MolecularDefects inHumanSevereCombinedImmunodeficiency andApproaches toImmuneReconstitution

Transplantation of hematopoietic stem cells in human severe combined immunodeficiency: longterm outcomes

Severe combined immunodeficiency (SCID) is a syndrome of diverse genetic cause characterized by profound deficiencies of T- and B-cell function and, in some types, also of NK cells and function. Mutations in thirteen different genes have been found to cause this condition, which is uniformly fatal in the first 2 years of life unless immune reconstitution can be accomplished. In the 42 years since the first bone marrow transplant was given in 1968, the standard treatment for all forms of SCID has been allogeneic bone marrow transplantation. Both HLA-identical unfractionated and T-cell-depleted HLA-haploidentical bone marrow transplants have been very successful in effecting immune reconstitution, especially if performed in the first 3.5 months of life and without pre-transplant chemotherapy. This paper summarizes the longterm outcome, according to molecular type, of 166 consecutive SCID infants given non-conditioned related donor bone marrow transplants at this institution over the past 28.3 years and reviews published reports of longterm outcomes of transplants in SCID performed at other centers.

Herpes simplex virus encephalitis in a patient with complete TLR3 deficiency: TLR3 is otherwise redundant in protective immunity

A new autosomal recessive form of complete TLR3 deficiency reveals that human TLR3 is nonredundant in immunity against herpes simplex virus 1 in the central nervous system (CNS) but redundant in host defense against viruses outside the CNS.

Autosomal dominant TLR3 deficiency has been identified as a genetic etiology of childhood herpes simplex virus 1 (HSV-1) encephalitis (HSE). This defect is partial, as it results in impaired, but not abolished induction of IFN-β and -λ in fibroblasts in response to TLR3 stimulation. The apparently normal resistance of these patients to other infections, viral illnesses in particular, may thus result from residual TLR3 responses. We report here an autosomal recessive form of complete TLR3 deficiency in a young man who developed HSE in childhood but remained normally resistant to other infections. This patient is compound heterozygous for two loss-of-function TLR3 alleles, resulting in an absence of response to TLR3 activation by polyinosinic-polycytidylic acid (poly(I:C)) and related agonists in his fibroblasts. Moreover, upon infection of the patient’s fibroblasts with HSV-1, the impairment of IFN-β and -λ production resulted in high levels of viral replication and cell death. In contrast, the patient’s peripheral blood mononuclear cells responded normally to poly(I:C) and to all viruses tested, including HSV-1. Consistently, various TLR3-deficient leukocytes from the patient, including CD14⁺ and/or CD16⁺ monocytes, plasmacytoid dendritic cells, and in vitro derived monocyte-derived macrophages, responded normally to both poly(I:C) and HSV-1, with the induction of antiviral IFN production. These findings identify a new genetic etiology for childhood HSE, indicating that TLR3-mediated immunity is essential for protective immunity to HSV-1 in the central nervous system (CNS) during primary infection in childhood, in at least some patients. They also indicate that human TLR3 is largely redundant for responses to double-stranded RNA and HSV-1 in various leukocytes, probably accounting for the redundancy of TLR3 for host defense against viruses, including HSV-1, outside the CNS.

Neonatal screening for severe combined immunodeficiency caused by an adenosine deaminase defect: a reliable and inexpensive method using tandem mass spectrometry

BACKGROUND

Adenosine deaminase (ADA)-severe combined immunodeficiency (SCID) is an SCID caused by a defect in the enzyme adenosine deaminase. It is usually fatal in infancy because of severe recurrent infections. When diagnosis is made, permanent damage caused by infections or by metabolites is often present. Gene therapy, bone marrow transplantation, or enzyme therapy might be effective if performed early. ADA-SCID complies with all the criteria for inclusion in a newborn screening program. However, screening methods are still expensive or provide a non-negligible number of indeterminate results.

OBJECTIVE

The aim of the present study was to develop a simple, reliable, and inexpensive method for diagnosis of ADA-SCID by using dried blood spot (DBS) samples taken at birth. Cost per test was calculated, including the cost for reagents, equipment, and operators.

METHODS

DBS samples from 4 patients with genetically confirmed ADA-SCID and 12,020 DBS samples from healthy newborns were examined. Adenosine and 2'-deoxyadenosine were tested by using tandem mass spectrometry (PCT EP2010/070517).

RESULTS

The mean levels of adenosine and 2'-deoxyadenosine were 7.8 ± 3.1 and 8.5 ± 6.0 μmol/L, respectively, in affected children; adenosine was found at 0.23 ± 0.09 μmol/L, whereas 2'-deoxyadenosine was never detected in healthy control subjects (adenosine: P < 10(-6) [95% confidence limit, 7.59-7.78] and 2'-deoxyadenosine: P < 10(-6) [95% confidence limit, 8.65-8.82] for control subjects vs patients with ADA-SCID). No indeterminate or false-positive results were found. Cost per test was €0.01 ($0.013). A pilot population-based newborn screening for ADA-SCID has started in Tuscany, Italy.

CONCLUSION

Tandem mass spectrometry can be used for diagnosis of one of the most frequent form of SCID at a negligible cost.

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.

Hematopoietic stem cell transplantation for CD3δ deficiency

BACKGROUND

CD3δ deficiency is a fatal form of severe combined immunodeficiency that can be cured by hematopoietic stem cell transplantation (HSCT). The presence of a thymus loaded with T-cell progenitors in patients with CD3δ deficiency may require special considerations in choosing the regimen of conditioning and the type of HSCT.

OBJECTIVES

To study the outcome of CD3δ deficiency by using various modalities of stem cell transplantation.

METHODS

We analyzed data on 13 patients with CD3δ deficiency who underwent HSCT in 7 centers. HSCT was performed by using different sources of donor stem cells as well as various conditioning regimens.

RESULTS

One patient received stem cells from a matched related donor and survived after a second transplant, needing substantial conditioning in order to engraft. Only 2 of 7 other patients who received a mismatched related donor transplant survived; 2 of them had no conditioning, whereas the others received various combinations of conditioning regimens. Engraftment of T cells in the survivors appears incomplete. Three other patients who received stem cells from a matched unrelated donor survived and enjoyed full immune reconstitution. Two patients received unrelated cord blood without conditioning. One of them has had a partial but stable engraftment, whereas the other engrafted well but is only 12 months after HSCT. We also report here for the first time that patients with CD3δ deficiency can present with typical features of Omenn syndrome.

