Evaluation of A/Alaska/6/77 (H3N2) cold-adapted recombinant viruses derived from A/Ann Arbor/6/60 cold-adapted donor virus in adult seronegative volunteers

Antibody Responsiveness to Influenza: What Drives It?

The induction of a specific antibody response has long been accepted as a serological hallmark of recent infection or antigen exposure. Much of our understanding of the influenza antibody response has been derived from studying antibodies that target the hemagglutinin (HA) protein. However, growing evidence points to limitations associated with this approach. In this review, we aim to highlight the issue of antibody non-responsiveness after influenza virus infection and vaccination. We will then provide an overview of the major factors known to influence antibody responsiveness to influenza after infection and vaccination. We discuss the biological factors such as age, sex, influence of prior immunity, genetics, and some chronic infections that may affect the induction of influenza antibody responses. We also discuss the technical factors, such as assay choices, strain variations, and viral properties that may influence the sensitivity of the assays used to measure influenza antibodies. Understanding these factors will hopefully provide a more comprehensive picture of what influenza immunogenicity and protection means, which will be important in our effort to improve influenza vaccines.

A Dose-finding Study of a Wild-type Influenza A(H3N2) Virus in a Healthy Volunteer Human Challenge Model

BACKGROUND

The development of vaccines and therapeutics has relied on healthy volunteer influenza challenge studies. A validated human infection model with wild-type A(H1N1)pdm09 was reported previously. Our objective was to characterize a wild-type influenza A/Bethesda/MM1/H3N2 challenge virus in healthy volunteers.

METHODS

Participants received a single dose of a cell-based, reverse-genetics, Good Manufacturing Practices-produced wild-type influenza A(H3N2)2011 virus intranasally and were isolated at the National Institutes of Health Clinical Center for ≥9 days. Dose escalation was performed from 104 to 107 TCID50 (50% tissue culture infectious dose). Viral shedding and clinical disease were evaluated daily.

RESULTS

Of 37 participants challenged, 16 (43%) had viral shedding and 27 (73%) developed symptoms, with 12 (32%) participants experiencing mild to moderate influenza disease (MMID), defined as shedding and symptoms. Only participants receiving 106 and 107 TCID50 experienced MMID at 44% and 40%, respectively. Symptom severity peaked on day 3, whereas most viral shedding occurred 1-2 days after challenge. Only 10 (29%) participants had a ≥4-fold rise in hemagglutination inhibition antibody titer after challenge.

CONCLUSIONS

The A/Bethesda/MM1/H3N2 challenge virus safely induced MMID in healthy volunteers, but caused less MMID than the A(H1N1)pdm09 challenge virus even at the highest dose. There was less detection of shedding though the incidence of symptoms was similar to A(H1N1)pdm09. Fewer serum anti-hemagglutinin (HA) antibody responses with less MMID indicate that preexisting immunity factors other than anti-HA antibody may limit shedding in healthy volunteers. This A/Bethesda/MM1/H3N2 challenge virus can be utilized in future studies to further explore pathogenesis and immunity and to evaluate vaccine candidates.

CLINICAL TRIALS REGISTRATION

NCT02594189.

A Rapid Blood Test To Determine the Active Status and Duration of Acute Viral Infection

The ability to rapidly detect and diagnose acute viral infections is crucial for infectious disease control and management. Serology testing for the presence of virus-elicited antibodies in blood is one of the methods used commonly for clinical diagnosis of viral infections. However, standard serology-based tests have a significant limitation: they cannot easily distinguish active from past, historical infections. As a result, it is difficult to determine whether a patient is currently infected with a virus or not, and on an optimal course of action, based off of positive serology testing responses. Here, we report a nanoparticle-enabled blood test that can help overcome this major challenge. The new test is based on the analysis of virus-elicited immunoglobulin G (IgG) antibody present in the protein corona of a gold nanoparticle surface upon mixing the gold nanoparticles with blood sera. Studies conducted on mouse models of influenza A virus infection show that the test gives positive responses only in the presence of a recent acute viral infection, approximately between day 14 and day 21 following the infection, and becomes negative thereafter. When used together with the traditional serology testing, the nanoparticle test can determine clearly whether a positive serology response is due to a recent or historical viral infection. This new blood test can provide critical clinical information needed to optimize further treatment and/or to determine if further quarantining should be continued.

