Phospholipid-anchored and transmembrane versions of either decay-accelerating factor or membrane cofactor protein show equal efficiency in protection from complement-mediated cell damage

Molecular events contributing to successful pediatric cardiac transplantation in HLA sensitized recipients

Antibodies to HLA resulting in positive cytotoxicity crossmatch are generally considered a contraindication for cardiac transplantation. However, cardiac transplantations have been performed in children by reducing the Abs and modifying immunosuppression. To identify mechanisms leading to allograft acceptance in the presence of Abs to donor HLA, we analyzed priming events in endothelial cells (EC) by incubating with sera containing low levels of anti-HLA followed by saturating concentration of anti-HLA. Pre-transplant sera were obtained from children with low levels of Abs to HLA who underwent transplantation. EC were selected for donor HLA and exposed to sera for 72 h (priming), followed by saturating concentrations of anti-HLA (challenge). Priming of EC with sera induced the phosphatidylinositol 3-kinase/Akt mediated by the BMP4/WNT pathway and subsequent challenge with panel reactive antibody sera increased survival genes Bcl2 and Heme oxygenase-1, decreased adhesion molecules, induced complement inhibitory proteins and reduced pro-inflammatory cytokines. In contrast, EC which did not express donor HLA showed decreased anti-apoptotic genes. Primed EC, upon challenge with anti-HLA, results in increased survival genes, decreased adhesion molecules, induction of complement inhibitory proteins, and downregulation of pro-inflammatory cytokines which may result in accommodation of pediatric cardiac allografts despite HLA sensitization.

Novel oncolytic viral therapies in patients with thoracic malignancies

Oncolytic virotherapy is the use of replication-competent viruses to treat malignancies. The potential of oncolytic virotherapy as an approach to cancer therapy is based on historical evidence that certain viral infections can cause spontaneous remission of both hematologic and solid tumor malignancies. Oncolytic virotherapy may eliminate cancer cells through either direct oncolysis of infected tumor cells or indirect immune-mediated oncolysis of uninfected tumor cells. Recent advances in oncolytic virotherapy include the development of a wide variety of genetically attenuated RNA viruses with precise cellular tropism and the identification of cell-surface receptors that facilitate viral transfer to the tissue of interest. Current research is also focused on targeting metastatic disease by sustaining the release of progeny viruses from infected tumor cells and understanding indirect tumor cell killing through immune-mediated mechanisms of virotherapy. The purpose of this review is to critically evaluate recent evidence on the clinical development of tissue-specific viruses capable of targeting tumor cells and eliciting secondary immune responses in lung cancers and mesothelioma.

Generation of a felinized swine endothelial cell line by expression of feline decay-accelerating factor

Embryonic stem cell research has facilitated the generation of many cell types for the production of tissues and organs for both humans and companion animals. Because ≥30% of pet cats suffer from chronic kidney disease (CKD), xenotransplantation between pigs and cats has been studied. For a successful pig to cat xenotransplant, the immune reaction must be overcome, especially hyperacute rejection. In this study, we isolated the gene for feline decay-accelerating factor (fDAF), an inhibitor of complement proteins, and transfected a swine endothelial cell line with fDAF to "felinize" the pig cells. These fDAF-expressing cells were resistant to feline serum containing anti-pig antibodies, suggesting that felinized pig cells were resistant to hyperacute rejection. Our results suggest that a "felinized" pig kidney can be generated for the treatment of CKD in cats in the future.

The role of decay accelerating factor in environmentally induced and idiopathic systemic autoimmune disease

Decay accelerating factor (DAF) plays a complex role in the immune system through complement-dependent and -independent regulation of innate and adaptive immunity. Over the past five years there has been accumulating evidence for a significant role of DAF in negatively regulating adaptive T-cell responses and autoimmunity in both humans and experimental models. This review discusses the relationship between DAF and the complement system and highlights major advances in our understanding of the biology of DAF in human disease, particularly systemic lupus erythematosus. The role of DAF in regulation of idiopathic and environmentally induced systemic autoimmunity is discussed including studies showing that reduction or absence of DAF is associated with autoimmunity. In contrast, DAF-mediated T cell activation leads to cytokine expression consistent with T regulatory cells. This is supported by studies showing that interaction between DAF and its molecular partner, CD97, modifies expression of autoimmunity promoting cytokines. These observations are used to develop a hypothetical model to explain how DAF expression may impact T cell differentiation via interaction with CD97 leading to T regulatory cells, increased production of IL-10, and immune tolerance.

Pilicides inhibit the FGL chaperone/usher assisted biogenesis of the Dr fimbrial polyadhesin from uropathogenic Escherichia coli

BACKGROUND

The global spread of bacterial resistance has given rise to a growing interest in new anti-bacterial agents with a new strategy of action. Pilicides are derivatives of ring-fused 2-pyridones which block the formation of the pili/fimbriae crucial to bacterial pathogenesis. They impair by means of a chaperone-usher pathway conserved in the Gram-negative bacteria of adhesive structures biogenesis. Pili/fimbriae of this type belong to two subfamilies, FGS and FGL, which differ in the details of their assembly mechanism. The data published to date have shown that pilicides inhibit biogenesis of type 1 and P pili of the FGS type which are encoded by uropathogenic E. coli strains.

RESULTS

We evaluated the anti-bacterial activity of literature pilicides as blockers of the assembly of a model example of FGL-type adhesive structures--the Dr fimbriae encoded by a dra gene cluster of uropathogenic Escherichia coli strains. In comparison to the strain grown without pilicide, the Dr⁺ bacteria cultivated in the presence of the 3.5 mM concentration of pilicides resulted in a reduction of 75 to 87% in the adherence properties to CHO cells expressing Dr fimbrial DAF receptor protein. Using quantitative assays, we determined the amount of Dr fimbriae in the bacteria cultivated in the presence of 3.5 mM of pilicides to be reduced by 75 to 81%. The inhibition effect of pilicides is concentration dependent, which is a crucial property for their use as potential anti-bacterial agents. The data presented in this article indicate that pilicides in mM concentration effectively inhibit the adherence of Dr⁺ bacteria to the host cells--the crucial, initial step in bacterial pathogenesis.

CONCLUSIONS

Structural analysis of the DraB chaperone clearly showed it to be a model of the FGL subfamily of chaperones. This permits us to conclude that analyzed pilicides in mM concentration are effective inhibitors of the assembly of adhesins belonging to the Dr family, and more speculatively, of other FGL-type adhesive organelles. The presented data and those published so far permit to speculate that based on the conservation of chaperone-usher pathway in Gram-negative bacteria , the pilicides are potential anti-bacterial agents with activity against numerous pathogens, the virulence of which is dependent on the adhesive structures of the chaperone-usher type.

In vivo incorporation of an azide-labeled sugar analog to detect mammalian glycosylphosphatidylinositol molecules isolated from the cell surface

N-Acetylgalactosamine (GalNAc) linked to the first mannose of glycosylphosphatidylinositol (GPI) core has been previously reported to be heterogeneously present on some mammalian GPI-anchored proteins. Here we present a method for profiling GalNAc-containing GPI-anchored proteins in mammalian cells by metabolic labeling with tetraacetylated N-azidoacetylgalactosamine (GalNAz) followed by biotinylation of the incorporated sugar analog. We have labeled both endogenous and recombinant GPI-anchored proteins with GalNAz, and demonstrated that the azide-activated sugar gets incorporated into the GPI glycan, likely as an unsubstituted side branch of the core structure. GalNAz was detected only on GPI molecules attached to proteins, and not on GPI precursors, indicating that GalNAc modification takes place after the GPI anchor is transferred to protein. We have highlighted the utility of this cell labeling approach by demonstrating the ability to examine specific GalNAc-containing GPI-anchored proteins isolated non-destructively from separate membrane domains (apical and basolateral) in polarized epithelial cells. This study represents the first demonstration of site-specific in vivo labeling of a GPI moiety with a synthetic sugar analog.

