Immunogenicity of poxvirus A16/G9 entry–fusion subcomplex and its restriction by A56/K2 protein informs vaccine design

The finding that antibodies binding to G9 and A16 were elicited by soluble proteins but were undetectable or minimal in the blood of mice, monkeys and humans that had been vaccinated with the JYNNEOS MVA vaccine or infected with pathogenic VACV, CPXV or MPXV was intriguing. The latter findings align with a recent report of low A16/G9 antibody levels elicited by MVA and by an earlier study of antibodies elicited by recipients of the DryVax smallpox vaccine. Several factors may contribute to a generally low immune response to the EFC proteins, including the assembly of MVs in the cytoplasm resulting in the absence of EFC proteins displayed in the plasma membrane, the shielding of MV proteins by the EV membrane, and the 10–20-fold lower abundance of EFC proteins relative to the major attachment proteins. The concealment of the EFC by an additional membrane in extracellular virus and the binding of fusion suppressors to A16/G9 constitute immune evasion mechanisms unique to poxviruses. For these reasons, recombinant mRNA and virus-like particle vaccines that elicit protective antibodies to multiple proteins including A16/G9 and attenuated live OPXVs that express secreted EFC proteins or that have deletions of the A56/K2 genes may have advantages over current vaccines.