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

The Arg279Gln [corrected] substitution in the adenovirus type 11p (Ad11p) fiber knob abolishes EDTA-resistant binding to A549 and CHO-CD46 cells, converting the phenotype to that of Ad7p

The major determinant of adenovirus (Ad) attachment to host cells is the C-terminal knob domain of the trimeric fiber protein. Ad type 11p (Ad11p; species B2) in contrast to Ad7p (species B1) utilizes at least two different cellular attachment receptors, designated sBAR (species B adenovirus receptor) and sB2AR (species B2 adenovirus receptor). CD46 has recently been identified as one of the Ad11p attachment receptors. However, CD46 did not seem to constitute a functional receptor for Ad7p. Although Ad7p shares high knob amino acid identity with Ad11p, Ad7p is deficient in binding to both sB2AR and CD46. To determine what regions of the Ad11p fiber knob are necessary for sB2AR-CD46 interaction, we constructed recombinant fiber knobs (rFK) with Ad11p/Ad7p chimeras and Ad11p sequences having a single amino acid substitution from Ad7p. Binding of the constructs to A549 and CHO-CD46 BC1 isoform-expressing cells was analyzed by flow cytometry. Our results indicate that an Arg279Gln [corrected] substitution is sufficient to convert the Ad11p receptor-interaction phenotype to that of Ad7p and abolish sB2AR and CD46 interaction. Also a Glu279Arg substitution in Ad7p rFKs increases CD46 binding. Thus, the lateral HI loop of the Ad11p fiber knob seems to be the key determinant for Ad11p sB2AR-CD46 interaction. This result is comparable to another non-coxsackie-adenovirus receptor binding Ad (Ad37p), where substitution of one amino acid abolishes virus-cell interaction. In conjunction with previous results, our findings also strongly suggest that sB2AR is equivalent to CD46.

Adenovirus Vectors Based on Human Adenovirus Type 19a Have High Potential for Human Muscle-Directed Gene Therapy

Until recently, adenovirus-based gene therapy has been almost exclusively based on human adenovirus serotype 5 (Ad5). The aim of this study was to systematically compare the efficiency of transduction of primary muscle cells from various species by two adenoviral vectors from subgroups C and D. Transduction of a panel of myoblasts demonstrated a striking specificity of an Ad19a-based replication-defective E1-deleted vector (Ad19aEGFP) for human cells, whereas the Ad5-based vector had high affinity for nonhuman primate myoblasts. Transgene expression correlated well with cell-associated vector genomes. Up to 6.59% of the initially applied Ad19aEGFP vector particles were taken up by human myoblasts, as compared with 0.1% of the corresponding Ad5 vector. Remarkably, Ad19aEGFP but not Ad5EGFP efficiently transduced differentiated human myotubes, an in vitro model for skeletal muscle transduction. Uptake of Ad19aEGFP vector particles in human myotubes was 12-fold more efficient than that of Ad5EGFP. Moreover, both vectors demonstrated an early block at the level of vector uptake in mouse myoblasts and rat L6 cells. Investigation of the underlying mechanism for binding and uptake of the two vectors by human myoblasts showed high susceptibility for Ad19a to neuraminidase and wheat germ agglutinin (WGA) lectin, whereas Ad5-mediated transduction was dependent on binding to the coxsackie-adenovirus receptor (CAR) and sensitive to soluble RGD peptide and heparin. Our study offers insights into species-dependent factors that determine Ad tropism and, moreover, provides a basis for application of the novel Ad19a-based vector for gene transfer into human skeletal muscle.

CD46 is a cellular receptor for all species B adenoviruses except types 3 and 7

The 51 human adenovirus serotypes are divided into six species (A to F). Adenovirus serotypes from all species except species B utilize the coxsackie-adenovirus receptor for attachment to host cells in vitro. Species B adenoviruses primarily cause ocular and respiratory tract infections, but certain serotypes are also associated with renal disease. We have previously demonstrated that adenovirus type 11 (species B) uses CD46 (membrane cofactor protein) as a cellular receptor instead of the coxsackie-adenovirus receptor (A. Segerman et al., J. Virol. 77:9183-9191, 2003). In the present study, we found that transfection with human CD46 cDNA rendered poorly permissive Chinese hamster ovary cells more permissive to infection by all species B adenovirus serotypes except adenovirus types 3 and 7. Moreover, rabbit antiserum against human CD46 blocked or efficiently inhibited all species B serotypes except adenovirus types 3 and 7 from infecting human A549 cells. We also sequenced the gene encoding the fiber protein of adenovirus type 50 (species B) and compared it with the corresponding amino acid sequences from selected serotypes, including all other serotypes of species B. From the results obtained, we conclude that CD46 is a major cellular receptor on A549 cells for all species B adenoviruses except types 3 and 7.

