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

Adenovirus entry: Stability, uncoating, and nuclear import

Adenoviruses (AdVs) are widespread in vertebrates. They infect the respiratory and gastrointestinal tracts, the eyes, heart, liver, and kidney, and are lethal to immunosuppressed people. Mastadenoviruses infecting mammals comprise several hundred different types, and many specifically infect humans. Human adenoviruses are the most widely used vectors in clinical applications, including cancer treatment and COVID-19 vaccination. AdV vectors are physically and genetically stable and generally safe in humans. The particles have an icosahedral coat and a nucleoprotein core with a DNA genome. We describe the concept of AdV cell entry and highlight recent advances in cytoplasmic transport, uncoating, and nuclear import of the viral DNA. We highlight a recently discovered "linchpin" function of the virion protein V ensuring cytoplasmic particle stability, which is relaxed at the nuclear pore complex by cues from the E3 ubiquitin ligase Mind bomb 1 (MIB1) and the proteasome triggering disruption. Capsid disruption by kinesin motor proteins and microtubules exposes the linchpin and renders protein V a target for MIB1 ubiquitination, which dissociates V from viral DNA and enhances DNA nuclear import. These advances uncover mechanisms controlling capsid stability and premature uncoating and provide insight into nuclear transport of nucleic acids.

Immune priming using DC- and T cell-targeting gene therapy sensitizes both treated and distant B16 tumors to checkpoint inhibition

Immune checkpoint inhibitors have revolutionized the treatment of metastatic melanoma, but most tumors show resistance. Resistance is connected to a non-T cell inflamed phenotype partially caused by a lack of functional dendritic cells (DCs) that are crucial for T cell priming. Herein, we investigated whether the adenoviral gene vehicle mLOAd703 carrying both DC- and T cell-activating genes can lead to inflammation in a B16-CD46 model and thereby overcome resistance to checkpoint inhibition therapy. B16-CD46 cells were injected subcutaneously in one or both flanks of immunocompetent C57BL/6J mice. mLOAd703 treatments were given intratumorally alone or in combination with intraperitoneal checkpoint inhibition therapy (anti-PD-1, anti-PD-L1, or anti-TIM-3). Tumor, lymph node, spleen, and serum samples were analyzed for the presence of immune cells and cytokines/chemokines. B16-CD46 tumors were non-inflamed and resistant to checkpoint blockade. In contrast, mLOAd703 treatment led to infiltration of the tumor by CD8⁺ T cells, natural killer (NK) cells, and CD103⁺ DCs, accompanied by a systemic increase of pro-inflammatory cytokines interferon γ (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-27 (IL-27). This response was even more pronounced after combining the virus with checkpoint therapy, in particular with anti-PD-L1 and anti-TIM-3, leading to further reduced tumor growth in injected lesions. Moreover, anti-PD-L1 combination also facilitated abscopal responses in non-injected lesions.

The RGD-binding integrins αvβ6 and αvβ8 are receptors for mouse adenovirus-1 and -3 infection

Mammalian adenoviruses (AdVs) comprise more than ~350 types including over 100 human (HAdVs) and just three mouse AdVs (MAdVs). While most HAdVs initiate infection by high affinity/avidity binding of their fiber knob (FK) protein to either coxsackievirus AdV receptor (CAR), CD46 or desmoglein (DSG)-2, MAdV-1 (M1) infection requires arginine-glycine-aspartate (RGD) binding integrins. To identify the receptors mediating MAdV infection we generated five novel reporter viruses for MAdV-1/-2/-3 (M1, M2, M3) transducing permissive murine (m) CMT-93 cells, but not B16 mouse melanoma cells expressing mCAR, human (h) CD46 or hDSG-2. Recombinant M1 or M3 FKs cross-blocked M1 and M3 but not M2 infections. Profiling of murine and human cells expressing RGD-binding integrins suggested that αvβ6 and αvβ8 heterodimers are associated with M1 and M3 infections. Ectopic expression of mβ6 in B16 cells strongly enhanced M1 and M3 binding, infection, and progeny production comparable with mαvβ6-positive CMT-93 cells, whereas mβ8 expressing cells were more permissive to M1 than M3. Anti-integrin antibodies potently blocked M1 and M3 binding and infection of CMT-93 cells and hαvβ8-positive M000216 cells. Soluble integrin αvβ6, and synthetic peptides containing the RGDLXXL sequence derived from FK-M1, FK-M3 and foot and mouth disease virus coat protein strongly interfered with M1/M3 infections, in agreement with high affinity interactions of FK-M1/FK-M3 with αvβ6/αvβ8, determined by surface plasmon resonance measurements. Molecular docking simulations of ternary complexes revealed a bent conformation of RGDLXXL-containing FK-M3 peptides on the subunit interface of αvβ6/β8, where the distal leucine residue dips into a hydrophobic pocket of β6/8, the arginine residue ionically engages αv aspartate215, and the aspartate residue coordinates a divalent cation in αvβ6/β8. Together, the RGDLXXL-bearing FKs are part of an essential mechanism for M1/M3 infection engaging murine and human αvβ6/8 integrins. These integrins are highly conserved in other mammals, and may favour cross-species virus transmission.

