The influenza virus enigma.

Both seasonal and pandemic influenza continue to challenge both scientists and clinicians. Drug-resistant H1N1 influenza viruses have dominated the 2009 flu season, and the H5N1 avian influenza virus continues to kill both people and poultry in Eurasia. Here, we discuss the pathogenesis and transmissibility of influenza viruses and we emphasize the need to find better predictors of both seasonal and potentially pandemic influenza.

Priming Vaccination With Influenza Virus H5 Hemagglutinin Antigen Significantly Increases the Duration of T cell Responses Induced by a Heterologous H5 Booster Vaccination.

BACKGROUND: Influenza A(H5N1) virus and other avian influenza virus strains represent major pandemic threats. Like all influenza A virus strains, A(H5N1) viruses evolve rapidly. Innovative immunization strategies are needed to induce cross-protective immunity. METHODS: Subjects primed with clade 1 H5 antigen, with or without adjuvant, and H5-naive individuals were boosted with clade 2 H5 antigen. The impact of priming on T cells capable of both proliferation and cytokine production after antigen restimulation was assessed. RESULTS: Subjects previously vaccinated with clade 1 H5 antigen developed significantly enhanced clade 2 H5 cross-reactive T cell responses detectable 6 months after vaccination with clade 2 H5 antigen. Priming dose (15 µg vs 45 or 90 µg) had no effect on magnitude of heterotypic H5 T cell responses. In contrast, age at priming negatively modulated both the magnitude and duration of heterotypic H5 T cell responses. Elderly subjects developed significantly less heterotypic H5 T cell boosting, predominantly for T cells capable of cytokine production. Adjuvant had a positive albeit weaker effect than age. The magnitude of CD4(+) interferon-γ producing T cells correlated with H5 antibody responses. CONCLUSIONS: H5 heterotypic priming prior to onset of an A(H5N1) pandemic may increase magnitude and duration of immunity against a newly drifted pandemic H5 virus.

Increased acid stability of the hemagglutinin protein enhances H5N1 influenza virus growth in the upper respiratory tract but is insufficient for transmission in ferrets.

Influenza virus entry is mediated by the acidic-pH-induced activation of hemagglutinin (HA) protein. Here, we investigated how a decrease in the HA activation pH (an increase in acid stability) influences the properties of highly pathogenic H5N1 influenza virus in mammalian hosts. We generated isogenic A/Vietnam/1203/2004 (H5N1) (VN1203) viruses containing either wild-type HA protein (activation pH 6.0) or an HA2-K58I point mutation (K to I at position 58) (activation pH 5.5). The VN1203-HA2-K58I virus had replication kinetics similar to those of wild-type VN1203 in MDCK and normal human bronchial epithelial cells and yet had reduced growth in human alveolar A549 cells, which were found to have a higher endosomal pH than MDCK cells. Wild-type and HA2-K58I viruses promoted similar levels of morbidity and mortality in C57BL/6J mice and ferrets, and neither virus transmitted efficiently to naive contact cage-mate ferrets. The acid-stabilizing HA2-K58I mutation, which diminishes H5N1 replication and transmission in ducks, increased the virus load in the ferret nasal cavity early during infection while simultaneously reducing the virus load in the lungs. Overall, a single, acid-stabilizing mutation was found to enhance the growth of an H5N1 influenza virus in the mammalian upper respiratory tract, and yet it was insufficient to enable contact transmission in ferrets in the absence of additional mutations that confer α(2,6) receptor binding specificity and remove a critical N-linked glycosylation site. The information provided here on the contribution of HA acid stability to H5N1 influenza virus fitness and transmissibility in mammals in the background of a non-laboratory-adapted virus provides essential information for the surveillance and assessment of the pandemic potential of currently circulating H5N1 viruses.

The polymerase complex genes contribute to the high virulence of the human H5N1 influenza virus isolate A/Vietnam/1203/04.

