Background. Live attenuated influenza vaccine (LAIV) and inactivated influenza vaccine (IIV) are available for children. Local and systemic immunity induced by LAIV followed a month later by LAIV and IIV followed by LAIV were investigated with virus recovery after LAIV doses as surrogates for protection against influenza on natural exposure.
Methods. Fifteen children received IIV followed by LAIV, 13 an initial dose of LAIV, and 11 a second dose of LAIV. The studies were done during autumn 2009 and autumn 2010 with the same seasonal vaccine (A/California/07/09 [H1N1], A/Perth/16/09 [H3N2], B/Brisbane/60/08).
Results. Twenty-eight of 39 possible influenza viral strains were recovered after the initial dose of LAIV. When LAIV followed IIV, 21 of 45 viral strains were identified. When compared to primary LAIV infection, the decreased frequency of shedding with the IIV-LAIV schedule was significant (P = .023). With LAIV-LAIV, the fewest viral strains were recovered (3/33)—numbers significantly lower (P < .001) than shedding after initial LAIV and after IIV-LAIV (P < .001). Serum hemagglutination inhibition antibody responses were more frequent after IIV than LAIV (P = .02). In contrast, more mucosal immunoglobulin A responses were seen with LAIV.
Conclusions. LAIV priming induces greater inhibition of virus recovery on LAIV challenge than IIV priming. The correlate(s) of protection are the subject of ongoing analysis.
Clinical Trials Registration. NCT01246999.
influenza; vaccines; children
H3N8 influenza viruses are a commonly found subtype in wild birds, usually causing mild or no disease in infected birds. However, they have crossed the species barrier and have been associated with outbreaks in dogs, pigs, donkeys, and seals and therefore pose a threat to humans. A live attenuated, cold-adapted (ca) H3N8 vaccine virus was generated by reverse genetics using the wild-type (wt) hemagglutinin (HA) and neuraminidase (NA) genes from the A/blue-winged teal/Texas/Sg-00079/2007 (H3N8) (tl/TX/079/07) wt virus and the six internal protein gene segments from the ca influenza A virus vaccine donor strain, A/Ann Arbor/6/60 ca (H2N2), the backbone of the licensed seasonal live attenuated influenza vaccine. One dose of the tl/TX/079/07 ca vaccine induced a robust neutralizing antibody response against the homologous (tl/TX/079/07) and two heterologous influenza viruses, including the recently emerged A/harbor seal/New Hampshire/179629/2011 (H3N8) and A/northern pintail/Alaska/44228-129/2006 (H3N8) viruses, and conferred robust protection against the homologous and heterologous influenza viruses. We also analyzed human sera against the tl/TX/079/07 H3N8 avian influenza virus and observed low but detectable antibody reactivity in elderly subjects, suggesting that older H3N2 influenza viruses confer some cross-reactive antibody. The latter observation was confirmed in a ferret study. The safety, immunogenicity, and efficacy of the tl/TX/079/07 ca vaccine in mice and ferrets support further evaluation of this vaccine in humans for use in the event of transmission of an H3N8 avian influenza virus to humans. The human and ferret serology data suggest that a single dose of the vaccine may be sufficient in older subjects.
IMPORTANCE Although natural infection of humans with an avian H3N8 influenza virus has not yet been reported, this influenza virus subtype has already crossed the species barrier and productively infected mammals. Pandemic preparedness is an important public health priority. Therefore, we generated a live attenuated avian H3N8 vaccine candidate and demonstrated that a single dose of the vaccine was highly immunogenic and protected mice and ferrets against homologous and heterologous H3N8 avian viruses.
The development of an H2N2 vaccine is a priority in pandemic preparedness planning. We previously showed that a single dose of a cold-adapted (ca) H2N2 live attenuated influenza vaccine (LAIV) based on the influenza A/Ann Arbor/6/60 (AA ca) virus was immunogenic and efficacious in mice and ferrets. However, in a Phase I clinical trial, viral replication was restricted and immunogenicity was poor. In this study, we compared the replication of four H2N2 LAIV candidate viruses, AA ca, A/Tecumseh/3/67 (TEC67 ca), and two variants of A/Japan/305/57 (JAP57 ca) in three non-human primate (NHP) species: African green monkeys (AGM), cynomolgus macaques (CM) and rhesus macaques (RM). One JAP57 ca virus had glutamine and glycine at HA amino acid positions 226 and 228 (Q-G) that binds to α2-3 linked sialic acids, and one had leucine and serine that binds to α2-3 and α2-6 linked residues (L-S). The replication of all ca viruses was restricted, with low titers detected in the upper respiratory tract of all NHP species, however replication was detected in significantly more CMs than AGMs. The JAP57 ca Q-G and TEC67 ca viruses replicated in a significantly higher percentage of NHPs than the AA ca virus, with the TEC67 ca virus recovered from the greatest percentage of animals. Altering the receptor specificity of the JAP57 ca virus from α2-3 to both α2-3 and α2-6 linked sialic acid residues did not significantly increase the number of animals infected or the titer to which the virus replicated. Taken together, our data show that in NHPs the AA ca virus more closely reflects the human experience than mice or ferret studies. We suggest that CMs and RMs may be the preferred species for evaluating H2N2 LAIV viruses, and the TEC67 ca virus may be the most promising H2N2 LAIV candidate for further evaluation.
