Molecular Evidence of Genome Editing in a Mouse Model of Immunodeficiency

Genome editing is the introduction of directed modifications in the genome, a process boosted to therapeutic levels by designer nucleases. Building on the experience of ex vivo gene therapy for severe combined immunodeficiencies, it is likely that genome editing of haematopoietic stem/progenitor cells (HSPC) for correction of inherited blood diseases will be an early clinical application. We show molecular evidence of gene correction in a mouse model of primary immunodeficiency. In vitro experiments in DNA-dependent protein kinase catalytic subunit severe combined immunodeficiency (Prkdc scid) fibroblasts using designed zinc finger nucleases (ZFN) and a repair template demonstrated molecular and functional correction of the defect. Following transplantation of ex vivo gene-edited Prkdc scid HSPC, some of the recipient animals carried the expected genomic signature of ZFN-driven gene correction. In some primary and secondary transplant recipients we detected double-positive CD4/CD8 T-cells in thymus and single-positive T-cells in blood, but no other evidence of immune reconstitution. However, the leakiness of this model is a confounding factor for the interpretation of the possible T-cell reconstitution. Our results provide support for the feasibility of rescuing inherited blood disease by ex vivo genome editing followed by transplantation, and highlight some of the challenges.

Enhancement of mouse hematopoietic stem/progenitor cell function via transient gene delivery using integration-deficient lentiviral vectors

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Integration-deficient vectors (IdLVs) express genes transiently in dividing stem cells.

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Hematopoietic stem/progenitor cells (HSPCs) can be programmed using IdLVs.

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HOXB4 or Angptl3 expression from IdLVs improves engraftment of transplanted HSPCs.

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Short-term gene delivery avoids the side effects associated with constitutive expression.

Integration-deficient lentiviruses (IdLVs) deliver genes effectively to tissues but are lost rapidly from dividing cells. This property can be harnessed to express transgenes transiently to manipulate cell biology. Here, we demonstrate the utility of short-term gene expression to improve functional potency of hematopoietic stem and progenitor cells (HSPCs) during transplantation by delivering HOXB4 and Angptl3 using IdLVs to enhance the engraftment of HSPCs. Constitutive overexpression of either of these genes is likely to be undesirable, but the transient nature of IdLVs reduces this risk and those associated with unsolicited gene expression in daughter cells. Transient expression led to increased multilineage hematopoietic engraftment in in vivo competitive repopulation assays without the side effects reported in constitutive overexpression models. Adult stem cell fate has not been programmed previously using IdLVs, but we demonstrate that these transient gene expression tools can produce clinically relevant alterations or be applied to investigate basic biology.

Long-term persistence of a polyclonal T cell repertoire after gene therapy for X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the common cytokine receptor γ chain. These mutations classically lead to complete absence of functional T and natural killer cell lineages as well as to intrinsically compromised B cell function. Although human leukocyte antigen (HLA)-matched hematopoietic stem cell transplantation (HSCT) is highly successful in SCID-X1 patients, HLA-mismatched procedures can be associated with prolonged immunodeficiency, graft-versus-host disease, and increased overall mortality. Here, 10 children were treated with autologous CD34(+) hematopoietic stem and progenitor cells transduced with a conventional gammaretroviral vector. The patients did not receive myelosuppressive conditioning and were monitored for immunological recovery after cell infusion. All patients were alive after a median follow-up of 80 months (range, 54 to 107 months), and a functional polyclonal T cell repertoire was restored in all patients. Humoral immunity only partially recovered but was sufficient in some patients to allow for withdrawal of immunoglobulin replacement; however, three patients developed antibiotic-responsive acute pulmonary infection after discontinuation of antibiotic prophylaxis and/or immunoglobulin replacement. One patient developed acute T cell acute lymphoblastic leukemia because of up-regulated expression of the proto-oncogene LMO-2 from insertional mutagenesis, but maintained a polyclonal T cell repertoire through chemotherapy and entered remission. Therefore, gene therapy for SCID-X1 without myelosuppressive conditioning effectively restored T cell immunity and was associated with high survival rates for up to 9 years. Further studies using vectors designed to limit mutagenesis and strategies to enhance B cell reconstitution are warranted to define the role of this treatment modality alongside conventional HSCT for SCID-X1.

Hematopoietic stem cell gene therapy for adenosine deaminase-deficient severe combined immunodeficiency leads to long-term immunological recovery and metabolic correction

Genetic defects in the purine salvage enzyme adenosine deaminase (ADA) lead to severe combined immunodeficiency (SCID) with profound depletion of T, B, and natural killer cell lineages. Human leukocyte antigen-matched allogeneic hematopoietic stem cell transplantation (HSCT) offers a successful treatment option. However, individuals who lack a matched donor must receive mismatched transplants, which are associated with considerable morbidity and mortality. Enzyme replacement therapy (ERT) for ADA-SCID is available, but the associated suboptimal correction of immunological defects leaves patients susceptible to infection. Here, six children were treated with autologous CD34-positive hematopoietic bone marrow stem and progenitor cells transduced with a conventional gammaretroviral vector encoding the human ADA gene. All patients stopped ERT and received mild chemotherapy before infusion of gene-modified cells. All patients survived, with a median follow-up of 43 months (range, 24 to 84 months). Four of the six patients recovered immune function as a result of engraftment of gene-corrected cells. In two patients, treatment failed because of disease-specific and technical reasons: Both restarted ERT and remain well. Of the four reconstituted patients, three remained off enzyme replacement. Moreover, three of these four patients discontinued immunoglobulin replacement, and all showed effective metabolic detoxification. All patients remained free of infection, and two cleared problematic persistent cytomegalovirus infection. There were no adverse leukemic side effects. Thus, gene therapy for ADA-SCID is safe, with effective immunological and metabolic correction, and may offer a viable alternative to conventional unrelated donor HSCT.

Biochemical correction of X-CGD by a novel chimeric promoter regulating high levels of transgene expression in myeloid cells

X-linked chronic granulomatous disease (X-CGD) is a primary immunodeficiency caused by mutations in the CYBB gene encoding the phagocyte nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase catalytic subunit gp91(phox). A recent clinical trial for X-CGD using a spleen focus-forming virus (SFFV)-based γ-retroviral vector has demonstrated clear therapeutic benefits in several patients although complicated by enhancer-mediated mutagenesis and diminution of effectiveness over time due to silencing of the viral long terminal repeat (LTR). To improve safety and efficacy, we have designed a lentiviral vector that directs transgene expression primarily in myeloid cells. To this end, we created a synthetic chimeric promoter that contains binding sites for myeloid transcription factors CAAT box enhancer-binding family proteins (C/EBPs) and PU.1, which are highly expressed during granulocytic differentiation. As predicted, the chimeric promoter regulated higher reporter gene expression in myeloid than in nonmyeloid cells, and in human hematopoietic progenitors upon granulocytic differentiation. In a murine model of stem cell gene therapy for X-CGD, the chimeric vector resulted in high levels of gp91(phox) expression in committed myeloid cells and granulocytes, and restored normal NADPH-oxidase activity. These findings were recapitulated in human neutrophils derived from transduced X-CGD CD34(+) cells in vivo, and suggest that the chimeric promoter will have utility for gene therapy of myeloid lineage disorders such as CGD.

Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients

X-linked SCID (SCID-X1) is amenable to correction by gene therapy using conventional gammaretroviral vectors. Here, we describe the occurrence of clonal T cell acute lymphoblastic leukemia (T-ALL) promoted by insertional mutagenesis in a completed gene therapy trial of 10 SCID-X1 patients. Integration of the vector in an antisense orientation 35 kb upstream of the protooncogene LIM domain only 2 (LMO2) caused overexpression of LMO2 in the leukemic clone. However, leukemogenesis was likely precipitated by the acquisition of other genetic abnormalities unrelated to vector insertion, including a gain-of-function mutation in NOTCH1, deletion of the tumor suppressor gene locus cyclin-dependent kinase 2A (CDKN2A), and translocation of the TCR-beta region to the STIL-TAL1 locus. These findings highlight a general toxicity of endogenous gammaretroviral enhancer elements and also identify a combinatorial process during leukemic evolution that will be important for risk stratification and for future protocol design.

Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients

X-linked SCID (SCID-X1) is amenable to correction by gene therapy using conventional gammaretroviral vectors. Here, we describe the occurrence of clonal T cell acute lymphoblastic leukemia (T-ALL) promoted by insertional mutagenesis in a completed gene therapy trial of 10 SCID-X1 patients. Integration of the vector in an antisense orientation 35 kb upstream of the protooncogene LIM domain only 2 (LMO2) caused overexpression of LMO2 in the leukemic clone. However, leukemogenesis was likely precipitated by the acquisition of other genetic abnormalities unrelated to vector insertion, including a gain-of-function mutation in NOTCH1, deletion of the tumor suppressor gene locus cyclin-dependent kinase 2A (CDKN2A), and translocation of the TCR-beta region to the STIL-TAL1 locus. These findings highlight a general toxicity of endogenous gammaretroviral enhancer elements and also identify a combinatorial process during leukemic evolution that will be important for risk stratification and for future protocol design.

Gene therapy of inherited immunodeficiencies

BACKGROUND

Primary immunodeficiencies (PID) are a group of inherited diseases that affect the development or activity of the immune system. In severe cases allogeneic haematopoietic stem cell transplantation has proved to be a successful curative modality but it is limited by toxicity and reduced efficacy in mismatched donor settings.

OBJECTIVE

Gene therapy for PID has been developed as an alternative strategy and has entered the clinical arena. In this review we discuss the outcomes of recent gene therapy trials and some of the problems that remain to be tackled.

METHODS

Results from clinical trials for X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase deficient SCID (ADA-SCID), and X-linked chronic granulomatous disease (X-CGD) are discussed. In addition, other conditions are highlighted such as the Wiskott Aldrich Syndrome (WAS) for which gene therapy has shown considerable promise in preclinical studies, and are currently being translated into novel clinical approaches.

RESULTS/CONCLUSION

Whilst these encouraging results demonstrate that gene therapy can be used successfully to treat monogenic PID, the occurrence of vector-related side effects has highlighted the need for accurate assessment of the associated risks and a requirement for improvements in vector design.

Improvement of migratory defects in a murine model of Wiskott-Aldrich syndrome gene therapy

Wiskott-Aldrich syndrome (WAS) is an X-linked hematological disease characterized by immunodeficiency, eczema, and thrombocytopaenia, and shows promise for treatment with hematopoietic stem cell gene therapy. The immunopathology of WAS is attributable at least in part to defects of cell migration and localization as a result of chemotactic, adhesive, and chemokinetic defects. Whereas previous studies using either gammaretroviral or lentiviral vectors have demonstrated variable correction of T-cell proliferation and dendritic cell (DC) cytoarchitecture, we have used a lentiviral vector expressing an eGFP-WASp fusion protein to test the potential for restoration of cell migratory defects. Multilineage expression of the fusion transgene was present for up to 10 months after primary engraftment, and also in secondary recipients analyzed after a further 9 months. Transduced bone marrow-derived dendritic cells (BMDCs) demonstrated recovery of podosome numbers and turnover, while B cells, BMDCs, and Langerhans cells (LCs) exhibited enhanced chemotactic responses to specific stimuli. As an indication of functionality in vivo, splenic marginal zone B cells and a cutaneous contact hypersensitivity (CHS) response to dinitrofluorobenzene (DNFB) were both partially restored. These proof of principle experiments demonstrate that WAS protein (WASp) transgene expression can be successfully maintained long term in primary and secondary recipients, and that it is associated with a significant repair of migratory defects.

Bone morphogenetic protein 4 modulates c-Kit expression and differentiation potential in murine embryonic aorta-gonad-mesonephros haematopoiesis in vitro

The transforming growth factor-β-related factor bone morphogenetic protein 4 (BMP4) is expressed in the human embryonic aorta-gonad-mesonephros (AGM) coincident with the emergence of haematopoietic cells and influences postnatal mammalian haematopoietic stem cells in vitro. To investigate the role of BMP4 in mammalian embryonic haematopoiesis, cells were isolated from murine AGM and two populations of CD34⁺ cells with different levels of c-Kit expression and multipotency were identified. CD34⁺/c-Kit^high cells express CD45 and are haematopoietic-restricted progenitors. In contrast, CD34⁺/c-Kit^low cells are Flk1+/CD45^neg and generate adherent colonies in ex vivo culture that resemble haemangioblast colonies identified in other systems. The addition of BMP4 to AGM cells resulted in expansion of the CD34⁺/c-Kit^low cell pool within 48 h, via a combination of down modulation of the c-Kit receptor in CD34⁺/c-Kit^high cells and proliferation. In long-term culture, BMP4 increased the growth/survival of CD34⁺/c-Kit^high haematopoietic progenitors, effects that were blocked by BMP inhibitors. CD34⁺/c-Kit^high progenitors cultured with BMP4 also generated adherent colonies typical of c-Kit^low cells. These results suggest that BMP4 regulates c-Kit expression and differentiation potential in CD34⁺ AGM cells and supports a role for BMP signalling in the maintenance of multipotency during embryonic haematopoiesis, providing an insight into stem cell homeostasis within the mammalian haematopoietic niche.

