RELA haploinsufficiency in CD4 lymphoproliferative disease with autoimmune cytopenias

[A20 haploinsufficiency: what do clinicians need to know?]

A20 Haploinsufficiency (HA20) is a monogenic autoinflammatory disease associated with an autosomal dominant mutation in the TNFAIP3 gene. It induces a defect in the inactivation of the pro-inflammatory NF-κB pathway. Less than 200 cases have been described worldwide. The clinical picture of the disease is essentially based on the association of recurrent fever and/or biologic inflammatory syndrome, aphtosis, often bipolar, and cutaneous folliculitis. However, the clinical spectrum of HA20 is very broad, including gastrointestinal (mainly colonic ulceration), articular, cutaneous, pericardial and lymph node involvement, as well as frequent association with organ-specific or non-specific autoimmune manifestations and/or autoantibodies, including antinuclear antibodies and anti-dsDNA. As a result, the diagnosis of a number of systemic or organic disorders, most notably Behçet's disease, Crohn's disease, and sometimes even systemic lupus, has been corrected to HA20 by molecular research for a heterozygous mutation with functional deficiency of TNFAIP3. Although the first signs of the disease often appear in the first years of life, the diagnosis is often made in adulthood and requires the involvement of both paediatric and adult physicians. Treatment for HA20 is not codified and relies on conventional or biological immunomodulators and immunosuppressants adapted to the patient's symptomatology. This review highlights the enormous diagnostic challenges in this autoinflammatory disease.

NF-κB subunits RelA and c-Rel selectively control CD4+ T cell function in multiple sclerosis and cancer

The outcome of cancer and autoimmunity is often dictated by the effector functions of CD4+ conventional T cells (Tconv). Although activation of the NF-κB signaling pathway has long been implicated in Tconv biology, the cell-autonomous roles of the separate NF-κB transcription-factor subunits are unknown. Here, we dissected the contributions of the canonical NF-κB subunits RelA and c-Rel to Tconv function. RelA, rather than c-Rel, regulated Tconv activation and cytokine production at steady-state and was required for polarization toward the TH17 lineage in vitro. Accordingly, RelA-deficient mice were fully protected against neuroinflammation in a model of multiple sclerosis due to defective transition to a pathogenic TH17 gene-expression program. Conversely, Tconv-restricted ablation of c-Rel impaired their function in the microenvironment of transplanted tumors, resulting in enhanced cancer burden. Moreover, Tconv required c-Rel for the response to PD-1-blockade therapy. Our data reveal distinct roles for canonical NF-κB subunits in different disease contexts, paving the way for subunit-targeted immunotherapies.

The NF-κB RelA transcription factor is not required for CD8+ T-cell function in acute viral infection and cancer

CD8+ T cells are critical mediators of pathogen clearance and anti-tumor immunity. Although signaling pathways leading to the activation of NF-κB transcription factors have crucial functions in the regulation of immune responses, the CD8+ T cell-autonomous roles of the different NF-κB subunits, are still unresolved. Here, we investigated the function of the ubiquitously expressed transcription factor RelA in CD8+ T-cell biology using a novel mouse model and gene-edited human cells. We found that CD8+ T cell-specific ablation of RelA markedly altered the transcriptome of ex vivo stimulated cells, but maintained the proliferative capacity of both mouse and human cells. In contrast, in vivo experiments showed that RelA deficiency did not affect the CD8+ T-cell response to acute viral infection or transplanted tumors. Our data suggest that in CD8+ T cells, RelA is dispensable for their protective activity in pathological contexts.

Nucleotide metabolism, leukodystrophies, and CNS pathology

The balance between a protective and a destructive immune response can be precarious, as exemplified by inborn errors in nucleotide metabolism. This class of inherited disorders, which mimics infection, can result in systemic injury and severe neurologic outcomes. The most common of these disorders is Aicardi Goutières syndrome (AGS). AGS results in a phenotype similar to "TORCH" infections (Toxoplasma gondii, Other [Zika virus (ZIKV), human immunodeficiency virus (HIV)], Rubella virus, human Cytomegalovirus [HCMV], and Herpesviruses), but with sustained inflammation and ongoing potential for complications. AGS was first described in the early 1980s as familial clusters of "TORCH" infections, with severe neurology impairment, microcephaly, and basal ganglia calcifications (Aicardi & Goutières, Ann Neurol, 1984;15:49-54) and was associated with chronic cerebrospinal fluid (CSF) lymphocytosis and elevated type I interferon levels (Goutières et al., Ann Neurol, 1998;44:900-907). Since its first description, the clinical spectrum of AGS has dramatically expanded from the initial cohorts of children with severe impairment to including individuals with average intelligence and mild spastic paraparesis. This broad spectrum of potential clinical manifestations can result in a delayed diagnosis, which families cite as a major stressor. Additionally, a timely diagnosis is increasingly critical with emerging therapies targeting the interferon signaling pathway. Despite the many gains in understanding about AGS, there are still many gaps in our understanding of the cell-type drivers of pathology and characterization of modifying variables that influence clinical outcomes and achievement of timely diagnosis.

