Characterization of a factor formed in the course of adenovirus infection of tissue cultures causing detachment of cells from glass

The Adenovirus Death Protein - a small membrane protein controls cell lysis and disease

Human adenoviruses (HAdVs) cause widespread acute and persistent infections. Infections are usually mild and controlled by humoral and cell-based immunity. Reactivation of persistently infected immune cells can lead to a life-threatening disease in immunocompromised individuals, especially children and transplant recipients. To date, no effective therapy or vaccine against HAdV disease is available to the public. HAdV-C2 and C5 are the best-studied of more than 100 HAdV types. They persist in infected cells and release their progeny by host cell lysis to neighbouring cells and fluids, a process facilitated by the adenovirus death protein (ADP). ADP consists of about 100 amino acids and harbours a single membrane-spanning domain. It undergoes post-translational processing in endoplasmic reticulum and Golgi compartments, before localizing to the inner nuclear membrane. Here, we discuss the current knowledge on how ADP induces membrane rupture. Membrane rupture is essential for both progression of disease and efficacy of therapeutic viruses in clinical applications, in particular oncolytic therapy.

Oncolytic adenoviruses: A thorny path to glioma cure

Glioblastoma Multiforme (GBM) is a rapidly progressing brain tumor. Despite the relatively low percentage of cancer patients with glioma diagnoses, recent statistics indicate that the number of glioma patients may have increased over the past decade. Current therapeutic options for glioma patients include tumor resection, chemotherapy, and concomitant radiation therapy with an average survival of approximately 16 months. The rapid progression of gliomas has spurred the development of novel treatment options, such as cancer gene therapy and oncolytic virotherapy. Preclinical testing of oncolytic adenoviruses using glioma models revealed both positive and negative sides of the virotherapy approach. Here we present a detailed overview of the glioma virotherapy field and discuss auxiliary therapeutic strategies with the potential for augmenting clinical efficacy of GBM virotherapy treatment.

Poliovirus-induced Cellular Injury

Protein leakage was used as a quantitative measure of poliovirus-induced cellular injury under suspended cell culture conditions. The requirements for protein leakage were studied in detail and it was established that events early in the infectious cycle which depend upon viral protein synthesis were responsible for cell damage. Extralysosomal beta-glucuronidase appeared in infected cells before the onset of protein leakage and release of newly synthesized virus. Hydrocortisone treatment of infected cells resulted in only a slight delay in the release of beta-glucuronidase from lysosomes and protein and virus from cells. These results suggest that events associated with poliovirus synthesis trigger the release of lysosomal hydrolases which in turn injure the plasma membrane, allowing cytoplasmic proteins and virus to leak out of the cell.

Further characterization of the adenovirus erythrocyte receptor-modifying factor

Agglutinability of human erythrocytes for 3 hemagglutinating adenoviruses was markedly reduced by pretreatment of red cells with a factor present in tissue cultures which had been infected with adenovirus types 1, 2,4, or 15. The factor responsible for erythrocyte receptor modification was non-dialyzable and unaffected by the action of ribonuclease, desoxyribonuclease, trypsin, chymotrypsin, or ether. The factor was smaller, more thermostable, and separable from the infectious virus. Erythrocyte receptor modification was found to be a function of time and temperature. Titers of erythrocyte receptor-modifying activity were not diminished by successive exposures to fresh erythrocytes. Erythrocytes treated with erythrocyte receptor-modifying factor suspensions failed to significantly adsorb test virus hemagglutinin. Inhibition of erythrocyte receptor modifying-activity of the adenovirus suspensions by rabbit antiserum was type-specific.

