Activation of tumor antigen-specific cytotoxic T lymphocytes (CTLs) by human dendritic cells infected with an attenuated influenza A virus expressing a CTL epitope derived from the HER-2/neu proto-oncogene

From threat to cure: understanding of virus-induced cell death leads to highly immunogenic oncolytic influenza viruses

Oncolytic viruses constitute an emerging strategy in immunomodulatory cancer treatment. The first oncolytic virus, Talimogene laherparepvec (T-VEC), based on herpes simplex virus 1 (HSV-1), was approved by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) in 2015. The field of oncolytic virotherapy is still in its beginnings, since many promising viruses remain only superficially explored. Influenza A virus causes a highly immunogenic acute infection but never leads to a chronic disease. While oncolytic influenza A viruses are in preclinical development, they have not made the transition into clinical practice yet. Recent insights into different types of cell death caused by influenza A virus infection illuminate novel possibilities of enhancing its therapeutic effect. Genetic engineering and experience in influenza A virus vaccine development allow safe application of the virus in patients. In this review we give a summary of efforts undertaken to develop oncolytic influenza A viruses. We discuss strategies for targeting viral replication to cancerous lesions and arming them with immunogenic transgenes. We furthermore describe which modes of cell death are induced by influenza A virus infection and how these insights may be utilized to optimize influenza A virus-based oncolytic virus design.

Exposure to Low-Dose Radiation Enhanced the Antitumor Effect of a Dendritic Cell Vaccine

The unsatisfactory clinical efficacy of dendritic cell (DC)-based cancer vaccines prepared by conventional methods is partly due to their insufficient capacity for migration. Our previous study showed that exposure to low-dose radiation (LDR) at a dose of 0.2 Gy promoted DC migration in vitro. The present study further investigates whether exposure to LDR at a dose of 0.2 Gy during the DC vaccine preparation could increase the antitumor effect of DC vaccines derived from mouse bone marrow. Our results showed that the migratory capacities of DCs were significantly increased after exposure to LDR. Furthermore, exposure to LDR resulted in an increased ability of DCs to induce T-cell proliferation, and the cytotoxic effect of cytotoxic T lymphocytes (CTLs) primed by the DCs exposed to LDR was significantly enhanced. An in vivo study using a mouse transplanted tumor model showed that subcutaneous injections of a DC vaccine exposed to LDR led to an increased mouse survival rate, infiltration of CTLs into tumor tissue, and apoptosis of tumor cells, which were accompanied by significant upregulation of serum interferon γ and interleukin 12. These results indicate that exposing DCs to LDR during the DC vaccine preparation is an effective approach to enhance its antitumor effect.

Cytoprotective effects of high dose of α-galactosylceramide against activation-induced CD4+ T and CD8+ T cell death as an adjuvant

OBJECTIVE

To investigate the cytoprotective effects of high dose of α-galactosylceramide (α-GC) on the activation-induced CD4+ T and CD8+ T cell death.

METHODS

Experimental autoimmune encephalomyelitis (EAE) was induced using adoptive transfer of MOGCD4+ cells treated using α-GC into recipient C57BL/6 mice while the MOGCD4+ cells treated using 0.5% polysorbate were set as vehicle group, based on which to investigate the effects of α-GC on activation induced CD4+ T cell death. Additionally, an EG7 tumor-bearing mice model is established using adoptive transfer of CD8+ T cells, based on which to investigate the effect of α-GC on the apoptosis of CD8+ T cells.

RESULTS

A higher induction rate was noticed after adoptive transfer of MOGCD4+ cells treated using α-GC together with the severity of EAE compared with the conventional methods. Longer survival duration was noted in the green fluorescent protein (GFP) labeled MOGT in the α-GC group compared with the vehicle group (P < 0.05). Severe inflammatory cell infiltration and myelinoclasis was noted in the white matter of nervous system in the α-GC group. In the EG7 tumor model, more adoptive CD8+ T cells were survived in α-GC group compared with that of vehicle group. The growth of tumor mass was significantly inhibited in α-GC group.

CONCLUSIONS

high dose of α-GC could be used as an adjuvant for inhibiting activation-induced CD4+ T and CD8+ T cell death. Our study could provide helpful information for the development of adoptive cell therapy with reduced programmed cell death.

