Gene structure of human CD59 and demonstration that discrete mRNAs are generated by alternative polyadenylation

A Promoter Polymorphism in the CD59 Complement Regulatory Protein Gene in Donor Lungs Correlates With a Higher Risk for Chronic Rejection After Lung Transplantation

Complement activation leads primarily to membrane attack complex formation and subsequent target cell lysis. Protection against self-damage is regulated by complement regulatory proteins, including CD46, CD55, and CD59. Within their promoter regions, single-nucleotide polymorphisms (SNPs) are present that could influence transcription. We analyzed these SNPs and investigated their influence on protein expression levels. A single SNP configuration in the promoter region of CD59 was found correlating with lower CD59 expression on lung endothelial cells (p = 0.016) and monocytes (p = 0.013). Lung endothelial cells with this SNP configuration secreted more profibrotic cytokine IL-6 (p = 0.047) and fibroblast growth factor β (p = 0.036) on exposure to sublytic complement activation than cells with the opposing configuration, whereas monocytes were more susceptible to antibody-mediated complement lysis (p < 0.0001). Analysis of 137 lung transplant donors indicated that this CD59 SNP configuration correlates with impaired long-term survival (p = 0.094) and a significantly higher incidence of bronchiolitis obliterans syndrome (p = 0.046) in the recipient. These findings support a role for complement in the pathogenesis of this posttransplant complication and are the first to show a deleterious association of a donor CD59 promoter polymorphism in lung transplantation.

Membrane-bound complement regulatory proteins as biomarkers and potential therapeutic targets for SLE

For the last two decades, there had been remarkable advancement in understanding the role of complement regulatory proteins in autoimmune disorders and importance of complement inhibitors as therapeutics. Systemic lupus erythematosus is a prototype of systemic autoimmune disorders. The disease, though rare, is potentially fatal and afflicts women at their reproductive age. It is a complex disease with multiorgan involvement, and each patient presents with a different set of symptoms. The diagnosis is often difficult and is based on the diagnostic criteria set by the American Rheumatology Association. Presence of antinuclear antibodies and more specifically antidouble-stranded DNA indicates SLE. Since the disease is multifactorial and its phenotypes are highly heterogeneous, there is a need to identify multiple noninvasive biomarkers for SLE. Lack of validated biomarkers for SLE disease activity or response to treatment is a barrier to the efficient management of the disease, drug discovery, as well as development of new therapeutics. Recent studies with gene knockout mice have suggested that membrane-bound complement regulatory proteins (CRPs) may critically determine the sensitivity of host tissues to complement injury in autoimmune and inflammatory disorders. Case-controlled and followup studies carried out in our laboratory suggest an intimate relation between the level of DAF, MCP, CR1, and CD59 transcripts and the disease activity in SLE. Based on comparative evaluation of our data on these four membrane-bound complement regulatory proteins, we envisaged CR1 and MCP transcripts as putative noninvasive disease activity markers and the respective proteins as therapeutic targets for SLE. Following is a brief appraisal on membrane-bound complement regulatory proteins DAF, MCP, CR1, and CD59 as biomarkers and therapeutic targets for SLE.

Gecko CD59 is implicated in proximodistal identity during tail regeneration

Several adult reptiles, such as Gekko japonicus, have the ability to precisely re-create a missing tail after amputation. To ascertain the associated acquisition of positional information from blastemal cells and the underlying molecular mechanism of tail regeneration, a candidate molecule CD59 was isolated from gecko. CD59 transcripts displayed a graded expression in the adult gecko spinal cord with the highest level in the anterior segment, with a stable expression along the normal tail. After tail amputation, CD59 transcripts in the spinal cord proximal to the injury sites increased markedly at 1 day and 2 weeks; whereas in the regenerating blastema, strong CD59 positive signals were detected in the blastemal cells anterior to the blastema, with a gradual decrease along the proximodistal (PD) axis. When treated with RA following amputation, CD59 transcripts in the blastema were up-regulated. PD confrontation assays revealed that the proximal blastema engulfed the distal one after in vitro culture, and rabbit-anti human CD59 antibody was able to block this PD engulfment. Overexpression of the CD59 during tail regeneration causes distal blastemal cells to translocate to a more proximal location. Our results suggest that position identity is not restricted to amphibian limb regeneration, but has already been established in tail blastema of reptiles. The CD59, a cell surface molecule, acted as a determinant of proximal–distal cell identity.

