JAK inhibitors CP-690550 ((3R,4R)-3-[4-methyl-3-[N-methyl-N-(7H-pyrrolo[2,3-d]pyrimidin-4-yl)amino]piperidin-1-yl]-3-oxopropionitrile) and CMP-6 (tetracyclic pyridone 2-t-butyl-9-fluoro-3,6-dihydro-7H-benz[h]-imidaz[4,5-f]isoquinoline-7-one), both of which bind in the ATP-binding cavities of both JAK isozymes in orientations similar to that observed in crystal structures of JAK1 and JAK2. Additionally, a complete thermodynamic characterization of JAK/CP-690550 complex formation was completed by isothermal titration calorimetry, indicating the critical role of the nitrile group from the CP-690550 compound. Finally, computational analysis using WaterMap further highlights the critical positioning of the CP-690550 nitrile group in the displacement of an unfavorable water molecule beneath the glycine-rich loop. Taken together, the data emphasize the outstanding properties of the kinome-selective JAK inhibitor CP-690550, as well as the challenges in obtaining JAK isozyme-selective inhibitors due to the overall structural and sequence similarities between the TYK2, JAK1, JAK2 and JAK3 isozymes. Nevertheless, subtle amino acid variations of residues lining the ligand-binding cavity of the JAK enzymes, as well as the global positioning of the glycine-rich loop, might provide the initial clues to obtaining JAK-isozyme selective inhibitors.

Immunodeficiency due to mutations in ORAI1 and STIM1

Lymphocyte activation requires Ca(2+) influx through specialized Ca(2+) channels in the plasma membrane. In T cells the predominant Ca(2+) channel is the Ca(2+) release activated Ca(2+) (CRAC) channel encoded by the gene ORAI1. ORAI1 is activated by stromal interaction molecule (STIM) 1 that is localized in the ER where it senses the concentration of stored Ca(2+). Following antigen binding to immunoreceptors such as the TCR, ER Ca(2+) stores are depleted, STIM1 is activated and ORAI1-CRAC channels open resulting in what is referred to as store-operated Ca(2+) entry (SOCE). Mutations in ORAI1 and STIM1 genes in human patients that lead to expression of non-functional ORAI1 or complete lack of ORAI1 or STIM1 protein are associated with a unique clinical phenotype that is characterized by immunodeficiency, muscular hypotonia and anhydrotic ectodermal dysplasia, as well as, in the case of STIM1 deficiency, autoimmunity and lymphoproliferative disease. The immunodeficiency in these patients is due to a severe defect in T cell activation but not in lymphocyte development. This review describes the immunological and non-immunological phenotypes of patients with defects in SOCE and CRAC channel function and discusses them in the context of similar immunodeficiency diseases and animal models of ORAI1 and STIM1 function.

The enigma of the role of protein inhibitor of activated STAT3 (PIAS3) in the immune response

Protein inhibitor of activated STAT3 (PIAS3), the main cellular inhibitor of signal transducers and activator of transcription 3 (STAT3), has been described as a modulator of DNA binding transcription factors. The exploration of the emerging roles of PIAS3 in immune regulation is a growing and fascinating field. Recent discoveries have shed new light on the key role of PIAS3 in the regulation of transcriptional activity, and on the molecular mechanism involved. These findings suggest that the known functions of this signalling molecule are merely the "tip of the iceberg". This article reviews the challenging questions regarding the link between PIAS3 and the intracellular signalling in immune cells. Some of the known functions of PIAS3 that potentially modulate key proteins in the immune system will also be discussed.

B cell-intrinsic signaling through IL-21 receptor and STAT3 is required for establishing long-lived antibody responses in humans

Engagement of cytokine receptors by specific ligands activate Janus kinase–signal transducer and activator of transcription (STAT) signaling pathways. The exact roles of STATs in human lymphocyte behavior remain incompletely defined. Interleukin (IL)-21 activates STAT1 and STAT3 and has emerged as a potent regulator of B cell differentiation. We have studied patients with inactivating mutations in STAT1 or STAT3 to dissect their contribution to B cell function in vivo and in response to IL-21 in vitro. STAT3 mutations dramatically reduced the number of functional, antigen (Ag)-specific memory B cells and abolished the ability of IL-21 to induce naive B cells to differentiate into plasma cells (PCs). This resulted from impaired activation of the molecular machinery required for PC generation. In contrast, STAT1 deficiency had no effect on memory B cell formation in vivo or IL-21–induced immunoglobulin secretion in vitro. Thus, STAT3 plays a critical role in generating effector B cells from naive precursors in humans. STAT3-activating cytokines such as IL-21 thus underpin Ag-specific humoral immune responses and provide a mechanism for the functional antibody deficit in STAT3-deficient patients.

