Adenovirus type 11 uses CD46 as a cellular receptor

Crystal structure of species D adenovirus fiber knobs and their sialic acid binding sites

Adenovirus serotype 37 (Ad37) belongs to species D and can cause epidemic keratoconjunctivitis, whereas the closely related Ad19p does not. Primary cell attachment by adenoviruses is mediated through receptor binding of the knob domain of the fiber protein. The knobs of Ad37 and Ad19p differ at only two positions, Lys240Glu and Asn340Asp. We report the high-resolution crystal structures of the Ad37 and Ad19p knobs, both native and in complex with sialic acid, which has been proposed as a receptor for Ad37. Overall, the Ad37 and Ad19p knobs are very similar to previously reported knob structures, especially to that of Ad5, which binds the coxsackievirus-adenovirus receptor (CAR). Ad37 and Ad19p knobs are structurally identical with the exception of the changed side chains and are structurally most similar to CAR-binding knobs (e.g., that of Ad5) rather than non-CAR-binding knobs (e.g., that of Ad3). The two mutations in Ad19p result in a partial loss of the exceptionally high positive surface charge of the Ad37 knob but do not affect sialic acid binding. This site is located on the top of the trimer and binds both alpha(2,3) and alpha(2,6)-linked sialyl-lactose, although only the sialic acid residue makes direct contact. Amino acid alignment suggests that the sialic acid binding site is conserved in several species D serotypes. Our results show that the altered viral tropism and cell binding of Ad19p relative to those of Ad37 are not explained by a different binding ability toward sialyl-lactose.

The Identification of an Adenovirus Receptor by Using Affinity Capture and Mass Spectrometry

A tandem mass spectrometry--based approach is demonstrated for detecting a receptor for Ad37, one of the causative agents for epidemic keratoconjunctivitis. Partial purification of membrane glycoproteins was performed by using lectin-affinity chromatography and SDS-PAGE. Gel bands that were shown to bind Ad37 by using Viral Overlay Protein Blot Assay (VOPBA) were excised, proteolyzed and analyzed by using nanoLC-MS/MS to identify putative receptors contained in a mixture of proteins. Four candidate receptors were identified among approximately 50 proteins based on a search against a protein database. Inhibition of gene delivery mediated by an Ad37 vector, with antibodies against the glycoproteins identified by tandem mass spectrometry, strongly indicated that Membrane Cofactor Protein (MCP), a member of the complement regulatory family of proteins, is the receptor. This rapid and sensitive MS/MS-based strategy is perceived to have wide potential applications for the detection of viral receptors.

CD46: A complement regulator and pathogen receptor that mediates links between innate and acquired immune function

In the last 10 years, the human cell-surface molecule, CD46, has evolved from 'just another complement regulator' to a receptor for a striking array of pathogens. CD46 not only protects cells from complement-mediated attack and facilitates infection by a large number of pathogens, but also exerts complex effects on cellular immune function. It has been proposed that CD46 links innate and adaptive immunity by affecting cellular immune function in response to complement binding, and the role of CD46 in the pathogenesis of many infectious pathogens is now the subject of intense investigation. So far, the flood of information that implicates CD46 in modifying a host response to measles, Neisseria, human herpes virus 6, and pathogens that activate complement has not yet been matched with a comprehensive understanding of the molecular mechanisms by which CD46 affects immune function. This review summarizes the evidence that points to a significant role for CD46 in a range of pathological processes and describes how CD46 might exert its effects by altering signal transduction and antigen presentation pathways.

Gene transduction and cell entry pathway of fiber-modified adenovirus type 5 vectors carrying novel endocytic peptide ligands selected on human tracheal glandular cells

