Early interactions between animal viruses and the host cell: relevance to viral vaccines

Precision therapeutic targets for COVID-19

Beginning in late 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged as a novel pathogen that causes coronavirus disease 2019 (COVID-19). SARS-CoV-2 has infected more than 111 million people worldwide and caused over 2.47 million deaths. Individuals infected with SARS-CoV-2 show symptoms of fever, cough, dyspnea, and fatigue with severe cases that can develop into pneumonia, myocarditis, acute respiratory distress syndrome, hypercoagulability, and even multi-organ failure. Current clinical management consists largely of supportive care as commonly administered treatments, including convalescent plasma, remdesivir, and high-dose glucocorticoids. These have demonstrated modest benefits in a small subset of hospitalized patients, with only dexamethasone showing demonstrable efficacy in reducing mortality and length of hospitalization. At this time, no SARS-CoV-2-specific antiviral drugs are available, although several vaccines have been approved for use in recent months. In this review, we will evaluate the efficacy of preclinical and clinical drugs that precisely target three different, essential steps of the SARS-CoV-2 replication cycle: the spike protein during entry, main protease (MPro) during proteolytic activation, and RNA-dependent RNA polymerase (RdRp) during transcription. We will assess the advantages and limitations of drugs that precisely target evolutionarily well-conserved domains, which are less likely to mutate, and therefore less likely to escape the effects of these drugs. We propose that a multi-drug cocktail targeting precise proteins, critical to the viral replication cycle, such as spike protein, MPro, and RdRp, will be the most effective strategy of inhibiting SARS-CoV-2 replication and limiting its spread in the general population.

Nuclear import of adenovirus DNA in vitro involves the nuclear protein import pathway and hsc70

Adenovirus, a respiratory virus with a double-stranded DNA genome, replicates in the nuclei of mammalian cells. We have developed a cytosol-dependent in vitro assay utilizing adenovirus nucleocapsids to examine the requirements for adenovirus docking to the nuclear pore complex and for DNA import into the nucleus. Our assay reveals that adenovirus DNA import is blocked by a competitive excess of classical protein nuclear localization sequences and other inhibitors of nuclear protein import and indicates that this process is dependent on hsc70. Previous work revealed that the hexon (coat) protein of adenovirus is the only major protein on the surface of the adenovirus nucleocapsid that docks at the nuclear pore complex. This, together with our finding that in vitro nuclear import of hexon is inhibited by an excess of classical nuclear localization sequences, suggests a role for the hexon protein in adenovirus DNA import. However, recombinant transport factors that are sufficient for hexon import in permeabilized cells do not support DNA import, indicating that there are other as yet unidentified factors required for this process.

Neutralization of influenza virus by low concentrations of hemagglutinin-specific polymeric immunoglobulin A inhibits viral fusion activity, but activation of the ribonucleoprotein is also inhibited

High concentrations of hemagglutinin-specific neutralizing polymeric monoclonal immunoglobulin A (IgA) inhibit attachment of the majority of type A influenza virus virions to cell monolayers and tracheal epithelium (H. P. Taylor and N. J. Dimmock, J. Exp. Med. 161:198-209, 1985; M. C. Outlaw and N. J. Dimmock, J. Gen. Virol. 71:69-76, 1990). A minority of virions attaches but is not infectious. Here, we report that a different mechanism operates when influenza virus A/Puerto Rico/8/34 (H1N1) is neutralized by low concentrations of monoclonal polymeric IgA or when A/fowl plague virus/Rostock/34 (H7N1) is neutralized by low concentrations of polyclonal rat secretory IgA. Under these conditions, neutralized virus attaches to cells and is taken up by them. However, upon entering the cell, the nucleoprotein (NP) of neutralized virus is found in the perinuclear cytoplasm, whereas NP from nonneutralized virus is concentrated in the nucleus itself. Further data show that the low-pH-mediated cell fusion activity of virions is inhibited by IgA in proportion to loss of infectivity. The possibilities that neutralization by low amounts of polymeric IgA is caused by inhibition of the virion fusion activity and that the aberrant distribution of NP from neutralized virus results from its failure to escape from the endosomal system were investigated by using A/PR/8/34 and the fusogenic agent polyethylene glycol (PEG) at pH 5.4. A/PR/8/34 attached to cells at 4 degrees C, with minimal internalization of the virus; treatment with PEG at pH 5.4 and 4 degrees C for 1 min led to infectious fusion of nonneutralized virus with the plasma membrane and, under these conditions, was more efficient than PEG at pH 7 or medium at pH 5.4. Neutralized virus which was attached to cells and treated with acidified PEG appeared to undergo primary and secondary uncoating, with its NP protein becoming concentrated in the nucleus and M1 becoming concentrated in the perinuclear cytoplasm. Although the distribution of NP and M1 was indistinguishable from infectious virus, infectivity was not restored. Thus, even when IgA-induced inhibition of fusion is reversed, virus is still neutralized. We suggest that infectious influenza virus undergoes an activation stage which may be the relaxation of the ribonucleoprotein structure needed to permit transcription or may be the removal of M1 bound to the ribonucleoprotein.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of entry of a xenotropic MMuLV-derived recombinant retrovirus into porcine cells using the expression of the reporter nlslacZ gene

A xenotropic Moloney murine leukemia virus-derived recombinant retrovirus (MMuLVSVnlslacZ) has been utilized to study the mechanism of virus entry into endothelial and epithelial porcine cells. In the genome of this recombinant retrovirus, the nlslacZ reporter gene is under the transcriptional control of both LTR and SV40 early promoter. The entry of the retrovirus has been determined from the expression of this transduced reporter gene after its integration into the infected cells. This allows the detection of a very low level of viral infection and hence entry of the virus. Exposure of the virus-cell mixture to acidic pH (less than 6) during the early phase of interaction reduces the level of internalization. Cellular infection in presence of weak bases, ammonium chloride and amantadine and an ionophore monensin at concentrations sufficient to neutralize the endosomal pH does not modify the extent of viral entry into the cells. The results indicate that the entry of the recombinant retrovirus into porcine cells takes place by a pH-independent viral membrane-cell plasma membrane fusion mechanism.