Nucleuc acid synthesis in types 4 and 5 adenovirus-infected HeLa cells

Type 4 adenovirus infection of HeLa cells effected a marked increase in synthesis of the saline-soluble DNA fraction, but not the host-cell DNA (the water-soluble fraction). This was demonstrated by the marked increase in specific activity of saline-soluble DNA but not water-soluble DNA when P³²-inorganic phosphate or sodium formate-C¹⁴ was employed. When these isotopes were used to label cells before viral infection rather than during the process of viral propagation, the saline-soluble DNA from infected cells had a specific activity of 10 to 20 per cent less than that of uninfected cells, indicating that the saline-soluble DNA was synthesized both from prelabeled precursors of the cell pools and unlabeled materials from the medium. Saline-soluble DNA began to increase between 10 to 12 hours after viral infection and 3 to 4 hours before appearance of newly propagated infectious virus. The specific activity of the acid-soluble pool of infected cells also increased between 10 to 12 hours after viral inoculation when sodium formate-C¹⁴ was used as a radioisotope. When P³²-inorganic phosphate was utilized, the specific activity of infected-cell RNA was increased approximately the same relative amount as when total RNA was determined chemically; i.e., 30 to 40 per cent. With type 5 adenovirus, not only did a 3- to 5-fold increase in saline-soluble DNA occur, but also an increase was measured in specific activity of RNA when P³²-inorganic phosphate was used.

A human cell line selected for resistance to adenovirus infection has reduced levels of the virus receptor

To investigate determinants of host cell susceptibility to infection, cells partially resistant to infection were selected from the rare cells which remained adherent after infection of a culture of A549 cells with Ad2RAE, a mutant of adenovirus type 2 whose vertex capsomers lack an Arg-Gly-Asp (RGD) sequence which mediates binding of wild-type virus to integrins. Integrins promote the internalization of attached virions, whereas adsorption itself results from binding of the viral fibers to an unidentified cellular receptor. Following three rounds of selection, a persistently infected culture was established in which virus replication was detected in approximately 5% of the cells. Uninfected cells were readily cloned from the culture, indicating that at any particular time the majority of cells in the culture were uninfected. The resistance of one clone of uninfected cells to infection was correlated with a 10-fold reduction in the concentration of fiber receptors on these cells compared with the parental A549 cell line, indicating that efficiency of virus adsorption depends on the receptor concentration. Surprisingly, the rate at which host cells internalized RGD-negative virus also was strongly dependent on the fiber receptor concentration. While internalization of wild-type virus is promoted by the binding of integrins to the penton base RGD sequence, these results suggest that virus also can enter cells by an alternate pathway which requires binding of virions to multiple fiber receptors.

Vitronectin receptor antibodies inhibit infection of HeLa and A549 cells by adenovirus type 12 but not by adenovirus type 2

The penton base gene from adenovirus type 12 (Ad12) was sequenced and encodes a 497-residue polypeptide, 74 residues shorter than the penton base from Ad2. The Ad2 and Ad12 proteins are highly conserved at the amino- and carboxy-terminal ends but diverge radically in the central region, where 63 residues are missing from the Ad12 sequence. Conserved within this variable region is the sequence Arg-Gly-Asp (RGD), which, in the Ad2 penton base, binds to integrins in the target cell membrane, enhancing the rate or the efficiency of infection. The Ad12 penton base was expressed in Escherichia coli, and the purified refolded protein assembled in vitro with Ad2 fibers. In contrast to the Ad2 penton base, the Ad12 protein failed to cause the rounding of adherent cells or to promote attachment of HeLa S3 suspension cells; however, A549 cells did attach to surfaces coated with either protein and pretreatment of the cells with an integrin alpha v beta 5 monoclonal antibody reduced attachment to background levels. Treatment of HeLa and A549 cells with integrin alpha v beta 3 or alpha v beta 5 monoclonal antibodies or with an RGD-containing fragment of the Ad2 penton base protein inhibited infection by Ad12 but had no effect on and in some cases enhanced infection by Ad2. Purified Ad2 fiber protein reduced the binding of radiolabeled Ad2 and Ad12 virions to HeLa and A549 cells nearly to background levels, but the concentrations of fiber that strongly inhibited infection by Ad2 only weakly inhibited Ad12 infection. These data suggest that alpha v-containing integrins alone may be sufficient to support infection by Ad12 and that this pathway is not efficiently used by Ad2.

