Cause Of Severe Combined Immunodeficiency Research Articles

Reticular Dysgenesis-Associated Adenylate Kinase 2 Deficiency Impairs Purine Metabolism and Ribosomal Biogenesis during Myelopoiesis

Reticular Dysgenesis (RD) is a particularly grave form of severe combined immunodeficiency (SCID), characterized by maturation arrest of both myeloid and lymphoid lineages paired with sensorineural hearing loss. RD is caused by biallelic mutations in the mitochondrial enzyme adenylate kinase 2 (AK2). AK2 catalyzes the phosphorylation of adenosine monophosphate (AMP) to adenosine diphosphate (ADP) in the mitochondrial intermembrane space. Using a CRISPR/Cas9 AK2 biallelic knock out model in human hematopoietic stem and progenitor cells (HSPCs), we have shown that AK2 -/- HSPCs mimic the neutrophil maturation defect in RD patients. Mitochondrial respiration is compromised in AK2 -/- HSPCs, which leads to a decreased NAD +/NADH ratio resulting in reductive stress. Metabolomics analysis by LC-MS/MS showed a significant accumulation of AMP, along with increased AMP/ADP and AMP/ATP ratios in AK2 -/- HSPCs, suggesting that purine metabolism is compromised by AK2 deficiency. Purine metabolism defects, such as deficiencies in adenosine deaminase (ADA) and purine nucleotide phosphorylase (PNP), have long been recognized as a cause of SCID. Furthermore, pharmacological interference with purine metabolism is a highly effective antiproliferative strategy in cancer therapy. In this study, we sought to investigate whether impaired purine metabolism contributes to the myelopoietic defect caused by AK2 deficiency.ResultsWe explored how purine metabolism affects myelopoiesis by differentiating HSPCs in media containing no nucleosides (nucleoside-), mixed nucleosides (nucleoside+) or adenosine only (adenosine+). We observed no difference in proliferation or neutrophil maturation between nucleosides- and nucleoside+ media for both control and AK2 -/- HSPCs, suggesting that AK2 -/- HSPCs do not rely on exogenous nucleosides. Interestingly, control HSPCs cultured in adenosine+ media showed severe proliferation and neutrophil maturation defects that mimic AK2 deficiency, suggesting that purine imbalance is detrimental to myelopoiesis.Previous metabolomics analysis showed a significant accumulation of inosine monophosphate (IMP) in AK2 -/- HSPCs. Since IMP can be produced through AMP deamination by AMPD, we asked whether the IMP accumulation in AK2 -/- HSPCs is caused by converting excess AMP to IMP. An LC-MS/MS analysis showed that AMPD inhibitor (AMPDi) treatment significantly lowered IMP levels and increased AMP levels in AK2 -/- HSPCs, indicating that AMP deamination is a major source of IMP accumulation in AK2 -/- HSPCs. Furthermore, AMPDi treatment did not improve, but rather slightly aggravated neutrophil differentiation in AK2 -/- HSPCs, suggesting that the AK2 -/- neutrophil maturation defect is not caused by IMP accumulation. This raises the possibility that AK2 -/- HSPCs employ AMP deamination as a mechanism to curtail the toxicity of excess AMP.Since purine is a building block of RNA, and ribosomal RNA (rRNA) constitutes >85% of cellular RNA content, we asked whether rRNA synthesis is compromised by AK2 deficiency. Pyronin Y staining showed a significant decrease in rRNA content in AK2 -/- HSPCs. Nascent peptide synthesis rate was also decreased in AK2 -/- HSPCs, as quantified by OP-puromycin uptake. These findings are corroborated by RNA-seq analysis of AK2 -/- and control HSPCs, which showed that ribosomal subunits, ribosomal biogenesis and ribonucleoprotein complex assembly are among the top down-regulated pathways. The data suggest that defective purine metabolism in AK2 -/- HSPCs impairs ribosomal biogenesis and protein synthesis.ConclusionOur studies showed that purine imbalance in HSPCs impairs myeloid proliferation and neutrophil maturation. AK2 depletion in HSPCs leads to AMP accumulation and defective ribosomal biogenesis. AK2 -/- HSPCs convert excess AMP to IMP, possibly as a means to mitigate AMP toxicity. However, AMP deamination activities alone are not sufficient to lower AMP levels to those of control HSPCs. We are currently testing whether boosting 5'-nucleotidase activities (cNIA, cN1B and cNII) in AK2 -/- HSPCs can decrease AMP levels and rescue the neutrophil maturation defect. As purine metabolic defects are associated with diverse immune and non-immune abnormalities, further understanding of how purine metabolism governs differentiation of human HSPCs will enable us to develop novel therapeutic strategies for RD and other purine disorders. DisclosuresPorteus: CRISPR Therapeutics: Current equity holder in publicly-traded company; Allogene Therapeutics: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Versant Ventures: Consultancy; Ziopharm: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Graphite Bio: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees. Morrison: Garuda Therapeutics: Other: founder and SAB member ; Kojin Therapeutics: Other: SAB member ; Frequency Therapeutics: Other: SAB member ; Ona Terapeutics: Other: SAB member ; Protein Fluidics: Other: SAB member .