Inborn errors of immunity underlying defective T-cell memory

PURPOSE OF REVIEW

T-cell memory is a complex process not well understood involving specific steps, pathways and different T-cell subpopulations. Inborn errors of immunity (IEIs) represent unique models to decipher some of these requirements in humans. More than 500 different IEIs have been reported to date, and recently a subgroup of monogenic disorders characterized by memory T-cell defects has emerged, providing novel insights into the pathways of T-cell memory generation and maintenance, although this new knowledge is mostly restricted to peripheral blood T-cell memory populations.

RECENT FINDINGS

This review draws up an inventory of the main and recent IEIs associated with T-cell memory defects and their mice models, with a particular focus on the nuclear factor kappa B (NF-κB) signalling pathway, including the scaffold protein capping protein regulator and myosin 1 linker 2 (CARMIL2) and the T-cell co-stimulatory molecules CD28 and OX-40. Besides NF-κB, IKZF1 (IKAROS), a key transcription factor of haematopoiesis and STAT3-dependent interleukin-6 signals involving the transcription factor ZNF341 also appear to be important for the generation of T cell memory. Somatic reversion mosaicism in memory T cells is documented for several gene defects supporting the critical role of these factors in the development of memory T cells with a potential clinical benefit.

SUMMARY

Systematic examination of T-cell memory subsets could be helpful in the diagnosis of IEIs.

How to Build a Fire: The Genetics of Autoinflammatory Diseases

Systemic autoinflammatory diseases (SAIDs) are a heterogeneous group of disorders caused by excess activation of the innate immune system in an antigen-independent manner. Starting with the discovery of the causal gene for familial Mediterranean fever, more than 50 monogenic SAIDs have been described. These discoveries, paired with advances in immunology and genomics, have allowed our understanding of these diseases to improve drastically in the last decade. The genetic causes of SAIDs are complex and include both germline and somatic pathogenic variants that affect various inflammatory signaling pathways. We provide an overview of the acquired SAIDs from a genetic perspective and summarize the clinical phenotypes and mechanism(s) of inflammation, aiming to provide a comprehensive understanding of the pathogenesis of autoinflammatory diseases.

Human RELA dominant-negative mutations underlie type I interferonopathy with autoinflammation and autoimmunity

Inborn errors of the NF-κB pathways underlie various clinical phenotypes in humans. Heterozygous germline loss-of-expression and loss-of-function mutations in RELA underlie RELA haploinsufficiency, which results in TNF-dependent chronic mucocutaneous ulceration and autoimmune hematological disorders. We here report six patients from five families with additional autoinflammatory and autoimmune manifestations. These patients are heterozygous for RELA mutations, all of which are in the 3' segment of the gene and create a premature stop codon. Truncated and loss-of-function RelA proteins are expressed in the patients' cells and exert a dominant-negative effect. Enhanced expression of TLR7 and MYD88 mRNA in plasmacytoid dendritic cells (pDCs) and non-pDC myeloid cells results in enhanced TLR7-driven secretion of type I/III interferons (IFNs) and interferon-stimulated gene expression in patient-derived leukocytes. Dominant-negative mutations in RELA thus underlie a novel form of type I interferonopathy with systemic autoinflammatory and autoimmune manifestations due to excessive IFN production, probably triggered by otherwise non-pathogenic TLR ligands.

