Anti-CD45 PBD-based antibody-drug conjugates are effective targeted conditioning agents for gene therapy and stem cell transplant

Stem cell gene therapy and hematopoietic stem cell transplantation (SCT) require conditioning to ablate the recipient's hematopoietic stem cells (HSCs) and create a niche for gene-corrected/donor HSCs. Conventional conditioning agents are non-specific, leading to off-target toxicities and resulting in significant morbidity and mortality. We developed tissue-specific anti-human CD45 antibody-drug conjugates (ADCs), using rat IgG2b anti-human CD45 antibody clones YTH24.5 and YTH54.12, conjugated to cytotoxic pyrrolobenzodiazepine (PBD) dimer payloads with cleavable (SG3249) or non-cleavable (SG3376) linkers. In vitro, these ADCs internalized to lysosomes for drug release, resulting in potent and specific killing of human CD45+ cells. In humanized NSG mice, the ADCs completely ablated human HSCs without toxicity to non-hematopoietic tissues, enabling successful engraftment of gene-modified autologous and allogeneic human HSCs. The ADCs also delayed leukemia onset and improved survival in CD45+ tumor models. These data provide proof of concept that conditioning with anti-human CD45-PBD ADCs allows engraftment of donor/gene-corrected HSCs with minimal toxicity to non-hematopoietic tissues. Our anti-CD45-PBDs or similar agents could potentially shift the paradigm in transplantation medicine that intensive chemo/radiotherapy is required for HSC engraftment after gene therapy and allogeneic SCT. Targeted conditioning both improve the safety and minimize late effects of these procedures, which would greatly increase their applicability.

Targeting the central nervous system in lysosomal storage diseases: Strategies to deliver therapeutics across the blood-brain barrier

Lysosomal storage diseases (LSDs) are multisystem inherited metabolic disorders caused by dysfunctional lysosomal activity, resulting in the accumulation of undegraded macromolecules in a variety of organs/tissues, including the central nervous system (CNS). Treatments include enzyme replacement therapy, stem/progenitor cell transplantation, and in vivo gene therapy. However, these treatments are not fully effective in treating the CNS as neither enzymes, stem cells, nor viral vectors efficiently cross the blood-brain barrier. Here, we review the latest advancements in improving delivery of different therapeutic agents to the CNS and comment upon outstanding questions in the field of neurological LSDs.

Lentiviral Gene Transfer Corrects Immune Abnormalities in XIAP Deficiency

BACKGROUND

X-linked inhibitor of apoptosis protein (XIAP) deficiency is a severe immunodeficiency with clinical features including hemophagocytic lymphohistiocytosis (HLH) and inflammatory bowel disease (IBD) due to defective NOD2 responses. Management includes immunomodulatory therapies and hematopoietic stem cell transplant (HSCT). However, this cohort is particularly susceptible to the chemotherapeutic regimens and acutely affected by graft-vs-host disease (GvHD), driving poor long-term survival in transplanted patients. Autologous HSC gene therapy could offer an alternative treatment option and would abrogate the risks of alloreactivity.

METHODS

Hematopoietic progenitor (Lin-ve) cells from XIAPy/- mice were transduced with a lentiviral vector encoding human XIAP cDNA before transplantation into irradiated XIAP y/- recipients. After 12 weeks animals were challenged with the dectin-1 ligand curdlan and recovery of innate immune function was evaluated though analysis of inflammatory cytokines, body weight, and splenomegaly. XIAP patient-derived CD14+ monocytes were transduced with the same vector and functional recovery was demonstrated using in vitro L18-MDP/NOD2 assays.

RESULTS

In treated XIAPy/- mice, ~40% engraftment of gene-corrected Lin-ve cells led to significant recovery of weight loss, splenomegaly, and inflammatory cytokine responses to curdlan, comparable to wild-type mice. Serum IL-6, IL-10, MCP-1, and TNF were significantly reduced 2-h post-curdlan administration in non-corrected XIAPy/- mice compared to wild-type and gene-corrected animals. Appropriate reduction of inflammatory responses was observed in gene-corrected mice, whereas non-corrected mice developed an inflammatory profile 9 days post-curdlan challenge. In gene-corrected patient CD14+ monocytes, TNF responses were restored following NOD2 activation with L18-MDP.

CONCLUSION

Gene correction of HSCs recovers XIAP-dependent immune defects and could offer a treatment option for patients with XIAP deficiency.

Retrospective, Landmark Analysis of Long-term Adult Morbidity Following Allogeneic HSCT for Inborn Errors of Immunity in Infancy and Childhood

Purpose

Allogeneic hematopoietic stem cell transplant (HSCT) remains the treatment of choice for patients with inborn errors of immunity (IEI). There is little published medical outcome data assessing late medical complications following transition to adult care. We sought to document event-free survival (EFS) in transplanted IEI patients reaching adulthood and describe common late-onset medical complications and factors influencing EFS.

Methods

In this landmark analysis, 83 adults surviving 5 years or more following prior HSCT in childhood for IEI were recruited. The primary endpoint was event-free survival, defined as time post-first HSCT to graft failure, graft rejection, chronic infection, life-threatening or recurrent infections, malignancy, significant autoimmune disease, moderate to severe GVHD or major organ dysfunction. All events occurring less than 5 years post-HSCT were excluded.

Results

EFS was 51% for the whole cohort at a median of 20 years post HSCT. Multivariable analysis identified age at transplant and whole blood chimerism as independent predictors of long-term EFS. Year of HSCT, donor, conditioning intensity and underlying diagnosis had no significant impact on EFS. 59 events occurring beyond 5 years post-HSCT were documented in 37 patients (45% cohort). A total of 25 patients (30% cohort) experienced ongoing significant complications requiring active medical intervention at last follow-up.

Conclusion

Although most patients achieved excellent, durable immune reconstitution with infrequent transplant-related complications, very late complications are common and associated with mixed chimerism post-HSCT. Early intervention to correct mixed chimerism may improve long-term outcomes and adult health following HSCT for IEI in childhood.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10875-022-01278-6.