Programmed nanoparticles for combined immunomodulation, antigen presentation and tracking of immunotherapeutic cells

We report programmed nanoparticles (pNPs) that can tailor the immunotherapeutic function of primary bone marrow-derived dendritic cells (BMDCs) by ex vivo combined immunomodulation and track the in vivo migration of them after injection into body. Because DCs are the most effective antigen-presenting cells (APCs) that are able to present the antigens to T cells that contribute to tumor rejection, the maturation and monitoring of therapeutic DCs are essential for the efficient cancer immunotherapy. For combined immunomodulation of DCs, poly (lactic-co-glycolic acid) (PLGA) NPs containing both small interfering RNA (siRNA) for the knock-down of immune-suppressor gene (signal transducer and activator of transcription-3, STAT3) of DCs and an immune response modifier (imiquimod, R837) for the activation of DCs through the toll-like receptor 7 (TLR7) were developed. To deliver tumor antigen-specific information to DCs ex vivo and track the migration of DCs in vivo, another type of PLGA NPs containing tumor model antigen (ovalbumin, OVA) and near-infrared (NIR) fluorophores (indocyanine green, ICG) were also fabricated. These pNPs were taken up efficiently by DCs and various cytokines were expressed in matured DCs. DCs treated with pNPs also efficiently presented antigen-peptide to CD8 OVA 1.3 T cells through cross-presentation. Immunization of mice with these pNPs-treated DCs induced OVA-specific cytotoxic T lymphocytes (CTL) activity against the EG7-OVA tumor model and inhibited tumor growth efficiently. In addition, the migration of PLGA NPs-treated DCs to lymph nodes was monitored by NIR imaging technique. These multifunctional pNPs represent a promising technology for the combined immunomodulation and antigen-specific tumor therapy.

Engineering influenza viral vectors

The influenza virus is a respiratory pathogen with a negative-sense, segmented RNA genome. Construction of recombinant influenza viruses in the laboratory was reported starting in the 1980s. Within a short period of time, pioneer researchers had devised methods that made it possible to construct influenza viral vectors from cDNA plasmid systems. Herein, we discuss the evolution of influenza virus reverse genetics, from helper virus-dependent systems, to helper virus-independent 17-plasmid systems, and all the way to 3- and 1- plasmid systems. Successes in the modification of different gene segments for various applications, including vaccine and gene therapies are highlighted.

Serum-free microcarrier based production of replication deficient influenza vaccine candidate virus lacking NS1 using Vero cells

BACKGROUND

Influenza virus is a major health concern that has huge impacts on the human society, and vaccination remains as one of the most effective ways to mitigate this disease. Comparing the two types of commercially available Influenza vaccine, the live attenuated virus vaccine is more cross-reactive and easier to administer than the traditional inactivated vaccines. One promising live attenuated Influenza vaccine that has completed Phase I clinical trial is deltaFLU, a deletion mutant lacking the viral Nonstructural Protein 1 (NS1) gene. As a consequence of this gene deletion, this mutant virus can only propagate effectively in cells with a deficient interferon-mediated antiviral response. To demonstrate the manufacturability of this vaccine candidate, a batch bioreactor production process using adherent Vero cells on microcarriers in commercially available animal-component free, serum-free media is described.

RESULTS

Five commercially available animal-component free, serum-free media (SFM) were evaluated for growth of Vero cells in agitated Cytodex 1 spinner flask microcarrier cultures. EX-CELL Vero SFM achieved the highest cell concentration of 2.6 × 10^6 cells/ml, whereas other SFM achieved about 1.2 × 10^6 cells/ml. Time points for infection between the late exponential and stationary phases of cell growth had no significant effect in the final virus titres. A virus yield of 7.6 Log10 TCID50/ml was achieved using trypsin concentration of 10 μg/ml and MOI of 0.001. The Influenza vaccine production process was scaled up to a 3 liter controlled stirred tank bioreactor to achieve a cell density of 2.7 × 10^6 cells/ml and virus titre of 8.3 Log10 TCID50/ml. Finally, the bioreactor system was tested for the production of the corresponding wild type H1N1 Influenza virus, which is conventionally used in the production of inactivated vaccine. High virus titres of up to 10 Log10 TCID50/ml were achieved.

CONCLUSIONS

We describe for the first time the production of Influenza viruses using Vero cells in commercially available animal-component free, serum-free medium. This work can be used as a basis for efficient production of attenuated as well as wild type Influenza virus for research and vaccine production.