Mutations that alter an Arg-Gly-Asp (RGD) sequence in the adenovirus type 2 penton base protein abolish its cell-rounding activity and delay virus reproduction in flat cells

The adenovirus penton base protein has a cell rounding activity and may lyse endosomes during virus entry into the cytoplasm. We found that penton base that was expressed in Escherichia coli also caused cell rounding and that cells adhered to polystyrene wells that were coated with the protein. Mutant analysis showed that both properties required an Arg-Gly-Asp (RGD) sequence at residues 340 to 342 of penton base. In flat adherent cells, virus mutants with amino acid substitutions in the RGD sequence were delayed in virus reproduction and in the onset of viral DNA synthesis. In nonadherent or poorly spread cells, the kinetics of mutant virus reproduction were similar to those of wild-type adenovirus type 2. Expression of the mutant phenotype exclusively in the flat cells that we tested supports a model in which penton base interacts with an RGD-directed cell adhesion molecule during adenovirus uptake or uncoating.

Epithelium-specific response of cultured keratinocytes to infection with adenovirus type 2

Adenoviruses are pathogenic for certain stratified squamous epithelia. The sites most frequently involved are the upper respiratory tract and oropharynx. Adenovirus infections of the epidermis are quite rare. We examined the virus-cell interactions of adenovirus type 2 (Ad2) and cultured human keratinocytes grown from a variety of body sites. Our intent was to explore the nature of the apparent epithelium-specific susceptibility to Ad2. In brief, we found that in vitro viral susceptibility of the keratinocytes could be reliably predicted based on whether the cells originated from an epidermal or oropharyngeal surface. Ad2 proceeded through a complete vegetative cycle when used to infect cultured keratinocytes from oropharyngeal sites (e.g., gingiva and soft palate). In contrast, Ad2 infection was severely restricted in keratinocytes from epidermal sites (e.g., foreskin, abdomen, and buttock). These results demonstrate that the in vitro response to infection with Ad2 reflects in vivo tissue-specific susceptibility. In vivo, cervical epithelium is rarely infected with Ad2 and yet in culture, cervical keratinocytes were fully permissive for Ad2 replication. We propose that the permissive or nonpermissive response to Ad2 may be regulated by a particular aspect of cell phenotype. Because the permissive responses seen in this study were all generated in keratinocytes from mucosal sites, it is possible the in vitro response to Ad2 reflects inherent differences between mucosal and epidermal keratinocytes.

Progressive reorganization of the host cell cytoskeleton during adenovirus infection

Infection of cells with adenovirus lead to a characteristic reorganization of all cytoskeleton systems, starting with alterations at the microtubuli of the cells. During this progress, the flat, extended, and polar morphology of the cytoskeleton became nonpolar and rounder. These rearrangements were initiated before the appearance of adenovirus structural proteins hexon and fiber, as well as before the shutoff of host protein synthesis. We conclude that these alterations reflect a specific reorganization rather than an unorganized breakdown of the cell during adenovirus infection.

Neutralization of adenovirus infectivity and cytotoxin in various cell cultures

The neutralization of human adenovirus 5 and 11 by homologous and heterologous rabbit antisera was determined by CPE inhibition in various cell cultures (HeLa, HEL, Vero, secondary kidney cells from cercopithecus, rabbit, mouse), or in HeLa cells made impermissive by IUdR inhibition. The results concerning sensitivity and specificity were similar in all cases. Crude and purified virus showed similar neutralization. Immunofluorescence neutralization in HeLa cell cultures gave similar results; this method is suitable for demonstrating subtle immunological relations between adenovirus types. The neutralization of the early cytopathic factor ('cytotoxin') showed a pattern of cross-reactivity different from the virion; the cytotoxin was found to be active in part of the cell cultures only. It is concluded from the results that the virus function(s) blocked by antibody appear to be identical for the replicative cycle in infection and for the initiation of the abortive infection in non-permissive cells. Hence, either kind of cells may be used for neutralization tests.

