Nucleic acid vaccines: tasks and tactics

Nanovaccines: A game changing approach in the fight against infectious diseases

The field of nanotechnology has revolutionised global attempts to prevent, treat, and eradicate infectious diseases in the foreseen future. Nanovaccines have proven to be a valuable pawn in this novel technology. Nanovaccines are made up of nanoparticles that are associated with or prepared with components that can stimulate the host's immune system. In addition to their delivery capabilities, the nanocarriers have been demonstrated to possess intrinsic adjuvant properties, working as immune cell stimulators. Thus, nanovaccines have the potential to promote rapid as well as long-lasting humoral and cellular immunity. The nanovaccines have several possible benefits, including site-specific antigen delivery, increased antigen bioavailability, and a diminished adverse effect profile. To avail these benefits, several nanoparticle-based vaccines are being developed, including virus-like particles, liposomes, polymeric nanoparticles, nanogels, lipid nanoparticles, emulsion vaccines, exomes, and inorganic nanoparticles. Inspired by their distinctive properties, researchers are working on the development of nanovaccines for a variety of applications, such as cancer immunotherapy and infectious diseases. Although a few challenges still need to be overcome, such as modulation of the nanoparticle pharmacokinetics to avoid rapid elimination from the bloodstream by the reticuloendothelial system, The future prospects of this technology are also assuring, with multiple options such as personalised vaccines, needle-free formulations, and combination nanovaccines with several promising candidates.

Envelope virus-mimetic nanovaccines by hybridizing bioengineered cell membranes with bacterial vesicles

Emerging threats of rapid spread highly lethal infectious diseases highlight the urgent need of vaccine development. Here, we describe the preparation of envelope virus-mimetic nanovaccines by hybridizing bioengineered cell membranes with bacterial vesicles. Membranes acquired from bioengineered cells overexpressing viral antigens are fused with bacterial outer membrane vesicles to develop hybrid nanovesicles. Because of the presence of intact viral antigenic proteins with natural conformation bound to lipid bilayer and pathogen-associated molecular patterns, hybrid nanovesicles can strikingly promote antigen uptake, processing and presentation by dendritic cells. Immunization with envelope virus-mimetic nanovaccines shows significantly enhanced maturation and activation of dendritic cells, which elicit robust humoral and cellular immune responses in mice. By virtue of their artificial characteristic and absence of loaded adjuvants, these biomimetic nanovaccines exhibit favorable biosafety. Our work demonstrates the effectiveness of envelope virus-mimetic nanovaccines to boost antigen-specific immunity and proposes a simple yet versatile platform to prepare antiviral vaccines.

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Nanovaccines were hybridized by bioengineered cell membranes and bacterial vesicles

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Nanovaccines possessed intact viral antigens with natural conformation

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Nanovaccines promoted antigen uptake, processing, and presentation by dendritic cells

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Nanovaccines elicited robust humoral and cellular immune responses in mice

A CpG-adjuvanted intranasal enterovirus 71 vaccine elicits mucosal and systemic immune responses and protects human SCARB2-transgenic mice against lethal challenge

Enterovirus 71 (EV71) is an aetiological agent responsible for seasonal epidemics of hand-foot-and-mouth disease, which causes considerable mortality among young children. Mucosal vaccines can efficiently induce secretory IgA at mucosal surfaces and thereby prevent or limit infection at the site of virus entry. CpG oligodeoxynucleotides (ODNs), which resemble bacterial DNA, can induce the innate immune response through activation of Toll-like receptor 9. Here, we used CpG ODNs as adjuvants to investigate an EV71 mucosal vaccine in mice. In the EV71 + CpG group, the EV71-specific IgG and IgA titres in the serum, nasal wash, bronchoalveolar lavage fluid, and faeces were substantially higher than those in the EV71- and phosphate-buffered saline-treated groups. Moreover, the number of EV71-specific IgG- and IgA-producing cells was also higher in the EV71 + CpG group. Furthermore, T-cell proliferative responses and interleukin-17 secretion were markedly increased when CpG-adjuvanted EV71 was delivered intranasally. More importantly, the induced antibodies neutralised infection by EV71 of the C2 genotype and crossneutralised infection by EV71 of the B4 and B5 genotypes. Lastly, human scavenger receptor class B, member 2-transgenic mice intranasally immunised with the CpG-adjuvanted EV71 vaccine resisted a subsequent lethal challenge with EV71, indicating that CpG was an effective intranasal adjuvant for EV71 mucosal-vaccine development.

