The hamster as an experimental animal for the study of influenza. I. The role of antibody in protection

Comparative Pathology of Animal Models for Influenza A Virus Infection

Animal models are essential for studying disease pathogenesis and to test the efficacy and safety of new vaccines and therapeutics. For most diseases, there is no single model that can recapitulate all features of the human condition, so it is vital to understand the advantages and disadvantages of each. The purpose of this review is to describe popular comparative animal models, including mice, ferrets, hamsters, and non-human primates (NHPs), that are being used to study clinical and pathological changes caused by influenza A virus infection with the aim to aid in appropriate model selection for disease modeling.

Syrian Hamster as an Animal Model for the Study of Human Influenza Virus Infection

Ferrets and mice are frequently used as animal models for influenza research. However, ferrets are demanding in terms of housing space and handling, whereas mice are not naturally susceptible to infection with human influenza A or B viruses. Therefore, prior adaptation of human viruses is required for their use in mice. In addition, there are no mouse-adapted variants of the recent H3N2 viruses, because these viruses do not replicate well in mice. In this study, we investigated the susceptibility of Syrian hamsters to influenza viruses with a view to using the hamster model as an alternative to the mouse model. We found that hamsters are sensitive to influenza viruses, including the recent H3N2 viruses, without adaptation. Although the hamsters did not show weight loss or clinical signs of H3N2 virus infection, we observed pathogenic effects in the respiratory tracts of the infected animals. All of the H3N2 viruses tested replicated in the respiratory organs of the hamsters, and some of them were detected in the nasal washes of infected animals. Moreover, a 2009 pandemic (pdm09) virus and a seasonal H1N1 virus, as well as one of the two H3N2 viruses, but not a type B virus, were transmissible by the airborne route in these hamsters. Hamsters thus have the potential to be a small-animal model for the study of influenza virus infection, including studies of the pathogenicity of H3N2 viruses and other strains, as well as for use in H1N1 virus transmission studies.IMPORTANCE We found that Syrian hamsters are susceptible to human influenza viruses, including the recent H3N2 viruses, without adaptation. We also found that a pdm09 virus and a seasonal H1N1 virus, as well as one of the H3N2 viruses, but not a type B virus tested, are transmitted by the airborne route in these hamsters. Syrian hamsters thus have the potential to be used as a small-animal model for the study of human influenza viruses.

Animal models for influenza viruses: implications for universal vaccine development

Influenza virus infections are a significant cause of morbidity and mortality in the human population. Depending on the virulence of the influenza virus strain, as well as the immunological status of the infected individual, the severity of the respiratory disease may range from sub-clinical or mild symptoms to severe pneumonia that can sometimes lead to death. Vaccines remain the primary public health measure in reducing the influenza burden. Though the first influenza vaccine preparation was licensed more than 60 years ago, current research efforts seek to develop novel vaccination strategies with improved immunogenicity, effectiveness, and breadth of protection. Animal models of influenza have been essential in facilitating studies aimed at understanding viral factors that affect pathogenesis and contribute to disease or transmission. Among others, mice, ferrets, pigs, and nonhuman primates have been used to study influenza virus infection in vivo, as well as to do pre-clinical testing of novel vaccine approaches. Here we discuss and compare the unique advantages and limitations of each model.

Lipid vesicle-entrapped influenza A antigen modulates the influenza A-specific human antibody response in immune reconstituted SCID-human mice

This study was designed to investigate the capacity of purified influenza antigen in the presence and absence of adjuvant to induce human antibody responses in human-PBL-SCID mice. Non-ionic surfactant vesicles (NISV) were used as adjuvant as they have been shown to promote the development of Th1 responses in mouse studies. Human peripheral blood lymphocyte-SCID mice were inoculated with either purified influenza antigen (A/Texas, H3N2) or influenza antigen entrapped in NISV. Both vaccinated groups produced significantly higher plasma levels of influenza-specific human IgG when individually compared with non-vaccinated controls. However, similar comparisons revealed that specific IgM levels were significantly higher only in the group challenged with purified antigen. Further analysis of IgG subclasses also demonstrated an adjuvant-dependent dichotomy in the responses of the vaccine groups when compared with non-vaccinated controls. Thus, only influenza-specific IgG1 antibodies (associated with Th1 responses in humans) were significantly increased above control levels using antigen with adjuvant, while both this subclass and antigen-specific IgG4 (Th2 associated) were significantly increased with antigen alone. These results illustrate the suitability of this model for use in human vaccination studies and demonstrates that influenza antigen applied with NISV selectively promotes only Th1 responses, unlike free antigen which also promotes Th2 responses in vivo.

