Alternatively spliced RNAs encode several isoforms of CD46 (MCP), a regulator of complement activation

Outside in: Roles of complement in autophagy

The complement system is a well-characterized cascade of extracellular serum proteins that is activated by pathogens and unwanted waste material. Products of activated complement signal to the host cells via cell surface receptors, eliciting responses such as removal of the stimulus by phagocytosis. The complement system therefore functions as a warning system, resulting in removal of unwanted material. This review describes how extracellular activation of the complement system can also trigger autophagic responses within cells, up-regulating protective homeostatic autophagy in response to perceived stress, but also initiating targeted anti-microbial autophagy in order to kill intracellular cytoinvasive pathogens. In particular, we will focus on recent discoveries that indicate that complement may also have roles in detection and autophagy-mediated disposal of unwanted materials within the intracellular environment. We therefore summarize the current evidence for complement involvement in autophagy, both by transducing signals across the cell membrane, as well as roles within the cellular environment. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Regulation of anti-microbial autophagy by factors of the complement system

The complement system is a major component of innate immunity that participates in the defense of the host against a myriad of pathogenic microorganisms. Activation of complement allows for both local inflammatory response and physical elimination of microbes through phagocytosis or lysis. The system is highly efficient and is therefore finely regulated. In addition to these well-established properties, recent works have revealed that components of the complement system can be involved in a variety of other functions including in autophagy, the conserved mechanism that allows for the targeting and degradation of cytosolic materials by the lysosomal pathway after confining them into specialized organelles called autophagosomes. Besides impacting cell death, development or metabolism, the complement factors-autophagy connection can greatly modulate the cell autonomous, anti-microbial activity of autophagy: xenophagy. Both surface receptor-ligand interactions and intracellular interactions are involved in the modulation of the autophagic response to intracellular microbes by complement factors. Here, we review works that relate to the recently discovered connections between factors of the complement system and the functioning of autophagy in the context of host-pathogen relationship.

Autophagy in Measles Virus Infection

Autophagy is a biological process that helps cells to recycle obsolete cellular components and which greatly contributes to maintaining cellular integrity in response to environmental stress factors. Autophagy is also among the first lines of cellular defense against invading microorganisms, including viruses. The autophagic destruction of invading pathogens, a process referred to as xenophagy, involves cytosolic autophagy receptors, such as p62/SQSTM1 (Sequestosome 1) or NDP52/CALCOCO2 (Nuclear Dot 52 KDa Protein/Calcium Binding And Coiled-Coil Domain 2), which bind to microbial components and target them towards growing autophagosomes for degradation. However, most, if not all, infectious viruses have evolved molecular tricks to escape from xenophagy. Many viruses even use autophagy, part of the autophagy pathway or some autophagy-associated proteins, to improve their infectious potential. In this regard, the measles virus, responsible for epidemic measles, has a unique interface with autophagy as the virus can induce multiple rounds of autophagy in the course of infection. These successive waves of autophagy result from distinct molecular pathways and seem associated with anti- and/or pro-measles virus consequences. In this review, we describe what the autophagy–measles virus interplay has taught us about both the biology of the virus and the mechanistic orchestration of autophagy.

Divergent tropism of HHV-6AGS and HHV-6BPL1 in T cells expressing different CD46 isoform patterns

CD46 is a receptor for HHV-6A, but its role as a receptor for HHV-6B is controversial. The significance of CD46 isoforms for HHV-6A and HHV-6B tropism is unknown. HHV-6A_GS was able to initiate transcription of the viral genes U7 and U23 in the CD46⁺CD134^- T-cell lines Peer, Jurkat, Molt3, and SupT1, whereas HHV-6B_PL1 was only able to do so in Molt3 and SupT1, which expressed a CD46 isoform pattern different from Peer and Jurkat. The HHV-6B_PL1-susceptible T-cell lines were characterized by low expression of the CD46 isoform BC2 and domination of isoforms containing the cytoplasmic tail, CYT-1. A HHV-6B_PL1 susceptible cell line, Be13, changed over time its CD46 isoform pattern to resemble Peer and Jurkat and concomitantly lost its susceptibility to HHV-6B_PL1 but not HHV-6A_GS infection. We propose that isoforms of CD46 impact on HHV-6B infection and thereby in part explain the distinct tropism of HHV-6A_GS and HHV-6B_PL1.

Characterization of the Regulation of CD46 RNA Alternative Splicing

Here we present a detailed analysis of the alternative splicing regulation of human CD46, which generates different isoforms with distinct functions. CD46 is a ubiquitous membrane protein that protects host cells from complement and plays other roles in immunity, autophagy, and cell adhesion. CD46 deficiency causes an autoimmune disorder, and this protein is also involved in pathogen infection and cancer. Before this study, the mechanisms of CD46 alternative splicing remained unexplored even though dysregulation of this process has been associated with autoimmune diseases. We proved that the 5' splice sites of CD46 cassette exons 7 and 8 encoding extracellular domains are defined by noncanonical mechanisms of base pairing to U1 small nuclear RNA. Next we characterized the regulation of CD46 cassette exon 13, whose inclusion or skipping generates different cytoplasmic tails with distinct functions. Using splicing minigenes, we identified multiple exonic and intronic splicing enhancers and silencers that regulate exon 13 inclusion via trans-acting splicing factors like PTBP1 and TIAL1. Interestingly, a common splicing activator such as SRSF1 appears to repress CD46 exon 13 inclusion. We also report that expression of CD46 mRNA isoforms is further regulated by non-sense-mediated mRNA decay and transcription speed. Finally, we successfully manipulated CD46 exon 13 inclusion using antisense oligonucleotides, opening up opportunities for functional studies of the isoforms as well as for therapeutics for autoimmune diseases. This study provides insight into CD46 alternative splicing regulation with implications for its function in the immune system and for genetic disease.

A CD46-like molecule functional in teleost fish represents an ancestral form of membrane-bound regulators of complement activation

In the complement system, the regulators of complement activation (RCA) play crucial roles in controlling excessive complement activation and in protecting host cell from misdirected attack of complement. Several members of RCA family have been cloned from cyclostome and bony fish species and classified into soluble and membrane-bound type as in mammalian RCA factors. Complement-regulatory functions have been described only for soluble RCA of lamprey and barred sand bass; however, little is known on the biological function of the membrane-bound RCA proteins in the lower vertebrates. In this study, a membrane-bound RCA protein, designated teleost complement-regulatory membrane protein (Tecrem), was cloned and characterized for its complement-regulatory roles. Carp Tecrem, an ortholog of a zebrafish type 2 RCA, ZCR1, consists of four short consensus repeat modules, a serine/threonine/proline-rich domain, a transmembrane region, and a cytoplasmic domain, from the N terminus, as does mammalian CD46. Tecrem showed a ubiquitous mRNA expression in carp tissues, agreeing well with the putative regulatory role in complement activation. A recombinant Chinese hamster ovary cell line bearing carp Tecrem showed a significantly higher tolerance against lytic activity of carp complement and less deposition of C3-S, the major C3 isotypes acting on the target cell, than control Chinese hamster ovary (mock transfectant). Anti-Tecrem mAb enhanced the depositions of carp C3 and two C4 isotypes on autologous erythrocytes. Thus, the present findings provide the evidence of complement regulation by a membrane-bound group 2 RCA in bony fish, implying the host-cell protection is an evolutionarily conserved mechanism in regulation of the complement system.

