Establishment of a CPER reverse genetics system for Powassan virus defines attenuating NS1 glycosylation sites and an infectious NS1-GFP11 reporter virus

Passage-attenuated Powassan virus LI9P protects mice from lethal LI9 challenge and links envelope residue D308 to neurovirulence

Powassan virus (POWV) is an emergent tick-borne flavivirus that causes lethal encephalitic disease and chronic neurologic deficits in surviving patients. POWV-LI9 is a tick-derived isolate that causes neurovirulent disease and age-dependent lethality in mice. Serial passage of VeroE6 cells infected with LI9 resulted in eight amino acid changes in a POWV strain LI9P. LI9P fails to cause neurological sequelae, or lethality in C57BL/6 mice yet elicits neutralizing POWV antibody responses and protects mice from lethal LI9 challenge. Analysis revealed that LI9, but not LI9P, is present at high levels in the CNS, suggesting that LI9P is restricted from neuroinvasion or CNS replication. LI9 and LI9P are distinguished by a D308N envelope change within a domain associated with cell attachment. We evaluated the roles of Env-Domain III residue changes in LI9 virulence and LI9P attenuation using recombinant POWVs (recPOWVs) generated by reverse genetics. Remarkably, mutating D308N in LI9 completely abolished viral lethality and neuroinvasion in 50-week-old mice, reflecting the avirulent phenotype of LI9P. Analysis of the reciprocal N308D change in LI9P only partially restored neuroinvasion and lethality to the LI9P-N308D mutant, indicating that further LI9P residue changes contribute to LI9P attenuation. Consistent with differences in neuroinvasion, we found that rapid LI9P RNA synthesis and corresponding early IFN induction may contribute to LI9P clearance. Collectively, these findings define D308 as a determinant of POWV neuroinvasion and lethality, suggest potential mechanisms for restricted LI9P CNS entry, and reveal passage-attenuated LI9P as a candidate POWV vaccine platform.

IMPORTANCE

Powassan virus (POWV) infection causes a 10% lethal encephalitis, resulting in chronic neurological symptoms in half of survivors. POWV is transmitted in as short as 15 min following tick attachment, demonstrating the need for the development of POWV vaccines and therapeutics. Mechanisms of POWV neurovirulence remain to be defined to inform vaccine and therapeutic design. Cell culture passage has successfully been used to generate live-attenuated flavivirus vaccines. Accordingly, we serially passaged POWV LI9-infected VeroE6 cells and isolated an attenuated POWV strain, LI9P, that fails to cause neurologic sequelae or murine lethality. LI9P elicits neutralizing antibody responses, protects mice from a lethal WT POWV challenge, and is a potential POWV vaccine. Analysis of attenuating mutations in LI9P revealed that changing envelope residue D308N alone in LI9 prevents POWV neurovirulence and lethality in immunocompetent mice. Altogether, this study defines viral determinants of POWV pathogenesis and attenuating mutations that inform the development of live-attenuated POWV vaccines.

Imaging of viral replication in live cells by using split fluorescent protein-tagged reporter flaviviruses

The knowledge on the life cycle of flaviviruses is still incomplete, and no direct-acting antivirals against their infections are clinically available. Herein, by screening via a Zika virus (ZIKV) replicon assay, we found that the N-terminus of NS2A exhibited great tolerance to the insertions of different split fluorescent proteins (split-FPs). Furthermore, both ZIKV and dengue virus encoding a split-FP-tagged NS2A propagated efficiently, and the split-FP-tagged ZIKVs had good genetic stability. Robust green fluorescence was observed in the reporter cell lines infected with these viruses and the fluorescence responded to anti-flavivirus chemicals with high specificity and sensitivity. Moreover, the sites of viral RNA replication were illuminated in live cells. Interestingly, by blocking viral RNA synthesis with an NS5 inhibitor, we found a correlation between the morphological characteristics of potential replication organelles and RNA amplification, highlighting that the NS2A-tagged viruses are of great value for the in-depth understanding of flavivirus replication mechanisms.