M202 PERSISTENT MTHFD1-DEFICIENT SEVERE COMBINED IMMUNODEFICIENCY DESPITE FOLINIC ACID SUPPLEMENTATION; HEMATOPOIETIC ALLOGENEIC TRANSPLANTATION REQUIRED

Methylenetetrahydrofolate dehydrogenase 1 (MTHFD1) deficiency is a rare cause of severe combined immunodeficiency (SCID); it presents with megaloblastic anemia and sometimes atypical hemolytic uremic syndrome. In reports of four prior cases, treatment with folic/folinic acid corrected T-cell deficiency and reversed SCID.

A Large Cohort of RAG1/2-Deficient SCID Patients-Clinical, Immunological, and Prognostic Analysis.

Severe combined immunodeficiency (SCID) is a fatal disorder resulting from various genetic defects. In the Middle East, where consanguineous marriage is prevalent, autosomal recessive mutations in recombination-activating genes (RAG) are a leading cause of SCID. We present a large cohort of SCID patients due to RAG1 or RAG2 mutations. Twenty-six patients with RAG1 or RAG2 deficiency, diagnosed at Sheba Medical Center, were retrospectively investigated. Clinical presentation, immunologic phenotype, genetic analysis, treatment, and outcome were analyzed. Majority of patients were referred from the Palestinian Authority. Most patients were males of Muslim Arab descent, 77% were born to consanguineous parents, and 65% had family history of immunodeficiency. Nearly all patients suffered from various infections before turning 2 months old, eight patients (31%) presented with Omenn and Omenn-like syndrome, and three patients (11%) had maternal engraftment. Notably, seven patients (27%) suffered from vaccine-derived infections, including a rare case of measles encephalitis. Nineteen patients underwent hematopoietic stem cell transplantation (HSCT) at a median age of 6 months, with a successful outcome for 72% of them. Genetic analysis revealed 11 different mutations (7 RAG2, 4 RAG1), two of them novel. Consanguineous marriages account for a genetic "founder effect." SCID is a pediatric emergency that dictates immediate precautions and curative treatment with HSCT. Due to lack of newborn screening for SCID within the Palestinian population, most patients in this cohort were diagnosed upon clinical symptoms, which led to a delayed diagnosis, harmful administration of contra-indicated live vaccines, delay in HSCT, and poor outcome.

Narrative review of severe combined immunodeficiency—a purine metabolism disorder

Severe combined immunodeficiency (SCID) is a primary immune deficiency that is a pediatric emergency. Left untreated, patients will die prior to 2 years of age from overwhelming infection. There are many known causes of SCID, but this review focuses on adenosine deaminase (ADA) deficiency. ADA is a ubiquitous “housekeeping” enzyme, and its deficiency leads to buildup of toxic metabolites, which preferentially affects lymphocytes but also causes non-immune manifestations of disease. While previously diagnosed based on recurrent, severe, or unusual infection, SCID is now being diagnosed during its initial asymptomatic period thanks to recent implementation of T cell receptor excision circle (TREC) newborn screening, thus greatly improving survival of patients as treatment can occur before the onset of infection. To date, TREC screening has shown 100% sensitivity for SCID but also has brought to light other causes of T cell lymphopenia that were previously lesser known. ADA-deficient SCID can be treated through enzyme replacement therapy (ERT), hematopoietic stem cell transplantation (HSCT), or gene therapy, and advances continue to be made every day that improve cost-effectiveness and efficacy of these therapies and make them more widely available to patients.

