Avian cells expressing the murine Mx1 protein are resistant to influenza virus infection

Genetic insertion of mouse Myxovirus-resistance gene 1 increases innate resistance against both high and low pathogenic avian influenza virus by significantly decreasing replication in chicken DF1 cell line

Avian influenza virus (AIV) is a constant threat to animal health with recent global outbreaks resulting in the death of hundreds of millions of birds with spillover into mammals. Myxovirus-resistance (Mx) proteins are key mediators of the antiviral response that block virus replication. Mouse (Mu) Mx (Mx1) is a strong antiviral protein that interacts with the viral nucleoprotein to inhibit polymerase function. The ability of avian Mx1 to inhibit AIV is unclear. In these studies, Mu Mx1 was stably introduced into chicken DF1 cells to enhance the immune response against AIV. Following infection, titers of AIV were significantly decreased in cells expressing Mu Mx1. In addition, considerably less cytopathic effect (CPE) and matrix protein staining was observed in gene-edited cells expressing Mu Mx1, suggesting Mu Mx1 is broadly effective against multiple AIV subtypes. This work provides foundational studies for use of gene-editing to enhance innate disease resistance against AIV.

microRNA control of interferons and interferon induced anti-viral activity

Interferons (IFNs) are cytokines that are spontaneously produced in response to virus infection. They act by binding to IFN-receptors (IFN-R), which trigger JAK/STAT cell signalling and the subsequent induction of hundreds of IFN-inducible genes, including both protein-coding and microRNA genes. IFN-induced genes then act synergistically to prevent virus replication and create an anti-viral state. miRNA are therefore integral to the innate response to virus infection and are important components of IFN-mediated biology. On the other hand viruses also encode miRNAs that in some cases interfere directly with the IFN response to infection. This review summarizes the important roles of miRNAs in virus infection acting both as IFN-stimulated anti-viral molecules and as critical regulators of IFNs and IFN-stimulated genes. It also highlights how recent knowledge in RNA editing influence miRNA control of virus infection.

In Vivo modulation of the innate response to pneumovirus by type-I and -III interferon-induced Bos taurus Mx1

The respiratory syncytial virus (RSV) is a major pathogen of the human species. This pneumovirus is a prominent cause of airway morbidity in children and maintains an excessive hospitalization rate despite decades of research. As involvement of a genetic vulnerability is a possibility supported by recent data, we addressed the question of whether the Mx gene products, the typical target of which consists in single-stranded negative-polarity RNA viruses, could alter the course of pneumovirus-associated disease in vivo. Wild-type and Bos taurus Mx1-expressing transgenic FVB/J mice were inoculated with the mouse counterpart and closest phylogenetic relative of RSV, pneumonia virus of mice. Survival data and follow-up of body weight, histological scores, lung virus spread, and lung viral load unequivocally showed that the viral infection was severely repressed in Mx-transgenic mice, thus suggesting that pneumoviruses belong to the antiviral spectrum of mammalian Mx GTPases. Elucidating the underlying mechanisms at the molecular level could reveal critical information for the development of new anti-RSV molecules.

Mx is dispensable for interferon-mediated resistance of chicken cells against influenza A virus

The type I interferon (IFN) system plays an important role in antiviral defense against influenza A viruses (FLUAV), which are natural chicken pathogens. Studies of mice identified the Mx1 protein as a key effector molecule of the IFN-induced antiviral state against FLUAV. Chicken Mx genes are highly polymorphic, and recent studies suggested that an Asn/Ser polymorphism at amino acid position 631 determines the antiviral activity of the chicken Mx protein. By employing chicken embryo fibroblasts with defined Mx-631 polymorphisms and retroviral vectors for the expression of Mx isoforms in chicken cells and embryonated eggs, we show here that neither the 631Asn nor the 631Ser variant of chicken Mx was able to confer antiviral protection against several lowly and highly pathogenic FLUAV strains. Using a short interfering RNA (siRNA)-mediated knockdown approach, we noted that the antiviral effect of type I IFN in chicken cells was not dependent on Mx, suggesting that some other IFN-induced factors must contribute to the inhibition of FLUAV in chicken cells. Finally, we found that both isoforms of chicken Mx protein appear to lack GTPase activity, which might explain the observed lack of antiviral activity.