Recombinant Ad35 adenoviral proteins as potent modulators of human T cell activation

The protein CD46 protects cells from complement attack by regulating cleavage of C3b and C3d. CD46 also regulates the adaptive immune response by controlling T cell activation and differentiation. Co-engagement of the T cell receptor and CD46 notably drives T cell differentiation by switching production of IFNγ to secretion of anti-inflammatory IL-10. This regulatory pathway is altered in several chronic inflammatory diseases highlighting its key role for immune homeostasis. The manipulation of the CD46 pathway may therefore provide a powerful means to regulate immune responses. Herein, we investigated the effect of recombinant proteins derived from the fiber knob of the adenovirus serotype 35 (Ad35) that uses CD46 as its entry receptor, on human T cell activation. We compared the effects of Ad35K++, engineered to exhibit enhanced affinity to CD46, and of Ad35K-, mutated in the binding site for CD46. Ad35K++ profoundly affects T cell activation by decreasing the levels of CD46 at the surface of primary T cells, and impairing T cell co-activation, shown by decreased CD25 expression, reduced proliferation and lower secretion of IL-10 and IFNγ. In contrast, Ad35K- acts a potent coactivator of T cells, enhancing T cell proliferation and cytokine production. These data show that recombinant Ad35 proteins are potent modulators of human T cell activation, and support their further development as potential drugs targeting T cell responses. This article is protected by copyright. All rights reserved.

Functional premature polyadenylation signals and aberrant splicing within a recombinant protein coding sequence limit expression

Recombinant glycoproteins can be produced at high levels in permanently transfected mammalian cells using expression vectors with strong viral promoters. CHO-K1 cell lines developed to produce the recombinant complement activator blocking protein, CAB-2 (a fusion of membrane co-factor protein, MCP, and decay accelerating factor, DAF), showed unexpectedly low expression. Northern blot analysis revealed that in addition to the expected 2300 base CAB-2 mRNA species, these cell lines expressed 790 and 1500 base mRNA species accounting for ~50% and ~10% of the total CAB-2 mRNA, respectively. RT-PCR studies established that the 1500 base species resulted from aberrant splicing from within the DAF region of the CAB-2 coding sequence to a site within the 3' untranslated region. 3' RACE analysis confirmed that the 790 base species resulted from premature polyadenylation at an AATAAA site within the MCP coding region of CAB-2. Another prematurely polyadenylated species, not observed on Northern blots, was observed in the DAF region by 3' RACE. Analysis of human tissues and cell lines revealed that these internal polyadenylation signals in native MCP and DAF coding regions also generated prematurely polyadenylated mRNAs. Genetic modification of these functional RNA processing elements within the CAB-2 gene eliminated the aberrant mRNA species and significantly increased recombinant CAB-2 expression. These results illustrate that protein expression can be limited by aberrant mRNA processing and demonstrate the importance of identifying and eliminating these mRNA processing signals from within coding DNA to maximize recombinant protein expression.

Vaccination with adenovirus serotypes 35, 26, and 48 elicits higher levels of innate cytokine responses than adenovirus serotype 5 in rhesus monkeys

Adenovirus (Ad) vaccine vectors have proven highly immunogenic in multiple experimental models, but the innate immune responses induced by these vectors remain poorly characterized. Here we report innate cytokine responses to 5 different Ad vectors in 26 rhesus monkeys. Vaccination with adenovirus serotype 35 (Ad35), Ad26, and Ad48 induced substantially higher levels of antiviral (gamma interferon [IFN-γ], 10-kDa gamma interferon-induced protein [IP-10]) and proinflammatory (interleukin 1 receptor antagonist [IL-1RA], IL-6) cytokines than vaccination with Ad5 on day 1 following immunization. In vitro studies with capsid chimeric vectors and receptor-blocking monoclonal antibodies suggested that fiber-receptor interactions, as well as other capsid components, were critical for triggering these innate responses. Moreover, multiple cell populations, including dendritic cells, monocytes/macrophages, and T lymphocytes, contributed to these innate cytokine profiles. These data demonstrate that Ad35, Ad26, and Ad48, which utilize CD46 as their primary cellular receptor, induce significantly greater innate cytokine responses than Ad5, which uses the coxsackievirus and adenovirus receptor (CAR). These differences in innate triggering result in markedly different immunologic milieus for the subsequent generation of adaptive immune responses by these vaccine vectors.

Antiviral RNAi: translating science towards therapeutic success

Viruses continuously evolve to contend with an ever-changing environment that involves transmission between hosts and sometimes species, immune responses, and in some cases therapeutic interventions. Given the high mutation rate of viruses relative to the timescales of host evolution and drug development, novel drug classes that are readily screened and translated to the clinic are needed. RNA interference (RNAi)-a natural mechanism for specific degradation of target RNAs that is conserved from plants to invertebrates and vertebrates-can potentially be harnessed to yield therapies with extensive specificity, ease of design, and broad application. In this review, we discuss basic mechanisms of action and therapeutic applications of RNAi, including design considerations and areas for future development in the field.

CD46 processing: a means of expression

CD46 is a ubiquitously expressed type I transmembrane protein, first identified as a regulator of complement activation, and later as an entry receptor for a variety of pathogens. The last decade has also revealed the role of CD46 in regulating the adaptive immune response, acting as an additional costimulatory molecule for human T cells and inducing their differentiation into Tr1 cells, a subset of regulatory T cells. Interestingly, CD46 regulatory pathways are defective in T cells from patients with multiple sclerosis, asthma and rheumatoid arthritis, illustrating its importance in regulating T cell homeostasis. Indeed, CD46 expression at the cell surface is tightly regulated in many different cell types, highlighting its importance in several biological processes. Notably, CD46 is the target of enzymatic processing, being cleaved by metalloproteinases and by the presenilin/gamma secretase complex. This processing is required for its functions, at least in T cells. This review will summarize the latest updates on the regulation of CD46 expression and on its effects on T cell activation.

Defining the role of CD46, CD80 and CD86 in mediating adenovirus type 3 fiber interactions with host cells

Attachment of human adenoviruses (Ads) to host cells is mediated by the interaction of the fiber protein of the capsid with specific cell-surface molecules. For one of the species B adenoviruses, Ad3, the mechanism of binding to cells remains to be defined. Several previous reports have proposed CD46, CD80 or CD86 as possible Ad3 fiber attachment molecules. In this study, CD80 and CD86 were not found to mediate Ad3 fiber binding or Ad3-EGFP transduction of cells. Low levels of Ad3-EGFP transduction of a CHO cell line expressing relatively high levels of CD46 were detected which might suggest a role for CD46 in facilitating Ad3: cell interactions, in the absence of other attachment molecules. Anti-CD46 antibodies and siRNAs had almost no effect on Ad3 fiber binding or Ad3-EGFP transduction of HeLa cells. However, treatment of A549 cells with CD46 siRNA resulted in some decrease of transduction with Ad3-EGFP.

