Inhibition of influenza C viruses by human MxA protein.

Human MxA protein was analyzed for its ability to inhibit the replication of different influenza C viruses. Three laboratory derivatives of viral strain C/Ann Arbor/1/50 were investigated, namely the parental wild-type virus C/AA-wt, the persistent variant C/AA-pi and the highly cytopathogenic variant C/AA-cyt. In addition, strain C/Paris/214/91 isolated from an influenza patient was used.

A persistent variant of influenza C virus fails to interact with actin filaments during viral assembly.

C/AA-pi virus, a variant of influenza C/Ann Arbor/1/50 virus, establishes persistent infections in MDCK cells, characterized by low levels of progeny production. During viral assembly, nucleoprotein (NP) was found homogeneously distributed over cytoplasmic and nuclear compartments and matrix (M) protein was likewise localized in a barely structured fashion. In contrast, infections with nonpersistent influenza A, B and C viruses produced cytoplasmic granular structures, which typically consisted of colocalized NP and M proteins.

The ORF, regulated synthesis, and persistence-specific variation of influenza C viral NS1 protein.

The open reading frame (ORF) and the regulated synthesis of the influenza C viral NS1 protein were analyzed in view of viruses possessing different biological activities. We provide evidence for a 246-amino-acid NS1-ORF, encoded by five viral strains and variants. Prokaryotic expression of the prototype NS1-ORF resulted in a product of 27 kDa, confirming the predicted molecular weight.

Persistent infection with an influenza C virus variant is dominantly established in the presence of the parental wild-type virus.

Two influenza C viruses were used for double-infection experiments to investigate the dominance of their phenotypes. The wild-type virus (C/AA-wt) had been characterized by its short-lived productive cycle, whereas a distinct variant derived from it (C/AA-pi) was demonstrated to persist in long-term passages of infected MDCK cultures. Here we show that the persistent virus C/AA-pi is capable of replicating in the presence of abundant amounts of wild-type virus: the persistent virus could be diluted to 10(-9) within wild-type inoculum, still developing a stable form of persistence.

A highly cytopathogenic influenza C virus variant induces apoptosis in cell culture.

An influenza C virus variant, C/AA-cyt, was identified as the agent responsible for highly effective induction of cytopathogenicity in MDCK cells. The cytopathogenic effect was manifested by cell rounding, cell shrinkage and foci of cell destruction leading finally to disruption of the monolayer in a virus dose-dependent manner. Virus-induced cytopathogenicity was suppressed by temperatures nonpermissive for virus replication.

Nucleotide-specific PCR for molecular virus typing.

Nucleotide sequence studies detected a double-point mutation in the genomic RNA segment 4 (nt 871 and 872) of the persistent variant C/AA-pi of influenza C/Ann Arbor/1/50 virus. The 3'-end-points of two distinct PCR primers were positioned exactly at this genome location and thereby adjusted the priming determinant complementary to the varied strain or to its wild-type counterpart. Consequently, positive RT-PCR products strictly referred to one of the two viruses examined, in both cases, using either virion or infected-cell RNA templates.