Expression, purification, characterization and crystallization of non- and phosphorylated states of JAK2 and JAK3 kinase domain

Janus-associated kinases (JAKs) play critical roles in cytokine signaling, and have emerged as viable therapeutic targets in inflammation and oncology related diseases. To date, targeting JAK proteins with highly selective inhibitor compounds have remained elusive. We have expressed the active kinase domains for both JAK2 and JAK3 and devised purification protocols to resolve the non-, mono- (Y1007) and diphosphorylated (Y1007 and Y1008) states of JAK2 and non- and monophosphorylated states of JAK3 (Y980). An optimal purified protein yield of 20, 29 and 69mg per 20L cell culture was obtained for the three JAK2 forms, respectively, and 12.2 and 2.3mg per 10L fermentation for the two JAK3 forms allowing detailed biochemical and biophysical studies. To monitor the purification process we developed a novel HPLC activity assay where a sequential order of phosphorylation was observed whereby the first tyrosine residue was completely phosphorylated prior to phosphorylation of the tandem tyrosine residue. A Caliper-based microfluidics assay was used to determine the kinetic parameters (K(m) and k(cat)) for each phosphorylated state, showing that monophosphorylated (Y1007) JAK2 enzyme activity increased 9-fold over that of the nonphosphorylated species, and increased an additional 6-fold for the diphosphorylated (Y1007/Y1008) species, while phosphorylation of JAK3 resulted in a negligible increase in activity. Moreover, crystal structures have been generated for each isolated state of JAK2 and JAK3 with resolutions better than 2.4A. The generation of these reagents has enabled kinetic and structural characterization to inform the design of potent and selective inhibitors of the JAK family.

TH17 cells and regulatory T cells in primary immunodeficiency diseases

After activation by unique cytokines, CD4(+) naive T cells differentiate into lineages of helper/effector (T(H)) and regulatory T (Treg) cells that are characterized by distinct developmental pathways and unique biologic functions. The trusted binary system of T(H)1 and T(H)2 has been expanded to include the IL-17-producing T(H)17 cell lineage, which plays a role in immune responses to infectious agents and maintenance of autoimmune diseases. Acting as counterbalance, Treg cells maintain peripheral tolerance and protect the host from autoaggressive lymphocytes. T(H)1 cells produce IFN-gamma and are involved in cell-mediated immunity, T(H)2 cells produce IL-4 and contribute to humoral immunity, T(H)17 cells generate IL-17 and play an important role in immune responses to fungi and extracellular pathogens, and forkhead box protein 3-positive (FOXP3(+)) Treg cells secrete TGF-beta and IL-10 and downregulate effector T cells. Autosomal dominant hyper-IgE syndrome, a rare primary immunodeficiency disorder, is caused by hypomorphic heterozygous mutations of signal transducer and activator of transcription 3 (STAT3), preventing T(H)17 lineage differentiation and increasing susceptibility to Staphylococcus and Candida species infections. Mutations in the FOXP3 gene interfere with Treg cell development and cause immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome. Other single-gene defects resulting in reduced Treg cell function include CD25, signal transducer and activator of transcription 5b, autoimmune regulator, and Wiskott-Aldrich syndrome protein. These observations emphasize the importance of functionally distinct T-cell lineages in maintaining a balanced innate and cognate immune system.

Loss of STAT5 causes liver fibrosis and cancer development through increased TGF-{beta} and STAT3 activation