Transductional targeting of adenovirus vectors for gene therapy

Cancer gene therapy approaches will derive considerable benefit from adenovirus (Ad) vectors capable of self-directed localization to neoplastic disease or immunomodulatory targets in vivo. The ablation of native Ad tropism coupled with active targeting modalities has demonstrated that innate gene delivery efficiency may be retained while circumventing Ad dependence on its primary cellular receptor, the coxsackie and Ad receptor. Herein, we describe advances in Ad targeting that are predicated on a fundamental understanding of vector/cell interplay. Further, we propose strategies by which existing paradigms, such as nanotechnology, may be combined with Ad vectors to form advanced delivery vehicles with multiple functions.

Crystal structure of enteric adenovirus serotype 41 short fiber head

Human enteric adenoviruses of species F contain two fibers in the same virion, a long fiber which binds to coxsackievirus and adenovirus receptor (CAR) and a short fiber of unknown function. We have determined the high-resolution crystal structure of the short fiber head of human adenovirus serotype 41 (Ad41). The short fiber head has the characteristic fold of other known fiber heads but has three unusual features. First, it has much shorter loops between the beta-strands. Second, one of the usually well-ordered beta-strands on the distal face of the fiber head is highly disordered and this same region is sensitive to digestion with pepsin, an enzyme occurring naturally in the intestinal tract, the physiological environment of Ad41. Third, the AB loop has a deletion giving it a distinct conformation incompatible with CAR binding.

Human adenovirus type 37 and the BALB/c mouse: progress toward a restricted adenovirus keratitis model (an American Ophthalmological Society thesis)

PURPOSE

To establish a mouse model of adenovirus keratitis in order to study innate immune mechanisms in the adenovirus-infected cornea.

METHODS

Balb/c 3T3 fibroblasts were inoculated with human adenovirus (HAdV) serotypes 8, 19, or 37 and observed for cytopathic effect. Viral growth titers were performed, and apoptosis was measured by TUNEL assay. Viral and host cytokine gene expression was assessed by RT-PCR in cultured Balb/c 3T3 fibroblasts and in the corneas of virus-injected Balb/c mice. Western blot analysis was performed to detect cell signaling in the virus-infected cornea.

RESULTS

Only HAdV37 induced cytopathic effect in mouse cells. Viral gene expression was limited, and viral replication was not detected. Apoptotic cell death in HAdV37-infected Balb/c cells was evident 48 and 72 hours postinfection (P < .01). MCP-1, IL-6, KC, and IP-10 mRNA levels were increased maximally by 8.4, 9.6, 10.5, and 20.0-fold, respectively, at 30 to 90 minutes after HAdV37 infection. Similar cytokine elevations were observed in the corneas of Balb/c mice 4 hours after stromal injection of HAdV37, when viral gene expression for the viral capsid protein IIIa was not detected. Western blot showed increased phosphorylation of ERK1/2 at 4 and 24 hours after corneal infection.

CONCLUSIONS

Despite limited viral gene expression, HAdV37 infection of Balb/c 3T3 fibroblasts results in increased proinflammatory gene expression. A similar pattern of cytokine expression in the corneas of HAdV37-infected Balb/c mice suggests the mouse adenoviral keratitis model may be useful for the study of early innate immune responses in the adenovirus-infected corneal stroma.

Oncolytic adenoviruses - selective retargeting to tumor cells

Virotherapy is an approach for the treatment of cancer, in which the replicating virus itself is the anticancer agent. Virotherapy exploits the lytic property of virus replication to kill tumor cells. As this approach relies on viral replication, the virus can self-amplify and spread in the tumor from an initial infection of only a few cells. The success of this approach is fundamentally based on the ability to deliver the replication-competent viral genome to target cells with a requisite level of efficiency. With virotherapy, while a number of transcriptional retargeting strategies have been utilized to restrict viral replication to tumor cells, this review will focus primarily on transductional retargeting strategies, whereby oncolytic viruses can be designed to selectively infect tumor cells. Using the adenoviral vector paradigm, there are three broad strategies useful for viral retargeting. One strategy uses heterologous retargeting ligands that are bispecific in that they bind both to the viral vector as well as to a cell surface target. A second strategy uses genetically modified viral vectors in which a cellular retargeting ligand is incorporated. A third strategy involves the construction of chimeric recombinant vectors, in which a capsid protein from one virus is exchanged for that of another. These transductional retargeting strategies have the potential for reducing deleterious side effects, and increasing the therapeutic index of virotherapeutic agents.