Influenza A virus polymerase inhibits type I interferon induction by binding to interferon beta promoter stimulator 1

Type I interferons (IFNs) are known to be critical factors in the activation of host antiviral responses and are also important in protection from influenza A virus infection. Especially, the RIG-I- and IPS-1-mediated intracellular type I IFN-inducing pathway is essential in the activation of antiviral responses in cells infected by influenza A virus. Previously, it has been reported that influenza A virus NS1 is involved in the inhibition of this pathway. We show in this report that the influenza A virus utilizes another critical inhibitory mechanism in this pathway. In fact, the viral polymerase complex exhibited an inhibitory activity on IFNβ promoter activation mediated by RIG-I and IPS-1, and this activity was not competitive with the function of NS1. Co-immunoprecipitation analysis revealed that each polymerase subunit bound to IPS-1 in mammalian cells, and each subunit inhibited the activation of IFNβ promoter by IPS-1 independently. In addition, by a combinational expression of each polymerase subunit, IPS-1-induced activation of IFNβ promoter was more efficiently inhibited by the expression of PB2 or PB2-containing complex. Moreover, the expression of PB2 inhibited the transcription of the endogenous IFNβ gene induced after influenza A virus infection. These findings demonstrate that the viral polymerase plays an important role for regulating host anti-viral response through the binding to IPS-1 and inhibition of IFNβ production.

A single substitution in amino acid 184 of the NP protein alters the replication and pathogenicity of H5N1 avian influenza viruses in chickens

Changes in the NP gene of H5N1 highly pathogenic avian influenza (HPAI) viruses have previously been shown to affect viral replication, alter host gene expression levels and affect mean death times in infected chickens. Five amino acids at positions 22, 184, 400, 406, and 423 were different between the two recombinant viruses studied. In this study, we individually mutated the five amino acids that differed and determined that the difference in virus pathogenicity after NP gene exchange was a result of an alanine to lysine change at position 184 of the NP protein. Infection with viruses containing a lysine at NP 184 induced earlier mortality in chickens, increased virus titers and nitric oxide levels in tissues, and resulted in up-regulated host immune genes, such as alpha-interferon (IFN-alpha), gamma-interferon (IFN-gamma), orthomyxovirus resistance gene 1 (Mx1), and inducible nitric oxide synthase (iNOS). This study underlines the importance of the NP in avian influenza virus replication and pathogenicity.

Evolutionary repercussions of avian culling on host resistance and influenza virulence

BACKGROUND

Keeping pandemic influenza at bay is a global health priority. Of particular concern is the continued spread of the influenza subtype H5N1 in avian populations and the increasing frequency of transmission to humans. To decrease this threat, mass culling is the principal strategy for eradicating influenza in avian populations. Although culling has a crucial short-term epidemiological benefit, evolutionary repercussions on reservoir hosts and on the viral population have not been considered.

METHODS AND FINDINGS

To explore the epidemiological and evolutionary repercussions of mass avian culling, we combine population genetics and epidemiological influenza dynamics in a mathematical model parameterized by clinical, epidemiological, and poultry data. We model the virulence level of influenza and the selection on a dominant allele that confers resistance against influenza [1, 2] in a poultry population. Our findings indicate that culling impedes the evolution of avian host resistance against influenza. On the pathogen side of the coevolutionary race between pathogen and host, culling selects for heightened virulence and transmissibility of influenza.

CONCLUSIONS

Mass culling achieves a short-term benefit at the expense of long-term detriments: a more genetically susceptible host population, ultimately greater mortality, and elevated influenza virulence.

NP, PB1, and PB2 viral genes contribute to altered replication of H5N1 avian influenza viruses in chickens

The virulence determinants for highly pathogenic avian influenza viruses (AIVs) are considered multigenic, although the best characterized virulence factor is the hemagglutinin (HA) cleavage site. The capability of influenza viruses to reassort gene segments is one potential way for new viruses to emerge with different virulence characteristics. To evaluate the role of other gene segments in virulence, we used reverse genetics to generate two H5N1 recombinant viruses with differing pathogenicity in chickens. Single-gene reassortants were used to determine which viral genes contribute to the altered virulence. Exchange of the PB1, PB2, and NP genes impacted replication of the reassortant viruses while also affecting the expression of specific host genes. Disruption of the parental virus' functional polymerase complexes by exchanging PB1 or PB2 genes decreased viral replication in tissues and consequently the pathogenicity of the viruses. In contrast, exchanging the NP gene greatly increased viral replication and expanded tissue tropism, thus resulting in decreased mean death times. Infection with the NP reassortant virus also resulted in the upregulation of gamma interferon and inducible nitric oxide synthase gene expression. In addition to the impact of PB1, PB2, and NP on viral replication, the HA, NS, and M genes also contributed to the pathogenesis of the reassortant viruses. While the pathogenesis of AIVs in chickens is clearly dependent on the interaction of multiple gene products, we have shown that single-gene reassortment events are sufficient to alter the virulence of AIVs in chickens.

