Research Progress on the Pathogenic Mechanism of Human Adenovirus

The adenovirus (AdV) family consists of two genera, mammalian AdV and avian AdV, of which human adenovirus (HAdV) is a member of the mammalian AdV genus. Rowe et al. isolated and cultured a new virus from the atrophic tonsil tissue of healthy people in 1953, which was the first discovery of HAdV in human history. Hilleman et al. isolated the same virus from the throat lavage fluid of patients with acute respiratory tract infection in 1954. In 1956, BALDUCCI et al. named it HAdV based on the fact that the virus often exists in glands and was first isolated from glandular tissue. Initially, HAdV was divided into 69 HAdV serotypes based on its ability to be neutralized by specific animal antisera. Based on its ability to agglutinate human, rat and monkey red blood cells and its carcinogenicity in rodents, HAdV can be further subdivided into 7 subgroups (A~G). There are some specific correlations between different subgroups and their tissue tropism and clinical characteristics. Subgroups B1, C and E mainly cause respiratory diseases, while subgroups B, D and E can induce eye diseases. Subgroup F is the cause of gastroenteritis, and subgroup B2 mainly infects the urinary tract, bladder and kidneys.

Figure 1. Schematic of the HAdV-5 virion. (Sources: Jennings MR, et al.; 2023)

HAdV is a non-enveloped double-stranded DNA virus that often infects the human respiratory tract, gastrointestinal tract, urinary tract and conjunctiva, causing disease in related parts. HAdV infection outbreaks usually occur in healthy children, adults or people living in crowded or closed places. When patients with impaired immune function (such as congenital immunodeficiency syndrome, human immunodeficiency virus (HIV) infection, and organ transplant infection with HAdV) are infected, their clinical manifestations are often more severe and diffuse infection is stronger. HAdV basic structural characterization According to the results of X-ray and transmission electron microscopy, HAdV is a non-enveloped double-stranded DNA virus with a diameter of about 90-100 nm and a genome length of about 36 kb, which can encode more than 40 different proteins. The AdV genome contains the early transcribed E1A, E1B, E2A, E2B, E3, and E4 genes, as well as the late expressed HAdV assembly-related L1 (pⅢa), L2 (pⅤ, pⅦ, pⅩ, Penton), L3 (pⅥ, Hexon, EP), L4 (pⅧ), and L5 genes. The HAdV virus particle is spherical in structure and has no envelope. The capsid contains the virus core, which is composed of naked double-stranded linear DNA and core protein. The capsid is composed of 252 shell particles and has an icosahedral symmetrical structure. Each face of the icosahedron is composed of 12 repeated copies of pseudo-hexagonal trimer hexons, and the 12 corner vertices are formed by pentamers. Each oligomer has one or more copies of non-covalently bound trimer fiber spikes. Each oligomer is composed of 4 loops (loop1-4), and the base of the oligomer contains P1 and P2 parts. The fibrous protrusion is a linear protrusion extending from the surface of the capsid to the base of each penton. The length is about 8.5-76.5 nm, including the head region and the handle structure fiber. Its length depends on the number of β-helix repeats on the axis and the type of ADV. The length and shape of the fiber, as well as the sequence variation of the three major capsid protein hypervariable regions, at least partially determine the entry pathways and immunological characteristics of different serotypes.

HAdV has 13 structural proteins, including 3 major capsid proteins (trimeric hexon, penton oligomer, fibrous protrusions), 6 core proteins (V, VII, Mu, TP, IV, Caspases) and 4 minor capsid proteins (IIIa, VIII, VI, IX). The function of major capsid proteins is to mediate the process of viral internalization, while minor capsid proteins help stabilize viral particles. Both are involved in the process of viral uncoating and endosome destruction. The purpose of core proteins is to transfer viral DNA to the cell nucleus, so that viral DNA proliferates with the replication of host cells, and assists in the assembly of viral DNA and capsid.