Direct IL-6 Signals Maximize Protective Secondary CD4 T Cell Responses against Influenza

Memory T cells can often respond against pathogens that have evaded neutralizing Abs and are thus key to vaccine-induced protection, yet the signals needed to optimize their responses are unclear. In this study, we identify a dramatic and selective requirement for IL-6 to achieve optimal memory CD4 T cell recall following heterosubtypic influenza A virus (IAV) challenge of mice primed previously with wild-type or attenuated IAV strains. Through analysis of endogenous T cell responses and adoptive transfer of IAV-specific memory T cell populations, we find that without IL-6, CD4+, but not CD8+, secondary effector populations expand less and have blunted function and antiviral impact. Early and direct IL-6 signals to memory CD4 T cells are required to program maximal secondary effector responses at the site of infection during heterosubtypic challenge, indicating a novel role for a costimulatory cytokine in recall responses.

Connecting within and between-hosts dynamics in the influenza infection-staged epidemiological models with behavior change

Influenza viruses are a major public health problem worldwide. Although influenza has been extensively researched, there are still many aspects that are not fully understood such as the effects of within and between-hosts dynamics and their impact on behavior change. Here, we develop mathematical models with multiple infection stages and estimate parameters based on within-host data to investigate the impact of behavior change on influenza dynamics. We divide the infected population into three and four groups based on the age of the infection, which corresponds to viral load shedding. We consider within-host data on viral shedding to estimate the length and force of infection of the different infectivity stages. Our results show that behavior changes, due to exogenous events (e.g., media coverage) and disease symptoms, are effective in delaying and lowering an epidemic peak. We show that the dynamics of viral shedding and symptoms, during the infection, are key features when considering epidemic prevention strategies. This study improves our understanding of the spread of influenza virus infection in the population and provides information about the impact of emergent behavior and its connection to the within and between-hosts dynamics.

Comparative study of influenza virus replication in MDCK cells and in primary cells derived from adenoids and airway epithelium

Although clinical trials with human subjects are essential for determination of safety, infectivity, and immunogenicity, it is desirable to know in advance the infectiousness of potential candidate live attenuated influenza vaccine strains for human use. We compared the replication kinetics of wild-type and live attenuated influenza viruses, including H1N1, H3N2, H9N2, and B strains, in Madin-Darby canine kidney (MDCK) cells, primary epithelial cells derived from human adenoids, and human bronchial epithelium (NHBE cells). Our data showed that despite the fact that all tissue culture models lack a functional adaptive immune system, differentiated cultures of human epithelium exhibited the greatest restriction for all H1N1, H3N2, and B vaccine viruses studied among three cell types tested and the best correlation with their levels of attenuation seen in clinical trials with humans. In contrast, the data obtained with MDCK cells were the least predictive of restricted viral replication of live attenuated vaccine viruses in humans. We were able to detect a statistically significant difference between the replication abilities of the U.S. (A/Ann Arbor/6/60) and Russian (A/Leningrad/134/17/57) cold-adapted vaccine donor strains in NHBE cultures. Since live attenuated pandemic influenza vaccines may potentially express a hemagglutinin and neuraminidase from a non-human influenza virus, we assessed which of the three cell cultures could be used to optimally evaluate the infectivity and cellular tropism of viruses derived from different hosts. Among the three cell types tested, NHBE cultures most adequately reflected the infectivity and cellular tropism of influenza virus strains with different receptor specificities. NHBE cultures could be considered for use as a screening step for evaluating the restricted replication of influenza vaccine candidates.

Immunity to the conserved influenza nucleoprotein reduces susceptibility to secondary bacterial infections

Influenza causes >250,000 deaths annually in the industrialized world, and bacterial infections frequently cause secondary illnesses during influenza outbreaks, including pneumonia, bronchitis, sinusitis, and otitis media. In this study, we demonstrate that cross-reactive immunity to mismatched influenza strains can reduce susceptibility to secondary bacterial infections, even though this fails to prevent influenza infection. Specifically, infecting mice with H3N2 influenza before challenging with mismatched H1N1 influenza reduces susceptibility to either Gram-positive Streptococcus pneumoniae or Gram-negative Klebsiella pneumoniae. Vaccinating mice with the highly conserved nucleoprotein of influenza also reduces H1N1-induced susceptibility to lethal bacterial infections. Both T cells and Abs contribute to defense against influenza-induced bacterial diseases; influenza cross-reactive T cells reduce viral titers, whereas Abs to nucleoprotein suppress induction of inflammation in the lung. These findings suggest that nonneutralizing influenza vaccines that fail to prevent influenza infection may nevertheless protect the public from secondary bacterial diseases when neutralizing vaccines are not available.