CONCLUSIONS

HSCT is a successful treatment for patients with CD3δ deficiency. The small number of patients in this report prevents definitive statements on the importance of survival factors, but several are suggested: (1) HLA-matched donor transplants are associated with superior reconstitution and survival than are mismatched donor transplants; (2) substantial conditioning appears necessary; and (3) early diagnosis and absence of opportunistic infections may affect outcome.

IL-7R expression and IL-7 signaling confer a distinct phenotype on developing human B-lineage cells

IL-7 is an important cytokine for lymphocyte differentiation. Similar to what occurs in vivo, human CD19⁺ cells developing in human/murine xenogeneic cultures show differential expression of the IL-7 receptor α (IL-7Rα) chain (CD127). We now describe the relationship between CD127 expression/signaling and Ig gene rearrangement. In the present study, < 10% of CD19⁺CD127⁺ and CD19⁺CD127⁻ populations had complete VDJ(H) rearrangements. IGH locus conformation measurements by 3D FISH revealed that CD127⁺ and CD127⁻ cells were less contracted than pediatric BM pro-B cells that actively rearrange the IGH locus. Complete IGH rearrangements in CD127⁺ and CD127⁻ cells had smaller CDR3 lengths and fewer N-nucleotide insertions than pediatric BM B-lineage cells. Despite the paucity of VDJ(H) rearrangements, microarray analysis indicated that CD127⁺ cells resembled large pre-B cells, which is consistent with their low level of Ig light-chain rearrangements. Unexpectedly, CD127⁻ cells showed extensive Ig light-chain rearrangements in the absence of IGH rearrangements and resembled small pre-B cells. Neutralization of IL-7 in xenogeneic cultures led to an increase in Ig light-chain rearrangements in CD127⁺ cells, but no change in complete IGH rearrangements. We conclude that IL-7-mediated suppression of premature Ig light-chain rearrangement is the most definitive function yet described for IL-7 in human B-cell development.

Congenital neutropenia: diagnosis, molecular bases and patient management

The term congenital neutropenia encompasses a family of neutropenic disorders, both permanent and intermittent, severe (<0.5 G/l) or mild (between 0.5-1.5 G/l), which may also affect other organ systems such as the pancreas, central nervous system, heart, muscle and skin. Neutropenia can lead to life-threatening pyogenic infections, acute gingivostomatitis and chronic parodontal disease, and each successive infection may leave permanent sequelae. The risk of infection is roughly inversely proportional to the circulating polymorphonuclear neutrophil count and is particularly high at counts below 0.2 G/l.When neutropenia is detected, an attempt should be made to establish the etiology, distinguishing between acquired forms (the most frequent, including post viral neutropenia and auto immune neutropenia) and congenital forms that may either be isolated or part of a complex genetic disease.Except for ethnic neutropenia, which is a frequent but mild congenital form, probably with polygenic inheritance, all other forms of congenital neutropenia are extremely rare and have monogenic inheritance, which may be X-linked or autosomal, recessive or dominant.About half the forms of congenital neutropenia with no extra-hematopoietic manifestations and normal adaptive immunity are due to neutrophil elastase (ELANE) mutations. Some patients have severe permanent neutropenia and frequent infections early in life, while others have mild intermittent neutropenia.Congenital neutropenia may also be associated with a wide range of organ dysfunctions, as for example in Shwachman-Diamond syndrome (associated with pancreatic insufficiency) and glycogen storage disease type Ib (associated with a glycogen storage syndrome). So far, the molecular bases of 12 neutropenic disorders have been identified.Treatment of severe chronic neutropenia should focus on prevention of infections. It includes antimicrobial prophylaxis, generally with trimethoprim-sulfamethoxazole, and also granulocyte-colony-stimulating factor (G-CSF). G-CSF has considerably improved these patients' outlook. It is usually well tolerated, but potential adverse effects include thrombocytopenia, glomerulonephritis, vasculitis and osteoporosis. Long-term treatment with G-CSF, especially at high doses, augments the spontaneous risk of leukemia in patients with congenital neutropenia.

PID comes full circle: applications of V(D)J recombination excision circles in research, diagnostics and newborn screening of primary immunodeficiency disorders

The vast majority of patients suffering from a primary immunodeficiency (PID) have defects in their T- and/or B-cell compartments. Despite advances in molecular diagnostics, in many patients no underlying genetic defect has been identified. B- and T-lymphocytes are unique in their ability to create a receptor by genomic rearrangement of their antigen receptor genes via V(D)J recombination. During this process, stable circular excision products are formed that do not replicate when the cell proliferates. Excision circles can be reliably quantified using real-time quantitative (RQ-)PCR techniques. Frequently occurring δREC-ψJα T-cell receptor excision circles (TRECs) have been used to assess thymic output and intronRSS-Kde recombination excision circles (KREC) to quantify B-cell replication history. In this perspective, we describe how TRECs and KRECs are formed during precursor - T- and B-cell differentiation, respectively. Furthermore, we discuss new insights obtained with TRECs and KRECs and specifically how these excision circles can be applied to support therapy monitoring, patient classification and newborn screening of PID.

Gene therapy for primary immunodeficiencies

The concept of gene therapy emerged as a way of correcting monogenic inherited diseases by introducing a normal copy of the mutated gene into at least some of the patients' cells. Although this concept has turned out to be quite complicated to implement, it is in the field of primary immunodeficiencies (PIDs) that proof of feasibility has been undoubtedly achieved. There is now a strong rationale in support of gene therapy for at least some PIDs, as discussed in this article.

Molecular diagnosis of severe combined immunodeficiency--identification of IL2RG, JAK3, IL7R, DCLRE1C, RAG1, and RAG2 mutations in a cohort of Chinese and Southeast Asian children

Severe combined immunodeficiencies (SCID) are a group of rare inherited disorders with profound defects in T cell and B cell immunity. From 2005 to 2010, our unit performed testing for IL2RG, JAK3, IL7R, RAG1, RAG2, DCLRE1C, LIG4, AK2, and ZAP70 mutations in 42 Chinese and Southeast Asian infants with SCID adopting a candidate gene approach, based on patient's gender, immune phenotype, and inheritance pattern. Mutations were identified in 26 patients, including IL2RG (n = 19), IL7R (n = 2), JAK3 (n = 2), RAG1 (n = 1), RAG2 (n = 1), and DCLRE1C (n = 1). Among 12 patients who underwent hematopoietic stem cell transplantation, eight patients survived. Complications and morbidities during transplant period were significant, especially disseminated bacillus Calmette-Guérin disease which was often difficult to control. This is the first cohort study on SCID in the Chinese and Southeast Asian population, based on a multi-centered collaborative research network. The foremost issue is service provision for early detection, diagnosis, management, and definitive treatment for patients with SCID. National management guidelines for SCID should be established, and research into an efficient platform for genetic diagnosis is needed.

