Expression of glycosylphosphatidylinositol-anchored CD59 on target cells enhances human NK cell-mediated cytotoxicity

NK cell-mediated cytotoxicity of target cells is the result of a balance between the activating and inhibitory signals provided by their respective ligand-receptor interactions. In our current study, we have investigated the significance of CD59 on human target cells in modulating this process. A range of CD59 site-specific Abs were used in NK cytotoxicity blocking studies against the CD59-expressing K562 target cell line. Significantly reduced cytotoxicity was observed in the presence of Abs previously shown to lack blocking capacity for C-mediated lysis. We investigated the consequences for alternative membrane attachment modalities, namely bis-myristoylated-peptidyl (BiMP) and GPI anchoring, on CD59-negative U937 cells. Expression of GPI-anchored CD59 either via transfection or incorporation rendered U937 targets more susceptible to NK cytotoxicity, whereas incorporation of CD59 via a BiMP anchor to similar levels did not alter susceptibility to NK cytotoxicity. Localization of both BiMP- and GPI-anchored CD59 proteins was shown to be within the lipid raft microdomain. A role for the GPI anchor and independence from glycosylation status was confirmed by expression of transmembrane-anchored CD59 or unglycosylated CD59 and by testing in NK cytotoxicity assays. To investigate mechanisms, we compared the signaling capacity of the various forms of expressed and incorporated CD59 following Ab cross-linking in calcium flux assays. GPI-anchored CD59, with or without glycosylation, mediated activation events, whereas CD59 forms lacking the GPI anchor did not. The data show that the increased susceptibility of target cells expressing CD59 to NK cytotoxicity requires GPI anchor-mediating signaling events, likely mediated by interactions between GPI-anchored CD59 on targets and NK receptors.

Removal of the glycosylphosphatidylinositol anchor from PrP(Sc) by cathepsin D does not reduce prion infectivity

According to the protein-only hypothesis of prion propagation, prions are composed principally of PrP(Sc), an abnormal conformational isoform of the prion protein, which, like its normal cellular precursor (PrP(C)), has a GPI (glycosylphosphatidylinositol) anchor at the C-terminus. To date, elucidating the role of this anchor on the infectivity of prion preparations has not been possible because of the resistance of PrP(Sc) to the activity of PI-PLC (phosphoinositide-specific phospholipase C), an enzyme which removes the GPI moiety from PrP(C). Removal of the GPI anchor from PrP(Sc) requires denaturation before treatment with PI-PLC, a process that also abolishes infectivity. To circumvent this problem, we have removed the GPI anchor from PrP(Sc) in RML (Rocky Mountain Laboratory)-prion-infected murine brain homogenate using the aspartic endoprotease cathepsin D. This enzyme eliminates a short sequence at the C-terminal end of PrP to which the GPI anchor is attached. We found that this modification has no effect (i) on an in vitro amplification model of PrP(Sc), (ii) on the prion titre as determined by a highly sensitive N2a-cell based bioassay, or (iii) in a mouse bioassay. These results show that the GPI anchor has little or no role in either the propagation of PrP(Sc) or on prion infectivity.

Immunoselection by natural killer cells of PIGA mutant cells missing stress-inducible ULBP

The mechanism by which paroxysmal nocturnal hemoglobinuria (PNH) clones expand is unknown. PNH clones harbor PIGA mutations and do not synthesize glycosylphosphatidylinositol (GPI), resulting in deficiency of GPI-linked membrane proteins. GPI-deficient blood cells often expand in patients with aplastic anemia who sustain immune-mediated marrow injury putatively induced by cytotoxic cells, hence suggesting that the injury allows PNH clones to expand selectively. We previously reported that leukemic K562 cells preferentially survived natural killer (NK) cell-mediated cytotoxicity in vitro when they acquired PIGA mutations. We herein show that the survival is ascribable to the deficiency of stress-inducible GPI-linked membrane proteins ULBP1 and ULBP2, which activate NK and T cells. The ULBPs were detected on GPI-expressing but not on GPI-deficient K562 cells. In the presence of antibodies to either the ULBPs or their receptor NKG2D on NK cells, GPI-expressing cells were as less NK sensitive as GPI-deficient cells. NK cells therefore spared ULBP-deficient cells in vitro. The ULBPs were identified only on GPI-expressing blood cells of a proportion of patients with PNH but none of healthy individuals. Granulocytes of the patients partly underwent killing by autologous cytotoxic cells, implying ULBP-associated blood cell injury. In this setting, the lack of ULBPs may allow immunoselection of PNH clones.