Modeling within-host dynamics of influenza virus infection including immune responses

Influenza virus infection remains a public health problem worldwide. The mechanisms underlying viral control during an uncomplicated influenza virus infection are not fully understood. Here, we developed a mathematical model including both innate and adaptive immune responses to study the within-host dynamics of equine influenza virus infection in horses. By comparing modeling predictions with both interferon and viral kinetic data, we examined the relative roles of target cell availability, and innate and adaptive immune responses in controlling the virus. Our results show that the rapid and substantial viral decline (about 2 to 4 logs within 1 day) after the peak can be explained by the killing of infected cells mediated by interferon activated cells, such as natural killer cells, during the innate immune response. After the viral load declines to a lower level, the loss of interferon-induced antiviral effect and an increased availability of target cells due to loss of the antiviral state can explain the observed short phase of viral plateau in which the viral level remains unchanged or even experiences a minor second peak in some animals. An adaptive immune response is needed in our model to explain the eventual viral clearance. This study provides a quantitative understanding of the biological factors that can explain the viral and interferon kinetics during a typical influenza virus infection.

Influenza, commonly referred to as the flu, is a contagious respiratory illness caused by influenza virus infections. Although most infected subjects with intact immune systems are able to clear the virus without developing serious flu complications, the mechanisms underlying viral control are not fully understood. In this paper, we address this question by developing mathematical models that include both innate and adaptive immune responses, and fitting them to experimental data from horses infected with equine influenza virus. We find that the innate immune response, such as natural killer cell-mediated infected cell killing and interferon's antiviral effect, can explain the first rapid viral decline and subsequent second peak viremia, and that the adaptive immune response is needed to eventually clear the virus. This study improves our understanding of influenza virus dynamics and may provide more information for future research in influenza pathogenesis, treatment, and vaccination.

Live attenuated influenza vaccine (LAIV) impacts innate and adaptive immune responses

Influenza A infection induces a massive inflammatory response in the lungs that leads to significant illness and increases the susceptibility to secondary bacterial pneumonia. The most efficient way to prevent influenza infection is through vaccination. While inactivated vaccines induce protective levels of serum antibodies to influenza hemaglutinin (HA) and neuraminidase (NA) surface proteins, these are strain specific and offer little protection against heterosubtypic influenza viruses. In contrast, live attenuated influenza vaccines (LAIVs) induce a T cell response in addition to antibody responses against HA and NA surface proteins. Importantly, LAIV vaccination induces a response in a mouse model that protects against illness due to heterosubtypic influenza strains. While it is not completely clear what is the mechanism of action of LAIV heterosubtypic protection in humans, it has been shown that LAIV induces heterosubtypic protection in mice that is dependent upon a Type 1 immune response and requires CD8 T cells. In this study, we show that LAIV-induced immunity leads to significantly reduced viral titers and inflammatory responses in the lungs of mice following heterosubtypic infection. Not only are viral titers reduced in LAIV vaccinated mice, the amounts of inflammatory cytokines and chemokines in lung tissue are significantly lower. Additionally, we show that LAIV vaccination of healthy adults also induces a robust Type 1 memory response including the production of chemokines and cytokines involved in T cell activation and recruitment. Thus, our results indicate that LAIV vaccination functions by inducing immune memory which can act to modulate the immune response to subsequent heterosubtypic challenge by influencing both innate and adaptive responses.

Evaluation of a virus derived from MDCK cells infected persistently with influenza A virus as a potential live-attenuated vaccine candidate in the mouse model

A temperature-sensitive mutant virus unable to replicate at 38 degrees C was recovered from passage 189 (IVpi-189) of Madin-Darby canine kidney cells infected persistently with influenza A. Immunofluorescent staining of the IVpi-189 virus-infected cells revealed disrupted transport of the matrix (M) 1 protein into the nucleus at non-permissive temperatures, resulting in retention of the nucleoprotein (NP) in the nucleus. Upon comparison with the parental influenza A E61-24-P15 strain used to establish persistent infection, amino acid exchanges were found in the M1 protein of IVpi-189 virus; arginine to glutamine at position 72 and threonine to alanine at position 139. When mice were inoculated intranasally with IVpi-189 virus, virus growth in the lungs was restrained and terminated rapidly. Prior intranasal inoculation with only a small dose of IVpi-189 virus induced humoral and cellular immune responses and protected mice against subsequent virulent virus challenge. These results indicate that IVpi-189 virus, an avirulent temperature-sensitive mutant, is a promising candidate for use as a live-attenuated vaccine.

