Comparison of single versus booster dose of influenza vaccination on humoral and cellular immune responses in older adults

This study compared the immune response to the standard single-dose (SD) of influenza vaccine to a booster dose (BD) re-vaccination given 16 weeks after the initial dose. While seroprotection rates following vaccination were similar, T-cell responses were more optimally stimulated in the SD versus the BD group. SD lead to a greater than 10-fold decline in ex vivo interleukin-10 (IL-10) levels (P < .0001) and a corresponding significant increase in the interferon-gamma (IFN-gamma) to IL-10 ratio. Although BD had no further effect on IL-10 production, the IFN-gamma:IL-10 ratio declined in the BD group (P < .001, A/H3N2 and B strains). In the SD group only, IFN-gamma:IL-10 ratios significantly correlated with serum antibody titers (R = .37 - .50, P < or = .01) and ex vivo granzyme B (Grz B) levels (R = .50-.65, P < or = .001). Following vaccination, granzyme B levels were significantly higher in the SD compared to the BD group (P < or= .0002). These results suggest that SD influenza vaccine produces Th1 and CTL responses while BD may produce a Th2 response that poorly stimulates the CTL response.     

Effect of congestive heart failure on humoral and ex vivo cellular immune responses to influenza vaccination in older adults

This study examined the effect of congestive heart failure (CHF) on immune responses to influenza vaccination (2000-2001 preparation) in three groups of older adults including healthy, Class II and Class III/IV CHF. Serum antibody titers measured by hemagglutination inhibition (HI), and interferon-gamma (IFN-gamma), interleukin-10 (IL-10) and granzyme B (GrzB) levels in ex vivo virus-activated mononuclear cell cultures showed significant responses from pre-vaccination to 4 and 12 weeks post-vaccination (P<0.01). There was a trend for lower GrzB and higher IFN-gamma and IL-10 levels in healthy versus CHF groups (P<0.06) for all viral strains at 4 weeks. HI titers did not differ between groups. In the regression model, Grz B levels were significantly predicted by the IFN-gamma:IL-10 ratio and performance on the 6 min Walk Test; age and CHF dropped out of the model. In conclusion, CHF in older adults predicts GrzB responses to influenza vaccination due to cytokine and physical ability differences.     

Antibody responses after dose-sparing intradermal influenza vaccination

Reduced-dose intradermal (ID) influenza vaccination is an attractive approach to increase availability of vaccine supply in an event of vaccine shortage. We conducted a randomized open-label study, in which 500 subjects were randomly assigned to receive an ID injection of 0.1 ml dose of inactivated split-virion influenza vaccine or an IM injection of 0.5 ml dose. The subjects who had hemagglutination inhibition (HI) antibody titer of at least 1:40 at day 28 post-vaccination in ID and IM groups were 93.3% versus 98.0% for influenza A(H1N1) virus, 86.3% versus 95.0% for A(H3N2) virus, and 43.5% versus 57.0% for influenza B virus. Subjects in the ID group had an increase in geometric mean titer by a factor of 16 for the H1N1 strain, 8 for the H3N2 strain, and 2 for the B strain on day 28, as compared with respective increase in the IM group of 31, 20, and 3. Local reactions were significantly more frequent among subjects in the ID group than those in the IM group, but the reactions were mild and transient. In this study, ID administration of one-fifth dose of influenza vaccine elicited significantly lower levels of antibody response as compared to full-dose IM injection. However, the antibody responses elicited by the ID vaccination were still sufficiently high to meet the requirement guidelines of the European Committee for Proprietary Medicinal Products (CPMP) for the annual relicensure of influenza vaccines.     

Unique cellular and humoral immunogenicity profiles generated by aerosol,intranasal, or parenteral vaccination in rhesus macaques

Respiratory mucosa immunization is capable of eliciting both local and distal mucosal immune responses; it is a potentially powerful yet largely unused modality for vaccination against respiratory diseases. Targeting the lower versus upper airways by aerosol delivery alters the immunogenicity profile of a vaccine, although the full extent of this impact is not well characterized. We set out to define the cellular and humoral response profiles elicited by immunization via intranasal, small aerosol droplets, and large aerosol droplets. We compared responses following adenovirus-vectored vaccination by these routes in macaques, either for the generation of primary immune responses or for the boosting of previously primed systemic responses. Aerosol delivery (4 or 10 μm diameter droplets, addressing lower or upper airways, respectively) generated the highest magnitude lung CD4 and CD8 T-cell responses, reaching 10–30% vaccine-specific levels in bronchoalveolar lavage cells. In contrast, intranasal delivery was less immunogenic with >10-fold lower peak lung T-cell responses. Systemic (blood) T-cell responses were only observed following 4 μm aerosol (and parenteral) immunization, while all delivery routes elicited similar humoral responses. These data demonstrate distinct immune response profiles with each respiratory tract vaccination modality and suggest that small droplet aerosol offers several immunological advantages over other respiratory routes.     