A novel pathogenic mutation on Interleukin-7 receptor leading to severe combined immunodeficiency identified with newborn screening and whole exome sequencing

Patients with severe combined immunodeficiency (SCID), which is caused by genetic defects in immune-related genes involved in the development or activation of the adaptive immune system, often died in infancy due to severe infections before definite molecular diagnosis could be made. Although recent improvement in early diagnosis has been achieved by newborn screening, the genetic basis of many of the patients is still unknown. Here we performed whole exome sequencing (WES) to investigate the underlying genetic causes of SCID in a proband identified with newborn screening. Inheritance of the mutation was confirmed with targeted sequencing of the parents. Homozygosity mapping from the WES was used to investigate the consanguinity of the parents. Immunoblotting was used to confirm the loss of expression of the mutant protein. A novel homozygous frameshift mutation of IL7R was identified through WES. Both parents are carriers for this 1-bp deletion. HLA typing and exome-wide homozygous stretch mapping suggested that the parents are consanguineous. Immunoblotting showed no expression of IL7Rα isoform in the whole blood sample of the proband. The proband received peripheral blood stem cell transplantation and her general condition became stable. Our results suggest that IL7R is essential for T cell development but dispensable for the development of certain human NK cells B cells and suggest that WES can be a useful tool for precise genetic diagnosis of SCID following newborn screening in the index patient without the need to screen other members of the whole family.

Newborn Screening for Severe Combined Immunodeficiency in Israel

Newborn screening (NBS) programs for severe combined immunodeficiency (SCID), the most severe type of primary immunodeficiency, are being implemented in more and more countries with every passing year. Since October 2015, SCID screening via T cell receptor excision circle (TREC) quantification in dried blood spots (DBS) has been part of the Israeli NBS program. As an NBS program in its infancy, SCID screening is still evolving, making gathering input from the various programs crucial for compiling an ideal screening algorithm. The relatively high rate of consanguineous marriages in Israel, especially among non-Jews, correlates with an increased incidence of SCID. The Israeli algorithm uses a commercial kit and consists of a two-Guthrie card confirmation system prior to referral to a national immunology center. Preliminary data from the first year and a half of SCID screening in Israel has identified a surprisingly high prevalence of DNA cross-link repair protein 1c (DCLRE1C; ARTEMIS) mutations as the cause of SCID in Israel. The clinically unbiased nature of SCID screening helps unearth mild/leaky SCID phenotypes, resulting in a better understanding of true SCID prevalence and etiology.

Clinical, immunologic, and genetic characteristics of RAG mutations in 15 Chinese patients with SCID and Omenn syndrome.

Mutations in Recombination Activating Genes (RAG1 and RAG2) are common genetic causes of severe combined immunodeficiency (SCID) and Omenn syndrome (OS). The clinical, immunologic, and genetic characteristics of RAG mutations in Chinese patients with SCID or OS have not been studied in detail. In this research, 22 RAG mutations were identified in 15 Chinese patients, including 10 novel mutations in RAG1 (R108X, M630T, E510X, S666P, E669K, C730Y, A857V, K847E, L922PfsX7, and L1025FfsX39) and 4 in RAG2 (R73C, I427GfsX12, P432L, and 311insL). L1025FfsX39 is a potential RAG1 hot-spot mutation in the Chinese population. The distribution of RAG1 mutations rather than mutation type seemed to differ between SCID and OS patients. The thymic output of T lymphocytes, TCR rearrangement, and T cell proliferation were severely impaired in RAG mutant patients. These findings will contribute to the early diagnosis and treatment of SCID and OS to a certain extent.

Tandem mass spectrometric determination of purine metabolites and adenosine deaminase activity for newborn screening of ADA–SCID