Metabolite and thymocyte development defects in ADA-SCID mice receiving enzyme replacement therapy

Deficiency of adenosine deaminase (ADA, EC3.5.4.4), a housekeeping enzyme intrinsic to the purine salvage pathway, leads to severe combined immunodeficiency (SCID) both in humans and mice. Lack of ADA results in the intracellular accumulation of toxic metabolites which have effects on T cell development and function. While untreated ADA-SCID is a fatal disorder, there are different therapeutic options available to restore ADA activity and reconstitute a functioning immune system, including enzyme replacement therapy (ERT). Administration of ERT in the form of pegylated bovine ADA (PEG-ADA) has proved a life-saving though non-curative treatment for ADA-SCID patients. However, in many patients treated with PEG-ADA, there is suboptimal immune recovery with low T and B cell numbers. Here, we show reduced thymus cellularity in ADA-SCID mice despite weekly PEG-ADA treatment. This was associated with lack of effective adenosine (Ado) detoxification in the thymus. We also show that thymocyte development in ADA-deficient thymi is arrested at the DN3-to-DN4 stage transition with thymocytes undergoing dATP-induced apoptosis rather than defective TCRβ rearrangement or β-selection. Our studies demonstrate at a detailed level that exogenous once-a-week enzyme replacement does not fully correct intra-thymic metabolic or immunological abnormalities associated with ADA deficiency.

Long-Term Immune Recovery After Hematopoietic Stem Cell Transplantation for ADA Deficiency: a Single-Center Experience

Unconditioned hematopoietic stem cell transplantation (HSCT) is the recommended treatment for patients with adenosine deaminase (ADA)-deficient severe combined immunodeficiency with an HLA-matched sibling donor (MSD) or family donor (MFD). Improved overall survival (OS) has been reported compared to the use of unrelated donors, and previous studies have demonstrated that adequate cellular and humoral immune recovery can be achieved even in the absence of conditioning. Detailed insight of the long-term outcome is still limited. We aim to address this by studying a large single-center cohort of 28 adenosine deaminase-deficient patients who underwent a total of 31 HSCT procedures, of which more than half were unconditioned. We report an OS of 85.7% and event-free survival of 71% for the entire cohort, with no statistically significant differences after procedures using related or unrelated HLA-matched donors. We find that donor engraftment in the myeloid compartment is significantly diminished in unconditioned procedures, which typically use a MSD or MFD. This is associated with poor metabolic correction and more frequent failure to discontinue immunoglobulin replacement therapy. Approximately one in four patients receiving an unconditioned procedure required a second procedure, whereas the use of reduced intensity conditioning (RIC) prior to allogeneic transplantation improves the long-term outcome by achieving better myeloid engraftment, humoral immune recovery, and metabolic correction. Further longitudinal studies are needed to optimize future management and guidelines, but our findings support a potential role for the routine use of RIC in most ADA-deficient patients receiving an HLA-identical hematopoietic stem cell transplant, even when a MSD or MFD is available.

Autologous Ex Vivo Lentiviral Gene Therapy for Adenosine Deaminase Deficiency

BACKGROUND

Severe combined immunodeficiency due to adenosine deaminase (ADA) deficiency (ADA-SCID) is a rare and life-threatening primary immunodeficiency.

METHODS

We treated 50 patients with ADA-SCID (30 in the United States and 20 in the United Kingdom) with an investigational gene therapy composed of autologous CD34+ hematopoietic stem and progenitor cells (HSPCs) transduced ex vivo with a self-inactivating lentiviral vector encoding human ADA. Data from the two U.S. studies (in which fresh and cryopreserved formulations were used) at 24 months of follow-up were analyzed alongside data from the U.K. study (in which a fresh formulation was used) at 36 months of follow-up.

RESULTS

Overall survival was 100% in all studies up to 24 and 36 months. Event-free survival (in the absence of reinitiation of enzyme-replacement therapy or rescue allogeneic hematopoietic stem-cell transplantation) was 97% (U.S. studies) and 100% (U.K. study) at 12 months; 97% and 95%, respectively, at 24 months; and 95% (U.K. study) at 36 months. Engraftment of genetically modified HSPCs persisted in 29 of 30 patients in the U.S. studies and in 19 of 20 patients in the U.K. study. Patients had sustained metabolic detoxification and normalization of ADA activity levels. Immune reconstitution was robust, with 90% of the patients in the U.S. studies and 100% of those in the U.K. study discontinuing immunoglobulin-replacement therapy by 24 months and 36 months, respectively. No evidence of monoclonal expansion, leukoproliferative complications, or emergence of replication-competent lentivirus was noted, and no events of autoimmunity or graft-versus-host disease occurred. Most adverse events were of low grade.

CONCLUSIONS

Treatment of ADA-SCID with ex vivo lentiviral HSPC gene therapy resulted in high overall and event-free survival with sustained ADA expression, metabolic correction, and functional immune reconstitution. (Funded by the National Institutes of Health and others; ClinicalTrials.gov numbers, NCT01852071, NCT02999984, and NCT01380990.).

The transformative potential of HSC gene therapy as a genetic medicine

Hematopoietic stem cells (HSCs) are precursor cells that give rise to blood, immune and tissue-resident progeny in humans. Their position at the starting point of hematopoiesis offers a unique therapeutic opportunity to treat certain hematologic diseases by implementing corrective changes that are subsequently directed through to multiple cell lineages. Attempts to exploit HSCs clinically have evolved over recent decades, from initial approaches that focused on transplantation of healthy donor allogeneic HSCs to treat rare inherited monogenic hematologic disorders, to more contemporary genetic modification of autologous HSCs offering the promise of benefits to a wider range of diseases. We are on the cusp of an exciting new era as the transformative potential of HSC gene therapy to offer durable delivery of gene-corrected cells to a range of tissues and organs, including the central nervous system, is beginning to be realized. This article reviews the rationale for targeting HSCs, the approaches that have been used to date for delivering therapeutic genes to these cells, and the latest technological breakthroughs in manufacturing and vector design. The challenges faced by the biotechnology cell and gene therapy sector in the commercialization of HSC gene therapy are also discussed.

Long-term lymphoid progenitors independently sustain naïve T and NK cell production in humans

Our mathematical model of integration site data in clinical gene therapy supported the existence of long-term lymphoid progenitors capable of surviving independently from hematopoietic stem cells. To date, no experimental setting has been available to validate this prediction. We here report evidence of a population of lymphoid progenitors capable of independently maintaining T and NK cell production for 15 years in humans. The gene therapy patients of this study lack vector-positive myeloid/B cells indicating absence of engineered stem cells but retain gene marking in both T and NK. Decades after treatment, we can still detect and analyse transduced naïve T cells whose production is likely maintained by a population of long-term lymphoid progenitors. By tracking insertional clonal markers overtime, we suggest that these progenitors can support both T and NK cell production. Identification of these long-term lymphoid progenitors could be utilised for the development of next generation gene- and cancer-immunotherapies.

