Activating mutations in PIK3CD disrupt the differentiation and function of human and murine CD4+ T cells

TH2-driven manifestations of inborn errors of immunity

Monogenic lesions in pathways critical for effector functions responsible for immune surveillance, protection against autoinflammation, and appropriate responses to allergens and microorganisms underlie the pathophysiology of inborn errors of immunity (IEI). Variants in cytokine production, cytokine signaling, epithelial barrier function, antigen presentation, receptor signaling, and cellular processes and metabolism can drive autoimmunity, immunodeficiency, and/or allergic inflammation. Identification of these variants has improved our understanding of the role that many of these proteins play in skewing toward TH2-related allergic inflammation. Early-onset or atypical atopic disease, often in conjunction with immunodeficiency and/or autoimmunity, should raise suspicion for an IEI. This becomes a diagnostic dilemma if the initial clinical presentation is solely allergic inflammation, especially when the prevalence of allergic diseases is becoming more common. Genetic sequencing is necessary for IEI diagnosis and is helpful for early recognition and implementation of targeted treatment, if available. Although genetic evaluation is not feasible for all patients with atopy, identifying atopic patients with molecular immune abnormalities may be helpful for diagnostic, therapeutic, and prognostic purposes. In this review, we focus on IEI associated with TH2-driven allergic manifestations and classify them on the basis of the affected molecular pathways and predominant clinical manifestations.

Activated PI3Kδ Specifically Perturbs Mouse Regulatory T Cell Homeostasis and Function Leading to Immune Dysregulation

FOXP3+ regulatory T cells (Treg) are required for maintaining immune tolerance and preventing systemic autoimmunity. PI3Kδ is required for normal Treg development and function. However, the impacts of dysregulated PI3Kδ signaling on Treg function remain incompletely understood. In this study, we used a conditional mouse model of activated PI3Kδ syndrome to investigate the role of altered PI3Kδ signaling specifically within the Treg compartment. Activated mice expressing a PIK3CD gain-of-function mutation (aPIK3CD) specifically within the Treg compartment exhibited weight loss and evidence for chronic inflammation, as demonstrated by increased memory/effector CD4+ and CD8+ T cells with enhanced IFN-γ secretion, spontaneous germinal center responses, and production of broad-spectrum autoantibodies. Intriguingly, aPIK3CD facilitated Treg precursor development within the thymus and an increase in peripheral Treg numbers. Peripheral Treg, however, exhibited an altered phenotype, including increased PD-1 expression and reduced competitive fitness. Consistent with these findings, Treg-specific aPIK3CD mice mounted an elevated humoral response following immunization with a T cell-dependent Ag, which correlated with a decrease in follicular Treg. Taken together, these findings demonstrate that an optimal threshold of PI3Kδ activity is critical for Treg homeostasis and function, suggesting that PI3Kδ signaling in Treg might be therapeutically targeted to either augment or inhibit immune responses.

How to treat monogenic SLE?

Systemic lupus erythematosus is a rare and life-threatening autoimmune disease characterized by autoantibodies against double-stranded DNA, with an immunopathology that remains partially unclear. New insights into the disease have been provided by the discovery of key mutations leading to the development of monogenic SLE, occurring in the context of early-onset disease, syndromic lupus, or familial clustering. The increased frequency of discovering these mutations in recent years, thanks to the advent of genetic screening, has greatly enhanced our understanding of the immunopathogenesis of SLE. These monogenic defects include defective clearance of apoptotic bodies, abnormalities in nucleic acid sensing, activation of the type-I interferon pathway, and the breakdown of tolerance through B or T cell activation or lymphocyte proliferation due to anomalies in TLR signalling and/or NFκB pathway overactivation. The translation of genetic discoveries into therapeutic strategies is presented here, within the framework of personalized therapy.

Phenotypic and Immunological Characterization of Patients with Activated PI3Kδ Syndrome 1 Presenting with Autoimmunity

PURPOSE

Autoimmunity is a significant feature of APDS1 patients. We aimed to explore the pathogenic immune phenotype and possible mechanisms of autoimmunity in APDS1 patients.

METHODS

The clinical records and laboratory data of 42 APDS1 patients were reviewed. Immunophenotypes were evaluated by multiparametric flow cytometry. Autoantibodies were detected via antigen microarray analysis.

RESULTS

A total of 42 children with PIK3CD gene mutations were enrolled. Immunological tests revealed increased proportions of effector memory cells (86%) and central memory cells (59%) among CD4+ T cells; increased proportions of effector memory cells (83%) and terminally differentiated effector memory T cells (38%) among CD8+ T cells. Fewer CD3+ T cells and B cells and higher IgG levels were reported in patients with autoimmunity. The proportion of Tregs was decreased, and the proportions of Th9, Tfh, and Tfr cells were increased in APDS1 patients. Among APDS1 patients, higher proportion of Th2 and Tfr cells were found in those with autoimmunity. The proportions of CD11c+ B and CD21lo B cells in patients with autoimmunity were significantly increased. Antigen microarray analysis revealed a wide range of IgG/IgM autoantibodies in patients with APDS1. In patients with autoimmunity, the proportion of Tfr might be positively correlated with autoantibodies.

CONCLUSIONS

The pathogenic immune phenotype of APDS1 patients included (1) deceased CD3+ T-cell and B-cell counts and increased IgG levels in patients with autoimmunity, (2) an imbalanced T helper cell subset, (3) increased proportions of autoreactive B cells, and (4) distinct autoantibody reactivities in patients with autoimmunity.

The differentiation courses of the Tfh cells: a new perspective on autoimmune disease pathogenesis and treatment

The follicular helper T cells are derived from CD4+T cells, promoting the formation of germinal centers and assisting B cells to produce antibodies. This review describes the differentiation process of Tfh cells from the perspectives of the initiation, maturation, migration, efficacy, and subset classification of Tfh cells, and correlates it with autoimmune disease, to provide information for researchers to fully understand Tfh cells and provide further research ideas to manage immune-related diseases.

