Extending the clinical and immunological phenotype of human interleukin-21 receptor deficiency

Interleukin-21 receptor signaling promotes metabolic dysfunction-associated steatohepatitis-driven hepatocellular carcinoma by inducing immunosuppressive IgA+ B cells

BACKGROUND

Dysregulation of immune surveillance is tightly linked to the development of metabolic dysfunction-associated steatohepatitis (MASH)-driven hepatocellular carcinoma (HCC); however, its underlying mechanisms remain unclear. Herein, we aimed to determine the role of interleukin-21 receptor (IL-21R) in MASH-driven HCC.

METHODS

The clinical significance of IL-21R was assessed in human HCC specimens using immunohistochemistry staining. Furthermore, the expression of IL-21R in mice was assessed in the STAM model. Thereafter, two different MASH-driven HCC mouse models were applied between IL-21R-deficient mice and wild type controls to explore the role of IL-21R in MASH-driven HCC. To further elucidate the potential mechanisms by which IL-21R affected MASH-driven HCC, whole transcriptome sequencing, flow cytometry and adoptive lymphocyte transfer were performed. Finally, flow cytometry, enzyme-linked immunosorbent assay, immunofluorescent staining, chromatin immunoprecipitation assay and western blotting were conducted to explore the mechanism by which IL-21R induced IgA+ B cells.

RESULTS

HCC patients with high IL-21R expression exhibited poor relapse-free survival, advanced TNM stage and severe steatosis. Additionally, IL-21R was demonstrated to be upregulated in mouse liver tumors. Particularly, ablation of IL-21R impeded MASH-driven hepatocarcinogenesis with dramatically reduction of lipid accumulation. Moreover, cytotoxic CD8+ T lymphocyte activation was enhanced in the absence of IL-21R due to the reduction of immunosuppressive IgA+ B cells. Mechanistically, the IL-21R-STAT1-c-Jun/c-Fos regulatory axis was activated in MASH-driven HCC and thus promoted the transcription of Igha, resulting in the induction of IgA+ B cells.

CONCLUSIONS

IL-21R plays a cancer-promoting role by inducing IgA+ B cells in MASH-driven hepatocarcinogenesis. Targeting IL-21R signaling represents a potential therapeutic strategy for cancer therapy.

The trajectory of human B-cell function, immune deficiency, and allergy revealed by inborn errors of immunity

The essential role of B cells is to produce protective immunoglobulins (Ig) that recognize, neutralize, and clear invading pathogens. This results from the integration of signals provided by pathogens or vaccines and the stimulatory microenvironment within sites of immune activation, such as secondary lymphoid tissues, that drive mature B cells to differentiate into memory B cells and antibody (Ab)-secreting plasma cells. In this context, B cells undergo several molecular events including Ig class switching and somatic hypermutation that results in the production of high-affinity Ag-specific Abs of different classes, enabling effective pathogen neutralization and long-lived humoral immunity. However, perturbations to these key signaling pathways underpin immune dyscrasias including immune deficiency and autoimmunity or allergy. Inborn errors of immunity that disrupt critical immune pathways have identified non-redundant requirements for eliciting and maintaining humoral immune memory but concomitantly prevent immune dysregulation. Here, we will discuss our studies on human B cells, and how our investigation of cytokine signaling in B cells have identified fundamental requirements for memory B-cell formation, Ab production as well as regulating Ig class switching in the context of protective versus allergic immune responses.

T-cell help in the germinal center: homing in on the role of IL-21

Interleukin 21 (IL-21) is a pleiotropic cytokine that is overproduced in multiple autoimmune settings. Provision of IL-21 from follicular helper T cells is an important component of T-cell help within germinal centers (GC), and the last few years have seen a resurgence of interest in IL-21 biology in the context of the GC environment. While it has been more than a decade since T cell-derived IL-21 was found to upregulate B-cell expression of the GC master transcription factor B-cell lymphoma 6 (Bcl-6) and to promote GC expansion, several recent studies have collectively delivered significant new insights into how this cytokine shapes GC B-cell selection, proliferation, and fate choice. It is now clear that IL-21 plays an important role in GC zonal polarization by contributing to light zone GC B-cell positive selection for dark zone entry as well as by promoting cyclin D3-dependent dark zone inertial cycling. While it has been established that IL-21 can contribute to the modulation of GC output by aiding the generation of antibody-secreting cells (ASC), recent studies have now revealed how IL-21 signal strength shapes the fate choice between GC cycle re-entry and ASC differentiation in vivo. Both provision of IL-21 and sensitivity to this cytokine are finely tuned within the GC environment, and dysregulation of this pathway in autoimmune settings could alter the threshold for germinal center B-cell selection and differentiation, potentially promoting autoreactive B-cell responses.