Adenovirus Entry: From Infection to Immunity

More than 80 different adenovirus (AdV) types infect humans through the respiratory, ocular, or gastrointestinal tracts. They cause acute clinical mani-festations or persist under humoral and cell-based immunity. Immuno-suppressed individuals are at risk of death from an AdV infection. Concepts about cell entry of AdV build on strong foundations from molecular and cellular biology-and increasingly physical virology. Here, we discuss how virions enter and deliver their genome into the nucleus of epithelial cells. This process breaks open the virion at distinct sites because the particle has nonisometric mechanical strength and reacts to specific host factors along the entry pathway. We further describe how macrophages and dendritic cells resist AdV infection yet enhance productive entry into polarized epithelial cells. A deep understanding of the viral mechanisms and cell biological and biophysical principles will continue to unravel how epithelial and antigen-presenting cells respond to AdVs and control inflammation and persistence in pathology and therapy.

IL-6 Signaling Blockade during CD40-Mediated Immune Activation Favors Antitumor Factors by Reducing TGF-β, Collagen Type I, and PD-L1/PD-1

IL-6 plays a role in cancer pathogenesis via its connection to proteins involved in the formation of desmoplastic stroma and to immunosuppression by driving differentiation of myeloid suppressor cells together with TGF-β. Inhibition of IL-6 signaling in the tumor microenvironment may, thus, limit desmoplasia and myeloid suppressor cell differentiation. CD40 signaling can further revert myeloid cell differentiation toward antitumor active phenotypes. Hence, the simultaneous use of IL-6 blockade with CD40 stimuli may tilt the tumor microenvironment to promote antitumor immune responses. In this paper, we evaluated the mechanisms of LOAd713, an oncolytic adenovirus designed to block IL-6R signaling and to provide myeloid cell activation via a trimerized membrane-bound isoleucine zipper (TMZ) CD40L. LOAd713-infected pancreatic cancer cells were killed by oncolysis, whereas infection of stellate cells reduced factors involved in stroma formation, including TGF-β-1 and collagen type I. Virus infection prevented IL-6/GM-CSF-mediated differentiation of myeloid suppressors, but not CD163 macrophages, whereas infection of dendritic cells led to upregulation of maturation markers, including CD83, CD86, IL-12p70, and IFN-γ. Further, IL-6R blockade prevented upregulation of programed death ligand 1 (PD-L1) and PD-1 on the stimulated dendritic cells. These results suggest that LOAd713 can kill infected tumor cells and has the capacity to affect the tumor microenvironment by stimulating stellate cells and myeloid suppressors with TMZ-CD40L and IL-6R blockade. Gene transfer of murine TMZ-CD40L prolonged survival in an animal model. LOAd713 may be an interesting therapeutic option for cancers connected to IL-6 signaling, such as pancreatic cancer.

Glycomics and Proteomics Approaches to Investigate Early Adenovirus-Host Cell Interactions

Adenoviruses as most viruses rely on glycan and protein interactions to attach to and enter susceptible host cells. The Adenoviridae family comprises more than 80 human types and they differ in their attachment factor and receptor usage, which likely contributes to the diverse tropism of the different types. In the past years, methods to systematically identify glycan and protein interactions have advanced. In particular sensitivity, speed and coverage of mass spectrometric analyses allow for high-throughput identification of glycans and peptides separated by liquid chromatography. Also, developments in glycan microarray technologies have led to targeted, high-throughput screening and identification of glycan-based receptors. The mapping of cell surface interactions of the diverse adenovirus types has implications for cell, tissue, and species tropism as well as drug development. Here we review known adenovirus interactions with glycan- and protein-based receptors, as well as glycomics and proteomics strategies to identify yet elusive virus receptors and attachment factors. We finally discuss challenges, bottlenecks, and future research directions in the field of non-enveloped virus entry into host cells.

CD46 is a potent co-stimulatory receptor for expansion of human IFN-γ-producing CD8+ T cells

Similar to CD4⁺ T cells, precursor CD8⁺ T cells are thought to depend on a co-stimulatory signal through CD28 for proliferation and differentiation into effector cells. CD46 is another co-stimulatory receptor that promotes differentiation of CD4⁺ T-helper cells type 1 (Th1 cells) into a regulatory phenotype with a switch from IFN-γ towards IL-10-secretion over time. Whether CD46 exerts a similar function on CD8⁺ T cells remains to be fully elucidated. Here, we demonstrate that CD46 co-stimulation induced secretion of IFN-γ as well as expansion of IFN-γ-secreting CD8⁺ T cells. In contrast to CD46 co-stimulation of CD4⁺ T cells, CD8⁺ T cells did not differentiate into a regulatory IL-10-secreting phenotype. This demonstrates that CD46 is a co-stimulatory receptor on CD8⁺ T cells, and that it exerts separate functions during CD4⁺ and CD8⁺ T-cell differentiation.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

Co-option of Membrane Wounding Enables Virus Penetration into Cells

During cell entry, non-enveloped viruses undergo partial uncoating to expose membrane lytic proteins for gaining access to the cytoplasm. We report that adenovirus uses membrane piercing to induce and hijack cellular wound removal processes that facilitate further membrane disruption and infection. Incoming adenovirus stimulates calcium influx and lysosomal exocytosis, a membrane repair mechanism resulting in release of acid sphingomyelinase (ASMase) and degradation of sphingomyelin to ceramide lipids in the plasma membrane. Lysosomal exocytosis is triggered by small plasma membrane lesions induced by the viral membrane lytic protein-VI, which is exposed upon mechanical cues from virus receptors, followed by virus endocytosis into leaky endosomes. Chemical inhibition or RNA interference of ASMase slows virus endocytosis, inhibits virus escape to the cytosol, and reduces infection. Ceramide enhances binding of protein-VI to lipid membranes and protein-VI-induced membrane rupture. Thus, adenovirus uses a positive feedback loop between virus uncoating and lipid signaling for efficient membrane penetration.