The immunological potency and therapeutic potential of a prototype dual vaccine against influenza and Alzheimer's disease

Background

Numerous pre-clinical studies and clinical trials demonstrated that induction of antibodies to the β-amyloid peptide of 42 residues (Aβ₄₂) elicits therapeutic effects in Alzheimer's disease (AD). However, an active vaccination strategy based on full length Aβ₄₂ is currently hampered by elicitation of T cell pathological autoreactivity. We attempt to improve vaccine efficacy by creating a novel chimeric flu vaccine expressing the small immunodominant B cell epitope of Aβ₄₂. We hypothesized that in elderly people with pre-existing memory Th cells specific to influenza this dual vaccine will simultaneously boost anti-influenza immunity and induce production of therapeutically active anti-Aβ antibodies.

Methods

Plasmid-based reverse genetics system was used for the rescue of recombinant influenza virus containing immunodominant B cell epitopes of Aβ₄₂ (Aβ_1-7/10).

Results

Two chimeric flu viruses expressing either 7 or 10 aa of Aβ₄₂ (flu-Aβ_1-7 or flu-Aβ_1-10) were generated and tested in mice as conventional inactivated vaccines. We demonstrated that this dual vaccine induced therapeutically potent anti-Aβ antibodies and anti-influenza antibodies in mice.

Conclusion

We suggest that this strategy might be beneficial for treatment of AD patients as well as for prevention of development of AD pathology in pre-symptomatic individuals while concurrently boosting immunity against influenza.

A peptidoglycan monomer with the glutamine to serine change and basic peptides bind in silico to TLR-2 (403-455)

Bacterial cell wall polysaccharides, such as PGN, bind and activate TLR-2, NOD2 and PGRP on monocytes/macrophages and activate inflammation. We found that the peptides containing basic amino acids (cations) at N -terminus and tyrosine at C-terminus interfered with activating ability of PGN. This finding is significant because the ECD of TLR-2 in vivo encounters a large number of proteins or peptides. Some should bind ECD and "pre-form" TLR-2 to respond or not to its activators, although they cannot activate TLR-2 alone. TLR-2 is receptor for a large number of ligands, including lipopeptides and bacterial cell wall glycoproteins. A binding site for lipopeptides has been identified; however, a binding site for soluble or multimeric PGN has not been proposed. To identify the candidate binding sites of peptides and PGN on TLR-2, we modeled docking of peptides and of the PGN monomer (PGN-S-monomer) to extracellular domain (ECD-TLR-2) of the unbound TLR-2. Quantification, in silico, of free energy of binding (DG) identified 2 close sites for peptides and PGN. The PGN-S-monomer binding site is between amino acids TLR-2, 404-430 or more closely TLR-2, 417-428. The peptide-binding site is between amino acids TLR-2, 434-455. Molecular models show PGN-S-monomer inserts its N -acetyl-glucosamine (NAG) deep in the TLR-2 coil, while its terminal lysine interacts with inside (Glu(403)) and outside pocket (Tyr(378)). Peptides insert their two N -terminal arginines or their C-terminal tyrosines in the TLR-2 coil. PGN did not bind the lipopeptide-binding site in the TLR-2. It can bind the C-terminus, 572-586 (DG = 0.026 kcal), of "lipopeptide-bound" TLR-2. An additional, low-affinity PGN-binding site is TLR-2 (227-237). MTP, MDP, and lysine-less PGN bind to TLR-2, 87-113. This is the first report identifying candidate binding sites of monomer PGN and peptides on TLR-2. Experimental verification of our findings is needed to create synthetic adjuvant for vaccines. Such synthetic PGN can direct both adjuvant and cancer antigen to TLR-2.

Generation of replication-competent recombinant influenza A viruses carrying a reporter gene harbored in the neuraminidase segment

Replication-competent influenza viruses carrying reporter genes are of great use for basic research, screening of antiviral drugs, and neutralizing of antibodies. In this study, two recombinant influenza A viruses with a neuraminidase (NA) segment harboring enhanced green fluorescent protein (EGFP) in the background of A/PR/8/34 (PR8) were generated. The viral RNA (vRNA)-specific packaging signals for NA were largely retained for efficient packaging. An "autocleave" 2A peptide sequence, which was inserted at the N terminus or the COOH terminus of NA to link with EGFP, enabled NA and EGFP to be expressed monocistronically. Further analysis demonstrated that both viruses, named rPR8-EGFP+NA and rPR8-NA+EGPF, although with some characteristic differences in growth and EGFP expression, could replicate in noncomplementary cells and propagate to large quantities while maintaining genome stability after multiple passages in embryonated eggs. These replication-competent influenza viruses carrying reporter genes are a great addition to the tool set for developing antiviral therapeutics and vaccines and for in vivo studies of viral dissemination and pathogenicity.