Site-specific cleavage of CD59 mRNA by endoplasmic reticulum-localized ribonuclease, IRE1

IRE1, an ER-localized transmembrane-RNase, plays a central role in ER stress response. Upon ER stress, IRE1 induces various adaptive genes through the processing of mRNA encoding the transcription factor XBP1. Moreover, it was recently reported that in fly IRE1 attenuates the expression of several genes by cleaving mRNAs, but it has been unclear whether such a mechanism also exists in mammal. In this study, we searched for IRE1alpha-cleaved mRNAs in mammalian cells and identified human CD59 (complement defense 59) mRNA as a novel cleavage target. In addition, the expression of CD59 was significantly attenuated by overexpression of IRE1alpha or ER stress. These results suggest that IRE1alpha-mediated mRNA cleavage functions even in mammals as a common system to regulate gene expression.

Molecular and functional characterization of a CD59 analogue from large yellow croaker Pseudosciana crocea

CD59 is a widely distributed membrane-bound inhibitor of the cytolytic membrane attack complex (MAC) of complement. Here, the cDNA of a CD59 analogue was cloned from large yellow croaker (Pseudosciana crocea), a marine fish (LycCD59), by expressed sequence tags (EST) and RACE techniques. The open reading frame (ORF) of 351 nucleotides (nt) of LycCD59 encodes a polypeptide of 117 amino acids (aa), which includes a putative 20-aa NH(2)-signal peptide and a 97-aa coding region with a putative GPI-anchoring site at Asn(71). The deduced LycCD59 protein shared the structural feature of mammalian CD59, including a conserved cysteine skeleton responsible for the formation of disulfide bonds, and a similar pattern of hydrophobic termini. RT-PCR analysis showed that LycCD59 mRNA was broadly expressed in various tissues examined, except for intestine. And Northern blot analysis revealed a single LycCD59 transcript of approximately 1.0kb. LycCD59 expression in blood, spleen, and kidney was significantly up-regulated during 24h of induction with poly(I:C) or inactivated trivalent bacterial vaccine as determined by a relative quantitative real-time PCR analysis, and a coordinated up-regulation of LycCD59 and complement C3 and C7 mRNA was also found in these three tissues post-induction although their up-regulation pattern and extent were somewhat different in various tissues with poly(I:C) or bacterial vaccine. The recombinant protein of LycCD59 produced in E. coli was shown to significantly inhibit the erythrocyte lysis of tilapia (Oreochromis niloticus) in an in vitro hemolytic system, which was mediated by serum from large yellow croaker and tilapia, respectively, but not from mouse and chicken, suggesting that LycCD59 has a species-selective inhibition of complement activation. These results represent the first functional identification of a CD59 analogue in teleost fish, strongly suggesting the presence of regulatory mechanism for terminal complement pathway in teleost fish.

Cloning of a CD59-like gene in rainbow trout

CD59, the major inhibitor of the complement membrane attack complex, is an 18-20 kDa glycoprotein, linked to the membrane via a glycosylphosphatidylinositol (GPI)-anchor. It restricts binding of C9 to the C5b-8 complex, preventing the formation of the complement membrane attack complex C5b-9. In this study we report the cloning of a second CD59-like gene in the rainbow trout, Oncorhynchus mykiss (referred to as CD59-2 and the previously deposited trout CD59-like gene as CD59-1). Trout CD59-2 is 56% identical to CD59-1 at the amino acid level. Both of trout CD59s show the highest identity score (54%) with putative CD59-like molecules from other teleost, and the overall identity with their mammalian orthologs is less than 30%. Trout CD59s are expressed in brain, heart, intestine, kidney, liver and spleen. Particularly, CD59-2 is abundant in trout brain, while CD59-1 seems to be absent in the trout spleen. Moreover, both of trout CD59 genes seems to be present as a single copy in trout genome.