Modulating the PI3K Signalling Pathway in Activated PI3K Delta Syndrome: a Clinical Perspective

Activated phosphoinositide-3-kinase (PI3K) δ syndrome (APDS) is an inborn error of immunity characterised by immune dysregulation. Since the discovery of genetic mutations resulting in PI3Kδ overactivation, treatment of APDS patients has begun to focus on modulation of the PI3K pathway in addition to supportive therapies. The mTOR inhibitor sirolimus has been used effectively for some clinical manifestations of this condition, however the arrival of specific PI3Kδ inhibitor leniolisib has shown promising early results and may provide a more targeted approach. This review summarizes key aspects of PI3K pathway biology and discusses potential options for nuanced modulation of the PI3K pathway in APDS from a clinical perspective, highlighting differences from PI3K inhibition in haematological malignancies.

Activated PI3Kδ specifically perturbs mouse Treg homeostasis and function leading to immune dysregulation

Foxp3+ regulatory T cells (Treg) are required for maintaining immune tolerance and preventing systemic autoimmunity. PI3Kδ is required for normal Treg development and function. However, the impacts of dysregulated PI3Kδ signaling on Treg function remain incompletely understood. Here, we used a conditional mouse model of activated PI3Kδ syndrome (APDS) to investigate the role of altered PI3Kδ signaling specifically within the Treg compartment. Aged mice expressing a PIK3CD gain-of-function mutation (aPIK3CD) specifically within the Treg compartment exhibited weight loss and evidence for chronic inflammation as demonstrated by increased memory/effector CD4+ and CD8+ T cells with enhanced IFN-γ secretion, spontaneous germinal center responses and production of broad-spectrum autoantibodies. Intriguingly, aPIK3CD facilitated Treg precursor development within the thymus and an increase in peripheral Treg numbers. Peripheral Treg, however, exhibited an altered phenotype including increased PD1 expression and reduced competitive fitness. Consistent with these findings, Treg specific-aPIK3CD mice mounted an elevated humoral response following immunization with a T-cell dependent antigen, that correlated with a decrease in follicular Treg. Taken together, these findings demonstrate that an optimal threshold of PI3Kδ activity is critical for Treg homeostasis and function, suggesting that PI3Kδ signaling in Treg might be therapeutically targeted to either augment or inhibit immune responses.

Monogenic forms of common variable immunodeficiency and implications on target therapeutic approaches

PURPOSE OF REVIEW

Common variable immunodeficiency (CVID) is the most common symptomatic inborn error of immunity. The disorder is characterized by variable clinical and immunological manifestations, and, in a small minority of patients, a monogenic cause may be identified. In this review, we focalized on three different monogenic forms of CVID-like disease.

RECENT FINDINGS

Activated phosphoinositide 3-kinase delta syndrome (APDS) is a rare disorder characterized by hyperactivated class I phosphatidylinositol-3 kinase (PI3K) pathway. Affected patients present with respiratory infectious episodes, impaired viral clearance and lymphoproliferation. Recently, a direct PI3K inhibitor has been approved and it showed encouraging results both in controlling clinical and immunological manifestations of the disease. On the other hand, patients with defects in CTLA-4 or LRBA gene present with life-threatening immune dysregulation, autoimmunity and lymphocytic infiltration of multiple organs. Abatacept, a soluble cytotoxic T lymphocyte antigen 4 (CTLA-4) fusion protein that acts as a costimulation modulator, has been widely implemented for affected patients with good results as bridge treatment.

SUMMARY

Understanding the biological basis of CVID is important not only for enriching our knowledge of the human immune system, but also for setting the basis for potential targeted treatments in this disorder.

The dysfunction of Tfh cells promotes pediatric recurrent respiratory tract infections development by interfering humoral immune responses

Recurrent respiratory tract infections (RRTIs) are one of the most common pediatric diseases. Although the pathogenesis of pediatric RRTIs remains unknown, ineffective B cell-dominated humoral immunity has been considered as the core mechanism. During the course of pediatric RRTIs, B cell-dominated humoral immunity has changed from "protector" of respiratory system to "bystander" of respiratory tract infections. Under physiological condition, Tfh cells are essential for B cell-dominated humoral immunity, including regulating GC formation, promoting memory B cell (MB)/plasma cell (PC) differentiation, inducting immunoglobulin (Ig) class switching, and selecting affinity-matured antibodies. However, in disease states, Tfh cells are dysfunctional, which can be reflected by phenotypes and cytokine production. Tfh cell dysfunctions can cause the disorders of B cell-dominated humoral immunity, such as promoting B cell presented apoptosis, abrogating total Ig production, reducing MB/PC populations, and delaying affinity maturation of antigens-specific antibodies. In this review, we focused on the functions of B and Tfh cells in the homeostasis of respiratory system, and specifically discussed the disorders of humoral immunity and aberrant Tfh cell responses in the disease process of pediatric RRTIs. We hoped to provide some clues for the prevention and treatment of pediatric RRTIs.

Human PIK3R1 mutations disrupt lymphocyte differentiation to cause activated PI3Kδ syndrome 2

Heterozygous loss-of-function (LOF) mutations in PIK3R1 (encoding phosphatidylinositol 3-kinase [PI3K] regulatory subunits) cause activated PI3Kδ syndrome 2 (APDS2), which has a similar clinical profile to APDS1, caused by heterozygous gain-of-function (GOF) mutations in PIK3CD (encoding the PI3K p110δ catalytic subunit). While several studies have established how PIK3CD GOF leads to immune dysregulation, less is known about how PIK3R1 LOF mutations alter cellular function. By studying a novel CRISPR/Cas9 mouse model and patients' immune cells, we determined how PIK3R1 LOF alters cellular function. We observed some overlap in cellular defects in APDS1 and APDS2, including decreased intrinsic B cell class switching and defective Tfh cell function. However, we also identified unique APDS2 phenotypes including defective expansion and affinity maturation of Pik3r1 LOF B cells following immunization, and decreased survival of Pik3r1 LOF pups. Further, we observed clear differences in the way Pik3r1 LOF and Pik3cd GOF altered signaling. Together these results demonstrate crucial differences between these two genetic etiologies.

T-helper-2 cells and atopic disease: lessons learnt from inborn errors of immunity

Inborn errors of immunity (IEI) are caused by monogenic variants that affect the host response to bacterial, viral, and fungal pathogens. As such, individuals with IEI often present with severe, recurrent, and life-threatening infections. However, the spectrum of disease due to IEI is very broad and extends to include autoimmunity, malignancy, and atopic diseases such as eczema, atopic dermatitis, and food and environmental allergies. Here, I review IEI that affect cytokine signaling pathways that dysregulate CD4⁺ T-cell differentiation, resulting in increased T-helper-2 (Th2) cell development, function, and pathogenicity. These are elegant examples of how rare IEI can provide unique insights into more common pathologies such as allergic disease that are impacting the general population at increased frequency.