Efficient regulation of gene expression using self-contained fiber-modified adenovirus vectors containing the tet-off system

Previously, we developed single adenovirus (Ad) vectors that contained the gene of interest in the E1 deletion region and the transactivator gene for the tetracycline-controllable expression system in the E3 deletion region. In the present study, we improved the Ad vector-mediated tetracycline-controllable expression system by the fiber modification of Ad. We developed fiber-modified Ad vectors containing the tet-off system, which are effective in overcoming the limitations of conventional Ad vectors, specifically their inefficient gene transfer into cells lacking the primary receptor, the coxsackievirus and adenovirus receptor (CAR). Ad vectors containing the tet-off system with an Arg-Gly-Asp (RGD) peptide in the HI loop of the fiber knob or the Ad type 35 fiber greatly improved transduction efficiency (more than 1-2-log orders) into the cells lacking CAR expression but expressing alphav integrin or CD46, respectively. They exhibited vastly higher regulation of gene expression by doxycycline. The combination of fiber-modified Ad vectors and the tetracycline-controllable expression system should offer a powerful tool for gene therapy and gene transfer experiment.

Cellular receptor for Pixuna virus in chicken embryonic fibroblasts

In this study, we describe the isolation and partial characterization of a Pixuna virus receptor, which is a component of a plasma membrane fraction of chicken embryo fibroblast (CEF). Polyclonal antiserum was prepared from rabbits immunized with the membrane fraction. Said polyclonal antiserum reacted in a similar way as monoclonal antibodies raised against the membrane fraction. Both antisera were able to prevent CEF and Vero cells from infection with Pixuna virus. Immunofluorescence studies suggested that the receptors found in the fibroblasts and in the Vero cells shared at least some epitopes. The Western blot analysis of the purified membrane fraction antigens, which reacted with the monoclonal and polyclonal antibodies, detected a double band with a molecular mass of approximately 60 kDa. Not only immunofluorescence staining but also electron and immunoelectron microscopy studies evidenced the receptor localization in the plasma membrane. In this manner, we reported the isolation and partial characterization of a new Pixuna virus receptor in the plasma membrane of chicken embryo fibroblasts in culture. The data obtained demonstrated the receptor significance for the penetration of Pixuna virus into fibroblasts and mammalian cell and the related importance of designing new antiviral drugs by blocking the mechanism of receptor penetration of the virus into the cells.

CD46-utilizing adenoviruses inhibit C/EBPbeta-dependent expression of proinflammatory cytokines

The majority of adenovirus serotypes utilize the coxsackievirus-adenovirus receptor (CAR) for virus-host cell attachment, but subgroup B and subgroup D (adenovirus type 37 [Ad37]) viruses recognize CD46. CD46 is a ubiquitously expressed receptor that serves as a cofactor for the inactivation of the complement components C3b and C4b, and it also serves as a receptor for diverse microbial pathogens. A reported consequence of CD46 engagement is a reduced capability of human immune cells to express interleukin-12 (IL-12), a cytokine involved in both the innate and adaptive immune responses. Studies were thus undertaken to determine whether CD46-utilizing Ads alter the expression of proinflammatory cytokines. Subgroup B (Ad16 and -35) and Ad37, but not Ad2 or -5, significantly reduced IL-12 production by human peripheral blood mononuclear cells stimulated with gamma interferon (IFN-gamma) and lipopolysaccharide. IL-12 mRNA (p35 and p40 subunits) levels as well as other cytokine mRNA levels (IL-1alpha and -beta, IL-1Ra, and IL-6) were decreased upon interaction with CD46-utilizing Ads. Analysis of transcription factor activity required for cytokine expression indicated that CD46-utilizing Ads preferentially inhibited IFN-gamma-induced C/EBPbeta protein expression, consequently reducing its ability to form DNA complexes. Interference with IFN-gamma signaling events by CD46-utilizing Ads, but not CAR-utilizing Ads, reveals a potentially critical difference in the host immune response against distinct Ad vectors, a situation that has implications for gene delivery and vaccine development.

Antiviral cyclic D,L-alpha-peptides: targeting a general biochemical pathway in virus infections

Diverse virus families have evolved to exploit the acidification of endosomal compartments to gain entry into cells. We describe a supramolecular approach for selectively targeting and inhibiting viral infections through this central biochemical pathway. Using adenovirus as a model non-enveloped virus, we have determined that an eight-residue cyclic D,L-alpha-peptide, selected from a directed combinatorial library, can specifically prevent the development of low pH in endocytic vesicles, arrest the escape of virions from the endosome, and abrogate adenovirus infection without an apparent adverse effect on cell viability. The likely generality of this approach against other pH-dependent viral infections is supported by the inhibition of type-A influenza virus escape from endosomes in the presence of the same peptide. Our studies suggest that self-assembling cyclic D,L-alpha-peptides hold considerable potential as a new rational supramolecular approach toward the design and discovery of broad-spectrum antiviral agents.

