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 K_D (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-Å 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.     

Deletion of the fiber gene induces the storage of hexon and penton base proteins in PML/Sp100-containing inclusions during adenovirus infection

The present study has documented changes in the in situ distribution of viral DNA and capsid proteins in 293 cells infected with fiber gene-deleted adenoviruses. It shows that infection results in the intense production of progeny viruses which appear morphologically intact although they are devoid of fiber-coding sequence in their genome and hence of fiber protein in their capsid. The data confirm, therefore, that fiber protein is not essential for the assembly of progeny viruses. The main contribution of our observations concerns specific intranuclear structures induced by infection with either wild-type or fiber gene-deleted viruses. These clear amorphous inclusions contain two cellular proteins, PML and Sp100, which in non-infected cells co-localize to a special type of nuclear bodies. PML and Sp100 nuclear bodies appear to directly modulate or to be altered in a wide variety of situations including viral infections, cell death and transformation. In cells infected with fiber gene-deleted viruses, the clear amorphous inclusions now accumulate non-used hexon and penton base proteins, whereas the absence of fiber protein prevents the assembly of capsid proteins in crystallin arrays. Taken together, the data suggest that the clear amorphous inclusions may correspond to storage sites of structural and regulatory proteins. Consequently, these virus-induced structures may promote the productive cycle of adenoviruses by regulating the amount of over-produced viral proteins and the shutoff of the host cell metabolism.     

Immunogenicity of recombinant adenovirus serotype 35 vaccine in the presence of pre-existing anti-Ad5 immunity

The high prevalence of pre-existing immunity to adenovirus serotype 5 (Ad5) in human populations may substantially limit the immunogenicity and clinical utility of recombinant Ad5 vector-based vaccines for HIV-1 and other pathogens. A potential solution to this problem is to use vaccine vectors derived from adenovirus (Ad) serotypes that are rare in humans, such as Ad35. However, cross-reactive immune responses between heterologous Ad serotypes have been described and could prove a major limitation of this strategy. In particular, the extent of immunologic cross-reactivity between Ad5 and Ad35 has not previously been determined. In this study we investigate the impact of pre-existing anti-Ad5 immunity on the immunogenicity of candidate rAd5 and rAd35 vaccines expressing SIV Gag in mice. Anti-Ad5 immunity at levels typically found in humans dramatically blunted the immunogenicity of rAd5-Gag. In contrast, even high levels of anti-Ad5 immunity did not substantially suppress Gag-specific cellular immune responses elicited by rAd35-Gag. Low levels of cross-reactive Ad5/Ad35-specific CD4(+) T lymphocyte responses were observed, but were insufficient to suppress vaccine immunogenicity. These data demonstrate the potential utility of Ad35 as a candidate vaccine vector that is minimally suppressed by anti-Ad5 immunity. Moreover, these studies suggest that using Ad vectors derived from immunologically distinct serotypes may be an effective and general strategy to overcome the suppressive effects of pre-existing anti-Ad immunity.     

PEGylated adenovirus vectors containing RGD peptides on the tip of PEG show high transduction efficiency and antibody evasion ability