Case Report: Successful Treatment With Monoclonal Antibodies in One APDS Patient With Prolonged SARS-CoV-2 Infection Not Responsive to Previous Lines of Treatment

We described the case of a patient affected by activated PI3K-kinase delta syndrome (APDS) and a long-lasting and pauci-symptomatic SARS-CoV-2 infection, treated with multiple therapeutic agents including remdesivir and SARS-CoV-2-neutralizing monoclonal antibodies. We detected the clearance of the virus 105 days from the first positive swab and 7 days after monoclonal antibody administration. At genotyping, the SARS-CoV-2 virus resulted as wild type on all samples tested. This case shows the monoclonal antibodies’ good tolerability and efficacy in reducing viral shedding in long-lasting infections refractory to other treatments.

Germinal Center-Related G Protein-Coupled Receptors in Antibody-Mediated Autoimmune Skin Diseases: from Basic Research to Clinical Trials

Germinal center (GC) reaction greatly contributes to the humoral immune response, which begins in lymph nodes or other secondary lymphoid organs after follicular B cells are activated by T-dependent antigens. The GCs then serve as a platform for follicular B cells to complete clonal expansion and somatic hypermutation and then interact with follicular dendritic cells (FDC) and follicular helper T cells (Tfh). Through the interaction between the immune cells, significant processes of the humoral immune response are accomplished, such as antibody affinity maturation, class switching, and production of memory B cells and plasma cells. Cell positioning during the GC reaction is mainly mediated by the chemokine receptors and lipid receptors, which both belong to G protein-coupled receptors (GPCRs) family. There are some orphan GPCRs whose endogenous ligands are unclear yet contribute to the regulation of GC reaction as well. This review will give an introduction on the ligands and functions of two types of GC-relating GPCRs-chemokine receptors like CXCR4 and CXCR5, as well as emerging de-orphanized GPCRs like GPR183, GPR174, and P2RY8. The roles these GPCRs play in several antibody-mediated autoimmune skin diseases will be also discussed, including systemic lupus erythematosus (SLE), pemphigus, scleroderma, and dermatomyositis. Besides, GPCRs are excellent drug targets due to the unique structure and vital functions. Therefore, this review is aimed at providing readers with a focused knowledge about the role that GPCRs play in GC reaction, as well as in provoking the development of GPCR-targeting agents for immune-mediated diseases besides autoimmune diseases.

PI3K Isoforms in B Cells

Phosphatidylinositol-3-kinases (PI3K) control many aspects of cellular activation and differentiation and play an important role in B cells biology. Three different classes of PI3K have been described, all of which are expressed in B cells. However, it is the class IA PI3Ks, and the p110δ catalytic subunit in particular, which seem to play the most critical role in B cells. Here we discuss the important role that class IA PI3K plays in B cell development, activation and differentiation, as well as examine what is known about the other classes of PI3Ks in B cells.

Activated phosphoinositide 3-dinase delta syndrome (APDS): An update

Activated phosphoinositide 3-kinase delta syndrome (APDS) is a recently described form of inborn error of immunity (IEI) caused by heterozygous mutations in PIK3CD or PIK3R1 genes, respectively, encoding leukocyte-restricted catalytic p110δ subunit and the ubiquitously expressed regulatory p85 α subunit of the phosphoinositide 3-kinase δ (PI3Kδ). The first described patients with respiratory infections, hypogammaglobulinemia with normal to elevated IgM serum levels, lymphopenia, and lymphoproliferation. Since the original description, it is becoming evident that the onset of disease may be somewhat variable over time, both in terms of age at presentation and in terms of clinical and immunological complications. In many cases, patients are referred to various specialists such as hematologists, rheumatologists, gastroenterologists, and others, before an immunological evaluation is performed, leading to delay in diagnosis, which negatively affects their prognosis. The significant heterogeneity in the clinical and immunological features affecting APDS patients requires awareness among clinicians since good results with p110δ inhibitors have been reported, certainly ameliorating these patients' quality of life and prognosis.

Activated PI3Kδ signals compromise plasma cell survival via limiting autophagy and increasing ER stress

While phosphatidylinositide 3-kinase delta (PI3Kδ) plays a critical role in humoral immunity, the requirement for PI3Kδ signaling in plasma cells remains poorly understood. Here, we used a conditional mouse model of activated PI3Kδ syndrome (APDS), to interrogate the function of PI3Kδ in plasma cell biology. Mice expressing a PIK3CD gain-of-function mutation (aPIK3CD) in B cells generated increased numbers of memory B cells and mounted an enhanced secondary response but exhibited a rapid decay of antibody levels over time. Consistent with these findings, aPIK3CD expression markedly impaired plasma cell generation, and expression of aPIK3CD intrinsically in plasma cells was sufficient to diminish humoral responses. Mechanistically, aPIK3CD disrupted ER proteostasis and autophagy, which led to increased plasma cell death. Notably, this defect was driven primarily by elevated mTORC1 signaling and modulated by treatment with PI3Kδ-specific inhibitors. Our findings establish an essential role for PI3Kδ in plasma cell homeostasis and suggest that modulating PI3Kδ activity may be useful for promoting and/or thwarting specific immune responses.

Immune Activation Induces Telomeric DNA Damage and Promotes Short-Lived Effector T Cell Differentiation in Chronic HCV Infection

BACKGROUND AND AIMS

Hepatitis C virus (HCV) leads to a high rate of chronic infection and T cell dysfunction. Although it is well known that chronic antigenic stimulation is a driving force for impaired T cell functions, the precise mechanisms underlying immune activation-induced T cell dysfunctions during HCV infection remain elusive.

APPROACH AND RESULTS

Here, we demonstrated that circulating CD4⁺ T cells from patients who are chronically HCV-infected exhibit an immune activation status, as evidenced by the overexpression of cell activation markers human leukocyte antigen-antigen D-related, glucose transporter 1, granzyme B, and the short-lived effector marker CD127^- killer cell lectin-like receptor G1⁺ . In contrast, the expression of stem cell-like transcription factor T cell factor 1 and telomeric repeat-binding factor 2 (TRF2) are significantly reduced in CD4⁺ T cells from patients who are chronically HCV-infected compared with healthy participants (HP). Mechanistic studies revealed that CD4⁺ T cells from participants with HCV exhibit phosphoinositide 3-kinase/Akt/mammalian target of rapamycin signaling hyperactivation on T cell receptor stimulation, promoting proinflammatory effector cell differentiation, telomeric DNA damage, and cellular apoptosis. Inhibition of Akt signaling during T cell activation preserved the precursor memory cell population and prevented inflammatory effector cell expansion, DNA damage, and apoptotic death. Moreover, knockdown of TRF2 reduced HP T cell stemness and triggered telomeric DNA damage and cellular apoptosis, whereas overexpression of TRF2 in CD4 T cells prevented telomeric DNA damage.

