Defects in T-cell-mediated immunity to influenza virus in murine Wiskott-Aldrich syndrome are corrected by oncoretroviral vector-mediated gene transfer into repopulating hematopoietic cells

Hematopoietic Stem Cell Therapy for Wiskott-Aldrich Syndrome: Improved Outcome and Quality of Life

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder caused by mutations in the WAS gene resulting in congenital thrombocytopenia, eczema, recurrent infections and an increased incidence of autoimmune diseases and malignancies. Without curative therapies, affected patients have diminished life expectancy and reduced quality of life. Since WAS protein (WASP) is constitutively expressed only in hematopoietic stem cell-derived lineages, hematopoietic stem cell transplantation (HSCT) and gene therapy (GT) are well suited to correct the hematologic and immunologic defects. Advances in high-resolution HLA typing, new techniques to prevent GvHD allowing the use of haploidentical donors, and the introduction of reduced intensity conditioning regimens with myeloablative features have increased overall survival (OS) to over 90%. The development of GT for WAS has provided basic knowledge into vector selection and random integration of various viral vectors into the genome, with the possibility of inducing leukemogenesis. After trials and errors, inactivating lentiviral vectors carrying the WAS gene were successfully evaluated in clinical trials, demonstrating cure of the disease except for insufficient resolution of the platelet defect. Thus, 50 years of clinical evaluation, genetic exploration and extensive clinical trials, a lethal syndrome has turned into a curable disorder.

Wiskott-Aldrich syndrome protein may be critical for CD8+ T cell function following MCMV infection

Wiskott-Aldrich syndrome (WAS) patients are characterized by immunodeficiency and viral infections. T cells derived from WAS patients and WAS protein (WASP)-deficient mice have various defects. However, whether WASP plays a role in immune control of cytomegalovirus (CMV) infection remains unclear. We analyzed the distribution of CD8⁺ T subsets and the pathological damage to various organs and tissues in MCMV infected Was knockout (KO) mice. A relatively high number of MCMV-specific cytotoxic T cells (CTLs) were observed in the spleen of Was KO mice. In MCMV infected Was KO mice, the late differentiated CD8⁺ T subset (CD27^-CD28^-) decreased in lungs, compared with those in the spleen and peripheral blood. Additionally, we found that the most severe pathological lesions occurred in the lungs, the main target organ of MCMV infection. By stimulating the spleen-derived CD8⁺ T lymphocytes of Was KO mice, we found that IL-2 and granzyme B production declined compared with that in wild- type mice. Moreover, the number of apoptotic CD8⁺ T cells increased in Was KO mice compared with the number in wild-type mice. Therefore, our results demonstrate that WASP may be involved in regulating cytotoxic function and apoptosis in CD8⁺ T cells following MCMV infection, which is supported by the distribution and memory compartment of MCMV-specific T cells in MCMV infected WAS mice.

Deletion of Wiskott-Aldrich syndrome protein triggers Rac2 activity and increased cross-presentation by dendritic cells

Wiskott-Aldrich syndrome (WAS) is caused by loss-of-function mutations in the WASp gene. Decreased cellular responses in WASp-deficient cells have been interpreted to mean that WASp directly regulates these responses in WASp-sufficient cells. Here, we identify an exception to this concept and show that WASp-deficient dendritic cells have increased activation of Rac2 that support cross-presentation to CD8(+) T cells. Using two different skin pathology models, WASp-deficient mice show an accumulation of dendritic cells in the skin and increased expansion of IFNγ-producing CD8(+) T cells in the draining lymph node and spleen. Specific deletion of WASp in dendritic cells leads to marked expansion of CD8(+) T cells at the expense of CD4(+) T cells. WASp-deficient dendritic cells induce increased cross-presentation to CD8(+) T cells by activating Rac2 that maintains a near neutral pH of phagosomes. Our data reveals an intricate balance between activation of WASp and Rac2 signalling pathways in dendritic cells.

Abnormalities of follicular helper T-cell number and function in Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome protein (WASp) is a hematopoietic-specific regulator of actin nucleation. Wiskott-Aldrich syndrome (WAS) patients show immunodeficiencies, most of which have been attributed to defective T-cell functions. T follicular helper (Tfh) cells are the major CD4(+) T-cell subset with specialized B-cell helper capabilities. Aberrant Tfh cells activities are involved in immunopathologies such as autoimmunity, immunodeficiencies, and lymphomas. We found that in WAS patients, the number of circulating Tfh cells was significantly reduced due to reduced proliferation and increased apoptosis, and Tfh cells were Th2 and Th17 polarized. The expression of inducible costimulator (ICOS) in circulating Tfh cells was higher in WAS patients than in controls. BCL6 expression was decreased in total CD4(+) T and Tfh cells of WAS patients. Mirroring the results in patients, the frequency of Tfh cells in WAS knockout (KO) mice was decreased, as was the frequency of BCL6(+) Tfh cells, but the frequency of ICOS(+) Tfh cells was increased. Using WAS chimera mice, we found that the number of ICOS(+) Tfh cells was decreased in WAS chimera mice, indicating that the increase in ICOS(+) Tfh cells in WAS KO mice was cell extrinsic. The data from in vivo CD4(+) naive T-cell adoptive transfer mice as well as in vitro coculture of naive B and Tfh cells showed that the defective function of WASp-deficient Tfh cells was T-cell intrinsic. Consistent findings in both WAS patients and WAS KO mice suggested an essential role for WASp in the development and memory response of Tfh cells and that WASp deficiency causes a deficient differentiation defect in Tfh cells by downregulating the transcription level of BCL6.