BACKGROUND: PEGylation of adenovirus vectors (Ads) is an attractive strategy in gene therapy. Although many types of PEGylated Ad (PEG-Ads), which exhibit antibody evasion activity and long plasma half-life, have been developed, their entry into cells has been prevented by steric hindrance by polyethylene glycol (PEG) chains. Likewise, sufficient gene expression for medical treatment could not be achieved. METHODS: A set of PEG-Ads, which have different PEG modification rates, was constructed, and gene expression was evaluated using A549 cells. A novel PEGylated Ad (RGD-PEG-Ad), which contained RGD (Arg-Gly-Asp) peptides on the tip of PEG, was developed. We evaluated gene expression both in Coxsackie-adenovirus receptor (CAR)-positive as well as -negative cells, and in vivo gene expression was also determined. Furthermore, the antibody evasion ability and the specificity of infection exhibited by this RGD-PEG-Ad were also evaluated. RESULTS: Whereas PEG-Ads decreased gene expression in CAR-positive cells, RGD-PEG-Ad enhanced gene expression notably, to a level about 200-fold higher than that of PEG-Ads. Moreover, gene expression of RGD-PEG-Ad was almost equal to that of Ad-RGD, which contains an RGD-motif in the fiber and exhibits among the highest gene expression of CAR-positive and -negative cells. Furthermore, although Ad-RGD gene expression decreased remarkably in the presence of anti-Ad antiserum, RGD-PEG-Ad maintained its activity against antibodies. In vivo experiments also demonstrated that the modification of Ads with RGD-PEG induced efficient gene expression. CONCLUSIONS: In the present study, we demonstrated that a new strategy, which combined integrin-targeting the RGD peptide on the tip of PEG and modified the Ad using this material, could enhance gene expression in both CAR-positive and -negative cells. At the same time, this novel PEGylated Ad maintained strong protective activity against antibodies. This strategy could also be easily modified for developing other vectors using other targeting molecules.     

Model of the trimeric fiber and its interactions with the pentameric penton base of human adenovirus by cryo-electron microscopy

Adenovirus invades host cells by first binding to host receptors through a trimeric fiber, which contains three domains: a receptor-binding knob domain, a long flexible shaft domain, and a penton base-attachment tail domain. Although the structure of the knob domain associated with a portion of the shaft has been solved by X-ray crystallography, the in situ structure of the fiber in the virion is not known; thus, it remains a mystery how the trimeric fiber attaches to its underlying pentameric penton base. By high-resolution cryo-electron microscopy, we have determined the structure of the human adenovirus type 5 (Ad5) to 3.6-Å resolution and have reported the full atomic models for its capsid proteins, but not for the fiber whose density cannot be directly interpreted due to symmetry mismatch with the penton base. Here, we report the determination of the Ad5 fiber structure and its mode of attachment to the pentameric penton base by using an integrative approach of multi-resolution filtering, homology modeling, computational simulation of mismatched symmetries, and fitting of atomic models into cryo-electron microscopy density maps. Our structure reveals that the interactions between the trimeric fiber and the pentameric penton base are mediated by a hydrophobic ring on the top surface of the penton base and three flexible tails inserted into three of the five available grooves formed by neighboring subunits of penton base. These interaction sites provide the molecular basis for the symmetry mismatch and can be targeted for optimizing adenovirus for gene therapy applications.     

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

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

Ubiquitination of both adeno-associated virus type 2 and 5 capsid proteins affects the transduction efficiency of recombinant vectors

In the presence of complementing adeno-associated virus type 2 (AAV-2) Rep proteins, AAV-2 genomes can be pseudotyped with the AAV-5 capsid to assemble infectious virions. Using this pseudotyping strategy, the involvement of the ubiquitin-proteasome system in AAV-5 and AAV-2 capsid-mediated infections was compared. A recombinant AAV-2 (rAAV-2) proviral luciferase construct was packaged into both AAV-2 and AAV-5 capsid particles, and transduction efficiencies in a number of cell lines were compared. Using luciferase expression as the end point, we demonstrated that coadministration of the viruses with proteasome inhibitors not only increased the transduction efficiency of rAAV-2, as previously reported, but also augmented rAAV-5-mediated gene transfer. Increased transgene expression was independent of viral genome stability, since there was no significant difference in the amounts of internalized viral DNA in the presence or absence of proteasome inhibitors. Western blot assays of immunoprecipitated viral capsid proteins from infected HeLa cell lysates and in vitro reconstitution experiments revealed evidence for ubiquitin conjugation of both AAV-2 and AAV-5 capsids. Interestingly, heat-denatured virus particles were preferential substrates for in vitro ubiquitination, suggesting that endosomal processing of the viral capsid proteins is a prelude to ubiquitination. Furthermore, ubiquitination may be a signal for processing of the capsid at the time of virion disassembly. These studies suggest that the previously reported influences of the ubiquitin-proteasome system on rAAV-2 transduction are also active for rAAV-5 and provide a clearer mechanistic framework for understanding the functional significance of ubiquitination.     

