Normal polymorphic variations and transcription of the decay accelerating factor gene in paroxysmal nocturnal hemoglobinuria cells

The complement alternative pathway in paroxysmal nocturnal hemoglobinuria: From a pathogenic mechanism to a therapeutic target

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare clonal, not malignant, hematological disease characterized by intravascular hemolysis, thrombophilia and bone marrow failure. While this latter presentation is due to a T-cell mediated auto-immune disorder resembling acquired aplastic anemia, the first two clinical presentations are largely driven by the complement pathway. Indeed, PNH is characterized by a broad impairment of complement regulation on affected cells, which is due to the lack of the complement regulators CD55 and CD59. The deficiency of these two proteins from PNH blood cells is due to the somatic mutation in the phosphatidylinositol N-acetylglucosaminyltransferase subunit A gene causing the disease, which impairs the surface expression of all proteins linked via the glycosylphosphatidylinositol anchor. The lack of the complement regulators CD55 and CD59 on PNH erythrocytes accounts for the hallmark of PNH, which is the chronic, complement-mediated intravascular hemolysis. This hemolysis results from the impaired regulation of the alternative pathway upstream in the complement cascade, as well as of the downstream terminal pathway. PNH represented the first indication for the development of anti-complement agents, and the therapeutic interception of the complement cascade at the level of C5 led to remarkable changes in the natural history of the disease. Nevertheless, the clinical use of an inhibitor of the terminal pathway highlighted the broader derangement of complement regulation in PNH, shedding light on the pivotal role of the complement alternative pathway. Here we review the current understanding of the role of the alternative pathway in PNH, including the emergence of C3-mediated extravascular hemolysis in PNH patients on anti-C5 therapies. These observations provide the rationale for the development of novel complement inhibitors for the treatment of PNH. Recent preclinical and clinical data on proximal complement inhibitors intercepting the alternative pathway with the aim of improving the treatment of PNH are discussed, together with their clinical implications which are animating a lively debate in the scientific community.

Discovering C3 targeting therapies for paroxysmal nocturnal hemoglobinuria: Achievements and pitfalls

The treatment of paroxysmal nocturnal hemoglobinuria (PNH) was revolutionized by the introduction of the anti-C5 agent eculizumab, which resulted in sustained control of intravascular hemolysis, leading to transfusion avoidance and hemoglobin stabilization in at least half of all patients. Nevertheless, extravascular hemolysis mediated by C3 has emerged as inescapable phenomenon in PNH patients on anti-C5 treatment, frequently limiting its hematological benefit. More than 10 years ago we postulated that therapeutic interception of the complement cascade at the level of C3 should improve the clinical response in PNH. Compstatin is a 13-residue disulfide-bridged peptide binding to both human C3 and C3b, eventually disabling the formation of C3 convertases and thereby preventing complement activation via all three of its activating pathways. Several generations of compstatin analogs have been tested in vitro, and their clinical evaluation has begun in PNH and other complement-mediated diseases. Pegcetacoplan, a pegylated form of the compstatin analog POT-4, has been investigated in two phase I/II and one phase III study in PNH patients. In the phase III study, PNH patients with residual anemia already on eculizumab were randomized to receive either pegcetacoplan or eculizumab in a head-to-head comparison. At week 16, pegcetacoplan was superior to eculizumab in terms of hemoglobin change from baseline (the primary endpoint), as well as in other secondary endpoints tracking intravascular and extravascular hemolysis. Pegcetacoplan showed a good safety profile, even though breakthrough hemolysis emerged as a possible risk requiring additional attention. Here we review all the available data regarding this innovative treatment that has recently been approved for the treatment of PNH.

How we('ll) treat paroxysmal nocturnal haemoglobinuria: diving into the future

Paroxysmal nocturnal haemoglobinuria (PNH) is characterized by complement-mediated intravascular haemolysis, severe thrombophilia and bone marrow failure. While for patients with bone marrow failure the treatment follows that of immune-mediated aplastic anaemia, that of classic, haemolytic PNH is based on anti-complement medication. The anti-C5 monoclonal antibody eculizumab has revolutionized treatment, resulting in control of intravascular haemolysis and thromboembolic risk, with improved long-term survival. Novel strategies of complement inhibition are emerging. New anti-C5 agents reproduce the safety and efficacy of eculizumab, with improved patient convenience. Proximal complement inhibitors have been developed to address C3-mediated extra-vascular haemolysis and seem to improve haematological response.

Anti-complement Treatment for Paroxysmal Nocturnal Hemoglobinuria: Time for Proximal Complement Inhibition? A Position Paper From the SAAWP of the EBMT

The treatment of paroxysmal nocturnal hemoglobinuria has been revolutionized by the introduction of the anti-C5 agent eculizumab; however, eculizumab is not the cure for Paroxysmal nocturnal hemoglobinuria (PNH), and room for improvement remains. Indeed, the hematological benefit during eculizumab treatment for PNH is very heterogeneous among patients, and different response categories can be identified. Complete normalization of hemoglobin (complete and major hematological response), is seen in no more than one third of patients, while the remaining continue to experience some degree of anemia (good and partial hematological responses), in some cases requiring regular red blood cell transfusions (minor hematological response). Different factors contribute to residual anemia during eculizumab treatment: underlying bone marrow dysfunction, residual intravascular hemolysis and the emergence of C3-mediated extravascular hemolysis. These two latter pathogenic mechanisms are the target of novel strategies of anti-complement treatments, which can be split into terminal and proximal complement inhibitors. Many novel terminal complement inhibitors are now in clinical development: they all target C5 (as eculizumab), potentially paralleling the efficacy and safety profile of eculizumab. Possible advantages over eculizumab are long-lasting activity and subcutaneous self-administration. However, novel anti-C5 agents do not improve hematological response to eculizumab, even if some seem associated with a lower risk of breakthrough hemolysis caused by pharmacokinetic reasons (it remains unclear whether more effective inhibition of C5 is possible and clinically beneficial). Indeed, proximal inhibitors are designed to interfere with early phases of complement activation, eventually preventing C3-mediated extravascular hemolysis in addition to intravascular hemolysis. At the moment there are three strategies of proximal complement inhibition: anti-C3 agents, anti-factor D agents and anti-factor B agents. These agents are available either subcutaneously or orally, and have been investigated in monotherapy or in association with eculizumab in PNH patients. Preliminary data clearly demonstrate that proximal complement inhibition is pharmacologically feasible and apparently safe, and may drastically improve the hematological response to complement inhibition in PNH. Indeed, we envision a new scenario of therapeutic complement inhibition, where proximal inhibitors (either anti-C3, anti-FD or anti-FB) may prove effective for the treatment of PNH, either in monotherapy or in combination with anti-C5 agents, eventually leading to drastic improvement of hematological response.

