Jak3, severe combined immunodeficiency, and a new class of immunosuppressive drugs

Protein kinases: drug targets for immunological disorders

Protein kinases play a major role in cellular activation processes, including signal transduction by diverse immunoreceptors. Given their roles in cell growth and death and in the production of inflammatory mediators, targeting kinases has proven to be an effective treatment strategy, initially as anticancer therapies, but shortly thereafter in immune-mediated diseases. Herein, we provide an overview of the status of small molecule inhibitors specifically generated to target protein kinases relevant to immune cell function, with an emphasis on those approved for the treatment of immune-mediated diseases. The development of inhibitors of Janus kinases that target cytokine receptor signalling has been a particularly active area, with Janus kinase inhibitors being approved for the treatment of multiple autoimmune and allergic diseases as well as COVID-19. In addition, TEC family kinase inhibitors (including Bruton's tyrosine kinase inhibitors) targeting antigen receptor signalling have been approved for haematological malignancies and graft versus host disease. This experience provides multiple important lessons regarding the importance (or not) of selectivity and the limits to which genetic information informs efficacy and safety. Many new agents are being generated, along with new approaches for targeting kinases.

JAK-STAT inhibitors in Immune mediated diseases: An Overview

For any biological response, transmission of extracellular signals to the nucleus is required for DNA transcription and gene expression. In that respect, cytokines/chemokines are well-known inflammatory agents which play a critical role in signalling pathways by activating the Janus kinase-signal transducers and activators of transcription (JAK-STAT) signalling proteins (Janus kinase-signal transducers and activators of transcription) which are a group of intracellular kinase molecules. Cytokines are a category of small proteins (∼5-25 kDa) that play a major role in cell signalling and are major drivers of an autoimmune response. Here we will discuss the role of Janus kinase-signal transducers and activators of transcription kinase cascades in the inflammatory-proliferative cascades of autoimmune disease and about the recent progress in the development of oral synthetic Janus kinase inhibitors (JAKi) and their therapeutic efficacies in dermatologic and systemic autoimmune diseases. Therapeutic efficacy of Janus kinase inhibitors is now well established in the treatment of array of autoimmune and inflammatory disease: spondylarthritis with a special focus on psoriatic arthritis (PsA) and its dermatologic manifestations (psoriasis) and ankylosing spondylitis (AS), atopic dermatitis (AD), alopecia areata (AA), rheumatoid arthritis (RA) and inflammatory bowel disease (IBD). In addition to the first-generation Janus kinase inhibitors several new-generation Janus kinase inhibitors are currently being evaluated. It is expected that these Janus kinase inhibitors likely have higher potency and less adverse effects as compared to their predecessors. Here we have discussed: (1) the functional significance of the Janus kinase-signal transducers and activators of transcription kinase cascades in the inflammatory-proliferative processes of autoimmune diseases and its cellular/molecular mechanisms and (2) progress in the development of oral synthetic Janus kinase inhibitors and their therapeutic efficacies in several systemic and cutaneous autoimmune diseases.

Janus kinase inhibitors for the treatment of psoriatic arthritis

Psoriatic arthritis (PsA), a systemic disease, has multi-domain musculoskeletal pathologies along with dermatological manifestations. The current recommendations and the standard of care for the treatment of PsA is to address the domain-based pathologies and the disease severity of the six clinical domains unique to PsA, namely, arthritis of the large and small joints, skin involvement, nail involvement enthesitis, dactylitis and axial disease. With currently available therapies, there are good numbers of primary/secondary non-responders and there are added concerns because of intolerance and adverse effects. In that respect, JAK/STAT inhibitors bring new options for many such patients with psoriasis and PsA. Here, we will discuss currently approved JAK inhibitors for PsA and the others which are in different phases of development, including the TYK2 inhibitors.

JAKs and STATs from a Clinical Perspective: Loss-of-Function Mutations, Gain-of-Function Mutations, and Their Multidimensional Consequences

The JAK/STAT signaling pathway plays a key role in cytokine signaling and is involved in development, immunity, and tumorigenesis for nearly any cell. At first glance, the JAK/STAT signaling pathway appears to be straightforward. However, on closer examination, the factors influencing the JAK/STAT signaling activity, such as cytokine diversity, receptor profile, overlapping JAK and STAT specificity among non-redundant functions of the JAK/STAT complexes, positive regulators (e.g., cooperating transcription factors), and negative regulators (e.g., SOCS, PIAS, PTP), demonstrate the complexity of the pathway's architecture, which can be quickly disturbed by mutations. The JAK/STAT signaling pathway has been, and still is, subject of basic research and offers an enormous potential for the development of new methods of personalized medicine and thus the translation of basic molecular research into clinical practice beyond the use of JAK inhibitors. Gain-of-function and loss-of-function mutations in the three immunologically particularly relevant signal transducers STAT1, STAT3, and STAT6 as well as JAK1 and JAK3 present themselves through individual phenotypic clinical pictures. The established, traditional paradigm of loss-of-function mutations leading to immunodeficiency and gain-of-function mutation leading to autoimmunity breaks down and a more differentiated picture of disease patterns evolve. This review is intended to provide an overview of these specific syndromes from a clinical perspective and to summarize current findings on pathomechanism, symptoms, immunological features, and therapeutic options of STAT1, STAT3, STAT6, JAK1, and JAK3 loss-of-function and gain-of-function diseases.

The role of upadacitinib in the treatment of ulcerative colitis

The JAK signaling pathway plays a major role in the immunopathology of autoimmune diseases, including inflammatory bowel disease. JAK enzymes provide novel targets for rapidly effective inflammatory bowel disease therapy, particularly in ulcerative colitis. Upadacitinib is a targeted JAK1 inhibitor. In multiple phase III clinical trials, upadacitinib has demonstrated significant improvement in clinical and endoscopic outcomes and quality of life for patients with moderate-to-severe ulcerative colitis. In this drug evaluation we describe the role of the JAK signaling pathway in ulcerative colitis, the mechanism of action of upadacitinib and the current clinical evidence for its use in ulcerative colitis; we also review its safety and tolerability, including for special populations.

