Heterologous Prime-Boost Immunization Strategies Using Varicella-Zoster Virus gE mRNA Vaccine and Adjuvanted Protein Subunit Vaccine Triggered Superior Cell Immune Response in Middle-Aged Mice

Purpose: Heterologous immunization using different vaccine platforms has been demonstrated as an efficient strategy to enhance antigen-specific immune responses. In this study, we performed a head-to-head comparison of both humoral and cellular immune response induced by different prime-boost immunization regimens of mRNA vaccine and adjuvanted protein subunit vaccine against varicella-zoster virus (VZV) in middle-aged mice, aiming to get a better understanding of the influence of vaccination schedule on immune response. Methods: VZV glycoprotein (gE) mRNA was synthesized and encapsulated into SM-102-based lipid nanoparticles (LNPs). VZVprimed middle-aged C57BL/6 mice were then subjected to homologous and heterologous prime-boost immunization strategies using VZV gE mRNA vaccine (RNA-gE) and protein subunit vaccine (PS-gE). The antigen-specific antibodies were evaluated using enzyme-linked immunosorbent assay (ELISA) analysis. Additionally, cell-mediated immunity (CMI) was detected using ELISPOT assay and flow cytometry. Besides, in vivo safety profiles were also evaluated and compared. Results: The mRNA-loaded lipid nanoparticles had a hydrodynamic diameter of approximately 130 nm and a polydispersity index of 0.156. Total IgG antibody levels exhibited no significant differences among different immunization strategies. However, mice received 2xRNA-gE or RNA-gE>PS-gE showed a lower IgG1/IgG2c ratio than those received 2xPS-gE and PS-gE> RNA-gE. The CMI response induced by 2xRNA-gE or RNA-gE>PS-gE was significantly stronger than that induced by 2xPS-gE and PS-gE> RNA-gE. The safety evaluation indicated that both mRNA vaccine and protein vaccine induced a transient body weight loss in mice. Furthermore, the protein vaccine produced a notable inflammatory response at the injection sites, while the mRNA vaccine showed no observable inflammation. Conclusion: The heterologous prime-boost strategy has demonstrated that an mRNA-primed immunization regimen can induce a better cellmediated immune response than a protein subunit-primed regimen in middle-aged mice. These findings provide valuable insights into the design and optimization of VZV vaccines with the potentials to broaden varicella vaccination strategies in the future.