IL-15 transpresentation promotes both human T-cell reconstitution and T-cell-dependent antibody responses in vivo

Cytokine immunotherapies targeting T lymphocytes are attractive clinical interventions against viruses and tumors. In the mouse, the homeostasis of memory α/β CD8(+) T cells and natural killer (NK) cells is significantly improved with increased IL-15 bioavailability. In contrast, the role of "transpresented" IL-15 on human T-cell development and homeostasis in vivo is unknown. We found that both CD8 and CD4 T cells in human immune system (HIS) mice are highly sensitive to transpresented IL-15 in vivo, with both naïve (CD62L(+)CD45RA(+)) and memory phenotype (CD62L(-)CD45RO(+)) subsets being significantly increased following IL-15 "boosting." The unexpected global improvement in human T-cell homeostasis involved enhanced proliferation and survival of both naïve and memory phenotype peripheral T cells, which potentiated B-cell responses by increasing the frequency of antigen-specific responses following immunization. Transpresented IL-15 did not modify T-cell activation patterns or alter the global T-cell receptor (TCR) repertoire diversity. Our results indicate an unexpected effect of IL-15 on human T cells in vivo, in particular on CD4(+) T cells. As IL-15 promotes human peripheral T-cell homeostasis and increases the frequency of neutralizing antibody responses in HIS mice, IL-15 immunotherapy could be envisaged as a unique approach to improve vaccine responses in the clinical setting.

Gene therapy for primary immunodeficiencies: looking ahead, toward gene correction

Allogeneic hematopoietic stem cell transplantation is the treatment of choice for severe primary immunodeficiencies (PIDs). For patients lacking an HLA-identical donor, gene therapy is an attractive therapeutic option. Approaches based on insertion of a functional gene by using viral vectors have provided proof of concept for the ability of gene therapy to cure PIDs. However, leukemic transformation as a result of insertional mutagenesis has been observed, prompting development of novel approaches based on introduction of DNA double-strand breaks into the endogenous locus to achieve gene correction, or into a safe genomic location ("safe harbor"). Homing endonucleases and zinc finger nucleases are target-specific endonucleases that induce site-specific DNA double-strand breaks, facilitating homologous recombination around their target sites to achieve gene correction or gene insertion into safe harbors. An alternative approach to achieve site-specific insertion of functional genes is based on transposons, DNA elements that spontaneously translocate from a specific chromosomal location to another. These novel tools may lead to efficient and safer strategies to achieve gene therapy for PIDs and other disorders.

Neonatal diagnosis of severe combined immunodeficiency leads to significantly improved survival outcome: the case for newborn screening

Severe combined immunodeficiency (SCID) carries a poor prognosis without definitive treatment by hematopoietic stem cell transplantation. The outcome for transplantation varies and is dependent on donor status and the condition of the child at the time of transplantation. Diagnosis at birth may allow for better protection of SCID babies from infection and improve transplantation outcome. In this comparative study conducted at the 2 designated SCID transplantation centers in the United Kingdom, we show that SCID babies diagnosed at birth because of a positive family history have a significantly improved outcome compared with the first presenting family member. The overall improved survival of more than 90% is related to a reduced rate of infection and significantly improved transplantation outcome irrespective of donor choice, conditioning regimen used, and underlying genetic diagnosis. Neonatal screening for SCID would significantly improve the outcome in this otherwise potentially devastating condition.

Insulin effect on glucose transport in thymocytes and splenocytes from rats with metabolic syndrome

Metabolic syndrome (MS) may comprise several clinical conditions such as obesity, diabetes and inflammatory disorders, which are characterized by metabolic imbalances. The study of glucose transport and regulation by insulin in lymphocytes is important, since the way they increase inflammation and susceptibility to infections are common in MS. We studied glucose internalization in isolated thymocytes and splenocytes, its regulation by insulin, and the role of three glucose transporters (Gluts) in control and in MS rats. Control glucose internalization and insulin responses were lower in splenocytes than in thymocytes. Control and insulin-induced glucose internalization in thymocytes declined with age, while transport by splenocyte continued to respond to insulin. Control thymocyte glucose internalization was blocked by antibodies against Glut 1 and 4, while the insulin response also was blocked by an anti-Glut 3 antibody. On four month old control and insulin-induced response, splenocyte transport was only blocked by Glut 1 and 4 antibodies. At six months splenocyte glucose internalization depended on Glut 1 and was less sensitive to the effects of an anti-Glut 4 antibody. In MS splenocytes the capacity of anti-Glut 1 antibodies to inhibit control and insulin-dependent glucose transport was less significant, and we found that in MS rats, glucose internalization was dependent on Glut 3 and Glut 4. In summary, the altered metabolic state present in MS rats shows signs of modulation of glucose internalization by the Glut1, Glut 3 and Glut 4 transporters, compared with its own age control.

Early vs. delayed diagnosis of severe combined immunodeficiency: a family perspective survey

Infants affected with severe combined immunodeficiency (SCID) are susceptible to severe and recurrent infections and do not survive unless provided with immune reconstituting treatments. In the absence of population-based newborn screening, infants with SCID who do not have an affected older relative are ascertained only after they have developed infections. However, only limited data are available from the perspective of patients and families to indicate what proportion of SCID cases might benefit from earlier detection by pre-symptomatic screening, whether adequate treatment facilities are available, and how screening could improve SCID treatment outcomes. A survey of parents of children with SCID evaluated family history, pre- and post-diagnosis events, outcomes, and impact of SCID on families. Affected infants diagnosed with SCID as neonates had better survival, demonstrating the potential benefit of universal newborn screening.