H2N2; influenza; vaccine; LAIV; non-human primate; monkey
Vaccination is the most effective strategy for prevention and control of influenza. Timely production and deployment of seasonal influenza vaccines is based on an understanding of the epidemiology of influenza and on global disease and virologic surveillance. Experience with seasonal influenza vaccines guided the initial development of pandemic influenza vaccines. A large investment in pandemic influenza vaccines in the last decade has resulted in much progress and a body of information that can now be applied to refine the established paradigm. Critical and complementary considerations for pandemic influenza vaccines include improved assessment of the pandemic potential of animal influenza viruses, proactive development and deployment of pandemic influenza vaccines, and application of novel platforms and strategies for vaccine production and administration.
antigenic mismatch; avian influenza; pandemic influenza; prepandemic vaccine; prime boost
New vaccine technologies are being investigated for their ability to elicit broadly cross-protective immunity against a range of influenza viruses. We compared the efficacies of two intranasally delivered nonreplicating influenza virus vaccines (H1 and H5 S-FLU) that are based on the suppression of the hemagglutinin signal sequence, with the corresponding H1N1 and H5N1 cold-adapted (ca) live attenuated influenza virus vaccines in mice and ferrets. Administration of two doses of H1 or H5 S-FLU vaccines protected mice and ferrets from lethal challenge with homologous, heterologous, and heterosubtypic influenza viruses, and two doses of S-FLU and ca vaccines yielded comparable effects. Importantly, when ferrets immunized with one dose of H1 S-FLU or ca vaccine were challenged with the homologous H1N1 virus, the challenge virus failed to transmit to naive ferrets by the airborne route. S-FLU technology can be rapidly applied to any emerging influenza virus, and the promising preclinical data support further evaluation in humans.
Influenza viruses continue to represent a global public health threat, and cross-protective vaccines are needed to prevent seasonal and pandemic influenza. Currently licensed influenza vaccines are based on immunity to the hemagglutinin protein that is highly variable. However, T cell responses directed against highly conserved viral proteins contribute to clearance of the virus and confer broadly cross-reactive and protective immune responses against a range of influenza viruses. In this study, two nonreplicating pseudotyped influenza virus vaccines were compared with their corresponding live attenuated influenza virus vaccines, and both elicited robust protection against homologous and heterosubtypic challenge in mice and ferrets, making them promising candidates for further evaluation in humans.
The emergence of Middle East respiratory syndrome coronavirus (MERS-CoV) as a cause of severe respiratory disease highlights the need for effective approaches to CoV vaccine development. Efforts focused solely on the receptor-binding domain (RBD) of the viral Spike (S) glycoprotein may not optimize neutralizing antibody (NAb) responses. Here we show that immunogens based on full-length S DNA and S1 subunit protein elicit robust serum-neutralizing activity against several MERS-CoV strains in mice and non-human primates. Serological analysis and isolation of murine monoclonal antibodies revealed that immunization elicits NAbs to RBD and, non-RBD portions of S1 and S2 subunit. Multiple neutralization mechanisms were demonstrated by solving the atomic structure of a NAb-RBD complex, through sequencing of neutralization escape viruses and by constructing MERS-CoV S variants for serological assays. Immunization of rhesus macaques confers protection against MERS-CoV-induced radiographic pneumonia, as assessed using computerized tomography, supporting this strategy as a promising approach for MERS-CoV vaccine development.
Unmet need exists for a vaccine against Middle East respiratory syndrome coronavirus (MERS-CoV). Here the authors report the establishment and evaluation, in mice and primates, of a series of MERS-CoV immunogens and show that they can serve as promising leads for vaccine development.
