Immune cytopenia post-cord transplant in Hurler syndrome is a forme fruste of graft rejection.

Umbilical cord blood (UCB) is the preferred donor cell source for children with Hurler syndrome undergoing transplant, and its use has been associated with improved "engrafted survival" rates. However, as in other pediatric recipients of UCB transplants for nonmalignant disease, immune-mediated cytopenia (IMC) is a significant complication. This article describes 8 episodes of IMC in 36 patients with Hurler syndrome undergoing UCB transplant. The incidence of IMC was increased in those with a higher preconditioning absolute lymphocyte count and in those conditioned with fludarabine-containing regimens rather than cyclophosphamide, and it included red cell alloantibodies directed at cord blood group antigens that are novel to the recipient. In several cases, IMC was a precursor to immune-mediated complete graft rejection. We describe IMC as part of a spectrum of graft rejection by a residual competent host immune system and a forme fruste of complete graft rejection.

Adeno-Associated Virus Vectors and Stem Cells: Friends or Foes?

The infusion of healthy stem cells into a patient-termed "stem-cell therapy"-has shown great promise for the treatment of genetic and non-genetic diseases, including mucopolysaccharidosis type 1, Parkinson's disease, multiple sclerosis, numerous immunodeficiency disorders, and aplastic anemia. Stem cells for cell therapy can be collected from the patient (autologous) or collected from another "healthy" individual (allogeneic). The use of allogenic stem cells is accompanied with the potentially fatal risk that the transplanted donor T cells will reject the patient's cells-a process termed "graft-versus-host disease." Therefore, the use of autologous stem cells is preferred, at least from the immunological perspective. However, an obvious drawback is that inherently as "self," they contain the disease mutation. As such, autologous cells for use in cell therapies often require genetic "correction" (i.e., gene addition or editing) prior to cell infusion and therefore the requirement for some form of nucleic acid delivery, which sets the stage for the AAV controversy discussed herein. Despite being the most clinically applied gene delivery context to date, unlike other more concerning integrating and non-integrating vectors such as retroviruses and adenovirus, those based on adeno-associated virus (AAV) have not been employed in the clinic. Furthermore, published data regarding AAV vector transduction of stem cells are inconsistent in regards to vector transduction efficiency, while the pendulum swings far in the other direction with demonstrations of AAV vector-induced toxicity in undifferentiated cells. The variation present in the literature examining the transduction efficiency of AAV vectors in stem cells may be due to numerous factors, including inconsistencies in stem-cell collection, cell culture, vector preparation, and/or transduction conditions. This review summarizes the controversy surrounding AAV vector transduction of stem cells, hopefully setting the stage for future elucidation and eventual therapeutic applications.

Effect of anti-laronidase antibodies on efficacy and safety of laronidase enzyme replacement therapy for MPS I: A comprehensive meta-analysis of pooled data from multiple studies.

Enzyme replacement therapy (ERT) with laronidase has an important role in the treatment of patients with mucopolysaccharidosis type I (MPS I). Laronidase is safe and has demonstrated effectiveness in terms of stabilizing or improving conventional clinical and laboratory markers of the disease. However, like most ERTs, laronidase produces an anti-drug IgG antibody response in more than 90% of patients during the first few months of treatment. Preclinical data from the MPS I canine model suggest that anti-drug antibodies (ADA) impair enzyme uptake in target tissues. In patients, the effects on tissue glycosaminoglycan (GAG) clearance are difficult to assess directly but data from clinical studies have suggested an association between ADA and both a reduced pharmacodynamic response and hypersensitivity reactions. This comprehensive meta-analysis of pooled data from patients in three clinical studies of laronidase (including one study with an extension) was undertaken to provide a more robust assessment of the relationship between the ADA response to laronidase, clinical and laboratory markers of MPS I, and hypersensitivity reactions. The meta-analysis demonstrated an inverse relationship between the ADA response and the percent reduction in urinary GAG (uGAG) levels. However, no relationships between the ADA response and changes in percent predicted forced vital capacity and six-minute walk test were seen. The study also re-assayed stored serum samples from the original trials with a novel method to determine the inhibitory effect of ADA. Patients with higher ADA exposure over time were found to have higher inhibition of enzyme uptake into cells. High ADA exposure can result in a commensurate level of enzyme uptake inhibition that decreases the pharmacodynamic effect of the exogenously administered therapeutic enzyme, but with no clear effect on clinical efficacy.