Network Approaches to Uncover Pathogenesis and Therapeutic Targets of Inflammatory Bowel Diseases

Inflammatory bowel diseases (IBD) are currently recognized to involve chronic intestinal inflammation in genetically susceptible individuals. Patients with IBD mainly develop gastrointestinal inflammation, but it is sometimes accompanied by extraintestinal manifestations such as arthritis, erythema nodosum, episcleritis, pyoderma gangrenosum, uveitis, and primary sclerosing cholangitis. These clinical aspects imply the importance of interorgan networks in IBD. In the gastrointestinal tract, immune cells are influenced by multiple local environmental factors including microbiota, dietary environment, and intercellular networks, which further alter molecular networks in immune cells. Therefore, deciphering networks at interorgan, intercellular, and intracellular levels should help to obtain a comprehensive understanding of IBD. This review focuses on the intestinal immune system, which governs the physiological and pathological functions of the digestive system in harmony with the other organs.

Selective JAK1 inhibitors for the treatment of inflammatory bowel disease

Janus kinase (JAK) inhibitors, also known as jakinibs, are third-generation oral small molecules that have expanded the therapeutic options for the management of chronic inflammatory diseases, including inflammatory bowel disease (IBD). Tofacitinib, a pan-JAK inhibitor, has spearheaded the new JAK class for IBD treatment. Unfortunately, serious adverse effects, including cardiovascular complications such as pulmonary embolism and venous thromboembolism or even death from any cause, have been reported for tofacitinib. However, it is anticipated that next-generation selective JAK inhibitors may limit the development of serious adverse events, leading to a safer treatment course with these novel targeted therapies. Nevertheless, although this drug class was recently introduced, following the launch of second-generation biologics in the late 1990s, it is breaking new ground and has been shown to efficiently modulate complex cytokine-driven inflammation in both preclinical models and human studies. Herein, we review the clinical opportunities for targeting JAK1 signaling in the pathophysiology of IBD, the biology and chemistry underpinning these target-selective compounds, and their mechanisms of actions. We also discuss the potential for these inhibitors in efforts to balance their benefits and harms.

Novel Therapeutic Approaches to Psoriasis and Risk of Infectious Disease

Psoriasis is a chronic immune-mediated skin and joint disease, with a plethora of comorbidities, characterized by a certain genetic predisposition, and a complex pathogenesis based on the IL-23/IL-17 pathway. There is no doubt that the patients affected by psoriasis are more susceptible to infections as well as that the risk of infection is higher in psoriatic subjects than in the general population. The advent of biotechnological agents on the therapeutic arsenal actually available for the treatment of moderate-to-severe patients, given the fact that the severity of the disease is a predictor of the level of infectious risk, has raised the question of whether these ‘new’ drugs could be considered a safer option and how they can be used in selected cases. Old and newer strategies in cases of chronic infectious conditions are reviewed under the light of clinical trials and other studies present in literature.

Janus kinase-signal transducers and activators of transcription cell signaling in Spondyloarthritis: rationale and evidence for JAK inhibition

PURPOSE OF REVIEW

The Janus kinase-signal transducers and activators of transcription (JAK-STAT) signaling proteins represent a group of intracellular kinase molecules that play a central role in the signaling pathways induced by cytokines, chemokines, and certain growth factors associated with systemic and local inflammation of autoimmune diseases including in Spondyloarthritis (SpA). Here, we will discuss (i) the functional significance of the JAK-STAT kinase cascades in the inflammatory-proliferative processes of SpA and its cellular/molecular mechanisms (ii) progress in the development of oral synthetic JAK inhibitors (JAKi) and their therapeutic efficacies in SpA.

RECENT FINDINGS

Development JAKi is a fast-moving field in the medical science. Several new-generation JAKi are being identified for psoriatic arthritis and ankylosing spondylitis. It is expected these JAKi likely to have higher potency and less adverse effects.

SUMMARY

Here, we are providing an updated review on the significance of JAK-STAT signaling proteins in SpA with an emphasis on new-generation of JAK-STAT inhibitors for the treatment of SpA.

Failure of tofacitinib to achieve an objective response in a DDX3X-MLLT10 T-lymphoblastic leukemia with activating JAK3 mutations

T-cell lymphoblastic lymphoma/T-cell acute lymphoblastic leukemia (T-LBL/T ALL) is an aggressive hematological malignancy arising from malignant transformation of T-cell progenitors with poor prognosis in adult patients. Outcomes are particularly dismal in the relapsed/refractory setting, and therapeutic options are limited in this context. Genomic profiling has shown frequent aberrations in the JAK-STAT pathway, including recurrent mutations in JAK3 (15%–20% of T-ALL cases), suggesting that JAK kinase inhibition may be a promising therapeutic approach. Activating JAK3 mutations are capable of transforming cytokine-dependent progenitor cells in vitro and causing T-ALL-like disease when expressed in hematopoietic progenitors in vivo. We describe a case of relapsed T-ALL in an adult patient, with two JAK3 activating mutations identified by whole-exome sequencing (WES), leading to hypothesis-based treatment with the JAK1 and JAK3 inhibitor, tofacitinib, following failure of salvage chemotherapy reinduction. Despite the molecularly targeted rationale, tofacitinib did not induce an objective clinical response. Our report suggests that the presence of activating JAK3 mutations does not necessarily confer sensitivity to pharmacological JAK3 inhibition.

JAK Inhibitors: What Is New?

PURPOSE OF THE REVIEW

Three Janus kinase (JAK) inhibitors are approved for rheumatoid arthritis (RA) with a fourth awaiting approval. Multiple clinical trial results with these molecules have recently been reported. These were the first small molecule oral targeted synthetic DMARDs (tsDMARDs) to be approved for RA.

RECENT FINDINGS

Preclinical studies have suggested differential affinity for JAK isoform inhibition but it is not presently clear that there is any difference in efficacy in the clinic with these therapies. Preliminary data has suggested that filgotinib may have a modestly different safety profile but lacking direct comparisons, this will be difficult to confirm. Long-term safety studies have suggested similar safety signals to biologics although a possible signal for VTE/PE risk has been noted with tofacitinib and baricitinib. Having an oral small molecule such as the JAK inhibitors with similar or better efficacy than biologics has been a major advance in RA treatment.

