Human Adenovirus Vaccines

Adenovirus Types

Within the genus Mastadenovirus , HAdVs are grouped into seven species (A through G) containing numerous unique types. Defining type uniqueness has been a matter of some professional debate with some virologists recognizing 51 unique serotypes through complex classical immunotyping techniques and others recognizing more than 110 unique genotypes through partial or full genome sequencing ( http://hadvwg.gmu.edu/ ). Adenoviruses may be described through their host, their species, their numeric type, and sometimes their subtype (stratified by restriction enzyme digests or phylogenetic data). HAdVs affect most organ systems and individual HAdV types often have preferred tissue tropisms that lead to specific syndromes ( Table 11.1 ).

Virology

AdVs are nonenveloped, double-stranded DNA viruses belonging to the genus Mastadenovirus , family Adenoviridae . HAdVs were originally classified by species (A–G) through hemagglutination properties and by serotype (1–51) with horse or rabbit antisera ( Table 11.2 ). ^{, ,} Different strains within serotypes can be further distinguished by whole-genome restriction-enzyme digest patterns. However, as serum typing reagents have become unavailable and sequencing costs have decreased, full genome sequencing has become the method of choice for classifying HAdV types. This has been used to describe as many as 111 unique HAdV types ( http://hadvwg.gmu.edu/ ).

AdVs have an estimated diameter of approximately 920 Å. The viruses have an icosahedral capsid shell that is made up of 240 hexon bases, 12 penton bases, and 12 fibers that are associated with the penton bases ( Fig. 11.1 ). Four minor proteins (IIIa, VI, VIII, and IX) add to the complexity of the capsid.

AdV hexons are both type-specific and species-specific antigens, primarily inducing species-specific complement-fixing antibodies, whereas the fibers are especially active in hemagglutination. ^, The fibers also evoke type-specific antibodies, vary in length among human strains, and are sometimes absent in particular animal strains. The genome core of the virus is composed of five more proteins (V, VII, u, Iva2, and terminal protein) and a single molecule of linear, double-stranded DNA of 26 × 106 to 45 × 106 molecular weight. The G + C base compositions of the human virus genomes range from 47% to 60%.

AdVs are unusually stable to physical and chemical agents, as well as adverse pH, and can survive for long periods outside the host, making them available for transmission to others. They can be destroyed by heat at 56°C for 30 minutes, UV irradiation, 0.25% sodium dodecyl sulfate, chlorine at 0.5 µg/mL, and formalin, but are resistant to ether and chloroform.

AdVs replicate in the cell nucleus and tend to be very host-specific. However, there is mounting evidence that AdVs cross-species and occasionally humans and animals have exchanged AdVs.

Some species A HAdVs have been determined to be oncogenic in animals and to transform cell lines, but oncogenicity has not been observed in humans. Genetic recombination between HAdV types may occur. Sometimes, these recombinant strains or other emergent novel adenovirus strains lead to epidemics. A number of recent examples have been reported in the medical literature. ^,

Pathogenesis as It Relates to Prevention

HAdV infections can be asymptomatic or cause disease depending on the route of inoculation, the type of virus, and the immune state of the host. Respiratory infection is presumed to result from inhalation of aerosolized virus, whereas ocular infection, gastroenteritis, and nosocomial infections may arise from fomites, water, or fecal–oral contact. Reactivation of latent HAdV is also believed to occur. Some 50% to 80% of surgically removed tonsils have detectable HAdV DNA by PCR, suggesting that these viruses may remain in a latent state for years. ^, Virus has been also isolated from lymphocytes, kidney, blood, cerebrospinal fluid, and most body organs. ^{, , ,} In the lungs, extensive pathology has been found with microscopic necrosis of the tracheal and bronchial epithelium. Acidophilic intranuclear inclusions are seen in bronchial epithelial cells in addition to the basophilic masses of cells surrounded by clear halos, which may indicate aggregations of viral material. A mononuclear infiltrate, rosette formation, and focal necrosis of mucous glands are characteristically seen.