Combined Immunodeficiency Caused by a Novel Nonsense Mutation in LCK

Mutations affecting T-cell receptor (TCR) signaling typically cause combined immunodeficiency (CID) due to varying degrees of disturbed T-cell homeostasis and differentiation. Here, we describe two cousins with CID due to a novel nonsense mutation in LCK and investigate the effect of this novel nonsense mutation on TCR signaling, T-cell function, and differentiation. Patients underwent clinical, genetic, and immunological investigations. The effect was addressed in primary cells and LCK-deficient T-cell lines after expression of mutated LCK. RESULTS: Both patients primarily presented with infections in early infancy. The LCK mutation led to reduced expression of a truncated LCK protein lacking a substantial part of the kinase domain and two critical regulatory tyrosine residues. T cells were oligoclonal, and especially naïve CD4 and CD8 T-cell counts were reduced, but regulatory and memory including circulating follicular helper T cells were less severely affected. A diagnostic hallmark of this immunodeficiency is the reduced surface expression of CD4. Despite severely impaired TCR signaling mTOR activation was partially preserved in patients' T cells. LCK-deficient T-cell lines reconstituted with mutant LCK corroborated partially preserved signaling. Despite detectable differentiation of memory and effector T cells, their function was severely disturbed. NK cell cytotoxicity was unaffected. Residual TCR signaling in LCK deficiency allows for reduced, but detectable T-cell differentiation, while T-cell function is severely disturbed. Our findings expand the previous report on one single patient on the central role of LCK in human T-cell development and function.

IL-21 shapes germinal center polarization via light zone B cell selection and cyclin D3 upregulation

Germinal center (GC) dysregulation has been widely reported in the context of autoimmunity. Here, we show that interleukin 21 (IL-21), the archetypal follicular helper T cell (Tfh) cytokine, shapes the scale and polarization of spontaneous chronic autoimmune as well as transient immunization-induced GC. We find that IL-21 receptor deficiency results in smaller GC that are profoundly skewed toward a light zone GC B cell phenotype and that IL-21 plays a key role in selection of light zone GC B cells for entry to the dark zone. Light zone skewing has been previously reported in mice lacking the cell cycle regulator cyclin D3. We demonstrate that IL-21 triggers cyclin D3 upregulation in GC B cells, thereby tuning dark zone inertial cell cycling. Lastly, we identify Foxo1 regulation as a link between IL-21 signaling and GC dark zone formation. These findings reveal new biological roles for IL-21 within GC and have implications for autoimmune settings where IL-21 is overproduced.

Inherited human ZNF341 deficiency

Typical hyper-IgE syndromes (HIES) are caused by autosomal-dominant-negative (DN) variants of STAT3 (Signal Transducer And Activator Of Transcription 3) or IL6ST (Interleukin 6 Cytokine Family Signal Transducer), biallelic partial loss-of-function (LOF) variants of IL6ST, or biallelic complete LOF variants of ZNF341 (Zinc Finger Protein 341). Including the two new cases described in this review, only 20 patients with autosomal-recessive (AR) ZNF341 deficiency have ever been reported. Patients with AR ZNF341 deficiency have clinical and immunological phenotypes resembling those of patients with autosomal-dominant STAT3 deficiency, but with a usually milder clinical presentation and lower NK (Natural Killer) cell counts. ZNF341-deficient cells have 50% the normal level of STAT3 in the resting state. However, as there is no clear evidence that STAT3 haploinsufficiency causes HIES, this decrease alone is probably insufficient to explain the HIES phenotype observed in the ZNF341-deficient patients. The combination of decreased basal expression level and impaired autoinduction of STAT3 observed in ZNF341-deficient lymphocytes is considered a more likely pathophysiological mechanism. We review here what is currently known about the ZNF341 gene and ZNF341 deficiency, and briefly discuss possible roles for this protein in addition to its control of STAT3 activity.

The Th17/IL-17 Axis and Host Defense Against Fungal Infections

Chronic mucocutaneous candidiasis (CMC) was recognized as a primary immunodeficiency in the early 1970s. However, for almost 40 years, its genetic etiology remained unknown. The progressive molecular and cellular description of inborn errors of immunity (IEI) with syndromic CMC pointed toward a possible role of IL-17-mediated immunity in protecting against fungal infection and CMC. Since 2011, novel IEI affecting either the response to or production of IL-17A and/or IL-17F (IL-17A/F) in patients with isolated or syndromic CMC provided formal proof of the pivotal role of the IL-17 axis in mucocutaneous immunity to Candida spp, and, to a lesser extent, to Staphylococcus aureus in humans. In contrast, IL-17-mediated immunity seems largely redundant against other common microbes in humans. In this review, we outline the current knowledge of IEI associated with impaired IL-17A/F-mediated immunity, highlighting our current understanding of the role of IL-17A/F in human immunity.