Erythrocyte surface glycosylphosphatidyl inositol anchored receptor for the malaria parasite

Parasitophorous vacuole formation is a critical step for the successful invasion of host erythrocytes by the malaria parasite. Rhoptry proteins are believed to have essential roles in vacuole formation, although their biological roles are poorly understood. To understand the molecular interactions between parasite rhoptry proteins and the erythrocyte during invasion, we have characterized the binding specificity of the high molecular mass rhoptry protein (RhopH) complex to erythrocytes using the rodent malaria parasite, Plasmodium yoelii. RhopH complex binding to erythrocytes was species-specific, observed with mouse but not rabbit or human erythrocytes. Binding is abolished following treatment of erythrocytes with trypsin or chymotrypsin. Because host cell cholesterol-rich membrane domains are recruited into the nascent parasitophorous vacuole, we evaluated a possible role of RhopH complex binding to the cholesterol-rich membrane domain-associated glycosylphosphatidyl inositol (GPI)-anchored protein. Using chimeric mice harboring GPI-deficient erythrocytes, RhopH complex binding to GPI-deficient mouse erythrocytes was undetectable, indicating involvement of GPI-anchored protein in PyRhopH complex binding. Furthermore, a significant reduction of P. yoelii parasite infection of GPI-deficient erythrocytes was observed in vivo, probably due to inefficient invasion. We conclude that the major erythrocyte receptor for PyRhopH complex is a protein attached to the erythrocyte surface via GPI-anchor and that GPI-deficient erythrocytes are resistant to P. yoelii invasion.

PIG-A mutations in normal hematopoiesis

Paroxysmal nocturnal hemoglobinuria (PNH) is caused by phosphatidylinositol glycan-class A (PIG-A) mutations in hematopoietic stem cells (HSCs). PIG-A mutations have been found in granulocytes from most healthy individuals, suggesting that these spontaneous PIG-A mutations are important in the pathogenesis of PNH. It remains unclear if these PIG-A mutations have relevance to those found in PNH. We isolated CD34+ progenitors from 4 patients with PNH and 27 controls. The frequency of PIG-A mutant progenitors was determined by assaying for colony-forming cells (CFCs) in methylcellulose containing toxic doses of aerolysin (1 x 10(-9) M). Glycosylphosphatidylinositol (GPI)-anchored proteins serve as receptors for aerolysin; thus, PNH cells are resistant to aerolysin. The frequency of aerolysin resistant CFC was 14.7 +/- 4.0 x 10(-6) in the bone marrow of healthy donors and was 57.0 +/- 6.7 x 10(-6) from mobilized peripheral blood. DNA was extracted from individual day-14 aerolysin-resistant CFCs and the PIG-A gene was sequenced to determine clonality. Aerolysin-resistant CFCs from patients with PNH exhibited clonal PIG-A mutations. In contrast, PIG-A mutations in the CFCs from controls were polyclonal, and did not involve T cells. Our data confirm the finding that PIG-A mutations are relatively common in normal hematopoiesis; however, the finding suggests that these mutations occur in differentiated progenitors rather than HSCs.

Large granular lymphocyte (LGL)-like clonal expansions in paroxysmal nocturnal hemoglobinuria (PNH) patients

In paroxysmal nocturnal hemoglobinuria (PNH), clonal expansion of glycosylphosphatidylinositol-anchored proteins (GPI-AP)-deficient cells leads to a syndrome characterized by hemolytic anemia, marrow failure, and venous thrombosis. PNH is closely related to aplastic anemia and may share its immune pathophysiology. In vivo expansion of dominant T-cell clones can reflect an antigen-driven immune response but may also represent autonomous proliferation, such as in large granular lymphocytic (LGL)-leukemia. T-cell clonality can be assessed by a combination of T-cell receptor (TCR) flow cytometry and complementarity-determining-region-3 (CDR3) molecular analysis. We studied 24 PNH patients for evidence of in vivo dominant T-cell responses by flow cytometry; TCR-Vbeta-specific expansions were identified in all patients. In four cases, extreme expansions of one Vbeta-subset of CD8+/CD28-/CD56+ (effector) phenotype mimicked subclinical LGL-disease. The monoclonality of these expansions was inferred from unique CDR3-size peak distributions and sequencing of dominant clonotypes. We conclude that the molecular analysis of TCR-beta chain may demonstrate clonal LGL-like expansions at unexpected frequency in PNH patients. Our observations blur the classical boundaries between different bone marrow failure syndromes such as AA, PNH, and LGL, and support the hypothesis that in PNH, the mutant clone may expand as a result of an immune-escape from antigen-driven lymphocyte attack on hematopoietic progenitors.