HAdVs can interact with cells in three ways. A lytic infection may take place during which the virus completes an entire replicative cycle resulting in cell death. Between 10 ⁵ and 10 ⁶ progeny viruses per cell are produced, of which only 1–5% are actually infectious. The second type of interaction is chronic or latent infection, in which small amounts of virus are produced, and the cell survives. Viral shedding from the gastrointestinal tract may occur for years. In fact, reactivation of such latent infection likely explains much HAdV disease in the severely compromised. Correspondingly, monitoring viral loads in the stool of transplant patients has predicted disseminated disease and been useful in guiding antiviral therapy. In addition to aerosolization, intestinal shedding of HAdV is an important factor to consider in the prevention of nosocomial spread in hospitals and chronic care homes. ^{, ,} Persistent infection has been reported in epithelial cells from monkeys. Lymphoid cells are thought to be the reservoir for these persistent infections. ^, The third type of interaction is oncogenic transformation, whereby the viral DNA integrates into the host genome and replicates with the cellular host DNA, but only the early steps in the viral cycle occur and no infectious virions are produced.

The genes from adenoviruses are expressed in the cell nucleus in two phases: “early” (E), which precedes viral DNA replication, and “late.” Early genes encode proteins that function to thwart immunosurveillance, especially those from the E3 transcription unit. The late genes primarily encode viral structural proteins. The functions of the E1 proteins include the induction of DNA synthesis in quiescent cells, immortalization of primary cells in cooperation with activated ras or with the E1B proteins, transactivation of delayed early genes, induction or repression of several cellular genes, and induction of apoptosis. These proteins modulate the sensitivity of HAdV-infected cells to tumor necrosis factor (TNF), a key inflammatory cytokine with antiviral properties. None of the E3 genes are required for adenovirus replication in cultured cells, but several of the E3-coded proteins (10.4 K, 14.5 K, and 14.7 K) inhibit TNF cytolysis. ^{, ,} Because a major function of TNF may be to prevent viral replication, the inhibition of TNF by these viral proteins may be a significant mechanism of pathogenesis.

Another significant E3-coded protein is Gp 19 K. This glycoprotein is located in the endoplasmic reticulum and forms a complex with class I antigens of the major histocompatibility complex (MHC), preventing cells from being killed by cytotoxic T lymphocytes. A cotton rat animal model was used by Ginsberg and colleagues and Ginsberg and Prince to study the pathogenesis of HAdV-2 and -5, which cause pneumonia similar to that seen in humans. ^, Two phases of infection were seen; the initial phase, characterized by the infiltration of monocytes and neutrophils, and a later phase associated with the infiltration of lymphocytes. The pathology seemed to reflect the response by host immune defenses to viral infection. The Gp 19 K markedly reduced the transport of the class I MHC to the surface of the infected cells and impeded the attack of cytotoxic T cells. ^{, ,} It is now known that only the early genes are required to induce the complete pathogenesis of adenovirus infection in cotton rats. Although several cytokines, such as TNF-α, interleukin-1, and interleukin-6, were elaborated during the first 2–3 days of the infection in the cotton rat model, only TNF-α had a major role in pathogeneis. Steroids almost completely eliminated the pneumonic inflammatory response to infection.

Pathology caused by latent infection with HAdVs has been linked to chronic obstructive pulmonary disease (COPD). ^, Some have suggested that childhood viral diseases represent an independent risk factor for COPD. The adenoviral E1A proteins can stimulate the transcription of many heterologous viral and cellular genes. These proteins have the ability to interact with the DNA binding domains of several cellular transcription factors and activate a wide variety of genes. ^, The HAdV genome has been detected in the lungs of more patients with COPD than in control subjects. E1A proteins are expressed in epithelial cells of human lung tissue, and by increasing the expression of several genes important in controlling the inflammatory process, these may contribute to the pathogenesis of COPD. The events described may amplify the airway inflammation associated with cigarette smoking.

The isolation and cloning of a 46-kDa protein HAdV receptor, which mediates attachment and infection of species B viruses, may facilitate the development of new strategies to limit HAdV disease. The more common receptor is referred to as CAR. This protein has been identified as the receptor for the coxsackie B virus and HAdVs. Recently, another receptor, desmoglein 2, was identified for HAdV-3, -7, -11, and -14.

Laboratory Diagnosis

HAdV infections generally cannot be diagnosed on clinical grounds alone because the clinical signs and symptoms of these infections are variable and often resemble those caused by other pathogens. Qualified personnel and laboratory support are necessary to accurately diagnose HAdV infections. Considerations include specimen type and timing, collection and storage procedures, types of laboratory tests performed, and availability of appropriate diagnostic assays.