Susceptibility to SARS-CoV-2 of Cell Lines and Substrates Commonly Used to Diagnose and Isolate Influenza and Other Viruses

Co-infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other viruses has been reported. We evaluated cell lines commonly used to isolate viruses and diagnose related diseases for their susceptibility to SARS-CoV-2. Although multiple kidney cell lines from monkeys were susceptible to SARS-CoV-2, we found many cell types derived from humans, dogs, minks, cats, mice, and chicken were not. We analyzed MDCK cells, which are most commonly used for surveillance and study of influenza viruses, and found that they were not susceptible to SARS-CoV-2. The low expression level of the angiotensin converting enzyme 2 receptor and lower receptor affinity to SARS-CoV-2 spike, which could be overcome by overexpression of canine angiotensin converting enzyme 2 in trans, strengthened the cellular barrier to productive infection. Moreover, a D614G mutation in the spike protein did not appear to affect SARS-CoV-2 cell tropism. Our findings should help avert inadvertent propagation of SARS-CoV-2 from diagnostic cell lines.

Vaccine-Derived Poliovirus Outbreaks and Events - Three Provinces, Democratic Republic of the Congo, 2017

The last confirmed wild poliovirus (WPV) case in Democratic Republic of the Congo (DRC) had paralysis onset in December 2011 (1). DRC has had cases of vaccine-derived polioviruses (VDPVs) documented since 2004 (Table 1) (1-6). After an outbreak of 30 circulating VDPV type 2 (cVDPV2) cases during 2011-2012, only five VDPV2 cases were reported during 2013-2016 (Table 1) (1-6). VDPVs can emerge from oral poliovirus vaccine (OPV types 1, 2, or 3; Sabin) polioviruses that have genetically mutated resulting in reversion to neurovirulence. This process occurs during extensive person-to-person transmission in populations with low immunity or after extended replication in the intestines of immune-deficient persons following vaccination (1-6). During 2017 (as of March 8, 2018), 25 VDPV cases were reported in three provinces in DRC: in Tanganyika province, an emergence with one VDPV2 case (pending final classification) in Kabalo health zone and an emergence with one ambiguous VDPV type 1 (aVDPV1) case in Ankoro health zone; in Maniema province, an emergence with two cVDPV2 cases; and in Haut Lomami province, an emergence with 20 cVDPV2 cases that originated in Haut Lomami province and later spread to Tanganyika province (hereafter referred to as the Haut Lomami outbreak area) and an emergence with one aVDPV type 2 (aVDPV2) case in Lwamba health zone (Table 1) (Figure) (6). Outbreak response supplementary immunization activities (SIAs) were conducted during June-December 2017 (Table 2) (6). Because of limitations in surveillance and suboptimal SIA quality and geographic scope, cVDPV2 circulation is likely continuing in 2018, requiring additional SIAs. DRC health officials and Global Polio Eradication Initiative (GPEI) partners are increasing human and financial resources to improve all aspects of outbreak response.

The coxsackievirus B 3C protease cleaves MAVS and TRIF to attenuate host type I interferon and apoptotic signaling

The host innate immune response to viral infections often involves the activation of parallel pattern recognition receptor (PRR) pathways that converge on the induction of type I interferons (IFNs). Several viruses have evolved sophisticated mechanisms to attenuate antiviral host signaling by directly interfering with the activation and/or downstream signaling events associated with PRR signal propagation. Here we show that the 3C(pro) cysteine protease of coxsackievirus B3 (CVB3) cleaves the innate immune adaptor molecules mitochondrial antiviral signaling protein (MAVS) and Toll/IL-1 receptor domain-containing adaptor inducing interferon-beta (TRIF) as a mechanism to escape host immunity. We found that MAVS and TRIF were cleaved in CVB3-infected cells in culture. CVB3-induced cleavage of MAVS and TRIF required the cysteine protease activity of 3C(pro), occurred at specific sites and within specialized domains of each molecule, and inhibited both the type I IFN and apoptotic signaling downstream of these adaptors. 3C(pro)-mediated MAVS cleavage occurred within its proline-rich region, led to its relocalization from the mitochondrial membrane, and ablated its downstream signaling. We further show that 3C(pro) cleaves both the N- and C-terminal domains of TRIF and localizes with TRIF to signalosome complexes within the cytoplasm. Taken together, these data show that CVB3 has evolved a mechanism to suppress host antiviral signal propagation by directly cleaving two key adaptor molecules associated with innate immune recognition.