Signal Integration during T Lymphocyte Activation and Function: Lessons from the Wiskott-Aldrich Syndrome

Over the last decades, research dedicated to the molecular and cellular mechanisms underlying primary immunodeficiencies (PID) has helped to understand the etiology of many of these diseases and to develop novel therapeutic approaches. Beyond these aspects, PID are also studied because they offer invaluable natural genetic tools to dissect the human immune system. In this review, we highlight the research that has focused over the last 20 years on T lymphocytes from Wiskott–Aldrich syndrome (WAS) patients. WAS T lymphocytes are defective for the WAS protein (WASP), a regulator of actin cytoskeleton remodeling. Therefore, study of WAS T lymphocytes has helped to grasp that many steps of T lymphocyte activation and function depend on the crosstalk between membrane receptors and the actin cytoskeleton. These steps include motility, immunological synapse assembly, and signaling, as well as the implementation of helper, regulatory, or cytotoxic effector functions. The recent concept that WASP also works as a regulator of transcription within the nucleus is an illustration of the complexity of signal integration in T lymphocytes. Finally, this review will discuss how further study of WAS may contribute to solve novel challenges of T lymphocyte biology.

Wiskott-Aldrich syndrome protein-mediated actin dynamics control type-I interferon production in plasmacytoid dendritic cells

Wiskott-Aldrich Syndrome protein (WASp)–mediated actin polymerization controls intracellular trafficking and compartmentalization of TLR9 ligands in plasmacytoid dendritic cells.

Mutations in Wiskott-Aldrich syndrome (WAS) protein (WASp), a regulator of actin dynamics in hematopoietic cells, cause WAS, an X-linked primary immunodeficiency characterized by recurrent infections and a marked predisposition to develop autoimmune disorders. The mechanisms that link actin alterations to the autoimmune phenotype are still poorly understood. We show that chronic activation of plasmacytoid dendritic cells (pDCs) and elevated type-I interferon (IFN) levels play a role in WAS autoimmunity. WAS patients display increased expression of type-I IFN genes and their inducible targets, alteration in pDCs numbers, and hyperresponsiveness to TLR9. Importantly, ablating IFN-I signaling in WASp null mice rescued chronic activation of conventional DCs, splenomegaly, and colitis. Using WASp-deficient mice, we demonstrated that WASp null pDCs are intrinsically more responsive to multimeric agonist of TLR9 and constitutively secrete type-I IFN but become progressively tolerant to further stimulation. By acute silencing of WASp and actin inhibitors, we show that WASp-mediated actin polymerization controls intracellular trafficking and compartmentalization of TLR9 ligands in pDCs restraining exaggerated activation of the TLR9–IFN-α pathway. Together, these data highlight the role of actin dynamics in pDC innate functions and imply the pDC–IFN-α axis as a player in the onset of autoimmune phenomena in WAS disease.

Reduced type I interferon production by dendritic cells and weakened antiviral immunity in patients with Wiskott-Aldrich syndrome protein deficiency

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency caused by absence of Wiskott-Aldrich syndrome protein (WASP) expression, resulting in defective function of many immune cell lineages and susceptibility to severe bacterial, viral, and fungal infections. Despite a significant proportion of patients with WAS having recurrent viral infections, surprisingly little is known about the effects of WASP deficiency on antiviral immunity.

OBJECTIVE

We sought to evaluate the antiviral immune response in patients with WASP deficiency in vivo.

METHODS

Viral clearance and associated immunopathology were measured after infection of WASP-deficient (WAS KO) mice with lymphocytic choriomeningitis virus (LCMV). Induction of antiviral CD8(+) T-cell immunity and cytotoxicity was documented in WAS KO mice by means of temporal enumeration of total and antigen-specific T-cell numbers. Type I interferon (IFN-I) production was measured in serum in response to LCMV challenge and characterized in vivo by using IFN-I reporter mice crossed with WAS KO mice.

RESULTS

WAS KO mice showed reduced viral clearance and enhanced immunopathology during LCMV infection. This was attributed to both an intrinsic CD8(+) T-cell defect and defective priming of CD8(+) T cells by dendritic cells (DCs). IFN-I production by WAS KO DCs was reduced both in vivo and in vitro.

CONCLUSIONS

These studies use a well-characterized model of persistence-prone viral infection to reveal a critical deficiency of CD8(+) T-cell responses in murine WASP deficiency, in which abrogated production of IFN-I by DCs might play an important contributory role. These findings might help us to understand the immunodeficiency of WAS.

Ubiquitous high-level gene expression in hematopoietic lineages provides effective lentiviral gene therapy of murine Wiskott-Aldrich syndrome

The immunodeficiency disorder Wiskott-Aldrich syndrome (WAS) leads to life-threatening hematopoietic cell dysfunction. We used WAS protein (WASp)-deficient mice to analyze the in vivo efficacy of lentiviral (LV) vectors using either a viral-derived promoter, MND, or the human proximal WAS promoter (WS1.6) for human WASp expression. Transplantation of stem cells transduced with MND-huWASp LV resulted in sustained, endogenous levels of WASp in all hematopoietic lineages, progressive selection for WASp+ T, natural killer T and B cells, rescue of T-cell proliferation and cytokine production, and substantial restoration of marginal zone (MZ) B cells. In contrast, WS1.6-huWASp LV recipients exhibited subendogenous WASp expression in all cell types with only partial selection of WASp+ T cells and limited correction in MZ B-cell numbers. In parallel, WS1.6-huWASp LV recipients exhibited an altered B-cell compartment, including higher numbers of λ-light-chain+ naive B cells, development of self-reactive CD11c+FAS+ B cells, and evidence for spontaneous germinal center (GC) responses. These observations correlated with B-cell hyperactivity and increased titers of immunoglobulin (Ig)G2c autoantibodies, suggesting that partial gene correction may predispose toward autoimmunity. Our findings identify the advantages and disadvantages associated with each vector and suggest further clinical development of the MND-huWASp LV for a future clinical trial for WAS.

