Oral immunization with a novel attenuated Salmonella Typhimurium encoding influenza HA, M2e and NA antigens protects chickens against H7N9 infection
© The Author(s) 2018
Received: 30 October 2017
Accepted: 18 January 2018
Published: 1 February 2018
Attenuated Salmonella strains constitute a promising technology for the development of efficient protein-based influenza vaccines. H7N9, a low pathogenic avian influenza (LPAI) virus, is a major public health concern and currently there are no effective vaccines against this subtype. Herein, we constructed a novel attenuated Salmonella Typhimurium strain for the delivery and expression of H7N9 hemagglutinin (HA), neuraminidase (NA) or the conserved extracellular domain of the matrix protein 2 (M2e). We demonstrated that the constructed Salmonella strains exhibited efficient HA, NA and M2e expressions, respectively, and the constructs were safe and immunogenic in chickens. Our results showed that chickens immunized once orally with Salmonella (Sal) mutants encoding HA (Sal-HA), M2e (Sal-M2e) or NA (Sal-NA), administered either alone or in combination, induced both antigen-specific humoral and cell mediated immune (CMI) responses, and protected chickens against the lethal H7N9 challenge. However, chickens immunized with Sal-HA+Sal-M2e+Sal-NA vaccine constructs exhibited efficient mucosal and CMI responses compared to the chickens that received only Sal-HA, Sal-M2e or Sal-M2e+Sal-NA vaccine. Further, chickens immunized with Sal-HA+Sal-M2e+Sal-NA constructs cleared H7N9 infection at a faster rate compared to the chickens that were vaccinated with Sal-HA, Sal-M2e or Sal-M2e+Sal-NA, as indicated by the reduced viral shedding in cloacal swabs of the immunized chickens. We conclude that this vaccination strategy, based on HA, M2e and NA, stimulated efficient induction of immune protection against the lethal H7N9 LPAI virus and, therefore, further studies are warranted to develop this approach as a potential prophylaxis against LPAI viruses affecting poultry birds.
Avian influenza viruses, classified as highly pathogenic avian influenza (HPAI) or low-pathogenicity avian influenza (LPAI), cause huge economical losses to the poultry industry annually [1, 2]. Outbreaks of HPAI and LPAI viruses belonging to H7 subtype in chickens have been reported in the past [3–5], and infections caused by H7N9 LPAI virus have not only infected poultry birds but also humans as well, with limited person to person transmission [6, 7]. The World Health Organization (WHO) has identified H7N9 virus as an unusually dangerous virus for humans, and infection caused by H7N9 virus is a major public health concern as it is unlikely that there will be pre-existing immunity against this subtype in the human population . Humans infected with H7N9 virus mostly result in severe respiratory illness, with a mortality of roughly 30% . The H7N9 virus possess characteristic features related to the human adaptation, for instance mutations in the HA and PB2 proteins, which facilitate the virus binding to and replicating in the respiratory tract [7, 10]. Most of the H7N9 vaccines showed low immunogenicity and induction of non-protective hemagglutination-inhibiting antibody titers [11, 12]. Thus, potent and effective vaccines must be available to prevent the occurrence of H7N9 infection in humans. Recently, a complete protection against the lethal challenge of novel H7N9 virus with heterologous inactivated H7 vaccine was observed in mice . Although effective in controlling the H7N9 infection, these inactivated influenza vaccines require a large supply of specific-pathogen free (SPF) embryonated eggs and a long timeline that could be threatened during an influenza pandemic affecting both animal and human population . Thus, effective vaccination strategies should be in place which not only induce protective immune responses but concurrently allows easier manipulation and faster production of vaccines, and can provide broader spectrum of protection against the heterologous strains of influenza viruses . Earlier studies have reported that live attenuated Salmonella-based vaccines carrying various heterologous bacterial or viral antigens elicit efficient protective mucosal and cell mediated immune (CMI) responses . Salmonella-based vaccination strategy is highly economical and allows for a quick response to novel influenza viruses, as it eliminates the need for specific pathogen free (SPF) embryonated eggs required for the production of conventional influenza vaccines. Moreover, this strategy allows the differentiation of vaccinated from infected animals as antigenically and immunologically relevant antigens are used to construct the vaccine strain.