Structure and metabolism of adenovirus RNAs containing sequences from the fiber gene

The body of adenovirus fiber messenger RNA is specified by viral r-strand co-ordinates 86.2 to 91.2. Since this mRNA is transcribed from the major late promoter at map position 16, nuclear precursors to the mRNA could be as large as 84% of the length of the 35,000 nucleotide genome. This study identified and characterized polyadenylated nuclear RNAs that contain fiber sequences and therefore are possible processing intermediates. These nuclear RNAs were characterized by hybridization of [³H]RNA preparations and by electron microscopy of RNA-DNA hybrids. Three size classes of RNAs containing fiber sequences were identified: (1) a 22 S species maps from 86.2 to 90.3. This RNA has essentially the same co-ordinates as fiber mRNA. (2) Two 28 S species have co-ordinates of 80.1 to 90.4 and 85.9 to 96.9, respectively. Thus one species has a 5′ terminus coincident with that of the mRNA body, and one has a 3′ terminus coincident with that of the 3′ end of the mRNA body. The polyadenylated terminus at 96.9 does not coincide with the 3′ end of any known mRNA. (3) There are at least two 35 S species. The 3′ end of one species is coincident with that of fiber mRNA. The 3′ terminus of the second RNA is at approximately 96.9.The labeling kinetics of each of these polyadenylated nuclear RNAs were investigated. In continuous label experiments, the two 35 S RNAs and the 85.9 to 96.9 28 S RNA became uniformly labeled in approximately 60 minutes. The 22 S RNA and the 80.1 to 90.4 28 S species continued to accumulate for at least several hours. These results are consistent with a precursor function for the 35 S RNAs and the 85.9 to 96.9 28 S species. The structures of the putative precursors imply that processing of the 3′ end is not a prerequisite for 5′ cleavage.     

Pseudopackaging of adenovirus type 5 genomes into capsids containing the hexon proteins of adenovirus serotypes B, D, or E

Adenoviruses (Ad) show promise as a vector system for gene delivery in vivo. However, a major challenge in the development of Ad vectors is the circumvention of the host immune responses to Ad infection, including both the host cytotoxic T-cell response and the humoral response resulting in neutralizing antibodies. One method to circumvent the effect of neutralizing antibodies against an Ad vector is to use different Ad serotypes to deliver the transgene of interest. This approach has been demonstrated with Ad genomes of highly related members of subgroup C. However, it is not known whether an Ad5-based vector DNA molecule can be packaged into capsids of evolutionarily more divergent adenoviruses. The aim of these studies was to determine if capsids containing hexon proteins from other Ad subgroups could package the Ad5 genome. A genetic approach utilizing an Ad5 temperature-sensitive (ts) mutant with a mutation in the hexon protein was used. When grown at the nonpermissive temperature, Ad5 ts147 replicates normally, providing a source of Ad5 DNA for virus assembly, but does not produce virus particles due to the hexon protein mutation. Coinfection of Ad5 ts147 with a wild-type virus of other Ad serotypes (Ad3, Ad4, or Ad9), which supply functional hexon proteins, resulted in the pseudopackaging of the Ad5 DNA genome. Furthermore, the pseudopackaged Ad5 DNA virions obtained in the coinfections were infectious. Therefore, switching hexons did not impair the infectivity or uncoating process of the pseudopackaged virion. Since hexon protein is a major antigenic determinant of the Ad capsid, this approach may prove useful to reduce the antigenicity of therapeutic Ad vectors and allow repeated vector administration.     

