Enterovirus A71 does not meet the uncoating receptor SCARB2 at the cell surface.

Enterovirus A71 (EV-A71) infection involves a variety of receptors. Among them, two transmembrane protein receptors have been investigated in detail and shown to be critical for infection: P-selectin glycoprotein ligand-1 (PSGL-1) in lymphocytes (Jurkat cells), and scavenger receptor class B member 2 (SCARB2) in rhabdomyosarcoma (RD) cells. PSGL-1 and SCARB2 have been reported to be expressed on the surface of Jurkat and RD cells, respectively. In the work reported here, we investigated the roles of PSGL-1 and SCARB2 in the process of EV-A71 entry. We first examined the expression of SCARB2 in Jurkat cells, and detected it within the cytoplasm, but not on the cell surface. Further, using PSGL-1 and SCARB2 knockout cells, we found that although both PSGL-1 and SCARB2 are essential for virus infection of Jurkat cells, virus attachment to these cells requires only PSGL-1. These results led us to evaluate the cell surface expression and the roles of SCARB2 in other EV-A71-susceptible cell lines. Surprisingly, in contrast to the results of previous studies, we found that SCARB2 is absent from the surface of RD cells and other susceptible cell lines we examined, and that although SCARB2 is essential for infection of these cells, it is dispensable for virus attachment. These results indicate that a receptor other than SCARB2 is responsible for virus attachment to the cell and probably for internalization of virions, not only in Jurkat cells but also in RD cells and other EV-A71-susceptible cells. SCARB2 is highly concentrated in lysosomes and late endosomes, where it is likely to trigger acid-dependent uncoating of virions, the critical final step of the entry process. Our results suggest that the essential interactions between EV-A71 and SCARB2 occur, not at the cell surface, but within the cell.

An efficient trans complementation system for in vivo replication of defective poliovirus mutants.

The picornavirus genome encodes a large, single polyprotein that is processed by viral proteases to form an active replication complex. The replication complex is formed with the viral genome, host proteins, and viral proteins that are produced/translated directly from each of the viral genomes (viral proteins provided in cis). Efficient complementation in vivo of replication complex formation by viral proteins provided in trans, thus exogenous or ectopically expressed viral proteins, remains to be demonstrated. Here, we report an efficient trans complementation system for the replication of defective poliovirus (PV) mutants by a viral polyprotein precursor in HEK293 cells. Viral 3AB in the polyprotein, but not 2BC, was processed exclusively in cis. Replication of a defective PV replicon mutant, with a disrupted cleavage site for viral 3Cpro protease between 3Cpro and 3Dpol (3C/D[A/G] mutant) could be rescued by a viral polyprotein provided in trans. Only a defect of 3Dpol activity of the replicon could be rescued in trans; inactivating mutations in 2CATPase/hel, 3B, and 3Cpro of the replicon completely abrogated the trans-rescued replication. An intact N-terminus of the 3Cpro domain of the 3CDpro provided in trans was essential for the trans-active function. By using this trans complementation system, a high-titer defective PV pseudovirus (PVpv) (>107 infectious units per mL) could be produced with the defective mutants, whose replication was completely dependent on trans complementation. This work reveals potential roles of exogenous viral proteins in PV replication and offers insights into protein/protein interaction during picornavirus infection. IMPORTANCE: Viral polyprotein processing is an elaborately controlled step by viral proteases encoded in the polyprotein; fully processed proteins and processing intermediates need to be correctly produced for replication, which can be detrimentally affected even by a small modification of the polyprotein. Purified/isolated viral proteins can retain their enzymatic activities required for viral replication, such as protease, helicase, polymerase, etc. However, when these proteins of picornavirus are exogenously provided (provided in trans) to the viral replication complex with a defective viral genome, replication is generally not rescued/complemented, suggesting the importance of viral proteins endogenously provided (provided in cis) to the replication complex. In this study, I discovered that only the viral polymerase activity of poliovirus (PV) (the typical member of picornavirus family) could be efficiently rescued by exogenously expressed viral proteins. The current study reveals potential roles for exogenous viral proteins in viral replication and offers insights into interactions during picornavirus infection.

Nasal and pharyngeal mucosal immunity to poliovirus in children following routine immunization with inactivated polio vaccine (IPV) in the United States of America.

BACKGROUND: Although polioviruses (PVs) replicate in lymphoid tissue of both the pharynx and ileum, research on polio vaccine-induced mucosal immunity has predominantly focused on intestinal neutralizing and binding antibody levels measured in stool. METHODS: To investigate the extent to which routine immunization with intramuscularly injected inactivated polio vaccine (IPV) may induce nasal and pharyngeal mucosal immunity, we measured PV type-specific neutralization and immunoglobulin (Ig) G, IgA, and IgM levels in nasal secretions, adenoid cell supernatants, and sera collected from 12 children, aged 2 to 5 years, undergoing planned adenoidectomies. All participants were routinely immunized with IPV and had no known contact with live PVs. RESULTS: PV-specific mucosal neutralization was detected in nasal and adenoid samples, mostly from children who had previously received four IPV doses. Across the three PV serotypes, both nasal (Spearman's rho ≥ 0.87, p≤0.0003 for all) and adenoid (Spearman's rho ≥0.57, p≤0.05 for all) neutralization titers correlated with serum neutralization titers. In this small study sample, there was insufficient evidence to determine which Ig isotype(s) was correlated with neutralization. CONCLUSIONS: Our findings provide policy-relevant evidence that routine immunization with IPV may induce nasal and pharyngeal mucosal immunity. The observed correlations of nasal and pharyngeal mucosal neutralization with serum neutralization contrast with previous observations of distinct intestinal and serum responses to PV vaccines. Further research is warranted to determine which antibody isotype(s) correlate with polio vaccine-induced nasal and pharyngeal mucosal neutralizing activity and to understand the differences from intestinal mucosal immunity.