Nonrandom X chromosome inactivation in B cells from carriers of X chromosome-linked severe combined immunodeficiency

Epithelial Cell Transformation and Senescence as Indicators of Genome Aging: Current Advances and Unanswered Questions

The recent advances in deciphering the human genome allow us to understand and evaluate the mechanisms of human genome age-associated transformations, which are largely unclear. Genome sequencing techniques assure comprehensive mapping of human genetics; however, understanding of gene functional interactions, specifically of time/age-dependent modifications, remain challenging. The age of the genome is defined by the sum of individual (inherited) and acquired genomic traits, based on internal and external factors that impact ontogenesis from the moment of egg fertilization and embryonic development. The biological part of genomic age opens a new perspective for intervention. The discovery of single cell-based mechanisms for genetic change indicates the possibility of influencing aging and associated disease burden, as well as metabolism. Cell populations with transformed genetic background were shown to serve as the origin of common diseases during extended life expectancy (superaging). Consequently, age-related cell transformation leads to cancer and cell degeneration (senescence). This article aims to describe current advances in the genomic mechanisms of senescence and its role in the spatiotemporal spread of epithelial clones and cell evolution.

Progressive B Cell Loss in Revertant X-SCID

We report the case of a patient with X-linked severe combined immunodeficiency (X-SCID) who survived for over 20 years without hematopoietic stem cell transplantation (HSCT) because of a somatic reversion mutation. An important feature of this rare case included the strategy to validate the pathogenicity of a variant of the IL2RG gene when the T and B cell lineages comprised only revertant cells. We studied the X-inactivation of sorted T cells from the mother to show that the pathogenic variant was indeed the cause of his SCID. One interesting feature was a progressive loss of B cells over 20 years. CyTOF (cytometry time of flight) analysis of bone marrow offered a potential explanation of the B cell failure, with expansions of progenitor populations that suggest a developmental block. Another interesting feature was that the patient bore extensive granulomatous disease and skin cancers that contained T cells, despite severe T cell lymphopenia in the blood. Finally, the patient had a few hundred T cells on presentation but his TCRs comprised a very limited repertoire, supporting the important conclusion that repertoire size trumps numbers of T cells.

A statistical measure for the skewness of X chromosome inactivation based on family trios

BACKGROUND

X chromosome inactivation (XCI) is an important gene regulation mechanism in females to equalize the expression levels of X chromosome between two sexes. Generally, one of two X chromosomes in females is randomly chosen to be inactivated. Nonrandom XCI (XCI skewing) is also observed in females, which has been reported to play an important role in many X-linked diseases. However, there is no statistical measure available for the degree of the XCI skewing based on family data in population genetics.

RESULTS

In this article, we propose a statistical approach to measure the degree of the XCI skewing based on family trios, which is represented by a ratio of two genotypic relative risks in females. The point estimate of the ratio is obtained from the maximum likelihood estimates of two genotypic relative risks. When parental genotypes are missing in some family trios, the expectation-conditional-maximization algorithm is adopted to obtain the corresponding maximum likelihood estimates. Further, the confidence interval of the ratio is derived based on the likelihood ratio test. Simulation results show that the likelihood-based confidence interval has an accurate coverage probability under the situations considered. Also, we apply our proposed method to the rheumatoid arthritis data from USA for its practical use, and find out that a locus, rs2238907, may undergo the XCI skewing against the at-risk allele. But this needs to be further confirmed by molecular genetics.

CONCLUSIONS

The proposed statistical measure for the skewness of XCI is applicable to complete family trio data or family trio data with some paternal genotypes missing. The likelihood-based confidence interval has an accurate coverage probability under the situations considered. Therefore, our proposed statistical measure is generally recommended in practice for discovering the potential loci which undergo the XCI skewing.

Exome sequence identified a c.320A > G ALG13 variant in a female with infantile epileptic encephalopathy with normal glycosylation and random X inactivation: Review of the literature