H5N1 influenza viruses transmitted from poultry to humans in Asia cause high mortality and pose a pandemic threat. Viral genes important for cell tropism and replication efficiency must be identified to elucidate and target virulence factors. We applied reverse genetics to generate H5N1 reassortants combining genes of lethal A/Vietnam/1203/04 (VN1203), a fatal human case isolate, and nonlethal A/chicken/Vietnam/C58/04 (CH58) and tested their pathogenicity in ferrets and mice. The viruses' hemagglutinins have six amino acids differences, identical cleavage sites, and avian-like alpha-(2,3)-linked receptor specificity. Surprisingly, exchanging hemagglutinin and neuraminidase genes did not alter pathogenicity, but substituting CH58 polymerase genes completely attenuated VN1203 virulence and reduced viral polymerase activity. CH58's NS gene partially attenuated VN1203 in ferrets but not in mice. Our findings suggest that for high virulence in mammalian species an avian H5N1 virus with a cleavable hemagglutinin requires adaptive changes in polymerase genes to overcome the species barrier. Thus, novel antivirals targeting polymerase proteins should be developed.

The NIH Human Microbiome Project.

The Human Microbiome Project (HMP), funded as an initiative of the NIH Roadmap for Biomedical Research (http://nihroadmap.nih.gov), is a multi-component community resource. The goals of the HMP are: (1) to take advantage of new, high-throughput technologies to characterize the human microbiome more fully by studying samples from multiple body sites from each of at least 250 "normal" volunteers; (2) to determine whether there are associations between changes in the microbiome and health/disease by studying several different medical conditions; and (3) to provide both a standardized data resource and new technological approaches to enable such studies to be undertaken broadly in the scientific community. The ethical, legal, and social implications of such research are being systematically studied as well. The ultimate objective of the HMP is to demonstrate that there are opportunities to improve human health through monitoring or manipulation of the human microbiome. The history and implementation of this new program are described here.

Changes in H5N1 influenza virus hemagglutinin receptor binding domain affect systemic spread.

The HA of influenza virus is a receptor-binding and fusion protein that is required to initiate infection. The HA receptor-binding domain determines the species of sialyl receptors recognized by influenza viruses. Here, we demonstrate that changes in the HA receptor-binding domain alter the ability of the H5N1 virus to spread systemically in mice. The A/Vietnam/1203/04 (VN1203) and A/Hong Kong/213/03 (HK213) viruses are consistently lethal to domestic chickens but differ in their pathogenicity to mammals. Insertion of the VN1203 HA and neuraminidase (NA) genes into recombinant HK213 virus expanded its tissue tropism and increased its lethality in mice; conversely, insertion of HK213 HA and NA genes into recombinant VN1203 virus decreased its systemic spread and lethality. The VN1203 and HK213 HAs differ by 10 aa, and HK213 HA has shown greater binding affinity for synthetic alpha2,6-linked sialyl receptor. Introduction of an S227N change and removal of N-linked glycosylation at residue 158 increased the alpha2,6-binding affinity of VN1203 HA. Recombinant VN1203 virus carrying the S227N change alone or with the residue-158 glycosylation site removed showed reduced lethality and systemic spread in mice but not in domestic chickens. Wild-type VN1203 virus exhibited the greatest efficiency in systemic spread after intramuscular inoculation and in infection of mouse bone marrow-derived dendritic cells and conventional pulmonary dendritic cells. These results show that VN1203 HA glycoprotein confers pathogenicity by facilitating systemic spread in mice; they also suggest that a minor change in receptor binding domain may modulate the virulence of H5N1 viruses.

The pH of activation of the hemagglutinin protein regulates H5N1 influenza virus pathogenicity and transmissibility in ducks.

While the molecular mechanism of membrane fusion by the influenza virus hemagglutinin (HA) protein has been studied extensively in vitro, the role of acid-dependent HA protein activation in virus replication, pathogenesis, and transmission in vivo has not been characterized. To investigate the biological significance of the pH of activation of the HA protein, we compared the properties of four recombinant viruses with altered HA protein acid stability to those of wild-type influenza virus A/chicken/Vietnam/C58/04 (H5N1) in vitro and in mallards. Membrane fusion by wild-type virus was activated at pH 5.9. Wild-type virus had a calculated environmental persistence of 62 days and caused extensive morbidity, mortality, shedding, and transmission in mallards. An N114K mutation that increased the pH of HA activation by 0.5 unit resulted in decreased replication, genetic stability, and environmental stability. Changes of +0.4 and -0.5 unit in the pH of activation by Y23H and K58I mutations, respectively, reduced weight loss, mortality, shedding, and transmission in mallards. An H24Q mutation that decreased the pH of activation by 0.3 unit resulted in weight loss, mortality, clinical symptoms, and shedding similar to those of the wild type. However, the HA-H24(1)Q virus was shed more extensively into drinking water and persisted longer in the environment. The pH of activation of the H5 HA protein plays a key role in the propagation of H5N1 influenza viruses in ducks and may be a novel molecular factor in the ecology of influenza viruses. The data also demonstrate that H5N1 neuraminidase activity increases the pH of activation of the HA protein in vitro.