Gammaretrovirus-mediated correction of SCID-X1 is associated with skewed vector integration site distribution in vivo

We treated 10 children with X-linked SCID (SCID-X1) using gammaretrovirus-mediated gene transfer. Those with sufficient follow-up were found to have recovered substantial immunity in the absence of any serious adverse events up to 5 years after treatment. To determine the influence of vector integration on lymphoid reconstitution, we compared retroviral integration sites (RISs) from peripheral blood CD3(+) T lymphocytes of 5 patients taken between 9 and 30 months after transplantation with transduced CD34(+) progenitor cells derived from 1 further patient and 1 healthy donor. Integration occurred preferentially in gene regions on either side of transcription start sites, was clustered, and correlated with the expression level in CD34(+) progenitors during transduction. In contrast to those in CD34(+) cells, RISs recovered from engrafted CD3(+) T cells were significantly overrepresented within or near genes encoding proteins with kinase or transferase activity or involved in phosphorus metabolism. Although gross patterns of gene expression were unchanged in transduced cells, the divergence of RIS target frequency between transduced progenitor cells and post-thymic T lymphocytes indicates that vector integration influences cell survival, engraftment, or proliferation.

Unregulated actin polymerization by WASp causes defects of mitosis and cytokinesis in X-linked neutropenia

Specific mutations in the human gene encoding the Wiskott-Aldrich syndrome protein (WASp) that compromise normal auto-inhibition of WASp result in unregulated activation of the actin-related protein 2/3 complex and increased actin polymerizing activity. These activating mutations are associated with an X-linked form of neutropenia with an intrinsic failure of myelopoiesis and an increase in the incidence of cytogenetic abnormalities. To study the underlying mechanisms, active mutant WASp(I294T) was expressed by gene transfer. This caused enhanced and delocalized actin polymerization throughout the cell, decreased proliferation, and increased apoptosis. Cells became binucleated, suggesting a failure of cytokinesis, and micronuclei were formed, indicative of genomic instability. Live cell imaging demonstrated a delay in mitosis from prometaphase to anaphase and confirmed that multinucleation was a result of aborted cytokinesis. During mitosis, filamentous actin was abnormally localized around the spindle and chromosomes throughout their alignment and separation, and it accumulated within the cleavage furrow around the spindle midzone. These findings reveal a novel mechanism for inhibition of myelopoiesis through defective mitosis and cytokinesis due to hyperactivation and mislocalization of actin polymerization.

Immunotherapy for neuroblastoma using syngeneic fibroblasts transfected with IL-2 and IL-12

Cytokine-modified tumour cells have been used in clinical trials for immunotherapy of neuroblastoma, but primary tumour cells from surgical biopsies are difficult to culture. Autologous fibroblasts, however, are straightforward to manipulate in culture and easy to transfect using nonviral or viral vectors. Here we have compared the antitumour effect of fibroblasts and tumour cells transfected ex vivo to coexpress interleukin-2 (IL-2) and IL-12 in a syngeneic mouse model of neuroblastoma. Coinjection of cytokine-modified fibroblasts with Neuro-2A tumour cells abolished their in vivo tumorigenicity. Treatment of established tumours with three intratumoral doses of transfected fibroblasts showed a significant therapeutic effect with reduced growth or complete eradication of tumours in 90% of mice, associated with extensive leukocyte infiltration. Splenocytes recovered from vaccinated mice showed enhanced IL-2 production following Neuro-2A coculture, and increased cytotoxicity against Neuro-2A targets compared with controls. Furthermore, 100% of the tumour-free mice exhibited immune memory against tumour cells when rechallenged three months later. The potency of transfected fibroblasts was equivalent to that of tumour cells in all experiments. We conclude that syngeneic fibroblasts cotransfected with IL-2 and IL-12 mediate therapeutic effects against established disease, and are capable of generating immunological memory. Furthermore, as they are easier to recover and manipulate than autologous tumour cells, fibroblasts provide an attractive alternative immunotherapeutic strategy for the treatment of neuroblastoma.

Generation of Trispecific Cytotoxic T Cells Recognizing Cytomegalovirus, Adenovirus, and Epstein-Barr Virus

Cytomegalovirus (CMV), adenovirus (Ad), and Epstein-Barr virus (EBV) are a major cause of morbidity and mortality after allogeneic stem cell transplantation (SCT). Adoptive immunotherapy with donor-derived cytotoxic T cells (CTLs) directed against EBV or CMV prevents the clinical manifestations of these viruses. We have designed a protocol for the simultaneous generation of polyclonal CTL specific for CMV, Ad, and EBV, which could be used to restore immunity to multiple viruses after SCT. EBV-transformed lymphoblastoid cell lines (LCLs), transduced with an adenoviral vector carrying a transgene for the immunodominant CMV antigen pp65 (Ad5f35-pp65GFP), were used to stimulate peripheral blood mononuclear cells in 6 normal donors. We detected the simultaneous presence of CD8 CTL recognizing peptide epitopes from all 3 viruses by pentamer staining. Enzyme-linked immunospot assays demonstrated a median 29-fold (8 to 248), 47-fold (2 to 137), or 18-fold (5 to 29) increase in cells secreting interferon-gamma in response to CMV, adenoviral, or EBV antigens, respectively, compared with unmanipulated peripheral blood mononuclear cell, with concomitant loss of alloreactivity. The CTL lines showed cytotoxicity against autologous LCL alone and increased cytotoxicity to autologous LCLs pulsed with CMV pp65 peptides or infected with Ad. In summary, we have developed a protocol for the generation of CTL with trivirus specificity, enabling adoptive transfer of CTL recognizing multiple viruses to restore cellular immunity after SCT.