Congenital Disorders of Glycosylation (CDG) are new and rapidly expanding neurometabolic disorders with multisystem involvements, broad phenotypic manifestations, and variable severity. The majority results from a defect of one of the steps involved with protein or lipid N-glycosylation pathway. Almost all are inherited in autosomal recessive patterns with a few exceptions such as the X-linked ALG13. Mutations of ALG13 are reported, so far in only 10 patients, all were ascertained through exome/genome sequencing. Specifically, the ALG13 c.320A > G (p.Asn107Ser) variant was reported only in females and in all were de novo mutations. These findings may suggest an X-linked dominant inheritance of this mutation with embryonic male lethality. These patients presented with severe infantile epileptic encephalopathy, global developmental delay, and multisystem abnormalities. Only two of these females had glycosylation studies done, and both showed normal pattern of glycosylated serum transferrin isoforms, and none had their X-chromosome inactivation patterns studied. Here, we report on another female patient who is heterozygous for the same ALG13 c.320A > G (p.Asn107Ser) variant. She presented with infantile spasms, epileptic encephalopathy, hypsarrhythmia, hypotonia, developmental delay, intellectual disability, abnormal coagulation profile, feeding problems, hypotonia, and dysmorphic features. The diagnosis of CGD was suspected clinically, but glycosylation studies were done twice and showed normal patterns on both occasions. Her X-inactivation study was also done and, surprisingly, showed a random pattern of X-inactivation, with no evidence of skewness.

XLP: clinical features and molecular etiology due to mutations in SH2D1A encoding SAP

X-linked lymphoproliferative disease (XLP) is a rare primary immunodeficiency affecting approximately 1-2 per 1 million males. A key feature of XLP is the exquisite sensitivity of affected individuals to disease induced following EBV infection. However, patients can also develop hypogammaglobulinemia and B-cell lymphoma independently of exposure to EBV. XLP is caused by loss-of function mutations in SH2D1A, which encodes the intracellular adaptor molecule SAP. SAP is predominantly expressed in T cells and NK cells, and functions to regulate signal transduction pathways downstream of the SLAM family of surface receptors to control CD4+ T cell (and by extension B cells), CD8+ T cell and NK cell function, as well as the development of NKT cells. The study of XLP had shed substantial light on the requirements for lymphocyte differentiation and immune regulation, which in turn have the potential to be translated into novel treatments for not only XLP patients but individuals affected by EBV-induced disease, impaired humoral immunity and malignancy.

IL-21 is the primary common γ chain-binding cytokine required for human B-cell differentiation in vivo

SCID resulting from mutations in IL2RG or JAK3 is characterized by lack of T and natural killer cells; B cells are present in normal number, but antibody responses are defective. Hematopoietic cell transplantation (HCT) is curative for SCID. However, B-cell dysfunction persists in a substantial proportion of patients. We hypothesized that impaired B-cell responses after HCT in IL2RG/JAK3 deficiency results from poor donor B-cell engraftment and defective γc-dependent cytokine signaling in host B cells. To test this, and to identify which γc cytokine(s) is critical for humoral immunity, we studied 28 transplanted patients with IL2RG/JAK3 deficiency. Lack of donor B-cell engraftment associated with persistent humoral dysfunction and significantly reduced memory B cells. B-cell proliferation induced by CD40L alone or together with CpG, anti-Ig, IL-4, IL-10, or IL-13 was comparable in healthy controls and in post-HCT SCID patients, irrespective of their chimerism status. However, in vitro stimulation with CD40L/IL-21 induced B-cell proliferation, plasmablast differentiation, and antibody secretion in patients with donor B cells, but not in patients with autologous B cells. These data imply that IL-21-mediated signaling is critical for long-lived humoral immunity and to restore antibody responses in IL2RG/JAK3-deficient patients after HCT. Furthermore, in vitro stimulation with CD40L/IL-21 can predict in vivo B-cell immunity in IL2RG/JAK3 SCID after transplantation.

B cell-intrinsic signaling through IL-21 receptor and STAT3 is required for establishing long-lived antibody responses in humans

Engagement of cytokine receptors by specific ligands activate Janus kinase–signal transducer and activator of transcription (STAT) signaling pathways. The exact roles of STATs in human lymphocyte behavior remain incompletely defined. Interleukin (IL)-21 activates STAT1 and STAT3 and has emerged as a potent regulator of B cell differentiation. We have studied patients with inactivating mutations in STAT1 or STAT3 to dissect their contribution to B cell function in vivo and in response to IL-21 in vitro. STAT3 mutations dramatically reduced the number of functional, antigen (Ag)-specific memory B cells and abolished the ability of IL-21 to induce naive B cells to differentiate into plasma cells (PCs). This resulted from impaired activation of the molecular machinery required for PC generation. In contrast, STAT1 deficiency had no effect on memory B cell formation in vivo or IL-21–induced immunoglobulin secretion in vitro. Thus, STAT3 plays a critical role in generating effector B cells from naive precursors in humans. STAT3-activating cytokines such as IL-21 thus underpin Ag-specific humoral immune responses and provide a mechanism for the functional antibody deficit in STAT3-deficient patients.