Attenuated influenza virus vaccines with modified NS1 proteins

The development of reverse genetics techniques allowing the rescue of influenza virus from plasmid DNA has opened up the possibility of inserting mutations into the genome of this virus for the generation of novel live attenuated influenza virus vaccines. Modifications introduced into the viral NS1 gene via reverse genetics have resulted in attenuated influenza viruses with promising vaccine potential. One of the main functions of the NS1 protein of influenza virus is the inhibition of the innate host type I interferon-mediated antiviral response. Upon viral infection, influenza viruses with modified NS1 genes induce a robust local type I interferon response that limits their replication, resulting in disease attenuation in different animal models. Nevertheless, these viruses can be grown to high titers in cell- and egg-based substrates with deficiencies in the type I IFN system. Intranasal inoculation of mice, pigs, horses, and macaques with NS1-modified influenza virus strains induced robust humoral and cellular immune responses, and generated immune protection against challenge with wild-type virus. This protective response was not limited to homologous strains of influenza viruses, as reduced replication of heterologous strains was also demonstrated in animals vaccinated with NS1-modified viruses, indicating the induction of a broad cross-neutralizing response by these vaccine candidates. The immunogenicity of NS1-modified viruses correlated with enhanced activation of antigen-presenting cells. While further studies on their safety and efficacy are still needed, the results obtained so far indicate that NS1-modified viruses could represent a new generation of improved influenza virus vaccines, and they suggest that modifying viral interferon antagonists in other virus families is a promising strategy for the generation of live attenuated virus vaccines.

Taxol Increases the Amount and T Cell–Activating Ability of Self-Immune Stimulatory Multimolecular Complexes Found in Ovarian Cancer Cells

It has been proposed that chemotherapy enhances tumor antigen (TA)-specific immunity. The molecular form of TA from ovarian tumor that activates cellular immunity is unknown. We report here identification of a novel molecular form of immunogenic TA for CD8(+) cells named self-immune stimulatory multimolecular complexes (ISMMC). ISMMC consist of a molecular complex of polyosome/ribosome-bound ubiquitinated nascent HER-2 polypeptides. This complex is chaperoned by heat shock protein Gp96, which mediates ISMMC uptake by antigen-presenting cells through the scavenger receptor CD91. RNAs in ISMMC stimulate immature dendritic cells to secrete interleukin 12 and induce IFN-gamma in peripheral blood mononuclear cells. ISMMC dissociate, retrotranslocate from the lysosome to cytoplasm, and are processed to peptides by the proteasome. At subpharmacologic doses, Taxol increased the amount of ISMMC by three to four times and modified their composition by inducing the attachment of cochaperones of HSP70, such as the mitotic-phase phosphoprotein 11J. On a total protein basis, Taxol induced ISMMC, expanded more CD8(+) cells, activated more CD56(+) NKG2D(+) cells to produce IFN-gamma, and were more potent inducers of high T-cell receptor density Perforin(+) cells than native ISMMC and peptide E75. Elucidation of the composition of ISMMC and identification of adducts formed by Taxol should be important for developing molecular cancer vaccines.

Recombinant influenza virus vectors

Newly optimized reverse genetics techniques have allowed influenza researchers to generate recombinant influenza viruses expressing mutant viral proteins, as well as foreign proteins. Approaches include the insertion of noninfluenza epitopes and polypeptides into viral glycoproteins, foreign open reading frames as additional segments, and the fusion of independent proteins into viral genes encoding glycoproteins or the nonstructural protein 1. These genetically engineered viruses have been demonstrated to be good viral vectors for mounting B- and T-cell responses and are attractive candidates for vaccine development. As the molecular biology of influenza viral infection is more fully understood, influenza vectors can be concurrently manipulated to produce designed chimeric viruses, unveiling the possibility of a prosperous future with cheap, effective and safe vaccines against different human diseases.