Blood transcriptome analysis in a buck-ewe hybrid points towards an nuclear factor-kappa B lymphoproliferative autoimmune disorder

Mammal hybridization is a speciation mechanism and an evolutionary driver. Goat-sheep, especially buck-ewe hybrids, are very rare with only one case reported in 2016, which is the subject of the work presented here. Blood transcriptome analysis revealed that the hybrid largely deviated from imprinting schemes previously described in sheep and other mammals. Furthermore, transcriptome regulation seems to differ from the parent transcriptomes, which is most likely a product of partially incompatible imprinting mechanisms from two closely related species. To gain a deeper understanding of hybridization in mammals we re-analyzed the RNA sequencing data of the buck-ewe hybrid and its parents. We found parent-of-origin-specific expression of genes that functionally clustered, which we explain with the Dobzhansky-Muller incompatibility (DMI) model. According to the DMI model, proteins which interact have a high probability of being barrier loci and hence are prone to monoallelic expression. We discovered enrichment of genes uniquely expressed by the buck-ewe hybrid, which implicate that it suffered from an NF-κB lymphoproliferative autoimmune disorder. Similar findings were reported in the F1 generation of hybrid mice. We propose that hybridization of two related species may lead to an autoimmune phenotype, due to immunoglobulin incompatibilities and incomplete silencing of barrier loci.

NFκB pathway dysregulation due to reduced RelB expression leads to severe autoimmune disorders and declining immunity

BACKGROUND

Genetic aberrations in the NFκB pathway lead to primary immunodeficiencies with various degrees of severity. We previously demonstrated that complete ablation of the RelB transcription factor, a key component of the alternative pathway, results in an early manifested combined immunodeficiency requiring stem cell transplantation.

OBJECTIVE

To study the molecular basis of a progressive severe autoimmunity and immunodeficiency in three patients.

METHODS

Whole exome sequencing was performed to identify the genetic defect. Molecular and cellular techniques were utilized to assess the variant impact on NFκB signaling, canonical and alternative pathway crosstalk, as well as the resultant effects on immune function.

RESULTS

Patients presented with multiple autoimmune progressive severe manifestations encompassing the liver, gut, lung, and skin, becoming debilitating in the second decade of life. This was accompanied by a deterioration of the immune system, demonstrating an age-related decline in naïve T cells and responses to mitogens, accompanied by a gradual loss of all circulating CD19+ cells. Whole exome sequencing identified a novel homozygous c. C1091T (P364L) transition in RELB. The P364L RelB protein was unstable, with extremely low expression, but retained some function and could be transiently and partially upregulated following Toll-like receptor stimulation. Stimulation of P364L patient fibroblasts resulted in a marked rise in a cluster of pro-inflammatory hyper-expressed transcripts consistent with the removal of RelB inhibitory effect on RelA function. This is likely the main driver of autoimmune manifestations in these patients.

CONCLUSION

Incomplete loss of RelB provided a unique opportunity to gain insights into NFκB's pathway interactions as well as the pathogenesis of autoimmunity. The P364L RelB mutation leads to gradual decline in immune function with progression of severe debilitating autoimmunity.

Case report: Novel variants in RELA associated with familial Behcet's-like disease

RELA haploinsufficiency is a recently described autoinflammatory condition presenting with intermittent fevers and mucocutaneous ulcerations. The RELA gene encodes the p65 protein, one of five NF-κB family transcription factors. As RELA is an essential regulator of mucosal homeostasis, haploinsufficiency leads to decreased NF-κB signaling which promotes TNF-driven mucosal apoptosis with impaired epithelial recovery. Thus far, only eight cases have been reported in the literature. Here, we report four families with three novel and one previously described pathogenic variant in RELA. These four families included 23 affected individuals for which genetic testing was available in 16. Almost half of these patients had been previously diagnosed with more common rheumatologic entities (such as Behcet's Disease; BD) prior to the discovery of their pathogenic RELA variants. The most common clinical features were orogenital ulcers, rash, joint inflammation, and fever. The least common were conjunctivitis and recurrent infections. Clinical variability was remarkable even among familial cases, and incomplete penetrance was observed. Patients in our series were treated with a variety of medications, and benefit was observed with glucocorticoids, colchicine, and TNF inhibitors. Altogether, our work adds to the current literature and doubles the number of reported cases with RELA-Associated Inflammatory Disease (RAID). It reaffirms the central importance of the NF-κB pathway in immunity and inflammation, as well as the important regulatory role of RELA in mucosal homeostasis. RELA associated inflammatory disease should be considered in all patients with BD, particularly those with early onset and/or with a strong family history.

Case report and review of the literature: immune dysregulation in a large familial cohort due to a novel pathogenic RELA variant

OBJECTIVE

To explore and define the molecular cause(s) of a multi-generational kindred affected by Bechet's-like mucocutaneous ulcerations and immune dysregulation.

