Antibody-triggered dissociation of adenovirus penton capsomer

Humanization and characterization of a murine monoclonal neutralizing antibody against human adenovirus 7

Human adenovirus type 7 (HAdV7) is commonly associated with febrile acute respiratory disease (ARD) outbreaks. We have reported that 10G12, a mouse monoclonal antibody (mAb) specifically recognizing and neutralizing HAdV7, is a promising candidate for humanization. In this study, we engineered the six variants of 10G12 with increased degree of humanization and investigated their biological activity. The humanized monoclonal antibody (mAb) 10G12-M2 was shown to retain the parental antibody's high binding affinity, specificity and potent efficacy of viral suppression. The mAb 10G12-M2 recognized a conformational neutralization epitope of the hexon protein. Complex structure-based molecular docking simulation showed that the hexon protein formed several interactions with 10G12-M2, including hydrogen bonds and salt bridges interaction. Physicochemical properties analysis of 10G12-M2 demonstrated that it is stable and desirable lead candidate. In general, 10G12-M2 had excellent biological activity after humanization combined with the potential for use in prophylactic or therapeutic applications against HAdV7.

Significance of Preexisting Vector Immunity and Activation of Innate Responses for Adenoviral Vector-Based Therapy

An adenoviral (AdV)-based vector system is a promising platform for vaccine development and gene therapy applications. Administration of an AdV vector elicits robust innate immunity, leading to the development of humoral and cellular immune responses against the vector and the transgene antigen, if applicable. The use of high doses (10¹¹-10¹³ virus particles) of an AdV vector, especially for gene therapy applications, could lead to vector toxicity due to excessive levels of innate immune responses, vector interactions with blood factors, or high levels of vector transduction in the liver and spleen. Additionally, the high prevalence of AdV infections in humans or the first inoculation with the AdV vector result in the development of vector-specific immune responses, popularly known as preexisting vector immunity. It significantly reduces the vector efficiency following the use of an AdV vector that is prone to preexisting vector immunity. Several approaches have been developed to overcome this problem. The utilization of rare human AdV types or nonhuman AdVs is the primary strategy to evade preexisting vector immunity. The use of heterologous viral vectors, capsid modification, and vector encapsulation are alternative methods to evade vector immunity. The vectors can be optimized for clinical applications with comprehensive knowledge of AdV vector immunity, toxicity, and circumvention strategies.

Neutralizing antibody blocks adenovirus infection by arresting microtubule-dependent cytoplasmic transport

Neutralizing antibodies are commonly elicited by viral infection. Most antibodies that have been characterized block early stages of virus entry that occur before membrane penetration, whereas inhibition of late stages in entry that occurs after membrane penetration has been poorly characterized. Here we provide evidence that the neutralizing antihexon monoclonal antibody 9C12 inhibits adenovirus infection by blocking microtubule-dependent translocation of the virus to the microtubule-organizing center following endosome penetration. These studies identify a previously undescribed mechanism by which neutralizing antibodies block virus infection, a situation that may be relevant for other nonenveloped viruses that use microtubule-dependent transport during cell entry.

Structural and biochemical characterization of a human adenovirus 2/12 penton base chimera

The vertex of the adenoviral capsid is formed by the penton, a complex of two proteins, the pentameric penton base and the trimeric fiber protein. The penton contains all necessary components for viral attachment and entry into the host cell. After initial attachment via the head domain of the fiber protein, the penton base interacts with cellular integrins through an Arg-Gly-Asp (RGD) motif located in a hypervariable surface loop, triggering virus internalization. In order to investigate the structural and functional role of this region, we replaced the hypervariable loop of serotype 2 with the corresponding, but much shorter, loop of serotype 12 and compared it to the wild type. Here, we report the 3.6 A crystal structure of a human adenovirus 2/12 penton base chimera crystallized as a dodecamer. The structure is generally similar to human adenovirus 2 penton base, with the main differences localized to the fiber protein-binding site. Fluorescence anisotropy assays using a trimeric fiber protein mimetic called the minifiber and wild-type human adenovirus 2 and chimeric penton base demonstrate that fiber protein binding is independent of the hypervariable loop, with a K(d) for fiber binding estimated in the 1-2 microm range. Interestingly, competition assays using labeled and unlabeled minifiber demonstrated virtually irreversible binding to the penton base, which we ascribe to a conformational change, on the basis of comparisons of all available penton base structures.

