Efficacy of influenza haemagglutinin and nucleoprotein as protective antigens against influenza virus infection in mice

Effect of an Adenovirus-Vectored Universal Influenza Virus Vaccine on Pulmonary Pathophysiology in a Mouse Model

Current influenza vaccines, live attenuated or inactivated, do not protect against antigenically novel influenza A viruses (IAVs) of pandemic potential, which has driven interest in the development of universal influenza vaccines. Universal influenza vaccine candidates targeting highly conserved antigens of IAV nucleoprotein (NP) are promising as vaccines that induce T cell immunity, but concerns have been raised about the safety of inducing robust CD8 T cell responses in the lungs. Using a mouse model, we systematically evaluated effects of recombinant adenovirus vectors (rAd) expressing IAV NP (A/NP-rAd) or influenza B virus (IBV) NP (B/NP-rAd) on pulmonary inflammation and function after vaccination and following live IAV challenge. After A/NP-rAd or B/NP-rAd vaccination, female mice exhibited robust systemic and pulmonary vaccine-specific B cell and T cell responses and experienced no morbidity (e.g., body mass loss). Both in vivo pulmonary function testing and lung histopathology scoring revealed minimal adverse effects of intranasal rAd vaccination compared with unvaccinated mice. After IAV challenge, A/NP-rAd-vaccinated mice experienced significantly less morbidity, had lower pulmonary virus titers, and developed less pulmonary inflammation than unvaccinated or B/NP-rAd-vaccinated mice. Based on analysis of pulmonary physiology using detailed testing not previously applied to the question of T cell damage, mice protected by vaccination also had better lung function than controls. Results provide evidence that, in this model, adenoviral universal influenza vaccine does not damage pulmonary tissue. In addition, adaptive immunity, in particular, T cell immunity in the lungs, does not cause damage when restimulated but instead mitigates pulmonary damage following IAV infection.IMPORTANCE Respiratory viruses can emerge and spread rapidly before vaccines are available. It would be a tremendous advance to use vaccines that protect against whole categories of viruses, such as universal influenza vaccines, without the need to predict which virus will emerge. The nucleoprotein (NP) of influenza virus provides a target conserved among strains and is a dominant T cell target. In animals, vaccination to NP generates powerful T cell immunity and long-lasting protection against diverse influenza strains. Concerns have been raised, but not evaluated experimentally, that potent local T cell responses might damage the lungs. We analyzed lung function in detail in the setting of such a vaccination. Despite CD8 T cell responses in the lungs, lungs were not damaged and functioned normally after vaccination alone and were protected upon subsequent infection. This precedent provides important support for vaccines based on T cell-mediated protection, currently being considered for both influenza and SARS-CoV-2 vaccines.

Universal Influenza Vaccines: Progress in Achieving Broad Cross-Protection In Vivo

Despite all we have learned since 1918 about influenza virus and immunity, available influenza vaccines remain inadequate to control outbreaks of unexpected strains. Universal vaccines not requiring strain matching would be a major improvement. Their composition would be independent of predicting circulating viruses and thus potentially effective against unexpected drift or pandemic strains. This commentary explores progress with candidate universal vaccines based on various target antigens. Candidates include vaccines based on conserved viral proteins such as nucleoprotein and matrix, on the conserved hemagglutinin (HA) stem, and various combinations. Discussion covers the differing evidence for each candidate vaccine demonstrating protection in animals against influenza viruses of widely divergent HA subtypes and groups; durability of protection; routes of administration, including mucosal, providing local immunity; and reduction of transmission. Human trials of some candidate universal vaccines have been completed or are underway. Interestingly, the HA stem, like nucleoprotein and matrix, induces immunity that permits some virus replication and emergence of escape mutants fit enough to cause disease. Vaccination with multiple target antigens will thus have advantages over use of single antigens. Ultimately, a universal vaccine providing long-term protection against all influenza virus strains might contribute to pandemic control and routine vaccination.