DAF in diabetic patients is subject to glycation/inactivation at its active site residues

Decay accelerating factor (DAF or CD55) is a cell associated C3 and C5 convertase regulator originally described in terms of protection of self-cells from systemic complement but now known to modulate adaptive T cell responses. It is expressed on all cell types. We investigated whether nonenzymatic glycation could impair its function and potentially be relevant to complications of diabetes mellitus and other conditions that result in nonenzymatic glycation including cancer, Alzheimer's disease, and aging. Immunoblots of affinity-purified DAF from erythrocytes of patients with diabetes showed pentosidine, glyoxal-AGEs, carboxymethyllysine, and argpyrimidine. HPLC/MS analyses of glucose modified DAF localized the sites of AGE modifications to K125 adjacent to K126, K127 at the junction of CCPs2-3 and spatially near R96, and R100, all identified as being critical for DAF's function. Functional analyses of glucose or ribose treated DAF protein showed profound loss of its regulatory activity. The data argue that de-regulated activation of systemic complement and de-regulated activation of T cells and leukocytes could result from non-enzymatic glycation of DAF.

Altered lipid raft composition and defective cell death signal transduction in glycosylphosphatidylinositol anchor-deficient PIG-A mutant cells

Paroxysmal nocturnal haemoglobinuria (PNH) is a clonal disorder of haematopoietic stem cells caused by somatic PIGA mutations, resulting in a deficiency in glycosylphosphatidylinositol-anchored proteins (GPI-AP). Because GPI-AP associate with lipid rafts (LR), lack of GPI-AP on PNH cells may result in alterations in LR-dependent signalling. Conversely, PNH cells are a suitable model for investigating LR biology. LR from paired, wild-type GPI(+), and mutant GPI(-) cell lines (K562 and TF1) were isolated and analysed; GPI(-) LR contained important anti-apoptotic proteins, not found in LR from GPI(+) cells. When methyl-beta-cyclodextrin (MbetaCD) was utilized to probe for functional differences between normal and GPI(-) LR, increased levels of phospho-p38 mitogen-activated protein kinase (MAPK), and phospho-p65 nuclear factor NF-kappaB were found in control and GPI(-) cells respectively. Subsequent experiments addressing the inhibition of phosphoinositide-3-kinase (PI3K) suggest that the PI3K/AKT pathway may be responsible for the resistance of K562 GPI(-)cells to negative effects of MbetaCD. In addition, transduction of tumour necrosis factor-alpha (TNF-alpha) signals in a LR-dependent fashion increased induction of p38 MAPK in GPI(+) and increased pro-survival NF-kappaB levels in K562 GPI(-) cells. Therefore, we suggest that the altered LR-dependent signalling in PNH-like cells may induce different responses to pro-inflammatory cytokines from those observed in cells with intact GPI-AP.

Decay accelerating factor can control T cell differentiation into IFN-gamma-producing effector cells via regulating local C5a-induced IL-12 production

A newly recognized link between the complement system and adaptive immunity is that decay accelerating factor (DAF), a cell surface C3/C5 convertase regulator, exerts control over T cell responses. Extending these results, we show that cultures of Marilyn TCR-transgenic T cells stimulated with DAF-deficient (Daf1(-/-)) APCs produce significantly more IL-12, C5a, and IFN-gamma compared with cultures containing wild-type APCs. DAF-regulated IL-12 production and subsequent T cell differentiation into IFN-gamma-producing effectors was prevented by the deficiency of either C3 or C5a receptor (C5aR) in the APC, demonstrating a link between DAF, local complement activation, IL-12, and T cell-produced IFN-gamma. Bone marrow chimera experiments verified that bone marrow cell-expressed C5aR is required for optimal differentiation into IFN-gamma-producing effector T cells. Overall, our results indicate that APC-expressed DAF regulates local production/activation of C5a following cognate T cell/APC interactions. Through binding to its receptor on APCs the C5a up-regulates IL-12 production, this in turn, contributes to directing T cell differentiation toward an IFN-gamma-producing phenotype. The findings have implications for design of therapies aimed at altering pathologic T cell immunity.

Structure-based mapping of DAF active site residues that accelerate the decay of C3 convertases

Focused complement activation on foreign targets depends on regulatory proteins that decay the bimolecular C3 convertases. Although this process is central to complement control, how the convertases engage and disassemble is not established. The second and third complement control protein (CCP) modules of the cell surface regulator, decay-accelerating factor (DAF, CD55), comprise the simplest structure mediating this activity. Positioning the functional effects of 31 substitution mutants of DAF CCP2 to -4 on partial structures was previously reported. In light of the high resolution crystal structure of the DAF four-CCP functional region, we now reexamine the effects of these and 40 additional mutations. Moreover, we map six monoclonal antibody epitopes and overlap their effects with those of the amino acid substitutions. The data indicate that the interaction of DAF with the convertases is mediated predominantly by two patches approximately 13 A apart, one centered around Arg69 and Arg96 on CCP2 and the other around Phe148 and Leu171 on CCP3. These patches on the same face of the adjacent modules bracket an intermodular linker of critical length (16 A.) Although the key DAF residues in these patches are present or there are conservative substitutions in all other C3 convertase regulators that mediate decay acceleration and/or provide factor I-cofactor activity, the linker region is highly conserved only in the former. Intra-CCP regions also differ. Linker region comparisons suggest that the active CCPs of the decay accelerators are extended, whereas those of the cofactors are tilted. Intra-CCP comparisons suggest that the two classes of regulators bind different regions on their respective ligands.

Erythroid involvement in CD36 deficiency

OBJECTIVE

The CD36 molecule is expressed in platelets, monocytes, erythroblasts, and other different tissues. The two types of platelet CD36 deficiency, types I and II, are associated with the absence and presence of CD36 on monocytes, respectively. To clarify the involvement of the erythroid lineage in CD36 deficiency, we investigated the phenotype and RNA expression of CD36.

MATERIALS AND METHODS

CD36 expression was examined in 296 patients with several cardiovascular diseases in our outpatient clinic. There were 12 patients with type I deficiency and 16 with type II CD36 deficiency. A bone marrow sample was examined in five type I and four type II patients. Expression of CD36 mRNA was examined in burst-forming unit-erythroid (BFU-E). The sequences of reverse transcriptase polymerase chain reaction (RT-PCR) products of the CD36 mRNA from monocytes were examined.