Enhanced responses of glycosylphosphatidylinositol anchor-deficient T lymphocytes

The functions of GPI-anchored proteins in T lymphocyte activation have been controversial. This issue was addressed by studying the responses of T lymphocytes from T lymphocyte-specific GPI anchor-deficient mice to different stimuli that normally allow coligation of TCR and GPI-anchored proteins. Stimulation of GPI anchor-deficient T lymphocytes with ConA induced 2-fold higher proliferative responses than did normal cells. In response to allogeneic stimulation, proliferation of GPI anchor-deficient T lymphocytes was enhanced 2- to 3-fold. The response to ConA of a GPI anchor-deficient anti-OVA T lymphocyte clone generated from these mice was approximately 3-fold higher than that of cells from the same clone in which GPI anchor expression was restored by retroviral transduction. The response of the GPI anchor-deficient cloned anti-OVA T lymphocytes to antigenic stimulation was similar to that of the retrovirally restored cells. These results indicate that coligation with GPI-anchored proteins counteracts the response to TCR stimulation by ConA or alloantigen but not protein Ag.

Endosomes, glycosomes, and glycosylphosphatidylinositol catabolism in Leishmania major

Glycosylphosphatidylinositols (GPIs) serve as membrane anchors of polysaccharides and proteins in the protozoan parasite Leishmania major. Free GPIs that are not attached to macromolecules are present in L. major as intermediates of protein-GPI and polysaccharide-GPI synthesis or as terminal glycolipids. The importance of the intracellular location of GPIs in vivo for functions of the glycolipids is not appreciated. To examine the roles of intracellular free GPI pools for attachment to polypeptide, a GPI-specific phospholipase C (GPI-PLCp) from Trypanosoma brucei was used to probe trafficking of GPI pools inside L. major. The locations of GPIs were determined, and their catabolism by GPI-PLCp was analyzed with respect to the intracellular location of the enzyme. GPIs accumulated on the endo-lysosomal system, where GPI-PLCp was also detected. A peptide motif [CS][CS]-x(0,2)-G-x(1)-C-x(2,3)-S-x(3)-L formed part of an endosome targeting signal for GPI-PLCp. Mutations of the endosome targeting motif caused GPI-PLCp to associate with glycosomes (peroxisomes). Endosomal GPI-PLCp caused a deficiency of protein-GPI in L. major, whereas glycosomal GPI-PLCp failed to produce the GPI deficiency. We surmise that (i) endo-lysosomal GPIs are important for biogenesis of GPI-anchored proteins in L. major; (ii) sequestration of GPI-PLCp to glycosomes protects free protein-GPIs from cleavage by the phospholipase. In T. brucei, protein-GPIs are concentrated at the endoplasmic reticulum, separated from GPI-PLCp. These observations support a model in which glycosome sequestration of a catabolic GPI-PLCp preserves free protein-GPIs in vivo.

GPI-anchor deficiency in myeloid cells causes impaired FcgammaR effector functions

Signaling by transmembrane immunoglobulin G (IgG)-Fc receptors (FcgammaRs) in response to ligand involves association with membrane microdomains that contain glycosyl phosphatidylinositol (GPI)-anchored proteins. Recent in vitro studies showed enhancement of FcgammaR signaling by forced monoclonal antibody-mediated cocrosslinking with various GPI-anchored proteins. Here, the possibility that GPI-anchored proteins are involved in normal physiologic FcgammaR effector functions in response to a model ligand was studied using myeloid-specific GPI-anchor-deficient mice, generated by Cre-loxP conditional targeting. GPI-anchor-deficient primary myeloid cells exhibited normal FcgammaR expression and binding or endocytosis of IgG-immune complexes (IgG-ICs). Strikingly, after stimulation with IgG-ICs, tumor necrosis factor-alpha release, dendritic cell maturation, and antigen presentation were strongly reduced by GPI-anchor deficiency. Tyrosine phosphorylation of the FcR gamma-chain in response to IgG-IC was impaired in GPI-anchor-deficient cells. Myeloid GPI-anchor deficiency resulted in attenuated in vivo inflammatory processes during IgG-IC-mediated alveolitis. This study provides the first genetic evidence for an essential role of GPI-anchored proteins in physiologic FcgammaR effector functions in vitro and in vivo.