METHODS

Whole genome sequencing and confirmatory Sanger sequencing were performed. Components of the NFκB pathway were quantified by immunoblotting, and function was assessed by cytokine production (IL-6, TNF-α, IL-1β) after lipopolysaccharide (LPS) stimulation. Detailed immunophenotyping of T-cell and B-cell subsets was performed in four patients from this cohort.

RESULTS

A novel variant in the RELA gene, p. Tyr349LeufsTer13, was identified. This variant results in premature truncation of the protein before the serine (S) 536 residue, a key phosphorylation site, resulting in enhanced degradation of the p65 protein. Immunoblotting revealed significantly decreased phosphorylated [p]p65 and pIκBα. The decrease in [p]p65 may suggest reduced heterodimer formation between p50/p65 (NFκB1/RelA). Immunophenotyping revealed decreased naïve T cells, increased memory T cells, and expanded senescent T-cell populations in one patient (P1). P1 also had substantially higher IL-6 and TNF-α levels post-stimulation compared with the other three patients.

CONCLUSION

Family members with this novel RELA variant have a clinical phenotype similar to other reported RELA cases with predominant chronic mucocutaneous ulceration; however, the clinical phenotype broadens to include Behçet's syndrome and IBD. Here we describe the clinical, immunological and genetic evaluation of a large kindred to further expand identification of patients with autosomal dominant RELA deficiency, facilitating earlier diagnosis and intervention. The functional impairment of the canonical NFκB pathway suggests that this variant is causal for the clinical phenotype in these patients.

New primary immunodeficiencies 2021 context and future

PURPOSE OF REVIEW

Primary immunodeficiency diseases (PIDs), also called inborn errors of immunity (IEI), are genetic disorders classically characterized by an increased susceptibility to infection and/or disruption in the regulation of an immunologic pathway. This review summarizes and highlights the new IEI disorders in the IUIS 2019 report and 2020 interim report and discusses the directions for the future management of PIDs.

RECENT FINDINGS

Since 2017, the International Union of Immunologic Societies (IUIS) IEI committee has updated the IUIS classification of IEIs with 88 new gene defects and 75 new immune disorders. The increased utilization of genetic testing and advances in the strategic evaluation of genetic variants have identified, not only novel IEI disorders, but additional genetic causes for known IEI disorders. Investigation of potential immune susceptibilities during the ongoing COVID-19 pandemic suggests that defects in Type I interferon signalling may underlie more severe disease.

SUMMARY

The rapid discovery of new IEIs reflects the growing trend of applying genetic testing modalities as part of medical diagnosis and management.In turn, elucidating the pathophysiology of these novel IEIs have enhanced our understanding of how genetic mutations can modulate the immune system and their consequential effect on human health and disease.

NF-κB: At the Borders of Autoimmunity and Inflammation

The transcription factor NF-κB regulates multiple aspects of innate and adaptive immune functions and serves as a pivotal mediator of inflammatory response. In the first part of this review, we discuss the NF-κB inducers, signaling pathways, and regulators involved in immune homeostasis as well as detail the importance of post-translational regulation by ubiquitination in NF-κB function. We also indicate the stages of central and peripheral tolerance where NF-κB plays a fundamental role. With respect to central tolerance, we detail how NF-κB regulates medullary thymic epithelial cell (mTEC) development, homeostasis, and function. Moreover, we elaborate on its role in the migration of double-positive (DP) thymocytes from the thymic cortex to the medulla. With respect to peripheral tolerance, we outline how NF-κB contributes to the inactivation and destruction of autoreactive T and B lymphocytes as well as the differentiation of CD4⁺-T cell subsets that are implicated in immune tolerance. In the latter half of the review, we describe the contribution of NF-κB to the pathogenesis of autoimmunity and autoinflammation. The recent discovery of mutations involving components of the pathway has both deepened our understanding of autoimmune disease and informed new therapeutic approaches to treat these illnesses.