Biochemical characterization of PRV-1, a novel hematopoietic cell surface receptor, which is overexpressed in polycythemia rubra vera

The cDNA for polycythemia rubra vera 1 (PRV-1), a novel hematopoietic receptor, was recently cloned by virtue of its overexpression in patients with polycythemia vera. PRV-1 is a member of the uPAR/CD59/Ly6 family of cell surface receptors, which share a common cysteine-rich domain and are tethered to the cell surface via a glycosylphosphatidylinositol (GPI) link. We have determined the intron-exon structure of the PRV1 gene and show that the locus is structurally intact in patients with polycythemia vera. Thus, PRV-1 overexpression in these patients is not due to rearrangement or structural alteration of the gene. Northern blot analysis detects multiple PRV-1 transcripts. Here we show that these transcripts arise from alternative polyadenylation and encode the same protein. Biochemical analysis reveals that PRV-1 is N-glycosylated and embedded in the cell membrane by a lipid anchor, like other members of this family. Moreover, PRV-1 is shed from the cell surface because soluble protein can be detected in cell supernatants. Fluorescence-activated cell sorting analysis of stably transfected cells revealed that PRV-1 is recognized by antibodies directed against the neutrophil antigen NB1/CD177. Flow cytometry of bone marrow and peripheral blood of both healthy donors and patients with polycythemia vera showed that PRV-1 protein is expressed on myeloid cells of the granulocytic lineage. However, unlike the significant difference in PRV-1 expression observed on the mRNA level, the amount of PRV-1 protein on the cell surface is not consistently elevated in patients with polycythemia vera compared with healthy controls. Therefore, quantification of PRV-1 surface expression cannot be used for the diagnosis of polycythemia vera.

Gamma-ray mutagenesis studies in a new human-hamster hybrid, A(L)CD59(+/-), which has two human chromosomes 11 but is hemizygous for the CD59 gene

Kraemer, S. M., Vannais, D. B., Kronenberg, A., Ueno, A. and Waldren, C. A. Gamma-Ray Mutagenesis Studies in a New Human-Hamster Hybrid, A(L)CD59(+/-), which has Two Human Chromosomes 11 but is Hemizygous for the CD59 Gene. Radiat. Res. 156, 10-19 (2001). We have developed a human-CHO hybrid cell line, named A(L)CD59(+/-), which has two copies of human chromosome 11 but is hemizygous for the CD59 gene and the CD59 cell surface antigen that it encodes. Our previous studies used the A(L) and A(L)C hybrids that respectively contain one or two sets of CHO chromosomes plus a single copy of human chromosome 11. The CD59 gene at 11p13.5 and the CD59 antigen encoded by it are the principal markers used in our mutagenesis studies. The hybrid A(L)CD59(+/-) contains two copies of human chromosome 11, only one of which carries the CD59 gene. The incidence of CD59 (-) mutants (formerly called S1(-)) induced by (137)Cs gamma rays is about fivefold greater in A(L)CD59(+/-) cells than in A(L) cells. Evidence is presented that this increase in mutant yield is due to the increased induction of certain classes of large chromosomal mutations that are lethal to A(L) cells but are tolerated in the A(L)CD59(+/-) hybrid. In addition, significantly more of the CD59 (-) mutants induced by (137)Cs gamma rays in A(L)CD59(+/-) cells display chromosomal instability than in A(L) cells. On the other hand, the yield of gamma-ray-induced CD59 (-) mutants in A(L)CD59(+/-) cells is half that of the A(L)C hybrid, which also tolerates very large mutations but has only one copy of human chromosome 11. We interpret the difference in mutability as evidence that repair processes involving the homologous chromosomes 11 play a role in determining mutant yields. The A(L)CD59(+/-) hybrid provides a useful new tool for quantifying mutagenesis and shedding light on mechanisms of genetic instability and mutagenesis.

Membrane complement regulatory proteins: insight from animal studies and relevance to human diseases

The complement system plays an important role in host defense. However, if not properly regulated, activated complement can also cause significant damage to host tissues. To prevent complement-mediated autologous tissue damage, host cells express a number of membrane-bound complement regulatory proteins. These include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46) and CD59. Recent studies of membrane complement regulatory proteins from various animal species have revealed similarities as well as significant differences from the corresponding human proteins. In this review, we summarize recent advances in this area and contrast the structure, function and tissue distribution of membrane complement regulatory proteins in human and nonprimate mammalian species. We also discuss how the characterization of the animal proteins has provided important clues and might continue to show relevance to the pathogenesis and therapeutics of a number of human diseases.

