Adenovirus Replication
المؤلف:
Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia
المصدر:
Jawetz, Melnick, & Adelberg’s Medical Microbiology
الجزء والصفحة:
28e , p463-466
2025-11-02
40
Adenoviruses replicate well only in cells of epithelial origin. The replicative cycle is divided into early and late events. The carefully regulated course of sequential events in the adeno virus cycle is shown in Figure 1. The distinction between early and late events is not absolute in infected cells; early genes continue to be expressed throughout the cycle; a few genes begin to be expressed at “intermediate” times; and low levels of late gene transcription may occur soon after infection.
Fig1. Time course of adenovirus replication cycle. The time between infection and the first appearance of progeny virus is the eclipse period. Note the sequential regulation of specific events in the virus replication cycle. PFU means plaque-forming unit, a measure of infectious virus. (Courtesy of M Green.)
A. Virus Attachment, Penetration, and Uncoating
The virus attaches to cells via the fiber structures. The host cell receptor for some serotypes is CAR (coxsackie— adenovirus receptor), a member of the immunoglobulin gene superfamily. The interaction of the penton base with cellular integrins after attachment promotes the internalization step. Adsorption and internalization are separate steps in the adenovirus infection process, requiring the interaction of fiber and penton proteins with different cellular target proteins. Adsorbed virus is internalized into endosomes; the majority of particles (∼90%) move rapidly from endosomes into the cytosol (half-life ∼5 minutes) by a process triggered by the acidic pH of the endosome. Microtubules are prob ably involved in the transport of virus particles across the cytoplasm to the nucleus. Uncoating commences in the cytoplasm and is completed in the nucleus, with release of viral DNA perhaps occurring at the nuclear membrane. Uncoating is an organized, sequential process that systematically breaks down the stabilizing interactions that were established during maturation of the virus particle.
B. Early Events
The steps that occur before the onset of viral DNA synthesis are defined as early events. The goals of the early events are to induce the host cell to enter the S phase of the cell cycle to create conditions conducive to viral replication, to express viral functions that protect the infected cell from host defense mechanisms, and to synthesize viral gene products needed for viral DNA replication.
The early (“E”) transcripts come from seven widely separated regions of the viral genome and from both viral DNA strands. More than 20 early proteins, many of which are nonstructural and are involved in viral DNA replication, are synthesized in adenovirus-infected cells. The E1A early gene is especially important; it must be expressed for the other early regions to be transcribed. Modulation of the cell cycle is accomplished by the E1A gene products. The E1B early region encodes proteins that block cell death (apoptosis) occurring as a result of E1A functions; this is necessary to prevent premature cell death that would adversely affect virus yields. The E1A and E1B regions contain the only adenovirus genes necessary for cell transformation; those gene products bind cellular proteins (eg, pRb, p300, and p53) that regulate cell cycle progression. The early proteins are represented by the 75-kDa DNA-binding protein shown in Figure 1.
C. Replication of Viral DNA and Late Events
Viral DNA replication takes place in the nucleus. The virus encoded, covalently linked terminal protein functions as a primer for initiation of viral DNA synthesis.
Late events begin concomitantly with the onset of viral DNA synthesis. The major late promoter controls the expression of the late (“L”) genes coding for viral structural proteins. There is a single large primary transcript (∼29,000 nucleotides in length) that is processed by splicing to generate at least 18 different late messenger RNAs (mRNAs). These mRNAs are grouped (L1–L5) based on the utilization of common poly (A) addition sites. The processed transcripts are transported to the cytoplasm, where the viral proteins are synthesized.
Although host genes continue to be transcribed in the nucleus late in the course of infection, few host genetic sequences are transported to the cytoplasm. A complex involving the E1B 55-kDa polypeptide and the E4 34-kDa polypeptide inhibits the cytoplasmic accumulation of cellular mRNAs and facilitates accumulation of viral mRNAs, per haps by relocalizing a cellular factor required for mRNA transport. Very large amounts of viral structural proteins are subsequently made.
It is noteworthy that studies with adenovirus hexon mRNA led to the profound discovery that eukaryotic mRNAs are usually not colinear with their genes but are spliced products of separated coding regions in the genomic DNA.
