Interferon-gamma inhibits the intrahepatocytic development of malaria parasites in vitro

In this study, we examined the activity of recombinant interferon (IFN)-gamma against Plasmodium berghei exoerythrocytic forms (EEF) grown in vitro within the highly differentiated human hepatoma cell line HEPG2. We assayed the effect of IFN-gamma on parasite growth by DNA hybridization using a P. berghei specific DNA probe. The specific activity of IFN-gamma against EEF is very high, and depends upon the time of lymphokine addition. When IFN-gamma is added to HEPG2 cells containing intracellular EEF, 6 hr after sporozoite invasion, parasite DNA replication is inhibited by approximately 75% at 10(3) U/ml and 50% at 1 U/ml. This treatment can either abolish or greatly reduce the infectivity of EEF for mice. When added earlier, 3 hr after completion of sporozoite invasion, IFN-gamma inhibits parasite replication to an even greater degree. The highest levels of inhibition were obtained when IFN-gamma was added 6 hr prior to sporozoite invasion (100% inhibition at 10(2) U/ml, approximately 55% inhibition at 0.1 U/ml, and 17% inhibition at 0.001 U/ml). We found that HEPG2 cells express approximately 44,000 surface receptors for IFN-gamma. These data are consistent with the view that IFN-gamma exerts its antimalarial activity by binding to surface receptors on hepatocytes and inducing intracellular changes unfavorable for parasite development. Tryptophan starvation does not appear to be involved in this process. These findings also support the idea that IFN-gamma, released from immune T cells upon encountering sporozoite antigen, may be an important effector mechanism in sterile immunity to sporozoite challenge.

Relationship between decay accelerating factor deficiency, diminished acetylcholinesterase activity, and defective terminal complement pathway restriction in paroxysmal nocturnal hemoglobinuria erythrocytes

Paroxysmal nocturnal hemoglobinuria (PNH) erythrocytes exhibit abnormalities in decay accelerating factor (DAF), acetylcholinesterase, and resistance to autologous C5b-9 attack. To investigate the nature of the lesion underlying PNH cells, we examined the relationship of these abnormalities to one another. Analyses of DAF in acetylcholinesterase-negative erythrocytes revealed that these two abnormalities involve functionally independent molecules, coincide precisely in the same cell populations, and are similarly expressed in PNH II and more complement-sensitive PNH III erythrocytes. The DAF and acetylcholinesterase deficiencies contrast with the C3b/C4b receptor (CR1) deficit, which is less profound and similarly distributed in complement-insensitive cell populations. Hemolytic studies showed that defective resistance to autologous C5b-9 attack is mediated by another mechanism. Whereas reconstitution of PNH II erythrocytes with DAF completely corrected their complement sensitivity, DAF reconstitution of PNH III erythrocytes restored their ability to circumvent C3b uptake but had no effect on their heightened susceptibility to reactive lysis. Assays of complement-insensitive (PNH I) erythrocytes surviving after reactive lysis disclosed partial DAF and acetylcholinesterase deficits. These findings indicate that the PNH lesion involves multiple membrane components and that PNH I erythrocytes are also abnormal.

Mechanism of escape of exoerythrocytic forms (EEF) of malaria parasites from the inhibitory effects of interferon-gamma

We have studied the mechanism of inhibition by interferon-gamma (IFN-gamma) of the development of exoerythrocytic forms (EEF) of Plasmodium berghei in the livers of rats. At the time corresponding to the maximum development of EEF (44 hr after injection of sporozoites), the livers of the IFN-gamma-treated rats contained less parasite DNA as compared with controls. Twenty-four to 72 hr later, the livers of both groups of animals were free of parasites; that is, IFN-gamma treatment does not delay the development of the EEF. The decrease in parasite DNA observed in the IFN-gamma-treated rats was due to a diminution in the number, but not the size, of EEF. It appears, therefore, that treatment with the lymphokine either destroys the parasites or does not affect their replication. To study the mechanism of resistance to IFN-gamma of a small population of EEF, we subjected the parasites to four cycles of selection by IFN-gamma. The parasites from the "selected" and "nonselected" populations were equally susceptible to inhibition by IFN-gamma, indicating that the escape from IFN-gamma activity is not inherited.