Toxicopathology of the Developing Immune System

Developmental immunotoxicity (DIT) has gained attention with the recognition that environmental chemicals can potentially affect the developing immune system and the incidence of childhood allergic diseases. Preclinical safety assessment of pharmaceuticals for men and women of childbearing potential as well as for pediatric and juvenile indications may require DIT assessments. Draft documents from environmental and chemical regulatory agencies propose strategies that use the rat as a test species and incorporate histopathology and functional testing as endpoints. While there are no guidelines for DIT assessment of pharmaceuticals, current discussions suggest that combining immunotoxicity and developmental and reproductive toxicology studies may serve this purpose. Knowledge of the principles and applications of DIT will facilitate participation in strategy development and effective conduct of relevant studies.

Guidelines for implementation of population-based newborn screening for severe combined immunodeficiency

Severe combined immunodeficiency (SCID) is a Primary Immune Deficiency that is under consideration for population-based newborn screening (NBS) by many NBS programs, and has recently been recommended for inclusion in the US uniform panel of newborn screening conditions. A marker of SCID, the T cell receptor excision circle (TREC), is detectable in the newborn dried blood spot using a unique molecular assay as a primary screen. The New England Newborn Screening Program developed and validated a multiplex TREC assay in which both the TREC analyte and an internal control are acquired from a single punch and run in the same reaction. Massachusetts then implemented a statewide pilot SCID NBS program. The authors describe the rationale for a pilot SCID NBS program, a comprehensive strategy for successful implementation, the screening test algorithm, the screening follow-up algorithm and preliminary experience based on statewide screening in the first year. The Massachusetts experience demonstrates that SCID NBS is a program that can be implemented on a population basis with reasonable rates of false positives.

The density of CD10 corresponds to commitment and progression in the human B lymphoid lineage

BACKGROUND

Requirements for human B lymphopoiesis are still poorly understood, and that has hampered investigation of differentiation events. For example, there are few cell surface antigens that can be used as milestones of lineage progression. The CD10 ectoenzyme is one such marker and has been used to define CLP, but we found substantial tissue specific variations in CD10 levels, and there was no information about how that corresponded to differentiation options.

METHODOLOGY/PRINCIPAL FINDINGS

The aim of the present study was to use recently developed culture methods to assess the nature and differentiation potential of progenitors sorted according to CD10 density from umbilical cord blood (CB), adult bone marrow (BM) or G-CSF mobilized peripheral blood (PB). Many CD34(+) cells in BM express high levels of CD10, while low or low/negative CD10 densities were found on CD34(+) cells in CB or G-CSF mobilized PB, respectively. The relative abundance of CD10(Lo) versus CD10(Hi) cells only accounts for some CB versus BM differences. Almost all of the CD34(+) CD10(Hi) cells expressed CD19 and lymphocyte transcription factors and corresponded to loss of myeloid potential. A high degree of immunoglobulin D(H)-J(H) gene rearrangements was characteristic only of the CD10(Hi) subset. In contrast, the CD34(+) CD10(Lo) progenitors efficiently produced plasmacytoid and conventional dendritic cells as well as myeloid cells. These findings suggest a positive correlation between CD10 density and degree of differentiation. Although freshly isolated CD34(+) CD10(Hi) cells were in cycle, those from CB or BM expanded poorly in culture, suggesting regulators of populations remain to be discovered.

CONCLUSIONS/SIGNIFICANCE

Steps in human B lymphopoiesis have not been sufficiently studied, and we now show that increased CD10 expression corresponds to differentiation potential and stage. CD34(+) CD10(Hi) progenitors are obviously in the B lineage but may have progressed beyond the point where they can be expanded in culture.

Hematopoietic cell transplantation for Wiskott-Aldrich syndrome: advances in biology and future directions for treatment

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by a triad of diagnostic clinical elements: immunodeficiency, eczema, and hemorrhage caused by thrombocytopenia with small-sized platelets. The formal proof that hematopoietic cell transplantation (HCT) could be used to cure WAS revealed a requirement for both immunosuppression and myelosuppression that still underlies the standard approach to curative therapy today. The current short- and long-term toxicities of HCT are the main stumbling block for the ability to cure every patient with WAS and X-linked thrombocytopenia, and much remains to be done.

Haploidentical bone marrow transplantation in primary immune deficiency: stem cell selection and manipulation

Since the early 1980s T-cell depletion has allowed haploidentical bone marrow transplantation to be performed in patients with primary immunodeficiency for whom a matched sibling donor was not available, without causing severe graft versus host disease (GVHD). This review article presents the available data in the literature on survival, GVHD, and immune reconstitution in different categories of patients, with special emphasis on the impact of different T-cell depletion methods.

Why newborn screening for severe combined immunodeficiency is essential: a case report

Physicians caring for infants in the first months of life need to know the normal ranges for absolute lymphocyte counts (ALCs) during that age. Any ALC <2500/microL is potentially pathogenic in early infancy and should be evaluated. We report the case of a 4-month-old white girl with a 2-month history of an oral ulcer, intermittent fever, recurrent otitis, decreased appetite, weight loss, and a new respiratory illness with hypoxemia. She had been in an in-home day care since birth. The patient's primary care physician had seen her frequently and obtained blood counts, but her persistent lymphopenia had not been appreciated. The infant was ultimately diagnosed with T(-)B(-)NK(+) (lacking both B and T lymphocytes and having primarily natural killer [NK] cells), recombinase-activating gene 2 (RAG2)-deficient severe combined immunodeficiency (SCID). However, because she had already developed 2 difficult-to-treat viral infections (parainfluenza 3 and adenovirus), she did not survive long enough to receive a bone marrow transplant. Newborn screening would not only have made the diagnosis at birth but would have led to measures to protect her from becoming infected before she could receive a transplant. Newborn screening would also reveal the true incidence of SCID and define the range of conditions characterized by severely impaired T-cell development. Until screening for SCID and other T-cell defects becomes available for all neonates (either by quantifying T-cell receptor excision circles in Guthrie spots or using other tests that quantify T cells), all pediatricians should know the normal range for ALCs according to age. Recognition of the characteristic lymphopenia of SCID can facilitate early diagnosis.