JAK3 restrains inflammatory responses and protects against periodontal disease through Wnt3a signaling

Homeostasis between pro- and anti- inflammatory responses induced by bacteria is critical for the maintenance of health. In the oral cavity, pro-inflammatory mechanisms induced by pathogenic bacteria are well-established; however, the anti-inflammatory responses that act to restrain innate responses remain poorly characterized. Here, we demonstrate that infection with the periodontal pathogen Porphyromonas gingivalis enhances the activity of Janus kinase 3 (JAK3) in innate immune cells, and subsequently phospho-inactivates Nedd4-2, an ubiquitin E3 ligase. In turn, Wingless-INT (Wnt) 3 (Wnt3) ubiquitination is decreased, while total protein levels are enhanced, leading to a reduction in pro-inflammatory cytokine levels. In contrast, JAK3 or Wnt3a inhibition robustly enhances nuclear factor kappa-light-chain-enhancer of activated B cells activity and the production of pro-inflammatory cytokines in P. gingivalis-stimulated innate immune cells. Moreover, using gain- and loss-of-function approaches, we demonstrate that downstream molecules of Wnt3a signaling, including Dvl3 and β-catenin, are responsible for the negative regulatory role of Wnt3a. In addition, using an in vivo P. gingivalis-mediated periodontal disease model, we show that JAK3 inhibition enhances infiltration of inflammatory cells, reduces expression of Wnt3a and Dvl3 in P. gingivalis-infected gingival tissues, and increases disease severity. Together, our results reveal a new anti-inflammatory role for JAK3 in innate immune cells and show that the underlying signaling pathway involves Nedd4-2-mediated Wnt3a ubiquitination.

New insights into the novel anti-inflammatory mode of action of glucocorticoids

Inflammation is a physiological intrinsic host response to injury meant for removal of noxious stimuli and maintenance of homeostasis. It is a defensive body mechanism that involves immune cells, blood vessels and molecular mediators of inflammation. Glucocorticoids (GCs) are steroidal hormones responsible for regulation of homeostatic and metabolic functions of body. Synthetic GCs are the most useful anti-inflammatory drugs used for the treatment of chronic inflammatory diseases such as asthma, chronic obstructive pulmonary disease (COPD), allergies, multiple sclerosis, tendinitis, lupus, atopic dermatitis, ulcerative colitis, rheumatoid arthritis and osteoarthritis whereas, the long term use of GCs are associated with many side effects. The anti-inflammatory and immunosuppressive (desired) effects of GCs are usually mediated by transrepression mechanism whereas; the metabolic and toxic (undesired) effects are usually manifested by transactivation mechanism. Though GCs are most potent anti-inflammatory and immunosuppressive drugs, the common problem associated with their use is GC resistance. Several research studies are rising to comprehend these mechanisms, which would be helpful in improving the GC resistance in asthma and COPD patients. This review aims to focus on identification of new drug targets in inflammation which will be helpful in the resolution of inflammation. The ample understanding of GC mechanisms of action helps in the development of novel anti-inflammatory drugs for the treatment of inflammatory and autoimmune disease with reduced side effects and minimal toxicity.

Emerging Topical and Systemic JAK Inhibitors in Dermatology

Accumulating data on cellular and molecular pathways help to develop novel therapeutic strategies in skin inflammation and autoimmunity. Examples are psoriasis and atopic dermatitis, two clinically and immunologically well-defined disorders. Here, the elucidation of key pathogenic factors such as IL-17A/IL-23 on the one hand and IL-4/IL-13 on the other hand profoundly changed our therapeutic practice. The knowledge on intracellular pathways and governing factors is shifting our attention to new druggable molecules. Multiple cytokine receptors signal through Janus kinases (JAKs) and associated signal transducer and activators of transcription (STATs). Inhibition of JAKs can simultaneously block the function of multiple cytokines. Therefore, JAK inhibitors (JAKi) are emerging as a new class of drugs, which in dermatology can either be used systemically as oral drugs or locally in topical formulations. Inhibition of JAKs has been shown to be effective in various skin disorders. The first oral JAKi have been recently approved for the treatment of rheumatoid arthritis and psoriatic arthritis. Currently, multiple inhibitors of the JAK/STAT pathway are being investigated for skin diseases like alopecia areata, atopic dermatitis, dermatomyositis, graft-versus-host-disease, hidradenitis suppurativa, lichen planus, lupus erythematosus, psoriasis, and vitiligo. Here, we aim to discuss the immunological basis and current stage of development of JAKi in dermatology.

JAK/STAT signaling in regulation of innate lymphoid cells: The gods before the guardians

Immunity to pathogens is ensured through integration of early responses mediated by innate cells and late effector functions taking place after terminal differentiation of adaptive lymphocytes. In this context, innate lymphoid cells (ILCs) and adaptive T cells represent a clear example of how prototypical effector functions, including polarized expression of cytokines and/or cytotoxic activity, can occur with overlapping modalities but different timing. The ability of ILCs to provide early protection relies on their poised epigenetic state, which determines their propensity to quickly respond to cytokines and to activate specific patterns of signal-dependent transcription factors. Cytokines activating the Janus kinases (JAKs) and members of the signal transducer and activator of transcription (STAT) pathway are key regulators of lymphoid development and sustain the processes underlying T-cell activation and differentiation. The role of the JAK/STAT pathway has been recently extended to several aspects of ILC biology. Here, we discuss how JAK/STAT signals affect ILC development and effector functions in the context of immune responses, highlighting the molecular mechanisms involved in regulation of gene expression as well as the potential of targeting the JAK/STAT pathway in inflammatory pathologies.

Opportunities and challenges for the development of covalent chemical immunomodulators

Compounds that react irreversibly with cysteines have reemerged as potent and selective tools for altering protein function, serving as chemical probes and even clinically approved drugs. The exquisite sensitivity of human immune cell signaling pathways to oxidative stress indicates the likely, yet still underexploited, general utility of covalent probes for selective chemical immunomodulation. Here, we provide an overview of immunomodulatory cysteines, including identification of electrophilic compounds available to label these residues. We focus our discussion on three protein classes essential for cell signaling, which span the 'druggability' spectrum from amenable to chemical probes (kinases), somewhat druggable (proteases), to inaccessible (phosphatases). Using existing inhibitors as a guide, we identify general strategies to guide the development of covalent probes for selected undruggable classes of proteins and propose the application of such compounds to alter immune cell functions.