Mechanism of escape of exoerythrocytic forms (EEF) of malaria parasites from the inhibitory effects of interferon-gamma

We have studied the mechanism of inhibition by interferon-gamma (IFN-gamma) of the development of exoerythrocytic forms (EEF) of Plasmodium berghei in the livers of rats. At the time corresponding to the maximum development of EEF (44 hr after injection of sporozoites), the livers of the IFN-gamma-treated rats contained less parasite DNA as compared with controls. Twenty-four to 72 hr later, the livers of both groups of animals were free of parasites; that is, IFN-gamma treatment does not delay the development of the EEF. The decrease in parasite DNA observed in the IFN-gamma-treated rats was due to a diminution in the number, but not the size, of EEF. It appears, therefore, that treatment with the lymphokine either destroys the parasites or does not affect their replication. To study the mechanism of resistance to IFN-gamma of a small population of EEF, we subjected the parasites to four cycles of selection by IFN-gamma. The parasites from the "selected" and "nonselected" populations were equally susceptible to inhibition by IFN-gamma, indicating that the escape from IFN-gamma activity is not inherited.

A high m.w. form of decay-accelerating factor (DAF-2) exhibits size abnormalities in paroxysmal nocturnal hemoglobinuria erythrocytes

Decay-accelerating factor (DAF) is a 70,000 Mr membrane protein that inhibits the amplification of the complement cascade on cell surfaces. Monoclonal antibodies against different epitopes of the 70,000 Mr DAF (DAF-1) recognize a second band at the position of 140,000 Mr on a Western blot of total red cell ghost proteins or partially pure DAF subjected to electrophoresis under denaturing conditions. Like DAF-1, this polypeptide (DAF-2) has the ability to accelerate decay of the C3 convertase, C4b2a, and to reincorporate into red cell membranes. A population of erythrocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) lack DAF-1 and also DAF-2. In addition, in some patients' red cells bearing DAF-1 of normal Mr, DAF-2 is 5,000 to 10,000 Mr smaller than normal. The structural basis for these differences in size of DAF and its PNH variants is unknown.

A high m.w. form of decay-accelerating factor (DAF-2) exhibits size abnormalities in paroxysmal nocturnal hemoglobinuria erythrocytes

Decay-accelerating factor (DAF) is a 70,000 Mr membrane protein that inhibits the amplification of the complement cascade on cell surfaces. Monoclonal antibodies against different epitopes of the 70,000 Mr DAF (DAF-1) recognize a second band at the position of 140,000 Mr on a Western blot of total red cell ghost proteins or partially pure DAF subjected to electrophoresis under denaturing conditions. Like DAF-1, this polypeptide (DAF-2) has the ability to accelerate decay of the C3 convertase, C4b2a, and to reincorporate into red cell membranes. A population of erythrocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) lack DAF-1 and also DAF-2. In addition, in some patients' red cells bearing DAF-1 of normal Mr, DAF-2 is 5,000 to 10,000 Mr smaller than normal. The structural basis for these differences in size of DAF and its PNH variants is unknown.

Expression in yeast of a Plasmodium vivax antigen of potential use in a human malaria vaccine

DNA coding for 234 amino acids of the circumsporozoite (CS) protein of Plasmodium vivax was incorporated into yeast expression vectors. The DNA encoded all the repeat domain and codons for a highly conserved sequence, KLKQP, found in CS proteins from all malaria parasites. Yeast cells transformed with these autonomously replicating plasmids expressed, upon induction, high levels of the CS polypeptide. The malaria antigen was purified in good yields from yeast extracts and was injected into mice using alum as adjuvant. The antibodies recognized the authentic CS protein, and at high dilutions, they inhibited the invasion of hepatocytes by sporozoites in vitro.

Identification of the complement decay-accelerating factor (DAF) on epithelium and glandular cells and in body fluids