Role of Src kinases in mobilization of glycosylphosphatidylinositol-anchored decay-accelerating factor by Dr fimbria-positive adhering bacteria

Afa/Dr fimbriae constitute the major virulence factor of diffusely adhering Escherichia coli (Afa/Dr DAEC). After recognizing membrane-bound signaling receptors, they trigger cell responses. One of these receptors is the human decay-accelerating factor (hDAF). It has previously been reported that the binding of Afa/Dr fimbriae to hDAF quickly induces recruitment of hDAF around adhering bacteria. The aim of our study is to analyze the role of Src kinases in the Dr fimbria-induced recruitment of hDAF. Using biochemical methods and confocal microscopy followed by 3-dimensional (3D) analysis, we have shown that the activation and cell membrane targeting of Src kinases are necessary for the recruitment and organization of hDAF around adhering bacteria. We identified c-Src to be the specific kinase involved in this process. Using a set of Src-green fluorescent protein mutants, we showed that the catalytic activity and the Src homology 2 (SH2) and SH3 domains of the Src kinases are necessary for Dr fimbria-induced hDAF mobilization to occur. In addition, using mutated Dr fimbriae and a set of mutated hDAFs in which each of the complement control protein (CCP) domains had successively been deleted, we found that the aspartic acids at position 54 in the Dr fimbriae and in CCP domain 4 of hDAF played pivotal roles in the mobilization of the Src kinases and hDAF, respectively.

Development of a recombinant CHO cell model for the investigation of CAR and DAF role during early steps of echovirus 6 infection

The early steps of echovirus 6 (E6) infection remain poorly understood and the only described receptor for haemagglutinating E6 strains is the decay accelerating factor (DAF). There is, however, accumulating evidence suggesting that E6 interaction with DAF is necessary but not sufficient for infection. In this report, we investigated the role of the coxsackie-adenovirus-receptor (CAR) as a potential DAF co-receptor during E6 infection. Using stably transfected Chinese Hamster Ovary (CHO) cells expressing CAR and DAF receptors, we found that DAF expression allowed attachment of both haemagglutinating and non-haemagglutinating E6 strains but was not sufficient for promoting E6 cell entry. Interestingly, the co-expression of DAF and CAR rendered 0.1-0.2% of cells permissive to some E6 strains' infection. Although our results did not show a major role of the CAR/DAF cooperation for E6 infection, it nevertheless indicated the use of CAR in the cell entry step of some minor E6 quasispecies. Moreover, the present report validates the use of recombinant CHO cells as valuable cellular model for the further characterisation of E6 receptors.

Inhibiting complement activation on cells at the step of C3 cleavage

Nearly half of the proteins in the complement system serve in regulation. Control at the central step of C3 activation is provided by an orchestrated interplay of membrane and plasma regulators. A model system employing Chinese hamster ovary (CHO) cells transfected with human regulators was employed to assist in making functional comparisons. Also, in this experimental setup, the pathway and magnitude of complement activation can be varied while monitoring C4b/C3b deposition and cleavage as well as cytotoxicity. This review describes lessons learned and the application of this model for functionally characterizing mutations in regulators associated with atypical hemolytic uremic syndrome.

C3b deposition on human erythrocytes induces the formation of a membrane skeleton-linked protein complex

Decay-accelerating factor (DAF, also known as CD55), a glycosylphosphatidylinositol-linked (GPI-linked) plasma membrane protein, protects autologous cells from complement-mediated damage by inhibiting complement component 3 (C3) activation. An important physical property of GPI-anchored complement regulatory proteins such as DAF is their ability to translate laterally in the plasma membrane. Here, we used single-particle tracking and tether-pulling experiments to measure DAF lateral diffusion, lateral confinement, and membrane skeletal associations in human erythrocyte membranes. In native membranes, most DAF molecules exhibited Brownian lateral diffusion. Fluid-phase complement activation caused deposition of C3b, one of the products of C3 cleavage, onto erythrocyte glycophorin A (GPA). We then determined that DAF, C3b, GPA, and band 3 molecules were laterally immobilized in the membranes of complement-treated cells, and GPA was physically associated with the membrane skeleton. Mass spectrometry analysis further showed that band 3, alpha-spectrin, beta-spectrin, and ankyrin were present in a complex with C3b and GPA in complement-treated cells. C3b deposition was also associated with a substantial increase in erythrocyte membrane stiffness and/or viscosity. We therefore suggest that complement activation stimulates the formation of a membrane skeleton-linked DAF-C3b-GPA-band 3 complex on the erythrocyte surface. This complex may promote the removal of senescent erythrocytes from the circulation.

Escherichia coli DraE adhesin-associated bacterial internalization by epithelial cells is promoted independently by decay-accelerating factor and carcinoembryonic antigen-related cell adhesion molecule binding and does not require the DraD invasin

The Dr family of Escherichia coli adhesins are virulence factors associated with diarrhea and urinary tract infections. Dr fimbriae are comprised of two subunits. DraE/AfaE represents the major structural, antigenic, and adhesive subunit, which recognizes decay-accelerating factor (DAF) and carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) CEA, CEACAM1, CEACAM3, and CEACAM6 as binding receptors. The DraD/AfaD subunit caps fimbriae and has been implicated in the entry of Dr-fimbriated E. coli into host cells. In this study, we demonstrate that DAF or CEACAM receptors independently promote DraE-mediated internalization of E. coli by CHO cell transfectants expressing these receptors. We also found that DraE-positive recombinant bacteria adhere to and are internalized by primary human bladder epithelial cells which express DAF and CEACAMs. DraE-mediated bacterial internalization by bladder cells was inhibited by agents which disrupt lipid rafts, microtubules, and phosphatidylinositol 3-kinase (PI3K) activity. Immunofluorescence confocal microscopic examination of epithelial cells detected considerable recruitment of caveolin, beta(1) integrin, phosphorylated ezrin, phosphorylated PI3K, and tubulin, but not F-actin, by cell-associated bacteria. Finally, we demonstrate that the DraD subunit, previously implicated as an "invasin," is not required for beta(1) integrin recruitment or bacterial internalization.

Sialic acid is a cellular receptor for coxsackievirus A24 variant, an emerging virus with pandemic potential

Binding to target cell receptors is a critical step in the virus life cycle. Coxsackievirus A24 variant (CVA24v) has pandemic potential and is a major cause of acute hemorrhagic conjunctivitis, but its cellular receptor has hitherto been unknown. Here we show that CVA24v fails to bind to and infect CHO cells defective in sialic acid expression. Binding of CVA24v to and infection of corneal epithelial cells are efficiently inhibited by treating cells with a sialic acid-cleaving enzyme or sialic acid-binding lectins and by treatment of the virus with soluble, multivalent sialic acid. Protease treatment of cells efficiently inhibited virus binding, suggesting that the receptor is a sialylated glycoprotein. Like enterovirus type 70 and influenza A virus, CVA24v can cause pandemics. Remarkably, all three viruses use the same receptor. Since several unrelated viruses with tropism for the eye use this receptor, sialic acid-based antiviral drugs that prevent virus entry may be useful for topical treatment of such infections.

The short consensus repeats 1 and 2, not the cytoplasmic domain, of human CD46 are crucial for infection of subgroup B adenovirus serotype 35

Human CD46 (membrane cofactor protein) has recently been identified to be an attachment receptor for subgroup B adenoviruses (Ads); however, the precise interaction between human CD46 and subgroup B Ads are just beginning to be understood. In this study, to characterize the interaction between human CD46 and subgroup B Ads, varieties of mutant CD46 were tested for their ability to act as a receptor for Ad serotype 35 (Ad35), which belongs to subgroup B. In addition, we determined Ad35 vector-mediated transgene expression and cellular uptake of Ad35 vectors in the presence of a set of anti-CD46 antibodies. Our data demonstrated that the short consensus repeats (SCRs) 1 and 2 in human CD46 are important for interaction with Ad35, whereas the cytoplasmic domain of human CD46 was found not to be required for the function as an Ad35 receptor. Rather, a complete deletion of the cytoplasmic domain of human CD46 increased the transduction efficiencies of Ad35 vectors. This information should help in elucidation of the mechanism of subgroup B Ad infection, as well in the improvement of the subgroup B Ad vectors.