The novel adjuvant IC31 strongly improves influenza vaccine-specific cellular and humoral immune responses in young adult and aged mice

The compromised immune responses in the elderly as well as the threat of pandemic influenza necessitate the development of improved influenza vaccines. This study provides evidence that IC31, a two-component synthetic adjuvant signalling through TLR-9, augments humoral and cellular immune responses to seasonal influenza vaccines. Experiments performed in young adult mice showed increased HI titres and higher levels of IgG2a antibodies that were accompanied by the induction of IFN-gamma producing CD4(+) T cells after single vaccination with reduced doses of vaccine antigens, even 200 days after single immunisation. Importantly, similar effects were seen in aged mice, although most pronounced upon booster immunisation. Thus, IC31 fulfils important criteria of novel influenza vaccine adjuvants.     

Long-lasting humoral and cellular immune responses and mucosal dissemination after intramuscular DNA immunization

Naïve Indian rhesus macaques were immunized with a mixture of optimized plasmid DNAs expressing several SIV antigens using in vivo electroporation via the intramuscular route. The animals were monitored for the development of SIV-specific systemic (blood) and mucosal (bronchoalveolar lavage) cellular and humoral immune responses. The immune responses were of great magnitude, broad (Gag, Pol, Nef, Tat and Vif), long-lasting (up to 90 weeks post third vaccination) and were boosted with each subsequent immunization, even after an extended 90-week rest period. The SIV-specific cellular immune responses were consistently more abundant in bronchoalveolar lavage (BAL) than in blood, and were characterized as predominantly effector memory CD4⁺ and CD8⁺ T cells in BAL and as both central and effector memory T cells in blood. SIV-specific T cells containing Granzyme B were readily detected in both blood and BAL, suggesting the presence of effector cells with cytolytic potential. DNA vaccination also elicited long-lasting systemic and mucosal humoral immune responses, including the induction of Gag-specific IgA. The combination of optimized DNA vectors and improved intramuscular delivery by in vivo electroporation has the potential to elicit both cellular and humoral responses and dissemination to the periphery, and thus to improve DNA immunization efficacy.     

Sindbis virus vectors elicit hemagglutinin-specific humoral and cellular immune responses and offer a dose-sparing strategy for vaccination

We report here on the use of a Sindbis virus-based DNA-launch RNA replicon vector (pSIN-HA) that expresses influenza hemagglutinin (HA) as an immunogen. Immunization of mice with pSIN-HA generated anti-HA antibody and CTL responses and resulted in lower lung viral titers after influenza challenge when compared to controls. Importantly, immunization with a low dose of pSIN-HA mediated significantly reduced lung viral titers following challenge at 43 weeks after the final immunization. In contrast, immunization with a non-replicon DNA vector expressing HA failed to mediate reduced lung viral titer at the same dose. This demonstrated the dose-sparing capacity of the SIN vector system and its ability to stimulate long-term memory responses, properties that are highly desirable in any vaccine formulation.     

Cellular and humoral responses to tetanus vaccination in Gabonese children

Protection to tetanus is often not optimal in developing countries due to incomplete vaccination schemes, or decreased efficacy of vaccination. In this study we investigated the immunological response to tetanus booster vaccination in school children living in a semi-urban or in a rural area of Gabon. Tetanus-specific total IgG as well as antibody subclasses of the IgG1, IgG2, IgG3 and IgG4 isotype and the avidity of the dominating IgG1 subclass were determined both before and 1 month after the booster vaccination. In addition, tetanus-specific cytokine responses were determined. We found a polarization towards a T helper 1 (Th1) profile in the semi-urban children, whereas the cytokine responses of the rural children showed a T helper 2 (Th2) skewed response. Furthermore, tetanus-specific antibodies of the different IgG subclasses were all increased upon a tetanus booster vaccination and levels of IgG1 and IgG3 were higher in the rural children. In conclusion, a tetanus booster vaccination induced a stronger Th2 over Th1 cytokine profile to tetanus toxoid (TT) in rural children who showed the highest levels of IgG1 and IgG3 anti-TT antibody responses.     