CONCLUSIONS

These results suggest that modulation of immune activation through inhibiting Akt signaling and protecting telomeres through enhancing TRF2 expression may open therapeutic strategies to fine tune the adaptive immune responses in the setting of persistent immune activation and inflammation during chronic HCV infection.

Autoimmune Cytopenia as an Early and Initial Presenting Manifestation in Activated PI3 Kinase Delta Syndrome: Case Report and Review

Activated PI3 kinase delta syndrome (APDS) is a combined immunodeficiency characterized by recurrent sinopulmonary infections, increased risk of herpesvirus infections, lymphoproliferation, autoimmunity, and increased risk of lymphoid malignancies. Gain-of-function mutations in PIK3CD and PIK3R1 result in increased phosphoinositide-3-kinase-delta activity which causes hyperactivation of lymphocytes and abnormal development and activation of T and B cells. Cytopenias are the most common autoimmune process occurring in patients with APDS and typically occur as a later manifestation of the disease. Here we present a female patient with an early autoimmune hemolytic anemia, hepatosplenomegaly, and frequent infections presenting in infancy, followed by development of significant lymphadenopathy before her diagnosis with APDS type 1. She had significant improvement in her infectious history with immunoglobulin replacement, and control of autoimmune hemolytic anemia with initiation of sirolimus after her diagnosis with APDS type 1. We utilize this case to review the literature on APDS and present the novel finding of early-onset autoimmune disease in the setting of APDS. Autoimmune cytopenias are seen in many primary immunodeficiencies, and workup of autoimmune cytopenias in young patients should include evaluation for underlying immune disorder.

Inborn errors of immunity: Recent progress

Recent advances in the field of inborn errors of immunity (IEIs) have been wide in scope, including progress in mechanisms of disease, diagnosis, and management. New gene defects affecting the immune response continue to be reported, as many as 26 in the year 2020. It was noted that the presentation of IEIs might not include recurrent infections in 9% of cases, and that current diagnostic methods can identify molecular causes in 92% of patients with severe combined immunodeficiency. Progress in immunopathogenesis explained mechanisms leading to symptoms of autosomal-recessive hyper-IgE syndrome. There was an emphasis on research in primary antibody deficiencies. The benefit of antibiotic prophylaxis to reduce the frequency of infections was demonstrated in these patients. The regimen of rituximab and azathioprine or mycophenolate was proven effective for chronic granulocytic interstitial pneumonia. The efficacy and adverse events of hematopoietic stem cell transplant in different IEI conditions were reported, as well as different strategies to improve outcomes, supporting its use in immunodeficiency and immunodysregulatory syndromes. The recent pandemic of coronavirus disease 2019 affected patients with IEIs, in particular those with deficiency in the interferon-mediated activation of the immune response. Initial data suggest that coronavirus disease 2019 vaccines might elicit anti-coronavirus disease 2019-neutralizing antibody responses in some patients with IEI conditions.

Intrinsic Defects in B Cell Development and Differentiation, T Cell Exhaustion and Altered Unconventional T Cell Generation Characterize Human Adenosine Deaminase Type 2 Deficiency

PURPOSE

Deficiency of adenosine deaminase type 2 (ADA2) (DADA2) is a rare inborn error of immunity caused by deleterious biallelic mutations in ADA2. Clinical manifestations are diverse, ranging from severe vasculopathy with lacunar strokes to immunodeficiency with viral infections, hypogammaglobulinemia and bone marrow failure. Limited data are available on the phenotype and function of leukocytes from DADA2 patients. The aim of this study was to perform in-depth immunophenotyping and functional analysis of the impact of DADA2 on human lymphocytes.

METHODS

In-depth immunophenotyping and functional analyses were performed on ten patients with confirmed DADA2 and compared to heterozygous carriers of pathogenic ADA2 mutations and normal healthy controls.

RESULTS

The median age of the patients was 10 years (mean 20.7 years, range 1-44 years). Four out of ten patients were on treatment with steroids and/or etanercept or other immunosuppressives. We confirmed a defect in terminal B cell differentiation in DADA2 and reveal a block in B cell development in the bone marrow at the pro-B to pre-B cell stage. We also show impaired differentiation of CD4⁺ and CD8⁺ memory T cells, accelerated exhaustion/senescence, and impaired survival and granzyme production by ADA2 deficient CD8⁺ T cells. Unconventional T cells (i.e. iNKT, MAIT, Vδ2⁺ γδT) were diminished whereas pro-inflammatory monocytes and CD56^bright immature NK cells were increased. Expression of the IFN-induced lectin SIGLEC1 was increased on all monocyte subsets in DADA2 patients compared to healthy donors. Interestingly, the phenotype and function of lymphocytes from healthy heterozygous carriers were often intermediate to that of healthy donors and ADA2-deficient patients.

CONCLUSION

Extended immunophenotyping in DADA2 patients shows a complex immunophenotype. Our findings provide insight into the cellular mechanisms underlying some of the complex and heterogenous clinical features of DADA2. More research is needed to design targeted therapy to prevent viral infections in these patients with excessive inflammation as the overarching phenotype.