RESULTS

As expected, CD36 was deficient in erythroblasts from all five patients with type I deficiency. CD36 was present in erythroblasts from three of the four with type II deficiency, suggesting that their abnormality is restricted to platelets (type IIa). CD36 was unexpectedly absent from erythroblasts of a single type II patient (type IIb). CD36-specific mRNA was identified in BFU-E from each of two normals, six type I, and six type II patients, including type IIb. The sequences of RT-PCR products of the CD36 mRNA in a patient with type IIa and another with type IIb showed homozygous wild alleles.

CONCLUSION

The findings provide evidence for further heterogeneity among CD36-deficient individuals and the existence of a basic principle mechanism of type II, such as glycosylation abnormality.

Characterization of the active sites in decay-accelerating factor

Decay-accelerating factor (DAF) is a complement regulator that dissociates autologous C3 convertases, which assemble on self cell surfaces. Its activity resides in the last three of its four complement control protein repeats (CCP2-4). Previous modeling on the nuclear magnetic resonance structure of CCP15-16 in the serum C3 convertase regulator factor H proposed a positively charged surface area on CCP2 extending into CCP3, and hydrophobic moieties between CCPs 2 and 3 as being primary convertase-interactive sites. To map the residues providing for the activity of DAF, we analyzed the functions of 31 primarily alanine substitution mutants based in part on this model. Replacing R69, R96, R100, and K127 in the positively charged CCP2-3 groove or hydrophobic F148 and L171 in CCP3 markedly impaired the function of DAF in both activation pathways. Significantly, mutations of K126 and F169 and of R206 and R212 in downstream CCP4 selectively reduced alternative pathway activity without affecting classical pathway activity. Rhesus macaque DAF has all the above human critical residues except for F169, which is an L, and its CCPs exhibited full activity against the human classical pathway C3 convertase. The recombinants whose function was preferentially impaired against the alternative pathway C3bBb compared with the classical pathway C4b2a were tested in classical pathway C5 convertase (C4b2a3b) assays. The effects on C4b2a and C4b2a3b were comparable, indicating that DAF functions similarly on the two enzymes. When CCP2-3 of DAF were oriented according to the crystal structure of CCP1-2 of membrane cofactor protein, the essential residues formed a contiguous region, suggesting a similar spatial relationship.

Farming for spare body parts: silk purse or sow's ear

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Conservation in decay accelerating factor (DAF) structure among primates

The decay accelerating factor (DAF, CD55) protects self cells from activation of autologous complement on their surfaces. It functions to disable the C3 convertases, the central amplification enzymes of the cascade. Its active site(s) are contained within four approximately 60 amino acid long units, termed complement control protein repeats (CCPs), which are suspended above the cell surface on a 68 amino acid long serine/threonine (S/T)-rich cushion that derives from three exons. We previously proposed a molecular model of human DAF's four CCPs in which certain amino acids were postulated to be recognition sites for the interaction between DAF and the C3 convertases. In the current study, we characterized DAF in five non-human primates: the great apes, gorilla and common chimpanzee, and the Old World monkeys: hamadryas baboon, Rhesus macaque, and patas monkey. Amino acid homology to human DAF was approximately 98% for the two great apes and 83% for the three Old World monkeys. The above cited putative ligand interactive residues were found to be fully conserved in all of the non-human primates, although there were amino acid changes outside of these areas. In the chimpanzee, alternative splicing of the S/T region was found potentially to be the source of multiple protein isoforms in erythrocytes, whereas in the patas monkey, similar alternative splicing was observed but only one protein band was seen. Interestingly, a Rhesus macaque was found to exhibit a phenomenon paralleling the human Cromer Dr(a-) blood group, in which a 44-base pair deletion in CCP3 leads to a frameshift and early STOP codon.

IFN-gamma and IFN-alpha posttranscriptionally down-regulate the IL-4-induced IL-4 receptor gene expression

As Th1 and Th2 cytokines, IFN-gamma/alpha and IL-4 counterregulate diverse immune functions. In particular, IFN-gamma and IFN-alpha have been reported to markedly suppress the IL-4-induced IgE production and type II IgE receptor (FcepsilonRII/CD23) expression. Because modulation of IL-4R may be an important mechanism in the regulation of IL-4 response, we have investigated the effect of IFN-gamma/alpha on IL-4R expression and signal transduction mechanisms involved in this process. In human mononuclear cells and B cells isolated from tonsil or peripheral blood, IL-4 up-regulates IL-4R(alpha) expression at surface protein and mRNA levels, and the IL-4-induced IL-4R(alpha) is significantly down-regulated by both IFN-gamma and IFN-alpha to a similar extent. The inhibitory effects of IFN-gamma/alpha on the IL-4R mRNA expression require a lag period of about 8 h, and are sensitive to cycloheximide treatment, which suggests that the suppressive effect of IFNs on IL-4R gene expression is a secondary response requiring de novo synthesis of IFN-induced factors. Under such conditions that the inhibitory effects of IFNs are observed, IFNs do not affect the IL-4-induced STAT6 activation and IL-4R transcription, as analyzed by EMSA and nuclear run-on assays, respectively. Subsequently, mRNA stability studies have indicated that the action of IFN-gamma/alpha is primarily mediated by an accelerated decay of IL-4-induced IL-4R mRNA. Thus, it appears that, as already shown in the case of the IL-4-induced FcepsilonRII regulation, posttranscriptional inhibition of IL-4-inducible genes by mRNA destabilization is a common mechanism by which type I and II IFNs antagonize the IL-4 response in human immune cells.

Structure/function studies of human decay-accelerating factor

The decay-accelerating factor (DAF) contains four complement control protein repeats (CCPs) with a single N-linked glycan positioned between CCPs 1 and 2. In previous studies we found that the classical pathway regulatory activity of DAF resides in CCPs 2 and 3 while its alternative pathway regulatory activity resides in CCPs 2, 3 and 4. Molecular modelling of the protein predicted that a positively charged surface area on CCPs 2 and 3 (including KKK125-127) and nearby exposed hydrophobic residues (L147F148) on CCP3 may function as ligand-binding sites. To assess the roles of the N-linked glycan and the above two sets of amino acids in the function of DAF, we mutated N61 to Q, KKK125-127 to TTT and L147F148 to SS. Following expression of the mutated cDNAs in Chinese hamster ovary cells, the glycosylphosphatidylinositol (GPI)-anchored mutant proteins were affinity purified and their functions were assessed. In initial assays, the proteins were incorporated into sheep and rabbit erythrocytes and the effects of the mutations on regulation of classical and alternative C3 convertase activity were quantified by measuring C3b deposition. Since DAF also functions on C5 convertases, comparative haemolytic assays of cells bearing each mutant protein were performed. Finally, to establish if spatial orientation between DAF and the convertases on the cell surface played any role in the observed effects, fluid-phase C3a generation assays were performed. All three assays gave equivalent results and showed that the N-linked glycan of DAF is not involved in its regulatory function; that L147F148 in a hydrophobic area of CCP3 is essential in both classical and alternative pathway C3 convertase regulation; and that KKK125-127 in the positively charged pocket between CCPs 2 and 3 is necessary for the regulatory activity of DAF on the alternative pathway C3 convertase but plays a lesser role in its activity on the classical pathway enzyme.