Attachment of class B CpG ODN onto DOTAP/DC-chol liposome in nasal vaccine formulations augments antigen-specific immune responses in mice

Background

To overcome infectious diseases, the development of mucosal vaccines would be an effective strategy, since mucosal surfaces are the entry site for most pathogens. In general, protein antigens show inherently poor immunogenicity when administered by the mucosal route. Therefore, co-administration of an appropriate mucosal adjuvant is required to exert immune responses toward pathogen-derived antigens effectively. However, the development of a safe and effective mucosal adjuvant system is still challenging. Although, recent studies reported that oligodeoxynucleotides (ODNs) containing immunostimulatory CpG motifs (CpG ODNs) act as potent mucosal adjuvants and are useful in the formulation of nasal vaccines, there are some disadvantages. For instance, the administration of phosphorothioate (PS)-modified CpG ODNs can induce adverse systemic effects, such as splenomegaly, in a dose-dependent manner. Therefore, a reduced dose of CpG ODN might be crucial when used as vaccine adjuvant for clinical purposes. Therefore, we prepared a CpG ODN-loaded cationic liposome, and evaluated its mucosal adjuvant activity.

Results

We prepared a CpG ODN-loaded DOTAP/DC-chol liposome that was stable during our experiments, by mixing CpG ODNs and liposomes at an N/P ratio of 4. Further, we demonstrated that the attachment of class B CpG ODN to the DOTAP/DC-chol liposomes synergistically enhanced antigen-specific IgA production in the nasal area than that induced by CpG ODN and DOTAP/DC-chol liposomes alone. The endpoint titers were more than tenfolds higher than that induced by either single CpG ODN or single DOTAP/DC-chol liposomes. Additionally, although serum IgG1 responses (indicated as a Th2 response) remained unchanged for DOTAP/DC-chol liposomes and CpG ODN-loaded DOTAP/DC-chol liposomes, the CpG ODN-loaded DOTAP/DC-chol liposomes synergistically induced the production of serum IgG2a (indicated as a Th1 response) than that by the individual liposomes.

Conclusions

We conclude that the advantage of using DOTAP/DC-chol liposome harboring CpG ODN is it induces both antigen-specific mucosal IgA responses and balanced Th1/Th2 responses. Therefore, such a combination enables us to resolve the adverse effects of using CpG ODNs (as a mucosal adjuvant) by reducing the overall dose of CpG ODNs. Further, the biodegradable and essentially non-antigenic nature of the liposomes makes it superior than the other existing mucosal adjuvants.

Targeting antigen to MHC Class I and Class II antigen presentation pathways for malaria DNA vaccines

An effective malaria vaccine which protects against all stages of Plasmodium infection may need to elicit robust CD8(+) and CD4(+) T cell and antibody responses. To achieve this, we have investigated strategies designed to improve the immunogenicity of DNA vaccines encoding the Plasmodium yoelii pre-erythrocytic stage antigens PyCSP and PyHEP17, by targeting the encoded proteins to the MHC Classes I and II processing and presentation pathways. For enhancement of CD8(+) T cell responses, we targeted the antigens for degradation by the ubiquitin (Ub)/proteosome pathway following the N-terminal rule. We constructed plasmids containing PyCSP or PyHEP17 genes fused to the Ub gene: plasmids where the N-terminal antigen residues were mutated from the stabilizing amino acid methionine to destabilizing arginine, plasmids where the C-terminal residues of Ub were mutated from glycine to alanine, and plasmids in which the potential hydrophobic leader sequences of the antigens were deleted. For enhancement of CD4(+) T cell and antibody responses, we targeted the antigens for degradation by the endosomal/lysosomal pathway by linking the antigen to the lysosome-associated membrane protein (LAMP). We found that immunization with DNA vaccine encoding PyHEP17 fused to Ub and bearing arginine induced higher IFN-gamma, cytotoxic and proliferative T cell responses than unmodified vaccines. However, no effect was seen for PyCSP using the same targeting strategies. Regarding Class II antigen targeting, fusion to LAMP did not enhance antibody responses to either PyHEP17 or PyCSP, and resulted in a marginal increase in lymphoproliferative CD4(+) T cell responses. Our data highlight the antigen dependence of immune enhancement strategies that target antigen to the MHC Class I and II pathways for vaccine development.

Poly-beta amino ester-containing microparticles enhance the activity of nonviral genetic vaccines

Current nonviral genetic vaccine systems are less effective than viral vaccines, particularly in cancer systems where epitopes can be weakly immunogenic and antigen-presenting cell processing and presentation to T cells is down-regulated. A promising nonviral delivery method for genetic vaccines involves microencapsulation of antigen-encoding DNA, because such particles protect plasmid payloads and target them to phagocytic antigen-presenting cells. However, conventional microparticle formulations composed of poly lactic-co-glycolic acid take too long to release encapsulated payload and fail to induce high levels of target gene expression. Here, we describe a microparticle-based DNA delivery system composed of a degradable, pH-sensitive poly-beta amino ester and poly lactic-co-glycolic acid. These formulations generate an increase of 3-5 orders of magnitude in transfection efficiency and are potent activators of dendritic cells in vitro. When used as vaccines in vivo, these microparticle formulations, unlike conventional formulations, induce antigen-specific rejection of transplanted syngenic tumor cells.

