REVIEW URRENT C OPINION Human IL-21 and IL-21R deficiencies: two novel entities of primary immunodeficiency Daniel Kotlarz a, Natalia Zie˛tara b, Joshua D. Milner c, and Christoph Klein a Purpose of review This review highlights the recent identification of human interleukin-21 (IL-21) and interleukin-21 receptor (IL-21R) deficiencies as novel entities of primary immunodeficiency. Recent findings We recently described the first patients with IL-21R deficiency who had cryptosporidial infections associated with chronic cholangitis and liver disease. All IL-21R-deficient patients suffered from recurrent respiratory tract infections. Immunological work-up revealed impaired B cell proliferation and immunoglobulin class-switch, reduced T cell effector functions, and variable natural killer cell dysfunctions. Recently, these findings have been extended by the discovery of one patient with a mutation in the IL21 gene. This patient predominantly manifested with very early onset inflammatory bowel disease and recurrent respiratory infections. Laboratory examination showed reduced circulating B cells and impaired B cell class-switch. Summary Human IL-21 and IL-21R deficiencies cause severe, primary immunodeficiency reminiscent of common variable immunodeficiency. Early diagnosis is critical to prevent life-threatening complications, such as secondary liver failure. In view of the critical role of IL-21 in controlling immune homeostasis, early hematopoietic stem cell transplantation might be considered as therapeutic intervention in affected children. Keywords cryptosporidiosis, hematopoietic stem cell transplantation, inflammatory bowel disease, interleukin-21, primary immunodeficiency INTRODUCTION Primary immunodeficiencies (PID) represent a heterogeneous group of diseases leading to global or selective susceptibility to infections [1,2]. More than 200 monogenic defects of the human immune system have been identified [3 ]; yet for many PID the molecular cause remains unknown. The common g chain (gc) cytokine family consists of the following members: interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21 . These cytokines play a fundamental role in the development, survival, proliferation, differentiation, and function of the cells of the immune system, as highlighted by the identification of immunodeficient patients with mutations in IL2RG (TBþNK XSCID [5,6]), IL2RA (severe immune dysregulation ), IL7R (TBþNKþ SCID ), and JAK3 (TBþNK SCID [9,10]). To date, no monogenic defects in IL4R, IL9R, IL15R, or the respective cytokines have been reported. Patients with deficiencies in gc-related cytokine receptors suffer from life-threatening common & www.co-pediatrics.com and opportunistic infections and die within the first year of life without effective treatment, such as hematopoietic stem cell transplantation (HSCT) or gene therapy [11,12]. In XSCID patients, defective IL-7 and IL-15 signaling results in abrogated T and natural killer (NK) cell development and function; however, the cause of B cell deficiency remained largely elusive . Of note, it has been documented that IL-21-mediated signaling may contribute to the a Department of Pediatrics, Dr. von Hauner Children’s Hospital, Ludwig Maximilians University, Munich, bInstitute of Immunology, Hannover Medical School, Hannover, Germany and cLaboratory of Allergic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, USA Correspondence to Christoph Klein, MD, PhD, Dr. von Hauner Children’s Hospital, Ludwig Maximilians University, Lindwurmstr. 