High-throughput multiplexed T-cell-receptor excision circle quantitative PCR assay with internal controls for detection of severe combined immunodeficiency in population-based newborn screening

BACKGROUND

Real-time quantitative PCR (qPCR) targeting a specific marker of functional T cells, the T-cell-receptor excision circle (TREC), detects the absence of functional T cells and has a demonstrated clinical validity for detecting severe combined immunodeficiency (SCID) in infants. There is need for a qPCR TREC assay with an internal control to monitor DNA quality and the relative cellular content of the particular dried blood spot punch sampled in each reaction. The utility of the qPCR TREC assay would also be far improved if more tests could be performed on the same newborn screening sample.

METHODS

We approached the multiplexing of qPCR for TREC by attenuating the reaction for the reference gene, with focus on maintaining tight quality assurance for reproducible slopes and for prevention of sample-to-sample cross contamination. Statewide newborn screening for SCID using the multiplexed assay was implemented, and quality-assurance data were recorded.

RESULTS

The multiplex qPCR TREC assay showed nearly 100% amplification efficiency for each of the TREC and reference sequences, clinical validity for multiple forms of SCID, and an analytic limit of detection consistent with prevention of contamination. The eluate and residual ghost from a 3.2-mm dried blood spot could be used as source material for multiplexed immunoassays and multiplexed DNA tests (Multiplex Plus), with no disruption to the multiplex TREC qPCR.

CONCLUSIONS

Population-based SCID newborn screening programs should consider multiplexing for quality assurance purposes. Potential benefits of using Multiplex Plus include the ability to perform multianalyte profiling.

Thymus microenvironment in human primary immunodeficiency diseases

PURPOSE OF REVIEW

Severe combined immunodeficiencies represent a heterogeneous group of genetic disorders affecting genes of both early and late steps in lymphocytes development, a process tightly controlled by thymic epithelial cells. Detailed analysis of thymic morphology aids to the assessment of the severity of the immune disorder and may be critical to the understanding of the role of the genetic defects in the pathophysiology of these diseases. In this review, we highlight recent advancements in the characterization of the thymic microenvironment in primary immunodeficiencies.

RECENT FINDINGS

Crosstalk between thymocytes and thymic epithelial cells is essential to preserve thymic architecture and function, and therefore to promote T-cell maturation and development of self-tolerance. Early severe defects in T-cell development result in profound abnormalities of thymic epithelial cells differentiation with loss of AIRE expression and severe reduction of thymic dendritic and T-regulatory cells. Differently, later defects in T-cell development that are permissive for normal thymocytes development allow cortico-medullary differentiation with partially preserved AIRE expression and dendritic/T-regulatory cells distribution. Hypomorphic mutations in the same genes partially permissive to T-cell development may result in a more complex phenotype with immunodysreactivity and peculiar thymic alterations.

SUMMARY

Although the molecular and genetic bases of primary immunodeficiencies directly aid to both diagnosis and management of the patients, the detailed analysis of thymic morphology critically contributes to unveil the pathophysiology of these diseases.

A self-inactivating lentiviral vector for SCID-X1 gene therapy that does not activate LMO2 expression in human T cells

To develop safer and more effective vectors for gene therapy of X-linked severe combined immunodeficiency (SCID-X1), we have evaluated new self-inactivating lentiviral vectors based on the HIV virus. The CL20i4-hgamma(c)-Revgen vector contains the entire human common gamma chain (gamma(c)) genomic sequence driven by the gamma(c) promoter. The CL20i4-EF1alpha-hgamma(c)OPT vector uses a promoter fragment from the eukaryotic elongation factor alpha (EF1alpha) gene to express a codon-optimized human gamma(c) cDNA. Both vectors contain a 400-bp insulator fragment from the chicken beta-globin locus within the self-inactivating long-terminal repeat. Transduction of bone marrow cells using either of these vectors restored T, B, and natural killer lymphocyte development and function in a mouse SCID-X1 transplantation model. Transduction of human CD34(+) bone marrow cells from SCID-X1 patients with either vector restored T-cell development in an in vitro assay. In safety studies using a Jurkat LMO2 activation assay, only the CL20i4-EF1alpha-hgamma(c)OPT vector lacked the ability to transactivate LMO2 protein expression, whereas the CL20i4-hgamma(c)-Revgen vector significantly activated LMO2 protein expression. In addition, the CL20i4-EF1alpha-hgamma(c)OPT vector has not caused any tumors in transplanted mice. We conclude that the CL20i4-EF1alpha-hgamma(c)OPT vector may be suitable for testing in a clinical trial based on these preclinical demonstrations of efficacy and safety.

Primary immunodeficiency diseases: relevant aspects for pulmonologists

Primary immunodeficiency diseases comprise a genetically heterogeneous group of disorders that affect distinct components of the innate and adaptive immune system, such as neutrophils, macrophages, dendritic cells, complement proteins and natural killer cells, as well as T and B lymphocytes. The study of these diseases has provided essential insights into the functioning of the immune system. Primary immunodeficiency diseases have been linked to over 120 different genes, abnormalities in which account for approximately 180 different forms of these diseases. Patients with primary immunodeficiency diseases are most often recognized because of their increased susceptibility to infections. However, these patients can also present with a variety of other manifestations, such as autoimmune diseases, inflammatory diseases and cancer. The purpose of this article is to update the main aspects of primary immunodeficiency diseases, especially regarding the clinical manifestations related to the diagnosis, emphasizing the need for the early recognition of warning signs for these diseases.

Delayed presentation of severe combined immunodeficiency due to prolonged maternal T cell engraftment

Severe combined immunodeficiency (SCID) is a primary immunodeficiency disorder with heterogenous genetic etiologies. We describe a typical case in a 9-year-old boy that was masked by a clinically functional maternal T cell engraftment leading to late presentation with Pneumocystis jiroveci pneumonia and cytomegalovirus infection, probably following exhaustion of maternally engrafted cells. Based on immunological findings, he had a T- B+SCID phenotype.This report suggests that in rare cases, engrafted maternal T cell might persist for long time leading to partial constitution of immune function and delayed clinical presentation of SCID.

Implementing routine testing for severe combined immunodeficiency within Wisconsin's newborn screening program

Severe combined immunodeficiency (SCID) is the result of genetic defects that impair normal T-cell development. SCID babies typically appear normal at birth, but acquire multiple life-threatening infections within a few months. Early diagnosis and treatment with a bone-marrow transplant markedly improves long-term outcomes. On January 1, 2008, the newborn screening (NBS) program in Wisconsin became the first in the world to routinely test all newborns for SCID. A realtime quantitative polymerase chain reaction assay measures T-cell receptor excision circles (TRECs), which are formed during the maturation of normal T-cells. A lack or very low number of TRECs is consistent with T-cell lymphopenia. The development and validation of the TREC assay and the results of the first year of screening have been published. This article describes the process used to add SCID to the NBS panel, the establishment of follow-up capacity, and the integration of SCID screening into routine NBS workflows. The development of this expanded NBS program is described so that other states might benefit from the processes used in Wisconsin.