The molecular mechanisms underlying the development of hepatocellular carcinoma are not fully understood. Liver-specific signal transducer and activator of transcription (STAT) 5A/B–null mice (STAT5-LKO) were treated with carbon tetrachloride (CCl₄), and histological analyses revealed liver fibrosis and tumors. Transforming growth factor (TGF)–β levels and STAT3 activity were elevated in liver tissue from STAT5-LKO mice upon CCl₄ treatment. To define the molecular link between STAT5 silencing and TGF-β up-regulation, as well as STAT3 activation, we examined STAT5-null mouse embryonic fibroblasts and primary hepatocytes. These cells displayed elevated TGF-β protein levels, whereas messenger RNA levels remained almost unchanged. Protease inhibitor studies revealed that STAT5 deficiency enhanced the stability of mature TGF-β. Immunoprecipitation and immunohistochemistry analyses demonstrated that STAT5, through its N-terminal sequences, could bind to TGF-β and that retroviral-mediated overexpression of STAT5 decreased TGF-β levels. To confirm the in vivo significance of the N-terminal domain of STAT5, we treated mice that expressed STAT5 lacking the N terminus (STAT5-ΔN) with CCl₄. STAT5-ΔN mice developed CCl₄-induced liver fibrosis but no tumors. In conclusion, loss of STAT5 results in elevated TGF-β levels and enhanced growth hormone–induced STAT3 activity. We propose that a deregulated STAT5–TGF-β–STAT3 network contributes to the development of chronic liver disease.

Genetic syndromic immunodeficiencies with antibody defects

This article reviews the major syndromic immunodeficiencies with significant antibody defects, many of which may require intravenous immunogammaglobulin therapy. The authors define syndromic immunodeficiency as an illness associated with a characteristic group of phenotypic abnormalities or laboratory features that comprise a recognizable syndrome. Many are familial with a defined inheritance pattern. Immunodeficiency may not be a major part of the illness and may not be present in all patients; thus, these conditions differ from primary immunodeficiency syndromes, in which immune abnormalities are a consistent and prominent feature of their disease.

Immune dysregulation in primary immunodeficiency disorders

The past several years have brought an increased awareness of the prevalence of autoimmunity and immune dysregulation among patients who have primary immunodeficiency disorders (PIDD). The recent clinical and molecular definition of PIDD, in which the primary defect is in the immunoregulatory compartment of the immune system, has offered insight into the basic mechanisms of immune tolerance, which has provided new targets and new techniques to study immune tolerance in PIDD. Many of these studies have focused on the presence and function of regulatory T (T(REG)) cells in PIDD, particularly since the discovery of murine and human syndromes associated with T(REG) deficiency. This article focuses on the current state of knowledge regarding the role of T(REG) in various PIDD that have clinical features indicative of dysregulated immunity.

Interpretation of cytokine signaling through the transcription factors STAT5A and STAT5B

Transcription factors from the family of Signal Transducers and Activators of Transcription (STAT) are activated by numerous cytokines. Two members of this family, STAT5A and STAT5B (collectively called STAT5), have gained prominence in that they are activated by a wide variety of cytokines such as interleukins, erythropoietin, growth hormone, and prolactin. Furthermore, constitutive STAT5 activation is observed in the majority of leukemias and many solid tumors. Inactivation studies in mice as well as human mutations have provided insight into many of STAT5's functions. Disruption of cytokine signaling through STAT5 results in a variety of cell-specific effects, ranging from a defective immune system and impaired erythropoiesis, the complete absence of mammary development during pregnancy, to aberrant liver function. On a molecular level, STAT5 has been linked to cell specification, proliferation, differentiation, and survival. Evidence is growing that the diverse outcomes of STAT5 signaling are not only determined by the expression of specific receptors but also by the interaction of STAT5 with cofactors and the cell-specific activity of members of the SOCS family, which negatively regulate STAT function. In this review, we focus on emerging concepts and challenges in the field of Janus kinase (JAK)-STAT5 signaling. First, we discuss unique functions of STAT5 in three distinct systems: mammary epithelial cells, hepatocytes, and regulatory T cells. Second, we present an example of how STAT5 can achieve cell specificity in hepatocytes through a physical and functional interaction with the glucocorticoid receptor. Third, we focus on the relevance of STAT5 in the development and progression of leukemia. Next, we discuss lessons derived from human mutations and disease. Finally, we address an emerging issue that the interpretation of experiments from STAT5-deficient mice and cells might be compromised as these cells might reroute and reprogram cytokine signals to the "wrong" STATs and thus acquire inappropriate cues. We propose that mice with mutations in various components of the JAK-STAT signaling pathway are living laboratories, which will provide insight into the versatility of signaling hardware and the adaptability of the software.