Lentiviral vectors containing an enhancer-less ubiquitously acting chromatin opening element (UCOE) provide highly reproducible and stable transgene expression in hematopoietic cells

Ubiquitously acting chromatin opening elements (UCOEs) consist of methylation-free CpG islands encompassing dual divergently transcribed promoters of housekeeping genes that have been shown to confer resistance to transcriptional silencing and to produce consistent and stable transgene expression in tissue culture systems. To develop improved strategies for hematopoietic cell gene therapy, we have assessed the potential of the novel human HNRPA2B1-CBX3 UCOE (A2UCOE) within the context of a self-inactivating (SIN) lentiviral vector. Unlike viral promoters, the enhancer-less A2UCOE gave rise to populations of cells that expressed a reporter transgene at a highly reproducible level. The efficiency of expression per vector genome was also markedly increased in vivo compared with vectors incorporating either spleen focus-forming virus (SFFV) or cytomegalovirus (CMV) promoters, suggesting a relative resistance to silencing. Furthermore, an A2UCOE-IL2RG vector fully restored the IL-2 signaling pathway within IL2RG-deficient human cells in vitro and successfully rescued the X-linked severe combined immunodeficiency (SCID-X1) phenotype in a mouse model of this disease. These data indicate that the A2UCOE displays highly reliable transcriptional activity within a lentiviral vector, largely overcoming insertion-site position effects and giving rise to therapeutically relevant levels of gene expression. These properties are achieved in the absence of classic enhancer activity and therefore may confer a high safety profile.

Lentiviral Vectors for T-cell Suicide Gene Therapy: Preservation of T-cell Effector Function After Cytokine-mediated Transduction

Retroviral transfer of the Herpes Simplex thymidine kinase (HSVTK) suicide gene to donor T cells has been used as a safety strategy against graft-versus-host disease following allogeneic stem cell transplantation. The feasibility of this strategy in human studies has been demonstrated, but a number of limitations have become apparent. Preactivation of donor lymphocytes using mitogens or monoclonal antibodies is essential for retroviral transduction, but can compromise subsequent T-cell function in vivo. We report the application of lentiviral vectors for transduction of T cells in cytokine culture, without activation through the T-cell receptor. Using vectors encoding either enhanced green fluorescent protein or a truncated CD34/mutant HSVTK fusion selection/suicide construct, we investigated the properties of T cells after gene modification. We found that following cytokine stimulation, a fraction of T cells undergoes division, and transgene expression occurred predominantly in these cells. Antiviral and alloreactive responses were preserved in these populations, and in contrast to fully activated T cells, there was minimal perturbation of regulatory T-cell numbers. We conclude that the use of interleukin-7 for lentiviral transduction offers the greatest potential for gene transfer to T cells without loss of function, and is favored for the clinical production of suicide gene modified T cells.

Capture and generation of adenovirus specific T cells for adoptive immunotherapy

Adenoviral infections represent a major cause of morbidity and mortality following haematopoietic stem cell transplantation. Current anti-viral agents are virostatic and it is evident that elimination of adenovirus (ADV) infection is only achieved by recovery of cellular immunity. Using an interferon-gamma (IFN-gamma) secretion and capture assay to isolate ADV-specific T cells, followed by a 2 week expansion and restimulation protocol, we generated ADV T cells that may be used for cellular immunotherapy. In contrast to virus-specific T cells for cytomegalovirus or Epstein-Barr virus, the ADV response was dominated by CD4(+) T cells and the majority of captured cells exhibited an effector/memory immunophenotype. Highly specific antigen responses were demonstrated by intracellular IFN-gamma expression and cytotoxicity assays when the expanded cells underwent restimulation with ADV-pulsed target cells. Although T cells were initially generated in response to ADV species C, the expanded populations also showed strong activity against ADV species B, suggesting cross-reactivity across ADV species; a finding that has important clinical consequences in the paediatric setting, where the majority of infections are caused by ADV type B and C. The protocols can be readily translated to generate ADV-specific T cells suitable for clinical use and offer an effective immunotherapeutic strategy to control ADV infection.

Two novel activating mutations in the Wiskott-Aldrich syndrome protein result in congenital neutropenia

Severe congenital neutropenia (SCN) is characterized by neutropenia, recurrent bacterial infections, and maturation arrest in the bone marrow. Although many cases have mutations in the ELA2 gene encoding neutrophil elastase, a significant proportion remain undefined at a molecular level. A mutation (Leu270Pro) in the gene encoding the Wiskott-Aldrich syndrome protein (WASp) resulting in an X-linked SCN kindred has been reported. We therefore screened the WAS gene in 14 young SCN males with wild-type ELA2 and identified 2 with novel mutations, one who presented with myelodysplasia (Ile294Thr) and the other with classic SCN (Ser270Pro). Both patients had defects of immunologic function including a generalized reduction of lymphoid and natural killer cell numbers, reduced lymphocyte proliferation, and abrogated phagocyte activity. In vitro culture of bone marrow progenitors demonstrated a profound reduction in neutrophil production and increased levels of apoptosis, consistent with an intrinsic disturbance of normal myeloid differentiation as the cause of the neutropenia. Both mutations resulted in increased WASp activity and produced marked abnormalities of cytoskeletal structure and dynamics. Furthermore, these results also suggest a novel cause of myelodysplasia and that male children with myelodysplasia and disturbance of immunologic function should be screened for such mutations.

Successful reconstitution of immunity in ADA-SCID by stem cell gene therapy following cessation of PEG-ADA and use of mild preconditioning

Gene therapy is a promising treatment option for monogenic diseases, but success has been seen in only a handful of studies thus far. We now document successful reconstitution of immune function in a child with the adenosine deaminase (ADA)-deficient form of severe combined immunodeficiency (SCID) following hematopoietic stem cell (HSC) gene therapy. An ADA-SCID child who showed a poor response to PEG-ADA enzyme replacement was enrolled into the clinical study. Following cessation of enzyme replacement therapy, autologous CD34(+) HSCs were transduced with an ADA-expressing gammaretroviral vector. Gene-modified cells were reinfused following one dose of preconditioning chemotherapy. Two years after the procedure, immunological and biochemical correction has been maintained with progressive increase in lymphocyte numbers, reinitiation of thymopoiesis, and systemic detoxification of ADA metabolites. Sustained vector marking with detection of polyclonal vector integration sites in multiple cell lineages and detection of ADA activity in red blood cells suggests transduction of early hematopoietic progenitors. No serious side effects were seen either as a result of the conditioning procedure or due to retroviral insertion. Gene therapy is an effective treatment option for the treatment of ADA-SCID.

Gene therapy: has it delivered what it promised?

Gene therapy was hailed in the late 1970s and early 1980s as the great ‘miracle cure’ for a wide range of diseases. With the increasing genetic knowledge and skills available it was possible to envisage a world where every genetically defined disease would be amenable to some sort of genetic intervention and potential cure. Although we have continued to make remarkable progress in understanding the genetic basis of disease, particularly through the success of the human genome project, many feel that the therapeutic improvements that were expected have failed to materialize.