Cytokine-mediated STAT-dependent pathways underpinning human B-cell differentiation and function

B cells are fundamental to host defence against infectious diseases; indeed, the ability of humans to elicit robust antibody responses following exposure to foreign antigens underpins long-lived humoral immunity and serological memory, as well as the success of most currently administered vaccines. However, B cells also have a dark side - they can cause myriad diseases, including autoimmunity, atopy, allergy and malignancy. Thus, it is critical to understand the molecular requirements for generating effective, high-affinity, specific immune responses following natural infection or vaccination, as well as for constraining B-cell function to mitigate B-cell-mediated immune dyscrasias. In this review, we discuss recent developments that have been derived from the identification and detailed analysis of individuals with inborn errors of immunity that disrupt cytokine signalling, resulting in immune dysregulatory conditions. These studies have defined fundamental cytokine/cytokine receptor/signal transducer and activator of transcription (STAT) signalling pathways that are critical for the generation and maintenance of human memory B-cell and plasma cell subsets during host defence, as well as revealed mechanisms of disease pathogenesis causing immune deficiency, autoimmunity and atopy. More importantly, these studies have identified molecules that could be targeted to either enhance humoral immunity in the settings of infection or vaccination, or attenuate humoral immunity that contributes to antibody-mediated autoimmunity or allergy.

Identification of novel NFKB1 and ICOS frameshift variants in patients with CVID

Common variable immunodeficiency (CVID) is a 'late-onset' primary immunodeficiency characterized by variable manifestations and genetic heterogeneity. A monogenic cause of CVID has been reported in 10% of patients. In this study, we identified two novel pathogenic variants implicated in monogenic CVID by whole exome sequencing (WES) analysis: a heterozygous nuclear factor κB subunit 1 (NFKB1) p.G686fs mutation and a homozygous inducible T-cell co-stimulator (ICOS) p.L96Sfs mutation. The predicted crystal models indicated premature truncation of the two mutated proteins. Both variants were demonstrated as loss-of-function mutations and were associated with overlapped manifestations of respiratory fungal infection and splenomegaly. We further performed a detailed assessment of immunologic phenotypes and impaired lymphocyte functions in patients. Moreover, we discovered an association between monoclonal T-large granular lymphocyte proliferation and ICOS-deficient CVID for the first time. These observations lead to a new perspective on the underlying genetic heterogeneity of CVID.

Combined immunodeficiency with marginal zone lymphoma due to a novel homozygous mutation in IL-21R gene and successful treatment with hematopoietic stem cell transplantation

Mutations in the interleukin-21 receptor (IL-21R) gene are recently defined as primary immunodeficiency diseases. IL-21R defects result in combined immunodeficiency by affecting the functions of innate and adaptive immune system components.A six-year-old girl was admitted to our hospital with complaints of chronic diarrhea that started after the newborn period and generalized rash over the last three months. She had severe respiratory distress due to Cytomegalovirus (CMV) pneumonia requiring mechanical ventilation and was diagnosed as combined immunodeficiency at another hospital at the age of four. Her physical examination on admission revealed erythematous rash on cheeks, extremities, gluteal region, and lymph node enlargements in cervical, axillary, and inguinal regions. CMV DNA and stool Cryptosporidium parvum were positive. Marginal zone lymphoma -negative for Epstein-Bar virus- was reported in the lymph node biopsy. Targeted next-generation sequencing Ion AmpliSeq™ primary immunodeficiency panel revealed a novel homozygous IL21R c.132delC (p.Ser45fs) mutation.This case is presented to emphasize that IL21R defects should be considered in the differential diagnosis of the patients with recurrent respiratory infections, chronic diarrhea, C. parvum infection, chronic liver disease, sclerosing cholangitis, and malignancy where early hematopoietic stem cell transplantation (HSCT) is life-saving. A total of eight cases with IL21R gene defects have been reported so far. The significance of this case is that it is the first case of malignancy among the published IL-21R deficient patients successfully treated with HSCT.

Immune Response in Pneumocystis Infections According to the Host Immune System Status

The host immune response is critical in Pneumocystis pneumonia (PCP). Immunocompetent hosts can eliminate the fungus without symptoms, while immunodeficient hosts develop PCP with an unsuitable excessive inflammatory response leading to lung damage. From studies based on rodent models or clinical studies, this review aimed to better understand the pathophysiology of Pneumocystis infection by analysing the role of immune cells, mostly lymphocytes, according to the immune status of the infected host. Hence, this review first describes the immune physiological response in infected immunocompetent hosts that are able to eliminate the fungus. The objective of the second part is to identify the immune elements required for the control of the fungus, focusing on specific immune deficiencies. Finally, the third part concentrates on the effect of the different immune elements in immunocompromised subjects during PCP, to better understand which cells are detrimental, and which, on the contrary, are beneficial once the disease has started. This work highlights that the immune response associated with a favourable outcome of the infection may differ according to the immune status of the host. In the case of immunocompetency, a close communication between B cells and TCD4 within tertiary lymphocyte structures appears critical to activate M2 macrophages without much inflammation. Conversely, in the case of immunodeficiency, a pro-inflammatory response including Th1 CD4, cytotoxic CD8, NK cells, and IFNγ release seems beneficial for M1 macrophage activation, despite the impact of inflammation on lung tissue.