A human multi-epitope recombinant vaccinia virus as a universal T cell vaccine candidate against influenza virus

There is a need to develop a universal vaccine against influenza virus infection to avoid developing new formulations of a seasonal vaccine each year. Many of the vaccine strategies for a universal vaccine target strain-conserved influenza virus proteins, such as the matrix, polymerase, and nucleoproteins, rather than the surface hemagglutinin and neuraminidase proteins. In addition, non-disease-causing viral vectors are a popular choice as a delivery system for the influenza virus antigens. As a proof-of-concept, we have designed a novel influenza virus immunogen based on the NP backbone containing human T cell epitopes for M1, NS1, NP, PB1 and PA proteins (referred as NPmix) as well as a construct containing the conserved regions of influenza virus neuraminidase (N-terminal) and hemagglutinin (C-terminal) (referred as NA-HA). DNA vectors and vaccinia virus recombinants expressing NPmix (WR-NP) or both NPmix plus NA-HA (WR-flu) in the cytosol were tested in a heterologous DNA-prime/vaccinia virus-boost vaccine regimen in mice. We observed an increase in the number of influenza virus-specific IFNγ-secreting splenocytes, composed of populations marked by CD4(+) and CD8(+) T cells producing IFNγ or TNFα. Upon challenge with influenza virus, the vaccinated mice exhibited decreased viral load in the lungs and a delay in mortality. These findings suggest that DNA prime/poxvirus boost with human multi-epitope recombinant influenza virus proteins is a valid approach for a general T-cell vaccine to protect against influenza virus infection.

Cell-mediated protection in influenza infection

Current vaccine strategies against influenza focus on generating robust antibody responses. Because of the high degree of antigenic drift among circulating influenza strains over the course of a year, vaccine strains must be reformulated specifically for each influenza season. The time delay from isolating the pandemic strain to large-scale vaccine production would be detrimental in a pandemic situation. A vaccine approach based on cell-mediated immunity that avoids some of these drawbacks is discussed here. Specifically, cell-mediated responses typically focus on peptides from internal influenza proteins, which are far less susceptible to antigenic variation. We review the literature on the role of CD4+ and CD8+ T cell-mediated immunity in influenza infection and the available data on the role of these responses in protection from highly pathogenic influenza infection. We discuss the advantages of developing a vaccine based on cell-mediated immune responses toward highly pathogenic influenza virus and potential problems arising from immune pressure.

Immunization with influenza A NP-expressing vaccinia virus recombinant protects mice against experimental infection with human and avian influenza viruses

Two-fold immunization of Balb/c mice with a vaccinia virus recombinant expressing the NP protein of influenza A/PR8/34 (H1N1) virus under the control of a strong synthetic promoter induced specific antibodies and protected animals against low-dose challenge by mouse-adapted heterosubtypic variants of human A/Aichi2/68 (H3N2) and avian A/Mallard/Pennsylvania/10218/84 (H5N2) influenza virus strains. The surviving immunized animals had lower anti-hemagglutinin antibody titers compared to non-immunized mice. There was no difference in viral titers in lungs of immunized and non-immunized animals that succumbed to the infection. In order to try to increase immune system presentation of NP-protein-derived peptides, and thereby increase their immunogenicity, we constructed another vaccinia-based NP-expressing recombinant containing a rapid proteolysis signal covalently bound to the NP protein. This sequence, derived from the mouse ornithine decarboxylase gene has been shown to increase degradation of various proteins. However, we found that when used as part of a recombinant NP, this signal neither increased its proteolytic degradation, nor was it more efficient in the induction of a protective response against influenza infection.