Identification and functional characterization of a new gene encoding the mouse terminal complement inhibitor CD59

CD59 is a 18- to 20-kDa, GPI-anchored membrane protein that functions as a key regulator of the terminal step of the complement activation cascade. It restricts binding of C9 to the C5b-8 complex, thereby preventing the formation of the membrane attack complex (C5b-9 of complement). A single human CD59 gene has been identified, and corresponding genetic homologues from rat, mouse, and pig have been characterized in previous studies. In this study, we report the discovery and functional characterization of a separate cd59 gene in the mouse (referred to as cd59b, the previously characterized mouse cd59 gene as cd59a). Mouse cd59b is 85% and 63% identical to cd59a at the nucleotide and amino acid level, respectively. In cDNA transfection experiments with Chinese hamster ovary cells, peptide-tagged cd59b was detected on the cell surface by flow cytometry and was shown to be susceptible to phosphatidylinositol-specific phospholipase C cleavage. Chinese hamster ovary cells expressing cd59b were significantly more resistant than control cells to human and mouse complement-mediated lysis. These results suggest that cd59b encodes a GPI-anchored protein that is functionally active as a membrane attack complex inhibitor. Northern blot analysis revealed that cd59b is expressed selectively in the mouse testis. In contrast, the major transcript of cd59a was shown to be expressed at high levels in the heart, kidney, liver, and lung, but only minimally in the testis. These results revealed the existence of two distinct cd59 genes in the mouse that are differentially regulated and that may have nonoverlapping physiological functions in vivo.

Functionally significant secondary structure of the simian virus 40 late polyadenylation signal

The structure of the highly efficient simian virus 40 late polyadenylation signal (LPA signal) is more complex than those of most known mammalian polyadenylation signals. It contains efficiency elements both upstream and downstream of the AAUAAA region, and the downstream region contains three defined elements (two U-rich elements and one G-rich element) instead of the single U- or GU-rich element found in most polyadenylation signals. Since many reports have indicated that the secondary structure in RNA may play a significant role in RNA processing, we have used nuclease structure analysis techniques to determine the secondary structure of the LPA signal. We find that the LPA signal has a functionally significant secondary structure. Much of the region upstream of AAUAAA is sensitive to single-strand-specific nucleases. The region downstream of AAUAAA has both double- and single-stranded characteristics. Both U-rich elements are predominately sensitive to the double-strand-specific nuclease RNase V(1), while the G-rich element is primarily single stranded. The U-rich element closest to AAUAAA contains four distinct RNase V(1)-sensitive regions, which we have designated structural region 1 (SR1), SR2, SR3, and SR4. Linker scanning mutants in the downstream region were analyzed both for structure and for function by in vitro cleavage analyses. These data show that the ability of the downstream region, particularly SR3, to form double-stranded structures correlates with efficient in vitro cleavage. We discuss the possibility that secondary structure downstream of the AAUAAA may be important for the functions of polyadenylation signals in general.

mRNA expression of complement components and regulators in rat arterial smooth muscle cells

The presence of C5b-9 complexes, some complement regulators, and abundant cytokines in atherosclerotic lesions has been reported. However, it is unclear whether these complement-associated proteins are produced by vascular smooth muscle cells (SMCs) and how they are influenced by the cytokines. In the present study, we demonstrated, by the reverse transcription-polymerase chain reaction method, the mRNA expression of complement components (C3, C4, and C5) and membrane regulators (decay-accelerating factor, membrane cofactor protein, Crry, and CD59) in cultured SMCs derived from the rat carotid artery. The expression of C9 mRNA was also induced upon stimulation by interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and/or lipopolysaccharide (LPS). Northern blot analysis showed that the mRNA expression of C3, C4, DAF and Crry was up-regulated, but that of CD59 was down-regulated by IFN-gamma, TNF-alpha and/or LPS alone or by synergy. The increase of C3 mRNA by TNF-alpha or LPS and that of C4 mRNA by IFN-gamma was induced in a dose-dependent manner. The results indicate that the arterial SMCs of rat have the ability to produce complement components and regulators, which is affected by cytokines and/or LPS. Since atherosclerosis is characterized by the intimal proliferation of SMCs, the complement system including its regulators may be involved in the pathogenesis of the disease.

