Human influenza vaccines and assessment of immunogenicity

Mucosal Vaccination: A Promising Alternative Against Flaviviruses

The Flaviviridae are a family of positive-sense, single-stranded RNA enveloped viruses, and their members belong to a single genus, Flavivirus. Flaviviruses are found in mosquitoes and ticks; they are etiological agents of: dengue fever, Japanese encephalitis, West Nile virus infection, Zika virus infection, tick-borne encephalitis, and yellow fever, among others. Only a few flavivirus vaccines have been licensed for use in humans: yellow fever, dengue fever, Japanese encephalitis, tick-borne encephalitis, and Kyasanur forest disease. However, improvement is necessary in vaccination strategies and in understanding of the immunological mechanisms involved either in the infection or after vaccination. This is especially important in dengue, due to the immunological complexity of its four serotypes, cross-reactive responses, antibody-dependent enhancement, and immunological interference. In this context, mucosal vaccines represent a promising alternative against flaviviruses. Mucosal vaccination has several advantages, as inducing long-term protective immunity in both mucosal and parenteral tissues. It constitutes a friendly route of antigen administration because it is needle-free and allows for a variety of antigen delivery systems. This has promoted the development of several ways to stimulate immunity through the direct administration of antigens (e.g., inactivated virus, attenuated virus, subunits, and DNA), non-replicating vectors (e.g., nanoparticles, liposomes, bacterial ghosts, and defective-replication viral vectors), and replicating vectors (e.g., Salmonella enterica, Lactococcus lactis, Saccharomyces cerevisiae, and viral vectors). Because of these characteristics, mucosal vaccination has been explored for immunoprophylaxis against pathogens that enter the host through mucosae or parenteral areas. It is suitable against flaviviruses because this type of immunization can stimulate the parenteral responses required after bites from flavivirus-infected insects. This review focuses on the advantages of mucosal vaccine candidates against the most relevant flaviviruses in either humans or animals, providing supporting data on the feasibility of this administration route for future clinical trials.

Evaluation of the immune response of a H7N9 candidate vaccine virus derived from the fifth wave A/Guangdong/17SF003/2016

Highly pathogenic influenza H7N9 viruses that emerged in the fifth wave of H7N9 outbreak pose a risk to human health. The World Health Organization has updated the candidate vaccine viruses for H7N9 viruses recently. In this study, we evaluated the immune response to an updated H7N9 candidate vaccine virus, which derived from the highly pathogenic A/Guangdong/17SF003/2016 (GD/16) in mice and rhesus macaques. GD/16 vaccination elicited robust neutralizing, virus-specific immunoglobulin G antibodies and effective protection, but poor hemagglutination inhibition antibody titers. Furthermore, mouse and rhesus macaque serum raised against the previous H7N9 CVV A/Anhui/1/2013 (AH/13) were tested for its cross-reactivity to GD/16 virus. We found that although AH/13-immune serum has poor hemagglutination inhibition reactivity against GD/16 virus, AH/13 elicit efficient cross-neutralizing antibodies and in vivo protection against GD/16. Further studies showed that the hemagglutinin of GD/16 has strong receptor binding avidity, which might be associated with the decreased hemagglutination inhibition assay sensitivity. This study underscores the point that receptor binding avidity should be taken into account when performing quantitative interpretation of hemagglutination inhibition data. A combination of multiple serological assays is required for accurate vaccine evaluation and antigenic analysis of influenza viruses.

Comparison of hemagglutination inhibition, single radial hemolysis, virus neutralization assays, and ELISA to detect antibody levels against seasonal influenza viruses

Background

The immunological response to influenza vaccine and/or natural infection is evaluated by serological techniques, the most common being hemagglutination inhibition (HI), single radial hemolysis (SRH), and virus neutralization assays, which is commonly used in a micro‐neutralization (MN) format. ELISA is not officially required; however, this assay is able to measure different class‐specific antibodies. The four assays identify different sets or subsets of antibodies.

Objectives

The aim of this study was to establish the correlation among four serological assays using four seasonal influenza strains.

Methods

The HI, SRH, MN assays, and ELISA were performed on four seasonal influenza strains.

Results

A strong positive correlation was found between HI and MN and between SRH and MN assays for influenza A strains. The B strains also showed good correlations among the three assays. A positive correlation was also found between ELISA and the “classical” assays for all strains. Concerning the correlates of protection, as defined by HI ≥ 40 and SRH ≥ 25 mm², good agreement was observed for the influenza A strains. By contrast, the agreement for the B strains was very low.

Conclusions

There is a positive strong correlation among the four serological assays for both A and B strains, especially for the HI and MN assays. There is good agreement on correlates of protection between HI and SRH assays for the A strains, but very low agreement for the B strains, suggesting higher sensitivity of SRH than HI assay in detecting antibodies against the influenza B viruses.