Age-dependent Powassan virus lethality is linked to glial cell activation and divergent neuroinflammatory cytokine responses in a murine model

Powassan virus (POWV) is an emergent tick-borne flavivirus that causes fatal encephalitis in the elderly and long-term neurologic sequelae in survivors. How age contributes to severe POWV encephalitis remains an enigma, and no animal models have assessed age-dependent POWV neuropathology. Inoculating C57BL/6 mice with a POWV strain (LI9) currently circulating in Ixodes ticks resulted in age-dependent POWV lethality 10-20 dpi. POWV infection of 50-week-old mice was 82% fatal with lethality sequentially reduced by age to 7.1% in 10-week-old mice. POWV LI9 was neuroinvasive in mice of all ages, causing acute spongiform CNS pathology and reactive gliosis 5-15 dpi that persisted in survivors 30 dpi. High CNS viral loads were found in all mice 10 dpi. However, by 15 dpi, viral loads decreased by 2-4 logs in 10- to 40-week-old mice, while remaining at high levels in 50-week-old mice. Age-dependent differences in CNS viral loads 15 dpi occurred concomitantly with striking changes in CNS cytokine responses. In the CNS of 50-week-old mice, POWV induced Th1-type cytokines (IFNγ, IL-2, IL-12, IL-4, TNFα, IL-6), suggesting a neurodegenerative pro-inflammatory M1 microglial program. By contrast, in 10-week-old mice, POWV-induced Th2-type cytokines (IL-10, TGFβ, IL-4) were consistent with a neuroprotective M2 microglial phenotype. These findings correlate age-dependent CNS cytokine responses and viral loads with POWV lethality and suggest potential neuroinflammatory therapeutic targets. Our results establish the age-dependent lethality of POWV in a murine model that mirrors human POWV severity and long-term CNS pathology in the elderly.

IMPORTANCE

Powassan virus is an emerging tick-borne flavivirus causing lethal encephalitis in aged individuals. We reveal an age-dependent POWV murine model that mirrors human POWV encephalitis and long-term CNS damage in the elderly. We found that POWV is neuroinvasive and directs reactive gliosis in all age mice, but at acute stages selectively induces pro-inflammatory Th1 cytokine responses in 50-week-old mice and neuroprotective Th2 cytokine responses in 10-week-old mice. Our findings associate CNS viral loads and divergent cytokine responses with age-dependent POWV lethality and survival outcomes. Responses of young mice suggest potential therapeutic targets and approaches for preventing severe POWV encephalitis that may be broadly applicable to other neurodegenerative diseases. Our age-dependent murine POWV model permits analysis of vaccines that prevent POWV lethality, and therapeutics that resolve severe POWV encephalitis.

A rapid and versatile reverse genetics approach for generating recombinant positive-strand RNA viruses that use IRES-mediated translation

Reverse genetics systems have played a central role in developing recombinant viruses for a wide spectrum of virus research. The circular polymerase extension reaction (CPER) method has been applied to studying positive-strand RNA viruses, allowing researchers to bypass molecular cloning of viral cDNA clones and thus leading to the rapid generation of recombinant viruses. However, thus far, the CPER protocol has only been established using cap-dependent RNA viruses. Here, we demonstrate that a modified version of the CPER method can be successfully applied to positive-strand RNA viruses that use cap-independent, internal ribosomal entry site (IRES)-mediated translation. As a proof-of-concept, we employed mammalian viruses with different types (classes I, II, and III) of IRES to optimize the CPER method. Using the hepatitis C virus (HCV, class III), we found that inclusion in the CPER assembly of an RNA polymerase I promoter and terminator, instead of those from polymerase II, allowed greater viral production. This approach was also successful in generating recombinant bovine viral diarrhea virus (class III) following transfection of MDBK/293T co-cultures to overcome low transfection efficiency. In addition, we successfully generated the recombinant viruses from clinical specimens. Our modified CPER could be used for producing hepatitis A virus (HAV, type I) as well as de novo generation of encephalomyocarditis virus (type II). Finally, we generated recombinant HCV and HAV reporter viruses that exhibited replication comparable to that of the wild-type parental viruses. The recombinant HAV reporter virus helped evaluate antivirals. Taking the findings together, this study offers methodological advances in virology.

IMPORTANCE

The lack of versatility of reverse genetics systems remains a bottleneck in viral research. Especially when (re-)emerging viruses reach pandemic levels, rapid characterization and establishment of effective countermeasures using recombinant viruses are beneficial in disease control. Indeed, numerous studies have attempted to establish and improve the methods. The circular polymerase extension reaction (CPER) method has overcome major obstacles in generating recombinant viruses. However, this method has not yet been examined for positive-strand RNA viruses that use cap-independent, internal ribosome entry site-mediated translation. Here, we engineered a suitable gene cassette to expand the CPER method for all positive-strand RNA viruses. Furthermore, we overcame the difficulty of generating recombinant viruses because of low transfection efficiency. Using this modified method, we also successfully generated reporter viruses and recombinant viruses from a field sample without virus isolation. Taking these findings together, our adapted methodology is an innovative technology that could help advance virologic research.