Biomarker responses correlate with antibody status in mucopolysaccharidosis type I patients on long-term enzyme replacement therapy.

BACKGROUND: Antibody formation can interfere with effects of enzyme replacement therapy (ERT) in lysosomal storage diseases. Biomarkers are used as surrogate marker for disease burden in MPS I, but large systematic studies evaluating the response of biomarkers to ERT are lacking. We, for the first time, investigated the response of a large panel of biomarkers to long term ERT in MPS I patients and correlate these responses with antibody formation and antibody mediated cellular uptake inhibition. METHODS: A total of 428 blood and urine samples were collected during long-term ERT in 24 MPS I patients and an extensive set of biomarkers was analyzed, including heparan sulfate (HS) and dermatan sulfate (DS) derived disaccharides; total urinary GAGs (DMBu); urinary DS:CS ratio and serum heparin co-factor II thrombin levels (HCII-T). IgG antibody titers and the effect of antibodies on cellular uptake of the enzyme were determined for 23 patients. RESULTS: Median follow-up was 2.3 years. In blood, HS reached normal levels more frequently than DS (50% vs 12.5%, p=0.001), though normalization could take several years. DMBu normalized more rapidly than disaccharide levels in urine (p=0.02). Nineteen patients (83%) developed high antibody titers. Significant antibody-mediated inhibition of enzyme uptake was observed in 8 patients (35%), and this correlated strongly with a poorer biomarker response for HS and DS in blood and urine as well as for DMBu, DS:CS-ratio and HCII-T (all p<0.006). CONCLUSIONS: This study shows that, despite a response of all studied biomarkers to initiation of ERT, some biomarkers were less responsive than others, suggesting residual disease activity. In addition, the correlation of cellular uptake inhibitory antibodies with a decreased biomarker response demonstrates a functional role of these antibodies which may have important clinical consequences.

Unrelated CD3/CD19-depleted peripheral stem cell transplantation for Hurler syndrome.

For patients with mucopolysaccharidosis type IH (MPS1-H; Hurler syndrome), early allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of choice. One boy and one girl aged 20.5 and 22 months, respectively, with MPS1-H received a conditioning regimen consisting of thiotepa, fludarabine, treosulfan, and ATG. Grafts were peripheral blood stem cells from unrelated donors (10/12 and 11/11 matched), that were manipulated by CD3/CD19 depletion and contained 20.3 and 28.2 × 10(6) CD34+ cells/kg body weight, respectively. Both patients achieved stable hematopoietic engraftment and stable donor chimerism. Neither acute or chronic graft-versus-host disease (GVHD) nor other severe transplant-related complications occurred. At a follow-up of 48 and 37 months, both patients are alive and well with normal levels of α-L-iduronidase and have made major neurodevelopmental progress. Treosulfan-based conditioning offers the advantage of reduced toxicity; the use of unrelated CD3/CD19-depleted peripheral stem cell grafts allows transfusion of high CD34+ cell numbers together with a "tailored" number of CD3+ cells as well as engraftment facilitating cells in order to achieve rapid hematopoietic engraftment while reducing the risk of graft rejection and GVHD. This regimen might be an additional option when unrelated donor HSCT is considered for a patient with MPS1-H.

Mesenchymal stem cells do not prevent antibody responses against human α-L-iduronidase when used to treat mucopolysaccharidosis type I.