Background: Screening newborns for severe combined immunodeficiency (SCID) aims for early identification and treatment of the affected newborns. Adenosine deaminase (ADA) deficiency, a defect in the purine metabolic pathway, is a major cause of SCID and is characterized by the accumulation of adenosine (Ado) and deoxyadenosine (dAdo) in dried blood spots (DBSs). If left untreated, infants with this disorder are at risk of life-threatening infections. Analysis of T-cell receptor excision circles (TRECs) in DBS samples is the gold-standard screening method. However, TREC analysis is insufficient to determine SCID etiology, and a fraction of ADA–SCID may not be detected.Methods: We used the original DBS screening sample to measure Ado, dAdo, and ADA activity. Erythro-9-(2-hydroxy-3-nonyl) adenine was used as an ADA inhibitor to imitate ADA deficiency, making it possible to create quality control material with pathological enzyme activity and metabolite levels. Quantification was achieved by tandem mass spectrometric analysis with a run time of 2.5 min.Results: The 95th percentile reference intervals (n = 588) of Ado and dAdo were 0.9–3.0 and 0.1–0.4 µmol/L, respectively. The 95th percentile reference interval (n = 200) of ADA activity using13C10,15N5Ado and15N5dAdo as substrates were 0.8–1.6 and 0.4–0.7 pmol/DBS, respectively. In confirmed ADA patients (n = 4), Ado and dAdo were significantly elevated, whereas ADA activity was almost absent.Conclusion: These novel methods are applied, in our lab, to samples with low TRECs, with no false negative or false positives encountered to date. The potential of using these methods as a primary screening approach for ADA–SCID is in the process of validation.Statement of novelty: New mass spectrometric methods to simultaneously measure adenosine, deoxyadenosine, guanosine, and deoxguanosine, as well as ADA activity in neonatal DBS samples have been developed. This methodology highlights the metabolic nature of ADA–SCID and complements TREC analysis by providing additional biochemical information.

New insights into the metabolic and nutritional determinants of severe combined immunodeficiency.

Human mutations in MTHFD1 have recently been identified in patients with severe combined immunodeficiency (SCID). SCID results from inborn errors of metabolism that cause impaired T- and B-cell proliferation and function. One of the most common causes of SCID is adenosine deaminase (ADA) deficiency, which ultimately inhibits DNA synthesis and cell division. MTHFD1 has been shown to translocate to the nucleus during S-phase of the cell cycle; this localization is critical for synthesis of thymidyate (dTMP or the “T” base in DNA) and subsequent progression through the cell cycle and cell proliferation. Identification of MTHFD1 mutations that are associated with SCID highlights the potential importance of adequate dTMP synthesis in the etiology of SCID.

Hypomorphic Janus kinase 3 mutations result in a spectrum of immune defects, including partial maternal T-cell engraftment

Mutations in Janus kinase 3 (JAK3) are a cause of severe combined immunodeficiency, but hypomorphic JAK3 defects can result in a milder clinical phenotype, with residual development and function of autologous T cells. Maternal T-cell engraftment is a common finding in infants with severe combined immunodeficiency but is not typically observed in patients with residual T-cell development. We sought to study in detail the molecular, cellular, and humoral immune phenotype and function of 3 patients with hypomorphic JAK3 mutations. We analyzed the distribution and function of T and B lymphocytes in 3 patients and studied the invitro and invivo responses of maternal T lymphocytes in 1 patient with maternal T-cell engraftment and residual production of autologous Tlymphocytes. B cells were present in normal numbers but with abnormal distribution of marginal zone-like and memory Bcells. B-cell differentiation to plasmablasts invitro in response to CD40 ligand and IL-21 was abolished. In 2 patients the T-cell repertoire was moderately restricted. Surprisingly, 1 patient showed coexistence of maternal and autologous T lymphocytes. By using an mAb recognizing the maternal noninherited HLA-A2 antigen, we found that autologous cells progressively accumulated invivo but did not compete with maternal cells invitro. The study of 3 patients with hypomorphic JAK3 mutations suggests that terminal B-cell maturation/differentiation requires intact JAK3 function, even if partially functioning T lymphocytes are present. Maternal T-cell engraftment can occur in patients with JAK3 mutations despite the presence of autologous T cells.

Severe Combined Immunodeficiency Resulting From Mutations in MTHFD1

Folate and vitamin B(12) metabolism are essential for de novo purine synthesis, and several defects in these pathways have been associated with immunodeficiency. Here we describe the occurrence of severe combined immunodeficiency (SCID) with megaloblastic anemia, leukopenia, atypical hemolytic uremic syndrome, and neurologic abnormalities in which hydroxocobalamin and folate therapy provided partial immune reconstitution. Whole exome sequencing identified compound heterozygous mutations in the MTHFD1 gene, which encodes a trifunctional protein essential for processing of single-carbon folate derivatives. We now report the immunologic details of this novel genetic cause of SCID and the response to targeted metabolic supplementation therapies. This finding expands the known metabolic causes of SCID and presents an important diagnostic consideration given the positive impact of therapy.