Electrostatic Interactions between Complement Regulator CD46(SCR1-2) and Adenovirus Ad11/Ad21 Fiber Protein Knob

Adenoviruses bind to a variety of human cells to cause infection. Both the B2 adenovirus 11 and B1 adenovirus 21 use protein knobs to bind to complement regulator CD46(SCR1-2) in order to gain entry into host cells. In each complex, the two proteins are highly negatively charged but bind to each other at an interface with oppositely charged surface patches. We computationally generated single-alanine mutants of charged residues in the complexes CD46(SCR1-2)-Ad11k and CD46(SCR1-2)-Ad21k. We used electrostatic clustering and Poisson-Boltzmann free energy calculations to propose a hypothesis on the role of electrostatics in association. Our results delineate specific interfacial electrostatic interactions that are critical for association in both CD46(SCR1-2)-Ad11k and CD46(SCR1-2)-Ad21k. These results will serve as a predictive tool in the selection of mutants with desired binding affinity in experimental mutagenesis studies. This study will also serve as a foundation for the design of inhibitors to treat adenovirus infections.

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.

Innate immunity to adenovirus

Human adenoviruses are the most widely used vectors in gene medicine, with applications ranging from oncolytic therapies to vaccinations, but adenovirus vectors are not without side effects. In addition, natural adenoviruses pose severe risks for immunocompromised people, yet infections are usually mild and self-limiting in immunocompetent individuals. Here we describe how adenoviruses are recognized by the host innate defense system during entry and replication in immune and nonimmune cells. Innate defense protects the host and represents a major barrier to using adenoviruses as therapeutic interventions in humans. Innate response against adenoviruses involves intrinsic factors present at constant levels, and innate factors mounted by the host cell upon viral challenge. These factors exert antiviral effects by directly binding to viruses or viral components, or shield the virus, for example, soluble factors, such as blood clotting components, the complement system, preexisting immunoglobulins, or defensins. In addition, Toll-like receptors and lectins in the plasma membrane and endosomes are intrinsic factors against adenoviruses. Important innate factors restricting adenovirus in the cytosol are tripartite motif-containing proteins, nucleotide-binding oligomerization domain-like inflammatory receptors, and DNA sensors triggering interferon, such as DEAD (Asp-Glu-Ala-Asp) box polypeptide 41 and cyclic guanosine monophosphate-adenosine monophosphate synthase. Adenovirus tunes the function of antiviral autophagy, and counters innate defense by virtue of its early proteins E1A, E1B, E3, and E4 and two virus-associated noncoding RNAs VA-I and VA-II. We conclude by discussing strategies to engineer adenovirus vectors with attenuated innate responses and enhanced delivery features.

Emerging roles for ubiquitin in adenovirus cell entry

Adenovirus relies on numerous interactions between viral and host cell proteins to efficiently enter cells. Undoubtedly, post-translational modifications of host and cellular proteins can impact the efficiency of this cell entry process. Ubiquitylation, once simply thought of as a modification targeting proteins for proteasomal degradation, is now known to regulate protein trafficking within cells, protein-protein interactions and cell signalling pathways. Accumulating evidence suggests that protein ubiquitylation can influence all stages of the life cycle of other viruses such as cell entry, replication and egress. Until recently, the influence of ubiquitylation has only been documented during adenovirus replication. This review highlights the most recent evidence demonstrating direct engagement of host ubiquitylation and SUMOylation machinery by adenovirus during cell entry. Additionally, potential roles for host protein ubiquitylation and the potential for adenovirus regulation of host ubiquitylation machinery during cell entry are discussed.

Attenuation of CD4+ T-cell function by human adenovirus type 35 is mediated by the knob protein

The complement-regulatory protein CD46 is the primary receptor for human adenovirus type 35 (HAdV-35) and can regulate human immune-cell activation. CD4(+) T-cells are critical for initiating and maintaining adaptive immunity elicited by infection or vaccination. It was reported previously that HAdV-35 can bind these cells and suppress their activation. The data reported here demonstrate that recombinant trimeric HAdV-35 knob proteins alone can induce CD46 receptor downregulation and inhibit interleukin-2 production and proliferation of human CD4(+) T-cells in vitro similarly to mAbs specific to the CD46 region bound by HAdV-35 knobs. A mutant knob protein with increased affinity for CD46 compared with the wild-type knob caused equivalent effects. In contrast, a CD46-binding-deficient mutant knob protein did not inhibit T-cell activation. Thus, the capacity of HAdV-35 to attenuate human CD4(+) T-cell activation depends predominantly on knob interactions with CD46 and can occur independently of infection.

