Circular RNA vaccines with long-term lymph node-targeting delivery stability after lyophilization induce potent and persistent immune responses
Authors: Wan Jiawu , Wang Zongmei , Wang Lingli , Wu Liqin , Zhang Chengguang , Zhou Ming , Fu Zhen F. , Zhao Ling 
Published: December 2023

ABSTRACT
Several recent attempts to improve the stability and immunogenicity of messenger RNA (mRNA) vaccines include the use of circular RNA (circRNA), targeted delivery, and lyophilization. However, these research directions have often been pursued independently, ignoring the impact of the modification of lipid nanoparticle-encapsulated mRNA vaccines on their targeted delivery after lyophilization and on their subsequent immunogenicity. Here, we develop a circRNA vaccine targeting lymph nodes that expresses rabies virus glycoprotein (G), termed circRNA-G. Mannose modification is introduced directly into the process of synthesizing PEG lipids, and the resulting PEG-mannose can be used in the preparation of mannose-LNPs (mLNPs) that target dendritic cells, thereby promoting the specific distribution of circRNA-G to lymph nodes (mLNP-circRNA-G). We demonstrated that mLNP-circRNA-G has continuous antigen availability that promotes the generation of T follicular helper cells, germinal center B cells, long-lived plasma cells, and memory B cells in mice. Importantly, the vaccine with this targeting modification remained stable for at least 24 weeks of storage at 4℃ after lyophilization, and its immunogenicity was also maintained. Notably, this strategy also enhances the antibody production of the SARS-CoV-2 trimeric receptor-binding domain circRNA vaccine and the stability of immunogenicity after lyophilization. In summary, this study provides a general platform for the design of lyophilized vaccines with targeted stability, demonstrating the potential of lymph node-targeting circRNAs as next-generation vaccines.

IMPORTANCE
messenger RNA (mRNA) vaccines are a key technology in combating existing and emerging infectious diseases. However, the inherent instability of mRNA and the nonspecificity of lipid nanoparticle-encapsulated (LNP) delivery systems result in the need for cold storage and a relatively short-duration immune response to mRNA vaccines. Herein, we develop a novel vaccine in the form of circRNAs encapsulated in LNPs, and the circular structure of the circRNAs enhances their stability. Lyophilization is considered the most effective method for the long-term preservation of RNA vaccines. However, this process may result in irreversible damage to the nanoparticles, particularly the potential disruption of targeting modifications on LNPs. During the selection of lymph node-targeting ligands, we found that LNPs modified with mannose maintained their physical properties almost unchanged after lyophilization. Additionally, the targeting specificity and immunogenicity remained unaffected. In contrast, even with the addition of cryoprotectants such as sucrose, the physical properties of LNPs were impaired, leading to an obvious decrease in immunogenicity. This may be attributed to the protective role of mannose on the surface of LNPs during lyophilization. Freshly prepared and lyophilized mLNP-circRNA vaccines elicited comparable immune responses in both the rabies virus model and the SARS-CoV-2 model. Our data demonstrated that mLNP-circRNA vaccines elicit robust immune responses while improving stability after lyophilization, with no compromise in tissue targeting specificity. Therefore, mannose-modified LNP-circRNA vaccines represent a promising vaccine design strategy.