Developments in transgenic technology: applications for medicine

Recent advances in the efficiency of transgenic technology have important implications for medicine. The production of therapeutic proteins from animal bioreactors is well established and the first products are close to market. The genetic modification of pigs to improve their suitability as organ donors for xenotransplantation has been initiated, but many challenges remain. The use of transgenesis, in combination with the method of RNA interference to knock down gene expression, has been proposed as a method for making animals resistant to viral diseases, which could reduce the likelihood of transmission to humans. Here, the latest developments in transgenic technology and their applications relevant to medicine and human health will be discussed.

Resistance of Paramyxoviridae to Type I Interferon-InducedBos taurusMx1 Dynamin

Typical targets of type I interferon (IFN)-induced antiviral Mx proteins known to date have been shown to share a common profile: single-stranded negative-sense RNA viruses. Among them, human MxA is known to interfere with the replication of measles, human, and bovine parainfluenza-3 viruses (BoPi3V), that is, three members of the Paramyxoviridae family. Recently, bovine Mx1 protein (BoMx1) was included in the group of Mx proteins with authenticated antiviral potential, as it dramatically represses the replication of vesicular stomatitis virus (VSV). As replication in bovine cells of Pi3, respiratory syncytial (RS), and Sendai (Se) viruses, all members of the same family, is known to be reduced on IFN-alpha incorporation into the culture medium, it was hypothesized that the BoMx1 pathway possibly was involved, its antiviral spectrum thus probably extending to Paramyxoviridae. In this study, probing of BoMx1-inhibiting effects was carried out by infecting a transgenic Vero cell line that allows tightly regulated conditional expression of BoMx1 after doxycycline treatment with a wide array of Paramyxoviridae. Expressing and nonexpressing cells displayed similar viability, cytopathic effects (CPEs), and amounts of infectious virus yields, whatever the infecting virus or the multiplicity of infection (moi) imposed. It is, therefore, concluded that BoMx1 does not interfere with Paramyxoviridae.

Molecular cloning of the pufferfish (Takifugu rubripes) Mx gene and functional characterization of its promoter

Mx proteins are members of a family of interferon-inducible genes that are expressed by cells in response to viral infection. They are important determinants of innate immunity against viral infection in vertebrates. We cloned the pufferfish ( Takifugu rubripes) Mx gene and sequenced 80 kb from the Mx locus. The Fugu Mx gene spans 3.4 kb from the transcription start site to the polyadenylation signal, and is made up of 12 exons and 11 introns. The protein sequence encoded by the Fugu Mx gene is 77%, 48%, and 51% identical to that of trout Mx1, chicken Mx, and mouse Mx1 genes, respectively. The Fugu Mx gene is expressed in a variety of tissues, with high expression detected in the heart, gill, kidney, intestine, and brain. Analysis of the 5'-flanking sequence of the gene showed the presence of two interferon-stimulated response elements (ISRE) at positions -51 to 38 and -97 to 85, relative to the transcription start site. The Fugu Mx promoter was inducible by human IFN-beta in the human hepatoma (Huh7) cells and by polyinosinic: polycytidilic acid in the top minnow hepatoma (PLHC-1) cells. Deletion analysis of the promoter showed that both ISREs contributed to inducibility. These results demonstrate that the molecular mechanisms involved in Mx gene regulation are conserved between fish and mammals.