Systematic evidence review of newborn screening and treatment of severe combined immunodeficiency

CONTEXT

Severe combined immunodeficiency (SCID) is a group of disorders that leads to early childhood death as a result of severe infections. Recent research has addressed potential newborn screening for SCID.

OBJECTIVE

To conduct a systematic review of the evidence for newborn screening for SCID, including test characteristics, treatment efficacy, and cost-effectiveness.

METHODS

We searched Medline and the OVID In-Process & Other Non-Indexed Citations databases. We excluded articles if they were reviews, editorials or other opinion pieces, or case series of fewer than 4 patients or if they contained only adult subjects or nonhuman data. The remaining articles were systematically evaluated, and data were abstracted by 2 independent reviewers using standardized tools. For topics that lacked published evidence, we interviewed experts in the field.

RESULTS

The initial search resulted in 719 articles. Twenty-six met inclusion criteria. The results of several small studies suggested that screening for SCID is possible. Interviews revealed that 2 states have begun pilot screening programs. Evidence from large case series indicates that children receiving early stem-cell transplant for SCID have improved outcomes compared with children who were treated later. There is some inconclusive evidence regarding the need for donor-recipient matching and use of pretransplant chemotherapy. Few data on the cost-effectiveness of a SCID-screening program.

CONCLUSIONS

Evidence indicates the benefits of early treatment of SCID and the possibility of population-based newborn screening. Better information on optimal treatment and the costs of treatment and screening would benefit policy makers deciding among competing health care priorities.

B-cell function in severe combined immunodeficiency after stem cell or gene therapy: a review

Although bone marrow transplantation has resulted in life-saving T-cell reconstitution in infants with severe combined immunodeficiency (SCID), correction of B-cell function has been more problematic. This review examines B-cell reconstitution results presented in 19 reports from the United States and Europe on posttransplantation immune reconstitution in patients with SCID over the past 2 decades. The analysis considered whether pretransplantation conditioning regimens were used, the overall survival rate, the percentage with donor B-cell chimerism, the percentage with B-cell function, and the percentage of survivors requiring immunoglobulin replacement. The survival rates were higher at those centers that did not use pretransplantation conditioning or posttransplantation graft-versus-host disease prophylaxis. The percentage of survivors with B-cell chimerism, function, or both was higher and the percentage requiring immunoglobulin replacement was lower at those centers that used pretransplantation conditioning. However, there were substantial numbers of patients requiring immunoglobulin replacement at all centers. Thus pretransplantation conditioning does not guarantee that B-cell function will develop. Because most infants with SCID either present with serious infections or are given diagnoses as newborns, one must decide whether there is justification for using agents that compromise innate immunity and have intrinsic toxicities to gain B-cell immune reconstitution.

Immunodeficiency due to mutations in ORAI1 and STIM1

Lymphocyte activation requires Ca(2+) influx through specialized Ca(2+) channels in the plasma membrane. In T cells the predominant Ca(2+) channel is the Ca(2+) release activated Ca(2+) (CRAC) channel encoded by the gene ORAI1. ORAI1 is activated by stromal interaction molecule (STIM) 1 that is localized in the ER where it senses the concentration of stored Ca(2+). Following antigen binding to immunoreceptors such as the TCR, ER Ca(2+) stores are depleted, STIM1 is activated and ORAI1-CRAC channels open resulting in what is referred to as store-operated Ca(2+) entry (SOCE). Mutations in ORAI1 and STIM1 genes in human patients that lead to expression of non-functional ORAI1 or complete lack of ORAI1 or STIM1 protein are associated with a unique clinical phenotype that is characterized by immunodeficiency, muscular hypotonia and anhydrotic ectodermal dysplasia, as well as, in the case of STIM1 deficiency, autoimmunity and lymphoproliferative disease. The immunodeficiency in these patients is due to a severe defect in T cell activation but not in lymphocyte development. This review describes the immunological and non-immunological phenotypes of patients with defects in SOCE and CRAC channel function and discusses them in the context of similar immunodeficiency diseases and animal models of ORAI1 and STIM1 function.

More than just SCID--the phenotypic range of combined immunodeficiencies associated with mutations in the recombinase activating genes (RAG) 1 and 2

Combined immunodeficiencies with impaired numbers and function of T- and B-cells can be attributed to defects in the recombinase activating genes (RAG). The products of these genes, the RAG1 and 2 proteins, are key players in the V(D)J recombination process leading to the assembly of antigen receptor genes. Complete RAG deficiency (RAGD) with no V(D)J (<1% recombination activity of wild type) is associated with classical SCID and absence of T- and B-cells. In RAGD with residual V(D)J activity (>1% recombination activity of wild type), several clinical and immunological subtypes have been described: RAGD with skin inflammation and alphabeta T-cell expansion (classical Omenn syndrome), RAGD with skin inflammation and without T-cell expansion (incomplete Omenn syndrome), RAGD with gammadelta T-cell expansion and RAGD with granulomas. Engraftment of maternal T-cells can add to variation in phenotype. The potential role of epigenetic factors that influence the emergence of these phenotypes is discussed. Thorough assessment and interpretation of clinical and immunological findings will guide treatment modalities as intense as hematopoietic stem cell transplantation.

Bone marrow transplantation for primary immunodeficiency diseases

Advances in immunology have led to a breathtaking expansion of recognized primary immunodeficiency diseases (PID) with over 120 disease-related genes identified. In North America alone more than 1000 children have received allogeneic blood or marrow transplant over the past 30 years, with the majority surviving long term. This review presents results and highlights challenges and notable advances, including novel less toxic conditioning regimens, to transplant the more common and severe forms of PID. HLA-matched sibling donors remain the ideal option, however, advances in living donor unrelated HSCT and banked umbilical cord blood grafts provide hope for all children with severe PID.