Monolayers of cystic fibrosis transmembrane conductance regulator (CFTR)-deficient human tracheal glandular cells (CF-KM4) were subjected to phage biopanning, and cell-internalized phages were isolated and sequenced, in order to identify CF-KM4-specific peptide ligands that would confer upon adenovirus type 5 (Ad5) vector a novel cell target specificity and/or higher efficiency of gene delivery into airway cells of patients with cystic fibrosis (CF). Three different ligands, corresponding to prototypes of the most represented families of phagotopes recovered from intracellular phages, were designed and individually inserted into Ad5-green fluorescent protein (GFP) (AdGFP) vectors at the extremities of short fiber shafts (seven repeats [R7]) terminated by scissile knobs. Only one vector, carrying the decapeptide GHPRQMSHVY (abbreviated as QM10), showed an enhanced gene transduction of CF-KM4 cells compared to control nonliganded vector with fibers of the same length (AdGFP-R7-knob). The enhancement in gene transfer efficiency was not specific to CF-KM4 cells but was observed in other mammalian cell lines tested. The QM10-liganded vector was referred to as AdGFP-QM10-knob in its knobbed version and as AdGFP-QM10 in its proteolytically deknobbed version. AdGFP-QM10 was found to transduce cells with a higher efficiency than its knob-bearing version, AdGFP-QM10-knob. Consistent with this, competition experiments indicated that the presence of knob domains was not an absolute requirement for cell attachment of the QM10-liganded vector and that the knobless AdGFP-QM10 used alternative cell-binding domains on its capsid, including penton base capsomer, via a site(s) different from its RGD motifs. The QM10-mediated effect on gene transduction seemed to take place at the step of endocytosis in both quantitative and qualitative manners. Virions of AdGFP-QM10 were endocytosed in higher numbers than virions of the control vector and were directed to a compartment different from the early endosomes targeted by members of species C Ad. AdGFP-QM10 was found to accumulate in late endosomal and low-pH compartments, suggesting that QM10 acted as an endocytic ligand of the lysosomal pathway. These results validated the concept of detargeting and retargeting Ad vectors via our deknobbing system and redirecting Ad vectors to an alternative endocytic pathway via a peptide ligand inserted in the fiber shaft domain.

Biodistribution of radioiodinated adenovirus fiber protein knob domain after intravenous injection in mice

The knob domains from the fiber proteins of adenovirus serotypes 2 and 12 were labeled with radioiodine and then injected into the bloodstreams of mice. Knob proteins with functional binding sites for the coxsackie and adenovirus receptor (CAR) were cleared rapidly from the circulation, with radioactivity appearing predominantly in the stomach, while knob mutants unable to bind to CAR remained in the blood circulation for a prolonged period. The clearance of radiolabeled wild-type knob from the blood was slowed by coinjecting an excess of unlabeled wild-type knob protein. An earlier study showed that (99m)Tc-labeled knob protein with intact CAR-binding activity also cleared rapidly from the blood circulation of mice, with radioactivity accumulating predominantly in the liver (K. R. Zinn et al., Gene Ther. 5:798-808, 1998). Together these results suggest that rapid clearance of knob protein from the blood results from specific binding to CAR in the liver and that the bound knob then enters a degradative pathway. The elevated levels of radioiodine in the stomach observed in our experiments are consistent with deiodination of labeled knob by dehalogenases in hepatocyte microsomes and uptake of the resultant free radioiodine by Na/I symporters in the gastric mucosa. Although CAR has been shown to localize in tight junctions of polarized epithelial cells, where it functions in intercellular adhesion, the results of our study suggest that a subset of CAR molecules in the liver is highly accessible to ligands in the blood and able to rapidly deliver bound ligand to an intracellular degradative compartment.

Adenovirus fibre exchange alters cell tropism in vitro but not transgene-specific T CD8+ immune responses in vivo

Gene transfer with recombinant adenoviruses (rAds) is a powerful means of inducing an immune response against a transgene product. However, little is known about the mechanisms that underlie the induction of the immune response after intramuscular inoculation of adenovirus and, in particular, the relative role of the different cell types transduced. Several studies have suggested that CD8+ cytotoxic T lymphocyte responses elicited after inoculation of adenoviruses (Ads) are induced both by direct transduction of antigen presenting cells (APCs) and by cross-priming. In the present study, a library of fibre-chimeric rAds was screened in order to identify rAds with distinct capacities to express transgene product in murine cell types naturally found in muscle, i.e. myoblasts, endothelial cells (both representing non-APCs) and dendritic cells (representing APCs). Four selected pseudotypes, differing in their ability to infect muscular cells were used to immunize C57BL/6 mice. The relationship between the capacity to transduce non-APC or APC in vitro and the ability to induce humoral and cellular responses against the beta-galactosidase antigen after intramuscular inoculation were studied. Results indicate that CD8+ T cell responses against the beta-galactosidase antigen were similar after inoculation of the four viruses, thus revealing no direct relationship with their ability to transduce myoblasts, endothelial cells or dendritic cells in vitro.