Measles virus and CD46

Measles virus (MV) was isolated in 1954 (Enders and Peeble 1954). It is among the most contagious of viruses and a leading cause of mortality in children in developing countries (Murray and Lopez 1997; Griffin 2001; Bryce et al. 2005). Despite intense research over decades on the biology and pathogenesis of the virus and the successful development in 1963 of an effective MV vaccine (Cutts and Markowitz 1994), cell entry receptor(s) for MV remained unidentified until 1993. Two independent studies showed that transfection of nonsusceptible rodent cells with human CD46 renders these cells permissive to infection with the Edmonston and Halle vaccine strains of measles virus (Dorig et al. 1993; Naniche et al. 1993). A key finding in these investigations was that MV binding and infection was inhibited by monoclonal and polyclonal antibodies to CD46. These reports established CD46 as a MV cell entry receptor. This chapter summarizes the role of CD46 in measles virus infection.

T-cell regulation by CD46 and its relevance in multiple sclerosis

CD46 is a complement regulatory molecule expressed on every cell type, except for erythrocytes. While initially described as a regulator of complement activity, it later became a 'magnet for pathogens', binding to several viruses and bacteria. More recently, an alternative role for such complement molecules has emerged: they do regulate T-cell immunity, affecting T-cell proliferation and differentiation. In particular, CD46 stimulation induces Tr1 cells, regulatory T cells characterized by massive production of interleukin-10 (IL-10), a potent anti-inflammatory cytokine. Hence, CD46 is likely to control inflammation. Indeed, data from CD46 transgenic mice highlight a role for CD46 in inflammation, with antagonist roles depending on the cytoplasmic tail being expressed. Furthermore, recent data have shown that CD46 is defective in multiple sclerosis, IL-10 production being severely impaired in these patients. This lack of IL-10 production probably participates in the inflammation observed in patients with multiple sclerosis. This review will summarize the data on CD46 and T cells, and how CD46 is likely involved in multiple sclerosis.

Down-regulation of CD55 and CD46 expression by anti-sense phosphorothioate oligonucleotides (S-ODNs) sensitizes tumour cells to complement attack

Overexpression of one or more membrane-bound complement regulatory proteins (mCRPs) protects tumour cells against complement-mediated clearance by the autologous humoral immune response and is also considered as a barrier for successful immunotherapy with monoclonal anti-tumour antibodies. Neutralization of mCRPs by blocking antibodies, enzymatic removal or cytokine-mediated down-regulation has been shown to sensitize tumour cells to complement attack. In our study we applied, for the first time, anti-sense phosphorothioate oligonucleotides (S-ODNs) to knock down the expression of the mCRPs CD55 and CD46 with the aim of exploiting complement more effectively for tumour cell damage. Potent anti-sense oligonucleotides against CD55 and CD46 were identified by screening various target sequences (n = 10) for each regulator. S-ODN anti-CD55(687) reduced CD55 protein expression up to 84% and CD46 protein expression was inhibited up to 76% by S-ODN anti-CD46(85). Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed a similar reduction of the CD55 and CD46 mRNA levels, which argues for an RNAse H-dependent anti-sense mechanism. T47D, A549 and PC3 cells, representing breast, lung and prostate carcinoma, were used for functional studies. Dependent on the particular cell line, anti-sense-based inhibition of mCRP expression enhanced complement-dependent cytolysis (CDC) up to 42% for CD55 and up to 40% for CD46, and the combined inhibition of both regulators yielded further additive effects in T47D cells. C3 opsonization of CD55/CD46-deficient tumour cells was also clearly enhanced upon mCRP suppression. Due to the clinical applicability of S-ODNs, the anti-sense approach described in this study may offer an additional alternative to improve the efficacy of antibody- and complement-based cancer immunotherapy.

Localization of regions in CD46 that interact with adenovirus

A variety of pathogens use CD46, a ubiquitously expressed membrane protein that regulates complement activation, as a cellular attachment receptor. While the CD46 binding sites of several pathogens, including measles virus, Neisseria gonorrhea, and human herpesvirus 6, have been described, the region of CD46 responsible for adenovirus binding has not been determined. In this study, we used competition experiments with known CD46 ligands, CD46-specific antibodies, and a set of CD46 mutants to localize the binding domain for the group B adenovirus serotype 35 (Ad35). Our results show that Ad35 competes with measles virus for binding to CD46 but not with complement protein C3b. We further show that this interaction is a protein-protein interaction and that N glycosylations do not critically contribute to infection with Ad35 fiber-containing Ad vectors. Our data demonstrate that the native conformation of the CCP2 domain is crucial for Ad35 binding and that the substitution of amino acids at positions 130 to 135 or 152 to 156 completely abolishes the receptor function of CD46. These regions localize to the same planar face of CD46 and likely form an extended adenovirus binding surface, since no single amino acid substitution within these areas eliminates virus binding. Finally, we demonstrate that the infection with a virus possessing human group B serotype Ad11 fibers is also mediated by the CCP2 domain. This information is important to better characterize the mechanisms of the receptor recognition by adenovirus relative to other pathogens that interact with CD46, and it may help in the design of antiviral therapeutics against adenovirus serotypes that use CD46 as a primary cellular attachment receptor.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

Ligand binding determines whether CD46 is internalized by clathrin-coated pits or macropinocytosis

CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.

CD46 in meningococcal disease

The human-specific bacterial pathogen Neisseria meningitidis is a major cause of sepsis and/or meningitis. The pili of N. meningitidis interact with CD46, a human cell-surface protein involved in regulation of complement activation. Transgenic mice expressing human CD46 were susceptible to meningococcal disease, because bacteria crossed the blood-brain barrier in these mice. Development of disease was more efficient with piliated bacteria after intranasal, but not intraperitoneal, challenge of CD46 transgenic mice, suggesting that human CD46 facilitates pilus-dependent interactions at the epithelial mucosa. Hence, the human CD46 transgenic mice model is a potentially useful tool for studying pathogenesis and for vaccine development against meningococcal disease.

Characterization of human membrane cofactor protein (MCP; CD46) on spermatozoa

Membrane cofactor protein (MCP; CD46) is a complement regulator widely expressed as four isoforms that arise via alternative splicing. On human spermatozoa, MCP is expressed on the inner acrosomal membrane and alterations of spermatozoa MCP may be associated with infertility. In rodents, expression of MCP is largely restricted to the testes. MCP on human spermatozoa has a unique M(r) pattern that we have investigated. We also characterized MCP expression in mice transgenic (tg) for human MCP. Human MCP expression in the tg mice mimics the human pattern in that it is located on the inner acrosomal membrane and has a faster M(r) than MCP expressed elsewhere. Sequencing of RT-PCR products from the testis indicates that there is not a unique male reproductive tissue specific cytoplasmic tail. Instead, human spermatozoa express MCP bearing cytoplasmic tail two, which is also utilized in most other tissues and contains several signaling motifs. Further, using N-glycosidases, we demonstrate that the unique lower molecular weight of MCP on spermatozoa is secondary to a modification in the N-linked sugars. Specifically, as the spermatozoa mature, but before they reach the epididymis, the three N-linked sugars of MCP are trimmed to less complex structures. While the purpose of this deglycosylation is unknown, we propose that it is a common feature of proteins expressed on the plasma and inner acrosomal membranes of spermatozoa and hypothesize that it is a spermatozoa specific event critical for facilitating sperm-egg interactions.