Lipid-Based Particles: Versatile Delivery Systems for Mucosal Vaccination against Infection

Vaccination is the process of administering immunogenic formulations in order to induce or harness antigen (Ag)-specific antibody and T cell responses in order to protect against infections. Important successes have been obtained in protecting individuals against many deleterious pathological situations after parenteral vaccination. However, one of the major limitations of the current vaccination strategies is the administration route that may not be optimal for the induction of immunity at the site of pathogen entry, i.e., mucosal surfaces. It is now well documented that immune responses along the genital, respiratory, or gastrointestinal tracts have to be elicited locally to ensure efficient trafficking of effector and memory B and T cells to mucosal tissues. Moreover, needle-free mucosal delivery of vaccines is advantageous in terms of safety, compliance, and ease of administration. However, the quest for mucosal vaccines is challenging due to (1) the fact that Ag sampling has to be performed across the epithelium through a relatively limited number of portals of entry; (2) the deleterious acidic and proteolytic environment of the mucosae that affect the stability, integrity, and retention time of the applied Ags; and (3) the tolerogenic environment of mucosae, which requires the addition of adjuvants to elicit efficient effector immune responses. Until now, only few mucosally applicable vaccine formulations have been developed and successfully tested. In animal models and clinical trials, the use of lipidic structures such as liposomes, virosomes, immune stimulating complexes, gas-filled microbubbles and emulsions has proven efficient for the mucosal delivery of associated Ags and the induction of local and systemic immune reponses. Such particles are suitable for mucosal delivery because they protect the associated payload from degradation and deliver concentrated amounts of Ags via specialized sampling cells (microfold cells) within the mucosal epithelium to underlying antigen-presenting cells. The review aims at summarizing recent development in the field of mucosal vaccination using lipid-based particles. The modularity ensured by tailoring the lipidic design and content of particles, and their known safety as already established in humans, make the continuing appraisal of these vaccine candidates a promising development in the field of targeted mucosal vaccination.

Antibody-Dependent Cell-Mediated Cytotoxicity to Hemagglutinin of Influenza A Viruses After Influenza Vaccination in Humans

Background.  Detection of neutralizing antibodies (nAbs) to influenza A virus hemagglutinin (HA) antigens by conventional serological assays is currently the main immune correlate of protection for influenza vaccines However, current prepandemic avian influenza vaccines are poorly immunogenic in inducing nAbs despite considerable protection conferred. Recent studies show that Ab-dependent cell-mediated cytotoxicity (ADCC) to HA antigens are readily detectable in the sera of healthy individuals and patients with influenza infection. Methods.  Virus neutralization and ADCC activities of serum samples from individuals who received either seasonal or a stock-piled H5N1 avian influenza vaccine were evaluated by hemagglutination inhibition assay, microneutralization assay, and an improved ADCC natural killer (NK) cell activation assay. Results.  Immunization with inactivated seasonal influenza vaccine led to strong expansion of both nAbs and ADCC-mediating antibodies (adccAbs) to H3 antigen of the vaccine virus in 24 postvaccination human sera. In sharp contrast, 18 individuals vaccinated with the adjuvanted H5N1 avian influenza vaccine mounted H5-specific antibodies with strong ADCC activities despite moderate virus neutralization capacity. Strength of HA-specific ADCC activities is largely associated with the titers of HA-binding antibodies and not with the fine antigenic specificity of anti-HA nAbs. Conclusions.  Detection of both nAbs and adccAbs may better reflect protective capacity of HA-specific antibodies induced by avian influenza vaccines.

Influenza immunology evaluation and correlates of protection: a focus on vaccines

Vaccination is the most effective method of controlling seasonal influenza infections and preventing possible pandemic events. Although influenza vaccines have been licensed and used for decades, the potential correlates of protection induced by these vaccines are still a matter of discussion. Currently, inactivated vaccines are the most common and the haemagglutination inhibition antibody titer is regarded as an immunological correlate of protection and the best available parameter for predicting protection from influenza infection. However, the assay shows some limitations, such as its low sensitivity to B and avian strains and inter-laboratory variability. Additional assays and next-generation vaccines have been evaluated to overcome the limitations of the traditional serological techniques and to elicit broad immune responses, underlining the need to revise the current correlates of protection. The aim of this review is to provide an overview of the current scenario regarding the immunological evaluation and correlates of protection of influenza vaccines.