Deletion of the gene encoding the adenovirus 5 early region 1b 21,000-molecular-weight polypeptide leads to degradation of viral and host cell DNA.

The adenovirus 5 mutant H5dl337 lacks 146 base pairs within early region 1B. The deletion removes a portion of the region encoding the E1B 21,000-molecular-weight (21K) polypeptide, but does not disturb the E1B-55K/17K coding region. The virus is slightly defective for growth in cultured HeLa cells, in which its final yield is reduced ca. 10-fold compared with wild-type virus. The mutant displays a striking phenotype in HeLa cells. The onset of cytopathic effect is dramatically accelerated, and both host cell and viral DNAs are extensively degraded late after infection. This defect has been described previously for a variety of adenovirus mutants and has been termed a cytocidal (cyt) phenotype. H5dl337 serves to map this defect to the loss of E1B-21K polypeptide function. In addition to its defect in the productive growth cycle, H5dl337 is unable to transform rat cells at normal efficiency.     

Effect of preexisting immunity to adenovirus human serotype 5 antigens on the immune responses of nonhuman primates to vaccine regimens based on human- or chimpanzee-derived adenovirus vectors

In this study we compared a prime-boost regimen with two serologically distinct replication-defective adenovirus (Ad) vectors derived from chimpanzee serotypes C68 and C1 expressing Gag, Pol, gp140, and Nef of human immunodeficiency virus type 1 with a regimen in which replication-defective Ad vectors of the human serotype 5 (AdHu5) were given twice. Experiments were conducted in rhesus macaques that had or had not been preexposed to antigens of AdHu5. There was no significant difference in T-cell responses tested from peripheral blood of the different groups, although responses were overall highest in nonpreexposed animals immunized with the chimpanzee Ad vectors. Preexisting immunity to AdHu5 completely inhibited induction of transgene product-specific antibodies by the AdHu5 vectors without affecting antibody responses to the chimpanzee vectors. Upon euthanasia, T-cell responses were tested from a number of tissues. Preexisting immunity to AdHu5, commonly found in humans, changed the homing pattern of vaccine-induced T cells. In AdHu5-preexposed animals vaccinated with the chimpanzee Ad vectors, frequencies of transgene-specific T cells were higher in spleens than in blood, and in most preexposed animals vaccinated either with AdHu5 vectors or chimpanzee adenovirus vectors, frequencies of such T cells were exceptionally high in livers. The latter results indicate that analysis of T-cell responses solely from blood mononuclear cells of vaccine recipients may not suffice to compare the potencies of different vaccine regimens.     

Immunogenicity of heterologous prime-boost regimens involving recombinant adenovirus serotype 11 (Ad11) and Ad35 vaccine vectors in the presence of anti-ad5 immunity

The high prevalence of preexisting immunity to adenovirus serotype 5 (Ad5) in human populations will likely limit the immunogenicity and clinical utility of recombinant Ad5 (rAd5) vector-based vaccines for human immunodeficiency virus type 1 and other pathogens. A potential solution to this problem is to utilize rAd vaccine vectors derived from rare Ad serotypes such as Ad35 and Ad11. We have previously reported that rAd35 vectors were immunogenic in the presence of anti-Ad5 immunity, but the immunogenicity of heterologous rAd prime-boost regimens and the extent that cross-reactive anti-vector immunity may limit this approach have not been fully explored. Here we assess the immunogenicity of heterologous vaccine regimens involving rAd5, rAd35, and novel rAd11 vectors expressing simian immunodeficiency virus Gag in mice both with and without anti-Ad5 immunity. Heterologous rAd prime-boost regimens proved significantly more immunogenic than homologous regimens, as expected. Importantly, all regimens that included rAd5 were markedly suppressed by anti-Ad5 immunity. In contrast, rAd35-rAd11 and rAd11-rAd35 regimens elicited high-frequency immune responses both in the presence and in the absence of anti-Ad5 immunity, although we also detected clear cross-reactive Ad35/Ad11-specific humoral and cellular immune responses. Nevertheless, these data suggest the potential utility of heterologous rAd prime-boost vaccine regimens using vectors derived from rare human Ad serotypes.     