The expansion of human T-bethighCD21low B cells is T cell dependent

Accumulation of human CD21low B cells in peripheral blood is a hallmark of chronic activation of the adaptive immune system in certain infections and autoimmune disorders. The molecular pathways underpinning the development, function, and fate of these CD21low B cells remain incompletely characterized. Here, combined transcriptomic and chromatin accessibility analyses supported a prominent role for the transcription factor T-bet in the transcriptional regulation of these T-bethighCD21low B cells. Investigating essential signals for generating these cells in vitro established that B cell receptor (BCR)/interferon-γ receptor (IFNγR) costimulation induced the highest levels of T-bet expression and enabled their differentiation during cell cultures with Toll-like receptor (TLR) ligand or CD40L/interleukin-21 (IL-21) stimulation. Low proportions of CD21low B cells in peripheral blood from patients with defined inborn errors of immunity (IEI), because of mutations affecting canonical NF-κB, CD40, and IL-21 receptor or IL-12/IFNγ/IFNγ receptor/signal transducer and activator of transcription 1 (STAT1) signaling, substantiated the essential roles of BCR- and certain T cell–derived signals in the in vivo expansion of T-bethighCD21low B cells. Disturbed TLR signaling due to MyD88 or IRAK4 deficiency was not associated with reduced CD21low B cell proportions. The expansion of human T-bethighCD21low B cells correlated with an expansion of circulating T follicular helper 1 (cTfh1) and T peripheral helper (Tph) cells, identifying potential sources of CD40L, IL-21, and IFNγ signals. Thus, we identified important pathways to target autoreactive T-bethighCD21low B cells in human autoimmune conditions, where these cells are linked to pathogenesis and disease progression.

PI3Kδ coordinates transcriptional, chromatin, and metabolic changes to promote effector CD8+ T cells at the expense of central memory

Patients with activated phosphatidylinositol 3-kinase delta (PI3Kδ) syndrome (APDS) present with sinopulmonary infections, lymphadenopathy, and cytomegalvirus (CMV) and/or Epstein-Barr virus (EBV) viremia, yet why patients fail to clear certain chronic viral infections remains incompletely understood. Using patient samples and a mouse model (Pik3cdE1020K/+ mice), we demonstrate that, upon activation, Pik3cdE1020K/+ CD8+ T cells exhibit exaggerated features of effector populations both in vitro and after viral infection that are associated with increased Fas-mediated apoptosis due to sustained FoxO1 phosphorylation and Fasl derepression, enhanced mTORC1 and c-Myc signatures, metabolic perturbations, and an altered chromatin landscape. Conversely, Pik3cdE1020K/+ CD8+ cells fail to sustain expression of proteins critical for central memory, including TCF1. Strikingly, activated Pik3cdE1020K/+ CD8+ cells exhibit altered transcriptional and epigenetic circuits characterized by pronounced interleukin-2 (IL-2)/STAT5 signatures and heightened IL-2 responses that prevent differentiation to memory-like cells in IL-15. Our data position PI3Kδ as integrating multiple signaling nodes that promote CD8+ T cell effector differentiation, providing insight into phenotypes of patients with APDS.

PI3K inhibitors are finally coming of age

Overactive phosphoinositide 3-kinase (PI3K) in cancer and immune dysregulation has spurred extensive efforts to develop therapeutic PI3K inhibitors. Although progress has been hampered by issues such as poor drug tolerance and drug resistance, several PI3K inhibitors have now received regulatory approval - the PI3Kα isoform-selective inhibitor alpelisib for the treatment of breast cancer and inhibitors mainly aimed at the leukocyte-enriched PI3Kδ in B cell malignancies. In addition to targeting cancer cell-intrinsic PI3K activity, emerging evidence highlights the potential of PI3K inhibitors in cancer immunotherapy. This Review summarizes key discoveries that aid the clinical translation of PI3Kα and PI3Kδ inhibitors, highlighting lessons learnt and future opportunities.

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.

Phosphatidylinositol 3-kinase signaling and immune regulation: insights into disease pathogenesis and clinical implications

INTRODUCTION

Phosphatidylinositol 3-kinase (PI3K) is a lipid kinase that plays a fundamental role in cell survival, metabolism, proliferation and differentiation. Thus, balanced PI3K signalling is critical for multiple aspects of human health. The discovery that germline variants in genes in the PI3K pathway caused inborn errors of immunity highlighted the non-redundant role of these signalling proteins in the human immune system. The subsequent identification and characterisation of >300 individuals with a novel immune dysregulatory disorder, termed activated PI3K-delta syndrome (APDS), has reinforced the status of PI3K as a key pathway regulating immune function. Studies of APDS have demonstrated that dysregulated PI3K function is disruptive for immune cell development, activation, differentiation, effector function and self-tolerance, which are all important in supporting effective, long-term immune responses.

AREAS COVERED

In this review, we recount recent findings regarding humans with germline variants in PI3K genes and discuss the underlying cellular and molecular pathologies, with a focus on implications for therapy in APDS patients.

EXPERT OPINION

Modulating PI3K immune cell signalling by offers opportunities for therapeutic interventions in settings of immunodeficiency, autoimmunity and malignancy, but also highlights potential adverse events that may result from overt pharmacological or intrinsic inhibition of PI3K function.

Case Report: Activating PIK3CD Mutation in Patients Presenting With Granulomatosis With Polyangiitis

Activated phosphoinositide 3-kinase δ syndrome (APDS) is an autosomal dominant primary immunodeficiency caused by gain-of-function (GOF) mutations in PIK3CD or PIK3R1 genes. The phenotypes of APDS are highly variable, ranging from asymptomatic adults to profound immunodeficiency causing early death in childhood. Herein, we reported two pediatric patients with APDS presented with recurrent lung infections, sinusitis, hematuria, and positive anti-neutrophil cytoplasmic antibody (ANCA), previously diagnosed as granulomatosis with polyangiitis (GPA). Bronchoscopy showed mucosal nodule lymphoid hyperplasia in the entire airway. Many inflammatory cells infiltrated around the airway and in the lung parenchyma, and numbers of CD3⁺ T cells and CD20⁺ B cells were significantly increased, especially CD3+ T cells. Whole exome sequencing showed that they had the E1021K (c.3061 G >A) mutation in the PIK3CD gene. These are the first reported cases of APDS presenting as childhood-onset GPA. Pediatricians should suspect of APDS in the differential diagnosis of children who present with GPA-like symptoms. Additionally, timely and repeated bronchoscopies could contribute to providing an important diagnostic clue for APDS.