Severe combined immunodeficiency associated with nephrogenic diabetes insipidus and a deletion in the Xq28 region

We evaluated a baby boy with severe combined immunodeficiency (SCID) and X-linked nephrogenic diabetes insipidus (NDI). This patient had less than 10% CD3+ T cells, almost all of which were positive for CD4 and CD45RO. Genetic studies demonstrated a 34.4 kb deletion at Xq28 which included AVPR2, the gene responsible for NDI; ARHGAP4, a hematopoietic specific gene encoding a GTPase-activating protein; and a highly conserved segment of DNA between ARHGAP4 and ARD1A, a gene involved in the response to hypoxia. Other patients with NDI, but without immunodeficiency, have had deletions that remove all ARHGAP4 except exon 1; however, no other patients have had deletions of the highly conserved intragenic region between ARHGAP4 and ARD1A. X chromosome inactivation studies, done on sorted cells from the mother and grandmother of the patient, carriers of the deletion, demonstrated exclusive use of the non-mutant X chromosome as the active X in CD4 and CD8 T cells. Surprisingly, NK cells, monocytes and neutrophils from these women demonstrated preferential use of the mutant X chromosome as the active X. These results are consistent with an X-linked form of SCID, due to the loss of regulatory elements that control the response to hypoxia in hematopoietic cells.

Interleukin-21: a modulator of lymphoid proliferation, apoptosis and differentiation

The interleukin-21 (IL-21)-IL-21-receptor system was discovered in 2000. It was immediately of great interest because of the homology of IL-21 to IL-2, IL-4 and IL-15, and of the IL-21-receptor subunit IL-21R to the beta-subunit of the IL-2 receptor, and because the IL-21 receptor also contains the common cytokine-receptor gamma-chain, the protein that is mutated in X-linked severe combined immunodeficiency. As we discuss, IL-21 has pleiotropic actions, from augmenting the proliferation of T cells and driving the differentiation of B cells into memory cells and terminally differentiated plasma cells to augmenting the activity of natural killer cells. Moreover, it has antitumour activity and might have a role in the development of autoimmunity, so these findings have implications for the treatment of cancer and autoimmune diseases.

Cytokines and immunodeficiency diseases: critical roles of the gamma(c)-dependent cytokines interleukins 2, 4, 7, 9, 15, and 21, and their signaling pathways

In this review, we discuss the role of cytokines and their signaling pathways in immunodeficiency. We focus primarily on severe combined immunodeficiency (SCID) diseases as the most severe forms of primary immunodeficiencies, reviewing the different genetic causes of these diseases. We focus in particular on the range of forms of SCID that result from defects in cytokine-signaling pathways. The most common form of SCID, X-linked SCID, results from mutations in the common cytokine receptor gamma-chain, which is shared by the receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21, underscoring that X-linked SCID is indeed a disease of defective cytokine signaling. We also review the signaling pathways used by these cytokines and the phenotypes in humans and mice with defects in the cytokines or signaling pathways. We also briefly discuss other cytokines, such as interferon-gamma and IL-12, where mutations in the ligand or receptor or signaling components also cause clinical disease in humans.

Gene therapy in infants with severe combined immunodeficiency

Severe combined immunodeficiencies (SCID) are rare disorders that represent paediatric medical emergencies, as the outcome for affected patients can easily be fatal unless proper treatment is performed. The only curative treatment for SCID is reconstitution of the patient's immunity. For more than 30 years, allogeneic bone marrow transplantation (BMT) has been extremely successful for SCID. However, BMT often results in only incomplete restoration of B cell function in treated patients, especially when haploidentical donors are used. In addition, BMT can be associated with severe complications such as graft-versus-host disease (GVHD). Alternative forms of therapy for SCID are therefore desirable. Genetic correction of peripheral T lymphocytes and/or haematopoietic stem cells (HSCs) by retrovirally mediated gene transfer has been attempted for patients with SCID due to adenosine deaminase deficiency, the first genetic disease targeted in clinical gene therapy trials with very limited success, overall. After these pioneer trials, recent progress has led to significant improvement of gene transfer techniques and better understanding of HSC biology which has culminated in the recent success of a gene therapy trial for patients affected with X-linked SCID (X-SCID). In this trial, patients with X-SCID received autologous bone marrow stem/progenitor cells which had been retrovirally transduced with a therapeutic gene. Based on the current follow-up, the overall efficacy of this gene therapy procedure is to be considered similar to or even better than that achievable by allogeneic BMT, because patients were not exposed to the risks of GVHD. Although these exciting results have clearly demonstrated that gene therapy is a feasible therapeutic option for X-SCID, they have also raised important questions regarding the long-term outcome of this experimental procedure and the possibility of translating this success into applications for other forms of SCID.