4, D-80337 Munich, Germany. Tel: +49 89 4400 57701; fax: +49 89 4400 57702; e-mail: firstname.lastname@example.org Curr Opin Pediatr 2014, 26:704–712 DOI:10.1097/MOP.0000000000000160 Volume 26 Number 6 December 2014 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. IL-21 and IL-21R deficiencies Kotlarz et al. KEY POINTS IL-21 is a master regulator of function and homeostasis of the immune system. Human IL-21 and IL-21R deficiencies are life-threatening PID, which may manifest with variable phenotypes, including VEO-IBD, liver diseases, and susceptibility to opportunistic infections. HSCT may represent a potential curative therapeutic strategy to prevent the fatal consequences of human IL-21 and IL-21R deficiencies. & defective B lymphocyte function in children with XSCID . Recently, Kotlarz et al. [15 ] and Salzer et al. [16 ] have identified mutations in the IL21R and IL21 genes, respectively, as novel causes of PID. & and activator of transcription (STAT) pathway . IL-21 mainly activates STAT3 and STAT1, but also transiently phosphorylates STAT5A and STAT5B  (Fig. 1). The dominant activation of STAT1 and STAT3 makes IL-21 distinctive among cytokines of the gc family. The phosphorylation of STATs leads to their dimerization and nuclear translocation, subsequently inducing transcriptional activity of the following genes: Cyclins a/b/e, Gzma, Gzmb, Ifng, Cxcr3, Cxcr6, Bcl3, Jak3, Il10, Bim, Bcl6, Maf, Prdm1, Rorgt, Eomes, Socs1, Socs3, Il21, and IL21r [22 ,23–25]. Accordingly, IL-21 plays a critical role in the regulation of cell cycle, cellular activation, migration, cell survival, cytotoxicity, and its own expression. In addition to the JAK-STAT pathway, IL-21 can also transduce signals via the phosphoinositide-3-kinase/Akt  and rat sarcoma/mitogenactivated protein kinase pathways  (Fig. 1). & INTERLEUKIN-21 AND INTERLEUKIN-21 RECEPTOR The interleukin-21 receptor (IL-21R) was discovered in 2000 by two independent groups [17,18]. These studies showed that the IL-21R has a predicted open reading frame with properties of a type I cytokine receptor, such as a four-a-helical bundle structure and an ‘up-up-down-down’ configuration. Furthermore, the extracellular domain of the IL-21R includes the highly conserved WSXWS signature motif that is common to type I cytokine receptors. Subsequently, expression cloning of the IL-21R in BaF3 cells has led to the identification of its ligand, termed IL-21, which shares structural homology to IL-2, IL-4, and IL-15 . Recently, Hamming et al.  have reported the crystal structure of the extracellular domain of the human IL-21R complexed to IL-21. The authors could demonstrate that the IL-21/IL-21R complex has considerable structural resemblance to other members of the gc family. Furthermore, the authors highlighted that the WSXWS motif and N-linked glycosylation of the IL-21R are critical for the protein folding and localization at the plasma membrane. INTERLEUKIN-21-MEDIATED SIGNAL TRANSDUCTION IL-21 mediates signals via the heterodimeric IL-21R complex that is composed of a unique a-chain of the IL-21R and the gc . Similar to the other gc-related cytokines, IL-21 predominantly mediates signaling via the Janus kinase (JAK)-signal transducer EXPRESSION AND PLEIOTROPIC IMMUNOMODULATORY ACTIONS OF THE INTERLEUKIN-21/INTERLEUKIN-21 RECEPTOR Analysis of the tissue distribution of IL-21R demonstrated that it is primarily expressed in lymphohematopoietic cells, such as T, B, NK, and myeloid cells [22 ]. The expression of IL-21R on cells of the immune system is tightly regulated depending on their activation status and environmental context. Furthermore, expression of the IL-21R in nonhematopoietic cells has been documented in epithelial cells [27–29], keratinocytes , and fibroblasts [30,31]. In contrast to the wide expression of the IL-21R on lymphoid and myeloid lineages, production of IL-21 appears to be limited to T cell subsets, such as NKT cells , CD8þ T cells , and activated CD4þ T cells, in particular T helper 17 and T follicular helper cells [19,34]. Consistent with the broad cellular distribution of the IL-21R, IL-21 exerts its biological effects on essentially all cells of the innate and adaptive immune system. In particular, it has been documented that IL-21 supports the following immunomodulatory actions: proliferation and survival of CD4þ T cells , differentiation and immunoglobulin class-switch of B cells [14,35,36], proliferation, expansion, survival, and cytotoxicity of CD8þ T cells [25,37–41], development of T follicular