Lentiviral Hematopoietic Stem Cell Gene Therapy Rescues Clinical Phenotypes in a Murine Model of Pompe Disease

Pompe disease is a lysosomal storage disorder caused by malfunctions of the acid alpha-glucosidase (GAA) enzyme with a consequent toxic accumulation of glycogen in cells. Muscle wasting and hypertrophic cardiomyopathy are the most common clinical signs that can lead to cardiac and respiratory failure within the first year of age in the more severe infantile forms. Currently available treatments have significant limitations and are not curative, highlighting a need for the development of alternative therapies. In this study, we investigated the use of a clinically relevant lentiviral vector to deliver systemically GAA through genetic modification of hematopoietic stem and progenitor cells (HSPCs). The overexpression of GAA in human HSPCs did not exert any toxic effect on this cell population, which conserved its stem cell capacity in xenograft experiments. In a murine model of Pompe disease treated at young age, we observed phenotypic correction of heart and muscle function with a significant reduction of glycogen accumulation in tissues after 6 months of treatment. These findings suggest that lentiviral-mediated HSPC gene therapy can be a safe alternative therapy for Pompe disease.

Successful Preclinical Development of Gene Therapy for Recombinase-Activating Gene-1-Deficient SCID

Recombinase-activating gene-1 (RAG1)-deficient severe combined immunodeficiency (SCID) patients lack B and T lymphocytes due to the inability to rearrange immunoglobulin and T cell receptor genes. Gene therapy is an alternative for those RAG1-SCID patients who lack a suitable bone marrow donor. We designed lentiviral vectors with different internal promoters driving codon-optimized RAG1 to ensure optimal expression. We used Rag1 -/- mice as a preclinical model for RAG1-SCID to assess the efficacy of the various vectors. We observed that B and T cell reconstitution directly correlated with RAG1 expression. Mice with low RAG1 expression showed poor immune reconstitution; however, higher expression resulted in phenotypic and functional lymphocyte reconstitution comparable to mice receiving wild-type stem cells. No signs of genotoxicity were found. Additionally, RAG1-SCID patient CD34+ cells transduced with our clinical RAG1 vector and transplanted into NSG mice led to improved human B and T cell development. Considering this efficacy outcome, together with favorable safety data, these results substantiate the need for a clinical trial for RAG1-SCID.

Lentiviral gene therapy for X-linked chronic granulomatous disease

Chronic granulomatous disease (CGD) is a rare inherited disorder of phagocytic cells1,2. We report the initial results of nine severely affected X-linked CGD (X-CGD) patients who received ex vivo autologous CD34+ hematopoietic stem and progenitor cell-based lentiviral gene therapy following myeloablative conditioning in first-in-human studies (trial registry nos. NCT02234934 and NCT01855685). The primary objectives were to assess the safety and evaluate the efficacy and stability of biochemical and functional reconstitution in the progeny of engrafted cells at 12 months. The secondary objectives included the evaluation of augmented immunity against bacterial and fungal infection, as well as assessment of hematopoietic stem cell transduction and engraftment. Two enrolled patients died within 3 months of treatment from pre-existing comorbidities. At 12 months, six of the seven surviving patients demonstrated stable vector copy numbers (0.4-1.8 copies per neutrophil) and the persistence of 16-46% oxidase-positive neutrophils. There was no molecular evidence of either clonal dysregulation or transgene silencing. Surviving patients have had no new CGD-related infections, and six have been able to discontinue CGD-related antibiotic prophylaxis. The primary objective was met in six of the nine patients at 12 months follow-up, suggesting that autologous gene therapy is a promising approach for CGD patients.

Consensus approach for the management of severe combined immune deficiency caused by adenosine deaminase deficiency

Inherited defects in adenosine deaminase (ADA) cause a subtype of severe combined immunodeficiency (SCID) known as severe combined immune deficiency caused by adenosine deaminase defects (ADA-SCID). Most affected infants can receive a diagnosis while still asymptomatic by using an SCID newborn screening test, allowing early initiation of therapy. We review the evidence currently available and propose a consensus management strategy. In addition to treatment of the immune deficiency seen in patients with ADA-SCID, patients should be followed for specific noninfectious respiratory, neurological, and biochemical complications associated with ADA deficiency. All patients should initially receive enzyme replacement therapy (ERT), followed by definitive treatment with either of 2 equal first-line options. If an HLA-matched sibling donor or HLA-matched family donor is available, allogeneic hematopoietic stem cell transplantation (HSCT) should be pursued. The excellent safety and efficacy observed in more than 100 patients with ADA-SCID who received gammaretrovirus- or lentivirus-mediated autologous hematopoietic stem cell gene therapy (HSC-GT) since 2000 now positions HSC-GT as an equal alternative. If HLA-matched sibling donor/HLA-matched family donor HSCT or HSC-GT are not available or have failed, ERT can be continued or reinstituted, and HSCT with alternative donors should be considered. The outcomes of novel HSCT, ERT, and HSC-GT strategies should be evaluated prospectively in "real-life" conditions to further inform these management guidelines.

T-cell gene therapy for perforin deficiency corrects cytotoxicity defects and prevents hemophagocytic lymphohistiocytosis manifestations

BACKGROUND

Mutations in the perforin 1 (PRF1) gene account for up to 58% of familial hemophagocytic lymphohistiocytosis syndromes. The resulting defects in effector cell cytotoxicity lead to hypercytokinemia and hyperactivation with inflammation in various organs.

OBJECTIVE

We sought to determine whether autologous gene-corrected T cells can restore cytotoxic function, reduce disease activity, and prevent hemophagocytic lymphohistiocytosis (HLH) symptoms in in vivo models.

METHODS

We developed a gammaretroviral vector to transduce murine CD8 T cells in the Prf-/- mouse model. To verify functional correction of Prf-/- CD8 T cells in vivo, we used a lymphocytic choriomeningitis virus (LCMV) epitope-transfected murine lung carcinoma cell tumor model. Furthermore, we challenged gene-corrected and uncorrected mice with LCMV. One patient sample was transduced with a PRF1-encoding lentiviral vector to study restoration of cytotoxicity in human cells.

RESULTS

We demonstrated efficient engraftment and functional reconstitution of cytotoxicity after intravenous administration of gene-corrected Prf-/- CD8 T cells into Prf-/- mice. In the tumor model infusion of Prf-/- gene-corrected CD8 T cells eliminated the tumor as efficiently as transplantation of wild-type CD8 T cells. Similarly, mice reconstituted with gene-corrected Prf-/- CD8 T cells displayed complete protection from the HLH phenotype after infection with LCMV. Patients' cells showed correction of cytotoxicity in human CD8 T cells after transduction.

CONCLUSION

These data demonstrate the potential application of T-cell gene therapy in reconstituting cytotoxic function and protection against HLH in the setting of perforin deficiency.