Previously, we have constructed an O antigen intact Salmonella-hemagglutinin (HA) consensus (H7N3, H7N7 and H7N9 sequence) based vaccine that conferred significant protection against the lethal heterologous H7N1 infection in chicken model . The HA is the most abundant integral viral envelope protein and is the major target for generating protective immunity; however, HA is continuously subjected to antigenic variations either through antigenic drift (point mutations) or rapidly through reassortment with another divergent virus (antigenic shift). Consequently, the immunity generated against one vaccine strain is only protective against another strain that shares antigenically related proteins. Similar to HA, the matrix protein 2 (M2) is an integral transmembrane protein of influenza A viruses . The ectodomain of M2 (M2e), 23 amino acid residues, has been remarkably conserved in all human influenza A strains, and animal experiments have demonstrated that M2e-specific antibodies can provide cross protective immunity against infections caused by different types of influenza A subtypes . Recently, Schotsaert et al. has shown that neuraminidase (NA) and M2e-based immunization strategies can induce long-lasting cross-protective immunity against influenza A viruses . In accordance with this notion, in this study, we constructed O antigen deficient attenuated auxotrophic mutant of Salmonella Typhimurium expressing and secreting HA, M2 ectodomain (M2e) or NA of H7N9 virus, and evaluated the efficacy of these Salmonella-based influenza vaccines in chicken model.
Our results show that chickens immunized once orally with Salmonella mutants encoding HA (Sal-HA), M2e (Sal-M2e) or NA (Sal-NA), administered either alone or in combination, induced both humoral and CMI responses, and protected the chickens against the lethal H7N9 challenge. Our results further demonstrate that the chickens immunized with a co-mix of Salmonella mutants encoding HA, M2e or NA proteins showed higher protective immunity than the chickens vaccinated with Sal-HA, Sal-M2e or Sal-M2e+Sal-NA based vaccine.
Materials and methods
Virus and cell line
The tissue culture infective dose (TCID50) of H7N9 influenza virus, cultivated in the allantoic cavity of SPF embryonated eggs, was calculated in Madin Darby Canine Kidney (MDCK) cells as described previously .
Construction of O antigen deficient attenuated auxotrophic Salmonella Typhimurium mutants expressing HA, M2e tetramer or NA antigens
Bacterial strains and plasmids used in this study
∆lon, ∆cpxR, ∆asd, wbaP mutant of S. Typhimurium
JOL1800 with pMMP65
JOL1800 with pMMP65-4M2e
JOL1800 with pMMP65-HA1 plasmid
JOl1800 with pMMP65-NA
IPTG-inducible, T7 expression vector, C-terminal 6× His tag, KanR
asd+, pBRori, lactamase signal sequence based periplasmic secretion plasmid, 6 His tag, high copy number plasmid
Immunization and challenge studies of chickens
All animal experimentation work was approved by the Chonbuk National University Animal Ethics Committee (CBU 2014-1-0038) and the chicken experiments were carried out according to the guidelines of the Korean Council on Animal Care. One-day-old female layer chickens (Corporation of Join hatchery, Republic of Korea) were maintained under standard conditions and provided antibiotic-free food and water ad libitum. Two weeks later, the chickens were randomly divided into five groups (n = 12) and immunized orally with Salmonella-based influenza vaccines. Group 1 (Sal-vector) received JOL1837 (JOL1800 carrying an empty pMMP65 vector), group 2 (Sal-HA) received JOL2030, group 3 (Sal-M2e) received JOL1913, group 4 (Sal-M2e-NA) received a co-mix of JOL1913 and JOL2052, and group 5 received a co-mix of JOL2030, JOL1913 and JOL2052. The 109 colony forming units (CFU) of each bacterial strain were used for inoculation in this study. Serum and intestinal wash samples were collected on the day of vaccination (pre-vaccination) and weekly thereafter to assess the HA, M2e and NA specific systemic IgG and mucosal IgA humoral responses.
Four weeks post-vaccination, all the immunized and the control chickens were intranasally challenged with 1.6 × 104 TCID50 of H7N9 virus and the cloacal swabs were collected through day 1–day 6 post-challenge to determine the viral load by qRT-PCR assay as described previously . For histological analyses, three chickens in each group were sacrificed on day 4 post-challenge. Lung and intestinal tissues were aseptically collected and fixed with 10% formalin, embedded in paraffin and cut into 5 μm thin sections. These sections were subsequently stained with hematoxylin and eosin (H&E) for microscopic examination.