Decay-accelerating factor (DAF) is a 70 kD membrane regulatory protein that prevents the activation of autologous complement on cell surfaces. Using immunohistochemical methods and a radioimmunometric assay based on mAbs to DAF, we found large amounts of membrane-associated DAF antigen on the epithelial surface of cornea, conjunctiva, oral and gastrointestinal mucosa, exocrine glands, renal tubules, ureter and bladder, cervical and uterine mucosa, and pleural, pericardial and synovial serosa. Additionally, we detected soluble DAF antigen in plasma, tears, saliva, and urine, as well as in synovial and cerebrospinal fluids. While plasma, tear, and saliva DAF are larger than erythrocyte (Ehu) membrane DAF by Western blot analysis, urine DAF is slightly smaller (67,000) in Mr. Unlike purified Ehu DAF, however, urine DAF is unable to incorporate into the membrane of red cells. Although its inhibitory activity on the complement enzyme C3-convertase is lower than that of Ehu DAF, it is comparable to that of serum C4 binding protein (C4bp). Biosynthetic studies using cultured foreskin epithelium and Hela cells disclosed DAF levels (approximately 2 X 10(5) molecules/cell) exceeding those on blood cells. In addition, these studies revealed the synthesis of two DAF species, one with apparent Mr corresponding to that of epithelial cell membrane DAF and the other to urine DAF, suggesting that the urine DAF variant arises from adjacent epithelium. The function of DAF in body fluids is unknown, but the observation that urine DAF has C4bp-(or factor H-)like activity shows that it could inhibit the fluid phase activation of the cascade.

Isolation of decay accelerating factor (DAF) by a two-step procedure and determination of its N-terminal sequence

Decay-accelerating factor (DAF) from human red cell membranes was purified by a two-step procedure involving anion exchange and immunoaffinity chromatography. The DAF preparations were purified to homogeneity as judged by silver staining. In several experiments, the final product yields were approximately 23% of the total DAF present in the initial membrane extracts. The purified DAF retained its ability to inhibit the classical pathway C3-convertase and to reincorporate into cell membranes. An amino-terminal sequence was obtained by gas-phase sequencing. Rabbit antibodies to a synthetic peptide representing part of this sequence reacted with purified reduced membrane DAF by Western blotting and by a solid-phase immunoradiometric assay.

Use of a DNA probe to measure the neutralization of Plasmodium berghei sporozoites by a monoclonal antibody

A specific DNA probe has been used to quantify the neutralizing effects of monoclonal antibodies (3D11) against the circumsporozoite protein of Plasmodium berghei sporozoites. The amount of parasite DNA was measured in the livers of Norway Brown rats at the peak of proliferation of the exoerythrocytic forms (EEF). In vitro treatment of 1.5 X 10(5) sporozoites with 0.36 microgram/0.5 ml of whole 3D11 IgG neutralized about 90% of the sporozoite infectivity. When the dose was 3.6 micrograms no signal was detected, indicating that less than ten sporozoites developed into EEF in the liver. In contrast, 3.6 micrograms of Fab obtained from 3D11 neutralized sporozoite infectivity by only 60%. Although the neutralizing effect of 3D11 was very marked, the infected rats developed parasitemias after a prolonged delay in patency, suggesting that a small proportion of sporozoites was resistant to the effects of 3D11. The sporozoites were subjected to four cycles of 3D11-mediated selection, each one involving treatment of sporozoites with the antibodies, injection of the mixture into rats, infection of hamsters with blood stage parasites obtained from the rats, feeding of Anopheles stephensi on these hamsters, and obtaining sporozoites from the salivary glands of the infected mosquitoes. After four cycles of selection, the susceptibility of the resulting sporozoites to different concentrations of 3D11 was compared with that of nonselected sporozoites. No differences were detected, indicating that the capacity of a few sporozoites to escape the neutralizing effect of 3D11 antibodies is not inherited.

Use of a DNA probe to measure the neutralization of Plasmodium berghei sporozoites by a monoclonal antibody

A specific DNA probe has been used to quantify the neutralizing effects of monoclonal antibodies (3D11) against the circumsporozoite protein of Plasmodium berghei sporozoites. The amount of parasite DNA was measured in the livers of Norway Brown rats at the peak of proliferation of the exoerythrocytic forms (EEF). In vitro treatment of 1.5 X 10(5) sporozoites with 0.36 microgram/0.5 ml of whole 3D11 IgG neutralized about 90% of the sporozoite infectivity. When the dose was 3.6 micrograms no signal was detected, indicating that less than ten sporozoites developed into EEF in the liver. In contrast, 3.6 micrograms of Fab obtained from 3D11 neutralized sporozoite infectivity by only 60%. Although the neutralizing effect of 3D11 was very marked, the infected rats developed parasitemias after a prolonged delay in patency, suggesting that a small proportion of sporozoites was resistant to the effects of 3D11. The sporozoites were subjected to four cycles of 3D11-mediated selection, each one involving treatment of sporozoites with the antibodies, injection of the mixture into rats, infection of hamsters with blood stage parasites obtained from the rats, feeding of Anopheles stephensi on these hamsters, and obtaining sporozoites from the salivary glands of the infected mosquitoes. After four cycles of selection, the susceptibility of the resulting sporozoites to different concentrations of 3D11 was compared with that of nonselected sporozoites. No differences were detected, indicating that the capacity of a few sporozoites to escape the neutralizing effect of 3D11 antibodies is not inherited.