Profound protection against respiratory challenge with a lethal H7N7 influenza A virus by increasing the magnitude of CD8(+) T-cell memory

The recall of CD8(+) T-cell memory established by infecting H-2(b) mice with an H1N1 influenza A virus provided a measure of protection against an extremely virulent H7N7 virus. The numbers of CD8(+) effector and memory T cells specific for the shared, immunodominant D(b)NP(366) epitope were greatly increased subsequent to the H7N7 challenge, and though lung titers remained as high as those in naive controls for 5 days or more, the virus was cleared more rapidly. Expanding the CD8(+) memory T-cell pool (<0.5 to >10%) by sequential priming with two different influenza A viruses (H3N2-->H1N1) gave much better protection. Though the H7N7 virus initially grew to equivalent titers in the lungs of naive and double-primed mice, the replicative phase was substantially controlled within 3 days. This tertiary H7N7 challenge caused little increase in the magnitude of the CD8(+) D(b)NP(366)(+) T-cell pool, and only a portion of the memory population in the lymphoid tissue could be shown to proliferate. The great majority of the CD8(+) D(b)NP(366)(+) set that localized to the infected respiratory tract had, however, cycled at least once, though recent cell division was shown not to be a prerequisite for T-cell extravasation. The selective induction of CD8(+) T-cell memory can thus greatly limit the damage caused by a virulent influenza A virus, with the extent of protection being directly related to the number of available responders. Furthermore, a large pool of CD8(+) memory T cells may be only partially utilized to deal with a potentially lethal influenza infection.

Strategies for inducing protection against avian influenza A virus subtypes with DNA vaccines

The cross-species transfer of a H5N1 influenza virus from birds to humans, and the systemic spread of this virus in mice, has accelerated the efforts to devise protective strategies against lethal influenza viruses. DNA vaccination with the highly conserved nucleoprotein gene appears to provide cross protection against influenza A viruses in murine models. Whether such vaccines would protect human hosts against different influenza A viruses, including strains with pandemic potential, is unclear. Our aim in this study is to evaluate the ability of a combination DNA vaccine consisting of two plasmids encoding the HA genes from two different subtypes and a DNA vaccine encoding the viral nucleoprotein gene from a H5 virus to induce protection against highly lethal infection caused by H5 and H7 influenza viruses in chickens. Chickens given a single dose of plasmids expressing H5 and H7 hemagglutinins protected the birds from infection by either subtype. However, birds immunized with nucleoprotein DNA and challenged with either A/Ck/Vic/1/85(H7N7) or A/Ty/Ir/1/83 (H5N8) showed definite signs of infection, suggesting inadequate immunity against viral infection. Fifty percent of the nucleoprotein DNA immunized birds survived infection by influenza A/Ty/Ir/1/83 (H5N8) virus (virus of same subtype) while 42% survived infection by influenza A/Ck/Vic/1/85/(H7N7) virus (virus of a different subtype). These studies demonstrate that immunization with DNA encoding a type-specific gene may not be effective against either homologous or heterologous strains of virus, particularly if the challenge virus causes a highly lethal infection. However, the combination of HA subtype vaccines are effective against lethal infection caused by viruses expressing any of the HA subtypes used in the combination preparation.

Vaccination with DNA encoding internal proteins of influenza virus does not require CD8(+) cytotoxic T lymphocytes: either CD4(+) or CD8(+) T cells can promote survival and recovery after challenge

