Enterovirus capsid interactions with decay-accelerating factor mediate lytic cell infection

Receptors and Host Factors for Enterovirus Infection: Implications for Cancer Therapy

Enteroviruses, with their diverse clinical manifestations ranging from mild or asymptomatic infections to severe diseases such as poliomyelitis and viral myocarditis, present a public health threat. However, they can also be used as oncolytic agents. This review shows the intricate relationship between enteroviruses and host cell factors. Enteroviruses utilize specific receptors and coreceptors for cell entry that are critical for infection and subsequent viral replication. These receptors, many of which are glycoproteins, facilitate virus binding, capsid destabilization, and internalization into cells, and their expression defines virus tropism towards various types of cells. Since enteroviruses can exploit different receptors, they have high oncolytic potential for personalized cancer therapy, as exemplified by the antitumor activity of certain enterovirus strains including the bioselected non-pathogenic Echovirus type 7/Rigvir, approved for melanoma treatment. Dissecting the roles of individual receptors in the entry of enteroviruses can provide valuable insights into their potential in cancer therapy. This review discusses the application of gene-targeting techniques such as CRISPR/Cas9 technology to investigate the impact of the loss of a particular receptor on the attachment of the virus and its subsequent internalization. It also summarizes the data on their expression in various types of cancer. By understanding how enteroviruses interact with specific cellular receptors, researchers can develop more effective regimens of treatment, offering hope for more targeted and efficient therapeutic strategies.

Development of intravenously administered synthetic RNA virus immunotherapy for the treatment of cancer

The therapeutic effectiveness of oncolytic viruses (OVs) delivered intravenously is limited by the development of neutralizing antibody responses against the virus. To circumvent this limitation and to enable repeated systemic administration of OVs, here we develop Synthetic RNA viruses consisting of a viral RNA genome (vRNA) formulated within lipid nanoparticles. For two Synthetic RNA virus drug candidates, Seneca Valley virus (SVV) and Coxsackievirus A21, we demonstrate vRNA delivery and replication, virus assembly, spread and lysis of tumor cells leading to potent anti-tumor efficacy, even in the presence of OV neutralizing antibodies in the bloodstream. Synthetic-SVV replication in tumors promotes immune cell infiltration, remodeling of the tumor microenvironment, and enhances the activity of anti-PD-1 checkpoint inhibitor. In mouse and non-human primates, Synthetic-SVV is well tolerated reaching exposure well above the requirement for anti-tumor activity. Altogether, the Synthetic RNA virus platform provides an approach that enables repeat intravenous administration of viral immunotherapy.

Structure of Human Enterovirus 70 and Its Inhibition by Capsid-Binding Compounds

Enterovirus 70 (EV70) is a human pathogen belonging to the family Picornaviridae. EV70 is transmitted by eye secretions and causes acute hemorrhagic conjunctivitis, a serious eye disease. Despite the severity of the disease caused by EV70, its structure is unknown. Here, we present the structures of the EV70 virion, altered particle, and empty capsid determined by cryo-electron microscopy. The capsid of EV70 is composed of the subunits VP1, VP2, VP3, and VP4. The partially collapsed hydrophobic pocket located in VP1 of the EV70 virion is not occupied by a pocket factor, which is commonly present in other enteroviruses. Nevertheless, we show that the pocket can be targeted by the antiviral compounds WIN51711 and pleconaril, which block virus infection. The inhibitors prevent genome release by stabilizing EV70 particles. Knowledge of the structures of complexes of EV70 with inhibitors will enable the development of capsid-binding therapeutics against this virus.

IMPORTANCE Globally distributed enterovirus 70 (EV70) causes local outbreaks of acute hemorrhagic conjunctivitis. The discharge from infected eyes enables the high-efficiency transmission of EV70 in overcrowded areas with low hygienic standards. Currently, only symptomatic treatments are available. We determined the structures of EV70 in its native form, the genome release intermediate, and the empty capsid resulting from genome release. Furthermore, we elucidated the structures of EV70 in complex with two inhibitors that block virus infection, and we describe the mechanism of their binding to the virus capsid. These results enable the development of therapeutics against EV70.