Live-attenuated influenza viruses as delivery vectors for Chlamydia vaccines

Effective delivery systems are needed to design efficacious vaccines against the obligate intracellular bacterial pathogen, Chlamydia trachomatis. Potentially effective delivery vehicles should promote the induction of adequate levels of mucosal T-cell and antibody responses that mediate long-term protective immunity. Antigen targeting to the nasal-associated lymphoid tissue (NALT) is effective for inducing high levels of specific immune effectors in the genital mucosa, and therefore suitable for vaccine delivery against genital chlamydial infection. We tested the hypothesis that live attenuated influenza A viruses are effective viral vectors for intranasal delivery of subunit vaccines against genital chlamydial infection. Recombinant influenza A/PR8/34 (H1N1) viruses were generated by insertion of immunodominant T-cell epitopes from chlamydial major outer membrane protein into the stalk region of the neuraminidase gene. Intranasal immunization of mice with viral recombinants resulted in a strong T helper 1 (Th1) response against intact chlamydial elementary bodies. Also, immunized mice enjoyed a significant state of protective immunity (P > 0.002) by shedding less chlamydiae and rapidly clearing the infection. Furthermore, a high frequency of Chlamydia-specific Th1 was measured in the genital mucosal and systemic draining lymphoid tissues within 24 hr after challenge of vaccinated mice. Moreover, multiple epitope delivery provided a vaccine advantage over single recombinants. Besides, long-term protective immunity correlated with the preservation of a robustly high frequency of specific Th1 cells and elevated immunoglobulin G2a in genital secretions. Because live attenuated influenza virus vaccines are safe and acceptable for human use, they may provide a new and reliable approach to deliver efficacious vaccines against sexually transmitted diseases.

Prostate tumor cells infected with a recombinant influenza virus expressing a truncated NS1 protein activate cytolytic CD8+ cells to recognize noninfected tumor cells

Many viral oncolytic approaches against cancer are based on the ability of specific viruses to replicate in tumors expressing components of the constitutively activated Ras/mitogen-activated protein kinase (MAPK) pathways and/or inhibited or dysregulated alpha/beta interferon (IFN-alpha/beta) response pathways. A major issue when considering these approaches is their applicability to tumors that lack activated Ras. To identify the effector mechanisms activated by oncolytic viruses, we investigated inhibition of proliferation of the prostate cancer line LNCap by the recombinant TR-NS1 influenza A virus, a genetically attenuated influenza A/PR8/34 virus expressing a truncated nonstructural protein (NS1) of 126 amino acids. LNCap cells lack constitutively activated MAPK, extracellular signal-regulated kinase (ERK), and p38 and are resistant to death by IFN-alpha. Truncation of the NS1 protein of influenza viruses is known to result in viral attenuation due to a reduced ability of the NS1 to inhibit the IFN-alpha/beta response. Infection with TR-NS1 virus rapidly activated ERK-1 more than ERK-2 in LNCap cells. Importantly, TR-NS1 virus infection transiently inhibited cell proliferation and induced apoptosis in LNCap cells. Addition of peripheral blood mononuclear cells (PBMC) and interleukin 12 (IL-12) to TR-NS1 virus-infected LNCap cells (TR-NS1-LNCap) resulted in faster elimination of TR-NS1-LNCap cells compared with LNCap cells. Moreover, TR-NS1-LNCap cells induced IFN-gamma in PBMC. The levels of IFN-gamma were amplified by IL-12. TR-NS1-LNCap cells also induced tumor-lytic cytotoxic T lymphocytes (CTL). These CTL lysed noninfected LNCap cells in a CD8-dependent manner. Activation of cellular immunity to tumor cells by viruses is an intriguing effector pathway, which should be especially significant for elimination of human tumors that lack activated Ras.

Recombinant influenza A viruses harboring optimized dicistronic NA segment with an extended native 5' terminal sequence: induction of heterospecific B and T cell responses in mice

We generated novel recombinant influenza A viruses (vNA38) harboring dicistronic NA segments with an extended native 5' terminal sequence of 70 nucleotides comprised of the last 42 nucleotides of the NA ORF and the 5' noncoding region (5' NCR). vNA38 viruses replicated stably and more efficiently than vNA35 viruses with a dicistronic NA segment comprised of the native 5' NCR only, that we described previously (Vieira Machado, A., Naffakh, N., van der Werf, S., Escriou, N., 2003. Expression of a foreign gene by stable recombinant influenza viruses harboring a dicistronic genomic segment with an internal promoter. Virology 313, 235-249). In addition, vNA38 viruses drove the expression of higher levels of encoded heterologous proteins than corresponding vNA35 viruses, both in cell culture and in the pulmonary tissue of infected mice. These data demonstrate that a sequence overlapping 5' coding and noncoding regions of the NA segment determines efficient replication and/or propagation of the vRNA. Intranasal immunization of mice with live vNA38 viruses induced B and T cell responses specific for the heterologous protein expressed, establishing the usefulness of such recombinant influenza viruses with a dicistronic segment for the development of live bivalent vaccines.