Characterization of pigs transgenic for human decay-accelerating factor

BACKGROUND

To prevent the central role played by complement activation in the hyperacute rejection of pig organs transplanted into primates, pigs transgenic for human decay-accelerating factor (HDAF) have recently been produced. The data presented here extend previous immunohistochemical findings by documenting the immunological characterization and the levels of expression of HDAF in these transgenic pigs.

METHODS

Animals from 30 independently derived lines were included in this study. HDAF expression was characterized by immunoprecipitation and epitope mapping. Quantitative analysis was performed by radiometric assays followed by Scatchard analysis and by double-determinant radioimmunoassay. Deposition of iC3b on porcine aortic endothelial cells was determined by radioimmunoassay. DNA slot-blot analysis and densitometric scanning were used to evaluate HDAF transgene copy number.

RESULTS

The integrity of HDAF expressed by these transgenic pigs could be demonstrated. HDAF was present in 72% of the organs analyzed, although considerable variation in expression occurred, both between animals and within the same pig. High levels of HDAF on porcine aortic endothelial cells resulted in iC3b deposition at levels as low as that detected on human endothelial cells. Twenty-six organs expressed levels of HDAF greater than those observed in the equivalent human tissue. HDAF expression did not correlate with the number of copies of the transgene incorporated into the porcine genome.

CONCLUSIONS

Transgenic pigs, which express levels of functional HDAF even greater than those observed in humans, have successfully been produced. Pigs transgenic for human complement inhibiting molecules could represent a source of organs for future clinical xenotransplantation.

Mechanism of intravascular hemolysis in paroxysmal nocturnal hemoglobinuria (PNH)

Paroxysmal nocturnal hemoglobinuria (PNH) hemolysis requires both intravascular complement activation and affected erythrocytes susceptible to complement. This susceptibility is explained by a deficiency in complement regulatory membrane proteins that are attached to the membrane by a glycosylphosphatidylinositol (GPI) anchor. Affected cells lack a series of GPI-anchored membrane proteins with various functions. The lack is caused by a synthetic defect of the anchor due to an impaired transfer of N-acetylglucosamine to phosphatidylinositol which is an early metabolic precursor in the anchor synthesis. Moreover, PIG-A gene responsible for the membrane defect was recently cloned. Further, a possible mechanism of complement activation has been proposed, especially for an infection-induced hemolytic precipitation which is clinically crucial. Thus, the molecular events, leading to intravascular hemolysis characteristic of PNH, has been virtually clarified. Next major concern is the nature of PIG-A: How does PIG-A explain the complex pathophysiology of PNH which exhibits various clinical manifestations?

Role of phosphatidylinositol-linked proteins in paroxysmal nocturnal hemoglobinuria pathogenesis

Patients with paroxysmal nocturnal hemoglobinuria have one or more mutant hematopoietic stem cell clones deficient in glycosylphosphatidylinositol (GPI)-anchor synthesis owing to somatic mutations in the X-linked gene PIG-A. The progeny of mutant stem cells dominates the peripheral blood. The presence of a large number of GPI-anchor deficient, complement-sensitive erythrocytes leads to hemolytic anemia. The somatic mutations in PIG-A are small, various, and widely distributed in the coding regions and splice sites, indicating they occur randomly. Profiles of the mutations vary geographically, suggesting the presence of mutagen-induced mutations. The clonal dominance by the mutants does not seem to be solely due to the PIG-A mutation but may be caused either by autonomous expansion of the mutants due to a combination of the PIG-A mutation and some other genetic change(s) or by selection that preferentially suppresses normal stem cells.

Establishment of a human T-cell line with deficient surface expression of glycosylphosphatidylinositol (GPI)-anchored proteins from a patient with paroxysmal nocturnal haemoglobinuria

A novel interleukin-2 dependent T-cell line, PMT-2Y, was established from the peripheral blood of a patient with paroxysmal nocturnal haemoglobinuria (PNH) by human T lymphotropic virus type I (HTLV-I)-mediated transformation. PMT-2Y cells are positive for CD2, CD3, CD4, CD25, T cell receptor alpha beta and HLA-DR, but negative for CD1, CD7, CD8, CD19 and CD20, indicating that the clone belongs to a helper/inducer subset of T cells. PMT-2Y cells have the monoclonal integration of HTLV-I proviral DNA, suggesting that they derived from a single clone. Moreover, they lack surface expression of complement regulatory proteins such as DAF (CD55) and CD59, that are the most important glycosylphosphatidylinositol (GPI)-anchored membrane proteins defective in haemopoietic cells of patients with PNH. Northern blot analysis, however, revealed the production of normal levels of DAF mRNAs. Thus, PMT-2Y is derived from a PNH T cell clone and may be a useful model to study PNH.

Additional GPI-anchored glycoproteins on human platelets that are absent or deficient in paroxysmal nocturnal haemoglobinuria

In order to detect novel glycophosphatidylinositol (GPI)-anchored platelet proteins, human platelets were incubated with PI-specific phospholipase C (PI-PLC) and the supernatant was analysed by PAGE and silver-staining for additional protein bands. PI-PLC treatment resulted in the appearance of at least two additional novel GPI-linked glycoproteins (GP), GP500 and GP175, in the supernatant. Their presence on the platelet plasma membrane surface was demonstrated by periodate/[3H]borohydride surface-labelling. Activation of platelets did not enhance the amount of GP500 and GP175 that could be cleaved by PI-PLC. In Triton X-114 phase partitioning of platelet membranes the membrane form of GP175, mfGP175, was in the Triton phase while mfGP500 was found in the water phase. Neither GP500 nor GP175 were present in the supernatant of surface-labelled platelets treated with PI-PLC from 4 patients, diagnosed as having paroxysmal nocturnal haemoglobinuria (PNH), but the supernatant from platelets from healthy volunteers treated the same way contained both.