GPI-defective monocytes from paroxysmal nocturnal hemoglobinuria patients show impaired in vitro dendritic cell differentiation

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal, acquired hematopoietic disorder characterized by a phosphatidylinositol (PI) glycan-A gene mutation, which impairs the synthesis of the glycosyl-PI (GPI) anchor, thus causing the absence of all GPI-linked proteins on the membrane of the clonal-defective cells. The presence of a consistent GPI-defective monocyte compartment is a common feature in PNH patients. To investigate the functional behavior of this population, we analyzed its in vitro differentiation ability toward functional dendritic cells (DCs). Our data indicate that GPI-defective monocytes from PNH patients are unable to undergo full DC differentiation in vitro after granulocyte macrophage-colony stimulating factor and recombinant interleukin (IL)-4 treatment. In this context, the GPI-defective DC population shows mannose receptor expression, high levels of the CD86 molecule, and impaired CD1a up-regulation. The analysis of lipopolysaccharide and CD40-dependent, functional pathways in these DCs revealed a strong decrease in tumor necrosis factor alpha and IL-12 production. Finally, GPI-defective DCs showed a severe impairment in delivering accessory signals for T cell receptor-dependent T cell proliferation.

Frequent HPRT mutations in paroxysmal nocturnal haemoglobinuria reflect T cell clonal expansion, not genomic instability

Paroxysmal nocturnal haemoglobinuria (PNH) results from acquired mutations in the PIG-A gene of an haematopoietic stem cell, leading to defective biosynthesis of glycosylphosphatidylinositol (GPI) anchors and deficient expression of GPI-anchored proteins on the surface of the cell's progeny. Some laboratory and clinical findings have suggested genomic instability to be intrinsic in PNH; this possibility has been supported by mutation analysis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene abnormalities. However, the HPRT assay examines lymphocytes in peripheral blood (PB), and T cells may be related to the pathophysiology of PNH. We analysed the molecular and functional features of HPRT mutants in PB mononuclear cells from eleven PNH patients. CD8 T cells predominated in these samples; approximately half of the CD8 cells lacked GPI-anchored protein expression, while only a small proportion of CD4 cells appeared to derive from the PNH clone. The HPRT mutant frequency (Mf) in T lymphocytes from PNH patients was significantly higher than in healthy controls. The majority of the mutant T lymphocyte clones were of CD4 phenotype, and they had phenotypically normal GPI-anchored protein expression. In PNH patients, the majority of HPRT mutant clones were contained within the Vbeta2 T cell receptor (TCR) subfamily, which was oligoclonal by complementarity-determining region three (CDR3) size analysis. Our results are more consistent with detection of uniform populations of expanded T cell clones, which presumably acquired HPRT mutations during antigen-driven cell proliferation, and not due to an increased Mf in PNH. HPRT mutant analysis does not support underlying genomic instability in PNH.

A novel mechanism of complement-independent clearance of red cells deficient in glycosyl phosphatidylinositol–linked proteins

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic anemia characterized by the increased sensitivity of red blood cells (RBCs) to complement, leading to intravascular hemolysis and hemoglobinuria. PNH is due to the expansion of a cell clone that has acquired a mutation in the PIGA gene. Mice with targeted Piga gene inactivation genetically mimic the human disease and have phosphatidylinositol glycan class A-negative (PIGA-) RBCs with a reduced half-life in circulation. Although PIGA-RBCs are hypersensitive to complement in vitro, their complement sensitivity in vivo is barely detectable. Here we show that the shortened survival of PIGA- RBCs is independent of complement either by using inhibitory C5 antibodies or by transfusion into C5-, C4-, C3-, or factor B-deficient mice. Splenectomy or high-dose cortisone treatment had no effect on the shorter survival of PIGA- RBCs. However, treatment with liposome-encapsulated clodronate, an agent that depletes macrophages in vivo, normalized the half-life of PIGA- RBCs. This indicates that the shortened survival of PIGA- RBCs is due to a novel pathway of PIGA- RBC clearance that is mediated by macrophages, but occurs independently of complement. Future investigations will show whether this novel pathway of PIGA- RBC destruction identified in mice may also operate in patients with PNH. (Blood. 2004;103:2827-2834)

