Productive and non-productive phases during long-term persistence of influenza C virus

Host Range, Biology, and Species Specificity of Seven-Segmented Influenza Viruses-A Comparative Review on Influenza C and D

Other than genome structure, influenza C (ICV), and D (IDV) viruses with seven-segmented genomes are biologically different from the eight-segmented influenza A (IAV), and B (IBV) viruses concerning the presence of hemagglutinin-esterase fusion protein, which combines the function of hemagglutinin and neuraminidase responsible for receptor-binding, fusion, and receptor-destroying enzymatic activities, respectively. Whereas ICV with humans as primary hosts emerged nearly 74 years ago, IDV, a distant relative of ICV, was isolated in 2011, with bovines as the primary host. Despite its initial emergence in swine, IDV has turned out to be a transboundary bovine pathogen and a broader host range, similar to influenza A viruses (IAV). The receptor specificities of ICV and IDV determine the host range and the species specificity. The recent findings of the presence of the IDV genome in the human respiratory sample, and high traffic human environments indicate its public health significance. Conversely, the presence of ICV in pigs and cattle also raises the possibility of gene segment interactions/virus reassortment between ICV and IDV where these viruses co-exist. This review is a holistic approach to discuss the ecology of seven-segmented influenza viruses by focusing on what is known so far on the host range, seroepidemiology, biology, receptor, phylodynamics, species specificity, and cross-species transmission of the ICV and IDV.

Influenza A virus derived from persistently virus-infected cells shows attenuated cytotoxicity in cultured cells but virulent pathogenicity in mice

The IVpi-43 strain of influenza A virus, a progeny virus derived from persistently virus-infected Madin-Darby canine kidney (MDCK) cells, showed a more attenuated nature in cytopathology in cultured cells than the parental wild-type influenza virus (IVwt) that was used for establishment of the virus carrier culture. Upon infection of MDCK cells, growth of the IVpi-43 virus was restrained with an impaired synthesis of virus structural proteins in the cells. Apoptosis induced by IVpi-43 virus was confined at a low level. The IVpi-43 virus was able to easily cause persistent infection in fresh MDCK cells. In contrast to the in vitro phenotype, the IVpi-43 virus proved highly virulent in mice, with massive and broadly disseminated virus multiplication in the lungs. It was suggested that impaired activity of the neuraminidase molecule of the IVpi-43 virus was responsible for the delayed and faint appearance of apoptosis in the IVpi-43 virus-infected respiratory cells, which made it possible for the virus to replicate for a longer period and to spread to a broader area of the lungs and that abundant numbers of the virus-infected lung cells were killed within a short period by the subsequently established virus-specific immune responses, leading to unrecoverable serious pneumonia.

Conformational maturation of the nucleoprotein synthesized in influenza C virus-infected cells

The conformational maturation of the influenza C virus nucleoprotein (NP) synthesized in infected cells was investigated. Monoclonal antibodies (mAbs) that have previously been characterized [Sugawara, K., Nishimura, H., Hongo, S., Kitame, F., Nakamura, K., 1991. Antigenic characterization of the nucleoprotein and matrix protein of influenza C virus with monoclonal antibodies. J. Gen. Virol. 72, 103-109] enabled this molecular maturation to be detected. Both pulse-labeled and chased NPs could equally retain high reactivity with H31 mAb recognizing a linear epitope on the NP molecule. However, pulse-labeled NP showed three- to four-fold lower reactivity with H27 mAb recognizing a conformational epitope, compared to chased NP. Sedimentation analyses by sucrose gradient centrifugation revealed that the mature NP could readily participate in nucleocapsid formation while the immature NP was free. The immature NP was rapidly transported into the nucleus and its maturation seemed to occur after or during translocation into the nucleus. A single expression of NP cDNA in COS-1 cells demonstrated that the NP maturation was an intrinsic feature of the NP molecule without relation to other viral components.

Varied persistent life cycles of Borna disease virus in a human oligodendroglioma cell line

Borna disease virus (BDV) establishes a persistent infection in the central nervous system of vertebrate animal species as well as in tissue cultures. In an attempt to characterize the life cycle of BDV in persistently infected cultured cells, we developed 30 clones by single-cell cloning from a human oligodendroglioma (OL) cell line after infection with BDV. According to the percentage of cells expressing the BDV major proteins, p40 (nucleoprotein) and p24 (phosphoprotein), the clones were classified into two types: type I (>20%) and type II (<20%). mRNAs corresponding to both proteins were detected by in situ hybridization (ISH) in a percentage of cells consistent with that for the protein expression in the two types. Surprisingly, ISH for the detection of the genomic RNA, mainly in type II, revealed a significantly larger cell population harboring the genomic RNA than that with the protein as well as the mRNA expression. By recloning from type II primary cell clones, the same phenotype was confirmed in the secondary cell clones obtained: i.e., low percentage of protein-positive cells and higher percentage of cells harboring the genomic RNA. After nerve growth factor treatment, the two types of clones showed increases in the percentage of cells expressing BDV-specific proteins that reached 80% in type II clones, in addition to increased expression levels per cell. Such enhancement might have been mediated by the activation of the mitogen-activated protein kinase in the clones as revealed by the detection of activated ERK1/2. Thus, our findings show that BDV may have established a persistent infection at low levels of viral expression in OL cells with the possibility of a latent infection.

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. Multiplication of all four viruses was suppressed in MxA-expressing Vero cells, as indicated by a decrease in viral RNA synthesis, viral protein synthesis, virion production and induction of a cytopathic effect. Inhibition correlated with the level of MxA expression. Furthermore, inhibition was independent of cell clone-specific differences in expression of virus receptors, as demonstrated by receptor reconstitution experiments. Thus, human MxA protein has antiviral activity against influenza C viruses.

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. Studies on the in vitro interaction between NP and M proteins revealed identical binding capacities comparing influenza C wild-type virus with the persistent variant. Cytochalasin D treatment of infected cells demonstrated that NP protein of the wild-type virus, but not of the persistent variant, was distinctly associated with cellular actin filaments. Moreover, the assembly characteristics of wild-type virus were modulated in the presence of recombinant persistent-type NP protein towards a behaviour similar to persistent infection. Cell type specificity was particularly illustrated in C/AA-pi virus-infected Vero cells, which did not support viral persistence, but produced granular wild-type-like complexes. Thus, interaction between NP, M and actin proteins (i) is a basic part of the viral assembly process, (ii) is dominantly modulated by NP protein and (iii) is specifically altered in the case of persistent infection.

Rescue of a viral gene from VERO cells latently infected with influenza virus B/Lee/40

By growing VERO cells infected with 5 PFU/cell of influenza virus B/Lee/40, a latently infected culture was readily established (L/V cells). The cells continued to multiply stably, excreting a small amount of virus in the beginning, which sharply declined according to cell division to undetectable level by day 9. However, nucleotide sequences for all the 8 genes of B/Lee/40 as well as their mRNAs were amplified from L/V cells on day 50 or later by RT-PCR. Moreover, from the 95-day-old L/V cells, a persisting NP gene of B/Lee/40 was rescued into infectious virus particles upon superinfection with homotypic influenza virus B/Yamagata/1/73.

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. Artificial virus mixtures could easily be distinguished by this method in a subsequent qualitative gel analysis. PCR annealing conditions and control reactions were optimized, for the monitoring of influenza virus isolates throughout multifold passages. Thus, sequence diversity in just two neighbouring nucleotides is sufficient to determine whether or not successful PCR amplification takes place, and this method can be used as a reliable means of virus strain differentiation.