Evaluation of genetic immunization adjuvants to improve the effectiveness of a human immunodeficiency virus type 2 (HIV-2) envelope DNA vaccine

In an effort to develop a more effective genetic immunization strategy for HIV, we developed an HIV-2 env DNA vaccine and evaluated three adjuvant formulations. The gp140 gene from HIV-2(UC2 )was synthesized using mammalian codons and cloned into a plasmid vector that expresses eukaryotic genes at high levels. We found that after three immunizations in mice, a novel cationic liposome formulation (Vaxfectin) was superior at inducing systemic and mucosal antibody responses compared to a naked DNA, a controlled release device (an Alzet minipump) and polysaccharide microparticles made from chitosan (P = 0.027). Vaxfectin also induced higher levels of systemic antibodies for each isotype and IgG subclass as well as levels of HIV-2-specific mucosal IgA (P = 0.034). When different routes of immunization were used with the Vaxfectin formulation, gp140-specific systemic antibody responses were highest by the intradermal route, mucosal antibody responses were highest by the intramuscular route, while the intranasal route was the least effective. These results suggest that this cationic liposome formulation is an important adjuvant to improve the effectiveness of genetic immunization strategies for AIDS, and that multiple routes of immunization should be employed for optimal efficacy for HIV vaccine candidates.

Anti-mycobacterial immunity induced by a single injection of M. leprae Hsp65-encoding plasmid DNA in biodegradable microparticles

A single sub-cutaneous injection of a plasmid DNA encoding a mycobacterial heat shock protein 65 (Hsp65) entrapped in biodegradable poly(lactic-co-glycolic acid) microspheres produced high titers of antibodies, measured 5 months after the injection in BALB/c mice. Splenocytes secreted IFN-gamma and exerted an anti-bacterial effect on macrophages infected in vitro with Mycobacterium tuberculosis. The results are encouraging with regard to obtaining good compliance and vaccination coverage with candidate plasmid DNA vaccines, especially in developing countries.

Enhanced immunogenicity of DNA fusion vaccine encoding secreted hepatitis B surface antigen and chemokine RANTES

To increase the potency of DNA vaccines, we constructed genetic fusion vaccines encoding antigen, secretion signal, and/or chemokine RANTES. The DNA vaccines encoding secreted hepatitis B surface antigen (HBsAg) were constructed by inserting HBsAg gene into an expression vector with an endoplasmic reticulum (ER)-targeting secretory signal sequence. The plasmid encoding secretory HBsAg (pER/HBs) was fused to cDNA of RANTES, generating pER/HBs/R. For comparison, HBsAg genes were cloned into pVAX1 vector with no signal sequence (pHBs), and further linked to the N-terminus of RANTES (pHBs/R). Immunofluorescence study showed the cytoplasmic localization of HBsAg protein expressed from pHBs and pHBs/R, but not from pER/HBs and pER/HBs/R at 48 h after transfection. In mice, RANTES-fused DNA vaccines more effectively elicited the levels of HBsAg-specific IgG antibodies than pHBs. All the DNA vaccines induced higher levels of IgG(2a) rather than IgG(1) antibodies. Of RANTES-fused vaccines, pER/HBs/R encoding the secreted fusion protein revealed much higher humoral and CD8(+) T cell-stimulating responses compared to pHBs/R. These results suggest that the immunogenicity of DNA vaccines could be enhanced by genetic fusion to a secretory signal peptide sequence and RANTES.

Enhanced immunogenicity of human papillomavirus 16 L1 genetic vaccines fused to an ER-targeting secretory signal peptide and RANTES

To increase the potency of human papillomavirus (HPV) DNA vaccines, we constructed a series of HPV16 L1 vaccines genetically fused with a secretion signal and/or immune cell-recruiting RANTES. The DNA vaccines encoding secretory HPV L1 were constructed by inserting HPV L1 gene into a vector with an ER-targeting secretory signal sequence. The expression plasmid encoding secretory HPV L1 (pER/L1) was fused with cDNA of RANTES, generating pER/L1/R. For comparison, HPV L1 genes were cloned into pVAX1 vector with no signal sequence (pL1), and further linked to the N-terminus (pL1/R) or C-terminus of RANTES (pR/L1). The secretion of L1 proteins was observed in the pER/L1, pER/L1/R, and pR/L1-transfected cells, except the pL1/R-transfected group. Cytoplasmic localization of L1 protein was observed in the cells transfected with pL1/R, but not with pER/L1/R at 48 h after transfection. In mice, RANTES-fused vaccines more effectively elicited the levels of HPV16 L1-specific IgG and IgG2a antibodies than pL1. Of RANTES-fused vaccines, pER/L1/R encoding the secreted fusion protein induced the highest humoral and CD8(+) T-cell-stimulating responses. These results suggest that the immunogenicity of HPV L1 DNA vaccines could be enhanced by genetic fusion to a chemokine and secretory signal peptide sequences.