A new aspect of the molecular pathogenesis of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematologic disorder which is manifest by complement-mediated hemolysis, venous thrombosis, and bone marrow failure. Complement-mediated hemolysis in PNH is explained by the deficiency of glycosylphosphatidylinositol (GPI)-anchored proteins, CD55 and CD59 on erythrocyte surfaces. All the PNH patients had phosphatidylinositol glycan-class A (PIG-A) gene abnormalities in various cell types, indicating that PIG-A gene mutations cause the defects in GPI-anchored proteins that are essential for the pathogenesis of PNH. In addition, a PIG-A gene abnormality results in a PNH clone. Bone marrow failure causes cytopenias associated with a proliferative decrease of its hematopoietic stem cells and appears to be related to a pre-leukemic state. Although it is unclear how a PNH clone expands in bone marrow, it is considered that the most important hypothesis implicates negative selection of a PNH clone, but it does not explain the changes in the clinical features at the terminal stage of PNH. Recently, it has been suggested that an immune mechanism, in an HLA-restricted manner, plays an important role in the occurrence or selection of a PNH clone and GPI may be a target for cytotoxic-T lymphocytes. Also, it has been indicated that the Wilms' tumor gene (WT1) product is related to a PNH clone, but the significance of WT1 expression is not clear because of the functional diversity of the gene. To elucidate this problem, it is important to know the pathophysiology of bone marrow failure in detail and how bone marrow failure affects hematopoietic stem cells and immune mechanisms in bone marrow failure syndromes.

Shared and individual specificities of immunodominant cytotoxic T-cell clones in paroxysmal nocturnal hemoglobinuria as determined by molecular analysis

OBJECTIVE

Similar immune mechanisms have been suggested to operate in aplastic anemia (AA) and paroxysmal nocturnal hemoglobinuria (PNH), and the presence of PNH clones in AA may indicate that an immune reaction directed against hematopoietic stem cells may be responsible for the immune selection pressure leading to PNH evolution. We previously described expansions of selective cytotoxic T-lymphocyte (CTL) clones in AA patients.

MATERIALS AND METHODS

We applied a molecular analysis of the T-cell receptor repertoire to study the characteristics of CTL response in patients with various forms of PNH. Immunodominant T-cell clones were detected using combined flow cytometric and molecular analysis of the variable beta (VB) chain and CDR3 representation, followed by determination of the frequency of individual CDR3 clonotypes. Clonotypic polymerase chain reaction (PCR) was performed to establish clonotypic utilization pattern.

RESULTS

In patients with a past history of AA, and when subgrouped by current blood counts as "hypoproliferative" PNH patients (in contrast to purely hemolytic form of PNH), more pronounced skewing of VB family utilization was found, consistent with T-cell responses involving several immunodominant CTL clones. Sequences of the PNH-derived clonotypes were used to design PCR-based assays for the utilization analysis of individual clones in PNH patients. The clonotypic distribution pattern established by this PCR method indicated that immunodominant T-cell specificities were shared between some patients but also may be found at low frequencies in controls.

CONCLUSION

Analysis of the CDR3 sequence pattern as a marker for expanded immunodominant clonotypes may have an application in the study of T-cell responses in PNH.

SETH1 and SETH2, two components of the glycosylphosphatidylinositol anchor biosynthetic pathway, are required for pollen germination and tube growth in Arabidopsis

Glycosylphosphatidylinositol (GPI) anchoring provides an alternative to transmembrane domains for anchoring proteins to the cell surface in eukaryotes. GPI anchors are synthesized in the endoplasmic reticulum via the sequential addition of monosaccharides, fatty acids, and phosphoethanolamines to phosphatidylinositol. Deficiencies in GPI biosynthesis lead to embryonic lethality in animals and to conditional lethality in eukaryotic microbes by blocking cell growth, cell division, or morphogenesis. We report the genetic and phenotypic analysis of insertional mutations disrupting SETH1 and SETH2, which encode Arabidopsis homologs of two conserved proteins involved in the first step of the GPI biosynthetic pathway. seth1 and seth2 mutations specifically block male transmission and pollen function. This results from reduced pollen germination and tube growth, which are associated with abnormal callose deposition. This finding suggests an essential role for GPI anchor biosynthesis in pollen tube wall deposition or metabolism. Using transcriptomic and proteomic approaches, we identified 47 genes that encode potential GPI-anchored proteins that are expressed in pollen and demonstrated that at least 11 of these proteins are associated with pollen membranes by GPI anchoring. Many of the identified candidate proteins are homologous with proteins involved in cell wall synthesis and remodeling or intercellular signaling and adhesion, and they likely play important roles in the establishment and maintenance of polarized pollen tube growth.