Species-specific RT-PCR amplification of human enteroviruses: a tool for rapid species identification of uncharacterized enteroviruses

The 65 serotypes of human enteroviruses are classified into four species, Human enterovirus (HEV) A to D, based largely on phylogenetic relationships in multiple genome regions. The 3'-non-translated region of enteroviruses is highly conserved within a species but highly divergent between species. From this information, species-specific RT-PCR primers were developed that can be used to rapidly screen collections of enterovirus isolates to identify species of interest. The four primer pairs were 100 % specific when tested against enterovirus prototype strains and panels of isolates of known serotype (a total of 193 isolates). For evaluation in a typical application, the species-specific primers were used to screen 186 previously uncharacterized non-polio enterovirus isolates. The HEV-B primers amplified 68.3 % of isolates, while the HEV-A and HEV-C primers accounted for 9.7 and 11.3 % of isolates, respectively; no isolates were amplified with the HEV-D primers. Twelve isolates (6.5 %) were amplified by more than one primer set and eight isolates (4.3 %) were not amplified by any of the four primer pairs. Serotypes were identified by partial sequencing of the VP1 capsid gene, and in every case sequencing confirmed that the species-specific PCR result was correct; the isolates that were amplified by more than one species-specific primer pair were mixtures of two (11 isolates) or three (one isolate) species of viruses. The eight isolates that were not amplified by the species-specific primers comprised four new serotypes (EV76, EV89, EV90 and EV91) that appear to be unique members of HEV-A based on VP1, 3D and 3'-non-translated region sequences.

Influenza virus ns1 protein induces apoptosis in cultured cells

The importance of influenza viruses as worldwide pathogens in humans, domestic animals, and poultry is well recognized. Discerning how influenza viruses interact with the host at a cellular level is crucial for a better understanding of viral pathogenesis. Influenza viruses induce apoptosis through mechanisms involving the interplay of cellular and viral factors that may depend on the cell type. However, it is unclear which viral genes induce apoptosis. In these studies, we show that the expression of the nonstructural (NS) gene of influenza A virus is sufficient to induce apoptosis in MDCK and HeLa cells. Further studies showed that the multimerization domain of the NS1 protein but not the effector domain is required for apoptosis. However, this mutation is not sufficient to inhibit apoptosis using whole virus.

Mink lung epithelial cells: unique cell line that supports influenza A and B virus replication

We have demonstrated for the first time that a mink lung epithelial cell line (Mv1Lu) supports the replication of influenza A and B viruses, including the recently isolated H5N1 avian and human Hong Kong strains, to titers comparable to those in MDCK cells. These results suggest that Mv1Lu cells might serve as an alternative system for the isolation and cultivation of influenza A and B viruses and may be useful for vaccine development.

Coadministration of DNA encoding interleukin-6 and hemagglutinin confers protection from influenza virus challenge in mice

This study was conducted to investigate whether Accell gene gun coadministration of DNA encoding human interleukin-6 (IL-6) would enhance protective immune responses in mice to an equine influenza A virus hemagglutinin (HA) DNA vaccine. Mice that received HA DNA alone exhibited accelerated clearance of homologous challenge virus but were not protected from infection. In contrast, mice that received both HA and IL-6 DNA had no detectable virus in their lungs after challenge. These results strongly support the use of IL-6 as a cytokine adjuvant in DNA vaccination.

Immunogenicity and efficacy of baculovirus-expressed and DNA-based equine influenza virus hemagglutinin vaccines in mice

Two fundamentally different approaches to vaccination of BALB/c mice with the hemagglutinin (HA) of A/Equine/Kentucky/1/81 (H3N8) (Eq/KY) were evaluated, that is, administration of HA protein vs administration of HA-encoding DNA. Each vaccine was tested for its immunogenicity and ability to provide protection from homologous virus challenge. HA protein was synthesized in vitro by infection of Sf21 insect cells with a recombinant baculovirus. Intranasal administration of this vaccine induced virus-specific antibodies, as measured by enzyme-linked immunosorbent assay (ELISA), but did not induce virus neutralizing (VN) antibodies. This route of administration provided partial protection from virus challenge, but interestingly, this protection was completely abrogated, rather than enhanced, by co-administration of 10 micrograms of cholera holotoxin. As a second approach, mice were directly vaccinated in vivo by Accell gene gun delivery of plasmid DNA encoding the Eq/KY HA gene. This approach induced VN antibodies as well as virus-specific ELISA antibodies. When two doses of DNA vaccine were administered 3 weeks apart, mice were not protected from challenge, although they cleared the infection more rapidly than control mice. However, when the second DNA vaccination was delayed until 9 weeks after the first, 9 out of 10 vaccinated mice were completely protected. These results indicate that the time between initial and booster DNA vaccinations may be an important variable in determining DNA vaccination efficacy.