Large Rab GTPases: Novel Membrane Trafficking Regulators with a Calcium Sensor and Functional Domains

Rab GTPases are major coordinators of intracellular membrane trafficking, including vesicle transport, membrane fission, tethering, docking, and fusion events. Rab GTPases are roughly divided into two groups: conventional “small” Rab GTPases and atypical “large” Rab GTPases that have been recently reported. Some members of large Rab GTPases in mammals include Rab44, Rab45/RASEF, and Rab46. The genes of these large Rab GTPases commonly encode an amino-terminal EF-hand domain, coiled-coil domain, and the carboxyl-terminal Rab GTPase domain. A common feature of large Rab GTPases is that they express several isoforms in cells. For instance, Rab44’s two isoforms have similar functions, but exhibit differential localization. The long form of Rab45 (Rab45-L) is abundantly distributed in epithelial cells. The short form of Rab45 (Rab45-S) is predominantly present in the testes. Both Rab46 (CRACR2A-L) and the short isoform lacking the Rab domain (CRACR2A-S) are expressed in T cells, whereas Rab46 is only distributed in endothelial cells. Although evidence regarding the function of large Rab GTPases has been accumulating recently, there are only a limited number of studies. Here, we report the recent findings on the large Rab GTPase family concerning their function in membrane trafficking, cell differentiation, related diseases, and knockout mouse phenotypes.

Two patients with chronic mucocutaneous candidiasis caused by TRAF3IP2 deficiency

BACKGROUND

TRAF3 interacting protein 2 (TRAF3IP2) (Act1) is an adapter protein that interacts with IL-17R via its similar expression to fibroblast growth factor genes and IL-17R domain and coordinates 2 separate proinflammatory pathways following IL-17 cytokine stimulation.

OBJECTIVE

We sought to elucidate the immunologic consequences of TRAF3IP2 homozygous mutations to improve treatments for immunodeficiency patients with chronic mucocutaneous candidiasis.

METHODS

We describe 2 patients presenting with chronic mucocutaneous candidiasis who harbor biallelic nonsense mutations in TRAF3IP2. The cellular and molecular features of this genetic defect were assessed using in vitro cytokine assays and protein analysis.

RESULTS

We show that the homozygous mutation causes complete loss of protein expression. We also show that the absence of TRAF3IP2 was associated with a defective response to combined IL-2/IL-25 (IL-17E) stimulation.

CONCLUSIONS

Failure to initiate normal signaling downstream of IL-17R engagement likely contributes to the patients' recurrent fungal infections. These findings add to our molecular understanding of genetic defects affecting this critical pathway of antifungal immunity.

Comprehensive Targeted Sequencing Identifies Monogenic Disorders in Patients With Early-onset Refractory Diarrhea

OBJECTIVES

Causes of early-onset refractory diarrhea include exudative diarrhea associated with very early-onset inflammatory bowel diseases, osmotic or secretory diarrhea, and protein-losing enteropathy. Monogenic disorders are included in these diseases, yet a comprehensive genetic analysis has not been fully established.

METHODS

We established targeted gene panels covering all responsible genes for early-onset diarrhea. In total, 108 patients from 15 institutions were enrolled in this study. We collected clinical data from all patients. Seventy-three patients with exudative diarrhea, 4 with osmotic or secretory diarrhea and 8 with protein-losing enteropathy were subjected to genetic analysis.

RESULTS

A total of 15 out of the 108 enrolled patients (13.9%) were identified as monogenic. We identified 1 patient with RELA, 2 with TNFAIP3, 1 with CTLA4, 1 with SLCO2A1, 4 with XIAP, 3 with IL10RA, 1 with HPS1, 1 with FOXP3, and 1 with CYBB gene mutations. We also identified 1 patient with NFKB2 and 1 with TERT mutations from the gene panel for primary immunodeficiency syndromes. The patient with refractory diarrhea caused by heterozygous truncated RelA protein expression is the first case identified worldwide, and functional analysis revealed that the mutation affected nuclear factor kappa B signaling. Genotypes were significantly associated with the clinical and pathological findings in each patient.

CONCLUSIONS

We identified variable monogenic diseases in the patients and found that genes responsible for primary immunodeficiency diseases were frequently involved in molecular pathogenesis. Comprehensive genetic analysis was useful for accurate molecular diagnosis, understanding of underlying pathogenesis, and selecting the optimal treatment for patients with early-onset refractory diarrhea.An infographic for this article is available at: http://links.lww.com/MPG/B853.