B cell-intrinsic deficiency of the Wiskott-Aldrich syndrome protein (WASp) causes severe abnormalities of the peripheral B-cell compartment in mice

Wiskott Aldrich syndrome (WAS) is caused by mutations in the WAS gene that encodes for a protein (WASp) involved in cytoskeleton organization in hematopoietic cells. Several distinctive abnormalities of T, B, and natural killer lymphocytes; dendritic cells; and phagocytes have been found in WASp-deficient patients and mice; however, the in vivo consequence of WASp deficiency within individual blood cell lineages has not been definitively evaluated. By conditional gene deletion we have generated mice with selective deficiency of WASp in the B-cell lineage (B/WcKO mice). We show that this is sufficient to cause a severe reduction of marginal zone B cells and inability to respond to type II T-independent Ags, thereby recapitulating phenotypic features of complete WASp deficiency. In addition, B/WcKO mice showed prominent signs of B-cell dysregulation, as indicated by an increase in serum IgM levels, expansion of germinal center B cells and plasma cells, and elevated autoantibody production. These findings are accompanied by hyperproliferation of WASp-deficient follicular and germinal center B cells in heterozygous B/WcKO mice in vivo and excessive differentiation of WASp-deficient B cells into class-switched plasmablasts in vitro, suggesting that WASp-dependent B cell-intrinsic mechanisms critically contribute to WAS-associated autoimmunity.

Foamy virus vector-mediated gene correction of a mouse model of Wiskott-Aldrich syndrome

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by eczema, thrombocytopenia and immunodeficiency. Hematopoietic cell transplantation can cure the disease and gene therapy is being tested as an alternative treatment option. In this study, we assessed the use of foamy virus (FV) vectors as a gene transfer system for WAS, using a Was knockout (KO) mouse model. Preliminary experiments using FV vectors expressing the green fluorescent protein under the transcriptional control of the endogenous WAS promoter or a ubiquitously acting chromatin opening element allowed us to define transduction conditions resulting in high (>40%) and long-term in-vivo marking of blood cells after transplantation. In following experiments, Was KO mice were treated with FV vectors containing the human WAS complementary DNA (cDNA). Transplanted animals expressed the WAS protein (WASp) in T and B lymphocytes, as well as platelets and showed restoration of both T-cell receptor-mediated responses and B-cell migration. We also observed recovery of platelet adhesion and podosome formation in dendritic cells (DCs) of treated mice. These data demonstrate that FV vectors can be effective for hematopoietic stem cell (HSC)-directed gene correction of WAS.

Transduction of human primitive repopulating hematopoietic cells with lentiviral vectors pseudotyped with various envelope proteins

Lentiviral vectors are useful for transducing primitive hematopoietic cells. We examined four envelope proteins for their ability to mediate lentiviral transduction of mobilized human CD34(+) peripheral blood cells. Lentiviral particles encoding green fluorescent protein (GFP) were pseudotyped with the vesicular stomatitis virus envelope glycoprotein (VSV-G), the amphotropic (AMPHO) murine leukemia virus envelope protein, the endogenous feline leukemia viral envelope protein or the feline leukemia virus type C envelope protein. Because the relative amount of genome RNA per ml was similar for each pseudotype, we transduced CD34(+) cells with a fixed volume of each vector preparation. Following an overnight transduction, CD34(+) cells were transplanted into immunodeficient mice which were sacrificed 12 weeks later. The average percentages of engrafted human CD45(+) cells in total bone marrow were comparable to that of the control, mock-transduced group (37-45%). Lenti-particles pseudotyped with the VSV-G envelope protein transduced engrafting cells two- to tenfold better than particles pseudotyped with any of the gamma-retroviral envelope proteins. There was no correlation between receptor mRNA levels for the gamma-retroviral vectors and transduction efficiency of primitive hematopoietic cells. These results support the use of the VSV-G envelope protein for the development of lentiviral producer cell lines for manufacture of clinical-grade vector.

Contributions of Wiskott-Aldrich syndrome family cytoskeletal regulatory adapters to immune regulation

Cytoskeletal structure and dynamic rearrangement are integrally involved in coupling external stimuli to the orchestrated network of molecular interactions and cellular responses required for T-cell effector function. Members of the Wiskott-Aldrich syndrome protein (WASp) family are now widely recognized as cytoskeletal scaffolding adapters that coordinate the transmission of stimulatory signals to downstream induction of actin remodeling and cytoskeletal-dependent T-cell responses. In this review, we discuss the structural and functional properties of the WASp family members, with an emphasis on the roles of these proteins in the molecular pathways underpinning T-cell activation. The contributions of WASp family proteins and the cytoskeletal reorganization they evoke to expression of specific T-cell effector functions and the implications of such activity to normal immune responses and to the immunologic deficits manifested by Wiskott-Aldrich syndrome patients are also described.

The thrombocytopenia of WAS: a familial form of ITP?

In the first report of the concurrent immunodeficiency, thrombocytopenia, and eczema that we now call the Wiskott-Aldrich Syndrome (WAS), Alfred Wiskott asked whether it could be a familial form of Werlhof's disease (now called ITP). This review summarizes what is known about platelet production, consumption, and function in clinical and murine WAS. Both platelet production and consumption are affected by WASP deficiency. Likely molecular mechanisms have been identified for the former process, but remain problematic for the latter. Recent data in a murine model suggest that WASP deficiency could increase both the incidence of antiplatelet antibodies and susceptibility to their enhancement of platelet consumption. Wiskott's original speculation about the relationship between WAS and ITP may need to be reconsidered.

Wiskott-Aldrich Syndrome: Immunodeficiency resulting from defective cell migration and impaired immunostimulatory activation

Regulation of the actin cytoskeleton is crucial for many aspects of correct and cooperative functioning of immune cells, such as migration, antigen uptake and cell activation. The Wiskott-Aldrich Syndrome protein (WASp) is an important regulator of actin cytoskeletal rearrangements and lack of this protein results in impaired immune function. This review discusses recent new insights of the role of WASp at molecular and cellular level and evaluates how WASp deficiency affects important immunological features and how defective immune cell function contributes to compromised host defence.