A Novel Viral System for Generating Antigen-Specific T Cells

Dendritic cell (DC)-based vaccines are increasingly used for the treatment of patients with malignancies. Although these vaccines are typically safe, consistent and lasting generation of tumor-specific immunity has been rarely demonstrated. Improved methods for delivering tumor Ags to DCs and approaches for overcoming tolerance or immune suppression to self-Ags are critical for improving immunotherapy. Viral vectors may address both of these issues, as they can be used to deliver intact tumor Ags to DCs, and have been shown to inhibit the suppression mediated by CD4+CD25+ regulatory T cells. We have evaluated the potential use of Venezuelan equine encephalitis virus replicon particles (VRPs) for in vitro Ag delivery to human monocyte-derived DCs. VRPs efficiently transduced immature human DCs in vitro, with approximately 50% of immature DCs expressing a vector-driven Ag at 12 h postinfection. VRP infection of immature DCs was superior to TNF-alpha treatment at inducing phenotypic maturation of DCs, and was comparable to LPS stimulation. Additionally, VRP-infected DC cultures secreted substantial amounts of the proinflammatory cytokines IL-6, TNF-alpha, and IFN-alpha. Finally, DCs transduced with a VRP encoding the influenza matrix protein (FMP) stimulated 50% greater expansion of FMP-specific CD8+ CTL when compared with TNF-alpha-matured DCs pulsed with an HLA-A*0201-restricted FMP peptide. Thus, VRPs can be used to deliver Ags to DCs resulting in potent stimulation of Ag-specific CTL. These findings provide the rationale for future studies evaluating the efficacy of VRP-transduced DCs for tumor immunotherapy.

HER-2: A biomarker at the crossroads of breast cancer immunotherapy and molecular medicine

The oncoprotein encoded by the HER-2 oncogene is a member of the HER family of receptor tyrosine kinases and is actually the first successfully exploited target molecule in new biomolecular therapies of solid tumors. The association of HER-2 overexpression with human tumors, its extracellular accessibility, as well as its involvement in tumor aggressiveness are all factors that make this receptor an appropriate target for tumor-specific therapy. In addition, HER-2 overexpression fosters its immunogenicity, as shown by the frequency of B and T cell-mediated responses against this oncoprotein in cancer patients, and it is being investigated as a promising molecule for either passive and active immunotherapy strategies. This review summarizes a number of immune intervention approaches that target HER-2 in breast cancer.

A recombinant influenza A virus expressing an RNA-binding-defective NS1 protein induces high levels of beta interferon and is attenuated in mice

Previously we found that the amino-terminal region of the NS1 protein of influenza A virus plays a key role in preventing the induction of beta interferon (IFN-beta) in virus-infected cells. This region is characterized by its ability to bind to different RNA species, including double-stranded RNA (dsRNA), a known potent inducer of IFNs. In order to investigate whether the NS1 RNA-binding activity is required for its IFN antagonist properties, we have generated a recombinant influenza A virus which expresses a mutant NS1 protein defective in dsRNA binding. For this purpose, we substituted alanines for two basic amino acids within NS1 (R38 and K41) that were previously found to be required for RNA binding. Cells infected with the resulting recombinant virus showed increased IFN-beta production, demonstrating that these two amino acids play a critical role in the inhibition of IFN production by the NS1 protein during viral infection. In addition, this virus grew to lower titers than wild-type virus in MDCK cells, and it was attenuated in mice. Interestingly, passaging in MDCK cells resulted in the selection of a mutant virus containing a third mutation at amino acid residue 42 of the NS1 protein (S42G). This mutation did not result in a gain in dsRNA-binding activity by the NS1 protein, as measured by an in vitro assay. Nevertheless, the NS1 R38AK41AS42G mutant virus was able to replicate in MDCK cells to titers close to those of wild-type virus. This mutant virus had intermediate virulence in mice, between those of the wild-type and parental NS1 R38AK41A viruses. These results suggest not only that the IFN antagonist properties of the NS1 protein depend on its ability to bind dsRNA but also that they can be modulated by amino acid residues not involved in RNA binding.