Vitronectin: a possible determinant of adenovirus type 19 tropism for human corneal epithelium

PURPOSE

Adenoviruses typically demonstrate specific tissue tropisms, as in the association of Ad19 with epidemic keratoconjunctivitis. We sought to determine factors that might influence the apparent tropism of Ad19 for the cornea.

DESIGN

Laboratory investigation.

METHODS

Adenovirus serotypes Ad2, 5, 9, 10, 11, 13, and 19 were compared for their capacity to replicate in human corneal epithelial cells (HCECs) in culture. Organotypically cultured human corneas were infected with Ad19 or Ad2, and viral titers were compared after 7 days. Replication of both viruses was compared in HCECs cultured on various extracellular matrices. Western blot analysis and immunohistochemistry were applied to human donor corneas and HCECs.

RESULTS

One week after infection of HCEC monolayer cultures, Ad2 titers were significantly higher than any of the other viruses tested (P <.05). In organotypic corneal cultures, Ad19 titers were significantly higher than Ad2 (P = .0003). Ad2 replication in HCECs equaled or exceeded that of Ad19 on all extracellular matrices except vitronectin, where Ad2 replication was reduced and Ad19 replication enhanced (P <.0001). Vitronectin was detected by immunohistochemistry within the corneal epithelial basement membranes of human donor corneas. Increased alpha(v) integrin expression and greater tyrosine kinase phosphorylation in HCECs cultured on vitronectin were demonstrated by Western blot analysis.

CONCLUSIONS

In vitro, vitronectin enhances growth of Ad19, possibly by up-regulation of receptor alpha(v) integrins and increased activity of tyrosine kinases necessary for adenoviral internalization. We hypothesize that differential tissue tropisms for adenoviruses may derive in part from tissue-specific extracellular matrix expression.

The influence of adenovirus fiber structure and function on vector development for gene therapy

The collective attributes of adenoviruses (Ads), including ease of accomplishing replication deficiency, readily achievable high titers, encoding of large expression cassettes, efficiency of gene delivery to most cell types, and well-characterized biology, have made Ads, particularly Ad serotype 5 (Ad 5), some of the most utilized vectors for gene delivery. In recent years, however, it has become apparent that additional aspects of basic Ad virology must be uncovered for this vector system to succeed in the clinic. While local gene delivery is generally efficient, the broad tropism of Ad 5 and its tendency to home to the liver after systemic administration have proved to be limitations for other therapeutic applications, such as the treatment of disseminated cancers and cardiovascular disease. This has refocused research into the biology of Ad capsid components, particularly the main tropism determinant, the fiber/penton base complex, and their influence on transduction of selected cell types in vivo.

Human-like immune responses in CD46 transgenic mice

Neisseria meningitidis is a major cause of sepsis and/or meningitis. These bacteria normally cause disease only in humans, however, mice expressing human CD46 are susceptible to meningococcal disease. To explain the sensitivity of CD46 transgenic mice to meningococci, we evaluated early immune responses. Stimulation of TNF, IL-6, and IL-10 was stronger in CD46 transgenic mice compared with nontransgenic mice, and resembled human responses. In CD46 transgenic mice, bacterial clearance in blood started at later time points, and neutrophil numbers in blood were lower compared with nontransgenic mice. Further, elevated levels of activated microglia cells and cyclooxygenase-2 were observed in brain of infected CD46 transgenic mice. Intraperitoneal administration of meningococci lead to increased levels of macrophages only in the i.p. cavity of CD46 transgenic mice. Most of the responses were impaired or absent using LPS-deficient meningococci, showing the importance of LPS in the early immune response to meningococcal infection. Taken together, these data demonstrate that responses in mice expressing human CD46 mimic human meningococcal disease in many aspects, and demonstrate novel important links between CD46 and the innate immune system.