helper cells and germinal center formation [42–49], differentiation of T helper 17 cells [50–52], survival, proliferation, granular morphology, cytotoxicity, and cytokine production of NKT cells , proliferation, differentiation, and cytotoxicity of NK cells [53,54], suppression of regulatory T cells , 1040-8703 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins & www.co-pediatrics.com 705 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Allergy, immunology and related disorders IL-21 γc IL-21R JAK3 JAK1 P P P Cytoplasm P P STAT5 STAT1 MAPK PI3K P STAT3 STAT3 P P STAT3 Target genes Nucleus FIGURE 1. Interleukin-21 (IL-21)-mediated signal transduction. Binding of IL-21 to the heterodimeric receptor complex consisting of the interleukin-21 receptor (IL-21R) and common gamma chain leads to activation of JAK1 and JAK3. These kinases phosphorylate STAT1, STAT3, and STAT5 leading to STAT dimerization and translocation to the nucleus with subsequent binding to target gene regulatory elements. In addition to the JAK/STAT pathway, IL-21 activates also PI3K and MAPK pathways. JAK, Janus kinase; MAPK, mitogen-activated protein kinases; PI3K, phosphoinositide-3-kinase; STAT, signal transducers and activators of transcription. Adapted and modified from [22 ]. & antigen presentation and apoptosis of dendritic cells , phagocytosis of macrophages , and autocrine regulation of its own expression . The pleiotropic immunomodulatory effects of IL-21 are summarized in Fig. 2. LOSS-OF-FUNCTION MUTATIONS IN THE INTERLEUKIN-21 RECEPTOR GENE AS A NOVEL CAUSE OF PRIMARY IMMUNODEFICIENCY Using whole exome and targeted sequencing based on cellular screening assays, Kotlarz et al. [15 ] have recently identified novel loss-of-function mutations in the human IL21R gene in two unrelated consanguineous families with two affected siblings each. & 706 www.co-pediatrics.com Clinically, the patients manifested with recurrent respiratory and gastrointestinal infections. In particular, all patients showed cryptosporidial infections associated with chronic cholangitis and liver disease. Detailed demographic, genetic, clinical, and immunological information for the individual patients are provided in Table 1. Of note, the index patient 1 and patient 2 had originally been diagnosed with ‘idiopathic liver fibrosis’ and were referred for liver transplantation. The underlying PID had not been clinically recognized in patient 1. He had to undergo liver transplantation because of end-stage liver disease but died shortly after the procedure due to infectious complications and multiorgan failure. A critical clinical review of both cases revealed the presence of Cryptosporidium Volume 26 Number 6 December 2014 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. IL-21 and IL-21R deficiencies Kotlarz et al. Cytotoxicity Proliferation Survival Cytokines Differentiation Expansion Differentiation GC firmation TH 17 TFH NKT Cytotoxicity Proliferation Survival IFN-γ Perforin CD8 Proliferation Differentiation Cytotoxicity Resistance to Treg supp. CD4 IL-21 DC Mϕ ag presentation Phagocytosis NK Treg Cytotoxicity Maturation Differentiation Survival B Proliferation, Plasma cell differentiation lg class-switch lg production Apoptosis FIGURE 2. Pleiotropic actions of interleukin-21 (IL-21) on the immune system. The major sources of IL-21 are CD4þ T cells, including T follicular helper cells (TFH) and TH17 cells, as well as NK T cells and CD8þ T cells. IL-21 can act in autocrine fashion on these populations or can have multiple effects on other lymphoid [NK cells, B cells, regulatory T (Treg) cells] as well as myeloid lymphoid [NK cells, B cells, regulatory T cells (Treg) cells] as well as myeloid (DC, dendritic cells; Mw, macrophages) cells. Other known cellular targets of IL-21 include epithelial cells, fibroblasts, and keratinocytes (not depicted). Immunomodulatory effects of IL-21 on each population are listed. Adapted and modified from [22 ]. & species in the bile duct system and duodenum of both patients, triggering an extensive