Recent advances in understanding the pathophysiology of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a severe X-linked immunodeficiency caused by mutations in the gene encoding for WASP, a key regulator of signaling and cytoskeletal reorganization in hematopoietic cells. Mutations in WASP result in a wide spectrum of clinical manifestations ranging from the relatively mild X-linked thrombocytopenia to the classic full-blown WAS phenotype characterized by thrombocytopenia, immunodeficiency, eczema, and high susceptibility to developing tumors and autoimmune manifestations. The life expectancy of patients affected by severe WAS is reduced, unless they are successfully cured by bone marrow transplantation from related identical or matched unrelated donors. Because many patients lack a compatible bone marrow donor, the administration of WAS gene–corrected autologous hematopoietic stem cells could represent an alternative therapeutic approach. In the present review, we focus on recent progress in understanding the molecular and cellular mechanisms contributing to the pathophysiology of WAS. Although molecular and cellular studies have extensively analyzed the mechanisms leading to defects in T, B, and dendritic cells, the basis of autoimmunity and thrombocytopenia still remains poorly understood. A full understanding of these mechanisms is still needed to further implement new therapeutic strategies for this peculiar immunodeficiency.

Evidence for long-term efficacy and safety of gene therapy for Wiskott-Aldrich syndrome in preclinical models

Wiskott-Aldrich Syndrome (WAS) is a life-threatening X-linked disease characterized by immunodeficiency, thrombocytopenia, autoimmunity, and malignancies. Gene therapy could represent a therapeutic option for patients lacking a suitable bone marrow (BM) donor. In this study, we analyzed the long-term outcome of WAS gene therapy mediated by a clinically compatible lentiviral vector (LV) in a large cohort of was(null) mice. We demonstrated stable and full donor engraftment and Wiskott-Aldrich Syndrome protein (WASP) expression in various hematopoietic lineages, up to 12 months after gene therapy. Importantly, we observed a selective advantage for T and B lymphocytes expressing transgenic WASP. T-cell receptor (TCR)-driven T-cell activation, as well as B-cell's ability to migrate in response to CXCL13, was fully restored. Safety was evaluated throughout the long-term follow-up of primary and secondary recipients of WAS gene therapy. WAS gene therapy did not affect the lifespan of treated animals. Both hematopoietic and nonhematopoietic tumors arose, but we excluded the association with gene therapy in all cases. Demonstration of long-term efficacy and safety of WAS gene therapy mediated by a clinically applicable LV is a key step toward the implementation of a gene therapy clinical trial for WAS.

Ten years of gene therapy for primary immune deficiencies

Gene therapy with hematopoietic stem cells (HSC) is an attractive therapeutic strategy for several forms of primary immunodeficiencies. Current approaches are based on ex vivo gene transfer of the therapeutic gene into autologous HSC by vector-mediated gene transfer. In the past decade, substantial progress has been achieved in the treatment of severe combined immundeficiencies (SCID)-X1, adenosine deaminase (ADA)-deficient SCID, and chronic granulomatous disease (CGD). Results of the SCID gene therapy trials have shown long-term restoration of immune competence and clinical benefit in over 30 patients. The inclusion of reduced-dose conditioning in the ADA-SCID has allowed the engraftment of multipotent gene-corrected HSC at substantial level. In the CGD trial significant engraftment and transgene expression were observed, but the therapeutic effect was transient. The occurrence of adverse events related to insertional mutagenesis in the SCID-X1 and CGD trial has highlighted the limitations of current retroviral vector technology. For future applications the risk-benefit evaluation should include the type of vector employed, the disease background and the nature of the transgene. The use of self-inactivating lentiviral vectors will provide significant advantages in terms of natural gene regulation and reduction in the potential for adverse mutagenic events. Following recent advances in preclinical studies, lentiviral vectors are now being translated into new clinical approaches, such as Wiskott-Aldrich Syndrome.

WASP confers selective advantage for specific hematopoietic cell populations and serves a unique role in marginal zone B-cell homeostasis and function

Development of hematopoietic cells depends on a dynamic actin cytoskeleton. Here we demonstrate that expression of the cytoskeletal regulator WASP, mutated in the Wiskott-Aldrich syndrome, provides selective advantage for the development of naturally occurring regulatory T cells, natural killer T cells, CD4(+) and CD8(+) T lymphocytes, marginal zone (MZ) B cells, MZ macrophages, and platelets. To define the relative contribution of MZ B cells and MZ macrophages for MZ development, we generated wild-type and WASP-deficient bone marrow chimeric mice, with full restoration of the MZ. However, even in the presence of MZ macrophages, only 10% of MZ B cells were of WASP-deficient origin. We show that WASP-deficient MZ B cells hyperproliferate in vivo and fail to respond to sphingosine-1-phosphate, a crucial chemoattractant for MZ B-cell positioning. Abnormalities of the MZ compartment in WASP(-/-) mice lead to aberrant uptake of Staphylococcus aureus and to a reduced immune response to TNP-Ficoll. Moreover, WASP-deficient mice have increased levels of "natural" IgM antibodies. Our findings reveal that WASP regulates both development and function of hematopoietic cells. We demonstrate that WASP deficiency leads to an aberrant MZ that may affect responses to blood-borne pathogens and peripheral B-cell tolerance.

Gene therapy for primary immunodeficiencies

Primary immunodeficiencies are a group of disorders that are highly amenable to gene therapy because of their defined pathophysiology and the accessibility of the hematopoietic system to molecular intervention. The development of this new therapeutic modality has been driven by the established morbidity and mortality associated with conventional allogeneic stem cell transplantation, particularly in the human leukocyte antigen-mismatched setting. Recently, several clinical studies have shown that gamma retroviral gene transfer technology can produce major beneficial therapeutic effects, but, as for all cellular and pharmacologic treatment approaches, with a finite potential for toxicity. Newer developments in vector design showing promise in overcoming these issues are likely to establish gene therapy as an efficacious strategy for many forms of primary immunodeficiencies.

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.

Wiskott-Aldrich syndrome

PURPOSE OF REVIEW

Wiskott-Aldrich syndrome is caused by mutations of the Wiskott-Aldrich syndrome protein gene, which codes for a cytoplasmic protein with multiple functions. This review will focus on recent progress in understanding the molecular basis of Wiskott-Aldrich syndrome and its ramifications for the cure of this lethal disease.