Equine influenza viruses (EIV) are responsible for rapidly spreading outbreaks of respiratory disease in horses. Although natural infections of humans with EIV have not been reported, experimental inoculation of humans with these viruses can lead to a productive infection and elicit a neutralizing antibody response. Moreover, EIV have crossed the species barrier to infect dogs, pigs, and camels and therefore may also pose a threat to humans. Based on serologic cross-reactivity of H3N8 EIV from different lineages and sublineages, A/equine/Georgia/1/1981 (eq/GA/81) was selected to produce a live attenuated candidate vaccine by reverse genetics with the hemagglutinin and neuraminidase genes of the eq/GA/81 wild-type (wt) virus and the six internal protein genes of the cold-adapted (ca) A/Ann Arbor/6/60 (H2N2) vaccine donor virus, which is the backbone of the licensed seasonal live attenuated influenza vaccine. In both mice and ferrets, intranasal administration of a single dose of the eq/GA/81 ca vaccine virus induced neutralizing antibodies and conferred complete protection from homologous wt virus challenge in the upper respiratory tract. One dose of the eq/GA/81 ca vaccine also induced neutralizing antibodies and conferred complete protection in mice and nearly complete protection in ferrets upon heterologous challenge with the H3N8 (eq/Newmarket/03) wt virus. These data support further evaluation of the eq/GA/81 ca vaccine in humans for use in the event of transmission of an equine H3N8 influenza virus to humans.
IMPORTANCE Equine influenza viruses have crossed the species barrier to infect other mammals such as dogs, pigs, and camels and therefore may also pose a threat to humans. We believe that it is important to develop vaccines against equine influenza viruses in the event that an EIV evolves, adapts, and spreads in humans, causing disease. We generated a live attenuated H3N8 vaccine candidate and demonstrated that the vaccine was immunogenic and protected mice and ferrets against homologous and heterologous EIV.
Background. Highly pathogenic avian influenza A(H5N1) causes severe infections in humans. We generated 2 influenza A(H5N1) live attenuated influenza vaccines for pandemic use (pLAIVs), but they failed to elicit a primary immune response. Our objective was to determine whether the vaccines primed or established long-lasting immunity that could be detected by administration of inactivated subvirion influenza A(H5N1) vaccine (ISIV).
Methods. The following groups were invited to participate in the study: persons who previously received influenza A(H5N1) pLAIV; persons who previously received an irrelevant influenza A(H7N3) pLAIV; and community members who were naive to influenza A(H5N1) and LAIV. LAIV-experienced subjects received a single 45-μg dose of influenza A(H5N1) ISIV. Influenza A(H5N1)– and LAIV-naive subjects received either 1 or 2 doses of ISIV.
Results. In subjects who had previously received antigenically matched influenza A(H5N1) pLAIV followed by 1 dose of ISIV compared with those who were naive to influenza A(H5N1) and LAIV and received 2 doses of ISIV, we observed an increased frequency of antibody response (82% vs 50%, by the hemagglutination inhibition assay) and a significantly higher antibody titer (112 vs 76; P = .04). The affinity of antibody and breadth of cross-clade neutralization was also enhanced in influenza A(H5N1) pLAIV–primed subjects.
Conclusions. ISIV administration unmasked long-lasting immunity in influenza A(H5N1) pLAIV recipients, with a rapid, high-titer, high-quality antibody response that was broadly cross-reactive across several influenza A(H5N1) clades.
Clinical Trials Registration. NCT01109329.
H5N1; avian influenza; live attenuated; vaccine
Although lymphopenia is a hallmark of severe infection with highly pathogenic H5N1 and the newly emerged H7N9 influenza viruses in humans, the mechanism(s) by which lethal H5N1 viruses cause lymphopenia in mammalian hosts remains poorly understood. Because influenza-specific T cell responses are initiated in the lung draining lymph nodes, and lymphocytes subsequently traffic to the lungs or peripheral circulation, we compared the immune responses in the lung draining lymph nodes following infection with a lethal A/HK/483/97 or non-lethal A/HK/486/97 (H5N1) virus in a mouse model. We found that lethal H5N1, but not non-lethal H5N1 virus infection in mice enhances Fas ligand (FasL) expression on plasmacytoid dendritic cells (pDCs), resulting in apoptosis of influenza-specific CD8+ T cells via a Fas-FasL mediated pathway. We also found that pDCs, but not other DC subsets, preferentially accumulate in the lung draining lymph nodes of lethal H5N1 virus-infected mice and that the induction of FasL expression on pDCs correlates with high levels of IL-12p40 monomer/homodimer in the lung draining lymph nodes. Our data suggest that one of the mechanisms of lymphopenia associated with lethal H5N1 virus infection involves a deleterious role for pDCs.