Avidity binding of human adenovirus serotypes 3 and 7 to the membrane cofactor CD46 triggers infection

The species B human adenoviruses (HAdVs) infect cells upon attaching to CD46 or desmoglein 2 (DSG-2) by one or several of their 12 fiber knob trimers (FKs). To test whether DSG-2 and CD46 simultaneously serve as virus receptors for adenovirus type 3 (Ad3), we performed individual and combined CD46/DSG-2 loss-of-function studies in human lung A549 and 16HBE14o cells. Our results suggest that in these cells, DSG-2 functions as a major attachment receptor for Ad3, whereas CD46 exerts a minor contribution to virus attachment and uptake in the range of ∼10%. However, in other cells the role of CD46 may be more pronounced depending on, e.g., the expression levels of the receptors. To test if avidity allows Ad3/7 to use CD46 as a receptor, we performed gain-of-function studies. The cell surface levels of ectopically expressed CD46 in CHO or human M010119 melanoma cells lacking DSG-2 positively correlated with Ad3/7 infections, while Ad11/35 infections depended on CD46 but less on CD46 levels. Antibody-cross-linked soluble CD46 blocked Ad3/7/11/35 infections, while soluble CD46 alone blocked Ad11/35 but not Ad3/7. Soluble Ad3/7-FKs poorly inhibited Ad3/7 infection of CHO-CD46 cells, illustrating that Ad3/7-FKs bind with low affinity to CD46. This was confirmed by Biacore studies. Ad3/7-FK binding to immobilized CD46 at low density was not detected, unlike that of Ad11/35-FK. At higher CD46 densities, however, Ad3/7-FK bound to CD46 with only 15-fold-higher dissociation constants than those of Ad11/35-FK. These data show that an avidity mechanism for Ad3/7 binding to CD46 leads to infection of CD46-positive cells.

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.

Adenovirus type-35 vectors block human CD4+ T-cell activation via CD46 ligation

Recombinant adenoviruses (rAds) based on types 5 (rAd5) and 35 (rAd35) have emerged as important vaccine delivery vectors in clinical testing for a variety of pathogens. A major difference between these vectors is their binding to cellular receptors used for infection. Whereas rAd5 binds coxsackie-adenovirus receptor (CAR), rAd35 binds the complement regulatory protein CD46. Although rAd35 infected and phenotypically matured human blood dendritic cells (DCs) more efficiently than rAd5, we show here that rAd35 markedly suppressed DC-induced activation of naive CD4(+) T cells. rAd35 specifically blocked both DCs and anti-CD3/CD28 mAb-induced naive T-cell proliferation and IL-2 production. This effect was also observed in CD4(+) memory T cells but to a lesser extent. The suppression occurred by rAd35 binding to CD46 on T cells and was independent of infection. CD46 engagement with mAb mimicked the effects of rAd35 and also led to deficient NF-κB nuclear translocation. In contrast, rAd5 and rAd35 vectors with ablated CD46 binding did not inhibit T-cell activation. Our findings provide insights into the basic biology of adenoviruses and indicate that CD46 binding may have an impact on the generation of primary CD4(+) T-cell responses by Ad35.

Unity and diversity in the human adenoviruses: exploiting alternative entry pathways for gene therapy

Human Ads (adenoviruses) have been extensively utilized for the development of vectors for gene transfer, as they infect many cell types and do not integrate their genome into host-cell chromosomes. In addition, they have been widely studied as cytolytic viruses, termed oncolytic adenoviruses in cancer therapy. Ads are non-enveloped viruses with a linear double-stranded DNA genome of 30–38 kb which encodes 30–40 genes. At least 52 human Ad serotypes have been identified and classified into seven species, A–G. The Ad capsid has icosahedral symmetry and is composed of 252 capsomers, of which 240 are located on the facets of the capsid and consist of a trimeric hexon protein and the remaining 12 capsomers, the pentons, are at the vertices and comprise the penton base and projecting fibre protein. The entry of Ads into human cells is a two-step process. In the first step, the fibre protein mediates a primary interaction with the cell, effectively tethering the virus particle to the cell surface via a cellular attachment protein. The penton base then interacts with cell-surface integrins, leading to virus internalization. This interaction of the fibre protein with a number of cell-surface molecules appears to be important in determining the tropism of adenoviruses. Ads from all species, except species B and certain serotypes of species D, utilize CAR (coxsackie and adenovirus receptor) as their primary cellular-attachment protein, whereas most species B Ads use CD46, a complement regulatory protein. Such species-specific differences, as well as adaptations or modifications of Ads required for applications in gene therapy, form the major focus of the present review.