Deletion and insertion mutations in early region 1a of type 5 adenovirus that produce cold-sensitive or defective phenotypes for transformation

On the basis of earlier findings showing that H5hr1 (hr1) is cold sensitive for transformation, a series of mutants were constructed so that they contained deletions or insertions in different sites of early region 1a (E1a) to ascertain: (i) whether the cold-sensitive phenotype of hr1 was the result of the identified single-base pair deletion of nucleotide 1,055 or due to a missense mutation at another site and (ii) what region and how much of the E1a 51-kilodalton protein is actually required to produce cell transformation. A mutant, H5dl101 (dl101), was constructed to contain a 5-base pair deletion of nucleotides 1,008 to 1,012, which produced a frameshift and a subsequent stop codon at nucleotide 1,241. This mutant, which should encode a truncated 33-kilodalton protein in place of the wild-type 51-kilodalton protein, had a cold-sensitive phenotype for transformation essentially identical to hr1. Consonant with this finding, a mutant (H5in106) engineered to contain a 16-base pair insertion initiated after nucleotide 1,009, with a stop codon beginning at the newly inserted nucleotide 1,013, also had a cold-sensitive phenotype like hr1 and dl101. It is striking, however, that a mutant (H5dl105) with a 69-base pair deletion beginning at nucleotide 1,003, and having a stop codon at nucleotide 1,544, was totally defective for transformation at any temperature. Transfection studies with plasmids containing the E1a or E1a and E1b genes of sub309, hr1, and dl101 further revealed that the cold-sensitive transformation phenotype observed could be exhibited in the absence of viral E1b gene expression.     

Reduction of natural adenovirus tropism to the liver by both ablation of fiber-coxsackievirus and adenovirus receptor interaction and use of replaceable short fiber

The initial recognition and binding of adenovirus vector to the host cell surface is mediated by interaction between the adenovirus fiber knob protein and its receptor, the coxsackievirus and adenovirus receptor (CAR). This natural tropism of adenovirus vector needs to be ablated in order to achieve targeted gene transfer. To this end, we noted that adenovirus serotype 40 (Ad40) contains two distinct long and short fibers; the short fiber is unable to recognize CAR, while the long fiber binds CAR. We generated adenovirus serotype 5-based mutants with chimeric Ad40-derived fibers, which were composed of either long or short shafts together with CAR binding or nonbinding knobs. The capacity of these adenovirus mutants for in vitro and in vivo gene transfer to liver cells was examined. In the case of primary human hepatocytes displaying a high expression level of CAR and alphav integrin, both CAR binding ability and fiber shaft length played important roles in efficient transduction. Most significantly, the high transduction efficiency observed in the liver and spleen following intravenous administration of adenovirus vector was dramatically reduced by both ablation of fiber-CAR interaction and the use of replaceable short fiber. In other tissues displaying a low level of transduction, no significant differences in transduction efficiency were observed among adenovirus vector mutants. Furthermore, incorporation of a 7-lysine-residue motif at the C-terminal end of CAR-nonbinding short fiber efficiently achieved transduction of target cells via the heparan-containing receptor. Our results demonstrated that the natural tropism of adenovirus in vivo is influenced not only by fiber-CAR interaction but also by fiber shaft length. Furthermore, our strategy may be useful for retargeting adenovirus to particular tumors and tissue types with specific receptors.     

