A Quadrivalent mRNA immunization elicits potent immune responses against vaccinia and monkeypox viral antigens - a step closer to a broad orthopoxvirus vaccine

The global outbreak of the 2022 monkeypox virus infection of human raised the public health concerns of the threat of human-to-human transmission of zoonotic diseases. Given the evidence that other orthopoxviruses including cowpox and camelpox were also reported infectious to human, and that the reemerging risk of smallpox as a bioterrorist or accidental laboratory escape exists, there is an urgent need to develop a poxvirus vaccine with a broad protection of orthopoxviruses to stockpile for future emergency. Extensive studies of vaccinia virus (VACV) suggested that multiple VACV antigens, such as A27, L1, A33 and B5, showed high level similarity in terms of immunogenicity to their respective homologous antigens of other orthopoxviruses. These findings paved the ground for VACV antigens to be used as potential vaccine targets for development of a universal poxvirus vaccine. In this study, we construct a novel poxvirus vaccine candidate, mRNA-ALAB-LNP, encoding four vaccinia viral antigens A27, L1, A33 and B5. Strong anti-L1-specific antibody and moderate anti-A33-, anti-A27- and anti-B5-specific antibody responses were induced in mice after a single immunization. The antibody responses to all four antigens were significantly boosted after the second shot with all IgG titers >5 logs and highest being anti-A33 IgG. The high level of binding antibodies showed potent neutralizing capability against vaccinia virus. Specific IFN-{gamma} responses were detected to all four antigens with the highest cellular response being that induced by the same antigen, A33. When evaluating the cross reactivity, equivalent or better serum IgG responses were seen in responses to corresponding monkeypox antigens A35, M1, A29 and B6, in comparison to vaccinia antigens. Apparently, the mRNA vaccine encoding four vaccinia antigens induced immunity not only to vaccinia virus but also to monkeypox, suggesting that the mRNA-ALAB may be a candidate for potential vaccine development against infection of monkeypox, smallpox and possibly other orthopoxviruses.