Extrapulmonary manifestations of adenovirus type 7 pneumonia simulating Reye syndrome and the possible role of an adenovirus toxin

Three children developed extensive extrapulmonary disease in the course of fatal adenovirus type 7 pneumonia. Several clinical features, including the unexpected onset of coma, suggested the development of Reye syndrome, but biochemical and histopathologic findings were inconsistent with this diagnosis. Virologic and pathologic studies did not reveal evidence of extrapulmonary adenovirus infection, despite clinical involvement of the liver, skeletal muscle, and central nervous system. The detection in premortem sera from all three patients of adenovirus penton antigen, known to be cytotoxic in vitro, suggests a possible mechanism for the production of extrapulmonary pathology in the absence of extrapulmonary virus infection.

Immunological and other biological characteristics of pentons of human adenoviruses

Comparative hemagglutination-enhancement (HE) tests demonstrated diversified patterns of antigenic specificities both in the fiber and vertex capsomer part of pentons of human adenovirus types 3, 11 (subgroup I), 9, 15 (II), 1, 2, 4, 5, 6 (III), and 12. All fibers contained a type-specific antigen. Subgroup II and III fibers, in addition, contained specificities both unique for each subgroup and also common to the two subgroups. Fibers of serotypes 4 and 12 displayed a somewhat deviating behavior. All vertex capsomers tested shared a group-specific part. This was the only antigenic specificity demonstrable for serotype 12. Maximal penton HE titers of all sera were reached in tests with incomplete hemagglutinin of type 11. In addition, maximal HE activity of sera against individual serotypes also was recorded against pentons of other members of the same subgroup. Antigen characteristics of vertex capsomers of type 4 indicated a closer relationship to subgroup I than to subgroup III. The toxin activity of pentons was more sensitive to trypsin treatment than their capacity to function as incomplete hemagglutinin. Homotypic antipenton sera, unabsorbed or absorbed with homotypic fibers to remove antibodies against this component, and, to a varying extent, also heterotypic antipenton sera could neutralize toxin activity. Antifiber sera could neutralize toxin activity of pentons carrying short fibers (10 nm, type 3) but not of those carrying long fibers (28 to 31 nm, type 2). It is concluded that toxin activity is carried by a specific part of vertex capsomers and that cell detachment can be brought about via a direct contact between this component and cell membranes. Fiber-mediated attachment does not seem to be necessary for this biological activity to become expressed.

Persistent cyclic herpes simplex virus infection in vitro. II. Localization of virus, degree of cell destruction, and mechanisms of virus transmission

Hampar, Berge (National Institute of Dental Research, Bethesda, Md.). Persistent cyclic herpes simplex virus infection in vitro. II. Localization of virus, degree of cell destruction, and mechanisms of virus transmission. J. Bacteriol. 91:1959-1964. 1966. The localization of virus, degree of cell destruction, and mechanisms of virus transmission in persistent herpes simplex virus-infected cultures were studied. The major fraction of infectious virus was associated with the medium and a minor fraction was associated with the attached cells. Virus in the medium was further separable into a sedimentable (cellular) fraction and a nonsedimentable (extracellular) fraction. The sedimentable fraction was comprised of cellular debris, most of which appeared to contain viral antigen, and intact cells of which less than 10% contained infectious virus. Cell destruction during the cycle involved more than 99.9% of the maximal number of cells present. Infection could be transmitted by extracellular virus, cell-to-cell transfer, and reattachment of infectious cellular material. The results indicated that transmission by reattachment was probably mediated through the cellular debris rather than the intact cells.