Growth hormone enhances thymic function in HIV-1-infected adults

Growth hormone (GH) is an underappreciated but important regulator of T cell development that can reverse age-related declines in thymopoiesis in rodents. Here, we report findings of a prospective randomized study examining the effects of GH on the immune system of HIV-1-infected adults. GH treatment was associated with increased thymic mass. In addition, GH treatment enhanced thymic output, as measured by both the frequency of T cell receptor rearrangement excision circles in circulating T cells and the numbers of circulating naive and total CD4(+) T cells. These findings provide compelling evidence that GH induces de novo T cell production and may, accordingly, facilitate CD4(+) T cell recovery in HIV-1-infected adults. Further, these randomized, prospective data have shown that thymic involution can be pharmacologically reversed in humans, suggesting that immune-based therapies could be used to enhance thymopoiesis in immunodeficient individuals.

Regulatory T cells in primary immunodeficiency diseases

PURPOSE OF REVIEW

Several primary immune deficiency disorders are associated with autoimmunity and malignancy, suggesting a state of immune dysregulation. Here, we review the role of regulatory T-cell deficits in mediating the immune dysregulation associated with certain primary immune deficiency disorder syndromes.

RECENT FINDINGS

Systematic studies in primary immune deficiency disorders and their associated animal models have led to an increased understanding of both central and peripheral tolerance mechanisms, and in particular have yielded new insights into regulatory T-cell function, development and maintenance.

SUMMARY

Single-gene defects identified in patients with multiple autoimmune phenomena have defined new primary immune deficiency disorder syndromes in which the primary deficit is in the establishment or maintenance of immune tolerance. The disorder that has been most informative with regard to understanding the function and development of regulatory T cells is forkhead box P3 deficiency, known as immune dysregulation, polyendocrinopathy, enteropathy and X-linked syndrome in humans and Scurfy in the mouse. Recent studies in patients with other primary immune deficiency disorders, including autoimmune polyendocrinopathy, candidiasis and ectodermal dystrophy syndrome, CD25 deficiency, STAT5b deficiency, and Wiskott-Aldrich syndrome, have added to our understanding of regulatory T-cell biology. The study of patients with rare primary immune deficiency disorder syndromes provides an unparalleled opportunity to understand mechanisms of autoimmunity and immune tolerance in humans.

Genetic disorders in the GH IGF-I axis in mouse and man

Animal knockout experiments have offered the opportunity to study genes that play a role in growth and development. In the last few years, reports of patients with genetic defects in GH-IGF-I axis have greatly increased our knowledge of genetically determined causes of short stature. We will present the animal data and human reports of genetic disorders in the GH-IGF-I axis in order to describe the role of the GH-IGF-I axis in intrauterine and postnatal growth. In addition, the effects of the GH-IGF-I axis on the development and function of different organ systems such as brain, inner ear, eye, skeleton, glucose homeostasis, gonadal function, and immune system will be discussed. The number of patients with genetic defects in the GH-IGF-I axis is small, and a systematic diagnostic approach and selective genetic analysis in a patient with short stature are essential to identify more patients. Finally, the implications of a genetic defect in the GH-IGF-I axis for the patient and the therapeutic options will be discussed.

Growth hormone insensitivity and severe short stature in siblings: a novel mutation at the exon 13-intron 13 junction of the STAT5b gene

BACKGROUND/AIMS

Growth hormone insensitivity (GHI) is characterized by severe short stature, high serum growth hormone (GH), low serum IGF-I and IGFBP-3 levels and is classically associated with genetic defects of the GH receptor (GHR). Recently, mutations of the STAT5b gene have been identified and shown to be associated with GHI and severe IGF deficiency. We investigated 2 sisters from a consanguineous family from Kuwait, with clinical and biochemical features of GHI, in whom no molecular defects in the GHR were identified.

METHODS

Serum and DNA were analyzed.

RESULTS

In addition to GHI, siblings 2 and 1 presented with, respectively, a diagnosis of juvenile idiopathic arthritis and recurrent pulmonary infections. Molecular analysis of the STAT5b gene revealed a novel homozygous deletion of a G at the junction of exon 13-intron 13. The parents, who are of normal height, were heterozygous for the mutation.

CONCLUSIONS

This is the first STAT5b defect to be identified in siblings, further supporting the autosomal recessive mode of transmission of STAT5b deficiency. The results affirm that defective STAT5b is an etiology for IGF deficiency and the GHI phenotype, and emphasize the importance of considering this diagnosis in patients with IGF deficiency, especially when associated with diverse immunological problems.