Structural Similarities in the Cellular Receptors Used by Adenovirus and Reovirus

Adenovirus and reovirus are nonenveloped viruses that engage cell-surface receptors using filamentous attachment proteins with head-and-tail morphology. The coxsackievirus and adenovirus receptor (CAR) and reovirus receptor junctional adhesion molecule 1 (JAM1) are immunoglobulin superfamily members that form homodimers stabilized by ionic and hydrophobic contacts between their N-terminal immunoglobulin-like domains. Both proteins are expressed at regions of cell-cell contact and contain sequences in their cytoplasmic tails that anchor the proteins to the actin cytoskeleton. Like CAR and JAM1, the attachment proteins of adenovirus and reovirus, fiber and sigma1, respectively, also share key structural features. Both fiber and sigma1 have defined regions of flexibility within the tail, which is constructed in part using an unusual triple beta-spiral motif. The head domains of both proteins are formed by an 8-stranded beta-barrel with identical beta-strand connectivity. Strikingly, both adenovirus fiber and reovirus 1 engage their receptors by interacting with sequences that also mediate formation of receptor homodimers. Therefore, while adenovirus and reovirus belong to different virus families and have few overall properties in common, the observed similarities between the receptors and attachment proteins of these viruses suggest a conserved mechanism of attachment and an evolutionary relationship.

Analysis of adenovirus sequestration in the liver, transduction of hepatic cells, and innate toxicity after injection of fiber-modified vectors

After intravenous administration, adenovirus (Ad) vectors are predominantly sequestered by the liver. Delineating the mechanisms for Ad accumulation in the liver is crucial for a better understanding of Ad clearance and Ad-associated innate toxicity. To help address these issues, in this study, we used Ad vectors with different fiber shaft lengths and either coxsackievirus-Ad receptor (CAR)-interacting Ad serotype 9 (Ad9) or non-CAR-interacting Ad35 fiber knob domains. We analyzed the kinetics of Ad vector accumulation in the liver, uptake into hepatocytes and Kupffer cells, and induction of cytokine expression and release in response to systemic vector application. Immediately after intravenous injection, all Ad vectors accumulated equally efficiently in the liver; however, only genomes of long-shafted Ads were maintained in the liver tissue over time. We found that Kupffer cell uptake of long-shafted Ads was mediated by the fiber knob domain and was CAR independent. The short-shafted Ads were unable to efficiently interact with hepatocellular receptors and were not taken up by Kupffer cells. Moreover, our studies indicated that Kupffer cells were not the major reservoir for the observed accumulation of Ads (used in this study) in the liver within the first 30 min after virus infusion. The lower level of liver cell transduction by short-shafted Ads correlated with a significantly reduced inflammatory anti-Ad response as well as liver damage induced by the systemic administration of these vectors. This study contributes to a better understanding of the biology of systemically applied Ad and will help in designing safer vectors that can efficiently transduce target tissues.

Abolition of hCAR-dependent cell tropism using fiber knobs of Atadenovirus serotypes

Most adenoviral vectors use in gene therapy protocols derive from species C. However, expression of the primary receptor (human Coxsackievirus and Adenovirus receptor, hCAR) for these AdV is variable on cancer cells. In vivo targeting of a therapeutic gene to specific cells has then become a major issue in gene therapy. The Ad fiber protein largely determines viral tropism through interaction with specific receptors. Hereto, we constructed a set of HAdV5 vectors carrying chimeric fibers with knob domains from nonhuman AdV, namely from the FAdV-1 (Aviadenovirus), DAdV-1, and BAdV-4 (Atadenovirus). Correspondents viruses were produced using an established new HEK293 cell line, which express the HAdV2 fiber. Recombinant HAdV harboring chimeric fibers constituted of the N-terminal domain of HAdV2, and knob domain of bovine adenovirus type 4 (BAdV-4) demonstrated the greatest reduction in fiber-mediated gene transfer into human cells expressing the hCAR. Moreover, this vector infects with a better efficiency than vector with wild-type fiber, the Chinese Hamster Ovarian (CHO) and SKOV3 cell lines, both from ovarian origin, hamster and human, respectively. These studies support the concept that changing the fiber knob domain to ablate hCAR interaction should result in a de- or retargeted adenoviral vector. The adenoviral vector with the chimeric HAdV2/BAdV-4 fiber lacking hCAR interaction and with an ovarian cell tropism could be a nice candidate to elaborate vectors for ovarian tumor therapy.