Evaluation of the haemopoietic reservoir in de novo haemolytic paroxysmal nocturnal haemoglobinuria

Paroxysmal nocturnal haemoglobinuria (PNH) is an acquired clonal disorder of the haemopoietic stem cell (HSC). The pathogenetic link with bone marrow failure is well recognized; however, the process of clonal expansion of the glycosylphosphatidylinositol (GPI)-deficient cells over normal haemopoiesis remains unclear. We have carried out detailed analysis of the stem cell population in 10 patients with de novo haemolytic PNH using the long-term culture-initiating cells (LTC-IC) assay in parallel with measurements of CD34+ cells and mature haemopoietic progenitors, granulocyte-macrophage colony-forming unit (CFU-GM) and CFU-erythroid [burst-forming units erythroid (BFU-E) + CFU granulocyte/erythroid/macrophage/megakaryocyte (GEMM)]. All patients had hypercellular bone marrows with erythroid hyperplasia, normal blood counts or mild peripheral blood cytopenias, increased reticulocyte counts and evidence of deficient GPI-anchored proteins. We found a significant reduction in the LTC-IC frequency in the CD34+ compartment of PNH patients (mean 2, range 1.3-3.0; n=6) compared with normal donors (mean 13, range 5.2-45.5; n=21) (P<0.0001). Furthermore, there was a significant reduction in the erythroid compartment [CFU-E/105 bone marrow mononuclear cells (BMMC) and CFU-E/105 CD34+ cells] of PNH patients, but no significant difference in the granulocyte-monocyte precursors (CFU-GM/105 BMMC or CFU-GM/105 CD34+ cells) compared with normal donors, suggesting that there is a defect in the early stem cell pool in PNH patients without clinical or haematological evidence of bone marrow failure.

Severe telomere shortening in patients with paroxysmal nocturnal hemoglobinuria affects both GPI- and GPI+ hematopoiesis

A most distinctive feature of paroxysmal nocturnal hemoglobinuria (PNH) is that in each patient glycosylphosphatidylinositol-negative (GPI-) and GPI+ hematopoietic stem cells (HSCs) coexist, and both contribute to hematopoiesis. Telomere size correlates inversely with the cell division history of HSCs. In 10 patients with hemolytic PNH the telomeres in sorted GPI- granulocytes were shorter than in sorted GPI+ granulocytes in 4 cases, comparable in 2 cases, and longer in the remaining 4 cases. Furthermore, the telomeres of both GPI- and GPI+ hematopoietic cells were markedly shortened compared with age-matched controls. The short telomeres in the GPI- cells probably reflect the large number of cell divisions required for the progeny of a single cell to contribute a large proportion of hematopoiesis. The short telomeres of the GPI+ cells indicate that the residual hematopoiesis contributed by these cells is not normal. This epigenetic change is an additional feature shared by PNH and aplastic anemia.

Glycosylphosphatidylinositol-dependent protein trafficking in bloodstream stage Trypanosoma brucei

We have previously demonstrated that glycosylphosphatidylinositol (GPI) anchors strongly influence protein trafficking in the procyclic insect stage of Trypanosoma brucei (M. A. McDowell, D. A. Ransom, and J. D. Bangs, Biochem. J. 335:681-689, 1998), where GPI-minus variant surface glycoprotein (VSG) reporters have greatly reduced rates of endoplasmic reticulum (ER) exit but are ultimately secreted. We now demonstrate that GPI-dependent trafficking also occurs in pathogenic bloodstream trypanosomes. However, unlike in procyclic trypanosomes, truncated VSGs lacking C-terminal GPI-addition signals are not secreted but are mistargeted to the lysosome and degraded. Failure to export these reporters is not due to a deficiency in secretion of these cells since the N-terminal ATPase domain of the endogenous ER protein BiP is efficiently secreted from transgenic cell lines. Velocity sedimentation experiments indicate that GPI-minus VSG dimerizes similarly to wild-type VSG, suggesting that degradation is not due to ER quality control mechanisms. However, GPI-minus VSGs are fully protected from degradation by the cysteine protease inhibitor FMK024, a potent inhibitor of the major lysosomal protease trypanopain. Immunofluorescence of cells incubated with FMK024 demonstrates that GPI-minus VSG colocalizes with p67, a lysosomal marker. These data suggest that in the absence of a GPI anchor, VSG is mistargeted to the lysosome and subsequently degraded. Our findings indicate that GPI-dependent transport is a general feature of secretory trafficking in both stages of the life cycle. A working model is proposed in which GPI valence regulates progression in the secretory pathway of bloodstream stage trypanosomes.