RECENT FINDINGS

The discovery of the causative gene has revealed a spectrum of clinical phenotypes demonstrating a strong genotype/phenotype correlation. The discovery of unique functional domains of Wiskott-Aldrich syndrome protein has been instrumental in defining mechanisms that control activation of Wiskott-Aldrich syndrome protein. Long-term follow up of patients undergoing hematopoietic stem cell transplantation has led to important modifications of the procedure. Studies of Wiskott-Aldrich syndrome protein-deficient cell lines and wasp-knockout mice have paved the way for possible gene therapy.

SUMMARY

Wiskott-Aldrich syndrome protein gene mutations result in four clinical phenotypes: classic Wiskott-Aldrich syndrome and X-linked thrombocytopenia, intermittent thrombocytopenia and neutropenia. Wiskott-Aldrich syndrome protein is a signaling molecule and instrumental for cognate and innate immunity, cell motility and protection against autoimmune disease. The success of hematopoietic stem cell transplantation is related to the recipient's age, donor selection, the conditioning regimen and the extent of reconstitution. Since Wiskott-Aldrich syndrome protein is expressed exclusively in hematopoietic stem cells, and because Wiskott-Aldrich syndrome protein exerts a strong selective pressure, gene therapy is expected to cure the disease.

Current understanding of the Wiskott-Aldrich syndrome and prospects for gene therapy

Gene therapy, based on the transplantation of genetically corrected autologous hematopoietic stem cells (HSCs), has proven to be an effective therapeutic approach as an alternative to allogenic HSC transplantation for the cure of severe combined immunodeficiencies (SCID). In this article, the recent preclinical studies aiming towards gene therapy trials for the Wiskott-Aldrich syndrome (WAS), a life-threatening immunodeficiency characterized by infections, hemorrhages, autoimmune disorders and lymphomas, will be reviewed. An update of the safety and efficacy data obtained in studies performed in murine disease models and in cells from WAS patients will be presented. Based on these data and on the clinical results of the recent trials for SCID, the most critical issues regarding the implementation of a gene therapy approach for WAS will be discussed.

Suppression of clonal dominance in cultured human lymphoid cells by addition of the cHS4 insulator to a lentiviral vector

Lentiviral vectors efficiently transduce quiescent stem cells and are being evaluated for gene therapy of blood dis-orders. The risk of genotoxicity as a result of insertional mutagenesis is an important safety consideration. The hy-persensitive site 4 insulator from the chicken beta-globin locus (cHS4) possesses chromatin bar-rier and enhancer-blocking functions. A control lentiviral vector encoding green fluorescent protein was compared with a vector in which the cHS4 insulator element flanked the green fluorescent protein expression cassette in single cell isolates of transduced human T cells (Jurkat) after 9 days in culture. The insulator had minimal effect on mean fluorescent intensity and only modestly reduced the variability of green fluorescent protein expression among indi-vidual single cell isolates. Most unique integration sites were within genes, but the insulator-containing vector had a moderate predilection to integrate near the transcriptional start site compared with the control vector. Clonal domi-nance developed in cultures of cells containing the integrated vector genomes, as reflected by the recovery of mul-tiple single cell isolates containing the same integration site. We infer that certain integrations conferred a prolifera-tive or survival advantage by affecting gene expression through insertional mutagenesis, leading to this clonal dominance. This effect was diminished by including the insulator element in the vector genome.

Isolation and characterization of hematopoietic progenitor cells in canine bone marrow

For ultimate diagnoses of canine leukemia or malignant lymphoma, we sought to isolate hematopoietic progenitor cells (HPCs) from canine bone marrow (BM) using physiological phenotypes. Canine BM cells were separated by equilibrium discontinued density centrifugation, and HPCs, detected by in vitro colony formation, were significantly enriched in the relatively low density (LD) fraction. In flow cytometry, many CD34 or MHC class II expressing cells were detected in the LD fraction, but these were not significantly enriched. When the LD cells were separated, using a cell-sorting method, into cells with high affinity of wheat germ agglutinin (WGAhigh) and cells with WGAlow, almost all multipotent HPCs (MHPCs) and HPCs committed to myeloid lineage were found in the WGAhigh population. When the WGAhigh population was further stained for rhodamin 123, almost all MHPCs were included in the dull population (Rhlow), but not in the bright one (Rhhigh). Morphologically, most Rhlow cells were round, blastic cells containing a large nucleus with nucleoli and narrow cytoplasm. Based on these results, we suggest that all of the MHPCs in canine BM show the Rhlow WGAhigh LD phenotype, and may contain hematopoietic stem cells, which are the primitive HPCs.

Defective Th1 cytokine gene transcription in CD4+ and CD8+ T cells from Wiskott-Aldrich syndrome patients

Wiskott-Aldrich syndrome (WAS) protein (WASP) plays a key role in TCR-mediated activation and immunological synapse formation. However, the effects of WASP deficiency on effector functions of human CD4+ and CD8+ T cells remain to be determined. In this study, we report that TCR/CD28-driven proliferation and secretion of IL-2, IFN-gamma, and TNF-alpha are strongly reduced in CD8+ T cells from WAS patients, compared with healthy donor CD8+ T cells. Furthermore, WAS CD4+ T cells secrete low levels of IL-2 and fail to produce IFN-gamma and TNF-alpha, while the production of IL-4, IL-5, and IL-10 is only minimally affected. Defective IL-2 and IFN-gamma production persists after culture of naive WAS CD4+ T cells in Th1-polarizing conditions. The defect in Th1 cytokine production by WAS CD4+ and CD8+ T cells is also present at the transcriptional level, as shown by reduced IL-2 and IFN-gamma mRNA transcripts after TCR/CD28 triggering. The reduced transcription of Th1 cytokine genes in WAS CD4+ T cells is associated with a defective induction of T-bet mRNA and a reduction in the early nuclear recruitment of NFAT-1, while the defective activation of WAS CD8+ T cells correlates with reduced nuclear recruitment of both NFAT-1 and NFAT-2. Together, our data indicate that WASP regulates the transcriptional activation of T cells and is required specifically for Th1 cytokine production.

