2772. HCMV gB Ectodomain Subunit and gB mRNA Vaccines Reduce AD-3 Immunodominance and Elicit More Durable Antibody Responses Than gB/MF59 Immunization

Abstract Background A vaccine to prevent maternal acquisition of human cytomegalovirus (HCMV) during pregnancy is one potential strategy to reduce the incidence of congenital disease. The MF59-adjuvanted glycoprotein B (gB/MF59) protein subunit vaccine is the most efficacious tested to-date, though achieved only 50% efficacy in phase 2 trial. We previously identified that gB/MF59 vaccination elicited poor heterologous virus neutralization and an immunodominant response against non-neutralizing/cytosolic antigenic domain 3 (AD-3) (Figure 1). Thus, we sought novel gB vaccination strategies to improve functional antibody responses and reduce AD-3 immunodominance. Methods Groups of juvenile New Zealand White rabbits (n = 6) were administered 3 sequential doses of gB protein with an MF59-like squalene adjuvant IM, gB ectodomain protein (lacking AD-3) + squalene adjuvant IM, or lipid nanoparticle (LNP)-packaged nucleoside-modified mRNA encoding gB ID. Results The AD-3 immunodominant IgG response seen in human vaccinees was closely mimicked in rabbits, with 78% of binding antibodies directed against this region in the gB protein group compared with 1% and 46% in the ectodomain and mRNA-LNP-vaccinated groups respectively (Figure 2). All vaccines were highly immunogenic with similar kinetics and comparable peak gB-binding/functional antibody responses. However, both ectodomain and mRNA-LNP-immunized rabbits exhibited enhanced durability of IgG binding to gB protein (P = 0.04 and 0.02, respectively), and the mRNA-LNP group had more durable binding of cell membrane-associated gB (P < 0.001) (Figure 3). Additionally, ectodomain and mRNA-LNP-vaccinated rabbits had increased durability of antibodies targeting neutralizing epitopes AD-4 and AD-5 (P < 0.01). Finally, low-magnitude gB-specific T-cell activity was observed in the gB protein and mRNA-LNP groups, though not in ectodomain-vaccinated rabbits. Conclusion Altogether these data suggest that gB ectodomain subunit and gB mRNA-LNP vaccine formulations reduced targeting of non-neutralizing epitope AD-3 and elicited more durable IgG responses than gB protein vaccination. These next-generation HCMV vaccine candidates aiming to improve upon the partial efficacy of gB/MF59 vaccination should be further evaluated in preclinical models. Disclosures All authors: No reported disclosures.