Beyond monogenetic rare variants: tackling the low rate of genetic diagnoses in predominantly antibody deficiency

Predominantly antibody deficiency (PAD) is the most prevalent form of primary immunodeficiency, and is characterized by broad clinical, immunological and genetic heterogeneity. Utilizing the current gold standard of whole exome sequencing for diagnosis, pathogenic gene variants are only identified in less than 20% of patients. While elucidation of the causal genes underlying PAD has provided many insights into the cellular and molecular mechanisms underpinning disease pathogenesis, many other genes may remain as yet undefined to enable definitive diagnosis, prognostic monitoring and targeted therapy of patients. Considering that many patients display a relatively late onset of disease presentation in their 2nd or 3rd decade of life, it is questionable whether a single genetic lesion underlies disease in all patients. Potentially, combined effects of other gene variants and/or non-genetic factors, including specific infections can drive disease presentation. In this review, we define (1) the clinical and immunological variability of PAD, (2) consider how genetic defects identified in PAD have given insight into B-cell immunobiology, (3) address recent technological advances in genomics and the challenges associated with identifying causal variants, and (4) discuss how functional validation of variants of unknown significance could potentially be translated into increased diagnostic rates, improved prognostic monitoring and personalized medicine for PAD patients. A multidisciplinary approach will be the key to curtailing the early mortality and high morbidity rates in this immune disorder.

Eosinophilia/Hypereosinophilia in the Setting of Reactive and Idiopathic Causes, Well-Defined Myeloid or Lymphoid Leukemias, or Germline Disorders

OBJECTIVES

To report the findings of the 2019 Society for Hematopathology/European Association for Haematopathology Workshop within the categories of reactive eosinophilia, hypereosinophilic syndrome (HES), germline disorders with eosinophilia (GDE), and myeloid and lymphoid neoplasms associated with eosinophilia (excluding entities covered by other studies in this series).

METHODS

The workshop panel reviewed 109 cases, assigned consensus diagnosis, and created diagnosis-specific sessions.

RESULTS

The most frequent diagnosis was reactive eosinophilia (35), followed by acute leukemia (24). Myeloproliferative neoplasms (MPNs) received 17 submissions, including chronic eosinophilic leukemia, not otherwise specified (CEL, NOS). Myelodysplastic syndrome (MDS), MDS/MPN, and therapy-related myeloid neoplasms received 11, while GDE and HES received 12 and 11 submissions, respectively.

CONCLUSIONS

Hypereosinophilia and HES are defined by specific clinical and laboratory criteria. Eosinophilia is commonly reactive. An acute leukemic onset with eosinophilia may suggest core-binding factor acute myeloid leukemia, blast phase of chronic myeloid leukemia, BCR-ABL1-positive leukemia, or t(5;14) B-lymphoblastic leukemia. Eosinophilia is rare in MDS but common in MDS/MPN. CEL, NOS is a clinically aggressive MPN with eosinophilia as the dominant feature. Bone marrow morphology and cytogenetic and/or molecular clonality may distinguish CEL from HES. Molecular testing helps to better subclassify myeloid neoplasms with eosinophilia and to identify patients for targeted treatments.

Case Report: EBV Chronic Infection and Lymphoproliferation in Four APDS Patients: The Challenge of Proper Characterization, Therapy, and Follow-Up

Activated PI3K-kinase Delta Syndrome (APDS) is an autosomal-dominant primary immunodeficiency (PID) caused by the constitutive activation of the PI3Kδ kinase. The consequent hyperactivation of the PI3K-Akt-mTOR pathway leads to an impaired T- and B-cells differentiation and function, causing progressive lymphopenia, hypogammaglobulinemia and hyper IgM. Patients with APDS show recurrent sinopulmonary and chronic herpes virus infections, immune dysregulation manifestations, including cytopenia, arthritis, inflammatory enteropathy, and a predisposition to persistent non-neoplastic splenomegaly/lymphoproliferation and lymphoma. The recurrence of the lymphoproliferative disorder and the difficulties in the proper definition of malignancy on histological examination represents the main challenge in the clinical management of APDS patients, since a prompt and correct diagnosis is needed to avoid major complications. Targeted therapies with PI3Kδ-Akt-mTOR pathway pharmacologic inhibitors (i.e., Rapamycin, Theophylline, PI3K inhibitors) represent a good therapeutic strategy. They can also be used as bridge therapies when HSCT is required in order to control refractory symptoms. Indeed, treated patients showed a good tolerance, improved immunologic phenotype and reduced incidence/severity of immune dysregulation manifestations. Here, we describe our experience in the management of four patients, one male affected with APDS1 (P1) and the other three, a male and two females, with APDS2 (P2, P3, P4) presenting with chronic EBV replication, recurrent episodes of immune dysregulation manifestations and lymphomas. These cases highlighted the importance of a tailored and close follow-up, including serial endoscopic and lymph nodes biopsies control to detect a prompt and correct diagnosis and offer the best therapeutic strategy.

Activated PI3Kinase Delta Syndrome-A Multifaceted Disease

Autosomal dominant gain-of-function mutations in the PIK3CD gene encoding the catalytic subunit p110δ of phosphoinositide 3-kinase-δ (PI3K-δ) or autosomal dominant loss-of-function mutations in the PIK3R1 gene encoding the p85α, p55α and p50α regulatory subunits cause Activated PI3-kinase-δ syndrome (APDS; referred as type 1 APDS and type 2 APDS, respectively). Consequences of these mutations are PI3K-δ hyperactivity. Clinical presentation described for both types of APDS patients is very variable, ranging from mild or asymptomatic features to profound combined immunodeficiency. Massive lymphoproliferation, bronchiectasis, increased susceptibility to bacterial and viral infections and, at a lesser extent, auto-immune manifestations and occurrence of cancer, especially B cell lymphoma, have been described for both types of APDS patients. Here, we review clinical presentation and treatment options as well as fundamental immunological and biological features associated to PI3K-δ increased signaling.

Identification of microRNA-Associated-ceRNA Networks Regulating Crop Milk Production in Pigeon (Columba livia)

Pigeon belongs to altrices. Squab cannot forage independently. Nutrition can only be obtained from crop milk secreted by male and female pigeon. miRNA could regulate many biological events. However, the roles of miRNA and ceRNA in regulating crop milk production are still unknown. In this study, we investigated the miRNAs expression profile of female pigeon crop, explored the potential key genes, and found the regulatory mechanisms of crop milk production. A total of 71 miRNAs were identified differentially expressed significantly. Meanwhile, miR-20b-5p, miR-146b-5p, miR-21-5p, and miR-26b-5p were found to be the key miRNAs regulating lactation. Target genes of these miRNAs participated mainly in cell development; protein and lipid synthesis; and ion signaling processes, such as cell-cell adhesion, epithelial cell morphogenesis, calcium signaling pathway, protein digestion, and absorption. In the ceRNA network, miR-193-5p was located in the central position, and miR-193-5p/CREBRF/LOC110355588, miR-460b-5p/GRHL2/MSTRG.132954, and miR-193-5p/PIK3CD/LOC110355588 regulatory axes were believed to affect lactation. Collectively, our findings enriched the miRNA expression profile of pigeon and provided novel insights into the microRNA-associated-ceRNA networks regulating crop milk production in pigeon.

