Human severe combined immunodeficiency: genetic, phenotypic, and functional diversity in one hundred eight infants

Lymphoid development and function in X-linked severe combined immunodeficiency mice after stem cell gene therapy

Mutations of the common gamma chain (gammac) of cytokine receptors cause X-linked severe combined immunodeficiency (XSCID), a candidate disease for gene therapy. Using an XSCID murine model, we have tested the feasibility of stem cell gene correction. XSCID bone marrow (BM) cells were transduced with a retroviral vector expressing the murine gammac (mgammac) and engrafted in irradiated XSCID animals. Transplanted mice developed mature B cells, naive T cells, and mature natural killer (NK) cells, all of which were virtually absent in untreated mice. The mgammac transgene was detected in all treated mice, and we could demonstrate mgammac expression in newly developed lymphocytes at both the RNA and protein level. In addition, treated mice showed T cell proliferation responses to mitogens and production of antigen-specific antibodies upon immunization. Four of seven treated animals showed a clear increase of the transgene positive cells, suggesting in vivo selective advantage for gene-corrected cells. Altogether, these results show that retroviral-mediated gene transfer can improve murine XSCID and suggest that similar strategies may prove beneficial in human clinical trials.

Natural variation in immune responsiveness, with special reference to immunodeficiency and promoter polymorphism in class II MHC genes

This review deals with natural selection operating on heterozygotes as a key factor controlling (a) the frequency of immunodeficiencies, and (b) promoter polymorphism in MHC class II genes. The known difference in frequency distribution of X-linked and autosomal deficiencies lend support to this possibility, and suggest that the frequency of neonatal defect may rise as old-established equlibria between entry and exit of deleterious mutations change. MHC class II gene promoters differ in their capacity to favor Th1 (or reciprocally Th2) responses, thus suggesting that promoter polymorphism is sustained by the greater flexibility in response that this confers on heterozygotes.

Fetal hematopoietic stem cell transplantation

In utero hematopoietic stem cell transplantation (IUHSCTx) is a promising approach for the treatment of a potentially large number of fetuses affected by congenital hematologic disorders. With technical and molecular advances in prenatal diagnosis, the majority of these diseases can now be diagnosed early in gestation, allowing consideration of prenatal treatment. In addition, technical advances in fetal imaging and intervention make it possible to perform the transplants with relatively minimal risk. It, therefore, stands to reason that there is increasing interest in performing in utero hematopoietic stem cell transplantation at many fetal treatment centers. Although the approach remains experimentally promising, expansion of clinical application will depend on improved understanding of the biological barriers to engraftment in the fetus as well as the development of effective clinical strategies based on the hematopoietic biology of individual disorders. This article presents the current status of this emerging therapeutic approach.

Restoration of Lymphoid Populations in a Murine Model of X-Linked Severe Combined Immunodeficiency by a Gene-Therapy Approach

X-linked severe combined immunodeficiency (XSCID) is a life-threatening syndrome in which both cellular and humoral immunity are profoundly compromised. This disease results from mutations in theIL2RG gene, which encodes the common cytokine receptor γ chain, γc. Previously, we generated γc-deficient mice as a murine model of XSCID. We have now used lethally irradiated γc-deficient mice to evaluate a gene therapeutic approach for treatment of this disease. Transfer of the human γc gene to repopulating hematopoietic stem cells using an ecotropic retrovirus resulted in an increase in T cells, B cells, natural killer (NK) cells, and intestinal intraepithelial lymphocytes, as well as normalization of the CD4:CD8 T-cell ratio and of serum Ig levels. In addition, the restored cells could proliferate in response to interleukin-2 (IL-2). Thus, our results provide added support that gene therapy is a feasible therapeutic strategy for XSCID. Moreover, because we used a vector directing expression of human γc to correct a defect in γc-deficient mice, these data also indicate that human γc can cooperate with the distinctive cytokine receptor chains such as IL-2Rβ and IL-7R to mediate responses to murine cytokines in vivo.