Essential Kinases and Transcriptional Regulators and Their Roles in Autoimmunity

Kinases and transcriptional regulators are fundamental components of cell signaling that are expressed on many types of immune cells which are involved in secretion of cytokines, cell proliferation, differentiation, and apoptosis. Both play important roles in biological responses in health as well as in illnesses such as the autoimmune diseases which comprise at least 80 disorders. These diseases are caused by complex genetic and environmental interactions that lead to a breakage of immunologic tolerance and a disruption of the balance between self-reactive cells and regulatory cells. Kinases or transcriptional regulatory factors often have an abnormal expression in the autoimmune cells that participate in the pathogenesis of autoimmune disease. These abnormally expressed kinases or transcriptional regulators can over-activate the function of self-reactive cells to produce inflammatory cytokines or down-regulate the activity of regulatory cells, thus causing autoimmune diseases. In this review we introduce five kinds of kinase and transcriptional regulator related to autoimmune diseases, namely, members of the Janus kinase (JAK) family (JAK3 and/or tyrosine kinase 2 (TYK2)), fork head box protein 3 (Foxp3), the retinoic acid-related orphan receptor gamma t (RORγt), and T-box expressed in T cells (T-bet) factors. We also provide a mechanistic insight into how these kinases and transcriptional regulators affect the function of the immune cells related to autoimmune diseases, as well as a description of a current drug design targeting these kinases and transcriptional regulators. Understanding their exact role helps offer new therapies for control of the inflammatory responses that could lead to clinical improvement of the autoimmune diseases.

Hyperactivation of Oncogenic JAK3 Mutants Depend on ATP Binding to the Pseudokinase Domain

Janus kinase 3 (JAK3) tyrosine kinase has a central role in the control of lymphopoiesis, and mutations in JAK3 can lead to either severe combined immunodeficiency or leukemia and lymphomas. JAK3 associates with the common gamma chain (γc) receptor and functions in a heteromeric signaling pair with JAK1. In IL-2 signaling JAK1 is the effector kinase for STAT5 phosphorylation but the precise molecular regulatory mechanisms of JAK1 and JAK3 and their individual domains are not known. The pseudokinase domain (JAK homology 2, JH2) of JAK3 is of particular interest as approximately half of clinical JAK3 mutations cluster into it.

In this study, we investigated the role of JH2s of JAK1 and JAK3 in IL-2R signaling and show that STAT5 activation requires both JH1 and JH2 of JAK1, while both JH1 and JH2 in JAK3 are specifically required for the cytokine-induction of cellular signaling. Characterization of recombinant JAK3 JH2 in thermal shift assay shows an unstable protein domain, which is strongly stabilized by ATP binding. Unexpectedly, nucleotide binding to JAK3 JH2 was found to be cation-independent. JAK3 JH2 showed higher nucleotide binding affinity in MANT-ATP and fluorescent polarization competition assays compared to the other JAK JH2s. Analysis of the functional role of ATP binding in JAK3 JH2 in cells and in zebrafish showed that disruption of ATP binding suppresses ligand-independent activation of clinical JAK3 gain-of-function mutations residing in either JH2 or JH1 but does not inhibit constitutive activation of oncogenic JAK1. ATP-binding site mutations in JAK3 JH2 do not, however, abrogate normal IL-2 signaling making them distinct from JH2 deletion or kinase-deficient JAK3. These findings underline the importance of JAK3 JH2 for cellular signaling in both ligand-dependent and in gain-of-function mutation-induced activation. Furthermore, they identify the JH2 ATP-binding site as a key regulatory region for oncogenic JAK3 signaling, and thus a potential target for therapeutic modulation.

Development, Optimization, and Structure-Activity Relationships of Covalent-Reversible JAK3 Inhibitors Based on a Tricyclic Imidazo[5,4- d]pyrrolo[2,3- b]pyridine Scaffold

Janus kinases are major drivers of immune signaling and have been the focus of anti-inflammatory drug discovery for more than a decade. Because of the invariable colocalization of JAK1 and JAK3 at cytokine receptors, the question if selective JAK3 inhibition is sufficient to effectively block downstream signaling has been highly controversial. Recently, we discovered the covalent-reversible JAK3 inhibitor FM-381 (23) featuring high isoform and kinome selectivity. Crystallography revealed that this inhibitor induces an unprecedented binding pocket by interactions of a nitrile substituent with arginine residues in JAK3. Herein, we describe detailed structure-activity relationships necessary for induction of the arginine pocket and the impact of this structural change on potency, isoform selectivity, and efficacy in cellular models. Furthermore, we evaluated the stability of this novel inhibitor class in in vitro metabolic assays and were able to demonstrate an adequate stability of key compound 23 for in vivo use.

Immunologic effects of chronic administration of tofacitinib, a Janus kinase inhibitor, in cynomolgus monkeys and rats - Comparison of juvenile and adult responses

Tofacitinib, an oral Janus kinase (JAK) inhibitor for treatment of rheumatoid arthritis, targets JAK1, JAK3, and to a lesser extent JAK2 and TYK2. JAK1/3 inhibition impairs gamma common chain cytokine receptor signaling, important in lymphocyte development, homeostasis and function. Adult and juvenile cynomolgus monkey and rat studies were conducted and the impact of tofacitinib on immune parameters (lymphoid tissues and lymphocyte subsets) and function (T-dependent antibody response (TDAR), mitogen-induced T cell proliferation) assessed. Tofacitinib administration decreased circulating T cells and NK cells in juvenile and adult animals of both species. B cell decreases were observed only in rats. These changes and decreased lymphoid tissue cellularity are consistent with the expected pharmacology of tofacitinib. No differences were observed between juvenile and adult animals, either in terms of doses at which effects were observed or differential effects on immune endpoints. Lymphomas were observed in three adult monkeys. Tofacitinib impaired the primary TDAR in juvenile monkeys, although a recall response was generated. Complete or partial reversal of the effects on the immune system was observed.

Janus kinase/signal transducer and activator of transcription pathways in spondyloarthritis

PURPOSE OF REVIEW

Cytokines are major drivers of autoimmunity, and biologic agents targeting cytokines have revolutionized the treatment of immune-mediated diseases. Janus kinase/signal transducer and activator of transcription (JAK-STAT) pathway represents a group of several intracellular molecules with a key role in signal pathways activated by growth factors and cytokines. These kinase proteins are associated with the signaling process of multiple key cytokines, which regulates various T-cell subpopulations and their effector cytokines. Development of novel drugs to inhibit this kinase cascade is an emerging field in clinical immunology. Thus, it is essential to have insights about the regulatory role of the JAK-STAT cytokine signaling in relation to autoimmune diseases and its applications in spondyloarthritis.

RECENT FINDINGS

JAK-STAT kinase signaling proteins have been extensively studied in rheumatoid arthritis. Initial observations suggest that JAK-STAT kinase signaling cascade regulates activation and proliferation of the IL17 effector memory T cells and thus has a potential role in the pathogenesis of psoriasis, psoriatic arthritis and ankylosing spondylitis.