Transplantation immunology: solid organ and bone marrow

Development of the field of organ and tissue transplantation has accelerated remarkably since the human MHC was discovered in 1967. Matching of donor and recipient for MHC antigens has been shown to have a significant positive effect on graft acceptance. The roles of the different components of the immune system involved in the tolerance or rejection of grafts and in graft-versus-host disease have been clarified. These components include antibodies, antigen-presenting cells, helper and cytotoxic T-cell subsets, immune cell-surface molecules, signaling mechanisms, and cytokines. The development of pharmacologic and biological agents that interfere with the alloimmune response has had a crucial role in the success of organ transplantation. Combinations of these agents work synergistically, leading to lower doses of immunosuppressive drugs and reduced toxicity. Reports of significant numbers of successful solid-organ transplantations include those of the kidneys, liver, heart, and lung. The use of bone marrow transplantation for hematologic diseases, particularly hematologic malignancies and primary immunodeficiencies, has become the treatment of choice in many of these conditions. Other sources of hematopoietic stem cells are also being used, and diverse immunosuppressive drug regimens of reduced intensity are being proposed to circumvent the mortality associated with the toxicity of these drugs. Gene therapy to correct inherited diseases by means of infusion of gene-modified autologous hematopoietic stem cells has shown efficacy in 2 forms of severe combined immunodeficiency, providing an alternative to allogeneic tissue transplantation.

Molecular assessment of thymus capabilities in the evaluation of T-cell immunodeficiency

T-cell immunodeficiency may pose a diagnostic challenge to clinicians, especially when the basic T-cell immune workup is not sufficiently informative. An intensive assessment of thymus capabilities that involves either measuring the recent thymic emigrant cells or analyzing the T-cell receptor (TCR) repertoire is often required to estimate the severity and nature of the immune disorder. A comprehensive T-cell immune workup, including TCR excision circles (TRECs) and TCR repertoire analyses, was performed in three patients with various degrees of severity of T-cell immunodeficiency. All three patients had normal peripheral CD3+ T lymphocytes. TCR repertoire analysis revealed oligoclonal (patient 1), restricted (patient 2), and near-normal (patient 3) patterns. TREC quantification was significantly reduced in patients 1 and 2 but normal in patient 3. Based on clinical features at presentation and at follow-up, and supported by the results of immunologic studies, patients 1 and 2 were diagnosed as having significant T-cell immunodeficiency and patient 3 as having T-cell immunocompetence. Assessment of thymus capabilities by TRECs and TCR repertoire analyses is helpful in diagnosing patients with T-cell immunodeficiency and should be part of the evaluation of every patient suspected of having that condition.

Improving cellular therapy for primary immune deficiency diseases: recognition, diagnosis, and management

More than 20 North American academic centers account for the majority of hematopoietic stem cell transplantation (HCT) procedures for primary immunodeficiency diseases (PIDs), with smaller numbers performed at additional sites. Given the importance of a timely diagnosis of these rare diseases and the diversity of practice sites, there is a need for guidance as to best practices in management of patients with PIDs before, during, and in follow-up for definitive treatment. In this conference report of immune deficiency experts and HCT physicians who care for patients with PIDs, we present expert guidance for (1) PID diagnoses that are indications for HCT, including severe combined immunodeficiency disease (SCID), combined immunodeficiency disease, and other non-SCID diseases; (2) the critical importance of a high degree of suspicion of the primary care physician and timeliness of diagnosis for PIDs; (3) the need for rapid referral to an immune deficiency expert, center with experience in HCT, or both for patients with PIDs; (4) medical management of a child with suspicion of SCID/combined immunodeficiency disease while confirming the diagnosis, including infectious disease management and workup; (5) the posttransplantation follow-up visit schedule; (6) antimicrobial prophylaxis after transplantation, including gamma globulin administration; and (7) important indications for return to the transplantation center after discharge. Finally, we discuss the role of high-quality databases in treatment of PIDs and HCT as an element of the infrastructure that will be needed for productive multicenter clinical trials in these rare diseases.

Primary immunodeficiencies

In the last years, advances in molecular genetics and immunology have resulted in the identification of a growing number of genes causing primary immunodeficiencies (PIDs) in human subjects and a better understanding of the pathophysiology of these disorders. Characterization of the molecular mechanisms of PIDs has also facilitated the development of novel diagnostic assays based on analysis of the expression of the protein encoded by the PID-specific gene. Pilot newborn screening programs for the identification of infants with severe combined immunodeficiency have been initiated. Finally, significant advances have been made in the treatment of PIDs based on the use of subcutaneous immunoglobulins, hematopoietic cell transplantation from unrelated donors and cord blood, and gene therapy. In this review we will discuss the pathogenesis, diagnosis, and treatment of PIDs, with special attention to recent advances in the field.

Identification of severe combined immunodeficiency by T-cell receptor excision circles quantification using neonatal guthrie cards

OBJECTIVE

To assess the feasibility of T-cell receptor excision circles (TRECs) quantification for neonatal mass screening of severe combined immunodeficiency (SCID).

STUDY DESIGN

Real-time PCR based quantification of TRECs for 471 healthy control patients and 18 patients with SCID with various genetic abnormalities (IL2RG, JAK3, ADA, LIG4, RAG1) were performed, including patients with maternal T-cell engraftment (n = 4) and leaky T cells (n = 3).

RESULTS

TRECs were detectable in all normal neonatal Guthrie cards (n = 326) at the levels of 10(4) to 10(5) copies/microg DNA. In contrast, TRECs were extremely low in all neonatal Guthrie cards (n = 15) and peripheral blood (n = 14) from patients with SCID, including those with maternal T-cell engraftment or leaky T cells with hypomorphic RAG1 mutations or LIG4 deficiency. There were no false-positive or negative results in this study.

CONCLUSION

TRECs quantification can be used as a neonatal mass screening for patients with SCID.

Mutation analysis in primary immunodeficiency diseases: case studies

PURPOSE OF REVIEW

The application of mutation analysis is becoming an integral part of the complete evaluation of patients with primary immunodeficiencies, and as such, clinicians caring for these patients must develop a better understanding of the utility and challenges of this important laboratory technology.

RECENT FINDINGS

Genomic DNA sequencing is currently the standard approach used to characterize a possible gene mutation causing a specific primary immunodeficiency. There are clinical situations in which this approach is revealing of a genetic defect and other circumstances in which this generates a false-positive or false-negative result. One case study is presented that reviews a straightforward analysis that clarifies the genetic basis of a primary immunodeficiency, and four cases are presented that required additional studies to clarify the underlying basis of the immunodeficiency. In the latter circumstances, the rationale for additional studies is outlined and the outcome of these is presented.