Dendritic cell responses to early murine cytomegalovirus infection: subset functional specialization and differential regulation by interferon alpha/beta.

Differentiation of dendritic cells (DCs) into particular subsets may act to shape innate and adaptive immune responses, but little is known about how this occurs during infections. Plasmacytoid dendritic cells (PDCs) are major producers of interferon (IFN)-alpha/beta in response to many viruses. Here, the functions of these and other splenic DC subsets are further analyzed after in vivo infection with murine cytomegalovirus (MCMV). Viral challenge induced PDC maturation, their production of high levels of innate cytokines, and their ability to activate natural killer (NK) cells. The conditions also licensed PDCs to efficiently activate CD8 T cells in vitro. Non-plasmacytoid DCs induced T lymphocyte activation in vitro. As MCMV preferentially infected CD8alpha+ DCs, however, restricted access to antigens may limit plasmacytoid and CD11b+ DC contribution to CD8 T cell activation. IFN-alpha/beta regulated multiple DC responses, limiting viral replication in all DC and IL-12 production especially in the CD11b+ subset but promoting PDC accumulation and CD8alpha+ DC maturation. Thus, during defense against a viral infection, PDCs appear specialized for initiation of innate, and as a result of their production of IFN-alpha/beta, regulate other DCs for induction of adaptive immunity. Therefore, they may orchestrate the DC subsets to shape endogenous immune responses to viruses.

Amino acid residues in the fusion peptide pocket regulate the pH of activation of the H5N1 influenza virus hemagglutinin protein.

The receptor specificity and cleavability of the hemagglutinin (HA) protein have been shown to regulate influenza A virus transmissibility and pathogenicity, but little is known about how its pH of activation contributes to these important biological properties. To identify amino acid residues that regulate the acid stability of the HA protein of H5N1 influenza viruses, we performed a mutational analysis of the HA protein of the moderately pathogenic A/chicken/Vietnam/C58/04 (H5N1) virus. Nineteen HA proteins containing point mutations in the HA2 coiled-coil domain or in an HA1 histidine or basic patch were generated. Wild-type and mutant HA plasmids were transiently transfected in cell culture and analyzed for total protein expression, surface expression, cleavage efficiency, pH of fusion, and pH of conformational change. Four mutations to residues in the fusion peptide pocket, Y23H and H24Q in the HA1 subunit and E105K and N114K in the HA2 subunit, and a K58I mutation in the HA2 coiled-coil domain significantly altered the pH of activation of the H5 HA protein. In some cases, the magnitude and direction of changes of individual mutations in the H5 HA protein differed considerably from similar mutations in other influenza A virus HA subtypes. Introduction of Y23H, H24Q, K58I, and N114K mutations into recombinant viruses resulted in virus-expressed HA proteins with similar shifts in the pH of fusion. Overall, the data show that residues comprising the fusion peptide pocket are important in triggering pH-dependent activation of the H5 HA protein.

Three amino acid changes in PB1-F2 of highly pathogenic H5N1 avian influenza virus affect pathogenicity in mallard ducks.

Despite reports that the PB1-F2 protein contributes to influenza virus pathogenicity in the mouse model, little is known about its significance in avian hosts. In our previous study, the A/Vietnam/1203/04 (H5N1) wild-type virus (wtVN1203) was more lethal to mallard ducks than a reverse genetics (rg)-derived VN1203. In search of potential viral factors responsible for this discrepancy, we found that synonymous mutations (SMs) had been inadvertently introduced into three genes of the rgVN1203 (rgVN1203/SM-3). Of 11 SMs in the PB1 gene, three resided in the PB1-F2 open reading frame, caused amino acid (aa) substitutions in the PB1-F2 protein, and reduced virus lethality in mallard ducks. The wtVN1203 and recombinant viruses with repairs to these three aa's (rgVN1203/R-PB1-F2) or with repairs to all 11 SMs (rgVN1203/R-PB1) were significantly more pathogenic than rgVN1203/SM-3. In cultured cells, repairing three mutations in PB1-F2 increased viral polymerase activity and expression levels of viral RNA.