SUMMARY

Here, we provide an overview of the clinical rheumatologists about the significance of JAK-STAT signaling system in rheumatic diseases and introduce the potential application of JAK and STAT inhibitors in spondyloarthritis.

Jak3 deficiency blocks innate lymphoid cell development

Loss-of-function mutations in the tyrosine kinase JAK3 cause autosomal recessive severe combined immunodeficiency (SCID). Defects in this form of SCID are restricted to the immune system, which led to the development of immunosuppressive JAK inhibitors. We find that the B6.Cg-Nr1d1^tm1Ven/LazJ mouse line purchased from Jackson Laboratories harbors a spontaneous mutation in Jak3, generating a SCID phenotype and an inability to generate antigen-independent professional cytokine-producing innate lymphoid cells (ILCs). Mechanistically, Jak3 deficiency blocks ILC differentiation in the bone marrow at the ILC precursor and the pre-NK cell progenitor. We further demonstrate that the pan-JAK inhibitor tofacitinib and the specific JAK3 inhibitor PF-06651600 impair the ability of human intraepithelial ILC1 (iILC1) to produce IFN-γ, without affecting ILC3 production of IL-22. Both inhibitors impaired the proliferation of iILC1 and ILC3 and differentiation of human ILC in vitro. Tofacitinib is currently approved for the treatment of moderate-to-severely active rheumatoid arthritis. Both tofacitinib and PF-06651600 are currently in clinical trials for several other immune-mediated conditions. Our data suggest that therapeutic inhibition of JAK may also impact ILCs and, to some extent, underlie clinical efficacy.

Malignancy and Janus Kinase Inhibition

The use of biologics such as anti-tumor necrosis factor and oral Janus kinase inhibitors have revolutionized the treatment of rheumatoid arthritis (RA). The risk of malignancies such as lymphomas, lung cancer, and nonmelanoma skin cancers (NMSCs) is greater in patients with RA compared with the general population. The incidence of all malignancy (excluding NMSC) was similar in tofacitinib users compared with the general population. The rates of overall and site-specific malignancies in patients with RA treated with tofacitinib are similar to what is expected in the RA population and not different from disease-modifying antirheumatic drugs and biologics.

Recent advances in JAK3 inhibition: Isoform selectivity by covalent cysteine targeting

Janus kinases (JAKs) are a family of four cytosolic protein kinases with a high degree of structural similarity. Due to its very restricted role in immune regulation, JAK3 was promoted as an excellent target for immunosuppression for more than a decade, but clinical validation of this concept is still elusive. During the last years, speculation arose that kinase activity of JAK1, which cooperates with JAK3 in cytokine receptor signaling, may have a dominant role over the one of JAK3. Until recently, however, this issue could not be appropriately addressed due to a lack of highly isoform-selective tool compounds. With the recent resurgence of covalent drugs, targeting of a specific cysteine that distinguishes JAK3 from other JAK family members became an attractive design option. By applying this strategy, a set of JAK3 inhibitors with excellent selectivity against other JAK isoforms and the kinome was developed during the last three years and used to decipher JAK3-dependent signaling. The data obtained with these tool compounds demonstrates that selective JAK3 inhibition is sufficient to block downstream signaling. Since one of these inhibitors is currently under evaluation in phase II clinical studies against several inflammatory disorders, it will soon become apparent whether selective JAK3 inhibition translates into clinical efficacy.

The Selective JAK1/3-Inhibitor R507 Mitigates Obliterative Airway Disease Both With Systemic Administration and Aerosol Inhalation

BACKGROUND

The efficacy of selective Janus kinase 1/3 inhibitor R507 to prevent obliterative airway disease was analyzed in preclinical airway transplantation models.

METHODS

Orthotopic trachea transplantations were performed between Lewis donors and Brown Norway rat recipients. Oral everolimus (4 mg/kg once per day) or oral respective inhaled R507 (60 mg/kg twice per day, each) was used for immunosuppression. Grafts were retrieved after 6 or 60 days. Toxicity and anti-inflammatory effects of R507 were analyzed on human airway epithelial cells.

RESULTS

In 6-day animals, oral and inhaled R507 more potently diminished mononuclear graft infiltration than everolimus and preserved ciliated pseudostratified columnar respiratory epithelium. Everolimus and R507 similarly suppressed systemic cellular and humoral immune activation. In untreated rats, marked obliterative airway disease developed over 60 days. Oral and inhaled R507 was significantly more effective in reducing airway obliteration and preserved the morphology of the airway epithelium. Luciferase-positive donors revealed that a substantial amount of smooth muscle cells within the obliterative tissue was of donor origin. Only everolimus but not R507, adversely altered kidney function and lipid profiles. The R507 aerosol did not show airway toxicity in vitro but effectively suppressed activation of inflammatory signaling pathways induced by IL-1β.

CONCLUSIONS

The Janus kinase 1/3 inhibitor R507 is a very well-tolerated immunosuppressant that similarly diminished obliterative airway disease with systemic or inhaled administration.

Future therapeutic targets in rheumatoid arthritis?

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by persistent joint inflammation. Without adequate treatment, patients with RA will develop joint deformity and progressive functional impairment. With the implementation of treat-to-target strategies and availability of biologic therapies, the outcomes for patients with RA have significantly improved. However, the unmet need in the treatment of RA remains high as some patients do not respond sufficiently to the currently available agents, remission is not always achieved and refractory disease is not uncommon. With better understanding of the pathophysiology of RA, new therapeutic approaches are emerging. Apart from more selective Janus kinase inhibition, there is a great interest in the granulocyte macrophage-colony stimulating factor pathway, Bruton's tyrosine kinase pathway, phosphoinositide-3-kinase pathway, neural stimulation and dendritic cell-based therapeutics. In this review, we will discuss the therapeutic potential of these novel approaches.

