Hantavirus Regulation of Type I Interferon Responses

Abstract

Hantaviruses primarily infect human endothelial cells (ECs) and cause two highly lethal human diseases. Early addition of Type I interferon (IFN) to ECs blocks hantavirus replication and thus for hantaviruses to be pathogenic they need to prevent early interferon induction. PHV replication is blocked in human ECs, but not inhibited in IFN deficient VeroE6 cells and consistent with this, infecting ECs with PHV results in the early induction of IFNβ and an array of interferon stimulated genes (ISGs). In contrast, ANDV, HTNV, NY-1V and TULV hantaviruses, inhibit early ISG induction and successfully replicate within human ECs. Hantavirus inhibition of IFN responses has been attributed to several viral proteins including regulation by the Gn proteins cytoplasmic tail (Gn-T). The Gn-T interferes with the formation of STING-TBK1-TRAF3 complexes required for IRF3 activation and IFN induction, while the PHV Gn-T fails to alter this complex or regulate IFN induction. These findings indicate that interfering with early IFN induction is necessary for hantaviruses to replicate in human ECs, and suggest that additional determinants are required for hantaviruses to be pathogenic. The mechanism by which Gn-Ts disrupt IFN signaling is likely to reveal potential therapeutic interventions and suggest protein targets for attenuating hantaviruses.

Hantavirus GnT Elements Mediate TRAF3 Binding and Inhibit RIG-I/TBK1-Directed Beta Interferon Transcription by Blocking IRF3 Phosphorylation

“…Gc contains a short (Ͻ10-residue) cytoplasmic tail, while Gn contains a 142-residue cytoplasmic tail (GnT) with several potential functions in the viral life cycle (5). The GnT directs binding of viral nucleocapsid complexes, has matrix protein functions that nucleate viral assembly and budding (23), and with the exception of the PHV GnT, regulates RIG-I-directed beta interferon (IFN-␤) induction (19,20,(24)(25)(26).…”

“…Hantavirus GnGc and GnT proteins have been shown to regulate IFN induction in response to the expression of pathwayspecific RIG-I and TBK1 inducers, suggesting a mechanism for early IFN regulation within hantavirus-infected endothelial cells (19,20,24,25). Expression of GnGc or GnT proteins from NY-1V, ANDV, TULV, and HTNV, but not PHV, results in the regulation of RIG-I/TBK1-directed ISRE, B, and IFN-␤ transcriptional responses (19)(20)(21).…”

“…Degrons at the C termini of NY-1V, HTNV, and ANDV GnTs direct ubiquitination and degradation of GnTs and further suggest a means by which GnTs might regulate ubiquitination-sensitive TRAF3-TBK1 complexes (26). However, the TULV GnT regulates RIG-I/TBK1-directed IFN-␤ induction yet lacks a degron domain (24,25). These findings question whether degrons are required for TRAF3 binding and whether TBK1 inhibition is a function of discrete elements within hantavirus GnTs.…”

“…Using pathway-specific inducers, ANDV GnGc and GnT proteins also inhibit RIG-I-and TBK1-directed ISRE, B, and IFN-␤ transcription and IRF3 phosphorylation (Fig. 1, 2, and 10), while N proteins from NY-1V and PHV have no effect on RIG-I-induced ISRE or IFN induction (19,20,24,25). One study indicates that the ANDV GnGc as well as SNV and ANDV N proteins fail to regulate Sendai virus (SeV)-induced IFN responses (68), but when coexpressed, the ANDV N and GnGc were partially inhibitory (68).…”

“…Depending on the proteins, cells, and IFN assays used to evaluate regulation, several hantavirus proteins or complexes have been suggested to regulate IFN induction (19,20,24,25,47,67,68). Reports demonstrate that the NY-1V, TULV, or SNV GnGc proteins or GnTs inhibit IFN induction and that inhibition is blocked downstream of RIG-I and upstream of constitutively activated IRF3, at the level of the TBK1 complex (19,20,24,25). Using pathway-specific inducers, ANDV GnGc and GnT proteins also inhibit RIG-I-and TBK1-directed ISRE, B, and IFN-␤ transcription and IRF3 phosphorylation (Fig.…”