Human herpesvirus 6 variant A but not variant B induces fusion from without in a variety of human cells through a human herpesvirus 6 entry receptor, CD46

Human herpesvirus 6 (HHV-6) is a lymphotropic betaherpesvirus that productively infects T cells and monocytes. HHV-6 isolates can be differentiated into two groups, variants A and B (HHV-6A and HHV-6B). Here, we show a functional difference between HHV-6A and -6B in that HHV-6A induced syncytium formation of diverse human cells but HHV-6B did not. The syncytium formation induced by HHV-6A was observed 2 h after infection; moreover, it was found in the presence of cycloheximide, indicating that HHV-6A induced fusion from without (FFWO) in the target cells. Furthermore, the fusion event was dependent on the expression of the HHV-6 entry receptor, CD46, on the target cell membrane. In addition, we determined that short consensus repeat 2 (SCR2), -3, and -4 of the CD46 ectodomain were essential for the formation of the virus-induced syncytia. Monoclonal antibodies against glycoproteins B and H of HHV-6A inhibited the fusion event, indicating that the syncytium formation induced by HHV-6A required glycoproteins H and B. These findings suggest that FFWO, which HHV-6A induced in a variety of cell lines, may play an important role in the pathogenesis of HHV-6A, not only in lymphocytes but also in various tissues, because CD46 is expressed ubiquitously in human tissues.

Molecular assembly of CD46 with CD9, alpha3-beta1 integrin and protein tyrosine phosphatase SHP-1 in human macrophages through differentiation by GM-CSF

Human CD46, formerly membrane cofactor protein (MCP), binds and inactivates complement C3b and serves as a receptor for measles virus (MV), thereby protecting cells from homologous complement and sustaining systemic viral infection. CD46 on activated macrophages (Mphi) but not intact monocytes is presumed to be the factor responsible for virus-mediated immune modulation including down-regulation of IL-12 production. As CD46 is expressed on both Mphi and monocytes, the molecular mechanisms responsible for these distinct immune responses remain largely unknown. Here, we found that peripheral blood monocytes treated for 5--8 days with GM-CSF (i.e. mature Mphi) acquired the capacity to assemble CD9, alpha3-beta1 integrin and the tyrosine phosphatase SHP-1 with their CD46. Prior to this maturation stage, Mphi expressed sufficient amounts of CD9 and CD46 but showed no such complex formation, and as in intact monocytes MV replication was markedly suppressed. By flow cytometry and confocal microscopy, the complex was found to assemble on the surface in cells treated with approximately 6 days with GM-CSF but not for approximately 2 days. Notably, an alternative MV receptor SLAM CDw150 was neither expressed nor recruited to this complex throughout GM-CSF-mediated Mphi differentiation. These responses and molecular links were not reproduced in the hamster cell line CHO expressing human CD46 although these cells acquired high susceptibility to MV. Based on these observations, MV susceptibility in human myeloid lineages appears not to be as simple as that observed in human CD46-transfected non-myeloid cells. The molecular complex involving CD46 may confer high MV permissiveness leading to immune modulation in Mphi.

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.

A human CD46 transgenic pig model system for the study of discordant xenotransplantation

BACKGROUND

The chronic shortage in the supply of human organs available for allotransplantation has turned attention toward the use of animals as potential donors, with pigs as the most likely species under consideration. Hyperacute rejection, the initial and immediate barrier to a pig-to-primate xenograft, has been addressed by generation of transgenic pigs that express the human membrane-bound complement-regulatory proteins CD59 and/or CD55. Difficulty has been encountered in generation of transgenic animals that express a third membrane-bound complement-regulatory protein, CD46.

METHODS

We have generated transgenic animals by using a large genomic construct that encompasses the entire human CD46 gene.

RESULTS

We report the first description of transgenic mice and pigs that express high levels of human CD46 in a cell and tissue type-specific manner, resembling patterns of endogenous CD46 expression observed in human tissues. Furthermore, when human CD46 transgenic porcine hearts were transplanted into baboons, the grafts did not succumb to hyperacute rejection, and survival extended for up to 23 days. Under the same conditions, nontransgenic grafts underwent hyperacute rejection within 90 min.

CONCLUSIONS

This is the first report to describe generation of transgenic pigs that express human CD46, and the first in vivo demonstration of the ability of human CD46 expressed on pig organs to regulate complement activation and overcome hyperacute rejection upon transplantation of a vascularized organ into nonhuman primates.

Functional modulation of human macrophages through CD46 (measles virus receptor): production of IL-12 p40 and nitric oxide in association with recruitment of protein-tyrosine phosphatase SHP-1 to CD46

Human CD46, formerly membrane cofactor protein, binds and inactivates complement C3b and serves as a receptor for measles virus (MV), thereby protecting cells from homologous complement and sustaining systemic measles infection. Suppression of cell-mediated immunity, including down-regulation of IL-12 production, has been reported on macrophages (Mphi) by cross-linking their CD46. The intracellular events responsible for these immune responses, however, remain unknown. In this study, we found that 6- to 8-day GM-CSF-treated peripheral blood monocytes acquired the capacity to recruit protein-tyrosine phosphatase SHP-1 to their CD46 and concomitantly were able to produce IL-12 p40 and NO. These responses were induced by stimulation with mAbs F(ab')(2) against CD46 that block MV binding or by a wild-type MV strain Kohno MV strain (KO; UV treated or untreated) that was reported to induce early phase CD46 down-regulation. Direct ligation of CD46 by these reagents, but not intracellular MV replication, was required for these cellular responses. Interestingly, the KO strain failed to replicate in the 6- to 8-day GM-CSF-cultured Mphi, while other MV strains replicated to form syncytia under the same conditions. When stimulated with the KO strain, rapid and transient dissociation of SHP-1 from CD46 was observed. These and previous results provide strong evidence that CD46 serves as a signal modulatory molecule and that the properties of ligands determine suppression or activation of an innate immune system at a specific maturation stage of human Mphi.

Inhibition of hyperacute transplant rejection by soluble proteins with the functional domains of CD46 and FcgammaRII

BACKGROUND

Recombinant soluble forms of complement regulatory molecules, including the human complement regulatory protein CD46 (rsCD46), have been shown to inhibit hyperacute transplant rejection (HAR) and protect against complement-mediated inflammatory tissue damage. Similarly, recombinant soluble forms of the immunoglobulin receptor FcgammaRII (rsFcgammaRII) can attenuate antibody-mediated inflammatory responses. We have produced and tested the function of novel recombinant chimeric proteins that incorporate the functional domains of both CD46 (membrane cofactor protein, MCP) and the low affinity human IgG receptor FcgammaRII (CD32).

METHODS

Two recombinant soluble chimeric proteins (CD46:FcR and FcR:CD46) were designed and produced using a human cell expression system. Their ability to protect cells against complement-mediated lysis (through the CD46 domain) and bind human IgG (through the Fc receptor domain) was assessed in vitro. They were also tested in vivo in the rat reverse passive Arthus reaction and a murine model of hyperacute cardiac transplant rejection.

RESULTS

In vitro, the functional domains of the chimeric proteins each retained their activity. In vivo, the serum half-life of the recombinant chimeric proteins in mice was more than either rsCD46 or rsFcgammaRII. In the rat reverse passive Arthus reaction, intradermal injection of each recombinant protein substantially reduced inflammatory skin edema (>50%) and polymorphonuclear neutrophil infiltration (>90%). In the hyperacute rejection model, i.v. treatment with FcR:CD46 prevented complement-mediated rejection, macroscopic bruising, edema, and thrombosis more effectively than rsCD46.