Selective JAK3 Inhibitors with a Covalent Reversible Binding Mode Targeting a New Induced Fit Binding Pocket

Janus kinases (JAKs) are a family of cytoplasmatic tyrosine kinases that are attractive targets for the development of anti-inflammatory drugs given their roles in cytokine signaling. One question regarding JAKs and their inhibitors that remains under intensive debate is whether JAK inhibitors should be isoform selective. Since JAK3 functions are restricted to immune cells, an isoform-selective inhibitor for JAK3 could be especially valuable to achieve clinically more useful and precise effects. However, the high degree of structural conservation makes isoform-selective targeting a challenging task. Here, we present picomolar inhibitors with unprecedented kinome-wide selectivity for JAK3. Selectivity was achieved by concurrent covalent reversible targeting of a JAK3-specific cysteine residue and a ligand-induced binding pocket. We confirmed that in vitro activity and selectivity translate well into the cellular environment and suggest that our inhibitors are powerful tools to elucidate JAK3-specific functions.

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Identification and characterization of novel covalent reversible JAK3 inhibitors

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Picomolar affinities along with both high isoform and kinome selectivity is achieved

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Covalent-reversible interaction and a new induced binding pocket confirmed by X-ray structures

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High potency and selectivity are successfully proven in cellular models

Efficacy of tofacitinib in treatment of alopecia universalis in two patients

BACKGROUND

Autoimmune-triggered non-scarring hair loss is a feature of alopecia areata (AA). Initially patchy and often self-limited, severe hair loss forms include the complete loss of scalp hair or alopecia totalis (AT) and complete loss of all hair or alopecia universalis (AU). For AT and AU a reliable treatment has remained elusive. The targeted kinase inhibitor tofacitinib, in current use for treatment of other immune diseases, has been hypothesized as a viable option for AA, AT and AU therapy and a few case reports support this.

OBJECTIVE

Our study aims to provide evidence for the effectiveness of tofacitinib in the treatment of AU.

METHODS

Two patients diagnosed with long-term AU were prescribed tofacitinib citrate at a dosage of 5 mg twice daily and observed for eight months.

RESULTS

In the first patient, beard growth was significant by 3 months of treatment. By 6 months of treatment, hair growth was apparent throughout the entire body. By 8 months of treatment, scalp hair continued to grow longer and thicker. In addition, eyelashes and eyebrows were established. In the second patient, a noticeable increase in scalp hair was present just 1 month into treatment. By 4 months into treatment, significant scalp regrowth was observed as well as eyelash, eyebrow and beard regrowth. Axillary hair regrowth and isolated leg hair was noted by 8 months.

CONCLUSION

In our patients, tofacitinib successfully alleviated AU in the absence of significant adverse side-effects. We recommend that further study be required to establish safety and confirm efficacy.

Innate immune regulation by STAT-mediated transcriptional mechanisms

The term innate immunity typically refers to a quick but non-specific host defense response against invading pathogens. The innate immune system comprises particular immune cell populations, epithelial barriers, and numerous secretory mediators including cytokines, chemokines, and defense peptides. Innate immune cells are also now recognized to play important contributing roles in cancer and pathological inflammatory conditions. Innate immunity relies on rapid signal transduction elicited upon pathogen recognition via pattern recognition receptors (PRRs) and cell:cell communication conducted by soluble mediators, including cytokines. A majority of cytokines involved in innate immune signaling use a molecular cascade encompassing receptor-associated Jak protein tyrosine kinases and STAT (signal transducer and activator of transcription) transcriptional regulators. Here, we focus on roles for STAT proteins in three major innate immune subsets: neutrophils, macrophages, and dendritic cells (DCs). While knowledge in this area is only now emerging, understanding the molecular regulation of these cell types is necessary for developing new approaches to treat human disorders such as inflammatory conditions, autoimmunity, and cancer.

A novel water-soluble benzothiazole derivative BD926 inhibits human activated T cell proliferation by down-regulating the STAT5 activation

Immunosuppressants are widely used for treatment of T cell-mediated autoimmune diseases and allogeneic graft rejection. However, because of the toxicity and tolerance of these drugs, novel immunosuppressants are urgently needed. We synthesized a series of novel water-soluble benzothiazole derivatives and found that BD926 [sodium 2-(benzo[d]thiazol-2-yl)-4,5,6,7-tetrahydro-2H-indazol-3-olate] had potent immunosuppressive activity. Treatment with BD926 significantly inhibited anti-CD3/anti-CD28 and alloantigen-induced human T cell proliferation as well as IL2-stimulated activated T cell proliferation in a dose-dependent manner in vitro. BD926 had no obvious cytotoxicity against human resting T cells, IL-4 treated activated T cells and fibroblast-like synoviocytes in our experimental conditions. Furthermore, BD926 induced cell cycle arrest at G0/G1 phase and inhibited the cyclin D3 and CDK 6 expression in activated T cells. BD926 inhibited the STAT5, but not Akt and p70S6K, phosphorylation in a dose-dependent manner in the IL-2-treated activated T cells. Interestingly, BD926 inhibited IFN-γ, IL-6 and IL-17, but not IL-2, IL-4 and IL-10, production in activated T cells. Finally, treatment with BD926 reduced delayed-type hypersensitivity in mice in a dose-dependent manner. Collectively, these data suggest that BD926 may be a lead compound for the design and development of new immunosuppressants for the intervention of allograft rejection and autoimmune diseases.

Combined immunodeficiency evolving into predominant CD4+ lymphopenia caused by somatic chimerism in JAK3

Purpose

Idiopathic CD4 lymphopenia constitutes a heterogeneous group of immunodeficiencies with characteristically low CD4+ T-cell counts with largely unknown genetic etiology. We here sought to determine the underlying molecular cause in an index family with two patients suffering from combined immunodeficiency that evolved into predominant CD4+ lymphopenia. The more severely affected index patient also presented with selective antibody deficiency against bacterial polysaccharide antigens.

Methods

For the genetic analysis, we used combined homozygosity mapping and exome sequencing. Functional assays included immunoblot analysis, flow cytometry and TCR Vβ spectratyping.

Results

A novel homozygous missense mutation was revealed in the kinase domain of JAK3 (c.T3196C, p.Cys1066Arg). Further analysis showed revertant chimerism in CD8+ T-cells in both patients. The additional presence of revertant CD4+ T-cells was associated with a milder clinical and immunological phenotype in the second patient, although the role somatic chimerism plays in amelioration of disease phenotype is uncertain, as presence of revertant cells had no effect on residual CD4 cell JAK3 signaling function. Residual activity of JAK3-dependent STAT3 and STAT5 signaling was also found in immortalized B-cell lines indicating a hypomorphic nature of the described mutation which likely contributes to the milder clinical phenotype.