Macropinocytotic uptake and infection of human epithelial cells with species B2 adenovirus type 35

Human adenovirus serotype 35 (HAdV-35; here referred to as Ad35) causes kidney and urinary tract infections and infects respiratory organs of immunocompromised individuals. Unlike other adenoviruses, Ad35 has a low seroprevalence, which makes Ad35-based vectors promising candidates for gene therapy. Ad35 utilizes CD46 and integrins as receptors for infection of epithelial and hematopoietic cells. Here we show that infectious entry of Ad35 into HeLa cells, human kidney HK-2 cells, and normal human lung fibroblasts strongly depended on CD46 and integrins but not heparan sulfate and variably required the large GTPase dynamin. Ad35 infections were independent of expression of the carboxy-terminal domain of AP180, which effectively blocks clathrin-mediated uptake. Ad35 infections were inhibited by small chemicals against serine/threonine kinase Pak1 (p21-activated kinase), protein kinase C (PKC), sodium-proton exchangers, actin, and acidic organelles. Remarkably, the F-actin inhibitor jasplakinolide, the Pak1 inhibitor IPA-3, or the sodium-proton exchange inhibitor 5-(N-ethyl-N-isopropyl) amiloride (EIPA) blocked endocytic uptake of Ad35. Dominant-negative proteins or small interfering RNAs against factors driving macropinocytosis, including the small GTPase Rac1, Pak1, or the Pak1 effector C-terminal binding protein 1 (CtBP1), potently inhibited Ad35 infection. Confocal laser scanning microscopy, electron microscopy, and live cell imaging showed that Ad35 colocalized with fluid-phase markers in large endocytic structures that were positive for CD46, alphanu integrins, and also CtBP1. Our results extend earlier observations with HAdV-3 (Ad3) and establish macropinocytosis as an infectious pathway for species B human adenoviruses in epithelial and hematopoietic cells.

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.

Replication-competent Ad11p vector (RCAd11p) efficiently transduces and replicates in hormone-refractory metastatic prostate cancer cells

Selective replication-competent adenovirus serotype 5 vectors have been used for prostate cancer therapy. Unfortunately, gene transfer is inefficient because hormone-refractory metastatic prostate cancer cells have minimal coxsackievirus-adenovirus receptor expression. Vectors based on species B adenoviruses are attractive tools for use in human gene therapy because the viruses have low seroprevalence and they have efficient transduction capacity. Most species B adenoviruses use ubiquitously expressed complement-regulatory CD46 protein as a cellular receptor. Here we report the transduction efficacy and oncolytic capacity of a replication-competent Ad11p (RCAd11p) vector in human prostate cancer cells. Green fluorescent protein was efficiently expressed in a dose-dependent manner in PC-3 and DU 145 cells derived from metastasis of prostate cancer to bone and brain, respectively. However, transduction was less effective in LNCaP cells derived from prostate cancer metastasis to lymph nodes. The oncolytic capacity of the RCAd11p vector was 100 times higher in PC-3 cells than in the two other cell lines. The oncolysis was independent of the level of expression of p53 in the cells or on the absence of E1B55k expression in the vector. In vivo experiments revealed significant growth inhibition of PC-3 tumors in the xenograft mouse group treated with RCAd11p vector or Ad11pwt in comparison with the untreated control group. Thus, we have demonstrated that RCAd11p vector intrinsically possesses oncolytic properties, which were active in targeting tumor cells. Consequently, the novel RCAd11p vector has great potential for the treatment of incurable metastatic prostate disease.

Insights into adenovirus host cell interactions from structural studies

Human adenoviruses cause a significant number of acute respiratory, enteric and ocular infections, however they have also served as useful model systems for uncovering fundamental aspects of cell and molecular biology. In addition, replication-defective forms of adenovirus are being used in gene transfer and vaccine clinical trials. Over the past decade, steady advances in structural biology techniques have helped reveal important insights into the earliest events in the adenovirus life cycle as well as virus interactions with components of the host immune system. This review highlights the continuing use of structure-based approaches to uncover the molecular features of adenovirus-host interactions.

An arginine switch in the species B adenovirus knob determines high-affinity engagement of cellular receptor CD46

Adenoviruses (Ads) are icosahedral, nonenveloped viruses with a double-stranded DNA genome. The 51 known Ad serotypes exhibit profound variations in cell tropism and disease types. The number of observed Ad infections is steadily increasing, sometimes leading to fatal outcomes even in healthy individuals. Species B Ads can cause kidney infections, hemorrhagic cystitis, and severe respiratory infections, and most of them use the membrane cofactor protein CD46 as a cellular receptor. The crystal structure of the human Ad type 11 (Ad11) knob complexed with CD46 is known; however, the determinants of CD46 binding in related species B Ads remain unclear. We report here a structural and functional analysis of the Ad11 knob, as well as the Ad7 and Ad14 knobs, which are closely related in sequence to the Ad11 knob but have altered CD46-binding properties. The comparison of the structures of the three knobs, which we determined at very high resolution, provides a platform for understanding these differences and allows us to propose a mechanism for productive high-affinity engagement of CD46. At the center of this mechanism is an Ad knob arginine that needs to switch its orientation in order to engage CD46 with high affinity. Quantum chemical calculations showed that the CD46-binding affinity of Ad11 is significantly higher than that of Ad7. Thus, while Ad7 and Ad14 also bind CD46, the affinity and kinetics of these interactions suggest that these Ads are unlikely to use CD46 productively. The proposed mechanism is likely to determine the receptor usage of all CD46-binding Ads.

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.