Pathogenesis of Afa/Dr diffusely adhering Escherichia coli

Over the last few years, dramatic increases in our knowledge about diffusely adhering Escherichia coli (DAEC) pathogenesis have taken place. The typical class of DAEC includes E. coli strains harboring AfaE-I, AfaE-II, AfaE-III, AfaE-V, Dr, Dr-II, F1845, and NFA-I adhesins (Afa/Dr DAEC); these strains (i) have an identical genetic organization and (ii) allow binding to human decay-accelerating factor (DAF) (Afa/Dr(DAF) subclass) or carcinoembryonic antigen (CEA) (Afa/Dr(CEA) subclass). The atypical class of DAEC includes two subclasses of strains; the atypical subclass 1 includes E. coli strains that express AfaE-VII, AfaE-VIII, AAF-I, AAF-II, and AAF-III adhesins, which (i) have an identical genetic organization and (ii) do not bind to human DAF, and the atypical subclass 2 includes E. coli strains that harbor Afa/Dr adhesins or others adhesins promoting diffuse adhesion, together with pathogenicity islands such as the LEE pathogenicity island (DA-EPEC). In this review, the focus is on Afa/Dr DAEC strains that have been found to be associated with urinary tract infections and with enteric infection. The review aims to provide a broad overview and update of the virulence aspects of these intriguing pathogens. Epidemiological studies, diagnostic techniques, characteristic molecular features of Afa/Dr operons, and the respective role of Afa/Dr adhesins and invasins in pathogenesis are described. Following the recognition of membrane-bound receptors, including type IV collagen, DAF, CEACAM1, CEA, and CEACAM6, by Afa/Dr adhesins, activation of signal transduction pathways leads to structural and functional injuries at brush border and junctional domains and to proinflammatory responses in polarized intestinal cells. In addition, uropathogenic Afa/Dr DAEC strains, following recognition of beta(1) integrin as a receptor, enter epithelial cells by a zipper-like, raft- and microtubule-dependent mechanism. Finally, the presence of other, unknown virulence factors and the way that an Afa/Dr DAEC strain emerges from the human intestinal microbiota as a "silent pathogen" are discussed.

Raft Partitioning and Dynamic Behavior of Human Placental Alkaline Phosphatase in Giant Unilamellar Vesicles

Much attention has recently been drawn to the hypothesis that cellular membranes organize in functionalized platforms called rafts, enriched in sphingolipids and cholesterol. The notion that glycosylphosphatidylinositol (GPI)-anchored proteins are strongly associated with rafts is based on their insolubility in nonionic detergents. However, detergent-based methodologies for identifying raft association are indirect and potentially prone to artifacts. On the other hand, rafts have proven to be difficult to visualize and investigate in living cells. A number of studies have demonstrated that model membranes provide a valuable tool for elucidating some of the raft properties. Here, we present a model membrane system based on domain-forming giant unilamellar vesicles (GUVs), in which the GPI-anchored protein, human placental alkaline phosphatase (PLAP), has been functionally reconstituted. Raft morphology, protein raft partitioning, and dynamic behavior have been characterized by fluorescence confocal microscopy and fluorescence correlation spectroscopy (FCS). Approximately 20-30% of PLAP associate with sphingomyelin-enriched domains. The affinity of PLAP for the liquid-ordered (l(o)) phase is compared to that of a nonraft protein, bacteriorhodopsin. Next, detergent extraction was carried out on PLAP-containing GUVs as a function of temperature, to relate the lipid and protein organization in distinct phases of the GUVs to the composition of detergent resistant membranes (DRMs). Finally, antibody-mediated cross-linking of PLAP induces a shift of its partition coefficient in favor of the l(o) phase.

Vaccination with purified Dr Fimbriae reduces mortality associated with chronic urinary tract infection due to Escherichia coli bearing Dr adhesin

The vaccination of C3H/HeJ mice with Escherichia coli Dr fimbrial antigen reduced mortality associated with an experimental urinary tract infection due to a homologous strain bearing Dr adhesin. Immune sera with high titers of anti-Dr antibody inhibited bacterial binding to bladders and kidneys but did not affect the rate of renal colonization.

Biological activity, membrane-targeting modification, and crystallization of soluble human decay accelerating factor expressed in E. coli

Decay-accelerating factor (DAF, CD55) is a glycophosphatidyl inositol-anchored glycoprotein that regulates the activity of C3 and C5 convertases. In addition to understanding the mechanism of complement inhibition by DAF through structural studies, there is also an interest in the possible therapeutic potential of the molecule. In this report we describe the cloning, expression in Escherichia coli, isolation and membrane-targeting modification of the four short consensus repeat domains of soluble human DAF with an additional C-terminal cysteine residue to permit site-specific modification. The purified refolded recombinant protein was active against both classical and alternative pathway assays of complement activation and had similar biological activity to soluble human DAF expressed in Pichia pastoris. Modification with a membrane-localizing peptide restored cell binding and gave a large increase in antihemolytic potency. These data suggested that the recombinant DAF was correctly folded and suitable for structural studies as well as being the basis for a DAF-derived therapeutic. Crystals of the E. coli-derived protein were obtained and diffracted to 2.2 A, thus permitting the first detailed X-ray crystallography studies on a functionally active human complement regulator protein with direct therapeutic potential.

Epithelial invasion by Escherichia coli bearing Dr fimbriae is controlled by nitric oxide-regulated expression of CD55

We previously reported that inhibition of nitric oxide (NO) increases the rate of bacteremia and maternal mortality in pregnant rats with uterine infection by Escherichia coli expressing the Dr fimbria (Dr(+)). Epithelial binding and invasion by Dr(+) E. coli has also been shown to be dependent upon the expression level of the cellular receptor decay-accelerating factor (DAF; CD55). Here, we hypothesize that NO-related severity of infection could be mediated by changes in DAF expression and in the rate of epithelial invasion. The cellular basis of NO effects on epithelial invasion with Dr(+) E. coli was studied using Ishikawa endometrial carcinoma cells as an in vitro model of the human endometrial epithelium. Initially, we show that Ishikawa cells produce NO and express both NO synthase enzymes, NOS II and NOS III, and DAF protein. We next tested the abilities of both Dr(+) E. coli and a Dr(-) E. coli mutant to invade Ishikawa cells, and invasion was seen only with Dr(+) E. coli. Invasion by Dr(+) E. coli was decreased by elevated NO production and increased by NO inhibition. Elevated NO production significantly decreased DAF protein and mRNA expression in Ishikawa cells in a time- and dose-dependent manner. Here, we propose that in vitro invasion of an epithelial cell line is directly related to NO-regulated expression of DAF. The significance of NO-regulated receptor-ligand invasion is that it may represent a novel unrecognized phenomenon of epithelial defense against infection.

Differential recognition of members of the carcinoembryonic antigen family by Afa/Dr adhesins of diffusely adhering Escherichia coli (Afa/Dr DAEC)

Little is known about the molecular bases underlying the virulence of diffusely adhering Escherichia coli (DAEC) harbouring the Afa/Dr family of adhesins. These adhesins recognize as receptors the GPI-anchored proteins CD55 (decay-accelerating factor, DAF) and CD66e (carcinoembryonic antigen, CEA). CD66e is a member of the CEA-related cell adhesion molecules (CEACAM) family, comprising seven members. We analysed the interactions of Afa/Dr DAEC with the CEACAMs using CEACAM-expressing CHO and HeLa cells. The results demonstrate that only E. coli expressing a subfamily of Afa/Dr adhesins, named here Afa/Dr-I, including Dr, F1845 and AfaE-III adhesins, bound onto CHO cells expressing CEACAM1, CEA or CEACAM6. Whereas all the Afa/Dr adhesins elicit recruitment of CD55 around adhering bacteria, only the Afa/Dr-I subfamily elicits the recruitment of CEACAM1, CEA and CEACAM6. In addition, although CEACAM3 is not recognized as a receptor by the subfamily of Afa/Dr adhesins, it is recruited around bacteria in HeLa cells. The recruited CEACAM1, CEA and CEACAM6 around adhering bacteria resist totally or in part a detergent extraction, whereas the recruited CEACAM3 does not. Finally, the results show that recognition of CEA and CEACAM6, but not CEACAM1, is accompanied by tight attachment to bacteria of cell surface microvilli-like extensions, which are elongated. Moreover, recognition of CEA is accompanied by an activation of the Rho GTPase Cdc42 and by a phosphorylation of ERM, which in turn elicit the observed cell surface microvilli-like extensions.