Mucopolysaccharidosis type I (MPSI) is an autosomal recessive disease that leads to systemic lysosomal storage, which is caused by the absence of α-L-iduronidase (IDUA). Enzyme replacement therapy is recognized as the best therapeutic option for MPSI; however, high titers of anti-IDUA antibody have frequently been observed. Due to the immunosuppressant properties of MSC, we hypothesized that MSC modified with the IDUA gene would be able to produce IDUA for a long period of time. Sleeping Beauty transposon vectors were used to modify MSC because these are basically less-immunogenic plasmids. For cell transplantation, 4×10(6) MSC-KO-IDUA cells (MSC from KO mice modified with IDUA) were injected into the peritoneum of KO-mice three times over intervals of more than one month. The total IDUA activities from MSC-KO-IDUA before cell transplantation were 9.6, 120 and 179 U for the first, second and third injections, respectively. Only after the second cell transplantation, more than one unit of IDUA activity was detected in the blood of 3 mice for 2 days. After the third cell transplantation, a high titer of anti-IDUA antibody was detected in all of the treated mice. Anti-IDUA antibody response was also detected in C57Bl/6 mice treated with MSC-WT-IDUA. The antibody titers were high and comparable to mice that were immunized by electroporation. MSC-transplanted mice had high levels of TNF-alpha and infiltrates in the renal glomeruli. The spreading of the transplanted MSC into the peritoneum of other organs was confirmed after injection of 111In-labeled MSC. In conclusion, the antibody response against IDUA could not be avoided by MSC. On the contrary, these cells worked as an adjuvant that favored IDUA immunization. Therefore, the humoral immunosuppressant property of MSC is questionable and indicates the danger of using MSC as a source for the production of exogenous proteins to treat monogenic diseases.

Osteoimmunology in mucopolysaccharidoses type I, II, VI and VII. Immunological regulation of the osteoarticular system in the course of metabolic inflammation.

BACKGROUND: Mucopolysaccharidoses (MPSs) are rare genetic diseases caused by a deficient activity of one of the lysosomal enzymes involved in the glycosaminoglycan (GAG) breakdown pathway. These metabolic blocks lead to the accumulation of GAGs in various organs and tissues, resulting in a multisystemic clinical picture. The pathological GAG accumulation begins a cascade of interrelated responses: metabolic, inflammatory and immunological with systemic effects. Metabolic inflammation, secondary to GAG storage, is a significant cause of osteoarticular symptoms in MPS disorders. OBJECTIVE AND METHOD: The aim of this review is to present recent progress in the understanding of the role of inflammatory and immune processes in the pathophysiology of osteoarticular symptoms in MPS disorders and potential therapeutic interventions based on published reports in MPS patients and studies in animal models. RESULTS AND CONCLUSIONS: The immune and skeletal systems have a number of shared regulatory molecules and many relationships between bone disorders and aberrant immune responses in MPS can be explained by osteoimmunology. The treatment options currently available are not sufficiently effective in the prevention, inhibition and treatment of osteoarticular symptoms in MPS disease. A lot can be learnt from interactions between skeletal and immune systems in autoimmune diseases such as rheumatoid arthritis (RA) and similarities between RA and MPS point to the possibility of using the experience with RA in the treatment of MPS in the future. The use of different anti-inflammatory drugs requires further study, but it seems to be an important direction for new therapeutic options for MPS patients.

Characterisation of the T cell and dendritic cell repertoire in a murine model of mucopolysaccharidosis I (MPS I).