In vitro and in vivo properties of adenovirus vectors with increased affinity to CD46

Gene transfer vectors containing adenovirus (Ad) serotype 35 (Ad35) fibers have shown promise for cancer and stem cell gene therapy. In this study, we attempted to improve the in vitro and in vivo infection properties of these vectors by increasing their affinity to the Ad35 fiber receptor CD46. We constructed Ad vectors containing either the wild-type Ad35 fiber knob (Ad5/35) or Ad35 knob mutants with 4-fold- and 60-fold-higher affinity to CD46 (Ad5/35+ and Ad5/35++, respectively). In in vitro studies with cell lines, the higher affinities of Ad5/35+ and Ad5/35++ to CD46 did not translate into correspondingly higher transduction efficiencies, regardless of the CD46 receptor density present on cells. However, in vivo, in a mouse model with preestablished CD46(high) liver metastases, intravenous injection of Ad5/35++ resulted in more-efficient tumor cell transduction. We conclude that Ad5/35 vectors with increased affinity to CD46 have an advantage in competing with non-CD46-mediated sequestration of vector particles after intravenous injection.

Structural variations in species B adenovirus fibers impact CD46 association

A majority of species B adenoviruses (Ads) use CD46 as their primary receptor; however, the precise mechanisms involved in the binding of different Ad types to CD46 have not been resolved. Although previous studies indicate close similarities between two members of species B2 Ads in their usage of CD46, our current investigations revealed a surprisingly low CD46 binding affinity of the species B1 Ad16 fiber knob (equilibrium dissociation constant of 437 nM). We determined the crystal structure of the Ad16 fiber knob and constructed a model of this protein in complex with CD46. A comparison of this model to that of the CD46-Ad11 complex revealed structural differences in the FG and IJ loops that are part of the CD46 binding site. An analysis of a panel of recombinant fiber knobs with mutations targeting these regions in Ad16 and Ad11 uncovered a major contribution of the FG loop on CD46 binding. Two extra residues in the FG loop of the Ad16 fiber significantly reduce receptor interaction. Although avidity effects permit the use of CD46 on host cells by Ad16, virus binding occurs with lower efficiency than with B2 Ad types. The longer FG loop of the Ad16 fiber knob also is shared by other species B1 Ad fibers and, thus, may contribute to the low CD46 binding efficiencies observed for these Ad types. Our findings provide a better understanding of how different Ad types associate with CD46 and could aid in the selection of specific Ad fibers for more efficient Ad gene delivery vectors.

Identification of CD46 binding sites within the adenovirus serotype 35 fiber knob

Species B human adenoviruses (Ads) are often associated with fatal illnesses in immunocompromised individuals. Recently, species B Ads, most of which use the ubiquitously expressed complement regulatory protein CD46 as a primary attachment receptor, have gained interest for use as gene therapy vectors. In this study, we focused on species B Ad serotype 35 (Ad35), whose trimeric fiber knob domain binds to three CD46 molecules with a KD (equilibrium dissociation constant) of 15.5 nM. To study the Ad35 knob-CD46 interaction, we generated an expression library of Ad35 knobs with random mutations and screened it for CD46 binding. We identified four critical residues (Phe242, Arg279, Ser282, and Glu302) which, when mutated, ablated Ad35 knob binding to CD46 without affecting knob trimerization. The functional importance of the identified residues was validated in surface plasmon resonance and competition binding studies. To model the Ad35 knob-CD46 interaction, we resolved the Ad35 knob structure at 2-A resolution by X-ray crystallography and overlaid it onto the existing structure for Ad11-CD46 interaction. According to our model, all identified Ad35 residues are in regions that interact with CD46, whereby one CD46 molecule binds between two knob monomers. This mode of interaction might have potential consequences for CD46 signaling and intracellular trafficking of Ad35. Our findings are also fundamental for better characterization of species B Ads and design of antiviral drugs, as well as for application of species B Ads as in vivo and in vitro gene transfer vectors.

Species B adenovirus serotypes 3, 7, 11 and 35 share similar binding sites on the membrane cofactor protein CD46 receptor

We recently characterized the domains of the human cofactor protein CD46 involved in binding species B2 adenovirus (Ad) serotype 35. Here, the CD46 binding determinants are mapped for the species B1 Ad serotypes 3 and 7 and for the species B2 Ad11. Ad3, 7 and 11 bound and transduced CD46-positive rodent BHK cells at levels similar to Ad35. By using antibody-blocking experiments, hybrid CD46–CD4 receptor constructs and CD46 single point mutants, it is shown that Ad3, 7 and 11 share many of the Ad35-binding features on CD46. Both CD46 short consensus repeat domains SCR I and SCR II were necessary and sufficient for optimal binding and transgene expression, provided that they were positioned at an appropriate distance from the cell membrane. Similar to Ad35, most of the putative binding residues of Ad3, 7 and 11 were located on the same glycan-free, solvent-exposed face of the SCR I or SCR II domains, largely overlapping with the binding surface of the recently solved fiber knob Ad11–SCR I–II three-dimensional structure. Differences between species B1 and B2 Ads were documented with competition experiments based on anti-CD46 antibodies directed against epitopes flanking the putative Ad-binding sites, and with competition experiments based on soluble CD46 protein. It is concluded that the B1 and B2 species of Ad engage CD46 through similar binding surfaces.