Ferrets are a valuable model for influenza virus pathogenesis, virus transmission, and antiviral therapy studies. However, the contributions of the volume of inoculum administered and the ferret's respiratory tract anatomy to disease outcome have not been explored. We noted variations in clinical disease outcomes and the volume of inoculum administered and investigated these differences by administering two influenza viruses (A/California/07/2009 [H1N1 pandemic] and A/Minnesota/11/2010 [H3N2 variant]) to ferrets intranasally at a dose of 106 50% tissue culture infective doses in a range of inoculum volumes (0.2, 0.5, or 1.0 ml) and followed viral replication, clinical disease, and pathology over 6 days. Clinical illness and respiratory tract pathology were the most severe and most consistent when the viruses were administered in a volume of 1.0 ml. Using a modified micro-computed tomography imaging method and examining gross specimens, we found that the right main-stem bronchus was consistently larger in diameter than the left main-stem bronchus, though the latter was longer and straighter. These anatomic features likely influence the distribution of the inoculum in the lower respiratory tract. A 1.0-ml volume of inoculum is optimal for delivery of virus to the lower respiratory tract of ferrets, particularly when evaluation of clinical disease is desired. Furthermore, we highlight important anatomical features of the ferret lung that influence the kinetics of viral replication, clinical disease severity, and lung pathology.
IMPORTANCE Ferrets are a valuable model for influenza virus pathogenesis, virus transmission, and antiviral therapy studies. Clinical disease in ferrets is an important parameter in evaluating the virulence of novel influenza viruses, and findings are extrapolated to virulence in humans. Therefore, it is highly desirable that the data from different laboratories be accurate and reproducible. We have found that, even when the same virus was administered at similar doses, different investigators reported a range of clinical disease outcomes, from asymptomatic infection to severe weight loss, ocular and nasal discharge, sneezing, and lethargy. We found that a wide range of inoculum volumes was used to experimentally infect ferrets, and we sought to determine whether the variations in disease outcome were the result of the volume of inoculum administered. These data highlight some less explored features of the model, methods of experimental infection, and clinical disease outcomes in a research setting.
Live attenuated influenza vaccines in the United States are derived from a human virus that is temperature sensitive (ts), characterized by restricted (≥100-fold) replication at 39°C. The ts genetic signature (ts sig) has been mapped to 5 loci in 3 genes: PB1 (391E, 581G, and 661T), PB2 (265S), and NP (34G). However, when transferred into avian and swine influenza viruses, only partial ts and attenuation phenotypes occur. To investigate the reason for this, we introduced the ts sig into the human origin virus A/WSN/33 (WSN), the avian-origin virus A/Vietnam/1203/04 (VN04), and the swine origin triple-reassortant 2009 pandemic H1N1 virus A/California/07/2009 (CA07), which contains gene segments from human, avian, and swine viruses. The VN04ts
sig and CA07ts
sig viruses replicated efficiently in Madin-Darby canine kidney (MDCK) cells at 39°C, but the replication of WSNts
sig was restricted ≥100-fold compared to that at 33°C. Reassortant CA07ts
sig viruses were generated with individual polymerase gene segments from WSN, and vice versa. Only ts sig viruses with a PB2 gene segment derived from WSN were restricted in replication ≥100-fold at 39°C. In ferrets, the CA07ts
sig virus replicated in the upper and lower respiratory tract, but the replication of a reassortant CA07ts
sig virus with a WSN PB2 gene was severely restricted in the lungs. Taken together, these data suggest that the origin of the PB2 gene segment influences the ts phenotype in vitro and attenuation in vivo. This could have implications for the design of novel live vaccines against animal origin influenza viruses.
IMPORTANCE Live attenuated influenza vaccines (LAIVs) on temperature-sensitive (ts) backbones derived from animal origin influenza viruses are being sought for use in the poultry and swine industries and to protect people against animal origin influenza. However, inserting the ts genetic signature from a licensed LAIV backbone fails to fully attenuate these viruses. Our data indicate this is associated with the presence of a PB2 gene segment derived from an avian influenza virus. We show that a reassortant 2009 pandemic H1N1 virus with the ts signature from a licensed LAIV donor virus is ts in vitro and attenuated in vivo when the PB2 gene is derived from a human origin virus but not from an avian virus. Our study provides information that could benefit the rational design of alternative LAIV backbones against animal origin influenza viruses.