Membrane cofactor protein is a receptor for adenoviruses associated with epidemic keratoconjunctivitis

Subgroup D adenovirus (Ad) types 8, 19, and 37 (Ad8, -19, and -37, respectively) are causative agents of epidemic keratoconjunctivitis and genital tract infections. Previous studies showed that Ad37 binds to a 50-kDa membrane glycoprotein expressed on human ocular (conjunctival) cells. To identify and characterize the role of the 50-kDa glycoprotein in Ad37 infection, we partially purified this molecule from solubilized Chang C conjunctival cell membranes by using lentil lectin chromatography and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Liquid chromatography coupled to nano-electrospray ionization-tandem mass spectrometry was subsequently used to identify four Ad37 receptor candidates: CD46, CD87, CD98, and CD147. Immunodepletion analyses demonstrated that the 50-kDa protein is identical to CD46 (also known as membrane cofactor protein). The Ad37, but not Ad5, fiber knob bound to the extracellular domain of CD46, demonstrating a direct interaction of an Ad37 capsid protein with CD46. An antibody specific for the N-terminal 19 amino acids of CD46 also blocked Ad37 infection of human cervical carcinoma and conjunctival cells, indicating a requirement for CD46 in infection. Finally, expression of a 50-kDa isoform of human CD46 in a CD46-null cell line increased cell binding by wild-type Ad37 and gene delivery by an Ad vector pseudotyped with the Ad37 fiber, but not by a vector bearing the Ad5 fiber. Together, these studies demonstrate that CD46 serves as an attachment receptor for Ad37 and shed further light on the cell entry pathway of subgroup D Ads.

Adenovirus type 7 induces interleukin-8 in a lung slice model and requires activation of Erk

Adenovirus (Ad), particularly Ad type 7 (Ad7), causes severe lung infection and pneumonia. Initially, Ad causes neutrophilic inflammation of the distal airways and alveoli. Interleukin-8 (IL-8) is the major lung neutrophil chemotaxin, and we have shown that Ad7 induces IL-8 release from the A549 alveolar epithelial cell line. We sought to determine whether ex vivo human and bovine lung tissue containing primary pneumocytes could be used as a more accurate and relevant model to study Ad acute inflammation. We found that cultured lung tissue preserved normal lung architecture for more than 10 days. IL-8 was generated upon exposure of the lung organ culture to Ad7. IL-8 production required activation of the Ras/Erk pathway, since a pharmacological inhibitor blocked the appearance of IL-8 in the medium. Both human and bovine lung explants supported replication of Ad7, and immunohistochemistry experiments demonstrated the presence of the Ad hexon antigen within alveolar epithelial cells. These findings show that our novel human lung organ culture accurately reproduces the in vivo infectious disease process. Thus, this organ culture model represents a valuable tool for studying the acute innate immune response to respiratory infections.

Adenovirus serotype 3 utilizes CD80 (B7.1) and CD86 (B7.2) as cellular attachment receptors

Most viruses exploit a variety of host cellular proteins as primary cellular attachment receptors in the context of successful execution of infection. Furthermore, many viral agents have evolved precise mechanisms to subvert host immune recognition to achieve persistence. Herein we present data indicating that adenovirus (Ad) serotype 3 utilizes CD80 (B7.1) and CD86 (B7.2) as cellular attachment receptors. CD80 and CD86 are co-stimulatory molecules that are present on mature dendritic cells and B lymphocytes and are involved in stimulating T-lymphocyte activation. To our knowledge, this is one of the first demonstrations of a virus utilizing immunologic accessory molecules as a primary means of cellular entry. This finding suggests a mechanism whereby viral exploitation of these proteins as receptors may achieve both goals of cellular entry and evading the immune system.