HIV--Leishmania infantum co-infection: humoral and cellular immune responses to the parasite after chemotherapy

Specific serum antibodies, peripheral blood T-cell subsets, cellular response in vitro to soluble Leishmania antigens, phenotype of stimulated cells, and serum levels of tumour necrosis factor (TNF)-alpha and transforming growth factor (TGF)-beta 1 were studied in Spain in 17 patients co-infected with HIV and Leishmania infantum who had been previously treated with pentavalent antimony. Both humoral and cellular responses to Leishmania sp. appeared diminished, 8 out of 17 patients were positive by indirect immunofluorescence, and immunoblotting detected heterogeneous antibody-binding pattern in 11 out of 13 subjects. A blastogenesis test was positive in 4 cases; 2 of them presented proliferation of CD4+ cells while CD8+ cells proliferated in the other 2 patients. Serum levels of TNF-alpha were similar to those observed in patients infected with HIV only, while serum levels of TGF-beta 1 were significantly lower in the co-infected patients. The inability of antibody response to control the parasite and the absence of specific T-cell immunity to Leishmania sp. would explain the high frequency of relapses reported in these patients. The decreased levels of TGF-beta 1 could have an important role in the interaction between the 2 pathogens.     

Poor immune responses to influenza vaccination in infants

Twenty-two and 37 infants and young children received two doses of influenza HA vaccine before the 2001-2002 influenza season and before the 2002-2003 season, respectively. Two or three serial specimens were obtained, before and 1 month after the first vaccination as well as 1 month after the second vaccination. Infants showed a significantly poor HI antibody rise and lymphocyte response compared with young children aged > or =12 months. Time kinetics of the lymphoproliferative responses to influenza antigen among young children varied whereas their activities in infants were typically negative before immunization and increased after vaccination. Infants responded poorly to HA influenza vaccine compared with young children.     

Proinflammatory cytokine responses correspond with subjective side effects after influenza virus vaccination

BackgroundThough typically mild, side effects to the influenza virus vaccine are common and may contribute to negative perceptions including the belief that the vaccine can cause the flu. However, the extent to which subjective symptoms correspond with biological response indicators is poorly understood.MethodsThis study examined associations among subjective side effects (soreness at the site of injection and illness-like symptoms), serum proinflammatory cytokines and body temperature a baseline, 1, 2, and 3 days following receipt of trivalent inactivated influenza vaccine (IIV3) in a sample of 56 women 18–40 years in age.ResultsIn relation to local reactions, women reporting being very sore at the injection site at 1 day post-vaccination exhibited greater increases in serum TNF-α and MIF in the days following vaccination compared to those with no or mild soreness. In addition, higher basal body temperature was observed in this group compared to other groups (98.7 °F versus 98.0–98.1°). In relation to systemic reactions, women endorsing illness-like symptoms (headache, fatigue, nausea, sore throat, dizziness, achiness, or mild fever) exhibited marginally higher IL-6 at baseline (p = 0.055) and greater increases in serum MIF at 2 days post-vaccination than those reporting no systemic symptoms. Associations of systemic symptoms with inflammatory responses were not accounted for by concomitant local reactions. As expected, antibody responses to the vaccine were highly similar in women regardless of local or systemic symptoms.ConclusionsThese results are consistent with the notion that subjective reports of local and systemic reactions following vaccination may be predicted by and correspond with biological indicators of inflammatory status, but are not meaningful predictors of antibody responses. To improve adherence to vaccine recommendations, clinicians should provide assurance that such symptoms may be related to normal mild inflammatory responses to the vaccine and do not reflect immunogenicity.     