Polymorphisms and the differential antiviral activity of the chicken Mx gene

The nucleotide sequence of chicken Mx cDNA was reported earlier using the White Leghorn breed in Germany, but it showed no enhanced resistance to viruses. In this study, the nucleotide sequences of chicken Mx cDNA were determined in many breeds. A total of 25 nucleotide substitutions, of which 14 were deduced to cause amino acid exchanges, were detected, suggesting that the chicken Mx gene is very polymorphic. Transfected cell clones expressing chicken Mx mRNA were established after the Mx cDNA was constructed with an expression vector and introduced into mouse 3T3 cells, and the Mx genes from some breeds were demonstrated to confer positive antiviral responses to influenza virus and vesicular stomatitis virus. On the basis of the comparison among the antiviral activities associated with many Mx variations, a specific amino acid substitution at position 631 (Ser to Asn) was considered to determine the antivirally positive or negative Mx gene. Thus, a single amino acid substitution influences the antiviral activity of Mx in domesticated chickens.

Interferon-inducible Mx proteins in fish

Mx proteins are members of a family of interferon-inducible genes expressed when cells are treated with double-stranded RNA or virus infection. These proteins are important components of the antiviral response and form the first line of the body's defense against virus infections. The exact mechanism of action for these proteins has not been discovered, but mice missing the Mx genes are extremely sensitive to influenza virus infection. Mammals have between two and three Mx genes whose functions may vary with regard to the inhibition of a specific virus, cellular localization, and activity. The cDNA of three rainbow trout Mx proteins has been cloned and a comparison of their sequences with that of avian and mammalian species reveals striking conservation of domains. They all maintain the tripartite ATP/GTP binding domain and the dynamin family signature in the amino terminal half of the protein. In the carboxyl terminal half of the Mx proteins are the localization signals and the leucine zipper motifs which account for the trimerization of Mx in the cell. Like the rat and human Mx proteins, the different trout Mx proteins exhibit distinctly different immunohistochemical staining patterns in cells transfected with plasmids expressing RBTMx1, RBTMx2, or RBTMx3. To date, the antiviral function of the trout Mx proteins has not been satisfactorily established.

Replication-competent retroviral vectors for expressing genes in avian cells in vitro and in vivo

Replication-competent retroviral vectors based on Rous sarcoma virus (RSV) are becoming increasingly popular for expressing genes in both primary cell cultures and embryonic chick tissues in ovo. In this article, we review the features of RSV and its life cycle that make it suitable for use as a vector. We describe the design and use of the RCAS and RCAS (BP) series of vectors, which are currently the most widely used RSV-based vectors, illustrating both their strengths and weakness. Finally, we outline laboratory protocols suitable for the banding of these retroviral vectors.

Antiviral activity of alpha interferon in Sindbis virus-infected cells is restored by anti-E2 monoclonal antibody treatment

Pretreatment of AT3 rat prostatic carcinoma cells expressing the inhibitor of apoptosis bcl-2 (AT3-bcl-2 cells) with alpha interferon (IFN-alpha) affected replication of a virulent strain of Sindbis virus (SV) but did not protect against virus-induced cell death. Treatment of cells with IFN-alpha late during infection affected ongoing SV replication very little. Previous studies have shown that cross-linking of the viral glycoprotein E2 with antibody delays the inhibition of K+ influx by improving the function of Na+K+ATPase and the Na(+)-K(+)-2Cl-cotransport system in SV-infected cells (P. Després, J. W. Griffin, and D. E. Griffin, J. Virol. 69:7006-7014, 1995). In these studies, we have shown that treatment of infected cells with anti-E2 monoclonal antibody also restored the ability of IFN-alpha to induce antiviral activity in infected cells late during infection. The very low rate of virus release in SV-infected cells treated simultaneously with anti-E2 monoclonal antibody and IFN-alpha was postulated to be linked to inhibition of virus maturation. Synergistic effects of antibody and IFN-alpha are likely to be important for control of SV replication in vivo.

Transgenic chickens--methods and potential applications

The development of techniques for the genetic manipulation of poultry has lagged behind the technology available in mammalian systems, although several different approaches are being taken to overcome the problems associated with the manipulation of avian embryos. Several methods being developed for generating transgenic chickens are giving promising results, and the production of transgenic chickens by DNA microinjection has recently been demonstrated. Exploitation of this technology, in both basic and applied research, is now a possibility, and many applications of transgenic technology to poultry breeding and novel uses of transgenic chickens have been suggested.

Mx proteins: GTPases with antiviral activity

Mx proteins are synthesized in interferon-treated vertebrate cells. They have attracted much attention because some of them can block the multiplication of influenza A virus and certain other negative-stranded RNA viruses. Recently, Mx proteins have been shown to be GTPases with significant homology to dynamins and yeast VPS1, enzymes involved in intracellular protein trafficking. Several biochemical properties of dynamin and VPS1 are similar to those of Mx, promoting new speculation about how Mx proteins might interfere with virus multiplication.