CONCLUSIONS

CD46/FcgammaRII bifunctional proteins have an improved ability to control complement-mediated hyperacute graft rejection and have therapeutic potential in other conditions involving antibody-mediated inflammation.

The 3'-UT of the ubiquitous mRNA of human CD46 confers selective suppression of protein production in murine cells

Mice express CD46 protein and its approximately 1.5-kb mRNA only in the testicular germ cells, unlike primates and pigs which ubiquitously express CD46 and its approximately 4 kb mRNA. Human CD46 is not well expressed in transgenic mice carrying human CD46 cDNA. To analyze the mechanism of regulation of human CD46 expression in mouse cells, we cloned the long (ubiquitous approximately 4 kb, L-form) and short ( approximately 1.5 kb, S-form) forms of human CD46 cDNA whose size difference is due to a stretch of the 3'-UT. Transfection of either cDNA resulted in marked S-form-dependent protein generation in all mouse cell lines tested. In contrast, there were virtually no differences in protein synthesis between S- and L-form cDNA in the simian and swine cell lines. Quantitative mRNA analyses and luciferase reporter gene assays suggested that one major cause of this interspecies discrepancy is transcriptional regulation, i. e. selective suppression of the 4-kb mRNA leading to low levels of protein synthesis. Although other mechanisms such as mRNA stability and translational regulation may lead to the low expression levels of L-form-derived CD46 in mice, the silencer activity in the L-form 3'-UT appears to function in human CD46 transcriptional regulation in mice.

Morbillivirus downregulation of CD46

There is evidence that CD46 (membrane cofactor protein) is a cellular receptor for vaccine and laboratory-passaged strains of measles virus (MV). Following infection with these MV strains, CD46 is downregulated from the cell surface, and consequent complement-mediated lysis has been shown to occur upon infection of a human monocytic cell line. The MV hemagglutinin (H) protein alone is capable of inducing this downregulation. Some wild-type strains of MV fail to downregulate CD46, despite infection being prevented by anti-CD46 antibodies. In this study we show that CD46 is also downregulated to the same extent by wild-type, vaccine, and laboratory-passaged strains of rinderpest virus (RPV), although CD46 did not appear to be the receptor for RPV. Expression of the RPV H protein by a nonreplicating adenovirus vector was also found to cause this downregulation. A vaccine strain of peste des petits ruminants virus caused slight downregulation of CD46 in infected Vero cells, while wild-type and vaccine strains of canine distemper virus and a wild-type strain of dolphin morbillivirus failed to downregulate CD46. Downregulation of CD46 can, therefore, be a function independent of the use of this protein as a virus receptor.

Expression of human complement regulatory protein CD46 restricts measles virus replication in mouse macrophages

Measles virus (MV) can infect mouse macrophages to cause a prolonged non-cytopathic infection that produces low levels of infectious virus for days. We have generated RAW264.7 mouse macrophages expressing human CD46, a cell surface complement regulatory protein that serves as a receptor for laboratory-adapted strains of MV. Laboratory-adapted MV strains efficiently enter the CD46-positive mouse macrophages to cause a cytopathic infection with extensive multinucleated cells and pseudopodia-like extensions. However, MV infection of mouse macrophages through CD46 is self-limiting. Both viral protein synthesis and infectious virus production are abruptly terminated after the second day of infection. This novel virus-cell interaction is seen only in mouse macrophages but not in mouse or hamster fibroblasts expressing human CD46. The possible role of CD46 in macrophage antiviral response restricting MV replication is discussed.

A novel isoform of human membrane cofactor protein (CD46) mRNA generated by intron retention

The reverse transcription polymerase chain reaction (RT-PCR) with primers specific for each of the 14 exons of the human complement regulatory protein membrane cofactor protein (MCP;CD46) has been utilized to determine MCP mRNA transcript expression in peripheral blood mononuclear cells (PBMC). An additional transcript of a larger size than predicted was consistently detected in reactions with a sense primer for exon 7, that encodes the first alternatively spliced serine-threonine-rich region (ST-A), together with an antisense exon 12 primer, RT-PCR with primers for other exons both 5' and 3' of exon 7 further showed that these MCP transcripts contain additional sequences immediately both 5' and 3' to the exon 7-encoded sequence. Comparison of genomic DNA with cDNA by PCR, in combination with sequence analysis, demonstrated the presence of the complete invariant sequences of both introns adjacent to exon 7, i.e. intron 6 (411 bp) and intron 7 (127 bp). RT-PCR using primers specific for the intron 6 sequence, together with Southern and Northern blotting using an intron 6-specific probe, confirmed retention of this intron within a novel 4.8-kb mRNA transcript in human PBMC. Due to the presence of a stop codon within intron 6, translation would result in a novel truncated MCP isoform (MCPi) containing the four invariant short consensus repeat (SCR) regions and a unique C-terminal 39 amino acid transmembrane and cytoplasmic tail region that may promote endoplasmic reticulum retention.

Post-translational modification and intracellular localization of a splice product of CD46 cloned from human testis: role of the intracellular domains in O-glycosylation

We obtained a unique CD46 cDNA, STc/CY4, from the human testis, the predicted amino acid sequence of which suggested the presence of a novel isoform of CD46. This message was present predominantly in the testis, and the predicted isoform possessed a short (11 amino acids) transmembrane section (TM) and an unidentified cytoplasmic tail (CY). When expressed in Chinese hamster ovary (CHO) cells, this CD46 isoform underwent no O-glycosylation and was mostly retained in the endoplasmic reticulum. This unusual behaviour of the new isoform was due in part to the short TM and the unusual sequences of the CY. The molecular mass of this isoform was 42,000, approximately 20,000 smaller than conventional CD46. These properties of the STc/CY4 isoform were similar to those of sperm CD46. The only difference between sperm CD46 and the STc/CY4 isoform expressed on CHO cells was that only the latter possessed N-linked sugars of high mannose types. Since the STc/CY4 isoform may behave like sperm CD46 in cellular localization and post-translational modification, studies of sperm-egg interassociation were performed using hamster eggs and CHO cell clones expressing various isoforms including the STc/CY4. Rosette formation was seen most effectively between hamster eggs and STc/CY4-expressing CHO cells. These results infer that O-glycosylation perturbs CD46-mediated sperm-binding to eggs and thus sperm CD46 lacking O-linked sugars can serve as an adhesion molecule. The possible role of CD46 in fertilization and the structural differences between sperm and conventional CD46 are discussed.