Interaction of penton base Arg-Gly-Asp motifs with integrins is crucial for adenovirus serotype 35 vector transduction in human hematopoietic cells

Most subgroup B adenoviruses (Ads), including adenovirus (Ad) serotype 35 (Ad35), bind to human CD46 as a receptor; however, the infection processes of subgroup B Ads following attachment to CD46 remain to be elucidated. Subgroup B Ads possess Arg-Gly-Asp (RGD) motifs in the penton base, similarly to subgroup C Ad serotypes 2 and 5. In this study, we examined the role of penton base RGD motifs in Ad35 vector-mediated transduction in human hematopoietic cells. Inhibition of interaction between integrins and the RGD motifs by divalent cation chelation and a synthetic RGD peptide reduced the transduction efficiencies of Ad35 vectors; however, the amounts of cell-associated vector DNA of Ad35 vectors at 4 or 37 degrees C were not decreased by divalent cation chelation or the RGD peptide. Mutation of penton base RGD motifs reduced the transduction efficiencies of Ad35 vectors, although the amounts of cell-associated vector DNA of Ad35 vectors at 4 or 37 degrees C were not altered by mutation of penton base RGD motifs in Ad35 vectors. Furthermore, preincubation with several types of anti-integrin antibodies significantly inhibited Ad35 vector-mediated transduction. These results suggest that interaction between integrins and penton base RGD motifs plays a crucial role in Ad35 vector-mediated transduction in hematopoietic cells, probably in the post-internalization steps.

Downregulation of human CD46 by adenovirus serotype 35 vectors

Human CD46 (membrane cofactor protein), which serves as a receptor for a variety of pathogens, including strains of measles virus, human herpesvirus type 6 and Neisseria, is rapidly downregulated from the cell surface following infection by these pathogens. Here, we report that replication-incompetent adenovirus (Ad) serotype 35 (Ad35) vectors, which belong to subgroup B and recognize human CD46 as a receptor, downregulate CD46 following infection. A decline in the surface expression of CD46 in human peripheral blood mononuclear cells was detectable 6 h after infection, and reached maximum (72%) 12 h after infection. Ad35 vector-induced downregulation of surface CD46 levels gradually recovered after the removal of Ad35 vectors, however, complete recovery of CD46 expression was not observed even at 96 h after removal. The surface expression of CD46 was also reduced after incubation with fiber-substituted Ad serotype 5 (Ad5) vectors bearing Ad35 fiber proteins, ultraviolet-irradiated Ad35, vectors and recombinant Ad35 fiber knob proteins; in contrast, conventional Ad5 vectors did not induce surface CD46 downregulation, suggesting that the fiber knob protein of Ad35 plays a crucial role in the downregulation of surface CD46 density. These results have important implications for gene therapy using CD46-utilizing Ad vectors and for the pathogenesis of Ads that interact with CD46.

Adenovirus type 11 binding alters the conformation of its receptor CD46

Adenoviruses (Ads) are important human pathogens and valuable gene delivery vehicles. We report here the crystal structure of the species B Ad11 knob complexed with the Ad11-binding region of its receptor CD46. The conformation of bound CD46 differs profoundly from its unbound state, with the bent surface structure straightened into an elongated rod. This mechanism of interaction is likely to be conserved among many pathogens that target CD46 or related molecules.

[Characterization of adenovirus serotype 35 vectors using genetically modified animals and non-human primates]

Recombinant Adenovirus (Ad) vectors are considered to be a promising gene delivery vehicle of high utility because they are easy to construct, can be produced at high titers, and efficiently transduce various types of cells. Ad vectors commonly used in the world, including clinical trials, are composed of Ad serotype 5 (Ad5), which belongs to subgroup C. In recent years, however, it has become apparent that Ad5 vectors have some drawbacks, such as high seroprevalence of anti-Ad5 antibodies in adults and low transduction efficiencies of Ad5 vectors in cells lacking a primary receptor for Ad5, coxsackievirus and adenovirus receptor (CAR). To overcome these limitations of Ad5 vectors, we have developed a novel type of Ad vector, which is composed of Ad serotype 35 (Ad35), belonging to subgroup B. Ad35 vectors recognize human CD46, not CAR, as a cellular receptor for infection. Human CD46 is expressed in almost all of human cells, leading to a broad tropism of Ad35 vectors to human cells, in contrast, expression of rodent CD46 is limited to the testis. Therefore, in vivo transduction properties of Ad35 vectors are not appropriately evaluated in normal mice. In order to evaluate the in vivo transduction properties of Ad35 vectors, Ad35 vectors were applied to human CD46-transgenic mice and nonhuman primates, which express CD46 in a similar pattern to humans. The data obtained using CD46-transgenic mice and nonhuman primates would provide valuable information towards clinical applications of Ad35 vectors.

A new group B adenovirus receptor is expressed at high levels on human stem and tumor cells

CD46 is used by human group B adenoviruses (Ads) as a high-affinity attachment receptor. Here we show evidence that several group B Ads utilize an additional receptor for infection of human cells, which is different from CD46. We tentatively named this receptor receptor X. Competition studies with unlabeled and labeled Ads, recombinant Ad fiber knobs, and soluble CD46 and CD46 antibodies revealed three different subgroups of group B Ads, in terms of their receptor usage. Group I (Ad16, -21, -35, and -50) nearly exclusively uses CD46. Group II (Ad3, -7p, and -14) utilizes receptor X and not CD46. Group III (Ad11p) uses both CD46 and the alternative receptor X. Interaction of group II and III Ads with receptor X occurs via the fiber knob. Receptor X is an abundantly expressed glycoprotein that interacts with group II and III Ads at relatively low affinity in a Ca(2+)-dependent manner. This receptor is expressed at high levels on human mesenchymal and undifferentiated embryonic stem cells, as well as on human cancer cell lines. These findings have practical implications for stem cell and gene therapy.