A critical role for IL-21 in regulating immunoglobulin production

The cytokine interleukin-21 (IL-21) is closely related to IL-2 and IL-15, and their receptors all share the common cytokine receptor gamma chain, gammac, which is mutated in humans with X-linked severe combined immunodeficiency disease (XSCID). We demonstrate that, although mice deficient in the receptor for IL-21 (IL-21R) have normal lymphoid development, after immunization, these animals have higher production of the immunoglobulin IgE, but lower IgG1, than wild-type animals. Mice lacking both IL-4 and IL-21R exhibited a significantly more pronounced phenotype, with dysgammaglobulinemia, characterized primarily by a severely impaired IgG response. Thus, IL-21 has a significant influence on the regulation of B cell function in vivo and cooperates with IL-4. This suggests that these gammac-dependent cytokines may be those whose inactivation is primarily responsible for the B cell defect in humans with XSCID.

The role of common gamma chain in human monocytes in vivo; evaluation from the studies of X-linked severe combined immunodeficiency (X-SCID) carriers and X-SCID patients who underwent cord blood stem cell transplantation

Expression of common gamma chain (gammac) on monocytes was studied in five carriers of X-linked severe combined immunodeficiency (X-SCID) and two X-SCID patients who underwent cord blood stem cell transplantation (CBSCT). Flow cytometric analysis revealed that both gammac-negative and positive monocytes co-existed in X-SCID carriers, whereas no gammac-negative T, B or NK cells were observed in them. Clonal analysis and reverse transcription polymerase chain reaction studies revealed that 13.2-45.0% of monocytes from these carriers expressed the mutant gammac message. X-SCID patients who received CBSCT persistently possessed the majority of gammac-negative monocytes with a good clinical course. These results, together, may indicate that gammac is not essential for monocyte development/function in vivo.

Reduced memory B-cell populations in boys with B-cell dysfunction after bone marrow transplantation for X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency (XSCID) is a lethal disease resulting in death in infancy. In many instances, haploidentical bone marrow transplantation (BMT) offers reconstitution of T-cell immunity alone, with residual hypogammaglobulinaemia. The exact nature of B-cell dysfunction in these patients is unclear, although differentiation arrest of the B cells is a potential explanation. To ascertain the differentiation status of peripheral blood B lymphocytes from XSCID patients after BMT, the surface expression of CD19, CD10, CD34, CD5, serum immunoglogulin (sIg)M, sIgD, sIgG and CD27 on these B cells was investigated using three-colour flow cytometry. CD27 is a marker of memory B cells. Populations of CD19+IgM-D- B cells, CD19+IgM-only, CD19+IgG+CD27+ and CD19+IgM+ CD27+ B cells were found to be diminished in the XSCID patients after BMT with persistent hypogammaglobulinaemia, compared with both post-BMT patients with B-cell function and age-matched normal controls. This indicated the lack of CD19+IgM-D- B cells, which represent Ig isotype-switched B cells, as well as CD19+IgM-only and CD19+IgG+CD27+ or CD19+IgM+CD27+ memory B-cell populations. Interaction between CD27 and its ligand CD70 has been shown to induce IgG and IgM production by CD27+ B cells. Therefore, the lack of CD27/70 interaction is a probable explanation for the hypogammaglobulinaemia in these patients after BMT.

Self-selection by genetically modified committed lymphocyte precursors reverses the phenotype of JAK3-deficient mice without myeloablation

Janus kinase 3 (JAK3) is an essential component of cytokine receptor signal transduction pathways required for normal lymphocyte development and function. JAK3 deficiency in both mice and humans results in severe combined immunodeficiency (SCID) and increased susceptibility to opportunistic infections. We have previously shown that JAK3 gene transfer into irradiated recipients could restore immune function. However, since this toxic conditioning would be undesirable for infants in a clinical application, we have tested whether immune function could be restored in nonmyeloablated JAK3-deficient (-/-) mice. Murine JAK3 retroviral vectors were transduced into hematopoietic stem cells from the livers of newborn JAK3(-/-) mice. These cells were then injected intraperitoneally into nonirradiated JAK3(-/-) neonates. Transduced cells were detectable in these mice at time points 4 to 6 months after injection and resulted in significant correction of T and B lymphocyte numbers and circulating immunoglobulin (Ig) levels. After immune challenge with a dose of influenza A virus that was lethal to nonmanipulated JAK3(-/-) mice, mice injected with transduced cells showed development of circulating virus-specific IgG and enhanced survival. This work shows that the large selective advantage for JAK3-corrected lymphoid cells may be sufficient to overcome the need for myeloablative conditioning in JAK3 gene therapy protocols.