Induction of Fc gamma R-III (CD16) expression on neutrophils affected by paroxysmal nocturnal haemoglobinuria by administration of granulocyte colony-stimulating factor

The inducibility of glycosyl-phosphatidylinositol (GPI)-anchored proteins on affected paroxysmal nocturnal haemoglobinuria (PNH) neutrophils (PMN) after both in vitro and in vivo stimulation was investigated. Fc gamma R-III (CD16), decay-accelerating factor (DAF/CD55) and 20 kD homologous restriction factor (HRF20/CD59) were demonstrated to be concurrently deficient on unstimulated defective PNH PMN. Upon in vitro stimulation with either N-formyl-methionyl-leucyl-phenylalanine (fMLP), zymosan-activated serum (ZAS), or recombinant human granulocyte colony-stimulation factor (G-CSF), neither CD16 nor CD55 expression was induced on defective PNH PMN. G-CSF was administered to two patients with PNH when their conditions were complicated by bacterial infections, or to prevent infections associated with the extraction of teeth or cataract surgery. CD16 expression was induced on the defective PNH PMN in both cases during the administration of G-CSF, but the expression of CD55 and CD59 was not. CD16, induced on the defective PNH PMN during the administration of G-CSF, was phosphatidylinositol-specific phospholipase C (PIPLC)-sensitive, implying that it had GPI-linkage to the membranes. The patients treated with G-CSF recovered from infection or evaded infection. These observations suggest that a deficiency of GPI-anchored proteins is not always seen in defective PNH blood cells, at least under certain stimulation conditions.

Defective and normal haematopoietic stem cells in paroxysmal nocturnal haemoglobinuria

The expression of decay-accelerating factor (DAF or CD55) and CD59 during haematopoietic cell development in bone marrow aspirates of two patients with paroxysmal nocturnal haemoglobinuria (PNH) was compared with that in normal bone marrow by five-dimensional flow cytometry. In contrast to early uncommitted haematopoietic progenitor cells (CD34+, CD38-) in normal bone marrow which uniformly express DAF and CD59, the majority of CD34+, CD38- cells in both patients' marrow exhibited the absence of the two proteins. In both specimens, however, subpopulations of CD34+, CD38- cells expressing DAF and CD59 were detectable, indicative of the presence of two lines of haematopoiesis, one abnormal and the other normal. Concurrent abnormal and normal haematopoietic development was further evident by the presence of subpopulations of DAF-, CD59- and DAF+, CD59+ cells along the differentiation and maturation pathways of the myeloid (CD33+, CD15(-)-->CD33+-->++, CD15+), the erythroid (CD45dim, CD71dim-->CD45-, CD71++), and the B-lymphoid cell lineages (CD10++, CD20(-)-->CD10-, CD20++). While the majority of cells differentiating into and maturing along each cell lineage lacked DAF and CD59, the majority of mature B (CD20++, CD10-) and T-lymphocytes lymphocytes (CD3+) expressed both proteins suggestive of the presence of lymphocytes with a long life span which were generated from normal haematopoietic progenitors before the onset of the disease. The detection of distinct sets of CD34+, CD38(-)--> + progenitor cells which are DAF+, CD59+ or DAF-, CD59- in marrow of PNH patients has relevance for the treatment of PNH. Cells with the phenotype CD34+, CD38-, DAF+, CD59+ are capable of self renewal and represent potential candidates for autologous bone marrow transplantation following depletion of CD34+, CD38-, DAF-, CD59- cells.

Specific defect in N-acetylglucosamine incorporation in the biosynthesis of the glycosylphosphatidylinositol anchor in cloned cell lines from patients with paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal disorder arising in a multipotent hemopoietic stem cell. PNH manifests clinically with intravascular hemolysis resulting from an increased sensitivity of the red cells belonging to the PNH clone to complement-mediated lysis. Numerous studies have shown that surface proteins anchored to the membrane via a glycosylphosphatidylinositol (GPI) anchor (including proteins protecting the cell from complement) are deficient on the cells of the PNH clone, leading to the notion that GPI-anchor biosynthesis may be abnormal in these cells. To investigate the biochemical defect underlying PNH we have used lymphoblastoid cell lines (LCLs) with the PNH phenotype obtained by Epstein-Barr virus immortalization of lymphocytes from nine patients with PNH. By labeling cells with myo-[3H]inositol we have found that PNH LCLs produce phosphatidylinositol normally. By contrast, PNH LCLs fail to incorporate [3H]mannose into GPI anchor precursors. When cell-free extracts of PNH LCLs and normal LCLs obtained from the same patients (and expected therefore to be isogeneic except for the PNH mutation) were incubated with uridine diphospho-N-acetyl[3H]glucosamine (UDP-[3H]GlcNAc), we observed complete failure or marked reduction in the production of N-acetylglucosaminyl(alpha-1,6)phosphatidylinositol and glucosaminyl(alpha-1,6)phosphatidylinositol by the PNH LCLs in all cases. These findings pinpoint the block in PNH at an early stage in the biosynthesis of the GPI anchor, suggesting that the defective enzyme is UDP-GlcNAc:phosphatidylinositol-alpha-1,6-N- acetylglucosaminyltransferase. The existence of PNH type III cells and type II cells is probably explained by the transferase deficiency being total or partial, respectively.

Expression of decay-accelerating factor and CD59 in lymphocyte subsets of healthy individuals and paroxysmal nocturnal hemoglobinuria patients

The expression of phosphatidylinositol (PI)-anchored complement-regulatory membrane proteins on circulating blood cells has been well clarified; however, the PI proteins on lymphocyte subsets have not been fully analyzed yet. We examined the expression of decay-accelerating factor (DAF) and CD59 on the T lymphocytes (CD2+, CD3+, CD4+, and CD8+) and CD20+ B lymphocytes in ten healthy volunteers and 12 paroxysmal nocturnal hemoglobinuria (PNH) patients by cytofluorometry. In healthy controls, each subset of lymphocytes showed a small population of cells weakly positive and a large population of cells strongly positive for DAF and CD59, while erythrocytes showed a single population of cells positive for the PI proteins. The two-population expression of DAF was most distinctive in CD8+ T cells among the subsets. In PNH, each subset of lymphocytes showed a moderately higher population of cells weakly positive and a smaller population of cells strongly positive for the membrane proteins compared with those in the healthy controls. Moreover, in some PNH cases, a negative population for the proteins was found in all subsets. Thus the analysis of PI-anchored proteins on lymphocytes subsets (especially CD8+ T cells) was considered to be of diagnostic value in PNH patients who receive blood transfusion after hemolytic attack of affected erythrocytes. Furthermore, the two-population expression of PI proteins in normal lymphocytes suggests that membrane PI protein would be a new subset marker of lymphocytes.