Insights into neutralization of animal viruses gained from study of influenza virus

It has long been known that the binding of antibodies to viruses can result in a loss of infectivity, or neutralization, but little is understood of the mechanism or mechanisms of this process. This is probably because neutralization is a multifactorial phenomenon depending upon the nature of the virus itself, the particular antigenic site involved, the isotype of immunoglobulin and the ratio of virus to immunoglobulin (see below). Thus not only is it likely that neutralization of one virus will differ from another but that changing the circumstances of neutralization can change the mechanism itself. To give coherence to the topic we are concentrating this review on one virus, influenza type A which is itself well studied and reasonably well understood [1–3]. Reviews of the older literature can be found in references 4 to 7.

Interaction of African swine fever virus with macrophages

Morphological data obtained by electron microscopy have shown that African swine fever virus adapted to VERO cells enters swine macrophages, its natural host cell, by a mechanism of receptor-mediated endocytosis. Binding studies with 3H-labeled virus and competition experiments with UV-inactivated virus have shown that the virus entry that leads to a productive infection in swine macrophages is mediated by saturable binding sites on the plasma membrane. The virus also penetrated into rabbit macrophages that do not produce infectious virus and initiated the synthesis of some early viral proteins; however, the viral replication cycle was aborted since viral DNA synthesis did not occur. The interaction of ASF virus particles with rabbit macrophages was mediated by nonsaturable binding sites, suggesting that the lack of specific receptors in these cells may be related to the absence of a productive infection. A similar abortive infection was detected in macrophages from other virus-resistant animal species.

The antiviral drug amantadine has a direct inhibitory effect on T-lymphocytes

We investigated the effect of the antiviral drug amantadine (AmTd) on polyclonal activation of thymic-dependent (T) and thymic-independent (B) lymphocytes from normal mice. In the present studies, T-lymphocytes are defined by their response to concanavalin A (Con A) and B-lymphocytes by their response to lipopolysaccharide (LPS). Polyclonal activator-induced lymphocyte proliferation was assessed by quantifying cellular incorporation of tritiated thymidine. The results show that, in a dose-dependent manner, AmTd exhibits at least 2-fold greater inhibitory activity against Con A-responding T-cells than against LPS-responding B-cells. Further, several findings demonstrate that AmTd has a direct inhibitory effect on T-lymphocytes. First, AmTd pulse treatment of isolated T-cells, but not accessory cells, abolished the T-cell response to Con A. Second, AmTd pulse treatment of the cytotoxic T-lymphocyte line, CTLL-2, markedly reduced their ability to undergo IL-2-induced proliferation. Third, proliferation of T-cells which had already undergone activation by ConA was inhibited by AmTd. Further, the finding that addition of IL-1, IL-2 or both to cultures failed to reverse inhibition of the response to ConA argues that AmTd did not interfere with endogenous production of these lymphokines. Possible implications of these findings are discussed.

The entry of African swine fever virus into Vero cells

The entry of African swine fever virus into Vero cells has been investigated by both biochemical and morphological techniques. A quantitative electron microscopy analysis of the early steps of the infection has shown that African swine fever virus enters Vero cells by a receptor-mediated endocytosis mechanism. The internalization of virus particles is a temperature- and energy-dependent process, since it did not take place at 4 degrees or in the presence of NaF and 2,4-dinitrophenol. To determine the involvement of acidic intracellular vacuoles in the virus entry pathway we have tested the effect of lysosomotropic agents in the infection. Chloroquine, dansylcadaverine, amantadine, methylamine, and ammonium chloride inhibited African swine fever virus production in Vero cells. Dansylcadaverine and chloroquine did not inhibit virus adsorption and internalization; however, in the presence of these drugs, virus particles were retained in cytoplasmic vacuoles and early viral RNA and protein synthesis were not detected, indicating that these compounds inhibit an early step in the infectious cycle, probably the uncoating of the virus particle.

Neutralisation of HIV isolates by anti-idiotypic antibodies which mimic the T4 (CD4) epitope: a potential AIDS vaccine

Polyclonal anti-idiotypic antibodies were raised in mice against anti-Leu3a, a mouse monoclonal anti-human T4 (CD4) antibody that blocks the in-vitro binding of human immunodeficiency virus (HIV) to the CD4 molecule. The anti-idiotypes recognized anti-Leu3a but not OKT4, an anti-human T4 antibody that does not inhibit HIV binding to CD4. The anti-idiotypes specifically reacted with the HIV envelope glycoprotein in solid-phase immunoassays. More importantly, the anti-idiotypes neutralised three distinct isolates of HIV-1 and one isolate of HIV-2 in a syncytial inhibition assay. These results have implications for a potential AIDS vaccine of anti-CD4 preparations to induce an anti-idiotypic response with the capacity to bind HIV at its receptor site.