High level transcription of the complement regulatory protein CD59 requires an enhancer located in intron 1

CD59 is a complement regulatory protein and may also act as a signal-transducing molecule. CD59 transgenic mice have been generated using a CD59 minigene (CD59 minigene-1). Although this minigene contained a 4.6-kilobase pair 5'-flanking region from the human CD59 gene as a promoter, the expression levels of the CD59 mRNA were substantially lower than those observed in humans, suggesting that CD59 gene expression might also require other transcriptional regulatory elements such as an enhancer. To investigate the transcriptional regulation of the CD59 gene, we used three cell lines that express CD59 at different levels. We have identified DNase I-hypersensitive sites in intron 1 in HeLa cells, which express CD59 at high levels, but not in Jurkat (intermediate level) or Raji cells (low level). Furthermore, cell line-specific enhancer activity was detected in a fragment containing these DNase I-hypersensitive sites. The CD59 enhancer was mapped to between -1155 and -888 upstream of the 5'-end of exon 2. To investigate the enhancer activity in vivo, a new CD59 minigene was constructed by the addition of the enhancer fragment into CD59 minigene-1. High expressor CD59 transgenic mice were generated using the new minigene.

Expression and regulation by interferon-gamma of the membrane-bound complement regulators CD46 (MCP), CD55 (DAF) and CD59 in gastrointestinal tumours

The membrane-bound complement inhibitors CD46 (membrane cofactor protein), CD55 (decay-accelerating factor) and CD59 (protectin) protect tumour cells against lysis by activated complement. In this study, a total of 14 (3 gastric, 3 colonic and 8 pancreatic) gastrointestinal tumour cell lines were examined for the expression of CD46, CD55 and CD59 with respect to the regulatory efficacy of interferon-gamma (IFN-gamma). The effects of IFN-gamma on mRNA and protein expression levels of CD46, CD55 and CD59 were evaluated by Northern blot hybridisation, RT-PCR, flow cytometry and immunostaining. In unstimulated cell lines, CD46 and CD59 transcripts were expressed at comparable levels, whereas the basal expression of CD55 mRNA was heterogeneous. The complement inhibitor proteins were detected in all cell lines using specific antibodies. Additional immunohistochemical stainings of gastrointestinal tissue specimens supported these findings. IFN-gamma evoked a weak induction of certain transcripts in a subset of the cell lines. Upregulation of protein expression was only observed in HT29 cells for CD55 and CD59 and was accompanied by a marked increase of the corresponding transcripts. We conclude that membrane-bound complement inhibitors are broadly expressed in gastrointestinal tumour cells and vary in their susceptibility to IFN-gamma. Thus, they may be involved in tumour escape mechanisms in gastric, pancreatic and colorectal cancer.

Molecular Cloning and Functional Characterization of the Pig Analogue of CD59: Relevance to Xenotransplantation

In this work, we report the cloning of the cDNA for the porcine analogue of human CD59. Degenerate primers, derived from the N-terminal sequence of pig erythrocyte CD59, were used to obtain the corresponding cDNA sequence. From this sequence, gene-specific primers were designed and used to amplify the 3′ and 5′ ends of the cDNA using the rapid amplification of cDNA ends (RACE) method. The complete 768-bp cDNA so obtained consisted of a 84-bp 5′ untranslated region, a 26-amino-acid NH2-signal peptide, a 98-amino-acid coding region, including putative N-glycosylation sites and a glycosylphosphatidylinositol-anchoring signal, and a 312-bp 3′ untranslated region. The mature protein sequence was 48% identical to human CD59 at the amino acid level. Northern blot analysis revealed several distinct CD59 transcripts, and a variability in expression levels of the different transcripts in the panel of tissues screened. Stable expression of pig CD59 in a CD59-negative human cell line conferred protection against lysis by complement from pig and several other species. Separate expression of pig and human CD59 at similar levels in the same cell line allowed a direct functional comparison between these two analogues. Pig CD59 and human CD59 showed similar activity in inhibiting lysis by complement from all species tested; in particular, expressed pig CD59 efficiently inhibited lysis by human complement. The relevance of these data to current work in the engineering of pig organs for xenotransplantation is discussed.

Complement regulation

Because of its strong potential for generating inflammation and causing tissue destruction the complement system has to be kept strictly under control. Cells of the host need special protection against the cytolytic complement system. This paper will describe how inappropriate activation of complement in the fluid phase is prevented and how viable human blood cells defend themselves against being destroyed and cleared away by the complement system. Since disturbances in complement regulation occasionally result in disease a brief reference will be made to two of the syndromes caused by complement regulator deficiency, hereditary angioedema (HAE) and paroxysmal nocturnal hemoglobinuria (PNH).