The influence of calcium and reactive oxygen species on influenza virus-induced apoptosis

In previous studies we observed that influenza A and B viruses induce apoptosis in Madin-Darby canine kidney (MDCK) cells and that this apoptosis is blocked by expression of bcl-2. Using a well-characterized, highly virulent, avian influenza virus, A/Turkey/Ontario/7732/66 (H5N9) (Ty/Ont), we sought to better understand this system. We investigated the influence of two cellular factors that are known to function in other models of apoptosis inhibited by bcl-2, calcium (Ca(2+)) and reactive oxygen species (ROS). Although Ca(2+) chelators generally inhibit apoptosis, treatment of MDCK cells with either an extracellular chelator, EGTA, or an intracellular chelator, BAPTAAM, induced apoptosis instead and enhanced Ty/Ont-induced apoptosis. Conversely, treatment with an ionophore, ionomycin, blocked the viral-induced apoptosis. In terms of ROS, neither treatment with antioxidants, N(2) flushing to induce hypoxia, nor nigericin (a compound which, like bcl-2, stabilizes the mitochondrial membrane potential against the effects of ROS and subsequent Ca(2+) dysregulation) were able to block Ty/Ont-induced apoptosis. Therefore, it is likely that ROS play little, if any, role in influenza-induced apoptosis in MDCK cells and the influence of Ca(2+) appears to be opposite to that in the majority of other more classical models of apoptosis.

Apoptosis: a mechanism of cell killing by influenza A and B viruses

In previous studies, we observed that the virulent avian influenza A virus A/Turkey/Ontario/7732/66 (Ty/Ont) induced severe lymphoid depletion in vivo and rapidly killed an avian lymphocyte cell line (RP9) in vitro. In examining the mechanism of cell killing by this virus, we found that Ty/Ont induced fragmentation of the RP9 cellular DNA into a 200-bp ladder and caused ultrastructural changes characteristic of apoptotic cell death by 5 h after infection. We next determined that the ability to induce apoptosis was not unique to Ty/Ont. In fact, a variety of influenza A viruses (avian, equine, swine, and human), as well as human influenza B viruses, induced DNA fragmentation in a permissive mammalian cell line, Madin-Darby canine kidney (MDCK), and this correlated with the development of a cytopathic effect during viral infection. Since the proto-oncogene bcl-2 is a known inhibitor of apoptosis, we transfected MDCK cells with the human bcl-2 gene; these stably transfected cells (MDCKbcl-2) did not undergo DNA fragmentation after virus infection. In addition, cytotoxicity assays at 48 to 72 h after virus infection showed a high level of cell viability for MDCKbcl-2 compared with a markedly lower level of viability for MDCK cells. These studies indicate that influenza A and B viruses induce apoptosis in cell cultures; thus, apoptosis may represent a general mechanism of cell death in hosts infected with influenza viruses.

Overlapping cytotoxic T-lymphocyte and B-cell antigenic sites on the influenza virus H5 hemagglutinin

To define the recognition site of cytotoxic T lymphocytes (CTLs) on influenza virus H5 hemagglutinin (HA), an H5 HA-specific CTL clone was examined for the ability to recognize monoclonal antibody-selected HA variants of influenza virus A/Turkey/Ontario/7732/66 (H5N9). On the basis of 51Cr release assays with the variants, a CTL epitope was located near residue 168 of H5 HA. To define the epitope more precisely, a series of overlapping peptides corresponding to this region was synthesized and tested for CTL recognition. The minimum peptide recognized by the CTL clone encompassed residues 158 to 169 of H5 HA. Relative to the H3 HA three-dimensional structure, this CTL epitope is located near the distal tip of the HA molecule, also known as a major B-cell epitope on H3 HA. A single mutation at residue 168 (Lys to Glu) in the H5 HA variants abolished CTL recognition; this same amino acid was shown previously to be critical for B-cell recognition (M. Philpott, C. Hioe, M. Sheerar, and V. S. Hinshaw, J. Virol. 64:2941-2947, 1990). Additionally, mutations within this region of the HA molecule were associated with attenuation of the highly virulent A/Turkey/Ontario/7732/66 (H5N9) (M. Philpott, B. C. Easterday, and V.S. Hinshaw, J. Virol. 63:3453-3458, 1989). When tested for recognition of other H5 viruses, the CTL clone recognized the HA of A/Turkey/Ireland/1378/83 (H5N8) but not that of A/Chicken/Pennsylvania/1370/83 (H5N2), even though these viruses contain identical HA amino acid 158-to-169 sequences. These results suggest that differences outside the CTL epitope affected CTL recognition of the intact HA molecule. The H5 HA site defined in these studies is, therefore, important in both CTL and B-cell recognition, as well as the pathogenesis of the virus.