Comparison of polystyrene nanoparticles and UV-inactivated antigen-displaying adenovirus for vaccine delivery in mice
© Johrden et al.; licensee BioMed Central Ltd. 2013
Received: 27 September 2012
Accepted: 21 March 2013
Published: 5 April 2013
Inert nanoparticles are attracting attention as carriers for protein-based vaccines. Here we evaluate the immunogenicity of the model antigen ovalbumin delivered on polystyrene particles and directly compare particulate delivery with adenovirus-based immunization.
Mice were vaccinated with soluble ovalbumin, ovalbumin-coated polystyrene particles of different sizes, or an adenovirus-based expression-display vector that encodes and displays a pIX-ovalbumin fusion protein. Antibody responses were clearly higher when ovalbumin was administered on polystyrene particles compared to soluble protein administration, regardless of the particle size. Compared to adenovirus-based immunization, antibody levels were lower if an equivalent amount of protein was delivered, and no cellular immune response was detectable.
We demonstrate in a side-by-side comparison that inert nanoparticles allow for the reduction of the administered antigen amount compared to immunization with soluble protein and induce strongly enhanced antibody responses, but responses are lower compared to adenovirus-based immunization.
Protein and peptide vaccines are generally regarded as safe; however, in most cases they require the co-administration of adjuvants to be effective, which may in turn cause adverse events . Therefore it is desirable to develop safe and clinically effective non-adjuvanted vaccines, and one approach is the use of nano- or microparticles coated with antigen, which can enhance immunogenicity compared to soluble antigen [2–4]. In this study we analyzed polystyrene particles coated with the model antigen ovalbumin for their potency at inducing humoral and cellular responses. As adenoviral (Ad) vectors are very popular for vaccine development because of their high immunogenicity and strong immune responses to the delivered vaccine antigens, we made side-by-side comparisons of coated polystyrene particles of varying sizes with an Ad expression-display vector, which we showed before to induce especially good CD4+ T cell and antibody responses .
Carboxyl-modified polystyrene beads of 24, 60, 93, 220 and 340 nm in diameter (BangsLabs/Polysciences, Fishers, IN) were loaded with recombinant ovalbumin (Sigma, Munich, Germany) by the carbodiimide method , and antigen loading was verified by dot blot and enzyme-linked immunosorbent assay (ELISA; data not shown). For immunization experiments, 8–9 weeks old C57BL/6 mice (Élevage Janvier, Le Genest St Isle, France) were used. Mice were treated in accordance with the regulations and guidelines of the Institutional Animal Care and Use Committee of the Ruhr University Bochum, Germany.
To analyze whether the induction of antibodies by protein-loaded polystyrene particles is dependent on CD4+ T cell help, mice were immunized twice with 93 nm polystyrene particles delivering the intermediate amount of 500 ng ovalbumin, and depleted of CD4+ T cells around both time points of immunization by intraperitoneal injection of a CD4 antibody . After mice were depleted of CD4+ cells during vaccination, no IgG1 antibodies were detectable (data not shown), suggesting the necessity of CD4+ T cell help for antibody induction.
Adenoviral vectors are very popular tools for vaccine development and they are known to be highly immunogenic . We compared immune responses induced by polystyrene particles loaded with ovalbumin with an adenoviral ovalbumin expression-display vector; this Ad5 vector Ad.pIXova encodes a fusion protein of the capsid protein IX and ovalbumin, and hence displays ovalbumin on the particle . The ovalbumin content of Ad.pIXova was determined by ELISA to be 50 ng per 1010 viral particles (vp; data not shown). To include a control for the influence of in vivo expression of ovalbumin from the adenoviral vector, a UV-inactivated  Ad.pIXova (UV-AdpIXova) was included. Mice were immunized twice in a three-week interval by intradermal injection into the hind footpads with 50 ng soluble ovalbumin, 50 ng ovalbumin coupled to 24 nm, 93 nm, or 340 nm polystyrene particles, or 1010 vp Ad.pIXova or UV-AdpIXova, equaling a protein delivery of 50 ng ovalbumin; immune responses were analyzed three weeks after the second immunization.
For the analysis of cellular immune responses, splenocytes were isolated and restimulated in vitro with ovalbumin-derived peptides [11, 12], and flow-cytometric analysis of intracellular cytokine levels was performed. The numbers of IFNγ producing CD8+ (Figure 2C) and CD4+ T cells (Figure 2D) were highest in mice immunized with Ad.pIXova, and significantly lower when mice were immunized with the UV-inactivated UV-AdpIXova or polystyrene particles carrying 50 ng ovalbumin. While there is a tendency to higher CD8+ T cell levels in mice immunized with UV-AdpIXova than polystyrene particles, the levels are comparable in these groups and underline the importance of antigen expression by the Ad vector for strong CTL induction, as well as for induction of CD4+ T cell responses, as was described before . A similar trend was observed when the concentration of IL4 was analyzed in the supernatant of restimulated splenocytes, although the levels were rather low in all groups (data not shown).
Comparing the immune responses induced by adenovirus-based immunization with immune responses induced by polystyrene particles loaded with a high amount of protein, i.e. 5 μg, it can be observed that the antibody levels that can be reached by immunization with protein-coated 93 nm particles are actually comparable in strength. It has been reported before that the particle size determines the induced immune response (reviewed in ), and our findings confirm this, as the best antibody response was found for the 93 nm particles, while only the smallest, 24 nm particles, that can most easily enter lymphatic structures on their own , could induce a significant, albeit low, CD8+ T cell response (data not shown).
Our results indicate that antigen coupling to polystyrene particles, although they are immunologically inert, induces enhanced antibody responses in comparison to immunization with soluble protein, and thus allows for the reduction of protein amount used for protein-based immunization. In contrast to adenoviral vectors, delivery on polystyrene particles does not change the antibody profile compared to protein immunization, which consists predominantly of IgG1 subtype antibodies.
We confirmed that polystyrene particles themselves are not immunogenic, having no particular effect on DC activation and cytokine production, thus they should be associated with very little risk of inducing autoimmunity as recently described for other vaccine adjuvants  (and reviewed in ). Rather, the enhancing effect of polystyrene particle delivery seems to rely mainly on the particulate nature and the size of the particles, as well as on the presentation of the antigen in an ordered array. Polystyrene particles are inert but not degradable, however, biodegradable particles such as dextran particles should be comparably adequate protein carriers and appropriate for use in humans. While the cellular immune response to adenoviral vectors was much higher than to antigen-coated polystyrene particles, in cases where protection can be conferred by humoral immunity, or for protein delivery in prime-boost combinations with viral vectors, particle-mediated delivery is an attractive option to efficiently and safely enhance immune responses to protein-based vaccines.
This work was supported by grants from the Deutsche Forschungsgemeinschaft (DFG grants GRK 1045 to OW, WI 1329/6-1 to WB) and by a grant from the Krebshilfe to OW.
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