Decay-Accelerating Factor Deficiency Increases Susceptibility to Dextran Sulfate Sodium-Induced Colitis: Role for Complement in Inflammatory Bowel Disease

Decay-accelerating factor (DAF or CD55) is expressed on colonic epithelial cells but its function in the mucosa is unknown. In humans, a proportion of DAF-deficient (Cromer INAB) patients develop inflammatory bowel disease (IBD). To evaluate how DAF deficiency may contribute to gut inflammation and thus could play a role in IBD pathogenesis, we compared the severity of dextran sulfate sodium-induced colitis in Daf1 gene-targeted and control mice. Seven days after consuming 3% dextran sulfate sodium in their drinking water, Daf1(-/-) mice suffered markedly greater weight loss (-24.7 +/- 7.5% vs -14.2% +/- 4.9%), exhibited uniformly bloody diarrhea as compared with soft stool in control mice, developed shortened colons, and had larger spleens. Histological examination of distal colons showed massively increased neutrophilic and mononuclear cell infiltration, greater epithelial cell destruction, and increased ulcerations. Cytokine production in organ cultures of colonic explants showed increased levels of IL-12 and IL-6. Fourteen days after switching back to regular water, in contrast to the Daf1(+/+) controls which showed little stool abnormality, all Daf1(-/-) mice continued to have diarrhea. Organ culture cytokine measurements at this time point, i.e., the end of the recovery phase, showed markedly increased levels of IL-10 (6-fold), IL-12 (4-fold), and IL-6 (2-fold), as well as TNF-alpha (>10-fold) compared with the controls. Our findings argue that, as shown for IL-10 in IL-10(-/-) mice and IL-2 in IL-2(-/-) mice, DAF control of complement additionally is important in regulating gut homeostasis and consequently its activity may participate in protecting against IBD.

Endocytosis of oxidized low density lipoprotein through scavenger receptor CD36 utilizes a lipid raft pathway that does not require caveolin-1

The scavenger receptor CD36 binds a diverse array of ligands, including thrombospondin-1, oxidized low density lipoprotein (OxLDL), fatty acids, anionic phospholipids, and apoptotic cells. CD36 has been reported to be present in lipid rafts/caveolae, but little is known about the membrane trafficking of this protein at baseline or following ligand binding. Here, we determined that expression of CD36 in Chinese hamster ovary (CHO) cells and endogenous expression of CD36 in C32 cells led to a homogeneous distribution of the protein on the plasma membrane, as judged by confocal fluorescence microscopy. This homogeneous pattern was observed both by anti-CD36 antibody staining and by live cell imaging of CHO cells expressing a chimeric CD36-green fluorescent protein construct. In contrast, caveolin-1 displayed its usual punctate surface distribution. Correspondingly, dual labeling of CD36 and caveolin-1 showed essentially no overlap, neither by immunofluorescence light microscopy nor by immunogold electron microscopy. Furthermore, isolation of lipid rafts by sucrose gradient ultracentrifugation of cold Triton X-100 cell lysates yielded both CD36 and caveolin-1, but immunoprecipitates of caveolin-1 did not contain CD36. Binding of Ox-LDL led to internalization of CD36 and OxLDL into endosomal structures that did not contain caveolin-1 or transferrin but that co-internalized the glycosyl-phosphatidylinositol-anchored protein decay accelerating factor, a lipid raft protein. Furthermore, expression of CD36 in the caveolin-1-negative KB cell line is sufficient for OxLDL-induced internalization of CD36, indicating that caveolin-1 is not required for this endocytic process. Taken together, these data demonstrate that at steady state, CD36 is localized in lipid rafts but not in caveolae, and that binding of OxLDL to CD36 leads to endocytosis through a lipid raft pathway that is distinct from the clathrin-mediated or caveolin internalization pathways.

Chronic progression of tubulointerstitial damage in proteinuric renal disease is mediated by complement activation: a therapeutic role for complement inhibitors?

The mechanisms by which increased urinary protein concentrations lead to nephrotoxic injury are certain to be multifactorial and involve complex interactions between numerous pathways of cellular damage mediated by both cellular and humoral pathways. These may include a major role for the podocyte in glomerular diseases leading to chronic renal failure, the loss of microvascular endothelium, the albumin-induced upregulation of renal cytokines and growth factors that promote tubulointerstitial injury by inflammation and fibrogenesis, and the role of complement-mediated tubulointerstitial injury due to proteinuria. This review will focus on the last mechanism, and emphasize recent studies implicating a primary role for activation of complement in proteinuric urine as the principal mediator of tubulointerstitial damage and progressive renal disease in various experimental animal models of nephrosis. It will be our contention that intraluminal activation of the terminal complement cascade leading to the formation of the C5b-9 membrane attack complex is the principal mediator of chronic progressive interstitial damage and progressive renal failure irrespective of the type of primary glomerular injury. This paradigm has important implications for the potential therapeutic role of complement inhibitors that are currently being developed.

The Afa/Dr adhesins of diffusely adhering Escherichia coli stimulate interleukin-8 secretion, activate mitogen-activated protein kinases, and promote polymorphonuclear transepithelial migration in T84 polarized epithelial cells

Afa/Dr diffusely adhering Escherichia coli (Afa/Dr DAEC) strains cause symptomatic urinary tract and intestinal infections. The proinflammatory effects of Afa/Dr DAEC strains in vitro have been not investigated to date. In the present study, we used confluent polarized monolayers of intestinal cell line T84 to evaluate the consequences of epithelial infection by Afa/Dr DAEC strains in terms of proinflammatory response. Polymorphonuclear leukocyte (PMNL) migration across the epithelial barrier was induced after incubation of the T84 monolayers with the wild-type Afa/Dr DAEC strain C1845 harboring the fimbrial F1845 adhesin and strain IH11128 harboring the Dr hemagglutinin, and the E. coli laboratory strain HB101 was transformed with the pSSS1 plasmid, producing Afa/Dr F1845 adhesin. PMNL migrations were correlated with a basolateral secretion of interleukin-8 by T84 cells and were abolished after incubation of epithelial cells with an anti-decay accelerating factor (DAF) antibody that recognized the short consensus repeat 3 domain of DAF (monoclonal antibody 1H4). Moreover, Afa/Dr DAEC strains induced tyrosine phosphorylation of several T84 proteins and activated the mitogen-activated protein kinases (ERK1/2 mitogen-activated protein, P38, and Jun-C kinases). These data demonstrated for the first time that, in vitro, Afa/Dr DAEC strains exert a proinflammatory signal in intestinal epithelial cells.

Expression and purification of functional, recombinant Trypanosoma cruzi complement regulatory protein

The complement regulatory protein (CRP) of Trypanosoma cruzi is a developmentally regulated glycosylphosphatidylinositol (GPI)-anchored membrane protein that protects the parasite from complement-mediated killing, and is an important virulence determinant of the microorganism. CRP binds human complement components C3b and C4b to restrict activation of the complement cascade. Here, we report production of functional, recombinant T. cruzi CRP in mammalian cells and a one-step purification of the recombinant protein. Exchange of the crp DNA sequence encoding the carboxy-terminal GPI signal sequence with the corresponding sequence of decay accelerating factor (DAF) was necessary for recognition, cleavage, and addition of GPI in mammalian cells. CRP production was assessed in two mammalian cell lines with crp-daf gene expression driven by three different transcription control regions: Rous sarcoma virus long terminal repeat, cytomegalovirus (CMV) immediate early gene, and chicken beta-actin promoter/CMV enhancer. We present evidence that CRP produced in transfected Chinese hamster Ovary (CHO) cells was functional and protected the cells from complement-mediated lysis. To facilitate purification of the recombinant protein, a hexahistidyl tag was incorporated at 3(') end of the cDNA upstream of the GPI anchor addition sequence. An additional histidine fusion construct was made that allowed for secretion and recovery of recombinant protein from culture supernatant fluid. Both membrane and secreted forms of the protein were purified in one step by nickel nitrilotriacetic acid. The production and purification of functionally active CRP in a non-infectious expression system will allow for structure and function studies aimed at identifying the active site(s) of this protein.