Time lines of infection and disease in human influenza: a review of volunteer challenge studies

The dynamics of viral shedding and symptoms following influenza virus infection are key factors when considering epidemic control measures. The authors reviewed published studies describing the course of influenza virus infection in placebo-treated and untreated volunteers challenged with wild-type influenza virus. A total of 56 different studies with 1,280 healthy participants were considered. Viral shedding increased sharply between 0.5 and 1 day after challenge and consistently peaked on day 2. The duration of viral shedding averaged over 375 participants was 4.80 days (95% confidence interval: 4.31, 5.29). The frequency of symptomatic infection was 66.9% (95% confidence interval: 58.3, 74.5). Fever was observed in 37.0% of A/H1N1, 40.6% of A/H3N2 (p = 0.86), and 7.5% of B infections (p = 0.001). The total symptoms scores increased on day 1 and peaked on day 3. Systemic symptoms peaked on day 2. No such data exist for children or elderly subjects, but epidemiologic studies suggest that the natural history might differ. The present analysis confirms prior expert opinion on the duration of viral shedding or the frequency of asymptomatic influenza infection, extends prior knowledge on the dynamics of viral shedding and symptoms, and provides original results on the frequency of respiratory symptoms or fever.

Priming with cold-adapted influenza A does not prevent infection but elicits long-lived protection against supralethal challenge with heterosubtypic virus

We show in this study several novel features of T cell-based heterosubtypic immunity against the influenza A virus in mice. First, T cell-mediated heterosubtypic protection against lethal challenge can be generated by a very low priming dose. Second, it becomes effective within 5-6 days. Third, it provides protection against a very high dose challenge for >70 days. Also novel is the finding that strong, long-lasting, heterosubtypic protection can be elicited by priming with attenuated cold-adapted strains. We demonstrate that priming does not prevent infection of the lungs following challenge, but leads to earlier clearance of the virus and 100% survival after otherwise lethal challenge. Protection is dependent on CD8 T cells, and we show that CD4 and CD8 T cells reactive to conserved epitopes of the core proteins of the challenge virus are present after priming. Our results suggest that intranasal vaccination with cold-adapted, attenuated live virus has the potential to provide effective emergency protection against emerging influenza strains for several months.

Genetic and phenotypic stability of cold-adapted influenza viruses in a trivalent vaccine administered to children in a day care setting

The genetic and phenotypic stability of viruses isolated from young children following intranasal administration of the trivalent live-attenuated influenza virus vaccine (LAIV, marketed in the United States as FluMist) was evaluated by determination of genomic sequence and assessment of the cold-adapted (ca), temperature-sensitive (ts) and attenuated (att) phenotypes. The complete genomic sequence was determined for 56 independent isolates obtained from children following vaccination (21 type A/H1N1, 12 A/H3N2, 1 A/H3N1 and 22 type B viruses), 20% of which had no nucleotide misincorporations compared with administered vaccine. The remaining isolates had from one to seven changes per genome. None of the observed misincorporations resulted in predicted amino acid codon substitutions at sites previously shown to contribute to the ca, ts or att phenotypes, and all vaccine-derived isolates retained ca and ts phenotypes consistent with the observation that none of the vaccine recipients displayed distinctive symptoms. The results indicate that LAIV strains undergo very limited genetic change following replication in vaccine recipients and that those changes did not affect vaccine attenuation.

Effect of antipyretic therapy on the duration of illness in experimental influenza A, Shigella sonnei, and Rickettsia rickettsii infections

STUDY OBJECTIVES

To determine whether antipyretic therapy prolongs the course of experimental influenza A, Shigella sonnei, and Rickettsia rickettsii infections.

DESIGN

Retrospective observational study.

SETTING

University Center for Vaccine Development.

SUBJECTS

Fifty-four volunteers with experimentally induced influenza A, 45 with S. sonnei, and 21 with R. rickettsii infections participated.

INTERVENTIONS

Subjects from the six influenza A studies were challenged intranasally. If they met certain criteria, they were offered aspirin or acetaminophen for symptomatic relief. Subjects from the three Shigella studies were challenged with the bacteria and then given trimethoprimsulfamethoxazole. Acetaminophen also could be administered. In the one R. rickettsii trial, subjects were inoculated intradermally and treated with tetracycline. Again, acetaminophen was administered for symptomatic relief.

MEASUREMENTS AND MAIN RESULTS

Data, excerpted from subjects' study records, were evaluated using Wilcoxon tests, Spearman's correlation coefficients, and multiple regression analysis. Two-tailed hypotheses with a p value of 0.05 were used for all of the analyses. There was a striking correlation between antipyretic therapy and duration of illness in subjects infected with influenza A and S. sonnei, but not R. rickettsii.

CONCLUSIONS

Multivariate analysis suggested that antipyretic therapy prolonged illness in subjects infected with influenza A, but its use was the result of prolonged illness in those infected with S. sonnei. The precise nature of these relationships requires a prospective, randomized, placebo-controlled trial.