Lentiviral vectors targeting WASp expression to hematopoietic cells, efficiently transduce and correct cells from WAS patients

Gene therapy has been proposed as a potential treatment for Wiskott-Aldrich syndrome (WAS), a severe primary immune deficiency characterized by multiple hematopoietic-specific cellular defects. In order to develop an optimal lentiviral gene transfer cassette for this application, we compared the performance of several internal promoters in a variety of cell lineages from human WAS patients. Vectors using endogenous promoters derived from short (0.5 kb) or long (1.6 kb) 5' flanking sequences of the WAS gene, expressed the transgene in T, B, dendritic cells as well as CD34(+) progenitor cells, but functioned poorly in non-hematopoietic cells. Defects of T-cell proliferation and interleukin-2 production, and the cytoskeletal anomalies in WAS dendritic cells were also corrected. The levels of reconstitution were comparable to those obtained following transduction with similar lentiviral vectors incorporating constitutive PGK-1, EF1-alpha promoters or the spleen focus forming virus gammaretroviral LTR. Thus, native regulatory sequences target the expression of the therapeutic WAS transgene to the hematopoietic system, as is naturally the case for WAS, and are effective for correction of multiple cellular defects. These vectors may have significant advantages for clinical application in terms of natural gene regulation, and reduction in the potential for adverse mutagenic events.

Stem cell transplantation for the Wiskott-Aldrich syndrome: a single-center experience confirms efficacy of matched unrelated donor transplantation

The treatment of Wiskott-Aldrich syndrome (WAS), a once uniformly fatal disorder, has evolved considerably as the use of hematopoietic stem cell transplant has become more widespread. For the majority of patients who lack an human leukocyte antigen-identical sibling, closely matched unrelated donor bone marrow transplant (MUD BMT) at an early age is an excellent option that nevertheless is not uniformly chosen. We retrospectively analyzed our experience with transplantation in 23 patients with WAS from 1990 to 2005 at the University of Brescia, Italy, of whom 16 received MUD BMT. Myeloablative chemotherapy was well tolerated with median neutrophil engraftment at day 18, and no cases of grade III or IV graft-vs-host disease. Overall survival was very good with 78.2% (18/23) of the whole cohort and 81.2% (13/16) of MUD BMT recipients surviving. Among 18 survivors, full donor engraftment was detected in 12 patients, and stable mixed chimerism in all blood lineages in four patients. Deaths were limited to patients who had received mismatched related BMT or who had severe clinical symptomatology at the time of transplantation, further emphasizing the safety and efficacy of MUD BMT when performed early in the clinical course of WAS.

Retroviral WASP gene transfer into human hematopoietic stem cells reconstitutes the actin cytoskeleton in myeloid progeny cells differentiated in vitro

OBJECTIVE

Wiskott-Aldrich syndrome (WAS) is a primary immunodeficiency disorder characterized by recurrent infections, autoimmunity, microthrombocytopenia, and susceptibility to malignant tumors. Compared with the conventional treatment using allogeneic bone marrow transplantation, hematopoietic stem cell gene therapy might offer more specific and less toxic therapeutic options.

METHODS

We investigated retroviral WAS protein (WASP) gene transfer to assess functional correction and potential toxicities in human CD34(+) cells from WAS patients and healthy individuals, respectively.

RESULTS

WASP mRNA and protein levels were restored in CD14(+) cells derived from WASP-transduced hematopoietic stem cells. Functional reconstitution in WASP-transduced myeloid cells was documented by podosome formation and Fc gamma R-mediated phagocytosis. Importantly, overexpression of WASP in CD34(+) cells from healthy donors did not cause any discernible toxic effects.

CONCLUSIONS

Our studies document the feasibility of WASP gene transfer into human CD34(+) cells and suggest that the phenotype of WASP-deficient myeloid cells can be restored upon retroviral gene transfer.

Perspectives of gene therapy for primary immunodeficiencies

PURPOSE OF REVIEW

Standard therapies for patients with severe primary immunodeficiencies include bone marrow transplantation and, for adenosine deaminase deficiency, enzyme replacement. In the last decade, gene therapy has been developed as an alternative for these conditions. We summarize the recent advances in gene therapy for primary immunodeficiencies and discuss the unexpected occurrence of leukemia in a gene therapy trial for X-linked severe combined immunodeficiency.

RECENT FINDINGS

Eight of 10 infants with X-linked severe combined immunodeficiency who received autologous hematopoietic stem cells transduced with a retroviral vector carrying the IL2RG complementary DNA achieved immune reconstitution. However, the two youngest patients developed leukemic expansions of gene-corrected cells. The first case had proliferation of a gamma delta T cell clone, and the second case had three alpha beta T cell clones derived from a single transduced progenitor. Leukemic cells in both patients aberrantly expressed the LIM domain only-2 transcription factor due to retroviral vector insertions in this locus. After receiving anti-leukemic treatment one patient achieved a lasting remission, but the other relapsed. Four adenosine deaminase deficient severe combined immunodeficiency patients also developed functional immunity after receiving autologous hematopoietic stem cells transduced with the adenosine deaminase gene complementary DNA following submyeloablative chemotherapy. Chronic granulomatous disease, Wiskott-Aldrich syndrome, JAK3 deficiency and RAG2 deficiency are other immunodeficiencies being studied as candidates for gene therapy.

SUMMARY

Gene therapy is a promising therapeutic option for some primary immunodeficiencies, especially when cells expressing the correct gene have a selective advantage. More clinical trials with closer patient monitoring are under way to define which patients may benefit from this approach, and strategies are being developed to understand and ultimately reduce the risk of leukemia secondary to retroviral vector insertion.