In vivo competitive studies between normal and common gamma chain-defective bone marrow cells: implications for gene therapy

Corrective gene transfer into hematopoietic stem cells (HSCs) is being investigated as therapy for X-linked severe combined immunodeficiency (XSCID) and it is hoped that selective advantage of gene-corrected HSCs will help in achieving full immune reconstitution after treatment. Lines of evidence from the results of allogeneic bone marrow transplantation in patients with XSCID support this hypothesis that, however, has not been rigorously tested in an experimental system. We studied the competition kinetics between normal and XSCID bone marrow (BM) cells using a murine bone marrow transplantation (BMT) model. For easy chimerism determination, we used genetic marking with retrovirus-mediated expression of the enhanced green fluorescent protein (EGFP). We found that XSCID BM cells were able to compete with normal BM cells for engraftment of myeloid lineages in a dose-dependent manner, whereas we observed selective repopulation of T, B, and NK cells deriving from normal BM cells. This was true despite the evidence of competitive engraftment of XSCID lineage marker-negative/c-Kit-positive (Lin-/c-Kit+) cells in the bone marrow of treated animals. From these results we extrapolate that genetic correction of XSCID HSCs will result in selective advantage of gene-corrected lymphoid lineages with consequent restoration of lymphocyte populations and high probability of clinical benefit.

B-cell function in canine X-linked severe combined immunodeficiency

Canine X-linked severe combined immunodeficiency (XSCID) is due to mutations in the common gamma (gammac) subunit of the IL-2, IL-4, IL-7, IL-9 and IL-15 receptors and has a similar clinical phenotype to human XSCID. We have previously shown that the block in T-cell development is more profound in XSCID dogs than in genetically engineered gamma c-deficient mice. In this study we evaluated the B-cell function in XSCID dogs. In contrast to the marked decrease in peripheral B-cells in gamma c-deficient mice, XSCID dogs have increased proportions and numbers of peripheral B-cells as observed in XSCID boys. Canine XSCID B-cells do not proliferate following stimulation with the T-cell-dependent B-cell mitogen, pokeweed mitogen (PWM); however, they proliferate normally in response to the T-cell-independent B-cell mitogen, formalin-fixed, heat-killed Staphylococcus aureus. Canine XSCID B-cells are capable of producing IgM but are incapable of normal class-switching to IgG antibody production as demonstrated by in vitro stimulation with PWM and immunization with the T-cell-dependent antigen, bacteriophage PhiX174. Similar results have been reported for XSCID boys. Thus, it appears that gamma c-dependent cytokines have differing roles in human and canine B-cell development than in the mouse making the XSCID dog a valuable model for studying the role of these cytokines in B-cell development and function.

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.

Long-Term Chimerism and B-Cell Function After Bone Marrow Transplantation in Patients With Severe Combined Immunodeficiency With B Cells: A Single-Center Study of 22 Patients

We retrospectively analyzed the B-cell function and leukocyte chimerism of 22 patients with severe combined immunodeficiency with B cells (B+ SCID) who survived more than 2 years after bone marrow transplantation (BMT) to determine the possible consequences of BMT procedures, leukocyte chimerism, and SCID molecular deficit on B-cell function outcome. Circulating T cells were of donor origin in all patients. In recipients of HLA-identical BMT (n = 5), monocytes were of host origin in 5 and B cells were of host origin in 4 and of mixed origin in 1. In recipients of HLA haploidentical T-cell–depleted BMT (n = 17), B cells and monocytes were of host origin in 14 and of donor origin in 3. Engraftment of B cells was found to be associated with normal B-cell function. In contrast, 10 of 18 patients with host B cells still require Ig substitution. Conditioning regimen (ie, 8 mg/kg busulfan and 200 mg/kg cyclophosphamide) was shown neither to promote B-cell and monocyte engraftment nor to affect B-cell function. Eight patients with B cells of host origin had normal B-cell function. Evidence for functional host B cells was further provided in 3 informative cases by Ig allotype determination and by the detection, in 5 studied cases, of host CD27+ memory B cells as in age-matched controls. These results strongly suggest that, in some transplanted patients, host B cells can cooperate with donor T cells to fully mature in Ig-producing cells.

Bone marrow transplantation for canine X-linked severe combined immunodeficiency

Canine X-linked severe combined immunodeficiency (XSCID) is due to mutations in the common gamma chain which is a subunit of the receptors of IL-2, IL-4, IL-7, IL-9 and IL-15. Bone marrow transplantation (BMT) of human XSCID patients without pretransplant conditioning (cytoablation) results in engraftment of donor T-cells and reconstitution of T-cell function but engraftment of few, if any, donor B cells with resultant poor reconstitution of humoral immune function. In this study, we show that XSCID dogs can be transplanted with allogeneic bone marrow cells resulting in engraftment of both donor B and T cells and reconstitution of full systemic immune function including normal humoral immune function without the need for cytoablation.