Evaluation of bovine, cold-adapted human, and wild-type human parainfluenza type 3 viruses in adult volunteers and in chimpanzees

In an attempt to evaluate the level of attenuation of live parainfluenza type 3 virus (PIV3) vaccine candidates, we compared the responses of partially immune adult volunteers inoculated intranasally with 10(6) to 10(7) 50% tissue culture infective dose (TCID50) of bovine PIV3 (n = 18) or cold-adapted (ca) PIV3 (n = 37) with those of 28 adults administered 10(6) to 10(7) TCID50 of wild-type PIV3. The candidate vaccine viruses and the wild-type virus were avirulent and poorly infectious for these adults even though all of them had a low level of nasal antibodies to PIV3. To determine whether the ca PIV3 was attenuated, we then administered 10(4) TCID50 of ca PIV3 (cold-passage 12) or wild-type PIV3 intranasally and intratracheally to two fully susceptible chimpanzees, respectively, and challenged the four primates with wild-type virus 1 month later. Compared with wild-type virus, which caused upper respiratory tract illness, the ca PIV3 was highly attenuated and manifested a 500-fold reduction in virus replication in both the upper and lower respiratory tracts of the two immunized animals. Despite restriction of virus replication, infection with ca PIV3 conferred a high level of protective immunity against challenge with wild-type virus. The ca PIV3 which had been passaged 12 times at 20 degrees C did not retain its ts phenotype. These findings indicate that ca PIV3 may be a promising vaccine candidate for human beings if a passage level can be identified that is genetically stable, satisfactorily attenuated, and immunogenic.

Analysis of virus and host factors in a study of A/Peking/2/79 (H3N2) cold-adapted vaccine recombinant in which vaccine-associated illness occurred in normal volunteers

Live attenuated cold-adapted influenza vaccine is undergoing evaluation in man. Several strains have proven to be safe, immunogenic, nontransmissible, and protective against experimental challenge. In this study of A/Peking/2/79(H3N2), with six internal genes from the cold-adapted (Ca) parent A/Ann Arbor/6/60(H2N2), we encountered at the highest input multiplicity, 28% illness rate among individuals infected with vaccine. Reversion to wild type and excessive viral replication did not occur. Physical characteristics of the vaccine were similar to nonreactogenic vaccine A/Washington/897/80(H3N2). At ten- and 100-fold lower input multiplicities, infection frequency was maintained, but reactions did not occur. We compared the observations in this study with those made in a similar study of A/Scotland/840/74(H3N2), a cold-adapted vaccine with five genes from the Ca parent in which reactogenicity also was noted. The dose of vaccine virus in relation to tissue culture infectious doses required to infect 50% of susceptibles (HID50) was proportionally lower for both A/Peking/2/79(H3N2) and A/Scotland/80(H3N2). Hence, when the vaccine was undiluted the recipients were inoculated with more than 100 HID50. We concluded that the very high input could be avoided if vaccines were screened beginning at 1/1,000 of maximum titers. Ca vaccines must be safe before they undergo field trials.

Four viral genes independently contribute to attenuation of live influenza A/Ann Arbor/6/60 (H2N2) cold-adapted reassortant virus vaccines

Clinical studies previously demonstrated that live influenza A virus vaccines derived by genetic reassortment from the mating of influenza A/Ann Arbor/6/60 (H2N2) cold-adapted (ca) donor virus with epidemic wild-type influenza A viruses are reproducibly safe, infectious, immunogenic, and efficacious in the prevention of illness caused by challenge with virulent wild-type virus. These influenza A reassortant virus vaccines also express the ca and temperature sensitivity (ts) phenotypes in vitro, but the genes of the ca virus parent which specify the ca, ts, and attenuation (att) phenotypes have not adequately been defined. To identify the genes associated with each of these phenotypes, we isolated six single-gene substitution reassortant viruses, each of which inherited only one RNA segment from the ca parent virus and the remaining seven RNA segments from the A/Korea/1/82 (H3N2) wild-type virus parent. These were evaluated in vitro for their ca and ts phenotypes and in ferrets, hamsters, and seronegative adult volunteers for the att phenotype. We found that the polymerase PA gene of the ca parent specifies the ca phenotype and that the PB2 and PB1 genes independently specify the ts phenotype. The PA, M, PB2, and PB1 genes of the ca donor virus each contribute to the att phenotype. The finding that four genes of the ca donor virus contribute to the att phenotype provides a partial explanation for the observed phenotypic stability of ca reassortant viruses following replication in humans.