SUMMARY

The identification of a gene mutation as the underlying basis of a primary immunodeficiency begins with the evaluation of the clinical presentation focusing on the infection history so as to develop a differential diagnosis including potential genetic causes. The next step is to obtain specific laboratory studies, including immunologic function evaluation, and, based on these findings, to proceed with DNA sequencing of one or several selected candidate genes. Genomic DNA sequencing has certain limitations, and alternative follow-up approaches may be necessary to establish the molecular basis of the primary immunodeficiency in a given patient.

Long-term clinical outcome of patients with severe combined immunodeficiency who received related donor bone marrow transplants without pretransplant chemotherapy or post-transplant GVHD prophylaxis

OBJECTIVE

To determine long-term health benefits of nonablative bone marrow transplantation for severe combined immunodeficiency (SCID), we investigated our cohort of 161 related donor bone marrow-transplanted patients with SCID. Only 16 (10%) had HLA-identical donors.

STUDY DESIGN

All 124 survivors were sent questionnaires about their current clinical statuses. Details from clinic visits were also compiled. One hundred eleven patients (90%) were reached. We compared outcomes of patients transplanted before and after 3.5 months of life and by molecular defect.

RESULTS

The overall survival rate was 77%, but the rate for the 48 infants transplanted in the first 3.5 months of life was 94%, compared with 70% for the 113 transplanted after 3.5 months (P = .002). Twenty-eight (76%) of the 37 deceased patients died of viral infections present at diagnosis. One or more clinical problems were reported to have been present in the past 2 years in 71 (64%) of the survivors, although 95 (86%) were considered healthy by their families.

CONCLUSIONS

Most patients with SCID transplanted with related donor marrow without pretransplant chemotherapy have done well in the long term, but those transplanted at <3.5 months of age had a superior survival rate, a lower rate of clinical problems, less need for booster transplants, and better nutritional status.

Outcomes of patients with severe combined immunodeficiency treated with hematopoietic stem cell transplantation with and without preconditioning

BACKGROUND

The effect of pretransplantation conditioning on the long-term outcomes of patients receiving hematopoietic stem cell transplantation for severe combined immunodeficiency (SCID) has not been completely determined.

OBJECTIVE

We sought to assess the outcomes of 23 mostly conditioned patients with SCID and compare their outcomes with those of 25 previously reported nonconditioned patients with SCID who underwent transplantation.

METHODS

In the present study we reviewed the medical records of these 23 consecutive, mostly conditioned patients with SCID who underwent transplantation between 1998 and 2007.

RESULTS

Eighteen patients (median age at transplantation, 10 months; range, 0.8-108 months) received haploidentical mismatched related donor, matched unrelated donor, or mismatched unrelated donor transplants, 17 of whom received pretransplantation conditioning (with 1 not conditioned); 13 (72%) patients engrafted with donor cells and survive at a median of 3.8 years (range, 1.8-9.8 year); 5 (38%) of 13 patients require intravenous immunoglobulin; and 6 of 6 age-eligible children attend school. Of 5 recipients (median age at transplantation, 7 months; range, 2-23 months) of matched related donor transplants, all 5 engrafted and survive at a median of 7.5 years (range, 1.5-9.5 year), 1 recipient requires intravenous immunoglobulin, and 3 of 3 age-eligible children attend school. Gene mutations were known in 16 cases: mutation in the common gamma chain of the IL-2 receptor (IL2RG) in 7 patients, mutation in the alpha chain of the IL-7 receptor (IL7RA) in 4 patients, mutation in the recombinase-activating gene (RAG1) in 2 patients, adenosine deaminase deficiency (ADA) in 2 patients, and adenylate kinase 2 (AK2) in 1 patient. Early outcomes and quality of life of the previous nonconditioned versus the present conditioned cohorts were not statistically different, but longer-term follow-up is necessary for confirmation.

CONCLUSIONS

Hematopoietic stem cell transplantation in patients with SCID results in engraftment, long-term survival, and a good quality of life for the majority of patients with or without pretransplantation conditioning.

Molecular analysis of T-B-NK+ severe combined immunodeficiency and Omenn syndrome cases in Saudi Arabia

Background

Children with Severe Combined Immunodeficiency (SCID) lack autologous T lymphocytes and present with multiple infections early in infancy. Omenn syndrome is characterized by the sole emergence of oligoclonal auto-reactive T lymphocytes, resulting in erythroderma and enteropathy. Omenn syndrome (OS) shares the genetic aetiology of T^-B^-NK⁺ SCID, with mutations in RAG1, RAG2, or DCLRE1C.

Methods

Patients diagnosed with T^-B^-NK⁺ SCID or phenotypes suggestive of Omenn syndrome were investigated by molecular genetic studies using gene tightly linked microsatellite markers followed by direct sequencing of the coding regions and splice sites of the respective candidate genes.

Results

We report the molecular genetic basis of T^-B^-NK⁺ SCID in 22 patients and of OS in seven patients all of Arab descent from Saudi Arabia. Among the SCID patients, six (from four families) displayed four homozygous missense mutations in RAG1 including V433M, R624H, R394W, and R559S. Another four patients (from three familes) showed 3 novel homozygous RAG2 mutations including K127X, S18X, and Q4X; all of which predict unique premature truncations of RAG2 protein. Among Omenn patients, four (from two families) have S401P and R396H mutations in RAG1, and a fifth patient has a novel I444M mutation in RAG2. Seven other patients (six SCID and one OS) showed a gross deletion in exons 1-3 in DCLRE1C. Altogether, mutations in RAG1/2 and DCLRE1C account for around 50% and 25%, respectively, in our study cohort, a proportion much higher than in previous reported series. Seven (24%) patients lack a known genetic aetiology, strongly suggesting that they carry mutations in novel genes associated with SCID and Omenn disorders that are yet to be discovered in the Saudi population.

Conclusion

Mutation-free patients who lack a known genetic aetiology are likely to carry mutations in the regulatory elements in the SCID-causing genes or in novel genes that are yet to be discovered. Our efforts are underway to investigate this possibility by applying the whole genome scans on these cases via the use of Affymetrix high density DNA SNP chips in addition to homozygosity mapping.