The human membrane cofactor CD46 is a receptor for species B adenovirus serotype 3

Many human adenovirus (Ad) serotypes use the coxsackie B virus-Ad receptor (CAR). Recently, CD46 was suggested to be a receptor of species B Ad serotype 11 (Ad11), Ad14, Ad16, Ad21, Ad35, and Ad50. Using Sindbis virus-mediated cDNA library expression, we identify here the membrane cofactor protein CD46 as a surface receptor of species B Ad3. All four major CD46 transcripts and one minor CD46 transcript expressed in nucleated human cells were isolated. Rodent BHK cells stably expressing the BC1 form of CD46 bound radiolabeled Ad3 with a dissociation constant of 0.3 nM, identical to that of CD46-positive HeLa cells expressing twice as many Ad3 binding sites. Pull-down experiments with recombinant Ad3 fibers and a soluble form of the CD46 extracellular domain linked to the Fc portion of human immunoglobulin G (CD46ex-Fc) indicated direct interactions of the Ad3 fiber knob with CD46ex-Fc but not CARex-Fc (Fc-linked extracellular domain of CAR). Ad3 colocalized with cell surface CD46 in both rodent and human cells at the light and electron microscopy levels. Anti-CD46 antibodies and CD46ex-Fc inhibited Ad3 binding to CD46-expressing BHK cells more than 10-fold and to human cells 2-fold. In CD46-expressing BHK cells, wild-type Ad3 and a chimeric Ad consisting of the Ad5 capsid and the Ad3 fiber elicited dose-dependent cytopathic effects and transgene expression, albeit less efficiently than in human cells. Together, our results show that all of the major splice forms of CD46 are predominant and functional binding sites of Ad3 on CD46-expressing rodent and human cells but may not be the sole receptor of species B Ads on human cells. These results have implications for understanding viral pathogenesis and therapeutic gene delivery.

Mutations in CD46, a complement regulatory protein, predispose to atypical HUS

Membrane cofactor protein (MCP, CD46) is a widely expressed transmembrane complement regulator. As does the soluble regulator factor H, it inhibits complement activation by inactivating the C3b that is deposited on target membranes. Factor H mutations have been described in 15-30% of patients with atypical haemolytic uraemic syndrome (HUS). Recent studies have identified mutations in the MCP gene in four families. In one, a heterozygous deletion resulted in the intracellular retention of the mutant protein. In another, a different heterozygous deletion led to a premature stop codon and the loss of the C-terminus. In the other two, a substitution (S206P) resulted in cell-surface expression but inefficient inactivation of surface-bound C3b. These findings provide further evidence that complement dysregulation predisposes to the development of HUS.

DNA sequencing and analysis of the right-hand part of the genome of the unique bovine adenovirus type 10

The prototype strain of bovine adenovirus (BAdV) type 10 and four additional isolates that were indistinguishable in serum-neutralization tests have been shown to have remarkable variation in their genome size and restriction maps. In the present study, more than 40 % of the DNA sequence of the BAdV-10 isolate with the longest genome was determined. A biased base composition resulting in low (<41 %) GC content was noticed. Analysis of the genes of the DNA-binding protein, 100K, 33K, pVIII and fibre proteins, as well as early regions E3 and E4, which are encoded by the genome fragment examined, confirmed that BAdV-10 is different from the other known BAdV types regarding its phylogenetic distance and the organization of its exceptionally short E3 region, apparently containing only two genes. A comparative analysis of the E3 and E4 regions of BAdV-10 with various animal adenoviruses revealed interesting features accounting for the very short genome of BAdV-10. In the examined BAdV-10 isolate, duplicated sequences were localized in and around the fibre gene. Since BAdV-10 appears to be pathogenic to cattle and is genetically distant from the other BAdVs, we suggest that BAdV-10 is not a genuine bovine virus, but has recently switched host and is now undergoing an adaptation process in its new host. In accordance with this hypothesis, the remarkable predominance of AT-rich codons along with the variable fibre gene might be signs of adaptation.