Transgenic pigs carrying cDNA copies encoding the murine Mx1 protein which confers resistance to influenza virus infection

An important aspect of gene transfer into farm animals is the improvement of disease resistance. The mouse Mx1 protein is known to be sufficient to confer resistance to influenza viruses. Gene constructs containing the mouse Mx1 cDNA controlled by the human metallothionein IIA promoter (hMTIIA::Mx), the SV40 early enhancer/promoter region (SV40::Mx) and the mouse Mx1 promoter (mMx::Mx) were transferred into pigs. The results of the gene transfer experiments with the hMTIIA::Mx and the SV40::Mx constructs indicate that the permanent high-level synthesis of Mx1 might be deleterious to the organism: the gene transfer efficiency was surprisingly low, and all transgenic piglets born had rearrangements in their transgene copies that abolished protein synthesis. The use of the interferon (IFN)- and virus-inducible mMx::Mx construct resulted in normal gene transfer efficiency. Two transgenic pig lines could be established which expressed IFN-inducible mouse Mx1 mRNA. Extensive protein analysis did not detect mouse Mx1 in IFN-treated transgenic animals.

Nuclear localization of mouse Mx1 protein is necessary for inhibition of influenza virus

The interferon-induced Mx1 protein of mice confers selective resistance to influenza virus. It inhibits viral mRNA synthesis in the nucleus of influenza virus-infected cells. The related human MxA protein is localized in the cytoplasm and can inhibit influenza virus and vesicular stomatitis virus but not other viruses. MxA blocks a poorly defined cytoplasmic multiplication step of influenza virus that follows primary transcription of the viral genome. We previously showed that nuclear variants of MxA that carry an artificial nuclear translocation signal were also active against influenza virus. However, these variants blocked primary transcription of influenza virus. In the present study, we addressed the question of whether cytoplasmic forms of Mx1 were capable of mimicking the antiviral action of MxA by determining the antiviral activities of mutant mouse Mx1 protein. Cytoplasmic Mx1(E614), which differs from wild-type Mx1 by a single amino acid substitution in its nuclear transport signal, failed to inhibit the multiplication of influenza virus and vesicular stomatitis virus. Relocation of Mx1(E614) to the nucleus with the help of the simian virus 40 large T nuclear translocation signal attached to its amino terminus restored the influenza virus-inhibiting activity. Other changes in the carboxy-terminal region of Mx1 also abolished transport to the nucleus and simultaneously abolished antiviral activity. One of these variants, Mx1/A, gained activity against influenza virus upon relocation to the nucleus. These results demonstrate that unlike human MxA, the mouse Mx1 protein can function only in the nucleus. This finding has important implications regarding the mechanistic details of Mx protein action.

Domains 1 and 2 of ICAM-1 are sufficient to bind human rhinoviruses

The intercellular adhesion molecule-1 (ICAM-1) receptor was expressed in primary chicken embryo cells using a retroviral vector and shown to specifically bind major group human rhinoviruses (HRVs). A truncated, membrane-bound ICAM-1 protein containing N-terminal domains 1, 2, and 3 retained the ability to bind virus whereas proteins containing domains 1 and 2 or domain 1 were not expressed under these conditions. Soluble forms of ICAM-1 proteins were expressed to circumvent the reduced expression levels of shorter ICAM-1 truncations. Full-length and truncated ICAM-1 molecules containing only domains 1 and 2 were capable of neutralizing HRV binding to cells. Soluble receptors containing only domain 1 could not be recovered. Mutants of ICAM-1 lacking carbohydrate attachment sites were constructed and shown to have no effect on the ability of ICAM-1 to bind HRVs. In addition, ICAM-1 proteins expressed in the presence of tunicamycin also retained their virus binding capability. These data suggest that the N-terminal two domains of ICAM-1 are sufficient for virus interaction and that carbohydrates do not play a major role in virus binding.