Genetic retargeting of adenovirus: novel strategy employing "deknobbing" of the fiber

For efficient and versatile use of adenovirus (Ad) as an in vivo gene therapy vector, modulation of the viral tropism is highly desirable. In this study, a novel method to genetically alter the Ad fiber tropism is described. The knob and the last 15 shaft repeats of the fiber gene were deleted and replaced with an external trimerization motif and a new cell-binding ligand, in this case the integrin-binding motif RGD. The corresponding recombinant fiber retained the basic biological functions of the natural fiber, i.e., trimerization, nuclear import, penton formation, and ligand binding. The recombinant fiber bound to integrins but failed to react with antiknob antibody. For virus production, the recombinant fiber gene was rescued into the Ad genome at the exact position of the wild-type (WT) fiber to make use of the native regulation of fiber expression. The recombinant virus Ad5/FibR7-RGD yielded plaques on 293 cells, but the spread through the monolayer was two to three times delayed compared to WT, and the ratio of infectious to physical particles was 20 times lower. Studies on virus tropism showed that Ad5/FibR7-RGD was able to infect cells which did not express the coxsackie-adenovirus receptor (CAR), but did express integrins. Ad5/FibR7-RGD virus infectivity was unchanged in the presence of antiknob antibody, which neutralized the WT virus. Ad5/FibR7-RGD virus showed an expanded tropism, which is useful when gene transfer to cells not expressing CAR is needed. The described method should also make possible the construction of Ad genetically retargeted via ligands other than RGD.

Reversed-phase high-performance liquid chromatographic assay for the adenovirus type 5 proteome

An RP-HPLC assay was developed for a recombinant adenovirus type 5. During chromatography, intact adenovirus dissociated into its structural components (DNA and proteins) and the viral proteome was separated yielding a characteristic fingerprint. The individual components were identified by matrix-assisted laser desorption ionization time-of-flight mass spectroscopy, N-terminal sequencing and amino acid composition. The assay was utilized to measure adenovirus particle concentration through quantification of structural proteins. Each structural protein provided independent measurement of virus concentration allowing verification of accuracy. The assay sensitivity is at or below 2 x 10(8) particles. Contrary to the benchmark spectrophotometric assay, the RP-HPLC assay was shown to be insensitive to contaminants common for partially purified adenovirus preparations.

Immune response to recombinant capsid proteins of adenovirus in humans: antifiber and anti-penton base antibodies have a synergistic effect on neutralizing activity

Replication-deficient adenovirus used in humans for gene therapy induces a strong immune response to the vector, resulting in transient recombinant protein expression and the blocking of gene transfer upon a second administration. Therefore, in this study we examined in detail the capsid-specific humoral immune response in sera of patients with lung cancer who had been given one dose of a replication-defective adenovirus. We analyzed the immune response to the three major components of the viral capsid, hexon (Hx), penton base (Pb), and fiber (Fi). A longitudinal study of the humoral response assayed on adenovirus particle-coated enzyme-linked immunosorbent assay plates showed that patients had preexisting immunity to adenovirus prior to the administration of adenovirus-beta-gal. The level of the response increased in three patients after adenovirus administration and remained at a maximum after three months. One patient had a strong immune response to adenovirus prior to treatment, and this response was unaffected by adenovirus administration. Sera collected from the patients were assayed for recognition of each individual viral capsid protein to determine more precisely the molecular basis of the humoral immune response. Clear differences existed in the humoral response to the three major components of the viral capsid in serum from humans. Sequential appearance of these antibodies was observed: anti-Fi antibodies appeared first, followed by anti-Pb antibodies and then by anti-Hx antibodies. Moreover, anti-Fi antibodies preferentially recognized the native trimeric form of Fi protein, suggesting that they recognized conformational epitopes. Our results showed that sera with no neutralizing activity contained only anti-Fi antibodies. In contrast, neutralizing activity was only obtained with sera containing anti-Fi and anti-Pb antibodies. More importantly, we showed that anti-native Fi and anti-Pb antibodies had a synergistic effect on neutralization. The application of these conclusions to human gene therapy with recombinant adenovirus should lead to the development of strategies to overcome the formation of such neutralization antibodies, which have been shown to limit the efficacy of gene transfer in humans.