The threat of future influenza pandemics and their potential for rapid spread, morbidity, and mortality has led to the development of pandemic vaccines. We generated seven reassortant pandemic live attenuated influenza vaccines (pLAIVs) with the hemagglutinin (HA) and neuraminidase (NA) genes derived from animal influenza viruses on the backbone of the six internal protein gene segments of the temperature sensitive, cold-adapted (ca) A/Ann Arbor/60 (H2N2) virus (AA/60 ca) of the licensed seasonal LAIV. The pLAIV viruses were moderately to highly restricted in replication in seronegative adults; we sought to determine the biological basis for this restriction. Avian influenza viruses generally replicate at higher temperatures than human influenza viruses and, although they shared the same backbone, the pLAIV viruses had a lower shutoff temperature than seasonal LAIV viruses, suggesting that the HA and NA influence the degree of temperature sensitivity. The pH of HA activation of highly pathogenic avian influenza viruses was greater than human and low-pathogenicity avian influenza viruses, as reported by others. However, pLAIV viruses had a consistently higher pH of HA activation and reduced HA thermostability compared to the corresponding wild-type parental viruses. From studies with single-gene reassortant viruses bearing one gene segment from the AA/60 ca virus in recombinant H5N1 or pH1N1 viruses, we found that the lower HA thermal stability and increased pH of HA activation were associated with the AA/60 M gene. Together, the impaired HA acid and thermal stability and temperature sensitivity likely contributed to the restricted replication of the pLAIV viruses we observed in seronegative adults.
IMPORTANCE There is increasing evidence that the HA stability of influenza viruses depends on the virus strain and host species and that HA stability can influence replication, virulence, and transmission of influenza A viruses in different species. We investigated the HA stability of pandemic live attenuated influenza vaccine (pLAIV) viruses and observed that the pLAIV viruses consistently had a less stable HA than the corresponding wild-type influenza viruses. The reduced HA stability and temperature sensitivity of the pLAIV viruses may account for their restricted replication in clinical trials.
Live attenuated cold-adapted (ca) H5N1, H7N3, H6N1, and H9N2 influenza vaccine viruses replicated in the respiratory tract of mice and ferrets, and 2 doses of vaccines were immunogenic and protected these animals from challenge infection with homologous and heterologous wild-type (wt) viruses of the corresponding subtypes. However, when these vaccine candidates were evaluated in phase I clinical trials, there were inconsistencies between the observations in animal models and in humans. The vaccine viruses did not replicate well and immune responses were variable in humans, even though the study subjects were seronegative with respect to the vaccine viruses before vaccination. Therefore, we sought a model that would better reflect the findings in humans and evaluated African green monkeys (AGMs) as a nonhuman primate model. The distribution of sialic acid (SA) receptors in the respiratory tract of AGMs was similar to that in humans. We evaluated the replication of wt and ca viruses of avian influenza (AI) virus subtypes H5N1, H6N1, H7N3, and H9N2 in the respiratory tract of AGMs. All of the wt viruses replicated efficiently, while replication of the ca vaccine viruses was restricted to the upper respiratory tract. Interestingly, the patterns and sites of virus replication differed among the different subtypes. We also evaluated the immunogenicity and protective efficacy of H5N1, H6N1, H7N3, and H9N2 ca vaccines. Protection from wt virus challenge correlated well with the level of serum neutralizing antibodies. Immune responses were slightly better when vaccine was delivered by both intranasal and intratracheal delivery than when it was delivered intranasally by sprayer. We conclude that live attenuated pandemic influenza virus vaccines replicate similarly in AGMs and human subjects and that AGMs may be a useful model to evaluate the replication of ca vaccine candidates.
IMPORTANCE Ferrets and mice are commonly used for preclinical evaluation of influenza vaccines. However, we observed significant inconsistencies between observations in humans and in these animal models. We used African green monkeys (AGMs) as a nonhuman primate (NHP) model for a comprehensive and comparative evaluation of pairs of wild-type and pandemic live attenuated influenza virus vaccines (pLAIV) representing four subtypes of avian influenza viruses and found that pLAIVs replicate similarly in AGMs and humans and that AGMs can be useful for evaluation of the protective efficacy of pLAIV.
In a phase I clinical trial, a H5N1 pandemic live attenuated influenza virus (pLAIV) VN2004 vaccine bearing avian influenza H5N1 hemagglutinin (HA) and NA genes on the A/Ann Arbor cold-adapted vaccine backbone displayed very restricted replication. We evaluated T cell responses to H5N1 pLAIV vaccination and assessed pre-existing T cell responses to determine whether they were associated with restricted replication of the H5N1 pLAIV.