Comparative studies of wild-type and cold-mutant (temperature-sensitive) influenza virus: detection of mutations in all genes of the A/Ann Arbor/6/60 (H2N2) mutant vaccine donor strain

Direct biochemical evidence has been obtained for the existence of mutations in all eight RNA segments of the A/Ann Arbor/6/60 cold-adapted (ca) mutant influenza virus strain as compared with its wild-type (wt) progenitor. Polyacrylamide gel electrophoresis (PAGE) of viral RNA revealed a change in the electrophoretic migration of RNA 2 (PB1). T1 oligonucleotide mapping revealed changes in two polymerase genes (the PB2 and PA genes), the hemagglutinin (HA) gene and the nucleoprotein (NP) gene. Analysis of S1 nuclease-treated RNA hybrids on polyacrylamide gels detected changes in the HA and neuraminidase (NA) genes. Partial DNA sequence analysis demonstrated a base sequence change in the matrix (M) protein gene that predicts an amino acid change in the M2 protein and a silent mutation in the non-structural (NS) protein gene. In addition, analysis of viral polypeptides by PAGE has so far revealed changes in the viral protein, PA. These findings directly demonstrate the existence of multiple mutations in the ca vaccine strain, a property that may provide reliably and stably attenuated vaccines that derive their six internal genes from the ca A/Ann Arbor/6/60 donor strain.

Cold-adapted reassortants of influenza A virus: pathogenicity of A/Ann Arbor/6/60 X A/Alaska/6/77 reassortant viruses in vivo and in vitro

Cold-adapted reassortants of A/Ann Arbor/6/60 X A/Alaska/6/77 viruses made in MDCK cells have recently been assessed genotypically and for temperature-sensitive and cold-adapted phenotypes. These reassortants were used to infect ferrets and hamsters and to inoculate organ cultures of hamster tracheal rings, in order to assess their degree of virulence. Virulence in the three model systems corresponded quite well, and a correlation between loss of virulence and particular A/AA/6/60 genes present in the reassortants was noted. Two different reassortants containing either RNA 2 or RNA 5 (NA gene) alone from A/AA/6/60 showed little attenuation from the wild-type parent. A reassortant containing both RNA 2 and the NA gene from A/AA/6/60 and all remaining wild-type genes showed some small decrease in virulence compared to the wild-type virus. However a reassortant containing these two A/AA/6/60 genes and RNA 3 as an additional gene from this parent, had a level of attenuation comparable to that of the cold-adapted virus.

Development and characterization of cold-adapted viruses for use as live virus vaccines

Representative viruses from twelve RNA and two DNA virus genera have been successfully adapted to growth at sub-optimal temperature (cold-adapted). In almost every case, there was a correlation between acquisition of the cold-adaptation phenotype and loss of virulence in the normal host whether animal or man. Overall, the best method of cold adaptation to develop a live virus vaccine line appeared to be a stepwise lowering of the growth temperature allowing time for multiple lesions to occur and/or be selected. In addition, the starting virus should be a recent isolate not as yet adapted to a tissue culture host and the cold-adaptation process should then occur in a host heterologous to the virus' normal host. These viruses have been reviewed in the light of their cold-adaptation method and successful production of an attenuated line as virus vaccine candidate. Finally, detailed information is presented for the cold-adaptation process in influenza virus.

Attenuation of wild-type human influenza A virus by acquisition of the PA polymerase and matrix protein genes of influenza A/Ann Arbor/6/60 cold-adapted donor virus

Wild-type influenza A viruses can be attenuated for humans by the acquisition of genes from the A/Ann Arbor/6/60 cold-adapted (ca) donor virus. Six-gene reassortants, that is, viruses containing the hemagglutinin and neuraminidase surface glycoprotein genes of the wild-type virus and the six remaining RNA segments of the ca donor virus, are consistently attenuated for humans. During the production of a six-gene reassortant virus containing the surface glycoproteins of the A/Washington/897/80 (H3N2) wild-type virus, a reassortant virus was isolated that contained RNA segments 3 (coding for the polymerase PA protein) and 7 (coding for matrix [M] proteins) from the ca parent and all other genes from the wild-type virus. This reassortant virus is referred to as a two-gene reassortant. Because the gene or set of genes responsible for the attenuation of ca reassortant viruses has not been defined, we evaluated the two-gene reassortant for level of replication and level of virulence in ferrets and in humans, and we compared its characteristics to those of a six-gene reassortant virus derived from the same two parents. The two-gene reassortant virus infected each of 14 adult seronegative (serum hemagglutination inhibition titer of less than or equal to 1:8) volunteers when administered intranasally at a dose of 10(7) 50% tissue culture infectious doses, yet it did not produce illness. The level of replication of the two-gene reassortant virus in the upper respiratory tract was equivalent to that of the six-gene reassortant virus. This demonstrates that transfer of the A/Ann Arbor/6/60 ca PA polymerase and M genes is sufficient to confer the attenuation phenotype on wild-type influenza A viruses. In the context of previous observations, these results suggest that the A/Ann Arbor/6/60 ca donor virus PA polymerase gene plays a major role in the attenuation of ca reassortant viruses.