Structural properties of the glycoplasmanylinositol anchor phospholipid of the complement membrane attack complex inhibitor CD59

CD59, the membrane regulator of autologous C5b-9 channel formation, exhibits variable sensitivity to cleavage by phosphatidylinositol-specific phospholipase C (PI-PLC), an enzyme that releases glyco-inositolphospholipid (GPI)-anchored proteins from cell surfaces. To determine whether the GPI-anchor phospholipid of CD59 is similar to that of decay-accelerating factor (DAF) and whether variation in its structure underlies its variable enzyme susceptibility, the GPI anchors of the two proteins expressed on erythrocytes, polymorphonuclear and mononuclear leucocytes were compared in situ and after purification. Flow cytometric analyses of PI-PLC-treated cells showed parallel cell type specific release of both proteins as a function of enzyme concentration. Non-denaturing PAGE analyses of alkaline/hydroxylamine-treated proteins (affinity-purified from [125I]-surface-labelled cells) provided evidence for (i) comparable proportions of GPI-anchor acylation, and (ii) alkali-resistant rather than alkali-sensitive lipid substituents in erythrocytes. These findings argue that the differential C5b-9 sensitivity that distinguishes paroxysmal nocturnal haemoglobinuria II and III erythrocytes does not derive from expression of CD59 molecules with alternative GPI-anchor phospholipid structures.

Differential expression of glycosylphosphatidylinositol-anchored proteins in a murine T cell hybridoma mutant producing limiting amounts of the glycolipid core. Implications for paroxysmal nocturnal hemoglobinuria

A T cell hybridoma mutant, which expressed a markedly reduced level of glycosylphosphatidylinositol (GPI)-anchored proteins on the cell surface, was characterized. The surface expression level of Thy-1 was approximately 17% of the wild-type level, whereas the surface expression of Ly-6A was approximately 2.4% of the wild-type level. We show here that these cells synthesized limiting amounts of the GPI core and that the underlying defect in these cells was an inability to synthesize dolichyl phosphate mannose (Dol-P-Man) at the normal level. The defect in Ly-6A expression could be partially corrected by tunicamycin, which blocked the biosynthesis of N-linked oligosaccharide precursors and shunted Dol-P-Man to the GPI pathway. Full restoration of Thy-1 and Ly-6A expression, however, required the stable transfection of a yeast Dol-P-Man synthase gene into the mutants. These results revealed that when the GPI core is limiting, there is a differential transfer of the available GPI core to proteins that contain GPI-anchor attachment sequences. Our findings also have implications for the elucidation of the defects in paroxysmal nocturnal hemoglobinuria.

Decreased expression of Fc gamma RIII mRNA in leukemic granulocytes

Morphologically mature granulocytes from patients with chronic myeloid leukemia show significant impairment in their ability to internalize aggregated IgG, a ligand that is rapidly phagocytosed by normal human granulocytes. With a view to understand the molecular basis of this defect, normal and leukemic granulocytes were examined for the steady-state levels of mRNA for Fc gamma RIII, a membrane-associated receptor that initially binds and traps the IgG-opsonized antigens. Northern blot analyses revealed that the level of the specific mRNA in CML granulocytes was between 0.08 and 0.69 times that seen in the normal granulocytes. This could be one of the contributory factors for the observed endocytic defect in the leukemic granulocytes.

The erythrocytes in paroxysmal nocturnal haemoglobinuria of intermediate sensitivity to complement lysis

The sensitivity to lysis by complement of the erythrocytes of 56 patients with paroxysmal nocturnal haemoglobinuria (PNH) was compared to the membrane expression of decay accelerating factor (DAF, CD55), membrane inhibitor of reactive lysis (MIRL, CD59) and acetylcholinesterase (AChE). Most patients (36/50 72% in whom the analysis could be made) appeared to have erythrocytes of intermediate sensitivity to complement in the blood. These cells appeared as a discrete population of cells (PNH II cells), as a 'tail' of cells slightly less sensitive than the predominant PNH III cells (previously called PNH IIIb cells), or as a continuous spectrum of cells sensitive to complement. The PNH III cells totally lacked all three proteins (DAF, MIRL, AChE) by flow cytometric analysis whereas PNH I cells appeared to have normal or nearly normal amounts of each. The cells of intermediate sensitivity (PNH II) had coordinately decreased expression of all three proteins; the level of expression of DAF and MIRL paralleled the sensitivity of the cells to the haemolytic action of complement.

Characterization of the decay-accelerating factor gene promoter region

Decay-accelerating factor (DAF) expression modulates susceptibility of cells to autologous complement attack. To characterize the regulatory region controlling DAF gene transcription, genomic DNA extending from 815 base pairs (bp) upstream to approximately 4 kilobases downstream of DAF's AUG codon (designated +1) was cloned and sequenced. The 5' flanking sequence showed 59-76% G + C content (-355 to +1), at least one GC box(es) (-135 to -131), and variable length sequences (from -629 to -285) conforming to the motifs TCCTCC and TCn. Nuclease S1 digestions and primer extensions localized a major transcriptional start site to -82/-81, 38 bp downstream of a possible TATA variant, (A)TTTAA. In COS cell transfections, the sequence encompassing -815 to -67 functioned 2.5% as efficiently as the Rous sarcoma virus 3' long terminal repeat, but following deletion upstream of -355 its activity increased approximately 4-fold. Two octanucleotides exhibiting partial homology to phorbol 12-myristate 13-acetate (PMA) and cAMP responsive elements (PREs and CREs, respectively) were detected, and the respective modulators enhanced transcriptional efficiency 2- and approximately 10-fold, respectively. Thus, the DAF gene promoter (i) exhibits sequences resembling both conventional and unconventional transcriptional control elements, (ii) possesses a region with negative regulatory activity, and (iii) responds to PMA and cAMP induction presumably via PRE- and CRE-like enhancer elements.