A single amino acid change in the hemagglutinin protein of measles virus determines its ability to bind CD46 and reveals another receptor on marmoset B cells

This paper provides evidence for a measles virus receptor other than CD46 on transformed marmoset and human B cells. We first showed that most tissues of marmosets are missing the SCR1 domain of CD46, which is essential for the binding of Edmonston measles virus, a laboratory strain that has been propagated in Vero monkey kidney cells. In spite of this deletion, the common marmoset was shown to be susceptible to infections by wild-type isolates of measles virus, although they did not support Edmonston measles virus production. As one would expect from these results, measles virus could not be propagated in owl monkey or marmoset kidney cell lines, but surprisingly, both a wild-type isolate (Montefiore 89) and the Edmonston laboratory strain of measles virus grew efficiently in B95-8 marmoset B cells. In addition, antibodies directed against CD46 had no effect on wild-type infections of marmoset B cells and only partially inhibited the replication of the Edmonston laboratory strain in the same cells. A direct binding assay with insect cells expressing the hemagglutinin (H) proteins of either the Edmonston or Montefiore 89 measles virus strains was used to probe the receptors on these B cells. Insect cells expressing Edmonston H but not the wild-type H bound to rodent cells with CD46 on their surface. On the other hand, both the Montefiore 89 H and Edmonston H proteins adhered to marmoset and human B cells. Most wild-type H proteins have asparagine residues at position 481 and can be converted to a CD46-binding phenotype by replacement of the residue with tyrosine. Similarly, the Edmonston H protein did not bind CD46 when its Tyr481 was converted to asparagine. However, this mutation did not affect the ability of Edmonston H to bind marmoset and human B cells. The preceding results provide evidence, through the use of a direct binding assay, that a second receptor for measles virus is present on primate B cells.

Artificial mutations and natural variations in the CD46 molecules from human and monkey cells define regions important for measles virus binding

CD46 was previously shown to be a primate-specific receptor for the Edmonston strain of measles virus. This receptor consists of four short consensus regions (SCR1 to SCR4) which normally function in complement regulation. Measles virus has recently been shown to interact with SCR1 and SCR2. In this study, receptors on different types of monkey erythrocytes were employed as "natural mutant proteins" to further define the virus binding regions of CD46. Erythrocytes from African green monkeys and rhesus macaques hemagglutinate in the presence of measles virus, while baboon erythrocytes were the least efficient of the Old World monkey cells used in these assays. Subsequent studies demonstrated that the SCR2 domain of baboon CD46 contained an Arg-to-Gln mutation at amino acid position 103 which accounted for reduced hemagglutination activity. Surprisingly, none of the New World monkey erythrocytes hemagglutinated in the presence of virus. Sequencing of cDNAs derived from the lymphocytes of these New World monkeys and analysis of their erythrocytes with SCR1-specific polyclonal antibodies indicated that the SCR1 domain was deleted in these cells. Additional experiments, which used 35 different site-specific mutations inserted into CD46, were performed to complement the preceding studies. The effects of these artificial mutations were documented with a convenient binding assay using insect cells expressing the measles virus hemagglutinin. Mutations which mimicked the change found in baboon CD46 or another which deleted the SCR2 glycosylation site reduced binding substantially. Another mutation which altered GluArg to AlaAla at positions 58 and 59, totally abolished binding. Finally, the epitopes for two monoclonal antibodies which inhibit measles virus attachment were mapped to the same regions implicated by mutagenesis.

Transgenic expression of a CD46 (membrane cofactor protein) minigene: studies of xenotransplantation and measles virus infection

CD46 (membrane cofactor protein) is a human cell-surface regulator of activated complement and a receptor for the measles virus. A CD46 transgenic mouse line with an expression pattern similar to that of human tissues has been produced, to develop an animal model of (i) the control of complement activation by complement regulators in hyperacute rejection of xenografts, and (ii) measles virus infection. The mouse line was made using a CD46 minigene that includes promoter sequence and the first two introns of genomic CD46, which was coinjected into mouse ova with chicken lysozyme matrix attachment region DNA. A high level of CD46 expression in homozygotic transgenic mice was obtained with spleen cells having approximately 75% of the level found on human peripheral blood mononuclear cells. CD46 was detected in all tissues examined by immunohistochemistry, radioimmunoassay and Western blotting, showing that these mice were suitable for transplantation and measles virus infection studies. It also indicated that the transgene included the important regulatory elements of the CD46 promoter. Transgenic spleen cells were significantly protected in vitro from human complement activated by either the classical or alternative pathways and from alternative pathway rat complement. Furthermore, transgenic mouse hearts transplanted to rats regulated complement deposition in an in vivo model of antibody-dependent hyperacute xenograft rejection. Similar to human lymphocytes, transgenic lymphoblasts could be infected in vitro with measles virus; infected cells expressed viral proteins and produced infectious viral particles. The data demonstrate the suitability of this minigene for obtaining high-level CD46 expression sufficient for enhanced resistance of transgenic cells to complement attack and for obtaining wide tissue distribution of CD46, analogous to human tissues and, therefore, useful for comparative studies.

The CD46 transmembrane domain is required for efficient formation of measles-virus-mediated syncytium

Two phosphatidylinositol (PI)-anchored versions of a measles virus (MV) receptor membrane cofactor protein (MCP; CD46) were generated by fusing the extracellular domain of MCP to the decay-accelerating factor (DAF; CD55) or its PI anchor. The PI-anchored forms of MCP expressed on Chinese hamster ovary cells, otherwise non-permissive to MV, conferred a smaller MV cytopathic effect than a wild-type MCP, a Ser/Thr-rich domain-deletion mutant and a cytoplasmic tail-deletion mutant of MCP. Therefore the differences in MV receptor properties between the two PI-anchored and three transmembrane forms were investigated. The PI-anchored forms were predominantly expressed on microvilli as in DAF, whereas the other transmembrane forms were found on intracellular membranes. The PI-anchored forms conferred high MV-binding capacity compared with the transmembrane versions. MV replication was, however, severely suppressed in cells expressing the PI-anchored forms, resulting in ineffective syncytium formation. In contrast, cell-to-cell fusion occurred efficiently after co-transfection of cDNA species encoding MV-H. MV-F and any version of MCP. Thus the PI-anchored forms, despite showing sufficient MV binding and cell-to-cell fusion competence together with MV-H and MV-F, mediate inefficient MV entry or replication, which causes severe suppression of the MV cytopathic effect. A biased receptor distribution on microvilli might participate in the selection of a low MV uptake pathway in the PI-anchored forms of MCP. Taken together, the transmembrane portion of MCP is a critical factor for effective virus-cell fusion and the subsequent MV replication.

Identification of a cytoplasmic Tyr-X-X-Leu motif essential for down regulation of the human cell receptor CD46 in persistent measles virus infection

To investigate the sequence requirements for measles virus (MV)-induced receptor down regulation, we transfected the human CD46 gene into simian cells persistently infected by the Biken strain of MV. Surface expression of CD46 is drastically reduced in these cells. Deletion analysis has shown that the juxtamembrane region of the CD46 cytoplasmic domain is essential for down regulation. Deleting a Tyr-Arg-Tyr-Leu sequence in this region or changing these residues to Ala prevents CD46 down regulation from the infected cell surface. Alanine-scanning mutagenesis has identified two amino acid residues, Tyr and Leu, forming a Tyr-X-X-Leu motif critical for CD46 down regulation. Mutations that prevent CD46 down regulation enhance syncytium formation. These results indicate that CD46 down regulation limits the cytopathic effects in a persistent MV infection and that CD46 down regulation requires a cytoplasmic Tyr-X-X-Leu sequence which resembles known motifs for membrane protein trafficking and receptor signalling.