BACKGROUND: Mucopolysaccharidosis I (MPS I) is a metabolic disorder caused by α-L-Iduronidase (IDUA) deficiency, resulting in lysosomal accumulation of heparan (HS) and dermatan sulphate (DS). This has been reported in microglia, yet currently the effect of IDUA deficiency on T cells and dendritic cells (DC) and their functionality in disease pathogenesis remains unclear. METHODS: Peripheral blood was collected from 3 month old C57BL/6 MPS I (n = 11) and wildtype (WT) (n = 6) mice. T cell and DC phenotype and functional characteristics were identified by flow cytometry. RESULTS: MPS I mice exhibited a reduction in DC (p = <0.001) along with CD8+ cytotoxic (p = 0.01) and CD4+ T helper (p = 0.032) cells, compared to WT controls. MPS I DC displayed a significant decrease in cell surface CD123 (p = 0.02) and CD86 (p = 0.006) expression. Furthermore, CD45RB expression was significantly reduced on T helper cells in the MPS I population (p = 0.019). CONCLUSION: We report a reduction in circulating DC and T cells in the MPS I mouse; indicative of adaptive immune dysfunction. DC reduction may occur in response to down-regulation of the IL-3 receptor (CD123), necessary for DC survival. We also report down-regulation of cell surface CD86, a molecule required for T cell co-stimulation. T helper cell down-regulation of CD45RB is redolent of an anti-inflammatory phenotype with poor proliferative capacity. The definitive causes of our findings and the consequences and role that these findings play in the pathogenesis of MPS are unclear, but may be in response to lysosomal storage of unmetabolized HS and DS.

Specific antibody titer alters the effectiveness of intrathecal enzyme replacement therapy in canine mucopolysaccharidosis I.

Intrathecal enzyme replacement therapy is an experimental option to treat central nervous system disease due to lysosomal storage. Previous work shows that MPS I dogs receiving enzyme replacement with recombinant human alpha-l-iduronidase into the cisterna magna showed normal brain glycosaminoglycan (GAG) storage after three or four doses. We analyzed MPS I dogs that received intrathecal enzyme in a previous study using an assay that detects only pathologic GAG (pGAG). To quantify pGAG in MPS I, the assay measures only those GAG which display terminal iduronic acid residues on their non-reducing ends. Mean cortical brain pGAG in six untreated MPS I dogs was 60.9±5.93 pmol/mg wet weight, and was 3.83±2.64 in eight normal or unaffected carrier animals (p<0.001). Intrathecal enzyme replacement significantly reduced pGAG storage in all treated animals. Dogs with low anti-iduronidase antibody titers showed normalization or near-normalization of pGAG in the brain (mean 8.17±6.17, n=7), while in dogs with higher titers, pGAG was reduced but not normal (mean 21.9±6.02, n=4). Intrathecal enzyme therapy also led to a mean 69% reduction in cerebrospinal fluid pGAG (from 83.8±26.3 to 27.2±12.3 pmol/ml CSF). The effect was measurable one month after each dose and did not differ with antibody titer. Prevention of the immune response to enzyme may improve the efficacy of intrathecal enzyme replacement therapy for brain disease due to MPS I.

Hematopoietic stem cell transplantation improves the high incidence of neutralizing allo-antibodies observed in Hurler's syndrome after pharmacological enzyme replacement therapy.

BACKGROUND: Mucopolysaccharidosis type I is caused by deficiency of α-L-iduronidase. Currently available treatment options include an allogeneic hematopoietic stem cell transplant and enzyme replacement therapy. Exogenous enzyme therapy appears promising but the benefits may be attenuated, at least in some patients, by the development of an immune response to the delivered enzyme. The incidence and impact of alloimmune responses in these patients remain unknown. DESIGN AND METHODS: We developed an immunoglobulin G enzyme-linked immunosorbent assay as well as in vitro catalytic enzyme inhibition and cellular uptake inhibition assays and quantified enzyme inhibition by allo-antibodies. We determined the impact of these antibodies in eight patients who received enzyme therapy before and during hematopoietic stem cell transplantation. In addition, 20 patients who had previously received an allogeneic stem cell transplant were tested to evaluate this treatment as an immune tolerance induction mechanism. RESULTS: High titer immune responses were seen in 87.5% (7/8) patients following exposure to α-L-iduronidase. These patients exhibited catalytic enzyme inhibition (5/8), uptake inhibition of catalytically active enzyme (6/8) or both (4/8). High antibody titers generally preceded elevation of previously described biomarkers of disease progression. The median time to development of immune tolerance was 101 days (range, 26-137) after transplantation. All 20 patients, including those with mixed chimerism (22%), tested 1 year after transplantation were tolerized despite normal enzyme levels. CONCLUSIONS: We found a high incidence of neutralizing antibodies in patients with mucopolysaccharidosis type I treated with enzyme replacement therapy. We also found that allogeneic hematopoietic stem cell transplantation was an effective and rapid immune tolerance induction strategy.