Restoration of Lymphoid Populations in a Murine Model of X-Linked Severe Combined Immunodeficiency by a Gene-Therapy Approach

X-linked severe combined immunodeficiency (XSCID) is a life-threatening syndrome in which both cellular and humoral immunity are profoundly compromised. This disease results from mutations in theIL2RG gene, which encodes the common cytokine receptor γ chain, γc. Previously, we generated γc-deficient mice as a murine model of XSCID. We have now used lethally irradiated γc-deficient mice to evaluate a gene therapeutic approach for treatment of this disease. Transfer of the human γc gene to repopulating hematopoietic stem cells using an ecotropic retrovirus resulted in an increase in T cells, B cells, natural killer (NK) cells, and intestinal intraepithelial lymphocytes, as well as normalization of the CD4:CD8 T-cell ratio and of serum Ig levels. In addition, the restored cells could proliferate in response to interleukin-2 (IL-2). Thus, our results provide added support that gene therapy is a feasible therapeutic strategy for XSCID. Moreover, because we used a vector directing expression of human γc to correct a defect in γc-deficient mice, these data also indicate that human γc can cooperate with the distinctive cytokine receptor chains such as IL-2Rβ and IL-7R to mediate responses to murine cytokines in vivo.

Targeting Janus kinase 3 in mast cells prevents immediate hypersensitivity reactions and anaphylaxis

Janus kinase 3 (JAK3), a member of the Janus family protein-tyrosine kinases, is expressed in mast cells, and its enzymatic activity is enhanced by IgE receptor/FcepsilonRI cross-linking. Selective inhibition of JAK3 in mast cells with 4-(4'-hydroxylphenyl)-amino-6, 7-dimethoxyquinazoline) (WHI-P131) blocked the phospholipase C activation, calcium mobilization, and activation of microtubule-associated protein kinase after lgE receptor/FcepsilonRI cross-linking. Treatment of IgE-sensitized rodent as well as human mast cells with WHI-P131 effectively inhibited the activation-associated morphological changes, degranulation, and proinflammatory mediator release after specific antigen challenge without affecting the functional integrity of the distal secretory machinery. In vivo administration of the JAK3 inhibitor WHI-P131 prevented mast cell degranulation and development of cutaneous as well as systemic fatal anaphylaxis in mice at nontoxic dose levels. Thus, JAK3 plays a pivotal role in IgE receptor/FcepsilonRI-mediated mast cell responses, and targeting JAK3 with a specific inhibitor, such as WHI-P131, may provide the basis for new and effective treatment as well as prevention programs for mast cell-mediated allergic reactions.

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 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-7R+/IL-3R+ 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

Expansion 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-7R+/IL-3R+ 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.

IL-10 Transgenic Mice Present a Defect in T Cell Development Reminiscent to SCID Patients

To analyze the effect of IL-10 overexpressed by APCs as observed in some SCID patients, we have expressed the human IL-10 cDNA under the control of the murine MHC class II promoter in transgenic mice. Similar to SCID patients, these mice presented a defect in T cell maturation characterized by a rapid thymic aplasia that started after birth. The blockage in T cell maturation was strictly restricted to TCR-αβ T cells as the absolute number of thymic dendritic, TCR-γδ and NK1.1 T cells were equivalent to control littermates. Crossing IL-10 transgenic mice with TCR transgenic mice or treatment with staphylococcal enterotoxin B showed that the defect was not related to the impairment of positive or negative selection. However, repopulating of IL-10 transgenic mouse-fetal thymic organ culture with different stages of triple negative T cells isolated from control mice showed that the blockage occurred specifically at the pre-T cell stage and was reverted by treatment with blocking anti-IL-10 mAbs. These results demonstrate that IL-10 regulates T cell maturation and that dysregulation of IL-10 expression can lead to severe T cell immunodeficiency.