A polyvalent influenza DNA vaccine applied by needle-free intradermal delivery induces cross-reactive humoral and cellular immune responses in pigs

BackgroundPigs are natural hosts for influenza A viruses, and the infection is widely prevalent in swine herds throughout the world. Current commercial influenza vaccines for pigs induce a narrow immune response and are not very effective against antigenically diverse viruses. To control influenza in pigs, the development of more effective swine influenza vaccines inducing broader cross-protective immune responses is needed. Previously, we have shown that a polyvalent influenza DNA vaccine using vectors containing antibiotic resistance genes induced a broadly protective immune response in pigs and ferrets using intradermal injection followed by electroporation. However, this vaccination approach is not practical in large swine herds, and DNA vaccine vectors containing antibiotic resistance genes are undesirable.ObjectivesTo investigate the immunogenicity of an optimized version of our preceding polyvalent DNA vaccine, characterized by a next-generation expression vector without antibiotic resistance markers and delivered by a convenient needle-free intradermal application approach.MethodsThe humoral and cellular immune responses induced by three different doses of the optimized DNA vaccine were evaluated in groups of five to six pigs. The DNA vaccine consisted of six selected influenza genes of pandemic origin, including internally expressed matrix and nucleoprotein and externally expressed hemagglutinin and neuraminidase.ResultsNeedle-free vaccination of growing pigs with the optimized DNA vaccine resulted in specific, dose-dependent immunity down to the lowest dose (200 μg DNA/vaccination). Both the antibody-mediated and the recall lymphocyte immune responses demonstrated high reactivity against vaccine-specific strains and cross-reactivity to vaccine-heterologous strains.ConclusionThe results suggest that polyvalent DNA influenza vaccination may provide a strong tool for broad protection against swine influenza strains threatening animal as well as public health. In addition, the needle-free administration technique used for this DNA vaccine will provide an easy and practical approach for the large-scale vaccination of swine.     

Factors influencing cellular immune responses to feline immunodeficiency virus induced by DNA vaccination

Virus-specific effector cytotoxic T lymphocytes (CTL) were elicited in the peripheral blood of domestic cats following a single intramuscular inoculation of replication defective feline immunodeficiency virus proviral DNA (FIVDeltaRT). Higher levels of virus-specific cytolysis were observed in the blood when cats were co-inoculated with feline gamma-interferon (IFN) DNA. The responses declined by 12 weeks following the first DNA inoculation and were, with the exception of FIV Gag-specific responses in some cats, refractory to repeated DNA inoculations. Nevertheless, a significant proportion of the cats were protected from challenge with homologous virus. The effects of interval between inoculations, route of DNA delivery, and promoter used to regulate viral gene expression on the induction of virus-specific CTLs were evaluated. The highest levels of virus-specific lysis were recorded following intramuscular co-inoculation of FIVDeltaRT and gamma-IFN DNA, where FIV gene expression was under the control of a cytomegalovirus (CMV) promoter. However, the highest levels of protection were observed using the viral 5'LTR as the promoter. These results suggest that a single intramuscular inoculation of FIVDeltaRT DNA together with gamma-IFN DNA may be sufficient to induce virus-specific CTLs and protection.     

Influenza viruses and cross-reactivity in healthy adults: humoral and cellular immunity induced by seasonal 2007/2008 influenza vaccination against vaccine antigens and 2009 A(H1N1) pandemic influenza virus

We analyzed humoral and cellular immune responses against vaccine antigens and the new A(H1N1) virus in healthy adults before and after immunization with the 2007/2008 commercially available trivalent subunit MF59-adjuvanted influenza vaccine during the Fall 2007, prior to the emergence of the new virus. Antibody titers were significantly boosted only against the three vaccine antigens. Seasonal vaccination boosted pre-existing cellular responses upon stimulation of peripheral blood mononuclear cells not only with the homologous three vaccine antigens, but also with the heterologous new 2009 A(H1N1) and with a highly conserved peptide present in the stalk region of hemagglutinin (HA). These results show that cross-reactive cell responses against the new virus were present before the circulation of the virus and were boosted by seasonal vaccination. The cross-reactivity of cellular responses might, at least in part, explain the low pathogenicity of the new pandemic virus. The finding of cellular immunity, that can be increased by seasonal vaccination, against the conserved HA peptide, underline the potential use, in human vaccines, of conserved peptides of the stalk region of HA characterized by broad immunogenicity in experimental systems.     