Minicircle DNA-based gene therapy coupled with immune modulation permits long-term expression of α-L-iduronidase in mice with mucopolysaccharidosis type I.

Mucopolysaccharidosis type I (MPS I) is a lysosomal storage disease characterized by mutations to the α-L-iduronidase (IDUA) gene resulting in inactivation of the IDUA enzyme. The loss of IDUA protein results in the progressive accumulation of glycosaminoglycans within the lysosomes resulting in severe, multi-organ system pathology. Gene replacement strategies have relied on the use of viral or nonviral gene delivery systems. Drawbacks to these include laborious production procedures, poor efficacy due to plasmid-borne gene silencing, and the risk of insertional mutagenesis. This report demonstrates the efficacy of a nonintegrating, minicircle (MC) DNA vector that is resistant to epigenetic gene silencing in vivo. To achieve sustained expression of the immunogenic IDUA protein we investigated the use of a tissue-specific promoter in conjunction with microRNA target sequences. The inclusion of microRNA target sequences resulted in a slight improvement in long-term expression compared to their absence. However, immune modulation by costimulatory blockade was required and permitted for IDUA expression in MPS I mice that resulted in the biochemical correction of pathology in all of the organs analyzed. MC gene delivery combined with costimulatory pathway blockade maximizes safety, efficacy, and sustained gene expression and is a new approach in the treatment of lysosomal storage disease.

Development and maturation of invariant NKT cells in the presence of lysosomal engulfment.

A defect in invariant NKT (iNKT) cell selection was hypothesized in lysosomal storage disorders (LSD). Accumulation of glycosphingolipids (GSL) in LSD could influence lipid loading and/or presentation causing entrapment of endogenous ligand(s) within storage bodies or competition of the selecting ligand(s) by stored lipids for CD1d binding. However, when we analyzed the iNKT cell compartment in newly tested LSD animal models that accumulate GSL, glycoaminoglycans or both, we observed a defective iNKT cell selection only in animals affected by multiple sulfatase deficiency, in which a generalized aberrant T-cell development, rather than a pure iNKT defect, was present. Mice with single lysosomal enzyme deficiencies had normal iNKT cell development. Thus, GSL/glycoaminoglycans storage and lysosomal engulfment are not sufficient for affecting iNKT cell development. Rather, lipid ligand(s) or storage compounds, which are affected in those LSD lacking mature iNKT cells, might indeed be relevant for iNKT cell selection.

A dose-optimization trial of laronidase (Aldurazyme) in patients with mucopolysaccharidosis I.

Recombinant human alpha-l-iduronidase (Aldurazyme), laronidase) is approved as an enzyme replacement therapy to treat the lysosomal storage disorder, mucopolysaccharidosis type I (MPS I) at a dose of 0.58 mg/kg by once-weekly intravenous infusion. To assess whether alternate dosing regimens might provide a better reduction in lysosomal storage, a 26-week, randomized, open-label, multinational dose-optimization trial was conducted. The pharmacodynamic effect and safety of the approved laronidase dose was compared to three alternative regimens (1.2mg/kg every 2 weeks; 1.2mg/kg every week; 1.8 mg/kg every 2 weeks) among 33 MPS I patients. The four treatment regimens showed no significant differences in the reduction of urinary glycosaminoglycan excretion or liver volume. Laronidase had an acceptable safety profile in all dose regimen groups. Infusion-associated reactions were the most common drug-related adverse events across dose regimens (by patient incidence), and included pyrexia (21%), vomiting (15%), rash (15%), and urticaria (12%). Patients in the approved dose group had the lowest incidence of drug-related adverse events (38% vs. 63-75%) and infusion-associated reactions (25% vs. 25-63%). There was one death: a patient with acute bronchitis died of respiratory failure 6h after completing the first laronidase infusion. The approved 0.58 mg/kg/week laronidase dose regimen provided near-maximal reductions in glycosaminoglycan storage and the best benefit-to-risk ratio. The 1.2mg/kg every 2 weeks regimen may be an acceptable alternative for patients with difficulty receiving weekly infusions, but the long-term effects of this regimen are unknown.