Human Herpesvirus 6 and Measles Virus Employ Distinct CD46 Domains for Receptor Function

We employed a quantitative cell fusion assay to identify structural domains of CD46 required for its function as a receptor for human herpesvirus 6 (HHV-6). We examined the activities of recombinant variants of CD46, including different isoforms as well as engineered truncations and molecular chimeras with decay-accelerating factor, a related protein in the family of regulators of complement activation (RCA). We observed strong receptor activity for all four CD46 isoforms, which differ in the membrane-proximal extracellular and cytoplasmic domains, indicating that the critical determinants for HHV-6 receptor activity reside outside the C-terminal portion of CD46. Analysis of the short consensus repeat (SCR) regions that comprise most of the extracellular portion of CD46 indicated a strong dependence on SCRs 2 and 3 and no requirement for SCRs 1 or 4. Fusion-inhibition studies with SCR-specific monoclonal antibodies supported the essential role of SCRs 2 and 3 in HHV-6 receptor activity. These findings contrast markedly with fusion mediated by measles virus glycoproteins for which we observed a strict dependence on SCRs 1 and 2, consistent with previous reports. These results expand the emerging notion that CD46 and other members of the RCA family are co-opted in distinct manners by different infectious pathogens.

Identification of amino acids in the Dr adhesin required for binding to decay-accelerating factor

Members of the Dr family of adhesins of Escherichia coli recognize as a receptor the Dr(a) blood-group antigen present on the complement regulatory and signalling molecule, decay-accelerating factor (DAF). One member of this family, the Dr haemagglutinin, also binds to a second receptor, type IV collagen. Structure/function information regarding these adhesins has been limited and domains directly involved in the interaction with DAF have not been determined. We devised a strategy to identify amino acids in the Dr haemagglutinin that are specifically involved in the interaction with DAF. The gene encoding the adhesive subunit, draE, was subjected to random mutagenesis and used to complement a strain defective for its expression. The resulting mutants were enriched and screened to obtain those that do not bind to DAF, but retain binding to type IV collagen. Individual amino acid changes at positions 10, 63, 65, 75, 77, 79 and 131 of the mature DraE sequence significantly reduced the ability of the DraE adhesin to bind DAF, but not collagen. Over half of the mutants obtained had substitutions within amino acids 63-81. Analysis of predicted structures of DraE suggest that these proximal residues may cluster to form a binding domain for DAF.

Solving the organ shortage: potential strategies and the likelihood of success

The discrepancy between the demand and supply of organs for clinical transplantation remains a major problem. The current incidence of end-stage renal disease results in a patient population that doubles every decade. However, there have been no advancements in developing a comparable increase in the number of available allografts. There are three potential approaches to solving the shortage. In the near-term, the development of technology to access the pool of warm ischemically damaged organs may represent a solution. To achieve this goal, it will be necessary to develop technology that mimics the physiologic processes of wound repair. Alternatively, it has been proposed that an increased supply of organs can be developed with xenografts. To make xenotransplantation a clinical reality, it will be necessary to overcome the barriers that exist in nature between the species. Recent work in the area of stem cell research has provided evidence supporting the potential of generating biohybrid organs. A major undertaking of this emerging field will be to develop the ability to define and control the differentiation processes involved in organ specificity. The following is a review of the current status and relative issues involved with these three potential approaches to solving the organ shortage.

The major structural subunits of Dr and F1845 fimbriae are adhesins

Fimbrial adhesins mediate the attachment of pathogenic Escherichia coli to various host tissues leading to the development of disease. The Dr hemagglutinin and F1845 fimbriae belong to the Dr family of adhesins, which is associated with urinary tract infections and diarrheal disease. These adhesins bind to the Dr(a) blood-group antigen present on decay-accelerating factor (DAF). The Dr hemagglutinin is unique in this family since it also binds to type IV collagen and its binding is inhibited by the presence of chloramphenicol. We have purified the major structural subunits of Dr and F1845 fimbriae, DraE and DaaE, as fusions to maltose-binding protein and to oligohistidine tags and examined their binding to erythrocytes, Chinese hamster ovary cell transfectants expressing DAF, and a DAF fusion protein. The DraE and DaaE fusion proteins bind to the DAF receptor in a specific manner resembling the distinct phenotypes of the corresponding Dr and F1845 fimbriae. In contrast to binding studies with the DAF receptor, the DraE fusion proteins did not bind to type IV collagen.

Recombinant transmembrane CD59 (CD59-TM) confers complement resistance to GPI-anchored protein defective melanoma cells

Protectin (CD59) is a glycosylphosphatidylinositol (GPI)-anchored cell membrane glycoprotein, broadly expressed on melanocytic cells, that represents the main restriction factor of complement (C)-mediated lysis of human melanoma cells. Levels of CD59 expression may impair the clinical efficacy of C-activating monoclonal antibodies (mAb); thus, we investigated the molecular mechanisms underlying the lack of CD59 expression in selected melanoma cells. Serological and biochemical analyses showed that MeWo melanoma cells expressed CD59 neither at cell surface nor at cytoplasmic levels; however, no critical mutations were identified in their CD59 mRNA. Consistently, MeWo CD59 cDNA (MeWo-CD59) was appropriately translated when transfected into the CD59-positive Mel 100 melanoma cells, and into the CD59-negative Nalm-6 pre-B leukemia cells that acquired resistance to C. In contrast, transfection of MeWo cells with CD59 cDNA from Mel 275 melanoma cells did not induce CD59 expression; however, their transfection with the CD59-TM chimeric construct, obtained by replacing the GPI-anchoring signal of MeWo-CD59 with the transmembrane tail of the human low-density lipoprotein receptor, induced the expression of a C-protective transmembrane form of CD59. These data, together with the absent expression of additional GPI-anchored proteins (i.e., CD55), suggest that defects in the biosynthesis and/or processing of GPI-anchored proteins underlie the lack of CD59 expression in MeWo cells. Further unveiling of the molecular mechanism that turns off CD59 expression in human melanoma cells will help to set-up more effective therapeutic strategies utilizing C-activating mAb in melanoma patients.

Attachment of Neisseria gonorrhoeae to the cellular pilus receptor CD46: identification of domains important for bacterial adherence

Pili of Neisseria gonorrhoeae mediate binding of the bacteria to human host cells. Membrane cofactor protein (MCP or CD46), a human cell-surface protein involved in regulation of complement activation, acts as a cellular pilus receptor. In this work, we examined which domains of CD46 mediate bacterial adherence. The CD46 expression was quantified and characterized in human epithelial cell lines. N. gonorrhoeae showed the highest adherence to ME180 cells, which have BC1 as the dominant phenotype. The BC isoforms of CD46 were expressed in all cell lines tested. The adherence was not enhanced by high expression of other isoforms, showing that the BC domain of CD46 is important in adherence of N. gonorrhoeae to human cells. To characterize the pilus-binding site within the CD46 molecule, a set of CD46-BC1 deletion constructs were transfected into COS-7 cells. Piliated N. gonorrhoeae attached well to CD46-BC1-expressing COS-7 cells. We show that the complement control protein repeat 3 (CCP-3) and the serine-threonine-proline (STP)-rich domain of CD46 are important for efficient adherence to host cells. Further, partial deletion of the cytoplasmic tail of CD46 results in low bacterial binding, indicating that the cytoplasmic tail takes part in the process of establishing a stable interaction between N. gonorrhoeae and host cells.

Cooperation between decay-accelerating factor and membrane cofactor protein in protecting cells from autologous complement attack

Decay-accelerating factor (DAF or CD55) and membrane cofactor protein (MCP or CD46) function intrinsically in the membranes of self cells to prevent activation of autologous complement on their surfaces. How these two regulatory proteins cooperate on self-cell surfaces to inhibit autologous complement attack is unknown. In this study, a GPI-anchored form of MCP was generated. The ability of this recombinant protein and that of naturally GPI-anchored DAF to incorporate into cell membranes then was exploited to examine the combined functions of DAF and MCP in regulating complement intermediates assembled from purified alternative pathway components on rabbit erythrocytes. Quantitative studies with complement-coated rabbit erythrocyte intermediates constituted with each protein individually or the two proteins together demonstrated that DAF and MCP synergize the actions of each other in preventing C3b deposition on the cell surface. Further analyses showed that MCP's ability to catalyze the factor I-mediated cleavage of cell-bound C3b is inhibited in the presence of factors B and D and is restored when DAF is incorporated into the cells. Thus, the activities of DAF and MCP, when present together, are greater than the sum of the two proteins individually, and DAF is required for MCP to catalyze the cleavage of cell-bound C3b in the presence of excess factors B and D. These data are relevant to xenotransplantation, pharmacological inhibition of complement in inflammatory diseases, and evasion of tumor cells from humoral immune responses.