DNA vaccination offers the advantages of viral gene expression within host cells without the risks of infectious virus. Like viral vaccines, DNA vaccines encoding internal influenza virus proteins can induce immunity to conserved epitopes and so may defend the host against a broad range of viral variants. CD8(+) cytotoxic T lymphocytes (CTL) have been described as essential effectors in protection by influenza nucleoprotein (NP), although a lesser role of CD4(+) cells has been reported. We immunized mice with plasmids encoding influenza virus NP and matrix (M). NP + M DNA allowed B6 mice to survive otherwise lethal challenge infection, but did not protect B6-beta(2)m(-/-) mice defective in CD8(+) CTL. However, this does not prove CTL are required, because beta(2)m(-/-) mice have multiple immune abnormalities. We used acute T cell depletion in vivo to identify effectors critical for defense against challenge infection. Since lung lymphocytes are relevant to virus clearance, surface phenotypes and cytolytic activity of lung lymphocytes were analyzed in depleted animals, along with lethal challenge studies. Depletion of either CD4(+) or CD8(+) T cells in NP + M DNA-immunized BALB/c mice during the challenge period did not significantly decrease survival, while simultaneous depletion of CD4(+) and CD8(+) cells or depletion of all CD90(+) cells completely abrogated survival. We conclude that T cell immunity induced by NP + M DNA vaccination is responsible for immune defense, but CD8(+) T cells are not essential in the active response to this vaccination. Either CD4(+) or CD8(+) T cells can promote survival and recovery in the absence of the other subset.

Contribution of virus-specific CD8+ cytotoxic T cells to virus clearance or pathologic manifestations of influenza virus infection in a T cell receptor transgenic mouse model

The ability of influenza virus to evade immune surveillance by neutralizing antibodies (Abs) directed against its variable surface antigens provides a challenge to the development of effective vaccines. CD8+ cytotoxic T lymphocytes (CTLs) restricted by class I major histocompatibility complex molecules are important in establishing immunity to influenza virus because they recognize internal viral proteins which are conserved between multiple viral strains. In contrast, protective Abs are strain-specific. However, the precise role of effector CD8+ CTLs in protection from influenza virus infection, critical for understanding disease pathogenesis, has not been well defined. In transgenic mice with a very high frequency of antiinfluenza CTL precursors, but without protective Abs, CD8+ CTLs conferred protection against low dose viral challenge, but exacerbated viral pathology and caused mortality at high viral dose. The data suggest a dual role for CD8+ CTLs against influenza, which may present a challenge to the development of effective CTL vaccines. Effector mechanisms used by CD8+ CTLs in orchestrating clearance of virus and recovery from experimental influenza infection, or potentiation of lethal pathology, are discussed.

Mechanism of Protective Immunity Against Influenza Virus Infection in Mice Without Antibodies

There is considerable interest in developing viral vaccines intended to induce T cell immunity, especially cytotoxic CD8+ T lymphocytes, when Abs are not protective or are too narrow in viral strain specificity. We have studied protective immunity in doubly inactivated (DI) mice devoid of Abs and mature B cells. When infected with influenza B virus, these mice cleared the virus in a process dependent upon CD8+ T lymphocytes. Cytotoxic activity was detected in lung lymphocytes of DI mice after primary or secondary infection, and was abrogated by depletion of CD8+ cells in vivo. Challenge experiments showed that DI mice could be protected by immunization against reinfection 1 mo later, and protection was virus specific. Depletion of CD4+ or CD8+ T cells in vivo during the challenge period partially abrogated, and depletion of both subsets completely abrogated, the protection. This indicates that both CD4+ and CD8+ T cells are required effectors in the optimal control of virus replication. Thus, when Abs fail to protect against varying challenge viruses, as is the case with variant strains of influenza and HIV, there is hope that T cells might be able to act alone.

Influenza-specific immunity induced by recombinant Listeria monocytogenes vaccines

In this study, we evaluate two Listeria monocytogenes strains that express influenza nucleoprotein (NP) sequences for their ability to protect against challenge with influenza-virus. The construction of one strain, which expresses only the Kd restricted NP epitope (NP 147-155), is described in this study; the other strain, which expresses the full NP sequence in the form of a fusion protein, has been described previously. The ability of the two strains to present the Kd restricted NP epitope in vitro and induce NP-specific CTL in vivo is also described. Mice immunized by the intravenous route with either strain cleared a subsequent (3 weeks post-immunization) influenza virus infection more rapidly as indicated by reduced virus titers in the lungs 5 days after challenge. Efficacy of both recombinant L. monocytogenes strains as vaccines in this system was equivalent and equal to that of recombinant vaccinia expressing NP.