Deletion of the glucose-6-phosphate dehydrogenase gene KlZWF1 affects both fermentative and respiratory metabolism in Kluyveromyces lactis

In Kluyveromyces lactis, the pentose phosphate pathway is an alternative route for the dissimilation of glucose. The first enzyme of the pathway is the glucose-6-phosphate dehydrogenase (G6PDH), encoded by KlZWF1. We isolated this gene and examined its role. Like ZWF1 of Saccharomyces cerevisiae, KlZWF1 was constitutively expressed, and its deletion led to increased sensitivity to hydrogen peroxide on glucose, but unlike the case for S. cerevisiae, the Klzwf1Delta strain had a reduced biomass yield on fermentative carbon sources as well as on lactate and glycerol. In addition, the reduced yield on glucose was associated with low ethanol production and decreased oxygen consumption, indicating that this gene is required for both fermentation and respiration. On ethanol, however, the mutant showed an increased biomass yield. Moreover, on this substrate, wild-type cells showed an additional band of activity that might correspond to a dimeric form of G6PDH. The partial dimerization of the G6PDH tetramer on ethanol suggested the production of an NADPH excess that was negative for biomass yield.     

Adenovirus vectors containing chimeric type 5 and type 35 fiber proteins exhibit altered and expanded tropism and increase the size limit of foreign genes

Adenovirus (Ad) fiber proteins are responsible for the initial attachment of the virion to the cell membrane. Most Ad vectors currently in use are based on the Ad type 5 (Ad5), which belong to subgroup C, and use the coxsackievirus and adenovirus receptors (CAR) as the initial receptor. Ad35, which belongs to subgroup B, recognizes unknown receptor(s) other than CAR. In this study, the feasibility of the Ad vector containing Ad5/35 chimeric fiber protein was examined in a wide variety of cell types, such as CAR-positive or -negative human tumor cells, rodent cells, and blood cells (a total of 20 cell types), and in mice in vivo. Transduction data suggested that the Ad vectors containing the Ad5/35 chimeric fiber protein exhibited altered and expanded tropism when compared with the Ad5-based vector. The chimeric vector also allows the packaging of larger foreign DNAs than the conventional Ad5-based vector, which can package approximately 8.1-8.2 kb of foreign DNA. The chimeric vector containing approximately 8.8 kb of foreign DNA was generated without affecting the viral growth rate and titer. These results suggested that inclusion of the Ad35 fiber protein into the Ad5-based vector could lead to an improved efficiency in gene therapy and in gene transfer experiments, especially for the cells lacking in sufficient CAR expression.     

Deletion of Epstein-Barr virus BFLF2 leads to impaired viral DNA packaging and primary egress as well as to the production of defective viral particles

Previous genetic and biochemical studies performed with several members of the Alphaherpesvirus subfamily have shown that the UL31 and UL34 proteins are essential components of the molecular machinery that mediates the primary egress of newly assembled capsids across the nuclear membrane. Further, there is substantial evidence that BFLF2 and BFRF1, the respective positional homologs of UL31 and UL34 in the Epstein-Barr virus (EBV) genome, are also their functional homologs, i.e., that the UL31/UL34 pathway is common to distant herpesviruses. However, the low degree of protein sequence identity between UL31 and BFLF2 would argue against such a hypothesis. To further clarify this issue, we have constructed a recombinant EBV strain devoid of BFLF2 (DeltaBFLF2) and show that BFLF2 is crucial for efficient virus production but not for lytic DNA replication or B-cell transformation. This defective phenotype could be efficiently restored by trans complementation with a BFLF2 expression plasmid. Detailed analysis of replicating cells by electron microscopy revealed that, as expected, DeltaBFLF2 viruses not only failed to egress from the nucleus but also showed defective DNA packaging. Nonfunctional primary egress did not, however, impair the production and extracellular release of enveloped but empty viral particles that comprised L particles containing tegument-like structures and a few virus-like particles carrying empty capsids. The DeltaBFLF2 and DeltaUL31 phenotypes therefore only partly overlap, from which we infer that BFLF2 and UL31 have substantially diverged during evolution to fulfil related but distinct functions.