Hemagglutination inhibition assay and virus neutralization assay
Immunized sera showing values below X values are positive for neutralization activity and the serum neutralization titers are calculated as log10 of the reciprocal of the last serum dilution that neutralized the H7N9 virus activity by 50%.
Systemic IgG and mucosal IgA specific antibody responses
Lymphocyte proliferation assay
The cell mediated immunity elicited by vaccination was evaluated by a lymphocyte proliferation test as described previously . Two weeks post-immunization, in vitro proliferation of vaccinated peripheral blood mononuclear cells (PBMCs) in response to the inactivated H7N9 recall antigen was measured by a MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] based assay as described in Hyoung et al. .
PBMCs restimulated with inactivated H7N9 virus were harvested after 24 h and total RNA isolated from stimulated cells was analysed for IFN-γ, IL-4 and IL-17 gene expressions by qRT-PCR assay as described previously .
All the obtained data was analysed using GraphPad prism 6.00 program (San Diego, CA, USA). One way ANOVA with Tukey’s multiple comparison test was used to compare the data between different groups. p < 0.05 were considered statistically significant.
Construction of a HA, M2e or NA based influenza vaccine delivered by an O antigen deficient attenuated auxotrophic mutant of Salmonella Typhimurium
The construction of Salmonella-HA and M2e-based influenza vaccines is described elsewhere [15, 18]. To construct the Salmonella-NA based influenza vaccine, the codon optimized NA gene sequence was cloned in frame into pMMP65 expression vector as described earlier . The presence of NA gene in pMMP65 vector was confirmed by digestion of recombinant clones with EcoR1 and HindIII to release a fragment of 1425 bp size. The recombinant plasmid, pMMP65-NA, was transformed into JOL1800 strain for expression of NA protein as described previously , and the resultant clone was designated as JOL2052. Western blot analysis with a polyclonal NA antibody (catalog#, NBP2-41279, Novus Biologicals USA) showed an immunoreactive band corresponding to the ∼58 kDa, the expected size of our protein of interest, and thus confirmed the NA expression (Additional file 2).
Immunization with HA, M2e and NA antigens delivered by Salmonella system induces protective antibody responses
We also analysed VN titers among immunized chickens by serum microneutralization assay. Our results showed that VN titers were significantly (p < 0.05) higher in immunized chickens as compared to the control group (Figure 1B). Among immunized groups, chickens vaccinated with Salmonella-M2e based vaccine showed significantly (p < 0.05) lower VN titers compared to the groups immunized with Salmonella-HA, Salmonella-M2e-NA or Salmonella-HA-M2e-NA based vaccine. However, no statistical significant difference was observed when VNT were compared among Sal-HA, Sal-M2e-NA and Sal-HA-M2e-NA vaccinated groups. These findings suggest that Salmonella-based influenza vaccines have potential to elicit significant neutralizing antibody responses which reflect the level of protective immunity against influenza challenges.
Immunization with HA, M2e and NA antigens delivered by Salmonella system elicits efficient systemic and mucosal antibody responses
Chickens vaccinated with either Salmonella-HA or Salmonella-HA-M2e-NA based vaccine showed significantly (P < 0.05) higher HA-specific systemic IgG responses as compared to the control chicken group (Figure 2A). The IgG levels were detected at 14th day post-immunization, which maintained till 6th week, and the responses were comparable in both the immunized groups. Chickens immunized with either Salmonella-M2e-NA or Salmonella-HA-M2e-NA based vaccine elicited significantly (p < 0.05) higher NA-specific IgG responses compared to the control group (Figure 2B). The IgG levels were detected at 14th day post-immunization, which peaked at 6th week in both the vaccinated groups (Figure 2B). Our results further demonstrated that the HA and the NA-specific IgG levels were comparable among immunized groups (p > 0.05). Chickens immunized with either Salmonella-M2e, Salmonella-M2e-NA or Salmonella-HA-M2e-NA based vaccine elicited significantly (p < 0.05) higher M2e-specific IgG responses compared to the control group (Figure 2C). The IgG levels were detected at 14th day post-immunization, which peaked at 28th day and then maintained till 6th week. There were no statistically significantly differences in M2e-specific IgG responses among the immunized groups, however, the M2e-specific responses were significantly (p < 0.05) higher at 4 and 6th week post-immunization compared to the HA and the NA-specific IgG responses (Figure 2C).