Oral immunization with a replication-deficient recombinant vaccinia virus protects mice against influenza

Mice immunized with two intragastrically administered doses of a replication-deficient recombinant vaccinia virus containing the hemagglutinin and nucleoprotein genes from H1N1 influenza virus developed serum anti-H1 immunoglobulin G (IgG) antibody that completely protected the lungs from challenge with H1N1. Almost all of the mice given two intragastric doses also developed mucosal anti-H1 IgA antibody, and those with high anti-H1 IgA titers had completely protected noses. Intramuscular injection of the vaccine protected the lungs but not the noses from challenge. We also found that the vaccine enhanced recovery from infection caused by a shifted (H3N2) influenza virus, probably through the induction of nucleoprotein-specific cytotoxic T-lymphocyte activity. A replication-deficient, orally administered, enteric-coated, vaccinia virus-vectored vaccine might safely protect humans against influenza.

Protection of mice against an influenza virus infection by oral vaccination with viral nucleoprotein incorporated into immunostimulating complexes

Influenza A virus nucleoprotein (NP) was integrated into immunostimulating complexes (ISCOMs) after attachment of bacterial lipopolysaccharide to the antigen. Oral immunization with these NP-ISCOMs protected mice fully against an otherwise lethal challenge infection with an unrelated influenza virus subtype without the appearance of severe clinical signs or extensive pathological lesions in the lungs. Mice immunized with analogous bovine serum albumine-incorporated ISCOMs all died. After oral immunization, high titers of NP-specific antibodies, particularly IgA, could be detected in the bronchoalveolar fluid and in the blood serum. No cytotoxic lymphocytes could be demonstrated in the spleens or the lungs of vaccinated mice, and no anti-NP antibody-dependent cytolysis of infected host cells was mediated by complement or in the form of an antibody-dependent cell cytotoxicity. However, a vigorous delayed-type hypersensitivity reaction was produced after probing vaccinated animals with purified NP. No comparable protective immunity or antibody response was induced by a strictly intragastric administration of NP-ISCOMs. It appears, therefore, that the general and local immune response in the lungs was primarily stimulated through contact of NP-ISCOMs with the mucous membrane of the oro-pharyngeal cavity and that cytotoxic effects did not play a major role for the establishment of the protective immunity. Partial protection against a lethal challenge was observed in chickens immunized with NP-ISCOMs in the drinking water.

Primary pulmonary cytotoxic T lymphocytes induced by immunization with a vaccinia virus recombinant expressing influenza A virus nucleoprotein peptide do not protect mice against challenge

The nucleoprotein (NP) of influenza A virus is the dominant antigen recognized by influenza virus-specific cytotoxic T lymphocytes (CTLs), and adoptive transfer of NP-specific CTLs protects mice from influenza A virus infection. BALB/c mouse cells (H-2d) recognize a single Kd-restricted CTL epitope of NP consisting of amino acids 147 to 155. In the present study, mice were immunized with various vaccinia virus recombinant viruses to examine the effect of the induction of primary pulmonary CTLs on resistance to challenge with influenza A/Puerto Rico/8/34 virus. The minigene ESNP(147-155)-VAC construct, composed of a signal sequence from the adenovirus E3/19K glycoprotein (designated ES) and expressing the 9-amino-acid NP natural determinant (amino acids 147 to 155) preceded by an alanine residue, a similar minigene NP(Met 147-155)-VAC lacking ES, and a full-length NP-VAC recombinant of influenza virus were analyzed. The two minigene NP-VAC recombinants induced a greater primary pulmonary CTL response than the full-length NP-VAC recombinant. However, NP-specific CTLs induced by immunization with ESNP(147-155)-VAC did not decrease peak virus titer or accelerate clearance of virus in the lungs of mice challenged intranasally with A/PR/8/34. Furthermore, NP-specific CTLs induced by immunization did not protect mice challenged intranasally with a lethal dose of A/PR/8/34. Sequence analysis of the NP CTL epitope of A/PR/8/34 challenge virus obtained from lungs after 8 days of replication in ESNP(147-155)-VAC-immunized mice showed identity with that of the input virus, demonstrating that an escape mutant had not emerged during replication in vivo. Thus, in contrast to adoptively transferred CTLs, pulmonary NP-specific CTLs induced by recombinant vaccinia virus immunization do not have protective in vivo antiviral activity against influenza virus infection.