Junctional adhesion molecule a serves as a receptor for prototype and field-isolate strains of mammalian reovirus

Reovirus infections are initiated by the binding of viral attachment protein sigma1 to receptors on the surface of host cells. The sigma1 protein is an elongated fiber comprised of an N-terminal tail that inserts into the virion and a C-terminal head that extends from the virion surface. The prototype reovirus strains type 1 Lang/53 (T1L/53) and type 3 Dearing/55 (T3D/55) use junctional adhesion molecule A (JAM-A) as a receptor. The C-terminal half of the T3D/55 sigma1 protein interacts directly with JAM-A, but the determinants of receptor-binding specificity have not been identified. In this study, we investigated whether JAM-A also mediates the attachment of the prototype reovirus strain type 2 Jones/55 (T2J/55) and a panel of field-isolate strains representing each of the three serotypes. Antibodies specific for JAM-A were capable of inhibiting infections of HeLa cells by T1L/53, T2J/55, and T3D/55, demonstrating that strains of all three serotypes use JAM-A as a receptor. To corroborate these findings, we introduced JAM-A or the structurally related JAM family members JAM-B and JAM-C into Chinese hamster ovary cells, which are poorly permissive for reovirus infection. Both prototype and field-isolate reovirus strains were capable of infecting cells transfected with JAM-A but not those transfected with JAM-B or JAM-C. A sequence analysis of the sigma1-encoding S1 gene segment of the strains chosen for study revealed little conservation in the deduced sigma1 amino acid sequences among the three serotypes. This contrasts markedly with the observed sequence variability within each serotype, which is confined to a small number of amino acids. Mapping of these residues onto the crystal structure of sigma1 identified regions of conservation and variability, suggesting a likely mode of JAM-A binding via a conserved surface at the base of the sigma1 head domain.

The distal short consensus repeats 1 and 2 of the membrane cofactor protein CD46 and their distance from the cell membrane determine productive entry of species B adenovirus serotype 35

The human regulator of complement activation membrane cofactor protein (CD46) has recently been identified as an attachment receptor for most species B adenoviruses (Ads), including Ad type 3 (Ad3), Ad11, and Ad35, as well as species D Ad37. To characterize the interaction between Ad35 and CD46, hybrid receptors composed of different CD46 short consensus repeat (SCR) domains fused to immunoglobulin-like domains of CD4 and a set of 36 CD46 mutants containing semiconservative changes of single amino acids within SCR domains I and II were tested in binding and in Ad35-mediated luciferase transduction assays. In addition, anti-CD46 antibodies and soluble polypeptides constituting various CD46 domains were used in binding inhibition studies. Our data indicate that (i) CD46 SCR I or SCR II alone confers low but significant Ad35 binding; (ii) the presence of SCR I and II is required for optimal binding and transgene expression; (iii) transduction efficiencies equivalent to that of full-length CD46 are obtained if SCR I and II are at an appropriate distance from the cell membrane; (iv) ablation of the N-glycan attached to SCR I has no influence on receptor function, whereas ablation of the SCR II N-glycan results in about a two- to threefold reduction of binding and transgene expression; (v) most putative Ad35 binding residues are located on the same solvent-exposed face of the SCR I or SCR II domain, which are twisted by about 90 degrees ; and (vi) the putative Ad35 binding sites partly overlap with the measles virus binding surface.

Adenovirus protein VI mediates membrane disruption following capsid disassembly

In contrast to enveloped viruses, the mechanisms involved in membrane penetration by nonenveloped viruses are not as well understood. In these studies, we determined the relationship between adenovirus (Ad) capsid disassembly and the development of membrane lytic activity. Exposure to low pH or heating induced conformational changes in wild-type Ad but not in temperature-sensitive Ad (ts1) particles that fail to escape the early endosome. Wild-type Ad but not ts1 particles permeabilized model membranes (liposomes) and facilitated the cytosolic delivery of a ribotoxin. Alterations in wild-type Ad capsids were associated with the exposure of a pH-independent membrane lytic factor. Unexpectedly, this factor was identified as protein VI, a 22-kDa cement protein located beneath the peripentonal hexons in the viral capsid. Recombinant protein VI and preprotein VI, but not a deletion mutant lacking an N-terminal amphipathic alpha-helix, possessed membrane lytic activity similar to partially disassembled virions. A new model of Ad entry is proposed based on our present observations of capsid disassembly and membrane penetration.