Human cell receptor CD46 is down regulated through recognition of a membrane-proximal region of the cytoplasmic domain in persistent measles virus infection

Monkey cells persistently infected by measles virus (MV) Biken strain (Biken-CV-1 cells) showed no cytopathic effects and lacked surface expression of a homolog of human cell receptor, membrane cofactor protein CD46. Transfection of a human CD46 gene into these cells induced extensive cell fusion, indicating that down regulation of the endogenous CD46 homolog was essential for the maintenance of a noncytopathic mode of infection. Surface expression of the exogenously introduced human CD46 was also drastically down regulated in the persistently infected cells compared with uninfected cells. The down regulation was specific for CD46 and did not affect surface expression of exogenously introduced CD4. Exogenous human CD46 was synthesized efficiently in the persistently infected cells, but it did not accumulate on the cell surface. Fusion of Biken-CV-1 cells required the extracellular hemagglutinin (H-protein)-binding domain but not the cytoplasmic domain. Replacing the transmembrane and cytoplasmic domains of CD46 with a glycosylphosphatidylinositol anchor did not prevent cell fusion but completely alleviated down regulation of the glycosylphosphatidylinositol-anchored CD46 in Biken-CV-1 cells. Deletion analyses revealed that the membrane-distal sequences of the CD46 cytoplasmic domain were not only unnecessary but also inhibitory for CD46 down regulation. By contrast, the six amino acid residues proximal to the membrane contained a sequence required for CD46 down regulation in the persistently infected cells. These results indicate that CD46 is down regulated in the persistently infected cells by a mechanism that recognizes a membrane-proximal sequence in the CD46 cytoplasmic domain.

Translation is enhanced after silent nucleotide substitutions in A+T- rich sequences of the coding region of CD46 cDNA

Specific sequences in the coding region of CD46 (membrane cofactor protein) transcripts have been shown to have a marked effect on translation. Two A+T-rich regions of CD46 cDNA were altered by mutation without changing the CD46 amino acid sequence (silent nucleotide substitution). In one region, the A+T content was reduced from 78% to 55% and in the other a putative polyadenylation addition sequence was disrupted. In each example, mutated sequences transfected into COS-7 cells produced significantly more soluble or cell surface protein (up to a 20-fold increase) than wild-type sequences. The amount of cellular plasmid DNA and CD46 mRNA was not increased, suggesting that the effect was not due to increased transfection efficiency, or transcript synthesis or stability. Biosynthetically labelled transfected cells showed an increase in translation rate but cell-free in vitro translation studies demonstrated that wild-type and mutated transcripts were translated with similar efficiency. The data show that translation of CD46 is affected by specific mRNA coding sequences, 400-540 bases from the initiation codon, and suggest that these sequences require the structural integrity of the cell to exert their effect.

A functional analysis of recombinant soluble CD46 in vivo and a comparison with recombinant soluble forms of CD55 and CD35 in vitro

The human cell surface complement regulatory proteins CD46 (MCP), CD55 (DAF) and CD35 (CR1) protect autologous cells from complement-mediated damage by inhibiting C3 and C5 convertases. This regulatory potential has previously been exploited in the treatment of some models of inflammatory injury by the generation of recombinant soluble (rs) proteins, such as rsCD55 and rsCD35 . More recently, we have shown that rsCD46 inhibits complement activation in the fluid phase. In this report, the ability of rsCD46, rsD55 and rsCD35 to regulate human complement activation mediated by the classical pathway in vitro was clearly demonstrated by all three soluble proteins; however, rsCD35 was a more effective inhibitor than either rsCD46 or rsCD55. A combination of rsCD46+ rsCD55 was more potent than either of these proteins alone. Cell lysis via alternative pathway activation in vitro was efficiently regulated by rsCD46 and rsCD35 to a similar extent, whereas rsCD55 was not effective. Assays of rsCD46 in vivo have previously not been possible due to difficulties in expressing sufficient quantities of protein. This limitation has been overcome and now we report the ability of rsCD46 to inhibit immune complex-mediated inflammation in a rat using the reverse passive Arthus reaction model. Administration of rsCD46 significantly reduced the size of lesion, and histological examination showed a reduction in inflammatory infiltrate and edema. These data suggest that rsCD46, in addition to rsCd55 and rsCD35, may be useful a therapeutic agent.

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.

High expression of membrane cofactor protein of complement (CD46) in human leukaemia cell lines: implication of an alternatively spliced form containing the STA domain in CD46 up-regulation

Human membrane cofactor protein (MCP, CD46) is a receptor for the measles virus and serves as a complement regulator which protects host cells from autologous complement attack. MCP is highly polymorphic due to a variety of mRNA splice products. The levels of MCP expression on T and myeloid cell lines are usually two-eightfold higher than those on their normal counterparts, whereas Burkitt's lymphoma B cell lines express less MCP than B cell lineages carrying no EB virus. The molecule has a Ser/Thr-rich (ST) domain adjacent to the functional domain, namely short consensus repeats (SCR). The ST domain and a cytoplasmic tail (CYT) contribute to the MCP polymorphism. The ST domain is encoded by three exons (A, B and C) and major ST isoforms are STABC, STBC and STC. The authors investigated the relationship between the expression levels and isoform usage of MCP by flow cytometry using specific antibodies against STA and STC, by reverse transcriptase-polymerase chain reaction (RT-PCR) with size markers for each splice variant, and by RT-PCR/Southern blotting using a specific probe for STA. The results were (1) the profiles of mean shifts of myeloid and T cell lines were STC < STA on flow cytometry while those of B cell lines and normal blood cells were STA < STC; (2) all cell lines tested by RT-PCR expressed the messages for the isoforms STBC/CYT1, STC/CYT1, STBC/CYT2, and STC/CYT2. The band for STABC/CYT2 overlapped that for STC/CYT1, and the band for STABC/CYT1 was marginal in all cell lines examined; (3) semi-quantitative analysis of the STABC isoforms by Southern blotting indicated the presence of high levels of the STABC messages in myeloid and T-cell lines in comparison with B lymphoid cells and normal leucocytes. Thus, the quantity of MCP expressed parallels the STABC message level, which is up-regulated in T and myeloid leukaemia cell lines.

Human lymphocytes are more susceptible to measles virus than granulocytes, which is attributable to the phenotypic differences of their membrane cofactor protein (CD46)

Membrane cofactor protein (MCP, CD46) of the complement system is a measles virus (MV) receptor. Human lymphocytes express a heavily glycosylated (H) and a lightly glycosylated (L) form of MCP, which confers a two-band profile on SDS-PAGE the ratio of which is controlled genetically and organ-specifically. In contrast, granulocytes express a single heavily glycosylated form regardless of lymphocyte MCP phenotype. We investigated susceptibility to MV of granulocytes and lymphocytes from individuals with different lymphocyte MCP phenotypes. In any individual, granulocytes were > 10-fold less susceptible to MV than lymphocytes, and the lymphocytes with predominant H form were generally less susceptible to those with an increasing amount of L form. Thus, lymphocytes always exhibit high susceptibility to MV compared to granulocytes in all individuals. This finding may explain the lymphopenia and immunosuppression observed secondary to MV infection.