Transfer of gene-corrected T cells corrects humoral and cytotoxic defects in patients with X-linked lymphoproliferative disease

BACKGROUND

X-linked lymphoproliferative disease 1 arises from mutations in the SH2D1A gene encoding SLAM-associated protein (SAP), an adaptor protein expressed in T, natural killer (NK), and NKT cells. Defects lead to abnormalities of T-cell and NK cell cytotoxicity and T cell-dependent humoral function. Clinical manifestations include hemophagocytic lymphohistiocytosis, lymphoma, and dysgammaglobulinemia. Curative treatment is limited to hematopoietic stem cell transplantation, with outcomes reliant on a good donor match.

OBJECTIVES

Because most symptoms arise from defective T-cell function, we investigated whether transfer of SAP gene-corrected T cells could reconstitute known effector cell defects.

METHODS

CD3+ lymphocytes from Sap-deficient mice were transduced with a gammaretroviral vector encoding human SAP cDNA before transfer into sublethally irradiated Sap-deficient recipients. After immunization with the T-dependent antigen 4-hydroxy-3-nitrophenylacetly chicken gammaglobulin (NP-CGG), recovery of humoral function was evaluated through germinal center formation and antigen-specific responses. To efficiently transduce CD3+ cells from patients, we generated an equivalent lentiviral SAP vector. Functional recovery was demonstrated by using in vitro cytotoxicity and T follicular helper cell function assays alongside tumor clearance in an in vivo lymphoblastoid cell line lymphoma xenograft model.

RESULTS

In Sap-deficient mice 20% to 40% engraftment of gene-modified T cells led to significant recovery of germinal center formation and NP-specific antibody responses. Gene-corrected T cells from patients demonstrated improved cytotoxicity and T follicular helper cell function in vitro. Adoptive transfer of gene-corrected cytotoxic T lymphocytes from patients reduced tumor burden to a level comparable with that seen in healthy donor cytotoxic T lymphocytes in an in vivo lymphoma model.

CONCLUSIONS

These data demonstrate that autologous T-cell gene therapy corrects SAP-dependent defects and might offer an alternative therapeutic option for patients with X-linked lymphoproliferative disease 1.

The 2017 IUIS Phenotypic Classification for Primary Immunodeficiencies

Since the 1990s, the International Union of Immunological Societies (IUIS) PID expert committee (EC), now called Inborn Errors of Immunity Committee, has published every other year a classification of the inborn errors of immunity. This complete catalog serves as a reference for immunologists and researchers worldwide. However, it was unadapted for clinicians at the bedside. For those, the IUIS PID EC is now publishing a phenotypical classification since 2013, which proved to be more user-friendly. There are now 320 single-gene inborn errors of immunity underlying phenotypes as diverse as infection, malignancy, allergy, auto-immunity, and auto-inflammation. We herein propose the revised 2017 phenotypic classification, based on the accompanying 2017 IUIS Inborn Errors of Immunity Committee classification.

International Union of Immunological Societies: 2017 Primary Immunodeficiency Diseases Committee Report on Inborn Errors of Immunity

Beginning in 1970, a committee was constituted under the auspices of the World Health Organization (WHO) to catalog primary immunodeficiencies. Twenty years later, the International Union of Immunological Societies (IUIS) took the remit of this committee. The current report details the categorization and listing of 354 (as of February 2017) inborn errors of immunity. The growth and increasing complexity of the field have been impressive, encompassing an increasing variety of conditions, and the classification described here will serve as a critical reference for immunologists and researchers worldwide.

Hematopoietic Stem-Cell Gene Therapy for Cerebral Adrenoleukodystrophy

BACKGROUND

In X-linked adrenoleukodystrophy, mutations in ABCD1 lead to loss of function of the ALD protein. Cerebral adrenoleukodystrophy is characterized by demyelination and neurodegeneration. Disease progression, which leads to loss of neurologic function and death, can be halted only with allogeneic hematopoietic stem-cell transplantation.

METHODS

We enrolled boys with cerebral adrenoleukodystrophy in a single-group, open-label, phase 2-3 safety and efficacy study. Patients were required to have early-stage disease and gadolinium enhancement on magnetic resonance imaging (MRI) at screening. The investigational therapy involved infusion of autologous CD34+ cells transduced with the elivaldogene tavalentivec (Lenti-D) lentiviral vector. In this interim analysis, patients were assessed for the occurrence of graft-versus-host disease, death, and major functional disabilities, as well as changes in neurologic function and in the extent of lesions on MRI. The primary end point was being alive and having no major functional disability at 24 months after infusion.

RESULTS

A total of 17 boys received Lenti-D gene therapy. At the time of the interim analysis, the median follow-up was 29.4 months (range, 21.6 to 42.0). All the patients had gene-marked cells after engraftment, with no evidence of preferential integration near known oncogenes or clonal outgrowth. Measurable ALD protein was observed in all the patients. No treatment-related death or graft-versus-host disease had been reported; 15 of the 17 patients (88%) were alive and free of major functional disability, with minimal clinical symptoms. One patient, who had had rapid neurologic deterioration, had died from disease progression. Another patient, who had had evidence of disease progression on MRI, had withdrawn from the study to undergo allogeneic stem-cell transplantation and later died from transplantation-related complications.

CONCLUSIONS

Early results of this study suggest that Lenti-D gene therapy may be a safe and effective alternative to allogeneic stem-cell transplantation in boys with early-stage cerebral adrenoleukodystrophy. Additional follow-up is needed to fully assess the duration of response and long-term safety. (Funded by Bluebird Bio and others; STARBEAM ClinicalTrials.gov number, NCT01896102 ; ClinicalTrialsRegister.eu number, 2011-001953-10 .).

Lentiviral Vectors with Cellular Promoters Correct Anemia and Lethal Bone Marrow Failure in a Mouse Model for Diamond-Blackfan Anemia

Diamond-Blackfan anemia is a congenital erythroid hypoplasia and is associated with physical malformations and a predisposition to cancer. Twenty-five percent of patients with Diamond-Blackfan anemia have mutations in a gene encoding ribosomal protein S19 (RPS19). Through overexpression of RPS19 using a lentiviral vector with the spleen focus-forming virus promoter, we demonstrated that the Diamond-Blackfan anemia phenotype can be successfully treated in Rps19-deficient mice. In our present study, we assessed the efficacy of a clinically relevant promoter, the human elongation factor 1α short promoter, with or without the locus control region of the β-globin gene for treatment of RPS19-deficient Diamond-Blackfan anemia. The findings demonstrate that these vectors rescue the proliferation defect and improve erythroid development of transduced RPS19-deficient bone marrow cells. Remarkably, bone marrow failure and severe anemia in Rps19-deficient mice was cured with enforced expression of RPS19 driven by the elongation factor 1α short promoter. We also demonstrate that RPS19-deficient bone marrow cells can be transduced and these cells have the capacity to repopulate bone marrow in long-term reconstituted mice. Our results collectively demonstrate the feasibility to cure RPS19-deficient Diamond-Blackfan anemia using lentiviral vectors with cellular promoters that possess a reduced risk of insertional mutagenesis.