Dr(a-) polymorphism of decay accelerating factor. Biochemical, functional, and molecular characterization and production of allele-specific transfectants

The Dra antigen belongs to the Cromer-related blood group system, a series of antigens on decay accelerating factor (DAF), a glycosyl-phosphatidylinositol-anchored membrane protein that protects host cells from complement-mediated damage. We studied the rare inherited Dr(a-) phenotype to ascertain the associated biochemical and functional changes in DAF and to characterize the basis for this polymorphism. Radioimmunoassay assay and flow cytometric analysis of Dr(a-) erythrocytes demonstrated 40% of normal surface expression of DAF but normal levels of several other glycosyl-phosphatidylinositol-anchored proteins, distinguishing this phenotype from that of paroxysmal nocturnal hemoglobinuria. Western blots confirmed this reduced DAF expression and indicated a slightly faster mobility of the molecule on SDS-PAGE. Despite the reduced DAF expression, Dr(a-) erythrocytes functioned normally in the complement lysis sensitivity assay. Utilization of the polymerase chain reaction to amplify mononuclear cell genomic DNA from three unrelated Dr(a-) individuals demonstrated that a point mutation underlies the Dr(a-) phenotype: a C to T change in nucleotide 649 resulting in a serine165 to leucine change. This defines the Drb allele of DAF, which can be distinguished from Dra by a Taq I restriction fragment length polymorphism. We created transfected Chinese hamster ovary cell lines expressing either the Dra or the Drb allelic form of DAF. These allele-specific transfectants were tested by inhibition of hemagglutination or flow cytometry and confirmed the specificity of anti-Dra alloantisera. The allele-specific transfectants could form the basis of a new serological approach to immunohematology.

Assembly and deacetylation of N-acetylglucosaminyl-plasmanylinositol in normal and affected paroxysmal nocturnal hemoglobinuria cells

Decay-accelerating factor (DAF) is anchored in cell membranes by a glycosyl-plasmanylinositol (GPI) moiety that is transferred to it en bloc in the rough endoplasmic reticulum. To analyze the biochemical reactions involved in preassembly of this structure, a human hematopoietic cell-free system was employed. Incubation of cell extracts with UDP-[3H]GlcNAc and butanol partitioning of reaction mixtures yielded two products similar in TLC mobility to intermediates described in Trypanosoma brucei. Both species were sensitive to Bacillus thuringiensis phosphatidylinositol-specific phospholipase C, indicative of association of [3H]GlcNAc label with a plasmanylinositol-containing acceptor. In contrast to trypanosome intermediates, which contain phosphatidylinositol (1,2-diacylglycerophosphoinositol), however, alkali treatment and phospholipase A2 digestion generated butanol-phase products characteristic of glycosylated plasmanylinositol (1-alkyl-2-acylglycerophosphoinositol). Kinetic and pulse-chase experiments indicated that the slower-migrating species was a product of the faster and that it, but not the faster, was sensitive to both GPI-specific phospholipase D and nitrous acid deamination, consistent with conversion of GlcNAc- to GlcN-plasmanylinositol. Accordingly, acetic anhydride acetylation retransformed the slower species back to the faster. Further incubation with cell extracts converted the slower species into more polar products. Lysates of normal and of affected blood leukocytes from two paroxysmal nocturnal hemoglobinuria (PNH) patients supported assembly of the two intermediates within 1 min. Thus, the initial enzymes mediating human GPI-anchor assembly are GlcNAc-plasmanylinositol transferase and GlcNAc-plasmanylinositol deacetylase, their substrates contain plasmanylinositols, and the products of their activities are normal in affected PNH cells.

Preferential expression of human Fc gamma RIIIPMN (CD16) in paroxysmal nocturnal hemoglobinuria. Discordant expression of glycosyl phosphatidylinositol-linked proteins

The isoform of Fc gamma RIII (CD16) expressed on PMN has a GPI membrane anchor, and in paroxysmal nocturnal hemoglobinuria (PNH) there is a deficiency in Fc gamma RIII expression on PMN. Contrary to expectation, however, CD16 expression is preserved (albeit at reduced levels) in all affected PNH PMN that completely lack the GPI-anchored proteins DAF (CD55) and CD59. Fc gamma RIII negative PMN are not observed in any of the six PNH patients examined in this study. Analysis of the molecular weight of both glycosylated and deglycosylated Fc gamma RIII from PMN with reduced Fc gamma RIII expression indicates no variations in size relative to normal donor Fc gamma RIIIPMN. Indeed, the Fc gamma RIII expressed at intermediate levels is phosphatidylinositol-specific phospholipase C (PI-PLC)-sensitive. Thus, there is no evidence suggestive of expression of a transmembrane isoform and all data indicate that Fc gamma RIIIPMN on affected cells in PNH is a GPI-linked isoform. With Fc gamma RIIIPMN expression preserved at reduced levels on affected cells in PNH, PMN from PNH patients retain the capacity to internalize the Fc gamma RIIIPMN-specific probe E-ConA (at reduced levels) as well as IgG-opsonized erythrocytes. Reduced expression of GPI-anchored molecules on PNH PMN is not restricted to Fc gamma RIIIPMN since intermediate levels of CD59 were observed in the PNH PMN that were decay-accelerating factor (DAF)-negative and Fc gamma RIIIPMN intermediate. In addition, discordant expression of GPI-linked molecules in individual cells is not restricted to PMN since DAF+/CD14- monocytes were observed in one PNH patient. These data suggest that, when analyzed on an individual cell level, the GPI anchor defect in PNH is not absolute and must involve either a hierarchy of access of different protein molecules to available GPI anchors, distinct anchor biochemistries for the different proteins, or differential regulation of protein-anchor assembly.

[Paroxysmal nocturnal hemoglobinuria]

Paroxysmal nocturnal hemoglobinuria, first described in the late 19th century, is an acquired disorder characterized by hemoglobinemia and hemoglobinuria. The major clinical manifestation of PNH is chronic intravascular hemolysis of various severity. Patients-mostly young adults - may also present with episodes of abdominal or back pain. Common cause of death is thrombosis especially of the hepatic veins. Granulocytopenia and thrombocytopenia may be the initial manifestation of PNH, indicating that the disorder is a primary bone-marrow disease, affecting not only the erythrocytes but also other peripheral blood cells and the haematopoietic stem cell. The course of the disease is variable. Partial complete recovery was described, but also fatal thrombosis. The major phenotypic expression of PNH is an increased susceptibility of the erythrocytes to the lytic action of complement in vitro. The enhanced complement susceptibility is most probably due to membrane defects: two membrane proteins regulating the complement cascade in PNH cells were missing, the decay-accelerating factor, DAF, inhibiting the activation of the lytic complement complex and the C8 binding protein, C8bp, which interferes with the lytic process. Aside from the lack of the complement regulators also other membrane defects have been described (e.g. of acetylcholinesterase or alkaline phosphatase). The proteins as well as DAF and C8bp are linked to the cell membrane via a phosphatidylinositol (PI) anchor, leading to the speculation that the disease results from a deficiency in the post-translational PI anchoring mechanism. The diagnosis of PNH is based on the Hamtest, but will be extended to the quantitation of the above described membrane proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of aberrant decay-accelerating factor proteins by affected paroxysmal nocturnal hemoglobinuria leukocytes