Recombinant fowlpox virus vaccines for poultry

The intensive poultry industries rely heavily upon the use of vaccines for disease control. Viral vector based vaccines offer new avenues for the development of vaccines for effective disease control in poultry. Techniques developed for the construction of recombinant vaccinia viruses have been readily adapted to the construction of recombinant viruses based on fowlpox virus (rFPV). The ability to insert several genes into the large genome of fowlpox may enable the development of multivalent vaccines and vaccines incorporating immune response modifiers such as lymphokines. Newcastle disease, avian influenza, infectious bursal disease and Marek's disease antigens expressed by rFPV have been shown to be effective vaccines in poultry. None appear, however, to provide a substantial improvement in vaccine efficacy. Recombinant FPV will be a valuable adjunct to conventional vaccines currently in widespread use. Whether rFPV or other vector based vaccines can circumvent the problems of vaccination in the presence of high maternally derived antibodies is yet to be resolved. The observation that avipoxvirus recombinants may be suitable for the vaccination of non-avian species provides an added dimension to vaccines based on FPV or other avipoxviruses. Recombinant FPV will find a useful role in poultry disease control when used in conjunction with conventional vaccines.

Comparison of humoral immune responses to trivalent influenza split vaccine in young, middle-aged and elderly people

A total of 296 volunteers in five different groups were immunized with one dose of the commercial 1991-1992 trivalent split influenza vaccine formulation A/Singapore/6/86 (H1N1), A/Beijing/353/89 (H3N2) and B/Yamagata/16/88. The groups differed in age (young adults, middle-aged and elderly) and history of previous vaccination. Antibodies were determined in pre- and postvaccination sera by haemagglutination inhibition assay and the results were evaluated as geometric mean titre, mean fold antibody increase, protection and response rates. No significant age-related differences among the protection rates were found. The proportion of vaccinees with antibodies > or = 40 ranged between 70 and 95%. Compared with the H3N2 and B components the antibody response to the H1N1-component was low. Residents of a nursing home fully vaccinated the previous year developed 7.6-8.4-fold antibody increases and showed 96-100% protection rates.

The cytolytic activity of pulmonary CD8+ lymphocytes, induced by infection with a vaccinia virus recombinant expressing the M2 protein of respiratory syncytial virus (RSV), correlates with resistance to RSV infection in mice

Previous studies demonstrated that the pulmonary resistance to respiratory syncytial virus (RSV) challenge induced by immunization with a recombinant vaccinia virus expressing the M2 protein of RSV (vac-M2) was significantly greater 9 days after immunization than at 28 days and was mediated predominantly by CD8+ T cells. In this study, we have extended these findings and sought to determine whether the level of CD8+ cytotoxic T-lymphocyte (CTL) activity measured in vitro correlates with the resistance to RSV challenge in vivo. Three lines of evidence documented an association between the presence of pulmonary CTL activity and resistance to RSV challenge. First, vac-M2 immunization induced pulmonary CD8+ CTL activity and pulmonary resistance to RSV infection in BALB/c (H-2d) mice, whereas significant levels of pulmonary CTL activity and resistance to RSV infection were not seen in BALB.K (H-2k) or BALB.B (H-2b) mice. Second, pulmonary CD8+ CTL activity was not induced by infection with other vaccinia virus-RSV recombinants that did not induce resistance to RSV challenge. Third, the peak of pulmonary CTL activity correlated with the peak of resistance to RSV replication (day 6), with little resistance being observed 45 days after immunization. An accelerated clearance of virus was not observed when mice were challenged with RSV 45 days after immunization with vac-M2. The results indicate that resistance to RSV induced by immunization with vac-M2 is mainly mediated by primary pulmonary CTLs and that this resistance decreases to very low levels within 2 months following immunization. The implications for inclusion of CTL epitopes into RSV vaccines are discussed in the context of these observations.