Thymus transplantation for complete DiGeorge syndrome: European experience

Background

Thymus transplantation is a promising strategy for the treatment of athymic complete DiGeorge syndrome (cDGS).

Methods

Twelve patients with cDGS underwent transplantation with allogeneic cultured thymus.

Objective

We sought to confirm and extend the results previously obtained in a single center.

Results

Two patients died of pre-existing viral infections without having thymopoiesis, and 1 late death occurred from autoimmune thrombocytopenia. One infant had septic shock shortly after transplantation, resulting in graft loss and the need for a second transplant. Evidence of thymopoiesis developed from 5 to 6 months after transplantation in 10 patients. Median circulating naive CD4 counts were 44 × 10⁶/L (range, 11-440 × 10⁶/L) and 200 × 10⁶/L (range, 5-310 × 10⁶/L) at 12 and 24 months after transplantation and T-cell receptor excision circles were 2,238/10⁶ T cells (range, 320-8,807/10⁶ T cells) and 4,184/10⁶ T cells (range, 1,582-24,596/10⁶ T cells). Counts did not usually reach normal levels for age, but patients were able to clear pre-existing infections and those acquired later. At a median of 49 months (range, 22-80 months), 8 have ceased prophylactic antimicrobials, and 5 have ceased immunoglobulin replacement. Histologic confirmation of thymopoiesis was seen in 7 of 11 patients undergoing biopsy of transplanted tissue, including 5 showing full maturation through to the terminal stage of Hassall body formation. Autoimmune regulator expression was also demonstrated. Autoimmune complications were seen in 7 of 12 patients. In 2 patients early transient autoimmune hemolysis settled after treatment and did not recur. The other 5 experienced ongoing autoimmune problems, including thyroiditis (3), hemolysis (1), thrombocytopenia (4), and neutropenia (1).

Conclusions

This study confirms the previous reports that thymus transplantation can reconstitute T cells in patients with cDGS but with frequent autoimmune complications in survivors.

A prospective study on the natural history of patients with profound combined immunodeficiency: An interim analysis

BACKGROUND

Absent T-cell immunity resulting in life-threatening infections provides a clear rationale for hematopoetic stem cell transplantation (HSCT) in patients with severe combined immunodeficiency (SCID). Combined immunodeficiencies (CIDs) and "atypical" SCID show reduced, not absent T-cell immunity. If associated with infections or autoimmunity, they represent profound combined immunodeficiency (P-CID), for which outcome data are insufficient for unambiguous early transplant decisions.

OBJECTIVES

We sought to compare natural histories of severity-matched patients with/without subsequent transplantation and to determine whether immunologic and/or clinical parameters may be predictive for outcome.

METHODS

In this prospective and retrospective observational study, we recruited nontransplanted patients with P-CID aged 1 to 16 years to compare natural histories of severity-matched patients with/without subsequent transplantation and to determine whether immunologic and/or clinical parameters may be predictive for outcome.

RESULTS

A total of 51 patients were recruited (median age, 9.6 years). Thirteen of 51 had a genetic diagnosis of "atypical" SCID and 14 of 51 of CID. About half of the patients had less than 10% naive T cells, reduced/absent T-cell proliferation, and at least 1 significant clinical event/year, demonstrating their profound immunodeficiency. Nineteen patients (37%) underwent transplantation within 1 year of enrolment, and 5 of 51 patients died. Analysis of the HSCT decisions revealed the anticipated heterogeneity, favoring an ongoing prospective matched-pair analysis of patients with similar disease severity with or without transplantation. Importantly, so far neither the genetic diagnosis nor basic measurements of T-cell immunity were good predictors of disease evolution.

CONCLUSIONS

The P-CID study for the first time characterizes a group of patients with nontypical SCID T-cell deficiencies from a therapeutic perspective. Because genetic and basic T-cell parameters provide limited guidance, prospective data from this study will be a helpful resource for guiding the difficult HSCT decisions in patients with P-CID.

Mutations in linker for activation of T cells (LAT) lead to a novel form of severe combined immunodeficiency

BACKGROUND

Signaling through the T-cell receptor (TCR) is critical for T-cell development and function. Linker for activation of T cells (LAT) is a transmembrane adaptor signaling molecule that is part of the TCR complex and essential for T-cell development, as demonstrated by LAT-deficient mice, which show a complete lack of peripheral T cells.

OBJECTIVE

We describe a pedigree affected by a severe combined immunodeficiency phenotype with absent T cells and normal B-cell and natural killer cell numbers. A novel homozygous frameshift mutation in the gene encoding for LAT was identified in this kindred.

METHODS

Genetic, molecular, and functional analyses were used to identify and characterize the LAT defect. Clinical and immunologic analysis of patients was also performed and reported.

RESULTS

Homozygosity mapping was used to identify potential defective genes. Sanger sequencing of the LAT gene showed a mutation that resulted in a premature stop codon and protein truncation leading to complete loss of function and loss of expression of LAT in the affected family members. We also demonstrate loss of LAT expression and lack of TCR signaling restoration in LAT-deficient cell lines reconstituted with a synthetic LAT gene bearing this severe combined immunodeficiency mutation.

CONCLUSION

For the first time, the results of this study show that inherited LAT deficiency should be considered in patients with combined immunodeficiency with T-cell abnormalities.

Hematopoietic stem cell transplantation outcomes for 11 patients with dedicator of cytokinesis 8 deficiency

BACKGROUND

Dedicator of cytokinesis 8 (DOCK8) deficiency can be cured by allogeneic hematopoietic stem cell transplantation (HSCT). Reports of outcomes are still limited.

OBJECTIVE

We sought to analyze the results of HSCT in patients with DOCK8 deficiency and report whether approaches resulting in mixed chimerism result in clinically relevant immune reconstitution.

METHODS

We performed a retrospective chart review of 11 patients with DOCK8 deficiency and measured DOCK8 expression and cytokine production.

RESULTS

Of 11 patients, 7 received HSCT from related and 4 from unrelated donors; 9 patients received busulfan-based conditioning regimens. Survival was excellent (10 [91%] of 11 patients alive), including a patient who had undergone liver transplantation. Patients showed significant improvements in the frequency and severity of infections. Although eczema resolved in all, food allergies and high IgE levels persisted in some patients. Lymphopenia, eosinophilia, low numbers of naive CD8(+) T cells and switched memory B cells, and TH1/TH2 cytokine imbalance improved in most patients. Although the 8 matched related or unrelated donor recipients had full donor chimerism, all 3 recipients of mismatched unrelated donor HSCT had high levels of donor T-cell chimerism and low B-cell and myeloid cell chimerism (0% to 46%). Almost all switched memory B cells were of donor origin. All patients, including those with mixed chimerism, mounted robust antibody responses to vaccination.