Immunopotentiation of humoral and cellular responses to inactivated influenza vaccines by two different adjuvants with potential for human use

Two quite different adjuvants, currently under development for use in humans, have been examined for their effects on the magnitude and type of immunity elicited in response to inactivated influenza vaccine. Immunostimulating complexes (ISCOM™ adjuvant) contain the saponin ISCOPREP™ 703, and SPT is an oil-in-water emulsion of squalane, non-ionic block copolymer (L121) and Tween 80. Influenza virus vaccines formulated in either adjuvant were far superior to the non-adjuvanted aqueous vaccine in eliciting antibody and T-cell responses in mice, particularly at lower doses of antigen. In addition, the vaccines containing adjuvant were superior in eliciting protective immunity. One of the shortcomings of the unadjuvanted inactivated influenza vaccine was its inability to elicit a primary proliferative T-cell response. However, after one dose of either adjuvaned vaccine, strong proliferative responses were achieved. We also show that subcutaneous vaccination with inactivated vaccines is capable of modulating the isotype profile of antibody secreting cells generated in the lungs of mice in response to intranasal challenge with live virus. In this system, the isotype of antibody elicited after challenge of mice that had received ISCOM vaccine more closely mimicked that of animals vaccinated with live virus.     

Enhanced humoral and cell-mediated immune responses after immunization with trivalent influenza vaccine adjuvanted with cationic liposomes

The recent pandemic caused by new influenza A (H1N1) has emphasized the need for improved influenza vaccines with enhanced immune responses that ideally include longlived humoral and CMI responses and mediate a broad protection. This study demonstrates that administration of trivalent influenza vaccine (TIV) with the cationic liposome adjuvant system CAF01 enhances the humoral immune response as measured by hemagglutinin inhibition titers and influenza-specific serum antibody titers, and promote a strong Th1 response with augmented levels of IL-1β, IL-2, IL-12, IFN-γ and TNF-α. Furthermore, high levels of IL-17 are detected in agreement with CAF01's ability to promote TH17 responses. Importantly, the Th1/Th17 cytokine profile is still maintained 20 weeks after the last vaccination. The CAF01 adjuvanted influenza vaccine reduces weight loss and temperature decrease and results in complete survival of mice challenged with the drifted H1N1 influenza strain A/PR/8/34. Overall, the results suggest that CAF01 is a potent adjuvant system for future, improved influenza vaccines.     

DNA vaccination against bovine viral diarrhoea virus induces humoral and cellular responses in cattle with evidence for protection against viral challenge

The immune response induced by a DNA construct expressing the E2 envelope glycoprotein of bovine viral diarrhoea virus (BVDV) was studied in cattle. Four groups of five calves, were immunised by intradermal injection with a total of 1mg of plasmid DNA on each of two occasions, with a 3-week dose interval. Group 1 received non-coding plasmid DNA only (control), group 2 received the E2 coding plasmid (0.5mg) plus non-coding plasmid DNA (0.5mg) and groups 3 and 4 received the E2 coding plasmid plus plasmid encoding either bovine interleukin 2 (IL-2) or granulocyte macrophage colony stimulating factor (GM-CSF) respectively. Two weeks after the final immunisation, all calves were challenged by intranasal inoculation with 5 x 10(6) TCID(50) of homologous virus. On the day of challenge, neutralising antibodies were detectable in 13 of 15 vaccinated calves (one animal in each of groups 3 and 4 remained seronegative at this point). Thereafter, a strong anamnestic serological response was evident in all vaccinated animals. Furthermore, T-cell proliferation following in vitro re-stimulation with BVDV antigen was significantly elevated in the cytokine adjuvanted groups. This enhancement of BVDV specific immune responses in vaccinated animals was reflected in the clinical responses observed post-challenge. In particular, reduced febrile responses provided evidence of a disease sparing effect of vaccination. Significantly, whilst a transient viraemia was detected in all control animals following challenge, no virus was isolated from the leucocytes from 8 out of the 15 vaccinated animals. In groups 2 and 4, three animals remained virus free, although virus was isolated from two animals in each group at a single time point, while in group 3, three out of five animals had detectable viraemia.In summary, the administration of a DNA vaccine encoding only the E2 glycoprotein of BVDV induced a disease sparing effect in vaccinated calves following challenge and protected more than half of the vaccinated animals from detectable viraemia.     