Efficacy of gene therapy for Wiskott-Aldrich syndrome using a WAS promoter/cDNA-containing lentiviral vector and nonlethal irradiation

Wiskott-Aldrich syndrome (WAS) is a life-threatening X-linked primary immunodeficiency characterized by infections, hemorrhages, autoimmune disorders, and lymphomas. Transplantation of genetically corrected autologous hematopoietic stem cells (HSCs) could represent an alternative treatment to allogeneic HSC transplantation, the latter being often associated with severe complications. We used WAS-/- mice to test the efficacy of a gene therapy approach based on nonlethal irradiation followed by transplantation of WAS-/- HSCs transduced with lentiviral vectors encoding the WAS protein (WASP) from either the ubiquitous PGK promoter or the tissue- specific WAS promoter. The procedure resulted in significant levels of engraftment of WASP-expressing T cells, B cells, platelets, and myeloid cells. T cells harbored one or two vector copies and displayed partial to full correction of T cell receptor-driven interleukin-2 production and proliferation. In addition, polymerization of F-actin and localization of WASP at the site of the immunological synapse were restored. The treatment was well tolerated and no pathology was detected by systematic blood analysis and autopsy. The efficacy of WAS gene transfer into HSCs, using the WAS promoter-containing lentiviral vector, combined with nonlethal irradiation provides a strong rationale for the development of gene therapy for WAS patients.

WASP and the phenotypic range associated with deficiency

PURPOSE OF REVIEW

This review reports on the range of clinical phenotypes that are caused by mutations in the Wiskott-Aldrich Syndrome Protein (WASP) gene. The basis of genotype-phenotype correlation in Wiskott-Aldrich syndrome (WAS) is discussed, with regard to expression of the WAS protein (WASp) and of the effects of WASP mutations on WASp function. Advances in preclinical models of gene therapy for WAS are presented.

RECENT FINDINGS

Two recent studies have supported genotype-phenotype correlation in WAS and in related X-linked thrombocytopenia. Expression of the WASp was found to be the best predictor of clinical phenotype. Investigation of autoimmune manifestations associated with WAS has shown that autoimmune hemolytic anemia and elevated serum IgM associate with a more severe clinical course. Finally, while results of hematopoietic stem cell transplantation for WAS continue to improve, several studies have shown the potential benefit of novel therapeutic approaches based on gene transfer. In particular, use of lentiviral vector-driven expression of the WASP gene under autologous promoter sequences has been found to result in increased targeting of hematopoietic stem cells, higher levels of WASp expression, and improved reconstitution of immune function.

SUMMARY

Availability of tools that allow analysis of WASp expression has provided evidence for a genotype-phenotype correlation in patients with WASP gene defects. Protein expression is an important prognostic indicator. The molecular and cellular abnormalities of WAS-associated defects are being identified, and significant advances in vector-mediated gene transfer have opened possibilities for the treatment of WAS based on gene therapy.

Survival of the fittest: in vivo selection and stem cell gene therapy

Stem cell gene therapy has long been limited by low gene transfer efficiency to hematopoietic stem cells. Recent years have witnessed clinical success in select diseases such as X-linked severe combined immunodeficiency (SCID) and ADA deficiency. Arguably, the single most important factor responsible for the increased efficacy of these recent protocols is the fact that the genetic correction provided a selective in vivo survival advantage. Since, for most diseases, there will be no selective advantage of gene-corrected cells, there has been a significant effort to arm vectors with a survival advantage. Two-gene vectors can be used to introduce the therapeutic gene and a selectable marker gene. Efficient in vivo selection strategies have been demonstrated in clinically relevant large-animal models. Mutant forms of the DNA repair-enzyme methylguanine methyltransferase in particular have allowed for efficient in vivo selection and have achieved sustained marking with virtually 100% gene-modified cells in large animals, and with clinically acceptable toxicity. Translation of these strategies to the clinical setting is imminent. Here, we review how in vivo selection strategies can be used to make stem cell gene therapy applicable to the treatment of a wider scope of genetic diseases and patients.

Gene therapy in primary immunodeficiencies

Primary immunodeficiencies are a group of disorders that are highly amenable to gene therapy due to their defined molecular biology and pathophysiology. The development of this new therapeutic modality has been driven by the established morbidity and mortality associated with conventional allogeneic stem cell transplantation, particularly in the human leukocyte antigen-mismatched setting. Recently, several clinical studies have demonstrated that conventional gene transfer technology can produce major beneficial therapeutic effects, but as for all cellular and pharmacologic treatment approaches, with a finite potential for toxicity. New strategies to overcome these issues are likely to establish gene therapy as an efficacious strategy for many forms of primary immunodeficiencies.

WASP- mice exhibit defective immune responses to influenza A virus, Streptococcus pneumoniae, and Mycobacterium bovis BCG

OBJECTIVE

To quantify the immune response of WASP- mice to three different pathogens: influenza A virus, Streptococcus pneumoniae, and Mycobacterium bovis.

METHODS

Primary and secondary T-cell responses to influenza A virus were quantified via tetramer assays. Viral clearance from lung was also measured. Lethality of intranasal inoculation with luminescent S. pneumoniae was assessed by dose escalation and direct luminescence imaging. After intravenous inoculation with M. bovis, residual mycobacteria in lung, liver, and spleen were measured by standard culture methods.

RESULTS

The reduced secondary T-cell response to influenza A virus correlates with a relative but not absolute loss of splenic T and B cells similar to that seen in clinical Wiskott-Aldrich Syndrome (WAS), and slower clearance of virus from lung. The reduced magnitude of the secondary T-cell response correlates with a progressive loss of influenza-specific T cells after primary inoculation. WASP- mice show an increased susceptibility to lethal pneumonia after intranasal inoculation with S. pneumoniae, which is among the most frequent causes of clinical complications in WAS patients. WASP- mice clear M. bovis bacille Calmette-Guerin (BCG) more slowly from lung, liver, and spleen. Bone marrow-derived macrophages, however, show normal ex vivo cytokine secretion in response to M. bovis.

CONCLUSIONS

These results demonstrate that WASP- mice are functionally immunodeficient in regard to three different pathogens, and provide relevant end points for the study of treatment modalities in this model. They also suggest a specific physiologic mechanism, failure to accumulate memory T cells, for at least one of the defective immune responses.