Immunogenicity and safety of Intanza(®)/IDflu(®) intradermal influenza vaccine in South Korean adults: a multicenter, randomized trial

Intanza(®)/IDflu(®) (Sanofi Pasteur, Lyon, France) is an intradermal inactivated trivalent influenza vaccine developed as an alternative to intramuscular influenza vaccine. The objective of this study was to confirm the immunogenicity and safety of Intanza/IDflu in South Korean adults. In a phase IV multicenter trial, South Korean adults 18-59 y old (n = 120) and ≥ 60 y old (n = 120) were randomized 1:1 to receive a single dose of Intanza/IDflu (9 µg for 18-59 y, 15 µg for ≥ 60 y) or trivalent intramuscular vaccine (Vaxigrip(®) 15 µg, Sanofi Pasteur, Lyon, France). Blood was collected on pre-vaccination (day 0) and on day 21. Hemagglutination inhibition titers, seroprotection rates and seroconversion rates were determined on day 21. Geometric mean titers, seroprotection and seroconversion rates were similar between the intradermal and intramuscular vaccines in both age groups for all three vaccine strains (A/H1N1, A/H3N2 and B). Both vaccines met Committee for Medicinal Products for Human Use criteria for all three strains. Solicited systemic reactions of the intradermal groups were generally mild, transient, and similar to those of the intramuscular groups. Solicited injection site reactions were more frequent in the intradermal groups but were mostly mild, transient, and consisted mainly of pain, erythema, and pruritus. No treatment-related serious adverse events or other safety concerns were reported. These results confirm that Intanza/IDflu is an effective and well-tolerated alternative to IM influenza vaccination. (Clinicaltrials.gov NCT ID: NCT01215669).

Molecular basis of live-attenuated influenza virus

Human influenza is a seasonal disease associated with significant morbidity and mortality. The most effective means for controlling infection and thereby reducing morbidity and mortality is vaccination with a three inactivated influenza virus strains mixture, or by intranasal administration of a group of three different live attenuated influenza vaccine strains. Comparing to the inactivated vaccine, the attenuated live viruses allow better elicitation of a long-lasting and broader immune (humoral and cellular) response that represents a naturally occurring transient infection. The cold-adapted (ca) influenza A/AA/6/60 (H2N2) (AA ca) virus is the backbone for the live attenuated trivalent seasonal influenza vaccine licensed in the United States. Similarly, the influenza A components of live-attenuated vaccines used in Russia have been prepared as reassortants of the cold-adapted (ca) H2N2 viruses, A/Leningrad/134/17/57-ca (Len/17) and A/Leningrad/134/47/57-ca (Len/47) along with virulent epidemic strains. However, the mechanism of temperature-sensitive attenuation is largely elusive. To understand how modification at genetic level of influenza virus would result in attenuation of human influenza virus A/PR/8/34 (H1N1,A/PR8), we investigated the involvement of key mutations in the PB1 and/or PB2 genes in attenuation of influenza virus in vitro and in vivo. We have demonstrated that a few of residues in PB1 and PB2 are critical for the phenotypes of live attenuated, temperature sensitive influenza viruses by minigenome assay and real-time PCR. The information of these mutation loci could be used for elucidation of mechanism of temperature-sensitive attenuation and as a new strategy for influenza vaccine development.

Validation of the modified hemagglutination inhibition assay (mHAI), a robust and sensitive serological test for analysis of influenza virus-specific immune response

BACKGROUND

The hemagglutination inhibition assay (HAI) is universally regarded as the gold standard in influenza virus serology. Nevertheless, difficulties in titre readouts are common and interlaboratory variations are frequently reported.

OBJECTIVE

We developed and validated the modified HAI to facilitate reliable, accurate and reproducible analysis of sera derived from influenza vaccination studies.

STUDY DESIGN

Clinical and preclinical serum samples, NIBSC reference sera and seasonal influenza virus type A (H1N1 and H3N2) and type B antigens were employed to validate the mHAI. Moreover, pandemic virus strains (H5N1 and H1N1pdm09) were used to prove assay robustness.

RESULTS

Utilisation of a 0.08% solution of stabilised human erythrocytes, assay buffer containing bovine serum albumin and microscopical plate readout are the major differences between the modified and standard HAI assay protocols. Validation experiments revealed that the mHAI is linear, specific and up to eightfold more sensitive than the standard HAI. In 95.6% of all measurements mHAI titres were precisely measured irrespective of the assay day, run or operator. Moreover, 96.4% (H1N1) or 95.2% (H3N2 and B), respectively, of all serum samples were determined within one dilution step of the nominal values for spiked samples. Finally, the mHAI results remained unaffected by variations in virus antigens, erythrocytes, reagents, laboratory location, sample storage conditions or matrix components.

CONCLUSION

The modified HAI is easy to analyse, requires only a single source of erythrocytes and allows utilisation of numerous influenza virus antigens, also including virus strains which are difficult to handle by the standard HAI (e.g. H3N2, H5N1 and H1N1pdm09).