ELISPOT assays were performed using pools of overlapping peptides spanning the entire H5N1 proteome and the HA proteins of relevant seasonal H1N1 and H3N2 viruses. We tested stored peripheral blood mononuclear cells (PBMCs) from 21 study subjects who received two doses of the H5N1 pLAIV. The PBMCs were collected 1 day before and 7 days after the first and second pLAIV vaccine doses, respectively.
T cell responses to conserved internal proteins M and NP were significantly boosted by vaccination (p = 0.036). In addition, H5N1 pLAIV appeared to preferentially stimulate and boost pre-existing seasonal influenza virus HA-specific T cell responses that showed low cross-reactivity with the H5 HA. We confirmed this observation by T cell cloning and identified a novel HA-specific epitope. However, we did not find any evidence that pre-existing T cells prevented pLAIV replication and take.
We found that cross-reactive T cell responses could be boosted by pLAIV regardless of the induction of antibody. The impact of the “original antigenic sin” phenomenon in a subset of volunteers, with preferential expansion of seasonal influenza-specific but not H5N1-specific T cell responses merits further investigation.
influenza; H5N1; vaccine; T cells; LAIV; peptide; epitope; antigenic sin
The spread of highly pathogenic avian H5N1 influenza viruses since 1997 and their virulence for poultry and humans has raised concerns about their potential to cause an influenza pandemic. Vaccines offer the most viable means to combat a pandemic threat. However, it will be a challenge to produce, distribute and implement a new vaccine if a pandemic spreads rapidly. Therefore, efforts are being undertaken to develop pandemic vaccines that use less antigen and induce cross-protective and long-lasting responses, that can be administered as soon as a pandemic is declared or possibly even before, in order to prime the population and allow for a rapid and protective antibody response. In the last few years, several vaccine manufacturers have developed candidate pandemic and pre-pandemic vaccines, based on reverse genetics and have improved the immunogenicity by formulating these vaccines with different adjuvants. Some of the important and consistent observations from clinical studies with H5N1 vaccines are as follows: two doses of inactivated vaccine are generally necessary to elicit the level of immunity required to meet licensure criteria, less antigen can be used if an oil-in-water adjuvant is included, in general antibody titers decline rapidly but can be boosted with additional doses of vaccine and if high titers of antibody are elicited, cross-reactivity against other clades is observed. Prime-boost strategies elicit a more robust immune response. In this review, we discuss data from clinical trials with a variety of H5N1 influenza vaccines. We also describe studies conducted in animal models to explore the possibility of reassortment between pandemic live attenuated vaccine candidates and seasonal influenza viruses, since this is an important consideration for the use of live vaccines in a pandemic setting.
H5N1; vaccine strategies; pandemic; adjuvants; clinical trials
Live attenuated H7N9 influenza vaccine viruses that possess the hemagglutinin (HA) and neuraminidase (NA) gene segments from the newly emerged wild-type (wt) A/Anhui/1/2013 (H7N9) and six internal protein gene segments from the cold-adapted influenza virus A/Ann Arbor/6/60 (AA ca) were generated by reverse genetics. The reassortant virus containing the original wt A/Anhui/1/2013 HA and NA sequences replicated poorly in eggs. Multiple variants with amino acid substitutions in the HA head domain that improved viral growth were identified by viral passage in eggs and MDCK cells. The selected vaccine virus containing two amino acid changes (N133D/G198E) in the HA improved viral titer by more than 10-fold (reached a titer of 108.6 fluorescent focus units/ml) without affecting viral antigenicity. Introduction of these amino acid changes into an H7N9 PR8 reassortant virus also significantly improved viral titers and HA protein yield in eggs. The H7N9 ca vaccine virus was immunogenic in ferrets. A single dose of vaccine conferred complete protection of ferrets from homologous wt A/Anhui/1/2013 (H7N9) and nearly complete protection from heterologous wt A/Netherlands/219/2003 (H7N7) challenge infection. Therefore, this H7N9 live attenuated influenza vaccine (LAIV) candidate has been selected for vaccine manufacture and clinical evaluation to protect humans from wt H7N9 virus infection.
IMPORTANCE In response to the recent avian H7N9 influenza virus infection in humans, we developed a live attenuated H7N9 influenza vaccine (LAIV) with two amino acid substitutions in the viral HA protein that improved vaccine yield by 10-fold in chicken embryonated eggs, the substrate for vaccine manufacture. The two amino acids also improved the antigen yield for inactivated H7N9 vaccines, demonstrating that this finding could great facilitate the efficiency of H7N9 vaccine manufacture. The candidate H7N9 LAIV was immunogenic and protected ferrets against homologous and heterologous wild-type H7 virus challenge, making it suitable for use in protecting humans from H7 infection.