Immunogenicity of influenza and HSV-1 mixed antigen ISCOMs in mice

Immunostimulating complexes (ISCOMs) were prepared with mixtures of antigens from influenza A virus (A/PR/8/34 or A/Sichuan/2/87) and herpes simplex virus type 1 (HSV-1), and were characterised by enzyme linked immunosorbent assay (ELISA) and electron microscopy using double-labelling immunogold techniques employing monoclonal antibodies to influenza or HSV-1 glycoproteins. The immunogenicity of the mixed antigen ISCOMs was evaluated in mice, following administration by the subcutaneous route, by measuring the total and subclass IgG antibody responses. Protection of these animals against challenge with live influenza A/Sichuan virus or live HSV-1, was compared with that induced by immunization with aqueous mixed antigen preparations. It was found that relatively high humoral responses to both influenza and HSV antigens, and increased levels of protection to both influenza and HSV viruses were elicited in mice receiving the mixed antigen ISCOM preparation compared to those observed in animals receiving the mixed aqueous subunit preparation. The findings also indicate that antigens from more than one virus can be used in an ISCOM formulation to produce immunity and protection.

The IgA and subclass IgG responses and protection in mice immunised with influenza antigens administered as ISCOMS, with FCA, ALH or as infectious virus

Comparative studies on the local IgA, and circulating IgG subclass antibody responses of mice to A/Sichuan/2/87 (H3N2) influenza virus surface antigens administered with different carrier or delivery systems by the parenteral route, were carried out. The results obtained were compared with the responses observed following live influenza virus infection, and the protection afforded to these animals by these various preparations determined. Infection with live virus elicited early and high levels of protection against homologous virus challenge and this correlated with both local IgA and circulating IgG2a antibody levels. When incorporated into immunostimulating complexes (ISCOMS), A/Sichuan surface antigens promoted high levels of local IgA and circulating IgG1 antibody, and achieved a more rapid and more solid immunity against homologous virus challenge infection, than that elicited by the same surface antigens administered alone or together with Freund's complete adjuvant or alhydrogel.

Use of single radial haemolysis for assessing antibody response to influenza virus vaccines in animals

The value of the single radial haemolysis (SRH) test as a possible replacement for the haemagglutination-inhibition (HAI) test for the estimation of antibodies against influenza was assessed in three animal models. The serum antibody response was measured by both assay systems; correlation of the two tests was assessed using regression analysis. The study showed that when the response to a single immunisation was determined, the ferret model gave satisfactory correlation of SRH and HAI, whilst in the mouse and hamster models poor correlation was observed. Correlation was only improved in the mouse model when an immunisation schedule that mimicked the human situation of a background exposure to different strains of influenza virus was used. Since influenza vaccine efficacy is usually assessed in animals using a single immunisation we suggest that the SRH is not acceptable for use in either hamsters or mice, but is acceptable where the ferret model is involved.

Interpretation of responses and protective levels of antibody against attenuated influenza A viruses using single radial haemolysis

Antibody determinations against H3N2 and H1N1 type A influenza viruses were carried out on paired sera obtained from volunteers taking part in influenza virus vaccine studies, using both the haemagglutination-inhibition (HI) and single radial haemolysis (SRH) test. Good correlation between the HI and SRH test was found for both H3N2 and H1N1 antibody and the zone area increases corresponding to significant SRH antibody rises determined for both virus strains. In both H3N2 and H1N1 vaccine studies, intranasal infection of the volunteers with live attenuated viruses was involved and by the measurement of HI and SRH antibodies prior to and following infection, levels of antibody equating with protection against the infecting viruses could be estimated. For the HI test the antibody titres associated with 50% protection were 42 for H1N1, and 44 for H3N2 viruses; for the SRH test, 50% protection was associated with zone areas of 20.0-25.0 mm2 for both H1N1 and H3N2 viruses.

Detection and characterization of natural killer cells in Syrian golden hamsters

Natural killer (NK) cell activity was observed in the peripheral blood and spleen of 8- to 10-week-old Syrian golden hamsters, but not in the bone marrow or thymus. Low, but significant, levels of cytotoxicity were also observed in mesenteric and axillary lymph nodes and cells harvested from the peritoneum. Cytotoxicity, in a 4-hr 51Cr-release assay, was found to be nylon wool non-adherent and was significantly reduced by treatment with trypsin or incubation at 37 degrees C for 18 hr. Natural cytotoxicity was shown to be low at 1 week of age, but increased to a maximum at 8 weeks and was maintained into old age. Correlation was observed between peripheral blood cytotoxicity and the presence of large, often granular, lymphocytes following fractionation of effector cells by Percoll discontinuous gradient separation. These findings are compared with previous studies in hamsters, where the results of longer-term cytotoxicity tests differ in some respects to those of the present study.