Recombinant cold-adapted attenuated influenza A vaccines for use in children: reactogenicity and antigenic activity of cold-adapted recombinants and analysis of isolates from the vaccinees

Reactogenicity and antigenic activity of recombinants obtained by crossing cold-adapted donor of attenuation A/Leningrad/134/47/57 with wild-type influenza virus strains A/Leningrad/322/79(H1N1) and A/Bangkok/1/79(H3N2) were studied. The recombinants were areactogenic when administered as an intranasal spray to children aged 3 to 15, including those who lacked or had only low titers of pre-existing anti-hemagglutinin and anti-neuraminidase antibody in their blood. After two administrations of vaccines at a 3-week interval, both strains induced antibody in 75 to 95% of the children. On coinfection of chicken embryos with both recombinants, only weak interference was observed. Administration to children of the bivalent vaccine containing H1N1 and H3N2 recombinants induced efficient production of antibody to H1 and H3 hemagglutinins and N1 and N2 neuraminidases without adverse reactions. The recombinants studied were genetically stable as judged by retention of the temperature-sensitive phenotypes and a lack of reversion of the genes carrying temperature-sensitive mutations in all of the reisolates from vaccinated children.

Serological evaluation of an influenza A virus cold-adapted reassortant live vaccine, CR-37 (H1N1), in Japanese adult volunteers

A cold-adapted influenza A virus, CR-37 (H1N1), derived from genetic reassortment between A/Ann Arbor/6/60 (H2N2) cold-adapted variant virus and A/California/10/78 (H1N1) wild-type virus, was tested in Japanese adult volunteer. The CR-37 live virus preparation induced only low-grade clinical reactions in volunteers for the first 3-4 days after inoculation. Two vaccinees who did not show any antibody changes became febrile (over 38.0 degrees C). Skin tests using the vaccine preparation and uninfected allantoic fluid were performed, and indicated that one of these two vaccines was positive for the CR-37 vaccine preparation. A high proportion of the vaccinees whose sera had a haemagglutination-inhibition (HI) antibody titre against the vaccine strain of less than or equal to 64 before inoculation, seroconverted in both HI and neuraminidase-inhibition (NAI) antibody titrations, and only a few seroconverted in the titration of antibody against type-specific internal antigens. The serological examinations against heterotypic H1N1 variants indicated that the cold-adapted live influenza virus vaccine could induce a broad spectrum of HI antibody reactivity and immunity of long duration.

Advantage of live attenuated cold-adapted influenza A virus over inactivated vaccine for A/Washington/80 (H3N2) wild-type virus infection

The efficacy of live attenuated cold-adapted (ca) reassortant influenza virus vaccine against experimental challenge with homologous wild-type virus 5 to 8 weeks after vaccination was compared with that of licensed inactivated vaccine in 81 seronegative (haemagglutination-inhibition antibody titre less than or equal to 1:8) college students. At a dose of 10(7.5) 50% tissue culture infectious dose (TCID50) (70 HID50, human 50% infectious doses) the live virus vaccine, given intranasally, completely protected against illness caused by wild-type virus, whereas the inactivated vaccine, administered intramuscularly, provided 72% protection. Wild-type virus was recovered from only 13% of live virus vaccinees (10(7.5) TCID50 dose of ca virus) compared with 63% of inactivated virus vaccinees and the few infected live virus vaccinees shed 1000 times less wild-type virus than did infected inactivated virus vaccinees or unvaccinated controls. This striking reduction in virus shedding suggests that influenza transmission may be more efficiently interrupted with live than with inactivated virus vaccination.

In vivo and in vitro hamster models in the assessment of virulence of recombinant influenza viruses

The virulence of five wild-type influenza A viruses and 14 recombinant viruses, prepared from the cold adapted A/Ann Arbor/6/60 virus and various wild-type viruses, was studied by two methods. Firstly, the viruses were inoculated into hamsters, and the titres present in the lungs and turbinates at 1, 3 and 4 days post-infection were measured. Secondly, the effect of five wild-type and ten recombinant viruses on the ciliated epithelium of in vitro hamster tracheal organ cultures was examined. The results obtained were assessed with reference to the known virulence of the viruses for human volunteers. The results showed that virus strains virulent for man grew to higher titres in hamster lungs and turbinates than attenuated strains; and that virulent strains destroyed the ciliary activity of hamster tracheal organ cultures more quickly and to a greater extent than attenuated strains. Comparison of the results with the known virulence of viruses tested for man suggests that the reduced ability of virus to grow in hamster lung tissue and the relatively little effect on ciliary activity may be used as markers of virus attenuation; however, the growth of virus in hamster turbinates overlaps for virulent and attenuated strains and therefore was not considered a useful marker of virulence.