Molecular regulation and dysregulation of T follicular helper cells - learning from inborn errors of immunity

The production of high-affinity antibodies is a key feature of the vertebrate immune system. Antibodies neutralize and clear pathogens, thereby protecting against infectious diseases. However, dysregulated production of antibodies can cause immune pathologies, such as autoimmunity and immune deficiency. Long-lived humoral immunity depends on B-cell help provided by T follicular helper (Tfh) cells, which support the differentiation of antigen (Ag)-specific B cells into memory and plasma cells. Tfh cells are generated from naïve CD4⁺ T cells following the receipt of inputs from various cell surface receptors, and can undergo further differentiation into subsets with specialised effector functions to induce and maintain serological memory. While genetically modified mice have provided great understanding of the requirements for generating Tfh cells, it is critical that requirements for human Tfh cell generation and function are also established. Key insights into the molecular requirements for human Tfh cells have been elucidated from the systematic analysis of humans with monogenic germline variants that cause inborn errors of immunity characterised by impaired humoral immunity following infection or vaccination or immune dysregulation and autoimmunity. In this review we will discuss how studying rare 'experiments of nature' has enabled discovery of non-redundant molecules and pathways necessary for Tfh cell generation, differentiation, regulation and function in humans, and how these findings inform us about basic and clinical immunology.

Case Report: Interleukin-2 Receptor Common Gamma Chain Defect Presented as a Hyper-IgE Syndrome

X-linked severe combined immunodeficiency (X-SCID) is caused by mutations of IL2RG, the gene encoding the interleukin common gamma chain (IL-2Rγ or γc) of cytokine receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Hypomorphic mutations of IL2RG may cause combined immunodeficiencies with atypical clinical and immunological presentations. Here, we report a clinical, immunological, and functional characterization of a missense mutation in exon 1 (c.115G>A; p. Asp39Asn) of IL2RG in a 7-year-old boy. The patient suffered from recurrent sinopulmonary infections and refractory eczema. His total lymphocyte counts have remained normal despite skewed T cell subsets, with a pronounced serum IgE elevation. Surface expression of IL-2Rγ was reduced on his lymphocytes. Signal transducer and activator of transcription (STAT) phosphorylation in response to IL-2, IL-4, and IL-7 showed a partially preserved receptor function. T-cell proliferation in response to mitogens and anti-CD3/anti-CD28 monoclonal antibodies was significantly reduced. Further analysis revealed a decreased percentage of CD4⁺ T cells capable of secreting IFN-γ, but not IL-4 or IL-17. Studies on the functional consequences of IL-2Rγ variants are important to get more insight into the pathogenesis of atypical phenotypes which may lay the ground for novel therapeutic strategies.

Genomic Spectrum and Phenotypic Heterogeneity of Human IL-21 Receptor Deficiency

Biallelic inactivating mutations in IL21R causes a combined immunodeficiency that is often complicated by cryptosporidium infections. While eight IL-21R-deficient patients have been reported previously, the natural course, immune characteristics of disease, and response to hematopoietic stem cell transplantation (HSCT) remain to be comprehensively examined. In our study, we have collected clinical histories of 13 patients with IL-21R deficiency from eight families across seven centers worldwide, including five novel patients identified by exome or NGS panel sequencing. Eight unique mutations in IL21R were identified in these patients, including two novel mutations. Median age at disease onset was 2.5 years (0.5–7 years). The main clinical manifestations were recurrent bacterial (84.6%), fungal (46.2%), and viral (38.5%) infections; cryptosporidiosis-associated cholangitis (46.2%); and asthma (23.1%). Inflammatory skin diseases (15.3%) and recurrent anaphylaxis (7.9%) constitute novel phenotypes of this combined immunodeficiency. Most patients exhibited hypogammaglobulinemia and reduced proportions of memory B cells, circulating T follicular helper cells, MAIT cells and terminally differentiated NK cells. However, IgE levels were elevated in 50% of IL-21R-deficient patients. Overall survival following HSCT (6 patients, mean follow-up 1.8 year) was 33.3%, with pre-existing organ damage constituting a negative prognostic factor. Mortality of non-transplanted patients (n = 7) was 57.1%. Our detailed analysis of the largest cohort of IL-21R-deficient patients to date provides in-depth clinical, immunological and immunophenotypic features of these patients, thereby establishing critical non-redundant functions of IL-21/IL-21R signaling in lymphocyte differentiation, humoral immunity and host defense against infection, and mechanisms of disease pathogenesis due to IL-21R deficiency. Outcome following HSCT depends on prior chronic infections and organ damage, which should thus be considered as early as possible following molecular diagnosis.