The hypothesis of original antigenic sin (OAS) states that the imprint established by an individual's first influenza virus infection governs the antibody response thereafter. Subsequent influenza virus infection results in an antibody response against the original infecting virus and an impaired immune response against the newer influenza virus. The purpose of our study was to seek evidence of OAS after infection or vaccination with the 2009 pandemic H1N1 (2009 pH1N1) virus in ferrets and humans previously infected with H1N1 viruses with various antigenic distances from the 2009 pH1N1 virus, including viruses from 1935 through 1999. In ferrets, seasonal H1N1 priming did not diminish the antibody response to infection or vaccination with the 2009 pH1N1 virus, nor did it diminish the T-cell response, indicating the absence of OAS in seasonal H1N1 virus-primed ferrets. Analysis of paired samples of human serum taken before and after vaccination with a monovalent inactivated 2009 pH1N1 vaccine showed a significantly greater-fold rise in the titer of antibody against the 2009 pH1N1 virus than against H1N1 viruses that circulated during the childhood of each subject. Thus, prior experience with H1N1 viruses did not result in an impairment of the antibody response against the 2009 pH1N1 vaccine. Our data from ferrets and humans suggest that prior exposure to H1N1 viruses did not impair the immune response against the 2009 pH1N1 virus.
Human influenza viruses predominantly bind α2,6 linked sialic acid (SA) while avian viruses bind α2,3 SA-containing complex glycans. Virulence and tissue tropism of influenza viruses have been ascribed to this binding preference. We generated 2009 pandemic H1N1 (pH1N1) viruses with either predominant α2,3 or α2,6 SA binding and evaluated these viruses in mice and ferrets. The α2,3 pH1N1 virus had similar virulence in mice and replicated to similar titers in the respiratory tract of mice and ferrets as the α2,6 and WT pH1N1 viruses. Immunohistochemical analysis determined that all viruses infected similar cell types in ferret lungs. There is increasing evidence that receptor specificity of influenza viruses is more complex than the binary model of α2,6 and α2,3 SA binding and our data suggest that influenza viruses use a wide range of SA moieties to infect host cells.
Influenza Virus; Tissue Tropism; Virulence; Receptor Specificity; Replication
H2 influenza viruses have not circulated in humans since 1968, and therefore a significant portion of the population would be susceptible to infection should H2 influenza viruses reemerge. H2 influenza viruses continue to circulate in avian reservoirs worldwide, and these reservoirs are a potential source from which these viruses could emerge. Three reassortant cold-adapted (ca) H2 pandemic influenza vaccine candidates with hemagglutinin (HA) and neuraminidase (NA) genes derived from the wild-type A/Japan/305/1957 (H2N2) (Jap/57), A/mallard/6750/1978 (H2N2) (mal/78), or A/swine/MO/4296424/2006 (H2N3) (sw/06) viruses and the internal protein gene segments from the A/Ann Arbor/6/60 ca virus were generated by plasmid-based reverse genetics (Jap/57 ca, mal/78 ca, and sw/06 ca, respectively). The vaccine candidates exhibited the in vitro phenotypes of temperature sensitivity and cold adaptation and were restricted in replication in the respiratory tract of ferrets. In mice and ferrets, the vaccines elicited neutralizing antibodies and conferred protection against homologous wild-type virus challenge. Of the three candidates, the sw/06 ca vaccine elicited cross-reactive antibodies and provided significant protection against the greatest number of heterologous viruses. These observations suggest that the sw/06 ca vaccine should be further evaluated in a clinical trial as an H2 pandemic influenza vaccine candidate.
IMPORTANCE Influenza pandemics arise when novel influenza viruses are introduced into a population with little prior immunity to the new virus and often result in higher rates of illness and death than annual seasonal influenza epidemics. An influenza H2 subtype virus caused a pandemic in 1957, and H2 viruses circulated in humans till 1968. H2 influenza viruses continue to circulate in birds, and the development of an H2 influenza vaccine candidate is therefore considered a priority in preparing for future pandemics. However, we cannot predict whether a human H2 virus will reemerge or a novel avian H2 virus will emerge. We identified three viruses as suitable candidates for further evaluation as vaccines to protect against H2 influenza viruses and evaluated the immune responses and protection that these three vaccines provided in mice and ferrets.