Retroviral vectors: new applications for an old tool

Retroviral vectors (RVs) have been used for stable gene transfer into mammalian cells for more than 20 years. The most popular RVs are those derived from the Moloney murine leukaemia virus (MoMLV). One of their main limitations is their inability to transduce noncycling cells. However, they have a relatively simple genome and structure, are easy to use, and are relatively safe for in vivo applications. For the last two decades, the artificial evolution of RVs has paralleled evolution in their applications, which now include those as diverse as the generation of transgenic animals, the stable delivery of small interfering RNA (siRNA) and gene therapy clinical trials. Recent reports of two successful gene therapy clinical trials in patients with severe immunodeficiency disease in France and Italy, and the development of T-cell acute leukaemia in two of 10 children participating in one of these clinical trials, demonstrate the great potential of RVs, but also some potential risks which may be intrinsically associated with their use. Basic aspects of RVs and vector production were reviewed in detail in a previous supplement of this journal. This article will first summarize some general aspects of retroviruses and RVs. Thereafter, recent developments in gene therapy using RVs, novel applications such as stable RNA interference and some other recent issues related to retroviral integration, including clonality studies after haematopoietic stem cell transplantation, retroviral tagging and insertional oncogenesis will be discussed.

A lentiviral vector encoding the human Wiskott–Aldrich syndrome protein corrects immune and cytoskeletal defects in WASP knockout mice

Wiskott-Aldrich syndrome (WAS) is an immune deficiency with thrombopenia resulting from mutations in the WASP gene. This gene normally encodes the Wiskott-Aldrich syndrome protein (WASP), a major cytoskeletal regulator expressed in hematopoietic cells. Gene therapy is a promising option for the treatment of WAS, requiring that clinically applicable WASP gene transfer vectors demonstrate efficacy in preclinical studies. Here, we describe a self-inactivating HIV-1-derived lentiviral vector encoding human WASP and show that it effectively transduced bone marrow progenitor cells of WASP knockout (WKO) mice. Transplantation of these transduced cells into lethally irradiated WKO recipients led to stable expression of WASP and correction of immune, inflammatory and cytoskeletal defects. Splenic T-cell proliferation was restored, podosomes were reinstated on bone-marrow-derived dendritic cells and colon inflammation was reduced. This shows for the first time (a) that cytoskeletal defects can be corrected in WKO mice, (b) that human WASP is biologically active in mice and (c) that a lentiviral vector is effective to express human WASP in vivo over several months. These data support further development of such lentiviral vectors for the gene therapy of WAS.

X-linked immunodeficiencies

Recent advances in molecular genetics have allowed identification of at least seven genes involved in X-linked immunodeficiencies. This has resulted not only in improved diagnostic possibilities but also in a better understanding of the pathophysiology of these disorders. In some cases, mutations in the same gene have been shown to cause distinct clinical and immunologic phenotypes, demonstrating a strong genotype-phenotype correlation. Identification of the molecular basis of these diseases has permitted creation of disease-specific registries, with a better characterization of the clinical and immunologic features associated with the various forms of X-linked immunodeficiencies. Additionally, gene therapy has been attempted in X-linked severe combined immune deficiency (XSCID), with clear evidence of successful correction of the pathology, and the appearance of severe adverse effects.

Lentiviral Vector-Mediated Gene Transfer in T Cells from Wiskott–Aldrich Syndrome Patients Leads to Functional Correction

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency with a median survival below the age of 20 due to infections, severe hemorrhage, and lymphomas. Transplantation of hematopoietic stem cells from HLA-identical sibling donors is a resolutive treatment, but is available for a minority of patients. Transplantation of genetically corrected autologous hematopoietic stem cells or T cells could represent an alternative treatment applicable to all patients. We investigated whether WAS gene transfer with MMLV-based oncoretroviral and HIV-based lentiviral vectors could restore normal functions of patients' T cells. T cells transduced either with lentiviral vectors expressing the WAS protein (WASP) from the ubiquitous PGK promoter or the tissue-specific WASP promoter or with an oncoretroviral vector expressing WASP from the LTR, reached normal levels of WASP with correction of functional defects, including proliferation, IL-2 production, and lipid raft upregulation. Lentiviral vectors transduced T cells from WAS patients at higher rates, compared to oncoretroviral vectors, and efficiently transduced both activated and naive WAS T cells. Furthermore, a selective growth advantage of T cells corrected with the lentiviral vectors was demonstrated. The observation that lentiviral vector-mediated gene transfer results in correction of T cell defects in vitro supports their application for gene therapy in WAS patients.

Mechanisms of WASp-mediated hematologic and immunologic disease

The Wiskott-Aldrich syndrome protein (WASp) is a key regulator of actin polymerization in hematopoietic cells. The dynamic nature of cytoskeletal changes during a variety of cellular processes demands complex mechanisms for coordinated integration of input signals, precise localization within the cell, and regulated activation of the Arp2/3 complex. Mutations in the Wiskott-Aldrich syndrome gene either inhibit or dysregulate normal WASp function, resulting in clinical diseases with complex and disparate phenotypes. This review highlights recent advances that have enhanced our understanding of the mechanisms by which these molecular defects cause hematologic and immunologic disease.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.

Wiskott—Aldrich Syndrome: a model for defective actin reorganization, cell trafficking and synapse formation

Wiskott-Aldrich Syndrome (WAS) is an X-linked immunodeficiency characterized by thrombocytopenia with small platelets, eczema, recurrent infections, autoimmune disorders and increased incidence of malignancies. Classic WAS, and a milder form, X-linked thrombocytopenia, are caused by mutations of the WAS protein (WASP) gene. Recent investigations have provided evidence that WASP and several related proteins are involved in the reorganization of the actin cytoskeleton by activating Arp2/3-mediated actin polymerization. This function is controlled by the small GTPase Cdc42, which regulates the autoinhibitory loop formation of WASP. In addition, WASP is involved in cytoplasmic signaling via its interaction with a variety of adaptor molecules. Mutation analysis of large cohorts of WAS/X-linked thrombocytopenia patients has provided evidence for a strong correlation between phenotype and genotype.