Identification of X-linked severe combined immunodeficiency by mutation analysis of blood and hair roots

Severe combined immunodeficiency is a heterogenous syndrome of varied genetic origins of which the X-linked type is the commonest (XSCID). The most sensitive method for diagnosis of XSCID in the absence of X-linked inheritance pattern is by mutation analysis. In this report we have performed mutation analysis in 13 unrelated boys transplanted (BMT) for SCID without a known cause to determine the frequency of XSCID. Five boys had an affected male relative. We also assessed the utility of hair roots for children without pre-transplant blood stored for mutation analysis since donor genotype was expressed in peripheral blood post BMT. Screening was performed by analysis of single-strand conformational polymorphism (SSCP) followed by sequencing of candidate exons. Mutations were found in 11 cases, of which six were sporadic, and maternal mosaicism was found in one family. Three mothers of the six sporadic cases were identified as carriers. The majority (6/8) of boys with SCID had gammac deficiency despite the absence of X-linked inheritance pattern. The significant frequency of de novo mutations and the occurrence of maternal germline mosaicism highlights the importance of mutation analysis. The strategy of using DNA from hair roots was particularly valuable where no pre-transplant blood was stored. Characterization of the mutations will also enable research into the correction of these genetic defects.

Janus kinases and their role in growth and disease

Janus kinases (JAK) play a crucial role in the initial steps of cytokine signaling. Each of the four members (JAK1, JAK2, JAK3, TYK2) of this non-receptor tyrosine kinase family is indispensable for the effects of distinct cytokines. Moreover, recent reports have added to our knowledge on their highly specific functions: JAK3 knockout mice and JAK3 deficient patients cannot signal through the interleukin-2,4,7,9, or 15 receptors and suffer from severe combined immunodeficiency (SCID). JAK1 and JAK2 knockout mice do not survive, their cells again showing distinct patterns of cytokine signaling deficits. At the other end of the spectrum, JAK fusion proteins have been shown to play a role in leukemias. In addition, a new class of JAK-specific inhibitors was described by several groups, the CIS/SOCS/Jab family. This review on the rapidly growing field focuses on JAK function and regulation, and on their emerging role in development and human disease.

Southwestern Athabaskan (Navajo and Apache) genetic diseases

PURPOSE

Four apparently unique disorders are known among the Southwestern Athabasan Amerindians, i.e., the Navajo and Apache; they are Athabaskan severe combined immunodeficiency, Navajo neuropathy, Navajo poikiloderma, and Athabaskan brainstem dysgenesis. This study reviews background information on Athabaskan groups and clinical descriptions of these recessive disorders.

METHODS

The major clinical findings of these four disorders are reviewed. In addition, the findings of epidemiological surveys are included where available.

RESULTS

Although the importance of genetic bottlenecks in increasing the frequency of rare, sometimes unique, autosomal recessive disorders is known for a number of populations, similar phenomena among Native Americans seem to be less well known.

CONCLUSION

As many more Native Americans move off the Reservation, the awareness of susceptibility to particular genetic diseases needs to be more widely disseminated.

Kinetics of T-cell development of umbilical cord blood transplantation in severe T-cell immunodeficiency disorders

BACKGROUND

Hematopoietic stem-cell transplantation is the treatment of choice for severe primary T-cell immunodeficiencies. When an HLA-identical sibling donor is not available, an alternative donor stem-cell source is needed. In primary T-cell immunodeficiencies, T-cell-depleted HLA-haploidentical bone marrow transplantation has been particularly successful in reconstituting the T-cell immune system in many of the severe combined immunodeficiency syndrome types. However, there are some problems associated with this preparation as a stem donor source, such as increased resistance to engraftment, a long period of time for T-cell engraftment to occur, and failure to engraft B cells and B-cell functions. These problems can be especially troublesome if the patient is infected before the transplantation.

OBJECTIVE

Umbilical cord blood was evaluated as a stem-cell source for immune reconstitution in children with severe primary T-cell immunodeficiency disorders, such as severe combined immunodeficiency syndrome, reticular dysgenesis, thymic dysplasia, and combined immunodeficiency disease, when a matched sibling donor was unavailable.