Release of host-derived membrane vesicles following pilus-mediated adhesion of Neisseria gonorrhoeae

Following attachment of Neisseria gonorrhoeae to human epithelial cell lines, the cellular pilus receptor CD46 is shed from the cell and accumulates in the media. In this report, we assess Neisseria-induced alterations in CD46 surface distribution and characterize this complement regulatory protein following its release from the infected cell. Within 3 h of attachment of gonococci to human epithelial cell lines, CD46 is enriched beneath sites of microcolony adhesion. By 6 h post infection, differential ultracentrifugation of culture media from ME-180 monolayers resulted in sedimentation of structurally and functionally intact CD46. Electron microscopy of these 100,000 g pellets revealed 30-200 nm vesicles. These vesicles likely originated from the host cell as they contained additional host cell surface proteins including CD55 and the epidermal growth factor receptor. Further, these vesicles were visualized by quick-freeze, deep-etch electron microscopy in association with the surface of infected ME-180 cells and with pili of adherent gonococci. Like CD46 shedding, CD46 redistribution and vesicle release were insensitive to colchicine and cytochalasin-D but dependent on expression of the pilus retraction protein PilT. This vesiculation may represent a host cell defence response in which surface proteins that are commonly exploited by pathogens, such as CD46, are removed from the cell.

Fiber-modified adenovirus vectors mediate efficient gene transfer into undifferentiated and adipogenic-differentiated human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are considered a source of cells for regenerative medicine, and cell and gene therapy. Efficient gene transfer into hMSCs is essential for basic investigations into cellular differentiation and developmental biology, and for therapeutic applications in gene-modified regenerative medicine. In the present study, we optimized the transduction of hMSCs by means of fiber-modified adenovirus (Ad) vectors. Among the various types of Ad vectors tested, the polylysine modification of the C-terminal of the fiber knob most markedly improved the efficiency of hMSC transduction. At 300 vector particles per cell of polylysine-modified Ad vectors, more than 95% of the hMSCs expressed transgene. In this condition, polylysine-modified Ad vectors mediated 460-fold more transgene activity than the conventional Ad vectors. Ad vectors containing the Ad type 35 fiber or an Arg-Gly-Asp (RGD) peptide in the fiber knob mediated 130 or 16 times, respectively, the transgene activity mediated by the conventional Ad vectors. We also examined the efficiency of transduction into adipogenic-differentiated hMSCs. In this latter case, only Ad vectors containing the Ad type 35 fiber showed efficient gene expression. These results showed that fiber-modified Ad vectors could become a potent tool for basic research into, and the therapeutic application of, hMSCs and adipogenic-differentiated hMSCs.

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.

CryoEM Structure at 9Å Resolution of an Adenovirus Vector Targeted to Hematopoietic Cells

We report a sub-nanometer resolution cryo-electron microscopy (cryoEM) structural analysis of an adenoviral vector, Ad35F, comprised of an adenovirus type 5 (Ad5) capsid pseudo-typed with an Ad35 fiber. This vector transduces human hematopoietic cells via association of its fiber protein with CD46, a member of the complement regulatory protein family. Major advances in data acquisition and image processing allowed a significant improvement in resolution compared to earlier structures. Analysis of the cryoEM density was enhanced by docking the crystal structures of both the hexon and penton base capsid proteins. CryoEM density was observed for hexon residues missing from the crystal structure that include hypervariable regions and the epitope of a neutralizing monoclonal antibody. Within the penton base, density was observed for the integrin-binding RGD loop missing from the crystal structure and for the flexible beta ribbon of the variable loop on the side of the penton base. The Ad35 fiber is flexible, consistent with the sequence insert in the third beta-spiral repeat. On the inner capsid surface density is revealed at the base of the hexons and below the penton base. A revised model is presented for protein IX within the virion. Well-defined density was assigned to a conserved domain in the N terminus of protein IX required for incorporation into the virion. For the C-terminal domain of protein IX two alternate conformations are proposed, either binding on the capsid surface or extending away from the capsid. This model is consistent with the tolerance of the C terminus for inserted ligands and its potential use in vector retargeting. This structural study increases our knowledge of Ad capsid assembly, antibody neutralization mechanisms, and may aid further improvements in gene delivery to important human cell types.

Stampidine: a selective oculo-genital microbicide

Adenoviruses (ADVs) are causative agents of severe and extremely contagious ocular and genital infections associated with conjunctivitis, genital ulcers and urethritis. Yet, no functional antiviral compounds are currently available against adenoviral infections. We discovered halogen-substituted phenyl phosphoramidate derivatives of stavudine (STV/d4T) as a new class of dual-function anti-human immunodeficiency virus (HIV) agents with potent and selective anti-ADV activity. The lead compound, stampidine [5'-(4-bromophenyl methoxyalaninylphosphate)-2',3'-didehydro-3'-deoxythymidine], was the most potent non-toxic dual-function antiviral agent. Stampidine displayed remarkable in vitro and in vivo anti-HIV activity against drug-sensitive and drug-resistant HIV strains. Stampidine was non-cytotoxic and nonirritating to mucosal epithelial cells. Several preclinical studies conducted thus far, suggest that stampidine has clinical potential as a dual-function topical agent for the prevention and/or effective treatment of oculo-genital ADV/HIV infections.