Pathogenesis of selective expansion of PNH clones

Hemolysis, a characteristic of paroxysmal nocturnal hemoglobinuria (PNH), is caused by the expansion of an affected stem cell with a mutation of the PIG-A gene. Increasing evidence has shown that the presence of the PIG-A mutation alone does not induce the expansion. Two theories have been proposed. One, the growth advantage hypothesis, is supported by current data indicating the presence of several intrinsic alterations that might confer a proliferative advantage to PNH clones over normal cells. Alternatively, the PIG-A mutation might confer a relative survival advantage to PNH clones. This theory is supported by clinical observation indicating that PIG-A mutant cells survive immune-mediated bone marrow injury in patients with aplastic anemia, PNH, and myelodysplastic syndromes. The latter theory is also supported by current experimental data indicating that PIG-A mutant cells are relatively resistant to cytotoxic attack by natural killer cells and cytotoxic T-lymphocytes. The 2 theories appear complementary rather than mutually exclusive. Rapid progress in this field can be expected in the near future.

Detection of erythrocytes deficient in glycosylphosphatidyl-inositol anchored membrane proteins in patients with paroxysmal nocturnal hemoglobinuria by the toxin HEC secreted by Aeromonas hydrophila J-1

OBJECTIVE

To study the feasibility of diagnosing paroxysmal nocturnal hemoglobinuria (PNH) with toxin HEC, the abbreviation of hemolytic, entreotoxigenicity and cytotoxity secreted by Aeromonas hydrophila J-1.

METHODS

The crude toxin HEC was extracted from the culture medium of Aeromonas hydrophila J-1 by precipitating with saturated (NH(4))(2)SO(4) and then purified through DEAE52. Purified toxin HEC is different from Aerolysin in molecular weight and necessity of activation. Crude toxin is prepared possessed same effect as purified ones. This crude toxin was used to act on red blood cells (RBCs) from patients with PNH, non-PNH anemia, and normal persons. Absorbance at 630 nm was measured to quantitate the extent of hemolysis. Toxin HEC treated and untreated RBCs were both stained with anti-CD59 monoclonal antibody and FITC labeled goat-anti-mouse IgG. The percentage of CD59(+) cells was detected by flow cytometry (FCM).

RESULTS

After toxin HEC treatment, RBCs from PNH patients showed resistance to the toxin hemolysis, which was negatively related to the percentage of CD59(+) cells, while RBCs from normal persons and non-PNH anemic patient were nearly totally lysed.

CONCLUSION

Detection of RBCs resistance to toxin HEC can be used for the diagnosis of PNH.

The glycosylphosphatidylinositol anchor and paroxysmal nocturnal haemoglobinuria/aplasia model

Paroxysmal nocturnal haemoglobinuria (PNH) is unique because it is an acquired haemolytic anaemia, resulting from an intrinsic red cell membrane disorder. The disease has been shown to be due to a somatic mutation of the phosphatidylinositol glycan complementation class A (pig-a) gene at the level of the haemopoietic stem cell. The defect in synthesis of the glycosylphosphatidylinositol (GPI) anchor results in a deficiency of all proteins that are GPI-bound to red cell, leucocyte and platelet membranes. The function of these proteins is extremely varied but a critical role is the protection of the cell from complement and it is the unopposed action of the complement cascade that results in the intravascular haemolysis and venous thrombosis which are hallmarks of the disease. The relationship between PNH and aplastic anaemia remains intriguing. It appears likely that an insult to a haemopoietic progenitor alters it in such a way that it becomes vulnerable to immune-mediated attack by cytotoxic T cells and/or cytokines. This attack requires one or more GPI-anchored molecules to be effective. Thus a GPI-negative clone would be at a relative advantage, and it is the balance between bone marrow impairment and proliferation of the GPI-negative clone(s) that determines the clinical picture. Prospects for molecular therapy continue to improve. Cell-to-cell transfer of GPI-linked proteins has been demonstrated in murine studies and recombinant CD59 has been expressed on GPI-deficient lymphocytes in vitro. Gene therapy remains a tantalising possibility, although a greater understanding of the pathophysiology of PNH is required, as well as advances in gene therapy techniques, before such an approach can be seriously considered.