Alternative poly(A) site selection in complex transcription units: means to an end?

Many genes have been described and characterized which result in alternative polyadenylation site use at the 3'-end of their mRNAs based on the cellular environment. In this survey and summary article 95 genes are discussed in which alternative polyadenylation is a consequence of tandem arrays of poly(A) signals within a single 3'-untranslated region. An additional 31 genes are described in which polyadenylation at a promoter-proximal site competes with a splicing reaction to influence expression of multiple mRNAs. Some have a composite internal/terminal exon which can be differentially processed. Others contain alternative 3'-terminal exons, the first of which can be skipped in some cells. In some cases the mRNAs formed from these three classes of genes are differentially processed from the primary transcript during the cell cycle or in a tissue-specific or developmentally specific pattern. Immunoglobulin heavy chain genes have composite exons; regulated production of two different Ig mRNAs has been shown to involve B cell stage-specific changes in trans -acting factors involved in formation of the active polyadenylation complex. Changes in the activity of some of these same factors occur during viral infection and take-over of the cellular machinery, suggesting the potential applicability of at least some aspects of the Ig model. The differential expression of a number of genes that undergo alternative poly(A) site choice or polyadenylation/splicing competition could be regulated at the level of amounts and activities of either generic or tissue-specific polyadenylation factors and/or splicing factors.

Characterization of transgenic pigs expressing functionally active human CD59 on cardiac endothelium

The critical shortage of human donor organs has generated interest in the potential for porcine to human xenotransplantation. The initial immunological barrier to xenotransplantation is hyperacute rejection, which is mediated by xenoreactive antibodies and complement, and results in rapid and irreversible tissue destruction. While endogenous complement regulatory proteins (CRPs) protect cells from injury caused by autologous complement, they are relatively species specific and most likely ineffectual in this setting. This has led to the hypothesis that expression of human CRPs in transgenic pigs may affect susceptibility to complement-mediated tissue injury in a porcine-to-human xenograft. Using specific lines of transgenic pigs that express low levels of human CD59, a CRP that acts at the terminal stage of the complement cascade, we present evidence that shows that the human CD59 protein inhibits membrane attack complex assembly and reduces tissue damage when the heart is transplanted to a baboon. Examination by immunohistochemistry of transgenic porcine hearts after transplantation revealed markedly reduced deposition of C5b and MAC, but a similar level of C3 deposition as compared with transplanted control hearts. This finding supports the concept that the species specific function of CRPs contributes to the humoral barrier to xenotransplantation and, given the low level of human CD59 protein expression in the porcine heart, argues that the human protein contributes a unique rather than an additive function in regulation of complement in a xenogeneic setting.

Control of the complement system

The complement system has developed a remarkably simple but elegant manner of regulating itself. It has faced and successfully dealt with how to facilitate activation on a microbe while preventing the same on host tissue. It solved this problem primarily by creating a series of secreted and membrane-regulatory proteins that prevent two highly undesirable events: activation in the fluid phase (no target) and on host tissue (inappropriate target). Also, if not checked, even on an appropriate target, the system would go to exhaustion and have nothing left for the next microbe. Therefore, the complement enzymes have an intrinsic instability and the fluid-phase control proteins play a major role in limiting activation in time. The symmetry of the regulatory process between fluid phase and membrane inhibitors at the C4/C3 step of amplification and convertase formation as well as at the MAC steps are particularly striking features of the self/nonself discrimination system. The use of glycolipid anchored proteins on membranes to decay enzymes and block membrane insertion events is unlikely to be by chance. Finally, it is economical for the cofactor regulatory activity to produce derivatives of C3b that now specifically engage additional receptors. Likewise, C1-Inh leads to C1q remaining on the immune complex to interact with the C1q receptor. Thus the complement system is designed to allow rapid, efficient, unimpeded activation on an appropriate foreign target while regulatory proteins intervene to prevent three undesirable consequences of complement activation: excessive activation on a single target, fluid phase activation, and activation on self.