Characterization of adenovirus subgenus D fiber genes

The fiber genes of human adenovirus types 8, 9, 15, and two genome types of intermediate adenovirus 15/H9, all belonging to subgenus D, were sequenced and the nucleotide and predicted amino acid sequences compared. The analyzed peptide sequences were consistent with the structural domains described for other adenoviruses: an amino-terminal tail region, an intervening shaft region composed of eight repeating 15-amino-acid segments, and a carboxy-terminal knob. Furthermore, several highly conserved sequences could be confirmed. The fiber genes of AV9 and the AV15/H9 intermediate strains were 100% identical, not only in the coding region, but also in the presented 5'- and 3'-flanking regions. These findings support the hypothesis that intermediate strains arise by recombination of two serologically different parent viruses. The predicted AV8, AV9, and AV15/H9 polypeptides showed an overall homology of 92%. In the knob regions, AV8 and AV9; AV15/H9 intermediate strains revealed nine mismatches. AV8 is the main causative agent of epidemic keratoconjunctivitis in man, whereas AV9 and the AV15/H9 intermediate strains only infrequently cause acute follicular conjunctivitis. Since the knob is the component responsible for interaction with the cell receptor, these differences in the receptor binding sites should be expected to play an important role in the differences in pathogenicity.

Deletion analysis of functional domains in baculovirus-expressed adenovirus type 2 fiber

Various forms of Ad2 fiber were expressed in insect cells using recombinant baculoviruses and phenotypically characterized with respect to the following properties: trimerization, binding to penton base, nuclear targeting, and glycosylation. The morphology and dimensions of full-length fiber produced by invertebrate cells were indistinguishable from those observed in extracts from lytically infected mammalian cells. The domain required for trimer formation was mapped to the C-terminus, between amino acids 541 and 582. The N-terminal domain, between amino acids 1 and 16, negatively influenced the trimerization efficiency. Fiber gene products reduced to the shaft portion of the fiber capsomer formed significant amounts of stable dimers. Recognition with penton base only occurred with trimeric forms of fiber and was apparently not affected by deletion of the first 60 amino acids from the N-terminus. Fiber deleted of the Met1-Gly60 sequence was found to localize within the nucleus at levels similar to those of full-length fiber. All recombinant fibers, including tail-and-know-deleted forms, were found to be glycosylated using three separate assays, (i) in vivo labeling with [3H]glucosamine, (ii) binding to WGA, and (iii) reaction with monoclonal antibody RL2 directed against O-GlcNAc-containing glycopeptide. This implied that Ad2 fiber is a substrate for GlcNAc O-seryl transferase in insect cell cytoplasm and that at least one major glycosylation site is located in the shaft domain, between Met61 and Asn410.

Fiber sequence heterogeneity in subgroup F adenoviruses

The fiber gene of adenovirus type 41 was sequenced and compared to the fiber gene sequence of adenovirus type 40 (A. H. Kidd and M. J. Erasmus, 1989, Virology 172, 134-144), the other known member of subgroup F. The open reading frame, from map units 87 through 92 with transcription from the r-strand, comprised 1686 bases and was 45 bases longer than its counterpart on the Ad40 genome. The 45-base difference appears to have resulted from a block deletion on the Ad40 sequence. Apart from this one region, the Ad40 and Ad41 fiber genes showed remarkably high homology (95.6%), indicating a relatively recent evolutionary divergence. The deduced amino acid sequence of the Ad41 fiber polypeptide was analyzed according to the model of N. M. Green et al. (1983, EMBO J. 2, 1357-1365) for the structure of the adenovirus fiber. Ad41 had one more 15-residue repeat in the shaft region than Ad40, there being 22 repeat motifs. A detailed study of various Ad40 and Ad41 strains with proven genome differences indicated that the 15-amino acid difference in polypeptide length at the 14th repeat motif is a type-specific difference among the subgroup F adenoviruses. However, two uncommon Ad41 strains belonging to 2 of the 16 Ad41 genome types tested had a 15-amino acid block deletion which was different to that of the Ad40 polypeptide. The implication from this work is that the Ad40 fiber gene probably arose from its Ad41 counterpart, but the fiber gene sequences of both types of subgroup F adenovirus are so similar that genetic recombination between strains could occur with some frequency.