Human-murine chimeras of ICAM-1 identify amino acid residues critical for rhinovirus and antibody binding

Human ICAM-1 is the cellular receptor for the major group of human rhinoviruses (HRVs). Previous studies have suggested that the N-terminal domain of ICAM-1 is critical for binding of the major group rhinoviruses. To further define the residues within domain 1 that are involved in virus binding, we constructed an extensive series of ICAM-1 cDNAs containing single and multiple amino acid residue substitutions. In each case, substitutions involved replacement of the human amino acids with those found in murine ICAM-1 to minimize conformational effects. To facilitate the mutagenesis process, a synthetic gene encompassing the first two domains of ICAM-1 was constructed which incorporated 27 additional restriction sites to allow mutagenesis by oligonucleotide replacement. Each of the new constructs was placed into a Rous sarcoma virus vector and expressed in primary chicken embryo fibroblast cells. Binding assays were performed with six major group HRVs, including one high-affinity binding mutant of HRV-14, and two monoclonal antibodies. Results indicated that different serotypes displayed a range of sensitivities to various amino acid substitutions. Amino acid residues of ICAM-1 showing the greatest effect on virus and antibody binding included Pro-28, Lys-29, Leu-30, Leu-37, Lys-40, Ser-67, and Pro-70.

Avian retroviral expression of luciferase

Biologically active replication-competent (subgroups A, B, and C) and replication-defective Rous sarcoma virus-derived vectors containing the cDNA encoding firefly luciferase as a reporter gene were constructed. In these retroviral vectors, luciferase is expressed from a spliced subgenomic mRNA. A biologically active replication-defective UR2 virus-derived vector expressing the reporter gene as a gag-luciferase fusion protein from an unspliced genomic mRNA was also constructed. The luciferase reporter gene was used because it lacks homology with chicken genomic sequences and because a rapid and sensitive direct enzymatic assay is available to monitor luciferase expression in retrovirus-infected cells. The levels of luciferase expression in luciferase recombinant retrovirus-infected chicken embryo fibroblasts are greater than 10(3) higher than that detected in uninfected cells or in cells infected with retroviral vectors carrying other genes. Endpoint dilution titration experiments demonstrated that one infected cell can be detected in a background of 10(3) uninfected cells. The vectors are stable in tissue culture and high level expression of the unselected luciferase reporter gene is maintained. The vectors were used to express luciferase in chicken embryos, demonstrating the potential utility of luciferase as a reporter in vivo.

Antiviral actions of interferon. Interferon-regulated cellular proteins and their surprisingly selective antiviral activities

Considerable progress has been made in the understanding of the molecular biology of the human interferon system. The genes encoding the interferons, their receptors, and the proteins that mediate many of their biological effects have been molecularly cloned and characterized. The availability of complete cDNA clones of components of the interferon systems has contributed significantly to our understanding of both the biology and the biochemistry of the antiviral actions of interferons. At the biological level, the antiviral effects of interferon may be viewed to be virus-type nonspecific. That is, treatment of cells with one type or even subspecies of interferon often leads to the generation of an antiviral state effective against a wide array of different RNA and DNA animal viruses. However, at the biochemical level, the antiviral action of interferon is often virus-type selective. That is, the apparent molecular mechanism which is primarily responsible for the inhibition of virus replication may differ considerably between virus types, and even host cells. For example, the IFN-regulated Mx protein selectively inhibits influenza virus but not other viruses when constitutively expressed in mouse cells. The IFN-regulated 2',5'-oligoadenylate synthetase selectively inhibits EMC and mengo viruses, two picornaviruses, but not viruses of other families when constitutively expressed in transfected cells. Some viruses are typically insensitive to the antiviral effects of interferon, both in cell culture and in intact animals. This lack of sensitivity to IFN may result from a virus-mediated direct antagonism of the interferon system. For example, in the case of adenovirus, the activation of the IFN-regulated RNA-dependent P1/elF-2 protein kinase is blocked by the virus-associated VA RNA. The relative sensitivity to interferon of different animal viruses varies appreciably. All three of the basic components required to measure an antiviral response may play a role in determining the relative effectiveness of the antiviral response: the species of interferon administered; the kind of cell treated; and, the type of virus used to challenge the interferon-treated host cell. Thus, the relative sensitivity to interferon observed for a particular interferon-cell-virus combination is likely the result of the equilibrium between the many agonists and antagonists which contribute to the overall response. That is, the relative sensitivity of a virus to the inhibitory action of IFN is governed by the qualitative nature and quantitative amount of the individual IFN-regulated cell proteins that may collectively contribute to the inhibition of virus replication.(ABSTRACT TRUNCATED AT 400 WORDS)