Amantadine-oseltamivir combination therapy for H5N1 influenza virus infection in mice.

BACKGROUND: The clinical management of H5N1 influenza virus infection in humans remains unclear. Combination chemotherapy with drugs that target different viral proteins might be more effective than monotherapy. METHODS: BALB/c mice were treated by oral gavage for 5 days with amantadine (1.5, 15 or 30 mg/kg/day) and oseltamivir (1 or 10 mg/kg/day) separately or in combination. Mice were challenged 24 h after initiation of treatment with 10 mouse 50% lethal doses of either amantadine-sensitive (having S31 in the M2 protein) or amantadine-resistant (having N31 in the M2 protein) recombinant A/Vietnam/1203/04 (H5N1) virus. RESULTS: Combination treatment with amantadine (15 or 30 mg/kg/day) and oseltamivir (10 mg/kg/day) provided greater protection (60% and 90%, respectively) against lethal infection with amantadine-sensitive H5N1 virus than did monotherapy. Moreover, spread of the virus to the brain was prevented by both combination regimens. The efficacy of the drug combinations against amantadine-resistant H5N1 virus was comparable to that of oseltamivir alone. Oseltamivir produced a dose-dependent effect against both recombinant H5N1 viruses (P < 0.05) but did not provide complete protection against lethal infection. Importantly, no mutations in the HA, NA and M2 proteins were detected when the two drugs were used in combination. CONCLUSIONS: Combination chemotherapy provided a survival advantage over single-agent treatment of mice inoculated with neurotropic H5N1 influenza virus. This strategy might be an option for the control of pandemic influenza viruses that are sensitive to amantadine. Combinations that include other drugs should be explored.

Inhibition of the cytokine response does not protect against lethal H5N1 influenza infection.

Because proinflammatory cytokines are markedly elevated during H5N1 influenza virus infection, the "cytokine storm" is hypothesized to be the main cause of mortality. Here, we demonstrate that mice deficient in the hallmark inflammatory cytokines TNF-alpha, IL-6, or CC chemokine ligand 2 succumb to infection with A/Vietnam/1203/04 (H5N1) virus, as do wild-type mice treated with glucocorticoids for suppression of cytokines. Because cytokine inhibition does not protect against death, therapies that target the virus rather than cytokines may be preferable.

Neuraminidase inhibitor-resistant recombinant A/Vietnam/1203/04 (H5N1) influenza viruses retain their replication efficiency and pathogenicity in vitro and in vivo.

Effective antiviral drugs are essential for early control of an influenza pandemic. It is therefore crucial to evaluate the possible threat posed by neuraminidase (NA) inhibitor-resistant influenza viruses with pandemic potential. Four NA mutations (E119G, H274Y, R292K, and N294S) that have been reported to confer resistance to NA inhibitors were each introduced into recombinant A/Vietnam/1203/04 (VN1203) H5N1 influenza virus. For comparison, the same mutations were introduced into recombinant A/Puerto Rico/8/34 (PR8) H1N1 influenza virus. The E119G and R292K mutations significantly compromised viral growth in vitro, but the H274Y and N294S mutations were stably maintained in VN1203 and PR8 viruses. In both backgrounds, the H274Y and N294S mutations conferred resistance to oseltamivir carboxylate (50% inhibitory concentration [IC(50)] increases, >250-fold and >20-fold, respectively), and the N294S mutation reduced susceptibility to zanamivir (IC(50) increase, >3.0-fold). Although the H274Y and N294S mutations did not compromise the replication efficiency of VN1203 or PR8 viruses in vitro, these mutations slightly reduced the lethality of PR8 virus in mice. However, the VN1203 virus carrying either the H274Y or N294S mutation exhibited lethality similar to that of the wild-type VN1203 virus. The different enzyme kinetic parameters (V(max) and K(m)) of avian-like VN1203 NA and human-like PR8 NA suggest that resistance-associated NA mutations can cause different levels of functional loss in NA glycoproteins of the same subtype. Our results suggest that NA inhibitor-resistant H5N1 variants may retain the high pathogenicity of the wild-type virus in mammalian species. Patients receiving NA inhibitors for H5N1 influenza virus infection should be closely monitored for the emergence of resistant variants.

Mx1 gene protects mice against the highly lethal human H5N1 influenza virus.