CD34+ cells from paroxysmal nocturnal hemoglobinuria (PNH) patients are deficient in surface expression of cellular prion protein (PrPc)

Cellular prion protein (PrP(c)) is a glycosylphosphatidylinositol (GPI)-anchored protein (GPI-AP) constitutively expressed by neurons but also in hematopoietic cells. In trasmissible spongiform encephalopathies, the protease-resistant form of prion (PrP (s c)) converts the host PrP(c) into the pathologic form. We have investigated PrP(c) expression in hematopoietic cells from paroxysmal nocturnal hemoglobinuria (PNH). In this disease, due to somatic mutations in PIG-A gene, biosynthesis of the (GPI)-anchor is impaired and affected cells lack membrane expression of all GPI-AP. Normal and PNH hematopoietic progenitors and paired wild-type (WT) and PIG-A mutant cell lines were used for analysis of intracellular and surface PrP(c) expression using flow cytometry and Western blot.By flow cytometry, PrP(c) was constitutively present on normal CD34(+) cells, including more immature CD38(dim) cells, as well as hematopoietic cell lines. Similar results were obtained in purified CD34(+). Phospholipase C treatment confirmed that PrP(c) was expressed on the membrane via the GPI-anchor. In PNH patients, GPI-AP-deficient CD34(+) cells lacked PrP(c) membrane expression. PIG-A-mutated cell lines (Jurkat, K562, C(EBV), A(EBV)), in contrast to their normal counterparts, did not express surface PrP(c). However, we detected intracellular PrP(c) at approximately equivalent levels in both normal and PIG-A-mutated cells using intracellular flow cytometry and Western blotting. Cells and cell lines with PNH phenotype together with their normal counterparts may be a suitable system to explore the function of membrane PrP(c) in the hematopoietic system. Conversely, PrP(c) is a good model to elucidate the fate of GPI-AP in PIG-A-deficient cells.

Inefficient response of T lymphocytes to glycosylphosphatidylinositol anchor-negative cells: implications for paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a hematopoietic stem cell disorder in which clonal cells defective in glycosylphosphatidylinositol (GPI) biosynthesis are expanded, leading to complement-mediated hemolysis. PNH is often associated with bone marrow suppressive conditions, such as aplastic anemia. One hypothetical mechanism for the clonal expansion of GPI(-) cells in PNH is that the mutant cells escape attack by autoreactive cytotoxic cells that are thought to be responsible for aplastic anemia. Here we studied 2 model systems. First, we made pairs of GPI(+) and GPI(-) EL4 cells that expressed major histocompatibility complex (MHC) class II molecules and various types of ovalbumin. When the GPI-anchored form of ovalbumin was expressed on GPI(+) and GPI(-) cells, only the GPI(+) cells presented ovalbumin to ovalbumin-specific CD4(+) T cells, indicating that if a putative autoantigen recognized by cytotoxic cells is a GPI-anchored protein, GPI(-) cells are less sensitive to cytotoxic cells. Second, antigen-specific as well as alloreactive CD4(+) T cells responded less efficiently to GPI(-) than GPI(+) cells in proliferation assays. In vivo, when GPI(-) and GPI(+) fetal liver cells, and CD4(+) T cells alloreactive to them, were cotransplanted into irradiated hosts, the contribution of GPI(-) cells in peripheral blood cells was significantly higher than that of GPI(+) cells. The results obtained with the second model suggest that certain GPI-anchored protein on target cells is important for recognition by T cells. These results provide the first experimental evidence for the hypothesis that GPI(-) cells escape from immunologic attack.

The effect of GPI-anchor deficiency on apoptosis in mice carrying a Piga gene mutation in hematopoietic cells

Glycosyl phosphatidylinositol (GPI) anchors are used by a variety of proteins to link to the cell surface. GPI-anchored proteins are deficient on a proportion of blood cells from patients with paroxysmal nocturnal hemoglobinuria. This is caused by the expansion of a cell clone that has acquired a mutation in a gene, PIGA, which is essential in the synthesis of GPI anchors. The nature of the growth/survival advantage permitting the expansion of PIGA(-) cells is unknown. A decreased susceptibility to apoptosis has been found in blood cells from patients, but the contribution of the PIGA gene mutation to this finding remained controversial. Therefore, we investigated apoptosis in mice that harbor a targeted Piga gene mutation in hematopoietic cells. When exposed to a variety of apoptotic stimuli, apoptosis in PIGA(-) thymocytes, granulocytes, and hematopoietic progenitor cells was similar to apoptosis induced in PIGA(+) cells from the same mouse or from wild-type controls. Similarly, whole-body gamma-irradiation did not produce an in vivo survival advantage of PIGA(-) hematopoietic stem cells. Our findings imply that a Piga gene mutation does not alter susceptibility to cell death, indicating that other factors in addition to the PIGA gene mutation are necessary to promote the clonal outgrowth of PIGA(-) cells.