Molecular requirements for human lymphopoiesis as defined by inborn errors of immunity

Hematopoietic stem cells (HSCs) are the progenitor cells that give rise to the diverse repertoire of all immune cells. As they differentiate, HSCs yield a series of cell states that undergo gradual commitment to become mature blood cells. Studies of hematopoiesis in murine models have provided critical insights about the lineage relationships among stem cells, progenitors, and mature cells, and these have guided investigations of the molecular basis for these distinct developmental stages. Primary immune deficiencies are caused by inborn errors of immunity that result in immune dysfunction and subsequent susceptibility to severe and recurrent infection(s). Over the last decade there has been a dramatic increase in the number and depth of the molecular, cellular, and clinical characterization of such genetically defined causes of immune dysfunction. Patients harboring inborn errors of immunity thus represent a unique resource to improve our understanding of the multilayered and complex mechanisms underlying lymphocyte development in humans. These breakthrough discoveries not only enable significant advances in the diagnosis of such rare and complex conditions but also provide substantial improvement in the development of personalized treatments. Here, we will discuss the clinical, cellular, and molecular phenotypes, and treatments of selected inborn errors of immunity that impede, either intrinsically or extrinsically, the development of B- or T-cells at different stages.

Regulation of the germinal center and humoral immunity by interleukin-21

Here we review the critical and non-redundant functions of IL-21 in regulating humoral immune responses. We particularly focus on studies in natura—from individuals from inborn errors of immunity that impact on IL-21 production and/or function.

Cytokines play critical roles in regulating the development, survival, differentiation, and function of immune cells. Cytokines exert their function by binding specific receptor complexes on the surface of immune cells and activating intracellular signaling pathways, thereby resulting in induction of specific transcription factors and regulated expression of target genes. While the function of cytokines is often fundamental for the generation of robust and effective immunity following infection or vaccination, aberrant production or function of cytokines can underpin immunopathology. IL-21 is a pleiotropic cytokine produced predominantly by CD4⁺ T cells. Gene-targeting studies in mice, in vitro analyses of human and murine lymphocytes, and the recent discoveries and analyses of humans with germline loss-of-function mutations in IL21 or IL21R have revealed diverse roles of IL-21 in immune regulation and effector function. This review will focus on recent advances in IL-21 biology that have highlighted its critical role in T cell–dependent B cell activation, germinal center reactions, and humoral immunity and how impaired responses to, or production of, IL-21 can lead to immune dysregulation.

Dominant-negative mutations in human IL6ST underlie hyper-IgE syndrome

Autosomal dominant hyper-IgE syndrome (AD-HIES) is typically caused by dominant-negative (DN) STAT3 mutations. Patients suffer from cold staphylococcal lesions and mucocutaneous candidiasis, severe allergy, and skeletal abnormalities. We report 12 patients from 8 unrelated kindreds with AD-HIES due to DN IL6ST mutations. We identified seven different truncating mutations, one of which was recurrent. The mutant alleles encode GP130 receptors bearing the transmembrane domain but lacking both the recycling motif and all four STAT3-recruiting tyrosine residues. Upon overexpression, the mutant proteins accumulate at the cell surface and are loss of function and DN for cellular responses to IL-6, IL-11, LIF, and OSM. Moreover, the patients' heterozygous leukocytes and fibroblasts respond poorly to IL-6 and IL-11. Consistently, patients with STAT3 and IL6ST mutations display infectious and allergic manifestations of IL-6R deficiency, and some of the skeletal abnormalities of IL-11R deficiency. DN STAT3 and IL6ST mutations thus appear to underlie clinical phenocopies through impairment of the IL-6 and IL-11 response pathways.

Genetics in community-acquired pneumonia

PURPOSE OF REVIEW

Host defense against community-acquired pneumonia depends on an intact innate and acquired immune system. This review analyses the correlation between specific defects and polymorphisms of immunity genes with susceptibility for pneumonia.

RECENT FINDINGS

Mutations in BTK, Bruton's tyrosine kinase, lead to X-linked agammaglobulinemia, a disease characterized by recurrent respiratory tract infections, including pneumonia. BTK inhibitors, which are used for treatment of leukemia, have pneumonia as side effect. Polymorphisms in B lymphocyte growth and differentiation factors, including IL-6 and IL-10, Fcg RIIa receptors, as well as genetic variants of ACE, angiotensin-converting enzyme, also are associated with increased susceptibility for pneumonia.

SUMMARY

Delineation of underlying genetic defects and polymorphisms may add in diagnosis, therapy, and prognosis of community-acquired pneumonia. In case of humoral immunodeficiency, antibody replacement therapy may be indicated.