Mutant JAK3 FERMents ATLL

Loss-of-function mutations in Janus kinase 3 (JAK3) are an underlying cause of severe combined immunodeficiency (SCID), whereas hyperactive JAK3 mutants have been identified in hematolocigal malignancies.1–3 In this issue of Blood, Elliot et al place JAK3 again under the magnifying lens and describe novel gain-of-function mutations in the FERM (Founding members: band 4.1, Ezrin, Radixin, and Moesin) domain of JAK3 in adult patients with T-cell leukemia/lymphoma (ATLL).4

Severe combined immunodeficiency in Frisian Water Dogs caused by a RAG1 mutation

Mortality of pups at 8-12 weeks of age was frequently observed in Frisian Water Dogs. Blood parameters and clinical signs of newborns from three litters were monitored. Three pups from two litters showed strongly reduced levels of immunoglobulins and lymphocytes. These dogs were euthanized after first display of disease. Concurrent clinical and pathological features were consistent with a diagnosis of severe combined immunodeficiency (SCID). Defective V(D)J recombination is one of the causes of SCID in humans and animals. Eight genes involved in V(D)J recombination were investigated by segregation analysis of closely located microsatellite markers and by DNA sequence analysis. A nonsense mutation in the gene coding for V(D)J recombination factor RAG1 was identified in DNA from the cases at a position similar to that of nonsense mutations found in human SCID. It was concluded that SCID due to a mutation of RAG1 led to the high mortality.

Beware the lymphopenia: A case of severe combined immunodeficiency

We present a case of a 2-month-old boy with partially treated meningitis and suspected Pneumocystis carinii pneumonia. A full blood count revealed profound lymphopenia. The child was diagnosed with adenosine deaminase deficiency, a rare cause of severe combined immunodeficiency (SCID). SCID is an immunological emergency and must be considered in any lymphopaenic infant with opportunistic infection. We discuss adenosine deaminase-deficient SCID, which can involve multiple systems and in which other treatment options apart from bone marrow transplant are available.

Mutations in CHD7 in patients with CHARGE syndrome cause T–B + natural killer cell + severe combined immune deficiency and may cause Omenn-like syndrome

More than 11 genetic causes of severe combined immunodeficiency (SCID) have been identified, affecting development and/or function of T lymphocytes, and sometimes B lymphocytes and natural killer (NK) cells. Deletion of 22q11.2 is associated with immunodeficiency, although less than 1% of cases are associated with T-B + NK + SCID phenotype. Severe immunodeficiency with CHARGE syndrome has been noted only rarely Omenn syndrome is a rare autosomal recessive form of SCID with erythroderma, hepatosplenomegaly, lymphadenopathy and alopecia. Hypomorphic recombination activating genes 1 and 2 mutations were first described in patients with Omenn syndrome. More recently, defects in Artemis, RMRP, IL7Ralpha and common gamma chain genes have been described. We describe four patients with mutations in CHD7, who had clinical features of CHARGE syndrome and who had T-B + NK + SCID (two patients) or clinical features consistent with Omenn syndrome (two patients). Immunodeficiency in patients with DiGeorge syndrome is well recognized--CHARGE syndrome should now be added to the causes of T-B + NK + SCID, and mutations in the CHD7 gene may be associated with Omenn-like syndrome.

Jak3 and the pathogenesis of severe combined immunodeficiency

The discovery that Jak3 mutations are a significant cause of severe combined immunodeficiency (SCID), a rare inherited defect characterized by lymphopenia, has provided valuable insights into the functions of Jak3 in lymphoid development and function. The current therapy for patients suffering from Jak3 SCID is hematopoetic stem cell transplantation, although gene therapy trials have also been performed. In lieu of crystal structure data, these patient-derived mutations have aided in the elucidation of the functions of various structural components of Jak3. By virtue of its requirement for lymphoid functions, Jak3 makes a tantalizing target for immunosuppression and anti-cancer therapy. Herein, we discuss the normal actions of the γ c cytokines, the pathogenesis and treatment protocols for SCID, and finally, the production of a new, selective Jak3 inhibitor capable of preventing transplant rejection in two animal models. Further study of Jak3 will hopefully provide insights into the clinical treatment of patients suffering from immune-mediated diseases.