Antigenicity in hamsters of inactivated vaccines prepared from recombinant influenza viruses

Inactivated vaccines prepared form influenza virus strains obtained by the recombination of A/PR/8/34 (H1N1) or A/FM/1/47 (H1N1) viruses with A/Victoria/3/75 (H3N2) virus, were tested for their antigenicity in hamsters. The parental origin of the genes of each cloned recombinant virus was determined by polyacrylamide gel electrophoresis, and vaccines prepared from each strain by concentration, purification on sucrose density gradients and inactivation with formalin. All the recombinant strains used in these studies possessed surface haemagglutinin and neuraminidase antigens derived from the A/Victoria/75 parent strain. On inoculation into hamsters, at equivalent concentrations, these vaccines varied in their ability to induce haemagglutination-inhibiting (HI) antibodies in the serum. This variation was not dependent on concentration and was observed using neutralization and single radial haemolysis, as well as HI. The possible reasons for the findings are discussed.

Use of the enzyme-linked immunosorbent assay (ELISA) for the estimation of serum antibodies in an influenza virus vaccine study

The value of the enzyme-linked immunosorbent assay (ELISA) for determining the serum antibody responses of volunteers following immunisation with various inactivated influenza virus vaccines was assessed, and the incidence of seroconversions, as measured by both haemagglutination-inhibition (HI) and ELISA response of the volunteers determined. ELISA was found to be more sensitive than the HI test in detecting serum antibodies, but was also less specific under the conditions used. With regard to efficacy, the whole virus vaccine proved to be more effective in inducing serum antibody in an unprimed population than either tween-ether split or subunit adsorbed vaccines, but the reverse situation held when the population was primed with respect to the antigen concerned.

Growth of influenza A viruses in hamsters

The growth characteristics of four temperature-sensitive or cold-adapted recombinant influenza viruses and eight recombinant influenza viruses derived in other ways, together with the wild-type, parent viruses of these strains, were tested in hamster lungs and turbinates and in embryonated eggs at different temperatures for their replicative ability. The results showed that although the temperature-sensitive and cold-adapted recombinant viruses replicated to considerably lower titres than their wild-type parent virus strains in hamster lung and at 37 degrees C in embryonated eggs, no similar pattern of growth was observed for the group of A/PR 8 and A/Okuda recombinant influenza viruses studied in these systems. The hamster model is not therefore generally applicable as a marker for attenuated influenza virus vaccine strains.

Influenza virus infection of the guinea pig: immune response and resistance

Guinea pigs were inoculated by intranasal inoculation with unadapted, influenza virus A/England/42/72, and virus was recovered from nasal washings between 3 and 10 days post-inoculation. Infected animals did not exhibit a febrile response to infection, did not produce local antibody and produced only relatively low levels of serum antibody. However, they developed delayed-type hypersensitivity to influenza virus, demonstrable by both skin tests and macrophage migration inhibition tests, which was similar to that of man. The relevance of the influenza virus specific delayed hypersensitivity in immunity to infection was examined in this model. Guinea pigs previously infected with virus or passively immunized with hyperimmune serum were relatively resistant to reinfection with influenza virus A/England/42/72. Inoculation of guinea pigs with spleen cells from immune donor animals, together with or without immune serum, did not give or enhance resistance to challenge virus infection. The results do not suggest a role for delayed hypersensitivity response in immunity to influenza virus infection.

Growth of some attenuated influenza viruses in hamster tracheal organ cultures

Hamster tracheal organ cultures were infected with different influenza viruses and the metabolic activity measured using a tetrazolium reduction assay. In addition, relative ciliary activity was observed, virus multiplication measured, and histological studies were made. The hamster organ cultures proved relatively simple to set up and were sensitive to infection with influenza viruses; and the tetrazolium reduction was a reliable objective measure of relative tissue damage correlating well with histological and virological findings as well as with visual assessment of ciliary activities.

Growth of some attenuated influenza A viruses in hamsters

The replication of three attenuated influenza viruses in SPF hamsters inoculated by aerosol was investigated. Strains A2-Eng, A2-Victoria, ts-1-[E] A2-Victoria recombinant, and the RIT 4050 strain all multiplied to high titers in hamster lungs and induced both primary and secondary serological responses. Recombinant strain, A/AA/6/60-A2-Vic (CR22) multiplied to lower titers in hamsters. Differences were observed between the infectivity of different strains for hamsters, the wild-type viruses A2-Eng and A2-Vic being the most infective.

The hamster as an experimental animal for the study of influenza. II. The role of spleen cells in protection

Hamsters previously infected by influenza viruses, have been shown to have a cell-mediated immune response, as measured by the macrophage migration inhibition test. The participation of spleen cells in the protection of recipients against homologous influenza virus infection was also demonstrated using adoptive transfer experiments. However, the protection achieved by spleen cell transfer was marginal and not observed in every animal. The time at which the spleen cells were transferred following infection, and their number, affected the outcome. Evidence suggesting that transferred spleen cells protected recipient hamsters through specific antibody is presented.