Receptor-mediated regulation of plasminogen activator function: plasminogen activation by two directly membrane-anchored forms of urokinase

The generation of the broad specificity serine protease plasmin in the pericellular environment is regulated by binding of the urokinase-type plasminogen activator (uPA) to its specific glycosylphosphatidylinositol (GPI)-anchored cell-surface receptor, uPAR. This interaction potentiates the reciprocal activation of the cell-associated zymogens pro-uPA and plasminogen. To further study the role of uPAR in this mechanism, we have expressed two directly membrane-anchored chimeric forms of uPA, one anchored by a C-terminal GPI-moiety (GPI-uPA), the other with a C-terminal transmembrane peptide (TM-uPA). These were expressed in the monocyte-like cell lines U937 and THP-1, which are excellent models for kinetic and mechanistic studies of cell-surface plasminogen activation. In both cell-lines, GPI-uPA activated cell-associated plasminogen with characteristics both qualitatively and quantitatively indistinguishable from those of uPAR-bound uPA. By contrast, TM-uPA activated cell-associated plasminogen less efficiently. This was due to effects on the K, for plasminogen activation (which was increased up to five-fold) and the efficiency of pro-uPA activation (which was decreased approximately four-fold). These observations suggest that uPAR serves two essential roles in mediating efficient cell-surface plasminogen activation. In addition to confining uPA to the cell-surface, the GPI-anchor plays an important role by increasing accessibility to substrate plasminogen and, thus, enhancing catalysis. However, the data also demonstrate that, in the presence of an alternative mechanism for uPA localization, uPAR is dispensable and, therefore, unlikely to participate in any additional interactions that may be necessary for the efficiency of this proteolytic system. In these experiments zymogen pro-uPA was unexpectedly found to be constitutively activated when expressed in THP-1 cells, suggesting the presence of an alternative plasmin-independent proteolytic activation mechanism in these cells.

Pigs express multiple forms of decay-accelerating factor (CD55), all of which contain only three short consensus repeats

We report the cloning of cDNAs encoding multiple isoforms of the pig analogue of human decay-accelerating factor (DAF; CD55). Screening of a pig muscle cDNA library using a human DAF probe identified a single clone that encoded a DAF-like molecule comprising three short consensus repeats (SCR) homologous with the amino-terminal three SCR in human DAF, a serine/threonine-rich (ST) region, and sequence compatible with a transmembrane domain and cytoplasmic tail. Northern blot and RT-PCR analysis showed that pig DAF was expressed in a wide range of tissues. Additional isoforms of DAF were sought using RT-PCR and 3'-rapid amplification of cDNA ends followed by sequencing. Isoforms containing a GPI anchor and with differing lengths of ST region were identified; no isoform containing a fourth SCR was found. Cloning of the GPI-anchored isoform from granulocytes confirmed that it was identical with the original transmembrane isoform through the three SCR and first portion of ST and was derived from a frame shift caused by splicing out 176 bp of sequence. A panel of mAbs was generated and used to analyze the distribution and anchoring of pig DAF in circulating cells. Pig DAF was expressed on all circulating cells and was transmembrane anchored on erythrocytes, but completely or partially GPI anchored on granulocytes and mononuclear cells. The transmembrane isoform of pig DAF was expressed on Chinese hamster ovary cells and was shown to affect regulatory activity for the classical pathway of human complement, but was only marginally active against pig serum.

Recruitment of CD55 and CD66e brush border-associated glycosylphosphatidylinositol-anchored proteins by members of the Afa/Dr diffusely adhering family of Escherichia coli that infect the human polarized intestinal Caco-2/TC7 cells

The Afa/Dr family of diffusely adhering Escherichia coli (Afa/Dr DAEC) includes bacteria expressing afimbrial adhesins (AFA), Dr hemagglutinin, and fimbrial F1845 adhesin. We show that infection of human intestinal Caco-2/TC7 cells by the Afa/Dr DAEC strains C1845 and IH11128 is followed by clustering of CD55 around adhering bacteria. Mapping of CD55 epitopes involved in CD55 clustering by Afa/Dr DAEC was conducted using CD55 deletion mutants expressed by stable transfection in CHO cells. Deletion in the short consensus repeat 1 (SCR1) domain abolished Afa/Dr DAEC-induced CD55 clustering. In contrast, deletion in the SCR4 domain does not modify Afa/Dr DAEC-induced CD55 clustering. We show that the brush border-associated glycosylphosphatidylinositol (GPI)-anchored protein CD66e (carcinoembryonic antigen) is recruited by the Afa/Dr DAEC strains C1845 and IH11128. This conclusion is based on the observations that (i) infection of Caco-2/TC7 cells by Afa/Dr DAEC strains is followed by clustering of CD66e around adhering bacteria and (ii) Afa/Dr DAEC strains bound efficiently to stably transfected HeLa cells expressing CD66e, accompanied by CD66e clustering around adhering bacteria. Inhibition assay using monoclonal antibodies directed against CD55 SCR domains, and polyclonal anti-CD55 and anti-CD66e antibodies demonstrate that CD55 and CD66e function as a receptors for the C1845 and IH11128 bacteria. Moreover, using structural draE gene mutants, we found that a mutant in which cysteine replaced aspartic acid at position 54 displayed conserved binding capacity but failed to induce CD55 and CD66e clustering. Taken together, these data give new insights into the mechanisms by which Afa/Dr DAEC induces adhesin-dependent cross talk in the human polarized intestinal epithelial cells by mobilizing brush border-associated GPI-anchored proteins known to function as transducing molecules.

Role of decay-accelerating factor domains and anchorage in internalization of Dr-fimbriated Escherichia coli

Dr-fimbriated Escherichia coli capable of invading epithelial cells recognizes human decay-accelerating factor (DAF) as its cellular receptor. The role of extracellular domains and the glycosylphosphatidylinositol anchor of DAF in the process of internalization of Dr(+) E. coli was characterized in a cell-cell interaction model. Binding of Dr(+) E. coli to the short consensus repeat 3 domain of DAF expressed by Chinese hamster ovary cells was critical for internalization to occur. Deletion of short consensus repeat 3 domain or replacement of Ser(165) by Leu in this domain, or the use of a monoclonal antibody to this region abolished internalization. Replacing the glycosylphosphatidylinositol anchor of DAF with the transmembrane anchor of membrane cofactor protein or HLA-B44 resulted in abolition or reduction of internalization respectively. Cells expressing glycosylphosphatidylinositol-anchored DAF but not the transmembrane-anchored DAF internalized Dr(+) E. coli through a glycolipid pathway, since the former cells were more sensitive to inhibition by methyl-beta-cyclodextrin, a sterol-chelating agent. Electron microscopic studies revealed that the intracellular vacuoles containing the internalized Dr(+) E. coli were morphologically distinct between the anchor variants of DAF. The cells expressing glycosylphosphatidylinositol-anchored DAF contained a single bacterium in tight-fitting vacuoles, while the cells expressing transmembrane-anchored DAF contained multiple (two or three) bacteria in spacious phagosomes. This finding suggests that distinct postendocytic events operate in the cells expressing anchor variants of DAF. We provide direct evidence for the DAF-mediated internalization of Dr(+) E. coli and demonstrate the significance of the glycosylphosphatidylinositol anchor, which determines the ability and efficiency of the internalization event.