We also measured mucosal IgA responses in immunized chickens at 28th day post-immunization (Figures 2D–F). Our results showed that chicken vaccinated with Salmonella-HA-M2e-NA based vaccine showed significantly (p < 0.05) higher HA (Figure 2D), M2e (Figure 2E) and NA (Figure 2F) specific IgA levels compared to the other immunization groups. Further, chickens immunized with either Salmonella-M2e or Salmonella-M2e-NA based vaccine elicited significantly (p < 0.05) higher M2e specific IgA responses compared to the control group (Figure 2F). Although chickens immunized with either Salmonella-M2e-NA or Salmonella-HA based vaccine showed slight increase in NA and HA-specific IgA mucosal responses, respectively, but the responses were statistically non-significant (p > 0.05) compared to the control chicken group.
Immunization with HA, M2e and NA antigens delivered by Salmonella system activates efficient CMI responses
Immunization with HA, M2e and NA antigens delivered by Salmonella system protects chickens against lethal H7N9 challenge
The present study demonstrates the potential of Salmonella-based influenza vaccines to elicitate efficient antigen-specific immune responses and protection against a lethal H7N9 infection in chicken model. A major obstacle in influenza vaccine development is the extent of genetic diversity among influenza strains and therefore, neutralizing antibodies elicited by one vaccine strain provides little or no protection against the heterologous strain . In accordance with this notion, we generated HA and NA consensus sequences to centralize the immunogenicity of the vaccine antigen that might provide cross protection against various circulating strains of influenza viruses. Another approach to achieve a broad spectrum of protection is the designing of vaccines based on the highly conserved viral antigens that are subjected to minor antigenic variations. M2e is highly conserved target and an attractive choice for the development of a universal influenza A vaccine [22, 23]. In the present study, we used four tandem repeats of M2e to increase the immunogenicity of M2e antigen . Earlier studies show that M2e-based vaccines induced certain level of cross protective immunity in animal models, but partial protection against the lethal influenza challenges [23, 24]. The coadministration of M2e with either HA or NA is an effective strategy to induce broadly cross-reactive anti-M2e specific antibodies. Previous study shows that immunization with Freund’s adjuvanted M2e antigen failed to induce M2e-specific antibody responses, but by administering HA-M2e conjugate readily elicited M2e specific antibodies . Thus in the present study, we used M2e either alone or in combination with HA and NA proteins. Previously, we show that Salmonella-HA consensus based vaccine provided efficient protection against heterologous H7N1 infection . Herein, we report that immunization with Salmonella mutants encoding HA, NA and M2e antigens, respectively, administered either alone or in combination, provided significant protection against the lethal H7N9 infection, however, the protective efficacy was higher in Salmonella-HA-M2e-NA vaccine compared to the Salmonella-M2e or Salmonella-M2e-NA based vaccine. Salmonella-based vaccination strategy has certain advantages over conventional influenza vaccines. This vaccination strategy is readily and rapidly amenable to modifications once the circulating strains are identified, and Salmonella-based vaccines can be rapidly produced in large quantities without the need of specific cell culture conditions required for the production of inactivated whole virus based influenza vaccines. Further, Salmonella system acts as a natural adjuvant and delivers efficient maturation signals to dendritic cells, which is essential for the development of effective antigen-specific adaptive immune responses [18, 26, 27].