Reconstitution of Early Lymphoid Proliferation and Immune Function in Jak3-Deficient Mice by Interleukin-3

Expansion of early lymphoid progenitors requires interleukin-7 (IL-7), which functions through gamma(c)-mediated receptor activation of Jak3. Jak3 deficiency is a cause of severe combined immunodeficiency (SCID) in humans and mice. IL-3 activates many of the same signaling pathways as IL-7, such as Stat5, but achieves this effect through the activation of Jak2 rather than Jak3. We hypothesized that expansion of an IL-7-responsive precursor population through a Jak3-independent pathway using IL-3 may stimulate early lymphoid progenitors and restore lymphopoiesis in Jak3(-/-) mice. Newborn Jak3(-/-) mice that were injected with IL-3 demonstrated thymic enlargement, a 2- to 20-fold increase in thymocyte numbers, and up to a 10-fold expansion in the number of CD4(+), CD8(+), and B220(+)/IgM(+) splenic lymphocytes, consistent with an effect upon an early lymphoid progenitor population. In contrast to control mice, IL-3-treated Jak3(-/-) mice challenged with the allogeneic major histocompatibility complex (MHC) class I-bearing tumor P815 developed a specific CD8-dependent cytotoxic T lymphocyte (CTL) response. IL-3-treated mice also mounted influenza-specific CTL responses and survival was prolonged. The beneficial effects of IL-3 are proposed to be produced by stimulation of a lymphoid precursor population of IL-7Ralpha(+)/IL-3Ralpha(+) cells that we identified in wild-type bone marrow. In vitro, we show that an early IL-7R(+) lymphoid progenitor population expresses IL-3R and proliferates in response to IL-3 and that IL-3 activates Stat5 comparably to IL-7. Clinically, IL-3 may therefore be useful treatment for X-linked and Jak3-deficient SCID patients who lack bone marrow donors.

Reconstitution of Early Lymphoid Proliferation and Immune Function in Jak3-Deficient Mice by Interleukin-3

AbstractExpansion of early lymphoid progenitors requires interleukin-7 (IL-7), which functions through γc-mediated receptor activation of Jak3. Jak3 deficiency is a cause of severe combined immunodeficiency (SCID) in humans and mice. IL-3 activates many of the same signaling pathways as IL-7, such as Stat5, but achieves this effect through the activation of Jak2 rather than Jak3. We hypothesized that expansion of an IL-7–responsive precursor population through a Jak3-independent pathway using IL-3 may stimulate early lymphoid progenitors and restore lymphopoiesis in Jak3−/− mice. Newborn Jak3−/− mice that were injected with IL-3 demonstrated thymic enlargement, a 2- to 20-fold increase in thymocyte numbers, and up to a 10-fold expansion in the number of CD4+, CD8+, and B220+/IgM+ splenic lymphocytes, consistent with an effect upon an early lymphoid progenitor population. In contrast to control mice, IL-3–treated Jak3−/− mice challenged with the allogeneic major histocompatibility complex (MHC) class I-bearing tumor P815 developed a specific CD8-dependent cytotoxic T lymphocyte (CTL) response. IL-3–treated mice also mounted influenza-specific CTL responses and survival was prolonged. The beneficial effects of IL-3 are proposed to be produced by stimulation of a lymphoid precursor population of IL-7Rα+/IL-3Rα+ cells that we identified in wild-type bone marrow. In vitro, we show that an early IL-7R+ lymphoid progenitor population expresses IL-3R and proliferates in response to IL-3 and that IL-3 activates Stat5 comparably to IL-7. Clinically, IL-3 may therefore be useful treatment for X-linked and Jak3-deficient SCID patients who lack bone marrow donors.