METHODS

From January 1996 through July 1997, 6 children received unrelated cord blood stem-cell transplantation after a preparative regimen for the treatment of combined immunodeficiency diseases. The patients ranged in age from 2 weeks to 6 years. The cord blood units were 3 of 6 HLA antigen matches in 2 children, 4 of 6 HLA antigen matches in 3 children, and 5 of 6 HLA antigen matches in 1 child, with molecular HLA-DR mismatch in 3 of the children.

RESULTS

The average time for neutrophil engraftment (absolute neutrophil count, >500/mm3) was 12 days (range, 10 to 15 days), and the average time for platelet engraftment (platelet count, >20,000/mm3) was 36 days (range, 24 to 50 days). In a patient with reticular dysgenesis, the first transplant failed to engraft but fully engrafted after a second unrelated donor cord blood transplantation. Five of 6 patients exhibited grade I graft-versus-host disease (GvHD), although 1 child experienced grade IV skin and gut GvHD. Immunologic reconstitution demonstrated that cord blood stem-cell transplantation resulted in consistent and stable T-cell, B-cell, and natural killer-cell development. The kinetics of recovery of phenotypic expression and function of T cells occurred between 60 to 100 days and that of natural killer cells at approximately 180 days. B cells engrafted early, and a study of functional B-cell antibody responses revealed that 2 of 2 patients in whom intravenous immune globulin was discontinued have low detectable antibody responses to tetanus and diphtheria toxoid immunizations more than 1 year after the transplantation.

CONCLUSIONS

Unrelated umbilical donor cord blood is an excellent source of stem cells for transplantation of children with immune deficiency disorders. Benefits include rapid and reliable recovery of immune function, low risk of GvHD, and low viral transmission rate.

Genetic and biochemical evidence for a critical role of Janus kinase (JAK)-3 in mast cell-mediated type I hypersensitivity reactions

We investigated the role of JAK3 in IgE receptor/FcepsilonRI-mediated mast cell responses. IgE/antigen induced degranulation and mediator release were substantially reduced with Jak3-/- mast cells from JAK3-null mice that were generated by targeted disruption of Jak3 gene in embryonic stem cells. Further, treatment of mast cells with 3'bromo-4'-hydroxylphenyl)-amino-6,7-dimethoxyquinazoline (WHI-P154), a potent inhibitor of JAK3, inhibited degranulation and proinflammatory mediator release after IgE receptor/ FcepsilonRI crosslinking. Thus, JAK3 plays a pivotal role in IgE receptor/ FcepsilonRI-mediated mast cell responses and targeting JAK3 may provide the basis for new and effective treatment as well as prevention programs for mast cell-mediated allergic reactions.

Severe combined immunodeficiency: otolaryngological presentation and management

In order to increase the awareness of otolaryngologists of severe combined immunodeficiency syndrome (SCIDS) so they may contribute to an earlier diagnosis of this disorder, we performed a retrospective chart review of a multicenter series from 2 children's hospital medical centers. Eighteen cases were identified, and 14 had an otolaryngological presentation. The average age of presentation was 3.3 months, and 72% were males. Most cases were inherited in an X-linked fashion. Five patients had thrush; 4 had recurrent otitis media. Other otolaryngological presentations included cough, mouth ulcers, pharyngitis, mastoiditis, and bilateral neck abscess. The most severe form of immunodeficiency, SCIDS is a rare condition that involves a disorder in both T and B cell functions. The manifestations involving the head and neck include recurrent upper respiratory tract infections, otitis media, thrush, oral ulcers, and abscesses. It is important that SCIDS be considered in any infant with recurrences of these common infections.