Cholesterol is required for endocytosis and endosomal escape of adenovirus type 2

The species C adenovirus type 2 (Ad2) and Ad5 bind the coxsackievirus B Ad receptor and alphav integrin coreceptors and enter epithelial cells by clathrin-mediated endocytosis. This pathway is rapid and efficient. It leads to cell activation and the cholesterol-dependent formation of macropinosomes. Macropinosomes are triggered to release their contents when incoming Ad2 escapes from endosomes. Here, we show that cholesterol extraction of epithelial cells by methyl-beta-cyclodextrin (mbetaCD) treatment reduced Ad5-mediated luciferase expression approximately 4-fold. The addition of cholesterol to normal cells increased gene expression in a dose-dependent manner up to threefold, but it did not restore gene expression in mbetaCD-treated cells. mbetaCD had no effect in the presence of excess cholesterol, indicating that the inhibition of gene expression was due specifically to cholesterol depletion. Cholesterol depletion inhibited rapid Ad2 endocytosis, endosomal escape, and nuclear targeting, consistent with the notion that clathrin-dependent endocytosis of Ad2 is cholesterol dependent. In cholesterol-reduced cells, Ad2 internalized at a low rate, suggestive of an alternative, clathrin-independent, low-capacity entry pathway. While exogenous cholesterol completely restored rapid Ad2 endocytosis, macropinocytosis, and macropinosome disruption, it did not, surprisingly, restore viral escape from endosomes. Our results indicate that macropinosome disruption and endosomal escape of Ad2 are independent events in cells depleted of and then refilled with cholesterol, suggesting that viral escape from endosomes requires lipid-controlled membrane homeostasis, trafficking, or signaling.

Structural and functional analysis of integrin alpha2I domain interaction with echovirus 1

Integrins are cell surface receptors for several microbial pathogens including echovirus 1 (EV1), a picornavirus. Cryo-electron microscopy revealed that the functional domain (alpha(2)I) of human alpha(2)beta(1) integrin binds to a surface depression on the EV1 capsid. This three-dimensional structure of EV1 bound to alpha(2)I domain provides the first structural details of an integrin interacting with a picornavirus. The model indicates that alpha(2)beta(1) integrin cannot simultaneously bind both EV1 and the physiological ligand collagen. Compared with collagen binding to the alpha(2)I domain, the virus binds with a 10-fold higher affinity but in vitro uncoating of EV1 was not observed as a result of attachment of alpha(2)I. A molecular model, constructed on the basis of the EV1-integrin complex, shows that multiple alpha(2)beta(1) heterodimers can bind at adjacent sites around the virus 5-fold symmetry axes without steric hindrance. In agreement with this, virus attachment to alpha(2)beta(1) integrin on the cell surface was found to result in integrin clustering, which can give rise to signaling and facilitate the initiation of the viral entry process that takes place via caveolae-mediated endocytosis.

CD46 is a cellular receptor for group B adenoviruses

Group B adenoviruses, a subgenus of human Adenoviridae, are associated with a variety of often-fatal illnesses in immunocompromised individuals, including bone marrow transplant recipients and cancer and AIDS patients. Recently, group B adenovirus derivatives have gained interest as attractive gene therapy vectors because they can transduce target tissues, such as hematopoietic stem cells, dendritic cells and malignant tumor cells, that are refractory to infection by commonly used adenoviral vectors. Whereas many adenoviruses infect cells through the coxsackievirus and adenovirus receptor (CAR), group B adenoviruses use an alternate, as-yet-unidentified cellular attachment receptor. Using mass spectrometric analysis of proteins interacting with a group B fiber, we identified human CD46 as a cellular attachment receptor for most group B adenoviruses. We show that ectopic expression of human CD46 rendered nonhuman cells susceptible to infection with group B viruses in vitro and in vivo. In addition, both siRNA-mediated knockdown of CD46 and a soluble form of CD46 blocked infection of human cell lines and primary human cells. The discovery that group B adenoviruses use CD46, a ubiquitously expressed complement regulatory protein, as a cellular attachment receptor elucidates the diverse clinical manifestation of group B virus infections, and bears directly on the application of these vectors for gene therapy.