Activated Phosphoinositide 3-Kinase Delta Syndrome 1: Clinical and Immunological Data from an Italian Cohort of Patients

Activated phosphoinositide 3-kinase delta syndrome 1 (APDS-1) is a recently described inborn error of immunity caused by monoallelic gain-of-function mutations in the PIK3CD gene. We reviewed for the first time medical records and laboratory data of eight Italian APDS-1 patients. Recurrent sinopulmonary infections were the most common clinical feature at onset of disease. Seven patients presented lymphoproliferative disease, at onset or during follow-up, one of which resembled hemophagocytic lymphohistiocytosis (HLH). Genetic analysis of the PIK3CD gene revealed three novel mutations: functional testing confirmed their activating nature. In the remaining patients, the previously reported variants p.E1021K (n = 4) and p.E525A (n = 1) were identified. Six patients were started on immunoglobulin replacement treatment (IgRT). One patient successfully underwent hematopoietic stem cell transplantation (HSCT), with good chimerism and no GVHD at 21 months post-HSCT. APDS-1 is a combined immune deficiency with a wide variety of clinical manifestations and a complex immunological presentation. Besides IgRT, specific therapies targeting the PI3Kδ pathway will most likely become a valid aid for the amelioration of patients’ clinical management and their quality of life.

Control of T lymphocyte fate decisions by PI3K signaling

Virtually all aspects of T and B lymphocyte development, homeostasis, activation, and effector function are impacted by the interaction of their clonally distributed antigen receptors with antigens encountered in their respective environments. Antigen receptors mediate their effects by modulating intracellular signaling pathways that ultimately impinge on the cytoskeleton, bioenergetic pathways, transcription, and translation. Although these signaling pathways are rather well described at this point, especially those steps that are most receptor-proximal, how such pathways contribute to more quantitative aspects of lymphocyte function is still being elucidated. One of the signaling pathways that appears to be involved in this “tuning” process is controlled by the lipid kinase PI3K. Here we review recent key findings regarding both the triggering/enhancement of PI3K signals (via BCAP and ICOS) as well as their regulation (via PIK3IP1 and PHLPP) and how these signals integrate and determine cellular processes. Lymphocytes display tremendous functional plasticity, adjusting their metabolism and gene expression programs to specific conditions depending on their tissue of residence and the nature of the infectious threat to which they are responding. We give an overview of recent findings that have contributed to this model, with a focus on T cells, including what has been learned from patients with gain-of-function mutations in PI3K as well as lessons from cancer immunotherapy approaches.

Increased activation of PI3 kinase-δ predisposes to B-cell lymphoma

Activated phosphatidylinositol 3-kinase-δ (PI3K-δ) syndrome (APDS) is a rare primary combined immunodeficiency caused by either dominant gain-of-function mutations in the PIK3CD gene encoding the catalytic subunit p110δ of PI3K-δ (referred to as type 1 APDS) or dominant loss-of-function mutations in the PIK3R1 gene encoding the p85α, p55α, and p50α regulatory subunits (type 2 APDS). In types 1 and 2 APDS, the PI3K-δ hyperactivity resulting from the gene mutations leads to similar clinical presentations, characterized by increased susceptibility to bacterial and viral infections and (to a lesser extent) autoimmune manifestations. A hallmark of this disease is lymphoproliferation, which may even be life threatening and require repeated surgical treatment. A major complication of APDS is malignancy (especially B-cell lymphomas), which greatly worsens the prognosis. Here, we review the different neoplastic conditions observed in patients with APDS and discuss the uncontrolled PI3K-δ activity in B and T cells that leads to malignant transformation.

Activated PI3Kδ breaches multiple B cell tolerance checkpoints and causes autoantibody production

In patients, gain-of-function (GOF) mutations in PIK3CD break tolerance, causing highly penetrant secretion of autoreactive IgM. Mouse models reveal that Pik3cd GOF subverts the response to self-antigen, preventing the induction of anergy and instead stimulating plasmablast and GC formation.

Antibody-mediated autoimmune diseases are a major health burden. However, our understanding of how self-reactive B cells escape self-tolerance checkpoints to secrete pathogenic autoantibodies remains incomplete. Here, we demonstrate that patients with monogenic immune dysregulation caused by gain-of-function mutations in PIK3CD, encoding the p110δ catalytic subunit of phosphoinositide 3-kinase (PI3K), have highly penetrant secretion of autoreactive IgM antibodies. In mice with the corresponding heterozygous Pik3cd activating mutation, self-reactive B cells exhibit a cell-autonomous subversion of their response to self-antigen: instead of becoming tolerized and repressed from secreting autoantibody, Pik3cd gain-of-function B cells are activated by self-antigen to form plasmablasts that secrete high titers of germline-encoded IgM autoantibody and hypermutating germinal center B cells. However, within the germinal center, peripheral tolerance was still enforced, and there was selection against B cells with high affinity for self-antigen. These data show that the strength of PI3K signaling is a key regulator of pregerminal center B cell self-tolerance and thus represents a druggable pathway to treat antibody-mediated autoimmunity.

The Clinical Immunogenomics Research Consortium Australasia (CIRCA): a Distributed Network Model for Genomic Healthcare Delivery

The Clinical Immunogenomics Research Consortium Australasia (CIRCA) crowdsources expertise in medicine, genomics, data science, and fundamental biology to diagnose and treat patients with rare inborn errors of immunity. This distributed network model operates free of geographic borders and allows rapid progression through the full research/translation/clinical management pipeline, from initial gene variant discovery, through functional validation, and on to precision mechanism-based treatment of patients throughout Australia and New Zealand. The model is scalable and applicable to other rare diseases where clinical experience and scientific know-how are limited, and enables efficient delivery of genomics for all.

Rare genetic variants in systemic autoimmunity

Autoimmune disease is a substantial cause of morbidity and is strongly influenced by genetic risk. Extensive efforts have characterized the overall genetic basis of many autoimmune diseases, typically by investigation of common variants. While these common variants have modest effects and may cumulatively predispose to disease, it is also increasingly apparent that rare variants have significantly greater effect on phenotype and are likely to contribute to autoimmune disease. Recent advances have illustrated the next fundamental step in elucidating the genetic basis of autoimmunity, moving beyond association to demonstrate the functional consequences of these variants.