Elicitation of influenza-specific neutralizing antibody responses following infection/vaccination in peripheral blood circulation strongly correlates with the recovery from the clinical disease and protection from subsequent influenza infection . The present study demonstrates that immunization with Salmonella-HA and Salmonella-HA-M2e-NA based vaccines elicited higher levels of neutralizing antibody responses compared to the Salmonella-M2e-NA and Salmonella-M2e immunized groups and this correlates well with the challenge studies. Our immunization study shows that Salmonella-HA-M2e-NA based vaccine elicited higher protective immunity compared to the Salmonella-HA, Salmonella-M2e or Salmonella-M2e-NA vaccine, as indicated by the reduced viral shedding in the cloacal samples. Several studies demonstrated that mucosal immunization could result in the induction of higher immune responses not only at the site of stimulation but also in remote external secretions, and mucosal IgA antibodies were more effective and cross-protective against viral infection than systemic IgG antibodies [28, 29]. The induction of antigen-specific secretory IgA antibodies following infection/vaccination at mucosal surfaces prevent viral attachment to epithelial cells and infection of the host cells subsequently [30, 31]. In the present study, we found that chickens vaccinated with Salmonella-HA-M2e-NA based vaccine elicited higher mucosal HA, M2e and NA-specific IgA responses compared to the chickens immunized with Salmonella-HA, Salmonella-M2e or Salmonella-M2e-NA based vaccine. This might explain why Salmoenlla-HA-M2e-NA vaccinated chickens show higher protection rate against the lethal H7N7 virus infection. Thus, our data supports the conclusion that orally administered Salmonella-based vaccines can enhance systemic as well as mucosal immune responses, and the co-administration of HA, M2e and NA antigens improves mucosal and protective immune responses in chickens. Studies have shown that M2e specific antibodies mediate cross protection against the influenza A virus infections . Thus, our vaccination strategy could provide broad spectrum of protection against various influenza subtypes and therefore warrants further research in this regard.
The induction of cellular immune responses following influenza infection are necessary for viral clearance from the lungs and are, therefore, critical for chickens to recover from the clinical disease . In the present study, the Salmonella-based HA, NA and M2e vaccines not only generated efficient humoral responses but also stimulated high level of antigen-specific cellular responses. Previous study have shown that immunization with Salmonella-based HA and NA vaccination elicits efficient cellular immune responses and protects the mice against the homologous and heterologous H5N1 and H1N1 challenges, respectively . Our results show that vaccinated PBMCs stimulated with recall inactivated H7N9 antigen resulted in higher proliferative responses compared to the control chickens. Further, we evaluated the recall cytokine responses in vaccinated PBMCs. The nature of the cytokines secreted after vaccination or in vitro restimulation of cells is an important parameter to define the type of immunity stimulated. Our data demonstrates that chickens immunized with Salmonella-HA-M2e-NA based vaccine showed higher IFN-γ, a Th1 cytokine, and IL-17, a proinflammatory cytokine, recall responses compared to the Salmonella-HA, Salmonella-M2e or Salmonella-M2e-NA vaccinated chickens. IFN-γ is critical in the development of CMI responses, especially cytotoxic CD8+ responses, which are more potent and effective in clearance of viral infections [33, 34]. This might explain why Salmonella-HA-M2e-NA immunized chickens cleared H7N9 infection at a faster rate than the other immunized groups. The IL-17 plays important and protective roles at host epithelial and mucosal barriers against certain pathogens . A report by Wang et al. shows that IL-17 deficient mice exhibit markedly increased weight loss, more pronounced lung immunopathology, and significantly reduced survival rates upon infection with lethal H5N1 virus . Another report has shown that IL-17 can protect mice against lethal H1N1 (A/Puerto Rico/8/34) and H3N2 (A/Alaska/6/77) virus challenges . This might further explain why Salmonella-HA-M2e-NA vaccinated chickens showed enhanced protection against the lethal H7N7 influenza virus challenge. We also observed an increment in IL-10, the Th2 cytokine; however, the IL-10 levels were significantly lower compared to IL-17 and IFN-γ, indicating elicitation of Th1 type of immunity, which is important for the clearance of viral infections. These findings clearly indicate that Salmonella-based influenza vaccines can stimulate efficient Th1 type of immunity and can protect the hosts from pathogenic influenza infections. Our results are in agreement to the previously published reports [18, 32].
In summary, we show that Salmonella-based influenza vaccines elicitate efficient induction of antigen-specific humoral and CMI responses, and offered significant protection against the LPAI H7N9 infection in chickens. Thus, Salmonella-based vaccination strategy could be used as a potential prophylactic approach against LPAI viruses, which generally emerge to HPAI viruses affecting both animal and human population. Further, this approach can be readily and rapidly adjusted to the changes in the circulating field strains of influenza virus and, therefore, this promising technology can overcome the limitations usually associated with the conventional inactivated influenza vaccines, including SPF embryonated eggs and a long time line that could be threatened during an influenza pandemic.
The authors declare that they have no competing interests.
The work was designed by JHL and IAH. Experiments and sampling were done by IAH and JHK. Manuscript draft preparation was done by IAH. Statistical analysis was done by JHK and IAH. All authors read and approved the final manuscript.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MISP) (No. 2013R1A4A1069486).
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