Human CD59 expressed in transgenic mouse hearts inhibits the activation of complement

Porcine-to-human xenotransplantation offers a potential solution to the critical shortage of human organs. The major immunological barrier to xenotransplantation between these species is a rapid rejection process mediated by preformed natural antibodies and complement. Xenogeneic organ grafts are especially susceptible to complement mediated injury because complement regulatory proteins, which ordinarily protect cells from inadvertent injury during the activation of complement, function poorly in regulating activation of heterologous complement. Removal of xenoreactive antibodies or systemic inhibition of complement activity has been shown to prolong graft survival. As an alternative to the systemic inhibition of complement activity, we have established a model system using transgenic animals to test whether the expression of human membrane bound complement regulatory proteins on mouse endothelial cells can inhibit the activation of human complement. CD59, which acts at the terminal stage of complement activation by inhibiting the formation of the membrane attack complex, was used as a paradigm for this model. A CD59 construct containing the putative CD59 gene promoter linked to the CD59 coding region was used to demonstrate expression of the human CD59 protein in various tissues of transgenic mice, including endothelial cells in the heart. In addition, we show that the transgenic CD59 protein is biologically active as determined by the ability to inhibit the formation of membrane attack complex in transgenic mouse hearts perfused ex vivo with human plasma. These results demonstrate that expression of membrane bound complement regulatory proteins can achieve complement inhibition in a xenogeneic organ and suggest that this approach may be useful for successful xenotransplantation between discordant species.

Functional activity of the membrane-associated complement inhibitor CD59 in a pig-to-human in vitro model for hyperacute xenograft rejection

Hyperacute rejection triggered by activation of the recipient's complement system represents the major barrier to successful xenotransplantation. Transfer of human membrane-associated complement regulators to donor organs has been suggested as one strategy to interfere with complement-mediated hyperacute xenograft rejection. Pigs are discussed as potential organ donors. We therefore investigated a putative protective function of the membrane-bound complement inhibitor CD59 in a pig-to-human in vitro model of hyperacute xenograft rejection. Aortic porcine endothelial cells were transfected with human CD59 cDNA. Expression of human CD59 was demonstrated by cytofluorimetric and RNA analysis. Removal of CD59 from the cell surface by phosphatidylinositol-specific phospholipase C (PI-PLC) demonstrated its production as a glycosyl phosphatidylinositol (GPI)-anchored protein. Functional activity of the transfected CD59 was tested by a lactate dehydrogenase (LDH) release assay for complement-mediated lysis. Porcine endothelial cells expressing human CD59 were significantly protected from lysis by human serum complement compared with CD59- cells. The protective effect was abolished by preincubating the cells with anti-CD59 antibodies or PI-PLC. We calculated by Scatchard analysis that the established CD59+ cell line expressed a CD59 level comparable to that of human endothelial cells. Our results recommend the production of pigs transgenic for CD59.

CD59: its role in complement regulation and potential for therapeutic use

CD59 regulates complement activation cascade at the final step, inhibiting formation of membrane attack complex (MAC). This protein, being anchored to the cell membrane via glycosyl phosphatidyl inositol (GPI), is expressed ubiquitously on cells which are in contact with body fluids containing components. Recently, MAC formation has been reported to play an important role in pathogenesis of inflammatory diseases such as ischemia or autoimmune diseases. In this review, we describe the structure and biological activities of CD59, the pathogenic role of MAC formation, and discuss application of soluble molecules of CD59 for therapeutic use.

The human E48 antigen, highly homologous to the murine Ly-6 antigen ThB, is a GPI-anchored molecule apparently involved in keratinocyte cell-cell adhesion

The E48 antigen, a putative human homologue of the 20-kD protein present in desmosomal preparations of bovine muzzle, and formerly called desmoglein III (dg4), is a promising target antigen for antibody-based therapy of squamous cell carcinoma in man. To anticipate the effect of high antibody dose treatment, and to evaluate the possible biological involvement of the antigen in carcinogenesis, we set out to molecularly characterize the antigen. A cDNA clone encoding the E48 antigen was isolated by expression cloning in COS cells. Sequence analysis revealed that the clone contained an open reading frame of 128 amino acids, encoding a core protein of 13,286 kD. Database searching showed that the E48 antigen has a high level of sequence similarity with the mouse ThB antigen, a member of the Ly-6 antigen family. Phosphatidylinositol-specific (PI-specific) phospholipase-C treatment indicated that the E48 antigen is glycosylphosphatidylinositol-anchored (GPI-anchored) to the plasma membrane. The gene encoding the E48 antigen is a single copy gene, located on human chromosome 8 in the 8q24-qter region. The expression of the gene is confined to keratinocytes and squamous tumor cells. The putative mouse homologue, the ThB antigen, originally identified as an antigen on cells of the lymphocyte lineage, was shown to be highly expressed in squamous mouse epithelia. Moreover, the ThB expression level is in keratinocytes, in contrast to that in lymphocytes, not mouse strain related. Transfection of mouse SV40-polyoma transformed mouse NIH/3T3 cells with the E48 cDNA confirmed that the antigen is likely to be involved in cell-cell adhesion.