Conclusions

We here present the first case of revertant mosaicism in JAK3 deficiency, manifesting as combined immunodeficiency evolving into predominant CD4+ lymphopenia. Revertant chimerism or hypomorphic mutations in genes typically associated with more severe T-cell deficiency should be considered when assessing patients with milder forms of combined immunodeficiencies.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-014-0088-2) contains supplementary material, which is available to authorized users.

Targeting the JAK/STAT pathway in cytotoxic T lymphocytes (CTL) by Next Generation Sequencing (NGS)

Next Generation Sequencing (NGS), together with our evolving knowledge of genes and disease, is likely to change the current practice of medicine and public health by facilitating more accurate, sophisticated, and cost-effective genetic testing. Here, we propose a new molecular approach by using MiSeq Sequencing System (Illumina) to investigate the presence of mutations/variants in genes of JAK/STAT pathway involved in different cytotoxic T lymphocytes (CTL)-mediated immune disorders and to develop and validate new and less expensive molecular protocol based on Next Generation Sequencing.

Protein kinase inhibitors in the treatment of inflammatory and autoimmune diseases

Protein kinases mediate protein phosphorylation, which is a fundamental component of cell signalling, with crucial roles in most signal transduction cascades: from controlling cell growth and proliferation to the initiation and regulation of immunological responses. Aberrant kinase activity is implicated in an increasing number of diseases, with more than 400 human diseases now linked either directly or indirectly to protein kinases. Protein kinases are therefore regarded as highly important drug targets, and are the subject of intensive research activity. The success of small molecule kinase inhibitors in the treatment of cancer, coupled with a greater understanding of inflammatory signalling cascades, has led to kinase inhibitors taking centre stage in the pursuit for new anti-inflammatory agents for the treatment of immune-mediated diseases. Herein we discuss the main classes of kinase inhibitors; namely Janus kinase (JAK), mitogen-activated protein kinase (MAPK) and spleen tyrosine kinase (Syk) inhibitors. We provide a mechanistic insight into how these inhibitors interfere with kinase signalling pathways and discuss the clinical successes and failures in the implementation of kinase-directed therapeutics in the context of inflammatory and autoimmune disorders.

Inhibition of Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling pathway in rheumatoid synovial fibroblasts using small molecule compounds

Janus kinase (JAK) inhibitors have been developed as anti-inflammatory agents and have demonstrated clinical efficacy in rheumatoid arthritis (RA). We investigated if JAK-3-selective inhibition alone could disrupt cytokine signalling in rheumatoid synovial fibroblasts. In-vitro studies were performed using synovial fibroblasts isolated from patients with RA. Levels of activated JAK and signal transducer and activator of transcription (STAT) proteins were detected by immunoblot analysis. Target-gene expression levels were measured by reverse transcription-polymerase chain reaction (RT-PCR) or real-time PCR. The JAK inhibitors CP-690,550 and INCB028050 both suppressed activation of JAK-1/-2/-3 and downstream STAT-1/-3/-5, as well as the expression levels of target proinflammatory genes (MCP-I, SAA1/2) in oncostatin-M (OSM)-stimulated rheumatoid synovial fibroblasts. In contrast, the JAK-3-selective inhibitor, PF-956980, suppressed STAT-1/-5 activation but did not affect STAT-3 activation in OSM-stimulated rheumatoid synovial fibroblasts. In addition, PF-956980 significantly suppressed MCP-1 gene expression, but did not block SAA1/2 gene expression in OSM-stimulated rheumatoid synovial fibroblasts. These data suggest that JAK-3-selective inhibition alone is insufficient to control STAT-3-dependent signalling in rheumatoid synovial fibroblasts, and inhibition of JAKs, including JAK-1/-2, is needed to control the proinflammatory cascade in RA.

Jakpot! New small molecules in autoimmune and inflammatory diseases

Cytokines are key mediators of the development and homeostasis of haematopoietic cells, critical for host defense, but also for the development of autoimmune and inflammatory diseases such as psoriasis or rheumatoid arthritis (RA). Blocking cytokines activity by interfering with the ligand-receptor association has been successfully employed to treat several immune disorders. A subgroup of cytokines signals through receptors requiring the association with a family of cytoplasmic protein tyrosine kinases known as Janus kinases (Jaks). Jaks have recently gained significant attention as therapeutic targets in inflammation and autoimmunity, and several Jak inhibitory small molecules have been developed. The first two Jak inhibitors, tofacitinib and ruxolitinib, have been approved for the treatment of RA and primary myelofibrosis, respectively. Efficacy and safety data suggest that some of these oral Jak inhibitors as well as their topical formulations may soon enter the daily clinical practice for treating patients with psoriasis, lupus erythematosus or other inflammatory skin diseases. While biologics typically target one single cytokine, these new immunomodulators can inhibit signals from multiple cytokines intra-cellularly and therefore could be useful when other therapies are ineffective. Thus, Jak inhibitors may replace some traditional immunosuppressive agents and help patients not responding to previous therapies.

Therapeutic targeting of the Jak/STAT pathway

Antibodies that block cytokine function provide a powerful therapeutic tool especially for the treatment of autoimmune diseases. Cytokines are a group of small hydrophilic glycoproteins that bind their receptors on the cell surface and subsequently activate intracellular signalling cascades, such as the JAK/STAT pathway. A bulk of evidence has demonstrated that genetic mutations in signalling molecules can cause immunodeficiencies and malignant cell growth. As a result, several drug companies have begun to develop therapeutics that inhibit the function of JAK tyrosine kinases. Currently, two JAK inhibitors, tofacitinib and ruxolitinib, are used in the clinic for treating rheumatoid arthritis and myeloproliferative diseases, respectively. Inhibiting JAK function has been shown to efficiently prevent the uncontrolled growth of cancerous cells and to harness overly active immune cells. In the future, other small molecule compounds are likely to come into clinical use, and intense work is ongoing to develop inhibitors that specifically target the constitutively active mutant JAKs. This MiniReview will summarize the basic features of the JAK/STAT pathway, its role in human disease and the therapeutic potential of JAK/STAT inhibitors.