Canine X-linked severe combined immunodeficiency

Canine X-linked severe combined immunodeficiency (XSCID) is due to mutations in the common gamma (gamma c) subunit of the IL-2, IL-4, IL-7, IL-9 and IL-15 receptors. The most striking clinical feature is a failure to thrive or 'stunted' growth. Recurrent or chronic infections begin at the time of decline of maternal antibody, usually between six and eight weeks of age. Affected dogs rarely survive past three to four months of age. The major pathologic feature of canine XSCID is a small, dysplastic thymus. Grossly identifiable lymph nodes, tonsils, and Peyer's patches are absent in XSCID dogs. During the neonatal period, XSCID dogs have few, if any, peripheral T cells and increased number of peripheral B cells. Some XSCID dogs do develop phenotypically mature, nonfunctional T cells with age, however, the absolute number of peripheral T cells remain significantly decreased compared to age-matched normal dogs. An interesting finding is that as soon as T cells begin to appear in XSCID dogs they rapidly switch from a CD45RA+ (naive) phenotype to a CD45RA- (activated or memory phenotype). One of the characteristic findings in XSCID dogs is an absent or markedly depressed blastogenic response of T cells in response to stimulation through the T cell receptor and when the necessary second messengers for cellular proliferation are directly provided that by-pass signals delivered through ligand-receptor interaction. The proliferative defect is due to the inability of T cells to express a functional IL-2 receptor. Canine XSCID B cells do not proliferate following stimulation with T cell-dependent B cell mitogens, however, they proliferate normally in response to T cell-independent B cell mitogens. Canine XSCID B cells are capable of producing IgM but are incapable of class-switching to IgG antibody production following immunization with the T cell-dependent neoantigen, bacteriophage phiX174. The number of thymocytes in the XSCID thymus is approximately 0.3% of the thymocytes present in the thymus of age-matched normal dogs. The proportion of CD4-CD8- thymocytes in XSCID dogs is increased 3.5-fold and the CD4+CD8+ population is decreased 2.3-fold. These findings demonstrate that (1) a functional gamma c is required for normal B and T cell function, (2) early T cell development is highly dependent upon a functional gamma c, and (3) B cell development can occur through a gamma c-independent pathway.

Immunodeficiency due to a unique protracted developmental delay in the B-cell lineage

A unique immune deficiency in a 24-month-old male characterized by a transient but protracted developmental delay in the B-cell lineage is reported. Significant deficiencies in the number of B cells in the blood, the concentrations of immunoglobulins in the serum, and the titers of antibodies to T-dependent and T-independent antigens resolved spontaneously by the age of 39 months in a sequence that duplicated the normal development of the B-cell lineage: blood B cells followed by immunoglobulin M (IgM), IgG, IgA, and specific IgG antibodies to T-independent antigens (pneumococcal polysaccharides). Because of the sequence of recovery, the disorder could have been confused with other defects in humoral immunity, depending on when in the course of disease immunologic studies were conducted. Investigations of X-chromosome polymorphisms suggested that the disorder was not X linked in that the mother appeared to have identical X chromosomes. An autosomal recessive disorder involving a gene that controls B-cell development and maturation seems more likely. In summary, this case appears to be a novel protracted delay in the development of the B-cell lineage, possibly due to an autosomal recessive genetic defect.

Multiple gamma c-dependent cytokines regulate T-cell development

Mutations in the common gamma chain (gamma c) of cytokine receptors account for human X-linked severe combined immunodeficiency disease. gamma c contributes to ligand binding and signaling as a component of five cytokine receptors: interleukin-2-receptor (IL-2R), IL-4R, IL-7R, IL-9R and IL-15R. Here, Thomas Malek and colleagues discuss the contribution of individual gamma c-dependent cytokines in both conventional and intraepithelial T-cell development.

Inherited cytokine and cytokine receptor deficiencies in man

Over the past decades, cytokines and their receptors have been shown to play a decisive role in the differentiation of both innate and adaptive immunity. The essential roles of cytokine/receptor pathways in vivo, however, have remained elusive and poorly defined. In some cases, primary immunodeficiency syndromes have provided the natural models in which the role of cytokines and their receptors in the development and function of the immune system have been elucidated. Animal models of cytokine/receptor deficiencies generated through gene targeting have also played a decisive factor in identifying the true biological roles of cytokine/receptor pathways. The end result of these approaches has been an enormous advance in our understanding of the cytokine control of normal and pathological human conditions, as well as the advent of new diagnostic tools and novel therapies.

