(209c) Improving Immune Responses to Nanoparticle-Based Vaccines Using Injectable Hydrogel Delivery

The control of antigen delivery rates show great potential to enhance neutralizing antibody and germinal center responses. In this study, we examined the immunological effects to the slow release of a nanoparticle vaccine by encapsulating it in a thermally-sensitive hydrogel depot. This vaccine has been designed to elicit a response to Coxiella burnetti, a bacterium that causes Q-fever and is described by the CDC as a potential bioterrorism agent, but for which there is currently no FDA-approved vaccine. In the nanoparticle vaccine design, we used the SpyTag/SpyCatcher (ST/SC) approach to conjugate an immunodominant antigen from C. burnetii (CBU1910) and an adjuvant (CpG) onto E2 protein nanoparticles (E2). The final nanoparticle assembly with antigen and adjuvant (CBU1910-CpG-E2) was approximately 32 nm in diameter, which is within the optimal DC-uptake size range. The hydrogel material for the controlled release of the nanoparticle vaccine was poly(D,L-lactide-co-glycolide)-b-poly(ethyleneglycol)-b-poly(D,L-lactide-co-glycolide) (PLGA-PEG-PLGA), a copolymer which can be mixed with antigen and adjuvants as an injectable solution at room temperature but after injection forms a hydrogel upon physiological temperature. Using an IVIS imaging system, we showed that the slow release of fluorescently-labeled nanoparticles from the hydrogel depot material lasted up to eight weeks in vivo. The gelling temperature of the hydrogel was not sensitive to the concentration of the protein antigen within the dose range, and the protein released from the depot material retained its natural primary and secondary structures. After in vivo immunization, we observed that stronger IgG, IgG1, and IgG2c responses were elicited with CBU1910-CpG-E2 vaccine nanoparticles that were embedded in the PLGA-PEG-PLGA hydrogel, relative to the free CBU1910 antigen (no nanoparticle, no adjuvant), the CBU1910 antigen embedded in hydrogel, and the CBU1910-CpG-E2 nanoparticle without hydrogel. The group for CBU1910-CpG-E2 in the hydrogel also demonstrated a more balanced IgG1/IgG2c ratio when compared with the control groups, a feature which is important for resisting C. burnetii infection due to the need for both Th2- and Th1-mediated responses. Our results point towards the potential of using hydrogels in vaccine delivery to modulate and improve immune responses.