CONCLUSION

Allogeneic HSCT ameliorated the infectious and atopic symptoms of patients with DOCK8 deficiency. In patients with mixed chimerism, selective advantage for donor-derived T cells and switched memory B cells promoted restoration of cellular and humoral immunity and protection against opportunistic infection.

Treating Immunodeficiency through HSC Gene Therapy

Haematopoietic stem cell (HSC) gene therapy has been successfully employed as a therapeutic option to treat specific inherited immune deficiencies, including severe combined immune deficiencies (SCID) over the past two decades. Initial clinical trials using first-generation gamma-retroviral vectors to transfer corrective DNA demonstrated clinical benefit for patients, but were associated with leukemogenesis in a number of cases. Safer vectors have since been developed, affording comparable efficacy with an improved biosafety profile. These vectors are now in Phase I/II clinical trials for a number of immune disorders with more preclinical studies underway. Targeted gene editing allowing precise DNA correction via platforms such as ZFNs, TALENs and CRISPR/Cas9 may now offer promising strategies to improve the safety and efficacy of gene therapy in the future.

Autologous Transplantation of Amniotic Fluid-Derived Mesenchymal Stem Cells into Sheep Fetuses

Long-term engraftment and phenotype correction has been difficult to achieve in humans after in utero stem cell transplantation mainly because of allogeneic rejection. Autologous cells could be obtained during gestation from the amniotic fluid with minimal risk for the fetus and the mother. Using a sheep model, we explored the possibility of using amniotic fluid mesenchymal stem cells (AFMSCs) for autologous in utero stem cell/gene therapy. We collected amniotic fluid (AF) under ultrasound-guided amniocentesis in early gestation pregnant sheep ( n = 9, 58 days of gestation, term = 145 days). AFMSCs were isolated and expanded in all sampled fetal sheep. Those cells were transduced using an HIV vector encoding enhanced green fluorescent protein (GFP) with 63.2% (range 38.3-96.2%) transduction efficiency rate. After expansion, transduced AFMSCs were injected into the peritoneal cavity of each donor fetal sheep at 76 days under ultrasound guidance. One ewe miscarried twin fetuses after amniocentesis. Intraperitoneal injection was successful in the remaining 7 fetal sheep giving a 78% survival for the full procedure. Tissues were sampled at postmortem examination 2 weeks later. PCR analysis detected GFP-positive cells in fetal tissues including liver, heart, placenta, membrane, umbilical cord, adrenal gland, and muscle. GFP protein was detected in these tissues by Western blotting and further confirmed by cytofluorimetric and immunofluorescence analyses. This is the first demonstration of autologous stem cell transplantation in the fetus using AFMSCs. Autologous cells derived from AF showed widespread organ migration and could offer an alternative way to ameliorate prenatal congenital disease.

Spectrum of mutations in a cohort of UK patients with ADA deficient SCID: Segregation of genotypes with specific ethnicities

Severe combined immunodeficiency (SCID) arises from a number of different genetic defects, one of the most common being mutations in the gene encoding adenosine deaminase (ADA). In the UK, ADA deficient SCID compromises approximately 20% of all known cases of SCID. We carried out a retrospective analysis of the ADA gene in 46 known ADA deficient SCID patients on whom DNA had been stored. Here, we report a high frequency of two previously reported mutations and provide a link between the mutations and patient ethnicity within our patient cohort. We also report on 9 novel mutations that have been previously unreported.

Gene therapy for haemophagocytic lymphohistiocytosis

Haemophagocytic lymphohistiocytosis (HLH) describes a severe and often fatal immunodysregulatory disorder caused primarily by the uncontrolled activation and proliferation of T cells and macrophages. A number of genetic defects mainly involving defective granule exocytosis and effector cell cytotoxicity have been identified and well characterised at the molecular and cellular level. These conditions have limited therapeutic options and given the predominant restriction of the causative gene to the haematopoietic system, they have become attractive targets for haematopoietic cell gene therapy. Pre clinical studies in murine models of HLH due to perforin deficiency have shown correction of the disease phenotype as a result of autologous haematopoietic stem cell (HSC) gene transfer using lentiviral vectors. In a murine model of X-linked lymphoproliferative disease (XLP1), HSC gene transfer is able to correct the immunological manifestations of the disease. These encouraging murine studies have led to further work to develop clinically applicable strategies. An alternative approach is to correct defective T cells as this approach is safer than HSC gene therapy and may allow early control of the HLH through the engraftment of functional gene modified effector T cells. Both strategies are now in development and a gene therapy option for certain genetic forms of HLH may soon enter clinical trials.

Severe combined immunodeficiency: recent developments and guidance on clinical management

Severe combined immunodeficiency (SCID) is a rare but important condition. Affected infants are born with profound abnormalities of immune cell function that lead to severe and recurrent infection that are almost always fatal in the first year of life without treatment. Infants with SCID are often initially seen by general paediatricians in the hospital care setting, and the recognition of the cardinal features of the disease and alertness to specific laboratory parameters are important in making an early diagnosis. There is also increasing interest in newborn screening for SCID, which has the potential to significantly improve outcome through early diagnosis and implementation of prophylactic medications. Definitive treatments such as haematopoietic stem cell transplantation and gene therapy have also made major advances over the last decade and again promise to improve the overall outcome for SCID with reduced long-term toxicities. In this review, we highlight some of the major advances in diagnosis and management of the disease, but we also want to emphasise the important role of the general paediatrician in making an early diagnosis and in ongoing management.

Outcomes following gene therapy in patients with severe Wiskott-Aldrich syndrome

IMPORTANCE

Wiskott-Aldrich syndrome is a rare primary immunodeficiency associated with severe microthrombocytopenia. Partially HLA antigen-matched allogeneic hematopoietic stem cell (HSC) transplantation is often curative but is associated with significant comorbidity.

OBJECTIVE

To assess the outcomes and safety of autologous HSC gene therapy in Wiskott-Aldrich syndrome.

DESIGN, SETTING, AND PARTICIPANTS

Gene-corrected autologous HSCs were infused in 7 consecutive patients with severe Wiskott-Aldrich syndrome lacking HLA antigen-matched related or unrelated HSC donors (age range, 0.8-15.5 years; mean, 7 years) following myeloablative conditioning. Patients were enrolled in France and England and treated between December 2010 and January 2014. Follow-up of patients in this intermediate analysis ranged from 9 to 42 months.

INTERVENTION

A single infusion of gene-modified CD34+ cells with an advanced lentiviral vector.