immunological workup in patient 2. Routine immunological analyses of peripheral blood lymphocytes revealed normal numbers of T, B, and NK cells in all patients. However, the index patient 2 showed reduced immunoglobulin classswitched IgDCD27þIgGþ B cells and increased naı̈ve IgMþIgDþ B cells [15 ]. Similarly, patient 3, patient 4, and two unpublished patients (patient 5 and patient 6) exhibited a significant reduction of CD20þCD10CD27þ memory B cells and elevated frequencies of IgMþIgDþ B cells [58 ,59 ], indicating a deficiency in immunoglobulin isotype classswitch in IL-21R-deficient children. Moreover, all published patients showed elevated IgE levels and reduced IgG levels were detected in patient 2, patient 4, and patient 6 [15 ]. It has been previously shown that IL-21-induced STAT3 signaling is critical for activation of PRDM1, a master regulator of isotype class-switch and B cell differentiation . As an adjunctive mechanism in IL-21-induced B cell differentiation, Berglund et al. [61 ] have recently & & & & & shown that IL-21R-mediated STAT3 activation induces CD25 (IL-2Ra) expression on naı̈ve B cells, thus sensitizing their responsiveness to IL-2. Detailed experimental studies confirmed aberrant IL-21-mediated activation of STAT signaling resulting in impaired IL-21-related functions of lymphoid cells. The IL-21R-deficient patients 1–4 were characterized by B cell defects (proliferation and immunoglobulin class-switch), T cell deficiency (reduced T cell proliferation in response to specific antigens, specific antibody responses to immunization, and cytokine production), and variable dysfunctions of NK cell cytotoxicity (Table 1). In contrast to deficiencies in other gc-related cytokines, such as IL2RG [5,6] and IL7R , lossof-function mutations in the IL21R gene appear to be not associated with severe combined immunodeficiency. However, IL-21R-deficient patients exhibited increased susceptibility to infections and multiorgan complications with high morbidity and mortality in childhood. Accordingly, patient 1 had to undergo orthotopic liver transplantation because of progressive chronic cholangitis, biliary 1040-8703 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins www.co-pediatrics.com 707 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. 708 Sex M F M M Patient Pat 1a Pat 2b Pat 3 www.co-pediatrics.com Pat 4c Latin American/ Hispanic, Colombia Latin American/ Hispanic, Colombia Lebanese, Lebanon Lebanese, Lebanon Ethnicity, Country of origin Yes Yes Yes Yes Consanguinity 8 13 n/a n/a Age of diagnosis (years) IL21R, c.240_ 245delCTGCCA, p.Cys81_His82del IL21R, c.240_245 delCTGCCA, p.Cys81_His82del IL21R, c.G602T, p.Arg201Leu IL21R, c.G602T, p.Arg201Leu Mutation (Gene; Genotype) 4 7 4 2 Age of onset (years) Recurrent otitis media, multiple episodes of pneumonia (Pseudomonas aeruginosa, Pneumocystis jirovecii, Streptococcus pneumoniae); cryptosporidiosis associated with liver disease, chronic diarrhea; failure to thrive; other infectious complications (persistent norovirus infection, several episodes of bacteremia, hematogenous dissemination of Mycobacterium massiliense, systemic Candida albicans infections) Recurrent otitis media, multiple episodes of pneumonia (Pseudomonas aeruginosa, Pneumocystis jirovecii); cryptosporidiosis associated with liver disease Recurrent pneumonia and sinusitis; cryptosporidiosis associated with liver disease/fibrosis, chronic cholangitis, chronic diarrhea; failure to thrive; other infectious complications (Helicobacter pylori-associated gastritis, esophageal candidiasis) Chronic upper respiratory infections; cryptosporidiosis associated with chronic cholangitis, biliary fibrosis, end-stage liver disease Clinical manifestations IVIG (at age of 7 years) IVIG (at age of 11 years) IgE "", IgG #; memory CD20þ CD10CD27þ B cells #, IgDþIgMþ B cells ", antibody responses to immunization (DTT, TT, PPSV23) #; T cell proliferation (TT) #; defective augmentation of CD8þ T cell proliferation; B cell proliferation and Ig classswitching # IgE slightly increased, IgG #; CD8þ T cells #; memory CD20þ CD10CD27þ B cells #, IgDþIgMþ B cells "; antibody response to immunization (PPSV23) #; T cell proliferation (PHA, ConA, TT, Candida) #; defective augmentation of CD8þ T cell proliferation; B cell proliferation and Ig classswitching #; NK cell cytotoxicity # HLA-identical HSCT (died because of infectious complications, multiorgan failure, and graft rejection on day þ84) Liver transplantation at the age of 4 years (died because of infectious complications and multiorgan failure on day þ542) Treatment IgE ", lgG below-normal; classswitch CD19þIgDIgGþ CD27þ B cells #, IgDþIgMþ B cells "; antibody responses to immunization (DTT, TT, Hib) #; T cell proliferation (DTT, TT, Candida) #; T cell effector cytokine production #; B cell proliferation and Ig classswitching #; NK cell cytotoxicity # IgM ", IgE "; CD19 B cells " Immunological findings Table 1. Synopsis of the clinical and immunological phenotypes of IL-21-deficient and IL-21R-deficient patients Kotlarz et al. & [15 ] Kotlarz et al. & [15 ] Kotlarz et al. & [15 ] Kotlarz et al. & [15 ] Reference Allergy, immunology and related disorders Volume 26 Number 6 December 2014 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. 1040-8703 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins M M F Pat 6d Pat 7 Pat 8 Palestinian, Israel Turkish, Turkey Turkish, Turkey n/p Yes Yes Yes n/p 8 10 20 15.5 IL21R, c.G602A, p.Arg201Gln IL21, c.T147C, p.Leu49Pro IL21R, c.153-1G>T (splice site) IL21R, p.Trp138Ser 2 4 6 n/p Recurrent otitis media and pneumonia (Pneumocystis jirovecii, interstitial) Recurrent upper and lower respiratory tract infections; VEO-IBD with persistent nonbloody diarrhea, failure to thrive, and recurrent oral aphthous ulcers Pulmonary tuberculosis; hepatic steatosis, hepatosplenomegaly; lymphadenopathy; discoid lupus/chronic inflammatory skin disease with T-cell infiltrate; other infectious complications (invasive fungal infection, CMV keratoconjunctivitis/retinitis) n/p HSCT (at the age of 8 years) IVIG, trimethoprimsulfamthoxazole, mesalazine, enteral nutrition IgE ", IgG #; isohemagglutin #; circulating CD19þ #, marginal zone CD19þIgDCD27þ #, class-switch CD19þIgDCD27þ B cells #, transitional CD19þCD38hi CD23 B cells "; T cell proliferation (TT) #; antibody responses to immunization (HepB, BCG) # IgG #, IgM "; B cell proliferation and Ig class-switching # IVIG, Bactrim prophylaxis, HSCT at age of 20 years (died soon after post-HSCT complications) n/p Leukopenia; IgG, IgA #, IgM above-normal; B cells #; class switch B cells # Full phenotype not reported except: memory CD20þ CD10CD27þ B cells #, IgDþIgMþ B cells "; defective augmentation of CD8þ T cell proliferation K. Warnatz, personal communication Salzer et al. & [16 ] J. Milner and I. Barlan, personal communication Deenick et al. & [58 ]; Ives & et al. [59 ] BCG, Bacillus Calmette–Guérin; ConA, concanavalin A; DTT, diphtheria toxoid; F, female; HepB, hepatitis B; Hib; haemophilis influenza type B; HLA, human leukocyte antigen; HSCT, hematopoietic stem cell transplantation; IVIG, intravenous immunoglobulin replacement; M, male; n/a, not applicable; n/p, not published; Pat, patient; PHA, phytohaemagluttinin; PPSV23, Pneumococcal Polysaccharide Vaccine, 23-valent; tbc, to be confirmed; TT, tetanus toxoid; VEO-IBD, very early onset inflammatory bowel disease. a Died because of infectious complications and multiorgan failure on day þ542 after orthotopic liver transplant at the age of 5 years. b Died because of infectious complications and multiorgan failure on day þ84 after HLA-identical HSCT at the age of 11 years. c Died because of infectious complications, progressive fulminant liver failure, and hepatorenal syndrome at the age of 8 years. d Died because of HSCT-related complications at the age of 20 years. n/p Pat 5 IL-21 and IL-21R deficiencies Kotlarz et al. www.co-pediatrics.com 709 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Allergy, immunology and related disorders fibrosis, and end-stage