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.

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

Mutation analysis by a non-radioactive single-strand conformation polymorphism assay in nine families with X-linked severe combined immunodeficiency (SCIDX1)

X-linked severe combined immunodeficiency (SCIDX1) is an inherited disease characterized by profound abnormalities of cell-mediated and humoral immunity. Patients with SCIDX1 have defects in the common cytokine receptor gamma chain gene (IL2RG) that encodes a shared, essential component of the receptors for interleukin-2 (IL-2), IL-4, IL-7, IL-9 and IL-15. We have characterized nine SCIDX1 families by using a DNA-based, non-radioactive screening method and DNA sequencing. Nine different mutations were found, scattered from exon 1 to exon 5 of the IL2RG gene. Two of these mutations have been previously identified in other unrelated patients; the other seven are novel mutations that differ from all of the 95 already reported in the IL2RG mutation data base. In addition to describing novel mutations in the IL2RG gene, this study shows that the knowledge of the genetic defect and the use of an efficient, non-radioactive, and rapid screening approach have important implications for prenatal and postnatal diagnosis, carrier female identification, and possibly prenatal therapy.

An X chromosome gene regulates hematopoietic stem cell kinetics

Females are natural mosaics for X chromosome-linked genes. As X chromosome inactivation occurs randomly, the ratio of parental phenotypes among blood cells is approximately 1:1. Recently, however, ratios of greater than 3:1 have been observed in 38-56% of women over age 60. This could result from a depletion of hematopoietic stem cells (HSCs) with aging (and the maintenance of hematopoiesis by a few residual clones) or from myelodysplasia (the dominance of a neoplastic clone). Each possibility has major implications for chemotherapy and for transplantation in elderly patients. We report similar findings in longitudinal studies of female Safari cats and demonstrate that the excessive skewing that develops with aging results from a third mechanism that has no pathologic consequence, hemizygous selection. We show that there is a competitive advantage for all HSCs with a specific X chromosome phenotype and, thus, demonstrate that an X chromosome gene (or genes) regulates HSC replication, differentiation, and/or survival.

Canine X-linked severe combined immunodeficiency. A model for investigating the requirement for the common gamma chain (gamma c) in human lymphocyte development and function

Our laboratory has identified and characterized an X-linked severe combined immunodeficiency (XSCID) in dogs that is due to mutations in the common gamma (gamma c) subunit of the interleukin-2 (IL2), IL4, IL7, IL9, and IL15 receptors. Canine XSCID, unlike genetically engineered gamma c-deficient mice, has a clinical and immunologic phenotype virtually identical to human XSCID. It appears that species-specific differences exist in the role of the gamma c and its associated cytokines in mice compared to their role in humans and dogs, suggesting gamma c-deficient dogs may be a more relevant model for studying the role of the gamma c in humans. We are utilizing this model for a variety of studies to address: 1. Fundamental questions concerning the role of the gamma c in cytokine regulation and lymphocyte development. 2. The pathogenesis of XSCID. 3. Strategies for improving bone marrow transplantation outcome. 4. Development and evaluation of strategies for gene therapy. 5. Human hematopoietic stem cell development.

Full Immunologic Reconstitution Following Nonconditioned Bone Marrow Transplantation for Canine X-Linked Severe Combined Immunodeficiency

Bone marrow transplantation in human X-linked severe combined immunodeficiency (XSCID) without pretransplant conditioning results in engraftment of donor T cells and reconstitution of T-cell function but engraftment of few, if any, donor B cells and poor reconstitution of humoral immune function. Since bone marrow transplantation remains the most effective treatment of XSCID patients, better strategies are necessary to achieve optimum long-term results. Canine XSCID, like human XSCID, is due to mutations in the common γ chain (γc) gene and has clinical and immunologic features identical to those of human XSCID, making it a true homolog of the human disease. We have successfully performed bone marrow transplantation in three XSCID dogs without pretransplant conditioning, using untreated bone marrow cells from mixed lymphocyte culture–nonreactive normal littermates. Unlike the experience in human XSCID patients, all three dogs engrafted both donor B and T cells and attained full reconstitution of immunologic function. Normal percentages of T cells and T-cell mitogenic responses were attained by 3 months posttransplant. CD3+ T cells after transplantation expressed the CD45RA isoform indicating that the cells were recent thymic emigrants derived from immature progenitors. Serum IgG levels were within normal range by 5 months posttransplant. Immunization with the T-dependent antigen, bacteriophage φX174, demonstrated normal antibody titers, immunologic memory, and class-switching. Polymerase chain reaction (PCR) analysis of the γc locus showed that 100% of circulating T cells and 30% to 50% of circulating B cells were donor-derived. None of the dogs developed clinically evident graft-versus-host disease (GVHD). Thus, canine XSCID provides a model to determine the optimal conditions for bone marrow transplantation in human patients, and to develop and test strategies for somatic gene therapy.