Characterization of mouse DAF on transfectant cells using monoclonal antibodies which recognize different epitopes

Several membrane proteins prevent host cells from homologous complement attack. In humans, one such protein, decay-accelerating factor (DAF), exists as two isoforms, a GPI anchored form and a secreted form, which are generated by alternative splicing. DAF in mouse is also expressed as two isoforms, a GPI anchored form (GPI-DAF) and a transmembrane form (TM-DAF), which are produced from two separate genes. In this study, we transfected cDNA of mouse GPI-DAF or TM-DAF into Chinese hamster ovary (CHO) cells. Both isoforms of DAF on CHO cells were shown to regulate mouse complement C3 deposition mediated by the classical and alternative pathways and the inhibitory activity of both isoforms was species restricted. The two mouse DAF isoforms were effective against rat complement but not against human and guinea pig complement. Furthermore, we produced hamster mAbs to mouse DAF using GPI-DAF transfectant cells and established seven unique mAbs (RIKO-1-7). Western blotting analysis using RIKO-3, which reacts with both GPI-DAF and TM-DAF, and RIKO-4, which is an anti-GPI-DAF specific mAb, indicated that GPI-DAF was expressed on erythrocytes, spleen and testis, and that TM-DAF was expressed only in testis.

Molecular and functional analysis of mouse decay accelerating factor (CD55)

Molecular cloning of mouse decay accelerating factor (DAF; CD55) predicted two forms of the molecule, one transmembrane (TM) and the other glycosylphosphatidylinositol (GPI)-anchored; these are encoded by separate genes termed Daf-GPI and Daf-TM. In the present study several additional isoforms of mouse DAF, generated by alternative splicing from these genes, are described. Northern-blot analysis of RNA and reverse transcriptase-PCR from various tissues indicated that spleen and testis expressed high levels of DAF, which comprised several species. These species were cloned and sequence analysis revealed various novel forms in addition to those previously reported. Two novel forms were derived from the Daf-TM gene but the transmembrane sequence defined previously was replaced by a unique GPI-anchor addition sequence; one clone also had part of the serine/threonine/proline (STP) region deleted. A third clone, encoding a transmembrane protein, was also derived from this gene but the entire STP region was deleted. A fourth clone, derived from the Daf-GPI gene, contained a novel C-terminal sequence, suggestive of a secreted form of the protein. Two DAF cDNAs (TM and GPI-anchored) were stably expressed in Chinese hamster ovary cells. When these cells were attacked with mouse or rat complement and analysed for C3b deposition, DAF-transfected cells had greatly reduced C3b deposition compared with controls. Transfection with DAF also conferred protection from complement in a cell-lysis assay, and a soluble, recombinant form of mouse DAF inhibited complement in a haemolytic assay.

Complement-protected amphotropic retroviruses from murine packaging cells

The application of retroviruses generated from murine cells for in vivo gene therapy is restricted primarily because of the rapid inactivation of these viruses by the human complement system. To circumvent this disadvantageous property of murine retroviruses we have generated infectious amphotropic retroviruses that exhibit strong protection against human complement attack. The membrane of these viruses contains a fusion protein, DAFF2A, that is composed of the catalytic domain of the human complement regulatory protein (CRP) decay-accelerating factor (DAF) and the envelope protein of the amphotropic murine leukemia virus (MuLV) 4070A (EnvA). The fusion of two other CRPs, MCP and CD59, to the same amphotropic Env moiety did not lead to equivalent results. The fusion protein DAFF2A was stably expressed in mouse NIH 3T3-based helper cells and independently identified with either alpha-DAF MAb or alpha-Env PAb on the cell membrane. Western blot analysis confirmed the expected molecular weight of the fusion protein. Viral titers obtained from NIH 3T3 helper cell pools were 5 x 10(5) CFU for wild-type amphotropic EnvA virus and 1 x 10(5) CFU for DAFF2A virus, respectively. By blocking the catalytic domain of DAF by pretreatment with alpha-DAF MAb DAFF2A, recombinant virions could be converted to wild-type with respect to sensitivity against human serum. Since the method for producing virions that are protected against human serum should be applicable to any cell type it offers a novel tool for human in vivo gene therapy.

Human CD46 enhances nitric oxide production in mouse macrophages in response to measles virus infection in the presence of gamma interferon: dependence on the CD46 cytoplasmic domains

CD46 is a transmembrane complement regulatory protein widely expressed on nucleated human cells. Laboratory-adapted strains of measles virus (MV) bind to the extracellular domains of CD46 to enter human cells. The cytoplasmic portion of CD46 consists of a common juxtamembrane region and different distal sequences called Cyt1 and Cyt2. The biological functions of these cytoplasmic sequences are unknown. In this study, we show that expression of human CD46 with the Cyt1 cytoplasmic domain in mouse macrophages enhances production of nitric oxide (NO) in response to MV infection in the presence of gamma interferon (IFN-gamma). Human CD46 does not increase the basal levels of NO production in mouse macrophages and does not augment NO production induced by double-stranded polyribonucleotides. Replacing the cytoplasmic domain of human CD46 with Cyt2 reduces MV and IFN-gamma-induced NO production in mouse macrophages. Deleting the entire cytoplasmic domains of human CD46 does not prevent MV infection but markedly attenuates NO production in response to MV and IFN-gamma. Mouse macrophages expressing a tailless human CD46 mutant are more susceptible to MV infection and produce 2 to 3 orders of magnitude more infectious virus than mouse macrophages expressing human CD46 with intact cytoplasmic domains. These results reveal a novel function of CD46 dependent on the cytoplasmic domains (especially Cyt1), which augments NO production in macrophages. These findings may have significant implications for roles of CD46 in innate immunity and MV pathogenesis.

Complement activation and expression of membrane regulators in the middle ear mucosa in otitis media with effusion

The aetiopathogenesis of chronic otitis media with effusion (OME) in children is not yet fully understood. OME is characterized by metaplasia of the epithelium and accumulation of sticky, glue-like effusion in the middle ear containing different mediators of inflammation, including activation fragments of the complement system. Here we examined whether the fluid phase complement activation is reflected in the middle ear mucosa and how the mucosa is protected against the cytolytic activity of complement. Mucosal biopsies from 18 middle ears of children with a history of chronic OME were taken. The biopsies were analysed by immunofluorescence microscopy after staining for complement fragments iC3b/C3c, C3d and C9, and regulators membrane cofactor protein (MCP; CD46), decay-accelerating factor (DAF; CD55) and protectin (CD59). There was a strong staining for iC3b/C3c, and a weaker one for C3d and C9 on the surface of the middle ear epithelial cells of OME patients but not in controls without OME. MCP was expressed on the hyperplastic three to four outer cell layers of the epithelium, while CD59 was expressed throughout the middle ear mucosa. The results suggest a strong ongoing complement activation and consequent inflammation in the middle ear cavity. Unrestricted complement damage of the epithelial lining is prevented by the strong expression of MCP and CD59.

Coxsackievirus and adenovirus receptor cytoplasmic and transmembrane domains are not essential for coxsackievirus and adenovirus infection

Coxsackievirus and adenovirus receptor (CAR) from which the cytoplasmic domain had been deleted and glycosylphosphatidylinositol (GPI)-anchored CAR lacking both transmembrane and cytoplasmic domains were both capable of facilitating adenovirus 5-mediated gene delivery and infection by coxsackievirus B3. These results indicate that the CAR extracellular domain is sufficient to permit virus attachment and entry and that the presence of a GPI anchor does not prevent infection.

Expression and functional analysis of glycosyl-phosphatidyl inositol-linked CD46 in transgenic mice

BACKGROUND

Complement activation plays a pivotal role in hyperacute xenograft rejection. In humans, activation of complement is regulated by a number of cell surface regulatory proteins. Membrane cofactor protein (CD46) is one such regulator that protects cells by acting as a cofactor for the factor I-mediated cleavage of C3b and C4b. Transgenic animals expressing human CD46 may provide organs that are resistant to complement attack. However, attempts to generate mice expressing human CD46 using cDNA-based constructs have been largely unsuccessful.

METHODS

Transgenic mice expressing a glycosylphosphatidyl inositol (GPI)-linked form of CD46 were generated by microinjection of a hybrid CD46/CD55 cDNA under the control of the human intercellular adhesion molecule-2 promoter. Expression of CD46-GPI on the vascular endothelium was determined by immunohistochemistry. The ability of CD46-GPI to protect mouse tissues from human complement attack was determined using an ex vivo isolated perfused heart model.

