Optical and biological evaluation of upconverting Gd2O3:Tb3+/Er3+ particles as microcarriers of a Zika virus antigenic peptide

New adjuvants are required in the field of vaccinology to enhance the immunogenicity and efficacy of subunit vaccines, which constitute a safer approach to protect against infectious diseases compared to whole pathogen-based vaccines. In the present study, rare-earth upconverting microparticles were explored as possible antigen carriers with adjuvant properties by using a peptide from the Zika virus, an emerging pathogen to which no vaccines are available. The Gd2O3:Tb3+/Er3+ (UpGO) carriers were prepared by hydrothermal synthesis and modified by a reaction with (3-Aminopropyl)triethoxysilane (APTES), in order to introduce reactive amine groups at their surfaces to anchor the ZK2 peptide. The system was fully characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), photoluminescence spectroscopy (PL), Fourier-transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS). The results showed a stable colloidal system of APTES-UpGO with a high loading capacity of up to 0.480 mu g of ZK2 peptide per mu g of microparticles. For the in vivo studies, the system was administered to mice by the subcutaneous route and enhanced humoral responses were observed when UpGO was used as the antigen carrier as compared with the group immunized with the peptide alone. The immune response showed a Th2-polarization as revealed IgG subclass analysis. The present study provides new insights into the use of lanthanides-based materials as antigen carriers during vaccine development.