We investigated the importance of the host Mx1 gene in protection against highly pathogenic H5N1 avian influenza virus. Mice expressing the Mx1 gene survived infection with the lethal human H5N1 isolate A/Vietnam/1203/04 and with reassortants combining its genes with those of the non-lethal virus A/chicken/Vietnam/C58/04, while all Mx1-/- mice succumbed. Mx1-expressing mice showed lower organ virus titers, fewer lesions, and less pulmonary inflammation. Our data support the hypothesis that Mx1 expression protects mice against the high pathogenicity of H5N1 virus through inhibition of viral polymerase activity ultimately resulting in reduced viral growth and spread. Drugs that mimic this mechanism may be protective in humans.

Modulation of STAT1 protein levels: a mechanism shaping CD8 T-cell responses in vivo.

Type 1 interferons (IFNs) are induced in vivo, administered therapeutically, and potential targets for amelioration of autoimmune diseases. The cytokines mediate profound antiproliferative effects. Signal transducer and activator of transcription 1 (STAT1)-dependent signaling pathways are required for inhibition of proliferation, and viral infections can elicit high levels of type 1 IFNs as well as total STAT1 protein expression. Thus, a mechanism must be in place to help antigen-specific T cells overcome IFN-induced inhibition of proliferation. The studies reported here demonstrate that total CD8 T-cell proliferation in the presence of IFNs, ex vivo in response to cytokines and in vivo during viral infection, is inhibited through a STAT1-dependent mechanism. In contrast, major proportions of antigen-specific CD8, but not CD4, T cells are rendered less sensitive to this inhibition, express lower endogenous levels of total STAT1, and are selectively proliferating in the presence of type 1 IFN, at key times after viral challenge. Taken together, these novel results show that differential STAT1 expression is used by the immune system to modify cytokine-mediated effects on T-cell expansion and have implications for the consequences of therapeutic intervention in cytokine function.

Cross-protectiveness and immunogenicity of influenza A/Duck/Singapore/3/97(H5) vaccines against infection with A/Vietnam/1203/04(H5N1) virus in ferrets.

Ferrets were immunized with two 7- mu g doses of hemagglutinin from inactivated whole-virus vaccines containing the hemagglutinin gene of A/Duck/Singapore/3/97(H5N3) then inoculated with a lethal dose of A/Vietnam/1203/04(H5N1) (Viet/1203/04). Serum samples did not react with Viet/1203/04 in hemagglutination-inhibition (HI) or virus-neutralization (VN) tests. All vaccinated ferrets survived the challenge, whereas all mock-immunized ferrets died. Immunized ferrets had significantly lower virus titers in the upper respiratory tract and less-severe disease. Vaccine generated from antigenically different H5 virus protects against infection by a highly pathogenic H5 strain. Neither HI nor VN testing provides correlates of cross-protection in ferrets.

Critical role for STAT4 activation by type 1 interferons in the interferon-gamma response to viral infection.

Interferons (IFNs) are essential for host defense. Although the antiviral effects of the type 1 IFNs IFN-alpha and IFN-beta (IFN-alpha/beta) have been established, their immunoregulatory functions, especially their ability to regulate IFN-gamma production, are poorly understood. Here we show that IFN-alpha/beta activate STAT4 directly (STAT, signal transducers and activators of transcription) and that this is required for IFN-gamma production during viral infections of mice, in concert with T cell receptor-derived signals. In contrast, STAT1 appears to negatively regulate IFN-alpha/beta induction of IFN-gamma. Thus, type 1 IFNs, in addition to interleukin-12, provide pathways for innate regulation of adaptive immunity, and their immunoregulatory functions are controlled by modulating the activity of individual STATs.

T-bet regulates the terminal maturation and homeostasis of NK and Valpha14i NKT cells.

Natural killer (NK) and CD1d-restricted Valpha14i natural killer T (NKT) cells play a critical early role in host defense. Here we show that mice with a targeted deletion of T-bet, a T-box transcription factor required for Th1 cell differentiation, have a profound, stem cell-intrinsic defect in their ability to generate mature NK and Valpha14i NKT cells. Both cell types fail to complete normal terminal maturation and are present in decreased numbers in peripheral lymphoid organs of T-bet(-/-) mice. T-bet expression is regulated during NK cell differentiation by NK-activating receptors and cytokines known to control NK development and effector function. Our results identify T-bet as a key factor in the terminal maturation and peripheral homeostasis of NK and Valpha14i NKT cells.