Effective gene therapy with nonintegrating lentiviral vectors

Retroviral and lentiviral vector integration into host-cell chromosomes carries with it a finite chance of causing insertional mutagenesis. This risk has been highlighted by the induction of malignancy in mouse models, and development of lymphoproliferative disease in three individuals with severe combined immunodeficiency-X1 (refs. 2,3). Therefore, a key challenge for clinical therapies based on retroviral vectors is to achieve stable transgene expression while minimizing insertional mutagenesis. Recent in vitro studies have shown that integration-deficient lentiviral vectors can mediate stable transduction. With similar vectors, we now show efficient and sustained transgene expression in vivo in rodent ocular and brain tissues. We also show substantial rescue of clinically relevant rodent models of retinal degeneration. Therefore, the high efficiency of gene transfer and expression mediated by lentiviruses can be harnessed in vivo without a requirement for vector integration. For therapeutic application to postmitotic tissues, this system substantially reduces the risk of insertional mutagenesis.

Gene therapy of X-linked severe combined immunodeficiency by use of a pseudotyped gammaretroviral vector

BACKGROUND

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the common cytokine-receptor gamma chain (gamma(c)), resulting in disruption of development of T lymphocytes and natural-killer cells. B-lymphocyte function is also intrinsically compromised. Allogeneic bone-marrow transplantation is successful if HLA-matched family donors are available, but HLA-mismatched procedures are associated with substantial morbidity and mortality. We investigated the application of somatic gene therapy by use of a gibbon-ape-leukaemia-virus pseudotyped gammaretroviral vector.

METHODS

Four children with SCID-X1 were enrolled. Autologous CD34-positive haemopoietic bone-marrow stem cells were transduced ex vivo and returned to the patients without preceding cytoreductive chemotherapy. The patients were monitored for integration and expression of the gamma(c) vector and for functional immunological recovery.

FINDINGS

All patients have shown substantial improvements in clinical and immunological features, and prophylactic medication could be withdrawn in two. No serious adverse events have been recorded. T cells responded normally to mitogenic and antigenic stimuli, and the T-cell-receptor (TCR) repertoire was highly diverse. Where assessable, humoral immunity, in terms of antibody production, was also restored and associated with increasing rates of somatic mutation in immunoglobulin genes.

INTERPRETATION

Gene therapy for SCID-X1 is a highly effective strategy for restoration of functional cellular and humoral immunity.

Impaired dendritic-cell homing in vivo in the absence of Wiskott-Aldrich syndrome protein

Regulated migration and spatial localization of dendritic cells (DCs) are critical events during the initiation of physiologic immune responses and maintenance of tolerance. Here we have used cells deficient in the Wiskott-Aldrich syndrome protein (WASp) to demonstrate the importance of dynamic remodeling of the actin cytoskeleton for these trafficking processes to occur in vitro and in vivo. On fibronectin-coated surfaces, WASp-null immature murine DCs exhibited defects both of attachment and detachment, resulting in impaired net translocation compared with normal cells. The chemokinetic response to CCL21, which is critical for normal lymphatic trafficking, was also abrogated in the absence of WASp. In vivo in both fluorescein isothiocyanate (FITC) and oxazolone contact hypersensitivity models, WASp-null Langerhans cell (LC) migration was compromised, as judged by exit from the skin as well as by homing to the draining lymph node (LN). Furthermore, following systemic challenge with lipopolysaccharide (LPS) or toxoplasma-derived antigen, WASp-null DCs showed incomplete redistribution to T-cell areas in the spleen. Instead, they were retained ectopically in the marginal zone. DC trafficking in vivo is therefore dependent on a normally regulated actin cytoskeleton, which performs an essential function during maintenance of physiologic immunity and when disturbed may contribute significantly to the immunopathology of Wiskott-Aldrich Syndrome.

Enhancer-deleted retroviral vectors restore high levels of superoxide generation in a mouse model of CGD

BACKGROUND

Retroviral vectors possess many advantages for use in gene therapy protocols, especially within the haematopoietic system. A number of attendant problems, however, still limit their safety in clinical application. The effects of the enhancer present in the retroviral long terminal repeat (LTR) are a major concern for the clinical usage of such vectors, as they can exert a powerful regulatory influence on the genes that surround them.

METHODS

To improve the safety and widen the applicability of retroviral vectors for use in gene therapy we have developed an enhancer-deleted (Delta-LTR) retroviral vector that retained high titre and demonstrated transcriptional activity in myeloid cells.

RESULTS

When used to correct a mouse model of autosomal recessive chronic granulomatous disease, the Delta-LTR vectors gave acceptable levels of gene transfer to mouse bone marrow cells. Evidence for a slight preferential expression in myeloid cells was obtained with all the vectors studied. Nitroblue tetrazolium assay of superoxide generation in mouse bone marrow derived haematopoietic colonies revealed that transduction with Delta-LTR vectors could restore functional NADPH oxidase to cells from these animals. Superoxide assay of peripheral blood confirmed that, although relatively low numbers of cells were transduced, the Delta-LTR vector was capable of reconstituting very high levels of oxidase activity, comparable to that obtained from normal cells.

CONCLUSIONS

The Delta-LTR vector described here could provide the basis for a new generation of retroviral vectors with improved safety.

SAP mediates specific cytotoxic T-cell functions in X-linked lymphoproliferative disease

Cytotoxic T cells (CTLs) and natural killer cells play a major role in the immune response to Epstein-Barr virus (EBV) infection. In X-linked lymphoproliferative (XLP) disease, a severe immunodeficiency, immunodysregulatory phenomena are observed following EBV infection, suggesting that defects exist in these effector populations. The gene defective in XLP is SAP (signaling lymphocytic activation molecule [SLAM]–associated protein), an adaptor protein that mediates signals through SLAM and other immunoglobulin superfamily receptors including 2B4. We generated EBV-specific T-cell lines from controls and XLP patients and examined CTL function in response to different stimuli. We show that XLP patients can generate EBV–T-cell lines that are phenotypically similar to those from controls. XLP patient EBV–T-cell lines showed a significant decrease in interferon-gamma (IFN-γ) production in response to 2B4 and autologous EBV-transformed lymphoblastoid cell line (LCL) stimulation but not in response to SLAM. Furthermore, XLP EBV–T-cell lines demonstrated markedly decreased cytotoxic activity against autologous LCLs. By retroviral gene transfer of the SAP gene into XLP EBV–T-cell lines, we show reconstitution of IFN-γ production and of cytotoxic activity confirming SAP-dependent defects. These studies demonstrate that in XLP the lack of SAP affects specific signaling pathways resulting in severe disruption of CTL function.