Complexity and ultrastructure of infectious extracellular vesicles from cells infected by non-enveloped virus

Enteroviruses support cell-to-cell viral transmission prior to their canonical lytic spread of virus. Poliovirus (PV), a prototype for human pathogenic positive-sense RNA enteroviruses, and picornaviruses in general, transport multiple virions en bloc via infectious extracellular vesicles, 100~1000 nm in diameter, secreted from host cells. Using biochemical and biophysical methods we identify multiple components in secreted microvesicles, including mature PV virions; positive-sense genomic and negative-sense replicative, template viral RNA; essential viral replication proteins; and cellular proteins. Using cryo-electron tomography, we visualize the near-native three-dimensional architecture of secreted infectious microvesicles containing both virions and a unique morphological component that we describe as a mat-like structure. While the composition of these mat-like structures is not yet known, based on our biochemical data they are expected to be comprised of unencapsidated RNA and proteins. In addition to infectious microvesicles, CD9-positive exosomes released from PV-infected cells are also infectious and transport virions. Thus, our data show that, prior to cell lysis, non-enveloped viruses are secreted within infectious vesicles that also transport viral unencapsidated RNAs, viral and host proteins. Understanding the structure and function of these infectious particles helps elucidate the mechanism by which extracellular vesicles contribute to the spread of non-enveloped virus infection.

Respiratory protease/antiprotease balance determines susceptibility to viral infection and can be modified by nutritional antioxidants

The respiratory epithelium functions as a central orchestrator to initiate and organize responses to inhaled stimuli. Proteases and antiproteases are secreted from the respiratory epithelium and are involved in respiratory homeostasis. Modifications to the protease/antiprotease balance can lead to the development of lung diseases such as emphysema or chronic obstructive pulmonary disease. Furthermore, altered protease/antiprotease balance, in favor for increased protease activity, is associated with increased susceptibility to respiratory viral infections such as influenza virus. However, nutritional antioxidants induce antiprotease expression/secretion and decrease protease expression/activity, to protect against viral infection. As such, this review will elucidate the impact of this balance in the context of respiratory viral infection and lung disease, to further highlight the role epithelial cell-derived proteases and antiproteases contribute to respiratory immune function. Furthermore, this review will offer the use of nutritional antioxidants as possible therapeutics to boost respiratory mucosal responses and/or protect against infection.

Role of class I human leukocyte antigen molecules in early steps of echovirus infection of rhabdomyosarcoma cells

Several echoviruses use decay accelerating factor (DAF) as a cell surface receptor. However, most of them require additional cell surface coreceptors. We investigated the respective roles of DAF and class I human leukocyte antigen (HLA) molecules in the early steps of the echovirus 11 (EV11) lifecycle in rhabdomyosarcoma (RD) cells. EV11 infection was inhibited at an early stage by anti-beta2-microglobulin (beta2m) and anti-HLA monoclonal antibodies and by a soluble monochain HLA class I molecule. Expression of class I HLA molecules restored the early steps of the EV11 lifecycle, but its expression was not sufficient for EV11 replication and particle production. Expression of HLA class I molecules was associated with leukocyte cell line permissiveness to EV11 infection. In conclusion, HLA class I molecules are involved in the early steps of EV11 infection of RD cells and appear to participate in a complex interplay of surface molecules acting as coreceptors, including DAF.

dsRNA formed as an intermediate during Coxsackievirus infection does not induce NO production in a β-cell line with or without addition of IFN-γ

Virus infection is one environmental factor that has been implicated as a precipitating event initiating beta-cell damage during the development of type 1 diabetes. One aim of this study was to investigate how permissive an insulin-producing beta-cell line, RINm5F, is to enterovirus (EV) infections. A second aim was to study if the viral replicative intermediate, double-stranded RNA (dsRNA), together with IFN-gamma results in nitric oxide (NO) production. Monolayer cultures of RINm5F cells were not permissive to infection with seven different strains of EV. However, when the growth pattern of the beta-cell line changed and the cells started to grow as free-floating RIN cell clusters (RCC), all EV strains replicated. Immunostaining for the Coxsackie-adenovirus-receptor (CAR) detected the protein on the free-floating RIN cell clusters, but not on the RINm5F cells cultured as a monolayer of beta-cells. This shows that the CAR expression can change and/or the CAR protein can be redistributed on the cell surface as a consequence of altered growth pattern thus allowing viral replication in a previously non-permissive beta-cell line. As expected, NO production was significantly increased (p<0.05) by addition of synthetic dsRNA and IFN-gamma to the RCC. In contrast, the dsRNA formed during virus infection with a Coxsackievirus B4 strain (E2) with or without addition of IFN-gamma did not induce NO production in these cells. This indicates that synthetic dsRNA does not mimic a real viral infection in that respect, and suggests an NO-independent mechanism for virus-induced beta-cell damage.