Hematopoietic stem-cell transplantation for the treatment of severe combined immunodeficiency

BACKGROUND

Since 1968 it has been known that bone marrow transplantation can ameliorate severe combined immunodeficiency, but data on the long-term efficacy of this treatment are limited. We prospectively studied immunologic function in 89 consecutive infants with severe combined immunodeficiency who received hematopoietic stem-cell transplants at Duke University Medical Center between May 1982 and September 1998.

METHODS

Serum immunoglobulin levels and lymphocyte phenotypes and function were assessed and genetic analyses performed according to standard methods. Bone marrow was depleted of T cells by agglutination with soybean lectin and by sheep-erythrocyte rosetting before transplantation.

RESULTS

Seventy-seven of the infants received T-cell-depleted, HLA-haploidentical parental marrow, and 12 received HLA-identical marrow from a related donor; 3 of the recipients of haploidentical marrow also received placental-blood transplants from unrelated donors. Except for two patients who received placental blood, none of the recipients received chemotherapy before transplantation or prophylaxis against graft-versus-host disease. Of the 89 infants, 72 (81 percent) were still alive 3 months to 16.5 years after transplantation, including all of the 12 who received HLA-identical marrow, 60 of the 77 (78 percent) who were given haploidentical marrow, and 2 of the 3 (67 percent) who received both haploidentical marrow and placental blood. T-cell function became normal within two weeks after transplantation in the patients who received unfractionated HLA-identical marrow but usually not until three to four months after transplantation in those who received T-cell-depleted marrow. At the time of the most recent evaluation, all but 4 of the 72 survivors had normal T-cell function, and all the T cells in their blood were of donor origin. B-cell function remained abnormal in many of the recipients of haploidentical marrow. In 26 children (5 recipients of HLA-identical marrow and 21 recipients of haploidentical marrow) between 2 percent and 100 percent of B cells were of donor origin. Forty-five of the 72 children were receiving intravenous immune globulin.

CONCLUSIONS

Transplantation of marrow from a related donor is a life-saving and life-sustaining treatment for patients with any type of severe combined immunodeficiency, even when there is no HLA-identical donor.

A novel mutant gammac chain from a patient with typical phenotype of X-linked severe combined immunodeficiency (SCID) has partial signalling function for mediating IL-2 and IL-4 receptor action

Mutations of the common gamma (gammac) chain result in X-linked SCID (X-SCID), which is characterized by the reduction in number or absence of peripheral blood T cells and natural killer (NK) cells, with retention of normal numbers of B cells. In the present study we describe a novel mutant gammac chain of an X-SCID patient with a typical X-SCID phenotype. This mutant receptor subunit is able to associate with Jak3 to transduce a weak signal. The Jak3-specific action is demonstrated by the induction of gene expression through the haematopoietin receptor response element (HRRE) by IL-2 and IL-4 in the experimental model of transiently transfected hepatoma cells over-expressing Jak3. This result suggests that a threshold in the gammac-Jak3 interaction determines the X-SCID phenotype.

Functional Role of Interleukin-4 (IL-4) and IL-7 in the Development of X-Linked Severe Combined Immunodeficiency

X-linked severe combined immunodeficiency (X-SCID) is characterized by an absent or diminished number of T cells and natural-killer (NK) cells with a normal or elevated number of B cells, and results from mutations of the γc chain. The γc chain is shared by interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. Recently, a survival signal through the IL-7 receptor  (IL-7R) chain was shown to be important for T-cell development in mice and was suggested to contribute to the X-SCID phenotype. In the present study, we examined function of a mutant γc chain (A156V) isolated from an X-SCID patient and found that T cells expressing the mutant γc chain were selectively impaired in their responses to IL-4 or IL-7 compared with the wild-type γc chain expressing cells although responses to IL-2 or IL-15 were relatively maintained. The result shows that IL-4– and/or IL-7–induced signaling through the γc chain is critical for T-cell development and plays an important role in the development of the X-SCID phenotype.