Heterogeneity of Liver Disease in Common Variable Immunodeficiency Disorders

Common variable immunodeficiency (CVID) is the most frequent primary immunodeficiency (PID) in adulthood and is characterized by severe reduction of immunoglobulin serum levels and impaired antibody production in response to vaccines and pathogens. Beyond the susceptibility to infections, CVID encompasses a wide spectrum of clinical manifestations related to a complex immune dysregulation that also affects liver. Although about 50% CVID patients present persistently deranged liver function, burden, and nature of liver involvement have not been systematically investigated in most cohort studies published in the last decades. Therefore, the prevalence of liver disease in CVID widely varies depending on the study design and the sampling criteria. This review seeks to summarize the evidence about the most relevant causes of liver involvement in CVID, including nodular regenerative hyperplasia (NRH), infections and malignancies. We also describe the clinical features of liver disease in some monogenic forms of PID included in the clinical spectrum of CVID as ICOS, NFKB1, NFKB2, CTLA-4, PI3Kδ pathway, ADA2, and IL21-R genetic defects. Finally, we discuss the clinical applications of the various diagnostic tools and the possible therapeutic approaches for the management of liver involvement in the context of CVID.

Regulation of B cell responses by distinct populations of CD4 T cells

Maturation of B cells in Germinal Centers (GC) is a hallmark in adaptive immunity and the basis of successful vaccines that protect us against lethal infections. Nonetheless, vaccination efficacy is very much reduced in aged population and against highly mutagenic viruses. Therefore, it is key to understand how B cell selection takes place in GC in order to develop new and fully protective vaccines. The cellular mechanisms that control selection of GC B cells are performed by different T cell populations. On one side, cognate entanglement of B cells with T follicular helper (Tfh) cells through cytokines and co-stimulatory signals promotes survival, proliferation, mutagenesis and terminal differentiation of GC B cells. On the other hand, regulatory T cells have also been reported within GC and interfere with T cell help for antibody production. These cells have been classified as a distinct T cell sub-population called T Follicular regulatory cells (Tfr). In this review, we investigate the phenotype, function and differentiation of these two cell populations. In addition, based on the different functions of these cell subsets, we highlight the open questions surrounding their heterogeneity.

The Common Cytokine Receptor γ Chain Family of Cytokines

Interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21 form a family of cytokines based on their sharing the common cytokine receptor γ chain (γc), which was originally discovered as the third receptor component of the IL-2 receptor, IL-2Rγ. The IL2RG gene is located on the X chromosome and is mutated in humans with X-linked severe combined immunodeficiency (XSCID). The breadth of the defects in XSCID could not be explained solely by defects in IL-2 signaling, and it is now clear that γc is a shared receptor component of the six cytokines noted above, making XSCID a disease of defective cytokine signaling. Janus kinase (JAK)3 associates with γc, and JAK3-deficient SCID phenocopies XSCID, findings that served to stimulate the development of JAK3 inhibitors as immunosuppressants. γc family cytokines collectively control broad aspects of lymphocyte development, growth, differentiation, and survival, and these cytokines are clinically important, related to allergic and autoimmune diseases and cancer as well as immunodeficiency. In this review, we discuss the actions of these cytokines, their critical biological roles and signaling pathways, focusing mainly on JAK/STAT (signal transducers and activators of transcription) signaling, and how this information is now being used in clinical therapeutic efforts.

Evaluating IL-21 as a Potential Therapeutic Target in Crohn's Disease

Background and Aim

Interleukin-21 (IL-21) is primarily a T cell-derived cytokine; it is upregulated in patients with Crohn's Disease (CD) and could be a potential new therapeutic target in CD.

Methods

In human material, IL-21 and IL-21R expression was investigated by in situ hybridization (ISH) and immunohistochemistry (IHC) in noninflammatory bowel disease (non-IBD) controls and patients with CD. The pathologic role of IL-21 was examined in murine models of T cell-dependent and T cell-independent colitis, either with a neutralizing monoclonal antibody against IL-21 or with the transfer of CD4⁺CD45RB^highIL-21R^-/- T cells. Colonic pathology was examined by endoscopy, histopathology, IHC, ELISA, and Luminex.

Results

In the human intestine, IL-21 and IL-21R mRNA and protein-expressing cells were observed in the mucosa, in lymphoid aggregates of submucosa in non-IBD controls, and in lymphoid aggregates of muscularis externa in patients with CD. IL-21 expression was most abundant in germinal centers (GCs) of the lymphoid aggregates, and IL-21R expression assessed semiquantitatively, was significantly higher in patients with CD compared to non-IBD controls. Following prophylactic and interventive anti-IL-21 mAb treatment in the adoptive transfer (AdTr) model, clinical and pathological parameters were significantly reduced. The most persistent finding was a reduction in colonic infiltrating neutrophils. As well, Rag2^-/- mice receiving CD4⁺CD45RB^highIL-21R^-/- T cells developed less severe colitis compared to Rag2^-/- mice receiving CD4⁺CD45RB^highIL-21R^+/+ T cells. No effect of reduced IL-21 signalling was observed in T cell-independent colitis.