Interaction with coxsackievirus and adenovirus receptor, but not with decay-accelerating factor (DAF), induces A-particle formation in a DAF-binding coxsackievirus B3 isolate

Although many coxsackie B viruses interact with decay accelerating factor (DAF), attachment to DAF by itself is not sufficient to initiate infection. We examined the early events in infection that follow virus interaction with DAF, and with the coxsackievirus and adenovirus receptor (CAR). Interaction with soluble CAR in a cell-free system, or with CAR on the surfaces of transfected cells, induced the formation of A particles; interaction with soluble or cell surface DAF did not. The results suggest that CAR, but not DAF, is capable of initiating the conformational changes in the viral capsid that lead to release of viral nucleic acid.

Novel role for decay-accelerating factor in coxsackievirus A21-mediated cell infectivity

Decay-accelerating factor (DAF) is involved in the cell membrane attachment of many human enteroviruses. Presently, further specific active roles of DAF in mediating productive cell infection and in the pathogenesis of natural enterovirus infection are poorly understood. In an attempt to more fully understand the role of DAF in lytic cell infection we examined the specific interactions of the prototype strain of coxsackievirus A21 (CVA21) with surface-expressed DAF. Investigations into discrete DAF-CVA21 interactions focused on viral binding; viral particle elution with respect to the parameters of time, temperature, and pH; and subsequent cell infection. Radiolabeled-virus binding assays revealed that peak elution of CVA21 from DAF occurred within 15 min of initial attachment and that the DAF-eluted virus increased in a linear fashion with respect to temperature and pH. CVA21 eluted from endogenous surface-expressed DAF was highly infectious, in contrast to CVA21 eluted from intercellular adhesion molecule 1 (ICAM-1), which retained little to no infectivity. Using an adenovirus transduction system, we demonstrate that CVA21 can remain infectious for up to 24 h after DAF binding and is capable of initiating a multicycle lytic infection upon delayed ICAM-1 surface expression. Taken together, the data suggest that a major role of DAF in cell infection by the prototype strain of CVA21 is to provide membrane concentration of infectious virions, effectively increasing viral interactions with endogenous or induced ICAM-1.

Enhanced cellular receptor usage by a bioselected variant of coxsackievirus a21

Decay-accelerating factor (DAF) functions as cell attachment receptor for a wide range of human enteroviruses. The Kuykendall prototype strain of coxsackievirus A21 (CVA21) attaches to DAF but requires interactions with intercellular cell adhesion molecule 1 (ICAM-1) to infect cells. We show here that a bioselected variant of CVA21 (CVA21-DAFv) generated by multiple passages in DAF-expressing, ICAM-1-negative rhabdomyosarcoma (RD) cells acquired the capacity to induce rapid and complete lysis of ICAM-1-deficient cells while retaining the capacity to bind ICAM-1. CVA21-DAFv binding to DAF on RD cells mediated lytic infection and was inhibited by either antibody blockade with a specific anti-DAF SCR1 monoclonal antibody (MAb) or soluble human DAF. Despite being bioselected in RD cells, CVA21-DAFv was able to lytically infect an additional ICAM-1-negative cancer cell line via DAF interactions alone. The finding that radiolabeled CVA21-DAFv virions are less readily eluted from surface-expressed DAF than are parental CVA21 virions during a competitive epitope challenge by an anti-DAF SCR1 MAb suggests that interactions between CVA21-DAFv and DAF are of higher affinity than those of the parental strain. Nucleotide sequence analysis of the capsid-coding region of the CVA21-DAFv revealed the presence of two amino acid substitutions in capsid protein VP3 (R96H and E101A), possibly conferring the enhanced DAF-binding phenotype of CVA21-DAFv. These residues are predicted to be embedded at the interface of VP1, VP2, and VP3 and are postulated to enhance the affinity of DAF interaction occurring outside the capsid canyon. Taken together, the data clearly demonstrate an enhanced DAF-using phenotype and expanded receptor utilization of CVA21-DAFv compared to the parental strain, further highlighting that capsid interactions with DAF alone facilitate rapid multicycle lytic cell infection.