In vitro dendritic cell infection by pseudotyped adenoviral vectors does not correlate with their in vivo immunogenicity

Expression of antigens in dendritic cells (DC) can stimulate protective immunity against both viral infection and tumor growth, making them important targets for gene therapy. In-vitro-generated DC are commonly used in gene delivery studies with the assumption that the results will correlate with in vivo activity. Adenovirus Type 5 (Ad5) vectors have been widely used with DC, but these cells lack the primary receptor (CAR) used by Ad5 and are poorly infected. We investigated the use of Ad5 vector particles pseudotyped with fibers from other Ad serotypes in DC targeting. Several fiber proteins, including those from Ad16 (Subgroup B) and Ad37 (Subgroup D), conferred dramatically increased in vitro infection. Surprisingly, neither dendritic cell infection nor the immune response to an Ad-delivered antigen was improved when the modified viruses were tested in vivo. These results underscore the importance of using appropriate animal models in gene delivery studies.

Integrin alphavbeta3 is a coreceptor for human cytomegalovirus

Human cytomegalovirus (HCMV) is a widespread opportunistic pathogen that causes birth defects in newborns and severe disease in immunocompromised individuals. The broad tropism of HCMV infection suggests that it uses multiple receptors. We recently showed that the epidermal growth factor receptor (EGFR) serves as a receptor for HCMV. Here we show that HCMV also uses integrin alphavbeta3 as a coreceptor. Upon infection, HCMV glycoproteins gB and gH independently bind to EGFR and alphavbeta3, respectively, to initiate viral entry and signaling. Alphavbeta3 then translocates to lipid rafts where it interacts with EGFR to induce coordinated signaling. The coordination between EGFR and alphavbeta3 is essential for the early events of HCMV infection, including viral entry, RhoA downregulation, stress-fiber disassembly and viral nuclear trafficking. Our findings support a model in which EGFR and alphavbeta3 work together as coreceptors for HCMV entry and signaling. This discovery is fundamental to understanding HCMV pathogenesis and developing treatment strategies targeted to viral receptors.

The structure of the human adenovirus 2 penton

The adenovirus penton, a noncovalent complex of the pentameric penton base and trimeric fiber proteins, comprises the vertices of the adenovirus capsid and contains all necessary components for viral attachment and internalization. The 3.3 A resolution crystal structure of human adenovirus 2 (hAd2) penton base shows that the monomer has a basal jellyroll domain and a distal irregular domain formed by two long insertions, a similar topology to the adenovirus hexon. The Arg-Gly-Asp (RGD) motif, required for interactions with cellular integrins, occurs on a flexible surface loop. The complex of penton base with bound N-terminal fiber peptide, determined at 3.5 A resolution, shows that the universal fiber motif FNPVYPY binds at the interface of adjacent penton base monomers and results in a localized structural rearrangement in the insertion domain of the penton base. These results give insight into the structure and assembly of the adenovirus capsid and will be of use for gene-therapy applications.

Deletion of penton RGD motifs affects the efficiency of both the internalization and the endosome escape of viral particles containing adenovirus serotype 5 or 35 fiber knobs

Adenovirus (Ad) vectors are widely used for gene delivery in vitro and in vivo. A solid understanding of the biology of this virus is imperative for the development of novel, effective, and safe vectors. For the group C adenovirus serotypes 2 and 5 that use CAR as a primary attachment receptor, it is known that the penton base RGD motifs interact with cellular integrins and that this interaction promotes virus internalization. However, the RGD motif's impact on the efficiency of postinternalization steps, such as the escape of the virus particle from the endosome, is less defined. Furthermore, the role of penton-integrin interactions remains unknown for new vectors possessing group B Ad fiber knobs that use CD46 as a primary virus attachment receptor. In this study, we used vectors with the RGD motif deleted that contained Ad5 and B-group Ad35 fiber knobs and long fiber shafts and studied the role of RGD-integrin interactions in virus internalization and endosome escape. The deletion of the RGD motif in the penton base did not affect virus attachment, regardless of the type of cellular receptor used for attachment. RGD motif deletion, however, significantly reduced the rate of virus internalization for both the Ad5 and Ad35 fiber knob-containing vectors. This study also demonstrates the role of penton RGD motifs in facilitating the endosome escape step of virus infection and indicates that penton-integrin interactions are involved in internalization of capsid-chimeric CD46-interacting Ads with long fiber shafts.