Immunology of neonatal gene transfer.

Gene therapy could result in the permanent correction or amelioration of the clinical manifestations of many genetic diseases. However, immune responses to the therapeutic protein pose a significant hurdle for successful gene therapy. Problematic immune responses can include the development of a cytotoxic T lymphocyte (CTL) response that results in the destruction of genetically-modified cells and/or the formation of antibodies directed against the therapeutic protein. One approach to avoid an immune response is to perform gene therapy in newborns, which takes advantage of the fact that the immune system is relatively immature at birth. This approach has been highly effective in mice, and has resulted in stable expression without antibody formation for proteins that are highly immunogenic after transfer to adults. High levels of expression after neonatal gene therapy were more effective at inducing tolerance than low levels of expression in mice, which suggests that high antigen levels are more efficient at inducing tolerance. A criticism of this approach is that the murine immune system is less mature at birth than the immune systems of larger animals. Indeed, neonatal gene therapy to cats with mucopolysaccharidosis I resulted in a CTL response that destroyed expressing cells. Nevertheless, the immune system was still relatively immature, as transient administration of a single immunosuppressive agent at the time of neonatal gene therapy resulted in stable expression. Neonatal administration can reduce, but not eliminate, immune responses after gene therapy.

Pre-stem cell transplantation enzyme replacement therapy in Hurler syndrome does not lead to significant antibody formation or delayed recovery of the endogenous enzyme post-transplant: a case report.

Combined enzyme replacement therapy (ERT) and stem cell transplant (SCT) were done for a two year old boy with severe Hurler syndrome(HS) with the aim to decrease transplant related complications. He tolerated both the procedures well without any major complications. Urine glycosaminoglycans (GAGs) decreased post-transplant and child has improved clinically and neurologically. Insignificant titers of the anti-iduronidase antibodies which developed post-transplant did not affect the transplant outcome or the endogenous recovery of the alpha-L-iduronidase enzyme.

Improvements in mucopolysaccharidosis I mice after adult retroviral vector-mediated gene therapy with immunomodulation.

Mucopolysaccharidosis I (MPS I) is caused by deficient alpha-L-iduronidase (IDUA) activity and results in the accumulation of glycosaminoglycans and multisystemic disease. Gene therapy could program cells to secrete mannose 6-phosphate-modified IDUA, and enzyme in blood could be taken up by other cells. Neonatal retroviral vector (RV)-mediated gene therapy has been shown to reduce the manifestations of murine MPS I; however, intravenous injection of RV into adults was ineffective owing to a cytotoxic T lymphocyte (CTL) response against transduced cells. In this study, prolonged inhibition of CD28 signaling with CTLA4-Ig, or transient administration of CTLA4-Ig with an anti-CD40 ligand antibody or with an anti-CD4 antibody, resulted in stable expression in most mice that received RV as adults. Mice with stable expression had 81 +/- 41U/ml IDUA activity in serum. This resulted in reductions in bone disease, improvements in hearing and vision, and reductions in biochemical and pathological evidence of lysosomal storage in most organs. Improvements in brain were likely due to diffusion of enzyme from blood. However, aortic disease was refractory to treatment. This demonstrates that most manifestations of MPS I can be prevented using adult gene therapy if an immune response is blocked.

Rheumatologic aspects of lysosomal storage diseases.