Paroxysmal nocturnal hemoglobinuria (PNH) leukocytes fail to express decay-accelerating factor (DAF) but contain DAF mRNA transcripts resembling those in normal cells. To further investigate the nature of the DAF defect in affected cells, patients' polymorphonuclear and mononuclear leukocytes (PMN and MNC) were biosynthetically labeled and newly synthesized DAF proteins examined. Analyses of greater than 98% surface DAF-negative PMN and MNC from a patient with PNH III erythrocytes showed precursor DAF protein approximately 3 kD smaller in each cell type than in normal cells. The proportion of precursor to mature (O-glycosylated) DAF protein was increased and soluble DAF protein was detected in the medium. Studies of 70-80% surface DAF-negative PMN and MNC from four patients with type II erythrocytes showed mixtures of the 3 kD smaller and normal DAF precursors. Partitioning with Triton X-114 detergent and biosynthetic labeling with the anchor precursor [3H]ethanolamine indicated that the abnormal peptides lacked glycosyl-inositolphospholipid membrane-anchoring structures. Thus, in PNH cells nascent DAF polypeptides are synthesized. Some of the abnormal pro-DAF molecules are processed in the Golgi and some are released extracellularly.

Paroxysmal nocturnal haemoglobinuria with coexisting deficiency of the ninth component of complement: lack of massive haemolytic attack

A 47-year-old woman with paroxysmal nocturnal haemoglobinuria (PNH) was found to have an inherited deficiency in the ninth complement component (C9). In complement-sensitivity lysis tests, 80% of her erythrocytes were markedly complement-sensitive (PNH-III). Laser cytofluorimetry with a monoclonal antibody against decay-accelerating factor (DAF) revealed that 95% of her erythrocytes were DAF-negative. Surprisingly, she has suffered only mild haemolysis and has never experienced massive spontaneous haemolysis. Gross haemoglobinuria and jaundice occurred only after receiving postoperative transfusion of whole blood. In her serum, C9 was not detectable either by immunological or by functional assays. Both the Ham test and the sugar water test using normal human serum or plasma yielded marked haemolysis of the patient's erythrocytes. When the patient's serum or plasma was used, only a trace of lysis was detected. Addition of purified human C9 to her plasma fully restored haemolysis. These observations indicated that C9 may play a critical role in haemolytic attacks in patients with PNH and that characteristic haemolysis in PNH may be tempered by coexisting C9 deficiency.

Studies on the sensitivity to complement-mediated lysis of erythrocytes (Inab phenotype) with a deficiency of DAF (decay accelerating factor)

No episodes of clinically significant in vivo haemolysis have been reported in individuals with a novel form of decay accelerating factor (DAF) deficiency (Inab phenotype), nor do functional in vitro assays for complement-mediated haemolysis show the extreme sensitivity to lysis characteristic of paroxysmal nocturnal haemoglobinuria (PNH) erythrocytes. DAF appears to be totally deficient in the Inab erythrocytes as judged by immunochemical and functional assays. Unlike PNH, the only other described DAF deficiency (where several other phosphatidylinositol (PI)-linked membrane proteins are also absent), the only protein lacking from Inab erythrocytes appears to be DAF. The Inab phenotype seems to be an inherited specific defect in DAF whereas PNH is an acquired defect in the mechanism of insertion of PI-linked proteins into cell membranes. These findings support the view that susceptibility of PNH erythrocytes to in vivo and in vitro complement-mediated haemolysis is not due simply to DAF deficiency but to either the combined lack of several membrane proteins or to deficiency of other regulatory proteins such as the membrane attack complex inhibitor/homologous restriction factor (MIP/HRF). The findings also raise questions as to the role of erythrocyte DAF.

Studies on the defect which causes absence of decay accelerating factor (DAF) from the peripheral blood cells of an individual with the Inab phenotype

1. We have studied the peripheral blood cells of an individual with the Inab phenotype who is deficient in decay accelerating factor (DAF). 2. In contrast with the situation in paroxysmal nocturnal haemoglobinuria, membranes from peripheral blood cells of the Inab phenotype individual lack DAF, but retain the other glycosylphosphatidylinositol-linked proteins acetylcholinesterase and LFA-3. 3. Unlike normal Epstein-Barr-virus-transformed lymphoblastoid cell lines (EBV-LCL), DAF was not expressed on EBV-LCL derived from peripheral blood lymphocytes of the Inab individual. 4. No differences in the DAF gene of normal and Inab phenotype individuals could be detected by Southern blotting studies. 5. EBV-LCL derived from the Inab individual had a gross reduction in the level of DAF mRNA compared with normal EBV-LCL. 6. Our results suggest that the DAF gene in the Inab phenotype contains a mutation which affects the transcription or processing of DAF mRNA.

Heightened complement sensitivity of acquired immunodeficiency syndrome lymphocytes related to diminished expression of decay-accelerating factor

Although the human immunodeficiency virus can induce cytopathic changes in human lymphocytes in vitro, the mechanism(s) underlying progressive lymphopenia in patients with AIDS and AIDS-related complex has not been elucidated. To investigate this issue, peripheral blood lymphocytes of AIDS and AIDS-related complex patients and healthy control subjects were examined for their ability to resist homologous complement-mediated lysis. Upon sensitization with monoclonal antibodies to major histocompatibility complex class I antigen, as much as 48% lysis of patients' cells was observed in as little as a 1:32 dilution of human serum compared to 18 +/- 8% (mean +/- SD) lysis of controls' cells even in a 1:8 dilution of human serum. To investigate the mechanism of the abnormal complement sensitivity, AIDS and AIDS-related complex cells were analyzed for expression of decay-accelerating factor (DAF), a complement regulatory protein that functions intrinsically in blood cell membranes to prevent complement activation on their surfaces. Flow cytometric assays using anti-DAF monoclonal antibodies demonstrated that patients' lymphocytes and monocytes were DAF-deficient, in contrast to their polymorphonuclear leukocytes, which showed normal DAF levels. Expression of DAF was diminished on CD4+ as well as CD8+ T-lymphocyte subpopulations as opposed to expression of CD3, which was comparable in patients and controls. Incubation of normal lymphocytes with anti-DAF monoclonal antibodies or phosphatidylinositol-specific phospholipase C, an enzyme that cleaves DAF, enhanced lysis. Conversely, reconstitution of patients' cells with exogenous DAF reduced their lysis. The findings of heightened complement sensitivity and DAF deficiency of patients' lymphocytes in vitro suggest the possibility that the DAF deficit may contribute to the progressive lymphopenia of AIDS in vivo.