Proposal for a new nomenclature of disease-modifying antirheumatic drugs

In light of the recent emergence of new therapeutics for rheumatoid arthritis, such as kinase inhibitors and biosimilars, a new nomenclature for disease-modifying antirheumatic drugs (DMARDs), which are currently often classified as synthetic (or chemical) DMARDs (sDMARDS) and biological DMARDs (bDMARDs), may be needed. We propose to divide the latter into biological original and biosimilar DMARDs (boDMARDs and bsDMARDs, respectively, such as abatacept, adalimumab, anakinra, certolizumab pegol, etanercept, golimumab, infliximab, rituximab or tocilizumab, but also emerging ones like clazakizumab, ixekizumab, sarilumab, secukinumab or sirukumab) and the former into conventional synthetic and targeted synthetic DMARDs (csDMARDs and tsDMARDs, respectively). tsDMARDs would then constitute only those that were specifically developed to target a particular molecular structure (such as tofacitinib, fostamatinib, baricitinib or apremilast, or agents not focused primarily on rheumatic diseases, such as imatinib or ibrutinib), while csDMARDs would comprise the traditional drugs (such as methotrexate, sulfasalazine, leflunomide, hydroxychloroquine, gold salts and others). The proposed nomenclature may provide means to group and distinguish the different types of DMARDs in clinical studies and review articles.

Promising new treatments for psoriasis

Psoriasis is a chronic, proliferative, and inflammatory skin disease affecting 2-3% of the population and is characterized by red plaques with white scales. Psoriasis is a disease that can affect many aspects of professional and social life. Currently, several treatments are available to help control psoriasis such as methotrexate, ciclosporin, and oral retinoids. However, the available treatments are only able to relieve the symptoms and lives of individuals. The discovery of new immunological factors and a better understanding of psoriasis have turned to the use of immunological pathways and could develop new biological drugs against specific immunological elements that cause psoriasis. Biological drugs are less toxic to the body and more effective than traditional therapies. Thus, they should improve the quality of life of patients with psoriasis. This review describes new psoriasis treatments, which are on the market or currently in clinical trials that are being used to treat moderate-to-severe plaque psoriasis. In addition, this paper describes the characteristics and mechanisms in detail. In general, biological drugs are well tolerated and appear to be an effective alternative to conventional therapies. However, their effectiveness and long-term side effects need to be further researched.

Lead optimization of a 4-aminopyridine benzamide scaffold to identify potent, selective, and orally bioavailable TYK2 inhibitors

Herein we report our lead optimization effort to identify potent, selective, and orally bioavailable TYK2 inhibitors, starting with lead molecule 3. We used structure-based design to discover 2,6-dichloro-4-cyanophenyl and (1R,2R)-2-fluorocyclopropylamide modifications, each of which exhibited improved TYK2 potency and JAK1 and JAK2 selectivity relative to 3. Further optimization eventually led to compound 37 that showed good TYK2 enzyme and interleukin-12 (IL-12) cell potency, as well as acceptable cellular JAK1 and JAK2 selectivity and excellent oral exposure in mice. When tested in a mouse IL-12 PK/PD model, compound 37 showed statistically significant knockdown of cytokine interferon-γ (IFNγ), suggesting that selective inhibition of TYK2 kinase activity might be sufficient to block the IL-12 pathway in vivo.

The kinase inhibitor tofacitinib in patients with rheumatoid arthritis: latest findings and clinical potential

Macrophages, T and B cells, and neutrophils concentrate mainly into the synovial tissue of rheumatoid arthritis (RA) patients and produce several inflammatory mediators including cytokines. More recently, small molecule inhibitors of signalling mediators which have intracellular targets (mainly in T and B cells) such as the Janus kinase (JAK) family of tyrosine kinases have been introduced in RA treatment. The JAK family consist of four types: JAK1, JAK2, JAK3 and TyK2. In particular, JAK3 is the only JAK family member that associates with just one cytokine receptor, the common gamma chain, which is exclusively used by the receptors for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 critically involved in T and natural killer (NK)-cell development, and B-cell function and proliferation. Tofacitinib is one of the first JAK inhibitors tested and mainly interacts with JAK1 and JAK3. Four phase II (one A and three B dose-ranging) trials in RA patients, lasting from 6 to 24 weeks, achieved significant improvements of American College of Rheumatology 20% improvement criteria (ACR20) and Disease Activity Score in 28 joints using the C-reactive protein level (DAS28-CRP) or erythrocyte sedimentation rate (DAS-ESR; in one study that analysed this), as early as week 2 and sustained at week 24 in two studies. Doses ranged from 1, 3, 5, 10, 15, 20 up to 30 mg and were administered orally twice a day. ACR20 response rates for dosages ≥3 mg were found to be significantly (p ≤ 0.05) greater than those for placebo in all phase II studies. In general, the major adverse effects included liver test elevation, neutropenia, lipid and creatinine elevation and increased incidence of infections. More recently, RA patients randomly assigned to 5 or 10 mg of tofacitinib twice daily, in both 6- and 12-month phase III trials, achieved a significantly higher ACR20 than those receiving placebo. Adverse events occurred more frequently with tofacitinib than with placebo, and included pulmonary tuberculosis and other serious infections. The balance of efficacy and safety of tofacitinib compared with standard of care therapy is bringing this first orally available biological disease-modifying antirheumatic drug (DMARD) a step closer for RA patients.

A reversion of an IL2RG mutation in combined immunodeficiency providing competitive advantage to the majority of CD8+ T cells

Mutations in the common gamma chain (γc, CD132, encoded by the IL2RG gene) can lead to B(+)T(-)NK(-) X-linked severe combined immunodeficiency, as a consequence of unresponsiveness to γc-cytokines such as interleukins-2, -7 and -15. Hypomorphic mutations in CD132 may cause combined immunodeficiencies with a variety of clinical presentations. We analyzed peripheral blood mononuclear cells of a 6-year-old boy with normal lymphocyte counts, who suffered from recurrent pneumonia and disseminated mollusca contagiosa. Since proliferative responses of T cells and NK cells to γc -cytokines were severely impaired, we performed IL2RG gene analysis, showing a heterozygous mutation in the presence of a single X-chromosome. Interestingly, an IL2RG reversion to normal predominated in both naïve and antigen-primed CD8(+) T cells and increased over time. Only the revertant CD8(+) T cells showed normal expression of CD132 and the various CD8(+) T cell populations had a different T-cell receptor repertoire. Finally, a fraction of γδ(+) T cells and differentiated CD4(+)CD27(-) effector-memory T cells carried the reversion, whereas NK or B cells were repeatedly negative. In conclusion, in a patient with a novel IL2RG mutation, gene-reverted CD8(+) T cells accumulated over time. Our data indicate that selective outgrowth of particular T-cell subsets may occur following reversion at the level of committed T progenitor cells.