Lysosomal storage diseases are rare metabolic disorders, some of which can now be treated using enzyme replacement therapies. Because the time point of treatment initiation significantly influences the outcome in Gaucher disease, Fabry disease, and mucopolysaccharidosis type I, early diagnosis is of utmost importance. All three disorders can present with musculoskeletal symptoms in early stages, therefore, the rheumatologist may be the first to be contacted by these patients. Here, we present three characteristic lysosomal storage disease cases to increase awareness in the rheumatological community of the typical symptom constellations associated with these rare but treatable disorders.

Mucopolysaccharidosis I cats mount a cytotoxic T lymphocyte response after neonatal gene therapy that can be blocked with CTLA4-Ig.

Although gene therapy has reduced manifestations of genetic diseases, immune responses can abrogate the effect. One approach to inducing tolerance is to perform gene transfer in newborns when the immune system is immature. We demonstrate here that the dose of retroviral vector (RV) is important in mice, as mucopolysaccharidosis I (MPS I) mice that received neonatal intravenous gene therapy with a high dose of a canine alpha-L-iduronidase (cIDUA)-expressing RV had stable expression, while those that received a low dose did not. It was unclear, however, if neonatal transfer with any dose could induce tolerance in large animals. Therefore, newborn MPS I cats were injected intravenously with the RV expressing cIDUA. Although this resulted in high serum IDUA activity due to secretion by transduced cells, expression fell due to a CTL response. Cats that transiently received the immunosuppressive agent CTLA4-Ig did not develop a CTL response. In contrast, MPS I dogs, which can respond immunologically to canine IDUA, had stable serum IDUA activity after neonatal gene therapy. We conclude that cats, but not dogs, mount a potent CTL response to canine IDUA after neonatal gene therapy, which can be prevented with transient CTLA4-Ig.

Common antigenicity for two glycosidases.

Enzyme replacement therapy (ERT) has proven to be an effective therapy for some lysosomal storage disorder (LSD) patients. A potential complication during ERT is the generation of an immune response against the replacement protein. We have investigated the antigenicity of two distantly related glycosidases, alpha-glucosidase (Pompe disease or glycogen storage disease type II, GSD II), and alpha-L-iduronidase (Hurler syndrome, mucopolysaccharidosis type I, MPS I). The linear sequence epitope reactivity of affinity purified polyclonal antibodies to recombinant human alpha-glucosidase and alpha-L-iduronidase was defined, to both glycosidases. The polyclonal antibodies exhibited some cross-reactive epitopes on the two proteins. Moreover, a monoclonal antibody to the active site of alpha-glucosidase showed cross-reactivity with a catalytic structural element of alpha-L-iduronidase. In a previous study, in MPS I patients who developed an immune response to ERT, this same site on alpha-L-iduronidase was highly antigenic and the last to tolerise following repeated enzyme infusions. We conclude that glycosidases can exhibit cross-reactive epitopes, and infer that this may relate to common structural elements associated with their active sites.

Enzyme replacement therapy in mucopolysaccharidosis type I.

UNLABELLED: Mucopolysaccharidosis (MPS) type I is a lysosomal storage disorder caused by deficiency of the enzyme alpha-L-iduronidase (IDUA), which presents with a wide spectrum of phenotypes. Recently, enzyme replacement therapy (ERT) became available for patients with MPS I and has been demonstrated to be safe and effective in patients with the milder Hurler-Scheie and Scheie phenotypes. Treatment for 26 weeks with recombinant human IDUA (laronidase) has been shown to significantly increase the percentage of predicted normal forced vital capacity and the distance walked in the 6-minute walk test. There was also a clear reduction in the volume of the liver and the levels of urinary glycosaminoglycan excretion. The drug was generally well tolerated. There were no drug-related severe adverse events, and although the majority of patients developed IgG antibodies, these declined by the end of the study. CONCLUSION: ERT seems to be a very promising new therapeutic regimen for patients with MPS I, especially for those with the less severe variants. However, as laronidase does not cross the blood-brain barrier it will probably not influence the central nervous manifestations in the most severely affected patients with the Hurler phenotype, although it may improve general lung and heart function, making bone marrow transplantation easier to tolerate.