Cloning of decay-accelerating factor suggests novel use of splicing to generate two proteins

Decay-accelerating factor (DAF), a glycoprotein that is anchored to the cell membrane by phosphatidylinositol, binds activated complement fragments C3b and C4b, thereby inhibiting amplification of the complement cascade on host cell membranes. Here, we report the molecular cloning of human DAF from HeLa cells. Analysis of DAF complementary DNAs revealed two classes of DAF messenger RNA, one apparently derived from the other by a splicing event that causes a coding frameshift near the C terminus. The apparent 'intron' sequence contains an Alu family member and encodes contiguous protein sequence. Two DAF proteins are therefore possible, having divergent C-terminal domains which differ in their hydrophobicity. Both mRNAs are found on polysomes, suggesting that both are translated. We propose that the major (90%) spliced DAF mRNA encodes membrane-bound DAF whereas the minor (10%) unspliced DAF mRNA may encode secreted DAF and we present expression data supporting this. The deduced DAF sequence contains four repeating units homologous to a consensus repeat found in a recently described family of complement proteins.

Decay-accelerating factor (DAF) shares a common carbohydrate determinant with the variant surface glycoprotein (VSG) of the African Trypanosoma brucei

Decay-accelerating factor (DAF) is an integral membrane protein that inhibits amplification of the complement cascade on the cell surface. We and other investigators have shown that DAF is part of a newly characterized family of proteins that are anchored to the cell membrane by phosphatidylinositol (PI). The group includes the variant surface glycoprotein (VSG) of African trypanosomes, the p63 protein of Leishmania, acetylcholinesterase (AChE), alkaline phosphatase, Thy-1, 5'-nucleotidase, and RT6.2--an alloantigen from rat T cells. The structure of the membrane anchor has been best characterized for VSG, but chemical studies of the membrane anchors of AChE and Thy-1 suggest that similar glycolipid moieties anchor these proteins to the cell surface. In the VSG, the membrane anchor consists of an ethanolamine linked covalently to an oligosaccharide and glucosamine; the entire complex is anchored to the cell membrane by PI. Immunologically, this glycolipid defines an epitope, the cross-reacting determinant (CRD), that is only revealed after removal of the diacyl glycerol anchor by a phospholipase C. By Western blotting, we show here that DAF-S (DAF released from the membrane by PI-specific phospholipase C [PIPLC]) also contains CRD. Using a newly developed immunoradiometric assay (IRMA) in which the solid-phase capturing antibody is a monoclonal antibody to DAF and the second antibody is anti-CRD, we have been able to quantitate DAF-S. By IRMA, we show that the reaction between anti-CRD and DAF-S is specific, since the binding is competitively inhibited only by the soluble form of the VSG. These observations further support the concept that the glycolipid anchors of this new family of proteins have similar structures. DAF is also found as a soluble protein in various tissue fluids as well as in Hela cell supernatants. No evidence for the presence of the CRD epitope was found on these proteins, suggesting that these forms of DAF are not released from the surface of cells by endogenous phospholipases.

Decay-accelerating factor (DAF) shares a common carbohydrate determinant with the variant surface glycoprotein (VSG) of the African Trypanosoma brucei