Mendelian diseases of dysregulated canonical NF-κB signaling: From immunodeficiency to inflammation

NF-κB is a master transcription factor that activates the expression of target genes in response to various stimulatory signals. Activated NF-κB mediates a plethora of diverse functions including innate and adaptive immune responses, inflammation, cell proliferation, and NF-κB is regulated through interactions with IκB inhibitory proteins, which are in turn regulated by the inhibitor of κB kinase (IKK) complex. Together, these 3 components form the core of the NF-κB signalosomes that have cell-specific functions which are dependent on the interactions with other signaling molecules and pathways. The activity of NF-κB pathway is also regulated by a variety of post-translational modifications including phosphorylation and ubiquitination by Lys63, Met1, and Lys48 ubiquitin chains. The physiologic role of NF-κB is best studied in the immune system due to discovery of many human diseases caused by pathogenic variants in various proteins that constitute the NF-κB pathway. These disease-causing variants can act either as gain-of-function (GoF) or loss-of-function (LoF) and depending on the function of mutated protein, can cause either immunodeficiency or systemic inflammation. Typically, pathogenic missense variants act as GoF and they lead to increased activity in the pathway. LoF variants can be inherited as recessive or dominant alleles and can cause either a decrease or an increase in pathway activity. Dominantly inherited LoF variants often result in haploinsufficiency of inhibitory proteins. Here, we review human Mendelian immunologic diseases, which results from mutations in different molecules in the canonical NF-κB pathway and surprisingly present with a continuum of clinical features including immunodeficiency, atopy, autoimmunity, and autoinflammation.

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.

Hereditary systemic autoinflammatory diseases and Schnitzler's syndrome

The systemic autoinflammatory diseases are disorders of the innate immune system distinguished by severe inflammation resulting from dysregulation of the innate immune system. Hereditary fever syndromes, such as FMF, TNF receptor-associated periodic syndrome, cryopyrin-associated periodic syndromes and mevalonate kinase deficiency, were the first group of systemic autoinflammatory diseases for which a genetic basis was established, between 1999 and 2001. Currently according to the latest report of the international union of immunological societies, 37 separate monogenic disorders were classified as autoinflammatory. In addition to the abovementioned monogenic conditions, we describe Schnitzler's syndrome, a well-defined, acquired autoinflammatory condition without a clear genetic basis. For the purposes of this review, we discuss several conditions defined by the latest consensus process as systemic autoinflammatory diseases. We focus on those disorders where recent studies have contributed to further phenotypic characterization or had an impact on clinical management.

Human Inborn Errors of Immunity: 2019 Update on the Classification from the International Union of Immunological Societies Expert Committee

We report the updated classification of Inborn Errors of Immunity/Primary Immunodeficiencies, compiled by the International Union of Immunological Societies Expert Committee. This report documents the key clinical and laboratory features of 430 inborn errors of immunity, including 64 gene defects that have either been discovered in the past 2 years since the previous update (published January 2018) or were characterized earlier but have since been confirmed or expanded upon in subsequent studies. The application of next-generation sequencing continues to expedite the rapid identification of novel gene defects, rare or common; broaden the immunological and clinical phenotypes of conditions arising from known gene defects and even known variants; and implement gene-specific therapies. These advances are contributing to greater understanding of the molecular, cellular, and immunological mechanisms of disease, thereby enhancing immunological knowledge while improving the management of patients and their families. This report serves as a valuable resource for the molecular diagnosis of individuals with heritable immunological disorders and also for the scientific dissection of cellular and molecular mechanisms underlying inborn errors of immunity and related human diseases.

Molecular Classification of Primary Immunodeficiencies of T Lymphocytes

Proper regulation of the immune system is required for protection against pathogens and preventing autoimmune disorders. Inborn errors of the immune system due to inherited or de novo germline mutations can lead to the loss of protective immunity, aberrant immune homeostasis, and the development of autoimmune disease, or combinations of these. Forward genetic screens involving clinical material from patients with primary immunodeficiencies (PIDs) can vary in severity from life-threatening disease affecting multiple cell types and organs to relatively mild disease with susceptibility to a limited range of pathogens or mild autoimmune conditions. As central mediators of innate and adaptive immune responses, T cells are critical orchestrators and effectors of the immune response. As such, several PIDs result from loss of or altered T cell function. PID-associated functional defects range from complete absence of T cell development to uncontrolled effector cell activation. Furthermore, the gene products of known PID causal genes are involved in diverse molecular pathways ranging from T cell receptor signaling to regulators of protein glycosylation. Identification of the molecular and biochemical cause of PIDs can not only guide the course of treatment for patients, but also inform our understanding of the basic biology behind T cell function. In this chapter, we review PIDs with known genetic causes that intrinsically affect T cell function with particular focus on perturbations of biochemical pathways.