The impact of retroviral suicide gene transduction procedures on T cells

Retroviral vectors encoding the herpes simplex thymidine kinase gene have been used to render T cells sensitive to the prodrug ganciclovir. Such genetically modified T cells have been used in clinical trials for their graft-versus-leukaemia effects following allogeneic haematopoietic stem cell transplantation. In the event of graft-versus-host disease (GVHD) the cells were susceptible to elimination through exposure to ganciclovir. We have investigated the impact of T-cell activation, required for successful retrovirus-mediated gene transfer, on T-cell receptor repertoire profile, subset distribution and antiviral potential. Using a combination of antibodies against CD3 and CD28, T cells were transduced at high efficiency when exposed to retrovirus between 48 and 72 h later. Lymphocytes had undergone up to seven cycles of cell division by the end of the procedure. Although the T-cell receptor Vbeta repertoire was not altered after retroviral transduction, there were notable shifts in subset profiles with an increased proportion of CD45RO cells in transduced populations. T cells continued to proliferate for several days after transduction and were difficult to sustain under the extended culture conditions required to generate virus-specific T cells. These observations may explain the lower than expected levels of GVHD and poor antiviral immunity reported in recent trials.

Enhancement of integrin-mediated transfection of haematopoietic cells with a synthetic vector system

The aim of the present study was to develop and assess an integrin-targeted synthetic vector system for the transfection of haematopoietic cell lines and dendritic cells. The vector consists of a cationic liposome, Lipofectin (L), a peptide that both targets integrins and binds to DNA (I) and plasmid DNA (D). These components interact electrostatically to form the LID vector complex. Transfection conditions were optimized for the ratio of vector components, the amount of DNA and transfection incubation time. The kinetic analysis of transgene expression revealed a peak of activity at about 24 h, followed by a rapid decline over the next 48 h. Targeted gene delivery was demonstrated by comparing transfected luciferase reporter gene levels using LID complexes containing integrin-targeting peptide sequences with a control peptide. In addition, transfection levels of integrin-targeted LID complexes were significantly enhanced by treatment of cells with PMA, which was also shown to activate integrin receptors and enhance binding to fibronectin. Under optimized conditions transfection efficiencies of 19% for TF-1 cells, 28% for Jurkat cells and 10% for primary dendritic cells were achieved. The LID vector may thus find application for gene-transfer experiments in haematopoietic cell lines and for the development of genetic vaccines using transfected dendritic cells.

Improved antitumour immunity in murine neuroblastoma using a combination of IL-2 and IL-12

Neuroblastoma immunotherapy using cytokine-modified tumour cells has been tested in clinical trials. However, because of the complex nature of antitumour immune responses, a number of therapies may be required for complete tumour eradication and generation of systemic immunity. We report here the improved antitumour effect of two cytokines, interleukin-2 (IL-2) and interleukin-12 (IL-12), when coexpressed by neuroblastoma cell lines. Initially, transfection of human and mouse neuroblastoma cell lines resulted in high expression levels of biologically active IL-2 and IL-12 in vitro. These cytokines when expressed by transfected Neuro-2A cells completely abolished their in vivo tumorigenicity in a syngeneic neuroblastoma model. Vaccination of established tumours with IL-12-producing cells exhibited a clear effect with reduced tumour growth in the presence of IL-2. In vivo depletion studies showed that CD4(+) and CD8(+) T cells mediate the response against cytokine-producing cells. These results suggest that IL-2 and IL-12, when cotransfected in tumour cells, are effective against established disease and provide a promising immunotherapeutic approach for the treatment of neuroblastoma.

T cell transduction and suicide with an enhanced mutant thymidine kinase

Retroviral transfer of Herpes simplex virus thymidine kinase to T cells has been used to confer sensitivity to the antiviral agent ganciclovir. This has allowed therapeutic approaches to be developed in which T cells mediating graft-versus-host disease after bone marrow transplantation can be selectively eliminated by the administration of ganciclovir. Although the strategy has been shown to be generally successful in early clinical trials, there are concerns about possible resistance to ganciclovir and the risk of myelosuppressive side-effects at the doses required to induce T cell suicide. We have incorporated the enhanced mutant HSV-TKSR39 into retroviral vectors tailored to exhibit high levels of expression in T cells and have used protocols optimized for the transduction and selection of primary lymphocytes. We demonstrate that leukemic and primary T cells can be efficiently transduced and highly enriched under conditions that should be readily adaptable for clinical use. T cells carrying HSV-TKSR39 were inhibited by exposure to ganciclovir at concentrations an order of magnitude below those required for wild-type HSV-TK. The less toxic agent aciclovir also eliminated T cells transduced with HSV-TKSR39 (but not HSV-TK), underlining the increased therapeutic potential of the mutant suicide gene system in the bone marrow transplantation setting.

High-level transduction and gene expression in hematopoietic repopulating cells using a human immunodeficiency [correction of imunodeficiency] virus type 1-based lentiviral vector containing an internal spleen focus forming virus promoter

Prolonged exposure of human hematopoietic stem cells (HSC) to growth factors for efficient transduction by murine oncoretroviral vectors has major detrimental effects on repopulating activity. In this study, we have used a vesicular stomatitis virus G envelope protein (VSV-G)-pseudotyped human immunodeficiency virus type 1 (HIV-1) lentiviral-based vector system to transduce cord blood (CB) CD34+ cells over a limited time period (< or =24 hours). Under these conditions, significant gene marking was observed in engrafted human lymphoid, myeloid, and progenitor cells in all transplanted Severe Combined Immunodeficient (SCID) mice. To enhance the level of gene expression in hematopoietic cells, we also generated a series of lentiviral vectors incorporating the spleen focus forming virus (SFFV) long terminal repeat (LTR) sequences, and the Woodchuck hepatitis virus posttranscriptional regulatory element (WPRE). By including the central polypurine tract (cPPT) sequence of HIV-1 we were then able to achieve high levels of transduction (over 80%) and gene expression in vivo after a single exposure to viral supernatant. These results demonstrate that lentiviral vectors are highly effective for gene transfer to human HSC, and that SFFV regulatory sequences can be successfully incorporated to enhance the long-term expression of a transgene in primary human hematopoietic cells in vivo.