Phenotypic characterization of patients with activated PI3Kδ syndrome 1 presenting with features of systemic lupus erythematosus

Activated phosphoinositide 3-kinase δ syndrome 1 (APDS1) is a primary immunodeficiency disease caused by gain-of-function mutations in PIK3CD. Clinical features of autoimmune disease have been reported in patients with APDS1. In this study, we reported three patients with APDS1 presenting with systemic lupus erythematosus (SLE) phenotype. The clinical manifestations included recurrent respiratory tract infection, lymphoproliferation, Coombs-positive hemolytic anemia, decreased complement fractions, positive antinuclear antibodies, renal complications related to SLE associated diseases, which met the clinical spectrum of APDS1 and the classification criteria of SLE. The immunological phenotype included an inversion in the CD4:CD8 ratio, an increase in both non-circulating Tfh CD4⁺ memory T and circulating Tfh populations, a low level of recent thymic emigrant T cells, overexpression of CD57 on T cells, and a decrease in B cells with fewer antibody class switch recombination. These phenotypes detected in patients with APDS1 presenting with SLE were resemble that in patients with APDS1 presenting without SLE. Meanwhile, we described the effect of glucocorticoids and rapamycin therapy on patients with APDS1. The phosphorylation of S6 at Ser235/236 was inhibited in patients with APDS1 who underwent glucocorticoids therapy, including two who presented with SLE phenotype. The phosphorylation of AKT at Ser473 and phosphorylation of S6 at Ser235/236 were inhibited in other patients with APDS1 who underwent rapamycin therapy. Here, we showed the coexistence of immunodeficiency and SLE phenotype in APDS1, and the inhibition of rapamycin in activated Akt-mTOR signaling pathway.

Molecular and cellular mechanisms underlying defective antibody responses

Primary immune deficiency is caused by genetic mutations that result in immune dysfunction and subsequent susceptibility to infection. Over the last decade there has been a dramatic increase in the number of genetically defined causes of immune deficiency including those which affect B-cell function. This has not only identified critical nonredundant pathways that control the generation of protective antibody responses but also revealed that immunodeficiency and autoimmunity are often closely linked. Here we explore the molecular and cellular mechanisms of these rare monogenic conditions that disrupt antibody production, which also have implications for understanding the causes of more common polygenic immune dysfunction.

Hyperactive PI3Kδ predisposes naive T cells to activation via aerobic glycolysis programs

Activated phosphoinositide 3-kinase δ syndrome (APDS) is an autosomal-dominant combined immunodeficiency disorder resulting from pathogenic gain-of-function (GOF) mutations in the PIK3CD gene. Patients with APDS display abnormal T cell homeostasis. However, the mechanisms by which PIK3CD GOF contributes to this feature remain unknown. Here, with a cohort of children with PIK3CD GOF mutations from multiple regions of China and a corresponding CRISPR/Cas9 gene-edited mouse model, we reported that hyperactive PI3Kδ disrupted T_Naive cell homeostasis in the periphery by intrinsically promoting the growth, proliferation, and activation of T_Naive cells. Our results showed that PIK3CD GOF resulted in loss of the quiescence-associated gene expression profile in naive T cells and promoted naive T cells to overgrow, hyperproliferate and acquire an activated functional status. Naive PIK3CD GOF T cells exhibited an enhanced glycolytic capacity and reduced mitochondrial respiration in the resting or activated state. Blocking glycolysis abrogated the abnormal splenic T cell pool and reversed the overactivated phenotype induced by PIK3CD GOF in vivo and in vitro. These results suggest that enhanced aerobic glycolysis is required for PIK3CD GOF-induced overactivation of naive T cells and provide a potential therapeutic approach for targeting glycolysis to treat patients with APDS as well as other immune disorders.

Case Study: Mechanism for Increased Follicular Helper T Cell Development in Activated PI3K Delta Syndrome

Gain-of-function variants in p110δ, the catalytic subunit of phosphatidylinositol 3-kinase (PI3K) expressed in lymphocytes, cause activated PI3-kinase δ syndrome (APDS), a primary immunodeficiency that is characterized by recurrent infections, viremia, lymphadenopathy, and autoimmunity. The mechanism of autoimmunity in APDS has not been well-understood. Here, we show the profound skewing of peripheral CD4+ T cells to a T follicular helper (TFH) phenotype in a patient with APDS bearing a novel p110δ variant, Y524S. We also saw a diminishment of transient Foxp3 expression in activated T cells. Mechanistic studies revealed that both the new variant and a previously described, pathogenic variant (E81K) enhanced an interaction between intracellular Osteopontin and p85α. This interaction had been shown in mice to promote TFH differentiation. Our results demonstrate a new influence of PI3K on human T cell differentiation that is unrelated to its lipid-kinase activity and suggest that TFH should be monitored in APDS patients.

Immune Dysregulation and Disease Pathogenesis due to Activating Mutations in PIK3CD-the Goldilocks' Effect

"This porridge is too hot!" she exclaimed. So, she tasted the porridge from the second bowl. "This porridge is too cold," she said. So, she tasted the last bowl of porridge. "Ahhh, this porridge is just right," she said happily and she ate it all up. While this describes the adventures of Goldilocks in the classic fairytale "The Story of Goldilocks and the Three Bears," it is an ideal analogy for the need for balanced signaling mediated by phosphatidylinositol-3-kinase (PI3K), a key signaling hub in immune cells. Either too little or too much PI3K activity is deleterious, even pathogenic-it needs to be "just right"! This has been elegantly demonstrated by the identification of inborn errors of immunity in key components of the PI3K pathway, and the impact of these mutations on immune regulation. Detailed analyses of patients with germline activating mutations in PIK3CD, as well as the parallel generation of novel murine models of this disease, have shed substantial light on the role of PI3K in lymphocyte development and differentiation, and mechanisms of disease pathogenesis resulting not only from PIK3CD mutations but genetic lesions in other components of the PI3K pathway. Furthermore, by being able to pharmacologically target PI3K, these monogenic conditions have provided opportunities for the implementation of precision medicine as a therapy, as well as to gain further insight into the consequences of modulating the PI3K pathway in clinical settings.