Vaccination is the most effective way to reduce the impact of epidemic as well as pandemic influenza. However, the licensed inactivated influenza vaccine induces strain-specific immunity and must be updated annually. When novel viruses appear, matched vaccines are not likely to be available in time for the first wave of a pandemic. Yet, the enormous diversity of influenza A viruses in nature makes it impossible to predict which subtype or strain will cause the next pandemic. Several recent scientific advances have generated renewed enthusiasm and hope for universal vaccines that will induce broad protection from a range of influenza viruses.
Influenza virus; Universal vaccine
The substitution of glutamic acid (E) for lysine (K) at position 627 of the PB2 protein of avian H5N1 viruses has been identified as a virulence and host range determinant for infection of mammals. Here, we report that the E-to-K host-adaptive mutation in the PB2 gene appeared from day 4 and 5 along the respiratory tracts of mice and was complete by day 6 postinoculation. This mutation correlated with efficient replication of the virus in mice.
Background. Dengue virus (DENV) causes hundreds of millions of infections annually. Four dengue serotypes exist, and previous infection with one serotype increases the likelihood of severe disease with a second, heterotypic DENV infection.
Methods. In a randomized, placebo-controlled study, the safety and immunogenicity of 4 different admixtures of a live attenuated tetravalent (LATV) dengue vaccine were evaluated in 113 flavivirus-naive adults. Serum neutralizing antibody levels to all 4 dengue viruses were measured on days 0, 28, 42, and 180.
Results. A single dose of each LATV admixture induced a trivalent or better neutralizing antibody response in 75%–90% of vaccinees. There was no significant difference in the incidence of adverse events between vaccinees and placebo-recipients other than rash. A trivalent or better response correlated with rash and with non-black race (P < .0001). Black race was significantly associated with a reduced incidence of vaccine viremia.
Conclusions. TV003 induced a trivalent or greater antibody response in 90% of flavivirus-naive vaccinees and is a promising candidate for the prevention of dengue. Race was identified as a factor influencing the infectivity of the LATV viruses, reflecting observations of the effect of race on disease severity in natural dengue infection.
Clinical Trials Registration NCT01072786.
dengue vaccine; live attenuated tetravalent; clinical trial
Reassortment of influenza viral RNA (vRNA) segments in co-infected cells can lead to the emergence of viruses with pandemic potential. Replication of influenza vRNA occurs in the nucleus of infected cells, while progeny virions bud from the plasma membrane. However, the intracellular mechanics of vRNA assembly into progeny virions is not well understood. Here we used recent advances in microscopy to explore vRNA assembly and transport during a productive infection. We visualized four distinct vRNA segments within a single cell using fluorescent in situ hybridization (FISH) and observed that foci containing more than one vRNA segment were found at the external nuclear periphery, suggesting that vRNA segments are not exported to the cytoplasm individually. Although many cytoplasmic foci contain multiple vRNA segments, not all vRNA species are present in every focus, indicating that assembly of all eight vRNA segments does not occur prior to export from the nucleus. To extend the observations made in fixed cells, we used a virus that encodes GFP fused to the viral polymerase acidic (PA) protein (WSN PA-GFP) to explore the dynamics of vRNA assembly in live cells during a productive infection. Since WSN PA-GFP colocalizes with viral nucleoprotein and influenza vRNA segments, we used it as a surrogate for visualizing vRNA transport in 3D and at high speed by inverted selective-plane illumination microscopy. We observed cytoplasmic PA-GFP foci colocalizing and traveling together en route to the plasma membrane. Our data strongly support a model in which vRNA segments are exported from the nucleus as complexes that assemble en route to the plasma membrane through dynamic colocalization events in the cytoplasm.
Influenza A viruses, containing eight single stranded RNA segments, cause seasonal epidemics and occasional pandemics. Reassortment of the influenza viral genome in co-infected cells confers an evolutionary advantage for the virus, and can result in viruses with pandemic potential like the 2009 pandemic H1N1 and 2013 H7N9 virus. Replication of the viral genome occurs in the nucleus of the host cell and the progeny viral RNA (vRNA) segments must be transported to the plasma membrane for budding. The dynamics of vRNA assembly into progeny virions remains unknown. We used novel techniques to visualize the 3D-localization of four distinct vRNA segments in an infected cell and a fluorescent virus to visualize vRNA transport during a productive infection to determine where, when and how assembly occurs. Our data suggest that vRNA segments are exported from the nucleus as subcomplexes that undergo additional assembly en route to the plasma membrane through dynamic fusion events of vRNA-containing cytoplasmic foci. These observations have broad implications for understanding the intracellular requirements behind reassortment of influenza viruses and may lead to the development of new antiviral targets.