liver disease. He died because of severe infectious complications and multiorgan failure on day þ542 posttransplant. In view of the refractory course and suspected immunodeficiency, an allogeneic HSCT was performed in patient 2 prior to genetic diagnosis. She developed multiple complications associated with reactivation of viral infections and fulminant cryptosporidiosis. She required a second HSCT due to graft rejection but died shortly after the procedure suffering from multiorgan failure. Patient 4 died of infectious complications, progressive fulminant liver failure, and hepatorenal syndrome. Patient 6 succumbed because of HSCT-related complications at the age of 20 years. LOSS-OF-FUNCTION MUTATION IN THE INTERLEUKIN-21 GENE & Salzer et al. [16 ] have recently identified a homozygous loss-of-function mutation in the IL21 gene in patient 7 from a consanguineous Turkish family. Similar to the identified IL-21R-deficient patients, laboratory examinations demonstrated increased IgE levels and reduced IgG levels (Table 1). Immunological analysis revealed reduced numbers of circulating (CD19þ), marginal zone-like (CD19þIgDþ CD27þ), and class-switched memory (CD19þIgD CD27þ) B cells, whereas transitional (CD19þCD38hi CD23) B cells were increased. Patient 7 showed normal T cell proliferation upon stimulation with various stimuli, but not in response to tetanus toxoid. The immunological findings in patient 7, in particular the defective B cell development, are reminiscent of common variable immunodeficiency (CVID). Of note, IL21 has been already considered as a candidate gene for patients with CVID or selective IgA deficiency . Borte et al.  demonstrated that stimulation of B cells from CVID and IgA-deficient patients with IL-21, IL-4, and antiCD40 could prevent apoptosis and induce isotype class-switch. On the basis of these findings, they have performed Sanger sequencing of IL21 in a cohort of 16 unrelated CVID patients; however, no mutation in the coding region of IL21 was detected. Therefore, targeted gene sequencing of IL21 may be considered in a larger cohort of CVID patients. Of note, IL-21 deficiency was associated with severe very early onset inflammatory bowel disease (VEO-IBD) in the first year of life. Patient 7 showed recurrent and severe upper and lower respiratory tract infections, indicative for PID, that may have been masked by severe VEO-IBD in his two deceased siblings lacking genetic diagnosis [16 ]. Furthermore, patient 7 showed no signs of cholangitis and was negative for Cryptosporidium species in feces. In contrast, only two IL-21R-deficient patients & 710 www.co-pediatrics.com (patient 2 and patient 4) had a history of mild chronic diarrhea that may also been triggered by gastrointestinal infections, such as cryptosporidiosis. As IBD represents a complex and multifactorial disorder, the IBD-like phenotype in patient 7 may be attributed to environmental factors or differences in the intestinal microbiota [63,64]. In accordance with IL-21 deficiency, a number of monogenetic immunoregulatory disorders have been already associated with IBD-like phenotypes, such as Wiskott–Aldrich syndrome, immunodysregulation polyendocrinopathy enteropathy X-linked syndrome, chronic granulomatous disease, and X-linked lymphoproliferative syndrome, as reviewed in . These PID emphasize that a variable penetrance of the IBD-like phenotype can be observed under conditions of immunodeficiency and that gene– environment interactions play a critical role in the manifestation of IBD. Therefore, the identification of additional patients is required to define the full phenotype of IL-21-deficient patients and to determine the penetrance of VEO-IBD. PERSPECTIVES IL-21 and IL-21R deficiencies are life-threatening PID that may manifest with variable phenotypes, such as liver disease (IL-21R deficiency) and VEO-IBD (IL-21 deficiency). Interestingly, another IL-21R-deficient patient (patient 6) has recently been identified with yet more unique features, including liver disease in the absence of