Functional Role of Interleukin-4 (IL-4) and IL-7 in the Development of X-Linked Severe Combined Immunodeficiency

X-linked severe combined immunodeficiency (X-SCID) is characterized by an absent or diminished number of T cells and natural-killer (NK) cells with a normal or elevated number of B cells, and results from mutations of the γc chain. The γc chain is shared by interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. Recently, a survival signal through the IL-7 receptor  (IL-7R) chain was shown to be important for T-cell development in mice and was suggested to contribute to the X-SCID phenotype. In the present study, we examined function of a mutant γc chain (A156V) isolated from an X-SCID patient and found that T cells expressing the mutant γc chain were selectively impaired in their responses to IL-4 or IL-7 compared with the wild-type γc chain expressing cells although responses to IL-2 or IL-15 were relatively maintained. The result shows that IL-4– and/or IL-7–induced signaling through the γc chain is critical for T-cell development and plays an important role in the development of the X-SCID phenotype.

Virus-specific immunity after gene therapy in a murine model of severe combined immunodeficiency

Human severe combined immunodeficiency (SCID) can be caused by defects in Janus kinase 3 (JAK3)-dependent cytokine signaling pathways. As a result, patients are at high risk of life-threatening infection. A JAK3 -/- SCID mouse model for the human disease has been used to test whether transplant with retrovirally transduced bone marrow (BM) cells (JAK3 BMT) could restore immunity to an influenza A virus. The immune responses also were compared directly with those for mice transplanted with wild-type BM (+/+ BMT). After infection, approximately 90% of the JAK3 BMT or +/+ BMT mice survived, whereas all of the JAK3 -/- mice died within 29 days. Normal levels of influenza-specific IgG were present in plasma from JAK3 BMT mice at 14 days after respiratory challenge, indicating restoration of B cell function. Influenza-specific CD4(+) and CD8(+) T cells were detected in the spleen and lymph nodes, and virus-specific CD8(+) effectors localized to the lungs of the JAK3 BMT mice. The kinetics of the specific host response correlated with complete clearance of the virus within 2 weeks of the initial exposure. By contrast, the JAK3 -/- mice did not show any evidence of viral immunity and were unable to control this viral pneumonia. Retroviral-mediated JAK3 gene transfer thus restores diverse aspects of cellular and humoral immunity and has obvious potential for human autologous BMT.

Primary immunodeficiency disorders

The primary immunodeficiency diseases are a relatively rare group of congenital disorders that are linked by the expression of an excessive number, duration, or severity of infections. The clinical features of most of the primary immunodeficiency diseases have been well described by astute physicians over several decades and have provided important clues to our basic understanding of human immunology. In contrast, the genetic basis and potential life-saving therapies for many of these disorders have been established only over the past few years. These recent advances have resulted in the prognosis of many of these disorders being largely dependent on their rapid recognition and treatment. Increased awareness of the differentiating epidemiologic, clinical, laboratory and genetic features of these diseases hold the promise of both furthering our understanding of basic human immunology and providing improved care for this challenging group of patients.

Defective IL7R expression in T(-)B(+)NK(+) severe combined immunodeficiency

Severe combined immunodeficiency (SCID) is caused by multiple genetic defects. The most common form of SCID, X-linked SCID (XSCID), results from mutations in IL2RG (ref. 4), which encodes the common cytokine receptor gamma chain (gamma(c)) that is shared by the IL-2, IL-4, IL-7, IL-9 and IL-15 receptors. In XSCID and SCID resulting from mutations in JAK3, which encodes a Janus family tyrosine kinase that couples to gamma(c) and is required for gamma(c)-dependent signalling, T- and natural killer (NK)-cells are decreased but B-cell numbers are normal (T(-)B(+)NK(-)SCID). Some SCID patients lack T cells but retain NK cells. Given diminished T-cell development in Il7- or Il7r-deficient mice and that Il/7r-deficient mice have NK cells, we hypothesized that T(-)B(+)NK(+) SCID might result from defective IL-7 signalling, although apparent differences in the role of the IL-7/IL-7R pathway in humans and mice in T-cell and B-cell development have been suggested. We now demonstrate that defective IL7R expression causes T(-)B(+)NK(+) SCID, indicating that the T-cell, but not the NK-cell, defect in XSCID results from inactivation of IL-7Ralpha signalling.