D. Viral Assembly and Maturation
Virion morphogenesis occurs in the nucleus. Each hexon capsomere is a trimer of identical polypeptides. The penton is composed of five penton base polypeptides and three fiber polypeptides. A late L4-encoded “scaffold protein” assists in the aggregation of hexon polypeptides but is not part of the final structure.
Capsomeres self-assemble into empty-shell capsids in the nucleus. Naked DNA then enters the preformed capsid. A cis-acting DNA element near the end of the viral chromosome serves as a packaging signal, necessary for the DNA–capsid recognition event. Another viral scaffolding protein, encoded in the L1 group, facilitates DNA encapsidation. Finally, pre cursor core proteins are cleaved, which allows the particle to tighten its configuration, and the pentons are added. A virus encoded cysteine proteinase functions in some cleavages of precursor proteins. The mature particle is then stable, infectious, and resistant to nucleases. The adenovirus infectious cycle takes about 24 hours. The assembly process is inefficient; about 80% of hexon capsomeres, and 90% of viral DNA are not used. Nevertheless, about 100,000 virus particles are produced per cell. Structural proteins associated with mature virus particles are catalogued in Figure 2B.
Fig2. Models of the adenovirus virion. A: A three-dimensional image reconstruction of an intact adenovirus particle showing fibers projecting from the penton bases. (Reproduced with permission from Liu H, Wu L, Zhou ZH: Model of the trimeric fiber and its interactions with the pentameric penton base of human adenovirus by cryo-electron microscopy. J Mol Biol 2011;406:764. [Graphical abstract.] Copyright Elsevier.) B: A stylized section of the adenovirus particle showing polypeptide components and DNA. No real section of the icosahedral virion would contain all components. Virion constituents are designated by their polypeptide numbers with the exception of the terminal protein (TP). (Reproduced with permission from Stewart PL, Burnett RM: Adenovirus structure as revealed by x-ray crystallography, electron microscopy and difference imaging. Jpn J Appl Phys 1993;32:1342.)
E. Virus Effects on Host Defense Mechanisms
Adenoviruses encode several gene products that counter antiviral host defense mechanisms. The small, abundant VA RNAs afford protection from the antiviral effect of interferon by preventing activation of an interferon-inducible kinase that phosphorylates and inactivates eukaryotic initiation factor 2. Adenovirus E3 region proteins, which are nonessential for viral growth in tissue culture, inhibit cytolysis of infected cells by host responses. The E3 gp19-kDa protein blocks movement of the major histocompatibility complex class I antigen to the cell surface, thereby protecting the infected cell from cytotoxic T-lymphocyte-mediated lysis. Other E3-encoded proteins block induction of cytolysis by the cytokine tumor necrosis factor α.
F. Virus Effects on Cells
Adenoviruses are cytopathic for human cell cultures, particularly primary kidney and epithelial cell lines. The cytopathic effect usually consists of marked rounding, enlargement, and aggregation of affected cells into grape-like clusters. The infected cells do not lyse even though they round up and detach from the glass surface on which they have been grown.
In cells infected with some adenovirus types, rounded intranuclear inclusions containing DNA are seen (Figure 3). Such nuclear inclusions may be mistaken for those of cytomegalovirus, but adenovirus infections do not induce syncytia or multinucleated giant cells. Although the cytologic changes are not pathognomonic for adenoviruses, they are helpful for diagnostic purposes in tissue culture and biopsy specimens.
Fig3. Adenovirus cytopathology in human tissue. Tubular epithelial cells with basophilic inclusion bodies from a patient with necrotizing tubulointerstitial nephritis (450×). (Courtesy of M lto.)
Virus particles in the nucleus frequently exhibit crystal line arrangements. Cells infected with group B viruses also contain crystals composed of protein without nucleic acid. Virus particles remain within the cell after the cycle is complete and the cell is dead.
Species C adenoviruses establish latent infections in tonsils and adenoids of children, predominantly in T lymphocytes. Specimens from most young children contain viral DNA; however, it is less commonly detected in tissues from adolescents and adults. Adenovirus types 1, 2, and 5 are most commonly detected. Productive virus replication is rare in lymphocytes.
Human adenoviruses exhibit a narrow host range. When cells derived from species other than humans are infected, the human adenoviruses usually undergo an abortive replication cycle and no infectious progeny are produced.
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