Sequence characterization of the adenovirus 40 fiber gene

The fiber gene of human adenovirus type 40 has been characterized. The 6.1-kbp EcoRI fragment C of the Ad40 genome, from map units 74 through 92, was cloned and the right-most 2.8 kbp from 84 map units was sequenced. By analogy with Ad2, this region would be expected to contain the gene specifying the Ad40 fiber polypeptide. Sequencing revealed an open reading frame of 1641 bases on the r-strand, the first 53 bases of which had marked homology with the corresponding L5 (fiber) regions of Ad2 (77.2%), Ad5 (75.0%), and Ad3 (64.3%). In addition, base positions 1114 to 1146 of this open reading frame had 85% homology with base positions 1198 to 1230 of the Ad2 fiber gene. The predicted polypeptide sequence of 547 amino acids showed marked homology with the Ad2, Ad5, and Ad3 fiber polypeptides in two regions, in the first 55 amino acids from the N-terminus and from amino acids 372 through 382. Analysis of hydrophobic amino acid positions revealed a repeating pattern of approximately 15 residues between positions 42 and 374, with 21 repeats. The sequence of the Ad40 polypeptide thus fits the model of Green et al. [1983), EMBO J. 2, 1357-1365) for the structure of the adenovirus fiber, but is 35 amino acids shorter than the Ad2 fiber polypeptide, with one less 15-residue repeat in the shaft region. According to this model, the regions of highest homology between the Ad40 fiber polypeptide and those of Ad2, Ad5, and Ad3 correspond to the tail of the shaft and the base of the knob. The results of this analysis are in agreement with previously published EM data on the fiber length of subgroup F adenoviruses.

Crystallization, enzymatic cleavage, and the polarity of the adenovirus type 2 fiber

Crystals of the fiber protein of adenovirus type 2 have been grown. Analysis of these crystals (type I crystals) showed that they were composed of fiber polypeptide with a lower apparent molecular weight (60 kDa) than that of the soluble or virion-incorporated fiber (62 kDa). N-terminal sequencing revealed that the fiber polypeptide chain of 60 kDa was cleaved at tyrosine17 from the N-end. The C-terminus remained intact. Assays with protease inhibitors suggested that the spontaneous cleavage of the fiber occurring upon its crystallization was due to a cellular, calcium-dependent, chymotrypsin-like protease co-purifying with the fiber and activated during hydroxyapatite chromatography. Crystallization of fiber purified in the presence of chymostatin provided crystals of a different structure under the electron microscope (crystals of type II), composed of 62-kDa fiber polypeptide units. The 62-kDa fiber from the type II crystals, as well as the 62-kDa fiber isolated from infected cell extracts, were able to associate with the penton base in vitro to form a penton capsomer. The 60-kDa fiber has lost this capacity. The accessibility of the N- and C-termini of the fiber inside the penton structure was probed by anti-peptide sera after limited proteolysis. The results are consistent with a polarity of the fiber in which its N-terminus is oriented toward the penton base, the C-terminal domain corresponding to the distal knob.

Molecular composition of the adenovirus type 2 virion

The representation of the different structural polypeptides within the adenovirus virion has been accurately determined, and the particle molecular weight has been derived. A stoichiometric analysis was performed with [35S]methionine as a radiolabel, and analytical sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to separate the polypeptides. The recently available sequence of the adenovirus type 2 genome was used to determine the number of methionines in each polypeptide. The resulting relative representation was placed on an absolute scale by using the known number of hexon polypeptides per virion. The analysis provides new information on the composition of the vertex region, which has been the subject of some controversy. Penton base was found to be present in 60 copies, distributed as pentamers at each of the 12 vertices. Three fiber monomers were associated with one penton base to form the penton complex. Polypeptide IX was present in 240 copies per virion and 12 copies per group-of-nine hexons, supporting a model proposed earlier for the distribution of this protein. The location of polypeptide IX explains the dissociation of the virus outer capsid into groups-of-nine hexons. The penton base was microheterogeneous, and the relative amounts suggest that the symmetry mismatch, which occurs within the penton complex between base and fiber, is resolved by the synthesis of penton base polypeptides from two closely spaced start codons.