Decay-accelerating factor (DAF) is an integral membrane protein that inhibits amplification of the complement cascade on the cell surface. We and other investigators have shown that DAF is part of a newly characterized family of proteins that are anchored to the cell membrane by phosphatidylinositol (PI). The group includes the variant surface glycoprotein (VSG) of African trypanosomes, the p63 protein of Leishmania, acetylcholinesterase (AChE), alkaline phosphatase, Thy-1, 5'-nucleotidase, and RT6.2--an alloantigen from rat T cells. The structure of the membrane anchor has been best characterized for VSG, but chemical studies of the membrane anchors of AChE and Thy-1 suggest that similar glycolipid moieties anchor these proteins to the cell surface. In the VSG, the membrane anchor consists of an ethanolamine linked covalently to an oligosaccharide and glucosamine; the entire complex is anchored to the cell membrane by PI. Immunologically, this glycolipid defines an epitope, the cross-reacting determinant (CRD), that is only revealed after removal of the diacyl glycerol anchor by a phospholipase C. By Western blotting, we show here that DAF-S (DAF released from the membrane by PI-specific phospholipase C [PIPLC]) also contains CRD. Using a newly developed immunoradiometric assay (IRMA) in which the solid-phase capturing antibody is a monoclonal antibody to DAF and the second antibody is anti-CRD, we have been able to quantitate DAF-S. By IRMA, we show that the reaction between anti-CRD and DAF-S is specific, since the binding is competitively inhibited only by the soluble form of the VSG. These observations further support the concept that the glycolipid anchors of this new family of proteins have similar structures. DAF is also found as a soluble protein in various tissue fluids as well as in Hela cell supernatants. No evidence for the presence of the CRD epitope was found on these proteins, suggesting that these forms of DAF are not released from the surface of cells by endogenous phospholipases.

Research toward malaria vaccines

Malaria exacts a toll of disease to people in the Tropics that seems incomprehensible to those only familiar with medicine and human health in the developed world. The methods of molecular biology, immunology, and cell biology are now being used to develop an antimalarial vaccine. The Plasmodium parasites that cause malaria have many stages in their life cycle. Each stage is antigenically distinct and potentially could be interrupted by different vaccines. However, achieving complete protection by vaccination may require a better understanding of the complexities of B- and T-cell priming in natural infections and the development of an appropriate adjuvant for use in humans.

Circumsporozoite protein of Plasmodium berghei: gene cloning and identification of the immunodominant epitopes

The gene encoding the circumsporozoite (CS) protein of the rodent malaria parasite Plasmodium berghei was cloned and characterized. A cDNA library made from P. berghei sporozoite RNA was screened with a monoclonal antibody for expression of CS protein epitopes. The resulting cDNA clone was used to isolate the CS protein gene from a lambda library containing parasite blood-stage DNA. The CS protein gene contains a central region encoding two types of tandemly repeated amino acid units, flanked by nonrepeated regions encoding amino- and carboxy-terminal signal and anchorlike sequences, respectively. One of the central repeated amino acid unit types contains the immunodominant epitopes.

Membrane-bound C4b interacts endogenously with complement receptor CR1 of human red cells

Activation of the classical complement pathway on the membrane of autologous cells results in the deposition of C4b on their surface and in the assembly of the C3 convertase C4b2a, one of the amplifying enzymes of the cascade. Here we study the sequence of events leading to irreversible inactivation of the potentially harmful C4b bound to human red cells. We show that deposited C4b interacts endogenously with complement receptor type 1 (CR1) present on the membrane of the same red cell. Complexes containing CR1 and C4b are found in extracts of membranes of C4b-bearing red cells after treatment of the intact cells with a bifunctional crosslinking reagent. The amount of complexed CR1 increases with the number of deposited C4b molecules. Only small amounts of free CR1 are observed on red cells bearing as few as 1,900 molecules of C4b, suggesting that the binding avidity between C4b and endogenous CR1 is high. In agreement with this observation, we find that the deposited C4b inhibits the exogenous cofactor activity of the red cell CR1 for the factor I-mediated cleavage of target-bound clustered C3b. The C4b bound to the human red cells is cleaved by the serum enzyme C3b/C4b inactivator (factor I) and a large fragment (C4c) is released in the incubation medium. The cleavage is totally inhibited by mAbs against CR1, showing that the complement receptor is an essential cofactor for the activity of I. When the number of bound C4b per red cell is relatively small (less than 1,000 molecules) the substrate for the enzymatic activity of factor I is mostly or exclusively the C4b bound endogenously to CR1. Indeed, the kinetics or the extent of cleavage of C4b are not affected by greatly augmenting the concentration of exogenous CR1 or of C4b-bearing red cells in the incubation mixture, thereby increasing the frequency of collisions between CR1 on the surface of one cell with C4b deposited on the membrane of a different cell. On the basis of the present and prior observations, we speculate that both DAF and CR1 act endogenously to inactivate the function of autologous red cell-bound C4b and prevent the progression of the cascade. DAF binding prevents the formation of the C3 convertase, C4b2a. The cleavage and irreversible inactivation of C4b only occurs after the concerted activities of endogenous CR1 and serum factor I.(ABSTRACT TRUNCATED AT 400 WORDS)