Biology of the interleukin-2 receptor

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Adenosine deaminase deficiency: genotype-phenotype correlations based on expressed activity of 29 mutant alleles

Adenosine deaminase (ADA) deficiency causes lymphopenia and immunodeficiency due to toxic effects of its substrates. Most patients are infants with severe combined immunodeficiency disease (SCID), but others are diagnosed later in childhood (delayed onset) or as adults (late onset); healthy individuals with "partial" ADA deficiency have been identified. More than 50 ADA mutations are known; most patients are heteroallelic, and most alleles are rare. To analyze the relationship of genotype to phenotype, we quantitated the expression of 29 amino acid sequence-altering alleles in the ADA-deleted Escherichia coli strain SO3834. Expressed ADA activity of wild-type and mutant alleles ranged over five orders of magnitude. The 26 disease-associated alleles expressed 0.001%-0.6% of wild-type activity, versus 5%-28% for 3 alleles from "partials." We related these data to the clinical phenotypes and erythrocyte deoxyadenosine nucleotide (dAXP) levels of 52 patients (49 immunodeficient and 3 with partial deficiency) who had 43 genotypes derived from 42 different mutations, including 28 of the expressed alleles. We reduced this complexity to 13 "genotype categories," ranked according to the potential of their constituent alleles to provide ADA activity. Of 31 SCID patients, 28 fell into 3 genotype categories that could express <=0.05% of wild-type ADA activity. Only 2 of 21 patients with delayed, late-onset, or partial phenotypes had one of these "severe" genotypes. Among 37 patients for whom pretreatment metabolic data were available, we found a strong inverse correlation between red-cell dAXP level and total ADA activity expressed by each patient's alleles in SO3834. Our system provides a quantitative framework and ranking system for relating genotype to phenotype.

Mosaicism in X-linked severe combined immunodeficiency

We performed gamma c gene analysis of a boy with severe combined immunodeficiency whose brother died of the same condition. A base pair deletion in exon 6 was found, which was absent in his mother, indicating maternal mosaicism with important implications for genetic counseling and demonstrating the importance of mutation analysis in boys with severe combined immunodeficiency.

Role of JAK3 in CD40-Mediated Signaling

CD40 is a member of the tumor necrosis factor receptor family and plays an important role in B-cell survival, growth, differentiation, and isotype switching. Recently, CD40 has been shown to associate with JAK3, a member of the family of Janus Kinases, which are nonreceptor protein kinases involved in intracellular signaling mediated by cytokines and growth factors. To investigate the role of JAK3 in CD40-mediated signaling, we studied the effect of CD40 stimulation on B-cell proliferation, IgE isotype switching, and upregulation of surface expression of CD23, ICAM-1, CD80, and LT-α in JAK3-deficient patients. Our studies show that stimulation of B cells with monoclonal antibody to CD40 in the presence of interleukin-4 (IL-4) or IL-13 resulted in similar responses in JAK3-deficient patients and normal controls. This suggests that JAK3 is not essential for CD40-mediated B-cell proliferation, isotype switching, and upregulation of CD23, ICAM-1, CD80, and LT-α surface expression.

Role of JAK3 in CD40-Mediated Signaling

CD40 is a member of the tumor necrosis factor receptor family and plays an important role in B-cell survival, growth, differentiation, and isotype switching. Recently, CD40 has been shown to associate with JAK3, a member of the family of Janus Kinases, which are nonreceptor protein kinases involved in intracellular signaling mediated by cytokines and growth factors. To investigate the role of JAK3 in CD40-mediated signaling, we studied the effect of CD40 stimulation on B-cell proliferation, IgE isotype switching, and upregulation of surface expression of CD23, ICAM-1, CD80, and LT-α in JAK3-deficient patients. Our studies show that stimulation of B cells with monoclonal antibody to CD40 in the presence of interleukin-4 (IL-4) or IL-13 resulted in similar responses in JAK3-deficient patients and normal controls. This suggests that JAK3 is not essential for CD40-mediated B-cell proliferation, isotype switching, and upregulation of CD23, ICAM-1, CD80, and LT-α surface expression.