Fetal haemopoietic cells display enhanced migration across endothelium

Fetal haemopoietic cells continually circulate and migrate into tissues, and thus may have specialized homing capabilities. In this study we investigated the in vitro features of haemopoietic cells in fetal blood and liver which are relevant to homing and engraftment. Fetal cells were examined for long-term culture-initiating cell (LTC-IC) and progenitor content, adhesion molecule expression, cell cycle behaviour and transendothelial migratory activity. The LTC-IC content of fetal CD34+ cells is similar to that of CD34+ cells from cord and adult mobilized blood. In contrast to adult and cord blood CD34+ cells, fetal CD34+ cells were actively cycling (11.0 +/- 1.7% and 28 +/- 1.1% of fetal blood and liver CD34+ cells, respectively, in S+G2M, P < 0.001, compared with cord and adult cells). The striking finding was that fetal haemopoietic cells (both LTC-ICs and committed progenitors) displayed significantly higher levels of migration across endothelium (P < 0.05 compared with cord, P < 0.01 compared with adult blood and bone marrow CD34+ cells), which were further increased by chemokines and growth factors. The superior migratory activity of fetal haemopoietic cells may underlie a more efficient homing ability, in keeping with their physiological role.

Non-viral, integrin-mediated gene transfer into fibroblasts from patients with lysosomal storage diseases

BACKGROUND

Non-viral vectors consisting of Lipofectin/integrin-targeting peptide/DNA (LID) complexes have great potential for gene therapy, as they are safe, simple, and able to package large DNA molecules. In this study, these vectors were evaluated in vitro for the therapy of lysosomal storage disorders.

METHODS

Non-viral vectors were designed to deliver therapeutic genes by integrin-mediated uptake into fibroblasts from patients with the lysosomal storage disorders fucosidosis and Fabry disease, which result from deficiencies of alpha-L-fucosidase and alpha-galactosidase A, respectively. The vectors consisted of a complex (LID) of Lipofectin and a peptide containing an integrin-targeting domain and a poly-lysine domain to which was bound plasmid DNA, containing alpha-L-fucosidase (LID-alpha-Fuc) or alpha-galactosidase A (LID-alpha-Gal).

RESULTS

Patients' fibroblasts transfected with LID-alpha-Fuc and LID-alpha-Gal produced the corresponding enzyme at levels which were 10-40% of the total activity in cultures of normal fibroblasts. However, 95-98% of this activity was secreted. Transfection of endothelial cells, the main target cells in Fabry disease, with an LID-alpha-Gal produced a total alpha-galactosidase activity 65% higher than that in untransfected cultures after 6 days, 67% of the activity being secreted. Although transfection of fibroblasts with LID complexes also caused small changes in the distribution of endogenous lysosomal enzymes, it did not appear to affect the viability of the cells.

CONCLUSIONS

The integrin-mediated transfer of genes encoding lysosomal enzymes into cells results in the secretion of large amounts of normal enzyme that could be taken up by other cells. This could be a useful strategy for enzyme-replacement therapy.

Defective expression of the interleukin-2/interleukin-15 receptor beta subunit leads to a natural killer cell-deficient form of severe combined immunodeficiency

Development of T and natural killer (NK) cells is critically dependent on cytokine signaling, and defects in cytokine receptor complex subunits have been shown to result in severe combined immunodeficiency (SCID) syndromes in humans and in murine models. An infant boy had typical clinical features of SCID and was found to lack NK cells in his peripheral circulation. Molecular analysis did not reveal abnormalities in his gammac or JAK-3 genes, and he was investigated for defects in the interleukin-15 (IL-15) receptor complex because functional IL-15 signaling is essential for NK cell development. Expression of the IL-2R/IL-15Rbeta chain was significantly reduced in the patient's peripheral blood mononuclear cells (PBMCs) by immunoblot, flow cytometry, and Northern blot analysis. Furthermore, IL-2 stimulation of PBMCs showed only minimal tyrosine phosphorylation of JAK-3. These data demonstrate that defects in IL-2R/1L-15Rbeta expression can lead to a unique NK-deficient SCID immunophenotype. (Blood. 2001;98:877-879)

Protein assays for diagnosis of Wiskott-Aldrich syndrome and X-linked thrombocytopenia

Mutations in the gene encoding the Wiskott-Aldrich syndrome protein (WASp) give rise to Wiskott-Aldrich syndrome (WAS), a condition that exhibits a wide spectrum of clinical severity. Patients may develop mild thrombocytopenia or suffer from a wide range of associated disorders including eczema, immune dysfunction, autoimmune disease and malignancy. The clinical diagnosis of Wiskott-Aldrich syndrome (WAS) can be difficult and is usually supported by the detection of WASp gene mutations using genetic analysis. Recently, protein-based assays have been used to demonstrate the absence of WASp in patients known to have WASp gene mutations. We have now reversed this approach and report on the use of immunoblot assays to rapidly diagnose WAS in 13 patients. There was a complete absence of WASp in 10 out of 13 patients and an abnormal protein form was detected in the remaining three patients. In all cases, subsequent genetic analysis confirmed the presence of a WASp gene mutation. We believe that protein-based assays should be employed as the first line of investigation in the diagnosis of WAS spectrum disorders.

Cutting edge: the Wiskott-Aldrich syndrome protein is required for efficient phagocytosis of apoptotic cells

Phagocytosis of apoptotic cells by macrophages and dendritic cells is necessary for clearance of proinflammatory debris and for presentation of viral, tumor, and self Ags. While a number of receptors involved in the cognate recognition of apoptotic cells by phagocytes have been identified, the signaling events that result in internalization remain poorly understood. Here we demonstrate that clearance of apoptotic cells is accompanied by recruitment of the Wiskott-Aldrich syndrome (WAS) protein to the phagocytic cup and that it's absence results in delayed phagocytosis both in vitro and in vivo. Therefore, we propose that WAS protein plays an important and nonredundant role in the safe removal of apoptotic cells and that deficiency contributes significantly to the immune dysregulation of WAS. The efficiency of apoptotic cell clearance may be a key determinant in the suppression of tissue inflammation and prevention of autoimmunity.

Rapid protein-based assays for the diagnosis of T-B+ severe combined immunodeficiency

The severe combined immunodeficiencies (SCID) are a heterogeneous group of conditions arising from a variety of molecular defects. The X-linked form of SCID (X-SCID) is caused by defects in the common gamma chain (gammac), and is characterized by a T-B+NK- immunophenotype. This lymphocyte profile is seen in an autosomal recessive form of SCID caused by mutations in the JAK3 molecule. Thus, X-SCID and JAK3-deficient SCID are clinically and immunologically indistinguishable. Knowledge of the precise molecular defect is essential for antenatal diagnosis, carrier testing and for treatment using somatic gene therapy. To identify the molecular defect in children presenting with a T-B+NK- form of SCID, we have developed rapid assays based on flow cytometric analysis of gammac, immunoblotting for JAK3 and gammac, and detection of interleukin-2 (IL-2)-induced tyrosine phosphorylation of JAK3. Sixteen T-B+NK- SCID patients from 15 families were examined. Nine had no detectable gammac, four had abnormal gammac expression and no IL-2-induced JAK3 tyrosine phosphorylation, and one had normal gammac expression but no IL-2-induced JAK3 tyrosine phosphorylation, although JAK3 was present. All these patients had mutations identified in their gammac gene. Two patients exhibited normal gammac expression, but JAK3 was not detected by immunoblotting and these patients were confirmed as having JAK3 gene mutations. Thus, these protein-based assays have led to rapid molecular diagnoses in T-B+ SCID that have subsequently been confirmed by genetic analysis.