The polymorphic human TLX-B/CD46/MCP system and its implications in transplantation and reproduction

TLX antigens have been found on most peripheral blood cells, trophoblasts, seminal vesicle cells and sperms. These antigens seem to be associated with the membrane cofactor protein (MCP) and the CD46 antigen. Alloantibodies to TLX antigens with Fc tau RII-blocking features were obtained by transfusion of leucocytes or platelets. Preliminary population studies revealed that alloantibodies to TLX/CD46/MCP recognize four overlapping specificities. The terminology TLX-B was introduced with specificities TLX-B1, B2, B3, B4 and frequencies obtained in the population were: 38%, 46%, 42% and 26%, respectively. Family studies showed an independent segregation of the TLX and HLA alleles. At the cellular protein on trophoblast, the alloantibody detected a glycoprotein of 66-67 kDa molecular mass, which may correspond to the alpha chain of the TLX/CD46/MCP isotypes. A direct association of the alloantibody with Fc tau RII could be excluded thus its FcR blocking feature is probably based on an indirect functional effect. After transfusion and in pregnancy the induction of TLX alloantibody production depended on the mismatching in the TLX/CD46/MCP phenotypes. Probable associations were revealed in the case of recurrent habitual abortion between the lack of Fc tau R blocking antibody production and the matched TLX specificities of the couples. After transfusion, TLX alloantibody production with Fc tau R and MLR blocking function was induced only when the recipient was lacking the TLX specificities expressed on the donor cells. Suppression of MLR was found only when TLX specificity in sera corresponded to the TLX specificity of the effector cell. The immunopathological importance of these findings in transplantation and reproductive medicine has yet to be clarified.

CD46, a primate-specific receptor for measles virus

Measles virus normally infects only primate cells. The receptor for measles virus has recently been shown to be the complement regulator CD46, also known as membrane cofactor protein. Transfection of rodent cells with human CD46 renders them susceptible to the virus, suggesting that transgenic animals may prove useful for testing antiviral agents and vaccines.

Characterization of cofactor activity for factor I: cleavage of complement C4 in human syncytiotrophoblast microvilli

To coexist with complement, human tissues express membrane-integrated regulatory proteins that inhibit the activity of autologous complement on cell surfaces. Certain of these complement regulatory proteins act as obligatory cofactors for proteolytic inactivation of activated C4(C4b) by factor I. Extraembryonic tissues and in particular trophoblasts constitute an interface at risk from maternal complement during pregnancy. The present study examined syncytiotrophoblast plasma membrane (STM) cofactor activity for cleavage of immobilized methylamine-treated complement component C4(C4ma), a C4b analog by factor I. Membrane cofactor protein (MCP or CD46) provided most of the cofactor activity in STM preparations. Minor cofactor activity was derived from C4 binding protein that was firmly bound to STM. Cofactor activity for cleavage of C4ma at its two sites for factor I was enhanced at higher concentrations of STM and at lower concentrations cleavage at a C terminal site predominated. Soluble cofactor activity was present in STM preparations and was provided by 65 KDa, 55 KDa and 50 KDa soluble species of MCP that lacked amphiphilic properties. These results are consistent with a major role for MCP in regulation of C4 activity on the maternal-facing surfaces of extraembryonic tissues during human development. Soluble MCP may provide additional fluid phase complement regulatory activity in the maternotrophoblastic zone.

The human CD46 molecule is a receptor for measles virus (Edmonston strain)

Measles virus normally causes disease in human beings, and the host range of this virus may be determined by a specific receptor on the surface of primate cells. Human-rodent somatic cell hybrids were tested for their ability to bind measles virus, and only cells that contained human chromosome 1 were capable of binding virus. A study of lymphocyte markers suggested that the complement regulator known either as membrane cofactor protein or CD46 was the measles virus receptor. We proved this hypothesis by demonstrating that hamster cell lines that expressed human CD46 could subsequently bind virus. Furthermore, infected CD46+ cells produced syncytia and viral proteins. Finally, polyclonal antisera against CD46 inhibited virus binding and infection. These results prove that human CD46 permits cells both to bind measles virus and to support infection.

Polymorphic expression of CD46 protein isoforms due to tissue-specific RNA splicing

CD46 is a member of the regulators of complement activation (RCA) family and serves to protect autologous cells from complement mediated lysis. The CD46 gene consists of 14 exons and extensive RNA splicing produces protein isoforms of different molecular weight. Predominant protein isoforms of 66 and 56 kDa arise from splicing in or out of exon 8 which encodes a region rich in serine, threonine and proline residues known to be heavily O-glycosylated. An inherited allelic polymorphism controls the relative expression of these isoforms in PBL and other tissues. This study has analysed an independent and overriding tissue specific regulation of CD46 splicing. Salivary gland and kidney produce RNA transcripts that preferentially include exon 8, giving rise to the 66 kDa protein species, while exon 8 is spliced out in brain tissue to give the 56 kDa protein. The cytoplasmic tail of CD46 is encoded by either exon 13 (CYT 1) or exon 14 (CYT 2). There is a preferential deletion of exon 13 from transcripts in salivary gland, kidney and brain to encode a protein containing cytoplasmic tail CYT 2. This preferential production of the CYT 2 tail is contrary to that seen on peripheral blood lymphocytes where equivalent expression of both CYT 1 and CYT 2 is observed. Our results suggest that while the splicing of exons within most cells is controlled by nucleotide sequences within or close to the CD46 gene (i.e. cis-regulation), splicing in tissues such as salivary gland, kidney and brain is regulated by trans-splicing factors encoded by another gene(s).

Membrane cofactor protein (MCP, CD46) in seminal plasma and on spermatozoa in normal and "sterile" subjects

A sperm protein of molecular mass 43 kDa (the spermatozoa membrane cofactor protein, smMCP) and a seminal plasma protein of 60 kDa (ssMCP) were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting with four monoclonal antibodies (mAb) against membrane cofactor protein (MCP, CD46). These proteins served as factor I cofactors for the cleavage of methylamine-treated C3 (C3ma), the activity of which was blocked by M75, an MCP cofactor-activity-blocking mAb. Thus, these semen proteins are antigenic and functional homologous of MCP. On SDS-PAGE analysis these MCP migrated as single-band proteins which differed from the two-band forms of MCP expressed on other cells. smMCP was N-glycosylated but not O-glycosylated, while ssMCP was O-glycosylated: after deglycosylation of these proteins bands were detected at 38-40 kDa and 43 kDa on SDS-PAGE, respectively. These semen MCP are therefore, structurally different from the conventional MCP. ssMCP in both normal and "sterile" subject groups was determined by sandwich enzyme-linked immunosorbent assay. Seminal plasma in the two groups contained 250-700 ng/ml ssMCP. The difference between the two groups was marginal, although samples from normal subjects tended to show higher concentrations of ssMCP than samples from "sterile" subjects. No molecular difference was observed with ssMCP and smMCP in the two groups by SDS-PAGE/immunoblotting analysis. Immunohistochemical analysis suggested that MCP was positive in glandular epithelial cells and the lumen of the prostate, and in most intra-lumen cells of the testis. Using antibody M177, solubilized prostate and testis were analyzed by immunoblotting and compared with other cell MCP. The major band of MCP in the testis, but not in the prostate, was of 60 kDa, which aligned with ssMCP. No band of testis or prostate MCP, however, aligned with smMCP. ssMCP may be produced in the testis, while the origin of smMCP remains unknown. We hypothesize that ssMCP is important in the survival of spermatozoa, protecting them against local secretion of immunoglobulin and complement in the female genital tract, and that smMCP, which is expressed on acrosome-reacted spermatozoa, plays an essential role in the interaction of spermatozoa with oocytes.