detectable cryptosporidiosis, lymphoproliferation, fungal disease, and inflammatory skin disease (J. Milner and I. Barlan, personal communication). Furthermore, an additional IL-21R-deficient patient (patient 8) with a missense mutation, affecting the same codon of IL21R as in patient 1 and patient 2, has been recently discovered (K. Warnatz, personal communication). The clinical findings predominantly included pneumonia and recurrent otitis media, but notably no cryptosporidial infections or liver disease could be detected. Therefore, further studies and identification of additional patients are required to define the full phenotype of both IL-21 and IL-21R deficiencies. The studies highlighted in this review indicate that early diagnosis is of critical importance for IL-21-deficient and IL-21R-deficient patients to prevent life-threatening infections and multiorgan complications. In view of the critical role of IL-21 in modulating immune homeostasis, HSCT may be advocated as a therapeutic option. However, future studies are required to assess the potential and consequences of this strategy. In cases of IL-21 deficiency, administration of recombinant IL-21 may be considered as an alternative, especially Volume 26 Number 6 December 2014 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. IL-21 and IL-21R deficiencies Kotlarz et al. when the clinical status of the patient is incompatible with HSCT. Several preclinical studies and clinical trials have already documented the feasibility and safety of the administration of recombinant IL-21 in the treatment of solid tumors and hematological malignancies (reviewed in [22 ]), thus providing basic knowledge on established dosing regimens. & CONCLUSION PID are a heterogeneous group of rare, chronic disorders that affect the development and function of the immune system. Advances in immunology and genetics have resulted in the identification of more than 200 PID. Previous studies on PID have provided fundamental insights into key factors controlling immune homeostasis in a natural ecosystem. A better understanding of the underlying pathomechanisms is critical to make a rational choice of the available therapeutic strategies for children with intractable diseases. Furthermore, this knowledge will improve diagnostic processes and may help to develop new genetic-tailored therapies to cure affected children. The characterization of IL-21 and IL-21R deficiency highlights that PID can manifest with a multitude of clinical presentations, making it difficult to diagnose the underlying immunodeficiency. As exemplified by the identification of IL-21-deficient and IL-21R-deficient patients, nextgeneration sequencing, in particular whole exome sequencing, serves as an unbiased and powerful technique that may be implemented in routine clinical diagnostics in the near future. Both IL-21 and IL-21R deficiencies must still be completely characterized with respect to the clinical and immunological phenotype. Therefore, we hope that this review will raise the awareness of the medical and scientific community for PID caused by mutations in IL21 or IL21R genes. Acknowledgements We thank the interdisciplinary medical teams involved in the care of the described patients. Furthermore, we are grateful to all patients and their families for participating in genetic studies. In particular, we thank Drs. Isil Barlan, Polina Stepensky, and Klaus Warnatz for their personal communication on newly identified IL-21R patients. This work was partially supported by grants from the DFG (KFO250, SFB914, Gottfried-Wilhelm-Leibniz Program), BMBF (PID-NET), and Reinhard-Frank Stiftung (fellowship to D. Kotlarz). This work was sponsored in part by the intramural research program of the NIAID, NIH. Conflicts of interest There are no conflicts of interest. REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Casanova JL, Abel L. Primary immunodeficiencies: a field in its infancy. Science 2007; 317:617–619. 2. Fischer A. 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