Characterization of the humoral and cellular immune responses against hepatitis C virus core induced by DNA-based immunization

Hepatitis C Virus (HCV) causes most cases of posttransfusion hepatitis. Chronic HCV infection is highly related to chronic hepatitis, cirrhosis and hepatocellular carcinoma. Current therapies are only minimally effective and no vaccine has been developed. DNA-based immunization could be of prophylactic and therapeutic value for HCV infection. By intramuscular inoculation in BALB/c mice with an HCV recombinant plasmid pCI-HCV-C, we found significant levels of IgM antibody, but no significant IgG rise. After boost the immunized mice with recombinant HCV-core protein (cp1-10; 1-164aa), the anticore IgG, verified by Western-blotting, rose rapidly, which was two weeks earlier than that with control plasmid. Spleen cells from pCI-HCV-C immunized mice gave higher proliferation index (PI) than control (P < 0.05). The PI of cp1-10 boosted mice was even higher. Proliferation blocking assay with mAb proved the responding cell to be of CD4+ CD8- phenotype, supporting specific priming of T helper cells. A 51Cr-releasing CTL assay specific for HCV-core was developed, and a specific CTL response against HCV-core was demonstrated in both pCI-HCV-C immunized mice and mice boosted with cp1-10. Strong cytotoxic activity against peptide-pulsed p815 cells (H-2d), but not EL-4 cells (H-2b), suggested MHC class I restriction of the CTL activity. Blocking of CTL with mAb proved the effector cells to be of CD4- CD8+. Three CTL epitopes in HCV-core protein were demonstrated. We failed to detect CTL when immunized only with core protein. The results suggested that vaccination with HCV-core derived DNA sequences could be an effective method to induce humoral and cellular immune responses to HCV.     

Genetically defined race,but not sex,is associated with higher humoral and cellular immune responses to measles vaccination

In addition to host genetic and environmental factors, variations in immune responses to vaccination are influenced by demographic variables, such as race and sex. The influence of genetic race and sex on measles vaccine responses is not well understood, yet important for the development of much-needed improved measles vaccines with lower failure rates. We assessed associations between genetically defined race and sex with measles humoral and cellular immunity after measles vaccination in three independent and geographically distinct cohorts totaling 2872 healthy racially diverse children, older adolescents, and young adults. We found no associations between biological sex and either humoral or cellular immunity to measles vaccine, and no correlation between humoral and cellular immunity in these study subjects. Genetically defined race was, however, significantly associated with both measles vaccine-induced humoral and cellular immune responses, with subjects genetically classified as having African-American ancestry demonstrating significantly higher antibody and cell-mediated immune responses relative to subjects of Caucasian ancestry. This information may be useful in designing novel measles vaccines that are optimally effective across human genetic backgrounds.     

Protective cellular responses elicited by vaccination with influenza nucleoprotein delivered by a live recombinant attenuated Salmonella vaccine

Orally administered recombinant attenuated Salmonella vaccines (RASVs) elicit humoral and mucosal immune responses against the immunizing antigen. The challenge in developing an effective vaccine against a virus or an intracellular bacterium delivered by RASVs is to introduce the protective antigen inside the host cell cytoplasm for presentation to MHC-I molecules for an efficient cell mediated immune response. To target the influenza nucleoprotein (NP) into the host cell cytosol, we constructed a regulated delayed lysis in vivo RASV strain χ11246(pYA4858) encoding influenza NP with a chromosomal deletion of the sifA gene to enable it to escape from the endosome prior to lysis. Oral immunization of mice with χ11246(pYA4858) (SifA⁻) with 3 booster immunizations resulted in complete protection (100%) against a lethal influenza virus (rWSN) challenge (100 LD₅₀) compared to 25% survival of mice immunized with the isogenic χ11017(pYA4858) (SifA⁺) strain. Reducing the number of booster immunizations with χ11246(pYA4858) from 3 to 2 resulted in 66% survival of mice challenged with rWSN (100 LD₅₀). Immunization with χ11246(pYA4858) via different routes provided protection in 80% orally, 100% intranasally and 100% intraperitoneally immunized mice against rWSN (100 LD₅₀). A Th1 type immune response was elicited against influenza NP in all experiments. IFN-γ secreting NP_147-155 specific T cells were not found to be correlated with protection. The role of antigen-specific CD8⁺ T cells remains to be determined. To conclude, we showed that Salmonella can be designed to deliver antigen(s) to the host cell cytosol for presumably class I presentation for the induction of protective immune responses.