Respiratory enterovirus D68: virology, clinical surveillance, host-pathogen interactions, and therapeutic prospects

Figure 1. Diagrammatic representation of EV-D68 architecture. The virion of EV-D68 consists of an icosahedral capsid shell surrounding a single strand of a positive-sense RNA genome, with a diameter around 30 nm. Its capsid is composed of 60 protomers, each comprising four structural proteins designated VP1 through VP4. Five protomers assemble to form a pentamer, and twelve such pentamers then form the icosahedral shell, concomitantly packaging the viral RNA genome. Within mature virions, VP4 is located internally, whereas VP1, VP2, and VP3 comprise the external surface of the particle. At the capsid vertices, these latter proteins converge to establish alternating three-fold and five-fold symmetry axes. Near the five-fold axis vertices, a conserved structural feature termed the “canyon” is present. At the base of this canyon, each VP1 subunit contains a hydrophobic pocket that accommodates a host-derived lipid-like molecule, commonly referred to as the “pocket factor.” VPg, virus encoded protein.

Figure 2. Schematic illustration of the enterovirus genome and polyprotein processing. The genome of EV-D68 is a 7.4 kb single stranded positive-sense RNA. It comprises a single open reading frame (ORF) flanked by 5’ and 3’ untranslated regions (UTRs), which contain an internal ribosome entry site (IRES) and a polyadenylate tail, respectively. The polyprotein encoded within the enterovirus genome undergoes sequential proteolytic cleavage mediated by the viral proteases 2Apro and 3Cpro, leading to the production of 4 mature structural proteins (VP1-VP4) and 7 mature non-structural proteins (2A-2C, 3A-3D).

Figure 3. Depiction of the life cycle of EV-D68. EV-D68 initiates infection by attaching to the host cell membrane through interactions with specific receptors, including MFSD6, ICAM5, and sialic acid, facilitating its internalization via endocytosis (steps ①-②). Subsequently, the viral particle releases its genomic RNA into the cytoplasm through pores formed in the endosomal membrane (step ③). The viral RNA undergoes translation followed by co-translational proteolytic cleavage, producing eleven principal proteins. These include structural proteins VP1 through VP4, which are essential for capsid assembly, and non-structural proteins 2A through 3D, which are involved in viral RNA replication (step ④). Genome replication occurs on specialized membranous replication organelles, beginning with the synthesis of a negative-strand (−) RNA intermediate. This (−) RNA serves as a template for the production of new positive-strand (+) RNA genomes by the viral RNA-dependent RNA polymerase (3D) (step ⑤). Finally, the newly synthesized viral RNA and capsid proteins are assembled into mature virions, which are subsequently released from the host cell either via extracellular vesicles or through direct…

Figure 4. Interactions between EV-D68 and the host innate immune responses. Upon EV-D68 infection, toll-like receptors (TLR), such as TLR3 and TLR7, along with retinoic acid-inducible gene I-like receptors (RLRs), including RIG-I and MDA5, recognize viral RNA or its replicative intermediates. This facilitates the recruitment of downstream adaptors proteins, namely MyD88/TRIF for TLRs and MAVS for RLRs. Subsequently, these adaptor proteins initiate the nuclear translocation of transcription factors IRF3/7 and NF-κB, leading to the induction of interferons (IFNs) and interleukins (ILs). Besides, EV-D68 infection induces mitochondrial damage, releasing mitochondrial DNA into the cytoplasm and activating STING-mediated antiviral signaling. Additionally, EV-D68 3Cpro cleaves host NLRP1 at its N-terminus, thereby releasing the C-terminal fragment to initiate inflammasome activation. To promote EV-D68 propagation in host cells, viral factors such as 2Apro, 3Cpro and VP3, are known to target TLR, RLR, STING pathways to counteract antiviral innate immunity. RIG-I, retinoic acid-inducible gene I; MDA5, melanoma differentiation-associated protein 5; MyD88, myeloid differentiation primary response 88;…