Different virulence of influenza A virus strains and susceptibility to pneumococcal otitis media in chinchillas

We have previously shown that chinchillas infected with a multiply passaged laboratory strain of influenza A/NWS/33 (H1N1) develop negative middle-ear pressure; polymorphonuclear leukocyte oxidative, bactericidal, and chemotactic dysfunction; and increased susceptibility to pneumococcal otitis media. Because influenza A virus strains show different virulence in humans, three such strains were compared in the chinchilla model. Negative middle-ear pressure and tympanic membrane inflammation developed significantly more often in chinchillas infected with wild-type H3N2 virus than with either wild-type H1N1 virus or an attenuated, cold-adapted H3N2 vaccine strain, CR29. Marked depression in polymorphonuclear leukocyte chemiluminescent activity also developed significantly more often in H3N2 infected animals than in H1N1- or CR29-infected animals. Intranasal challenge of influenza virus-infected animals with type 7 Streptococcus pneumoniae resulted in a significantly greater occurrence of pneumococcal otitis media in H3N2-infected animals than in H1N1-, CR29-, or non-influenza-infected control animals. Clearance of pneumococci from nasal washings of animals infected with wild-type H3N2 was significantly delayed in comparison with the other groups. Thus, the previously demonstrated increased susceptibility to otitis media among children infected with H3N2 influenza virus may relate to the capacity of this strain to induce negative middle-ear pressure, polymorphonuclear leukocyte dysfunction, and alteration in the mucosal clearance of pneumococci.

Dose response of A/Alaska/6/77 (H3N2) cold-adapted reassortant vaccine virus in adult volunteers: role of local antibody in resistance to infection with vaccine virus

An attenuated influenza A candidate vaccine virus, derived from the A/Ann Arbor/6/60 (H2N2) cold-adapted (ca) donor virus and the A/Alaska/6/77 (H3N2) wild-type virus, was evaluated in adult seronegative volunteers (serum hemagglutination-inhibiting antibody titer, less than or equal to 1:8) for level of attenuation, infectivity, antigenicity, and genetic stability. Four groups with similar preinoculation mean titers of serum and nasal wash antibodies were inoculated intranasally with 10(4.5), 10(5.5), 10(6.5), or 10(7.5) 50% tissue culture infectious doses (TCID50) of the ca reassortant virus, and eight other seronegative adult volunteers received the wild-type virus. Only 2 of 66 vaccinees developed fever or mild and brief systemic or upper respiratory tract illness or both. Both volunteers with vaccine-related reactions received the highest dose (10(7.5) TCID50) of ca virus, which indicates that the vaccine retains some mild reactogenicity at a high dosage. In contrast, four of eight volunteers infected with the wild-type virus became ill. Each of the 54 isolates tested retained the temperature-sensitive phenotype of the vaccine virus. Thus, the ca reassortant was genetically stable and attenuated at 10(4.5) to 10(7.5) TCID50 for seronegative adults. The 50% human infective dose of ca virus was approximately 10(5.3) TCID50. Ten and one hundred 50% human infectious doses infected 73 and 83% of vaccinees, respectively, and approximately 75% developed an immunological response at these doses. The failure of the vaccine virus to infect some volunteers was correlated with the presence of pre-inoculation nasal wash immunoglobulin A hemagglutinin antibody.

Secretory and systemic immunological response in children infected with live attenuated influenza A virus vaccines

An enzyme-linked immunosorbent assay was used to measure isotype-specific antibody to purified hemagglutinin (HA) of influenza A virus, using serum and nasal-wash specimens from young children undergoing primary infection with live cold-adapted influenza A/Alaska/77 (H3N2) or A/Hong Kong/77 (H1N1) candidate vaccine virus. The serum antibody response followed the pattern expected for a primary viral infection. Each of 17 vaccinated children had a serum immunoglobulin G (IgG) HA antibody response, 16 had an IgM antibody response, and 13 had an IgA antibody response. Nasal-wash HA antibody was detected in the IgA, IgM, and IgG isotypes. Of the 17 vaccinated children, 14 had an IgA response, 13 had an IgM response, and 9 had an IgG response. Most of the IgA and IgM HA antibody was actively secreted locally, whereas only some of the IgG HA antibody could be shown to be actively secreted into the respiratory tract. There was a good correlation between the level of nasal-wash antibodies measured by the HA-specific IgA enzyme-linked immunosorbent assay and by a plaque neutralization assay. These data indicate that intranasal vaccination of susceptible children with live, attenuated, cold-adapted influenza A viruses efficiently stimulates both systemic and local antibody responses.