MAIN OUTCOMES AND MEASURES

Primary outcomes were improvement at 24 months in eczema, frequency and severity of infections, bleeding tendency, and autoimmunity and reduction in disease-related days of hospitalization. Secondary outcomes were improvement in immunological and hematological characteristics and evidence of safety through vector integration analysis.

RESULTS

Six of the 7 patients were alive at the time of last follow-up (mean and median follow-up, 28 months and 27 months, respectively) and showed sustained clinical benefit. One patient died 7 months after treatment of preexisting drug-resistant herpes virus infection. Eczema and susceptibility to infections resolved in all 6 patients. Autoimmunity improved in 5 of 5 patients. No severe bleeding episodes were recorded after treatment, and at last follow-up, all 6 surviving patients were free of blood product support and thrombopoietic agonists. Hospitalization days were reduced from a median of 25 days during the 2 years before treatment to a median of 0 days during the 2 years after treatment. All 6 surviving patients exhibited high-level, stable engraftment of functionally corrected lymphoid cells. The degree of myeloid cell engraftment and of platelet reconstitution correlated with the dose of gene-corrected cells administered. No evidence of vector-related toxicity was observed clinically or by molecular analysis.

CONCLUSIONS AND RELEVANCE

This study demonstrated the feasibility of the use of gene therapy in patients with Wiskott-Aldrich syndrome. Controlled trials with larger numbers of patients are necessary to assess long-term outcomes and safety.

Adenosine deaminase deficient severe combined immunodeficiency presenting as atypical haemolytic uraemic syndrome

PURPOSE

Adenosine deaminase (ADA) deficiency is a systemic disorder of purine metabolism. Deficiency of the purine salvage enzyme ADA leads to the build-up of the toxic metabolites, deoxyadenosine triphosphate and deoxyadenosine. ADA is ubiquitously expressed in all tissues of the body but most profoundly affects lymphocyte development and function leading to severe combined immunodeficiency (SCID). Unlike most other forms of SCID, ADA deficiency also results in non-immunologic manifestations. Associations between ADA deficiency and sensorineural hearing loss, behavioural abnormalities, non-infectious pulmonary disease and skeletal dysplasia are all recognised, and affect the long term outcome for these patients. Identification of new non-immunological manifestations and clinical presentations of ADA deficiency is essential to allow early optimisation of supportive care.

METHODS AND RESULTS

Here we report four patients with ADA deficiency whose presenting feature was haemolytic uremic syndrome (HUS). 3 of 4 patients were diagnosed with ADA deficiency only after developing HUS, and one diagnosis was made post mortem, after a sibling was diagnosed with SCID. Shiga-toxigenic organisms were not isolated from any of the patients. 2 patients made a good recovery from their HUS with supportive treatment and initiation of PEG-ADA. Both remain well on enzyme replacement with mild or no residual renal impairment.

CONCLUSIONS

Clinicians should be aware of this previously unreported non-immunologic manifestation of ADA deficiency.

Multicenter experience in hematopoietic stem cell transplantation for serious complications of common variable immunodeficiency

BACKGROUND

Common variable immunodeficiency (CVID) is usually well controlled with immunoglobulin substitution and immunomodulatory drugs. A subgroup of patients has a complicated disease course with high mortality. For these patients, investigation of more invasive, potentially curative treatments, such as allogeneic hematopoietic stem cell transplantation (HSCT), is warranted.

OBJECTIVE

We sought to define the outcomes of HSCT for patients with CVID.

METHODS

Retrospective data were collected from 14 centers worldwide on patients with CVID receiving HSCT between 1993 and 2012.

RESULTS

Twenty-five patients with CVID, which was defined according to international criteria, aged 8 to 50 years at the time of transplantation were included in the study. The indication for HSCT was immunologic dysregulation in the majority of patients. The overall survival rate was 48%, and the survival rate for patients undergoing transplantation for lymphoma was 83%. The major causes of death were treatment-refractory graft-versus-host disease accompanied by poor immune reconstitution and infectious complications. Immunoglobulin substitution was stopped in 50% of surviving patients. In 92% of surviving patients, the condition constituting the indication for HSCT resolved.

CONCLUSION

This multicenter study demonstrated that HSCT in patients with CVID was beneficial in most surviving patients; however, there was a high mortality associated with the procedure. Therefore this therapeutic approach should only be considered in carefully selected patients in whom there has been extensive characterization of the immunologic and/or genetic defect underlying the CVID diagnosis. Criteria for patient selection, refinement of the transplantation protocol, and timing are needed for an improved outcome.

Gene-ectomy: gene ablation with CRISPR/Cas9 in human hematopoietic cells

CRISPR/Cas9 has recently been introduced as a gene editing tool and shows considerable promise. In this issue of Cell Stem Cell, Mandal et al. (2014) show efficient CRISPR/Cas9-mediated ablation of the CCR5 and B2M genes in primary human hematopoietic cells, two editing strategies that are potentially translatable into clinical application.

A modified γ-retrovirus vector for X-linked severe combined immunodeficiency

BACKGROUND

In previous clinical trials involving children with X-linked severe combined immunodeficiency (SCID-X1), a Moloney murine leukemia virus-based γ-retrovirus vector expressing interleukin-2 receptor γ-chain (γc) complementary DNA successfully restored immunity in most patients but resulted in vector-induced leukemia through enhancer-mediated mutagenesis in 25% of patients. We assessed the efficacy and safety of a self-inactivating retrovirus for the treatment of SCID-X1.

METHODS

We enrolled nine boys with SCID-X1 in parallel trials in Europe and the United States to evaluate treatment with a self-inactivating (SIN) γ-retrovirus vector containing deletions in viral enhancer sequences expressing γc (SIN-γc).

RESULTS

All patients received bone marrow-derived CD34+ cells transduced with the SIN-γc vector, without preparative conditioning. After 12.1 to 38.7 months of follow-up, eight of the nine children were still alive. One patient died from an overwhelming adenoviral infection before reconstitution with genetically modified T cells. Of the remaining eight patients, seven had recovery of peripheral-blood T cells that were functional and led to resolution of infections. The patients remained healthy thereafter. The kinetics of CD3+ T-cell recovery was not significantly different from that observed in previous trials. Assessment of insertion sites in peripheral blood from patients in the current trial as compared with those in previous trials revealed significantly less clustering of insertion sites within LMO2, MECOM, and other lymphoid proto-oncogenes in our patients.

CONCLUSIONS

This modified γ-retrovirus vector was found to retain efficacy in the treatment of SCID-X1. The long-term effect of this therapy on leukemogenesis remains unknown. (Funded by the National Institutes of Health and others; ClinicalTrials.gov numbers, NCT01410019, NCT01175239, and NCT01129544.).