Molecular and biochemical characterization of JAK3 deficiency in a patient with severe combined immunodeficiency over 20 years after bone marrow transplantation: implications for treatment

Severe combined immunodeficiency (SCID) comprises a heterogenous group of disorders that are fatal unless treated by bone marrow transplantation (BMT). The most common form of SCID (T-B+ SCID) is due to mutations of either the common gamma chain (gammac) or of gammac-coupled JAK3 kinase. We report an unusual JAK3 defect in a female who was successfully treated > 20 years ago with a BMT using her HLA-identical father as the donor. Persistence of genetically and biochemically defective autologous B cells, associated with reconstitution of cellular and humoral immunity, suggests that integrity of the gammac-JAK3 signalling pathway is not strictly required for immunoglobulin production.

Retroviral Marking of Canine Bone Marrow: Long-Term, High-Level Expression of Human Interleukin-2 Receptor Common Gamma Chain in Canine Lymphocytes

Optimization of retroviral gene transfer into hematopoietic cells of the dog will facilitate gene therapy of canine X-linked severe combined immunodeficiency (XSCID) and in turn advance similar efforts to treat human XSCID. Both canine and human XSCID are caused by defects in the common γ chain, γc, of receptors for interleukin-2 and other cytokines. In this study, normal dogs were given retrovirally transduced bone marrow cells with and without preharvest mobilization by the canine growth factors granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF). Harvey sarcoma virus and Moloney murine leukemia virus constructs were used, both containing cDNA encoding human γc. The Harvey-based vector transduced into cytokine-primed marrow yielded persistent detectable provirus in bone marrow and blood and expression of human γc on peripheral lymphocytes. In three dogs, human γc expression disappeared after 19 to 34 weeks but reappeared and was sustained, in one dog beyond 16 months posttransplantation, upon immunosuppression with cyclosporin A and prednisone, with up to 25% of lymphocytes expressing human γc. The long-term expression of human γc in a high proportion of normal canine lymphocytes predicts that retrovirus-mediated gene correction of hematopoietic cells may prove to be of clinical benefit in humans affected with this XSCID.

This is a US government work. There are no restrictions on its use.

Retroviral Marking of Canine Bone Marrow: Long-Term, High-Level Expression of Human Interleukin-2 Receptor Common Gamma Chain in Canine Lymphocytes

Optimization of retroviral gene transfer into hematopoietic cells of the dog will facilitate gene therapy of canine X-linked severe combined immunodeficiency (XSCID) and in turn advance similar efforts to treat human XSCID. Both canine and human XSCID are caused by defects in the common γ chain, γc, of receptors for interleukin-2 and other cytokines. In this study, normal dogs were given retrovirally transduced bone marrow cells with and without preharvest mobilization by the canine growth factors granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF). Harvey sarcoma virus and Moloney murine leukemia virus constructs were used, both containing cDNA encoding human γc. The Harvey-based vector transduced into cytokine-primed marrow yielded persistent detectable provirus in bone marrow and blood and expression of human γc on peripheral lymphocytes. In three dogs, human γc expression disappeared after 19 to 34 weeks but reappeared and was sustained, in one dog beyond 16 months posttransplantation, upon immunosuppression with cyclosporin A and prednisone, with up to 25% of lymphocytes expressing human γc. The long-term expression of human γc in a high proportion of normal canine lymphocytes predicts that retrovirus-mediated gene correction of hematopoietic cells may prove to be of clinical benefit in humans affected with this XSCID.

This is a US government work. There are no restrictions on its use.