Monoclonal anti-gametocyte antibodies identify an antigen present in all blood stages of Plasmodium falciparum

Two polypeptides of 150 and 130 kDa present in all asexual and sexual blood stages of Plasmodium falciparum have been identified with anti-gametocyte monoclonal antibodies. The apparent molecular mass of these antigens is identical in different developmental stages of the parasite and in different isolates. These antigens are released in the culture supernatant during the process of schizogony and are also detected in the sera of patients undergoing a primary P. falciparum infection. Antibodies against these antigens occur in sera of a large percentage of children and most adults living in malaria-endemic areas, suggesting that they are highly immunogenic. The anti-gametocyte monoclonal antibodies react with a synthetic peptide (Glu-Glu-Asn-Val)4, present in antigen Pf155 [Perlmann, H. et al. (1984) J. Exp. Med. 159, 1686-1704] and in the ring-infected erythrocyte surface antigen [Coppel, R.L. et al. (1984) Nature 310, 789-792], indicating that these polypeptides are closely related. In contrast, two glycophorin-binding proteins of similar molecular mass [Perkins, M.E. (1984) J. Exp. Med. 160, 788-798] appear to be entirely distinct from the presently described antigens. We failed to observe any in vitro inhibitory activity of the monoclonal antibodies on merozoite invasion and on gametocyte infectivity.

Inhibition of development of exoerythrocytic forms of malaria parasites by gamma-interferon

A specific DNA probe was used to study the effect of recombinant rat, mouse, and human gamma-interferon (gamma-IFN) on the course of sporozoite-induced malaria infections. In mice and rats infected with sporozoites of Plasmodium berghei, mouse and rat gamma-IFN's strongly inhibited the development of the exoerythrocytic forms in the liver liver cells of the hosts, but not the development of the erythrocytic stages. The degree of inhibition of the exoerythrocytic forms was proportional to the dose of gamma-IFN administered, but was independent of the number of sporozoites used for challenge. A 30 percent reduction in the development of exoerythrocytic forms in rat liver was achieved when 150 units (about 15 nanograms of protein) of rat gamma-IFN were injected a few hours before sporozoite challenge; the reduction was 90 percent or more with higher doses of gamma-IFN. The effect was less pronounced if the gamma-IFN was administered 18 hours before or a few hours after challenge. Human gamma-IFN also diminished the parasitemia in chimpanzees infected with sporozoites of the human malaria parasite Plasmodium vivax. The target of gamma-IFN activity may be the infected hepatocytes themselves, as shown by in vitro experiments in which small doses of the human lymphokine inhibited the development of exoerythrocytic forms of Plasmodium berghei in a human hepatoma cell line. These results suggest that immunologically induced interferon may be involved in controlling malaria infection under natural conditions.

Release of decay-accelerating factor (DAF) from the cell membrane by phosphatidylinositol-specific phospholipase C (PIPLC). Selective modification of a complement regulatory protein

Decay-accelerating factor (DAF) is a 70,000 Mr membrane protein that inhibits amplification of the complement cascade on the cell surface, and protects cells from damage. Purified DAF can be reincorporated into the membrane of red cells and is functional. DAF is deficient in paroxysmal nocturnal hemoglobinuria (PNH), a disease characterized by increased sensitivity of erythrocytes to complement lysis. We show here that DAF is part of a newly described family of membrane proteins anchored to the lipid bilayer by means of phosphatidylinositol (PI). Treatment with PI-specific phospholipase C (PIPLC) releases 70-80, 60, and 10% of cell surface DAF from mononuclear cells, neutrophils, and erythrocytes, respectively. The PIPLC-released DAF (DAF-S) is slightly smaller (67,000 Mr) than the membrane form. DAF and DAF-S cannot be distinguished antigenically. Furthermore, DAF-S has lost its ability to significantly inhibit the C3-convertase, as well as its ability to incorporate into cell membranes. Since DAF can only inhibit C3-convertase endogenously, i.e., within the membrane of the same cell, it is likely that the loss of activity of DAF-S is causally related to its inability to reincorporate in the lipid bilayer. As shown by others, the complement-sensitive red cells from PNH patients lack acetylcholinesterase, which is also anchored to the membrane by PI (9). Thus it is possible that the molecular defect in PNH lies in the biosynthetic pathways leading to the attachment of PI to the polypeptide chains, in the transport of these proteins to the surface, or in their release by the action of endogenous phospholipases. From a practical standpoint the specific release of DAF by PIPLC could facilitate killing of tumor cells by amplifying the effects of the complement cascade on the surface of antibody-sensitized cells.