Ecto-ADA in the development of theimmune system

The causes of severe combined immunodeficiency (SCID) due to the congenital defect of adenosine deaminase (ADA) are not known. To explain ADA-related SCID, a number of abnormalities derived from accumulation of ADA substrates, adenosine and deoxyadenosine, have been hypothesized. In revisiting all the possible hypotheses, Resta and Thompson[1xResta, R.R. and Thompson, L.F. Immunol. Today. 1997; 18: 371–3740Abstract | Full Text PDF | PubMed | Scopus (52)See all References[1]focus on an impairment of thymic development due to adenosine accumulation. This impairment results from activation of adenosine receptors coupled to adenylate cyclase, which subsequently leads to apoptosis. With our current knowledge of the effects of ADA deficiency we think that all the possibilities are open and that probably more than one single mechanism will be necessary to explain the diverse clinical manifestations of the disease.The article by Resta and Thompson makes no reference to the fact that ADA appears on the cell as an ecto-enzyme (ecto-ADA)[2xFranco, R., Valenzuela, A., Lluis, C., and Blanco, J. Immunol. Rev. 1998; 161: 27–42Crossref | PubMed | Scopus (129)See all References[2], which may be involved in the pathogenesis of ADA-related SCID. In fact, in peripheral nucleated blood cells, ecto-ADA is found on the majority of monocytes and B cells, and on 10–20% of T cells[3xAran, J.M., Colomer, D., Matutes, E., Vives Corrons, J.L., and Franco, R. J. Histochem. Cytochem. 1991; 39: 1001–1008Crossref | PubMedSee all References[3]. Ecto-ADA expression is enhanced in T-cell activation and its presence is necessary for complete T-cell activation[4xMartin, M., Huguet, J., Centelles, J.J., and Franco, R. J. Immunol. 1995; 155: 4630–4643PubMedSee all References[4]. The physiological role of ecto-ADA is related to its ability to degrade extracellular adenosine[5xDong, R.P., Kameoka, J., Hegen, M. et al. J. Immunol. 1996; 156: 1349–1355PubMedSee all References[5]. In addition, ADA has an extra-enzymatic behavior in that it can transduce signals through its interaction with CD26 on lymphocytes[4xMartin, M., Huguet, J., Centelles, J.J., and Franco, R. J. Immunol. 1995; 155: 4630–4643PubMedSee all References[4]or through its interaction with A1 adenosine receptors (A1R) on nonlymphoid cells[6xCiruela, F., Saura, C., Canela, E.I., Mallol, J., Lluis, C., and Franco, R. FEBS Lett. 1996; 380: 219–223Abstract | Full Text PDF | PubMed | Scopus (116)See all References[6].It is not yet known to what extent ecto-ADA is involved in the organogenesis of the immune system and in maturation of T and B cells in humans. In support of a specific role for ecto-ADA in these events, ADA-replacement therapies for ADA-deficient children, such as infusion of bovine ADA coupled to polyethylene glycol, have proved useful even though the enzyme remains extracellularly located. Based on the existence of ADA–CD26 signaling pathways in lymphocytes, we postulate a role for ecto-ADA in cell-to-cell contacts which would be important for the development of lymphoid tissues and for maturation of lymphocytes[2xFranco, R., Valenzuela, A., Lluis, C., and Blanco, J. Immunol. Rev. 1998; 161: 27–42Crossref | PubMed | Scopus (129)See all References[2].If ecto-ADA facilitates information exchange from cell to cell, it is possible that in lymphoid tissues ecto-ADA mediates specific interactions either between lymphocytes (T or B) or between lymphocytes and accessory cells. The quality of the information provided by this ecto-ADA would depend on which molecules make contact with the ecto-enzyme in each of the two communicating cells. Possible contacts would be: CD26–ADA–CD26, CD26–ADA–A1R or A1R–ADA–A1R. Additional contacts are possible since not all ecto-ADA molecules co-localize with CD26 on lymphocytes (C. Lluis et al., unpublished). Quantitatively, the strength of the signal would depend upon the concentration of ecto-ADA available. This raises interesting possibilities for ecto-ADA function, since cell-to-cell contacts are of relevance in the development and functionality of lymphoid tissues.

Restoration of lymphocyte function in Janus kinase 3-deficient mice by retroviral-mediated gene transfer.

Janus kinase-3 (JAK3) deficiency has recently been identified as a cause of severe combined immunodeficiency (SCID) in humans. We used a mouse model of Jak3-deficient SCID to test a gene therapy approach for treatment of this disease. Transfer of a Jak3 retroviral vector to repopulating hematopoietic stem cells resulted in increased numbers of T and B lymphocytes, reversal of hypogammaglobulinemia, restoration of T-cell activation upon stimulation with mitogens, and development of an antigen-specific immune response after immunization. Analysis for vector copy number in lymphoid and myeloid populations showed a large in vivo selective advantage for Jak3-expressing lymphoid cells. These results show that gene replacement is a feasible treatment strategy for this disease and that naturally occurring in vivo selection of corrected cells is an important advantage of this approach.