Melanoma cultures show different susceptibility towards E1A-, E1B-19 kDa- and fiber-modified replication-competent adenoviruses

Replicating adenovirus (Ad) vectors with tumour tissue specificity hold great promise for treatment of cancer. We have recently constructed a conditionally replicating Ad5 AdDeltaEP-TETP inducing tumour regression in a xenograft mouse model. For further improvement of this vector, we introduced four genetic modifications and analysed the viral cytotoxicity in a large panel of melanoma cell lines and patient-derived melanoma cells. (1) The antiapoptotic gene E1B-19 kDa (Delta19 mutant) was deleted increasing the cytolytic activity in 18 of 21 melanoma cells. (2) Introduction of the E1A 122-129 deletion (Delta24 mutant), suggested to attenuate viral replication in cell cycle-arrested cells, did not abrogate this activity and increased the cytolytic activity in two of 21 melanoma cells. (3) We inserted an RGD sequence into the fiber to extend viral tropism to alphav integrin-expressing cells, and (4) swapped the fiber with the Ad35 fiber (F35) enhancing the tropism to malignant melanoma cells expressing CD46. The RGD-fiber modification strongly increased cytolysis in all of the 11 CAR-low melanoma cells. The F35 fiber-chimeric vector boosted the cytotoxicity in nine of 11 cells. Our results show that rational engineering additively enhances the cytolytic potential of Ad vectors, a prerequisite for the development of patient-customized viral therapies.

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

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

CD46 Represents a Target for Adenoviral Gene Therapy of Malignant Glioma

Malignant gliomas remain refractory to adenovirus serotype 5 (Ad5) gene therapy because of the lack of the primary adenoviral receptor, the coxsackie-adenovirus receptor (CAR), on tumor cells. To bypass the dependence on CAR, we investigated the expression of adenovirus serotype 3 (Ad3) receptor, or CD46, on glioma cells. First, we analyzed the expression of CD46 by RT-PCR on primary and passaged glioma cells. We then performed immunofluorescence studies to examine protein expression of CAR and CD46 on the same tumor lines. Finally, we constructed a replication-defective Ad vector that binds to CD46 and contains a luciferase transgenic cassette in place of the deleted E1 region: Ad5/3 (containing tail/shaft domain of Ad5 and knob domain of Ad3). These vectors were analyzed in vitro and in vivo against malignant glioma and compared with wild-type Ad5 or control vector Ad3/5 (containing tail of Ad5, shaft of Ad3, and knob of Ad5). The chimeric vector Ad5/3 showed a significant increase in the transduction efficiency of glioma tumor cells. At the same time, blocking the CD46 receptor caused a 65% inhibition of adenoviral infection when using Ad5/3. Taken together, these results indicate that CD46 is overexpressed by malignant glioma. Retargeting to the Ad3 receptor enhances gene transfer and offers a novel target for gene therapy of malignant brain tumors.

Emerging roles and new functions of CD46

In the past 20 years, our understanding of the workings of complement regulatory protein, CD46 (membrane cofactor protein), has grown as has the impressive list of pathogens interacting with this membrane-bound complement inhibitor. Referred to as a "pathogen magnet," CD46 serves as a receptor for seven human pathogens. Initially discovered as a widely expressed C3b- and C4b-binding protein, it was subsequently shown to be a cofactor for the serine protease factor I to inactivate by limited proteolysis these two opsonins and components of the convertases. The involvement of CD46 in reproductive processes continues to be an emerging story. It is a protector of placental tissue, but it may also play a more direct role in reproduction through its expression on the inner acrosomal membrane of spermatozoa. Cross-linking CD46 with antibodies or natural or pathogenic ligands induces rapid turnover and signaling events. In this regard, much attention is currently focused on generating human T lymphocyte regulatory cells by cross-linking CD46. Finally, highlighting its importance in protecting cells against excessive complement activation is the discovery that even a heterozygous deficiency of CD46 predisposes to hemolytic uremic syndrome.

The nucleotide sequence and a first generation gene transfer vector of species B human adenovirus serotype 3

Human adenovirus (Ad) serotype 3 causes respiratory infections. It is considered highly virulent, accounting for about 13% of all Ad isolates. We report here the complete Ad3 DNA sequence of 35,343 base pairs (GenBank accession DQ086466). Ad3 shares 96.43% nucleotide identity with Ad7, another virulent subspecies B1 serotype, and 82.56 and 62.75% identity with the less virulent species B2 Ad11 and species C Ad5, respectively. The genomic organization of Ad3 is similar to the other human Ads comprising five early transcription units, E1A, E1B, E2, E3, and E4, two delayed early units IX and IVa2, and the major late unit, in total 39 putative and 7 hypothetical open reading frames. A recombinant E1-deleted Ad3 was generated on a bacterial artificial chromosome. This prototypic virus efficiently transduced CD46-positive rodent and human cells. Our results will help in clarifying the biology and pathology of adenoviruses and enhance therapeutic applications of viral vectors in clinical settings.