RESULTS

Three founder animals expressing CD46-GPI were identified. Histological analysis showed strong and uniform expression of CD46-GPI on the vascular endothelium of all organs examined. Ex vivo perfusion of transgenic mouse hearts with human plasma showed a reduction in C3c deposition and a slightly prolonged function compared with controls.

CONCLUSIONS

High-level expression of CD46-GPI was achieved in transgenic mice by using a modified cDNA-based construct. The CD46-GPI was functional, providing some protection from complement-mediated damage in the ex vivo model, and may be useful in xenotransplantation if expressed in combination with CD55 and CD59.

Short consensus repeat domain 1 of decay-accelerating factor is required for enterovirus 70 binding

Enterovirus 70 (EV70), like several other human enteroviruses, can utilize decay-accelerating factor (DAF [CD55]) as an attachment protein. Using chimeric molecules composed of different combinations of the short consensus repeat domains (SCRs) of DAF and membrane cofactor protein (CD46), we show that sequences in SCR1 of DAF are essential for EV70 binding. Of the human enteroviruses that can bind to DAF, only EV70 and coxsackievirus A21 require sequences in SCR1 for this interaction.

Membrane Cofactor Protein: Importance of N- and O-Glycosylation for Complement Regulatory Function

Membrane cofactor protein (MCP; CD46) is a type 1 membrane glycoprotein that inhibits complement activation on host cells. It also is a measles virus (MV) receptor, an adherence factor for group A Streptococcus pyogenes, and a cellular pilus receptor for pathogenic Neisseria. The amino terminus of MCP consists of four complement control protein (CCP) repeats, three of which (CCP-1, -2, and -4) possess N-glycans. Immediately following the CCP modules is an alternatively spliced region for extensive O-glycosylation (termed the STP domain). Previous studies established that the N-glycan of CCP-2 is essential for MV binding and infection and that the splicing variants of the STP domain not only affect MV binding and fusion, but also differentially protect against complement-mediated cytolysis. In this report, we dissect the role of these carbohydrates on complement regulatory function. We constructed, expressed, and characterized proteins deleting these carbohydrates. For MCP-mediated protection against cytolysis, the N-glycans of CCP-2 and -4 were necessary, the STP segment influenced but was not essential, and the N-glycan of CCP-1 was not required. In addition, the rate and magnitude of cell surface cleavage of C4b to C4c and C4d by MCP and factor I correlated with cytoprotection. These studies expand the structure-function understanding of the active sites of MCP and elucidate an important role for carbohydrates in its function, a finding consistent with their conservation in the MCP of other species.

Reduction of the major swine xenoantigen, the alpha-galactosyl epitope by transfection of the alpha2,3-sialyltransferase gene

alpha2,3-Sialyltransferase represents a putative enzyme that reduces the Galalpha1-3Gal beta1-4GlcNAc-R (the alpha-galactosyl epitope) by intracellular competition with alpha1,3-galactosyltransferase for a common acceptor substrate. This study demonstrates that the overexpression of the alpha2,3-sialyltransferase gene suppresses the antigenicity of swine endothelial cells to human natural antibodies by 77% relative to control cells and by 30% relative to cells transfected with alpha1,2-fucosyltransferase, and in addition, it reduces the complement-mediated cell lysis by 75% compared with control cells and by 22% compared with cells transfected with alpha1, 2-fucosyltransferase. The mechanism by which the alpha-galactosyl epitope was reduced was also studied. Suppression of alpha1, 3-galactosyltransferase activity by 30-63% was observed in the transfectants with alpha2,3-sialyltransferase, and mRNA expression of the alpha1,3-galactosyltransferase gene was reduced as well. The data suggest that the alpha2,3-sialyltransferase effectively reduced the alpha-galactosyl epitope as well as or better than the alpha1, 2-fucosyltransferase did and that the reduction of the alpha-galactosyl epitope is due not only to substrate competition but also to an overall reduction of endogenous alpha1, 3-galactosyltransferase enzyme activity.

Molecular cloning of membrane cofactor protein (MCP; CD46) on B95a cell, an Epstein-Barr virus-transformed marmoset B cell line: B95a-MCP is susceptible to infection by the CAM, but not the Nagahata strain of the measles virus

Measles virus (MV) infects not only human beings but also some simian species. The MV receptor on Vero cells (a cell line established from African Green monkey kidney cells) and human cells has been shown to be the membrane cofactor protein MCP/CD46, which is an inhibitor of autologous complement (C) activation. B95a, an Epstein-Barr virus (EBV)-transformed marmoset B cell line, is a simian cell line used for MV selection and is much more susceptible to MV than Vero cells. In the present study, we isolated cDNAs encoding MCP homologues from B95a cDNA library and assessed whether B95a-MCP is responsible for the high susceptibility of B95a to MV. The deduced amino acid sequence of the cDNA of B95a-MCP was 76% identical to that of human-MCP, and the recombinant B95a-MCP exerts C inhibitor activity. Although CAM, a vaccine strain of MV, infected Chinese hamster ovary (CHO) cells expressing B95a-MCP, Nagahata strain, a wild type of MV, failed to infect the CHO transfectants, suggesting that additional membrane molecules of B95a are responsible for the high susceptibility of B95a to the Nagahata strain.

Functional Differences Among Multiple Isoforms of Guinea Pig Decay-Accelerating Factor

Decay-accelerating factor (DAF, CD55) is a membrane inhibitor that protects host cells from the autologous C-mediated attack. The guinea pig homologue of DAF consists of multiple isoforms generated by alternative splicing from a single copy gene. These isoforms are mainly comprised of a glycosylphosphatidylinositol (GPI)-anchored form and a transmembrane form (TM) that is not present in human DAF. Both forms occur in at least three variations that differ in the length of the Ser/Thr-rich region (termed ST-a, ST-ab, and ST-abc). We have transfected cDNAs of the six major isoforms into Chinese hamster ovary cells, and their functional differences were evaluated in inhibition of C-mediated cytolysis and C3 deposition, using the transfectants expressing DAF at the same level on cell membranes. The degree of inhibition in both the classical and alternative pathways differed according to the length of the ST region in the order of abc > ab > a in both GPI and TM forms. When GPI and TM forms were compared, those with the ab or abc variation exhibited almost the same activity, whereas a-TM was less efficient than a-GPI. Although several isoforms are expressed constitutively in most of tissues, spermatozoa preferentially express the abc-GPI isoform, suggesting that this isoform offers effective protection to spermatozoa in the female genital tract.

Relative contributions of decay accelerating factor (DAF), membrane cofactor protein (MCP) and CD59 in the protection of melanocytes from homologous complement

Complement regulatory molecules, membrane cofactor protein (MCP), decay accelerating factor (DAF) and CD59, protect body cells from autologous complement. They have wide tissue distribution but nothing is known about the expression of these molecules on human melanocytes. Since melanocytes are lysed in the lesional skin of patients with a depigmentary disorder vitiligo, it is important to compare the protection offered by complement regulatory molecules to melanocytes present in normal and vitiligo epidermis, against autologous complement. From this point of view, we investigated the differential expression of MCP, DAF and CD59 on normal cultured human melanocytes and assessed their individual contribution in the protection of these cells against complement-mediated damage. Flow cytometric analysis showed that MCP and DAF but not CD59 were expressed on cultured melanocytes. When heat inactivated sera of patients with vitiligo were used as a source of anti-melanocyte antibody to sensitize melanocytes, and guinea pig serum (GpS) or normal human serum (NHS) as a source of complement, GpS was found to be more effective in causing the lysis of melanocytes than NHS. When melanocytes were sensitized with autoantibody as well as F(ab')2 fragment of either anti-MCP or anti-DAF and subsequently incubated with NHS or GpS, both antibody fragments increased the killing of melanocytes by NHS as well as by GpS. F(ab')2 fragment of anti-DAF was much more effective in causing enhancement of lysis than that of anti-MCP. Thus, cultured normal human melanocytes express functionally active MCP and DAF but not CD59. Contribution of DAF in protecting melanocytes against complement attack was much more than that of MCP.