Conclusion

Our study shows that patients with CD have significant expression of IL-21 and IL-21R in the gut. As well, we show that neutralization of IL-21 in experimental T cell-driven colitis is associated with a reduction in clinical and pathological findings. This amelioration seems to be associated with a reduction in colon-infiltrating neutrophils.

New advances in understanding the host immune response to Pneumocystis

Pneumocystis jirovecii causes clinical pneumonia in immunocompromised hosts. Despite this, the inability to cultivate this organism in vitro has likely hindered the field in ascertaining the true impact of Pneumocystis in human infection. However the recent release of the genome as well as in advances in understanding host genetics, and other risk factors for infection and robust experimental models of disease have shed new light on the impact of this fungal pathogen as to better define populations at risk. This review will highlight these recent advances as well as highlight future needed areas of research.

Cytokine-Mediated Regulation of Human Lymphocyte Development and Function: Insights from Primary Immunodeficiencies

Cytokine-mediated intracellular signaling pathways are fundamental for the development, activation, and differentiation of lymphocytes. These distinct processes underlie protection against infectious diseases after natural infection with pathogens or immunization, thereby providing the host with long-lived immunological memory. In contrast, aberrant cytokine signaling can also result in conditions of immune dysregulation, such as early-onset autoimmunity. Thus, balanced signals provided by distinct cytokines, and delivered to specific cell subsets, are critical for immune homeostasis. The essential roles of cytokines in human immunity have been elegantly and repeatedly revealed by the discovery of individuals with mutations in cytokine ligands, receptors, and downstream transcription factors that cause primary immunodeficiency or autoimmune conditions. In this article, we review how the discovery and characterization of such individuals has identified nonredundant, and often highly specialized, functions of specific cytokines and immune cell subsets in human lymphocyte biology, host defense against infections, and immune regulation.

Citrobacter rodentium: a model enteropathogen for understanding the interplay of innate and adaptive components of type 3 immunity

Citrobacter rodentium is a natural murine intestinal pathogen that shares a core set of virulence factors with the related human pathogens enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC). C. rodentium is now the most widely used small animal model for studying the molecular underpinnings of EPEC and EHEC infections in vivo, including: enterocyte attachment; virulence; colonization resistance; and mucosal immunity. In this review, we discuss type 3 immunity in the context of C. rodentium infection and discuss recent publications that use this model to understand how the innate and adaptive components of immunity intersect to mediate host protection against enteric pathogens and maintain homeostasis with the microbiota.

IL-21 restricts T follicular regulatory T cell proliferation through Bcl-6 mediated inhibition of responsiveness to IL-2

T follicular regulatory (Tfr) cells control the magnitude and specificity of the germinal centre reaction, but how regulation is contained to ensure generation of high-affinity antibody is unknown. Here we show that this balance is maintained by the reciprocal influence of interleukin (IL)-2 and IL-21. The number of IL-2-dependent FoxP3+ regulatory T cells is increased in the peripheral blood of human patients with loss-of-function mutations in the IL-21 receptor (IL-21R). In mice, IL-21:IL-21R interactions influence the phenotype of T follicular cells, reducing the expression of CXCR4 and inhibiting the expansion of Tfr cells after T-cell-dependent immunization. The negative effect of IL-21 on Tfr cells in mice is cell intrinsic and associated with decreased expression of the high affinity IL-2 receptor (CD25). Bcl-6, expressed in abundance in Tfr cells, inhibits CD25 expression and IL-21-mediated inhibition of CD25 is Bcl-6 dependent. These findings identify a mechanism by which IL-21 reinforces humoral immunity by restricting Tfr cell proliferation.

Unique and shared signaling pathways cooperate to regulate the differentiation of human CD4+ T cells into distinct effector subsets

Tangye and collaborators use a series of mutants to elucidate the pathways required to generate distinct subsets of human effector CD4⁺ T cells.

Naive CD4⁺ T cells differentiate into specific effector subsets—Th1, Th2, Th17, and T follicular helper (Tfh)—that provide immunity against pathogen infection. The signaling pathways involved in generating these effector cells are partially known. However, the effects of mutations underlying human primary immunodeficiencies on these processes, and how they compromise specific immune responses, remain unresolved. By studying individuals with mutations in key signaling pathways, we identified nonredundant pathways regulating human CD4⁺ T cell differentiation in vitro. IL12Rβ1/TYK2 and IFN-γR/STAT1 function in a feed-forward loop to induce Th1 cells, whereas IL-21/IL-21R/STAT3 signaling is required for Th17, Tfh, and IL-10–secreting cells. IL12Rβ1/TYK2 and NEMO are also required for Th17 induction. Strikingly, gain-of-function STAT1 mutations recapitulated the impact of dominant-negative STAT3 mutations on Tfh and Th17 cells, revealing a putative inhibitory effect of hypermorphic STAT1 over STAT3. These findings provide mechanistic insight into the requirements for human T cell effector function, and explain clinical manifestations of these immunodeficient conditions. Furthermore, they identify molecules that could be targeted to modulate CD4⁺ T cell effector function in the settings of infection, vaccination, or immune dysregulation.