Genomic organization and chromosomal localization of the mouse synexin gene

We have isolated and characterized the gene encoding mouse synexin, which consists of 14 exons and spans approximately 30 kbp of genomic DNA. The protein's unique N-terminal domain is encoded by six exons, and the C-terminal tetrad repeat, the site of the membrane-fusion and ion-channel domain, is encoded by seven exons. The first exon encodes the 5'-untranslated region. Analysis of synexin-gene expression in different mouse tissues shows that mRNA with exon 6 is only present in brain, heart and skeletal muscle. mRNA lacking exon 6 is expressed in all tissues we have examined. The initiation site for transcription was determined by primer-extension analysis and S1 nuclease mapping. Sequence analysis of the 1.3 kb 5'-flanking region revealed that the promoter has a TATA box located at position -25 and a number of potential promoter and regulatory elements. A CCAAT motif was not observed but CCATT is located in an appropriate position for the CCAAT motif upstream from the transcription-initiation start site. In addition, the 5'-flanking region contains two sets of palindromic sequences. Finally, we have determined that the functional synexin gene (Anx7) is located on mouse chromosome 14 and that a pseudogene (Anx7-ps1) is located on chromosome 10.

Regulation of CD59 expression on the human endothelial cell line EA.hy 926

CD59 (protectin) is an 18-20-kDa inhibitor of the membrane attack complex of complement. It protects homologous cells from complement-mediated damage and has been shown to be present on the endothelial cell membranes both in vitro and in vivo. In this study we observed that the surface expression of CD59 on the cultured EA.hy 926 endothelial cell line can be up-regulated to an approximately threefold higher level after a 72-h stimulation by the protein kinase C inducers phorbol-12-myristate-13 acetate (PMA; 10 nM) and calcium ionophore, A23187 (100 nM). Similarly, an increase in the level of CD59 expression was seen by the protein kinase A inducer dibutyryl-cyclic adenosine monophosphate. In Northern blot analysis increases were observed in CD59 mRNA expression, particularly in the level of the longest 1.9-kb, 2.1-kb and 5.8-kb transcripts. A functional significance for the increased CD59 expression was implied by an observed increased resistance of the PMA-stimulated EA.hy 926 cells to complement-mediated cell lysis.

Membrane defence against complement lysis: the structure and biological properties of CD59

The complement system is an important branch of the innate immune response, constituting a first line of defence against invading microorganisms which activate complement via both antibody-dependent and -independent mechanisms. Activation of complement leads to (a) a direct attack upon the activating cell surface by assembly of the pore-forming membrane attack complex (MAC), and (b) the generation of inflammatory mediators which target and recruit other branches of the immune system. However, uncontrolled complement activation can lead to widespread tissue damage in the host, since certain of the activation products, notably the fragment C3b and the C5b-7 complex, can bind nonspecifically to any nearby cell membranes. Therefore it is important that complement activation is tightly regulated. Our own cells express a number of membrane-bound control proteins which limit complement activation at the cell surface and prevent accidental complement-mediated damage. These include decay-accelerating factor, complement receptor 1 and membrane cofactor protein, all of which are active at the level of C3/C5 convertase formation. Until recently, cell surface control of MAC assembly had been attributed to a single 65-kD membrane protein called homologous restriction factor (alternatively named C8-binding protein and MAC-inhibiting protein). However a second MAC-inhibiting protein has since been discovered and it is now clear that this protein plays a major role in the control of membrane attack. This review charts the rapid progress made in elucidating the protein and gene structure, and the mechanism of action of this most recently discovered complement inhibitor, CD59.