Primary immunodeficiency diseases: an update on the classification from the international union of immunological societies expert committee for primary immunodeficiency

We report the updated classification of primary immunodeficiencies (PIDs) compiled by the Expert Committee of the International Union of Immunological Societies. In comparison to the previous version, more than 30 new gene defects are reported in this updated version. In addition, we have added a table of acquired defects that are phenocopies of PIDs. For each disorder, the key clinical and laboratory features are provided. This classification is the most up-to-date catalog of all known PIDs and acts as a current reference of the knowledge of these conditions and is an important aid for the molecular diagnosis of patients with these rare diseases.

Reduced-intensity conditioning and HLA-matched haemopoietic stem-cell transplantation in patients with chronic granulomatous disease: a prospective multicentre study

BACKGROUND

In chronic granulomatous disease allogeneic haemopoietic stem-cell transplantation (HSCT) in adolescents and young adults and patients with high-risk disease is complicated by graft-failure, graft-versus-host disease (GVHD), and transplant-related mortality. We examined the effect of a reduced-intensity conditioning regimen designed to enhance myeloid engraftment and reduce organ toxicity in these patients.

METHODS

This prospective study was done at 16 centres in ten countries worldwide. Patients aged 0-40 years with chronic granulomatous disease were assessed and enrolled at the discretion of individual centres. Reduced-intensity conditioning consisted of high-dose fludarabine (30 mg/m(2) [infants <9 kg 1·2 mg/kg]; one dose per day on days -8 to -3), serotherapy (anti-thymocyte globulin [10 mg/kg, one dose per day on days -4 to -1; or thymoglobuline 2·5 mg/kg, one dose per day on days -5 to -3]; or low-dose alemtuzumab [<1 mg/kg on days -8 to -6]), and low-dose (50-72% of myeloablative dose) or targeted busulfan administration (recommended cumulative area under the curve: 45-65 mg/L × h). Busulfan was administered mainly intravenously and exceptionally orally from days -5 to -3. Intravenous busulfan was dosed according to weight-based recommendations and was administered in most centres (ten) twice daily over 4 h. Unmanipulated bone marrow or peripheral blood stem cells from HLA-matched related-donors or HLA-9/10 or HLA-10/10 matched unrelated-donors were infused. The primary endpoints were overall survival and event-free survival (EFS), probabilities of overall survival and EFS at 2 years, incidence of acute and chronic GVHD, achievement of at least 90% myeloid donor chimerism, and incidence of graft failure after at least 6 months of follow-up.

FINDINGS

56 patients (median age 12·7 years; IQR 6·8-17·3) with chronic granulomatous disease were enrolled from June 15, 2003, to Dec 15, 2012. 42 patients (75%) had high-risk features (ie, intractable infections and autoinflammation), 25 (45%) were adolescents and young adults (age 14-39 years). 21 HLA-matched related-donor and 35 HLA-matched unrelated-donor transplants were done. Median time to engraftment was 19 days (IQR 16-22) for neutrophils and 21 days (IQR 16-25) for platelets. At median follow-up of 21 months (IQR 13-35) overall survival was 93% (52 of 56) and EFS was 89% (50 of 56). The 2-year probability of overall survival was 96% (95% CI 86·46-99·09) and of EFS was 91% (79·78-96·17). Graft-failure occurred in 5% (three of 56) of patients. The cumulative incidence of acute GVHD of grade III-IV was 4% (two of 56) and of chronic graft-versus-host disease was 7% (four of 56). Stable (≥90%) myeloid donor chimerism was documented in 52 (93%) surviving patients.

INTERPRETATION

This reduced-intensity conditioning regimen is safe and efficacious in high-risk patients with chronic granulomatous disease.

FUNDING

None.

Pathogenesis and diagnosis of X-linked lymphoproliferative disease

X-linked lymphoproliferative syndrome (XLP) is a rare, often fatal, primary immunodeficiency that has profound and damaging effects on the immune system of affected individuals. It is characterized by a dysregulated immune response, most commonly to Epstein-Barr viral infection. The defective gene in this syndrome has been identified as SAP-SLAM (signaling lymphocyte activation molecule)-associated protein. It is an adapter molecule that is required for appropriate function of the SLAM-related receptors. There is now a greater understanding of the molecular associations and cellular pathogenesis of SAP and this review will summarize the most recent findings. Clinically, XLP may be difficult to diagnose as a result of its varied clinical phenotype, and protein and genetic assays are currently used to make a definitive diagnosis. With the advances in gene analysis and genomics technology, it is likely that better and more rapid diagnostic techniques will become available.

Hematopoietic SCT in Europe: data and trends in 2011

In all, 651 from 680 centers in 48 countries reported 35 660 hematopoietic SCT (HSCT) in 32 075 patients (13 470 allogeneic (42%), 18 605 autologous (58%)) to the 2011 survey. Main indications were: leukemias; 10 113 (32%; 94% allogeneic); lymphoid neoplasias; non-Hodgkin's lymphoma, Hodgkin's lymphoma, plasma cell disorders; 18 433 (57%; 12% allogeneic); solid tumours; 1573 (5%; 5% allogeneic); and non-malignant disorders; 1830 (6%; 92% allogeneic). There were more unrelated donors than HLA identical sibling donors (54% versus 39%); proportion of peripheral blood as stem cell source was 99% for autologous and 73% for allogeneic HSCT. Cord blood was only used in allogeneic transplants (6% of total). In the past 10 years, the overall number of transplants has increased by 53%. Allogeneic HSCT have doubled (from 7272 to 14 549) while, autologous have increased by 32% and continue to increase by about 1100 HSCT per year since 2001. In the past 2 years, an increase of >2000 HSCT per year was seen. Transplant activity is shown by team size. For allogeneic HSCT, we show use of reduced-intensity conditioning versus myeloablative conditioning across Europe and use of post-transplant donor lymphocyte infusions with considerable variation across different countries.

Current progress on gene therapy for primary immunodeficiencies

Primary immunodeficiencies have played a major role in the development of gene therapy for monogenic diseases of the bone marrow. The last decade has seen convincing evidence of long-term disease correction as a result of ex vivo viral vector-mediated gene transfer into autologous haematopoietic stem cells. The success of these early studies has been balanced by the development of vector-related insertional mutagenic events. More recently the use of alternative vector designs with self-inactivating designs, which have an improved safety profile has led to the initiation of a wave of new studies that are showing early signs of efficacy. The ongoing development of safer vector platforms and gene-correction technologies together with improvements in cell-transduction techniques and optimised conditioning regimes is likely to make gene therapy amenable for a greater number of PIDs. If long-term efficacy and safety are shown, gene therapy will become a standard treatment option for specific forms of PID.