Epidemiological Characteristics of HAdV Infection

HAdV can infect the human respiratory tract, gastrointestinal tract, urinary tract and conjunctiva and cause disease in related parts. When the human immune function is normal, the symptoms after infection with HAdV are mild, often self-healing, and self-limiting. There are many serotypes of HAdV. Different types of HAdV have different tissue tropisms, resulting in different clinical manifestations caused by different HAdV serotypes in each subgroup.

Mechanism of Tissue Damage Caused by HAdV

In the innate immune response, (1) the E1A gene of HAdV activates E2F1 and the subsequent upregulation of E2F1-induced miRNA-93 and miRNA-106b effectively inhibits p21 expression, leading to enhanced apoptosis and autophagy induced by p53, causing tissue cell damage. (2) After HAdV infects host cells, host cells will undergo adaptive autophagy, leading to disordered host cell autophagy regulation; at the same time, after HAdV enters the lysosome, it will change the conformation of the AdV capsid in an acidic environment, and the released acrosomal protein can destroy the endosomal membrane, jointly causing cell necrosis. (3) HAdV directly activates macrophages and dendritic cells (DCs) through Toll-like receptors (TLRs) and non-TLR-dependent pathways, promoting the synthesis and release of proinflammatory cytokines, promoting the maturation of interleukin (IL)-1β, and promoting the secretion of IL-6, tumor necrosis factor-α (TNF-α), and interferon-α (IFN-α). The production of IFN-α is strongly dependent on the cytoplasmic DNA sensor cyclic GMP-AMP synthase (CGAS) or the cytoplasmic DNA sensor TLR9 that recognizes unmethylated CpG double-stranded DNA. These cytokines further accelerate the infiltration of leukocytes, leading to tissue cell destruction and necrosis. (4) HAdV can activate the inflammatory mediator NALP3 in macrophages, which participates in the maturation of host cell phagosomes and phagolysosomes, ultimately causing tissue cell damage. HAdV can promote the release of high mobility group protein B1 (HMGB1) to the extracellular space, which binds to transmembrane proteins TLR and receptor for advanced glycosylation end products (RAGE) on the cell surface, activates intracellular MAPK, NF-κB and other cell pathways, and further promotes the synthesis and release of inflammatory factors such as IL-6, IL-8, and TNF-α, causing late inflammatory reactions and aggravating tissue cell damage. The adaptive immune response is as follows: (1) HAdV-induced activated cytotoxic T lymphocytes (CTL) promote target cell apoptosis by releasing perforin, serine ester alcohol and inducing the expression of death receptors FAS/FASL, dissolving host cells infected with HAdV and causing host cell damage. (2) HAdV infection promotes the formation of plasma cells. The Fc segment of IgE antibodies produced by plasma cells binds to mast cells, eosinophils, etc. When the body is exposed to allergens again, the allergens specifically bind to the Fab segment of IgE antibodies, promoting mast cells, eosinophils, etc. to release inflammatory mediators and cytokines. (3) HAdV infection causes a decrease in the ratio of CD4+/CD8+, further causing an imbalance in the ratio of Th1/Th2 subpopulations, which in turn causes an imbalance in cytokines in the body, promotes eosinophil infiltration, and causes variant inflammation. α-Gliadin is an α-Gliadin component known to activate celiac disease. The E1b protein of HAdV-12 has amino acid homology with α-Gliadin. When the body is infected with HAdV, antibodies to the E1b protein in adaptive immunity will cross-react with α-Gliadin, activate α-Gliadin, and ultimately cause the occurrence of celiac disease. HAdV infection can reduce the excitability of β-adrenergic nerve fibers, sensitize M cholinergic receptors, increase the excitability of parasympathetic cholinergic fibers and weaken the inhibitory non-adrenergic non-cholinergic (i-NANC) effect, thereby stimulating airway smooth muscle contraction, inhibiting relaxation, and ultimately inducing the occurrence of asthma. RNA molecules produced during the HAdV proliferation cycle can be recognized by TLRs and intracellular viral sensors such as protein catalytic enzymes (PKRs), thereby activating different signaling pathways and downstream effectors, the latter of which induce the production of MUC5AC mucin, leading to excessive secretion of airway epithelial mucus and aggravating asthma symptoms.