Endogenous association of decay-accelerating factor (DAF) with C4b and C3b on cell membranes

Decay-accelerating factor (DAF) is a membrane glycoprotein found on various cells that are in contact with complement. It inhibits the formation of the C3 convertases of the complement system, both the classic (C4b2a) and alternative (C3bBb) pathways. In this investigation, we used a homobifunctional cross-linking reagent to search for a DAF ligand on the surface of cells subjected to complement attack. We found that DAF forms complexes with C4b and C3b deposited on the same erythrocytes, but not with the physiologic degradation products of these complement fragments, that is, C4d or C3dg. Taken together with prior observations that DAF action is reversible, and DAF does not affect the structure of C4b or C3b, these findings suggest that DAF functions by competitively inhibiting the uptake of C2 or factor B, and preventing the assembly of the C3 convertases.

Endogenous association of decay-accelerating factor (DAF) with C4b and C3b on cell membranes

Decay-accelerating factor (DAF) is a membrane glycoprotein found on various cells that are in contact with complement. It inhibits the formation of the C3 convertases of the complement system, both the classic (C4b2a) and alternative (C3bBb) pathways. In this investigation, we used a homobifunctional cross-linking reagent to search for a DAF ligand on the surface of cells subjected to complement attack. We found that DAF forms complexes with C4b and C3b deposited on the same erythrocytes, but not with the physiologic degradation products of these complement fragments, that is, C4d or C3dg. Taken together with prior observations that DAF action is reversible, and DAF does not affect the structure of C4b or C3b, these findings suggest that DAF functions by competitively inhibiting the uptake of C2 or factor B, and preventing the assembly of the C3 convertases.

Infectivity of Plasmodium berghei sporozoites measured with a DNA probe

A 2.3 kb, 32P-labeled repetitive DNA probe of Plasmodium berghei was used to measure the amount of parasite DNA in the liver of Norway Brown rats and mice infected with sporozoites. Standard hybridization curves were obtained by probing different amounts (100 pg to 1 microgram) of P. berghei DNA immobilized on nitrocellulose filters. Host DNA did not interfere with hybridization specificity and sensitivity. A 100-fold increase in hepatic parasite DNA was detected between 25 h post-infection and the peak of parasite proliferation, detected at 44 h. The amount of parasite DNA increased with the number of injected sporozoites. At 5 h post-infection, a large proportion of parasite DNA was found in the spleen. However, this diminished with time and was negligible in amount at 25 h. A significant number of viable sporozoites were probably cleared in the spleen, since considerably more parasite DNA was found in the livers of splenectomized rats than in sham-operated counterparts. Although older rats develop much lower parasitemias upon inoculation of sporozoites, no significant differences were observed in the amount of parasite DNA in rats, 43 and 152 days old, injected with equal numbers of sporozoites. The higher resistance to malaria displayed by older rats is probably controlled by post-hepatic events. The infectivity of sporozoites for A/J mice was calculated to be about 1/20th that of Norway Brown rats.

Decay-accelerating factor is present on cultured human umbilical vein endothelial cells

Decay-accelerating factor (DAF) has been previously described only in cells of bone marrow origin where it serves as a negative modulator of complement activation. Using mAb against human DAF, we demonstrated the presence of DAF in human umbilical vein endothelial cells by immunofluorescence microscopy and flow cytometry. By means of an immunoradiometric assay we detected an average of 3.3 X 10(5) molecules of DAF on each cell. When immunoisolates were analyzed in Western blots, endothelial cell DAF comigrated with DAF purified from normal erythrocytes. DAF was synthesized by the endothelial cells since 35S-labeled DAF could be immunoisolated from HUVEC cultured in medium containing [35S]methionine. This is the first evidence for the presence of DAF in cells of extra-marrow origin. DAF may protect endothelial cells from complement-mediated injury.