Normal development of human fetal hematopoiesis between eight and seventeen weeks' gestation

OBJECTIVE

The aim of this study was to compare the hematologic compositions of fetal blood and liver and to phenotypically quantify the hematopoietic stem and progenitor cells during early human gestation.

STUDY DESIGN

Fifty fetal blood samples and 50 fetal livers were collected at 10 to 17 weeks' gestation and 8 to 17 weeks' gestation, respectively. Investigations included fetal blood cell counts, determinations of red blood cell index values, and flow cytometric analyses of mononuclear cells.

RESULTS

Fetal red blood cell, white blood cell, and platelet counts all increased with gestation, reflecting hematologic development. The proportion of normoblasts decreased dramatically with gestation. Individual mature red blood cells were larger and contained more hemoglobin during early gestation. Circulating and hepatic T lymphocytes increased in number shortly before the 13th week of gestation, which reflected thymic maturation. As a proportion fetal liver contained fewer T lymphocytes than did fetal blood (2.5% vs 18.6%; P =.003) but more CD34(+) hematopoietic stem and progenitor cells (17.5% vs 4.3%; P =. 004). As a proportion, fetal liver contained more of the primitive CD34(+) and CD38(-) hematopoietic stem and progenitor cells than did fetal blood (32% vs 17%; P =.04).

CONCLUSION

Both fetal blood and liver provide a rich source of hematopoietic stem and progenitor cells. Fetal liver provides a richer source of more primitive hematopoietic stem and progenitor cells than does fetal blood. For stem cell transplantation we suggest that fetal livers be collected before the 13th week of gestation, because T lymphocytes are present in much greater numbers in the fetal liver after this stage of gestation. Further, we suggest that in utero stem cell transplantations in fetuses with normal immune development should be performed before the 13th week of gestation.

Polarized expression of bone morphogenetic protein-4 in the human aorta-gonad-mesonephros region

In the mammal, definitive hematopoietic stem cells (HSCs) are first derived from mesodermal cells within a region of the embryonic para-aortic splanchnopleura known as the aorta-gonad-mesonephros (AGM). Within this region, HSCs are thought to arise from hemangioblast precursors located in the ventral wall of the dorsal aorta. However, the factors that regulate HSC development in vivo are still largely unknown. Bone morphogenetic protein (BMP)-4, a member of the transforming growth factor beta (TGF-beta) superfamily of growth factors, is a potent ventralizing factor and has been implicated in the commitment of embryonic mesodermal cells to a hematopoietic fate in a number of systems. In the human AGM, we find that BMP-4 is expressed at high levels, and with striking polarity, in a region of densely packed cells underlying intra-aortic hematopoietic clusters. In contrast, TGF-beta1 is expressed predominantly by hematopoietic cells within the clusters. These findings implicate both BMP-4 and TGF-beta1 in the initiation and regulation of hematopoiesis in the human AGM. Furthermore, the distribution of BMP-4 expression is highly suggestive of a direct role in the specification of human hematopoietic cells from embryonic mesoderm in vivo. (Blood. 2000;96:1591-1593)

Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.

Diagnosis of X-linked lymphoproliferative disease by analysis of SLAM-associated protein expression

X-linked lymphoproliferative disease (XLP) is an inherited immunodeficiency in which affected boys show abnormal responses to Epstein-Barr virus infection. The gene defective in XLP has been identified and designated SH2D1A and encodes a protein termed SLAM-associated protein (SAP). Mutation analysis in individuals with typical XLP presentations and family histories has only detected abnormalities in approximately 60% of patients. Thus, genetic analysis alone cannot confirm a diagnosis of XLP We have developed a SAP expression assay that can be used as a diagnostic indicator of XLP We show that SAP is constitutively expressed in normal individuals, in patients with severe sepsis and in patients with other primary immunodeficiencies. In six XLP patients, four with classical and two with atypical presentations, SAP expression was absent. In the latter two, who were previously assigned as having common variable immunodeficiency (CVID), the diagnosis of XLP was initially made using the protein expression assay. In two further patients in whom no mutation could be detected by genetic analysis, lack of SAP expression strongly suggests that these individuals have XLP. We therefore suggest that XLP should be suspected in certain boys previously diagnosed as having CVID and recommend that patients are investigated both by genetic analysis of SH2D1A and by expression of SAP protein.

Wiskott-Aldrich syndrome protein is necessary for efficient IgG-mediated phagocytosis

Interactions between the Wiskott-Aldrich (WAS) protein (WASp), small GTPases, and the cytoskeletal organizing complex Arp2/3 appear to be critical for the transduction of signals from the cell membrane to the actin cytoskeleton in hematopoietic cells. This study shows that Fcgamma-receptor (FcgammaR)-mediated phagocytosis is impaired in WASp-deficient peripheral blood monocytes, and that in macrophages, formation of the actin cup and local recruitment of tyrosine phosphorylated proteins is markedly attenuated. Results also show that, in normal macrophages, WASp itself is actively recruited to the cup, suggesting that assembly of this specialized cytoskeletal structure is dependent on its expression. (Blood. 2000;95:2943-2946)

Kinase mutant Btk results in atypical X-linked agammaglobulinaemia phenotype

X-linked agammaglobulinaemia (XLA) is a B cell humoral abnormality arising from mutations in the gene encoding Bruton's tyrosine kinase (Btk). The phenotype of XLA can be variable, with some individuals having a less severe immunophenotype, although in most cases this cannot be correlated with the Btk mutation or expression of Btk protein. In this study we describe clinical and immunological heterogeneity within the same pedigree. Analysis of the genetic defect identified a missense mutation in the kinase domain of Btk which, unusually, preserved Btk protein expression but at reduced levels, and also considerably diminished autophosphorylation activity. Structural analysis of the effect of this mutation on the kinase domain suggests that this mutation is not an integral part of the ATP or substrate binding domains but may affect the interaction of the kinase domain with its own kinase domain and other substrates. Together, these data may provide an explanation for the variable XLA phenotype.