Adenoviral Serotype 5 Vectors Pseudotyped with Fibers from Subgroup D Show Modified TropismIn VitroandIn Vivo

Adenovirus (Ad5) serotype 5 vectors are commonly used for gene transfer. Preclinical studies have shown that their application to systemic gene delivery, however, is limited by their highly efficient uptake in the liver, principally mediated by receptor-binding sites on the fiber shaft and knob domain. Using Ad to target other sites in vivo requires vectors that lack hepatic tropism. We therefore sought to exploit Ad family diversity to isolate vectors that possessed poor hepatic tropism. We pseudotyped the fibers from Ad16 (subgroup B; Ad5/16), Ad19p (subgroup D; Ad5/19p), and Ad37 (subgroup D; Ad5/37) onto Ad5 capsids and assessed infectivity profiles in vitro in multiple cell types and in vivo in rats. In rat, mouse, and human hepatocytes, Ad5/19p and Ad5/37 both possessed a striking lack of hepatic cell infectivity compared with Ad5. Both vectors were, however, able to transduce human vascular endothelial and smooth muscle cells with efficiencies equal to or greater than that of nonmodified Ad5. We evaluated liver uptake in 12-week-old male rats after intravenous injection. In contrast to a vector with the wild-type Ad5 fiber, Ad5, both Ad5/19p and Ad5/37 produced significantly less virion accumulation (measured at 1 hr and 5 days) and transgene expression in the liver. Thus, Ad5/19p and Ad5/37 may be useful platforms for the development of targeted Ad vectors.

Genome size and structure determine efficiency of postinternalization steps and gene transfer of capsid-modified adenovirus vectors in a cell-type-specific manner

Adenovirus serotype 5 (Ad5) vectors containing Ad B-group fibers have become increasingly popular as gene transfer vectors because they efficiently transduce human cell types that are relatively refractory to Ad5 infection. So far, most B-group fiber-containing vectors have been first-generation vectors, deleted of E1 and/or E3 genes. Transduction with these vectors, however, results in viral gene expression and is associated with cytotoxicity and immune responses against transduced cells. To circumvent these problems, we developed fiber-chimeric Ad vectors devoid of all viral genes that were produced either by the homologous recombination of first-generation vectors or by using the Cre/lox-based helper virus system. In this study we compared early steps of infection between first-generation (35-kb genome) and Ad vectors devoid of all viral genes with genome sizes of 28 kb and 12.6 kb. All vectors possessed an Ad35-derived fiber knob domain, which uses CD46 as a primary attachment receptor. Using immortalized human hematopoietic cell lines and primary human CD34-positive hematopoietic cells, we found that the Ad genome size did not affect the efficiency of virus attachment to and internalization into cells. Furthermore, independently of the genome length and structure, all vectors migrated to the nucleus through late endosomal and lysosomal cellular compartments. However, the vector containing the short 12.6-kb genome was unable to efficiently escape from endosomes and deliver its DNA into the nucleus. Moreover, compared to other vectors, these Ad particles were less stable and had an abnormal capsid protein composition, including a lack of capsid-stabilizing protein IX. Our data indicate that the size and structure of the packaged viral genomes can affect the integrity of Ad particles, which in turn results in lower infectivity of Ad vectors.

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.

Rat membrane cofactor protein (MCP; CD46) is expressed only in the acrosome of developing and mature spermatozoa and mediates binding to immobilized activated C3

The rat analogue of the complement regulator membrane cofactor protein (MCP; CD46) was recently cloned and analysis at the mRNA level suggested that expression was restricted to testis. In light of the proposed roles of human MCP in sperm-egg interaction, we undertook to analyze rat MCP expression at the protein level in order better to address its putative role in fertilization. Recombinant fusion proteins comprising antibody Fc and specific domains of rat MCP were generated and used to develop a monoclonal antibody, MM.1, specific for rat MCP. Immunohistochemistry using these reagents confirmed the reported testis-specific expression of MCP in sexually mature rats and demonstrated that MCP was expressed only by spermatozoa and their immediate precursors in spermiogenesis, spermatids. Prepubertal male rats did not express MCP, and there was no evidence of MCP expression at any site in the embryo. Spermatozoal MCP expression was restricted to the inner acrosomal membrane, exposed only after fixation or induction of the acrosome reaction. Acrosome-reacted but not unreacted spermatozoa bound methylamine-activated C3 immobilized on plastic. The retention of MCP at this subcellular site, which is probably crucial to sperm-egg interaction, and the functional demonstration of binding to activated C3 strengthen suggestions from human studies that MCP may play an important role in fertilization. The reagents and results described here will enable studies of the role of spermatozoal MCP in sperm-egg interaction using a relevant animal model system.