Naturally occurring primary deficiencies of the immune system

Naturally occurring genetic disorders of the immune system provide many models for the study of its development and function. In a way, their analysis complements the information provided by the generation of genetic defects in mice created using homologous recombination techniques. In this review, the recent findings made in three areas are focused upon deficiencies in T cell differentiation and in T lymphocyte activation, and on the control process of peripheral immune response.

The interleukin-2 receptor gamma chain: its role in the multiple cytokine receptor complexes and T cell development in XSCID

Interleukin 2 (IL-2), a T cell-derived cytokine, targets a variety of cells to induce their growth, differentiation, and functional activation. IL-2 inserts signals into the cells through IL-2 receptors expressed on cell surfaces to induce such actions. In humans, the functional IL-2 receptor consists of the subunit complexes of the alpha, beta and gamma chains, or the beta and gamma chains. The third component, the gamma chain, of IL-2 receptor plays a pivotal role in formation of the full-fledged IL-2 receptor, together with the beta chain, the gamma chain participates in increasing the IL-2 binding affinity and intracellular signal transduction. Moreover, the cytokine receptors for at least IL-2, IL-4, IL-7, IL-9, and IL-15 utilize the same gamma chain as an essential subunit. Interestingly, mutations of the gamma chain gene cause human X-linked severe combined immunodeficiency (XSCID) characterized by a complete or profound T cell defect. Among the cytokines sharing the gamma chain, at least IL-7 is essentially involved in early T cell development in the mouse organ culture system. The molecular identification of the gamma chain brought a grasp of the structures and functions of the cytokine receptor and an in-depth understanding of the cause of human XSCID. To investigate the mechanism of XSCID and development of gene therapy for XSCID, knockout mice for the gamma chain gene were produced that showed similar but not exactly the same phenotypes as human XSCID.

Genomic structure and PCR-SSCP analysis of the human CD40 ligand gene: its application to prenatal screening for X-linked hyper-IgM syndrome

To develop a general method for analysis of the mutation and prenatal diagnosis of X-linked hyper-IgM syndrome (XHM), the human CD40 ligand (hCD40L) gene was cloned and sequenced with special reference to the 5' and 3' flanking regions and exon/intron boundaries. The hCD40L gene consists of five exons and four introns, as already reported by others. Two major transcription initiation sites were identified at 67 bp and 64 bp upstream from the ATG initiation codon. The hCD40L mRNA transcripts terminated at 321 bp, 327 bp and 987 bp downstream from the TGA stop codon. Based on the intronic sequences, oligonucleotide primers were designed for amplifying the coding region of each exon separately. Polymerase chain reaction--single-strand conformational polymorphism (PCR-SSCP) analysis was successfully applied to screening for the defective hCD40L gene in a family with XHM. The nonsense mutation, Trp140 (TGG)-->stop (TAG) in exon 5, was found in the mother and an affected child. We also performed prenatal diagnosis by PCR-SSCP during the first trimester of pregnancy in this family.

Mutations of Jak-3 gene in patients with autosomal severe combined immune deficiency (SCID)

Severe combined immune deficiency (SCID) represents a heterogenous group of hereditary diseases. Mutations in the common gamma-chain (gamma c), which is part of several cytokine receptors including those for interleukin (IL)-2, IL-4, IL-7, IL-9 and IL-15, are responsible for X-linked SCID, which is usually associated with a lack of circulating T cells and the presence of B lymphocytes (T- B+ SCID). The gene(s) responsible for autosomal recessive T- B+ SCID is still unknown. The Jak-3 protein kinase has been found to associate with the gamma c-chain-containing cytokine receptors. Therefore Jak-3 or other STAT proteins with which it interacts are candidate genes for autosomal recessive T- B+ SCID. Here we investigate two unrelated T- B+ SCID patients (both from consanguineous parents) who have homozygous mutations in the gene for Jak-3. One patient carries a mutation (Tyr100-->Cys) in a conserved tyrosine residue in the JH7 domain of Jak-3 which is absent in more than 150 investigated chromosomes. The other patient carries a homozygous 151-base-pair deletion in the kinase-like domain, leading to a frameshift and premature termination. Both mutations resulted in markedly reduced levels of Jak-3. These findings show that abnormalities in the Jak/STAT signalling pathway can account for SCID in humans.