Role of T-cell epitope-based vaccine in prophylactic and therapeutic applications

Prophylactic and therapeutic vaccines against viral infections have advanced in recent years from attenuated live vaccines to subunit-based vaccines. An ideal prophylactic vaccine should mimic the natural immunity induced by an infection, in that it should generate long-lasting adaptive immunity. To complement subunit vaccines, which primarily target an antibody response, different methodologies are being investigated to develop vaccines capable of driving cellular immunity. T-cell epitope discovery is central to this concept. In this review, the significance of T-cell epitope-based vaccines for prophylactic and therapeutic applications is discussed. Additionally, methodologies for the discovery of T-cell epitopes, as well as recent developments in the clinical testing of these vaccines for various viral infections, are explained.

Differential serum cytokine responses to inactivated and live attenuated seasonal influenza vaccines

Despite vaccine efforts, influenza outbreaks pose a significant threat to global public health. There are two commercially available seasonal influenza vaccines in the United States: the trivalent inactivated vaccine (TIV), delivered parenterally, and the live attenuated influenza vaccine (LAIV), delivered intranasally. Although both vaccines are generally efficacious, the immunologic mechanisms which contribute to protective immunity are incompletely understood. Thus, we investigated the protracted effects of TIV and LAIV on serum cytokine profiles at 14 and 28 days post-vaccination (when antibody titers are peak) in healthy adults over two influenza seasons. Vaccination with TIV was associated with a small, yet significant, decrease in the levels of both IL-8 and TNF-α at 14 and 28 days post-vaccination. LAIV, however, had no impact on serum cytokine levels. Similarly, analysis of serum antibody titers indicated that TIV recipients had a significantly higher sero-response rate compared to LAIV recipients, as has been previously shown. Finally, we examined the relationship between baseline serum cytokine levels and antibody responses to TIV (LAIV recipients were excluded due to the poor sero-response rate). Interestingly, in TIV recipients pre-vaccination levels of IL-8 were higher in sero-responders compared to non-responders. Collectively, these data suggest that cytokines may influence vaccine outcomes and indicate that parenteral immunization with TIV induces a sustained, systemic cytokine response which lasts for weeks.

Inhibition of influenza A virus replication by antagonism of a PI3K-AKT-mTOR pathway member identified by gene-trap insertional mutagenesis

BACKGROUND

Host genes serving potential roles in virus replication may be exploited as novel antiviral targets.

METHODS

Small interfering RNA (siRNA)-mediated knockdown of host gene expression was used to validate candidate genes in screens against six unrelated viruses, most importantly influenza. A mouse model of influenza A virus infection was used to evaluate the efficacy of a candidate FDA-approved drug identified in the screening effort.

RESULTS

Several genes in the PI3K-AKT-mTOR pathway were found to support broad-spectrum viral replication in vitro by RNA interference. This led to the discovery that everolimus, an mTOR inhibitor, showed in vitro antiviral activity against cowpox, dengue type 2, influenza A, rhino- and respiratory syncytial viruses. In a lethal mouse infection model of influenza A (H1N1 and H5N1) virus infection, everolimus treatment (1 mg/kg/day) significantly delayed death but could not prevent mortality. Fourteen days of treatment was more beneficial in delaying the time to death than treatment for seven days. Pathological findings in everolimus-treated mice showed reduced lung haemorrhage and lung weights in response to infection.

CONCLUSIONS

These results provide proof of concept that cellular targets can be identified by gene knockout methods, and highlight the importance of the PI3K-AKT-mTOR pathway in supporting viral infections.

Mimotopes selected with neutralizing antibodies against multiple subtypes of influenza A

Background

The mimotopes of viruses are considered as the good targets for vaccine design. We prepared mimotopes against multiple subtypes of influenza A and evaluate their immune responses in flu virus challenged Balb/c mice.

Methods

The mimotopes of influenza A including pandemic H1N1, H3N2, H2N2 and H1N1 swine-origin influenza virus were screened by peptide phage display libraries, respectively. These mimotopes were engineered in one protein as multi- epitopes in Escherichia coli (E. coli) and purified. Balb/c mice were immunized using the multi-mimotopes protein and specific antibody responses were analyzed using hemagglutination inhibition (HI) assay and enzyme-linked immunosorbent assay (ELISA). The lung inflammation level was evaluated by hematoxylin and eosin (HE).

Results

Linear heptopeptide and dodecapeptide mimotopes were obtained for these influenza virus. The recombinant multi-mimotopes protein was a 73 kDa fusion protein. Comparing immunized infected groups with unimmunized infected subsets, significant differences were observed in the body weight loss and survival rate. The antiserum contained higher HI Ab titer against H1N1 virus and the lung inflammation level were significantly decreased in immunized infected groups.

Conclusions

Phage-displayed mimotopes against multiple subtypes of influenza A were accessible to the mouse immune system and triggered a humoral response to above virus.