Interference of maternal antibodies with the immune response of foals after vaccination against equine influenza

The purpose of the study was twofold. First, using two groups of 22 foals each, we investigated the extent to which maternal antibodies interfere with the humoral response against equine influenza. The foals were born to mares that had been vaccinated twice yearly against influenza since 1982. Foals of group I were vaccinated three times at early ages (12, 16, and 32 weeks of age), and foals of group II were likewise vaccinated but a later ages (24, 28, and 44 weeks of age). After the first and second vaccinations, neither group showed an increase in antibodies that inhibit haemagglutination. Group II foals, however, had a significantly stronger antibody response against nucleoprotein after the second vaccination than the foals of group I. After the third vaccination, group II foals had a significantly stronger and longer lasting antibody response against haemagglutinin than the foals of group I. However, the antibody response to nucleoprotein was comparable in both groups. Second, the foals of group II were studied to determine the persistence of maternal antibodies directed against a common nucleoprotein and the haemagglutinin of two strains of equine influenza A virus. Biological half-lives of 39, 32, and 33 days were calculated for maternal antibodies directed against haemagglutinin of strains H7N7 Prague and H3N8 Miami, and against the nucleoprotein respectively. Maternal antibody titres at the time of vaccination were closely related to the degree of interference with the immune response. Because even small amounts of maternal antibodies interfered with the efficacy of vaccination, we conclude that foals born to mares vaccinated more than once yearly against influenza virus should not be vaccinated before 24 weeks of age.

Raccoon poxvirus recombinants expressing the rabies virus nucleoprotein protect mice against lethal rabies virus infection

Raccoon poxvirus (RCN) recombinants expressing the rabies virus internal structural nucleoprotein (RCN-N) protected A/WySnJ mice against a lethal challenge with street rabies virus (SRV). Maximum survival was achieved following vaccination by tail scratch and footpad (FP) SRV challenge. RCN-N-vaccinated mice inoculated in the FP with SRV were resistant to infection for at least 54 weeks postvaccination. Protection was also elicited by RCN recombinants expressing the rabies virus glycoprotein (RCN-G). Vaccination with RCN-G evoked rabies virus neutralizing antibody. Rabies virus neutralizing antibody was not detected in RCN-N-vaccinated mice prior to or following SRV infection. Radioimmunoprecipitation assays showed that sera from RCN-N-vaccinated mice which survived SRV infection did not contain antibody to SRV structural protein G, M, or NS. The mechanism(s) of N-induced resistance appears to correlate with the failure of peripherally inoculated SRV to enter the central nervous system (CNS). Support for this correlation with resistance was documented by the observations that SRV-inoculated RCN-N-vaccinated mice did not develop clinical signs of CNS rabies virus infection, infectious SRV was not detected in the spinal cord or brain following FP challenge, and all RCN-N-vaccinated mice died following direct intracranial infection of the CNS with SRV. These results suggest that factors other than anti-G neutralizing antibody are important in resistance to rabies virus and that the N protein should be considered for incorporation with the G protein in recombinant vaccines.