Genes associated with common variable immunodeficiency: one diagnosis to rule them all?

Common variable immunodeficiency (CVID) is a primary antibody deficiency characterised by hypogammaglobulinaemia, impaired production of specific antibodies after immunisation and increased susceptibility to infections. CVID shows a considerable phenotypical and genetic heterogeneity. In contrast to many other primary immunodeficiencies, monogenic forms count for only 2-10% of patients with CVID. Genes that have been implicated in monogenic CVID include ICOS, TNFRSF13B (TACI), TNFRSF13C (BAFF-R), TNFSF12 (TWEAK), CD19, CD81, CR2 (CD21), MS4A1 (CD20), TNFRSF7 (CD27), IL21, IL21R, LRBA, CTLA4, PRKCD, PLCG2, NFKB1, NFKB2, PIK3CD, PIK3R1, VAV1, RAC2, BLK, IKZF1 (IKAROS) and IRF2BP2 With the increasing number of disease genes identified in CVID, it has become clear that CVID is an umbrella diagnosis and that many of these genetic defects cause distinct disease entities. Moreover, there is accumulating evidence that at least a subgroup of patients with CVID has a complex rather than a monogenic inheritance. This review aims to discuss current knowledge regarding the molecular genetic basis of CVID with an emphasis on the relationship with the clinical and immunological phenotype.

Human T Follicular Helper Cells in Primary Immunodeficiency: Quality Just as Important as Quantity

T follicular helper (Tfh) cells are a subset of effector CD4(+) T cells specialised to induce Ab production by B cells. This review highlights some of the recent advances in the field of human Tfh cells that have come from the study of primary immunodeficiencies. In particular it is increasingly evident that the quality of the Tfh cells that are generated, is just as important as the quantity.

IL-21 Signaling in Immunity

IL-21 is a type I cytokine produced by T cells and natural killer T cells that has pleiotropic actions on a wide range of immune and non-immune cell types. Since its discovery in 2000, extensive studies on the biological actions of IL-21 have been performed in vitro and in vivo. Recent reports describing patients with primary immunodeficiency caused by mutations of IL21 or IL21R have further deepened our knowledge of the role of this cytokine in host defense. Elucidation of the molecular mechanisms that mediate IL-21's actions has provided the rationale for targeting IL-21 and IL-21 downstream mediators for therapeutic purposes. The use of next-generation sequencing technology has provided further insights into the complexity of IL-21 signaling and has identified transcription factors and co-factors involved in mediating the actions of this cytokine. In this review, we discuss recent advances in the biology and signaling of IL-21 and how this knowledge can be potentially translated into clinical settings.

STAT3 is a critical cell-intrinsic regulator of human unconventional T cell numbers and function

Wilson et al. show that individuals with loss-of-function mutations in STAT3 have reduced numbers of peripheral blood MAIT and NKT cells, but not γδ T cells. Residual MAIT cells had normal expression of RORγt, but displayed impaired secretion of IL-17A and IL-17F.

Unconventional T cells such as γδ T cells, natural killer T cells (NKT cells) and mucosal-associated invariant T cells (MAIT cells) are a major component of the immune system; however, the cytokine signaling pathways that control their development and function in humans are unknown. Primary immunodeficiencies caused by single gene mutations provide a unique opportunity to investigate the role of specific molecules in regulating human lymphocyte development and function. We found that individuals with loss-of-function mutations in STAT3 had reduced numbers of peripheral blood MAIT and NKT but not γδ T cells. Analysis of STAT3 mosaic individuals revealed that this effect was cell intrinsic. Surprisingly, the residual STAT3-deficient MAIT cells expressed normal levels of the transcription factor RORγt. Despite this, they displayed a deficiency in secretion of IL-17A and IL-17F, but were able to secrete normal levels of cytokines such as IFNγ and TNF. The deficiency in MAIT and NKT cells in STAT3-deficient patients was mirrored by loss-of-function mutations in IL12RB1 and IL21R, respectively. Thus, these results reveal for the first time the essential role of STAT3 signaling downstream of IL-23R and IL-21R in controlling human MAIT and NKT cell numbers.

Enteroviral Infection in a Patient with BLNK Adaptor Protein Deficiency

B-cell linker (BLNK) protein is a non-redundant adaptor molecule in the signaling pathway activated by (pre) B-cell antigen receptor signals. We present two siblings with a homozygous deleterious frameshift mutation in BLNK, resulting in a block of B cell development in the bone marrow at the preB1 to preB2 stage, absence of circulating B cells and agammaglobulinemia. This is the first description of an enteroviral infection associated arthritis and dermatitis in a patient with BLNK deficiency.