Efficacy of nucleoprotein and haemagglutinin antigens expressed in fowlpox virus as vaccine for influenza in chickens

Fowlpox virus (FPV) recombinants expressing influenza virus H5 haemagglutinin (HA), nucleoprotein (NP) or co-expressing both of these antigens were tested for vaccine efficacy in chickens. Immunization with the recombinant FPV-HA was highly efficacious but provided no cross protection between subtypes. Bursectomy established that immunity against the H5 subtype was antibody-mediated despite the presence of very low levels of antibody in the vaccinated birds. Immunization with the recombinant FPV expressing the cross-reactive NP antigen did not provide protective immunity despite hyperimmunization and provided no benefit above HA expressed alone. The results suggest that the kinetics of viral replication outpaces immunity induced by NP.

Construction of a pigeonpox virus recombinant: expression of the Newcastle disease virus (NDV) fusion glycoprotein and protection of chickens against NDV challenge

A pigeonpox transfer plasmid was constructed by cloning a 2.5 kb DNA fragment containing the viral thymidine kinase (TK) gene in the psp65 plasmid. The vaccinia virus P11K promoter followed by the NDV fusion (F) gene was inserted in the TK gene. The F gene was transferred to the viral genome by homologous recombination in pigeonpox virus infected CEF cells, transfected with the recombinant plasmid. Recombinant viruses were selected with BUdR and screened for their ability to induce fusion between adjacent cells. Because of the unexpected growth advantage of the TK+ WT over the TK- recombinants, viral purification was needed to obtain stable recombinants expressing a glycosylated and cleaved F protein. Vaccination of chickens by the follicular method induced high anti-F antibody titers and good protection against challenge with the virulent Italian NDV strain. Half of the oculonasal vaccinated chickens showed anti F antibodies and also half of them were protected. Although protection seems to be correlated with antibody titers, no neutralizing antibodies were found.

Vaccinia virus-mediated damage of murine ovaries and protection by virus-expressed interleukin-2

Vaccinia virus was shown to replicate in the ovaries of normal inbred mice and cause sufficient damage to ovarian follicles to decrease fertility of the mice. The mouse-adapted strain, VV-WR, had a greater affinity for growth in ovaries than the vaccine strains, VV-Elstree, VV-NYBH or VV-Copenhagen. Virus reached the ovaries after intravenous or intraperitoneal inoculation, but not after subcutaneous inoculation in the foot pad. Interleukin-2 (IL-2), when expressed by a recombinant vaccinia virus was able to decrease the titre of virus in the ovaries and prevent infertility. Both non-specific (natural killer cells) and antigen-specific mechanisms were active within the ovaries and may play a role in the IL-2-mediated clearance of vaccinia virus.

An ELISA for detection of antibodies against influenza A nucleoprotein in humans and various animal species

A double antibody sandwich blocking ELISA, using a monoclonal antibody (MAb) against influenza A nucleoprotein (NP) was developed to detect antibodies against influenza. Collections of serum samples were obtained from human and various animal species. All influenza A subtypes induced antibodies against hemagglutinins and NP. A close correlation between titers of the hemagglutination inhibition (HI) test and the NP-ELISA was seen. Antibodies against influenza NP were demonstrated in serum samples from humans, ferrets, swine, horses, chickens, ducks, guinea pigs, mice, and seals. The serum samples were collected at intervals during prospective epidemiological studies, from experimental and natural infections, and vaccination studies. The decline of maternal antibodies was studied in swine and horses. The NP-ELISA enables rapid serological diagnosis and is suited for influenza A antibody screening, especially in species which harbor several influenza subtypes. The HI and neuraminidase inhibition tests, however, must still be used for subtyping.

Immunogenic properties of ISCOM prepared with influenza virus nucleoprotein

After covalent attachment of bacterial lipopolysaccharide to the nucleoprotein of influenza A virus, this water-soluble antigen could be incorporated firmly into ISCOM. This potent "immunostimulating complex" induced the production of high antibody titers in mice and could partially protect the animals from a lethal challenge infection. After immunization with ISCOM preparations NP-specific cytotoxic T cell activity could not be demonstrated.